TiO2 photocatalysis and related surface phenomena

An HREELS and TPD study of water on TiO2(110): the extent of molecular versus dissociative adsorption

Temperature-programmed desorption (TPD) and oxygen isotopic labeling studies were used to probe the dissociation of water on the (100) and (110) surfaces of TiO2 (rutile). Water TPD spectra from these two surfaces were distinctive. Three monolayer desorption states were observed for the (100) surface (at 205, 250, and 305 K), while only a single desorption state was observed for the (110) surface (at 270 K). TPD experiments on the surfaces enriched with 18O revealed that water desorbing in the 305 K state from TiO2(100) was isotopically scrambled with the lattice oxygen atoms, strongly suggesting that this TPD state resulted from recombination of surface hydroxyl groups. Isotopic scrambling was not observed for any other desorption state on either surface (in the absence of defects). Since very little water desorption occurred from the (110) surface in the temperature range in which exchange was observed on the (100) surface and since previous HREELS work (Henderson, M. A. Surf. Sci. 1996, 355, 151) indic...

Structural Sensitivity in the Dissociation of Water on TiO2 Single-Crystal Surfaces

Kinetics on the photoinduced hydrophilic conversion processes of the TiO2 surface was investigated. First, we show that there exist linear relationships between the reciprocal of the contact angle and the UV irradiation time and use the slope of this straight line as the hydrophilic conversion rate, which is independent of the initial value of the contact angle. Second, we examine the relationships between the reciprocal of the contact angle and the reconstruction of the surface hydroxyl groups and show that the reciprocal of the contact angle corresponds to the density of the surface hydroxyl groups reconstructed by UV irradiation. Finally, the dependence of the hydrophilic conversion on various parameters such as the incident UV intensity, the wavelength of irradiated light, and the concentration of the hole scavenger are quantitatively investigated with the proposed hydrophilic conversion rate. These results show that the photoinduced hydrophilic conversion proceeds competitively with the photocatalyti...

Quantitative Evaluation of the Photoinduced Hydrophilic Conversion Properties of TiO2 Thin Film Surfaces by the Reciprocal of Contact Angle

The relationships between morphology and Lewis acid and base character of surface sites of two types of titania powders (TiO2 P25 and TiO2 Merck) were studied by HRTEM and FTIR spectroscopy of adsorbed molecules. Electron micrographs revealed that TiO2 P25 microcrystals have a prismatic shape, mainly exposing (0 0 1) and (0 1 0) surface planes. TiO2 Merck powder, which exhibits a significantly lower specific surface area, appeared constituted by large roundish microcrystals. FTIR spectra of adsorbed CO indicated that Ti4+ ions exposed on (0 0 1) and (0 1 0) faces of TiO2 P25 particles are Lewis acid centres significantly stronger than those present on the surface of TiO2 Merck microcrystals. As in both cases the exposed cations are coordinated to five oxygen anions, the observed differences in Lewis acidity are ascribed to some difference in the geometric arrangement of the O2− ligands. Such difference in structure affects the basicity of these centres also. In fact, a fraction of O2− ions on the surface of TiO2 P25 behave as basic centres toward CO2 linearly adsorbed on neighbour Ti4+ centres, and then Lewis acid–base pairs can be recognised. By contrast, no basic activity towards CO2 was detected for the TiO2 Merck sample. The two titania powders exhibited different chemical behaviour in condition of high surface hydration also. Hydroxyl groups on the surface of hydrated TiO2 P25 are able to transform benzaldehyde molecules in hemiacetalic-like species, whereas C6H5CHO molecules are only weakly perturbed by interaction with the OH groups on TiO2 Merck particles. This feature could be related to the different photocatalytic behaviour in the oxidation of toluene in gas phase, where benzaldehyde was found as a relevant intermediate species.

Lewis acid and base sites at the surface of microcrystalline TiO2 anatase: relationships between surface morphology and chemical behaviour

Samples of rutile prepared by hydrolysis of TiCl4 and by combustion of Ti(isoPrO)4 have been investigated by infra-red spectroscopic techniques to provide information on their surface hydroxylation and hydration. After outgassing at ambient temperatures the rutile surface carries both hydroxyl ions and adsorbed molecular water. The depopulation of the surface caused by raising the outgassing temperature follows a pattern that suggests there are two types of surface site for the adsorbed molecular species. Complementary studies of pyridine adsorption indicate that the molecular water is held on the surface as a coordinating ligand and also show the presence of two types of hydroxyl species on the ambiently outgassed oxide. Further studies of the adsorption of thionyl chloride, sulphur dioxide and hydrogen chloride vapours support the contention that the surface hydroxyls are ionic in character. In the case of HCl adsorption, evidence for two types of coordinately bonded water was found after exposure of the oxide to the dry acid gas. It was also found that the sample prepared from TiCl4, which carried chlorine in its surface layers, was more readily dehydroxylated than the “chlorine free” material prepared from the metal alkoxide. The chlorine containing sample also aggregated on storage whereas the pure material did not.

P. Jones

Papers

Infra-red studies of rutile surfaces. Part 1