ARTICLE pubs.acs.org/JPCC
Copper Nanowires Array: Controllable Construction and Tunable Wettability Jie Li, Zheng Guo, Jin-Huai Liu,* and Xing-Jiu Huang* Research Center for Biomimetic Functional Materials and Sensing Devices, Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, P.R. China
bS Supporting Information ABSTRACT: Copper nanowires array have been constructed using a porous alumina template approach. The morphologies of nanowires array could be modulated by introducing Sn4+ ion except for adjusting the deposition time, dissolution time, and Cu2+ ion concentrations. Scanning electron microscopy, transmission electron microscopy, and X-ray diffractometer are employed to characterize and analyze as-synthesized samples. The results indicate that the component of obtained nanowires is not CuSn alloy but Cu. The wettability of as-prepared samples after silanization has been carefully investigated. It shows that the wettability has been greatly governed by their morphologies, which demonstrates the evolution of Wenzel to Cassie model. The relationship of the contact time with impact velocity has also been investigated.
’ INTRODUCTION The “lotus effect”, that when water droplets land on the surface of a natural lotus leaf they can move freely and roll off with dust particles and surface contaminants, has inspired a significant amount of research on wettability of solid surfaces, especially the realization of superhydrophobic behavior and selfcleaning properties.13 As an important standard of surface wettability, the contact angle is governed by many factors, such as surface free energy, chemical composition, and roughness of the surfaces.46 Recently, various approaches have been employed to modulate these factors to obtain the specific wettability of surface.7 For example, through decreasing surface free energy and increasing roughness, superhydrophobic surface with the contact angles in excess of 150° and the contact angle hysteresis of less than 5° could be realized.810 As increasing numbers of aspects about superhydrophobicity are applied, more and more applications, from windshields, self-cleaning windows, exterior paints for roof tiles, building, ships, and so on, to magnetic storage devices and micro-/nanoelectromechanical systems as well as controlling polluted liquid dropletss that contain relative protein, are emerging and calling for more study about superhydrophobic behavior.1114 Various materials, such as Si nanorods,15 ZnO nanowire films,1619 Cu(OH)2 nanowires film,20 and carbon nanotubes,21 are fabricated by glancing angle deposition,15 electrospinning,22 thermal oxidation,18 electroplating,23,24 chemical etching,25,26 chemical vapor deposition,27 thermal chemical vapor transport,19 and hydrothermal method.18 Many researchers have focused on producing hydrophobic surfaces by increasing roughness, and many ways are used to decrease the surface free energy, such as r 2011 American Chemical Society
chemical modification and coating the surface with polymeric monolayers or polymer fluorine. Although the fabrication of superhydrophobic surfaces has been extensively studied for numerous materials by various methods,2830 only a few studies involve realizing different degrees of wettability by adjusting and controlling the morphology. In this paper, a controllable fabrication of copper nanowires arrays with different typical morphologies based on an electrodeposition approach will be introduced, and how several factors, especially the second metal ions’ existence, affect the final topography will be specified. Good controllability and reproducibility was the first focus in this study. And surfaces with different water wettabilities were further prepared via alkylation reaction, realizing the fabrication of controllable superhydrophobic surfaces. Also, the dynamic impact behavior of a water droplet on different surfaces was investigated.
’ EXPERIMENTAL SECTION All the reagents and solvents, which were purchased from Shanghai Chemical Reagent Ltd. Co., are analytical grade and used as received without further purification. Preparation of Cu Nanowires Array. Anodic aluminum oxide (AAO) surrounded by metal Al, as the template in the electroless deposition process of Cu nanowires, was prepared by a two-step anodization process.27,31 The diameter of the channels and the spacing between pores were around 60 and 80 nm, Received: June 21, 2011 Revised: July 19, 2011 Published: July 21, 2011 16934
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Table 1. Ninety-Six Deposition Conditions, Subscripts Representing Different Dissolved Timesa Deposition Time proportion (Sn/Cu, mol/L)
20 h
30 h
40 h
60 h
2:1
0.025:0.0125
A1,2,3,4
G1,2,3,4
M1,2,3,4
S1,2,3,4
1:1
0.05:0.025 0.025:0.025
B1,2,3,4 C1,2,3,4
H1,2,3,4 I1,2,3,4
N1,2,3,4 O1,2,3,4
T1,2,3,4 U1,2,3,4
0.0125:0.0125
D1,2,3,4
J1,2,3,4
P1,2,3,4
V1,2,3,4
1:2
0.0125:0.025
E1,2,3,4
K1,2,3,4
Q1,2,3,4
W1,2,3,4
0.025:0.05
F1,2,3,4
L1,2,3,4
R1,2,3,4
X1,2,3,4
a
The subscripts 1, 2, 3, and 4 represent 20, 40, 60, and 80 min, respectively. A, B, C, ..., X represent serial numbers of the samples in the experiments.
respectively. Prior to the deposition of Cu nanowires, an Au film (about 100 nm thickness) was evaporated onto the bottom surface of the Al-surrounded AAO membrane with two-end through pores. To prepare Cu nanowires array, CuCl2 and SnCl4 aqueous solutions were infiltrated into an Au-coated native porous Al-surrounded AAO template at room temperature according to a previously reported approach.31 In this modified process, Sn4+ (SnCl4 aqueous solution) has been employed to modulate the growth of Cu nanowires and obtain the specific morphology of Cu nanowires array. Notably, a narrow ringshaped part of Al should be exposed before infilitration, when the Al-surrounded AAO template was held in a simple polymethyl methacrylate holder. With a specific time electroless deposition finished, the AAO template embedded with Cu nanowires was taken out from the holder, washed with deionized water several times, and dried at room temperature. Ninety-six different experimental conditions, as shown in Table 1, depending on concentration ratio and ion concentrations of Cu2+ and Sn4+, deposition time, and the dissolved time of AAO templates, are employed for the fabrication and treatment of samples. To obtain large-scale samples avoiding breakage, it was fixed on a piece of silicon wafer before the removal of AAO. Then the samples were immersed into 1 M NaOH aqueous solution for a specific time. Subsequently, it was rinsed with deionized water and dried at room temperature for further use. Characterization. The morphology and structure of copper nanowires were investigated by scanning electron microscopy (FESEM, FEI Quanta 200 FEG), transmission electron microscopy (TEM, JEM-200CX), and selected area electron diffraction (HRTEM/SAED, JEM 2010). X-ray diffraction (XRD) patterns of the samples were recorded on a Philips X’pert diffractometer (XRD, X’Pert Pro MPD) with Cu KR radiation (1.5418 Å). The contact angle measurements were performed on an OCA 20 (DataPhysics Instruments GmbH, Filderstadt) instrument at 25 °C using sessile drop fitting method for the static contact angle and needle in sessile drop for the dynamic contact angle. Prior to the measurements, the samples are chemically modified with fluoroalkysilane, which is preformed with 2 μL of ethanol solution of 20 mM 1H,1H,2H,2H-perfluorodecyltrichlorosilane (Alfa Aesar, USA) dropping on the substrates of the samples. Finally, the samples were dried at room temperature for 48 h, inducing a layer of perfluorosilane on the surface of the samples. The static contact angles are measured in at least 12 different locations for each sample.
Figure 1. (a) SEM image of the side cross section for Cu nanowires array; (b) TEM image of Cu nanowires; (c) high-magnification TEM image of individual Cu nanowire; and (d) X-ray diffraction pattern of asdeposited copper nanowires. The inset in panel c corresponds to the SAED pattern of the Cu nanowire.
’ RESULTS AND DISCUSSION After the deposition for 60 h in the 0.025:0.0125 M (Sn4+:Cu2+) solution and the removal of the AAO template by immersion into 1 M NaOH aqueous solution for 20 min, Cu nanowires arrays have been prepared. According to the side cross-sectional view shown in Figure 1a, it could be observed that the length of Cu nanowires is about 10 μm. On the basis of TEM images presented in Figure 1b, as-prepared nanowires are continuous and uniform in diameter along the entire length of the wires. The diameter is about 60 nm, which is in agreement with that of the AAO channel diameter. This result could be confirmed from the high-magnified TEM image of an individual nanowire shown in Figure 1c. An XRD pattern of the sample via the complete removal of AAO are shown in Figure 1d. Interestingly, it could be found that as-synthesized nanowires are pure Cu without any other Sn phase observed. This phenomenon is different from the previous reports concerning the preparation of metal alloys via the AAO template.31 The three diffraction peaks completely fit very well with Cu(111), Cu(200), and Cu(220), respectively. Furthermore, the SAED pattern suggests that Cu nanowires have single crystalline structure, which is shown in the inset of Figure 1c. Owing to the existence of Sn4+, the deposition of Cu nanowires is affected. Figure 2 shows the morphology evolution of Cu nanowires arrays with the increase of the Cu2+ relative concentration. Figure 2a describes the top view of the sample prepared at low relative concentration (Sn4+:Cu2+ = 0.025:0.0125 M). From the magnified SEM image shown in Figure 2d, it could be easily found that Cu nanowires stay compact and approximately vertical to the substrate. With the Cu2+ concentration increasing to 0.025 M, the nanowires shown in (b) and (e) of Figure 2 would be inclined to congregate together. However, at relative high concentration (Sn4+:Cu2+ = 0.0125:0.025 M) as-prepared nanowire array shows a honeycomb-like structure, which could be observed in Figure 2c,f. These results might be mainly ascribed to the increase of the 16935
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directions as the result of a concentration gradient.31 For pure diffusion in the solution, the flux of Cu atoms is described with Fick’s first law as follows, J ¼ D dC=dx
Figure 2. Three typical morphologies of Cu nanowires array: (a) and (d) SEM images corresponding to low- and high-magnification morphologies of sample a prepared under 0.025:0.0125 M (Sn4+:Cu2+). (b) and (e) SEM images corresponding to low- and high-magnification morphologies of sample b prepared under 0.025:0.025 M (Sn4+:Cu2+). (c) and (f) SEM images corresponding to low- and high-magnification morphologies of sample c prepared under 0.0125:0.025 M (Sn4+:Cu2+). All of the three samples were deposited for 60 h, and removed AAO template by immersion into 1 M NaOH aqueous solution for 20 min.
length of deposited Cu nanowires with the increase of the Cu2+ relative concentration. For the longer nanowires, it can be easily intertwined together from the flash evaporation of the water, attributed to the horizontal component of the water’s surface tension. But for the short nanowires, it could not easily be bent under the drive of the water’s surface tension. Although the nanowires arrays show different morphologies under different Sn4+ and Cu2+ concentrations, the component always remains unchanged. This has been concluded from their XRD patterns. (More information is shown in Figure S1). Thereof, the existence and concentration of tin ions would just manipulate the speed and length of forming copper nanowires and length, and further affected the morphology of the nanowires arrays after the removal of AAO. On the basis of the above results, the possible mechanism for the tunable effect of Sn4+ to prepare Cu nanowires can be speculated. When only CuCl2 solution is employed, the copper nanowires were formed, which is the same as the previous report.31 First, Cu2+ ions obtain electrons from around Al and turn Cu atoms, as chloride ions can break down the surface alumina layer on pure Al foil. Then Cu atoms accumulated at the bottom of the template, besides the majority that formed at the surface of the template. Therefore, the nanowires grow from both
where J is the flux of Cu atoms, D is the diffusion coefficient of Cu atoms, and dC/dx is the concentration gradient of Cu atoms. However, when Sn4+ ions were introduced into this case, owing to the lower redox potential of Al (Al3+/Al) than that of Sn4+/Sn, it is also favorable to the formation of Sn atoms in theory. In contrast to the redox potential of Cu2+/Cu, the potential of Sn4+/ Sn is lower. Accordingly, in the above reaction system, Cu2+ ions are more easily reduced. Maybe in this case some Sn atoms have been formed. Because of the existence of Cu2+ ions, the replacement reaction between Sn atoms and Cu2+ ions immediately occurs. This is the reason that as-prepared nanowires are only Cu nanowires, not alloy formed by Cu and Sn. Furthermore, the existence of Sn4+ could decrease the diffusion coefficient D to some extent, as Cu2+ ions would need more collisions to meet electrons and the diffusion of Cu atoms also become increasingly difficult. When the concentration of Sn4+ ion is relatively high, the flux of Cu atoms becomes rather low, and the growth speed decreased too. Accordingly, the diffusion of Cu atoms become easier and the length of nanowires increase with the decrease of the concentration of Sn4+ ions. Furthermore, deposition time, dissolution time, and ion concentrations are also important for the controllable formation of Cu nanowires (more details shown in Figures S2S8 in the Supporting Information). The difference of morphologies can result in different wettabilities, varying from normal hydrophobic surfaces to superhydrophobic ones. The images of static contact angle and the dynamic contact angles for the three typical morphologies of a CuO nanowires array displayed in Figure 2 are illustrated in Figure 3. (a), (b), and (c) of Figure 3 are their corresponding static contact angle photos. Where the nanowires corresponding to sample a are erect and tight, the corresponding static contact angle is around 89°, showing poor hydrophobicity. Compared with sample a, its static contact angle is about 148° for sample b, which is close to that of a superhydrophobic surface. For sample c, it shows the highest static contact angle, which is more than 170°, indicating the formation of a good superhydrophobic surface. On the basis of the above results, we could obviously conclude that the wettability greatly depends on the roughness. That is the morphologies of Cu nanowires array in our research case. Furthermore, to evaluate the contact angle hysteresis of samples a, b, and c, their dynamic contact angles have also been investigated via adding to and withdrawing from the deionized water droplet. The results are shown in (d), (e), and (f) of Figure 3, corresponding to samples a, b, and c, respectively. As shown in Figure 3d, the turning points a1 and a2 show the process of adding the deionized water droplet, and the average value of the series angles can be viewed as an advancing angle. Then the measurement of receding angles were done by withdrawing from the water droplet after a 2-s pause, as the part between point a3 and a4 shows. The volume of the water that was added to and withdrawn from the original water droplet was about 2.5 μL with a rate around 1 μL s1, while the original water droplet was about 5 μL. According to dynamic contact angle measurements, the results indicate that this morphology of sample a performed poor as a hydrophobic surface, with a contact angle hysteresis of around 15°. However, for the honeycomb-like morphology of 16936
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Figure 3. (a), (b), and (c) show static contact angle corresponding to samples a, b, and c, respectively; (d), (e), and (f) are dynamic contact angle plots corresponding to samples a, b, and c, respectively.
Figure 4. Snapshots of a water droplet impacting the surface. (a), (b), and (c) corresponding to samples a, b, and c, respectively.
sample c, the rolling angle is less than 1°, indicating good superhydrophobic behaviors. So it can be concluded that the morphologies of nanowires turn from erect to intertwined with the length of Cu nanowires gradually increased, leading to the wettability from poor hydrophobicity to superhydrophobicity. From sample a, sample b, to sample c, the static contact angle changes from 89° to 174°; the rolling contact angle changes from 15° to less than 1°. And the shape of plots changed from a
concave parabolic curve for sample a to a convex parabolic curve for sample b and sample c, with increasing slope coefficient. From the above analysis, it is clearly found that the different proportion of Cu2+/Sn4+ resulted in different morphologies with different wettabilities. In the following, a bouncing experiment by a water droplet free falling from a certain height onto the object surface was conducted in this study. When the liquid droplet impacts the surface with a certain velocity, it might change a liquidairsolid interface into a liquidsolid interface. 32 This experiment was done on the same samples in the formerly mentioned dynamic contact angle measurement experiment. Figure 4 shows the process of a water droplet hitting the surfaces and bouncing. The impact velocity for this case is ∼53.30 cm s1. As shown in Figure 4a corresponding to sample a, the droplet first deforms and flattens into a pancake at the time of 7.5 ms, and then it retracts and tries to rebound off the surface, as detailed in the images of 20 and 22.5 ms, respectively. After that, part of the droplet attached to the substrate, indicating a formation of a solidairliquid interface. As shown in Figure 4b corresponding to sample b, the droplet hitting the surface under an impact velocity of 53.30 cm s1 first deformed and then flattened into a pancake, and then easily retracted and bounced off the surface. Figure 4c indicates the droplet cost a shorter time to perform deformation, flattening and retracting on the honeycomb-like 16937
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Figure 5. (a) Contact time vs impact velocity plot for sample b and sample c, respectively. (b) First bouncing time vs impact velocity plot for sample b and sample c, respectively. The radius of the water droplet is about 0.985 mm. Before the experiments, the surfaces were first modified by 20 mM 1H,1H,2H,2H-perfluorodecyltrichlorosilane.
Figure 6. (a) SEM image of the sample a surface. (b) Top view of the model which imitates the sample morphology. (c) Pillar arrays which imitate the sample morphology, and the pillars consist of 15 to 20 of the highest nanowires; P is the distance between two pillars, H is the height of true tip of the supporting point, and D is the diameter of the pillars. (d) SEM image of the sample c surface. (e) Model which imitates the sample morphology, and P is the average diameter of the honeycomb-like unit; D is the average width between two honeycomb-like units.
surface for sample c. Compared with sample a, the droplet in sample c kept bouncing until it rolled off the surface; this is possibly because the droplet failed to penetrate the spaces among the nanowires and the air pocket that is responsible for keeping the superhydrophobicity intact. Furthermore, the impact behaviors of a water droplet with various velocities on the superhydrophobic surfaces of samples b and c have also been investigated. We conduct this experiment by dropping the water droplet with a volume of 8 μL from different heights. To further identify the stability of the honeycomb-like surface’s superhydrophobicity, each bounce with different impact velocity was recorded by high-speed camera, which helps us analyze the procedure of bouncing, especially the contact time. Figure 5a indicated the tendency of contact time as the impact velocity increased. For sample b, the contact time decreased as the
velocity increased, and when the impact velocity increased to a certain value, around 25 cm s1 in this case, the contact time remained constant. As for sample c, the contact time decreased as the impact velocity increased, and began to remain constant at around 30 cm s1. This further confirmed the good superhydrophobicity of the modified honeycomb-like surface. Figure 5b shows the corresponding first bouncing time in the air after the water droplet’s first bouncing. For samples b and c, their flying times for their droplets both increased as the impact velocity increased. The Wenzel model33 can be used to explain the situation of the first morphology, whose nanowires are tight and erect. Air could hardly enter into the gap among these nanowires, and liquid entered as a result, leading to relatively lower contact angles and higher rolling angles. When it comes to the second honeycomb16938
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like topography, air could easily immerse into the interspaces, better fitting the CassieBaxter model,34 resulting in relatively higher contact angles and much lower contact angle hysteresis (
