Luminescence characteristics of cobalt doped TiO2 nanoparticles

TiO2 nanoparticles doped with two different concentrations of Cobalt, 0.02 and 0.04 mol, are prepared by sol-gel method. The crystalline phase of the doped and undoped nanoparticles and particle sizes are observed with X-ray diffraction and transmission electron microscope. FTIR confirms the bonding interaction of Co2+ in TiO2 lattice framework. The UV absorption spectra of the doped material shows two absorption peaks in the visible region related to d-d electronic transitions of Co2+ in TiO2 lattice. Compared to undoped TiO2 nanoparticles, the cobalt doped samples show a red shift in the band gap. Steady state photoluminescence spectra give emission peaks related to oxygen defects. The decrease in the intensity ratio of UV/visible emission peaks confirms distortion of structural regularity and formation of defects after doping. The intensity ratio of different visible emission peaks is nearly same for undoped and 0.02 Co2+. However, this ratio decreases profoundly at 0.04 Co2+, due to concentration quenching effect. Photoluminescence excitation spectra, recorded at 598 nm emission wavelength, give different excitation peaks associated with oxygen vacancies and Co2+. Time resolved photoluminescence spectra give longer decay time for doped samples, indicating longer relaxation of conduction band electrons on the defect and on dopant sites. - Highlights: → Cobalt doping results in the increase in d-spacing and shifting of XRD peak position. → Decrease in photoluminescence intensity of TiO2 after doping indicates inhibition of electron-hole recombination. → Concentration quenching effect results in decrease in visible emission intensity ratio. → Excitation peaks related to oxygen defects and Co2+ are observed in photoluminescence excitation spectra. → Cobalt doped TiO2 has longer decay time compared to undoped TiO2.