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Fabrication of Superhydrophobic Surfaces from Binary Colloidal Assembly Gang Zhang,† Dayang Wang,*,† Zhong-Ze Gu,‡ and Helmuth Mo¨hwald† Max Planck Institute of Colloids and Interfaces, D-14424, Potsdam, Germany, and Department of Biological Science and Medical Engineering, Southeast University, 210096, Nanjing, People’s Republic of China Received May 4, 2005. In Final Form: July 8, 2005 In this work, superhydrophobic surfaces were derived from binary colloidal assemblies. CaCO3-loaded hydrogel spheres and silica or polystyrene ones were consecutively dip-coated on silicon wafers. The former assemblies were recruited as templates for the latter self-assembly. Due to the hydrophilicity difference between silicon wafers and CaCO3-loaded hydrogel spheres, the region selective localization of silica or polystyrene spheres leads to irregular binary structures with a hierarchical roughness. The subsequent modification with low surface energy molecules yields a superhydrophobic surface. The heating treatment may largely enhance the mechanical stability of the resulting binary structures, which allows regeneration of the surface superhydrophobicity, providing a good durability in practice.
Introduction The wettability of solid surfaces plays a pivotal role in nature, for example, self-cleaning surfaces of plants and insects, and in numerous industrial applications, for example, glass windows. The two crucial factors of manipulating the wettability of solid surfaces are their chemical composition and topography.1 The water contact angle (WCA) of flat hydrophobic surfaces is in the range of 90-120°, even when coated with a monolayer of perfectly close-hexagonal-packed -CF3 groups. Provided the surfaces are rough or microstructured, their maximum WCA values may exceed 150°, thus becoming superhydrophobic.2 To achieve superhydrophobicity, the surface structure can be regular or irregular. Regularly structured surfaces provide useful models for quantitatively evaluating the relationship between WCA and surface structures. From the point of view of practical applicability, irregularly structured surfaces provide the advantage of simplicity and low cost of fabrication. Recently, hierarchical regularly or irregularly structured surfaces, exhibiting roughness on various length scales, have gained increasing interest because they might theoretically make any substrates superhydrophobic despite their surface chemistry.2d,e The best example is the leaf of the sacred lotus. On the surface of the lotus leaves, one may observe cellular micron-papillae, being about 20-40 µm apart, and submicrometer wax crystals covering the entire epicuticular surface.3 Although the WCA of the wax is usually slightly above 90°, the twolength scale roughnesses and especially air enclosed between the water droplet and the wax crystals on the lotus leaves significantly reduce the solid/water interface, known as the “lotus effect”, leading to WCA higher than * Corresponding author. Fax: 49 331 5679202. E-mail: dayang@ mpikg-golm.mpg.de. † Max Planck Institute of Colloids and Interfaces. ‡ Southeast University. (1) Blossey, R. Nat. Mater. 2003, 2, 301. (2) (a) Bico, J.; Marzolin, C.; Quere, D. Europhys. Lett. 1999, 47, 220. (b) Lenz, P. Adv. Mater. 1999, 11, 1531. (c) Quere, D. Nat. Mater. 2003, 2, 457. (d) Herminghaus, S. Europhys. Lett. 2000, 52, 165. (e) Miwa, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watenabe, T. Langmuir 2000, 16, 5754. (3) Barthlott, W.; Neihuis, C. Planta 1997, 202, 1.
170°.3 Up to now, a variety of methods, such as lithographic patterning,2a,4 plasma etching,5 electrical deposition,6 phase separation,7 sol-gel synthesis,8 and electrostatic self-assembly,9 have been established to achieve the lotus effect. These existing techniques usually are complicated, expensive, and/or time-consuming. Herein, we present an alternative way to simply and inexpensively achieve superhydrophobic surfaces, constructed by hierarchical irregularly packed binary colloidal self-assembly. Monodisperse colloidal spheres usually self-assemble into highly ordered hexagonal or cubic close-packing arrays.10 Due to their long-range ordered structures, colloidal self-assemblies have been employed to pattern substrates.11 Nevertheless, there exist limited numbers of reports on exploring their surface wettability.12,13 Nakae and co-workers employed 50-500 µm steel balls as models to study the roughness influence on the surface wettability.12 Chen and co-workers most recently demonstrated that close-packed ordered arrays of monodisperse latex spheres may render the apparent WCA of their coated substrates close to 130°, independent of the sphere diameters.13 After these close-packed arrays were etched (4) O ¨ ner, D.; McCarthy, T. Langmuir 2000, 16, 7777. (5) (a) Tsujii, K.; Yamamoto, T.; Onda, T.; Shibuchi, S. Angew. Chem., Int. Ed. Engl. 1997, 36, 1011. (b) Li, H.; Wang, X.; Song, Y.; Liu, Y.; Li, Q.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2001, 40, 1743. (6) (a) Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem., Int. Ed. 2004, 43, 4338. (b) Li, M.; Zhai, J.; Liu, H.; Song, Y.; Jiang, L.; Zhu, D. J. Phys. Chem. B 2003, 107, 9954. (7) (a) Nakajima, A.; Abe, K.; Hashimoto, K.; Watanabe, T. Thin Solid Films 2000, 376, 140. (b) Minko, S.; Muller, M.; Motornov, M.; Nitschke, M.; Grundke, K.; Stamm, M. J. Am. Chem. Soc. 2003, 125, 3896. (c) Yabu, H.; Takebayashi, M.; Tanake, M.; Shimomura, M. Langmuir 2005, 21, 3235. (8) (a) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2003, 19, 5626. (b) Erbil, H. Y.; Demirel, A L.; Avci, Y.; Mert, O. Science 2003, 299, 1377. (9) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2001, 4, 1349. (10) For recent reviews, see: (a) Arsenault, A.; Fournier-Bidoz, S.; Hatton, B.; Mı´guez, H.; Te´treault, N.; Vekris, E.; Wong, S.; Yang, S.; Kitaev, V.; Ozin, G. A. J. Mater. Chem. 2004, 14, 781. (b) Wang, D.; Mo¨hwald, H. J. Mater. Chem. 2004, 14, 459. (11) For a review, see: Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 5599. (12) Nakae, H.; Inui, R.; Hirata, Y.; Saito, H. Acta Mater. 1998, 46, 2313. (13) Shiu, J.; Kuo, C.; Chen, P.; Mou, C. Chem. Mater. 2004, 16, 561.
10.1021/la0511945 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/27/2005
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Langmuir, Vol. 21, No. 20, 2005
Figure 1. Schematic illustration of the procedure of creating binary assemblies on silicon wafers by consecutively dip-coating the aqueous dispersions of CaCO3-PNIPAM particles and aqueous dispersions of silica or PS colloidal spheres.
into non-close ones by plasma, a superhydrophobic surface with a WCA of around 170° was obtained.13 Experimental Section Materials. N-Isopropyl acrylamide (NIPAM), N,N-methylenebisacrylamide (MBA), potassium persulfate (K2S2O8), calcium chloride (CaCl2), ammonium carbonate ((NH4)2CO3), (heptade-
Zhang et al. cafluoro-1,1,2,2,-tetrahydrodecyl) dimethyl chlorosilane (HFDC), and hexadecanethiol (HDT) were purchased from Sigma-Aldrich. The aqueous suspension of 296, 519 nm silica spheres and 270, 496 nm polystyrene (PS) spheres was purchased from Microparticles GmbH, Germany. NIPAM was purified by recrystallization from a toluene/hexane mixture (1:3). Other commercial materials were used without further purification. Silicon wafers were cleaned in a piranha solution, a mixture of 98% H2SO4 and 30% H2O2 with a volume ratio of 3:1. Mineralization of CaCO3 within Hydrogel Spheres. PNIPAM hydrogel spheres were prepared by a surfactant-free emulsion polymerization.14 The hydrodynamic diameter of PNIPAM spheres is 690 nm at pH 7 at room temperature, determined by dynamic light scattering (DLS). In-situ mineralization of CaCO3 within gel spheres was conducted on the basis of the method reported previously.14 In glass vials, the hydrogel spheres with or without loading of nanocrystals were well dispersed in 10 mM CaCl2 aqueous solution. The concentration of PNIPAM is 1.0 wt %. These vials were put in a closed desiccator where solid (NH4)2CO3 was placed as the CO2 vapor source. All mineralization experiments were carried out at room temperature. The resulting CaCO3-loaded poly(N-isopropylacrylamide) spheres are denoted as CaCO3-PNIPAM. Fabrication of Self-Assemblies of CaCO3-PNIPAM Composite Spheres. After being washed two times with deionized water, two-dimensional (2D) colloidal assemblies were produced on silicon wafers by dip-coating this aqueous dispersions of CaCO3-PNIPAM composite spheres (0.1-0.5 wt %) at a withdrawing speed of 1-10 µm/s. The separation distance between hydrogel spheres can be adjusted by the hydrogel spheres concentration and withdrawing speed during the dip coating. Suprehydrophobic Hierarchical Binary Structures. Figure 1 shows the procedure of constructing binary assemblies
Figure 2. (a) SEM image of the self-assembly of polydisperse 296 nm silica spheres, formed by dip-coating their 1.0 wt % aqueous dispersion at a withdrawing speed of 10 µm/s. (b) Photograph of a 5 µL water droplet residing on this silica self-assembly modified with HFDC, corresponding to a WCA of about 129°. (c) AFM image of the resulting surface and a cross-sectional profile along the line marked in the image.
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Figure 3. (a) SEM image of a 2D non-close-packing array of CaCO3-PNIPAM particles produced by dip-coating a 0.1 wt % aqueous particle dispersion on silicon wafers. (b) SEM image of binary colloidal assemblies produced by dip-coating a 1.0 wt % aqueous dispersion of 296 nm silica spheres on the non-close-packing arrays of CaCO3-PNIPAM particles. Low (c) and high (d) SEM images of binary colloidal assemblies obtained by dip-coating a 2.0 wt % aqueous dispersion of 296 nm silica spheres on a non-close-packing array of CaCO3-PNIPAM particles. The withdrawing speed used is 10 µm/s. consisting of CaCO3-PNIPAM composite particles and silica or PS ones. Binary packed structures were fabricated by dip-coating aqueous dispersions of 296 and 519 nm silica spheres and 270 and 468 nm PS spheres at a withdrawing speed of 10 µm/s. The colloidal dispersion concentrations ranged from 0.5 to 2.0 wt %, and the withdrawing speeds ranged from 10 to 20 µm/s. The surface modification of silicon wafers and those coated with silica spheres was implemented by immersing them in 1.0 wt % toluene solution of HFDC for 2 h, followed by washing two times with toluene. Prior to the modification of HDT, a 30 nm thick Au layer was deposited on the PS sphere-coated substrates. A HDT monolayer was formed on these surfaces after immersion in 2 mM ethanol solution of HDT for 15 min and then washing two times with ethanol. The surface cleaning of the uncoated and coated silicon wafers was performed via a plasma cleaner (Harrick, PDC-32G) at a pressure of 0.2 mbar and a power density of 100 W for 60 s. Characterization. DLS measurements were performed by using a commercial laser light scattering spectrometer (Malvern Autosizer 3000) with a 5 mW He-Ne laser. Scanning electron microscopy (SEM) images were recorded by means of a Gemini LEO 1550 instrument operated at 3 kV. The specimens were sputtering-coated with gold prior to SEM imaging. Atomic force microscopy (AFM) imaging was performed by using the Nanoscope Dimension 3100 system operating in tapping mode. Contact angles were measured with an optical contact angle meter at ambient temperature by a contact angle measuring system G10 apparatus (Kru¨ss, Germany). Water droplets (5 µL) were dropped carefully onto the sample surface, and the average value of five measurements, made at different positions of the same sample, was adopted as the average values of WCA of the substrates, summarized in Table 1.
Results and Discussion Self-Assembly of Silica Spheres. In our work, monolayers of silica spheres were formed on silicon wafers via
Table 1. Water Contact Angles of the Surfaces of Silicon Wafers and Those Coated with Colloidal Self-Assemblies substrate surfaces
unmodified modifieda
silicon wafer silica colloidal self-assemblies CaCO3-PNIPAM colloidal self-assemblies silica/CaCO3-PNIPAM binary assembliesb PS colloidal self-assembliesc PS/CaCO3-PNIPAM binary assembliesc
