(757a) Molecular Dynamics Simulation of the Thermodynamics of Bismuth Telluride Exfoliation in Ionic Liquids

Authors: 
Abedini, A., The University of Alabama
Ludwig, T., The University of Alabama
Zhang, Z., University of Alabama
Turner, C. H., University of Alabama
Bismuth Telluride (Bi2Te3) is a topological insulator that possesses unique thermoelectric properties at near room temperatures, and it is a valuable material for waste heat recovery and other niche applications.1 The crystal structure of Bi2Te3 contains quintuple sheets of Te1-Bi-Te2-Bi-Te3 bonded covalently with a thickness of approximately 1 nm. The bulk Bi2Te3 material is composed of these quintuple sheets stacked together with only dispersion interactions acting between the neighboring nanosheets. The thermoelectric performance of Bi2Te3 can be improved by separating the bulk material into two-dimensional (2D) nanosheets. When these materials are exfoliated into very thin layers, increased phonon scattering decreases the thermal conductivity, while their high crystallinity enhances their electrical conductivity. As a result, these materials possess high crystallinity, unaffected topological surface states, and high anisotropy, providing valuable thermoelectric properties.2, 3

In order to synthesize Bi2Te3 nanosheets, several different methods have been reported, such as vapor-phase decomposition4 and liquid-phase synthesis.1 Liquid-phase synthesis is preferred because of its low cost and simplicity. Ionic liquids (ILs) have been proposed as environmentally-friendly solvents and have previously been used to exfoliate layered materials such as graphene5 and Bi2Te3.6 ILs provide some unique advantages (versus volatile organic solvents), due to their low vapor pressure, high thermal stability, high ionic conductivity, and low melting points.

Molecular Dynamics (MD) simulations have been widely used to provide atomistic insight into the thermophysical properties of ILs and their interfacial interactions.7 In addition, MD simulations have been successfully used to model the exfoliation of bilayer graphene,5 hexagonal boron nitride (h-BN),8 and recently Bi2Te3.6 However, there is still a considerable lack of knowledge about how to design the most effective IL for exfoliating 2D nanomaterials. There must be a balance of interactions that allow an IL solvent to initialize the exfoliation mechanism and stabilize the individual nanosheets; this mechanism involves both dispersion and electrostatic interactions. For instance, while Bi2Te3 nanosheets are charge-neutral, Te atoms cover the basal plane of the quintuple sheets, which presents a negatively charged surface. Currently, these solid-liquid interfacial interactions between IL solvents and chalcogenide materials are not well characterized, and a clearer molecular-level description is needed.

In our study, we use MD simulations to model the initial exfoliation mechanisms of Bi2Te3 in several different imidazolium-based ILs. These imidazolium-based ILs have surface energies estimated to be 63-83 mJ/m2,9 which provide a commensurate match with the estimated surface energy of Bi2Te3 (63-81 mJ/m2)6. Eight different ILs are modeled, containing [C4mim+] and [C2mim+] as cations and [Tf2N-], [PF6-], [Br-], and [Cl-] as anions, using steered molecular dynamics (SMD) and the weighted histogram analysis method (WHAM). With these different solvents, several different exfoliation mechanisms are explored (peeling, pulling, and sliding), and the thermodynamics of the different exfoliation routes are quantified. Our simulations indicate that the peeling mechanism is the most probable mechanism for exfoliation, due to the minimal amount of required peeling force and from an analysis of the potential mean force (PMF). The exfoliation analysis is also complemented by an analysis of the fluid structure (density profiles and angle distributions) of each IL on the Bi2Te3 surface. Overall, the [C4mim+] [Tf2N-] solvent provides the lowest barrier for exfoliation, and its effectiveness can be traced to its unique cation and anion interactions with the surface of the Bi2Te3 layers.

References


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