Preparation And Electrical Properties Of Hierarchical Bismuth Selenide

Thermoelectric materials, a class of promising functional materials that can convet heat into electricity directly, have attracted researchers`great attention.They refrigeration technology. However, the performance of the thermoelectric materials has not been able to obtain a breakthrough in today’s theory and technical level, which limited the application of these materials. Researchers try to enhance thermoelectric properties by differnt ways. Among these methods, low dimension and complex nanostructure are considered to be a promising ways to improve the performance, and has been confined by experiments. In addition, the complex preparing process and low yield of thermoelectric materials is another aspect limited its application. Meanwhile, traditional techniques usually require harsh experimental conditions, long reaction time and toxic reducing agents or solvents which pose potential environmental and biological risks. Therefore, it is essential to develop environmentally benign approaches for fabricating novel nanomaterials, where utilization of renewable/biodegradable materials and environmentally harmful chemicals. Thus, different natural products and their derivative as surfactants can be applied to fabricate Bi2Se3with various nanostructures. According to the different effect factors, the formation mechanism of different nanostructured bismuth selenide is proposed. Furthermore, the effect of different nanostructures on electrical transport properties of the products has been investigated.Bi2Se3nanosheets have been successfully fabricated through microwave-assisted approach in the presence of ethylene glycol (EG) under1kW microwave power for1minute. The micro-structure and morphology of the as-prepared samples were investigated by XRD, FESEM, TEM, HRTEM, SAED and Raman spectroscopy techniques. Based on controlled experiments, a possible formation mechanism of Bi2Se3nanosheets was proposed. Morerover, the electriccal transport properties of the nanosheets are investigated by measuring the electrical conductivity and the Seebeck coefficient at temperature ranging from298to523K. The maximum power factor can reach157W m-1K-2at523K due to the ultrathin nature of the as-prepared sample.Bi2Se3microrods composed of nanoparticles have been successfully fabricated through self-sacrificial template microwave-assisted method applying ascorbic acid (AA) as a reducing agent and a soluble starch (SS) as surfactant. The structure and morphology of the obtained products were characterized by XRD, FESEM, TEM, HRTEM, SAED and Raman spectroscopy techniques. Based on the time-dependent experiments, a possible formation mechanism was proposed. The electrical transport properties were investigated by measuring the electrical conductivity and the Seebeck coefficient at a temperature range of298to523K and the maximum power factor can be as high as92W m-1K-2at523K.Bi2Se3stacking nanosheets composed of several thin nanosheets have been successfully fabricated through a―green‖method using sodium carboxymethyl cellulose (CMC) as a surfactant. The microstructure and morphology of the obtained product were characterized by XRD, FESEM, TEM, HRTEM, SAED and Raman spectroscopy techniques. Efects of the concentration of KOH and CMC amount on the morphology of the as-prepared samples were investigated. Based on time-resolved experiments and FESEM observations, a self-assembling-fusing process was proposed to explain the formation of the Bi2Se3stacking nanosheets. We found that the power factor could be tuned by energy filtering. The maximum power factor of179W m-1K-2can be obtained for the sample (CMC-3) using CMC3(viscosity,300-800mPa s) as surfactant at544K, which is about14%higher than that of the sample prepared with no CMC as surfactant.Tellurium (Te) doped bismuth selenide (Bi2Se3-xTex) nanosheets have been successfully synthesized by the microwave-assisted method in the presence of ethylene glycol (EG). The obtained products were characterized by XRD, FESEM, TEM, HRTEM, SAED, XPS and Raman spectroscopy techniques. The electrical transport properties of nanosheets were investigated by measuring the electrical conductivity and the Seebeck coefficient at temperatures ranging from298to523K. The power factor of Bi2Se3-xTex nanosheet vary with different doping concentrations of Te, and the maximum power factor can reach178W m-1K-2at523K for Bi2Se2.7Te0.3, indicating the potential application in thermoelectric devices.Bi2Se3microballs composed of many nanoparticles have been successfully fabricated through anion exchange process in the presence of ethylene glycol (EG) solution, applying L-cysteine asa reducing agent and sulfur source, sodium tartrate as a surfactant. Studying the different reaction steps and the effect of L-cysteine and sodium tartrate on the morphology, we reveal the formation mechanism of the Bi2Se3microballs is anion exchange mechanism. The electrical transport properties of the nanosheets were investigated by measuring the electrical conductivity and the Seebeck coefficient at temperatures ranging from298to573K. The power factor values of the Bi2Se3microballs can reach45W m-1K-2at523K, lower than those of other products.