| Energy shortage and environmental pollution are the main problems of the world. It is necessary to solve all these problems to realize the sustainable development, raise people's living standard and guarantee national security. Semiconductor photocatalysis supplied an ideal way of energy utilization and environmental treatment. Semiconductor photocatalyst can convert the low-density solar energy to electrical energy (dye-sensitized solar cells) or chemical energy (hydrogen energy). Hydrogen energy possesses several advantages, such as high energy density, environmental friendliness and renewability et al, and can be used as a future alternative to fossil fuels. Moreover, the organic pollutant in the natural environment can be converted into CO2and water or other useful raw materials by the photocatalyst, thus photocatalysis was considered as the most promising technology for improving environment. The application of semiconductor photocatalyst in the field of environmental treatment and clear energy conversion has become a research focus.In the past forty years, semiconductor photocatalyst have been studied extensively and deeply. Among the numerous photocatalysts, TiO2has become the most widely used semiconductor photocatalyst, which is due to its advantages such as non-toxic, chemical stability, photocorrosion resistance, high photocatalytic activity and low-cost et al. However, there are some problems during the practical applications of TiO2, including narrow range of photoresponse, poor quantum efficiency and recycling issue, which restrict the development of TiO2.In this thesis, the photoresponse range and photogenerated charge separation of TiO2was developed by means of microstructure modulation (including hierarchical structure and crystal facet regulation), surface loading and modification. There are two main parts in this thesis:(1) Microstructure Modulation:Hierarchical microspheres were synthesized, which include TiO2microspheres consisting of1-D nanotubes and SrTiO3hollow microspheres consisting of nanocubes. Based on the idea of crystal facet regulation, TiO2microspheres with high percentage of{001} facets were synthesized, and the highly enhanced photocatalytic efficiency was characterized. By studying the crystal facet stability of Cu2O in photocatalytic experiment, a model of charge-separation among crystal facets was proposed. Then, the study of charge-separation behavior among crystal facets was extended to TiO2, and we conclude that a synergetic effect between well-formed{001} and{101} facets is the determining factor for photoreactivity.(2) Visible-Light Expansion:Plasmonic photocatalyst M@TiO2(M=Au, Pt, Ag) and Glass-Microspheres/Ag/TiO2were synthesized by using the surface plasmon resonance (SPR) of noble-metal nanoparticles. Hydrogenated TiO2nanowire-microspheres were synthesized by titanate-assistant method, and the visible-light photocatalytic activity of the as-prepared material was systematically investigated.In chapter one, firstly we introduced the mechanism and latest progress of semiconductor photocatalysis, and we also briefly introduced the crystal structure and electronic structure of TiO2as well as the main problems restricting the practical application of TiO2. Secondly, we introduced the research progress that improving the photocatalytic performance of TiO2by means of microstructure modulation and visible-light expansion. Finally, the signification of topic selection, train of thought and outline of the thesis were summarized.In chapter two, we synthesized hierarchical microspheres by a simple and general method, which include TiO2microspheres consisting of1-D nanotubes and SrTiO3hollow microspheres consisting of regular nanocubes. Owing to their high surface area, open porous networks and excellent light scattering properties, the TiO2nanotube-microspheres exhibit a superior photocatalytic activity. Their porous structure has a high thermal stability. Hierarchical SrTiO3hollow microspheres, obtained by the same synthesis idea, possess a strong optical absorption capability, which is due to their unique hierarchical hollow structure and light scattering properties; thus these SrTiO3microspheres exhibit a superior photocatalytic activity in photoreduction of Cr(VI). To obtain detailed crystalline information of the sample, we carried out high-resolution TEM measurements, which show that these nanocubes are well-crystallized single crystal. On the basis of morphology-evolution observation and a series of characterization results, the formation process follows the dissolution-recrystallization mechanism.In chapter three, we developed a template-and surfactant-free alcohothermal method of synthesizing TiO2microspheres produced from anatase nanosheets. The crystalline information of the TiO2nanosheets were characterized by HRTEM, and the percentage of{001} facets was as high as83%. We systematically investigated the influence of reaction time and the amount of HF on the morphology of our samples. The photocatalytic property was evaluated by measuring the decomposition of organic dye. The surface stabilities and photocatalytic properties of Cu2O microcrystals were systematically investigated. On the basis of methyl orange (MO) decomposition and theoretical calculations, a novel model of charge separation among crystal faces was proposed. Then the study of charge-separation behavior among crystal facets was extended to TiO2. The percentage of{001} facets of TiO2microspheres can be tuned from82to45%by secondary hydrothermal treatment in different solutions. The photocatalytic properties were evaluated by investigating the photo-oxidation reactions for·OH radical generation and photoreduction reactions for hydrogen evolution. The XPS gives the surface composition difference of these samples. The adsorption energy of fluorine atoms over clean and EtOH-modified surfaces were calculated, which indicates that ethanol can effectively strengthen the adsorption of fluorine. We calculated the density of states (DOS) of TiO2by using first-principles DFT, and concluded that a synergetic effect between{001} and{101} facets is the determining factor for photoreactivity.In chapter four, the plasmonic photocatalysts M@TiO2(M=Au, Pt, Ag) were successfully fabricated by Ti3+-ion-assisted method. The presence of Ti3+in the irradiated TiO2samples is supported by EPR spectroscopy. The metal content of noble-metal@TiO2can be tuned by changing the amount of Ti3+, which in turn can be tuned by varying the irradiation time t. The photocatalytic oxidation of benzene in aqueous phenol was investigated with noble-metal@TiO2-microspheres as photocatalysts under visible light. Among the noble-metal@TiO2composites, Au@TiO2exhibits a high yield and selectivity for the catalytic oxidation of benzene to phenol in aqueous phenol under visible light. We systematically investigated the catalytic oxidation of benzene to phenol over Au@TiO2-microspheres with different loading amount, and discussed the probable mechanism for the photocatalytic oxidation of benzene under visible light in aqueous phenol over Au@TiO2, which involves the visible-light induced electron transfer from the Au NPs to the TiO2particle, the oxidation of phenoxy anions by electron-depleted Au NPs, and the oxidation of benzene to phenol by phenoxy free radicals. The sliver nanoparticles were loaded on the surface of glass-microspheres (GMS) by ion-exchange method, and TiO2was loaded on the GMS with the solution of Ti(OBu)4and ethanol as precursor, then the plasmonic photocatalyst GMS/Ag/TiO2was obtained. The samples were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV/Vis diffuse-reflectance spectra. MO was chosen to evaluate their photocatalytic activity under visible light.In chapter five, we synthesized surface-hydrogenated anatase TiO2(TiO2-H) nanowire-microspheres by annealing protonated titanate nanotube (H-TiNT) microspheres under hydrogen atmosphere, and these TiO2-H nanowire-microspheres consist of anatase nanowires with a tiny diameter of8nm. Based on surface analysis and absorption energy calculation, we concluded that both Ti-H and O-H bonds located at the surface of TiO2-H nanowire-microspheres. The as-prepared sample exhibits improved visible-light absorption and highly enhanced photocatalytic activity toward H2production from water and degradation of organic compounds. The visible-light photocatalytic activity of different TiO2samples was also examined by probing the formation of hydroxyl radicals (·OH). The experimental analyses and theoretical calculations lead us to propose a possible transition mechanism from H-TiNT to anatase TiO2-H. The highly enhanced photocatalytic activities of TiO2-H nanowire-microspheres are attributed to synergy of the improved optical absorption and efficient photogenerated electron-hole separation induced by the surface Ti-H bonds as well as the structure of1-D nanowires. Moreover, Ti3+seft-doped TiO2photocatalyst were synthesized by a Zn-assistant method, and the phase composition of the as-synthesized sample can be regulated by the ratio (rZT) of reactants. Furthermore, the photocatalytic activity towards decomposition of MO dye and formation of·OH can be tuned by varying the phase composition.In chapter six, we summarized our work and discussed the problems remained to be solved. At last, we made a plan for the future work and looked forward to the futurity.In summary, the photocatalytic property of materials can be greatly influenced by their microstructure and surface state. In this thesis, the constraints of TiO2photocatalyst, including narrow range of photoresponse, low quantum efficiency et al, are improved by means of microstructure modulation, surface loading and modification. And it is of great theoretical guiding significance for improving photocatalytic performance of TiO2and its practical application.
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