Bulk versus surface state contributions to the efficiency of perovskite titanate photoanodes

Abstract

Attempts have been made to enhance the photon efficiency of perovskite titanate electrodes during band gap illumination by increasing the bulk dielectric constant with the substitution of suitable isovalent ions in the lattice so that the Curie point is shifted to room temperature and by decreasing the ND values with controlled chemical reduction. The photoresponse showed only marginal improvements with these changes in the bulk characteristics. In comparison, the efficiency increased considerably when the electrode surfaces are treated by exposing to dilute acids, particularly HNO3 + HF, for extended periods of time. This is accompanied by the changes in Von to cathodic direction, red shift in the spectral dependence of photoresponse and marginal decrease of ND in the space charge layer. X-ray photoelectron spectra show that the treated electrode surfaces are not uniform with respect to oxygen/titanium ratios, hydroxyls and fluoride incorporated. The enhanced photon efficiency arises from the combined effect of eliminating the undesirable electron recombination centres in the space charge layer and the presence of heterogeneous surface regions leading to non-uniform potential distribution near the electrode surface. The results also point to the limitations of applying Schottky barrier model for semiconductor/electrolyte interface with higher concentrations of surface states. © 1988 the Indian Academy of Sciences.