A mixed-solvent strategy for efficient exfoliation of inorganic graphene analogues.

Layered two-dimensional (2D) nanomaterials such as graphene are a conceptually new class of materials that offers new access to low-dimensional physics. Besides wellknown graphene, inorganic graphene analogues (IGAs) such as layered transition metal dichalcogenides (e.g., MoS2 and WS2) [5–7] and boron nitride (BN) have been attracting rapidly increasing attention in the past few years. These IGAs were expected to exhibit unique properties and have great potential in applications like transistors, energy storage, thermal conductors, and topological insulators. Moreover, IGAs like MoS2 and WS2 have intrinsic band gap and high mobility, and may even compete with graphene in certain fields. However, investigations on IGAs have been significantly hindered by the practical difficulties in the preparation and assembly of these 2D nanomaterials. Only a few approaches to obtain few-layered IGAs have been reported. Mechanical exfoliation was first used to obtain layered IGAs from their bulk materials. Other approaches that are being explored include chemical synthesis and liquid exfoliation. Coleman et al. recently reported a surfactant-free liquid-exfoliation method which can produce few-layered nanosheets of IGAs dispersed in various organic solvents. Thermodynamic analysis suggested that, because of the high surface energy of IGAs, the best solvents are likely to have high boiling points. Using nonvolatile solvents makes it difficult to process IGAs into devices, due to the difficulties in the removal of solvent and the occurrence of aggregation during the slow solvent evaporation. To date, liquid exfoliation of layered MoS2 and WS2 in volatile solvent has met with very limited success. Herein we demonstrate a versatile and scaleable mixedsolvent strategy for liquid exfoliation of IGAs, including WS2, MoS2, and BN, in volatile solvents. By choosing solvents with appropriate composition, highly stable IGA suspensions can be obtained in low-boiling solvent mixtures, which can then be easily used in further applications. The dispersion of nanomaterials in liquids can be partially predicted by the theory of Hansen solubility parameters (HSP), which is a semi-empirical correlation developed to explain dissolution behavior. Three HSP parameters are used to describe the character of a solvent or material: dD, dP, and dH, which are the dispersive, polar, and hydrogen-bonding solubility parameters, respectively. The dissolution process is one of adaptation between the HSP parameters of solvents and solutes. The HSP distance Ra is used to evaluate the level of adaptation [Eq. (1)].