Synthesis And Properties Of Bi₂Se₃ Based Low-dimensional Nanostructures

Thermoelectric materials and technologies that can directly convert waste heat into electrical energy have important application prospects in the energy and environmental fields.The conversion efficiency of thermoelectric devices is mainly determined by the dimensionless figure of merit of thermoelectric materials ZT=S²?T/k,where S represents the Seebeck coefficient,?represents the conductivity,k is the thermal conductivity of the material,and T is the absolute temperature.As conventional thermoelectric materials,Bi₂Se₃ has low performance.This thesis used nanostructuring and doping strategies to promoting its thermoelectric performance,and the main results are as follows:Bi_2-xSb_xSe₃ nanoflowers self-assembled by nanosheets were prepared by solvothermal method.The Bi,Se and Sb sources were Bi?NO₃?₃·5H₂O,SeO₂,and Sb₂O₃,and cetyltrimethylammonium bromide?CTAB?was used as the surfactant.The mixture of nitrogen dimethylformamide?DMF?and oleic acid?OA?with a ratio of2:1 was used as the solvent.The phase and morphology of the samples were analyzed by powder X-ray diffraction?XRD?and scanning electron microscopy?SEM?.The results showed that the prepared samples were pure-phase trigonal Bi₂Se₃ with nanoflower morphology self-assembled by.Energy dispersive spectroscopy and X-ray photoelectron spectroscopy?XPS?were used to determine the composiition and chemical valence of the samples.The results showed that the prepared sample was composed of Sb,Bi,and Se.The chemical valences of Sb and Bi is+3 and Se exhibits-2 valence.The samples were pressed into plates by spark plasma sintering?SPS?for the test of thermoelectric performance.The results show that the Seebeck coefficient of the samples is negative,indicating N-type conductivity.The electrical conductivity of the sample was increased by nearly 13 times compared with that of the bulk material,and the thermal conductivity was reduced to 0.55 WmK^-1.The highest ZT value was 0.55 when the Sb doping content was 0.06 in the synthesized sample.Bi₂Se₃ thin films were prepared by high-vacuum thermal evaporation technique.High-purity Bi₂Se₃ powder?99.99%?was used as the evaporation source,and SiO₂was used as the substrate.By controlling the temperature of the evaporation source,the morphology of Bi2_Se₃ films was tuned from nanosheets to nanorod arrays.The phase analysis of the sample by XRD and the morphological analysis by the SEM show that the morphology and phase of the films were controlled.XPS results show that Bi has a valence of+3 and Se has a valence of-2,and the atomic ratio of Bi and Se is close to 2:3.The Raman scattering pattern shows two vibrational frequency models of Bi₂Se₃ with Raman activity.The analysis of the electrical properties of the samples shows that the power factors of Bi₂Se₃ films increased from 1.1?Wcm^-1K^-2to 1.7?Wcm^-1K^-2 when their morphology vary from nanosheet to nanorod arrays.The above results present some new Bi₂Se₃ nanomaterials and achieve the improvement for thermoelectric performance by the control on morphology and compositions,which is helpful to the research on promoting the thermoelectric performance of other thermoelectric materials.