Nitrogen concentration-dependent electronic, magnetic, and optical properties of anatase TiO 2 : Implications for visible-light photocatalysis from DFT calculations

In this work, first-principles Density Functional Theory (DFT) calculations within the GGA + U framework were performed to investigate substitutional nitrogen-doped anatase TiO 2 at nitrogen concentrations of 3.125, 6.25, and 12.5 at.%. Unlike previous studies, which mainly focused on a single doping concentration or a limited set of physical properties, this work provides a comprehensive investigation of the influence of nitrogen concentration on the structural, electronic, magnetic, and optical properties of anatase TiO 2 within a unified computational framework. The optimized structures confirm that the anatase phase remains structurally stable after nitrogen substitution, exhibiting only slight local distortions around the Ti–N bonds. Electronic structure calculations reveal that increasing the nitrogen concentration progressively reduces the band gap from 3.20 eV for pristine TiO 2 to 2.91, 2.819, and 2.625 eV for 3.125, 6.25, and 12.5 at.% N, respectively. This band-gap narrowing originates from the hybridization between N-2p and O-2p states, while the conduction band remains predominantly composed of Ti-3d states. The reduction of the band gap extends the optical absorption edge toward the visible region, enabling more efficient utilization of solar radiation and consequently enhancing the photocatalytic performance of TiO 2 under visible-light irradiation. Spin-polarized calculations further demonstrate that nitrogen substitution preserves the non-magnetic nature of anatase TiO 2 . Optical analyses reveal a pronounced red shift of the absorption edge, together with enhanced visible-light absorption and improved dielectric and optical responses as the nitrogen concentration increases. Among the investigated systems, the 12.5 at.% N-doped TiO 2 exhibits the strongest visible-light response owing to its narrowest band gap and highest absorption intensity in the visible range, making it the most promising candidate for photocatalytic applications. These findings provide a comprehensive theoretical understanding of the concentration-dependent effects of nitrogen doping on anatase TiO 2 and offer valuable guidelines for the rational design of high-performance visible-light-driven TiO 2 photocatalysts and advanced optoelectronic materials.