Silver Mirror Reaction as an Approach to Construct Superhydrophobic

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Silver Mirror Reaction as an Approach to Construct Superhydrophobic Surfaces with High Reflectivity Liyan Shen, Jian Ji,* and Jiacong Shen Department of Polymer Science and Engineering, Key Laboratory of Macromolecule Synthesis and Functionalzation of Minster of Education, Zhejiang UniVersity, 310027, Hangzhou, China ReceiVed June 11, 2008. ReVised Manuscript ReceiVed July 30, 2008 Superhydrophobic surfaces with high reflectivity might provide a promising self-cleaning approach in a wide variety of optical applications ranging from traffic to solar energy industries. However, the contradiction between the hierarchical micronanostructure and the high reflectivity is a challenge for superhydrophobic materials with high reflectivity. Here we report a facile method to fabricate a superhydrophobic silver film with reflectivity as high as that of polished silicon by carefully controlling the seed-induced silver mirror reaction.

Introduction Highly reflective materials play a pivotal role in a wide variety of optical technologies ranging from traffic to solar energy industries, but those reflective coatings may easily get contaminated, which not only reduces their reflectivity but also greatly increases cleaning costs. Superhydrophobic surfaces have aroused increasing interest in both fundamental research1-4 and industry applications5-9 because of their water-repellent and self-cleaning properties. Further research10-13 has demonstrated that hierarchical micronanostructures can both increase the contact angle (CA) and reduce the sliding angle (SA), which is essential to self-cleaning properties. Numerous methods, including electron/ laser etching,14,15 electrodeposition,16-19 layer-by-layer assembly,20-23 replacement reactions,24-26 and others,13,27-30 have * Corresponding author. E-mail: [email protected]. (1) Nosonovsky, M. Langmuir 2007, 23, 3157. (2) Lafuma, A.; Quéré, D. Nat. Mater. 2003, 2, 457. (3) Gao, L. C.; McCarthy, T. J. Langmuir 2007, 23, 3762. (4) Wang, S. T.; Liu, H. J.; Liu, D. S.; Ma, X. Y.; Fang, X. H.; Jiang, L. Angew. Chem., Int. Ed. 2007, 46, 3915. (5) Chen, C. H.; Cai, Q. J.; Tsai, C. L.; Chen, C. L.; Xiong, G. Y.; Yu, Y.; Ren, Z. F. Appl. Phys. Lett. 2007, 90. (6) Michielsen, S.; Lee, H. J. Langmuir 2007, 23, 6004. (7) Takei, G.; Nonogi, M.; Hibara, A.; Kitamori, T.; Kim, H. B. Lab Chip 2007, 7, 596. (8) Wang, T.; Hu, X. G.; Dong, S. J. Chem. Commun. 2007, 1849. (9) Miyauchi, Y.; Ding, B.; Shiratori, S. Nanotechnology 2006, 17, 5151. (10) Xiu, Y. H.; Zhu, L. B.; Hess, D. W.; Wong, C. P. Langmuir 2006, 22, 9676. (11) Fang, W. J.; Mayama, H.; Tsujii, K. J. Phys. Chem. B 2007, 111, 564. (12) Gao, L. C.; McCarthy, T. J. Langmuir 2006, 22, 2966. (13) Tung, P. H.; Kuo, S. W.; Jeong, K. U.; Cheng, S. Z. D.; Huang, C. F.; Chang, F. C. Macromol. Rapid Commun. 2007, 28, 271. (14) Lee, E. J.; Lee, H. M.; Li, Y.; Hong, L. Y.; Kim, D. P.; Cho, S. O. Macromol. Rapid Commun. 2007, 28, 246. (15) Gao, X. F.; Yao, X.; Jiang, L. Langmuir 2007, 23, 4886. (16) Shi, F.; Song, Y. Y.; Niu, H.; Xia, X. H.; Wang, Z. Q.; Zhang, X. Chem. Mater. 2006, 18, 1365. (17) Zhao, N.; Shi, F.; Wang, Z. Q.; Zhang, X. Langmuir 2005, 21, 4713. (18) 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. (19) Shi, F.; Wang, Z. Q.; Zhang, X. AdV. Mater. 2005, 17, 1005. (20) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349. (21) Zhang, L. B.; Chen, H.; Sun, J. Q.; Shen, J. C. Chem. Mater. 2007, 19, 948. (22) Zhai, L.; Berg, M. C.; Cebeci, F.; Kim, C. Y.; Milwid, J. M.; Rubner, M. F.; Cohen, R. E. Nano Lett. 2006, 6, 1213. (23) Ji, J.; Fu, J. H.; Shen, J. C. AdV. Mater. 2006, 18, 1441. (24) Qu, M. N.; Zhang, B. W.; Song, S. Y.; Chen, L.; Zhang, J. Y.; Cao, X. P. AdV. Funct. Mater. 2007, 17, 593. (25) Larmour, I. A.; Bell, S. E. J.; Saunders, G. C. Angew. Chem., Int. Ed. 2007, 46, 1710.

been explored to fabricate micronanostructures so as to obtain superhydrophobic surfaces. Nano- and/or microstructured aggregates of nanoparticles may induce other properties rather than superhydrophobicity alone.31 For example, when the size of the nanostructure is close to the wavelength range of visible light (400-750 nm), the nanostructure may have special optical properties.32-37 Gu and co-workers fabricated a uniform inverse opal film deriving both structural color and superhydrophobicity simultaneously.36 Nakajima et al.32 and Bravo et al.37 constructed transparent superhydrophobic surfaces by sublimation and layerby-layer processing, respectively. However, no research has ever been reported on the fabrication of superhydrophobic surfaces with high reflectivity. The silver mirror reaction has been and is still being used in the mirror and vacuum flask industries as an effective method for preparing highly reflective surfaces. Qu et al.38 and He et al.39 found that the silver mirror reaction on copper and aluminum surfaces could develop rough leaflike silver aggregates and 2D staggered silver nanosheets, respectively. Zhang and co-workers16,17,19 made noble metal aggregates via electrodeposition and then modified them with alkythiol to fabricate superhydrophobic surfaces. Herein, we want to see the possibility of fabricating a superhydrophobic surface with high reflectivity via the traditional silver mirror reaction. Gu and coworkers36 pointed out the contradiction between transparency and superhydrophobicity on their transparent superhydrophobic surfaces. The same contradiction between reflectivity and superhydrophobicity may exist. From the viewpoint of surface (26) Huang, Z. B.; Zhu, Y.; Zhang, J. H.; Yin, G. F. J. Phys. Chem. C 2007, 111, 6821. (27) Zhao, Y.; Lu, Q. H.; Chen, D. S.; Wei, Y. J. Mater. Chem. 2006, 16, 4504. (28) Zhao, N.; Zhang, X. Y.; Zhang, X. L.; Xu, J. ChemPhysChem 2007, 8, 1108. (29) Yang, L. L.; Bai, S.; Zhu, D. S.; Yang, Z. H.; Zhang, M. F.; Zhang, Z. F.; Chen, E. Q.; Cao, W. J. Phys. Chem. C 2007, 111, 431. (30) Wang, S. T.; Feng, L.; Jiang, L. AdV. Mater. 2006, 18, 767. (31) Callies, M.; Quéré, D. Soft Matter 2005, 1, 55. (32) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. AdV. Mater. 1999, 11, 1365. (33) Su, C. H.; Li, J.; Geng, H. B.; Wang, Q. J.; Chen, Q. M. Appl. Surf. Sci. 2006, 253, 2633. (34) Prevo, B. G.; Hon, E. W.; Velev, O. D. J. Mater. Chem. 2007, 17, 791. (35) Kim, M.; Kim, K.; Lee, N. Y.; Shin, K.; Kim, Y. S. Chem. Commun. 2007, 2237. (36) Gu, Z. Z.; Uetsuka, H.; Takahashi, K.; Nakajima, R.; Onishi, H.; Fujishima, A.; Sato, O. Angew. Chem., Int. Ed. 2003, 42, 894. (37) Bravo, J.; Zhai, L.; Wu, Z. Z.; Cohen, R. E.; Rubner, M. F. Langmuir 2007, 23, 7293. (38) Qu, L. T.; Dai, L. M. J. Phys. Chem. B 2005, 109, 13985. (39) He, Y.; Wu, X. F.; Lu, G. W.; Shi, G. Q. Nanotechnology 2005, 16, 791.

10.1021/la801774v CCC: $40.75  2008 American Chemical Society Published on Web 08/22/2008

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Figure 1. SEM images of silver nanoparticle deposition morphology from the silver mirror reaction at various reaction times: (a) 15, (b) 43, (c) 60, and (d) 80 min. (e, f) Magnified images of d at flat and aggregated positions, respectively. The insertions of a, b, and c are their magnified images. Scale bars in a, b, c, and d are 10 µm, and the scale bars in e and g and insertions are 500 nm.

roughness, hydrophobicity increases when the surface roughness increases, and reflectivity decreases as the surface roughness increases. Therefore, good control over surface roughness is essential to optimizing both reflectivity and superhydrophobicity. In this letter, we report a facile method for preparing superhydrophobic surfaces with high reflectivity by carefully controlling the seed-induced silver mirror reaction.

Results and Discussion Silver nanoparticle seeds were prepared according to the literature.40,41 Briefly, silver nitrate was chemically reduced by NaBH4 in the presence of trisodium citrate to stabilize the nanoparticles. The UV absorption at 387 nm indicated that Ag seeds with a diameter of about 20 nm were successfully prepared.40 The polyethyleneimine (PEI)-coated substrates were immersed in Ag seed solution for half an hour to absorb a layer of seeds accordingly.42-44 Then the traditional silver mirror reaction was carried out on these Ag-seed-modified substrates by immersing them in a silver plating solution (9.562 mM silver amino, 51.07 mM glucose, and 6.007 mM tartaric acid) for a certain amount of time. The silver nanoparticles grew immediately after the substrates were immersed in the silver plating solution. The surface morphology was monitored by scanning electron microscopy (SEM) investigations (Figure 1). The SEM images showed that silver nanoparticles grew rapidly at the beginning by increasing the particle size from about 20 nm to more than (40) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 617. (41) Henglein, A.; Giersig, M. J. Phys. Chem. B 1999, 103, 9533. (42) Jana, N.; Gearheart, R. L.; Murphy, C. J. AdV. Mater. 2001, 13, 1389. (43) Huo, S. J.; Xue, X. K.; Li, Q. X.; Xu, S. F.; Cai, W. B. J. Phys. Chem. B 2006, 110, 25721. (44) Aslan, K.; Lakowicz, J. R.; Geddes, C. D. J. Phys. Chem. B 2005, 109, 6247.

100 nm in only 15 min (Figure 1a) and then to 200-300 nm when the deposition time increased to 43 min (Figure 1b). These silver nanoparticles packed together to form flat films. After 43 min, some nanoparticle aggregates appeared (Figure 1c,d) and grew gradually to form “rose”like micronanohierarchical composites (Figure 1d,f). The seed-induced silver mirror reaction lasted for about 3 h before the solution became clear again. After the silver films were formed from the seed-induced silver mirror reaction, octadecanethiol was used as a low-surface-energy compound to tune hydrophilic surfaces to hydrophobic ones. The final wettability of the as-prepared films was examined by water contact angle tests. It is shown in Figure 2 that the CAs of the films increased as the immersion time increased. Obviously, there were two large jumps and two regions of leveling off. The first jump occurred just when the seeded substrates were placed into the silver plating solution as a result of the formation of a well-packed silver film from loosely assembled seeds. Then, when the reaction lasted for about 8 min, it was followed by a leveling off at about 130° as the films appeared to be flat even though the particle size still increased. Another jump of the CAs happened when the aggregated clusters appeared on the flat films and the immersion time was about 43 min: the water contact angle rose from 130 to 140° when the aggregated structures grew to be 1-2 µm and covered 20-30% of the surface; the contact angle further reached to 174° when one of the dimensions reached to almost 5 µm and the coverage increased to almost 50%. The critical immersion time was around 80 min. Thereafter, another leveling off took place. According to the Cassie-Baxter equation cos θ* ) f cos θ + f - 1, where θ is the intrinsic contact angle and θ* is the apparent contact angle, the liquid and solid contact areas can be calculated. Because the intrinsic contact angle of alky-terminated surfaces is equal to 110°, when the

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Figure 2. Water contact angles of silver films from seed-induced silver mirror reaction at various time after being coated with octadecanethiol. The insertions are water droplet images whose corresponding reaction times are 5, 8, 43, 60, 80, and 150 min from left to right.

apparent contact angle increases to 174° on the as-prepared silver films, the liquid and solid contact areas are calculated to be only 0.83% over the apparent contact area. That is, over 99% of the water was in contact with air at the liquid-solid interfaces. This large amount of air trapped between the solid substrates and water droplets ensures that these surfaces have a very low tilt angle, or else water droplets can role off the substrates easily (Supporting Information 1), which guarantees the surface to have remarkable self-cleaning properties. Further study also demonstrated that these superhydrophobic surfaces have water-repellent (Supporting Information 2) and drag-resistant (Supporting Information 3) properties. As shown in Supporting Information 1, a 10 µL water droplet can hardly sit on the sample, even when the sample is not tilted, and rolls away quickly. Because the water droplets rolled off so easily, this induced difficulties in determining some of the CAs. In this research, the extremely large CAs were tested with the water-affinity method suggested by Gao and McCarthy.45 The superhydrophobic silver films were lowered to a supported droplet and repetively contacted, compressed, and released from the droplet. Encouragingly, the silver films that we made via immersion for no less than 80 min passed the test for “180°” contact angle surfaces suggested by Gao and McCarthy.45 As illustrated in Supporting Information 2, an 8 µL water droplet was suspended on a microsyringe, and then the sample was lifted to contact the droplet tightly. Interestingly, it was difficult to pull the droplet down to the substrate, or the surface had almost no apparent water adhesion to the suspending droplet, indicating that our superhydrophobic surfaces were considered to be water-repellent.15 Furthermore, the superhydrophobic surfaces as prepared here have no apparent water adhesion and drag reduction as reported previously.15 As the sample was lifted to contact a droplet (8 µL) tightly suspended on a microsyringe (Supporting Information 3) and then moved reversibly from one end to another, the droplet remained intact without any apparent deformation, preserving the perfect sphericity and behavior just like a solid ball sliding on a smooth surface, besides a slight turning. More interestingly, this superhydrophobic silver surface exhibited mirrorlike properties. As indicated in Figure 3, the silver film fabricated from the seed-induced silver mirror reaction bore both high reflectivity and superhydrophobicity after modification with octadecanethiol. (45) Gao, L. C.; McCarthy, T. J. J. Am. Chem. Soc. 2006, 128, 9052.

The clear inverted image of characters in the left picture and the water droplet in the right picture certified the high reflectivity of the prepared silver films, and the spherical water droplet in the right picture confirmed the superhydrophobicity. To further understand the relationship between micronanohierarchical structure and reflectivity, we monitored the specular reflectivity of silver films prepared for various reaction times, also with various surface morphologies, by ellipsometry (M2000, J.A.Woollam Co., Inc.) in specular reflective mode. As shown in Figure 4, the reflectivity went up and then down as a function of the deposition time. Hozumi and co-workers46 performed the silver mirror reaction on aldehyde-modified substrates and got compactly packed silver films with 300 nm nanoparticles, which had mirrorlike properties. In our research, we obtained similar silver densely packed films, which had extremely high reflectivity. When the reaction time went from 0 to 15 min, the reflectivity quickly increased to the highest point, and the silver film grew from a loosely packed thin film to a much thicker one. Then, when the reaction went from 15 to 43 min, the reflectivity did not change much even though the silver nanoparticles grew much larger. These phenomena reconfirmed that high reflectivity should be ascribed to compactly packed silver nanoparticle flat films. Here, we defined the reflectivity of sputtered silver films as the reference; that is, we defined their reflectivity as 100%. However, as we had assumed before, the reflectivity decreased when some silver nanoparticle clusters were produced. As the reaction time went on for 60 and 80 min and even longer, the reflectivity decreased because silver nanoparticle aggregates appeared on the flat silver films and gradually grew larger, which caused light scattering and affected the reflectivity. Therefore, there does exist a contradiction between reflectivity and superhydrophobicity. On one hand, high reflectivity in visible light demands a surface with a very small roughness. Because the wavelength range is only 400-700 nm, the formation of G5 µm clusters can decline the visible light reflection and even the infrared light reflection. On the other hand, it is just the aggregates of a certain size (about 5 µm in this research) and coverage (about 50% in our study) that make the triple-phase contact line extremely discrete and induced the unbelievable superhydrophobicity. There is an optimized point, that is at about 80 min of silver mirror reaction. When about 50% (46) Hozumi, A.; Inagaki, M.; Shirahata, N. Surf. Sci. 2006, 600, 4044.

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Figure 3. Digital photograph of a silver film fabricated from the seed-induced silver mirror reaction for 80 min, indicating high reflectivity (left) and extraordinary superhydrophobicity (right).

been proven to be a good candidate for antibacteria use, this superhydrophobic silver reflector may open a new avenue to producing the reflective part of biomedical devices.

Conclusions

Figure 4. Reflectivity of the prepared silver film surfaces from the seed-induced silver mirror reaction at different wavelengths: 600 nm (circles) and 1700 nm (triangles). The straight dashed line indicates the reflectivity of polished silicon at 600 nm.

of the densely packed film was covered with 5 µm silver clusters, a highly reflective superhydrophobic surface was obtained. The reflectivity is about 49%, which is close to that of polished silicon wafers, as observed from Figure 4. Meanwhile, the reflectivity of infrared light was higher than that of visible light. This reflective superhydrophobic material may find use in most of the areas that ordinary reflective materials used, such as light collection, thermal insulation, traffic and transportation, and decoration. What makes a difference is that this material can keep itself free from contamination because it possesses a remarkable superhydrophobic, or self-cleaning, ability. Because nanosilver has already

Superhydrophobic surfaces with micronanostructures were fabricated via the seed-induced silver mirror reaction in this research. It is also demonstrated that the contradiction between high reflectivity and superhydrophobicity really exists in our system. The micronanostructured silver clusters that make a crucial contribution to the self-cleaning properties of superhydrophobic surfaces may reduce reflectivity. By carefully controlling the deposition time, we finally obtained a superhydrophobic silver film with high reflectivity, which is as high as that of a polished silicon wafer, and a water contact angle of 174°. The even larger reflectivity in the infrared range may be useful in the solar energy collecting and thermal insulation industries. Acknowledgment. Financial support from the Natural Science Foundation of China (NSFC- 20774082), the 863 National HighTech R&D Program (2006AA03Z329 and 2006AA03Z444), the Program for New Century Excellent Talents in University (NCET05-0527), and the Science and Technology Projects of Zhejiang Province (2007C24008) is gratefully acknowledged. Supporting Information Available: Obtained superhydrophobic surfaces with low sliding angle, water repellency, and drag reduction properties. This material is available free of charge via the Internet at http://pubs.acs.org. LA801774V