Interplay of Vanadium States and Oxygen Vacancies in the Structural and Optical Properties of TiO2:V Thin Films

In this work, we present the customized modifications in the morphology and optical properties of vanadium (V) doped TiO2 thin films sputter deposited on glass substrates at a growth rate of ∼0.6 Å/s at 500 °C. The sputtering targets of pure and V doped TiO2 with three concentrations of V (1.0, 1.5, and 2.0 atomic percentage (at. %)) were prepared from powders. XRD patterns confirmed the grown TiO2 films were anatase. In the doped TiO2 films, the crystallite size reduced by almost half when the V concentration increased from 0 to 2 at. % systematically. Incorporation of V in the TiO2 host lattice led to the enhanced growth of (211) planes, which significantly modified the grain geometry from the faceted to the elongated as observed in the SEM images and confirmed by structural simulation using VESTA code. The confinement of phonon modes was observed in the Raman spectra, which was attributed to the increased nonstoichiometry and enhanced asymmetry in bonding with increased V concentration. XPS spectra confirmed that the enhancement in the nonstoichiometry in TiO2 was due to V substitution in the structure. It was suggested that the difference in the valence states of Ti and V resulted in the suppression of equilibrium (101) planes and augmentation of nonequilibrium (211) planes, which modified the grain morphology of the TiO2 thin films. Photoluminescence (PL) spectroscopy clearly demonstrated the interplay of V defect states and O vacancy states. Pure TiO2 showed mainly green luminescence related to oxygen vacancies; however, addition of V clearly demonstrated orange and red emission bands due to incorporation in V3+ and V5+ states, which increased at a much faster rate than oxygen vacancies on further addition of V. The PL results complimented the XPS findings.