Investigation of photoelectrochemical oxidation of Fe2+ ions on porous nanocrystalline TiO2 electrodes using electrochemical quartz crystal microbalance

The photoelectrochemical oxidation of Fe2+ ions on porous nanocrystalline TiO2 electrodes was investigated using in situ electrochemical quartz crystal microbalance (QCM). It was found that the adsorption of Fe2+ ions at the surface of TiO2 electrodes strongly depended on the pH of the solution, and that the saturated amount of adsorbed Fe2+ ions was approximately 4.4 ions nm-2 at pH 4. The protons produced in photochemical reactions could cause the release of adsorbed Fe2+ ions from TiO2 surface, therefore, the frequency of QCM increased at the initial stage of illumination, during which the direct oxidation of adsorbed Fe2+ ions by photogenerated holes was dominant. When the frequency decreased, the major reaction was the photoelectrochemical deposition of Fe2O3 from the bulk solution via a hydroxyl radical-mediated oxidation pathway, whose efficiency depended on the Fe2+ ion concentration