Nanoscale

Volume 6 Number 22 21 November 2014 Pages 13257–14004

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ISSN 2040-3364

PAPER Y. Huttel et al. The ultimate step towards a tailored engineering of core@shell and core@shell@shell nanoparticles

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Cite this: Nanoscale, 2014, 6, 13483

The ultimate step towards a tailored engineering of core@shell and core@shell@shell nanoparticles† D. Llamosa,a M. Ruano,a L. Martínez,a A. Mayoral,b E. Roman,a M. García-Hernándeza and Y. Huttel*a Complex core@shell and core@shell@shell nanoparticles are systems that combine the functionalities of the inner core and outer shell materials together with new physico-chemical properties originated by their low (nano) dimensionality. Such nanoparticles are of prime importance in the fast growing field of nanotechno-

Received 27th May 2014, Accepted 5th August 2014

logy as building blocks for more sophisticated systems and a plethora of applications. Here, it is shown that

DOI: 10.1039/c4nr02913e

although conceptually simple a modified gas aggregation approach allows the one-step generation of wellcontrolled complex nanoparticles. In particular, it is demonstrated that the atoms of the core and the shell

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of the nanoparticles can be easily inverted, avoiding intrinsic constraints of chemical methods.

Introduction Nanoparticles (NPs) hold a special place in nanoscience and nanotechnology not only because of their particular properties resulting from their reduced dimensions, but also because they are promising building blocks for more complex materials in nanotechnology.1 Numerous examples have been reported of NPs with new magnetic,2–5 catalytic,6,7 magneto-optical or optical8–10 properties, etc, that differ from the bulk materials. Because of these new properties, NPs have been conquering increasing room in our lives; in 2010, more than 1000 products containing nanoparticles were commercially available.11 However, there is an increasing concern associated with the reproducible synthesis of nanomaterials.12 Within such a general frame, it appears that the development of fabrication methods for high quality NPs is a key issue to follow the increasing demand of complex multifunctional nanoparticles for advanced applications.13 Since the initial work of Haberland et al.14 the gas aggregation sources or ion cluster sources (ICS) have been modified in several different ways. Further information on the ICS can be found in the literature.15–17 The

a Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas (CSIC), C/Sor Juana Inés de la Cruz, 3, 28049 Madrid, Spain. E-mail: [email protected] b Laboratorio de Microscopías Avanzadas (LMA), Instituto de Nanociencia de Aragón (INA), Universidad de Zaragoza, c/Mariano Esquillor, Edificio I+D, 50018 Zaragoza, Spain † Electronic supplementary information (ESI) available: 1: Scheme illustrating the different strategies to grow nanoparticles with controlled chemical composition and structure. 2: Examples of TEM results on Ag@Au nanoparticles. 3: Magnetic measurements results on Co@Au and Au@Co nanoparticles. See DOI: 10.1039/c4nr02913e

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recent development of the multiple ion cluster source, MICS,18 opens a new range of possibilities because it is possible to grow complex NPs with adjustable chemical composition and size.19 Here, we show that the MICS represents a powerful tool to generate high quality complex NPs in a single step. In particular, we show that in addition to the possibility of growing alloyed NPs with controlled stoichiometry,19 the MICS can be used for the one-step generation of core@shell (CS) and core@shell@shell (CSS) nanoparticles. The proposed fabrication method allows the fine tuning and controlling of the structure and chemical composition that will define the properties and the functionalities of the NPs.

Experimental The nanoparticles were grown in a MICS, where the 3 magnetrons were loaded with high purity Co, Au and Ag targets.20 The MICS was connected to an UHV chamber with base pressure in the middle 10−9 mbar, where the NPs were deposited on TEM carbon coated grids. The magnetron efficiencies in terms of the number of nanoparticles per surface area per time were determined by AFM analysis, as described elsewhere.21,22 The working parameters of each magnetron (argon flux, applied power and distance from exit slit) were adjusted to produce the density of ions required to form the cores and the shells of predefined dimensions. Typically the applied powers were in the range of 4–10 Watt, the argon flux in the range of 5–40 sccm (standard cubic centimeter per minute) and the distances of the magnetrons from the exit slit in the range of 75–160 mm. More information regarding this new type of gas aggregation can be found in ESI 1.† Electron

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microscopy measurements were performed with a FEI-TITAN X-FEG transmission electron microscope used in STEM mode and operated at 300 kV and 120 kV. The images were acquired using a high angle annular dark field (HAADF) detector. In addition, the microscope was equipped with a monochromator, Gatan Energy Filter Tridiem 866 ERS, and a spherical aberration corrector (CEOS) for the electron probe, allowing an effective 0.08 nm spatial resolution. The column was also fitted with energy dispersive X-ray spectroscopy (EDS).

Results and discussion The versatility of the MICS is illustrated in Fig. 1, where it is shown that the combination of 3 elements (here Au, Ag and Co) is likely to lead to 25 different NP structures of interest. The combination of applied powers and positions of the magnetrons allows the generation of five major types of clusters in one single step under UHV conditions, namely α, β, χ, δ, ε, corresponding to single element NPs, homogeneously alloyed NPs with controlled stoichiometry, single core@shell NPs, complex core@shell NPS and core@shell@shell, respectively (see ESI 1†). ε NPs are composed of a core and 2 shells while NPs of type δ are formed by core (shell) resulting from the combination of the elements of 2 magnetrons and a shell (core) from the third magnetron. Although the formation of single element NPs (α type)21,22 is straightforward in gas aggregation sources and the formation of homogenously alloyed NPs (β type) using a MICS has been recently reported,19 more complex core@shell structures (χ, δ, ε types) have not been reported so far using MICS.

Fig. 1 Illustration of the structures that can be grown using a multiple ion cluster source equipped with 3 magnetrons. Note that in the case presented here, elements are single metal elements Au, Ag and Co. Replacing each element by complex alloys would allow the generation of more complex NPs.

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Designing and tailoring core@shell NPs (CSNPs) are key issues that lead to the generation of multifunctional NPs that combine the functionalities of individual elements into a single NP.23 To the best of our knowledge, the combination of more than 2 elements in an aggregation zone has been reported a limited number of times in the literature.12,19,24 Here, it is shown that χ and ε type NPs can be grown and the test elements are Co, Ag and Au. It is demonstrated that with 2 pairs of elements CSNPs can be generated in a single step using a MICS. Furthermore, it is demonstrated that inverting the positions of the magnetrons into the aggregation zone of the MICS results in the generation of the inverse CSNPs. In addition, we show that the smart combination of 3 elements leads to the formation of a core@shell@shell structure (CSSNP) with a well-defined core and 2 concentric shells. Fig. 2 presents the first example of CSNPs with an Ag core and an Au shell (Ag@Au) generated with the Ag magnetron placed behind the Au magnetron. The Ag seed NPs are then covered by Au in their way through the aggregation zone, resulting in the formation of an Ag@Au structure. Note that the formation of Ag@Au NPs in UHV would lead to non

Fig. 2 Core@shell Ag@Au and Co@Au nanoparticles. (a) Cs-corrected STEM image of a CSNP with a core of 5.6 nm Ag and a shell of 1.2 nm Au. The brighter Au planes in the outer shell are clearly distinguished form the Ag core. (b) Confirmation of the CS structure directly given by the EDS line scans performed along the line displayed in (a). (c) Cs-corrected STEM image of a CSNP with a core of 3.1 nm Co and a shell of 1.4 nm Au. (d) EDS line scans performed along the line displayed in (c).


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oxidized Ag due to the Au protective shell, i.e., preserving the properties of Ag. Fig. 2a displays a representative spherical aberration (Cs) corrected high-resolution scanning transmission electron microscopy (HRSTEM) image of an Ag@Au NP with icosahedral morphology. Although most of the NPs of the deposit presented the desired Ag@Au structure, it is worth mentioning that NPs with other structures ( pure Au, pure Ag and AuAg alloy) were detected (cf. ESI 2†). The mean size of the NPs is quite homogeneous (Fig. S2.1†); however, if further size control is desired, they could be mass selected using a quadrupole mass filter (not implemented here). This observation is extensive for all the structures presented in this work. The energy dispersive X-ray spectroscopy (EDS) line scan displayed in Fig. 2b clearly highlights the CS structure of the NP, where the Ag atoms are confined in the core and the Au atoms surround it forming a thin shell of 1.2 nm. Note that despite the miscibility of Ag and Au, a sharp interface between both elements is observed. Another example of the one-step generation of CSNPs is illustrated in Fig. 2c and d for Co core and Au shell (Co@Au). Fig. 2c shows a representative Cs-corrected HRSTEM image of a Co@Au NP. In this case, the composition of the nanoparticle is even more evident by simple observation due to the large Z difference between Au and Co. This CS structure is easily visualized in the EDS line scans displayed in Fig. 2d. As a general trend, the dimensions of the core and shell can be tuned through the power applied to the magnetrons, and hence the mass of the NPs can be adjusted in situ during the fabrication of the NPs. The magnetic properties of a multilayered Co@Au NPs deposit have been investigated using a superconducting quantum interference device (SQUID) magnetometer and displayed the expected behavior (cf. ESI 3†). In particular, the assembly of Co@Au NPs exhibited a low blocking temperature TB ≈ 3 K, which is in agreement with the small Co magnetic core (3.1 nm diameter) and the absence of exchange bias consistent with a uniform protective Au shell. As stated above, the versatility of the MICS also resides in the ability to generate the “inverse” structures of the CSNPs presented in Fig. 2. The Au@Ag and Au@Co NPs have been grown by inverting the positions of the magnetrons inside the aggregation zone. Fig. 3a displays a representative Cs-corrected HRSTEM image of the Au@Ag NPs. The crystalline Au core (with decahedral structure) appears brighter as a result of the higher scattering factor and the Ag shell is less bright due to the lower Z, where no crystalline order could be detected because of its oxidation during sample transfer from the MICS to the TEM. Similar results on the inverse CS Au@Co structure are presented in Fig. 3b. The crystalline Au core of less than 2 nm is clearly observed in the Cs-corrected HRSTEM image while the oxidized Co shell appears more crystalline than the ˉm space group symmetry of the Ag oxide, adopting the Fd3 Co3O4, observed for this particular case along the [211] orientation. Note that in these inverse CSNPs, the size of the Au core as well as the shells have been modified to illustrate the possibility to fine tune both the core size and the shell thickness of the nanoparticles.


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Fig. 3 Core@shell Au@Ag and Au@Co nanoparticles. (a) Representative Cs-corrected STEM image of a Au@Ag NP where the high crystalline nature of the Au core is distinguished by adopting decahedral shape. (b) Representative Cs-corrected STEM image of a Au@Co NP.

Fig. 4 Core@shell@shell Co@Ag@Au nanoparticles. (a) Representation of the complex Co@Ag@Au structure together with the expected EDS intensity profiles. (b) Cs-corrected STEM representative image of a Co@Ag@Au NP. (c) EDS line scan performed at the Co, Ag and Au, along the line depicted in (b). (d) EELS compositional analysis for the Co L3,2 edge. The dashed line represents the outer limit of the NP. (e) EELS map for the Ag M4,5 edge. (f ) STEM image together with the corresponding Co and Ag EELS concentration maps superimposed.

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The formation of the more complex coreCo@shellAg@ shellAu structure in one single step is also possible by placing, for instance, the Ag magnetron between the Co and Au magnetrons in such a manner that seeds Co NPs are first covered by Ag and subsequently “protected” by Au. This is illustrated in Fig. 4. Fig. 4a displays the representation of the expected CSS structure and its corresponding EDS analysis profile. The experimental results are presented in Fig. 4b–f. The Cs-corrected HRSTEM image is presented in Fig. 4b. The darker Co core is clearly distinguished at the center of the NP. However, for a neat identification of each part of such a complex structure we present the EDS line scans obtained for Co, Ag and Au along the line depicted in Fig. 4b. As it can be observed in Fig. 4c, the NPs present a clear Co core first surrounded by an Ag shell that is then covered by an Au shell. The CSSNP structure is further evidenced by the EELS maps performed at the Co-L3,2 (Fig. 4d) and Ag-M4,5 edges (Fig. 4e), which merged in Fig. 4f, provides the spatial distribution of the elements in the NP.

Conclusions In summary, an innovative method that enable the growth of complex spherical core@shell and core@shell@shell nanoparticles in a one step process using a multiple ion cluster source (MICS) has been presented. The inversion of the elements that constitute the core and the shell is performed by adjusting the relative positions of the magnetrons into the aggregation zone. Furthermore, core size and shell thickness can be tuned at the nano scale. Our proof-of-concept demonstrates that it is possible to grow high purity NPs with complex structures for future applications and to use the MICS as a tool for studying the fundamental properties of novel nanoparticles.

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Work was supported by the Spanish Ministerio de Ciencia e Innovación under projects MAT2011-29194-C02-02, MAT201127470-C02-02, CSD2007-00041 (NANOSELECT), and CSD200900013 (IMAGINE). D. Llamosa P. acknowledges financial support from Ministerio de Ciencia e Innovación under contract no. JAEPre-09-01925. M. R. acknowledges the FPI grant from the Spanish Ministerio de Economía y Competitividad. L. M. and E. R. acknowledge the Consejo Superior de Investigaciones Cientifícas (PIE 201160E085). The microscopy works have been conducted in the Laboratorio de Microscopias Avanzadas at Instituto de Nanociencia de Aragon – Universidad de Zaragoza, Spain. The European Union Seventh Framework Programme under grant agreement no. 312483 (ESTEEM2, Integrated Infrastructure Initiative) and no. 604391 (Graphene Flagship) is acknowledged.

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Electronic Supplementary Material (ESI) for Nanoscale. This journal is © The Royal Society of Chemistry 2014

Supporting Information 1 Scheme illustrating the different strategies to grow nanoparticles with controlled chemical composition and structure Daniel Llamosa Pérez, Manuel Ruano Díaz, Lidia Martínez, Alvaro Mayoral, Elisa Roman, Mar García-Hernández and Yves Huttel. The nanoparticles are generated using a so-called Multiple Ion Cluster Source. The generation process is illustrated in figure S1.1 where the 3 individual magnetrons are displayed together with their translators that allow the translation of the magnetrons inside the ultra-high vacuum aggregation zone.

Supporting information 1

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Figure S1.1. Schematic representation of the possible configurations of the magnetrons inside the Multiple Ion Cluster Source that allow the generation of nanoparticles of different types. The different strategies to grow complex NPs are also depicted. Nanoparticles of type α are those generated by a single magnetron while nanoparticles of type β are alloys formed by the elements generated by 2 or 3 magnetrons. ,  and δ nanoparticles are core@shell NPs that can be formed by the elements of 2 magnetrons () or 3 elements (, δ).  NPs are formed by a core and 2 shells while NPs of type δ are formed by core (shell) resulting from the combination of the elements of 2 magnetrons and a shell (core) from the third magnetron.


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Supporting Information 2 Examples of TEM results on Ag@Au nanoparticles Daniel Llamosa Pérez, Manuel Ruano Díaz, Lidia Martínez, Alvaro Mayoral, Elisa Roman, Mar García-Hernández and Yves Huttel. TEM measurements have been performed on nanoparticles deposited on ultrathin Carbon (< 3 nm) on carbon holey support films supported on Cu TEM grids. Representative TEM images are displayed in Figure S2.1.

a)

c)

b)

d)

Au

Ag@Au

Figure S2.1 Cs-corrected STEM-HAADF images of NPs Ag@Au recorded at different magnifications.


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In figure S2.1 a) and b) the lacey structure of the carbon support is clearly observed as well as the homogeneous nanoparticle distribution over the whole surface. The deposition time was 15 seconds and the deposition rate is approximately 2 NPs/µm2s. Such relatively low deposition rate is well suited for TEM grid deposition, but it can be increased several orders by changing the working parameters of the MICS. In figure S2.1 c) we have highlighted the pure Au nanoparticles by yellow squares. The pure Au nanoparticles can be distinguished from Ag@Au nanoparticles thanks to their higher intensity resulting from their higher density of electrons. High-resolution TEM inspection of pure Au and Ag@Au nanoparticles have confirmed that the nanoparticles could be recognized with modest magnification images as shown in the figure S2.1 c). At higher magnification, as displayed in figure S2.1 d) the distinction of pure Au NPs and Ag@Au NPs is even more straightforward since the bright shells of Au surrounding the Ag cores are distinguishable. Note that the size of pure Au NPs differs from the size of Ag@Au NPs as could be expected. Such size evolution as a function of NPs type allows the possibility to mass select the NPs using a quadrupole mass filter. Depending on the working parameters applied to each of the magnetrons (argon flux, position in the aggregation zone and applied power), different types and concentrations of nanoparticles were observed. The best case, 50% of the nanoparticles were Ag@Au as expected while the other nanoparticles were pure Au NPs (39%) and Janus type NPs (11%). We found that the proportions of nanoparticles with different structures were very dependent on the working parameters. As an example, we display in figure S2.2a, a high magnification image were the order of 16% of the nanoparticles were agglomerated and in figure S2.2b were 30% of nanoparticles displayed a Janus type structure. These different types of nanoparticles are obtained by modifying the working parameters of the MICS. A detailed study on the influence of the working


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parameters on the generation of different types of nanoparticles will be presented in a further contribution.

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Figure S2.2. a) Example of agglomerated and b) Janus type nanoparticles.


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Supporting Information 3 Magnetic measurements results on Co@Au and Au@Co nanoparticles Daniel Llamosa Pérez, Manuel Ruano Díaz, Lidia Martínez, Alvaro Mayoral, Elisa Roman, Mar García-Hernández and Yves Huttel. Magnetic measurements have been performed in a Superconducting Quantum Interference Device (SQUID) magnetometer from Quantum Design and equipped with a 5 Teslas coil. The samples consisted of multilayers of Co@Au and Au@Co nanoparticles in order to increase the magnetic response. The multilayers were deposited on Si(100) substrates and the diamagnetic contributions (in particular that coming from the Si substrates) have been carefully subtracted from the measured data. The magnetic field was applied parallel to the sample surface, and the samples were carefully demagnetized before measurements. In the case of field-cooled (FC) measurements the samples were cooled under a magnetic field of 1500 Oe. As expected, the assembly of Co@Au NPs exhibited a low blocking temperature TB ≈ 3 K (cf. Figure S3.1a)) in agreement with the small Co magnetic core (3.1 nm diameter). A ferromagnetic behavior was evidenced through the hysteresis loop measured at 2 K (cf. Figure S3.1 b)) where a coercive field Hc ≈ 120 Oe was observed. Also it can be seen in Figure S3.1 b) that the Field-Cooled (FC) hysteresis loop measured at 2 K is perfectly centered around zero magnetic field and hence revealing the absence of exchange bias. This is coherent with the presence of an uniform and protective Au shell that prevents the formation of cobalt oxide in contact with cobalt that would induce an exchange bias.

[1]

Note that the separation between the FC and ZFC branches in the

magnetization curves versus temperature (cf. figure S3.1a)) is at 10 K, i.e. at higher temperature than the blocking temperature. Such behavior could indicate that the NPs are still interacting despite the presence of the Au shells that keep the Co cores in non-contact.


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Figure S3.1. a) Magnetization versus temperature for an assembly of Co@Au nanoparticles. b) Field cooled hysteresis loop measured at 2 K. The results of the magnetic characterization of the inverse structure, i.e. Au@Co NPs are displayed in Figure S3.2. The measured magnetic properties are very similar to those reported by other authors on similar nanoparticle systems.

[2-3]

For this system, the

interpretation of the SQUID results is more complex since the Au@Co multilayer cannot be seen as an assembly of isolated NPs since the Co shells are in contact and, consequently, strongly interact rendering in magnetic patches with a larger effective diameter than that of a single shell. Note, that the Au@Co multilayer is rather close to a system where small Au nanoparticles are inserted into a porous cobalt matrix that can partially oxidise. Such


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oxidation is clearly observed throught the exchange bias effect measured in the field cooled hysteresis loop displayed in figure S3.2a. Note also that the magnetization curves are very different from the previous Co@Au multilayer. In particular there is no clear evidence of a blocking temperature that could be associated with the presence of nanoparticles.

a)

b)

Figure S3.2. a) Field cooled and zero field cooled hysteresis loops measured on a multilayer of Au@Co NPs. b) Corresponding field cooled and zero field cooled magnetization curves as a function of temperature.

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[3] T. Wen, K. M. Krishnan, J. Appl. Phys. 2011, 109, 07B515.


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