Diffusion and reaction pathways of water near fully hydrated TiO₂ surfaces from ab initio molecular dynamics

Ab initio molecular dynamics simulations are reported for water-embedded TiO₂ surfaces to determine the diffusive and reactive behavior at full hydration. A three-domain model is developed for six surfaces [rutile (110), (100), and (001), and anatase (101), (100), and (001)] which describes waters as "hard" (irreversibly bound to the surface), "soft" (with reduced mobility but orientation freedom near the surface), or "bulk." The model explains previous experimental data and provides a detailed picture of water diffusion near TiO₂ surfaces. Water reactivity is analyzed with a graph-theoretic approach that reveals a number of reaction pathways on TiO₂ which occur at full hydration, in addition to direct water splitting. Hydronium (H₃O⁺) is identified to be a key intermediate state, which facilitates water dissociation by proton hopping between intact and dissociated waters near the surfaces. These discoveries significantly improve the understanding of nanoscale water dynamics and reactivity at TiO₂ interfaces under ambient conditions.