Yolk/shell nanoparticles: classifications, synthesis ...

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Yolk/shell nanoparticles: classifications, synthesis, properties, and applications Rahul Purbia and Santanu Paria* Core/shell nanoparticles were first reported in the early 1990s with a simple spherical core and shell structure, but the area is gradually diversifying in multiple directions such as different shapes, multishells, yolk/shell etc., because of the development of different new properties of the materials, which are useful for several advanced applications. Among different sub-areas of core/shell nanoparticles, yolk/shell nanoparticles (YS NPs) have drawn significant attention in recent years because of their unique properties such as low density, large surface area, ease of interior core functionalization, a good molecular loading capacity in the void space, tunable interstitial void space, and a hollow outer shell. The YS NPs have better properties over simple core/shell or hollow NPs in various fields including biomedical, catalysis, sensors,

Received 14th July 2015, Accepted 21st October 2015

lithium batteries, adsorbents, DSSCs, microwave absorbers etc., mainly because of the presence of free

DOI: 10.1039/c5nr04729c

void space, porous hollow shell, and free core surface. This review presents an extensive classification of YS NPs based on their structures and types of materials, along with synthesis strategies, properties, and

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applications with which one would be able to draw a complete picture of this area.

1.

Introduction

The term ‘nanoparticle’ is defined as a particle of at least any one of the three possible dimensions within the range of Interfaces and Nanomaterials Laboratory, Department of Chemical Engineering, National Institute of Technology, Rourkela-769008, India. E-mail: [email protected], [email protected]; Fax: +91 661 246 2999

Rahul Purbia hails from Rajsamand, Rajasthan, India. He received his B. Tech.–M. Tech. integrated degree in Nanotechnology from the Centre for Converging Technologies, University of Rajasthan (2012), India. He then worked as a post graduate trainee under the guidance of Dr Neha Y. Hebalkar at the Centre for Nanomaterials in ARCI Hyderabad, India. Currently, he is pursuing his docRahul Purbia toral research under the supervision of Dr Santanu Paria at the Department of Chemical Engineering, National Institute of Technology, Rourkela, India. His main research interests include the synthesis of yolk/shell, hollow, and core/shell nanoparticles for different applications.

This journal is © The Royal Society of Chemistry 2015

1–100 nm. Nanoparticles (NPs) are considered as the building blocks for various applications of nanotechnology. Owing to this fact, NPs have been proved to be the backbone of almost all branches of nanoscience and engineering. Over the years, researchers have been exploring the advantages of structurally different NPs rather than those of simple spherical particles. To develop exciting new and advanced physicochemical properties of the nanomaterials, precise control over the size,

Santanu Paria is currently an Associate Professor of Department of Chemical Engineering at the National Institute of Technology, Rourkela, India. He obtained his Ph.D. degree in 2003 from Indian Institute of Technology, Bombay in Chemical Engineering and performed his postdoctoral studies at the Department of Chemical Engineering, Dalhousie University, Halifax, Canada. Prof. Paria Santanu Paria joined the National Institute of Technology in 2006, where his research interests include the synthesis of core/shell, doped, and yolk/shell nanoparticles for photocatalytic, antimicrobial, and molecular sensor applications; as well as colloids and interfacial phenomena. He has published more than 45 papers in different peer reviewed journals.

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shape, and morphology of NPs is becoming the prime focus for nanotechnologists. During the past few decades, there has been exponential growth in the development of different NPs with complex structures over the simple spherical structure. Thus, the appropriate design of different NPs along with the properties of materials is the prime objective of material scientists and engineers to obtain improved performances in several applications such as photovoltaics, chemical catalysis, photocatalysis, sensors, photonics, rechargeable batteries, biomedical, and so on. During the early stages of nanotechnology, NPs were given significant importance because of the higher surface to volume ratio and improved functionality compared to those of bulk materials. However, over a period of time, the advancement of NP research is moving towards the directions of anisotropic shape,1–6 hollow,7–12 core/shell (CS),13–15 yolk/ shell (YS),8,9,12,16–28 janus,29,30 composite,19 doped31–34 etc., because of their higher surface area, improved functionalizability, and other physicochemical properties. The hollow NPs are important for different applications mainly because of their high surface area and available inner void space. The core/shell (concentric multilayer nanoparticles consisting of two or more materials) NPs have also been developed gradually from the early 1990s onwards for many advanced applications to exhibit extraordinary or new properties that arise mainly from the synergism between two or more materials. As a result, for more than two decades, core/shell NPs have been intensively studied by several research groups owing to their improved optical, electronic, magnetic, and catalytic properties. Further, to bring the advantages of these two classes of morphology together, in late 2002 YS NPs were first reported.35,36 The YS NPs are defined as a hybrid structure (mixture of core/shell and hollow) where a core particle is encapsulated inside the hollow shell and may move freely inside the shell, generally represented as a core/void/shell. The YS nanostructures are also termed as movable core/shell or rattle-type nanostructures. 1.1.

Different terminologies of yolk/shell NPs

There are different terminologies used so far by several researchers to represent this class of materials. The different terminologies used by different researchers when they reported for the first time, as per as our knowledge, are presented below: (a) Nanorattle: in 2002, Hyeon and co-workers used the terminology of ‘nanorattels’,35 which was inspired by the rattle toy. As core nanoparticles were encapsulated inside the hollow shells they termed the structure as nanorattle. (b) Movable core/shell: the movable core terminology was first demonstrated by Xia and coworkers in 2003.36 They used the terminology where the core was entrapped inside the hollow shell and the core was movable when dispersed in a liquid and stick on the wall surface in air. (c) Yolk/shell: the yolk/shell terminology was first used by Alivisatos and coworkers37 in 2004; later in 2007 Xu’s38 group also used the same terminology, where the core particle behaved like a movable yolk inside the hollow shell. (d) Core/shell with hollow interiors: many researchers also introduced new terminology (core/shell with

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hollow interiors) deviating from the widely used terminologies mentioned before. However, all of the researchers demonstrated that the core is encapsulated inside the hollow shell and the core may move under the favorable conditions. In this review, we used the terminology of YS for consistency where the core is defined as a movable core. 1.2

Core/shell vs. YS NPs

The properties of core/shell NPs are mainly dependent on the sequence of core and shell, the sizes of both core and shell, and materials combination. In contrast, YS is a hybrid structure consisting of a movable core inside the hollow shell of the same or different materials. It can be derived from the core/sacrificial shell/outer shell structure via removal of the sacrificial layer. The basic difference between the core/shell and YS is the presence of void space derived from the sacrificial layer which gives rise to the multifunctional and other unique properties. Since the YS nanostructures are a hybrid of hollow and core/shell structures, they have the advantages of both structures together. As per the reported studies a few specific advantages of YS NPs over the core/shell NPs are listed here and discussed later in the respective sections. (i) The YS NPs can be synthesized even from a single material to enhance the specific surface area.39–49 (ii) The core surface is unblocked compared to the CS NPs; as a result, YS NPs provide more active sites and higher surface area.50–56 (iii) The void space is suitable to accommodate the guest molecules.39,57–60 (iv) The shell layer provides more active inner and outer surfaces.61–63 (v) The void space provides space for the expansion of core NPs in many applications.64–67 1.3

Types of YS NPs

The YS NPs can be broadly classified into ‘spherical’ and ‘nonspherical’ structures depending on their core and shell morphologies, without considering their material properties. In the first case, both the core and the shell are spherical in shape, and in the latter case at least one structure should be non-spherical. Further, the YS structures under the head of spherical shape can be classified into five distinct subcategories such as: (i) single core/shell, (ii) multi-cores/single shell, (iii) single core/multi-shell, (iv) multi-cores/shells, and (v) multi-shells or shell in shell. Different types of reported spherical YS NPs are shown in Fig. 1. Non-spherical YS NPs are also divided into two categories based on their geometry: (i) complete non-spherical YS NPs, where both the core and the shell have a non-spherical shape, (ii) partially non-spherical YS NPs, where either the core or the shell of the YS has a non-spherical shape. 1.4

Approaches for YS NPs synthesis

Synthesis approaches play a crucial role in the core size and shape, void space, outer shell thickness, shell porosity and shape to fabricate the YS NPs. Generally, all types of YS NPs are synthesized by the bottom-up approach because of the complexity of the structure. However, based on the synthesis sequences, the approaches are of two types, such as (i) core-toshell, where core/shell/shell type NPs are synthesized first and


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Fig. 1 Different classes of spherical YS NPs (a) Single core/shell. Reprinted with permission from ref. 68. Copyright 2012 from ESG Publiser, (b) Single core multi shells. Reprinted with permission from ref. 69. Copyright 2013 from Wiley-VCH Verlag, (c) Multiple cores single shell. Reprinted with permission from ref. 70. Copyright 2012 from Wiley-VCH Verlag, (d) Multi-cores/shells. Reprinted with permission from ref. 71. Copyright 2011 from American Chemical Society, (e) Multi shells. Reprinted with permission from ref. 72. Copyright 2013 from the Royal Society of Chemistry (f–j) Schematic presentations of the respective classes.

then a sacrificial shell layer is removed by a suitable method to obtain the final structure. In this approach, selective etching or calcination of the sacrificial template, galvanic replacement, Kirkendall effect, and Ostwald ripening are used. (ii) Shell-tocore, such as the ship-in-bottle method. In the second approach, hollow shells are synthesized first, and then core NPs are encapsulated inside the hollow shell. The first approach is widely used by several researchers while the second approach is fewer in number. As per the present status of this area, significant research efforts have already been made to develop maximum possible structures of YS NPs. However, still there are huge possibilities ahead to control the size and shape of movable cores and shells with appropriate synthesis methods, suitable functionalization of core and shell surfaces for desired applications, and selection of appropriate materials to obtain improved desired properties. 1.5

(DSSCs),43,159,160 microwave absorber161–165 etc. In catalytic applications, the presence of a mesoporous shell serves as a protective shield to protect the core catalyst NPs from the harsh environmental conditions (chemical and thermal) and also prevents the core from agglomeration. Similarly, as an electrocatalyst, the metallic shell on an electrocatalyst core (bimetallic YS NPs) exhibits high activity, high CO tolerance, and reduced loading of Pt catalysts as compared to conventional Pt catalysts with improved stability, electrical conductivity, reactivity, and optical and electronic properties.107–110,166,167 In biomedical applications, the large void space between the core and hollow shell is particularly suitable for encapsulation of guest species such as fluorescent and drug molecules with a specific target functionality. The void space also provides a buffering space for electroactive core material during the charging of lithium batteries. Similarly, there are several advantages for other specific applications which will be discussed later.

Importance of YS structure

The YS NPs are under a specific class of CS NPs, where the presence of void space makes them so special in several nanotechnology applications. The unique properties arise mainly from the movable core, hollow shell, and void space of the YS NPs as mentioned before. In general, the final YS structure shows individual or synergistic properties of core and shell materials. Additionally, the properties of YS NPs can also be modified via selection of desired core and shell materials, and tuning of other parameters such as core size, void space, shell thickness and porosity. The importance of YS NPs for selective applications is also summarized in some recent reviews.16,20,22,24,73 The YS NPs are important in various fields including biomedical,39,57,59,60,71,74–89 catalysis,42,50,56,70,90–110 sensors,63,111–117 lithium batteries,49,64–69,72,118–146 57,59,100,147–158 adsorbents, dye-sensitized solar cells


1.6

Scope of this review

A review plays an important role to bring the research advancements in a specific area under a single umbrella. After the first reported studies on YS in 2002, the development of this topic is gradually diversifying in different directions such as different shapes, multishells, multicores etc., because of the advantages of generation of different new properties with several important applications. While analyzing the research papers on YS NPs (synthesis, properties and applications), it has been found that both the quality and quantity of publications are increasing continuously over the last decade as presented in Fig. 2. In spite of the fact that research publications are increasing exponentially on this particular topic during the last two decades, there is no complete extensive review available so far based on materials classification on this upcoming

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Fig. 2 Publications per year for YS NPs during the period 2002 to 2015 and list of published documents in journals (data collected from SciFinder scholar database).

area. There are few short specific reviews on this topic available.20,73 In addition, there are some review articles or book chapters, in which a subsection discusses this topic.8,9,12,15–28,73 Most of the reviews specifically highlighted on silica-based structures and catalytic applications.23,26,27 But so far there has been no review available considering all reported materials with a proper classification. Finally, keeping in mind the importance of this advanced nanostructure with multifunctional behaviour for many advanced applications, it is expected that there is still a strong demand for an extensive review with the updated literature on YS NPs. The novelty of this manuscript over the few available published reviews lies in the organization of the structure based on the different classes of materials reported so far, which presents a comprehensive idea of the topic with all studied materials. While organizing different materials, we also discussed their structural aspects, synthesis strategies, and major applications such as energy, the environment, and bio-medical fields. Most frequently, we also compared the YS NPs with simple CS NPs to show the superiority of the YS structure.

2. Classification of YS NPs There are a variety of YS NPs reported so far with unique properties for several advanced applications. For a better understanding of this topic, a systematic organization of all available literature reports is highly essential. Because of this reason, we intend to discuss all possible types of YS structures here.

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Broadly, based on their shapes, these are classified into spherical and non-spherical structures. In a spherical structure, both the core and the shell are spherical in shape, and in the latter case at least one structure should be non-spherical. Then both structures are subdivided into several parts based on their core and shell materials properties. 2.1

Spherical shape YS NPs

The spherical shape YS structures, based on the presence of a number of cores and shells, can be divided into five distinct subcategories: (i) single core/shell nanostructures where a simple spherical core nanoparticle is encapsulated in a hollow shell, (ii) multi-core/single shell, where multiple cores are encapsulated in a single shell, (iii) single core/multi-shell, where a core is encapsulated inside multiple shells, (iv) multicore/shell, where multiple cores are encapsulated in multiple shells together, and (v) multi-shells, where multiple shells are present together one inside another. In this case, the core also has a hollow structure. 2.1.1 Single core/shell YS NPs. The YS NPs under this class are the simplest one made of either the same or different materials, one as the core and the other as the shell. A majority of the reported YS studies are under this category, where different types of materials with their maximum probable combinations are reported for their improved physicochemical properties with advanced applications. In this section, we have classified the materials broadly as organic and inorganic; then depending on the combinations of the core and shell materials,


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further we classified them into three groups: (i) inorganic/ inorganic, (ii) inorganic/polymer, and (iii) polymer/polymer. 2.1.1.1 Inorganic/inorganic YS NPs. The inorganic/ inorganic combinations of different materials have been most studied among the other two classes. This is mainly because of a wide range of unique optical, magnetic, electrical, photoelectrochemical, electrochemical, mechanical, thermal, and catalytic properties of different inorganic materials, which can be manipulated for a variety of suitable applications. The proper combination of these materials in the form of YS with tunable core size, void space, and shell thickness makes them important for advanced applications. In this category, most of the studies reported are on silica based YS NPs, because of their advantage as a sacrificial template and an easy synthesis process. As the majority of YS NPs are silica based, depending on the specific type of material YS NPs are again divided into the following sub-classes: (i) core with a silica shell and (ii) core without a silica shell. 2.1.1.1.1 Core with a silica shell. In the area of nanotechnology, among various types of inorganic materials, silica is one of the most studied materials because of its wide range of applications such as biomedical, chemical, and electronics, which has also been reflected in the form of YS nanostructures. As a result, several research articles and reviews have been published where the synthesis, properties and applications of simple or mesoporous silica NPs are highlighted.168,169 Subsequently, there are many studies reported on the use of a silica shell on metal, metal oxide, and many other inorganic core materials. After summarizing all these research and review papers it has been found that there are several advantages of the silica shell such as: (i) simplicity of the sol–gel reaction to synthesize silica, (ii) chemically inert and non-toxic nature (suitable for biological and chemical applications),168 (iii) coating on noble metal cores does not affect the LSPR (localized surface plasmon resonance) properties of noble metallic NPs because of the reduction of electromagnetic coupling between metallic nanoparticles,170 (iv) optical transparency towards electromagnetic radiation of the wavelength range 300–800 nm,26,170,171 (v) transparency towards a magnetic field,26 (vi) versatility in the design of diverse surface morphologies, and functionalization via surface modification for improved biocompatibility (important for drug-delivery applications),168,170,172–174 (vii) acting as a catalyst support (when the core acts as a catalyst), leading to high catalytic activity,175 (viii) increase in the suspension stability of core particles by reducing the bulk conductivity15 and increasing the steric repulsion,56 (ix) acting as a sacrificial layer for creating a YS structure,21,176 (x) tunable pore diameter leads to high specific surface area,55 and (xi) sinter stable nature or tolerance of high temperature.170 Owing to these advantages, the YS NPs with a silica shell are used in various specific fields such as catalysis,57,92,94,96,147,148,157,177–179 separation,57,147,148 170 57,81,83,87,88,92,180–183 sensors, biomedical, and microwave absorption.184 This section aims to provide a complete overview of the inorganic nanoparticle encapsulation into a hollow silica shell with new properties and applications. Since a wide range


Review

of inorganic core materials have been reported, based on the majority of available literature reports, core materials are broadly classified into two types: (i) metal and (ii) metal oxide. 2.1.1.1.1.1 Metal/silica YS NPs. The unique structural, optical, magnetic, catalytic, and quantum properties of pure metallic NPs are most useful in the fields of optoelectronics,185 chemical sensor,186 water purification,187 catalysis,188 and biomedical (diagnosis,189 sensor,186,190 therapy189). Among different metals, noble and magnetic metallic NPs are the most popular classes of metals used in the mentioned applications. The noble metals are corrosion resistant because of the lack of reactivity in the presence of humid and mild acidic conditions. Mostly, the optoelectronic properties of pure nanoscale noble metals make them unique for different applications as mentioned above. The historical background of metal NPs indicates that more than a century ago, the optical (light absorption and scattering) properties of colloidal suspensions of metals were first observed by Michael Faraday in 1857.191 Later in 1908, Mie proposed the interaction of spherical particles with electromagnetic waves, which explains the origin of surface plasmon resonance (SPR) in the extinction spectra and coloration of colloidal metallic particles;192 subsequently there has been a huge development in this area to date. On the other hand, the development of magnetic metallic NPs has also been increasing continuously in the last few decades, because of their suitability for different applications such as separable adsorbents,193,194 magnetically reusable catalysts,195–197 and biomedical (magnetic resonance imaging (MRI),198,199 hyperthermia,198 drug delivery200 and biosensing201,202). The ferromagnetic metals (Fe, Co, Ni) have two filled electrons in the outermost 4s shell and the unsaturated 3d shell, because of which they show unique magnetic, catalytic and physicochemical properties for different applications.203 Magnetic materials also exhibit superparamagnetic behavior below some critical size, in which each particle can be considered as a single magnetic domain, which is very useful for biomedical applications. These materials show magnetic properties only in the presence of external magnetic fields; however, no magnetism remains in the absence of external magnetic fields.204–208 To obtain the combined advantages of both materials, to overcome the disadvantages of naked metal NPs, and to develop new properties, the YS structures of these combinations were studied. These types of YS NPs have been prepared mostly from the hard56,94,157,170,178,179,209–215 and soft23,92,96,177,216,217 sacrificial templates, and galvanic replacement reactions.218,219 While analyzing the published literature on YS NPs, it has been found that a majority of them are on either noble23,47,55,56,92,94,96,157,170,177,178,210,211,213–215,217–219 or magnetic metals.148,179,209,216 So, the metal/silica YS NPs are discussed separately under two subclasses: (i) noble metal/ silica and (ii) magnetic metal/silica. 2.1.1.1.1.1.1 Noble metal/silica YS NPs. Under this class of YS, noble metals such as Au,23,47,56,92,94,96,157,170,177,211,214,215,217,218,220–223 Ag,219,222,224 Cu,219,225 Pt,210,226 and Pd178,213,223,227,228 are reported as cores

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within a hollow silica shell. Most of the researchers have studied the SPR and catalytic properties of noble metal/silica YS NPs. The optical properties of noble metal (Au, Ag) NPs are explained in terms of SPR, which is the resonant oscillation of conduction electrons at the interface between a negative and a positive permittivity material stimulated by incident light. In 1970, the optical and electronic properties of Au and Ag were explained for the first time in terms of SPR.229 Similarly, a localized surface plasmon (LSPR) is an optical phenomenon of the confinement of a surface plasmon (light wave) within metallic NPs of size comparable to or smaller than the wavelength of light used to excite the plasmon. The resonant frequency of LSPR strongly depends on the composition, size, geometry, dielectric environment, and particle–particle separation distance of NPs.230 In the case of noble metals (Ag and Au) because of the energy levels of d–d transitions, LSPR is exhibited in the visible range of the spectrum.231 It is also worth mentioning that Ag exhibits the sharpest and strongest bands among all noble metals. However, Au is preferred for biological applications mainly because of its inert nature and biocompatibility232 as well as better compatibility with thiol compounds, which in turn helps in immobilization of biomolecules. Another important finding on the optical properties of NPs was reported in the year 1974, when a strong Raman scattering from pyridine molecules in the vicinity of rough silver surfaces was observed.233 Later, this phenomenon became well known as Surface Enhanced Raman Scattering (SERS). These plasmonic properties of noble metal NPs in the form of YS structures were also studied with improved properties for different suitable applications. The SPR properties of Au NPs are highly dependent on the size, shape, and dielectric constant of the surrounding medium; as a result, the agglomeration of NPs in the application media significantly changes the SPR properties. The aggregation of NPs also leads to the coupling of LSPR and results in a new excitation band with lower energy, which in turn impacts the intensity of the SERS signal.170 However, the problem of agglomeration can be prevented significantly by making a YS structure with a porous silica shell.55,56,170,234 Furthermore, the aggregation of NPs can also be prevented by using any organic capping agents. However, these capping molecules mostly experience several problems such as chemical degradation, reaction with the metal, and being affected by environmental changes ( pH, temperature, ionic strength etc.); as a result, the particles start to aggregate again after some time.170,235 In contrast, encapsulation by a silica shell can solve this problem for a longer time.236 For example, in the presence of salt (NaCl) the LSPR band of Au NPs shifted to a lower extension band centered at ∼600 nm because of agglomeration. In contrast, the LSPR excitation band (centered at ∼520 nm) of Au/SiO2 YS NPs was unchanged in the presence of salt as shown in Fig. 3.170 Here, the silica shell physically impedes electromagnetic coupling between the metal cores. The SERS signal of bare Au NPs in the presence of 2-naphthalenethiol (2-NT) drastically changes with the progress of time. In contrast, there was no SERS signal observed for Au/SiO2

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Fig. 3 Assessment of the optical stability of bare and Au/SiO2 YS NPs in the absence and presence of 0.1 M NaCl. (a) When bare gold nanoparticles are exposed to NaCl, the LSPR centered at 520 nm decreases in intensity while a new band forms at longer wavelengths. (b) Au/SiO2 YS NPs are stable in the presence of NaCl. Reprinted with permission from ref. 170. Copyright 2008 from American Chemical Society.

core/shell NPs, because the molecules (2-NT) could not reach the metal core surface. However, the YS structure shows enhanced, reproducible, and steady SERS signals for at least a 2 h period.170 The effect of core size on the optical properties shows that the UV absorbance peak maximum shifts (from 594 to 587 nm) towards lower wavelength (blue shift) on decreasing the core size (from 104 to 43 nm), as shown in Fig. 4. This result also indicates the optical transparency of the silica layer as well as consistency with the Mie theory.56 On the other hand, silica shell thickness is also a very important parameter related to optical properties, which has not been reported for the YS structure. However, it has been reported for the core/ shell structure that the scattering becomes significant with the increase in silica shell thickness; which in turn masks the SPR band, eventually when the shell layer is too thick. A thin silica shell layer is necessary to obtain a good SERS effect as the strong electromagnetic field decreases exponentially on increasing the distance from the metal core surface to the outside of the silica shell.237 Tuning of silica shell thickness is also easy compared to other oxides or polymers. Apart from the optical properties, most of the studies on this class of YS NPs have focused on the improvement of different catalytic applications. Generally, noble metal NPs exhibit high catalytic activity for different chemical reactions, such as hydrogenation, oxidation–reduction, and reforming, owing to their unique physicochemical properties such as a high surface to volume ratio along with a large fraction of active atoms with dangling bonds exposed to the surface.


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Fig. 4 TEM images of Au/SiO2 YS NPs and silica hollow shells. Gold core diameters are (a, b) 104 ± 9 nm, (c, d) 67 ± 8 nm, and (e, f ) 43 ± 7 nm. The scale bars represent 200 nm (a, c, e, g) and 100 nm (b, d, f, h), (i) scheme for synthesis of Au/SiO2 YS NPs, ( j) UV-Vis spectra of Au/SiO2 core/shell and YS NPs, (k) UV-Vis extinction changes of Au/SiO2 colloidal dispersions in 2-propanol by adding quinoline. Reprinted with permission from ref. 56. Copyright 2008 from Wiley-VCH Verlag.

However, the use of these NPs in the actual processes suffers from many limitations such as aggregation, stability at high temperature due to the low melting point of smaller sized particles, lower catalytic activity and instability in a standard solution phase in the presence of target molecules, and so on. To avoid the bottlenecking of catalytic applications of pure metal NPs, metal/silica core/shell NPs were introduced.171 Core/shell NPs of similar noble metals with a silica shell show much improved activity in catalytic activity because of metal-support and higher temperature stability. However, the limitation of the core/shell structure is the blocking of the core surface by the silica shell, which in turn reduces the reaction rate because of lower contact area available between the metal surface and the reactant molecules.56 This problem has been solved by changing the simple core/shell structure to the YS structure. For instance, optical measurements (UV-Vis spectroscopy) of Au/SiO2 YS nanostructures support the permeability of silica shells and unblocked the core surface as shown in Fig. 4(k). In this specific case, the permeability of the silica shell was tested by adding a small amount of quinolone into colloidal dispersion of Au/SiO2 YS NPs in 2-propanol. Finally, the extinction peak maximum was gradually shifted to a longer wavelength and saturated at 600 nm in 30 min. At the same time, there was no peak shifting observed using Au/SiO2


core/shell NPs during the same time period. The reported results confirm that the core surfaces of YS NPs are accessible by the solvent molecules.56 The porous nature of the shell also extends the functionality of YS NPs. In the past decade, significant efforts have been made to design different noble metal/silica YS structures compared to those of bare metals or metal/silica core/shell NPs because of improved catalytic properties by preventing aggregation, providing more surface sites, and introducing an interface on the metal/silica. Additionally, this hollow silica shell also provides a protective shield to prevent catalytic poisoning, thermal stability of high-temperature catalytic reactions, and also provides a homogeneous environment.209 For example, Au/ SiO2 55,56,94,96,177 and Pd/SiO2 178 YS NPs have been used as catalysts with higher activity for different chemical reactions. The Au-based YS NPs have been used as catalysts for different reactions such as reduction of o-nitroaniline,55,94,96,220 p-nitrophenol,56 and oxidation of CO.177 Generally, the catalytic activity of Au NPs originated from the Au atoms present at the surface and the presence of the interface of support oxide/ Au.238–240 To date, several explanations have been proposed for the catalytic behaviour of gold NPs; at the same time, many catalytic behaviours also need to be explored further for gaining a better understanding.188,241 The conversion of a

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nitro compound to amine through hydrogenation reaction is a most promising and practical route for different industries such as pharmaceuticals, dyes, natural products etc.242 The hydrogenation reaction of the nitro compound was carried out with a supported gold catalyst in the presence of reducing agents such as NaBH4, hydrazine etc. The O atoms of the nitro group adsorb on the NP surface and interact with lowcoordinated Au atoms at the edges, which eventually further induce the selective hydrogenation reaction.242 To enhance the catalytic activity of Au/SiO2 YS NPs, the effects of different parameters such as porosity and thickness of shell layer, core size, and core surface functionality were reported in the presence of NaBH4.55,56,94,96,220 The porosity of the silica can be generated by the use of a pore generating reagent ( progen) during the process of sol–gel reaction. Long alkyl chain siloxanes such as octadecyltrimethoxysilane (C18TMS) can be used as a progen, which can be removed finally by calcination.55 It was observed that on increasing the concentration of C18TMS during the synthesis of Au/SiO2 YS NPs, the diffusion coefficient as well as turnover frequency (TOF) increase significantly for the catalytic reduction reaction of o-nitroaniline because of the presence of pores. Interestingly, the rate constant of o-nitroaniline reduction can be enhanced 2.4 times when the core Au surface was selectively functionalized by 3-mercaptopropionic acid because of the generation of a carboxylate ion on the surface.55 The mass transport phenomenon with different pore sizes was also supported experimentally with the help of diffusion kinetics of the silica shell. The diffusion coefficient and its dependence on the porosity of the silica shell can be understood from the changes in surface plasmon resonance of the metal core which is sensitive to the refractive index of the surrounding medium. For this purpose, the diffusion kinetics were analyzed by adding quinoline into the colloidal dispersion of Au/SiO2 YS NPs in 2-propanol. The UV-Vis extinction spectrum was gradually shifted from 598 to 604 nm by the exchange of 2-propanol to quinoline inside the silica shells, which confirmed the mass transport behaviour of the silica shell. The diffusion coefficient of the silica shell can be calculated from the extinction shift using eqn (1) ( peak position change versus time) with different porosities. D ¼ ðπR 2 =36Þ ðV f 2 =tÞ

ð1Þ

where (Vf2/t ) is the square of the line slope from the linear relationship between nt/n∞ and the square root of time. Vf is the volume fraction of 2-propanol escaping from the silica shell. D is the diffusion coefficient. R is the radius of the silica hollow shell (59.5 nm). The diffusion coefficient of silica shells increases monotonically from 5.9 × 10−19 to 2.1 × 10−18 m2 s−1 with the increase in the [C18 TMS]/[TEOS] ratio. These results reveal that the mass transport rate increases with the increase in shell porosity, which confirms full accessibility of the active metal core inside the hollow shell. The silica shells without C18TMS exhibit a negligible diffusion coefficient after calcination.55

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The porosity of the silica shell can also be tuned by the change in reaction temperature during the synthesis.215 In addition, core size dependent catalytic activity of Au/SiO2 was also demonstrated by partial etching of a gold core by KCN. The TOF for catalytic reduction of 4-nitrophenol increases 5 times as the core diameter decreases from 104 to 43 nm because of the presence of larger surface area or active sites.56 On the other hand, the oxidation of carbon monoxide at room temperature has become a very important reaction using supported gold catalysis since Haruta’s group first reported it in 1987.243 The Au/SiO2 YS NPs (with 3 nm Au core) synthesized in reverse microemulsion method were also demonstrated as good catalyst for CO oxidation.177 This type of NP also shows enhanced catalytic activity in the presence of moisture. As an example, the CO conversion increases from 0 to 85% as the temperature increases from 10 to 45 °C in the presence of moisture using the Au/SiO2 YS nanocatalyst. The catalytic activity was inhibited below 25 °C and increases three times in the presence of moisture above 25 °C. When the temperature is above 25 °C, the pores of the mesoporous silica shell open; as a result, the reactant gas can easily come into contact with the core Au surface; at the same time, moisture also promotes the conversion of CO.244 Apart from silica, porous silicates are also used as shells with metal cores, which show a periodic, size controllable pore system and high surface area. For example, Au/aluminosilicate YS NPs were reported for catalytic reduction of 4-nitrophenol in the presence of NaBH4 with high catalytic performances and recyclability in two-step reaction sequences of synthesizing benzimidazole derivatives.92,157 The acidity of aluminosilicate depends on the amount of aluminium present in frameworks as well as their structures.92 In the field of catalysis, Pd has a wide range of applications such as hydrogenation of aliphatic and aromatic organic chemicals, petroleum refining, selective hydrogenation of acetylene to ethylene, vinyl acetate production, and a catalyst for car exhaust gas. In fact, the Nobel Prize in chemistry (2010) was for the development of a procedure for ‘palladium-catalyzed cross couplings in organic synthesis’, which enables the synthesis of large carbon-based molecules. The Pd/SiO2 YS NPs with a porous shell structure were also synthesized in the presence of C18TMS and by partial etching of the silica shell, similar to that of gold.178 These Pd/SiO2 YS NPs showed an extremely high initial TOF (78 000 h−1) with more than 10 times reusability towards the Suzuki coupling reaction of various bromo- and chloro-benzene substituents with arylboronic acid.178 In spite of the fact that these particles have an extremely high catalytic activity, but the main bottleneck of industrial application is the large scale production. As production scalability is a critical issue, recently one research group proposed a novel method for the synthesis of 120–180 nm Pd/SiO2 YS NPs by the one-pot ultrasonic spray pyrolysis (USP) method.213 Another noble metal, Pt is also used as a core material for silica coated YS synthesis.210 Generally, the catalytic performance is measured by activity, selectivity, and stability during the reaction. Here, YS NPs show


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Reported metal/silica YS nanostructures with detailed description of core, shell, and sacrificial layer


Core

Shell

Sacrificial layer

Core/shell

Method

Precursor

Method

Precursor

Material

Removal

Au/SiO2

Polyol Frens Turkevich Galvanic Reduction (Citrate) Reduction (Igepal) Reduction (Citrate) Reduction (Citrate) Reduction (NaBH4) Reduction (NaBH4) Reduction (NaBH4) Thermal Spray pyrolysis Thermal Reduction (N2H4) Galvanic Thermal Reduction (N2H4) Thermal Thermal Reduction (Citrate)

HAuCl4 HAuCl4 HAuCl4 HAuCl4 HAuCl4 HAuCl4 HAuCl4 HAuCl4 HAuCl4 HAuCl4 HAuCl4 Pd(acac)2 Pd(NO3)2 Pt(acac)2 Cu(NO3)2 AgNO3 Ni(acac)2 Ni(NO3)2 Fe(acac)2 Co(acac)2 HAuCl4

Stöber Stöber Stöber Sol–gel Sol–gel Sol–gel Stöber Sol–gel Sol–gel Sol–gel Sol–gel TMOS + C18TMS Spray pyrolysis Sol–gel Sol–gel Sol–gel Sol–gel Sol–gel Sol–gel Sol–gel Stöber, Hydrothermal

TEOS TEOS TEOS TEOS TEOS TEOS TEOS TEOS TEOS TEOS TEOS TEOS TEOS TMOS PTMS PTMS TMOS + C18TMS TEOS TMOS + C18TMS TMOS + C18TMS TEOS, NaAlO2

Au SiO2 SiO2 Ag SiO2 Fe3O4 LSB + SDBS SiO2 PS-co-PVP W/O ME W/O ME TOPO V2O5 Ni Reduction Cu Ni W/O ME FeO CoO CTAB

KCN55,56 NaBH4215 HF47 Galvanic218 NaOH170 (NaBH4)211 Alcohol217 Hot water214 Calcination94 Alcohol96 Alcohol177 Calcination178 Melting213 HCl210 Ship-in-bottle219 Galvanic55,219 HCl209,179 Alcohol216 HCl209 Heating + HCl148,209 Alcohol92

Pd/SiO2 Pt/SiO2 Cu/SiO2 Ag/SiO2 Ni/SiO2 Fe/SiO2 Co/SiO2 Au/Al2SiO5

Microemulsion (ME), trioctylphosphine oxide (TOPO), polyvinylpyrrolidone (PVP), lauryl sulfonate betaine (LSB), sodium dodecyl benzene sulphonate (SDBS), polystyrene (PS), acetylacetonate ((acac)2), tetramethoxysilane (TMOS), n-octadecyltrimethoxysilan (C18TMS), tetraethyl orthosilicate (TEOS), phenyltrimethoxysilane (PTMS), cetyltrimethylammonium bromide (CTAB).

improved catalytic activity, size selectivity of reactants, and stability because of the higher surface area of the metal core and the porous nature of the shell. Different noble metal/silica YS NPs are listed in Table 1 as per the available literature. 2.1.1.1.1.1.2 Magnetic metal/silica YS NPs. The ferromagnetic metallic cores such as Fe,209 Co,148,209 and Ni179,209,216,245,246 with a hollow silica shell are used under this class of YS NPs. Since the transitional metal NPs are well known as active heterogeneous catalysts, the magnetic metallic NPs also form a significant part of that class, apart from their interesting biological applications (such as magnetic resonance imaging,198 drug delivery,247 sensors,248 bioseparation,149 therapy198 etc.). The NPs of magnetic metals are preferred as heterogeneous catalysts because these catalysts can be recycled and easily separated from the reaction mixture. However, bare magnetic NPs are not very useful because of their agglomeration tendency, reactivity towards reaction media etc. as mentioned before for noble metals. On the other hand, magnetic metal/silica combinations in the form of the YS structure are very useful because of remarkably improved properties such as magnetically separability, high recyclability or reusability, prevention of particle aggregation, and thermal stability without losing reactivity. Compared to noble metal based YS catalysts, the magnetic metals are cheap and reusable, which further reduces the cost of a process. It should also be noted that the outer silica shell provides a size selective catalytic reaction because of the tuneable porosity of the silica shell as discussed in section 2.1.1.1.1.1.1. In addition, the silica shell prevents the magnetic metal core from oxidization during high-temperature annealing like core/shell NPs, but YS provides a more


active surface of the core with the mesoporous shell. The catalytic properties of some magnetic metal YS NPs are discussed below. Among various other catalysts, nickel-based nanocatalysts are of significant interest for different applications of petroleum refining including hydro-treating (hydro-denitrogenation to reduce NOx and hydro-desulphurisation to reduce SOx), hydro-cracking, steam reforming, hydrogenation of vegetable oil, and many other useful chemical reactions. The major problems of the Ni-based catalyst’s deactivation are carbon formation on the catalyst surface and particle sintering at high temperature;249 however, these problems are also solved nicely by using the Ni/SiO2 YS structure.209 Fig. 5 shows that the cata-

Fig. 5 TEM images of Ni/SiO2 YS nanoreactors with core diameters of (a) 31 and (b) 24 nm after calcination. The bars represent 100 nm (a, b) and 20 nm (inset of b). Reprinted with permission from ref. 209. Copyright 2010 from the Royal Society of Chemistry.

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lyst particles maintained the YS structure without agglomeration even after heating at 500 °C under a H2 atmosphere and the particles are organized like a chain because of interparticle magnetic interactions. The catalytic studies of the steam methane reforming (SMR) reaction indicate that the YS NPs are stable at high temperatures up to 700 °C with average 31 nm core size. Additionally, the catalysts are easily separable from reaction media by magnetic separation and reusable up to five times without any loss of catalytic activity.209 Further, these catalysts are also used for heterogeneous hydrogen-transfer reactions of various aromatic ketones (acetophenone, propiophenone etc.). The catalyst (with 3 nm core and 17 nm outer diameter) showed high TOF (6000 h−1) for the reduction of acetophenone to 1-phenylethanol, and also the activity remains the same (90% conversion) even after six times reuse.179 Similarly, Co/SiO2 YS NPs were also reported for phenoxy-carbonyl catalysis reactions of iodobenzene and showed high activity and reusability.148 The characterization studies of these YS structures showed that the magnetic properties can also be carefully controlled by tuning the size of the magnetic core. The analysis of the magnetic hysteresis loops of different core sizes of Co/SiO2 YS NPs revealed that the saturation magnetization (Ms) increases with the increase

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in core size. The Ms and the magnetic coercivity (Hc) at 27 °C for Co/SiO2 YS NPs are reported to be 72 emu g−1 and 98 Oe for 14 nm core size; whereas they are 91 emu g−1 and 296 Oe for 32 nm core respectively as shown in Fig. 6. The low Ms value for the small sized core is mainly because of higher orientation disorder of the magnetic moments on the surface sites.148,250 Different types of magnetic metal/silica YS NPs are listed in Table 1. 2.1.1.1.1.2 Metal oxide/silica and silicates. Metal oxide (MO) nanoparticles play a very important role in the various fields of applications such as sensors, electronics, photovoltaics, catalysis, piezoelectric devices, fuel cells, surface coatings, and biomedical. Because of the wide range of important applications of MO NPs, different research groups are putting continuous efforts to achieve new or improved properties by changing the structure as well as proper combinations of materials. While analyzing the research papers on YS NPs of this category, it has been found that the core is mostly made of SiO2,23,39,41,47,57,74,251,252 Fe3O4,57,81,83,88,92,147,157,180–182,184,253–255 and TiO2;105 and at the same time the hollow shell structure is made of silica and metal (Mg,157 Al,92 Cu,184 Ni147) silicates. Based on these studies, these types of YS NPs are divided into magnetic and

Fig. 6 Magnetization-applied magnetic field (M–H) curves of (a) Co (14 nm diameter)/SiO2 and (b) Co (32 nm diameter)/SiO2 YS NPs: 5 K (*), 300 K (*), (c) separation of Co/SiO2 YS NPs by using a magnetic field, (d) TEM image of Co/SiO2 YS NPs (the bars represent 100 nm). Reprinted with permission from ref. 148. Copyright 2011 from Wiley-VCH Verlag.

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non-magnetic cores. As usual, this silica shell hinders flocculation and allows easy dispersion of such NPs in aqueous and biological buffer (PBS) solutions without any extra functionalization step. 2.1.1.1.1.2.1 Magnetic metal oxide/silica and silicate. The magnetic metal oxide NPs such as oxides of Fe, Co, Ni have been extensively exploited in the fields of data storage,256 catalysis,256 separation/adsorbents,150 bio-sensing,257 and biomedical applications such as targeted drug delivery,204,258 contrast agent in MRI204,258 and so on. Among them, iron oxides (magnetite,259 hematite260 and maghemite261) are most popular magnetic metal oxides because of their unique sizedependent magnetic properties known as superparamagnetism.204–208 The naked iron oxide NPs have a high flocculation tendency in the solution phase and also show high chemical reactivity towards air/aqueous solutions, which results in loss of magnetism and poor dispersibility. Therefore, an inert silica shell coating on the iron oxide NPs not only stabilizes the core in aqueous and biological buffer solutions via improving the dispersibility, but also can provide scope for further functionalization for various applications. The stabilization of YS NPs occurs by shielding the magnetic dipole moment interaction within the silica shell; at the same time, the repulsive coulomb force between the negatively charged silica shells also enhances the dispersion stability of

Review

NPs in aqueous media. Under this class, only Fe3O4 has been reported as a magnetic core in various YS studies. Similar to the magnetic metallic NPs, the magnetic metal oxide NPs are also used in recyclable catalysis,57,255 targeted drug delivery with magnetic controllable on–off reaction,57,81,83,87,88,92,180–183 and microwave absorption184 applications. Different approaches have been explored to create this type of YS nanostructure such as Ostwald ripening,88 hard templates (silica,87,157,184,215,253,262 carbon,81,83,255 polystyrene183), and soft templates57,92,180–182,217 The external silica shell layer on the magnetic core decreases the Ms of the YS structure compared to that of uncoated magnetic iron oxide NPs because of the presence of a non-magnetic silica shell. However, compared to the core/ shell structure, the YS structure shows a superior MS value. For example, Fe3O4/OA (oleic acid capped Fe3O4) NPs exhibit ferromagnetic behavior at 50 K with the Ms of 73.40 emu g−1, as shown in Fig. 7(a). When the core particles are coated with nonmagnetic silica shells, the values of Ms decrease to 2.96 and 5.01 emu g−1 for core/shell and YS respectively, as shown in Fig. 7(b). This result shows that the YS structures have a 40% higher Ms value than that of core/shell NPs.180 The Ms value of a particular shell can also be increased by increasing the Fe3O4 content in a YS structure.81 Instead of only magnetic particles, a magnetic silica core/silica263 YS structure was also

Fig. 7 Hysteresis loops at 50 and 300 K temperature of (a) Fe3O4/OA and (b) Fe3O4/silica core/shell and YS (H) NPs. TEM images of (c) Fe3O4/OA NPs, (d) Fe3O4/SiO2 core/shell, and (e) Fe3O4/SiO2 YS NPs. Reprinted with permission from ref. 180. Copyright 2011 from American Chemical Society.


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reported to obtain 1-D and 2-D self-assembled structures in the presence of external electric and magnetic fields. In the presence of an AC (alternate current) electric field, the particles are self-assembled to 1-D pearl chain like structure and in the presence of a magnetic field, a 2-D hexagonal close-packed morphology, as shown in Fig. 8.183 Interestingly, this study also shows that these particles can form a 2-D doublet structure in the presence of a magnetic field and again returned to the original state of the Brownian motion by turning off the magnetic field. The iron oxide NPs also play an important role in different areas of bio-medical applications such as MRI contrast enhancement, targeted drug delivery, and therapeutic purpose. In spite of several advantages, some limitations of iron oxide NPs are still a matter of concern such as stability, toxicity,264,265 surface functionalization, and biodegradation of relatively small size NPs in biological environments.266 Similarly, limited pore volume and high density also limit its application as in vivo drug delivery because of a low drug loading capacity. To overcome these problems, initially magnetic core/ porous silica shell NPs were attracting researchers’ attention.266–268 However, for drug delivery or other applications where loading capacity as well the availability of a core surface is important, the core/shell structure with a porous silica shell is not sufficient because of lower loading capacity. In that case, silica coating on magnetic oxide in the form of YS provides a high drug loading capacity because of its void space, biocompatibility, and ease of surface functionalization ability for drug delivery application. In an application oriented study, it was reported that Fe3O4/mesoporous silica YS struc-

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tures have not only a high ibuprofen drug loading (302 mg g−1) capacity but also a significantly high magnetization strength (>20 emu g−1). In many studies mesoporous shell structures were developed using C18TMS as an additive.81,88 In some studies, it has been shown that YS NPs loaded with drug have a higher cytotoxicity towards the targeted cell than that of a pure drug. As an example, doxorubicin hydrochloride (DOX, an anticancer drug) loaded Fe3O4/SiO2 YS NPs exhibited a somewhat higher cytotoxic effect towards the targeted cancer cell than that of pure DOX.81,83,180 In addition to a high drug loading capacity, this type of YS NP is also shown to be very effective for controlled target specific drug delivery via specific functionalization of the outer surface with specific-ligands having high specificity towards cancer cells such as proteins, peptides, antigens, cytokines etc.83,180,182,253 They are also very effective for pH sensitive drug release.180 As an example, Fe3O4/SiO2 YS NPs were functionalized with 1,2-cyclohexanedicarboxylic anhydride, where the carboxylic acid group of the linker molecule reaches the amine groups of DOX. This architecture shows 73.2% DOX release in 10 h at pH 5, whereas only 21.8% at pH 7.4; which could provide cancer cell-specific drug release properties with magnetic manipulation.180 Similarly, other compounds such as folic acid (FA)83 and polyethyleneimine (PEI)182 conjugated YS NPs are also used for magnetically driven targeted anticancer drug (DOX) and gene (siRNA) delivery to HeLa cells. In fact, FA conjugated Fe3O4/SiO2 YS loaded DOX exhibited a greater cytotoxicity towards HeLa cells than free DOX and only DOX loaded Fe3O4/SiO2 spheres due to the increase of cell uptake of anticancer drug delivery vehicles mediated by the FA

Fig. 8 Optical microscopy images of 1-D assembly of YS particles observed at 0 (a) and 30 s; (b) after initiation of an AC electric field; (c, d) 2-D hexagonally close-packed YS particles (doublet structure inset of d); (e, f ) TEM images of YS NPs. Reprinted with permission from ref. 183. Copyright 2013 from American Chemical Society.

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receptor.83 Some research groups have also reported that various alkaline polyelectrolytes such as PEI, diethylaminoethyl-dextran hydrochloride (DEAEDEX), poly-L-lysine hydrobromide (PLL) and poly(diallyldimethylammonium chloride) (PDDA) act as etching agents for silica to prepare the YS structure as well as an in situ functionalizing agent for drug delivery or other biological applications.253 Generally, due to their superparamagnetic nature, these YS NPs can be used as an MRI agent in radiation-free and penetration depth free modes of imaging, but their contrast is much poorer for optical imaging. However, incorporation of suitable fluorescent dyes such as fluorescein isothiocyanate (FITC)181,253 and rhodamine B isothiocyanate (RITC)81,83,182 or luminescent quantum dots inside Fe3O4/SiO2 YS NPs extends their applicability in the biomedical field with the combination of optical and magnetic imaging. The superparamagnetic Fe3O4/ SiO2 YS NPs are reported to be a very good MRI contrasting agent; at the same time, when the silica shell is modified with the FITC dye, the same particles can be used for fluorescent tracking (optical imaging) as shown in Fig. 9.181 This dual nature will be very useful for biomedical applications. Similarly, other fluorescent dye (RITC) doped Fe3O4/SiO2 YS with emission at 550 nm can also be used as fluorescent labelling agents.182 These types of YS NPs also show superiority in metal-silica supported catalytic activity because of their good selectivity, efficient recovery and recyclability. While magnetic/metal-silica YS NPs are mixed in solution, magnetic separation provides a convenient way to separate and recycle the catalyst particles by applying an appropriate external magnetic field. The magnetic core (Fe3O4) also provides much higher stability in various physicochemical environments including acidic and basic media, organic solvents, and high temperature and pressure.269 Additionally, some groups impregnated Au NPs inside the silica and silicate shells; the magnetic core also showed high catalytic activity and acted as a magnetically recyclable metal support catalyst for various reactions.57,147 Recently, microwave absorbing materials have also drawn significant attention in different applications of science and

Review

engineering such as microwave technology, telecommunication devices (mobile telephone, local area network system), and radar detection, which minimizes the electromagnetic reflection. These materials are a type of functional material which can dissipate electromagnetic (EM) waves by converting them into thermal energy. In spite of the fact that the magnetite (Fe3O4) NPs are good candidates for microwave absorption with low cost and strong absorption, their agglomeration tendency, high density, and narrow absorption bandwidth limit their use in further applications. Therefore, materials with light weight, small thickness, a wide absorption band, chemical stability, and excellent absorption efficiency are highly useful for this application. The coating of silica on magnetic NPs is a feasible way to achieve the above mentioned properties due to their unique structures, which exhibit a wider absorption spectrum range and lower reflection loss (RL) compared to single-layer absorbers.270–275 Here, mesoporous silica with large pores possesses a high dielectric constant which enhances the dielectric dissipation. On the other hand, the core/shell structure helps in electromagnetic attenuation due to the existing interfacial polarization, and the void space provides more active sites for reflection and scattering.184,270,276 For example, Fe3O4/Cu-silicate YS NPs with a thicker shell showed enhanced microwave absorption properties; as a result, these NPs can be used as an electromagnetic interference (EMI) shielding material. This microwave absorption property is mainly because of the good dielectric behaviour and effective complementarities between the dielectric loss and the magnetic loss, which originates from the synergistic effect of both the Fe3O4 cores and copper silicate shells. In spite of the fact that the microwave absorber performance of Fe3O4 is low, the performance can be increased by choosing a suitable shell such as metal-silicates. Again this performance increases on increasing the thickness of shell material as shown in Fig. 10.277 Fig. 11 shows the reflection loss data for the Fe3O4/EP (epoxy resin) and Fe3O4/Cu-silicates/EP composites. The values of maximum RL of 150, 330, and 450 nm Fe3O4 particles are −10.2, −10.7, and −11.1 dB at 7 GHz with a

Fig. 9 (a) Confocal microscopy image of HeLa cells uptake of Fe3O4/FITC-silica YS NPs; (b) T2-weighted MR images of a pellet of 1 × 10−5 cells treated with 40 or 80 mg mL−1 Fe3O4/FITC-silica YS NPs for 1 h, taken with a 4.7 T MR instrument; (c) TEM image of Fe3O4/FITC-silica YS NPs. Reprinted with permission from ref. 181. Copyright 2009 from the Royal Society of Chemistry.


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Fig. 10 TEM images of the Fe3O4/Cu-silicate YS microspheres with different Fe3O4 core sizes and copper silicate shell thicknesses: (a) ∼450 nm core, ∼63 nm shell, (b) ∼450 nm core, ∼75 nm shell, (c) ∼450 nm core, ∼90 nm shell, (d) ∼450 nm core, ∼106 nm shell, (e) ∼450 nm core, ∼55 nm shell, (f ) ∼450 nm core, ∼125 nm shell, (g) ∼150 nm core, ∼39 nm shell, and (h) ∼330 nm core, ∼59 nm shell. Reprinted with permission from ref. 184. Copyright 2013 from American Chemical Society.

Fig. 11 Microwave reflection loss curves of the EP composites containing the Fe3O4 particles and Fe3O4/Cu-silicate with different Fe3O4 core sizes and copper silicate shell thicknesses (MCS-1 (450/63 nm), MCS-3 (450/90 nm), MCS-6 (450/125 nm), MCS-7 (150/39 nm), MCS-8 (330/ 59 nm)) YS microspheres. Reprinted with permission from ref. 184. Copyright 2013 from American Chemical Society.

thickness of 2 mm, respectively, while the Fe3O4/Cu-silicate YS microspheres with 150, 330, and 450 nm Fe3O4 cores and 39, 59, and 63 nm copper silicate shells show maximum RL values of −13.2, −16.2, and −18.4 dB at 7 GHz with the same thickness, respectively. It can be seen that the Fe3O4/Cu-silicate/EP composites display enhanced microwave absorption properties in terms of both the maximum RL values and the absorption bandwidths compared with the Fe3O4/EP composites because of a larger specific area, high porosity, and synergistic effects of both the core and the shell.184,278 Additionally, the presence of void space of the YS structure provides more active sites for

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reflection and scattering of microwave, so microwave absorption properties improve in YS structures.184,270,271,279 2.1.1.1.1.2.2 Non-magnetic metal oxide/silica. In the category of non-magnetic metal oxide cores, only the two materials SiO2 23,39,41,47,57,74,251,252,280–282 and TiO2 105 have been studied. In fact, among these two materials most of the studies are on the silica/silica YS NPs. Since both the shell and the core are made of the same material, this type of YS structure is preferred over solid or hollow NPs mainly because of a very high available specific surface area for a particular size external diameter particle. A review article briefly described the soft template route to the synthesis of these types of YS NPs.23 Interestingly, the SiO2/SiO2 YS NPs have been drawing increasing attention for their bio-medical applications because of some specific properties such as: (i) a large surface area with a high pore volume, a tunable mesoporous structure, high chemical and mechanical stability, and biocompatibility for intelligent drug delivery,39,57 (ii) good adsorption and encapsulation ability of guest particles (therapeutic drugs) on the surface of the core and the shell with interstitial hollow space for co-delivery as drug carriers,39,57 (iii) during the synthesis of YS, the silica surface can be functionalized and bioconjugated together for targeted drug delivery, (iv) the outer shell prevents degradation of encapsulated drug molecules, (v) in vivo delivery of an encapsulated drug for cancer or tumour therapy via enhanced permeability and retention (EPR) effects,39,283,284 (vi) doping or encapsulation of the fluorescent molecules in a movable core is useful for optical monitoring of the delivery of the therapeutic drugs,39 and (vii) scalability is important for commercial applications.23,39,41,57,74,251,252 From the synthesis perspective, it has been found that most of the researchers used the modified Stöber method for the synthesis of silica YS NPs. In some study, in fact, it is reported that the method is highly scalable for the synthesis of different


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Fig. 12 TEM images of silica YS NPs with different sizes and core–shell ratios: (A) 645 nm (core 330 nm, shell 50 nm); (B) 330 nm (core 160 nm, shell 40 nm); (C) 260 nm (core 140 nm, shell 20 nm); (D) 110 nm (core 53 nm, shell 13 nm); (E) 95 nm (core 46 nm, shell 15 nm). (F) Hollow silica spheres of 170 nm diameter (shell 25 nm). Reprinted with permission from ref. 47. Copyright 2009 from Wiley-VCH Verlag.

outer diameter particles (95–645 nm) with tuneable shell thickness (10–50 nm) and core diameter (40–330 nm) as shown in Fig. 12.47 In that specific study, the sizes of YS NPs were controlled by changing the precursor concentrations (TEOS and ammonia) and speed of addition. These particles are also efficient for the optical imaging after encapsulation of the FITC (fluorescein isothiocyanate) fluorescent dye.47 These types of NPs also have a very good EPR effect which is extremely useful for targeted drug delivery.39 The dispersion stability (in water, saline, 5% glucose, and DMEM cell culture media) of these NPs was enhanced drastically (more than 1 month) after PEGylation. Additionally, PEGylated NPs have 32% (42 mg per 100 mg NPs) higher hydrophobic docetaxel (Dtxl) antitumor drug loading capacity; at the same time, the drug release rate slowed down. The maximum inhibitory concentration (IC50) of Dtxl encapsulated PEGylated YS NPs for human liver cancer cells (Hep-G2) was only 7% compared to that of free Dtxl after 72 h. Finally, these NPs were also tested for in vivo systematic toxicity against healthy mice (20 mg kg−1 three doses) and showed very low toxicity as shown in Fig. 13 (E–H).39 Fluorescent (FITC) labelled silica-PEG YS NPs were also demonstrated for monitoring the optical trafficking of human liver carcinoma cells of Hep-G2 by fluorescence microscopy as shown in Fig. 13(I–L).39 It has already been proved that these YS NPs have a high drug loading capacity; at the same time, they also have the multi-stage drug release ability, which is difficult to achieve for naked, core/shell, hollow, and composite nanoparticles. The silica/silica YS structure provides more sites to adsorb drug molecules for stepwise release. In a controlled drug release experiment using ibuprofen as the model drug, it was reported


that in the first stage drug molecules were rapidly released during 1–5 h mainly because of the adsorbed drug molecules from the external shell surface. Then in the second stage, the maximum release was within 5–8 h because of trapped molecules in the void space, and finally in the third stage molecules were released from the movable core during 24–72 h. The kinetics for the unique release behavior of ibuprofen from the YS structure can be predicted by a modified Freundlich kinetic model which has been successfully applied to the experimental data of adsorption by many researchers. This model can be presented as Mt/M∞ = Ktn, where Mt/M∞ is a fraction of the drug released at time t, K is the release rate coefficient and n is a constant. The kinetic calculated results are also very close to those of the experimental release curve at different stages.57 Further, another drug molecule such as R6G was also tested with silica YS NPs and shown to have a two-stage drug release process.41 Despite drug loading, when these YS NPs were used as a carrier for the protein or vaccine delivery system, they also showed a greater amount of protein (ovalbumin) loading efficiency compared to that of mesoporous and solid silica NPs.74 Apart from silica, only TiO2 has been reported as a movable core among other metal oxides in the hollow silica shell, where both silica and TiO2 were synthesized by the sol–gel method and PMAA as a sacrificial layer.23 These particles also showed good photocatalytic activity for degradation of methyl orange dye in the presence of UV light.105 Different metal oxide movable cores with silica hollow shells are listed in Table 2. 2.1.1.1.2 Core without a silica shell. Apart from silica, a wide variety of materials are also used as shells along with

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Fig. 13 (A) TEM image of silica YS NPs with a size of 125 nm, (B) TEM image of the silica YS NPs after PEGylation, (C) size distribution of silica YS and silica YS-PEG by dynamic light scattering, and (D) in vitro cumulative drug release profile of docetaxel from silica YS-PEG-Dtxl in neutral PBS medium at 37 °C. Cytotoxicity of YS-PEG, YS-PEG-Dtxl, and Dtxl on Hep-G2 cells by MTT assays. (E) Cell viability with concentrations of YS-PEG from 0.0625 to 1 mg mL−1 for 72 h. Inhibition rate of Dtxl and YS-PEG-Dtxl (concentration of Dtxl from 0.1 to 62.5 nM) on Hep-G2 cells for (F) 24 h and (G) 72 h, and (H) corresponding IC50 values for 24 and 72 h. Uptake and subcellular localization of the YS/FITC-PEG in Hep-G2 cells. (I–L) Colocalization of nucleus with YS/FITC-PEG. The cells were incubated with 100 g mL−1 of YS-PEG for 4 h, fixed, and then stained with DAPI. (I) Blue fluorescence shows nuclear staining with DAPI. (J) Green fluorescence shows the location of YS/FITC-PEG. (K) Overlaid image of (I) and (J), and (L) the corresponding transmission image. (M) Schematic diagram of the drug delivery system based on silica YS NPs. Reprinted with permission from ref. 39. Copyright 2010 from American Chemical Society.

different materials as cores. On the basis of the type of shell material, these NPs can be classified into two categories: (i) elements (metals, 44,108,109,285–295 nonmetals35,52,64,65,67–69,93,98,99,118,121,124,126,127,134,135,137–145,151,152,167,296–301) and (ii) compounds (metal oxides,40,42,43,45,46,48,49,63,101,119,123,125,128,131–133,154,160,166,302–309 sulphides,48,310,311 phosphates,312 hydroxides,107,313 and fluorides61). Here, all individual materials have their own advantages, which will be further discussed in their respective sections. 2.1.1.1.2.1 Elemental shell. According to the reported YS literature, the elemental shells can be either made of metallic44,108,109,285–295 or non-metallic35,52,64,65,67–69,93,98,99,118,121,124,126,127,134,135,137–145,151,152,167,296–301 materials. In non-metallic, studies are limited to only carbon material so far. So these types of YS can be further divided into metallic and carbon shells.

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2.1.1.1.2.1.1 Metallic shell. The metallic NPs are important from the fundamental and application perspectives because of their optical, surface plasmon, electronic, magnetic, and related properties; which make them suitable for the emerging applications in photonics, sensing, imaging, catalysis, and even in medicine.314,315 While analyzing the literature on metallic shells of YS NPs, it has been found that most of them are on metallic/metallic types and are made of mono or bi-metallic combinations. In general, metal/metal YS NPs are of greater interest than simple mono-metallic NPs because of their improvement in optical, magnetic and catalytic properties.44,108,109,285–291 Mostly, bi-metallic YS NPs are reported on different noble metals. Apart from the noble metals, magnetic metals are also reported under this class. In general, several efforts have been made to synthesize bi-metallic YS NPs with different combinations such as Au/Ag,286,291 Au/ Au,44,316 Au/Pt,108,317–319 Co/Au,290 Au/Pd,287 Ag/Cu,292 Au/Pd,62


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Review

Reported metal oxide/silica YS nanostructures with detailed description of core, shell, and sacrificial layer


Core

Shell

Sacrificial layer

Core/shell

Method

Precursor

Method

Precursor

Material

Removal

TiO2/SiO2 SiO2/SiO2

Sol–gel Sol–gel Sol–gel Sol–gel Sol–gel Sol–gel Sol–gel Pre-synthesized Hydrothermal Hydrolysis Aging Aging Thermal Thermal Thermal Thermal Hydrothermal Hydrolysis Hydrolysis Polyol

TBOT TEOS TEOS TEOS TEOS TEOS TEOS SiO2 Fe(ClO4)3 FeCl3 Fe(ClO4)3 FeCl3 Fe(acac)3 FeCl3 Fe(Co)5 FeCl3 Fe3Cl3 FeCl3 FeCl3 FeCl3 Fe3O4 NPs FeCl2 + FeCl3 Fe(NO3)2 FeCl3 FeCl3 FeCl3

Sol–gel Sol–gel Stöber Sol–gel Sol–gel Sol–gel Sol–gel Sol–gel Stöber Stöber Stöber Sol–gel Sol–gel Sol–gel Sol–gel Sol–gel Sol–gel Sol–gel Stöber Stöber Sol–gel Sol–gel Hydrothermal Hydrothermal Stöber process Hydrothermal

TEOS TEOS TEOS TEOS TEOS TEOS TEOS + APS TEOS TEOS + C18TMS TEOS TEOS TEOS + C18TMS TEOS TEOS TEOS TEOS TEOS + C18TMS TEOS TEOS TEOS TEOS APTMS + MPTMS SiO2 + Mg(NO3)2 NiCl2SiO2 TEOS, NaAlO2 SiO2Cu(NO3)2

PMAA SiO2 SiO2 FC4 + F127 PMMA + PS CTAB SiO2 SiO2 SiO2 LSB + SDBS SiO2 Organic-silica W/O ME FC4 + F127 W/O ME C C sphere C SiO2 SiO2 CTAB PS SiO2 SiO2 CTAB SiO2

Calcination105 NaOH41 HF39 Calcination57 Calcination251 Wash + heat252 HF74 HF47 NH3 87 Alcohol217 NaBH4 215 Calcination88 Alcohol181 Calcination57 Alcohol180 Calcination255 Calcination83 Calcination81 HF253 Electron beam254 Calcination182 Calcination183 Urea157 Urea262 Calcination92 NH4OH184

Fe2O3/SiO2 Fe3O4/SiO2

Fe3O4/MgSiO3 Fe3O4/NiSiO3− Fe3O4/Al2SiO5 Fe3O4/CuSiO3

Massart Hydrothermal Hydrolysis Hydrolysis Solvothermal

(3-Aminopropyl)trimethoxysilane (APTMS), 3-(mercaptopropyl)trimethoxy silane (MPTMS), poly methacrylic acid (PMMA), formaldehyde (FA), polystyrene (PS).

Au/Pd–Ag320,321 etc., to develop new properties for the related applications. These types of metallic/metallic YS NPs are easily prepared by galvanic replacement44,62,108,109,285–288,291,316,320,322 and Kirkendall reaction289,292 processes. The strong LSPR properties of noble metals are useful for optoelectronic and biomedical applications. At the same time, the SPR properties of noble metals NPs can be easily tailored by just tuning their sizes, shapes and compositions,234,323 which has been already discussed before under the metal/ silica section. While two metals as individuals or alloys are present in the form of the YS structure, the optical properties of the resultant structure are also better than those of the core/ shell structure. In the first reported study of alloy type YS NPs, both core and shell structures were made of a Au/Ag alloy via a galvanic replacement reaction between silver and chloroauric acid as shown in Fig. 14(a).286 The study showed the development of new optical features, which varied across a broad range of the optical spectrum extending to the visible and near infrared regions compared to conventional solid or hollow NPs. Additionally, the YS structure showed two distinct extinction peaks at ∼510 nm and another at the NIR region, similar to that of Au or Ag nanorods as shown in Fig. 14(d). These peaks are mainly because of the Au/Ag core and shell respectively. At the same time, by varying the void space and core diameter of the YS nanostructure, the extinction spectra can be tuned from the visible to NIR regions as shown in Fig. 14(d). The pure Au/Ag core/shell nanoparticles exhibit only one sharp


peak at 502 nm (curve a), but by increasing the inner diameter it extended to another peak at longer wavelength, which can be described from the Mie scattering of the Au/Ag shell. Later, a similar principle was also used for the synthesis of the Au core inside the Pt/Ag alloy shells.285 A small modification of this process was done to obtain pure Au/Au YS NPs, where the composition of the core/shell was controlled by varying the reaction temperature. In this case, AgCl formed via a replacement reaction between Ag and HAuCl4 was completely soluble in water at 100 °C and can be easily removed by aqueous NH4OH wash.44 Further, many combinations such as Au/ Au,44,316 Au/Pd,62 and Au/Pt108 were also synthesized via galvanic replacement reaction. Apart from a metal alloy, the combination of pure metal/metal YS NPs also exhibits different extinction peaks corresponding to the core and the shell. For example, when LSPR was measured individually, Au nanospheres and Pd nanoshells exhibit sharp and broad peaks, respectively, because of the presence of LSPR spectra as shown in Fig. 15(A and B). However, when they are present together in the form of a YS nanostructure their plasmonic spectra hybridized and led to a broadening of the LSPR spectrum with a dip between Au and Pd LSPR peaks as shown in Fig. 15(C and D), which was also supported by theoretical calculations using the discrete dipole approximation (DDA) technique with different sizes of YS NPs. This complex plasmonic behavior of YS can be well explained by the hybridization model as shown in Fig. 15(G). The Pd nanoshells exhibit plasmonic properties

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Fig. 14 (a) Schematic synthesis procedure for Au/Ag alloy YS NPs, (b) TEM and (c) SEM images of YS NPs, (d) UV-Vis-NIR extinction spectra of YS NPs that were prepared by titrating the same amount of Ag-coated Au/Ag colloids with different volumes of 1 mM HAuCl4 solution: (a) 0, (b) 0.4, (c) 0.5, and (d) 0.6 mL. All of the spectral curves were normalized against the intensities of peaks around 510 nm. (e) Extinction spectra calculated for a YS whose core and shell contained 75% of Au and 25% of Ag. The inner and outer diameters of the shell were fixed at 70 and 80 nm, respectively. The diameter of the core was varied from 20 to 50 nm. As a result, the spacing between the core and the inner surface of the shell was varied from 25 to 10 nm, respectively. Reprinted with permission from ref. 286. Copyright 2004 from American Chemical Society.

due to inner and outer surfaces, which is like a Pd nanosphere and nanocavity. When Au NPs are encapsulated inside the Pd shell, the plasmon mode of Au NPs hybridizes with the bonding and anti-bonding modes of the Pd nanoshell. The reason for broad LSPR with a dip can be understood on the basis of the hybridization model. Au/Pd YS NPs showed two bonding plasmon modes, one with lower energy and large values of dipole moment (a bright, super-radiant broad plasmon mode) and the other has high energy and low value of dipole moment (dark, sub-radiant narrow plasmon mode) as shown in Fig. 15(g). The interference between narrow dark and bright broad plasmon modes results in the formation of a dip in the LSPR spectrum of Au/Pd YS and a Fano resonance.62 Since the metallic NPs have very good catalytic activity towards various chemical reactions, the YS NPs of bimetallic

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combinations act as a very good catalyst, especially as an electrocatalyst compared to simple, core/shell, hollow, or composite NPs.108,109,289 Among several metals, platinum metal NPs are widely studied as electrocatalyst components for the anode (oxidation) and cathode (reduction) of the fuel cells because of their high electrochemical activity, chemical stability, high selectivity, and electrical conduction.324,325 As a specific example, platinum is an excellent electrocatalyst for DMFCs (direct methanol fuel cells), but its self-poisoning in the presence of CO or CO tolerance during the reaction limits its efficiency in commercial applications. It has also been reported that alloying of Pt with another metal element reduces the catalyst poisoning effect.326–329 Further, it is also established that the Au/Pt YS NPs enhance the electrocatalytic oxidation of methanol in DMFCs. In this unique YS nano-


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Fig. 15 LSPR extinction spectrum of (A) 55.5 ± 6.3 nm palladium hollow NPs experimentally measured (green spectrum) and 77.5 nm palladium hollow NPs with 5 nm wall thickness calculated by DDA technique (black spectrum). (B) 29.8 nm gold NPs experimentally measured (blue spectrum) and calculated theoretically (red spectrum). (C) Palladium–gold YS consist of 77.5 nm palladium nanoshell and 29.8 nm gold NP when the inside separation distance between them is 0 (green spectrum), 3 (cyan spectrum), and 15 nm (magenta spectrum) calculated theoretically. (D) Measured for palladium–gold YS of 29.8 ± 2.4 nm inside gold NPs and 77.5 ± 7.2 nm palladium outer nanoshells. (E) High-resolution TEM image of PdAu YS NPs of 40.8 nm diameter and 3.2 nm wall thickness; the inner AuNS is 17.1 nm in diameter. (F) Lattice-resolved image of PdAu YS. (G) Schematic diagram that describes the hybridization of the plasmon resonance. Reprinted with permission from ref. 62. Copyright 2014 from American Chemical Society.

structure, a hollow Pt shell provides high catalytic activity, and the Au/Pt combination works as a Pt-based alloy, which helps in CO tolerance. On the other hand, void space also allows methanol molecules to remain at the catalyst surface for long enough to be oxidized efficiently. So, the YS structure shows superiority in catalytic activity compared to that of Au/Pt core/ shell, hollow Pt, solid Pt NPs, commercial Pt/C and Au NPs by evaluating the electrochemical properties using cyclic voltammetry under the same conditions. The typical cyclic voltammograms of electrocatalytic oxidation of methanol show two oxidation peaks, the current peak for the forward scan (If ) is because of the oxidation of methanol and the current peak of the back scan (Ib) is because of removal of carbonaceous intermediates. The Au/Pt YS catalyst showed a much higher If/Ib ratio which indicates the higher CO tolerance, more effective oxidation of CH3OH molecules and higher electrochemically active surface area. To explore the effect of void space and Au core size on the electrocatalytic performance, different core


size and void space YS NPs were tested, and the results are shown in Fig. 16. The experimental results indicate that larger void space leads to better CO tolerance (If/Ib = 37.3) compared to the smaller space because the larger void space allows more reactant molecules for oxidation more efficiently. However, the catalytic activity decreases significantly with the increase in Au core size, mainly because of decreasing Pt concentration. So, small core size and larger void space in a YS structure are beneficial for the catalytic activity.108 A similar type of Au/Pd YS NP also exhibit significantly enhanced catalytic activity and improved CO tolerance over Pd NPs, core/shell, and Pt/C catalysts.109 On the other hand, Au/Pd YS NPs were also demonstrated as catalysts for the C2H5OH oxidation (anodic reaction) and the oxygen reduction reaction (cathodic reaction). These particles exhibit enhanced catalytic performance compared to that of Pd black NPs and Pt/C catalysts because of their porous structure, more active sites, and improved CO tolerance.109

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Fig. 16 The initial cyclic voltammograms of a Au–Pt YS NP catalyst with smaller space (a), bigger space (b), smaller Au core (c) and bigger Au core (d), the insets are the corresponding TEM images. Reprinted with permission from ref. 108. Copyright 2011 from the Royal Society of Chemistry.

The bimetallic noble metal NPs are also used as excellent candidates for sensor applications because their LSPR frequency depends on the dielectric constant of the surrounding medium. At the same time, bimetallic NPs also act as a good candidate for electrocatalyst sensors because of their potential for electrode modification by enhancing the electrode conductivity. The Cu-based YS NPs can also be used in the sensor field because of their low cost and good electrical conductivity. As an example of the electrochemical sensor, Ag/Cu YS NPs synthesized via the Kirkendall effect were reported as an electrode for the nonenzymatic glucose sensor, which showed a linear response and good selectivity in the concentration range of 0.1–4.8 mM.292 Similarly, Au/Pd–Ag YS NPs were functionalized with amination graphene (NH2-GS) and were also used as an electrochemical immune sensor for the detection of nuclear matrix protein 22 (NMP22). Here, the large surface area of NH2-GS could capture more YS NPs via covalent bonding with amino groups on graphene. On the other hand, the unique structure of YS provides more exposed active sites, which could increase the sensitivity and improve the electric signal. These YS facilitate the superior electron transfer from the redox center of the protein to the electrode surface compared to monometallic with a very low detection limit (3.3 pg ml−1) and a linear range between 0.01 and 18 ng ml−1. Apart from a good sensitivity and selectivity, they also show good reproducibility for the detection of NMP22 even after a long time.320 The application of bi-metallic YS NPs in drug delivery is also continuously increasing.330–332 To achieve multi-function-

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ality in a drug delivery system, combinations of the superparamagnetic metal with noble-metal YS NPs are more useful materials. They show a dual role as a magnetic/optical nanohybrid structure, which is very useful in the field of biomedical applications for optical and magnetic imaging.290 Besides the hybrid properties, the YS NPs with void space have an advantage of loading of guest molecules. On the other hand, plasmonic nanomaterials such as Au, Ag, and Cu, especially the NPs which convert near-infrared light to heat, have been used as a shell material with magnetic metallic cores, mainly for photo-thermal therapy. For example, an Au nanoshell is a thin shell of Au containing dielectric space at the core and exhibits optical absorbance in the NIR region. This optical absorption cross section of Au nanoshells (3.8 × 10−14 m2) is six orders of magnitude higher than that of the indocyanine green dye (1.66 × 10−20 m2) at 800 nm wavelength,333,334 which proves that this material is a much stronger NIR absorber and an effective candidate for a photothermal coupling agent. In addition, rigid nanoshell structures are less susceptible to chemical/thermal denaturation and photobleaching effects than conventional NIR dyes.333,334 So, the Au nanoshell works as a nanocarrier for the target-specific delivery of therapeutic agents because of its non-toxic nature, inertness, and tunable SPR properties towards the NIR region.335–342 The super-paramagnetic core and a metallic nanoshell with absorption in the NIR region in the form of the YS structure act as a photo-thermal agent with magnetic tracking for localized hyperthermia cancer therapy. For instance, a magnetic movable core encapsulated in a gold hollow shell


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proved to be a multifunctional material with good magnetic and NIR absorption properties. As shown in Fig. 17(h), the hysteresis loop of the YS NPs exhibits superparamagnetic behaviour at 300 K with a mass magnetization of 41.9 emu g−1; additionally, no coercivity (Hc) was observed even within an enlarged view of the loop measured by a superconducting quantum interference device (SQUID) magnetometer. The only gold nanoshell obtained from YS NPs after strong acid etching of the super-paramagnetic core also shows an absorption in the NIR region due to the unique hollow structure shown in Fig. 17(i). These hybrid YS NPs have excellent performance in potential biomedical applications such as optical imaging and in vivo MR tracking when used for a non-viral vector for gene delivery and transfection; mainly because of the superparamagnetic behavior of the core, the NIR absorbance of the hollow nanoshell, the presence of void space, and the positive surface potential.290 In addition to its unique properties, these YS NPs exhibit “peas in pods” like assemblies under an external magnetic field as shown in Fig. 17(a–f ). However, this type of combination may also be useful for bioseparation application. The different types of metal movable cores with metallic hollow shell materials are summarized in Table 3.

Review

2.1.1.1.2.1.2 Nonmetallic (carbon) shell. In this class, different core materials such as metal oxides (SnO2,124,127,141,143 Fe3O4,139,144,152,167,300,343,344 ZnO,138 NiO137 and α-Fe2O3 151), metals (Au,35,52,93,98,99,298 Ag,301 64,118,126,135,345,346 297 298 299,347 65,67,121,142,145,348 Sn, Pt, Se, Pd, Si, Fe349), non-metals (S,140,350 C351–354) and inter-metallic compounds (Cu5Sn6 68) have been reported inside the hollow carbon shell. These YS NPs were mostly synthesized by template assisted routes, where silica,35,52,64,65,67,93,98,99,121,126,127,139,141,144,145,151,152,296,299–301 carbon,68,69,118,142,143 MOF,137 Fe3O4 167 or polystyrene124,343 was used as a sacrificial layer. Carbon coating as a hollow shell could endow the core materials with new and novel properties, thus rendering them attractive for great potential applications in chemical catalysis52,93,98,99,167,299,300 and photocatalysis,297 bio-medical,151,343 adsorption/separation,151,152 super-capacitors,296 sacrificial templates,68,69,118,142,143 and electrodes in lithium batteries.64,65,67–69,118,121,124,126,127,135,137–145 The carbon hollow shell is used in various diverse applications because of its superior physicochemical properties such as: (i) high specific surface area, low density and large pore volume, (ii) chemical inertness and bio-compatibility, (iii) good mechanical stability and

Fig. 17 (a) Schematic illustration of the formation of ‘‘peas in a pod’’-like assemblies from nanospheres. (b) TEM image and (c–f ) a series of SEM images of the large-scale synthesized uniform pea-like assemblies of Co/Au YS nanospheres. (h) Quick response of hybrid nanospheres to a magnet (upper left inset) and the amplified M–H curve (lower right inset) of Co/Au YS nanospheres. (i) UV-Vis-NIR spectrum of Au nanocages. Reprinted with permission from ref. 290. Copyright 2010 from Wiley-VCH Verlag.


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Nanoscale Reported metal/metal YS nanostructures with detailed description of core, shell and sacrificial layer


Core

Shell

Sacrificial layer

Core/shell

Method

Precursor

Method

Precursor

Material

Removal

Au/Ag alloy Au/Au Au/Pt/Ag Co/Au Au/Ag Pd-, and Pt Au/Ag alloy Au–Pt Ag/Cu Au/Pd Au/Pd–Ag Au/M (M = Au, Pd, and Pt) Pd/MxCu1−x (M = Au, Pd, and Pt)

Reducing (EG) Reduction (Citrate) Reduction (Citrate) Reduction (NaBH4) Pre-synthesized Pre-synthesized Reducing (DMF) Reduction (Citrate) Reduction (Citrate) Reduction (NaBH4)

AgNO3 HAuCl4 HAuCl4 CoCl2 Au nanorod Au seed Au core AgNO3 HAuCl4 HAuCl4 HAuCl4

Galvanic Reduction (Citrate) Galvanic Reduction (NaBH4) Galvanic Reduction (ODA, TTD) Galvanic Reducing (DMF) Galvanic Galvanic Galvanic

HAuCl4 HAuCl4 HPt2Cl6, AgNO3 HAuCl4 Na2PdCl4, K2PtCl4 CH3CO2Ag H2PtCl6 Cu(NO3)2 (NH4)2PdCl4 AgNO3, K2PdCl4 H2PdCl4, H2PtCl6

Ag Ag Ag Co Ag Ag Ag Cu2O Ag Ag Ag

Galvanic286 Galvanic44,316 Galvanic285 Galvanic290 Galvanic288 Galvanic291 Galvanic108 Kirkendall effect292 Galvanic62 Galvanic320 Galvanic109

Reduction (AA)

Na2PdCl4

Galvanic

HAuCl4, K2PtCl4, Na2PdCl4

Cu

Galvanic287

Ascorbic acid (AA), 1,2-tetradecanediol (TTD), octadecylamine (ODA), dimethylformamide (DMF).

electrical conductivity35,52,68,69,93,118,121,124,126,127,135,151,296–299,355 for lithium-ion battery and electronic applications, and (iv) the hollow and mesoporous nature of carbon spheres is advantageous for mass diffusion and transport.151 The carbon shell combinations with metal and metal oxide cores are mostly used in energy storage applications, especially in the fields of lithium-ion and lithium–S batteries, where electrochemically active cores such as Fe3O4,139 NiO,137 ZnO,138 SnO2,69,124,127 Sn,64,118,126,135 Si,65,67,121,142,145 and Cu5Sn6 68 are encapsulated inside the hollow carbon shell. Different NPs (nano-carbon, alloys, metal-oxides, metal sulphides/nitrides etc.356,357) are used as electrodes for lithium batteries, which show good energy density and cyclic capacity. However, the volume changes during the lithiation/delithiation process cause pulverization, and loss of electronic contact with conductive additives and also lead to instability of the solid– electrolyte interphase (SEI), which reduces the overall electrochemical performance during cycling. To overcome these problems, initially an electro-active core was encapsulated by carbon in the form of the core/shell structure to improve the efficiency of lithium-ion batteries. However, the large volumetric expansion (∼300%) of the core during the process of lithiation/delithiation remained unsolved; as a result, after a few cycles of charging and discharging the electrode materials develop cracks, which in turn leads to a decrease in the charging efficiency of the electrode materials abruptly. In contrast, a hollow carbon shell in the form of the YS structure plays a dual role by increasing the electrical conductivity through intimate contact with the core as well as provides a physical buffer/void space for core expansion without damaging the shell during lithium insertion/extraction. In addition, the carbon shell also protects the core material against direct contact with the electrolyte which forms an unstable SEI layer. Because of these properties the YS structure is more suitable compared to the conventional core/shell and hollow morphology by improving the cycling performance, pulverization,

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and the life spans of anodes of lithium batteries.65,67,69,118,121,124,127,142,145 In the case of anodic materials, some elements such as Si and Sn were used extensively, because of their better alloying ability with lithium and higher theoretical specific capacity. Among these materials, Si has the highest specific capacity (3500 mA h g−1), which is ten times greater than the capacity of graphite.121 A magnified schematic of individual Si/C YS NPs shows that the Si NPs expand without breaking the carbon coating or disrupting the solid–electrolyte interphase layer on the outer surface of the shell shown in Fig. 18(i–k).121 As discussed before, the availability of void space of this type of NP is very important for the application of rechargeable batteries. Some researchers have also studied the effect of the void space of YS NPs on the cyclic properties of lithium-ion batteries.358 In the case of Si/carbon YS anode material, the higher void space/Si ratio leads to improved cyclic performance because of the availability of void space to accommodate the volume expansion.358 While most of the researchers focused on different types of YS structures, in a recent study, it has been shown that the Si/C YS in the form of a pomegranate structure is highly useful for the Li-ion battery. In this case, Si NPs are encapsulated by a conducting carbon shell which provides enough buffering space for expansion and contraction during lithiation and delithiation as shown in Fig. 18(l). To obtain the pomegranate structure, these hybrid NPs were encapsulated by a thicker carbon layer in micrometer-size pouches, which eventually act as a solid–electrolyte barrier. Finally, this resultant hierarchical arrangement shows some advantages: (i) the solid-electrolyte interphase remains stable and spatially confined, (ii) superiority in cyclability (97% capacity retention after 1000 cycles), (iii) lower electrode–electrolyte contact area, high coulombic efficiency (99.87%) and volumetric capacity (1270 mA h cm−3), and (iv) the cycling remains stable even when the areal capacity was increased to the level of commercial lithium-ion batteries (3.7 mA h cm−2).64 Similarly, Sn/C YS


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Fig. 18 SEM (a, c, e, and g) and TEM (b, d, f, and h) images of Si/C YS NPs with different particle sizes. (a–d) YS NPs with a diameter of 450 nm and (e–h) YS NPs with a diameter of 880 nm. Reprinted with permission from ref. 296. Copyright 2013 from American Chemical Society. Schematic of the YS materials design for battery application (i) a conventional slurry coated Si NP electrode. SEI on the surface of the Si NPs ruptures and reforms upon each Si NP during cycling, which causes the excessive growth of SEI and failure of the battery. The expansion of each Si NP also disrupts the microstructure of the electrode. ( j) A novel Si/void/C electrode, (k) the void space between each Si NP and the carbon coating layer allows the Si to expand without rupturing the coating layer, which ensures that a stable and thin SEI layer forms on the outer surface of the carbon. Also, the volume change of the Si NPs is accommodated in the void space and does not change the microstructure of the electrode. Reprinted with permission from ref. 121. Copyright 2012 from American Chemical Society. (l) Schematic and SEM images of the pomegranate-inspired design of Si/carbon YS for Liion battery application. Reprinted with permission from ref. 64. Copyright 2014 from Nature Publishing Group.

has also been shown to be a promising anode material for lithium batteries because of its much higher theoretical specific lithium storage capacity (992 mA h g−1) than that of already commercialized graphite (372 mA h g−1), along with all other advantages of the hollow carbon shell as mentioned before.135 The high lithium storage capacity of a Sn-based compound (SnO2, 790 mA h g−1)124,127,141,143 and an alloy (Cu5Sn6, 578 mA h g−1)68 inside the hollow carbon shell were also reported to be very good anodic materials. Various other metal oxides of high theoretical storage capacity Fe3O4 (924 mA h g−1),139 ZnO (978 mA h g−1),138 and NiO (718 mA h g−1)137 in the form of the YS structure have also received significant attention owing to their low cost, environment-benign nature for energy storage application. The reported studies on metal/C YS NPs clearly show that they are also useful for catalytic applications. In this case the carbon shell protects the metal core nanoparticle from agglomeration and sintering like a silica shell; as a result, catalytic efficiency increases.52,93,300 In addition, the tunable hydrophobic/hydrophilic nature of the carbon shell also helps in the diffusion of more hydrophobic/hydrophilic molecules into the core region of the catalyst, which increases the activity of the catalyst. For example, Au encapsulated in the hydro-


phobic carbon shell shows much faster reduction of hydrophobic nitrobenzene than that of hydrophilic 4-nitrophenol by NaBH4 mainly because of more diffusion of nitrobenzene towards the core surface, whereas the reverse is true for the hydrophilic carbon shell.98,322 The kinetic analysis of both reactions from the temporal decay in absorbance peaks of nitrobenzene and 4-nitrophenol indicates that the reactions follow first-order kinetics. The apparent rate constants were estimated from diffusion-coupled first order reaction kinetics using the slopes of straight lines. The rate constant calculated for the reduction of nitrobenzene (k = 5.2 × 10−3 s−1) was one order of magnitude higher than that of 4-nitrophenol (k = 4.8 × 10−4 s−1), which confirms that the hydrophobic carbon shell favors the diffusion of more hydrophobic nitrobenzene into the core region of YS NPs. The rate constant calculated for the reduction of 4-nitrophenol (k = 6.0 × 10−3 s−1) is higher than that of nitrobenzene (k = 2.8 × 10−34 s−1), which indicates that the hydrophilic carbon shell favors the diffusion of hydrophilic 4-nitrophenol into a core catalyst. So the selectivity can be easily altered by just changing the nature of the carbon shell. The reaction rate constants for bare Au NPs towards nitrobenzene (k = 3.6 × 10−3 s−1) and 4-nitrophenol (k = 3.5 × 10−3 s−1) indicate that Au NPs are not selective towards these reactants

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and the selectivity is mainly because of the shell structure.98 On the other hand, the porosity of the carbon shell (nonporous and mesoporous) also plays a critical role in the catalytic activity. The high porosity of the carbon shell in Au/C YS NPs increases the rate of reaction by diffusing more molecules inside the shell with thermal stability and high reusability, when used for catalytic reduction of 4-nitrophenol.99,300 Similarly, the thermosensitive diffusion of a PNIPA shell was also reported for the reduction of 4-nitrophenol and nitrobenzene in aqueous solution, where 4-NP reacts much faster at a low temperature, while the reduction of NB is preferred at higher temperature.322 In general, carbon is also used as a good adsorbent in the separation processes because of its porosity and high surface area; however, separation of nanoscale particles from the suspension after the adsorption process restricts its practical use. This problem can be easily solved by using a magnetic movable core inside the hollow carbon shell. For example, iron oxide (Fe3O4/γ-Fe2O3)/carbon YS NPs are reported to have very high adsorption capacity of bilirubin (146.5 mg g−1), which is an endogenous toxic molecule present in the human body, and finally the adsorbed particles can be separated by the magnetic field. The saturation magnetization of the magnetic core can also be tuned by changing the concentration of the iron precursor and carbonization temperature.151 In other sep-

aration studies, Fe3O4/C YS NPs also exhibit excellent reusability for the adsorption of pyrene over six adsorption–desorption cycles with fast adsorption rates (∼40 min to reach equilibrium) and high adsorption capacities (77.1 mg g−1).152 Different types of inorganic movable cores with carbon hollow shell YS materials are listed in Table 4. 2.1.1.1.2.2 Core/inorganic compound. While classifying the NPs based on shell materials, it has been found that a major fraction of YS NPs are made of different inorganic compounds as shells. The inorganic shell materials are broadly classified into: (i) metal oxides, and (ii) other compounds ( phosphates, sulphides, and fluorides). Since a majority of literature reports available are on the metal-oxide, depending on the core material type, we classify these structures into (a) non-metallic element/metal oxide,66 (b) metal/ metal oxide,50–53,116,360–372 (c) metal oxide/metal 40,42,43,45,46,48,49,63,101,106,119,123,125,128,131–133,154,160,166,302,304–309 oxide, and (d) other compounds. 2.1.1.1.2.2.1 Non-metallic element/metal oxide. Under this class only sulfur is reported as the core for the fabrication of electrode materials in lithium–sulphur batteries. In recent years, the demand for high performance rechargeable batteries in terms of high energy density and long-term cyclic performance for the application of different advanced devices is gradually increasing. In spite of the fact that there has been some

Table 4 Reported metal/carbon and metal oxide/carbon YS nanostructures with detailed description of core, shell, and sacrificial layer

Core method

Shell method

Sacrificial layer

Core/shell

Method

Precursor

Method

Precursor

Material

Removal

SnO2/C

Hydrothermal Hydrothermal Hydrothermal Solvothermal USP Reduction (Citrate) Reduction (Citrate) Sol–gel Sol–gel Pre-synthesized Hydrothermal Hydrothermal Reduction (FA) Reduction (EG) Commercial Commercial Commercial Decompose Carbonization Solvothermal Hydrothermal Hydrothermal Hydrothermal Reduction (Citrate) Pre-synthesized Hydrothermal Heating Pre-synthesized

K2SnO3 K2SnO3 SnCl4 K2SnO3 SnC2O4 HAuCl4 HAuCl4 Au/SiO2 Au/SiO2 Au NPs Na2SnO3 K2SnO3 AgNO3 H2PdCl4 Si NPs Si NPs Si NPs SiO2 phenolic resols FeCl3 FeCl3 FeCl3 FeCl3 FeCl3 Fe3O4 Ni(NO3)2 Zn3(C6H5O7)2 S NPs

Carbonization Carbonization Carbonization Carbonization Carbonization Carbonization CVD Carbonization Carbonization Carbonization Carbonization Carbonization Carbonization Carbonization Carbonization Carbonization Carbonization Carbonization Carbonization Carbonization Carbonization Carbonization Carbonization Carbonization Carbonization Carbonization Carbonization Carbonization

Glucose Glucose Starch RF Sucrose RF Toluene Dopamine PFA RF Glucose Urea RF PF resin RF Dopamine Sucrose Citric acid PF PF Glucose Dopamine EG RF PPy Sucrose Citrate THF

Hollow SnO2 SiO2 C SiO2 C SiO2 SiO2 SiO2 SiO2 SiO2 SiO2 C SiO2 SiO2 SiO2 SiO2 SiO2 Si SiO2 Pst SiO2 SiO2 SiO2 SiO2 PPy MOF Citrate C

Ship-in-bottle124 NaOH127 Calcination143 HF141 Combustion69 HF35 HF99 HF93 HF52 HF98 NaOH126 Calcination118 HF301 HF299 HF64 HF121,145,358 HF65 HF142 HF296 Calcination343 NaOH139 NaOH144 NH4OH300 NaOH152 Carbonization167 Calcination359 Calcination138 Calcination140

Au/C

Sn/C Ag/C Pd/C Si/C

C/C Fe3O4/C

NiO/C ZnO/C S/C

Polyfurfuryl alcohol (PFA), formaldehyde (FA), phenol-formaldehyde (PF), ethylene glycol (EG), polypyrrole (PPy), tetrahydrofuran (THF).

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progress on the high performance anodic materials (Si and Sn based) for rechargeable batteries in the last few years, the development is limited to the cathode materials. Generally, sulfur is a promising cathode material with a theoretical specific capacity of 1673 mA h g−1, which is 5 times that of existing materials (transition metals, phosphate etc.).373–375 Many significant research studies have been developed for lithium–sulfur batteries with different morphologies of sulfur cathode such as core/shell, composites and hollow nanostructures in recent years. It is also believed that the Li–S batteries may serve as next generation high-energy-density reduced-cost rechargeable batteries in terms of efficiency and cycle life.376 At the same time, many challenges also remain for research and development to obtain better cathode materials for Li–S batteries. When naked S NPs are used as the electrode material of Li–S batteries, there are three major drawbacks: (a) poor electronic conductivity, (b) dissolution of intermediate polysulfides, and (c) large volumetric expansion upon lithiation.66 Because of these shortcomings, new multifunctional sulfur-based materials are developed. In recent years, researchers have attempted to solve the first two problems by encapsulating sulfur with conductive materials including porous carbon,377–380 graphene oxide,381,382 and conductive polymers383,384 in the form of core/shell nanostructures. However, the third problem cannot be solved because of the large volumetric expansion of the sulfur core during the lithiation coupled with polysulfide dissolution, which in turn poses the problem of cracking or fracture on the shell layer during

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the core expansion. Finally, it has been found that the core/ shell morphology is ineffective to enhance the efficiency of Li– S batteries. Recently, S/TiO2 YS nanostructures have been reported for the first time for stable and prolonged cycling over 1000 cycles in Li–S batteries. The unique advantage of the YS structure is that it can accommodate the large volumetric expansion of sulfur during lithiation, and thus preserves the structural integrity of the shell to minimize polysulfide dissolution as shown in Fig. 19. Finally, the resultant YS structure shows an initial specific capacity of 1030 mA h g−1 at 0.5 C (coulomb) and a coulombic efficiency of 98.4% over 1000 cycles. On the other hand, it also exhibits capacity retentions of 88, 87, 81, and 67% at the end of 100, 200, 500, and 1000 cycles, respectively, which represents a better performance for long cycle Li–S batteries because of the effectiveness of the intact TiO2 shell in limiting polysulfide dissolution. In comparison with the YS structure, bare sulfur and S/TiO2 core/shell NPs show capacity retentions of 48 and 66% respectively, after 200 cycles, which indicates greater degree of polysulfide dissolution into the electrolyte; this YS structure exhibits the highest capacity retention.66 2.1.1.1.2.2.2 Metallic/metal oxide. Encapsulation of metal nanoparticles into the metal oxide hollow shell is extremely important because of improved chemical, catalytic, optical, and electronic properties of the resultant YS structure (metal core and metal oxide shell) for a wide range of potential applications such as antibacterial activity,364,385 50–52,360–362,365,366,369–371,385–388 catalysis, bio-medical,363,389 and

Fig. 19 (a–c) Schematic of the lithiation process in various sulfur-based nanostructure morphologies; (d) SEM image and (e) TEM image of assynthesized sulfur–TiO2 YS nanostructures. Reprinted with permission from ref. 66. Copyright 2013 from Nature Publishing Group.


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sensors.112,113,116,117,390 While they are (metal and metal oxide) present in a combination, the catalytic and optical properties of the metal core improve because of an enhancement of thermal stability and a change in the local dielectric environment around the metal surface. Similarly, the presence of metal improves the catalytic, optical, and electronic properties by increasing active reaction sites, improves visible light absorbance and light harvesting, and provides better charge separation, and a metal support interface of metal oxide. A proper combination of metal and metal oxides becomes an important consideration to obtain the improved desired properties for specific applications.111,114,385,391–396 Over the past few years, there have been many methods developed for the synthesis of YS nanostructures of metal/metal oxide NPs, because of which it is possible to achieve highly tuneable chemical, thermal, optical, and physical properties by tailoring the core and shell sizes and the material combination. Among all the studied metals, noble metals such as Au,50–53,112,116,117,365–370,386–388,390,397,398 Ag,113,361,364,385,389 Pd,360,361 and Pt362,363,388 are studied extensively because of their superior optical and catalytic properties. Further, among the noble metals, gold cores with metal oxide hollow shells have been mostly studied.50–53,112,116,117,365–370,386–388,390,397 Metal oxide coating on gold nanoparticles with void space enhances some important physical properties, such as the thermal stability by protecting the core material during thermal treatment, biocompatibility, and tuneable optical properties.50–53,116,365–370,397 Among the reported synthesis methods, sacrificial templates,50–53,112,113,117,360,365,366,368–370,372,385,386,388–390 Ostwald ripening,361,362,371,387,399 and Kirkendall reaction116,363,364 are well established for metal/metal oxide YS NPs synthesis. The noble metals are considered fairly inert towards the chemical reactions; however, the NPs of noble metals act as a good catalyst for several hydrogenation and oxidation reactions. Generally, a coating of metal oxide on the metal core prevents the agglomeration and increases thermal stability, similar to the silica shell as discussed in the previous section. Many researchers have reported a sinter-stable catalyst composed of a metal movable core inside different metal oxide hollow shells such as TiO2,365,366,369,370,387,398 ZrO2,53,368 and CeO2 386 which show a synergistic effect, thermally stability, lower agglomeration, higher mass transfer,400 more void space for reactant molecules, and finally high catalytic activity at high temperature. Fig. 20 shows that the catalytic efficiency of as-synthesized Au/ZrO2 is similar to that of high temperature (800 °C) treated Au/ZrO2. On the other hand, a crushed nonencapsulate Au/ZrO2 catalyst shows a substantial loss in catalytic activity.368 Similarly, metal cores are also encapsulated inside the TiO2 365,366,369,370,387 and CeO2 360,386 hollow shells for better catalytic properties by preventing sintering and aggregation. Metal/metal oxide YS nanostructures are also important for the modification of plasmonic optical properties of the metal core by tailoring the void space and alteration of the local dielectric environment,230 which can be used in the fields of

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Fig. 20 (a) General scheme of the synthetic process of Au/ZrO2 YS NPs, (b) TEM images of step by step synthesis of Au/SiO2/ZrO2 and Au/ ZrO2 YS NPs. (c) Catalyst performance of different samples of Au/ZrO2. Reprinted with permission from ref. 368. Copyright 2006 from WileyVCH Verlag.

bio-imaging, photo-thermal therapy, photocatalysis, optoelectronics, and plasmon-enhanced spectroscopy.367 As shown in Fig. 21(a), during the change in void space of the Au/Cu2O YS nanostructure, there is a dramatic colour change because of the change in optical properties. The results are supported by the change in extinction spectra as shown in Fig. 21(b). This change can be explained by Mie scattering theory. According to this theory, as the spacing between the Au core and the Cu2O shell increases, the plasmon resonance of the Au core progressively shifts towards the blue region and becomes very similar to the plasmon resonance of bare Au as shown in Fig. 21(c and d). This blue shift in plasmon resonance would be because of a significant decrease in the effective refractive index of the surrounding Au core.367 Similarly, the metal core also helps to enhance the photoelectrochemical properties of semiconductor metal oxide hollow shells for the applications of photocatalysts, photovoltaics, and remediation of organic contaminated pollutants. However, for photo-induced applications of NPs, most of the semiconductor NPs are weakly interacting with the visible light because of their wide band-gap. In the photocatalytic process, the light is absorbed first by the catalyst to initiate the charge transfer process by generating electron–hole pairs, which in turn leads to facilitating the electron and hole driven reduction and oxidation processes, respectively. The widely


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Fig. 21 (a) Photograph of colloidal suspensions of Au–Cu2O YS NPs with an average outer radius of 130 nm obtained at different reaction times during the symmetric hollowing of Cu2O shells. The right-most sample is bare Au colloids. (b) Experimentally measured extinction spectra of colloidal suspensions of Au/Cu2O YS NPs obtained at different reaction times: 2, 5, 10, 15, 20, 25, 30, 40, 50, 60, 75, and 90 min (from top to bottom). The bottom spectrum is corresponding to bare Au colloids. (c) Geometry of the Au–Cu2O composite particle employed for Mie scattering theory calculations. (d) Calculated extinction spectra of an Au/Cu2O composite particle with fixed R1 of 63 nm, fixed R3 of 130 nm, and varying R2 of 64, 65, 66, 68, 70, 75, 80, 86, 93, 100, and 110 nm (from top to bottom). The bottom curve is the calculated spectrum of a spherical Au nanoparticle (63 nm in radius). (e) Schematic illustration of the structural evolution of Au–Cu2O core–shell NPs during the hollowing of Cu2O shell. Reprinted with permission from ref. 367. Copyright 2011 from American Chemical Society.

used photocatalysts such as TiO2, ZnO etc. absorb light in the UV region of the electromagnetic spectrum because of their wide band gap; in that case, they can utilize only about 4% of sunlight. On the other hand, noble metal NPs can strongly absorb visible light because of the SPR effect. In addition, noble metal NPs can also act as an electron trap and active reaction sites. So employing metal NPs in the semiconductor shell could change the absorption of the catalyst in the visible region as well as charge separation. The metal/semiconductor metal oxide YS NPs also represent an important class of plasmonic photocatalyst materials; finally, these YS NPs exhibit the mixed properties of metals and semiconductors. The noble metal movable core absorbs visible light and acts as the active redox centre, while the semiconductor shell acts as a photocatalyst. Overall, this type of structure shows improved photocatalytic activity because of a better light-harvesting efficiency, better charge separation at the metal–metal oxide interface, and more absorption of visible light.366,367 In addition, more surface area and void space provides enough room for loading of reactant molecules and light-harvesting properties, respectively. As per the reported studies, various metals such as Au,366,386,388,398 Ag,385 Pt,362,388 and Pd360,361 NPs are encapsulated inside the semiconductor shells, which have plasmonic properties in the visible to near-infrared region. The Pt/CeO2 YS nanostructure was reported for the first time as a visible light-induced photocatalyst.362 The UV-visible spectra of Pt/


CeO2 YS NPs show a superior absorption ability in visible light compared to that of the core/shell and pure CeO2 NPs as shown in Fig. 22(a–d).362 Similarly, Au/TiO2 YS NPs were tested for photocatalytic hydrogen evolution, and exhibited a remarkable enhancement of H2 evolution activity; which is 7 times higher than commercial P25 TiO2 under UV light.388 When metal/metal oxide YS NPs were irradiated with visible light, plasmon excited electrons from the metal move into the metal oxide shell to reduce H+ to H2. In this situation, since the electrons from TiO2 are not excited under visible light, a low H2 evolution rate was observed because of only the contribution of metal core plasmon excited electrons. In contrast, when the metal/metal oxide YS NPs were irradiated with UV light, the excited electrons from TiO2 can easily transfer to the metal core and improve the charge separation within the structure, which in turn enhances the H2 evolution rate as shown in Fig. 22(e).388 Similarly, other metal/metal oxide combinations such as Au/TiO2, Pt/TiO2, and Au/Fe2O3 were also synthesized and used as plasmonic photocatalysts.388 Additionally, the strong antimicrobial properties of Ag, Ag/TiO2 YS NPs were also exploited as a good bactericide along with the photocatalytic activities in the presence of UV light exposure.385 The metal core with superparamagnetic oxide shells also provides good catalytic activity with magnetically induced separation of particles and recyclability. For instance, Pd/Fe3O4 YS NPs show very good catalytic activity for the reduction of 4-nitrophenol

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Fig. 22 (a, b) HRTEM images of Pt/CeO2 YS nanostructure, (c) UV-Vis diffuse reflectance spectra (DRS) of the as-prepared core–shell Pt/CeO2, Pt/ CeO2 YS, and blank-CeO2 and nanosized CeO2 powder, (d) photocatalysis mechanism of Pt/CeO2 under visible light. Reprinted with permission from ref. 362. Copyright 2011 from the Royal Society of Chemistry. (e) Schematic illustration of the proposed mechanism of YS NPs for H2 evolution under visible light and UV light. Reprinted with permission from ref. 388. Copyright 2014 from Elsevier Inc.

and Suzuki cross-coupling reactions, and at the same time, the catalyst can also be easily separated from the reaction mixture in the presence of a magnetic field.361 On the other hand, functionalized metal oxide magnetic nanoparticles have also the advantage of bacterial detection and separation from the products.401–404 As Ag has strong antimicrobial properties, the combination of Ag and functionalize magnetic nanoparticles in the form of the YS structure shows both antibacterial and bacterial capturing properties. For instance, Ag/Fe2O3 YS multifunctional nanostructures synthesized through the Kirkendall effect are reported for the detection, capturing, and detoxification of bacteria or pathogens as shown in Fig. 23(a and b).364 The YS NP surfaces are functionalized by using glucose because many bacteria use the mammalian cell surface’s carbohydrates as the anchor for attachment.404 After attachment of bacterial cells to glucose functionalized YS NPs, the magnetic shell helps to eliminate bacteria by magnetic separation and the silver nano core helps to kill the pathogen. The YS capturing efficiency of bacteria was measured with the help of the fluorescence intensity of E. coli ER2566 cells (ER2566 was constructed from the recombinant plasmids encoding enhanced green fluorescent

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protein) in the supernatant solution after the magnetic capture and separation. The fluorescent intensity of the supernatant decreases as the concentration of YS NPs increases because of the lowering of the number of bacteria. It is reported that the capturing efficiency (97%) becomes high at 64 µg mL−1 YS NPs concentration as shown in Fig. 23(c and d). The Ag/Fe2O3–glucose conjugated YS NPs are a unique multifunctional nanomaterial, which exhibit both high capture efficiency for detection and elimination of bacteria, and potent antibacterial activity as shown in Fig. 23(a).364 The semiconductor metal oxides (SnO2, ZnO, TiO2, WO3, In2O3 etc.) are also considered promising candidates for the gas sensing application because of their low cost and flexibility of synthesis methods. Since 1962 it has been known that adsorption or desorption of gas on the surface of metal oxide changes the conductivity of metal oxide; as a result, this property enables their use for gas sensing applications.405 When the semiconductor metal oxide-based sensors are exposed to air at elevated temperature, oxygen molecules from the air is chemisorbed on the surface of metal oxide and then ionized to oxygen species (O2−, O−, and O2−) by the excited free electrons from the conduction band of semiconductor metal oxide. As a


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Fig. 23 (a) Schematic working mechanism of Ag/Fe2O3–glucose YS NPs capturing bacterial by magnetic separation, (b) TEM images of Ag/Fe2O3 YS NPs, (c) the fluorescence analysis of the supernatants from E. coli ER2566 samples after treatment with different concentrations of Ag/Fe2O3–Glu conjugates (from top to down: 0, 0.5, 1.0, 2.0, 4.0, 8.0, 16.0, 32.0, 64.0, and 128.0 mg mL−1). (d) The analysis of bacterial capture efficiency based on fluorescence measurements, (e) TEM image of YS conjugates captured bacteria (E. coli.). Reprinted with permission from ref. 364. Copyright 2011 from the Royal Society of Chemistry.

result, band bending and the depletion layer (space charge region) were formed on the surface of metal oxide, which led to changes in resistance (reducing gases decrease the resistance and oxidizing gases increase the resistance). When sensing or target gases are introduced, the gas molecules react with oxygen species on the surface of metal oxide, and the change in resistance indicates the presence of target gas molecules. In metal/metal oxide YS NPs, an active metal core additionally serves as an effective adsorption site to bind and dissociate oxygen molecules, which in turn enhance the quantity of adsorbed oxygen species and finally improve the sensitivity.117,239,406 As an example, Au/ZnO YS NPs exhibits a high gas response (the ratio of the resistance of the sensor in fresh air (Ra) to that in the tested gases (Rg)) for 100 ppm acetone at 300 °C, which is 2–3 times that of pure hollow and solid ZnO nanosphere inner structures as shown in Fig. 24(b). This improvement in YS structures is mainly because of the presence of porous shell, void space, and more active sites for the catalytic effect of the Au core as shown in Fig. 24(f ).117 Similarly, Au and Ag-based YS NPs such as Au/SnO2 for CO gas,390 Au/NiO for H2S gas,112 and Ag/SnO2 for H2S gas113 exhibit a better sensing ability than that of hollow SnO2 and NiO spheres respectively, because of the additional catalytic effect of the metal core and enhanced electron depletion at the surface of the YS structure.112,113,390


Apart from pure metals, bi-metallic cores are also encapsulated inside the metal oxide hollow shell to achieve specific properties and applications. As an example, FePt/Fe2O3 YS NPs exhibit dual functions such as high cytotoxicity towards cancer cells and a strong MR contrast enhancement because of the FePt core and the Fe2O3 shell respectively. Compared to conventional treatment, this type of YS nanostructure leads to a new type of control drug delivery system, which serves both as an MRI agent and as a potent anticancer drug.363 Different metal movable cores with metal oxide hollow shells are listed in Table 5. 2.1.1.1.2.2.3 Metal oxide/metal oxide. These classes of YS nanostructures are either made of the same or different metal oxides. As mentioned before, metal oxide YS nanostructures are important among the various other inorganic/ inorganic combinations because of their suitability with an improved efficiency in various advanced applications such as photocatalysis,40,154,303,408–411 chemical 42,166,412–414 catalysis, sensors,63,125,305,415–417 lithium ion batteries,49,119,123,125,128,131–133,414,418–427 DSSCs,43,101,160,428,429 and microwave absorption.306,430 For better organization, we subdivide them into two parts: (i) single metal oxide and (ii) different metal oxide/metal oxide YS nanostructures. 2.1.1.1.2.2.3.1 Single metal oxide. The YS NPs constructed from the same metal oxide is possible to

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Fig. 24 (a, b) TEM images of Au/ZnO YS NPs, (c) responses of the sensor devices (Au/ZnO YS, hollow ZnO, solid ZnO) upon exposure to 100 ppm acetone at different working temperatures, gas sensing principles of (d) solid ZnO nanospheres, (e) hollow ZnO nanospheres, and (f ) Au/ZnO YS NPs. Reprinted with permission from ref. 117. Copyright 2014 from American Chemical Society.

Table 5

Reported metal/metal oxide YS nanostructures with detailed description of core, shell and sacrificial layer

Core

Shell

Sacrificial layer

Core/shell

Method

Precursor

Method

Precursor

Material

Removal

Au/TiO2

Au/TiO2/ZrO2 Au/Cu2O Au/ZnO Au/Fe2O3 Au/CeO2 Au/NiO Ag/TiO2 Ag/Y2O3:Yb3+,Tm3+ Pd/Fe3O4

Reduction (AA) Reduction (Citrate) Reduction (NaBH4) Reduction (Glucose) Reduction (Citrate) Reduction (Heating) Reduction (Citrate) Reduction (Citrate) Pre-synthesized Reducing (FA) Reduction (Glucose) Reduction (Glucose) Reduction (Glucose) Reduction (Glucose) Reduction (Glucose) Reduction (EG) Reduction (Citrate)

HAuCl4 HAuCl4 HAuCl4 HAuCl4 HAuCl4 AuPPh3Cl HAuCl4 HAuCl4 Au/TiO2 HAuCl4 HAuCl4 HAuCl4 HAuCl4 HAuCl4 AgNO3 AgNO3 K2PdCl4

Sol–gel Sol–gel Sol–gel Sol–gel Wet chemical Wet chemical Wet chemical Wet chemical Wet chemical Hydrolysis Sol–gel Thermal Hydrothermal Precipitation Sol–gel Hydrothermal Hydrolysis

CTAB SiO2 TiO2 C SiO2 SiO2 Au SiO2 SiO2 Cu2O C C C C SiO2 C Fe3O4

Water369 NaOH366,370 Ostwald ripening387 Calcination388 NaOH53,368 NaOH50 NaCN51 HF52 NaOH365 Ostwald ripening367 Calcination407 Calcination388 Calcination386 Calcination112 HF385 Calcination389 Ostwald ripening361

Pd/CeO2 Pt/CeO2 Pt/TiO2

Reduction (TOPO) Reduction (Citrate) Reduction (Glucose)

Pd(acac)2 H2PtCl6 H2PtCl6

Wet chemical Hydrothermal Sol–gel

TiF4 TBOT TiF4 TIP Zr(OBu)4 Zr(OBu)4 Zr(OBu)4 Zr(OBu)4 Zr(OBu)4 Cu(NO3)2 Zn(acac)2 FeCl3 Ce(NO3)3 Ni(NO3)2 TBOT Y(NO3)3 FeCl3 Fe(NO3)3 Ce(NO3)3 CeCl3 TIP

SiO2 CeO2 C

NaOH360 Ostwald ripening362 Calcination388

Au/ZrO2

Ethylene glycol (EG).

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synthesize via several techniques such as Ostwald ripening,40,42,43,45,46,48,49,63,101,119,128,133,160,302,303,305,308,409,411,413,416,423,428,429,431,432 spray pyrolysis,132,414,418,420,421,424–426 and sacrificial templates.125,131,309 Under this category, YS NPs of different materials have been reported such as TiO2,43,101,131,160,302,308,309,409,413,423,428,429,431–434 CO3O4,48 63,133,424 46,119,125 40,416,435 SnO2, γ-Fe2O3, ZnO, CuO2,45,305 42 417,418 49,128 CeO2, NiO, V 2 O5 , and complex transition metal oxides (LiMn2O4,132 CoMn2O4,425 ZnFe2O4,421 MnCo2O4,419 ZnWO4,411 CoFe2O4,422,436 NiFe2O4,436 CdFe2O4,436 CoMoO4 420 etc.). As mentioned before, the advantages of this type of nanostructure made of a single material over the solid or hollow structures are very high effective surface area, and utilization of void space between the core and shell for advanced applications.413,418,419,423,424,429,437–442,442–447 Semiconductor metal oxide NPs (TiO2, ZnO etc.) act as promising photocatalysts for many applications such as solar fuel production, pollutant degradation, and self-cleaning coatings since their discovery by Fujishima and Honda.448 However, in many cases, pure metal oxide NPs show lower efficiency in photo-electrochemical applications because of their lower reflection or scattering properties and higher band gap.43,101,106,160,428,429 The problems related to scattering and reflection have been solved by changing the simple NP structure to a unique YS architecture. The electrochemical characteristic of TiO2 YS nanostructures has been studied by many researchers43,101,106,160,428,429 because of their better performance. A comparative study shows the good photocatalytic activity and photoconversion efficiency of TiO2 YS NPs compared to those of pure nanoparticles and solid and hollow spheres as shown in Fig. 25(b).106 The higher efficiency of YS NPs is attributed to (i) multiple reflections of UV and visible light within the interior voids of the sphere, which in turn exhibit high diffuse reflectance in the visible region of 400–700 nm,43,101,160,428,429 and (ii) a better dye

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adsorption capacity because of their high surface area and void space.43,101,160,428,429 The electron impedance spectroscopy of YS NPs also confirms the longer lifetime of injected electrons than that of a simple TiO2 or P25 based photoanode.106 As a result, because of the above mentioned advantages, the TiO2 YS NPs are not only advantageous in photocatalysis409,411,431,432 but also in the DSSC applications.43,101,160,428,429,434 The facets and surface structure of the metal oxide shell can also be controlled in the YS structure,43,101 which in turn is advantageous for photocatalysis and photo-electrochemical applications. To enhance the photocatalytic activity under visible light and to prevent the recombination of photogenerated electrons and holes, many researchers also doped metals308,371,432 and non-metals308,433 in either the core or the shell. As shown in Fig. 26(a), the UV-Vis absorption spectra of Eu and N co-doped TiO2 exhibit absorbance in the visible region compared to pure NPs. The light absorption behaviour of doped nanoparticles in the visible region is mainly because of the lowering of the band gap; additionally, the interior void space of the YS structure also promotes the light absorbance properties because of multiple reflections within the sphere and its unique structure as shown in Fig. 26(b–e) for Bi doped TiO2,371 and also mentioned before. As presented in Fig. 26(b–e), Bi doping on TiO2 not only enhances the degradation efficiency of p-chlorophenol, but also shows no significant decrease in the degradation efficiency under visible light irradiation even after 10 times repetition.371 Similar to TiO2, ZnO YS nanostructures also show better photoluminescence and photocatalytic properties compared to those of solid and hollow ZnO nanospheres (Fig. 27) because of the maximum number of radiative defects at the interfaces of the inner hollow structures of YS NPs, multiple reflection, and the highest surface area.40

Fig. 25 (a) FE-SEM image of titania YS nanostructures, (b) comparison of photocatalytic activities of the titania spheres with solid, sphere-insphere, and hollow structure. Inset shows a schematic illustration of multi-reflections within the sphere-in-sphere structure. Reprinted with permission from ref. 106. Copyright 2007 from American Chemical Society.


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Fig. 26 (a) UV-Vis spectra of N–Eu-0.0, N–Eu-0.1, N–Eu-0.5 YS NPs, P-25 and N-P25 (the insert shows the optical photographs of all samples). Reprinted with permission from ref. 308. Copyright 2012 from Elsevier Inc. (b) SEM and TEM (inset) images of the TiO2 YS NPs, (c) UV-Vis DRS spectra of (a) TiO2, (b) 1.9% Bi2O3/TiO2, and (c) 1.9% BixTi1−xO2 samples calcined at 773 K. (d) Schematic illustrations of the light multi-reflections in the core–shell structured chamber of the BixTi1−xO2. Red dots = Bi3+, blue area = TiO2. (e) Recycling test of (a) 1.9% BixTi1−xO2 and (b) 1.9% Bi2O3/TiO2 catalysts. Reprinted with permission from ref. 371. Copyright 2009 from Elsevier Inc.

Fig. 27 The SEM and TEM images of (a, b) ZnO solid nanosphere, (c, d) ZnO YS nanospheres, (e, f ) ZnO hollow nanospheres. The inset in (a, b, c) display high magnification SEM images of nanospheres and (b, d, f ) shows SAED patterns of TEM images, (g) photoluminescence spectra, (h) photocatalytic activity of ZnO solid nanospheres, YS nanospheres and hollow nanospheres. Reprinted with permission from ref. 40. Copyright 2012 from the Royal Society of Chemistry.

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Apart from photocatalysis, metal oxide YS NPs also act as catalyst support for metal NPs. For this purpose, small sized metal NPs are usually deposited on the metal oxide YS surfaces, which not only provide high surface support and mechanical strength but also give thermal stability and recyclability to improve the catalytic activity of metal NPs. The metal NPs can be deposited on metal oxide NPs by many methods including impregnation, ion-exchange, reduction, precipitation etc.433,449 For instance, Au impregnated CeO2 YS NPs exhibit high catalytic activity for the reduction of p-nitrophenol to p-aminophenol compared to that with pure Au NPs and also shows good recyclability after running for 5 cycles with limited Au leaching. Here, the metal oxide YS structure not only stabilizes the metal NPs via preventing them from agglomeration and leaching, but also provides strong metal–support interaction and recylability.42,433 On the other hand, the transition metal oxides based on d0 (TiO2, V2O5, WO3 etc.) and d10 (ZnO, SnO2 etc.) electronic configurations are found to be more suitable for gas sensors because of their electronic properties. The reported studies on YS structures show that they are indeed superior for the sensor applications, specifically for gas sensors,63,125,416,417 over the pure or hollow NPs made of the same material. So, the metal oxide NPs are promising candidates for gas-sensing applications, where the change in conductivity is determined by the change in charge carrier concentration (electrons and holes)

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via adsorption of a gas molecule on the surface of metal oxides as discussed in the metal/metal oxide section. The changes in conductance occur because of the exposure to reducing gases towards n-type (conductance increase e.g. ZnO, SnO2) or p-type (conductance decrease e.g. CuO) semiconductor metal oxides and vice versa in the presence of oxidizing gases. The YS structure gives a strong response even in the presence of a low concentration of target gas molecules compared to that of hollow or solid NPs because of the higher surface area and large number density of available surface sites for adsorption of gas molecules. For instance, SnO2 YS NPs show higher surface area over hollow SnO2 NPs, due to their unique structure with the nano-sized spikes on the shells as shown in Fig. 28. This unique structure enhances the absorption of reactant gas molecules on the cores surface and inner–outer surfaces of the hollow shell, which exhibits three times better sensing response of ethanol vapour than hollow SnO2.63 Similarly, other metal oxide YS NPs such as Fe2O3 125 for acetone, Fe2O3 125 and ZnO416 for ethanol, and NiO417 for xylene also exhibit an improved sensing ability than pure and hollow NPs. Apart from gas sensors, semiconductor metal oxides can also be used for electrochemical bio-sensing by applying a standard potential between two electrodes. The electrochemical measurements demonstrated that Cu2O YS NPs have a high efficiency for the detection of dopamine molecules with a very low detection limit (1.0 × 10−7 M) because of

Fig. 28 (a) Responses to 50 ppm ethanol at different temperatures. (b) The dynamic response–recovery curves at different ethanol concentration (10–50 ppm). (c) Comparative sensing ability of the sea urchin-like SnO2 YS nanostructures and the common hollow SnO2 spheres corresponding. (d) Schematic diagram of the sensing mechanism. (e) TEM image of SnO2 sphere. Reprinted with permission from ref. 63. Copyright 2010 from Elsevier Inc.


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high surface area, which leads to high electro-catalytic efficiency for the oxidation of dopamine with improved electron transfer.305 Since the first reported study on nano-sized 3d transition metal oxides in a lithium ion battery as the negative electrode,450 there have been increasing research activities on metal oxide YS NPs as a negative electrode material to improve their energy density, cycle performance, and coulombic efficiency. Metal oxide YS NPs such as SnO2,133 TiO2,423 V2O5,49,128 and Fe2O3119,125 have been widely studied because of their high surface area, shorter Li+ diffusion path length, buffering space, and morphological stability, which possess superior lithium storage performance as discussed previously. Recently, complex transition metal oxide YS NPs (CoMn2O4,425 ZnFe2O4,421 MnCo2O4,419 CoFe2O4,422,436 NiFe2O4,436 436 420 CdFe2O4, CoMoO4 ) also attracted attention as anode materials over simple metal oxide YS NPs because of their high theoretical capacity, more efficient charge storage sites, and good cycling performance. At the same time, because of the development of easy synthesis techniques such as spray pyrolysis, the production and application of these materials on a large scale has become feasible in recent years.132,414,418,420,421,424–426 2.1.1.1.2.2.3.2 Different metal oxide/metal oxide. The combinations of mixed or dissimilar metal oxide YS NPs provide new physicochemical properties, which enable us to engineer this class of materials. Different combinations of dissimilar core/shell metal oxide/metal oxide YS such as α-Fe2O3/SnO2,123 Cu2O/Fe2O3,304 Fe3O4/SnO2,154 iron oxide/Y2O3,307 Fe3O4/ Co3O4,166 Fe3O4/TiO2,306,408 Co3O4/Al2O3,412 and Fe3O4/ ZrO2 415,430 have been studied by different research groups. The proper combinations are mostly selected aiming to specific novel applications. From the above-listed reported combinations, one can easily understand that mostly either the core or the shell is made of a magnetic material. These types of YS structures are mostly used in lithium storage,123 catalysis,154,166,408,412 sensor,415 bio-medical,307 and microwave-absorption enhancement306,430 applications. In the case of catalytic applications, separation of the catalyst is a major concern to make the process economical by re-using the catalyst. The combinations of magnetic materials as the core or the shell with a metal oxide catalyst shell or core in the form of YS NPs have attracted a great deal of attention because of recyclability of the catalyst by magnetic separation. As a specific example, a superparamagnetic iron oxide (SPIO) core with a mesoporous photocatalyst SnO2 shell acts as a magnetically recyclable photocatalyst, which exhibits high efficiency of photocatalytic degradation of rhodamine B and could be reused up to five times by magnetic separation.154 As shown in Fig. 29, the catalyst can be separated easily, and the catalyst can remain stable to maintain its high activity over a long time. At the same time, the morphology remains unaffected after 5 cycles of reuse as a photocatalyst as shown in Fig. 29(d). Similarly, Fe3O4/TiO2 YS NPs are also used as a magnetically recyclable photocatalyst for the degradation of rhodamine B dye.408 Apart from the photocatalysis, dissimilar metal oxide combinations such as Fe3O4/Co3O4 166 and Co3O4/Al2O3 412 in

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the form of YS NPs are very useful as electocatalysts in fuel cells for cathodic oxygen reduction and CO oxidation reactions, respectively. In addition to high surface area and more active sites present on the surface, both the core and the shell reduce O2 collaboratively in YS to enhance the catalytic performance.166,412 The combined magnetic manipulation and optical imaging of the superparamagnetic core with upconversion lanthanide oxide nanoshells provide an opportunity in dual imaging for targeted drug delivery and thermal therapy. These materials have a good combination of strong magnetic and excellent tuneable fluorescence properties. The fluorescent properties can also be tuned according to the thickness of lanthanide oxide nanoshells.307 Different transition metal oxide combinations can be used as an electrode for lithium-ion batteries. The YS structure avoids the pulverization problems and provides more space for lithiation/delithiation. For instance, α-Fe2O3/SnO2 YS NPs exhibit a much lower initial irreversible loss and a higher reversible capacity compared to those of SnO2 hollow spheres because of a synergistic effect between SnO2 and α-Fe2O3.123 The combination of the magnetic core and the dielectric shell is also useful for microwave absorption applications as discussed in the silica section. The microwave absorption properties of the YS nanostructure can be easily tuned by altering the core size, void volume, and shell thickness for a particular external diameter particle. For example, Fe3O4/TiO2 YS microspheres with different core sizes, interstitial void volumes, and shell thicknesses can be synthesized by simply varying the synthesis parameters as shown in Fig. 30.306 The size of the void can be easily tuned by controlling the thickness of the sacrificial template (SiO2). The shell thickness (TiO2) can be easily tuned by changing the concentration of the titanium precursor. The electromagnetic characterization results demonstrate that the as-synthesized Fe3O4/TiO2 YS microspheres exhibit significantly enhanced microwave absorption properties compared with pure Fe3O4 and Fe3O4/TiO2 core/shell microspheres, which is mainly because of the unique YS nanostructure with a large surface area, high porosity, more sites for the reflecting and scattering of electromagnetic waves, as well as synergistic effects between the functional Fe3O4 core and the TiO2 shell. On the other hand, void space in the YS structure can also enhance the microwave absorption properties by preventing the spread of electromagnetic waves and generating dissipation because of the existing impedance difference.162 These types of magnetic and dielectric YS structures are promising materials for microwave-absorption enhancement in many potential applications because of their unique composition and properties.306 For the high temperature applications, a thermally-stable microwave absorbing material has also been developed by encapsulating the Fe3O4 core inside the ZrO2 hollow shell where the thermal stability of the particles is developed because of the presence of a thick ZrO2 shell layer.430 Different metal oxide movable cores with metal oxide hollow shells are listed in Table 6. 2.1.1.1.2.2.4 Other compounds as a shell. Apart from metal oxides, a few researchers have studied also other com-


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Fig. 29 (A) UV-Vis spectroscopy changes of the RhB aqueous solution in the presence of the SPIO/SnO2 YS NPs under exposure to UV light. (B) Degradation rate of RhB in the dark and under UV light: (curve b) the as-prepared SPIO/SnO2 YS NPs, (curve a) without a photocatalyst. The inset is a digital photograph of (a) RhB aqueous solution before degradation, (b) the suspension of SPIO/SnO2 YS NPs and RhB aqueous solution before degradation, (c) the suspension of SPIO/SnO2 YS NPs and RhB aqueous solution after degradation, (d) the suspension of (c) separated by a magnet. (C) Five cycles of the photocatalytic degradation of SPIO/SnO2 YS NPs. (D) TEM image of SPIO/SnO2 YS NPs after five photocatalytic cycles. Reprinted with permission from ref. 154. Copyright 2012 from the Royal Society of Chemistry.

pounds such as sulfides, fluorides, and phosphides to construct YS nanostructures. The other compounds as a shell of a YS nanostructure are listed in Table 7. 2.1.1.2 Inorganic/polymer. In a broad sense, these YS NPs are classified based on the organic shell structure; however, practically the majority of the reported studies are on polymeric shells than other organics. Polymeric hollow nanospheres have attracted considerable research attention because of their unique properties including large inner volume, low density, easy surface functionality, high specific surface area, controlled permeability, and good flow ability for a variety of applications in the fields of chemistry, bio-medical, and materials science.58,97,453 The coating of polymeric hollow spheres on the inorganic movable core develops an approach to design new advanced functional nanostructures with improved properties for promising applications in many fields as mentioned before. The dual properties of inorganic and organic building components within a single material endow them with novel properties compared with the original host hollow shell and the movable core materials. In this category, different polymer hollow shells such as BzMA,36 MMA,454–456 Pyrrole,399,453,457–460 Styrene,461,462 EDOT,463 DMPA,464


NIPA,58,465–467 aniline,165 and DVB35,468 have been reported. Among all polymeric hollow shells, electro-active polymers such as PPy, PANI, PEDOT etc., have emerged as an important class of materials because of their natural conductive form, unique redox properties and high conductivity. These materials are specifically used in drug nano-carriers and catalysis applications for the controlled release of molecules at the right moment and at the right place guided by stimuli including temperature, ionic strength, pH, magnetic field, and so on. Now, based on the types of cores, the inorganic/ organic YS NPs can be further divided into metal and metal oxide cores. 2.1.1.2.1 Metal/polymer. Under this class, noble metallic cores such as Au35,36,97,455,458,466,467,469–473 and 453,457,461,463,464 Ag have been reported inside the hollow polymer shells. As already mentioned before, noble metallic nanoparticles are important because of their optical and catalytic properties. Under this class, many researchers encapsulate metal nanoparticles in polymeric hollow shells to improve catalytic and optical properties by preventing agglomeration. The metal/polymer YS NPs are very useful for bio-medical applications because of tuneable SPR properties of the metal

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Fig. 30 TEM images of the Fe3O4/TiO2 YS microspheres with different core sizes, interstitial void volumes, and shell thicknesses: (a) MTM-1, (b) MTM-2, (c) MTM-3, and (d) MTM-4. (e, f ) FESEM images of Fe3O4/TiO2 YS microspheres. Reprinted with permission from ref. 306. Copyright 2013 from the Royal Society of Chemistry.

core, and easy functionalization and biocompatibility of the polymer shell. The optical properties of the resultant YS structures in terms of surface plasmon resonance highly depend on the relative size of the core nanoparticles as mentioned before. A simple method has been reported to synthesize Ag/PPy YS NPs, where the core Ag was formed by photoreduction (UV light) of Ag+ inside the hollow PPy shell in the absence of additional reducing agents. The surface plasmon absorption of the Ag core in the Ag/PPy YS nanostructure is possible to tune in the range of 399–455 nm by simply changing the size of Ag core nanoparticles (20 ± 4, 36 ± 4, 50 ± 6, 60 ± 7 nm) as shown in Fig. 31. Apart from the optical properties, these particles may also be useful for new drug delivery applications.457 Similarly, the optical properties of Au/PNIPA YS NPs also show a red shift of SPR patterns (529, 547, 573 nm) with the increase in core sizes (15, 67, 95 nm).467 Additionally, the optical properties of the Au core can also be modulated by the volume

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transition of thermosensitive PNIPA shells because of changes in temperature. The incorporation of a noble metal (Au) nanoparticle as a movable core into a polymeric hollow sphere can also be used as an optical probe, which can monitor the diffusion of chemicals across the shell.36 For example, when Au/PBzMA YS NPs were separated from aqueous suspension (void space filled with water) and redispersed in quinoline, the surface plasmon resonance peak of Au cores was shifted towards longer wavelengths as shown in Fig. 32(a and b). This observation is attributed to the higher refractive index quinoline (1.62) diffused into the shell and the lower refractive index water (1.33) diffused out of the polymer shells, which is also consistent with Mie scattering theory calculation as shown in Fig. 32(d).36 Apart from the optical properties, the encapsulation of metal NPs in polymer shells (Au/PPy YS) also improves the thermal stability compared to pure PPy due to the strong interaction between PPy chains and inorganic nano-fillers.453


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Reported metal oxide/metal oxide YS nanostructures with detailed description of core, shell, and sacrificial layer


Core method

Shell method

Sacrificial layer

Core/shell

Method

Precursor

TiO2/TiO2

Hydrothermal Hydrothermal Hydrothermal Solvothermal Sol–gel Thermal Reducing (AA) Hydrothermal Hydrothermal Spray drying Hydrothermal Sol–gel Aerosol Hydrothermal Solvothermal USP Hydrolysis Wet chemical Hydrothermal Spray pyrolysis Spray pyrolysis Spray pyrolysis Spray pyrolysis Hydrothermal Hydrothermal Hydrothermal Spray pyrolysis Hydrothermal Spray pyrolysis Hydrothermal

TiCl4 TBOT Ti(SO4)2 TiF4 TIP FeSO4 Cu(CH3COO)2 Cu(Ac)2 SnSO4 C2O4Sn TiOSO4, Bi(NO3)3 Zn(NO3)2 [MeZnOCH2CH2OMe]4 Zn(CH3COO)2 Ce(NO3)3 LiNO3, Mn(NO3)2 VIP K3[Fe(CN)6] Co(NO3)2 Co(NO3)2, Mn(NO3) Zn(NO3)2, Fe(NO3)3 Ni(NO3)2 C4H6NiO4 Na2WO4, ZnSO4 NiSO4, (NH4)2Fe(SO4)2 CoSO4 (NH4)2Fe(SO4)2 Co(acac)2, Fe(NO3)3 CdSO4, (NH4)2Fe(SO4) MoO3, Co(NO3)2 Mn(CH3COO)2, Co(CH3COO)2 Zn(NO3)2, Mn(NO3)2 CO(NH2)2 FeCl3 FeCl3 FeCl3 FeCl3 FeCl3 CuSO4 Fe3O4 NPs

Hydrothermal Sol–gel Sol–gel Sol–gel Sol–gel Hydrothermal Heating Heating

Fe3O4 NPs FeSO4

Hydrothermal Hydrothermal

η-Fe2O3/η-Fe2O3 Cu2O/Cu2O SnO2/SnO2 BixTi1−xO2 ZnO/ZnO CeO2/CeO2 LiMn2O4/LiMn2O4 V2O5/V2O5 Fe2O3/Fe2O3 Co3O4/Co3O4 CoMn2O4/CoMn2O4 ZnFe2O4/ZnFe2O4 NiO/NiO ZnWO4/ZnWO4 NiFe2O4/NiFe2O4 CoFe2O4/CoFe2O4 CdFe2O4/CdFe2O4 CoMoO4/CoMoO4 MnCo2O4/MnCo2O4 ZnO–Mn3O4 Co3O4/Al2O3 Fe3O4/ZrO2 Fe3O4/TiO2 α-Fe2O3/SnO2 Cu2O/Fe2O3 Fe3O4/(Y2O3:Eu) Fe3O4/SnO2 Fe3O4/Co3O4

Spray pyrolysis Hydrothermal Solvothermal Solvothermal Thermal Solvothermal Thermal Reducing (Glucose)

Reducing (Citrate)

Method

Precursor

Al(NO3)3 Zr(OBu)4 Zr(OBu)4 TDAA TIP K2SnO3 FeCl3 (YA (NO3)3, (Eu(NO3)3 Na2SnO3 CoCl2,

Material

Removal

TiO2 TiO2 TiO2 TiO2 TiO2 Fe2O3 Cu2O Cu2O SnO2 C TiO2 ZnO Oxidation ZnO CeO2 C V2O5 Fe2O3 Co3O4 C C C C ZnWO4 NiFe2O4 CoFe2O4 C CdFe2O4 C MnCo2O4

Ostwald ripening160,302 Ostwald ripening131,413,423,428,451 Ostwald ripening106,429,431 Ostwald ripening309 Ostwald ripening43,101 Ostwald ripening46,125 Ostwald ripening305 Ostwald ripening45 Ostwald ripening63 Combustion424 Ostwald ripening371 Ostwald ripening40 Kirkendall effect435 Ostwald ripening416 Ostwald ripening42 Combustion132 Ostwald ripening49 Ostwald ripening119 Ostwald ripening48 Combustion425 Combustion421 Combustion418 Combustion417 Ostwald ripening411 Ostwald ripening436 Ostwald ripening436 Combustion422 Ostwald ripening436 Combustion420 Ostwald ripening414,452

C Al2O3 SiO2 PMAA SiO2 SiO2 SnO2 Fe(OH)x SiO2

Combustion426 Ostwald ripening412 NaOH430 Calcination415 NaOH408 KOH306 Ostwald ripening123 Replacement304 NaOH307

SiO2 Co3O4

NH4OH154 Ostwald ripening166

Titanium diisopropoxide bis(acetylacetonate) (TDAA), vanadium oxytriisopropoxide (VIP), ultrasonic spray pyrolysis (USP).

Table 7

Reported other compounds as a shell of YS nanostructures with detailed description of core, shell, and sacrificial layer

Core

Shell

Core/shell

Core

Precursor

ZnS/ZnS FePt/CoS2 SmF3/SmF3 Fe3O4/YPO4

Hydrothermal Hydrothermal Wet chemical Wet chemical

Zn(NO3)2 Pt(acac)2, Fe(CO)5 Sm(NO)3 FeCl3

Shell

Precursor

Oxidation

Co2(CO)8 + S

Hydrothermal

Y(NO3)3, NH4H2PO4

The encapsulation of metal NPs inside the pH or thermosensitive hollow polymer shells as YS has also drawn significant attention towards catalytic and drug delivery applications, which are also known as smart materials. In catalytic appli-


Sacrificial layer Material

Removal

ZnS Sulphides SmF3 SiO2

Ostwald ripening48 Kirkendall effect38 Ostwald ripening61 NaOH312

cations, the shell material of thermo-sensitive microgels reacts with external stimuli and alters the catalytic activity accordingly. This type of YS NP can be used as an active nanoreactor for the immobilization of metal nanoparticles and the catalytic

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Fig. 31 (a) Schematic procedure for the synthesis of Ag/PPy-CS YS NPs. (b) UV-Vis absorption spectra of PPy-CS hollow nanospheres and Ag/PPy YS NPs (Ag: 20 ± 4, 36 ± 4, 50 ± 6, 60 ± 7 nm). TEM images of (c) PPy-CS hollow nanospheres; (d) Ag/PPy-CS YS NPs (size of Ag 20 ± 4 nm); (e) Ag/ PPy YS NPs (size of Ag 36 ± 4 nm), (f ) Ag/PPy YS NPs (size of Ag 50 ± 6 nm); (g) Ag/PPy YS NPs (size of Ag 60 ± 7 nm). Reprinted with permission from ref. 457. Copyright 2005 from American Chemical Society.

activity of immobilized nanoparticles, which can be tuned by swelling and shrinkage of polymeric shells. For example, the porosity and hydrophobicity of Au/thermo-sensitive microgel (PNIPA) YS nanostructure’s shell can be tuned in a welldefined manner by the change in temperature.322 The catalytic activity of the immobilized metal core can also be tuned by the swelling and shrinkage behaviour of the thermo-sensitive microgel hollow shell, where the temperature acts as a trigger to control the selectivity of the catalyst core. In this case, the reduction of hydrophilic 4-nitrophenol by sodium borohydride occurs at a much faster rate at lower temperature, while the reduction of hydrophobic nitrobenzene is preferred at higher temperature.322,467 The YS structures have additional advantages over the core/shell structure such as the presence of void space, unblocked surface of the core, and permeability of the shell. 2.1.1.2.2 Metal oxide/polymer. Under this category, only Fe3O4 58,165,454,459,465,474,475 and SiO2 399,456,460 metal oxide NPs have been reported as the core inside polymeric hollow shells. The aggregation and chemical instability in the dispersion phase of magnetic Fe3O4 nanoparticles limit their utilization

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in many applications, which we have already discussed before. Similar to other inorganic shells, polymeric shells also provide stabilization to the magnetic core from oxidation and prevent aggregation of particles. The magnetic/polymer YS NPs have the following additional advantages in biomedical applications: (i) the thermo or pH responsive polymer shells show volume phase transition with the change in the external temperature or pH, which in turn helps in the controlled release of encapsulated drug molecules from the hollow shells,58,454,465,476 (ii) biocompatibility and easy functionalization ability of polymer shells make them suitable for drug delivery,58,454,465,476 (iii) the movable magnetic NP core of the YS nanostructure is useful in catalysis, and external magnetic field driven chemical separation and purifications,58,456,459 and (iv) the void space allows encapsulation of fluorescent organic dye and drug/guest molecules.58 In advanced biomedical applications, many special requirements need to be fulfilled such as targeted drug delivery, imaging and control release at the right moment in the right place guided by stimuli including temperature, ionic strength,


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Fig. 32 (A) UV-Vis absorption spectra of Au/PBzMA YS NPs in aqueous solution after they had been added into quinoline for different periods of time, (B) dependence of plasmon peak position observed for the same gold nanoparticles encapsulated with different configurations: as-received Au, Au/SiO2 core–shell, and Au/PBzMA YS NPs, (C) TEM image of Au/PBzMA YS NPs, (D) the surface plasmon absorption bands of gold particles were calculated using Mie theory adopted with different refractive index of surrounding media. Reprinted with permission from ref. 36. Copyright 2003 from American Chemical Society.

pH, magnetic field, and so on. As a result, magnetic movable core with stimuli-responsive hollow polymer shell coating have gained importance to achieve the above control on the above mentioned specific requirements by stimuli and magnetic field. For instance, Fe3O4/PNIPAM YS NPs incorporated with an organic fluorescent dye (FITC) exhibit a specific drug delivery system because of the presence of a magnetic core and a thermo-responsive PNIPAM hollow shell as shown in Fig. 33.58 Here, the magnetic cores provide magnetic manipulation of YS NPs with the help of the magnetic field, the fluorescent dye helps in optical imaging, void space helps in drug encapsulation, and the permeable thermosensitive PNIPAM shell triggers the release of drug molecules or biomolecules or chemical compounds by changing the external temperature. Similarly, pH sensitive polymer microspheres (N,N′-methylenebisacrylamide-co-methacrylic acid) with a movable magnetic core were also synthesized for targeted drug delivery applications.454,476 The encapsulation of SiO2 inside the PDVB and PPy polymers in the form of YS NPs is also useful in bio-


medical applications.399,454,456,460 When the magnetic metal oxide core is encapsulated inside the conductive shell, the resultant YS structure exhibits good dielectric and magnetic loss with high surface area, which can be beneficial for improving capacities in both electromagnetic shielding and microwave absorption applications.165,477–479 Apart from the metal and metal oxides, metal chalcogenides such as CdS NPs were also encapsulated inside the polystyrene shell.462 Different types of inorganic/polymer YS NPs are listed in Table 8. 2.1.1.3 Polymer/polymer. Under this category, both core and shell particles are made of polymeric materials; however only a few literature reports are available to date. Under this category, different YS NPs such as poly(DVB-co-AA)/poly(DVBco-AA),480 P(MAA-co-EGDMA),481 pyridinium polyelectrolyte/ PEGDMA,482 (P(MAAcoEGDMA)/PDVB,481 (P(MAAco481 EGDMA)/PEGDMA (P(MAAcoEGDMA))/P(NIPAM-coMAA),483 and PS/PS-co-PAM484 have been reported. This class of materials has promising applications in drug delivery and catalysis. Generally, the polymerization (emulsion and distilla-

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Fig. 33 (a) Schematic illustration of the fabrication of thermoresponsive Fe3O4/PNIPAM YS NPs with fluorescence-labelled magnetic cores. The stars represent the FITC molecules and the dark gray loop is a layer modified by APS. (b) TEM, AFM and SEM images of Fe3O4/SiO2 particles and, Fe3O4/SiO2/PNIPAM particles and Fe3O4/PNIPAM YS NPs. Reprinted with permission from ref. 58. Copyright 2005 from Wiley-VCH Verlag.

Table 8 Reported inorganic/polymer YS nanostructures with detailed description of core, shell, and sacrificial layer

Core

Shell

Sacrificial layer

Core/shell

Method

Precursor

Method

Precursor

Material

Removal

Au/PDVB Au/PBzMA Au/PMMA Au/PPy Au/PEGDMA Au/PNIPA

Reduction (Citrate)

HAuCl4 Au NPs HAuCl4 HAuCl4 HAuCl4 HAuCl4 HAuCl4 AgNO3 AgNO3 AgNO3 Fe3O4 NPs FeCl3, FeCl2 FeCl3, FeCl2 FeCl3, FeCl2 TEOS, Cd(CH3COO)2, NaSeSO3 TEOS Fe(SA)3 FeCl3, FeCl2, TEOS

Polymerization Polymerization Polymerization Polymerization Polymerization Polymerization Polymerization Polymerization Polymerization Polymerization Polymerization Polymerization Polymerization Polymerization Polymerization

DVB + AIBN BzMA MMA PPy EGDMA-co-MAA NIPA NIPA EDOT DMPA PPy PPy NIPAM NIPAM Aniline PPy

SiO2 SiO2 SiO2 Ag SiO2 SiO2 SiO2 Ag+ Ag+ SiO2 SiO2 SiO2 SiO2 Pst SiO2

HF35 HF36 HF455 NH3 458 HF97 NaOH467 NaOH322 Ship-in-bottle463 UV-irradiation464 HF453 NaOH459 NaOH58,476 HF465 Calcination165 NaOH399

Polymerization Polymerization Polymerization

PPy BTME, APTES MAA + MBA

Pst CTAB + FC4 PMMA

THF460 Ethanol474 Ethanol/water454

Na2S2O3, 3CdSO4

Polymerization

Styrene

W/O ME

Ethanol462

Ag/PEDOT Ag/PDMPA Ag/PPy Fe3O4/PPy Fe3O4/PNIPA

Reduction (NaBH4) Reduction (Citrate) Reduction (NaBH4) Reduction (Citrate) Reduction (Citrate) Reduction (NaBH4) UV-irradiation Reduction (Polyol)

SiO2/CdSe/PPy

Co-precipitation Co-precipitation Co-precipitation Stöber + precipitation

SiO2/PPy Fe3O4/organosilica Fe3O4/SiO2/ P(MBAAm-co-MAA) CdS/Ps

Sol–gel Thermal Co-precipitation, Sol–gel Irradiate γ-rays

N-isopropylacrylamide (NIPA), N,N-methylenebisacrylamide (MBA), methyl methacrylate (MMA), N,N′-methylenebisacrylamide (MBA), methacrylic acid (MAA), (3-aminopropyl)triethoxysilane (APTES), bis(triethoxysilyl)ethane (BTME), polypyrrole (PPy), 2-dimethoxy-2phenylacetophenone (DMPA), (3,4-ethylenedioxythiophene) (EDOT), ethylene glycol dimethacrylate (EGDMA), benzyl methacrylate (BzMA), 2,2′-azobisisobutyronitril (AIBN), divinylbenzene (DVB), acrylic acid (AA).

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tion precipitation) method is mostly used to synthesize different types of polymers with a variety of functional groups, where polymerization is started by the addition of an initiator. For example, the PDVB/PDVB YS NPs have been synthesized by the precipitation polymerization process of PDVB-co-AA as the core and the shell with AIBN (2,2′-azobisisobutyronitrile) as an initiator. The core/shell structure is finally transformed to the YS structure by dissolution of the non-crosslinking polymer PAA ( poly acrylic acid) in alkaline solution. Similarly, PE/PEGDMA,482 P(MAA-co-EGDMA)/PEGDMA,481 P(MAA-coEGDMA)/PDVB,481 and (P(MAA-co-EGDMA))/P(NIPAM-coMAA)483 NPs were also synthesized by distillation precipitation polymerization by dissolution of the silica middle layer as a sacrificial template. The PS/PS-co-PAM YS NPs have been reported by the surfactant-free emulsion polymerization method, which includes three steps. In the first step, pre-synthesized PS-co-PAM microspheres adsorbed the seed latex particles with a hydrophobic monomer mixture (St and DVB) by swelling. The second step includes the polymerization of an

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adsorbed entrapped monomer mixture and assembly of the coated microsphere into a sandwich like structure (PS-PAEMA/ PS-PAEMA-co-PAM) through phase separation. Finally, in the third step, the fabrication of PS/PS-co-PAM YS NPs is performed by dissolution of the seed by DMF etching as shown in Fig. 34(a). The multifunctional polymer/polymer YS NPs with multiple swelling or stimuli-responsive properties were also reported for the tumour-curing drug delivery process, where these particles exhibit a dual stimuli-response according to external stimuli. In this case, the traditional anticancer drugs without an engineered delivery system cannot differentiate between the cancerous cells and healthy cells; as a result, there is a fair chance to damage both cells. However, most cancerous tissues exhibit a lower extracellular pH value than normal tissues. While utilizing this behavior, a combination of multiple stimuli-responsive ( pH and thermo) polymers in the form of YS NPs is a good option for the delivery of drug at the targeted position. Now, thermo and pH-responsive dual characteristics of the YS

Fig. 34 (a) Schematic presentation of the synthesis of the PS/PS-co-PAM YS microspheres. Reprinted with permission from ref. 484. Copyright 2011 from American Chemical Society, (b) schematic illustration of the fabrication process of the multifunctional YS microspheres for controlled release of drug from the sandwich core/shell microspheres with a sacrificial silica interlayer. Reprinted with permission from ref. 483. Copyright 2014 from American Chemical Society.


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microspheres could induce the controlled release because of the difference in temperature and pH of tumour tissues and normal tissues. This indicates their ability for tumour environment-responsive drug delivery performance. For instance, the pH-sensitive polymer core (P(MAA-co-EGDMA) encapsulated in the pH-thermo sensitive polymer shell (P(NIPAM-co-MAA)) in the form of the YS structure. These YS NPs were also reported for drug delivery application, where the core and the shell were prepared by precipitation polymerization with silica as the sacrificial template as shown in Fig. 34(b). The movable core exhibits pH responsive stimuli behaviour and the hollow shell also acts as pH and temperature responsive stimuli, respectively. On the other hand, void space allows us to introduce more drug molecules. The uptake and in vitro release of DOX drugs have also been demonstrated from the smart pHinduced thermally responsive YS structure as a controlled drug release system with very low in vitro cytotoxicity, which exhibits the apparent tumour-environment-responsive controlled “on– off” drug release. Thus, this type of YS microsphere improves the DOX drug loading capacity and exhibits a more rational release behaviour, i.e., very low drug release at pH 7.4 but rapid drug release at reduced pH values (6.5 or 5.0) at 37 °C.483 Different polymer/polymer YS nanostructures studied so far are listed in Table 9. 2.1.2 Single core/multishells. In spite of the fact that most of the YS NPs studies have concentrated on single core/shell types, very little attention has been directed towards single core/multishell YS NPs. The available literature shows that this type of complex YS NP is very important for different applications because such structures exhibit not only a very high specific surface area, abundant inner voids, and multiphase heterogeneous interfaces, but also provide different shells with different functionality, and a controllable physical and chemical environment compared with single shell YS NPs. The single core multishelled reported nanostructures are synthesized using Ostwald ripening161,486 and sacrificial template layer (silica)214,476,487–490 approaches as shown in Fig. 35. However, there is still a great challenge to synthesise this type of nanostructure via a simple and easy approach for mass production. These advanced structures make them very useful for various applications such as drug delivery,476 catalysis,214,490 sensors,486 microwave absorption,161,487 and lithium ion batteries.489

Table 9

Based on the available literature, mostly oxide materials such as Cu2O/Cu2O/Cu2O,486 Fe3O4/TiO2/SnO2,490 Fe3O4/TiO2/ TiO2,488 Fe3O4/SnO2/SnO2,161 and Fe3O4/barium silicate/ barium titanium oxide487 are reported under this class. From the structural perspective, these NPs show much higher additional surface area compared to that of single YS NPs, which could be useful for catalysis,214,490 microwave absorption,161,487 and lithium ion batteries.489 For instance, facile hydrothermal etching assisted Fe3O4/TiO2/TiO2 double shell YS NPs act as superior photocatalytic and acid mediated Friedel–Crafts alkylation catalysts, which can also be reused after magnetic separation. The presence of multi-shells of TiO2 provides more surface area, highly open pores and more active sites on the ultrathin shell, which makes it more efficient for excellent catalysis application.488 The encapsulation of magnetic NPs (core) inside multidielectric shells (Fe3O4/SnO2) also improves microwave absorption properties compared to those of single YS NPs161 because of more surface area, porosity of the shell layer, and improved scattering or reflection of EM waves. Multiple core/shell/shell gradient interfaces improve electromagnetic attenuation because of the existing interfacial polarization. In the case of Fe3O4/SiO2/SiO2 NPs, the enhanced microwave absorption behavior is mainly because multishells of the dielectric material induce dielectric losses and the synergistic effect between the magnetic Fe3O4 cores and dielectric SnO2 shells.161,271,279,491–496 Similarly, Fe3O4 NPs are also encapsulated inside different dielectric barium silicate and barium titanium oxide shells for improving the microwave absorbing properties.487 While most of the studies have concentrated on two materials for the construction of the core/ multishell YS structure, one study has focused on core/multishell (up to 4 shells) YS NPs made of a single material (Cu2O).486 In this study, Cu2O/Cu2On YS nanostructures (where n is the number of Cu2O shells) were precisely synthesized with a variety of sizes and morphologies through multiple-step seed-mediated growth and the Ostwald ripening method at room temperature, where the symmetric and asymmetric Ostwald ripening hollowing processes were controlled by changing the stirring conditions. The overall spherical centricity or eccentricity of the particles can be controlled by varying the type of seed. The thickness of the shell, morphology, and overall size of the YS structure can be tuned by changing the

Reported polymer/polymer YS nanostructures with detailed description of core, shell, and sacrificial layer

Core

Shell

Sacrificial layer

Core/shell

Method

Precursor

Method

Precursor

Material

Removal

Poly(DVB-co-AA)/poly(DVB-co-AA)

Polymerization

DVB, AA

Polymerization

DVB, AA

PAA

Polymerization

EGDMA + MAA

SiO2

Polymerization Polymerization Polymerization Polymerization

Pyridinium Polyelectrolyte MAA-co-EGDMA Aniline, Styrene MAA + EGDMA PS

Alkaline ethanol480 HF482

Polymerization Sol–gel Polymerization Polymerization

MAA-co-EGDMA TEOS, HAuCl4 NIPAM + MAA PS-co-PAM

SiO2 SiO2 SiO2 DVB

HF481 NaOH485 HF483 DMF484

Pyridinium polyelectrolyte/PEGDMA P(MAA-co-EGDMA)/PDVB PS/PANI/Au/m-SiO2 (P(MAAco- EGDMA))/P(NIPAM-co-MAA) PS/PS-co-PAM

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Fig. 35 TEM images of single core/multi-shell (a) Au/SiO2. Reprinted with permission from ref. 214. Copyright 2004 from American Chemical Society. (b) Au/SnO2. Reprinted with permission from ref. 489. Copyright 2007 from Wiley-VCH Verlag, (c) Cu2O/Cu2O. Reprinted with permission from ref. 486. Copyright 2012 from American Chemical Society. (d) Fe3O4/TiO2. Reprinted with permission from ref. 488. Copyright 2011 from American Chemical Society, (e) Fe3O4/SnO2. Reprinted with permission from ref. 161. Copyright 2013 from American Chemical Society, (f ) SiO2/SnO2. Reprinted with permission from ref. 489. Copyright 2007 from Wiley-VCH Verlag.

amount of added seed, the reaction and ripening times. From the application perspective, this unique structure with thin shells also exhibits a better sensitivity for non-enzymatic glucose oxidation sensing application over single YS because of easy diffusion and fast charge transfer of the analyte. The optical properties can also be changed by tuning the structural configuration of YS as shown in Fig. 36. The optical properties show that the extinction spectra shift towards the blue wavelength for single shell structures, while a red shift for multishells with respect to Cu2O core excitation spectra is seen because of an increase in the sphere size and scattering contribution to the extinction from more thicker shells.486,497 Other metal oxide/metal oxides such as Fe3O4/SnO2 161 and Fe3O4/ TiO2 488 were also reported as double shell YS NPs. Apart from metal oxide/metal oxide, the metal (Au) core was also encapsulated in SiO2 214 and SnO2 489 multishells, which might be useful for sensors, catalysis, drug delivery, and bioseparation applications with improved properties. Inorganic/ organic YS NPs with multiorganic shells are also promising, especially for drug delivery applications because of the advantage of possibilities of multi-functionality in a single system. For example, double shells of pH (PMMA shell) and temperature (PNIPAM shell) responsive polymeric YS structures with movable magnetic cores (Fe3O4) are reported for the targeted drug delivery applications.476 These YS nanostructures rep-


resent an intelligent multifunctional drug delivery material to provide independent pH and temperature, as well as dual stimuli-responsive behaviour with magnetic-targeting function.476 Different types of single core multishell YS nanostructures are listed in Table 10. 2.1.3 Multicores/single shell. The gradual progress of research activities on the development of new YS nanostructures led to encapsulation of the same or different multicore nanoparticles into a single hollow shell for the development of new physical and chemical properties. So far, many multicore/single shell YS nanostructures have been reported; among them, mostly multicores of noble metals are encapsulated in silica, carbon, and metal oxide hollow shells. Different similar or dissimilar NPs such as Au,498–501 Pt,502 Pd,500 Au + Pt,110 Fe2O3,76 Fe3O4,158 Au + SiO2,70,503 Ag + C,504 Fe3O4 + Au,82,505 and Pd + SiO2 70 are encapsulated as multicores inside the silica shell. Apart from these, (SiO2 + Pt)/TiO2,103 Au/Fe3O4,506 Au/TiO2,95,507 (SiO2 + Pd + Au)/PMO,508 Sn/C,126 γ-Fe2O3/C,151 M (Pt, Ag, Sn, Fe, FeO)/C,136 and (Cu + ZnO)/C509 were also reported under this class. This class of materials is synthesized by Ostwald ripening,501 Kirkendall process,506,507 and sacrificial template70,82,95,103,110,126,151,158,498–500,504,505,508 techniques. The multiple core/single shell NPs exhibit novel, unique and tuneable optical, magnetic, electrical, and catalytic properties that originate from the collective interaction

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Fig. 36 TEM images of various Cu2O/Cu2O YS NPs: (a) truncated cube (starting seed), type A, (b) core–shell, type B, (c) YS, type C, (d) concentric double-shell core–shell, type D, (e) nonconcentric double-shell core–shell, type F, (f ) concentric double-shell YS with a movable core, type E, (g) nonconcentric double-shell YS with a movable core, type G, (h) concentric triple shell YS NPs, type J, and (i) concentric quadruple-shell YS with a movable core, type Q (The scale bar is 200 nm), ( j) extinction spectra of pristine Cu2O cubes and different types of complex (Cu2O/)n Cu2O structures. (A) Cu2O seed, (B) Cu2O centric core single shell, (C) Cu2O eccentric core single shell, (D) Cu2O centric core double shells, (E) Cu2O eccentric core double shells, (H) Cu2O centric core triple shell (P) Cu2O centric core quadruple shell. Reprinted with permission from ref. 486. Copyright 2012 from American Chemical Society.

Table 10 Reported single core multihollow shell YS nanostructures with detailed description of core, shell, and sacrificial layer

Core

Shell

Sacrificial layer

Core/shell

Method

Precursor

Method

Precursor

Material

Removal

Au/SiO2 Fe3O4/TiO2 + SnO2 Fe3O4/TiO2 SiO2/SnO2 Cu2O/Cu2O Fe3O4/SnO2 Fe3O4/BS/BTO Au/SnO2 Fe3O4/PMMA-PNIPAM

Reduction (Citrate) Aging Wet chemical Stöber Wet chemical Solvothermal Reduction (SA)

HAuCl4 FeCl3 FeCl3 TEOS CuCl2 FeCl3 FeCl3 Au–SiO2 FeCl3, FeCl2

Stöber Wet chemical Sol–gel Hydrothermal Wet chemical Hydrothermal Heating Wet chemical Polymerization

TEOS K2SnO3, TIP TBOT K2SnO3 Cu(NO3)2 K2SnO3 Ba(OH)2, TEOS, TBOT K2SnO3 NIPAM, MMA

SiO2 SiO2 SiO2 SiO2 Cu2O SnO2 SiO2 SiO2 SiO2

Hot water214 HF490 NaOH488 HF489 Ostwald ripening486 Ostwald ripening161 Na(acac)2 487 HF489 NaOH476

Co-precipitation

Sodium acetate (SA).

between the cores of the same or different materials, which makes them very useful for various applications such as drug delivery,76,82 catalysis,70,82,95,103,110,498,503–505,507–509 magnetically separable adsorbents,151,158 and lithium ion batteries.126,136 Considering the catalytic application, most of the studies have reported on different combinations such as metal/metal oxide, metal oxide/metal oxide, and metal oxide/carbon YS structures. The same metal multicore NPs provide better catalytic performance over single core YS NPs because of more available surface area present in multiple core particles. Multiple Au cores inside the hollow silica shell exhibit good catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol reaction compared to a single gold core as

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shown in Fig. 37(a–g).70 Similarly, multiple Au cores inside the hollow TiO2 shell also exhibit enhanced catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol reaction.95 Apart from the Au core, multiple Pd core NPs inside the periodic mesoporous organosilica shell also exhibit high conversion (92 to 100%) and excellent selectivity (>99%) for selective oxidation of various alcohols (benzyl alcohol, methyl substituted benzylic alcohol, cinnamyl alcohol etc.) to the corresponding aldehydes.508 The (Au + SiO2)/SiO2 YS NPs were also synthesized by impregnating SiO2/SiO2 YS NPs into HAuCl4 followed by simple heating.503 The sizes of Au cores were also easily controlled by changing the Au precursor concentration. This type of hetero core/SiO2 YS NP acts as a good supported metal catalyst for better catalytic reduction of 2-nitroaniline


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Fig. 37 TEM images of multiple Au NPs encapsulated inside SiO2/SiO2 YS NPs: (a) GESNs-1 (0.5 mM HAuCl4), (b) GESNs-2 (0.9 mM HAuCl4), (c) GESNs-3 (1.3 mM HAuCl4), time-dependent UV-Vis absorption spectral evolutions of the 4-nitrophenol reduction catalyzed by (d) GESNs-1; (e) GESNs-2; (f ) GESNs-3. (g) Their conversion rates during the catalytic reactions. Reprinted with permission from ref. 70. Copyright 2012 from WileyVCH Verlag. (h) Typical TEM of (SiO2 + Pt)/TiO2 YS, (i) typical SEM image of (SiO2 + Pt)/TiO2 YS NPs, (i) H2 gas evolution through photocatalytic splitting of water by (SiO2 + Pt)/TiO2 and SiO2/TiO2 as a function of time with schematic illustration of multiple reflections of light within an inner cavity of the YS structure. Reprinted with permission from ref. 103. Copyright 2012 from Wiley-VCH Verlag.

along with other advantages as discussed in the previous sections.503 The multiple Fe3O4 NPs as cores encapsulated inside SiO2 shells also provide good magnetic separability from a stable alcohol suspension.505 The presence of more than one metal core inside the hollow shell also improves the catalytic activity, where one core acts as a catalyst and the other as a cocatalyst. For instance, (Pt + Au)/SiO2 YS NPs exhibit strong electrocatalytic activity for methanol oxidation compared to that of Pt/SiO2 YS NPs because of the synergistic effect of Au and Pt NPs as a catalyst.110 Similarly, Cu NPs incorporated inside the ZnO/C YS NPs exhibit good catalytic activity for H2 generation by partial oxidation of methanol.509 In the photocatalytic application, multiple scattering of incident light is an important characteristic of the YS structure compared to that of a simple core/shell. On the other hand, the encapsulation of plasmonic NPs as the core inside the YS structure of a photocatalyst material also improves the efficiency of a photocatalyst because of a better charge transfer process at the semiconductor interface and absorption of visible light. The Pt NPs encapsulated in the SiO2/TiO2 YS structure show a better photo-induced H2 production efficiency compared to that of the SiO2/TiO2 core/shell struc-


ture, because of the presence of void space which indeed allows multiple reflection of light within the interior space; at the same time, Pt nanoparticles also act as a co-catalyst and improve the charge transfer process as shown in Fig. 37(h–j).103 Similarly, multiple Au NPs inside the hollow TiO2 shells also exhibit good photocatalytic activity for photodegradation of p-chlorophenol because of a decreased recombination rate of charge carriers and a strong absorption in the visible light wavelength range.507 The magnetic NPs are always important for bio-medical applications, especially when there is a requirement for targeted delivery, so there is no exception for this class of materials with a magnetic core for bio-medical applications. The metal NPs encapsulated inside the Fe3O4/SiO2 YS provide the plasmonic as well as magnetic properties from the core side and the bio-compatible/functionalization properties from the shell side of a single material, which can be useful for multimodal imaging and magnetically targeted drug delivery.82 Generally, it is very difficult to obtain high quantum yield (QY) in plasmonic–fluorescent composite NPs because of mutual non-radiative energy transfer between metal and fluorophores, which eventually gives a quenching effect on

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fluorescence intensity. In such a situation, these types of multicores/single shell YS structures additionally help to enhance the QY because of their unique composition and architecture. For instance, (Fe3O4 + Au)/fluorescein doped-SiO2 NPs exhibit enhanced QY compared to Fe3O4/fluorescein doped-SiO2; the resultant YS nanostructure shows magnetic, plasmonic, and fluorescent properties together in one material. The QY of fluorescein increases because of electromagnetic field coupling of Fe3O4 + Au nanoparticles, which can also be confirmed by the discrete dipole approximation (DDA) theory by calculating the electromagnetic near field around the multicore as shown in Fig. 38. Plasmonic metal placed near fluorophores induced the absorption and scattering of the electromagnetic energy radiated by fluorophores, which is called the “metal-enhanced fluorescent phenomenon”.82 While coming to the energy storage applications, it has been found that these multicore materials are also very useful. As an example of lithium-ion battery applications, when multiple electro-active metallic cores are encapsulated inside the hollow conductive carbon shell, the capacity and cyclic per-

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formance of anodic material improve compared to those of single metal/C YS NPs. For this purpose, various multimetallic cores such as Sn, Pt, Ag, and Fe were incorporated inside the carbon shell via ultrasonic spray synthesis, and to support the better performance in application, Sn/C YS NPs as the anode material for the lithium-ion battery showed good performance.126,136 Different types of multicore YS NPs reported so far are listed in Table 11. 2.1.4 Multicores/multi or complex shells. As we have seen in the previous sections that the multicores/single shell and single core/multishell NPs have much better properties than those of single YS structures, researchers have also attempted to test the performance of multicores/multishell YS structures. The multicore/multihollow shell structure provides a multifunctional complex YS nanostructure with a multi-level interior architecture. Compared to the traditional YS nanostructure, this type of unique complex nanostructure can provide additional multifunctionality by incorporating multimovable cores with multihollow shells, especially for catalytic applications. For instance, highly engineered bi-functional (mag-

Fig. 38 (a) TEM images of Fe3O4–Au/SiO2-fluorescein YS NPs, (b) fluorescence intensity of Fe3O4/silica-fluorescein and Fe3O4–Au/silica-fluorescein nanoparticles, (c) the model used for electromagnetic DDA calculations and the electric field distribution of Fe3O4 and Au nanoparticles (insets), of which the diameters are 15 and 5 nm, respectively. (d) The electric field distribution of Fe3O4–Au nanoparticle embedded in silica medium when the incident electromagnetic wave along the z direction is polarized along the symmetry axis and perpendicular to it (inset). Reprinted with permission from ref. 82. Copyright 2010 from American Chemical Society.

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Table 11 Reported multicores single hollow shell YS nanostructures with detailed description of core, shell, and sacrificial layer


Core

Shell

Sacrificial layer

Core/shell

Method

Precursor

Method

Precursor

Material

Removal

Au/SiO2

Reduction (Citrate) Reduction (NaBH4) Thermal Reduction (LSS) Reduction (Heating) Reduction (Heating) Reduction (Heating) Hydrothermal Reduction (LSS) Reduction (Heating) Reduction (FFA)

HAuCl4 HAuCl4 HAuCl4 HAuCl4 HAuCl4 HAuCl4 HAuCl4 CdCl2O8, H2Te H2PdCl4 H2PdCl4 AgNO3

Sol–gel Sol–gel

TEOS TEOS SiO2 YS TEOS TEOS TEOS TEOS TEOS TEOS TEOS RF resins, TEOS

Au–Cl complex MF core SiO2 LSS SiO2 Au SiO2 SiO2 LSS SiO2 C

Ostwald ripening501 NH4OH499 HF503 Calcination500 Aminosilane70 Ship-in-bottle498 Aminosilane70 NH3 510 Calcination500 Aminosilane70 Calcination504

Thermal

HAuCl4, Fe(CO)5 Au/SiO2 NPs K2PtCl6, TEOS FeCl3 Fe(NO3)2 SnCl4, FeCl2, H2PtCl6, AgNO3 Cu(NO3)2, Zn(NO3)6 Au NPs HAuCl4 HAuCl4 HAuCl4, H2PtCl6, Pd(NO)3 FeCl2 and FeCl3 Na2SnO3

TEOS + AEAP3 TEOS + BTME Ti(OPri)4 TEOS, FA C6H5Na3O7

Organo- SiO2 Organo- SiO2 SiO2 C SiO2 C

Calcination82 Calcination505 NaOH103 Calcination76 NaOH151 Combustion136

Carbonization Decompose Solvothermal Wet chemical Sol–gel

β-Cyclodextrin Fe(CO)5 TiOSO4 TBOT TEOS

Cu + ZnO Fe3O4 Ti4+ SiO2 CTAB + FC4

Ship-in-bottle509 Kirkendall effect506 Kirkendall effect507 NaOH95 Calcination508

Sol–gel Carbonization

TEOS Glucose

PS SiO2

Calcination158 NaOH126

CdTe/SiO2 Pd/SiO2 Ag/C/SiO2 Fe3O4–Au/SiO2 Fe3O4/Au/SiO2 SiO2/Pt/TiO2 Fe2O3/SiO2 α-Fe2O3/C M/C, (M - Sn, Pt, Ag, or Fe–FeO) Cu–ZnO/C Au/Fe3O4 Au/TiO2

Stöber Solvothermal Wet chemical USP Reduction (Thermal)

SiO2/Au, Pt, Pd

Solvothermal Reduction (Citrate) Heating

Fe3O4/SiO2 Sn/C

Solvothermal Reducing (Urea)

Sol–gel Sol–gel Hydrolyzed Sol–gel Sol–gel Sol–gel Sol–gel Polymerization, Stöber Sol–gel Sol–gel Sol–gel Stöber Carbonization Carbonization

N-Lauroylsarcosine sodium (LSS).

netic and catalytic) multiple Fe3O4 cores encapsulated inside noble metal (Pt, Ag, Pd) NPs embedded C shells have been reported for better catalytic activity and magnetic separability as shown in Fig. 39(a–d).153 In fact, using the same procedure, it is also possible to introduce noble metal NPs inside the shell, which can co-exist with Fe3O4 NPs. From the application perspective, the reported multifunctional catalyst is suitable for hydrogenation of nitrobenzene, where the noble metal inside the carbon shell acts as a supported catalyst and entrapped Fe3O4 nanoparticles in the hollow shell enhance the magnetic separation ability of the YS NPs for recycling of the catalysts. Additionally, different reactants can easily diffuse through shell materials into the void space where they can be catalysed with noble metal cores; at the same time other cores can provide magnetic functionality which will not affect the catalytic reaction. There is only one study reported so far related to the multicore/double shell YS structure speculating biological applications with improved mechanical, optical, and other properties.71,511,512 In this study, the first SnO2/SiO2 (double shell hollow core)/SiO2 shell in the shell YS structure was formed, then Au NPs were introduced into the void space between two silica layers, and finally SnO2 was reduced to Sn inside the inner SiO2 shell. The synthesis scheme and the TEM image of the YS structure are presented in Fig. 39(e and f ).71 The hollow silica provides more surface area compared to silica nanoparticles, which could be promising systems as drug delivery vehicles for loading more drug molecules.513–515


In addition to the inert nature and easy surface functionalization, the larger elastic modulus of hollow silica capsules also provides stability and mechanical strength.516–518 On the other hand, encapsulation of noble metal NPs implies huge opportunities for the catalytic nanoreactor, where different reactants can easily diffuse via the porous shell into the large interstitial space for catalytic contact with active cores. Similarly, encapsulation of a fluorescence dye or quantum dots provides optical properties to the multifunctional complex.81,83,181,182,253 This type of YS nanostructure is listed in Table 12. 2.1.5 Multiple shells or shell in shell. This class of materials is defined as encapsulation of hollow cores inside hollow shells; as a result, these particles are comprised of multiple concentric shells with different diameters. Some researchers have also used the terminology of onion or Russian doll structure,8,9,519,520 which makes this class suitable for potential applications. While considering the controlled drug delivery applications, these NPs are extremely important because of the presence of lots of void spacing between the shells to encapsulate distinct drug and guest species;59,60 at the same time they are superior for other applications too. Multishells of various compositions have been synthesized by many methods such as sacrificial templates, spray pyrolysis, vesicle mediated, and Ostwald ripening. So far, all the reported multishell YS structures have been made with metal oxides such as SiO2,59,60,521 TiO2,104,522,523 Fe3O4,523 Co3O4,129 Cu2O,115,524 TiO2/SnO2 159 and V2O5/SnO2.130 These

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Fig. 39 (a) SEM image of Fe3O4/h-C/Pt after cutting the hollow carbon spheres into hemispheres, (b, c) HAADF-STEM image of Fe3O4/h-C/noble metal YS NPs. Reprinted with permission from ref. 153. Copyright 2013 from Wiley-VCH Verlag, (e) schematic illustration of the fabrication of Sn/ SiO2/SiO2 complex YS structures, (f ) transmission electron microscopy (TEM) images of Sn/SiO2/SiO2 with decoration of Au nanoparticles. Reprinted with permission from ref. 71. Copyright 2011 from the Royal Society of Chemistry.

Table 12

Reported multicores/multishells or complex YS nanostructures with detailed description of core, shell, and sacrificial layer

Core

Shell

Sacrificial layer

Core/shell

Method

Precursor

Method

Precursor

Material

Removal

α-Fe2O3/SiO2 Fe3O4/C-Pt/Ag/Pd

Aging Thermal, polymerization

FeCl3 FeCl3, oleate emulsion

Stöber Reducing (thermal)

TEOS Pt(NH3)4·(NO3)2, Pd (NH3)Cl2, AgNO3

C Oleate emulsion

Calcination71 Calcination153

NPs exhibit additional advantages over traditional YS in drug delivery,59,60 DSSCs,159,522 lithium-ion batteries,129,130 gas sensing,115 and photocatalysis.104 In the case of photocatalytic application, multiple shells of semiconductor metal oxides allow multiple reflection of light because of more available void space, which enhances the photocatalytic and photoconversion efficiency. Multiple hollow TiO2 shells exhibit high photocatalytic activity compared to single shell TiO2 and pure NPs as shown in Fig. 40(a–c). The superior photocatalytic properties are attributed to multiple reflections of UV light within the void space of large multishells nanostructures.104 Similarly, because of the enhanced light harvesting efficiency, double shell TiO2 hollow spheres exhibit higher photoconversion efficiency than single shell TiO2 when tested as a photo-electrode for DSSC application.522 Similar to TiO2, other multishell semiconductor metal oxides can also be used as a photoanode in DSSCs to improve performance by enhanced harvesting of incident light. For example, SnO2/TiO2 multishell NPs exhibit an improved photo-

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conversion efficiency as photo-electrode material in DSSCs compared to that of SnO2/TiO2 core/shell NPs and TiO2 (P25) nanoparticles as shown in Fig. 40(d–h), because this structure exhibits much longer electron lifetime and better light-harvesting property, and also favours electron transport through a longer distance with less diffusive hindrance.159 Coming to the energy storage applications, as already discussed in previous sections the hollow nanostructured materials are very useful for lithium ion batteries because of their high surface area and shorter Li path length. Similarly, multiple shell metal oxide NPs also help to improve the lithium storage capacity of the lithium-ion battery as the anode material because of several reasons such as the small diffusion length of multishells, extra Li-ion storage in the inner cavity, and sufficient void buffer space for the volume expansion. To understand the importance of multishells, soft template assisted synthesis of Co3O4 spheres with single, double, and triple shells was tested as the anode material in the lithium-ion battery.129 The NPs with multiple shells exhibit


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Fig. 40 (a, b) Magnified TEM images of multi-shelled TiO2 hollow spheres, (c) comparisons of photocatalytic activities of multishells, single shell, and NPs after 40 min UV light irradiation. Inset shows a schematic illustration of multireflections within the multishells structure. Reprinted with permission from ref. 104. Copyright 2010 from the Royal Society of Chemistry, (d, f ) SEM images of SnO2 and TiO2–SnO2 multishells, (e, g) SEM and HRTEM images of SnO2 and TiO2–SnO2 multishells, (h) I–V characteristics of DSSCs with the photoelectrode films of TiO2–SnO2 MHSs, TiO2–nanoSnO2, SnO2 MHSs, nano-SnO2, and TiO2 (P25) nanoparticles. The inset illustrates the multiple reflecting and scattering of light in the multilayered hollow spheres. Reprinted with permission from ref. 159. Copyright 2009 from Wiley-VCH Verlag.

excellent cycle stability, a good rate capacity, and an enhanced lithium storage capacity compared to single shell materials. The double-shelled hollow structures, in particular, showed an exceptional capacity of 866 mA h g−1 over 50 cycles at a current rate of C/5 (completing the charge or discharge process in 5 h; 1 C = 890 mA g−1). This improvement is attributed to the synergetic effect of the small diffusion length of a multishell and sufficient void buffer space for the volume expansion.129 Similarly, a double shell V2O5 hollow sphere with a low ratio of SnO2 nanocrystals also exhibits a very high reversible capacity, excellent cycling performance, and a good rate capability when used as an anode in the lithium-ion battery. The reversible discharge capacity also increases in the presence of an active SnO2 component from 10 to 15 wt%.130 In the case of gas sensing applications, these particles show much improved sensitivity towards the sensing of gas molecules because of the much higher surface area of multiple shells compared to that of a similar simple YS structure. The importance of multilayered Cu2O was also reported for the sensing of ethanol vapour (100 ppm), where it has been shown


that the sensitivity of multilayered particles is 8.2 compared to the 1.5 for the same size solid particles because of high surface area and good porosity as shown in Fig. 41.115 It has also been tested that the multilayered crushed nanoparticles showed higher sensitivity than the solid particles but lower than the original multilayered particles. Some facile methods have also been reported for the synthesis of novel hollow structured Cu2O (single, double, triple or quadruple shelled) spheres by just adjusting the concentration of CTAB surfactant in the synthesis media, where the surfactant vesicle acts as a template for the formation of multilayered spheres.524 The multishell silica NPs were also synthesized by the selftemplated approach. Importantly, the shell to shell distance can also be tuned by controlling the etching period of the template without changing the overall size and interior core diameter.6 To test the enhancement of surface area, the BET surface areas of different particles are compared; it has been found that the surface area gradually increases linearly with increasing shell layers such as single (155.2 m2 g−1), double (243.9 m2 g−1), and triple (329.2 m2 g−1) with a better meso-

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Fig. 41 (a–d) SEM images with different magnifications and (e) dynamical response to 100 ppm ethanol of Cu2O sensors with hierarchical hollow microspheres, nanocrystallites, and solid microspheres with the average diameter of 500 nm. Reprinted with permission from ref. 115. Copyright 2007 from Wiley-VCH Verlag.

porous nature as shown in Fig. 42(a); which could be very useful in drug delivery and other applications. In this study, FITC fluorophores and anti-cancer doxorubicin (DOX) drugs were encapsulated in the 2nd and 3rd shell layer respectively, for the purpose of simultaneously imaging probe and chemotherapeutic agents by the appropriate selection of pH dependent molecules. Here, double shelled structures were synthesized first for entrapping the FITC dye and then the 3rd shell was fabricated to load the DOX drug, where the FITC exhibits poor release while DOX favours faster escape from multishelled silica nanospheres under acidic conditions as shown in Fig. 42(d and e). These DOX + FITC loaded multishell structures were used for the dual purpose of imaging and drug co-delivery on A549 cancer cells.60 Similarly, SiO2 multishell NPs were also prepared via a vesicle templated route; these particles exhibit a high loading capacity of the ibuprofen drug and also work as good adsorbents for the removal of the MB dye.59 Different types of hybrid metal oxides with tuneable composition were also reported and are also proposed as an important class because of multi-functionality. These hybrid metal oxide multishell NPs were synthesized via chemical transformation and continuous thermal treatment of the colloidal

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coordination polymer (metallo Schiff base with metal) through the cation exchange reaction as shown in Fig. 43.525 Firstly, spherical coordinating polymer particles containing metal ions were synthesized via a precipitation method. Then, the second metal was introduced through cation exchange to obtain sphere multishells. As usual, these types of multishell NPs of hybrid metal oxides can be used in catalysis, drug delivery, lithium-ion batteries, sensors, and nano-reactors, where the properties can be tuned by using the chemical composition. For example, the incorporation of drugs into hybrid particles can be achieved by mixing PEG-coated particles with drug molecules.526 The magnetic properties of hybrid metal oxide also depends on the chemical composition of the combination of shells, which can be tailored by a synthesis method reported by Moonhyun Oh and co-workers. Besides, the magnetic, photoluminescent, semiconductor, sensing and catalytic properties of the metal oxide can also be tailored with the tuneable hybrid metal oxide YS NPs.525 Similarly, the combinations of magnetic oxides with other metal oxide catalysts in the form of YS NPs have attracted great attention because of the recyclability of the catalyst by magnetic separation. The Fe3O4 double shell structures were also synthesized by using a


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Fig. 42 (a) BET analysis of single-, double-, and triple-shelled hollow silica nanospheres, (b) TEM images of triple-shelled silica nanoparticles, in vitro release profiles of (c) FITC and DOX molecules individually encapsulated in rattle-type triple-shelled silica nanostructures in PBS solution ( pH = 4.3 and 7.2) at 37 °C and (d) FITC encapsulated in rattle-type double-shelled silica nanostructures in PBS solution ( pH = 4.3 and 7.2) at 37 °C. Reprinted with permission from ref. 60. Copyright 2011 from Elsevier Inc.

sulphuric acid treated polymer as a sacrificial template.523 Different types of multihollow shell YS NPs are listed in Table 13. 2.2

Non-spherical or anisotropic YS NPs

During the last two or more decades, anisotropic or nonspherical nanoparticles have attracted researchers’ attention to a greater extent than spherical particles of the same material because of their better performance in several applications; in this regard, anisotropic YS structures are not an exception because of similar reasons. In the process of synthesis of the anisotropic NPs, surface energy, nucleation, and growth considerations are important to understand or predict the shapes of NPs. When nuclei are formed in the reaction media, they grow via addition of molecules to minimize the surface free energy; finally, the growth process depends on the nature and surface energy of nuclei. Generally, after the nucleation and growth process, nearly spherical shaped particles are formed over non-spherical shape because of the lowest surface energy of the spherical shape, whereas, for anisotropic shape, the growth results in a stable morphology by particle binding to the lower-index crystal planes because of the closest atomic


packing. The anisotropy can be controlled by tuning several parameters such as temperature, reduction potential, the reactant concentration, diffusion, solubility, the reaction rate, and the presence of stabilizer or capping agents.1–6 There are several methods reported in the literature such as sol–gel, polyol reaction, surfactant micelle-induced assembly, precipitation reaction, pyrolysis, and hydrothermal method to obtain different shapes of NPs2. While considering the YS structure, because of the complexity of the process, the variation of shape in YS is difficult and thus available structures are also limited. After summarizing the reported literature, it has been found that the shapes of YS nanostructures are limited to dumbbell,251 cubes,287 octahedron,288,304,502 rods,90,498,528 tubes,216,286,529 and spindle57,84,87,92,215,217,252,255,311,468,490,528 via different routes. Particularly, most of the reported studies have focused on the anisotropic noble metal YS NPs, as the anisotropic particles show different optical, catalytic, and other properties than spherical shaped particles because of the presence of additional face, corner, and edge atoms apart from the surface atoms. Additionally, other materials such as magnetic oxide/metal oxide YS NPs are also reported because of their easy synthesis routes to obtain different shapes.

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Fig. 43 Schematic processes for the preparation of multi ball-in-ball hybrid metal oxides: (I) CPP preparation using a precipitation method, (II) cation exchange reaction for composition transformation of CPPs, and (III) a final calcination processing of pre-prepared CPPs that engenders decomposition of the CPPs and formation of metal oxides. Reprinted with permission from ref. 525. Copyright 2011 from Wiley-VCH Verlag.

Table 13

Reported multishells or shell-in-shell YS nanostructures with detailed description of core, shell, and sacrificial layer

Core

Shell

Sacrificial layer

Core/shell

Core

Precursor

Shell

Precursor

Material

Removal

SiO2/SiO2

Sol–gel Sol–gel Hydrothermal Sol–gel Sol–gel Hydrothermal Sol–gel Sol–gel Wet chemical Solvothermal Chemical Spray pyrolysis Chemical Sol–gel Hydrothermal Hydrolysis

TEOS TEOS, TEOS TBOT TIP TiF4 FeCl2, FeCl3 TiCl4 VO(acac)2 Co(CH3COO)2 Co(OAc)2 Co(NO3)2 Zn(OAc)2 CuSO4 Cu(NO3)2 Fe(NO3)3, Co(NO3)2, Ni(NO3)2, Cu(NO3)2, Zn(NO3)2

Sol–gel Sol–gel Hydrothermal Sol–gel Sol–gel Hydrothermal Sol–gel Hydrothermal Wet chemical Solvothermal

TEOS TEOS TEOS TBOT TIP TiF4 FeCl2, FeCl3 SnCl4 SnCl4 Co(CH3COO)2

Pst + SiO2 CTAB + C12-OH FC4 and F127 Sulfonated sphere PS-DVB C Sulfonated sphere C SnO2 PVP Coordination polymer C Coordination polymer Cu2O Cu2O C

Calcination, NaOH60 Calcination521 Calcination59 Calcination523 Calcination104 Calcination522 Calcination523 Calcination159 Ostwald ripening130 Calcination129 Calcination525 Calcination72 Calcination525 Ostwald ripening524 Ostwald ripening115 Calcination527

TiO2/TiO2 Fe3O4/Fe3O4 TiO2/SnO2 V2O5/SnO2 Co3O4/Co3O4 ZnO/ZnO Cu2O/Cu2O α-Fe2O3 Co3O4, CuO, ZnO, ZnFe2O4, ZnO/ZnO/ ZnFe2O4/ZnO/ZnFe2O4.

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Further this class of YS NPs can be divided into two categories based on their geometry: (i) non-spherical YS and (ii) nonspherical core or shell, where either the core or the shell of the YS has a non-spherical shape. 2.2.1 Non-spherical YS NPs. Under this class, both core and shell structures have a non-spherical shape, where core and shell structures may have similar or dissimilar shapes. Generally to achieve a similar non-spherical core/shell structure, the sacrificial layer is coated uniformly on the pre-synthesized non-spherical core surface, and then the coating of the final outer layer. So, one can easily understand that the shape of the pre-synthesized core is essential to achieve a similar shaped YS. Under this category, different shapes such

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as ellipsoidal,57,84,87,92,215,217,252,255,311,468,490,528 rod,90,528 tubes,286 cubes287 and octahedron288,304 are reported. Specifically, the ellipsoidal α-Fe2O3 46,57,84,87,92,215,217,252,255,285,311,468,490 and anisotropic metal NPs90,287,288,304,528 are mostly pre-synthesized as a general core template and studied for the YS structure. The ellipsoidal shaped α-Fe2O3 NPs were mostly studied in recent years because α-Fe2O3 can be synthesized in large amounts with a wide range of sizes and shapes among all phases of iron oxides. Mostly, non-spherical α-Fe2O3 NPs are synthesized by thermal decomposition in the presence of a capping agent or a surfactant. On the other hand, it is well known that the hematite (α-Fe2O3) phase is not a good mag-

Fig. 44 (a) The synthesis procedure of Fe2O3/SiO2YS nanostructure by a spindle-shaped Fe2O3 template. Reprinted with permission from ref. 255. Copyright 2013 from the Royal Society of Chemistry, (b) schematic evolution from a Pd/Cu core/shell nanocube into a Pd/AuxCu1−x YS nanocage during galvanic replacement. Reprinted with permission from ref. 287. Copyright 2012 from Wiley-VCH Verlag, (c) schematic illustration of synthesis Ni/SiO2 YS NPs via a gas-induced elongation profile of reverse micelles over time. Reprinted with permission from ref. 216. Copyright 2012 from American Chemical Society, (d) schematic procedures for the preparation of hollow asymmetrical silica dumbbell YS NPs with an inner core and TEM image of asymmetrical silica dumbbells. Reprinted with permission from ref. 251. Copyright 2010 from American Chemical Society.


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netic material compared to maghemite and magnetite. Thus, to achieve good magnetic properties with similar high surface area and shape NPs, the anisotropic shaped α-Fe2O3 can be easily converted into other magnetic oxide phases such as (Fe3O4) by reduction with H2 at 300 °C, or even (γ-Fe2O3) by reduction with H2 and then re-oxidation with O2. For example, ellipsoidal α-Fe2O3/SiO2 and cocoon-shaped α-Fe2O3/SnO2 YS NPs were synthesized first. Then α-Fe2O3 was reduced to Fe3O4 by heat treatment in the presence of a H2 and N2 gas mixture without changing the initial shape.87,311 Here, ellipsoidal α-Fe2O3 was used as a core template and the non-spherical YS structure was prepared by depositing silica onto the α-Fe2O3 ellipsoidal core surface. Finally, the void space was achieved after either etching a silica layer in the presence of acid or alkaline solutions or by the removal of a non-metallic sacrificial layer after calcination to obtain ellipsoidal shaped YS NPs.57,84,87,92,215,217,252,255 Fig. 44(a) shows the formation of YS NPs after removal of a sacrificial layer by calcination. Similarly, ellipsoidal α-Fe2O3 was also used as a core template for different shell materials such as PDVB,468 double layer SnO2,311 and TiO2 490 to obtain final ellipsoidal shaped YS nanostructures by using silica as a middle or sacrificial layer. Interestingly, spherical, egg-like, olivary, elliptical, and shuttlelike shapes of YS NPs of η-Fe2O3 (both core and shell) were

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also synthesized by controlling the amount of multidentate ligand PVP ( poly(vinylpyrrolidone)) capping agent in solution, where PVP can affect the growth of NPs via the Ostwald ripening process, the final YS structures were obtained after removing the PVP layer by calcination. When the amount of PVP in solution was increased to 1.2, 1.7, 2.2, and 2.8 g (in 60 ml reaction vol.) without changing the reaction conditions, different shapes of η-Fe2O3 YS NPs such as egg-like (Fig. 45(a and b), olivary (Fig. 45(c and d), elliptical (Fig. 45(e and f ) and shuttlelike structures (Fig. 45(g and h) were obtained, respectively.46 It is also worth mentioning that the specific saturation magnetization of the elliptical shapes was higher than that of spherical YS η-Fe2O3 NPs. At the same time, the NPs cannot be magnetically saturated in the fields as high as 10 kOe because of difference in interior void space and pores on the surface of different morphologies.46 Thus, the magnetic properties also have significant effects on the shape of YS NPs. As mentioned before, while considering the metallic NPs, synthesis of noble metallic NPs with various shapes has been continuously reported, where the shapes of noble metal NPs were controlled by the nature of capping ligand or reaction conditions such as temperature, reduction potential, and so on.6,530 The non-spherical noble metal YS NPs were synthesized using non-spherical noble metal NPs as a core tem-

Fig. 45 (a, b) SEM and TEM images of YS structured η-Fe2O3 nanoparticles with egg-like shapes obtained by controlling the amount of PVP at 1.2 g. (c, d) SEM and TEM images of YS structured η-Fe2O3 nanoparticles with olivary shapes obtained by controlling the amount of PVP at 1.7 g. (e, f ) SEM and TEM images of YS structured η-Fe2O3 nanoparticles with elliptical shapes obtained by controlling the amount of PVP at 2.2 g. (g, h) SEM and TEM images of YS structured η-Fe2O3 nanoparticles with shuttle-like shapes obtained by controlling the amount of PVP at 2.8 g. For all samples, the reaction time in solution was controlled by 3 h. Reprinted with permission from ref. 46. Copyright 2013 from American Chemical Society. (i) A Au/Ag alloy and a Pd/Ag alloy for the inner and outer layer. Reprinted with permission from ref. 286. Copyright 2004 from American Chemical Society. ( j) TEM images of Pd/Cu YS nanocubes. Reprinted with permission from ref. 287. Copyright 2012 from Wiley-VCH Verlag, (k) Pd/PdxCu1−x YS nanocages. Reprinted with permission from ref. 287. Copyright 2012 from Wiley-VCH Verlag, (l) Pd- based hollow octahedral shape YS nanostructures with Au nanorods. Reprinted with permission from ref. 288. Copyright 2010 from Wiley-VCH Verlag.

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plate, where the shell metal precursor was chosen according to a suitable reduction potential, so that the shell structure can be formed by the galvanic replacement reaction by retaining the shape of the core as shown in Fig. 44(b). For instance, the tubular shaped Au–Ag alloy YS structure has been synthesized by using a galvanic replacement reaction as shown in Fig. 45(i), where the first Ag nanowire was synthesized by the polyol method, which acts as a core-shaped template. When HAuCl4 solution was added into the Ag nanowire, because of the galvanic replacement reaction, Au3+ ions were reduced to Au atoms and deposited as a shell layer by replacing Ag because of higher reduction potential.286 Further, a rod-shaped Au/Au YS

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structure was also reported by a similar method,90 where rodshaped Au/Au YS NPs exhibit the advantage of gold rods such as low plasmon resonance frequency, high polarizability, and small plasmon linewidth with high surface area because of the unique structure as shown in Fig. 46.90 Importantly, these structures also showed improved optical sensitivity compared to Au nanorods, solid spheres, and hollow spheres in the visible range of light and also provide chemical stability against dissolution and oxidation in an aqueous environment, which is required for plasmon sensing applications for biological systems. On the other hand, because of high surface area and more active sites, these materials have high catalytic activity

Fig. 46 Growth of gold YS NPs starting with gold nanorods (a), a silver layer is deposited (b). By means of reaction with Au ions (blue arrows), a shell of Ag–Au grows and then transforms into a cage with the dealloying of silver leading to a closed (c) or porous shell (d), depending on the amount of gold added. Representative TEM images corresponding to all the steps are shown on the bottom (scale bar is 50 nm). Extinction spectra corresponding to each step (e). While the silver coating leads to a blue-shift of the resonance wavelength, the replacement of the silver shell with a gold cage red-shifts the resonance wavelength. Stability of silver-coated particles, closed and porous YS in aqueous media. Spectra (left column) and TEM images of freshly prepared (middle) and aged particles (right). Black graphs – the freshly prepared particles and red – particles after aging for 21 days. (f ) Silver coated rods, (g) YS with a closed outer shell, (h) the porous gold YS. It is clear that porous YS maintain their resonance wavelength better than closed ones and silver coated rods. (i) Estimation of the increase of surface area in YS compared to nanorods. Dimensions of the rattle used for the calculations are shown. Total surface area of the nanorattle (21 500 nm²) is approximately 4 times bigger than that of the initial rod (5300 nm²). Reprinted with permission from ref. 90. Copyright 2009 from American Chemical Society.


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over Au nanorods when used for the reduction of p-nitrophenol to p-aminophenol.90 Many researchers have also reported similar non-spherical YS structures such as cages (Pd/ PdxCu1−x 287) and cubes (Pd/Cu,287 Cu2O/Fe(OH)x 304) using a galvanic replacement reaction as shown in Fig. 45( j and k). Similarly, the rod-shaped Au/SiO2 YS structure was also synthesized using silica etching after uniform coating on the core, which can be used in fluorescence imaging and NIR-controlled drug release after anti-cancer drug loading and fluorescent dye (RITC/FITC) or rare-earth up-conversion material (NaYF:Yb/Er/ Gd) modifications.528 The discussion of the previous section was on similar shaped core and shell YS NPs; however, it has been found that, depending on the geometry of the sacrificial layer, the shape of the shell can also be changed to differ from that of the core, to obtain different shapes of cores and shells. For example, nanorod/octahedron Au/Ag–M (M = Au, Pd, Pt) YS nanostructures have been reported using galvanic replacement,288 where the octahedron Ag layer acts as a shape template for Ag– M (M = Au, Pd, Pt) shells. It has been reported that when a small volume of AgNO3 (23 ml) was added to a pre-synthesized Au nanorod suspension, Ag was uniformly deposited on the surface of the Au nanorod. However, with the increase in the AgNO3 concentration, the Ag was deposited more on the sides of the surface of the Au nanorod and gives octahedron coating. Additionally, PVP, CTAB, and temperature also affect the shape of Ag layers because of different nucleation rates.288 2.2.2 Non-spherical core or shell YS NPs. In this type of anisotropic YS structure, either the core or the shell has a nonspherical shape. Different material combinations such as Ni/ SiO2,216,529 TiO2/SiO2,251 Fe3O4/SiO2 251 and Pt/SiO2 502 YS NPs have been reported, where the shell structure is non-spherical. Generally, non-spherical shells are prepared using hard251,502 and soft templated216,529 methods, where the template provides the shape of the shell structure. For instance, the tubular shaped SiO2 shell on a spherical Ni core was synthesized using the reverse-microemulsion technique as shown in Fig. 44(c). In this case, the shape of reverse micelles acts as the template for the final morphology and the void space formation was because of gas formation during synthesis as shown in Fig. 44(c).216,529 Here, cyclohexane and polyoxyethylene (10) cetyl ether were used as the oil phase and non-ionic surfactant, respectively, for the formation of microemulsion. The size of the reverse micelles is generally tuned easily by varying the water to surfactant ratio, which commands the nucleation of NPs inside the confined shape of the micelle to help achieve shape.216 Importantly, the tube lengths of the final YS structure strongly depend on the reaction parameters such as aging time before the addition of a silica precursor, reducing agent concentration (N2H2), and temperature.216,529 A sacrificial template method was also used to synthesize spherical/dumbbell shaped silica YS NPs as shown in Fig. 44(d). In this process, the silica core particles were first coated with cross-linked PMMA and then created a protrusion of polystyrene on the polymer coated core particles. After that, it was coated with silica shells, and the polymer layer was removed by heat treat-

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Nanoscale Table 14 Reported different shaped YS nanostructures and their core and shell compositions

Core/Shell

Movable core shape

Hollow shell shape

α-Fe2O3/SnO2311 α-Fe2O3/SiO2 84,215,217,252 α-Fe2O3/Al-silicate92 α-Fe2O3/SiO2 255 Fe2O3/SiO2 57 Au/SiO2 528 α-Fe2O3/PDVB468 Fe3O4/SiO2 87 Cu2O/Fe(OH)x 304 α-Fe2O3/TiO2 490 Au–Ag/Au–Ag alloy286 Pd/MxCu1−x 287 Au/Ag288 Au/Au90 α-Fe2O3/α-Fe2O3 46

Spindle Spindle Ellipsoidal Spindle Spindle Nanorod Ellipsoidal Ellipsoid Cube Spindle Tube Cubes, cages Nanorod Nanorod Spherical, egg-like, olivary, elliptical, shuttle-like Spherical NPs Spherical NPs

Spindle Spindle Ellipsoidal Spindle Spindle Rod shape Ellipsoidal Ellipsoid Octahedral Spindle Tube Cubes, cages Octahedrons Nanorod Spherical, egg-like, olivary, elliptical, shuttle-like Tube Dumbbells

Spherical NPs Rod

Octahedral Sphere

Ni/SiO2 216,529 TiO2/SiO2 251 Fe3O4/SiO2 Pt/SiO2 502 Au/SiO2 498

ment to obtain the silica dumbbell shell type YS nanostructure.251 A little different concept was also used to synthesize Pt/octahedron silica YS NPs, where H2PtCl6 and ammonia were mixed to form a complex ((NH4)2PtCl6) as a quasi-template for a octahedron shaped silica hollow shell.502 Then the quasi-template was removed partially by water wash and then calcined. During the water wash some small and uniform spherical Pt NPs were trapped inside the octahedron hollow silica shell to form YS NPs. In contrast, direct calcination without water wash results in decomposition of the Pt precursor to form big particles inside the silica shell. Here, this quasi-template provides many advantages over other hard templates such as easy removal by washing and in situ formation of cores avoids the pre-synthesis template process.502 Under the category of a non-spherical core, a Au nanorod/ silica YS structure has been reported, where the non-spherical Au core NPs were encapsulated inside a spherical hollow silica shell.498 In this method, first the Au precursor solution was encapsulated inside the pre-synthesized hollow silica shell and subsequently it was converted to Au NPs using a chemical reducing agent. Further, Au NPs inside the hollow shell were used as seed to induce the growth of Au nanorods.498 Different types of anisotropic YS nanostructures are summarized in Table 14.

3. Synthesis methods for YS NPs A majority of YS nanostructures were synthesized via different core/shell structures as templates. The synthesis of inorganic and organic core/shell NPs using different routes and chemical reactions has already been described extensively in our pre-


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vious review on core/shell nanoparticles.15 However, the extra and important feature to design the YS nanostructure is the creation of void space between the core and shell, which is an essential step to fabricate the YS nanostructures. In this section, we present a comprehensive review of synthesis methods of the YS nanostructures to create void space between the core and shell. Generally, synthesis approaches of the YS nanostructures are classified into two types: (i) core-to-shell and (ii) shell-to-core. In the former approach, first the core and then successive shell layers are synthesized; among the shell layers one or more layers may present as a sacrificial shell. In the latter approach, the hollow shells are synthesized first and then core nanoparticles are encapsulated inside the hollow shell. For a better understanding, we can divide the synthesis approach into 4 parts on the basis of the method.

3.1

Sacrificial template-assisted synthesis

From the introduction of the YS nanostructures, the sacrificial template based approaches have been receiving a lot of attention because of the simplicity of the process, ease to retain the core shape, control over the void space, and so on. The generation of void space by the removal of sacrificial layers can be classified into three types: (i) complete removal of the sacrificial layer, in which the sacrificial layer is made of different materials than that of the core and the shell, (ii) partial dissolution of the core layer, where the core surface is partially etched after the formation of the core/shell structure, and (iii) partial dissolution of the inner shell layer, where the inner shell layer is partially etched after the formation of the core/ shell structure. Thus, one can easily understand that the template removal or dissolution step is very important for the formation of YS nanostructures. Now based on the physical states of the materials, the sacrificial layers can be classified into (a) hard, where solid and rigid particles are used as a template, (b) soft types, the self-assembly of surfactant and organic molecules provides the template, (c) galvanic replacement, the redox reaction between two metals helps to create void space because of the difference in electrode potential, (d) the Kirkendall reaction, the void space is created because of a nonreciprocal mutual diffusion process through the interface of two materials. 3.1.1 Hard templates. Generally, the hard templates refer to solid rigid material layers, which can be easily removed through calcination, dissolution by solvents, and etching by acid or alkali solutions depending on the material properties. When removing the hard template by dissolution or calcination methods, special care is required to retain the external shell layer without collapsing. In some cases such as partial removal of core or shell layers and for the synthesis of multiple hollow shells, special care in the removal technique is very important. So far, different materials as hard templates have been reported, which include silica,35,36,39,41,47,50,52,53,58,77,87,97,147,154,157,162,212,214,310,322,360,365,366, 370,400,453,459,465,467,476,531–533 carbon,81,110,255 poly94,104,124,153,158,165,183,251,454,456,460,480,523,534 mers, Fe3O4,211


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gold,55,56 Ni,209,210 V2O5,213 MnO,535 Cu2O,536 sulphur,66 Ag,458 AgBr,457 β-FeOOH80 etc., to synthesise YS nanostructures. 3.1.1.1 Silica template. Silica is the most widely used hard template to design YS nanostructures because of its easy synthesis and removal technique. The SiO2 layer can be selectively or partially etched in the presence of hydrofluoric acid35,36,39,47,97,212,310,453,465,531,532 or alkaline solutions.41,50,52,53,58,77,87,147,154,157,162,214,322,360,365,366,370,400,459,467,476,533 The dissolution of SiO2 in HF is a well-known process in the glass and integrated circuit industries. The SiO2 hydrolyze in the presence of HF and form silanol on the surface and then form a water soluble H2SiF6 compound because of the high electronegativity of fluorine. The dissolution of silica by aqueous HF can be represented by the chemical reaction described in eqn (2).537–543 On the other hand, silica also dissolves in the presence of alkaline solutions such as soluble hydroxides and carbonates, and forms water soluble alkali silicates as shown in the chemical reaction in eqn (3).544 Comparing two dissolution methods, the HF mediated process is faster than that of alkali; however, handling of HF required several precautions and safety norms. SiO2ðsÞ þ 6HFðlÞ ! H2 SiF6ðaq:Þ þ 2H2 O

ð2Þ

SiO2ðsÞ þ 2NaOH ! Na2 SiO3ðaq:Þ þ H2 O

ð3Þ

The most commonly used sol–gel and Stöber methods are generally used for the synthesis of SiO2 layers. Using the Stöber method, two types of silica layers are synthesized: (i) pure inorganic silica and (ii) organic–inorganic hybrid silica. Generally, a pure silica layer is synthesized using a silica precursor (TEOS) by the Stöber method. On the other hand, organic–inorganic hybrid silica (organosilica) can be synthesized from the combined use of a silica precursor (TEOS) and organosilanes (C18TMS, APTES, APTMS, 3-MPTS, HDTES, EPTMS etc.) by the Stöber method. Thus, both types of silica have specific importance as a sacrificial template for different purposes. The organosilica layer is used when either the core or shell is formed with pure silica. Thus, during selective etching, organisilica will react faster with HF or alkaline solution than pure silica due to less dense and cross-linked structures and gives void space as shown in Fig. 47(a). In addition, the organosilica can also be calcined at high temperature to obtain the YS structure by the removal of the organic group. When the pure form of silica is used as a sacrificial template, it can be partially or completely removed by HF and alkaline solution as shown in Fig. 47(c). Similarly, the surface protecting etching of the silica layer is also used for the synthesis of porous YS NPs, where a SiO2 layer is pre-adsorbed with a protecting layer of PVP or other ligands as shown in Fig. 47(b). Here, PVP is used as a surface protecting agent of silica because its abundant carbonyl groups can interact with the hydroxyl groups on the SiO2 surface via strong hydrogen bonding. Thus, the interior part of the SiO2 layer only etches and the surface of shell is protected during the dissolution, which creates porosity on the silica shell.21,25,41,545

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Fig. 47 (a) Selective etching of an organosilicate layer. Reprinted with permission from ref. 47. Copyright 2009 from Wiley-VCH Verlag, (b) Surface protected etching of silica using PVP. Reprinted with permission from ref. 41. Copyright 2010 from Springer, (c) complete etching of a silica layer. Reprinted with permission from ref. 162. Copyright 2013 from Wiley-VCH Verlag.

A partial or complete removal technique of the silica layer depends on the nature of core and shell materials as shown in Fig. 47. The silica layer is removed completely to obtain the hollow space for the synthesis of nonsilica-based YS NPs such as Au/polymer,35 γ-Fe2O3/Y2O3,307,372 Au/ZrO2,50,52,53,400 Fe3O4/ TiO2,162 Fe3O4/YPO4,312 Fe3O4/SnO2,154 Au/carbon,52,93 Si/ carbon,121 C/C,134 Pd/C,299 Au/TiO2,365,370 Pd/CeO2 360 etc. On the other hand, for the synthesis of silica-based YS NPs, partial etching of silica is a useful method. Many researchers used partial etching of silica for the synthesis of Au/SiO2,170 Ni/SiO2,179 Pd/SiO2,178,531 SiO2/SiO2,41,47,215 Fe3O4/SiO2,87 SiO2/ TiO2103,546 YS NPs. The silica template is also useful to make silicate shells in YS nanostructures. During the silica etching, the noble metal and metal-oxide partially react with the silica to form products of silicates.77,92,157,547 3.1.1.2 Carbon template. The sacrificial hard shell layer of carbon is used as another important material for synthesizing the YS structure. Generally, the carbon template layer can be completely removed by calcination at high temperature in the presence of air. Several YS nanostructures such as Fe3O4/ SiO2,81,83 Fe2O3/SiO2,255 Au/SiO2,110 SnO2/C,124 Fe3O4/C,153 TiO2/TiO2 104 and SnO2/TiO2 159 have been synthesized by using the carbon layer as a sacrificial template and removed completely by calcination. In some approaches, presynthesized hollow carbon spheres are used as a sacrificial template, where core precursors diffuse inside to form the core particles and then the shell material is coated on the carbon shell. Finally,

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the carbon layer is removed by heat treatment to obtain the YS nanostructure. Utilization of pre-synthesized carbon spheres has the advantage of a tuneable size which can be controlled by tuning the reaction time, temperature, and the concentration of the carbon precursor. On the other hand, the carbon layer can also be formed by carbonization of organic compounds under hydrothermal conditions.76,104,110,124,153 In general, the carbon layer is synthesized easily by the hydrothermal treatment of the carbonaceous water soluble precursors such as glucose81,83,255 and urea,118 where the transformation of the carbon source to elemental carbon involves dehydration, polymerization, condensation, and carbonization steps. Initially intermolecular dehydration occurs under the hydrothermal conditions and forms organic compounds and organic acids. Thus, the hydronium ions are formed in acidic pH, which acts as a catalyst for degradation of the organic compound. Then, subsequent polymerization and condensation reactions form the final carbon nanosphere.548–552 3.1.1.3 Polymer template. Different polymers are also used as sacrificial templates for the design of YS nanostructures. The polymeric sacrificial layers are generally removed by calcination94,105,158,165,183,251,553 or by dissolution454,456,460,480,523,534,553 in the presence of an appropriate solvent. Here, the choice of the removal technique totally depends on the shell material as shown in Fig. 48. If the shell material is a polymer or organic, then the removal by calcina-


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Fig. 48 (a) Formation of TiO2/SiO2 YS NPs through removal of a sacrificial layer (PMMA) by calcination. Reprinted with permission from ref. 105. Copyright 2009 from American Chemical Society, (b) synthesis of SiO2/PPy YS NPs through removal of a sacrificial layer (PS) by the dissolution method. Reprinted with permission from ref. 460. Copyright 2007 from Elsevier Inc.

tion can be avoided; only a dissolution method with organic solvents is used.454,456,460,480,523,534,553 On the other hand, if the shell is made up of an inorganic layer, then the polymer template can be easily removed by the calcination process.94,105,158,165,183,251,553 Thus, many different YS materials such as SiO2/TiO2 ( polystyrene),553 Au/SiO2 ( polystyrene-co-poly(4-vinylpyridine),94 magnetic silica/silica ( polystyrene),183 SiO2/polypyrrole ( polystyrene),460 Fe3O4/SiO2/poly(MBAAm-co-MAA) (PMMA),454 poly(DVB-co-AA)/poly(DVB-coAA) ( poly(acrylic acid)),480 PS-co-PAEMAco- PVTES-co-PMAA ( polystyrene),534 Fe3O4/silica ( polystyrene),158 TiO2/TiO2 ( polystyrene),523 SiO2/TiO2 (PMMA, polystyrene),251 Au/SiO2 (PS-coP4VP),94 Fe3O4/PANi ( polystyrene),165 TiO2/SiO2 (PMMA)105 and silica/PDVB (PMAA)456 were also reported using the polymer as a sacrificial template (mentioned in parentheses). Mostly, the polymer layer was synthesized by the emulsion polymerization method. 3.1.1.4 Other materials. While silica, polymer, and carbon materials are mostly used as a sacrificial template for YS synthesis, many other inorganic materials such as Fe3O4,211 gold,55,56 Ni,209,210 V2O5,213 MnO,535 Cu2O,536 sulphur,66 Ag,458 AgBr,457 and β-FeOOH80 have also been reported. When metal oxides and metals are used as a sacrificial template, mostly acid (HNO3, H2SO4, HCl) and alkali leaching (NH4OH, NaOH)


techniques are used for complete removal of the sacrificial layer.5,56 The metal and metal oxides react with acid to form a soluble metal salt and hydrogen as shown in eqn (4) and (5). Apart from acidic dissolution, reductive dissolution can also be used to remove a metal oxide layer with a reducing agent as presented in eqn (6) and Fig. 49(a). For example, the Au/SiO2 YS NPs were prepared by reductive dissolution of Fe3O4 in the presence of HCl and NaBH4 as shown in eqn (5).211 Similarly, MnO was also selectively etched by NH2OH-based solutions (hydroxylamine hydrochloride), where dissolution of MnO occurs by reduction of Mn(IV) or Mn(III) to soluble Mn(II) as presented in eqn (7) and (8).535,554 Metal þ acid ! metal salt þ hydrogen

ð4Þ

Metal oxides þ acid ! metal salt þ water

ð5Þ

Fe3 O4 þ 2e þ 8Hþ ! 3Fe2þ þ 4H2 O

ð6Þ

MnO2 ðMnðivÞÞ þ 2NH3 OHþ ! Mn2þ þ N2 ðgÞ þ H2 O

ð7Þ

Mn2 O3 ðMnðiiiÞÞ þ 2NH3 OHþ þ 2Hþ ! Mn2þ þ N2 ðgÞ þ 5H2 O ð8Þ Some specific reagents are also used, such as leaching of gold by cyanide (cyanidation), which is a common process to

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a photoactive material, which absorbs photons in the UV-Vis wavelengths to generate electrons and hole pairs. The generated electron associated with interstitial Ag ions to produce Ag atoms as shown in eqn (12)–(14).558,559 The AgBr can also be dissolved in the presence of NH3 solution by forming silver– amine complexes as shown in eqn (15). The β-FeOOH80 was also used as a template, where it was converted into Fe3O4 by reduction. 2C6 H5 CH3 þ 2S ! C6 H5 CH : CHC6 H6 þ 2H2 S

ð10Þ

Ag þ 2NH3 ðaq:Þ ! ½AgðNH3 Þ2

ð11Þ

AgBr þ hν ! AgBr ðhþ þ e Þ

ð12Þ

e þ Ag

i

þ

! Agi 0

1 AgBr þ hν ! Ag þ Br2 2 AgBrðsÞ þ 2NH3 ðaq:Þ ! AgðNH3 Þ2 þ ðaq:Þ þ Br ðaq:Þ Fig. 49 (a) The synthesis of Au/h-SiO2 YS NPs by using Fe3O4 as a sacrificial layer. Reprinted with permission from ref. 211. Copyright 2010 from the Royal Society of Chemistry, (b) synthetic procedure of Au/SiO2 nanoreactor framework by using Au as a sacrificial template. Reprinted with permission from ref. 56. Copyright 2008 from Wiley-VCH Verlag, (c) schematic of the synthetic process that involves coating of sulphur nanoparticles with TiO2 to form sulphur/TiO2 core/shell nanostructures, followed by partial dissolution of sulfur in toluene to achieve the YS morphology. Reprinted with permission from ref. 66. Copyright 2013 from Nature Publishing Group.

dissolve Au as shown in Fig. 49(b). In this process, cyanide ions oxidize metallic gold and form a water soluble Au–CN coordination complex as shown in eqn (8). Using this reaction, the thickness of the dissolution layer can be controlled by controlling the concentration of added KCN as per the stoichiometry mentioned in eqn (9).55,56 4Au þ 8KCN þ O2 þ 2H2 O ! 4K½AuðCNÞ2 þ 4KOH

ð9Þ

Among non-metallic sacrificial layers, sulfur is also an important material, as the sulfur layer can be removed by both dissolution (CS2 and toluene) and calcination at low temperature.555–557 Recently some studies also reported the synthesis of the YS nanostructure by using the sulfur core as a sacrificial template, where the sulfur core dissolves partially in the presence toluene solution as shown in Fig. 49(c).66 The sulfur reacts with toluene and forms H2S gas as shown in eqn (10). Besides toluene, sulphur can also be removed using carbon disulfide, carbon tetrachloride, xylene, and benzene solution. We can also eliminate sulphur via the calcination at elevated temperature.555 The Ag template was also reported for the synthesis of Au/PPy YS NPs,458 where the Ag template was removed using NH3 solution by forming a soluble metal– amine complex compound as shown in eqn (11). Silver halides such as AgBr were also reported as a sacrificial template, where AgBr/PPy core/shell transforms into Ag/PPy YS through photoreduction of AgBr in the presence of UV light.457 AgBr is

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ð13Þ ð14Þ ð15Þ

In general, conventional hard template methods are usually time consuming because of the requirements of pre-designing or multiple step control, which may finally lead to difficulty to obtain more complex structures. Besides, these templates are also advantageous for many reasons such as easy synthesis, monodispersity, size variety, tuneability of components, bulk synthesis, and being easy to scale up for large-scale production. 3.1.2 Soft template based method. The hard template based processes are probably the most effective and are widely used over the soft templates to design YS nanostructures. However, these methods have several issues such as stability of core and shell structures during removal of the sacrificial layer (calcinations or dissolution) and a multistep process. Many applications including drug delivery, therapeutic delivery, bioimaging, and catalysis require a facile way to access void space to encapsulate the guest molecules. In the hard templated method, encapsulation of the guest molecules in the void space during shell formation is very difficult. These problems can be solved by the use of a soft template based strategy.8,12,20,23,73,560 The soft template based approach allows easy encapsulation of guest molecules inside the hollow shell. The soft templates are considered adsorbed surfactant layers23,47,57,84,86,107,129,135,217,252,290,369,474,500,504,508 and microemulsions96,177,180,181,216,461,462,464,510 for the synthesis of YS nanostructures. Usually, a soft template can be completely removed by various routes such as the acid extraction method,23 water or alcohol washing,96,129,181,217,252,461,462,464,474 and thermal annealing or calcination.57,84,107,180,216,500,504 The surfactants are organic amphiphilic molecules used in the NP synthesis process as template or capping agents to control the size and shape of the NPs. In general, adsorbed or self-assembled layers of surfactant molecules on the core surface, surfactant micelle, reverse micelle, vesicle, and microemulsions which contain a huge surfactant are generally used as a template. In the surfactant based template method,


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Fig. 50 (a) Schematic procedure used to generate YS nanostructures through a soft template. Reprinted with permission from ref. 217. Copyright 2009 from American Chemical Society, (b) procedure for the preparation of YS nanostructures with a mesoporous shell. FC4 = fluorocarbon surfactant, PPO = poly ( propylene oxide), PEO ( poly (ethylene oxide), TEOS (tetraethoxysilane). Reprinted with permission from ref. 57. Copyright 2010 from Wiley-VCH Verlag.

surfactant molecules are adsorbed on the core surface by electrostatic or van der Waals interactions and then the outer shell layer is deposited on that surface because of similar interactions. For example, a detailed process of the formation of YS nanostructures is shown in Fig. 50(a). First, the core NPs were dispersed in the aqueous solution of a suitable surfactant to induce the formation of vesicles with core NPs with the assistance of a vesicle-inducing agent such as a short alkyl amine or other small organic molecules. Then, the co-structure-directing agents (CSDAs), typically aminosilane such as APTES or TMAPS, were attached to the surface of the vesicles through electrostatic attraction. Finally, the silica shells were formed through the sol–gel process by aminosilane and TEOS. In the whole process, the aminosilane acts as both a vesicleinducing agent and a co-structure directing agent simultaneously. Thus, the core NPs are accommodated into the vesicles and the YS structure formation occurs through the replication of vesicles by silica shells.23,217 Finally, the surfactant layer was removed by the ethanol washing and acid extraction (acetonitrile solution containing ∼35% HCl) method. Similarly in another approach, FC4 (fluorocarbon surfactant) surfactant was used as the vesicle–core complex template for the synthesis SiO2/SiO2, Au/SiO2, Fe3O4/SiO2 YS nanostructures with a tuneable shell thickness as shown in Fig. 50(b), where the organic template was removed via calcination to obtain the hollow structure.57 In the case of surfactant-based soft templated methods, researchers are continuously exploring various surfactants to obtain their desire advantages. As another example, an aqueous mixture of lauryl sulfonate betaine (LSB, a zwitterionic surfactant) and sodium dodecyl benzenesulfonate (SDBS, an anionic surfactant) was also used as a soft template to synthesize YS nanostructures.217 For


designing this process, the molecular structure of the surfactant, the concentration, and the addition of a co-surfactant or other molecules are extremely important to control the morphological parameters of YS structures. As mentioned before, emulsions can also be used as a soft template to synthesize the YS structure. Generally, microemulsions are used as a template, since nanometer size droplets are formed in the microemulsion. A microemulsion is a thermodynamically stable phase which contains a mixture of oil, water, surfactant, and co-surfactants (if required). Depending on the process and selection of materials, both oil in water (O/W) and water in oil (W/O) emulsions are used. The synthesis of YS can be achieved in two steps by the emulsion process. Firstly, the core NPs were dispersed in the emulsion and then the shell layers were deposited around the interface between the emulsion droplets and the continuous phase of the microemulsion to generate YS structures. Many researchers have also demonstrated water in oil microemulsion as a soft template to prepare YS nanostructures.96,177,180,181,216,461,462,464,510 For example, the Au/SiO2 YS nanostructure was prepared using W/O microemulsion by encapsulating a gold precursor in a SiO2 template and further encapsulating the gold nanoparticles inside the SiO2 shell via reduction of the gold precursor by NaBH4.96,177 In another approach, the Fe3O4/SiO2 YS nanostructure was obtained by using Triton X-100 surfactant based W/O microemulsion shown in Fig. 51. However, this process is limited to the selective surfactant for the preparation of YS nanostructures.180 3.1.3 Galvanic replacement. Galvanic replacement reaction offers an efficient route for the synthesis of noble metallic or alloy type YS nanostructures with controllable hollow interiors, size, shape, composition, and morphologies;44,108,109,285–288,291

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Fig. 51 Schematic diagrams of the formation mechanism of Fe3O4/Silica YS NPs via W/O microemulsion. Reprinted with permission from ref. 180. Copyright 2011 from American Chemical Society.

this method is also called transmetalation. This process is a sacrificial template type technique based on the electrochemical potential of the used metals, where the template also acts as a reducing agent. The key point of galvanic replacement is the electrical potential difference between two metals, where one metal acts as the cathode and other metal as the anode. Galvanic replacement reaction occurs spontaneously when atoms of a metal (zero valance state) react with ions of another metal having a higher electrochemical potential in a solution phase. Generally, in the galvanic replacement process the lower standard electrode potential metal (A) is used as a sacrificial layer and the higher standard electrode potential metal (B) as a shell. In this process metal A is oxidized and B is reduced. The reaction can be expressed as follows: Anodic reaction : AðcoreÞ ! A ðsolutionÞ þ ne

ð16Þ

Cathodic reaction : Bmþ þ me ! B0 ðshellÞ

ð17Þ

nþ

Overall reaction : mA0 ðcoreÞ þ nBmþ ! nB0 ðshellÞ þ mAnþ ðsolutionÞ

ð18Þ

The standard reduction potentials of some metals are shown in Table 15. The core material is synthesized using a chemical reduction method. In this electrochemical process, partial or complete removal of the sacrificial template is dependent on the final YS structure. When the removal is partial the core material is synthesized first, and the finally A/B type YS structure is formed. In the case of complete removal, A/C type core/shell

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Table 15 Reduction potentials of commonly used metals for YS synthesis relative to the standard hydrogen electrode (SHE)

Reduction reaction

E0 (V vs. SHE)a

Co2+ + 2e− → Co Cu2+ + 2e− → Cu Ag+ + e− → Ag Pd2+ + 2e− → Pd Pt2+ + 2e− → Pt Au3+ + 3e− → Au

−0.28 0.34 0.80 0.95 1.18 1.50

a

For ideal conditions at 25 °C and 1 atm.

material is synthesized first and then after removal of the C layer it gives the A/B type YS structure. However, in this case, if there is an incomplete removal of the C layer, then the A/C type core/shell will be encapsulated inside the hollow shell layer of B. Similar to other methods, here also shell thickness, void space etc., can be controlled by changing the process parameters (reactant concentration, standard electrode potential, etc.). In this process, the properties of the shell depends on various parameters such as: (i) when more than one metal precursor is present in the presynthesized core media for galvanic replacement, the distribution of metals in the shell can be controlled by altering the order of addition of precursors. (ii) The core dissolution rate from different positions depends on the chemical reactivity of the respective sites and the shielding effect of the deposited shell layer, and the finally formed shell structures are in general of porous type. During the dissolution


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process there is a chance alloying too in the galvanic replacement. When metal B is deposited on A, in the initial period of the process alloying started within the thin shell layer. It has been reported that the kinetics of alloying obeys Fick’s second law of diffusion.561 Cx;t ¼ Ci Ci erf

x 1=2 2DT

ð19Þ

where Cx,t is the atomic concentration of metal B as a function of time (t ) and distance (x), Ci is the initial concentration, and D is the diffusion coefficient. Finally, the degree of alloying increases with increasing reaction temperature, thickening of the shell layer, and depression of the energy barrier to diffusion. Additionally, dealloying is also an important fact in the later stage of this process, to control porosity of the shell layer by removing one metal. In general, dealloying occurs in the presence of more metal ions involved in the galvanic replacement reaction or by etching (acid or alkali). The alloying and dealloying process depends on the stoichiometry of the oxidation number or the valence difference between A and B materials. As an example of the Au–Ag system, when HAuCl4 is used as a Au-precursor, one Au atom formed (reduced) by oxidation of three Ag atoms according to eqn (20). In this process, alloying continues till the lower electrochemical potential metal or sacrificial layer molecules (Ag) are present in excess or equivalent moles compared to that of the higher electrochemical potential (Au3+) metal ions. If a larger amount of HAuCl4

is added, in the presence of more Au3+, dealloying of the Au/Ag shell occurs by selective removal of Ag atoms from the alloyed shell, and finally pure Au shell forms. During the dealloying process, many small lattice vacancies are also generated, which form small holes on the shell as shown in Fig. 52. As an example of the valance effect, if Au+ ions are used in place of Au3+ ions with an Ag core, a thicker Au–Ag shell will form, as one Ag atom will dissolve per Au+ ion. Thus, the composition or removal of the sacrificial layer can be controlled by tuning the stoichiometric ratio or valence ratio of the molecules present in the sacrificial template and the outer layer precursor material. The first example of the synthesis of the YS nanostructure via a galvanic replacement reaction was reported for the synthesis of Au/Ag alloy YS nanostructures via a reaction between Ag and HAuCl4.286 In this study, Au was formed after reduction on the surface of the Ag template, as the reduction potential of AuCl4−/Au (0.99 V SHE) is higher than that of Ag+/Ag (0.8 V SHE). The shell layer was formed through the processes of nucleation and growth; the reaction is shown in eqn (20). In the Ag–Au system, the thickness of the void space was also controlled by varying the thickness of sacrificial Ag coating on the Au/Ag alloy core by changing the concentration of AgNO3 in the electrolysis plating bath. In this case, the morphology of the Au NPs is similar to the Ag template as shown in Fig. 53a.286 Similarly, many research groups have also used a galvanic replacement reaction to design Au/Au,44 Au/Pt–Ag,285 Au nanorod/Au,288 Au–Ag/Au,291 Au/Pt,108 Au/Pd,109 and Au/

Fig. 52 Schematic illustration of the morphological and structural changes at different stages of the galvanic replacement reaction between a Ag nanoparticle and HAuCl4 in an aqueous solution (step 1–3) alloying by retaining the morphology of the original template during a galvanic replacement reaction, (steps 4 and 5) the dealloying process, when the metal with a lower reduction potential is selectively removed from the alloyed wall and generates pores. Reprinted with permission from ref. 562. Copyright 2013 from Wiley-VCH Verlag.


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Fig. 53 (a) Schematic illustration of the procedure for preparing YS NPs consisting of a Au/Ag alloy core and a Au/Ag alloy shell, where two major steps are involved: (A) electroless deposition of pure Ag on the surface of an Au/Ag alloy NP; and (B) reaction of the resultant particle with an aqueous HAuCl4 solution to transform the coating of pure Ag into an Au/Ag alloy shell larger in size. Reprinted with permission from ref. 286. Copyright 2004 from American Chemical Society, (b) the galvanic replacement mechanism of the pure Au/Pt YS nanostructure. Reprinted with permission from ref. 108. Copyright 2011 from the Royal Society of Chemistry.

SiO2 218 YS NPs. Other than these, Pd/MxCu1−x alloy (M = Au, Pd, and Pt) YS nanostructure was also synthesized using Pd/Cu core/shell nanocubes as a template. While most of the studies used the Ag sacrificial layer, in contrast, Cu also offers a much lower reduction potential (0.34 versus 0.80 V for Ag) to facilitate the galvanic reaction more easily, even at room temperature.287 3AgðSÞ þ AuCl4 ðaq:Þ ! AuðSÞ þ 3Agþ ðaq:Þ þ 4Cl ðaq:Þ

ð20Þ

3.1.4 The Kirkendall reaction. The Kirkendall effect is a classical phenomenon in metallurgy, where vacancy (absence of atoms in a crystal structure) diffusion occurs at the interface between two metals because of the difference in diffusion rate between them. According to this concept, atomic diffusion in a metal does not occur directly, but through the vacancy exchange. The material with the higher diffusion coefficient will have a larger associated vacancy flux in it, so the net movement of vacancies will be from the material with the lower diffusion coefficient to the material with the higher diffusion coefficient. Finally, the condensation of excess vacancies can give rise to void formation near the original interface and within the faster diffusion side called ‘Kirkendall voids’.563–570 The Kirkendall effect is based on two principles: (i) atomic diffusion occurs via vacancies and (ii) each metal diffuses at a different mobility. This effect was first observed by Smigelskas and Kirkendall on the diffusion between zinc and brass.571,572 During the last decade, the Kirkendall effect has been recognized as one of the most useful methods to prepare hollow and YS NPs of metal oxides, sulfides and phosphides after it was first reported by Yin’s group in 2004, where Co NPs were converted into hollow cobalt oxide and chalcogenide (S, Se) NPs through a reaction with sulfur, oxygen, and selenium.565 This study demonstrated that at the metal/oxide or metal/

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sulphide interface, the outward diffusion of the cobalt atoms is much faster than the inward diffusion of oxygen or sulphur at high temperature, which gives rise to voids or vacancies during oxidation and sulfidation. In this case, there is formation of material bridges between the core and the shell, and this persists until the core is completely consumed. These bridges provide a fast transport path for outward diffusion of cobalt atoms that can then spread to the inner shell surface. This approach inspired several researchers to design oxide, phosphide, and chalcogenide YS nanostructures,289,311,363,364,506,507 where the void can develop through the Kirkendall effect. In this approach, when a metal is exposed to oxygen, phosphorus, sulfur, or selenium precursors at elevated temperature, it gives a diffusion couple. Thus, outward diffusion of metal NP cation species is quicker than inward diffusion of the anions species; then an inward flux of vacancies balances the diffusivity difference of the outward metal cation flux. Finally, condensation of excess vacancies can give rise to void formation at the interface of the faster diffusion side.563–570 The YS synthesis through the Kirkendall effect involves three major steps: (i) synthesis of core NPs, (ii) deposition of shell material on the core NPs, (iii) oxidation, phosphidation, or chalcogination of the shell to form a hollow shell by the Kirkendall reaction for diffusion of the outer layer to create void space at the interface. For example, the Kirkendall effect can be explained by the oxidation (a chemical conversion reaction) of the metal (Ni) NP process as shown in Fig. 54. During the oxidation process, a thin layer of oxide forms on the surface of metal NPs, which allows metal ions to diffuse more quickly towards the outside than oxygen atoms because of the larger ionic radius of anions than cations and finally leaves a void space inside.566,570 Using the same approach, a FePt/CoS2 YS nanostructure was synthesized, where the YS structure was formed after wet sulfidation of FePt/Co nanoparticles. Mostly, researchers have used the thermal oxidation process to synthesize YS nanostructures, which is a type of non-equilibrium inter-diffusion process. Similarly, multifunctional Au/Fe2O3, Ag/Fe2O3, and FePt/Fe2O3 YS nanostructures were also prepared from the oxidation of the iron shell on the respective core materials by means of the Kirkendall effect.363,506 3.2

Template free approach (Ostwald ripening)

The template-mediated synthesis methods of NPs have been used for several years to synthesize different size and shape nanoparticles to obtain proper control on different parameters such as size, shape, shell layer thickness etc. However, in many cases, the synthesis process required several steps based on the complexity of the desired structure. More specifically, while coming to the YS structure, removal of the sacrificial layer and its purification required a few more steps. Needless to say, the sacrificial layer material is completely undesirable and finally becomes a waste at the end of the process. In many cases, this sacrificial layer is either made of some precious materials (Au, Ag), or the removal process is environmentally harsh. A sacrificial layer removal process finally complicates


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Fig. 54 Formation of a single void at the core/shell interface causes asymmetrical outward diffusion of Ni, resulting in a non-uniform NiO shell thickness. Reprinted with permission from ref. 570. Copyright 2010 from American Chemical Society.

the overall process and increases the chance of impurity, agglomeration, damage of core or shell nanoparticles, and chances of shell collapsing. Finally, because of these reasons the overall costs of template-mediated methods are higher, while commercialized for the practical applications. So many researchers would prefer a one-step template-free process to synthesize YS nanostructures. As a result, many research groups have reported one step template-free methods for the synthesis of YS nanostructures based on the Ostwald ripening process.43,45,46,48,49,61,63,101,102,106,119,123,128,130,133,160,161,166,302,305, 308,361,362,399,486,501,524 Ostwald ripening is a physical phenomenon that refers to “the growth of larger crystals from those of smaller size which have a higher solubility than the larger ones”. In this process, small nanoparticles dissolve in solution and large nanoparticles because of minimization of surface energy. In this process, a void space will be generated when the core is made of aggregates of small NPs and the shell is made of larger NPs. When the small NPs are located at the central part of the spherical aggregates, it is called symmetric Ostwald ripening. On the other hand, if small nanoparticles are located asymmetrically on the spherical aggregates, then it is termed as an asymmetric Ostwald ripening process.8,20,573,574 To date, a wide variety of YS nanostructures have been synthesized using this template-free method. For instance, semiconductor ZnS and Co3O4 YS nanostructures are synthesized based on the symmetric and asymmetric Ostwald ripening processes as shown in Fig. 55. In this process, firstly, small nanoparticles were aggregated into a solid sphere. Then, during the recrystallization process, small crystallites of the solid sphere were dissolved and move towards the outer shell to make a void space. With the progress of time, the void space becomes larger and divides the solid sphere into two parts: one is movable core, and other is a hollow outer shell.48 Similarly, a simple one-pot template-free synthesis of Cu2O YS NPs


was also reported by symmetric and asymmetric Ostwald ripening. The synthesis was performed in aqueous Cu(CH3COO)2 solution, where the precursor was reduced by ascorbic acid at room temperature to form the Cu2O nuclei clusters. Firstly, these clusters are self-assembled into a solid sphere and then through the recrystallization process they form void space inside the solid sphere.305 The rare earth fluoride (SmF3) YS NPs were also synthesized by symmetric and asymmetric Ostwald ripening processes.61 The formation of α-Fe2O3/SnO2 YS nanostructures was also reported based on the inside-out Ostwald ripening mechanism, where the SnO2 nanoparticles were synthesized by hydrolysis of K2SnO3·3H2O around the α-Fe2O3 NPs to form a solid core/shell structure. Further, during the hydrothermal Ostwald ripening process, the small SnO2 nanoparticles dissolve and relocate to the outer shell because of a recrystallization process, and form the void space.123 Similarly, other researchers have also reported Fe3O4/ Co3O4,166 Fe3O4/SnO2,161 Pt/CeO2,362 and Au/Cu2O367 YS nanostructures. 3.3

Ultrasonic spray pyrolysis

Ultrasonic spray pyrolysis (USP) is a one-step, scalable and continuous approach for the fabrication of a diverse range of nanostructured materials including hollow and YS nanostructures by the evaporation and decomposition of precursor droplets in a reactor. It is a solution based process, where high-frequency ultrasound is passed through the liquid precursor solution to generate a liquid–gas interface (aerosol), which nebulized into micro-droplets. Finally, the generated droplets are carried by a gas flow into a furnace, where solvent evaporation and precursor decomposition take place to form the YS nanostructures. The volatile organic compounds form a layer on the NP surface and leave a void space to generate a YS structure during the heating process. The role of ultrasound in USP is to provide the phase isolation of one microdroplet reactor

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Fig. 55 (a) General process of Ostwald ripening. (b) Various schemes of Ostwald ripening for spherical colloidal aggregates: (1) core hollowing process; (2) symmetric Ostwald ripening for formation of a homogeneous core–shell structure; (3) asymmetric Ostwald ripening in formation of a semi-hollow core–shell structure; (4) a combination of 1 and 2. (c) A rotating core. (d) Mass center of a semi-hollow core–shell structure. (e) Proposed model for the formation of the void space between a core and a shell in scheme 2 of (b). Hashed lines: the cross-sectional plane of a sphere. Darker areas: larger and/or closely packed crystallites. Lighter areas: smaller and/or loosely packed crystallites. White areas: void space. Note that the above area illustrations are simplified, as actual transitions between two different areas should be much more gradual. (f ) TEM images of symmetric Ostwald ripening in Zns. (g) TEM images of symmetric Ostwald ripening in Co3O4. Reprinted with permission from ref. 48. Copyright 2005 from Wiley-VCH Verlag.

from another.575,576 Generally, this process involves the following steps: (i) micro-droplet generation using USP, (ii) evaporation of solvents in the heated zone, (iii) diffusion of reactants, (iv) reaction or precipitation, and (v) escape of any volatile components. This method has several advantages over conventional methods for the synthesis of YS nanostructures such as a one step process, no acid or alkali treatments for sacrificial layer dissolution, an inexpensive continuous process, environmentally friendly, low-cost initial reactants, control on the composition, and scope for scalability of production of YS/ hollow NPs. In addition, the facile control over chemical and physical compositions in the USP method makes it useful in the preparation of multicomponent or composite materials. For example, M/C (where M = multiple Sn, Pt, Ag, or Fe–FeO NPs) YS nanostructures have been prepared using ultrasonic spray pyrolysis of aqueous solutions containing sodium citrate and corresponding inorganic metal salts.136 Initially, the process involves the preparation of aqueous solutions contain-

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ing sodium citrate and inorganic metal salts used as a precursor. Then during the USP process, first inner nanoparticles are generated via reduction of metal salts with an organic reducing agent in the hot liquid droplets. The sodium citrate outer shell is formed due to the tendency of free sodium citrate molecules to move towards the periphery of the hot liquid droplets; then an outer shell (carbon) is formed via carbonization of the sodium citrate shell. Finally, the resultant M/C YS NPs are formed via removal of water-soluble by-products. In this approach, a carbon precursor/inorganic template composite is first formed, followed by carbonization, and then chemical leaching of the template material. Similarly, double shell SnO2 YS NPs were also synthesized by the decomposition of tin salt, and carbon was formed by the polymerization and carbonization of sucrose. Finally, after combustion of carbon layers at high temperature, a multi-shell YS nanostructure was produced as shown in Fig. 56(a).69 More complex structures such as binary (TiO2, TiO2/Al2O3), ternary (TiO2–Al2O3–ZrO2),


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Fig. 56 (a) Mechanism of the formation of the double-shelled SnO2 YS nanostructured powders. Reprinted with permission from ref. 69. Copyright 2013 from Wiley-VCH Verlag, (b) schematic diagrams of the formation of YS composite powders with quinary composition. Firstly, drying and decomposition of the droplets generated by the ultrasonic nebulizer in air produced the carbon–metal oxide particles. Further, sequentially combustion of the carbon–metal oxide powder produced the metal oxide YS structure. Reprinted with permission from ref. 577. Copyright 2013 from Wiley-VCH Verlag.

quaternary (TiO2–Al2O3–ZrO2–CeO2), and quinary (TiO2–Al2O3– ZrO2–CeO2–Y2O3) multi-shell YS nanostructures were also synthesized by using different metal salt precursors with the ultrasonic spray pyrolysis process, where carbon was removed by calcination as shown in Fig. 56(b).577 3.4

The ship in bottle method

Generally, in the sacrificial template-mediated methods, a core particle is sequentially coated with a template and then the shell material. Further, the sacrificial template is removed by dissolution or calcination to obtain YS nanostructures.


However, core materials may be affected by the dissolution or calcination depending on the sensitivity of the material, and the number of steps also increases depending on the complexity of the structure as mentioned before. In contrast, many researchers have used the ship in bottle approach for the synthesis of YS nanostructures in recent years.122,151,498,501,502,578 This is a simple approach for the synthesis of YS nanostructures. In this method, the core material precursor is encapsulated inside the pre-synthesized hollow shells first and then core particles are generated inside the hollow shells through a chemical reaction or by the self-assembly process to

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Fig. 57 (a) An illustration of the in situ confined growth of Au nanorods from Au nanosphere seeds inside hollow mesoporous silica shells with corresponding TEM images. Reprinted with permission from ref. 498. Copyright 2013 from Wiley-VCH Verlag, (b) illustration of the ship-in-bottle synthesis procedure of YS NPs: (1) introduction of the iron nitrate solution into the hollow core by vacuum nanocasting, (2) calcination into hematite particles of the core, (3) the introduction of the furfuryl alcohol into the mesoporous channels, and polymerization, (4) carbonization of the polyfurfuryl alcohol, and the reduction of hematite particles into magnetic particles, (5) removal of the silica template. Reprinted with permission from ref. 151. Copyright 2009 from Wiley-VCH Verlag.

form a final YS structure as shown in Fig. 57. This approach can be used for many materials such as metals, metal oxides, polymers, drugs molecules, and proteins. For instance, Au nanorod/SiO2 YS nanostructures were synthesized through the ship-in-bottle method as shown in Fig. 57(a).498,578 The synthesis method was based on encapsulation of gold seed inside the silica hollow shell, where the hollow silica shell was dispersed in an aqueous solution of gold precursor (HAuCl4), and further reduced to the Au nanoparticle in the presence of an ice-cold aqueous NaBH4 reducing agent. The Au nanoparticles formed outside the hollow shells were removed by centrifugation with water and the remaining Au nanoparticles inside the hollow shells were used as seed to induce the growth of Au nanorods.498,578 Similarly, Fe2O3/C,151 Fe2O3/silica,151 Au/ silica,501 Pt/silica,502 and SnO2/SiO2 122 YS nanostructures were also synthesized by the ship-in-bottle process by encapsulating a metal precursor inside the pre-existing hollow shell and a subsequent reduction of the metal precursor.

4. Emerging properties with applications 4.1

Biomedical applications

In recent years, applications of nanoparticles are extended to almost all fields of science and engineering. Among several fields, applications of nanoparticles in different areas of biomedical fields are rapidly growing and show significant future challenges. In fact, applications of YS NPs are not an exception in this area. The advantages of YS NPs in different biomedical applications have been discussed separately in the respective material sections. Unlike other nanoparticles, multifunctional-

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ity can be developed in one particle by using the YS morphology, which is eventually very important for biomedical applications. In the case of YS NPs, their structure, tuneable interior void space, appropriate combination of the core and the shell, and multi-functionality make them superior for biomedical applications including targeted drug/gene delivery, controlled release, bio-imaging, diagnostic, therapeutic agent, biosensor, antimicrobial activity, and so on. The NP-based drug delivery technique is a promising treatment for many diseases and therapy in bio-medical science. In this regard, silica is the most popular material for drug delivery applications because of its good bio-compatibility and easy bio-functionalization ability.174 Because of these reasons, mostly silica-based YS NPs are used for drug delivery and drug carrier applications with an enhanced drug loading capacity.39,57,59,60,71,74–88 Additionally, the magnetic and optical imaging properties are also achieved in in vivo and in vitro biological specimens by encapsulating magnetic materials,71,75,76,81,83,88,89 fluorescent molecules60,71,80,82,84,87 and quantum dots77 within the hollow silica shells, where magnetic NPs are used as MRI agents, and fluorescent dye or quantum dot NPs are used as optical agents. In some cases, dual imaging is also used for simultaneous magnetic and optical imaging.307,528 In the field of drug delivery, magnetic/ silica YS NPs have tremendous advantages for magnetic fieldinduced drug targeting or triggering86 and for use as an MRI agent363 because of additional magnetic properties and functionalities. Under this class, Fe3O4/SiO2 YS NPs have been reported for the drug delivery applications, where both drug loading capacity and a significant magnetization strength make it suitable as a multifunctional drug carrier.81,87,88 The FePt/CoS2 and FePt/Fe2O3 YS NPs were also reported as poten-


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tial nanostructured materials for anticancer drug delivery with MRI functionality; in this case, the presence of Pt acts as a potential anti-cancer drug like the well-known cancer drug cisplatin ( platinum-containing anti-cancer drug).579 In the case of the acidic environment of the secondary lysosomes, FePt cores are oxidized to metal ions in the presence of O2 inside the cells, and the unprotected iron promotes the disintegration of FePt to release platinum ions (Pt2+). The permeability of shells allows Pt ions to diffuse to the nucleus and mitochondria of the cell to damage the DNA helix chains and lead to the apoptosis of the cell as shown in Fig. 58(a).38,363 Apart from magnetic oxides, noble metal NPs have also been reported for bio-medical applications including optical imaging functionality and therapeutic agents. In particular, anisotropic noble metals are good options for the therapeutic agent. The plasmonic resonance of anisotropic noble metals can be tuned to the NIR region of the electromagnetic spectrum, where light is converted to heat under the irradiation of laser light. This thermal or heat energy can heat the surrounding environment, which can either be used for stimulating the encapsulated drug218 or photothermal therapy for cancer treatment.212 The gold nanoshells212 or nanorods288 could be the best option for therapeutic agent carriers because of their tuneable SPR properties towards NIR absorbance, which can act as a photo-thermal agent for localized hyperthermia cancer therapy. Additionally, the combination of materials such as magnetic, metal oxides, and polymers with a noble metal would provide many advantages in biomedicine. For this purpose a variety of YS NPs have been reported, where Au exhibits NIR absorption either as a core or a shell with multifunctionality.212,218,288

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Recently, smart stimuli-responsive polymer based YS NPs have been reported as a specific multifunctional drug delivery system, where the release of drug molecules can be triggered in the targeted organs and tissues with the desired parameters such as pH, temperature etc.58,218,454,465,467,476,483 For example, the Fe3O4/SiO2 YS NPs are particularly suitable for drug carriers. When the silica shell is functionalized with the carboxylic group, the NPs can easily react with the amine groups of DOX drug molecules and form amide linkers, which act as a pH triggered switch as shown in Fig. 58(b). At low pH, they released drug molecules into the solution. On the other hand, the superparamagnetic core responds in the presence of an external magnetic field, which provides an additional advantage for biomedical applications.58,180,454 Similarly, polymeric ( polymer/polymer) YS NPs were also reported for both pH and temperature stimuli-responsive drug delivery applications.476 On the other hand, the Co/Au290 and Fe3O4/SiO2 580 YS NPs were also reported for non-viral gene delivery to target the HeLa cells with optical imaging and magnetic tracking. 4.2

Antimicrobial applications

Bacterial infections are a major problem in the textile industry, hospitals, water disinfection, medicine, and food packaging,581,582 which in turn led to many serious human diseases and effects on human health. The YS NPs are also providing an effective solution for antibacterial activity with multi-functionality. As an example, multifunctional Ag/Fe2O3 YS NPs conjugated with glucose are effective for bacterial capturing, killing, and elimination applications, where the magnetic shell allows magnetic removal of attached bacteria from the media and the silver nano core helps to kill the pathogen.364

Fig. 58 (a) The cytotoxicity mechanism of FePt/CoS2 YS NPs for HeLa cells. Reprinted with permission from ref. 38. Copyright 2007 from American Chemical Society, (b) conjugation and release scheme of DOX molecules from pH stimuli-responsive YS NPs. Reprinted with permission from ref. 180. Copyright 2011 from American Chemical Society.


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Similarly, the combination of Ag and TiO2 as the YS structure exhibits a strong bactericide property in the presence of both light and dark conditions.385


4.3

Catalysis

Catalysis is an evergreen field for many chemical industries for the synthesis of different chemicals. So far, many strategies such as metal support on metal oxide, noble metal alloy, and core/shell structures have been developed for high performance chemical, photocatalysis, and electrocatalysis applications. Transition metal NPs such as Au, Ag, Pd, Pt, and Ni are well known because of their catalytic properties. Pure metal NPs may improve the catalytic performance to some level, but cannot avoid the problems of aggregation, chemical/ thermal stability, and separation after the completion of reactions for reuse. To solve these problems, researchers have encapsulated many different materials such as metal NPs (Au, Ag, Pt, Pd, and Fe), metal oxides (SiO2, TiO2, ZrO2, CeO2), and polymers in the form of YS structures to combine the individual properties of core and shell materials, which exhibit collective and synergistic effects to improve the catalytic efficiency.536 The YS structures show improved catalytic performance because of the following advantages: (i) the movable cores exhibit a high surface area and more active sites because of the presence of an unblocked surface, (ii) the porous shell prevents core NP agglomeration, provides thermal/chemical stability to the core, and acts as a selective membrane for the diffusion of reactants inside the YS, (iii) the void space accommodates more reactants inside the YS structure,55 (iv) encapsulation of noble metals in the mesoporous metal oxide shell acts as a support catalyst,42,360 and (v) replacement of single noble metals with multi- or alloy noble metals.110,508 The unique properties of YS have great importance in catalytic applications, which will be discussed below for different catalytic systems. 4.3.1 Chemical catalysis. In the case of nanoparticle based heterogeneous catalysis, different oxidation and reduction reactions such as the catalytic reduction of nitro compounds, oxidation of alcohols and CO have been employed as model reactions for the determination of catalytic activity. Generally, Au based YS NPs are used for the reduction of p-nitrophenol,42,50,56,70,90–99 2-nitroaniline,55,220,503 nitrobenzene;98 and catalytic oxidation of o-phenylenediamine,498,578 CO,51,52,177,365,368,370,387 and alcohols.473,508 Similarly, Ag based YS NPs are also used in the reduction of the nitro compounds.463 The Pd base YS NPs also exhibit high activity in Suzuki-coupling reaction178,531,547 and oxidation of alcohols.299 The Ni metal based YS structures are also used for methane reforming209 and hydrogen transfer for ketone179 reactions, which is advantageous over the expensive metal NPs. Similarly, Au NPs encapsulated inside metal oxides such as ZrO2, SiO2, CeO2, TiO2 etc. have been used as high temperature or sinter stable catalytic reactions such as hydrogenation and oxidation–reduction, where a metal core catalyst exhibits high catalytic activity. The incorporation of metal NPs inside the stimuli-responsive polymers has also been reported. Such YS

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structure’s catalytic properties could be tuned by the stimuliresponsive polymer functionality.322,467 4.3.2 Photocatalysis. Semiconductor metal oxides such as TiO2, ZnO, CeO2 etc. have been widely used for photocatalytic applications to degrade organic pollutants and for H2 production. The metal oxide/metal oxide combination of YS structures exhibits an enhancement in photocatalytic efficiency because of multiple light scattering in void space and a high available surface area.100–106 Interestingly, plasmonic metal encapsulation in semiconductor metal oxides called plasmonic catalysts has also been explored for enhanced photocatalytic applications, and show better absorption of visible light and the charge separation property.360,362,366,398,507,583 Similarly, some elements (Bi, Eu, N, Ag) encapsulated in semiconductor metal oxides also help in the enhancement of visible light absorbance and in the increase of charge carrier lifetime.308,371,433,507 Plasmonic metal/semiconductor metal oxide YS structures have been extensively employed for photocatalytic degradation of organic pollutants and H2 production. The magnetic material encapsulation inside the semiconductor metal oxide provides additional magnetic separation properties in the presence of an external magnetic field, which provides recyclability of the catalysts.154,490 4.3.3 Electro catalysis. So far, platinum has been mostly used as an electrocatalyst because of its superior catalytic activity and long-term operation stability. However, its expensiveness and poor CO-tolerance are the major concern in practical applications. Recently, intensive efforts have been devoted to the synthesis of bi-metallic or metal oxide YS electrocatalysts for fuel cells107–110 and ORR reactions.109,166,167 In comparison with the conventional electrode, the YS structure can provide high electrocatalytic activity, enhance the CO-tolerance, and also reduce the loading of a Pt catalyst. There are many reports on bimetallic YS NPs with remarkable activity as an electrocatalyst for the oxidation of methanol to CO2 at low temperature. For example, Au/Pt YS NPs exhibit superiority in catalytic activity compared to Au/Pt core/shell, hollow Pt, solid Pt nanoparticles, commercial Pt/C, and Au nanoparticles because of bimetallic synergetic effects and the presence of void space.108

4.4

SERS

The SERS has been one of the most powerful analytical techniques for sensitive and selective identification of molecular species since its discovery in the late 1970s.584–589 In recent years, many advanced noble plasmonic metals have been reported for biological and chemical SERS sensing, where the binding agent can be sensed by the change in the local refractive index around the NPs and the shift in the LSPR wavelength. Among all noble metals, Au, Ag, and Cu have attracted more attention in optical sensors because of their biocompatible nature and strong excitation of light in the visible to near infrared region because of the excitation of their plasmon oscillation.


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Generally, plasmonic properties of noble metal NPs are affected by the change in size or shape of the metal nanoparticles. So, when the nanoparticles are agglomerated in the suspension, it can affect the signal and intensity of SERS.234 In that case, to maintain the electromagnetic properties of metals and prevent aggregation of nanoparticles, the YS nanostructure with a noble metal core and a porous protective shell is a useful approach. Here, noble metal/silica YS NPs provide many advantages such as optical transparency, reduced electromagnetic coupling between the metallic cores, decreased oxidation rate of noble metals, porous permeable nature of the shell to allow reactant molecules inside, and protection of the core from degradation under harsh environmental conditions. The outer protective layer also prevents the adsorption of the unwanted molecules on their surface or extra signals in the spectrum.90,237,590 More interestingly, metal/SiO2 YS structures can produce active and stable SERS signals. For example, Au/ SiO2 YS NPs demonstrate good optical stability by the addition of small amounts of quinolone for SERS.237,590 Besides silica, many polymers and metal oxides are also used as shell materials, but silica provides more advantages because of its easy tuneability of thickness compared to other oxides or polymers. 4.5

Lithium batteries

Lithium batteries are presently the most dominant power source used to run small electronic devices in our daily life because of their high energy density. Initially, the Li–S battery was considered a promising candidate as a power source since 1960 after its discovery. However, since 1990, Li-ion batteries have dominated the battery market because of their improved stable electrochemistry and longer life span over Li–S batteries. However, the limited electrochemical stability of the electrolyte makes it difficult to increase the cathode operating voltage and capacity. Thus, research interest in Li–S batteries is rising again because sulphur as a cathode has a 5 times higher theoretical capacity than existing commercial graphite materials and has the capability of accommodating more ions.357,376,591–595 In recent years, extensive efforts have been devoted to the design of YS NP-based electrode materials in lithium batteries.49,64–69,72,118–146 With the unique structure and composition, YS structures have also shown dominant importance in the field of Li batteries because of their low pulverization and good conductivity compared to pure NPs, core/shell, and hollow NPs. The YS structures show an overall improved performance of lithium batteries because of the following advantages: (i) the core of such structures improves the rate capability as well as the energy density of the powders by increasing the weight fraction of the electrochemically active component, (ii) the void space between the core and shell can also serve as a buffering space for the electroactive core material during lithium insertion/extraction, which improves the cycling performance of the battery, (iii) the hollow conductive shell provides electrical conductivity and shows good elasticity to accommodate the effective strain of volume changes


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during Li+ insertion/extraction, (iv) the large surface area and lower diffusion distance, and (v) the hollow shell protects the core from outside environmental changes and agglomeration. Thus, the YS provides a suitable and advanced electrode material for lithium battery applications, and exhibits improved cyclic capacity and energy density. As a result, many combinations have been tried as anode material for Li-ion batteries. For example, Si has been widely used as one of the promising candidates for Li-ion battery anode material because of its high theoretical specific capacity (4200 mA h g−1). However, its low conductivity and fast decay in the cycling test due to large volume expansion (300%) during lithiation/delithiation limits its applicability in batteries. The unique Si/C YS structure with well-defined void space exhibits a high capacity (2800 mA h g−1 at C/10), a long cycle life (74% capacity retention after 1000 cycles), and a high columbic efficiency (99.84%) because of the presence of conductive carbon coating and freely expanding Si core. 4.6

Microwave absorber

Microwave absorber materials have been used in military applications for several decades for EMI reduction, antenna pattern shaping, and radar cross reduction. More recently, with the demand for wireless electronics and the movement to higher frequencies, microwave absorbers are used to reduce electromagnetic interference (EMI) inside wireless electronic assemblies. Generally, microwave absorber materials either convert the EM energy into thermal energy or dissipate EM waves through interference.162,596 Mostly, magnetite (Fe3O4) NPs are used as microwave absorber materials because of their magnetic properties, low cost, and strong absorption characteristics. Recently, many efforts have been made to design YS NPs for microwave absorber application,161–164 which exhibit improved magnetic and dielectric loss properties. The YS NPs exhibit unique properties such as low density, high surface area, and synergistic effects of movable core and shell. Mostly, the magnetic core and dielectric shell YS NPs have been reported with excellent microwave absorption performance, which showed lower reflection loss and a wider absorption frequency range.161–165 For instance, Fe3O4/TiO2 YS NPs have been reported as an attractive material for microwave absorption, where the maximum reflection loss value of YS can reach −37.6 db at 7 GHz with a thickness of 2 mm, compared to −10.5 db at 7 GHz with the same thickness of pure Fe3O4.162,163 The microwave absorption properties can also be tuned with different core sizes, interstitial void volumes, and shell thicknesses of YS NPs.162 Similarly, Fe3O4/SnO2 YS NPs exhibited the maximum reflection loss value of −36.5 db at 7 GHz at a thickness of 2 mm.161 4.7

Separation processes

Magnetically driven YS NPs exhibit advantages in the field of adsorption and separation because of a high surface area and a unique structure.57,59,100,147–158 Generally, most adsorbent shells are reported with magnetic cores, where the high surface area shell exhibits very good adsorption behaviour and

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the magnetic core helps in the separation of YS NPs from the solution phase. In this case, YS with metal oxide and carbon shells are reported as good adsorbents. Compared to other shells, carbon exhibits much higher capability for adsorption application because of its better stability in acid or alkaline media. Additionally, the mobility of magnetic NPs enhances the recyclability efficiency of adsorbents. For example, a very high adsorption capacity of bilirubin (146.5 mg g−1) was reported using iron oxide (Fe3O4/γ-Fe2O3)/C YS NPs. Similarly, Fe3O4/C YS NPs also exhibit excellent reusability for the adsorption of pyrene with high adsorption capacities (77.1 mg g−1).152 4.8

Gas sensors

There has been increasing attention on gas sensors in recent years because of growing awareness on environmental pollution. It has been found that significant efforts are being paid to design YS NPs for gas sensor applications with ultra-high sensitivity, selectivity, response, stability, and reproducibility. The high surface area of NPs is advantageous for gas sensing and is widely used for industrial applications. Mostly, the metal oxide semiconductor NPs are used as gas sensors based on the working principle of the change in electrical conductivity after the adsorption or desorption of target gas molecules. The YS morphology provides more surface area for adsorption of target gases, which improve the sensitivity, selectivity, and response. The semiconductor metal oxide YS

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NP-based gas sensors can be categorized into two groups based on their sensing mechanism: (i) semiconductor YS gas sensors, where multishell layers of metal oxide provide a high surface area for the sensing of the target gas, (ii) catalytic driven oxide gas sensors, where metal NPs are encapsulated inside the oxide shell to provide additional sensitivity because of the catalytic effects of metal NPs. Among these sensors, only semiconductor oxide-based sensors are widely used for gas sensing applications, which are made from similar or different metal oxide materials.63,115 The multishell structure of YS provides more sites for target molecule adsorption, which enhances the sensitivity and response of the sensor. The combination of metal with metal oxide gives new strategies for the design of advanced high-performance gas sensors with improved selectivity and sensitivity based on catalytic promotion. Particularly, the multishell layers of oxide prolong the retention time of the target gas molecule within the shell, which also helps in catalytic dissociation.116 The Pd/SnO2 YS NPs are reported as useful methyl benzene sensors (xylene and toluene vapor), where Pd loading enhances the gas response either by chemical sensitization (catalytic promotion) or electronic sensitization (decreasing the electron concentration in the sensing material).597 Similarly, Au/SnO2 YS NPs exhibit lower detection (3 ppm), faster response (0.3 s) and better selectivity towards CO sensing because of the catalytic effect of Au and enhanced electron depletion at the surface of the

Fig. 59 (a) Schematic illustration of multiple scattering and reflection of light inside void space, (b) diffuse reflectance spectra, (c) I–V characteristics, (d) IPCE spectra of DSSC photoanode composed with YS, NPs and P25 TiO2, respectively. Reprinted with permission from ref. 599. Copyright 2012 from the Royal Society of Chemistry.

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YS.111 Many other YS NP-based gas sensors are also reported for H2S,112–114 CO,111 ethanol,63,115 H2O2,116 and acetone117 gas sensing.


4.9

Dye-sensitized solar cells (DSSC)

The DSSC has been an attractive field of research for a new generation of photovoltaic devices because of its low cost and easy fabrication since 1991, when it was first reported by Michael Gratzel.598 The DSSC is considered a potential alternative renewable energy source in place of expensive siliconbased solar cells. The DSSC is a third generation photovoltaic solar cell that converts visible light into electrical energy. In the DSSC, the dye molecules serve as a photo-sensitizer, in which a photoexcited electron is injected into a wide band-gap semiconductor and conducted away by the semiconductor to the electrolyte and finally the electron comes back to the dye. The movement of the electron in the circuit creates energy which can be harvested by the battery or the super-capacitor. A photoanode composed of semiconductor NPs is the heart of the DSSC, which requires a high surface area for adsorption of more dye molecules. Surface area, electron transport and lightharvesting properties are important parameters to achieve high efficiency of DSSC. The unique YS structure helps to improve the efficiency of DSSC by enhancing both the surface area and light-harvesting properties.43,159,160,434 The multifunctional YS NP-based photoanode offers two possible ways to enhance the efficiency of DSSC: (i) a high surface area for a higher adsorption capacity of dye molecules and (ii) enhanced scattering of incident light. For example, the improved light harvesting and scattering properties of anatase TiO2 YS microspheres are attributed to the superior IPCE (60.4%) efficiency in visible light compared to simple TiO2 NPs (24.4%) and P25 (44.6%) as shown in Fig. 59. Similarly, TiO2 YS NPs also show higher overall photoconversion efficiency (6.01%) compared to those of TiO2 NPs (4.01%) and P25 (4.46%).599

5. Concluding remarks and future perspectives This review highlights a specific class (YS) of nanoparticles, which has become an eminent research area during the last one decade. The idea of this nanostructure was developed from the year 2002 onwards to obtain the distinct advantages of hollow and core/shell structures in the same structure. The area has been flourishing at an incredibly fast rate and already proved the breakthrough performances by the reported studies in almost all important areas of science and technology such as biomedical, catalysis, optical, energy storage, sensors, electronics, and so on. During the initial period, the research has mostly focused on the development of several YS structures, characterization, and properties of the developed materials; however, in recent years the studies have focused mostly on the applications to support the best performances over the contemporary nanostructures. So far, many attempts have


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been made to develop new properties by tuning several physicochemical parameters such as shell thickness, core size, void space, shapes of core and shell, the number of cores and shells, and combinations of materials. With the rapid progress of our modern lifestyle, the demand for several advanced materials with improved properties is limitless; in this respect, we hope that the YS nanostructures will contribute significantly to the development of the next generation of advanced materials. This review presents a state-of-the-art knowledge of YS nanostructures by covering all possible types of structures reported so far. The in-depth classifications based on the structures and materials along with their properties and applications would be able to draw a complete picture of this area. The literature reports cited in this review confirm that among the different types, single core/shell YS NPs are extensively studied probably because of their simple structure. Depending on the studied materials, the single core/shell YS NPs are classified into inorganic/inorganic (metal/silica, metal oxide/silica, metal/metal, metal/carbon, metal oxide/carbon, metal/metal oxide, metal oxide/metal oxide), inorganic/ polymer (metal/polymer, metal oxide/polymer), and polymer/ polymer. The inorganic/inorganic YS NPs are extensively studied because of the wide range of applications of inorganic materials. Specifically, metal (magnetic and noble) cores encapsulated inside the silica shells are extensively used for biological and catalytic applications in order to enhance the drug loading capacity, functionality, and biocompatibility. The optical and magnetic properties of metals are also improved by coating with the silica shell. Similarly, the magnetic oxides with silica shells are also used in bio-medical and microwave absorption applications for their improved magnetic, dielectric, and biocompatibility properties with multifunctionality. The non-magnetic metal oxides with silica shells are also used for catalytic and drug delivery applications. Metal/metal YS NPs show improved catalytic and optical properties, which are useful for the catalysis of different chemical reactions, sensors, and bio-medical applications. The cores made of inorganic materials including metals and metal oxides encapsulated inside the carbon shells are useful in the potential applications of catalysis, bio-medical, adsorption/separation, and electrodes of lithium batteries. Similarly, metal cores encapsulated in metal oxides are also useful for many applications such as antibacterial activity, catalysis, bio-medical, and sensors. The metal oxide YS NPs are also an important class, which are useful in the fields of gas sensors, catalysis, lithium batteries, DSSCs, microwave absorption, and bio-medical applications. Compared to inorganic/inorganic combinations, the polymeric YS NPs are less studied. In this category, some inorganic/polymer YS NPs are important in drug delivery and catalysis applications. Among the most studied polymeric YS NPs, the use of stimuli-responsive polymers as either a core or a shell is very important. In this class, the functionality or response of the stimuli polymers are controlled to obtain the desired properties by changing the surrounding local environment. In addition, some biocompatible polymers are also used

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as shell for drug delivery applications. Among different inorganic cores, metals (Au, Ag) and metal oxides (Fe3O4, SiO2) are most studied because of their optical, catalytic, magnetic, and multi-functional properties, while encapsulated by the polymeric shell. Under the class of inorganic/polymers, the magnetic cores with polymeric shells are useful for in vivo biomedical applications. Comparing all magnetic materials, superparamagnetic iron oxides are preferably used for bioimaging and drug delivery applications. Similarly, the polymer/polymer YS NPs are also reported for promising applications in drug delivery and catalysis. The developments are not only limited to single YS structures but also extended to complex YS structures with multicores and multihollow shells, whose structures also show much improved unique properties in different specific applications with multiple functionalities. Specifically, multicores/ single shell and multicores/shell YS NPs are promising candidates in catalytic and energy storage applications because of their multi-functionality behaviour with different materials. On the other hand, the multihollow shells in shell structures have also shown better applicability in controlled drug delivery, DSSC, lithium-ion battery electrodes, and photocatalysis because of the presence of lots of void space between shells and high surface area. In addition to multiple shells in the shell structure, a single core with multishells also shows multiphase heterogeneous interfaces and different functionality behaviour with the above mentioned properties. As shape dependent properties of nanomaterials are also extremely important in several applications, YS structures are also developed with various shapes. The non-spherical YS structures are synthesized with a variety of shapes, especially noble metals and metal oxides are extensively studied under this class because of their size and shape controllable properties. Anisotropic features in YS NPs make them a better candidate for tuning the optical, magnetic, and catalytic properties. The surface plasmon resonance of non-spherical noble metal YS NPs are used in diagnostic and analytical tools such as SERS. In general, sacrificial template and galvanic replacement methods are used to prepare anisotropic YS NPs. This review also attempts to summarize the synthesis mechanisms based on the available literature. Several synthesis strategies including different sacrificial templates, galvanic replacement reactions, Kirkendall reactions, spray pyrolysis, ship-in-bottle, and Ostwald ripening are important. In general, a better understanding of the synthesis methods helps to design YS NPs through a very easy route that have well controlled size and shape. With the progress of synthesis routes of NPs of well controlled size, shape, and void space, the applications are also diversifying in multiple dimensions including bio-medical, DSSCs, catalysis, environment remediation, microwave absorption, sensors, and lithium batteries, which are also discussed in brief. The multifunctional YS NPs are used as drug carriers, image contrast agents, and therapeutic agents. Similarly, the improved light harvesting, enhanced surface area, and larger electron lifetime properties of YS NPs are useful in the fields of photocatalysts and photo-

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electrodes of DSSCs. The higher surface area and superior catalytic properties of these materials make them suitable for sensor applications with better sensing ability and good selectivity. The YS NPs act as a good catalytic reactor, and also act as a sinter stable catalyst, and prevent the agglomeration problem with stability (thermal and oxidative). For energy storage applications, these YS NPs exhibit good electrochemical performance as an electrode material for lithium batteries in terms of high capacity and stability for lithium storage mainly because of the unique structure and the presence of void space. While analyzing the future perspectives in the light of the present review, it has been found that researchers from a wide range of science and engineering areas have already proved from their laboratory scale studies that the YS NPs seem to be much better than the core/shell and hollow nanoparticles. As the area is only a decade old with a significant number of high-standard publications, it is expected to have much more breakthrough development in the future. There is plenty of room available to develop new synthesis routes for easy synthesis of the YS structures, their suitable materials combinations, and breakthrough applications. The laboratory scale findings are not the ultimate fate of any new research, unless it is used on an industrial scale for the final product to be utilized in practical applications. In the coming years, it is expected that some materials should reach that stage. There is much scope for different areas of bio-related applications such as pharmaceutical products, diagnosis, sensors, and treatment. Some of the exciting materials should at least reach the clinical trial stage to obtain the final approval of bio-medical applications. We expect that the YS NPs would play a key role in saving human life in the future.

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