Ionic liquids for nano- and microstructures preparation. Part 1: Properties and multifunctional role

CIS-01564; No of Pages 16 Advances in Colloid and Interface Science xxx (2015) xxx–xxx

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Historical Perspective

Ionic liquids for nano- and microstructures preparation. Part 1: Properties multifunctional role Justyna Łuczak a,⁎, Marta Paszkiewicz b, Anna Krukowska b, Anna Malankowska a, Adriana Zaleska-Medynska a a b

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

a r t i c l e

i n f o

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

a b s t r a c t Ionic liquids (ILs) are a broad group of organic salts of varying structure and properties, used in energy conversion and storage, chemical analysis, separation processes, as well as in the preparation of particles in nano- and microscale. In material engineering, ionic liquids are applied to synthesize mainly metal nanoparticles and 3D semiconductor microparticles. They could generally serve as a structuring agent or as a reaction medium (solvent). This review deals with the resent progress in general understanding of the ILs role in particle growth and stabilization and the application of ionic liquids for nano- and microparticles synthesis. The first part of the paper is focused on the interactions between ionic liquids and growing particles. The stabilization of growing particles by steric hindrance, electrostatic interaction, solvation forces, viscous stabilization, and ability of ILs to serve as a soft template is detailed discussed. For the first time, the miscellaneous role of the ILs in nano- and microparticle preparation composed of metals as well as semiconductors is collected, and the formation mechanisms are graphically presented and discussed based on their structure and selected properties. The second part of the paper gives a comprehensive overview of recent experimental studies dealing with the applications of ionic liquids for preparation of metal and semiconductorbased nano- and microparticles. A wide spectrum of preparation routes using ionic liquids are presented, including precipitation, sol-gel technique, hydrothermal method, nanocasting, and microwave or ultrasound-mediated methods. © 2015 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . Structure, composition, and properties of ionic liquids . . . . . . Ionic interactions and nanostructural organization in ionic liquids . The role of ionic liquids in nano- and microstructures preparation . 4.1. Ionic liquid protective layer formation . . . . . . . . . . 4.1.1. Electrostatic stabilization by anion. . . . . . . . 4.1.2. Parallel coordination by the cation. . . . . . . . 4.1.3. Ionic liquid ion pair stabilization . . . . . . . . 4.1.4. Steric stabilization . . . . . . . . . . . . . . . 4.1.5. Stabilization by solvation forces . . . . . . . . . 4.2. Viscous stabilization . . . . . . . . . . . . . . . . . . 4.3. Stabilization by intermediates or by-products . . . . . . . 4.4. The role of the impurities . . . . . . . . . . . . . . . . 4.5. Stabilization by ligands—functionalization of the ionic liquid 4.6. Ionic liquids as source of F− anions generated in situ. . . . 4.7. Ionic liquid micellar systems . . . . . . . . . . . . . . 4.8. Ionic liquid as reducing agents . . . . . . . . . . . . . 4.9. Ionic liquid as “all in one” media. . . . . . . . . . . . .

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⁎ Corresponding author. Tel.: +48 58 347 13 65. E-mail address: [email protected] (J. Łuczak).

http://dx.doi.org/10.1016/j.cis.2015.08.006 0001-8686/© 2015 Elsevier B.V. All rights reserved.

Please cite this article as: Łuczak J, et al, Ionic liquids for nano- and microstructures preparation. Part 1: Properties multifunctional role, Adv Colloid Interface Sci (2015), http://dx.doi.org/10.1016/j.cis.2015.08.006

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4.10. Microemulsion systems based on the ionic liquids 5. Conclusions and outlook . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction The great interest in the synthesis, physicochemical properties, and applications of ionic liquids (ILs) is based on their unique and highly customizable properties. Ionic liquids are a group of salts composed only of ions and have a melting point below 100 °C [1]. The low vapor pressure and high polarity of these compounds have attracted a lot of attention, being utilized in a variety of applications. Considering a great practical relevance of ILs, there is already rich literature on different aspects of ionic liquids. The negligible vapor pressure and thermal stability make ILs valuable solvents in the separation processes such as CO2 capture [2], ionic liquid-based membranes [3], chromatographic and electrophoretic separation [4,5], liquid-liquid extraction [6], and liquid-solid state extraction [7] techniques. Suitable cation–anion coupling accompanied by high electrochemical stability, non-flammability, and ionic conductivity provides materials for energy applications by means of electrolytes for variety of batteries [8], membranes in fuel cells [9], or biomass dissolution and hydrolysis for biofuel production [10,11]. A few recent reviews were also focused on the use of IL in organic and inorganic synthesis [12], catalysis [13], and biocatalysis [14]. Several earlier reviews deal also with the ability of ILs to adsorb onto a metal surface and produce a protective film thereby working as lubricants [15] or surface active agents [16]. The applications of ionic liquids confined in nanopores, including experimental studies dealing with changes in the physicochemical properties of ionic liquids, such as thermal phase transition, stability, dynamical behavior, optical properties, etc., as well as recent theoretical studies highlighting the layering and structural heterogeneity of ionic liquids confined in nano-pores were discussed in detail in a review paper by Singh et al. [17]. Ionic liquids were also proposed as a novel alternative absorbent combined with refrigerant such as water, ammonia, alcohols, and hydrofluorocarbons and used as working pairs for an absorption refrigeration cycle, heat pump, and absorption power cycle. Imidazolium IL working pairs were described regarding the status of evaluation and selection methods, thermophysical property measurement, and modeling, as well as their future prospect assessments by Zheng et al. [18]. Several reviews also deal with selected properties of ILs, including transport properties [19], wetting behavior [20], particles self-assembly at IL-based interfaces [21], and toxicity and environmental fate [22]. According to their structure and structure-dependent properties, ionic liquids seem to be a useful and promising group of compounds in synthesis of metal and semiconductor nano- and microparticles. However, the following questions are still arising: What properties of ILs are crucial in the particle preparation? What are structural descriptors of ILs influencing size and shape of the particles? What role can ILs play in the nano- and microparticle preparation? What is possible mode of action of ILs in the reaction mixture? How do IL mediate the chemical reactions involving ILs? These aspects are crucial to select ILs and method of IL-assisted preparation method and understand ILs effect. In this regard in this review, we will summarize, describe, and schematically present the numerous ways that ionic liquids could interact with growing nano- and microparticles during their preparation routes and how these interactions affect the size and shape of finally formed solid particles. The possible roles of ILs in solid particle preparation are analyzed and discussed based on their structure and selected properties. Thus, in the next sections of this review, the structure and selected properties of ionic liquids are summarized taking into account solvents most often used in experimental and modeling studies, followed by a discussion of the types of

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interaction between ILs and growing particles and the synthesis of the multiple role that ionic liquids can play during preparation route. 2. Structure, composition, and properties of ionic liquids According to the literature data, ionic liquids are divided into two main groups: aprotic and protic ILs [1]. The aprotic ionic liquids, more popularly used and also known as “conventional” or “classical” ILs, are usually built of large, organic cations and smaller anions. Organic cations contain a positively charged nitrogen, phosphorus, or sulfur atom, e.g., derivatives of imidazolium, ammonium, pyridinium, sulfonium, phosphonium, and others, as shown in Table 1. Anions that form ionic liquids may be either inorganic or organic ions having a diffuse or protected negative charge. Commonly used inorganic anions are halides [Cl], [Br], hexafluorophosphate [PF6], tetrafluoroborate [BF4], and bis(trifluoromethanesulfonyl)imide [Tf2N], and typical organic anions are alkylsulfate [ASO4], alkylsulfonate [ASO3], p-toluenesulfonate [Tos], trifluoroacetate [CF3COO], etc. [23]. Protic ionic liquids are generally prepared through proton transfer from a Brönsted acid to a Brönsted base [1]. Selected cations and anions—ionic liquid components, together with their structures and abbreviations, are presented in Table 1. There is also an interest in task-specific ionic liquids, the compounds in which both cation and anion may be functionalized (e.g., fluorinated, chiral, and metal containing) being often designed for given applications [24]. It is worth mentioning, that various abbreviations for ionic liquid cations are commonly used in literature. For example, a 1-butyl3-methylimidazolium cation can be abbreviated as follows [BMIM], [bmim], [C4MIM], [C4mim], and [IM14], etc. In this paper, for most of the ionic liquids, we use acronyms of the [BMIM] type—that is, short, easily understood, and gives an indication of what chemical groups are present in the ionic liquid structure. For ammonium, sulfonium, and phosphonium salts, giving butyltrimethylammonium salt as an example, the acronym [N4111] is used. Ionic liquids have attracted much attention in the nano- and microsized material's preparation due to their valuable properties such as negligible vapor pressure, wide liquid range, good dissolving ability, high thermal stability, excellent microwave absorbing ability, electric conductivity, wide electrochemical window, non-flammability, etc. Additionally, the ability to modify the cation structure and anion selection and, as a consequence, to alter their physicochemical properties provides new media for solvation processes. A variety of ILs were already investigated for nanostructures preparation with respect to the cation type (imidazolium [25,26], ammonium [27,28], pyridinium [29,30], and phosphonium [31,32]), the chain length of the alkyl substituent in the heterocyclic cation (C2–C18) [33], and the structure of the anion ([Cl], [Br], [BF4], [PF6], [Tf2N]) [34, 35]. It was revealed that their useful properties can be applied in the effective preparation of nanomaterials and microstructures with controlled physical and structural properties, i.e., specific surface area, density of crystalline defects, particle size, and shape. Physicochemical properties of ILs relevant for nano- and microparticle preparation could be divided into the following groups: A. Melting and decomposition temperatures—low melting points are related to the difficulty of crystalline structure formation that results from the large, asymmetrical shape, and flexible ions with delocalized charge that constitute ILs. Unlike common organic fluids, the liquid range of the ILs may be as large as 200–300 °C. The thermal stability of ionic liquids is determined

Please cite this article as: Łuczak J, et al, Ionic liquids for nano- and microstructures preparation. Part 1: Properties multifunctional role, Adv Colloid Interface Sci (2015), http://dx.doi.org/10.1016/j.cis.2015.08.006

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Table 1 Selected cations and anions forming ionic liquids.

1-Alkyl-3-methylimidazolium [AMIM]

1-Alkylpyridinium [APy]

1,1-Dialkylpyrrolidinium [AAPyr]

Tetraalkylammonium [Nw,x,y,z]

Tetraalkylphosphonium [Pw,x,y,z]

N-alkyl-N-methylpiperidinium [AMPip] Cl−, Br−, I−

1,2-Dialkylpyrazolium [AAPyr]

N-alkylthiazolium [ATh]

Trialkylsulfonium [Sx,y,z]

N-alkyl-N-methylmorfolinium [AMMor]

Halides [Cl], [Br], [I]

Hexafluorophosphate [PF6]

Tetrafluoroborate [BF4]

bis(Trifluoromethanesulfonyl)imide [Tf2N]

bis(Perfluoroethanesulfonyl)imide [BETI]

Alkyl sulfate [RSO4]

Trifluoroacetate [TA]

Acetate [Ac]

Trifluoromethane-sulfonate [TfO]

Tosylate, p-toluenesulfonic acid [Tos] SCN−

Tetraalkylborate [BR4]

Methyl carbonate [MeCO3]

Dicyanamide [N(CN)2]

Hexafluoroantimonate [SbF6]

Thiocyanate [SCN]

Tetracyanoborate

Dimethylphosphate [DMP]

Tetrachloroaluminate, heptachloroaluminate [AlCl4], [Al2Cl7]

1,1-Dioxo-1,2-benzisothiazol-3-onate

[B(CN)4]

tris(Pentafluoroethyl)trifluorophosphate [(C2F5)3PF3], [FAP]

mainly by the nature of the anion in particular by hydrophobicity (H-bonding capacity) and hence often the nucleophilicity. Therefore, the decrease in nucleophilicity by fluorinating anion is directly related to increased thermal stability [36]. In this regard, the use of the ILs as solvents for colloidal particle preparation may be extended over wider temperature ranges in comparison to classical solvents and allows for their use at much higher temperatures, which may be beneficial, for example, in the preparation of iron oxide or alloys in conditions where the nanoparticles (NPs) size can be controlled by the preparation temperature or at lower temperatures in relation to solid state chemistry [37]. B. Polarity—quantitative estimation of the ILs polarity based on the various solvents parameters, dye sets, and polarity scales provides conclusions that polarity vary with different cation–anion combinations (different hydrogen bond donor/acceptor abilities and charge density). ILs are generally polar compounds with polarity higher than molecular solvents and similar to alcohols with medium chain length. Polar character can be increased by application of the functional groups or delocalized bonds in the ILs structure. C. Density—for ambient pressure and temperature conditions, the density of most ionic liquids is higher than water being in the 1.05–1.35 g·cm − 3 range [38]. As the exception, pyrrolidinium dicyanodiamide and guanidinium salts with density ranging from 0.9 to 0.97 g·cm [3] can be mentioned [39]. D. Transport properties—viscosity, diffusion coefficient, electric conductivity, and thermal conductivity are among the most relevant ones required for design and development of the methods for particle preparation [38]. These properties will influence chemical process when using ILs not only as reaction media but also as reagents or additives. One of the main characteristics of some ionic liquids is their high viscosity, which is larger than those of the conventional organic solvents commonly used in the chemical industry. The viscosity values of ILs are governed mainly by the ability of the ILs to

[(2-SO2PhCO)N]

form van der Waals interactions and hydrogen bonding. High viscosity can be considered as a factor limiting the diffusion of reagents in ILs as well as electrical and thermal conductivity of ILs, that is, mass and heat transfer during particle synthesis [38,23]. E. Surface and interfacial tensions—the general observation is that, at room temperature, ILs exhibit lower surface tension than water. However, it is significantly higher than that of conventional organic solvents [40]. In addition, ILs reveal much lower water/IL interfacial tension, in comparison with the air-liquid systems, that may be attributed to the surface activity of the imidazolium cation. The low surface tension is beneficial because it may lead to high nucleation rates, weak Ostwald ripening and, as a consequence, to small particle formation [41]. F. Surface activity—due to their amphiphilic characteristics, ionic liquids possess the ability to create organized structures in solutions (micelles, vesicles, lyotropic liquid crystals, gels, microemulsions) [42]. From a practical point of view, the formation of different types of dispersion systems enable the overcoming of the limitations of solvents to dissolve some materials inherently insoluble in ILs. Among the various ILs families, the 1-alkyl-3-methylimidazoliumbased salts are the most popular compounds used in the preparation of the colloidal particles. The most often chosen compounds are composed of [BMIM] cation and [BF4], [PF6], and [Tf2N] anions. This choice is related to their liquid state in the room temperature, relatively low viscosity, and thereby favorable transport properties, good solvent properties, and relatively low price. Moreover, these ionic liquids belong also to compounds most often used in experimental and theoretical studies and therefore constitute the most known subject of research. However, one have to remember that purification of the ILs is more complex than molecular solvents; therefore, ILs can be contaminated with unreacted substrates and/ or by-products. It is particularly important to be aware of that since impurities can significantly change certain properties. Thus,

Please cite this article as: Łuczak J, et al, Ionic liquids for nano- and microstructures preparation. Part 1: Properties multifunctional role, Adv Colloid Interface Sci (2015), http://dx.doi.org/10.1016/j.cis.2015.08.006

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the nature and content of impurities as well as the multiplicity of analytical techniques used in the studies are the cause of divergence of results. 3. Ionic interactions and nanostructural organization in ionic liquids Ionic liquids, being salts with a complex structure, undergo intrinsic ion–ion interactions (cation–cation, cation–anion, and anion–anion) as well as have the ability to interact with components of the reaction solution. Taking 1-alkyl-3-methylimidazolium-based compounds into account, the electronic structure of these heterocyclic cations consist of a delocalized electron configuration across the N1-C2-N3 atoms, a double bond between C4 and C5 at the opposite side of the ring, and a weak delocalization in the central region. As a consequence, carbon C2 is positively charged owing to the electron deficit in the C = N bond, resulting in acidity of the hydrogen atom C2-H (Fig. 1) [23,43]. From a macroscopic point of view, IL is a system which consists of interacting ions, characterized by molecular properties such as geometry and dipole moment, charge distribution and polarizability, hydrogen bond donating/accepting, and electron-pair donating/accepting capabilities, determining their solvent properties. The existing ionic interactions, enriched by cooperative network of hydrogen bonds, lead to the formation of the highly organized structure at the nanomolecular scale [44]. Therefore, ionic liquids are considered as a network of cations and anions not only interacting via electrostatic forces but also forming an extended hydrogen bond network [45]. The bigger the anion size ([Cl] b [PF6] b [Tf2N]), the higher is the diffuse nature of the ion negative charge, and as a consequence, the more delocalized is the charge. Therefore, the relative mutual position of the cations and anions in the IL network is different [46,44]. The position of highest probability for the [PF6 ] anion is the imidazolium ring, where the positive charge is concentrated, whereas the [Cl] anion (and analogously [Br] and [I]) are associated with the most acidic hydrogen in the cation C2-H [44]. Increasing anion size to [Tf2N] results in an increased number of lowest energy distributions and overlap of the anions and cations in the radial distribution, as shown in Fig. 2 [46]. The ability to form hydrogen bonds between ions distinguishes ILs from the simple salts or classical quaternary ammonium compounds [12]. Looking at ILs from a mesoscopic length scale, it was proposed by Dupont [45,47] that the monomeric unit of imidazolium IL is probably composed of one cation surrounded by at least three anions, and in turn, each anion may be surrounded by at least three imidazolium cations as schematically presented in Fig. 3. However, the number of anions (cations) that surround the cation (anion) can be changeable depending on the anion size, geometry, and the length of the alkyl substituent. Nevertheless, this hydrogen bonded network is a common feature of imidazolium salts in the liquid and solid state [23, 48]. As a consequence, a tridimensional network of ionic channels formed by (a) columns of anions and cations and (b) aggregates of alkyl chains in the cation is observed [48]. Moreover, for ionic liquids

Fig. 2. Probable distributions of (a) the anions and (b) the imidazolium cations around an imidazolium cation derived from the empirical potential structural refinement model for liquid 1,3-dimethylimidazolium salts (adapted from [46]).

with hydrocarbon chains equal or longer than four carbon atoms, the aggregation of the alkyl chains is observed providing nonpolar domains with nanometrical scale [48]. Interplay between Coulomb, van der Waals, π–π interactions, and hydrogen bonding leads to the segregation between polar and nonpolar spatial domains and the formation of distinct environments composed of a high-charge density network (the hydrophilic head groups) permeated by low-charge density regions (the hydrophobic alkyl chains) in the nanosized scale with a high degree of directionality. Such an organization was concluded from both experimental, as well as molecular dynamics simulations [50,51]. Exemplary three-dimensional visualization of that phenomenon adapted from Lopes [48] is presented in Fig. 4. For longer side chains, the nonpolar domains become larger and more connected/continuous, resulting in the swelling of the ionic network, in a manner analogous to systems exhibiting microphase separation [48]. Moreover, for ILs with a long side chain, due to increased Van der Waals interactions, liquid crystal phases might exist [52,53]. The structure and formation mechanism of ionic liquids clusters was systematically described in a previous review [54]. This internal heterogeneity cannot remain without consequences when precursors and eventually particles appear in the IL. The combination of possible interactions that ionic liquid are able to form must affect behavior of the precursor and other reagents. Solubility of the precursors will strongly depends on the affinity to the IL, being a result of the IL composition. Molecules of the solutes may be preferentially dissolved in distinct local environments due to selective interactions with certain parts of the ions according to their polarity that is hydrogen bonding formation and polarizability. The possibility to design ionic liquids with desired ability to solve reagents can be also used. Besides the nature of the

Fig. 1. Imidazolium ionic liquids interaction scheme.

Please cite this article as: Łuczak J, et al, Ionic liquids for nano- and microstructures preparation. Part 1: Properties multifunctional role, Adv Colloid Interface Sci (2015), http://dx.doi.org/10.1016/j.cis.2015.08.006

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Fig. 3. Schematic representation of the 1,3-dialkylimidazolium-based ionic liquid structure (a), bis(trifluoromethanesulfonyl)imide surrounded by three 1,3-dimethylimidazolium cations (b), packing diagrams for 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide [MMIM][Tf2N] presenting layered structure of separated nonpolar and polar regions (c) (panels b and c adapted from [49]).

ionic liquid, the dissolution of the precursors will also depend on the water and other impurities content that are common feature in the ILs. Unreacted substrates, e.g., methylimidazole and alkylamines, will decrease IL polarity, whereas water will have opposite effect. Ionic liquids being well-structured liquid phases may also strongly influence the morphology of the nanoparticles formed in it. It was proposed that the structural organization of ionic liquids can be used as “entropic driver”/the driving force of the spontaneous, well-defined, and extended ordering of nanostructures (the so-called IL effect) [41]. In this regard, the combination of the affinity to other molecules due to the hydrogen bond donating/accepting abilities, electrostatic, and van der Waals interactions as well as inherent interactions between ILs cation and anion makes ionic liquids a potential key tool in the preparation of a new generation of nanostructures with desired properties. 4. The role of ionic liquids in nano- and microstructures preparation The morphology of the nano- and microstructures, such as particle shape and size, porosity, density of crystalline defects, and surface area, could be controlled by selecting proper reactants and preparation route conditions (e.g., reagents, temperature, pressure, and reaction duration). Nano- and microstructures are usually obtained using variety of methods such as precipitation in solvents, sol-gel method, hydro- and solvothermal synthesis, preparation in the microemulsion system, microwave and sonochemical-enhanced methods [55,56], etc. In this regard, ionic liquids are considered as non-flammable, nonvolatile reaction media for particle preparation. However, in order to obtain stable particles with a high surface/volume ratio, the particle aggregation and agglomeration processes have to be limited and stability of the particles have to be controlled. Therefore, from an application point of view, suspensions of the colloids are

stabilized by protective agents to prevent aggregation and/or to facilitate reuse. This protection is essential due to the mutual attraction of the particles, and the potential to agglomerate with simultaneous loss of the properties associated with the colloidal state (e.g., loss of catalytic activity [57]). Stable nanostructures with a controlled size and composition are usually obtained by synthesis in the presence of protective agents or stabilizers with different structure such as amphiphilic compounds, polymers, dendrimers, ligands, as well as by application of the dispersion systems with nanosized domains like micelles or microemulsions. Thus, the structure and nature of ionic liquids is predetermined to stabilize and affect the growing nanoparticles. The composition of the ionic liquid and thereby their physicochemical properties strongly influence the size, morphology, and the other properties of the particles. For example, the smaller mean diameters of metal NPs can be related to the higher coordination ability of the IL anion [34]. However, the influence of the alkyl chain length in the IL cation, and at the same size of the nonpolar nanodomains in the supramolecular structure, on the size of the particles depends on the ionic or nonionic nature of the precursor [33]. Due to the amphiphilic structure (shown in Fig. 1) and inherent charge, ionic liquids can obviously protect particles and attenuate the electrostatic repulsion by charge and steric stabilization. Interestingly, there are some reports where the formation of the stable colloidal particles in the ILs in the absence of additional stabilizers was described [58]. However, the question is what other types of interactions ILs can form with particle surface (taking into account its ability to form multiple interactions) and what are general rules of ILs behavior? Considering ILs as fluids with a supramolecular structure and internal directionality, these compounds may be used at the same time as reaction environment, cosolvent, and structuring agent, creating a protective layer or dispersion systems and thereby directly influencing

Fig. 4. Visualization of the charged and nonpolar domains formed in the (a) 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6] and (b) 1-hexyl-3-methylimidazolium hexafluorophosphate [HMIM][PF6], where red are the atoms of the charged/polar regions, and green are the nonpolar ones (adapted from [48]).

Please cite this article as: Łuczak J, et al, Ionic liquids for nano- and microstructures preparation. Part 1: Properties multifunctional role, Adv Colloid Interface Sci (2015), http://dx.doi.org/10.1016/j.cis.2015.08.006

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particles size, morphology, and activity. The formation and stabilization of the structures in these well-organized supramolecular polar media must occur with the re-organization of the hydrogen bond network and nanoregions with polar and nonpolar properties [12]. Among the main advantages of using ILs as solvents their high polarity is usually given. In fact, polarity of ionic liquids is higher than most molecular solvents and not vary significantly with IL composition. Taking into account a huge number of possible ionic liquid's structures as well as a variety of colloidal particle preparation methods, better understanding of the roles that ionic liquid play during nano- and microparticles synthesis is crucial for designing new nanostructures with uniformity in size and distribution, controlled morphology, and as a consequence desired and improved properties. Over the past decade, significant progress have been made to understand the chemistry required to control the size and shape of nanoand microparticles prepared in the presence of ionic liquids. In this regard, according to the available literature, we summarized multifarious ways in which ionic liquids can interact, shape, and stabilize nano- and microparticles, and thereby multiple roles that ILs can play during colloidal structure preparation. First of all, we pointed out roles of the ionic liquids in the inorganic particles synthesis considering ILs as structuring agents and solvents. Then the most important types of interaction are presented and analyzed in the following subsections, in relation to the specific interactions and benefits for the size and shape of the particles. We discuss basic interactions providing colloidal stabilization, e.g., electrostatic interactions and steric hindrance, as well as specific ability of ILs to stabilize by coordination or intermediate structure formation. The application of ILs in a form of dispersive systems, e.g., micellar aggregates and microemulsions, was considered in terms of control of the particles size and shape. Moreover, we discussed the mode of action of ILs as a reducing agent as well as particle precursor in the context of limiting by-product formation. To facilitate understanding and to systematize possible modes of action of ionic liquids, we schematically presented them in Fig. 5. Shortly, in the nanostructure preparation processes, ionic liquids may play multiple roles, acting as follows: Structuring agent: 1. Ionic liquid protective layer formation: i. Electrostatic stabilization—the ionic liquid molecules form a supramolecular protective layer being in contact with nano- or microstructures formed due to internal cation–cation, anion– anion, and cation–anion interactions in the ILs and their affinity to particle surface. Owing to intrinsic charge, ILs can create an electrostatic stabilization of nano- and microparticles [59] through the following: • Anion moiety (Fig. 5a), • Parallel coordination by the cation(Fig. 5b), • Ionic liquid ion pair stabilization (Fig. 5c), ii. Steric stabilization—the IL cation and anion clusters, as well as intermediate IL species formed during reaction, contribute to a steric stabilization of the material thus controlling particle growth and inhibiting further agglomeration [30,60] (Fig. 5d), iii. Stabilization by solvation (structuring forces)—intrinsic supramolecular structure of the ionic liquid facilitate uniform shell layer formation surrounding surface of the particles (Fig. 5e), 2. Viscous stabilization—higher viscosity of the ILs results in decrease of the aggregation rate of colloidal particles when compared to water or classical organic solvents,

3. Stabilization by intermediates or by-products, e.g., N-heterocyclic carbene stabilization (Fig. 5f), 4. Stabilizing or destabilizing role of the ionic liquid impurities, 5. Stabilization by ligands—functionalization of the ionic liquid cation or anion in order to directly stabilize nanostructures, as shown in Fig. 5g, 6. Source of fluoride anions generated in situ—fluoride atoms required for the precipitation of the nanoparticles, e.g., YF3, EuF3, etc., may come from ILs due to hydrolysis of the salts possessing [BF4] and [PF6] anions in acidic conditions and elevated temperatures (Fig. 5h), 7. Micellar solutions—ionic liquids are able to self-assemble into organized structures in aqueous and nonaqueous solutions [28] as well as support micelle formation by conventional surfactants [61]. These micellar aggregates may be used as nanoreactors or soft templates for the controlled synthesis of nanostructures (Fig. 5i), 8. Agent promoting reduction—reduction of the precursor may be promoted by either the cation and/or anion of the imidazolium moiety or hydroxyl group in a functionalized IL; it can be oxidized with a parallel reduction of the nanostructure precursor, e.g., HAuCl4 [62] and AgNO3 [24], or in certain ILs, 1-hexyl-2 3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide [HMMIM][Tf2N], the precursor bis(2-methylallyl)(1,5,cyclooctadiene)ruthenium may undergo spontaneous reduction providing Ru nanoparticles [63], 9. ILs may be used as morphology templates, the choice of the salt composition determines the emerging morphology of the particles, e.g., the IL can promote the growth of flower-like, star-like, or even rod shaped ZnO [30], Au nanosheets [64], CoPt alloy nanorods [65], and Bi2S3 flowers [25], Au nanodendrites [26], and many others (examples of nanostructures are shown in Fig. 6 and more details are given in second part of review), Solvent: i. Reaction environment—nanostructures can be synthesized in situ in ionic liquids or in ionic liquid used as a cosolvent. Structure of the IL determine solubility of the reagents, heat, and mass transport affects the reaction rate and selectivity and may shift the equilibrium towards the product formation. Synthesis in ILs is possible under ambient conditions, at elevated and for some salts even at lower than room temperatures, in anhydrous, water-poor, and aqueous conditions, ii. Precursor of the nanostructures—ionic liquids can be used as an all-in-one medium, which means that IL can serve as a solvent/ template, stabilizing agent as well as a metal precursor for growth of nanostructures. Metal can be incorporated in both IL cation, e.g., [Zn(CH3NH2)4(Tf2N)2] [67] or anion structure, e.g., [BIM][AuCl4] [68] iii. IL-based microemulsion systems—ILs may be used as one of the phases in aqueous or nonaqueous microemulsions [69], and the formation of the microemulsions enables overcoming the limitations of ILs to dissolve some compounds. The application of the aqueous microemulsions is probably a more promising option from a green chemistry point of view since it excludes the use of volatile organic solvents as a nonpolar part of microemulsions (Fig. 5j), iv. due to the ionic character and high polarizability, ILs have an ability to absorb the microwave (MW) radiation that, combined with steric stabilization, provides an MW-IL approach (Microwave-assisted synthesis in combination with ionic liquids). Due to the ionic nature of the ILs, they

Please cite this article as: Łuczak J, et al, Ionic liquids for nano- and microstructures preparation. Part 1: Properties multifunctional role, Adv Colloid Interface Sci (2015), http://dx.doi.org/10.1016/j.cis.2015.08.006

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(a) Electrostatic stabilisation by anion

(d) Steric stabilisation

(b) Stabilization through parallel coordination by the cation

(e) Stabilisation by solvatation

(g) Stabilisation by ligands

(c) Ionic liquid ion pair stabilization

(f) N-heterocyclic carbens stabilisation

(h) Source of F- anions generated in-situ

(i) ILs micelle formation

(j) ILs in microemulsions system

Fig. 5. Multiple role of ionic liquids in the nanostructures synthesis or preparation.

Please cite this article as: Łuczak J, et al, Ionic liquids for nano- and microstructures preparation. Part 1: Properties multifunctional role, Adv Colloid Interface Sci (2015), http://dx.doi.org/10.1016/j.cis.2015.08.006

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Fig. 6. Examples of particles morphologies prepared using different ionic liquids (a) Au nanodendrites [26], (b) Bi2S3 flowers [25], and (c) Au polyhedral particles and plates [66].

effectively interact with MW radiation and the most beneficial aspect in the microwave-assisted preparations is the considerably shortened reaction time and higher yields compared to the conventional methods [70,71], v. Stable colloidal systems can be formed that can be used for high turnover rates, e.g., in catalytic hydrogenation [72,73], vi. The low surface tension of the ILs results in a high nucleation rates—generation of very small particles with weak Ostwald ripening [41].

4.1. Ionic liquid protective layer formation Some authors postulate the electric double layer formation by ionic liquids moieties on the particle surface [35,74–77,102] according to the electrostatic and van der Waals interactions analogously as it is present in most of the colloidal systems(classical Derjaguin– Landau–Verwey–Overbeek theory). This theory is usually employed to discuss the nanoparticles stabilization by classical surfactants such as quaternary ammonium compounds, for example; therefore, due to the analogues structure of surfactants and ionic liquids may be also in principle used for the description of the IL behavior. Despite plenty of work has been done, there is still controversial debate surrounding this area. 4.1.1. Electrostatic stabilization by anion An important step in understanding the interactions between particles and ionic liquids was performed using the density functional theory (DFT) method. Based on that calculations, stronger interactions were found between particles (e.g., Au, Fe, Pd) and anions when compared to imidazolium and ammonium cations, as shown schematically in Fig. 5a [35,74,75]. For example, for Au nanoparticles, chloride anions were found to have the highest binding energy followed by [TfO], [BF4], and [PF6] anions [35]. These observations were supported by numerous spectroscopic techniques [49,34,78,79,102] that were used to verify presence and characterize possible interactions of IL components with the surface of the particles, such as the X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), Fourier transform infrared spectroscopy (FTIR), small-angle X-ray scattering (SAXS), or surface-enhanced Raman spectroscopy (SERS), among others. The presence of the ionic liquids residues on the charged metal surfaces was confirmed experimentally, for example, for particles formed in the [BMIM][PF6], [BMIM][BF4], [BMIM][TfO], [EMIM][EtSO4] through F atoms (for [BF4] and [PF6]), or O atoms for [TfO] and [EtSO4] anions [76,77,102]. The electrostatic interactions result in the formation of the protective layer surrounding the NPs surface that is composed of IL anions gathered close to the nanoparticle surface balanced by cationic counter-ions. This electrical double layer, which thickness depends on the anion size (e.g., 2.8–4.0 nm for [BF4], [PF6], [TfO] anions), provides

the Coulombic repulsion between particles. However, unsymmetrical [Tf2N] anions adsorb at the surfaces in a slightly tilted orientation interacting mainly by SO2 groups (Fig. 7) and taking a cis conformation in the first layer [80]. This protective layer was also indicated to be composed of a semi-organized ionic species of the type ([BMIM]x[X]x + 1)−, where x = 2 for X = BF4 and x = 3 for X = PF6 [76]. The role of the ionic liquid in electrostatic stabilization of the metal and metal oxides particles with a positively charge surface, as an example, is recognize as an agent providing anions, thereby negatively charged first solvation shell, followed by a less ordered layer of cations, due to the presence of an intrinsic charge in the IL [77]. 4.1.2. Parallel coordination by the cation However, it was also revealed that particles can be coordinated by cations of the ionic liquid as schematically presented in Fig. 5b. For example, the [BMIM] cation interacts with surface of the Ag nanoparticles (in a flat configuration), forming a positively charged layer, whereas [BF4] anions do not interact chemically with the particle surface and create the second layer [78]. The thickness of the electrostatic layer formed due to the interactions between the silica surface, and the 1-ethyl-3methylimidazolium [EMIM] cation was found to be 0.5 nm and was related with the ethyl substituent placed nearly parallel to the surface normal. A second solvation layer (1.15 nm) is consistent with the diameter of the 1-ethyl-3-methylimidazolium acetate ionic liquid being tested [81]. 4.1.3. Ionic liquid ion pair stabilization On the contrary, some experimental and quantum chemical simulation studies reveal that both cation and anion of the ionic liquids, as ion pairs, interact with the surface of the metallic particles (Fig. 5c) [82]. For example, both ions in [EMIM][BF4] interact with Pd surface, with cation oriented close to the surface normal (dominating interaction) and anion interacting by two or three fluoride anions [83]. The interfacial region on the Pt [84], Ru [85] surface was observed to be one ion layer thick and orientation of the ions depends on the surface charge. At the negative surface, imidazolium rings lay parallel to the surface, thus

Fig. 7. Most probable orientation of the bis(trifluoromethanesulfonyl)imide anion on the surface of Al2O3/NiAl(110) in the submonolayer region (adapted from [80]).

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maximizing the attractive interaction. However, at positively charged surface, cation is to some extend repulsed from the surface [84]. Moreover, stronger interactions of IL with metal surfaces were observed for cations with ions located on one atom, e.g., ammonium, phosphonium ones, than for ion with delocalized charge. Side alkyl chains of cation as well as CF3 groups of the anion are preferentially directed a way from the nanoparticle surface [85]. Regardless of the composition of the solvation layers, ionic liquid definitely take part in the electrostatic stabilization of the particles; however, these types of interactions are not dominating. Electrostatic repulsion (a repulsive electrostatic potential Vele) combined with van der Waals attraction (attractive potential VvdW) constitute the total interparticle potential (Vtotal) according to the classical DLVO theory. Vtotal ¼ Vele þ VvdW A more detailed description of that theory accompanied by calculation Vtotal between silica and gold nanoparticles dispersed in various imidazolium ILs was presented in [86] and [87] and was found that the total interaction potential is negative over the whole distance range indicating attraction of the particles at all separations. The van der Waals attractive potential, being a driving force for agglomeration of the particles, was observed to be much lower when compared to aqueous or classical organic solvent dispersions, whereas the thickness of the double layer suppressed [86]. Nevertheless, according to the DLVO theory, particles are not effectively stabilized by electrostatic interaction due to high ionic strength of the ILs. To form stable colloidal particles in ILs, the additional steric repulsion was suggested to be necessary. Moreover, calculation of the time after which half of the Au particles form dimers (t1/2 = 0.085) reveal that Au NPs in [BMIM]-based ILs should agglomerate immediately. However, experimental results revealed some unexpected stability; therefore, this and other studies [88,89] suggest that additional repulsive forces must be present in ILs [87]. In this regard, three ways of repulsion between particles can be pointed: (i) electrostatic forces as described above, (ii) steric hindrance derived from adsorption or coordination of the unsymmetrical, bulky IL ions, and (iii) solvation (structuring) forces that are ILs layers located in the vicinity of the particle. The amphiphilic structure of most ionic liquids facilitates its adsorption on the particle surface. It is worth mentioning that not only ionic liquid cations (Fig. 1) but also anions may consist of a charged hydrophilic head group and a hydrophobic tail domain (e.g., octylsulfate anions). Hydrophobic substituents in these moieties may play a role of the protective group enhancing particles stability. 4.1.4. Steric stabilization Ion steric orientation on the surfaces is a consequence of the tendency of ionic liquids to interact preferentially by the charge moieties of the ions, with nonpolar fragments directed away from the surface (Fig. 5d) [81]. The presence of these forces were concluded, for example, for Au particles prepared in functionalized 1-triethylene glycol monomethyl ether-3-methylimidazolium methanesulfonate, where a protective layer of the imidazolium cations with ether-functionality directed to the bulk was observed [90]. The length of the alkyl substituent has a crucial effect on the conformation of the alkyl chain. An increase in the chain length results in a more ordered structure with a reduction in the number of gauche defects [91]. Therefore, by proper selection of the IL structure, interactions between ILs and particles can be influenced. 4.1.5. Stabilization by solvation forces The formation of the IL layers (Fig. 5e), which may be attributed to the solvation forces, were determined by many different experimental techniques, such as atomic force microscopy (AFM), surface forces

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measurements, dynamic light scattering (DLS), transmission electron microscopy (TEM), small-angle neutron scattering (SANS), etc. [81,89, 88,92]. Although most of these studies were performed for ionic liquids confined between two flat surfaces on the nanometer scale, solvation forces were confirmed in both protic and aprotic ILs on various surfaces [81,93]. The intrinsic supramolecular structure of the ionic liquid, resulted from cation–cation, cation–anion as well as anion–anion interactions, may facilitate uniform shell surrounding surface layer formation. These ions shells surrounding particles affect stability of the articles through structural forces. For example, ethylammonium nitrate forms at least five solvation layers at the silica surface. The thickness of the first layer (0.25 nm) suggests being rich in the ammonium cations. The elongation of the alkyl chain to carbon atoms or exchange cation type to imidazolium results in increase of the layers thickness and flexibility. A comparison of the cation types reveals that imidazolium cations are less effectively packed; therefore, less solvation layers in that IL were detected [81]. Moreover, the composition of the anion is also influencing since the large size of the FAP anion effectively impedes packing [93]. The 5-nm-layer thickness that may corresponds to about sever ion pair layers was determined for fluorocarbon modified silica particles in [BMIM][BF4]. It was proposed that interactions between fluorine atoms in [BF4] and the terminal hydrogen group on the surface coating facilitate solvation layer formation providing a steric barrier to keep particles apart [92]. The number of the layers, the orientation of the IL ions, and the binding strength depend on the surface charge, composition, and roughness as well as the composition of the IL moieties. Higher surface potentials results in stronger electrostatic interactions providing with a more compact first solvation layer, followed also by more effectively packed next layers, resulting in increased bulk structure [93]. Therefore, stabilization by ionic liquids cannot be completely explained by the DLVO theory since the stabilization in ILs is a more complicated phenomenon and may involve both the cation and the anion being in contact with colloidal structures, steric stabilization, as well as effect from the supramolecular network of ILs combined by hydrogen bonding, their properties mainly viscosity, intermediate species formation, and presence of impurities [35]. 4.2. Viscous stabilization It was already pointed in some studies that an extra stabilization may also come from the viscosity of the ionic liquid [86,87,29]. The aggregation rate of colloidal particles in ionic liquids appears to become smaller than that in water and classical organic solvents due to slower Brownian motion caused by high viscosity of the ILs that decreases mass transfer of the solute and finally agglomeration [86]. For example, the time after which half of the Au particles form dimers (t1/2) using the simplified Smoluchowski equation, t 1 =2 ¼

3η 4kB TNo

where η is the viscosity of the solvent, kB is the Boltzmann constant, T is the absolute temperature, and N0 is initial particle concentration, was calculated to quantify the instability of the Au particles according to the DVLO theory [87]. The aggregation rate constant for the dimer formation of an initially monodispersed particles in the diffusion controlled aggregation process, according to the Smoluchowski approximation (and taking into account the Stokes–Einstein relation) can be described by [29], ks ¼ 16πDR ¼

8kB T 3η

where D is the diffusion coefficient and R is the radius of the particle.

Please cite this article as: Łuczak J, et al, Ionic liquids for nano- and microstructures preparation. Part 1: Properties multifunctional role, Adv Colloid Interface Sci (2015), http://dx.doi.org/10.1016/j.cis.2015.08.006

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Fig. 8. Simplified mechanism of the N-heterocyclic carbenes formation in the presence of Ir nanoparticles proposed by Ott and Finke (adapted from [96]).

These assessments revealed that high viscosity of the ionic liquid bulk phase determine both parameters. The t1/2 is independent on the particle size and depends only on the viscosity of the IL bulk phase and the particle concentration [87], whereas kS only on viscosity of the bulk phase [29]. Indeed, coagulation and sedimentation of the gold nanoparticles were observed to be inversely proportional to the viscosity of the ionic liquid. For ionic liquids with different anions, the following trend in coagulation and sedimentation time was observed [N(CN)2] b [Tf2N] b [BF4] b [PF6] [87]. Interesting study was presented by Szilagyi et. al., who analyzed the influence of the IL/water content on the rate constant of model polystyrene latex particles in 1-butyl-3-methylimidazolium, 1-butyl-3methylpyridinium, and 1-butyl-1-methylpyrrolidinium ionic liquids with different anions [29]. In diluted IL aqueous solution, aggregation rate was low (and in agreement with DLVO theory) but increases dramatically when IL content rises. At higher IL/water molar ratio, the reverse trend was observed and correlated with increase of the viscosity of the bulk system, even though the agglomeration process was diffusion controlled (viscous stabilization). In the IL dispersion systems, where water constitutes only small additive, further aggregation rate was observed, confirming structural solvation role of the ILs as described above [29]. Theoretical evaluation of the properties and proper selection of ionic liquid structure followed by experimental verification enable the formation of the dispersion system in the controlled way, preventing agglomeration and stabilize particles. 4.3. Stabilization by intermediates or by-products The surface of the nanostructures can be also covered with some species originating from synthesis, such as carbenes, oxides, hydrides, carbonates, and residual ligands from the precursors that also may take part in the stabilization. From the IL role in the particle preparation point of view, the most important are N-heterocyclic carbenes (NHCs), electron-rich neutral σ-ligands that are formed in situ due to deprotonation of the imidazolium salts [34,63,94,95]. Since NHCs may interact with the majority of metals, these ligands were proposed play a role of the stabilizing agents. Ott [96] and Bernardi [34] observed that surface-ligand-coordinated NHCs derived from imidazolium cation are involved in the formation of the Ir particles in [BMIM][Tf2N] and [EMIM][EtSO4], as shown in Figs. 5f

and 8. NHC species were formed mainly at the most acidic C2 position in the imidazolium ring due to the strong interaction between the C2 hydrogen and the coordinating anions. A fast polarization of the C2-H bond enables the formation of the transient NHC species in the presence of metal in the reaction environment [34]. These results also confirmed that ionic liquid interacts with the metal surface as aggregates of the type rather than isolated ions. Some additional experiments by in situ labeling, spectroscopic, and electrospray ionization mass spectroscopy (ESI-MS) methods also confirmed carbene stabilization [63,94,95]. Moreover, since C4 or C5 positions of the imidazolium cation are also involved in this type of stabilization [97], chemical protection of the C2 position in the imidazolium cation does not stall carbene formation. The presence of hydrides [94] and oxide layers [98] was also detected on the surface some metal surfaces. Hydrides were found on the metal surface for particles prepared under reducing conditions in the presence of molecular hydrogen, whereas oxides were formed when particles contact with air. ILs were also found to act as agents that effectively protect the nanoparticle surface from oxidation that takes part more preferably in the air than in ILs. In this regard, Ni particles in the absence of NiO and Ni(OH)2 were prepared in aqueous solution of 1(3-aminopropyl)-2,3-dimethylimidazolium bromide and acetate [99]. Moreover, oxides were observed to disappear after a sputtering with Ar+, indicating that only the external Ru atoms undergo oxidation [98]. Nevertheless, hydrides and oxides may provide additional stabilization of the nanoparticles; however, they also change the nanoparticle surface compositions that have to be considered for specific application. 4.4. The role of the impurities Water, halogens, and 1-methylimidazole are common impurities that are often present in ionic liquids solvents and originate from the synthesis route. Therefore, there is also a considerable interest in the influence of the ILs impurities on the nano- and microparticles stabilization. An interesting, however indirect, observation was presented by Dash [100] and Redel [101], who revealed that even low levels of 1methylimidazole and 1-butylimidazole in the system may enhance the stability and activity of the Au nanoparticles synthesized directly in ILs. The addition of imidazole derivative inhibits aggregation of Au

Fig. 9. Schematic representation of the influence of impurities originating from synthesis route on the size of the Au nanoparticles in 1-butyl-3-methylimidazolium tetrafluoroborate (adapted from [102]).

Please cite this article as: Łuczak J, et al, Ionic liquids for nano- and microstructures preparation. Part 1: Properties multifunctional role, Adv Colloid Interface Sci (2015), http://dx.doi.org/10.1016/j.cis.2015.08.006

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nanoparticles that were seen in the pure [BMIM]-based ionic liquids with different anions. The role of the imidazole precursor may be related with the formation of the steric protective layer interacting with ionic liquid layers surrounding particle or scavenge acid by-products, thereby preventing product decomposition or side reactions. Contrary conclusions, however, were drawn by Lazarus and coauthors [102], who affirm that even 0.025 wt% of added 1-methylimidazolecan destabilize and/or affect the morphology of Ag nanoparticles in [BMIM][BF4]. Moreover, agglomerates, larger, and irregularly shaped nanoparticles were detected in the presence of water and chloride (Fig. 9). The presence of water, being a coordinating substrate solute, was revealed to have strongly negative impact on the stability of particles when prepared in ionic liquids as it was shown also in other studies [78,94,103]. Water was found to destroy the layered structure of ionic liquid ion layers surrounding the particle (Fig. 10) [78] as well as the rigid ionic channels of the IL due to the hydrogen bond formation with both cations and anions [94]. Interestingly, a direct study of role of water was also provided by group of Atkin [89]. The addition of even small amount of water destabilizes the dispersion of SiO2 particles by resulting in repulsion between well-organized solvation layers surrounding particles, thereby decreasing their number and strength [89].

4.5. Stabilization by ligands—functionalization of the ionic liquid However, ILs may be designed to contain functional groups that can directly stabilize nanostructures, as shown in Fig. 5g. Synthesis can be carried out in the functionalized IL, or these structures may be added as an extra stabilizing agent [104]. Indeed, nitrile functionalized ILs were found to constitute good media for generation of stable and monodispersed Pd [105,106] (Fig. 11), Ru [107] nanoparticles, and poly(1-vinyl-3-alkyl imidazolium) derivatives for Ag, Ni, Au [108], whereas thiol-functionalized ionic liquids for Au and Pt synthesis [109]. The additional stabilization of the nanostructures in ionic liquids dispersions may be also provided by an extra ligands. There are several examples of works where the addition of the ligand induces higher nanoparticle stabilization. This solution may constitute an alternative approach when ionic liquids do not efficiently act as both a solvent and a protective agent. However, when the additional compounds are introduced to the system in order to increase the particle stability, a balance between stability and particle activity must be considered since overprotection may result in loss of activity. Therefore, some examples of the ligands were proposed for the nanoparticle preparation ranging from ligands to polymeric materials: 2,2′-, 3,3′-, 4,4′-bipyridine [110], phenanthroline [111], polyvinylpyrrolidone-derived ionic liquid polymers [112], 2,4,6-tris(2-pyridyl)-s-triazine [113], and tetra-2pyridinylpyrazine [114].

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Fig. 11. Palladium nanoparticles stabilized by nitrile functionalized ionic liquid (adapted from [105]).

4.6. Ionic liquids as source of F− anions generated in situ It is well known that the tetrafluoroborate-based ILs are water-sensitive compounds and hydrolyze with the hydrofluoric acid formation under acidic, neutral, and basic conditions. In comparison, ionic liquids with [PF6] anion were found to be chemically and thermally more stable and hydrolysis occurs in acidic conditions or at temperatures above 343 K. These properties are usually seen as a serious drawback due to the loss of the IL and contamination of the reacting system by hydrolysis products, particularly hydrofluoric acid. However, this ability may be also intentionally utilized in reaction where the gradual release of fluoride anions is needed and crucial for specific structure formation. Several studies have shown that application of ionic liquids providing fluoride anions in situ enable the formation of particles with variety of shapes [115–119]. Fluorides offer multiple advantages as matrices for lanthanide fluoride nanoparticle formation due to their high transparency, wide band gap, and low energy phonons minimizing multiphonon relaxation. In this regard, the fluoride anions required for the precipitation of the nanoparticles may be provided by the application of the ILs with [BF4] or [PF6] anions (Fig. 5f) [115]. Some studies [115–117] have shown an influence of the IL structure and morphology, revealing that anion type mainly affects the size of lanthanide fluoride nanoparticles (LnF3), whereas their morphology is determined by the cation type. As such, in imidazolium derivatives, spherical and orthorhombic particles were formed, whereas in phosphonium and choline salts, rodlike ones were detected. Moreover, the same source of fluoride anions was also used for the formation of hierarchical, oriented one-dimensional (1D) TiO2 nanotubes during anodization reaction. The presence of the fluorides is crucial for building nanotubular structures, whereas geometry depends

Fig. 10. Schematic representation of the influence of water on the interactions between ionic liquid and silver surface (adapted from [78]).

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on the anodic potential, the anodization time, and particularly the water content in the ionic liquid [119]. High viscosity of ionic liquid can be beneficial for nanotube length evolution.

interconnected cylinders, lamellar phases), we can expect, in theory, that IL will also enable formation many possible nanosized structures. 4.8. Ionic liquid as reducing agents

4.7. Ionic liquid micellar systems Classical micellar systems, such as normal and reverse micelles, polymeric micelles, and self-assembling block copolymers, are often used as effective soft templates for the synthesis of nanostructures with a controlled size, oriented morphology, and low polydispersity due to the eligible core–shell (or different shape) structure. In this regard, micellar solutions based on the ionic liquids (Fig. 5i) were also applied for nanoparticle preparation with controlled size and shapes due to the combination of the unique chemical and physical properties of ILs and micellar template. However, up to date, only a few preliminary studies were performed in this direction, revealing the dependence of the particles size on the IL structure and concentration. The ability of micellar aggregates formed by polymeric ILs to control the size of gold nanoparticles and the role of ILs concentration was presented by Li at al [120]. At a lower AuCl− 4 /imidazolium molar ratio, the vesicle structures were observed with uniform nanosized gold particles (average 4 nm) distributed in the vesicle walls. However, the spherical micelles with much highly packed gold particles (10 nm) were obtained in the presence of higher AuCl− 4 and lower IL content. The lower size of the particles formed in the vesicles was explained by repulsive electrostatic interactions between IL cations that restrict particles aggregation in the vesicle walls [120]. The aqueous micellar solution of dodecyltrimethylammonium bromide was found to not only stabilize α-FeOOH nanoparticles but also miniaturize then when compared to those prepared without IL. Interestingly, it was concluded that the head groups of IL in aqueous solution were attached to hydroxyl anion groups around the iron core and hydrocarbon chains pointed outward, thereby forming self-assembly structure with the reverse micelle shape, as shown in Fig. 12 [27]. Particle size can be affected also by solvent type in the micellar solution. This was shown by Anouti et al. [28] in a study on gold nanoparticles prepared in the micellar solution of the octylammonium formate in water and dimethylformamide. Larger particles were seen (by TEM) to be formed in water than in dimethylformamide. In this regard, ionic liquid micellar systems as soft templates are still open case. Taking into account variety of self-assembly structures that micelles can take besides spherical micelles and vesicles (cylinders,

Most recently, a new method of metal nanoparticle formation in ionic liquids in the absence of classical reducing agents at low temperature and atmospheric pressure was proposed by Prechtl and coworkers [63] and soon developed also by other groups, for example [121]. This idea fits into a tendency to lower a number of reaction component minimizing potential contamination. It was revealed that the proper selection of cations and anions in the ILs enable to reduce certain metal precursors (both metal complexes or salts) with formation metal or metal oxide nanoparticles. This method was found to take place in anhydrous short chained imidazolium salts, dihydroxyl functionalized imidazolium derivatives, and asphosphonium ionic liquids with [Tf2N] [63], [PF6] [62], and bis(2,4,4-trimethylpentyl)phosphinate anions [121]. Mechanisms of that reactions is still open to debate; however, it was proposed that IL can act as a nucleophile and attack the precursor complex according to the reaction presented in Fig. 13a or undergo oxidation to a carbonyl group, as shown in Fig. 13b. Moreover, it was also revealed that due to the reduction reaction, imidazolium IL undergo oxidation, and thus, formed oxidation products such as ketones were found to acts as additional protecting agents [62]. The reducing process was observed to be slow and therefore provides small particles limiting fast particles growth. In comparison, the application of the non-functionalized ILs containing strongly coordinating [B(CN)4] anions or even [Cl] or [BF4] ones prevented a reduction of the precursors. Therefore, it seems that the presence of the certain anion or functional groups in the IL is crucial for this preparation route. 4.9. Ionic liquid as “all in one” media The final properties of the colloidal structures strongly depend on the chemical composition of the all reagents (also metal precursors) due to possible by-product formation. These species may positively influence the morphology and size by additional coordination and stabilization of the nanostructure. However, they may also have a negative impact on the properties of utility (catalytic, photocatalytic, magnetic, optical) due to ability to “poison” the product. Therefore, it was proposed to avoid reagents facilitating by-product formation, and as such, ionic liquids were used as an all-in-one medium for synthesis of inorganic materials. IL can serve as a solvent/template, stabilizing agent,

Fig. 12. Micellar ionic liquid solution as a template for a-FeOOH nanoparticle preparation (adapted from [27]).

Please cite this article as: Łuczak J, et al, Ionic liquids for nano- and microstructures preparation. Part 1: Properties multifunctional role, Adv Colloid Interface Sci (2015), http://dx.doi.org/10.1016/j.cis.2015.08.006

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Fig. 13. (a) Mechanism of the reductive elimination of the 1,5-cyclooctadiene (COD) ligand induced by the nucleophilic attack of the [Tf2N] anion (L L′ ligand/solvent-IL) (adapted from [63]). (b) Oxido-reduction mechanism of 1-(2,3-dihydroxypropyl)-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, M = Ag or Cu (adapted from [122]).

and metal precursor as well as reactant limiting the growth of the nanostructures. Some nanostructures of a variety of interesting shapes and different sizes were already prepared using various metal-containing ILs, as shown in Fig. 14. Metal can be incorporated in both IL cation, e.g., [Zn(CH3NH2)4(Tf2N)2] or anion structure, e.g., [BIM][AuCl4]. It was observed that morphology and size of the nanostructures can be differential and is strongly dependent on the nature of IL precursor– CuCl platelets (Fig. 14a) [123], ZuO flower-like structures (Fig. 14b) [67], Au polyhedral (Fig. 14c) [68].

nanoparticle size increases with the increase aqueous phase content at constant surfactant/IL molar ratio [125]. Evidences for potential of IL microemulsion systems to control the particles size and shape were also presented by Zhao [124] and Li [126]. The application of water-sensitive [BMIM][PF6] ionic liquid in aqueous microemulsions stabilized by TX-100 surfactant enables the formation of nanorods of SiO2, which was not succeeded previously in the classical microemulsion system. Moreover, [BMIM][BF4]/TX-100/benzene system provides smaller SiO2 particles with ellipsoidal shape when acidic conditions are used, whereas empty spheres in alkaline pH are found.

4.10. Microemulsion systems based on the ionic liquids 5. Conclusions and outlook The application of ionic liquid in a form of microemulsion systems, the dispersion of two immiscible fluids enable not only to control of the final shape and size of the nanostructures but also to overcome the limitations related with the solubility of the precursor (Fig. 5j). Again only few investigations were performed in order to apply ionic liquid-based microemulsions for nanoparticle preparation, where ILs were applied as all of the components—polar [124], nonpolar phase [125,126], and a surfactant-stabilizing dispersive system [127]. These solutions combine advantages of both microemulsions and ionic liquids. It is worth to mention that selection ionic liquid that is immiscible in water as a nonpolar phase, instead of a conventional volatile organic solvent, provides more safety systems with a new property that has a significant impact on conductivity on both phases. The application of ILs in a form of microemulsion system enables to overcome the main IL drawbacks: high viscosity and price. However, higher viscosity of ILs in comparison to molecular solvents can be beneficial since it may limit the coalescence of water droplets, thereby stabilizing dispersion. Moreover, the ability of ILs to interact with surfactants [61] enables penetration between surfactant molecules, which can increase rigidity of the interfacial film and thereby lead to a slower nanoparticles growth rate. Nevertheless, the effect of the IL microemulsion composition on the size and shape is up to date poorly known. It was observed that the higher IL content used as a surfactant provides higher number of nanoparticles with smaller diameter [127]. What is more, due to template character of nanodroplets, the

This review aims to recognize the impressive research efforts that have been devoted to the multifunctional role of ionic liquids applied to the synthesis of nanoparticles, nanomaterials, and oriented microstructures. We have discussed modes of action of ionic liquids as a soft template during particle preparation, as well as how they can stabilize growing nanoparticles through surface interactions. Ionic liquids can work as a structuring agent due to the formation of protective layers, by steric and electrostatic stabilization of growing nano- and microparticles, by viscous stabilization and solvation forces of colloidal structures, as well as a soft template through the growing nanoparticles around micelle formed of ILs. Literature data show that imidazolium cations, together with [BF4], [PF6], and [Tf2N] anions, are the most often used groups for nano- and microparticle preparation due to low melting temperature, low viscosity, and good transport properties. Additionally, it could be observed that most studies are focused on the application and interaction between ionic liquids and metal nanoparticles. Far less work was done in the case of semiconductor preparation using ILs. In some works, ionic liquids are used as an agent promoting the reduction of metal precursors. Cation and/or anion of the imidazolium moiety or hydroxyl group in functionalized IL can be oxidized with a simultaneous reduction of the nanostructure precursor. Moreover, ILs may be used as one of the phases in aqueous or nonaqueous microemulsion, and the application of this kind of microemulsion is a promising option from a green chemistry point of view since the

Fig. 14. Selected ionic liquid precursors used in the preparation of nanostructures (a) [DDPy]2[CuCl4], (b) [Zn(CH3NH2)4][Tf2N]2, and (c) [BIM][AuCl4].

Please cite this article as: Łuczak J, et al, Ionic liquids for nano- and microstructures preparation. Part 1: Properties multifunctional role, Adv Colloid Interface Sci (2015), http://dx.doi.org/10.1016/j.cis.2015.08.006

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usage of volatile organic solvents as a nonpolar part of microemulsions could be excluded. It was also found that the application of ILs as a reaction medium results in a high nucleation rate of metal nanoparticles due to the low surface tension of ILs, and it causes the generation of very fine particles. However, there is still a need to apply a computation method to predict the properties for new groups of ionic liquids, such as molecule geometry and dipole moment, charge distribution and polarizability, hydrogen bond donating/accepting, and electron-pair donating/ accepting capabilities. It should allow for the synthesis of new ionic liquids with suitable properties for the preparation of nanoscale materials. Thus, one of the major challenges for the scientific and industrial community involved in application of ILs for nanomaterial engineering is to integrate modeling, simulation, and informatics tools with theory and experiment and finally to transform the resulting information into knowledge useful in the processing and manufacturing of nano- and microparticles with desired features (e.g., size and architecture). A major area of future research would be the better understanding of interaction between ionic liquids and growing nanoparticles. Under this perspective, it is possible to conceive a materials-by-design approach, where materials are manipulated at the nanoscale, resulting in new systems with physicochemical properties tailored to meet industrial relevant issues and needs. Acknowledgments The author J.Ł. acknowledges funding from the National Science Center within program OPUS 3 (grant titled “Study on Aggregation Behavior of Nonionic Surfactants in Ionic Liquids,” contract no. UMO-2012/05/B/ ST4/02023). The author A.Z.-M. acknowledges funding from the National Science Centre within program OPUS 2 (grant titled “Preparation and Characteristics of Novel Three-Dimensional Semiconductor-Based Nanostructures Using a Template-Free Methods,” contract no. UMO2011/03/B/ST5/03243), for work described here. References [1] Mai NL, Ahn K, Koo Y-M. Methods for recovery of ionic liquids—a review. Process Biochem 2014;49:872–81. [2] Kumar S, Cho JH, Moon I. Ionic liquid-amine blends and CO2BOLs: prospective solvents for natural gas sweetening and CO2 capture technology—a review. Int J Greenhouse Gas Control 2014;20:87–116. [3] Gan Q, Zou Y, Rooney D, Nancarrow P, Thompson J, Liang L, et al. Theoretical and experimental correlations of gas dissolution, diffusion, and thermodynamic properties in determination of gas permeability and selectivity in supported ionic liquid membranes. Adv Colloid Interface Sci 2011;164:45–55. [4] Kapnissi-Christodoulou CP, Stavrou IJ, Mavroudi MC. Chiral ionic liquids in chromatographic and electrophoretic separations. J Chromatogr A 2014;1363:2–10. [5] Pino V, Afonso AM. Surface-bonded ionic liquid stationary phases in highperformance liquid chromatography—a review. Anal Chim Acta 2012;714:20–37. [6] Domínguez I, González EJ, Domínguez Á. Liquid extraction of aromatic/cyclic aliphatic hydrocarbon mixtures using ionic liquids as solvent: literature review and new experimental LLE data. Fuel Process Technol 2014;125:207–16. [7] Vidal L, Riekkola M-L, Canals A. Ionic liquid-modified materials for solid-phase extraction and separation: a review. Anal Chim Acta 2012;715:19–41. [8] MacFarlane DR, Tachikawa N, Forsyth M, Pringle JM, Howlett PC, Elliott GD, et al. Energy applications of ionic liquids. Energy Environ Sci 2014;7:232–50. [9] Díaz M, Ortiz A, Ortiz I. Progress in the use of ionic liquids as electrolyte membranes in fuel cells. J Membr Sci 2014;469:379–96. [10] Liu C-Z, Wang F, Stiles AR, Guo C. Ionic liquids for biofuel production: opportunities and challenges. Appl Energy 2012;92:406–14. [11] Vancov T, Alston A-S, Brown T, McIntosh S. Use of ionic liquids in converting lignocellulosic material to biofuels. Renew Energy 2012;45:1–6. [12] Dupont J, Scholten JD. On the structural and surface properties of transition-metal nanoparticles in ionic liquids. Chem Soc Rev 2010;39:1780–804. [13] Selvam T, Machoke A, Schwieger W. Supported ionic liquids on non-porous and porous inorganic materials—a topical review. Appl Catal A Gen 2012;445-446: 92–101. [14] Naushad M, Alothman ZA, Khan AB, Ali M. Effect of ionic liquid on activity, stability, and structure of enzymes: a review. Int J Biol Macromol 2012;51:555–60. [15] Somers A, Howlett P, MacFarlane D, Forsyth M. A review of ionic liquid. Lubricants 2013;1:3–21. [16] Łuczak J, Hupka J, Thöming J, Jungnickel C. Self-organization of imidazolium ionic liquids in aqueous solution. Colloids Surf, A 2008;329:125–33.

[17] Singh MP, Singh RK, Chandra S. Ionic liquids confined in porous matrices: physicochemical properties and applications. Prog Mater Sci 2014;64:73–120. [18] Zheng D, Dong L, Huang W, Wu X, Nie N. A review of imidazolium ionic liquids research and development towards working pair of absorption cycle. Renew Sustain Energy Rev 2014;37:47–68. [19] Wang X, Chi Y, Mu T. A review on the transport properties of ionic liquids. J Mol Liq 2014;193:262–6. [20] Delcheva I, Ralston J, Beattie DA, Krasowska M. Static and dynamic wetting behaviour of ionic liquids. Adv Colloid Interface Sci 2015;222:162–71. [21] Frost DS, Nofen EM, Dai LL. Particle self-assembly at ionic liquid-based interfaces. Adv Colloid Interf 2014;206:92–105. [22] Cvjetko Bubalo M, Radošević K, Radojčić Redovniković I, Halambek J, Gaurina Srček V. A brief overview of the potential environmental hazards of ionic liquids. Ecotoxicol Environ Saf 2014;99:1–12. [23] Weingärtner H. Understanding ionic liquids at the molecular level: facts, problems, and controversies. Angew Chem Int Ed 2008;47:654–70. [24] Choi S, Kim K-S, Yeon S-H, Cha J-H, Lee H, Kim C-J, et al. Fabrication of silver nanoparticles via self-regulated reduction by 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate. Korean J Chem Eng 2007;24:856–9. [25] Jiang J, Yu S-H, Yao W-T, Ge H, Zhang G-Z. Morphogenesis and crystallization of Bi2S3 nanostructures by an ionic liquid-assisted templating route: synthesis, formation mechanism, and properties. Chem Mater 2005;17:6094–100. [26] Qin Y, Song Y, Sun N, Zhao N, Li M, Qi L. Ionic liquid-assisted growth of singlecrystalline dendritic gold nanostructures with a three-fold symmetry. Chem Mater 2008;20:3965–72. [27] Jusoh R, Jalil AA, Triwahyono S, Idris A, Haron S, Sapawe N, et al. Synthesis of reverse micelle α-FeOOH nanoparticles in ionic liquid as an only electrolyte: inhibition of electron–hole pair recombination for efficient photoactivity. Appl Catal A Gen 2014;469:33–44. [28] Anouti M, Jacquemin J. Structuring reductive media containing protic ionic liquids and their application to the formation of metallic nanoparticles. Colloids Surf A 2014;445:1–11. [29] Szilagyi I, Szabo T, Desert A, Trefalt G, Oncsik T, Borkovec M. Particle aggregation mechanisms in ionic liquids. Phys Chem Chem Phys 2014;16:9515–24. [30] Yavari I, Mahjoub A, Kowsari E, Movahedi M. Synthesis of ZnO nanostructures with controlled morphology and size in ionic liquids. J Nanopart Res 2009;11: 861–8. [31] Banerjee A, Theron R, Scott RWJ. Design, synthesis, catalytic application, and strategic redispersion of plasmonic silver nanoparticles in ionic liquid media. J Mol Catal A Chem 2014;393:105–11. [32] Goharshadi EK, Abareshi M, Mehrkhah R, Samiee S, Moosavi M, Youssefi A, et al. Preparation, structural characterization, semiconductor and photoluminescent properties of zinc oxide nanoparticles in a phosphonium-based ionic liquid. Mater Sci Semicond Process 2011;14:69–72. [33] Migowski P, Zanchet D, Machado G, Gelesky MA, Teixeira SR, Dupont J. Nanostructures in ionic liquids: correlation of iridium nanoparticles' size and shape with imidazolium salts' structural organization and catalytic properties. Phys Chem Chem Phys 2010;12:6826–33. [34] Bernardi F, Scholten JD, Fecher GH, Dupont J, Morais J. Probing the chemical interaction between iridium nanoparticles and ionic liquid by XPS analysis. Chem Phys Lett 2009;479:113–6. [35] Redel E, Walter M, Thomann R, Vollmer C, Hussein L, Scherer H, et al. Stop-and-go, stepwise and “ligand-free” nucleation, nanocrystal growth and formation of AuNPs in ionic liquids (ILs). Chem Eur J 2009;15:10047–59. [36] Maton C, De Vos N, Stevens CV. Ionic liquid thermal stabilities: decomposition mechanisms and analysis tools. Chem Soc Rev 2013;42:5963–77. [37] Nan A, Liebscher J. Ionic Liquids as Advantageous Solvents for Preparation of Nanostructures. In: Handy S, editor. Applications of Ionic Liquids in Science and Technology. USA: Middle Tennessee State University; 2011. [38] Aparicio S, Atilhan M, Karadas F. Thermophysical properties of pure ionic liquids: review of present situation. Ind Eng Chem Res 2010;49:9580–95. [39] Zhang S, Sun N, He X, Lu X, Zhang X. Physical properties of ionic liquids: database and evaluation. J Phys Chem Ref Data 2006;35:1475–517. [40] Tariq M, Freire MG, Saramago B, Coutinho JAP, Lopes JNC, Rebelo LPN. Surface tension of ionic liquids and ionic liquid solutions. Chem Soc Rev 2012;41:829–68. [41] 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;43:4988–92. [42] Łuczak J, Markiewicz M, Thöming J, Hupka J, Jungnickel C. Influence of the Hofmeister anions on self-organization of 1-decyl-3-methylimidazolium chloride in aqueous solutions. J Colloid Interface Sci 2011;362:415–22. [43] Hunt PA, Kirchner B, Welton T. Characterizing the electronic structure of ionic liquids: an examination of the 1-butyl-3-methylimidazolium chloride ion pair. Chem Eur J 2006;12:6762–75. [44] Hardacre C, Holbrey JD, McMath SEJ, Bowron DT, Soper AK. Structure of molten 1,3dimethylimidazolium chloride using neutron diffraction. J Chem Phys 2003;118: 273–8. [45] Consorti CS, Suarez PAZ, de Souza RF, Burrow RA, Farrar DH, Lough AJ, et al. Identification of 1,3-dialkylimidazolium salt supramolecular aggregates in solution. J Phys Chem B 2005;109:4341–9. [46] Hardacre C, Holbrey JD, Nieuwenhuyzen M, Youngs TGA. Structure and solvation in ionic liquids. Acc Chem Res 2007;40:1146–55. [47] Dupont J. On the solid, liquid and solution structural organization of imidazolium ionic liquids. J Braz Chem Soc 2004;15:341–50. [48] Canongia Lopes JNA, Pádua AAH. Nanostructural organization in ionic liquids. J Phys Chem B 2006;110:3330–5.

Please cite this article as: Łuczak J, et al, Ionic liquids for nano- and microstructures preparation. Part 1: Properties multifunctional role, Adv Colloid Interface Sci (2015), http://dx.doi.org/10.1016/j.cis.2015.08.006

J. Łuczak et al. / Advances in Colloid and Interface Science xxx (2015) xxx–xxx [49] Holbrey JD, Reichert WM, Rogers RD. Crystal structures of imidazolium bis(trifluoromethanesulfonyl)imide ‘ionic liquid’ salts: the first organic salt with a cis-TFSI anion conformation. Dalton Trans 2004:2267–71. [50] Kirchner B, editor. Ionic Liquids. New York: Springer; 2009. [51] Russina O, Triolo A, Gontrani L, Caminiti R. Mesoscopic structural heterogeneities in room-temperature ionic liquids. J Phys Chem Lett 2011;3:27–33. [52] Goossens K, Lava K, Nockemann P, Van Hecke K, Van Meervelt L, Driesen K, et al. Pyrrolidinium ionic liquid crystals. Chem Eur J 2009;15:656–74. [53] Ji Y, Shi R, Wang Y, Saielli G. Effect of the Chain length on the structure of ionic liquids: from spatial heterogeneity to ionic liquid crystals. J Phys Chem B 2013;117: 1104–9. [54] Chen S, Zhang S, Liu X, Wang Y, Wang J, Dong K, et al. Ionic liquid clusters: structure, formation mechanism, and effect on the behavior of ionic liquids. Phys Chem Chem Phys 2014;16:5893–906. [55] Reszczynska J, Iwulska A, Sliwinski G, Zaleska A. Characterization and photocatalytic activity of rare earth metal-doped titanium dioxide. Physicochem Probl Miner Process 2012;48:201–8. [56] Fattakhova-Rohlfing D, Zaleska A, Bein T. Three-dimensional titanium dioxide nanomaterials. Chem Rev 2014;114:9487–558. [57] Roucoux A, Schulz J, Patin H. Reduced transition metal colloids: a novel family of reusable catalysts? Chem Rev 2002;102:3757–78. [58] Migowski P, Dupont J. Catalytic applications of metal nanoparticles in imidazolium ionic liquids. Chem Eur J 2007;13:32–9. [59] Dupont J, Fonseca GS, Umpierre AP, Fichtner PFP, Teixeira SR. Transition-metal nanoparticles in imidazolium ionic liquids: recyclable catalysts for biphasic hydrogenation reactions. J Am Chem Soc 2002;124:4228–9. [60] Kaper H, Willinger MG, Djerdj I, Gross S, Antonietti M, Smarsly BM. IL-assisted synthesis of V2O5 nanocomposites and VO2 nanosheets. J Mater Chem 2008;18: 5761–9. [61] Łuczak J, Latowska A, Hupka J. Micelle formation of Tween 20 nonionic surfactant in imidazolium ionic liquids. Colloids Surf A 2015;471:26–37. [62] Gao Y, Voigt A, Zhou M, Sundmacher K. Synthesis of single-crystal gold nano- and microprisms using a solvent-reductant-template ionic liquid. Eur J Inorg Chem 2008;24:3769–75. [63] Prechtl MHG, Campbell PS, Scholten JD, Fraser GB, Machado G, Santini CC, et al. Imidazolium ionic liquids and decomposition of organometallic complexes. Nanoscale 2010;2:2601–6. [64] Li Z, Liu Z, Zhang J, Han B, Du J, Gao Y, et al. Synthesis of single-crystal goldnanosheets of large size in ionic liquids. J Phys Chem B 2005;109:14445–8. [65] Wang Y, Yang H. Synthesis of CoPt nanorods in ionic liquids. J Am Chem Soc 2005; 127:5316–7. [66] 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;18:1008–14. [67] Zhu H, Huang J-F, Pan Z, Dai S. Ionothermal synthesis of hierarchical ZnO nano‐ structures from ionic-liquid precursors. Chem Mater 2006;18:4473–7. [68] Hsu S-J, Lin IJB. Synthesis of gold nanosheets through thermolysis of mixtures of long chain 1-alkylimidazole and hydrogen tetrachloroaurate (III). J Chin Chem Soc 2009;56:98–106. [69] Łuczak J, Hupka J. Studies on formation and percolation in ionic liquids/TX-100/ water microemulsions. J Mol Liq 2014;199:552–8. [70] Martínez-Palou R. Microwave-assisted synthesis using ionic liquids. Mol Divers 2010;14:3–25. [71] Hoffmann J, Nuchter M, Ondruschka B, Wasserscheid P. Ionic liquids and their heating behaviour during microwave irradiation—a state of the art report and challenge to assessment. Green Chem 2003;5:296–9. [72] Fonseca GS, Scholten JD, Dupont J. Iridium nanoparticles prepared in ionic liquids: an efficient catalytic system for the hydrogenation of ketones. Synlett 2004: 1525–8. [73] 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;347:1404–12. [74] Yuan X, Yan N, Katsyuba SA, Zvereva EE, Kou Y, Dyson PJ. A remarkable anion effect on palladium nanoparticle formation and stabilization in hydroxyl-functionalized ionic liquids. Phys Chem Chem Phys 2012;14:6026–33. [75] Mendonça ACF, Pádua AAH, Malfreyt P. Non-equilibrium molecular simulations of new ionic lubricants at metallic surfaces: prediction of the friction. J Chem Theory Comput 2013;9:1600–10. [76] Scheeren CW, Machado G, Teixeira SR, Morais J, Domingos JB, Dupont J. Synthesis and characterization of Pt(0) nanoparticles in imidazolium ionic liquids. J Phys Chem B 2006;110:13011–20. [77] Fonseca GS, Machado G, Teixeira SR, Fecher GH, Morais J, Alves MCM, et al. Synthesis and characterization of catalytic iridium nanoparticles in imidazolium ionic liquids. J Colloid Interface Sci 2006;301:193–204. [78] Rubim JC, Trindade FA, Gelesky MA, Aroca RF, Dupont J. Surface-enhanced vibrational spectroscopy of tetrafluoroborate 1-n-butyl-3-methylimidazolium (BMIBF4) ionic liquid on silver surfaces. J Phys Chem C 2008;112:19670–5. [79] An J, Wang D, Luo Q, Yuan X. Antimicrobial active silver nanoparticles and silver/ polystyrene core-shell nanoparticles prepared in room-temperature ionic liquid. Mater Sci Eng C 2009;29:1984–9. [80] Sobota M, Nikiforidis I, Hieringer W, Paape N, Happel M, Steinrück H-P, et al. Toward ionic-liquid-based model catalysis: growth, orientation, conformation, and interaction mechanism of the [Tf2N]–anion in [BMIM][Tf2N] thin films on a wellordered alumina surface. Langmuir 2010;26:7199–207. [81] Atkin R, Warr GG. Structure in confined room-temperature ionic liquids. J Phys Chem C 2007;111:5162–8.

15

[82] Zvereva EE, Grimme S, Katsyuba SA, Ermolaev VV, Arkhipova DA, Yan N, et al. Solvation and stabilization of palladium nanoparticles in phosphonium-based ionic liquids: a combined infrared spectroscopic and density functional theory study. Phys Chem Chem Phys 2014;16:20672–80. [83] Katsyuba SA, Zvereva EE, Yan N, Yuan X, Kou Y, Dyson PJ. Rationalization of solvation and stabilization of palladium nanoparticles in imidazolium-based ionic liquids by DFT and vibrational spectroscopy. Chem Phys Chem 2012;13: 1781–90. [84] Baldelli S. Surface structure at the ionic liquid—electrified metal interface. Acc Chem Res 2008;41:421–31. [85] Pensado AS, Pádua AAH. Angew Chem Int Ed 2011;50:8683–7. [86] Ueno K, Inaba A, Kondoh M, Watanabe M. Colloidal stability of bare and polymergrafted silica nanoparticles in ionic liquids. Langmuir 2008;24:5253–9. [87] Vanecht E, Binnemans K, Patskovsky S, Meunier M, Seo JW, Stappers L, et al. Stability of sputter-deposited gold nanoparticles in imidazolium ionic liquids. Phys Chem Chem Phys 2012;14:5662–71. [88] Ueno K, Watanabe M. From colloidal stability in ionic liquids to advanced soft materials using unique media. Langmuir 2011;27:9105–15. [89] Smith JA, Werzer O, Webber GB, Warr GG, Atkin R. Surprising particle stability and rapid sedimentation rates in an ionic liquid. J Phys Chem Lett 2010;1:64–8. [90] Schrekker HS, Gelesky MA, Stracke MP, Schrekker CML, Machado G, Teixeira SR, et al. J Colloid Interface Sci 2007;316:189–95. [91] Fitchett BD, Conboy JC. Structure of the room-temperature ionic liquid/SiO2 interface studied by sum-frequency vibrational spectroscopy. J Phys Chem B 2004; 108:20255–62. [92] Gao J, Ndong RS, Shiflett MB, Wagner NJ. Creating nanoparticle stability in ionic liquid [C4mim][BF4] by inducing solvation layering. ACS Nano 2015;9:3243–53. [93] Hayes R, Warr GG, Atkin R. At the interface: solvation and designing ionic liquids. Phys Chem Chem Phys 2010;12:1709–23. [94] Campbell PS, Santini CC, Bouchu D, Fenet B, Philippot K, Chaudret B, et al. A novel stabilisation model for ruthenium nanoparticles in imidazolium ionic liquids: in situ spectroscopic and labelling evidence. Phys Chem Chem Phys 2010;12: 4217–23. [95] Corilo YE, Nachtigall FM, Abdelnur PV, Ebeling G, Dupont J, Eberlin MN. Chargetagged N-heterocyclic carbenes. RSC Adv 2011;1:73–8. [96] Ott LS, Cline ML, Deetlefs M, Seddon KR, Finke RG. Nanoclusters in ionic liquids: evidence for N-heterocyclic carbene formation from imidazolium-based ionic liquids detected by 2H NMR. J Am Chem Soc 2005;127:5758–9. [97] Lebel H, Janes MK, Charette AB, Nolan SP. Structure and reactivity of “unusual” Nheterocyclic carbene (NHC) palladium complexes synthesized from imidazolium salts. J Am Chem Soc 2004;126:5046–7. [98] Silveira ET, Umpierre AP, Rossi LM, Machado G, Morais J, Soares GV, et al. The Partial hydrogenation of benzene to cyclohexene by nanoscale ruthenium catalysts in imidazolium ionic liquids. Chem Eur J 2004;10:3734–40. [99] Hu Y, Yu Y, Zhao X, Yang H, Feng B, Li H, et al. Catalytic hydrogenation of aromatic nitro compounds by functionalized ionic liquids-stabilized nickel nanoparticles in aqueous phase: the influence of anions. Sci China Chem 2010;53:1541–8. [100] Dash P, Scott RWJ. 1-Methylimidazole stabilization of gold nanoparticles in imidazolium ionic liquids. Chem Commun 2009:812–4. [101] Redel E, Thomann R, Janiak C. First correlation of nanoparticle size-dependent formation with the ionic liquid anion molecular volume. Inorg Chem 2007;47: 14–6. [102] Lazarus LL, Riche CT, Malmstadt N, Brutchey RL. The effect of ionic liquid impurities on the synthesis of silver nanoparticles. Langmuir 2012;28:15987–93. [103] Salas G, Podgorsek A, Campbell PS, Santini CC, Padua AAH, Costa Gomes MF, et al. Ruthenium nanoparticles in ionic liquids: structural and stability effects of polar solutes. Phys Chem Chem Phys 2011;13:13527–36. [104] Léger B, Denicourt-Nowicki A, Olivier-Bourbigou H, Roucoux A. Imidazoliumfunctionalized bipyridine derivatives: a promising family of ligands for catalytical Rh(0) colloids. Tetrahedron Lett 2009;50:6531–3. [105] Venkatesan R, Prechtl MHG, Scholten JD, Pezzi RP, Machado G, Dupont J. Palladium nanoparticle catalysts in ionic liquids: synthesis, characterisation and selective partial hydrogenation of alkynes to Z-alkenes. J Mater Chem 2011;21:3030–6. [106] Fei Z, Zhao D, Pieraccini D, Ang WH, Geldbach TJ, Scopelliti R, et al. Development of nitrile-functionalized ionic liquids for C—C coupling reactions: implication of carbene and nanoparticle catalysts. Organometallics 2007;26:1588–98. [107] Prechtl MHG, Scholten JD, Dupont J. Tuning the selectivity of ruthenium nanoscale catalysts with functionalised ionic liquids: hydrogenation of nitriles. J Mol Catal A Chem 2009;313:74–8. [108] Prabhu Charan KT, Pothanagandhi N, Vijayakrishna K, Sivaramakrishna A, Mecerreyes D, Sreedhar B. Poly(ionic liquids) as “smart” stabilizers for metal nanoparticles. Eur Polym J 2014;60:114–22. [109] Kim K-S, Demberelnyamba D, Lee H. Size-selective synthesis of gold and platinum nanoparticles using novel thiol-functionalized ionic liquids. Langmuir 2003;20: 556–60. [110] Léger B, Denicourt-Nowicki A, Olivier-Bourbigou H, Roucoux A. Rhodium nanocatalysts stabilized by various bipyridine ligands in nonaqueous ionic liquids: influence of the bipyridine coordination modes in arene catalytic hydrogenation. Inorg Chem 2008;47:9090–6. [111] Huang J, Jiang T, Han B, Gao H, Chang Y, Zhao G, et al. Hydrogenation of olefins using ligand-stabilized palladium nanoparticles in an ionic liquid. Chem Commun 2003:1654–5. [112] Yan N, Yuan Y, Dyson PJ. Rhodium nanoparticle catalysts stabilized with a polymer that enhances stability without compromising activity. Chem Commun 2011;47: 2529–31. [113] Leger B. Roucoux A. Google Patents: Olivier-Bourbigou H; 2013.

Please cite this article as: Łuczak J, et al, Ionic liquids for nano- and microstructures preparation. Part 1: Properties multifunctional role, Adv Colloid Interface Sci (2015), http://dx.doi.org/10.1016/j.cis.2015.08.006

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

[114] Léger B, Denicourt-Nowicki A, Olivier-Bourbigou H, Roucoux A. Rhodium colloidal suspensions stabilised by poly-N-donor ligands in non-aqueous ionic liquids: preliminary investigation into the catalytic hydrogenation of arenes. ChemSusChem 2008;1:984–7. [115] Lorbeer C, Cybińska J, Mudring A-V. Europium(III) fluoride nanoparticles from ionic liquids: structural, morphological, and luminescent properties. Cryst Growth Des 2011;11:1040–8. [116] Lorbeer C, Cybinska J, Mudring A-V. Facile preparation of quantum cutting GdF3: Eu3+ nanoparticles from ionic liquids. Chem Commun 2010;46:571–3. [117] Nuñez N, Ocaña M. An ionic liquid based synthesis method for uniform luminescent lanthanide fluoride nanoparticles. Nanotechnology 2007;18:455606. [118] John SE, Mohapatra SK, Misra M. Double-wall anodic titania nanotube arrays for water photooxidation. Langmuir 2009;25:8240–7. [119] Paramasivam I, Macak JM, Selvam T, Schmuki P. Electrochemical synthesis of selforganized TiO2 nanotubular structures using an ionic liquid (BMIM-BF4). Electrochim Acta 2008;54:643–8. [120] Li J, Liang J, Wu W, Zhang S, Zhang K, Zhou H. AuCl4−-responsive self-assembly of ionic liquid block copolymers for obtaining composite gold nanoparticles and polymeric micelles with controlled morphologies. New J Chem 2014;38:2508–13.

[121] Kalviri HA, Kerton FM. Synthesis of Pd nanocrystals in phosphonium ionic liquids without any external reducing agents. Green Chem 2011;13:681–6. [122] Darwich W, Gedig C, Srour H, Santini CC, Prechtl MHG. Single step synthesis of metallic nanoparticles using dihydroxyl functionalized ionic liquids as reductive agent. RSC Adv 2013;3:20324–31. [123] Taubert A. CuCl Nanoplatelets from an ionic liquid-crystal precursor. Angew Chem Int Ed 2004;43:5380–2. [124] Zhao M, Zheng L, Bai X, Li N, Yu L. Fabrication of silica nanoparticles and hollow spheres using ionic liquid microemulsion droplets as templates. Colloids Surf A 2009;346:229–36. [125] Serra A, Gomez E, Lopez-Barbera JF, Nogues J, Valles E. Green electrochemical template synthesis of CoPt nanoparticles with tunable size, composition, and magnetism from microemulsions using an ionic liquid (bmimPF6). ACS Nano 2014;8: 4630–9. [126] Li Z, Zhang J, Du J, Han B, Wang J, Li Z, et al. Colloids Surf A 2006;286:117–20. [127] Wu L-G, Shen J-N, Du C-H, Wang T, Teng Y, BVd Bruggen. Development of AgCl/ poly(MMA-co-AM) hybrid pervaporation membranes containing AgCl nanoparticles through synthesis of ionic liquid microemulsions. Sep Purif Technol 2013; 114:117–25.

Please cite this article as: Łuczak J, et al, Ionic liquids for nano- and microstructures preparation. Part 1: Properties multifunctional role, Adv Colloid Interface Sci (2015), http://dx.doi.org/10.1016/j.cis.2015.08.006