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.