Fabricating Superhydrophobic Surfaces via a Two-Step

Oct 1, 2013 - This work presents a template-free electrochemical route to producing superhydrophobic copper coatings with the water contact angle of 1...
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Fabricating Superhydrophobic Surfaces via a Two-Step Electrodeposition Technique A. Haghdoost and R. Pitchumani* Advanced Materials and Technologies Laboratory, Department of Mechanical Engineering, Virginia Tech, Blacksburg, Virginia 24061-0238, United States S Supporting Information *

ABSTRACT: This work presents a template-free electrochemical route to producing superhydrophobic copper coatings with the water contact angle of 160 ± 6° and contact angle hysteresis of 5 ± 2°. In this technique, copper deposit with multiscale surface features is formed through a two-step electrodeposition process in a concentrated copper sulfate bath. In the first step, applying a high overpotential results in the formation of structures with dense-branching morphology, which are loosely attached to the surface. In the second step, an additional thin layer of the deposit is formed by applying a low overpotential for a short time, which is used to reinforce the loosely attached branches on the surface. The work also presents a theoretical analysis of the effects of the fabrication parameters on the surface textures that cause the superhydrophobic characteristic of the deposit.

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INTRODUCTION Engineering micro- and nanoscale texture of the surfaces has received great interest in recent years due to its importance in both fundamental research and practical applications.1−4 In particular, a great focus on superhydrophobic surfaces is inspired by the lotus leaf effect.5 On these surfaces, a sessile droplet shows apparent contact angle higher than 150°; in addition, a dynamic droplet does not stick to these surfaces.6,7 Several methods were presented in the literature to demonstrate and measure the nonwetting characteristic of a superhydrophobic surface. For example, it is shown that a falling droplet bounces off after hitting a superhydrophobic surface.8 Moreover, low contact angle hysteresis, i.e., small difference between the contact angles at the front and back of a moving droplet, is another method to show nonwetting characteristic of a surface.13,9−11 It is already known that proper design of surface topographical features is required to support the aforementioned criteria for superhydrophobicity.12−14 In particular, roughness at two or more length scales has been presented as the cause of superhydrophobicity in the lotus leaf and other naturally superhydrophobic materials.15,16 A sessile drop interacting with a rough surface may fully wet the surface which is known as the Wenzel wetting regime (WE),17 or it may result in Cassie−Baxter regime (CB) in which air pockets are entrapped underneath the drop and a composite interface (liquid−solid−air)18 is formed. Reduction of the liquid−solid contact area in CB regime results in both high apparent contact angles (α) and low contact angle hysteresis; therefore, the CB regime supports the criteria of superhydrophobicity. © 2013 American Chemical Society

Many methods have been presented in the literature to fabricate superhydrophobic surfaces, such as sol−gel processing,19 self-assembly technique,20,21 electrospinning,22 hydrothermal synthesis23 laser etching,24 plasma etching,25 physical and chemical vapor deposition,26 anodic oxidation,27 and spray method.28 Some of the fabrication techniques presented in the literature for making superhydrophobic surfaces are potentially costly and time-consuming for practical implementation in applications. Moreover, for some of the applications, the effects of the superhydrophobic coating on thermal and electrical conductivity of the substrate should be minimized; therefore, insulating coatings presented in most of the previous works are not applicable. Among the conductive coatings, superhydrophobic copper coating has attracted considerable interest due to its high diathermanous and electric performance in addition to its good thermal and mechanical stability.29−31 In some of the techniques presented in the literature for making superhydrophobic copper surfaces, besides formation of surface topographical features, the chemistry of the copper surface is changed, which may result in undesirable changes in the essential surface properties.30 In addition, in some of these techniques, the topmost layer of the copper surface is transformed to a superhydrophobic film while for most of the applications, coating a substrate with an additional layer of a superhydrophobic copper film is desired. Received: September 11, 2013 Revised: September 26, 2013 Published: October 1, 2013 4183

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the deposit surface structure at the limiting condition is more appropriately attributed to the change in the overpotential. This work presents an experimental investigation on the effect of the overpotential on the surface texture of the copper deposit. It is shown that formation of a powdery deposit, which has been reported previously in the literature,40−44 occurs at a relatively high overpotential, far from the initiation of the limiting condition. The growth of this deposit is so unstable that morphological instabilities can grow both on the deposit surface and in the electrolyte adjacent to the deposit surface. Consequently, the deposit surface is covered by fractal-like structures with a densely branched morphology, which are either a part of the surface or loosely attached to the surface. Generally, loosely attached structures are formed beyond a certain stage in several unstable growth processes, but their formation during thermal solidification is the most wellinvestigated research area.51 In electrodeposition, this phenomenon is of much interest due to its application in making metallic powders with a high growth rate. 40−44 The experimental investigation presented in this work shows that the powdery deposit made from a concentrated electrolyte exhibits superhydrophobic property due to the re-entrant geometry of its surface structures. To reinforce the loosely attached structures on the deposit surface, an additional thin layer of the deposit is made in a subsequent step by applying a low overpotential for a short time. This two-step electrodeposition technique is used to directly make superhydrophobic copper coatings without the need for an additional layer of a low surface energy material. The presented technique is a potentially low-cost and simple approach for coating metallic surfaces with an enduring superhydrophobic film totally composed of copper. The article is organized as follows: The experimental methods and fabrication technique are described in the next section, followed by a presentation and discussion of the results of the study.

Electrodeposition is a well-known coating technique which can be utilized to make uniform coatings regardless of the surface size and shape.32 Template-assisted electrodeposition has been utilized previously to coat surfaces with a patterned layer of a superhydrophobic metal deposit.31,33,34 In this technique, texture of the deposit depends on the template structure and might be adversely affected by the template fabrication step. Because of this reason, template-free electrodeposition methods have gained more interest and applications. Wang et al.35 presented a method to directly deposit hierarchical spherical cupreous microstructures from an aqueous solution of Cu(NO3) by applying a constant overpotential and further hydrophobized the deposit surface by a self-assembled monolayer of n-dodecanethiol. Wang et al.36 used a two-step procedure to deposit a hydrophobic copper film: the first step at a low overpotential in order to create nucleation sites and the second step at a high overpotential for the growth of particles on the nucleation sites. Afterward, they enhanced the wettability of the copper deposit to the superhydrophobic region by applying a selfassembled monolayer of an n-alkanoic acid. Shirtcliffe et al.37 made a copper deposit with dense-branching morphology by applying a specific current density and further made the deposit superhydrophobic using a layer of a fluorocarbon coating. In the aforementioned works, morphological instabilities that are inherently made during electrodeposition38−40 are enhanced by applying an appropriate overpotential or current density. These instabilities provide the surface texture required for the hydrophobic state, but in all these works, superhydrophobicity is not obtained without applying an additional layer of a low surface energy material. The endurance of this additional layer strongly affects the superhydrophobic characteristic of these surfaces, and therefore, delamination of this layer is a serious issue for the superhydrophobic surfaces made using these techniques. Growth of morphological instabilities during electrodeposition can be explained using the models presented in the literature for the unstable growth process.40−44 Most of these models discuss the electrodeposition systems at the limiting condition, which corresponds to the mass transfer limitation of the system. After reaching the limiting condition, the concentration of the metal ion on the deposit surface falls to zero, and the distribution of the local current density or the local deposition rate is mainly controlled by mass transfer process in the electrolyte. Mass transfer favors the growth of the arbitrary protrusions of the surface and enhances the morphological instabilities of the deposit.45−48 Unlike mass transfer, surface tension acts to stabilize the growth process by relating surface curvature to the local surface potential. Based on this relation, the surface potential is higher at the curved locations of the deposit, and consequently, less growth rate is expected at these locations compared to the flat areas. The stabilizing effect of the surface tension is only effective for very small protrusions, but in general, surface texture of the deposit is defined by the competing effects of the mass transfer and surface tension.49 The mass transfer effect increases by increasing overpotential while surface tension at the interface of the metal−electrolyte decreases when higher overpotentials are applied.50 Mass-transfer-controlled deposition and low surface tension promote unstable growth process at the limiting condition. Since after reaching the limiting condition the current density remains constant with increasing overpotential, the change of

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EXPERIMENTAL METHODS

Reagents. Impurities other than the electrolyte species adversely influence the reproducibility of the experiment because they have uncontrolled effects on the mass transfer process in the bath and on the surface characteristic of the electrode.52 In order to mitigate the effects of impurities containing sulfur and hydrogen in the structure of copper deposit, which is formed in a copper sulfate bath, analytical grade CuSO4 and H2SO4 supplied by Fisher Chemicals (Pittsburgh, PA, USA) were used for accuracy of experimental results. In addition, deionized water with resistivity of 18 MΩ.cm (Millipore Corp., Billerica, MA, USA) was utilized for making all solutions. Electrodeposition Experiments. Electrodeposition was performed using an AUTOLAB PGSTAT128N potentiostat (ECO Chemie, Utrecht, The Netherlands). A traditional three-electrode system was employed in which a copper sheet and a platinum mesh were utilized as the reference electrode and anode, respectively. As used herein the term “substrate” refers to a surface, part, or item to which a coating is applied. Substrate plays the role of the cathode in the electrodeposition process. In this work, the substrate was a 0.4 × 0.6 cm2 silicon wafer covered by a smooth copper film using physical vapor deposition (PVD). In order to investigate the effect of the bath concentration on the wetting property of the deposit, electrolytes with different CuSO4 concentrations were used while the concentration of H2SO4 in those electrolytes was kept constant at 0.5 M. The electrolyte was bubbled for 15 min before fabricating each sample in order to remove oxygen. Moreover, the electrolyte was replaced by a new solution after fabricating three samples to avoid contamination. Since the reference electrode was composed of the same material as 4184

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the deposit, herein the term overpotential is used to refer to the electric voltage. A constant negative overpotential was applied to coat substrate with a copper film. However, in this paper, the overpotential, is referred to by its absolute value, η. The total amount of electrical charge (Q) was obtained from the current profile and utilized in the following equation to evaluate the deposit height (h): h=

QM nFρA

In this equation, n is the number of transferred electrons; n = 2 for copper deposition. F, A, M, and ρ are respectively the Faraday constant, surface area of the substrate, molecular weight, and density of the copper deposit. In order to investigate the effects of the overpotential on the deposit morphology, copper coatings with 30 μm thickness were deposited using different overpotentials in the range of 0.1−1.3 V. In addition, coatings with 100 μm thickness were fabricated to investigate the effect of the deposit height on the superhydrophobic characteristic of the coating. Characterization. The surface morphologies of the deposited coatings were examined using the FEI Quanta 600 scanning electron microscope (SEM). The PHI Quantera X-ray photoelectron spectroscopy (XPS) was utilized to obtain the chemical state of the deposit. In order to show the re-entrant geometry of the deposit topographical features, the cross section of one of the surface structures was revealed through a cross section formed by a focused ion beam using the FEI Helios 600 NanoLab. Measurement of the Wetting Characteristic. Droplets of deionized (DI) water of 4.5 μL volume were used in all wetting experiments. In order to investigate the effects of the overpotential and electrolyte concentration on the wetting characteristic of the deposit, the static contact angles of the droplet were obtained using the FTA 200 contact angle measurement system at room temperature. At each overpotential and electrolyte concentration, two samples were fabricated, and on each sample, three contact angle measurements were performed. Angles were deduced from the measurements using the ImageJ software.53,54 The contact angle hysteresis of a moving droplet on the superhydrophobic coating was measured five times using the same equipment. All results are reported as the mean number ± standard deviation. In order to further show the nonwetting characteristic of a superhydrophobic surface, full rebound of a falling droplet was captured at the speed of 3000 Hz by a XS-5 high-speed camera (IDT; Tallahassee, FL). Fabrication of the Superhydrophobic Coating. Figure 1 shows a schematic illustration of the two-step approach to fabricate a superhydrophobic coating. Figure 1a presents the incipient formation of the cauliflower-shaped structures on the deposit surface in the concentrated electrolyte of 1 M CuSO4, when overpotentials greater than 0.9 V in magnitude were applied. At these conditions, as explained in a later section, the interface between the electrolyte and the deposit is very unstable. At this level of instability, fractal structures such as those shown in Figure 1a are formed. The morphological instability grows with time or when a higher overpotential is applied. Beyond a certain stage, the branches that detach from the deposit can grow independently, and the surface of the deposit is covered by loosely attached structures, as seen in Figure 1b. The coating formed at the end of this first step of deposition will be referred to as one-layer deposit in the rest of the discussion. In the present approach, the branches are reinforced on the deposit surface by an additional thin layer of copper, shown by the thick line in Figure 1c, which is made by applying a low overpotential (0.15 V) for a short duration (10 s). The coating obtained from the two-step electrodeposition technique will be referred to as two-layer deposit in the reminder of the discussion. The coating shown in Figure 1a is hydrophobic while increasing morphological instabilities in Figures 1b,c enhances the wetting property of the coating to the superhydrophobic region.

Figure 1. Schematic illustration of the two-step deposition technique: (a) Cauliflower-shaped structures form on the deposit surface in a concentrated electrolyte of 1 M CuSO4 when overpotentials higher than 0.9 V are applied. (b) At a certain stage, with increasing time or with increasing overpotential, the branches appear detached from the surface. (c) The branches are reattached to the surface by depositing an additional thin layer (shown by the thick lines) of copper which is made by applying a low overpotential (0.15 V) for a short time (10 s).

copper coatings with 30 μm thickness at different overpotentials from an electrolyte of 1 M CuSO4 and 0.5 M H2SO4. Figures 2a−f portray the micrographs of the surface topographies of the one-layer deposits formed by applying overpotential in the range of 0.1 to 1.1 V, in increments of 0.2 V, while Figures 2g−l show the median values of the contact angles, the standard deviations, and the representative shape of a 4.5 μL water droplet on the deposited surfaces. The deposit surface features are observed to change gradually from needle-shaped structures at low overpotentials (η = 0.1 V) in Figure 2a to cauliflower-shaped structures at high overpotentials (η = 1.1 V) in Figure 2f. For an increase in overpotential from 0.1 to 0.7 V, a gradual transition from needle-shaped structures in Figures 2a and 2b to sphericalshaped structures in Figures 2c and 2d is observed. From Figures 2g−j, it is found that the needle- and spherical-shaped deposit surface textures (Figures 2a−d) have a negligible effect on the contact angle, which is observed to remain in the vicinity of 90°. The results obtained for the contact angles of these deposits agree with the value of 90° reported in the literature for smooth copper films exposed to the ambient conditions, which are always covered by a thin layer of Cu2O.55 At overpotentials greater than 0.7 V, morphological instabilities are enhanced and cauliflower-shaped surface features are formed (Figures 2e,f); as a result, the contact angle increases to larger values. As seen in Figures 2k,l, the coatings made by applying 0.9 and 1.1 V overpotentials exhibit contact angles of 135 ± 6° and 160 ± 6°, respectively. When a water droplet is brought in contact with the deposit made by applying 1.1 V overpotential (Figure 2f), it does not leave the syringe tip. Similar to what has been reported for the lotus leaves,10 the droplet is repelled by these surfaces and cannot be placed on the surface. This behavior shows the extreme nonwetting property of the coating which is further investigated by measuring contact angle hysteresis later on in this work. Similar to the lotus leaves, the areas of the deposit surface which allow the droplet to be placed on, such as the one shown in Figure 2l, may contain a point defect. The increase in the morphological instabilities by increasing overpotential can be

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RESULTS AND DISCUSSION The effect of the overpotential on the surface topography and contact angle of a one-layer deposit was studied by depositing 4185

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Figure 2. Micrographs (a−f) and the corresponding contact angles (g−l) of the deposits made by applying overpotentials of (a, g) 0.1 V, (b, h) 0.3 V, (c, i) 0.5 V, (d, j) 0.7 V, (e, k) 0.9 V, and (f, l) 1.1 V. The inset is a high-magnification micrograph of one of the florets on the deposit surface.

second layer of copper by applying a low overpotential (0.15 V) for a short duration (10 s) will fix the cauliflower-shaped branches on the deposit surface. Therefore, these branches are not removed from the surface when the tape is peeled off, as shown by the footprint of a two-layer deposit on a piece of tape in Figure 3b. Figure 4 shows the micrograph of the two-layer deposit. In this figure the cross section of one of the florets is revealed

explained by the model of unstable growth during electrodeposition presented by Aogaki et al.40 Based on this model, when deposit is made by applying larger overpotentials the growth time constant is smaller and consequently, at a particular deposition time, more morphological instabilities are formed. The inset displayed in Figure 2f is a highmagnification micrograph of one of the florets of the surface topography, which shows that these branches are covered by asperities of the smaller scale. This suggests the formation of fractal structures, resulting in the multiscale roughness on the coating deposited through the use of a high overpotential of 1.1 V, which may be considered as the reason for the superhydrophobic behavior of this coating. Figure 3a shows the footprint of a deposit surface made by applying 1.1 V overpotential on a piece of Scotch tape (3M; St Paul, MN). The cauliflower-shaped branches shown in Figure 2f are loosely attached to the surface and can be easily removed when the tape is peeled off from the surface. Depositing the

Figure 4. Micrograph of a two-layer deposit showing the cross section of one of the florets revealed through a cut made by ion beam.

through a cut made by ion beam. This cross section shows the reentrant geometry of the cauliflower-shaped structures which is considered as the main reason for their superhydrophobic characteristic.56 Comparing surface features of the two-layer deposit (Figure 4) with those of a one-layer deposit (Figure 2f) reveals that applying the thin layer of the deposit in the second step does not have a considerable effect on the surface topography. Figure 5 shows six successive snapshots obtained from the full rebound of a 4.5 μL droplet on the two-layer deposit made by applying an overpotential of 1.1 V for the formation of the

Figure 3. Photographs of the residue on a piece of transparent adhesive tape after a tape test of (a) one-layer deposit and (b) twolayer deposit. 4186

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Figure 5. Successive snapshots obtained from the full rebound of a 4.5 μL droplet on a coating made by the two-step electrodeposition technique.

Figure 6. XPS spectra of copper-colored deposits made by applying an overpotential of (a) 1.1 V and (b) 1.3 V. The representative drop shape and the corresponding contact angle obtained on the black coating are shown (c) just after its fabrication and (d) after 4 weeks of exposure in air.

first layer of coating followed by applying the second layer of copper at an overpotential of 0.15 V for 10 s. This bouncing sequence is similar to what has been reported for the dynamic behavior of a drop on a superhydrophobic surface in the literature.57 The snapshot at t = 0 ms shows the water droplet at the initial condition as it is about to hit the surface, and the snapshot at t = 5.3 ms shows the image of the water droplet when it impinges on the surface and starts to deform. The droplet is completely deformed as seen in Figure 3 at t = 8.0 ms. For impingement on the superhydrophobic surface, the kinetic energy of the droplet is restored into the surface energy, which allows the droplet to bounce off the surface as observed at t = 33.3 ms, and the droplet impinges on the surface again at t = 64 ms. In addition to the quantitative and qualitative observations on the wetting characteristics presented in Figures 2−5, it is also instructive to examine the surface chemical composition in order to show that the fabrication technique does not change the chemistry of the copper surface. At the ambient condition, copper always appears in one of two oxidation states, of which Cu2O is the most common state that is associated with the copper color. Further oxidation of copper produces CuO, which is easily distinguished from Cu2O because of its black color. One- and two-layer deposits shown in Figures 2a−f and Figure 4 have the copper color associated with the Cu2O oxidation state. Formation of a black CuO deposit was observed when overpotentials higher than 1.1 V were applied. In order to determine the chemical state of the copper- and black-colored deposits, XPS analysis has been carried out on two coatings made by applying 1.1 and 1.3 V overpotentials, and the corresponding spectra are shown in Figures 6a and 6b, respectively. The spectrum of the black deposit in Figure 6b shows high intensity shakeup satellites after the peaks. Moreover, these peaks are broader than the ones displayed in Figure 6a for the copper-colored Cu2O deposit. Based on the literature, high-intensity shakeup satellites and broad peaks are the main characteristics of the CuO spectrum which make it distinct from the spectrum of Cu2O.58 The electron micrograph of the black deposit is similar to that shown in Figure 2f for copper-colored Cu2O deposit obtained by applying an

overpotential of 1.1 V and is not depicted here. Furthermore, the representative drop shape, the average contact angle, and its standard deviation obtained on three different black coatings just after their fabrication have been displayed in Figure 6c. As this figure shows, the as-deposited black coating exhibits complete-wetting characteristic, with an average contact angle of 53°, despite its multiscale roughness. After 4 weeks, the wetting property of this deposit was found to change to being superhydrophobic, and the average contact angle of 162° was measured on the three samples, as shown in Figure 6d. The shift from a complete wetting characteristic to superhydrophobicity after several weeks at the ambient condition is in agreement with studies reported in the literature for the textured CuO surfaces.59 The change of the wetting characteristic of the black CuO deposit may be attributed to the explanation by Pei et al.,30 who reported a reversible superhydrophobicity to superhydrophilicity transition of textured CuO surfaces by alternation of ultraviolet (UV) irradiation and dark storage. In their work, this transition was attributed to the water and oxygen adsorption on the defective sites made on the surface by UV irradiation. During electrodeposition, as in UV radiation exposure, defects are formed on the deposit surface. While water adsorption on the surface defects is kinetically favorable, thermodynamically, the defective sites are more prone to adsorb oxygen. Initially, adsorption is mainly governed by the kinetic effect, as a result of which most of the defective sites are occupied by the hydroxyl group. Hydroxyl adsorption results in the superhydrophilic characteristic of the as-deposited black coating. Adsorption of the hydroxyl groups gradually shifts the surface to a thermodynamically unstable condition and provides the driving force for the oxygen adsorption. Therefore, the hydroxyl groups are replaced gradually by oxygen atoms. After several weeks, the defective sites are mainly occupied by oxygen, and the surface shows superhydrophobic characteristic. In addition to the oxygen adsorption on the defective sites of the CuO 4187

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Figure 7. Effect of the CuSO4 concentration in the electrolyte (for a fixed H2SO4 concentration of 0.5 M) on the contact angle of two-layer deposits of (a) 30 μm and (b) 100 μm thickness made by applying an overpotential of 1.1 V. The error bars represent the standard deviations.

Figure 8. Photographs of a water droplet attached to the syringe being slowly moved along the two-layer deposit made from the electrolyte with (a) 1 M and (b) 0.5 M CuSO4 concentration, by applying an overpotential of 1.1 V. The image in (c) shows that the droplet can be separated easily from the superhydrophobic surface made from the electrolyte with 1 M CuSO4 concentration.

morphological instabilities of the deposit.40 Based on this model, at higher bath concentrations, the growth time constant is smaller, and consequently, at a particular deposition time, more morphological instabilities are formed. As shown in Figures 7a and 7b, increasing electrolyte concentration from 0.1 to 0.5 M results in a 17% enhancement in the contact angle of the 30 μm deposit and a negligible change in the contact angle of the 100 μm deposit. In addition, if the bath concentration changes from 0.5 to 1 M, a greater increase in the deposit contact angle is obtained at the deposit height of 30 μm compared to the deposit height of 100 μm. Consequently, the effect of the electrolyte concentration on the contact angle is more evident at the smaller deposit heights. Furthermore, the contact angle of the 100 μm deposit is higher than that for the 30 μm deposit at all electrolyte concentrations shown in Figures 7a,b. These effects of the deposit height on the contact angle can be explained by the model of unstable growth during electrodeposition which shows that level of surface irregularity enhances at larger deposition times.60 During electrodeposition, like other unstable growth processes, protrusions develop with time, and the surface roughness is higher at larger deposit heights.39 Therefore, when deposition continues for a longer time and a thicker deposit is formed, the effect of the electrolyte concentration on the deposit contact angle is diminished, and a larger contact angle is observed. As Figures 7a,b show, both 30 and 100 μm coatings satisfy the contact angle requirement for superhydrophobicity (α > 150°), if they are made by applying 1.1 V overpotential in an electrolyte with 1 M CuSO4 concentration. During the first step deposition of these superhydrophobic coatings, current density

coating, XPS analysis shows a large amount of carbon adsorption on the surface of the black deposit. After 4 weeks, carbon content on the surface of the CuO and Cu2O coatings reaches to 50% and 23%, respectively. High carbon adsorption on the surface of the black CuO coating may be regarded as the second reason for the change in the wetting property of this coating after a long time exposure to the ambient environment. In addition to the effect of overpotential discussed so far, superhydrophobic characteristic of the deposit also depends on the bath concentration and deposit thickness. The effect of the bath concentration and deposit thickness on the contact angle of a two-layer deposit was studied by depositing copper coatings with 30 and 100 μm thickness at an overpotential of 1.1 V from the electrolytes with 0.1, 0.5, and 1 M CuSO4 concentration. The concentration of H2SO4 was kept constant in 0.5 M for all three electrolytes. At each bath concentration and deposit thickness, the contact angle of a 4.5 μL droplet was measured on two different samples, and on each sample, the measurement was repeated three times. The median values of the contact angles and the representative shapes of the droplet on the deposits made from the electrolytes with different bulk concentrations are shown in Figures 7a and 7b for deposit heights of 30 and 100 μm, respectively. As these figures display, the average contact angle increases from 103° to 160° and from 132° to 162° for the deposit heights of 30 and 100 μm, respectively, when the CuSO4 concentration in the electrolyte increases from 0.1 to 1 M. This effect of the electrolyte concentration on the deposit contact angle can be attributed to what has been shown previously in the model of unstable growth for the effect of bath concentration on the 4188

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Langmuir varies from zero to a limiting value. As explained in the Introduction, this limiting value corresponds to the mass transfer limitation of the process and is independent of the deposit height. At the aforementioned conditions which result in the formation of a superhydrophobic coating, an average limiting current density of 7553 ± 424 A/m2 was obtained from the experiments. In addition to the static contact angles (Figures 7a,b) and the full rebound of the droplet on the surface (Figure 5), contact angle hysteresis may be determined to further show the superhydrophobic characteristic of the two-layer deposit.10 To determine the contact angle hysteresis, a 4.5 μL droplet was formed with a syringe and brought in slight contact with the surface. Then water droplet, attached to the syringe, was slowly moved along the sample surface, and the contact angle hysteresis was deduced from the advancing and receding contact angles, which were measured at the front and back of the moving droplet, respectively. This measurement was repeated five times for a two-layer deposit with 30 μm thickness which was made by applying 1.1 V overpotential in a bath of 1 M CuSO 4 concentration (Figure 4). The representative shape of the droplet that is moved on this surface by the syringe is shown in Figure 8a. On this surface, the median value of the contact angle hysteresis and the standard deviation were obtained to be 5° and 2°, respectively. On this two-layer deposit, the droplet can be moved freely, and it can be separated from the surface easily as shown in Figure 8b. A similar behavior is observed when the droplet is moved faster on the surface. As presented in the video provided in the Supporting Information, the water droplet does not adhere to the surface even if it is pushed onto the surface; furthermore, it moves freely along the surface even at a high velocity. In contrast, a water droplet adheres to a nonsuperhydrophobic surface after the first contact and cannot be separated. Figure 8c shows an example of this behavior of the droplet on a two-layer deposit with 30 μm thickness which was made by applying 1.1 V overpotential in a bath of 0.5 M CuSO4 concentration. As this figure presents, when the syringe is moved, the droplet is not separated from the surface; instead, it is elongated and a large difference between the advancing and receding contact angles is observed. In this work, an experimental investigation for the effects of the overpotential and bath concentration on the contact angle of the copper deposit was presented. This investigation shows that superhydrophobic copper deposit can be obtained by applying a high overpotential in a concentrated electrolyte. Using the results of the experimental investigation, a novel twostep electrodeposition technique for fabricating superhydrophobic copper coatings was developed. The superhydrophobic characteristic of this coating can be attributed to the morphological instabilities formed during electrodeposition. In this work, the models presented previously for the unstable growth process during electrodeposition were used to explain the experimental results obtained for the effects of the bath concentration and overpotential on the deposit contact angle. This theoretical explanation provides the foundation for using the fabrication technique, presented in this work, in other electrodeposition systems besides copper. Application of this fabrication technique in making superhydrophobic coatings from materials other than copper will be considered in a future study.

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CONCLUSIONS

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ASSOCIATED CONTENT

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The results in this paper show that superhydrophobic copper coating can be obtained by electrodeposition from a concentrated electrolyte of 1 M CuSO4 if a high overpotential of 1.1 V is applied. At these conditions, the surface of the deposit is covered by loosely attached structures which are formed by the equiaxed growth mechanism. In this work, these structures were reinforced on the surface by an additional thin layer of copper, which is made by applying a low overpotential (0.15 V) for a short duration (10 s). Water contact angle of 160 ± 6° and contact angle hysteresis of 5 ± 2° were measured for the two-layer deposit, fabricated using this technique. In addition, studying effects of the overpotential and bath concentration on the deposit contact angle showed that superhydrophobic characteristic of the deposit diminishes if deposition occurs from the bath with concentrations less than 1 M or by applying overpotentials less than 1.1 V. The superhydrophobic characteristic of the deposit was attributed to the formation of the morphological instabilities which was theoretically explained by the models presented in the literature for the unstable growth process during electrodeposition.

S Supporting Information *

Video of droplet on the surface. 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]; Ph +1 540 231 1776 (R.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the assistance and inputs of Mr. Mehdi Kargar and Dr. William T. Reynolds in the work reported.

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REFERENCES

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