Enhanced remote photocatalytic oxidation on surface-fluorinated TiO2

The mobile nature of active oxygen species generated on the UV-illuminated TiO2 surface is now well-recognized. Surface oxidants not only migrate two-dimensionally but also desorb from the surface to be air-borne oxidants. The remote photocatalytic oxidation (PCO) of stearic acids over the surface-fluorinated TiO2 (F-TiO2) film was carried out in the ambient air to study the effects of fluorination on the desorption of oxidants from the surface. The F-TiO2 film was faced to a stearic acid-coated glass plate separated by a small gap (typically 30 mum), and the photocatalytic degradation of the stearic acid was monitored by Fourier transform infrared measurement or gas-chromatographic CO2 production analysis. Remote photocatalytic degradation of stearic acids was markedly faster with F-TiO2 than with the pure TiO2 film, which indicates that the generation of air-borne oxidants is enhanced over the F-TiO2 surface. The remote PCO activity was higher with a higher surface fluoride concentration, higher UV intensity, and smaller gap. The remote photocatalytic activity of F-TiO2 was maximal at a relative humidity of 50% and did not show any sign of deactivation with repeated reactions. The production of CO2 that evolved as a result of the remote PCO of stearic acids was enhanced when H2O2 vapor was present but was strongly inhibited in the presence of ammonia gas that should scavenge OH radicals. Judging from various evidences, the air-borne oxidants in remote PCO are most likely OH radicals and the surface fluorination of TiO2 seems to facilitate the desorption of OH radicals.