Photogeneration of Highly Amphiphilic TiO2 Surfaces

TLDR: In this article, the authors reported the details of the photoconvertible surface wettability of TiO2 surfaces and showed that the formation of a microstructured composite between hydrophilic and oleophilic phases, which results from the photogenerated Ti defects at definite sites, is considered to explain the unique feature.

Abstract: The discovery of photoinduced water splitting on TiO2 electrodes has prompted extensive research on TiO2 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 or photocatalytic reactions. Little research has been reported to clarify the effect of light on the properties of TiO2 surfaces. Very recently, we found that UV illumination of TiO2 materials can generate surfaces that display 0 contact angle for both water and oily liquids. 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 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 TiO2 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 . The contact angle of glycerol trioleate (GT), a main ingredient of edible oil, for the TiO2 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 TiO2 surface. Parallel experiments were performed using other liquid species, e.g., hexadecane, ethylene glycol, tetralin. Distinct contact angles resulted for the hydrophobic TiO2 surface. However, all of the liquids spread completely on a UV-illuminated TiO2 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 TiO2 surfaces in the form of either polycrystals or a single crystal, independent of their photocatalytic activities. Even after the TiO2 had been stored in the dark for a few days, the high amphiphilicity of the TiO2 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 adhesive forces between the solid and the liquid. For a certain liquid, the predominant contribution to the contact angle comes from the interfacial character of the solid material, which is related to its surface structure. Therefore, structural change of the TiO2 surface via UV illumination may play an important role in its unique wettability. Friction force microscopy (FFM) images supply information at a microscopic level to explain surface wettability. A rutile TiO2(110) single crystal was used since its wettability behavior (as induced by UV illumination) is analogous to anatase polycrystalline films. In addition, a flat surface is required for FFM measurement. Before UV illumination (Fig. 1a), no difference in contrast was observed for either the FFM image or the topographic image (not shown here), indicating microscopically homogeneous wettability on the surface. After UV illumination (Fig. 1b), however, a distribution of hydrophilic (bright) and oleophilic (dark) areas was clearly seen on the surface. Figure 1c shows a medium scale FFM image, corresponding to the framed area in Figure 1b, which illustrates hydrophilic domains with a regular rectangular shape in the range of 30±80 nm in size. A higher resolution topographic image (Fig. 1d) was also acquired, demonstrating that the hydrophilic domains are higher in position than the oleophilic areas. The image was viewed by rotating the sample stage by 45 with respect to Figure 1c, indicating that the characteristic structure is irrespective of the scanning direction. All the images show that the rectangular features align particularly along the [001] direction of the (110) single-crystal surface. As schematically illustrated in Figure 2A, oxygen bridging sites align along the same direction. It is widely accepted that the atomic coordinations at the TiO2 surface differ from those in the bulk since the atom arrangements are truncated on the surface. This gives rise to five-coordinated Ti atoms and two-coordinated O atoms, which are more energetically reactive than the six-coordinated Ti and