Precursor effects and formation mechanism of polyol-synthesized thermoelectric Bi₂Te₃

Polyol synthesis offers a controllable and scalable approach for producing high-performance thermoelectric materials such as bismuth telluride (Bi₂Te₃), providing more control over crystal growth and microstructure compared to conventional solid-state methods. The chemical nature of the selected precursors can strongly influence the reaction pathways, phase evolution, and resulting material properties. In this work, two polyol synthesis routes using Bi₂O₃ and Bi(NO₃)₃·5H₂O as bismuth precursors were systematically investigated to evaluate their influence on the structural evolution and thermoelectric performance of Bi₂Te₃. Comparative characterization and transport measurements reveal clear precursor-dependent variations in microstructure and anisotropic charge transport. Despite being undoped, both materials exhibit strong thermoelectric performance with the nitrate-derived sample achieving a peak figure of merit of zT = 1.27 at 432 K, and the oxide-derived material reaching zT = 1.10 at 333 K. Moreover, analysis of the nitrate route revealed the formation of a previously unreported bismuth complex, Bi₃(C₂H₄O₂)₄NO₃. Overall, these findings advance the mechanistic understanding of Bi₂Te₃ formation in polyol synthesis and highlight the importance of precursor selection as a key parameter for tailoring microstructure and optimizing thermoelectric performance.