water separation on structure-controllable Cu mesh: Transition of superhydrophilic-superoleophilic to superhydrophobic-superoleophilic without chemical modification

Accepted Manuscript Oil/water separation on structure-controllable Cu mesh: Transition of superhydrophilic-superoleophilic to superhydrophobic-superoleophilic without chemical modification

Kai Zhang, Hao Li, Xunqian Yin, Zhongwei Wang PII: DOI: Reference:

S0257-8972(18)31271-4 https://doi.org/10.1016/j.surfcoat.2018.11.061 SCT 24027

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

25 September 2018 3 November 2018 20 November 2018

Please cite this article as: Kai Zhang, Hao Li, Xunqian Yin, Zhongwei Wang , Oil/ water separation on structure-controllable Cu mesh: Transition of superhydrophilicsuperoleophilic to superhydrophobic-superoleophilic without chemical modification. Sct (2018), https://doi.org/10.1016/j.surfcoat.2018.11.061

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ACCEPTED MANUSCRIPT Oil/water separation on structure-controllable Cu mesh: transition of superhydrophilic-superoleophilic to superhydrophobic-superoleophilic without chemical modification Kai Zhang,2 Hao Li,1* Xunqian Yin,1 and Zhongwei Wang1 1 School of Material Science and Engineering, Shandong University of Science and

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Technology, Qingdao, 266590, China 2 ICD/LASMIS, University of Technology of Troyes, UMR 6281, CNRS, Troyes

Corresponding author: Dr. Hao Li

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School of Material Science and Engineering

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10004, France

Shandong University of Science and Technology

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Qingdao, 266590 P.R. China

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E-mail address: [email protected];

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ACCEPTED MANUSCRIPT Abstract: Four different surface morphologies, including needle-like, bamboo leaf-like, pine needle-like, and peony flower-like structures, were prepared on Cu mesh surface, and the existence of these different surface morphologies were due to the formation of Cu(OH)2 or CuO microstructures by controlling the oxidation time and oxidation temperature of the chemical

reaction.

These

freshly

prepared

Cu

meshes

all

exhibit

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superhydrophilic-superoleophilic after dried with a hair dryer. They all have excellent separation efficiency, and the separation efficiency for oil-remove (~96%) is higher than that

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of water-remove (~94%). Interestingly, these meshes with four different surface morphologies

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can transform from superhydrophilic-superoleophilic to superhydrophobic-superoleophilic just after storage in air for more than 20 days without any further chemical modification. This

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is because oxygen adsorption on the mesh surface contributes to air trapped into the microstructures. These superhydrophobic-superoleophilic copper meshes have good durability. to

the

investigation

of

separation

efficiency

of

the

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According

superhydrophobic-superoleophilic mesh, we find that the separation efficiency for oil-remove

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of the superhydrophobic-superoleophilic mesh (~99%) is higher than that of the

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superhydrophilic-superoleophilic mesh (~96%).

Keywords: Chemical treatment; Superhydrophilicity; Transition; Superhydrophobicity;

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Modifier-free; Durability; Oil/water separation.

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ACCEPTED MANUSCRIPT 1. Introduction Oil/water separation has become a global task owing to the increase of oily sewage [1]. Generally, based on the difference of specific gravity between oil and water, oil skimmers, centrifugation and membrane separation are devoted to the separation of oily wastewater [2]. However, most of the approaches require expensive equipment and device, complex steps or long

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processing time [3]. Therefore, a simple and universal method for valid removal of oil from water is necessary. Inspired by plants and insects in nature, such as lotus leaves [4], rose petals [5], water

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striders [6], superwetting surfaces have attracted tremendous attention owing to their properties such

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as self-cleaning [7], corrosion resistance [8,9], drag reduction [10], anti-icing [11]. Recent years, based on the difference of surface energy between oil and water, materials with superwetting surfaces

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for oil/water separation have drawn considerable attention and are considered eco-friendly, highly efficient, and lower cost [12,13].

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To date, many materials with superwetting surfaces for oil/water separation have been explored, such as metal mesh [14-17], foam [18,19], fabrics [20], sponge [21], and aerogel [22]. It is noted that

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the low surface energy and the hierarchical microstructure are two sufficient conditions for the

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meshes with superhydrophobic surfaces [23,24]. There are usually two ways to fabricate efficient oil/water separation mesh: one way is to fabricate the superhydrophilic-superoleophilic mesh [25]; another way is to fabricate the superhydrophobic-superoleophilic mesh [20]. For the

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superhydrophilic-superoleophilic mesh, it has the low contact angle for both water and oil, showing underwater superoleophobic and underoil superhydrophobic [26,27]. The water pre-wetted meshes

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are superhydrophilic and superoleophobic under water to enable water-remove. Meanwhile the oil pre-wetted meshes are superoleophilic and superhydrophobic under oil allowing oil-remove [28]. For the superhydrophobic-superoleophilic mesh, it has a larger contact angle for water, while exhibiting a low contact angle for oil. It results in that the water is blocked and the oil penetrates the mesh [29,30]. In addition, meshes with switchable surface wettability have also been developed for controllable oil/water separation, including the control of pH values [31], thermo-treatment [32], or multiple stimuli [33]. Copper becomes one of the most common engineering metal materials because of excellent

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ACCEPTED MANUSCRIPT thermal conductivity and malleability [34]. To obtain superhydrophobic surfaces on copper substrate, most researchers find ways to fabricate micro/nanostructured copper hydroxide or copper oxide on the copper surface, followed by modified with organic materials [35,36]. Achieving a superhydrophobic surface without chemical modification remains a great challenge. Interestingly, few reports showed that micro or nano CuO films can achieve superhydrophobicity without chemical

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modification [37,38]. For example, Archana et al. fabricated a multiscale superhydrophobic CuO/Cu(OH)2 film on copper substrate with a one-step solution-immersion process without chemical

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modification, and the superhydrophobic property and superhydrophilic property could transform

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under the change of plasma treatment and dark storage [39]. Wang et al. found the result that pristine superhydrophilic nanostructured CuO surface could spontaneously transform to superhydrophobic

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after exposed in air at room temperature for about 3 weeks because of the adsorption of oxygen molecules on the surface [40]. Until now, copper mesh was mainly with superamphiphilic

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(superhydrophilic and superoleophilic in air) micro/nanostructured copper hydroxide or copper oxide for oil/water separation [2,14]. However, the superhydrophobic copper mesh without chemical

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modification for oil-water separation has not been reported, particularly a comparative study of

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separation efficiency of the superhydrophobic-superoleophilic copper mesh and the superamphiphilic (superhydrophilic-superoleophilic) copper mesh. Herein, we prepared four different surface morphologies, including needle-like, bamboo

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leaf-like, pine needle-like, and peony flower-like structures, on Cu mesh by controlling the oxidation time and oxidation temperature of the chemical reaction. Wettability of these meshes was

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investigated, and these freshly prepared meshes all exhibit superhydrophilic-superoleophilic property after dried with a hair dryer. We then studied the separation efficiency of these superhydrophilic-superoleophilic meshes. After storage in air for more than 20 days, we investigated the

wettability

of

these

meshes

again

and

found

that

they

all

presented

superhydrophobic-superoleophilic property. Durability of these superhydrophobic-superoleophilic meshes was studied by some tests, including immersing into water, peeling test, heating treatment, and droplets with different pH. We investigated the separation efficiency of them again, and studied the reason for the transition of superhydrophilic-superoleophilic to superhydrophobic-superoleophilic

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ACCEPTED MANUSCRIPT without chemical modification by characterizing the surface morphology and chemical composition. In addition, we compared the separation efficiency of the superhydrophilic-superoleophilic mesh and the superhydrophobic-superoleophilic mesh. 2. Experimental section 2.1 Materials

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Hydrochloric acid (HCl, ~36%), sodium hydroxide (NaOH, AR), absolute ethanol (AR) were purchased from Xilong Chemical Co., Ltd. Ammonium persulfate ((NH4)2S2O8, AR) was purchased

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from Sinopharm Chemical Reagent Co., Ltd. All reagents were used as received without further

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purification. Deionized (DI) water was obtained using the deionized water equipment (YL-100A, ELION, China). Copper mesh (250 mesh, ~58 µm) and peanut oil were purchased from a local

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market. 2.2 Sample Preparation

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Copper mesh was cut into 4 × 4 cm2 samples, and then put into 10 mL of HCl (0.1 mol/L) for 10 min to remove organic contaminants and oxide layers. Subsequently, the copper meshes were

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ultrasonically cleaned in absolute ethanol and DI water for 10 min, respectively. Pre-cleaned copper

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meshes were immersed in a mixed alkaline solution of 50 mL, including NaOH (2.5 mol/L) and (NH4)2S2O8 (0.1 mol/L). These meshes were washed in DI water and dried in an oven of 80℃ for 30 min. To develop surface morphologies of the prepared copper meshes, they were immersed into a

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mixed alkaline solution under different conditions. Table 1 shows different experimental conditions

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for preparing S1, S2, S3, and S4.

Sample

Table 1. Experimental conditions for different samples.

Mixed alkaline solution

Temperature

Time

ID

(℃)

(min)

S1

Room temperature (21℃)

10

S2

2.5 mol/L of NaOH and

Room temperature (21℃)

30

S3

0.1 mol/L of (NH4)2S2O8

Room temperature (21℃)

50

40 ℃

30

S4

2.3 Characterization Oxidative mesh with different surface morphologies was characterized by a field-emission scanning electron microscopy (FE-SEM, NanoSEM450, FEI) with an operating voltage of 15 kV.

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ACCEPTED MANUSCRIPT XRD patterns were obtained using X-ray diffraction (XRD, Rigaku D/Max 2500PC, Japan) with Cu Kα radiation (λ= 1.5406 Å), an accelerating voltage of 40 kV, an applied current of 100 mA and 2θ of 10-95°. Contact angle (CA) was measured on a SL 200B contact angle system (KINO, America) at ambient temperature with a droplet volume of ∼5 μL. An average of three measurements at different positions of the sample surface was applied to calculate the final CA. Surface elements of these

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obtained meshes were investigated using an X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermo) equipped with a standard monochromator Al Kα source and the binding energy 285 eV of C

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1s in hydrocarbon as reference. There was no argon sputtering cleaning procedure was applied

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during the measurement. 2.4 Oil–water separation

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Various oils, including light oil of hexane, toluene, peanut oil, paraffin oil, silicone oil, and heavy oil of chloroform, dichloromethane were used for testing the efficacy of oil/water separation in

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many articles [41,42]. However, most of above oils are not conducive to the environment and researchers’ physical health. Therefore, we only choose peanut oil, light oil, for oil/water separation

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in this work. In addition, to make a better visual distinguish oil and water, water was colored by

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methyl orange. For superhydrophilic-superoleophilic copper meshes, it was pre-wetted by oil for oil-remove and pre-wetted by water for water-remove. For the superhydrophobic-superoleophilic copper meshes, it was just pre-wetted by oil for oil-remove. Oil/water separation efficiency (f) of

f 

V1 100% V0

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copper meshes was calculated according to the following equation [43]: (1)

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where V0 and V1 are the volumes of oil in oil-remove (water in water-remove) before and after separation, respectively.

3. Results and discussion 3.1. Surface morphology and composition of copper meshes. Surface morphologies of the pristine Cu mesh and the Cu meshes immersed into mixed alkaline solution under different conditions of Table 1 are shown in Figure 1, and the surface composition of them is investigated by XRD patterns (Figure 2). The pristine Cu mesh is smooth with the mesh pore of about 58 µm (Figure 1(a)), and the XRD pattern shows four strong peaks of Cu (Bare Cu mesh in 6

ACCEPTED MANUSCRIPT Figure 2). When the mesh was immersed into the mixed alkaline solution at room temperature for 10 min (S1), as shown in Figure 1(b), needle-like structures covered the mesh surface. XRD pattern of S1 in Figure 2 shows that several weak peaks associated with Cu(OH)2 (JCPDS card no. 00-003-0307) appeared on the mesh surface. The visible color of S1 is blue (Figure S1), which is the typical color of Cu(OH)2, confirming the formation of Cu(OH)2 after oxidation of 10 min at room

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temperature. When the oxidation time was 30 min at room temperature (S2), the width of needle-like structures increased and the surface morphology converted into bamboo leaf-like structures (Figure

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1(c)). The XRD pattern of S2 (Figure 2) still includes Cu(OH)2 as well as Cu, and the visible color of

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S2 is also blue (Figure S1). If the oxidation time was extended to 50 min (S3), as shown in Figure 1(d), the bamboo leaf-like structures gradually changed to pine needle-like structures. XRD pattern

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of S3 (Figure 2) shows that CuO (JCPDS card no. 01-089-2529) is formed on the mesh surface besides of Cu(OH)2 and Cu. The visible color of S3 is black (Figure S1), which is the typical color of

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CuO, showing the formation of CuO after oxidation of 50 min at room temperature. Once the temperature increased to 40 ℃ and the oxidation time was 30 min (S4), as shown in Figure 1(e), the

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peony flower-like structures were formed on the mesh surface. XRD pattern of S4 (Figure 2) shows

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that there are only CuO and Cu, while Cu(OH)2 disappears. Therefore, we obtained four different surface morphologies, including needle-like, bamboo leaf-like, pine needle-like, and peony flower-like structures, on the copper mesh surface. Meanwhile, the chemical composition of the

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oxidative copper mesh surface changed from Cu to Cu(OH)2, then to CuO with the increase of oxidation time or oxidation temperature, which was also confirmed by the visible color of copper

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meshes. The oxidation mechanism of Cu meshes immersed into the mixed alkaline solution of NaOH (2.5 mol/L) and (NH4)2S2O8 (0.1 mol/L) is depicted in equation (2) [44]. Then, Cu(OH)2 can transform into a more stable state of CuO (equation (3)) [45]. Cu  2OH 1  S2O82  Cu (OH )2  2SO4 2

(2)

Cu(OH )2  CuO  H 2O

(3)

Above results demonstrated that the surface morphology of copper meshes could be controlled by tuning the oxidation time or oxidation temperature of the oxidation process.

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Figure 1. SEM images of (a) bare Cu mesh, (b) S1, (c) S2, (d) S3, and (e) S4. Horizontal images are different

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magnification, and vertical images are the same magnification.

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Figure 2. XRD patterns of bare Cu mesh, S1, S2, S3, and S4.

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3.2. Wettability of superhydrophilic-superoleophilic copper meshes. Firstly, wettability of these untreated and treated copper meshes was evaluated in the air

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environment (Figure 3). Water and oil contact angles on the untreated copper mesh are 117.96° and

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67.27°, respectively. After oxidation treatment, when water and oil droplets were put onto S1, S2, S3, and S4, all droplets were instantaneously spread, with both water and oil contact angles approximately equal to 0°, showing superhydrophilic-superoleophilic (superamphiphilic). Afterwards,

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these treated meshes (S1, S2, S3, and S4) were immersed into peanut oil. As observed in Figure 4(a), dyed water droplets on these treated meshes under peat oil are all of spherical shapes, and contact

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angles of S1, S2, S3, and S4 are 153.29°, 153.12°, 154.07°, and 154.39°, respectively, exhibiting under-oil superhydrophobicity. Taking S4 as the example, as shown in Figure 4(b), the water droplet also can roll on the treated copper mesh, with the sliding angle about 7±1°, and the details were recorded in Video S1a and Video S1b. Reason for this disparity is that the peanut oil trapped in the microstructures of the superhydrophilic-superoleophilic mesh surface, minimizing the contact opportunity between water droplets and the mesh under oil. Wettability change of the water droplet in air and under oil can attribute to the following equation [28]:

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cos  wo 

 w cos w   o cos o  wo

(4)

In equation (4), θw, θo, and θwo are water contact angle in air, oil contact angle in air, and water contact angle in oil, respectively. γw, γo, and γwo are the interfacial tension of water-air, oil-air, and water-oil interfaces, respectively. When under-oil superhydrophobic, cosθwo<0, which means

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cosθw>0 and θw should be lower than 90°. According to above verifications, superhydrophilic

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materials in air should be suitable to achieve under-oil superhydrophobic property.

Figure 3. Photographs of the water droplet and water contact angle or oil droplet and oil contact angle on the

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freshly prepared copper meshes after dried with a hair dryer in air.

Figure 4. (a) Underoil−water droplet and the water contact angle on the freshly prepared four meshes after dried with a hair dryer. (b) A water droplet sliding on a slightly inclined oil pre-wetted S4 under oil.

3.3. Oil-water separation of superhydrophilic-superoleophilic copper meshes. Superhydrophilic-superoleophilic behavior of the prepared copper mesh was applied to gravity directed oil/water separation [46]. For these superhydrophilic-superoleophilic copper meshes, taking S4 as the example, it was first pre-wetted by peanut oil for oil-remove. Figure 5 and Video S2 depict the separation process that a mixture of oil/water, 5 mL of each (50%, v/v), is poured onto the

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as shown in Video S3a. Oil was collected firstly because it cannot penetrate the mesh. However, there is still oil left on the mesh, which hinders the penetration of water, resulting in poor separation.

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Therefore, to avoid the effect of residual oil, as shown in Figure 6 and Video S3b, we make water

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first out using a syringe. Water can easily penetrate the mesh pre-wetted with water under the influence of gravity, and oil can be collected because it cannot penetrate the mesh. By this way, the

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collected volumes of water and oil were both about 5 mL, indicating high separation efficiency. Figure 7 shows separation efficiency of these prepared meshes with different surface morphologies,

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including S1, S2, S3, and S4. It can be found that the separation efficiency of all meshes is high, and the separation efficiency for oil-remove (~96%) is higher than that of water-remove ((~94%)). In

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addition, as shown in Figure 8, the separation efficiency of the superhydrophilic-superoleophilic

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copper mesh was still larger than 92% even after 10 cycles, confirming excellent stability of the

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separation efficiency.

Figure 5. Photographs showing the separation status of S4 pre-wetted by peanut oil for oil-remove.

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Figure 6. Photographs showing the separation status of S4 pre-wetted by water for water-remove.

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Figure 7. Separation efficiency of the obtained copper meshes.

Figure 8. Separation efficiency variations of S4 (a) pre-wetted by peanut oil for oil-remove and (b) pre-wetted by water for water-remove during multi-cycles separation tests.

3.4. Wettability of superhydrophobic-superoleophilic copper meshes. It is intriguing that, after storage in air for more than 20 days, these four samples transformed from superhydrophilic to superhydrophobic without chemical modification. Next, we will take S1 and S4 as examples to study wettability of the as-stored copper meshes. As displayed in Figure 9(a), water droplets could stand on the as-stored S1 surface, showing almost spheres with the water contact angle of up to 153.56°, while peanut oil droplets are spreading on the mesh surface with the 12

ACCEPTED MANUSCRIPT oil contact angle close to 0°. When the superhydrophobic mesh was submerged into water, it showed clear silver mirror-like phenomenon. As shown in Video S4a, we make the mesh surface inclined, and water droplets can bounce off the mesh surface with no residues, indicating that water droplets can bounce off or roll easily over the inclined mesh. This is because air trapped among the microstructures of the mesh surface, resulting in a solid-liquid-air interface, which can prevent water

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permeation [37]. As shown in Figure 9(b), water droplets on the as-stored S4 mesh surface also show spheres with the water contact angle of about 154.27° and the peanut oil droplets are wetting on the

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mesh surface with the oil contact angle about 0°. We also can see clear silver mirror-like

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phenomenon (Figure 9(b)) and water droplets can bounce off the inclined mesh surface (Video S4b). Above results indicate that, after stored in air for adequate duration, these as-stored mesh surfaces

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have transformed from superhydrophilic to superhydrophobic without chemical modification.

Figure 9. Superhydrophobic behaviors of (a) S1 and (b) S4 after stored in air for more than 20 days: photographs

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of water droplets stay on the mesh surface, and the silver mirror-like phenomenon after immersed into water.

To investigate the transition progress of the wetting behavior from superhydrophilic to superhydrophobic without chemical modification, the water contact angle variation of S1 and S4 with time was measured as shown in Figure 10. When these two treated copper meshes stored in air longer than 12 days, the water contact angle increased to larger than 90°, switching to hydrophobic. After stored in air longer than 20 days, the water contact angle increased to over 150°, resulting in transition to superhydrophobicity.

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Figure 10. Water contact angle variation of (a) S1 and (b) S4 when they stored in air for diverse periods of time.

the

mechanism

of

wettability transition

from

superhydrophilic

to

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To understand

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superhydrophobic without chemical modification, as shown in Figure 11, we also take S1 and S4 as examples to compare surface morphologies and XRD patterns of the fresh and as-stored copper

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meshes. As shown in Figure 11(a), we can find that surface morphologies of the fresh and as-stored S1 are both needle-like structures and XRD patterns of S1 are both with Cu(OH)2 and Cu. As shown

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in Figure 11(b), the peony flower-like structures and the crystal structure (CuO and Cu) of fresh and as-stored S4 are also the same. Above results show that the surface morphology and the crystal

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structure of the obtained meshes has no change after stored in air more than 20 days. Therefore, the

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transformation from superhydrophilic to superhydrophobic without chemical modification has

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nothing to do with the change of the surface morphology and the crystal structure.

Figure 11. Comparison of surface morphologies and XRD patterns of the fresh and as-stored (more than 20 days) mesh surfaces: (a) S1 and (b) S4. 14

ACCEPTED MANUSCRIPT XPS spectra of as-stored S1 and S4 were investigated to further understand the reason for transformation from superhydrophilic to superhydrophobic without chemical modification. As shown in Cu 2p spectrum of Figure 12(a), it has peaks at 934.3 eV and 954.1 eV, corresponding to Cu 2p3/2 and Cu 2p1/2, indicating the existence of Cu2+ [47] on S1 surface, and this is due to the formation of Cu(OH)2 [44,48]. As shown in Cu 2p spectrum of Figure 12(b), it also has Cu 2p3/2 and Cu 2p1/2 on

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S4 surface, corresponding to Cu2+, and it arises from CuO [44,48]. Visible colors of S1 and S4 were still blue and black, respectively, after stored in air for more than 20 days, which verified the

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existence of Cu(OH)2 on S1 surface and the formation of CuO on S4 surface. The satellite structures

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at around 962.8 eV, 944.2 eV, and 942.2 eV binding energies confirm the presence of Cu2+ [30]. Apparently, above results demonstrate the existence of Cu(OH)2 on S1 surface and the formation of

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CuO on S4 surface, corresponding to XRD results. As displayed in O 1s spectrum of Figure 12 (a) and (b), the peak located at 531.1 eV belongs to the lattice O of CuO on S1 surface and Cu(OH)2 on

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S4 surface, respectively [49], while the peak at 529.6 eV is because of the adsorbed O both on S1 and S4 surfaces [40]. The adsorbed O can achieve the transition from superhydrophilic to

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of these as-stored mesh surfaces.

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superhydrophobic without modification because it contributes to air trapped into the microstructures

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Figure 12. XPS spectra (Cu 2p and O 1s) of the as-stored mesh surfaces: (a) S1 and (b) S4.

3.5. Durability of superhydrophobic-superoleophilic copper meshes.

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To examine mechanical durability of these obtained superhydrophobic-superoleophilic meshes,

test,

heating treatment

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taking S1 and S4 for examples, we performed some tests, including immersing into water, peeling and droplets

with different

pH,

on

meshes

[50-52]. Firstly,

superhydrophobic-superoleophilic meshes were immersed into water for different times. As shown in

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Figure 13(a), the water contact angle maintained stable, showing immersion into water has no obvious effect on the superhydrophobicity. Then, we investigated stability of these meshes using tape

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for a number of peeling tests. Figure 13(b) displays the effect of peeling times on water contact angles of the superhydrophobic-superoleophilic copper meshes. Water contact angles were still larger than 150° after 7 times of peel tests, and there was no damage on the meshes. These superhydrophobic-superoleophilic meshes were heated at different temperatures for 1 h. We measured water contact angles of these meshes after cooling, and found that the water contact angle also

remained

stable

larger

than

150°

(Figure

13(c)),

indicating

that

these

superhydrophobic-superoleophilic copper meshes had good thermal stability. Finally, we investigated contact angles of droplets with different pH on these superhydrophobic-superoleophilic copper

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ACCEPTED MANUSCRIPT meshes. Droplets with pH less than 7 were adjusted with H2SO4, and that greater than 7 were adjusted with NaOH. We can see that contact angles fluctuate above 150°, showing that these obtained superhydrophobic-superoleophilic meshes have good stability in whole pH conditions. Moreover, their superhydrophobicity maintained for more than 6 months after exposure to air,

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showing excellent long-term stability.

Figure 13. Effects of (a) immersing time in water, (b) peeling times, (c) heating temperature, and (d) droplets with

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different pH on water contact angles of S1 and S4.

3.6. Oil-water separation of superhydrophobic-superoleophilic copper meshes.

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Taking S4 for example, a mixture of oil/water, 5 mL of each, was poured onto the superhydrophobic-superoleophilic copper mesh, as shown in Figure 14 and Video S5. Oil droplets immediately spread and permeated through the copper mesh because of superoleophilicity, whereas the water droplet rolled down from the mesh due to its superhydrophobicity. After separation, the collected volumes of water and oil were both about 5 mL, indicating high separation efficiency. Comparison of the separation efficiency of S4 between superhydrophilic-superoleophilic and superhydrophobic-superoleophilic property, as shown in Figure 15(a), we find that the separation efficiency of superhydrophobic-superoleophilic property (~99%) is higher than that of the

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95%, confirming the excellent stability of the separation efficiency.

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Figure 14. Photographs showing the separation process of the superhydrophobic-superoleophilic S4 copper mesh.

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Figure 15. (a) Comparison of the separation efficiency between superhydrophilic-superoleophilic and

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superhydrophobic-superoleophilic copper meshes; (b) Separation efficiency of the superhydrophobic-superoleophilic mesh changed with the number of cycles.

4. Conclusions

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In summary, we have fabricated four superhydrophilic-superoleophilic copper meshes with different surface morphologies, including needle-like, bamboo leaf-like, pine needle-like, and peony

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flower-like structures, by controlling the oxidation time and oxidation temperature. They all have excellent separation efficiency, and the separation efficiency for oil-remove (~96%) is higher than that of water-remove (~94%). Interestingly, these copper meshes with four different surface morphologies

can

transform

from

superhydrophilic-superolephilic

to

superhydrophobic-superolephilic just after storage in air for more than 20 days without any further chemical modification. This is because the oxygen adsorption on the mesh surface contributes to air trapped among the microstructures. These superhydrophobic-superolephilic copper meshes have good durability. Moreover, we find that the separation efficiency for oil-remove of

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property

(~99%)

is

higher

than

that

of

the

superhydrophilic-superoleophilic property (~96%). Acknowledgments Authors acknowledge the financial support of the National Natural Science Foundation of China (No. 51075184) and Scientific Research Foundation of Shandong University of Science and

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Technology for Recruited Talents (2017RCJJ016). References

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ACCEPTED MANUSCRIPT surfaces

prepared

by

a

facile

one-step

solution-immersion

process:

transition

to

superhydrophilicity with oxygen plasma treatment, Journal of Physical Chemistry C 115 (2011) 18213-18220. [50] S. Zheng, C. Li, Q. Fu, W. Hu, T. Xiang, Q. Wang, M. Du, X. Liu, Z. Chen, Development of stable superhydrophobic coatings on aluminum surface for corrosion-resistant, self-cleaning, and

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combustion, Colloids and Surfaces A: Physicochemical and Engineering Aspects 492 (2016)

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110-118.

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ACCEPTED MANUSCRIPT Highlights 1. Four different surface morphologies, including needle-like, bamboo leaf-like, pine needle-like, and peony flower-like structures, were prepared on Cu meshes. 2. They all have excellent separation efficiency, with oil-remove (~96%) higher than that of water-remove (~94%).

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3. Superhydrophilic-superoleophilic meshes can transform to superhydrophobic-superoleophilic just after storage in air for more than 20 days without chemical modification.

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of the superhydrophilic-superoleophilic Cu mesh (~96%).

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4. Separation efficiency of the superhydrophobic-superoleophilic mesh (~99%) is higher than that

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