No abstract

Titanium dioxide (TiO2) displays photocatalytic behavior under near-ultraviolet (UV) illumination. In another scientific field, it is well understood that the excitation of localized plasmon polaritons on the surface of silver (Ag) nanoparticles (NPs) causes a tremendous increase of the near-field amplitude at well-defined wavelengths in the near UV. The exact resonance wavelength depends on the shape and the dielectric environment of the NPs. We expected that the photocatalytic behavior of TiO2 would be greatly boosted if it gets assisted by the enhanced near-field amplitudes of localized surface plasmon (LSP). Here we show that this is true indeed. We named this new phenomenon "plasmonic photocatalysis". The key to enable plasmonic photocatalysis is to deposit TiO2 on a NP comprising an Ag core covered with a silica (SiO2) shell to prevent oxidation of Ag by direct contact with TiO2. The most appropriate diameter for Ag NPs and thickness for the SiO2 shell giving rise to LSP in the near UV were estimated from Mie scattering theory. Upon implementing a device that took these design considerations into account, the measured photocatalytic activity under near UV illumination of such a plasmonic photocatalyst, monitored by decomposition of methylene blue, was enhanced by a factor of 7. The enhancement of the photocatalytic activity increases with a decreased thickness of the SiO2 shell. The plasmonic photocatalysis will be of use as a high performance photocatalyst in nearly all current applications but will be of particular importance for applications in locations of minimal light exposure.

Hydrophobicity and sliding behavior of water droplets were investigated on various hydrophobic pillarlike and groove structures prepared on a silicon wafer by dicing and subsequently coating with fluoroalkylsilane. The dominant hydrophobicity mode was changed from Wenzel's mode to Cassie's mode at a smaller roughness than that expected from the calculation based on the sinusoidal surface by Johnson and Dettre. The effect of water intrusion on the microstructure due to droplet weight was revealed to be an important factor governing the water sliding angle on the surface. In a comparison of the sliding behavior of water droplets over pillarlike and groove structures, it was demonstrated that a proper design of the surface with respect to shape and extent of the three-phase line is more effective than the increase of contact angles merely by decreasing the solid−water contact area.

Various superhydrophobic films having different surface roughnesses were prepared, and the relationships between the sliding angle, the contact angle, and the surface structure were investigated. In the highly hydrophobic region, the sliding angles of water droplets decreased with increasing contact angles. Microstructural observation revealed that surface structures that can trap air are important for the preparation of low-sliding-angle surfaces. We have also derived an equation that describes the relationship between sliding angles and contact angles on superhydrophobic surfaces with roughness. The results calculated on the basis of this equation agreed well with the experimental ones. Moreover, we have successfully prepared a transparent superhydrophobic film whose sliding angle is ∼1°for a 7 mg water droplet. On this film, there was almost no resistance to the sliding of water droplets. The film obtained satisfies the requirements of superhydrophobicity, transparency, and a low water sliding angle.

No abstract

The discovery of photoinduced water splitting on TiO 2 electrodes [1] has prompted extensive research on TiO 2 and other semiconductor materials, which have been widely adopted as potential substances for solar energy conversion and environmental purification. Most work has focused on improving the efficiency of energy conversion [2±5] or photocatalytic reactions. [6±12] Little research has been reported to clarify the effect of light on the properties of TiO 2 surfaces. Very recently, we found that UV illumination of TiO 2 materials can generate surfaces that display 0 contact angle for both water and oily liquids. [13] Following this finding, intensive research has been performed to explicate the mechanism of this unique amphiphilic surface character. In this communication, we report the details of the photoconvertible surface wettability. The formation of a microstructured composite between hydrophilic and oleophilic phases, which results from the photogenerated Ti 3+ defects at definite sites, is considered to account for this unique feature.The observation of the amphiphilic surfaces was initiated by the contact angle measurements of TiO 2 anatase thin films. The water contact angle for a freshly prepared film averaged 15±1. After the sample had been stored in the dark for 2 months, the water contact angle increased to 72±1. When a water droplet touched the UV-illuminated film, it spread immediately, leaving an irregular shape on the surface with a contact angle of 0±1. [13] The contact angle of glycerol trioleate (GT), a main ingredient of edible oil, for the TiO 2 surface was also measured. Prior to UV illumination, the GT contact angle averaged 10±1, indicating that the surface is hydrophobic and oleophilic. Surprisingly, after UV illumination a GT droplet also spread out, resulting in a contact angle of 0±1 when it touched the TiO 2 surface. Parallel experiments were performed using other liquid species, e.g., hexadecane, ethylene glycol, tetralin. Distinct contact angles resulted for the hydrophobic TiO 2 surface. However, all of the liquids spread completely on a UV-illuminated TiO 2 surface, with a contact angle of 0±1. This leads to the tremendous conclusion that UV illumination has created a surface that is both highly hydrophilic and highly oleophilic. The wettability change was observed on both anatase and rutile TiO 2 surfaces in the form of either polycrystals or a single crystal, independent of their photocatalytic activities. Even after the TiO 2 had been stored in the dark for a few days, the high amphiphilicity of the TiO 2 surface was maintained. A longer storage period induced a gradual increase in the water contact angle, revealing a surface wettability trend towards hydrophobicity. However, the high amphiphilicity was repeatedly regenerated by UV illumination.Surface wettability is generally denoted by the contact angle. According to Young's equation, the contact angle of a liquid drop on a solid surface results from the balance between the cohesive forces in the liquid and the adhesiv...

The photoinduced surface wettability conversion reactions of ZnO and TiO2 thin films were investigated by means of water contact angle measurement and X-ray photoelectron spectroscopy. Before ultraviolet (UV) illumination, ZnO and TiO2 films exhibited water contact angles of ∼109 and ∼54°, respectively. UV illumination turned both surfaces to highly hydrophilic with water contact angles smaller than 10°. Storage in the dark reconverted the highly hydrophilic films to their original states. Reversible surface wettability conversion reactions were achieved by alternate UV illumination and storage in the dark on both the films. The similar behaviors of wettability conversion observed on ZnO and TiO2 surfaces suggest that they follow a similar conversion mechanism. Preferential adsorption of water molecules on the photogenerated surface defective sites is ascribed to the formation of highly hydrophilic ZnO and TiO2 surfaces. Achievement of highly hydrophilic ZnO and TiO2 surfaces by high-temperature annealing and Ar+ sputtering provided supporting evidence for the explanation that surface defective sites play crucial roles in causing the surface wettability conversion reactions.

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 photocatalytic oxidation process on the TiO2 surface under UV irradiation. The comparison of the hydrophilic conversion rate with the rate for the reverse process clarifies that the critical contact angle is obtained when the reconstruction of the surface hydroxyl groups and their relaxation process are equilibrated under UV irradiation.