(476e) Simulation of Bismuth Telluride Exfoliation in an Ionic Liquid Solvent

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.