Ionic liquids for nano

CIS-01568; No of Pages 52 Advances in Colloid and Interface Science xxx (2015) xxx–xxx

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Historical perspective

Ionic liquids for nano- and microstructures preparation. Part 2: Application in synthesis Justyna Łuczak a, Marta Paszkiewicz b, Anna Krukowska b, Anna Malankowska a, Adriana Zaleska-Medynska a,b a b

Department of Chemical Technology, Faculty of Chemistry, Gdańsk University of Technology, G. Narutowicza 11/12, 80-233 Gdansk, Poland Department of Environmental Technology, Faculty of Chemistry, University of Gdańsk, Wita Stwosza 63, 80-308 Gdansk, Poland

a r t i c l e

i n f o

Available online xxxx Keywords: Ionic liquids Nanoparticle interaction Nanoparticle synthesis Nanomaterial synthesis Microstructure preparation

a b s t r a c t Ionic liquids (ILs) are widely applied to prepare metal nanoparticles and 3D semiconductor microparticles. Generally, they serve as a structuring agent or reaction medium (solvent), however it was also demonstrated that ILs can play a role of a co-solvent, metal precursor, reducing as well as surface modifying agent. The crucial role and possible types of interactions between ILs and growing particles have been presented in the Part 1 of this review paper. Part 2 of the paper gives a comprehensive overview of recent experimental studies dealing with application of ionic liquids for preparation of metal and semiconductor based nano- and microparticles. A wide spectrum of preparation routes using ionic liquids is presented, including precipitation, sol–gel technique, hydrothermal method, nanocasting and ray-mediated methods (microwave, ultrasound, UV-radiation and γ-radiation). It was found that ionic liquids formed of a 1-butyl-3-methylimidazolium [BMIM] combined with tetrafluoroborate [BF4], hexafluorophosphate [PF6], and bis(trifluoromethanesulfonyl)imide [Tf2N] are the most often used ILs in the synthesis of nano- and microparticles, due to their low melting temperature, low viscosity and good transportation properties. Nevertheless, examples of other IL classes with intrinsic nanoparticles stabilizing abilities such as phosphonium and ammonium derivatives are also presented. Experimental data revealed that structure of ILs (both anion and cation type) affects the size and shape of formed metal particles, and in some cases may even determine possibility of particles formation. The nature of the metal precursor determines its affinity to polar or nonpolar domains of ionic liquid, and therefore, the size of the nanoparticles depends on the size of these regions. Ability of ionic liquids to form varied extended interactions with particle precursor as well as other compounds presented in the reaction media (water, organic solvents etc.) provides nano- and microstructures with different morphologies (0D nanoparticles, 1D nanowires, rods, 2D layers, sheets, and 3D features of molecules). ILs interact efficiently with microwave irradiation, thus even small amount of IL can be employed to increase the dielectric constant of nonpolar solvents used in the synthesis. Thus, combining the advantages of ionic liquids and ray-mediated methods resulted in the development of new ionic liquid-assisted synthesis routes. One of the recently proposed approaches of semiconductor particles preparation is based on the adsorption of semiconductor precursor molecules at the surface of micelles built of ionic liquid molecules playing a role of a soft template for growing microparticles. © 2015 Elsevier B.V. All rights reserved.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Improvement of nano- and microparticles precursor solubility in ionic liquids . . . Precipitation in ionic liquids . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Effect of the anion type in the imidazolium ILs . . . . . . . . . . . . . . 3.2. Influence of the alkyl chain length in the imidazolium cation . . . . . . . 3.3. Effect of other factors . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Ionic liquid as an agent promoting reduction/decomposition of the precursor 3.5. Nanostructures from ionic liquid precursors . . . . . . . . . . . . . . . 3.6. Functionalized ionic liquids (FILs) in precipitation of the NPs . . . . . . . Microwave (MW) heating method in IL solutions . . . . . . . . . . . . . . . . 4.1. Effect of ionic liquid anion type . . . . . . . . . . . . . . . . . . . . . Sonochemical method . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microemulsion method . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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http://dx.doi.org/10.1016/j.cis.2015.08.010 0001-8686/© 2015 Elsevier B.V. All rights reserved.

Please cite this article as: Łuczak J, et al, Ionic liquids for nano- and microstructures preparation. Part 2: Application in synthesis, Adv Colloid Interface Sci (2015), http://dx.doi.org/10.1016/j.cis.2015.08.010

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J. Łuczak et al. / Advances in Colloid and Interface Science xxx (2015) xxx–xxx

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Sol–gel method . . . . . . . . . . Hydrothermal method . . . . . . . 8.1. Influence of the amount of IL . 8.2. IL as a structure directing agent 8.3. Order in addition of reagents . 9. Other methods . . . . . . . . . . . 9.1. Electrochemical methods . . . 9.2. Irradiation method . . . . . . 10. Conclusions . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . References . . . . . . . . . . . . . . .

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1. Introduction Metals and semiconductors in the form of nanoparticles (NPs) or microparticles (MPs) have a great potential to use in industrial processes (heterogeneous catalysis [1–3] and pigments [4,5]), electronics (high density data storage, quantum computers, quantum lasers, magnetic fluids), environmental technologies (water and waste water treatment, heterogeneous photocatalysis [6–9]), energy conversion and storage (solar cells [10,11], fuel cell catalysts [12,13], hydrogen storage materials [14,15], water photosplitting [16,17]), textile industry (selfcleaning textiles [18,19], microwave protective textiles [20] and electroconducting textiles), in biomedicine (drug controlled released [21,22] and drug delivery [23,24], biomarkers [25], cancer therapy [26], molecular tagging [27]), cosmetic and health care product manufacturing (sunscreens [28], antioxidants [29]), food production (food processing catalysts, food functional packaging [30]) and agriculture sector (fungicides). The large surface to volume ratio revealed by NPs is expected to show new catalytic, photocatalytic, optical and electronic properties. Three dimensional porous microstructures (such as microspheres, hollow microspheres or hierarchical structures) could be favorable for interface-related processes due to large surface area and density of surface defects [31]. Generally, metal nanoparticles could be obtained by chemical reduction of metal ions in aqueous solution, organic solvents or in microemulsion systems using chemical reducing agent [32,33], UV irradiation (photodeposition) [34,35] or gamma irradiation (radiolysis) [36–38]. The size of metal nanoparticles could be affected by the strength of reducing agent, UV irradiation intensity or γ-radiation dose as well as by the interaction with stabilizing agents. Weak reducing agents (e.g. hydrazine or citric acid) favor growing of NPs with larger size, while strong reducing agents (e.g. sodium borohydride or lithium borohydride) promote formation of ultrafine particles due to faster nucleation stage resulted in creation of much higher number of nuclei. Similarly, larger particle size and a low particle density are observed at low dose of γ-irradiation (e.g. 20 kGy) and a smaller particle size and a high particle density are observed at high dose (e.g. 60 kGy) [39]. Recent literature revealed that sol–gel route [40–42], hydrothermal [43,44] and solvothermal techniques are the most commonly used methods for preparation of nanoparticles and 3D microstructures built of semiconductors (mainly transition metal oxides, sulfides, selenides, tellurides, etc.). The chemistry involved in the sol–gel process is based on inorganic polymerization reaction that occurs in aqueous or nonaqueous system. Precursors are usually metal–organic compounds such as alkoxide: M(OR)x (OR = OCnH2n + 1) or metal salts as chlorides and sulfates. The conventional sol–gel process is based on the formation of oxo- bridges by hydrolysis and polycondensation of molecular precursors, while the “non-hydrolytic sol gel” is based on the condensation in non-aqueous media of chloride precursors with oxygen donors other than water (e.g. alkoxides, ethers, and alcohols). In case of conventional sol–gel process, the development of M–O–M chains is favored with low content of water, low hydrolysis rates, and excess of metal alkoxide in the reaction mixture and resulted in three-dimensional polymeric

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skeletons with close packing [45]. The formation of M(OH) x is favored with high hydrolysis rates for a medium amount of water and leads to insufficient development of three-dimensional polymeric skeleton and loosely packed first-ordered particles [45]. In a typical sol–gel process, e.g. employed for TiO2 formation, a colloidal suspension, or a sol, is formed through the acid-catalyzed hydrolysis of the precursors followed by polymerization reaction [45]. Complete polymerization and loss of solvent lead to the transition from the liquid sol into a solid gel phase [45]. In the final step, wet gels could be dried by evaporation producing so-called xerogels or using other techniques (e.g. supercritical or freeze-drying) forming so-called aerogels having lower density and higher surface area. The structure and the morphology of the resulting network strongly depend on the nature of precursors, the water content, the pH, the temperature, the solvent and the relative contribution of hydrolysis and polycondensation over the allowed reaction time [46]. Hydrothermal and solvothermal synthesis include a spectrum of techniques of semiconductor-based NPs and MPs crystallizing from high-temperature aqueous or organic solvent-based solutions at high vapor pressures. Hydrothermal and solvothermal route both can be used: to obtain mesoporous precipitate, as well as oriented structures such as nanotubes, nanorods, nanowires, nanobelts, porous nanoflakes, nanospheres, microspheres, hierarchical microspheres, hollow microspheres, and other hierarchical micro/nanostructures or flower-like structures. Although the complete synthesis route is very simple, every single step including the choice of semiconductor precursors, temperature of hydrothermal/solvothermal synthesis, type and concentration of additives, duration of thermal process, the subsequent post washing procedure and post calcination step play a crucial role in controlling the surface properties of final products (crystal structure, surface area, porosity, morphology, optical properties and catalytic or photocatalytic activity). Mesoporous or hollow structures could be obtained by soft or hard templated methods. The soft-templating method usually undergoes a co-assembly process of the semiconductor precursors and surfactant template at ambient temperature (sol–gel process), at increased temperature (hydrothermal or solvothermal route) or combining both type of process (sol–gel + solvothermal/hydrothermal). Evaporationinduced self-assembly process (EISA) was introduced by Stucky group as a very efficient alternative approach to obtain mesoporous transition metal oxides [47]. By slow alcohol evaporation, a controlled building of an inorganic network with nanocrystalline domains around the voids of the liquid-crystalline phase results in a well-defined hexagonally ordered TiO2 mesophase. According to Grosso et al., EISA process could be divided in four main steps: (1) fast evaporation of the solvent, (2) film water content equilibrium with the atmosphere humidity, (3) formation and stabilization of the hybrid mesophase, and (4) rigidification of the network by further condensation [48]. These four steps are either thermodynamically governed (i.e. steps 2 and 3) or kinetically governed (i.e. steps 1 and 4), and do not necessarily take place in a precise order and may overlap along the process of thin film deposition [48]. Amphiphilic block-copolymers, among them the mostly used



Pluronic surfactants, are generally used as soft templates to generate ordered mesoporosity in the films, following an EISA method [49–51]. Hard templating methods could be used to fabricate either macroporous semiconductor network [52,53], hierarchical structures [54, 55] or hollow structures, such as spheres [56,57], capsules [58], boxes [59] or tubes [60]. For the preparation of hollow spheres or capsules, semiconductor precursors are usually deposited on the core templates through electrostatic interaction at the first step, followed by removal of template core by calcination or solvent etching. The calcination step also plays an important role in the crystallization of the hollow shell. Hard templates used to fabricate porous semiconductors network, foam or hierarchical nanostructures include: silica nanoparticles [61,58], aluminum oxide [62,63], block copolymers [64,65], glucose [53], polymers or resin spheres [66,67], cellulosic substances [68], native starch [69], graphene nanosheets [70] or carbonaceous polysaccharide microspheres [71]. Thus, size and pore distribution in finally obtained semiconducting NPs or MPs could be controlled by structure and size of used template material. In the recent years ionic liquids (ILs) have been widely employed in preparation of metal and semiconductors built of nano- and microparticles with various shapes as shown in Fig. 1. As it was detailedly discussed in Part 1, ionic liquids could serve as stabilizing agent of growing particles by steric hindrance, electrostatic and viscous stabilization, solvation forces or as a template for the preparation of semiconductor particles. In this review, we will present the application of ionic liquids in numerous methods used for particles preparation. Ionic liquids are widely used to synthesize metal nanoparticles and in this regard the effect of the IL anion type, the length of alkyl chain in imidazolium cation as well as using ILs as an agent promoting reduction or decomposition of metal precursors is detailedly discussed. Decomposition of metal precursors resulted in nanoparticles formation carried out using microwaves, radiolysis or UV irradiation is given in the next sections. There are also some data considering application of ILs in solid particle preparation using sol–gel and hydrothermal methods. Thus in the last sections of this review, the structure and selected properties of semiconductor particles obtained in the assistance of ionic liquids are summarized. Conditions of the preparation route are related to applied

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ionic liquids as well as finally formed nano- or microstructure and summarized in the form of tables. In each section, we try to answer the following questions: What are structural descriptors of ILs influencing size and shape of the particles? What reaction conditions are crucial for preparation of the particles with expected size, morphology and properties? What is possible mode of action of ILs in the reaction mixture? How do ILs mediate the chemical reactions? 2. Improvement of nano- and microparticles precursor solubility in ionic liquids One of the problems that may hamper the wide application of certain ionic liquids as solvents in inorganic material preparation may be a low solubility of the inorganic salts (precursors). Solubility of the precursors in ILs strongly depends on the supramolecular structure of the ionic solvent. Understanding the ways in which the constituents of ionic liquids, i.e. the type of cation, its substitution, and the type of anion, interact with reagents is prerequisite to design an ionic liquidassisted preparation route of the materials with optimum performance [80]. In addition, properties of the precursor determine in which domain (polar/nonpolar) of ILs it will be preferentially distributed. In this regard, solubility of the precursors may be enhanced by several ways: • application of the strongly coordinating anions leads to formation of the anionic complexes (e.g. hexafluoroacetylacetonate [81])—the ionic nature of the metal complex increases the compatibility with the ionic liquids, • conversion of a precursor into a cationic Lewis acid complex by replacing of a precursor anion with a non-coordinating one being the same as the ionic liquid ion or having an affinity towards IL (e.g. cooper(II) trifluoromethanesulfonate and bisoxazoline dissolved in 1-ethyl-3methylimidazolium bis(trifluoromethanesulfonyl)imide [EMIM][Tf2N] [82]), • functionalization of the cation in the ionic liquid structure (e.g. 1cyanoalkyl-1-methylpyrrolidinium IL [83], protonated betaine IL [84–86], imidazolium, pyridinium, pyrrolidinium, piperidinium,

Fig. 1. Examples of nano- and microstructures with different shapes obtained in the presence of ionic liquids: SEM images and structures of (a) SnO2 spheres obtained by hydrothermal route with 1-buthyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] [72]; (b) ZnO nanosheet spheres obtained by microwave assisted method with glutamic tetrofluoroborate [HGlu][BF4] [73]; (c) γ-AlOOH flower-like structures prepared by 3-methyl-1-octylimidazolium chloride [OMIM][Cl]-assisted hydrothermal route [74]; (d) α-FeOOH hollow spheres prepared by microwave hydrothermal method in the presence of [BMIM][BF4] [75]; (e) Ag nanorods synthesized by polyol reduction in the presence of [BMIM][BF4] [76]; (f) Bi2WO3 nest-like microstructures prepared by [BMIM][BF4]-assisted hydrothermal method [77]; (g) CdS dendrites prepared in the presence of 1-n-butyl-3-methylimidazolium methylselenite [BMIM][SeO2(OCH3)] [78]; and (h) Ag/AgCl concave prepared using 1-butyl-3-methylimidazolium chloride [BMIM][Cl] [79].



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morpholinium, and quaternary ammonium salts functionalized with a carboxyl group [87]) enhance coordination ability of the IL. As an example incorporation of the Co ion in the metal–IL complex facilitated by the functionalized cation and coordinating ability of the anion is presented in Fig. 2, metal-containing ionic liquids are regarded as promising materials that combine the properties of ionic liquids with additional intrinsic magnetic, spectroscopic, or catalytic properties that depend on the incorporated metal ion [88], e.g. [BIM][AuCl4] [89] and [Zn(CH3NH2)4(Tf2N)2] [90], where [BIM] is 1-butylimidazolium cation, addition of co-solvent (e.g. hexane, polyethylene glycol, water) usually increases solubility of the inorganic metal salts, reduce the viscosity of the ILs and increase mass transfer of the compounds [91,92], application of the additional stabilizing agent, e.g. cetyltrimethylammonium bromide [93], oleyl acid [94], increasing temperature, application of the water in ionic liquid microemulsion systems [95,96].

Modification of the ionic liquid structure alters the volumes of the polar/non-polar domains in the ionic liquid, affinity of the precursor and consequently enables control of the size and shape of the nanostructures.

at high temperature and ambient pressures a term ‘ionothermal synthesis’ was proposed. Up to date four main ways of the ionic liquids usage in order to prepare metal nanoparticles were presented: a. chemical reduction of the metal precursor (such as: HAuCl4·3H2O, PdCl2, [Ir(cod)2][BF4], cod = 1,5-cyclooctadiene etc.) dissolved in ionic liquid by H2, NaBH4, H2N4∙H2O, trisodium citrate (Na3 citrate) [99–101]. Molecular hydrogen is often applied as a reducing agent due to its purity (Fig. 3a), b. decomposition of organometallic compounds in the zero oxidation state such as [Pt2(dba)3], [Ru(cod)(cot)] or [Ni(cod)2] (dba = dibenzylideneacetone, and cot = 1,3,5-cyclooctatriene), and Co2(CO)8, Ru3(CO)12 dispersed in ionic liquids [91,102–104], carried out without necessity of reducing agent application. For carbonyl precursors, byproducts are in a form of CO that is easily removed from the dispersion, thereby contamination is significantly reduced (Fig. 3b), c. nanoparticles transfer from other solvents (water, nonaqueous solvents) to ionic liquids Fig. 3c [105,106], d. metal sputter deposition in ionic liquids under reduced pressure without additional stabilizing agents (Fig. 3d) [107,108].

3.1. Effect of the anion type in the imidazolium ILs 3. Precipitation in ionic liquids Chemical methods of nano- and microstructure preparation such as the reduction of the metal precursors (usually salts or organometallic complexes) and decompositions of the complexes in solvents are probably the simplest and the most convenient ways to control the size and shape of the particles, and therefore the most investigated preparation route. The main factors that determine the properties of the product formed during reduction reaction, besides reaction conditions (temperature, pressure and metal precursor concentration), are the type of the metal precursor, reducing agent, and solvent [97]. Solvents play an important role in governing chemical reactions, not only are they involved in heat and mass transport, but – if chosen sensibly – they can increase selectivity, reaction rate, as well as shift of the equilibrium position towards the products. The precipitation of metals from aqueous or nonaqueous solutions is usually carried out by the chemical reduction by reducing agents such as gaseous H2, solvated MBH4 (M is alkali metal), hydrazine hydrate (N2H4·H2O), hydrazine dihydrochloride (N2H4·2HCl), ascorbic acid, and trisodium citrate (Na3 citrate). Like it was described in the Part 1 of this review [98], ionic liquid molecules form highly organized supramolecular network containing polar and nonpolar domains. Therefore, they appear as alternative solvent for the generation of a variety of size- and morphologycontrolled nano- and microstructures. For a reaction carried out in ILs

Up to date a lot of effort was put into determination of the structural descriptors of ILs influencing size and shape of the particles. Dupont and coworkers [99,109,110] presented a possibility to apply the 1-butyl-3methylimidazolium derivatives with different anions (mainly [BF4], [PF6], and trifluoromethanesulfonate [TfO]) as solvents for the formation of Ir, Rh, Ru and Pt nanoparticles among others. By reduction of the ionic precursors such as [Ir(cod)Cl]2, RhCl3·3H2O, Pt2(dba)3 and [Ru(cod)(cot)] by hydrogen (4 atm) at 75 °C, they obtained nanoparticles with a diameter of 1–4 nm, narrow size distribution, and irregular or sphere-like shape as schematically presented in Fig. 4 [99,109,110]. Investigation of the IL anion type effect on the nanoparticle size and distribution revealed that Ir NPs with an average diameter of 2.0 nm for [BMIM][PF 6], 2.6 nm for 1-butyl-3-methylimidazolium methylsulfate [BMIM][MeSO4], and 2.9 nm for [BMIM][BF4] can be obtained [100]. Moreover, application of 1-ethyl-3methylimidazolium ionic liquid with a more coordinating ethylsulfate anion [EtSO4] enables formation even smaller nanoparticles with mean size of 1.6 nm [33]. Similar observation was performed for Pt nanoparticles, and structures of around 2.3–2.5 nm were formed using as reaction media ILs composed of a more coordinated anion like [PF6], whereas larger particles with around 3.4 nm in mean diameter were formed in the [BMIM][BF4] salts [110,111]. Application of bis(trifluoromethanesulfonyl)imide [Tf2 N] anion

NTf 2

Tf 2N +

O6

N

O1

C

Co1 N2 N2 O6

O1

N

Co

N

C

+N NTf 2

Tf 2N

Fig. 2. Molecular structure of the solvated cobalt(II) bis(1-cyanomethyl-1-methylpyrrolidinium) bis(trifluoromethanesulfonyl)imide [Co(C1C1CNPyr)2(Tf2N)4] complex (adapted from [83]).



Fig. 3. The main ways of ionic liquid application in metal particles preparation (a) chemical reduction, (b) thermal decomposition, (c) NPs transfer from other solvents, (d) metal sputter deposition.

leads to formation of Ru NPs with smaller mean diameter when compared with [BF4] anion as observed by Prechtl et al. [112]. As a general conclusion, it was revealed that the relative size of the metal NPs can be related to the coordination ability of the IL anion. Analogous precipitation studies were subsequently carried out confirming that a variety of nanoparticles 2–10 nm in size can be prepared by reducing the metal precursor or by thermal decomposition of the

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organometallic compounds in ionic liquid media according to two first synthesis ways described above, e.g. In [113], Rh [114], Pt [111], Pd [115], Cu [116], Ag [117], Ru [118], Os [119], and Fe [119]. A new route of the Ag NPs preparation through the silver precursor (AgBF4) reduction by hydrogen in ILs and scavenging of the HBF4 formed during synthesis by 1-buthylimidazole [BIM] was described by Redel [120]. In this solution, 1-alkylimidazole compounds scavenge acid by products thereby preventing product decomposition or side reactions according to BASIL (Biphasic Acid Scavenging utilizing Ionic Liquids) process patented by BASF company [121]. Silver salts of AgBF4, AgPF6, AgTfO and [BMIM]-based ILs with appropriate anions were applied according to the reaction presented in Fig. 5a. Nanoparticles prepared with scavenger were found to have a narrower size distribution and to be more stable. The average size of the Ag NPs formed in the presence of [BIM] increases roughly linearly with the molecular volume of the IL anion as shown in Fig. 5b. A similar procedure was also applied by the same group for Au nanoparticles preparation [122], however the reaction was based on the thermal decomposition of the Au(CO)Cl or KAuCl4 precursors under argon in the presence of n-butylimidazole dispersed in [BMIM][BF4], [BMIM][TfO] and n-butyltrimethylimidazolium bis(trifluoromethanesulfonyl)imide [N4111][Tf2N]. The authors suggest that (NHC carbene)–Au intermediate species in [BMIM][Cl] ionic liquid are formed due to scavenging process, inhibiting aggregation, and preventing destabilization of the Au NPs in acidic environment (Fig. 6). The results confirmed earlier findings that anion molecular volume controls the range of the nanoparticle size [122]. Hydrophobic ionic liquids [BMIM][PF6] and [BMIM][Tf2N] were also applied as both reaction medium and reducing agent of HAuCl4·3H2O for Au precipitation as was shown by Gao et al. [123]. As a result uniform single-crystal Au nano- and microprisms were formed. By applying [BMIM][Tf2N], Au prisms with larger size of about 100 μm in diameter than in [BMIM][PF6] were obtained. What is more, it was concluded that hydrophilic ILs probably cannot provide an aggregation environment to form such structures since in [BMIM][Cl] and [BMIM][BF4] nano- and microprisms were not obtained [123]. Bussamara also observed that [BMIM][Tf2N] supports the nanoparticles synthesis (however this time Mn3O4 NPs), whereas in less hydrophobic [BMIM][PF6] and [BMIM][BF4], no indication of Mn3O4 formation was detected [124]. Application of the ILs with [Tf2N] large anion for preparation of the nanoparticles with an average size of b10 nm was presented to be possible by Wang and coauthors, who applied [BMIM][Tf2N] for Ag NPs synthesis [94]. However, this time an IL was combined with oleic acid due to low solubility of the NP precursor silver trifluoroacetate in the ionic liquid. As a result of the reaction carried out at 160–200 °C NPs with narrow size distribution (average size 4–6 nm) and an extended ordered hexagonal arrange precipitated from ionic liquid. A control experiment carried out without oleic acid revealed a crucial role of this substrate for reduction and precipitation of the NPs from IL solution. It was suggested that oleic acid forms complexes through the carboxyl acid group with silver precursor around growing particles with alkyl chains pointing out towards ionic liquid environment [94]. The same ionic liquid was also applied by Wang and Yang [93] who presented synthesis of the CoPt alloy nanoparticles and nanorods using platinum acetylacetonate Pt(acac)2 and cobalt acetylacetonate Co(acac)3 precursors, again in the presence of the stabilizing agent (cetyltrimethylammonium bromide CTAB). Nanoparticles with an average diameter of 5 nm were obtained when the Pt(acac)2:Co(acac)3 molar ratio was ≥1, whereas 8 nm in size nanorods (in a form of bundles) were produced when the Pt(acac)2:Co(acac)3 molar ratio was ≤1 [93]. The simple method of the KAuCl4 reduction by the less noble SnCl2 in 1-butyl-3-methylimidazolium ionic liquids with different anions [BF4], [PF6], and [TfO] was also proposed by Redel and coworkers [125]. In this case, the average diameter of the nanoparticles was regulated by a variation of the molar ratio of Au(III):Sn(II). The addition of the Au precursor in IL to excess of reducing agent/IL system results in the changes


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Fig. 4. Synthesis of Ir nanoparticles based on the reduction of [Ir(1,5-cyclooctadiene)Cl]2 by H2 in 1-butyl-3-methylimidazolium hexafluorophosphate (adapted from [110]).

Fig. 6. Gold nanoparticles preparation mechanism through the formation Au-heterocyclic carbene intermediate (adapted from [122]).

in color accordingly to Au particle size distribution (2.6–200 nm, from light-yellow to black-blue). It was found that Au NPs nucleation and growth can be stopped and resumed at different steps by the controlled addition of new portion of the Au precursor (“stop and go” method). Binding energy of the IL anion to Au NP surface calculated by density functional theory (DFT) was found to be weaker than that determined for typical stabilizers, and lower than Au–Au interaction. Therefore, a dynamic Au NPs growth mechanism is responsible for the formation and stabilization of the gold nanoparticles in ionic liquids. Interestingly, addition of the chloride anions to the reaction system, anions with high binding energy, inhibits Au NPs formation [125]. The influence of the ionic liquid anion type on the morphology of the Ag nanostructures was presented by Kim and co-workers [126]. This time however, reduction of the Ag precursor (AgNO3) was carried out in the ethylene glycol as a solvent, whereas the IL plays the role of the cosolvent. It was revealed that the application of the [BMIM][MeSO4] salt supports 1D alignment to nanowires, [BMIM][Cl] three-dimensional arrangements into cubic nanoparticles, whereas [BMIM][Br] octahedral shape structures. Dependence of the Ag morphology on anion moiety of the IL was also confirmed by using in the synthesis compounds bearing [MeSO4] anions and different cations ([EMIM] and 1-ethyl-3methylpyridinium [EMPy] cations), where similar nanowires were formed. TEM micrographs of samples taken after 10, 20, 30, and 60 min of the reaction, present the mechanism of the Ag nanowire growth. After 10 min of the reaction Ag nanoparticles with sizes of 2–10 nm (Fig. 7a) were observed to be formed in the presence of [BMIM][MeSO4]. As a next step, some of the Ag nanoparticles self-assembly (Fig. 7b) forming twinned structures (Fig. 7c) followed by self-organization of adjacent particles in an anisotropic fashion (Fig. 7d). Further increasing of the reaction time provides nanowires with smooth surfaces as shown in Fig. 7e, that fused together forming single crystals confirmed by XRD and selected-area electron diffraction (SAED) data. Finally, a bundle-like structure composed of several nanowires stacked upon one another is formed. Methylsulphate anions having a higher degree of directional polarizability than [Cl] or

[Br] are probably responsible for 1D alignment of the nanoparticles [126]. Similar in structure ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate [EMIM][EtSO4] was examined by Ryu and coworkers [127] as a template for anisotropic Au nanorod formation by the reduction of the HAuCl4 precursor. The synthetic approach relies on the seeded-growth method developed by the Murphy group [128]. This method starts with the spherical gold nanocrystals formation, called seed crystals, that are synthesized by the reduction of the HAuCl4 with a strong reducing agent as NaBH4. As a second step, the seed crystals were injected to the so called secondary-growth solution of [EMIM][EtSO4] containing Au+, Ag+ and ascorbic acid as a weak reducing agent according to the scheme presented in Fig. 8. Based on the research presented above, it was revealed that ILs provide a suitable medium for the controlled synthesis and stabilization of nanoparticles due to the coordination ability, size and hydrophobicity of the anions. 3.2. Influence of the alkyl chain length in the imidazolium cation Increasing of the ionic liquid organization range order by increasing the alkyl chain length in the cation was found to induce the formation of the NPs with smaller diameter and size-distribution as it was shown for example for Ni NPs by Migowski [129]. However, the mean diameter of Ni nanoparticles prepared in 1-alkyl-3 methylimidazolium derivatives [AMIM][Tf2N] IL with increasing alkyl chain length (C4–C16) displayed a minimum, therefore the dependency between increasing chain length and decreasing mean diameter is not valid for very long alkyl side chains. This phenomenon may be related to the effect known in literature as a “cut-off effect” [130]. In addition, the distance between Ni nanoparticles decreases progressively with increasing hydrocarbon substituent in the imidazolium cation as it was revealed by smallangle X-ray scattering (SAXS) analysis [129]. The influence of the chain length on the mean size of the nanostructures was also presented by Clavel and co-workers during the synthesis of cyano-bridged

Fig. 5. (a) Formation of Ag NPs by reduction of AgBF4 in [BMIM][BF4] using the 1-buthylimidazole as scavenger (adapted from [120]); (b) dependence of the Ag NP size on the molecular volume of the IL anions, where volume of [BF4] is 0.073 ± 0.009 nm3, [PF6] is 0.109 ± 0.008 nm3, [TfO] is 0.131 ± 0.015 nm3, and [Tf2N] is 0.232 ± 0.015 nm3 (adapted from [120]).



7

Fig. 7. TEM micrograph sequence of the Ag nanowire formation by the reduction of AgNO3 precursor in the presence of [BMIM][MeSO4] ionic liquid taken after (a) i (b) 10 min, (c) 20 min, (d) 30 min, and (e) 60 min (adapted from [126]).

molecule-based magnets of M3[Fe(CN)6]2 (M = Cu, Ni, Co), and Fe4[Fe(CN)6]3 [131]. By increasing the length of the N-alkyl chain from C4 to C10 in the imidazolium cation, the growing process was observed to be controlled to produce NPs with smaller size. The mean diameter of the nanostructures obtained in [BMIM][BF4] was observed to be approximately 3 nm, whereas elongation of the chain length in imidazolium cation to ten carbon atoms in 1-decyl-3-methylimidazolium tetrafluoroborate [DMIM][BF4] decreases average diameter to approximately 2 nm. It was concluded that the low IL/air surface tension results in a high nucleation rate and weak Ostwald ripening favoring the formation of the small nanoparticles, therefore using [DMIM][BF4] provides nanoparticles with smaller size [131]. To support an earlier described tendency, a longer alkyl chain at the imidazole ring of the ionic liquid impeded growth of the ZnO nanostructures [132]. For example, in [EMIM][BF4] salt, ZnO nanorods with length of about 500–1500 nm were formed, whereas in [BMIM][BF4] in the same reaction conditions shorter ones were obtained (200–600 nm). However, substitution of the C2 position in the imidazolium ring provides ZnO nanoparticles with size a distribution of 10–60 nm depending on the applied temperature. Due to possible electrostatic interactions, hydrogen bonding between C2–H in the imidazolium ring, and oxygen atoms of ZnO, ionic liquids can be adsorbed on the surface of growing nuclei. Therefore, a growth rate of the O2− terminated surface may be slowed down in comparison to other crystal surfaces, and growth of Zn2+-terminated face privileged [132]. Contrarily, Gutel et al. observed that the mean size of the Ru nanoparticles formed by decomposition of the Ru(cod)(cot) in [AMIM][Tf2N] ionic liquids increases with the side alkyl chain length as shown in Fig. 9 [102]. Again the size of the Ru nanoparticles obtained ILs with the shortest chain length [EMIM][Tf2N] does not fit with this relation. It was explained by polar properties of [EMIM][Tf2N] that do not generate enough non-polar domains needed for the NPs with smaller size preparation [102]. Method of cubic-shaped cobalt NPs preparation by thermal decomposition of [Co2(CO)8] in the presence of ILs dissolved in hexane was

proposed by Scariot and co-workers [91]. It was revealed that the formation of cubic Co NPs is related to the self-organizational ability of the ionic liquid solvent. Therefore, NPs are preferentially formed in the ILs with a longer substituent that is [DMIM] derivative instead of [BMIM], and what is more combined with [Tf 2 N], tris(pentafluoroethyl)trifluorophosphate [FAP] anions instead of [BF4 ] [103]. Reaction time seems also to be crucial since diameter of the cubes prepared in [DMIM][Tf 2 N] decreases in time (from about 88 nm at 15 min to 61 nm after 5 h), whereas the percentage of spherical nanoparticles increases [91]. These, in appearance, divergent results described above may be explained by the nature and concentration of the metal precursor, as well as size of the ionic liquid polar and nonpolar domains. Neutral metal precursors such as [Co2(CO)8], Ru(cod)(cot) tend to be dissolved in the IL nonpolar domains [133], and therefore, the size of NPs depends on the size of these regions, and increases with the elongation of the alkyl sidechain in the imidazolium cation. Therefore, they observed an increase in the size distribution of the nanostructures in imidazolium salts with longer chain length. However, the relative size of the metal NPs prepared based on the ionic precursor as for example M3[Fe(CN)6]2 is related to the volume of the polar nanoregions since the ionic precursor is located in the polar nanodomains of IL [134]. For that reason, the average diameter of the particles prepared from ionic precursors may exhibit opposite relation. The confirmation of these relations was presented by Migowski et al. who applied two types of Ir precursors, namely ionic [Ir(cod)2][BF4] and nonionic [Ir(cod)Cl]2. Iridium nanoparticles preparation was carried out in ionic liquids with different nature of the alkyl groups, that is [BMIM][BF4] and [DMIM][BF4]. In IL with shorter alkyl substituents and thus smaller volume of the nonpolar domains [BMIM][BF4], irregular nanoparticles with a spherical size were formed. In [DMIM][BF4],

Fig. 8. Seeded-growth synthesis of the Au nanorod particles in 1-ethyl-3methylimidazolium ethylsulfate (adapted from [127]).

Fig. 9. Relation between the number of carbon atoms in the alkyl substituent of the imidazolium cation and the Ru nanoparticle mean diameter (adapted from [102]).



8

Fig. 10. Illustration of growth process of gold dendrites on zinc plate and SEM images of gold dendrites grown in [BMIM][PF6] solution (adapted from [101]).

however, worm like nanoparticles, formed by the attachment of spherical NPs, were observed [134]. Influence of the chain length on the Ru NPs size was also investigated for decomposition of Ru(cod)(cot) in [AMIM][Tf2N]. However this time, the presence of 1-hexadecylamine, and 1-octyamine as additional stabilizing agent was also investigated. Interestingly, no correlation between the size of the alkyl chain in the imidazolium cation and the size of the NPs was found in contrary to abovedescribed studies. Regardless of the IL and amine used in the synthesis, NPs spherical in shape and similar in sizes in the range 1.1–1.3 nm were obtained. Moreover, the size of the NPs was found to be more smaller than NPs formed in the absence of ligand (2.3 nm), and prepared in THF with the same ligand (1.9–2.3 nm). These results may suggest synergistic effect of IL and amine in controlling the growth of the NPs, and that coordination of the NPs by amine constitutes a dominating factor in Ru NPs stabilization [135]. 3.3. Effect of other factors The comparison of an IL and water as a reaction medium was considered by Qiu and co-workers [101] based on the preparation of Au nanostructures on a Zn plate. The application of [BMIM][PF6] instead of water as a reaction medium was found to change the route of the crystal growth and the reaction rate due to the higher viscosity of IL (viscous stabilization), and as a consequence lower ion diffusivity. The synthesis method relies on the immersing of the zinc plate in a HAuCl4 solution (15 mM) in [BMIM][PF6] salt or aqueous solution and autoclaving it at 60 °C for 4.5 h. In IL solution, gold nuclei nanocrystals are preliminarily formed on the zinc substrate through a direct surface reaction according to the scheme presented in Fig. 10. As a consequence, the three-fold symmetrical, hyperbranched, single-crystalline gold dendrites are formed due to nanocrystal growth obtaining a three-order hierarchy. Moreover, decreasing of the HAuCl4 concentration influences shape of the dendrites providing with a four-branched, b110N-oriented

dendrites with a cross-like cross-section. A similar experiment carried out in an aqueous HAuCl4 solution resulted in formation of the AuZn alloy dendrites consisting of aggregated primary nanoparticles [101]. Taubert and co-workers used unusual ability of ILs to dissolve cellulose for the formation of micrometer sized gold particles in [BMIM][Cl]. It was revealed that cellulose acts not only as the reducing agent of the HAuCl4 precursor but also as particle template [136]. The presence of IL, having long-range ordered structure and acting as an organic template, as well as of cellulose, is of crucial importance for morphology and size control, providing crystallization into polyhedral particles or thick plates (Fig. 11). Particles that were obtained through this method seem to differ from other preparation routs, where usually large plates with smooth surfaces and small thickness are formed [137,138]. The higher the cellulose concentration and the lower the temperature, the lower the amount of the gold plates, and the samples contain mainly polyhedral gold crystals [136]. Polyaniline/gold nanocomposites were prepared by polymerization of aniline using HAuCl4 as the oxidant at the water/[BMIM][PF6] interface. During this reaction, reduction of the Au precursor was accompanied by the oxidative polymerization of aniline, leading to the formation of submicron sized composite particles of polyaniline and gold. The mechanism of the reaction is still not clear, however the authors suggest that first of all aniline undergo protonation in an acidic solution followed by an electrostatic complexation with AuCl− 4 anions, reduction of HAuCl4 and simultaneous oxidation of aniline. Therefore, polyaniline/gold nanocomposite were formed (as confirmed by X-ray photoelectron spectroscopy analysis) and transferred to the aqueous solution [139]. 3.4. Ionic liquid as an agent promoting reduction/decomposition of the precursor A method of the Ru NPs formation in ILs in the absence of classical reducing agents at low temperature and atmospheric pressure was

Fig. 11. SEM pictures of gold particles precipitated from 1-butyl-3-methylimidazolium chloride solution gat 110 °C containing (a) 20 mg of cellulose and (b) 100 mg of cellulose per g of IL (adapted from [136]).



9

Fig. 12. Proposed mechanism of the reductive elimination of the 1,5-cyclooctadiene ligand induced by the nucleophilic attack of the [Tf2N] anion (L Ľ ligand/solvent-IL) (adapted from [140]).

proposed by Prechtl and co-workers [140]. However, this method was found to take place preliminary only in imidazolium salts with [Tf2N], that acts as a nucleophile and attacks the Ru precursor complex according to the reaction presented in Fig. 12. It was proposed that during the first step of the reduction reaction, a ligand exchange takes place between 1,5-cyclooctadiene and N-heterocyclic carbenes formed in situ in ionic liquids. Formation of the carbenes enhances nucleophilicity of the [Tf2N] anion that fosters attack on the Ru precursor followed by nanoparticles formation. The application of the ILs containing strongly coordinating tetracyanoborate [B(CN) 4 ] anions prevented the reduction of the Ru complex, therefore it seems that the presence of the [Tf 2 N] anion is crucial for this preparation route. For decomposition of the [Ni(cod)2] precursor, also the ILs with short alkyl chains and [Tf2N] were necessary for the activation of the acidic protons on the imidazolium ring and then followed by the intermediate carbene formation. The proposed mechanism is presented in Fig. 13. An ability of [BMIM][PF6] to act parallelly as a solvent and reducing agent for the Au nano- and microstructure formation was presented by Gao and coworkers [123]. Interaction of the IL and HAuCl4·H2O precursor was attributed again to the carbene formation as well as hydrogen bonding between the chlorine atoms of the Au precursor and

protons in the imidazolium ring. The Fourier transform infrared spectroscopy (FTIR) analysis revealed that due to the reduction reaction, the imidazolium IL undergoes oxidation according to the reaction presented in Fig. 14. Growth of the formed Au nanoparticles is further controlled by the oxidation products, ketones, which act as a capping agent. The Au particles sinter then into single-crystal prisms, and successively grow as a result of the incorporating Au atom production during the subsequent reduction of the remaining precursor. The formation process is presented in Fig. 15. The attention was also turned to the inability of the hydrophilic ILs like [BMIM][Cl] or [BMIM][BF4] to play a role of the HAuCl4·H2O reducing agent. 3.5. Nanostructures from ionic liquid precursors Final properties of the nanostructures strongly depend on the chemical structure of the metal precursors due to possible by-product formation. These species may negatively influence the properties of utility (catalytic, photocatalytic, magnetic, optical). Therefore, during designing of the nanostructures synthesis method, the idea to avoid the presence of the “poisoning” species also appeared realized by considering ILs

Fig. 13. Proposed mechanism of the [Ni(cod)2] auto-decomposition in imidazolium ionic liquids (adapted from [140]).



10

Fig. 14. Ionic liquid as a reaction medium and reducing agent in Au structure formation (adapted from [123]).

Fig. 15. Illustration of the gold nano- and microprisms formation in the hydrophobic 1-butyl-3-methyimidazolium hexafluorophosphate (adapted from [123]).

Fig. 16. Structure of ionic liquid precursor (a), scanning electron micrographs of CuCl nanoplates obtained at 85 °C (b), and 105–140 °C (c) (adapted from [141]).

Fig. 17. The Zn-containing ionic liquid precursor (a), SEM images of ZnO nanostructures derived from the [Zn(CH3NH2)4(Tf2N)2] precursor at (b) 160 °C and (c) 70 °C (adapted from [90]).

Table 1 Gold nanostructures obtained by using different alkyl chain lengths at molar ratio [AHIM]/ HAuCl4 of 4:1 (adapted from [89]).

Chain length (n)

18

12

6

1

85/15a

60/40a

30/70a

10/90a

Plates particles %

a

Octahedra, pentagonal decahedra, and other ill-defined particles.

as an “all-in-one” medium for synthesis of inorganic materials. That means that IL can serve as a solvent/template, stabilizing agent, metal precursor as well as reactant for growth of the nanostructures. Metal can be incorporated in both IL cation e.g. [Zn(CH3NH2)4(Tf2N)2] or anion structure e.g. [BIM][AuCl4]. Synthesis of the CuCl nanostructures using Cu-containing ionic liquid precursor (Fig. 16) and 6-O-palmitoyl ascorbic acid was proposed by Taubert [141]. This preparation route provides CuCl plates with a uniform thickness of 220–260 nm and large range continuous structures of about 5–50 μm. Increase of the synthesis temperature results with smaller sized particles (typical in-plane dimensions 5–8 mm) with thickness exceeding 1 μm [141]. Further investigation revealed that Cu(II)–Cu(I) reduction



11

(a) [ODIM]/[HAuCl4]

aggregation

heating

= Au (0) nuclei

Au nanoplates = free [ODIM]

Micro-sized gold plates

(b) A [MIM]/[HAuCl4]

heating

aggregation

B = Au (0) nuclei

= free [MIM]

A = low molar ratio of [MIM], B = high molar ratio of [MIM] Fig. 18. Schematic representation of the steps in the formation of the Au nanostructures from Au-containing ionic liquid precursor (adapted from [89]).

is complete after about 25–30 min, however the first CuCl particles form after 3 min. Therefore, the reduction process and formation of primary CuCl particles are overlapping processes and CuCl primary particles are formed in the environment of the supramolecular IL structure containing polar and nonpolar domains [142]. Zhu and co-workers proposed the use of a newly developed IL system containing zinc metal ions for ZnO nanostructure formation by reduction with tetramethylammonium hydroxide. Morphology of ZnO was found to be strongly dependent on the nature of the ligands in the IL precursor, therefore nanoplates were formed of spherical particles growth in [Zn(CH3NH2) 4 (Tf2 N) 2 ] IL, flowerlike nanostructures in [Zn(C 2H5 NH2) 4 (Tf2 N)2 ], and close to spherical particles in [Zn(C8H17NH2)4(Tf2N)2] as shown in Fig. 17. Application of the long chain alkylamine as precursor ligands stabilizes ZnO nanoparticles by the surface energy minimization leading to the monodispersed nanoparticles formation. Short chained ligands due to the limited stabilization ability self-assemble into large-scale continuous structures, such as flowers or nanoplates. The proposed mechanism is based on the displacement and decomposition reactions to form ZnO: þ

ZnðalkylamineÞ4 þ xOH− ⟶ ZnðalkylamineÞ4−x ðOHÞxð2−xÞ þ ZnðalkylamineÞ4−x ðOHÞxð2−xÞ ⟶ ZnO þ 3alkylamine þ H2 O: 2þ

− x)+ depends Decomposition of the Zn(alkylamine)4 − x(OH)(2 x on the reaction temperature, therefore morphology of the particles depends not only on the ligands in the Zn precursor but also temperature. For example, an increase of the reaction temperature for [Zn(propylamine)4 (Tf 2N) 2] results in more symmetric structures

Fig. 19. Structure of the nitrile-functionalized 1-butyronitrile-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (a) and a ruthenium precursor (b) (adapted from [144]).

with less monocrystal petals. However, with the elongation of the alkylamine chain length effect of the temperature begins to be weaker [90]. Thermolysis of the Au-containing ionic liquids was applied by Lin and co-workers for Au nanostructure formation [89]. Ionic liquids [AHIM][AuCl4], that were obtained by mixing 1-alkylimidazole [AHIM] precursor with chloroauric acid (HAuCl4), were heated at 200 °C for 1 h giving Au micron-sized gold nanosheets and polyhedra. By changing of the molar ratio of these two components, morphology and particle size may be controlled. Short alkyl substituents in the imidazolium ring (e.g. methyl) favor formation of the regular octahedral, pentagonal bipyramidal particles with size distribution of few microns, whereas compounds with longer hydrocarbon chains provide platelet formation as shown in Table 1. It was suggested that C18 imidazolium derivatives self-assemble forming a supramolecular layered structure, where Au ions are placed in between layers. Due to heating, electron transfer from Cl− to Au3+ takes place, forming Au nuclei protected by long alkyl chains. An IL acts as a supramolecular template, Au precursor as well as capping agent favoring growth along b 111 N planes with formation microsized gold plates. ILs, with methyl substituent in the imidazolium ring, are not able to form a similar layered network, therefore rather lamellar structure is formed, and the creation of polyhedra is privileged. The proposed mechanism of the Au structure formation is shown in Fig. 18. 3.6. Functionalized ionic liquids (FILs) in precipitation of the NPs A further research aspect on the NPs precipitation from the ILs concerns the application of the functionalized ILs in order to directly stabilize nanostructures formed in the ionic liquid. The progress in the functionalized ILs synthesis and application appears as an important research direction to maintain the attractive properties of ILs as well as improve their ability to interact with precursors, nanostructures or other solutes. A direct preparation of FIL-stabilized nanoparticles may provide a new route to formation of the new functionalized materials, especially when prepared in a single step, one-pot procedure. In that way, both stability and chemical functionality to the nanoparticle products are provided, often without the need for further modification. It has been shown that a functionalized IL may act as: - additional stabilizer added to NPs dispersed in ionic liquid [143], - both reaction medium and direct stabilizer [144,145].


12


Fig. 20. SEM images showed ZnO obtained using different mass ratios of [HGlu][BF4]/NaOH: (a) 0:6, (b) 1:6, (c) 3:6, (a–c under microwave), and (d) 3:6, in the absence of microwave (adapted from [73]).

Usually only the cation is modified, however anions may be functionalized for a desired application. Ether-functionalized ionic liquid 1-triethylene glycol monomethyl ether-3-methylimidazolium methane sulfonate was proposed by Schrekker for Au nanoparticles preparation as both a solvent and protecting agent. Analysis performed by surfaceenhanced Raman spectroscopy (SERS) method confirmed the presence of the electrosteric stabilization of the Au nanoparticles by an IL [146]. Prechtl [144] and Venkatesan [145] applied a nitrile-functionalized 1butyronitrile-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [(BCN)MIM][Tf2N] IL (Fig. 19) for synthesis of the Ru NPs and Pd NPs, respectively. The nitrile-functionalized IL was found to have an ability to stabilize both type NPs via nitrile binding without blocking the active places of the product. In the synthesis route proposed by Prechtl, the nitrile group was maintained during the NPs synthesis, however after 5 days hydrogenation to primary amine was observed, followed by aminolysis, which produces the secondary imine IL and unsaturated product. This phenomenon was observed due to the usage hydrogen as a reducing agent [144]. Venkatesan and co-workers [145], however, compared different preparation methods namely hydrogen reduction and thermal treatment for Pd NPs formation using palladium acetate Pd(AcO)2 as a precursor. Thermal treatment of the Pd(OAc)2 provided better dispersed and immobilized NPs when compared with the hydrogen treatment. The application of hydrogen as a reducing agent is fast, which results in the high local concentration of Pd NPs leading to fast agglomeration [145]. The synthesis and application of a series of nitrile-functionalized imidazolium salts in a selected catalytic reactions were also presented by Fei and co-workers [147]. Yuan and co-workers [148] described preparation of Pd NPs through the thermal decomposition of Pd(OAc) 2 in a series of hydroxyl-functionalized ILs containing the 1-(2′-hydroxylethyl)3-methylimidazolium cation and various anions ([TfO], [Tf 2 N], [BF 4 ], [PF 6 ], trifluoroacetate [TA]). Analysis of the 1H NMR method revealed that acetate anions are likely to act as the reducing agent in the process, whereas not cation containing OH group. When compared to the non-functionalized IL, hydroxyl-functionalized ones provide

nanoparticles with smaller size (2.3–4.0 nm) and narrower size distribution. In addition, the remarkable anion effect on the decomposition rate of the Pd NPs formation was observed and follows the trend [Tf2N], [PF6] N [BF4] N [TfO] N [TA]. This relation is to some extent related with a nucleophilicity of the anion and its size. That is, the less nucleophilic and the bigger in size anion ([Tf2N], [PF6]), the larger particles are formed. This corroborates previous studies with non-functionalized ILs [148]. As a continuation Yuan et al. used the same hydroxyl-functionalized ILs containing [Tf2N] anion for bimetallic Au–Pd nanoparticles formation by co-decomposition of Pd(OAc)2 and Au(OAc)3. The core–shell structure of the bimetallic NPs was confirmed by XRD analysis revealing that Au atoms are mostly located in the NP core and the Pd atoms enriched at the surface [149]. Hu and co-workers [150] presented application of (2,3-dimethyl-1-[3-N,N-bis(2-pyridyl)-propylamido]imidazolium hexafluorophosphate ([BMMDPA][PF6])) as a ligand for palladium nanoparticles preparation in corresponding 1-butyl-2,3-dimethylimidazolium hexafluorophosphate ([BMMIM][PF6]). It was confirmed that the IL plays the role of the ligand and therefore effectively stabilizes, disperse, and modifies the Pd NP surface providing 5–6 nm nanoparticles in size [150]. More recently, the application of the methoxy-functionalized IL as a solvent and reducing agent for TiO 2 -RGO (reduced graphene oxide) preparation by TiCl4 hydrolysis at 100 and 120 °C was reported by Nagaraju et al. [151]. The proposed preparation method yield in 4 nm TiO 2 particles uniformly distributed on the surface of the graphene sheet with a BET surface area of 161 m2/g with the main pore size at 1.5 nm [151]. Leger and coworkers describe synthesis of a new imidazolium-monofunctionalized bipyridine ligand 4-(1methylimidazolium-3-yloctyl)-40-methyl-2,20-bipyridine bromide and effective stabilization of the Rh in [BMIM][PF6] according to the reaction [152]: TH F=½BMI½P F6

Bipyridine‐functionalized imidazolium IL1

NaBH4

TH F

RhCI3 3H2 O → Rhð0Þ=IL → Rhð0Þ=1=½BMI½PF6 :

Selected properties of the nanostructures formed in ionic liquids by precipitation method are presented in Table 2.


Compound

Precursor

Ionic liquid

Synthesis conditions

Characterization

Ir, Rh, Pt, Ru

[Ir(cod)Cl]2, [Ir(cod)2]BF4 RhCl3·3H2O, Pt2(dba)3 [Ru(cod)(cot)] where: cod is 1,5-cyclooctadiene, cot is 1,3,5-cyclooctatriene dba is dibenzylideneacetone

[BMIM][PF6], [BMIM[[BF4], [BMIM][TfO], [EMIM][EtSO4]

Hydrogen (4 atm) at 75 °C for 10–60 min, 2–24 h

Irregular shape, monomodal distribution, mean diameter 1–4 nm

Ir

[Ir(cod)2]BF4, [Ir(cod)Cl]2

[BMIM][BF4], [DMIM][BF4]

Hydrogen (4 atm) at 75 °C, 2 h Spherical NPs in [BMIM][BF4] 1.9–2.5 nm; worm-like in [DMIM][BF4] 1.9–3.6 nm

[134]

Ir, Ro, Co

Co2(CO)8, Rh6(CO)16, Ir4(CO)12

[BMIM][BF4], [BMIM][TfO], [N4111][Tf2N]

230 °C, Ar

[104]

Ir-NPs 1.3–3.6 nm; Rh-NPs 3.0–14

SEM/TEM images

Ref [99,100,109–111,33]

nm; Co-NPs 14 nm

Ni

[Ni(cod)2]

[AMIM][Tf2N], where A: C4–C16

Hydrogen (4 atm) at 75 °C, 30 min

5.9 ± 1.4 nm for [BMIM] [Tf2N] 5.6 ± 1.3 nm for [OMIM][Tf2N] 4.9 ± 0.9 nm for [DMIM][Tf2N] 5.1 ± 0.9 nm for [TDMIM][Tf2N] 5.5 ± 1.1 nm for [HDMIM][Tf2N]

[129]

Pd

[PdCl2(cod)], PdCl2, [Pd2(dba)3·CHCl3]

[BMIM][PF6]

Hydrogen (3 bar) at room temperature for [PdCl2(cod)], PdCl2 and [Pd2(dba)3·CHCl3] at 60 °C

[PdCl2(cod)] as a precursor — a star-like shaped particles with branches of 6–8 nm; for NPs formed from PdCl2 salt or from [Pd2(dba)3·CHCl3]) well-dispersed NPs with a mean size ca. 7 nm

[115]


(continued on next page)

13


Table 2 Selected properties of the nanostructures obtained by reduction or thermal decomposition of the metal precursors in ionic liquids.

14

Compound

Precursor

Ionic liquid


Characterization

SEM/TEM images

Ref

Pd

Pd(OAc)2 where: OAc-acetate

[(BCN)MIM][Tf2N] 1-butyronitrile-3-methylimidazoliumbis(trifluoromethanesulfonyl)imide acetone

120 °C, vacuum, 1 h

7.3 ± 2.2 nm

[145]

Pd

Pd(OAc)2

[BMMDPA][PF6] 2,3-dimethyl-1-[3-N,N-bis(2-pyridyl)propylamido]imidazolium hexafluorophosphate

Acetone, hydrogen (2 atm) at 60 °C for 15 min

5–6 nm

[150]

Pd

Pd(OAc)2

[HOEMIM][X], where [HOEMIM] is 1-(2'-hydroxylethyl)-3-methylimidazolium cation and X = [Tf2N], [PF6], [BF4], [OTf], [TA]

Acetonitrile, 15 min of stirring, 2.4 ± 0.5 nm for [HOEMIM][TfO] 120 °C for 30 min, vacuum 2.3 ± 0.4 nm, for [HOEMIM][TA] 3.3

[148]

± 0.6 nm for [HOEMIM][BF4] 3.1 ± 0.7 nm for [HOEMIM][PF6] 4.0 ± 0.6 nm for [HOEMIM][Tf2N]

Ru

[Ru(cod)(cot)]

[BMIM][Tf2N], [HMIM][Tf2N], [OMIM][Tf2N], [DMIM][Tf2N]

Hydrogen (3 atm) at 30 °C, 20 h, 1-octylamine, 1-hexadecylamine

Black colloidal suspensions, spherical in shape NPs, size 1.1–1.3 nm for all IL

[135]

Ru

[Ru(cod)(2-methylallyl)2]

[BMIM][BF4], [BMIM][Tf2N], [DMIM][BF4], [DMIM][Tf2N]

Hydrogen (4 atm) at 50 °C

[BMIM][BF4] — 2.9 ± 0.5 nm [BMIM][Tf2N] — 2.1 ± 0.5 nm [DMIM][BF4] — 2.7 ± 0.5 nm [DMIM][Tf2N] — 2.1 ± 0.5 nm

[112]

Ru, Ni

[Ru(cod)(2-methylallyl)2], [Ni(cod)2]

[BMIM][Tf2N], [BM2IM][Tf2N], [HM2IM][Tf2N], [(BCN)MIM][Tf2N] 1-butyronitrile-3-methylimidazolium bis(trifluoromethane-sulfonyl)imide

Hydrogen (4 atm) at 50 °C, Ar) Ru in [HM2IM][Tf2N] — 2.0 ± 0.3 nm; Ni in [BM2IM][Tf2N] — 7.0 ± 2.0 nm

[140,144]



Table 2 (continued)

Fe2(CO)9, Ru3(CO)12, Os3(CO)12

[BMIM][BF4]

250 °C, Ar, 6–12 h

Ru, Os 1.5–2.5 nm Fe 5.2 nm

[119]

RuO2

RuCl3

[BMIM][PF6]

NaBH4 50–60°C, Ar, 2 h

Black powder, orthorhombic crystals 2.5 ± 0.4 nm, carbon on the NP surface

[153]

Ag Ag/PS AgNO3 (PS—polystyrene)

[BMIM][BF4]

Na3 citrate

Regular shaped cubic Ag (5–15 nm) and Ag/PS core/shell (15–25 nm) nanoparticles

[117]

Ag

AgX, where : X = [BF4], [PF6], [TfO]

[BMIM][PF6] [BMIM][BF4] [BMIM][TfO] [N4111][Tf2N]

H2 4 atm at 85 °C, 2 h, n-butylimidazole as acid scavenger

2.8–26.1 nm

[120]

Ag

AgNO3

[BMIM][MeSO4] [BMIM][Cl] [BMIM][Br] in ethylene glycol solutions

Ethylene glycol, mixing for 15 min, then refluxed at 160 °C for 60 min

[BMIM][MeSO4] - nanowires with length of up to about 15 mm, [BMIM][Cl] - cubic NPs, [BMIM][Br] octahedral NPs

[126]

Ag

CF3COOAg

[BMIM][Tf2N]

160 °C, 200 °C, oleic acid, 20 min

4.1–6.1 nm, spherical nanoparticles with extended ordered hexagonal arrange

[94]



15


Ru, Fe, Os

16

Compound

Precursor

Ionic liquid


Characterization

Ag

AgNO3

[P6,6,6,14][Cl] tri(hexyl)tetradecyl-phosphonium halide

N2, 80 °C to dissolve reagents, cooling to 50 °C for LiBH4/THF addition for 2–3 min, vacuum treatment 80 °C

4.8 ± 0.9 nm, yellowishbrown NPs

SEM/TEM images

[154]

Ref

Au

HAuCl4·4H2O, Zn plate

[BMIM][PF6] as a solvent

60 °C, 4.5 h

Hyperbranched dendrites N10 μm in length, dendrite consists of a threefold symmetrical trunk with three groups of branches on the trunk in parallel, three-order structure

[101]

Au

HAuCl4·3H2O

[TEIM][MeSO4] 1-triethylene glycol monomethyl ether-3-methylimidazolium methanesulfonate

H2N4·H2O, 25 °C, Ar, 1 h or 72 h

Black suspension, mean diameter 7.5 nm

[146]

Au

HAuCl4·3H2O

[BMIM][PF6] [BMIM][Cl] [BMIM][Tf2N]

Ambient temperature and 90 °C, [BMIM][PF6], BMIM][Tf2N] as reaction medium and reducing agent

In [BMIM][PF6] prismatic particles with size distribution of 3–20 μm in diameter and 10–400 nm in thickness. In [BMIM][Tf2N] uniform, single-crystal Au with an even larger size of about 100 μm in diameter.

[123]

Au

HAuCl4

[EMIM][EtSO4]

0–85 °C, 2 h Na3 citrate, NaBH4 ascorbic acid

Na3citrate–nanocrystals, 9.4 ± 3 nm; Na3citrate, NaBH4, CTAB–nanoparticles 6.5 ± 2.1 nm ; NaBH4, CTAB, nanoparticles 3.9 ± 1.6 nm; ascorbic acid, AgNO3-nanorods up to 45.9 ± 6.2 nm

[127]

Au

HAuCl4·3H2O

[BMIM][Cl]

b220 °C, 20 h

Micrometer sized gold particles, two-dimensional plates and sheets, octahedral, polyhedral particles

[136]



Table 2 (continued)

Au foil

[BMIM][BF4] [N1113][Tf2N]

Au film spread on the glass plate, horizontally set in a sputter coater. Sputter deposition onto ILs was carried out with a current of 4.0 mA under a pressure of 20 Pa at room temperature.

Spherical NPs, 5.5 ± 0.86 nm in [BMIM][BF4], 1.9 ± 0.46 nm in [N1113][Tf2N]

[107]

AuPd/Au

HAuCl4

[BMIM][PF6] 1-methylimidazole

NaBH4

3.7 ± 0.8 nm 6.0 ± 2.9 nm

[155]

Au

Au(CO)Cl, KAuCl4

[BMIM][BF4] [BMIM][TfO] [BMIM][Tf2N] n-butylimidazole

Ar, 230 °C for 18 h

Au(CO)Cl as a precursor: [BMIM][BF4] — 1.8 ± 0.4 nm, [BMIM][TfO] — 130 ± 40 nm, [BMIM][Tf2N] — 350 ± 180 nm, KAuCl4 in [BMIM][BF4] — 1.1 ± 0.2 nm

[122]

Au

AuCl3

[PhPhIM][Br]bisbenzylimidazolium bromide

Thiol (16-mercaptohexadecanoic acid) solvents — CHCl3, CH3OH, PhOH, PhCH3, a mixture of CHCl3 and CH3OH, where Ph — phenyl group

Uniform gold nanoparticles 1–2 nm

[156]

Au

[ODHIM][AuCl4] 1-octadecylimidazolium tetrachloroaurate, [HDHIM][AuCl4] 1-hexadecylimidazolium tetrachloroaurate

[ODHIM][AuCl4] [HDHIM][AuCl4]

200 °C, 1 h

Triangular, hexagonal nanosheets, octahedra, pentagonal decahedra, and other ill-defined particles

[89]

Au–Pd

Au(OAc)3, Pd(OAc)2

[HOEMIM][Tf2N] 1-(2′-hydroxylethyl)-3methylimidazoliumbis (trifluoromethanesulfonyl)imide

120 °C, 2 h

3–5 nm Au–Pd nanoparticles, with Au atoms mostly located in the NP core, and Pd-enriched shell

[149]



17


Au

18

Compound

Precursor

Ionic liquid


Characterization

SEM/TEM images

Ref

Au/polyaniline

HAuCl4x3H2O

[BMIM][PF6]

Aniline dissolved I nIL was mixed with HAuCl4×3H2O in aqueous solution of HCl. Oxidative polymerization reaction take place at IL/water interface

Polyaniline/Au nanocomposites 450 ± 80 nm

[139]

YF3, EuF3, TbF3, Eu-doped YF3, Tb-doped YF3

Y(acac)3·XH2O, Y(OAc)3·XH2O, Y(NO3)3·6H2O, Eu(OAc)3, Eu(acac)3·3H2O, Eu(NO3)3·5H2O, Tb(acac)3, Tb(NO3)3 where: acac is acetylacetonate, OAc is acetate

Ethylene glycol/[BMIM][BF4] mixture, Ethanol/[BMIM][BF4] mixture, [BMIM][BF4] as cosolvent and fluoride source

120 °C; type of the precursor Type of solvent IL/Y (III) ratio

YF3, TbF3 NPs with orthorhombic structure and rhombic shape EuF3 less homogeneous hexagonal phase with ellipsoidal shape Y(acac)3 — 230 nm × 85 nm × 50 nm, Y(OAc)3 — 130 nm × 54 nm × 3 nm

[92]

Co

[Co2(CO)8]

[DMIM][Tf2N], [DMIM][FAP], [DMIM][BF4], [BMIM][Tf2N] in hexane

150 °C, 1 h

[DMIM][Tf2N] NPs with bimodal size distribution: cubic shape — 79 ± 17 nm, spherical shape — 11 ± 3 nm, [DMIM][FAP] only cubic NPs 52 ± 22 nm, [BMIM][BF4] spherical NPs — 7.7 ± 1.2 nm, [DMIM][BF4] spherical NPs — 4.5 ± 0.6 nm

[91,103]

CoPt

Co(acac)3, Pt(acac)2

[BMIM][Tf2N]

350 °C for 1 h, Ar

~5 nm (nanoparticles) ~8 nm (nanorods)

[93]

TiO2, TiO2–RGO (reduced graphene oxide)

TiCl4

[CH3OEMIM][X] 1-(2-methoxyethyl)-3methylimidazolium cation, X = [MeSO4], [BF4]

100 °C, 120 °C, 24 h

4 nm, BET 170 m2/g (TiO2), 162 m2/g (TiO2–RGO), TiO2 NPs are present on the surface of the graphene sheet

[151]

Cu3[Fe(CN)6]2, Ni3[Fe(CN)6]2, Co3[Fe(CN)6]2, Fe4[Fe(CN)6]3

[M(H2O)6](BF4)2], [M(H2O)6](NO3)2]

[BMIM][BF4] [DMIM][BF4]

[BMIM]3[Fe(CN)6] was synthesized by a metathesis reaction of [BMIM][BF4] and K3[Fe(CN)6][BMIM]3[Fe(CN)6] was mix with [M(H2O)6](X)2 in IL for 2 h at room temperature

The colloids obtained in [BMIM][BF4] present NPs size distributions of approximately 3 nm, whereas the mean size of the colloids obtained in [DMIM][BF4] 2 nm

[131]



Table 2 (continued)

M(CO)6

[BMIM][BF4] [BMIM][TfO] [N4111][Tf2N]

9–230 °C, 6–12 h, argon or air

1–1.5 ± 0.3 nm in [BMIM][BF4], 5.7 ± 2.1 nm in [BMIM][TfO], 70–150 ± 32–82 nm in [N4111][Tf2N]

[157]

ZnO

[Zn(L)4(Tf2N)2], where L-CH3NH2, C2H5NH2, C3H7NH2, C4H9NH2, C8H17NH2

[Zn(L)4(Tf2N)2]

[N1111][OH] in CH3OH, 110 °C

[Zn(CH3NH2)4(Tf2N)2]-nanoplates with thickness 50 nm forming spherical particles of 2–4 μm, [Zn(C2H5NH2)4(Tf2N)2]-flowerlike nanostructures — 5 μm, [Zn(C8H17NH2)4(Tf2N)2-particles similar to spherical 60–70 nm

[90]

ZnO

Zn(CH3COO)2·2H2O

[EMIM][BF4] [BMIM][BF4] [BMIM][BF4]

NaOH, 60–100 °C, 48 h

Average diameter: nanoparticles 10–60 nm nanorods 30–200 nm nanowires 30–40 nm

[132]

CuCl

[DPy][CuCl4]

[DPy][CuCl4]

6-O-palmitoyl ascorbic acid, 85–145 °C, 24 h

85 °C — platelets with a thickness of about 220–260 nm and a large range of in-plane sizes, from 5 to N50 …m, N105 °C — smaller and thicker particles in-plane dimensions 5–8 …m, thickness 250 nm to 1–1.2 …m

[141]

Cu

Cu(NO3)2·H2O

[BMIM][BF4]

NaBH4 in methanol

6.6 nm

[159]

Mn3O4

[Mn(acac)2] (acac — acetylacetonate)

[BMIM][Tf2N] [BMIM][PF6] [BMIM][BF4]

180 °C, 96 h, Ar

[BMIM][Tf2N] - from 9.9 ± 1.8 nm after 9 h to 13.9 ± 2.8 nm after 96 h

[124]

J. Łuczak et al. / Advances in Colloid and Interface Science xxx (2015) xxx–xxx 19


Co, Mo, W, CoO, MoO3, WO3

20


4. Microwave (MW) heating method in IL solutions Microwave-assisted synthesis involves heating of the dielectric materials through mainly molecular motion and ionic conduction by the action of electromagnetic waves of 300 MHz to 300 GHz. The singularity of microwave irradiation is heating directly inside the sample due to volumetric nature of the power dissipation in a dielectric. During the microwave heating, the instantaneously raised temperature and pressure, and high heating homogeneity can be achieved in this closed reaction system. ILs have been identified as beneficial solvents for microwave synthesis due to the ability to efficiently absorb microwave irradiation as a result of their high polarity, high dielectric constant and high ionic charge. In this regard, combining the advantages of ionic liquids and microwave heating resulted in the development of a new microwaveassisted ionic liquid method (MWIL) [160,161]. ILs interact efficiently with microwave irradiation through the ionic conduction mechanism, therefore even small amount of IL can be employed to increase the dielectric constant of nonpolar solvents playing the role of a doping agent. How it will be shown in this section, application of ionic liquid enable formation variety of interesting hierarchical nanostructures. Type of used ionic liquids as well as selected properties of structures obtained by MWIL synthesis are presented in Table 3. The simultaneous combination of IL and MW plays the key role in the formation shape and size of nanostructures by microwave-assisted ionic liquid method. Glutamic tetrofluoroborate [HGlu][BF4] was used as solvent for ZnO nanostructure preparation over microwave irradiation [73]. Synthesis in the absence of the IL provides the nanoneedlelike ZnO structures as shown in Fig. 20a–c, whereas without microwave radiation, the clew-like hierarchical ZnO nanosheet spheres were prepared (Fig. 20d). Structures obtained in the presence of [HGlu][BF4] with MW revealed that the ZnO particles have good crystallinity with the hexagonal quartzite structure. Moreover increase of the IL content in the reaction mixture decreases the size distribution confirming ability of IL to prevent agglomeration of the particles [73]. It certifies that the simultaneous combination of IL and MW allow fast and controllable synthesis of hierarchical nanostructures. Another important parameter in microwave-assisted ionic liquid method is polarity of the negatively charged surfaces of nanoparticles, which could control the direction of crystal growth. Application of the tetrabutylammonium hydroxide ionic liquid [N4444][OH] in combination with MW enable to reverse the polarity of the particles surface leading to formation not only nanoparticles but also nanoparticle-constructed spheres, plate-constructed stacks, rod-constructed flower-like particles and complex ZnO aggregates [162]. Increasing the amount of zinc acetate precursor and water results in the formation of more complicated in morphology structures starting with 0D nanoparticles or spheres, through 1D nanorods/2D nanoplates, to 3D flower-like structures [162]. Moreover, different amounts of water can change the pH and viscosity of the solution and thereby the diffusion rate of the ions, which also influence the crystallization process, resulting in changes of the structures. On the other hand, the amount of IL also determines the morphology of the product obtained by microwave-assisted ionic liquid method. It was shown that 3-methyl-1-octylimidazolium trifluoroacetate [OMIM][TA] [163] and [BMIM][BF4] [164] play a role of dispersing and stabilizing agents in CuO nanostructure preparation over the microwave-assisted route. By increasing the amount of [OMIM][TA] in the system, CuO nanostructures transformed from flower-like to leaflike (Fig. 21) [163]. Considering the growth of the [BMIM][BF4] content in the reaction medium, leaf-like CuO structures split into nanorods [164]. Also the temperature effect could modify the nanostructure morphology. Increase of the reaction temperature from 80 to 100 °C in the presence of [OMIM][TA], the leaf-like CuO nanosheets change to needle-like CuO nanowhiskers [163]. Again, the amount of [BMIM][Br] ionic liquid in ethylene glycol solution determines the morphology of the product and so the mixture of

Bi2Te3 nanoplates and nanorods changes to nanoplates with small flecks when the concentration of the IL increases [165]. What is more, Bi2Te3 structures prepared in the absence of the IL are mainly composed of irregular shapes with a wide size distribution. The probable reason of the hexagonal plate formation is that the molecules of the IL interact with the Bi2Te3 crystal planes likely along the c-axis [165]. Application of the [BMIM][BF4] ionic liquid for M2S3 (M = Bi, Sb) formation also in ethylene glycol solution provides to get longer and thinner Bi2S3 nanorods (diameter b 80 nm) in comparison with synthesis carried out without application of IL, where urchin like Bi2S3 structures (consisted of a nanorods with diameters ranging from 50 to 100 nm) were obtained. Taking Sb2S3 into account, synthesis in the presence of [BMIM][BF4] provides single crystalline nanorods with diameters of about 200 nm. In the absence of the IL, irregularly shaped single-crystalline Sb2S3 nanoplates were formed with diameters less than 80 nm [166]. Interestingly, application of the [BMIM][BF4] ionic liquid in ethylene diamine solution provides solely irregular Bi2S3 nanoplates whereas in the absence of IL, a mixture of nanoparticles and irregular nanoplates was formed. Different results were observed in ethylene glycol, wherein the absence of IL irregularly shaped nanoplates was obtained, however the addition of IL facilitates the formation of symmetrically hexagonally shaped nanoplates [167]. Ionic liquid containing [BF4] anion, readily hydrolyzing in the presence of water or at elevated temperatures, can be used as a fluoride source, for example, for lanthanide nanoparticles preparation (LnF3). Nano- and microcrystals with multimodal structures (hexagonal and orthorhombic) and morphologies (nanodisks, secondary aggregates constructed from nanoparticles and elongated nanoparticles) can be formed depending on the type of oxide taken to the synthesis [168]. Application as a precursor of lanthanide oxides containing La–Sm elements provides blocks with hexagonal nanodisk shape, that self-assemble into longer hierarchical nanocolumns (Fig. 22a). EuF3 was observed to form round and submicroplates composed of nanoparticles (Fig. 22b), which differs from the spindle-like structures represented by Gd–Tm, Y (Fig. 22c). Moreover, nuclei containing Yb–Lu were observed to grow forming elongated nanoparticles as shown in Fig. 22d. This indicates that the growth habits and orientations of rare earth atoms exhibit a distinct difference even though the reaction parameters and conditions are the same [168]. Sensitivity of the [BF4] anion towards a water and temperature enabling fluoride anion formation was also used to prepare hollow microspheres composed of alkaline earth metal fluorides MF2 (M = Mg, Ca, Sr) by the microwave route method [169]. The formation process of CaF2 double-shelled hollow microspheres self-assembled by polyhedral was proceeded by microwave heating at different temperatures. The microwave-assisted reaction between Ca2+ cations and F− anions (originating from [BMIM][BF4]) results in the formation of CaF2 nuclei that grow into unstable nanocrystals. The self-assembly process is directed by the oriented attachment mechanism. Products prepared at 50 °C were mainly composed of solid microspheres with diameters of 200–100 nm as a major morphology. Increasing the temperature to 60 °C resulted in the formation of solid microspheres whereas in 120 °C, core–shell microspheres were formed [169]. Besides semiconductors, some examples of the metal particles obtained by the microwave-assisted method were also reported. Vollmer et al. proposed rapid microwave-induced decomposition of metal-carbonyl precursors in the [BMIM][BF4] providing Ru, Rh, Ir nanoparticles (0.8 to 5 nm) [170]. An analogical method, however, in the functionalized IL 1-methyl-3-(3-carboxyethyl)-imidazolium tetrafluoroborate and for comparison in [BMIM][BF4] was used to synthesize Co and Mn nanoparticles [171]. The application of the functionalized IL enables the formation of smaller and better separated Co and Mn nanoparticles (1.6 ± 0.3 nm and 4.3 ± 1.0 nm, respectively), than in the presence of a non-functionalized one. This result is related with the presence of an additional functional group in the chain length of the IL providing extra stabilization as shown in Fig. 23.


Microwave heating Compound

Precursor

ZnO

IL

Role of IL

Preparation method and conditions

Surface properties

Zn(CH3COO)2·2H2O [HGlu][BF4]

Diluting agent and an agglomeration inhibitor

Additional compounds: deionized H2O, NaOH. Microwave radiation synthesis: 80 °C/10 min (1000 W). Aging: 20 h. Drying: 120 °C/5 h.

Hexagonal wurtzite structure. The average crystalline sizes: 30 and 18 nm.

[73]

ZnO

Zn(CH3COO)2·2H2O [N4444][OH]

Adsorbing microwave, solvent, reactant, template

Additional compounds: deionized H2O. Microwave radiation synthesis: 106 °C/1 min. Drying: 60 °C/5 h.

Hexagonal wurtzite structure. Morphologies: spheres, plate-constructed stacks, rod-constructed flower-like particles and complex ZnO aggregates.

[162]

CuO

CuCl2·2H2O

Dispersant and soft-template

Additional compounds: deionized H2O, NaOH. Microwave radiation synthesis: 80 °C/10 min. Drying under vacuum: 40

Crystal structure. The width, length, and thickness of leaf-like CuO nanosheets: 500 nm, 2–3 mm and 50–60

[163]

[OMIM][TA]

SEM/TEM image

Ref

nm.

°C/24 h.

CuO

Cu(CH3COO)2·2H2O [BMIM][BF4]

Surface-active stabilizer and solvent

Bi2Te3

Bi(NO3)3·5H2O Te powder

Morphological template

Additional compounds: distilled H2O, NaOH, EtOH. Microwave radiation synthesis: 74–96 °C/3.5–6 min (2.45 GHz, 120 W). Drying under vacuum: 70

The width, length and thickness of leaf-like, chrysanthemum-like, rod shapes of CuO nanosheets: 160–280 nm, 520–800 nm and 25–35 nm.

[164]

Rhombohedral structure. Morphologies from nanorods and nanoplates to regular hexagonal plates and nanoplates with small flecks. The edge and thickness size: 0.5–2

[165]


°C/12 h. [BMIM][Br]

Additional compounds: EG, KOH. Stirring: 2 h. Microwave radiation synthesis: 10 min in a cycling mode on for 40 s and off for 60 s (2.45 GHz, 800 W). Drying under vacuum: 60

μm, 100 nm.

°C/12 h. (continued on next page) 21


Table 3 Selected properties of structures obtained by microwave heating method using ionic liquids.

22

Microwave heating Compound

Precursor

IL

Role of IL


Surface properties

SEM/TEM image

Ref

M2S3 where: M = Bi, Sb

Bi2O3 Sb2O3 Na2S2O3·5H2O

[BMIM][BF4]

Structure template

Additional compounds: HClaq, EG or C2H7N. Microwave radiation synthesis: 190 °C/30 s or 10 min (Bi2S2/EG), 165 °C (Sb2S3/C2H7N). Drying.

Orthorhombic structure: Bi2S3 and Sb2S3.

[166]

Bi2Se3

Bi(NO3)3·5H2O Se powder

[BMIM][BF4]


Additional compounds: EDA or EG, HNO3,aq. Microwave radiation synthesis: 110 °C/20 min (EDA) or 180 °C/40 min (EG). Drying in vacuum: 60 °C.

Hexagonal structure. Bi2Se3 ([BMIM][BF4]/EDA) irregularly shaped nanosheets with thicknesses: 50–100 nm. Bi2Se3 ([BMIM][BF4]/EG) hexagonally shaped nanosheets.

[167]

LnF3 where: Ln = La–Lu, Y

RE2O3, Pr6O11, Tb4O7, Ce(NO3)3·6H2O

[BMIM][BF4]

Solvent, reactant and template

Additional compounds: deionized H2O, HNO3. Microwave radiation synthesis: 150 °C/20 min (EuF3 and YF3in different time) (300

Hexagonal structure of LnF3 (Ln: La–Sm) and orthorhombic crystal phase of LnF3 (Ln: Eu–Lu,Y).

[168]

Face-centered cubic structure (CaF2), tetragonal structure (MgF2) and face-centered cubic structure (SrF2). Double-shelled hollow microspheres with diameters: 1.5–3 μm (CaF2) and hollow microspheres with diameters 1–4 μm (MgF2).

[169]

W). Drying: 80 °C. MF2 where: M = Mg, Ca, Sr

Ca(NO3)2·4H2O Mg(NO3)2·6H2O Sr(NO3)2·4H2O

[BMIM][BF4]

Solvent and template

Additional compounds: deionized H2O, NaH2PO4×2H2O. Microwave-solvothermal synthesis: 120 °C/10 min (CaF3), 120 °C/10 min (SrF3), 150 °C/30 min (MgF3).

Ru, Rh, Ir

Ru3(CO)12 Rh6(CO)16 Ir6(CO)16

[BMIM][BF4]


Microwave radiation The average size of nanoparticle: b5 nm. synthesis: 3 min (10 W), under argon atmosphere.

[170]



Table 3 (continued)

Co2(CO)8 Mn2(CO)12

1-methyl-3-(3-carboxyethyl)imidazolium Stabilizer and solvent tetrafluoroborate [ECO2MIM][BF6] [BMIM][BF4]

Microwave radiation synthesis: 6 min (20 W).

Not crystalline structure. Co nanoparticles with diameter: 1.6–0.3 nm [ECO2MIM][BF6], 5.1–0.9 nm [BMIM][BF4]. Mn nanoparticles with diameter: 4.3–1.0 nm [ECO2MIM][BF6], 28.6–11.5 nm [BMIM][BF4].

[171]

CdS

Cd(CH3COO)2·2H2O [EMIM][EtSO4] CH3CSNH2

Hydrolysis-stabilizer and solvent

Additional compounds: deionized H2O. Microwave radiation synthesis: 55% output power, 4–6 min. Drying: 50 °C/24 h.

Cubic structure (nanospheres with aggregation of the nanoparticles). The mean crystallite sizes: 8, 6, and 4 nm (1:1 and 1:4 compositions H2O + IL with 4 min microwave irradiation).

[174]

Au

HAuCl4·4H2O

[EMIM][BF4] [EMIM][PF6]


Additional compounds: EtOH, C4H9Cl. Microwave radiation synthesis: 10 min/210 °C (126 W). Drying in vacuum: 40 °C.

Triangular and hexagonal structure. The average size of nanosheets: 30 μm.

[172]

Au

HAuCl4·3H2O

[BMIM][BF4] [EMIM][BF4] [BPy][BF4] [BMIM][Cl] [BMIM][Br] [BMIM][PF6] [BMIM][Tf2N] [BMIM][Tos]

Control the topology, template and capping

Microwave radiation synthesis: 200–290 °C/5 min.

Triangular, hexagonal, octahedral, decahedral structure. Nano- and microplates.

[173]



Co, Mn


24

Fig. 21. SEM images of various CuO structures prepared at different temperature conditions and volume of IL: (a) 80 °C, without [OMIM][TA]; (b) 80 °C, 2 ml [OMIM][TA]; (c) 80 °C, 3 ml [OMIM][TA]; (d) 100 °C, 3 ml [OMIM][TA] (adapted from [163]).

4.1. Effect of ionic liquid anion type The effect of IL structure (alkyl chain length, type of the cation and anion) was investigated by synthesized SnO2 microspheres in a hydrothermal process assisted by microwave in a presence of e.g. [BMIM][BF4], [BMIM][PF6], [BMIM][Cl], [OMIM][BF4] and 1butylpyridinium tetrafluoroborate [BPy][BF4] [72]. Only in the presence of [BMIM][BF4], [OMIM][BF4] and [BPy][BF4] salts, formation of the SnO2 microspheres with an average diameter 2.5 μm was detected, indicating the crucial role of the IL's anion in the SnO2 preparation. The authors concluded that the [PF6] anion is presumably too hydrophobic, whereas [Cl] can strongly interact with imidazolium cation and water, therefore interactions with SnO2 may be too weak to support particle nucleation and growth. Based on the time-dependent analysis of the particles structure by SEM technique, a multistep growth mechanism was proposed. According to the observations, firstly many small irregular nanoparticles were formed, followed by agglomeration into multispherejoint structures and reaching a self-organized critical state. Finally,

these multisphere-joint structures were broken due to high stress and these resultant fracture units recrystallize into microspheres [72]. The influence of the IL anion type on the morphology of Au nanoand microstructures was also investigated [172, 173]. Single-crystal Au nanosheets (triangular and hexagonal) with a size about 30 μm were formed by microwave irradiation in the presence of [BMIM][BF4] using HAuCl4·3H2O as a precursor. In this synthesis route, [BMIM][PF6] was used to enable formation of large-scale gold nanosheets without using any additional protecting agent [172]. The ILs[BMIM][BF4], [EMIM][BF4], [BPy][BF4], [BMIM][Cl], [BMIM][Br], [BMIM][PF6], [BMIM][Tf2N], and 1butyl-3-methylimidazolium tosylate [BMIM][Tos] were used for preparation of Au colloidal structures based on the HAuCl4·4H2O precursor. The anion of the IL controls the particle morphology by absorption on the metal crystal surface, whereas cation has protection ability and prevents aggregation of the particles. Different shapes are related with a variety of abilities of anions to coordinate the gold surface, leading to the different nucleation mechanisms, ([Cl], [Br] are strong donors of electrons, whereas [PF6] is a weak donor). Microwave heating of HAuCl4·4H2O in

Fig. 22. Formation mechanism of LaF3 nano- and microstructures with multiform morphologies (adapted from [168]).



25

Fig. 23. Stabilization modes of metal nanoparticles by functionalized IL cations, FG — functional group (adapted from [171]).

[BMIM][BF4] provides micrometer-sized octahedra with an edge length ≥ 5 μm as well as decahedral (or pentagonal bipyramidal) clusters up to 6.5 μm in dimension according to the mechanism shown in Fig. 24. The application of [BMIM][Tf2N] ILs results in hollow trapeziform crystals, hexagonal plates, and irregular holey crystal formation when the reaction was carried out at 200 °C, whereas micrometer-sized dendrites were synthesized at 290 °C. The reaction carried out in [BMIM][PF6] at different temperatures provides irregular particles with rough surfaces, whereas in [BMIM][Br], [BMIM][Cl] and [BMIM][Tos] at 200 °C irregular gold structures contaminated with carbon-like products were obtained [173].

5. Sonochemical method Sonochemical methods employ ultrasound irradiation — typically in the range from 20 kHz to 500 MHz to initiate or to alter chemical processes. Chemical effect of ultrasound comes from ultrasonic cavitation that is concerned with the formation, growth, and implosive collapse of bubbles in the liquid environment. Cavitational collapse produces intense local heating (~5000 K), high pressures (~1000 atm), and enormous heating and cooling rates (N109 K/s) [175]. When the ultrasound power is sufficiently high, the decompression during the rarefaction overcomes the intermolecular forces within the medium so that tiny bubbles are formed, which almost instantaneously collapse again. Cavitation bubbles are vacuum bubbles, which are created by a fast moving surface on one side and an inert liquid on the other. The resulting pressure differences serve to overcome the cohesion and adhesion forces within the liquid. In this regard Rabieh and coworkers [176] obtained nanosized ZnO particles (24 ± 7 nm) using [BMIM][Cl] ionic liquids under ultrasonic irradiation. The XRD patterns confirm the hexagonal wurtzite ZnO structure. The authors suggested that forming a spherical ZnO in the presence of the IL could be ascribed to electrostatic interactions between [BMIM] cations and the oxide ions of ZnO nucleus as well as strong hydrogen bonding between oxygen ions of ZnO and the hydrogen atom at position C2 of the imidazolium cation. These interactions might play an essential role in particle growth by protecting ZnO facets from growing faster [176]. Selected properties of

structures obtained by the sonochemical method carried in or in the presence of ionic liquids are collected in Table 4. Parra-Arciniega et al. [177] obtained spherical crystalline NiS and concluded that S2− ions of the semiconductor precursor combine with the [BMIM] cation of the ionic liquid through the electrostatic attraction. The dissociation of Ni(NO3)2 gives rise to solvated metal cations, along with nitrate anions. As soon as NiS nuclei are formed, they get coated by the IL, thereby controlling the growth [177]. Jiang et al. [178] obtained Bi2S3 flower nanostructures using [BMIM][BF4] as a reaction medium via sonochemical method. Flowers with a size of 3–5 μm were composed of nanowires with a diameter of 60–80 nm. The authors demonstrated that the shape evolution strongly depends on the reaction conditions, such as pH value and reaction time (see Fig. 25). With a prolonged aging time, the flowerlike structures tend to become loose and fall off from the mother flowers [178]. It was also proposed that micellar self-assembly structures formed from the hydrophilic and hydrophobic constituents of ILs in aqueous media [179–182] may serve as templates for Bi2S3 particles formation. With the addition of the reactants BiCl3 and acetothioamide (TAA), the solution becomes yellow, indicating the formation of [Bi–TAA] complex which associates with [BF4] groups (see Fig. 26), followed by crystallization of Bi2S3 particles on the surface of the vesicles. Moreover, interactions of ILs with other materials can lead to the formation of unique heterogeneous structures. Sabbaghan et al. [183] studied the effects of different ILs (1,4-diazabicyclo [2.2.2] octane-based IL (DABCO), [BMIM][Br] and imidazolium-based gemini IL) as a template on the morphology and 2D size of ZnO nanostructures (see Fig. 27). By using a longer alkyl chain at the DABCO cation or dicationic IL, uniform nanosheet and nanoleaf were formed. Due to the ability of IL cations precursor, cation–anion couples are created, to interact with Zn(OH)2− 4 and the strengths of this attraction depend on the cation structure (see Fig. 28). Cations of ILs can be stably adsorbed on the surface of ZnO nuclei and the activities generated on the ZnO surface will be greatly inhibited by the adsorbed cation. The longer the alkyl chain in the DABCO-based IL, the lower the solubility in water and increased interaction with ZnO nuclei (IL 3 template). The growth of ZnO particles into specific structure has been attributed to the π–π stack interactions between IL molecules

Fig. 24. Schematic of proposed growth process of the gold nanoplates (adapted from [173]).


26

Compound Precursor

IL

Role of IL


Sb2S3

SbCl3 Thioacetamide CH3CSNH2

[BMIM][BF4]

Solvent

Additional compounds: ethanol Orthorhombic phase. Ultrasound bath: Branson 1510 (70 W, 42 kHz, Diameter particle size 7 nm. 60 °C). Washing: absolute ethanol, distilled water and acetone. Calcination: at 6.8 kPa of pressure for 1 h at 200 °C.

ZnO

Zn(CH3COO)2·2H2O [BMIM][Cl]

ZnO

Zn(CH3COO)2·2H2O [EMIM][EtSO4] Solvent

CdS

SnO2

Morphological template Additional compounds: NaOH. Ultrasound bath: 10 min. Washing: distilled water. Drying: 100 °C for 2 h.

Surface properties

SEM/TEM image

Ref [184]

Hexagonal wurtzite phase. Average particles size 24 ± 7 nm.

[176]

Additional compounds: NaOH. Ultrasound bath: 60 min. Washing: distilled water, ethanol. Drying: 60 °C.

Wurtzite hexagonal phase. Small-sized nanocrystalline.

[185]

Cd(CH3COO)2·2H2O [EMIM][EtSO4] Solvent CH3CSNH2

Additional compounds: water. Ultrasound bath: 60 min. Washing: distilled water, ethanol. Drying: 50 °C for 24 h.

The cubic CdS crystal. The particle sizes 50–100 nm.

[186]

SnCl4·5H2O

Additional compounds: NaOH. Ultrasound bath: 60 min.

Tetragonal rutile phase. Dimension size about 30 nm.

[187]

[EMIM][EtSO4] Solvent



Table 4 Selected properties of structures obtained by the sonochemical method with using ionic liquids.

NiS

Ni(NO3)2·6H2O CH3CSNH2 (TAA)

[BMIM][BF4]

Solvent

Additional compounds: ethanol. Ultrasound bath: 20 kHz, 500 W, 10–30 min. Washing: ethanol. Drying: overnight in a vacuum oven at 80 °C.

Hexagonal structure. Spherical crystalline NiS particles.

[177]

Bi2S3

BiCl3 CH3CSNH2 (TAA)

[BMIM][BF4]

Template

Ultrasound bath: 5 min. Reaction in oven: 120 °C for 0.5 h. Washing: distilled water, ethanol.

The mixture of Bi2S3 phase and tetragonal structure BiOCl. Particle size 3–5 μm composed of nanowires with a diameter of 60–80 nm.

[178]

CuO

Cu(CH3COO)2·H2O

[BMIM][Tf2N]

Reaction medium

Additional compounds: NaOH. Ultrasound bath: 45 kHz, 60 for 24 h. Washing: water and ethanol. Drying: 90 °C for 4 h.

Monoclinic CuO. Nanocrystals with lengths from 30 to 100 nm and diameters of about 10 nm. The BET surface area 61.4 m2/g.

[188]



Washing: distilled water, ethanol. Drying: 50 °C.

28


and strong hydrogen bonds formed between the hydrogen atom at position 2 of the imidazole ring and the oxygen atoms of ZnO particles as schematically shown in Fig. 28 [183]. 6. Microemulsion method

Fig. 25. SEM images of the product obtained in an ionic liquid system after reaction at 120 °C for (a) 0.5 h, starting pH = 2, (b) 1 h, starting pH = 4, (c) 6 h, starting pH = 4, (d) 20 h. starting pH = 4 (adapted from [178]).

According to the definition, microemulsions are isotropic, macroscopically homogeneous, and thermodynamically stable solutions consisting of at least three components, particularly a polar phase (usually water), a nonpolar phase (usually oil) and a surfactant. Formed nanodroplets can take one of the following systems: (1) oil-swollen micelles dispersed in water as oil-in-water (O/W) microemulsion, or (2) water swollen micelles dispersed in oil as for water-in-oil (W/O) microemulsion, also called reverse microemulsion, controlled by the proportion of various components and the hydrophilic–lipophilic balance value of the surfactant used. These nanodroplets can serve both as nanoreactors (to carry out the chemical reactions), and as templates (to control the final size and shape of the particles) [189]. The application of ILs as one phase in aqueous microemulsion systems excludes the use of volatile organic solvents as a nonpolar part of microemulsions as well as enables overcoming the limitations of ILs to dissolve some compounds [190]. Nevertheless, using ILs as a polar

Fig. 26. Illustration of the formation mechanism of Bi2S3 flowers in ionic liquid solution (adapted from [178]).

Fig. 27. SEM images of ZnO nanostructures obtained (a) without IL, (b) with DABCO-based IL1 (structure presented on the picture), (c) with [BMIM][Br] being IL2, (d) with DABCO-based IL3 (structure presented on the picture), (e) with DABCO-based IL4 (structure presented on the picture), (f) with gemini IL (IL5, structure presented on the picture) (adapted from [183]).



29

Fig. 28. Schematic illustration of the 2D ZnO nanostructure formation in the ionic liquid assisted sonochemical approach (adapted from [183]).

Fig. 29. SEM images of the silica materials synthesized under (a) acidic, (b) alkaline conditions (adapted from [193]).

phase in nonaqueous systems is also investigated [191,192]. Type of used ILs as well as nanostructures together with their properties obtained by ionic liquid-based microemulsion systems are presented in Table 5. Based on available literature, it could be concluded that ionic liquids mainly serve as a continuous phase in microemulsion systems applied to prepare semiconductor and metal nano- and microparticles. Droplets of aqueous solution or oil in ionic liquids act as nanotemplates for nanoparticle growth. Zhao et al. reported preparation of SiO2 NPs with two different morphologies by a simple and novel method using droplets of nonaqueous [BMIM][BF4]/TX-100/benzene microemulsion as templates. The ellipsoidal nanoparticles with diameter ranges from 68 to 108 nm were prepared under acidic conditions, while hollow silica spheres (90–100 nm) were obtained under alkaline conditions (see Fig. 29) [193].

Fig. 30. Formation mechanism of silica materials under both acidic and alkaline conditions (adapted from [193]).


30

Compound

Precursor

IL

Role of IL


Surface properties

SiO2

Tetraethylorthosilicate (TEOS)

SiO2

TEOS

Ag

[BMIM][BF4] Continuous phase Silver perchlorate-AgClO4·H2O

CoPt

CoCl2 Na2PtCl6·6H2O

[BMIM][BF4] Continuous phase

Oil phase: benzene. Surfactant: TX-100. Additional compounds: NH3·H2O. Mixing: 24 h. Drying: 60 °C for two days. Calcination: 550 °C for 6 h.

Amorphous phase. Spherical particles with diameter in the range of 90–100 nm.

[193]

[BMIM][PF6] Continuous phase

Surfactant: TX-100. Additional compounds: acetonitrile, HCl. Mixing: 6 h. Drying: 60 °C for 3 days. Calcination: 550 °C for 4 h.

The diameter of rods 600 to 200 nm.

[194]

[BMIM][PF6] Continuous phase

SEM/TEM image

Ref

Surfactant: Tween 20. The average diameters of Ag particles 3.2 and Additional compounds: benzoin. 3.7 nm. Conditions: high-pressure (25 MPa) of CO2, microemulsions were irradiated by UV light (500 W high-pressure mercury lamp) for 6 h.

[196]

Surfactant: TX-100. Conditions: sonicated for 5 min under argon bubbling.

[198]

CoPt nanoparticles, in the 10–120 nm range.



Table 5 Selected properties of structures obtained by microemulsion method with using ionic liquids.


Fig. 31. SEM images of silica obtained at different conditions: (a) 0.15 ml HCl (w = 9) and 0.10 ml TEOS; (b) 0.10 ml HCl (w = 6) and 0.10 ml TEOS; (c) 0.05 ml HCl (w = 3) and 0.10 ml TEOS; (d) 0.10 ml HCl (w = 6) and 0.15 ml TEOS (adapted from ref. [194]).

The authors proposed formation mechanisms of the silica products under both acidic and alkaline conditions as shown in Fig. 30. Tetraethyl orthosilicate (TEOS) was firstly added to the microemulsion and dissolved into the benzene core of the microemulsion. Then HCl or NH3·H2O was added to the mixture and dispersed in the [BMIM][BF4] microemulsion. According to their hypothesis, catalytic ions could be located in the palisade layers of the microemulsion through diffusion process followed by hydrolysis and condensation reactions of TEOS molecules at the interface of the oil cores. Under acidic conditions, the silica nuclei are unstable and aggregate to form large units. Finally, the ellipsoidal silica nanoparticles are formed in the ionic liquid microemulsion system. However, under alkaline conditions, hydrolysis of TEOS undergoes in the palisade layer of microemulsion, and immediately condensation reaction takes place resulting in the subsequent formation of the monomers. Due to the collision and fusion of microemulsion droplets the obtained silica hollow spheres were obtained [193]. Porous silica microrods with nanosized pores were obtained by Li and coworkers [194] by the microemulsion method using the water/TX-100/ [BMIM][PF6] system. The influence of the water to surfactant molar ratio (w) as well as the TEOS content on the morphology of the final material was investigated, and results are shown in Fig. 31. By decreasing the amount of HCl in the system from 0.15 to 0.10 ml and 0.05 ml, the mean diameter of the rods decreased from 600 to 500 nm and 200 nm, respectively, while the effect of the amount of TEOS on the average diameter of the silica microrods is not considerable.

31

The TEOS used as a silica precursor hydrolyzed at the interface of the water droplet embedded in the microemulsion to form silica. In addition, [PF6] anions can also hydrolyze releasing F− influencing silica rod formation due to the reaction of HF with silica [194]. The effect of the length of alkyl chains on the size and stability of the Ag particles in water-in-ILs microemulsions was clarified by Harada et al. [34,195]. They proposed a method of Ag nanoparticle preparation using waterin-ionic liquid microemulsions consisting of the Tween 20, water, and hydrophilic ionic liquids, ([BMIM][BF4] or [OMIM][BF4]). Microemulsion formation was achieved by the dissolution of CO2 (5 MPa) into the IL that makes the continuous phase more hydrophobic, and resulting in a higher stability of Ag particles in the water droplets. The average diameters of the Ag nanoparticles prepared with [BMIM]][BF4] and [OMIM][BF4] were 3.2 and 3.7 nm, respectively. The change in size of water droplets (30–40 nm), which contains AgClO4 and Ag particles, was not noticeably observed during the formation of Ag nanoparticles [196]. Recently, Ag nanoparticles have been also synthesized by the photoreduction of AgNO3 in water-in-ionic liquid microemulsions consisting of nonionic surfactant Tween 20 or Triton X-100, water and [OMIM][PF6] [197]. The average diameters of the finally-grown Ag particles obtained under high pressure of CO2 in water/Tween20/ [OMIM][PF6] and water/TX-100/[OMIM][PF6] microemulsions were found to be 3.7 and 2.8 nm, respectively. They demonstrated that the diameter of the water droplets during Ag particle formation under high pressure of CO2 remained unchanged in the range of 33–37 nm due to their higher stability, whereas under ambient pressure the diameter drastically increases from 28 to 40 nm during the first 60 min of photoirradiation, resulting in the precipitation of larger Ag aggregates, especially in the case of water/[OMIM][PF6]/Tween 20 microemulsions [197]. 7. Sol–gel method In a typical sol–gel process, a colloidal suspension or a sol is formed from the hydrolysis and polymerization reactions of the precursors, which are usually inorganic metal salts or metal organic compounds. The most famous version of the sol–gel process is based on the processes of controlled hydrolysis of compounds, usually alkoxides M(OR)x (M = Si, Ti, Zr, V, Zn, Al, Sn, Ge, Mo, W, etc.) or corresponding chlorides, in an aqueous or organic medium (usually alcohol). Hydrolysis leads to a sol formation, a dispersion of colloidal particles in a liquid, and further condensation provides a gel, an interconnected, rigid and porous inorganic network enclosing a continuous liquid phase. This transformation is called the sol–gel transition. A sol is a dispersion of the solid particles (~0.1–1 μm) in a liquid where only the Brownian motions suspend the particles. A gel is a state where both liquid and solid are dispersed in each other, which presents a solid network containing liquid components. The amount of added water in the hydrolysis step and how the water is added, determines, whether the alkoxides are completely hydrolyzed or not and which oligomeric intermediate species are formed. A crucial role in the sol–gel process is played by the processes of solvent removal from the gel (drying). There are two possibilities to dry the gels. Upon removal of the pore liquid under hypercritical conditions,

Fig. 32. SEM images of synthesized ZnO nanoparticles from (a) [BMIM][BF4], (b) [EMIM][BF4] and (c) [BMIM][PF6] (adapted from [200]).


32


Fig. 33. Schematic representation of ZnO formation with the capping role of ionic liquid (adapted from [200]).

the network does not collapse and aerogels are produced. When the gel is dried under ambient conditions, shrinkage of the pores occurs, yielding a xerogel [175]. Gandhi et al. obtained ZnO nanoparticles by sol–gel method using [BMIM][BF4], [EMIM][BF4] and [BMIM][PF6] as the morphological template [199]. XRD analysis revealed that ZnO NPs have a hexagonal wurtzite structure exhibiting different morphologies of capsule like, flake like and rods like shapes for [BMIM][BF4], [EMIM][BF4] and [BMIM][PF6], respectively (see Fig. 32 and Table 6). It was assumed that the cation of ionic liquids used in this work interacts with the bulk by hydrogen bond or electrostatic force due to the sharing of the electron pair of hydrogen and carbon at position C2 of imidazole ring (see Fig. 33). Thus, it could resulted in π–π interactions or other non-covalent interactions between the imidazolium rings and growing nanostructures and adsorption of ionic liquid on the surface of the growing ZnO crystals, limiting the area of the forming material. Nanoparticles formed in the presence of [BMIM][BF4] were observed to have smaller size that was related with the presence of alkyl chain in the cation responsible for preventing of NPs from further growth due to the steric hindrance effect. The low surface tension of [BMIM][BF4] accelerates moderate interaction between the ZnO nuclei and [BF4], thereby increasing the nucleation at a much faster rate to produce smaller size. It was concluded that the shorter length of alkyl group in ([EMIM][BF4]) cation restricts steric effect and permits the nanoparticles to grow longer. Since [BMIM][PF6] is more hydrophobic and the hydrogen bonding strength between IL cation and [PF6] is weaker, the interaction between the formed nuclei and the anion is too weak to effectively serve for nucleation and growth. Therefore, in [BMIM][PF6] higher in size nanorods were formed [199]. Minami and coworkers

Fig. 34. SEM images of products by seeded dispersion sol–gel processes of Mg(NO3)2 (dropwise addition) at 70 °C in 2-propanol (a) and [BMIM][Cl] (b) using 15 M NH4OH in the presence of PS seed particles (adapted from [201]).

[201] prepared polystyrene PS/Mg(OH)2 particles (with diameter of ca. 380 nm) by seeded dispersion sol–gel process in [BMIM][Cl] in the presence of PS seeds. The particles had smooth surfaces and core–shell morphology consisting of PS core and Mg(OH)2 shell. When the authors used 2-propanol as a medium, the secondary nucleation of Mg(OH)2 was observed, and composite particles were not obtained (see Fig. 34) [201]. Choi and coworkers [202] obtained TiO2 particles via a sol–gel method using [BMIM][PF6]. The TiO2 particles had high surface area, controlled porosity, and narrow pore size distribution. No direct chemical bonding between [BMIM][PF6] and titanium tetraisopropoxide TTIP used as a precursor was observed at the first step of synthesis (see Fig. 35). After stabilizing the [BMIM][PF6]/TTIP mixture for 12 h, an obvious phase separation was observed due to the water immiscibility of [BMIM][PF6]. In step II, [BMIM][PF6] acts as a capping agent preventing direct hydrolysis of TTIP due to its water immiscibility. Solvents and [BMIM][PF6] were removed in step III of the reaction yielding TiO2 particles. Therefore, [BMIM][PF6] was observed to be an attractive tool for achieving longer aging time without shrinkage of the gel network in comparison to conventional molecular solvents that evaporate quickly before formation of a stable sol–gel network during the aging process [202]. Zhou and Antonietti [204] obtained titania nanocrystals in [BMIM][BF4] using TiCl4 as titania source. As a result well-defined, discrete, and porous TiO2 nanosponges, 70–100 nm in diameter, were formed. The TiCl4 hydrolyzed at the beginning of the reaction quickly, presumably into an amorphous TiO2 sol within IL, which gradually ripens into the well-defined TiO2 nanocrystals as the reaction time proceeds. The [BMIM][BF4] IL favors the reaction-limited aggregation of the TiO2 nanocrystals via specific sides, presumably even via epitaxial recognition [204]. Nakashima and Kimizuka proposed a method of hollow titania microsphere formation with diameters of 3–20 μm in [BMIM][PF6] and toluene by sol–gel method. The average diameter of the microspheres was dependent on the reaction temperature: it was 8.8, 10.8, and 14.0 μm at 15, 25, and 45 °C, respectively. As shown in Fig. 36, the molecules of Ti(OBu)4 precursor hydrolyzed selectively at the interface of the microdroplets. When the longer-chained [OMIM][BF4] was applied, hollow particles could be likewise obtained. If the water content in the ILis low (e.g. 2 wt.% water in [BMIM][PF6]) formation of rough, irregular titania microparticles is favored [205]. Yoo et al. obtained anatase mesoporous TiO2 particles using [BMIM][PF6] assisted sol–gel method. The XRD analysis showed anatase crystal phase. The BET surface area and specific pore volume of the TiO2 particles with heat treatment at 100 °C were observed to be about 273 m2/g and 0.308 cm3/g, respectively. The TEM analysis exhibits a


Compound

Precursor

IL

Cr, Mn, Fe, Co, Ni, Cu, Zn doped TiO2

Tetraisopropyl orthotitanate (TIPT) Cr(NO3)3 Mn(NO3)2 Fe(NO3)3 Co(NO3)2 Ni(NO3)2 Cu(NO3)2 Zn(NO3)2

ZnO

Role of IL


Surface properties

SEM/TEM image

Ref

2-Hydroxylethyl-ammonium Solvent medium formate [HEA][Fmt]

Additional compound: ethanol. Stirring: 2 h. Drying: 1 h, 100 °C. Calcination: 4 h at 500 °C.

More anatase than rutile for Zn, Fe, Cu, Co, Ni–TiO2. More rutile than anatase for Mn–TiO2 and Cr–TiO2.

-

[210]

(Zn(NO3)2·2H2O

[BMIM][BF4]


Additional compounds: water, NaOH. Aging: one day. Drying: 80 °C. Calcination: 2 h at 300 °C.

Hexagonal wurtzite structure. Crystallize size 28 nm. Particle size from 40 to 70 nm.

[199]

PS/Mg(OH2

Mg(NO3)2·6H2O

[BMIM][Cl]

Solvent medium

Additional compounds: polystyrene, methanol, NH4OH. Stirring: 70 °C for 24 h. Drying: room temperature under vacuum.

High crystallinity Mg(OH). Crystallize size 26 nm.

[201]

TiO2

Titanium tetraisopropoxide [BMIM][PF6] TTIP

Template

Additional compound: 2-propanol. Stirring: 30 min. Drying: 2 h at 100 °C.

The crystal size around 5 nm. The BET surface area 141 m2/g and pore volume of 0.046 cm3/g.

[202]

TiO2

TiCl4

[BMIM][BF4]

Solvent medium

Additional compounds: water, acetonitrile. Stirring: 80 °C for 12 h. Extracting IL: 50 °C for 8 h.

Particle size diameter 70–100 nm. The BET surface area 554 m2/g.

[204]

TiO2

Ti(OBu)4

[BMIM][PF6]

Solvent medium

Additional compounds: toluene, methanol. Stirring: 10 min. Centrifuged: 1000 rpm for 10 min. Drying: in vacuum.

Anatase microspheres. Diameter of the microspheres: 8.8, 10.8, and 14.0 μm at 15, 25, and 45 °C, respectively.

[205]



33


Table 6 Selected properties of structures obtained by sol–gel method with using ionic liquids.

34

Compound

Precursor

IL

Role of IL


Surface properties

SEM/TEM image

Ref

TiO2

TTIP

[BMIM][BF4]

Template solvent

Additional compounds: ethanol, Anatase crystal phase. hydrochloric acid. The BET surface area from 118 to 498 m2/g. Standing: for 24 h. Reaction: 80 °C for 6 h. Extracting IL: refluxing in acetonitrile. Drying: 50 °C under vacuum.

[211]

TiO2

TTIP

[BMIM][PF6]

Template

Additional compounds: isopropanol, H2O. Stirring: for 30 min. Drying: 100 °C for 2 h. Extracting IL: in acetonitrile.

Anatase crystal phase. The BET surface area decreased from 282 to 48 m2/g for 100 and 800 °C.

[206]

SiO2

Tetramethylortho-silicate TMOS

[HDMIM][Cl]

Template

Additional compounds: ethanol, HCl, poly(styrene). Standing: at room temperature Calcination: 550 °C for 5 h in air.

The macropore size 360 nm with a wall thickness of 100 nm. The BET surface area 244 m2/g.

[212]

SiO2

TMOS

[BMIM][BF4]

Template

Additional compound: HCl. Stirring: 2 h. Reaction: under vacuum at 60 °C. Standing: at 60 °C for 48 h. Extracting IL: with acetonitrile at 90 °C.

Wormlike mesopore of 2.5 nm in diameter and silica wall system of 2.5–3.1 nm in thickness. The surface BET area 801 m2/g.

[208]

SiO2

TMOS

[HDMIM][Cl]

Solvent and the structure template

Additional compounds: HCl, poly(styrene). Reaction: at 40 °C for 24 h. Calcination: 500 °C for 5 h under a flow of oxygen.

The average size of the voids about 175 nm. The BET surface 1340 m2/g.

[213]

CeO2

CeCl3·7H2O

[HDMIM][Cl]

Template

Additional compounds: ethanol, block copolymer. Stirring: 1 h. Calcination: 400 °C under air.

Spherical mesopores of 6 nm × 16 nm and wormlike pores 2.5–3 nm in diameter. The BET surface area 250–300 m2/g.

[207]

CeO2–TiO2

TIPT Ce(NO3)3·6H2O

[HEA][Fmt]

Solvent medium

Additional compounds: ethanol, formic acid. Stirring: 4 h. Drying: 12 h, 100 °C. Calcination: 4 h at various temperatures (500–700 °C).

Particle size 48 nm. BET surface area = 174.84 m2/g.

[214]



Table 6 (continued)


35

Fig. 35. Synthesis route of porous anatase crystalline TiO2 particles using sol–gel method modified with [BMIM][PF6] (adapted from [203]).

disordered wormhole-like pore structure. The high crystalline structure TiO2 could be attributed to the different interactions between water molecules and anions of IL. The [BMIM] cation should be arrayed into the opposite direction to [PF6] bonded with water and then they start to pileup and stack by possibly π–π interaction or other non-covalent interactions of imidazolium rings (see Fig. 37). The nature of the anion part of the IL is crucial factor due to its various affinities to form hydrogen bond with water which was increased in the order [PF6] b [BF4] b [TfO]. It was concluded that stronger hydrogen bonds result in blocking of the π–π interaction between the imidazolium rings and enhance hydrolysis in the presence of an excessive amount of water, producing particles with low surface area [206]. Brezesinski et al. [207] prepared mesoporous ceria using a suitable block copolymer ((CH2CH2CH2(CH)CH2CH3)79(OCH2–CH2)89OH), “KLE” and 1-hexadecyl-3-methylimidazolium chloride [HDMIM][Cl] as a template. Spherical mesopores of 6 nm × 16 nm and wormlike pores 2.5–3.0 nm in diameter were prepared. The BET surface area of such prepared structures was 250–300 m2/g. The special templating behavior of the KLE block copolymer family and presence of ionic liquid seem to play a crucial role as shown in Fig. 38. First, the IL template aggregates to wormlike micelles, tolerating the crystallization of ceria around them to a certain degree, which in clear contrast to alkyl ammonium salts such as CTAB. The model proposed in Fig. 38 for the bimodal pore structure indicates a special interaction behavior of the IL with the KLE mesophase. On the basis of the aforementioned model for the distribution of the IL and KLE, it is evident that the IL molecules form neither mixed micelles with KLE nor a separate phase. In contrast, the fact that the IL mesopores are located between the KLE mesopores suggests that the IL forms micelles (admicelles) adhering to the KLE micelles, which is in marked opposition to the interaction behavior of other surfactants with KLE. Zhou et al. [208] proposed the novel mechanism of formation of the wormhole-like mesoporous silica with a well-developed surface area (801 m2/g) using ILs as a template via the nanocasting technique. A short-chain [BMIM][BF4] ionic liquid was applied, salt which is not supposed to preferentially self-assemble into an ordered micelle structure or LC chase by the rearrangement of the hydrophobic and hydrophilic molecular chains in solution, such as long-chain surfactants [130]. Thus, proposed mechanism based on the so-called “hydrogen bond-co-π–π staking”, in which the hydrogen bonds between the [BF4] and SiO2 matrix in combination with the π–π interactions between the neighboring imidazolium rings, resulted in the mutual packing and formation of mesoporous SiO2 as shown in Fig. 39. Due to interaction between [BF4] ions and the silanol groups, the oriented arrangement of the [BF 4] anion along the pore walls takes place, as shown in Fig. 39. A different approach was proposed by Soll et al. [209]. They used polymerized ionic liquids (PILs) as a soft template for synthesis of meso- and macroporous silica. It was observed that the surface area depends on the amount of PIL template. Too low content of an ionic template resulted in the formation of isolated pores, whereas too high template concentration results in pore percolation in obtained silica.

8. Hydrothermal method Hydrothermal synthesis is a facile and prominent method for the formation of a wide diversity of organized structures, which are difficult to obtain with other available techniques. All reagents are mixed together, introduced into an autoclave, and a hydrothermal reaction can be carried out in different conditions of temperature and time, as schematically shown in Fig. 40 [215–221]. The hydrothermal preparation route can be modified by various factors, e.g. microwave irradiation or by application of the additional compound such as ILs. Up to date ionic liquid-assisted hydrothermal method was applied to synthesize a variety of nanoparticles in different reaction conditions as briefly presented in Table 7. Different types of ILs were used in hydrothermal syntheses playing a role of a co-solvent, template, modifier, structure directing or stabilizing agent. The attractive property of ILs for the preparation of the nanoparticles is mainly the surfactant-like nature [179,222], which inhibits the aggregation of the resultant particles. Moreover, ionic liquids by increasing of the dispersion viscosity and adsorption on the particles surface could limit the growth rate and stabilize the system. It was observed that presence and concentration of the ionic liquid in the reaction mixture significantly affect morphology of the particles, and therefore may allow one to prepare particles with desired properties [223–225]. Therefore, some studies on synthesis route of different types of semiconductors with usage of variety type of ionic liquids and numerous possible mechanisms of particle formation were reported [75,226–232]. Concerning this, it is difficult at this moment to develop general rules describing the effect, for example, of the structure of IL's cation or anion or reaction condition on the morphology and properties of the synthesized nanostructures. Following the literature, it could be concluded that the effect of IL addition as well as its content in the reaction mixture is mainly investigated 8.1. Influence of the amount of IL Close relationship between the amount of ionic liquid in the hydrothermal reaction system and the morphology of nanoparticles was clearly described for the preparation of FeOOH [75]. It was observed that dosage of [BMIM][BF4] lower than 0.5 ml resulted in β-FeOOH brick formation with average length of 1 μm, however higher amount of IL of 0.5–1 ml provided product consisted mainly of α-FeOOH hollow spheres with diameters close to 1 μm (Fig. 41). Another example of such a correlation was the synthesis of CuO nanoparticles. Experiments were carried out in the absence of [OMIM][TA] and with different IL contents in the reaction medium (2, 4 and 6 ml) [227]. It was found that a sample obtained in the absence of IL was mainly composed of many congregating, irregular CuO particles. When the amount of [OMIM][TA] was 2 ml, nanosheets with thickness of 70–80 nm were achieved. With an increase of the amount of [OMIM][TA], CuO nanosheets aggregated and self-assembled into peach stone-like architectures via the oriented attachment. Next example of the influence of IL content was also investigated in the [BMIM][BF4]-assisted synthesis of LiFePO4 [224]. Particles obtained without the IL displayed a plate-like shape with a thickness about 50 nm,


36


Fig. 36. SEM images of hollow TiO2 gel microspheres (a), illustration of the formation process of hollow TiO2 microspheres at the oil droplet/[BMIM] interface (b) (adapted from [205]).

Fig. 37. Scheme for the formation of mesoporous TiO2 particle (adapted from [206]).

whereas addition of 0.5 ml of [BMIM][BF4] provided samples with rectangular nanorod like morphology with diameter of 50 nm and length of 2– 3 nm. A further increase in the amount of IL led to reduction of the LiFePO4 particles dimension. This observation was explained by increased viscosity of the reaction system and low interfacial tension of the IL that results in a faster nucleation rate in comparison with growth in the initial stage of the synthesis. 8.2. IL as a structure directing agent This effect often occurs in hydrothermal synthesis when ionic liquid works on the principle of micelles formation [229,218]. The structure directing ability of ionic liquids was clearly revealed since [BMIM][Cl] enables formation of the multilayered, uniform MoS2 microspheres covered by many hollow vesicle monomers with diameter about 100 nm as shown in Fig. 42. In comparison, particles formed in the absence of IL

have non-uniform shapes. Again it was suggested that the ability of ILs to reduce surface tension of the aqueous solutions provides high nucleation rate, being faster than the microparticles growth rate. 8.3. Order in addition of reagents Great influence on the morphology of the particles has an order in adding reagents during the synthesis. It is important to note that the abovementioned dependence is described in a small number of publications despite the fact that it is a very significant factor, which could radically change the synthesis results. Zhang et al. synthesized CuO by two different routes as shown in Fig. 43 [231]. Addition of [BMIM][BF4] before NaOH to aqueous solution of Cu(NO3)2·3H2O resulted in a sheaf-like particles formation. The proposed mechanism involved Cu2+–[BMIM][BF4] complex formation due to the affinity of IL to the Cu2+ ions, with simultaneous decreased of the

Fig. 38. Illustration of the templating of the KLE Block Copolymer (Red/Blue Spherical Micelles) and Ionic Liquid (Gray Distorted Cylinders), (a) mesostructure after solidification and (b) final bimodal, mesoporous structure with walls composed of crystalline cerium oxide (the small lamellar domains correspond to the nanocrystals, while the pores are white) (adapted from [207]).



37

Fig. 39. Schematic illustration of the proposed hydrogen bond-co-π–π stack mechanism (adapted from [208]).

free Cu2+ cation concentration in the solution. When in the next step NaOH solution was introduced to the system, hydroxyl anions probably replaced IL molecules in the Cu2+–[BMIM][BF4] complex resulting in the slow generation of Cu(OH)2 particles. In the next step, due to the

nucleation and particle growth, Cu(OH)2 nanoribbons and subsequently sheaves of nanoribbons were formed (Fig. 43) [231]. However, dissolution of the Cu(NO3)2·3H2O in NaOH aqueous solution before addition of ionic liquid resulted in direct Cu(OH)2 formation and precipitation from the solution. Therefore, formation of the Cu2+–[BMIM][BF4] complex is limited, and formation of nuclei for the Cu(OH)2 nanoribbons is not facilitated. As a consequence, individual Cu(OH)2 nanoparticles were created providing the irregular CuO nanoplatelets [231]. 9. Other methods Much less studies were done in the field of IL-assisted synthesis of colloidal particles using electrochemical and irradiation methods (Table 8). 9.1. Electrochemical methods

Fig. 40. Block diagram of hydrothermal route used for nanoparticle preparation.

Electrochemical synthesis offer a suitable and universal route to obtain nanoparticles, thin films of metals hierarchical, oriented one-dimensional (1D) semiconductor nanostructures such as nanotubes etc. The advantages of the electrochemical methods compared with other solutions include: (a) rigid control of the film thickness, uniformity and deposition rate, (b) ability to control of the nanoparticles shapes, and (c) possibility of deposition particles/films on substrates with complex shape. Ionic liquids, being electrochemically stable, nonvolatile and highly conducting solvents, are proposed as a potential replacement of aqueous or nonaqueous electrolytes, especially the toxic ones, (such as cyanide bath) providing low-defect products. Ionic liquids in electrochemistry gain also interest since enable formation of the high-quality deposits of nanocrystalline metals with grain size adjusted by varying the electrochemical parameters of the process, composition of bath, application of pulsed electrodeposition. However, the electrodeposition processes in the presence of ionic liquids leads to lower deposition rate due to much higher viscosity of ionic liquids compared to aqueous- as well as nonaqueous based electrolytes. One of the biggest advantages of electroplating using ILs is elimination of problems associated with hydrogen ions evolution from electrochemical bath by applying ionic liquids as aprotic solvents which can alter the quality of the deposits obtained in solution. For example, the Ag nanowires and Ag-layer were obtained by electrodeposition in the air and [EMIM][TfO] using Ag(TfO) as a source of silver. The Ag-layer


38

Compound

Precursor

IL


Surface properties

Ag

AgNO3

poly(1-vinyl-3-ethyl imidazolium) Stabilizer, size controller bromide Poly[VEIM][Br],poly(1-vinyl-3-butyl imidazolium) bromide Poly[VBIM][Br], Poly[EPIIM][Br] Poly[VEIM][OH] Poly[VBIM][OH]

Role of IL

Additional compounds: NaBH4. Temp: 40 °C. Time: 12 h.

Average diameter: 10 nm — Poly[VEIM][Br], 40 nm — Poly[VBIM][Br], 75 nm — Poly[EPIIM][Br], 45 nm — Poly[VEIM][OH] , 60 nm — Poly[VBIM][OH]

[233]

y-Al2O3, y-AlOOH

AlCl3·6H2O

[BMIM][Cl]


Additional compounds: citric acid monohydrate, distilled water Temp: 170 °C. Time: 1; 2; 5; 10; 26; 36; 40; 48 h — product was y-Al2O3 Annealing: 550 °C for 3 h — product was y-AlOOH.

Cubic phase of y-Al2O3 and orthorhombic phase of y-AlOOH. Crystallite sizes of y-AlOOH and y-Al2O3: 7.1 and 5.2 nm. BET surface area of y-AlOOH and y-Al2O3: 220.6 and 240.5 m2/g.

[230]

BiOBr

Bi(NO3)3·5H2O

[BMIM][Br]

Solvent

Additional compounds: EG. Temp: 120, 140, 160, 180 °C. Time: 12 h.

Tetragonal phase. Particle sizes: 1–4 μm.

[234]

BiOBr

Bi(NO3)3·5H2O

[BMIM][Br]

Structure directing agent

Additional compounds: nitric acid, KBr, NaOH, deonized water. Temp: 120 °C. Time: 12 h.

Tetragonal phase. Nanoplates with 120–270 nm in width and 20–35 nm in thickness.

[228]

BiOCl–C3N4

BiNO35H2O

[HMIM][Cl]

Solvent

First obtained graphitic C3N4 by polycondensation of melamine. Additional compounds: glycol, NaCl, KCl. Temp: 140 °C Time: 24 h Different molar ratios BiOCl: C3N4

Tetragonal phase. BET surface area from 19.01 to 34.46 m2/g (depending on ratio of BiOCl:C3N4).

[235]

Bi2WO6

Bi(NO3)3

[BMIM][BF4]


Additional compounds: Na2WO4, deonized water. Temp: 160 °C. Time: 48 h.

Orthorhombic phase. Average particle diameter: 3–5 μm, the thickness of nanosheets: 16–20

[77]

nm.

SEM/TEM image

Ref



Table 7 Selected properties of structures obtained by hydrothermal method with using ionic liquids.

[BMIM][PF6]


Additional compounds: PVP, HMT, TAA, deonized water. Temp: 150 °C. Time: 0, 5; 7; 14; 20; 24 h.

Hexagonal phase. Average particle diameter 130 nm. BET surface area 31.40 m2/g.

[221]

CdS

CdCl2·2H2O

[BMIM][SCN]

Sulfur source

Additional compounds: distilled water. Temp: 200 °C. Time: 6; 12; 24 h. Different molar ratios: CdCl2·2H2O: [BMIM][SCN]. Different ph values: 2–6.

Hexagonal phase. Dendritic structure.

[236]

CdSe

Cd(NO3)2·4H2O

[HDMIM][Cl]


Additional compounds: Na2SeO3·5H2O, N2H4·H2O, deonized water. Temp: 120 °C. Time: 12 h.

Wurtzite phase. Dendritic structure. Diameter and length of the rodlike branches in the range of 10–50 and 50–200 nm.

[225]

CoS

CoCl2

[BMIM][SCN]

Sulfur source and capping Additional compounds: distilled water. ligand Temp: 200 °C. Time: 24 h.

Hexagonal phase. Average particle size 1–2 μm.

[237]

CuO

CuCl2·2H2O

[OMIM][TA], [HDMIM][TA], [BMIM][TA]

Capping reagent

Additional compounds: NaOH, distilled water. Temp: 100 °C. Time: 24 h.

Monoclinic phase. Peach stone-like CuO 3D architecture with a length of about 4 mm and a width of 3 mm ([OMIM][TA]).

[227]

Cu2S

CuCl2

[BMIM][SCN]

Capping agent

Additional compounds: ethanol, distilled water. Temp: 180 °C. Time: 24 h.

Cubic phase. Average particle diameter 2–5 μm.

[238]

CuSCN

CuCl2

[BMIM][SCN]

Capping agent

Additional compounds: ethanol, distilled water. Temp: 140 C. Time: 24 h.

Orthorhombic phase. Flower-like dendrites with length 1–2.5 μm, the diagonal at the center ~ 200 nm.

[238]


CdCl2

(continued on next page) 39

was composed of nodularshaped large agglomerated crystallites of 0.5– 1μm in size, where silver crystallites consist of small grains of sizes in


CdS

40

Compound

Precursor

IL

Role of IL


Surface properties

FeOOH

Fe(NO3)3·9H2O

[BMIM][BF4]


Additional compounds: urea, deonized water. Temp: 160 °C. Time: 30 min. Microwave-heated — product was α-FeOOH or β-FeOOH dependence on amount of IL. Heating α-FeOOH 275 °C for 1 h product was — α-Fe2O3. Adding to α-FeOOH, FeSO4x7H2O, NH3·H2O, 5 min. sonification — product was Fe3O4.

Orthorhombic phase of α-FeOOH. Particle diameters ~ 1

Fe–SiO2, SiO2

Fe(NO3)3·9H2O TEOS

[HDMIM][Br]


Additional compounds: NaF, ammonia, deionized water. Temp: 150, 170, 190 °C. Time: 3 d. Annealing: 550 °C for 6 h.

SEM/TEM image

Ref [75]

μm. Particle sizes of β-FeOOH ~1 μm or ~5 μm Hexagonal phase of α-Fe2O3. Particle diameters ~1 μm. Particle diameters of β-FeOOH ~500 nm.

Worm-like mesoporous structure of SiO2. Particle, diameters 400–500 nm. BET surface area 159.7–515.5 m2/g (depending on time of hydrothermal reaction and presence of NaF). BET surface area of Fe–SiO2 434.7–501.4

[239]

m2/g (depending on treatment: boiling, streaming, calcinating). LiFePO4

LiOH·H2O, FeSO4·7H2O, H3PO4

[BMIM][BF4]

Growth directing template agent and co-solvent

Additional compounds: ascorbic acid, ammonia, glucose Temp: 180 °C. Time: 0, 5; 1; 2; 4; 6; 8; 10; 20 h. Annealing: 600 °C for 4 h in the atmosphere of Ar. Different molar ratios: Li:Fe:P and different amounts of [BMIM][BF4].

β-MnO2

KMnO4

[BPy][BF4]

IL decreases the particle surface energy and controls the growth direction

Additional compounds: HCl, deonized water. Tetragonal phase. Particle sizes Temp: 150 °C. 1–5 μm (depending on amount Time: 1; 3; 6; 24; 36; 60 h. of IL and pH) Different amount of IL and pH.

[240]

MoS2

(NH4)2MoS4

[BMIM][Cl]


Additional compounds: HCl, N2H4·H2O, deonized water. Temp: 200 °C. Time: 24 h

[229]

MoS2

Na2MoO4·2H2O

[BMIM][BF4]


Additional compounds: CH3CSNH2, deonized Hexagonal phase. Average water. diameter of particles 2.1 μm. Temp: 240 °C. Time: 24 h. Annealing: 800 °C for 2 h in the atmosphere of N2 and H2.

Orthorhombic phase. Nanocubes with thickness ~50 nm and length from 50 nm to 3 μm (depending on amount of IL and time of hydrothermal reaction).

Turbostratic structure. Average diameters of particles 1–2 μm.

[224]

[218]



Table 7 (continued)

Y(NO3)·6H2O, Tb(NO3)·6H2O, NaMoO4

[MOIM][Cl]

Solvent

Additional compounds: distilled water. Temp: 180 °C. Time: 24 h.

Tetragonal structure. Dendritic shape.

[241]

NiO

NiSO4·6H2O

[BMIM][BF4]


Additional compounds: deionized water, HMT. Temp: 180 °C. Time: 20 h. Annealing: 500 °C for 2 h.

Hexagonal phase. Mean diameter of microspheres around 7 μm.

[242]

PbS

Pb(CH3COO)2

[BMIM][SCN]


Cubic phase. Dendritic structure.

[237]

TiO2

TiCl4

[DEAMI][Cl]

Solvent

Additional compounds: methyl imidazole, acetonitrile, ether Temp: 120 °C Time: 24 h Anneal: 400 °C for 3 h

Rutile phase. Size of particles ~

[215]

30 nm.

TiO2

TiO2

[BMIM][Cl]

Solvent

Additional compounds: H2O2, HNO3, water. Temp: 130 °C. Time: 48 h.

Anatase phase. Size of particles ~ 50–100 nm.

[232]

TiO2–CeO2

Ce(NO3)3 tetrabutyl orthotitanate (TBOT)

[HDMIM][Br]


Additional compounds: distilled water. Temp: 100 °C. Time: 48 h. Annealing: 550 °C for 2 h. Different molar proportions of Ce in the composition.

Anatase phase of TiO2 and cubic phase of CeO2. Crystal size 11–16

[243]

Additional compounds: distilled water. Temp: 100 °C. Time: 48 h. Annealing: 400, 800, 900 °C for 4 h. Different molar ratios of Zr in the composition.

Anatase phase of TiO2, ZrO2 is presented in separate phase than TiO2. Particle size 20–35 nm. BET surface area 170.6–193.8 m2/g (depending on %ZrO2 in composition).

TiO2–ZrO2

Zr(NO3)4·5H2O, TBOT

[HDMIM][Br]


nm. BET surface area from 150.4 to 198.3 m2/g (depending on %CeO2 in composition).


[244]


41


NaY(MoO4)2:Tb3+

42

Compound

Precursor

IL

Role of IL


Surface properties

TiO2-reduced graphene oxide (RGO)

TBOT graphene oxide

[BMIM][PF6]


Temp: 160 °C. Time: 4 h. Sonification for 30 min.

Anatase phase. Dendritic structure of particles. Crystalline sizes ~ 12 nm.

[245]

YBO3:Eu3+

Y(NO3)3·6H2O, Eu(NO3)3·6H2O, H3BO3

[MOIM][Cl]

Solvent

Additional compounds: NH3·H2O, distilled water. Temp: 180 °C. Time: 24 h. Different pH values.

Hexagonal phase of YBO3. Diameter of microspheres 2.5–4

[246]

Additional compounds: NaOH, distilled water. Temp: 170 °C. Time: 24 h. Different molar ratios Zn2+/Sn4+, different amounts of IL.

Tetragonal rutile structure of SnO2 and hexagonal wurtzite structure of ZnO.

[223]

Wurtzite phase. Spherical morphologies with diameters 1–2 μm.

[237]

ZnO/SnO2

Zn(NO3)2·2H2O, SnCl4

ditetrabutylammonium tartrate [TBA]2[L-Tar]


ZnS

Zn(CH3COO)2

[BMIM][SCN]


TiO2

TIP

1-Allyl-3-(butyl-4-sulfonyl) imidazolium hydrosulfate

Functionalization

SEM/TEM image

Ref

μm.

Additional compounds: KCl, MPTMS, ethanol Average particle diameter 700 Temp: 90 °C. nm. Time: 20 h. Rough surface with the adhesion of large quantities of IL in comparison to reference TiO2 prepared in the same conditions.

[247]



Table 7 (continued)


the nanometer size. Electrodeposition of the Ag nanowires starts on the gold cathode; nanowires have average diameters and lengths about 200 nm and 3 μm, respectively [248]. The gold nanoparticles prepared by electrodeposition in the presence of [BMIM][BF4] was found to have ability to improve the performance of the glassy electrode surface surface (have a more uniform appearance) [249]. Abedin et al [250] proposed a method of Cu and Al nanomaterials electrodeposition in 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate [BMPyr][TfO] and 1butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide [BMPyr][Tf 2N] on conventional solid electrodes without using additives. These solvents enable the formation of shiny, dense and adherent Al and Cu nanostructures with very fine crystallites (30 and 40 nm) on it. Adsorbtion of the IL cationon the substrates and growing nuclei hinder the further growth of crystallites [250]. Continuing research, Ni layer was prepared by electrodeposition from the ethylene glycol/choline chloride [EG]/[ChCl] ionic liquid at voltage of 1.0 V. The nanocrystalline Ni layer has an average grain size of about 6 nm and cubic crystal structure. Moreover, some Ni grains are found to grow on the brass grains, which make a strongly bonded interface [200]. 9.2. Irradiation method Photoreduction (or photoirradiation) process consists the direct reduction of metal ions providing metal particles under the UV light irradiation. Contrary to the typical chemical reduction, usage of hazardous reagents, working as reducing agent, is eliminated. The size and shape of metal nanoparticles are affected by the UV irradiation intensity as well as by the interaction with stabilizing agents. The main advantages of the photochemical synthesis of metal nanoparticles are: (a) elimination of usage of hazardous chemicals (clean process), (b) controllable formation in-situ of reducing agents, (c) possibility of NPs formation in different mediums including aqueous solution, organic solvents, emulsions, glasses, polymers films, and (d) better radiochemical stability than aqueous solution [251]. Photoirradiation solvates electrons and radicals in reaction medium (solvents), what affects formation of metal particles depending on the present of appropriate additives. Nanoparticles of Au were synthesized in the variety of ionic liquids by γ-rays and irradiation method using accelerated electron beams at 6 or 20 kGy [252]. The ILs containing [Tf2N] anion and different cations [BMIM], N-propyl-N-methylpiperidinium [PMPip], and [N1444], whereas as a precursor NaAuCl4·2H2O was applied. It was revealed that decomposition of ionic liquid by irradiation causes reductions of Au (III) to Au nanoparticles, that are easier formed in [N1444][Tf2N] followed by [PMPip][Tf2N], and [BMIM][Tf2N]. Contrary to most results regarding preparation of metal particles using radiolysis, spherical Au nanoparticles with particle size of 7.6 nm at 6 kGy and 26.4 nm at 20 kGy were received by accelerated electron beam irradiation. For comparison, the γ-ray method results in formation of smaller sized Au nanoparticles (2.9 nm at 6 kGy and 10.7 nm at 20 kGy) due to lower exposure dose rate [252]. Nanoparticles of Au (~122 nm mean diameter) were received via the reduction of NaAuCl4·2H2O in [BMIM][Tf2N] used as a solvent by low-energy electron beam irradiation. The reductive reaction was observed to take place at the surface of the ionic droplet. Electron images in Fig. 44 present IL droplets in the form of large black areas, where white parts correspond to Au nanoparticles generated with increasing irradiation time. An analysis of the images revealed that after 30 s, small particles were formed over the entire surface of the ionic liquid, after 60 s the aggregation of particles take place. Particles became longer and thicker with increasing irradiation time, which may happen probably due to convection of ionic liquid by electron beam irradiation [35]. It was observed that the type of the IL anion affected the size and morphology of the colloidal product. Some radical intermediates generated by the solvated electrons lead to the enhancement of the nanoparticle growth. As assumed, the anion strongly affected these properties.

43

For example the Ag particles, synthesized in [BF4], and [PF6]-based ionic liquids, were symmetric and had quasi-rounded shape, whereas in ILs containing the [Tf 2N] anion have a distorted and rugged shape. It was also noted that if the thickness of the IL layer surrounding the particle was smaller than the penetration length of the electron beam, the formation of secondary particles was suppressed [253]. Silver NPs deposition at the surface of graphene nanosheets was obtained using γ-irradiation in the presence of [EMIM][Ac] [254]. In the proposed method, silver ions and graphene oxide were reduced in one step. The difference in size of the silver NPs was not observed to differ much with using irradiation dose of 50 and 160 kGy. It was concluded that graphene oxide and silver ions were reduced by the electron generated from the radiolysis of water, since the [EMIM][Ac] serves as a scavenger for the oxidative OH radicals [254]. The ionic liquids [BMIM][BF4] were used for synthesis of Fe, Ru and Os nanoparticles by photolytic decomposition with Hg UV lamp (1000 W) in the range of 200–450 nm. Nanoparticles produced by photolysis give larger particles (from 2.9 nm to 10.7 nm diameter of particle) because of faster decomposition and growth process in the ionic liquid [119]. In summary, the formation of metal nanoparticles in ionic liquids by photoreduction has much effort to develop new structuring techniques to control their crystallinity, size and morphology 10. Conclusions Generally, it could be stated that ionic liquids are mainly used to form metal nanoparticles (both noble and transition metals) using chemical reduction, microwave, γ-irradiation or UV irradiation mediated methods. Imidazolium ILs, especially BMIM, are the most often used cations in the synthesis of nano- and microparticles, in conjunctions with anions such as: [BF4], [PF6], and [Tf2N], due to their low melting temperature, low viscosity and good transportation properties. In addition, chlorides and bromides are often applied due to their low prices, despite the fact that most often occur as a solid. Literature review revealed that the investigation focused on the interaction between growing/stabilizing particles and surrounding the IL is mostly carried out for metal nanoparticles, while they are limited in the case of semiconductor particles. Recent literature shows, that metal nanoparticles are popularly obtained by chemical reduction or decomposition of organometallic precursors in the presence of ILs. A wide range of research in this topic, allow estimating and generalizing the effect of cation and anion type on the size and morphology of metal NPs. Generally, it was observed that increasing in organization range order of ILs, by increasing alkyl chain length in the cation, results in decreasing polar domain formation, providing the NPs with smaller diameter and size-distribution, when ionic precursor is used [129,131]. However, if the nonionic metal precursor is dissolved in the IL non-polar domains, the size of NPs depends on the size of these regions, and increases with the elongation of the alkyl side chain in the imidazolium cation [133]. In addition, it was revealed that the relative size of the metal NPs can be related to the coordination ability of the IL anion. Nevertheless, it was concluded that anion of the IL rather controls the particle morphology by absorption on the metal crystals surface, whereas cation has protection ability and affects the NP size by preventing aggregation of the particles. Much less studies were done in the field of IL-assisted synthesis of semiconductor particles using sol–gel, hydrothermal and microemulsion method. Due to the broad spectrum of used ILs and the diversity of synthesized semiconductors, it is difficult at the moment to indicate clear dependence between the structure of ILs and morphology and size of formed semiconductor nano- or microparticles. However it could be clearly stated, that except the IL structure, the IL amount also determines the morphology of the product obtained by ionic liquid-assisted methods. One of the proposed approaches is based on the adsorption of


44

Electrochemical method Compound Precursor

IL

Role of IL


Surface properties

SEM/TEM image

Ref

Ag

Ag(TfO)

[EMIM][TfO]

Stabilizer and template

Potentiostatic electrodeposition: Ag layer (−0.3 V/2 h) Ag nanowires (−0.3 V/4 h)

Cubic structure. The average size of crystallites on layer: 35 nm. Ag nanowires with diameters and lengths: 200 nm, 3 μm.

[248]

Au

HAuCl4·3H2O

[BMIM][BF4]

Direct electron transfer reaction between proteins and electrode surface

Potentiostatic electrodeposition: Au nanoparticles (0.5 V/15 min/20 °C)

Cubic structure. The average size of Au nanoparticles: 100 nm.

[249]

Cu, Al

Cu(TfO)2, AlCl3

[BMPy][Tf2N], [BMPy][TfO]


Potentiostatic electrodeposition: Cu nanomaterials (−0.6 V/2 h/25 °C in Cu(TFO)2 /[BMPy][TfO]) Al nanomaterials (−0.45 V/2 h/100 °C in AlCl3/[BMPy][Tf2N])

The average size of Cu and Al crystallites: 30 and 40 nm.

[250]

Ni

NiCl2·6H2O

ethylene glycol/choline chloride ([EG]/[ChCl])


Potentiostatic electrodeposition: Ni layer (1.0–1.5 V/25 °C)

Cubic structure. The average size of Ni crystallites: 6 nm.

[200]



Table 8 Selected properties of the nanostructures obtained by ionic liquid-assisted electrochemical and irradiation methods.

Compound Precursor

IL

Role of IL


Surface properties

SEM/TEM image

Ref

Au

NaAuCl4·2H2O [N1444][Tf2N], [PMPip][Tf2N], [BMIM][Tf2N]


Spherical Au nanoparticles with size of: 7.6–26.4 nm Irradiation: Accelerated electron beam (4.8 MeV, 10 mA, (electron beam), 2.9–10.7 nm (γ-rays). 6 or 20 kGy) γ-rays (60Co, 1.17 and 1.33 MeV, 6 or 20 kGy)

Au

NaAuCl4·2H2O [BMIM][Tf2N]


Irradiation: accelerated electron beam (1 × 10−8 A, 10 kV)

The average size of Au nanoparticles: 122 nm.

[35]

Au

NaAuCl4·2H2O [EMIM][Tf2N] [BMIM][Tf2N] [HMIM][Tf2N] [EMIM][BF4] [BMIM][BF4] [OMIM][BF4] [BMIM][PF6] [OMIM][PF6]


Irradiation: accelerated electron beam (1 × 10−8 A, 20 kV)

The average size of Au nanoparticles: 28–76 nm.

[253]

Ag

AgNO3

[EMIM][AC]

Controller size and dispersion of NPs

Irradiation: γ-rays (60Co, 160 kGy)

The average size of Ag nanoparticles: 20 nm on the surface of reduced grapheme oxide.

[254]

Fe, Ru, Os

Fe2(CO)9 Ru3(CO)12 Os3(CO)12

[BMIM][BF4]


Irradiation: Hg-UV lamp (1000 W, 200–450 nm)

Nanoparticles with size of: 1.5–2.5 nm (Ru and Os), 4.2–5.2 nm (Cu).

[35]

[252]



Irradiation method

46


Fig. 41. SEM images of β-FeOOH bricks obtained with 0.1 ml IL (a); α-FeOOH hollow spheres obtained with 0.5 ml IL (b) (adapted from [75]).

Fig. 42. Schematic illustration of the formation mechanism of multiwall MoS2 hollow monomers (adapted from [229]).

semiconductor precursor molecules at the surface of micelles built of ionic liquid molecules. Thus, in the approach, ILs serve as a soft template for growing microparticles. Literature studies indicated that mainly spherical micelles are used as a template to form hollow sphere structures. Potentially, ILs could create also tubular and lamellar micelles, which could serve as a template for formation of nanotubes or nanosheet, respectively, as schematically shown in Fig. 45.

In addition, ability of ionic liquids to support micellar aggregate formation of nonionic and ionic surfactants in was also confirmed in the literature [255]. From a practical point of view, micellar IL systems may give an opportunity to overcome IL limitations to dissolve materials inherently insoluble in them. Thus, investigation focused on the formation of various shapes of micelles and application of them as soft templates should be taken under further consideration.

Fig. 43. Two routes of synthesis CuO and SEM images of sheaf-like CuO particles (a) and irregular CuO nanoplatelets (adapted from ref. [231]).



47

Summarizing, one of the major challenges for the scientific and industrial community, involved in the preparation of nano- and microparticles with desired morphology in the presence of ionic liquids (green technology approach), is better correlation between IL structure (anion and cation type) with their properties (charge density, viscosity and molecule conformation) and finally with the mechanism of interaction with growing nanoparticles. Ionic liquids could be a useful tool to interact with metals or semiconductor surface in controlled way, resulting in formation of desired morphology due to their amphiphilic properties, inherent charge, high dielectric constant and the ability to form CHN-carbenes. At this moment, there is a lack of theoretical estimation of IL properties used in NP and MP synthesis. Modern strategy, using idea built-by-design, should combine the theoretical prediction of IL properties with experimental verification of ability of theoretically selected ILs for synthesis of metal or semiconductor particles and finally leads to comprehensive modeling of the relationship between IL and NP structure. The most important conclusions were summarized in Table 9. Acknowledgment

Fig. 44. Images of the ionic liquid irradiated with an electron beam for a) 0 s, b) 90 s, c) 180 s, and d) 300 s (adapted from [35]).

The author, J.Ł., acknowledges funding from the National Science Center within program OPUS 3 (grant entitled: Study on aggregation behavior of nonionic surfactants in ionic liquids, contract No. UMO2012/05/B/ST4/02023). The author, A.Z.-M., acknowledges funding

Fig. 45. Potential shapes built of self-assembling molecules of ionic liquids.

Table 9 The most important conclusions regarding application of ILs in synthesis of metal and semiconductor particles. Type of particles

Metal nanoparticles

Semiconductor nano- and microparticles

Possible synthesis routes

Precipitation, microwave heating, sonochemical, microemulsion system, electrochemical, irradiation method Solvent, stabilizing agent, reducing agent The relative size of the metal NPs can be related to the coordination ability of the IL anion. Hydrophobic ILs provides better environment for agglomeration of metal atoms/clusters than hydrophilic ILs Increasing of the ILs organization range order by increasing the alkyl chain length in the cation favors formation of the NPs with smaller diameter and size-distribution. The relative size of the metal NPs prepared based on the ionic precursor could be related to the volume of the polar nanoregions since the ionic precursor is located in the polar nanodomains of IL. Long alkyl chains of imidazolium derivatives can form lamellar structure favors formation of micro-sized metal plates.

Sol gel, hydrothermal; solvothermal, microwave heating, microemulsion system, electrochemical Solvent, continuous phase (microemulsion system), stabilizing agent In the case of microwave heating method, increase of IL content in the reaction mixture decrease the size distribution confirming ability of IL to prevent agglomeration of the particles. The amount of ILs determines the morphology of the product obtained by MWIL, hydrothermal method. Longer alkyl chain of ILs cation aid to form structures such as nanosheet or nanoleaf due to interactions (the π–π stack interactions and strong hydrogen bonds) between IL molecule and semiconductor particles. ILs could prevent hydrolysis of precursors in the sol–gel method, resulted in longer aging and finally more porous structure. In the hydrothermal method, ILs by increasing of the dispersion viscosity and adsorption on the particles surface could limit the growth rate and stabilize growing particles. In the hydrothermal method the order of regents introduction onto reaction medium (eg. reagents/IL or IL/reagents) affected the morphology of growing particles. ILs could serve as a soft template during formation of hollow structure.

Possible application of ILs Most important general rules


48


from the National Science Centre within program OPUS 2 (grant entitled: Preparation and characteristics of novel three-dimensional semiconductor-based nanostructures using template-free methods, contract No.: UMO-2011/03/B/ST5/03243), for work described here.

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