Structure of a Water Monolayer on the Anatase TiO$_2$(101) Surface

TL;DR

This study uses first-principles calculations to clarify the structure of the water monolayer on anatase TiO$_2$(101), revealing a mix of dissociated and molecular water, and explaining experimental superstructures.

Contribution

It provides new insights into water adsorption energetics and the nature of the water monolayer on TiO$_2$(101) using advanced computational methods.

Findings

Evidence for a mix of dissociated and molecular water in the monolayer

Reduced energy cost of water dissociation via hydroxyl-water complexes

Explanation of observed superstructure in STM experiments

Abstract

Titanium dioxide (TiO$_2$) plays a central role in the study of artificial photosynthesis, owing to its ability to perform photocatalytic water splitting. Despite over four decades of intense research efforts in this area, there is still some debate over the nature of the first water monolayer on the technologically-relevant anatase TiO$_2$ (101) surface. In this work we use first-principles calculations to reverse-engineer the experimental high-resolution X-ray photoelectron spectra measured for this surface in [Walle et al., J. Phys. Chem. C 115, 9545 (2011)], and find evidence supporting the existence of a mix of dissociated and molecular water in the first monolayer. Using both semilocal and hybrid functional calculations we revise the current understanding of the adsorption energetics by showing that the energetic cost of water dissociation is reduced via the formation of a hydrogen-bonded hydroxyl-water complex. We also show that such a complex can provide an explanation of an unusual superstructure observed in high-resolution scanning tunneling microscopy experiments.