Anatase-to-rutile transition in co-doped TiO2 pigments

Abstract

In this paper, cobalt-doped titanium dioxide green ceramic pigments were synthesized by the polymeric precursor method. In order to investigate the obtainment of a crystalline rutile phase and particles in nanoscale, a chromophore doping was made with fixed concentrations of 1, 2, and 4 mol%, utilizing a calcination temperature of 600oC. The products were investigated by thermal analysis, X-ray diffraction, diffuse reflectance, scanning and transmission electron microscopies. The titanium dioxide:cobalt crystallite size was about 65–100 nm. A broad reflectance band around 540 nm (green region) was observed for all doped samples, indicating the appearance of a deep green pigment. Furthermore, the results have shown that cobalt-doped titanium dioxide promotes greater phase conversion of anatase to rutile. Samples submitted at a temperature of 600 °C, pure titanium dioxide contains 22% rutile, whereas titanium dioxide: 4% of cobalt contains 78% rutile. The mechanism of phase transition in presence of cobalt is attributed to the lower valences of the dopant (II or III) when compared to the titanium valence (IV). Ionic substitution leads to the generation of oxygen vacancies that promote phase transitions at lower temperatures than expected in pure titanium dioxide.