Photoelectrochemical properties of polycrystalline TiO2 electrodes: Anomalous photoeffects

We combined optical and impedance measurements with conventional electrochemical techniques to investigate the photoprocesses that occur on polycrystalline TiO₂ electrodes in the presence of reducible adsorbates in solution. The semiconductor-electrolyte junction is treated as a simple Schottky barrier. The photopotential, photocurrent and capacitance are described using this model. The observed anomalous large cathodic photoeffects on n-type TiO₂ result from either anodic photoproduction or the diffusion of the dissolved oxygen adsorbed on the electrode surface. Anomalous photocurrents occur together with relatively large dark currents, which are caused in n-type semiconductors by electron transfer from the conduction band to the interface via tunnelling through the space charge layer. The influence of the surface states on the determination of the flat-band potential by measurements of either the photocurrent or the impedance was also examined. It was found that reliable results could be obtained by scanning from cathodic to anodic potentials after prepolarization at -1.5 V. The value determined in this way for the flat-band potential was -0.5±0.05 V.