| Energy shortage and environmental pollution issues are of great concerns for the public and will keep increasing in the next few decades. In order to solve this problem, great efforts have been paid and various techniques have been developed including carbon adsorption, advanced oxidation processes, membrane filtration technique, microbiological or enzymatic decomposition, photocatalysis, etc. Among these, Photocatalytic technologies can be applied to convert solar energy into chemical energy, and is considered as an attractive strategy to tackle the energy and environmental challenges. However, the photocatalytic activity of conventional TiO2 is still far from practical use because the recombination of photogenerated electrons and holes is fast. Hence, the development of new highly efficient photocatalysts has emerged as one of the most essential topics for photocatalysis.The photocatalytic properties of semiconductor materials are determined by their structures, including the morphology, crystal structures and defects, electronic structure, surface and interface structures. Therefore, it can also provide a way to improve the photocatalytic performance of the photocatalyst by means of microstructure modulation. Since the photocatalytic performance of a semiconductor photocatalyst is influenced by photogenerated electron-hole separation and transport efficiency, the structures can be tuned to suppress the recombination rate of photogenerated electrons and holes so as to allow more electrons and holes to arrive at the surface for redox reaction.Bismuth-containing photocatalysts have attracted considerable interests due to their efficient photocatalytic activity, high stability, low cost, non-toxic and easy to handle. Up until now, various kinds of bismuth-base photocatalysts have been successfully synthesized and studied for photocatalysis in environment remediation, which exhibited excellent activities for degradation of pollutants, water splitting for hydrogen and oxygen production, heavy metal ions removal and present itself as a green-chemistry technology. However, from the point of view of industrialization and commercialization, many challenges remain in enhancing the photocatalytic efficiency of bismuth-containing photocatalysts. Thus, many efforts have been devoted to enhance the activity of bismuth-base photocatalysts, including the development of light-harvesting, design and synthesis of desired nanostructures, and doping with different elements. These strategies could improve the efficient separation of photogenerated carriers and increase photocatalytic efficiency of the semiconductor.Bismuth-containing semiconductor photocatalysts can serve more extensive applications in the environment remediation or energy conversion. Therefore, it is necessary to develop more efficient bismuth-containing semiconductor photocatalysts for further practical applications. In this dissertation, bismuth-containing semiconductor materials will be employed as main research object. The unique layered structures could favor the effective separation and transport of the photoinduced carriers. According to the structure-property correlation, we adopted microstructure modulation approaches, the crystal phase, morphology, doped, surface modification and hybrids etc., so as to promote separation efficiency of the charges and enhance the photocatalytic activity.In the first part of this dissertation, we briefly introduced the fundamental principles and main applications of semiconductor photocatalysis, and reviewed the latest progress in bismuth-containing semiconductor photocatalysts. We summarized the research status of photocatalytic technologies, including splitting water, environmental pollution controlor, and carbon dioxide reduction. Furthermore, some new ideas to imporve the efficiency of photocatalysts form elementary reactions process be discussed in detail, such as electronic structural features, crystal structures, defects, doping with different elements, interface composites, surface modification and hybrids. The significance, primary goal and the outline of the dissertation will be looked forward at the end of this chapter.In chapter two, we study the effect on photocatalytic performance of boron-doped BiVO4. Boron-doped monoclinic and tetragonal phases of BiVO4 were prepared by using urea-precipitation method, and the visible-light photocatalytic activities of pristine and B-doped BiVO4 for oxygen generation from water were compared, to find that B-doping enhances the photocatalytic activities of BiVO4.We probed why B-doping enhances the photocatalytic oxygen generation of BiVO4 by performing x-ray photoelectron, Raman and electrochemical impedance spectroscopy measurements and also by performing density functional calculations for model B-doped BiVO4 structures. This enhancement arises from the fact that B-doping creates BO4 tetrahedra, and that the BO4 tetrahedra are smaller in size than the VO4 tetrahedra. The O 2p-O 2p antibonding interactions are stronger in BO4 hence creating three localized defect levels per BO4 above the VBM of pristine m-BiV04. The lowest-lying level located about 0.17 eV above the VBM of pristine m-BiV04 is occupied and acts as a hole trap, which suppress the recombination of photoinduced carriers thus enhancing the photocatalytic activity of BiVO4. Our work suggests a general strategy for slowing down the recombination rate of photogenerated electrons and holes in transition-metal oxides by hole-trapping, which can be achieved when the small B3+ cations substitute for larger cations.In chapter three, we synthesized BiOBr-PVP hybrids via a simple PVP-assisted solvothermal process. The adsorption properties of as-prepared BiOBr-PVP hybrids were extensively investigated by adsorption experiments.According to the FT-IR characterizations, the vibration peaks for BiOBr-PVP and PVP are almost identical, which validate the presence of PVP in the as prepared hybrids. Furthermore, due to the donor-acceptor interactions between PVP and BiOBr, the zeta potential of BiOBr-PVP hybrids can be greatly increased, this could be helpful to improve the adsorption properties of cationic dyes. RhB dye was employed to evaluate the adsorption properties of as-prepared BiOBr-PVP hybrids, the maximum adsorption capacity for BiOBr-PVP hybrids is two times higher than that of pure BiOBr. Considering the specific surface area of BiOBr-PVP hybrids is only half of that of BiOBr, the enhanced adsorption capacity of RhB molecules on BiOBr-PVP hybrids can be mainly ascribed to the stronger electrostatic attractions between cationic RhB molecules. In order to further understand the interactions between adsorbate and adsorbent, the Langmuir and Freundlich adsorption models were utilized to analyze the RhB dye adsorption, and we can find the experimental data fit better to Langmuir adsorption model from fitted curves and parameters.In chapter four, we studied the synergistic effects of both high percentage of reactive crystal facets and strong adsorption capacity on the photocatalytic performance of the BiOBr-PVP hybrids. The corresponding adsorption mechanisms were investigated in detail. A hypothesis model for the enhanced adsorption was proposed.With the assistance of PVP, the thickness of the BiOBr nanosheet in BiOBr-PVP hybrids can be greatly reduced, which lead to the exposure of higher percentage of reactive (001) facets. According to the FT-IR and XPS characterizations, PVP was found to be adsorbed to BiOBr through a donor-acceptor interactions via C=O groups by transferring the nonbonding electron of O-atom in PVP to BiOBr in the hybrids, which increased the zeta values of the BiOBr-PVP hybrids with comparison to BiOBr. The increased negative zeta potential in BiOBr-PVP hybrids is beneficial for the adsorption of cationic dyes. Based on the above analysis, a hypothesis for the enhanced adsorption of RhB molecules on BiOBr-PVP hybrids was proposed. Owing to the electrostatic attractions between the negatively charged surfaces of BiOBr-PVP hybrids and the positively charged RhB molecules, the intimate contact could facilitate the charge transfer during the photocatalytic reactions, and improve the photocatalytic performances. Compared with BiOBr, the BiOBr-PVP hybrids has a higher photocatalytic reactive in both RhB dye and phenol degradations, which could be attributed to the synergistic effects of both the enhanced adsorption capacity and higher percentage of highly reactive (001) crystal facets exposed.In Chapter five, a new method for the treatment of paraquat wastewater was investigated. The wastewater resource utilization and through the photocatalytic technologies of organic pollutants in paraquat wastewater has conducted the preliminary research.However, the composition of paraquat wastewater is more complicated since the concentration of ammonium chloride and organic pollutants are higher, so we had to do some research and design a new type technique. The ammonium chloride efficiently remove is beneficial for the photocatalytic performances in paraquat wastewater treatment. Photocatalytic degradation of organic pollutants in paraquat wastewater with BiOBr-PVP and Degussa P25 were investigated. Photocatalytic technology had the advantages of easy operation, and high efficiency, therefore, it has a good prospect for practical application.In Chapter six, we summarized our research work, discussed the problems remained to be solved of the dissertation. At last, we proposed a research plan for future work.To sum up, the photocatalytic properties of semiconductor materials are determined by their microstructure. Therefore, it can also provide a way to improve the performance of the bismuth-containing photocatalyst by means of doped and surface modification, so as to promote separation efficiency of the photon-generated carrier and enhance the photocatalytic activity. The research in this dissertation suggests a new strategy for improving photocatalytic activity of semiconductor photocatalyst, which can promote the development of photocatalytic technology and broadened the wide range of practical applications in industrial wastewater treatment. |
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