Ionic Liquid-Based Low-Temperature Synthesis of Phase-Pure Tetradymite-Type Materials and Their Thermoelectric Properties

Abstract

Phase-pure crystalline Bi₂Se₃ and Bi₂Te₃ nanoparticles are formed in reactions of [C₄C₁Im]₃[Bi₃I₁₂] (C₄C₁Im = 1-butyl-3-methylimidazolium) with [C₄C₁Pyr][ESiMe₃] (E = Se or Te; C₄C₁Pyr = 1-butyl-1-methylpyrrolidinium) in the ionic liquid (IL) [C₄C₁Im]I. The resulting crystalline tetradymite-type nanoparticles exhibit stoichiometric Bi:E (E = Se or Te) molar ratios (2:3). Because all synthetic steps were performed under strict inert gas conditions, the surfaces of the Bi₂Se₃ and Bi₂Te₃ nanoparticles are free of metal oxide species. As proven by infrared and X-ray photoelectron spectroscopy analyses, the nanoparticle surfaces reveal only minor organic contamination from solvent residues ([C₄C₁Im]I). The nanomaterials show high Seebeck coefficients of -124 μV K^-1 (Bi₂Se₃) and -155 μV K^-1 (Bi₂Te₃) and feature high electrical conductivities (328 and 946 S cm^-1, respectively) at the highest tested temperature (240 °C). The corresponding thermal conductivities (0.8 and 2.3 W m^-1 K^-1, respectively, at 30 °C) are comparable to those of single crystals and recently reported ab initio calculations, which is in remarkable contrast to typical findings of nanograined bulk materials obtained from compacted nanoparticles. These findings emphasize the low level of impurities, surface contamination, and, in general, defects produced by the synthetic approach reported here. The figure of merit in the in-plane direction of the compacted pellets reached peak values 0.45 for Bi₂Se₃ and 0.4 for Bi₂Te₃.