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Stable Biomimetic Superhydrophobic Surfaces Fabricated by Polymer Replication Method from Hierarchically Structured Surfaces of Al Templates Yuwon Lee, Kuk-Youn Ju, and Jin-Kyu Lee* Department of Chemistry, Seoul National University, Seoul 151-747, Korea Received May 21, 2010. Revised Manuscript Received July 20, 2010 We have developed a simple, efficient, and highly reproducible method to fabricate the large-area biomimetic superhydrophobic polymer surfaces having hierarchical structures of micrometer-sized irregular steps and nanometersized fibrils. Commercial Al plates (99.0%) were etched using Beck’s dislocation etchant (mixture of HCl and HF) for different time periods in order to alter the structure of the etched Al surfaces from micrometer-sized to highly rough nanometer-sized irregular steps. These hierarchical structures could be easily replicated onto the surface of various thermoplastic polymer plates from the etched Al templates by applying heat and pressure; many polymer replicas without any significant deviations from each other could be duplicated from the same etched Al master templates. All of thermoplastic polymer replicas having hierarchical structures exhibited superhydrophobic properties with water contact angles of larger than 150°. Especially, the surfaces of the high-density polyethylene (HDPE) replicas having nanometersized curled strands exhibited superhydrophobicity with a static water contact angle of ∼160° and a sliding angle of less than 2°. These superhydrophobic HDPE replicas having nanometer-sized curled strands showed excellent stability after being exposed to various organic solvents and aqueous solutions of various pH.
1. Introduction In our day-to-day lives, we come across different types of natural and manmade objects with various types of surfaces.1 Consequently, considerable research has been performed to identify the phenomena that occur on these surfaces and to use these surfaces more effectively. Superhydrophobicity is commonly found in biological systems such as the leaves of plants,2,3 the feathers of birds,4,5 or the wings and legs of insects.5-8 These systems have superhydrophobic surfaces;the static contact angles of water on the surface is higher than 150° and their contact angle hysteresis is lower than 5°;and can undergo selfcleaning that facilitates the removal of dust and pollutants by water drops rolling off the surface; this self-cleaning mechanism is commonly known as the “lotus effect”.2,3 Thus, superhydrophobic surfaces can easily and completely protect themselves from various external contaminants without requiring any other specific cleaning mechanism. This property is governed by both the chemical composition of the surface material and the hierarchical structure comprised of nanostructures within the micrometerscale areas in the surface. Recently, the generation of superhydrophobic surfaces has become a topic of interest to scientists because of their importance in fundamental research9,10 and the biomimesis of these surfaces5 for technical and industrial applications. Artificial superhydrophobic surfaces have been produced *To whom correspondence should be addressed: e-mail
[email protected]; Fax þ82-2-882-1080; Tel þ82-2-879-2923.
(1) Blossey, R. Nature Mater. 2003, 2, 301. (2) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (3) Koch, K.; Bhushanb, B.; Barthlott, W. Soft Matter 2008, 4, 1943. (4) Bormashenko, E.; Bormashenko, Y.; Stein, T.; Whyman, G.; Bormashenko, E. J. Colloid Interface Sci. 2007, 311, 212. (5) Xia, F.; Jiang, L. Adv. Mater. 2008, 20, 2842. (6) Wagner, T.; Neinhuis, C.; Barthlott, W. Acta Zool. 2004, 20, 7665. (7) Lee, W.; Jin, M.-K.; Yoo, W. C.; Lee, J.-K. Langmuir 2004, 20, 7665. (8) Gao, X.; Jiang, L. Nature 2004, 432, 36. (9) de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827. € (10) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777.
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by altering the chemical composition of various surface materials (molecules,11,12 polymers,7,13-15 and metals and their oxides and alloys12,16-18) and by enhancing the roughness of the hierarchical structures using various techniques such as photholithography,10 soft lithography,19 self-assembly coating,11,20 plasma etching,21 and chemical etching.16-18 Although several types of artificial superhydrophobic surfaces have been fabricated using different approaches, most of these fabrication processes are complex, expensive, and time-consuming and yielding disposable surperhydrophobic surfaces,10,11,19,21 and the resistance of these surfaces to various contaminants, organic solvents, and a wide ranges of corrosive aqueous solutions of various pH values, which is essential to ascertain the real applications of the self-cleaning effect,22 has not been thoroughly investigated. Recently, we reported that Al templates with hierarchical structures, which were deliberately prepared by a technique that combines photolithography and anodization of Al, and duplicated polymer surfaces from them could be precisely controlled on the micrometer and nanometer scale.23 Measurement of the contact angle (11) Genzer, J.; Efimenko, K. Science 2000, 290, 2130. (12) Larmour, I. A.; Saunders, G. C.; Bell, S. E. J. Angew. Chem., Int. Ed. 2008, 47, 5043. (13) 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. (14) Han, W.; Wu, D.; Ming, W.; Niemantsverdriet, H.; Th€une, P. C. Langmuir 2006, 22, 7956. (15) Cho, W. K.; Choi, I. S. Adv. Funct. Mater. 2008, 18, 1089. (16) Guo, Z.; Zhou, F.; Hao, J.; Liu, W. J. Am. Chem. Soc. 2005, 127, 15670. (17) Qu, M.; Zhang, B.; Song, S.; Chen, L.; Zhang, J.; Cao, X. Adv. Funct. Mater. 2007, 17, 593. (18) Larmour, I. A.; Bell, S. E. J.; Saunders, G. C. Angew. Chem., Int. Ed. 2007, 46, 1710. (19) Jeong, H. E.; Lee, S. H.; Kim, J. K.; Suh, K. Y. Langmuir 2006, 22, 1640. (20) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349. (21) Mundo, R. D.; Palumbo, F.; d’Agostino, R. Langmuir 2008, 24, 5044. (22) Yan, H.; Kurogi, K.; Mayama, H.; Tsujii, K. Angew. Chem., Int. Ed. 2005, 44, 3453. (23) Lee, Y.; Park, S.-H.; Kim, K.-B.; Lee, J.-K. Adv. Mater. 2007, 19, 2330.
Published on Web 08/10/2010
DOI: 10.1021/la102057p
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of water droplets on these surfaces clearly revealed the cooperative effect of the micrometer- and nanometer-sized constituents of the hierarchical structures, even though the fabrication method is not simple and very time-consuming, and the large area surface cannot be easily obtained through this method. In this paper, as a completion of our previous results of the cooperative effect for the practical applications, we report a simple, highly efficient, and highly reproducible process for the fabrication of large-area biomimetic superhydrophobic polymeric surfaces that have hierarchical structures of micrometer-sized irregular steps and nanometer-sized curled strands, by employing the conventional wet etching method. Wet etching is basically a controlled corrosion process resulting from the electrolytic reaction between surface areas having different potentials and the etchant solution; this process was used to study the characteristics of the composition, structure, and properties of materials until about half a century ago.24 The wet etching method using several etchant solutions is now being utilized to produce rough metal surfaces with tunable wettability.16,17 We employed a Beck’s dislocation etchant to modify an Al surface; this is a well-known etching model that was used to analyze the plasticity, strength, and structure of grain boundaries in addition to metallographic information in the 1930s.24,25 The hierarchical structures on the Al surface, which were generated by wet etching using a Beck’s dislocation etchant, were easily replicated onto the surface of various thermoplastic polymer plates such as high-density polyethylene (HDPE), poly(ε-caprolactone) (PCL), polypropylene (PP), and polystyrene (PS) by using the etched Al surface as a master template for polymer replication under heat and pressure (Figure 1).7,23 Moreover, many thermoplastic polymer replicas without any significant deviations from each other could be duplicated from the same etched Al master template because replicas could be easily stripped from their templates by hand. All of the thermoplastic polymer replicas having hierarchical structures exhibited superhydrophobic properties with water contact angles of larger than 150°. In particular, the hierarchically structured surfaces of the HDPE replica exhibit excellent superhydrophobicity and have a high static water contact angle of ∼160° and a contact angle hysteresis of less than 2°. These polymer replicas with hierarchically structured surfaces showed excellent stability and durability when exposed to various organic solvents and a wide range of aqueous solutions of various pH values.
2. Experimental Section 2.1. Al Etching. Surface etching of commercial Al sheets (99.0%; area: 2 cm 3 cm; thickness: 3 mm) was carried out by immersing them in Beck’s dislocation etchant at room temperature for different etching times (10, 20, 30, 40, 50, and 60 s) to obtain nanometer- and micrometer-sized structures onto their surface. The Beck’s dislocation etchant is a mixture of HCl (37 wt %; Korea, Samchun), HF (50 wt %; J.T. Baker), and distilled water (HCl:HF:H2O = 28.1:2.2:69.7; weight ratio). Subsequently, the etched Al templates were carefully washed with distilled water and dried in an oven for 2 h at 120 °C. The Al sheets were then etched at low and high temperatures (0 and 70 °C) to confirm the effect of temperature on the etching process. Commercial Al sheets and high-purity Al sheets (99.999%; area: 1.5 cm 1.5 cm; thickness: 0.5 mm; Aldrich) were etched with either a 28.1 wt % aqueous HCl solution or a 2.2 wt % aqueous HF solution at various etching conditions to investigate the effects (24) Vander Voort, G. F. Metallography: Principles and Practice; McGraw-Hill: New York, 1984. (25) Qian, B.; Shen, Z. Langmuir 2005, 21, 9007.
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Figure 1. Schematic illustration of the fabrication of polymer replicas having various surface structures from etched Al templates with micrometer-sized irregular steps and nanometer-sized hierarchical structures by using the heat- and pressure-driven polymer replication method. of each component of Beck’s dislocation etchant on the Al etching process. 2.2. Polymer Replication. Various thermoplastic polymer plates (area: 2 cm 3 cm; thickness: 1.5 mm; typical Mw: 125 000 (HDPE), 65 000 (PCL), 190 000 (PP), and 192 000 (PS); Aldrich) were placed on top of the replication master templates, and hierarchical structures were easily replicated by applying heat and pressure (desired temperature (100-180 °C), ca. 12 kPa) on a hot plate for 10 min. Micrometer-sized irregular steps, nanometer-sized curled strands, and their mixed structures were generated on the thermoplastic polymer plates. After release of pressure and gradual cooling of the samples to room temperature, the polymer replicas were easily stripped from their templates by hand. The replication process could be carried out many times using the same replication master template.
2.3. Stability and Durability Tests for Superhydrophobic HDPE Replicas with Nanometer-Sized Curled Strands. Superhydrophobic HDPE replicas with nanometer-sized curled strands fabricated using Al templates etched for 60 s were treated for 5 days in various organic solvents (hexane, petroleum ether, toluene, chlorobenzene, methylene chloride, chloroform, tetrahydrofuran, ethanol, acetone, and methanol), the table water, and aqueous solutions of various pH (1-13). Subsequently, all of the treated superhydrophobic HDPE replicas were carefully blowdried using N2 gas, and their static water contact angles were measured. Furthermore, water droplets with different pH were placed on the superhydrophobic HDPE replicas by the sessile drop method. 2.4. Self-Cleaning Effect. Activated carbon powders (Aldrich) were uniformly spread on the surface of the superhydrophobic HDPE replica with nanometer-sized curled strands replicated from Al template etched for 60 s. Water was then added dropwise to the surface using a syringe. The activated carbon powders Langmuir 2010, 26(17), 14103–14110
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Figure 2. FE-SEM images of the Al master templates etched for various lengths of times (a, 10 s; b, 20 s; c, 30 s; d, 40 s; e, 50 s; f, 60 s) by using Beck’s dislocation etchant; insets show low-magnification images (2K). were removed by the water droplets on the surface of the superhydrophobic HDPE replica. 2.5. Characterizations. The topography of the etched Al templates and the thermoplastic polymer replicas was characterized using an FE-SEM (Hitachi S-4300). The water CAs of the thermoplastic polymer replicas were measured using a goniometer (Phoenix 300) under ambient conditions. The average water contact angle for each sample was obtained by measuring the CAs of the same sample at nine different positions and then averaging the resultant values. The self-cleaning property of the superhydrophobic HDPE replicas with nanometer-sized curled strands was measured using a digital camera (Olympus C-8080WZ).
3. Results and Discussion 3.1. Preparation of Etched Al Templates and Investigation of the Formation Mechanism of Surface Structures. Prior to the etching process, commercial Al sheets (99.0%; area: 2 cm 3 cm; thickness: 3 mm) were washed with acetone and deionized water to remove dust particles and organic compound. The pretreated Al sheets were then carefully immersed in the etchant solution comprising hydrochloric acid (HCl), hydrofluoric acid (HF), and deionized water (HCl:HF:H2O = 28.1:2.2:69.7; weight ratio), and Al etching was performed at room temperature for different lengths of time such as 10, 20, 30, 40, 50, and 60 s. After completion of the Al etching process, the etched Al templates were washed with deionized water several times, blow-dried with N2 gas, and then stored for future use as Al templates for polymer replication. The topographical structures of the flat and etched Al templates were analyzed using a field-emission scanning electron microscope (FE-SEM). Figure S1 shows the FE-SEM image of the surface of an untreated Al sheet (see Supporting Information). It can be seen that this sheet has a smooth and substantially flat structure over the entire surface. Treatment with an etchant for different lengths of time made the flat surface of the commercial Al sheets change to one with rough micrometer- to nanometer-sized structures, as shown in Figure 2. In the early stages of the Al etching process, i.e., within 10-20 s, micrometer-sized irregular steps were formed (Figure 2a,b), which gradually disappeared with prolonged etching (after 30-40 s) to yield nanometer-sized grooves (Figure 2c,d). The surface of the etched Al template completely transformed into one with a rough nanometer-size hierarchical structure upon further increase in the etching time (>50 s) (Figure 2e,f). The etching process usually involves attacking of the exposed defect sites on the Al surface and penetration of the etchant Langmuir 2010, 26(17), 14103–14110
through the imperfections in boundary of the Al crystal.24 This should lead to the Al surface having an almost unvarying structures, regardless of the etching time. If the etching had commenced at the defect sites and proceeded through the crystalline layers, the Al surface would have changed from one with rough nanometer-sized structures to one with larger micrometersized irregular steps. However, contrary to our initial expectations, the topography of the etched Al surfaces changed from micrometer-sized irregular steps to rough nanometer-sized structures as etching time increased. In order to understand this atypical phenomenon, the precise effect of temperature on the structure of the etched surface was investigated. Figure S2 shows the temperature change during an etching reaction. Although etching was commenced at room temperature (∼30 °C), the temperature of the solution near the Al surface was increased to ∼100 °C after 90 s. Therefore, it was assumed that the micrometer-sized irregular steps might be generated during the initial stages of the etching process. This could be because the etching reaction occurs selectively in the crystalline layers at room temperature, while it occurs more vigorously at some of the defect sites, including those that are at the edges of the steps after the reaction temperature is increased. The etching process was carried out at different temperatures, and the surface structures of the resulting Al templates were investigated. Micrometer-sized irregular steps were generated when the Al sheet was etched for 5 s at a low temperature (∼0 °C) in an ice bath (Figure 3, left), while nanometer-sized surface structures were generated when the Al sheet was etched for 5 s at a high temperature (70 °C) in a water bath (Figure 3, right). Furthermore, it was also observed that the conversion of the micrometer-sized irregular steps to nanometer-sized surface structures at a constant low temperature (0 °C) could be prevented, despite etching for a long time (>30 s). Similarly, the nanometer-sized surface structure could be preserved by carrying out etching at a high reaction temperature (70 °C), regardless of the reaction time (from 1 to 10 s). The nanometer-sized surface structures produced at a high temperature could easily be transformed to the micrometer-sized irregular steps by etching the surface at low temperatures for a short time. Similarly, the micrometer-sized irregular steps generated by low-temperature etching could also be easily reverted to the previous nanometer-sized surface structures by carrying out etching at high temperatures for a short time (Figure 3). Therefore, the transformation of the surface structure of the etched Al surface from the micrometer- to nanometer-sized structure and DOI: 10.1021/la102057p
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vice versa is reversible and can be performed repeatedly by simply controlling the reaction temperature. We also investigated the effect of each component in Beck’s dislocation etchant, which is an aqueous mixture solution of HCl and HF (HCl:HF:H2O = 28.1:2.2:69.7 ; weight ratio).24,25 Commercial Al sheets were etched with either a 28.1 wt % aqueous HCl solution or a 2.2 wt % aqueous HF solution, under the same etching conditions (room temperature, reaction time: 20 s; Figure S3). When using aqueous HCl solution, the surface of the Al sheet was not uniformly etched over the entire area of the Al sheet; i.e., etching was effective only in some specific portions (Figures S3 and S4). Furthermore, the time required for complete etching of surface of the Al sheet was very long, i.e., 120 s (Figure S4). On the other hand, the Al sheet was hardly etched by using the aqueous HF solution (Figure S3). Therefore, it can be assumed that in Beck’s dislocation etchant HF only promotes the initial etching of the Si impurity sites in low-purity Al (99.0%), where Si, which is homogeneously distributed on the entire surface of Al, is usually alloyed to enhance the mechanical properties of Al. This assumption is further supported by the experimental results, according to which the use of Beck’s dislocation etchant on high-purity Al (99.999%) (Figure S5) results in no uniform etching, dissimilar to the case of low-purity Al; further, this etchant generates several selectively etched spots on the Al surface similar to those observed on an Al surface etched using an aqueous HCl solution. Even when we employed lower purity
Figure 3. Effect of temperature on the Al etching process using Beck’s dislocation etchant. The topographic surface of the etched Al template is dependent on the Al etching reaction temperature. These FE-SEM images are etched Al templates having micrometer-size irregular steps (left) nanometer-sized curled strands (right). The transformation of the surface structure from the former to the latter and vice versa is reversible and can be performed by controlling the reaction temperature.
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(150°) and low CA hysteresis (157° and sliding angle
