In Situ Separation and Collection of Oil from Water Surface via a Novel

Jan 24, 2014 - (1, 2) A typical example is the blowout of a Deepwater Horizon oil well in the Gulf of ... for the separation of oil and water was repo...
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In Situ Separation and Collection of Oil from Water Surface via a Novel Superoleophilic and Superhydrophobic Oil Containment Boom Fajun Wang, Sheng Lei, Mingshan Xue, Junfei Ou, and Wen Li* School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, P. R. China S Supporting Information *

ABSTRACT: We have prepared a porous, superoleophilic and superhydrophobic miniature oil containment boom (MOCB) for the in situ separation and collection of oils from the surface of water. The MOCB was fabricated by a one-step electrodepositing of Cu2O film on Cu mesh surface without using low surface energy materials. Oils on water surface could be fast contained in the MOCB while water was completely repelled out of the MOCB, thus achieving the separation of oil from water surface. In addition, the contained oil in the MOCB could be in situ collected easily by a dropper, thus achieving the collection of oil. Moreover, the MOCB could be reused for many times in the oil−water separating process with large separation abilities more than 90%. The MOCB also possessed excellent water pressure resistance for about 164 mm water column and good corrosion resistance in simulating seawater. Therefore, the findings in the present study might offer a simple, fast, and low-cost method for the in situ separation and collection of oil spills on seawater surface.

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INTRODUCTION Recently, the pollution problems of environment and the ecosystem caused by marine oil spills have aroused a great deal of attentions.1,2 A typical example is the blowout of a Deepwater Horizon oil well in the Gulf of Mexico, which lasted for nearly 90 days and released more than 5 million barrels of crude oil to the marine and aquatic ecosystems.1 To combat the pollution of oil spills, a variety of measures have been used to clean oil from the water surface, including the use of oil skimmers,3 oil containment booms,4 in situ combustion,5 dispersants,6 biodegradation,7 and absorbent materials.8−15 Among them, the use of absorbent materials is considered a most desirable choice for the removal of oil from the surface of water.1 The traditional absorbent materials used in oil−water separation include activated carbon,8 exfoliated graphite,9,10 zeolites,11 organoclays,12,13 straw,14 wool fibers,15 etc. Although these absorbent materials have been applied in both research areas and practical applications due to their low cost and readily availability, they still have lots of limitations. For example, activated carbons are cheap and readily available from many companies; they have the problems such as pore clogging and high regeneration temperatures (800−850 °C). As a result, the recoverability and reusability of the activated carbons after its saturation with oil are difficult and means relatively high costs.16−18 Exfoliated graphite (EG) has been used as an absorbent material for the removal of oil spills due to its low density, environmental friendliness, and high porous structure. However, the absorption capacities of EG decrease apparently in the reused cycles, which limits its practical application.9,10 In addition, these absorbent materials are not superhydrophobic. © 2014 American Chemical Society

As a result, they can absorb both oil and water during the oil− water separation process, which might greatly decrease their separation selectivity and the final absorption capacity.19 Consequently, developing advanced oil−water separation technologies and materials which can selectively absorb oil while absolutely repel water are highly desirable. Recently, porous superhydrophobic materials have been demonstrated to realize a fast separation and absorption of oils due to their superoleophilicity and superhydrophobicity.20−23 The first example using the superhydrophobic and superoleophilic material for the separation of oil and water was reported by Jiang and co-workers.13 A coating mesh film with both superhydrophobic and superoleophilic properties was prepared by a spray-and-dry method. The water droplet was spontaneously rolling off from the mesh surface, while a diesel oil droplet permeated from the pores of the mesh thoroughly for a period less than 1 s, thus achieving the separation of water and oil.13 Based on this principle, numerous metallic meshes whose surfaces possess both superhydrophobicity and superoleophilicity have been prepared as filters for the purpose of oil−water separation. However, although these materials can separate oil and water from a mixture of water and oil, they cannot separate the spilled oil because the large volumes of oils on water surface must be first collected and then be poured into the mesh filter.23 Recently developed porous sponge systems, for example, carbon nanotube sponge,24 graphene-based sponge,25 and polyurethane sponge,26,27 which exhibit simultaReceived: October 8, 2013 Revised: January 1, 2014 Published: January 24, 2014 1281

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Figure 1. FESEM images of the surfaces of stainless steel mesh after different electrodeposition times: (a, b) 0 min; (c, d) 5 min; (e, f) 10 min; (g, h) 20 min; (i, j) 40 min; (k, l) 60 min. The insets in (a), (c), (e), (g), (i), and (k) show the optical images of water CA, and the insets in (d), (f), (h), (j), and (l) show the optical images of water SA.

neous superhydrophobicity and superoleophilicity have received considerable attention in oil−water separation application. Although they present high absorption capacity and recyclability, they were fabricated using expensive row materials and complicated synthesis procedures, limiting their practical applications. Cuprous oxide (Cu2O) films have generated extensive academic interest due to its potential applications in photovoltaic devices,28 catalysis,29 lithium-ion batteries,30 and so on. Over large area substrates Cu2O thin films can be prepared by various techniques, for example, chemical vapor deposition,31 thermal oxidation,32 and electrodeposition.33 Among the above methods, electrodeposition possesses many advantages such as simple, low cost, easy control, and large scale. However, the superhydrophobic properties of the surfaces based on Cu2O film have seldom been reported. In a representative study, Chang et al. prepared the superhydrophilic CuO film from the Cu wafer by thermal oxidation.34 The surface showed superhydrophobicity after being annealed in air at 100 °C for about 6 h due to the partial deoxidation of the outmost layer of CuO surfaces into Cu2O-like hydrophobic surfaces. However, the annealing method is very sensitive to temperature and time, resulting in bad repeatability in fabrication of superhydrophobic Cu2O surfaces. In this work, we fabricated a novel miniature oil containment boom (MOCB) that possesses porosity, superhydrophobicity, and superoleophilicity simultaneously. First, Cu2O film coated stainless steel (SS) mesh superhydrophobic surface was prepared by a one-step electrodeposition process without any surface modification. Then, the MOCB was fabricated by enfolding a sheet of superhydrophobic and superoleophilic mesh into a miniature box. The as-prepared MOCB floated freely on water surface. Once it made contact with oil that floated on water surface, the oil infiltrated through the walls of the MOCB automatically and finally gathered into the MOCB, while water was completely excluded outside the MOCB, thus achieving oil−water separation. In addition, the as-prepared

MOCB could selectively contain various kinds of oils in the MOCB while repelling water completely. The contained oil in the MOCB could be fast collected by a dropper. More importantly, the MOCB also showed high oil separation ability after 20 separation cycles. Therefore, the result offers a facile and low-cost method for the separation and collection of spilled oil from the water surface.

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EXPERIMENTAL SECTION

The 304 SS meshes were purchased from Hebei Anhua Hardware Product Co., Ltd., China. Copper sulfate pentahydrate (CuSO4· 5H2O), lactic acid (LA), hydrochloric acid (HCl), sodium hydroxide (NaOH), ethanol, and acetone were purchased from Shanghai Chemical Reagent Co., Ltd., China. All chemical reagents were analytical grade and used without further purification. Electrochemical deposition was performed in an electrochemical workstation (dc stabilized power supply LW10J5, Shanghai Liyou Electrification Co., Ltd., China) using a two-electrode system. Electrodeposition for the growth of Cu2O film was carried out on a SS mesh of 4.0 cm × 3.5 cm in a potentiostatic mode at room temperature. The mesh was cleaned thoroughly by 0.1 M HCl solution, acetone, deionized water, and ethanol for 2 min, respectively.Then it was immersed in a solution composed of 0.05 M CuSO4 and 0.8 M LA as chelating agent to stabilize the Cu2+ ion. The solution was adjusted to pH 8.7 with NaOH detected by a basic pH meter (FE20, Shanghai METTLER TOLEDO Apparatus Co., Ltd., China). The electrodeposition was carried out at 50 °C and 3 V in the two electrodes using a graphite sheet as the counter electrode. After deposition, the samples were rinsed with deionized water and alcohol, respectively, and then dried under atmosphere at 60 °C for 3 h. The structure and phase composition of the films were identified by X-ray diffraction (XRD, Bruker SMART APEX II, Germany) with a Cu Kα radiation source. The surface microstructures of the samples were measured by a NanoSEM-450 field emission scanning electron microscope (FESEM, FEI-Nova, America). The surface chemical compositions of the mesh samples were measured by a PHI-5702X-ray photoelectron spectroscopy (XPS). Water contact angles (CAs) and sliding angles (SAs) were measured with 9 μL of water using a Krüss 1282

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DSA 20 apparatus. The average value of five measurements was used as the final reported CA. The SAs were measured on a tilting stage. A sheet of superhydrophobic Cu2O mesh was enfolded into a miniature box [i.e., miniature oil containment boom (MOCB)] of 2.0 cm × 2.0 cm × 0.7 cm in size and was used for the measurement of oil separation ability. The oils used in the separation experiments were toluene, gasoline, diesel oil, kerosene, and motor oil. The oil separation ability was determined by volume measurements. A certain amount of toluene, v0, dyed with Sudan Black B was added on the surface of water (volume is 50 mL) in a beaker. Then the superhydrophobic MOCB was placed on the surface of the water containing oil. Oil was absorbed in the walls and bottom of the MOCB and then infiltrated in the MOCB rapidly. Oil in the box was collected by a dropper, and the volume of the contained oil, vcon, was measured. The oil separation ability of the superhydrophobic MOCB was calculated by the formula ks = vcon/v0 × 100%. After oil absorption, the MOCB was washed with acetone for three times and then dried by using a blow drier. After being cooled to room temperature, the MOCB was used for the recycled absorption experiments. The oil−water separation experiments were carried out by using 3.5 wt % NaCl aqueous solutions instead of water for the purpose of simulating seawater. Electrochemical corrosion behavior was performed using a computer-controlled workstation (CHI660C, CH Instruments, China). The aqueous solution of NaCl (3.5 wt %), platinum stick electrode, and saturated calomel electrode were used as electrolyte, counter electrode, and reference electrode, respectively. For polarization curves measurements, the potential was scanned at a rate of 10 mV s−1 at ambient temperature. The average value of three measurements of different samples was used as the final reported value.

Figure 2. Water CA and SA on the surface of SS mesh as a function of the electrodeposition time.

(10 min), 161.8 ± 2.1° (20 min), 163.4 ± 2.5° (40 min), and finally 163.5 ± 1.7° (60 min). While the SA on the surfaces decreases rapidly from none (5 min) to 71.5 ± 4.9° (10 min) and then to 6.8 ± 2.2° (20 min) at the beginning of the deposition process (5−20 min) and then decreases slowly from 3.9 ± 1.2° (40 min) to 2.7 ± 0.9° (60 min) at prolonged deposition time (Figure 1 insets and Figure 2). It should be noted that the SS surface possesses superhydrophobicity after the one-step electrodeposition process for only 20 min, with CA > 150° and SA < 10°. The as-prepared superhydrophobic mesh surface obtained after the deposition time of 20 min was chosen for further investigation due to its relatively low processing time. The adhesive property between Cu2O film and the SS mesh substrate was investigated by ultrasonication (500 W, 40 kHz) in water and toluene for 2 h, respectively, and then the water CAs of the resulting MOCB were measured.27 The variations of both the CA and SA of the MOCB are insignificant after ultrasonication in water and toluene for 2 h, indicating excellent adhesive property between Cu2O film and SS substrate. The crystal structure of the as-deposited film on the SS mesh was characterized by XRD measurements. The electrodeposition and growth of flower-like Cu2O film according to the following reactions:35

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RESULTS AND DISCUSSION Figure 1 shows the FE-SEM images of the SS mesh after different electrodeposition times. One can see that the surface morphologies varied apparently with the increasing of deposition time. The deposition processes have been carefully monitored. For the bare SS mesh, the wire surface is very smooth even at the magnified image (Figure 1b). When the deposition time is short (5 min), the surface of SS wire is mainly coated by uniform nanoparticles with about 200 nm in diameter and the surface becomes rough (Figure 1d). Increasing the deposition time to 10 min, the nanoparticles deposited on the wire surface become denser and the surface gets rougher (Figure 1f). Increasing the deposition time to 20 min, the nanoparticles agglomerate together and cauliflowerlike hierarchical structures with a typical diameter of about 3 μm were formed on the wire surface (Figure 1g,h). Further increasing the deposition time to 40 min, the size and density of the microflowers increase remarkably (Figure 1i). A microflower with a typical size of about 15 μm is composed by several smaller microflowers with diameters ranging from 3 to 8 μm (Figure 1j). For sample with deposition time close to 60 min, the pore sizes decrease obviously (Figure 1k0, and the wire surface is fully covered with blooming microflowers. In addition, it is observed that there are many pyramidal crystallites grow on the surface of microflowers, and the surface is pretty rough (Figure 1l). The electrodeposition time have significant influences on the surface microstructure and the wetting ability. The variations of the water CA and SA on the surface of SS mesh after different deposition time are shown in Figure 1 insets and Figure 2. The initial SS mesh has a very low CA of about 26.4 ± 3.4° and no SA (SA cannot be reported since the water droplets stick always to the surfaces independently from the tilting angles and completely unable to roll). As the deposition time gradually increases from 5 to 60 min, the CA on the as-prepared SS mesh surfaces increases from 127.4 ± 3.7° (5 min), to 138.5 ± 3.2°

Cu 2 + + e− → Cu+ −

2H 2O + 2e → H 2 + 2OH

(1) −

2Cu 2 + + 2e− + 2OH− → Cu 2O + H 2O

(2) (3)

The electrodeposition of Cu2O film on SS mesh surface can be divided into two steps. The Cu2+ ions were first reduced to Cu+ ions (see eq 1), and then the Cu+ ions were precipitated to form Cu2O film on the mesh surface due to the limitation of solubility of the Cu+ ions (see eq 3).36 In addition, the reduction of water would also occur (see eq 2). Because the Cu2O film could dissolve in acid, the electrolyte was adjusted to alkaline by using NaOH. Figure 3b shows that the marked peaks with 2θ values of 42.4°, 73.6°, and 77.4° correspond to (200), (311), and (222) planes of the crystalline Cu2O. All the diffraction peaks are consistent well with the face-centered cubic (FCC) symmetry of Cu2O (JCPDS, 05-0667) structure, indicating the product is pure FCC Cu2O.37,38 The peaks of Figure 3a, except for those marked with diamonds, can be ascribed to the stainless steel mesh substrate, just as Figure 3a. 1283

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between the water droplet and the mesh surface is so small and can be neglected, demonstrating the no-adhesive property of the as-prepared Cu2O film coated mesh surface. The measured CA on Cu2O film coated mesh surface can be interpreted by the model presented by Cassie and Baxter:45 cos θr = f1 cos θ − f2

(4)

where θr is the CA on a rough surface, θ is the intrinsic CA on a corresponding smooth surface, f1 and f 2 are the area fractions of liquid droplet makes contact with the solid surface and air, respectively, and f1 + f 2 = 1. In the present case, the water CA on the as-prepared superhydrophobic surface is 161.8 ± 2.1° and SA is 6.8 ± 2.2°. This equation indicates the larger f 2 represents a larger CA. It was reported that the Cu2O surface was hydrophobic, and water CA was approximately 110° at a surface with roughness about 10 nm.34 Because of the difficulty in fabrication of an absolutely smooth Cu2O surface, the CA value (θ = 110°) of the above relatively flatter Cu2O surface is used as a reference. Substituting the values of θr and θ into eq 4, we find the value of f 2 for the mesh surface is about 0.924, which means that about 92.4% of the rough surface area is covered by the air, and only 7.6% is in contact directly with water. It should be mentioned that the roughness of the Cu2O film coated SS mesh surface includes not only the roughness of Cu2O film (cauliflower-like hierarchical structures on the wire surface, see Figure 1g) but also the roughness of the mesh (including pores and wires, the pore seze is about 75 μm and the wire diameter is about 50 μm, see Figure 1h). In addition, the material of Cu2O is intrinsic hydrophobic.57 Therefore, in the present case, the binary rough structures on the mesh surface combined with the hydrophobic feature of Cu2O material gives rise to superhydrophobicity. The electrodeposited Cu2O film could be extended to other conducting mesh substrates, for example, Cu mesh. The variations of surface morphologies with the deposition time were investigated under the same deposition condition (see Supporting Information, Figure S1). It is observed that ball-like particles and particle aggregations with typical size ranging from 2 to 5 μm are formed on the wire surface in the magnified SEM images (Figure S1d,f,h,j,d). In addition, the growth of Cu2O film on Cu mesh is faster when compared with the growth on SS mesh. After the electrodeposition process for only 10 min, the surface exhibits superhydrophobicity (water CA = 164.3 ± 3.2° and SA = 5.8 ± 1.2°, Figure S1g,h). The hydrophobicity of Cu2O and hydrophilicity of CuO have been reported in the literature.34,46,47 It is well-known that most

Figure 3. XRD patterns for the stainless steel mesh (a) before electrodeposition and (b) after the electrodeposition time of 20 min.

Figure 4a is the XPS survey spectrum of the SS mesh surface after 20 min of electrodeposition. One can see that the elements of carbon, oxygen, and copper were detected by XPS measurement. The peaks of Cu 3p at 75.6 eV, Cu 3s at 125.1 eV, and Cu 2p doublet (Cu 2p3/2 and Cu 2p1/2) at 931−952 eV could be clearly observed. The O 1s at 532 eV corresponding to the metal oxide of Cu2O phase and the C 1s peak at 284.2 eV can be ascribed to the contamination of C form air contact.39 Figure 4b shows the high-resolution XPS spectra for Cu 2p. Two strong peaks centered at 931.7 and 951.7 eV, corresponding to the Cu+ double peaks of Cu 2p3/2 and Cu 2p1/2, are detected, which could confirm the main composition of the mesh surface is Cu2O.40,41 Moreover, there are two shakeup satellite peaks at the binding energy of 942.9 and 946.0 eV, respectively. The relatively lower peaks indicate the presence of a small amount of CuO on the sample surface.42−44 The above results also indicates that the successful electrochemical deposition Cu2O on the stainless steel mesh. To explain the variations of hydrophobicity of the stainless steel mesh with different electrodeposition time, the picture of CA and SA Vs different deposition time are shown in the Figure 1 insets. As the deposition time exceeding 20 min, a water droplet with volume of 4 μL cannot be placed on the mesh surface easily (Figure 1g,h) and spontaneously rolls off even when the surface is not inclined apparently. Gradually increasing the volume of water droplet to 9 μL, it drops from the syringe tip to the mesh surface due to gravity. The water drop could roll off from the mesh surface when the surface is slightly tilted (Figure 1i−l). Therefore, the adhesion force

Figure 4. XPS survey spectrum of as-prepared Cu2O surface (a) and high-resolution spectra of Cu 2p region (b). 1284

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which can be interpreted by the partially oxidation of Cu2O into CuO. The stability of the as-prepared mesh surface was also evaluated by storing the sample at ambient temperature, and then the water CA and SA were measured every 5 days. As shown in Figure 5b, water CAs and SAs remain nearly unchanged even after 30 days, with CA > 160° and SA < 4°, which demonstrated the long-term stability of its superhydrophobicity. The adhesive property between Cu2O and stainless steel of the superhydrophobic MOCB was investigated by ultrasonication (100 W, 40 kHz) in water and toluene for 2 h, respectively, and then the water CA and SA of the resulting MOCB are measured.27,47 No obvious changes of CA and SA of the sample can be observed after ultrasonic treatment in water and toluene for 2 h, indicating the adhesive force between Cu2O film and SS substrate is excellent. More interesting, we find that the as-prepared superhydrophobic Cu2O mesh has excellent superoleophilic and superhydrophobic properties. Figure 6 shows a MOCB with

of the metal oxides (including CuO) are hydrophilic due to their surface unsaturated metal and oxygen atoms that act as Lewis acid and base sites.48−51 There are two kinds of interactions between metal oxide substrate and surface water. One is between metal (metal oxide substrate) and O (surface H2O), and the other is between O (metal oxide substrate) and H (surface H2O).48,49,51 Moreover, the interaction between metal (metal oxide substrate) and O (surface H2O) is much more stable. There will also be hydrogen bonding network among water molecules. Water molecules wet a surface because they interact strongly with the substrate than among themselves and could not wet a surface because they interact weakly with the substrate than among themselves.52 According to the hard and soft acids and bases (HSAB) theory, Cu+ is a soft Lewis acid and H2O is a hard Lewis base; therefore, interactions between Cu2O and water are predicted to be weak.53 As a result, water molecules would not be bonded to the Cu2O surface but be bonded to each other. Therefore, Cu2O surface is hydrophobic. While Cu2+ is a borderline Lewis acid, its interaction with water (hard base) is predicted to be significantly higher than Cu+ (soft acid).54 For example, Cu2+ can form stable complex with water, i.e. [Cu(H2O)4]2+, while Cu+ cannot. Hence, water molecules interact strongly with CuO than among themselves. As a result, water molecules wet the CuO surface and the CuO surface is hydrophilic. The influence of temperature variation on the wettability of the superhydrophobic Cu2O mesh film was studied to assess the temperature stability of such a surface. The samples were treated at different temperatures for 1 h; after naturally cooled/ heated to room temperature, the water CA and SA were measured. Figure 5a shows that the temperature varies from

Figure 6. Image of a water droplet sits on the superhydrophobic surface of the MOCB and a toluene droplet diffuses in the pores of the same surface. The insets show the optical images of the CA of toluene (top) and water (bottom, dyed with methylene blue for a clear observation).

size of 2 cm × 2 cm × 0.7 cm, which was enfolded from a sheet of the Cu2O film coated superhydrophobic mesh (3.4 cm × 3.4 cm). We can see that a water droplet forms ball-like shape on the surface of the MOCB, but a drop of toluene diffuses rapidly in the pores of the same surface. The image shows the CA for water is about 162° (see the bottom inset) and for toluene is only 0° (see the top inset), which demonstrates the potential application in oil−water separation. There are chances that the Cu2O coatings peeled off from the MOCB due to mechanical bending and cause the leaking of water into the box. Therefore, the water resistance of the MOCB was examined by immersing the MOCB into water until its up edges close to water surface (see Figure S2). It is clearly observed that no water penetrates into the MOCB from the pores of the folding edges and the bottom of the MOCB. In addition, the Cu2O coating at the folding edges of MOCB was also observed by SEM Figure S3a shows the water droplets sits on the folding edges of the MOCB, demonstrating that the edges are still superhydrophobic after folding (see Figure S3 and Movie 1). Figures S3b−d

Figure 5. (a) Temperature and (b) time dependences of water CA and SA on the superhydrophobic Cu2O mesh surface.

−10 to 250 °C has negligible influence on both the CA and SA of the sample. Consequently, the water CAs of the mesh are always larger than 150° and SAs are lower than 10° after treated at various temperatures. However, after thermal treated at 350 °C, the CA of the surface decreases sharply from above 150° to about 123.7°, and at the same the SA increases sharply from about 6.5° to 180°. (The value of SA = 180° is not the true value of SA but only used as a comparison.) Meanwhile, the surface color of the stainless steel mesh changed from brick red into black thoroughly. The Cu2O film coated mesh surface loses its superhydrophobicity after heated at 300 °C for 1 h, 1285

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and the obtained toluene was about 2.9 mL, indicating a separating yield of the miniature box is 96.7%. Considering the experimental error, the residual amount of toluene can be attributed to the absorption of toluene within the walls and bottom of the box, the evaporation of toluene in the separating process, and a small amount of the residual of toluene on the water surface (Figure 7e). The water pressure tolerance of the MOCB is a key factor to determine its practical application. As can be seen in Figure 7c, after fully contained with toluene, the upper edge of the MOCB is flush with the water surface. This means that after the MOCB is fully filled with toluene, the bottom of the MOCB would bear a water pressure for about 7 mm water column (the height of the MOCB). The water pressure on the bottom of the MOCB increases with the increase of the height. Above the maximum height of the MOCB, water will penetrate into the MOCB from the pores of the bottom due to the high water pressure, resulting in the remixing of the water and oil, i.e., the failure of oil−water separation. Jiang et al. used capillary phenomenon to calculate the maximum water pressure value of a hydrophobic miniature copper mesh boat.55 Hence, the water pressure resistance of the MOCB can be calculated as55

show FESEM images of the unfolding surface of the MOCB. One can see that the shapes of the pores are squares and the wires are straight. As a comparison, FESEM images of the folding edge of the MOCB are shown in Figure S3e−g. It could be observed that the shapes of the pores at the folding edge are irregular and the wires are bent. However, rough structures of the pores and wires still remained, and the peeling off of Cu2O coating from the mesh surface is not observed. In order to investigate the oil−water separation properties of the MOCB from water surface, pictures of removal toluene from the water surface are displayed in Figure 7a−f. Figure 7a

h=−

2γ cos θ ρgR

(5)

where γ is the surface tension of water (γ = 72.75 mN m−1), θ is the CA between water and capillary, ρ is the density of water, g is the gravitational constant, and R is the diagonal line of the rectangular mesh pores.55 In the present case, the water CA (θ) of the superhydrophobic MOCB surface is 163.5°. In additional, the values of R can be calculated from the pore size of the mesh (pore size = 75 μm, see Figure 1g), which is about 106 μm. From eq 4 we find the values of h for the M are 13.3 cm. The actual water pressure resistance of the superhydrophobic MOCB surface is also measured (Figure S4). We find the maximum high of water column (h) that the MOCB could bear is about 16.4 cm. Importantly, the MOCB could bear such a pressure for a time larger than 5 h (Figure S4b). Hence, the as-prepared superhydrophobic mesh possesses excellent water pressure resistance and durability, which ensures the practical application of the MOCB. Moreover, the as-prepared MOCB could separate various kinds of oils from the water surface. Figure 8a shows the separation abilities of the MOCB for gasoline, kerosene, diesel

Figure 7. (a−f) Process of separating toluene (dyed with Sudan Black B) from water surface by using the MOCB.

shows the bare MOCB without loading can easily float on the water surface. Figure 7b shows 3.0 mL of toluene dyed with Sudan Black B is poured into a beaker containing 50 mL of water. The toluene floats on the surface of water due to its relatively lower density. Then the MOCB is placed on the surface of the toluene−water mixture. The toluene fast penetrates into the MOCB from the pores and finally gathered by the MOCB for a period less than 1 min (Figure 7c). The toluene is contained in the MOCB quickly while water is excluded completely (Figure 7d). Interestingly, as shown in Figure 7c, the MOCB keeps floating on the water surface even if its upper edge is flush with the water surface. The maximum volume of toluene that can be contained in the MOCB is about 2.8 mL. (The volume of the MOCB is 2.8 mL.) Therefore, only 2.8 mL of toluene can be contained in the MOCB, while the rest of toluene still floats on the water surface (Figure 7d). The contained toluene in the MOCB can be in situ collected easily by a dropper. Once a small amount of toluene is removed from the MOCB and provides a vacancy, the toluene outside of the MOCB will penetrate into the MOCB simultaneously and fill in the vacancy. Therefore, separation and collection of toluene from water surface can be achieved continuously and independent of the initial volume of toluene. After the contained toluene in the MOCB is taken out by a dropper, the MOCB floats on the water surface again without any water penetrating (Figure 7e). Finally, nearly all of the toluene is removed from the water surface after we take the MOCB away (Figure 7f). The toluene is collected into a measuring cylinder

Figure 8. Separation ability of the MOCB: (a) for different oils in the first cycle; (b) for toluene after different separation cycles. 1286

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CONCLUSIONS In summary, we demonstrated a simple one-step electrodeposition process to fabricate a novel porous, superhydrophobic and superoleophilic miniature oil containment boom (MOCB) based on Cu2O film coated SS mesh and their in situ oil separation properties from the water surface. The MOCB could contain various of oils from the water surface while repels water absolutely. The contained oil could be in situ and easily collected by a dropper, and the collected oil could be reused. Particularly, the MOCB could be used in the repeated oil−water separation cycles for more than 20 times while still keeping high oil separation abilities. Moreover, the MOCB also exhibited desired properties such as excellent water pressure resistance and good corrosion resistance, which were essential for its practical application in marine oil spill cleanup. Therefore, the MOCB might have broad industry applications, such as high efficiency in situ oil−water separation device for the oil spills on the water surface or other functional Cu2Obased materials with self-cleaning properties.

oil, and motor oil. It is found that the separation abilities for different kinds of oils are all above 90% after the first separation cycle. Importantly, the MOCB can be used in the oil−water separation process repeatedly for many times, which is another important characteristic for its practical application. The collected oil (2.9 mL of toluene) is used in the next oil− water separation cycle. Therefore, the volume of toluene (v0) used in the subsequent separating cycles gradually decreased with the increasing of recycle times, while the absorbed toluene in the walls and bottom of the MOCB remains nearly unchanged. As a result, the separation ability decreases from 96.7% to about 91% after recycled used 20 times. Therefore, ignoring the influence of the evaporation of toluene and a small amount of the residual of toluene on the water surface, the decrease of separation ability can be interpreted by the absorption of oils in the walls and bottom of the mesh. As can be seen from Figure 8b, the separation ability of the MOCB shows a slight decrease with the increase of separation cycles. However, even after 20 separation cycles, the oil separation ability of the MOCB keeps high value more than 90%, exhibiting excellent reproducibility. The corrosion resistance of the superhydrophobic surface of the MOCB is another important factor to determine its practical application. In this paper, the corrosion resistance of the Cu2O film coated SS mesh surface has been investigated by Tafel plots measurement. Figure S5 shows the potentiodynamic polarization curves (see the Supporting Information) of the uncoated SS mesh and the Cu2O film coated SS mesh after immersing the samples in 3.5 wt % aqueous solution of NaCl for 1 h. For the uncoated SS mesh sample, the corrosion current density (Icorr) is 3.54 × 10−6 A/cm2. After the deposition of Cu2O film, the Icorr of the sample decreased to a low value of about 6.51 × 10−8 A/cm2, which is much lower than the uncoated SS mesh (above 2 orders of magnitude). It is generally believed that the lower the polarization current the coating has, the better corrosion resistance it possesses.56 So we conclude that the coated Cu2O film could prevent the SS mesh from being corrupt in the corrosive medium, thus increasing the corrosion resistance of the SS mesh. The improving corrosion resistance of the SS mesh can be interpreted by the water-repellent properties of the sample. As revealed by Cassie’s equation, nearly 92.4% of the surface area of the Cu2O coated SS mesh is covered by the air and only 7.6% is in contact with water. As a result, the corrosive solution cannot reach the SS mesh surface easily due to the obstruction of the “air pocket”.57 Thus, the corrosion resistance of the initial uncoated SS mesh is increased effectively. The corrosion durability of the Cu2O-coated mesh surface is also studied by immersing the samples into the 3.5 wt % aqueous solution of NaCl for different immersing time (from 1 to 10 days), and then the water CA and SA were measured at room temperature. The results are depicted in Figure S6 (see the Supporting Information). One can see that the CAs decreases and SAs increases with the increase of immerging time. However, even after immersion treatment for 7 days, the sample still exhibits its superhydrophobicy: water CAs remains high value (more than 150°) and the SAs is low (lower than 10°). In addition, the superoleophilicity of the Cu2O film coated SS mesh film also remains unchanged. The result demonstrated that the hydrophobic Cu2O film coating possesses good corrosion performance in neutral NaCl solutions.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1: FESEM images of the surfaces of Cu mesh after different electrodeposition times; Figure S2: image of the MOCB immersed into water until its up edges close to water surface; Figure S3: image of water droplets with ball-like shape sit on the folding edges of the MOCB, FESEM images of the surface at the bottom, and the folding edges of the MOCB; Figure S4: experiments of water pressure resistance of the asprepared superhydrophobic surface of Cu2O film coated SS mesh; Figure S5: potentiodynamic polarization curves of uncoated SS mesh and Cu2O film coated SS mesh; Figure S6: water CA measurements of the superhydrophobic mesh surface after imersing in 3.5 wt % aqueous solution of NaCl for different times. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel +86-791-6453210; fax +86-791-6453210; e-mail wenl@ ualberta.ca (W.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge with pleasure the financial support of this work by the National Natural Science Foundation of China (Grants 51263018, 21103084, and 21203089), International S&T Cooperation Program of China (Grant 2012DFA51200), Science and technology Supporting Plan of Jiangxi Province, Social Development Field (Grant 20122BBG70165), and Jiangxi Provincial Department of Education (Grant GJJ12424).

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REFERENCES

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