Water Splitting by Nanocrystalline TiO₂ in a Complete Photoelectrochemical Cell Exhibits Efficiencies Limited by Charge Recombination

We report on the mechanism of water splitting by TiO₂ in the absence of chemical scavengers in a fully functional photoelectrochemical (PEC) cell. The application of a positive potential to a nanocrystalline-TiO₂ film is shown to lead to the formation of long-lived holes which oxidize water on the milliseconds time scale. These first time-resolved studies of a nanocrystalline-TiO₂ (nc-TiO₂) film in a complete PEC cell also showed that all of the long-lived photoholes go on to generate O₂, and that there are no major branching inefficiencies in the catalysis itself, which appears to be operating at efficiencies close to 100%. The overall quantum yield of oxygen production under pulsed illumination (355 nm) was found to be ∼8% at excitation densities of 4.4 photons per particle. Under all conditions examined, electron−hole recombination was found to be the dominant loss pathway.