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Langmuir 2006, 22, 9676-9681
Biomimetic Creation of Hierarchical Surface Structures by Combining Colloidal Self-Assembly and Au Sputter Deposition Yonghao Xiu,†,‡ Lingbo Zhu,†,‡ Dennis W. Hess,*,† and C. P. Wong*,‡ Departments of Chemical and Biomolecular Engineering and of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst DriVe, Atlanta, Georgia 30332-0245 ReceiVed June 13, 2006. In Final Form: August 18, 2006
Surfaces of hexagonally packed silica spheres have been functionalized with silanes containing different hydrocarbon or fluorocarbon chains. The resulting chemical and physical structures were studied to establish the effect of surface hydrophobicity on the measured contact angles on the rough surfaces. The results were used to assess the effects of surface modifications on the parameters in the Cassie equation. To achieve superhydrophobicity via a biomimetic approach, we created two-scale structures by first forming hexagonally packed SiO2 spheres, followed by Au deposition on the spheres and heat treatment to form Au nanoparticles on sphere surfaces. Contact angles over 160° were achieved. This work provides improved understanding of the effect of the surface roughness and solid surface fraction on superhydrophobicity.
1. Introduction
The Cassie equation for a heterogeneous (porous) surface
Research into the creation of superhydrophobic surfaces, especially with respect to mimicking the two-scale structure of lotus leaves, has received much attention. Recent studies suggest that topography at two length scales may not be necessary for superhydrophobicity. That is, even with only one nanoscale-size feature, if the solid surface fraction (relative to that of air) is very small, and the surface contact angle is smaller than 90°, superhydrophobicity can still be effectively achieved.1 However, such surfaces are not in a thermodynamically stable state. After extended times, water can still penetrate inside the nanostructures, indicating that surface hydrophobicity is required to achieve a stable state. With only micrometer-scale-size features, the superhydrophobicity can also be achieved on a macroscopic scale. However, when dealing with a microdroplet of water, it is not quite effective.2 In contrast, nature has designed lotus leaves with hierarchical length scales, thereby supplying reproducibility, requisite mechanical properties, and geometrical optimization for a nonfouling and self-cleaning surface. Also the biomimetic method of study from nature is a successful way of developing different applications of superhydrophobicity.3 The Wenzel equation describes the effect of the surface roughness on the water droplet contact angle:4,5
cos θA ) f1 cos θ1 + f1 - 1
cos θA ) r cos θ1
(1)
where θA is the apparent contact angle on the rough surface, r is the ratio of the actual solid/liquid contact area to its vertical projected area, and θ1 is the contact angle on a flat surface per Young’s equation. * To whom correspondence should be addressed. (C.P.W.) E-mail:
[email protected]. Phone: (404) 894-8391. (D.W.H.) E-mail:
[email protected]. Phone: (404) 894-5922. † Department of Chemical and Biomolecular Engineering. ‡ Department of Materials Science and Engineering. (1) Hosono, E.; et al. J. Am. Chem. Soc. 2005, 127 (39), 13458-13459. (2) Wier, K. A.; McCarthy, T. J. Langmuir 2006, 22 (6), 2433-2436. (3) Zhai, L.; et al. Nano Lett. 2006, 6 (6), 1213-1217. (4) Wenzel, R. N. Ind. Eng. Chem. 1936, 28 (8), 988-994. (5) Wenzel, R. N. J. Phys. Colloid Chem. 1949, 53, 1466-1467.
(2)
indicates that6 when the solid surface fraction f1 is decreased, the apparent contact angle can be increased. At the limit when f1 approaches zero, the surface contact angle approaches 180°. Considerable effort has been expended to create hierarchically structured surfaces that mimick the surface structure of lotus leaves. For example, superhydrophobic surfaces with hierarchical structures can be formed by a number of techniques, including the layer by layer process,7,8 phase-separation micromolding methods using soluble polymers,9 binary colloidal assemblies,10 and sol-gel silica,11 micellar solution casting of propyleneMMA diblock copolymer,12 polystyrene film preparation by the electrohydrodynamics method,13 carbon nanotube pattern generation,14,15 laser etching of poly(dimethylsiloxane) (PDMS) surfaces,16 galvanic cell corrosion of a copper foil with aqueous phosphorus acid solution,17 nanosphere lithography,18 adsorption of charged submicrometer polystyrene latex particles,19 etc. The relationship between surface structure and the contact angle and hysteresis was also widely studied.2,20,21 Despite the extensive work reported, we are not aware of studies that illustrate a clear relationship between superhydrophobicity and the surface twoscale roughness. (6) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546-551. (7) Jisr, R. M. H.; Rmaile, H.; Schlenoff, J. B. Angew. Chem., Int. Ed. 2005, 44 (5), 782-785. (8) Zhai, L.; et al. Nano Lett. 2004, 4 (7), 1349-1353. (9) Vogelaar, L.; Lammertink, R. G. H.; Wessling, M. Langmuir 2006, 22 (7), 3125-3130. (10) Zhang, G.; et al. Langmuir 2005, 21 (20), 9143-9148. (11) Shang, H. M.; et al. Thin Solid Films 2005, 472 (1-2), 37-43. (12) Xie, Q.; et al. AdV. Mater. (Weinheim, Ger.) 2004, 16 (20), 1830-1833. (13) Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem., Int. Ed. 2004, 43 (33), 43384341. (14) Lau, K. K. S.; et al. Nano Lett. 2003, 3 (12), 1701-1705. (15) Zhu, L.; et al. Langmuir 2005, 21 (24), 11208-11212. (16) Jin, M.; et al. Macromol. Rapid Commun. 2005, 26 (22), 1805-1809. (17) Li, Y.; Shi, G. J. Phys. Chem. B 2005, 109 (50), 23787-23793. (18) Shiu, J. Y.; et al. Chem. Mater. 2004, 16 (4), 561-564. (19) Takeshita, N.; et al. Langmuir 2004, 20 (19), 8131-8136. (20) Gao, L. C.; McCarthy, T. J. Langmuir 2006, 22 (7), 2966-2967. (21) Gao, L.; McCarthy, T. J. Langmuir 2006, 22 (14), 5998-6000.
10.1021/la061698i CCC: $33.50 © 2006 American Chemical Society Published on Web 10/05/2006
Hierarchical Surface Structure Biomimetic Creation
Colloidal crystals can be formed on a substrate from monodisperse nanospheres, e.g., SiO2 spheres and polystyrene spheres, as a result of assembly into arrays of hexagonal closepacked or face-centered cubic (fcc) layers. In this scenario, the as-formed surface is rough due to the regular arrangement of the monodisperse spheres. Although this surface is not superhydrophobic even after a surface treatment with hydrophobic surface modification agents,18 superhydrophobicity can be achieved by imparting nanoscale structures on/in the surface to achieve multiscale roughness. Furthermore, this surface can serve as an ideal model surface for the investigation of the effect of the surface roughness on superhydrophobicity. In this paper, we use monodisperse silica particles to form a thin layer on a glass slide. After deposition of a Au layer on top of the spheres followed by heat treatment to form Au nanoparticles, surface nanoroughness was generated. By controlling the Au deposition conditions, the necessary nanoroughness can be effectively designed into the surface. Such tunability of the structures furthers the understanding of the effect of the surface nanostructure on superhydrophobicity and suggests specific ways by which superhydrophobic surface properties such as mechanical stability and durability can be achieved. 2. Experimental Section 2.1. Materials. All solvents and chemicals were reagent grade and were filtered through 0.22 µm syringe filters (Gelman) except for tetraethyloxysilane (TEOS; 99%, Aldrich), which was distilled under vacuum conditions before use. Absolute ethanol was obtained from VWR, and 29% ammonium hydroxide was purchased from Fisher. Ultrapure water (18.2 MΩ cm-1) was used directly from a Milli-Q water system. Microscope slides (75 × 25 × 1 mm, Fisher) were cut in half lengthwise and used as substrates, cleaned in piranha solution (2:1 (v/v) mixture of 96% sulfuric acid and 30% aqueous hydrogen peroxide) at 80 °C for 30 min, and subsequently rinsed extensively with deionized water and ethanol. The surface hydrophobicity modification agent trichloro(1H,1H,2H,2H-perfluorooctyl)silane (PFOS) was purchased from Aldrich. 2.2. Colloidal Silica Nanosphere Synthesis. Monodisperse silica nanospheres were synthesized following the Sto¨ber-Fink-Bohn method.22 Nanospheres with diameters ranging from 200 to 700 nm and a relative standard deviation of 150° with hysteresis of
