Water adsorption on clean and defective anatase TiO2(001) nanotube surfaces: a surface science approach

Nanostructuring TiO₂, e.g., by the formation of nanotubes (NTs) with large surface area is an efficient way to improve the photocatalytic performance of anatase TiO₂. In this respect, TiO₂ (001) NTs are promising candidates for hydrogen production via photocatalytic water splitting, as their visible-light-driven photocatalytic response can be tuned by N and S dopant atoms. As water is necessarily present in operando, it is essential to simulate not only the pure material but also the interface with at least a film of water molecules, together with its molecular and chemical dynamics. Using a 2D slab representation of the nanotube which takes into account the strain and partial curvature of the NT, we employ ab initio molecular dynamics simulations to study the adsorption of thin films of water ( 1ML and 2ML) on anatase TiO₂ (001) nanotubes. The effect of anion doping on adsorption and surface chemistry is investigated by substituting O atoms by N and S impurities on the NT slab surface. While an S dopant weakens the interaction of the surface with water, which adsorbs molecularly, an N impurity renders the surface more reactive to water with a surface-assisted proton transfer taking place in the water film, and the formation of an OH^- group and a NH2⁺ cation on the surface.