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Wettability of Photoresponsive Titanium Dioxide Surfaces N. Stevens, C. I. Priest, R. Sedev, and J. Ralston* Ian Wark Research Institute, University of South Australia, Mawson Lakes, South Australia 5095, Australia Received July 22, 2002. In Final Form: November 18, 2002 According to K. Hashimoto and co-workers, the wettability of TiO2 surfaces can be altered by irradiation. Surfaces become hydrophilic (water contact angle, ∼0°) after UV irradiation and gradually revert to a more hydrophobic state (contact angles, 50-70°) when left in the dark or exposed to visible light. Such changes have been observed for both anatase and rutile surfaces (single crystals and polycrystals) and presumably are not directly related to the photocatalytic activity of TiO2. We report here similar changes of the contact angle of water on flat titania wafers and on densely packed layers of micron-sized titania particles. Wettability changes can be reversibly cycled, and the effect is rather robust. The hydrophobic-to-hydrophilic conversion is faster than the inverse one (at comparable UV and visible light intensities). The contact angle change observed on wafers (∼50°) is twice as large as that on the particle layer, but this is mainly related to the effect of roughness. Using a photomask, we have patterned successfully regular arrays of hydrophobic circles (10 µm in diameter) on a hydrophilic TiO2 matrix and vice versa. These findings are of significant interest for the design of intelligent surfaces even though the detailed mechanism of the transition is yet to be elucidated.
Introduction The wettability of materials plays an important role in nature and technology. There is a growing necessity to understand and possibly control the contact angle between a liquid and a solid surface. Since devices are progressively miniaturized, capillary forces and hence contact angle are becoming more and more important. Surfactants are widely used to modify interfacial properties, but they often adsorb on different interfaces, not just the target one, could modify the bulk solution properties, and may interact with other components thus creating side effects. This explains the strong interest in solid surfaces whose wettability can be controlled reversibly by an external stimulus (e.g., radiation, electric field, etc.). Recently Hashimoto and co-workers have published extensively on the hydrophilic-hydrophobic conversion of flat titanium dioxide layers induced by irradiating the dry sample with light of a specific wavelength.1-8 The contact angle of water on a clean TiO2 surface can be repeatedly cycled between practically zero (after UV irradiation1-8) and 50-60° (after irradiation with visible light or storage in the dark4,6,8). The wettability changes occur for both anatase and rutile surfaces (single crystal4 or polycrystalline5) and appear to be independent of their photocatalytic activity.5 * Corresponding author. Tel: +61 8 8302 3066. Fax: +61 8 8302 3683. E-mail:
[email protected]. (1) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (2) Sakai, N.; Wang, R.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Langmuir 1998, 14, 5918. (3) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Adv. Mater. 1998, 10, 135. (4) Wang, R.; Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 1999, 103, 2188. (5) Miyauchi, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Chem. Mater. 2000, 12, 3. (6) Sun, R.-D.; Nakajima, A.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 2001, 105, 1984. (7) Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 2001, 105, 3023. (8) Miyauchi, M.; Kieda, N.; Hishita, S.; Mitsuhashi, T.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Chem. Mater. 2001, Preprint.
These findings are rather unexpected. The TiO2 surface as a metal oxide surface is a high-energy one and therefore should be fully wetted by most liquids.9,10 However, reports about the contact angle of water on a clean (freshly prepared) titania surface are contradictory: 72°,1 60°,11 52°,12 50°,2 15°,3,13 less than 10°,14 and 0°.15 A useful measure of the wettability of high-energy surfaces is the heat of wetting (defined as the difference between the total surface energy of the wetted and dry solid16). The heat of wetting of fully hydroxylated amorphous silica in water is 160 mJ/m2,17 and the surface of clean silica is undoubtedly hydrophilic. The heat of wetting of anatase in water is 520 mJ/m2,18 thus confirming the above expectation. A very simple, if somewhat crude, explanation can be given to the influence of light irradiation as follows. Organic contaminants are always accumulating at highenergy surfaces,19 and TiO2 is no exception to this rule.15 Under UV irradiation of the (photocatalytic) TiO2 surface, these are decomposed. The surface becomes more wettable simply because it is cleaner. During storage in a laboratory atmosphere, contaminants are gradually adsorbing at the surface, thus making it more hydrophobic (titania surfaces are easily contaminated within few days;15 as a matter of fact, they remain hydrophilic only a few minutes after cleaning14). Hashimoto and co-workers argued that this is not the case since (i) the contact angle before and after cleaning (9) Zisman, W. A. Adv. Chem. Ser. 1964, 43, 1. (10) de Gennes, P.-G. Rev. Mod. Phys. 1985, 57, 827. (11) Lee, H. Y.; Park, Y. H.; Ko, K. H. Langmuir 2000, 16, 7289. (12) Utriainen, M.; Leijala, A.; Niinisto, L.; Matero, R. Anal. Chem. 1999, 71, 2452. (13) Yu, J.; Zhaoa, X.; Zhaoa, Q.; Wang, G. Mater. Chem. Phys. 2001, 68, 253. (14) Tosatti, S.; Michel, R.; Textor, M.; Spencer, N. D. Langmuir 2002, 18, 3537. (15) Takeda, S.; Fukawa, M.; Hayashi, Y.; Matsumoto, K. Thin Solid Films 1999, 339, 220. (16) Neumann, A. W.; Good, R. J. Surf. Colloid Sci. 1979, 11, 31. (17) Taylor, J. A. G.; Hockey, J. A.; Pethica, B. A. J. Phys. Chem. 1966, 70, 2169. (18) Jaycock, M. J.; Parfitt, G. D. Chemistry of Interfaces; E. Horwood: Chichester, U.K., 1981. (19) Clean Surfaces; Goldfinger, G., Ed.; Marcel Dekker: New York, 1970.
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a hydrophobic TiO2 surface in ethanol and water was practically unchanged (51° and 48°, respectively),2 (ii) ultrasonication in water increases the contact angle,2 (iii) the conversion is faster when carried in an oxygen atmosphere,4,6 and (iv) SrTiO3 like TiO2 has photocatalytic properties but does not become more hydrophilic after UV irradiation.5 Thus conflicting reports about the wettability of TiO2 surfaces can be found in the literature and these are yet to be reconciled. In this paper, we describe experiments that confirm the occurrence of a reversible hydrophilichydrophobic transition induced by light irradiation. The wettability of titania wafers and of silica plates densely covered with small titania particles can be reproducibly cycled by consecutively exposing the samples to UV and visible light, respectively. Materials and Methods Silicon wafers covered with a 100 nm layer of titania were kindly supplied by Dr. R. A. Hayes (Philips Research Laboratories, The Netherlands). They were cut to size (20 × 10 mm) and cleaned with a CO2 snow-jet gun (Applied Surface Technologies, NJ) to remove any debris left from the cutting process. They were then immersed in hot 30% KOH solution for 30 s, rinsed with copious amounts of water (resistivity, ∼18 MΩ cm; ELGA), and finally dried in a stream of filtered nitrogen. Titania particles were synthesized by hydrolyzing titanium tetraethoxide (Aldrich).20 The reaction was carried with 50 mM Ti(OC2H5)4 and 5 M water in ethanol chilled in an ice bath. Particles were separated by centrifugation, rinsed with water, and suspended at pH ) 4. Clean silica plates were dip-coated (3-5 times) with the titania suspension at a withdrawal speed of 50 µm/s. The surfaces were exposed to light of different wavelengths using a spectral irradiatior (MM3, Bunkoh-Keiki Co., Japan). Some of the surfaces were entirely illuminated. Others were irradiated through photomasks (DSTO, South Australia). The photomasks consisted of a regular array of chromium circles (diameter 10 µm) spaced differently (their area fraction ranged from 30 to 70%) on a quartz plate. Imaging was performed on a NanoScope III atomic force microscope (AFM; Digital Instruments, Santa Barbara, CA) in tapping mode. Silicon SPM cantilevers (Pointprobe NCH, Nanosensors) were used to obtain topographic information about the surfaces. Root-mean-square (rms) roughness and actual surface area were obtained with the AFM running software. Contact angle measurements were performed by the sessile drop method. A small water droplet (diameter, 2-3 mm) was advanced on the surface using a motorized syringe. Digital images (624 × 580 pixels, 256 gray levels) of the droplet silhouette were captured with a progressive scan CCD camera (JAI CV-M10BX, Japan). The contact angle was determined by numerically drawing a tangent close to the edge of the droplet. All glassware was cleaned in hot concentrated KOH solution, rinsed with pure water, and dried in a nitrogen stream. All experiments were carried out in a dust-free environment (clean room) at room temperature (23 °C).
Results and Discussion Tapping mode atomic force microscopy was used to image the titania plates (Figure 1a) and the silica plates covered with titania particles (Figure 1b). The rms roughness of the titania wafer was 0.4 nm, and the peak-to-valley height was 1.5 nm (estimated over an area of 25 µm2). The Wenzel ratio calculated as the ratio of the actual and projected areas was r ) 1.04. For the particle-covered surface, the rms roughness was 100 nm, the peak-to-valley height was 282 nm, and the Wenzel ratio was 1.39 (over an area of 100 µm2). (20) Barringer, E. A.; Bowen, H. K. J. Am. Ceram. Soc. 1982, 65, C-199.
Figure 1. AFM images of (a) a titania wafer and (b) a silica plate covered with titania particles. The AFM was used in tapping mode (scan rate, 1.0 Hz). The height scale is (a) 15 nm and (b) 600 nm. The diameter of the titania particles is about 0.5 µm.
The advancing contact angle of water on smooth titania wafers, which were stored in a dark box for several months, was usually in the range of 50-70°. These values did not change appreciably after sonicating the samples in ethanol for 40 min or, alternatively, stirring in hot 30% KOH for 30 s (in contrast, 30 s of plasma cleaning made the surfaces completely hydrophilic; this method was not further employed). The contact angle hysteresis on these wafers was about 15°, suggesting these rather smooth surfaces are not chemically homogeneous. The advancing contact angle decreased after exposing the sample to UV light (247 nm) and subsequently increased when the surface was irradiated with visible (466 nm) light (Figure 2). The contact angle decrease after UV exposure can be fitted with an exponential function:
θ ) θ0 exp(-kUVt)
(1)
The rate constant is kUV ) 0.16 min-1 (the fit is shown with a solid line in Figure 2). This value is in excellent agreement with the value 0.15 min-1, which we obtained
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Figure 2. Advancing contact angle on titania wafers after different lengths of exposure to UV (b) and visible (O) light. The trends are well described by exponential functions (eqs 1 and 2).
Figure 4. Advancing contact angle of water on a silica wafer coated with titania particles (diameter, ∼1 µm) after consecutive irradiation with visible (λ ) 466 nm) and UV (λ ) 247 nm) light (b). The wettability changes are practically reversible over the four cycles shown. A clean silica wafer processed in parallel is consistently hydrophilic (0).
a decreased contact angle, θ, should be observed on a rough surface of the same intrinsic quality:10
cosθ ) r cos θ0
Figure 3. Advancing contact angle of water on two (b and O) titania wafers after consecutive irradiation with visible (λ ) 466 nm) and UV (λ ) 247 nm) light. The wettability changes are practically reversible over the four cycles shown.
by fitting eq 1 to the data from Figure 2 of ref 5. The UV radiation intensity in both cases was 1 mW/cm2 (it has been confirmed that the conversion proceeds faster at higher intensity2,4). The contact angle decrease observed after the sample was exposed to visible light was also fitted with an exponent:
θ ) θ0[1 - exp(-kvist)]
(2)
The rate constant is significantly lower in this case, kvis ) 0.05 min-1, even though the intensity of the visible radiation was 12 mW/cm2. As seen from Figure 2, both hydrophilic and hydrophobic conversions are fully achieved after 40 min of irradiation. These saturated values of the advancing contact angles obtained after consecutive exposure to UV and visible light are plotted in Figure 3 for two titania wafers. Irradiation with visible light (λ ) 466 nm) makes the surface moderately hydrophobic (water contact angle, ∼50°) while UV irradiation (λ ) 247 nm) reverts the contact angle to practically zero. The effect is quite robust: during the four consecutive cycles shown in Figure 3, the upper contact angle limit decreased by only a few degrees. The results shown in Figure 3 are in very good agreement with those of Miyauchi et al.8 The advancing contact angles of water on silica wafers coated with titania particles are shown in Figure 4. Reversible wettability changes are also induced in this case, though the magnitude of the effect is smaller. Since the surface is rough (r > 1) and the contact angle on the flat wafers is acute (θ0 < 90°), according to Wenzel
(3)
Thus θ ) arccos[1.39 cos(50°)] ≈ 27°, and therefore we attribute the decreased magnitude of the contact angle change mainly to the effect of surface roughness. A flat, clean silica plate was treated in parallel with the titania surfaces, and the result is included in Figure 4. Obviously the wettability changes are specific for the titania surface, and therefore contamination can be excluded as an immediate cause for these changes (the silica plate remains hydrophilic, that is, clean, during the cyclic treatment). The robustness of the effect was further explored by preparing patterned titania surfaces. The wafers were initially irradiated with visible (UV) light to make them hydrophobic (hydrophilic). Subsequently they were irradiated with UV (visible) light through a photomask such that only the area outside a regular array of chromium circles (10 µm) is exposed. The result was an array of hydrophobic (hydrophilic) circles surrounded by a hydrophilic (hydrophobic) matrix. The quality of the patterning and the difference between the two instances are clearly seen under a light microscope after blowing humidified nitrogen over the patterned wafers (Figure 5). The macroscopically observed contact angle on a smooth but heterogeneous surface is given by the Cassie equation:10
cos θ ) φ cos 0° + (1 - φ) cos θvis
(4)
where φ is the area fraction of the hydrophilic (UVirradiated) surface and θvis is the contact angle on the hydrophobic (irradiated with visible light) surface. Equation 4 indeed provides a fair description of the limited number of cases studied so far (Figure 6). There is little doubt that the surface hydroxyl groups are of paramount importance for the overall wettability of the TiO2 surface. Most probably, the irradiation-induced wettability conversion is related to the interchange of tilanol and tiloxane groups:
Wettability of Photoresponsive TiO2 Surfaces
Figure 5. Microphotographs showing water condensation on patterned titania wafers: (a) hydrophobic circles in a hydrophilic matrix and (b) hydrophilic circles in a hydrophobic matrix. The diameter of the circles is 10 µm, and their area fraction is 40%.
Figure 6. Cassie plot of the advancing contact angle on hydrophobic titania wafers patterned with hydrophilic circles (diameter, 10 µm) of area fraction φ (see eq 4).
This effect is reminiscent of the heat-induced dehydroxylation of silica surfaces.21 There is some Fourier (21) Vansant, E. F.; van der Voort, P.; Vrancken, K. C. Characterization and Chemical Modification of the Silica Surface; Elsevier: Amsterdam, 1995.
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transform infrared3,8 and X-ray photoelectron spectroscopy2,4,6,11 evidence that supports this hypothesis, but results are not entirely conclusive. Wang et al.1,3 have reported the presence of rectangular hydrophilic domains (30-80 nm) aligned on the crystal surface along a specific direction. However, the fact that we observe essentially the same contact angle changes on amorphous (by virtue of preparation20) titania particles indicates that the effect is probably more complex and perhaps more general. Recent studies have shown that the relation between photocatalytic activity and light-induced wettability changes is complex.22,23 Even more confusion was added by a recent paper24 in which the hydrophobicity (∼80°) of an anatase film was quickly recovered by simply rubbing the sample with paper. The elucidation of the exact mechanism will require further efforts. All of our experiments were carried out with water at pH ) 6, which is very close to the isoelectric point of titanium dioxide.20 However, away from this value the contact angle is pH dependent,25,26 and this neglected influence could account for at least some of the discrepancies between different measurements. Conclusion We have conclusively shown that titania surfaces can be alternatively made hydrophilic or hydrophobic by irradiation with UV or visible light. The conversion is reversible, and the magnitude of the effect is rather robust. For smooth titania wafers, we observed a change in the contact angle of water of about 50°. Similar cyclic conversion can be induced for closely packed micron-sized titania spheres. The effect is, however, reduced, due mainly to the roughness of the surface. By selective irradiation (using a photomask), arrays of hydrophobic or hydrophilic circles embedded in a hydrophilic or hydrophobic matrix were produced. The hydrophilic-to-hydrophobic conversion can be used to prepare a smooth heterogeneous surface and eventually change the pattern in a reversible fashion. Acknowledgment. Financial support for this project from the Australian Research Council Special Research Centre Scheme is gratefully acknowledged. LA020660C (22) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1. (23) Miyauchi, M.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Chem. Mater. 2002, 14, 2812. (24) Kamei, M.; Mitsuhashi, T. Surf. Sci. 2000, 463, L609. (25) Pinzari, F.; Ascarelli, P.; Cappelli, E.; Giorgi, R.; Turtu, S. Appl. Surf. Sci. 2000, 156, 1. (26) Fokkink, L. G. J.; Ralston, J. Colloids Surf. 1989, 36, 69.
