Study On The Structural And Electronic Properties Of Bismuth-doped Nano-NaTaO₃

Hydrogen is of great interest as an ideal and clean source of energy in the future,and concerned by most scientists.Water splitting using photocatalysts is an important technique for hydrogen production.Photo splitting of water to produce hydrogen is a perfect project of "Sunlight Economy".Research and exploitation of new photo-catalyst materials with catalytic activity for hydrogen evolution reaction (HER) is important and effective to reduce the loss of energy.It has been reported that the perovskite-type sodium tantalates,NaTaO₃ show reasonable activities for water splitting into H₂ and O₂ in a stoichiometric ratio under UV irradiation,which has become one of the most active research fields in recent years.In order to improve its photocatalytic ability and utilize solar energy more efficiently,sodium tantalates is doped with bismuth.Although there have been many reports on doped sodium tantalates,most of them deal with the degrading performance tests and the doping mechanism is based on hypothesis rather than science evidence.In essential,the electronic structure of sodium tantalates is the predominant property which governs the photocatalytic ability.However,only a very limited research have been concentrated on this subject.Therefore,it is necessary to obtain the detailed information about the elecronic structures of Bi-doped sodium tantalates. This thesis presents the study of structural,optical and electronic properties of Bi-doped sodium tantalates.Based on First Principles,the band structures of Bi-doped sodium tantalates are calculated and the model of electronic structure is also put forward.The emphasis is placed on the experimental and theoretical research of the electronic structure.Bi-doped sodium tantalates are prepared using modified hydrothermal method. The micro-structure and morphology of the doped samples are characterized by using X-ray Diffraction(XRD),transmission electron microscopy(TEM) and high resolution TEM.The result shows that Bi-doping makes the(110) and(112) peaks in the XRD patterns of samples shift toward the decreased 2θangles and broadened. The lattice distortion is introdued into the sodium tantalates cell and the grain growth is supressed according to the enlarged lattice parameters and reduced grainsizes calculated from XRD patterns.Combined XRD and HRTEM analysis,it is possible that Bi ion enters the NaTaO₃ lattice substitutionally and some amorphous material exists in the doped sample.The optical properties of Bi-doped sodium tantalates are determined by UV-VIS Diffuse Reflection Spectrum(UV-VIS) and Photoluminescence Spectroscopy(PL). The result reveals that the absorption edges of Bi-doped sodium tantalates show red-shift and the energy gap become smaller with the concentration of Bi.The PL of Bi-doped sodium tantalates samples show difference in the intensity and no new fluorescence of Bi compared with undoped sodium tantalates.The photodegrated of sodium tantalates and Bi-doped sodium tantalates are determined by degradationg methylene blue.The results showed nano sodium tantalates have strong photocatalyst activity,effected by the concentration of Bi.Based on first principles,the electronic structures of Bi-doped sodium tantalate are calculated.The result shows that Bi-doping influnces the energy band structure,the densities of states and electronic properties of sodium tantalates and the energy gap of Bi-doped sodium tantalates become smaller with the concentration of Bi compared with the undoped sodium tantalates.