Preparation And Photocatalytic Properties Of Bismuth Oxyhalide And Composites

At present, the semiconductor photocatalytic technology has become one of the effective methods to remove toxic pollutants in the environment of life, and has potential applications in the field of environmental governance and energy. Titania（TiO₂） is found to be an efficient photocatalyst and attracted great interest due to photostability, nontoxicity, and low-cost. However, TiO₂ with the wide band gap（3.0 eV for rutile and 3.2 eV for anatase） absorbs only ultraviolet light, and has a rapid recombination of photoinduced electrons and holes and a lower quantum efficiency, which limits the utilization of sunlight. Therefore, looking for new-type and high activity catalyst is the inevitable trend.Bismuth oxyhalides（BiOX, X=Cl, Br, I） are a class of ternary oxide semiconductor materials. BiOX exhibits excellent light absorbility and photostability. BiOX exhibits indirect transition and [Bi₂O₂]²⁺ slabs interleaved by double slabs of Cl atoms, which is beneficial for the effective separation and transition of photoexcited electron–hole pairs. This endows high photocatalytic activity. In this thesis, series of BiOX and BiOX based composite photocatalysts with high photocatalytic performances were synthesized through different strategies, including the control of morphology, modification with metal Bi nanoparticles, building BiOX/semiconductor heterojunction, and doping with metal ions. The influences of synthesis conditions on photocatalytic properties were studied. The generation and separation of electron–hole pairs, dominant active species, and photostability were discussed. Moreover, the possible mechanisms of enhanced photoactivity were proposed. This thesis mainly includes the following several aspects of research:（1） Bismuth oxychloride（BiOCl） was synthesized by a simple combustion method. Compared with other methods, combustion method has some advantages, such as time-saving, low equipment requirement, fast heating and energy-consuming. The experimental results show that the percentage of exposed {001} facets can be modulated through changing the added amount of ammonium chloride. BiOCl micro-sheet with high percentage of exposed {001} facets was obtained, which leads to enhanced photocatalytic properties.（2） Bi/BiOCl composite photocatalysts were synthesized by a simple one-pot combustion method. During the combustion synthesis, the reduction of Bi³⁺ to metal Bi was achieved. Bi doping leads to red-shift in the absorption band and increased visible light absorption. Moreover, the loaded Bi nanoparticles can trap the photo-generated electrons, resulting high separation ratio of electron-hole pairs. Thus, as-obtained Bi/BiOCl photocatalysts exhibit excellent visible-light photocatalytic properties.（3） Bi/BiOBr photocatalysts with different metal Bi contents were synthesized by a facile one-step hydrothermal method using ethanol-water mixed solution. As-synthesized Bi/BiOBr photocatalysts exhibits not only unique visible-light photocatalytic properties but also excellent photostability. This can be attributed to the formation of oxygen vacancies caused by the reduction condition in reaction system, leading to the decreased band gap and thus improved visible light absorption. Moreover, the introduction of Bi nanoparticles can accept the photo-generated electrons in BiOBr, resulting high separation ratio of electron-hole pairs and thus enhanced photocatalytic performance.（4） AgCl/BiOCl composite photocatalysts were synthesized by a simple one-pot combustion method. The formation of heterojunction improved the separation of photogenerated electrons and holes derived from the coupling effect of BiOCl and AgCl heterojunction, which was regarded as the main reason for the high photocatalytic activity.（5） BiPO₄/BiOBr photocatalysts were successfully prepared via a facile in situ chemical transformation method. During the H₃PO₄ treatment, BiPO₄ particles were obtained through the reaction of partial BiOBr and H₃PO₄, resulting in the formation of BiPO₄/BiOBr heterojunction. The significant enhancement of photocatalytic activity could be mainly ascribed to the formation of heterojunction structure of BiPO₄/BiOBr composite which can effectively improve electron–hole separation during photocatalytic process.（6） Fe-doped BiOCl photocatalysts were synthesized by a simple combustion method. The experimental results show that Fe³⁺ was successfully doped into the lattice of BiOCl, leading to the formation of impurity energy level in forbidden band and thus decreased energy band gap and improved visible light absorption. Moreover, Fe³⁺ can trap electrons and holes, which favors the separation of electrons and holes. As a consequence, Fe-doped BiOCl photocatalysts have excellent visible light photocatalytic properties.（7） Eu³⁺-doped BiOCl with different doping concentrations were synthesized by a simple combustion method. Compared with pure BiOCl, the Eu³⁺-doped BiOCl photocatalysts exhibits much higher photocatalytic activity for Rhodamine B under UV light irradiation. The experimental results show that Eu³⁺ was successfully doped into the lattice of BiOCl and an impurity energy level was formed in the forbidden band, resulting in the decreased band gap and thus improved visible light absorption. Moreover, Eu³⁺ as a trap for electrons can decrease the recombination of electrons and holes.