Article pubs.acs.org/Langmuir
Highly Efficient and Robust Oil/Water Separation Materials Based on Wire Mesh Coated by Reduced Graphene Oxide Jing Chen,*,† Kaiyong Li,*,‡ Haodong Zhang,† Jie Liu,§ Shuwang Wu,§ Qingrui Fan,§ and Han Xue§ †
Department of Chemistry, School of Science, Tianjin University of Science and Technology, Tianjin Economic and Technological Development Area Campus, No. 29, 13th Avenue, Tianjin Economic and Technological Development Area, Tianjin 300457, People’s Republic of China ‡ School of Materials Science and Engineering, Luoyang Institute of Science and Technology, Wangcheng Road, Luoyang 471023, People’s Republic of China § Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun North First Street 2, Beijing 100190, People’s Republic of China S Supporting Information *
ABSTRACT: We develop a simple approach for the preparation of oil/water separation material based on the reduced graphene oxide. First, the graphene oxide (GO) is coated on the commercially available wire mesh. The treatment of O2 plasma is exploited to open the pores from the back side using the wire mesh as a ready-made mask, and the GO-coated mesh is subjected to the thermal annealing at 200 °C for 2 h to form stable superhydrophobic reduced graphene oxide (RGO) coating. The asprepared mesh has excellent stability and reusability and the separation selectivity is above 98% for a variety of mixtures of oil and water. Meanwhile, the as-prepared RGO@mesh-300 shows stable and robust superhydrophobic properties including the stability of long-term storage, the resistance to high temperatures, high humidities, and mechanical abrasion. It is expected that this method of fabricating superhydrophobic materials can find more practical applications, especially in the oil/water separation.
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reaction for the separation of oil/water.33 However, most of the methods suffer from several limitations including the use of special materials, complicated equipment, and experimental expertise.34−36 Therefore, the strategies for the preparation of separation film exhibiting stable wettability through a facile process with the advantage of scalability are highly desired.37−39 Meanwhile, the mechanical strength, repeatability of use, and long-term stability in a wide range of humidity and temperature are also essential for oil/water separation. In this work, we present a facile method to prepare oil/water separation film, which can selectively separate oil and water. First, the commercially available wire mesh was coated by graphene oxide (GO). Then the GO-coated wire mesh was treated with O2 plasma from the back side to open pores, which were obstructed by the GO film previously coated onto the wire mesh. Subsequently, the GO coating was transformed into the reduced graphene oxide (RGO) by thermal anneal, rendering the wire mesh superhydrophobic. The detailed studies of microstructure and chemical composition of the
INTRODUCTION The gradually increasing number of oil spill and production of oil-containing wastewater pose a severe threat to our aquatic environment.1−5 Thus, oil/water separation is highly desirable in a variety of fields, such as the remediation of water pollution and supply of clean water.6−10 The development of materials used for oil/water separation is not only important for scientific research, but also for the industrial communities and environmental protection.11−13 Recently, the materials with special wettability used for oil/water separation have received significant attentions.14−25 Jiang and co-workers first reported that a superhydrophobic and superoleophilic coating based on stainless steel mesh modified by polytetrafluoroethylene was used to separation of oil/water.26 Zhang et al. prepared a series of carbon-based sorbents with three-dimensional architectures for water remediation.27−30 Wang et al. fabricated a miniature collector composed of a superhydrophobic copper mesh combined with a superoleophilic polyurethane sponge to adsorb and collect the oil spill.31 Chen et al. prepared a highly efficient mesh for oil/water separation based on a superhydrophobic and superoleophilic coating of thiol−ene hybrid via the simple two-step process.32 Wang et al. fabricated the stainless steel mesh modified with ZnO by the hydrothermal © 2017 American Chemical Society
Received: June 2, 2017 Revised: July 21, 2017 Published: August 25, 2017 9590
DOI: 10.1021/acs.langmuir.7b01856 Langmuir 2017, 33, 9590−9597
Article
Langmuir Scheme 1. Schematic Drawing of the Fabrication Procedure of the Film of RGO@mesh-x
Figure 1. (a) Raman spectra of GO, GO-coated mesh after O2 plasma treatment, and RGO@mesh-300, (b) XPS spectra of bare mesh, GO-coated mesh, and GO-coated mesh treated by O2 plasma, and RGO@mesh-300, (c) FT-IR spectra of GO-coated mesh and RGO@mesh-300, and (d) XRD patterns of GO-coated mesh and RGO@mesh-300. GO aqueous solution (2 mg/mL) was prepared via dissolution of GO flakes in the ultrapure water by stirring for 5 h. Then the GO solution was dropped onto the surfaces of wire meshes and GO-coated meshes were dried in the air at the room temperature (25 °C) for 24 h. Subsequently, O2 plasma treatments (SY-DT02S, plasma cleaning instrument, 40 kHz, 100 W) were applied from the back side of the GO-coated meshes to open the pores for 10−30 min, depending on the pore size of mesh to be used. Finally, the GO-coated wire meshes were placed in an oven with a temperature of 200 °C for 2 h. The asprepared RGO-coated wire meshes with the open pores were referred as RGO@mesh-x, where x is the mesh size. Characterization. The wettabilities of the original meshes, GOcoated meshes, and RGO@mesh-x were measured at the room temperature using a CA System (DSA100, Kruss Co., Germany) with a droplet of 4 μL. The morphology of the separation film was
separation material based on RGO was also performed. The RGO-coated wire mesh exhibits a good reusable stability in harsh environments and can be recycled. The oil/water separation wire mesh in this work shows a prospective potential for a scalable and low-cost material used for water and oil purification.
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EXPERIMENTAL SECTION
Preparation of RGO-Coated Wire Mesh. The commercially available stainless steel wire meshes with the different sizes of pores (from 100 to 500 meshes) were used for the fabrication of the oil/ water separation film. The GO flake was obtained from Energy Chemical of China. First, the wire meshes were sequentially ultrasonically cleaned by acetone, ethanol and deionized water. A 9591
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Figure 2. Photographs of (a) GO-coated mesh after O2 plasma treatment and (c) RGO@mesh-300, (b, d) water contact angles on the surfaces, as shown in (a) and (c), respectively, the images of (e)−(h) show SEM images of (e) bare wire mesh, (f) GO-coated mesh, (g) GO-coated mesh after O2 plasma treatment, and (h) RGO@mesh-300. The scale bar is 50 μm. investigated by scanning electron microscope (SEM, Hitachi, S-4800, Japan) at an accelerating voltage of 5.0 kV. Raman spectra were obtained with a Renishaw Raman microscope using a 514 nm wavelength laser. The chemical compositions of surfaces were characterized by X-ray photoelectron spectroscopy (XPS, ESCALab220i-XL). Oil/Water Separation Experiments. The as-prepared RGO@ mesh-x was mounted between a vertical glass tube with a diameter of 40 mm and a conical flask. The freshly prepared mixture of oil and water was poured onto the RGO@mesh-x and spontaneously permeated quickly by the gravity of liquids. To test the separation efficiency of the mixture of oil (diesel, gasoline, petroleum ether, and hexane) and water, the conical flask was inclined at 45° to make sure that the oil can be contacted with the mesh. The oil concentration of the original mixture of oil/water and the collected water after the separation was measured by infrared spectrometer oil content analyzer (CY2000, China).
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reduced graphene oxide (RGO) due to the low cost and simplified preparation process. As shown in Scheme 1, the ultrasonically cleaned wire mesh of stainless steel is coated by GO film by the drop-coating method. However, all of the apertures are obturated after the evaporation of solvents due to the high viscidity of the GO precursor. Then O2 plasma is used to open the pores on back side of the GO-coated wire meshes, in which wire mesh acts as a ready-made mask to protect the underlying GO film and the exposed region is etched away, forming GO-coated mesh with the open pores. Then the GO-coated mesh is placed in an oven at the temperature of 200 °C for 2 h, and the GO coating is thermally reduced to RGO coating, exhibiting superhydrophobicity. The as-obtained RGO@mesh-x is directly used for subsequent experiments without any surface modifications. Characterization of RGO@mesh-300. Raman spectra of both GO, GO-coated mesh after the treatment of O2 plasma, and RGO@mesh-300 are collected to probe the structural change. As shown in Figure 1a, GO presents two broad peaks at 1354 and 1599 cm−1, corresponding to D and G band, respectively. The peak of G band is attributed to an E2g mode of graphite associated with the vibration of sp2-bonded carbon atoms. The peak of D band is related to the vibrations of carbon atoms with dangling bonds in plane terminations of disordered graphite.41 Notably, the ratio of ID/IG of GO is about 0.89, indicating the presence of the abundant defects on the GO sheets. After O2 plasma treatment, the ratio of ID/IG increases to 1.01. The significantly increased peak of D band is due to an increase in the edge defects caused by the treatment of O2 plasma. In the most cases, the defects can be ascribed to the abundant oxygen-containing groups. After the reduction, the peaks of D and G band become slightly sharp. In addition, there is no band shift in the Raman spectrum of RGO. The ratio of D/G intensity of RGO (ID/IG = 0.96) is slightly larger than that of GO (ID/IG = 0.89), which can be explained as a decrease in the size of reduced graphene domains.42 X-ray photoelectron spectroscopy (XPS) is used to further study the changes in the chemical compositions of stainless
RESULTS AND DISCUSSION
Fabrication of RGO-Coated Mesh with the Open Pores. The development of advanced materials that can separate the mixture of oil and water via a selective, efficient, and eco-friendly manner has received significant attentions. However, an inexpensive and broadly applicable approach of designing superwetting materials for the effective separation of oil/water mixtures is highly desired. As a unique hydrophilic carbon-based material, GO can be synthesized from natural graphite on a large scale and conveniently obtained and the Hummer’s method is widely used in the lab for the preparation of GO. However, Kim et al. reported that at the room temperature the multilayer GO was a metastable material in the air, which would undergo the spontaneous chemical modifications and reduction with a relaxation time of about one month based on the combinations of experimental and density functional calculation.40 Thus, the thermal, chemical, and optical reduction of GO has been explored as a route to produce graphene-based materials with the desired properties. In this work, the thermal reduction is chosen to prepare the 9592
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Figure 3. Photographs showing the process of the oil/water separation of the as-prepared RGO@mesh-300: (a) before separation, (b) during the separating process, (c) after the separation, (d) the changes of flux and separation efficiency of the mixture of chloroform and water with different mesh sizes, (e) the separation efficiency and trace oil content in the collected water of RGO@mesh-300 for a variety of mixtures of oil and water, (f) the recyclability of the as-prepared RGO@mesh-300 used for the separation of the mixture of chloroform and water, and (g) the change of the flux of RGO@mesh-300 with increasing cycle number for the separation of the mixture of chloroform and water.
The high-resolution XPS spectrum for C 1s also consolidates the changes of the elements on the GO-coated mesh before and after the thermal reduction (see Figure S1 in the Supporting Information). Figure 1c shows the typical FT-IR spectra of GO-coated mesh and RGO@mesh-300. Before the thermal treatment, the stretching vibration band of C = O is at 1735 cm−1, and the stretching vibration bands C−O of epoxy and alkoxy are at 1157 and 1104 cm−1, respectively, demonstrating that the GO has abundant oxygen-containing groups. In the RGO spectrum, however, it is markedly different from that of GO, where the intensities of all the absorption bands correlated to the oxygencontaining groups decrease dramatically, resulting from the reduction of GO.
steel wire mesh before and after coating (Figure 1b). As shown in Figure 1b, the coating of GO on the stainless steel wire mesh can be evidenced by the increased ratio of carbon-to-oxygen (1.134 and 2.26 for pure stainless steel wire mesh and GOcoated mesh, respectively). As for stainless steel wire mesh, the signals of iron, manganese and chromium are detected clearly. The absence of iron, manganese, and chromium on the GOcoated mesh also confirms the formation of GO coating. After the treatment of O2 plasma, the ratio of carbon-to-oxygen decreases to 1.429 due to the O2 plasma etching on the exposed regions of the stainless steel wire mesh. After the thermal reduction, the intensity of O 1s peak of RGO@mesh-300 significantly decreases and the ratio of carbon-to-oxygen increases to 1.568 compared with that of GO-coated mesh after O2 plasma treatment, demonstrating the loss of oxygen. 9593
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separation, the as-prepared RGO@mesh-300 is first sandwiched between a vertical glass tube and a conical flask (Figure 3a−c). Then a mixture of chloroform and water (50% v/v of chloroform) is poured into the upper tube. The separation of oil and water is realized by the gravity under normal atmospheric pressure. It can be seen from Figure 3c, chloroform can easily pass through the mesh due to the superoleophilicity of the mesh, whereas the water (dyed blue for clarity) remains above the superhydrophobic RGO@mesh300. In order to obtain the optimum results, both flux and separation efficiency of a mixture of chloroform and water are first measured with different mesh sizes. As shown in Figure 3d, the flux increases with the increase of the pore size, indicating that the larger pore size leads to higher flux. However, it should be noted that the separation efficiency of RGO@mesh-300 is higher than that of the others due to the superhydrophobicity. Thus, the RGO@mesh-300 with the pore size of 50 μm is used to characterize the properties because of its higher flux and highest separation efficiency. To quantitatively evaluate the separation efficiency, the oil contents before and after separation were determined using an infrared spectrometer oil content analyzer. The separation efficiency of the asprepared RGO@mesh-300 for a variety of mixtures of oil and water is presented in Figure 3e, which is determined by the oil rejection coefficient R (%) according to the following equation:
The XRD patterns of the GO-coated mesh and RGO@mesh300 are given in Figure 1d. Instead of the diffraction peak at 2θ of 11.60° for GO, a new broadened diffraction peak at 2θ of 23.80° appears in the pattern of RGO, which is close to the position of the (002) peak of the graphite (2θ = 26.43°). The results demonstrate that the RGO is in an inhomogeneous crystalline state and has similar structure to the graphite. As shown in Figure 2a,b, the stainless steel mesh shows superhydrophilicity after coated by GO. The color of the GOcoated mesh is transformed into dark upon the thermal treatment (Figure 2c) due to the reduction of GO and the wettability changes from superhydrophilicity to superhydrophobicity (Figure 2d). The morphologies of the as-received stainless steel wire mesh, GO-coated wire mesh, GO-coated mesh with the open pores and RGO@mesh-300 are characterized by scanning electron microscopy (SEM). After coated by the GO, all the apertures are obturated (Figure 2f) due to the viscidity of the GO. The treatment of O2 plasma on the back side of mesh opens the pores on the GO-coated wire mesh (Figure 2g), and the GO can be retained on the surface of the wire mesh (Figure 2g). Though the GO solution is stable and GO-coated mesh can be used for separation of the oil/ water mixture, recent studies have found that at room temperature multilayer GO is metastable,40,43 which significantly affects the durability and robustness of the separation film. Thus, the thermal reduction is used to prepare the stable superhydrophobic/superoleophilic separation mesh (Figure 2h). According to recent results,44,45 a higher temperature is expected to increase the degree of reduction of GO, which in turn increases the hydrophobicity of the separation film. To evaluate the hydrophobicity of the RGO@mesh-x samples, we measure water contact angles of the RGO@mesh-x with different temperatures of thermal treatment (160, 180, and 200 °C) and pore sizes, which are measured to be 150, 75, 50, 35, and 20 μm for the samples of 100, 200, 300, 400, and 500 mesh. As shown in Figure S2a, the water contact angles of RGO@mesh-100 are 122°, 128°, and 135° after the thermal treatment for 2 h at 160, 180, and 200 °C, respectively. If the temperature of thermal reduction is less than 160 °C, the water contact angle on the RGO@mesh-x cannot be measured due to the complete absorbance of water into the film. Meanwhile, the higher temperature has little impact on the water contact angles on the RGO@mesh-300 (Figure S2b). In addition, the longer annealing time does not affect the contact angle of the RGO@ mesh-300 (Figure S2c). On the other hand, the largest water contact angle is about 153° on the RGO@mesh-300 with the pore size of 50 μm at the annealing temperature of 200 °C due to a much rougher surface than that of the others. In sharp comparison, the contact angles of oil on the films are nearly close to 0°. In general, the larger distinction in the contact angle between the water and oil on the surface, the easier the separation of mixture of the water and oil, and it can be inferred that the optimum size of pore of the original mesh is 50 μm, beyond which the superhydrophobicity cannot be realized. Tests of Oil/Water Separation. The functional surfaces with the special wettability show superiority for oil-containing water treatment due to their high separation efficiency and repeatable use. For the realization of oil/water separation, the superhydrophobicity serves as a closed gate to block permeation of water, whereas superoleophilicity is simultaneously also required to act as an open gate for oil to smoothly penetrate the mesh. To evaluate the capability of the oil/water
⎛ Cp ⎞ R(%) = ⎜1 − ⎟ × 100 C0 ⎠ ⎝
(1)
where C0 and Cp are the oil concentration of the original mixture of oil and water, and the collected water after the separation, respectively. The separation efficiencies of the asprepared meshes for a variety of mixtures of oil and water are above 98%, and the trace oil content in the collected water after the separation is less than 74 ppm (Figure 3e). Moreover, the as-prepared mesh can be recycled due to the robustness of the superhydrophobic coating. The recycling ability is investigated by using the mixture of chloroform and water. After being rinsed and dried, the mesh still exhibits superhydrophobicity and superoleophilicity. As shown in Figure 3f, after 20 separation cycles, the separation efficiency varies slightly during the repeated separations and remains to be above 98%, showing the excellent recyclability of the RGO coating. The fluxes of the RGO@mesh-300 for the separation of the mixture of chloroform and water during 20 cycles are presented in Figure 3g. After each separation of the mixture, the film was simply washed with ethanol to recover the flux. It can be seen that the flux decreases slightly with increasing cycle times and no sharp decline is observed. After 20 cycles, the flux still keeps a relative high level of 1752 L m−2 h−1, indicating an outstanding antifouling performance of the RGO@mesh-300, which is an important feature for the practical application. Simultaneously, the coating retains the properties of the original mesh such as hardness, porosity, and outstanding resistance to corrosive liquids. Moreover, the measurements of separation efficiencies of RGO@mesh-300 after the immersion in two types of solutions including alkaline and acid indicate that it is suitable for the practical and industrial applications (see Figure S3 in the Supporting Information). For the practical applications in a large scale, the coated meshes are required to stand a large amount of mixtures of oil and water without the penetration of water through the meshes. 9594
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Figure 4. Separation efficiency of RGO@mesh-300 at (a) different temperatures and (b) different relative humidities, (c) long-term stability of RGO@mesh-300 under ambient condition for 120 days, (d) the variation in the water contact angle and the separation efficiency of the RGO@ mesh-300 with sliding distance in the friction test. The inset shows the schematic illustration of the abrasion test. The RGO@mesh-300 is dragged back and forth with a speed and abrasion length of 2 cm s−1 and 10 cm, respectively.
the loss of water-repellency. Consequently, the abrasion test is adopted to investigate the mechanical stability of RGO@mesh300 in order to evaluate the robustness of the as-prepared meshes. The inset in Figure 4d illustrates the detailed process of the abrasion test. The sandpaper (1500 mesh) serves as an abrasive surface with the RGO@mesh-300 to be tested facing the abrasive material. The RGO@mesh-300 is dragged back and forth under a pressure of 4 kPa with a speed of 2 cm s−1 and sliding distance of 10 cm. Figure 4d shows that the water contact angle decreases from 153° to 137° after the abrasion distance of 600 cm, demonstrating that the RGO@mesh-300 partially loses the superhydrophobicity after the abrasion test. SEM images of RGO@mesh-300 after the abrasion distance of 300 and 600 cm show the changes of the morphologies (see Figure S4 in the Supporting Information). Although the superhydrophobicity of the RGO@mesh-300 deteriorates after the frictional test, the separation efficiency of the mixture of chloroform and water remains above 97% (Figure 4d). The superior performance of the RGO coating under harsh conditions ensures its usefulness in a broad range of relevant applications.
The liquid-holding capacity can be characterized by the intrusion pressure. It is measured by continuously adding water onto the meshes until the pressure of water reaches a threshold, beyond which water starts to penetrate the pores of the meshes. The theoretical intrusion pressure (Pt) can be calculated by using the Young−Laplace equation: Pt =
2γ cos θ d
(2)
where γ is the surface tension of water, θ is the contact angle of water on the surface, and d is the pore size of the mesh. Therefore, based on the water contact angle (153°), the surface tension of water (72.75 × 10−3 N m−1), and the pore size of the mesh (50 μm), the theoretical intrusion pressure is 2.5 kPa, which agrees with the practical intrusion pressure (2.4 kPa, about 300 mL water). The separation performance of the RGO@mesh-300 is also determined under different harsh conditions. As shown in the Figure 4a,b, the RGO@mesh-300 exhibits the long-term durability in a wide range of humidity (RH: 10−90%) and temperature (−20 to 300 °C). Furthermore, the superhydrophobicity of RGO@mesh-300 is evaluated over the different storage times under the ambient condition. There is no considerable change in the separation efficiency after the storage of 4 months (Figure 4c). This long-term stability is essential when the superhydrophobic materials are used in the practical applications. In addition, the mechanical stability as a key factor in determining the practical applications is investigated by the frictional test. It is well-known that the micro/nano-structures of the superhydrophobic coatings are easily damaged or destroyed under abrasive forces, leading to
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CONCLUSIONS In summary, a simple method is presented for fabricating the functional materials for oil/water separation via the thermal reduction of GO on the wire mesh. By controlling the pore size of the mesh, the separation efficiency can be maximized with the water contact angle of 153° on the RGO@mesh-300. The as-prepared RGO@mesh-300 can successfully and effectively separate a variety of mixtures of oil and water. Furthermore, the RGO@mesh-300 has outstanding stability and recyclability 9595
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during the process of oil/water separation. Meanwhile, the RGO@mesh-300 shows stable superhydrophobicity under long-term storage, excellent resistance to high temperature and high humidity, and the resistance to mechanical abrasion. Thus, the excellent stability of the RGO@mesh extends the fields of applications in the realistic environments, especially in the harsh conditions.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01856. High-resolution XPS spectra of C 1s for GO-coated mesh after O2 plasma treatment and RGO@mesh-300; the water contact angles on the RGO@mesh-x with the different mesh size after thermal treatment at 160, 180, and 200 °C, the changes of water contact angles on the RGO@mesh-300 with the temperature and heating time; SEM images of RGO@mesh-300 after the immersion in two corrosive solutions and the accompanying changes of the separation efficiency before and after the immersion in the acid and alkaline solutions; SEM images of RGO@ mesh-300 after the abrasion distance of 300 and 600 cm (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Jing Chen: 0000-0002-7228-0625 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Natural Science Foundation of China (Grant No. 21404081) and Beijing National Laboratory for Molecular Sciences (Grant No. 20140149).
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DOI: 10.1021/acs.langmuir.7b01856 Langmuir 2017, 33, 9590−9597
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DOI: 10.1021/acs.langmuir.7b01856 Langmuir 2017, 33, 9590−9597
