Superhydrophobic Bionic Surfaces with Hierarchical Microsphere

Nov 16, 2006 - Transferrable Superhydrophobic Surface Constructed by a Hexagonal CuI Powder without Modification by Low-Free-Energy Materials ..... Ji...
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Langmuir 2007, 23, 2169-2174

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Superhydrophobic Bionic Surfaces with Hierarchical Microsphere/ SWCNT Composite Arrays Yue Li,†,§,| Xing Jiu Huang,‡,| Sung Hwan Heo,† Cun Cheng Li,† Yang Kyu Choi,‡ Wei Ping Cai,§ and Sung Oh Cho*,† Department of Nuclear and Quantum Engineering, Korea AdVanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea, Department of Electrical Engineering and Computer Science, KAIST, Daejeon 305-701, Korea, and Key Lab of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, Anhui, P. R. China ReceiVed July 18, 2006. In Final Form: October 16, 2006 Superhydrophobic bionic surfaces with hierarchical micro/nano structures were synthesized by decorating singlewalled or multiwalled carbon nanotubes (CNTs) on monolayer polystyrene colloidal crystals using a wet chemical self-assembly technique and subsequent surface treatment with a low surface-energy material of fluoroalkylsilane. The bionic surfaces are based on the regularly ordered colloidal crystals, and thus the surfaces have a uniform superhydrophobic property on the whole surface. Moreover, the wettability of the bionic surface can be well controlled by changing the distribution density of CNTs or the size of polystyrene microspheres. The morphologies of the synthesized bionic surfaces bear much resemblance to natural lotus leaves, and the wettability exhibited remarkable superhydrophobicity with a water contact angle of about 165° and a sliding angle of 5°.

Introduction Lotus leaves have a strong superhydrophobicity with a water contact angle (CA) larger than 150° and a sliding angle (SA) less than 10°, resulting in the self-cleaning effect that removes contamination and dirt on their surfaces.1-3 This property can be widely used for preventing the adhesion of water and snow to windows or antennas, the reduction of drag friction, selfcleaning utensils, antioxidation coatings, and microfluidic devices.4,5 Bionic investigation revealed that the self-cleaning effect of a lotus leaf, the so-called lotus effect, is attributed to the hierarchically combined micro/nano structure of the surface and the low surface-energy material covered on the structure.1-3 Inspired by this, several bionic surfaces have been fabricated, including a rough polymer surface by argon plasma etching,6 nanopapilla-structured particles via a hydrothermal method,7 an aligned carbon nanotube (CNT) film or ZnO film by chemical vapor deposition,8 hierarchical structures by binary colloid assembly,9 a transparent boehmite/silica film by sublimation,10 a superhydrophobic rough surface by electrochemical deposition,11 polymer patterns by the polymerization on the etched silicon substrate,12 and stable bionic superhydrophobic surfaces by a solution-immersion process.13 * Corresponding author. E-mail: [email protected]. † Department of Nuclear and Quantum Engineering, KAIST. ‡ Department of Electrical Engineering and Computer Science, KAIST. § Institute of Solid State Physics, Chinese Academy of Science. | These two authors contributed equally to this work. (1) (a) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (b) Fu¨rstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Langmuir 2005, 21, 956. (2) (a) Sun, T.; Feng, L.; Feng, X.; Jiang, L. Acc. Chem. Res. 2005, 38, 644. (b) 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. (3) (a) Blossey, R. Nat. Mater. 2003, 2, 301. (b) Neinhuis, C.; Barthlott, W. Ann. Bot. (London) 1997, 79, 667. (4) Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132, 31. (5) (a) Takeshita, N.; Paradis, L. A.; Oner, D.; McCarthy, T. J.; Chen, W. Langmuir 2004, 20, 8131. (b) Nakajima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 7044. (c) Zhai, L.; Cebeci, F. C¸ .; Cohen, R. E.; Rubner, M. F. Nano. Lett. 2004, 4, 1349. (6) Chen, W.; Fadeev, A.; Hsieh, M.; O ¨ ner, D.; Youngblood, J.; McCarthy, T. Langmuir 1999, 15, 3395.

Recently, fabrication techniques of colloid crystals have been well developed because of the promising applications of colloidal crystals to optical gratings, optical filters, and antireflective surface coatings. Since two-dimensional monolayer colloidal crystals that consist of hexagonally close-packed microspheres provide surfaces with a regularly ordered and well-defined roughness, they may lead to an enhancement of the surface hydrophobicity. However, several studies indicate that such surface roughness is not sufficient to induce superhydrophobicity.14 Therefore, to achieve superhydrophobicity using monolayer colloidal crystals, a much rougher surface texture should be provided on the colloidal crystals. Here, we present an approach to create a superhydrophobic bionic surface based on monolayer colloidal crystals. The bionic surfaces were synthesized from hierarchically combined polystyrene (PS) microspheres and CNTs; the CNTs were decorated on monolayer PS colloidal crystals by a wet chemical selfassembly technique. Subsequently, the surfaces of the hierarchical composite arrays were chemically modified with a low surfaceenergy material, and, consequently, the surfaces took on a strong superhydrophobicity with a small SA. A few methods have been developed to deposit multiwalled CNTs (MWCNTs) on the surface of microspheres, including layer-by-layer assembly of (7) Wang, B. X.; Zhao, X. P. AdV. Funct. Mater. 2005, 15, 1815. (8) (a) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701. (b) Liu, H.; Feng, L.; Zhai, J.; Jiang, L.; Zhu, D. Langmuir 2004, 20, 5659. (9) (a) Zhang, G.; Wang, D. Y.; Gu Z.-Z.; Mo¨hwald, H. Langmuir 2005, 21, 9143. (b) Ming, W.; Wu, D.; van Benthem, R.; de With, G. Nano Lett. 2005, 5, 2298. (10) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. AdV. Mater. 1999, 11, 1365. (11) (a) Nicolas, M.; Guittard, F.; Ge´ribaldi, S. Langmuir 2006, 22, 3081. (b) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z.; Jiang, L.; Li, X. J. Am. Chem. Soc. 2004, 126, 3064. (c) Zhang, L.; Zhou, Z.; Cheng, B. DeSimone, J. M.; Samulski, E. T. Langmuir 2006, 22, 8576. (12) Xia, F.; Feng, L.; Wang, S.; Sun, T.; Song, W.; Jiang, W.; Jiang, L. AdV. Mater. 2006, 18, 432. (13) Wang, S. Feng, L.; Jiang, L. AdV. Mater. 2006, 18, 767. (14) Shiu, J.-Y.; Kuo, C.-W.; Chen, P.; Mou, C.-Y. Chem. Mater. 2004, 16, 561.

10.1021/la0620758 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/16/2006

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Figure 1. Schematic illustration of the fabrication process for a bionic surface with hierarchical microsphere/CNT structures.

CNTs on microspheres15a,b and chemical vapor deposition of CNTs on microparticles.15c However, to the best of our knowledge, wettability research on the hierarchical microsphere/CNT composite array films has never been carried out. The presented route exhibits well the concept of bionic fabrication because the morphology of the resultant products bears resemblance to that of natural lotus leaves, which consist of rugged, hierarchical micro/nano structures. The periodicity of the microspheres and the distribution density of CNTs can be easily changed, allowing the wettability of the surface to be controlled. Experimental Section Materials Preparation. Commercialized monodispersed PS spheres (Soken, Chemisnow, Japan) with different diameters were well-dispersed in deionized water to prepare the monolayer colloidal crystals. Both MWCNTs (average diameter: 10 nm; length: ∼10 µm) and single-walled CNTs (SWCNTs; average diameter: 1.4 nm; length: ∼20 µm) (Iljin Nanotech, Korea) were used in the experiments: SWCNTs allowed finer nanostructures than did MWCNTs. The CNTs were pretreated in a mixture of 3:1 concentrated sulfuric and nitric acids (98% and 70%, respectively) under ultrasonication for 8 h, which functionalizes the surfaces of CNTs with carboxylic acid (-COOH) for the following self-assembly of CNTs (Figure S1 in Supporting Information). Then, the CNTs with different concentration were redispersed into acetone by sonication. Synthesis of the Bionic Surface. The fabrication process of the bionic surfaces is illustrated in Figure 1. The monolayer PS colloidal crystals were uniformly prepared on well-cleaned glass substrates that were 2 × 2 cm in size by spin coating the PS colloidal suspension.16 The colloidal crystals were then heated at a temperature of 130 °C (above the glass transition temperature of 105 °C of the microspheres) for 40 min, which strongly increases the adherence of the PS crystals to the substrate.17 A gold layer of 30 nm thickness was then coated on the surface of the colloidal crystal by plasma sputtering deposition by controlling the deposition current and time. The gold layer on the substrate was cleaned using 0.05 M H2SO4 solution and double deionized water (DDW). The mercaptoethylamine solution was prepared by dissolving it in ethanol, and the concentration was adjusted to be 0.1 M. The sample was dipped into mercaptoethylamine solution for 3 h, followed by exhaustive washing using ethanol and DDW, and then dried in a stream of high-purity N2. By such a method, an amino group (-NH2) was introduced onto (15) (a) Correa-Duarte, M. A.; Kosiorek, A.; Kandulski, W., Giersig, M.; Liz-Marza´n, L. M. Chem. Mater. 2005, 17, 3268. (b) Correa-Duarte, M. A.; Kosiorek, A.; Kandulski, W.; Giersig, M.; Salgueirin˜o-Maceira, V. Small 2006, 2, 220. (c) Agrawal, S.; Kumar, A.; Frederich, M. J.; Ramanath, G. Small 2005, 1, 823. (16) (a) Huang, X.; Li, Y.; Im, H.; Yarimaga, O.; Kim, J.; Jang, D.; Cho, S.; Cai, W.; Choi, Y. Nanotechnology 2006, 17, 2988. (b) Li, Y.; Cai, W. P.; Cao, B. Q.; Duan, G. T.; Li, C. C.; Sun, F. Q.; Zeng, H. B. J. Mater. Chem. 2006, 16, 609. (c) Sun, F. Q.; Cai, W. P.; Li, Y.; Cao, B. Q.; Lei, Y.; Zhang, L. D. AdV. Funct. Mater. 2004, 14, 283, (d) Li, Y.; Cai, W. P.; Cao, B. Q.; Duan, G. T.; Sun, F. Q.; Li, C. C.; Jia, L. C. Nanotechnology 2006, 17, 238. (e) Li, Y.; Cai, W. P.; Duan, G. T.; Cao, B. Q.; Sun, F. Q.; Lu, F. J. Colloid Interface Sci. 2005, 287, 634. (17) (a) Mazur, S.; Beckerbauer, R.; Buckholz, J. Langmuir 1997, 13 (3), 4287. (b) Gates, B.; Park, S. H.; Xia, Y. N. AdV. Mater. 2000, 12, 653.

Figure 2. (a,b) FESEM images with different magnifications of the bionic surface with microsphere/SWCNTs composition arrays. (c) Feature picture of the bionic surface obtained with a tilting angle of 40°. (d) Surface microstructure of natural a lotus leaf. the gold surface by the bond of -Au-S- (Figure S2 in Supporting Information). When the prepared CNT solution was dropped on the surface of the colloidal crystal, the CNTs terminated with a carboxylic group (-COOH) could be self-assembled on the PS spheres due to the condensation reaction between the -COOH and -NH2 as well as the electrostatic attraction and van der Waals interactions between the CNTs.15a,18 The CNTs were then decorated on the surfaces of PS microspheres with almost uniform distribution density. As a result, hierarchical micro/nano bionic surfaces comprising PS microspheres and CNTs were created. Surface Treatment. To reduce the surface energy on the bionic structures, the surfaces of the as-prepared samples were chemically modified with fluoroalkylsilane. The samples were immersed in an ethanol solution of 20 mM 1H,1H,2H,2H-perfluorodecyltrichlorosilane (CF3(CF2)7(CH2)2SiCl3, Alfa Aesar, USA) for 30 min, followed by washing the samples in ethanol. Finally, the samples were dried in an oven at 120 °C for 30 min, inducing a layer of perfluorosilane on the surface of the samples. (18) (a) Liu, Z. F.; Shen, Z. Y.; Zhu, T.; Hou, S. F.; Ying, L. Z. Langmuir 2000, 63, 569. (b) Wu, B.; Zhang, J.; Wei, Z.; Cai, S. M.; Liu, Z. F. J. Phys. Chem. B 2001, 105, 5075.

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Figure 3. Shapes of water droplets on the fabricated bionic surface (a) before and (b) after the surface treatment. The corresponding CAs are 33° and 165°, respectively. (c) A photo of a water droplet on the modified hierarchical micro/nano structured surface taken by a CCD camera. Characterization. The morphologies of the samples were characterized with a field emission scanning electron microscope (FESEM, FEI XL30 FEG). X-ray electron spectroscopy (XPS) was performed on an ESCA 2000 (USA). The static water CA and SA were measured with a G10 (KRU ¨ SS Gmbh) CA meter at room temperature. The weight of the individual water droplet used for the static CA measurements was 3 mg. CA values were obtained by averaging five measurement results on different areas of the sample surface.

Figure 4. XPS data of a bionic microsphere/SWCNT composite array (a) before and (b) after chemical modification with 1H,1H,2H,2H-perfluorodecyltrichlorosilane.

Results and Discussion Figure 2a-c shows the FESEM images of the bionic surface fabricated by an SWCNT solution with a concentration of 2.0 mg/L and a colloidal monolayer with a periodicity of 5.0 µm. The images clearly show that dense SWCNTs were adsorbed on the PS microspheres and the sphere joints. Interestingly, the SWCNTs formed interlaced ‘net’ structures. Each wire of the net structure had a lateral size of ∼100 nm, indicating that several tens of SWCNTs were assembled to form a bundle of SWCNTs during the adsorption process.15,16 All of the hierarchical microsphere/SWCNT composite arrays took hexagonally closepacked and regularly ordered arrangements. The FESEM image of a natural lotus leaf surface is also shown for a comparison of the morphology (Figure 2d).2 As can be seen in the images, the synthesized hierarchical structure mimicked well the surface of a lotus leaf. The wettability of the synthesized hierarchical structures was investigated by both static water CA and SA. The water CA of the as-prepared sample with the hierarchical structure was measured to be 33 ( 1° (Figure 3a), exhibiting hydrophilicity. However, after the surface treatment with fluoroalkylsilane, the water CA of the sample was dramatically increased to 165 ( 0.9°, and the water droplet on the surface remained almost spherical (Figures 3b,c), indicating that the wettability of the surface was changed from hydrophilic to superhydrophobic due to the chemical treatment. XPS was carried out here to confirm the key information concerning the chemical state of the film surface. Before chemical modification, the XPS measurement for the microsphere/SWCNT composite array just detected the

C and O elements, which were from the SWCNT and the carboxyl group on the SWCNT surface, respectively, as shown in Figure 4a. Generally, the detecting depth of XPS is less than 10 nm, so the elements, such as Au, N, and S, originating from the gold layer and mercaptoethylamine cannot be detected. However, it has been demonstrated that SWCNTs terminated with a carboxylic group (-COOH) can be self-assembled onto a gold surface due to the condensation reaction between the -COOH and -NH2 as well as the electrostatic attraction and van der Waals interactions between the CNTs.15a,18 After chemical modification, fluoroalkylsilane, F, C, Si, and O elements can be found in the XPS measurement, as shown in Figure 4b. The F, C, and Si come from the modification, and the O element originates from the -COO- on the SWCNT surface. XPS analysis indicates that low free-energy materials have effectively modified on such a bionic surface. The reaction between fluoroalkylsine and SWCNTs is described in Figure 5. When the microsphere/ SWCNT composite array is dipped into the 1H,1H,2H,2Hperfluorodecyltrichlorosilane ethanol solution, a reaction takes place with liberation of HCl between the -SiCl3 of fluoroalkylsilane and the -COOH on the SWCNT surfaces, as Roig et al. reported.19 One thing to note is that the wettability of the fluorinated surface was changed when the surface was exposed to both water and air. The water CA decreased from 165° to 158° for 5 days when the fluorinated surface was constantly immersed (19) Roig, A.; Molins, E.; Rodrı´guez, E.; Martı´nez, S.; Moreno-Man˜as, M.; Vallribera, A. Chem. Commun. 2004, 2316.

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Figure 5. Schematic illustration of the reaction process of 1H,1H,2H,2H-perfluorodecyltrichlorosilane on the carboxyl-rich SWCNT surfaces.

Figure 7. FESEM image of the microsphere/SWCNT composite array at a reduced SWCNT concentration of 1.0 mg/L. The left inset is the magnified FESEM image of the structure, and the right inset shows the water droplet on the surface.

Figure 6. FESEM images of (a) PS sphere arrays with gold coating and (b) SWCNT film on a substrate. The insets on the left top of panels a and b are the water drop shapes on the as-prepared surface, and the corresponding CAs are 94° and 63°, respectively. The insets on right top of panels a and b are water droplet shapes after the surface treatment, and the CAs are 138° and 132°, respectively.

in water. However, when the fluorinated surface was exposed to air, the CA change was not so serious compared to the water exposure: the CA of the surface decreased from 165° to 161° after a 4 month exposure to air. (Figure S3 in Supporting Information). In addition, the surface exhibited a small SA of about 5°. In the process of CA measurement, we found it difficult to add a water droplet on the fabricated surface, demonstrating that the surface has very low adhesive force and very small CA hysteresis. The existence of superhydrophobicity with such a low SA and the difficulty in dropping water on the surface provide strong evidence of the lotus effect for the synthesized bionic surface with the hierarchically combined micro/nano structure. This property of the synthesized bionic surface can be widely applied to many interesting fields, such as liquid transportation without loss and micropump-needleless microfluidic devices.20,21 To identify the origin of the superhydrophobic property of the synthesized bionic surface, we investigated the CAs of a

Figure 8. A graph of water CA variations as a function of CNT concentration after chemical modification.

monolayer PS colloidal crystal without deposited CNTs and a SWCNT film on a flat substrate (Figure 6). The CA of the colloidal monolayer crystal was 94 ( 1° and increased to 138 ( 1° after the surface treatment (Figure 6a): the colloidal crystal was coated with gold for the chemical treatment. Moreover, the SWCNT film, which was prepared on a gold-coated flat silicon substrate by the wet chemical self-assembly technique, exhibited hydrophilicity with a water CA of 63 ( 1° and hydrophobicity with a CA of 132 ( 1° after the surface treatment (Figure 6b). These results demonstrate that neither a bare monolayer colloidal crystal nor a SWCNT film can produce superhydrophobicity, even though their surfaces were modified with low surface- energy materials. Therefore, the strong superhydrophobicity of the synthesized bionic surface originates from its unique hierarchical structure

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Figure 9. FESEM image of the microsphere/MWCNT composite arrays and the water droplet on the surface.

combined with the microscaled PS spheres and the nanoscaled net structure of CNTs. Before the surface modification, the wettability of the SWCNT film on the flat surface was hydrophilic (CA: 63 ( 1.3°); the hydrophilicity of the CNT film was caused by the -COOH functional group on the CNT surface. Evidently, the hierarchical microsphere/SWCNT composite arrays dramatically increase the surface roughness compared to both the monolayer colloidal crystal and the SWCNT film on the flat surface. According to the Wenzel model,22a the hydrophilicity of a hydrophilic surface and the hydrophobicity of a hydrophobic surface are enhanced as the surface roughness increases. This can explain why the water CA (33°) of the microsphere/SWCNT composite array is smaller than the CA (63°) of the SWCNT film on a flat substrate. When the hierarchical micro/nano bionic surfaces are modified with the low surface-energy material, the air can be trapped in grooves or interstices on the bionic surfaces.22 In this case, the model presented by Cassie and Baxter can be used for the description of wettability, which is described by22b

cos θr ) f1 cos θ - f2

(1)

where f1 is the area fraction of a water droplet in contact with a bionic surface, and f2 () 1 - f1) is the area fraction of a water droplet in contact with air on such a surface. Since the given water CAs of a flat graphite surface8 and the bionic surface modified with fluoroalkylsilane are 108° and 165°, respectively, f2 is calculated to be 0.95. This reveals that the synthesized bionic surface produces a large amount of air traps between the microstructure of the PS colloidal crystal and the nanostructure of the SWCNTs, and that such a strong superhydrophobicity of the bionic surface is mainly caused by the unique hierarchical micro/nano structures and the subsequent surface treatment. The wettability of the bionic surfaces fabricated by the presented method can be easily controlled by changing the morphologies of the hierarchical structures, that is, by changing (20) Liu, J.; Rinzler, A. G.; Dai, H. J.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias, F.; Shon, Y. S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253. (21) Jin, M. H.; Feng, X. J.; Xi, J. M.; Zhai, J.; Cho, K.; Feng, L.; Jiang, L. Macromol. Rapid Commun. 2005, 26, 1805. (22) (a) Wenzel, R. N. J. Phys. Colloid Chem. 1949, 53, 1446. (b) Cassie, A. B. D. Discuss. Faraday Soc. 1948, 3, 11. (23) (a) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622. (b) Wong, M. Y.; Kang, W. P.; Davidson, J. L.; Wisitsora, A.; Soh, K. L. Sens. Actuators B 2003, 93, 327.

the size of the microspheres or by tuning the distribution density of the CNTs. For a typical example, when the CNT concentration was reduced from 2.0 to 1.0 mg/L, the distribution density of CNTs on the microsphere was also decreased; some parts of the microspheres were not completely coated by SWCNTs, as displayed in Figure 7. This indicates that, at lower SWCNT concentrations, a smaller quantity of SWCNTs was assembled on the surfaces of PS microspheres during the wet chemical self-assembly process. Consequently, the surface roughness becomes lower, resulting in a smaller CA of 156 ( 1° after the surface treatment. The experimental results showed that the CAs decreased further upon continuously decreasing the SWCNT concentration. Additionally, if the SWCNT concentration is too much higher, the roughness of such a composite film will also decrease, leading to a decrease in the water CA. In our research, superhydrophobic bionic surfaces with a CA higher than 150° were obtained at the concentration range of 0.7-2.5 mg/L, as shown in Figure 8. Moreover, for the same concentration of CNTs (2.0 mg/L), when the PS sphere sizes were decreased from 5 µm to lower values (1.3 µm), the measured water CAs were reduced to 160 ( 1°. This suggests that, besides the CNT concentration, the wettability can be tuned by the PS sphere sizes. Just for colloidal monolayers, although the air volume fraction of sphere monolayers with different PS sizes is the same, after deposition of the same concentration CNTs, the roughness will decrease a little for a colloidal monolayer with a smaller size, further leading to a reduced water CA after chemical modification. In addition to SWCNTs, MWCNTs combined with monolayer colloidal crystals also took on superhydrophobicity. Figure 9 depicts a typical morphology of the hierarchical microsphere/MWCNT composite arrays and the water droplet shapes on the surface of the arrays. The hierarchical structure was fabricated from the MWCNT solution with a concentration of 2.0 mg/L and PS spheres 5.0 µm in diameter. MWCNTs were deposited on the microspheres using the same wet chemical selfassembly technique described in the Experimental Section. The MWCNTs also formed the “net” structure on the colloidal crystal, and the surface exhibited superhydrophobicity with a CA of 166° and an SA of 5° after surface treatment. It is worth noting that, compared to natural lotus leaves that have randomly distributed microstructures with nonuniform sizes,1-3 the synthesized bionic surfaces consist of regularly ordered microsphere/CNT composite arrays. Therefore, the fabricated bionic surfaces had a very uniform wettability on the whole surface: the standard deviation of the measured water CA was around 1° on the whole fabricated surface.

Conclusion A route to synthesize superhydrophobic bionic surfaces has been presented. The bionic surfaces were fabricated by depositing CNTs on PS colloidal crystals by the wet chemical self-assembling technique and the subsequent chemical treatment of the surface with fluoroalkylsilane. The morphologies of the synthesized bionic surfaces were very similar to those of the natural lotus leaves, which consist of rugged, hierarchical micro/nano structures, and thus the surfaces exhibited strong superhydrophobicity with a low SA after the surface treatment. The wettability of the bionic surface can be well controlled by changing the distribution density of CNTs on the colloidal monolayers as well as the periodicity of colloidal monolayers. The superhydrophobic bionic surfaces have many potential applications in the fields of microfluidic devices, bioseparation devices, and liquid transportation without loss. Moreover, CNTs with a continuous and homogeneous distribution on PS spheres have unique electrical

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and electrochemical properties, and such a rough surface with hierarchical microsphere/CNTs structures has a large specific area. Thus, the fabricated bionic structures can also be used for other devices such as gas sensors with good selectivity and high sensitivity.23 Acknowledgment. This work was supported by the Research Infrastructure Expansion project funded by the Korea Ministry of Science and Technology, the Brain Korea 21 project, the School of Information Technology supported by the Korea Advanced Institute of Science and Technology in 2006, and the

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National Natural Science Foundation of China (Grant No. 50601026). Acknowledgment. This section tagged Supporting Information Supporting Information Available: FTIR spectrum of CNTs pretreated in an acid mixture under ultrasonication for 8 h, XPS data of a mercaptoethylamine-modified gold surface, and the measured water CA of a fluorinated surface upon water exposure. This material is available free of charge via the Internet at http://pubs.acs.org. LA0620758