CrystEngComm

HIGHLIGHT

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are different from those of their bulk counterparts.1,2 There-

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rananomaterials are fabricated insolvents or water); nevertheless,solvents can be used and somemental problems. In this regardexplore novel and green mediato occur.3

As a new type of green alternao

anions. Due to the asymmetry of volume, ionic liquids have

aDepartment of Materials Chemistry, Key

Materials Chemistry and TKL of Metal and

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0. are being extensively investigated. Subsequently, ionic liquidshave been actively employed for the synthesis of a broadrange of inorganic materials, and many interesting inorganicmaterials with various properties have been fabricated. Notably,

College of Chemistry, Nankai University, Tianjin, PR China.

E-mail: zhwj@nankai.edu.cnb Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education,

State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University,

Changsha, PR China. E-mail: nanoelechem@hnu.edu.cn

Dupont's group prepared uniformliquid media.23 Kimizuka and Na

2550 | CrystEngComm, 2014, 16, 25502559 This journal is The R

c Department of Physics, The Chinese University of Hong Kong, Hong Kong,

PR Chinasolvents, ionic liquids (ILs) htion in organic chemistry andfore, the design and optimizthe preparation of nanomateritigated. As a result, importantthe synthesis of nanomaterisize and shape using varioussolution-phase methods. Geneion of synthetic methods forls have been intensively inves-rogress has been achieved ons with tailored composition,synthetic methods, especiallylly, in conventional synthesis,molecular solvents (organic

a limited number of molecularof them may cause environ-, it is still a big challenge tothat allow particular reactions

ative to conventional organicve found widespread applica-rganometallic catalysis exactly

some distinctive features, such as low melting point, negligiblevapor pressure, non-volatility, high thermal stability, and highionic conductivity. More importantly, these properties arestrongly dependent on the species of cations and anions;therefore, ionic liquids can be referred to as designedliquids with tunable properties by adjusting their cationsand anions. Mainly on account of these unique properties,ionic liquids show an increasing potential to innovate in thesynthesis techniques. Although there are many excellent char-acteristics as stated above, we here should point out that anyionic liquid can hardly take on all these characteristicstogether, and we should carefully check them for any givenionic liquid before use.

The first attempt at using ionic liquids as the reactionmedium instead of conventional molecular solvents for thesynthesis of inorganic materials was pioneered by Dai andco-workers in 2000.22 They introduced ionic liquids for thepreparation of porous silica gels termed as ionogels, which

Laboratory of Advanced Energy

Molecule-Based Material Chemistry,atCite this: CrystEngComm, 2014, 16,

2550

Received 24th June 2013,Accepted 1st November 2013

DOI: 10.1039/c3ce41203b

www.rsc.org/crystengcomm

inorganic nanom

Xiaochuan Duan,a Jianmin

Wet chemistry using ionic liquids

several types of metallic, metal ox

article reviews state-of-the-art re

versatile regent for the synthesis

explore the ability of ionic liquid

synthesis parameters, also denote

many advantages over traditional

of functional inorganic materials

most recent advances in ionother

choice of cations or anions of io

issues that should be addressed in

Introduction

Over the past few decades, nanoscience has experiencedexponential growth in its research activities, since nanoscalematerials can exhibit physical and chemical properties thatnic liquids in the synthesis ofterials

,*b Jiabiao Lianc and Wenjun Zheng*a

the medium has proven to be highly efficient for the preparation of

s, and other kinds of semiconductor nanostructures, and so on. This

rch activities in the field, focusing on the use of ionic liquids as a

various nanoparticle systems. We begin with a survey of choices to

act as a reactant, solvent, and surfactant, as a function of other

as ionic liquid precursors (or task-special ionic liquids), which offer

lution-phase methods. We then examine the design and fabrication

means of optimizing the effect models of ionic liquids. Many of the

l or ionic liquid-assisted synthesis have been realized by appropriate

liquids according to the need. This review also highlights crucial

ture research activities.

from the discovery of water-stable ionic liquids by Wilkes andZaworotko in 1992,4 whilst their use in inorganic synthesis isjust about to begin.521 Different from the conventionalmolecular solvents, ionic liquids are usually composed oflarger organic cations and smaller organic or inorganic

View Article OnlineView Journal | View IssueIr nanoparticles in ionickashinma reported the

oyal Society of Chemistry 2014

synthesis of hollow titania microspheres in a toluene/ionic especially in the past five years are highlighted here. In order tofacilitate discussion, the range of properties and functions ofionic liquids for the synthesis of inorganic nanomaterials hasbeen divided into two major aspects: the ionic liquid precursorand the effect model of ionic liquids.

Ionic liquid precursor

Since ionic liquids can serve as tailored solvents, they thusgive us an opportunity for designing the ionic liquid precur-sors according to the crystal structures, compositions, andcrystal habits of the target products. The ionic liquid precur-sors can act as the reactant and solvent for the reaction,as well as the template over the final inorganic material mor-phology at the same time. Accordingly, ionic liquids can bepromising all-in-one solvents for the synthesis of inorganicmaterials, which can make the reaction system simpler, andthus with easier control over the phases and morphologies ofthe final products. The ultimate goal of ionic liquid precursorresearch is to understand and design the task-special ionicliquids at the molecular level and to synthesize inorganicmaterials with the desired phase and shape. The hypothesisof ionic liquids as all-in-one solvents was first testedon cuprous chloride by Taubert and co-workers.27 In thatstudy, they introduced a protocol for the synthesis of CuClnanoplatelets from a Cu-containing IL 1 and 6-O-palmitoyl

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View Article Onlineliquid medium.24 Antonietti and Zhou obtained sphericalTiO2 aggregates composed of nanoparticles using an ionicliquid.25 Correspondingly, the concepts ionothermal andall-in-one for the synthesis of materials using an ionic liquidas the solvent were proposed in 2004 by the respective groupsof Morris and Taubert.26,27 It is worth mentioning thationothermal synthesis is quite different from hydro- or solvo-thermal conditions, which may lead to new materials withinteresting morphologies and that are not accessible by usingconventional organic solvents or water due to the uniquephysicochemical properties of the ionic liquids. Gradually, theadvantages of ionic liquids in inorganic synthetic procedureshave been realized, for instance: (i) ionic liquids have lowinterface tensions in spite of their polar features, resulting ina high nucleation rate; (ii) ionic liquids can form extendedhydrogen bond systems in the liquid state and are thereforehighly structured, and thus can further affect the structures ofresulting products; and (iii) as a tunable medium, ionic liquidsare immiscible with a number of organic solvents and canprovide a non-aqueous and polar alternative for two-phasesystems. Thus, it is rational to expect that the application ofILs may offer a wide variety of possibilities for the fabricationof nanomaterials and develop into a mainstream area in thefield of synthetic chemistry.

However, although great efforts have been made on con-trolling the crystal phase and morphology of inorganic mate-rials using ionic liquids, a consistent and fundamentalunderstanding of the effect between ionic liquids and prod-ucts has still not been achieved.2852 As a consequence of thissituation, most of the syntheses are not predicted and simplyuse an IL or a mixture of an IL with a conventional solventjust like a common surfactant, which does not sufficientlyexhibit the main advantages of ILs. There may be mainly tworeasons that have led to this status: (i) A molecular-basedunderstanding of the physicochemical properties of ILs is agreat challenge since different ionic liquids have differentphysicochemical properties, and since reliable parameters aresometimes not available, it is urgent to build up a systematicdatabase on the physicochemical properties of ILs. (ii) Theresearch on well-established rules and correlations betweenmolecular structures of the adopted ILs and the morphologiesof the resulting inorganic materials is limited.53 We believe thatthe realization of the general trends may be utilized in therational design of desired inorganic materials with the desiredpolymorph and the desired morphology using ILs. For thisregard, this review will focus on the rational design of ionicliquids and an understanding of the ionic liquid's effect at themolecular level based on solution-phase methods. Althoughthe use of ILs for the synthesis of inorganic nanomaterials hasbeen reviewed in some inspiring short accounts or perspec-tives,3,5,6,12,54,55 a relatively comprehensive and fundamentalreview of this subject is still lacking, especially considering therational design of ionic liquids at the molecular level. In addi-tion, as a burgeoning field, there are hundreds of relevantpapers, and only typical examples and key concepts proposedThis journal is The Royal Society of Chemistry 2014ascorbic acid 2. It was found that the mixtures of 1 and 2could form thermotropic liquid crystals with lamellar self-assembled structures and the plate morphology was thereforecaused (Fig. 1). Subsequently, a large range of inorganic

Fig. 1 (a) Components of the ionic liquid precursors used for CuClplatelet synthesis: Cu-containing IL 1 and 6-O-palmitoyl ascorbic acid2. (b) Optical micrograph (crossed polarizers) of a demixed ionic liquidprecursor.CrystEngComm, 2014, 16, 25502559 | 2551

nanomaterials with interesting phases and morphologies (Fe1.9F4.750.95H2O and FeF30.33H2O).59 The ionic liquid pre-

cursor [Bmim][BF4] played an important role in the formationof the fluoride nanomaterials, as illustrated in Fig. 2. Afterthe addition of Fe(NO3)30.33H2O powder to the [Bmim][BF4]medium, Fe3+ ions and NO3

ions could be coordinatedlydissolved by the BF4

anion and large imidazolium cation,respectively. Furthermore, the Fe3+BF4

interaction layer isexpectedly surrounded by imidazolium cations. In contrast,in the presence of water for hydration, the weakly coordinat-ing BF4

anion is prone to hydrolyze and form BF3H2O andF. Consequently, the solvated Fe3+ ion combines with the F

ion to form precipitated iron-based fluorides, whose nano-grains are well separated due to the interfacial tension. Inter-estingly, Cui's group has successfully synthesized high-quality

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View Article Onlinewere fabricated from various all-in-one ILs. Here, we presentsome selected examples to highlight how the ionic liquidprecursors work in the synthesis of inorganic nanomaterials.

[Bmim][BF4]

Apparently, 1-n-butyl-3-methylimidazolium tetrafluoroborate([Bmim][BF4]) is one of the most widely used ionic liquids inthe synthesis of inorganic nanomaterials, especially for inor-ganic fluorides. However, the [BF4]

counterions are unstablein aqueous solution at higher temperature, and will decom-pose thermally and hydrolyze slowly under the appropriateconditions to release F ions, which may be a drawback fororganic synthesis and catalysis but is extremely helpful forthe synthesis of fluoride nanomaterials. Compared withthe common fluoride sources HF, NaF, or NH4F, whichare often toxic and need additional templates or structure-directing agents to obtain better control of the final products,[Bmim][BF4] acting as a fluoride source is environmentally-friendly and operationally safe. In a typical synthesis, whenheating the reaction medium up to a sufficiently high tem-perature, the [BF4]

ions chemically transform into active ionsand are rapidly hydrolyzed with water molecules, which canbe formulated as: BF4

(IL) + H2O BF3H2O(IL) + F. In addi-

tion, [Bmim][BF4] can also serve as a soft-template in theformation of the resulting products, which has a significantinfluence on the structures and shapes of the samples involvingvarious mechanisms, including hydrogen bonding and stacking interactions, self-assembly mechanisms, electrostaticattraction, and so on. Based on the above analysis, [Bmim][BF4]as an ionic liquid precursor serves not only as a reagent toprovide the necessary fluoride source, but also as a solvent andsoft template for nanostructure control.

Yan and co-workers have introduced the [Bmim][BF4]-based route into the synthesis of novel spherical NaYF4nanoclusters with diameters ranging from 200 to 430 nmself-assembled by small nanoparticles.56 Their experimentalresults indicate that [Bmim][BF4] plays a key role in theformation NaYF4 nanocrystals and the diameters of thenanoclusters could be easily tuned just by changing theamount of the ionic liquid precursor. Using the same ionicliquid precursor, Lin's group has reported a fast, facile andenvironmentally-friendly microwave-assisted ionic liquidmethod to fabricate REF3 (RE = LaLu, Y) nano/microcrystalswith multiform crystal structures (hexagonal and orthorhombic)and morphologies (nanodisks, secondary aggregates constructedfrom nanoparticles, and elongated nanoparticles), where[Bmim][BF4] plays a multiple role as the reactant, solvent andtemplate in the reaction system.57 Similarly, Mudring andco-workers reported the microwave reaction of Ln(OAc)3xH2O,and in the ionic liquid [Bmim][BF4] allows the fast and efficientsynthesis of small, uniform, oxygen-free lanthanide nanofluorideswith excellent photophysical behaviour.58

Li and co-workers have reported a novel low-temperaturenon-aqueous based route using the [Bmim][BF4] mediumto fabricate well-defined iron-based fluoride nanomaterials2552 | CrystEngComm, 2014, 16, 25502559lanthanide-doped NaGdF4 upconversion nanocrystals in anewly-developed facile OA[Bmim][BF4] two-phase system(Fig. 3). Oil-dispersible cubic-phase NaGdF4 nanocrystals withultra-small size (~5 nm) and monodispersity were obtained inthe oleic acid (OA) phase. Meanwhile, water-soluble hexagonal-phase NaFdF4 nanocrystals were obtained in the same systemsimply by adopting an extremely facile method to complete thedual phase-transition (crystal-phase transition and OA-phase toIL-phase transition) simultaneously.60 Most recently, Zhu andco-workers have reported a microwave-assisted ionic liquidsolvothermal method to prepare CaF2 double-shelled hollowmicrospheres using [Bmim][BF4] as the ionic liquid precursor,which can also be extended to prepare hollow microspheres ofMgF2 and SrF2.

61 Their experimental results also demonstratedthat the concentration of the ionic liquid precursor playedan important role in the formation of double-shelled hollowmicrospheres self-assembled by polyhedral particles.

[Bmim][SeO2(OCH3)]

Recently, more and more research has focused on the con-trolled synthesis of metal selenides due to their remarkableproperties and potential applications. Up to now, only a fewSe sources have been developed for fabricating metal sele-nides. For example, Na2SeO3 was widely used due to its highactivity and good water solubility. However, Na2SeO3 couldreact with metal ions (Mn+) to form precipitates in somesystems. In order to make up uniform reaction conditions,Mn+ ions are usually transformed into stable complexes,

Fig. 2 Scheme of hydrated iron-based fluoride formation mechanismfrom ionic liquid precursor [Bmim][BF4].This journal is The Royal Society of Chemistry 2014

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View Article OnlineThis journal is The Royal Society of Chemistry 2014

which would make the reaction systems more complicated,and uncertain factors influence the morphology of the finalproducts. In view of this, we have recently designed 1-n-butyl-3-methylimidazolium methylselenite ([Bmim][SeO2(OCH3)])as a new Se precursor. For this ionic liquid, the reactivity ofthe anion ([SeO2(OCH3)]

ions) is similar to SeO32 ions, and

the formation of metal selenides can be formulated as (takinga divalent metal as an example): 2M2+ + 2[SeO2(OCH3)]

+3N2H4 + 2OH

2MSe + 3N2 + 2CH3OH + 6H2O. Moreimportantly, one oxygen among SeO3

2 ions is replaced bymethoxy, leading to the weaker polarizing capability of[SeO2(OCH3)]

, and thus Mn+ ions can exist as free ions in thesolution. In addition, the cation ([Bmim]+ ions) can serve as astabilizer: along with the anions, [Bmim]+ will also adsorb onthe resulting particle surfaces possibly driven by the electro-static attractions, which is similar to the classic, DLVO(DerjauginLandauVerweyOverbeek) type Coulombic repul-sion model. Under this guidance, our group has successfullyprepared various metal selenides with controlled phases andshapes, including ZnSe hollow nanospheres (Fig. 4),62 CdSenanospheres and nanodendrites,63 Cu2xSe nanocrystals andCuSe nanoflakes.64 It could be highly expected that this ionicliquid precursor would be used to prepare other metal sele-nide nanomaterials with novel morphologies.

Fig. 3 Schematic diagram showing the mechanism for the formationNaGdF4 nanocrystals from OA[Bmim][BF4] two-phase system.[Cnmim]X (X = Cl, Br, I)

Recently, more and more attention has paid to halides due totheir excellent optical and photocatalytic properties. Althoughthe synthesis of halides in ionic liquids or mixed solutionscontaining ionic liquids has just begun, this new type ofhalide source, ionic liquid precursor 1-butyl-3-methylimidazoliumhalide [Cnmim]X (X = Cl, Br, I), has exhibited distinctive fea-tures compared with other halide sources. Our group has suc-cessfully synthesized ultrathin BiOCl nanoflakes, nanoplatearrays and curved nanoplates via an ionothermal syntheticroute by using the ionic liquid 1-hexadecyl-3-methylimidazoliumchloride ([C16mim]Cl) as the ionic liquid precursor.

65 Alongwith the Cl ion, [C16mim]

+ ion will also be aligned and arrayedalong the BiOCl layer driven by the Coulomb coupling force. Itis reasonable to deduce that [C16mim]Cl, which is consideredas a supramolecular solvent involved ordered structure, isselectively adsorbed on the (001) plane of BiOCl to effectivelyinhibit crystalline growth in the [001] direction. Therefore,the growth of BiOCl crystals are inhibited along the c-axisto form ultrathin BiOCl nanoflakes. Hereafter, Huang andco-workers reported a facile, one-pot approach to the uniformBiOBr hollow microspheres in the ionic liquid precursor1-hexadecyl-3-methylimidazolium bromide ([C16mim]Br).

66 Thepossible formation mechanism is proposed in Fig. 5. Due tothe limited miscibility of the ionic liquid precursor ([C16mim]Br)and co-solvent 2-methoxyethanol, micronized ionic liquidemulsions are formed under vigorous stirring, which aresurrounded by 2-methoxyethanol containing Bi3+ ions. These

Fig. 4 Schematic illustration of the formation process of ZnSe hollownanospheres from ionic liquid precursor [Bmim][SeO2(OCH3)].CrystEngComm, 2014, 16, 25502559 | 2553

miniemulsions can perform as microreaction chambers, andthus Bi3+ ions react accordingly with [C16mim]Br at theminiemulsion interface to form BiOBr nuclei at elevatedtemperature. In the subsequent growth process, the BiOBrnanoparticles at the miniemulsion interface grow and assem-ble to construct the shell of the microspheres, retaining thehollow interiors. In the following study, they used differentionic liquid precursors, 1-butyl-3-methylimidazolium bromide([C4mim]Br), 1-octyl-3-methylimidazolium bromide ([C8mim]Br),1-dodecyl-3-methylimidazolium bromide ([C12mim]Br), and1-hexadecyl-3-methylimidazolium bromide ([C16mim]Br), as abromide source to prepare AgBr microcrystals with differentmorphologies.67 Structurally, the {001} facet growth of silver

absorption edge (band gap) with excellent chemical and

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View Article Onlinehalide crystals has been studied and {100} facets are easilyexposed due to the lower surface energy than that of {111}and {110} facets, leading to the cubic shape. However, whenintroducing the ionic liquid precursor into the reaction system,as the length of ionic liquid alkyl chain is increased, the sterichindrance of the ionic liquid will restrict the diffusion of Ag+

ions, and convex facets of the near-spherical AgBr are formed.Zhang and co-workers have investigated the photocatalysis

mechanism of BiOI and therefore demonstrated that thephotocatalytic activity of BiOI could be enhanced greatlyby the in situ modification of the ionic liquid precursor1-butyl-3-methylimidazolium iodide ([Bmim]I).68 They foundthat ionic liquid modification could trap the photo-excited

Fig. 5 Formation mechanism of BiOBr hollow microspheressynthesized by using ionic liquid precursor [C16mim]Br.2554 | CrystEngComm, 2014, 16, 25502559

electron at the conduction band of BiOI, thus inhibiting therecombination of photoinduced electronhole pairs, andleading to the enhancement of its photocatalytic activityon the degradation of organic pollutants. Recently, Li andco-workers have successfully synthesized BiOI uniformflower-like hollow microspheres with a single hole in theirsurface structures through an EG-assisted solvothermalprocess in the presence of the same ionic liquid precursor,[Bmim]I.69 Similar to the above formation mechanism,[Bmim]I can easily form ionic liquid micelles, resulting in theBiOI hollow microspheres through a self-assembly process.During the process, ionic liquid precursor [Bmim]I playsan important role as reactant, solvent and template. Subse-quently, they also successfully synthesized BiOBr uniformflower-like hollow microsphere and porous nanosphere struc-tures using a similar route in the presence of ionic liquid1-hexadecyl-3-methylimidazolium bromide ([C16mim]Br).

70

[Choline][H2PO4] and [Bmim][H2PO4]

Nanophosphates are of tremendous interest as optical mate-rials since they are known to combine a high energy-mechanical stability. Compared with other inorganic nano-materials, which have already been prepared based on anionic liquid all-in-one solvent, there are few studies on thesynthesis of nanophosphates using an ionic liquid precursor.Mudring and co-workers have presented a universal, fast, andfacile microwave synthesis process using an ionic liquid pre-cursor [choline][H2PO4] acting as both the reagent andsurface-modifying agent.71 In the typical synthesis, lantha-nide acetate hydrates can be converted to lanthanide singlephosphate (LnPO4) with the use of an ionic liquid precursor:Ln(OAc)3 + [choline][H2PO4] LnPO4 + 2HOAc +[choline][OAc]. Here, the ionic liquid precursor is trulymultifunctional: (1) it serves as reaction medium dueto its ionic, highly polarizable character it guarantees anexcellent microwave susceptibility; (2) it helps to control theresulting particle size and morphology; and (3) it acts as areaction partner. Most recently, our group has successfullysynthesized well-dispersed ferric giniite microcrystals withcontrolled sizes and shapes from ionic liquid precursorsusing 1-n-butyl-3-methylimidazolium dihydrogenphosphate([Bmim][H2PO4]) as the phosphate source.

72 The successof this synthesis relies on the concentration and compositionof the ionic liquid precursors. By adjusting the molar ratiosof Fe(NO3)39H2O to [Bmim][H2PO4] as well as the composi-tion of the ionic liquid precursors, we obtained uniformmicrostructures such as bipyramids exposing {111} facets,plates exposing {001} facets, hollow spheres, tetragonalhexadecahedra exposing {441} and {111} facets, and truncatedbipyramids with carved {001} facets. Our experimental resultshave demonstrated that [Bmim][H2PO4] plays an importantrole in stabilizing the {111} facets of ferric giniite crystals,leading to the different morphologies in the presence of ionicliquid precursors with different compositions, as illustratedin Fig. 6.

Tetrabutylammonium hydroxide (TBAH)

Li and Taubert have developed a new type of powerful ionicliquid precursor tetrabutylammonium hydroxide (TBAH) tosynthesize a series of inorganic nanomaterials with desiredmorphologies. First, they successfully highly hydrated TBAHas the ionic liquid precursor for the controlled fabrication ofzinc oxide mesocrystals with various shapes and sizes by sim-ply adjusting the zinc acetate concentrations (Fig. 7).73 In thefollowing study, they modified the synthesis method toobtain zinc oxide mesocrystals using the same ionic liquidprecursor at room temperature.74 Their results revealed that,unlike ZnO grown from aqueous solution, the particle numberin the TBAHwater mixtures decreases with prolonged reactiontime. Initially, numerous small particles precipitate, then theseprimary particles transform into fewer, but larger, mesocrystalscomposed of rod-like particles. Then, they proved that thisfacile synthesis route using TBAH as the ionic liquid precursorcan also be extended to the preparation of other inorganicmaterials.75 Simply by replacing the zinc acetate precursor withother metal acetates (M(OAc)2, M = Mn, Fe, Co, Ni, Cu, etc.), itThis journal is The Royal Society of Chemistry 2014

This journal is The Royal Society of Chemistry 2014

Fig. 6 Schematic illustration of the formation of ferric giniite crystals witwere formed originally from the truncated bipyramid seeds. The formationthe growth rate of the ferric giniite seeds under the assistance of the strong

Fig. 7 SEM images of ZnO mesocrystals precipitated at different zincacetate concentrations: (a) 10, (b) 16, and (c) 35 mg in the ionic liquidprecursor TBAH.

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View Article Onlineis possible to synthesize a wide variety of metal (hydr)oxideswith uniform size, morphology, and chemical composition.Compared with the previous synthesis of metal (hydr)oxides,this facile approach avoids the synthesis of complex organicprecursors. Rather, simple salts like metal acetates can be usedas precursors for the synthesis of inorganic materials with well-defined shapes. Later on, Li's group reported a facile routeusing TBAH as an efficient ionic liquid precursor to preparehollow ZnO mesocrystals with various morphologies, includingflower-like particles of which the rod is composed of ZnO nano-particle subunits, ZnO plates composed of ZnO rods whose tiphas a lotus leaf-like structure, and porous ZnO plates.76 Inaddition, they also investigated the effect of the chain length of

h various morphologies from an ionic liquid precursor: all the crystalsof different morphologies was mainly attributed to the differences inadsorption of [Bmim]+ ions on the {111} facets.CrystEngComm, 2014, 16, 25502559 | 2555

the cation of the ionic liquid precursor on the formation ofZnO. They successfully prepared different ZnO nanostructureswith uniform size and morphology from tetrabutylammoniumhydroxide (TBAH), tetraethylammonium hydroxide (TEAH),tetramethylammonium hydroxide (TMAH) and benzyltri-methylammonium hydroxide (BTMAH) ionic liquid precursorswith different chain lengths on the surface of zinc foil.77

Effect model of ionic liquids

Ionic liquids cannot be regarded as merely a green alterna-tive to conventional organic solvents. The most importantadvantage of using ionic liquids for the preparation of inor-ganic materials is that ionic liquids form extended hydrogenbond systems in the liquid state and are therefore highlystructured, which can be defined as supramolecular fluids.This property of structural organization makes ionic liquidssuitable for use as entropic drivers for the generation ofwell-defined nanostructures with extended order. Consider-ing that ionic liquids have both cations and anions, to obtaina molecular level perspective of this structural organization,

we first need an answer to a simple question: which ions

stage. Zhou and co-workers successfully synthesized mesoporousSiO2 using 1-butyl-3-methylimidazolium tetrafluoroborate

Fig. 8 (a) Surface structure of rutile cleaved along the [110] directionand schematic illustration of rutile (110)c(2 2)-[Emim]+ original cell;moreover, [Emim]+ ions locate in the ae sites, whereas [Emim]+ unitsare omitted for clarity. The large and small rectangles represent theoriginal cell and rutile cell, respectively. (b) Schematic illustration of aprojected view of [Emim]+ ions anchored onto the rutile (110) plane toform a tight coverage layer via the original cell.

Fig. 9 Schematic illustration of the hydrogen bond-co- stackmechanism for the formation of mesoporous SiO2.

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View Article OnlineCationic dominant

Several studies suggest that the cationic species of ionicliquids can interact with nanoparticles and stabilize them.Finke and co-workers reported that imidazolium-based ionicliquids can react with Ir nanoparticles to form surface-attachedN-heterocyclic carbenes, contributing to the stabilization of theIr nanoparticles.78 These carbene-ligand-stabilized nanoclusterscould also provide some stabilization to other transition-metalnanoclusters. Similarly, Bockstaller and co-workers preparedAu nanorods in 1-ethyl-3-methylimidazolium ethylsulfate([Emim][ES]) without the addition of stabilizing agents.79 Itwas proposed that the imidazolium cations seem to havedifferent binding affinities for different crystal facets of theAu nanoparticles, resulting in the formation of Au nanorods.Furthermore, Bouvy and co-workers systematically investigatethe cation effect of a series of pyrrolidinium-, imidazolium-,and quaternary amine-based ionic liquids on the resultingmorphologies of Au nanostructures.80 Their experimentalresults demonstrate that the ionic liquids favor the anisotropicgrowth of gold by acting as template agents.

Previously, our group has demonstrated that the interactionbetween imidazolium cations and TiO6 octahedra could be adecisive factor for the formation of the rutile phase in awater[Emim]Br composite system.81 It was found that[Emim]Br served as a capping agent based on its stronghydrogen-bonding and stacking interaction with the (110)facet of rutile and accordingly played a critical role in thecontrol of the phase and morphology of TiO2 nanoparticles. Inthe synthesis, we considered that there were mainly tworeasons for the formation of interactions between [Emim]Brand TiO2: (1) in the imidazole-based ionic liquid, the H atomat position-2 of the imidazole ring (C(2)H) has a positivecharge due to the delocalization of positive charge locatingon the imidazole ring, which enhances the ability of theH atom (C(2)H) to form hydrogen bonds between the O(rutilesurface)HC([Emim]+); (2) if [Emim]+ can vertically adsorb onthe (110) plane via a rutile(110)c(2 2)-[Emim]+ original cell(Fig. 8), the distance between [Emim]+ along the [001] directionis 0.592 nm, which is in accord with the mutual -stackingdistance between the aromatic rings. Thus, stacking inter-actions exist between the cations and [Emim]+ can form a rela-tively tight coverage layer on the rutile surface. As a result,[Emim]+ could form relatively strong interactions with TiO6octahedra, which could be a decisive factor for the formationof the rutile phase due to the spatial effect.

Anionic dominant

On the other hand, anionic species of ILs can also interactwith nanoparticles and play an important role in the growth(cations or anions) are closest to the liquidsolid interfacebetween ionic liquids and resulting materials? This is noteasy to answer, although great efforts have been carried outon related experiments or computer simulations.2556 | CrystEngComm, 2014, 16, 25502559([Bmim][BF4]) as the template and proposed hydrogen bondsformed between [BF4]

ions and the SiO2 surface togetherwith the stacking interaction of the neighboringimidazolium rings, leading to the mutual packing and forma-tion of mesoporous SiO2.

82 It was believed that [BF4] ions

interacted with the silanol groups and formed hydrogenbonds, which might induce the oriented arrangement of[BF4]

ions along the pore walls. Along with [BF4] ions, as

presented in Fig. 9, driven by the Coulomb coupling forcewith the anion, [Bmim]+ ions were also arrayed along thesilica. Moreover, the fluid state of [Bmim][BF4] facilitatedthe proposed relocation of molecules, which could bethen stabilized by the additional stacking interactionbetween the imidazolium rings of [Bmim][BF4]. Taubertand co-workers also found the strong anion effect on Aunanoparticle formation including sizes and shapes inimidazolium-based ionic liquids.83 They successfully synthe-sized Au nanoparticles in a set of ionic liquids based onthe same cation (1-ethyl-3-methyl-imidazolium) and three typesof anion, namely triflate ([Emim][TfO]), methanesulfonate([Emim][MS]), and ethyl sulfate ([Emim][ES]). With the MSanion, Au nanoparticles with diameters between 5 and 7 nmThis journal is The Royal Society of Chemistry 2014

form, which increasingly aggregate at higher reaction tempera-tures. With TfO, also small 57-nm particles form, but only atlow temperatures. With ES, polydisperse samples form at alltemperatures except for 160 C. These results demonstratedthat the anion of the ionic liquid has a strong influence on theparticle size, shape, and aggregation. Hong and co-workersindicated that hydrogen bonding can occur at the interfacebetween the anions of [C16mim]Cl and the building blocksof aluminum hydroxides in the synthesis of large-mesoporous-Al2O3.

84 In addition, Dupont and co-workers proposedthe coordination model composed of semi-organized[(DAI)m(X)mn]

n+[(DAI)m(X)mn]n supramolecular aggregates on

the formation of nanoparticles.12 Similar conclusions havebeen drawn by Sieffert and co-workers using MD simulation of1-butyl-3-methylimidazolium octylsulfate ([Bmim][OcSO4]) neara quartz surface.85 They concluded that the imidazolium ringand the octyl chain of the anion prefer to align near and parallelto the surface. These findings are very useful in understandingthe type of effect of ionic liquids on the formation ofnanostructures with different morphologies despite synthesis

choice of cations or anions when considering the different

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View Article Onlinemechanisms in different cases being not quite clear. It is highlyexpected that this understanding will improve with the accumu-lation of knowledge and the systematic design of experiments.

Most recently, our group has successfully prepared well-dispersed NH4-Dw and -AlOOH nanostructures with controlledmorphologies using the 1-butyl-2,3-dimethylimidazolium chloride([Bdmim]Cl)-assisted hydrothermal process.53 Based on theexperimental results, the ionic liquid effect models on thesynthesis of NH4-Dw and -AlOOH nanostructures can bedivided into cationic- or anionic-dominant effect models deter-mined by the different surface structure of the targets, as illus-trated in Fig. 10. Specifically, under the cationic dominantregime, ionic liquids mainly show a dispersion effect for NH4-Dw nanostructures; meanwhile, the anionic dominant modelcan induce -AlOOH particle self-assembly to form hierarchicalstructures. Under the guidance of the models proposed, theeffect of ionic liquids would be optimized by the appropriate

Fig. 10 Scheme for the different effect models of [Bdmim]Cl in thesynthesis of NH4-Dw and -AlOOH.This journal is The Royal Society of Chemistry 2014Acknowledgements

This work was financially supported by National NaturalScience Foundation of China (grant no. 21371101 and51302079) and the Young Teachers' Growth Plan of HunanUniversity (grant no. 2012-118).Summary and outlook

In summary, we have briefly highlighted the applications ofionic liquids in the preparation of inorganic nanomaterials.Compared with conventional molecular solvents, ionic liquidscannot be only regarded as a green alternative, but alsoprovide a powerful medium for the synthesis of inorganicnanomaterials with unique morphologies and controlledphases. As one of the most rapidly growing fields, one canenvision that there will certainly be intensified interest in thispromising direction, especially in the following aspects: onthe one hand, one of the most distinctive features of ionicliquids is that they can be treated as tailored solvents due totheir unlimited flexibility of combinations of anions andcations. So one can design the appropriate ionic liquid precur-sor according to the initial crystal structures, compositions,and crystal habits of target products. These precursors aremolecularly defined entities, which can serve as both the reac-tant and solvent for the reaction, and as the template over thefinal inorganic material morphology at the same time. Thisall-in-one synthesis route which can make the reactionsystem simpler, and thus giving more control over the phasesand morphologies of the final products. The ultimate goal ofionic liquid precursor research is to understand and design thetask-special ionic liquids at the molecular level and provide amore efficient strategy to synthesize inorganic materials withnovel structures and interesting properties. On the other hand,we believe that the synthesis of new inorganic materials shouldgo hand-in-hand with the development of understanding ofthe effect type of ionic liquids. Since the research on well-established rules and correlations between molecular struc-tures of the adopted ionic liquids and the morphologies of theresulting inorganic materials is limited, it is highly expectedthat this understanding will improve with the accumulation ofknowledge and the systematic design of experiments.

In short, we hope that this review will not only display therecent developments in inorganic synthesis using ionic liquids,but also hope to give the readers some inspirations to explorenovel and effective synthesis routes for the synthesis of inor-ganic nanomaterials with desired phases and morphologies.effect model with substrate surface. It is highly expected thatsuch effect models between ionic liquids and target productsare helpful for the understanding and rational design of ionicliquids consisting of specific functional groups, and will thusopen up new opportunities for the synthesis of inorganic nano-materials with novel morphology and improved properties.CrystEngComm, 2014, 16, 25502559 | 2557

33 X. Zhao, W. Jin, J. Cai, J. Ye, Z. Li, Y. Ma, J. Xie and L. Qi,

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