Effects of Nitrogen/Fluorine Codoping on Photocatalytic Rutile TiO₂ Crystal Studied by First-Principles Calculations

Nitrogen/fluorine codoping of rutile TiO₂ was recently reported to be effective for introducing visible-light absorption, and the resultant TiO₂:N,F worked efficiently as an O₂ evolution photocatalyst in a Z-scheme water-splitting system. Although an increase in the amount of nitrogen doped into rutile TiO₂ lattice in the presence of fluorine was experimentally demonstrated, the role of fluorine in the system remained unclear. Here, we report a computational study on TiO₂:N,F through the construction of supercell models with substitutional defects to reveal the atomic arrangement of the material and the electronic band structure. Calculations for all possible structures of nitrogen/fluorine and nitrogen/oxygen-vacancy relative positions revealed that the defect complexes were preferentially located on the (110) plane and that the distance between defects did not have a strong correlation with the formation energy. The present work also showed that although fluorine did not directly contribute to the narrowing of the band gap of TiO₂:N,F, the fluorine activity of the synthetic atmosphere promotes the formation of substitutional defect complexes of nitrogen/fluorine for anion sites. This eventually increases the amount of nitrogen incorporated into the rutile TiO₂ lattice and also results in reduction of the amount of oxygen vacancy, which is in qualitative agreement with our previous result of transient absorption measurement for rutile TiO₂:N,F. The role of fluorine in TiO₂:N,F is thus clarified through our systematic first-principles calculations.