N-Doped TiO₂: A First-Principles Study of Band Gap Reduction From 2.06 eV to 1.4603 eV

N-Doped TiO₂: A First-Principles Study of Band Gap Reduction From 2.06 eV to 1.4603 eV Note: TiO2 unit cell has doped with N atom. So the concentration of doping is too high (12.5%) and it caused drastic reduction in bandgap (from 2.2eV to 1.4603eV) which is not practical. Always a supercell is preferred for doping but in that to reduce time cost unit cell has dopped System Details Model: 12-atom TiO2 cell. Lattice Parameters: a= 3.78254A, b= 3.78254A, c= 9.61502A. Doping: Replaced Oxygen atom with Nitrogen. Doping Concentration: 12.5% N. Doping Type: Substitutional. Fermi Energy: 7.525 eV. Fig 1: N dopped TiO2 Unit Cell Main Results The band gap dropped significantly after doping due to high concentration of doping. Pure TiO2 : 2.06 eV. (Ref: https://next-gen.materialsproject.org/materials/mp 390?formula=TiO2#electronic_Structure) N doped TiO2 :1.4603 eV. (From calculations: https://drive.google.com/drive/folders/1HQTzn3cIaQdRaahBCGmVbTMSSDYHl9FZ?usp=sharing) Analysis of Figures Total Density of States (DOS) Figure 2 shows the general electronic state. You can see the material is still a semiconductor because there is a clear gap between the occupied states (left) and empty states (right). Figure 2: Total Density of States showing the overall electronic distribution. Projected DOS (PDOS) Figure 3 explains why the gap got smaller. The Red Line is the Nitrogen 2p contribution. It creates a new peak right at 0 eV (top of the valence band). This pushes the valence band up, shrinking the gap compared to pure Oxygen states. Figure 3: PDOS showing N-2p states (red) introducing a new energy level at the Fermi edge (0 eV). Band Structure Figure 4 confirms the exact gap value. The gap is 1.46 eV. Both the top of the valence band (0 eV) and the bottom of the conduction band align at the G (Gamma) point. This means it is now a Direct Band Gap, which is better for absorbing light. Figure 4: Band structure confirming a direct gap of nearly 1.4603 eV at the Gamma point. Conclusion Doping with 12.5% Nitrogen worked. It introduced new states at the top of the valence band, lowering the band gap to 1.46 eV. This makes the material much better for visible light applications compared to pure TiO₂. Raw Data: https://drive.google.com/drive/folders/1HQTzn3cIaQdRaahBCGmVbTMSSDYHl9FZ?usp=sharing