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Ionic liquids for synthesis of inorganic nanomaterials
Current Opinion in Solid State and Materials Science 12 (2008) 1–8
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Current Opinion in Solid State and Materials Science journal homepage: www.elsevier.com/locate/cossms
Ionic liquids for synthesis of inorganic nanomaterials Zhonghao Li a,*, Zhen Jia b, Yuxia Luan c, Tiancheng Mu d a
Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, School of Materials Science and Engineering, Shandong University, Jinan, Shandong Province 250061, PR China Department of Chemistry, Dezhou University, Dezhou, Shandong Province 253023, PR China c School of Pharmaceutical Sciences, Shandong University, Jinan, Shandong Province 250012, PR China d Department of Chemistry, Renmin University of China, Beijing, 100872, PR China b
a r t i c l e
i n f o
Article history: Received 5 September 2008 Accepted 6 January 2009
Keywords: Ionic liquids Inorganic nanomaterials
a b s t r a c t In this review, the recent development of synthesizing inorganic nanomaterials in ionic liquids (ILs) is outlined. There are encouraging results suggesting that the ionic liquid route can guide design, production, and application of inorganic nanomaterials across the range of size, shape, composition, and functionality. All examples clearly show that ILs add great value to inorganic nanomaterials area. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Recently, inorganic nanomaterials have attracted much attention for their unique chemical and physical characteristics compared with the bulk solids [1–6]. The interesting size and shape dependent properties of the inorganic nanomaterials make the scientists endeavor to prepare inorganic nanomaterials with controlled size and structure. The preparation of inorganic nanomaterials that possesses desired properties is a great challenging task. Usually water or traditional organic solvent are involved in the synthesis of inorganic nanomaterials. Each of the synthetic routes with these solvents has advantages and disadvantages, and new synthetic routes with other solvents are still necessary to be developed in view of both fundamental and application purposes. Room-temperature ionic liquids (ILs) are organic salts, composed of entirely ions, with low melting points of below 100 °C, sometimes as low as 96 °C [7]. The first finding of an IL with a melting point of 12 °C was reported in 1914 [8]. Subsequently, a succession of the interesting analogous compounds have been discovered. The most extensively studied type of ILs is the 1-alkyl-3methylimidazolium salts due to their air and moisture stability. The physicochemical properties of ILs depend on the species of cation, anion and the length of the lateral alkyl groups on the heterocyclic rings, therefore alternation of the anion or the length of the alkyl group allows finely tuning the physicochemical properties of ILs, such as viscosity, solvation, catalytic activity, hydrophobicity and melting point [9,10]. * Corresponding author. Fax: +86 531 88392315. E-mail address: [email protected] (Z. Li). 1359-0286/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2009.01.002
ILs possess a wide liquidus, in some cases excess of 400 °C and they are good solvents for a wide range of inorganic, organic and polymer materials. The combination of these individual reagents can dissolve in the same phase which makes the great application of IL in the related areas. The ILs’ unusual properties, such as high polarity, negligible vapor pressure, high ionic conductivity, and thermal stability make them as attractive environmentally benign solvents for organic chemical reactions, polymer synthesis, separations, and electrochemical applications [11–14]. Until now, the related books [15–17] and reviews [11,18–20] on this subject concerning with the chemical reactions and physicochemical properties of ILs have been extensively published. In recent years, the advantages of ILs in inorganic nanomaterials synthetic processes have been gradually realized and have received more and more attention due to their unique physical and chemical properties [21–25]. Various approaches to the fabrication of inorganic nanomaterials with unprecedented and sometimes unique structures and properties have been reported in ILs. The most interesting thing is that it cannot only reproduce conventional inorganic nanomaterials, but also provide pathways towards new inorganic nanomaterials with properties that cannot (or only with great difficulty) be made via conventional processes. The synthesis of inorganic nanomaterials in ILs is a rather new field, which has roughly emerged over the last 10 years [21–25]. Due to the importance and the quick development of ILs in inorganic nanomaterials, it seems timely to review the field on developments of inorganic nanomaterials synthesis in ILs. In this review, the recent advances in the synthesis of inorganic nanomaterials in ILs will be presented. The inorganic–organic hybrids such as metal–organic frameworks synthesized in ILs have been reviewed recently by Parnham and Morris [26], and will not be discussed here.
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2. Methods 2.1. Electrodeposition synthesis ILs are composed solely of ions, therefore it would be expected that ILs have high conductivities. ILs have reasonably good ionic conductivities compared with those of organic solvents/electrolyte systems [27]. The specific conductivity for ionic liquid can be up to 10 mS cm1 while that of water is 4 108 mS cm1 at room temperature. The conductivity of aqueous solution is related to the concentration of electrolyte. If the conductivity of aqueous solution reaches to the similar conductivity of ionic liquid, the electrolyte concentration in the aqueous solution must be increased. For example the specific conductivity of 0.1 M KCl aqueous solution is 13 mS cm1 which is similar to that of ionic liquid at room temperature. The conductivity of solution depends not only on the number of charge carriers but also on their mobility. The conductivity of ILs increases while the viscosity of ILs deceases as the increase of the temperature. Therefore, the ionic conductivities can be well controlled by temperature. The ILs have received extensive attention because of their large electrochemical windows. As ILs have the properties of wide potential windows, high solubility of metal salts, avoidance of water and high conductivity compared to non-aqueous solvents, they are excellent solvent for the electrodeposition. Electrochemical approaches have been among the first to be used for the fabrication of inorganic nanoparticles and nanostructured films in ILs. The properties of ILs opened the door to the electrodeposition of metals and semiconductors at room temperature, which was formerly only possible from high-temperature molten salts. For example, Al, Mg, Ti, Si, Ge and rare-earth elements related materials can be obtained from ILs [25,28–31]. Besides these, the electrodeposition of Ag, Cd, Cu and Sb in 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] or basic chloride containing 1ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF4] was also reported [32–35]. Recently it is demonstrated that electrodeposition and de-alloying process in IL is a facile route for constructing porous nanostructures. For example, Sun et al. obtained the nanoporous Au, Pt, and Ag structures by electrodeposition of a binary alloy such as gold-zinc, platinum-zinc and silver-zinc and subsequently electrochemical dealloy them in zinc chloride-1ethyl-3-methylimidazolium chloride [36–38]. Fig. 1 shows the microstructure of a platinum surface after anodic stripping of the PtZn surface alloy that was produced by electrodeposition of 0.72 C/cm2 zinc on the pure platinum in zinc chloride-1-ethyl-3methylimidazolium chloride [38]. In comparison with conventional molecular electrolyte solutions, ILs provide a more versatile environment for fabricating porous metals. Both the deposition and the de-alloying steps are performed in a single bath of
Fig. 1. De-alloyed Pt wires that were electrodeposited with Zn at 120 °C followed by de-alloying at 1.2 V to remove the surface PtZn alloy (reproduced from Ref. [38]).
relatively low temperature without using any other corrosive acids or bases. The zinc(II) species consumed during the deposition step is recovered during the de-alloying step, and therefore the IL is reusable. The electrodeposition and electrochemical de-alloying route opens the door for the fabrication of nanoporous metal materials which can be used for many areas like catalysis. This concept is definitely promising for the future application of ILs in synthesis of inorganic nanomaterials with designed porous structures. 2.2. Thermal synthesis ILs are known for their nonvolatile, nonflammable, and thermally stable properties. These properties make the reactions in open reactor possible. Recently, the term ‘‘ionothermal” has been used to describe reactions that are conducted in ILs at high temperature with ambient pressures. As a result, ionothermal reactions avoid the high pressure of hydrothermal or solvothermal reactions and eliminate safety problems related with high pressure. Cooper et al. have demonstrated that imidazolium-based ILs can be utilized to synthesize several phosphate-based microporous zeolites by an ionothermal process [39]. Dai et al. reported the use of a newly developed IL system containing zinc metal ions, which can serve as both solvents and metal-oxide precursors, for manufacturing nanostructured zinc oxide under the ionothermal condition [40]. The morphologies of ZnO are strongly dependent on the nature of the IL precursor (ligands), providing unique methodologies to control the growth conditions. Fig. 2 shows the flowerlike ZnO nanoparticles prepared in Zn(propylamine)4 (NTf2)2 [(NTf2) = N(SO2CF3)2] at 110 °C by an ionothermal process [40]. Recently, Yang et al. utilized the IL as a high-temperature environment for the synthesis of CoPt nanorods with 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, [BMIM] [Tf2N] [41]. Subsequently, combining the IL with the oleic acid as capping agent, Ag and Pt nanoparticles were successfully prepared [42]. These metal nanoparticles can be readily dispersed in conventional organic solvents and the synthetic approach can potentially facilitate the development of continuous production of high quality nanoparticles. The surface energy of the polar planes of inorganic crystals can be reduced by compensating the surface charge with a passivating reagent such as an IL due to the strong electrostatic interactions between the ions of the IL and the polar surfaces. According to this principle, Xie et al. synthesized ZnO hexagonal micro-pyramids by a successful control of the exposed polar surfaces with the assistance of an IL (R–COO2 R–NH3+) at high temperature in air [43]. In some cases, the IL solution has template effect for the formation of inorganic nanomaterials. For example, uniform Bi2S3 flowers have been synthesized in [BMIM][BF4] solu-
Fig. 2. Representative SEM images of ZnO from the Zn(propylamine)4–(NTf2)2 precursor at 110 °C (reproduced from Ref. [40]).
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Fig. 3. SEM images of the product obtained in an ionic liquid system after reaction at 120 °C for 0.5 h. (reproduced from Ref. [44]).
tion at low temperature and ambient atmosphere as shown in Fig. 3 [44]. If water and other solvent are mixed with ILs, the reaction in ambient condition with high temperature is difficult. However this can be solved by refluxing or solvothermal process. Han et al. synthesized mesoporous SrCO3 spheres with diameter ranging from 200 to 400 nm via refluxing the solution of SrCl26H2O and sodium hydroxide in 1,1,3,3-tetramethylguanidinium lactate (TMGL) at 140 °C in the presence of CO2 [45]. CaCl2 was used to synthesize CaCO3 at the same experimental conditions, resulting in hollow calcite spheres and peanut-like structure calcite whose shell thickness and core diameter were about 100 and 200 nm, respectively, [45]. LaCO3OH nanostructures were also synthesized in TMGL by a solvothermal process which indicates the role of the template effect of the IL [46]. Based on the fact that [BMIM]Cl is an effective solvent for cellulose at high temperature, Li et al. prepared gold microcrystal via reduction of HAuCl4 by cellulose in [BMIM]Cl under high temperatures as shown in Fig. 4 [47]. 2.3. All-in-one synthesis (solvent-template-reactant) Anions such as sulfate, phosphate, carbonate, chloride, and metal cations in ILs can be viewed as reactive IL precursors for the fabrication of inorganic nanomaterials. In these cases, the IL is not only a solvent (or at least a solvent which shows a more or less controlled template effect) but also acts as a molecular precursor with a well-defined composition, structure, and reactivity. These parameters can be exploited for the fabrication of uniformly structured inorganic nanomaterials with various properties. Such ILs can be viewed as solvent–reactant ILs. In case the components of an IL are appropriately chosen, ILs exhibit long-range order and liquid crystallinity [48]. These so-called
ionic liquid crystals (ILCs) often contain metal ions, which can be reacted as described above. Due to the fact that ILCs form ordered domains, within which the reaction occurs, ILCs provide a facile way of reaction control for the inorganic materials. ILCs can be viewed as true solvent–template–reactant systems, where the ILC is solvent, template, and reactant simultaneously for the fabrication of desired inorganic nanomaterials with defined morphology or chemical composition. Based on this, Taubert et al. developed a protocol for the controlled synthesis of CuCl nanoplatelets with a well-developed crystal habit, a tunable particle size and connectivity from the Cu-containing ILC [bis(dodecylpyridinium) tetrachlorocuprate(II)] and the reducing agent (6-O-palmitoyl ascorbic acid) [49–51]. Exchange of the ligand, metal cation, anion, and reducing agent can offer a universal pathway towards metal halide and metal (alloy) nanostructures with tunable composition and morphology [52]. Analogous to the long-chain ILC, the general ILs containing the reactant ions can also be used to prepare inorganic nanomaterials which is defined as ILPs. Zhu et al. reported the use of a newly developed IL system containing zinc metal ions, which can serve as both solvents and metal-oxide precursors, for manufacturing nanostructured zinc oxide under the ionothermal condition [40]. Li et al. have shown that the strongly hydrated IL tetrabutylammonium hydroxide (TBAH) is an ILP for the controlled fabrication of unique hollow zinc oxide mesocrystals [53]. The SEM image of sample obtained at 10 mg of zinc acetate/g of TBAH is shown in Fig. 5. The particles are up to ca. 10 lm long and consist of smaller nanoparticles. Furthermore, the larger particles usually contain a central channel, which run parallel to the main particle axis. These particles can be defined as hollow ZnO mesocrystals. The interaction of ionic liquid with the primary ZnO particle play an important role for the formation ZnO mesocrystals. The large tetrabutylammonium cation of TBAH can bond to the negatively charged surface O2 (0 0 0 1) of the primary ZnO particles and reverse the polarity of the negatively charged surfaces of primary particles and thereby prevent further growth and aids the aggregation of primary ZnO particles into large mesocrystals. The TBAH can also be used for constructing unusual nanostructured ZnO particles (lotus like plate, porous plates and nanoparticle aggregated flowerlike particles), ZnO/carbohydrate hybrid, other metal (hydr)oxides, and zinc silicate nanomaterials [54–58]. Besides the application of TBAH, another designed ILP (an alcohol modified IL) was used to synthesize gold nanoparticles by all-in-one route. This IL is able to reduce Au(III) salt to Au(0) particles directly due to the –OH group present in the IL, which avoids additional reducing agent for the reaction [59]. 2.4. Microwave heating synthesis
Fig. 4. SEM images of gold crystals prepared at 100 mg of cellulose per gram of 1butyl-3-methyl imidazolium chloride (reproduced from Ref. [47]).
Large positive ions with high polarizability in ILs make them excellent solvents for absorbing microwaves. Therefore, the use
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2.5. Sol–gel synthesis
Fig. 5. SEM images of samples precipitated at 10 mg of zinc acetate/g of TBAH. Scale bars are 2 lm. Insets are magnified views of the same samples. (reproduced from Ref. [53]).
of microwave heating in ILs for the synthesis of inorganic nanomaterials has advantages over other solvents. Zhu et al. successfully synthesized single-crystal tellurium nanowires using microwave heating in N-butylpyridinium tetrafluoroborate IL in the presence of polymer surfactant of poly(vinylpyrrolidone) (PVP) [60]. Li et al. synthesized large-size single-crystal gold nanosheets by microwave heating of HAuCl4 in [BMIM][BF4], without any additional template agent [61]. Subsequently, PbCrO4 rods or bundle-like Pb2CrO5, Bi2Se3 nanosheets, sulfide M2S3 nanorods, single-crystalline cryptomelane-type manganese oxide octahedral molecular sieve (OMS-2) nanoneedles and nanorods, metal fluorides such as FeF2, CoF2, ZnF2, LaF3, YF3, SrF2, metal-oxide like Fe2O3, carbon-coated core shell structured copper or nickel nanoparticles and CNTs/Pt or CNTs/Rh composites have been successfully fabricated by microwave-assisted synthesis method in ILs [62–68]. A very recent report by Liu et al. demonstrates that anatase TiO2 nanocolloids with uniform size and shape can be prepared via a microwave-assisted route in [BMIM][BF4](Fig. 6) [69]. The synthesized TiO2 nanocolloids are highly crystalline, low in Ti3+ defect, free of aggregation. Moreover, the size of TiO2 nanocolloids can be easily controlled. In these efforts, it demonstrated that the ILs work as both a solvent to absorb microwaves and capping agent or directing agent to result in the final morphology. The advantages of microwave process include: the synthesis is fast and simple; the reaction can be performed under atmospheric pressure even in a domestic microwave oven; no high pressure and high temperature apparatus are required. These advantages make microwave method a promising route for a fast and largescale synthesis of inorganic nanomaterials in ILs.
Some ILs provide hydrophobic regions and a high directional polarizability, which is oriented parallel or perpendicular to the dissolved species, as well as extended hydrogen-bond systems in the liquid state resulting in a highly structured frame. These characteristics encourage the preparation of well-defined and extended ordering of inorganic nanostructures [22–24]. A lot of work concerning the synthesis of inorganic nanomaterials is based on sol– gel route. The original use of ILs as solvents for inorganic sol–gel reactions is the acid-catalyzed sol–gel synthesis of silica aerogels [70]. It turned out that such aerogels can be dried without a supercritical drying procedure. Subsequently, 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6] was used to synthesize hollow TiO2 microspheres by interfacial sol–gel reaction [71]. The imidazolium molecules act not only as the solvent but also as stabilizers for the hollow microspheres. Zhou et al. used TiCl4 as the precursor to synthesize very small TiO2 nanocrystals in [BMIM][BF4] and obtained mesoporous spherical aggregates of TiO2 nanocrystals [72]. The synthesized titania aggregates exhibits structural mesoporosity with considerable high surface area and narrow pore size distribution, rendering the materials interesting for solar cell conversion, catalysis, and electronic devices. Smarsly et al. synthesized rutile nanostructures at low temperature by sol– gel chemistry in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [73]. Nanostructured anatase titanium oxide with wormlike pore structures and large surface areas were also obtained by replacing the TiCl4 with titanium isopropoxide in [BMIM][BF4] [74]. The as-prepared products exhibited high surface area, which ranged from ca. 118 to ca. 498 m2 g1 by adding various amounts of IL. Following this work, Li et al. synthesized porous aminopropylsilsesquioxane (APSS) with [BMIM][PF6] as a template solvent by a nonhydrolytic sol–gel method of 3-aminopropyltrimethoxysilane under mild temperature [75]. The results demonstrated that the IL was physically embodied in the silsesquioxane bulk instead of the chemical bonding. [BMIM][BF4] was also used as template for monolithic mesoporous silica with wormhole framework via a convenient nanocasting technique [76]. In contrast with the applied liquid crystal self-assembly of long-chain surfactants on the preparation of mesoporous nanostructures, a new so-called hydrogen bond-co-p–p stack mechanism was proposed to be responsible for the self-assembly of the IL in the reaction system for the formation of the wormlike mesopore. Both the hydrogen bonds formed between the [BF4] and silano group of silica gel and the p–p stack interaction of the neighboring imidazolium rings play crucial roles in the formation of the wormhole framework of mesporous silica. The proposed hydrogen bond-cop–p stack mechanism with the IL as template may open a new pathway to prepare mesoporous materials. Combination of hydrogen-bonding networks and polarity contrast (amphiphilicity) for ILs with a longer hydrophobic tail, very well-organized lyotropic
Fig. 6. (a) TEM image, (b and c) HRTEM images of the TiO2 sample prepared via microwave irradiation in [BMIM] [BF4] for 40 min, respectively (reproduced from Ref. [69]).
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phases are obtained for both the pure ILs as well as their mixtures with water, oils, and reactants. These oriented phases can be employed for material synthesis by using sol–gel synthesis. Zhou et al. successfully synthesized super-microporous silica with lamellar order via the nanocasting technique using 1-hexadecyl3-methylimidazolium chloride as a template [77]. The silica walls of the synthesized product were arranged parallel to each other, displaying a very regular structure with an interlayer distance of about 2.7 nm, a pore diameter of about 1.3 nm, and a wall system of 1.4 nm thickness. Following this work, they reported the synthesis of highly ordered monolithic super-microporous lamellar silica with a series of ILs, 1-alkyl-3-methylimidazolium chloride, as templates via a nanocasting technique [78]. It was found that the produced lamellar pore width varied in the range of 1.2–1.5 nm with the carbon chain lengths of the ILs. By simultaneous application of three-dimensional order polystyrene (PS) beads, an amphiphilic IL(1-hexadecyl-3-methylimidazolium-chloride) or block copolymer as templates, Zhou et al. and Smarsly et al. successfully synthesized hierarchically ordered bimodal or trimodal porous silica or ceria with a well-defined inverse opal microstructure and super-microporous lamellar nanostructure [79–81]. Recently, Lin et al. synthesized a series of mesoporous silica nanoparticle materials with various porous structures and particle shapes, such as spheres, ellipsoids, rods, and tubes, by using different ILs templates, such as 1-tetradecyl3-methylimidazolium bromide, 1-exadecyl-3-methylimidazolium bromide, 1-octadecyl-3- methylimidazolium bromide, and 1tetradecyloxymethyl-3-methylimidazolium chloride [82]. This work shows that the particle and pore morphology of mesoporous silica nanoparticle materials could be well tuned by using various room-temperature ILs as synthetic templates. Endres et al. reported the synthesis of porous aluminium oxide nanoparticles in the ILs like 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide by hydrolysis reaction combined with a thermal decomposition process [83]. One of the recent particular interesting work by Gedanken et al. demonstrated the rapid synthesis of monodispersed conducting solid microsilica spheres by hydrolyzing tetramethylorthosilicate (TMOS) and tetraethylorthosilicate (TEOS) in [BMIM][PF6], at room temperature as shown in Fig. 7 [84]. The conductivity is most probably due to a special oriented alignment of the molecules of IL along the surface. The conductivity of the silica spheres will hopefully gain importance for their applicability in electrochemistry and sensors. This work provides a new concept for synthesis of functional inorganic nanomaterials combined with conductivity property of ILs. This research area is still in its infancy stage, and there is a lot of work to be
Fig. 7. Silica spheres with rough surfaces as synthesized from TMOS in [BMIM][PF6] (reproduced from Ref. [84]).
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investigated and explored, which might be a promising area for constructing functional inorganic nanomaterials in ILs in future. 2.6. Emulsions or microemulsions synthesis Microemulsions are thermodynamically stable media formed by two or more immiscible liquids which are stabilized by surfactants [85]. These microheterogeneous systems can solubilize both polar and nonpolar substances, and they have been extensively applied in synthesis of inorganic nanomaterials [86,87]. Water is one of the most commonly used solvents and the microemulsions with an aqueous continuous phase or dispersed phase have been studied extensively. In recent years, some non-aqueous microemulsions, in which water is replaced by non-aqueous solvents, have attracted much interest because of some unique features. ILs can provide highly polar environments for inorganic nanomaterials synthesis. To explore applications of ILs, the incorporation of surfactants with/within ILs has been investigated by several groups in micellar solution and emulsion compositions [88]. Uniform CaF2 cubes and hollow rods with a well-defined crystal habit can be grown from emulsions of an IL (1-methyl-3-octyl imidazolium hexafluorophosphate) in aqueous CaCl2 solutions via the hydrolysis of the PF6 counterion of the IL (Fig. 8) [89]. The synthesized CaF2 hollow rods are quite interesting because this is the first time that such kind of materials is obtained upon the open literatures. Li et al. found that the IL microemulsions can be used to fabricate silica microrods by sol–gel reaction [90]. It was demonstrated that porous silica microrods with nano-sized pores could be prepared in IL microemulsions. These reports indicate a promising application of such microenvironment. However, the application of IL microemulsions or emulsions for inorganic nanomaterials is not well explored; there are still a lot of work to do for comparing the IL microemulsions or emulsions with the traditional solvents for inorganic nanomaterials synthesis.
Fig. 8. SEM image of CaF2 nanocrystals obtained from a 90% (v/v) emulsion with a 1 M CaCl2 solution: (a) overview showing a typical mixture of cubes and rods; (b and c) are typical rods that have broken up during SEM sample preparation (reproduced from Ref. [89]).
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2.7. Ionic liquid modified nanoparticles The physicochemical properties of ILs strongly depend on the species of anion, cation and the length of the lateral alkyl groups on the heterocyclic rings. Alternation of the anion or the length of the alkyl groups allows finely tuning the physicochemical properties of ILs, such as viscosity, solvation, catalytic activity, hydrophobicity and melting points. Some ILs are immiscible with water and organic solvents. Their mixtures are biphasic systems, which enables easy extraction of products from the IL. The miscibility with organic solvents or water is mostly dependent on the appropriate anions. Chujo et al. report the synthesis and functions of gold nanoparticles modified with IL (3,30 -[disulfanyl bis(hexane-1,6-diyl)]-bis(1-methyl-1H-imidazol-3-ium)dichloride) [91]. Hydrophilic and hydrophobic properties could be tuned by anion exchange of the IL moiety. The aggregation-induced colour changes of the gold nanoparticles in aqueous solutions can be used as an optical sensor for anions via anion exchange of the IL moiety. Also they demonstrated the phase transfer of the gold nanoparticles from aqueous media to IL just by changing the anions in the solution. Lee et al. report one-phase synthesis of gold and platinum nanoparticles using new thiol-functionalized ILs (TFILs) [92]. TFILs as stabilizing agents for gold and platinum nanoparticles were designed to have thiol groups on either the cation or anion and symmetrical or unsymmetrical positions only in the cation. The metal nanoparticles formed using TFILs are crystalline structures with uniform size distribution, which depends on the number and position of thiol groups in the IL. 2.8. Transition-metal nanoparticles for catalysis With the advances of nanochemistry it is possible to prepare ‘‘soluble” analogues of heterogeneous catalysts. Nanoparticles have properties intermediate between those of bulk and single particles. However, nanoparticles are solely kinetically stable and they should be prevented from aggregation into larger particles and bulk material. In general, the main methods used for the stabilization of nanoparticles in solution involve electrostatic or steric protection by the use of water-soluble polymers, quaternary ammonium salts, surfactants, or polyoxoanions. The intrinsic high charge plus the steric bulk of ILs can create an electrostatic and steric colloid type stabilization of nanoparticles. Therefore the in-situ synthesized transition-metal nanoparticles in IL could be used directly for catalysis. These transition-metal nanoparticles are stable in the IL and have good catalytic activity. Imidazolium ILs possess preorganized structures through mainly hydrogen bonds that induce structural directionality in opposition to classical salts in which the aggregates display mainly charge-ordering structures. They form extended hydrogen bond and p–p stacking network in the liquid state and are consequently highly structured, that is, they can be described as ‘‘supramolecular” fluids. This structural organization of imidazolium ILs can function as an ‘‘entropic driver” for spontaneous, well-defined, and extended ordering of nanostructures. Therefore, the unique combination of adaptability toward other molecules and phases associated with the strong H-bond-driven fluid structure makes imidazolium ILs ‘‘templates” for the formation of nanostructures. Moreover, it is possible that imidazolium-based ILs form surfaceattached carbenes, at least as transient species, and may also be responsible for the stabilization of zero-valent transition-metal nanoparticles in these fluids. These ILs are suitable mediums for the preparation, stabilization of transition-metal nanoparticles and hence for ideal recyclable biphasic catalytic systems for hydrogenation and C–C coupling [93–111]. Dupont et al. synthesized Ir and Pt nanoparticles directly for catalysis in [BMIM][PF6] and [BMIM][BF4] by hydrogen reducing method [102]. These particles
show a high catalysis activity for hydrogenation and C–C coupling reactions. In some cases the transition-metal nanoparticles, in particular those of Rh(0) and Pd(0), originally dispersed in the IL tend to aggregate into larger structures and loss of their catalytic activity. Dupont et al. found that laser irradiation of nanoparticles dissolved in ILs could generate transition-metal nanoparticles with a narrow size distribution [93]. By this method they prepared Ru, Pd nanoparticles with controlled size. This method can be used to restructure particles that tend to aggregate after use in catalysis. The nanoparticles isolated from the IL can be easily redispersed by simple laser radiation, and the formed colloidal solution is stable for weeks without any change in size. This route is promising for the recycle of the transition-metal nanoparticles for catalysis in ILs, which is significant in the continuous catalysis reactions. 3. Conclusion The recent development of synthesis of inorganic nanomaterials in ILs reviewed in this article clearly shows that ILs add great value to inorganic nanomaterials area. The unique adaptability and flexibility of ILs provide us with a new, flexible, and powerful tool for the fabrication of novel, interesting, and highly sophisticated nanostructures via chemical approaches. The application of the ILs in inorganic nanomaterials is still in its early stages. Further research on developing the approach and examining the breadth of the IL application is necessary. It suggests that the IL route can guide design, production, and application of inorganic nanomaterials across the range of sizes, shapes, compositions, and functionality. Further development and application of the ILs to the design and production of different inorganic nanomaterials will benefit the nanoscience community. Acknowledgements We gratefully acknowledge financial support from the National Natural Science Foundation of China (NSFC, No. 20803043 and 20803044), and the foundation of Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education. References [1] Alivisatos AP. Semiconductor clusters, nanocrystals, and quantum dots. Science 1996;271:933. [2] Henglein A. Small-particle research: physicochemical properties of extremely small colloidal metal and semiconductor particles. Chem Rev 1989;89:1861. [3] Zäch M, Hägglund C, Chakarov D, Kasemo B. Nanoscience and nanotechnology for advanced energy systems. Curr Opin Solid State Mater Sci 2006;10:132. [4] Kolasinski KW. Silicon nanostructures from electroless electrochemical etching. Curr Opin Solid State Mater Sci 2005;9:73. [5] Macak JM, Tsuchiya H, Ghicov A, Yasuda K, Hahn R, Bauer S, et al. TiO2 nanotubes: Self-organized electrochemical formation, properties and applications. Curr Opin Solid State Mater Sci 2007;11:3. [6] Kolasinski KW. Catalytic growth of nanowires: Vapor–liquid–solid, vapor– solid–solid, solution–liquid–solid and solid–liquid–solid growth. Curr Opin Solid State Mater Sci 2006;10:182. [7] Seddon KR, Stark A, Torres MJ. Influence of chloride, water, and organic solvents on the physical properties of ionic liquids. Pure Appl Chem 2000;72:2275. [8] P. Walden, Bull Acad Imper Sci [St. Petersburg] 1914; 1800.. [9] Bradley AE, Hardacre C, Holbrey JD, Johnston S, McMath SEJ, Nieuwenhuyzen M. Small-Angle X-ray Scattering Studies of Liquid Crystalline 1-Alkyl-3methylimidazolium Salts. Chem Mater 2002;14:629. [10] Ngo HL, LeCompte K, Hargens L, McEwen AB. Thermal properties of imidazolium ionic liquids. Thermochim Acta 2000;97:357. [11] Sheldon R. Catalytic reactions in ionic liquids. Chem Commun 2001:2339. [12] Huddleston JG, Willauer HD, Swatloski RP, Visser AE, Rogers RD. Room temperature ionic liquids as novel media for ‘clean’ liquid–liquid extraction. Chem Commun 1998:1765. [13] Dickinson EV, Williams ME, Hendrickson SM, Masui H, Murray RW. Hybrid redox polyether melts based on polyether-tailed counterions. J Am Chem Soc 1999;121:613. [14] Kubisa P. Application of ionic liquids as solvents for polymerization processes. Prog Polym Sci 2004;29:3.
Z. Li et al. / Current Opinion in Solid State and Materials Science 12 (2008) 1–8 [15] Wasserscheid P, Welton T. Ionic liquids in synthesis. New York: Wiley-VCH Weinheim; 2002. [16] Rogers RD, Seddon KR, Volkov S. Green Industrial Applications of Ionic Liquids. Dordrecht: Kluwer Academic Publisher; 2003. [17] Mamantov G, Popov A. Advanced in Nonaqueous Chemistry. New York: Wiley-VCH Weinheim; 1994. [18] Semeril D, Bruneau C, Dixneuf PH. Imidazolium and imidazolinium salts as carbene precursors or solvent for ruthenium-catalysed diene. Adv Synth Catal 2002;344:585. [19] Zhang MM, Kamavaram V, Reddy RG. New electrolytes for aluminum production: Ionic liquids. Jom J Min Met Mat S 2003;55:A54. [20] Gu YL, Peng JJ, Qiao K, Yang HZ, Shi F, Deng YQ. Room temperature ionic liquids and their applications in catalysis and organic reactions. Prog Chem 2003;15:222. [21] Taubert A. Inorganic materials synthesis – a bright future for ionic liquids?. Acta Chim Slov 2005;52:183. [22] Antonietti M, Kuang D, Smarsly B, Zhou Y. Ionic liquids for the convenient synthesis of functional nanoparticles and other inorganic nanostructures. Angew Chem Int Ed 2004;43:4988. [23] Zhou Y. Recent advances in ionic liquids for synthesis of inorganic nanomater. Curr Nanosci 2005;1:35. [24] Taubert A, Li Z. Inorganic materials from ionic liquids. Dalton Trans. 2007;7:723. [25] Endres F, Abedin SZE. Air and water stable ionic liquids in physical chemistry. Phys Chem Chem Phys 2006;8:2101. [26] Parnham Emily R, Morris Russell E. Ionothermal synthesis of zeolites, metal– organic frameworks, and inorganic–organic hybrids. Acc Chem Res 2007;40:1005. [27] Wasserscheid P, Welton T. Ionic Liquids in Synthesis. Weinheim: Wiley-VCH; 2003. [28] Endres F, Bukowski M, Hempelmann R, Natter H. Electrodeposition of Nanocrystalline Metals and Alloys from Ionic Liquids. Angew Chem Int Ed 2003;42:3428. [29] Mukhopadhyay I, Aravinda CL, Borissov D, Freyland W. Electrodeposition of Ti from TiCl4 in the ionic liquid l-methyl-3-butyl-imidazolium bis (trifluoro methyl sulfone) imide at room temperature: study on phase formation by in situ electrochemical scanning tunneling microscopy. Electrochim Acta 2005;50:1275. [30] Zein El Abedin S, Boressinko N, Endres F. Electrodeposition of nanoscale silicon in a room temperature ionic liquid. Electrochem Commun 2004;6:510. [31] Endres F. Electrodeposition of a thin germanium film on gold from a room temperature ionic liquid. Phys Chem Chem Phys 2001;3:3165. [32] He P, Liu HT, Li ZY, Liu Y, Xu XD, Li JH. Electrochemical deposition of silver in room-temperature ionic liquids and its surface-enhanced raman scattering effect. Langmuir 2004;20:10260. [33] Chen PY, Sun IW. Electrochemistry of Cd(II) in the basic 1-ethyl-3methylimidazolium chloride/tetrafluoroborate room temperature molten salt. Electrochim Acta 2000;45:3163. [34] Chen PY, Sun IW. Electrochemical study of copper in a basic 1-ethyl-3methylimidazolium tetrafluoroborate room temperature molten salt. Electrochim Acta 1999;44:441. [35] Yang MH, Sun IW. Electrodeposition of antimony in a water-stable 1-ethyl-3methylimidazolium chloride tetrafluoroborate room temperature ionic liquid. J Appl Electrochem 2003;33:1077. [36] Huang JF, Sun IW. Electrodeposition of PtZn in a Lewis acidic ZnCl2–1-ethyl3-methylimidazolium chloride ionic liquid. Electrochim Acta 2004;49:3251. [37] Huang JF, Sun IW. Fabrication and surface functionalization of nanoporous gold by electrochemical alloying/dealloying. Adv Funct Mater 2005;15:989. [38] Huang JF, Sun IW. Formation of nanoporous platinum by selective anodic dissolution of PtZn surface alloy in a lewis acidic zinc chloride-1-ethyl-3methylimidazolium chloride ionic liquid. Chem Mater 2004;16:1829. [39] Cooper ER, Andrews CD, Wheatley PS, Webb PB, Wormald P, Morris RE. Ionic liquids and eutectic mixtures as solvent and template in synthesis of zeolite analogues. Nature 2004;430:1012. [40] Zhu H, Huang J, Pan Z, Dai S. Ionothermal synthesis of hierarchical ZnO nanostructures from ionic-liquid precursors. Chem Mater 2006;18:4473. [41] Wang Y, Yang H. Synthesis of CoPt nanorods in ionic liquids. J Am Chem Soc 2005;127:5316. [42] Wang Y, Yang H. Oleic acid as the capping agent in the synthesis of noble metal nanoparticles in imidazolium-based ionic liquids. Chem Commun 2006:2545. [43] Zhou X, Xie Z, Jiang Z, Kuang Q, Zhang S, Xu T, et al. Formation of ZnO hexagonal micro-pyramids: a successful control of the exposed polar surfaces with the assistance of an ionic liquid. Chem Commun 2005:5572. [44] Jiang J, Yu SH, Yao WT, Ge H, Zhang GZ. Morphogenesis and crystallization of Bi2S3 nanostructures by an ionic liquid-assisted templating. Chem Mater 2005;17:6094. [45] Du J, Liu Z, Li Z, Han B, Huang Y, Zhang J. Synthesis of mesoporous SrCO3 spheres and hollow CaCO3 spheres in room-temperature ionic liquid. Micropor Mesopor Mater 2005;83:145. [46] Li Z, Zhang J, Du J, Gao H, Gao Y, Mu T, et al. Synthesis of LaCO3OH nanowires via a solvothermal process in the mixture of water and room-temperature ionic liquid. Mater Lett 2005;59:963. [47] Li Z, Friedrich A, Taubert A. Gold microcrystal synthesis via reduction of HAuCl4 by cellulose in the ionic liquid 1-butyl-3-methyl imidazolium chloride. J Mater Chem 2008;18:1008.
7
[48] Binnemans K. Ionic liquid crystals. Chem Rev 2005;105:4148. [49] Taubert A. CuCl nanoplatelets from an ionic liquid-crystal precursor. Angew Chem Int Ed 2004;43:5380. [50] Taubert A, Steiner P, Mantion A. Ionic liquid crystal precursors for inorganic particles: phase diagram and thermal properties of a CuCl nanoplatelet precursor. J Phys Chem B 2005;109:15542. [51] Taubert A, Palivan C, Casse O, Gozzo F, Schmitt B. Ionic liquid crystal precursors (ILCPs) for CuCl platelets: the origin of the exothermic peak in the DSC curves. J Phys Chem C 2007;111:4077. [52] Taubert A, Arbell I, Mecke A, Graf P. Photoreduction of a crystalline Au(III) complex: a solid-state approach to metallic nanostructures. Gold Bull 2006;39:205. [53] Li Z, Geßner A, Richters J, Kalden J, Voss T, Kubel C, et al. Hollow zinc oxide mesocrystals from an ionic liquid precursor (ILP). Adv Mater 2008;20:1279. [54] Mumalo-Djokic D, Stern WB, Taubert A. Zinc oxide/carbohydrate hybrid materials via mineralization of starch and cellulose in the strongly hydrated ionic liquid tetrabutylammonium hydroxide. Cryst Growth Des 2008;8:330. [55] Li Z, Shkilnyy A, Taubert A. Room-temperature ZnO mesocrystal formation in the hydrated ionic liquid precursor (ILP) tetrabutylammonium hydroxide (TBAH). Cryst Growth Des 2008;8:4526. [56] Li Z, Rabu P, Strauch P, Mantion A, Taubert A. Uniform metal (hydr)oxide particles from water/ionic liquid precursor (ILP) mixtures. Chem Eur J 2008;14:8409. [57] Li Z, Khimyak YZ, Taubert A. Lessons from a failed‘‘ experiment: zinc silicates with complex morphology by reaction of zinc acetate, the ionic liquid precursor (ILP) tetrabutylammonium hydroxide (TBAH), and glass. Materials 2008;1:3. [58] Li Z, Luan Y, Mu T. Unusual nanostructured ZnO particles from an ionic liquid precursor. Chem Comm 2009. doi:10.1039/B819505F. [59] Kim KS, Choi S, Cha JH, Yeon SH, Lee H. Facile one-pot synthesis of gold nanoparticles using alcohol ionic liquids. J Mater Chem 2006;16:1315. [60] Zhu YJ, Wang WW, Qi RJ, Hu XL. Microwave-assisted synthesis of singlecrystalline tellurium nanorods and nanowires in ionic liquids. Angew Chem Int Ed 2004;43:1410. [61] Li Z, Liu Z, Zhang J, Han B, Du J, Gao Y, et al. Synthesis of single-crystal gold nanosheets of large size in ionic liquids. J Phys Chem B 2005;109:14445. [62] Wang WW, Zhu YJ. Synthesis of PbCrO4 and Pb2CrO5 rods via a microwaveassisted ionic liquid method. Cryst Growth Des 2005;5:505. [63] Jiang Y, Zhu YJ, Cheng GF. Synthesis of Bi2Se3 nanosheets by microwave heating using an ionic liquid. Cryst Growth Des 2006;6:2174. [64] Jiang Y, Zhu YJ. microwave-assisted synthesis of sulfide M2S3 (M) Bi, Sb) nanorods using an ionic liquid. J Phys Chem B 2005;109:4361. [65] Yang LX, Zhu YJ, Wang WW, Tong H, Ruan ML. Synthesis and formation mechanism of nanoneedles and nanorods of manganese oxide octahedral molecular sieve using an ionic liquid. J Phys Chem B 2006;110:6609. [66] Jacob DS, Bitton L, Grinblat J, Felner I, Koltypin Y, Gedanken A. Are ionic liquids really a boon for the synthesis of inorganic materials? a general method for the fabrication of nanosized metal fluorides. Chem Mater 2006;18:3162. [67] Singh P, Katyal A, Kalra R, Chandra R. Copper nanoparticles in an ionic liquid: an efficient catalyst for the synthesis of bis-(4-hydroxy-2oxothiazolyl)methanes. Tetrahedron Lett 2008;49:727. [68] Liu Z, Sun Z, Han B, Zhang J, Huang J, Du J, et al. Microwave-assisted synthesis of Pt nanocrystals and deposition on carbon nanotubes in ionic liquids. J Nanosci Nanotech 2006;6:175. [69] Ding K, Miao Z, Liu Z, Zhang Z, Han B, An G, et al. Facile synthesis of high quality TiO2 nanocrystals in ionic liquid via a microwave-assisted process. J Am Chem Soc 2007;129:6362. [70] Dai S, Ju YH, Gao HJ, Lin JS, Pennycook SJ, Barnesc CE. Preparation of silica aerogel using ionic liquids as solvents. Chem Commun 2000:243. [71] Nakashima T, Kimizuka N. Interfacial synthesis of hollow TiO2 microspheres in ionic liquids. J Am Chem Soc 2003;125:6386. [72] Zhou Y, Antonietti M. Synthesis of very Small TiO2 nanocrystals in a roomtemperature ionic liquid and their self-assembly toward mesoporous spherical aggregates. J Am Chem Soc 2003;125:14960. [73] Kaper H, Endres F, Djerdj I, Antonietti M, Smarsly BM, Maier J, et al. Direct low-temperature synthesis of rutile nanostructures in ionic liquids. Small 2007;3:1753. [74] Liu Y, Li J, Wang M, Li Z, Liu H, He P, et al. Preparation and properties of nanostructure anatase TiO2 monoliths using 1-butyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquids as template solvents. Cryst Growth Des 2005;5:1643. [75] Liu Y, Wang M, Li Z, Liu H, He P, Li J. Preparation of porous aminopropylsilsesquioxane by a nonhydrolytic solgel method in ionic liquid solvent. Langmuir 2005;21:1618. [76] Zhou Y, Schattka JH, Antonietti M. Room-temperature ionic liquids as template to monolithic mesoporous silica with wormlike pores via a sol-gel nanocasting technique. Nano Lett 2004;4:477. [77] Zhou Y, Antonietti M. Preparation of highly ordered monolithic supermicroporous lamellar silica with a room-temperature ionic liquid as template via the nanocasting technique. Adv Mater 2003;15:1452. [78] Zhou Y, Antonietti M. A series of highly ordered, super-microporus, lamellar silicas prepared by nanocasting with ionic liquids. Chem Mater 2004;16:544. [79] Zhou Y, Antonietti M. A novel tailored bimodal porous silica with welldefined inverse opal microstructure and super-microporous lamellar nanostructure. Chem Commun 2003:2564.
8
Z. Li et al. / Current Opinion in Solid State and Materials Science 12 (2008) 1–8
[80] Kuang D, Brezesinski T, Smarsly B. Hierarchical porous silica materials with a trimodal pore system using surfactant templates. J Am Chem Soc 2004;126:10534. [81] Brezesinski T, Erpen C, Iimura K, Smarsly B. Mesostructured crystalline ceria with a bimodal pore system using block copolymers and ionic liquids as rational templates. Chem Mater 2005;17:1683. [82] Trewyn BG, Whitman CM, Lin VSY. Morphological control of roomtemperature ionic liquid templated mesoporous silica nanoparticles for controlled release of antibacterial agents. Nano Lett 2004;4:2139. [83] Farag Hala K, Endres Frank. Studies on the synthesis of nano-alumina in air and water stable ionic liquids. J Mater Chem 2008;18:442. [84] Jacob DS, Joseph A, Mallenahalli SP, Shanmugam S, Makhluf S, CalderonMoreno J, et al. Rapid synthesis in ionic liquids of room-temperatureconducting solid microsilica spheres. Angew Chem Int Ed 2005;44:6560. [85] Luan Y, Xu G, Yuan S, Xiao L, Zhang Z. Comparative studies of structurally similar surfactants: Sodium bis(2-ethylhexyl)phosphate and sodium bis(2ethylhexyl)sulfosuccinate. Langmuir 2002;18:8700. [86] Li Z, Du J, Zhang J, Mu T, Gao Y, Han B, et al. Synthesis of single crystal BaMoO4 nanofibers in CTAB reverse microemulsions. Mater Lett 2005;59:64. [87] Li Z, Zhang J, Du J, Han B, Mu T, Gao Y. Preparation and self-assembly of nanostructured BaCrO4 from CTAB reverse microemulsions. Mater Chem Phys 2005;91:40. [88] Gao H, Li J, Han B, Chen W, Zhang J, Zhang R, et al. Microemulsions with ionic liquid polar domains. Phys Chem Chem Phys 2004;6:2914. [89] Taubert A. (Sub)micron CaF2 cubes and hollow rods from ionic liquid emulsions. Acta Chim Slov 2005;52:168. [90] Li Z, Zhang J, Du J, Han B, Wang J. Preparation of silica microrods with nanosized pores in ionic liquid microemulsions. Colloids Surf A 2006;286:117. [91] Itoh H, Naka K, Chujo Y. Synthesis of gold nanoparticles modified with ionic liquid based on the imidazolium cation. J Am Chem Soc 2004;126:3026. [92] Kim K, Demberelnyamba D, Lee H. Langmuir Size-selective synthesis of gold and pPlatinum nanoparticles using novel thiol-functionalized ionic liquids. 2004;20:556.. [93] Gelesky MA, Umpierre AP, Machado G, Correia RRB, Magno WC, Morais J, et al. Laser-induced fragmentation of transition metal nanoparticles in ionic liquids. J Am Chem Soc 2005;127:4588. [94] Cassol CC, Umpierre AP, Machado G, Wolke SI, Dupont J. The role of Pd nanoparticles in ionic liquid in the Heck reaction. J Am Chem Soc 2005;127:3298. [95] Umpierre AP, Machado G, Fecher GH, Morais J, Dupont J. Selective hydrogenation of 1,3-butadiene to 1-Butene by Pd(0) nanoparticles embedded in imidazolium ionic liquids. Adv Synth Catal 2005;347:1404. [96] Fonseca GS, Silveira ET, Gelesky MA, Dupont J. Competitive hydrogenation of alkyl-substituted arenes by transition-metal nanoparticles: Correlation with the alkyl-steric effect. Adv Synth Catal 2005;347:847. [97] Fonseca GS, Scholten JD, Dupont J. Iridium nanoparticles prepared in ionic liquids: An efficient catalytic system for the hydrogenation of ketones. Synlett 2004:1525.
[98] Rossi LM, Dupont J, Machado G, Fichtner PFP, Radtke C, Baumvol IJR, et al. Ruthenium dioxide nanoparticles in ionic liquids: Synthesis, characterization and catalytic properties in hydrogenation of olefins and arenas. J Braz Chem Soc 2004;15:904. [99] Rossi LM, Machado G, Fichtner PFP, Teixeira SR, Dupont J. On the use of ruthenium dioxide in 1-n-butyl-3-methylimidazolium ionic liquids as catalyst precursor for hydrogenation reactions. Catal Lett 2004;92:149. [100] Scheeren CW, Machado G, Dupont J, Fichtner PFP, Texeira SR. Nanoscale Pt(0) particles prepared in imidazolium room temperature ionic liquids: Synthesis from an organometallic precursor, characterization, and catalytic properties in hydrogenation reactions. Inorg Chem 2003;42:4738. [101] Fonseca GS, Umpierre AP, Fichtner PFP, Teixeira SR, Dupont J. The use of imidazolium ionic liquids for the formation and stabilization of Ir-0 and Rh-0 nanoparticles: Efficient catalysts for the hydrogenation of arenas. Chem Eur J 2003;9:3263. [102] Dupont J, Fonseca GS, Umpierre AP, Fichtner PFP, Teixeira SR. Transitionmetal nanoparticles in imidazolium ionic liquids: Recycable catalysts for biphasic hydrogenation reactions. J Am Chem Soc 2002;124:4228. [103] Bruss AJ, Gelesky MA, Machado G, Dupont J. Rh(0) nanoparticles as catalyst precursors for the solventless hydroformylation of olefins. J Mol Catal A Chem 2006;252:212. [104] Zhao D, Fei Z, Geldbach Tilmann J, Scopelliti R, Dyson Paul J. Nitrilefunctionalized pyridinium ionic liquids: Synthesis, characterization, and their application in carbon - Carbon coupling reactions. J Am Chem Soc 2004;126:15876. [105] Calo V, Nacci A, Monopoli A, Montingelli F. Pd nanoparticles as efficient catalysts for Suzuki and Stille coupling reactions of aryl halides in ionic liquids. J Org Chem 2005;70:6040. [106] Calo V, Nacci A, Monopoli A, Fornaro A, Sabbatini L, Cioffi N, et al. Heck reaction catalyzed by nanosized palladium on chitosan in ionic liquids. Organometallics 2004;23:5154. [107] Calo V, Nacci A, Monopoli A, Laera S, Cioffi N. Pd nanoparticles catalyzed stereospecific synthesis of beta-aryl cinnamic esters in ionic liquids. J Org Chem 2003;68:2929. [108] Calo V, Nacci A, Monopoli A, Detomaso A, Iliade P. Pd nanoparticle catalyzed Heck arylation of 1,1-disubstituted alkenes in ionic liquids. Study on factors affecting the regioselectivity of the coupling process. Organometallics 2003;22:4193. [109] Anderson K, Cortinas Fernandez S, Hardacre C, Marr PC. Preparation of nanoparticulate metal catalysts in porous supports using an ionic liquid route; hydrogenation and C-C coupling. Inorg Chem Commun 2003;7:73. [110] Mu XD, Meng JQ, Li ZC, Kou Y. Rhodium nanoparticles stabilized by ionic copolymers in ionic liquids: Long lifetime nanocluster catalysts for benzene hydrogenation. J Am Chem Soc 2005;127:9694. [111] Gao SY, Zhang HJ, Wang XM, Mai WP, Peng CY, Ge LH. Palladium nanowires stabilized by thiol-functionalized ionic liquid: seed-mediated synthesis and heterogeneous catalyst for Sonogashira coupling reaction. Nanotechnology 2005;16:1234.
Contents lists available at ScienceDirect
Current Opinion in Solid State and Materials Science journal homepage: www.elsevier.com/locate/cossms
Ionic liquids for synthesis of inorganic nanomaterials Zhonghao Li a,*, Zhen Jia b, Yuxia Luan c, Tiancheng Mu d a
Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, School of Materials Science and Engineering, Shandong University, Jinan, Shandong Province 250061, PR China Department of Chemistry, Dezhou University, Dezhou, Shandong Province 253023, PR China c School of Pharmaceutical Sciences, Shandong University, Jinan, Shandong Province 250012, PR China d Department of Chemistry, Renmin University of China, Beijing, 100872, PR China b
a r t i c l e
i n f o
Article history: Received 5 September 2008 Accepted 6 January 2009
Keywords: Ionic liquids Inorganic nanomaterials
a b s t r a c t In this review, the recent development of synthesizing inorganic nanomaterials in ionic liquids (ILs) is outlined. There are encouraging results suggesting that the ionic liquid route can guide design, production, and application of inorganic nanomaterials across the range of size, shape, composition, and functionality. All examples clearly show that ILs add great value to inorganic nanomaterials area. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Recently, inorganic nanomaterials have attracted much attention for their unique chemical and physical characteristics compared with the bulk solids [1–6]. The interesting size and shape dependent properties of the inorganic nanomaterials make the scientists endeavor to prepare inorganic nanomaterials with controlled size and structure. The preparation of inorganic nanomaterials that possesses desired properties is a great challenging task. Usually water or traditional organic solvent are involved in the synthesis of inorganic nanomaterials. Each of the synthetic routes with these solvents has advantages and disadvantages, and new synthetic routes with other solvents are still necessary to be developed in view of both fundamental and application purposes. Room-temperature ionic liquids (ILs) are organic salts, composed of entirely ions, with low melting points of below 100 °C, sometimes as low as 96 °C [7]. The first finding of an IL with a melting point of 12 °C was reported in 1914 [8]. Subsequently, a succession of the interesting analogous compounds have been discovered. The most extensively studied type of ILs is the 1-alkyl-3methylimidazolium salts due to their air and moisture stability. The physicochemical properties of ILs depend on the species of cation, anion and the length of the lateral alkyl groups on the heterocyclic rings, therefore alternation of the anion or the length of the alkyl group allows finely tuning the physicochemical properties of ILs, such as viscosity, solvation, catalytic activity, hydrophobicity and melting point [9,10]. * Corresponding author. Fax: +86 531 88392315. E-mail address: [email protected] (Z. Li). 1359-0286/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2009.01.002
ILs possess a wide liquidus, in some cases excess of 400 °C and they are good solvents for a wide range of inorganic, organic and polymer materials. The combination of these individual reagents can dissolve in the same phase which makes the great application of IL in the related areas. The ILs’ unusual properties, such as high polarity, negligible vapor pressure, high ionic conductivity, and thermal stability make them as attractive environmentally benign solvents for organic chemical reactions, polymer synthesis, separations, and electrochemical applications [11–14]. Until now, the related books [15–17] and reviews [11,18–20] on this subject concerning with the chemical reactions and physicochemical properties of ILs have been extensively published. In recent years, the advantages of ILs in inorganic nanomaterials synthetic processes have been gradually realized and have received more and more attention due to their unique physical and chemical properties [21–25]. Various approaches to the fabrication of inorganic nanomaterials with unprecedented and sometimes unique structures and properties have been reported in ILs. The most interesting thing is that it cannot only reproduce conventional inorganic nanomaterials, but also provide pathways towards new inorganic nanomaterials with properties that cannot (or only with great difficulty) be made via conventional processes. The synthesis of inorganic nanomaterials in ILs is a rather new field, which has roughly emerged over the last 10 years [21–25]. Due to the importance and the quick development of ILs in inorganic nanomaterials, it seems timely to review the field on developments of inorganic nanomaterials synthesis in ILs. In this review, the recent advances in the synthesis of inorganic nanomaterials in ILs will be presented. The inorganic–organic hybrids such as metal–organic frameworks synthesized in ILs have been reviewed recently by Parnham and Morris [26], and will not be discussed here.
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2. Methods 2.1. Electrodeposition synthesis ILs are composed solely of ions, therefore it would be expected that ILs have high conductivities. ILs have reasonably good ionic conductivities compared with those of organic solvents/electrolyte systems [27]. The specific conductivity for ionic liquid can be up to 10 mS cm1 while that of water is 4 108 mS cm1 at room temperature. The conductivity of aqueous solution is related to the concentration of electrolyte. If the conductivity of aqueous solution reaches to the similar conductivity of ionic liquid, the electrolyte concentration in the aqueous solution must be increased. For example the specific conductivity of 0.1 M KCl aqueous solution is 13 mS cm1 which is similar to that of ionic liquid at room temperature. The conductivity of solution depends not only on the number of charge carriers but also on their mobility. The conductivity of ILs increases while the viscosity of ILs deceases as the increase of the temperature. Therefore, the ionic conductivities can be well controlled by temperature. The ILs have received extensive attention because of their large electrochemical windows. As ILs have the properties of wide potential windows, high solubility of metal salts, avoidance of water and high conductivity compared to non-aqueous solvents, they are excellent solvent for the electrodeposition. Electrochemical approaches have been among the first to be used for the fabrication of inorganic nanoparticles and nanostructured films in ILs. The properties of ILs opened the door to the electrodeposition of metals and semiconductors at room temperature, which was formerly only possible from high-temperature molten salts. For example, Al, Mg, Ti, Si, Ge and rare-earth elements related materials can be obtained from ILs [25,28–31]. Besides these, the electrodeposition of Ag, Cd, Cu and Sb in 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] or basic chloride containing 1ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF4] was also reported [32–35]. Recently it is demonstrated that electrodeposition and de-alloying process in IL is a facile route for constructing porous nanostructures. For example, Sun et al. obtained the nanoporous Au, Pt, and Ag structures by electrodeposition of a binary alloy such as gold-zinc, platinum-zinc and silver-zinc and subsequently electrochemical dealloy them in zinc chloride-1ethyl-3-methylimidazolium chloride [36–38]. Fig. 1 shows the microstructure of a platinum surface after anodic stripping of the PtZn surface alloy that was produced by electrodeposition of 0.72 C/cm2 zinc on the pure platinum in zinc chloride-1-ethyl-3methylimidazolium chloride [38]. In comparison with conventional molecular electrolyte solutions, ILs provide a more versatile environment for fabricating porous metals. Both the deposition and the de-alloying steps are performed in a single bath of
Fig. 1. De-alloyed Pt wires that were electrodeposited with Zn at 120 °C followed by de-alloying at 1.2 V to remove the surface PtZn alloy (reproduced from Ref. [38]).
relatively low temperature without using any other corrosive acids or bases. The zinc(II) species consumed during the deposition step is recovered during the de-alloying step, and therefore the IL is reusable. The electrodeposition and electrochemical de-alloying route opens the door for the fabrication of nanoporous metal materials which can be used for many areas like catalysis. This concept is definitely promising for the future application of ILs in synthesis of inorganic nanomaterials with designed porous structures. 2.2. Thermal synthesis ILs are known for their nonvolatile, nonflammable, and thermally stable properties. These properties make the reactions in open reactor possible. Recently, the term ‘‘ionothermal” has been used to describe reactions that are conducted in ILs at high temperature with ambient pressures. As a result, ionothermal reactions avoid the high pressure of hydrothermal or solvothermal reactions and eliminate safety problems related with high pressure. Cooper et al. have demonstrated that imidazolium-based ILs can be utilized to synthesize several phosphate-based microporous zeolites by an ionothermal process [39]. Dai et al. reported the use of a newly developed IL system containing zinc metal ions, which can serve as both solvents and metal-oxide precursors, for manufacturing nanostructured zinc oxide under the ionothermal condition [40]. The morphologies of ZnO are strongly dependent on the nature of the IL precursor (ligands), providing unique methodologies to control the growth conditions. Fig. 2 shows the flowerlike ZnO nanoparticles prepared in Zn(propylamine)4 (NTf2)2 [(NTf2) = N(SO2CF3)2] at 110 °C by an ionothermal process [40]. Recently, Yang et al. utilized the IL as a high-temperature environment for the synthesis of CoPt nanorods with 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, [BMIM] [Tf2N] [41]. Subsequently, combining the IL with the oleic acid as capping agent, Ag and Pt nanoparticles were successfully prepared [42]. These metal nanoparticles can be readily dispersed in conventional organic solvents and the synthetic approach can potentially facilitate the development of continuous production of high quality nanoparticles. The surface energy of the polar planes of inorganic crystals can be reduced by compensating the surface charge with a passivating reagent such as an IL due to the strong electrostatic interactions between the ions of the IL and the polar surfaces. According to this principle, Xie et al. synthesized ZnO hexagonal micro-pyramids by a successful control of the exposed polar surfaces with the assistance of an IL (R–COO2 R–NH3+) at high temperature in air [43]. In some cases, the IL solution has template effect for the formation of inorganic nanomaterials. For example, uniform Bi2S3 flowers have been synthesized in [BMIM][BF4] solu-
Fig. 2. Representative SEM images of ZnO from the Zn(propylamine)4–(NTf2)2 precursor at 110 °C (reproduced from Ref. [40]).
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Fig. 3. SEM images of the product obtained in an ionic liquid system after reaction at 120 °C for 0.5 h. (reproduced from Ref. [44]).
tion at low temperature and ambient atmosphere as shown in Fig. 3 [44]. If water and other solvent are mixed with ILs, the reaction in ambient condition with high temperature is difficult. However this can be solved by refluxing or solvothermal process. Han et al. synthesized mesoporous SrCO3 spheres with diameter ranging from 200 to 400 nm via refluxing the solution of SrCl26H2O and sodium hydroxide in 1,1,3,3-tetramethylguanidinium lactate (TMGL) at 140 °C in the presence of CO2 [45]. CaCl2 was used to synthesize CaCO3 at the same experimental conditions, resulting in hollow calcite spheres and peanut-like structure calcite whose shell thickness and core diameter were about 100 and 200 nm, respectively, [45]. LaCO3OH nanostructures were also synthesized in TMGL by a solvothermal process which indicates the role of the template effect of the IL [46]. Based on the fact that [BMIM]Cl is an effective solvent for cellulose at high temperature, Li et al. prepared gold microcrystal via reduction of HAuCl4 by cellulose in [BMIM]Cl under high temperatures as shown in Fig. 4 [47]. 2.3. All-in-one synthesis (solvent-template-reactant) Anions such as sulfate, phosphate, carbonate, chloride, and metal cations in ILs can be viewed as reactive IL precursors for the fabrication of inorganic nanomaterials. In these cases, the IL is not only a solvent (or at least a solvent which shows a more or less controlled template effect) but also acts as a molecular precursor with a well-defined composition, structure, and reactivity. These parameters can be exploited for the fabrication of uniformly structured inorganic nanomaterials with various properties. Such ILs can be viewed as solvent–reactant ILs. In case the components of an IL are appropriately chosen, ILs exhibit long-range order and liquid crystallinity [48]. These so-called
ionic liquid crystals (ILCs) often contain metal ions, which can be reacted as described above. Due to the fact that ILCs form ordered domains, within which the reaction occurs, ILCs provide a facile way of reaction control for the inorganic materials. ILCs can be viewed as true solvent–template–reactant systems, where the ILC is solvent, template, and reactant simultaneously for the fabrication of desired inorganic nanomaterials with defined morphology or chemical composition. Based on this, Taubert et al. developed a protocol for the controlled synthesis of CuCl nanoplatelets with a well-developed crystal habit, a tunable particle size and connectivity from the Cu-containing ILC [bis(dodecylpyridinium) tetrachlorocuprate(II)] and the reducing agent (6-O-palmitoyl ascorbic acid) [49–51]. Exchange of the ligand, metal cation, anion, and reducing agent can offer a universal pathway towards metal halide and metal (alloy) nanostructures with tunable composition and morphology [52]. Analogous to the long-chain ILC, the general ILs containing the reactant ions can also be used to prepare inorganic nanomaterials which is defined as ILPs. Zhu et al. reported the use of a newly developed IL system containing zinc metal ions, which can serve as both solvents and metal-oxide precursors, for manufacturing nanostructured zinc oxide under the ionothermal condition [40]. Li et al. have shown that the strongly hydrated IL tetrabutylammonium hydroxide (TBAH) is an ILP for the controlled fabrication of unique hollow zinc oxide mesocrystals [53]. The SEM image of sample obtained at 10 mg of zinc acetate/g of TBAH is shown in Fig. 5. The particles are up to ca. 10 lm long and consist of smaller nanoparticles. Furthermore, the larger particles usually contain a central channel, which run parallel to the main particle axis. These particles can be defined as hollow ZnO mesocrystals. The interaction of ionic liquid with the primary ZnO particle play an important role for the formation ZnO mesocrystals. The large tetrabutylammonium cation of TBAH can bond to the negatively charged surface O2 (0 0 0 1) of the primary ZnO particles and reverse the polarity of the negatively charged surfaces of primary particles and thereby prevent further growth and aids the aggregation of primary ZnO particles into large mesocrystals. The TBAH can also be used for constructing unusual nanostructured ZnO particles (lotus like plate, porous plates and nanoparticle aggregated flowerlike particles), ZnO/carbohydrate hybrid, other metal (hydr)oxides, and zinc silicate nanomaterials [54–58]. Besides the application of TBAH, another designed ILP (an alcohol modified IL) was used to synthesize gold nanoparticles by all-in-one route. This IL is able to reduce Au(III) salt to Au(0) particles directly due to the –OH group present in the IL, which avoids additional reducing agent for the reaction [59]. 2.4. Microwave heating synthesis
Fig. 4. SEM images of gold crystals prepared at 100 mg of cellulose per gram of 1butyl-3-methyl imidazolium chloride (reproduced from Ref. [47]).
Large positive ions with high polarizability in ILs make them excellent solvents for absorbing microwaves. Therefore, the use
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2.5. Sol–gel synthesis
Fig. 5. SEM images of samples precipitated at 10 mg of zinc acetate/g of TBAH. Scale bars are 2 lm. Insets are magnified views of the same samples. (reproduced from Ref. [53]).
of microwave heating in ILs for the synthesis of inorganic nanomaterials has advantages over other solvents. Zhu et al. successfully synthesized single-crystal tellurium nanowires using microwave heating in N-butylpyridinium tetrafluoroborate IL in the presence of polymer surfactant of poly(vinylpyrrolidone) (PVP) [60]. Li et al. synthesized large-size single-crystal gold nanosheets by microwave heating of HAuCl4 in [BMIM][BF4], without any additional template agent [61]. Subsequently, PbCrO4 rods or bundle-like Pb2CrO5, Bi2Se3 nanosheets, sulfide M2S3 nanorods, single-crystalline cryptomelane-type manganese oxide octahedral molecular sieve (OMS-2) nanoneedles and nanorods, metal fluorides such as FeF2, CoF2, ZnF2, LaF3, YF3, SrF2, metal-oxide like Fe2O3, carbon-coated core shell structured copper or nickel nanoparticles and CNTs/Pt or CNTs/Rh composites have been successfully fabricated by microwave-assisted synthesis method in ILs [62–68]. A very recent report by Liu et al. demonstrates that anatase TiO2 nanocolloids with uniform size and shape can be prepared via a microwave-assisted route in [BMIM][BF4](Fig. 6) [69]. The synthesized TiO2 nanocolloids are highly crystalline, low in Ti3+ defect, free of aggregation. Moreover, the size of TiO2 nanocolloids can be easily controlled. In these efforts, it demonstrated that the ILs work as both a solvent to absorb microwaves and capping agent or directing agent to result in the final morphology. The advantages of microwave process include: the synthesis is fast and simple; the reaction can be performed under atmospheric pressure even in a domestic microwave oven; no high pressure and high temperature apparatus are required. These advantages make microwave method a promising route for a fast and largescale synthesis of inorganic nanomaterials in ILs.
Some ILs provide hydrophobic regions and a high directional polarizability, which is oriented parallel or perpendicular to the dissolved species, as well as extended hydrogen-bond systems in the liquid state resulting in a highly structured frame. These characteristics encourage the preparation of well-defined and extended ordering of inorganic nanostructures [22–24]. A lot of work concerning the synthesis of inorganic nanomaterials is based on sol– gel route. The original use of ILs as solvents for inorganic sol–gel reactions is the acid-catalyzed sol–gel synthesis of silica aerogels [70]. It turned out that such aerogels can be dried without a supercritical drying procedure. Subsequently, 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6] was used to synthesize hollow TiO2 microspheres by interfacial sol–gel reaction [71]. The imidazolium molecules act not only as the solvent but also as stabilizers for the hollow microspheres. Zhou et al. used TiCl4 as the precursor to synthesize very small TiO2 nanocrystals in [BMIM][BF4] and obtained mesoporous spherical aggregates of TiO2 nanocrystals [72]. The synthesized titania aggregates exhibits structural mesoporosity with considerable high surface area and narrow pore size distribution, rendering the materials interesting for solar cell conversion, catalysis, and electronic devices. Smarsly et al. synthesized rutile nanostructures at low temperature by sol– gel chemistry in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [73]. Nanostructured anatase titanium oxide with wormlike pore structures and large surface areas were also obtained by replacing the TiCl4 with titanium isopropoxide in [BMIM][BF4] [74]. The as-prepared products exhibited high surface area, which ranged from ca. 118 to ca. 498 m2 g1 by adding various amounts of IL. Following this work, Li et al. synthesized porous aminopropylsilsesquioxane (APSS) with [BMIM][PF6] as a template solvent by a nonhydrolytic sol–gel method of 3-aminopropyltrimethoxysilane under mild temperature [75]. The results demonstrated that the IL was physically embodied in the silsesquioxane bulk instead of the chemical bonding. [BMIM][BF4] was also used as template for monolithic mesoporous silica with wormhole framework via a convenient nanocasting technique [76]. In contrast with the applied liquid crystal self-assembly of long-chain surfactants on the preparation of mesoporous nanostructures, a new so-called hydrogen bond-co-p–p stack mechanism was proposed to be responsible for the self-assembly of the IL in the reaction system for the formation of the wormlike mesopore. Both the hydrogen bonds formed between the [BF4] and silano group of silica gel and the p–p stack interaction of the neighboring imidazolium rings play crucial roles in the formation of the wormhole framework of mesporous silica. The proposed hydrogen bond-cop–p stack mechanism with the IL as template may open a new pathway to prepare mesoporous materials. Combination of hydrogen-bonding networks and polarity contrast (amphiphilicity) for ILs with a longer hydrophobic tail, very well-organized lyotropic
Fig. 6. (a) TEM image, (b and c) HRTEM images of the TiO2 sample prepared via microwave irradiation in [BMIM] [BF4] for 40 min, respectively (reproduced from Ref. [69]).
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phases are obtained for both the pure ILs as well as their mixtures with water, oils, and reactants. These oriented phases can be employed for material synthesis by using sol–gel synthesis. Zhou et al. successfully synthesized super-microporous silica with lamellar order via the nanocasting technique using 1-hexadecyl3-methylimidazolium chloride as a template [77]. The silica walls of the synthesized product were arranged parallel to each other, displaying a very regular structure with an interlayer distance of about 2.7 nm, a pore diameter of about 1.3 nm, and a wall system of 1.4 nm thickness. Following this work, they reported the synthesis of highly ordered monolithic super-microporous lamellar silica with a series of ILs, 1-alkyl-3-methylimidazolium chloride, as templates via a nanocasting technique [78]. It was found that the produced lamellar pore width varied in the range of 1.2–1.5 nm with the carbon chain lengths of the ILs. By simultaneous application of three-dimensional order polystyrene (PS) beads, an amphiphilic IL(1-hexadecyl-3-methylimidazolium-chloride) or block copolymer as templates, Zhou et al. and Smarsly et al. successfully synthesized hierarchically ordered bimodal or trimodal porous silica or ceria with a well-defined inverse opal microstructure and super-microporous lamellar nanostructure [79–81]. Recently, Lin et al. synthesized a series of mesoporous silica nanoparticle materials with various porous structures and particle shapes, such as spheres, ellipsoids, rods, and tubes, by using different ILs templates, such as 1-tetradecyl3-methylimidazolium bromide, 1-exadecyl-3-methylimidazolium bromide, 1-octadecyl-3- methylimidazolium bromide, and 1tetradecyloxymethyl-3-methylimidazolium chloride [82]. This work shows that the particle and pore morphology of mesoporous silica nanoparticle materials could be well tuned by using various room-temperature ILs as synthetic templates. Endres et al. reported the synthesis of porous aluminium oxide nanoparticles in the ILs like 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide by hydrolysis reaction combined with a thermal decomposition process [83]. One of the recent particular interesting work by Gedanken et al. demonstrated the rapid synthesis of monodispersed conducting solid microsilica spheres by hydrolyzing tetramethylorthosilicate (TMOS) and tetraethylorthosilicate (TEOS) in [BMIM][PF6], at room temperature as shown in Fig. 7 [84]. The conductivity is most probably due to a special oriented alignment of the molecules of IL along the surface. The conductivity of the silica spheres will hopefully gain importance for their applicability in electrochemistry and sensors. This work provides a new concept for synthesis of functional inorganic nanomaterials combined with conductivity property of ILs. This research area is still in its infancy stage, and there is a lot of work to be
Fig. 7. Silica spheres with rough surfaces as synthesized from TMOS in [BMIM][PF6] (reproduced from Ref. [84]).
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investigated and explored, which might be a promising area for constructing functional inorganic nanomaterials in ILs in future. 2.6. Emulsions or microemulsions synthesis Microemulsions are thermodynamically stable media formed by two or more immiscible liquids which are stabilized by surfactants [85]. These microheterogeneous systems can solubilize both polar and nonpolar substances, and they have been extensively applied in synthesis of inorganic nanomaterials [86,87]. Water is one of the most commonly used solvents and the microemulsions with an aqueous continuous phase or dispersed phase have been studied extensively. In recent years, some non-aqueous microemulsions, in which water is replaced by non-aqueous solvents, have attracted much interest because of some unique features. ILs can provide highly polar environments for inorganic nanomaterials synthesis. To explore applications of ILs, the incorporation of surfactants with/within ILs has been investigated by several groups in micellar solution and emulsion compositions [88]. Uniform CaF2 cubes and hollow rods with a well-defined crystal habit can be grown from emulsions of an IL (1-methyl-3-octyl imidazolium hexafluorophosphate) in aqueous CaCl2 solutions via the hydrolysis of the PF6 counterion of the IL (Fig. 8) [89]. The synthesized CaF2 hollow rods are quite interesting because this is the first time that such kind of materials is obtained upon the open literatures. Li et al. found that the IL microemulsions can be used to fabricate silica microrods by sol–gel reaction [90]. It was demonstrated that porous silica microrods with nano-sized pores could be prepared in IL microemulsions. These reports indicate a promising application of such microenvironment. However, the application of IL microemulsions or emulsions for inorganic nanomaterials is not well explored; there are still a lot of work to do for comparing the IL microemulsions or emulsions with the traditional solvents for inorganic nanomaterials synthesis.
Fig. 8. SEM image of CaF2 nanocrystals obtained from a 90% (v/v) emulsion with a 1 M CaCl2 solution: (a) overview showing a typical mixture of cubes and rods; (b and c) are typical rods that have broken up during SEM sample preparation (reproduced from Ref. [89]).
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2.7. Ionic liquid modified nanoparticles The physicochemical properties of ILs strongly depend on the species of anion, cation and the length of the lateral alkyl groups on the heterocyclic rings. Alternation of the anion or the length of the alkyl groups allows finely tuning the physicochemical properties of ILs, such as viscosity, solvation, catalytic activity, hydrophobicity and melting points. Some ILs are immiscible with water and organic solvents. Their mixtures are biphasic systems, which enables easy extraction of products from the IL. The miscibility with organic solvents or water is mostly dependent on the appropriate anions. Chujo et al. report the synthesis and functions of gold nanoparticles modified with IL (3,30 -[disulfanyl bis(hexane-1,6-diyl)]-bis(1-methyl-1H-imidazol-3-ium)dichloride) [91]. Hydrophilic and hydrophobic properties could be tuned by anion exchange of the IL moiety. The aggregation-induced colour changes of the gold nanoparticles in aqueous solutions can be used as an optical sensor for anions via anion exchange of the IL moiety. Also they demonstrated the phase transfer of the gold nanoparticles from aqueous media to IL just by changing the anions in the solution. Lee et al. report one-phase synthesis of gold and platinum nanoparticles using new thiol-functionalized ILs (TFILs) [92]. TFILs as stabilizing agents for gold and platinum nanoparticles were designed to have thiol groups on either the cation or anion and symmetrical or unsymmetrical positions only in the cation. The metal nanoparticles formed using TFILs are crystalline structures with uniform size distribution, which depends on the number and position of thiol groups in the IL. 2.8. Transition-metal nanoparticles for catalysis With the advances of nanochemistry it is possible to prepare ‘‘soluble” analogues of heterogeneous catalysts. Nanoparticles have properties intermediate between those of bulk and single particles. However, nanoparticles are solely kinetically stable and they should be prevented from aggregation into larger particles and bulk material. In general, the main methods used for the stabilization of nanoparticles in solution involve electrostatic or steric protection by the use of water-soluble polymers, quaternary ammonium salts, surfactants, or polyoxoanions. The intrinsic high charge plus the steric bulk of ILs can create an electrostatic and steric colloid type stabilization of nanoparticles. Therefore the in-situ synthesized transition-metal nanoparticles in IL could be used directly for catalysis. These transition-metal nanoparticles are stable in the IL and have good catalytic activity. Imidazolium ILs possess preorganized structures through mainly hydrogen bonds that induce structural directionality in opposition to classical salts in which the aggregates display mainly charge-ordering structures. They form extended hydrogen bond and p–p stacking network in the liquid state and are consequently highly structured, that is, they can be described as ‘‘supramolecular” fluids. This structural organization of imidazolium ILs can function as an ‘‘entropic driver” for spontaneous, well-defined, and extended ordering of nanostructures. Therefore, the unique combination of adaptability toward other molecules and phases associated with the strong H-bond-driven fluid structure makes imidazolium ILs ‘‘templates” for the formation of nanostructures. Moreover, it is possible that imidazolium-based ILs form surfaceattached carbenes, at least as transient species, and may also be responsible for the stabilization of zero-valent transition-metal nanoparticles in these fluids. These ILs are suitable mediums for the preparation, stabilization of transition-metal nanoparticles and hence for ideal recyclable biphasic catalytic systems for hydrogenation and C–C coupling [93–111]. Dupont et al. synthesized Ir and Pt nanoparticles directly for catalysis in [BMIM][PF6] and [BMIM][BF4] by hydrogen reducing method [102]. These particles
show a high catalysis activity for hydrogenation and C–C coupling reactions. In some cases the transition-metal nanoparticles, in particular those of Rh(0) and Pd(0), originally dispersed in the IL tend to aggregate into larger structures and loss of their catalytic activity. Dupont et al. found that laser irradiation of nanoparticles dissolved in ILs could generate transition-metal nanoparticles with a narrow size distribution [93]. By this method they prepared Ru, Pd nanoparticles with controlled size. This method can be used to restructure particles that tend to aggregate after use in catalysis. The nanoparticles isolated from the IL can be easily redispersed by simple laser radiation, and the formed colloidal solution is stable for weeks without any change in size. This route is promising for the recycle of the transition-metal nanoparticles for catalysis in ILs, which is significant in the continuous catalysis reactions. 3. Conclusion The recent development of synthesis of inorganic nanomaterials in ILs reviewed in this article clearly shows that ILs add great value to inorganic nanomaterials area. The unique adaptability and flexibility of ILs provide us with a new, flexible, and powerful tool for the fabrication of novel, interesting, and highly sophisticated nanostructures via chemical approaches. The application of the ILs in inorganic nanomaterials is still in its early stages. Further research on developing the approach and examining the breadth of the IL application is necessary. It suggests that the IL route can guide design, production, and application of inorganic nanomaterials across the range of sizes, shapes, compositions, and functionality. Further development and application of the ILs to the design and production of different inorganic nanomaterials will benefit the nanoscience community. Acknowledgements We gratefully acknowledge financial support from the National Natural Science Foundation of China (NSFC, No. 20803043 and 20803044), and the foundation of Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education. References [1] Alivisatos AP. Semiconductor clusters, nanocrystals, and quantum dots. Science 1996;271:933. [2] Henglein A. Small-particle research: physicochemical properties of extremely small colloidal metal and semiconductor particles. Chem Rev 1989;89:1861. [3] Zäch M, Hägglund C, Chakarov D, Kasemo B. Nanoscience and nanotechnology for advanced energy systems. Curr Opin Solid State Mater Sci 2006;10:132. [4] Kolasinski KW. Silicon nanostructures from electroless electrochemical etching. Curr Opin Solid State Mater Sci 2005;9:73. [5] Macak JM, Tsuchiya H, Ghicov A, Yasuda K, Hahn R, Bauer S, et al. TiO2 nanotubes: Self-organized electrochemical formation, properties and applications. Curr Opin Solid State Mater Sci 2007;11:3. [6] Kolasinski KW. Catalytic growth of nanowires: Vapor–liquid–solid, vapor– solid–solid, solution–liquid–solid and solid–liquid–solid growth. Curr Opin Solid State Mater Sci 2006;10:182. [7] Seddon KR, Stark A, Torres MJ. Influence of chloride, water, and organic solvents on the physical properties of ionic liquids. Pure Appl Chem 2000;72:2275. [8] P. Walden, Bull Acad Imper Sci [St. Petersburg] 1914; 1800.. [9] Bradley AE, Hardacre C, Holbrey JD, Johnston S, McMath SEJ, Nieuwenhuyzen M. Small-Angle X-ray Scattering Studies of Liquid Crystalline 1-Alkyl-3methylimidazolium Salts. Chem Mater 2002;14:629. [10] Ngo HL, LeCompte K, Hargens L, McEwen AB. Thermal properties of imidazolium ionic liquids. Thermochim Acta 2000;97:357. [11] Sheldon R. Catalytic reactions in ionic liquids. Chem Commun 2001:2339. [12] Huddleston JG, Willauer HD, Swatloski RP, Visser AE, Rogers RD. Room temperature ionic liquids as novel media for ‘clean’ liquid–liquid extraction. Chem Commun 1998:1765. [13] Dickinson EV, Williams ME, Hendrickson SM, Masui H, Murray RW. Hybrid redox polyether melts based on polyether-tailed counterions. J Am Chem Soc 1999;121:613. [14] Kubisa P. Application of ionic liquids as solvents for polymerization processes. Prog Polym Sci 2004;29:3.
Z. Li et al. / Current Opinion in Solid State and Materials Science 12 (2008) 1–8 [15] Wasserscheid P, Welton T. Ionic liquids in synthesis. New York: Wiley-VCH Weinheim; 2002. [16] Rogers RD, Seddon KR, Volkov S. Green Industrial Applications of Ionic Liquids. Dordrecht: Kluwer Academic Publisher; 2003. [17] Mamantov G, Popov A. Advanced in Nonaqueous Chemistry. New York: Wiley-VCH Weinheim; 1994. [18] Semeril D, Bruneau C, Dixneuf PH. Imidazolium and imidazolinium salts as carbene precursors or solvent for ruthenium-catalysed diene. Adv Synth Catal 2002;344:585. [19] Zhang MM, Kamavaram V, Reddy RG. New electrolytes for aluminum production: Ionic liquids. Jom J Min Met Mat S 2003;55:A54. [20] Gu YL, Peng JJ, Qiao K, Yang HZ, Shi F, Deng YQ. Room temperature ionic liquids and their applications in catalysis and organic reactions. Prog Chem 2003;15:222. [21] Taubert A. Inorganic materials synthesis – a bright future for ionic liquids?. Acta Chim Slov 2005;52:183. [22] Antonietti M, Kuang D, Smarsly B, Zhou Y. Ionic liquids for the convenient synthesis of functional nanoparticles and other inorganic nanostructures. Angew Chem Int Ed 2004;43:4988. [23] Zhou Y. Recent advances in ionic liquids for synthesis of inorganic nanomater. Curr Nanosci 2005;1:35. [24] Taubert A, Li Z. Inorganic materials from ionic liquids. Dalton Trans. 2007;7:723. [25] Endres F, Abedin SZE. Air and water stable ionic liquids in physical chemistry. Phys Chem Chem Phys 2006;8:2101. [26] Parnham Emily R, Morris Russell E. Ionothermal synthesis of zeolites, metal– organic frameworks, and inorganic–organic hybrids. Acc Chem Res 2007;40:1005. [27] Wasserscheid P, Welton T. Ionic Liquids in Synthesis. Weinheim: Wiley-VCH; 2003. [28] Endres F, Bukowski M, Hempelmann R, Natter H. Electrodeposition of Nanocrystalline Metals and Alloys from Ionic Liquids. Angew Chem Int Ed 2003;42:3428. [29] Mukhopadhyay I, Aravinda CL, Borissov D, Freyland W. Electrodeposition of Ti from TiCl4 in the ionic liquid l-methyl-3-butyl-imidazolium bis (trifluoro methyl sulfone) imide at room temperature: study on phase formation by in situ electrochemical scanning tunneling microscopy. Electrochim Acta 2005;50:1275. [30] Zein El Abedin S, Boressinko N, Endres F. Electrodeposition of nanoscale silicon in a room temperature ionic liquid. Electrochem Commun 2004;6:510. [31] Endres F. Electrodeposition of a thin germanium film on gold from a room temperature ionic liquid. Phys Chem Chem Phys 2001;3:3165. [32] He P, Liu HT, Li ZY, Liu Y, Xu XD, Li JH. Electrochemical deposition of silver in room-temperature ionic liquids and its surface-enhanced raman scattering effect. Langmuir 2004;20:10260. [33] Chen PY, Sun IW. Electrochemistry of Cd(II) in the basic 1-ethyl-3methylimidazolium chloride/tetrafluoroborate room temperature molten salt. Electrochim Acta 2000;45:3163. [34] Chen PY, Sun IW. Electrochemical study of copper in a basic 1-ethyl-3methylimidazolium tetrafluoroborate room temperature molten salt. Electrochim Acta 1999;44:441. [35] Yang MH, Sun IW. Electrodeposition of antimony in a water-stable 1-ethyl-3methylimidazolium chloride tetrafluoroborate room temperature ionic liquid. J Appl Electrochem 2003;33:1077. [36] Huang JF, Sun IW. Electrodeposition of PtZn in a Lewis acidic ZnCl2–1-ethyl3-methylimidazolium chloride ionic liquid. Electrochim Acta 2004;49:3251. [37] Huang JF, Sun IW. Fabrication and surface functionalization of nanoporous gold by electrochemical alloying/dealloying. Adv Funct Mater 2005;15:989. [38] Huang JF, Sun IW. Formation of nanoporous platinum by selective anodic dissolution of PtZn surface alloy in a lewis acidic zinc chloride-1-ethyl-3methylimidazolium chloride ionic liquid. Chem Mater 2004;16:1829. [39] Cooper ER, Andrews CD, Wheatley PS, Webb PB, Wormald P, Morris RE. Ionic liquids and eutectic mixtures as solvent and template in synthesis of zeolite analogues. Nature 2004;430:1012. [40] Zhu H, Huang J, Pan Z, Dai S. Ionothermal synthesis of hierarchical ZnO nanostructures from ionic-liquid precursors. Chem Mater 2006;18:4473. [41] Wang Y, Yang H. Synthesis of CoPt nanorods in ionic liquids. J Am Chem Soc 2005;127:5316. [42] Wang Y, Yang H. Oleic acid as the capping agent in the synthesis of noble metal nanoparticles in imidazolium-based ionic liquids. Chem Commun 2006:2545. [43] Zhou X, Xie Z, Jiang Z, Kuang Q, Zhang S, Xu T, et al. Formation of ZnO hexagonal micro-pyramids: a successful control of the exposed polar surfaces with the assistance of an ionic liquid. Chem Commun 2005:5572. [44] Jiang J, Yu SH, Yao WT, Ge H, Zhang GZ. Morphogenesis and crystallization of Bi2S3 nanostructures by an ionic liquid-assisted templating. Chem Mater 2005;17:6094. [45] Du J, Liu Z, Li Z, Han B, Huang Y, Zhang J. Synthesis of mesoporous SrCO3 spheres and hollow CaCO3 spheres in room-temperature ionic liquid. Micropor Mesopor Mater 2005;83:145. [46] Li Z, Zhang J, Du J, Gao H, Gao Y, Mu T, et al. Synthesis of LaCO3OH nanowires via a solvothermal process in the mixture of water and room-temperature ionic liquid. Mater Lett 2005;59:963. [47] Li Z, Friedrich A, Taubert A. Gold microcrystal synthesis via reduction of HAuCl4 by cellulose in the ionic liquid 1-butyl-3-methyl imidazolium chloride. J Mater Chem 2008;18:1008.
7
[48] Binnemans K. Ionic liquid crystals. Chem Rev 2005;105:4148. [49] Taubert A. CuCl nanoplatelets from an ionic liquid-crystal precursor. Angew Chem Int Ed 2004;43:5380. [50] Taubert A, Steiner P, Mantion A. Ionic liquid crystal precursors for inorganic particles: phase diagram and thermal properties of a CuCl nanoplatelet precursor. J Phys Chem B 2005;109:15542. [51] Taubert A, Palivan C, Casse O, Gozzo F, Schmitt B. Ionic liquid crystal precursors (ILCPs) for CuCl platelets: the origin of the exothermic peak in the DSC curves. J Phys Chem C 2007;111:4077. [52] Taubert A, Arbell I, Mecke A, Graf P. Photoreduction of a crystalline Au(III) complex: a solid-state approach to metallic nanostructures. Gold Bull 2006;39:205. [53] Li Z, Geßner A, Richters J, Kalden J, Voss T, Kubel C, et al. Hollow zinc oxide mesocrystals from an ionic liquid precursor (ILP). Adv Mater 2008;20:1279. [54] Mumalo-Djokic D, Stern WB, Taubert A. Zinc oxide/carbohydrate hybrid materials via mineralization of starch and cellulose in the strongly hydrated ionic liquid tetrabutylammonium hydroxide. Cryst Growth Des 2008;8:330. [55] Li Z, Shkilnyy A, Taubert A. Room-temperature ZnO mesocrystal formation in the hydrated ionic liquid precursor (ILP) tetrabutylammonium hydroxide (TBAH). Cryst Growth Des 2008;8:4526. [56] Li Z, Rabu P, Strauch P, Mantion A, Taubert A. Uniform metal (hydr)oxide particles from water/ionic liquid precursor (ILP) mixtures. Chem Eur J 2008;14:8409. [57] Li Z, Khimyak YZ, Taubert A. Lessons from a failed‘‘ experiment: zinc silicates with complex morphology by reaction of zinc acetate, the ionic liquid precursor (ILP) tetrabutylammonium hydroxide (TBAH), and glass. Materials 2008;1:3. [58] Li Z, Luan Y, Mu T. Unusual nanostructured ZnO particles from an ionic liquid precursor. Chem Comm 2009. doi:10.1039/B819505F. [59] Kim KS, Choi S, Cha JH, Yeon SH, Lee H. Facile one-pot synthesis of gold nanoparticles using alcohol ionic liquids. J Mater Chem 2006;16:1315. [60] Zhu YJ, Wang WW, Qi RJ, Hu XL. Microwave-assisted synthesis of singlecrystalline tellurium nanorods and nanowires in ionic liquids. Angew Chem Int Ed 2004;43:1410. [61] Li Z, Liu Z, Zhang J, Han B, Du J, Gao Y, et al. Synthesis of single-crystal gold nanosheets of large size in ionic liquids. J Phys Chem B 2005;109:14445. [62] Wang WW, Zhu YJ. Synthesis of PbCrO4 and Pb2CrO5 rods via a microwaveassisted ionic liquid method. Cryst Growth Des 2005;5:505. [63] Jiang Y, Zhu YJ, Cheng GF. Synthesis of Bi2Se3 nanosheets by microwave heating using an ionic liquid. Cryst Growth Des 2006;6:2174. [64] Jiang Y, Zhu YJ. microwave-assisted synthesis of sulfide M2S3 (M) Bi, Sb) nanorods using an ionic liquid. J Phys Chem B 2005;109:4361. [65] Yang LX, Zhu YJ, Wang WW, Tong H, Ruan ML. Synthesis and formation mechanism of nanoneedles and nanorods of manganese oxide octahedral molecular sieve using an ionic liquid. J Phys Chem B 2006;110:6609. [66] Jacob DS, Bitton L, Grinblat J, Felner I, Koltypin Y, Gedanken A. Are ionic liquids really a boon for the synthesis of inorganic materials? a general method for the fabrication of nanosized metal fluorides. Chem Mater 2006;18:3162. [67] Singh P, Katyal A, Kalra R, Chandra R. Copper nanoparticles in an ionic liquid: an efficient catalyst for the synthesis of bis-(4-hydroxy-2oxothiazolyl)methanes. Tetrahedron Lett 2008;49:727. [68] Liu Z, Sun Z, Han B, Zhang J, Huang J, Du J, et al. Microwave-assisted synthesis of Pt nanocrystals and deposition on carbon nanotubes in ionic liquids. J Nanosci Nanotech 2006;6:175. [69] Ding K, Miao Z, Liu Z, Zhang Z, Han B, An G, et al. Facile synthesis of high quality TiO2 nanocrystals in ionic liquid via a microwave-assisted process. J Am Chem Soc 2007;129:6362. [70] Dai S, Ju YH, Gao HJ, Lin JS, Pennycook SJ, Barnesc CE. Preparation of silica aerogel using ionic liquids as solvents. Chem Commun 2000:243. [71] Nakashima T, Kimizuka N. Interfacial synthesis of hollow TiO2 microspheres in ionic liquids. J Am Chem Soc 2003;125:6386. [72] Zhou Y, Antonietti M. Synthesis of very Small TiO2 nanocrystals in a roomtemperature ionic liquid and their self-assembly toward mesoporous spherical aggregates. J Am Chem Soc 2003;125:14960. [73] Kaper H, Endres F, Djerdj I, Antonietti M, Smarsly BM, Maier J, et al. Direct low-temperature synthesis of rutile nanostructures in ionic liquids. Small 2007;3:1753. [74] Liu Y, Li J, Wang M, Li Z, Liu H, He P, et al. Preparation and properties of nanostructure anatase TiO2 monoliths using 1-butyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquids as template solvents. Cryst Growth Des 2005;5:1643. [75] Liu Y, Wang M, Li Z, Liu H, He P, Li J. Preparation of porous aminopropylsilsesquioxane by a nonhydrolytic solgel method in ionic liquid solvent. Langmuir 2005;21:1618. [76] Zhou Y, Schattka JH, Antonietti M. Room-temperature ionic liquids as template to monolithic mesoporous silica with wormlike pores via a sol-gel nanocasting technique. Nano Lett 2004;4:477. [77] Zhou Y, Antonietti M. Preparation of highly ordered monolithic supermicroporous lamellar silica with a room-temperature ionic liquid as template via the nanocasting technique. Adv Mater 2003;15:1452. [78] Zhou Y, Antonietti M. A series of highly ordered, super-microporus, lamellar silicas prepared by nanocasting with ionic liquids. Chem Mater 2004;16:544. [79] Zhou Y, Antonietti M. A novel tailored bimodal porous silica with welldefined inverse opal microstructure and super-microporous lamellar nanostructure. Chem Commun 2003:2564.
8
Z. Li et al. / Current Opinion in Solid State and Materials Science 12 (2008) 1–8
[80] Kuang D, Brezesinski T, Smarsly B. Hierarchical porous silica materials with a trimodal pore system using surfactant templates. J Am Chem Soc 2004;126:10534. [81] Brezesinski T, Erpen C, Iimura K, Smarsly B. Mesostructured crystalline ceria with a bimodal pore system using block copolymers and ionic liquids as rational templates. Chem Mater 2005;17:1683. [82] Trewyn BG, Whitman CM, Lin VSY. Morphological control of roomtemperature ionic liquid templated mesoporous silica nanoparticles for controlled release of antibacterial agents. Nano Lett 2004;4:2139. [83] Farag Hala K, Endres Frank. Studies on the synthesis of nano-alumina in air and water stable ionic liquids. J Mater Chem 2008;18:442. [84] Jacob DS, Joseph A, Mallenahalli SP, Shanmugam S, Makhluf S, CalderonMoreno J, et al. Rapid synthesis in ionic liquids of room-temperatureconducting solid microsilica spheres. Angew Chem Int Ed 2005;44:6560. [85] Luan Y, Xu G, Yuan S, Xiao L, Zhang Z. Comparative studies of structurally similar surfactants: Sodium bis(2-ethylhexyl)phosphate and sodium bis(2ethylhexyl)sulfosuccinate. Langmuir 2002;18:8700. [86] Li Z, Du J, Zhang J, Mu T, Gao Y, Han B, et al. Synthesis of single crystal BaMoO4 nanofibers in CTAB reverse microemulsions. Mater Lett 2005;59:64. [87] Li Z, Zhang J, Du J, Han B, Mu T, Gao Y. Preparation and self-assembly of nanostructured BaCrO4 from CTAB reverse microemulsions. Mater Chem Phys 2005;91:40. [88] Gao H, Li J, Han B, Chen W, Zhang J, Zhang R, et al. Microemulsions with ionic liquid polar domains. Phys Chem Chem Phys 2004;6:2914. [89] Taubert A. (Sub)micron CaF2 cubes and hollow rods from ionic liquid emulsions. Acta Chim Slov 2005;52:168. [90] Li Z, Zhang J, Du J, Han B, Wang J. Preparation of silica microrods with nanosized pores in ionic liquid microemulsions. Colloids Surf A 2006;286:117. [91] Itoh H, Naka K, Chujo Y. Synthesis of gold nanoparticles modified with ionic liquid based on the imidazolium cation. J Am Chem Soc 2004;126:3026. [92] Kim K, Demberelnyamba D, Lee H. Langmuir Size-selective synthesis of gold and pPlatinum nanoparticles using novel thiol-functionalized ionic liquids. 2004;20:556.. [93] Gelesky MA, Umpierre AP, Machado G, Correia RRB, Magno WC, Morais J, et al. Laser-induced fragmentation of transition metal nanoparticles in ionic liquids. J Am Chem Soc 2005;127:4588. [94] Cassol CC, Umpierre AP, Machado G, Wolke SI, Dupont J. The role of Pd nanoparticles in ionic liquid in the Heck reaction. J Am Chem Soc 2005;127:3298. [95] Umpierre AP, Machado G, Fecher GH, Morais J, Dupont J. Selective hydrogenation of 1,3-butadiene to 1-Butene by Pd(0) nanoparticles embedded in imidazolium ionic liquids. Adv Synth Catal 2005;347:1404. [96] Fonseca GS, Silveira ET, Gelesky MA, Dupont J. Competitive hydrogenation of alkyl-substituted arenes by transition-metal nanoparticles: Correlation with the alkyl-steric effect. Adv Synth Catal 2005;347:847. [97] Fonseca GS, Scholten JD, Dupont J. Iridium nanoparticles prepared in ionic liquids: An efficient catalytic system for the hydrogenation of ketones. Synlett 2004:1525.
[98] Rossi LM, Dupont J, Machado G, Fichtner PFP, Radtke C, Baumvol IJR, et al. Ruthenium dioxide nanoparticles in ionic liquids: Synthesis, characterization and catalytic properties in hydrogenation of olefins and arenas. J Braz Chem Soc 2004;15:904. [99] Rossi LM, Machado G, Fichtner PFP, Teixeira SR, Dupont J. On the use of ruthenium dioxide in 1-n-butyl-3-methylimidazolium ionic liquids as catalyst precursor for hydrogenation reactions. Catal Lett 2004;92:149. [100] Scheeren CW, Machado G, Dupont J, Fichtner PFP, Texeira SR. Nanoscale Pt(0) particles prepared in imidazolium room temperature ionic liquids: Synthesis from an organometallic precursor, characterization, and catalytic properties in hydrogenation reactions. Inorg Chem 2003;42:4738. [101] Fonseca GS, Umpierre AP, Fichtner PFP, Teixeira SR, Dupont J. The use of imidazolium ionic liquids for the formation and stabilization of Ir-0 and Rh-0 nanoparticles: Efficient catalysts for the hydrogenation of arenas. Chem Eur J 2003;9:3263. [102] Dupont J, Fonseca GS, Umpierre AP, Fichtner PFP, Teixeira SR. Transitionmetal nanoparticles in imidazolium ionic liquids: Recycable catalysts for biphasic hydrogenation reactions. J Am Chem Soc 2002;124:4228. [103] Bruss AJ, Gelesky MA, Machado G, Dupont J. Rh(0) nanoparticles as catalyst precursors for the solventless hydroformylation of olefins. J Mol Catal A Chem 2006;252:212. [104] Zhao D, Fei Z, Geldbach Tilmann J, Scopelliti R, Dyson Paul J. Nitrilefunctionalized pyridinium ionic liquids: Synthesis, characterization, and their application in carbon - Carbon coupling reactions. J Am Chem Soc 2004;126:15876. [105] Calo V, Nacci A, Monopoli A, Montingelli F. Pd nanoparticles as efficient catalysts for Suzuki and Stille coupling reactions of aryl halides in ionic liquids. J Org Chem 2005;70:6040. [106] Calo V, Nacci A, Monopoli A, Fornaro A, Sabbatini L, Cioffi N, et al. Heck reaction catalyzed by nanosized palladium on chitosan in ionic liquids. Organometallics 2004;23:5154. [107] Calo V, Nacci A, Monopoli A, Laera S, Cioffi N. Pd nanoparticles catalyzed stereospecific synthesis of beta-aryl cinnamic esters in ionic liquids. J Org Chem 2003;68:2929. [108] Calo V, Nacci A, Monopoli A, Detomaso A, Iliade P. Pd nanoparticle catalyzed Heck arylation of 1,1-disubstituted alkenes in ionic liquids. Study on factors affecting the regioselectivity of the coupling process. Organometallics 2003;22:4193. [109] Anderson K, Cortinas Fernandez S, Hardacre C, Marr PC. Preparation of nanoparticulate metal catalysts in porous supports using an ionic liquid route; hydrogenation and C-C coupling. Inorg Chem Commun 2003;7:73. [110] Mu XD, Meng JQ, Li ZC, Kou Y. Rhodium nanoparticles stabilized by ionic copolymers in ionic liquids: Long lifetime nanocluster catalysts for benzene hydrogenation. J Am Chem Soc 2005;127:9694. [111] Gao SY, Zhang HJ, Wang XM, Mai WP, Peng CY, Ge LH. Palladium nanowires stabilized by thiol-functionalized ionic liquid: seed-mediated synthesis and heterogeneous catalyst for Sonogashira coupling reaction. Nanotechnology 2005;16:1234.