Impurity levels of iron-group ions in TiO2(II)

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Abstract

Photocurrent measurements on oxidized, single-crystal TiO2 undoped and doped with V, Cr, Mn or Fe were made both before and after exposure to white light from a tungsten lamp; they revealed a band of states centered at 2.0 eV below the conduction-band edge Ec that appears to be the band of surface states active in the photoelectrolysis of water. They also revealed a band of bulk states extending from near the valence-band edge Ev to about 1.9 eV below Ec. The ESR signals from V4+, Cr3+ and Fe3+ ions in oxidized samples and from Mn3+ ions in reduced samples were measured before and after exposure to white and monochromatic light; changes in the ESR signal intensities allow placement of the V4+: d1 level about 2.1 eV below Ec, the Cr3+: d3 level about 2.7 eV belo Ec, the Fe3+ level at or just below the valence band edge Ev which is 3.0 eV below Ec, the Fe2+ level approximately at Ec, the Mn3+ level about 1.9 eV below Ec. Comparison of experiments on oxidized and reduced samples suggests that the bulk states are associated with cation vacancies, and measurements with polarized light gave anisotropies that are qualitatively interpretable with a cation-vacancy model for the oxidized samples.

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