| With the increasing of global population, the world is suffering from a severe energy crisis brought on by overexploitation. On the other hand, the consumption of the fossil fuels also brought serious environmental pollution problems. It is urgent to deal with the increasingly severe energy shortage and environmental pollution issues. Since1972, Akira Fujishima found that TiO2electrode can be used to split water into H2and O2under irradiation, semiconductor photocatalysis has been considered to be the ideal means to solve environmental problems and energy crisis. 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.Currently, a wide range of semiconducting materials have been developed as photocatalysts, however, most of these materials with superior photocatalytic properties only response to UV light for their wider band gap than3eV, this greatly restricts the practical application of semiconductor photocatalysis. One of the primary means for the development of new visible light photocatalyst is modified the wide band-gap semiconductors to obtain visible light harvesting. As we all know, the most efficient way to expand visible response range of the semiconductor is narrowing its band-gap, which can achieved easily by doping, introducing defects into semiconductor or forming a solid solution. While some other means, such as combinations with narrow band gap semiconductor, noble metal loading and surface plasma photocatalysis, were also used to expand the light absorption range of semiconductor.In this thesis, our researches mainly focus on the band-gap engineering of TiO2and ZnO to obtain wider light absorption range. We studied the effect of the doping, defects and composition on the band gap and photocatalytic activity of TiO2and ZnO.In chapter1, we briefly introduced the principal mechanism and main applications of semiconductor photocatalysis. Then we reviewed the latest progress in visible light photocatalysts. The relationship between structures and photocatalytic performances of semiconductors was discussed. Finally, the significance, research ideas and the outline of this thesis were summarized.In chapter2, we discussed the effects of co-doping and self-doping on the band structure of TiO2, several TiO2materials with one dimension nanostructure were prepared.Firstly, visible-light response C, N-codoped TiO2nanotubes with high aspect ratios were prepared by a two-step method. First we synthesized TiO2nanotubes by an ion-exchange method, and then the nanotubes were calcined at different temperatures with melamine as nitrogen and carbon source. The results show that the nanotubular structure was destroyed when the calcinations temperature was higher than823K. Further increase the temperature to923K, TiO2was reduced to TiO. The photocatalytic activity of the codoped TiO2nanotubes/nanorods was evaluated by degradation of Rhodamine B under visible-light irradiation (>420nm). Compared with N doped P25, these codoped TiO2nanotubes/nanorods possess a superior photocatalytic activity, owing to the synergistic effects of the nitrogen and carbon co-doping.Secondly, we prepare Ti3+selfdoped TiO2materials with one dimension nanostructure using Ti2O3, TiB2and TiSi2as precursors respectively. The obtained samples with various precursors have different morphologies such as nanowire, nanobelts and nanosheets. X-ray photoelectron spectroscopy, Raman spectroscopy were employed to characterize the existence of the Ti3+in the sample. UV-vis diffusive reflectance spectroscopy confirmed the wider light absorption range of self-doped TiO2.In Chapter3, we studied the effects of oxygen vacancies on the band structure of ZnO, and the relationship of oxygen vacancies concentration and band gap of ZnO was discussed. With ZnO2as precursors, we introduced high concentration of oxygen vacancies into ZnO successfully. The obtained ZnO exhibits a yellow color and the absorption edge shifts to longer wavelength. Raman and XPS spectra reveal that the concentration of oxygen vacancies in the ZnO decreased when the samples are annealed at higher temperature in air. It is consistent with the theory calculation. The increasing of oxygen vacancies results in a narrowing bandgap, and increases the visible light absorption of the ZnO. The ZnO with oxygen vacancies are found to be efficient for photodecomposition of2,4-Dichlorophenol under visible light irradiation.In Chapter4, we studied the band-gap variation of (GaN)1-x(ZnO)x solid solutions with the change of its composition.(GaN)1-x(Zn0)x solid solutions are potential photocatalysts for water splitting and environmental decontamination under visible light. The solid solutions prepared by the traditional method are not effective because of their low Zn content (x<0.5). Here we show that Zn-rich solid solutions (-0.5<x<-0.8) are readily prepared by nitridating layered double hydroxides (LDHs) containing Zn2+and Ga3+ions, and the Zn content is easily adjusted by changing the Zn/Ga ratio of the LDH precursors. We studied the effects of nitridation time and temperature on the composition and band structure of (GaN)1-x(ZnO)x solid solutions. The band gap of (GaN)1-x(ZnO)x decreases gradually from2.60eV at x0.46to2.37eV at x=0.81. The Zn-rich solid solutions absorb strongly well above500nm, and these solutions loaded with1%Pt are found efficient in photoreducing Cr6+ions under visible light.In Chapter5, we prepared ZnFe2O4/ZnO and CdS/TiO2materials, studied the effects of semiconductor combination on photocatalytic activity and photogenerated charge separation.Firstly, we prepared a Zn/Fe/SO4LDHs as precursor, after ion exchange with CO32-, obtained the Zn/Fe/CO3LDHs which is more likely thermal decomposition. After annealed in air, the precursor transformed to ZnFe2O4/ZnO composites. As for the Zn2+and Fe3+ranged uniformly in every layer of the LDHs, the ZnO and ZnFe2O4are closer contacted in the composites. We studied the structure, morphology and optical property of the composite. The photocatalytic performance was evaluated by decomposition of MB and2,4-Dichlorophenol, and found that the composite has better photocatalytic activity than both ZnO and ZnFe2O4. The mechanism for the high photocatalytic activity and stability of the ZnFe2O4/ZnO nanostructured composite was also discussed.Secondly, Cd and Ti-containing material with hexagonal structure was also synthesized as a precursor to prepare CdS/TiO2materials. The precursor has a hexagonal plate-like structure, similar with LDHs, the Cd2+and Ti3+in the precursor ranged uniformly. After sulfuration by hydrothermal, we obtained CdS/TiO2composite materials with porous plate-like structure. We systematical studied the effect of pH, reagent on the structure of final products.In chapter6, we summarized our work, and discussed the problems remained to be solved. Finally, we made a plan and looked forward to the future work.In summary, the visible light photocatalytic performances of the semiconductors can be optimized through composition adjustment and structural design which could widen the absorption range and broaden band gap. In this thesis, we focused on the improvement of the visible light photocatalytic performances of TiO2and ZnO, and the research results have further deepened our understanding of the catalytic mechanism. |
PDF Full Text Request