Free energy of proton transfer at the water-TiO₂ interface from ab initio deep potential molecular dynamics

TiO₂ is a widely used photocatalyst in science and technology and its interface with water is important in fields ranging from geochemistry to biomedicine. Yet, it is still unclear whether water adsorbs in molecular or dissociated form on TiO₂ even for the case of well-defined crystalline surfaces. To address this issue, we simulated the TiO₂-water interface using molecular dynamics with an ab initio-based deep neural network potential. Our simulations show a dynamical equilibrium of molecular and dissociative adsorption of water on TiO₂. Water dissociates through a solvent-assisted concerted proton transfer to form a pair of short-lived hydroxyl groups on the TiO₂ surface. Molecular adsorption of water is ΔF = 8.0 ± 0.9 kJ mol^-1 lower in free energy than the dissociative adsorption, giving rise to a 5.6 ± 0.5% equilibrium water dissociation fraction at room temperature. Due to the relevance of surface hydroxyl groups to the surface chemistry of TiO₂, our model might be key to understanding phenomena ranging from surface functionalization to photocatalytic mechanisms.