Preparation And Catalytic Properties Of Novel Bi-Based Heterojunction Photocatalysts

With the rapid development of industrialization and urbanization, the huge consumption of traditional fossil fuels, such as coal, oil and gas, leads to the pollution for environment and serious consumption for energy, which have become challenges around the world. Based on this situation, the development of new technologies for expansion of new energy sources and treatment of pollution has become the focus of today’s society. Among the new energy, solar energy, as a renewable, widely distributed, clean and green energy, has being constantly studied and exploited for its potential applications. Photocatalytic technology can convert solar energy into chemicaland electrical energies via photocatalysis of semiconductor materials; it can degrade organic pollutants and reduce heavy metals into non-toxic ions. Thus, photocatalytic technology is considered to be one of the most effective ways of comprehensive utilization of solar energy. Photocatalytic materials are the core of photocatalytic technology, and inorganic semiconductor photocatalytic materials are very important part of photocatalytic materials. With the constant progress of science and research, bi-based semiconductor photocatalytic materials play an increasingly important role in the field of photocatalysis. BiOX (X=Cl, Br, and I) as an excellent semiconductor photocatalyst has aroused people’s wide attention in recent years. As one of the most important photocatalysts in BiOX (X=Cl, Br, and I), BiOCl has unique layered structure, and it consists of [Bi2O2]2+ ayer and double Cl- layers. Its structure, size, morphology, and surface property have important effects on its photocatalytic properties. However, forbidden band of BiOCl photocatalyst is about 3.5 eV. It has excellent photocatalytic activity under ultraviolet light, while its photocatalytic activity under visible light is not satisfactory. Therefore, how to improve the photocatalytic activity of BiOCl under visible light is the development direction of new bi-based photocatalyst. Studies have shown that introducing other metal catalytic active centers or compositing with other semiconductor materials are important ways to improve the visible light response of BiOCl photocatalyst.In the first part of this thesis, framework structured Bi-MOFs have been prepared and they were used as precursors to synthesize Bi/Bi-MOFs nanostructures with Bi nanoparticles evenly dispersed on the surface of Bi-MOFs via a H2 reduction method. Then, BiOCl/Bi heterojunctions grafted on the surface of Bi-MOFs were obtained by the reaction of Bi nanoparticles with FeCl3 solution. The obtained BiOCl/Bi/Bi-MOFs nanostructures have unique hierarchical heterojunction structure consisting of BiOCl/Bi heterojunction covering the surface of Bi-MOFs. By controlling the reaction time of Bi/Bi-MOFs with FeCl3 solution, the composition of BiOCl/Bi heterojunction can be optimized to improve their catalytic performance. The catalytic performances of BiOCl/Bi/Bi-MOFs nanostructures were evaluated using the reduction of Cr (Ⅵ) under ultraviolet and visible light as a model reaction. It is revealed that BiOCl/Bi/Bi-MOFs nanocatalytic materials have a remarkable performance on the reduction of Cr (Ⅵ). The Cr (Ⅵ) can be completely redeced into Cr (Ⅲ) within 50 min and 3 h under ultraviolet and visible light, respectively. Their photocatalytic performance under visible light is much superior to pure BiOCl materials. The higher photocatalytic activity is ascribed to the larger surface area of BiOCl/Bi/Bi-MOFs nanocatalytic materials and their stronger electron-hole separation efficiency induced by heterojunction of BiOCl/Bi.In the second part of this thesis, framework structured Bi-MOFs were also used to synthesize Bi/Bi-MOFs nanostructures via a high temperature calcination method, which was different from previous H2 reduction method in the first part. Then, following the similar procedures, BiOCl/Bi heterojunction grafted on the surface of Bi-MOFs were prepared through the reaction of Bi/Bi-MOFs with FeCl3 solution. The obtained BiOCl/Bi/Bi-MOFs nanostructures have the unique hierarchical structure with BiOCl/Bi heterojunction covering the surface of Bi-MOFs. The catalytic performance of BiOCl/Bi/Bi-MOFs was evaluated using the reduction of Cr (Ⅵ) under visible light as a model reaction. It is shown that BiOCl/Bi/Bi-MOFs photocatalysts have an excellent performance on the reduction of Cr (Ⅵ). The Cr (Ⅵ) can be reduced into Cr (Ⅲ) within 2.5 h under visible light and such performance is much superior to pure BiOCl materials. In comparison with the H2 reduction method presented in the first part of this thesis, the high temperature calcination method can largely improve the photocatalytic performance of BiOCl/Bi/Bi-MOFs photocatalysts.In the third part of this thesis, framework structured Bi-MOFs have similarly been used as precursors to synthesize Bi/Bi-MOFs nanostrcutres by calcining under 800 ℃, which then reacted with different ratios of FeCl3 and FeBr3 solutions to generate BiOCl/BiOBr with various ratios on the surface of Bi/Bi-MOFs. The obtained BiOCl/BiOBr/Bi-MOFs photocatalysts possess the unique hierarchical structure with BiOCl/BiOBr/Bi heterojunction covering the surface of Bi-MOFs. The catalytic performance of BiOCl/BiOBr/Bi-MOFs photocatalysts was evelated using the reduction of Cr (Ⅵ) under visible light as a model reaction. It is found that (BiOCl:BiOBr=1:2)/Bi-MOFs photocatalysts have a good performance on the reduction of Cr (Ⅵ). The Cr (Ⅵ) can be reduced into Cr (Ⅲ) within 4 h under visible light.