| At present, two prominent issues which modern sociaties face are the energy crisis and environmental pollutions that are in need to be solved urgently. Therefore, upon to these issues, the quest to develop and utilize the energy resources of noval and renewable features without harmful effect has been considered as an important task. Due to its potential advantage, photocatalysis has attracted more and more attention in recent years. The traditional semiconductor materials, i.e., TiO2and SrTiO3, have become typical candidate materials due to their low cost, chemical stability, innocuity, and thus have been widely used as photocatalysis. However, because of their wide band gap of～3.2eV, TiO2and SrTiO3can only absorb ultraviolet (UV) light with wave length<387nm. The UV light corresponds to about a fraction of5%across of solar energy, leading to very limited efficiency of optical absorption, and thus the wide band gap restricts their practical applications as photocatalysts. Thus, to further improve their photocatalytic activity efficiency by extending the light response to the visible portion, which accounts for43%of solar energy, is the a hot topic for studies on photocatalysis. Generally, doping with foreign elements is a major approach to narrow band gaps so as to enhance the visible light absorption efficiency. Experimentally, the researchers synthesize metal-and/or nonmetal-doped TiO2and SrTiO3with physical or chemical processes, and then investigate the effects of doping on the photocatalytic activity. It should be noted that, only visible light absorption does not guarantee satisfactory photocatalytic activity. The semiconductor photocatalysis process includes three major steps:(a) photogeneration of electron hole pairs after absorption of photons in photocatalysts,(b) separation of photogenerated carrier pairs, and consequent migration to the photocatalyst surface under the action of in-build field or diffusion,(c) redox reaction through electrons or holes transfer from surface to adsorbates. In regard to process (b), the recombination of photogenerated electrons and holes is inevitable, which limits the photocatalytic activity. The defect states induced by mono-doping can lead to the absorption of visible light. However, they may also act as recombination centers depressing the transfer of carriers to material surface and accelerating the photogenerated electron and hole to recombine, which may even result in a much lower photocatalytic activity than that of the corresponding defect-free material. Compared with mono-doping, codoping with different elements can not only enhance the defect concentrations and the stability, but also depress the recombination of photogenerated carriers, and thus improve the photocatalytic activity. In addition, transition metal deposition on oxides photocatalyst surface, regarded as one of the most effective strategies to enhance the photocatalytic efficiency, has also received a lot of attention due to the modification of the electronic structures and decreasing of the carriers recombination. Furthermore, the morphology and dimension of materials can also have important effects on photocatalytic behaviors. In three-dimensional case, besides doping, it is also an important issue in the fields of photocatalysis to search for new types of dopant-free materials (e.g., SrNbO3) with desired band structure for visible light photocatalytic activity. In two-dimensional case (surface), the photo-induced redox reactions occur at the surface of a catalyst. Experimentally, design and preparation of activity surface is an important issue. Theoretically, the structural properties and photocatalytic properties of clean (modified) active surface become a hot research topic. In one-dimensional case, i.e., TiO2nanotubes, because of its unique geometrical structure and electronic properties, have been widely used in the fields of photocatalysis and sensitized dye solar cell. However, the wide band gap also limits its response to the visible light. Therefore, it is necessary to modify the electronic structure of TiO2nanotubes to enhance its photocatalytic activity.In this dissertation, to understand the related mechanism of photocatalytic materials mentioned above, we have investigated the geometric and electronic structure of defect-free, metal and/or nonmetal doped, and surface decorated semiconductor photocatalytic materials, and also analyzed the relations between geometric structure and electronic structure and photocatalytic properties. The dissertation contains seven chapters. In the first chapter, we briefly present the background and research progress of several semiconductor photocatalysts in the fields of photocatalysis. In the second chapter, we introduce the density functional theory and several codes employed in the theoretical simulations. In the third chapter, we discuss the relation between electronic structure, optical transition matrix element and optical absorption of new type photocatalytic materials (SrNbO3, SrVO3and CaVO3), and present some noval ideas in the mechanism of photocatalysis. In the fourth chapter, the relationship between geometric, electronic properties and photocatalytic properties of Ag doped, Nb doped, and Ag/Nb codoped SrTiO3are studied in detail. In the fifth chapter, we investigate the structure, stability, and electronic properties of Ag incorporated TiO2(001) surface with high activity, and explain in detail the effect of Ag on photocatalytic activity of the (001) surface. In the sixth chapter, we study the photocatalytic properties of TiO2nanotube and the charge transfer between Au atom and N atom, and analyze the N/Au synergistic photocatalytic mechanism. In the seventh chapter, we summarize the contents in the dissertation, and present some open issues to be discussed in this field and future work. The main results and conclusions of this dissertation are summarized as follows:(1) Electronic structure and optical transition of three d1metallic oxides SrNbO3, SrVO3, and CaVO3were theoretically investigated employing conventional density functional theory (DFT) and partially self-consistent GW calculations. To evaluate the possibility of visible light absorption, the matrix elements for direct transitions between band edge states were studied. Our results indicated that among the three inversion symmetry structures, electron direct transition in visible light region can only occur in SrNbO3, which is ascribed to different parity of band edge wavefunctions due to the mixing of Sr d states with Nb eg states. In addition, the effective mass of photogenerated carriers in SrNbO3with isotropic characteristic is the smallest, which implies that the photogenerated carriers can transfer to the surface reaction sites more easily with less recombination. Therefore, SrNbO3should be of better photocatalytic performance. The present work should be beneficial to exploring the series of metallic perovskite photocatalysts.(2) Besides TiO2, SrTiO3(STO) is also one typical and effective photocatalytic material. STO is a wide band gap semiconductor similar to TiO2. Extending the light absorption to visible light region has also attracted lots of attention in enhancement of photocatalytic efficiency. Employing density functional theory, we studied the Ag-doped, Nb-doped and Ag/Nb-doped SrTiO3. The results indicated that the Ag4d states in the Ag-doped SrTiO3mainly locate at the top of valance band, and the hybridization with O2p narrows the band gap, which can improve the visible light photoactivity. In order to keep electrical neutrality, the Ag/Nb co-doped SrTiO3has been investigated. The results suggested that the introduction of Nb favors the incorporation of Ag and the band gap does not change sensibly, which is in agreement with the experimental observation.(3) The incorporation of Ag on (001) surface of anatase TiO2was systematically investigated by means of density functional theory to understand the Ag effects on the electronic structure and photocatalytic properties in Ag/TiO2composites. Several possible adsorptional, substitutional and interstitial sites with two different Ag concentrations at surface and subsurface layers were examined. Our results on the stability of various Ag-incorporated (001) surfaces indicated that the adsorptional site is favorable regardless of the oxygen conditions, while the substitutional site becomes more stable under the oxygen-rich condition. However, it is difficult to incorporate Ag onto the surface with high concentration, especially for substitutional sites in the limited range of oxygen chemical potential. The adsorption of Ag introduces gap states near or below the conduction band minimum and the Fermi level locates next to or merges in the conduction band, which can act as photo-generated electron trap centers and inhibit the recombination of electron-hole pairs. The electron transitions from the impurity levels to the levels above the Fermi level may be responsible for the small visible light absorption peak observed in experiment. Substitutional Ag introduces some localized gap states, while the Fermi level is pinned near the top of valence band, and the impurity states can trap the hole to suppress the recombination of photo-generated carriers. For the interstitial Ag in surface, the Fermi level locates at the bottom of conduction band, and the partially occupied states may also act as electron trap centers improving the photocatalytic efficiency.(4) The structural and electronic properties of N-doped, Au-adsorbed, and Au/N co-implanted TiO2nanotubes (NTs) were investigated by performing first-principle density functional theory calculations. For all the implanted configurations, the radius and bond distance do not change significantly compared to the pristine NTs. Our results indicated that the introduction of N into NTs is in favor of Au incorporation. It was found that Au pre-adsorption on the NTs can also enhance the N concentration in NTs. The synergistic stability can be probably attributed to the charge transfer between Au and N atoms. In co-implanted configurations, the empty N2p gap states are occupied by one electron from Au5s states. Thus, the associated electron transition among the valence band, the conduction band and the gap states results in the red-shift of light absorption. In addition, the occupation of N2p states can effectively decrease the photo-generated carrier combination. Therefore, the Au/N implanted NTs should be regarded as a promising photocatalytic material within visible light region.In this dissertation, we have investigated geometrical structure, stability and electronic properties of doped (or novel) three-dimensional (bulk), modified two-dimensional (surface) and co-implanted one-dimensional (nanotubes) materials. The relationship between the geometrical structure, electronic structure and photocatalytic properties was explored in detail. The critical influence of defects and impurities on the optical absorption and the role of synergistic interaction in photocatalytic activity were demonstrated. We also presented some possible strategies to enhance photocatalytic performance in defect-free, metal and/or nonmetal incorporated materials. Our results should provide some theoretical guidance for design and synthesis of efficient photocatalytic materials. |
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