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water separation
Accepted Manuscript Title: A Facile Method for the Fabrication of a Superhydrophobic Polydopamine-Coated Copper Foam for Oil/Water Separation Authors: Wei Zhou, Guangji Li, Liying Wang, Zhifeng Chen, Yinlei Lin PII: DOI: Reference:
S0169-4332(17)31001-2 http://dx.doi.org/doi:10.1016/j.apsusc.2017.04.004 APSUSC 35676
To appear in:
APSUSC
Received date: Revised date: Accepted date:
3-2-2017 15-3-2017 1-4-2017
Please cite this article as: Wei Zhou, Guangji Li, Liying Wang, Zhifeng Chen, Yinlei Lin, A Facile Method for the Fabrication of a Superhydrophobic Polydopamine-Coated Copper Foam for Oil/Water Separation, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.04.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A Facile Method for the Fabrication of a Superhydrophobic Polydopamine-Coated Copper Foam for Oil/Water Separation Wei Zhou, Guangji Li*, Liying Wang, Zhifeng Chen, Yinlei Lin* School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China *Corresponding authors: Guangji Li; Yinlei Lin E-mail addresses: [email protected] (G. Li); [email protected] (Y. Lin)
Graphical abstarct
Graphical Abstract (for review)
Highlights A superhydrophobic polydopamine-coated 3D copper foam was fabricated by a simple dip-coating method. The 3D copper foam surface exhibited hierarchical micro/nanostructures. The superhydrophobic copper foam could separate water from various oil/water mixtures with separation efficiency above 95%. The as-prepared foam showed good durability and stability.
ABSTRACT
A simple dip-coating method was explored to construct hierarchical structures on a 3D copper foam (CF) surface by combining the intrinsic properties of mussel-inspired polydopamine (PDA) and a 3D metal structure. The CF substrate was sequentially modified with PDA and Ag nanoparticles (NPs) and then coupled with n-dodecyl mercaptan (NDM) to create a durable superhydrophobic CF for oil/water separation. The morphology, chemical composition and wettability of the fabricated modified CF surface were characterized. The modified CF surface possesses an increased roughness and exhibits superhydrophobicity, with water contact angle values greater than 150°. The PDA coating on the CF surface can reduce silver ions and anchor the formed NPs onto the surface to construct the hierarchical structure of the superhydrophobic CF. Furthermore, the oil/water separation properties were also investigated. The modified CF can separate a series of oil/water mixtures with a high efficiency and relatively high intrusion pressure. More importantly, the modified CF retains its high efficiency after 30 repeated uses (more than 98% for a dodecane/water mixture), exhibiting excellent durability. The mechanism of oil/water separation is also discussed. The results of this study indicate that the modified CF can serve as a promising candidate for the large-scale separation of oily pollutants from water.
Keywords: Oil/water separation, Copper foam, Polydopamine, Superhydrophobicity 1. Introduction Oil pollution in water and separation technologies for oil/water mixtures have attracted extensive attention because of the increasing amounts of industrial oil-bearing wastewater and the occurrence of massive oil leaks, which have caused serious environmental and ecological problems around the world in recent decades [1-4]. Conventional oil/water separation technologies include degradation [5], in situ burning [6], and other physical separation methods [7, 8]. However, almost all of these traditional methods are environmentally unfriendly and inefficient, with high operating costs. Therefore, advanced techniques are required to separate various oil/water mixtures. Recent progress in special wettability has opened a new route for the development of novel materials and advanced techniques for oil/water separation [9-11]. Various biomimetic surfaces with both superhydrophobicity and superoleophilicity have been designed and prepared by imitating natural surface structures [12-14]. They have been shown to have broad applications in fields such as self-cleaning [15], anti-icing [16, 17], anti-fouling [18], anti-corrosion [19] and oil/water separation [20, 21]. In 2004, Feng et al. [22] reported the first example of a superhydrophobic mesh for oil/water separation.
Since
then,
various
materials
for
oil/water
separation
with
superhydrophobic surfaces have been designed and fabricated [23-27]. For matrix materials, 3D porous materials have become promising candidates for the removal of oil from water because of their high porosity and long z-direction dimensions [28]. Furthermore, 3D copper foam (CF) exhibits greater mechanical strength, less shrinkage and a longer working life than traditional 3D porous materials such as polymeric sponges and foams. For this reason, 3D CF is a promising substrate for use in the construction of materials for oil/water separation [29-31]. Great progress has been made in the design and preparation of high-performance superhydrophobic materials for oil/water separation. However, maintaining highly
efficient and stable performances for superhydrophobic materials prepared with different
substrates
remains
a
challenge.
In
addition,
the
hierarchical
micro/nanostructures essential for superhydrophobicity are fragile and vulnerable to damage during the oil/water separation process. An effective solution for this issue is to employ intermediates that strongly adhere to most substrates and provide a platform for subsequent modification of the substrate surface. For example, mussel adhesive proteins, whose main functional component is 3,4-dihydroxyphenyl-L-alanine (DOPA), can attach mussels and other bivalves to rocks or other wet substrates [32-35]. On the basis of these findings, Messersmith et al. [36] first introduced an adherent polydopamine (PDA) coating that can form on the surface of almost any type of material via the self-polymerization of dopamine. In addition, the obtained PDA contained catechol groups that can serve as both reducing agents for metal ions and anchors for the resultant metal nanoparticles (NPs) [37-39]. Although the specific adhesion mechanism of PDA is still not completely understood, the amazing adhesion and intrinsic properties of PDA bearing catechol groups have led us to pursue the use of this coating combined with a 3D CF substrate to construct a superhydrophobic CF material for oil/water separation. According to the aforementioned analyses, when using CF as a substrate, fabricating a stable and durable superhydrophobic surface is important for oil/water separation. The key to achieving this goal is the development of a practical route to combine PDA’s ability to strongly adhere to various substrate surfaces and PDA’s metal-ion reduction characteristics with the excellent mechanical strength of a 3D CF. Herein, we propose a three-step approach to fabricate a superhydrophobic surface using CF as a substrate, as shown in Scheme 1. First, PDA bearing catechol anchor groups is coated onto the substrate surface via a simple dip-coating method to obtain the reactive surface. Next, the surface is immersed in a AgNO3 solution, which results in the reduction of Ag NPs and allows them to bond on the reactive surface. The surface is then further modified with n-dodecyl mercaptan (NDM), which has a long alkyl chain, and the superhydrophobic CF is prepared. The morphologies and chemical compositions of different surfaces were characterized by field-emission scanning electron microscopy
(FE-SEM) and X-ray photoelectron spectroscopy (XPS), respectively. The wettability of the surfaces was evaluated and analyzed using water contact angle (WCA) measurements. Furthermore, the oil/water separation performance of the modified CF was investigated. On the basis of the results of this study, the superhydrophobic surface developed on the 3D CF substrate was expected to have excellent wetting stability and durable oil/water separation properties. 2. Experimental 2.1. Materials and chemicals A commercial CF (99.96%, 0.96 porosity, 20 mm × 20 mm × 2 mm) was purchased from Kunshan Jiayisheng Electronics Co., Ltd., China. Dopamine hydrochloride (DA, 99%), tris(hydroxymethyl)aminomethane (TRIS, 99.9%), silver nitrate (AgNO3, 99.8%), NDM (98%), methylene blue and oil red O (dye content 70%) were supplied by Aladdin Industrial Corporation. Hexane, octane, decane, dodecane and hexadecane were purchased from Shanghai Macklin Biochemical Co., Ltd. Sesame oil was purchased from a local supermarket. All chemicals were used as received without further purification. 2.2. Preparation of samples A commercial CF was first pretreated by washing with acetone, ethanol, 1 M HCl, and deionized (DI) water in an ultrasonic bath, successively, followed by drying with nitrogen. The dopamine solution (2 mg/mL) was prepared by dissolving DA in 10 mM TRIS-HCl buffer (pH 8.5), according to the reported literature [36]. The pretreated CF was immersed into a 2 mg/mL dopamine solution for 24 h and then washed with DI water several times in an ultrasonic bath at room temperature. The obtained PDAmodified CF is denoted by CF-PDA. Sequentially, the CF-PDA was immersed in a 10 mM AgNO3 solution for 10 h at room temperature to reduce the Ag+ ions to metallic Ag; the CF-PDA was then cleaned with water in an ultrasonic bash several times, resulting in the modified surface bearing Ag NPs, CF-PDA-Ag. Finally, the reactive compound bearing the hydrophobic chain NDM was bonded to CF-PDA-Ag by immersing the CFPDA-Ag into an NDM/ethanol solution (VNDM/Vethanol = 1/50) at room temperature for 30 min (denoted by CF-PDA-Ag-NDM).
For comparison purposes, a control sample with no Ag NPs, CF-PDA-NDM, was prepared by immersing CF-PDA into the NDM/ethanol solution at room temperature for 30 min; another control sample without the PDA coating, CF-Ag, was prepared by dipping CF into a 10 mM AgNO3 solution for 5 min. 2.3. Characterization of the modified copper foams The morphology of a series of modified CF surfaces was observed by FE-SEM (ZEISS Merlin, Germany). The surface chemistry of each sample was analyzed by XPS (Axis Ultra DLD spectrometer, UK). Static WCAs were measured at room temperature using an OCA 20 apparatus (Data-Physics, Germany). Each sample was measured three times at three random locations, and the average values of the WCAs were calculated. 2.4. Measurement of the oil/water separation properties A simple filter model was designed to evaluate the oil/water separation properties of the modified foam samples (measured with a homemade filtration device). A CF sample was embedded onto the square hole of the polymethyl methacrylate (PMMA) plates, which were mounted on the end of each PMMA tube, and the two tubes were fixed endon-end with clamps and positioned with a tilt angle of 45°. The oil/water mixture (moil/mwater = 1) was poured into the upper tube, and separation was achieved via gravity. The corresponding separation efficiency was defined as the ratio between the mass of the collected oil after separation and the mass of the oil before separation. The separation efficiency (η) was calculated according to Eq. (1): η (%) = (m0/m1) × 100
(1)
where m1 and m0 are the mass of the original oil in the oil/water mixtures and the mass of the oil after the separation process, respectively. The intrusion pressure p, which is another property of the separation ability, was determined by the maximum height of water that the foam could support in the vertical direction and was calculated using Eq. (2): p = ρghmax
(2)
where ρ is the density of the water, g is the acceleration of gravity, and hmax is the maximum height of water that the modified CF can withstand. 3. Results and discussion 3.1. The surface morphologies of the modified copper foam samples
Fig. 1 displays the SEM images of the different modified CF samples and pristine CF as a control sample. Compared with the smooth surface of the pristine CF shown in Fig. 1 (a), the surface of the CF-PDA was slightly rough but lacked an observable hierarchical micro/nanostructure, even when observed under high-magnification SEM (Fig. 1 (b)). The surface morphology of the CF-PDA-NDM, shown in Fig. 1 (c), was similar to that of the CF-PDA. A comparison of Fig. 1 (d) and Fig. 1 (e) reveals that the 3D structure of the CF-Ag, i.e., the CF directly treated with the AgNO3 solution, was extensively damaged, whereas the 3D structure of CF-PDA-Ag, i.e., the AgNO3-treated CF-PDA, did not change. In addition, an abundance of Ag NPs and their aggregates with diameters of 50 - 200 nm were observed on the surface of the CF-PDA-Ag. These NPs were not observed on the surface of the CF-PDA, and the micro/nanostructures formed by the Ag NPs and their aggregates remained on the surface after the CF-PDAAg was further treated with NDM, as shown in Fig. 1 (f). On the basis of these observations, the tiny change in the roughness of the CF-PDA is attributed to the PDA coating on the CF surface. The CF-PDA-Ag, i.e., the AgNO3treated CF-PDA, maintained its original 3D porous structure and exhibited obvious micro/nanostructuring due to the Ag NPs and their aggregates, which were produced by the reduction reaction between the PDA coating and Ag+ ions. In addition, the hierarchical micro/nanostructures on the CF-PDA-Ag surface did not substantially change in the subsequent process of impregnation with the NDM. This indicates that the PDA coating on the CF plays a critical role in protecting the 3D structure of the CF from the destruction caused by the displacement reaction between Cu and Ag+, which allows the generation of Ag NPs and the fixation of the NPs onto the CF-PDA-Ag surface. 3.2. Analysis of the surface chemical composition of the modified copper foams The XPS spectra in Fig. 2 reveal changes in the surface chemical compositions of the CF after the modification process. The XPS spectrum of CF showed only a C1s peak, an O1s peak and Cu2p peaks originating from the CF substrate. In comparison, a new N1s peak appeared at 399.8 eV in the XPS spectra of all of the modified CF samples with the PDA coating (CF-PDA, CF-PDA-Ag, and CF-PDA-Ag-NDM); this peak is attributed to the presence of the PDA coating [40]. In addition, Ag3d peaks at 368.2 (Ag 3d5/2) and 374.2 eV (Ag 3d3/2), corresponding to Ag0, were observed in the XPS spectra of CF-PDA-Ag and CF-PDA-Ag-NDM, as shown in Fig. 2 [41], and the S2p
signal at 163.2 eV from the NDM was detected on the surface of the CF-PDA-Ag-NDM [42]. Further investigations of the surface modification process of the CF were conducted via analysis of the high-resolution XPS spectra of CF-PDA in the C1s and N1s regions and the high-resolution C1s, Ag3d and S2p spectra of CF-PDA-Ag-NDM, as shown in Fig. 3. The peaks at 285.6 and 283.8 eV in Fig. 3 (a) are ascribed to carbon in the form of C-N, C-O and C=C; these components are attributed to the PDA coating. The main peak at 399.7 eV in the N1s spectrum of the CF-PDA is attributed to the R-NH-R and =N-R of the PDA layer, which suggests that the primary amine group was transformed into a secondary or tertiary amine group via the self-oxidation/polymerization of dopamine [43]. As illustrated in Fig. 3 (b), in addition to the characteristic Ag3d peaks of Ag0, the peaks at 368.5 and 367.1 eV are ascribed to Ag-O and Ag-S, which were produced by the interaction of Ag with the catechol anchor groups of PDA and the -SH groups of NDM, respectively [44]. The peak at 161.9 eV is attributed to Ag-S-R, which is from the interaction between Ag and NDM. In addition, the peak attributed to a dialkylsulfide (R-S-R) at 163.2 eV may be derived from the Michael addition reaction between the NDM and the PDA, and the C-S and R-SH peaks at 285.7 eV and 163.8 eV in Fig. 3 (b) are derived from the NDM and further demonstrate the presence of NDM [45, 46]. According to the aforementioned analyses of the XPS spectra, the PDA was successfully coated onto the surface of the CF sample through the self-polymerization of dopamine. Ag+ ions were reduced to Ag0 and deposited onto the surface of the CFPDA-Ag-NDM by chemical bonding (specifically, Ag-O). The SEM images also demonstrate that Ag NPs and their aggregates were deposited onto the surface of the CF-PDA-Ag-NDM. After the hydrophobic treatment, the NDM was confirmed to be grafted onto the surface of the CF-PDA-Ag-NDM. The results of the quantitative analysis of the surface chemical composition, shown in Table 1, also indicate that the CF substrate was sequentially coated with PDA, Ag NPs and NDM. The nitrogen and carbon contents increased as a result of the presence of the PDA coating in CF-PDA. The increase of the silver content in the CF-PDA-Ag and the sulfur content in the CFPDA-Ag-NDM also indicated that Ag and NDM were successively bonded to the surface of the CF-PDA.
3.3. Wettability of the modified copper foam samples The images displayed in Fig. 4 reflect the wettability of the different modified CF samples and an unmodified CF as a control. Fig. 4 (a) clearly shows that the pristine CF is hydrophilic, with a WCA of 79.6 ± 2°; by comparison, all of the modified CF surfaces exhibited various degrees of change in the wettability. The CF-PDA and CFPDA-Ag had lower WCAs than the CF control sample. Specifically, the WCA of the CF-PDA was almost 0°, as shown in Fig. 4 (b). The modified CF surfaces, prepared by bonding the NDM onto CF-PDA-NDM and CF-PDA-Ag-NDM, exhibited strong hydrophobicity. The CF-PDA-Ag-NDM surface was superhydrophobic and had a WCA of 153.1 ± 1.5° (Fig. 4 (e)). Compared with the pristine CF, the CF-PDA exhibited a substantially lower WCA, which is attributed to the numerous hydrophilic oxygen-containing groups of the PDA film. After NDM modification, the surface wettability of the samples transformed from hydrophilic to hydrophobic (Fig. 4 (b) and (d) and Fig. 4 (c) and (e)) because the NDMmodified samples have a low surface energy. In addition, the WCA of CF-PDA-AgNDM was higher than that of CF-PDA-NDM. With the combined analyses of the surface morphology and the surface chemical compositions of the different modified CFs, the aforementioned analysis indicates that the WCAs of the hydrophobic surfaces increased with increasing surface roughness, i.e., with the conversion of the hydrophobic CF-PDA-NDM into the superhydrophobic CF-PDA-Ag-NDM. More importantly, the transformation from the hydrophilic CF-PDA-Ag to the superhydrophobic CF-PDA-Ag-NDM was due to the low surface energy properties of NDM. The hierarchical structures on the modified CF and the low surface energy NDM coating were the main reasons for the superhydrophobicity of the CF-PDA-Ag-NDM.
3.4. Oil/water separation properties of the superhydrophobic copper foam Fig. 5 shows that water droplets (dyed with methylene blue) dropped onto the surface of the CF-PDA-Ag-NDM were repelled. Meanwhile, oil droplets (dyed with oil red O) quickly permeated through the CF-PDA-Ag-NDM. The properties of the oil and water droplets on the CF-PDA-Ag-NDM surface further confirmed that the modified CF possessed both superhydrophobicity and strong oleophilicity in air, which is useful for separating the oil/water mixtures.
Hence, the oil/water separation properties of the prepared CF-PDA-Ag-NDM were evaluated via measurements of its separation efficiency, intrusion pressure and durability for various oil/water mixtures. As shown in Fig. 6, a mixture of hexane (dyed with oil red O) and water (dyed with methylene blue) was poured onto the superhydrophobic CF, and hexane passed through the mesh with the driving force of gravity, whereas the water was blocked and remained in the upper PMMA tube.
The CF-PDA-Ag-NDM separation capability for a series of oil/water mixtures, including hexane/water, heptane/water, octane/water, dodecane/water and sesame oil/water, was investigated. As shown in Fig. 7 (a), the separation efficiency of the superhydrophobic CF for the selection of oil/water mixtures was greater than 95%. Particularly, the separation efficiency of the dodecane/water mixture reached 98.3%. The intrusion pressure (p) of the materials for oil/water separation was also important. As shown in Fig. 7 (b), the maximum height and p calculated using Eq. (2) for the water in our self-designed system were 14.4 ± 0.9 cm and 1.4 ± 0.1 kPa, respectively, which indicated that water cannot flow through the foam under the specific intrusion pressure.
In practical applications, the durability of the material is worth investigating in addition to the separation efficiency and intrusion pressure. Fig. 8 (a) illustrates that the CFPDA-Ag-NDM maintains a high separation efficiency (greater than 98%) and superhydrophobicity after 30 repeated cycles of use with the dodecane/water mixture, showing the stability of the wettability and separation efficiency. At the same time, the hierarchical morphology of the modified CF, shown in Fig. 8 (b), was not destroyed after 30 repeated cycles of use.
As stated in the preceding text, although the CF-PDA-Ag-NDM can separate a series of oil/water mixtures, the mechanism of separation by the modified CF is not clear. A proposed separation mechanism for an oil/water mixture is displayed in Fig. 9. The process of oil/water separation can be divided into two stages that correspond to the different separation mechanisms. In the initial stage of the oil/water separation, as shown in Fig. 9 (a), the water droplets that are dropped onto the CF-PDA-Ag-NDM surface are in the Cassie state and can be easily repelled because of the
superhydrophobicity of the modified foam surface. The oil droplets can permeate through the CF-PDA-Ag-NDM, as demonstrated in our study. During the oil/water separation process, the surface of the CF-PDA-Ag-NDM will be gradually wetted by oil because of its strong oleophilicity, and the concave part on the nanoscale rough surface of the CF-PDA-Ag-NDM will be filled with oil and form a smooth oleophilic surface. At this point, the mechanism of the oil/water separation may be different from the initial mechanism. When the water is dropped onto the relatively smooth surface of the oil-wetted modified foam, the water droplets cannot pass through the modified foam quickly because of the repulsion between the oil and water [47], as shown in Fig. 9 (b). Accordingly, the water droplets will temporarily be in a Wenzel state with a low WCA. When the separation process is complete, the oil attached to the modified foam surface can be removed via gravity, and the modified foam will exhibit superhydrophobicity again, which is confirmed by the plot of WCA versus Repeat Times in Fig. 8 (a). Therefore, although the hydrophobicity of the modified CF will decrease to a certain extent in the oil/water separation process, the strong oleophilicity of the modified CF allows it to adsorb oil and continue to repel water, achieving oil/water separation.
4. Conclusion In summary, a feasible dip-coating method was explored to prepare a durable superhydrophobic 3D CF with hierarchical micro/nanostructures for oil/water separation. The final modified product, CF-PDA-Ag-NDM, exhibits excellent superhydrophobicity, which was characterized by a WCA of 153.1 ± 1.5°. The morphologies and chemical compositions of the modified CF surfaces prepared via different dip-coating processes were examined using SEM and XPS. The results indicated that, as expected, the pristine CF can be sequentially modified with PDA, Ag NPs and NDM via the presented dip-coating method, and the PDA coating can reduce silver ions and anchor the formed Ag NPs onto the surface, which plays an important role in the construction of the hierarchical structure of the CF-PDA-Ag-NDM. Moreover, the modified foam CF-PDA-Ag-NDM exhibits a high efficiency, high intrusion pressure and good durability for oil/water separation. Its separation efficiency is greater than 95.0% for a series of oil/water mixtures, and its high intrusion pressure is as high as 1.4 ± 0.1 kPa, which enables the separation of large volumes of oil/water
mixtures. Additionally, CF-PDA-Ag-NDM maintains up to a 98% separation efficiency and stable wettability for a dodecane/water mixture. These results are of great significance for practical applications. Therefore, this modified 3D CF has great potential for practical use in various fields such as oil spill cleanup and wastewater management. Acknowledgements This work was supported by the Natural Science Foundation of Guangdong Province, China (Grant No. 2014A030313246) and Science and Technology Planning Project of Guangdong Province, China (Grant No. 2016A010103005).
References [1] S.B. Joye, Deepwater horizon, 5 years on, Science 349 (2015) 592-593. [2] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marinas, A.M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (2008) 301-310. [3] B. Dubansky, A. Whitehead, J.T. Miller, C.D. Rice, F. Galvez, Multitissue molecular, genomic, and developmental effects of the deepwater horizon oil spill on resident gulf killifish (fundulus grandis), Environ. Sci. Technol. 47 (2013) 5074-5082. [4] I.B. Ivshina, M.S. Kuyukina, A.V. Krivoruchko, A.A. Elkin, S.O. Makarov, C.J. Cunningham, T.A. Peshkur, R.M. Atlas, J.C. Philp, Oil spill problems and sustainable response strategies through new technologies, Environ. Sci.: Process Impacts 17 (2015) 1201-1219. [5] S. Kleindienst, J.H. Paul, S.B. Joye, Using dispersants after oil spills: impacts on the composition and activity of microbial communities, Nat. Rev. Microbiol. 13 (2015) 388-396. [6] J. Fritt-Rasmussen, S. Wegeberg, K. Gustavson, Review on burn residues from in situ burning of oil spills in relation to arctic waters, Water Air Soil Pollut. 226 (2015). [7] K. Gaaseidnes, J. Turbeville, Separation of oil and water in oil spill recovery operations, Pure Appl. Chem. 71 (1999) 95-101. [8] A.B. Nordvik, J.L. Simmons, K.R. Bitting, A. Lewis, T. Strom-Kristiansen, Oil and water separation in marine oil spill clean-up operations, Spill Sci. Technol. Bull. 3 (1996) 107-122. [9] B. Wang, W.X. Liang, Z.G. Guo, W.M. Liu, Biomimetic super-lyophobic and super-lyophilic materials applied for oil/water separation: a new strategy beyond nature, Chem. Soc. Rev. 44 (2015) 336-361. [10] Q. Ma, H. Cheng, A.G. Fane, R. Wang, H. Zhang, Recent development of advanced materials with special wettability for selective oil/water separation, Small 12 (2016) 2186-2202. [11] Z. Xue, Y. Cao, N. Liu, L. Feng, L. Jiang, Special wettable materials for oil/water separation, J. Mater. Chem. A 2 (2014) 2445-2460. [12] W. Barthlott, C. Neinhuis, Purity of the sacred lotus, or escape from contamination in biological surfaces, Planta 202 (1997) 1-8. [13] X. Gao, L. Jiang, Water-repellent legs of water striders, Nature 432 (2004) 36.
[14] S. Niu, B. Li, Z. Mu, M. Yang, J. Zhang, Z. Han, L. Ren, Excellent structure-based multifunction of morpho butterfly wings: a review, J. Bionic Eng. 12 (2015) 170-189. [15] S. Liu, X. Liu, S.S. Latthe, L. Gao, S. An, S.S. Yoon, B. Liu, R. Xing, Self-cleaning transparent superhydrophobic coatings through simple sol–gel processing of fluoroalkylsilane, Appl. Surf. Sci. 351 (2015) 897-903. [16] R. Liao, Z. Zuo, C. Guo, Y. Yuan, A. Zhuang, Fabrication of superhydrophobic surface on aluminum by continuous chemical etching and its anti-icing property, Appl. Surf. Sci. 317 (2014) 701-709. [17] N. Wang, D.S. Xiong, Y.L. Deng, Y. Shi, K. Wang, Mechanically robust superhydrophobic steel surface with anti-icing, uv-durability, and corrosion resistance properties, ACS Appl. Mater. Interfaces 7 (2015) 6260-6272. [18] G.D. Bixler, A. Theiss, B. Bhushan, S.C. Lee, Anti-fouling properties of microstructured surfaces bio-inspired by rice leaves and butterfly wings, J. Colloid Interface Sci. 419 (2014) 114-133. [19] H.M. Zhang, J. Yang, B.B. Chen, C. Liu, M.S. Zhang, C.S. Li, Fabrication of superhydrophobic textured steel surface for anti-corrosion and tribological properties, Appl. Surf. Sci. 359 (2015) 905910. [20] J. Liu, L. Wang, F. Guo, L. Hou, Y. Chen, J. Liu, N. Wang, Y. Zhao, L. Jiang, Opposite and complementary: a superhydrophobic–superhydrophilic integrated system for high-flux, highefficiency and continuous oil/water separation, J. Mater. Chem. A 4 (2016) 4365-4370. [21] P. Pi, K. Hou, C. Zhou, G. Li, X. Wen, S. Xu, J. Cheng, S. Wang, Superhydrophobic Cu2S@Cu2O film on copper surface fabricated by a facile chemical bath deposition method and its application in oil-water separation, Appl. Surf. Sci. 396 (2017) 566-573. [22] L. Feng, Z. Zhang, Z. Mai, Y. Ma, B. Liu, L. Jiang, D. Zhu, A super-hydrophobic and superoleophilic coating mesh film for the separation of oil and water, Angew. Chem. Int. Ed. Engl. 43 (2004) 2012-2014. [23] C.R. Crick, J.A. Gibbins, I.P. Parkin, Superhydrophobic polymer-coated copper-mesh; membranes for highly efficient oil–water separation, J. Mater. Chem. A 1 (2013) 5943-5948. [24] C.H. Xue, P.T. Ji, P. Zhang, Y.R. Li, S.T. Jia, Fabrication of superhydrophobic and superoleophilic textiles for oil-water separation, Appl. Surf. Sci. 284 (2013) 464-471. [25] S. Liu, Q. Xu, S.S. Latthe, A.B. Gurav, R. Xing, Superhydrophobic/superoleophilic magnetic polyurethane sponge for oil/water separation, RSC Adv. 5 (2015) 68293-68298. [26] H. Zhu, D. Chen, W. An, N. Li, Q. Xu, H. Li, J. He, J. Lu, A robust and cost-effective superhydrophobic graphene foam for efficient oil and organic solvent recovery, Small 11 (2015) 5222-5229. [27] X. Zhang, Z. Li, K. Liu, L. Jiang, Bioinspired multifunctional foam with self-cleaning and oil/water separation, Adv. Funct. Mater. 23 (2013) 2881-2886. [28] X. Gao, J. Zhou, R. Du, Z. Xie, S. Deng, R. Liu, Z. Liu, J. Zhang, Robust superhydrophobic foam: A graphdiyne-based hierarchical architecture for oil/water separation, Adv. Mater. 28 (2016) 168173. [29] D. Zang, C. Wu, R. Zhu, W. Zhang, X. Yu, Y. Zhang, Porous copper surfaces with improved superhydrophobicity under oil and their application in oil separation and capture from water, Chem. Commun. 49 (2013) 8410-8412. [30] J. Zhang, K. Ji, J. Chen, Y. Ding, Z. Dai, A three-dimensional porous metal foam with selectivewettability for oil–water separation, J. Mater. Sci. 50 (2015) 5371-5377. [31] J. Xu, J. Xu, Y. Cao, X. Ji, Y. Yan, Fabrication of non-flaking, superhydrophobic surfaces using a
one-step solution-immersion process on copper foams, Appl. Surf. Sci. 286 (2013) 220-227. [32] L. Wang, G. Li, Y. Lin, Z. Zhang, Z. Chen, S. Wu, A strategy for constructing anti-adhesion surfaces based on interfacial thiol–ene photoclick chemistry between DOPA derivatives with a catechol anchor group and zwitterionic betaine macromolecules, Polym. Chem. 7 (2016) 4964-4974. [33] B.P. Lee, K. Huang, F.N. Nunalee, K.R. Shull, P.B. Messersmith, Synthesis of 3,4dihydroxyphenylalanine (DOPA) containing monomers and their co-polymerization with PEGdiacrylate to form hydrogels, J. Biomater. Sci., Polym. Ed. 15 (2004) 449-464. [34] J.H. Waite, M.l. Tanzer, Polyphenolic substance of mytilus edulis: Novel adhesive containing Ldopa and hydroxyproline, Science 212 (1981) 1038-1040. [35] M. Yu, J. Hwang, T.J. Deming, Role of L-3,4-dihydroxyphenylalanine in mussel adhesive proteins, J. Am. Chem. Soc. 121 (1999) 5825-5826. [36] H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Mussel-inspired surface chemistry for multifunctional coatings, Science 318 (2007) 426-430. [37] Y. Liu, K. Ai, L. Lu, Polydopamine and its derivative materials: synthesis and promising applications in energy, environmental, and biomedical fields, Chem. Rev. 114 (2014) 5057-5115. [38] E. Faure, C. Falentin-Daudré, C. Jérôme, J. Lyskawa, D. Fournier, P. Woisel, C. Detrembleur, Catechols as versatile platforms in polymer chemistry, Prog. Polym. Sci. 38 (2013) 236-270. [39] Z. Xu, K. Miyazaki, T. Hori, Fabrication of polydopamine-coated superhydrophobic fabrics for oil/water separation and self-cleaning, Appl. Surf. Sci. 370 (2016) 243-251. [40] S.M. Kang, I. You, W.K. Cho, H.K. Shon, T.G. Lee, I.S. Choi, J.M. Karp, H. Lee, One-step modification of superhydrophobic surfaces by a mussel-inspired polymer coating, Angew. Chem. Int. Ed. Engl. 49 (2010) 9401-9404. [41] L. Fan, B. Li, J. Zhang, Antibioadhesive Superhydrophobic Syringe Needles Inspired by Mussels and Lotus Leafs, Adv. Mater. Interfaces 2 (2015) 1500019. [42] L. Zhang, J. Wu, Y. Wang, Y. Long, N. Zhao, J. Xu, Combination of bioinspiration: a general route to superhydrophobic particles, J. Am. Chem. Soc. 134 (2012) 9879-9881. [43] H. Gao, Y. Sun, J. Zhou, R. Xu, H. Duan, Mussel-inspired synthesis of polydopamine-functionalized graphene hydrogel as reusable adsorbents for water purification, ACS Appl. Mater. Interfaces 5 (2013) 425-432. [44] F. Liu, F. Sun, Q. Pan, Highly compressible and stretchable superhydrophobic coating inspired by bio-adhesion of marine mussels, J. Mater. Chem. A 2 (2014) 11365. [45] T. Laiho, J.A. Leiro, M.H. Heinonen, S.S. Mattila, J. Lukkari, Photoelectron spectroscopy study of irradiation damage and metal–sulfur bonds of thiol on silver and copper surfaces, J. Electron. Spectrosc. Relat. Phenom. 142 (2005) 105-112. [46] Y. Cao, X. Zhang, L. Tao, K. Li, Z. Xue, L. Feng, Y. Wei, Mussel-inspired chemistry and Michael addition reaction for efficient oil/water separation, ACS Appl. Mater. Interfaces 5 (2013) 4438-4442. [47] K. Li, X. Zeng, H. Li, X. Lai, H. Xie, Facile fabrication of superhydrophobic filtration fabric with honeycomb structures for the separation of water and oil, Mater. Lett. 120 (2014) 255-258.
Fig. 1. SEM images of a pristine CF as a control and different modified CF samples. Fig. 2. XPS spectra of the different modified CF samples and a pristine CF sample as a control. Fig. 3. The high-resolution XPS spectra in the C1s and N1s regions of CF-PDA (a) and the C1s, Ag3d and S2p regions of CF-PDA-Ag-NDM (b). Fig. 4. WCA images of the different modified CF samples. Fig. 5. Photographs of water (a) and oil (b) droplets on the surface of the CF-PDA-Ag-NDM. Fig. 6. Diagram of the oil/water mixture (a) before and (b) after separation. Fig. 7. (a) The separation efficiency of the CF-PDA-Ag-NDM for a selection of oil/water mixtures, and (b) the maximum water intrusion pressure of the CF-PDA-Ag-NDM. Fig. 8. (a) Separation efficiency and WCA of the CF-PDA-Ag-NDM after repeated separation tests with dodecane/water. (b) The SEM and WCA images of the CF-PDA-Ag-NDM after 30 repeated separation tests. Fig. 9. The state of the water droplets on the surface of the superhydrophobic copper foam and the corresponding schematic diagrams (a) in the initial stage of the oil/water separation and (b) during the oil/water separation process. Scheme 1. Schematic of the construction process of the superhydrophobic copper foam
Fig. 1.
Fig. 2.
Fig. 3.
Fig. 4.
Fig. 5.
Fig. 6
Fig.7
Fig 8
Fig. 9
Scheme 1.
Table 1. Elemental composition analysis of CF, CF-PDA, CF-PDA-Ag and CF-PDA-Ag-NDM surfaces based on their XPS spectra. Element content (wt%) Sample Cu2p
C1s
N1s
O1s
Ag3d
S2p
CF
62.22
28.38
0.63
7.11
0.00
1.65
CF-PDA
33.17
52.68
2.66
10.67
0.00
0.82
CF-PDA-Ag
25.96
46.86
0.52
5.21
20.80
0.65
CF-PDA-Ag-NDM
19.17
57.25
0.79
4.14
14.88
3.77
S0169-4332(17)31001-2 http://dx.doi.org/doi:10.1016/j.apsusc.2017.04.004 APSUSC 35676
To appear in:
APSUSC
Received date: Revised date: Accepted date:
3-2-2017 15-3-2017 1-4-2017
Please cite this article as: Wei Zhou, Guangji Li, Liying Wang, Zhifeng Chen, Yinlei Lin, A Facile Method for the Fabrication of a Superhydrophobic Polydopamine-Coated Copper Foam for Oil/Water Separation, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.04.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A Facile Method for the Fabrication of a Superhydrophobic Polydopamine-Coated Copper Foam for Oil/Water Separation Wei Zhou, Guangji Li*, Liying Wang, Zhifeng Chen, Yinlei Lin* School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China *Corresponding authors: Guangji Li; Yinlei Lin E-mail addresses: [email protected] (G. Li); [email protected] (Y. Lin)
Graphical abstarct
Graphical Abstract (for review)
Highlights A superhydrophobic polydopamine-coated 3D copper foam was fabricated by a simple dip-coating method. The 3D copper foam surface exhibited hierarchical micro/nanostructures. The superhydrophobic copper foam could separate water from various oil/water mixtures with separation efficiency above 95%. The as-prepared foam showed good durability and stability.
ABSTRACT
A simple dip-coating method was explored to construct hierarchical structures on a 3D copper foam (CF) surface by combining the intrinsic properties of mussel-inspired polydopamine (PDA) and a 3D metal structure. The CF substrate was sequentially modified with PDA and Ag nanoparticles (NPs) and then coupled with n-dodecyl mercaptan (NDM) to create a durable superhydrophobic CF for oil/water separation. The morphology, chemical composition and wettability of the fabricated modified CF surface were characterized. The modified CF surface possesses an increased roughness and exhibits superhydrophobicity, with water contact angle values greater than 150°. The PDA coating on the CF surface can reduce silver ions and anchor the formed NPs onto the surface to construct the hierarchical structure of the superhydrophobic CF. Furthermore, the oil/water separation properties were also investigated. The modified CF can separate a series of oil/water mixtures with a high efficiency and relatively high intrusion pressure. More importantly, the modified CF retains its high efficiency after 30 repeated uses (more than 98% for a dodecane/water mixture), exhibiting excellent durability. The mechanism of oil/water separation is also discussed. The results of this study indicate that the modified CF can serve as a promising candidate for the large-scale separation of oily pollutants from water.
Keywords: Oil/water separation, Copper foam, Polydopamine, Superhydrophobicity 1. Introduction Oil pollution in water and separation technologies for oil/water mixtures have attracted extensive attention because of the increasing amounts of industrial oil-bearing wastewater and the occurrence of massive oil leaks, which have caused serious environmental and ecological problems around the world in recent decades [1-4]. Conventional oil/water separation technologies include degradation [5], in situ burning [6], and other physical separation methods [7, 8]. However, almost all of these traditional methods are environmentally unfriendly and inefficient, with high operating costs. Therefore, advanced techniques are required to separate various oil/water mixtures. Recent progress in special wettability has opened a new route for the development of novel materials and advanced techniques for oil/water separation [9-11]. Various biomimetic surfaces with both superhydrophobicity and superoleophilicity have been designed and prepared by imitating natural surface structures [12-14]. They have been shown to have broad applications in fields such as self-cleaning [15], anti-icing [16, 17], anti-fouling [18], anti-corrosion [19] and oil/water separation [20, 21]. In 2004, Feng et al. [22] reported the first example of a superhydrophobic mesh for oil/water separation.
Since
then,
various
materials
for
oil/water
separation
with
superhydrophobic surfaces have been designed and fabricated [23-27]. For matrix materials, 3D porous materials have become promising candidates for the removal of oil from water because of their high porosity and long z-direction dimensions [28]. Furthermore, 3D copper foam (CF) exhibits greater mechanical strength, less shrinkage and a longer working life than traditional 3D porous materials such as polymeric sponges and foams. For this reason, 3D CF is a promising substrate for use in the construction of materials for oil/water separation [29-31]. Great progress has been made in the design and preparation of high-performance superhydrophobic materials for oil/water separation. However, maintaining highly
efficient and stable performances for superhydrophobic materials prepared with different
substrates
remains
a
challenge.
In
addition,
the
hierarchical
micro/nanostructures essential for superhydrophobicity are fragile and vulnerable to damage during the oil/water separation process. An effective solution for this issue is to employ intermediates that strongly adhere to most substrates and provide a platform for subsequent modification of the substrate surface. For example, mussel adhesive proteins, whose main functional component is 3,4-dihydroxyphenyl-L-alanine (DOPA), can attach mussels and other bivalves to rocks or other wet substrates [32-35]. On the basis of these findings, Messersmith et al. [36] first introduced an adherent polydopamine (PDA) coating that can form on the surface of almost any type of material via the self-polymerization of dopamine. In addition, the obtained PDA contained catechol groups that can serve as both reducing agents for metal ions and anchors for the resultant metal nanoparticles (NPs) [37-39]. Although the specific adhesion mechanism of PDA is still not completely understood, the amazing adhesion and intrinsic properties of PDA bearing catechol groups have led us to pursue the use of this coating combined with a 3D CF substrate to construct a superhydrophobic CF material for oil/water separation. According to the aforementioned analyses, when using CF as a substrate, fabricating a stable and durable superhydrophobic surface is important for oil/water separation. The key to achieving this goal is the development of a practical route to combine PDA’s ability to strongly adhere to various substrate surfaces and PDA’s metal-ion reduction characteristics with the excellent mechanical strength of a 3D CF. Herein, we propose a three-step approach to fabricate a superhydrophobic surface using CF as a substrate, as shown in Scheme 1. First, PDA bearing catechol anchor groups is coated onto the substrate surface via a simple dip-coating method to obtain the reactive surface. Next, the surface is immersed in a AgNO3 solution, which results in the reduction of Ag NPs and allows them to bond on the reactive surface. The surface is then further modified with n-dodecyl mercaptan (NDM), which has a long alkyl chain, and the superhydrophobic CF is prepared. The morphologies and chemical compositions of different surfaces were characterized by field-emission scanning electron microscopy
(FE-SEM) and X-ray photoelectron spectroscopy (XPS), respectively. The wettability of the surfaces was evaluated and analyzed using water contact angle (WCA) measurements. Furthermore, the oil/water separation performance of the modified CF was investigated. On the basis of the results of this study, the superhydrophobic surface developed on the 3D CF substrate was expected to have excellent wetting stability and durable oil/water separation properties. 2. Experimental 2.1. Materials and chemicals A commercial CF (99.96%, 0.96 porosity, 20 mm × 20 mm × 2 mm) was purchased from Kunshan Jiayisheng Electronics Co., Ltd., China. Dopamine hydrochloride (DA, 99%), tris(hydroxymethyl)aminomethane (TRIS, 99.9%), silver nitrate (AgNO3, 99.8%), NDM (98%), methylene blue and oil red O (dye content 70%) were supplied by Aladdin Industrial Corporation. Hexane, octane, decane, dodecane and hexadecane were purchased from Shanghai Macklin Biochemical Co., Ltd. Sesame oil was purchased from a local supermarket. All chemicals were used as received without further purification. 2.2. Preparation of samples A commercial CF was first pretreated by washing with acetone, ethanol, 1 M HCl, and deionized (DI) water in an ultrasonic bath, successively, followed by drying with nitrogen. The dopamine solution (2 mg/mL) was prepared by dissolving DA in 10 mM TRIS-HCl buffer (pH 8.5), according to the reported literature [36]. The pretreated CF was immersed into a 2 mg/mL dopamine solution for 24 h and then washed with DI water several times in an ultrasonic bath at room temperature. The obtained PDAmodified CF is denoted by CF-PDA. Sequentially, the CF-PDA was immersed in a 10 mM AgNO3 solution for 10 h at room temperature to reduce the Ag+ ions to metallic Ag; the CF-PDA was then cleaned with water in an ultrasonic bash several times, resulting in the modified surface bearing Ag NPs, CF-PDA-Ag. Finally, the reactive compound bearing the hydrophobic chain NDM was bonded to CF-PDA-Ag by immersing the CFPDA-Ag into an NDM/ethanol solution (VNDM/Vethanol = 1/50) at room temperature for 30 min (denoted by CF-PDA-Ag-NDM).
For comparison purposes, a control sample with no Ag NPs, CF-PDA-NDM, was prepared by immersing CF-PDA into the NDM/ethanol solution at room temperature for 30 min; another control sample without the PDA coating, CF-Ag, was prepared by dipping CF into a 10 mM AgNO3 solution for 5 min. 2.3. Characterization of the modified copper foams The morphology of a series of modified CF surfaces was observed by FE-SEM (ZEISS Merlin, Germany). The surface chemistry of each sample was analyzed by XPS (Axis Ultra DLD spectrometer, UK). Static WCAs were measured at room temperature using an OCA 20 apparatus (Data-Physics, Germany). Each sample was measured three times at three random locations, and the average values of the WCAs were calculated. 2.4. Measurement of the oil/water separation properties A simple filter model was designed to evaluate the oil/water separation properties of the modified foam samples (measured with a homemade filtration device). A CF sample was embedded onto the square hole of the polymethyl methacrylate (PMMA) plates, which were mounted on the end of each PMMA tube, and the two tubes were fixed endon-end with clamps and positioned with a tilt angle of 45°. The oil/water mixture (moil/mwater = 1) was poured into the upper tube, and separation was achieved via gravity. The corresponding separation efficiency was defined as the ratio between the mass of the collected oil after separation and the mass of the oil before separation. The separation efficiency (η) was calculated according to Eq. (1): η (%) = (m0/m1) × 100
(1)
where m1 and m0 are the mass of the original oil in the oil/water mixtures and the mass of the oil after the separation process, respectively. The intrusion pressure p, which is another property of the separation ability, was determined by the maximum height of water that the foam could support in the vertical direction and was calculated using Eq. (2): p = ρghmax
(2)
where ρ is the density of the water, g is the acceleration of gravity, and hmax is the maximum height of water that the modified CF can withstand. 3. Results and discussion 3.1. The surface morphologies of the modified copper foam samples
Fig. 1 displays the SEM images of the different modified CF samples and pristine CF as a control sample. Compared with the smooth surface of the pristine CF shown in Fig. 1 (a), the surface of the CF-PDA was slightly rough but lacked an observable hierarchical micro/nanostructure, even when observed under high-magnification SEM (Fig. 1 (b)). The surface morphology of the CF-PDA-NDM, shown in Fig. 1 (c), was similar to that of the CF-PDA. A comparison of Fig. 1 (d) and Fig. 1 (e) reveals that the 3D structure of the CF-Ag, i.e., the CF directly treated with the AgNO3 solution, was extensively damaged, whereas the 3D structure of CF-PDA-Ag, i.e., the AgNO3-treated CF-PDA, did not change. In addition, an abundance of Ag NPs and their aggregates with diameters of 50 - 200 nm were observed on the surface of the CF-PDA-Ag. These NPs were not observed on the surface of the CF-PDA, and the micro/nanostructures formed by the Ag NPs and their aggregates remained on the surface after the CF-PDAAg was further treated with NDM, as shown in Fig. 1 (f). On the basis of these observations, the tiny change in the roughness of the CF-PDA is attributed to the PDA coating on the CF surface. The CF-PDA-Ag, i.e., the AgNO3treated CF-PDA, maintained its original 3D porous structure and exhibited obvious micro/nanostructuring due to the Ag NPs and their aggregates, which were produced by the reduction reaction between the PDA coating and Ag+ ions. In addition, the hierarchical micro/nanostructures on the CF-PDA-Ag surface did not substantially change in the subsequent process of impregnation with the NDM. This indicates that the PDA coating on the CF plays a critical role in protecting the 3D structure of the CF from the destruction caused by the displacement reaction between Cu and Ag+, which allows the generation of Ag NPs and the fixation of the NPs onto the CF-PDA-Ag surface. 3.2. Analysis of the surface chemical composition of the modified copper foams The XPS spectra in Fig. 2 reveal changes in the surface chemical compositions of the CF after the modification process. The XPS spectrum of CF showed only a C1s peak, an O1s peak and Cu2p peaks originating from the CF substrate. In comparison, a new N1s peak appeared at 399.8 eV in the XPS spectra of all of the modified CF samples with the PDA coating (CF-PDA, CF-PDA-Ag, and CF-PDA-Ag-NDM); this peak is attributed to the presence of the PDA coating [40]. In addition, Ag3d peaks at 368.2 (Ag 3d5/2) and 374.2 eV (Ag 3d3/2), corresponding to Ag0, were observed in the XPS spectra of CF-PDA-Ag and CF-PDA-Ag-NDM, as shown in Fig. 2 [41], and the S2p
signal at 163.2 eV from the NDM was detected on the surface of the CF-PDA-Ag-NDM [42]. Further investigations of the surface modification process of the CF were conducted via analysis of the high-resolution XPS spectra of CF-PDA in the C1s and N1s regions and the high-resolution C1s, Ag3d and S2p spectra of CF-PDA-Ag-NDM, as shown in Fig. 3. The peaks at 285.6 and 283.8 eV in Fig. 3 (a) are ascribed to carbon in the form of C-N, C-O and C=C; these components are attributed to the PDA coating. The main peak at 399.7 eV in the N1s spectrum of the CF-PDA is attributed to the R-NH-R and =N-R of the PDA layer, which suggests that the primary amine group was transformed into a secondary or tertiary amine group via the self-oxidation/polymerization of dopamine [43]. As illustrated in Fig. 3 (b), in addition to the characteristic Ag3d peaks of Ag0, the peaks at 368.5 and 367.1 eV are ascribed to Ag-O and Ag-S, which were produced by the interaction of Ag with the catechol anchor groups of PDA and the -SH groups of NDM, respectively [44]. The peak at 161.9 eV is attributed to Ag-S-R, which is from the interaction between Ag and NDM. In addition, the peak attributed to a dialkylsulfide (R-S-R) at 163.2 eV may be derived from the Michael addition reaction between the NDM and the PDA, and the C-S and R-SH peaks at 285.7 eV and 163.8 eV in Fig. 3 (b) are derived from the NDM and further demonstrate the presence of NDM [45, 46]. According to the aforementioned analyses of the XPS spectra, the PDA was successfully coated onto the surface of the CF sample through the self-polymerization of dopamine. Ag+ ions were reduced to Ag0 and deposited onto the surface of the CFPDA-Ag-NDM by chemical bonding (specifically, Ag-O). The SEM images also demonstrate that Ag NPs and their aggregates were deposited onto the surface of the CF-PDA-Ag-NDM. After the hydrophobic treatment, the NDM was confirmed to be grafted onto the surface of the CF-PDA-Ag-NDM. The results of the quantitative analysis of the surface chemical composition, shown in Table 1, also indicate that the CF substrate was sequentially coated with PDA, Ag NPs and NDM. The nitrogen and carbon contents increased as a result of the presence of the PDA coating in CF-PDA. The increase of the silver content in the CF-PDA-Ag and the sulfur content in the CFPDA-Ag-NDM also indicated that Ag and NDM were successively bonded to the surface of the CF-PDA.
3.3. Wettability of the modified copper foam samples The images displayed in Fig. 4 reflect the wettability of the different modified CF samples and an unmodified CF as a control. Fig. 4 (a) clearly shows that the pristine CF is hydrophilic, with a WCA of 79.6 ± 2°; by comparison, all of the modified CF surfaces exhibited various degrees of change in the wettability. The CF-PDA and CFPDA-Ag had lower WCAs than the CF control sample. Specifically, the WCA of the CF-PDA was almost 0°, as shown in Fig. 4 (b). The modified CF surfaces, prepared by bonding the NDM onto CF-PDA-NDM and CF-PDA-Ag-NDM, exhibited strong hydrophobicity. The CF-PDA-Ag-NDM surface was superhydrophobic and had a WCA of 153.1 ± 1.5° (Fig. 4 (e)). Compared with the pristine CF, the CF-PDA exhibited a substantially lower WCA, which is attributed to the numerous hydrophilic oxygen-containing groups of the PDA film. After NDM modification, the surface wettability of the samples transformed from hydrophilic to hydrophobic (Fig. 4 (b) and (d) and Fig. 4 (c) and (e)) because the NDMmodified samples have a low surface energy. In addition, the WCA of CF-PDA-AgNDM was higher than that of CF-PDA-NDM. With the combined analyses of the surface morphology and the surface chemical compositions of the different modified CFs, the aforementioned analysis indicates that the WCAs of the hydrophobic surfaces increased with increasing surface roughness, i.e., with the conversion of the hydrophobic CF-PDA-NDM into the superhydrophobic CF-PDA-Ag-NDM. More importantly, the transformation from the hydrophilic CF-PDA-Ag to the superhydrophobic CF-PDA-Ag-NDM was due to the low surface energy properties of NDM. The hierarchical structures on the modified CF and the low surface energy NDM coating were the main reasons for the superhydrophobicity of the CF-PDA-Ag-NDM.
3.4. Oil/water separation properties of the superhydrophobic copper foam Fig. 5 shows that water droplets (dyed with methylene blue) dropped onto the surface of the CF-PDA-Ag-NDM were repelled. Meanwhile, oil droplets (dyed with oil red O) quickly permeated through the CF-PDA-Ag-NDM. The properties of the oil and water droplets on the CF-PDA-Ag-NDM surface further confirmed that the modified CF possessed both superhydrophobicity and strong oleophilicity in air, which is useful for separating the oil/water mixtures.
Hence, the oil/water separation properties of the prepared CF-PDA-Ag-NDM were evaluated via measurements of its separation efficiency, intrusion pressure and durability for various oil/water mixtures. As shown in Fig. 6, a mixture of hexane (dyed with oil red O) and water (dyed with methylene blue) was poured onto the superhydrophobic CF, and hexane passed through the mesh with the driving force of gravity, whereas the water was blocked and remained in the upper PMMA tube.
The CF-PDA-Ag-NDM separation capability for a series of oil/water mixtures, including hexane/water, heptane/water, octane/water, dodecane/water and sesame oil/water, was investigated. As shown in Fig. 7 (a), the separation efficiency of the superhydrophobic CF for the selection of oil/water mixtures was greater than 95%. Particularly, the separation efficiency of the dodecane/water mixture reached 98.3%. The intrusion pressure (p) of the materials for oil/water separation was also important. As shown in Fig. 7 (b), the maximum height and p calculated using Eq. (2) for the water in our self-designed system were 14.4 ± 0.9 cm and 1.4 ± 0.1 kPa, respectively, which indicated that water cannot flow through the foam under the specific intrusion pressure.
In practical applications, the durability of the material is worth investigating in addition to the separation efficiency and intrusion pressure. Fig. 8 (a) illustrates that the CFPDA-Ag-NDM maintains a high separation efficiency (greater than 98%) and superhydrophobicity after 30 repeated cycles of use with the dodecane/water mixture, showing the stability of the wettability and separation efficiency. At the same time, the hierarchical morphology of the modified CF, shown in Fig. 8 (b), was not destroyed after 30 repeated cycles of use.
As stated in the preceding text, although the CF-PDA-Ag-NDM can separate a series of oil/water mixtures, the mechanism of separation by the modified CF is not clear. A proposed separation mechanism for an oil/water mixture is displayed in Fig. 9. The process of oil/water separation can be divided into two stages that correspond to the different separation mechanisms. In the initial stage of the oil/water separation, as shown in Fig. 9 (a), the water droplets that are dropped onto the CF-PDA-Ag-NDM surface are in the Cassie state and can be easily repelled because of the
superhydrophobicity of the modified foam surface. The oil droplets can permeate through the CF-PDA-Ag-NDM, as demonstrated in our study. During the oil/water separation process, the surface of the CF-PDA-Ag-NDM will be gradually wetted by oil because of its strong oleophilicity, and the concave part on the nanoscale rough surface of the CF-PDA-Ag-NDM will be filled with oil and form a smooth oleophilic surface. At this point, the mechanism of the oil/water separation may be different from the initial mechanism. When the water is dropped onto the relatively smooth surface of the oil-wetted modified foam, the water droplets cannot pass through the modified foam quickly because of the repulsion between the oil and water [47], as shown in Fig. 9 (b). Accordingly, the water droplets will temporarily be in a Wenzel state with a low WCA. When the separation process is complete, the oil attached to the modified foam surface can be removed via gravity, and the modified foam will exhibit superhydrophobicity again, which is confirmed by the plot of WCA versus Repeat Times in Fig. 8 (a). Therefore, although the hydrophobicity of the modified CF will decrease to a certain extent in the oil/water separation process, the strong oleophilicity of the modified CF allows it to adsorb oil and continue to repel water, achieving oil/water separation.
4. Conclusion In summary, a feasible dip-coating method was explored to prepare a durable superhydrophobic 3D CF with hierarchical micro/nanostructures for oil/water separation. The final modified product, CF-PDA-Ag-NDM, exhibits excellent superhydrophobicity, which was characterized by a WCA of 153.1 ± 1.5°. The morphologies and chemical compositions of the modified CF surfaces prepared via different dip-coating processes were examined using SEM and XPS. The results indicated that, as expected, the pristine CF can be sequentially modified with PDA, Ag NPs and NDM via the presented dip-coating method, and the PDA coating can reduce silver ions and anchor the formed Ag NPs onto the surface, which plays an important role in the construction of the hierarchical structure of the CF-PDA-Ag-NDM. Moreover, the modified foam CF-PDA-Ag-NDM exhibits a high efficiency, high intrusion pressure and good durability for oil/water separation. Its separation efficiency is greater than 95.0% for a series of oil/water mixtures, and its high intrusion pressure is as high as 1.4 ± 0.1 kPa, which enables the separation of large volumes of oil/water
mixtures. Additionally, CF-PDA-Ag-NDM maintains up to a 98% separation efficiency and stable wettability for a dodecane/water mixture. These results are of great significance for practical applications. Therefore, this modified 3D CF has great potential for practical use in various fields such as oil spill cleanup and wastewater management. Acknowledgements This work was supported by the Natural Science Foundation of Guangdong Province, China (Grant No. 2014A030313246) and Science and Technology Planning Project of Guangdong Province, China (Grant No. 2016A010103005).
References [1] S.B. Joye, Deepwater horizon, 5 years on, Science 349 (2015) 592-593. [2] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marinas, A.M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (2008) 301-310. [3] B. Dubansky, A. Whitehead, J.T. Miller, C.D. Rice, F. Galvez, Multitissue molecular, genomic, and developmental effects of the deepwater horizon oil spill on resident gulf killifish (fundulus grandis), Environ. Sci. Technol. 47 (2013) 5074-5082. [4] I.B. Ivshina, M.S. Kuyukina, A.V. Krivoruchko, A.A. Elkin, S.O. Makarov, C.J. Cunningham, T.A. Peshkur, R.M. Atlas, J.C. Philp, Oil spill problems and sustainable response strategies through new technologies, Environ. Sci.: Process Impacts 17 (2015) 1201-1219. [5] S. Kleindienst, J.H. Paul, S.B. Joye, Using dispersants after oil spills: impacts on the composition and activity of microbial communities, Nat. Rev. Microbiol. 13 (2015) 388-396. [6] J. Fritt-Rasmussen, S. Wegeberg, K. Gustavson, Review on burn residues from in situ burning of oil spills in relation to arctic waters, Water Air Soil Pollut. 226 (2015). [7] K. Gaaseidnes, J. Turbeville, Separation of oil and water in oil spill recovery operations, Pure Appl. Chem. 71 (1999) 95-101. [8] A.B. Nordvik, J.L. Simmons, K.R. Bitting, A. Lewis, T. Strom-Kristiansen, Oil and water separation in marine oil spill clean-up operations, Spill Sci. Technol. Bull. 3 (1996) 107-122. [9] B. Wang, W.X. Liang, Z.G. Guo, W.M. Liu, Biomimetic super-lyophobic and super-lyophilic materials applied for oil/water separation: a new strategy beyond nature, Chem. Soc. Rev. 44 (2015) 336-361. [10] Q. Ma, H. Cheng, A.G. Fane, R. Wang, H. Zhang, Recent development of advanced materials with special wettability for selective oil/water separation, Small 12 (2016) 2186-2202. [11] Z. Xue, Y. Cao, N. Liu, L. Feng, L. Jiang, Special wettable materials for oil/water separation, J. Mater. Chem. A 2 (2014) 2445-2460. [12] W. Barthlott, C. Neinhuis, Purity of the sacred lotus, or escape from contamination in biological surfaces, Planta 202 (1997) 1-8. [13] X. Gao, L. Jiang, Water-repellent legs of water striders, Nature 432 (2004) 36.
[14] S. Niu, B. Li, Z. Mu, M. Yang, J. Zhang, Z. Han, L. Ren, Excellent structure-based multifunction of morpho butterfly wings: a review, J. Bionic Eng. 12 (2015) 170-189. [15] S. Liu, X. Liu, S.S. Latthe, L. Gao, S. An, S.S. Yoon, B. Liu, R. Xing, Self-cleaning transparent superhydrophobic coatings through simple sol–gel processing of fluoroalkylsilane, Appl. Surf. Sci. 351 (2015) 897-903. [16] R. Liao, Z. Zuo, C. Guo, Y. Yuan, A. Zhuang, Fabrication of superhydrophobic surface on aluminum by continuous chemical etching and its anti-icing property, Appl. Surf. Sci. 317 (2014) 701-709. [17] N. Wang, D.S. Xiong, Y.L. Deng, Y. Shi, K. Wang, Mechanically robust superhydrophobic steel surface with anti-icing, uv-durability, and corrosion resistance properties, ACS Appl. Mater. Interfaces 7 (2015) 6260-6272. [18] G.D. Bixler, A. Theiss, B. Bhushan, S.C. Lee, Anti-fouling properties of microstructured surfaces bio-inspired by rice leaves and butterfly wings, J. Colloid Interface Sci. 419 (2014) 114-133. [19] H.M. Zhang, J. Yang, B.B. Chen, C. Liu, M.S. Zhang, C.S. Li, Fabrication of superhydrophobic textured steel surface for anti-corrosion and tribological properties, Appl. Surf. Sci. 359 (2015) 905910. [20] J. Liu, L. Wang, F. Guo, L. Hou, Y. Chen, J. Liu, N. Wang, Y. Zhao, L. Jiang, Opposite and complementary: a superhydrophobic–superhydrophilic integrated system for high-flux, highefficiency and continuous oil/water separation, J. Mater. Chem. A 4 (2016) 4365-4370. [21] P. Pi, K. Hou, C. Zhou, G. Li, X. Wen, S. Xu, J. Cheng, S. Wang, Superhydrophobic Cu2S@Cu2O film on copper surface fabricated by a facile chemical bath deposition method and its application in oil-water separation, Appl. Surf. Sci. 396 (2017) 566-573. [22] L. Feng, Z. Zhang, Z. Mai, Y. Ma, B. Liu, L. Jiang, D. Zhu, A super-hydrophobic and superoleophilic coating mesh film for the separation of oil and water, Angew. Chem. Int. Ed. Engl. 43 (2004) 2012-2014. [23] C.R. Crick, J.A. Gibbins, I.P. Parkin, Superhydrophobic polymer-coated copper-mesh; membranes for highly efficient oil–water separation, J. Mater. Chem. A 1 (2013) 5943-5948. [24] C.H. Xue, P.T. Ji, P. Zhang, Y.R. Li, S.T. Jia, Fabrication of superhydrophobic and superoleophilic textiles for oil-water separation, Appl. Surf. Sci. 284 (2013) 464-471. [25] S. Liu, Q. Xu, S.S. Latthe, A.B. Gurav, R. Xing, Superhydrophobic/superoleophilic magnetic polyurethane sponge for oil/water separation, RSC Adv. 5 (2015) 68293-68298. [26] H. Zhu, D. Chen, W. An, N. Li, Q. Xu, H. Li, J. He, J. Lu, A robust and cost-effective superhydrophobic graphene foam for efficient oil and organic solvent recovery, Small 11 (2015) 5222-5229. [27] X. Zhang, Z. Li, K. Liu, L. Jiang, Bioinspired multifunctional foam with self-cleaning and oil/water separation, Adv. Funct. Mater. 23 (2013) 2881-2886. [28] X. Gao, J. Zhou, R. Du, Z. Xie, S. Deng, R. Liu, Z. Liu, J. Zhang, Robust superhydrophobic foam: A graphdiyne-based hierarchical architecture for oil/water separation, Adv. Mater. 28 (2016) 168173. [29] D. Zang, C. Wu, R. Zhu, W. Zhang, X. Yu, Y. Zhang, Porous copper surfaces with improved superhydrophobicity under oil and their application in oil separation and capture from water, Chem. Commun. 49 (2013) 8410-8412. [30] J. Zhang, K. Ji, J. Chen, Y. Ding, Z. Dai, A three-dimensional porous metal foam with selectivewettability for oil–water separation, J. Mater. Sci. 50 (2015) 5371-5377. [31] J. Xu, J. Xu, Y. Cao, X. Ji, Y. Yan, Fabrication of non-flaking, superhydrophobic surfaces using a
one-step solution-immersion process on copper foams, Appl. Surf. Sci. 286 (2013) 220-227. [32] L. Wang, G. Li, Y. Lin, Z. Zhang, Z. Chen, S. Wu, A strategy for constructing anti-adhesion surfaces based on interfacial thiol–ene photoclick chemistry between DOPA derivatives with a catechol anchor group and zwitterionic betaine macromolecules, Polym. Chem. 7 (2016) 4964-4974. [33] B.P. Lee, K. Huang, F.N. Nunalee, K.R. Shull, P.B. Messersmith, Synthesis of 3,4dihydroxyphenylalanine (DOPA) containing monomers and their co-polymerization with PEGdiacrylate to form hydrogels, J. Biomater. Sci., Polym. Ed. 15 (2004) 449-464. [34] J.H. Waite, M.l. Tanzer, Polyphenolic substance of mytilus edulis: Novel adhesive containing Ldopa and hydroxyproline, Science 212 (1981) 1038-1040. [35] M. Yu, J. Hwang, T.J. Deming, Role of L-3,4-dihydroxyphenylalanine in mussel adhesive proteins, J. Am. Chem. Soc. 121 (1999) 5825-5826. [36] H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Mussel-inspired surface chemistry for multifunctional coatings, Science 318 (2007) 426-430. [37] Y. Liu, K. Ai, L. Lu, Polydopamine and its derivative materials: synthesis and promising applications in energy, environmental, and biomedical fields, Chem. Rev. 114 (2014) 5057-5115. [38] E. Faure, C. Falentin-Daudré, C. Jérôme, J. Lyskawa, D. Fournier, P. Woisel, C. Detrembleur, Catechols as versatile platforms in polymer chemistry, Prog. Polym. Sci. 38 (2013) 236-270. [39] Z. Xu, K. Miyazaki, T. Hori, Fabrication of polydopamine-coated superhydrophobic fabrics for oil/water separation and self-cleaning, Appl. Surf. Sci. 370 (2016) 243-251. [40] S.M. Kang, I. You, W.K. Cho, H.K. Shon, T.G. Lee, I.S. Choi, J.M. Karp, H. Lee, One-step modification of superhydrophobic surfaces by a mussel-inspired polymer coating, Angew. Chem. Int. Ed. Engl. 49 (2010) 9401-9404. [41] L. Fan, B. Li, J. Zhang, Antibioadhesive Superhydrophobic Syringe Needles Inspired by Mussels and Lotus Leafs, Adv. Mater. Interfaces 2 (2015) 1500019. [42] L. Zhang, J. Wu, Y. Wang, Y. Long, N. Zhao, J. Xu, Combination of bioinspiration: a general route to superhydrophobic particles, J. Am. Chem. Soc. 134 (2012) 9879-9881. [43] H. Gao, Y. Sun, J. Zhou, R. Xu, H. Duan, Mussel-inspired synthesis of polydopamine-functionalized graphene hydrogel as reusable adsorbents for water purification, ACS Appl. Mater. Interfaces 5 (2013) 425-432. [44] F. Liu, F. Sun, Q. Pan, Highly compressible and stretchable superhydrophobic coating inspired by bio-adhesion of marine mussels, J. Mater. Chem. A 2 (2014) 11365. [45] T. Laiho, J.A. Leiro, M.H. Heinonen, S.S. Mattila, J. Lukkari, Photoelectron spectroscopy study of irradiation damage and metal–sulfur bonds of thiol on silver and copper surfaces, J. Electron. Spectrosc. Relat. Phenom. 142 (2005) 105-112. [46] Y. Cao, X. Zhang, L. Tao, K. Li, Z. Xue, L. Feng, Y. Wei, Mussel-inspired chemistry and Michael addition reaction for efficient oil/water separation, ACS Appl. Mater. Interfaces 5 (2013) 4438-4442. [47] K. Li, X. Zeng, H. Li, X. Lai, H. Xie, Facile fabrication of superhydrophobic filtration fabric with honeycomb structures for the separation of water and oil, Mater. Lett. 120 (2014) 255-258.
Fig. 1. SEM images of a pristine CF as a control and different modified CF samples. Fig. 2. XPS spectra of the different modified CF samples and a pristine CF sample as a control. Fig. 3. The high-resolution XPS spectra in the C1s and N1s regions of CF-PDA (a) and the C1s, Ag3d and S2p regions of CF-PDA-Ag-NDM (b). Fig. 4. WCA images of the different modified CF samples. Fig. 5. Photographs of water (a) and oil (b) droplets on the surface of the CF-PDA-Ag-NDM. Fig. 6. Diagram of the oil/water mixture (a) before and (b) after separation. Fig. 7. (a) The separation efficiency of the CF-PDA-Ag-NDM for a selection of oil/water mixtures, and (b) the maximum water intrusion pressure of the CF-PDA-Ag-NDM. Fig. 8. (a) Separation efficiency and WCA of the CF-PDA-Ag-NDM after repeated separation tests with dodecane/water. (b) The SEM and WCA images of the CF-PDA-Ag-NDM after 30 repeated separation tests. Fig. 9. The state of the water droplets on the surface of the superhydrophobic copper foam and the corresponding schematic diagrams (a) in the initial stage of the oil/water separation and (b) during the oil/water separation process. Scheme 1. Schematic of the construction process of the superhydrophobic copper foam
Fig. 1.
Fig. 2.
Fig. 3.
Fig. 4.
Fig. 5.
Fig. 6
Fig.7
Fig 8
Fig. 9
Scheme 1.
Table 1. Elemental composition analysis of CF, CF-PDA, CF-PDA-Ag and CF-PDA-Ag-NDM surfaces based on their XPS spectra. Element content (wt%) Sample Cu2p
C1s
N1s
O1s
Ag3d
S2p
CF
62.22
28.38
0.63
7.11
0.00
1.65
CF-PDA
33.17
52.68
2.66
10.67
0.00
0.82
CF-PDA-Ag
25.96
46.86
0.52
5.21
20.80
0.65
CF-PDA-Ag-NDM
19.17
57.25
0.79
4.14
14.88
3.77