Effect of iron-group ions on the UV absorption of TiO2

The UV absorption spectra of polydisperse rutile (TiO2) doped by transition metal cations Cr3+, Cu2+, Co2+ and Fe2+ are investigated at room temperature. It is shown that the fundamental absorption edge of TiO2 is determined by the two mechanisms of electron transitions: one of them represents a direct forbidden transition and the other an indirect allowed one. The fundamental band gap for the pure rutile is determined to be (delta)E = 3.013 eV. The absorption of Fe-doped TiO2 reveals spectral redistribution near the fundamental edge, while the absorption of Co-doped TiO2 is strictly similar to that of the pure rutile. Photocatalytic decomposition of organic safranine by means of TiO2 is also studied. It is found that the pure TiO2 has higher photocatalytic activity in comparison with the doped compounds. We suppose that the impregnation cations act as traps at the initial stage of reaction but their function changes to that of recombination centres as the photocatalytic process develops.