Photoelectron Spectroscopy Study of Stoichiometric and Reduced Anatase TiO₂(101) Surfaces: The Effect of Subsurface Defects on Water Adsorption at Near-Ambient Pressures

X-ray photoelectron (XPS) experiments at normal and grazing emission are performed, demonstrating the labile nature of the anatase TiO2(101) surface after argon cluster ion sputtering and the propensity of oxygen vacancies to migrate subsurface at room temperature. Near-ambient XPS (NAP-XPS) shows that molecular water adsorbs on the anatase TiO2(101) surface at pressures of 0.6 mbar and above, at room temperature, in a mixed molecular and dissociated state. Water adsorbs in a similar fashion on both sputtered and stoichiometric surfaces and reaches a saturation point between 0.6 and 1.8 mbar at room temperature. This means there is little difference in reactivity with regards to water adsorption on both sputtered and stoichiometric surfaces, giving credence to the theory that anatase has superior photocatalytic activity over rutile due to the tendency of oxygen vacancies to lie subsurface, therefore being able to contribute to photocatalysis without being quenched by adsorbates.