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A Dual Superlyophobic Copper Foam with Good Durability and Recyclability for High-flux, High-efficiency and Continuous Oil-Water Separation Wenting Zhou, Song Li, Yan Liu, Zhengzheng Xu, Sufeng Wei, Guo Yong Wang, Jianshe Lian, and Qing Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19853 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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ACS Applied Materials & Interfaces

Article type: Full Paper A Dual Superlyophobic Copper Foam with Good Durability and Recyclability for Highflux, High-efficiency and Continuous Oil-Water Separation

Wenting Zhou,† Song Li,† Yan Liu,‡ Zhengzheng Xu,† Sufeng Wei,§ Guoyong Wang,∗ , † Jianshe Lian, † and Qing Jiang†

†

Key Laboratory of Automobile Materials, Department of Materials Science and Engineering,

Jilin University, Changchun, 130025, PR China ‡

Key Laboratory of Bionic Engineering (Ministry of Education) and State Key Laboratory of

Automotive Simulation and Control, Jilin University, Changchun 130022, PR China §

Key Laboratory of Advanced Structural Materials, Changchun University of Technology,

Changchun, 130012, PR China

KEYWORDS: TiO2; superhydrophilic; superoleophobic; superoleophilic; superhydrophobic; oil-water separation

∗

Corresponding author. E-mail addresses: [email protected], Tel: +86 431 85095875; Fax: +86 431 85095876. 1 ACS Paragon Plus Environment

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ABSTRACT Traditional oil-water separation materials have to own ultra-high or ultra-low surface energy. Thus they can only be wetted by one of oil and water. Our experiment here demonstrates that the wettability in oil-water mixtures can be tuned by oil and water initially. Hierarchical voids are built on commercial copper foams with the help of hydrothermally synthesized titanium dioxide nanorods. The foams can be easily wetted by both oil and water. The water prewetted foams are superhydrophilic and superoleophobic under oil-water mixtures, meanwhile the oil prewetted foams are superoleophilic and superhydrophobic. In this paper, many kinds of water-oil mixtures were separated by two foams, prewetted by corresponding oil or water respectively, combining a straight tee in a high-flux, high efficiency, continuous mode. This research indicates that oil-water mixtures can be separated more eco-friendly and lower-cost.

INTRODUCTION The separation of oil-water mixtures is a hot academic topic and also extensively required in industry to solve the increasingly serious environmental pollution such as oil spill, discharge of industrial oily wastewater, marine antifouling, and oil recovery.1-7 Compared with the conventional methods based on the specific gravity difference between oil and water, such as oil skimmer, centrifuge, coalesce and floatation, an emerging method based on the specific surface energy difference between oil and water has drawn considerable attention and is thought of as more eco-friendly, high efficiency and lower-cost.2, 4, 8 It needs a surface which has ultra-opposite wettability between oil and water. That is to say, it is superhydrophobicsuperoleophilic surfaces or superhydrophilic-superoleophobic surfaces. Then, oil or water was removed from the immiscible oil-water systems by the surface as an absorbent or strainer.7, 914

For example, different kinds of superhydrophobic-superoleophilic sponges are evidenced as

satisfactory oil absorbents,15-17 and many kinds of superhydrophobic-superoleophilic coating mesh can be used as oil strainers.18

9, 19, 20

Besides the above “oil-removing” materials, 2

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superhydrophilic and underwater superoleophobic surfaces were also designed to remove water from oil lately.19, 21-29 In theory, the strainer method is continuous and should be more efficient than the absorbent method, but it hadn’t come true in practice for a long period.30 Since the strainer is gravitydriven,19, 23, 27, 29 it can only fairly smoothly work when the high specific gravity liquid in the mixtures pass through the strainer. Or it will sink down and block the separation. Even so, the lighter one will interrupt the separation later as it is blocked by the strainer. So the conventional single strainer oil-water separation devices also run in an intermittent mode to dump the intercepted liquid, which would inevitably depress the rate. Such drawback can only be circumvented by a straight tee onto two outlets of which two strainers with opposite wettability are fixed.31 In solid-oil-water systems, the water contact angle against oil and oil contact angle against water in principle sum up to 180° as they are supplementary to each other. From a thermodynamic point of view, underwater superoleophobicity and underoil superhydrophobicity have been understood as contradictory properties, and they are not expected to appear on one surface. Thus, the present continuous oil-water separation needs two kinds of strainers: one with high surface energy (fish-scale effect)32 and another with low surface energy (lotus effect)33,

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. This is very complicated and costly. Can one type of

strainers realize the continuous separation? Recently, Tian and Ras offered an access to such surface combing underoil superhydrophobicity and underwater superoleophobicity.35 It is reported that only if the surface is textured with re-entrant characteristic, and a well-defined intermediate surface chemistry (moderate water contact angle on a flat surface), can the texture be filled by both water and oil which are trapped in it and aren’t displaced by the other suspended liquid in the oil-water mixtures. Thus the surface also shows repellence to the second liquid. Herein, we describe a simple and general strategy to synthesize a dual superlyophobic surface in oil-water mixture and show how to use it for continuous and efficient oil-water 3 ACS Paragon Plus Environment


separation. Flat titanium dioxide (TiO2, TD) surfacecerate hydrophobicity with a water contact angle of 72°.36 As a proof of concept demonstration of the application, TD nanorods forests are in-situ grown on readily available copper foams (CFs) by hydrothermal deposition. The wettability of the as-prepared CFs with TD (TD-CFs) under oil-water systems can be tuned simply by immersion sequence: oil first or water first. By the help of a straight tee, one type of TD-CFs with different immersion history can separate oil-water mixtures continuously and efficiently regardless of their specific gravity difference.

RESULTS AND DISCUSSION The overall preparation route of TD-CFs and the usage of them for oil-water separation are schematically illustrated in Figure 1a. Firstly, a piece of commercial CF is etched by the hydrochloric acid aqueous solution to roughen the surface. And then it is immersed in the aqueous solution for hydrothermal synthesization of TD nanorods. During the hydrothermal process, the rough surface provides lots of heterogeneous nucleation sites for TD nanorods. TD nanorods nucleating on the substrate firmly anchor the surface of CF. The nanorods which shear one heterogeneous nucleation site should grow in the radial pattern forming grass-tuft like morphology. After the surface is totally covered by TD nanorods, the CF is both superhydrophilic and superoleophilic. So it can be easily wetted by oil and water. Two pieces of TD-CFs are selected to be wetted by water and the particular oil (the oil in the mixtures to be separated), respectively, and then are fixed onto the opposite outlets of an inverted tee mode straight tee. As the oil-water mixtures are loaded into the straight tee, both water and oil can touch each TD-CF regardless of their relative specific gravity, as shown by the right-side of Figure 1a. So water can continuously flow out from the water prewetted TD-CF and oil can continuously flow out from the oil prewetted TD-CF, respectively. The TD-CF is recyclable. It can return to the original state and can be reused to separate the same or other kinds of oil-water mixtures, after absorbed water is evaporated by the sun, and absorbed oil 4 ACS Paragon Plus Environment

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degrades rapidly into CO2 and H2O under UV radiation in the role of TD nanorods.37,38 As shown in Figures S1a-e, the commercial CF consists of millions of smooth belts with a dimension of about 60 µm × 60 µm, leaving macro-voids with an average diameter of 420 µm between them. Since grain boundaries are easier to be etched than inner crystals, millions of hills (with an average diameter of 0.7 µm) and valleys are formed on the smooth surface through chemical etching, as shown in Figure S1f. During the hydrothermal process, the hills can act as heterogeneous nucleating sites for TD nanorods and enhance the adhesion force between the nanorods and substrate. After hydrothermal reaction, dense TD nanorods totally cover all CF skeletons, as shown in Figure 1b. Carefully observing the high magnification SEM image shown in Figure 1c manifests that the nanorods on the surface are divided into millions of tufts and look like a turf. One tuft of nanorods is enlarged in the SEM image of Figure 1d. The cross-section SEM image (Figure 1e) of the TD nanorods coating reveals that each tuft of nanorods shears one “root”. The mean distance between roots is about 650 nm. Nanorods grow radially from the roots, and nanorods from different roots interweave with each other forming millions of taper micro-voids between the nanorods. The nanorods have an average diameter of 70 nm. In fact, each rod contains several parallel TD rods as proved by the HRTEM image in Figure 1f. The interference fringes with a spacing of 0.32 nm is clearly observed, which belongs to the interference of (110) crystallographic planes in rutile TD. The Raman spectrum in Figure 1g indicates that besides rutile phase, brookite phase is also included in the TD nanorods. Three typical peaks of rutile TD at 150cm-1, 447cm-1, 610cm-1 and four typical peaks of brookite TD at 126 cm-1, 246 cm-1, 320 cm-1, 363 cm-1 are shown in the Raman spectrum39. The XRD result in Figure S2 (in Supporting information) is consistent with the Raman spectra. Both Raman and XRD results confirm the successful deposition of TD on the foam.

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After decorated by TD, the CF is superhydrophilic. The microstructures of the TD-CF can be readily filled by water. The water contact angle (WCA) is approximately 0°, as shown in Figure 2a and Movie S1. The water droplets are absorbed by the TD-CF as soon (in less than 0.03s) as they touch the TD-CF. The superhydrophilicity endow the TD-CF very dramatically wicking action, as shown in Movie S2. As it touches the water surface, water is pulled upon the water surface along the TD-CF, which implies that water is able to displace air within the texture effortlessly. As a contrast, the commercial CF cannot even sink down because air is firmly stored in the macro-voids, endowing high buoyancy on the CF (Movie S2). As oil has lower surface tension than water, superhydrophilic surfaces are generally superoleophilic surface. As shown in Figure 2b and Movie S1, S2, diesel (which was used as an illustration) can also be absorbed by the TD-CF in a faster mode. As the water or oil prewetted TD-CF is submerged into another liquid, oil or water, the first liquid is firmly locked inside the texture and cannot be displaced by the second liquid. As shown in Movie S3, the submerged TD-CF can support a steady oil-water interface in the second liquid, and the second liquid suspends on the texture rather than intruding into the texture. After submerged into the second liquid, the prewetted TD-CF can continue to absorb the first liquid droplets, but the second liquid droplets slide away. With the aid of contact angle meter, the whole aforementioned processes were recorded in detail in Figures 2c-f and Movie S4. Water sliding angle under diesel and the diesel sliding angle under water are 7° and 8.5°, respectively. The TD-CF presents superhydrophobicity underoil with a WCA of 160°, and superoleophobicity underwater with an OCA of 159°, as shown in Figure 2g, 2h. In fact, the TD-CF presents superhydrophobicity under a series of oils including diesel, n-hexane, gasoline, toluene and chloroform, meanwhile it maintains the anti-wettability to these oil under water. All the second liquid droplet contact angles on the TD-CFs under the first liquid are larger than 150° and the second liquid droplet sliding angles on the TD-CFs under the first liquid are smaller than 10° as shown in Figure 2i, which implies that the TD-CFs achieve dual superlyophobicity and low adhesion. 6 ACS Paragon Plus Environment

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Two TD-CFs were prewetted by water and oil (diesel in Figure 3a and chloroform in Figure 3b), respectively. And then each of them was fixed onto the opposite outlet of an inverted tee mode straight tee. As shown in Figure 3a, b and Movie S5, regardless of the relative specific gravity of water and oil, water continuously flowed out of the water prewetted TD-CF, and oil continuously flowed out from the opposite one. Thus, this device realizes totally continuous water-oil separation. Owing to the large diameter of the aperture, water can pass through the TD-CFs in a high flux of 2.45×105 L m-2 h-1, while diesel has a flux of 0.65×105 L m-2 h-1. The loading flux of the straight tee device is 0.4×105 L m-2 h-1, lower than each of them but still acceptable. Theoretically, it should be limited by the slower one. In addition to the flux, separation efficiency is also an important fundamental indicator in the oil-water separation. The separation efficiencies of water and oil were also estimated by a general method and the detailed description can be seen in supporting information.21 The separation efficiencies of a series of oil-water mixtures were estimated. The results are shown in Figure 3c. The collection efficiencies of water or oil all outstrip 99% when the loading mixtures are 1 L. Such high efficiency can be maintained even after the 30 L diesel-water mixtures (which was used as an illustration) was separated, which implies that the high efficiency is durable. For the skeleton of CF is metallic copper, it strong enough to support the hydrostatic pressure during separation and resist abrasion in the usage. The tensile strength of TD-CF is 1.98 MPa, as seen in Figure S4, which is equal to the hydrostatic pressure produced by 202 m water column height and enough for oil-water separation. The abrasion resistance of the TDCF is evaluated and compared to TD decorated copper mesh by a wildly used method illustrated in Figure S5a and S5b.40 After each abrasion cycle, the WCAs underoil and OCAs underwater on both surfaces were measured, and the results are shown in Figure 4a and 4b. It is obvious in the results that both WCAs underoil and OCAs underwater of TD-CMs are all very susceptible to abrasion. The values of both contact angles decline dramatically. The 7 ACS Paragon Plus Environment


WCAs underoil and OCAs underwater decrease from 156° to 106° and from 152° to 100°, respectively. The TD-CMs almost lose superlyophobicity within 10 circles of abrasion. As a contrast, the TD-CFs still maintain dual superlyophobicity even after 50 circles of abrasion. Besides, the WCAs underoil and OCAs underwater decrease from 160° to 159° and from 156° to 147°, respectively. Before and after abrasion, the microstructures of the TD-CFs and TD-CMs were both observed by SEM, and the corresponding images are shown in Figure S5c-f. It is clear that the abrasion can severally damage the cuticles of the TD-CFs and TDCMs, as shown in Figure S5c, e, f. The scratches on the skeletons and the wires are visible. Benefiting from the bigger length in the z-direction and the strong support of skeleton, numerous replaceable surfaces in the TD-CF beneath the damaged cuticle are reserved (Figure S5d). Thus, the original wettability of the TD-CF is also almost preserved. The surfaces beneath the cuticle play more important role on oil-water separation. Although the abrasion cause about 10° decrease of OCA underwater, the separation efficiency after 50 cycles abrasion was still up to 99%. There is no doubt that the water prewetted foam can return to the original state after water completely evaporated under the sun. However, it is really a serious problem to remove the oil contamination from the oil prewetted foam, which is necessary for the recyclability. TD is a well-known photocatalyst which can mineralize pollutants to convert into CO2 and H2O in wastewater under UV light irradiation.37, 41 The oil contamination can be decomposed into CO2 and H2O under UV irradiation in the role of the as-prepared TD nanorods on CFs, as also proved by Figure S6a, b. Thus, the decorated TD coatings endow the entire CF self-cleaning ability on oil contamination. Such hypothesis was investigated on TD-CMs surfaces and the results are shown in Figure 4c. The absorbed diesel changes the wettability of the TD-CM from superhydrophilicity (WCA: 0°) to moderate wettability (WCA: 65°). After exposed to UV irradiation, the WCA in air gradually reduced to 0° in 6 hours, as shown in Figure 4c and Movie S6. That indicates the original wettability returns. The Fourier-transform infrared 8 ACS Paragon Plus Environment

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spectra (FT-IR) results in Figure 4d indicate the original surface chemistry also returns. The peaks at the wavelength of 2924 cm-1, 2853 cm-1 with -CH2-, -CH3 in diesel completely disappeared after UV irradiation. Such process, in fact, can repeat in many cycles, as shown in Figure 4e. Ultra-high contact angle, low adhesion, and drag reduction are all widely known as “Lotus effect”.33,

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All these properties are also necessary for the strainer to separate oil-water

mixtures, but in solid-oil-water systems contrast to Lotus leave in solid-liquid-air systems. The mechanism for “Lotus effect” is well clear: low surface energy adding hierarchical roughness. To be specific, the hierarchical structures promote the formation of air pockets and a composite solid-liquid-air interface, as well as the lowest contact area of an applied water droplet, resulting in “Lotus effect”. The technology used to create artificial “Lotus effect” has been grafted successfully on oil-water separation strainer. Jiang and co-workers have proved that artificial superhydrophobic surface can be underoil superhydrophobic, because under the water droplet, a similar composite tri-phase interface as in air is also built in an oil-water mixtures where only air is substituted by oil.19 Afterwards, lots of successful cases on oilwater separation have drawn lessons from this general phenomenon.19,

23, 27, 29

For a long

period, that was once considered as the only strategy for oil-water separation and another use of the superhydrophobic surface.42,43 According to our research results in this paper, the key point for an oil-water separation material is whether it has the ability to build the composite tri-phase interface. It includes two parts: one, liquids can fulfill the hierarchical roughness instead of air; two, the following liquid can suspend on the composite tri-phase interface rather than extrude the first liquid. Superhydrophobic surface can actualize oil-water separation, but is not a prerequisite. As shown in Figure S7a, we also modified the TD-CF with 0.5% 1H, 1H, 2H, 2H-perfluoroocctadecyltrichloorosilane (FDTS) to low its surface energy and get a superhydrophobic surface. Because of the ultralow surface energy, only oil can access the voids instead of air as shown in Figure S7a and Movie S7. The surface 9 ACS Paragon Plus Environment


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presents underoil superhydrophobic (Figure S7a) but underwater superoleophilic (Movie S7). Only oil can pass through the CF when it contacts with oil-water mixtures (Figure S7b and Movie S8). In this experiment, the TD surface has a moderate WCA of 72° on a flat surface. Both water and oil can access the voids instead of air. The nanorods create millions of taper micro-voids between them. We think such morphology is necessary for the dual superlyophobic foam to form a steady composite tri-phase interface in solid-oil-water systems, because the first accessed liquid can be locked inside. The image in Figure 5a, just illustrates the situation of the TD-CF after wetted by the first liquid. The voids including the micro- and nano-sized are all full-filled by the first liquid. If the following infused liquid is the same as the first liquid, they will merge into one. If the following infused liquid is different from the first one as shown in Figure 5b, the interfacial energy between them will prevent the second one accessing to the apertures. So both oil and water can pass through or be obstructed by the TD-CF, just depending on whether it is prewetted by oil or water. According to Laplace equation, the critical resistant pressure PC is related to the contact angle of the second liquid θ, the void radius r and the interfacial energy γ following the Equation (1) = 2 /

(1)

as shown in Figure 5b. As indicated by the Equation (1), it is obvious that the nano-voids are harder than the micro-voids to let the second liquid in. Assuming the contact angle of the first liquid is comparable to the second, the critical resistant pressure should be the same no matter the foam is prewetted by oil or water. The pressure is measured by the instrument shown in Figure 5c and 5d by water and diesel, respectively. The critical height of water and diesel are 3.1 cm and 4.8 cm, respectively. The pressure can be easily calculated by the height according to the Equation (2): = ℎ

(2)

(Where, P is the experimental intrusion pressure; ρ is the density of second liquid; g is the 10 ACS Paragon Plus Environment

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acceleration of gravity; h is the maximum of the second liquid), which is 2.7 Pa and 2.8 Pa, respectively. No matter using water or diesel, the measured pressure is almost the same. It implies that the illustration in Figure 5b is close to reality.

CONCLUSIONS Our work demonstrates that the complicated oil-water separation can be easily and continuously implemented with the help of two TD decorated commercial CFs and a straight tee. The hierarchical voids in the TD-CFs are very important. The macrovoids in CFs and the nano-sized voids created by interweaved TD nanorods can work together to let them be fulfilled by water and oil effortlessly. Then the first infused liquid can pass through the macrovoids while the second liquid is obstructed because of Laplace force. Our work implies that the harsh chemical modification to obtain high/low surface energy can be omitted for oilwater separation. Moderate wettability which lots of materials in nature own can be competent. Our work also indicates many kinds of materials can be used for oil-water separation by the help of hierarchical voids. Then many unique functional properties along with these materials, such like photodegradation of TD, can be involved in the oil-water separation strainer. EXPERIMENTAL SECTION Surface Etch. CF (Cu, 99.99%, Yi Yang CF metal New material Co., Ltd.) with a dimension of 4.5 cm × 3 cm × 0.15 cm and PPI of 110 acted as the substrate material. Firstly, it was put into the cupric chloride (CuCl2, 99%, Sinopharm Chemical Reagent Co., Ltd.) solution, which was prepared by mixing hydrochloric acid (12.5 ml) (HCl, 37%, SigmaAldrich Co., Ltd.), CuCl2 (12.5 g) and deionized water (87.5 ml) (H2O, 18.2 MΩ, Millipore Direct-Q System) at room temperature for 7 min. Then the residuary CuCl2 was removed through HCl (1 M) aqueous solution for 10 min. After that, it was cleaned with deionized water and ethanol, and then dried in air.44 11 ACS Paragon Plus Environment


Preparation of TD-CF. TD-CF was obtained through a simple one-step hydrothermal sedimentary process. Titanium trichloride (0.86 ml) (TiCl3, 15%, Sinopharm Chemical Reagent Co., Ltd.) solution was added to the saturated sodium chloride (35 ml) (NaCl, 99.5%, Tianjin Guangfu Technology Development Co., Ltd.) solution drop by drop to make TiCl3 (0.03 M) solution under magnetic stirring at the appropriate rate. Then urea (0.1 g) (H2NCONH2, 99%, Sinopharm Chemical Reagent Co., Ltd.) was dissolved in the TiCl3 solution. After agitating for 10 min, the solution (35 ml) was poured into a Teflon-lined stainless steel autoclave (50 ml) and placed in an oven at 160ºC for 3 h. The sample was removed when the temperature cooled down to the room temperature in the air and washed with deionized water, ethanol and then dried at room temperature. TD-CM is also prepared in the same procedure for comparison. Sample Characterization. The surface morphological structures of samples were all captured from a field emission scanning electron microscopy (FE-SEM, ZEISS SUPRA 40, operating at 15 kV). The Raman spectra were obtained by using WITec CRM200 confocal Raman microscopy system (WITec, Germany) with a laser wavelength of 420 nm. The phases were studied by a DX-2007diffractometer (XRD, Dandong Haoyuan Instrument Co., China. Cu Kα radiation: λ= 1.5419 Å). Water contact angles (WCAs) and oil contact angles (OCAs) were all measured via a CCD camera on a contact angle meter (OCA 20 dataphysics, Germany) at room temperature. And when water or oil droplets fell respectively on the samples in air, the rapid assimilation progress of water or oil droplets would be captured by the foregoing apparatus. The volume of each water or oil droplet was 5 µl. It is worth noting that every measurement result was an average value of at least five spots on the sample to eliminate the influence of accidental factors and get a relatively correct value. The details of oil-water separation were all recorded via digital photos and digital videos by a camera (Nikon, D5000, Japan). The uniaxial tensile specimens with gauge length of 15 mm, a


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thickness of 0.15 mm, and the width in the gauge length of 5 mm was performed on CSS1102C system at a rate of 0.2 mm/min.45 Oil-water Separation. An adjustable speed peristaltic pump (0~2500 ml/min, Zhongshan Gaoshuo Electronic Technology Co., Ltd.) was used to mix the separated water and oil, and continued to add the oil-water mixtures to the upper mouth of the straight tee for simulating the continuity of the separation process. In order to better distinguish between oil and water on the vision, the oil and water were colored by Sudan Ⅲ and methyl blue, respectively. When calculating the separation efficiencies, the concentrations were obtained skillfully by the help of the absorbance peaks of MB and in oil-water mixtures which were determined by ultraviolet-visible (UV–Vis) spectrophotometer (UV−6100 (PC), Shanghai MeiPuda Co., Ltd.).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: FESEM images of the morphology of CFs before and after etching; XRD patterns of the TD and the standard PDF Cards; V-visible absorbance spectra of different MB concentration aqueous solution and different Sudan Ⅲ concentration diesel solution; absorbance peak intensities as a function of MB concentration and Sudan Ⅲ concentration; engineering stressstrain curve of CF; schematic illustration of the abrasion test procedure; SEM images of the TD-CMs and TD-CFs morphology after 50 cycles sandpaper abrasion; UV-vis absorbance curves of MB aqueous solutions and Sudan Ⅲdiesel solutions after different exposure times; WCAs of FDTS modified TD-CF surfaces in air and under oil; a series of snapshots recording separation process of a traditional diesel-water mixtures separation device using one FDTS modified TD-CF 13 ACS Paragon Plus Environment


Movies S1−S8

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (Grant No. 51401083). Y. Liu is grateful for the financial support by the research grants (No.51371089, 51275555) and Program for Innovative Research Team (in Science and Technology) in University of Jilin Province. Q. Jiang acknowledges the financial support by the research grants (No. 51631004).

REFERENCES (1) Xue, C.-H.; Li, Y.-R.; Hou, J.-L.; Zhang, L.; Ma, J.-Z.; Jia, S.-T. Self-Roughened Superhydrophobic Coatings for Continuous Oil–Water Separation. J. Mater. Chem. A 2015, 3, 10248-10253. (2) Wang, B.; Liang, W.; Guo, Z.; Liu, W. Biomimetic Super-Lyophobic and Super-Lyophilic Materials Applied for Oil/Water Separation: A New Strategy Beyond Nature. Chem. Soc. Rev. 2015, 44, 336-361. (3) Fan, J.-B.; Song, Y.; Wang, S.; Meng, J.; Yang, G.; Guo, X.; Feng, L.; Jiang, L. Directly Coating Hydrogel on Filter Paper for Effective Oil-Water Separation in Highly Acidic, Alkaline, and Salty Environment. Adv. Funct. Mater. 2015, 25, 5368-5375. (4) Wang, X.; Yu, J.; Sun, G.; Ding, B. Electrospun Nanofibrous Materials: A Versatile Medium for Effective Oil/Water Separation. Mater. Today 2016, 19, 403-414.


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Figure 1. a) Illustration of the overall preparation route and usage of TD-CFs for highefficiency and continuous oil-water separation process. b-d) FESEM images on TD-CFs



surface morphology with different magnification. e) A cross-section FESEM image of TD coating. f) HRTEM image of TD nanorods. g) Raman spectra of TD-CF and commercial CF.


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Figure 2. a-b) Water (a) and oil (b) droplets dynamic spreading behavior on TD-CF in air. c) Water droplets dynamic spreading behavior on water prewetted TD-CF under oil. d) Oil droplets dynamic spreading behavior on oil prewetted TD-CF under water. e) A water droplet sliding on a slightly inclined oil prewetted TD-CF (7°) under oil. f) An oil droplet sliding on a slightly inclined water prewetted TD-CF (8.5°) under water. g) The water contact angle of diesel prewetted TD-CF under diesel. h) The diesel contact angle of water prewetted TD-CF under water. i) A series of oil contact angles and sliding angles on the surfaces of water prewetted TD-CF under water and water contact angles and sliding angles on the surfaces of a series of oil prewetted TD-CF under corresponding oil.


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Figure 3. a-b) A snapshot on the device separating an oil-water mixtures where water has a relative heavier specific gravity (a) and a relative lighter specific gravity (b). c) The separation efficiencies for a series of oil-water mixtures.



Figure 4. a-b) The evolution of the WCAs underoil (a) and OCAs underwater (b) on TD-CF and CM surfaces versus cycles. c) The WCAs evolution of diesel prewetted TD-CM under UV light irradiation. d) ATR-FTIR spectra of TD-CM, diesel prewetted TD-CM and diesel prewetted TD-CM after 6 h UV light irradiation. e) Cycles of variation of WCAs on TD-CM as it is prewetted by diesel and irradiated by UV light for 6 h.


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Figure 5. a) Illustration of the wetting state of TD-CF. b) Illustration of the situation when the second liquid is introduced. c-d) Measuring the intrusion pressure of diesel (c) and intrusion pressure of water (d).



Graphical Table of Contents


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Nanorods decorated copper foam has been proved to separate water-oil mixtures in two modes: water passing through as prewetted by water or oil passing through as prewetted by oil. A inverse straight tee, outlets of which are covered by a water and oil prewetted such foam, respectively, can continuously separate kinds of water-oil mixtures with high efficiency and high flux. 49x45mm (300 x 300 DPI)

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