Effect of Ionic Liquid Structure and Dynamics on Liquid-Assisted Exfoliation of Bismuth Telluride: A Molecular Dynamics Study

Bismuth telluride (Bi₂Te₃) is a thermoelectric material with a layered crystal structure.  Exfoliating bulk Bi₂Te₃ to produce 2D nanosheets is of great research interest due to the favorable properties of these nanosheets, which can potentially be harnessed for use in diverse applications such as thermoelectric energy capture, electronics, and heterogeneous catalysis.  The main barrier to scientific and commercial application of these 2D nanosheets is the lack of a scalable, economically feasible, environmentally benign exfoliation process.  One promising process is ionic liquid (IL) assisted exfoliation, which has been successfully experimentally tested in various materials, using various ILs (primarily alkylimidazolium based), at small scales.  ILs display diverse physical properties, which depend on the selection of both the anion and the cation.  This results in a wide variety of possible ILs, each with unique molecular (ion size, shape, charge distribution, and functional groups) and macroscopic (viscosity, density, melting point) characteristics.  Therefore, testing a great deal of ILs experimentally is inefficient and expensive.  A theoretical understanding of the molecular-level dynamics of the process could greatly accelerate the experimental search for an effective exfoliation process by providing guidelines for which features are favorable for exfoliation of a given material.  In this project, we apply molecular dynamics (MD) simulations to understand both the mechanism of IL-assisted exfoliation of Bi₂Te₃ and the IL solvation layer structure, using several alkylimidazolium-based ILs as solvents.  Results indicate that the relative sliding of adjacent layers caused by IL-induced forces plays an important role in initiating exfoliation.  Furthermore, the cation is primarily responsible for initiating layer sliding and separation, while the anion is less active.  The solvation layer structure is strongly ordered, with an oscillating cation-anion density profile and an oscillating charge density profile.  This strongly ordered solvation layer structure is hypothesized to stabilize dispersed 2D nanosheets.  Current work focuses on further examining effects of the solvation-layer structure and how it depends on cation and anion selection, gathering data on the energetics of various exfoliation mechanisms, and extending these results to more ionic liquid pairs.  These simulation and analysis results should accelerate the experimental search for an effective exfoliation process for the manufacture of Bi₂Te₃ 2D nanosheets.