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.