Stable anatase phase with a bandgap in visible light region by a charge compensated Ga–V (1:1) co-doping in TiO2

A series of charge compensated Ga–V co-doped TiO2 samples (Ti(1-x)(Ga0.5V0.5)xO2) have been synthesized by a modified sol-gel process. X-ray diffraction pattern shows that the anatase to rutile (A→R) onset temperature (TO) shifts to a higher temperature, whereas the complete phase transformation temperature (TC) shifts to a low-temperature region as compared to pure TiO2, due to Ga–V incorporation. Ga–V co-doping helps in the transformation of some smaller sized Ti4+ to a relatively larger Ti3+. In the anatase phase, oxygen content also increases with increasing doping concentration, which along with the larger size of Ti3+ results in lattice expansion and thereby delays the TO. In the rutile phase, oxygen vacancy increases with increasing doping concentration, which results in lattice contraction and accelerates phase transition. Grain growth process is hindered in the anatase phase (crystallites size reduces from ~15 nm (x = 0.00) to 8 nm (0.10)), whereas it is accelerated in the rutile phase as compared to pure TiO2. In both phases bandgap (Eg) reduces to the visible light region (anatase: Eg = 3.16 eV (x = 0.00) to 2.19 eV (x = 0.10) and rutile: 3.08 eV (x = 0.00) to 2.18 eV (x = 0.10)) in all co-doped samples. The tail of the absorption edge reveals lattice distortion and increase of Urbach energy proofs the same due to co-doping. All these changes (grain growth, phase transition, and optical properties) are due to lattice distortion created by the combined effect of substitution, interstitials, and oxygen vacancies due to Ga–V incorporation in TiO2.