Effects of nitrogen concentration on N-doped anatase TiO2: Density functional theory and Hubbard U analysis

To fully comprehend the photocatalytic mechanisms of anatase TiO2-xNx of various nitrogen concentrations, this study performed first principles calculations based on density functional theory, employing Hubbard U on-site correction, to evaluate the crystal structure, impurity formation energy, and electronic structure. An effective Hubbard U of 8.47 eV was adopted to correctly determine the band gap of pure anatase TiO2. The calculations show that increasing the concentration of nitrogen requires greater formation energy during the synthesis of N-doped TiO2. Under light nitrogen doping (<= 6.25 at.%), N isolated impurity states form above the top of valence band meanwhile the band gap does not change noticeably. Under heavy nitrogen doping (>= 8.33 at.%), a narrowing of the band gap and broadening of the valence band occur, which might explain the red shift at the edge of the optical absorption range observed in some experimental studies. These findings provide a reasonable explanation of recent experimental results. (C) 2012 Elsevier B. V. All rights reserved.