Role of oxygen vacancy in the plasma-treated TiO 2 photocatalyst with visible light activity for NO removal

Abstract

The photocatalytic activity for NO removal under an oxidative atmosphere has been studied over commercial TiO 2 and plasma-treated TiO 2 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 (NO 3 −) by photocatalytic oxidation over the TiO 2 powders, where NO 3 − was accumulated. No difference in the crystal structure, the crystallinity, and the specific surface area was observed between the raw TiO 2 and the plasma-treated TiO 2 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 TiO 2 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 NO 3 −. The appearance of the visible light activity in the plasma-treated TiO 2 photocatalyst was ascribed to the newly formed oxygen vacancy state between the valence and the conduction bands in the TiO 2 band structure.