No abstract

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.

Some co-catalyst nanoparticles can enhance the activity of photocatalysts due to prolonging the charge separation lifetime by promoting the electron or hole transfer. CuO particles were prepared from an aqueous solution of copper (II) nitrate at 351 K on a TiO2 surface by a photocatalytic reaction and heating at 573 or 673 K. The amount and size of the particles deposited during the photocatalytic reaction can be controlled by changing the amount of the irradiated photons. The CuO crystals with about 50−250 nm-sized particles were formed. Nitrate ions were reduced to nitrite ions in the solution by the photocatalytic activity of the TiO2, and water was simultaneously transformed into hydroxide ions. An increase in the basicity on the TiO2 surface induced formation of a copper hydroxide. The copper hydroxide was subsequently dehydrated and transformed into CuO by heating. The TiO2 loading of a small amount of CuO demonstrated a higher photocatalytic activity for methylene blue degradation compared to the original TiO2 due to the electron transfer from the TiO2 conduction bands to the CuO conduction band.