| Semiconductor photocatalysts have attracted great intrests due to its potential applications in the fields of energy and environment for water splitting into hydrogen and organic pollutants degradation during recent years.. But now, the practical application of most of semiconductor photocatalysts are restricted seriously in two issues:the large band gap and the low efficiency. How to solve these two problems and how to make semiconducting photocatalysts more available are still the main topic in the field.Sodium tantalum oxide (NaTaO3) is a kind of semiconductor, and the perovskite type of NaTaO3has been concerned widely because of its photocatalytic activity. However, the problem of band gap and efficiency still exist in NaTaO3. Firstly, because of the band gap (about4.0eV), it can only respond UV whose energy is less than5%of total energy of the sunlight, so the band gap of NaTaO3restricts the utilization of sunlight. Secondly, the efficiency is too low. Generally speaking, the photocatalytic reaction mainly consists of the following three steps:1, photocatalysts absorbs suitable photons, and then electrons are excited from the valence band to the conduction band. Moreover electron-hole pairs are formed inside the material.2, photo-generated electron-hole pairs separate, and then the electrons and holes migrate to the surface of material.3, when the electrons and holes reach the surface, the oxidation and reduction reaction occurs, such as splitting water into hydrogen or degrading organic pollutants. But the photo-generated electrons and holes recombination occurs both inside and surface of the material, and the recombination reduces the efficiency of the photocatalysts. Therefore, we should reduce the band gap of NaTaO3and restrict the recombination of photo-generated electron-hole to improving its photocatalytic property, and for this purpose a variety of research and attempts have been done. There are many methods to improve the properties of semiconductor photocatalysts, such as elements doping, the formation of solid solution and surface modification, and elements doping have been proved to have very good performance both in experiment and theoretical study. Therefore, a variety of metallic and non-metallic elements doped NaTaO3have been synthesized, and the influence of these elements to visible light absorption efficiency and photocatalytic activity of NaTaO3has been studied.In this work, we studied the stability, electronic structure, and photocatalytic properties of doping and co-doping NaTaO3as well as some reasonable explanation is given for some experimental phenomena. At the same time, the purpose of this article were intended to find a way that not only reduce the band gap of NaTaO3but also improve the efficiency of the photocatalysts by means of elements doping. The dissertation is divided into five chapters. In the first chapter, we introduce the mechanism of photocatalysts as well as the research background and progress of NaTaO3. In the second chapter, we present the density functional theory, and gave a brief description for the first-principles software packages. In the third chapter, the effects of N and La have on electric structure and a photocatalytic property of NaTaO3is given. In the forth chapter, we discuss the electronic structures and related optical properties of Bi/La-doped NaTaO3. In the fifth chapter, we summarized the research contents in this article and pointed out some theoretical problems that need to be solved in the future. The studied contents and main conclusions are listed as follows:(1) The introduction of N could move up the valance band maximum of NaTaO3, and then the red-shift of absorption spectrum is observed. The introduction of La could reduce the effective mass of the electrons and holes, which means La makes electrons and holes migrate faster to the surface of photocatalysts and improves the efficiency of photocatalysts. Moreover, N/La codoped NaTaO3maintaining the both characteristics of N and La, which means not only the band gap is reduced but also the photocatalytic efficiency is improved. And thus N/La codoped NaTaO3can be used as a valuable semiconducting photocatalysts.(2) The introduction of Bi could reduce the band gap of NaTaO3and make NaTaO3absorb visible light, and that has already been observed in experiment. The gap state is appeared in Bi/La codoped NaTaO3, which could also make the absorption spectrum red shift. At the mean time, the characteristics of La still exist which means the photocatalytic efficiency of Bi/La codoped NaTaO3is good. So, Bi/La codoped NaTaO3is also a valuable semiconducting photocatalysts. |
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