Langmuir 2004, 20, 6065-6067
6065
Notes Water Ultrarepellency Induced by Nanocolumnar ZnO Surface Xin-Tong Zhang,†,‡ Osamu Sato,† and Akira Fujishima*,† Kanagawa Academy of Science and Technology, KSP Building West 614, 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa 213-0012, Japan, and Institute of Physical Chemistry, School of Chemistry, Jilin University, 119 Jiefang Road, Changchun 130023, P. R. China Received March 1, 2004. In Final Form: April 12, 2004
Water repellency of solid surfaces that is important for many biological processes and numerous industrial applications has been found to be governed by both the chemical properties and the microtexture of the surfaces.1-5 Numerous methods have reportedly created ultrahydrophobic surfaces with static water contact angles (CA) higher than 150° by either enlarging the roughness of hydrophobic surfaces6-9 or modifying rough surfaces with materials of low surface free energy.10-13 However, large dynamic water CAs, i.e., both advancing angle (θA) and receding angle (θR), as well as small CA hysteresis termed as the difference between advancing and receding CA, are more important for practical water-repellent applications and should be considered for the designing of real water ultrarepellent surfaces.14,15 Up to now, there have been very few studies on ultrahydrophobic surfaces that reported that both water advancing and receding angles exceeded 150°.16 Here we report that a special surface microtexture comprised of ZnO nanocolumns prepared by a facial cathodic electrodeposition shows both advancing and receding CAs of water higher than 150° after being modified with a monolayer of fluoroalkylsilane (FAS). Furthermore, the columnar surface showed such a small * Corresponding author: E-mail:
[email protected]. Fax: +81-44-819-2038. Telephone: +81-44-819-2020. † Kanagawa Academy of Science and Technology. ‡ Jilin University. (1) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (2) Neinhuis, C.; Barthlott, W. Ann. Bot. 1997, 79, 667. (3) Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132, 31. (4) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. Adv. Mater. 2002, 14, 1857. (5) Blossey, R. Nature Mater. 2003, 2, 301 and references therein. (6) Onda, T.; Shibuchi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125. (7) Coulson, S. R.; Woodward, I.; Badyal, J. P. S.; Brewer, S. A.; Wills, C. J. Phys. Chem. B 2000, 104, 8836. (8) Li, H.; Wang, X.; Song, Y.; Liu, Y.; Li, Q.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2001, 40, 1743. (9) Feng, L.; Li, S.; Li, H.; Zhai, J.; Song, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2002, 41, 1221. (10) Gu, Z.-Z.; Uetsuka, H.; Takahashi, K.; Nakajima, R.; Onishi, H.; Fujishima, A.; Sato, O. Angew. Chem. 2003, 115, 922. (11) Nakajima, A.; Hashimoto, K.; Watanabe, T.; Takai, K.; Yamauchi, G.; Fujishima, A. Langmuir 2000, 16, 7044. (12) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Adv. Mater. 1999, 11, 1365. (13) Tadanaga, K.; Katata, N.; Minami, T. J. Am. Ceram. Soc. 1997, 80, 1040. (14) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; O ¨ ner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395. (15) O ¨ ner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (16) Bico, J.; Marzolin, C.; Que´re´, D. Europhys. Lett. 1999, 47, 220.
water CA hysteresis (∼8°) that water droplets could not adhered on the surface even when placed horizontally. This study is interesting and significant since remarkable water ultrarepellency has been created from ZnO nanocolumns by a simple but reproducible chemical method. In addition, we will show here that this ultrahydrophobic surface could be converted into an interesting ultrahydrophobic-ultrahydrophilic micropattern by ultraviolet light lithography based on the photocatalytic activity of ZnO. ZnO columnar films were prepared by cathodic electrodeposition in a standard three-electrode reactor at 70 °C, using an aqueous solution of 1 mM ZnCl2 (Wako) and 0.1 M KCl (Wako), saturated with O2 by bubbling.17,18 The electrodeposition of ZnO films was carried out at -1 V vs SCE, on a conductive glass substrate (1.5 × 3 cm2) covered by fluoride-doped tin oxide film (Asahi), using Pt as counter-electrode. O2 molecules were reduced to hydroxyl anions at the surface of conductive glass substrate, and then ZnO nanocolumns grew on the substrate by a precipitation reaction after the process of heterogeneous nucleation. The growth mechanism of ZnO nanocolumns by electrodeposition was possibly similar to that of the solution deposition of ZnO microrod array reported elsewhere,19 except that in the latter case hydroxyl anions were produced by chemical reaction instead of electrochemical reduction of O2 molecules. At deposition current of 0.8 mA/cm2, ZnO nanocolumns grew to ca. 1 µm in length in 60 min. The ZnO film was then rinsed with deionized water (Millipore, 18 MΩ‚cm), dried with dry N2. Figure 1 shows the electron micrographs (SEM, JEOL JSM5400) of the ZnO columnar film in low and high magnification, respectively. Columns with an average diameter of 125 nm stand on the substrate in a disordered way with a density of about 24 columns/µm2. The nanocolumns were hexagonal phase of ZnO as measured by X-ray diffraction. The cross section of each nanocolumn could be clearly identified as hexagonal in electron micrograph at higher magnification, which indicates the good crystallinity of ZnO columns. We treated the ZnO films with the refluxing vapor of heptadecafluorodecyltrimethoxysilane (Toshiba silicone) in anhydrous toluene (Wako) for 2 h to render the surface ultrahydrophobic. The refluxing was carried out under the protection of high-purity N2. This vapor deposition method ensured the formation of FAS monolayer on ZnO films.20 CAs of water on the films were measured with a contact angle meter (Dropmaster 500, Kyowa Interface Science). Dynamic advancing and receding angles were recorded with a digital camera and analyzed by the commercial FAMAS software automatically while water was added to and withdrawn from the drop, respectively. The value reported were averages of five measurements made on different positions of the surface. Figure 2 shows (17) Peulon, S.; Licot, D. Adv. Mater. 1996, 8, 166. (18) Konenkamp, R.; Boedecker, K.; Lux-Steiner, M. C.; Poschenrieder, M.; Zenia, F.; Levy-Clement, C.; Wagner, S. Appl. Phys. Lett. 2000, 16, 2575. (19) Vayssieres, L.; Keis, K.; Hagfeldt, A.; Lindquist, S.-E. Chem. Mater. 2001, 13, 4395. (20) Haller, I. J. Am. Chem. Soc. 1978, 100, 8050.
10.1021/la049471f CCC: $27.50 © 2004 American Chemical Society Published on Web 06/10/2004
6066
Langmuir, Vol. 20, No. 14, 2004
Notes
Figure 1. Electron micrographs of ZnO nanocolumns on SnO2:F substrate prepared by cathodic electrodeposition. The bars in parts a and b are 5 and 1 µm, respectively. Part b was taken with a tilt angle of 30° of the substrate.
Figure 2. Optical photograph of a water drop on the ZnO nanocolumnar surface modified with a monolayer of fluoroalkylsilane. Part a was taken when the drop was swollen, while part b was taken when the drop was shrunk on the surface.
the photographs of water droplet on FAS-modified ZnO columnar surface at advanced and receded state, respectively. The advancing angle of water on the surface was measured as 167.0 ( 0.7°, while the receding angle was 159.0 ( 0.8°. The low standard deviations in both advancing and receding angle demonstrate that the surface is uniform in ultrahydrophobicity. The CA hysteresis of water on the surface is only 8°. Any effort to measure the static contact angle of water on such a surface was failed by the sessile method since the water drop could not be put onto the surface. However, the static CA should be close to the advancing angle of 167° according to the literature.16 Water dropped onto the horizontal FASmodified ZnO columnar surface rolled spontaneously due to the very small value of CA hysteresis. In control experiments, we modified the polished surface of ZnO single crystal (0001 face, MTI Corp.) with a monolayer of FAS. This flat ZnO surface only showed a water contact angle of θA/θR ) 112°/82°. Thus, the ultrahydrophobic behavior of columnar FAS-modified ZnO film must come from its special surface texture. There are two ways for a water droplet to interact with such a columnar surface. One is that water droplets intrude among the FAS-modified ZnO nanocolumns that could be described by Wenzel’s model21 (eq 1)
cos θ# ) r cos θ
(1)
Here the apparent water CA (θ#) is enlarged from the intrinsic CA (θ) by the roughness (r) of the columnar film, since the intrinsic CA of FAS-modified ZnO surface is larger than 90°. The other way is that water droplets could not intrude among the FAS-modified ZnO nanocolumns and have to stand on the composite surface made from (21) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988.
ZnO and air that could be described by Cassie’s model22 (eq 2)
cos θ# ) f1 cos θ - f2
(2)
Here the apparent CA (θ#) is determined by the surface fraction of ZnO (f1), intrinsic CA (θ), and surface fraction of air (f2) together. The higher f2 is in Cassie’s formulas, the larger apparent contact angle θ# will be. It was reported that the Wenzel’s type surface showed very different adhesive properties from the Cassie-type surface.23 The hysteresis of contact angle is always large for the Wenzel surface, while small hysteresis of contact angle could be found on the Cassie surface.23 Therefore, the present ultrahydrophobic ZnO film belongs to the Cassie type instead of the Wenzel type due to its extremely small contact angle hysteresis (∼8°) of water. Although the average diameter of ZnO columns is only 125 nm, yet their packing density (ca. 24-columns/µm2) is high enough to keep water droplets on top of the composite surface. This results in a discontinuous three-phase (air-liquidsurface) contact line of a water droplet in contact with this surface and consequently rather small contact angle hysteresis.14,15 Dynamic CAs of water on such a surface are enlarged to more than 150° due to the vast surface fraction of air (f2) on the composite surface. Water CA on the FAS-modified ZnO columnar film showed negligible change after storage over half year, but was found to be reduced from 167 to
