Synthesis,Characterization,and Performance Of Bismuth-based Chalcogenides Nanomaterials

Renewable energy is important for the sustainable development of society and has been pursued by the government and scientist of the world.As a new renewable energy,thermoelectric,direct conversion energy from "heat" to "electricity”,can solve the issue of energy crisis and has attracted wide attention of scientific and industrial commmity.Bismuth-based chalcogenides,such as:Bi2Te3,Bi2Se3,are important thermoelectric material and has been studied extensively.It has been proved that the nanocrystallization of thermoelectric materials can improve thermoelectric performance effectively.Therefore,synthesis of BiTes and Bi2Se3 nanomaterials has become a key topic.This doctoral dissertation focused on the synthesis and characterization of Bi2Te3 and Bi2Se3 nanostructures.By analysing the effect of reaction temperature,NaOH concentration,surfactant amount,we can control the growth process and morphology evolution to obtain the single crystal hexagonal Bi2Te3 nanoplates,Bi2Se3 nanoplates,BiTe nanotubes with uniform size and morphology.These as-prepared Bi2Te3 nanoplates,Bi2Se3 nanoplates,BiTe nanotubes exhibited excellent thermoelectric performance,superior to their corresponding bulk materials and nanocrystallines.The main results are as follows:1.By analyzing the effect of reaction temperature,NaOH concentration,sur-factant,we can control the growth process and morphology evolution to obtain the single crystal hexagonal monodisperse Bi2Te3 nanoplates with uniform size and morphology via the solvo-thermal method.A probable growth mechanism about the Bi2Te3 single crystal nanoplates is proposed according to the experi-mental results.The as-prepaxed Bi2Te3 nanoplates were hot pressed to measure the thermoelectricity properties.Both the electrical conductivity(?)and Seebeck coefficient(S)in the temperature range of 300-530 K are superior to the Bi2Te3 bulk materials and nanocrystallines.2.By optimizing the Bi source(BiCl3 was replaced with Bi2O3),the Bi2Te3 single-crystal hexagonal nanoplates were synthesized efficiently with a distinct re-duction of reaction time from 36 h to 7 h.We also observed that the Bi2Te3 devel-oped from "Sundial" to "nanoplates" and finally to "Chinese Coin" morphology under different reaction time.A self-repair mechanism was proposed according to the morphology evolution of these three structures.The thermoelectricity perfor-mance measurement of hot pressed as-prepared Bi2Te3 nanoplates showed better electrical conductivity(?)and Seebeck coefficient(S)than that of Bi2Te3 bulk materials and nanocrystallines in the temperature range of 300-530 K.3.By replacing TeO2 with SeO2 only,we can synthesize the Bi2Se3 nanoplates successfully with unchanged others conditions.The effect of reaction temperature,NaOH concentration,surfactant on the morphology of Bi2Se3 nanostructures were discussed systematically.A probable growth mechanism for the monodisperse Bi2Se3 single-crystal hexagonal nanoplates is proposed on the basis of experimental observation.Micro-Raman spectra over the temperature range of 83-603 K were used to investigate the lattice dynamics of as-prepared single Bi2Se3 nanoplates.The first-order temperature coefficients Xi of different Raman modes is calculated to be xA1g1=1-258 × 10-2 cm-1/K,XEg2=-1.385 × 10-2 cm-1/K,XA1g2=-2.363 × 10-2 cm-1/K,respectively.Such a low temperature coefficient may favor to obtain high figure of merit(ZT)and indicate that Bi2Se3 nanoplates were used as excellent candidates of thermoelectric materials.4.1n order to further reduce the thermal conductivity and improve the ZT value,we optimize the growth conditions to synthesize BiTe nanotubes by a simple solvothermal method.To the best of our knowledge,this is the first report on the synthesis of BiTe nanotubes.We discussed the effect of PVP amount on the morphology of BiTe.Results show that the amount of PVP is a key factor to determine the formation of BiTe nanotubes and a minor deviation can destroy the tube-like morphology and stoichiometry of Bi:Te.Micro-Raman scattering spectra over a temperature range of 103-593 K were used to investigate the lattice dynamics of BiTe nanotubes.Both the 2Ag1and 1Eg1 modes shift evidently with reduced temperature.The line shape demonstrates a significant broadening with increased temperature.The temperature coefficient of Eg1,A1g1,Eg2,A1g2 modes were determined to be XEg1=1.85 × 10-5 cm-1/K,XA1g1=1.98 × 10-4 cm-1/K,XEg2=1.09 × 10-4 cm-1/K,XA1g2=2.34 × 10-4 cm-1/K,respectively.Such low temperature coefficient may favor to obtain high figure of merit(ZT)and indicate that BiTe nanotubes were used as excellent thermoelectric materials.