Abstract
Research on graphene-monolayers of carbon atoms arranged in a honeycomb lattice-is proceeding at a relentless pace as scientists of both experimental and theoretical bents seek to explore and exploit its superlative attributes, including giant intrinsic charge mobility, record-setting thermal conductivity, and high fracture strength and Young's modulus. Of course, fully exploiting the remarkable properties of graphene requires reliable, large-scale production methods which are non-oxidative and introduce minimal defects, criteria not fully satisfied by current approaches. A major advance in this direction is ionic liquid-assisted exfoliation and dispersion of graphite, leading to the isolation of few- and single-layered graphene sheets with yields two orders of magnitude higher than the earlier liquid-assisted exfoliation approaches using surface energy-matched solvents such as N-methyl-2-pyrrolidone (NMP). In this Minireview, we discuss the emerging use of ionic liquids for the practical exfoliation, dispersion, and modification of graphene nanosheets. These developments lay the foundation for strategies seeking to overcome the many challenges faced by current liquid-phase exfoliation approaches. Early computational and experimental results clearly indicate that these same approaches can readily be extended to inorganic graphene analogues (e.g., BN, MoX2 (X = S, Se, Te), WS2, TaSe2, NbSe2, NiTe2, and Bi2Te3) as well.
Highlights
Considered to be the Holy Grail for physicists and material scientists, the elusive material-graphene is touted to be the future silicon of the microelectronics industry
Consider these facts: (1) ionic liquids (ILs) have a versatile chemistry that can be molecularly tuned in many ways, as opposed to the conventional molecular solvents; see Fig. 2, for an illustration of ILs as a molecular toolkit; (2) a growing number of ILs exist which have essentially negligible vapour pressure and are thermally stable at high temperatures, radiation-resistant, non-flammable, and chemically inert; (3) imidazolium-based ILs have successfully served as dispersion media for single-walled carbon nanotubes[13] and IL-based bucky gels, a gelatinous paste of single-walled carbon nanotubes and ILs,[14] (4) ILs have surface tensions closely matching the surface energy of graphite; (5) ILs can allow homogeneous graphene nanosheets (GNs) dispersions via stabilization against re-aggregation by Coulomb repulsion of the surface charges introduced by the adsorbed IL
This review examines the ability of ILs for practical exfoliation, dispersion, and modification of GNs as well as GO, reduced GO (RGO), and inorganic graphene analogues (IGAs)
Summary
Considered to be the Holy Grail for physicists and material scientists, the elusive material-graphene is touted to be the future silicon of the microelectronics industry. This method is marred by certain drawbacks; primarily the production of poor quality of GNs due to the presence of residual solvents on the surface and the nature of the volatile organics used, which tend to be environmentally toxic With this in mind, ionic liquids (ILs)—molten salts comprising of polyatomic organic or inorganic ions that have melting temperatures below 100 °C—have a number of attractive features indicating that they would rise to this challenge.[12] Consider these facts: (1) ILs have a versatile chemistry that can be molecularly tuned in many ways, as opposed to the conventional molecular solvents; see Fig. 2, for an illustration of ILs as a molecular toolkit; (2) a growing number of ILs exist which have essentially negligible vapour pressure and are thermally stable at high temperatures, radiation-resistant, non-flammable, and chemically inert; (3) imidazolium-based ILs have successfully served as dispersion media for single-walled carbon nanotubes[13] and IL-based bucky gels, a gelatinous paste of single-walled carbon nanotubes and ILs,[14] (4) ILs have surface tensions closely matching the surface energy of graphite; (5) ILs can allow homogeneous GN dispersions via stabilization against re-aggregation by Coulomb repulsion of the surface charges introduced by the adsorbed (intrinsically charged) IL. Computer simulations, complementing and expanding upon experimental findings will be examined as both a screening tool and a method to provide molecular insights into the mechanism of ILs for exfoliation and dispersion
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