Transparent Superhydrophobic Thin Films with Self-Cleaning Properties

In Final Form: May 18, 2000. Transparent superhydrophobic thin films with TiO2 photocatalyst were prepared by utilizing a sublimation material and sub...
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Transparent Superhydrophobic Thin Films with Self-Cleaning Properties Akira Nakajima, Kazuhito Hashimoto,* and Toshiya Watanabe* Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan

Kennichi Takai Department of Mechanical Engineering, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan

Goro Yamauchi Department of Mechanical Engineering, Daido Institute of Technology, 2-21 Daido-cho, Minami-ku, Nagoya 457-8531, Japan

Akira Fujishima Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received February 3, 2000. In Final Form: May 18, 2000 Transparent superhydrophobic thin films with TiO2 photocatalyst were prepared by utilizing a sublimation material and subsequent coating of a (fluoroalkyl)silane. The transparency of the films decreased with increasing TiO2 concentration, which was attributed to the size difference of the starting materials. The film with only 2 wt % TiO2 maintained higher contact angle than the film without TiO2 after 1800-h outdoor exposure, the accumulation of stain being avoided due to TiO2 photocatalysis. The films prepared in this study are the first ones that satisfy the requirements of transparency, superhydrophobicity, and long lifetime simultaneously.

Introduction In a pond after a rainfall, we often see spherical water droplets on floating lotus leaves. This phenomenon is due to innumerable fine projections coated with water-repellent wax on the leaf.1 A surface with such excellent hydrophobicity has a high contact angle for water, and a surface whose water contact angle is greater than 150° is commonly called a superhydrophobic surface. This surface has attracted much interest from both industry and fundamental research. In fact, various methods for the processing of such surfaces are being investigated.2-14 For the preparation of superhydrophobic surfaces or * To whom all correspondence should be addressed. Prof. T. Watanabe: TEL: +81-3-5452-5332, FAX: +81-3-5452-5334, email: [email protected], Prof. K. Hashimoto: TEL: +81-3-5452-5080, FAX: +81-3-5452-5083, e-mail:kazuhito@ fchem.chem.t.u-tokyo.ac.jp. (1) Neinhuis, C.; Barthlott, W. Ann. Bot. 1997, 79, 667. (2) Dettre, R. H.; Johnson, R. E., Jr. Adv. Chem. Ser. 1963, 43, 136. (3) Washo, B. D. Org. Coat. Appl. Polym. Sci. Proc. 1982, 47, 69. (4) Morra, M.; Occhiello, E.; Garbassi, F. Langmuir 1989, 5, 872. (5) Kunugi, Y.; Nonaka, T.; Chong, Y.-B.; Watanabe, N. J. Electroanal. Chem. 1993, 353, 209. (6) Ogawa, K.; Soga, M.; Takada, Y.; Nakayama, I. Jpn. J. Appl. Phys. 1993, 32, L614. (7) Yamauchi, G.; Miller, J. D.; Saito, H.; Takai, K.; Ueda, T.; Takazawa, H.; Yamamoto, H.; Nislhi, S. Colloids Surf. A 1996, 116, 125. (8) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125. (9) Shibuichi, S.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512. (10) Tadanaga, K.; Katata, N.; Minami, T. J. Am. Ceram. Soc. 1997, 80, 3213.

films, the combination of surface roughness and low surface energy is required.15 From a viewpoint of surface roughness, hydrophobicity is competitive with its transparency because surface roughness becomes a scattering source of light. Thus, the precise control of roughness is required to satisfy both hydrophobicity and transparency. We have recently succeeded in developing a new process to prepare transparent superhydrophobic boehmite (A1OOH) or silica (SiO2) films by using aluminum acetylacetonate (Al(C5H7O2)3, sublimation temperature 193 °C)16 as a sublimation material.17 Either the boehmite or silica film is roughened by the sublimation of aluminum acetylacetonate after calcination. Then, transparent superhydrophobic films are obtained by subsequent coating of (fluoroalkyl)silane. However, the excellent hydrophobicity of these superhydrophobic surfaces gradually degrades, in general, during a long-time outdoor exposure due to the buildup of stain. This was a crucial and inevitable problem for artificially constructed superhydrophobic surfaces. Natu(11) Tadanaga, K.; Katata, N.; Minami, T. J. Am. Ceram. Soc. 1997, 80, 1040. (12) Hozumi, A.; Takai, O. Thin Solid Films 1997, 303, 222. (13) Takai, K.; Saito, H.; Yamauchi, G. Proceedings of the Composites: Design for Performance; Patrick S. Nicholson, Ed.; Eagle Press: Lake Louise, Canada, 1997; p 220. (14) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Oner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395. (15) Onda, K. Tsukuba Res. Consortium 1997, 4, 56. (16) Lange’s Handbook of Chemistry, 12th ed.; Dean, J. A., Ed.; McGraw-Hill Inc.: New York, 1979; p 414. (17) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Adv. Mater. 1999, 16, 1365.

10.1021/la000155k CCC: $19.00 © 2000 American Chemical Society Published on Web 07/25/2000

Transparent Superhydrophobic Thin Films

ral lotus leaves avoid this problem by continuous metabolism of their surface wax layer and maintain their hydrophobicity during their lifetime.1 Since a proper metabolic mechanism of lotus leaves is difficult to duplicate, practical applications of superhydrophobic surfaces have been limited. It is well-known that TiO2 photocatalysts have a strong oxidation power under UV illumination,18 and various organic stains accumulated on TiO2 thin films are oxidized finally into CO2.19,20 Moreover, we have also found that the surface of TiO2 becomes highly hydrophilic with a water contact angle of 0° under UV illumination.21,22 Stains adsorbed on the superhydrophilic surfaces are easily washed out by water. Due to the strong oxidation power and superhydrophilic properties of TiO2 under UV illumination, TiO2-coated substrates show very clear selfcleaning properties when they are exposed outdoor.23 In the present study, we have tried to prepare transparent superhydrophobic thin films with self-cleaning properties by adding photocatalytic TiO2 powders to the films. Experimental Section Titanium acetylacetonate (TiO(C5H7O2)2), which decomposes into TiO2 at -200 °C,24 was selected as a titanium source. Hereafter, aluminum acetylacetonate and titanium acetylacetonate are denoted as AACA and TACA, respectively. A commercial boehmite powder (DISPAL 18N4, Condea Chemie, Hamburg, Germany), reagent-grade AACA (Tokyo Kasei Co., Tokyo, Japan), and TACA (Tokyo Kasei) were mixed with ethanol (Wako Jyunyaku Co., Tokyo, Japan). The weight ratios of boehmite and AACA to ethanol were fixed as 0.0024 and 0.024, respectively. The ratio of TACA to ethanol was changed as 0, 0.0002, 0.002, 0.01, 0.015, and 0.02 to control the concentration of TiO2 in the film. The mixed suspensions were sonicated for 20 min. During the sonication, AACA dissolved into ethanol. The sonicated suspensions were coated on Pyrex glass plates (1 mm in thickness) by spin coating at 1500 rpm. The coated glass plates were dried at room temperature for a few minutes. During drying, the glass plates became opaque. Calcination of the glass plates was carried out on a hot plate at 500 °C for 20 s. White smoke was generated from the opaque films due to the sublimation of AACA, and the glass plates changed to transparent again during this heat treatment. Boehmite did not transform into γ-alumina under the above calcination condition. These coating and calcination procedures were repeated five times to coat the glass plates completely by the film materials. These processes yield the boehmite films containing 0, 2, 20, 55.6, 66, and 71.4 wt % TiO2. A methanol solution of (heptadecafluorodecyl)trimethoxysilane (CF3(CF2)7CH2CH2Si(OCH3)3, TSL8233, Toshiba Silicone Co., Tokyo, Japan, hereafter denoted as FAS-17) was hydrolyzed by the addition of a 3-fold molar excess of water at room temperature. This hydrolyzed silane solution was employed as a water-repellent agent. Pyrex glass plates with the roughened boehmite films were immersed in the silane solution at room temperature for 1 h, and subsequently heated at 140 °C for 1 h. The flow diagram of the film processing is shown in Figure 1. Water contact angle on the prepared film was measured by a contact angle meter (CA-X, Kyowa Interface Science, Saitama, (18) Kawai, T.; Sakata, T. Nature 1980, 286, 474. (19) Watanabe, T.; Kitamura, A.; Kojima, E.; Nakayama, C.; Hashimoto, K.; Fujishima, A. Photocatalytic Purification and Treatment of Water and Air; Ollis, D. E., Al-Ekabi, H., Eds.; Elsevier: Amsterdam/ London/New York/Tokyo, 1993; p 747. (20) Schwitzgebel, J.; Ekerdt, J. G.; Gericher, H.; Heller, A. J. Phys. Chem. 1995, 99, 5633. (21) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, R.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (22) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, R.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Adv. Mater. 1998, 10, 135. (23) Fujishima, A.; Hashimoto, K.; Watanabe, T. TiO2 Photocatalyst, Fundamentals and Applications; BKC Inc.: Tokyo, 1999; p 66. (24) Kagaku Binran, 3rd ed.; Chemical Society of Japan, Ed.; Maruzen Inc.: Tokyo, 1984; pp 1-195.

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Figure 1. Flow diagram of the processing of transparent superhydrophobic film with TiO2. Japan). The amount of water droplets used for the measurement was ∼3.80 µL. Average surface roughness (Ra) was evaluated by depth profiles from a laser profile micrometer (VF-7500, Keyence Co., Tokyo, Japan). Both contact angles and surface roughness were measured at five different points in ∼6 cm2. Particle size measurement and microstructure observation were carried out by a scanning electron microscope (SEM, S-4200, Hitachi Co., Tokyo, Japan). Average particle sizes and their distributions of the starting materials were obtained by the direct measurement of 50 particles from SEM micrographs of starting materials. Transmittance of visible light was evaluated by a UV-vis-NIR scanning spectrophotometer (UV-3 100PC, Shimadzu Co., Tokyo, Japan) using air as a reference. The kinetics of the decomposition of FAS-17 on the prepared films by TiO2 photocatalysts was evaluated by measuring contact angles under the UV illumination with an intensity of 1.7 mW/ cm2 by blacklight bulbs. The outdoor exposure test of the films was carried out at a rooftop (∼20 m from the ground level) of the building in Komaba, Tokyo, for 75 days from December 1998 to February 1999. The films were tilted at 45° for the outdoor exposure tests, and their contact angles were evaluated. This tilt angle was chosen as an intermediate and average condition between horizontal and perpendicular conditions.

Results and Discussion The particle sizes of boehmite and TACA were 24.5 (average) ( 7.l (standard deviation) nm and 91.4 ( 38.0 nm, respectively. These sizes were not changed significantly even after the calcination at 500 °C. Contact angles for the 6 boehmite films with the compositions of 0, 2, 20, 55.6, 66, and 71.4 wt % TiO2 were 148.1 ( 1.70, 150.2 ( 1.9, 151.1 ( 1.1, 152.9 ( 0.6, 153.9 ( 0.4, and 155.6 ( 1.0°, respectively. Their average surface roughnesses were 44 ( 4, 49 ( 5, 67 ( 13, 68 ( 13, 70 ( 15, and 109 ( 58 nm, respectively. Figure 2 shows a photograph of the droplet on a film containing 20 wt % TiO2. The shape of the water droplet was almost spherical. The SEM micrographs of the prepared films were shown in Figure 3. The films with less than 20 wt % TiO2 were composed of fine particles of several tens of nanometers, and had rough microstructure with pores of 100-300 nm, whereas the microstructure of the film became coarser with increasing concentration of TiO2. Figure 4 shows the transmittance of the prepared films in the range of visible wavelength. When TiO2 concentration is less than 20 wt %, transmittance of the film itself is almost 100%. The transmittance was decreased with increasing TiO2 concentration. The changes in microstructure and surface roughness with increasing TiO2 concentration are attributed to the increase of the amount of large TACA particles that decompose into TiO2

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Figure 2. Water droplet on the transparent superhydrophobic film with 20 wt % TiO2.

Nakajima et al.

Figure 5. Change of water contact angle for the films under UV illumination.

Figure 6. Apparent change of the surface energy by wetting per unit time.

Figure 3. SEM micrographs of prepared superhydrophobic films: (a) 0% TiO2; (b) 20% TiO2; (c) 55.6% TiO2; (d) 71.4% TiO2.

nificant with increasing illumination time and TiO2 concentration. The decrease of contact angle during UV illumination shown in Figure 5 is due to the decomposition of FAS-17 by photocatalytic reaction. This contact angle decrease can be converted to the apparent change of the surface energy by wetting per unit time by the following procedure. The work of adhesion (Wa) as the change of surface energy due to wetting is defined as the following equation:25

Wa ) γl(1 + cos θ) Here γl is a free surface energy of liquid and θ is a contact angle of liquid on the solid. Thus, the apparent change of the surface energy by wetting per unit time is evaluated as follows:

{Wa(t1) - Wa(t2)}/(t1 - t2) ) γl(cos θ1 - cos θ2)/(t1 - t2)

Figure 4. Transmittance of prepared superhydrophobic films in the visible wavelength range.

during the calcination. Because of the increase of the light scattering due to the microstructure change, the transmittance of visible light was decreased with increasing TiO2 concentration. However, the surface roughness is low enough for the scattering of visible light when TiO2 is less than 20 wt %. Figure 5 shows the change of water contact angle for these films under UV illumination. The contact angle of the film with 2 wt % TiO2 was higher than 140° even after the UV illumination with the intensity of 1.7 mW/cm2 for 800 h. The degradation of hydrophobicity became sig-

Here θ1 and θ2 are the contact angles at time tl and t2, respectively. Since the decrease of contact angle involves the change of contact area, this calculation does not give a net value but an approximated one. Figure 6 shows the results calculated from the degradation data shown in Figure 5. These values were calculated from the period where θ1 is ∼145° and θ2 is ∼135°. This figure indicates that the degradation of hydrophobicity becomes remarkable when the concentration of TiO2 in the film is higher than a certain value, ∼55 wt %. Besides the increase of surface roughness, another possible reason for this result is the appearance of the unscreened surface of TiO2. Aim and Goff calculated the coordination number of the large sphere (25) Principles of Colloid and Surface Chemistry; Hiemenz, P. C., Ed.; Marcel Dekker: New York, 1986; p 314.

Transparent Superhydrophobic Thin Films

Figure 7. Change of contact angles during outdoor exposure.

in a bimodal mixture versus the particle size ratio.26 On the basis of their calculation, ∼70 small spheres are expected to be coordinatable around a large sphere when the sphere size ratio is four. Assuming spherical particle morphology and densities for TiO2 of 3.9 g/cm3 and boehmite of 3.0 g/cm3, the numbers of boehmite particles for one TACA particle are 4204, 333, 67, 43, and 33 for the films with 2, 20, 55.6, 66, and 71.4 wt % TiO2, respectively. This result indicates that TiO2 particles which are not coordinated by boehmite particles should appear when the concentration of TiO2 in the film is higher than ∼55 wt %, leading to a conspicuous increase of the decomposition rate of FAS-17. The results of the outdoor exposure test are shown in Figure 7. When the concentration of TiO2 was increased to 20 wt %, the decomposition of FAS-17 becomes significant, and the contact angle of the film decreases. The contact angle of the film without TiO2 also decreased gradually and became ∼100° after 1800-h outdoor exposure. This is due to the accumulation of stains, which could be confirmed clearly by the eye. However, the film with 2 wt % TiO2 maintained higher contact angles and kept clean even after the 1800-h outdoor exposure. One of the possible explanations for this result is the difference of kinetic balance of the decomposition between stain and FAS-17. This balance will depend on the concentration of TiO2 and particle sizes of the starting materials. Thus, it will be important to obtain the optimum balance by controlling these key factors for the proper performance of the films. It is noteworthy that only 2 wt % TiO2 provides selfcleaning properties for the superhydrophobic thin films. Although various factors affect the result of outdoor exposure, we propose several mechanisms that might be responsible for the overall self-cleaning performance of the superhydrophobic thin films with TiO2. Radical species are produced by the UV illumination to TiO2. The photoproduced radical species may diffuse through the FAS-17 coating and decompose the stains or react them to form hydrophilic stains. The hydrophilic stains are easily rinsed off the surface by exposure to rainwater. The diffusion distance of photoproduced radical species (mainly •OH) from TiO2 will be expected to be rather long for the surfaces coated with FAS-17 because the C-F bondings are resistant against the photocatalytic decomposition.27 Thus, a long diffusion distance of the radical species produced on TiO2 may contribute the effective decomposition of the stains on the FAS-17 coating. The bare TiO2 surface is expected to contaminate readily because of the high surface energy of TiO2 relative to the (26) Aim, R. B.; Le Goff, P. Powder Technol. 1968, 2, 1. (27) Sopyan, I.; Watanabe, M.; Murasawa, S.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1996, 415, 183.

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low surface energy of the contaminants. Moreover, we recently showed that the TiO2 surface becomes highly amphiphilic under UV illlumination.20,21 In other words, both hydrophilic and oleophilic stains adsorb much easier on TiO2 than on FAS-17 because the FAS-17-coated surface is both hydrophobic and oleophobic. Therefore, it is expected that stains do not stick on the FAS-17-coated surface strongly and effectively collected on the surface of TiO2 photocatalyst during outdoor exposure. The stains collected on the TiO2 surface are either decomposed by strong oxidation power or washed out by the penetrating water into the interface between stains and highly hydrophilic TiO2 surface. The photoinduced amphiphilic property of TiO2 may also contribute the total self- cleaning performance. Although the surface coated with fluorocarbon shows low surface energy, various stains stick on the surface from ambient air. One of the expected driving forces for the initial contamination is electrostatic effects. These stains will mainly accumulate at the trough parts in the superhydrophobic surface with roughness. Recently, Nosaka et al. reported that the decay rate of the surface charge of the TiO2 thin films is increased by UV illumination.28 Although the mechanism of this phenomenon is still under discussion, one plausible mechanism is water adsorption on the amphiphilic TiO2 surface. This report suggests that static electricity on the superhydrophobic surface is decreased by the addition of TiO2 photocatalyst, and this effect has already been confirmed by different superhydrophobic films combined with TiO2.29 The decrease of static electricity may also contribute total selfcleaning performance. Detailed investigation is necessary to evaluate the contribution of each mechanism for the total self-cleaning performance of the transparent superhydrophobic films with TiO2. Conclusions In the present study, transparent superhydrophobic films with TiO2 photocatalysts were prepared. Microstructure and surface roughness of the films are changed with increasing TiO2 concentration, which was attributed the size difference of the starting materials. The contact angle of the film containing 2 wt % TiO2 was higher than 140° even after UV illumination with the intensity of 1.7 mW/cm2 for 800 h. The film with 2 wt % TiO2 maintained higher contact angles than the film with 0 wt % TiO2 after 1800-h outdoor exposure. The films prepared in this study are the first ones that satisfy the requirements of transparency, superhydrophobicity, and long lifetime simultaneously. It is noteworthy that the self-cleaning effect is achieved by the addition of only 2 wt % TiO2. This type of film provides a great opportunity for the development of long-lifetime, water-shedding transparent coatings for various industrial items. Acknowledgment. We are grateful to Prof. D. A. Tryk for the careful reading of the manuscript. Financial support by a Grant-in-Aid for Scientific Research on Priority Area of Electrochemistry of Ordered Interfaces from the Ministry of Education, Science, Sports, and Culture of Japan is gratefully acknowledged. LA000155K (28) Koizumi, S.; Igarashi, R.; Nosaka, Y. Abstracts of the Fall Meeting of the Electrochemical Society of Japan; Electrochemical Society of Japan: Tokyo, 1998; p 33. (29) Miwa, M. Sliding Behavior of Water Droplet on the Hydrophobic Surfaces. Masters Thesis, The University of Tokyo, 2000; p 98.