Preparation, Characterization And Their Photocatalytic Performances Of Modified Bi₂Wo₆Photocatalysts

Bi₂Wo₆was regarded as a narrow band gap semiconductor with a band energy of2.7eV， exhibiting the excellent visible-light-induced photocatalytic performance.However, in general, a larger crystalline size of Bi₂Wo₆was obtained owing to its largemolecular weight, which resulted in a higher recombination rate of photo-inducedcharge carriers. Thus, the photocatalytic activity of Bi₂Wo₆was lower than thetheoretical value in practice use. To obtain the highly active Bi₂Wo₆photocatalyst, inthis work, the modification of Bi₂Wo₆was carried out by the doping of differentnonmetallic elements of I, S, N and C via a hydrothermal method. The crystalline phasecomposition, microstructure and optical properties of modified Bi₂Wo₆and thechemical states of doping elements were investigated by XRD、SEM、TEM、FT-IR、Raman、XPS and UV-Vis diffuse reflectance spectra. Moreover, the effects of differentmodification methods on the photocatalytic performances were also discussed.The substitution of lattice oxygen was very difficult to be carried out owing to thelarge atomic radius of iodine species. In this study, the mechanism of the surfaceco-doping of I2and I-was proposed by the analysis of the chemical states of iodinespecies. The photo-absorption property and the photo-degradation of RhB indicated thatI2and I-as the surface dopant not only improved the visible-light photo-responsivity,but also enhanced the separation of photo-induced electrons and holes efficiently,resulting in the enhanced visible-light-induced photocatalytic activity.The atomic radius of sulfur is smaller than that of iodine, but is larger than that ofoxygen. Therefore, only one small amount of sulfur was doped into the lattice ofBi₂Wo₆, the other of sufur was used to form the crystalline phase of Bi2S3. Bi2S3as asemiconductor with a narrow band gap, would result in the narrowing of band gap bythe photosensitization for Bi₂Wo₆. The improved visible-light photoresponsivity andphotocatalytic activity were caused by the synergetic effect of sulfur-doping andBi2S3/Bi₂Wo₆heterojunction.Different from the cases of iodine-doping and sulfur-doping, the substitutionaldoping was prone to be carried out by the substitution of the lattice oxygen atom withnitrogen atom owing to the similar atomic radius of nitrogen and sulfur elements. The nitrogen-doping of Bi₂Wo₆was carried out by using urea as a nitrogen source,improving the visible-light-induced photocatalytic activity. The TiO₂/N-BWOheterojunction was formed by the coating of TiO₂on the surface of nitrogen-dopedBi₂Wo₆particles, resulting in the enhanced ultraviolet-light and visible-light-inducedphotocatalytic activities. Nitrogen doping resulted in the narrowing of band gap, whilethe TiO₂/N-BWO heterojunction inhibited the recombination of photo-induced chargecarriers efficiently.Although the atomic radius of carbon species is slightly larger than that of nitrogenspecies, the lattice oxygen atom was also substituted efficiently by carbon atom owingto the similar atomic radius of carbon and oxygen elements. The doping modification ofBi₂Wo₆was carried out using graphene as a carbon source. The carbon doping andgraphene/Bi₂Wo₆heterojunction was formed together. The substitutional doping ofcarbon species was obtained efficiently by avoiding the disadvantage of nitrogen sourcein the procedure of nitrogen doping. The narrowing of band gap was caused by thecarbon doping, improving the visible-light photo-responsivity. The efficient transfer rateof photo-induced electrons was caused by the heterojunction of graphene/Bi₂Wo₆.