TiO2 photocatalysis and related surface phenomena

Photoinduced reactivity of titanium dioxide

The photocatalytic activity for NO removal under an oxidative atmosphere has been studied over commercial TiO 2 and plasma-treated TiO 2 powders. By the plasma treatment, the photocatalytic activity for NO removal appeared in the visible light region up to 600 nm without a decrease in the ultraviolet light activity. It was found that the NO was removed as nitrate (NO 3 − ) by photocatalytic oxidation over the TiO 2 powders, where NO 3 − was accumulated. No difference in the crystal structure, the crystallinity, and the specific surface area was observed between the raw TiO 2 and the plasma-treated TiO 2 photocatalysts. In electron spin resonance (ESR) measurements, a sharp signal at g =2.004, which was identified as the electrons trapped on oxygen vacancies, was detected only for plasma-treated TiO 2 under visible light irradiation. The saturated intensity of the ESR signal at g =2.004 was proportional to the removal percentage of nitrogen oxides, suggesting that the number of trapped electrons determined the activity for the photocatalytic oxidation of NO to NO 3 − . The appearance of the visible light activity in the plasma-treated TiO 2 photocatalyst was ascribed to the newly formed oxygen vacancy state between the valence and the conduction bands in the TiO 2 band structure.

Role of oxygen vacancy in the plasma-treated TiO2 photocatalyst with visible light activity for NO removal

Self-cleaning methods currently employed are based on understanding of the functions, structures, and principles of various objects found in living nature. Three types of surfaces, including superhydrophobic, superoleophobic, and superhydrophilic, offer solutions to keep a surface clean. In this review article, an overview of self-cleaning surfaces inspired by nine biological objects is provided: lotus leaves, rice leaves, cicada wings, butterfly wings, snail shell, fish scale, shark skin, pitcher plant, and photosynthesis. These surfaces exhibit special properties such as low adhesion, low drag, anisotropic wetting, anti-reflection, directional adhesion, anti-fouling, photocatalysis, self-sterilizing, and anti-fogging. We discuss the differences between the superhydrophobic and superhydrophilic surfaces and perspectives for self-cleaning surfaces in the future.

Bioinspired self-cleaning surfaces with superhydrophobicity, superoleophobicity, and superhydrophilicity

The photocatalytic degradation of phenol, chosen as an aromatic model molecule, has been performed at room temperature (20°C) in contact with a suspended mixture of titania and of activated carbon (AC). Non-additive adsorption capacities were observed when the solids were mixed, and this was ascribed to a strong interaction, involving half of the surface of titania and ca. 14% of that of AC. A synergy effect was observed with an increase of the first order rate constant by a factor of 2.5. As for neat titania, the same main intermediate products (hydroquinone and benzoquinone) were found but in much smaller quantities and during a much smaller lifetime, suggesting that the same reaction mechanism occurred in the presence of photoinactive AC. The synergy effect was ascribed to an extended adsorption of phenol on AC followed by a transfer to titania where it is photocatalytically degraded. The synergy effect could not be improved by previous physical treatments of the solid mixture such as grinding and sonication. Some phenol remained adsorbed on AC when no traces of organic compounds were detected in the purified water. This adsorbed phenol could be destroyed by illuminated titania while maintaining UV-irradiation. This combined photocatalytic system may appear as a new performing one, more efficient with a shorter time necessary for decontaminating diluted used waters.

Koji Takeuchi

Papers

Removal of low concentration nitrogen oxides through photoassisted heterogeneous catalysis