Preparation And Characterization Of Bi,Te And Their Alloy Nanowires And PEDOT Composite Thermoelectric Films

Nanowires are considered as the most promising thermoelectric materials with large Seebeck coefficients and low thermal conductivity due to the quantum confinement and enhanced boundary scattering of phonons.In order to obtain high-purity and high-quality nanowires,a self-template growth method has been developed in the master thesis.Using nanoparticles as self-templates with the same composition as nanowires,we have successfully synthesized bismuth nanowires,tellurium nanowires and bismuth telluride nanowires through the self-template growth method.The factors influencing the growth of these nanowires were explored.The phase composition,microstructure and growth mechanism were studied with XRD,FESEM and TEM.The preparation processes of these nanowires were optimized.Since the polymer poly（3,4-ethylenedioxythiophene）（PEDOT）has good conductivity,stability and workability,the PEDOT/Bi and PEDOT:PSS/Bi2Te3composite thin films have been prepared via a dip coating method.The morphology and microstructure of these composite films were characterized with SEM and FTIR.The effects of Bi and Bi2Te3 nanowires on the electrical conductivity and Seebeck coefficient of the composite films were investigated.The main results are as follows.（1）The high-purity bismuth nanowires with 40–50 nm in diameter and several tens of micrometers in length were prepared by using Bi seeds as self-templates via the self-template growth method.The synthesis processes of Bi nanowires including the temperature,the reductant and Bi seeds were explored.It was discovered that the nucleation rate of Bi nuclei could be adjusted by precisely controlling the temperature,and the optimized temperature was about 140℃.Selecting an appropriate reductant can help maintain a low nucleation rate of Bi nuclei,and the suitable reductant for synthesizing high-purity Bi nanowires was ethylene glycol（EG）.Using Bi seeds as the templates to provide the centers for the epitaxial growth of Bi nuclei was very useful for the formation of nanowires.The growth direction of Bi nanowires was confirmed to be parallel to[]direction.The evolution process from Bi seeds to nanoparticles,nanoclusters,nanorods,and nanowires was suggested to describe the formation of Bi nanowires.（2）The high-quality tellurium nanowires with a diameter of 40–50 nm and a length of several tens of micrometers were prepared by using Te seeds as self-templates via the self-template growth method.The effects of the temperature,the surfactant and Te seeds on the growth of Te nanowires were investigated.The nucleation rate of Te nuclei can be adjusted by precisely controlling the temperature,and the optimized temperature was about 140℃.It was very important for the formation of nanowires to use Te seeds as the templates because they may provide the centers for the epitaxial growth of Te nuclei.An appropriate surfactant can induce the coordination between Te and PVP chain and reduce the aggregation of Te nuclei.The suitable surfactant for synthesizing Te nanowires with high aspect ratio was PVP K90.The growth direction of Te nanowires is demonstrated to be parallel to[0001]direction.The evolution process from Te seeds to nanoparticles,nanorods,and nanowires was also suggested to describe the formation of Te nanowires.（3）The high-purity bismuth telluride nanowires with a diameter of 50–70 nm and a length of several to sevral tens micrometers were successfully synthesized by using Te nanowires as self-templates through self-template growth method.The experimental results showed that Te nanowire templates could prevent the multicenter growth into spherical nanoparticles of Bi2Te3 crystal nucleus,which played a vital role in the formation of Bi₂Te₃ nanowires.It was discovered that the axis direction of Bi₂Te₃ nanowires was parallel to[]direction.The formation process of Bi₂Te₃nanowires was that Bi nuclei spread and alloyed with Te nanowire templates.（4）The PEDOT:PSS/Bi and PEDOT:PSS/Bi₂Te₃ nanowire composite thin films were prepared by the dip coating method.The effects of the morphology and microstructure of the composite films and the contents of nanowires on the electrical transport properties were studied.The electrical conductivity of PEDOT:PSS thin film is about 42.7 S/cm at room temperature.The electrical conductivity of PEDOT:PSS/Bi and PEDOT:PSS/Bi2Te3 composite thin films first increased and then decreased with the increasing of the contents of Bi and Bi2Te3 nanowires.The maximum electrical conductivity of PEDOT:PSS/Bi composite films reached 95.9S/cm when the content of Bi nanowires was 0.4 wt%,increased by 125%as compared to that of PEDOT:PSS films.The highest electrical conductivity of PEDOT:PSS/Bi₂Te₃ composite films reached 82.6 S/cm when the content of Bi₂Te₃nanowires was 0.6 wt%,increased by 93%as compared to that of PEDOT:PSS films.However,the presence of Bi and Bi₂Te₃ nanowires had almost no influence on the Seebeck coefficient.