Engineering Reduced Graphene Oxide Aerogel Produced by Effective

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Engineering reduced graphene oxide aerogel produced by effective #-ray radiation-induced self-assembly and its application for continuous oil–water separation Yalei He, Jihao Li, Kou Luo, Linfan Li, Jingbo Chen, and Jingye Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00073 • Publication Date (Web): 18 Mar 2016 Downloaded from http://pubs.acs.org on March 21, 2016

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Industrial & Engineering Chemistry Research

Engineering reduced graphene oxide aerogel produced by effective γ-ray radiation-induced self-assembly and its application for continuous oil–water separation Yalei He†‡§, Jihao Li†§, Kou Luo†, Linfan Li†, Jingbo Chen‡*, and Jingye Li†* †

CAS Center for Excellence on TMSR Energy System, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai, 201800, P. R. China.

‡

School of Materials Science & Engineering, Zhengzhou University, Zhengzhou 450002, P. R. China.

Supporting Information ABSTRACT: Reduced graphene oxide aerogel (RGO aerogel) is successfully synthesized via simultaneous reduction and self-assembly of graphene oxide (GO) sheets under γ-ray irradiation. As the resulting RGO aerogel possesses an interconnected macroporous structure with strong hydrophobicity and oleophilicity, it has great potential in oil–water separation. Adsorption–distillation and adsorption–combustion are both utilized to treat oil floating on water or oil sinking below the water surface. More importantly, a simple device has been developed to continuously adsorb and collect floating oil, and it has shown great promise for practical application.

1. INTRODUCTION Integration of graphene nano-sheets (Gs) into macroscopic structures is a very important way for their application and has received increasing interest. Graphene-based aerogel (GA), owing to its highly porous structure and large accessible surface area, has received tremendous attention in many 1 2 fields such as energy storage and conversion, catalysts, sen3 4 sors, and environmental remediation, especially oil adsorp5 tion and water purification. Owing to the rapidly increasing utilization and transportation of oils and chemical solvents, oil leakages and organic pollutants have attracted much attention considering the severe pollution of waters and living environment as well as 6 the significant loss of non-renewable resources. Among current strategies to tackle these problems, creating efficient absorbents for the separation of organic pollutants from water is regarded as an easy and practical approach. Traditional absorbents used for oil–water separation include many inorganic mineral materials with interconnected micropores, 7 7 8 such as zeolites, sepiolite, expanded perlite, activated car9 10 11 bon, sawdust, and expanded graphite. As a new form of carbon-based absorbents, GA shows very high absorption capacity and good recyclability on account of its ultralight, 12 porous and quite strong structure.

Conventional methods for preparing GA can be classified into two categories: self-assembly and template-directed 13 methods. The first category mainly includes direct selfassembly of graphene oxide (GO) and self-assembly induced 14 15 by hydrothermalreduction, chemicalreduction, or cross16 linkingagents. During a typical reduction process, the interaction force between GO sheets transforms from electrostatic repulsion into π–π attraction owing to the elimination of oxygen-functionalized groups and the restoration of conjugated regions, leading to the self-assembly of graphene 17 sheets; the resulting graphene hydrogel then goes through 18 vacuum freeze-drying or supercritical drying to form GA. Representative template-directed methods include chemical 19 vapor deposition (CVD) and template-directed assembly. As an efficient nuclear technique, γ-ray irradiation has 20 been widely used in sterilization and material processing. The most crucial characteristic of irradiation is the uniform generation of powerful oxidation or reduction species (or both oxidation and reduction species) that can accelerate the redox reaction of various molecules at any temperature and 21 any state without a catalyst. Moreover, regardless of the size and thickness of the three-dimensional (3D) object, it can be treated by γ-ray irradiation with strong penetrating power, which is very advantageous and convenient for industrializa22 tion. Consequently, γ-ray irradiation is considered as a safe, cost-effective, and environment-friendly process. In recent years, γ-ray irradiation has been used to prepare modified 23 multiwalled carbon nanotubes (MWCNTs) and well24 dispersed polymer decorated GO. Nevertheless, there has been no report on the utilization of nuclear techniques to prepare GA until now. Herein, a simple but effective approach for preparing reduced graphene oxide aerogel (RGO aerogel) via γ-ray irradiation is firstly presented. In short, GO in an EDA–H2O mixture goes through reduction and self-assembly simultaneously to form RGO hydrogel under γ-ray irradiation and oxygenfree conditions. Light-weight, hydrophobic, and oleophilic RGO aerogel is then obtained after a simple freeze-drying process. According to the characterization of both chemical and physical structures, RGO aerogel exhibits a highly porous structure with high C/O ratio and lipophilicity, indicat-

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ing that it is a promising material candidate for oil adsorption and water purification because it can absorb various organic liquids with high capacity and recyclability. In addition, when a pipette tip connected with a peristaltic pump through a length of flexible plastic tube is inserted in RGO

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aerogel floating on water, continuous adsorption of the floating oil can be realized without any water uptake. This unique application shows great potential in quick and ready removal and collection of organic pollutants or leaked oil floating on water.

Scheme 1. Overview of the synthesis of RGO aerogel via γ-ray irradiation.

Figure 1. (a) AFM image of original GO sheets. (b, c) SEM images of the interior of RGO aerogel with different magnification. (d) FT-IR spectra of GO, GO-i, and RGO aerogel. (e) Wide-scan XPS spectra of GO, GO-i and RGO aerogel. (f, g) C1s XPS spectra of GO and RGO aerogel. (h) XRD patterns of graphite, GO, and RGO aerogel. (i) TGA curves of graphite, GO, and RGO aerogel.

2. EXPERIMENTAL SECTION 2.1 Preparation of RGO aerogel: GO was prepared from natural graphite flakes on the basis of a modified Hummers’ method as mentioned in previous work. The complete preparation procedure of RGO aerogel is divided into three main

steps, as illustrated in Scheme 1. First, a mixture of aqueous –1 GO suspension (10 mL, about 2mg mL ) and EDA (200 mg corresponding to GO/EDA ratio of 1:10) was magnetically stirred. It was then deoxygenated by nitrogen bubbling for 10 min before sealing in a vial. Next, the mixture of GO in

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EDA–H2O was irradiated by γ-rays in a Co cell (2.11×10 Bq, located in SINAP, China) at room temperature about 25 °C. During this step, a cylindrical RGO hydrogel suspending in the solution was obtained. After irradiation, the unreacted EDA and extra ions were removed by alternate dialysis with HCl, water, and ethanol for more than five cycles. Finally, the resulting RGO hydrogel was freeze-dried to yield RGO aerogel. 2.2 Characterization of RGO aerogel: The surface features of GO were observed by an Atomic Force Microscope (AFM, Digital Nanoscope IIIa Multimode SPM). Scanning electron micrographs (SEM) were recorded on a field-emission scanning electron microscope (FE-SEM, JEOL, JSM-6700F) at 10 kV to observe the morphologies of graphene aerogel. Transmission electron micrographs (TEM) were also obtained on a high-resolution transmission electron microscopy (HR-TEM, JEOL, JEM-3000F).Contact angle analysis was measured on a KSV INSTITUTION (Attention Company) instrument and the volume of the water drop was 5 μL. Each sample was measured for three times on different positions and the average value was obtained. Fourier transform infrared spectra (FT-IR) were collected using KBr pellets on the transmission module of Thermo Nicolet Avatar 370 FT-IR spectrometer at –1 4 cm resolution and 32 scans. X-ray photoelectron spectroscopy (XPS) was performed on a SHIMADZU Kratos AXIS Ultra DLD XPS instrument equipped with a monochromated Al Kα X-ray source. High resolution scans were acquired with 40 eV pass energy, wide-scan survey spectra were acquired with 160 eV pass energy. X-ray diffraction (XRD) patterns were radiation (λ= 1.54 Å) at a generator voltage of 40 kV and a generator current of 50mA. All experiments were carried out in the reflection mode at ambient temperature with 2θ in the range of 5° to 50°. The scanning speed was set at 8° –1 min . TGA curves were recorded on a NETZSCH TG 209 F3 Tarus-Thermo-Microbalance. The samples were heated from –1 –1 50 °C to 700 °C at 10 °C min under 20 mL min of nitrogen purging. All tests were maintained at 100 °C for 5 min to eliminate the influence of absorbed water. Raman spectra was obtained from Raman spectrometer (RENISHAW inVia plus laser) with excitation wavelength about 532 nm. Conductivity was calculated according to the four probe method. And compression test was conducted on a INSTRON 5966 universal testing machine. Electric conductivity was tested on a ST2253C four-probe meter (Suzhou Jingge Electronic Co., LTD).

croscopy (AFM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) were utilized. According to the AFM image in Figure 1a and the TEM and SEM images in Figure S2, the GO sheets were quite uniform and flat, with an average lateral width of about 20μm and an average thickness of about 0.99nm. After the reduction and self-assembly induced by γ-ray irradiation, the cross section of RGO aerogel showed a honeycomb-like 3D porous structure with micrometer-scale voids and walls constructed by the stacking of RGO sheets in Figure 1b–c. The partial overlapping or coalescing of flexible sheets stemming from the 2 partial restoration of sp regions and the π–π interaction resulted in the formation of physical cross-linking sites of the 15 frameworks. Fourier-transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) are effective analytical techniques that are widely used to study the chemical structure of graphene-based materials. Figure 1d shows the FT-IR spectra of GO, GO-i (namely GO without EDA irradiated by γ-ray as a control group), and RGO aerogel. In the spectrum of GO, characteristic peaks of oxygen-containing functional groups appear: a peak related to carbonyl and carboxyl –1 groups appears at 1720 cm , a peak attributed to hydroxyl –1 groups appears at 1400 cm , a peak corresponding to epoxyl –1 appears at 1222 cm , and a peak for alkoxyl appears at 1053 –1 25 cm . As shown in its FT-IR spectrum, abundant oxygencontaining groups remained in GO-i. While there are dramatically fewer oxygen-related peaks in the spectrum of RGO aerogel, especially with an absence of peaks at 1720, 1400, and –1 1053 cm , new peaks corresponding to C=N, N–H, and C–N –1 show up at 1668, 1555, and 1200 cm , respectively.

As shown in Scheme 1, RGO aerogel is so light that it can stand on a feather and cause no observable deformation to the feather. The average density of the resulting RGO aerogel, which included the density of the air occupied in the pores, –3 was about 3.5-4.0 mg cm , obtained by simply dividing the weight by the volume. The average density of RGO aerogel increased nearly linearly as the concentration of GO suspension was increased (Figure S1, Supporting information).

XPS was employed to characterize both the chemical state and atomic concentration of each element. In the wide-scan XPS spectra after normalization, shown in Figure 1e, the intensity of peaks related to C and O at 285 and 530eV, respectively, is almost the same for both GO and GO-i. When the spectrum of RGO aerogel is compared to the spectra of GO and GO-i, the peak of O decreases sharply while the peak of N intensifies slightly. More precisely, as shown in Table S1, the molar ratio of C and O (C/O ratio) increased from 2.12 for GO to 8.90 for RGO aerogel, indicating effective reduction of GO. Both FT-IR and XPS analysis revealed that GO could not be reduced by γ-ray irradiation in the absence of EDA. To further understand the change in functional groups from GO to RGO aerogel, it was necessary to analyze the narrow-scan C1s spectrum. In Figure 1f, the four peaks of GO centered at 284.8, 287.0, 288.6, and 290.1 eV are assigned to C–C/C=C in aromatic rings, C–O in epoxyl and alkoxy, C=O in carbonyl, 26 and O–C=O in carboxyl groups, respectively. For RGO aerogel in Figure 1g, the XPS peaks related to oxygen decrease dramatically and the peak intensity of C–C/C=C in aromatic rings rises relatively. Meanwhile, two new peaks belonging to C–NH2 and C–NH(R) appear at 285.7 and 287.4 eV, suggesting successful EDA-mediated and γ-ray irradiation-assisted 27 reduction of GO. The narrow-scan N1s spectrum of RGO aerogel in Figure S3 also presents two peaks at 399.6 and 401.7 eV, which correspond to the amide groups and the amine groups, respectively.

To investigate the evolution in structure and morphology as GO was transformed into RGO aerogel, atomic force mi-

The X-ray diffraction (XRD) patterns of GO and RGO aerogel are shown in Figure 1h. In addition to a weak broad

3. RESULTS AND DISCUSSION 3.1 Synthesis and Characterizations of RGO aerogel

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peak close to the typical diffraction peak of graphite at 26.5° in the pattern of GO, a peak at 10.7°corresponds to the typi28 cal diffraction peak of GO with d-spacing of about 0.83 nm. The d-spacing of RGO aerogel was calculated to be 0.37 nm, which is much lower than that of GO, but still slightly higher than the d-spacing of pristine graphite owing to the incomplete reduction and the introduction of new functional 29 groups. Moreover, the broadened peak of RGO aerogel can be attributed to the relatively short domain order or turbostratic arrangement of the stacked sheets that consisted of 30 a few layers. Raman spectra was employed to further confirm the results of XRD. As shown in Fig. S4, the full-width at half maximum of G band becomes narrower, indicating a higher graphitization degree. And the intensity ratio ID/IG of RGO aerogel is about 1.29, slightly higher than that of GO 2 (about 1.12), suggesting the decrease in the average sp domain size and the formation of large amounts of new smaller 31, 32 graphitic domains with higher density of defects. Thermogravimetric analysis (TGA) was employed to evaluate the thermal stability in Figure 1i. Most of the weight loss of GO occurred at temperatures between 150 and 250 °C as the labile functional groups were eliminated, and the total weight loss was up to 60% when the temperature reached 700 °C. But for RGO aerogel, the total weight loss was only about 25% at 700 °C, demonstrating its good thermostability 33 even though it was still inferior to that of natural graphite. The electrical conductivity of RGO aerogel at original state -1 34 is about 1 S m measured by four-probe method. When RGO aerogel was compressed under pressure about 10 Mpa, -1 its electrical conductivity increased to about 6 S m . As shown in Figure S5, the Young’s Modulus of RGO aerogel is calculated to be about 31 kPa. This value is quite high and RGO aerogel does not show good compressibility. 3.2 Reaction Mechanism According to the radiation chemistry of water, γ-ray irradiation can decompose water molecules into both oxidative −

+

(·OH, H2O2, H3O ) and reductive ( eaq , H·, H2) species, as shown in Equation (1), among which ·OH is a dominating −

and strong oxidative radical, while eaq and H· are active and powerful reducing agents:

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H2O → eaq , H·, ·OH, H3O+, H2, H2O2 −

(1)

Based on the results of FT-IR, XPS, and XRD analyses of GO, GO-i, and RGO aerogel, we speculate that EDA played an important role as a radical scavenger like alcohols in the reduction and self-assembly of GO. The reactions between ·OH radicals and EDA that took place mainly at the

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α-carbon position and the amine group are shown as follows: 36

•

NH2(CH2)2NH2 + ·OH →NH2CH2CHNH2 + H2O (2) •

NH2(CH2)2NH2 + ·OH →NH2(CH2)2NH + H2O (3) Therefore, the strong oxidative radical ·OH turned into reductive species during these reactions. All reductive species reacting with functional groups on the surface of GO resulted in the elimination of oxygen-containing groups and the selfassembly of reduced graphene oxide (RGO) with the introduction of nitrogen-related groups during the γ-ray irradiation process. 3.3 Microstructure, property and sorption of organic solvents The microstructure is essential to the characterization of a material. Figure 2a–b show the SEM images of the surface and cross section of RGO aerogel. Obviously, the surface was quite smooth except for some wrinkles. The interior structure is consisted of micropores, which imparts RGO aerogel with strong hydrophobicity; the contact angle of a water droplet on the cross section of RGO aerogel was about 135°, which is higher than the water contact angle of 122° on the surface. The removal of hydrophilic groups and the roughness of the structure contributed to this high hydrophobicity. In particular, the cross section of RGO aerogel showed strong adhesion to the water droplet even when the glass slide was turned upside down, as demonstrated in Figure 2c. This “sticky” character is similar to the “petal effect” of rose pet37 als, namely the high contact angle and high contact-angle hysteresis, which can be treated as a transitional state be38 tween the Wenzel and Cassie states. As mentioned above, the RGO aerogel produced from γ-ray irradiation of GO had a hierarchical structure with microscale pores and nanoscale wrinkles or folds of graphene sheets. In general, the micro roughness features that permit water penetration tend to result in high adhesion, whereas the nanoroughness features that cannot be impregnated by water lead to high contact 39 angles. With its porous structure and high hydrophobicity, RGO aerogel shows great potential for oil sorption. As shown in Figure 2d, 2 mL of n-dodecane dye with Oil Red floating on the water could be quickly and completely absorbed by an intact RGO aerogel that weighed about 16 mg, with density ρ –3 ≈ 3.5 mg cm , which is consistent with the activity seen in Movie S1. RGO aerogel could also quickly absorb organic liquids with high density that sank below the water surface without any adsorption of water, as shown in Figure 2e and Movie S2.

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Figure 2. SEM images of the (a) surface and (b) cross section of RGO aerogel. (c) Snapshots of a water droplet with volume of about 5 μL on a cross section of RGO aerogel. (d) Snapshots of the removal process of n-dodecane (dyed with Oil Red) floating on water. (e) Snapshots of the removal process of tetrachloromethane (dyed with Oil Red) sinking below the water surface. (f) The relationship between Qwt and the density of different organic liquids (1: n-hexane, 2: n-dodecane, 3: ethanol, 4: turpentine oil, 5: N-methyl pyrrolidone, 6: ethylene glycol, 7: dichloromethane, 8: tetrachloromethane). (g) Adsorption-distillation cycles of RGO aerogel using n-hexane. (h) Adsorption-combustion cycles of RGO aerogel using n-dodecane.

As shown in Figure 2f, the absorption capacity (Qwt, defined as the ratio of the weight after full absorption to the weight –3 of the aerogel) of RGO aerogel (ρ≈4.0 mg cm ) varied from 100 to 280 g/g showing strong linear dependence on the density of the target organic liquid. The adsorption capacity is about 214 g/g assuming that the density of organic liquid is 1 -3 g cm . These values are higher than polymeric foams and 40-43 some graphene-based aerogels (< 100 g/g). The adsorption-distillation performance tended to be stable except for the first cycle whose Qwt was slightly higher, as shown in Figure 2g. The absorbed organic liquids could be distilled

and collected for cyclic utilization; the most important advantage of distillation is that this method will not cause any damage to the chemical and physical structure of RGO aerogel when recycling the valuable organic liquids, but it is time-consuming. Meanwhile, adsorption–combustion cycles can also be employed as a simple and fast approach to dispose the leaked oil in water, although the technique is not very economical. Therefore, the absorption–combustion cycles were performed repeatedly, as shown in Figure 2h, with a small loss in Qwt after 5 cycles.

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Figure 3. (a)Combustion process of n-dodecane (dyed with Oil Red) floating on water, with RGO aerogel serving as a wick. (b) Wide-scan XPS spectra of RGO aerogel and RGO-burned. (c) XRD patterns of RGO aerogel, RGO-burned, and graphite. (d) TGA curves of RGO aerogel, RGO-burned, and graphite. Considering the high porosity of the aerogel, which allowed for quick removal of heat and high adsorption of organic solvents, combined with its low density and high hydrophobicity, we removed organic solvents floating on water by simple combustion in our study (Figure 3a).The aerogel floating on the interface of water and the solvent could continuously absorb the organic solvent and stably maintain the combustion, which is consistent with the behavior shown in Movie S3. In comparison, the thin organic solvent layer could not be directly ignited, highlighting the importance of RGO aerogel as a collector and supporter of continuous combustion. At the same time, we also investigated the changes in RGO aerogel before and after combustion. In the wide-scan XPS spectra in Figure 3b, the peak of N element disappeared after combustion as expected, signifying the decomposition of organic groups associated with N. The XRD patterns in Figure 3c show that the diffraction peak of RGO-burned (namely RGO aerogel treated with combustion) is much closer to graphite’s diffraction peak than that of RGO aerogel because the functional groups were removed during the combustion. The TGA curves in Figure 3d demonstrate that

the thermal stability of RGO-burned became much better than that of RGO aerogel even though it was still inferior to that of graphite. Continuous combustion is a simple way to treat leaked oil; however, the collecting and recycling of this oil is more beneficial, not only economically, but also by being more environmentally friendly. Therefore, we developed a simple equipment using RGO aerogel, shown in Figure 4a and b, to continuously collect n-dodecane (dyed with Oil Red) floating on water without any water uptake. The entire collecting process did not stop until all the floating oil was cleared away, which is consistent with the result shown in Movie S4. RGO aerogel served as a smart filter screen that only selectively absorbed organic liquids without absorbing any water, as shown in Movie S4 and in Figure 4c and d. The excellent hydrophobicity or oleophilicity and the porous structure are two main reasons for RGO aerogel’s suitability for this kind of application. Moreover, this treatment will not cause any destruction to the macrostructure of RGO aerogel during the continuous adsorption of organic liquids, which is superior to conventional treatments like distillation, combustion, and squeezing, thus adding to its potential for actual application.

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Figure 4. (a) Diagram illustrating the continuous collection process via RGO aerogel and a peristaltic pump. (b) From left to right: original state before oil collection; effective process of continuous oil collection; final state when the floating oil was completely eliminated. Snapshots of the final state of (c) RGO aerogel and (d) the container.

4. CONCLUSION In summary,we successfully developed a novel and simple method of reduction and self-assembly induced by γ-ray irradiation to prepare graphene aerogel. This environmentally friendly and convenient method is well suited for volume production. The synthesized porous RGO aerogel showed high hydrophobility, which is favorable for the adsorption of organic solvents, with Qwt of up to 280 depending on the density of the organic solvents. Meanwhile, owing to the hydrophobicity and good thermostability of the aerogel, it could be used to treat organic liquids or oil through absorption–distillation cycles, adsorption–combustion cycles, or continual and recurrent combustion as a porous carbon wick floating on water. Furthermore, a set of simple equipment was designed for the continuous collection of floating oil, which is very convenient and simple for practical application. Therefore，the successful synthesis of RGO aerogel indicates that γ-ray irradiation is an easy and efficient technique to prepare 3Dgraphene aerogels, which will provide insight for further development of graphene-based nanocomposites aerogels.

Four videos of the absorption of the oils floating on water(Movie 1)The adsorption of oil sinking in the water with the aerogel(Movie 2), burning off the oil slick using aerogel as a wick(Movie 3) and the collection of oil slick with aerogel as a smart filter screen(Movie 4).(AVI)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (J.Y.Li); * E-mail: [email protected](J.B.Chen)

Author Contributions §Miss.Y.L. He and Dr. J.H. Li contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors acknowledge the ﬁnancial support from National Natural Science Foundation of China (No.11505270), and Shanghai Municipal Commission for Science and Technology (No.15ZR1448300).

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Engineering Reduced Graphene Oxide Aerogel Produced by Effective

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