pubs.acs.org/Langmuir © 2009 American Chemical Society
Fabrication of Biomimetic Superhydrophobic Surface on Engineering Materials by a Simple Electroless Galvanic Deposition Method Xianghui Xu,†,‡ Zhaozhu Zhang,*,† and Jin Yang†,‡ †
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China and ‡Graduate School, Chinese Academy of Sciences, Beijing 100039, PR China Received August 20, 2009. Revised Manuscript Received November 22, 2009 We have reported an easy means in this paper to imitate the “lotus leaf” by constructing a superhydrophobic surface through a process combining both electroless galvanic deposition and self-assembly of n-octadecanethiol. Superhydrophobicity with a static water contact angle of about 169 ( 2° and a sliding angle of 0 ( 2° was achieved. Both the surface chemical compositions and morphological structures were analyzed. We have obtained a feather-like surface structure, and the thickness of the Ag film is about 10-30 μm. The stability of the superhydrophobic surface was tested under the following three conditions: (1) pH value from 1 to 13; (2) after freezing treatment at -20 °C; (3) at ambient temperature. It shows a notable stability in that the contact angle of the sample still remained higher than 150° in different conditions. It can be concluded that our approach can provide an alternative way to fabricate stable superhydrophobic materials.
Introduction In nature, there exist some amazing superhydrophobic living organisms, such as lotus leaves1,2 and water-strider legs.3,4 Enlightened by these phenomena, biomimetic technology is increasingly drawing more and more attention. Recently, superhydrophobic surfaces, which have a contact angle (CA) bigger than 150°, have drawn much attention from both fundamental research and practical applications.5-8 Superhydrophobic surfaces exhibit self-cleaning, anticorrosive, and antipolluting characteristics. So far, how to fabricate a superhydrophobic surface can be mainly reduced to the following two methods: either create a rough structure on a hydrophobic material surface or modify the rough surface with a special low surface energy material.9-13 Many methods have been used to fabricate a lotus-like surface, *Corresponding author. Fax: 86-931-4968098. E-mail address: zzzhang@ lzb.ac.cn(Z.-Z. Zhang).
(1) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (2) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (3) Wei, P. J.; Chen, S. C.; Lin, J. F. Langmuir 2009, 25, 1526–1528. (4) Gao, X. F.; Jiang, L. Nature 2004, 432, 36. (5) Chundera, A.; Etcheverryb, K.; Londec, G.; Chob, H. J.; Zhaia, L. Colloids Surf., A 2009, 333, 187–193. (6) Guo, Z. G.; Fang, J.; Wang, L. B.; Liu, W. M. Thin Solid Films 2007, 515, 7190–7194. (7) Larmour, I. A.; Saunders, G. C.; Bell, S. E. J. New J. Chem. 2008, 32, 1215– 1220. (8) Srinivasan, S.; Praveen, V. K.; Philip, R.; Ajayaghosh, A. Angew. Chem., Int. Ed. 2008, 47, 5750–5754. (9) Qu, M. N.; Zhang, B. W.; Song, S. Y.; Chen, L.; Zhang, J. Y.; Cao, X. P. Adv. Funct. Mater. 2007, 17, 593–596. (10) Zhang, W. X.; Wen, X. G.; Yang, S. H. Inorg. Chem. 2003, 42, 5005–5014. (11) Nicolas, M.; Guittard, F.; Gribaldi, S. Angew. Chem., Int. Ed. 2006, 45, 2251–2254. (12) Zhao, N.; Weng, L. H.; Zhang, X. Y.; Xie, Q. D.; Zhang, X. L.; Xu, J. ChemPhysChem. 2006, 7, 824–827. (13) Wang, S. T.; Jiang, L. Adv. Mater. 2007, 19, 3423–3424. (14) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Chabrol, G.; Paul, C. C. Adv. Mater. 2004, 16, 1929–1932. (15) Zhao, N.; Xu, J.; Xie, Q. D.; Weng, L. H.; Guo, X. L.; Zhang, X. L.; Shi, L. H. Macromol. Rapid Commun. 2005, 26, 1075–1080. (16) Fresnais, J.; Chapel, J. P.; Poncin-Epaillard, F. Surf. Coat. Technol. 2006, 200, 5296–5305.
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such as electrochemical reaction and deposition,14 phase separation,15 plasma etching and polymerization,16 etching and lithography,17 sol-gel,18 LBL and colloidal assembly.19 Luo et al.20 have fabricated bionic poly(tetrafluoroethylene)/poly(phenylene sulfide) (PTFE/PPS) superhydrophobic coatings with a porous network. Jiang et al.21 have developed a novel means for mimicking nature by constructing a bionic superhydrophobic surface through a solution-immersion process, which is a simple and effective complement to traditional approaches and can be easily applied to other metals. Acatay et al.22 have developed tunable, superhydrophobically stable polymeric surfaces by electrospinning, which has three levels of roughness with the higher CA. Compared with the methods described above, electroless galvanic deposition is an environmentally friendly, efficient, and low-cost method for introducing double roughness on metal substrates. Larmour et al.23 have used the electroless galvanic deposition to coat a metal substrate with a textured layer of a second metal, after modification by fluorine-containing materials; they obtained a high CA of approximate 173°. A novel kind of miniature wire boat was made from superhydrophobic copper meshes by electroless deposition and then treated with n-dodecanoic acid.24 Due to the superhydrophobic property, the boats can float freely on a water surface. Learning from this, we want to deposit a second metal on copper and self-assemble a film of cheap, low surface energy material to replace the expensive fluorine-containing materials. In this research, we report a facile way to obtain a superhydrophobic surface by electroless galvanic deposition of nano (17) Qian, B. T.; She, Z. Q. Langmuir 2005, 21, 9007–9009. (18) Wu, X. D.; Zheng, L. J.; Wu, D. Langmuir 2005, 21, 2665–2667. (19) L. Zhai, F. C.; Cebeci, R. E.; Cohen, M. F. Nano. Lett. 2004, 4, 1349–1353. (20) Luo, Z.; Zhang, Z.; Hu, L.; Liu, W.; Guo, Z.; Zhang, H.; Wang, W. Adv. Mater. 2008, 20, 970–974. (21) Wang, S. T,; Feng, L.; Jian, L. Adv. Mater. 2006, 18, 767–770. (22) Acatay, K.; Simsek, E.; Ow-Yang, C.; Menceloglu, Y. Z. Angew. Chem., Int. Ed. 2004, 43, 5210–5213. (23) Larmour, I. A.; Bell, S. E. J.; Saunders, G. C. Angew. Chem., Int. Ed. 2007, 46, 1710–1712. (24) Pan, Q. M.; Wang, M ACS Appl. Mater. Interfaces 2009, 1, 420–423.
Published on Web 12/11/2009
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Ag film on metal substrate and then prepare a self-assembled monolayer (SAM) of n-octadecanethiol. The stable Ag-S bonding formed between the nano Ag and the thiol.25 The as-prepared surface shows an extreme nonwetting property. It is stable at different pH values from 1 to13. After the superhydrophobic sample was kept at -20 °C for one week, the contact angles are still larger than 150°. The method used in our paper can broaden the application of superhydrophobic materials in different conditions.
Experimental Section Copper plate sized 3.5 cm 1 cm 0.8 cm (width length thickness) was polished. The substrate surface was rinsed with acetone and deionized water, respectively. The operation is as follows: 0.1 g silver nitrate was dissolved in 30 mL deionized water. The substrates were dipped into the as-prepared solution (0.020 mol dm-3) for 10, 20, 30, or 60 s, depending on the experiment (see figure captions for specific deposition time). The substrate was then washed with a great deal of deionized water and dried in air. Finally, the substrates were dipped into ethanol solution of n-octadecanethiol (0.010 mol dm-3) for 5 min in order to selfassemble a film of low surface energy material. The plates were dried in a flow of nitrogen. The morphological structures of the as-prepared surface were examined by field emission scanning electron microscopy (FESEM, JSM-6701F). The microstructure is determined by an X-ray diffractometer (XRD) (Philips Corp., The Netherlands) operating with Cu KR radiation at a continuous scanning mode and ω angle of 1.0°. The surface chemical compositions of the asprepared sample were analyzed by VGESCALAB210 X-ray photoelectron spectroscopy (XPS). The sessile drop method was used for water contact angle measurements with a CA-A contact angle meter (Kyowa Scientific Company, Ltd., Japan) at ambient temperature. Water droplets (about 5 μL) were dropped carefully onto the surface. The average CA value was determined by measuring five times at different positions of the same sample.
Results and Discussion Contact Angle Measurements. The surface wettability of the as-prepared substrates has been studied by CA measurements. The plate after deposition of Ag for 30 s shows a high contact angle (CA) of 169 ( 2°. Water droplets are hardly able to stick to the surface, as indicated by a sliding angle of around 0 ( 2°on the surface, allowing droplets to roll off quite easily. On one of the asprepared samples, the water droplet was bouncing on the surface and finally rolled off quickly without any adhesion (the impact velocity of the water droplet is around 0.88 m s-1), which is an ideal model of self-cleaning. What is really important is that these plates were put flat on the test bed without any tilting. The rolling process of the water droplet on a horizontal superhydrophobic substrate was shown in Figure 1a. We also measured the static CAs of droplets after impact as a function of the impact velocity by taking droplets of 5 μL as the probe liquid for the surface of deposited Ag for 30 and 60 s, respectively. The results are shown in Figure 1b. It is seen that, after impact with 0.88 and 1.25 m/s, the advancing CAs slightly decreased. However, they still remained higher than 150°, and the droplets could easily slide on this surface. It may result from the surface chemical compositions and morphological structures of the substrate, which will be discussed later. X-ray Diffraction (XRD). The nonwetting property of the as-prepared substrates results from both the surface chemical (25) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; et al. J. Am. Chem. Soc. 1989, 11, 321–335.
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Figure 1. (a) Rolling process of the water droplet on a horizontal superhydrophobic film (the surface deposited Ag for 30 s, the mean time interval is about 0.03s). (b) Impact effect on the contact angle for water droplets of 5 μL falling at a height of about 40 mm and 80 mm (impact velocities are 0.88 and 1.25 m/s) impacting the superhydrophobic surface. (The x-axis label refers to surfaces prepared by deposition of Ag for 30 and 60 s.)
Figure 2. XRD spectra of copper plate before and after deposition of Ag for 30 s.
compositions and morphological structures. The surface chemical compositions are analyzed by XRD and XPS. Figure 2 is the product’s XRD pattern. Comparing the XRD pattern of the copper before and after surface deposition of Ag for 30 s, we see DOI: 10.1021/la9031128
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Figure 3. XPS spectra after surface deposition of Ag for 30 s: (a) survey spectra, (b) XPS Ag 3d spectra, (c) XPS S 2p spectra.
Figure 4. FESEM images of the cross section of the sample after deposition of Ag for 30 and 60 s.
four new peaks appearing in the region 35-80°. The four new diffraction peaks are indexed as (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes of face-centered cubic (fcc) Ag (a = 4.09, JCPDS no. 4-783), which are consistent with the XRD results of previous works,26,27 indicating that there was a film of Ag deposited on the copper surface. X-ray Photoelectron Spectroscopy (XPS). The XPS analysis shown in Figure 3. The Ag peaks shown in Figure 3b, appearing at 368.1 eV and 374.1 eV, which demonstrated that a film of Ag deposited on the copper substrate. The peaks located in 162.3 eV and 163.5 eV are attributed to the S of the n-octadecanethiol (Figure 3c). The strong interactions between the S atom in n-octadecanethiol and the Ag atom deposited on the copper plate obey the principle of hard and soft acids and bases and thus form a stable Ag-S bond, finally producing a stable monolayer on top of the copper plate.28 The -SH group is combined with the Ag atom on the surface and the hydrophobic long alkyl chain pointed outside. Field Emission Scanning Electron Microscopy (FESEM). The topography of the Cu surface, electroless galvanic deposited film of Ag, was examined by FESEM. Figure 4 shows the image of a cross section of the superhydrophobic surface with deposited Ag for 30 and 60 s. The average Ag film thickness changed as the deposition time increased. The average Ag film thickness is about 10.03 and 23.09 μm for 30 and 60 s, respectively. It is clearly shown in Figure 5c that the Ag film deposited for 30 s has a feather-like (26) Sun, Y.; Xia, Y. Adv. Mater. 2002, 14, 833–837. (27) Tang, S.; Meng, X.; Lu, H.; Zhu, S. Mater. Chem. Phys. 2009, 116, 464–468. (28) Dubios, L. H.; Nuzzo, R. G. Synthesis, Annu. Rev. Phys. Chem. 1992, 43, 437–463.
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Figure 5. Effect of the deposition time on the morphology of the substrate after deposition of Ag for (a) 10 s, (b) 20 s, (c) 30 s, and (d) 60 s; (e,f) larger views of c and d; contact angle shown in inset picture.
structure. We can see from the image that there is a trunk formed by nanosized Ag particles. All the branches grow sideways from the trunk; forming a parallel and periodic array, which implies that the silver crystals grow along preferred directions. Each branch seems to be a replica of the main trunk. Figure 5e shows the larger magnification of the Ag “feather”. It is obvious that the Ag particles grown on both the trunk and branch are within 100 nm. It is due to the surface roughness that the water droplet can easily bounce on this surface and finally roll off. The threephase contact lines (air-liquid-solid) on the surface are contorted and unstable, which can trap sufficient air to prevent water from intruding into the spaces. The trapped air formed a film of Langmuir 2010, 26(5), 3654–3658
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air cushion, which can keep the water droplet away from the surface, and finally results in large contact angles. The formation of the dendritic silver nanostructure can be explained as a process of initial reduction-nucleationadsorption-growth-branching-growth. It is suggested that the dendritic structures result from oriented attachment of the silver nanoparticles.29 The silver particles attach to one another and grow onto the copper plate. The shape of silver nanostructures is controlled by the strength of reducing agents in chemical reduction systems. For example, isotropic silver particles are obtained with a slow reductant, while fast reductants induce anisotropic growth with the appearance of dendrites.30 In this systerm, the copper plate is a relatively fast reductant, which is a key to obtaining the dendrites nanostructures. Deposition time is another important factor to obtain Ag film with different morphology (Figure 5). After depositing Ag for 10 s, there are nanoparticles grown as the “feather” array, but without clear “feathers”, shown in Figure 5a. Figure 5b shows the morphology of the surface deposited for 20 s, the particle become bigger than the sample deposited for 10 s. It looks like a coralline structure. The surface structures of samples deposited for 30 s (Figure 5c) and 60 s (Figure 5d) have already formed the “feather” figure, but the differences are subtle, what are shown in Figure 5e and f are the enlarged pictures of Figure 5c and d. The branch of the sample treated for 30 s is formed by nanoparticles in an array (50-100 nm). However, the branch of the sample deposited for 60 s is of straight claviform on which small papillae are dispersed. The special surface structure plays an important part in the superhydrophobic property. The CA of the sample varied as the deposition time changed, which was shown in the inset image of Figure 5. From the picture of CA, we can see that depositing Ag for 30 s is the optimal condition to obtain a superhydrophobic surface with the biggest contact angle value. Chemical Stability of the Superhydrophobic Surface. Defrosting in a refrigeration system or solving outdoor anti-icing problems of materials in cold winter requires that superhydrophobic surfaces at low temperature have certain stability. Under the low-temperature conditions, once the vapor meets the hydrophilic surface, it will spread and frost and ice up, and it is difficult to remove them. When the vapor meets a superhydrophobic surface, it will condense into water droplets, which makes the surface contact area relatively small. Under such conditions, even if the vapor ices up, it will remain spherical. When reaching a certain scale, it is easier to remove the ice under gravity conditions. So, the chemical stability of the prepared surface maintained at -20 °C was researched. First, water droplets were dropped carefully onto the superhydrophobic surfaces. Then, these samples were maintained at -20 °C for one week. After one week, the samples were taken out to room temperature. The water contact angle was measured and shown in Figure 6a. It can be seen that the CAs change over a small range as freezing times increase, and all CAs are higher than 150°. We can see from the images (inset in Figure 6) of the sample after being held at -20 °C for 168 h that the water droplet froze but was still in the shape of a whole sphere. When the sample was taken out to room temperature, the frozen water droplet thawed but still remained as a whole sphere (inset image). Additionally, the CAs of a water droplet with pH from 1 to 13 on the prepared substrates are measured and shown in Figure 6b. The CA is almost unchanged over a wide range of pH values. Such an observation indicates that the substrate has good stability. This (29) Mdluli, P. S.; Revaprasadu, N. Mater. Lett. 2009, 63, 447–450. (30) Tang, S.; Meng, X.; Lu, H.; Zhu, S. Mater. Chem. Phys. 2009, 116, 464–468.
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Figure 6. Variation of the apparent contact angle of the as-prepared superhydrophobic surface with different freezing times at -20 °C (a) and contact angles of water droplets with different pH values on the prepared superhydrophobic surface (b). Inset picture shows the frozen water droplet on the surface. (The surface was prepared by deposition of Ag for 30 s.)
Figure 7. S 2p spectra of the surface Ag (30 s deposition) that was stored at ambient conditions for one week and one month.
is due to the intrinsic chemical stability of thiol and the bonding between the -SH group and the Ag atom. Stability of Ag-Thiol SAMs at Ambient Conditions. Several research groups31,32 have argued that the gold-thiolate bond is easily and rapidly oxidized under ambient conditions, adversely affecting SAM quality and structure. In this study, we discussed the stability of Ag-thiol SAMs at ambient conditions. The sample analyzed by XPS in Figure 3 was after storage at ambient conditions for one week, and there is no oxidized sulfur peak located at 166.5 eV as described in previous literature.33 The XPS of the sample deposition of Ag for 30 s that was stored at ambient conditions for one month, which was shown in Figure 7, (31) Schoenfisch, M. H.; Pemberton, J. E. J. Am. Chem. Soc. 1998, 120, 4502– 4513. (32) Willey, T. M.; Vance, A. L.; van Buren, T.; Bostedt, C.; Terminello, L. J.; Fadley, C. S. Surf. Sci. 2005, 576, 188–196. (33) Lee, M. T.; Hsueh, C. C.; Freund, M. S.; Ferguson, G. S. Langmuir 1998, 14, 6419.
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still has no oxidized sulfur peak. Tarlov and Newman34 suggested that no sulfonate species were detected with static secondary ion mass spectrometry from any SAMs formed in air saturated thiol/ ethanol adsorbate solutions when analyzed immediately following removal from solution. In fact, no evidence for oxidation of SAMs stored in such solutions for periods up to 2 months was observed, suggesting that any sulfonates formed are continuously displaced by thiol molecules. Accroding to the statement of Tarlov and Newman, there are probably many thiol molecules on the outmost film of the superhydrophobic surface, and any sulfonates formed from air exposure are continuously displaced by thiol molecules. So, in the XPS of the as-prepared sample, we cannot detect the oxidized sulfur peak. Further study of the stability of Ag-thiol SAMs at ambient conditions will be done later. (34) Tarlov, M. J.; Newman, J. G. Langmuir 1992, 8, 1398–1405.
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Conclusions In this paper, we have utilized a novel means to imitate “lotus leaf” by constructing a superhydrophobic surface through a process combining both electroless galvanic deposition and selfassembly of n-octadecanethiol, which is time-saving and inexpensive as well as convenient. The surfaces remain superhydrophobic over a wide range of pH and temperature. Acknowledgment. The authors acknowledge the financial support of the National 973 Project of China (Grant No. 2007CB607601) and the National Science Foundation of China (Grant No. 50835009 and Grant No. 50721062). Note Added after ASAP Publication. This article was released ASAP on December 11, 2009. The text of the article and Figure 1 were replaced with new versions, and the article was reposted on January 6, 2010.
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