Enhancing oxygen vacancy photocatalytic efficiency of bismuth tungstate using In-doped W site

Abstract

Pure and In-doped orthorhombic Bi<sub>2</sub>WO<sub>6</sub> are synthesized by sol-gel method through using raw materials Bi(NO<sub>3</sub>)<sub>3</sub>·5H<sub>2</sub>O, In(NO<sub>3</sub>)<sub>3</sub>·6H<sub>2</sub>O, (NH<sub>4</sub>)<sub>2</sub>WO<sub>4</sub> and surfactants citric acid, polyethylene glycol. All samples are in pure phase without impurity phase as indicated by X-ray diffraction characterization. The In-doped sample degradation efficiency for rhodamine B is higher than that for pure phase with the optimal content 7% mole ratio. Because indium impurity adhering to Bi<sub>2</sub>WO<sub>6</sub> nucleus surface may affect the crystallization range, the sample morphology gradually becomes fluffy and regular, which is reveled through scanning electron microscopy analysis. This morphology change plays an important role in electron-hole transport process as well as contact area of carrier and organic molecule. Using X-ray photoelectron spectroscopy (XPS) characterization and Gaussian fitting, it is found that the O 1s XPS peak of pure and In-doped sample each contain three peak sites. The low energy peak around 530 eV originates from W—O and Bi—O bond. The high peak is ascribed to lattice oxygen defect and its intensity is enhanced gradually with the increase of In content. Thus the increase of oxygen vacancies is the main reason for this photocatalytic performance improvement. Comparing with the impurity-free sample, the visible absorption of In-doped Bi<sub>2</sub>WO<sub>6</sub> is enhanced and the corresponding band gap slightly decreases, which is indicated by diffraction reflection spectroscopy measurement. The reduction of forbidden band width further enhances the photocatalytic performance. After configuration relaxation and self-consistence calculation, the formation energy obtained from a single oxygen vacancy model is less than those from the Bi1<sub>In</sub> + V<sub>O</sub> and the Bi2<sub>In</sub> + V<sub>O</sub> co-doping models, and greater than the W<sub>In</sub> + V<sub>O</sub> formation energy. This result indicates that indium replacing W site can promote the generating of oxygen vacancies. The calculation of the 18%-hybridization function electronic structure shows that the Bi<sub>2</sub>WO<sub>6</sub> has indirect band gap semi-conduction with energy gap 2.76 eV, which is consistent with the experimental value 2.79 eV. A series of new local states appears in the band gap and near conduction band bottom based on the oxygen vacancy model. These local states promote light absorption and enhance photocatalytic performance. In conclusion, the enhanced photocatalytic performance of Bi<sub>2</sub>WO<sub>6</sub> is attributed to the indium entering into the tungsten site rather than the bismuth site as indicated by the experimental and theoretical result.