Superhydrophobic and Superoleophilic Miniature Device for the

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Superhydrophobic and Superoleophilic Miniature Device for the Collection of Oils from Water Surfaces Fajun Wang, Sheng Lei, Mingshan Xue, Junfei Ou, Changquan Li, and Wen Li* School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, P. R. China S Supporting Information *

ABSTRACT: We develop a simple strategy for the collection of oil spill using a miniature device. The device was enfolded from a sheet of copper mesh with surface superhydrophobicity and superoleophilicity. The copper mesh was fabricated through a simple thermal oxidation and surface modification process. The miniature device adsorbed various oils from the surface of oil− water mixture in its pores while repelling water completely. After the pores were full of oil, only oil could infiltrate into the device through the pores automatically and was finally concentrated into the device, showing its oil− water separating ability. Importantly, the concentrated oil in the device could be taken away by a dropper or pumped out, thus achieving the collection of oil for reuse. Additionally, after the concentrated oil was taken away, the device could be used in the next oil−water separating cycle without apparent decrease of its separation ability. Therefore, the findings in this work may offer a new strategy for the collection and reuse of various kinds of oils and other organic compounds from water surfaces.

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mesh film exhibiting both superhydrophobicity and superoleophilicity by spraying polytetrafluoroethylene (PTFE) precursor onto the stainless steel mesh surface. Oils permeates the mesh less than 1 s while water rolls away from the mesh very quickly, thus demonstrating its oil−water separation properties.17 Similarly, numerous superhydrophobic and superoleophilic meshes have been fabricated to separate oil−water mixtures by allowing oils to pass through the pores of the mesh while completely preventing water from penetrating.12,17−21 Although these mesh materials could separate oils from oil− water mixture efficiently, they could not separate oils from the water surface because the oil must be first gathered from the water surface and then be poured onto the mesh surface for filtering.16 Apart from the mesh materials, other superhydrophobic materials with micro- and/or nanosized porous microstructures have more recently been proposed for the selective separation of oil and various organic components from water. For example, Zhang and co-workers synthesized superhydrophobic nanoporous polydivinylbenzene materials.22 The nanoporous polydivinylbenzene could absorb organic compounds with high absorptive capacity while repelling both liquid and gaseous water. This material has potential in chemical accident treatment and environmental protection. Nguyen et al. fabricated a superhydrophilic and superoleophilic sponge by anchoring graphene nanosheets on the skeletons of a melamine

INTRODUCTION Oil spill accidents have significant influence on both the environment and our economies.1−3 An extreme example of an oil spill occurred in the Gulf of Mexico in 2010, releasing crude oils to the Gulf of Mexico for over 90 days, during which nearly 800 km of shorelines was polluted. The common methods used to deal with oil spill include the use of oil absorbent materials, in-situ burning, skimmers and booms, and dispersants.1,2,4 Although these methods succeeded in their practical application on the oil spill, they suffered considerable limitations, such as low separation ability, poor recyclability, environmental harmfulness, and waste of the spilled oil.1,4 For example, traditional absorbent materials have problems such as the recollection of the adsorbed oil from the absorbent materials and the apparent decrease of separating ability, implying the poor reusability of these absorbent materials.1,5−7 Exfoliated graphite possesses the advantages of low density, high porous structure, and environmental friendliness, but the absorption capacity of exfoliated graphite decreases in the reused cycles, which is a disadvantage for its practical use.8,9 Additionally, most of the traditional absorbent materials do not possess superhydrophobic properties. As a result, they can absorb water and oil simultaneously, which can greatly decrease their separation efficiency.10 Recently, porous materials with superhydrophobicity and superoleophilicity have received extensive attention due to their potential application in the field of oil−water separation because only oil can pass through the porous structures of the materials, while water is compelely repelled by the materials.11−16 For example, Jiang et al. prepared a coating © 2014 American Chemical Society

Received: January 12, 2014 Revised: March 4, 2014 Published: March 6, 2014 6344

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Figure 1. FE-SEM images of the as-received Cu mesh [(a), (b), and (c), scale bar = 100 μm, 20 μm, and 500 nm, respectively], CuO mesh [(d), (e), and (f), scale bar = 100 μm, 20 μm, and 500 nm, respectively], and PA modified CuO mesh [(g), (h), and (i), scale bar = 100 μm, 20 μm, and 500 nm, respectively] at different magnification [(a), (d), and (g): 800; (b), (e), and (h): 4000; (c), (f), and (i): 100 000].

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sponge, which could be used in the selective separation of various kinds of oils and organic solvents from water with excellent recyclability.23 Calcagnile et al. fabricated a mutifunctional composite foam by the modification of commercial polyurethane foam with PTFE particles and iron oxide nanoparticles. The foam exhibited superhydrophobicity, superoleophibicity, and magnetism simultaneously and could be driven by a magnetic field to pass through the areas with spilled oil for the collection of large areas of oil.16 Similar works were also reported by Zhu et al. and Chu et al.24,25 However, in the above studies, the creating of oil containment booms using these materials and their oil−water separation properties have seldom been reported. In the present study, we report the oil−water separation and collection properties of a novel miniature device. The device was fabricated from a superhydrophobic and superoleophilic copper mesh via a simple method at low cost. The miniature device can float on the surface of water due to its superhydrophobicity. As soon as it meets the spill oil, the oils immediately permeate into the device and finally concentrated in the device, while water is completely excluded, which means the separation of oil from water. The miniature device can concentrate different types of oils from the water surface rapidly with high separation efficiency. In addition, the collection of the separated oils could be easily achieved for reuse by a dropper. Furthermore, the miniautre device is stable in simulated seawater and can be repeatedly used in different separation process for oil−water mixtures. Therefore, the finding of this work may provide a facile method for the cleanup, collection, and reuse of oils and other organic chemicals from the surfaces of water.

EXPERIMENTAL SECTION

Copper mesh was provided by Anhua Fardware Product Co., Ltd. (Hebei, China). Palmitic acid (PA), toluene, and Sudan Black B (SBB) were purchased from Shanghai Chemical Reagent Co. Ltd. (Shanghai, China). Diesel oil (10#), gasoline (93#), and aviation kerosene (1#) were proviede by Datang Petroleum Industry Co. Ltd. (Shanghai, China). The copper mesh was heated at 400 °C for 20 min at air atmosphere and naturally cooled to about 25 °C. Afterward, the mesh was immersed into an ethanol solution of 0.1 M palmitic acid (PA) at 30 °C for about 30 min. After being cleaned in ethanol and distilled water, the obtained meshes were subjected to further characterization. The surface microstructures of the initial Cu mesh, the CuO mesh, and the CuO mesh modified with PA were observed by a field emission scanning electron microscope (FESEM, FEI NanoSEM-450, America) at 5−15 kV. The infrared spectra were obtained using a Bruker IFS66 V/S spectrometer. The XRD patterns of the Cu mesh and the CuO mesh were measured by an X-ray diffractometer (X’Pert-Pro, Philips Corp.). The XPS spectra were obtained by an X-ray photoelectron spectrometer (XPS, Thermo ESCALAB 250, America). Water contact angles (WCAs) were obtained with 4 μL of water using a contact angle analyzer (Krüss DSA100) at ambient temperature. Water sliding angles (WSAs) were obtained by gradually inclining the tilting stage until the water droplet began to roll. Each of the reported WCAs and WSAs was the average values of five measurements at different position of the same surface. Tafel plots were obtained by a CHI660C electrochemical workstation (CH Instruments, China). 6345

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has high phase purity. The CuO film on Cu wire substrates can be simply formed by the thermal oxidation of the Cu mesh in air at 400 °C for 20 min. The color of the Cu mesh changes from yellow to black, and the Cu mesh is oxidized to form CuO film on Cu wire surface. The diffraction peaks at 2θ values of 32.8°, 35.9°, 39.0°, 49.1°, 53.6°, 62.1°, and 66.2° presented in Figure 2b indicate the monoclinic structured of copper oxide (JCPDS card no. 05-0661).26,27 In addition, characteristic peaks of impurities, i.e., Cu2O (JCPDS no. 41-0254) and Cu, are also observed in Figure 2b. The peaks of Cu component are attributed to the incomplete oxidation of Cu mesh, and the peaks of Cu2O impurity could be interpreted as the partial deoxidation of the upmost layer of the CuO surface resulting in a CuO-like surface,28 i.e., 2CuO → Cu2O + 1/2O2. The reaction involved in the PA modification procedures can be described as follows:

The miniature box (4.5 × 3 × 1.5 cm) was folded from the PA modified CuO mesh and was used for the investigation of its oil−water separation properties. Toluene, kerosene, diesel oil, and gasoline were chosen to simulating the oil spill. The oil absorption capacity (ka) of the miniature box was determined by the formula ka = (m2 − m1)/(m1) × 100%, where m1 is the weight of the bare box and m2 is the weight of the box after oil absorption. The volume of absorbed oil, vabs, was calculated by the formula vabs= mabs × ρ, where ρ is the density of the various solvents. The oil separation ability of the box was determined by the formula ks = (vabs + vcon)/v0 × 100%, where v0 is initial volume of toluene deliberately poured into the water surface and vcon is the volume of the concentrated oil. All the experiments were conducted at room temperature.

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RESULTS AND DISCUSSION The surface topographical images of the initial copper mesh, copper mesh after thermal treatment (marked as CuO mesh hereafter), and PA modified CuO mesh are examined by FESEM. As shown in Figure 1a, the wire diameter of the initial mesh is about 45 μm and the pore size is about 74 μm. After thermal treatment, the average wire diameter increases lightly and the pore size decreases accordingly (Figure 1d). In addition, the wire surface of initial copper mesh is smooth even from the magnified images (Figure 1b,c). However, after oxidized, the copper wire substrates indicated the formation of cauliflower-like microstructures with diameters of about 5−10 μm (Figure 1e). Movever, it is seen from the maginified SEM image (Figure 1f) that the size of the CuO microcrystal ranges from 200 to 500 nm, indicating the submicrometer structures of the oxidized wire surface. The FE-SEM images of PA modified CuO mesh are shown in Figure 1g−i. Comparing Figure 1g−i with Figure 1d−f, the surface microstructures of CuO mesh remain nearly unchanged before and after PA treatment. Hence, micro/submicro binary rough structures are constructed on the surface of PA modified CuO mesh, which could have signigicant influence on its wetting behavior.12,18 The composition and phase purity of the as-received Cu mesh and its oxidized product were examined by X-ray diffraction analysis. As shown in Figure 2a, the diffraction peaks at 2θ values of 43.4°, 50.3°, and 73.9° could be assigned to the cubic structure of copper (JCPDS card no. 04-0836). The characteristic peaks of CuO and Cu2O cannot be seen in the XRD patterns, demonstrating that the as-received Cu mesh

CuO + 2C16O33COOH → (C16O33COO)2 Cu + H 2O

The chemical composition of (C16O33COO)2Cu on the CuO mesh surface could be verified by FT-IR and XPS measurement. Figure 3 shows the FT-IR spectra of powders scraped

Figure 3. FT-IR spectra of powders scraped from the CuO mesh (a) before and (b) after PA treatment.

from the CuO mesh before and after PA treatment. In the spectrum of powders scraped from PA modified CuO mesh, peaks at around 1541 and 1456 cm−1 could be attributed to m(COO)asym and m(COO)sym, respectively, demonstrating the formation of palmitate.29 The surface compositions of the initial Cu mesh, CuO mesh, and CuO mesh after PA modification were also analyzed by XPS. Figure 4a shows the survey spectrum of the three mesh samples. One can see that the peak of Cu 2p at 933 eV, peak of O 1s at 530 eV, peak of C 1s at 283−284 eV, peak of Cu 3p at 74−77 eV, and peak of Cu 3s at 123 eV are observed in the XPS survey spetrum.30 The O 1s peak of the Cu mesh is attributed to the oxidation of the sample, and the C 1s peaks of Cu mesh and CuO mesh are attributed to the air contamination on the sample surfaces, respectively.30,31 In addition, the intensity of the C 1s peak of the PA modified CuO mesh is significantly higher than both of the other two samples, which is attributed to the chemical bonded copper stearate on the CuO mesh surface. Figure 4b shows the high-resolution XPS scan spectrum of Cu 2p region of these samples. The sample of Cu mesh shows the peak of Cu 2p1/2 at 952.2 and Cu 2p3/2 and 932.4 eV can be regarded as Cu0.32 The peaks at 953.4−953.5 and 933.4−933.6 eV of the CuO mesh and the PA modified CuO mesh are attributed to

Figure 2. XRD patterns of as-received copper mesh (a) and the oxidized copper mesh (b). 6346

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Figure 4. XPS spectra of Cu mesh, CuO mesh, and CuO mesh modified with PA: (a) survey spectrum; (b) scan spectrum of the Cu 2p region.

Cu 2p1/2 and Cu 2p3/2 of Cu2+,33,34 respectively, which demonstrates the surface composition of CuO. The results of FT-IR and XPS measurement confirmed the successful modification of PA on CuO mesh surface. The outmost layer of (C16O33COO)2Cu on the CuO mesh surface after PA modification can be further verified by contact angle measurement. Figure 5 shows a series of images of

an apparent CA less than 10° (inset of Figure 5c) and demonstrating superhydrophilicity. Importantly, the superhydrophilicity of the CuO mesh transited to superhydrophobicity after PA treatment, exhibiting a WCA of 163.3° and a WSA of about 7.3°. This observation can be explained by eq 1 derived by Cassie and Baxter which could be used to understand the superhydrophobicity of the mesh surface.12,13,35 cos θr = f1 cos θ − f2

(1)

In eq 1, θ (=120°) is the intrinsic contact angle of a water droplet on a smooth surface of PA, θr (=163.3°) is the actual contact angle of a water droplet on a rough surface of PA, and f1 and f 2 are the area fractions of water droplet in contact with the PA modified CuO mesh surface and air, respectively ( f1 + f 2 = 1). From eq 1, we can easily deduce that the θr increases with the increasing of f 2. According to eq 1, the value of f 2 is calculated to be 0.92, demonstrating that 92% surface area of the PA modified mesh was covered by air, and only 8% surface area made contact with water. Apart form the static wetting behavior (reflected by static contact angle), the dynamic wetting behavior (reflected by contact angle hysteresis and sliding angle) is another important parameter in oil−water separation process.36 The PA modified CuO mesh surface shows a contact angle hysteresis (CAH) of about 9° (CAH = θadv of 164 ± 2° − θrec of 155 ± 2°) and a sliding angle of about 7.3°. Water droplet rolls off the surface rapidly without any residues, which ensures that the obtained superhydrophobic mesh only absorbs oils while repelling water completely. The oil absorption properties of the as-prepared superhydrophobic CuO mesh from water surface were investigated, and the absorption process was presented in Figure 6. Colored toluene droplets were dropped onto the surface of water, and then a sheet of PA modified CuO mesh was closed to the toluene droplet. The toluene droplet was absorbed in the pores of the superhydrophobic mesh immediatedly (Figure 6c). After the absorption, no toluene could be observed on the water surface (Figure 6d). The total absorption time is less than 1 s. The result indicated that the superhydrophobic CuO mesh could be used as a good absorbent material for the selective absorbption of oil from a water surface. The PA modified CuO mesh could absorb 30% of its own weight of toluene from water surface. However, this absorption capacitor is too low to resolve practical oil spill accidents. In

Figure 5. Optical images of (a) toluene (dyed with SBB for clear observation) and water droplets on the surface of the as-received Cu mesh, and the inset shows the WCA; (b) water droplets stick to the surface of Cu mesh when it is vertically placed; (c) droplets of toluene and water on the surface of CuO mesh, and the insets show corresponding contact angles; and (d) droplets of toluene and water on the surface of PA modified CuO mesh, and the insets show the WCA and WSA. Scale bar = 1 cm.

toluene and water droplets on the surface of Cu mesh, CuO mesh, and PA modified CuO mesh. All of the three kinds of mesh show similar wetting behavior with toluene, while exhibiting different wettability with water. As seen in Figure 5a, a toluene droplet first spread on the surface of the Cu mesh and then permeated the mesh as the toluene was dropwise added onto the surface of the mesh. Similar phenomena were also observed on both the surfaces of CuO mesh and PA modified CuO mesh, showing superoleophilicity of these mesh surfaces. Additionally, the initial Cu mesh is hydrophobic, with a WCA of about 131° (inset of Figure 5a), and the water droplets adhere to the surface even when the mesh is placed vertically (Figure 5b). However, after it was oxidized, the water droplet is absorbed into the mesh via capillary action, showing 6347

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(bottom) also increased. If the actual water pressure on the device is higher than the maximum water pressure of the device, not only oils but also water would penetrate into the device, which results in the remixing of the separating oil and the water. The maximum water pressure that the superhydrophobic mesh could bear was also measured using a homemade testing system (Supporting Information, Figure S1). The miniature box could bear a water pressure about 18.4 cm water column, showing excellent water-repellent properties.37 This guarantees the complete separation of oils from water. Figure 8 shows the concentration and collection process of a toluene−water mixture by using the as-prepared miniature

Figure 6. Optical images of oil absorption process: (a) colored toluene droplets were dropped onto the surface of water; (b) a sheet of PA modified CuO mesh with a size of 3.5 cm × 4 cm was closed the toluene droplet; (c) the toluene droplet was automatically absorbed in the pores of the superhydrophobic mesh; (d) after the absorption, no obvious toluene could be observed on the water surface. Scale bar = 1 cm.

addition, the absorbed oil in the pores of the mesh could not be easily collected and then be reused. Therefore, developing a new oil recovery approach to recover the absorbed oil, especially the method for in-situ recovery of oils from water surfaces, is still necessary. In order to improve the oil separating ability of the superhydrophobic mesh, a sheet of mesh was folded into a miniature box (4.5 × 3 × 1.5 cm, Figure 7) to

Figure 8. Photographs of (a) a 10 mL toluene colored with Sudan Black B, (b−e) process of separating toluene from water using the as prepared miniature box, and (f) the collected toluene was 8.4 mL. Scale bar = 2 cm.

device. A total of 10 mL of toluene (Figure 8a) and 50 mL of water were mixed in a culture dish to simulating the spilled oil. The toluene floated on the water surface immediately (Figure 8b). Then, the box was held to make contact with the toluene. It was observed that toluene was soon absorbed into the pores of the box, then permeated the pores (Figure 8c), and was finally contained in the miniature box (Figure 8d,e) because of capillary force and gravity. The separation process is not more than 2 min. It should be noted that as toluene gradually gathered in the box, the draft of the box became deeper and deeper due to the gravity. However, only toluene would penetrate into the box while water was excluded completely even if the upper edges of the box approached the water surface. The excellent properties of oil−water separation of the miniature box were attributed to the excellent superoleophilicity and superhydrophobicity of the miniature box and its superior tolerance of water pressure (see Figure 7b). More importantly, the contained toluene could be collected facilely with a dropper or a pump. Figure 8f shows the collected toluene volume is 8.4 mL, with the addition of the absorbed oil by the device (1.2 mL), we obtained the separation ability of the miniature device for toluene is 96%. The rest of toluene could be attributed to the residual (see Figure 8e) and the volatilization of toluene. In addition, the initial volume of toluene is only 10 mL, which is lower than the volume of the miniature device [volume = 14 mL (3.5 cm × 4 cm × 1 cm)]. In this case, all of the toluene could be contained in the box (Figure 8e). When the initial volume of toluene is larger than

Figure 7. Photographs of (a) a superhydrophobic CuO mesh box floating on water surface and (b) the superhydrophobic CuO box was immersed in water until the upper edge of the box was closed to water surface. Scale bar = 1 cm in (a) and (b).

create an oil containment boom. Figure 7a shows the superhydrophobic miniature box floats freely on water surface. Figure 7b shows the miniature box was immersed in water until its upper edges are closed to (even slightly below) water surface, which means that the water cannot penetrate into the mesh under this water pressure.37 In fact, the tolerance of water pressure for a superhydrophobic miniature box is a key factor for oil−water separation application. The device gradually sunk with the toluene gradually concentrated into the device due to gravity. Consequently, the water pressure on the device 6348

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Figure 9. Effect of oil type and oil−water absorption cycle on (a) the absorption capacity and (b) the separation ability.

14 mL, which is larger than the volume of the device, only 14 mL of toluene can be immediately separated from water surface and contained in the box. However, with the toluene gradually taken out from the box, the rest of toluene automatically and continuously infiltrated into the box via capillary action and gravitational effects. Moreover, the oil separation ability is independent of the initial volume of toluene. Figure S2a (see Supporting Information) shows a photograph of the miniature box with toluene absorbed in its walls and bottom. The absorbed toluene in the miniature box could be completely excluded into acetone. The weight of the initial box was 1.4732 g. After the device was used for 20 cycles in oil−water separation for various oils, the device was cleaned in acetone, dried, and then weighed. The weight of the box was 1.4729 g for toluene, 1.4735 g for gasoline, 1.4728 for diesel oil, and 1.4733 g for kerosene, which was nearly unchanged and demonstrated its stability. In addition, the miniature box restored its original appearance, and its superhydrophobicity also remained unchanged (Figure S2b,c). The above results show that the miniature box can be used in a new oil absorption and separation cycle. After the toluene on water surface was successfully separated and then collected by using the miniature superhydrophobic device, other types of commercially available industrial oils (i.e., kerosene, diesel oil, and gasoline) were also investigated. The absorption capacities (ka) and the separation abilities (ks) of the miniature device for different types of oils are shown in Figure 9. As seen in Figure 9a, the ka of the miniature device for different types of oils were all in the range of 20−30 wt %, which was mainly dependent on the densities of the oils. In addition, one can see from Figure 9b that the ks for various oils are all more than 90%. Another important property of the miniature device is that it could be used repeatedly. After being cleaned in acetone and dried in room temperature, the miniature device was reused. It was found that even after 20 times of oil−water separations, the miniature device always exhibited the oil separation ability higher than 20 wt % and the oil separation ability higher than 90% after each separation cycle, which demonstrated its reproducibility. The oil−water absorption and separation experiments were also conducted in NaCl aqueous solutions (3.5 wt %, artificial seawater). It was found that the miniature box worked equal well in the salt water, indicating that the as-prepared device can be used for oil spill cleanup in case of an oil spill accident.

The corrosion resistance of the miniature device is also an important factor for its practical applications. If the miniature device is used in the marine environment, the seawater might corrode the copper mesh and the CuO film or destroy the out layer of (C16O33COO)2Cu. As a result, the superhydrophobicity and superoleophilicity could disappear, leading to the loss of oil−water separation ability. Tafel plots measurement was used to investigate the corrosion resistance of the samples.38 The initial Cu mesh sample was also used as a comparison (see the Supporting Information Figure S3). As shown in Figure S3, one can see that the corrosion current density (Icorr) of the initial Cu mesh is 4.49 × 10−5 A/cm2 and the corrosion potential (Ecorr) is −0.4608 mV. After oxidation and modified with PA, the Icorr is reduced by nearly 2 orders of magnitude to 5.11 × 10−7 A/cm2, and Ecorr increases to −0.4386 V. It is reported in the literature that lower corrosion current density means lower corrosion dynamic rate and higher corrosion potential denotes a lower corrosion thermodynamical tendency.39,40 The enhanced corrosion resistance of the PA modified CuO mesh could be attributed to the following two factors. On one hand, as discussed in the previous section, only 8% surface area of the mesh made contact directly with water, while 92% surface area of the mesh was covered by air. Therefore, the corrosive solution could hardly approach to the mesh surface because of the obstruction of air pockets. On the other hand, the outmost film of copper stearate could also provide a protection of the CuO and Cu surface from corrosion.41

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CONCLUSIONS We have developed a novel functional miniature device based on a superhydrophobic and superoleophilic mesh for the potential application in the field of separating oil from the water surface. The miniature device was prepared via a simple twostep thermal oxidation followed by surface modification process. The miniature device could in situ adsorb, contain, and collect a variety of oils from the water surface via capillary action and gravitational effects. In addition, the superhydrophobic miniature device could be repeatedly used in the separating process without using any complicated regeneration process while still keeping high separation abilities, which shows its good reproducibility. Moreover, high water resistance (about 18.4 cm) and enhanced corrosion resistance in artificial seawater may also provide strong guarantees for the practical application of our device. Therefore, this work will provide a 6349

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new strategy for the facile collection of spilled oil and the leakage of other organic chemicals with low surface tension from the water surface.

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ASSOCIATED CONTENT

S Supporting Information *

Water pressure resistance measurement of the PA modified CuO surface; photographs of the miniature box absorbed with toluene, the same miniature box restored its original appearance after desorption of toluene; polarization curves of the initial Cu mesh and PA modified CuO mesh. 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]; Tel (Fax) +86 791 86453210 (W.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the International S&T Cooperation Program of China (Grant No. 2012DFA51200).

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REFERENCES

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