Preparation Of Nanostructured Bi₂Te₃ Based Thermoelectric Materials By Sonoelectrochemistry And Low Temperature Chemical Reduction

Bismuth telluride (Bi2Te3) based alloys are known as the best thermoelectric materials with the highest dimensionless figure of merit close to unity currently available near room temperature. It has been proved by theory and experiments that the thermoelectric performance of nano-structured material could be enhanced due to the quantum-size effects. The figure of merit of thermoelectric materials can be significantly improved if the materials are nanostructured, since the thermal conductivity can be decreased more significantly than the electric conductivity of the materials. Therefore, many preparation methods have been done to make thermoelectric materials nano-sized, such as thin films, nanowires and nano-sized powder. However, it isn't good ideal to synthesize thermoelectric materials by the use of traditional techniques, since those conditions of the preparation are generally quite complex and comparatively harsh such as the high temperature and high pressure. In the present work, two different novel and simple preparation methods, sonoelectrochemistry and low temperature aqueous chemical reduction, had been applied to synthesize nanostructured Bi2Te3 thermoelectric materials. Some important results of the present work are listed as follows.(1) Bi2Te3 nanoparticles with diameters approximately 20 nm were prepared by sonoelectrochemical method using the precursors of Bi(NO3)3 and TeO2 at room temperature. The effect of additive, current density, ultrasound power and time during the sonoelectrochemical synthesis on the morphology, sizes and compositions of Bi2Te3 nanoparticles had been investigated. The as-prepared samples were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectrometer (EDS), X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscope (FESEM).It was found that the morphology and size of particles were greatly affected by additive or by changing ultrasound power and time, but compositions of particles were weakly affected. Nearly stoichiometric Bi2Te3 nanoparticles could be produced by controlling the composition of solution and current density. When the current density is 5 mA/cm2, the Te atom percentage is approximately 60%, mostly approaching the component of Bi2Te3. XRD results showed that the sample exhibited preferred orientation along (015) plane, which was different from that of electrodeposited Bi2Te3 film. The synthesize process included two steps which involved the reduction of Bi3+ and HTeO2+ ions to Bi2Te3 on Ti horn, and dispersion of Bi2Te3 into electrolyte by the cavitation effect of the ultrasound.(2) Single crystalline Bi2Te3 nanorods with diameters of ~30 nm and lengths of ~400 nm were successfully prepared by low temperature chemical reduction. In this process, the solution was kept static at 50℃for 24 h with Bi(NO3)3 and TeO2 as reactants, NaBH4 as reductant, Brij56 as regulator of crystal growth. The structures of Bi2Te3 nanorods were characterized by XRD, X-ray fluorescence (XRF), SEM, transmission electron microscope (TEM), high-resolution transmission electron microscope (HRTEM). Results showed that the temperature of reaction and the concentration of the surfactant Brij56 had important influence on the morphology and size of synthesized sample.