The nature of water nucleation sites on TiO2(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 < T < 800 K) was used to study the adsorption of water on rutile TiO2(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 similar to 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.