Anodized Bi₂O₃ film prepared in NaOH and oxalic acid and the photocatalytic activity in organic dye degradation

Anodic oxidation was utilized to prepare Bi₂O₃ film on pure bismuth foil in NaOH and oxalic acid electrolytes. In NaOH electrolyte, β-phase Bi₂O₃ film with a flat surface was prepared. In oxalic acid, β and ω mixed-phase Bi₂O₃ film consisting of nanorods was formed. Although both the film showed intense visible light absorption, the sample prepared in NaOH showed an overall shift of VBM and CBM towards negative potential compared the sample prepared in oxalic acid. The shift weakened the oxidizing power of photo-generated holes; however, it made it possible to form \({}^{ \cdot }{\rm O}_{2}^{ - }\) species through one-electron transfer reaction between photo-generated electrons and oxygen. It enabled the conversion of the key oxidizing species from ^·OH radicals to \({}^{ \cdot }{\rm O}_{2}^{ - }\) radicals in the film prepared in NaOH. Although the film prepared in NaOH possessed higher charge separation efficiency, charge carrier transfer resistance, and greater amounts of ^·OH and \({}^{ \cdot }{\rm O}_{2}^{ - }\) species, the film prepared in oxalic acid showed a slightly higher photocatalytic activity in the degradation of MB dye under visible light irradiation. This is explained by the fact that the latter had a greater specific surface area than the former. In addition, mixed-phase effect may also help to the photocatalytic activity improvement.