Tunable reactivity of anatase TiO₂(101) by oxygen defects towards explicit aqueous H₂O ad-layers with implications for surface passivation: a DFT study

A comprehensive understanding of surface reactivity at the atomic level is key to processes at the interfaces between solids and electrolytes in terms of corrosion, oxidation, catalysis, etc. By explicitly including H₂O adlayers, the interactions between aqueous environment and oxygen vacancy (Vo) are investigated using density functional theory at the TiO₂(101)/H₂O interface. The H₂O monomer is preferentially molecularly adsorbed on TiO₂ in a flat mode with an adsorption energy of -0.67 eV. With increasing coverage, the adsorption energy increases by <0.2 eV, which is mainly due to covalent O-Ti bonds at low coverages, while synergistic O-Ti and hydrogen bonding occurs at high coverages. While no spontaneous H₂O dissociation occurs, it can be enhanced by oxygen vacancies (Vo), tensile strain, and possibly solvation. In addition, the formation and diffusion of Vo is nearly prohibited on perfect anatase or with pre-adsorbed O, leading to re-passivation. However, the reaction barrier of Vo can be reduced by the formation of a charged O₂ dimer. Pre-adsorbed H contributes to de-passivation by significantly reducing the diffusion barrier for Vo. The fundamental insights about the correlation between the reactivities of TiO₂(101) and the stability of Vo have far-reaching implications for electrochemical corrosion and catalysis.