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 photocur rent-potential curves similar to those for single-crystal n-TiO2 elect rodes, though the TiO2 particles are insulating and simply deposited o n conductive substrates without making any ohmic contact. The reason f or such an apparently curious phenomenon has been investigated by exam ining 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), ind icating 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 ab sence of it, especially in acidic solutions. The large Delta U implies that a photocurrent starts to flow by a positive shift of the electro de potential from U-fb or, in other words, by formation of certain ban d inclination in some TiO2 particles. Two plausible models, extended S chottky junction and Bardeen-type junction, are proposed for the elect rical junction at the TiO2 particle/substrate interface, and it is sug gested that the Bardeen-type junction is more plausible, explaining al l the experimental results.