Mechanism of hydrogen modification of titanium-dioxide

Recent explanations of the enhancement of the electrical properties of hydrogen-modified anatase-TiO2 propose mid-band gap states just below the conduction band and relate these to the creation of a disordered surface layer on an unmodified crystalline core. In this paper, the focus is on hydrogen modification of rutile-TiO2, which also acquires enhanced electrical properties. Oxygen-deficient TiO2–x was produced by exposing rutile to hydrogen at temperatures up to 730 °C. The desorption of hydrogen was studied by mass spectrometry measurements at temperatures up to 730 °C. All the evidence gathered in this new study is consistent with the absorption of hydrogen into the interior of the rutile particle. Re-examination of published x-ray diffraction results does not reveal evidence for a disordered surface layer on hydrogen-modified rutile. It therefore appears that the explanation of enhanced electrical properties owing to surface-only processes is incomplete, especially for rutile, but probably for anatase as well.