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Fabrication of Superhydrophobic Surfaces with Hierarchical Structure through a Solution-Immersion Process on Copper and Galvanized Iron Substrates Wenguo Xu,*,† Hongqin Liu,† Shixiang Lu,† Jinming Xi,‡ and Yanbin Wang§ Institute for Chemical Physics, School of Science, Beijing Institute of Technology, Beijing 100081, P. R. China, Center of Molecular Sciences, Institute of the Chemistry Chinese Academy of Sciences, Beijing 100080, P. R. China, and Institute of Industrial Product Inspection, Chinese Academy of Inspection and Quarantine, Beijing 100025, P. R. China ReceiVed February 26, 2008. ReVised Manuscript ReceiVed July 10, 2008 Superhydrophobic surfaces were obtained on copper and galvanized iron substrates by means of a simple solutionimmersion process: immersing the clean metal substrates into a methanol solution of hydrolyzed 1H,1H,2H,2Hperfluorooctyltrichlorosilane (CF3(CF2)5(CH2)2SiCl3, FOTMS) for 3-4 days at room temperature and then heated at 130 °C in air for 1 h. Both of the resulting surfaces have a high water contact angle (CA) of larger than 150.0° as well as a small sliding angle (SA) of less than 5°. The formation and structure of the superhydrophobic surfaces were characterized by means of scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectrometry (EDX). SEM images showed that both of the resulting surfaces exhibited special hierarchical structure. The special hierarchical structure along with the low surface energy leads to the high surface superhydrophobicity.
Introduction The idea of superhydrophobicity was introduced six decades ago by A. Cassie, who was interested in enhancement of water repellency.1,2 This amazing water repellency has been used in the textile industry ever since. Due to potential applications in a variety of technological areas such as microfluidic devices, textile industries, and possibly anti-icing applications, superhydrophobic surfaces have attracted extensive interest in the past decade.3,4 As is well known, the wettability of a solid surface is governed by both the chemical composition and the geometrical microstructure of the surface.5 If a surface is rough or microtextured with a low interfacial free energy, the CA of water can reach almost 180° and the surface will remain unwetted.6 For example, the well-known lotus leaf has a water CA larger than 161.0° and a SA of about 2°,1,7-9 which accounts for its unique self-cleaning properties. As a natural model for the design of synthetic superhydrophobic films, lotus leaves have been studied extensively.3,9 Numerous studies have confirmed that the hierarchical micro- and nanostructure along with a low surface energy material, the wax layer, causes the surface of lotus leaves to have a very high CA, a low SA, and the self-cleaning effect.10 * To whom correspondence should be addressed. Phone: +86 10 68913125. Fax: +86 10 68912631. E-mail:
[email protected]. † Beijing Institute of Technology. ‡ Institute of the Chemistry Chinese Academy of Sciences. § Chinese Academy of Inspection and Quarantine.
(1) Cassie, A. B. D.; Baxter, S. Nature (London) 1945, 155, 21. (2) Michielsen, S.; Lee, H. J. Langmuir 2007, 23, 6004–6010. (3) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1–8. (4) Wu, X.; Shi, G. J. Phys. Chem. B 2006, 110, 11247–11252. (5) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988–994. (6) Lafuma, A.; Que´re´, D. Nat. Mater. 2003, 2, 457–460. (7) Neinhuis, C.; Barthlott, W. Ann. Bot. 1997, 79, 667–677. (8) Zhu, Y.; Zhang, J. C.; Zheng, Y. M.; Huang, Z. B.; Feng, L.; Jiang, L. AdV. Funct. Mater. 2006, 16, 568–574. (9) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. AdV. Mater. 2002, 14, 1857–1860. (10) Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132, 31–41.
In general, a superhydrophobic surface can be prepared by combination of reducing the surface energy and enhancing the surface roughness. Thus far, many different techniques have been used to fabricate artificial superhydrophobic surfaces with these unusual characteristics such as self-cleaning, anticorrosion, etc.; many methods have been developed to fabricate superhydrophobic surfaces such as laser etching,11 plasma etching,12 physical and chemical vapor deposition,13 anodic oxidation,14 electrochemical reaction and deposition,15 sol-gel processing,16 electrospinning,17 layer-by-layer deposition,18 etc. However, many of these methods have certain limitations such as severe conditions, complex process control, special equipment, and poor durability. In addition, among recently reported results, metallic superhydrophobic surfaces have attracted a great deal of interest because of their technological importance and easy control of morphology with a variety of fabrication methods.19 Herein, we report on a new simple method for fabrication of superhydrophobic films on copper and galvanized iron substrates. Among the various materials used for fabrication of selfassembled films, alkylsilane is promising for practical applications because of its marked mechanical and chemical stability, which (11) (a) Jin, M. H.; Feng, X. J.; Xi, J. M.; Zhai, J.; Cho, K. W.; Feng, L.; Jiang, L. Macromol. Rapid Commun. 2005, 26, 1805–1809. (b) Song, X. Y.; Zhai, J.; Wang, Y. L.; Jiang, L. J. Phys. Chem. B 2005, 109, 4048–4052. (12) (a) Shiu, J.-Y.; Kuo, C.-W.; Chen, P. L. Proc, SPIE-Int. Soc. Opt. Eng. 2005, 5648, 325–332. (b) Teshima, K.; Sugimura, H.; Inoue, Y.; Takai, O.; Takano, A. Appl. Surf. Sci. 2005, 244, 619–622. (13) Huang, L.; Lau, S. P.; Yang, H. Y.; Leong, E. S. P.; Yu, S. F.; Prawer, S. J. Phys. Chem. B 2005, 109, 7746–7748. (14) (a) Narita, M.; Kasuga, T.; Kiyotani, A. J. Jpn. Inst. Light. Met. 2000, 50, 594–597. (b) Thieme, M.; Frenzel, R.; Schmidt, S.; Simon, F.; Hennig, A.; Worch, H.; Lunkwitz, K.; Scharnweber, D. AdV. Eng. Mater. 2001, 3, 691–695. (15) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z. Q. J. Am. Chem. Soc. 2004, 126, 3064–3065. (16) Feng, X. J.; Feng, L.; Jin, M. H.; Zhai, J.; Jiang, L.; Zhu, D. B. J. Am. Chem. Soc. 2004, 126, 62–63. (17) Ma, M.; Hill, R. M.; Lowery, J. L.; Fridrikh, S. V.; Rutledge, G. C. Langmuir 2005, 21, 5549–5554. (18) Shi, F.; Wang, Z. Q.; Zhang, X. AdV. Mater. 2005, 17, 1005–1009. (19) Bormashenko, D.; Stein, T.; Whyman, G.; Bormashenko, Y.; Pogreb, R. Langmuir 2006, 22, 9982.
10.1021/la800613d CCC: $40.75 2008 American Chemical Society Published on Web 09/06/2008
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Figure 1. SEM images of the thin film on copper substrate prepared with different immersion times for (a) 2 and (b) 3 days. (c) Magnified image of the thin film from b. (d) High-magnification image of a microsphere from c showing its structure.
is attributed to strong immobilization through siloxane bonding.20 Furthermore, the self-assembly technique for alkylsilane is one of the efficient strategies to introduce low surface energy compounds on the solid surface, which can effectively enhance surface hydrophobicity.21 There have been some reports about realization of superhydrophobicity through cooperation of alkylsilane self-assembly with surface roughening.22 However, in the present paper, we obtained superhydrophobic surfaces through a simple solution-immersion process by utilizing the reaction between the hydrolyzed solution of FOTMS (the fluorinated analog of alkylsilane) and metallic surfaces in which both surface roughness and low surface energy were obtained.
Experimental Section Materials and Sample Preparation. Copper specimens (copper, 90%; zinc, 10%; Tianjin Kermel Chemical Reagent Co., China) with a size of 1.0 cm × 1.0 cm × 0.01 cm were cleaned ultrasonically with ethanol (99.5%, AR, Beijing Fine Chemical. Co. LTD, China) and distilled water. After that, the specimens were first dried at ambient temperature and then at 80 °C in air. After drying they were washed ultrasonically with 13 wt % HNO3 (65%-68%, AR, Beijing Fine Chemical. Co. LTD, China) for 15 min; subsequently, the specimens were rinsed with distilled water and dried at ambient temperature and then 80 °C in air. (20) (a) Fadeev, A. Y.; Helmy, R.; Marcinko, S. Langmuir 2002, 18, 7521– 7529. (b) Sugimura, H.; Hozumi, A.; Kameyama, T.; Takai, O. Surf. Interface Anal. 2002, 34, 550–554. (c) McArthur, E. A.; Ye, T.; Cross, J. P.; Petoud, S.; Borguet, E. J. Am. Chem. Soc. 2004, 126, 2260–2261. (21) (a) Fadeev, A. Y.; McCarthy, T. J. Langmuir 2000, 16, 7268–7274. (b) Genzer, J.; Efimenko, K. Science 2000, 290, 2130–2133. (22) (a) Nakajima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 7044– 7047. (b) Teshima, K.; Sugimura, H.; Inoue, Y.; Takai, O.; Takano, A. Langmuir 2003, 19, 10624–10627. (c) Sun, T.; Wang, G.; Liu, H.; Feng, L.; Jiang, L.; Zhu, D. J. Am. Chem. Soc. 2003, 125, 14996–14997.
Galvanized iron specimens (Tianjin Kermel Chemical Reagent Co., China) with a size of 1.0 cm × 1.0 cm × 0.01 cm were cleaned ultrasonically with ethanol and distilled water. After that, the specimens were first dried at ambient temperature and then at 80 °C in air. Subsequently, a methanol (99.7%, AR, Beijing Fine Chemical. Co. LTD, China) solution of FOTMS (Sigma-Aldrich, USA) was hydrolyzed by addition of a 3-fold molar excess of water at room temperature. Then the dried metallic specimens were immersed in the hydrolyzed FOTMS solution for 3-4 days at room temperature. The immersed specimens were rinsed with distilled water and finally heated at 130 °C in air for 1 h. Evaluation of the Stability of the Surface to Different Aqueous Solutions and Organic Solvents. The prepared films on copper substrates were immersed in a sodium hydroxide solution of pH 13, a hydrochloric acid solution of pH 2, and toluene for 5 days at 25 °C and then dried at 130 °C in air for 1 h. The CA was measured for the dried films. Surface Characterization. The surface morphology was observed using a SEM (X650, Hitachi, Japan), and the corresponding element distributions of the surface were determined by EDX. The elemental and chemical compositions of the superhydrophobic surface were also analyzed by XPS, which was carried out on a PHI 5300 X-ray photoelectron spectrometer (Physical Electronics, USA), using 250 W Mg KR (hν )1253.6 eV) X-ray as the excitation source. The XPS spectra were collected in a constant analyzer energy mode at a chamber pressure of 10-7 Pa and pass energy of 44.75 at 0.1 eV/ step. The binding energy of contaminated carbon (C1s 284.6 eV) was used as the reference. The resolution for the measurement of the binding energy is about (0.2 eV. The infrared spectra were obtained as KBr pellets using Nicolet FTIR Nexus 870 infrared spectrometer. The sessile drop method was used for CA measurements using a Dataphysics OCA20 CA system at ambient temperature. The average CA value was obtained by measuring more than five different positions for the same sample.
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Figure 2. Possible reactions between the oxidized copper surface and hydrolyzed FOTMS (M represents Cu).
Results and Discussion Figure 1 shows SEM images of the prepared film on copper substrate. We carefully monitored this solution-immersion process. When the immersion time is short (2 days), one can see that the microscale hillocks and fine nanogrooves form a special hierarchical structure on the resulting surface (Figure 1a). Upon increasing the immersion time to 3 days, it is clearly shown that the resulting surface is densely covered by many inerratic microspheres with an average diameter of about 1.5-3 µm (Figure 1b and 1c). Seen from the magnified SEM images (Figure 1c and 1d), it is very interesting that submicroscopic structures, the uniformly dispersed nanoparticles with an average diameter of around 50 nm on the surface of microspheres, are also clearly observed in the resulting surface. That is, a hierarchical structure, containing micro- and nanoscale structure, is formed on copper substrate. Moreover, this special structure formed on copper substrate is somewhat similar to that of lotus leaf. It is thought that this morphology may give rise to the superhydrophobic behavior. The formation mechanism of this special structure can be explained by the following discussions: first, we find the pH value of the hydrolyzed FOTMS solution is about 1-2. As chlorosilanes are rapidly hydrolyzed by water and form silanols23
CF3(CF2)5(CH2)2SiCl3+3H2O f CF3(CF2)5(CH2)2Si(OH)3 + 3HCl (1) it is believed that the acidic environment of the solution is mainly attributed to hydrochloric acid. In addition, it is known that copper is readily oxidized in air or under humid conditions at room temperature.24,25 In the absence of any added oxidant, O2 is (23) Van Ooij, W. J.; Zhang, C.; Zhang Jun, Q. Proc. Symp. AdV. Corros. Protect. Org. Coatings 1998, 97. (24) (a) Lo´pez-Delgado, A.; Cano, E.; Bastidas, J. M.; Lo´pez, F. A. J. Mater. Sci. 2001, 36, 5203. (b) Carbonell, L.; Ratchev, P.; Caluwaerts, R.; Van Hove, M.; Verlinden, B.; Maex, K. Microelectron. Eng. 2002, 64, 63–71. (25) Mora, N.; Cano, E.; Mora, E. M.; Bastidas, J. M. Biomaterials 2002, 23, 667.
necessary for oxidation of copper24,26 and copper can be naturally oxidized by dissolved oxygen in ethanol solution. Thus, the hydrolyzed FOTMS solution provides an acidic environment for copper oxidization, and the acid may catalyze this oxidization process.24 Thus, in the presence of hydrochloric acid, the spontaneous oxidation reaction can be accelerated. Then interfacial condensation reactions may occur between hydroxyl groups (-Cu-OH) present on the oxidized copper surface and silanols (-Si-OH) to form a covalent bond.27 From Figure 2a it is observed that except for the condensation reaction between the oxidized copper surface and silanol, the horizontal condensation reaction among silanols will also occur, resulting in formation of the film. However, not all of the silanols can occur in the condensation reaction, some hydroxyl groups of silanols (-Si-OH) will exist on the film surface (Figure 2b). Meanwhile, the surface also can induce vertical polymerization to form grafted polysiloxane (Figure 2c). In addition, the selfpolymerization competition reaction among the hydrolyzed FOTMS is also likely to occur. In a word, above-mentioned factors will debase the degree of molecular order of the film.21a,22a Perhaps it leads to the different size of microspheres. The formation mechanism of the structure on the substrate can be identified with FTIR, XPS, and EDX measurements. The Fourier transform IR (FTIR) spectra are shown in Figure 3. The wide band appearing at 3409.3 cm-1 corresponds to the stretching vibration of coordinated -OH, which exists on the film surface (Figure 2b). In addition, the double band at about 1220.5 cm-1 corresponds to the stretching vibration of C-F. The band at 1066.0 cm-1 can be assigned to the stretching vibration of Si-O. The weak band near 942 cm-1 is likely contributed by the -Si-O-Cu- vibration,28 further suggesting that the coating films are covalently bonded to Cu (Figure 2). The formation mechanism can further be identified with XPS measurements. Figure 4a shows the XPS spectra of the copper (26) (a) Wen, X. G.; Xie, Y. T.; Choi, C. L.; Wan, K. C.; Li, X. Y.; Yang, S. H. Langmuir 2005, 21, 4729. (b) Wang, S. T.; Feng, L.; Jiang, L. AdV. Mater. 2006, 18, 767–770. (27) Subramanian, V.; Vnooij, W. J. Surf. Eng. 1999, 15, 1–5. (28) Li, Y. S.; Vecchio, N. E. Spectrochim. Acta, Part A 2007, 67, 598–603.
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Figure 3. FTIR spectra of the prepared film on copper substrate.
element for the superhydrophobic film. The peaks located at 942.6 (Cu2p3/2) and 963.2 eV (Cu2p1/2) are attributed to the Cu2p region of metallic copper.29 The other two new peaks located at 951.2 (Cu2p3/2) and 970.2 eV (Cu2p1/2), which can be implied to be compositional change on copper substrate before and after immersion,29b,30 should be assigned to the Cu element in -Si-O-Cu-. From Figure 4b the F1s peaks at 696.0 and 696.3 eV can be assigned to the F element in -CF3 and -CF2, respectively. From Figure 4c it is seen that the O1s profile is asymmetric, implying that at least two types of oxygen species are present in the near surface region. The peaks at 539.7 eV can be indexed to the O in Si-O, whereas the weaker shoulder absorbance at about 539.3 eV may be due to oxygen in the surface hydroxyl groups. In addition, according to eq 1, the -Si-Cl bond in FOTMS is easily hydrolyzed, but this hydrolysis reaction is incomplete. Thus, the Cl2p peak at 206.7 eV (Figure 4d) can be assigned to the Cl element in the remaining unhydrolyzed -Si-Cl bond. The EDX spectrum of the distributed elements on the treated copper surface, shown in Figure 5, reveals that the surface mainly consists of C, O, F, Si, Cl, Cu, and Zn elements with a ratio of 8.25:5.28:13.87:1.00:0.88:2.38:0.27. It indicates that after treatment the surface is mainly composed of the hydrophobic part (-(CH2)2(CF2)5CF3) of the hydrolyzed FOTMS, which has very low surface free energy. It is well known that fluorine is the most effective element for lowering the surface free energy because it has a small atomic radius and the biggest electronegativity among all atoms; so, it forms a stable covalent bond with carbon, resulting in a surface with low surface energy.31 However, even a flat surface with the lowest surface free energy, that of closest hexagonal packed -CF3 groups, shows a water CA of only 119°,32 far from superhydrophobicity. The situation is quite different when the surface is rough, and micro/nanostructures are essential for superhydrophobicity.33 Regarding the surface morphology shown in Figure 1, the resulting surface exhibits a special hierarchical structure similar to that of lotus leaf, which is rough enough to trap air, along with the low surface free energy, leads to superhydrophobicity. The surface wettability of the prepared film on copper substrate has been studied by CA measurements. As shown in Figure 6a, (29) (a) Przepiorski, J.; Morawski, A. W.; Oya, A. Chem. Mater. 2003, 15, 862–865. (b) Guo, Z. G.; Fang, J.; Wang, L. B.; Liu, W. M. Thin Solid Films 2007, 515, 7190–7194. (30) Lamprecht, E.; Watkins, G. M.; Brown, M. E. Thermochim. Acta 2006, 446, 91–100. (31) Shang, H. M.; Wang, Y.; Limmer, S. J. Thin Solid Films 2005, 472, 37–43. (32) (a) Nishino, T.; Meguro, M.; Nakamae, K.; Matsushita, M.; Ueda, Y. Langmuir 1999, 5, 4321. (b) Murase, H.; Fujibayashi, T. Prog. Org. Coat. 1997, 31, 97–104. (33) Sun, T.; Feng, L.; Gao, X.; Jiang, L. Acc. Chem. Res. 2005, 38, 644.
Figure 4. XPS spectra for the superhydrophobic film on copper substrate: (a) Cu region, (b) F region, (c) O region, (d) Cl region.
the water CAs of the substrates vary quite significantly depending upon the immersion time. When copper was immersed for 3 days, the water CA reaches a maximum value of about 155.3° (Figure 6b) as well as a small SA of about 3° (Figure 6c), implying that this method can be used for preparation of a superhydrophobic surface. The high water CA and the low SA indicated that a water droplet was hardly able to stick to the surface, allowing droplets to roll off quite easily. The low SA value, which reflects the difference between the advancing and reducing angles,
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Figure 7. (a) Shape of a water droplet (2 µL) on the resulting surface of galvanized iron substrate. (b) Sliding water droplet (3 µL) on a surface tilted at 4°.
Figure 5. EDX spectra of the distributed elements for the superhydrophobic film on copper substrate.
Figure 6. (a) Dynamic water CA measurements on the surfaces of the duration of the solution-immersion process at room temperature. (b) Shape of a water droplet (3 µL) on the resulting surface. (c) Sliding water droplet (3 µL) on a surface tilted at 3°.
provides further evidence for the superhydrophobicity of this surface. The wetting behavior of the resulting surface can be expressed by the Cassie and Baxter equation34
cos θr ) f1 cos θ - f2
(2)
Here θr and θ represent the water CAs on rough and smooth surfaces, respectively; f1 and f2 are the fractions of the solid surface and air in contact with water, respectively. Equation 2 has been recently modified to account for the local surface roughness on the wetted area as follows35,36
cos θr ) rf f cos θ + f - 1
(3)
where f is the fraction of the projected area of the solid surface wetted by water (thus, f2 )1 - f ) and rf is the surface roughness of the wetted area. For the surface mainly composed of FOTMS (θ > 90° for a smooth FOTMS surface), since rf >1 for a rough surface, which, in comparison with a smooth wetted area, can (34) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546–561. (35) Marmur, A. Langmuir 2003, 19, 8343. (36) Michielsen, S.; Lee, H. J. Langmuir 2007, 23, 6004.
enhance the surface hydrophobicity. Clearly, the prepared films mainly composed of FOTMS with the special hierarchical structure, consisting of microspheres and nanoparticles, will have even higher values of rf. Once rf reaches a certain level, air may become trapped within the interstices of the microspheres underneath a water droplet, which would further enhance the surface hydrophobicity and lead to the high surface superhydrophobicity. For the investigation on the feasibility of this simple strategy in the case of other metallic substrates, galvanized iron was examined. It was found that the superhydrophobic surface could also be obtained easily. As shown in Figure 7, it exhibited superhydrophobicity with a water CA of about 159.0° (Figure 7a) as well as a small SA of about 4° (Figure 7b). Figure 8 shows SEM images of the prepared film on galvanized iron substrate. One can see that the resulting surface exhibits honeycomb-like architectures, which are composed of uniformly dispersed pits with a width of about 0.2 µm. Furthermore, the beautiful flowerlike submicroscopic structures over the whole substrate, which can enhance the surface roughness, are also clearly observed (Figure 8a and 8b). As seen from the magnified SEM image (Figure 8b), the special flower-like structure is composed of nanosheets, which are about 10 nm thick, 100-200 nm wide. This observation shows that the micro- and nanoscale hierarchical structure is also formed on galvanized iron substrate. As is well known there are large numbers of dislocation defects in common crystalline metals. These dislocation sites, due to possessing relatively high energy, are prone to destroy, and thus, when attacked by chemical etchants, they would be dissolved first. When a crystal face is exposed to a dislocation etchant, pits are often formed on the surface.37 According to ref 21, hydrochloric acid can be used not only as a macrostructure etchant but also as a dislocation etchant of Zn. Clearly, the uniformly dispersed pits are formed by etching of the grains along the boundaries. However, the formation mechanism of the flowerlike structure is not very clear. Thus, the zinc surface of galvanized iron may first be etched with hydrochloric acid in the hydrolyzed FOTMS solution and then the etched surfaces are modified by silanols through the siloxane covalent bond.38 The EDX spectrum (Figure s1, Supporting Information) of the distributed elements on the treated galvanized iron substrate reveals that the surface mainly consists of C, O, F, Si, Cl, and Zn elements with a ratio of 7.83:7.48:13.69:1.00:1.06:3.89. Above EDX result supports the idea that hydrophobic groups have been successfully introduced to the metal surface. In the same way, the special hierarchical structure along with the slow surface energy leads to the surface high superhydrophobicity. In addition, the environmental stability and durability of the superhydrophobic surface on copper substrate have been (37) (a) Vander Voort, G. F. Metallography: Principles and Practice; McGrawHill: New York, 1984. (b) Gilman, J. J.; Johnston, W. G.; Sears, G. W. J. Appl. Phys. 1958, 29, 747–754. (38) Qian, B. T.; Shen, Z. Q. Langmuir 2005, 21, 9007–9009.
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Figure 8. SEM images of the thin film on galvanized iron substrate: (a) overview of the film and (b) high-magnification image of the flowerlike submicroscopic structure.
investigated. After several months of storage in air, the value of the CA did not change, which indicated that the superhydrophobicity of the hierarchical structures is stable in air. At the same time, after being placed in a sodium hydroxide solution of pH 13, a hydrochloric acid solution of pH 2 and organic solvent (toluene) for 5 days at 25 °C, the CA also almost had no change.
Conclusions In summary, the stable superhydrophobic surface on copper substrate with the special hierarchical structure, consisting of microspheres and nanoparticles, has been successfully fabricated through a simple and effective solution-immersion process. It is very significant that the superhydrophobicity is well maintained in acidic and basic solutions as well as in some organic solvents. This method may offer a simple and convenient strategy for fabricating superhydrophobic surface on other metal substrate. We tested galvanized iron substrate and the same effect is observed. The micro- and nanoscale hierarchical structure is also formed on galvanized iron substrate. The resulting surface
exhibits honeycomb-like architectures composed of uniformly dispersed pits, and over the whole substrate there are also beautiful flower-like submicroscopic structures, which are composed of nanosheets. Therefore, the method holds great prospect for constructing micro- and nanoscale hierarchical structures endowed with unique properties, such as resistance to corrosion and selfcleaning, for practical applications in various fields. Further work will focus on the detailed formation mechanism and on the extension of this technique to other substrates. Acknowledgment. We gratefully thank the National Natural Science Foundation of China (No. 20773014/B030202) and the 111 Project B07012 of China for their support of this work. Supporting Information Available: EDX spectrum of the distributed elements for the superhydrophobic film on galvanized iron substrate. This material is available free of charge via the Internet at http://pubs.acs.org. LA800613D
