Anodic behaviour of vanadium pentoxide under illumination

The anodic behaviour of n-type vanadium pentoxide (V2O5) single crystal electrodes was investigated under band-gap illumination in acid solutions of pH 1 -- 5. It was shown that the potentials (E*), at which anodic photocurrents (i*) were observable, coincided with the flat-band potentials (E sub(fb)) obtained from capacity measurements throughout the pH range. A Tafel region (A) and a succeeding pseudo-saturation region (B) were found in the i*-potential curves, irrespective of electrolyte pH. The transfer of holes from the electrode surface to some species on the solution side, and the diffusion of holes from the interior of the electrode toward the surface were proposed as the rate determining steps of the photo-electrochemical reaction for regions (A) and (B) respectively. In both regions, it was considered that the recombination of electrons and holes in the space-charge layer was concerned. From the pH dependence of i* on the Tafel lines at constant (E-E sub(fb)) (i.e., at constant potential drop across the space-charge layer), the hole-capturing species, which had been thought to play catalytic roles in the oxygen evolution reaction, were identified with the V(V) ions of VO2+ (pH<2) or H3V2O7-(pH>2) dissolved from the electrode materials. Furthermore, it was shown that the photocurrents were increased by the addition of iodide ions (I-) to the electrolytes, because the specific adsorption of I- caused a negative shift of E*, and accordingly of E sub(fb). (auth.)