Wet Environment Effects for Ethanol and Water Adsorption on Anatase TiO2 (101) Surfaces

Titanium dioxide exhibits superior photocatalytic properties, mainly occurring in liquid environments through molecular adsorptions and dissociations at the solid/liquid interface. The presence of these wet environments is often neglected when performing ab initio calculations for the interaction between the adsorbed molecules and the TiO2 surface. In this study, we consider two solvents, that is, water and ethanol, and show that the proper inclusion of the wet environment in the methodological scheme is fundamental for obtaining reliable results. Our calculations are based on structure predictions at a density functional theory level for molecules interacting with the perfect and defective anatase (101) surface under both vacuum and wet conditions. A soft-sphere implicit solvation model is used to describe the polar character of the two solvents. As a result, we find that surface oxygen vacancies become energetically favorable with respect to subsurface vacancies at the solid/liquid interface. This aspect is confirmed by ab initio molecular dynamics simulations with explicit water molecules. Ethanol molecules are able to strongly passivate these vacancies, whereas water molecules only weakly interact with the (101) surface, allowing the coexistence of surface vacancy defects and adsorbed species. The infrared and photoluminescence spectra of anatase nanoparticles predominantly exposing (101) surfaces dispersed in water and ethanol support the predicted molecule-surface interactions, validating the whole computational paradigm. The combined analysis allows for a better interpretation of TiO2 processes in wet environments based on improved computational models with implicit solvation features.