Phys. Status Solidi A 208, No. 10, 2325–2327 (2011) / DOI 10.1002/pssa.201084166 p s sa

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applications and materials science

A green and fast way forreduction of graphene oxide in

acidic aqueous solution via microwave assistance

Ting Lu1, Likun Pan*,1, Chunyang Nie1, Zhiwei Zhao2, and Zhuo Sun1

1Engineering Research Center for Nanophotonics and Advanced Instrument, Ministry of Education, Department of Physics,

East China Normal University, Shanghai 200062, P.R. China2School of Electronic Science and Engineering, Southeast University, Nanjing 210096, P.R. China

Received 15 November 2010, revised 6 March 2011, accepted 12 March 2011

Published online 12 August 2011

Keywords exfoliated GO, inorganic acid, microwave

*Corresponding author: e-mail lkpan@phy.ecnu.edu.cn, Phone: þ86-21-62234132, Fax: þ86-21-62234321

A facile and pollution-free method was applied to prepare

graphene nanosheets by reducing graphene oxide in acidic

aqueous solution without any toxic chemicals via microwave

heating. The surface morphology, structure, and composition

characterizationswere carriedout using atomic forcemicroscopy,

field-emission scanning electron microscopy, transmission elec-

tron microscopy, energy-dispersive X-ray spectroscopy, Fourier

transform infrared spectroscopy, and Raman spectroscopy,

respectively. The results showed that high-quality and large-area

reduced graphene nanosheets were obtained.

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1 Introduction Graphene, a two-dimensional honey-comb nanocarbon sheet, has attracted much attention inrecent years owing to its excellent electronic, capacitive andmechanical properties, superior chemical stability, and highspecific surface area [1–9], which promises applications invarious fields, such as transparent conductive films, lithium-ion batteries, supercapacitors, photovoltaic cells, and fieldeffect transistors. Graphene can be prepared via reduction ofgraphene oxide (GO) such as chemical reduction usinghydrazine or NaBH4 [10, 11], UV-assisted photocatalyticreduction, [12] and high-temperature annealing reduction[13–15]. However, in most of the reported methods,hazardous reducing agents, high temperatures, or longprocessing times are required. Therefore, producing gra-phene on a large scale using facile and practical reducingmethods is still a challenge.

As an inexpensive, quick, versatile technique, micro-wave can heat the reactant to a high temperature in a shorttime by transferring energy selectively to microwave-absorbing polar solvents with a simultaneous increase inself-generated pressure inside the sealed reaction vessel [16–18], and it has been successfully applied in the reduction ofGO. Li et al. [19] reported an ultrafast dry microwavesynthesis of graphene in very short duration (<5 s) withoutusing reducing agents. Chen et al. [20] presented a quick and

mild thermal reduction ofGO to graphenewith the assistanceofmicrowave in amixed solution ofN,N-dimethylacetamideand water. Murugan et al. [21] reported a facile microwave-assisted solvothermal reduction of exfoliated GO withnontoxic solvents to obtain graphene and graphene-polyani-line nanocomposite within a short reaction time (severalminutes) and at relatively low temperature (180–300 8C).However, most of these works reduced the GO using organicagents or in alkaline solutions. The microwave-assistedsynthesis of graphene in acidic solution has seldom beenreported so far. In this work, we report a green and fastmethod to synthesize reduced graphene nanosheet (RGS) bymicrowave-assisted reduction of GO dispersion in acidicaqueous solution without any toxic chemicals using amicrowave synthesis system.

2 Experimental GO was prepared and purified bymodified Hummers method [22]. Approximately 1.5 g ofgraphite powder was put into an 80 8C solution consisting of24ml concentrated nitric acid and 12ml sulfuric acid andkept for 4 h. The mixture was cooled to room temperatureand diluted with deionized (DI) water and left overnight.Then, the suspensionwas filtrated andwashedwithDIwater.After that, the reaction vessel was immersed in an ice bath,and potassium permanganate and concentrated sulfuric acid

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2326 T. Lu et al.: Green and fast way for reduction of graphene oxide in acidic aqueous solutionp

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Figure 1 (online color at: www.pss-a.com) GO and RGS suspen-sions prepared at different pH values.

Figure 3 FESEM images of RGS at (a) low magnification and (b)high magnification; (c) TEM image of RGS, inset is high magnifi-cation TEM image; (d) EDS spectrum of RGS.

Figure 2 (online color at: www.pss-a.com)AFM image of RGS intapping mode: topography image and height profiles obtained frompositions indicated by different lines.

were added slowly. Successively, themixturewas stirred andleft for 2 h. And then, after the dilution with DI water, 30%H2O2 was added to the mixture, and the color of the mixturechanged into brilliant yellow along with bubbling. Finally,the mixture was filtered and washed with HCl aqueoussolution (1:10 in volume) to remove residualmetal ions, thenwashed with DI water.

NaOH solutions were dropped into the highly acidic GOsuspension solutions to adjust the pH value to be 1, 3, or 5.Subsequently, the solution was heated at 150 8C with amicrowave irradiation power of 80W for 10min using amicrowave system (Explorer48, CEM Co.). As shown inFig. 1, the suspension evolved from yellow brown or yellowblack into a black solution, indicating that GO has beentransformed to RGS.

The surface morphology, structure, and composition ofRGS were characterized by transmission electron micro-scopy (TEM, JEM-2010F), field-emission scanning electronmicroscopy (FESEM, Hitachi S-4700), atomic force micro-scopy (AFM, SPI 3800N), energy-dispersive X-ray spec-troscopy (EDS, JEM-2010F), Raman spectroscopy(LABRAM HRUV-VISIBLE, resolution: 1 cm�1), respect-ively. Fourier transform infrared (FTIR) spectra of RGS andGO were obtained using a FTIR spectrometer (Nexus 670).

3 Results and discussion AFM was used to charac-terize the as-prepared RGS deposited on a hydrophilic-treated silicon substrate by simply drop casting of dilute RGSethanol dispersion. From Fig. 2, it can be observed that RGSwith large area was formed. Height profiles show thethickness of RGSwas 2.7–3.7 nm on average, correspondingto three or five layers based on a theoretical value of 0.78 nmfor single-layer graphene and the thickness contribution fromresidual oxygen-containing groups on the faces [22, 23].

Figure 3a and b shows themorphology ofRGS at low andhigh magnification by FESEM. The corrugated and scrolledsheets resemble crumpled silk veil waves in the low-magnification image. RGS layers interact with each otherto form an open pore system to enhance the specific surfaceratio. And the sheets present transparent and velvet surfacesin the high-magnification image. Figure 3c illustrates theTEM image of RGS. It is clearly observed that transparentultrathin RGSs with folds lay on copper grid. The EDSmeasurement in Fig. 3d was used to analyze the C/O ratio ofthe as-prepared RGS. It is found that the ratio of C/O is

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�9.12, which is close to the value for RGS obtained byconventional hydrazine reduction (�10) [24]. This indicatesthat the exfoliation and deoxygenation of GO can be realizedduring this facile and pollution-free process. The existence ofCu peaks is ascribed to the use of copper grid for dispersionof the sample during the EDS measurement.

Figure 4 presents the FTIR spectra of RGS and GO.There is a great decrease in the intensities of C––O(1631 cm�1) and C–O (1078 cm�1) stretching vibrationpeaks in RGS as compared to those in GO. O–H (3415 cm�1)vibration is largely contributed from molecular H2O. Theintensity of the C––C (1401 cm�1) stretching peak is almostunchanged in both samples, implying that the structure ofgraphite was little destroyed after microwave treatment. Theresult suggests that microwave-assisted reduction in acidicsolution is an effectivemethod to remove oxygen-containinggroups of GO.

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Phys. Status Solidi A 208, No. 10 (2011) 2327

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Figure 4 FTIR spectra of GO and RGS.

Figure 5 Raman spectrum of RGS.

Figure 5 exhibits a typical Raman spectrum of RGS. Thespectrum is dominated by two intensity peaks at 1350 and1593 cm�1, which are referred to as D line and G line,respectively. The D line is attributed to the presence ofamorphous carbons or defects in curved graphite sheets,while the G line shows the presence of tubular structure. A2D band with frequency (2693 cm�1) close to twice that ofthe D band is also observed, which is an intrinsic property ofthe graphene. The ratio of the area intensity of theD band andG band (ID/IG) is related to the amount of disorder in thecarbon products [25]. The value of ID/IG is 0.98< 1,indicating a high quality of as-synthesized RGS.

4 Conclusions In summary, we have demonstrated asimple method to prepare RGS by microwave-assistedreduction of GO in acidic aqueous solution. This methodallows a facile and rapid reduction in aqueous media withoutthe need for highly toxic chemicals with strong reducingproperties or reducing-gas atmospheres at high tempera-tures. Themorphology and structural characterizations showthat high-quality RGSs with three or four carbon layers areobtained.

Acknowledgements Thisworkwas supportedby theSpecialProject for Nanotechnology of Shanghai (No. 0952nm02200).

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