Synthesis of High Purity Bismuth Telluride (Bi₂Te₃) Nanostructures by Co-precipitation Process and Annealing Under Hydrazine Vapor: Structural and Thermoelectric Studies

High-purity bismuth telluride (Bi₂Te₃) nanostructures have been synthesized by a chemical reduction method with two processes: (1) co-precipitation by NaBH₄ and (2) annealing under hydrazine vapor and nitrogen gas atmosphere at T = 300°C. Based on the use of nitric acid (HNO₃) in the preparation of precursor solutions from primary materials, two syntheses were designed to prepare the Bi₂Te₃ nanostructures. Then, the structural, optical, and thermoelectric properties and the surface morphology of the obtained nanoparticles from the two syntheses were investigated. The X-ray diffraction analysis results revealed the formation of Bi₂Te₃ nanostructures in both syntheses. The values of the crystallite size and micro-strains of the Bi₂Te₃ nanostructure were obtained from the average Williamson–Hall (Langford) analysis and Scherrer methods. Field-emission scanning electron microscopy micrographs of the Bi₂Te₃ nanostructures showed hexagonal nanocrystals with different nano-sizes. Energy dispersive x-ray spectroscopy analysis confirmed the formation of the Bi₂Te₃ compound and the high purity of the synthesized nanostructures. The Bi₂Te₃ nanostructure produced in the second synthesis (HNO₃-Bi&Te) had a higher Seebeck coefficient (56.3 \(\mu \mathrm{V}/\mathrm{K}\)) and lower resistance (9 \(\mathrm{k}{\Omega }\)). The amount of ZT in this optimal synthesis (HNO₃-Bi&Te) increased by 6.5 times. The results of the Seebeck experiment mostly showed p-type conductivity for the two syntheses. Fourier-transform infrared spectroscopy analysis showed very low absorption of the NH₂ group on the Bi₂Te₃ nanostructured surface. This study showed that annealing in the presence of hydrazine vapor has major influences on the formation of the bismuth telluride binary phase.