Durability evaluation of superhydrophobic copper foams for long-term oil-water separation

Applied Surface Science 407 (2017) 145–155

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Durability evaluation of superhydrophobic copper foams for long-term oil-water separation Haiyan Zhu a , Lin Gao a , Xinquan Yu a,∗ , Caihua Liang b , Youfa Zhang a,∗ a b

Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing, 211189, PR China School of Energy and Environment, Southeast University, Nanjing 210096, PR China

a r t i c l e

i n f o

Article history: Received 20 December 2016 Received in revised form 11 February 2017 Accepted 20 February 2017 Available online 21 February 2017 Keywords: Superhydrophobicity Copper foam Mechanical durability Recycling stability Oil-water separation

a b s t r a c t Superhydrophobic three-dimensional porous composites with good mechanical stability and high efficiency are promising candidates for oil-water separation application. Hence, several superhydrophobic copper foams were fabricated via the in situ growth of patterned Cu(OH)2 nanoneedles or ZnO nanocrystals (e.g. ZnO nanocones and ZnO nanorods) on the skeleton and followed by chemically modification. All the superhydrophobic copper foams showed efficient oil-water separation ability, especially the samples with ZnO nanorods arrays on the pre-nanostructured skeleton. The durability of superhydrophobic copper foams were then evaluated. Although the superhydrophobic samples kept separation efficiency higher than 95% after cycled evaluation, the pre-roughened copper foams exhibited the best performance against various damage among the samples. Microstructural evolution revealed that the coverage of the copper skeleton became from smooth swelling micro-crystals into rough nano-crystals after the pretreatment of electrodepositing copper nanoparticles. The rough nanocrystals could not only avoid the formation of loose hierarchical structure, but also improve the binding force between patterned nanorods and the matrix. The fabricated closely-patterned ZnO nanorods could thus remain stable under the damage compared to others, presenting great mechanical robustness. Furthermore, we achieved a long-term efficient oil-water separation using the durable foams by periodic removal of residual oil in nanostructure gaps. © 2017 Elsevier B.V. All rights reserved.

1. Introduction In recent years, with the development of economy and society, the output of oily wastewater was increasing sharply. The separation of oil-water mixtures was becoming imperative due to its serious threat to environment and human society [1–5].Traditional techniques were employed to remove oil from water, such as in situ burning [6], gravity separation [7], air flotation [8], coalescence and flocculation [9], bioremediation [10], adsorption [11]. But these methods were often suffered from limitations including high costs, complex separation instruments, low separation efficiency, and secondary pollutants, etc. More recently, take inspiration from nature, such as lotus leaves [12] and desert beetle [13], the superwetting materials have been fabricated and used for oil-water separation successfully, which achieved by constructing rough surfaces and modifying with low-surface energy materials [14]. With their high separation efficiency, great selective separation per-

∗ Corresponding authors. E-mail addresses: [email protected], [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.apsusc.2017.02.184 0169-4332/© 2017 Elsevier B.V. All rights reserved.

formance, excellent recycling property and environment friendly, superwetting materials have become a research hotspot in the oilwater separation field. In general, there were three types [15] of separation materials which classified as: “oil-removing” type materials [16–18] with superhydrophobic/superoleophilic properties [19] which could filtrate or adsorb oil selectively, “water-removing” type materials [20] with superhydrophilic/superoleophobic properties which could filtrate or adsorb water selectively, and smart separation materials [21–24]. Based on this, various materials including metal meshes, porous sponge materials, fabrics, fluoro-polymers, aerogels, and nanoparticles have been developed to fabricate superwetting materials for oil-water separation. Among numerous separation materials with superwetting property, three-dimensional (3D) porous materials [25] have attracted great attention due to their huge surface area, light weight, and well-developed porous structure, excellent strength, low-cost, and facile preparation processes. In particular, 3D porous metallic materials(e.g., copper foam [26] and nickel foam [27]) compared with those 3D porous organic materials (e.g., polyurethane sponge [28]), did not need mechanical handling (squeezing or compression) to recycle oil or water after one

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absorption cycle, which enlarged the application for continuous separation in large-area oily wastewater. Except for oil-water separation property, the mechanical durability[29] and the durability of oil-water separation for physical abrasion were essential for practical separation application owing to the complex wastewater environment. The durability of superwetting materials were mainly correlated to the stability of their surface structures [30]. The destructive power of the physical abrasion, especially huge flow impact [31] and the abrasion of dirt particles which existed in oily wastewater, could destroy the surface structures of material surfaces, resulting in the separation performance and recyclability degradation or loss. Therefore, it was urgent demand to develop robust superwetting materials for oil-water separation with high separation efficiency, low energy consumption, stable separation abilities and outstanding mechanical durability even in complex environments. Various approaches, e.g., dip-coating [32], vapor-phase deposition [33], in situ synthesis [34] and chemically etching [35], have been used for preparing superwetting surfaces on the skeleton of 3D porous materials. To the best of our knowledge, few reports [26] have systematically investigated the mechanical durability of copper foams with superhydrophobic properties for oil-water separation. Herein, four kinds of patterned nanostructures (Cu(OH)2 nanoneedles, flower cluster-like ZnO nanocones and patterned ZnO nanorods on smooth micro-crystals closely-patterned ZnO nanorods on rough Cu nanocrystals) on three-dimensional porous copper foam surfaces with robust superhydrophobicity and superoleophilicity were fabricated through different approaches, which mainly involved simple electrodeposition, seed layer growth and low-temperature hydrothermal reaction, and followed by chemical modification. These copper foams showed great mechanical durability after mechanical resistance tests containing water impacting, sandy ethanol washing, sonication in ethanol and continuous oil-water separation. Moreover, all the as-prepared skeleton hierarchical nanostructure copper foams showed efficient and selective oil-water separation ability, which will be promising candidates for practical oil-water separation application under harsh circumstances. 2. Experiments 2.1. Materials Copper foam (average pore diameter: 450 ␮m, pore number: 80 PPI) was supplied by Alantum Advanced Technology Materials (Dalian) Co., Ltd., China. 1H,1H,2H,2Hzinc acetate dihydrate perfluorodecyltriethoxysilane, (Zn(CH3 COO)2 ·2H2 O), zinc nitrate hexahydrate (Zn(NO3 )2 ·6H2 O), ethanolamine (NH2 CH2 CH2 OH), ethylene glycol monomethyl ether (CH3 OCH2 CH2 OH), polyethylene glycol 4000 (HO(CH2 CH2 O)n H), sodium hydroxide (NaOH), sulfuric acid (H2 SO4 ), copper sulfate pentahydrate (CuSO4 ·5H2 O), potassium hydroxide(KOH), acetone, and ethanol were obtained from China National Medicines Co., Ltd. All chemicals were used as received without further purification.

electrolyte was 1.0 M NaOH aqueous solution. Cu(OH)2 nanoneedles were electrochemically grown at a constant current density of 6 mA/cm2 for 10 min. After anodization, they were rinsed and dried in a vacuum oven. 2.2.2. ZnO nanocones Flower cluster-like ZnO nanocones were fabricated on copper foams using a one-step method. Liquid phase growth solution of ZnO nanocones was the 0.25 M alkali zincate ions [Zn(OH)4 ]2− solution, which prepared by dropping 20 mL of 4.0 M KOH aqueous solution into 20 mL of 0.5 M Zn(NO3 )2 ·6H2 O aqueous solution under stirring [36]. Cleaned copper foams were placed in 40 mL of [Zn(OH)4 ]2− solution, then sealed in a beaker. After reaction at 35 ◦ C for 8 h, the samples were thoroughly rinsed with DI water and then dried. 2.2.3. ZnO nanorod arrays Patterned ZnO nanorod arrays were fabricated on smooth copper foams using a two-step process, combining compact seeding layer fabrication on smooth copper foam skeleton and patterned nanorods growth by a solution phase route. At first, the smooth copper foams were coated with precursor seed sol of 0.3 M zinc acetate dihydrate which contained ethylene glycol monomethyl ether, ethanolamine and polyethylene glycol 4000 by dip-coating method, and then high-temperature annealing in an argon atmosphere furnace at 450 ◦ C for 2 h [37]. Next, the patterned ZnO nanorods on seeded copper foams were fabricated. The method was the same as ZnO nanocones, as mentioned previously. After synthesis, the samples were rinsed with DI water and dried in a vacuum oven. 2.2.4. Cu/ZnO nanorod arrays Closely-patterned ZnO nanorod arrays on rough nanocrystals were fabricated on copper foams via a two-step process, combining simple electrodeposition [38] and ZnO nanorods growth by a two-step route, as illustrated in Scheme 1. First, roughened copper foams (as shown in Fig. A.1) which coated by copper nanoparticles (Cu NPs) was obtained by electrochemically depositing a layer of Cu NPs at a 2.5 V voltage for 300 s at room temperature. The working electrode was copper foam and cathodes were two copper sheets. The electrolyte was an aqueous solution composed of 0.2 M CuSO4 ·5H2 O and 1.5 M H2 SO4 . Second, closely-patterned ZnO nanorod arrays on roughened copper foams were finally obtained by a two-step route. The fabrication details of ZnO nanorod arrays were same as the patterned ZnO nanorods on smooth copper foams. 2.3. Fabrication of superhydrophobic surfaces The superhydrophobic copper foams coated with nanocrystals were fabricated by chemical vapor deposition (CVD) technique using 1 mL of 1H,1H,2H,2H-perfluorodecyltriethoxysilane under vacuum conditions at 120 ◦ C for 8 h in a vacuum oven. The superhydrophobic copper foams were finally obtained.

2.2. Preparation of nanostructures on copper foams

2.4. Characterization

The copper foams (4 cm × 4 cm × 0.16 cm) were sequentially washed with ethanol, acetone, 1.0 M HCl, and deionized water(DI) for approximately 5 min each under ultrasonication to remove surface oxide and organic contaminants. Finally, the samples were dried at 60 ◦ C.

Scanning electron microscopy (SEM) images were taken by a Sirion field-emission scanning electron microscope(FEI-SEM) at 20 kV. X-ray diffraction (XRD) patterns were performed with an X-ray diffractometer model D8-Discover (Bruker) with Cu K␣ radiation (␭¼ 1.5418 Å). The static contact angles (SCA) and rolling angles (RA) were measured on an OCA 15Pro machine (DataPhysics, Germany) at ambient temperature. The SCA values were measured from 5.0 ␮L DI water droplets and averaged five different positions on each sample. 10 ␮L water droplets were used for

2.2.1. Cu(OH)2 nanoneedles The cleaned copper foams were used as the working electrode. The counter electrode was a pure copper sheet (99.9%) and the

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Scheme 1. Schematic illustration of the routes of superhydrophobic nanocrystal arrays coating on the copper foam skeleton surfaces.

rolling angle (RA) measurement. The RA values were averaged three measurements of the same sample.

where C0 and C1 were the original water contents in the isooctanewater mixture and water contents in permeate before and after each three times separation, respectively.

2.5. Mechanical durability tests Mechanical durability of the substrates was evaluated by sandy ethanol washing, high-pressure water impacting [39], sonication in ethanol and continuous oil-water separation. For sandy ethanol washing tests, the samples were washed with 0.2 g/mL sands (80 mesh) ethanol solution under magnetic stirring (1500 rpm) at room temperature (Fig. A.3). The water impacting apparatus was homebuilt as shown in Fig. A.4. A water pipe (inner diameter, 5 mm) were fixed at a 10 cm height off the samples which tilted at 30◦ , and then jetted for a certain period of time at about 5 m/s of flow velocity. The sonication in ethanol test was that copper foam samples were worn ultrasonically in ethanol at 40 kHz. The SCA and RA of the as-prepared copper foams after mechanical tests were measured to estimate the mechanical stability of the samples. 2.6. Oil-water separation tests The as-prepared copper foams were fixed between two glass tubes and placed vertically (Fig. 6). The inner diameter of the tube was 35 mm. The oil-water mixtures (50% v/v) were poured onto the as-prepared copper foams. The separation was achieved by gravity drive solely. The oil-water separation efficiency (S(%)) of the samples were calculated using the following equation: S(%) = ((C 0 − C 1 )/C 0 ) × 100%

(1)

3. Results and discussion 3.1. Characterization of the composition and structure Four kinds of three-dimensional porous copper foams with superhydrophobicity were fabricated via different approaches. The digital photos of pure copper foams and the four kinds of as-prepared copper foams were shown in Fig. A.2. The surface morphology of copper foams coated with patterned nanostructures after surface modification was investigated by SEM (Fig. 1). Fig. 1(a1) exhibited a uniform array of pine-like nanoneedles over copper foam surfaces anodized in 1.0 M NaOH at room temperature. Some nanoneedles form micron-scale evenly distributed star-like aggregates, which were close to the dimension of the papillose protrusions on lotus leaves. The enlarged image (Fig. 1(a2)) indicates that the nanoneedles had a length of about several microns. The tip diameters of nanoneedles were about 30 nm and the bottom diameters were about 280 nm (Fig. 1(a3)). The nanoneedles were composed of orthorhombic-phase Cu(OH)2 crystals according to a typical XRD pattern based on the existence of (020), (021) and (002) peaks (JCPDS. 13-420), except for the Cu peaks marked with asterisks (Fig. 2). Fig. 1(b1–b3) shows the SEM images of the as-synthesized nanocones products on the copper foam surfaces. The lowmagnification SEM images (Fig. 1(b1)) displayed that star-like

Fig. 1. SEM images of (a1–a3) Cu(OH)2 nanoneedles fabricated by a anodic oxidation method, (b1–b3) ZnO nanocones prepared by one-step low temperature liquid growth method, (c1–c3) ZnO nanorods fabricated via a two-step process, combining seeding fabrication and nanorods growth by a solution phase route, (d1–d3) Cu/ZnO nanorods prepared via two-step methods, including surface pre-roughing through electrodeposition and nanorods growth by a two-step route, (a1–d1) and (a2–d2)were low and high magnifications, respectively. (a3–d3) were the cross-section morphology.

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Fig. 2. XRD patterns for the four different kinds of as-prepared copper foams.

flower cluster microcrystals (≈8 ␮m) evenly distributed on the substrate surface. The high-magnification SEM images (Fig. 1(b2)) and cross sectional view (Fig. 1(b3)) clearly showed that the microcrystals were composed of nanocone arrays (2–3 ␮m in length, 50–100 nm in top diameter and 150–200 nm in bottom diameter). These images clearly indicated the presence of binary structures with both micro- and nano-meter scales. The XRD pattern of the star-like flower cluster microcrystals (as shown in Fig. 2) indicated that the nanocone crystals were hexagonal wurtzite ZnO (JCPDS. 36-1451), which was the only detectable crystallographic phase, apart from the peaks attributed to the copper foam substrate. It indicated that uniform star-like ZnO nano-flower structure was successfully prepared on the metal curved surface of copper foams. Fig. 1(c1–c3) shows the morphology and structural characterization of the patterned nanorod arrays prepared on seeded copper foams via a two-step process. As shown in Fig. 1(c1–c2), the seeded copper foam surfaces were completely coated with uniform-size flocky arrays. Fig. 1(c3) is an enlarged cross sectional view of the nanorods arrays, which oriented perpendicularly to the substrate. It was clear that the nanorods are typically 1.8–2 ␮m in length, about 80 nm in diameter at the top parts, and 100–120 nm in diameter at the bottom parts. The presence of ZnO nanorods on copper foams was confirmed by XRD as shown in Fig. 2. The XRD pattern indicated that all of the diffraction peaks were in good agreement with hexagonal phase of wurtzite-type ZnO (JCPDS 36-1451). It indicated that dense ZnO nanorods arrays were successfully prepared on the metal curved surface of copper foams. After being electroplated in a solution containing 0.2 M CuSO4 ·5H2 O and 1.5 M H2 SO4 at a 2.5 V voltage at room temperature, Cu nanoparticles were deposited on the copper foam substrate (Fig. A.1(a and b)). As shown in Fig. A.1(b), the size of the Cu nanocrystals ranged from hundreds of nanometers to several microns, indicating that copper foam surfaces became rough after electro-deposition. The roughened copper foams were then treated through two-step processes, combining seeding layer fabrication and low-temperature solution-phase growth. Fig. 1(d1–d3) showed the SEM images of the resulting closely-patterned rod-like crystals on the roughened substrate. The copper foam surfaces were covered with well-aligned and dense patterned nanorods. From the high magnification images in Fig. 1(d2–d3), the top and bottom diameters of nanorods were about 80 nm and 120 nm, respectively. The length scale of nanorods was about 2 ␮m. Fig. 2 showed the XRD patterns of the copper foams after being roughened and two-step growth treated. It was seen that the as-received sample was composed of cubic-phase Cu (JCPDS. 85-1326) and hexagonal-phase

Fig. 3. The static water contact angles (water and isooctane selected as the model oil) and rolling angles of superhydrophobic and superoleophilicity Cu(OH)2 nanoneedles, ZnO nanocones (ZnO NCs), ZnO nanorods (ZnO NRs) and Cu/ZnO nanorods (ZnO NRs) which coated on the copper foam surfaces, respectively. Insets were the water and isooctane static contact angle photographs of the four kinds of nanostructures after fluorosilane modification.

ZnO (JCPDS. 36-1451). Moreover, no characteristic peaks assigned to impurities. It indicated that closely-patterned ZnO nanorods arrays were successfully prepared on the rough curved surface of copper foams. Fig. 3 shows the formation of superhydrophobic and superoleophilicity surfaces for the four kinds of patterned nano-arrays after fluorosilane modification. Clearly, the water contact angles of these superhydrophobic copper foams were all larger than 155◦ and the oil (isooctane selected as the model oil) contact angles were all about 0◦ . In particular, the closely-patterned Cu/ZnO nanorods arrays owned the best superhydrophobic performance. 3.2. Mechanical durability of the as-prepared copper foams surface structures One important limitation for practical applications of superhydrophobic surface was their low mechanical stability. Based on the wetting theory and reported experimental data, it was seemed that surface roughness played a vital role in superhydrophobic surfaces. Micro-nanostructures of superhydrophobic surfaces were always broken through some approaches such as mechanical wear, chemical corrosion and environmental aging. Thus, the mechanical stability of the surface structures was studied by means of sandy ethanol washing, water impacting, ultrasonic in ethanol and continuous oil-water separation tests. Mechanical durability of superhydrophobic copper foam surfaces was researched by measuring the static water contact angles (SCAs), rolling angles (RAs) and oil-water separation efficiency. 3.2.1. Sandy ethanol washing test In practical oil-water separation applications, superhydrophobic surfaces needed to survive various harsh conditions. Simulated wastewater containing sands, sand washing tests were performed to investigate the mechanical durability of the four different nanostructures. The samples were washed in the ethanol solution containing sand grains (80 mesh, 0.2 mg/mL) under magnetic stirring (1500 rpm) at room temperature. The schematic diagram of the home-built setups for sandy ethanol washing test was showed in Fig. A.3. The variations of SCA and RA values during sandy ethanol washing tests were showed in Fig. 4(a). For the smooth copper foams coated with Cu(OH)2 nanoneedles, the SCA was changed from original 158.2◦ to 148.7◦ , and the RA was

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Fig. 4. The changes of static contact angles and rolling angles of the four different kinds of as-prepared copper foams with increasing sandy ethanol washing time (a), water impacting time (b), ultrasonic in ethanol time (c), respectively.

Fig. 5. SEM images of surface micro-nanostructure morphology on copper foam surfaces after 15 min sandy ethanol washing tests. (a1) Cu(OH)2 nanoneedles fabricated by a anodic oxidation method, (b1) ZnO nanocones prepared by one-step low temperature liquid growth method, (c1) ZnO nanorods fabricated via a two-step process, combining seeding fabrication and nanorods growth by a solution phase route, (d1) Cu/ZnO nanorods prepared via two-step methods, including surface pre-roughing through electrodeposition and nanorods growth by a two-step route. (a2–d2) were high magnification of top view. (a3–d3) were the cross-section morphology.

increased from 7.2◦ to 15.7◦ after 15 min sandy ethanol washing (Fig. 4(a)), which mainly attributed to the surface morphology being damaged during washing. It can be proved by SEM. As shown in Fig. 5(a1), A part of Cu(OH)2 nanoneedles on the copper foam skeleton protuberant parts were abraded and shortened, even peeled off. However, the micro-nanostructures on the copper foam skeleton deboss or near skeleton protuberant remained intact. And the Cu(OH)2 crystal residues were still possessed of micro-nano binary structures (Fig. 5(a2–a3)). Since most of superhydrophobic surfaces have not been destroyed, the copper foams kept high hydrophobic property even after 15 min sand washing. Analogously, the SCA values changes of ZnO nanocones star-like flower clusters was from 157.5◦ to 145.3◦ . As shown in Fig. 5(b1–b3), A lot of superhydrophobic ZnO nanocones flower clusters were worn off integrally. The nanostructures of some residual ZnO nanocones were also broken. Because the damage degree of nanostructures was the most serious among four kinds of samples, the mechanical durability of ZnO nanocones star-like flower clusters was the worst. The smooth copper foam surfaces coated with ZnO nanorods arrays (SCA, 150.2◦ ; RA, 14.2◦ , after 15 min tests) remained better hydrophobicity than Cu(OH)2 nanoneedles and ZnO nanocones as shown in Fig. 4(a). It can be observed that a small part of monolithlike ZnO nanorods arrays was peeled off and the length of residual ZnO nanorods was shorter in Fig. 5(c1–c3). The results indicated that the ZnO nanorods arrays on smooth copper foam surfaces had better mechanical durability than Cu(OH)2 nanoneedles and ZnO nanocones. The reason may be that microstructure of ZnO nanorods

was more dense and the binding force between nanorods and copper foam matrix was stronger. The rough copper foam surfaces coated with closely-patterned ZnO nanorods arrays (i.e., Cu/ZnO nanorods) was still superhydrophobic (SCA, 152.1◦ ; RA, 9.8◦ ) after 15 min sandy ethanol washing tests. There was no very obvious difference of surface microtopography after 15 min tests for closely-patterned Cu/ZnO nanorods arrays (Fig. 5(d1)). It meant that closely-patterned Cu/ZnO nanorods showed outstanding mechanical stability. Comparison with ZnO nanorods arrays on smooth copper foam surfaces, closely-patterned ZnO nanorods arrays on rough copper foam surfaces were more dense (Fig. 7(a)) and could keep its superhydrophobic property even after long time sandy ethanol washing (Fig. 5(d3)). The tips of closely-patterned ZnO nanorods and fluorosilane on the tips were broken by the sand in ethanol after strong rubbing. However, the ZnO nanorods underneath was still remained, because the scale of nanorods gaps (<500 nm) (Fig. 5(d2)) was much smaller than rubbing sand particles (≈200 ␮m), providing an effective shielding action to prevent the bottom of nanorods and the fluorosilane on bottom parts surfaces from peeling off [40]. 3.2.2. Water impacting test Variation of the SCA and RA of the four kinds of superhydrophobic surfaces after water impacting tests were shown in Fig. 4(b). Clearly, the superhydrophobic copper foam surfaces coated with closely-patterned Cu/ZnO nanorods showed the best hydrophobic stability after 30 min water impacting, with a slight SCA decrease

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(from 164.5◦ to 150.5◦ ) and only a small RA increase (from 3.2◦ to 8.7◦ ). A similar SCA decrease was seen on the surfaces of copper foams with Cu(OH)2 nanoneedles and ZnO nanorods, respectively. Nevertheless, in comparison with ZnO nanorods (from 7.2◦ to 13.5◦ ), the RA increase amplitude of Cu(OH)2 nanoneedles (from 4.2◦ to 20.1◦ ) was bigger. For ZnO nanocones, the SCA of copper foam surfaces decreased below 140◦ (from 157.5◦ to 139.4◦ ) and the RA increased to 29◦ after 30 min water impacting, indicating the loss of superhydrophobicity and the self-cleaning effect. Water droplets would stick to the ZnO nanocones surface when the sample was tilted at a small angle (<30◦ ). As shown in Fig. A.5, the surface morphology of the copper foams were examined by SEM after 30 min water impacting tests. Obviously, the abrasion on these copper foam surfaces which caused by water impact, resulted in apparent micro/nano- twoscale structure nonreversible deterioration. For surfaces of Cu(OH)2 nanoneedles, the roughness changed significantly, as shown in Fig. A.5(a1–a3). Fig. A.5(a1) shows Cu(OH)2 nanoneedles surface was more smooth than before water impacting. Owing to the fact that lots of nanoneedles structures on the copper foam surfaces were seriously damaged, especially on the skeletal protrusion. The initial long nanoneedles were ground short as shown in the enlarged view (Fig. A.5(a2–a3)). In comparison, ZnO nanocones on copper foam skeletons were damaged more severely (Fig. A. 5(b1–b3)). Plenty of star-like flower cluster nanocones fell off from the substrate surface and A part of the copper foam skeleton was exposed (Fig. A.5(b1)). It was supposed that the interfacial binding affinity of flower cluster-like ZnO nanocones with matrix was very low. In addition, the top parts of remaining ZnO nanocones were worn down after long time water jetting (Fig. A.5(b2–b3)). These reasons aforementioned lead the worst wetting performance of ZnO nanocones among four kinds of micro-nanostructures. Before water impacting, the ZnO nanorods uniformly covered the smooth copper foam surfaces (Fig. 1(c1)). After tests, the tops of some ZnO nanorods and a fraction of ZnO nanorods structures on the substrate surface were removed, but the overwhelming majority of the nanorods were retained (Fig. A.5(c1–c2)). Therefore, this kind nanorods structure could maintain high hydrophobicity. Interestingly, the microstructures of closely-patterned Cu/ZnO nanorods were still intact, only some ZnO nanorods tips were abraded after 30 min test (Fig. A.5(d1–d2)). closely-patterned ZnO nanorods were tightly attached to the roughed copper foam matrix (Fig. A.5(d3)) compared to ZnO nanorods on the smooth copper foam surfaces (Fig. A.5(c3)). Results could be interpreted by the fact that Cu nanoparticles which electrodeposited on the copper foam surfaces increased the copper foam surface roughness and the

anchor points between ZnO nanorods and copper foam substrates. Thereby, the interfacial combination between ZnO nanorods and the rough matrix was improved, as well as the mechanical durability of the superhydrophobic copper foam surfaces. 3.2.3. Ultrasonic in ethanol test The changes in SCA and RA values of the four kinds of copper foams after ultrasonic in ethanol tests (40 kHz–250 W) were showed in Fig. 4(c). The SCA values of the four kinds of superhydrophobic copper foams were all decreased and the RA values were all increased during ultrasonic treatment in ethanol. The SCA decreased and RA increased might be caused by strong cavitation effect during ultrasonic in ethanol tests, causing cracks and shedding of micro-nanostructures on copper foam surfaces (Fig. A.6). After undergoing a long period of ultrasonic in ethanol, the SCA and RA values of Cu(OH)2 nanoneedles changed from 158.2◦ to 147.9◦ and 7.2◦ to 17.8◦ , respectively. The cause of these changes was that the Cu(OH)2 nanoneedles were broken and the microstructure surface was smoothed by ultrasonic wave forces (Fig. A.6(a1–a3)). Similarly, the changes in the values of SCA and RA of ZnO nanocones were more obvious with treatment time increasing. The SCA decreased to141.3◦ and the RA increased to 24.7◦ after 100 min tests. This wettability change can be ascribed to the entirety loose and partial loss of the surface micro-nano flower-like structure, as supported by the SEM images in Fig. A.6(b1–b3). However, The SCA values of patterned ZnO nanorods and closely-patterned Cu/ZnO nanorods still remained above 150◦ after 100 min, while the RA of patterned ZnO nanorods increased from 4.2◦ to 13.0◦ , and that of Cu/ZnO nanorods increased from 3.2◦ to 8.9◦ (Fig. 4(c)), indicating their excellent durability against ultrasonic effect. As a comparison (Fig. A.6(c1–c3)), closely-patterned Cu/ZnO nanorods arrays had the best superhydrophobic performance in terms of mechanical durability. The reason may be that there were no obvious interfacial debonding and large-area nanostructures peeling off on copper foam surfaces which coated with closely-patterned Cu/ZnO nanorods (Fig. A.6(d1–d3)). 3.3. Oil-water separation properties Considering the porosity and superhydrophobicity of copper foams, we used the copper foams coated with patterned nanostructures for oil-water separation test. The oil-water separation processes were carried out in a simple home-made device as shown in Fig. 6. A mixture of 20 mL of heavy oil colored with oil red and 20 mL of water colored with methylene blue was poured slowly into a glass tube. Tacking the superhydrophobic copper foams

Fig. 6. Heavy oil-water separation (a) and light oil-water separation (b) processes using the superhydrophobic copper foams coated with closely-patterned Cu/ZnO nanorods arrays. Oils (heavy oil and light oil) and water are colored by oil red and methylene blue, respectively.

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Fig. 7. The array average surface density and the array average nanocrystals spacing of Cu(OH)2 nanoneedles, patterned ZnO nanorods (ZnO NRs) and closely-patterned Cu/ZnO nanorods (ZnO NRs) (a). Water droplet impact pressures (b) and water intrusion pressures (c) of the four kinds of superhydrophobic copper foams.

coated with closely-patterned Cu/ZnO nanorods arrays as an example, when the oil (heavy oil) density was larger than the water density, the Cu/ZnO nanorods arrays were superhydrophobic and superoleophilic to selectively remove oil from the heavy oil-water mixture by the gravity-driven effect (Fig. 6(a)). When the oil (light oil) density was smaller than water density, due to the superhydrophobicity of Cu/ZnO nanorods arrays and the capillary force of porous structures of copper foams, the light oil was selectively to remove into the glass tube below though the copper foam from the light oil-water mixture, while the water was retained above the surface of the copper foam (Fig. 6(b)). In addition, the as-prepared copper foams exhibited a high separation efficiency of above 98% for heavy or light oil-water mixtures (Fig. 6(b)). This revealed that the superhydrophobic copper foams coated with patterned nanostructure arrays owned great oil-water separation performance. To further research the separation ability of the superhydrophobic copper foams coated with patterned nanostructure arrays, water droplet impact pressure and water intrusion pressure were measured. Water droplets with a certain volume were dropped from a certain height to impact the superhydrophobic copper foams. The impact dynamic pressure (Pd )can be obtained by using the Eq. (2) [16]:





2 Pd = Vmax /2

(2)

where  was water density and Vmax was the maximum of water droplet impact velocity. The intrusion pressure of water flowing through the as-prepared copper foams was measured, which indicated the maximum height of water that the copper foams could support. The intrusion pressure was provided by the weight of water, therefore, the intrusion pressure (Pi ) values were calculated according to the Eq. (3) [41]: P i = ghmax

(3)

where  was the density of water, g was gravity acceleration, and hmax was the maximum height of water which copper foams could support. The intrusion pressures and the impact dynamic pressures for water on the four kinds of as-prepared copper foams in this work were enumerated in Fig. 7(b) and Fig. 7(c). It meant that water cannot permeate through the superhydrophobic copper foams coated with nanostructures below the intrusion pressures, and great impact resistance of the four kinds of copper foams coated with patterned nanostructures for oil-water separation. 3.3.1. Recycling stability of continuous oil-water separation The superhydrophobic recycling stability of the as-prepared copper foams for long time separation was an important parameter to assess its potential reuse application of oil-water separation. In

this work, the four kinds of superhydrophobic porous copper foams coated with different nanostructures intended to be used to separate oil from oil-water mixture. The water SCA, RA and separation efficiency were measured after each oil-water separation cycle to evaluate the recycling stability. When the isooctane-water mixture (20:20 v/v) was poured onto as-prepared copper foams, isooctane quickly permeated through superhydrophobic copper foams and rapidly flowed into the glass tube below. The SCA and RA variation on copper foam surfaces with oil-water separation cycles was presented in Fig. 8(a). Clearly, the water SCA values of the several copper foams decreased slightly and all remained highly hydrophobic after 30 separation cycles. In particular, the SCA values of closely-patterned Cu/ZnO nanorods were always greater than 150◦ and the RA values were always lower than 10◦ after 30 cycles. This indicated that the four kinds of as-prepared copper foams had great mechanical durability of superhydrophobicity. The oil-water separation efficiency was determined by measuring the content of water before and after separation. As described in Fig. 8(b), the separation efficiency of the four kinds of samples got a slight decrease from >99% to about 95% after undergoing 30 times recycling separation, indicated that the as-prepared copper foams were capable of being reused to separate oil from oil water mixture for a long time. After the oil-water separation tests, the contaminated copper foams could be also easily cleaned with ethanol and dried for reuse, indicated significant recyclability of as-prepared copper foams with different micro-nanostructure.

3.3.2. The reasons of deterioration of continuous oil-water separation property The SEM observation showed that there were no noticeable change of the surface morphology of as-prepared copper foams after 30 times recycling separation (Fig. A.7(a1–d1)). It demonstrated that the four kinds of nanoscale structures (Figs. A.7(a2–d2) and A.7(a3–d3)) on copper foam surfaces were stable and can be tolerant to damage in recycling separation tests. However, as shown in Fig. 9b, there were obvious oil residue in the Cu(OH)2 nanostructure clearance after 30 times recycling separation contrast to that before separating (Fig. 9a). Thus, it was inferred that oil residue may be the major reason of the superhydrophobic degradation. For demonstrating this point, isooctane (density ≈ 0.69 kg L−1 at 20 ◦ C, boiling point = 99.3 ◦ C) was selected as a model oil to contaminate the surface of the as-prepared copper foams. After 30 times isooctanewater (50% v/v) mixture continuous separation, the four kinds of as-prepared copper foams were heated to remove the oil residue at 150 ◦ C for 30 min in a vacuum oven. Upon heat treatment, isooctane was volatilized and pyrolyzed, the four kinds of samples all exhibited superhydrophobic with SCA > 152◦ and RA < 10◦ , which were better than the ones before heat treatment. Meanwhile, the

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Fig. 8. The changes of static contact angles and rolling angles (a) and the oil-water separation efficiency (b) of the four different kinds of as-prepared copper foams with recycling separation times.

Fig. 9. SEM images of Cu(OH)2 nanoneedles surface micro-nanostructure morphology before (a) and after (b) 30 times recycling oil-water separation tests. (c) were the changes of surface wettability and separation efficiency of Cu(OH)2 nanoneedles, flower cluster-like ZnO nanocones (ZnO NCs), patterned ZnO nanorods (ZnO NRs) and closely-patterned Cu/ZnO nanorods (ZnO NRs) which coated on the copper foam surfaces before and after 30 min pyrolysis treatments for removing the residual oil which storing in the nanostructure gaps at 150 ◦ C.

separation efficiencies of Cu(OH)2 nanoneedles and ZnO nanocones were recovered to more than 98%, and the separation efficiencies of ZnO nanorods and Cu/ZnO nanorods were back to more than 99% (Fig. 9c). Therefore, it was speculated that, oil residue which storing in the nanostructure gaps mainly caused superhydrophobic and separation performance reversible deterioration during continue oil-water separation process. This fact also proved the outstanding recyclability and mechanical durability of as-prepared copper foams for the uptake of oils from oily wastewater.

3.4. The mechanism of mechanical durability The deterioration in the surface superhydrophobic properties of the four kinds of copper foams under mechanical wear was studied by means of the static contact angles, rolling angles and oil-water separation efficiency. Different mechanical durability mechanisms of the structured copper foam surfaces responsible for deterioration of superhydrophobic performance and oil-water separation property were described as shown in Scheme 2. The original Cu(OH)2 nanoneedles had a length of 3–6 ␮m (Fig. 1(a1–a3)). Under the action of the external force, Cu(OH)2 nanoneedles were loose and fractured, even peeling off (Fig. 5(a1–a3), Fig. A.5–A.6(a1–a3) and Scheme 2). The nanostructures of the original ZnO nanocones with pentagram flower cluster-like microcrystals (≈8 ␮m) (Fig. 1(b1–b3)) were interfacial debonding, fractured, even much of the flower cluster-like nanocones peeled off (Fig. 5(b1–b3), Fig. A.5–A.6(b1–b3) and Scheme 2). The original ZnO nanorods (1.8–2 ␮m in length) were uniform-size flocky, completely coated on copper foam surfaces (Fig. 1(c1–c3)). After mechanical wear,

a part of patterned ZnO nanorods nanostructures was interfacial debonding, fractured, and peeled off (Fig. 5(c1–c3), Fig. A.5–A.6(c1–c3) and Scheme 2). For Cu/ZnO nanorods, Cu nanoparticles were electrodeposited on the copper foam substrate (Fig. A.1), then covered with densely packed and well-aligned ZnO nanorods (about 2 ␮m in length, Fig. 1(d1–d3)). The rough Cu nanocrystals could not only avoid the formation of loose hierarchical ZnO nanostructures, but also improve the binding force between the patterned ZnO nanorods and the matrix. Hence, only a few of closely-patterned Cu/ZnO nanorods were fractured and a small part was peeled off (Figs. 5(d1–d3), A.5–A.6(d1–d3) and Scheme 2) after mechanical wear (e.g. sand washing, water impacting and ultrasonic treatment). In contrast, the microstructures of flower cluster-like ZnO nanocones on the copper foam surfaces were the sparsest and the loosest. Meanwhile, the top size of the flower cluster-like ZnO nanocones was big and the size of bottom was small, forming an inverted triangle structure. The binding strength of this type of structure with copper foam matrix was the lowest, brought about large area of ZnO nanocones peeling off. Thus, the mechanical durability of ZnO nanocones coating was the worst of the four. The nanostructure of Cu(OH)2 nanoneedles was similar to that of ZnO nanorods, but the Cu(OH)2 nanoneedles were looser than ZnO nanorods and the Cu(OH)2 nanoneedles spacing was larger (Fig. 7(a)), leading to the difference in capillary force [42] of the both nanostructures. Thus, the nanostructures and superhydrophobic properties of and Cu(OH)2 nanoneedles were more likely to be damaged by mechanical external force. Capillary pressure PC , namely, the required pressure which yielded by air-water inter-

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Scheme 2. Schematically illustrated surface abrasion effect on the four kinds of nanostructures (Cu(OH)2 nanoneedles, flower cluster-like ZnO nanocones, closely-patterned ZnO nanorods on rough Cu nano-crystals and patterned ZnO nanorods on smooth micro-crystals, respectively) which coated on the skeleton surface of three-dimensional porous copper foams. Under the action of the mechanical external force, the four kinds of nanocrystals have suffered from different forms of damage.

Scheme 3. Schematic image of oil penetration into the nano-arrays during separation. (a) undergoing recycling oil-water separation, (b) water repellency of the nanoarrays (Fe , external impulse force; Pc , capillary force), (c) oil pollution inside the space of the nano-arrays, (d) removal of residual oil by heat treatment to restore the superhydrophobicity and separation performance of copper foams.

facial area to resist water into the micro-nanostructure gaps, was given by the Eq. (4) [43]:

⎡ ⎢

Pc = ⎣ 

⎤

−4cos0

2

1 + b⁄a

−1

⎥ LV ⎦ a

(4)

where a was nanocrystal width and b was spacing of nanocrystals (as shown in Scheme 3b),  LV was the air-water surface energy, 72 mN/m. According to Eq. (4), capillary pressure (shown in Scheme 3b) decreases with the nanocrystals spacing increasing and nanocrystal width decreasing. As shown in Fig. 7(a), Cu(OH)2 nanoneedles array was looser than patterned ZnO nanorods array, and the array average nanocrystals spacing was larger. Thus, capillary force resisted external impulse force (Fe , shown in Scheme 3) of patterned ZnO nanorods was bigger than Cu(OH)2 nanoneedles. In the same

way, closely-patterned Cu/ZnO nanorods on rough substrate had stronger capillary force than ZnO nanorods on smooth copper foam surfaces. Therefore, when undergoing recycling oil-water separation, closely-patterned Cu/ZnO nanorods had the best mechanical durability and separation performance. Simultaneously, the water droplets could be bounce off because of the superhydrophobic property. However, the four kinds of superhydrophobic nanostructure arrays were not oleophobic, there would be oil residues in the nanocrystals gaps (shown in Scheme 3c) to lead the superhydrophobic and separation performance degradation (Fig. 8) after many times separation. In our experiments, the contaminated samples were heated to pyrolyzed the residual oil at a certain temperature. After heat-treatment, the superhydrophobic and separation performance of the nano-arrays were restored (Fig. 9c). Thus, we achieved a long-term efficient oil-water separation using the durable copper foams by periodic removal of the residual oil in the nanostructure gaps.

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4. Conclusions In summary, four kinds of nanostructure on the metal curved surface of porous copper foams were successfully fabricated through different methods including simple electrodeposition, seed layer growth and low-temperature hydrothermal reaction, and followed by chemically modification. The resultant samples exhibited superior superhydrophobicity with high water SCA (158–165◦ ) and low RA (<7◦ ). The as-prepared copper foams could withstand severe mechanical abrasion with great mechanical durability, which investigated by the mechanical resistance tests containing water jetting, sand washing, ultrasonic treatment and continuous oil-water separation. What was more, copper foams which deliberately selected not only endowed the nanocrystals with excellent mechanical durability, but also integrated its own outstanding porosity and high supporting strength simultaneously, importance for practical application. It was noted that, Cu NPs electroplated on the base copper foams surface provided much more powerful mechanical durability compared to several other nanostructures. It might be that Cu NPs improved the interfacial combination of ZnO nanorods with copper foams substrate. Because of the low-cost materials, simple preparation processes, excellent performance and lasting recyclability, the robust superhydrophobic copper foams will be a promising candidate for practical oil-water separation application under harsh circumstances. Acknowledgments The project was supported by the National Natural Science Foundation of China (Grants 51671055, 51676033), the China National Key R&D Program (2016YFC0700304), the National Natural Science Foundation of Jiangsu Province (BK20151135). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2017.02. 184. References [1] M.W. Lee, S. An, S.S. Latthe, C. Lee, S. Hong, S.S. Yoon, Electrospun polystyrene nanofiber membrane with superhydrophobicity and superoleophilicity for selective separation of sater and low viscous oil, ACS Appl. Mater. Inter. 5 (2013) 10597–10604. [2] H. Li, Y.S. Li, Q.Z. Liu, ZnO nanorod array-coated mesh film for the separation of water and oil, Nanoscale Res. Lett. 8 (2013) 1–6. [3] S.Y. Zhang, F. Lu, L. Tao, N. Liu, C.R. Gao, L. Feng, Y. Wei, Bio-inspired anti-oil-fouling chitosan-coated mesh for oil/water separation suitable for broad pH range and hyper-saline environments, ACS Appl. Mater. Inter. 5 (2013) 11971–11976. [4] G. Hayase, K. Kanamori, M. Fukuchi, H. Kaji, K. Nakanishi, Facile synthesis of marshmallow-like macroporous gels usable under harsh conditions for the separation of oil and water, Angew. Chem. Int. Ed. 52 (2013) 1986–1989. [5] F.J. Wang, S. Lei, M.S. Xue, J.F. Ou, C.Q. Li, W. Li, Superhydrophobic and superoleophilic miniature device for the collection of oils from water surfaces, J. Phys. Chem. C 118 (2014) 6344–6351. [6] J. Schaum, M. Cohen, S. Perry, R. Artz, R. Draxler, J.B. Frithsen, D. Heist, M. Lorber, L. Phillips, Screening level assessment of risks due to dioxin emissions from burning oil from the BP Deepwater Horizon Gulf of Mexico spill, Environ. Sci. Technol. 44 (2010) 9383–9389. [7] L. Feng, Z.Y. Zhang, Z.H. Mai, Y.M. Ma, B.Q. Liu, L. Jiang, D.B. Zhu, A super-hydrophobic and super-oleophilic coating mesh film for the separation of oil and water, Angew. Chem. Int. Ed. 43 (2004) 2012–2014. [8] A.A. Al-Shamrani, A. James, H. Xiao, Destabilisation of oil-water emulsions and separation by dissolved air flotation, Water Res. 36 (2002) 1503–1512. [9] W.L. Kang, L.M. Guo, H.M. Fan, L.W. Meng, Y.H. Li, Flocculation, coalescence and migration of dispersed phase droplets and oil-water separation in heavy oil emulsion, J. Petrol. Sci. Eng. 81 (2012) 177–181. [10] R. Boopathy, S. Shields, S. Nunna, Biodegradation of crude oil from the BP oil spill in the marsh sediments of Southeast Louisiana USA0, Appl. Biochem. Biotechnol. 167 (2012) 1560–1568.

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