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Pressure-Proof Superhydrophobic Films from Flexible Carbon Nanotube/Polymer Coatings Chih-Feng Wang,*,† Wei-Yan Chen,† Huy-Zu Cheng,† and Shen-Li Fu‡ Department of Materials Science and Engineering, I-Shou UniVersity, Kaohsiung, 840, Taiwan, and Department of Electronic Engineering, I-Shou UniVersity, Kaohsiung, 840, Taiwan ReceiVed: May 26, 2010; ReVised Manuscript ReceiVed: August 6, 2010
In this paper, we report a simple and inexpensive method for fabricating stable superhydrophobic films from flexible carbon nanotube (CNT)/polymer coatings. The CNT/polymer coatings, which do not contain any fluorinated compounds, maintain their superhydrophobicity after bending and pressing and display excellent environmental stability. Furthermore, these superhydrophobic films can be coated onto glass, metals, and polymers, resulting in water-repellent, self-cleaning surfaces exhibiting high water contact angles. Such superhydrophobic coatings have potential applications in bioseparation, microfluidic devices, liquid transportation, and nonwetting surfaces. Introduction Water repellency is a fundamental characteristic influencing a solid surface’s applications.1 Generally, the degree to which a solid surface repels a liquid depends upon two factors: its chemical composition and its topographical microstructures. Chemical modification alone, with fluoropolymeric coatings or fluorinated silane layers, can lead to water contact angles of flat surfaces reaching up to 120°.2 This approach is, however, inadequate for fabricating superhydrophobic surfaces, which usually possess water contact angles greater than 150°.3,4 Combining surface roughness on both the micro- and nanoscales with a low-surface-energy material is the key to obtaining superhydrophobicity.5,6 Two hypotheses have been proposed to explain this effect. The Wenzel model states that the surface roughness increases the surface area of the solid and, thereby, enhances its hydrophobicity;7 the Cassie-Baxter model states that the air trapped within the grooves beneath the liquid leads to superhydrophobic behavior because the drop sits partially on air.8 The superhydrophobic surfaces of lotus leaves, with high water contact angles (161.0 ( 2.7°) and low sliding angles (2°), cause water drops to bead and roll off the surface, thereby causing extreme water repellency and self-cleaning characteristics (the pollutants are easily removed by the rolling droplets). Such surface properties are used in many fields,9-11 including self-cleaning utensils, microfluidic systems, and microelectronic devices. Carbon nanotubes (CNTs) have found enormous applicability in most areas of science and engineering because of their excellent electronic, mechanical, and chemical properties.12-14 Superhydrophobic phenomena have been observed for aligned and nonaligned CNT films, notably when functionalized with perfluoroalkyl groups,15 after plasma treatment,16 for CNT/ polymer composites prepared through solvent processing,17 after layer-by-layer self-assembly of polymers and CNTs,18 for anisotropic structures of aligned CNT (ACNT) films,19 for aligned CNTs coated with ZnO thin films,20 and for CNT films prepared using other approaches.21-23 Herein, we introduce a simple method for fabricating pressure-proof superhydrophobic * To whom correspondence should be addressed. E-mail: cfwang@ isu.edu.tw. Tel: 886-7-6577711-3129. Fax: 886-7-6578444. † Department of Materials Science and Engineering. ‡ Department of Electronic Engineering.
films from flexible CNT/polymer coatings. We prepared these multifunctional CNT/polymer films under ambient conditions using readily available raw materials and laboratory equipment. The superhydrophobic films can be coated on glass, metals, and plastics, resulting in water-repellent, self-cleaning surfaces exhibiting high water contact angles (ca. 167°) and low sliding angles (lower than 3°). Furthermore, using commercially available raw materials makes it possible to fabricate large area films at a very low cost. Experimental Section Sylgard 184, a poly(dimethylsiloxane) (PDMS), was supplied by Dow Corning. Multiwalled carbon nanotubes (MWCNTs, average diameter ) 20-40 nm, length ) 5-15 µm) were purchased from Conyuan Biochemical Technology. Anodized aluminum oxide (AAO) membranes (Anodisc 0.2 µm, diameter ) 21 mm, thickness ) 60 µm, average pore diameter ) ca. 200 nm) were obtained from Whatman. Water that had been purified through reverse osmosis was further purified using a Millipore Milli-Q system (reverse osmosis, ion exchange, and filtration steps; 1018 Ω/cm). A suspension of MWCNTs in ethanol (1 mg/mL) was sonicated for 30 min. A film of MWCNT film on an AAO membrane was prepared through filtration of the MWCNT/EtOH suspension. Superhydrophobic coatings were prepared on Al foil, a glass slide, and polymer substrates through a two-step process. First, PDMS (0.1 g) was mixed with a curing agent (0.01 g) in toluene (or n-hexane, 10 mL), and then the solution was spin-coated for 45 s onto Al foil, a glass slide, or a polymer substrate using a photoresist spinner operated at 1500 rpm. Subsequently, the MWCNT/EtOH suspension was sprayed onto the PDMS surface, which was positioned on a heating plate held at ca. 90 °C and then cured in an oven at 150 °C for 1 h. Tests of pressure-proof properties were performed using two methods: uniaxial (single-ended) pressing of the superhydrophobic Al foil (20 mm × 20 mm) under various pressures (ranging from 3.56 to 32.0 ksi) or cold isostatic pressing of the sample at 40.0 ksi. Uniaxial pressing was conducted using a Carver Laboratory Press, model C. Cold isostatic pressing (model 57-6020, National Forge Company, Irvine, PA) was performed inside an evacuated polyethylene bag. The film’s durability to organic solvents was evaluated by immersing it in
10.1021/jp1047985 2010 American Chemical Society Published on Web 08/25/2010
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Figure 1. (a) Superhydrophobic coating on an Al foil. The scale bar is 1 cm. (b) Profile of a water drop on the as-prepared superhydrophobic surface. (c) Superhydrophobic coating on a glass substrate. (d) Superhydrophobic coatings on poly(methylmethacrylate) (PMMA) and polycarbonate (PC) substrates.
an organic solvent and then drying at 60 °C. After being immersed in diesel oil, the film was washed with tetrahydrofuran and dried as above. The contact angle was measured on the dried film. The microstructure of the CNT/polymer film was characterized using a Hitachi S-4700 scanning electron microscope (SEM) operated at an acceleration voltage of 5 kV; the specimen was coated with a thin layer of Pt/Pd prior to observation. The contact angle and sliding angle were measured using an FDSA MagicDroplet-100 contact angle goniometer. The static contact angle and the sliding angle were obtained from the 5 µL drop. Water droplets of various values of pH were placed carefully onto the superhydrophobic CNT/polymer films. Each of the reported contact angles represents the average of six measurements. Results and Discussion The PDMS was spin-coated onto an Al foil (or a glass slide) and then cured at 150 °C for 1 h, providing a smooth surface. We prepared an MWCNT film on an AAO membrane through filtration of an MWCNT/EtOH suspension. The water contact angles of the PDMS coating and the MWCNTs coating are 95 ( 2° and 143 ( 9°, respectively; although hydrophobic, these values are too low for the materials to be classified as superhydrophobic. Combining PDMS with MWCNTs provided superhydrophobic coatings on the Al foil samples. The MWCNT/EtOH suspensions were sprayed onto the PDMS-modified Al foils that were placed on a heating plate and then cured in an oven. The as-prepared MWCNT/PDMS coating in Figure 1a possessed a high water contact angle (167 ( 2°, Figure 1b) and small sliding angle (lower than 3°). On this superhydrophobic surface, water droplets possessed near-spherical shapes and rolled off with ease. We also coated the superhydrophobic onto glass (Figure 1c) and polymer substrates (Figure 1d), consequently, resulting in water-repellent, self-cleaning properties as on the metal surfaces. Moreover, the free-standing MWCNT/PDMS coatings also possessed superhydrophobicity (Supporting Information). Figure 2a-c presents top-view SEM images of the asprepared superhydrophobic surface on the Al foil. This superhydrophobic surface possessed a surface roughness on both the micro- and nanoscales (i.e., binary structures). Many microislands (5-20 µm) were present on this surface in a random distribution (Figure 2a). Figure 2b provides a higher-magnifica-
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Figure 2. (a) Large-area SEM image of the superhydrophobic surface on an Al foil. The scale bar is 50 µm. (b) Enlarged view of a microisland in (a). The scale bar is 1 µm. (c) SEM image of the lower surface of the superhydrophobic film. The scale bar is 1 µm. (d) A superhydrophobic Al foil in water, viewed perpendicular to the surface and past the critical angle.
tion image of a single island, in which hierarchical structures existed in the form of branchlike nanostructures having diameters of 20-40 nm. Nanostructures were also present on the lower surface of the superhydrophobic film (Figure 2c). Such a structure dramatically increases surface roughnesses and leads to a composite interface24 in which air becomes trapped within the grooves beneath the liquid, thereby inducing superhydrophobicity (Cassie-Baxter model). Figure 2d reveals the change in appearance of the as-prepared superhydrophobic surface after immersion in water. When we immersed a matte-black treated Al foil in water and viewed it at an oblique angle, it appeared as a silver mirror. The critical angle was 48 ( 1°, similar to that for reflection at a normal water-air boundary (48.6°).25 The small sliding angle and the critical angle both suggest that these films were Cassie-Baxter surfaces. Jiang et al.26 previously reported that nanostructured ACNTs can initially exhibit a high water contact angle (132°), but then they can become wetted after several minutes of contact with water. Lau et al.20 also noted that the water contact angle of the ACNT template decreases linearly over time, from an initial value of 146° to 0° after 15 min. Unlike ACNT surfaces, our MWCNT/PDMS coatings exhibited stable superhydrophobicity. Figure 3 reveals the time dependence of the water contact angle for our MWCNT/PDMS coatings. The contact angle of water droplets on the superhydrophobic surface remained constant for over 30 min (the water droplet could still be moved easily at that time), suggesting stable superhydrophobicity. Contact angle hysteresis is an important criterion for characterizing the hydrophobicity of solid surfaces. For an ideal surface, there exists only one contact angle (i.e., a true equilibrium contact angle). For a real surface, however, several contact angles may be observed, which results in contact angle hysteresis. The difference between the advancing and receding angles provides the extent of hysteresis. The hysteresis is more important in determining the true hydrophobicity than is achieving a maximum contact angle. The contact angle hysteresis can be regarded as the force required to move a liquid droplet across a surface.27 When the contact angle hysteresis is small or negligible, only a very small force is required to move the droplet. The
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Figure 3. Time dependence of the water contact angle for the MWCNT/PDMS coating.
Figure 5. (a) SEM image of the superhydrophobic film subjected to a pressure of 32.0 ksi. The scale bar is 50 µm. (b) Enlarged view of (a). The scale bar is 500 nm.
Figure 4. (a) Photographs of water droplets on bent areas of the films. (b) SEM image of the bent area of the superhydrophobic surface. The scale bar is 25 µm. (c) Enlarged view of a crack in (b). The scale bar is 5 µm. (d) SEM image of the lower surface in (c). The scale bar is 500 nm.
observation of the contact angle changing with time in Figure 3 (because of evaporation, the angle became a receding angle) determines a Cassie-Baxter state of comparable receding angle for the water drop on our superhydrophobic surfaces. The photographs of the fabricated samples in Figure 4a demonstrate their good flexibility. Interestingly, the bent area of the coating retained its superhydrophobicity; the spherical nature of the water droplet was sustained on the superhydrophobic surface, thereby assuring a high contact angle (ca. 168°). A water droplet placed onto the bent area rolled off the surface in less than 30 ms, which was beyond the time limit of our contact angle instrument. Thus, the contact angle hysteresis of this kind of surface was very low. Figure 4b-d presents SEM images of the bent area of the superhydrophobic surface at different magnifications; we observe the appearance of some cracks of various lengths. Magnified SEM images of the cracks revealed a network nanostructure (with diameters ranging from 20 to 40 nm), similar to the microstructure of unbent surfaces. Thus, both the bent and the unbent areas of the MWCNT/PDMS coatings possess binary structures and superhydrophobicity, analogous to lotus leaves.
These MWCNT/PDMS coating surfaces were very stable under extreme environmental conditions. In tests of their pressure-proof properties, the coated Al foils were pressed uniaxially (single-ended) under various pressures (from 3.56 to 32.0 ksi). We recorded SEM micrographs after each pressureproof test to study the changes in the structures of the CNTs after such harsh treatments. Figure 5a presents a representative SEM micrograph taken after performing a pressure-proof test (at a pressure of 32.0 ksi). Compared with the image of the original film (Figure 2a), the surface morphology changed such that the microislands possessed reduced diameters and heights. Nevertheless, the surface retained its branchlike nanostructures (Figure 5b). Figure 6a displays the water contact angle on the pressed superhydrophobic film that had been subjected to a pressure of 32.0 ksi. To our surprise, the droplets remained spherical with a high contact angle (166 ( 1°), and they were easily moved when tilting the surface slightly. In fact, such superhydrophobic behavior existed over the entire pressure range, as revealed in Figure 6a. We observed similar results for the coated Al foils after performing cold isostatic pressing tests. These observations indicate that the microstructures of the pressed MWCNT/PDMS coatings were sufficient to imitate the superhydrophobic character of natural self-cleaning surfaces. To best of our knowledge, these films are the first examples of pressure-proof superhydrophobic surfaces. Figure 6b displays the relationship between the water contact angles on the composite surface and the pH. We found that superhydrophobic behavior extended over the entire pH range from 1 to 14; that is, the surfaces possessed superhydrophobic properties not only for pure water but also under corrosive acidic and basic conditions. We also evaluated the effects of treatment with organic solvents and diesel oil on the wettability on the contact angles of pure water; Figure 6c reveals the contact angles of the superhydrophobic MWCNT/PDMS films upon treatment with various organic solvents and diesel oil. The contact angle of the films remained nearly unchanged after such treatment. Finally, we measured the superhydrophobicity under outdoor conditions. We adhered (twin adhesive tap) a superhydrophobic Al foil onto the roof of a car (Figure 7a) and then drove the car at various temperatures (from 23 to 33 °C), speeds (from 30 to 110 km/h), and weather conditions (sunny, cloudy, and rainy
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Figure 7. Photographs of a superhydrophobic Al foil (a) attached to the roof of a car and (b) after outdoor testing for 2 weeks.
exhibiting superhydrophobicity. These substrates were sufficiently robust that bending and pressing processes did not affect their superhydrophobicity. The as-prepared surfaces possessed high contact and very low sliding angles, allowing the rolling of water droplets and, thus, suggesting the self-cleaning ability for aqueous solutions of various values of pH. The nonfluorine superhydrophobic MWCNTs/PDMS coatings also possess marvelous environmental stability to both organic solvent treatments and outdoor tests in terms of the water CA. Our results are considered significant in terms of their importance to academic research and industrial applications. Acknowledgment. This study was supported financially by the National Science Council, Taiwan, Republic of China, under contracts NSC 98-2221-E-214-001, NSC 97-2221-E-214-013, and NSC 98-2218-E-214-001. Supporting Information Available: Video clips showing continuous roll-off of water droplets on the superhydrophobic surface. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 6. (a) Contact angle plotted with respect to the pressure applied to the film. (b) Water contact angle on the superhydrophobic surfaces plotted with respect to the pH. (c) Durability of the superhydrophobic films after treatment with organic solvents and diesel oil.
days). The films remained stable after 2 weeks under these outdoor conditions, with nearly no changes in its superhydrophobic and self-cleaning properties (Figure 7b). Indeed, these films remained stable for 1 year under ambient atmospheric conditions. Conclusions We have developed a simple and inexpensive method for the fabrication of flexible multifunctional MWCNT/PDMS coatings
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