Double-Scale Roughness and Superhydrophobicity on Metalized

Mar 5, 2009 - Double-Scale Roughness and Superhydrophobicity on Metalized Toray. Carbon Fiber Paper. S. Bliznakov, Y. Liu, and N. Dimitrov*...
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Double-Scale Roughness and Superhydrophobicity on Metalized Toray Carbon Fiber Paper S. Bliznakov, Y. Liu, and N. Dimitrov* Department of Chemistry, State University of New York at Binghamton, P.O. Box 6000, Binghamton, New York 13902-6000

J. Garnica, and R. Sedev* Ian Wark Research Institute, University of South Australia, Adelaide, Mawson Lakes SA 5095, Australia Received November 28, 2008. Revised Manuscript Received January 23, 2009 A simple, inexpensive route for the fabrication of a superhydrophobic metal surface is described. Carboncarbon composite paper (Toray TGP-H) is electroplated with copper. The copper layer is made hydrophobic by self-assembling a monolayer of dodecanethiol. The surface topography required to induce superhydrophobic behavior is achieved by varying the plating bath composition (Cl-, PEG, and SPS additives) and the time of deposition (effective thickness of the Cu layer). The surface morphology created by the original arrangement of the carbon fibers in the Toray paper (diameter 8 μm, spacing 30 μm) does not produce superhydrophobic behavior. This is true for both continuous and incomplete copper coatings. Truly superhydrophobic behavior (large contact angles, 160-165°, and very small contact angle hysteresis, 2 to 3°) is achieved when a continuous copper layer is deposited on the carbon fibers and also a second micrometer-range roughness is developed as a result of the formation of small copper crystallites (size ∼1 μm).

Introduction Surfaces on which liquids form large contact angles are called hydrophobic. Although the term hydrophobic implies a net repulsion between the liquid and the solid, it is in fact the adhesion within the liquid that is stronger than the attraction to the solid and thus prevents the liquid from spreading on that particular surface.1-3 The concept of wettability is best encapsulated within the Young equation,1,2 which relates the equilibrium contact angle, θ0, to the interfacial tensions, γij, of the three interfaces meeting at the contact line: cos θ0 ¼

γSV -γSL γ þ γSV -γSL ¼ -1 þ LV ¼ γLV γLV -1 þ 2

WA WC

ð1Þ

In this equation, γSV, γSL, and γLV are the interfacial tensions of the solid/vapor, solid/liquid, and liquid/vapor interfaces (macroscopic quantities accounting for the intermolecular interactions at the three interfaces); WA and WC are the work of adhesion and cohesion (the amount of reversible work required to split/create a unit area of the interface). The balance of adhesion and cohesion is strongly influenced by *Corresponding author. [email protected], rossen.sedev@unisa. edu.au (1) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; Wiley: New York, 1997. (2) de Gennes, P. G.; Brochard-Wyard, F.; Quere, D. Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves, Springer: New York, 2004. (3) van Oss, C. J. Interfacial Forces in Aqueous Media, 2nd ed.; Taylor & Francis: Boca Raton, FL, 2006. (4) Zisman, W. A. Adv. Chem. Ser. 1964, 43, 1. (5) de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827.

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the chemical nature of the two materials.1-5 It has been found that the most hydrophobic surface is one covered with closely packed perfluoromethyl groups.4,6 A further decrease in adhesion (i.e., higher contact angle) is possible only if the surface is also microstructured.7-12 The effect of roughness on the equilibrium contact angle is described by the Wenzel equation:1,2 cos θW ¼ r cos θ0

ð2Þ

In practice, however, the influence of roughness is twofold; not only the equilibrium angle, θW, but also the contact angle hysteresis increases.1,5,11 Contact angle hysteresis is more often than not considered to be a nonequilibrium phenomenon.13-15 The appearance of two distinct experimental values for the static contact angle (advancing and receding) is related to metastable states developing as a result of the complex shape of the liquid droplet and the contact line in particular.1,13,16 When surface roughness exceeds a certain threshold value, hydrophobicity strongly increases because the solid/liquid interface becomes discontinuous. This (6) Nishino, T.; Meguro, M.; Nakamae, K.; Matsushita, M.; Ueda, Y. Langmuir 1999, 15, 4321. (7) Dettre, R. H.; Johnson, R. E.Jr. Adv. Chem. Ser. 1964, 43, 136. (8) Johnson, R. E.Jr; Dettre, R. H. Adv. Chem. Ser. 1964, 43, 112. (9) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, 457. (10) Shibuichi, S.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512. (11) Qu :: ere, D. Physica A 2002, 313, 32. (12) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (13) Blake, T. D. In Surfactants; Tadros, T. F., Ed.; Academic Press: London, 1984. (14) Everett, D. H. Pure Appl. Chem. 1980, 52, 1279. (15) Good, R. J. J. Adhes. Sci. Technol. 1992, 6, 1269. (16) Neumann, A. W.; Spelt, J. K. Applied Surface Thermodynamics; Marcel Dekker: New York, 1996; p 646.

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situation is described by the Cassie equation:1,2 cos θC ¼ φ cos θ1 þ ð1 -φÞcos θ2

ð3Þ

Thus, the equilibrium contact angle, θC, is determined by the area fractions of the patches (φ and 1 - φ) and their intrinsic wettability (as given by θ1 and θ2). All contact angles in eqs 1-3 are equilibrium quantities because they are derived through free-energy minimization.1,2 They are also apparent contact angles because, under equilibrium conditions, they can be measured with available experimental techniques.17 For superhydrophobic surfaces θ2  180° (an air film separates the liquid and the solid) and for small φ, a large equilibrium contact angle is predicted by eq 3. Unlike the Wenzel state, in the Cassie state it is possible for contact angle hysteresis to decrease drastically simply because of the diminished influence of the solid-liquid junctions.18,19 Thus, true superhydrophobicity is related to the Cassie rather than the Wenzel state.9,20 In practice, the aim is to obtain a surface roughness that, after a suitable surface modification, generates superhydrophobic behavior on that surface. Myriad experimental approaches have been suggested during the past decade.21-25 Furthermore, as surface fabrication at the microlevel and nanolevel has rapidly matured, the task has shifted from just preparing a superhydrophobic surface to producing a robust one and at a reasonable cost. It has been found that on surfaces containing two or more roughness scales the superhydrophobic effect is more robust.20,23,26-34 Nature provides some stunning examples (e.g., plant leaves,35 water strider legs,36 and butterfly wings37). A hydrophobic coating whose roughness has a high fractal dimension induces superhydrophobic behavior.10,38 Double-scale roughness further decreases the local adhesion at the solid-liquid junctions,20,39 (17) Neumann, A. W.; Good, R. J. Surf. Colloid Sci. 1979, 11, 31. (18) Quere, D.; Lafuma, A.; Bico, J. Nanotechnology 2003, 14, 1109. (19) McHale, G.; Shirtcliffe, N. J.; Newton, M. I. Langmuir 2004, 20, 10146. (20) Gao, L.; McCarthy, T. J. Langmuir 2006, 22, 2966. (21) Ma, M.; Hill, R. M. Curr. Opin. Colloid Interface Sci. 2006, 11, 193. (22) Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132, 31. (23) Nakanishi, T.; Michinobu, T.; Yoshida, K.; Shirahata, N.; Ariga, K.; Mohwald, H.; Kurth, D. G. Adv. Mater. 2008, 20, 443. (24) Feng, X.; Jiang, L. Adv. Mater. 2006, 18, 3063. :: (25) Muller, F.; Michel, W.; Schlicht, V.; Tietze, A.; Winter, P. SelfCleaning Surfaces Using the Lotus Effect. In Handbook for Cleaning/ Decontamination of Surfaces; Johansson, I., Somasundaran, P., Eds.; Elsevier: Amsterdam, 2007; pp 791. (26) Xiu, Y.; Zhu, L.; Hess, D. W.; Wong, C. P. Langmuir 2006, 22, 9676. (27) Tsai, P.-S.; Yang, Y.-M.; Lee, Y.-L. Nanotechnology 2007, 18, 465604/1. (28) Bok, H.-M.; Shin, T.-Y.; Park, S. Chem. Mater. 2008, 20, 2247. (29) Kim, D.; Kim, J.; Park, H. C.; Lee, K.-H.; Hwang, W. J. Micromech. Microeng. 2008, 18, 015019/1. (30) Cortese, B.; D’Amone, S.; Manca, M.; Viola, I.; Cingolani, R.; Gigli, G. Langmuir 2008, 24, 2712. (31) Manoudis, P. N.; Karapanagiotis, I.; Tsakalof, A.; Zuburtikudis, I.; Panayiotou, C. Langmuir 2008, 24, 11225. (32) Bormashenko, E.; Stein, T.; Whyman, G.; Pogreb, R.; Sutovsky, S.; Danoch, Y.; Shoham, Y.; Bormashenko, Y.; Sorokov, B.; Aurbach, D. J. Adhes. Sci. Technol. 2008, 22, 379. (33) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Chabrol, G.; Perry, C. C. Adv. Mater. 2004, 16, 1929. (34) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Oener, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395. (35) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (36) Feng, X.-Q.; Gao, X.; Wu, Z.; Jiang, L.; Zheng, Q.-S. Langmuir 2007, 23, 4892. (37) Zheng, Y.; Gao, X.; Jiang, L. Soft Matter 2007, 3, 178. (38) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125. (39) Herminghaus, S. Europhys. Lett. 2000, 52, 165.

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but a complete account of the effect is not available. In spite of their availability and engineering importance, metal surfaces have received limited attention.40-47 The possible benefits are very significant, and the situation is changing quickly. In this work, we describe an inexpensive copper electroplating method for carbon fiber paper (Toray) featuring an established roughness length scale and its subsequent surface modification with a thiol-based self-assembled monolayer. The metallization not only plays a key role in obtaining a continuous coating but, based on a variety of plating parameters, also enables the generation of a second roughness length scale that promotes superhydrophobic behavior. The original surface morphology of the Toray paper (roughness length scale on the order of 10 μm) does not induce superhydrophobic behavior. However, its combination with a second micrometer-range roughness (electroplated copper crystallites, size ∼1 μm) is quite effective.

Materials and Methods Carbon-carbon composite paper (Toray TGP-H) is made of stacking and crossing carbon fibers “Torayca” together (Figure 1a). The fibers featuring high tensile strength and high modulus are firmly connected by carbon.48 Advanced properties such as high mechanical strength, conductivity, and gas permeability qualify the Toray paper (TP) as suitable for use as a gas diffusion layer in fuel cell applications.49-53 The TP is a commercial product available in both untreated and Teflon-treated (5-60%) forms.51 Applying Teflon that renders the TP hydrophobic was found to enhance the performance of TP as a catalyst support in gas-diffusion electrodes without compromising its characteristics.54 Currently, different brands of TP are widely used as electrode materials for phosphoric acid fuel cells (PAFC) and polymer electrolyte fuel cells (PEFC). Toray paper TGP-H-090 used in this study was 5% Teflontreated with a thickness of 280 μm.48 The TP consists of crisscrossing carbon fibers with a diameter of about 8 μm and empty spaces of various shapes with an average size of about 30 μm.52 This structure of TP features a porosity of 78% and an average surface roughness of 8 μm.48 The Teflon-treated TP is hydrophobic and can support water droplets with a high contact angle; see Figure 1b. The resistivity of TP is relatively low and increases moderately after Teflon treatment (from about 90 mΩ 3 cm at 0% up to 230 mΩ 3 cm at 40% PTFE).55 Most probably the Teflon coating is uneven and discontinuous. This allows the TP to be electroplated. (40) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2005, 21, 937. :: (41) Furstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Langmuir 2005, 21, 956. (42) Xu, W.; Liu, H.; Lu, S.; Xi, J.; Wang, Y. Langmuir 2008, 24, 10895. (43) Qian, B.; Shen, Z. Langmuir 2005, 21, 9007. (44) Guo, Z.; Fang, J.; Wang, L.; Liu, W. Thin Solid Films 2007, 515, 7190. (45) Safaee, A.; Sarkar, D. K.; Farzaneh, M. Appl. Surf. Sci. 2008, 254, 2493. (46) Wang, S.; Song, Y.; Jiang, L. Nanotechnology 2007, 18, 015103/1. (47) Huang, Z.; Zhu, Y.; Zhang, J.; Yin, G. J. Phys. Chem. C 2007, 111, 6821. (48) fuelcell.com2008, http://www.fuelcell.com/techsheets/TORAY. (49) Lu, G. Q.; Wang, C. Y. J. Power Sources 2004, 134, 33. (50) Kangasniemi, K. H.; Condit, D. A.; Jarvi, T. D. J. Electrochem. Soc. 2004, 151, E125. (51) Benziger, J.; Nehlsen, J.; Blackwell, D.; Brennan, T.; Itescu, J. J. Membr. Sci. 2005, 261, 98. (52) Gallagher, K. G.; Darling, R. M.; Patterson, T. W.; Perry, M. L. J. Electrochem. Soc. 2008, 155, B1225. (53) Gostick, J. T.; Fowler, M. W.; Ioannidis, M. A.; Pritzker, M. D.; Volfkovich, Y. M.; Sakars, A. J. Power Sources 2006, 156, 375. (54) Kindler, A.; Yen, S.-P.HSPES Membrane Electrode Assembly.U.S. Patent 6,136,463, October 24, 2000. (55) Lobato, J.; Canizares, P.; Rodrigo, M. A.; Ruiz-Lopez, C.; Linares, J. J. J. Appl. Electrochem. 2008, 38, 793.

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Figure 1. (a) SEM image of plain Toray paper (TP). (b) Advancing and (c) receding contact angles of water on a TP surface (needle diameter 0.46 mm).

Plating baths with different compositions were used in the present work in order to investigate the influence of the additives on the quality and morphology of the copper deposited on a TP support. Plating was performed in a standard three-electrode electrochemical cell. The working electrode was a square piece of 5% Teflon-treated TP of size 1 cm2. The counter electrode was a phosphorized Cu anode (0.04-0.06% P, Kocour). A pure Cu wire (99.99%, Surepure Chemetals) served as a reference electrode. Before immersion into the electrolyte, the counter and reference electrodes were etched in 8 M nitric acid, rinsed thoroughly with Barnstead Nanopure water (resistivity >18.2 Ω 3 cm), and dried in high-purity nitrogen gas (less than 1 ppb O2, CO, CO2, and H2O). The phosphorized Cu anode was additionally filmed prior to each plating experiment following a procedure described previously.56,57 Plating solutions were formulated as combinations of 0.26 M CuSO4 3 5H2O (Sigma-Aldrich), 2 M H2SO4 (doubly distilled, GFS), 1.13 mM HCl (doubly distilled, Sigma-Aldrich), 0.02 mM polyethylene glycol 4000 (PEG) (Fluka), 0.01 mM bis (3-sulfopropyl) disulfide (SPS) (Rachig); see Table 1. All chemicals were of the highest purity grade available and were used as provided by the vendors. A PC-based potentiostat/galvanostat (EC Epsilon, BASi) was used in galvanostatic mode. A current density of 20 mA/cm2 was applied for 1000 s in order to obtain copper deposits of a similar thickness from all solutions listed in Table 1. The thickness of the deposit was calculated from Faraday’s law assuming that the copper was deposited on a flat surface. After being plated, the samples were rinsed thoroughly with Barnstead Nanopure water and dried with high-purity nitrogen. The metallized specimens were then immersed in a 1 mM solution of 1-dodecanethiol (Sigma-Aldrich) in ethanol for 16 h in order to form a self-assembled monolayer.58 Finally, the samples were rinsed with ethanol and dried in a high-purity stream of nitrogen. The surface morphology of the samples (bare and Cu-covered Toray paper) was characterized by scanning electron microscopy. A high-resolution FEG SEM (Zeiss Supra 55 VP) coupled with an in-lens detector at an accelerating voltage of 10 kV and a working distance of 4 mm was used to characterize the samples. The wettability of the samples was characterized by contact angle measurements. Advancing and receding contact angles of water were measured with the sessile drop technique. A small droplet of pure water (diameter 2 to 3 mm) was deposited on the surface. Its volume was manually increased (or decreased) in order to make (56) Liu, Y.; Wang, J.; Yin, L.; Kondos, P.; Parks, C.; Borgesen, P.; Henderson, D. W.; Bliznakov, S.; Cotts, E. J.; Dimitrov, N. ECTC Proc. 2008, 2105. (57) Liu, Y.; Wang, J.; Yin, L.; Kondos, P.; Parks, C.; Borgesen, P.; Henderson, D. W.; Cotts, E. J.; Dimitrov, N. J. Appl. Electrochem. 2008, 38, 1695. (58) Ulman, A. Self-Assembled Monolayers of Thiols.; Academic Press: 1998; Vol. 24, p 278.

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Table 1. Designation and Composition of the Copper Plating Solutions solution a b c d

no additives Cl Cl + PEG Cl + PEG + SPS

CuSO4 3 5H2O

H2SO4

HCl

PEG

SPS

√ √ √ √

√ √ √ √

√ √ √

√ √

√

the contact line advance (or recede), and then a picture of the static droplet was recorded (CAM100, KSV). The contact angles were determined from the silhouette of the droplet using public domain image-processing software (ImageJ, NIH) with the DropSnake plugin (EPFL, Switzerland).

Results The TP is hydrophobic and rough (Figure 1a). The advancing contact angle of water measured on the Teflon-treated Toray paper is very high (160°, Figure 1b), but the receding contact angle is practically zero (Figure 1c). Thus, the contact angle hysteresis, θA - θR, is very large, and the TP surface is hydrophobic but not superhydrophobic. To achieve superhydrophobicity, the TP was metallized with copper and then covered with a dodecanethiol self-assembled monolayer (SAM). Experimentation has shown that the attachment of dodecanethiol to the plain (nonmetallized) TP is negligible, as expected from the surface chemistry, and has no effect on its wettability. The presence of additives such as accelerators, suppressors, and levelers is of crucial importance for the surface morphology of the deposited copper.56,57 To the best of our knowledge, this work represents the first attempt to metalize TP. Whereas the effect of the original TP hydrophobicity on the galvanostatic metallization was not considered herein, the impact of the plating bath additives on the quality and morphology of the Cu deposits was studied by chronopotentiometry and FE SEM. In Figure 2, the absolute values of the overpotential (as measured versus a Cu wire quasi-reference electrode) are plotted as a function of time at i = 20 mA/cm2. The i value was calculated using the geometric area of the electrodes. Under these conditions, the electrode overpotentials measured in the presence of Cl- ions, PEG, and/or SPS additives can be compared to those measured in a solution containing only Cu2+ and sulfuric acid. Copper grown from an additive-free solution forms a relatively smooth surface coating (Figure 3a). A distinctly Langmuir 2009, 25(8), 4760–4766

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different morphology was seen when chloride ions were present in the plating solution (Figure 3b). Polydisperse Cu crystallites with well-defined shapes are deposited onto the carbon surface. The carbon fibers are decorated with small separate copper crystallites with their sharp edges pointing out. When Cu is deposited from a solution containing both PEG and Cl- ions (Figure 3c), a much smoother surface is formed. It is, however, rougher than the coating grown in additive-free solution. When Cu was deposited from a solution containing all three additives (Cl- + PEG + SPS) (Figure 3d), the roughness is similar to that seen in Figure 3b (Cl- only). The Cu crystallites, however, are clearly faceted and significantly smaller, with an average size of about 3 μm. The water contact angles measured on Cu-coated and SAM-modified TP samples are presented in Figure 4. In all

cases, the advancing contact angle is within the range of 150160° and is essentially identical to the one measured on plain TP (Figure 1b). However, the receding contact angle is strongly affected, and all samples plated with additives (b-d) show negligible hysteresis and are thereby truly superhydrophobic. The morphology of the copper deposit, with respect to superhydrophobicity, appears to be optimal for plating solutions c and d. The other parameter of key importance is the thickness of the Cu coating. The plating bath containing SPS, PEG, and chloride ions as additives was selected to study the influence of the copper deposit thickness on surface morphology and the wettability of the Cu-coated SAM-modified Toray carbon fiber paper. The copper layer thickness was estimated from the overall charge accumulated during the plating by assuming that no morphology evolution took place.

Figure 2. Overpotential transients registered during Cu electrodeposition on TP in plating baths of different composition (current density 20 mA/cm2).

Figure 4. Water contact angles measured on Cu-coated and SAMmodified TP (plating conditions listed in Table 1).

Figure 3. SEM images of Cu-coated TP at 20 mA/cm2 for 1000 s in solutions a-d (Table 1). Langmuir 2009, 25(8), 4760–4766

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Figure 5. SEM images of Cu-coated TP in Cl- + PEG + SPS solution. Thickness: (a) 2, (b) 8, and (c) 16 μm. (manual and electrochemical polishing), SAM-modified, polycrystalline Cu surface (θA = 122°, θA - θR ≈ 40°) and a nonmetallized TP surface (θA = 160°, θA - θR ≈ 160°).

Discussion To consider a surface superhydrophobic, both advancing and receding contact angles should be large, and their difference (the contact angle hysteresis) should be small.9,20 The advancing angle on plain TP was 160°, and the receding one was effectively zero (Figure 1). Thus, the surface of the plain TP is hydrophobic but not superhydrophobic. The high advancing contact angle can be explained with the Cassie equation (eq 3), which for a porous surface (θ2 = 180°) is written as Figure 6. Water contact angles on a polished polycrystalline Cu surface and Cu-plated and SAM-modified TP with various Cu deposit thicknesses.

The surface morphology of the coating strongly depends on the thickness (2, 8, and 16 μm) of the copper deposit (Figure 5). The thinner (Figure 5a) and the thickest (Figure 5c) deposits are smoother than the intermediate one (Figure 5b), but only in the last case does Cu entirely cover the fiber surface. The contact angles measured on Cu-coated and SAMmodified TP samples with different deposit thicknesses are presented in Figure 6. The advancing contact angle is always high (155-160°). The receding contact angle is gradually increasing with the thickness of the Cu coating layer, and the contact angle hysteresis is within 10° for coatings of intermediate thickness. It appears that a 6-8 μm copper deposit is optimal from the point of view of inducing superhydrophobic behavior. These results are contrasted with the wettability of a smooth 4764

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cos θ ¼ ð1 þ cos θ0 Þφ -1

ð4Þ

where θ is the contact angle measured in the experiment (apparent contact angle) and θ0 is the local contact angle of water on Teflon. (Under equilibrium conditions, θ0 should be the Young contact angle for that system.) Local contact angles cannot be assessed with conventional contact angle techniques, and we take θ0 = 122° (a typical value for a Teflon-like coating59,60). The area fraction can be estimated from the porosity:48 φ = 1 - 0.78 = 0.22 or alternatively from the fiber (8 μm48) and pore (30 μm52) size φ = 8/(8 + 30) = 0.21. With these values, the angle calculated with eq 4 is 154°, in plausible agreement with the experimental value. The receding angle, however, is practically zero (Figure 1c). This substrate is rough and heterogeneous (Figure 1a) and the contact line would pin on various defects; therefore, the contact angle (59) Sedev, R.; Fabretto, M.; Ralston, J. J. Adhes. 2004, 80, 497. (60) Extrand, C. W. Langmuir 2006, 22, 1711.

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hysteresis will be large. This could be due to the presence of sharp edges (strong pinning of the contact line), incomplete Teflon coating (hydrophilic patches), or simply defects of higher energy. Only a few literature values of contact angles on TP are available for comparison. A 170° advancing contact angle of water was reported,51 in good agreement with our measurement, but much lower values, 96-98°, were also published.52 In both cases, the hysteresis recorded was considerable, 40-50°, but much less than that found in our experiments (Figure 1). Large water contact angles (151-160°) were found on Teflon-treated TP (samples with 10, 20, and 40% PTFE showed statistically identical values). The angles on untreated TP were 95-100°, but hysteresis was not noted.55 A 146° contact angle of mercury53 was reported, but that value seems unrealistically high. The Young equation (eq 1) can be combined with the Fowkes’ expression to obtain1,3

cos θ0 ¼ -1 þ 2

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi γLW LV γSV γLV

ð5Þ

where γLW LV is the nonpolar (Lifshitz-van der Waals component3) of the surface tension of the liquid and γLV is the total surface tension of the liquid (i.e., the experimentally measurable quantity). It was also assumed that the surface tension of the solid is entirely nonpolar, and γLW SV  γSV (a plausible hypothesis for a carbon or Teflon surface3,4). With eq 5, θ0 = 122° (for water) used in the estimation of eq 4 corresponds to the published value, 13.5 mJ/m2, of the surface tension of TP61 (typical values of γSV for Teflon-like surfaces range between 12.4 and 18.5 mJ/m2 62). However, θ0 = 112° (for mercury, after correction for the Cassie effect through eq 353) would imply a γSV about 8 times higher than that and therefore appears to be incorrect. It is quite common for contact angles (especially receding ones) measured on apparently similar systems to differ significantly between laboratories because the exact values are very sensitive to minor differences in pretreatment, conditioning, and contamination.17,63,64 Given the importance of the water wettability of Toray paper, further efforts should be made to elucidate the problem. The influence of the bath chemistry on the copper deposition kinetics, shown in Figure 2, has also been studied on flat Cu electrodes in our recent papers.56,57 There is no significant qualitative difference in the shape of the overpotential transients in either case. The higher overpotential values, measured on TP electrodes, are mainly due to the 2-fold-higher current density applied in this case. Generally, in all solution chemistries the overpotential increases initially and then slowly decreases until a steady-state situation is established. According to the analysis presented elsewhere,57 the steady state, depicted in Figure 2, features a dependence of the overpotential on current density consistent with charge-transfer-limited kinetics (no diffusion limitations) that is described by the Butler-Volmer formalism. The overpotential transient, registered in the electrolyte in the (61) Mathur, R. B.; Maheshwari, P. H.; Dhami, T. L.; Tandon, R. P. Electrochim. Acta 2007, 52, 4809. (62) Quinn, A.; Sedev, R.; Ralston, J. J. Phys. Chem. B 2003, 107, 1163. (63) Sedev, R. V.; Petrov, J. G.; Neumann, A. W. J. Colloid Interface Sci. 1996, 180, 36. (64) Lam, C. N. C.; Kim, N.; Hui, D.; Kwok, D. Y.; Hair, M. L.; Neumann, A. W. Colloids Surf., A 2001, 189, 265.

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presence of only Cl- as an additive, is an exception to this rule; the overpotential slowly reaches the value recorded in additivefree solution and then decreases with time. A possible explanation is that in the very beginning, when there is no copper deposited, Cl- does not adsorb on the carbon surface and therefore does not perform its catalytic role during the electrodeposition of Cu. Thus, the overpotential reaches values identical to these recorded in the additive-free solution. Latter, as copper gradually coats the carbon fibers, Cl- adsorbs on the freshly deposited Cu layer and thereby drives the overpotential to lower values. Most likely, a similar phenomenon occurs during copper deposition with all additives present. In addition to that, there is a competition between the CuCl, PEG-Cl-CuI, and CuI-SPS complexes on the electrode surface during deposition in a bath containing all additives.65,66 This also affects the shape of the overpotential transients shown in Figure 2. Unlike the other solutions, the bath containing Cl- + PEG blocks the reaction sites and suppresses the overall reaction rate, which for deposition at constant current density results in an increasing overpotential (Figure 2). Stating once again that no correlation between the original TP hydrophobicity and the metallization process was sought in this work, the solution chemistry undoubtedly plays a key role in the copper coating of the carbon fibers. In our procedure, the carbon fibers are metallized and then hydrophobized by the attachment of the thiol self-assembled monolayer. The metallization is a necessary step because the thiol does not attach to the plain TP. This also suggests that our approach is applicable to other microstructured surfaces. It should be noted that an incomplete copper deposit, as in Figure 5a, results in a surface that is not superhydrophobic (contact angle hysteresis ≈ 50°). It is therefore necessary to obtain a continuous hydrophobic coating of the microfibers in order to induce superhydrophobic behavior. The evolution of the copper deposit shown in Figure 5 is interesting in itself. At shorter time (small equivalent thickness as in Figure 5a), the deposit is smooth but does not cover the fiber surface entirely. At intermediate times of deposition (Figure 5b), the fibers are fully coated with Cu and the coating has a specific microroughness. At longer deposition time (Figure 5c), the coating has become thicker and very smooth. Most likely the deposition process follows a 3D growth mode. Deposition is initiated by the nucleation of separated Cu clusters that then grow and form crystallites. Their shape and size strongly depend on the chemistry of the plating bath (Figure 4). When these crystallites reach a certain critical size, superconformal deposition (owing to the selected bath chemistry67,68) takes over, and the crystallites gradually merge. Thus, a smoothing of the coating surface is eventually observed (Figure 5c). A detailed discussion of the copper growth mechanisms in the presence of various additives is given elsewhere.67-69 As noted above, the first condition for obtaining truly superhydrophobic behavior is to deposit a continuous coating. The second condition is to develop a smaller-length-scale (65) Walker, M. L.; Richter, L. J.; Moffat, T. P. J. Electrochem. Soc. 2006, 153, C557. (66) Walker, M. L.; Richter, L. J.; Moffat, T. P. J. Electrochem. Soc. 2007, 154, D277. (67) Moffat, T. P.; Wheeler, D.; Edelstein, M. D.; Josell, D. IBM J. Res. Dev. 2005, 49, 19. (68) Moffat, T. P.; Wheeler, D.; Kim, S. K.; Josell, D. Electrochim. Acta 2007, 53, 145. (69) Vereecken, ; Binstead, R. A.; Deligianni, H.; Andricacos, P. C. IBM J. Res. Dev. 2005, 49, 3.

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roughness. This is particularly clear in Figure 3. Even if the microfibers are coated continuously with copper and therefore hydrophobized by the SAM (Figure 3a), the macroscopic surface of the Toray paper does not show superhydrophobic behavior (Figure 4). Only when a significant roughness due to the deposition of discrete crystallites is present on the surface (Figure 3b-d) does it then display truly hydrophobic behavior (hysteresis ≈ 0°, Figure 4b-d). If the copper layer is thick and smooth, as in Figure 5c, it is again inefficient at producing a superhydrophobic surface (Figure 6). Note that the thickness of the Cu layer does not impinge on the intrinsic wettability of the surface. (Wettability is very surface-sensitive4,70 and is determined by the alkyl chains of the self-assembled monolayer.) The Cu thickness, however, affects the morphology of the deposit. The latter develops as copper deposition proceeds; therefore, the thickness is an indirect but convenient indicator of the surface topography. As demonstrated above, the large advancing contact angle can be explained by the morphology of the hydrophobic surface. It is the receding contact angle that is extremely sensitive to the small-scale roughness. We speculate that the local shape of the contact line is strongly affected. In corners and around edges and defects, the curvature of the contact line and the liquid surface is higher and this may cause deeper penetration into the pores and holes of the substrate. This effectively improves the local adhesion at the solid-liquid junctions.20 The small-scale roughness prevents the contact line from bending, and it assumes a shape that is rather close to the equilibrium one. The pinning of the receding contact line is considerably reduced. In this case, even a small external force is likely to shift the contact line into the next local minimum of the free energy (i.e., the major reason for contact angle hysteresis is being removed). (70) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 5897.

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It is apparent from our results that roughness on the order of 10 μm (we do not distinguish between the fiber diameter and the fiber spacing, which are of a similar order of magnitude;Figure 1a) does not induce superhydrophobicity but only large advancing contact angles. Genuine superhydrophobic behavior is observed whenever a second length scale on the order of only a few micrometers is developed. It is now accepted that double-scale roughness is more effective at generating superhydrophobic behavior.20,33,39 Recent work attempts to design surface preparation so as to incorporate two (or more) distinct levels of roughness.23,26-32 Intriguingly, in spite of the vastly different chemistry and geometry of the systems studied, a length scale on the order of a couple of micrometers is always present.

Conclusions Teflon-treated Toray carbon paper is hydrophobic. It can be made superhydrophobic by metallization with copper and subsequent surface modification with dodecanethiol. The morphology of a copper deposit can be fine- tuned through additives (Cl-, PEG, and SPS) and time of deposition (effective thickness of the Cu layer). To obtain superhydrophobic behavior, one must achieve a continuous copper coating of the carbon fibers and develop a micrometer-range roughness (∼1 μm). Coating the microporous Toray paper with a rough copper deposit and modifying it with a self-assembled monolayer is a simple and inexpensive route to the fabrication of a superhydrophobic metal surface. Acknowledgment. S.B., Y.L., and N.D. acknowledge the support of this work by the National Science Foundation (DMR-0603019). J.G. and R.S. acknowledge financial support from the Australian Research Council through the International Linkage scheme (LX0562056).

Langmuir 2009, 25(8), 4760–4766