Mechanistic aspects of the photooxidation of water at the n-TiO/aqueous interface: optically induced transients as a kinetic probe

The method of optically induced photocurrent transients is a powerful tool for probing the mechanisms of photoinduced charge transfer at the semiconductor/electrolyte interface. Using this technique on the TiO₂/aqueous interface, we conclude that water oxidation occurs via an outer-sphere process for electrolyte pH <12 and an inner-sphere mechanism at more basic pH's. Hydrogen isotope effects on the observed transient substantiate this finding. These isotope effects along with selective poisoning studies suggest a ″Ti-OH″ surface species as the key intermediate for the inner-sphere channel, along with the cleavage of a hydrogen bond as the rate-limiting process (H/D kinetic isotope effect of ≈ 10) in basic electrolyte. These studies further indicate a strong crystal face dependence on the observed photoelectrochemistry.