Room temperature bilayer water structures on a rutile TiO₂(110) surface: hydrophobic or hydrophilic?

The lack of understanding of the molecular-scale water adsorbed on TiO₂ surfaces under ambient conditions has become a major obstacle for solving the long-time scientific and applications issues, such as the photo-induced wetting phenomenon and designing novel advanced TiO₂-based materials. Here, with the molecular dynamics simulation, we identified an ordered water bilayer structure with a two-dimensional hydrogen bonding network on a rutile TiO₂(110) surface at ambient temperature, corroborated by vibrational sum-frequency generation spectroscopy. The reduced number of hydrogen bonds between the water bilayer and water droplet results in a notable water contact angle (25 ± 5°) of the pristine TiO₂ surface. This surface hydrophobicity can be enhanced by the adsorption of the formate/acetate molecules, and diminishes with dissociated H₂O molecules. Our new physical framework well explained the long-time controversy on the origin of the hydrophobicity/hydrophilicity of the TiO₂ surface, thus help understanding the efficiency of TiO₂ devices in producing electrical energy of solar cells and the photo-oxidation of organic pollutants.