Observation of Photoinduced Proton Transfer between the Titania Surface and Dye Molecule

The photocatalyst titania film surface acquires a high hydrophilicity after it is exposed to UV light, which is induced by changes in the densities of the surface hydroxyl groups and charge. A xanthene dye, fluorescein, was deposited from a solution onto a titania film after UV irradiation in order to probe the titania surface change. The change in the surface acidity was confirmed by the ratio of dianion to monoanion of fluorescein by Raman spectroscopy. The ratio increased by UV irradiation, indicating that the surface became more basic. Transient absorption spectroscopy revealed the transformation from the monoanion to dianion via the excited states, that is, photoinduced proton transfer from fluorescein to the titania surface. The UV irradiation increases the basic hydroxyl groups on the titania surface, which accepts protons from the water molecules or proton donors on the surface and has a positive charge.