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 enhancement of the signal intensity of water molecularly adsorbed on TiO 2 films could be observed on UV irradiation by 1 H NMR spectroscopy, which has been difficult to elucidate so far with alternative spectroscopic techniques such as X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and infrared reflection absorption spectroscopy (IRAS). It was also confirmed that the enhanced NMR signals resulted from the photoinduced hydrophilic conversion of the film surface decreased back to the original state with the recovery to the less hydrophilic state of the TiO 2 film surface when the film was kept in the dark.