Mechanisms of water photooxidation at n-TiO2 rutile single crystal oriented electrodes under UV illumination in competition with photocorrosion

Photoetching is known to compete with water photooxidation at n-TiO₂ rutile electrodes in contact with aqueous H₂SO₄ solutions under UV illumination and anodic bias. A mechanism based on the generation of bridging hydroxyl species from the adsorption of water molecules at photoinduced bridging oxygen vacancies is proposed in order to explain the competition between both photoreactions. This mechanism, designated as Redox Photooxidation (RP) Mechanism, correlates the atomic arrangement of the TiO₂ surface with its photocatalytic activity, considering that the first step for water photooxidation is the photogeneration of bridging oxygen/hydroxyl radicals associated with intrinsic bandgap surface states, via inelastic transfer of free valence band holes to bridging oxygen/hydroxyl groups, depending on the electrolyte pH. The critical distance between adjacent bridging oxygen/hydroxyl radicals allows their covalent bonding with generation of surface-bound peroxide species, which are further photooxidized leading to oxygen evolution. The RP mechanism allows to explain literature experimental results concerning surface modifications of n-TiO₂ rutile during photoetching in competition with water photooxidation, as well as their dependence on crystal orientation. The photogeneration of chemisorbed peroxo species, intermediates of the oxygen evolution reaction, detected by MIRIR spectroscopy, as well as experimental results obtained from PL and DEMS experiments are also interpreted in the light of the RP mechanism. A comparative analysis with the nucleophilic attack (NA) Mechanism, an alternative model proposed recently to explain photoelectrochemical water oxidation at n-TiO₂ rutile, is presented.