Ionic liquid and nanoparticle hybrid systems: Emerging applications
Graphical abstract
Introduction
Ionic liquids (ILs) are low-melting salts [1] typically consisting of organic cations and organic or inorganic anions [2]. ILs with melting point below room temperature are often called room temperature ionic liquids (RTILs) [3]. Ionic liquids exhibit several useful features such as negligible vapor pressure, good thermal stability, high ionic conductivity, broad electrochemical potential windows, good solubility and high synthetic flexibility [4], [5]. While they are in liquid state, it is important to note that ionic liquids are not simple liquids. Their ions are generally asymmetric, with delocalized electrostatic charges [6]. The molecular forces and interactions in ionic liquids are more complicated than those of classical salts [7]. The combination of strong Coulombic interactions and weak directional interactions (including hydrogen bonding, cation-π, and van der Waals inductive and dispersion interactions) induces the formation of nano-scale structures in ILs and in IL/molecular solvent or IL/solute mixtures [2], [8], [9]. For those ionic liquids with cations incorporating long alkyl side-chains, the alkyl chains can segregate to form nonpolar domains, while other parts of the ionic liquid form polar domains [10]. With their highly hydrogen-bonded networks and nanodomains, ionic liquids can facilitate the dissolution of various substances, where the solutes tend to localize into the nano-scale domains for which they have higher affinity [11], [12], [13]. Ionic liquids are also used as reaction media, playing the role of solvent and/or template [14], [15], [16], [17], [18]. Polymerized ionic liquids or poly(ionic liquid)s (PILs) have been applied as reaction media [19] and in the areas of energy harvesting and storage, catalysis, and separations as a result of combining properties of polymers and ionic liquids [20].
Nanoparticles, including nanocrystals, nanoclusters, nanometals, nanotubes, etc., exhibit unique physical properties that give rise to many applications in areas such as catalysis [21], nanocomposites [22], [23], [24], electrochemistry [25], biotechnology [26], chemical analysis/sensing [27], diagnostics [28], drug delivery and medicine [29], [30], [31]. Novel properties such as electronic and optical emanate from the nanoparticle large surface area-to-volume ratio and the quantum confinement effect, which are both consequence of their nano-scale size dimensions [32], [33], [34].
Ionic liquids and nanoparticles can form together various hybrid structures, depending on a balance of intra- and intermolecular interactions between them. For example, ionic liquids serve as solvent media for colloidal dispersions [35], facilitating the dispersion of metal nanoparticles [36], nanostructured inorganic particles [37], graphene [38] and carbon nanotubes [39], [40], assisting the dissolution of cellulose [41] and dispersing the as-obtained nanocellulose [42]. Some colloidal particles can be stably suspended in ionic liquids without the need to add classical stabilizers such as surfactants and/or polymers [35]. Ionic liquids, contributing electrostatic forces on the surface of nanoparticles, can also be utilized as colloidal stabilizers for nanoparticles synthesized in aqueous solution [43], [44].
Novel functional materials comprising nanoparticle and ionic liquid composites can be generated with improved features, sharing properties of both ionic liquids and nanoscale materials [45]. Notable synergistic effects have been observed in materials that combine ionic liquids and nanoparticles [46]. On the one hand, nanoparticle properties such as thermal stability, catalytic efficiency, adsorption efficiency, and electrical and electrochemical response can be modified and improved through physical/chemical surface modification with ionic liquids [47]. On the other hand, due to the interactions between ionic liquids and nanoparticles, the physicochemical and/or electrochemical properties of the ionic liquid and nanoparticle hybrid can be superior to those of the ionic liquid alone [45], [48]. As an example, the diffusion coefficient and electrical conductivity of 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) were enhanced by 35% and 65%, respectively, with the addition of 0.08 wt.% copper nanoparticles encapsulated within carbon shell (Cu@C) [48]. Ionic liquid and nanoparticle hybrid structures are usually thermally and chemically stable even in extreme conditions. The unique characteristics of ionic liquid and nanoparticle hybrids show promise in various applications, including catalysis, electrochemistry and separations.
This review addresses emerging applications of ionic liquid and nanoparticle hybrid materials from the perspective of the relationship between material nanostructure and function. A few reviews have previously discussed applications of ionic liquid and nanoparticle hybrid materials, focusing on one specific field of application and the optimization of operating conditions [49], [50], [51]. However, these reviews did not focus on aspects of the material structure that result in desired properties and consequently facilitate applications in various fields. Other reviews have addressed specific types of ionic liquid and nanoparticle hybrid, e.g., inorganic nanoparticles covalently linked by imidazolium moieties [45]. We have recently reviewed the organization of nanoparticles in ionic liquids in various modes that depend on a balance of intermolecular interactions [52]. The present review provides a timely overview on select applications of ionic liquid and nanoparticle hybrid materials with a focus on their structure-based properties. We first discuss several types of ionic liquid and nanoparticle hybrid materials, including colloidal dispersions, colloidal gels and glasses, and ionic liquid-grafted nanoparticles. We then review the better-studied applications of ionic liquid and nanoparticle hybrid materials in the fields of catalysis, electrochemistry, and separations. For catalysis applications, nanoparticles play an important role as the catalysts, while ionic liquids provide stabilization for better dispersion of the catalysts. Ionic liquid-functionalized nanoparticles are often applied as heterogeneous catalysts. The catalytic properties of nanoparticles as well as the ion and electron conductivity of both ionic liquid and nanoparticle components benefit electrochemical processes. For separation process, nanoparticles with their high surface area-to-volume ratio absorb effectively solutes or gases, while ionic liquids with their unique intermolecular interactions also interact with the solutes or gases. For each application area, we point out synergies observed in common types of nanoparticle and ionic liquid hybrid structures.
Section snippets
Structure and interactions in ionic liquid and nanoparticle hybrids
Intermolecular interactions in ionic liquid and nanoparticle hybrid systems act in tandem to organize materials with various structures. Stable colloidal dispersions, colloidal gels or colloidal glasses can be achieved by suspending nanoparticles into ionic liquids. Ionic liquids form a protective layer surrounding nanoparticles, improving their chemical and thermal stability. Ionic liquids can bond covalently on nanoparticle surfaces, thus combining the desired properties of ionic liquid and
Catalysis applications
Homogeneous catalysts, providing better yields and selectivities compared to traditional heterogeneous catalysts under mild reaction conditions, have been extensively developed on a laboratory scale [115]. However, the industrial-scale use of homogeneous catalysis is limited due to the difficulty of separating the products and recovering the catalysts. To overcome such challenges, more efficient catalytic systems have been explored that combine the advantages of both homogeneous and
Electrochemical applications
On the basis of properties that include high ionic conductivity, nonflammability, and electrochemical and thermal stability, ionic liquids have a great potential as electrolytes in electrochemical devices like in batteries [134], fuel cells [135], solar cells [136], and sensors [137] [138], [139]. By joining nanoparticles with ionic liquids, the high electron conductivity of nanoparticles combines with the good proton-transfer property of ionic liquids, with additional benefits to
Separations applications
Having multiple interactions with a variety of solutes, with the most important interactions being dipolarity, hydrogen bond basicity, and dispersion forces [172], ionic liquids have been reported to offer remarkable advantages in the separation of a wide variety of mixtures [173]. Furthermore, their unique properties of designable structures and good thermal stability and low volatility render ionic liquids ideal candidates for separation technologies [47]. Imidazolium-based ionic liquids,
Conclusions
Various types of structured ionic liquid and nanoparticle hybrids have been developed on the basis of a balance of several intermolecular interactions. These hybrids can combine novel properties of both nanoparticles and ionic liquids which benefit potential applications. In this review we address synergistic effects in ionic liquid and nanoparticle hybrids in the context of emerging applications of such hybrids in the fields of catalysis, electrochemistry, and separations.
In order to better
Acknowledgements
We thank the U.S. National Science Foundation for supporting research in our laboratory in the area of ionic liquids (CBET 1033878 and 1159981).
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