Role of oxygen vacancy in the plasma-treated TiO2 photocatalyst with visible light activity for NO removal

The photocatalytic activity for NO removal under an oxidative atmosphere has been studied over commercial TiO2 and plasma-treated TiO2 powders. By the plasma treatment, the photocatalytic activity for NO removal appeared in the visible light region up to 600 nm without a decrease in the ultraviolet light activity. It was found that the NO was removed as nitrate (NO3-) by photocatalytic oxidation over the TiO2 powders, where NO3- was accumulated. No difference in the crystal structure, the crystallinity, and the specific surface area was observed between the raw TiO2 and the plasma-treated TiO2 photocatalysts. In electron spin resonance (ESR) measurements, a sharp signal at g = 2.004, which was identified as the electrons trapped on oxygen vacancies, was detected only for plasma-treated TiO2 under visible light irradiation. The saturated intensity of the ESR signal at g = 2.004 was proportional to the removal percentage of nitrogen oxides, suggesting that the number of trapped electrons determined the activity for the photocatalytic oxidation of NO to NO3-. The appearance of the visible light activity in the plasma-treated TiO2 photocatalyst was ascribed to the newly formed oxygen vacancy state between the valence and the conduction bands in the TiO2 band structure. (C) 2000 Elsevier Science B.V. All rights reserved.