| Photocatalysis is catching more and more attention in the field of environmental decontamination and renewable clean energy production. Among the heterogeneous photocatalysts,TiO2is the most widely used because of its nontoxicity, easy availability and stability. However, due to the high recombination rate of photogenerated electron-hole pairs and poor visible light absorption [TiO2is only responsive to ultraviolet irradiation that only accounts for a small proportion of sunlight (ca.4%)], the quantum efficiency of TiO2is very low, which essentially limits its large-scale practical application. Plenty of approaches have been proposed to promote the visible light driven photocatalytic activity, among which the non-metal-doping, such as C-or N-doping, was extensively studied and demonstrated to be an effective way. Recently, it was reported in "Science2011,331,746" that the hydrogenation can essentially improve the sunlight driven photocatalytic property of TiO2. For water splitting under sunlight, the hydrogen yield of hydrogenated TiO2is about two orders higher than those of most photocatalysts, and the solar energy conversion efficiency reaches to24%. This approach supplies novel ideas for the development of photocatalysis, and the related theoretical studies need to be carried out continuously. In addition to TiO2, many other photocatalytic materials were also developed. For example, the sillenite Bi12GeO20, Bi12SiO20and Bi12TiO20present excellent photocatalytic activity for degradation of organic pollutants, and the Cd1-xZnxS solid solution shows better activity for water splitting.In this dissertation, we studied the stability, optical absorption and photocatalytic properties of the non-mental-doped TiO2, hydrogenated TiO2, sillenite Bi-based materials and Cd1-xZnxS solid solution. We gave some reasonable explanations for some important experimental phenomena, proposed some novel ideas to improve the photocatalytic properties, and unraveled some mistakes in people's understandings of previous experimental and theoretical studies. The dissertation is divided into five chapters. In the first chapter, we briefly present the mechanism and development of photocatalysis and the main content of this dissertation. In the second chapter, we introduced the density functional theory and gave a brief description for the first-principles software packages. In the third chapter, we elaborated the electronic structures and related properties of non-mental-doped, hydrogenated and oxygen deficient TiO2, and in the fourth chapter, we elucidated the electronic structures and related properties of sillenite Bi-based materials and Cd1-xZnxS solid solution. In the fifth chapter, we summarized the research contents and innovations in this dissertation and pointed out some main problems that need to be solved urgently as well as the further research directions. The main research work and contents are listed as follows:(1) The origin of visible light absorption and photocatalytic activity of nonmetal doped anatase TiO2were investigated in details in this work based on density functional theory calculations. Our results indicate that the electronegativity is of considerable impact on the band structures, which determines the relative positions of impurity states induced by the doping species, and further influences the optical absorption and photocatalytic activities of doped TiO2. The effect of charge balance on the electronic structure was also discussed, and it was found that the charge-balance structures may be more efficient for visible light photocatalytic activities. In addition, the edge positions of conduction band and valence band, which determine the ability of a semiconductor to transfer photoexcited electrons to species adsorbed on its surface, were predicted as well. The results may provide a reference to further experimental studies.(2) The conflict of understandings on experimental results about chemical and optical properties of C-doped TiO2, which has been overlooked for a long time but is essential for studies of the basic properties of this material, is unraveled. It is shown that in anatase TiO2the doped C and O atoms can easily couple with each other at typical synthesis conditions due to the large binding energy and small energy barrier, while in rutile phase, the coupling can hardly occur. The characteristics of the structures are elaborated in detail, which provides insights into the chemical, optical, and magnetic properties of C-doped TiO2.(3) Disorder-engineered nanophase anatase TiO2through hydrogenation has been demonstrated to exhibit substantial solar-driven photocatalytic activities (X. Chen, L. Liu, P. Y. Yu, S. S. Mao, Science2011,331,746), while the detailed image of the disorder is unclear, and the role of the hydrogenation as well as the mechanism of high photoactivity is still ambiguous. Based on first-principles calculations, we find by taking into account the synergic effect of Ti-H and O-H bonds that hydrogen atoms can be chemically absorbed both on Ti5c and O2c atoms for (101),(001) and (100) surfaces, while previous studies predicted that chemical absorption of H on both Ti5c and O2c only takes place on the (001) surface due to overlooking the synergic effect. The hydrogenation induces obvious lattice distortions on (101) and (100) surfaces of nanoparticles enhancing the intraband coupling within the valence band, while the (001) surface is not largely affected. Different from the previous understanding that the lattice disorder accounts for the induced mid-gap states while the hydrogen only stabilizes the lattice disorders by passivating their dangling bonds, we find that the adatoms not only induce the lattice disorders but also interact strongly with the Ti3d and O2p states, resulting in a considerable contribution to the mid-gap states. The optical absorption is dramatically red shifted due to the mid-gap states and the photogenerated electron-hole separation is substantially promoted as a result of electron-hole flow between different facets of hydrogenated nanoparticles, which may account for the exceptional high energy conversion efficiency under solar irradiation. Even more interestingly, we find that hydrogenation reverses the redox behavior of different surfaces of nanoparticles, which provides new hints that one can tune the photoexcited electron-hole flow between different surfaces of nanoparticles in accordance to one's request by appropriate chemical surface treatment. We believe that band-offset-engineering between different facets of nanocrystals can be an effective way to facilitate energy conversion efficiency and should be applicable to other nanophase materials.(4) Based on first-principles electronic structure calculations we find that the bridging oxygen vacancies on the (110) surface is energetically more favorable and may be responsible for the unexpected ferromagnetism in undoped rutile TiO2. Our results show that the ferromagnetism mainly originates from the partially occupied d orbitals of the low-charge-state Ti ions converted from Ti4+ions induced by the surface oxygen vacancies. The second-nearest neighbors of these ions (fivefold coordinated Ti) also contribute to the total magnetic moments. The spins induced by the local oxygen vacancies prefer a ferromagnetic arrangement.(5) Bi12MxO20±δ (hereafter BMO, M=Ti, Si, Ge) materials, which have been used as ferroelectric materials, actuators, capacitors, dielectric and photorefractive materials, catch the attention of the scientists as photocatalysts and exhibit high photocatalytic activities in many experiments. However, seldom work has been performed on the geometric and electronic properties of the BMO structures and little is known about the effect of alkaline earth metal-doping (AE-doping) on them. In this study, the pure and AE-doped BMO structures are investigated systematically for the first time by performing first-principles calculations. The electronic structures of the three BMO show that they should have high diffusion and low recombination rate of photogenerated electron-hole pairs. Alkaline earth ions could easily be doped into the three BMO structures under O-rich growth conditions, and the doping red shifts the absorption edge of the BMO with its reduction ability unchanged. This new AE-doped materials could act as a kind of excellent candidate for the visible-light-activated photocatalysis and need further study.(6) As an excellent band gap engineering material, the Cd]1-xZnxS solid solution is found to be an efficient visible light response photocatalyst for water splitting, but few theoretical studies have been performed on it. A better characterization of the composition dependence of the physical and optical properties of this material and a thorough understanding of the bandgap-variation mechanism are necessary to optimize the design of high-efficient photocatalysts. In order to get an insight into these problems, we systematically investigated the crystal structure, the phase stability, and the electronic structures of Cd1-xZnxS solid solution by means of density functional theory calculations. The most energetically favorable arrangement of the Cd, Zn, S atoms and the structural disorder of the solid solution are revealed. The phase diagram of the Cd1-xZnxS solid solution is calculated based on regular-solution model and compared with the experimental data. This is the first report on the calculated phase diagram of this solid solution, and can give guidance for the experimental synthesis of this material. Furthermore, the variation of the electronic structures versus x and its mechanism are elaborated in detail, and the experimental bandgap as a function of x is well predicted. Our findings provide important insights into the experimentally observed structural and electronic properties, and can give theoretical guidelines for the further design of the Cd1-xZnxS solid solution. |
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