Facile Route Toward Mechanically Stable Superhydrophobic Copper

Jan 2, 2012 - 26 mm width × 1 mm thickness) with a soft tape. By means of ..... (17) Xiu, Y.; Liu, Y.; Hess, D. W.; Wong, C. P. Nanotechnology 2010,...
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Facile Route Toward Mechanically Stable Superhydrophobic Copper Using Oxidation−Reduction Induced Morphology Changes Seung-Mo Lee,*,† Kwang-Seop Kim,† Eckhard Pippel,‡ Sangmin Kim,† Jae-Hyun Kim,† and Hak-Joo Lee† †

Nano-Mechanical Systems Research Division, Korea Institute of Machinery & Materials (KIMM), 156 Gajungbukno, Yuseong-gu, Daejeon, 305-343, Korea ‡ Max Planck Institute of Microstructure Physics, Weinberg 2, D-06120 Halle, Germany S Supporting Information *

ABSTRACT: We present that the morphological change induced by an oxidation−reduction reaction can be effectively employed for easily producing superhydrophobic copper without any use of hydrophobic agents. By thermal oxidation of copper, needle-shaped copper oxide nanowires (NWs) were grown on the copper substrate. Subsequent reduction led to wavy copper NW structures, which exhibited superhydrophobic properties (contact angle of over 160° and sliding angle of less than 2°). We found that the water adhesion behavior on a surface could markedly vary with the shape of miniature structures of the surface, presumably due to the pinning effect. Microstructure, element, and surface analyses indicated that the outermost layer (∼Å) of the resulting copper NWs is mainly composed of Cu2O. Furthermore, the resulting superhydrophobic copper structures were observed to have good mechanical stability as well as chemical stability. This simple and scalable method could potentially be adopted in industry or academia where research on anticorrosion, antibiofouling, or drag reduction of copper-based objects is performed.

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INTRODUCTION Copper (Cu) is an omnipresent material around us. Copper was the first mineral that was mined and along with tin brought about the Bronze Age. As the ages and technology have progressed, copper usage has rapidly increased throughout the world laying down the foundations for the industry as we know it today.1 Since copper resists corrosion and conducts heat as well as electricity well, the examples of copper usages can be easily found everywhere, such as plumbing for water distribution through building, roofing, and electrical/electronic devices. Particularly, copper has for a long time been used as sheathing and antibiofouling surfaces to underwater hull parts of ships or netting materials in the aquaculture industry to protect against corrosion and permanent attaching of sessile marine creatures (e.g., mussels and barnacles), respectively.2 Provided that the bulk metallic copper with water-repelling (superhydrophobic) lotus effect3 can be easily produced and the property shows high sustainability as well as mechanical/ chemical stability against harsh environmental conditions, it can surely offer huge opportunities in the area of corrosionresistance enhancement,4−6 antibiofouling,7 and drag reduction8 for marine vehicles including other applications, such as roofing and plumbing. Inspired by lotus leaves and to expand the applications of copper, until now, a myriad of synthetic approaches have been tried to produce copper with superhydrophobic surfaces.9−14 However, most of the approaches have been based on the sequential processes of hydrophobic treatments using surface coupling agents on the rough surfaces, © 2012 American Chemical Society

and the practicality is largely impeded by the poor mechanical stability of those resulting surfaces.15−18 Herein, we demonstrate that changes in the surface morphology, caused by a simple oxidation−reduction process of a bulk metallic copper (Figure 1), can be straightforwardly utilized to produce a superhydrophobic copper, which is covered with copper nanowires (NWs) and exhibits good mechanical stability as well as chemical stability under acidic, basic, and ionic conditions.

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EXPERIMENTAL SECTION Growth of Copper Oxide NWs and Copper NWs. Copper foils (99.8% purity and 25 μm thickness, No.13382, Alfa Aesar) were cut into ∼20 mm × 30 mm pieces and cleaned by sonication in anhydrous ethanol for 60 s, prior to thermal oxidation. Then the foils were loaded into a quartz tube and heated in air at 430 °C for 4 h to obtain copper oxide NWs. Subsequently, the copper foils with copper oxide NWs were rapidly (CuO/R)/slowly (CuO/S) cooled to room temperature (RT), respectively, or a reduction was continuously conducted under H2 with a steady Ar (50 sccm) gas stream at 200 °C for 2 h. Finally, the copper foils with copper NWs were cooled rapidly (Cu/R)/slowly (Cu/S) to RT, respectively. The Received: November 14, 2011 Revised: January 1, 2012 Published: January 2, 2012 2781

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Figure 1. Schematic illustration of the preparation process of rough and superhydrophobic copper surfaces. In this experiment, we used oxidation and reduction temperatures of 430 and 200 °C, respectively (see details in the Experimental Section). To evade and minimize exfoliation caused by the difference of thermal expansion between a bulk copper and an oxidized copper oxide or a reduced copper layer, the cooling rates were controlled (see the text).

The backscattered Raman light is diffracted by an 1800 g/mm grating and detected by a charged coupled device camera. The spectral distance between adjacent channels is ∼0.5 cm−1. The spectra were recorded in a wavenumber range of 100−1100 cm−1 under the extended range configuration. For wide-angle X-ray diffraction (WAXD) analysis, the copper oxide NW (CuO/S) and copper NW (Cu/S) samples were carefully attached to a regular slide glass (76 mm length × 26 mm width × 1 mm thickness) with a soft tape. By means of a conventional laboratory wide-angle X-ray diffractometer (Ru200B with 40 kV and 80 mA, Rigaku) with Ni-filtered Cu Kα (λ = 1.5421 Å) radiation, diffraction profiles were measured in θ2θ scans. First, WAXD patterns of all samples were measured in an angular range of 10° < 2θ < 80° with a step size of 0.02°. The scan speed was 1 °/min. For each sample, to confirm the reproducibility of the patterns under the same conditions, the measurement was repeated at least three times. After measuring diffraction patterns of each sample, the background diffraction pattern was subtracted from each pattern, and graphical tasks were performed with ORIGIN 7.5. Mechanical Stability Test. Test Using Microtribometry. For the evaluation of mechanical stabilities of the prepared copper oxide NW (CuO/S) and copper NW (Cu/S) samples, a home-built microtribometry was used (Figure S1, Supporting Information). A polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning) lens was used as the counterpart material. The PDMS lens was fabricated by a molding method. After a mixture of degassed prepolymer and initiator (10:1) was prepared, it was poured onto a fused-silica mold with a radius of curvature of 10.3 mm and then cured in an oven at 75 °C overnight. The cured PDMS was readily detached from the lens without a release agent. The microtribometry was installed in a chamber where the ambient conditions of relative humidity and gas could be controlled. As described in Figure 7a and 7b, the sample was moved upward to contact the PDMS lens by controlling the z-axis stage. Then the sample was pressed against the lens until the load reached the predetermined maximum load (300 mN), and contact was maintained for a dwell time. Subsequently, the sample was detached from the lens at a constant unloading velocity by lowering the z-axis

detailed processing procedures and conditions can be found in Figure 1 and Table 1. Table 1. Sample Numbering and Processing Conditions sample Cu/N CuO/R CuO/S Cu/R Cu/S

path S0→S1→S2→ S3→S5 S0→S1→S2→ S4→S6 S0→S1→S2→ S4→S7→S8 S0→S1→S2→ S4→S7→S9

ΔtO [min]

ΔtC1 [min]

Δt R [min]

ΔtC2 [min]

240

30 (S2→ S5) 180 (S2→ S6) 60 (S2→ S4) 60 (S2→ S4)

120 (S4→ S7) 120 (S4→ S7)

60 (S7→ S8) 10 (S7→ S9)

240 240 240

Characterization. The static water contact angles (CAs) and hysteresis were measured with an optical contact angle meter (SEO, Phoenix-300) at RT. The volume of the used individual water or other solution drops was 5 μL. CAs were measured at a minimum of 10 different positions of the samples, and the values were averaged. The morphologies and microstructures of as-prepared copper oxide NWs and copper NW samples were examined by a scanning electron microscope (SEM) [JEOL (JSM-6340F) and FEI (Novanano200)]. For the energy-dispersive X-ray spectroscopy (EDX) examinations and high-resolution transmission electron microscope (HR-TEM) observations, the TEM samples were prepared with the following procedure: vials containing a piece of copper oxide NWs or copper NWs (∼5 mm × 5 mm) and ethanol were placed in an ultrasonic bath for at least 1 min. Immediately after sonication, one drop of each mixture was deposited onto lacey carbon TEM grids (Plano) and allowed to dry. Then, the investigation was carried out with a FEI TITAN 80-300 (300 kV) microscope. X-ray photoelectron spectroscopy (XPS) analysis was carried out with Thermo VG Scientific ESCA 2000. Micro-Raman Spectroscopy data were obtained using a Horiba Jobin Yvon LabRam HR800 UV spectrometer with a laser line at 633 nm (He−Ne laser). To avoid local heating effects, the laser power density was kept below 100 μW/μm2. 2782

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Figure 2. Surface morphology, element and wetting behavior analyses of copper each after oxidation, and reduction using representative samples, CuO/S (a and b) and Cu/S (c and d). (a,b) and (c,d) show SEM images at different magnifications of copper each after thermal oxidation (CuO/S) and reduction (Cu/S), respectively. The insets in (a,c) and (b,d) show EDX spectra of CuO/S, Cu/S, and water drops of 5 μL on each surface, respectively.

pure copper in air, and the NWs grow spontaneously in diverse sizes without a catalyst during the oxidation of copper at temperatures ranging from 400 to 600 °C.19,20 Also, the copper oxide can be readily reduced to copper by reducing agents like H2, CH4, C2H5OH, etc.21,22 These reports suggested a hypothesis that the copper oxide NWs grown by thermal oxidation, under chemical reduction, could be transformed to metallic copper NWs with notable changes of the surface morphology arising from the phase transformation, which presumably enhances surface roughness of copper, eventually leading to superhydrophobicity, as described in Figure 1. In the following, we will demonstrate that our hypothetical route is realizable in a straightforward way. First, the well-cleaned copper foil specimen was loaded into a quartz tube furnace and then heated in air at 430 °C, for 4 h. After heating, the surface of the copper foil was tarnished (it appeared black) when visually inspected (Figure S1a and S1b, Supporting Information). SEM investigations indicated that the entire surface of the copper foil is covered with well-aligned copper oxide NWs (Figure 2a and 2b). It is well established that the diameters of the NWs can easily be modulated by varying the heating conditions (like temperature and duration).19,20 To transform the copper oxide NWs into metallic copper NWs, the sample was heated in a reducing environment (i.e., H2 at 200 °C for 2 h). Complete transformation of oxide NWs into metallic copper NWs in a reducing environment could be confirmed by EDX analysis (see inset in Figure 2 and Figure S2, Supporting Information). The resulting copper foil was tinged with red upon visual inspection (Figure S1c and d, Supporting Information). The

stage. For each sample, the process of detaching from the PDMS lens was recorded at 500 fps using a high-speed camera (Fastcam 1024PCI, Photron). The movies are shown in the Supporting Information. After finishing each test, the surface of the used PDMS lens was observed with an optical microscope to examine any worn-off flake from the counter surface. Adhesion Property Test. On the basis of testing procedures described in method B of ASTM D 3359-09ε2, the adhesion test was performed. As an alternative to the ASTM-designated tape, ‘Permacel P99’ tape (currently not distributed in the market anymore), a ‘Scotch 810 Magic Tape (3M)’ was used for the test. The tape was pressed against the samples (CuO/S and Cu/S). After peeling off the tape from the samples, the surfaces of the tape and samples were examined using an optical microscope. Hardness Test. On the basis of testing procedures described in the ASTM D3363-05(2011)ε1 test method, the hardness test was performed. A sharp 3H grade pencil (STAEDTLER) was employed as a testing tool. After scratching the surface of the Cu/S and CuO/S samples using the 3H pencil, the resulting surfaces were examined using the optical microscope.

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RESULTS AND DISCUSSION Morphology Changes and Corresponding Wetting Behaviors. Copper has two oxidation states, i.e., +1 and +2. Consequently, copper is known to form two semiconducting oxides, namely, cuprous (Cu2O) and cupric oxide (CuO), having cubic and monoclinic crystal structures, respectively. It was reported that the copper oxide NWs with mixtures of CuO and Cu2O can be easily grown by simple thermal oxidation of 2783

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Figure 3. Microstructure analyses by HR-TEM. (a and b) show the HR-TEM images of a single straight copper oxide NW (CuO/S) and a single wavy copper NW (Cu/S) together with corresponding FFT images, respectively. The CuO/S was found to be a mixture of CuO and Cu2O. In contrast, Cu/S exhibited a typical microstructure of pure copper with elusive clues to the presence of other structures.

originally straight and smooth copper oxide NWs were changed into wavy copper structures with shorter and wider shapes (Figure 2c and 2d) due to volume shrinkage induced by the reaction of oxygen to water. Due to the significantly enhanced surface roughness, the static water contact angles (CAs, θ) were observed to be highly increased on both surfaces,22−25 i.e., θCuO/S = 114.2 ± 3.1° (after oxidation) and θCu/S = 162.8 ± 3.8° (after reduction), as compared to the contact angle of the native copper foil (θCu/N = 78.1 ± 2.2°, see Figure S3 (Supporting Information); sample numberings and detailed processing conditions can be found in Table 1). Notwithstanding the increases of the static CAs, the water drops’ sliding behavior on both surfaces was observed to be rather different. Namely, the water sliding angle on the Cu/S surface was less than 2°, while the water drops on the CuO/S never rolled off or detached, even when the surface was turned upside down (inset in Figure 2b). The observed differences are presumably related to surface roughness changes as well as the intrinsic wetting properties26 of Cu, CuO, and CuO2, as can be deduced from microstructure analyses (Figure 3) done by HRTEM. Microstructure and Surface Analysis. When the copper is thermally oxidized in air and the resulting oxidized copper is subsequently reduced in a H2 atmosphere, the entire reaction can be summarized as follows.19,20 Oxidation:

4Cu + O2 → 2Cu2O

(1)

2Cu2O + O2 → 4CuO

(2)

Reduction:

2CuO + H2 → Cu2O + H2O

(3)

Cu2O + H2 → 2Cu + H2O

(4)

The CuO forms in a two-step process, as described in reactions 1 and 2. Namely, the Cu2O is formed first as a major product, and subsequently CuO is formed with Cu2O serving as a precursor, i.e., Cu → Cu2O → CuO. It means that the grown copper oxide NWs can be a mixture of CuO and Cu2O. Indeed, the HR-TEM investigation using a single copper oxide NW indicated that the resulting copper oxides are composed of CuO and Cu2O (Figure 3a). The interplanar distances between atomic planes measured from HR-TEM and FFT (fast Fourier transform) images were 2.32 and 2.45 Å, which correspond well with the interplane distances of CuO(220) and Cu2O(111) planes, respectively.27 Meanwhile, in the case of reduction (reactions 3 and 4), the sequence is known to be opposite to the oxidation, i.e., CuO → Cu2O → Cu, although the direct reduction was reported to be possible depending on the reduction conditions, i.e., CuO + H2 → Cu + H2O.28 While Cu2O can be directly reduced to Cu in H2, CuO is reduced to Cu via Cu2O. Hence, the resulting copper after reduction can contain small amounts of Cu2O, similar to the case of an oxidation. However, the HR-TEM and FFT images of a single copper NW after reduction indicated that the copper oxides are fully reduced to copper with a trace amount of Cu2O and CuO (Figure 3b). Moreover, the estimated lattice spacing, 1.81 Å, corresponded well to that of Cu(200) planes. However, the WAXD pattern of the reduced copper on a large copper foil 2784

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Figure 5. Schematically illustrated wetting behavior of a water droplet. The pinning effect presumably occurs more dominantly in the water drop on the needle-shaped copper oxide NW surface (left, CuO/S) than on the wavy copper NW surface (right, Cu/S). The detailed discussion can be found in the text.

weak signal of oxygen by EDX also in the reduced copper structures (Figure 2c and Figure S2b, Supporting Information)similar to the WAXD results. A diffraction peak associated with Cu2O was observed at 2θ ≈ 36.5° (Figure 4a). The peak was very weak but well above the background level of the spectra. Considering the fact that copper reacts with oxygen when it is exposed to air, resulting in the formation of copper oxide known as patina, the observed Cu2O peak is likely related with the patina formed over time rather than the Cu2O residues by unfinished reduction. In particular, among patinas, the red patina is known to be bright or dark red and to consist of cuprous oxide (Cu2O) by reduction of the cupric oxide (CuO) formed initially.29 Indeed, the color of our sample was dark red when WAXD was measured (Figures S1c and S1d, Supporting Information).30 Taking into account that the average copper oxide film thickness formed on a copper surface by low-temperature oxidation is known to be around a few Ångtroms22 and X-ray irradiation has a large penetration depth, thereby possibly inducing strong signals of the far below surface, the comparatively weak peak intensity of Cu2O was reasonable. In contrast to WAXD results, Raman spectroscopy and XPS measurements with high surface sensitivity showed more distinct evidence of the presence of Cu2O on the surface (Figure 4b and 4c). As can be recognized from the evolution behavior of vibrational modes associated with CuO (293, 340, and 625 cm−1)31 and Cu2O (218 cm−1)32 in Figure 4b, it appeared that the native copper (Cu/N) contains a small amount of CuO and Cu2O, and a greater amount of both oxides are present in the reduced copper (Cu/S), probably in the outermost surface layer. From XPS data, the relative amounts of CuO and Cu2O contained in copper allowed rough estimation before and after the oxidation−reduction process. Particularly, the appreciable inversion of the intensity ratio between Cu1+ and Cu2+ was obviously observed. As can be recognized from Figure 4c, the ratio (Cu1+/Cu2+)Cu/S was much greater than (Cu1+/Cu2+)Cu/N. In other words, the observed peaks related with the Cu1+ 2p3/2 and 2p1/2 (at 932.1 and 952.4 eV in Cu/S spectrum) were much stronger than the peaks related with Cu2+ 2p3/2 and 2p1/2. This implies that the chemical valence of Cu is mainly +1 with a small amount of +2 after reduction. It is likely that the outermost surface layer of the wavy copper NWs produced by reduction is composed of mainly Cu2O rather than CuO. Origin of the Differing Wetting Behaviors. Up to now, we presented that the morphological changes of copper (straight needle-shaped copper oxide NWs → wavy copper NWs) induced by the oxidation−reduction process give rise to superhydrophobicity of the resulting copper. More importantly, by crystallographic and compositional analyses, we found that the outermost surface layer of the resulting wavy copper NWs

Figure 4. Compositional analyses by means of WAXD, Raman spectroscopy, and XPS. (a and b) WAXD spectra and Raman shift data, respectively. (c) XPS spectra in the Cu 2p core level region. On the basis of the literature,33 Cu1+/2+ 2p3/2 and 2p1/2 peaks were fitted into Gaussian functions.

exhibited crystal structures slightly differing from HR-TEM results (Figure 4a). Presumably, in HR-TEM, the lattice planes of the very thin and tiny Cu2O crystals or layers were not able to be seen, as they disappear in the stronger contrast of the much thicker Cu lattice structure. Moreover, we detected a 2785

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when the surface becomes rough; the new CA becomes larger than the original (θW > θ). However, a hydrophilic surface (θ < 90°) becomes more hydrophilic when roughened; its new CA lowers (θW < θ). Cassie and Baxter, on the other hand, stated that if the liquid is suspended on the top of a surface structure θ (CA on flat surface) changes to θCB, cos θCB = f cos θ + (1 − f) cos 180°, where f is the area fraction of the solid that touches the liquid. Liquid in the Cassie−Baxter state is more mobile than in the Wenzel state and moreover can show superhydrophobic behavior on both the intrinsically hydrophilic and the hydrophobic surface. In our case, the water drop on the needle-shaped copper oxide NW surface shows hydrophobic nature (θCuO/S ≈ 114.2°) with sticking (high adhesion force), while the drop on the wavy copper NW surface shows superhydrophobic nature (θCu/S ≈ 162.8°) with a very weak adhesion force (sliding angle of less than 2°). In view of the fact that the outermost surface layers of both surfaces are observed to be mainly composed of intrinsically hydrophobic Cu2O,34 the surface roughness is believed to be a leading factor of such different wetting behaviors, which is likely related to both the Cassie−Baxter and the Wenzel state. It is known that the static CA can be drastically changed depending on the geometry of the surface roughness (hemisperically topped cylinder, pyramid, general squre pillar, etc.), spacings of the individual features, and radius/weight/falling velocity of the used dropet, which are intimately related to the transition of wetting from the Cassie− Baxter regime to the Wenzel regime.35,36 In our case, considering the fact that the effects of the chemical makeup in both samples are almost equivalent, the differences of the static CA are believed to be clearly ascribed to the surface morphology effect. Namely, while a water drop on the Cu/S still remains in the Cassie−Baxter regime, the specific geometric parameters of the CuO/S surface presumably lead to the transition of the water wetting from the Cassie−Baxter to Wenzel regime, thereby inducing lower static CA (θCuO/S ≈ 114.2°) than the CA on Cu/S (θCu/S ≈ 162.8°). In addition, these geometric parameters likely give rise to the marked contrasts in water adhesion force on both surfaces, which are thought to result from the pinning effect37 by individual geometrical features rather than the petal effect38 by the scale dependence of micro- and nanohierarchical structures, as depicted in Figure 5. In other words, presumably the water droplets on wavy copper NWs are not able to wet the nanostructure spaces between the wavy NWs (Cassie−Baxter regime). This allows enclosure of air inside the spaces, causing a heterogeneous surface composed of both air and solid. Consequently, the adhesive force between the water and the solid surface is likely very low, allowing the water to roll off easily. In contrast, the needle structures on copper oxide NWs are thought to allow the water to impregnate the spaces between those needles, thereby leading to pinning and resulting in a high adhesive force between the water and needles (Wenzel regime). With the variation of the processing conditions, the morphology control of the copper oxide NWs could overcome the surmisable pinning effect, consequently leading to superhydrophobicity with weak adhesion forces. As a further interesting phenomenon, the transitional behavior (superhydrophilicity to superhydrophobicity transition with the time elapsed) of both copper oxide NW34 and copper NW surfaces was observed. Namely, both surfaces in the as-prepared state initially showed superhydrophilicity. However, copper oxide and copper NW surfaces turned hydrophobic and superhydrophobic as time elapsed (after a few weeks) under

Figure 6. Wetting behaviors of the copper NW sample (Cu/S) surface under diverse conditions. (a) Time dependence of the static water contact angle on the Cu/S surface. (b) Comparison of contact angles of the fresh water and seawater on the on Cu/S surface. (c) The relationship between pH values and contact angles on the Cu/S surface.

is likely to be dominantly composed of Cu2O. In general, the hydrophilicity or hydrophobicity of a surface arises from its chemical makeup. This property can be enhanced by the control of the surface roughness.23−25 The effect of the surface roughness on the wetting behavior has been predicted with the Wenzel model23,24 and the Cassie−Baxter model.25 Wenzel determined that when the liquid is in intimate contact with surface structures θ will change to θW, cos θW = r cos θ, where r is defined as the ratio of the actual area to the projected area (roughness factor). Wenzel’s equation shows that surface structures amplify the natural tendency of the surface. Namely, a hydrophobic surface (θ > 90°) becomes more hydrophobic 2786

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Figure 7. Mechanical stability test using a home-built microtribometer. (a and b) A testing procedure and representative force versus time profile during the test, respectively. (c−e) An optical micrograph of the PDMS lens surface before testing (c) and after testing of the CuO/S sample (d) and Cu/S sample (e), respectively. While many flakes were observed on the lens after testing of the CuO/S sample, the Cu/S sample was as clear as the PDMS lens surface before testing.

atmospheric conditions, respectively (Figure S4, Supporting Information). Chang et al.34 stated that the involved chemistry is related to the partial deoxidation of the outermost surface layer (i.e., CuO → Cu2O, in the case of copper oxide NW surfaces), and the deoxidation can be greatly accelerated by simple annealing. Given that the surface layers of both copper oxide NW and copper NW surfaces were observed to be composed of Cu2O and those surfaces showed wetting behavior similar to the results by Chang et al., the partial deoxidationrelated mechanism is believed to be a reasonable scenario of this phenomenon. Wetting Behaviors of the Cu/S under Diverse Conditions. To confirm the feasibility that the as-prepared copper NW sample (Cu/S) with the saturated superhydrophobicity could be utilized in the industrial fields, some additional characterizations were carried out in terms of chemical stability in diverse conditions. First, a water droplet (5 μL) was gently put on the surface of the Cu/S with the saturated superhydrophobicity (Figure S4, Supporting Infor-

mation) and then the variation of the static water CA was examined with time. As can be seen in Figure 6a, the time dependence of water CAs for Cu/S appeared to be minor. CA variation seemed to be in a measurement error range, and CAs were measured to be almost unchanged for a few hours (CAs were observed to still be almost constant even until complete evaporation of the water droplet in air occurs). Second, the static CAs of fresh water and seawater on the Cu/S surface were measured (Figure 6b). Interestingly, the surface of the Cu/S was observed to be repellent to fresh water as well as real seawater. Similar repelling behaviors were also observed when the boiled fresh or seawater droplets with the temperature of around 75 °C were used. As a next characterization, the liquid droplets with different pH values (pH = 2.18−13.41) were placed on the Cu/S surface, and the changes of CAs were precisely examined. As can be recognized from Figure 6c depicting the relationship between the pH values of the used droplets and the corresponding static CAs, the CAs, surprisingly, showed marginal variations regardless of whether 2787

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Figure 9. Hardness measurement results evaluated by the pencil scratch test performed by referring to the ASTM D3363-05(2011)ε1 standard test method. After scratching the (a) the CuO/S and (b) the Cu/S surfaces using a 3H grade pencil (STAEDTLER), the resulting surfaces were examined using the optical microscope.

Figure 8. Adhesion property of the copper oxide NW sample (CuO/ S) and the copper NW sample (Cu/S) evaluated by the tape test performed by referring to the ASTM D 3359-09ε2 standard test method. (a−c) Optical microscope images of the tape surfaces (Scotch 810 Magic Tape, 3M): (a) bare tape, (b) after test of the CuO/S, and (c) after test of the Cu/S, respectively.

Figure 10. Constructed water strider model (0.1 g of weight and 3 cm of body length). Oxidation and reduction process has been applied to commercially available copper wire. After the process, the initially smooth surface of the copper wire turned into a rough surface covered with wavy copper NWs (right SEM image).

the used droplets are acidic or basic. These intriguing results imply that the Cu/S surface could retain its superhydrophobicity to the liquids with high salinity as well as the liquids with corrosive effect. Mechanical Stability. Although the mechanical stability must be considered and is of primary importance for the practical application of a superhydrophobic surface, this aspect has received relatively low attention.18 As already described in Figure S1 (Supporting Information), the foreseeable problem was exfoliation which can occur at the interface between the copper substrate and the oxidized/reduced surface layer. By controlling the cooling rates, the natural exfoliation problem could be evaded. Since the mechanical durability of the micro/ nanosized surface structures and their superhydrophobicity is a

critical factor to ensure practical usage, qualitative examination of the mechanical stability was performed first by means of home-built microtribometry (Figure S5, Supporting Information).39 As illustrated in Figure 7, a soft PDMS (polydimethylsiloxane) lens was employed as a counterpart material to ensure conformal contact of the tested surfaces with copper oxide NWs (CuO/S) or copper NWs (Cu/O). Those surfaces were pressed against the lens until the load reached the predetermined maximum value (300 mN). The contact was maintained for a dwell time (60 s). Subsequently, the sample was detached from the lens at a constant unloading velocity, and then the surface of the PDMS lens was examined under an optical microscope to examine any worn-off flake from the 2788

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counter surfaces. As can be seen in Figure 7c, 7d, and 7e, while the CuO/S were found to be mechanically rather weak (Figure 7d), the Cu/S appeared to have superior mechanical stability (Figure 7e) under mechanical adhesion and showed unchanged superhydrophobic nature. For the reaffirmation of the mechanical stability, an adhesion test and hardness test were further carried out based on testing procedures described in ASTM D3363-05(2011)ε1 and ASTM D 3359-09ε2 standards, respectively. First, from the adhesion test using Scotch tape, it was observed that the Cu/S was less damaged by the tape (only a small portion of the surface layer on the Cu/S was removed, as can be seen in Figure 8), and most of the surface structures were well preserved, thereby still retaining superhydrophobicity, after performing the adhesion test. In contrast, in the case of the CuO/S, as can be seen in Figure 8b, almost the whole surface layer was removed by the Scotch tape. Second, the hardness test results indicated that the CuO/S is rather brittle, thereby easily being broken by the pencil pressure (Figure 9a), and the Cu/S is likely deformed retaining the surface structures, rather than broken by the pencil pressure (Figure 9b). Thus, the results of the mechanical and chemical stability clearly support that the as-prepared superhydrophobic copper NW sample (Cu/S) is stable enough to be possibly used in the environmental conditions. This simple process producing superhydrophobic copper surfaces with good mechanical/chemical stability has inspired us to apply the process to other copper-based substrates with more complicated geometry than the rather simple copper foil. For instance, commercially available copper wires (∼500 μm thick) have been applied to this process, and the treated copper wires have prompted us to construct a water strider model (Figure 10).40 It is known that to float on water the water striders utilize the high surface tension of water and long superhydrophobic legs, which have unique hierarchical micro/ nanostructures composed of the fine nanoscale grooves on a microsetae. Our model was 0.1 g of weight (a real water strider weighs ∼0.01 g), and the resulting copper wire fully covered with hairy copper NWs showed good durability.

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ASSOCIATED CONTENT

S Supporting Information *

Additional data associated with this article are included. Two high speed movies are included, showing detaching processes of copper oxide NW or copper NW samples from a PDMS lens during the mechanical stability test. 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] or [email protected]. Telephone: +82-42-868-7659. Fax: +82-42-868-7884.

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ACKNOWLEDGMENTS We thank Dr. Mato Knez in Max Planck Institute of Microstructure Physics, Germany, and Dr. Woo Lee and Dr. Chang Soo Kim in Korea Research Institute of Standards and Science, Korea, for helpful discussions and arrangement of WAXD measurement. This research has received financial support from the Korean Ministry of Education, Science, and Technology (grant 2009K000179) and Korean Ministry of Knowledge Economy (grant 10033309).

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REFERENCES

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CONCLUSIONS

In summary, we demonstrated that, without the use of any hydrophobizing agent, copper with superhydrophobicity and good mechanical/chemical stability can be readily produced by simply oxidizing and subsequently reducing. We found that the water adhesion behavior on a surface could vary with the shape of the miniature structures (needle-shaped NWs or wavy NWs) presumably due to the pinning effect, although the outermost surface is composed of the same ingredient. Industrially, if our method is applied to diverse copper-based conduits, the liquid will flow with minimal contact with the tube interior wall, thereby reducing drag forces and energy costs. In most industrialized countries, the costs of metal corrosion have been reported to amount to at least 2−3% of the gross national product per annum.41 Copper is one of the most widely used metals in daily life. We demonstrated that, based on our approach, plain coppers can be easily altered into superhydrophobic coppers stable with corrosive liquids. Therefore, our approach appears considerable as a mean for a corrosionpreventive strategy of copper, although the practical applicability in the industry remains to be seen. 2789

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