Correlation of oxidative and reductive dye bleaching on TiO2 photocatalyst films

A series of titania films of different thicknesses (60–420 nm), and therefore activities, are produced using a sol–gel method and assessed for photocatalytic activity using three test methods. The first two involve the photo-oxidative bleaching of common dyes, namely methylene blue (MB), a cationic thiazine dye, and Acid Orange 7 (AO7), an anionic, azo dye, in aerated aqueous solution. The third test uses an ink comprising, amongst other things, a redox dye, Resazurin (Rz), and an easily oxidized species, glycerol. When deposited on a titania film and irradiated with UV light the colour of this Rz ink (initially blue) changes to pink due to the titania-sensitized reduction of Rz. The initial rates of destruction of MB and AO7 and the initial rate of reduction of Rz depend directly upon the thickness of the titania films, indicating a strong correlation between the different tests. The significance of these results is discussed in the context that the MB test is currently being considered by International Organization for Standardization (ISO) as a standard method for assessing the photocatalytic activities of the surfaces. Compared with MB, Rz ink test is much faster and easier to implement, which suggests that there would be significant advantages to employing the Rz test, instead of another dye test, for assessing the photocatalytic thin films activities.