Superoleophilic

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Biological and Environmental Phenomena at the Interface

The Facile Preparation of Ag coated Superhydrophobic/Superoleophilic Mesh for Efficient Oil/Water Separation with Excellent Corrosion Resistance Zhiping Du, Peng Ding, Xiumei Tai, zihe Pan, and Hengquan Yang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00640 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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The Facile Preparation of Ag coated Superhydrophobic/Superoleophilic Mesh for Efficient Oil/Water Separation with Excellent Corrosion Resistance Zhiping Du,1,2,* Peng Ding,2 Xiumei Tai,2 Zihe Pan1 and Hengquan Yang3 1

Institute of Resources and Environment Engineering, Shanxi University, Taiyuan 030006, China

2

China Research Institute of Daily Chemical Industry, Taiyuan 030001, China

3

College of Chemistry & Chemical Engineering, Shanxi University, Taiyuan 030006, China

To whom correspondence should be addressed e-mail: [email protected] 1

ACS Paragon Plus Environment

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ABSTRACT We present the facile preparation of a superhydrophobic-oleophilic stainless steel mesh with excellent oil/water separation efficiency and resistance to corrosion through HF etching, Ag nanoparticles coating, and stearic acid (STA) modification to construct a superhydrophobic micro/nanohierarchical structure. The surface of the treated mesh exhibits superhydrophobicity, with a water contact angle of 152°, and superoleophilicity, with an oil contact angle of 0°. The effects of variation in HF etching time and Ag nano-particles coating on surface wettability were explored. The treated mesh demonstrated very high separation efficiency, as high as 98 % for the optimal preparation, on a series of oil/water mixtures. The durability of the treated mesh was tested by repeated separation of kerosene/water mixtures, separation efficiency remaining higher than 97 % after 40 cycles. In addition, the mesh exhibited excellent chemical resistance to both acidic and alkaline conditions, good wearing in hot water. The improved superhydrophobic-oleophilic mesh represents a feasible and realistic oil/water separation methodology even under harsh conditions, and it could have a wide application in industrial processes.

Keywords: Superhydrophobic; Superoleophilic; Chemical etching; Nano-Ag; Oil/water separation.

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Introduction High levels of production of industrial oil-containing wastewater and frequent crude oil spillages have historically caused serious water pollution.1 Over the past two decades, a variety of technologies, including gravity separation,2 membrane filtration,3 membrane desalination, adsorption and electrochemical separation4 have been developed to separate oils from water. Among these techniques, membrane separation is utilized most extensively due to its high efficiency, relatively low cost and ease of operation. But most membranes lose their separation efficiency in hot water (＞ 50 ºC ) or strong corrosive liquid separation due to the reaction/etching between corrosive liquids (such as acidic or alkaline solutions) and the substrate, which restricts their applications. An increasing demanding on the fabrication of special oil/water separation materials which can resist corrosion is particularly urgent. Recently, scientists have carried out several studies on oil/water separation in acidic, basic and hot water mixtures using special wettability mesh.5-13 Surface structure and composition are regarded as two critical factors in developing superhydrophobic or superhydrophilic surfaces that proper micro/nano structure significantly enhances the wettability to super-wetting state. Thus the construction of micro/nano hierarchical structure is utilized extensively to delivery special wettability. Fan et al8 prepared a superhydrophilic/underwater superoleophobic filter paper through polyvinyl alcohol (PVA) coating onto cellulose filter paper by immersing cellulose filter paper in PVA solution for 2 h and then immersed into glutaraldehyde solutions for 24 h. The as-prepared filter paper showed high efficiency for oil/water separation in rigid environment such as acidic, alkaline and salty. Nevertheless, the long-time fabrication causes the low fabrication efficiency and the mechanical property of the pre-prepared filter papers is under concern. Wan et al9 introduced a superhydrophobic multiwalled carbon nanotubes (MWCNT) based membrane by spray coating and the treatment of fulurocarbons showing highly oil/water separation efficiency under 60 °C. Though the MWCNT membrane can maintain its superhydrophobicity in hot water, its oil/water separation 3


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performances under acidic and alkaline are lack of sufficient study. Li et al10 reported the collection of oil from hot water (~92 °C), strongly acidic, alkaline and salty solutions through coating candle soot (CS) by placing the stainless steel mesh on top of the candle flame and moving forward and backward for certain times and then spray coated by hydrophobic-silica to deliver the superhydrophobicity. The as-prepared mesh showed high hot water repellency and anti-corrosion performances. But the bonding strength of CS and silica with stainless steel mesh was weak thus CS and silica could easily washed away due to the stainless steel mesh was fairly smooth. To simplify the fabrication process and enhance the stability, anti-corrosion property and durability, a superhydrophobic-oleophilic mesh with micro/nano hierarchical structure through HFetching, Ag nano-particles coating and stearic acid (STA) modification is developed inspired by the micro/nano structure and outstanding superhydrophobicity of lotus leaves. The efficiency of oil/water separation, corrosion resistance, stability and durability of our prepared mesh are investigated. The method presented in this work provides theoretical and practical guidance for the application of such meshes to real-world oil/water separation even under harsh conditions. Experimental Materials. Stainless steel mesh (500 mesh size, 1 square inches has 500 aperture) is purchased from Anping Baiman mesh Co. Ltd. Hydrofluoric acid, stearic acid (STA,CH3(CH2)16COOH), AgNO3, ammonia, HCl, NaOH, NaCl, ethanol, acetone, atoleine, gasoline, heptane, hexane, kerosene and toluene are purchased from Tian jin Kermel Chemical Reagent Co. (China). Sudan I and methylene blue dyes are purchased from Sinopharm Chemical Reagent Co. All chemicals are used as received without further purification. Fabrication of superhydrophobic/superoleophilic mesh. In order to fabricate the nano-Ag coated superhydrophobic/superoleophilic mesh, the stainless steel meshes must be cleaned thoroughly with acetone, ethanol and deionized water sequentially to remove any residual impurities via ultra-sonication. The preparation process is schemed as Figure 1. Initially, the pre-cleaned 4


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meshes was immersed in a 40 % HF solution at ambient temperature for chemical etching to generate microscale rough structures on the stainless steel mesh. We named HF etched mesh HF-[X]; X referring to the etching time (minutes). After chemical etching, the HF-[X] was rinsed with deionized water and immersed in 0.1 mol L-1 [Ag(NH3)2]+ solution (25 % NH3·H2O is added drop wise into 0.1 mol L-1AgNO3, forming a transparent [Ag(NH3)2]+ solution) at ambient temperature, to obtain Agcoated nano-scale rough structures. We refer to this HF etched and Ag nano-particles coated mesh as HF-[X]@Ag-[Y] mesh; Y indicating the Ag coating time (minutes). After Ag nano-particles coating, the meshes were immersed into an 0.1 mol L-1 STA solution for 60 min at room temperature to reduce their surface energy. Because 60 min has been proved long enough for STA to be grafted onto the surface of the Ag nano-particles and HF etched stainless steel mesh, the duration of STA modification was fixed at 60 min for all samples. Therefore, by way of example, a mesh etched with HF for 5 minutes, coated with Ag nano-particles for 5 minutes, and then STA treated for 60 minutes would be referred to as HF-5@Ag-5@STA. Last but not least, the treated stainless steel mesh was ultrasonically cleaned in alcohol and deionized water for 10 minutes, followed by drying in a vacuum oven at 80 °C for 15 min.

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Figure 1. Schematic illustration of the fabrication of the superhydrophobic/superoleophilic stainless steel mesh.

Characterization. The surface morphology of mesh surfaces is examined by scanning electron microscopy (SEM, Quanta 250, FEI, American). The wettability is evaluated by Drop shape analyzer (DSA25, Kruss, Germany) in terms of water contact angle. The volume of the water droplet in all measurements is 5 µL. The final water contact angle value is reported as the mean of 5 measurements obtained in 5 different locations. The chemical composition of the meshes is characterized by X-ray diffractometry (XRD-6000, Japan), X-ray photoelectron spectroscopy (XPS, SPECS XR50, Japan) and attenuated total reflection Fourier transform infrared spectrophotometry (ATR-FTIR, BRUKER VERTEX70, Germany). Oil/water separation. A series of oils and organic solvents, including atoleine, gasoline, heptane, hexane, kerosene, and toluene are used in this study, which are dyed by Sudan I while water is colored using methylene blue. The oil/water mixtures (1:1, Voil=Vwater=10 mL) are poured onto the 6


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HF-etched, Ag-coated and STA-modified stainless steel meshes. The only driving force during the separation process is gravity. The separation efficiency was calculated according to eq (1): η= (m1/m0) × 100 %

eq (1)

Where, m0 and m1are the volume of the oil before and after the separation process, respectively. Hot water repellency and anti-corrosion performances. Corrosion resistance was tested by separating kerosene and a series of corrosive liquids including; 1M HCl, 1M NaOH, 1M CuCl2 and hot water (~90 °C). HCl, NaOH and hot water are dyed with methylene blue while kerosene is colored with Sudan I. The oil and corrosive liquid mixtures (1:1, Voil=Vwater=10 mL) are poured onto the HF-etched, Ag-coated and STA-modified stainless steel meshes. The only driving force during the separation process is gravity. The separation efficiency is calculated according to eq (1). Durability is also tested through calculation of the dependence of water contact angle and separation efficiency on the duration of their exposure to air. We calculate water contact angle and separation efficiency of the treated mesh every 4 days and tested it for 28 days continuously. Results and discussion The fabrication of nano-Ag coated stainless steel mesh was carried out by sonication cleaning, then etched in HF solution with varied etching time, immersed in [Ag(NH3)2]+ solution and modified by steric acid to reduce surface energy. In this study, HF etching was utilized to control micro-scale cavities for desired deposition of Ag nano-particles. The surface morphologies of the stainless steel meshes before and after HF etching are characterized by SEM. Figure 2 shows that the surface of the original stainless steel wires is smooth (Figure 2a and e). After HF etching for 10 min, there are some independent holes visible on the stainless steel wires and the surface has become rough in comparison to the non-etched meshes (Figure 2b and f). After 25 min etching (Figure 2c and g), it can be seen that there are plenty of lump-like bulges due to the reaction between F- and the stainless steel wires, generating micro-scale rough structures. This result indicates that the stainless steel meshes chemically etched by HF show typical micro-scale irregular-shaped plateaus and lump7


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like structures with sizes in the range of 1-2 µm. The superhydrophobic-oleophilic mesh surface becomes rougher with increased etching time. This substantially increased roughness plays a critical role in enhancing wettability, allowing the water droplets to wet and spread quickly on these surfaces. Although plenty of lump-like bulges were formed on the surface of the stainless steel wires after etching for 30 min (Figure 2d and h), the strength of the stainless steel wires was reduced due to corrosion by the HF solution (Figure S1). Accordingly, to balance the roughness and strength of our stainless steel wires, the optimum etching time was selected as 25 min.

Figure 2. Low and high resolution SEM images of stainless steel mesh: (a) and (e) original mesh; (b) and (f) HF-10; (c) and (g) HF-25; (d) and (h) HF-30.

Figure 3 shows the morphology of HF-25@Ag-5 mesh. Given a reaction time of 5 min, spherelike Ag nano-particles formed, which were homogeneously dispersed on the HF-25 mesh surfaces forming a micro/nano scale hierarchical structure (Figure 3a and 3b).

Figure 3. Low and high resolution SEM images of HF-25@Ag-5 mesh. 8


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XRD characterization was carried out to confirm the coated particles. Figure 4a shows the XRD patterns of the original and HF-25@Ag-5@STA mesh, three diffraction peaks at 43.62°, 50.78°, 74.68° are the characteristic peaks of Fe(111), Fe (200), Fe (220) which agree with the standard card (JCPDS Card, No. 033-0397),14 indicating that the main element of the original mesh is Fe. The peaks at 38.18°, 44.09°, 64.36° and 77.43° are the characteristic peaks of Ag (111), Ag (200), Ag (220), Ag (311), in agreement with the standard card (JCPDS Card, no. 04-0783),15 indicating that Ag nano-particles were coated onto the HF-25 mesh successfully: Fe + 3[Ag(NH3)2]+ + 3OH- = Fe(OH)3 + 3Ag↓ + 6NH3 (b)

30

40

50

eq (2). HF-25@Ag-5@STA mesh

Transimittance/%

Fe (220)

Ag (220)

Fe (200)

Fe (220)

Fe (111)

20

Fe (200)

Ag (111)

Fe (111) Ag (200)

HF-25@Ag-5@STA mesh original mesh

Ag (311)

(a)

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3400 2920

2852 1620

stearic acid 3430 2917 2849

60

70

4000

80

3500

2θ/°

3000

2500

1703

-1

2000

1500

Wavenumber/cm

Figure 4. (a) XRD pattern of the original and HF-25@Ag-5@STA mesh surfaces; (b) ATR-FTIR spectra of stearic acid and HF-25@Ag-5@STA mesh surfaces.

The ATR-FTIR spectra(Figure 4b) of STA and HF-25@Ag-5@STA mesh shows that the free stretching vibrations of the carbonyl groups at 1703 cm-1 disappear and a new group appears at 1620 cm-1 owing to the stretching vibration of the C=O bond which is caused by hydrogen bonding. A new chemical structure (iron stearate) is formed by Fe3+ and COO- leading to the absorption peak C=O shifting to a lower frequency position. The absorption peaks at 2917 cm-1 and 2849 cm-1 correspond to -CH3, whereas the peaks at 2852 cm-1, 2920 cm-1correspond to -CH2. The absorption peaks at 3430 cm-1 correspond to O–H stretching vibrations of the stearic acid molecule, while O–H stretching vibrations of the STA-modified mesh obviously shifted to a lower wavenumber (3400 cm9


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1

) indicating the formation of intra and intermolecular hydrogen bonds. The ATR-FTIR spectra

indicate that the STA molecule bonds to the stainless steel mesh surfaces. The combination of the micro-nano structure and the low surface energy molecules contribute to the observed superhydrophobicity. The surface composition of the HF-25@Ag-5@STA mesh was investigated using XPS (Figure 5). High resolution spectra of C 1s, O 1s, Fe 2p and Ag 3d were performed. As shown in Figure 5a, the C 1s peak can be deconvoluted into three peaks located at 288.5, 286 and 284.7 eV, which are typical signals of carbon atoms of O-C=O, C-O and C-C (or C-H) groups, respectively.16, 17 Figure 5b shows the O 1s high resolution spectra and the binding energy peaks at 531.4 eV and 532.6 eV which correspond to the O-C=O group.18 For the HF-25@Ag-5@STA mesh, the detection of an intense O-C=O signal was evidence that STA, of low surface energy, was immobilized at the mesh surface. Figure 5c is the high resolution spectrum of Fe 2p, the appearance of four typical peaks at binding energy 706.7 eV, 720 eV, 710.7 eV and 724.3 eV belong to Fe 2p3/2, Fe 2p1/2, Fe2O3 2p3/2 and Fe2O3 2p1/2 respectively,19 verifying that Fe is the main component of the mesh. Figure 5d shows the Ag 3d high-resolution spectrum, the appearance of two typical peaks with binding energy 367.9 eV and 374 eV belonging to Ag 3d5/2 and Ag 3d3/2, which confirm that Ag has been coated onto the mesh. The results of SEM, XRD and XPS confirmed that when the HF-25 mesh was immersed into 0.1 mol L-1 [Ag(NH3)2]+ solution, Fe reacts with [Ag(NH3)2]+ and OH- ions formed a large number of Ag molecules [eq (2)]. Ag was deposited on the surfaces of the stainless steel mesh and filled some cavities formed by the micro-nano scale lump-like bulge structures.

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(a)

(b) 4000

C 1s

O 1s

6000

Intensity

Intensity

C-C or C-H 4000

C-O

C=O

O-C=O

3000

Fe2O3

O-C=O

2000

2000 1000

290

288

286

284

282

536

280

534

532

Binding Energy/eV (c) Fe2O3 2p1/2

530

528

526

Binding Energy/eV (d)

Fe 2p

4000

Ag 3d Ag 3d5/2

1400

Fe2O3 2p3/2

Fe 2p1/2

Intensity

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3500

Fe 2p3/2

Ag 3d3/2

1200

1000

3000 800

732

728

724

720

716

712

708

704

700

378

376

374

372

370

368

366

364

362

Binding Energy/eV

Binding Energy/eV

Figure 5. XPS spectra of HF-25@Ag-5@STA mesh surfaces: (a) C1s; (b) O1s; (c) Fe2p; (d) Ag3d.

The effects of the coated Ag nano-particles on surface wettability were investigated by measuring the water contact angles. When the HF-25 mesh was immersed into [Ag(NH3)2]+ solution for 1 min, as shown in Figure 6a, a very limited number of Ag nano-particles were observed on the micro-scale lump-like rough structures. Prolonging the immersed time to 3 min, many Ag nanoparticles can be found on the skeleton of the mesh and the Ag nano-particles grew to a larger size (Figure 6b). Given a reaction time of 5 min, sphere-like Ag nano-particles formed and homogeneously dispersed on the HF-25 mesh surface forming a micro/nano scale hierarchical structure (Figure 6c). At the same time, the water contact angle increased from 147° to 152° as the reaction time increased from 1 min to 5 min (Figure 7). These findings shows that immersion time increasing leading to an apparent change in surface morphology that Ag nano-particles deposited 11


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onto the rough surface of HF-25 mesh and filled some of the cavities. Furthermore, the wettability crossed from hydrophobicity to superhydrophobicity, which we attribute to trapped air in the cavities forming a barrier between the solid and the liquid. This air cushion decreases the contact area between the water-drop and the mesh surface. According to Young’s20 equation eq (3):

cos θ =

γ γ

γ

eq (3)

S=-∆G = γ −γ − γ =γ (cos θ-1)

eq (4)

θ is the water contact angle on ideal surface, in this paper, the original mesh surface is assumed to be an ideal surface with a water contact angle of θ, θ=120°. γ , γ

and γ are the surface energy of

solid-vapor, solid-liquid and liquid-vapor phase. ∆G is Gibbs free energy. S is spreading coefficient. S﹥0, liquids spread and wet the solid surface. S﹤0, liquids droplet cannot wet the solid surface and form semi-spherical droplets. Putting the measured θ (θ=120°) into eq (4), γ ﹥0, S=-1.5γ ﹤0, thus the water tends to form semi-spherical droplets. In practice, the surface of a solid material wouldn’t be absolutely smooth, the stainless steel mesh surface was rougher after etching with HF and coating with Ag nano-particles. Due to the existence of the air cushion, liquid will not reach the deeper parts of the rough surface directly. By employing the Cassie-Baxter21 equation eq (5): cos θr=f·(1+cos θ)-1

eq (5)

where θr is apparent contact angle with rough surface, θ is the intrinsic contact angle at the solid-liquid interface, f is the percentage of total contact area of the solid-liquid interface and vaporliquid interface surface, and f1﹤1. when θ﹥90°, θr will increasing with decreasing f. In other words, when a hydrophobic surface (θ1 ﹥ 90 ° ) is consistent with the Cassie-Baxter model, the greater roughness results in decreased solid-liquid contact. Thereby we obtain a higher apparent contact angle. According to the wettability of the mesh, the suitable immersion time was fixed at 5 min. 12


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Figure 6. SEM images of stainless steel mesh: (a) HF-25@Ag-1; (b) HF-25@Ag-3; (c) HF-25@Ag-5, respectively.

Figure 7. Relationship between water contact, sliding angle and Ag-coating time after STA-modification.

The adhesion properties are important for oil/water separation meshes due to oil adhesion would contaminate the mesh leading to the loss of selectivity and oil/water separation ability. The adhesion behaviors of the HF-25@Ag-5@STA superhydrophobic mesh were assayed by separating a 5 µL water droplet from the surface. The dynamic process of approach, contact, deformation, and departure were recorded as shown in Figure 8. It can be seen that water droplet can easily and completely depart from the surface even when the droplet was compressed onto the sample platform, indicating that the HF-25@Ag-5@STA mesh has significant anti-adhesion properties. The adhesion behavior of water droplets at the surfaces of original, HF-25, and HF-25@Ag-5 meshes were carried out as shown in Figures S2, S3 and S4. When a 5 µL water droplet contact the original mesh surface, the water droplet adhere to the mesh and maintain a semi-spherical shape with a water contact angle 13


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of 120°. When a 5 µL water droplet contacts the HF-25, and HF-25@Ag-5 meshes surface, it adheres to the mesh and spreads over the mesh quickly.

Figure 8. Anti-adhesion behavior of water droplets coming into contact with the superhydrophobic surface of the HF-25@Ag-5@STA mesh (the direction of the arrow indicates the movement of the water-droplet).

Separation

of

oil/water

mixtures.

The

HF-25@Ag-5@STA

mesh

shows

both

superhydrophobicity and superoleophilicity, which made it an ideal candidate for the separation of oil/water mixtures. In contrast to water, oil is able to penetrate the mesh due to the capillarity effect and van der Waals attraction. To collect oil from a water/oil mixture, a simple oil/water separation setup was designed: the mesh was folded into a 3D structure as shown in Figure 9, placed on an erlenmeyer flask and then 20 mL mixtures of kerosene dyed with Sudan I and water dyed using methylene blue (Voil=Vwater=10 mL) were poured onto the HF-25@Ag-5@STA mesh. As shown in Figure 9(a-c), kerosene rapidly passed through the mesh and dropped into the erlenmeyer flask under the force of gravity, whereas water was repelled by the mesh. No additional force was applied during the separation process. As shown in Figure 9e, almost 9.8 mL of kerosene and 9.7 mL of water were collected and the separation efficiency was up to 98 %, which is considered very efficient. To prove that the separation process was due to the special wettability of the mesh rather than any another process, after separation, we kept the coloured water on the mesh for 1h then measured the volume of water, the result shows that the water was still unable to pass through the mesh and the volume of water remained 9.8 mL, proving that our mesh has great superhydrophobicity and superoleophilicity.

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Figure 9. Separation of a kerosene/water mixture. (a-b) water was dyed using methylene blue and oil was dyed using Sudan I and mixed before separation. (c-d) separation of the kerosene/water mixture using the HF-25@Ag5@STAmesh. (e) after separation the water and oil volume remained almost invariant.

Considering the growing levels of oil pollution, a collection device should be able to collect a variety of different oils from the oil/water mixture. Atoleine, gasoline, heptane, hexane, kerosene and toluene were used to imitate a plausible series of pollutants. As shown in Figure 10a, the separation efficiency of the HF-25@Ag-5@STA mesh remained above 96 % even for atoleine whose viscosity was largest, implying that the HF-25@Ag-5@STA mesh could be used in many complicated situations. The separation efficiency of kerosene/water mixtures versus cycle number was also investigated. As shown in Figure 10b, the prepared mesh still retained its superhydrophobic properties and high separation efficiency (above 97 %) after 40 cycles, which indicates the potential for prolonged reuse of our HF-25@Ag-5@STA mesh.

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Figure 10. (a) The separation efficiency for oil/water mixtures; (b) the separation efficiency for kerosene/water mixtures versus cycle number.

In practice, oil/water separation meshes might be used in harsh environments, stability to corrosion is essential. 1M HCl, 1M NaOH, 1M CuCl2 and hot water (about 90 °C) were chosen to imitate such harsh conditions. When mixtures of kerosene and 1M HCl (Figure 11a), 1M NaOH (Figure 11b), 1M CuCl2 (Figure 11c), hot water (90 °C, Figure 11d), were poured onto the HF25@Ag-5@STA mesh, HCl, NaOH, CuCl2 and hot water were repelled on the surface while the kerosene still penetrated the mesh quickly due to its superoleophilic properties. The separation efficiency of the HF-25@Ag-5@STA mesh remained ~98 %, implying that the prepared mesh could be used in harsh environments and industry.

before

before

after

after

16


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before

before

after

after

Figure 11. Separation by the treated mesh of mixtures of kerosene and various harsh solutions: (a) 1M HCl; (b) 1M NaOH; (c) 1M CuCl2; (d). hot water (about 90 °C).

The durability of the superhydrophobic-oleophilic HF-25@Ag-5@STA mesh was investigated. As shown in Figure 12a, when separating mixtures of kerosene and water with different pH values (ranging from 0 to 14), the separation efficiency had a slight fluctuation but still remained above 98%, while the water contact angle changed appreciably but remained greater than 140°, indicating high hydrophobicity and corrosion resistance. The relationship between the contact angle, separation efficiency and the exposure time on the HF-25@Ag-5@STA mesh was also studied. As shown in Figure 12b, after an exposure time of 28 days, the water contact angle remained above 140° and the separation efficiency remained above 96 %, indicating high hydrophobicity and excellent corrosion resistance. Although the contact angle and separation efficiency decreased slowly over time during the exposure process, it maintained a very high hydrophobicity, which indicated the HF-25@Ag5@STA mesh has excellent stability under various environmental conditions.

98 150 96 145 94 140 ■ separation efficiency

92

● contact angle

135

150

98

145 96

140 135

94 130 92

■ separation efficiency

125

● contact angle

120 90

130

90 0

2

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Figure 12. Dependence of separation efficiency and water contact angle on:(a) the pH of the water; (b) exposure time in air.

Conclusions A superhydrophobic-oleophilic stainless steel mesh was fabricated using a facile immersion strategy. A micro/nano-hierarchical rough structure was obtained by etching with HF, followed by coating with Ag nano-particles. After modification with stearic acid, the treated stainless steel mesh showed both superhydrophobicity and superoleophilicity. The treated mesh has a water contact angle of 152°, a sliding angle of 14° and an oil contact angle of 0°. The prepared mesh can separate a series of oils, including atoleine, gasoline, heptane, hexane, kerosene and toluene from oil/water mixtures with a separation efficiency of greater than 98 % and the separation efficiency remained 97 % after 40 cycles for separation of a kerosene/water mixture. In addition, the HF-25@Ag-5@STA mesh exhibited excellent stability when collecting oil from various corrosive solutions after 28 days. The possible reason for the durable and admirable anti-corrosion property may be relying on the mechanical stable stainless steel mesh, the hierarchical structure coated via Ag nano-particles and the superhydrophobicity. Therefore, the HF-25@Ag-5@STA superhydrophobic-oleophilic stainless steel mesh could realistically be applied in industrial oil/water separation even under harsh conditions.

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Acknowledgements This research was supported by the National Key Research ＆ Development Plan (#2017YFB0308704) and the Nature Science Fund of ShanXi Province (#2015011014-1, #201605D211008). We would like to thank the Jialan Foundation for financial support (Jala 2015). The authors thank Lijuan Wei for the assistance with Figure edition.

Supporting Information We have provided six figures and 3 movies for our supporting information.

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Table of contents graphic

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Superoleophilic

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