Aqueous Chemical Synthesis and Consolidation of Size-Controlled Bi2Te3 Nanoparticles for Low-Cost and High-Performance Thermoelectric Materials

Bi2Te3 nanoparticles (NPs) were synthesized with controlled mean diameters of 58 nm, 82 nm, and 100 nm using an aqueous chemical reduction, in which ascorbic acid was used instead of the commonly employed toxic reducing agent. In general, organic capping agents remained on the Bi2Te3 NP surfaces, which prevented the sintering of Bi2Te3 NPs and affected their thermoelectric properties. Not only the capping agent, but also water from the synthesis process, remained on the Bi2Te3 NPs even after their consolidation by spark plasma sintering. Consequently, evaporation of the water led to the collapse of sintered Bi2Te3 NPs when heated above 100°C. After the complete removal of the surface impurities and water, the sintered Bi2Te3 NPs became stable. To achieve enhanced thermoelectric properties, a high relative density of ∼ 96% was achieved in the sintered Bi2Te3 NPs without large grain growth by optimizing the sintering temperature and holding time. Subsequently, their thermoelectric properties showed that zT of the sintered Bi2Te3 NPs 100 nm in size is higher (0.41 at 390 K) than those of smaller sizes (58 nm and 82 nm). Finally, the effect of grain size, particle size and density on their thermoelectric properties is discussed.