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Light-induced amphiphilic surfaces

Abstract

The ability to control the surface wettability of solid substrates is important in many situations. Here we report the photogeneration of a highly amphiphilic (both hydrophilic and oleophilic) titanium dioxide surface. The unique character of this surface is ascribed to the microstructured composition of hydrophilic and oleophilic phases, produced by ultraviolet irradiation. The result is a TiO2-coated glass which is antifogging and self-cleaning.

Main

We prepared a thin TiO2 polycrystalline film from anatase sol on a glass substrate, which was annealed at 773 K. The film showed a water-contact angle of 72°±1° before ultraviolet irradiation (Fig. 1a). After irradiation, water droplets spread out on the film, resulting in a contact angle of 0°±1° (Fig. 1b). This change in wettability was clearer when a TiO2-coated glass was exposed to water vapour. Without ultraviolet irradiation the glass fogged (Fig. 1c), but on irradiation the glass became transparent (Fig. 1d), a remarkable antifogging effect.

Figure 1: a, A hydrophobic surface before ultraviolet irradiation.
figure 1

b, A highly hydrophilic surface on ultraviolet irradiation. c, Exposure of a hydrophobic TiO2-coated glass to water vapour. The formation of fog (small water droplets) hindered the view of the text on paper placed behind the glass. d, Creation by ultraviolet irradiation of an antifogging surface. The high hydrophilicity prevents the formation of water droplets, making the text clearly visible.

We also measured the contact angle using oily liquids (such as glycerol trioreate and hexadecane). We found distinct contact angles for the TiO2 film under normal conditions, but all the liquids spread across the surface on ultraviolet irradiation, with contact angles of 0°±1°. Irradiation created a surface that was highly hydrophilic and highly oleophilic. We observed the wettability change on both anatase and rutile TiO2 surfaces of polycrystals or single crystals, independent of their photocatalytic activities. Even after storage 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, high amphiphilicity was repeatedly regenerated by ultraviolet irradiation.

The contact angle gives information about the macroscopic surface wettability. To gain information about surface wettability at a microscopic level we used friction force microscopy (FFM)1. A rutile TiO2(110) single crystal was used to make these measurements because of its flat surface, a requirement for FFM. Before ultraviolet irradiation (Fig. 2a) we observed no difference in contrast, indicating microscopically homogeneous wettability. After irradiation, (Fig. 2b) hydrophilic (bright) and oleophilic (dark) areas of size 30-80 nm were clearly seen. Higher resolution images (Fig. 2b inset) show hydrophilic domains with regular rectangular shapes, aligned along the [001] direction of the (110) crystal surface, irrespective of the scanning direction. A gradual reversion to a lack of contrast was observed during the storage of the film in the dark. We conclude that the nanoscale separation between the hydrophilic and the oleophilic phases accounts for the highly amphiphilic character of the TiO2 surface.

Figure 2: FFM images of a rutile TiO2(110) single crystal surface.
figure 2

a, A 5×5 μm2 image before ultraviolet irradiation. b, The same surface after irradiation. Inset, topographic image (245×245 nm) acquired by rotating the sample stage through 45° to the large-scale image. The tip of the Si3N4 cantilever is hydrophilic, so hydrophilic areas are bright and hydrophobic areas are dark.

In combination with Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy studies, we propose the following model. Ultraviolet irradiation may create surface oxygen vacancies at bridging sites, resulting in the conversion of relevant Ti4+ sites to Ti3+ sites2 which are favourable for dissociative water adsorption3,4. These defects presumably influence the affinity to chemisorbed water of their surrounding sites, forming hydrophilic domains, whereas the rest of the surface remains oleophilic, as seen by FFM (Fig. 2b). The rectangular hydrophilic domains are areas where dissociative water is adsorbed, associated with oxygen vacancies that are preferentially photogenerated along the [001] direction of the (110) plane; the same direction in which oxygen bridging sites align5. Because a liquid droplet is substantially larger than the hydrophilic (or oleophilic) domain, it instantaneously spreads on such a surface, thereby resembling a two-dimensional capillary phenomenon. Microscopically, the hydrophilic and oleophilic areas are distinguishable, but macroscopically, the TiO2 surface exhibits high amphiphilicity. On long-term storage in the dark, the chemisorbed hydroxyl groups are replaced with oxygen from the air3.

Such highly amphiphilic surfaces have many practical applications. Besides the antifogging effect (Fig. 1d), they are self-cleaning. Various substrates coated with TiO2 films, regardless of their photocatalytic activities, showed obviously cleaner surfaces than those without TiO2 coating after being hung outdoors for six months. Ultraviolet irradiation from sunlight is sufficient to maintain the amphiphilic surface, so that hydrophilic or oleophilic contaminants on the surfaces are easily removed by rain. This unique amphiphilic surface should be superior to a monohydrophilic or mono-oleophilic surface. The present system is independent of conventional photocatalytic reactions6, but the combination of these two surface characters give TiO2 materials a promising future.

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Wang, R., Hashimoto, K., Fujishima, A. et al. Light-induced amphiphilic surfaces. Nature 388, 431–432 (1997). https://doi.org/10.1038/41233

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