Formation of Superhydrophobic Surfaces by Biomimetic Silicification

Sung Min Kang , Bang Sook Lee , Sang-gi Lee , Insung S. Choi. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2008 313-314, 150-153 ...
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Langmuir 2006, 22, 11208-11213

Formation of Superhydrophobic Surfaces by Biomimetic Silicification and Fluorination Woo Kyung Cho, Sung Min Kang, Dong Jin Kim, Sung Ho Yang, and Insung S. Choi* Department of Chemistry and School of Molecular Science (BK21), Center for Molecular Design and Synthesis, KAIST, Daejeon 305-701, Korea ReceiVed July 26, 2006. In Final Form: September 25, 2006 The amazing water repellency of many biological surfaces, exemplified by lotus leaves, has recently received a great deal of interest. These surfaces, called superhydrophobic surfaces, exhibit water contact angles larger than 150° and a low contact angle hysteresis because of both their low surface energy and heterogeneously rough structures. In this paper, we suggest a biomimetic method, “biosilicification”, for generating heterogeneously rough structures and fabricating superhydrophobic surfaces. The superhydrophobic surface was prepared by a combination of the formation of heterogeneously rough, nanosphere-like silica structures through biosilicification and the formation of self-assembled monolayers of fluorosilane on the surface. The resulting surface exhibited the water contact angle of 160.1° and the very low water contact angle hysteresis of only 2.3°, which are definite characteristics of superhydrophobic surfaces. The superhydrophobic property of our system probably resulted from the air trapped in the rough surface. The wetting behavior on the surface was in the heterogeneous regime, which was totally supported by Cassie-Baxter equation.

Introduction Nature has established a very effective way to deal with contaminating particles. The leaves of certain plants, such as lotus, indian cress, and lady’s mantle, exhibit the amazing waterrepellent property, and collectively this property is called “lotus effect”.1 The unusual wetting property of these leaves was found to be directly related with superhydrophobicity of surfaces. Not only do the surfaces show extremely low affinity against water, but they also have the ability to make water droplets roll off completely. As a result of the superhydrophobic property, dirt particles are washed off from the leaves effectively. Since the origin of the lotus effect was suggested, superhydrophobic surfaces have attracted a great deal of attention in both fundamental research and practical applications, such as selfcleaning coatings for windows or painted surfaces, microfluidics, house commodities, and so forth.2 The studies on lotus leaves, the most famous superhydrophobic surface found in nature, reveal that the surface of lotus leaves is composed of heterogeneously rough hills and valleys and is coated with hydrophobic waxy materials.1 The hills and valleys ensure that the surface contact area available to water is low, while the hydrophobic waxy materials prevent water from penetrating into the valleys. The net result is that water cannot wet the surface of lotus leaves, and spherically formed water droplets on the surface roll off. Therefore, the fundamental mechanism of the lotus effect suggests that both the chemical modification (leading to the low surface energy) and the surface structure (surface roughness) are important factors in determining hydrophobicity of surfaces. The chemical modification of flat surfaces with fluoropolymeric coatings or silane layers typically leads to water contact angle * Author to whom correspondence should be addressed. E-mail: [email protected]. (1) (a) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (b) Otten, A.; Herminghaus, S. Langmuir 2004, 20, 2405. (2) (a) Blossey, R. Nat. Mater. 2003, 2, 301. (a) Yabu, H.; Shimomura, M. Chem. Mater. 2005, 17, 5231.

of up to 120° at its maximum,3 which implies that the surface roughness could be a crucial factor in increasing the water contact angle over 120°. In other words, superhydrophobic surfaces are realized when both the low surface energy and the high degree of surface roughness are sufficiently satisfied. In this context, artificial superhydrophobic surfaces have been prepared mainly by either coating a rough surface with low surface-energy molecules or fabricating rough structures of hydrophobic materials.4-17 For example, the electrochemical deposition and layer-by-layer (LBL) methods were combined with hydrophobic coating to generate superhydrophobic gold4a and silver4b surfaces. The superhydrophobic porous microsphere/nanofiber composites5a (3) Nishino, T.; Meguro, M.; Nakamae, K.; Matsushita, M. Ueda, Y. Langmuir 1999, 15, 4321. (4) (a) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z.; Jiang, L.; Li, X. J. Am. Chem. Soc. 2004, 126, 3064. (b) Zhao, N.; Shi, F.; Wang, Z.; Zhang, X. Langmuir 2005, 21, 4713. (c) Jiang, Y.; Wang, Z.; Yu, X.; Shi, F.; Xu, H.; Zhang, X.; Smet, M.; Dehaen, W. Langmuir 2005, 21, 1986. (5) (a) Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem., Int. Ed. 2004, 43, 4338. (b) Feng, L.; Li, S.; Li, H.; Zhai, J.; Song, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2002, 41, 1221. (6) (a) Zhang, J.; Huang, W.; Han, Y. Langmuir 2006, 22, 2946. (b) Zhu, L.; Xiu, Y.; Xu, J.; Tamirisa, P. A.; Hess, D. W.; Wong, C.-P. Langmuir 2005, 21, 11208. (c) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Chabrol, G.; Perry, C. C. AdV. Mater. 2004, 16, 1929. (7) Sun, M.; Luo, C.; Xu, L.; Ji, H.; Ouyang, Q.; Yu, D.; Chen, Y. Langmuir 2005, 21, 8978. (8) Shiu, J.-Y.; Kuo, C.-W.; Chen, P.; Mou, C.-Y. Chem. Mater. 2004, 16, 561. (9) Vogelaar, L.; Lammertink, R. G. H.; Wessling, M. Langmuir 2006, 22, 3125. (10) Qian, B.; Shen, Z. Langmuir 2005, 21, 9007. (11) (a) Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L. Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 357. (b) Jiang, W.; Wang, G.; He, Y.; Wang, X.; An, Y.; Song, Y.; Jiang, L. Chem. Commun. 2005, 3550. (c) Fu, Q.; Rao, R.; Basame, S. B.; Keller, D. J.; Artyushkova, K.; Fulghum, J. E.; Lo´pez, G. P. J. Am. Chem. Soc. 2004, 126, 8904. (12) He, B.; Patankar, N. A.; Lee, J. Langmuir 2003, 19, 4999. (13) Sun, T.; Feng, L.; Gao, X.; Jiang, L. Acc. Chem. Res. 2005, 38, 644. (14) Han, J. T.; Xu, X.; Cho, K. Langmuir 2005, 21, 6662. (15) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349. (16) Takeshita, N.; Paradis, L. A.; O ¨ ner, D.; McCarthy, T. J.; Chen, W. Langmuir 2004, 20, 8131. (17) (a) Zhang, J.; Li, J.; Han, Y. Macromol. Rapid Commun. 2004, 25, 1105. (b) Lu, X.; Zhang, C.; Han, Y. Macromol. Rapid Commun. 2004, 25, 1606.

10.1021/la062191a CCC: $33.50 © 2006 American Chemical Society Published on Web 11/10/2006

Formation of Superhydrophobic Surfaces

and nanofibrous polymeric structures5b were also fabricated by using hydrophobic materials. In this paper, we introduce a biomimetic approach, “biosilicification”, to the generation of heterogeneously rough surfaces. Biosilicification, found in diatoms18a and glass sponges,18b occurs under ambient conditions at slightly acidic pH values, and the naturally occurring silica structures are precisely controlled at the nanometer scale. Polyamines are known to affect silica formation by catalyzing siloxane-bond formation and by acting as flocculating agents,19 and silica nanospheres have been produced biomimetically by using various polyamines,20 such as poly-L-lysine,21 poly(allylamine hydrochloride),22 amine-terminated dendrimers,23 and others.24 Recently, our group reported that silica thin films could be formed biomimetically at surfaces by using a tertiary amine-containing polymer, poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA), as a synthetic counterpart to naturally occurring, biosilica-forming peptides.25a,b In this study, we used a quaternized PDMAEMA to generate heterogeneously rough silica films, and we fabricated a superhydrophobic surface by coating the heterogeneously rough surface with fluorosilane. Experimental Section Materials. Copper(I) bromide (CuBr, 99.999%, Aldrich), 2,2′dipyridyl (g99%, Aldrich), 2-(dimethylamino)ethyl methacrylate (DMAEMA, 98%, Aldrich), aluminum oxide (Al2O3, activated, basic, Brockmann I, standard grade, ∼150 mesh, 58 Å, Sigma-Aldrich), bromoethane (C2H5Br, 99+%, Sigma-Aldrich), nitromethane (CH3NO2, 96%, Sigma-Aldrich), sodium fluoride (NaF, A.C.S. reagent, g99%, Aldrich), hydrochloric acid (HCl, 35%, Junsei), tetramethyl orthosilicate (TMOS, 99+%, Aldrich), absolute ethanol (99.8%, Merck), absolute methanol (99.9%, Merck), acetone (HPLC grade, Merck), dichloromethane (HPLC grade, Merck), and (tridecafluoro1,1,2,2-tetrahydrooctyl)trichlorosilane (Gelest, Inc.) were used as received. Ultrapure water (18.3 MΩ/cm) from the Human Ultrapure System (Human Corp., Korea) was used. The polymerization initiator, (BrC(CH3)2COO(CH2)11S)2, was synthesized by following the reported procedure.26 Formation of Poly((2-dimethylamino)ethyl methacrylate) (PDMAEMA) Films. The self-assembled monolayers (SAMs) presenting the polymerization initiator were formed by immersing a freshly prepared, gold-coated (with a titanium adhesion layer of 5 nm and thermally evaporated gold layer of 100 nm) silicon wafer in a 1 mM ethanolic solution of (BrC(CH3)2COO(CH2)11S)2 overnight at room temperature. After the formation of SAMs, the gold substrate was rinsed with ethanol several times and then was dried under a stream of argon. The formation of SAMs was confirmed by polarized (18) (a) Round, F. E.; Crawford, R. M.; Mann, D. G. The Diatoms: Biology & Morphology of the Genera; Cambridge University Press: Cambridge, U.K., 1990. (b) Sundar, V. C.; Yablon, A. D.; Grazul, J. L.; Ilan, M.; Aizenberg, J. Nature (London) 2003, 424, 899. (19) (a) Mizutani, T.; Nagase, H.; Fujiwara, N.; Ogoshi, H. Bull. Chem. Soc. Jpn. 1998, 71, 2017. (b) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (20) (a) Sumper, M. Angew. Chem., Int. Ed. 2004, 43, 2251. (b) Sumper, M.; Lorenz, S.; Brunner, E. Angew. Chem., Int. Ed. 2004, 42, 5192. (c) Kro¨ger, N.; Deutzmann, R.; Bergsdorf, C.; Sumper, M. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 14133. (d) Kro¨ger, N.; Lorenz, S.; Brunner, E.; Sumper, M. Science 2002, 298, 584. (e) Kro¨ger, N.; Deutzmann, R.; Sumper, M. Science 1999, 286, 1129. (21) Patwardhan, S. V.; Mukherjee, N.; Clarson, S. J. Inorg. Organomet. Polym. 2001, 11, 193. (22) (a) Brunner, E.; Lutz, K.; Sumper, M. Phys. Chem. Chem. Phys. 2004, 6, 854. (b) Patwardhan, S. V.; Clarson, S. J. Mater. Sci. Eng. 2003, 23, 495. (23) Knecht, M. R.; Wright, D. W. Langmuir 2004, 20, 4728. (24) Patwardhan, S. V.; Clarson, S. J. Silicon Chem. 2002, 1, 207. (25) (a) Kim, D. J.; Lee, K.-B.; Chi, Y. S.; Kim, W.-J.; Paik, H.-j.; Choi, I. S. Langmuir 2004, 20, 7904. (b) Kim, D. J.; Lee, K.-B.; Lee, T. G.; Shon, H. K.; Kim, W.-J.; Paik, H-j.; Choi, I. S. Small 2005, 1, 992. (c) Lee, S. B.; Koepsel, R. R.; Morley, S. W.; Matyjaszewski, K.; Sun, Y.; Russell, A. J. Biomacromolecules 2004, 5, 877. (26) Shah, R. R.; Merreceyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Hedrick, J. L. Macromolecules 2000, 33, 597.

Langmuir, Vol. 22, No. 26, 2006 11209 infrared external reflectance spectroscopy: 1737 (CdO), 1465 (-CH2-), and 1169 cm-1 (C-O). Before surface-initiated, atom transfer radical polymerization (SIATRP), the monomer, DMAEMA was purified by column chromatography on basic aluminum oxide. The initiator-coated gold substrate was placed in a Schlenk flask, and then the Schlenk flask was degassed under vacuum and was purged with argon. CuBr (0.0143 g, 0.1 mmol) and 2,2′-dipyridyl (0.0312 g, 0.2 mmol) were added to another flask, and the mixture was degassed under vacuum and was purged with argon. To the Schlenk flask containing the catalysts, degassed water (10 mL) and the purified DMAEMA (1.6869 mL, 10 mmol) were added. The resulting solution was transferred to the Schlenk flask containing the initiator-coated gold substrate by using a syringe. The mixture was stirred for 4 h at room temperature, and the PDMAEMA-coated gold substrate was taken, rinsed with water and methanol, and dried under a stream of argon. Quaternization of PDMAEMA Films and Anion Exchange. The PDMAEMA-coated gold substrate was placed in a flask, and then nitromethane (5 mL) and bromoethane (5 mL) were added to the flask. Quaternization was carried out for 24 h at room temperature, and the gold substrate was taken, rinsed with dichloromethane, acetone, and ethanol, and finally dried under a stream of argon. For the anion exchange, the gold substrate that had the quaternized PDMAEMA film, q-PDMAEMA, was placed in a conical tube, and 5-mL aqueous solution of sodium fluoride (50 mM) was added. The conical tube was then shaken for 24 h at room temperature. The gold substrate was taken, rinsed with water several times, and dried under a stream of argon. The anion exchange yielded a gold substrate coated with fluoride-exchanged q-PDMAEMA, q[F-]-PDMAEMA. Biomimetic Silicification. Monosilicic acid was independently formed by shaking a HCl (0.1 mM) solution of tetramethyl orthosilicate (TMOS, 100 mM) for 30 min at room temperature. The resulting solution (5 mL) was added to a conical tube containing the q[F-]-PDMAEMA-coated gold substrate. After 1 h, the substrate was taken, rinsed with water several times, and dried under a stream of argon. Silanization. Prior to silanization, the rough silica thin film on the gold substrate was oxidized by an oxygen plasma cleaner (Harrick PDC-002, medium setting) for 1 min to maximize -OH groups. The oxidized substrate was placed in a desiccator under vacuum for 1 h with a vial containing a few drops of (tridecafluoro-1,1,2,2tetrahydrooctyl)trichlorosilane. Characterizations. The thickness of monolayer and polymeric films was measured with a Gaertner L116s ellipsometer (Gaertner Scientific Corporation, IL) equipped with a He-Ne laser (632.8 nm) at a 70° angle of incidence. A refractive index of 1.46 was used for all the films. The X-ray photoelectron spectroscopy (XPS) study was performed with a VG-Scientific ESCALAB 250 spectrometer (United Kingdom) with a monochromatized Al KR X-ray source (1486.6 eV). Emitted photoelectrons were detected by a multichannel detector at a takeoff angle of 90° relative to the surface. During the measurements, the base pressure was 10-9-10-10 Torr. Survey spectra were obtained at a resolution of 1 eV from one scan and high-resolution spectra were acquired at a resolution of 0.05 eV from five scans. Atomic force microscopy (AFM) imaging was performed in a tapping mode on a Nanoscope IIIa multimode scanning probe microscope (Veeco, United States) with a tapping mode etched silicon probe (TESP). Field-emission scanning electron microscopy (FE-SEM) micrographs were obtained with Philips XL30s and Hitachi S-4800 that were equipped with a thermally assisted field emission gun. Contact angle measurements were performed using a DSA-10 goniometer (Kru¨ss, Germany). Dynamic advancing (θadv) and receding (θrec) water contact angles were determined by tilting experiment.27 Contact angles were measured at five different locations on each sample, and average values are reported in this paper. (27) (a) McHale, G.; Shirtcliffe, N. J.; Newton, M. I. Langmuir 2004, 20, 10146. (b) Ulman, A. An Introduction to Ultrathin Organic films; Academic Press: Boston, MA, 1991.

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Figure 1. The procedure for the formation of heterogeneously rough silica films.

Results and Discussion Formation of Quaternized PDMAEMA Films. The biosilicification found in diatoms18a was proposed to be achieved by specific interactions between peptides called silaffins and silicic acid derivatives. Silaffins contain long-chain polyamines that are usually methylated tertiary amines. Previously, we chose a tertiary amine-containing polymer, poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA), as a synthetic counterpart to silaffins and biomimetically generated silica thin films and their micropatterns at surfaces.25a,b The proposed mechanisms on biosilicification suggest that protonated (or positively charged) polyamines form complexes with negatively charged oligomeric silicic acid derivatives, and the complexation facilitates the polycondensation of the silicic acid derivatives.20b Therefore, we reasoned that positively charged, quaternary amine-containing polymers also can be used for in-vitro silica film formation. Quaternary amine-containing polymers are simply generated by the quaternization of PDMAEMA, and after the quaternization, the counteranions could be changed with other anions. In this paper, we report that thin films of quaternized PDMAEMA also induced the formation of silica thin films, and heterogeneously rough silica films were generated with fluoride as a counteranion. The procedure of the formation of heterogeneously rough silica thin films on gold substrates is depicted in Figure 1. The PDMAEMA film was generated by a combination of the formation of SAMs terminating in a polymerization initiator, 2-bromo-2-methyl propionyl group, and surface-initiated, atom

Figure 2. Wide-scan XPS spectra: (a) PDMAEMA film, (b) q-PDMAEMA film, (c) q[F-]-PDMAEMA film, and (d) silica film. High-resolution XPS spectra: (e) Br 3d and (f) F 1s regions acquired from (i) q-PDMAEMA film and (ii) q[F-]-PDMAEMA film.

transfer radical polymerization (SI-ATRP) of 2-(dimethylamino)ethyl methacrylate (DMAEMA).25a,b After 4-h SI-ATRP, a 144nm-thick uniform polymeric film was obtained. The formation of the PDMAEMA film on the gold substrate was confirmed by X-ray photoelectron spectroscopy (XPS): the XPS spectrum showed peaks at 284.6 (C 1s), 398.6 (N 1s), and 530.8 eV (O 1s) (Figure 2a). PDMAEMA contains quaternizable dimethylamino groups and the quaternization has been used for the generation of antibacterial surfaces.25c We also quaternized PDMAEMA with bromoethane, because our initial attempt to quaternize the dimethylamino group with methyl iodide deteriorated the integrity

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Figure 3. The water contact angles of (a) PDMAEMA film and (b) q-PDMAEMA film.

of the SAMs of alkanethiolates. The quaternization greatly changed chemical and physical properties of the polymeric films. First, we observed the increase of the thickness after quaternization. The thickness of the quaternized PDMAEMA (qPDMAEMA) film was measured to be about 200 nm (56-nm increase) by ellipsometry. It was previously reported that the quaternization of poly(vinylpridine) by n-butyl bromide induced the increase of the thickness of polymeric films grafted onto silicon surfaces, and the increase of the thickness was suggested to be caused by the augmentation of molecular weight of repeating units because of linkage of the n-butyl chains to the polymer and the incorporation of bromide as a counteranion into the polymeric layer.28 Therefore, the increase of the thickness observed in our system indirectly indicates successful quaternization of PDMAEMA films. Second, the quaternization resulted in the change of water contact angles. The water contact angle of the PDMAEMA film was 53°, but that of the q-PDMAEMA film was reduced to 38° (Figure 3). The water contact angle decreased because the quaternization created a charged surface that was more hydrophilic than the PDMAEMA film. The XPS spectrum also confirmed the successful quaternization of PDMAEMA films. In addition to the peaks of C 1s, N 1s, and O 1s observed in the XPS spectrum of the PDMAEMA film, new peaks at 253.3 (Br 3s), 187.3 (Br 3p1/2), 180.4 (Br 3p3/2), 67.7 (Br 3d3/2), 66.7 (Br 3d5/2), and 2.3 eV (Br 4d) appeared after quaternization (Figure 2b). In principle, there are two sources of Br peaks. One is the terminal bromide group of PDMAEMA and q-PDMAEMA, and the other is the bromide anion as a counteranion of the quaternary ammonium group. While we did not observe any Br peaks in the XPS spectrum of the PDMAEMA film probably because of the relatively low density of Br, we clearly observed Br peaks after quaternization. Therefore, we concluded that the Br peaks came from the bromide anion and the dimethylamino group was successfully quaternized by bromoethane. Anion Exchange. The counteranion (Br-) in the q-PDMAEMA film could be exchanged directly at the surface with other anions to tune the physicochemical properties of the surface. As a related work, we have previously reported that the counteranions of dialkylimidazolium salts anchored onto gold29a and Si/SiO2 surfaces29b and carbon nanotubes29c were exchanged with various anions to tailor their water wettability/miscibility. In this work, we exchanged the bromide anion in the qPDMAEMA film with fluoride anion (F-) because it is known that the fluoride anion facilitates the silica formation.30 After anion exchange with fluoride, the XPS peaks from bromide disappeared and a new peak was observed at 683.6 eV (F 1s), although the F peak was weak in the wide-scan XPS spectrum (Figure 2c). The high-resolution XPS spectra of Br 3d (28) Biesalski, M.; Ruhe, J. Macromolecules 1999, 32, 2309. (29) (a) Lee, B. S.; Chi, Y. S.; Lee, J. K.; Choi, I. S.; Song, C. E.; Namgoong, S. K.; Lee, S.-g. J. Am. Chem. Soc. 2004, 126, 480. (b) Chi, Y. S.; Lee, J. K.; Lee, S.-g.; Choi, I. S. Langmuir 2004, 20, 3024. (c) Park, M. J.; Lee, J. K.; Lee, B. S.; Lee, Y.-W.; Choi, I. S.; Lee, S.-g. Chem. Mater. 2006, 18, 1546.

Figure 4. The cross-sectional FE-SEM micrographs of (a) qPDMAEMA film and (b) q[F-]-PDMAEMA film. The scale bar is 200 nm. The AFM images of (c) q-PDMAEMA film and (d) q[F-]PDMAEMA film.

and F 1s regions clearly showed that the Br 3d peaks disappeared and a new F 1s peak certainly appeared after anion exchange with fluoride (Figures 2e and f). The thickness of the polymeric film decreased after anion exchange: while the thickness of the q-PDMAEMA film was about 200 nm, the thickness of the fluoride-exchanged q-PDMAEMA (q[F-]-PDMAEMA) film was 138 nm. The decrease of the thickness was also verified by crosssectional field emission scanning electron microscopy (FE-SEM) (Figures 4a and b). The height of the q-PDMAEMA film was 200 nm, and the height decreased to 132 nm after anion exchange with fluoride. The surface morphology was also altered by anion exchange. The atomic force microscopy (AFM) image of the q-PDMAEMA film showed a relatively uniform polymeric film with small protrusions (Figure 4c). The root-mean-square (rms) roughness was 1.057 nm. In contrast, the q[F-]-PDMAEMA film did not contain any small protrusions, and the surface became rougher as a whole (Figure 4d). The rms value was 3.816 nm. We believe that the disappearance of small protrusions was related with the decrease of the thickness of the q[F-]-PDMAEMA film.31 Biomimetic Silicification. For the silicification, monosilicic acid was independently prepared by shaking a HCl solution of TMOS for 30 min at room temperature. The substrate coated with the q[F-]-PDMAEMA film was immersed in the prepared monosilicic acid solution. The silicification was performed for 1 h at room temperature. After silicification, new peaks at 153.5 (Si 2s) and 102.3 eV (Si 2p) were observed in the XPS spectrum (Figure 2d). The morphology of the silica surface was characterized by FE-SEM. As shown in Figure 5a, the silica film was composed of heterogeneously rough, nanosphere-like structures. (30) (a) Reale, E.; Leyva, A.; Corma, A.; Martı´nez, C.; Garcı´a, H.; Rey, F. J. Mater. Chem. 2005, 15, 1742. (b) Lesaint, C.; Lebeau, B.; Marichal, C.; Patarin, J.; Zana, R. Langmuir 2005, 21, 8923. (c) Okabe, A.; Fukushima, T.; Ariga, K.; Niki, M.; Aida, T. J. Am. Chem. Soc. 2004, 126, 9013. (d) Holmes, R. R. Chem. ReV. 1990, 90, 17. (e) Schmidt-Winkel, P.; Yang, P.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. AdV. Mater. 1999, 11, 303. (f) Kim, W. J.; Yoo, J. C.; Hayhurst, D. T. Microporous Mesoporous Mater. 2002, 49, 125. (31) It was reported that anions played a crucial role in the biomimetic formation of silica nanoparticles in solution.20b We are currently investigating the effect of anions on the thicknesses and morphologies of the q-PDMAEMA and silica films. The results will be published separately.

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Figure 5. Heterogeneously rough silica films: (a) FE-SEM and (b) cross-sectional FE-SEM.

Figure 6. Heterogeneously rough silica films: (a) AFM images and (b) static water contact angle of the fluorinated rough silica surface.

The thickness of the silica film was measured by the crosssectional FE-SEM micrograph because it was not feasible to measure the thickness by ellipsometry (Figure 5b). The thickness of the silica film was 330 nm, which was about 200-nm increase compared with the thickness of the q[F-]-PDMAEMA film. The AFM image coincided with the FE-SEM results (Figure 6a). The rms roughness greatly increased to 38.71 nm after silicification. Our previous work on the biomimetic formation of silica films was based on the use of the PDMAEMA film as a silica-forming synthetic polymer, which did not involve the quaternization and anion exchange.25a,b In that case, tertiary amines of PDMAEMA were probably protonated and phosphate anions would act as a counteranion, because the silica formation was performed in slightly acidic phosphate buffer solution (pH 5.5). The rms roughness of the formed silica film was 6.4 nm, and the surface was smoother than the structure generated in this study. Therefore, we concluded that the fluoride anion played a crucial role in the generation of the heterogeneously rough silica surface.31 Fluorination. The formed silica structures were composed of ∼50-nm silica nanostructures on top of larger structures, which are reminiscent of the structures of lotus leaves. The water contact angle was measured to be 15° because of the hydrophilic nature

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of silica. The generation of a superhydrophobic surface was accomplished by the formation of SAMs of fluorosilane ((tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane) on the rough silica nanostructures. Superhydrophobic surfaces should meet two criteria: very high water contact angles (>150°) and very low contact angle hysteresis (i.e., very low roll-off angle).32 After fluorination, the static water contact angle greatly increased to 160.1° from 15° (Figure 6b). Dynamic advancing (θadv) and receding (θrec) water contact angles were determined by the tilting experiment.27 When a surface on which a water droplet rests is tilted slightly, the droplet remains at rest with different contact angles at each side of the droplet. The difference in forces per unit length at the two sides of the droplet is proportional to γLV(cos θL - cos θU), where γLV is the liquid-vapor surface tension, and θL and θU are the contact angles at the lower and upper sides of the droplet, respectively. When the upper angle reaches the receding angle and the lower angle does the advancing angle, the droplet just begins to move. The difference of angles, ∆θH ) (θadv - θrec), is defined as the contact angle hysteresis. The water contact angle hysteresis of the fluorinated surface was calculated to be only 2.3° (the advancing water contact angle: 161.2°; the receding water contact angle: 158.9°). The rolling-off of a water droplet on the q[F-]PDMAEMA film with a tilting angle of 5° was photographed (Figure 7) and also captured as a media file. Photographs were taken at 0.2-s intervals. The spherical water droplet rolled off extremely fast on the fluorinated surface, definitely showing the characteristic of superhydrophobic surfaces (see a media file in the Supporting Information). The superhydrophobic silica surface that we fabricated was very stable under ambient conditions and the static water contact angle kept unchanged after 8 months. However, the surface was relatively weak under mechanical force: we immersed the fluorinated silica substrate into water and sonicated the mixture. For the first 5 min, we did not observe any change of the waterrepellent property of the silica surface. Peeling-off of the film started to be observed in a part of the surface after 10-min sonication. After 30-min sonication, the surface was damaged significantly and showed the decreased static water contact angle of 54°. Theoretical Considerations. It is generally known that the surface roughness plays an important role in determining the wetting behavior of solid surfaces. The water contact angle of solid surfaces can be elevated by increasing surface roughness within a special size range33 because the air trapped between the solid surface and the water droplet can minimize the contact area. The trapped air is suggested to be an important factor to hydrophobicity because the water contact angle of air is considered to be 180°. Cassie and Baxter proposed an equation to describe the relationship between the surface wettability and the surface roughness.34

cos θr ) f1 cos θs - f2 where θr and θs are the water contact angles of a rough surface and a smooth surface, respectively. f1 and f2 are the fractions of a solid surface and air in contact with a water droplet, respectively (32) (a) Nun, E.; Oles, M.; Schleich, B. Macromol. Symp. 2002, 187, 677. (b) Kijlstra, J.; Reihs, K.; Klamt, A. Colloids Surf., A 2002, 206, 521. (c) Extrand, C. W. Langmuir 2002, 18, 7991. (d) Marmur, A. Langmuir 2003, 19, 8343. (e) Marmur, A. Langmuir 2004, 20, 3517. (33) (a) Lenz, P. AdV. Mater. 1999, 11, 1531. (b) Abbott, N. L.; Folkers, J. P.; Whitesides, G. M. Science 1992, 257, 1380. (c) Bico, J.; Tordeux, C.; Que´re´, D. Europhys. Lett. 2001, 55, 214. (34) Cassie, A. B. D.; Baxter, S. Trans. Faraday. Soc. 1944, 40, 546.

Formation of Superhydrophobic Surfaces

Langmuir, Vol. 22, No. 26, 2006 11213

In our system, we modeled a flat silicon wafer as a smooth surface. A flat silicon surface was chemically coated by the formation of SAMs of the same silane used for coating the rough silica structure, (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane. The water contact angle of the fluorinated, flat silicon surface was 117°. From this value and the water contact angle of the fluorinated rough silica surface (160.1°), f2 was calculated to be 0.8906. Since f2 is the air fraction, the value of 0.8906 indicates that the superhydrophobic property of our system probably resulted from the air trapped in the rough surface, and the wetting behavior was heterogeneous.35 The heterogeneous wetting behavior of our system coincided with that of lotus leaves. Several reports on the simulation of lotus leaves showed that the lotus leaves tend to be located in the heterogeneous wetting regime described by the Cassie-Baxter equation.32d,e,34,36

Conclusions In summary, we utilized the biomimetic silicification to generate heterogeneously rough silica structures at surfaces and subsequently fabricated the superhydrophobic silica surface by fluorination. The resulting silica surface exhibited the water contact angle as high as 160.1° and very low water contact angle hysteresis (2.3°). We found that the selection of fluoride as a counteranion of q-PDMAEMA films was the important factor for the formation of the heterogeneously rough silica structures in our system. This report is the first example of the use of quaternary amine-containing polymers for biomimetic formation of silica films, and we believe that the system demonstrated would give a potential of controlling the structures of silica films at the nanometer scale. Acknowledgment. This work was supported by the National R&D Project for Nano Science and Technology. The ellipsometer was purchased by a research fund from the Center for Molecular Design and Synthesis and the atomic force microscope by the Basic Research Program of Korea Science and Engineering Foundation (R08-2003-000-10533-0).

Figure 7. Photographs of a water droplet on the q[F-]-PDMAEMA film with a tilting angle of 5°. The unit of the ruler is millimeter. The rolling-off of a water droplet was also captured as a media file, and the file was provided in the Supporting Information.

(f1 + f2 ) 1). From the equation, a very hydrophobic surface is realized if f2 (air fraction) is large enough.

Supporting Information Available: A media file of the rollingoff of a water droplet. This material is available free of charge via the Internet at http://pubs.acs.org. LA062191A

(35) The wetting behavior on rough surfaces may assume one of two regimes. One is the homogeneous wetting regime, where the water droplets completely penetrate into the rough grooves. The other is the heterogeneous wetting regime, in which air is trapped underneath the liquid inside the rough grooves.32e (36) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988.