Synthesis And Photocatalytic Properties Of BiVO₄and Its Composited Materials

Photocatalysis is one kind of clear and environmental friendly technology used for pollutant degradation, water splitting and CO2reduction by photocatalysts under light irradiation. And the technology is of great value and importance in alleviating the crisis of the energy shortage and environmental pollution. The crucial factor to improvement of photocatalytic efficiency lies in the development of semiconductor catalysts. To overcome the limitations of narrow optical absorption range and rapid recombination of charge carriers of traditional photocatalyst (TiO2), it is urgent to discover and develop new materials with extended visible light response and high quantum yield. With the bandgap energy of about2.4eV, BiVO4is proved to possess good absorption in visible light region. And due to the proper positions of conduction and valence band (vs NHE), it is extensively applied in splitting water, decomposing organic pollutants and reducing CO2. However, the photocatalytic activity of BiVO4without any modification is not impressive because of excessive electron-hole recombination, poor charge transportation and so on. Thus, studies on the modification of BiVO4by various strategies such as morphology control, semiconductor coupling, element doping and dye sensitization are of great significance to realize its application in photocatalysis.In this thesis, studies are focused on morphology control and semiconductor coupling for BiVO4. Firstly, we studied the impact of pH values to the morphology of BiVO4in a hydrothermal process. Then with V2O5nanobelts and BiOBr hollow spheres used as precursors, we synthesized BiVO4with special morphologies and characterized properties by X-ray diffraction, Scanning electron microscope, RhB degradation. Secondly, we prepared BiVO4@β-AgVO3and BiOBr@BiVO4composite, respectively. Characterization of as-prepared products was performed and the possible mechanism of separation of charge carriers was proposed based on the energy band calculation.In chapter1, introduction of photocatalytic background was given. At first, we explained the mechanism and application of photocatalysis and analyzed the major limitations of traditional photacatalyst. Then the necessity of development of visible-light responsive materials was proposed. Next, we introduced the electronic and crystal structure, synthetic methods (hydrothermal method), research status and modification of BiVO4. Finally, we put forward the significance of our research and gave the main contents of the thesis.In chapter2, we studied the morphology control method for BiVO4. BiVO4was obtained by a CTAB-assisted hydrothermal method, and its morphology was controlled by adjusting the pH values of suspensions. The impact of pH value on crystal structure and morphology was analyzed by the characterization performed on the products. Next, V2O5nanobelts and BiOBr hollow spheres were used for precursors to prepare polyhedron-like BiVO4and BiVO4nanoparticles self-assembly spheres, respectively. Through characterization, we described the crystal structure, morphology and optical properties of as-prepared BiVO4. The photocatalytic activities were measured by decomposing RhB, and we concluded that activities can be improved by morphology control. At last, we summarized and evaluated the effectiveness of BiVO4modification by morphology control method.In chapter3, synthetic method and properties of BiVO4@β-AgVO3composite were comprehensively studied.β-AgVO3nanowires were firstly prepared by a simple hydrothermal method. Then, BiVO4@β-AgVO3composite was synthesized by in-situ growth of BiVO4nanoparticles on the surface of β-AgVO3nanowires. In the procedure, β-AgVO3nanowires not only provided V sources but also immobilized BiVO4nanoparticles to inhibit their aggregation. The properties and activities of as-prepared products were analyzed and evaluated, and BiVO4@β-AgVO3composite exhibited enhanced photocatalytic activity for degradation of RhB under visible light irradiation. We calculated the energy band levels of BiV04and β-AgVO3, further explained the possible separation mechanism of photoinduced electrons and holes in this system. Moreover, we controlled the amount of Bi sources during the in-situ process and optimized the photocatalytic activity of BiVO4@β-AgVO3composite by adjusting the content of BiVO4in the composite.In chapter4, we took BiOBr as a choice to form composite with BiVO4. BiOBr@BiVO4composite was synthesized by a reflux process. In this experiment, CTAB was added to provide Br sources for BiOBr nanoplates. And as a surfactant, CTAB played an important role in facilitating the formation of monoclinic BiVO4. Therefore, the heterostructure with closely touched interfaces was obtained between BiOBr and BiVO4. The structure, morphology, BET specific surface areas and optical absorption properties of products were measured. To explain photocatalytic activities of as-prepared samples, we gave the possible separation mechanism of charge carriers for BiOBr@BiVO4composite in terms of energy band structure. At the end, we assessed the experimental method and proposed the proper optimization scheme.In chapter5, a detailed summary and assessment to all the research in this thesis was presented and achieved conclusions and results were analyzed. Moreover, we discussed potential problems and put forward the promising prospect for further study.