Water structure, dynamics and reactivity on a TiO2-nanoparticle surface: new insights from ab initio molecular dynamics

Abstract

Water structure, dynamics and reactivity at the surface of a small TiO₂-nanoparticle fully immersed in water was investigated by an ab initio molecular dynamics simulation. Several modes of water binding were identified by assigning each atom to an atom type, representing a distinct chemical environment in the ab initio ensemble, and then computing radial distribution functions between the atom types. Surface reactivity was investigated by monitoring how populations of atom types change during the simulation. In order to acquire further insight, electron densities for a set of representative system snapshots were analyzed using an atoms-in-molecules approach. Our results reveal that water dissociation, where a water molecule splits at a bridging oxygen site to form a hydroxyl group and a protonated oxygen bridge, can occur by a mechanism involving transfer of a proton over several water molecules. The hydroxyl group and protonated oxygen bridge formed in the process persist (on a 10 ps time scale) and the hydroxyl group undergoes exchange using a mechanism similar to the one responsible for water dissociation. Rotational and translational dynamics of water molecules around the nanoparticle were analyzed in terms of reorientational time correlation functions and mean square displacement. While reorientation of water O–H vectors decreases quickly in the proximity of the nanoparticle surface, translational diffusion slows down more gradually. Our results give new insight into water structure, dynamics and reactivity on TiO₂-nanoparticle surfaces and suggest that water dissociation on curved TiO₂-nanoparticle surfaces can occur via more complex mechanisms than those previously identified for flat defect-free surfaces.