Akira Nakajima

Recent Studies on Super-Hydrophobic Films

Summary.u2002Films whose water contact angle is higher than 150° have been the subject of great interest and enthusiastic study in recent years. These films, which are called super-hydrophobic, are fabricated by combining appropriate surface roughness with surfaces of low surface energy. Here we briefly review the fundamental theories on the wettability of a hydrophobic rough solid surface, together with recent works on the processing and properties of super-hydrophobic films. Though the practical application of these films is still limited, the obstacles to application are gradually being surmounted, providing a great opportunity for the development of various industrial products.

Photocatalytic activity and photoinduced hydrophilicity of titanium dioxide coated glass

Antifogging mirror or self-cleaning glass can be realized utilizing photoinduced hydrophilicity of titanium dioxide. Application fields of functional titanium dioxide coating is now expanding rapidly not only in applications for glass but also in applications for polymer, metal and ceramic materials. The high hydrophilic surface of TiO2 is interesting also on the standpoint of the basic photon related surface science of titanium dioxide. In order to understand the photoinduced hydrophilic conversion on titanium dioxide coating in details, it is inevitably necessary to understand the relationship between the photo reaction and the surface crystal structure. In this paper, photoinduced hydrophilic conversion was evaluated on the different crystal faces of rutile single crystal and also polycrystalline anatase titanium dioxide to clarify the dependence of the crystal structure on the photoinduced hydrophilic conversion.

Preparation of hard super-hydrophobic films with visible light transmission

Hard super-hydrophobic thin films were prepared combining a phase separation of tetraethyl orthosilicate (TEOS) induced by the addition of an acrylic polymer, and subsequent fluoroalkylsilane (FAS) coating. The microstructure of the films was crater-like, and the structural dimension depended on the degree of the phase separation. The dominant mechanism of the hydrophobicity of the film was changed from Wenzel’s mode to Cassie’s mode with increasing surface roughness. The film obtained by proper composition possessed high transmittance in the visible wavelength range and its hardness was almost at the same level as normal silica-based hard coatings.

Reversible wettability control of TiO2 surface by light irradiation

Abstract Titanium dioxide (TiO 2 ) thin films with various surface morphologies were prepared on the substrates of (1xa00xa00) plane of strontium titanate by a metalorganic chemical vapor deposition process. These TiO 2 thin films became highly hydrophilic by ultraviolet (UV) irradiation, and returned to the initial relatively hydrophobic state by visible-light (VIS) irradiation. The hydrophobic–hydrophilic conversion induced by UV irradiation, is due to the increase in dissociated water adsorption on the film surface. In contrast, the conversion from hydrophilic to hydrophobic by VIS irradiation is caused by the elimination of water adsorbed on the surface due to the heat generated. Changes of water contact angle between hydrophilic states and hydrophobic ones strongly depended on the roughness of the film surface. The wettability of the TiO 2 surface was enabled to convert between hydrophilic and hydrophobic reversibly by alternative UV and VIS irradiation.

Cross-interference of RLR and TLR signaling pathways modulates antibacterial T cell responses

Although the mechanisms by which innate pathogen-recognition receptors enhance adaptive immune responses are increasingly well understood, whether signaling events from distinct classes of receptors affect each other in modulating adaptive immunity remains unclear. We found here that the activation of cytosolic RIG-I-like receptors (RLRs) resulted in the selective suppression of transcription of the gene encoding the p40 subunit of interleukin 12 (Il12b) that was effectively induced by the activation of Toll-like receptors (TLRs). The RLR-activated transcription factor IRF3 bound dominantly, relative to IRF5, to the Il12b promoter, where it interfered with the TLR-induced assembly of a productive transcription-factor complex. The activation of RLRs in mice attenuated TLR-induced responses of the T helper type 1 cell (TH1 cell) and interleukin 17–producing helper T cell (TH17 cell) subset types and, consequently, viral infection of mice caused death at sublethal doses of bacterial infection. The innate immune receptor cross-interference we describe may have implications for infection-associated clinical episodes.

Effect of repeated photo-illumination on the wettability conversion of titanium dioxide

Abstract The effect of the repeated UV illumination on the photo-induced wettability conversion of TiO2 surface was investigated using rutile single crystal. The hydrophilicizing rate was increased by the repeated UV illumination and this effect was more remarkable on (0xa00xa01) than (1xa01xa00) surface. This crystal plane dependence can be attributed to the differences of the efficiencies of oxygen vacancy creation and the degrees of resultant structural distortion between these two surfaces. These data suggest that the hydrophilicizing process of TiO2 is a kind of photo-corrosion process occurring at the surface.

TiO2-coated optical fiber bundles used as a photocatalytic filter for decomposition of gaseous organic compounds

Abstract An optical fiber-based, filter-type photocatalytic reactor was designed and the decomposition reaction of gas phase isopropanol was studied using a recirculating reaction system. Compared to the conventional TiO 2 -coated glass plate or honeycomb monolith, both higher efficiency of light utility and larger processing capacity were obtained. This good combination was achieved by employing both high input light intensities (1.6×10 16 to 2.6×10 17 xa0quantaxa0cm −2 xa0s −1 ) and a large number of glass fibers (18xa0000 pieces, 125xa0μm diameter) in the reactor, which makes it possible for the reaction to occur with a high reaction rates while maintaining a high quantum efficiency. These characteristics can be understood by considering a TiO 2 -coated single fiber as a micro-reactor distributed with low photon flux, and the corresponding fiber bundles being composed of large number of such micro-reactors.

Decomposition of gas-phase octamethyltrisiloxane on TiO2 thin film photocatalysts—catalytic activity, deactivation, and regeneration

Abstract The photocatalyzed decomposition reaction of gas-phase octamethyltrisiloxane (OMTS) on TiO 2 thin films was investigated in a batch-type photocatalytic reactor. The effectiveness of TiO 2 photocatalyst for decomposing OMTS was demonstrated by the decrease of OMTS concentration and increase of carbon dioxide concentration with illumination time as monitored using a gas chromatography. However, the photocatalytic activity was found to decrease with increasing reaction time, and catalyst deactivation was finally observed on the TiO 2 film that suffered from a long-time reaction. Formation and accumulation of hydroxylated SiO x ( x =3 ∼ 4) on the irradiated TiO 2 surface were detected by X-ray photoelectron spectroscopic (XPS) measurements. An equivalent amount of ∼7 monolayers of OMTS was estimated to be decomposed on the fully deactivated TiO 2 photocatalyst as calculated from the saturated adsorption value and the total decomposed amount of OMTS. It was found that thus formed SiO x can be easily removed by treatment in an alkaline solution for a short period of time without influencing the underlying TiO 2 film. Regeneration of the photocatalytic activity of the SiO x -removed TiO 2 thin film was confirmed by the re-decomposition of OMTS under the same experimental conditions.

Preferential adsorption of amino-terminated silane in a binary mixed self-assembled monolayer.

The composition and structure of a binary mixed self-assembled monolayer (SAM) of 3-aminopropyltriethoxysilane (APS, NH(2)(CH(2))(3)Si(OCH(2)CH(3))(3)) and octadecyltrimethoxysilane (ODS, CH(3)(CH(2))(17)Si(OCH(3))(3)) on a silicon oxide surface have been characterized by water contact-angle measurements, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and sum frequency generation (SFG) vibrational spectroscopy. XPS demonstrated that APS in the mixed SAM is significantly enriched in comparison to that in solution, indicating the preferential adsorption of APS during the SAM formation. AFM observations showed that the mixed SAM becomes rougher. SFG revealed that the coadsorption of APS induced a conformation disordering in the ODS molecules present in the mixed SAM. The surface enrichment of APS has been explained in terms of differences in the surface adsorption rates of the two components as well as in the self-congregation states of APS molecules in the bulk solution. Furthermore, the structure of the water molecules on the mixed SAM surface in contact with the aqueous solutions at different pHs has also been studied. The results indicate that the mixed-SAM modified surface is positively charged at pH < 5 and negatively charged at pH > 7.

Effect of dew condensation on the wettability of rough hydrophobic surfaces coated with two different silanes.

Dew condensation effects on the wettability of rough and smooth coatings of two fluoroalkylsilanes (FAS3 and FAS17) were investigated by controlling the temperature. Contact angles of the coatings decreased concomitantly with decreasing surface temperature. Inflection points in the temperature dependence of contact angles were observed at the dew point. They were attributable to the change of the interfacial free energy of the solid-gas interface by water adsorption. The contact angle decrease suggested a mode transition from Cassie to Wenzel on the rough surface, and resulted from the surface wettability change and the increase of the condensation amount of water. The contact angle change by increasing temperature from -6 degrees C revealed that the Wenzel mode is more stable than Cassies mode.