Photoinduced processes of adsorbed and associated dye molecules in mesoporous titania coatings

Abstract Studying photodegradation in porous semiconductor thin coatings by optical spectroscopy measurements (using dye model molecules) at the air-solid interface can be a simple model system relevant for such applications as self-cleaning coatings and dye-sensitized solar cells. Still, the interpretation of the results can be difficult, which is further complicated by the possibility of dye association processes. In this work, mesoporous titania coatings with different pore structures of 77–291 nm thickness and 30–45% porosity were prepared by sol-gel dip coating method on solid substrates, and impregnated in dye (rhodamine 6G, methylene blue) solutions. Dye photodegradation was studied under UV and visible light at the air-solid interface. Dye molecules adsorbed in the pore structure in monomer and associated forms: it was found that the association and dynamics of dye molecules depended on the pore sizes, and had an important role during photodegradation processes. Dye association was observed to occur during irradiation in the coatings with the highest pore thickness, while this process was inhibited in the coatings containing smaller pores. Associated forms of the investigated dyes showed higher photostability compared to the monomer. The degradation of rhodamine 6G monomers showed a two-step first order reaction, as interpreted by the Julson-Ollis model.