The influence of cobalt doping on photocatalytic nano-titania: Crystal chemistry and amorphicity

Photocatalysts of nominal composition (Ti1−xCox)O2−δ with 0.001⩽x⩽0.05 were prepared via a sol–gel technique followed by air firing (200–1000 °C). The incorporation of cobalt inhibited crystal growth and slightly raised the anatase to rutile transformation temperature (∼700 °C). An amorphous component was invariably significant with the maximum content (41–53 wt%) appearing simultaneously with the removal of anatase, suggesting that rutile crystallizes via an aperiodic structure. While the introduction of cobalt shifted the apparent band gap to visible light energies this did not enhance performance as there was limited miscibility of cobalt in titania, non-catalytic secondary phases were present, and active Ti3+ sites were displaced by cobalt.