Pathway of Photocatalytic Oxygen Evolution on Aqueous TiO₂ Anatase and Insights into the Different Activities of Anatase and Rutile

The photocatalytic oxidation of water to molecular oxygen is a key step toward the conversion of solar energy to fuels. Understanding the detailed mechanism and kinetics of this reaction is important for the development of robust catalysts with improved efficiency. TiO₂ is one of the best-known photocatalysts as well as a model system for the study of the oxygen evolution reaction (OER). Here we use hybrid density functional based energetic calculations and first-principles molecular dynamics simulations to investigate the pathway and kinetics of the OER on the majority (101) surface of anatase TiO₂ in a water environment. Our results show that terminal Ti-OH groups are stable intermediates at the aqueous (101) interface, in accord with the experimental observation that OH radicals are efficiently produced on anatase. Oxidation of Ti-OH gives rise to a second stable intermediate, a surface-bridging peroxo dimer ((O₂^2-)_br) composed of one water and one surface lattice oxygen atom, consistent with the surface peroxo intermediates revealed by "in situ" measurements on rutile. Our calculations further predict that molecular oxygen evolves directly from (O₂^2-)_br through a concerted two-electron transfer, thus leading to oxygen exchange between TiO₂ and the adsorbed species. Oxygen exchange is found to be negligible on rutile, so that different OER pathways are likely to be operative on the two main TiO₂ polymorphs. This difference could explain the observed lower OER activity of anatase relative to rutile.