DFT Investigation on the Electronic and Water Adsorption Properties of Pristine and N-Doped TiO₂ Nanotubes for Photocatalytic Water Splitting Applications

Experimental studies have shown the production of hydrogen through a photocatalytic water splitting process using a titanium dioxide nanotube (TiO₂NT) as a photoelectrode. In this study, a theoretical model of pristine and nitrogen-doped TiO₂NT based on a TiO₂ anatase (101) surface is presented. Spin unrestricted density functional theory calculations were performed to provide a detailed description of the geometries, electronic properties, and adsorption of water (H₂O) on pristine and N-doped TiO₂NT. The calculations show that doping with N will improve the photocatalytic properties of TiO₂NT in two ways: First, the energy barrier of the dissociation reaction of water into hydroxyl radical and hydrogen atom is reduced; and second, the defect-induced states above the valence band lowers the band gap which will result in enhanced visible-light-driven photoactivity. Based on the position of the Fermi level relative to the defect induced energy levels, an optimal doping concentration of around 1.4% is proposed, which is in good agreement with experimental results. This study provides an atomic/molecular level understanding of the photocatalytic water splitting process and may serve as a groundwork for the rational design of more efficient photocatalysts.