Vanishing of Current-Doubling Effect in Photooxidation of 2-Propanol on TiO₂ in Solutions Containing Fe(III) Ions

When organic compounds such as 2-propanol are in aqueous solutions and are photooxidized on the surface of semiconductor electrodes, the current quantum efficiency can reach close to 200% (current-doubling effect). However, we found that the current-doubling effect in the photooxidation of 2-propanol on TiO 2 electrodes disappeared in solutions containing Fe(III) ions. From the analyses of the products, we found that acetone was produced in quantitative correlation to the photocurrent, and that practically no Fe(II) ions were generated in the solution. We also found that Fe(II) ions were not oxidized at a TiO 2 photoanode in a solution containing Fe(III) ions, while they were oxidized in quantitative correlation to the photocurrent in the solution without Fe(III) ions. These results refuted the possibility that Fe(III) ions accepted electrons from the oxidation intermediates of 2-propanol. The plausible reason for the disappearance of the current-doubling effect is that 2-propanol is oxidized by two holes on the Fe(III)-adsorbed TiO 2 surface.