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Fabrication of Porous Hierarchical Polymer/Ceramic Composites by Electron Irradiation of Organic/Inorganic Polymers: Route to a Highly Durable, Large-Area Superhydrophobic Coating Eun Je Lee, Jae Joon Kim, and Sung Oh Cho* Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea Received January 8, 2010. Revised Manuscript Received January 25, 2010 Polymer/ceramic composite films with micro- and nanocombined hierarchical structures are fabricated by electron irradiation of poly(methyl methacrylate) (PMMA) microspheres/silicone grease. Electron irradiation induces volume contraction of PMMA microspheres and simultaneously transforms silicone grease into a ceramic material of silicon oxycarbide with many nanobumps. As a result, highly porous structures that consist of micrometer-sized pores and microparticles decorated with nanobumps are created. The fabricated films with the porous hierarchical structure exhibit good superhydrophobicity with excellent self-cleaning and antiadhesion properties after surface treatment with fluorosilane. In addition, the porous hierarchical structures are covered with silicon oxycarbide, and thus the superhydrophobic coatings have high hardness and strong adhesion to the substrate. The presented technique provides a straightforward route to producing large-area, mechanically robust superhydrophobic films on various substrate materials.

Introduction Peculiar surfaces that water cannot wet easily, so-called superhydrophobic surfaces with a water contact angle (CA) larger than 150°, are frequently observed in nature, and examples include lotus leaves1 and water striders’ legs.2 Superhydrophobic surfaces exhibit special properties, including self-cleaning, antifouling, antiadhesion, and antioxidation.1,3-5 Because of these interesting properties, the fabrication of artificial superhydrophobic surfaces has attracted strong interest with respect to various scientific and industrial applications such as microfluidic devices,6,7 lab-on-a-chip devices,8,9 biosurfaces,10,11 textiles,12,13 batteries, and fuel cells.14,15 Superhydrophobic surfaces result from a combination of high surface roughness and low surface free energy. Because a material’s surface can be easily modified by chemical treatment to have a low surface free energy, the realization of a sufficiently high surface roughness is crucial to the fabrication of a superhydrophobic *Corresponding author. E-mail: [email protected].

(1) Barthlott, K.; Neinhuis, C. Planta 1997, 202, 1–8. (2) Gao, X.; Jiang, L. Nature 2004, 432, 36. (3) Ma, Z.; Hong, Y.; Ma, L.; Su, M. Langmuir 2009, 25, 5446–5450. (4) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, 457–460. (5) Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132, 31–41. (6) Hong, X.; Gao, X.; Jiang, L. J. Am. Chem. Soc. 2007, 129, 1478–1479. (7) Joseph, P.; Cottin-Bizonne, C.; Benoıˆ t, J.-M.; Ybert, C.; Journet, C.; Tabeling, P.; Bocquet, L. Phys. Rev. Lett. 2006, 97, 156104. (8) Zhao, Y.; Cho, S. K. Lap Chip 2006, 6, 137–144. (9) Takei, G.; Nonogi, M.; Hibara, A.; Kitamori, T.; Kim, H.-B. Lap Chip 2007, 7, 596–602. (10) Ostuni, E.; Chen, C. S.; Ingber, D. E.; Whitesides, G. M. Langmuir 2001, 17, 2828–2834. (11) Wang, Y.; Sims, C. E.; Marc, P.; Bachman, M.; Li, G. P.; Allbritton, N. L. Langmuir 2006, 22, 8257–8262. (12) Zimmermann, J.; Reifler, F. A.; Fortunato, G.; Gerhardt, L.-C.; Seeger, S. Adv. Funct. Mater. 2008, 18, 3662–3669. (13) Xu, B.; Cai, Z. Appl. Surf. Sci. 2008, 254, 5899–5904. (14) Lifton, V. A.; Taylor, J. A.; Vyas, B.; Kolodner, P.; Cirelli, R.; Basavanhally, N.; Papazian, A.; Frahm, R.; Simon, S.; Krupenkin, T. Appl. Phys. Lett. 2008, 93, 043112. (15) Li, W.; Wang, X.; Chen, Z.; Waje, M.; Yan, Y. Langmuir 2005, 21, 9386– 9389.

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surface. Such high surface roughness for superhydrophobicity is mostly prepared by micro/nano combined hierarchical structures. Recently, many approaches to fabricating superhydrophobic surfaces have been reported, including layer-by-layer self-assembly,16,17 chemical vapor deposition,12,18 lithographic techniques,19,20 sol-gel methods,21,22 electrochemical methods,23,24 hydrothermal synthesis,25,26 and colloidal-crystal-based methods.27,28 For practical applications, a simple, cost-effective route to preparing large-scale superhydrophobic surfaces is required. Furthermore, such superhydrophobic coating films should be able to be fabricated on various substrate materials. High durability of the coating is also mandatory for long-time operation, especially because a surface with high roughness generally has poorer mechanical strength than a flat surface. However, few fabrication methods that satisfy all of these requirements have been reported. In our previous study, we reported that a hierarchical structure comprising micropores and nanopores could be created on the surface of a thin silicone grease film by electron irradiation and that the surface was rendered superhydrophobic or superhydrophilic (16) Zhai, L.; Cebeci, F. C-.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349–1353. (17) Liao, K.-S.; Wan, A.; Batteas, J. D.; Bergbreiter, D. E. Langmuir 2008, 24, 4245–4253. (18) Artus, G. R. J.; Jung, S.; Zimmermann, J.; Gautschi, H.-P.; Marquardt, K.; Seeger, S. Adv. Mater. 2006, 18, 2758–2762. (19) Min, W.-L.; Jiang, B.; Jiang, P. Adv. Mater. 2008, 20, 3914–3918. (20) Dorrer, C.; R€uhe, J. Adv. Mater. 2008, 20, 159–163. (21) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377– 1380. (22) Han, J. T.; Lee, D. H.; Ryu, C. Y.; Cho, K. J. Am. Chem. Soc. 2004, 126, 4796–4797. (23) Yu, X.; Wang, Z.; Jiang, Y.; Zhang, X. Langmuir 2006, 22, 4483–4486. (24) Shi, F.; Niu, J.; Liu, J.; Liu, F.; Wang, Z.; Feng, X.-Q.; Zhang, X. Adv. Mater. 2007, 19, 2257–2261. (25) Shi, F.; Chen, X.; Wang, L.; Niu, J.; Yu, J.; Wang, Z.; Zhang, X. Chem. Mater. 2005, 17, 6177–6180. (26) Shi, F.; Niu, J.; Liu, Z.; Wang, Z.; Smet, M.; Dehaen, W.; Qiu, Y.; Zhang, X. Langmuir 2007, 23, 1253–1257. (27) Li, Y.; Huang, X. J.; Heo, S. H.; Li, C. C.; Choi, Y. K.; Cai, W. P.; Cho, S. O. Langmuir 2007, 23, 2169–2174. (28) Li, Y.; Lee, E. J.; Cho, S. O. J. Phys. Chem. C 2007, 111, 14813–14817.

Published on Web 02/01/2010

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by chemical treatment.29 For the formation of such a hierarchical structure, the pristine silicone grease film must have high viscosity and the thickness of the film must be precisely controlled within a certain range. However, a viscous silicone grease film with uniform thickness could not be prepared on a large scale, thus preventing the fabrication of a large-area superhydrophobic film. Here, we present a facile, straightforward route to fabricating highly durable, large-area superhydrophobic coatings on diverse substrates by electron irradiation of a poly(methyl methacrylate) (PMMA) microsphere/silicone grease film. Only by irradiating a precursor film with an electron beam can a unique porous hierarchical structure composed of a polymer/ceramic composite be produced. The formation of this unique hierarchical structure is attributed to the decomposition of organic PMMA and the transformation of the inorganic polymeric material of silicone grease into a ceramic material by electron irradiation. The fabricated films exhibit excellent superhydrophobic and mechanical properties. The precursor PMMA/silicone grease film can be readily prepared on a large scale by a wet process because the precursor is made of polymeric materials. Thus, if a large precursor film is only electron irradiated then large-area superhydrophobic coatings can be straightforwardly fabricated by this approach.

Scheme 1. Schematic Illustration of the Fabrication of the Porous Hierarchical Structures by Electron Irradiation of the PMMA/Silicone Grease Filma

Experimental Section PMMA spheres (Soken, MP-1000, MX-180, MX-500, MX1000, and MX-2000), silicone grease (Dow Corning high vacuum grease), and PDMS (M w = 300 000 g 3 mol-1, Shin-Etsu Chemical Co., KF-96H) were used for the preparation of precursor materials. A PMMA colloidal solution (60 wt %) made of the PMMA spheres and ethanol was spin coated onto a silicon wafer (4 in. in diameter) at 700 rpm for 1 min. A silicone grease solution (10 wt %) was prepared using hexane as a solvent. This solution was additionally spin coated onto a multilayer of PMMA spheres at 2500 rpm for 1 min. The prepared precursor films were irradiated with an electron beam generated from a thermionic electron gun.30 The irradiation process was carried out at ambient temperature under a pressure of less than 2  105 Torr. The energy of the electron beam irradiating the samples was fixed at 50 keV, and the electron beam diameter was ∼12 cm. The total electron fluence was 2.9  1018 cm-2. During irradiation, the sample substrate was cooled with water to prevent possible melting of the PMMA microspheres by the electron beam. The morphologies of the precursor and electron-irradiated films were characterized with a field-emission scanning electron microscope (FESEM, Hitachi S-4800). The change in chemical composition of the films induced by electron irradiation was investigated with X-ray photoelectron spectroscopy (XPS) using Mg and Al KR X-ray sources in a Sigma Probe (Thermo VG) spectrometer. The XPS spectra were curve fitted with a mixed Gaussian-Lorentzian shape using the analysis software of XPSPEAK.31 A Shirley function was used to remove the background prior to curve fitting. All of the XPS spectra were charge compensated to C 1s at 284.6 eV.32,33 After electron irradiation, the film surfaces were chemically modified with fluorosilane [(heptadecafluoro-1,1,2,2-tetrahydrodecyl) trichlorosilane] (Alfa Aesar). The samples were immersed in a hexane solution of 10 mM fluorosilane for 30 min and (29) Lee, E. J.; Lee, H. M.; Li, Y.; Hong, L. Y.; Kim, D. P.; Cho, S. O. Macromol. Rapid Commun. 2007, 28, 246–251. (30) Lee, H. M.; Kim, Y. N.; Kim, B. H.; Kim, S. O.; Cho, S. O. Adv. Mater. 2008, 20, 2094–2098. (31) Kwok, R. W. M. XPSPEAK, version 4.1. (32) Youn, B.-H.; Huh, C.-S. Surf. Interface Anal. 2003, 35, 445–449. (33) Schnyder, B.; Lippert, T.; K€otz, R.; Wokaun, A.; Graubner, V.-M.; Nuyken, O. Surf. Sci. 2003, 532-535, 1067–1071.

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a (a) Spin coating of the PMMA colloidal solution. (b) Spin coating of the silicone grease solution. (c) Electron irradiation process.

dried at room temperature. The static water CA was measured with a CA measurement system (SEO Co., Ltd., Phoenix 300 Plus). The volume of the water drop used for the CA measurement was 4 μL. The pencil hardness of electron-irradiated films was determined by the method described in ASTM D 3363, a standard test method for film hardness by the pencil test. The tape test was carried out according to ASTM D 3359-02, a standard test for measuring adhesion by the tape test.

Results and Discussion The fabrication process of the porous hierarchical structure is illustrated in Scheme 1. PMMA microspheres are spin coated onto a substrate, and subsequently a solution of silicone grease is spin coated onto the microspheres, forming an organic/inorganic polymeric film. The mixture film is irradiated with an electron beam. This single-step irradiation process creates a hierarchical structure with micrometer-sized pores, of which the walls consist DOI: 10.1021/la100094y

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Figure 1. Photograph of a PMMA/silicone grease mixture film fabricated on a silicon wafer with a diameter of 4 in. (a) before electron irradiation and (b) after electron irradiation.

Figure 2. FESEM images of (a) the multilayer of PMMA microspheres, (b) the multilayer of PMMA microspheres with a silicone grease coating, and (c) the porous hierarchical structure formed after electron irradiation. (d) Magnified image of the wall of the porous hierarchical structure in image c. Scale bars are 100 μm for images a-c and 10 μm for image d.

of interconnected microspheres that are decorated with many nanobumps. Figure 1a displays an image of a PMMA/silicone grease film prepared on a 4 in. silicon wafer. Because both PMMA and silicone grease are polymeric materials, a large-area film could be easily fabricated by a solution-based spin-coating process. However, precursor films of 4 in. diameter were used in this research because the electron irradiation experiments were carried out with a compact homemade electron beam device that had a limited capability in terms of beam size (maximum diameter of ∼12 cm).30 The precursor film was initially white, but its color changed to brown after electron irradiation (Figure 1b). The morphologies of the films before and after electron irradiation were investigated with a field-emission scanning electron microscope (FESEM). Close-packed multilayer PMMA microspheres were formed on the substrate when the PMMA colloidal solution was spin coated (Figure 2a). The close-packed structure, however, is not a regularly ordered structure such as 3026 DOI: 10.1021/la100094y

hexagonal close-packed or cubic close-packed structures. After an additional coating of silicone grease, the PMMA microspheres were covered and connected with viscous grease materials (Figure 2b) but the close-packed microsphere structure was largely preserved. When the precursor film was electron irradiated, a unique structure such as that shown in Figure 2c was formed. Interestingly, the size of the PMMA microspheres was reduced from their original diameter of 20 μm to ∼15 μm. The microspheres were separated from one another because of the size reduction. We observed that the microspheres were covered with a hard material and that this material firmly interconnected the separated microspheres. In the spaces between the separated micropsheres, micrometer-sized pores were formed. Moreover, the microspheres had many nanometer-sized protuberances on their surfaces. Therefore, after electron irradiation of the PMMA microspheres/silicone grease, a porous hierarchical structure composed of interconnected microspheres whose surfaces had many nanobumps was created. Langmuir 2010, 26(5), 3024–3030

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Figure 3. Water CAs of the PMMA/silicone grease films measured (a) before electron irradiation, (b) after electron irradiation, and (c) after surface treatment of the irradiated film with fluorosilane.

The chemical structure of the material that covers the microspheres was analyzed using X-ray photoelectron spectroscopy (XPS). The XPS survey spectra of both the pristine and irradiated PMMA/silicone grease film showed silicon (Si), oxygen (O), and carbon (C) peaks. However, the relative atomic ratio of O to Si was considerably changed (Figure S1a). Moreover, the Si 2p peak shifted from 102.1 eV to a higher value of 103.0 eV (Figure S1b). This indicates that the material covering the PMMA microspheres is silicon oxycarbide, which is a network structure of SiOxC4 - x (0 e x e 4) (details in Figure S1) that is transformed from PDMS in the silicone grease by electron irradiation.34 Silicon oxycarbide is a ceramic material that has excellent thermomechanical properties and high thermochemical stability.35,36 To understand the formation mechanism of the porous hierarchical structure, we carried out the following experiments. First, multilayer PMMA microspheres without an additional silicone grease coating were irradiated with an electron beam. Size reduction of the microspheres was also observed in this case: the average size of the microspheres was reduced from 20 μm to ∼15 μm by electron irradiation (Figure S2a). However, the closepacked structure of the microspheres was retained, hence no micropores were produced. The size reduction of the PMMA microspheres can be explained by the decomposition of the polymeric material due to electron irradiation.37 Similar size reduction behavior was observed in our previous electron irradiation experiments with polystyrene nanospheres.38 PMMA is known as a radiation-degrading polymer, which means that the polymeric materials are decomposed and then removed in the form of volatile elements by electron irradiation.37 As a result, the volume and correspondingly the size of the microspheres decrease as the electron fluence is increased. Second, instead of silicone grease, pure PDMS was coated onto multilayer PMMA microspheres. Silicone grease is mainly composed of inorganic PDMS polymer and silica nanoparticles with an average size of ∼100 nm.29 Thus, the main difference between PDMS and silicone grease lies in whether the material contains silica nanoparticles. When a PMMA/PDMS film was electron irradiated, a microporous structure similar to that shown in Figure 2c was produced: separated microspheres were interconnected with one another by a hard material and micrometer-sized pores were formed between the microspheres. However, no nanometer-sized protuberances were found on the microspheres (34) Cho, S. O.; Lee, E. J.; Lee, H. M.; Kim, J. G.; Kim, Y. J. Adv. Mater. 2006, 18, 60–65. (35) Kleebe, H.-J.; Turquat, C.; Soraru, G. D. J. Am. Ceram. Soc. 2001, 84, 1073–1080. (36) Pantano, C. G.; Sing, A. K.; Zhang, H. J. Sol-Gel Sci. Technol. 1999, 14, 7–25. (37) Cho, S. O.; Jun, H. Y.; Ahn, S. K. Adv. Mater. 2005, 17, 120–125. (38) Li, Y.; Lee, E. J.; Cai, W.; Kim, K. Y.; Cho, S. O. ACS Nano 2008, 2, 1108– 1112.

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(Figure S2b), which is different from the structure shown in Figure 2c. Therefore, the nanobumps on the surfaces of microspheres are attributed to the silica nanoparticles originally embedded in silicone grease. On the basis of these experimental results and the surface characterization results, we could explain the formation mechanism of the porous hierarchical structure. If a film of PMMA microspheres and silicone grease is irradiated with an electron beam, the PMMA sphere size is decreased by irradiation and simultaneously PDMS in silicone grease is transformed into a ceramic material of silicon oxycarbide. If no material is coated onto the microspheres, then the close-packed structure of the microspheres is preserved although the size of each microsphere is reduced by irradiation, as shown in Figure S2a. This is because the microspheres shift their positions while the microspheres are slowly contracted by the irradiation. However, if silicone grease is coated onto the microspheres, then a hard ceramic material is produced on the microspheres by the irradiation. Because of the coverage of the hard material, the microspheres can largely retain their original positions in spite of the volume shrinkage. The spaces between the microspheres where the microspheres are contracted become empty; consequently, micropores are created. In addition, during the transformation of silicone grease to silicon oxycarbide, polymeric materials of PDMS in silicone grease are decomposed by electron irradiation. It was revealed in our previous study that molecular bonds of PDMS are broken by electron irradiation and, as a consequence, volatile elements such as hydrogen and hydrocarbon are easily removed from the irradiated PDMS.29 Because of the decomposition of PDMS, the silica nanoparticles embedded in silicone grease can be exposed to the surface of microspheres. However, the silica nanoparticles are tightly bound to the microspheres because the silica nanoparticles are covered with a hard silicon oxycarbide film. Therefore, electron irradiation of the PMMA microspheres/ silicone grease film leads to the formation of a porous hierarchical structure where PMMA microspheres decorated with silica nanoparticles are coated with silicon oxycarbide, as shown in Figure 2c. The water wettabilities of PMMA/silicone grease films before and after electron irradiation were investigated by measuring water CAs. The water CA of a pristine PMMA/silicone grease film was ∼110° (Figure 3a), which is close to that of a pure PDMS surface, ∼105°. The surface of a pristine film is covered with silicone grease, and the main component of silicone grease is PDMS. Hence, the water wettability of a pristine PMMA/silicone grease mixture film is similar to that of a PDMS surface. The small increase in the water CA of the mixture film compared to that of a pure PDMS surface was caused by the former not being smooth but instead having a rough surface induced by closedpacked microspheres. However, even though such a microtexture was produced, the surface roughness produced by the microspheres DOI: 10.1021/la100094y

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alone is not high enough to increase the water CA considerably. In contrast, the porous hierarchical structure fabricated by electron irradiation of PMMA/silicone grease exhibited a water CA of 54°, indicating that the hydrophobic PMMA/silicone grease surface turned into a hydrophilic surface after irradiation (Figure 3b). Although the surface roughness of the porous hierarchical structure was dramatically increased compared to that of the pristine film, the structure did not exhibit a superhydrophobic property. This is because the surface material was changed from hydrophobic PDMS to hydrophilic silicon oxycarbide.39,40 However, when the porous hierarchical structure was treated with lowsurface-energy fluorosilane, the water CA of the surface was remarkably increased to 169°, indicating that the surface became superhydrophobic (Figure 3c). The variation of the water CAs measured across the entire 4 in. film was within (1°, revealing that the prepared superhydrophobic surface has excellent uniformity in terms of wettability. The fabricated superhydrophobic films also showed excellent antiadhesion properties. Many water droplets with the same volume of 10 μL were placed on a prepared film (Figure 4a), and the film was then slowly stirred using a rotating plate. As stirring commenced, all of the water droplets freely moved back and forth on the surface and finally rolled off of the surface (Figure 4b-d). This reflects that the superhydrophobic surface fabricated by the presented approach has good antiadhesion properties with respect to water. Furthermore, the prepared superhydrophobic films had a good self-cleaning effect. White polymer powder was heavily sprinkled onto a fabricated film, and the film was then tilted by 2°. When water was placed on the inclined surface dropwise, the polymer powder was cleaned off by the water droplets rolling on the surface (Figure S3). Water droplets were initially transparent (lefthand droplets in Figure S3b) but became opaque (righthand droplets in Figure S3b) because of the adhesion of the white power to the water droplets. These results demonstrate the excellent antiadhesion properties and self-cleaning effect of the fabricated films. The wettability of the porous hierarchical structure can be controlled by changing the size of the PMMA microspheres. The water CA of the fabricated surface increased as the PMMA sphere size was increased. When PMMA spheres with a diameter of 400 nm were used, the water CA of the surface was 140° (Figure 5a). As the PMMA microsphere size was increased from 2 to 10 μm, the CA of the surface was increased from 158 to 167° (Figure 5b-d). Regardless, if micrometer-sized PMMA spheres were used, then the fabricated surfaces always exhibited superhydrophobicity. Additionally, superhydrophobic films can be fabricated not only on a silicon substrate but also on copper and glass substrates. Similar values of water CAs were achieved regardless of the substrates if the same sphere size was used (Figure S4). This suggests that the fabrication technique presented here can be applicable to coating a superhydrophobic film onto diverse material surfaces. One thing to note here is the importance of the nanobumps decorating the PMMA microspheres to obtain a superhydrophobic surface. We already showed that a microporous structure without nanobumps on the PMMA microspheres can be fabricated by electron irradiation of a PMMA/PDMS mixture, as shown in Figure S2b. This microporous structure exhibited only hydrophobicity with a maximum water CA of 141° even though the surface was treated with fluorosilane (inset of Figure S2b). (39) Socha, R. P.; Laajalehto, K.; Nowak, P. Surf. Interface Anal. 2002, 34, 413– 417. (40) Walkiewicz-Pietrzykowska, A.; Espinos, J. P.; Gonzalez-Elipe, A. R. J. Vac. Sci. Technol., A 2006, 24, 988–994.

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Figure 4. Movement of water droplets on a superhyphobic film when the film was slowly rotated. Photographs were captured (a) just before rotation and (b-d) after 5, 6, and 9 s, respectively.

This indicates that the microporous structure without nanobumps on the microspheres did not have sufficient surface roughness to obtain superhydrophobicity and that the presence of nanobumps on the microspheres is crucial to realizing the superhydrophobic property. As shown in Figure 3a, a pristine PMMA microsphere/silicone grease film is hydrophobic. However, we found that the surface wettability of the pristine film was very unstable. A water droplet (4 μL) dropped onto the pristine film surface gradually permeated the film, and the CA slowly decreased with time (Figure S5). This is because PMMA spheres are very weakly bound to one another; consequently, the close-packed sphere structure is broken even by Langmuir 2010, 26(5), 3024–3030

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Figure 5. FESEM images of the porous hierarchical structures with different PMMA sphere sizes: (a) 400 nm, (b) 2 μm, (c) 5 μm, and (d) 10 μm. (Insets) CAs of corresponding surfaces after surface treatment with fluorosilane. Scale bars are 50 μm.

the weight of such a small water droplet. Viscous silicone grease on the microspheres does not provide a binding force to prevent breakage of the close-packed structure. However, we observed that the surface of the fabricated film with a porous hierarchical structure was very hard and that water droplets on the surface stably kept their shape. For the determination of the hardness of the films, a pencil hardness test was performed. The pencil hardness of the pristine PMMA/silicone grease film was substantially lower than 6B, the minimum hardness level as determined by ASTM D 3363. However, the irradiated film had a pencil hardness of 2H, indicating that the hardness of the film was remarkably enhanced after electron irradiation. This is because the viscous silicone grease covering the surface of the film was transformed to a ceramic material of silicon oxycarbide by electron irradiation. Additionally, the adhesion strength between the film and the substrate was measured by a tape test (ASTM D 3359-02). The fabricated porous hierarchical film exhibited an adhesion strength of 5B, which is the highest adhesion rating defined by ASTM D 3359-02. This result means that 0% of the film is removed by successive attachment and detachment of the tape on the film. Furthermore, the porous hierarchical film on the silicon substrate was sonicated in an ultrasonic generator with a power of 300 W to estimate the adhesion strength between the fabricated film and the substrate. The film was neither detached nor damaged by sonication for 20 min. Even after sonication, the film completely recovered its original superhydrophobic property: the water CA of the sonicated film was also 169°, which is the same as that before sonication. All of these results reflect that the porous Langmuir 2010, 26(5), 3024–3030

hierarchical film fabricated by electron irradiation has good mechanical properties under harsh conditions.

Conclusions We have presented a straightforward route to producing a large-area superhydrophobic coating film based on an electron irradiation technique. By irradiating a mixture comprising PMMA microspheres and silicone grease with an electron beam, we could create a highly porous micro/nano combined hierarchical polymer/ ceramic composite. The formation of the porous hierarchical structure is caused by the volume reduction of PMMA micrpospheres and the transformation of silicone grease to silicon oxycarbide by electron irradiation. When the fabricated film was treated with fluorosilane, the surface exhibited superhydrophobicity with excellent antiadhesion and self-cleaning properties. The superhydrophobic films showed very uniform wetting across the entire film surface. Because of the formation of the silicon oxycarbide coating, the fabricated superhydrophobic films were mechanically robust and exhibited strong adhesion to the substrate. Moreover, the superhydrophobic coating film could be fabricated on the surfaces of various materials. We believe that the fabrication technique presented here provides a promising approach to preparing large-area superhydrophobic films. The irradiation technique is a straightforward one-step process for the fabrication of a rough hierarchical structure. In addition, the precursor materials used in this approach are inexpensive and widely used polymeric materials. Precursor films can be easily prepared on a large scale by a solution-based process. Moreover, commercialized electron irradiation devices can provide DOI: 10.1021/la100094y

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a meter-scale electron irradiation area and high electron dose rate under ambient conditions. Electron irradiation is a parallel process, and thus the hierarchical structures are simultaneously fabricated on all of the surface regions that are irradiated with an electron beam. Therefore, if large-scale precursor films are irradiated with a sufficiently large electron beam, then large-area superhydrophobic surfaces can be prepared cheaply and rapidly by the irradiation approach. In addition, the irradiation technique can also be used to fabricate other polymer-ceramic composite materials or ceramic protective coatings on soft matter for application to fields requiring high mechanical and thermal properties.

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Acknowledgment. This work was supported by the MEST/ KOSEF (nuclear R&D program, 20090062475). Supporting Information Available: XPS spectra of pristine and irradiated PMMA/silicone grease films. FESEM images of an electron-irradiated PMMA microsphere film and a PMMA/PDMS film. Photograph showing the self-cleaning effect of the superhydrophobic film. FESEM images of porous hierarchical structures fabricated on copper and glass substrates. CA change in PMMA/silicone grease film. This material is available free of charge via the Internet at http:// pubs.acs.org.

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