Facile Fabrication of a Superhydrophobic Cu Surface via a Selective

Jul 30, 2012 - The Cu surface with a dual-scale roughness has been prepared via a facile solution-phase etching route by the H2O2/HCl etchants...
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Facile Fabrication of a Superhydrophobic Cu Surface via a Selective Etching of High-Energy Facets Lijun Liu,* Feiyan Xu, and Lin Ma College of Chemistry and Chemical Engineering, Wuhan Textile University, Wuhan 430073, People’s Republic of China S Supporting Information *

ABSTRACT: The Cu surface with a dual-scale roughness has been prepared via a facile solution-phase etching route by the H2O2/HCl etchants. The selective etching of the high-energy {110} facets occurs at an ultralow rate of the redox etching reaction. The resultant surface is composed of many polyhedral microprotrusions and nanomastoids on the microprotrusions, exhibiting the binary micro/nanostructures. After hydrophobization, the resultant surface exhibits a water contact angle of 170° and a sliding angle of ∼2.8° for a 5 μL droplet. The combination of the dual-scale roughness and the low surface energy of the adsorbed stearic acid accounts for the superhydrophobicity. Such a superhydrophobic Cu surface has an excellent nonsticking behavior and anticorrosion against electrolyte solution. It also keeps its superhydrophobic ability after a long-time ultrasonication or abrasion test. Our work may shed light on the selective etching of other metal surfaces to create designed dual-scale roughness for superhydrophobicity.

1. INTRODUCTION The superhydrophobic surfaces have become a hot topic in recent material research fields.1 Inspired by natural models, for example, the lotus leaf, many artificial superhydrophobic surfaces have been prepared by increasing the surface roughness and decreasing the surface free energy.2−4 As an essential engineering material, Cu with a superhydrophobic surface has attracted great interest because of its potential and practical applications in oil−water separation,5 water pressure resistances,6,7 microfluidic devices,8 anticorrosion,9,10 and so forth. Therefore, considerable effort has been devoted to preparing a superhydrophobic Cu surface with various methods. Generally, the “coating” and “etching” strategies are usually adopted to introduce the dual-scale roughness on the Cu surface.11 By means of “coating”, a hierarchically rough film covers or grows out of the Cu substrate. This strategy mainly includes surface coating,12,13 electrodeposition,14−16 electroless galvanic deposition,17 heterogeneous nucleation, growth of another phase on a Cu substrate,18−27 and so on. Nevertheless, the as-prepared superhydrophobic film usually has no robust adhesion to the substrate and can peel off when subjected to heat or external mechanical forces, for example, ultrasonication, since there is an obvious phase interface between the rough film and the substrate. On the contrary, the “etching” method can avoid the above disadvantages. By means of “etching”, the hierarchical structures on the surface have the same phase as the substrate does. Thus, such superhydrophobic surfaces possess superior mechanical stability. Moreover, the etching process has advantages of inexpensiveness, simplicity, and easy scaling-up. However, the anisotropic etching of Cu foil to attain superhydrophobicity is seldom reported. It is known the Cu © 2012 American Chemical Society

foil is polycrystalline, and the grains are distributed randomly. The etching behavior preferentially occurs on the randomly distributed facets or grain boundaries.28,29 It usually leads to an irregular surface texture, which is hard to meet the structural demand for a superhydrophobic surface. Recently, the selective etching of the specific crystalline facets has been employed to synthesize nanomaterials with controlled morphologies.30−32 It is believed that the etching rate of highenergy facets is higher than that of low-energy ones.33 Thus, the final shapes of the nanomaterials can be kinetically controlled. Herein, we select a specific Cu foil as the substrate that has high-energy {110} facets exposed on the surface. By means of the selective etching of {110} facets, the superhydrophobic Cu surface is prepared by carefully modifying the redox kinetics in H2O2/HCl solution. Such a superhydrophobic surface exhibits a robust nonsticky behavior and corrosion resistance against electrolyte solution. This work may provide a facile strategy to construct controllable micro/nanostructures for superhydrophobic metal surfaces by selective etching of the specific highenergy facets.

2. EXPERIMENTAL SECTION The Cu foil (99.5 wt %) was purchased from Sinopharm Chemical Reagent Co., Ltd. in China. All the reagents were of analytical grade and used without further purification. In a typical process, a clean Cu foil (2 × 3 cm2) was immersed in the mixed solution (20 mL) composed of 0.5 wt % H2O2 and 2 M HCl at 60 °C for 1 h. After being rinsed with deionized water and ethanol, the resultant Cu foil was dipped in an Received: March 23, 2012 Revised: July 27, 2012 Published: July 30, 2012 18722

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The lattice constant, a, calculated from the XRD patterns is 3.610 Å, which is consistent with the standard data (a = 3.615 Å). The results confirm that the Cu substrate before and after being etched is of pure Cu and that no oxides exist on the Cu substrate. Interestingly, the peak intensity (I) of (220) facets of the bare Cu foil is overwhelmingly stronger than the reported value according to JCPDS file no. 04-0836, which means that the copper foil used here has a dominant {110} crystallographic texture and its basal surface is bounded mainly by the oriented (220) facets. The intensity ratios of I111/I220 and I200/I220 of the bare Cu foil are 0 and 0.18, while those of the etched one increase up to 0.09 and 0.28, respectively. This strongly indicates that a part of (111) and (200) facets expose gradually on the Cu surface during the etching process, implying that a selective and anisotropic etching of (220) facets occurs. Figure 1b is the EDX spectrum of the etched Cu foil. It reveals that only the Cu element exists in the final product, consistent with the XRD analysis. Figure 2 shows the SEM images of the Cu surface with different magnifications. The panoramic morphology shows

ethanol solution of 5 mM stearic acid (STA) for 0.5 h and dried at ambient temperature. The morphology of the Cu foil was observed using an Hitachi S4800 field emission scanning electron microscope (FESEM). The element composition was characterized by a Horiba EX-250 X-ray energy-dispersive (EDX) spectrometer associated with FESEM. The X-ray diffraction (XRD) patterns of the samples were recorded with a Rigaku D/Max-2000 diffractometer equipped with a Cu Kα radiation source (λ = 0.15418 nm). The contact angles (CAs) were measured on a Krüss contact angle instrument (Easydrop DSA 20) at ambient temperature using the Laplace−Young equation fitting method for the static CAs. The CAs reported here were the mean values measured with a ca. 5 μL water droplets at five different positions on each sample. The polarization curve measurements were performed in 3.5 wt % NaCl solution by a CHI660C electrochemical workstation (Shanghai CH Instruments) at room temperature. The superhydrophobic Cu foil and a platinum plate were used as the working and counter electrodes, respectively. A saturated calomel electrode (SCE) was used as the reference electrode. The Cu samples were immersed in the NaCl solution for 30 min, allowing the system to be stabilized, and polarization curves were subsequently measured with respect to the opencircuit potential (OCP) at a scanning rate of 1 mV·s−1.

3. RESULTS AND DISCUSSION Figure 1a shows the XRD patterns of the bare and etched Cu foils. The sharp and intensive peak located at 2θ = 74.13° is assigned to the (220) diffraction lattice plane of face-centered cubic (fcc) Cu. The weak ones located at 2θ = 50.43° and 43.28° are attributed to (200) and (111) facets, respectively.

Figure 2. SEM images of the resultant Cu surface prepared via the typical synthetic procedure.

that the surface is composed of many convex polyhedral protrusions with diameters ranging from 0.5 to 2 μm (Figure 2a). Many cavities with sizes of hundreds of nanometers exist among the microprotrusions. Further observation indicates that such polyhedral microprotrusions have several polyhedral mastoids at the nanoscale on their tips (Figure 2b). The results mean that the Cu surface possesses a complex morphology with two levels of hierarchy, the first level having the dimension of micrometers and the other having the dimension of nanometers. Such a dual-scale hierarchical structure favors the superhydrophobicity of the Cu surface. The wettability of the resultant surfaces is characterized in detail with static water CA measurements. The as-received Cu surface is hydrophilic with a water CA of 62° (Figure 3a). After

Figure 1. (a) XRD patterns of the bare and etched Cu foil prepared via the typical etching process. (b) EDX spectrum of the resultant etched Cu foil. 18723

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Cu surfaces show very different water CAs (insets of Figure 4). The CAs of the Cu surfaces obtained at various t of 10, 25, and 40 min are 143, 158, and 165°, respectively. Clearly, the CA increases with the enhanced roughness at the prolonged reaction time, highly indicating that the dual-scale roughness on the Cu surface contributes to the superhydrophobicity. The results are well consistent with the theoretical analysis suggested by Marmur that the binary micro/nanostructures can efficiently reduce the triple contact line and the contact area between the water droplet and the rough surface.34 We believe that the etching process is initiated by the redox reaction between metallic Cu and H2O2 in the acid solution. The redox reaction has a strong thermodynamic tendency (E⊖ = 1.434 V), because the H2O2/H2O pairs and Cu2+/Cu pairs have remarkably different electrode potentials (eqs 1−3). As the etching process goes on, the etchant solution gradually turns blue, that is, the color of the aqueous solution containing Cu2+ ions.

Figure 3. Photographs of the 5 μL water droplet on (a) the bare Cu foil, (b) the etched Cu surface, and (c) the etched Cu surface after STA modification.

being treated for 1 h, the Cu foil shows the superhydrophilic property. The water CA on this highly wettable surface decreases to ca. 2° with 1.60 s (Figure 3b). After the modification with STA, on the contrary, the resultant surface is so superhydrophobic that a 5 μL water droplet sitting on the surface exhibits an almost perfect sphere with a water CA of 170° (Figure 3c). The robustly superhydrophobic behavior is mainly attributed to the dual-scale roughness with micro/ nanostructures on the etched Cu surface, which can be further evidenced by the following time-dependent experiments. It is observed that the water CAs of the resultant Cu surface increase with increasing reaction time (t). For the smooth Cu surface modified with STA (Figure 4a), the CA is only 105°,

Cu → Cu 2 + + 2e

E ⊖(Cu 2 +/Cu) = 0.342 V

(1)

H 2O2 + 2e + 2H+ → 2H 2O E ⊖(H 2O2 /H 2O) = 1.776 V Cu + H 2O2 + 2H+ → Cu 2 + + 2H 2O

(2)

E ⊖ = 1.434 V (3)

According to eq 2, the oxidant ability of H2O2 can be improved with an increased concentration of HCl ([HCl]), as the elevated [HCl] can increase the electrode potential of the H2O2/H2O pairs. As a result, the morphologies of the etched Cu foil can be easily tailored by means of varying the [HCl] (Figure 5). When [HCl] is less than 0.5 M, no obvious etching process occurs within 1 h of reaction time, since the color of the

Figure 4. SEM images of Cu surfaces at various reaction times of (a) 0, (b) 10, (c) 25, and (d) 40 min. The insets are the representative photographs of the 5 μL water droplet on each surface after STA modification.

suggesting that the surface roughness is necessary for superhydrophobicity. At t = 10 min, the obtained Cu surface is only composed of nanoscale polyhedral protrusions without obvious binary micro/nanostructures (Figure 4b). At t = 25 min, some protrusions grow larger and have sizes of ca. 1 μm. Such microprotrusions keep their polyhedral shape and have several nanosteps on the tips (Figure 4c), indicating the appearance of the dual-scale surface textures. At t = 40 min, more microprotrusions with hierarchical micro/nanostructures appear (Figure 4d). When t is increased to 1 h, the Cu surface is composed of well-shaped polyhedral micro- and nanoprotrusions and possesses the dual-scale roughness shown in Figure 2. After being modified with STA, such time-dependent

Figure 5. SEM images of the resultant Cu foil treated at various [HCl] of (a) 1 and (b) 4 M while other conditions are kept constant. 18724

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solution stays unchanged and the resultant Cu foil keeps its thickness in comparison with the untreated one. At [HCl] = 1 M, an obvious etching process occurs as the etchant solution turns blue. The convex polyhedral protrusions and cavities with sizes ranging from 0.5 to 2 μm appear on the Cu substrate (Figure 5a). When [HCl] is increased to 2 M, the resultant surface possesses a dual-scale roughness (Figure 2). When [HCl] is increased up to 4 M, the etching reaction occurs more acutely and the etching rate is much higher. The obtained Cu foil has a significant decrease in thickness, and there are etched holes with depths of several micrometers in the surface (Figure 5b). The whole surface has no regular surface texture. This is because the 4 M [HCl] severely strengthens the oxidizing ability of the H2O2. From the above analysis, we believe that a lower etching rate favors the formation of the well-shaped binary micro/nanostructures on the Cu foil. In general, the surface energy and etching kinetics are the two key factors determining the final morphology of the Cu foil.29 The atoms on the high-energy facets are thermodynamically instable. They can be easily attacked by the H2O2 molecules and dissolve during the etching process in order to minimize the total surface energy. At the ultralow etching rate, an anisotropic etching process occurs. In this case, various facets have remarkably different etching rates, because the etchants have enough time to search and attack the Cu atoms on the high-energy facets. It is proved that the sequence of the crystalline surface energy for the fcc structure is γ{111} < γ{100} < γ{110}.35 The Cu foil used here is bounded mainly by high-energy {110} facets, as evidenced by the XRD analysis. Thus, the atoms on the {110} facets prefer to dissolve, and the etching rate of {110} facets is much higher than that of {111} or {100}. Namely, the etching direction occurs preferentially along the ⟨110⟩ direction, forming polyhedral micro/nanoprotrusions (Figures 2 and 5a) mainly bounded with {111} facets to acquire a lower surface free energy.36 On the contrary, if the etching rate is increased, the quasi-equilibrium etching condition will be broken and the etching rates of various facets would become comparable; that is, all the facets have similar etching rates. No selective or anisotropic etching behavior occurs. In this case, the as-etched Cu foil would show an irregular and uncontrollable surface structure (Figure 5b). According to the above analyses, the selective etching mechanism on the Cu foil is illustrated as Figure 6.

Figure 7. Sequential photographs of a 4 μL water droplet (a) suspended on a syringe, (b) slightly and (c) severely contacted with the lifting surface, and (d) departing from the lowering surface. The arrows represent the substrate’s moving direction.

apparent adhesion to the water droplet. Moreover, the water droplet remains an almost perfect sphere no matter whether it contacts with the substrate slightly or severely. Even if the needle is inserted into the water droplet deeply (Figure 7c), the droplet can also keep its spherical shape and depart from the substrate easily without any of the water remaining (Figure 7d). The results show that the adhesive force between the droplet and the superhydrophobic Cu surface is extremely feeble and may be ignored. Figure 8 shows the sliding behavior of a 5 μL water droplet on the resultant surface. The water droplet cannot stably sit on

Figure 8. Snapshot photographs of a 5 μL water droplet (a) suspended on a syringe and (b) rolling off the superhydrophobic surface as the substrate is tilted by ∼2.8°.

the superhydrophobic Cu surface tilted by ∼2.8°. It rolls off immediately without an obvious hysteresis. Generally, the hysteresis of a superhydrophobic surface is determined by the metastable state energy and the barrier energy for the droplet to move from one metastable state to another one.38−40 The resultant “roll-off” superhydrophobic Cu surface has the dualscale roughness. It possesses a high metastable state energy and low barrier energy, since such surface textures increase the triple contact line and decrease the wetted surface fraction.41 Accordingly, the water droplet sitting on the surface becomes instable and rolls off in search of a low-energy metastable state, causing roll-off with a low sliding angle of ∼2.8°. The superhydrophobicity can be explained by the Cassie− Baxter model.42 The superhydrophobic surface can be considered as a kind of porous medium composed of air

Figure 6. Schematic illustration for the formation of polyhedral microprotrusions via the selective etching of high-energy {110} facets.

The nonsticking behavior of the superhydrophobic Cu surface is characterized according to the method proposed first by McCarthy.37 Figure 7 shows the approach, contact, and departure processes of a 4 μL water droplet with respect to the Cu superhydrophobic surface. The droplet is first suspended on the needle due to gravity (Figure 7a), and then contacted with the lifting Cu substrate (Figure 7b). We find that the suspending droplet cannot be pulled down to the Cu surface in all cases, which means that the surface has almost no 18725

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pockets.43 As for the composite surface, water droplets sit mainly on such air pockets and roll off easily when the surface is tilted slightly. The apparent CA (θA) is given by cos θA = f1 cos θ − f2

After the abrasion test, some tips of the convex polyhedral protrusions are worn off and the water CAs have a mean value of 153° (Figure S3, Supporting Information). The decrease of the CA can be ascribed to the partial loss of the surface roughness and the adsorbed STA molecules. The chemical stability of the superhydrophobic surface is also characterized by immersing the sample in aqueous solution with various pH values. After the immersion for 72 h, the resultant superhydrophobic surface shows a CA of more than 158° for all pH solutions (Figure S4, Supporting Information).

(4)

where θ (105°) is the CA of the smooth Cu surface modified with STA; f1 and f 2 are the fractions of solid and air surfaces in contact with water (i.e., f1 + f 2 = 1), respectively. This equation indicates that increasing the fraction of air (f 2) can increase the CA of the rough surface (θA). According to eq 4, the f 2 value of the superhydrophobic Cu surface is estimated to be 0.980, which suggests that the air occupies about 98.0% of the contact area between the water droplet and the Cu surface. This further proves that the dual-scale roughness on the Cu foil accounts for its superhydrophobic behaviors. The ability of the superhydrophobic surface to protect the Cu foil from corrosion in 3.5 wt % NaCl solution is investigated by potentiodynamic curve measurements. In a typical polarization curve, a lower corrosion current density (icorr) or a higher corrosion potential (Ecorr) corresponds to a lower corrosion rate and a better corrosion resistance.44−46 The difference in the polarization curves between the bare Cu foil and the superhydrophobic Cu foil are shown in Figure 9. The

4. CONCLUSIONS In summary, the superhydrophobic Cu surface has been fabricated by means of selective etching of high-energy {110} facets at an ultralow etching rate. Such an anisotropic etching behavior results in the formation of the dual-scale convex polyhedral protrusions with regular shapes on the Cu surface. The resultant surface demonstrates superhydrophobicity with a water CA of 170° and a sliding angle of ∼2.8° after STA modification. Such a superhydrophobic surface exhibits a robust nonsticking behavior, corrosion resistance against electrolyte solutions, good mechanical stability, and chemical stability. These properties are favorable for the potential applications of Cu materials for various industrial fields, such as the coated Cu foil in air-conditioning facilities or other rigorous circumstances.

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ASSOCIATED CONTENT

S Supporting Information *

The photographs of the 5 μL water droplets on the resultant superhydrophobic surface after ultrasonic treatment and abrasion test, schematic illustration of the abrasion test, SEM image of the superhydrophobic Cu surface after abrasion test, and relationship between the pH values and the CAs of the resultant superhydrophobic surface. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 9. Potentiodynamic curves of the bare and superhydrophobic Cu foil in 3.5 wt % NaCl solution.

AUTHOR INFORMATION

Corresponding Author

*Phone: +86-27-59367343. Fax: +86-27-59367343. E-mail: [email protected].

Ecorr positively increases from −0.311 V for the bare Cu foil to −0.232 V for the superhydrophobic one. The positive shift of the Ecorr could be linked to an improvement in the protective properties of the superhydrophobic surface on the Cu foil. The icorr was obtained from the intersection of extrapolated anodic and cathodic Tafel lines in the polarization curves at Ecorr. The icorr of the superhydrophobic Cu foil (2.972 × 10−6 A·cm−2) decreases by 2 orders of magnitude as compared with that of the untreated one (1.081 × 10−4 A·cm−2). It is believed that the air trapped in micro- and nanoscale surface cavities behaves as a dielectric for a pure parallel plate capacitor.47 Such an air dielectric can inhibit the electron transfer between the electrolyte and the Cu substrate and dramatically improve the corrosion resistance of the Cu substrate. The mechanical strength of the superhydrophobic Cu surface is evaluated by an ultrasonic treatment in water. After the ultrasonication for 1 h, the resultant surface exhibits a mean CA of 165° (Figure S1, Supporting Information). We have conducted an abrasion test to evaluate the mechanical robustness of the superhydrophobic surface, as described in Figure S2 (Supporting Information).48,49 Briefly, the Cu foil is subjected to a normal pressure (∼5 kPa) and slides on a common cotton fabric (40s × 40s) by 25 cm in one direction.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support of this work by the Instructional Project of China National Textile and Apparel Council (No. 2011054).

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REFERENCES

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