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Mar 8, 2011 - Chinese Academy of Inspection and Quarantine, Beijing, 100123, P. R. China. bS Supporting Information. 'INTRODUCTION. Controllable ...
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Photocontrollable Water Permeation on the Micro/Nanoscale Hierarchical Structured ZnO Mesh Films Dongliang Tian,† Xiaofang Zhang,§ Jin Zhai,*,† and Lei Jiang*,†,‡ †

School of Chemistry and Environment, Beihang University, Beijing 100191, P. R. China Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China § Chinese Academy of Inspection and Quarantine, Beijing, 100123, P. R. China ‡

bS Supporting Information ABSTRACT: Most research of responsive surfaces mainly focus on the wettability transition on different solid substrate surfaces, but the dynamic properties of the micro/nanostructure-enhanced responsive wettability on microscale pore arrays are lacking and still remain a challenge. Here we report the photocontrollable water permeation on micro/nanoscale hierarchical structured ZnO-coated stainless steel mesh films. Especially, for aligned ZnO nanorod array-coated stainless steel mesh film, the film shows good water permeability under irradiation, while it is impermeable to water after dark storage. A detailed investigation indicates that the special nanostructure and the appropriate size of the microscale mesh pores play a crucial role in the excellent controllability over water permeation. The excellent controllability of water permeation on this film is promising in various important applications such as filtration, microreactor, and micro/nano fluidic devices. This work may provide interesting insight into the design of novel functional devices that are relevant to surface wettability.

’ INTRODUCTION Controllable surface wettability of solid substrates has aroused great interest for their importance in fundamental research and industrial applications. As reported, chemical composition and geometrical structure are two main factors to govern surface wettability. Recently, responsive materials, for their intrinsic reaction to environmental stimuli such as light irradiation,1-9 electric fields,10-18 thermal treatment,19 pH,20,21 and solvent treatment,22 have been extensively used as controllable surfaces. Because the contact angle (CA) change of the smooth responsive materials surfaces is usually very limited, micro/nanostructures have been introduced to enhance the responsive wettability between superhydrophilicity (CA < 5) and superhydrophobicity (CA > 150).3-6 For example, surface wettability of the photoresponsive materials such as ZnO, TiO2, SnO2, and WO3 switching between superhydrophilicity and superhydrophobicity have been reported.1 Because of the excellent intelligent controllability between the two extremes of wettability, this kind of materials are promising to be used in many important fields, such as microreactors, lab-on-chip devices, and micro/nanofluidic devices. However, most research mainly focus on the wettability transition between superhydrophilicity and superhydrophobicity on different solid substrate surfaces. Moreover, although the oil and r 2011 American Chemical Society

water separation films based on the mesh surfaces with intrinsic both superhydrophobic and superoleophilic properties has been reported,23-27 the properties, e.g., the controllable water permeation, of the micro/nanostructure-enhanced responsive wettability on microscale pore array such as mesh films, and the corresponding application research are lacking,28,29 which is still a challenge. As known, water permeation process is ubiquitous in living systems, production and life. The study of controllable water permeation process has significant implications for the understanding of biological activities and the design of novel micro/ nanofluidic devices or machines, and also has a wide range of potential applications.30-34 The controllable water permeation process is not only governed by the intrinsic wettability properties of the materials, but also more relevant to the controllable wetability, which is greatly influenced by their geometrical structure of the surface. Here we report a photocontrollable water permeation process on the ZnO-coated stainless steel mesh film with special hierarchical Received: December 27, 2010 Revised: January 29, 2011 Published: March 08, 2011 4265

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Langmuir micro/nanostructures. Namely, the film is hydrophobic and water can not penetrate the mesh film after dark storage because of the large negative capillary effect, while the film is hydrophilic and water can spread out quickly and permeate the mesh film under ultraviolet (UV) irradiation. This work is promising to gear up the application of the film in controllable filtration and microreactor, lab-on-chip devices, micro/nanofluidic devices, and so on.

’ EXPERIMENTAL SECTION Preparation of the Micro/Nanoscale Hierarchical Structured ZnO Films. The original stainless steel mesh films (or glass substrate) were sequentially cleaned with detergent, deionized water, ethanol, acetone, and deionized water before being dried at 80 C for 1 h. The aligned ZnO nanorod array-coated mesh film (or glass substrate) was prepared via a two-step solution approach. First, ZnO sol, prepared according to the method of literature,35 was dip coating onto the stainless steel mesh substrate (or glass substrate) three times and annealed at 420 C to prepare a 100-200 nm thick film of crystal seeds. The asprepared substrate was vertically suspended in an aqueous solution of zinc nitrate hydrate (0.02 M) and methenamine (0.02 M) at 85 C for 15 h. Then, it was removed from the solution, rinsed with deionized water and dried at 80 C for 2 h. The microscale hierarchical structured ZnOcoated mesh (or glass) film was prepared by placing the as-prepared ZnO crystal seeds mesh film horizontally into the bottom of an aqueous solution of zinc nitrate hydrate (0.03 M) and methenamine (0.03 M) at 85 C for 15 h, after that, the mesh film was turned over and another 15 h were needed. Then, it was removed from the solution, rinsed with deionized water and dried at 80 C for 2 h. These as-prepared micro/ nanoscale hierarchical structured ZnO films were kept in the dark for 2 weeks before being tested. Instruments and Characterization. SEM images were obtained using a JEOL JSM-6700F SEM at 3.0 kV. X-ray powder diffraction (XRD) patterns were recorded with a Mac Science MXPAHF18 X-ray diffractometer using Cu KR radiation. CAs were measured on a Dataphysics OCA20 CA system at ambient temperature. A 3 μL water droplet (γLV = 72 mN m-1) is used in all the water CA measurements. At least five different positions were measured and averaged to get a reliable value for the same sample. All of the UV irradiation and dark storage conditions are given as follows: UV irradiation on every one of the ZnO-coated mesh films is for ∼0.5 h (a 500 W Hg lamp with a filter centered at 365 ( 10 nm was used as the light source), and the distance between UV light source and sample is ∼20 cm. After the UV irradiated films were placed in the dark for 7 days (or under atmosphere of O2 for 2 days), a new superhydrophobicity or hydrophobicity of the films was obtained again for each sample.

’ RESULTS AND DISCUSSION In order to realize the photoinduced water permeation process, photoresponsive materials with special surface structure are necessary. Micro/nanoscale hierarchical structured ZnO-coated pore array surface (i.e., mesh film) is a good candidate for this purpose, because ZnO is a wide-band gap semiconductor with excellent surface wettability switching performance, and its pore array structure can also exhibit that wetting easily occurs in direction parallel to the microscale pore array under irradiation due to the capillary effect. In this study, the micro/nanoscale hierarchical structured ZnO is grown on the stainless steel mesh films. Figure 1 shows the scanning electron microscope (SEM) images, schematic diagrams of the as-prepared smooth and micro/nanoscale hierarchical structured ZnO on the stainless steel mesh films. The original mesh films are knitted by stainless steel

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wires used as substrates (see Figure S1 in the Supporting Information). Compared with the smooth ZnO-coated mesh film (Figure 1a), the top view of the SEM images indicates that the aligned ZnO nanorod arrays (Figure 1b) are grown radially and exhibited uniformly along the entire length of every wire with the diameter in the range ∼50-150 nm, and well preserved the cylindrical shape. All of the ZnO nanorods have a hexagonal cross-section with the length of ∼2 μm (see Figure S2 in Supporting Information). As confirmed by X-ray diffraction (XRD) with a remarkably enhanced (002) peak, the surface of the films is the (001) plane of the nanorods. Similarly, the flowerlike ZnO nanorods microstructures (Figure 1c) are also distributed along the wire and keep the cylindrical shape. The branched nanostructures consisted of several nanorods with the diameter in the range ∼500-1000 nm and the length of several micrometers. The sharp X-ray diffraction (XRD) peak at (100), (002), (101), (102), and (110) can be deduced that the samples are well crystallized hexagonal wurtzite-type. Noticeably, the pores of all the micro/nanoscale hierarchical structured ZnO mesh films remain almost the same as the original mesh, and the micro/ nanostructure can enhance the wetability (hydrophobicity after dark storage and hydrophilicity under irradiation),36,37 and then the water permeation process of the mesh films is feasible. As reported before, the CA variation of the responsive wetability on the ZnO film can be realized by alternation of UV irradiation and dark storage,3 which is also demonstrated on the aligned ZnO nanorod arrays-coated glass surface in this work (Figure 2a). To illustrate the photocontrollable water permeation process, the aligned ZnO nanorod arrays were introduced to the stainless steel mesh film with the square pores of approximate 48 μm. The CA measurement experiments (as mentioned above in the Experimental Section) show that this mesh film is superhydrophobic with the CA of ∼152 (top left side of Figure 2b) after dark storage, water drop stand on the film stably and can not penetrate through the film spontaneously because of the strong negative capillary effect. While the CA on the corresponding surface shows superhydrophilicity under UV irradiation, and the water drop would easily penetrate the film when it is in contact with the film (top right side of Figure 2b). The spreading and penetration process is very quick, and the water drop will drop down when more water is added (bottom right side of Figure 2b). This indicates that the film behaves very good permeability under UV irradiation. After dark storage, the film is superhydrophobic again, and a CA as large as about ∼152 is reached. Accordingly, water permeation can be well controlled through adjusting UV irradiation and dark storage due to the different wettabilities that the superhydrophilicity (good permeability) under UV irradiation and the superhydrophobicity (impermeability) after dark storage. Compared with the aligned ZnO nanorod arrays-coated the stainless steel mesh film, suitable photoresponsive wetability change of the smooth and flower-like nanorods microstructure ZnO-coated stainless steel mesh films was also observed, but it is very limited. After dark storage, the smooth ZnO-coated stainless steel mesh film shows hydrophobicity with a CA of about ∼112 (left side of Figure 2c), while the CA on the corresponding smooth surface shows hydrophilicity under UV irradiation, and the water drop would easily penetrate the film when it is in contact with the film (right side of Figure 2c). Similarly, the flower-like ZnO nanorods microstructure-coated stainless steel mesh film shows hydrophobicity with a CA of about ∼138 (left side of Figure 2d) after dark storage, while the CA on corresponding surface shows hydrophilicity under UV irradiation, and 4266

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Figure 1. SEM images, schematic diagrams, and XRD patterns of the as-prepared smooth and micro/nanoscale hierarchical structured ZnO on the stainless steel mesh films: (a) smooth structure; (b) aligned nanorod arrays; (c) flower-like nanorod microstructure. Insets show the corresponding schematic diagrams of the respective cross-section of the smooth and micro/nanoscale hierarchical structured ZnO-coated stainless steel wires. The inner circularity (in blue) represents the stainless steel wire and the outer ring, radial or branchlike pattern (in yellow) is the corresponding smooth and micro/nanoscale hierarchical structured ZnO layer.

the water drop would also easily penetrate the film when it is in contact with the film (right side of Figure 2d). Because water CAs on such smooth and flower-like ZnO nanorods microstructurecoated mesh films are much lower than that of the aligned ZnO nanorod arrays-coated mesh film, thus these mesh films are not high hydrophobic enough to prevent water from leaking if small water pressure exists or under vibration even after dark storage, i. e., water drop can not stand on these film stably. Upon considering the good controllability of the water permeation, the aligned ZnO nanorod arrays-coated the stainless steel mesh film is chosen for further study. Further experiments of repeatedly cycling under irradiation and after dark storage show that the aligned ZnO nanorod arrayscoated the stainless steel mesh has good transformation and repeatability between superhydrophilicity and superhydrophobicity (Figure 3). It reveals the feasibility of this film to be successfully utilized in practical systems, which is proven by a simple smart filtration device. As shown in Figure 4, a piece of the aligned ZnO nanorod arrays-coated stainless steel mesh film was placed and fixed to the bottom of a glass tube. After dark storage, water can not penetrate the mesh film (left side in Figure 4);

while under UV irradiation, water can easily penetrate through the mesh film and water drops were observed to drop down immediately when they were added from the upper side of the glass tube (the right side in Figure 4). For the photoinduced water permeation device above-mentioned, the responsive static and dynamic wettability change of ZnO-coated mesh film are the two important factors that induced by the cooperation of photocontrollable surface chemical component change and the micro/nanostructures of the mesh film. On one hand, ZnO, as a photoresponsive material, is coated to the mesh surface. On such surface, water and oxygen may compete to dissociatively adsorb on the surface oxygen vacancies by alternation of UV irradiation and dark storage,3 which leads to photoresponsive wettability. On the other hand, as indicated by Cassie36 and Wenzel37 equations, the surface roughness induced by the special nanostructures on the rough stainless steel mesh film can enhance not only the hydrophilicity under UV irradiation but also the hydrophobicity after dark storage, which will result in amplified responsive wettability switching between superhydrophilicity and superhydrophobicity. More importantly, the micro/nanostructures will also greatly influence the 4267

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Figure 4. Schematic diagrams of the photocontrollable water permeation device. The device is composed of a glass tube with an aligned ZnO nanorod array-coated stainless mesh film at one end (inset photograph on the left), and a triangular flask as a container to collect the water. Water can not penetrate the rough mesh film after dark storage (left), while water penetrates the rough mesh film easily under UV irradiation, and no water can be supported by the mesh film (right).

Figure 2. Photocontrollable wettability response of ZnO film and water permeation on micro/nanoscale hierarchical structured ZnO-coated stainless steel mesh films. (a) Water CA photographs of the aligned ZnO nanorod array films before (left) and after (right) UV irradiation. Photocontrollable water permeation on (b) aligned nanorod arrays, (c) smooth, and (d) flower-like nanorod microstructured ZnO-coated stainless steel mesh films. After dark storage (left), the film is hydrophobic, especially, aligned nanorod arrays-structured ZnO-coated mesh film is superhydrophobic with a CA of about 152, and a water drop can not penetrate the film because of the large negative capillary effect, while water can spread out quickly and permeate the mesh film under UV irradiation (right). When more water is dropped, the water droplet can drop down.

Figure 3. Reversible water CAs transition of the aligned ZnO nanorod array-coated stainless mesh film under the alternation of UV irradiation and dark storage.

dynamic properties of wettability38 at both UV irradiation and after dark storage. Water prefers to fill the nanostructures first on a hydrophilic rough surface (under UV irradiation) because of a much larger capillary effect than for the usual mesh pores on the micrometer scale,39 while after dark storage, such nanostructures

Figure 5. Schematic diagrams of the wetting model of the as-prepared aligned ZnO nanorod array-coated mesh films. (a) When the liquid is still on the mesh films, the meniscus of the liquid is stable, i.e., the intrinsic contact angle θ0, is maintained regardless of the three-phase contact line position due to the surface curvature. (b) With increasing applied pressure or reducing intrinsic contact angle, the three-phase contact line position (defined as R) of the meniscus changes as long as the advancing contact angle θA = θa,0 of the film is satisfied, and the liquid persists in moving to the maximum position of the pore that the liquid can be confined. The position, O, is the center of the spherical cap that describes the meniscus. O1 and O2 are the cross-section center of the stainless steel wires, O1O2 = d. A and B are the three-phase contact line positions. AF is the tangent to the meniscus. AE is the tangent of the circle of the cross-section of the stainless steel wires. AG is an extension of O1A. R is the radius of meniscus. Thus, — HAO = θ - — EAO — FAH = 3π/2 - R - θ.

can bring a sufficient proportion of trapped air to the surface, and largely improve the repellent force to water because of the size dependence of the capillary effect and the great increase in the total length of the air/water/solid triple contact line. Accordingly, both the permeability under UV irradiation and the antipermeation capacity after dark storage can be remarkably enhanced by suitable micro/nanostructures. To further thoroughly understand the mechanism of the photocontrollable water permeation of the ZnO-coated stainless steel mesh films, we model the process in Figure 5 on the assumption that the pores are arranged approximately in a regular square array. From Figure 5a, it is clear that the meniscus of the 4268

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Figure 6. Water contact angles of the ZnO-coated mesh films under UV irradiation (d-f) and after dark storage (a-c)as a function of the pore size of corresponding original stainless steel mesh films. The ZnOcoated mesh films: (a and d) aligned nanorod arrays-structured ZnO; (b and e) flower-like nanorod microstructured ZnO; (c and f) smoothstructured ZnO.

liquid is stable when the liquid is still on the mesh films. The position of the three-phase contact line (defined by R) of the meniscus is mainly governed by the differential pressure, ΔP, across the meniscus, and the relationship is given as:40,41 ΔP ¼

2γLV 2γ cosð3π=2 - R - θ0 Þ ¼ ¼ LV d=2 - r sin R R - lγLV ðcos θA Þ=A

ð1Þ

where γLV is the surface tension of the liquid-vapor interface, R is the radius of meniscus, d is the cross-section center distance of the stainless steel wires, l is the circumference of the pore, A is the cross-sectional area of the pore, θ0 is the intrinsic contact angle of liquid on the surface, and θA is the advancing contact angle of liquid on the surface. The intrinsic contact angle θ0 is maintained regardless of the three-phase contact line position due to the surface curvature as R changes. With increasing applied pressure or reducing intrinsic contact angle, the three-phase contact line position of the meniscus changes as long as the advancing contact angle θA = θa,0 of the film is satisfied, and the liquid persists in moving to the maximum position of the pore that the liquid can be confined (Figure 5b). It can also be deduced that as R becomes larger than π - θ (the position of the planar meniscus), the pressure differential across the meniscus increases with increasing R and will reach a maximum value before R reaches 180. When the applied pressure ΔP increases further, the liquid will overcome the bound of the mesh pores and the liquid permeation process occurs, that is, the wettability transit from Cassie to Wenzel state. In general, with the advancing contact angle θA = θa,0 increase, much larger applied pressure is needed to realize the liquid permeation process. In this work, the advancing contact angle θA = θa,0 of the aligned ZnO nanorod arrays, the flower-like ZnO nanorods microstructure and the smooth ZnO-coated mesh film are ∼154, ∼ 139, and ∼113, respectively. As a result, the applied pressure that needed to transit the liquid from Cassie to Wenzel state and realize the liquid permeation decreases in turn, namely, the liquid on the aligned ZnO nanorod arrays-coated mesh film is more stable than the other two structured films after dark storage. While the three types of ZnO-coated mesh films are all hydrophilic (CAs are all less than 20) under light irradiation, the advancing contact angle θA = θa,0 decrease considerably, and the

mesh film can not sustain a little water, especially for aligned ZnO nanorod arrays-coated mesh film (CAs are all less than 5), and good penetration effect can be achieved. Therefore, the effect of the aligned ZnO nanorod arrays-coated mesh film can be seemed as the advancing CA of the surface increase after dark storage, as a result, water pressure of the mesh film can support is higher. Calculated by the eq 1, the theoretical value of maximum water column that the aligned ZnO nanorod arrays-coated mesh film can support is about 50 cm. And the aligned nanorod arrays can also decrease the advancing CA of the surface under light irradiation to achieve good penetration effect. Therefore, the aligned ZnO nanorod arrays-coated stainless steel mesh films can be used as a controllable and stable water permeation device. To achieve the suitable micro/nanoscale hierarchical structure, various ZnO-coated rough stainless steel mesh films with different original mesh pore sizes were fabricated, and their corresponding wettability under UV irradiation and after dark storage were investigated. Figure 6 shows that the water CAs of the ZnO-coated mesh films under UV irradiation and after dark storage as a function of the pore size of the corresponding original stainless steel mesh films. Although all films exhibit superhydrophilicity and good permeability when UV irradiation, the hydrophobicity of the corresponding films after dark storage shows a remarkable dependence on pore size. Meantime, the pore size of the films does not change with UV irradiation (see Figure S3 in Supporting Information). Within experimental and instrumental error, only the aligned ZnO nanorod array-coated mesh films with pore sizes of ∼50 μm can exhibit superhydrophobicity after dark storage. Below 50 μm, the lack of superhydrophobicity can be explained by the insufficient proportion of the air/water interface. Whereas above 50 μm, because the mesh pore size is too large, the hydrophobic force that is provided by the nanostructured stainless steel thread may not be enough to support the water drop, decreased hydrophobicity is observed when the size of the mesh pores increases further. Because the largest difference between the wettability under UV irradiation and after dark storage is favorable for good controllability of the device, it can be inferred that ∼50 μm is the optimum pore size in the original mesh film; values above or below this range will lead to a decrease in controllability. Therefore, not only the nanostructures but also the suitable size of the micrometer-scale mesh pores is important in obtaining the best controllability of this device, that is, the nanostructures may efficiently enhance both the permeability under UV irradiation and the impermeability after dark storage, and the suitable size of the mesh pores may help to optimize the performance of the device.

’ CONCLUSIONS In conclusion, this work has demonstrated the photocontrollable water permeation on micro/nanoscale hierarchical structured ZnO-coated stainless steel mesh film. Especially, for aligned ZnO nanorod array-coated stainless steel mesh films, the film shows good water permeability under UV irradiation, while it is impermeable to water after dark storage. The special nanostructures and the appropriate size of the microscale mesh pores play a crucial role in the excellent controllability over water permeation on this film. The excellent controllability and repeatability of water permeation process on this film show that it is promising in various important applications including controllable filtration, microreactor, micro/nanofluidic devices, and so 4269

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Langmuir on. This work may also provide interesting insight into the design of novel functional devices that are relevant to surface wettability.

’ ASSOCIATED CONTENT

bS

Supporting Information. SEM images and pore size and pore size distribution of the ZnO-coated mesh films.This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: (L.J.) [email protected]; (J.Z.) [email protected].

’ ACKNOWLEDGMENT The authors thank the financial support by National Natural Science Fundation (21003006, 20974113, 20601005, 20971010), National Research Fund for Fundamental Key Projects (2010CB934700, 2009CB930404, 2007CB936403), and the Fundamental Research Funds for the Central Universities (YWF-10-01B16, YWF-10-01-C10). The Chinese Academy of Sciences is gratefully acknowledged. ’ REFERENCES (1) Wang, S. T.; Song, Y. L.; Jiang, L. J. Photochem. Photobiol. C: Photochem. Rev. 2007, 8, 18. (2) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (3) Feng, X. J.; Feng, L.; Jin, M. H.; Zhai, J.; Jiang, L.; Zhu, D. B. J. Am. Chem. Soc. 2004, 126, 62. (4) Lim, H. S.; Han, J. T.; Kwak, D.; Jin, M. H.; Cho, K. J. Am. Chem. Soc. 2006, 128, 14458. (5) Zhang, X. T.; Sato, O.; Fujishima, A. Langmuir 2004, 20, 6065. (6) Feng, C. L.; Zhang, Y. J.; Jin, J.; Song, Y. L.; Xie, L. Y.; Qu, G. R.; Jiang, L.; Zhu, D. B. Langmuir 2001, 17, 4593. (7) Athanassiou, A.; Lygeraki, M. I.; Pisignano, D.; Lakiotaki, K.; Varda, M.; Mele, E.; Fotakis, C.; Cingolani, R.; Anastasiadis, S. H. Langmuir 2006, 22, 2329. (8) Irie, H.; Ping, T. S.; Shibata, T.; Hashimoto, K. Electrochem. SolidState Lett. 2005, 8, D23. (9) Rosario, R.; Gust, D.; Garcia, A. A.; Hayes, M.; Taraci, J. L.; Clement, T.; Dailey, J. W.; Picraux, S. T. J. Phys. Chem. B 2004, 108, 12640. (10) Prins, M. W. J.; Welters, W. J. J.; Weekamp, J. W. Science 2001, 291, 277. (11) Krupenkin, T. N.; Taylor, J. A.; Schneider, T. M.; Yang, S. Langmuir 2004, 20, 3824. (12) Zhu, L. B.; Xu, J. W.; Xiu, Y. H.; Sun, Y. Y.; Hess, D. W.; Wong, C. P. J. Phys. Chem. B 2006, 110, 15945. (13) Wang, Z. K.; Ci, L. J.; Chen, L.; Nayak, S.; Ajayan, P. M.; Koratkar, N. Nano Lett. 2007, 7, 697. (14) Mugele, F.; Baret, J. C. J. Phys.: Condens. Matter 2005, 17, R705. (15) Verplanck, N.; Galopin, E.; Camart, J. C.; Thomy, V.; Coffinier, Y.; Boukherrou, R. Nano Lett. 2007, 7, 813. (16) Dhindsa, M. S.; Smith, N. R.; Heikenfeld, J.; Rack, P. D.; Fowlkes, J. D.; Doktycz, M. J.; Melechko, A. V.; Simpson, M. L. Langmuir 2006, 22, 9030. (17) McHale, G.; Herbertson, D. L.; Elliott, S. J.; Shirtcliffe, N. J.; Newton, M. I. Langmuir 2007, 23, 918. (18) Tian, D. L.; Chen, Q. W.; Nie, F.-Q.; Xu, J. J.; Song, Y. L.; Jiang, L. Adv. Mater. 2009, 21, 3744. (19) Crevoisier, G. D.; Fabre, P.; Corpart, J. M.; Leibler, L. Science 1999, 285, 1246.

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