Evaluation of photocatalytic activity by dye decomposition

A novel rapid evaluation method for the photocatalytic activity of TiO2 thin films was developed. An organic dye with a polyvinyl alcohol (PVA) binder was spin coated on the TiO2 thin film, and the decrease in the absorbance of the dye's absorption peak during UV light irradiation was measured. Acid Blue 9 (Brilliant Blue FCF; CI-42090) could be used as the probe, while Methylene Blue (CI-52015) was not applicable to this method because of the reversible color change after the UV irradiation was stopped. PVA has virtually no interaction with oxidizing radicals, therefore, it is regarded as a simple binder holding dye molecules in the coated dye-PVA film. It was found that the ambient humidity during the UV irradiation strongly accelerates the discoloration rate of the dye, probably due to the increase in the photogenerated oxidizing radicals on the TiO2 surface. This dye discoloration could be explained by the one-dimensional diffusion model with a first order reaction.