Analysis of the photocurrent transient behaviour associated with flatband potential shifts during water splitting at n-TiO 2 electrodes

Abstract

The effect of local pH changes and surface hole accumulation on the photocurrent transients observed during water photoelectrolysis at TiO 2 electrodes has been studied. The diffusion of pH-determining electrolyte species is found to affect not only the time-dependent photocurrent decay but also the photocurrent vs. potential dependence. Whereas the instantaneous initial photocurrent follows the theoretically expected dependence on the band-bending, the stationary photocurrent deviates from the predicted behaviour, depending on the electrolyte pH. With the help of the rotating disk electrode technique, the ratio between the stationary and the initial photocurrent was studied as a function of the rotation rate, at both pH 10.5 and 12. Variations of band bending, associated with V fb shifts due to local pH changes, are invoked to explain the observed behaviour. At sufficiently high and low pH values, where photocurrents are not limited by the diffusion of OH − and H + electrolyte species, the transients are due exclusively to V fb shifts associated with accumulation of photogenerated holes at the semiconductor surface. These results are consistent with those obtained recently in our laboratory from electrolyte electroreflectance experiments, which allowed for the first time direct measurement of the V fb shifts occurring during photoelectrochemical water splitting.