Periodic hybrid-DFT study on the N-doped TiO2 (001) nanotubes as solar water splitting catalysts: A comparison with the rutile and anatase bulk phases

TiO2 is a routinely used catalyst due to its photocatalytic activity. The main problem of TiO2 is its large band gap. Doping metal and non-metal atoms in TiO2 crystals reduce the band gap. In this work, first-principle calculations by the full-potential numeric atom-center orbital theory were used to investigate detailed electronic properties of N-doped one monolayer (1 ML) and two monolayers (2 ML) (4,4), (8,8) and (16,16) TiO2 (001) nanotubes. The proposed model first validated by comparing its data with the experimental available data for rutile and anatase bulk phases, and then was used to predict the electronic properties of proposed nanotubes. PBE0 hybrid method was used to align the valence and conduction bands and to predict their standard reduction potential. In addition, different routine generalized gradient approximation (GGA) and meta-GGA exchange-correlation functionals were used to show how much their errors are in predicting the band gaps of nanotubes. Hybrid data showed that a 2 ML (8,8) nanotube is a proper candidate for photocatalyst not only for its band gap but also from its formation energy point of view. Contrary to the N-doped rutile and anatase phases, there is no electron-hole recombination center for the N-doped 2 ML (8,8) TiO2 nanotube.