| In recent years, due to the energy shortages and environmental problems, it is urgent to explore new sources of clean energy. In terms of the huge energy capacity, the environ-mental friendly property and the renewable feature, hydrogen is a promising fuel source which is attractive as an alternative to fossil fuels. However, the amount of pure hydrogen existed in nature is low due to its low density and high chemical activity, so it must come from other resources, such as water. The possible way to get hydrogen from water is the photocatalytic technology by semiconductor.The photocatalytic technology, which has attracted considerable interests from physicists, chemists and materials scientists all over the world, not only splits water into hydrogen and oxygen but also decomposes organic matters into carbon dioxide and wa-ter. Therefore, the photocatalytic technology has become an effective way to solve the energy shortage and environmental issues. SrTiO3, as a photocatalyst, can split water into hydrogen and oxygen by the radiation of photons with the energy of the band gap.Unfortunately, the band gap of SrTiO3is so wide (3.2eV) that only the ultraviolet light can be absorbed, which is only about5%of the solar energy. For improving the solar energy efficiency, the band structure has to be modified so that the visible light, which is about43%of the solar energy, can be absorbed. Extensive investigations have been carried out in the effort to extend the light absorption edge towards the visible region. An efficient way is the doping of foreign elements to reduce the band gap, or create new donor and acceptor levels in the gap. For example, Cr-doping can reduce the absorption edge to the visible light region due to the introduction of the gap levels; N-La codoping has high activity under the visible light illumination and can prevent the creation of oxygen vacancies as the La3+and N3-ions keep the charge balance.In this thesis, the band structure of SrTiO3with Nb-C-Nb codoping is studied by the first-principle calculations. We also studied the electronic structure of the TiO2-terminated SrTiO3(100) surface with and without Cr doping. Furthermore, the changes of energy bands under the adsorption of water molecules on SrTiO3(100) surface with and without Cr doping were investigated.In Chapter1, we introduce the general mechanisms of the photocatalytic technol-ogy, review the progress of the research on SrTiO3photocatalyst, and outline our main work and the content of this thesis. In chapter2, we introduce the theoretical back-ground of this thesis, including the Hartree-Fock approximation, the density-functional theory, the energy band structure theory and some program packages employed in our research. In Chapter3, we detailed study the electronic structure of SrTiO3with C-and Nb-monodoping, C-Nb codoping and Nb-C-Nb codoping. The doping changes the band structure of SrTiO3and the properties of the optical absorption. In chapter4, we study the geometry structure and electronic structure of the TiO2-terminated (001) surface of SrTiO3with and without Cr doping. In Chapter5, we study the geometry structure and electronic structure of the adsorption of water molecule onto the TiO2-terminated (100) surface of SrTiO3. In chapter6, we summary the total topic and look forward to the future of the first-principles studied on the photocatalytic topic.The main work and content in this thesis are listed as follows:1. Our study on the C-doped SrTiO3by the first principle calculations shows that the band gap is narrower than that of the undoped structure. When C is doped, there are three subbands shifted from the valance band to the band gap, because the energy of C2p states is higher than that of O2p states. Since the C4-needs two more electrons than the O2-does, the subbands are partially occupied which can act as the recombination centers of electron-hole pairs. Our formation entropy calculation shows that the C-doping is difficult to form due to its p-type doping nature. We also find the Nb doping does not affect the band structure, but shifts the Fermi level into the conduction band as the Nb5+releases one more electron than the Ti4+does. Therefore, we employ the Nb and C codoping to compensate the electrons. We also find that the Nb-C-Nb codoping can reduce the energy cost of the C doping due to the larger attractive interaction between Nb5+and C4-2. When a TiO2-terminated (100) surface on SrTiO3is cleaved, the first TiO2plane relaxes towards inside while the second SrO plane relaxes towards outside. The different relaxation distance between the O and Ti atoms in the surface results in the surface rumpling. The electronic band structure calculations show that there is a split of upmost valance band which is about0.72eV, and this dispersion curve consists of the2px and2py states of the surface oxygen atom. Therefore, this band can be considered as a surface band. The electron density difference shows that the Sr atoms in the second layer are polarized, the electrons shift inward. For Cr doped system, the Cr atom relaxes inward more than that of the Ti atom due to the stronger interaction between the Cr and O. The Sr atoms in the second layer are also polarized, and the electrons of Sr atoms shift away from the Cr atom due to the more electrons of Cr than that of Ti. When Cr is doped, there are three gap levels introduced lying near the bottom of the conduction band. The Fermi level is pinned at the gap levels. This results in the visible light absorption.3. The water molecule adsorbed on the SrTiO3is an exothermic process. From the total energy calculations we found that the O of water is preferable to adsorbs on the top site of the Ti atom. The1b1orbital is the closest one to the Fermi level and interacts strongly with Ti3d states. Since the1b1orbital is perpendicular to the water molecule plane, the water molecule prefers to parallel to the surface in order to maximize the interaction with Ti3d states. The interaction between the1b1states and the Ti3d states results the1b1state broadened and spitted to create more levels. The energies of the new states are higher than the energy of1b1state while lower than the Fermi level. This means that there are electrons transferred from the lower states to the higher states. A careful inspection of Kohn-Sham orbitals reveals that the new states are O p-like orbitals and most of them are perpendicular to the surface. Because the plane of water molecule has oblique orientation, the states in the regions between the two H-O bond increase. When Cr is doped, we found that the water molecule is preferable to adsorb to the Ti atom which is far away to the Cr atom. From the adsorbed energy calculations, we deduce that the Cr dopant can promote the water adsorption. The other point is that the Cr dopant can shift the light absorption edge to the visible region. |
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