Article pubs.acs.org/Langmuir
Self-Movement of Water Droplet at the Gradient Nanostructure of Cu Fabricated Using Bipolar Electrochemistry Najmeh Dorri, Paria Shahbazi, and Abolfazl Kiani* Department of Chemistry, University of Isfahan, Isfahan 81744-73441, Iran S Supporting Information *
ABSTRACT: This Article reports on gradient electrodeposition of copper on the surface of a bipolar electrode (BPE). The formation mechanism of the as-fabricated gradient nanostructure is discussed, and the effects of time, potential, and concentration of CuSO4 solution on the morphology of the deposited structures are investigated. Scanning electron microscopy (SEM) is used to visualize the morphology of the deposited Cu at different positions of the BPE. By scanning from the cathodic pole to the midpoint of the BPE, three distinct structures are observed; (i) nanodendrites, (ii) nanodendrites in the vicinity of nanoparticles, and (iii) nanoparticles. The BPE surface was characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements. The contact angle measurement of a water droplet reveals a surface with gradient wettability. Modification of the as-electrodeposited Cu surface with 1-dodecanethiol provides self-movement of the water droplet.
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INTRODUCTION In recent years, there has been an increasing amount of literature on producing surfaces with spatially varying chemical and physical properties. These gradients are useful in different experimental situations, where it is desired to probe the effect of various surface chemistries in a single experiment. They especially apply in the biomedical field and for studying wetting phenomena.1 Several methods have been described for fabricating gradient surfaces. The most widely used methods for preparing gradients have been reviewed by Ruardy et al.2 and Morgenthaler et al.3 Bipolar electrochemistry seems promising among the various methods for fabrication of gradient surfaces. The gradient in electrochemical reaction rate at various locations of a bipolar electrode could be utilized in the fabrication of a surface gradient called bipolar patterning.4 Fuchigami et al. successfully demonstrated the bipolar patterning of poly(3-methylthiophene) (PMT) film. Furthermore, they reported the detailed studies on bipolar doping of PMT and other conducting polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(aniline) (PANI). Their report included the evidence of gradient composition by optical and spectroelectrochemical studies and local elemental analysis.5 The principles of bipolar electrochemistry have been described extensively, and several interesting applications have been reported.6−11 Briefly, in a bipolar system, a driving potential is applied across a cell containing an electrolyte and an electronic conductor which results in a potential drop within the solution. The electronic conductor acts as an alternative path for current passing through the solution because of its less resistance.12 If the potential differences between the solution © 2014 American Chemical Society
and the two ends of the electronic conductor (bipolar electrode, BPE) are sufficiently high, then faradic reactions occur at the poles of the BPE.13 Any conducting material can basically serve as a BPE, and no advanced laboratory equipment is needed to access the method. Opposed to the common electrochemical cells, in bipolar electrochemistry external electric field in the solution is used to manage the interfacial potential between the BPE and the solution. Since the potential difference between a point on the BPE and the solution varies laterally along the surface, rates of the reaction will vary accordingly and thus electrochemical reaction gradient is present on a BPE. Chemical composition gradient of CdS has been reported using bipolar electrochemistry.14 In another report, a thiolated self-assembled monolayer (SAM) previously deposited on the BPE was partially desorbed using bipolar system. The resulting SAM exhibited a density gradient along the BPE surface that tracked the strength of the local electric field.12 Shannon and Ramaswamy have reported codeposited Ag and Au onto stainless steel BPEs, forming compositional gradients. They then used a SERS active SAM and collected spectra at various points along the BPE cathode to determine the ideal composition for SERS enhancement.15 Bradley and co-workers have also carried out several experiments related to Cu bipolar electrodeposition16,17 Bohn and co-workers used unipolar electrochemistry to deposit Cu on a very thin Au electrode.18 They have successfully reported the selective deposition of Cu Received: September 21, 2013 Revised: January 2, 2014 Published: January 13, 2014 1376
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RESULTS AND DISCUSSION The configuration of the designed and used bipolar electrochemical setup is depicted in Scheme 1. The electrochemical
on some portion of the Au electrode by in-plane electrochemical potential gradient method. The procedure resulted in a deposition with a transition region and consequently a chemical gradient at the surface. However, no gradient was reported within the deposited Cu. Herein we report a simple approach for fabricating a gradient Cu deposition with a resulting wettability gradient using bipolar electrodeposition. Drop motion in a wettability gradient has potential application in condensation heat transfer, in microfluidics, for the removal of debris in inkjet printing,19 and for motility in vitro.20 In the present study, the hydrophobic and hydrophilic properties of the electrodeposited Cu on the surface of the bipolar electrode have been investigated. These parameters obtained by measuring contact angle of water droplets on various parts of the cathodic pole. In this way, we were interested to see the possibility of fabricating a gradient hydrophobic substrate with the ability to move water droplet along it.
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Scheme 1. Schematic Configuration of Bipolar Electrochemical Setup Designed and Used in This Studya
EXPERIMENTAL SECTION
Double distilled water was used throughout. The deposition solutions were prepared from CuSO4 (purity: 99%) and H2SO4 (purity: 98%). All the BPEs were made using Cu foil (purity: 99.5%). Pt electrodes were used as driving electrodes. HNO3 (purity: 65%) was used for cleaning the Pt electrodes after each experiment. The Cu foils were cleaned by ultrasonication sequentially in 0.5 M HCl (36.5%), acetone and double distilled water, each for 10 min. All the materials were from Merck Company. All the experiments were performed in a home-built Teflon cell with two reservoirs with dimensions of 2 × 2 × 1 cm3. The connection between two reservoirs was made via a groove. The length of the groove was 4.5 cm, and the total length of the cell was 8.5 cm. Two Pt electrodes were placed in the reservoirs and were connected to a DC power supply (MASTECH DC Power Supply HY3005F-3) to provide the necessary driving potential. The Cu foil (BPE) with dimensions of 4.5 × 0.75 cm2 was placed in the groove of the cell, and a Teflon wall was fixed over the middle. This wall reduces migration of the ions and consequently results in lower solution conductivity,13 which favors a higher degree of faradic depolarization in BPE. The experiment performed with and without the Teflon separator revealed that the wall was necessary for obtaining a better surface in terms of gradient. The width of the Teflon wall in the middle of the BPE was 0.76 cm. A 0.4 cm section of the wall was located in the groove (the height of the groove was a bit longer) (see the Supporting Information). The length and width of the BPE were exactly equal to those of groove size. All the experiments were carried out at room temperature. Surface modification was carried out by immersing the patterned BPE in an ethanol solution of 1-dodecanethiol (5 mM) for about 7 min, then washing with absolute ethanol, and finally drying in air. Scanning electron micrographs were taken using a scanning electron microscope (Philips model XL30). The wettability of the samples was examined by measuring the water contact angle. The indicator drop images were stored via a SONY Handycam model HDR-PJ50E/BE36. Static contact angles were measured by the drop analysis software using 5 μL drops of double distilled water placed on the samples from needle of a HPLC microsyringe. For testing the ability of the gradient surface in rolling off water, an 8 μL drop was used. X-ray photoelectron spectroscopy (XPS) analysis was performed with a TWIN ANOD XR3E2 X-RAY SOURCE SYSTEMS German model X-RAY 8025-BesTec instrument equipped with an Al Kα X-ray source at energy of 1486.6 eV and a concentric hemispherical energy analyzer (Specs EA 10 Plus) operating in a vacuum better than 10−7 Pa. All binding energy values were calibrated by fixing the C1s core level to 284.5 eV. All of the peaks were deconvoluted using SDP software (version 4.1) with 90% Gaussian/10% Lorentzian peak fitting.
a
The scheme is not in scale. The lengths of arrows represent potential difference between solution and BPE (ΔE). The potential difference between the solution and BPE is greatest at the BPE’s extremity, which causes formation of dendrites to dominate in this region.
deposition of Cu was performed by applying potential to the driving electrodes. Table S1 in the Supporting Information summarizes different conditions in which electrodeposition was performed. Top view of four of the as-electrodeposited structures at different potentials is shown in Figure 1. It was found that when the potential increased from 4 to 9 V, better deposit was fabricated in terms of the gradient length. At high potentials, 7
Figure 1. Top-view images of electrodeposited Cu at the cathodic pole of BPE. Electrodeposition was performed from 5 mM CuSO4 and 0.1 M H2SO4 solution for 30 min applying driving potentials of 4, 6, 7, and 9 V. 1377
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and 9 V, a foamlike structure is formed in the first part of the cathodic pole, where the highest possible potential difference is experienced. The acid (H2SO4) present in the solution is a source of excess hydrogen ions and production of hydrogen bubbles that change the structure of the deposit. As presented in Scheme 1, a potential difference between the BPE and solution exists laterally along the BPE. In all SEM figures presented in this report, three or four distinct electrodeposition regions are identified along the principal axis of BPE by visual inspection. These regions are labeled A, B, C, and D starting from the first part of cathodic pole located near the Pt driving electrode. In this region, the potential difference between BPE and solution (ΔE) is at its maximum value (Figure 1). As mentioned before, different locations of BPE experience different ΔE. Near the middle part of BPE, the ΔE is at its lowest value which is close to the thermodynamic equilibrium state. All the details about nanostructures that formed at different conditions are presented in Table S1 in the Supporting Information. It should be noted that oxidation of Cu takes places at the anodic pole of BPE without formation of any surface gradient in this location. The morphology of crystals depends on how far its formation is from an equilibrium condition.21−23 Here, it translates in high overpotential for electrodeposition. Increasing the driving force for crystallization (potential in this situation) results in crystals shapes varying from polyhedrons to various hierarchical structures like dendrites.21 So, one can conclude that nanoparticles and nanodendrites are the dominant structures at low and high ΔE locations in BPE, respectively. At the region between the two mentioned extreme potentials, a combination of these two structures is formed. Diffusion limiting aggregation (DLA) and oriented attachment are two main mechanisms that explain the growth process of dendritic structures.24,25 By scanning from the cathodic poles to the midpoint of BPEs, the evolutionary mechanism of crystal growth can be observed. Accordingly, nanoparticles in the last part of cathodic poles are most likely formed via a Volmer−Weber mechanism.26 The layer of copper nanoparitcles or nanoclusters and the layer of the synchronized copper dendrites which exist in the middle point of cathodic poles were named Volmer−Weber (VW) layer and DLA layer, respectively. It is assumed that the DLA layer is derived from the continuous aggregation growth of small particles on the VW layer (Figure 2).27 Top view images (Figure 1, Supporting Information Figures S6 and S10) show that, at high potential, high concentration, and long time, a deposition with a better gradient in terms of length is formed. Wetting Behavior. Superhydrophobic surfaces with a water contact angle greater than 150° and sliding angle smaller than
10° which can roll off water droplets have attracted much attention because of their different applications in selfcleaning, 28−30 anticontamination, 31 and anticorrosion fields.32,33 On the other hand, Whitesides and Chaudhury have shown that a hysteresis in contact angle smaller than 10° is required for self-movement of drop on the surface.34 They produced a spatial gradient surface on the polished silicon wafer. It was generated by exposing the wafer to the diffusing front of a vapor of decyltrichlorosilane. The as-prepared surface displayed a gradient of hydrophobicity, causing a droplet of water to move with an average velocity of 1−2 mm/s when the surface was tilted by 15°. Further research has revealed that hierarchical micronanostructures can both increase the contact angle and reduce the sliding angle (SA).35−39 Various methods have been used to fabricate this structure, like electrodeposition,40−43 electron/ laser etching,44,45 and other methods.38,46−49 On the other hand, the hydrophobicity of solid surfaces also can be enhanced by chemical modification with materials of lower surface energy.50,51 Zhang and co-workers made noble metal aggregates via electrodeposition and then modification of them with an alkylthiol to fabricate superhydrophobic surfaces.52−54 Cassie−Baxter equation (eq 1) is one of the models that describe the behavior of liquid droplet on the rough solid surfaces.51 cos θ = fs1 (1 + cos θ0) − 1
(1)
where fs1 is the fraction of interface areas of solid/water on the surface. So smaller surface roughness results in increasing fs1 and decreasing water contact angle.51 Self-Assembly of Thiols on the As-Prepared Gradient Nanostructure and Its Effect on Drop Movement. For the next step, we searched for the wetting behavior of the asprepared surface at optimum condition. Figure S14A shows a 5 μL drop on the surface of the bare copper foil. The static contact angle for a drop at this surface was determined to be ∼52° (Table 1). The contact angles of different parts of the Table 1. Contact Angles (CAs) of Water Droplet at Different Substrates contact angle (deg) substrate
region
naked bare Cu foil naked Cu foil modified with 1-dodecanethiol Cu foil with gradient structure
Cu foil with gradient structure modified with 1-dodecanethiol
CAleft
CAright
∼52 in all regions ∼97 in all regions B 132.24 120.70 C 69.13 54.67 D 29.20 24.11 droplet rolled off quite easily along the gradient surface; this induced difficulty in determining some of the CAs
cathodic pole were measured at a BPE which was fabricated with the condition of 7 V, electrodeposition time of 30 min, and CuSO4 concentration of 5 mM. The shape of a water droplet on the surface of the electrodeposited Cu on the cathodic pole of BPE is shown in Figure 3. The initial part of this pole strongly repelled water and the droplet did not fall onto this region, so the contact angle was measured by beginning from region B. These results show that the first part (part B) is hydrophobic and two other parts (parts C and D) are hydrophilic. In addition, contact angles are different for the
Figure 2. (A) BPE formed by applying a potential of 7 V in 3 mM CuSO4 and 0.1 M H2SO4 solution, for 30 min. (A1) High magnification SEM image of Cu nanostructure in (A). 1378
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Figure 4. XRD pattern of as fabricated Cu surface by applying a driving voltage of 7 V for 30 min in a 5 mM CuSO4 + 0.1 M H2SO4 solution. Figure 3. Shape of water droplet at different locations of the electrodeposited Cu structure on the cathodic pole of BPE. The applied driving potential was 7 V, and the solution was 5 mM CuSO4 in 0.1 M H2SO4. The electrodeposition was performed for 30 min.
left and right part of each droplet (Table 1). On the other hand, one can see a decrease in contact angle going from the first part of the cathodic pole to the middle point of BPE (Table 1). Contact angles of water droplets on the fabricated surfaces at other conditions (according to Table S1) are shown in Table S2. The change of the static contact angle θ along the surface is due to the change of the surface free energies of the solid− liquid (γSL) and the solid−gas (γSG) interfaces.55 The reason for this behavior could be searched in the presence of gradient structures on the surface. The gradient deposited structure consists of dendritic structure on the first part with the most roughness on the surface. Dendritic structures have more empty spaces between branches and leaves, so they can trap more air and have reduced fs1. As a result, they are predicted to have the most contact angles. By going to the middle point of the cathodic pole, the popularity of dendritic structures decreases and finally disappears completely on the last part of the pole. These results confirm fabrication of a surface that has gradient wettability along its surface. The XPS and X-ray diffraction (XRD) analyses were performed to obtain more insight into chemical composition of the fabricated surface. Figure 4 shows the XRD patterns of copper deposited by applying a driving potential of 7 V. The diffraction peaks at 51° and 59° corresponded to copper, and peaks at 42° and 49° corresponded to Cu2O. The electrode was subjected to more detailed study by performing the XPS analysis, and the oxidation states of copper have been identified. Figure 5 shows the high resolution Cu 2p3/2, O 1s, and core level spectra. The Cu 2p3/2 core level spectrum shows a main peak envelope curve-fitted into three components centered at 932.40, 933.30, and 935 eV (Figure 5a). The peak at 932.40 eV could be attributed to either metallic copper or Cu2O (C in Figure 5a).56 The component at 935 eV arises from the presence of Cu(OH)2 at the electrode surface (A in Figure 5a). The identification of the Cu components, especially the need for an additional peak for CuO, can be confirmed by the analysis of O 1s core level spectra presented in Figure 5b. The curve fitting of O 1s spectrum of the electrode yielded three main peaks at binding energies of 530.60, 531.70, and 532.90
Figure 5. XPS of (a) Cu 2p3/2 and (b) O 1s. Cu was deposited at 7 V for 30 min from a 5 mM CuSO4 + 0.1 M H2SO4 solution. The peaks D and E in panel (a) are satellite peaks.
eV, which can be assigned to Cu2O, Cu(OH)2, and CO/ adsorbed H2O, respectively (B−D in Figure 5b).56 As expected, the surface is not only composed of Cu(0) but other oxidation states of Cu, Cu(I), and Cu(II) are also present. This is understandable considering the standard electrode potential of Cu (+0.34 V) which indicates easy oxidation of Cu. It has been shown that the hydrophobicity of copper surface is increased after surface oxidation.57Meanwhile, no gradient in wettability expected to occur merely by surface oxidation of copper. Also It has been reported that Cu(OH)2 is hydrophilic and could be turned into a hydrophobic surface by covering with a low surface energy molecule through selfassembly. The possible explanation for the absence of CuO 1379
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peak in XRD spectrum could be the amorphous structure of CuO. For the preparation of superhydrophobic surfaces or films, the combination of creating a rough structure on a hydrophobic surface (contact angle > 90°) and lowering the surface energy by chemical modification is required.58 In our study, water droplet cannot roll off on the as-prepared gradient. This is due to the presence of an energy barrier, analogous to the activation energy of a chemical reaction that prevents spontaneous conversion to products. This behavior is similar to the petal effect phenomenon. On the other hand, the large contact angle of the lotus is based on the epicuticula wax and the micrometerscale papillae structure of the leaf. The epicuticula wax provides the lowest surface free energy. Thus, we modified the surface of the as-deposited structure with 1-dodecanethiol, an alkylterminated compound, which could decrease the attachment property of a water droplet.59For this purpose, the as-prepared deposit was immersed in ethanolic solution of 1-dodecanethiol for 7 min. This was the shortest time for wetability behavior and self-movement of a water droplet. Immersing the asfabricated deposit into the ethanolic solution of 1-dodecanethiol causes the structure to be covered with a monolayer of thiol molecule with low surface energy.60 Modifying the gradient surface with the thiol solution is similar to the superhydrophobic waxy layer on the leaves of lotus.59 To observe possible change in morphology of the deposited Cu structure after being covered with 1-dodecanethiol, the SEM images of the electrode were recorded (Figure S15). The SEM images show small changes in morphology of the deposited structure, especially in region B. One can see that some branches are adhered together to form a surface with higher roughness. Shi and Wu reported the fabrication of copper hydroxide nanoneedles on copper foil.61 They found that the appearance of as-prepared nanoneedle surfaces was changed when the foil was immersed in an ethanolic solution of ndodecanethiol. After this treatment, an 8 μL water droplet was placed onto the first part of the cathodic pole. Droplets cannot penetrate into the grooves. They could hardly stick to the surface and rolled off quite easily along the gradient surface without even a slight sliding angle (see the Supporting Information). A speed of about 18 mm/s was obtained for self-movement of the water droplet. The drop stops near the end part of the cathodic pole. This drop can hardly sit on the sample, even when the sample is not tilted, and rolled away quickly, making contact angle measurement impossible (Table 1). Time lapsed images of the process are shown in Figure 6. To gain confidence about whether the surface of the bare copper foils that immersed in the ethanolic solution of the thiol has the ability to move the droplet, a bare copper foil was treated in a similar manner with the ethanolic solution of 1dodecanethiol for 7 min. The static contact angle of this surface in all locations was determined to be ∼97° (Table 1) (Figure S14B), confirming a hydrophobic surface. An 8 μL droplet was also tested, but it was attached to the surface and did not show any movement. According to the above results, one can conclude that neither preparation of a gradient rough surface with bipolar electrochemistry nor modification of a bare surface with a monolayer of 1-dodecanethiol suffice for rolling off the droplet, and both conditions are necessary for the rolling purpose. We performed an additional experiment to see whether the droplet movement originates from SAMs with different packing densities on different parts of BPE, or if it is
Figure 6. Photographs of water droplet self-movement on the surface of as-fabricated gradient Cu structure. The pictures were extracted from a movie showing this behavior. The length of BPE is 4.5 cm. Time is calculated using the speed of water droplet movement and its position at the surface.
more directly related to the microscopic morphology. We measured the static contact angle of the naked copper foil after being covered with 1-dodecanethiol in a stepwise manner. A naked copper sheet with the same size as the cathodic pole of BPE was placed in a small container. Then a 5 mM ethanolic solution of 1-dodecanethiol was added slowly into the container, so that the level of the solution was increased gradually as time elapsed in three steps during 7 min. In this way, we expect self-assembly of 1-dodecanethiol with different density at different parts of the naked Cu sheet. The modified Cu sheet was then subjected to measuring static contact angle of water droplet at three parts (A, B, C). The measured contact angles at different parts of the modified Cu sheet show no significant difference (Figure 7). Comparing the measured contact angles with that obtained on a naked Cu sheet (∼52°) confirms fabrication of a surface with higher hydrophobicity. The density of adsorbed 1-dodecanethiol on the gradient nanostructure of Cu may differ from that on the naked Cu sheet, but gradient nanostructure plays the main role in movement of water droplet.
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CONCLUSIONS We have reported the gradient electrochemical deposition of copper using bipolar electrochemistry. Three distinct zones of electrodeposition along the principal axis of the BPE were identified by visual inspection and SEM imaging. It was shown that parameters such as time, potential, and concentration of the reagents affect the shape of the deposited structure on the substrate. It was observed that dendrite shape nanostructures are dominant at the near edge of the BPE where ΔE is largest. One could see a decrease in density of dendritic structures by scanning the length of the deposited structure at the BPE from the edge of the cathodic pole to the middle of the pole. Meanwhile, the irregular particles exhibiting faceted surfaces become dominant toward the middle of the pole. In the middle of the BPE, dendrites disappear completely and only irregular nanostructures are visible. It is notable that variation of 1380
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Figure 7. Photographs of a 5 μL water droplet on the surface of Cu sheet modified with 1-dodecanethiol in a stepwise manner. The Cu sheet was in contact with ethanol solution of 1-dodecanthiol for (A) 7 min, (B) 5 min, and (C) 2.5 min. CA = contact angle.
hydrophobicity along the length of the cathodic pole results in a surface with gradient wettability. Modification of the surface of the as-deposited gradient nanostructure with 1-dodecanthiol provides a surface on which water droplets can move spontaneously. Given the fact that this method is simple, convenient, and easy to scale up, we expect that it opens a new avenue for manufacturing gradient superhydrophobic surfaces for various fundamental studies and industrial applications.
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ASSOCIATED CONTENT
S Supporting Information *
SEM images of the fabricated surface under different conditions; a movie showing water droplet movement on the fabricated gradient surface; a movie showing water droplet behavior at the surface of the naked substrate. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions
The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ABBREVIATIONS BPE, bipolar electrode; BPEs, bipolar electrodes; ΔE, the potential difference between the solution and BPE REFERENCES
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(43) Shi, F.; Wang, Z. Q.; Zhang, X. Adv. Mater. 2005, 17, 1005. (44) Lee, E. J.; Lee, H. M.; Li, Y.; Hong, L. Y.; Kim, D. P.; Cho, S. O. Macromol. Rapid Commun. 2007, 28, 246. (45) Gao, X. F.; Yao, X.; Jiang, L. Langmuir 2007, 23, 4886. (46) Zhao, Y.; Lu, Q. H.; Chen, D. S.; Wei, Y. J. Mater. Chem. 2006, 16, 4504. (47) Zhao, N.; Zhang, X. Y.; Zhang, X. L.; Xu, J. ChemPhysChem 2007, 8, 1108. (48) 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. (49) Wang, S. T.; Feng, L.; Jiang, L. Adv. Mater. 2006, 18, 767. (50) Zhao, Y.; Lu, Q.; Chen, D.; Wei, Y. J. Mater. Chem. 2006, 16, 4504. (51) She, Z.; Li, Q.; Wang, Z.; Li, L.; Chen, F.; Zhou, J. ACS Appl. Mater. Interfaces 2012, 4, 4348. (52) Shi, F.; Song, Y. Y.; Niu, H.; Xia, X. H.; Wang, Z. Q.; Zhang, X. Chem. Mater. 2006, 18, 1365. (53) Zhao, N.; Shi, F.; Wang, Z. Q.; Zhang, X. Langmuir 2005, 21, 4713. (54) Shi, F.; Wang, Z. Q.; Zhang, X. Adv. Mater. 2005, 17, 1005. (55) Zielke, P. C.; Subramanian, R. S.; Szymczyk, J. A.; McLaughlin, J. B. PAMM Proc. Appl. Math. Mech. 2003, 2, 390. (56) Ghodbane, O.; Roué, L.; Bélanger, D. Chem. Mater. 2008, 20, 3495. (57) Zhang, Y.; Yu, X.; Zhou, Q.; Chen, F.; Li, K. Appl. Surf. Sci. 2010, 256, 1883. (58) Liu, H.; Feng, L.; Zhai, J.; Jiang, L.; Zhu, D. Langmuir 2004, 20, 5659. (59) Feng, J.; Qin, Z.; Yao, S. Langmuir 2012, 28, 6067. (60) Gu, C.; Zhang, J.; Tu, J. J. Colloid Interface Sci. 2010, 352, 573. (61) Wu, X.; Shi, G. J. Phys. Chem. B 2006, 110, 11247.
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Self-Movement of Water Droplet at the Gradient Nanostructure of Cu Fabricated Using Bipolar Electrochemistry Najmeh Dorri, Paria Shahbazi, and Abolfazl Kiani* Department of Chemistry, University of Isfahan, Isfahan 81744-73441, Iran S Supporting Information *
ABSTRACT: This Article reports on gradient electrodeposition of copper on the surface of a bipolar electrode (BPE). The formation mechanism of the as-fabricated gradient nanostructure is discussed, and the effects of time, potential, and concentration of CuSO4 solution on the morphology of the deposited structures are investigated. Scanning electron microscopy (SEM) is used to visualize the morphology of the deposited Cu at different positions of the BPE. By scanning from the cathodic pole to the midpoint of the BPE, three distinct structures are observed; (i) nanodendrites, (ii) nanodendrites in the vicinity of nanoparticles, and (iii) nanoparticles. The BPE surface was characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements. The contact angle measurement of a water droplet reveals a surface with gradient wettability. Modification of the as-electrodeposited Cu surface with 1-dodecanethiol provides self-movement of the water droplet.
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INTRODUCTION In recent years, there has been an increasing amount of literature on producing surfaces with spatially varying chemical and physical properties. These gradients are useful in different experimental situations, where it is desired to probe the effect of various surface chemistries in a single experiment. They especially apply in the biomedical field and for studying wetting phenomena.1 Several methods have been described for fabricating gradient surfaces. The most widely used methods for preparing gradients have been reviewed by Ruardy et al.2 and Morgenthaler et al.3 Bipolar electrochemistry seems promising among the various methods for fabrication of gradient surfaces. The gradient in electrochemical reaction rate at various locations of a bipolar electrode could be utilized in the fabrication of a surface gradient called bipolar patterning.4 Fuchigami et al. successfully demonstrated the bipolar patterning of poly(3-methylthiophene) (PMT) film. Furthermore, they reported the detailed studies on bipolar doping of PMT and other conducting polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(aniline) (PANI). Their report included the evidence of gradient composition by optical and spectroelectrochemical studies and local elemental analysis.5 The principles of bipolar electrochemistry have been described extensively, and several interesting applications have been reported.6−11 Briefly, in a bipolar system, a driving potential is applied across a cell containing an electrolyte and an electronic conductor which results in a potential drop within the solution. The electronic conductor acts as an alternative path for current passing through the solution because of its less resistance.12 If the potential differences between the solution © 2014 American Chemical Society
and the two ends of the electronic conductor (bipolar electrode, BPE) are sufficiently high, then faradic reactions occur at the poles of the BPE.13 Any conducting material can basically serve as a BPE, and no advanced laboratory equipment is needed to access the method. Opposed to the common electrochemical cells, in bipolar electrochemistry external electric field in the solution is used to manage the interfacial potential between the BPE and the solution. Since the potential difference between a point on the BPE and the solution varies laterally along the surface, rates of the reaction will vary accordingly and thus electrochemical reaction gradient is present on a BPE. Chemical composition gradient of CdS has been reported using bipolar electrochemistry.14 In another report, a thiolated self-assembled monolayer (SAM) previously deposited on the BPE was partially desorbed using bipolar system. The resulting SAM exhibited a density gradient along the BPE surface that tracked the strength of the local electric field.12 Shannon and Ramaswamy have reported codeposited Ag and Au onto stainless steel BPEs, forming compositional gradients. They then used a SERS active SAM and collected spectra at various points along the BPE cathode to determine the ideal composition for SERS enhancement.15 Bradley and co-workers have also carried out several experiments related to Cu bipolar electrodeposition16,17 Bohn and co-workers used unipolar electrochemistry to deposit Cu on a very thin Au electrode.18 They have successfully reported the selective deposition of Cu Received: September 21, 2013 Revised: January 2, 2014 Published: January 13, 2014 1376
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RESULTS AND DISCUSSION The configuration of the designed and used bipolar electrochemical setup is depicted in Scheme 1. The electrochemical
on some portion of the Au electrode by in-plane electrochemical potential gradient method. The procedure resulted in a deposition with a transition region and consequently a chemical gradient at the surface. However, no gradient was reported within the deposited Cu. Herein we report a simple approach for fabricating a gradient Cu deposition with a resulting wettability gradient using bipolar electrodeposition. Drop motion in a wettability gradient has potential application in condensation heat transfer, in microfluidics, for the removal of debris in inkjet printing,19 and for motility in vitro.20 In the present study, the hydrophobic and hydrophilic properties of the electrodeposited Cu on the surface of the bipolar electrode have been investigated. These parameters obtained by measuring contact angle of water droplets on various parts of the cathodic pole. In this way, we were interested to see the possibility of fabricating a gradient hydrophobic substrate with the ability to move water droplet along it.
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Scheme 1. Schematic Configuration of Bipolar Electrochemical Setup Designed and Used in This Studya
EXPERIMENTAL SECTION
Double distilled water was used throughout. The deposition solutions were prepared from CuSO4 (purity: 99%) and H2SO4 (purity: 98%). All the BPEs were made using Cu foil (purity: 99.5%). Pt electrodes were used as driving electrodes. HNO3 (purity: 65%) was used for cleaning the Pt electrodes after each experiment. The Cu foils were cleaned by ultrasonication sequentially in 0.5 M HCl (36.5%), acetone and double distilled water, each for 10 min. All the materials were from Merck Company. All the experiments were performed in a home-built Teflon cell with two reservoirs with dimensions of 2 × 2 × 1 cm3. The connection between two reservoirs was made via a groove. The length of the groove was 4.5 cm, and the total length of the cell was 8.5 cm. Two Pt electrodes were placed in the reservoirs and were connected to a DC power supply (MASTECH DC Power Supply HY3005F-3) to provide the necessary driving potential. The Cu foil (BPE) with dimensions of 4.5 × 0.75 cm2 was placed in the groove of the cell, and a Teflon wall was fixed over the middle. This wall reduces migration of the ions and consequently results in lower solution conductivity,13 which favors a higher degree of faradic depolarization in BPE. The experiment performed with and without the Teflon separator revealed that the wall was necessary for obtaining a better surface in terms of gradient. The width of the Teflon wall in the middle of the BPE was 0.76 cm. A 0.4 cm section of the wall was located in the groove (the height of the groove was a bit longer) (see the Supporting Information). The length and width of the BPE were exactly equal to those of groove size. All the experiments were carried out at room temperature. Surface modification was carried out by immersing the patterned BPE in an ethanol solution of 1-dodecanethiol (5 mM) for about 7 min, then washing with absolute ethanol, and finally drying in air. Scanning electron micrographs were taken using a scanning electron microscope (Philips model XL30). The wettability of the samples was examined by measuring the water contact angle. The indicator drop images were stored via a SONY Handycam model HDR-PJ50E/BE36. Static contact angles were measured by the drop analysis software using 5 μL drops of double distilled water placed on the samples from needle of a HPLC microsyringe. For testing the ability of the gradient surface in rolling off water, an 8 μL drop was used. X-ray photoelectron spectroscopy (XPS) analysis was performed with a TWIN ANOD XR3E2 X-RAY SOURCE SYSTEMS German model X-RAY 8025-BesTec instrument equipped with an Al Kα X-ray source at energy of 1486.6 eV and a concentric hemispherical energy analyzer (Specs EA 10 Plus) operating in a vacuum better than 10−7 Pa. All binding energy values were calibrated by fixing the C1s core level to 284.5 eV. All of the peaks were deconvoluted using SDP software (version 4.1) with 90% Gaussian/10% Lorentzian peak fitting.
a
The scheme is not in scale. The lengths of arrows represent potential difference between solution and BPE (ΔE). The potential difference between the solution and BPE is greatest at the BPE’s extremity, which causes formation of dendrites to dominate in this region.
deposition of Cu was performed by applying potential to the driving electrodes. Table S1 in the Supporting Information summarizes different conditions in which electrodeposition was performed. Top view of four of the as-electrodeposited structures at different potentials is shown in Figure 1. It was found that when the potential increased from 4 to 9 V, better deposit was fabricated in terms of the gradient length. At high potentials, 7
Figure 1. Top-view images of electrodeposited Cu at the cathodic pole of BPE. Electrodeposition was performed from 5 mM CuSO4 and 0.1 M H2SO4 solution for 30 min applying driving potentials of 4, 6, 7, and 9 V. 1377
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and 9 V, a foamlike structure is formed in the first part of the cathodic pole, where the highest possible potential difference is experienced. The acid (H2SO4) present in the solution is a source of excess hydrogen ions and production of hydrogen bubbles that change the structure of the deposit. As presented in Scheme 1, a potential difference between the BPE and solution exists laterally along the BPE. In all SEM figures presented in this report, three or four distinct electrodeposition regions are identified along the principal axis of BPE by visual inspection. These regions are labeled A, B, C, and D starting from the first part of cathodic pole located near the Pt driving electrode. In this region, the potential difference between BPE and solution (ΔE) is at its maximum value (Figure 1). As mentioned before, different locations of BPE experience different ΔE. Near the middle part of BPE, the ΔE is at its lowest value which is close to the thermodynamic equilibrium state. All the details about nanostructures that formed at different conditions are presented in Table S1 in the Supporting Information. It should be noted that oxidation of Cu takes places at the anodic pole of BPE without formation of any surface gradient in this location. The morphology of crystals depends on how far its formation is from an equilibrium condition.21−23 Here, it translates in high overpotential for electrodeposition. Increasing the driving force for crystallization (potential in this situation) results in crystals shapes varying from polyhedrons to various hierarchical structures like dendrites.21 So, one can conclude that nanoparticles and nanodendrites are the dominant structures at low and high ΔE locations in BPE, respectively. At the region between the two mentioned extreme potentials, a combination of these two structures is formed. Diffusion limiting aggregation (DLA) and oriented attachment are two main mechanisms that explain the growth process of dendritic structures.24,25 By scanning from the cathodic poles to the midpoint of BPEs, the evolutionary mechanism of crystal growth can be observed. Accordingly, nanoparticles in the last part of cathodic poles are most likely formed via a Volmer−Weber mechanism.26 The layer of copper nanoparitcles or nanoclusters and the layer of the synchronized copper dendrites which exist in the middle point of cathodic poles were named Volmer−Weber (VW) layer and DLA layer, respectively. It is assumed that the DLA layer is derived from the continuous aggregation growth of small particles on the VW layer (Figure 2).27 Top view images (Figure 1, Supporting Information Figures S6 and S10) show that, at high potential, high concentration, and long time, a deposition with a better gradient in terms of length is formed. Wetting Behavior. Superhydrophobic surfaces with a water contact angle greater than 150° and sliding angle smaller than
10° which can roll off water droplets have attracted much attention because of their different applications in selfcleaning, 28−30 anticontamination, 31 and anticorrosion fields.32,33 On the other hand, Whitesides and Chaudhury have shown that a hysteresis in contact angle smaller than 10° is required for self-movement of drop on the surface.34 They produced a spatial gradient surface on the polished silicon wafer. It was generated by exposing the wafer to the diffusing front of a vapor of decyltrichlorosilane. The as-prepared surface displayed a gradient of hydrophobicity, causing a droplet of water to move with an average velocity of 1−2 mm/s when the surface was tilted by 15°. Further research has revealed that hierarchical micronanostructures can both increase the contact angle and reduce the sliding angle (SA).35−39 Various methods have been used to fabricate this structure, like electrodeposition,40−43 electron/ laser etching,44,45 and other methods.38,46−49 On the other hand, the hydrophobicity of solid surfaces also can be enhanced by chemical modification with materials of lower surface energy.50,51 Zhang and co-workers made noble metal aggregates via electrodeposition and then modification of them with an alkylthiol to fabricate superhydrophobic surfaces.52−54 Cassie−Baxter equation (eq 1) is one of the models that describe the behavior of liquid droplet on the rough solid surfaces.51 cos θ = fs1 (1 + cos θ0) − 1
(1)
where fs1 is the fraction of interface areas of solid/water on the surface. So smaller surface roughness results in increasing fs1 and decreasing water contact angle.51 Self-Assembly of Thiols on the As-Prepared Gradient Nanostructure and Its Effect on Drop Movement. For the next step, we searched for the wetting behavior of the asprepared surface at optimum condition. Figure S14A shows a 5 μL drop on the surface of the bare copper foil. The static contact angle for a drop at this surface was determined to be ∼52° (Table 1). The contact angles of different parts of the Table 1. Contact Angles (CAs) of Water Droplet at Different Substrates contact angle (deg) substrate
region
naked bare Cu foil naked Cu foil modified with 1-dodecanethiol Cu foil with gradient structure
Cu foil with gradient structure modified with 1-dodecanethiol
CAleft
CAright
∼52 in all regions ∼97 in all regions B 132.24 120.70 C 69.13 54.67 D 29.20 24.11 droplet rolled off quite easily along the gradient surface; this induced difficulty in determining some of the CAs
cathodic pole were measured at a BPE which was fabricated with the condition of 7 V, electrodeposition time of 30 min, and CuSO4 concentration of 5 mM. The shape of a water droplet on the surface of the electrodeposited Cu on the cathodic pole of BPE is shown in Figure 3. The initial part of this pole strongly repelled water and the droplet did not fall onto this region, so the contact angle was measured by beginning from region B. These results show that the first part (part B) is hydrophobic and two other parts (parts C and D) are hydrophilic. In addition, contact angles are different for the
Figure 2. (A) BPE formed by applying a potential of 7 V in 3 mM CuSO4 and 0.1 M H2SO4 solution, for 30 min. (A1) High magnification SEM image of Cu nanostructure in (A). 1378
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Figure 4. XRD pattern of as fabricated Cu surface by applying a driving voltage of 7 V for 30 min in a 5 mM CuSO4 + 0.1 M H2SO4 solution. Figure 3. Shape of water droplet at different locations of the electrodeposited Cu structure on the cathodic pole of BPE. The applied driving potential was 7 V, and the solution was 5 mM CuSO4 in 0.1 M H2SO4. The electrodeposition was performed for 30 min.
left and right part of each droplet (Table 1). On the other hand, one can see a decrease in contact angle going from the first part of the cathodic pole to the middle point of BPE (Table 1). Contact angles of water droplets on the fabricated surfaces at other conditions (according to Table S1) are shown in Table S2. The change of the static contact angle θ along the surface is due to the change of the surface free energies of the solid− liquid (γSL) and the solid−gas (γSG) interfaces.55 The reason for this behavior could be searched in the presence of gradient structures on the surface. The gradient deposited structure consists of dendritic structure on the first part with the most roughness on the surface. Dendritic structures have more empty spaces between branches and leaves, so they can trap more air and have reduced fs1. As a result, they are predicted to have the most contact angles. By going to the middle point of the cathodic pole, the popularity of dendritic structures decreases and finally disappears completely on the last part of the pole. These results confirm fabrication of a surface that has gradient wettability along its surface. The XPS and X-ray diffraction (XRD) analyses were performed to obtain more insight into chemical composition of the fabricated surface. Figure 4 shows the XRD patterns of copper deposited by applying a driving potential of 7 V. The diffraction peaks at 51° and 59° corresponded to copper, and peaks at 42° and 49° corresponded to Cu2O. The electrode was subjected to more detailed study by performing the XPS analysis, and the oxidation states of copper have been identified. Figure 5 shows the high resolution Cu 2p3/2, O 1s, and core level spectra. The Cu 2p3/2 core level spectrum shows a main peak envelope curve-fitted into three components centered at 932.40, 933.30, and 935 eV (Figure 5a). The peak at 932.40 eV could be attributed to either metallic copper or Cu2O (C in Figure 5a).56 The component at 935 eV arises from the presence of Cu(OH)2 at the electrode surface (A in Figure 5a). The identification of the Cu components, especially the need for an additional peak for CuO, can be confirmed by the analysis of O 1s core level spectra presented in Figure 5b. The curve fitting of O 1s spectrum of the electrode yielded three main peaks at binding energies of 530.60, 531.70, and 532.90
Figure 5. XPS of (a) Cu 2p3/2 and (b) O 1s. Cu was deposited at 7 V for 30 min from a 5 mM CuSO4 + 0.1 M H2SO4 solution. The peaks D and E in panel (a) are satellite peaks.
eV, which can be assigned to Cu2O, Cu(OH)2, and CO/ adsorbed H2O, respectively (B−D in Figure 5b).56 As expected, the surface is not only composed of Cu(0) but other oxidation states of Cu, Cu(I), and Cu(II) are also present. This is understandable considering the standard electrode potential of Cu (+0.34 V) which indicates easy oxidation of Cu. It has been shown that the hydrophobicity of copper surface is increased after surface oxidation.57Meanwhile, no gradient in wettability expected to occur merely by surface oxidation of copper. Also It has been reported that Cu(OH)2 is hydrophilic and could be turned into a hydrophobic surface by covering with a low surface energy molecule through selfassembly. The possible explanation for the absence of CuO 1379
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peak in XRD spectrum could be the amorphous structure of CuO. For the preparation of superhydrophobic surfaces or films, the combination of creating a rough structure on a hydrophobic surface (contact angle > 90°) and lowering the surface energy by chemical modification is required.58 In our study, water droplet cannot roll off on the as-prepared gradient. This is due to the presence of an energy barrier, analogous to the activation energy of a chemical reaction that prevents spontaneous conversion to products. This behavior is similar to the petal effect phenomenon. On the other hand, the large contact angle of the lotus is based on the epicuticula wax and the micrometerscale papillae structure of the leaf. The epicuticula wax provides the lowest surface free energy. Thus, we modified the surface of the as-deposited structure with 1-dodecanethiol, an alkylterminated compound, which could decrease the attachment property of a water droplet.59For this purpose, the as-prepared deposit was immersed in ethanolic solution of 1-dodecanethiol for 7 min. This was the shortest time for wetability behavior and self-movement of a water droplet. Immersing the asfabricated deposit into the ethanolic solution of 1-dodecanethiol causes the structure to be covered with a monolayer of thiol molecule with low surface energy.60 Modifying the gradient surface with the thiol solution is similar to the superhydrophobic waxy layer on the leaves of lotus.59 To observe possible change in morphology of the deposited Cu structure after being covered with 1-dodecanethiol, the SEM images of the electrode were recorded (Figure S15). The SEM images show small changes in morphology of the deposited structure, especially in region B. One can see that some branches are adhered together to form a surface with higher roughness. Shi and Wu reported the fabrication of copper hydroxide nanoneedles on copper foil.61 They found that the appearance of as-prepared nanoneedle surfaces was changed when the foil was immersed in an ethanolic solution of ndodecanethiol. After this treatment, an 8 μL water droplet was placed onto the first part of the cathodic pole. Droplets cannot penetrate into the grooves. They could hardly stick to the surface and rolled off quite easily along the gradient surface without even a slight sliding angle (see the Supporting Information). A speed of about 18 mm/s was obtained for self-movement of the water droplet. The drop stops near the end part of the cathodic pole. This drop can hardly sit on the sample, even when the sample is not tilted, and rolled away quickly, making contact angle measurement impossible (Table 1). Time lapsed images of the process are shown in Figure 6. To gain confidence about whether the surface of the bare copper foils that immersed in the ethanolic solution of the thiol has the ability to move the droplet, a bare copper foil was treated in a similar manner with the ethanolic solution of 1dodecanethiol for 7 min. The static contact angle of this surface in all locations was determined to be ∼97° (Table 1) (Figure S14B), confirming a hydrophobic surface. An 8 μL droplet was also tested, but it was attached to the surface and did not show any movement. According to the above results, one can conclude that neither preparation of a gradient rough surface with bipolar electrochemistry nor modification of a bare surface with a monolayer of 1-dodecanethiol suffice for rolling off the droplet, and both conditions are necessary for the rolling purpose. We performed an additional experiment to see whether the droplet movement originates from SAMs with different packing densities on different parts of BPE, or if it is
Figure 6. Photographs of water droplet self-movement on the surface of as-fabricated gradient Cu structure. The pictures were extracted from a movie showing this behavior. The length of BPE is 4.5 cm. Time is calculated using the speed of water droplet movement and its position at the surface.
more directly related to the microscopic morphology. We measured the static contact angle of the naked copper foil after being covered with 1-dodecanethiol in a stepwise manner. A naked copper sheet with the same size as the cathodic pole of BPE was placed in a small container. Then a 5 mM ethanolic solution of 1-dodecanethiol was added slowly into the container, so that the level of the solution was increased gradually as time elapsed in three steps during 7 min. In this way, we expect self-assembly of 1-dodecanethiol with different density at different parts of the naked Cu sheet. The modified Cu sheet was then subjected to measuring static contact angle of water droplet at three parts (A, B, C). The measured contact angles at different parts of the modified Cu sheet show no significant difference (Figure 7). Comparing the measured contact angles with that obtained on a naked Cu sheet (∼52°) confirms fabrication of a surface with higher hydrophobicity. The density of adsorbed 1-dodecanethiol on the gradient nanostructure of Cu may differ from that on the naked Cu sheet, but gradient nanostructure plays the main role in movement of water droplet.
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CONCLUSIONS We have reported the gradient electrochemical deposition of copper using bipolar electrochemistry. Three distinct zones of electrodeposition along the principal axis of the BPE were identified by visual inspection and SEM imaging. It was shown that parameters such as time, potential, and concentration of the reagents affect the shape of the deposited structure on the substrate. It was observed that dendrite shape nanostructures are dominant at the near edge of the BPE where ΔE is largest. One could see a decrease in density of dendritic structures by scanning the length of the deposited structure at the BPE from the edge of the cathodic pole to the middle of the pole. Meanwhile, the irregular particles exhibiting faceted surfaces become dominant toward the middle of the pole. In the middle of the BPE, dendrites disappear completely and only irregular nanostructures are visible. It is notable that variation of 1380
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Figure 7. Photographs of a 5 μL water droplet on the surface of Cu sheet modified with 1-dodecanethiol in a stepwise manner. The Cu sheet was in contact with ethanol solution of 1-dodecanthiol for (A) 7 min, (B) 5 min, and (C) 2.5 min. CA = contact angle.
hydrophobicity along the length of the cathodic pole results in a surface with gradient wettability. Modification of the surface of the as-deposited gradient nanostructure with 1-dodecanthiol provides a surface on which water droplets can move spontaneously. Given the fact that this method is simple, convenient, and easy to scale up, we expect that it opens a new avenue for manufacturing gradient superhydrophobic surfaces for various fundamental studies and industrial applications.
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ASSOCIATED CONTENT
S Supporting Information *
SEM images of the fabricated surface under different conditions; a movie showing water droplet movement on the fabricated gradient surface; a movie showing water droplet behavior at the surface of the naked substrate. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions
The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ABBREVIATIONS BPE, bipolar electrode; BPEs, bipolar electrodes; ΔE, the potential difference between the solution and BPE REFERENCES
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