Explaining the High Photocatalytic Activity of a Mixed Phase TiO2: A Combined Effect of O2 and Crystallinity

The synergistic effect between anatase and rutile TiO2 particles for organic photodegradation in an aerated aqueous suspension has been widely reported, ascribed to the charge transfer between two oxides increasing the efficiency of charge separation for the surface reaction. In this work, we have found different explanation. Four anatase and rutile samples, pretreated at different temperatures, were used as a starting material, and the biphase oxide was prepared by mixing them at various ratios in an alcoholic aqueous solvent, followed by sintering at 450 degrees C. For phenol photodegradation under air, the synergistic effect was observed with one set of the mixed oxides but not with others. When the reaction was carried out under N-2 in the presence of AgNO3, no synergistic effect was observed with any mixed oxide. For methylene blue photodegradation under air, the synergistic effect was also observed as reported, but such an effect was absent when dye adsorption on the oxide was taken into account. The difference in activity among differently prepared samples correlates well with the combined effect of oxide crystallinity and its sorption capacity toward O-2. We propose that the observed synergistic effect is due to O-2 transfer from anatase phase to rutile that explores the masked photoactivity of rutile particles.