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Fabrication of Superhydrophobic Surfaces from Microstructured ZnO-Based Surfaces via a Wet-Chemical Route Xuedong Wu,* Lijun Zheng, and Dan Wu Department of Chemistry, School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China Received January 31, 2005 The fabrication of a superhydrophobic surface is demonstrated via a wet chemical route, and this method offers advantages of being cleanroom free, cost efficiency, and wide applicability. The preferable growth of ZnO crystalline forms a microstructured surface, and a variety of alkanoic acids were adopted to tune the surface wettability. Although all surfaces show an advancing contact angle greater than 150°, they substantially differ in the wetting mechanisms. It is found that only when the length of alkanoic acid is greater than 16, the microstructured surface shows a stable superhydrophobicity, in which the Cassie state dominates. While for those moderate-length alkanoic acids (C8-C14), their corresponding surfaces have a tendency to fall into the Wenzel state and display a great contact angle hysteresis.
With the continuous developments of micro/nanotechnologies, people have learned much about the state-ofthe-art microstructures from natural materials1 and have been trying to find out approaches to mimic these natural materials.2 The demands of tailored surfaces, such as the superhydrophobic surface, also motivate people to explore the physical and chemical mechanisms of material properties at the micro- or nanoscale. Chemically, the wettability of a flat solid surface is governed by the free energy of the surface material, and the known lowest free energy material is a fluorinated surface. The contact angle (CA) of water on such a flat surface is no more than 120°. However, the CA of water on some natural tissues, such as lotus leaves or Lepidoptera wings, can reach as high as 160° or above, and a water droplet can freely roll off on these surfaces without leaving any trace of beads.3 It is widely accepted that this superhydrophobicity is resulted from the particularly rough microstructure of these natural surfaces. This superhydrophobicity has been tremendously attracted in both fundamental studies and practical applications.2,4 Two assumptions, the Wenzel model5 and Cassie model,6 were put forward to explain the impact of surface roughness on the wettability (socalled “lotus effect”). Some scientists have demonstrated fabrications of artificial superhydrophobic surfaces via lithography-based etching,7 deposition techniques,8 and template-based extrusion.2,9 Nevertheless, most of these * To whom correspondence should be addressed. Tel.: +86-2154742814. Fax: +86-21-54741297. E-mail:
[email protected]. (1) (a) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, 457-460. (b) Blossey, R. Nat. Mater. 2003, 2, 301-306. (c) Gu, Z.-Z.; Uetsuka, H.; Takahashi, K.; Nakajima, R.; Onishi, H.; Fujishima, A.; Sato, O. Angew. Chem., Int. Ed. 2003, 42 (8), 894-897. (2) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, B. Adv. Mater. 2002, 14, 1857-1860. (3) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1-8. (4) (a) Shibuchi, S.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512-19517. (b) Yoshimitsu, Z.; Nakajima, A.; Watanable, T.; Hashimoto, K. Langmuir 2002, 18, 5818-5822. (5) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988-994. (6) Cassie, A.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546-551. (7) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777-7782. (8) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z.; Jiang, L.; Li, X. J. Am. Chem. Soc. 2004, 126, 3064-3065. (b)Wu, Y.; Sugimura, H.; Inoue, Y.; Takai, O. Chem. Vap. Deposition 2002, 8, 47-50. (9) Feng, L.; Song, Y.; Zhai, J.; Liu, B.; Xu, J.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2003, 42 (7), 800-802. (b) Jiang, L. Modern Scientific Instruments (in Chinese) 2003, 3, 6-10.
reported approaches are costly thanks to rigorous experimental conditions, such as clean-room facilities, or are restricted in some certain application. Feng10 et al. also developed a solution approach to obtain a microstructured surface, which showed a reversible transition between superhydrophobicity and superhydrophilicity. Some facile fabrications of bionic rough surfaces were reported by polymer casting.11 Recently, microtextured ZnO surfaces were introduced to create superhydrophobic surfaces via chemo- or electro-deposition.12 In this paper, we describe a novel method to fabricate superhydrophobic surfaces by a wet chemical route, which is simple and straightforward. This method substantially alleviates the usage of clean-room facilities and is more cost efficient, compared with conventional photolithography-based processes. It also shows a very good experimental reproducibility and can produce microstructured surfaces at a large scale. Moreover, this method can be applied to a variety of substrates, such as silicon wafer, glass, and even polymer surfaces, regardless of their irregular shapes or curved surfaces. This advantage allows the superhydrophobic surface to be utilized in more practical applications, such as aircraft, navy, and some special decorations. Here, the production of superhydrophobic surface is comprised of two steps. First, the ZnO microstructured surface was formed in a basic solution of Zn2+ at a facile temperature. Then the as-prepared rough surface was modified by organic self-assembled monolayers (SAMs) to obtain a superhydrophobic surface. Briefly, the sonication cleaned glass slide (or silicon wafer) was immersed into a 100-mL aqueous solution, which contained 0.01 mol of Zn(NO3)2, 0.02 mol of NH4Cl, 0.01 mol of urea, and 5 mL of 25% ammonia. The system was rapidly heated (10 °C/min) to 90 °C and kept for 1 h, and then the slide was rinsed by distilled water and was blown to dry with N2 at room temperature. Figure 1a,b (10) Feng, X.; Feng, L.; Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. J. Am. Chem. Soc. 2004, 126, 62-63. (11) (a) Xie, Q.; Xu, J.; Feng, L.; Jiang, L.; Tang, W.; Luo, X.; Han, C. C. Adv. Mater. 2004, 16, 302-305. (b) Xie, Q.; Fan, G.; Zhao, N.; Guo, X.; Xu J.; Dong, J.; Zhang, L.; Zhang, Y.; Han, C. C. Adv. Mater. 2004, 16, 1830-1833. (12) (a) Li, M.; Zhai, J.; Liu, H.; Song, Y.; Jiang, L.; Zhu, D. J. Phys. Chem. B 2003, 107, 9954-9957. (b) Liu, H.; Feng, L.; Zhai, J.; Jiang, L.; Zhu, D. Langmuir 2004, 20, 5659-5661. (c) Zhang, X.; Sato, O.; Fujishima, A. Langmuir 2004, 20, 6065-6067.
10.1021/la050275y CCC: $30.25 © 2005 American Chemical Society Published on Web 03/02/2005
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Figure 1. FESEM images of the as-prepared ZnO microstructured surface: (a) at low magnification; (b) at high magnification; (c) a close-up picture of the hexagonal ZnO nanorod; and (d) cross-sectional view.
Figure 2. Pictures captured from the goniometer for a water droplet on ZnO microstructured surfaces: (a) advancing and (b) receding on a C18 acid modified surface; (c) advancing and (d) receding on a C8 acid modified surface, after the 24-h wicking test.
shows the typical field emission scanning electron microscopy (FESEM) images of the as-prepared ZnO microstructured surface at low and high magnifications, respectively. It can be observed that the ZnO nanorods form a uniform and dense film at a large scale on the substrate, and these rods have diameters ranging from 400 to 600 nm. The high-magnification picture, as shown in Figure 1c, clearly reveals that the hexagonal structure of the ZnO nanorods. The cross-sectional view (Figure 1d) displays that these nanorods are grown perpendicularly from the substrate, and their length is around 3-4 µm. The XRD pattern indicates that the as-prepared ZnO nanorods have a typical Zincite structure (see Supporting Information). All positions of the diffraction peaks are identical to the standard hexagonal ZnO crystallography (JCPDS card no. 36-1451), except that the strongest peak is located in the (002) crystal plane. This indicates that an anisotropic growth of preferable orientation occurred under such experimental conditions. The growth mechanism of ZnO arrays on the substrate can be interpreted as an epitaxial growth.13 The ZnO nuclei were first deposited on the substrate and formed a seed layer. Here, the couple of ammonia/ammonium chloride might serve as a baffling agent to maintain a stable pH environment in this process, concerning that the growth of ZnO is very (13) Zhang, Y.; Jia, H.; Yu, D. J. Phys. D: Appl. Phys. 2004, 37, 413-415.
Letters
sensitive to the pH value. In addition, the ammonium ions also collaborate in the crystallization via being adsorbed on the surface of ZnO nuclei to promote the growth of crystallites. Urea is employed as a crystallization guide agent, and it was found that the ZnO oriented arrays could not be obtained without urea. The as-prepared ZnO microstructured surfaces were subsequently modified by a variety of SAMs. It is verified that various surface reactions can be performed on the as-prepared rough surface based on the chemistry of ZnO. This approach provides an alternative approach to control the surface wettability, instead of tailoring the geometric parameters by microfabrication techniques. A series of alkanoic acids, in terms of their chain length including n-octanoic acid (C8 acid), n-dodecanoic acid (C12 acid), n-tetradecanoic acid (C14 acid), n-hexadecanoic acid (C16 acid), and n-octadecanoic acid (C18 acid), were adopted to tune the surface wettability and study the transition of the superhydrophobic states. The Fourier transform infrared spectra of both free C18 acid and the corresponding modified ZnO surface provide the evidence for the formation of SAMs. There are two strong peaks at 2848 cm-1 (νCH2) and 2916 cm-1 (νCH3) in both spectra, which indicate the existence of the longchain aliphatic groups on the surface. Two peaks can be found in C18 acid, one at 1701 cm-1 (νCdO) and the other at 2674 cm-1 (a shoulder peak owing to the dimer form of free carboxylic acid). These two peaks disappear in the surface sample, and two new peaks are detected at 1539 and 1465 cm-1, which can be denoted as the νas and νs of carboxylate ion, respectively. Furthermore, it can be inferred that the stearate ion coordinates with Zn2+ in the bidentate form regarding to the ∆ν(νas-νs) of RCOO-.14 From above, we can speculate that the alkanoic acids were chemically bonded to the surface Zn2+ species, and, therefore, the ZnO microstructured surface was covered by a monolayer of organic molecules with their nonpolar tails exposed to air. All these modified surfaces have advancing CAs (θA) greater than 150° based on the instant measurements, as we expected (see Supporting Information). However, it is noticeable that receding CAs (θR) are seriously dependent upon the chain length of alkanoic acids. For instance, the C18 acid modified surface has a high receding CA of 152°, which is very close to its advancing CA, as shown in Figure 2a,b. The surface will keep its super-hydrophobic feature even after a long time wicking test, and a water droplet can roll off the surface freely. From Figure 2c,d, the receding CA of the C8 acid modified surface is dramatically reduced after the wicking test, which hints at a physically distinctive wetting mechanism from that of the C18 acid modified surface. As a matter of fact, the variation between θA and θR (so-called hysteresis of CAs, ∆θ) can virtually reflect the states of surperhydrophobicity. Basically, a great ∆θ, for example, in the case of the C8 acid modified surface, is a characteristic feature of the Wenzel state.1a,15 In many cases, Cassie and Wenzel states might coexist, and it can be assumed that the C8 acid modified surface undergoes a transition from the Cassie state to the Wenzel state during the wicking test. Scheme 1 illustrates the irreversible transition from the Cassie state to the Wenzel state. In the case of the Cassie state, an air pocket is trapped in the groove of the textured surface, and the water droplet is “sitting” on the surface. When the droplet (14) Chen, H. G.; Wu, X. D.; Yu, Q. Q.; Yang, S. R.; Wang, D. P.; Shen, W. Z. Chin. J. Chem. 2002, 20, 1467-1471. (15) (a) Ramos, S. M. M.; Charlaix, E.; Benyagoub, A. Surf. Sci. 2003, 540, 355-362. (b) Ishino, C.; Okumura, K.; Quere, D. Europhys. Lett. 2004, 68 (3), 419-425.
Letters Scheme 1. Irreversible Transition from the Cassie State to the Wenzel State by the Wicking Test
tends to shrink off, energy is required to overcome the adhesion of the real solid/liquid interface, which is much less than that of the apparent solid/liquid contact surface.6,16 However, this state is not sufficiently stable for those surfaces modified by moderate length acids (C8C14). The water will invade into or fill up the grooves during the wicking test. Consequently, the droplet suffers an extra solid/liquid interface, which is much more than (16) Zheng, L. J.; Wu, X. D.; Lou, Z.; Wu, D. Chin. Sci. Bull. 2004, 49 (17), 1779-1787.
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the apparent solid/liquid surface,5,16 when it tends to shrink off. At this stage, the vapor/liquid interface will be energetically prone to be diminished and the solid/liquid surface will keep pinned until the three-phase line recedes. Although there is still lack of insightful understanding for the receding CA of the rough surface, it should be stressed in particular that dynamic behaviors should be taken into account as a very essential feature for the construction of a superhydrophobic surface. Acknowledgment. The project is financially supported by the National Natural Science Foundation of China (No. 20306014) and Shanghai Natural Science Foundation (No. 04ZR14085). Prof. Jeffrey S. Moore at University of Illinois at Urbana Champaign is specially appreciated for his invaluable help to inspire this work. Supporting Information Available: Details for the experiments and some characterization of the samples. This material is available free of charge via the Internet at http://pubs.acs.org. LA050275Y
