In Situ Probing of Photoinduced Hydrophilicity on Titania Surface Using Dye Molecules

Abstract

The titania film surface exhibits superhydrophilicity after UV irradiation due to its photocatalytic function. This is caused by a change in the density of the surface hydroxyl groups, which affects the surface charge density. Titania films were prepared to observe the change in the surface charge during UV irradiation. The amounts of some xanthene dyes adsorbed and deposited on the titania films after UV irradiation were evaluated as a function of the irradiation time. The increase in the adsorption and deposition amounts of the more negative dye versus the UV irradiation time was greater. These results indicated that the titania surface charge became more positive by the UV irradiation. It was reported that basic hydroxyl groups formed on the titania particle surface during the UV irradiation. The surface dissociates hydroxyl ions into the water phase and has a positive charge, which was supported by the present study.

Observation of Excited State Proton Transfer between the Titania Surface and Dye Molecule by Time-Resolved Fluorescence Spectroscopy

“…The spectroscopic measurements of the fluorescein deposited onto the titania surface proposes that the UV irradiation increases the basic OH groups on the titania surface, which accepts protons from the proton donor, fluorescein monoanion, on the surface and has a positive charge. − …”

“…The amount of the deposited dye was estimated by the UV–vis spectroscopy, which was ca. 6 μmol cm –2 on the titania film before the UV irradiation . A Ti:sapphire femtosecond pulse laser through an optical parametric amplifier (350 nm, 1 mW) and streak scope spectroscopic system were used for the time-resolved fluorescence measurements of the dye deposited onto the silica and titania films.…”

“…Recently, it is becoming important that protons are effectively supplied to the active sites on the photocatalyst surface, such as the cocatalyst, and reduced by the separated electrons. , The authors et al reported that the phosphonate groups grafted on the photocatalyst surface promoted the proton transport from the water phase to the cocatalyst and then enhanced the hydrogen evolution during light irradiation, which was proved by electrochemical analyses and light-intensity-dependence of the photocatalysis . On the other hand, the surface of the photocatalyst titania film gradually acquires a high hydrophilicity during UV light irradiation. , This property is reported to be induced by an increase in the densities of the surface basic hydroxyl groups. , In our previous study, X-ray photoelectron spectroscopy revealed that the ratios of the OH group to the Ti on the titania film were 0.73 and 0.78 before and after irradiation for 5 h, respectively . With respect to the photocatalyst surface species, the behavior of proton or hydroxyl ions on the surface significantly affects the activity .…”

“…In our previous study, one of the xanthene dyes, fluorescein, deposited from an aqueous solution onto a titania film probed its surface change after UV irradiation. , The constant of the proton dissociation equilibrium between the monoanion and dianion of fluorescein is p K a 6.43, which is suitable for probing such a titania particle surface because its acidity corresponds to the condition around pH 7 in water. − This dye enables one to estimate the change in the surface acidity from the ratio of the dianion to monoanion of fluorescein, which consequently increased with the UV irradiation time, indicating that the surface gradually became more basic. , Furthermore, transient absorption spectroscopy is a very powerful means to elucidate fast photoinduced processes, − and it actually demonstrated the proton dissociation of the monoanion to the dianion via the excited states on the titania surface, i.e., photoinduced proton transfer from the fluorescein to the titania surface, using the probe molecules . Time-resolved fluorescence spectroscopy using the probe molecules is also a convenient method for observation of the fast photoinduced processes when the molecules are fluorescent species. ,− The fluorescence method is simpler and more sensitive than the transient absorption method.…”

Formation of CuO on TiO2 Surface Using its Photocatalytic Activity

“…The hydroxides were subsequently dehydrated and transformed into the oxides. On the other hand, the UV irradiation caused a slight increase in the density of the basic OH groups on the TiO 2 surface [18][19][20]. Even if there is a slight increase in the basicity it promotes hydrolysis of the TEOS and subsequent polymerization of its products, then the silica layers are deposited on the TiO 2 surface [17].…”