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

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 Bi₂Te_3.⁶ 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 Bi₂Te_3.⁶ 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

(1) Takashiri, M.; Kai, S.; Wada, K.; Takasugi, S.; Tomita, K. Role of stirring assist during solvothermal synthesis for preparing single-crystal bismuth telluride hexagonal nanoplates. Materials Chemistry and Physics 2016, 173, 213-218.

(2) Liang, Y.; Wang, W.; Zeng, B.; Zhang, G.; He, Q.; Fu, J. Influence of NaOH on the formation and morphology of Bi2Te3 nanostructures in a solvothermal process: From hexagonal nanoplates to nanorings. Materials Chemistry and Physics 2011, 129, 90-98.

(3) Li, Z.; Teng, R.; Zheng, S.; Zhang, Y.; Huang, T.; Lu, G. Single-crystalline Bi2Te3 nanosheets with uniform morphology via a simple, efficient, and high-yield microwave-assisted synthesis. Journal of Crystal Growth 2014, 406, 104-110.

(4) Guo, L.; Ivey, B. C.; Aglan, A.; Tang, C.; Song, J.; Turner, C. H.; Frazier, R. M.; Gupta, A.; Wang, H. T. Vapor Phase Growth of Bismuth Telluride Nanoplatelets on Flexible Polyimide Films. ECS Solid State Letters 2012, 2, P19-P21.

(5) Kamath, G.; Baker, G. A. In silico free energy predictions for ionic liquid-assisted exfoliation of a graphene bilayer into individual graphene nanosheets. Phys Chem Chem Phys 2012, 14, 7929-7933.

(6) Ludwig, T.; Guo, L.; McCrary, P.; Zhang, Z.; Gordon, H.; Quan, H.; Stanton, M.; Frazier, R. M.; Rogers, R. D.; Wang, H. T.; Turner, C. H. Mechanism of bismuth telluride exfoliation in an ionic liquid solvent. Langmuir 2015, 31, 3644-3652.

(7) Lopes, J. N. C.; Deschamps, J.; Padua, A. A. H. Modeling ionic liquids using a systematic all-atom force field. Journal of Physical Chemistry B 2004, 108, 2038-2047.

(8) Kamath, G.; Baker, G. A. Are ionic liquids suitable media for boron nitride exfoliation and dispersion? Insight via molecular dynamics. RSC Advances 2013, 3, 8197.

(9) Morishita, T.; Okamoto, H.; Katagiri, Y.; Matsushita, M.; Fukumori, K. A high-yield ionic liquid-promoted synthesis of boron nitride nanosheets by direct exfoliation. Chem Commun 2015, 51, 12068-12071.