Visible-Light Absorption and Band Gap Engineering in Ni–Ta Co-Doped Anatase TiO ₂ : A DFT Study

Comparative models were developed for pristine, Ni-doped, Ta-doped, and Ni–Ta codoped systems. Pristine anatase TiO 2 exhibits a band gap of 2.12 eV, Ni doping reduces the band gap dramatically to 1.02 eV by introducing two isolated Ni 3d impurity states above the valence band, narrowing the band gap and extending absorption into the visible region; however, these unoccupied states act as recombination centers that limit photocatalytic efficiency. In contrast, Ta doping introduces 5d states just below the conduction band, shifts the Fermi level upward, and reduces the band gap to ~1.80 eV, indicating n-type conductivity. Notably, co-doping with Ni and Ta further reduces the band gap to ~ 1.24 eV relative to Pure Tio2, while simultaneously transforming the Ni 3d states into occupied ones. This suppresses recombination and improves charge carrier separation, enhancing visible-light photocatalytic activity. Optical analyses reveal that Ni doping produces the strongest visible-light absorption intensity, whereas Ni–Ta codoping broadens the absorption range and enhances overall optical utilization. The negative formation energies obtained for all doped configurations confirm that Ni-, Ta-, and Ni–Ta-doped TiO 2 systems are thermodynamically stable under oxygen-rich conditions. Charge density and electrostatic potential analyses further reveal localized charge redistribution and internal field gradients, which promote efficient charge separation. These results suggest that Ni–Ta co-doping is a promising strategy for designing stable and highly efficient TiO 2 -based photocatalysts for solar-driven applications.