Acid-Resistant BiVO4 Photoanodes: Insolubility Control by Solvents and Weak W Diffusion in the Lattice.

We have revealed for the first time that BiVO4 photoanodes can be used even in strong acid media by mixing organic solvents into the electrolyte and depositing multilayers with a WO3 bottom layer. In general, the BiVO4 photoanodes are photocorrosive, especially in acid solutions. However, this shortcoming has been overcome using a combination of the two aforementioned modifications. We deduced that the contribution of each mixing organic solvent for the anti-photocorrosion of BiVO4 in sulfuric acid solutions can be evaluated on the basis of a new empirical indicator that incorporates molecular density, the Hansen solubility parameter, and molecular polarizability. Acetone and tert-butyl alcohol were especially promising solvents for stabilizing BiVO4 in acid media. We confirmed that the mixed organic solvents stabilized surface-emergent Bi oxide species as a passivation layer, which was generated via multilayering with a WO3 bottom layer. During heat treatment in the fabrication process, W weakly diffused into the BiVO4 layer and a Bi oxide layer was formed on the outermost surface because of the Bi segregation that arose from the charge compensation between W6+ and V5+ in the BiVO4 lattice. The surface Bi oxide layer, which was protected by the mixed organic solvents, steadily served as a passivation layer for anti-photocorrosion of the underlying BiVO4 layer. We have confirmed that the BiVO4/WO3 photoanodes in acetone-mixed aqueous sulfuric acid solution reliably functioned for a photoelectrochemical reaction under simulated sunlight illumination, and photoelectrochemical production of S2O82- ions was confirmed under light irradiation at λ > 480 nm. These results suggest that the BiVO4-based photoanodes have significant potential for use in acid media in conjunction with very straightforward modifications.