Local ordering and electronic signatures of submonolayer water on anatase TiO₂(101)

The interaction of water with metal oxide surfaces is of fundamental importance to various fields of science, ranging from geophysics to catalysis and biochemistry. In particular, the discovery that TiO"2 photocatalyses the dissociation of water has triggered broad interest and intensive studies of water adsorption on TiO₂ over decades. So far, these studies have mostly focused on the (110) surface of the most stable polymorph of TiO ₂, rutile, whereas it is the metastable anatase form that is generally considered photocatalytically more efficient. The present combined experimental (scanning tunnelling microscopy) and theoretical (density functional theory and first-principles molecular dynamics) study gives atomic-scale insights into the adsorption of water on anatase (101), the most frequently exposed surface of this TiO₂ polymorph. Water adsorbs as an intact monomer with a computed binding energy of 730meV. The charge rearrangement at the molecule-anatase interface affects the adsorption of further water molecules, resulting in short-range repulsive and attractive interactions along the [010] and directions, respectively, and a locally ordered (2×2) superstructure of molecular water.