NANO LETTERS
Stable Superhydrophobic Coatings from Polyelectrolyte Multilayers
2004 Vol. 4, No. 7 1349-1353
Lei Zhai,‡ Fevzi C¸ . Cebeci,‡,§ Robert E. Cohen,*,† and Michael F. Rubner*,‡ Departments of Chemical Engineering and Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and Department of Chemistry, Istanbul Technical UniVersity, 34469, Istanbul, Turkey Received April 9, 2004; Revised Manuscript Received May 5, 2004
ABSTRACT The present study demonstrates that the superhydrophobic behavior of the lotus leaf structure can be mimicked by creating a honeycomb-like polyelectrolyte multilayer surface overcoated with silica nanoparticles. Superhydrophobicity was achieved by coating this highly textured multilayer surface with a semifluorinated silane. The surface maintains its superhydrophobic character even after extended immersion in water. The key structural elements needed to create stable, superhydrophobic coatings from polyelectrolyte multilayers are discussed.
Many plants in nature including the lotus leaf exhibit the unusual wetting characteristic of superhydrophobicity.1,2 A superhydrophobic surface is the one that can bead off water droplets completely; such surfaces exhibit water droplet advancing contact angles of 150° or higher. In addition, the contact angle hysteresis is very low,3,4 producing a surface on which water droplets simply roll off. A self-cleaning surface results since the rolling water droplets remove dirt and debris. Nature accomplishes this fascinating effect through the use of a surface topography that presents at least two different length scales to the outside environment. The surface of the lotus leaf, for example, is textured with 3-10 micron-sized hills and valleys that are decorated with nanometer-sized particles of a hydrophobic wax-like material. The hills and valleys ensure that the surface contact area available to water is very low while the hydrophobic nanoparticles prevent penetration of water into the valleys. The net result is that water cannot wet the surface and therefore forms nearly spherical water droplets, leading to superhydrophobic surfaces. It is well known that increasing the roughness of a hydrophobic surface can increase its hydrophobicity dramatically.5-7 Two distinct wetting behaviors have been observed depending upon the nature and extent of the surface roughness. The Wenzel5,6 state describes a roughness regime in which water droplets become pinned to the surface and remain pinned, even when the film is tilted to a high angle. In this case, water is able to penetrate into the surface cavities. In contrast, the Cassie state7 describes a roughness * Corresponding authors. E-mail:
[email protected];
[email protected]. † Department of Chemical Engineering, MIT. ‡ Department of Materials Science and Engineering, MIT. § Department of Chemistry, Istanbul Technical University. 10.1021/nl049463j CCC: $27.50 Published on Web 05/18/2004
© 2004 American Chemical Society
regime in which water does not penetrate into the surface cavities. In this state, water droplets sit partially on surface air pockets and slide/roll easily when the film is tilted even only a few degrees. In both cases, it is possible to create surfaces with advancing water droplet contact angles in excess of 150°; however, only in the Cassie state does the surface also exhibit a low contact angle hysteresis (receding contact angle only a few degrees less than the advancing contact angle). Johnson and Dettre simulated the water contact angle for idealized sinusoidal hydrophobic surfaces with various roughness.8 They showed that, in the roughness regime where Wenzel’s mode is dominant, both the contact angle and contact angle hysteresis increase as the surface roughness increases. However, when the roughness factor exceeds a critical level, the contact angle continues to increase while the hysteresis starts decreasing. This decrease in hysteresis results from a change in the dominant wetting behavior from the Wenzel state to the Cassie state due to an increase of the amount of air trapped at the interface between the surface and water droplet. Thus, if stable, low hysteresis, superhydrophobic surfaces are desired, it is necessary to create the critical level of surface roughness needed to establish the Cassie state. Synthetic superhydrophobic surfaces have been fabricated through various approaches, including creating a rough surface covered with low surface energy molecules,9-16 roughening the surface of hydrophobic materials,17-19 and generating well-ordered microstructured surfaces with a small ratio of the liquid-solid contact area.3,4,20 Most of the methods disclosed to date, however, are either expensive, substrate limited, require the use of harsh chemical treatments, or cannot be easily scaled-up to create large-area
uniform coatings. Thus, many of these methods are not readily suitable for the coating of the surfaces of complex substrates such as the channels of a microfluidic device, fiber surfaces, or the intricate shapes of a bioimplant such as a stent. It is well known that the layer-by-layer processing of polyelectrolyte multilayers can be utilized to fabricate conformal thin film coatings with molecular level control over film thickness and chemistry. A coating of this type can be applied to any surface amenable to the water-based layer-by-layer (LbL) adsorption process used to construct these polyelectrolyte multilayers, including the inside surfaces of complex objects. It is therefore of interest from a both scientific and technological standpoint to determine if superhydrophobic surfaces can be created from multilayer films. Such conformable superhydrophobic surfaces would have applications as antifouling, self-cleaning and water resistant coatings and as coatings for microfluidic channels and biosensors, to name a few. Recently, Shiratori and co-workers21 disclosed the first example of a superhydrophobic coating created from polyelectrolyte multilayer films. In their work, the layer-by-layer process was used to assemble a polyelectrolyte multilayer containing SiO2 nanoparticles. The film was then heated to 650 °C to remove the polyelectrolytes and to create the surface texture needed for superhydrophobic behavior. Since then, an alternative approach to creating superhydrophobic surfaces from multilayers was reported by Zhang and coworkers.22 In this case, dendritic gold clusters were electrochemically deposited onto indium tin oxide (ITO) electrodes covered with a polyelectrolyte multilayer film; the surface showed superhydrophobicity after the deposition of a ndodecanethiol monolayer. The electrochemical deposition process used to create these films, however, limits the application of this method. We now demonstrate a relatively simple process for creating highly stable, superhydrophobic coatings from polyelectrolyte multilayer films with structures that mimic the surface structure of the lotus leaf. We previously found that multilayers assembled from poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) with the PAH dipping solution at a pH of 7.5 or 8.5 and the PAA dipping solution at a pH of 3.5 (PAH/PAA 7.5/3.5 or 8.5/3.5) formed microporous structures when treated at pH 2.4 followed by a deionized water rinse.23,24 The surface roughness of these films after treatment is below 100 nm; a roughness not sufficient to promote stable superhydrophobic behavior. We have now discovered that, by using an appropriate combination of acidic treatments, PAH/PAA 8.5/3.5 films can be induced to form pores on the order of 10 microns and a honeycomb-like structure on the surface. The surface roughness of such films can be more than 400 nm, making them ideally suited for use as the high roughness template of a superhydrophobic surface. As will be discussed, two key processing elements enable the formation of these honeycomb-like surfaces. First, it is important that the film not be rinsed with water after low pH treatment, and second, a staged low pH treatment protocol is better than a single low pH treatment. 1350
Figure 1. SEM images of (PAH/PAA)100.5 films after a single acid treatment (A) and after a combined acid treatment (B).
Figure 1A shows an SEM image of a porous PAH/PAA 8.5/3.5 film containing 100.5 assembled bilayers that was created by a single low pH treatment (6 h immersion in a pH 2.3 solution, with no water rinse). The resultant film exhibits surface pores on the order of 0.5-2 microns and an RMS surface roughness of about 100 nm. In sharp contrast, a 100.5 bilayer of PAH/PAA 8.5/3.5 porous film created by a combination of two low pH treatments (i.e., a 2 h immersion in a pH 2.7 solution followed by a 4 h immersion in a pH 2.3 solution, with no water rinse) is about 440 nm (Figure 1B). This film exhibits a honeycomb-like texture with sharp ridges and surface pores as large as 10 microns. If a water rinse (pH about 5.5) was used after these low pH treatments, the surface roughness decreases dramatically. To mimic the lotus leaf effect, it is necessary to create a surface texture with both micron- and nanoscale surface roughness. Thus, the two porous PAH/PAA 8.5/3.5 films with micron-scale surface roughness were used as templates for nanoparticle deposition. Dynamic contact angle measurements were carried out on these two different structures after each treatment step to reveal the relationship between contact Nano Lett., Vol. 4, No. 7, 2004
Figure 2. Contact angles measured from structure A and structure B. (Contact angles were measured from (1) dense films, (2) crosslinked porous films, and (3) fully treated porous films.) Advancing contact angles are presented with black bars and receding contact angles are presented with gray bars. Films with nanoparticles are marked with asterisks Scheme 1. Treatments for Forming Superhydrophobic Films from Polyelectrolyte Multilayers
angle and surface morphology/chemistry. Prior to the nanoparticle treatment and silane treatment described below, both structures were thermally cross-linked25 at 180 °C for 2 h to preserve desirable surface morphological features throughout the subsequent processing steps. Nanoscale texture was introduced by depositing 50 nm SiO2 nanoparticles onto the surfaces via alternating dipping of the substrates into an aqueous suspension of the negatively charged nanoparticles and an aqueous PAH solution, followed by a final dipping of the substrates into the nanoparticle suspension. The surfaces were then modified by a chemical vapor deposition (CVD) of (tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane (semifluorinated silane) followed by a 2 h heating at 180 °C to remove unreacted semifluorinated silane. These final steps render the entire surface hydrophobic. The entire process for forming superhydrophobic films is outlined in Scheme 1. Figure 2 shows the dynamic contact angles measured from structure A (Figure 1A) and structure B (Figure 1B) at various stages of the sequential treatment that leads to a superhydrophobic surface. Both advancing and receding contact angles were measured from (1) as-assembled dense Nano Lett., Vol. 4, No. 7, 2004
films, (2) cross-linked microporous films, and (3) fully treated microporous films, all with and without the surface bound SiO2 nanoparticles. It is evident that at each treatment step, the advancing contact angle measured from structure B is always larger than that measured from structure A, which is consistent with the predictions of both the Wenzel and Cassie models, i.e., with the same chemical composition, rougher surfaces exhibit larger advancing contact angles. The deposition of hydrophilic silica nanoparticles onto the surfaces of both cross-linked structures decreases the advancing contact angle since the surfaces were covered with more wettable hydrophilic groups. After the deposition of the semifluorinated silane and heating, all surfaces changed from hydrophilic to hydrophobic with large advancing contact angles (>120°). Superhydrophobic character (advancing contact angle > 150°) is observed from all surfaces except structure A without nanoparticles. Most importantly, however, a low contact angle hysteresis is only exhibited by structure B coated with nanoparticles. In this case, water droplets freely roll off the surface without becoming pinned, even after sitting on the surface for long times. In contrast, water droplets on the surfaces of structure A coated with nanoparticles and on the surface of structure B without nanoparticles start pinning after sitting on the surface for a couple of minutes, suggesting a transition from the Cassie state to the Wenzel state. These results show that both the microstructure created by the combined acid treatments and the nanostructure induced by the deposition of silica nanoparticles are necessary to create stable superhydrophobic surfaces. The top image in Figure 3 shows a high resolution SEM image of the final superhydrophobic surface fabricated from structure B. This image clearly shows that the nanoparticles decorate the surface of the micropores, forming a two-level structure that conceptually mimics the lotus leaf surface. The bottom image in Figure 3 shows a water droplet on this surface (advancing contact angle: 172°). The lowest angle needed to induce sliding of a 4 mg water droplet was less than 2°, which suggests a very small contact angle hysteresis with essentially no pinning of the water droplet. Remarkably, this surface remained superhydrophobic after being immersed in water for at least a week or in a high humidity environment for at least a month. This is in contrast to the superhydrophobic surface created from structure A (A3* in Figure 2), which lost its superhydrophobic character after a brief immersion in water. X-ray photoelectron spectroscopy (XPS) confirmed that the superhydrophobic surface of structure B was modified with the semifluorinated silane (Figure 4). The XPS spectrum of the cross-linked porous multilayer film with silica nanoparticles showed no detectable fluorine peaks (Figure 4A), whereas the spectrum of the semifluorinated silane-coated film displayed a strong fluorine peak at 688 ev (Figure 4B). This strong fluorine peak suggests the formation of a polymerized silane film on the surface that is thicker than a single monolayer. The silicon peaks located at 156 ev and 103 ev in Figure 4A are attributed to the silica nanoparticles deposited on the surface of the cross-linked multilayer film. 1351
Figure 3. (A) SEM image of the fully treated structure B with silica nanoparticles. (B) Water droplet on this superhydrophobic surface.
After the CVD of (tridecafluoro-1,1,2,2-tetrahydrooctyl)-1trichlorosilane and the subsequent thermal treatment, the intensity of these peaks increases due to the formation of the polymerized silane film (Figure 4B). In addition to reacting with the silica nanoparticles, the semifluorinated silane can also react with free amine groups on the surface of the cross-linked multilayer film. This was confirmed by examining the XPS spectrum of a porous cross-linked film without nanoparticles after CVD and thermal treatment: a strong fluorine peak located at 688 ev was also observed in this case. This work provides a straightforward procedure for creating stable superhydrophobic coatings from polyelectrolyte multilayers. Using this approach, it is possible to coat any substrate or object amenable to the layer-by-layer deposition process; essentially all surfaces. The procedures discussed in this communication can be optimized further. For example, with slight modifications, we are now able to create stable, superhydrophobic coatings from PAH/PAA multilayers with as few as 20 bilayers and with shorter treatment and crosslinking times (results to be published). By using thinner films, we are moving toward the possibility of creating transparent superhydrophobic coatings. The possibility of creating a superhydrophobic coating that mimics the lotus-leaf structure 1352
Figure 4. XPS spectra of structure B surface before (A) and after (B) semifluorinated silane treatment.
by simply carrying out a low pH treatment of a PAH/PAA multilayer previously assembled with all necessary ingredients (nanoparticles and hydrophobic polymer) is also under investigation. In addition, we have also found that the surface of structure B exhibits superhydrophilic behavior when coated with 3.5 bilayers of silica nanoparticles (nearly instantaneous wetting by water, contact angles less than 5°). Thus, we can use this approach to create both superhydrophobic and superhydrophilic behavior. This means that it is possible to create coatings with identical surface morphologies but with dramatically different wetting characteristics. The creation of patterned surfaces with superhydrophobic and superhydrophilic regions is currently under investigation and may lead to planar microfluidic devices. In conclusion, we have mimicked the two-level surface texture of the well-known lotus leaf by using suitably designed porous polyelectrolyte multilayers, silica nanoparticles, and a simple hydrophobic surface treatment. The net result is a stable superhydrophobic surface with high contact angles and low contact angle hysteresis. This approach provides a simple water-based process for making conformal superhydrophobic coatings. Acknowledgment. This work was supported in part by the DARPA BOSS Program and the MRSEC Program of Nano Lett., Vol. 4, No. 7, 2004
the National Science Foundation under award number DMR 02-13282. This work also made use of the Shared CMSE Experimental Facilities supported in part by the MRSEC Program of the National Science Foundation under award number DMR 02-13282. F.C. acknowledges the Istanbul Technical University President Office Grant to Support Long Term Research Activities Abroad For Researchers. Supporting Information Available: Detailed information on the preparation of the superhydrophobic surfaces from polyelectrolyte multilayers. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Neinhuis, C.; Barthlott, W. Ann. Bot. 1997, 79, 677. (2) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (3) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Oner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395. (4) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (5) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (6) Wenzel, R. N. J. Phys. Colloid Chem. 1949, 53, 1466. (7) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (8) Johnson, R. E.; Dettre, R. H. AdV. Chem. Ser. 1963, 43, 112. (9) 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. (10) 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.
Nano Lett., Vol. 4, No. 7, 2004
(11) Feng, L.; Song, Y.; Zhai, J.; Liu, B.; Xu, J.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2003, 42, 800. (12) Feng, L.; Li, S.; Li, H.; Zhai, J.; Song, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2002, 41, 1221. (13) Li, H.; Wang, X.; Song, Y.; Liu, Y.; Li, Q.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2001, 40, 1743. (14) Thieme, M.; Frenzel, R.; Schmidt, S.; Simon, F.; Hennig, A.; Worch, H.; Lunkwitz, K.; Scharnweber, D. AdV. Eng. Mater. 2001, 3, 691. (15) Youngblood, J. P.; McCarthy, T. J. Macromolecules 1999, 32, 6800. (16) Tadanaga, K.; Morinaga, J.; Matsuda, A.; Minami, T. Chem. Mater. 2000, 12, 590. (17) Morra, M.; Occhiello, E.; Garbassi, F. Langmuir 1989, 5, 872. (18) Morra, M.; Occhiello, E.; Garbassi, F. J. Colloid Inerface Sci. 1989, 132, 504. (19) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377. (20) Shiu, J.-Y.; Kuo, C.-W.; Chen, P.; Mou, C.-Y. Chem. Mater. 2004, 16, 561. (21) Soeno, T.; Inokuchi, K.; Shiratori, S. Trans. Mater. Res. Soc. Jpn. 2003, 28, 1207. (22) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z.; Jiang, L.; Li, X. J. Am. Chem. Soc. 2004, 126, 3064. (23) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017. (24) Hiller, J.; Mendelsohn, J. D.; Rubner, M. F. Nature Materials 2002, 11, 59. (25) Dai, J.; Jensen, A. W.; Mohanty, D. K.; Erndt, J.; Bruening, M. L. Langmuir 2001, 17, 931.
NL049463J
1353
