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Langmuir 1998, 14, 5918-5920
Effect of Ultrasonic Treatment on Highly Hydrophilic TiO2 Surfaces Nobuyuki Sakai,† Rong Wang,†,§ Akira Fujishima,*,† Toshiya Watanabe,‡,⊥ and Kazuhito Hashimoto‡ Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8904, Japan Received May 27, 1998. In Final Form: July 24, 1998 Glass surfaces coated with polycrystalline titanium dioxide (TiO2) films were found to exhibit a 0° water contact angle when the surfaces were illuminated with UV light in the air. This highly hydrophilic surface was maintained for more than 1 week in the dark in air. However, ultrasonic treatment in pure water decreased the degree of surface hydrophilicity, yielding a contact angle of approximately 11°. X-ray photoelectron spectroscopic measurements indicated that hydroxyl groups and molecular water adsorption, which govern the surface wettability, were partially removed from the surface by the ultrasonic treatment. The effect of ultrasonic treatment was ascribed to the generation of OH radicals that reoxidized the photoreduced surface, accompanied by the removal of surface-adsorbed water. This has been confirmed by adding acrylamide, a typical OH radical scavenger, to pure water to effectively suppress the hydrophilicto-hydrophobic reconversion on the TiO2 surface.
Introduction Titanium dioxide (TiO2) has been studied far more than other semiconductor photocatalysts due to its strong oxidizing power, chemical inertness, and nontoxicity. As a phenomenon that is distinct from conventional TiO2 photocatalytic oxidation reactions of adsorbed molecules on surfaces,1-5 we have recently found that UV illumination of TiO2 materials can produce a highly amphiphilic surface.6,7 The production of such a unique surface is attributed to the formation of a microstructured distribution of hydrophilic and oleophilic phases.6,7 We have explained the existence of hydrophilic sites in terms of the photoreduction of Ti4+ sites on the hydrophobic TiO2 surface to the Ti3+ oxidation state.7,8 We have also reported that such amphiphilic surfaces have antifogging and selfcleaning properties. Because a light-induced amphiphilic surface can revert back to the hydrophobic state after long-term storage in the dark, it is of great importance to control the reconversion process for actual applications. Here, we have focused our attention on changes in the hydrophilicity that can occur in the dark as well as under UV illumination. * To whom correspondence should be addressed. † Department of Applied Chemistry, The University of Tokyo. ‡ Research Center for Advanced Science and Technology, The University of Tokyo. § Current address: Chemical Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545. ⊥ Permanent address: Research & Development Division, TOTO Ltd., 2-8-1, Honson, Chigasaki-shi, Kanagawa 253-0042, Japan. (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (2) In Photocatalysis Fundamentals and Applications; Serpone, N., Pelizzetti, E., Eds.; Wiley-Interscience: Amsterdam, 1989. (3) In Photocatalytic Purification and Treatment of Water and Air; Ollis, D. E., Al-Ekabi, H., Eds.; Elsevier: Amsterdam, 1993. (4) Ikeda, K.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. B 1997, 101, 2617. (5) Ohko, Y.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. A 1997, 101, 8057. (6) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (7) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Adv. Mater. 1998, 10, 135. (8) Wang, R.; Sakai, N.; Hashimoto, K.; Fujishima, A.; Watanabe, T. Submitted for publication in J Phys. Chem. B.
Figure 1. Change of water contact angle of a TiO2-coated substrate upon UV illumination with an intensity of 0.58 mW/ cm2 (sOs) and 40 mW/cm2 (- -0- -), respectively.
Experimental Section TiO2 polycrystalline thin films (0.3 µm) were coated on glass substrates, which were prepared from TiO2 anatase sol by a spincoating method and annealed in the air at 500 °C. The surface wettability was evaluated by use of a contact angle meter (CA-X, Kyowakaimenkagaku Co. Ltd.), giving an experimental error of (1°. UV illumination was carried out using a fluorescent blacklight bulb and an Hg-Xe lamp to obtain UV light intensities of 0.58 and 40 mW/cm2, respectively. The ultrasound source was a 45-kHz generator (100 W output). The temperature of the bath was maintained in the range of 20-25 °C during sonication. X-ray photoelectron spectra (XPS) were acquired using a PerkinElmer Model 5600 X-ray photoelectron spectrometer. MgKR radiation was used, and the photoelectrons were collected at a takeoff angle of 45° with respect to the film surface normal.
Results and Discussion Contact angle (advancing) measurements were conducted to examine the surface wettability of the TiO2 thin films. Figure 1 shows the time dependence of the water contact angle of the film upon UV illumination with light of two different intensities. The film, which had been thoroughly cleaned with acetone and water, yielded an initial water contact angle of 50°, indicative of a relatively hydrophobic surface. When lower intensity UV light (0.58 mW/cm2) was shone on the surface for 2.5 h, the surface
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Effect of Ultrasonic Treatment on TiO2 Surfaces
Figure 2. Change of water contact angle subjected to alternate ultrasonic treatment (in pure water, sbs) and UV illumination (in the air, - -O- -); stored in the dark without sonication (‚‚‚ 4‚‚‚).
gradually converted to a hydrophilic state and finally the contact angle reached 0°. Using UV light with an intensity of 40 mW/cm2, the conversion from hydrophobic to highly hydrophilic was complete within 10 min. As we have reported previously, the hydrophobic to hydrophilic conversion is explained by assuming that surface Ti4+ sites are photoreduced to the Ti3+ state.6-8 This process is essentially the same as the surface reduction processes of Ti4+ to Ti3+ induced by Ar ion sputtering,9,11 electron beam exposure,10,11 and high-energy UV light.12 Stored in the dark for long times (>2 months), the surface gradually reconverts to the hydrophobic state. Two possible explanations for this hydrophilic-to-hydrophobic change are (1) chemical or morphological changes of the surface structure itself and/or (2) adsorption of organic contaminants on the surface. To clarify the reasons for the hydrophilic-hydrophobic conversion, a hydrophobic sample with a contact angle of 51.0°, which was obtained by having stored a hydrophilic sample with a contact angle of 0° in the dark for more than 2 months, was thoroughly cleaned in pure acetone, followed by cleaning in ultrapure water with sonication. The water contact angle for this surface changed only slightly, to 48.0°, ruling out the possibility that the hydrophobicity is due to surface contamination during storage. When a typical highly hydrophilic TiO2 thin film was sonicated at a frequency of 45 kHz in pure water, the water contact angle increased from 0° to 11°. When a similar hydrophilic sample was simply stored in the dark without sonication, it required approximately 3 weeks for the contact angle to increase to 11°. When this surface (with a contact angle of 11°) was illuminated with UV light again, it returned to the highly hydrophilic state (with a contact angle of 0°). Subjected to cycles of alternating ultrasound and UV illumination, the water contact angle for the TiO2 surface switched between 0° and 11° ( 1°, repeatedly, as shown in Figure 2. The sonication effect was also examined using X-ray photoelectron spectroscopy (XPS). Figure 3 shows O 1s spectra for a TiO2 polycrystalline thin film. The spectrum for the highly hydrophilic state (solid line) exhibits a broad shoulder to the high binding energy side of the main O 1s peak. The shoulder was fitted with two bands, which are associated with dissociatively adsorbed water on the TiO2 (9) Hugenschmidt, M. B.; Gamble, L.; Campbell, C. T. Surf. Sci. 1994, 302, 329. (10) Wang, L.-Q.; Baer, D. R.; Engelhard, M. H. Surf. Sci. 1994, 320, 295. (11) Wang, L.-Q.; Baer, D. R.; Engelhard, M. H.; Shultz, A. N. Surf. Sci. 1995, 344, 237. (12) Shultz, A. N.; Jang, W.; Hetherington, W. M., III; Baer, D. R.; Wang, L.-Q.; Engelhard, M. H. Surf. Sci. 1995, 339, 114.
Langmuir, Vol. 14, No. 20, 1998 5919
Figure 3. O 1s XPS spectra of an anatase TiO2 polycrystalline film on glass. Solid line, a highly hydrophilic surface; dashed line, same surface after ultrasonic treatment in pure water; dotted line, further irradiated by UV light in the air.
surface9,10 as well as physically adsorbed molecular water7,11 on the dissociatively adsorbed OH groups. After the ultrasonic treatment of the surface (dashed line), the shoulder decreased remarkably, showing that a portion of the adsorbed water was removed from the surface upon sonication. The effect of atmospheric oxygen during the sample transfer procedure was negligible in this case, since the XPS spectra showed little change even after the sample was stored in ambient air for the same period as that for the sonication. It should be noted that the C 1s spectral region (not shown) showed a very small peak, indicating a lack of surface organic contamination. On the basis of the above results, we conclude that the hydrophilic-hydrophobic reconversion is not due to the contamination of the surface but to the change of the surface itself. Because it is considered that the hydrophobic-to-hydrophilic change is accompanied by the surface reduction from Ti4+ to Ti3+,9-12 the removal of the adsorbed water via sonication, resulting in the hydrophilicto-hydrophobic change of the surface, is most likely associated with the reoxidation of the surface from Ti3+ to Ti4+ by O2. It is known that ultrasonic treatment in pure water creates OH radicals,13 whose oxidizing power is much stronger than that of molecular O2. It is therefore reasonable to assume that the photoreduced TiO2 surface, with hydroxyl groups and physisorbed molecular water is reoxidized to its original electronic structure by ultrasonic treatment, accompanied with removal of water from the surface. To confirm the effect of OH radicals on the hydrophilichydrophobic reconversion, acrylamide, a typical scavenger for OH radicals,14,15 was added to the water during the sonication of the highly hydrophilic TiO2 thin films. Figure 5 shows the change of the water contact angle of the TiO2 thin films upon sonication in 5, 1, and 0.01 M acrylamide aqueous solutions and in pure water, followed by washing in pure water. The rate of the water contact angle increase was drastically lowered by increasing the acrylamide concentration. This result clearly shows that the OH radicals, generated by sonication of water, play a major role in the hydrophilic-hydrophobic reconversion of the TiO2 surface. When the hydrophilic sample was dipped into the acrylamide solution, the water contact angle (0° ( 1°) did not change measurably. In addition, XPS spectra (C 1s) showed an absence of carbon contamination after sonication in the acrylamide solution. Since the ultrasonic treatment is carried out in pure water, water desorption and adsorption processes are (13) Henglein, A. Ultrasonics 1987, 25, 6. (14) Weissler, A. J. Am. Chem. Soc. 1959, 81, 1077. (15) Makino, K.; Mossoba, M. M.; Riesz, P. J. Phys. Chem. 1983, 87, 1369.
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Figure 4. Hydrophilic-hydrophobic reconversion upon storage of the highly hydrophilic sample in the dark after ultrasonic treatment (sOs) and without ultrasonic treatment (- -b- -). Ultrasonic treatment was performed in pure water.
competing, resulting in an equilibrium state. This is probably the reason the water contact angle saturates at approximately 11° in water, as shown in Figure 2. When the treated sample was stored in the air in the dark, the water contact angle increased much faster than that without ultrasonic treatment (Figure 4), indicating that the sonicated surface has a tendency to become further oxidized. There is also the possibility that sonication at 45 kHz could dislodge particles from the sintered film, exposing fresh hydrophobic surfaces. However, while this process can certainly occur with extensive treatment, this is probably not a significant influence on the contact angle. The fact that the increase in the contact angle during ultrasonication became small (reaching a final value of only ∼2°) as the acrylamide concentration was increased excludes this possibility. Conclusions These results show that the hydrophilic-to-hydrophobic conversion is due to chemical and/or morphological changes of the surface structure. Specifically, we conclude that this process involves the oxidation of the TiO2 surface itself. Hence, the hydrophobic-to-hydrophilic process can be considered to involve the reduction of the TiO2 surface itself, and thus, this reaction may be thought of as a
Sakai et al.
Figure 5. Effect of ultrasonic treatment in acrylamide aqueous solution on the hydrophilic-hydrophobic reconversion. The concentrations of acrylamide were 0 M (sOs), 0.01 M (‚‚‚4‚‚‚), 1.0 M (- -2- -), and 5.0 M (- -b- -), respectively.
photochromic reaction (see, e.g., ref 16). This aspect is now being examined in more detail. Note that the surface wettability of the anatase TiO2 polycrystalline film was balanced at a water contact angle of 11° ( 1° upon ultrasonic treatment in pure water. We have also observed that the ultrasonic treatment can reconvert the water contact angle of a rutile single crystal from 0° to 65° ( 1°.17 We suppose that the extent of surface wettability reconversion involves such factors as the surface energy, affinity to water adsorption, coverage of surface defect sites, distribution of exposed crystal faces, and so forth, all of which require further investigation. The current work offers an effective approach for the control of the hydrophilic-hydrophobic reconversion process in practical applications. Acknowledgment. We express gratitude to Prof. D. A. Tryk for careful reading of the manuscript. We gratefully acknowledge support by the Ministry of Education, Science, and Culture of Japan. LA980623E (16) Howe, R. F.; Gra¨tzel, M. J. Phys. Chem. 1985, 89, 4495. (17) Sakai, N.; Wang, R.; Hashimoto, K.; Fujishima, A.; Watanabe, T. In First International Symposium on Environmental Issues with Materials and Processes for the Electronics and Semiconductor Industries/1998; The Electrochemical Society Proceedings Series; The Electrochemical Society: Pennington, NJ, in press.
