Structure and Reactivity of Water-covered Anatase TiO2 (001) Surface

We systematically studied water adsorption and oxidation on the anatase TiO2(001)-(1×1) surface using first-principles calculations. Water firstly adsorbs on the surface in a dissociative state and then in a molecular state, as water coverage increases. The geometric propertise of all adsorption strucutres suggest that the dissociative water molecules can induce stress release in the (001) surface at low coverage, reducing reactivity of the surface and thus leading to molecular adsorption of water on the surface at high coverage. The adsorption energy (or the surface energy) monotonously increases (or decreases) with increase of the coverage, which further confirms that water, irrespectively of dissociative or molecular state, can improve stability of the (001) surface and reduce its activity. We deeply investigated mechanism of the oxygen evolution reaction (OER) on the water-covered (001) surface. A new water-assisted OER pathway is identified on the (001) surface, wchic includes the sequential transfer of protons from molecular water and surface hydroxyls, and the O-O coupling processes. During the OER pathway, the O-O coupling step exhibits the largest thermodynamic energy and highest energy barrier, clarifying that it is the rate-determining step in the whole pathway. Our findings provide new insights into strong dependence of water adsorption modes on the coverage for the anatase TiO2(001) surface and may explain the high oxidation activity of TiO2(001) surface in the aqueous environments typical of TiO2 photocatalysis.