Quantitative characterization of hydroxyl radicals produced by various photocatalysts

Hydroxyl radicals (center dot OH) have been deemed to be the major active species during the photocatalytic oxidation reaction. In this study, center dot OH produced on various semiconductor photocatalysts in aqueous solution under Xenon lamp irradiation was quantitatively investigated by the photoluminescence (PL) technique using coumarin (COU) as a probe molecule. The results indicated that the formation rate of center dot OH on the surface of irradiated commercial Degussa P25 (P25) was much higher than that of other semiconductor. The pH values of the solution and phase structure of TiO2 significantly influenced the production rate of center dot OH. The acidic pH environment of the solutions and bi-phase structure (anatase and ruffle) of TiO2 were beneficial to enhancing the formation rate of center dot OH. In addition, the formation rate of center dot OH on anatase TiO2 and P25 was much faster than that of center dot OH on the other semiconductors (such as rutile TiO2, ZnO, WO3, CdS, Bi2WO4 and BiOCl, etc.). A new concept OH-index was first proposed to compare photocatalytic activity of photocatalysts, which would provide new insight into the investigation of semiconductor photocatalysts. (C) 2011 Elsevier Inc. All rights reserved.