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Versatile fabrication of the magnetic polymer-based graphene foam and applications for oil–water separation
Accepted Manuscript Title: Versatile Fabrication of the Magnetic Polymer-based Graphene Foam and Applications for Oil-Water Separation Author: Can Liu Jin Yang Yongcai Tang Linting Yin Hua Tang Changsheng Li PII: DOI: Reference:
S0927-7757(14)00924-8 http://dx.doi.org/doi:10.1016/j.colsurfa.2014.12.005 COLSUA 19585
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
24-9-2014 2-12-2014 7-12-2014
Please cite this article as: C. Liu, J. Yang, Y. Tang, L. Yin, H. Tang, C. Li, Versatile Fabrication of the Magnetic Polymer-based Graphene Foam and Applications for OilWater Separation, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2014), http://dx.doi.org/10.1016/j.colsurfa.2014.12.005 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.
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Graphical abstract
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Highlights
Magnetic polymer-based graphene foam was fabricated by a facile synthesis process.
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Graphene was assembled on PU skeletons using ferrous ions as reducing agent.
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The foam exhibited superhydrophobicity and superoleophilicity.
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The foam had high oil-absorption capacity and excellent reusability.
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Versatile Fabrication of the Magnetic Polymer-based Graphene Foam and Applications for Oil-Water Separation
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Can Liu, Jin Yang, Yongcai Tang, Linting Yin, Hua Tang, Changsheng Li* School of Materials Science and Engineering, Jiangsu University, Zhenjiang, 212013, P.R. China
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Abstract
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The magnetic polymer-based graphene foam (MPG) for oil-water separation was fabricated by the synergistic effects of the deposition of Fe3O4 nanoparticles on graphene
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sheets and the self-assembly of graphene on polyurethane (PU) sponge. Graphene sheets
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were assembled on the surface of PU skeletons using ferrous ions as reducing agent, which can improve the adhesion of graphene on PU skeletons. The resulting MPG exhibited
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superhydrophobicity and superoleophilicity with the water contact angle of 158 ± 1° and the
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oil contact angle of 0°. A broad variety of oils and organic solvents can be removed from oilwater mixtures under manipulation by a magnet bar with high absorption capacity and selectivity. Significantly, the MPG can remain high absorption capacity for oil/water separation after several absorption/desorption-cycles, demonstrating its good reusability. Therefore, the findings of this study offered a facile approach for the cleanup of pollution by crude oil, petroleum products, and toxic organic solvents. Keywords: Graphene; Superhydrophobic; Oil-water separation; Magnetic; Reusability
* E-mail address: [email protected]. 2
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1. Introduction Pollution by crude oil, petroleum products, and toxic organic solvents caused severe
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environmental and ecological problems in recent years [1,2]. Demand for finding new materials that can sequester and remove pollutants became highly imperative. Recently,
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materials with superhydrophobicity and superoleophilicity have been developed for oil-water
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separation because they can absorb oil while repel water completely [3-5]. Especially, three-
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dimensional (3D) superhydrophobic and superoleophilic materials, such as silica aerogels [68], carbon nanotube sponges [9,10], and nanowire membranes [11-14], have attracted
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considerable interest due to the high absorption capacity and selectivity. However, the
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methods for creating these materials typically use expensive materials or severe conditions,
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and their porous structures are easily destroyed during the post-treatment process, limiting the
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application of 3D superhydrophobic and superoleophilic materials in the oil-water separation. Therefore, it would be a challenge to develop 3D superhydrophobic and superoleophilic materials with high absorption capacity, low cost, excellent flexibility, and environmental friendliness.
As two-dimensional carbon materials, graphene and its derivatives have received
considerable attentions in various research fields and have become “star” materials in recent years due to their remarkable physicochemical properties [15,16]. Besides their excellent physical and chemical properties, graphene materials have recently been reported that their 3
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various derivatives show exciting hydrophobic properties [17-21]. Moreover, various 3D graphene-based areogels with superhydrophobicity and superoleophilicity for oil-water
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separation have been fabricated via the hydrothermal method or freeze-dried method [19-21].
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However, there are several drawbacks for graphene-based areogels to separate oil-water mixtures, such as the poor mechanical performances and complexity of drying process. To
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solve these shortcomings, polymer-based graphene sponges for oil-water separation have
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been fabricated by physical absorption of graphene sheets on 3D polymer skeleton to improve the mechanical properties [22,23]. However, the absorbed graphene sheets can be
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peeled off during the separation and post-treatment processes, which limits the reusability of
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polymer-based graphene sponges.
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In this paper, we develop a facile and versatile approach to fabricate magnetic polymer-
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based graphene foam (MPG) with superhydrophobicity and superoleophilicity via the hydrothermal method. In the synthetic process, graphene sheets were assembled on the surface of polyurethane (PU) skeletons, using ferrous ions as reducing agent to improve the adhesion of graphene on PU skeletons. A broad variety of oils and organic solvents can be absorbed from oil-water mixtures using the prepared MPG with high absorption capacity and selectivity under manipulation in the magnetic field. Moreover, the MPG can remain high absorption capacity for oil/water separation after several absorption/desorption-cycles,
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demonstrating the good reusability. The details of the preparation, characterization and performances of MPG were described herein.
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2. Experimental 2.1. Materials
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PU sponges were purchased from Tuopu daily chemicals (China) Co., Ltd. Graphene
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oxide (GO) powder was synthesized from natural graphite powder by a modified Hummers method. Paraffin oil, n-hexadecane, hexane, octane, heptanes, FeSO4·7H2O and
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octadecyltrichlorosilane (95%) were all purchased from Sinopharm Chemical Reagent Co.,
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Ltd. Lubricating oil and hydraulic oil were purchased from Sinopec Lubricant Co., Ltd.
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2.2 Sample Preparation
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used as received.
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Peanut oil was supplied by Luhua Group. All the chemicals used were of analytical grade and
In the typical process, PU sponges were ultrasonically treated with deionized water and
ethanol for 30 min and dried at 60 °C. The pH value of GO solution (20 mL, 2 mg/mL) was adjusted with ammonia (1 M) to 11. Then 1 mmol FeSO4·7H2O was quickly added into this solution and vigorously stirred for 15 min. The mixture was poured into an 80 mL Teflon vessel, and the cleaned PU sponge was immersed into the mixture by a squeezing and vacuum degassing procedure. After heated at 90 °C for 12 h, a blackish green product was taken out from the vessel and immersed in distilled water 1 h for cleaning. The product was
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taken out and dried to obtain MPG. Subsequently, the MPG was dipped in a hexane solution of 2% (v/v) octadecyltrichlorosilane (OTS) for 10 min and dried at 80 °C for 1 h.
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2.3 Measurements of oil absorption
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The mixtures of n-hexadecane and water were used to test the oil-water separation capacity of the as-prepared MPG. N-hexadecane was dyed by oil red O. The oil-absorption
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capacity of MPG was calculated by the ratio between the maximum absorbed oil quantity moil
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and the foams mass mf (i.e., moil/mf). Then the volume-based absorption capacity was given by the equation Voil/Vf = (moilρf)/(mfρoil), where ρf and ρoil were the densities of the MPG and
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the oils, respectively.
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2.4 Reusability
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The extrusion method was used to remove most of the absorbed oil, and then immersed
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the sample in ethanol to remove the remaining oil. The MPG can be reused into the oilabsorption procedure after drying in oven directly. The absorption/desorption-cycle was repeated for eight times to evaluate the reusability of the MPG. 2.5 Characterization
The X-ray diffraction (XRD) patterns were obtained on a D8 advance (Bruker-AXS) X-
ray diffractometer with Cu Kα radiation (λ¼1.5406Å) at a scanning rate of 0.02/s. Magnetic properties of the MPGs were investigated on a magnetometer (Quantum Design MPMS XL). Field-emission
scanning
electron
microscopy
(JSM-7001F
SEM)
and
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transmission electron microscopy (JEM-100CX II TEM) were used to observe the surface morphology and structure of the MPGs. Apparent contact angle and sliding angle
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measurements were performed using a KSV CM20 apparatus at ambient temperature. 5 μL
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deionized water was dropped on the samples using an automatically dispense controller, and the contact angles were determined automatically by using the Laplace–Young fitting
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algorithm. The average contact angle values were obtained by measuring the sample at five
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different positions of the substrate. A droplet of 5 μL was used as the probing liquid for sliding angle measurement. Contact angle hysteresis was observed by measuring the
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difference between advancing angle and receding angle, which were collected with adding
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and withdrawing from the droplet, respectively. All of the images were captured with a
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3. Results and discussion
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traditional digital camera (Canon SX50 HS).
In order to obtain magnetic foams, a commercial PU sponge is used as a template and
dipped into a mixture of GO and FeSO4 solution. The magnetic Fe3O4 nanoparticles are generated through a serious of oxidation reduction reaction and co-precipitation reaction [21]. The synthesis mechanism is illustrated in Figure 1. The ferrous ions Fe2+ tended to disperse to the GO sheets by electrostatic interactions, which can be oxidized into ferric ions Fe3+ effectively by the oxygen-containing functional groups on the GO surface [21]. At high pH value, the oxidized Fe3+ ions and Fe2+ ions could coprecipitate to form Fe3O4 nanoparticles 7
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which are decorated on the reduced GO surface simultaneously, similar to the formation of graphene decorated with other nanoparticles [24-27]. During the reduction procedure, the
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oxygen-functionalized groups of GO can be removed and assembled on the PU skeletons to
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form the MPG.
After a series of reactions, monolithic MPG with 3D interconnected network was
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obtained (Figure 2a). Fe3O4 nanoparticles on the surface of PU sponge can be confirmed by
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XRD measurements. As shown in Figure 2b, all of the diffraction peaks are indexed to the magnetic Fe3O4 phase. Moreover, the magnetic property of MPG allows it capable of being
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manipulated by a magnetic bar (Figure 2c). To investigate the magnetic property of the as-
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obtained MPG, the magnetic hysteresis curve of the foam is recorded at room temperature. As
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shown in Figure 2d, the sample showed ferromagnetic properties, revealed by the magnified
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plot in the inset: the coercive force of 38.6 Oe and a remanent magnetization of 1.8 emu/g. The saturation magnetization (Ms) of the MPG is 24.1 emu/g due to the bulk embedded Fe3O4 nanoparticles.
The surface morphology of the original PU sponge is shown in Figure 3a and b. It can be
seen that the original PU sponge exhibits the interconnected and porous 3D network structure with the smooth skeletons. After being assembled graphene and Fe3O4 nanoparticles on the skeletons, the morphology of MPG is similar to that of PU sponges, but the PU skeletons exhibit a rough texture (Figure 3c,d). Meanwhile, high magnification SEM and TEM images 8
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indicate that numerous Fe3O4 nanoparticles are decorated on the surface of graphene sheets (Figure 3e,f). The SEM observations also illustrate that the chemical reaction occurred on the
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frame did not cause any damage to the PU sponge. In order to study the stability of the MPG,
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we performed a series of experiments to investigate the adhesion between PU sponge and graphene. As shown in Figure 4(a-f), the sample could restore to its original appearance after
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severe compression without any graphene sheets exfoliation. Additionally, when the MPG
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was ultrasonically treated in ethanol for 1 h (53 KHz, 180 W), there were not any nanoparticles appearing in ethanol. The stability of MPG may be attributed to the assembly of
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graphene sheets on the skeletons of PU sponge using ferrous ions as reducing agent, similar
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to the formation of 3D graphene-based foams [19].
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The as-obtained MPG exhibits superhydrophobicity and superoleophilicity with the
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water contact angle of 152 ± 1° and the oil contact angle of 0°, but water droplets can be pinned to the MPG surface with contact angle hysteresis of about 85 ± 3° due to the hydrophilicity of Fe3O4 nanoparticles. After simple modification with OTS [28], water droplets can roll off the MPG surface easily with apparent contact angle of 158 ± 1° and contact angle hysteresis of 5 ± 1°, and this modification has no influence on its superoleophilicity (Figure 5a,b). A water droplet retains nearly spherical on the surface of MPG, as well as an n-hexadecane droplet is absorbed inside instantly (Figure 5c). The water droplet on the MPG surface is in Cassie-Baxter state and a composite solid-liquid-air 9
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interface forms underneath the water, leading to the water droplet keep spherical shape. As shown in Figure 5d, there was a thick layer of bubble on the MPG surface when it was
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immersed in water, which is a signature of the composite solid-liquid-air interface.
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Additionally, the hexadecane on the MPG surface is in Wenzel state and a fully wetted interface results in the absorption of hexadecane. The phenomena of water repellency and oil
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absorption are very important to the oil-water separation of MPG.
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The oil-water separation experiments were performed using the as-prepared MPG. As shown in Figure 5e-h, the MPG is taken by a magnet bar and absorbs n-hexadecane
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completely from the oil-water mixture within 7 seconds. Indicatively, the MPG can be
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manipulated to the oil-polluted region by a magnet bar due to its magnetic properties. It
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undoubtedly provides a facile and energy-saving method to collect oils from oil-polluted area.
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Additionally, the photographs in Figure 6(a-d) exhibited the rapid process of the MPG absorbing a thick layer of n-hexadecane on the water surface. The absorbed oils can be readily removed from the MPG through a simple mechanical extrusion method due to its excellent mechanical flexibility (Figure 6e). The absorption capacity of the MPG was also investigated by absorbing a variety of oils, including lubricating oil, paraffin oil, hydraulic oil, peanut oil, n-hexadecane, hexane, octane, and heptane. As shown in Figure 6f, the massbased absorption capacities for these oils were in the range of 9-27 times of its own weight, depending on density, viscosity and surface tension of the absorbed liquids [28]. Not only the 10
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mass-based absorption capacity was comparable to those of other polymer-based counterparts [29-31], but also it showed considerable volume-based capacities that it all reaches to higher
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than 70%, indicating that majority of its space is used for oil-storage (Figure 6g).
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Furthermore, the regeneration capacity of MPG was investigated as an important index of a promising oil-water separation material [21]. After the oil/water separation experiment,
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the contaminated MPG was rinsed thoroughly by ethanol to remove the absorbed oil.
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Subsequently, the clean MPG was dried and the superhydrophobicity could be easily recovered. The line in Figure 7 shows the relationship between the absorption capacity and
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the recycle number. Because of its robust and stable porous structure, the MPG has an
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absorption capacity higher than 90% of the first maximum after eight absorption/desorption-
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cycles. The water contact angle on the MPG decreased slightly from 158 ± 1° to 155 ± 2°
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after eight separation cycles, which might be caused by the residual oil adhered to the MPG. As shown in Figure 7 inset, water droplets still retain nearly spherical on the surface of MPG. Therefore, the MPG with superhydrophobicity, superoleophilicity, magnetic actuation, high stability and facile synthesis method could become a promising oil-absorbent material in the field of oil-water separation. 4. Conclusions In summary, we demonstrated a facile and versatile method to fabricate the MPG with superhydrophobicity and superoleophilicity for the absorption of oils or organic solvents 11
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from water. PU sponges were used as framework to increase the mechanical flexibility of graphene framework due to its economic accessibility. The assembly of graphene sheets on
Meanwhile,
the
as-obtained
MPG
exhibits
superhydrophobicity
and
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skeletons.
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the skeletons of PU sponge was realized to improve the adhesion of graphene on PU
superoleophilicity with the water contact angle of 158 ± 1° and the oil contact angle of 0°.
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Thus the MPG can absorb a broad variety of oils and organic solvents from oil-water
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mixtures expeditiously under manipulation by a magnet bar with high absorption capacity and selectivity. Moreover, after several cycles of absorption/desorption, the MPG still
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exhibited a high absorption capacity due to the robust and stable porous structure. This facile
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and inexpensive fabricating process will provide a novel pathway for a promising candidate
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Acknowledgements
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for applications in the field of oil-water separation.
This work was supported by National Natural Science Foundation of China (51275213),
Natural Science Foundation of Jiangsu Province, China (BK20140562), and China Postdoctoral Science Foundation (2013M530235). References
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Figure captions Figure 1. Schematic illustration of the formation mechanism of the MPG.
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Figure 2. Optical image (a) and XRD pattern (b) of the as-prepared MPG. The image of the
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MPG manipulated by a magnet bar (c) and the room-temperature magnetization hysteresis curve of the as-prepared MPG (d).
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Figure 3. SEM images of the original PU sponge (a, b) and as-prepared MPG (c-e). TEM
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images of graphene sheets decorated with Fe3O4 nanoparticles (f).
Figure 4. The compression-recovery process of the as-prepared MPG (a-f).
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Figure 5. Apparent contact angles (a, b) and optical image (c) of water and n-hexadecane
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droplets on the surface of the MPG. The image of the MPG immersed in water by an external
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force (d). The MPG rapidly removed n-hexadecane from the mixture of water and n-
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hexadecane under magnetic field (e-h).
Figure 6. The process of a thick layer of n-hexadecane being absorbed by the MPG (a-e). Mass-based (f) and volume-based (g) absorption capacities of the MPG for oils and nonpolar solvents.
Figure 7. Regeneration capacity of the MPG for adsorbing hexane and n-hexadecane. Inset is the picture of water droplets on the surface of the MPG that was reused for eight times.
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S0927-7757(14)00924-8 http://dx.doi.org/doi:10.1016/j.colsurfa.2014.12.005 COLSUA 19585
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
24-9-2014 2-12-2014 7-12-2014
Please cite this article as: C. Liu, J. Yang, Y. Tang, L. Yin, H. Tang, C. Li, Versatile Fabrication of the Magnetic Polymer-based Graphene Foam and Applications for OilWater Separation, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2014), http://dx.doi.org/10.1016/j.colsurfa.2014.12.005 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.
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Graphical abstract
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Highlights
Magnetic polymer-based graphene foam was fabricated by a facile synthesis process.
an
Graphene was assembled on PU skeletons using ferrous ions as reducing agent.
M
The foam exhibited superhydrophobicity and superoleophilicity.
Ac ce p
te
d
The foam had high oil-absorption capacity and excellent reusability.
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Page 1 of 25
Versatile Fabrication of the Magnetic Polymer-based Graphene Foam and Applications for Oil-Water Separation
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Can Liu, Jin Yang, Yongcai Tang, Linting Yin, Hua Tang, Changsheng Li* School of Materials Science and Engineering, Jiangsu University, Zhenjiang, 212013, P.R. China
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Abstract
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The magnetic polymer-based graphene foam (MPG) for oil-water separation was fabricated by the synergistic effects of the deposition of Fe3O4 nanoparticles on graphene
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sheets and the self-assembly of graphene on polyurethane (PU) sponge. Graphene sheets
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were assembled on the surface of PU skeletons using ferrous ions as reducing agent, which can improve the adhesion of graphene on PU skeletons. The resulting MPG exhibited
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superhydrophobicity and superoleophilicity with the water contact angle of 158 ± 1° and the
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oil contact angle of 0°. A broad variety of oils and organic solvents can be removed from oilwater mixtures under manipulation by a magnet bar with high absorption capacity and selectivity. Significantly, the MPG can remain high absorption capacity for oil/water separation after several absorption/desorption-cycles, demonstrating its good reusability. Therefore, the findings of this study offered a facile approach for the cleanup of pollution by crude oil, petroleum products, and toxic organic solvents. Keywords: Graphene; Superhydrophobic; Oil-water separation; Magnetic; Reusability
* E-mail address: [email protected]. 2
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1. Introduction Pollution by crude oil, petroleum products, and toxic organic solvents caused severe
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environmental and ecological problems in recent years [1,2]. Demand for finding new materials that can sequester and remove pollutants became highly imperative. Recently,
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materials with superhydrophobicity and superoleophilicity have been developed for oil-water
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separation because they can absorb oil while repel water completely [3-5]. Especially, three-
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dimensional (3D) superhydrophobic and superoleophilic materials, such as silica aerogels [68], carbon nanotube sponges [9,10], and nanowire membranes [11-14], have attracted
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considerable interest due to the high absorption capacity and selectivity. However, the
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methods for creating these materials typically use expensive materials or severe conditions,
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and their porous structures are easily destroyed during the post-treatment process, limiting the
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application of 3D superhydrophobic and superoleophilic materials in the oil-water separation. Therefore, it would be a challenge to develop 3D superhydrophobic and superoleophilic materials with high absorption capacity, low cost, excellent flexibility, and environmental friendliness.
As two-dimensional carbon materials, graphene and its derivatives have received
considerable attentions in various research fields and have become “star” materials in recent years due to their remarkable physicochemical properties [15,16]. Besides their excellent physical and chemical properties, graphene materials have recently been reported that their 3
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various derivatives show exciting hydrophobic properties [17-21]. Moreover, various 3D graphene-based areogels with superhydrophobicity and superoleophilicity for oil-water
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separation have been fabricated via the hydrothermal method or freeze-dried method [19-21].
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However, there are several drawbacks for graphene-based areogels to separate oil-water mixtures, such as the poor mechanical performances and complexity of drying process. To
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solve these shortcomings, polymer-based graphene sponges for oil-water separation have
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been fabricated by physical absorption of graphene sheets on 3D polymer skeleton to improve the mechanical properties [22,23]. However, the absorbed graphene sheets can be
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peeled off during the separation and post-treatment processes, which limits the reusability of
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polymer-based graphene sponges.
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In this paper, we develop a facile and versatile approach to fabricate magnetic polymer-
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based graphene foam (MPG) with superhydrophobicity and superoleophilicity via the hydrothermal method. In the synthetic process, graphene sheets were assembled on the surface of polyurethane (PU) skeletons, using ferrous ions as reducing agent to improve the adhesion of graphene on PU skeletons. A broad variety of oils and organic solvents can be absorbed from oil-water mixtures using the prepared MPG with high absorption capacity and selectivity under manipulation in the magnetic field. Moreover, the MPG can remain high absorption capacity for oil/water separation after several absorption/desorption-cycles,
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demonstrating the good reusability. The details of the preparation, characterization and performances of MPG were described herein.
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2. Experimental 2.1. Materials
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PU sponges were purchased from Tuopu daily chemicals (China) Co., Ltd. Graphene
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oxide (GO) powder was synthesized from natural graphite powder by a modified Hummers method. Paraffin oil, n-hexadecane, hexane, octane, heptanes, FeSO4·7H2O and
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octadecyltrichlorosilane (95%) were all purchased from Sinopharm Chemical Reagent Co.,
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Ltd. Lubricating oil and hydraulic oil were purchased from Sinopec Lubricant Co., Ltd.
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2.2 Sample Preparation
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used as received.
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Peanut oil was supplied by Luhua Group. All the chemicals used were of analytical grade and
In the typical process, PU sponges were ultrasonically treated with deionized water and
ethanol for 30 min and dried at 60 °C. The pH value of GO solution (20 mL, 2 mg/mL) was adjusted with ammonia (1 M) to 11. Then 1 mmol FeSO4·7H2O was quickly added into this solution and vigorously stirred for 15 min. The mixture was poured into an 80 mL Teflon vessel, and the cleaned PU sponge was immersed into the mixture by a squeezing and vacuum degassing procedure. After heated at 90 °C for 12 h, a blackish green product was taken out from the vessel and immersed in distilled water 1 h for cleaning. The product was
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taken out and dried to obtain MPG. Subsequently, the MPG was dipped in a hexane solution of 2% (v/v) octadecyltrichlorosilane (OTS) for 10 min and dried at 80 °C for 1 h.
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2.3 Measurements of oil absorption
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The mixtures of n-hexadecane and water were used to test the oil-water separation capacity of the as-prepared MPG. N-hexadecane was dyed by oil red O. The oil-absorption
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capacity of MPG was calculated by the ratio between the maximum absorbed oil quantity moil
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and the foams mass mf (i.e., moil/mf). Then the volume-based absorption capacity was given by the equation Voil/Vf = (moilρf)/(mfρoil), where ρf and ρoil were the densities of the MPG and
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the oils, respectively.
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2.4 Reusability
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The extrusion method was used to remove most of the absorbed oil, and then immersed
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the sample in ethanol to remove the remaining oil. The MPG can be reused into the oilabsorption procedure after drying in oven directly. The absorption/desorption-cycle was repeated for eight times to evaluate the reusability of the MPG. 2.5 Characterization
The X-ray diffraction (XRD) patterns were obtained on a D8 advance (Bruker-AXS) X-
ray diffractometer with Cu Kα radiation (λ¼1.5406Å) at a scanning rate of 0.02/s. Magnetic properties of the MPGs were investigated on a magnetometer (Quantum Design MPMS XL). Field-emission
scanning
electron
microscopy
(JSM-7001F
SEM)
and
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transmission electron microscopy (JEM-100CX II TEM) were used to observe the surface morphology and structure of the MPGs. Apparent contact angle and sliding angle
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measurements were performed using a KSV CM20 apparatus at ambient temperature. 5 μL
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deionized water was dropped on the samples using an automatically dispense controller, and the contact angles were determined automatically by using the Laplace–Young fitting
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algorithm. The average contact angle values were obtained by measuring the sample at five
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different positions of the substrate. A droplet of 5 μL was used as the probing liquid for sliding angle measurement. Contact angle hysteresis was observed by measuring the
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difference between advancing angle and receding angle, which were collected with adding
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and withdrawing from the droplet, respectively. All of the images were captured with a
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3. Results and discussion
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traditional digital camera (Canon SX50 HS).
In order to obtain magnetic foams, a commercial PU sponge is used as a template and
dipped into a mixture of GO and FeSO4 solution. The magnetic Fe3O4 nanoparticles are generated through a serious of oxidation reduction reaction and co-precipitation reaction [21]. The synthesis mechanism is illustrated in Figure 1. The ferrous ions Fe2+ tended to disperse to the GO sheets by electrostatic interactions, which can be oxidized into ferric ions Fe3+ effectively by the oxygen-containing functional groups on the GO surface [21]. At high pH value, the oxidized Fe3+ ions and Fe2+ ions could coprecipitate to form Fe3O4 nanoparticles 7
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which are decorated on the reduced GO surface simultaneously, similar to the formation of graphene decorated with other nanoparticles [24-27]. During the reduction procedure, the
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oxygen-functionalized groups of GO can be removed and assembled on the PU skeletons to
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form the MPG.
After a series of reactions, monolithic MPG with 3D interconnected network was
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obtained (Figure 2a). Fe3O4 nanoparticles on the surface of PU sponge can be confirmed by
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XRD measurements. As shown in Figure 2b, all of the diffraction peaks are indexed to the magnetic Fe3O4 phase. Moreover, the magnetic property of MPG allows it capable of being
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manipulated by a magnetic bar (Figure 2c). To investigate the magnetic property of the as-
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obtained MPG, the magnetic hysteresis curve of the foam is recorded at room temperature. As
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shown in Figure 2d, the sample showed ferromagnetic properties, revealed by the magnified
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plot in the inset: the coercive force of 38.6 Oe and a remanent magnetization of 1.8 emu/g. The saturation magnetization (Ms) of the MPG is 24.1 emu/g due to the bulk embedded Fe3O4 nanoparticles.
The surface morphology of the original PU sponge is shown in Figure 3a and b. It can be
seen that the original PU sponge exhibits the interconnected and porous 3D network structure with the smooth skeletons. After being assembled graphene and Fe3O4 nanoparticles on the skeletons, the morphology of MPG is similar to that of PU sponges, but the PU skeletons exhibit a rough texture (Figure 3c,d). Meanwhile, high magnification SEM and TEM images 8
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indicate that numerous Fe3O4 nanoparticles are decorated on the surface of graphene sheets (Figure 3e,f). The SEM observations also illustrate that the chemical reaction occurred on the
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frame did not cause any damage to the PU sponge. In order to study the stability of the MPG,
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we performed a series of experiments to investigate the adhesion between PU sponge and graphene. As shown in Figure 4(a-f), the sample could restore to its original appearance after
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severe compression without any graphene sheets exfoliation. Additionally, when the MPG
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was ultrasonically treated in ethanol for 1 h (53 KHz, 180 W), there were not any nanoparticles appearing in ethanol. The stability of MPG may be attributed to the assembly of
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graphene sheets on the skeletons of PU sponge using ferrous ions as reducing agent, similar
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to the formation of 3D graphene-based foams [19].
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The as-obtained MPG exhibits superhydrophobicity and superoleophilicity with the
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water contact angle of 152 ± 1° and the oil contact angle of 0°, but water droplets can be pinned to the MPG surface with contact angle hysteresis of about 85 ± 3° due to the hydrophilicity of Fe3O4 nanoparticles. After simple modification with OTS [28], water droplets can roll off the MPG surface easily with apparent contact angle of 158 ± 1° and contact angle hysteresis of 5 ± 1°, and this modification has no influence on its superoleophilicity (Figure 5a,b). A water droplet retains nearly spherical on the surface of MPG, as well as an n-hexadecane droplet is absorbed inside instantly (Figure 5c). The water droplet on the MPG surface is in Cassie-Baxter state and a composite solid-liquid-air 9
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interface forms underneath the water, leading to the water droplet keep spherical shape. As shown in Figure 5d, there was a thick layer of bubble on the MPG surface when it was
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immersed in water, which is a signature of the composite solid-liquid-air interface.
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Additionally, the hexadecane on the MPG surface is in Wenzel state and a fully wetted interface results in the absorption of hexadecane. The phenomena of water repellency and oil
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absorption are very important to the oil-water separation of MPG.
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The oil-water separation experiments were performed using the as-prepared MPG. As shown in Figure 5e-h, the MPG is taken by a magnet bar and absorbs n-hexadecane
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completely from the oil-water mixture within 7 seconds. Indicatively, the MPG can be
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manipulated to the oil-polluted region by a magnet bar due to its magnetic properties. It
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undoubtedly provides a facile and energy-saving method to collect oils from oil-polluted area.
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Additionally, the photographs in Figure 6(a-d) exhibited the rapid process of the MPG absorbing a thick layer of n-hexadecane on the water surface. The absorbed oils can be readily removed from the MPG through a simple mechanical extrusion method due to its excellent mechanical flexibility (Figure 6e). The absorption capacity of the MPG was also investigated by absorbing a variety of oils, including lubricating oil, paraffin oil, hydraulic oil, peanut oil, n-hexadecane, hexane, octane, and heptane. As shown in Figure 6f, the massbased absorption capacities for these oils were in the range of 9-27 times of its own weight, depending on density, viscosity and surface tension of the absorbed liquids [28]. Not only the 10
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mass-based absorption capacity was comparable to those of other polymer-based counterparts [29-31], but also it showed considerable volume-based capacities that it all reaches to higher
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than 70%, indicating that majority of its space is used for oil-storage (Figure 6g).
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Furthermore, the regeneration capacity of MPG was investigated as an important index of a promising oil-water separation material [21]. After the oil/water separation experiment,
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the contaminated MPG was rinsed thoroughly by ethanol to remove the absorbed oil.
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Subsequently, the clean MPG was dried and the superhydrophobicity could be easily recovered. The line in Figure 7 shows the relationship between the absorption capacity and
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the recycle number. Because of its robust and stable porous structure, the MPG has an
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absorption capacity higher than 90% of the first maximum after eight absorption/desorption-
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cycles. The water contact angle on the MPG decreased slightly from 158 ± 1° to 155 ± 2°
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after eight separation cycles, which might be caused by the residual oil adhered to the MPG. As shown in Figure 7 inset, water droplets still retain nearly spherical on the surface of MPG. Therefore, the MPG with superhydrophobicity, superoleophilicity, magnetic actuation, high stability and facile synthesis method could become a promising oil-absorbent material in the field of oil-water separation. 4. Conclusions In summary, we demonstrated a facile and versatile method to fabricate the MPG with superhydrophobicity and superoleophilicity for the absorption of oils or organic solvents 11
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from water. PU sponges were used as framework to increase the mechanical flexibility of graphene framework due to its economic accessibility. The assembly of graphene sheets on
Meanwhile,
the
as-obtained
MPG
exhibits
superhydrophobicity
and
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skeletons.
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the skeletons of PU sponge was realized to improve the adhesion of graphene on PU
superoleophilicity with the water contact angle of 158 ± 1° and the oil contact angle of 0°.
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Thus the MPG can absorb a broad variety of oils and organic solvents from oil-water
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mixtures expeditiously under manipulation by a magnet bar with high absorption capacity and selectivity. Moreover, after several cycles of absorption/desorption, the MPG still
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exhibited a high absorption capacity due to the robust and stable porous structure. This facile
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and inexpensive fabricating process will provide a novel pathway for a promising candidate
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Acknowledgements
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for applications in the field of oil-water separation.
This work was supported by National Natural Science Foundation of China (51275213),
Natural Science Foundation of Jiangsu Province, China (BK20140562), and China Postdoctoral Science Foundation (2013M530235). References
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Figure captions Figure 1. Schematic illustration of the formation mechanism of the MPG.
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Figure 2. Optical image (a) and XRD pattern (b) of the as-prepared MPG. The image of the
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MPG manipulated by a magnet bar (c) and the room-temperature magnetization hysteresis curve of the as-prepared MPG (d).
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Figure 3. SEM images of the original PU sponge (a, b) and as-prepared MPG (c-e). TEM
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images of graphene sheets decorated with Fe3O4 nanoparticles (f).
Figure 4. The compression-recovery process of the as-prepared MPG (a-f).
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Figure 5. Apparent contact angles (a, b) and optical image (c) of water and n-hexadecane
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droplets on the surface of the MPG. The image of the MPG immersed in water by an external
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force (d). The MPG rapidly removed n-hexadecane from the mixture of water and n-
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hexadecane under magnetic field (e-h).
Figure 6. The process of a thick layer of n-hexadecane being absorbed by the MPG (a-e). Mass-based (f) and volume-based (g) absorption capacities of the MPG for oils and nonpolar solvents.
Figure 7. Regeneration capacity of the MPG for adsorbing hexane and n-hexadecane. Inset is the picture of water droplets on the surface of the MPG that was reused for eight times.
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