Synthesis And Photocatalytic Properties Of Several Bismuth-containing Micro/Nano-structures

The fast development of human society has caused the rapid consumption of resources as well as environmental crisis. Photocatalysis, especially semiconductor photocatalysis, which can utilize the abundant solar energy for organic contaminants photodegradation, is of great significance to solve the environmental emergency. TiO2has been one of the main most extensive research topics in semiconductor photocatalysis. However, semiconductors such as TiO2cannot be widely applied because of their intrinsic large band gap and rapid recombination of the photon-generated carriers. For the practical application, it is the inevitable trend that exploiting efficient photocatalysts with visible light irradiation. In recent years, bismuth-containing semiconductors exhibit great potential applications in water splitting and degradation of organic pollutions under visible-light-response because of their low valence bands and high chemical stability, thus become more and more attractive. Researchers have already undertaken wide-ranging studies of bismuth-containing semiconductors photocatalysis.In this work, we have improved the light absorption region and the separation and migration of photon-generated carriers of bismuth-containing semiconductors (Bi2WO6and BiVO4) by morphologies control, non-metal ions doping and composite semiconductor photocatalysts, and thus the visible-light-driven photocatalytic performance. The hydro/solvo-thermal method is the main synthesis approach in our work. After detailed structure characterization, the samples were measured by degradation of RhB under visible-light irradiation. We have obtained the following conclusions.We have synthesized Bi2WO6photocatalyst with different morphologies by controlling solvent kinds and reaction parameters under solvothermal method. Bi2WO63D hierarchical structures assembled by porous nanoplates were prepared in ethylene glycol and water solvent, the unique morphology of which endows the sample with high photocatalytic activity under visible light irradiation. A possible three-stage formation mechanism was proposed for the construction of3D hierarchical structures, and Ostwald ripening was responsible for the formation of meso-pores. Furthermore, Bi2WO6hollow structures consisting of nanoparticles with diameters of10-20nm were synthesized by a facile and reproducible solvothermal method in the presence of ethylene glycol and ethanol. The proper reaction temperature and duration were beneficial to the formation of hollow structures. Meanwhile, the proposed Ostwald ripening was the key mechanism for the formation of hollow structures.Bi2WO63D hierarchical structures and Bi2WO6hollow structures both have excellent visible light response photocatalytic properties and high cycling stability. Further analysis indicated that the Bi2WO6hollow structures possessed higher photocatalytic activity due to their higher BET surface area, smaller building blocks and higher mobility of carriers compared with Bi2WO63D hierarchical structures.BiVO4ellipsoid-like structures were prepared by mixed-solvothermal method. The sample showed lower photocatalytic activity under visible light irradiation due to its low surface area and high recombination of photo-carriers. We have tried to use the following methods to improve their photocatalytic activity.(1) Calcination. The appropriate calcination temperature could increase the crystallinity of catalysts, and decrease their inner defects, and then improve their photocatalytic activity.(2) Cation doping. We have synthesized F-doped BiVO4samples using NH4F as the fluoride source by one-step solvothermal process. Small amount of F could incorporate into the lattice of BiVO4and then improve its photocatalytic activity, and the reason was that the lattice distortion and impurity energy level induced by F ion could enlarge the light absorption region and decrease the recombination of photo-carriers. However, excess amount of NH4F would lead to the surface adsorption of F ion, which changed their morphology and decrease their photocatalytic activity.(3) Morphology controllable synthesis. We found that the morphology of the sample can be turned from ellipsoid-like to sphere by introducing the surfactant Na2EDTA2H2O, accompanied with the decreased size of building-blocks. The compared experiment indicated the synergistic effect of EDTA and EG molecular, which mainly adjust the aspect ratio and size of building-block of the catalysts. The degradation results revealed the improvement of photocatalytic activity by morphology controllable process under visible-light irradiation.By using Bi2WO63D hierarchical structures as Bi source and template, we have successfully synthesized n-n type Bi2S3/Bi2WO6composites. By the sensitization of Bi2S3with narrow band gap, the photo-carriers could be separated and migrated quickly, and then the photocatalytic activity was improved. In addition, p-n type BiOCl/BiVO4composites was prepared using BiVO4ellipsoid-like structures as Bi source and template. The results of photocatalytic performance indicated that the composites showed higher activity under visible light irradiation; and its activity was further improved under simulated sunlight irradiation. Upon analyzed the band gap position and p-n junction, the build-in internal electric field in the p-n junction not only increased the migration of photo-carriers but also inhibited their recombination. The migrations of photogenerated carriers can be promoted by the internally formed electric field. BiVO4can be excited under visible light irradiation, and the excited photo-holes can migrate from the valence band of n-type BiVO4to that of p-type BiOCl; on the other hand, more and more photo-carriers could be motivated and migrated because BiVO4and BiOCl both can be excited under simulated sunlight irradiation, leading to an enhanced photocatalytic activity.