Synthesis And Photocatalytic Activity Of Bismuth Tungstate-based Heterojunction Catalysts

It is possible to converse the low density solar energy to chemistry and electricity energyby the photocatalytic technologies, which provide great potential for the applications insterilization, remediation of environment, H2production by water splitting and dye-sensitizedsolar cell, and so on. However, the photocatalytic technologies still have many problems.Among them, the low quantum efficiency and difficult to be recovered have become majorobstacles for further application. Therefore, to design and develop highly efficient and easy tobe recovered photocatalyts has become the focus of current research. Generally, highly activephotocatalysts have the features of narrow band gap, high quantum efficiency, large specificsurface area, high stability and can be easy recovered. However, in fact, many programs ofimproving photocatalytic activity are still unable to meet the above points. In this dissertation,we focus on development of bismuth tungstate-based heterojunctions as photocatalysts, usinghydrothermal and solvothermal methods to control their composition and morphology.The main contents were discussed as follows:1. The Bi₂O₃/Bi₂WO₆heterojunction photocatalysts were prepared by a two-stepsolvothermal process using Bi(NO3)3-ethylene glycol solution as Bi source. The catalystswere characterized by XRD, SEM, TEM, XPS, DRS, BET. The possible formationmechanism for the architectures was discussed. The result show that the heterostructurecatalysts are composed of Bi₂O₃nanoparticles as a modifier and the3D Bi₂WO₆microspheresas the substrate. Bi₂O₃nanoparticles with diameter of about10-15nm were tightly grown onthe lateral surface of the Bi₂WO₆microspheres. The hierarchical Bi₂O₃/Bi₂WO₆microspheresexhibit higher photocatalytic activity than the single phase Bi₂WO₆or Bi₂O₃for thedegradation of rhodamine B under visible light illumination. The enhancement of thephotocatalytic activity of the Bi₂O₃/Bi₂WO₆heterojunction catalysts can be ascribed to theirimproved light absorption property and the reduced recombination of the photoexcitedelectrons and holes during the photocatalytic reaction. The effect of loading level of Bi₂O₃onthe catalytic performance of the heterojunction catalysts was also investigated. The optimalcontent of Bi₂O₃is3wt%. The Bi₂O₃/Bi₂WO₆heterojunction photocatalysts are essentiallystable during the photocatalytic process.2. In order to avoid the decrease of the surface area of photocatalysts by two-step methodfor preparing the heterojunction, the second chapter continues to develop new approach toprepare the hierarchical Bi₂O₃/Bi₂WO₆heterostructure photocatalyst with large surface area.Hierarchical nanostructured Bi₂WO₆micro-clews were synthesized by a solvothermal process with mixed solvents and formaldehyde. The hierarchical Bi₂WO₆micro-clews, withan average diameter of ca.1.0μm, consisting nano-sheets assembled in a special fashion.Based on systematic experiments, the formation mechanism of the as-preparedthree-dimensional (3D) nanostructured Bi₂WO₆micro-clews (3D-Bi₂WO₆) was discussed. Inorder to further improve the photocatalytic activity, the hierarchical3D-Bi₂WO₆was furthermodified by Bi₂O₃nanoparticles through deposition and annealing of BiOI on their surface asan intermediate. During the degradation process of RhB solution, Bi₂O₃/Bi₂WO₆hetero-junction shows higher photocatalytic than the pure Bi₂WO₆and Bi₂O₃.3. In order to further improve the photocatalytic activity of Bi₂O₃/Bi₂WO₆, aheterojunction hollow structure consisting Bi₂WO₆nanoplatelets and Bi₂O₃nanoparticles wassuccessfully prepared by a one-step solvothermal process. Bi₂O₃/Bi₂WO₆heterojunctionpossesses high quantum efficiency, large surface area, stability and can be easy recovered. Onthe basis of ESEM, TEM observation and XRD analysis of the samples synthesized atdifferent reaction stages, a possible growth mechanism was proposed for the growth of hollowBi₂O₃/Bi₂WO₆heterostructures. The photocatalytic activity of the hollow Bi₂O₃/Bi₂WO₆composites was evaluated by degradation of rhodamine B under visible-light irradiation (λ>400nm). The results indicate that the hollow Bi₂O₃/Bi₂WO₆composites exhibit much higherphotocatalytic activity than the pure Bi₂WO₆and Bi₂O₃. The improved photocatalyticperformance can be ascribed to presence of Bi₂O₃in the Bi₂WO₆framework, the hierarchicalhollow structure with good permeability and large surface area. More importantly, the hollowBi₂O₃/Bi₂WO₆composites are not only high stable but also easy to be separated by simplesedimentation. This work provides a facile, rapid, low-energy consumption route to preparenovel heterojunction photocatalysts.4. The novel heterojunction WO3/Bi₂WO₆photocatalysts were successfully prepared bya facile hydrothermal preparation strategy. The as-prepared sample is composed of WO3nanoparticles growing on the primary Bi₂WO₆nanoplatelets. The different morphology ofWO3/Bi₂WO₆can be obtained by adjusting the amount of nitric acid and Na2WO4·2H2O.Because of the visible light response and appropriate band edges of Bi₂WO₆and WO3, theWO3/Bi₂WO₆hierarchical heterojunction exhibits high photocatalytic activity over that of thepure Bi₂WO₆and WO3towards the decomposition of rhodamine B (RhB) under visible lightirradiation. This can be ascribed to the interconnected interface of WO3and Bi₂WO₆heterostructure, which leads to the high transfer rate of photoinduced charge carriers. Moreimportantly, the hierarchical WO3/Bi₂WO₆heterojunction is not only highly stable but alsovery easy to be separated by simple filtration for reuse. 5. The heterojunction Bi₂WO₆/g-C₃N₄photocatalysts were successfully synthesized by afacile solvothermal method. The composition, morphology and light absorption of theas-prepared products were characterized by XRD, IR, SEM, TEM and DRS. The results showthat Bi₂WO₆and g-C₃N₄form heterojunction and part of the g-C₃N₄nanoplates are separatedby the Bi₂WO₆particles. More importantly, the Bi₂WO₆/g-C₃N₄exhibits high photocatalyticactivity than the pure Bi₂WO₆and g-C₃N₄towards degradation of RhB under visible lightirradiation. The excellent photocatalytic performance of the Bi₂WO₆/g-C₃N₄heterojunction isascribed to its high quantum efficient and large surface area. In addition, the Bi₂WO₆/g-C₃N₄is stable and easy to be recovered during the photocatalytic reaction. The Bi₂WO₆/g-C₃N₄heterojunction photocatalyst shows strong adsorption capacity and high quantum efficiency.Moreover, modification of g-C₃N₄by the Bi₂WO₆also promotes the separation of thephotocatalyst from the reaction solution for recycling. The present work not only gives insightinto understanding the heterojunction photocatalyst but also provides a new way to peel thebulk g-C₃N₄.