Enhancing gas sensing and optoelectronic properties of rutile TiO₂ via transition metal doping: a DFT + U study

This research employs DFT + U calculations to investigate the impact of doping with Co, Ni, Cu, and Zn on the structural, electronic, optical, and gas sensing characteristics of rutile TiO₂. The doping process results in lattice expansion, with optimized lattice constants increasing from 4.630 Å (a) and 2.980 Å (c) in pure TiO₂ to as much as 4.681 Å and 3.143 Å in Zn-doped systems. This expansion is explained by the variation in atomic radii between Ti and the doped atoms. A reduction in the bandgap is noted across all doped systems (pure: 3.04 eV; Co: 2.51 eV; Cu: 2.63 eV; Ni: 2.66 eV; Zn: 2.56 eV), which enhances the absorption of visible light and promotes p-type conductivity. The analysis of the PDOS indicates significant hybridization among the Ti-3d, O-2p, and dopant 3d orbitals, as well as magnetic properties in the Co, Ni, and Cu-doped systems. The optical properties, including the dielectric function and absorption spectra, further indicate a redshift in all transition metal (TM) doped systems. The gas sensing evaluation shows enhanced detection of CO and NO, with Zn-doping achieving superior selectivity for CO with an adsorption energy of -0.949 eV, while Ni-doping demonstrates a strong affinity for both CO (-0.726 eV) and NO (-0.639 eV), making it suitable for multi-gas detection. Doped rutile TiO₂ systems exhibit enhanced sensitivity, stability, and reusability, positioning them as promising candidates for optoelectronic and gas sensing applications.