Chem. Mater. 2006, 18, 1365-1368
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Facile Method To Fabricate a Large-Scale Superhydrophobic Surface by Galvanic Cell Reaction Feng Shi,† Yanyan Song,‡ Jia Niu,† Xinghua Xia,*,‡ Zhiqiang Wang,† and Xi Zhang*,† Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua UniVersity, Beijing 100084, People’s Republic of China, and Key Lab of Analytical Chemistry for Life Science, Department of Chemistry, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed NoVember 14, 2005. ReVised Manuscript ReceiVed January 19, 2006
This paper describes the use of galvanic cell reaction as a facile method to chemically deposit Ag nanostructures on the p-silicon wafer on a large scale. When the Ag covered silicon wafer is further modified with a self-assembled monolayer of n-dodecanethiol, a superhydrophobic surface can be obtained with a contact angle of about 154° and a tilt angle lower than 5°.
Introduction The lotus effect is the self-cleaning property of lotus leaves, and it is due to the cooperative effect of surface roughness and low-surface-energy coatings, which was first reported by Barthlott and Neinhuis.1,2 Since then, Jiang and co-workers further demonstrated that the unique self-cleaning property of lotus leaves was also coming from the cooperation of microstructures and nanostructures on these surfaces.3,4 On the basis of the above understanding of the selfcleaning property of lotus leaves, many methods have been developed to mimic the superhydrophobic or superhydrophilic coatings, such as electrochemical deposition on a polyelectrolyte multilayer,5-9 in situ hydrothermal synthesis of roselike zeolite films,10 chemical vapor deposition, arrays of carbon nanotubes, reconformation of polymers, plasma fluorination, the sol-gel method, TiO2 coating by UV irradiation, and others.11-16 However, many of these methods * To whom correspondence should be addressed. mail.tsinghua.edu.cn (X.Z.);
[email protected] (X.X.). † Tsinghua University. ‡ Nanjing University.
E-mail:
xi@
(1) Johnson, R. E.; Dettre, R. H. AdV. Chem. Ser. 1964, 43, 12. (2) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (3) Feng, L.; Li, S. H.; Li, Y. S.; Li, H. J.; Zhang, L. J.; Zhai, J.; Song, L. Y.; Liu, B. Q.; Jiang, L.; Zhu, D. B. AdV. Mater. 2002, 14, 1857. (4) Sun, T. L.; Feng, L.; Gao, X. F.; Jiang, L. Acc. Chem. Res. 2005, 38, 644. (5) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z. Q.; Jiang, L.; Li, X. Y. J. Am. Chem. Soc. 2004, 126, 3064. (6) Shi, F.; Wang, Z. Q.; Zhang, X. AdV. Mater. 2005, 17, 1005. (7) Yu, X.; Wang, Z. Q.; Jiang, Y. G.; Shi, F.; Zhang, X. AdV. Mater. 2005, 17, 1289. (8) Zhao, N.; Shi, F.; Wang, Z. Q.; Zhang, X. Langmuir 2005, 21, 4713. (9) Jiang, Y. G.; Wang, Z. Q.; Yu, X.; Shi, F.; Xu, H. P.; Zhang, X.; Smet, M.; Dehaen, W. Langmuir 2005, 21, 1986. (10) Shi, F.; Chen, X. X.; Wang, L. Y.; Niu, J.; Yu, J. H.; Wang, Z. Q.; Zhang, X. Chem. Mater. 2005, 17, 6177. (11) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349. (12) Tsujii, K.; Yamamoto, T.; Onda, T.; Shibuichi, S. Angew. Chem., Int. Ed. Engl. 1997, 36, 1011. (13) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. AdV. Mater. 1999, 11, 1365. (14) Tadanaga, T.; Morinaga, J.; Matsuda, A.; Minami, T. Chem. Mater. 2000, 12, 590.
depend on the expensive instruments, and the prepared surface is mechanically fragile; for example, a physical friction with tweezers will damage the surface, which then loses its self-cleaning property. Herein, we describe a facile method for fabricating stable superhydrophobic coatings on a large scale, which is accomplished by immersing silicon wafers into a mixed solution of HF and AgNO3 and does not need any instruments. The galvanic cell reaction is an irreversible chemical reaction that generates electricity.17-19 It has been used to produce metal nanostructures on the silicon wafer substrate through a reaction between the silicon wafer and the metallic ion in HF solution. It is a local electroless method, which can be carried out by simply immersing a silicon wafer into a mixed solution of metallic ion and HF. Here we demonstrated that this method could be a simple and fast way to fabricate metal nanostructures on the silicon wafer substrate. Moreover, by carefully selecting the experimental conditions, we can control the density and morphology of metal nanostructures. After further chemisorption of a self-assembled monolayer of n-dodecanethiol, the as-prepared metal nanostructures on the silicon wafer substrate exhibit stable and large-scale superhydrophobic properties. This may lead to a novel and facile method for the fabrication of self-cleaning coatings on the silicon wafer. Such superhydrophobic coatings on the silicon wafer20 could be used in the formation of a nanobattery by separating the electrolyte from the electrode, which may lead to a novel battery with high power densities, a long shelf life, instantaneous fast ramp-up to full power, scalability, and chemistry-independent functionality. (15) 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. (16) Woodward, I.; Schofield, W. C. E.; Roucoules, V.; Badyal, J. P. S. Langmuir 2003, 19, 3432. (17) Xia, X. H.; Ashruf, C. M. A.; French P. J.; Kelly, J. J. Chem. Mater. 2000, 12, 1671. (18) Peng, K. Q.; Yan, Y. J.; Gao, S. P.; Zhu, J. AdV. Mater. 2002, 14, 1164. (19) Peng, K. Q.; Huang, Z. P.; Zhu, J. AdV. Mater. 2004, 16, 73. (20) Lifton, V. A.; Simon, S.; Frahm, R. E. Bell Labs Tech. J. 2005, 10, 81.
10.1021/cm052502n CCC: $33.50 © 2006 American Chemical Society Published on Web 02/09/2006
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Figure 2. FE-SEM images of Ag aggregates deposited on a p-silicon (100) wafer by galvanic cell reaction in an aqueous solution containing 20 mM AgNO3 and 5 M HF. The deposition times were 2 (a) and 10 min (b).
Figure 1. FE-SEM images of Ag aggregates deposited on a p-silicon (100) wafer by galvanic cell reaction in an aqueous solution containing 2 mM AgNO3 and 5 M HF. The deposition times were 2 (a), 10 (b), 30 (c), and 60 min (d).
Experimental Section The fabrication procedure of Ag nanostructures is described as follows: first, a p-silicon (100) wafer (GRINM semiconductor Materials Co., Ltd., Beijing, China) was cut into 1 cm × 1.5 cm squares and cleaned by ultrasonic washing in acetone and ethanol, respectively; second, the substrate was immersed into an aqueous solution of AgNO3 (2 mM) and HF (5 M) at 45 °C in the dark. In this process, local galvanic cells were formed between the p-silicon (100) wafer and the Ag ions in a HF solution. The control experiment was done by a similar process, only changing the concentration of the AgNO3 from 2 mM to 5 mM. The above process can be finished in 1 h. Finally, the Ag nanostructure covered substrate was washed with pure water and dried with nitrogen. The morphologies of the Ag nanostructures were characterized by a field-emission scanning electron microscope (FE-SEM; JEOL JSM6700F scanning electron microscope, at 10.0 kV). Surface modification was carried out by immersing the asprepared substrate in an ethanol solution of n-dodecanethiol (1 × 10-3 M) overnight, washed in turn with ethanol and water and dried with nitrogen. Then, the static contact angles and title angle of the as-prepared surfaces were measured with a commercial instrument (OCA 20, DataPhysics Instruments GmbH, Filderstadt). A distilled water droplet (drop volume 4 µL) was used as the indicator in the experiment to characterize the wetting property of the as-prepared self-cleaning surfaces.
Results and Discussion The morphology of the Ag nanostructures formed on the p-silicon (100) wafer was carefully observed with a FE-SEM. From the FE-SEM images, we can see that, with 2 min of deposition, a Ag layer was formed on the substrate with high coverage and the Ag layer was flat and had very little branches, as shown Figure 1a. With 10 min of deposition, the sizes of Ag nanostructures increased and three-dimension structures started to form, which increased the roughness of the p-silicon surface, as shown in Figure 1b. When the reaction time was extended further to 30 min, the threedimensional branches became much larger, as shown in Figure 1c. In Figure 1d, with a deposition time of 60 min, the whole surface was covered with microscale coral-like Ag aggregates bearing nanoscale branches. Moreover, the
Figure 3. Cross-sectional image of Ag aggregates deposited on a p-silicon (100) wafer by galvanic cell reaction. Image a was obtained from an aqueous solution containing 2 mM AgNO3 and 5 M HF (deposition time was 60 min). Image b was obtained from the aqueous solution containing 20 mM AgNO3 and 5 M HF (deposition time was 10 min). The two corner insets are zoomed-out images, accordingly.
thickness of the Ag structures also increased. These observations suggest that the size, density, and morphology of the Ag nanostructures can be controlled through the deposition time. We then wondered whether the size, density, and morphology of the Ag aggregates can be controlled through changing the concentration of AgNO3 in the reaction. To do so, we set up the experiment in the same condition as before except increased the concentration of AgNO3 from 2 mM to 20 mM. With 20 mM AgNO3, after 2 min of deposition, many Ag microlines with branches were formed, as shown in Figure 2a. This suggests that at higher AgNO3 concentration the deposition rate increased. A wafer surface fully covered with Ag microstructures with branches was obtained after a 10 min deposition, as shown in Figure 2b. By comparing the surface morphology of the Ag aggregates in Figures 2 and 1, we concluded that, under a higher concentration of AgNO3, a surface with higher roughness can be obtained in a very short time, suggesting that the concentration of the AgNO3 can strongly affect the morphology of the Ag aggregates. The thickness of the Ag aggregates deposited on a p-silicon (100) wafer by galvanic cell reaction was measured by the cross-sectional image of the FE-SEM. When the concentration of the AgNO3 is 2 mM, the cross-sectional image of the Ag aggregates, as shown in Figure 3a, shows that the Ag aggregates are three-dimensional leave-like structures. After increasing the concentration of AgNO3 from 2 mM to 20 mM, the thickness of the Ag aggregates increases from less than 4 µm to more than 10 µm. Moreover, the morphology of the Ag structures also changes from leavelike structures to multilayer structures of the Ag microlines with branches. This suggests that, when increasing the concentration of AgNO3, the thickness of the Ag nanostructures increases and the morphology also changes. This is
Superhydrophobic Surface Fabrication
Figure 4. Photo images of water droplets on the surface of the branchlike Ag aggregate, which was deposited for 10 min in the aqueous solution containing 20 mM AgNO3 and 5 M HF (drop volume 4 µL). The contact angle measurements (a) before modification with n-dodecanethiol and (b) after modification. Parts c and d show tilt angle measurements on the modified substrate.
because the galvanic cell reaction is a first-order kinetic reaction; increasing the concentration of the reactants increases the rate of the reaction, and the thickness of the Ag nanostructures increases correspondingly. Up to now, we can fabricated various surface morphologies through galvanic cell reaction by adjusting the deposition time and the concentration of AgNO3; then we wondered whether the prepared surface was superhydrophobic. Thus, the surface property of the silicon wafer is characterized by a contact angle instrument. However, the contact angle measurements indicated that the surface covered with a branch-like Ag aggregate is not superhydrophobic enough. Even on the roughest surface, which was deposited for 10 min in the aqueous solution containing 20 mM AgNO3 and 5 M HF, the contact angle is only about 67°, as shown in Figure 4a (drop volume 4 µL). This suggests that only a suitable surface morphology is not enough to obtain a superhydrophobic surface; the surface needs further modification with low-surface-energy materials. Both the surface roughness and low-surface-energy coating are prerequisite for synthesizing superhydrophobicity. To introduce low-surface-energy materials on the as-prepared surface of Ag aggregates, we immersed the substrate in an ethanol solution of n-dodecanethiol (1 × 10-3 M) overnight to coat a self-assembled monolayer of surface-active molecule. We then characterized the surface property of the silicon wafer by a contact angle instrument using a water droplet (drop volume 4 µL) as the indicator. When the concentration of the AgNO3 was 2 mM, we observed that the contact angle increased with the deposition time, as shown in Figure 5 with full squares. After the deposition time exceeded 40 min, the contact angle reached a constant value, at about 154°, suggesting that a superhydrophobic surface can be obtained using this method. The increasing value of the contact angle on the modified surface is consistent with the increasing surface roughness and the increasing sizes of Ag aggregates formed at different
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Figure 5. Dynamic water contact angle measurements on the surface of branch-like Ag aggregates as a function of the duration of deposition on a p-silicon (100) wafer by galvanic cell reaction. The full squares are the contact angle data that were obtained from the aqueous solution containing 2 mM AgNO3 and 5 M HF; the full circles were obtained from the aqueous solution containing 20 mM AgNO3 and 5 M HF.
deposition times, as indicated in Figure 1a-d. When the concentration of AgNO3 was increased from 2 mM to 20 mM, we observed that even a 2 min deposition of Ag aggregates can form a superhydrophobic surface with a contact angle of about 152°, as shown in Figure 5 with full circles. For a 10 min deposition, the contact angle of the surface was about 154°, and the shape of the water droplet is shown in Figure 4b. Herein, we should point out that the tilt angle of this as-prepared surface was lower than 3°, as shown in Figure 4c,d. The tilt angle reflects the difference between the advancing and the receding contact angles. The low tilt angle indicates that the water droplet can roll down the surface easily. Moreover, from the cross-sectional image of Figure 3b, we can see that the thickness of the Ag microlines with branches is more than 10 µm. The superhydrophobic surface with this thick coating should be more stable than the other artificial superhydrophobic surfaces. The stability of the superhydrophobic surface is crucial in terms of practical application. Most of the artificial self-cleaning surfaces are unstable under physical friction. It means that the superhydrophobic property can be lost in the regions with indentation. However, the silver superhydrophobic coatings we reported can keep their superhydrophobic property in the regions with indentation, which were formed purposely by physical friction with tweezers. To fully understand the superhydrophobicity of the surface of branch-like Ag aggregates modified with n-dodecanethiol, we describe the contact angle in terms of the Cassie equation: 21 cos θ r ) f1 cos θ - f2. θr (154°) and θ (110°) are the contact angles of the self-assembled monolayer of ndodecanethiol on a rough surface with branch-like Ag aggregates and on a smooth Ag surface, respectively; f1 and f2 are the fractional interfacial areas of the branch-like Ag aggregates and of the air in the interspaces among the branchlike Ag aggregates, respectively (i.e., f1 + f2 ) 1). It is easy to deduce from this equation that increasing the fraction of air (f2) increases the contact angle of the rough surface (θr). (21) Cassie, A. B. D. Trans. Faraday Soc. 1948, 44, 11.
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According to the equation, the f2 value of the rough surface with branch-like Ag aggregates is estimated to be 0.85. This means that the larger fraction of air among the interspaces of the branch-like Ag aggregates leads to the superhydrophobicity of the surface. In other words, the surface morphology plays a very important role in attaining a superhydrophobic surface. In conclusion, we have developed a facile method for the fabrication of a stable superhydrophobic surface on the p-silicon (100) wafer substrate through a galvanic cell reaction. The present study represents a model system to generate a rough surface on silicon; however, a similar concept could be extended to different substrates. For example, we may employ a similar concept to fabricate metal
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nanostructures through other known galvanic cell reactions as well as solar cell reaction on different substrates. On one hand, many interesting nanostructures of noble or heavy metals, for example, Au and Pt, prepared in this way can be used as matrixes for surface enhanced Raman spectroscopy, surface catalysis, and so on. On the other hand, to modify the as-prepared rough surface with different low-surfaceenergy coatings may lead to adjusting surface wetting properties from superhydophobicity to superhydrophilicity. Acknowledgment. The authors thank National Natural Science Foundation of China (20334010, 20473045, 20125515, 20299030, 20574040, 20573042, 50573042) for financial support. CM052502N
