Nature of electrical junction at the TiO2 substrate interface for particulate TiO2 film electrodes in aqueous electrolytes

Particulate TiO2 film electrodes in aqueous electrolytes show photocurrent-potential curves similar to those for single-crystal n-TiO2 electrodes, though the TiO2 particles are insulating and simply deposited on conductive substrates without making any ohmic contact. The reason for such an apparently curious phenomenon has been investigated by examining the effect of changing the work function of the substrate (e phi(w)). The onset potential of photocurrent (U-on) for particulate TiO2 film electrodes remains nearly the same, irrespective of e phi(w), indicating that a simple Schottky junction model cannot be applied to the TiO2/substrate contact. On the other hand, the deviation (Delta U) of U-on from the flat-band potential of single-crystal n-TiO2 electrodes (U-fb) is nearly zero in the presence of ethanol, but large in the absence of it, especially in acidic solutions. The large Delta U implies that a photocurrent starts to flow by a positive shift of the electrode potential from U-fb or, in other words, by formation of certain band inclination in some TiO2 particles. Two plausible models, extended Schottky junction and Bardeen-type junction, are proposed for the electrical junction at the TiO2 particle/substrate interface, and it is suggested that the Bardeen-type junction is more plausible, explaining all the experimental results.