Gianmario Martra

FTIR study of adsorption and photodegradation of L-α-alanine on TiO2 powder

An infrared study of the adsorption of L-α-alanine (L-Ala) on TiO2 powder in aqueous suspension was carried out. When the amino acid is in the zwitterionic form and the TiO2 surface is positively charged, experimental results indicated that L-Ala molecules are adsorbed on titania through the carboxylated group, which gives rise to a bridged structure with the positive centers (protonated hydroxyls) of the TiO2 surface. Adsorbed amino acid was photodegradated in model conditions by exposing the sample to UV light through the quartz walls of the IR cell. The amine function was found to be preferentially removed from the adsorbed L-Ala molecules during the initial period of UV irradiation, and then the complete mineralization of the rest of the molecular structure occurred. This surface process agrees with the results of the analysis of the species that diffuse from the surface of the photocatalyst into the bulk of the liquid medium in the actual photodegradation carried out with TiO2 suspended in aqueous solutions of L-Ala.

Investigations of the Photoinduced Superhydrophilicity of the TiO2 Photocatalyst Surface by Near-Infrared Spectroscopy

When H2O molecules desorb from the TiO2 surfaces during UV light irradiation, the vibrational spectroscopies, FT-IR and NIR, revealed that the H-bond interaction between H2O molecules decreased. This observation clearly suggests that the photo-induced high wettability of TiO2 surface is closely related to the decrease in the surface tension of H2O clusters to form H2O thin layers. The partial elimination of the hydrocarbons from the TiO2 surfaces by photocatalytic oxidation was seen to be another major factor, providing free spaces on the surface where the H2O clusters could spill over and spread out to form the thin H2O layers. The superhydrophilic properties of the TiO2 photocatalyst surfaces by a lessening in the amount of H2O adsorbed on the surfaces can be expressed as hydrophobic compared with the initial state before UV light irradiation. Moreover, the temperature changes of the TiO2 powder samples during UV light irradiation were found to show a good correspondence with the changes in the contact angle of H2O droplets on the TiO2 thin film surfaces. Especially, the time scale for hydrophilic conversion on the TiO2 surfaces under UV light irradiation was in good agreement with the decrease in the amount of H2O molecules adsorbed on the surfaces but not the amount of the hydrocarbons eliminated by the photocatalytic oxidation reactions, showing that the adsorption and desorption of H2O molecules are generally sensitive to the temperature changes of solid surfaces.