Photooxidation of Water on Pristine, S- and N-Doped TiO2(001) Nanotube Surfaces: A DFT + U Study

Using density functional theory calculations, we study the structure, energetics, and the photoelectrochemical oxidation of water on pristine, S-, N-, and (N + S)-doped anatase TiO 2 (001) nanotube (NT) surfaces. We found that water adsorbs molecularly on pristine and S-doped surfaces, while N doping promotes dissociative adsorption (both in the presence and absence of the S codopant) and leads to more favorable adsorbate–substrate interactions. Under photoelectrochemical conditions, OH groups are the most stable species on each surface with decreasing stability in the sequence (N + S) ≈ N → S → pristine. Surface Ti 5C are the active sites and the anion impurity sites are not structurally affected during the water oxidation reaction. Nanostructuring TiO 2 by forming three monolayer-thick (3 ML) TiO 2 (001) NT surfaces and subsequent anion doping yield an overpotential drop from 1.31 V on the flat (2D) TiO 2 (001) surface to 0.90, 0.71, 0.94, and 0.96 V on pristine, S-, N-, and (N + S)-doped nanotube surfaces, respectively. This reduction is a consequence of the strain-induced weakening of hydroxyl adsorption on the NT surfaces; the presence of an N dopant atom does not change the overpotential relative to the pristine nanotube, irrespective of the presence of a codoped S atom, while single S doping produces a slight decrease of the overpotential by 0.2 V. In all cases, the overpotential-determining step is the hydroxyl group dehydrogenation.