Heat treatment effect on the physical properties of cobalt doped TiO2 sol-gel materials

Cobalt doped and undoped TiO2 powders have been prepared by sol-gel technique and annealed at temperatures ranging from 400 °C to 1000 °C. The effects of annealing temperature on the structural, morphological and optical properties have been characterized by X-ray diffraction, transmission electron microscopy, electron energy-loss spectroscopy and diffuse reflectance spectroscopy. For all doped samples there is a general reduction of the band gap energy, in comparison with undoped samples prepared in the same conditions. More specifically, experimental results indicate that cobalt doping, occurring as Co2 + ion insertion into the TiO2 (Ti4 +) host lattice, inhibits the growth of the crystallites and delays the phase transformation from anatase to rutile. Moreover, at high temperature, a secondary phase (CoTiO3) is found to coexist with highly crystalline rutile. These structural characteristics are discussed in relation with the observed general trends for the optical properties.