Bulk bismuth telluride (Bi2Te3) and other chalcogenide materials have been widely used in thermoelectric applications since the 1950s. Recently, non-trivial topological surface states were discovered on the surfaces of these chalcogenides, providing promising electrical, thermal, and chemical properties. To utilize the unique charge transport, it is important to manufacture two-dimensional (2D) nanosheets with thicknesses of less than 5 nm (i.e., the penetration depth of topological surface states in Bi2Te3). Therefore, there is a need to develop a scalable, inexpensive, and environmentally-friendly process that can produce exfoliated Bi2Te3 2D nanosheets. However, separating 2D nanosheets from thick nanoplatelets is a formidable task. In the Bi2Te3 crystal, five covalently-bonded atomic layers in the order of Te(1)-Bi-Te(2)-Bi-Te(1) form a 1 nm thick charge-neutral quintuple sheet, while the bonding between adjacent quintuple sheets is a much weaker van der Waals interaction. Analogous to the exfoliation of graphite to produce graphene, Bi2Te32D nanosheets can be produced by breaking the weak inter-layer bonding between adjacent quintuple sheets.
Various methods for exfoliating Bi2Te3 have been reported, but solvent-assisted exfoliation by ionic liquid (IL) exfoliants is of particular interest for several reasons. Room temperature ILs, organic salts with melting points below 100 oC, are attractive exfoliants, due to their superior chemical and physical properties (e.g., low vapor pressure, high thermal stability, tunable viscosity, and high ionic conductivity), as well as the potential for solvent recyclability. However, the molecular-level interactions between an IL and Bi2Te3 (or other chalcogenide materials) have not been well characterized. This includes details about the electrostatic interactions between individual sites, the IL structure at the Bi2Te3surface, and the initial steps in the exfoliation mechanism. If this fundamental information is obtained, it would potentially enable the development of optimal exfoliation solvents and scalable manufacturing processes.
In this work, we have performed thorough molecular-level analyses of the exfoliation of Bi2Te3 in an ionic liquid solvent, in order to help understand the experimental exfoliation process. The ionic liquid 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) is used to exfoliate Bi2Te3 nanoplatelets. In both experiments and in molecular dynamics (MD) simulations, the Bi2Te3 nanoplatelets yield a stable dispersion of 2D nanosheets in the IL solvent, and our MD simulations provide molecular-level insight into the kinetics and thermodynamics of the exfoliation process. An analysis of the dynamics of the Bi2Te3 during exfoliation indicates that the relative translation (sliding apart) of adjacent layers caused by IL-induced forces plays an important role in the exfoliation mechanism. Moreover, an evaluation of the MD trajectories and electrostatic interactions indicates that the [C4mim]+ cation is primarily responsible for initiating the Bi2Te3 layer sliding and separation, while the Cl- anion is less active. Overall, our combined experimental and computational investigation highlights the effectiveness of IL-assisted exfoliation, and the underlying molecular-level insights should accelerate the development of future exfoliation techniques for producing 2D chalcogenide materials. Currently, additional IL combinations are being explored in order to enhance the exfoliation kinetics and thermodynamic stability.