An exploratory study on effect of the isomorphic replacement of Ti4+ ions by various metal ions on the light absorption character of TiO2

The electronic states of anatase and rutile TiO2 crystals and of TiO2 crystals with Ti4+ ions isomorphically replaced by various metal ions (V3+, V4+, V5+, Cr3+, Mn3+, Fe3+, Co3+, Ni2+, Ni3+, and Rh3+) were calculated using the DV-X alpha method, which is a density functional method. It was found that the band gap changed in parallel to the radius of the replacing ion, except for a few cases. This tendency agrees with an experimental result in the literature that the light absorption band of anatase shifts to a longer wavelength region, when metal ions are injected. Calculations showed that the replacement of Ti4+ by Ni3+ and V5+ gave the largest decreases in the band gap, suggesting that these metal ions are the most efficient replacing ions for TiO2. The isomorphic replacement resulted in the formation of new energy levels of the metal 3d orbitals in the forbidden band of TiO2. These levels seemed to interact with the 2p orbitals of oxide ions in TiO2, resulting in a decreased band gap. An increase in the degree of covalent bonds between the replacing ions and the oxide ions probably contributes to the decrease in the band gap. (C) 2001 Elsevier Science B.V. All rights reserved.