The Nature of Water Nucleation Sites on TiO<sub>2</sub>(110) Surfaces Revealed by Ambient Pressure X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy at ambient conditions of pressure (up to 1.5 Torr) and temperature (265 K < <i>T</i> < 800 K) was used to study the adsorption of water on rutile TiO<sub>2</sub>(110) under conditions of thermodynamic equilibrium. It was found that OH groups in bridging positions, normally present in small amounts due to residual O-vacancies, act as nucleation sites for subsequent water adsorption. The adsorption enthalpy of water binding to these sites is ∼70 kJ/mol, much stronger than that in the bulk liquid (45 kJ/mol). A model is proposed that relates the structure of the oxide surface to its hydrophilic character.