Photoelectrochemical behaviour of methanol oxidation at nanoporous TiO2 film electrodes

Abstract

A study of photoelectrochemical oxidation of methanol at nanoporous TiO2 film electrodes was carried out. The effect of variables such as applied potential, light intensity, methanol concentration and pH on the photoelectrochemical behaviour was investigated. It was found that the photoresponse was greatly influenced by these factors and a unique photocurrent/potential characteristic of the electrode in solution was observed which was similar to that of the single crystal semiconductor electrode. It was also found that the rate-limiting step for the overall photooxidation process varied depending on potential, light intensity and methanol surface coverage. In low potential region at a given light intensity and methanol concentration the migration of electron across the TiO2 film is the rate-limiting step. While in the relatively high potential region the interfacial charge transfer was found to be the rate-limiting step. A deviation of onset potential dependence on pH from the Nernstian relationship was also observed, and based on the energy band theory and the existence of deep electron traps an explanation was given. An unusual abrupt increase of photocurrent in strong basic methanol solution was observed, which was attributed to the existence of acidic hydroxyl group on TiO2 and its stronger adsorption to methanol molecule after its reaction with OH−, and the higher reactivity of the methanol thus adsorbed.