Concentration-dependent electronic and optical properties of Ti₁₋ₓNiₓO₂: an ab initio study

Abstract

Ni-doped rutile TiO₂ was studied by density functional theory to determine the effect of dopant concentration on its structural, electronic, and optical properties. Calculations were performed with the full-potential linearized augmented plane-wave method using GGA-PBE and GGA-mBJ exchange-correlation schemes. Substitutional Ni contents of 6.25%, 12.50%, 18.75%, and 25.00% were examined in the tetragonal rutile structure. The optimized lattice parameters decrease with increasing Ni concentration, indicating a progressive contraction of the crystal lattice after Ti substitution by Ni. The electronic structure shows the appearance of Ni-related impurity states within the forbidden region, which alter the excitation pathways of the host semiconductor. Although the calculated fundamental band gap increases with dopant content, the optical response is extended into the visible region because of transitions involving these intermediate states. The dielectric and absorption spectra also show optical anisotropy, with a stronger visible-light response as the Ni concentration increases. The results show that Ni incorporation changes the absorption mechanism of rutile TiO₂ and supports its potential for visible-light-responsive photocatalytic and optoelectronic applications.