Simple Synthesis of Graphene Oxide Using Ultrasonic Cleaner from ...

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Oct 26, 2014 - ABSTRACT: A simple method for the synthesis of graphene oxide (GO) from expanded graphite using an ultrasonic cleaner is described.
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Simple Synthesis of Graphene Oxide Using Ultrasonic Cleaner from Expanded Graphite Hongjun Yang,†,‡,§ Haiyan Li,† Jiali Zhai,† Lei Sun,† and Hongwen Yu*,† †

Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, 4888 Shengbei Road, Changchun 130102, China Shandong Key Laboratory of Eco-Environmental Science for the Yellow River Delta, Binzhou University, No. 391, 5th Yellow River Road, Binzhou City 256603, Shandong Province, China § University of Chinese Academy of Sciences, Beijing 100049, China ‡

ABSTRACT: A simple method for the synthesis of graphene oxide (GO) from expanded graphite using an ultrasonic cleaner is described. Due to the synergistic effect between intercalation (oxidation) and ultrasonication, this method showed strongly decreased demand for acid, time saving/less energy consuming, high productivity and without the release of toxic gas(es) than Hummers’ method. Single sheets of GO with layer spacing of ∼1 nm were obtained in less than 1 h. This procedure was safe and expected to be applicable for the scale production of GO. Both graphene oxide film and reduced graphene oxide (rGO) film showed good bending flexibility.

1. INTRODUCTION The graphene sheet, one-atom-thick two-dimensional layers of sp2-hybridized carbon, is a material of great interest. It has a large theoretical specific surface area (2630 m2 g−1), high intrinsic mobility (200 000 cm2 v−1 s−1), high Young’s modulus (∼1.0 TPa), thermal conductivity (∼5000 Wm−1 K−1), and its optical transmittance (∼97.7%). These properties make graphene a promising material for multifunctional composites, high performance electrodes for energy storage, catalyst, and many other potential applications. Although graphene sheets have been produced in small quantities through chemical vapor deposition,1−3 epitaxial growth,4 and mechanical peeling with Scotch tape,5 industrial scale manufacturing has only been possible with the use of graphite oxide, which is obtained through oxidation of graphite.6,7 Graphite oxide is then split apart into individual sheets either through rapid heating8−11 or microwave-assisted12 to yield functionalized graphene sheets (FGSs) with varying carbon-to-oxygen atomic ratios, C/O. Scalable preparation is desirable for future applications of graphene, which necessities the need for realizing such a process. As of today, the way we prepare graphene oxide (GO), as one effective precursor of graphene, mainly follows Hummers’ method or typically modified Hummers−Offeman method,6,11,13−15 which is time/oxidants/acid-consuming, low productivity, and involves the generation of the toxic gas(es) NO2, N2O4. The facile approach for large scale preparation of GO is expected to significantly promote the application of graphene in various fields. The sonication of a liquid results in sonic cavitation that creates localized “hotspots” with effective temperatures of 5000 K and lifetimes on the order of a few nanoseconds or less. As such, the chemical sonication has been used to promote or accelerate some chemical reaction. Expanded graphite is commercially available and its sizes can be optionally chosen. Herein, we report a fast and facile method for the preparation of GO following a modified Hummers−Offeman method using © 2014 American Chemical Society

ultrasonic cleaner from expanded graphite. It resulted in a preparation that strongly contributes to decreased demand for acid, time saving/less energy consuming, high productivity and without the release of toxic gas(es). Comparisons were also developed between both raw materials with different sizes. It is a promising method for large scale industrialization of graphene.

2. EXPERIMENTAL SECTION Materials and Reagents. Expanded graphite (EC1000 with an average particle size 10 μm and EC300 with an average particle size 30 μm) wwere purchased from Ito Kokuen Co., Ltd., Mieken, Japan. Other chemicals unless specifically noted were of analytical reagent grade (AR Grade) from Beijing Chemical Works. Preparation of Graphene Oxide and Reduced Graphene Oxide (rGO). We took some modifications into the Hummers−Offeman method6 and applied it for the preparation of graphene oxide from industrially expanded graphite. In a typical reaction, potassium permanganate (10 g) and expanded graphite (5 g) were stirred together to be homogeneous mixture. Then it was put into a bottom-round flask (500 mL) in ice−water bath. Concentric sulfuric acid (98%, 0 °C, 60 mL) was added with continuously stirring until the mixture became a uniform liquid paste. The mixture was still kept stirring until it became a foam-like paste (after around 20 min) at room temperature (25 °C), while it expanded several times in volume. Then deionized water (400 mL) was added, and the mixture was rapidly stirred to prevent effervescing. Then, the flask was heated to 90 °C using an ultrasonic cleaner and kept for 10 min to get a homogeneous suspension that was dark yellow in color. All the processes were treated using a table-top Received: Revised: Accepted: Published: 17878

September 10, 2014 October 23, 2014 October 26, 2014 October 26, 2014 dx.doi.org/10.1021/ie503586v | Ind. Eng. Chem. Res. 2014, 53, 17878−17883

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Figure 1. Images of GO cakes: (a) cake made from commercial EG (EC300); (b) cake made from commercial EG (EC1000).

Figure 2. (a) Typical AFM images of fully exfoliated GO sheets on freshly mica; (b) topographic image of GO sheets.

film-coated copper grid. Atomic force microscopy (AFM, Agilent series 5500 AFM instrument using the tapping mode at a scanning rate of 0.5 Hz), Raman spectroscopy (Raman, an in Via Raman microscope, Renishaw, with excitation wavelength at 532 nm), and infrared spectra (VERTEX 70 Fourier transform infrared (FTIR) spectrometer (Bruker)) were collected. Samples were degassed under vacuum at 60 °C overnight prior to analysis. X-ray diffraction (XRD) patterns of the samples were collected on a Bruker D8ADVANCE diffractometer (Germany) using Cu Kα (1.5406 Å) radiation. X-ray photoelectron spectrum (XPS) measurements were performed on an ESCALAB-MKII spectrometer (VG Co., United Kingdom) with Al Kα X-ray radiation as the X-ray source for excitation. Thermogravimetric analysis (TGA, TG/ DTA 6200, SII Exstar6000, with the heating rate of 5 °C per minute under N2 atmosphere) also was performed. Electrochemical Test. Electrochemical measurements were performed using an Autolab PDSTA302N electrochemical station with a typical three-electrode cell. To prepare the working electrodes, 2 mg of the GO/rGO samples was

ultrasonication cleaner (360 W). After that, the suspensions were filtered and GO cakes were obtained (Figure 1). Then, the GO was washed with deionized water and centrifuged (10000 rpm, 1 h) for several times to remove the remaining ions of oxidants and other inorganic impurities. For GO film, the as-obtained graphene oxide solution of certain volume was filtered through the polycarbonate membrane filter (pore size of 0.4 μm, diameter of 47 mm, Toyo Roshi Kaisha, Ltd.). Then the film was dried in a drying oven (80 °C). After that, the film was immersed in 70 °C Na2S2O4 solution (100 mg mL−1) as a reducing agent for 10 min. The GO film was changed from orange-red to deep gray. The rGO film was washed with deionized water several times to remove the reaction impurities and then dried in an oven (80 °C). Characterization. Transmission electron microscopy (TEM) images were taken by using a TECNAI G2 highresolution transmission electron microscope with an accelerating voltage of 200 kV. All TEM samples were created by depositing a drop of diluted suspensions in water on a carbon17879

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dispersed in a solution containing 1.8 mL of deionized water (18.2 MΩ) and 0.2 mg of 5 wt % Nafion aqueous solution. The mixture was ultrasonicated for 15 min to obtain a homogeneous catalyst ink. 15.00 μL of 1 mg· mL−1 GO/rGO dispersions was dropped onto a mirror polished glassy carbon electrode and dried in air at 60 °C. Pt/C (20 wt %) was deposited on the electrode under the same procedure. Normal cyclic voltammograms were performed from −0.1 v to +0.1 v with a sweep rate of 50 mv·s−1.

3. RESULTS AND DISCUSSION Intercalation and exfoliation are important processes for the synthesis of GO that determine the time-cost, yield, and the layers of GO. Because the expanded graphite has larger interlayer space than graphite, it is easy to overcome the van der Waals attractions between the adjacent layers and the intercalation will complete sooner. Moreover, it is cheap and readily available. In consideration of these factors, we choose expanded graphite as the starting material. Meanwhile, the ultrasonic vibration was applied in the preparation of GO. During ultrasonication, shear forces and cavitation act on the bulk material and induce exfoliation. What’s more, the synergistic effect between intercalation (oxidation) and ultrasonication made many advantages of this method. First, the intercalation efficiency and oxidation degree of graphite were improved, although without sodium nitrate, the single layer GO was obtained with high productivity (GO with layer spacing of ∼1 nm). Second, decreased demand for sulfuric acid and potassium permanganate. The ratio of sulfuric acid to graphite is reduced from 23:1 (v/w) (reported in literature, see refs 6, 11, and 13) to 12:1 (v/w). The ratio of potassium permanganate to graphite is reduced from 4:1 (w/w) (reported in literature, see refs 10−13) to 2:1 (w/w). Third, time saving/ less energy consuming. The whole process for synthesis of GO aqueous solution using this method takes less than 1 h, which is much less than the conventional (modified) Hummers method (4 h) and more than 24 h in literature (see ref 10). For the promoting mechanism of ultrasonication to oxidation-intercalation and exfoliating, the cavitation of bubbles generated by ultrasonication can accelerate the oxygen groups introduce into expanded graphite. It can furtherly reduce the van der Waals forces and expand the space between neighboring carbon sheets, which make the water molecules to penetrate into the interlayer space of graphite oxide easily. Then, the shear forces supplied by sonication can destroy the reduced van der Waals interaction and break apart the graphite oxide flakes, which are subsequently stabilized by the electrostatic repulsion and steric hindrance. To indicate the effect of material size, two types of expanded graphite, EC1000 and EC300, were used to prepare graphite oxide. Compared with EC1000, the GO aqueous solution with EC300 as the precursors contained some coarse particles, indicative of an insufficient oxidation/exfoliation of graphite. It should attribute to the smaller size of EC1000, which results in more reactive sites for the oxidant to attack. Following purification, the hydrophilic materials obtained were characterized. Atomic force microscopy (AFM) images (Figure 2) indicated that both materials were grossly similar, as the thicknesses of the GO layers were about 1.1 nm. The morphology of GO was also investigated by TEM. As seen from Figure 3, the copper grid substrates are covered with a number of thin films with many wrinkles, indicating that the GO sheet had been exfoliated.

Figure 3. TEM images of fully exfoliated GO sheets.

XRD patterns of expanded graphite (EG), GO, and rGO nanosheets are presented in Figure 4. The characteristic peak

Figure 4. XRD patterns of EG (EC1000), GO, and rGO.

(002) of EG at 2θ = 26.44° disappeared after oxidation, while an additional peak at 2θ = 11.58° was observed, which was corresponding to the (001) diffraction peak of GO. The interlayer distances changed from 3.4 to7.6 Å. The rGO shows a broad peak that can be fitted by using a Lorentzian function into three peaks centered at 2θ = 10.54°, 19.74°, and 22.18°, corresponding to interlayer distances of 8.3, 4.9, and 4.0 Å, respectively. For EG, the peak appears at 2θ = 26.44°, which corresponds to an interlayer distance of 3.4 Å. These XRD results are related to the exfoliation and reduction processes of GO and the processes of removing intercalated water molecules and the oxide groups. An X-ray photoelectron spectroscopy (XPS) analysis provided detailed information on the verification of EG, GO, and rGO (Figure 5a,b,c). For EC1000, the peak at 291.5 eV was assignable to the π→π* transition of the aromatic CC bonds. C 1s deconvolution spectrum of GO showed that four different peaks centered at 284.5, 286.6, 287.6, and 289.1 eV appeared, corresponding to CC/CC/CH in aromatic rings, CO, CO, and OCO groups, respectively. From EG to GO, the peaks for CO and CO were obviously increased. An additional peak for O−CO appears after oxidation. Compared rGO with GO, we can find that the peaks for oxygen functional groups were significantly reduced (blue 17880

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Figure 5. XPS patterns of expanded graphite EG (EC1000), GO, and rGO: (a) C 1s spectra of EG; (b) C 1s spectra of GO; (c) C 1s spectra of rGO.

and green curves) after reduction. These observations are strong indication of effective synthesis of GO and rGO. Raman spectra (Figure 6) show D peaks ∼1350 cm−1 and G peaks ∼1590 cm−1, confirming the lattice distortions. The D/G

Figure 7. FTIR images of EG (EC1000), GO, and rGO.

the same time, the characteristic peaks of OH becomes broadening and the bands shifted to the higher wavenumber. They are caused by the decrease of hydrogen bonds in rGO sample. The IR spectra of EC1000 and the rGO were of a similar shape except that the peak intensity of epoxy for EC1000 is higher. Further evidence for the formation of GO and rGO was provided by thermogravimetric analysis (TGA) in a nitrogen atmosphere, as shown in Figure 8. Weight loss (∼32 wt %) of the GO up to 230 °C could be primarily due to evaporation of water molecules and removal of labile oxygen functional groups. A comparatively small amount (10 wt %) of weight loss by rGO in this temperature region indicates that rGO contain less labile oxygen functional groups. The rGO (950 °C, ∼19% loss) showed less weight loss than GO (950 °C, ∼51% loss), whereas the EC1000 remained unchanged.

Figure 6. Raman spectra of EG (EC1000), GO, and rGO.

intensity ratio for rGO is larger than that for GO (1.40 for rGO and 0.88 for GO). This suggest that new (or more) graphitic domains are formed and the sp2 cluster number is increased.16 Fourier transform infrared spectroscopy (FTIR) (Figure 7) analysis showed the characteristic peaks of OH (νO−H at 3200−3400 cm−1), CO (νCO at 1733 cm−1), CC (νCC at 1615 cm−1), and CO (νC−O at 1274 and 1052 cm−1) for GO.17 In contrast, for rGO, the intensities of absorption bands of the oxygen functionalities (νO−H, νCO, and νC−O) decreased remarkably and a relatively sharp peak (νC−OH at 2930 cm−1) appeared. These all showed the success of reduction for GO. At 17881

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too large to conduct electricity. The rGO shows improved electrical properties with resistance around 4.1 × 102 Ω, which is reasonable due to its recovered graphitic carbon structure due to the reduction.

4. CONCLUSIONS GO with layer spacing of ∼1 nm was prepared from expanded graphite by ultrasound-assisted modified Hummers method. It was a fast and facile method, which strongly contributed to decreased demand for acid, time saving/less energy consuming, high productivity and without the release of toxic gas(es). It is expected that the title method could be applicable for large scale production of GO.



Figure 8. TGA curves of EG (EC1000), GO, and rGO.

AUTHOR INFORMATION

Corresponding Author

Bending testing (Figure 9) for graphene oxide film and reduced graphene oxide film showed that both of them had good bending flexibility.

*H. Yu. E-mail: [email protected]. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (No. 2177142), the Important Deployment Project of Chinese Academy of Sciences (KZZD-EW-TZ16) and Binzhou University research project (BZXYG18).



REFERENCES

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Figure 9. Images of bending testing for (a) graphene oxide film and (b) reduced graphene oxide film.

Graphene is known as an excellent conducting medium. However, for GO, the addition of O can destroy its carbon structure and reduce its conductivity. The conductivity of GO samples is tested using Current−voltage (I−V, Figure 10) plots. The measured resistance of GO is about 1.6 × 108 Ω, which is

Figure 10. Current−voltage (I−V) plots taken on the graphene oxide (GO) film and chemically reduced graphene oxide (rGO) paper. 17882

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