Effects of Co doping on structure and optical properties of TiO ₂ thin films prepared by sol-gel method

Co-doped TiO ₂ thin films were synthesized on quartz substrates by sol-gel method. Atomic force microscopy results indicate that the surfaces of the Ti _{1 - x}Co _xO ₂ (0 ≤ x ≤ 0.10) films become smooth and compact with increasing Co content. X-ray diffraction results show that all the films are rutile phase structure and Co doping leads to lattice contraction. X-ray photoelectron spectroscopy results reveal that the predominant oxidation state of Co is divalent. Peak positions of Raman-active modes (B _2g, A _1g and E _g) shift to lower frequency with increasing Co content. The refractive index n at 670 nm from transmittance spectra increases with increasing Co content. The OBG varies between 3.10 and 3.26 eV. Note that optical band gap (OBG) first increases and then decreases with increasing Co content, reaching its maximum value when x is 0.03. These results suggest that the increasing mechanism of OBG is related to the decrease of grain size, compressive stress, and reduction of rutile TiO ₂, and the decreasing mechanism of OBG is involved with defect and impurity. The competition of the two mechanisms leads to the strange change of OBG.