Facile Construction of Copper Mesh Surface from Superhydrophilic to Superhydrophobic for Various Oil-Water Separations

Xudong Zhang ^{1, #}, Yamin Pan ^1, 2, #, Junyang Zhao ¹, Xiaoqiong Hao ³, Yaming Wang ^{1, *}, Dirk W Schubert ², Chuntai Liu ¹, Changyu Shen ¹ and Xianhu Liu^{1, 2, *}

¹ College of Materials Science and Engineering; National Engineering Research Center for Advanced Polymer Processing Technology, Zhengzhou University, Zhengzhou 450001, China

² Institute of Polymer Materials, Friedrich-Alexander University Erlangen-Nuremberg, Martensstr. 7, 91058 Erlangen, Germany

³ School of Science, Xi'an Jiaotong University, 28 Xianningxi, Xi'an, Shaanxi 710049, China

*Email: xianhu.liu@zzu.edu.cn (X. Liu) or wangyaming@zzu.edu.cn (Y. Wang)

^#They contributed equally to this work.

 

Abstract

Specially wettability materials offer a new pathway for effective oil-water separation. However, these materials with desired wettability, simple process and low cost as well as high efficiency separation are still urgently needed. Herein, facile strategies, acid etching and PDMS coating, were reported for the construction of copper mesh surfaces with special wettability from superhydrophilic to superhydrophobic. In the first acid etching, copper mesh surface exhibits a superhydrophilic and underwater superoleophobic properties, and the etched copper mesh with high separation efficiencies (92%) was obtained. After PDMS coating, a low adhesion superoleophilic and superhydrophobic copper mesh shows high water contact angle of 153°, high flux (~31000 L*m^-2*h^-1), excellent anti-icing and self-cleaning performances. Additionally, after UVB, mechanical wear, hot water and acid/alkaline solution treating, the coated mesh still showed excellent water-repellency performance. Moreover, the coated mesh by connecting with a vacuum pump was used for continuous oil-water separation. This work provides not only a simple and efficient method to fabricate controllable wettability copper mesh with oil-recovery ability but also a new opportunity for potential applications in continuous oil-water separation.

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Table of Content

A facile method was proposed to constructe the copper mesh surface from superhydrophilic to superhydrophobic for various oil-water separations

 

 

Keywords

Low adhesion          Oil-water separation           Superhydrophobic           Superoleophilic

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1.Introduction

In the oil extraction process and industrial production, a lot of oil-water mixture is produced. If improperly handled, it may cause waste of resources, endanger human health and cause pollution to environment^{.1, 2} People solve this problem by using oil-water separation materials. These materials require different wettability for water and oil. Superhydrophobic materials have attracted research interests of many researchers with a broad application prospects in anti-icing, self-cleaning, oil-water separation and so on.^3-7 Specially wettability materials are mainly related to their surface roughness and surface free energy.⁸

Superhydrophobic mesh surfaces with the micro/nano-structures have been reported by chemical vapor deposition,⁴ spray deposition.⁹ hydrothermal method,¹⁰ foaming,^11-15 and so on^16-20. However, above methods usually need complex equipment and high cost. The chemical etching method can get a micro/nano-structure on the material surfaces with low cost, low energy consumption, no precision instruments required. ^{21, 22} Commercially available raw material copper mesh has low price and can be widely selected. Due to its own uniquely porous structure, it provided natural channels for the oil to be collected during the oil-water separation process. The filtration method is not affected by the amount of adsorption, easy to use, and no further separation of oil and separation materials. As is well known, micro/nano-structures and low surface energy substance simultaneously exist on the surface of material, the surface of the material exhibits superhydrophobic. PDMS (Polydimethylsiloxane) was commonly used for low surface energy modification, because of the contains low surface energy elements and group such as Si, C and methyl etc.²³ In this work PDMS was used to low surface energy modification of the substance.

In this work, a superhydrophobic and superoleophilic copper mesh with micro/nano- structures was reported via a rapid, low cost and highly effective solution process and low surface energy modification though coating with PDMS. The prepared copper mesh used for oil-water separation possessing high flux was fabricated with two-step micro/nano-structure formation and low surface energy modification. The key point in this research is to provide a simple method to create a micro/nano-structure which is through a simple displacement reaction. The prepared sample has a waterproof impact property while can be used for continuous oil-water separation.

2. Experimental

2.1 Materials

Copper meshes with pore diameters of 60 µm were bought from Shenzhen Ante Metal Wire Mesh Product Factory. Silver nitrate (AgNO₃) and PDMS with its curing agent were purchased at Sinopharm Group Chemical Reagent Co., LTD and Dow Corning, respectively. All other chemicals including anhydrous ethanol, hexane and xylene were bought from Tianjin Fuyu Fine Chemical Co., Ltd. All chemicals were used as received without further purification.

2.2 Preparation of specially wettable mesh

Copper meshes (200 mesh size) were first ultrasonically cleaned for 10 min in the solution of deionized water, alcohol, acetone and concentration of 20% sulfuric acid in order to remove the oxide and any organic layers. After drying, they were immersed in the solution of 0.01 M AgNO₃ and 100 mL ethanol at room temperature for 30 s. Then the meshes were rinsed with deionized water and dried in the blast oven at 80 °C for 20 min (Cu-Ag). For the coated meshes (Cu-Ag-P), Cu-Ag was immersed into a solution of 3 g PDMS and 100 mL xylene at room temperature for 10 s and dried at 80 °C for 120 min (Fig. S1).

2.3 Characterization

The surface structure of the rough surface was observed by a field-emission scanning electron microscope (Hitachi, S-4800, Japan, 20kV). Chemical composition data of films on copper mesh was characterized by EDS. Contact angle meter (SL200KS, KINO, USA) with a liquid droplet of 2 μL was used for water contact angle (WCA) and oil contact angle (OCA) test. Fourier Transform Infrared spectrophotometer (FT-IR, Nicolet 6700, USA) were used to examine the chemical composition. Droplet impact test was used a water droplet of 6 μL at a height of 6 cm, and a high-speed camera (Qianyan Lang 2F04, Junda Hi-Tech Co., Ltd.) captured the drop process at a frequency of 1000 frames.

2.4 Oil-water separation

Vacuum pump oil, silicone oil soybean oil, hexane and dichloromethane were colored with Sudan III. Cu-Ag-P was fixed on the filtration devices for the mixtures of oil and water (50% v/v) separation. The separation efficiency was calculated by G=(m₁/m₀)×100%, where m₀ and m₁ were the masses of the water before and after separation, respectively.

2. Results and discussion

Fig. 1 SEM of the (a) copper mesh, (b) etched copper mesh (Cu-Ag); EDS analysis of the surface element of the (c) copper mesh and (d) Cu-Ag.

The raw copper mesh knitted by wires has an average pore size of about 60 μm (Fig. 1a). From the SEM of copper mesh, we can find that in macroscopically its relatively smooth but further enlarged there are small gully structure which provides a larger area to reaction with AgNO₃. After etching, the silver particles attach the surface of wires evenly, and the surface of etched copper mesh (Cu-Ag) becomes roughness (Fig. 1b). As shown in the picture there are petal-like micro/nano-structure on the surface of copper wires which can significantly change the wetting properties of the material surface. The surface elements of the copper mesh were analyzed by EDS (Fig.1c). From the EDS spectra, the Cu, and Zn elements were found in the surface. Ag elements was found on the Cu-Ag surface (Fig.1d), which certificate that the micro/nano-structures were composition by Ag, and the surface was modified by the Ag.

Fig. 2 (a) OCA in air and (b) underwater of Cu-Ag; (c) WCA in air and (d) under cyclohexane of Cu-Ag; (e) The separation experiment of pump oil-water mixture by the Cu-Ag; (f) Oil-water separation efficiency in 10 cycles.

Cu-Ag displayed Superhydrophilic/Superlipophilic in air but underwater oleophobicity and under cyclohexane hydrophobic. This is attributed to the combination effect of the changed surface composition and the enhanced surface roughness. The (water or oil) droplet deposited on the Cu-Ag surface spread rapidly, indicating an OCA and WCA of 0° (Fig. 2a and c). When a trichloromethane or water was placed on the Cu-Ag in water (Fig. 2b) or cyclohexane (Fig. 2d), it still maintained sphere, showing excellent underwater oleophobicity or under cyclohexane hydrophobic with OCA of 132° and WCA 135°. Comparison with the WCA of the original copper mesh 120°, the Cu-Ag surface with the deposition of silver micro-nano structures form on the surface and the chemical composition of the surface change has changed. According to the Wenzel model with the increasing of the microstructure on the surface of a hydrophilic material while increases the hydrophilicity of the material. ^{24, 25}

Due to the superhydrophilicity and underwater oleophobicity of the Cu-Ag (pre-wetted by water), water permeated through it while pump oil was blocked, and no pump oil was visible in the separated water (Fig. 2e). Fig. S2 shows the oil-water separation mechanism of the Cu-Ag. The separation efficiencies of Cu-Ag are all above 92% during 10 cycles (Fig. 2f), indicating quite stable and durable superwettability of the etched copper meshes.²⁶

Fig. 3 (a) WCA and (b) OCA on the surface of Cu-Ag-P; (c) immersion the Cu-Ag-P in water; (d) photo of Cu-Ag-P and copper mesh in the water after shake; (e) water flow impact on Cu-Ag-P surface; (f) immersion the Cu-Ag-P in water (dyed by KMnO₄) by force; (g) self-cleaning of the Cu-Ag-P though the removal of CNT from the surface using water droplets; (h) Photographs of the water drops frozen on the surface of Cu-Ag-P.

After coated PDMS, the chemical composition (Fig. S3) and surface element (Fig. S4) results shows that low surface energy materials have been successfully combined. As shown in Fig. 3a, the water can retain a spherical state on the Cu-Ag-P surface. However, the oil droplets spread rapidly and exhibited superlipophilic properties (Fig. 3b). After the Cu-Ag-P immersed into water, a bright plastron layer can be observed (Fig. 3c) due to the total reflectance of light at air layer trapped on the surface. This air layer can even make the copper mesh and the Cu-Ag-P float on the calm water. However, when shake the beaker, copper mesh can sink easily while the Cu-Ag-P is still floating on the water (Fig. 3d).

The lower adhesion properties of the samples are important for self-cleaning and anti-fouling. Fig. 3e shows the impact of water flow on the Cu-Ag-P surface. The water flow completely reflected, and no water droplets remain in the impact position. In addition, after the Cu-Ag-P was completely immersed in water and removed from the water (Fig. 3f) for 3 times, there was no water droplets on the Cu-Ag-P surface. Low adhesion of Cu-Ag-P to water. To check the self-cleaning properties, self-cleaning tests were carried out. The water droplets on the copper mesh surface appearing as a hemisphere (Fig. 3g), and the carbon nanotubes (CNTs, as a conventional pollutant) were stayed in place. However, when the water dripped on the Cu-Ag-P surface, it rolled away to remove CNTs, leading a clean surface. Above results show that the Cu-Ag-P has good self-cleaning property.

The actual anti-icing performance of the Cu-Ag-P was studied via static experiments in low temperature environment (−4°C). In the static anti-icing experiment, a 10 μL water drop was put on the Cu-Ag-P. The freezing process was observed and recorded by a camera. (Figure 3h). The water drop can retain a spherical state on the Cu-Ag-P surface, after 12h the water droplets are completely frozen. This frozen water droplet still remains spherical. At the same time, we can also find other small spherical droplets that are frozen around it. This phenomenon can prove the low adhesion of the material surface.

Table 1. Surface tension of tested liquids.

Tested liquid	Surface tension (mN/m)	Dispersion component (mN/m)	Polar component (mN/m)
Water	72.8	21.8	51.0
Glycol	48.3	29	19.3

To prove this phenomenon, Owens formula were utilized to calculate the surface energy.²⁷ Water and glycol were chosen as tested liquids. Surface tension of the tested liquids was shown in Table1.

 $\mathit{1}\mathit{ }\mathit{+}\mathit{ }Cos{\theta }_{o\mathit{ }}\mathit{ }\mathit{=}\mathit{ }\mathit{2}\sqrt{{\gamma }_{sg}^d}\mathit{\left(}\raisebox{1ex}{$\sqrt{{\mathit{\gamma }}_{\mathit{l}\mathit{g}}^{\mathit{d}}}$}\!\left/ \!\raisebox{-1ex}{$\sqrt{{\mathit{\gamma }}_{\mathit{l}\mathit{g}}}$}\right.\mathit{\right)}\mathit{ }\mathit{+}\mathit{ }\mathit{2}\mathit{ }\sqrt{{\gamma }_{sg}^p}\mathit{\left(}\raisebox{1ex}{$\sqrt{{\mathit{\gamma }}_{\mathit{l}\mathit{g}}^{\mathit{p}}}$}\!\left/ \!\raisebox{-1ex}{$\sqrt{{\mathit{\gamma }}_{\mathit{l}\mathit{g}}}$}\right.\mathit{\right)}$               (1)

${\gamma }_{\mathit{l}\mathit{g}}^{\mathit{d}}$ and ${\gamma }_{lg}^p$  are the dispersion component and polar component of liquid surface tension, respectively. And ${\gamma }_{sg}^d$  and ${\gamma }_{sg}^p$  are the dispersion component and polar component of solid surface energy, respectively. ${\gamma }_{\mathit{l}\mathit{g}}$ is the liquid surface tension. The calculated results were shown in Table 2. From the results of the surface energy, the superhydrophobic copper mesh displayed energy decreased in comparison with the rough mesh and the blank mesh. This result was consistent with the contact angles of the modified samples.

Table 2. Results of calculated surface energy of the sample.

Sample	WCA (°)	Glycol CA (°)	${\mathit{\gamma }}_{\mathit{s}\mathit{g}}^{\mathit{d}}$ (mN/m)	${\mathit{\gamma }}_{\mathbf{s}\mathbf{g}}^{\mathbf{p}}$ (mN/m)	Surface energy (mN/m)
Copper mesh	127	100	19	0.7	19.7
Cu-Ag-P	151	130	5	0.7	5.7

 

Fig. 4 (a) Rebound height of water droplets vs. time. Insets show the time evolution of droplet-impact dynamics upon the Cu-Ag-P surface; (b) WCA and height (H is the height of the drop, H_o is the initial height before water droplets evaporate) during the evaporation of water droplets. Insets are the image of droplet evaporation on the (I) copper mesh and (II) Cu-Ag-P surface.

The impingement of water droplets on surface is inevitably during practical applications. To prove the anti-water impact performance, water droplets of 6 μL with a velocity of 1.09 m/s and fall height of 6 cm were used for water droplet impact experiment. The water hammer pressure (${\rho }_{\mathit{W}\mathit{H}}$ ) can be approximated by Eq. (2)