Water-Hydrogen-Polaron Coupling at Anatase TiO₂(101) Surfaces: A Hybrid Density Functional Theory Study

Defects and water generally coexist on the surfaces of reducible metal oxides for heterogeneous photocatalysis in aqueous environments, which makes quantification and understanding of their coupling essential for development of practical solutions. Here we explore and quantify the coupling between water (H₂O)- and hydrogen (H)-induced electron-polarons on the TiO₂ anatase (101) surface by means of first-principles simulations. Without H₂O, the hydrogen-induced electron-polaron localizes preferentially around the energetically favored subsurface H site. Its hopping barrier to neighboring sites in the subsurface is about 0.29 eV. Conversely, following H₂O adsorption, surface trapping of the electron-polaron becomes energetically favored, and the diffusion barrier from subsurface to surface decreases by 0.15 eV. H₂O adsorption is shown to be effective in decreasing the proton diffusion energy barrier within the same layer by reducing the polaron-proton coupling and promoting diffusion toward the subsurface in line with a recent experimental observation on water-dispersed anatase TiO₂ nanoparticles.