J. Phys. Chem. C 2007, 111, 6821-6825
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Stable Biomimetic Superhydrophobicity and Magnetization Film with Cu-Ferrite Nanorods Zhongbing Huang,*,† Ying Zhu,‡ Jihua Zhang,‡ and Guangfu Yin† School of Materials Sciences and Engineering, Sichuan UniVersity, Chengdu, China, and Institute of Chemistry, Chinese Academy of Sciences, Beijing, China ReceiVed: NoVember 27, 2006; In Final Form: March 2, 2007
We describe a simple and inexpensive method to produce lotus leaf-like Cu-ferrite films on copper alloy engineering materials fabricated via a sol-gel process and chemical modification. The films show stable superhydrophobicity and magnetization, even in many corrosive solutions, such as acidic or basic solutions, and also in oxidizing solutions. Water or aqueous solutions (pH ) 2-13) have contact angles of 156 ( 2 and 154 ( 2° on the treated surfaces of the Cu alloy, respectively. Such superhydrophobic films with magnetic properties will greatly extend the applications of Cu alloys under various weather and electromagnetic conditions.
Introduction Wettability is very important property governed by both the chemical composition and the geometrical structure of solid surfaces.1 A closely related phenomenon in nature is the lotus effect, referring to surfaces that are difficult to wet, which can be found in the leaves of lotus, rice, taro, etc.2 Superhydrophobic surfaces (with a water contact angle (CA) larger than 150°) and sliding angles (SA) smaller than 4° have been extensively investigated due to their importance in industrial applications.2-5 The micro- and nanoscale hierarchical structures on the surface of the lotus leaf (i.e., branch-like nanostructures on the top of micropapillae) contribute to this unique property.6-8 These surfaces usually have binary structures, which makes it possible to trap a large amount of air and to minimize the real contact area between surfaces and water droplets. Coating materials such as micro- or nanoscale films with a very low surface energy material on the surface is the key factor for constructing superhydrophobic surfaces. Copper alloy was selected because of its conductivity and diverse technological applications in the auto, aviation, and power transmission fields. According to this finding, biomimic superhydrophobic surfaces have been widely developed by constructing micro- and nanostructures on surfaces composed of various materials.9 On the other hand, there is a limit in the practical applications of Cu alloys due to its surface damage and the jam of electromagnetic waves. These limitations are a result of surface corrosion that is easily caused by acidic or basic ambiences that arise under certain weather conditions, such as acid rain and snow, and from surrounding contaminants. Cu ferrites can be applied in inductors operating at high frequencies because of their electrical resistivity. Bearing this in mind, we aim to biomimetically construct artificial superhydrophobic and magnetic surfaces on engineering materials that may extend their technical applications10 in preventing contamination, resisting water coalescence under a range of environmental conditions, * Corresponding author. Phone: 86 28 85413003; fax: 86 28 85413003; e-mail:
[email protected]. † Sichuan University. ‡ Chinese Academy of Sciences.
and avoiding jams from electromagnetic waves. To the best of our knowledge, there has been no report on this unique surface to date. Porous structures11 and micropatterned structures12 have also been used to prepare superhydrophobic surfaces. Most of the preparations involve strict conditions (such as harsh chemical treatment), expensive materials (e.g., nanotubes), and processing procedures including etching, plasma treatment, chemical vapor deposition, electrodeposition, and the use of a template. Therefore, applications of the superhydrophobic films prepared so far have been limited. Superhydrophobic surfaces were fabricated with engineering materials of copper alloy using hexamethylenetetramine (HMTA) and ethylene glycol (EG), a strong bidentate chelating agent to Cu2+ and Fe2+ ions with a high stability constant, as the capping reagents. The chemical composition of a copper alloy slide includes 96.1 wt % copper, 3.2 wt % zinc, 0.2 wt % lead, and 0.5 wt % nickel. Experimental Procedures The copper substrate was cleaned in an aqueous 1.0 HCl solution for about 2 min, followed by repeated rinsing with distilled water. After being dried under a N2 gas flow, a Cu slide covered by Cu-ferrite nanoparticles was placed in the reaction solution. A reaction containing 1 mmol of FeCl2‚2H2O, 0.5 mmol of CuCl2‚5H2O, 6 mmol of HMTA, 15 mmol of EG, and 10 mL of deionized water at 85 °C for 0.5 h gave the Cuferrite nanoparticles on a Cu substrate. After being dried under a N2 gas flow, the Cu slide covered by Cu-ferrite nanoparticles was vertically placed in the reaction solution. Cu-ferrite nanorods (NRs) were grown in 6 mmol of FeCl2‚2H2O, 3 mmol of CuCl2‚5H2O, 36 mmol of HMTA, 45 mmol of EG, and 30 mL of deionized water in a conventional reaction flask with a reflux condenser. The reaction temperature was 85 °C. The growth time was 10 h for the growth of NRs on a Cu substrate. Later, the substrates were removed from the solutions and rinsed with distilled water 3 times. Then, the substrates were dried in a vacuum oven at 70 °C for about 10 h. Finally, a layer of dodecafluorooctatriethoxysilane (FOS-12) was formed on the NRs film after immersion in a 3 mM ethanol solution of FOS12 for 24 h and then drying in a oven at 120 °C for 1 h to render the film surface hydrophobicity. To study the stability
10.1021/jp0678554 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/17/2007
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Figure 1. SEM images of (a) lotus, (b) nanorod film of Cu-ferrite by sol-gel process, and (c and d) magnification views of Cu-ferrite film.
under different aqueous solutions, the FOS-12-modified Cuferrite NR films were immerged in aqueous solutions with varying pH values for several days. The structural features of the FOS-12-modified Cu-ferrite NR films were verified using a JEOL-6700F field-emission scanning electron microscope. The structure of the NRs was characterized using HRTEM on a JEM-2010 TEM (JEOL, 200 kV). CAs and SAs were measured on a dataphysics OCA20 CA system at ambient temperature. Average CAs and SAs were obtained by measuring the same sample at five different positions. The magnetization of the Cu-ferrite NR films was characterized using a vibrating sample magnetometer (VSM, LDJ-9600) at room temperature. Precise pH values were measured using DELTA 320 pH meters at ambient temperature. Results and Discussion Surface Morphology of Cu-ferrite Films. In this paper, we focus on the synthesis of Cu-ferrite NRs and their flower-like papillae with a dilute NRs suspension for the fabrication of superhydrophobic surfaces. Many papillae with diameters ranging from 2 to 4 mm can be found on the surface of a NR array film (Figure 1b,c). Each of these papillae is composed of Cuferrite sword-like NRs with a 20-30 nm thickness and 60120 nm width. As can be seen from the magnified image of a single papilla (Figure 1d), the NRs grow like flowers from the substrate and are self-assembled into micro- and nanoscale hierarchical structures. These special structures on the film are somewhat similar to those on the surface of the self-cleaning lotus leaf (Figure 1a) and are expected to show unusual wettabilities after FOS-12 modification. Crystal Structure of Cu-ferrite. X-ray diffraction patterns (XRD) of as-deposited Cu-ferrite films show that obtained Cuferrite NRs possess the structure with good tetragonal cubic crystallinity (JCPDS Card File 34-0425). The peaks are relatively broad as compared with those of the bulk material, thus corroborating the small crystal size. In contrast to the standard pattern, high-resolution TEM (HRTEM) of the side
Figure 2. High-resolution TEM image of a Cu-ferrite nanorod wall at the side surface.
of the NR wall is shown in Figure 2. It can be seen that the NR is well-crystallized with clear lattice fringes parallel to the wall. The inter-plane distance of 0.304 nm for the (112) planes perpendicular to the rod axis can be readily resolved. Such a feature implies that the NRs grow along the (112) crystal plane with a preferred orientation. Surface Properties of FOS-12-Modified Cu-ferrite Films. The wettability was evaluated by the water CA measurement of the as-prepared films. Because of an abundance of hydroxyl groups on the surface of the Cu-ferrite NRs, their water CA is about 0°, showing that the packed arrays of NRs deposited on the Cu substrate have a superhydrophilicity. After the NR packed array film was modified with FOS-12, the water CAs were 156 ( 2.1°. Obviously, FOS-12 can easily react with the hydroxyl groups on the NR surface and decrease the free energy of the NR surface. Figure 1a,b also shows that the Cu-ferrite NRs align separately on the Cu substrate and that air can be present in the troughs between individual NRs. The hydrophobicity of a rough surface can be intensified by increasing the proportion of the air-water interface.1b As the CA on FOS-12-modified Cu alloy flat surfaces is 119° (Figure 3a), this result indicates that the array films were changed into the superhydrophobic surface. Moreover, a much smaller SA (less than 4.5° for the water droplet) was obtained on such a film (Figure 3b). Because two pictures were taken in 20 ms, we could calculate that the water droplet moved toward 0.2 mm. Obviously, water droplets can roll easily on a slightly tilted surface.
Magnetization Film with Cu-Ferrite Nanorods
Figure 3. (a) Shape of 2 µL water droplets on a FOS-12-modified Cu-ferrite film of a Cu alloy with a CA of 156.2°. (b) Sliding behavior of a water droplet on an as-prepared film of Cu alloy; the two pictures were taken in 20 ms.
Figure 4. Chemical stablility of a FOS-12-modified Cu-ferrite film.
Stable Superhydrophobicity of a FOS-12-Modified Cuferrite Film. The superhydrophobicity of the hierarchical structures is stable in air. After 60 days of storage in air, the values of the CA remained essentially constant, and there was essentially no change in the water CA. The sliding angle was lower than 4.5°, implying that the water droplets could be moved upward easily even when the surfaces were only slightly tilted. At the same time, the mechanical durability of the prepared surfaces also remained constant. In addition, the superhydrophobicity and magnetization of the nanorod films were maintained even when oxidizing solutions, such as hydrogen peroxide solutions, were dropped on it. These results were reproducible, even after the film was kept in an ambient environment for 2 months, which shows their excellent stability. Copper alloys, as an industry material of applications in the auto, aviation, and power transmission fields, can be in weak acidic or alkalescent environmental conditions. A key factor is their good environmental stability. After hydrophobic modification, Cu-ferrite NR arrays were immersed in various pH solutions, and the stability of the coating layer on the surface of the Cu-ferrite NR arrays was studied. FOS-12 coatings were bonded to the NRs’ surface via Si-O-Cu bonds and Si-OFe bonds. We know that the alkyl chains are stable and that the fluorocarbon chains are more stable than the alkyl chains. However, the substrate bonding part is the weak point of the modified coatings. Because the Si-O bond is easier to hydrolyze in alkaline solutions,18 it is necessary to increase the density of the hydrophobic coating to inhibit the permeation of water and OH ions through the FOS-12 coating.19 The FOS-12-modified Cu-ferrite NR film was dunked into various pH aqueous solutions for several days. Figure 4 shows that FOS-12-modified NR films are stable from pH 5 to 10 during the observed days. When immerged with a pH 13 aqueous solution and a hydrochloric acid solution of pH 2, the CAs are diminished in both cases continuously to typical values for Cu-ferrite. These
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Figure 5. Magnetic hysteresis curves measured at room temperature for the Cu-ferrite film.
results show that surface modification of Cu-ferrite NR films can withstand weak acidic or alkalescent environmental conditions for a long time and also withstand acidic or alkali unfavorable environmental conditions for a short time. Magnetic Properties of the FOS-12-Modified Cu-ferrite Film on Cu Alloy. Figure 5 shows magnetic hysteresic curves for typical samples measured at room temperature. Because most obtained Cu-ferrite NRs are perpendicular to the Cu alloy substrate, they are almost parallel to the magnetic field applied. It can be seen that when the field is applied perpendicular to the rods, the hysteresis curves exhibit a small coercivity (HC), a small remanence (MR), and a large saturation field (i.e., the field necessary to reach the saturation magnetization, MS). This indicates that the easy axis of the system is along NRs of Cuferrite, which is characteristic of polycrystalline NRs where the shape anisotropy dominates over the intrinsic magnetocrystalline anisotropy and thus dictates the magnetic behavior of the system. The Cu-ferrite film has a MS of 20.8 emu/g, and the rest has a magnetization of 2.78 emu/g, respectively. Both of them are lower than the 35.6 emu/g (MS) of the spinel ferrite CuFe2O4 nanoparticles.14 This may have resulted from a significant amount of organics and the FOS-12-modified layer, which lowers the volume ratio of the magnetic phase. Mechanism for Special Surface Properties. The obtained Cu-ferrite films have an excellent, stable superhydrophobicity over a wide pH range and magnetic property for a long period of time. There are two reasons for this: (i) the surface composition and (ii) the surface morphology. On the one hand, the rough Cu-ferrite films and the nanorod films modified by FOS-12 have different surface compositions. Energy-dispersive X-ray spectroscopy (EDX) analysis of the Cu-ferrite nanorods found that the oxide is superfluous, showing that there were large numbers of hydroxide groups on the surface of the nanorods.15 FOS-12 is easy to react with these hydroxide groups and form the FOS-12 thin layer with a low surface free energy. It can be deduced that the component in the top surface is mainly a low surface energy fluorocarbon,16 coming from the FOS-12 thin layer. On the other hand, regarding the surface morphology, the Cu-ferrite film exhibits a special hierarchical structure, which is rough enough to let air fill in the vacancies between individual nanostructures, so that the contact area between water and film can be minimized. The CA on the hierarchical surface structure of the Cu-ferrite film (θ′) can be expressed by eq 1.17
cos θ′ ) f 1 cos θ - f2
(1)
where θ′ is the apparent CA on the hierarchical structured surface, θ is the intrinsic CA on the corresponding flat surface, f1 is the fraction of the water-solid contact area, and f2 is the fraction of the water-air contact area (f1 + f2 ) 1). Figure 1
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SCHEME 1: Water Droplet Dripped on Up-End of NRs with Flower-like Packed Alignment
shows that the NRs in a papillae separate and grow on the substrate. Scheme 1 shows the case of water droplets dripped on the up-end of a flower-like packed alignment. From Figure 1c,d, we calculated f1 and f2 to be about 0.223 and 0.777, respectively.20 As the WCA on FOS-12-modified flat surfaces is 119°, the theoretical cos value of CA θ′ on the hierarchical structure surface derived from eq 1 is about -0.882, which is close to the experimental value for superhydrophobicity, -0.914 corresponding to a 156° WCA. Such larger f2 values are usually achieved by introducing air cavities or channels on the surface mainly via processes such as lithography, etching, and phase separation.2-5 All these results indicate that the modification of FOS-12 on Cu-ferrite nanorods and the micro- and nanoscale hierarchical surface structures, which are greatly similar to the lotus surface,2 results in superhydrophobicity of the Cu alloy. Controllable Surface Morphology and Surface Properties. It is important that the morphology can be well controlled by changing the time of the Cu-ferrite nanorod growth. A time ranging from 1 to 24 h allows for crystal growth processes. When the growth time is too short, there are only very fine nanorods formed, and no nanorod array films formed; when the growth time is too long, submicrorods will form. In other word, if the Cu-ferrite growth time is too short or too long, the hierarchical morphology of the nanorod films will not be obtained. The SEM images in Figure 6a-d show the variation in morphology as the growth time of the Cu-ferrite crystal increases (i.e., for 1, 3, 8, and 24 h, respectively). It can be seen that the number of nanorods decreases and that their size increases with increasing the growth time of the crystal in the sol-gel process. This is because the fine nanorods continuously form with increasing the crystal growth time. These fine diameter nanorods continue to grow as a bundle individually
Figure 7. Relationship between NRs growth time and CA (0, solid line) and MS (b, dotted line).
initially and eventually coalesce to form large diameter rods that can lower their surface energy.15 As the number of the small nanorods decreases, the nanorods become bigger, and the average distance between the nanorods also increases, as can be seen in Figure 6a,b. Meanwhile, the nanorods change from being dagger-like to being sword-like, and the surface of the nanorods becomes smooth when the growth time is 8 h (Figure 6c). Finally, the submicrorods appear for a growth time of 24 h (Figure 6d). As the surface morphology is a very important factor in the determination of surface properties, increasing the growth time can also control the superhydrophobicity and magnetization of the FOS-12-modified Cu-ferrite film. The magnetization depends strongly on the Cu-ferrite nanorod size. The dotted line in Figure 7indicates the relationship between the growth time and the saturation magnetization. The saturation magnetization of the film increases with the growth time of the Cu-ferrite nanorods. This is due to the increase of the crystal fraction of in large nanorods and the decrease of the noncrystal fraction. The solid line in Figure 7 shows the relationship between growth time and superhydrophobicity: the CA for water changes slightly from 156.5 ( 2.1 to 142 ( 2.4° because of the decrease in roughness caused by changing the crystal growth time. However, a surface with a high CA does not always show a low SA. The SA of these as-prepared films changes with changing crystal growth time, and a typical water droplet on
Figure 6. SEM images of Cu-ferrite films for different crystal growth times of (a) 1 h; (b) 3 h; (c) 8 h; and (d) 24 h.
Magnetization Film with Cu-Ferrite Nanorods the surface rolls off easily, even when there is little tilt on the surface, with films formed during 8 and 10 h. In contrast, a typical water drop placed on the films formed during 1 h begins to roll off until the film was tilted to 14° and more than that. The observed behavior of different SAs can be understood in terms of the contact line structure formed at the interfaces between solid, liquid, and air. For films with a low SA, it was expected that the three-phase contact lines were contorted on the surface and extremely unstable, owing to the hierarchical structure of the surface, which effectively traps sufficient air to reduce the sliding resistance drastically. For films with a high SA, however, the proper surface roughness makes the threephase contact lines change into solid-liquid contact lines on the surface, which are comparatively smooth and stable, resulting in the effective intrusion of water into the surface nanostructures because of the droplet’s weight. Accordingly, the experimental results show that the SA is strongly affected by the surface structure. Furthermore, it is known that other preparation parameters, such as crystal growth temperature, the ratio of the Fe ion and Cu ion, and the concentration of HMTA and EG, significantly influence the morphology of the Cu-ferrite nanorod array films and therefore affect the superhydrophobicity and magnetization of the film. Conclusion A FOS-12-modified Cu-ferrite nanorod film with a lotus leaflike structure was prepared by a simple sol-gel process on a large scale at low cost on copper alloys. The film is superhydrophobic, with both a high CA and a low SA, showing the self-cleaning effect of the lotus. It is very significant that the superhydrophobicity, self-cleaning effect, and magnetization are well-maintained over a wide range of pH conditions and also in an electromagnetic wave environment. The crystal growth time of Cu-ferrite can strongly influence the morphology of the NRs film, which thus displays different superhydrophobicities and magnetizations. This is the first example of a superhydrophobic surface with magnetization available in acidic and basic solutions, as well as in oxidizing solutions; this may give rise to new perspectives in practical applications under certain weather conditions. Acknowledgment. We gratefully acknowledge the Chinese Postdoctoral Science Foundation (Grant 2004036305) and the Chinese Academy of Sciences K. C. Wong Postdoctoral Fellowship for financial support of this project. We thank Prof. Lei Jiang of ICCAS for his constant instruction and support.
J. Phys. Chem. C, Vol. 111, No. 18, 2007 6825 References and Notes (1) (a) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (b) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (2) (a) Lau, K. K. S.; Bico, J.; Teol, K. B. K. Nano Lett. 2003, 3, 1701. (b) Miwa, M.; Nakajima, A.; Hashimoto, K. Langmuir 2000, 16, 5754. (c) Nakajima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 7044. (d) Nakajima, A.; Ujishima, A. AdV. Mater. 1999, 16, 1365. (e) Youngblood, J. P.; McCarthy, T. J. Macromolecules 1999, 32, 6800. (f) Bica, J.; Queed, M. Europhys. Lett. 1999, 47, 220. (g) Ma, M.; Hill, R. M.; Lowery, J. L. Langmuir 2005, 21, 5549-5554. (3) Parker, A. R.; Lawrence, C. R. Nature 2001, 414, 33. (4) (a) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125. (b) O ¨ ner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (c) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377. (5) (a) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanalbe, T. Nature 1997, 388, 431. (b) Bico, J.; Tordeux, C.; Que´re´, D. Europhys. Lett. 2001, 55, 214. (6) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (7) Neinhuis, C.; Barthlott, W. Ann. Bot. (London) 1997, 79, 667. (8) 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. (9) (a) Xie, Q.; Xu, J.; Feng, L.; Jiang, L.; Tang, W.; Luo, X.; Han, C. C. AdV. Mater. 2004, 16, 302. (b) Furstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Langmuir 2005, 21, 956. (10) (a) Herminghaus, S. Europhys. Lett. 2000, 52, 165. (b) Schmidt, D. L.; Coburn, C. E.; Benjamin, M. D. Nature 1994, 368, 39. (c) Patankar, N. A. Langmuir 2004, 20, 7097. (d) Shang, H.; Wang, Y.; Limmer, L. S. Thin Solid Films 2005, 472, 37. (e) Nakajima, A. K.; Hashimoto, K.; Watanable, T. Monatsh. Chem. 2001, 132, 31. (11) (a) Erbil, H. Y.; Demirel, A. L.; Newton, M. I.; Perry, C. C. Science 2003, 299, 1377. (b) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2003, 19, 5626. (c) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349. (12) (a) Han, J. T.; Lee, D. H.; Ryu, C. Y.; Cho, K. J. Am. Chem. Soc. 2004, 126, 4796. (b) Fu, Q.; Rao, G. V. R.; Basame, S. B.; Keller, D. J.; Artyushkova, K.; Fulghum, J. E.; Lopez, G. P. J. Am. Chem. Soc. 2004, 126, 8904. (c) Feng, L.; Yang, Z.; Zhai, J.; Jiang, L. Angew. Chem., Int. Ed. 2004, 43, 4338. (d) Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 357. (13) (a) Sun, S.; Zeng, H. J. Am. Chem. Soc. 2002, 124, 8204. (b) Sun, S.; Zeng, H.; Robinson, D. B.; Raoux, S.; Rice, P. M.; Wang, S. X.; Li, G. J. Am. Chem. Soc. 2004, 126, 273. (14) Goya, G. F.; Rechenberg, H. R.; Jiang, J. Z. J. Magn. Magn. Mater. 2000, 218, 221. (15) Huang, Z. B.; Zhu, Y.; Wang, S. T.; Yin, G. F. Cryst. Growth Des. 2006, 6, 1931. (16) Tadanga, K.; Morinaga, J.; Matsuda, A.; Minami, T. Chem. Mater. 2000, 12, 590. (17) Cassie, A. B. D. Trans. Faraday Soc. 1948, 44, 11. (18) Wasserman, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (19) Iimura, K.; Kato, T. Colloids Surf., A 2000, 171, 249. (20) Water droplets only dripped on the up-end of these NRs, and the area of the up-end was about 0.07 ( 0.04 µm2. Assuming that there is a papillae in 16 µm2, which contains 51 ( 10 Cu-ferrite NRs, the area of the up-end of NRs dripped by water droplets was about 51 × 0.07 ≈ 3.57 µm2. Thus, f1 and f2 are about 0.223 and 0.777, respectively.
