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Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (2011) 543– 548

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

jo ur nal homep a ge: www.elsev ier .com/ locate /co lsur fa

ne-step reduction of graphene oxide with l-glutathione

uan Anh Phama, Jeong Sik Kimb, Jeong Su Kima, Yeon Tae Jeonga,∗

Department of Image Science and Engineering, Pukyong National University, Busan, 608-739, Republic of KoreaDepartment of Chemistry, Dong-A University, Busan, 604-714, Republic of Korea

r t i c l e i n f o

rticle history:eceived 30 January 2011eceived in revised form 6 April 2011ccepted 8 May 2011vailable online 13 May 2011

eywords:

a b s t r a c t

The preparation of graphene nanosheets from graphene oxide by chemical reduction is one of theimportant topics in areas of nanotechnology because graphene-based nanomaterials have potentialapplications. Herein, we developed a green and facile approach to produce graphene by using an environ-mentally friendly reagent, namely, l-glutathione as a reducing agent. Graphene was prepared via one-stepreduction from graphene oxide under mild condition in aqueous solution. The resulting graphene wascharacterized using a range of analytical techniques. Fourier transform infrared spectroscopy and X-ray

raphene nanosheets-Glutathionehemical reductionnvironmentally friendly

photoelectron spectroscopy were used to study the changes in surface functionalities. X-ray diffractionwas used to investigate the crystallinity of graphene nanosheets whereas high resolution transmissionelectron microscopy and atomic force microscopy were employed to investigate the morphologies ofprepared graphene. Thermogravimetric analysis was used to characterize the thermal stability of thesamples on heating. The digital images provide a vivid observation on stable dispersions of graphene in

otic s

both water and polar apr

. Introduction

The recent increase of interest in carbon-based nanomaterialsas opened new pathways for the development of novel func-ional materials. Among them, the parent of all graphitic forms,raphene, has been emerging as a material of great interest dueo their remarkable physical, chemical, electrical properties [1–4].ver since their discovery in 2004 by Novoselov and Geim [5],esearchers have been exploring their potential in wide variety ofevice applications, including chemical sensors [6,7], catalyst sup-ort [8,9] and Li ion batteries [10,11]. While these protocols haveriggered burgeoning interest, the realization of these potentialpplications is limited due to difficulties in production individualraphene nanosheets. Hence, the development effective and scal-ble ways to prepare graphene is a great challenge for chemists.p till now, various methods to prepare graphene nanosheetsave been developed, including chemical vapor deposition [12,13],hemical reduction [14–17], ultrasonic exfoliation [18], epitaxialrowth [19]. Among these methods, the chemical reduction ofraphene oxide (GO) using reducing agents is the most versatilend easily scalable method to produce graphene in a bulk quan-ity because it has advantage of an inexpensive way. However,

he strong tendency of the monolayeric graphene nanosheets toorm irreversible agglomerates into multilayeric graphite throughtrong �–� stacking and van der Waals interaction presents a major

∗ Corresponding author. Tel.: +82 51 629 6411; fax: +82 51 629 6408.E-mail address: ytjeong@pknu.ac.kr (Y.T. Jeong).

927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2011.05.019

olvents.© 2011 Elsevier B.V. All rights reserved.

technical barrier for various potential areas because most of theiroutstanding properties are only associated with individual sheetsof two-dimensional hexagonally packed carbon lattice. Currently,although this obstacle can be overcome by chemical functionaliza-tion of GO with organic molecules or polymers followed by theirchemical reduction using hydrazine or its derivatives as reduc-ing agents [20,21]. However, the presence of foreign stabilizers isundersirable in the most case for practical applications. Further-more, hydrazine and its derivatives are very high poisonous andexplosive. Therefore, their use should be avoided in the produc-tion of graphene for their potential applications. One the basis ofthese observations, a new approach is essential to develop a “green”synthesis method for the production of graphene. However, tilldate, reports on chemical reduction of GO to produce grapheneusing environmentally friendly reagents are relatively rare. Forexample, Fernanderz-Merino et al. [22], have synthesized graphenenanosheets by using vitamin C as reducing agent. According to theirresults, stable suspensions of highly reduced GO can be preparednot only in water but also in some common organic solvent, suchas DMF and NMP. Recently, Zhu et al. [23], have developed a greenmethod to synthesis of graphene nanosheets based on reducingsugars.

Taking advantage of these literatures into account, herein, wedeveloped a simple approach for the production of graphenenanosheets by chemical reduction of GO using l-glutathione

reduced (GSH) as reducing agent in an aqueous solution undermild condition for the first time. GSH is a natural antioxidant inthe cells and many reactive oxygen species can be reduced by GSH[24]. The merit of this approach is that the oxidized product of

544 T.A. Pham et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (2011) 543– 548

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Fig. 1. FT-IR spectra of GO and graphene.

SH may play an important role as a capping agent to stabilizeeduced GO simultaneously because the terminal carboxylic acidsay supply enough negative charge and the electrostatic repulsion

an makes graphene nanosheets have a stable dispersion in bothqueous solution and polar aprotic solvents, thus avoiding theirgglomeration and precipitation. Most importantly, GSH itself andrepared graphene are inexpensive and environmentally friendly,hich open new opportunities for using graphene in a wide range

f potential applications.

. Experimental

.1. Materials

Graphite powder from Sigma–Aldrich was used to prepare GO.-Glutathione reduced and other chemicals were purchased fromigma–Aldrich and used as received.

.2. Synthesis of graphene nanosheets

Graphite oxide containing a large of oxygen functional groupsere synthesized from natural graphite by modified Hummers

Fig. 2. XRD patterns of pristine graphite, GO and graphene.

Fig. 3. High resolution C1s spectra of GO and graphene.

method according to our previous report [25]. In a typical syn-thetic process, natural graphite powder (2 g) was added to cooled(0 ◦C) H2SO4 (350 mL). Then, KMnO4 (8 g) and NaNO3 (1 g) wereadded gradually while stirring. The mixture was then transferredto a 30 ◦C water bath and stirred for 20 min. De-ionized water(250 mL) was slowly added and the temperature was increasedto 98 ◦C. The mixture was maintained at that temperature for30 min. The reaction was terminated by adding de-ionized water(500 mL) followed by adding 30% H2O2 solution (40 mL). Thecolor of the mixture changed to brilliant yellow, indicating theoxidation of pristine graphite-to-graphite oxide. Then, the mix-ture was filtered and washed with diluted HCl to remove metalions. Finally, the product was washed repeatedly with distilledwater until the pH was 7. The sample of graphite oxide wasobtained after drying. To prepare graphene oxide, the as-obtainedgraphite oxide was re-dispersed in distilled water to create ayellow–brown dispersion, and the exfoliation of graphite oxide togenerate GO sheets was achieved by ultrasonication for 30 min.The resultant aqueous dispersion of brown GO sheet was stable[26].

In the next step, a mixed aqueous solution containing GOsheets (0.1 mg mL−1) and GSH (2 mg mL−1) was first treatedusing ultrasonication for 1 h. The mixture was maintained at50 ◦C for 6 h. Then, the mixture was cooled to room temper-

ature and followed by another 1 h for ultrasonication. Finally,stably dispersed graphene nanosheets in aqueous media wereobtained.

T.A. Pham et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (2011) 543– 548 545

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Fig. 5. High resolution TEM images of graphene nanosheets showing a mixture of

Fig. 4. TGA curves of pristine graphite, GO and graphene.

.3. Characterization and technique

The changes in the surface chemical bondings and surface com-osition were characterized by using Fourier transform infraredpectroscopy (FT-IR) instrument (PerkinElmer Spectroscopy GX,SA), X-ray photoelectron spectroscopy (XPS) (Thermo VG Multilab000). The crystallographic states of the samples were determinedy a Philips X’pert-MPD system diffractometer (Netherland) whileransmission electron microscopy (TEM) images were recordedsing JEOL JEM 2010 instrument (Japan) to observe the nanoscaletructures. The topography of graphene nanosheets were studiedy AFM images (Digital Instruments, Nanoscope IIIA, multimodeM SPM). Thermal studies of the materials were carried outn a PerkinElmer (USA) Pyris 1 analyzer. Before the test, allamples were carefully grinded to fine powder form. The sam-les were scanned within the temperature range from 50 to00 ◦C at a heating rate of 10 ◦C min−1 under continuous nitrogenow.

. Results and discussion

The reduction of oxygen-containing functional groups of GOy GSH was confirmed by FT-IR spectroscopy. Fig. 1 shows FT-IRpectra of GO and graphene powder. In the spectrum of GO, the

Fig. 6. Tapping-mode AFM image and cross-section height profile of typical of grap

few layers graphene and their amorphous structure. The inset shows the corre-sponding selected area electron diffraction pattern (SAED).

broad band at 3227 cm−1 could be assigned to stretching of the–OH groups on the GO surface, while the band at 1713 cm−1 isassociated with stretching of the C O bond in carboxylic groups.Furthermore, the absorption peak at 1547 cm−1 can be assigned tothe skeletal vibration of unoxidized graphitic domains. In contrast,the FT-IR spectrum of graphene differs from that of GO. The intensi-ties of absorption peaks corresponding to oxygen functional groupsdecreased and these functional groups almost disappear and onlyremain a peak at 1534 cm−1 which relates to graphitic structure ofgraphene nanosheets. These results clearly confirmed that the oxy-gen functionalities were removed during chemical reduction usingGSH.

XRD is an effective method to investigate the interlayer changesand the crystalline properties of the synthesized material. Fig. 2shows the XRD patterns of pristine graphite, GO and graphene.The distance between two layers is an important parameter to

give the structural information of as-prepared graphene. The strongpeak in the XRD pattern of pristine graphite appears at 2� = 26.6◦,corresponding to the interlayer spacing of 0.335 nm while the

hene nanosheets. The sample was prepared on a freshly cleaved mica sheet.

546 T.A. Pham et al. / Colloids and Surfaces A: Physic

Fig. 7. Digital images of dispersed graphene in distilled water and various polarad

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prepared graphene, AFM image was recorded. Fig. 6 shows a typical

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protic solvents: (a) dispersion immediately after sonication and (b) dispersion twoays after sonication.

O pattern shows a characteristic peak at 2� = 11.8◦, correspond-ng to interlayer spacing of 0.75 nm, indicating the presence ofxygen-containing functional groups formed during oxidation.hese groups cause the GO sheets to stack more loosely, andhe interlayer spacing increases from 0.335 nm to 0.75 nm [27].

hereas the XRD pattern of graphene powder shows a weak androad diffraction peaks at 2� = 24.7◦, corresponding to the inter-

ayer spacing of 0.36 nm, which is significantly different from the

ristine graphite.

XPS is one of the surface analytical techniques, which can pro-ide useful information on the nature of the functional groups and

ig. 8. The synthesis route of graphene from GO: (a) each GSH releases a proton and reacorm graphene.

ochem. Eng. Aspects 384 (2011) 543– 548

also on the chemical reduction of GO. The C1s peak of both GOand graphene samples are shown in Fig. 3. The C1s spectrum of GOshows that there are three kinds of peaks assigned to oxygen func-tional groups, including hydroxyl, epoxy and carbonyl. The mainpeak of C1s located at 284.9 eV is attributed to the sp2 carbon ofC C bonding in the graphitic structure, whereas the other threepeaks at 286.5 eV, 287.8 eV, and 288.5 eV are assigned to carbonatoms in C–O, C O and O–C O functional groups, respectively. Incomparison to GO, the C1s XPS spectrum of GNS shows a significantdecrease of these signals, indicating effective deoxygenation of GOafter the chemical reduction.

The thermal stability of GO and prepared graphene were inves-tigated using TGA analysis. As can be seen in Fig. 4, TGA traces ofpristine graphite shows a negligible weight loss, which is only about2% of its total weight in the entire temperature range. Comparingwith the pristine graphite, GO shows much lower thermal stability.Although GO starts to lose weight upon heating below100 ◦C due tothe removal of physically adsorbed water, the main weight loss ofGO takes place around 200 ◦C and lose up to 45% of its total weight,presumably due to pyrolysis of the labile oxygen-containing func-tional groups present in the material, yielding CO, CO2 and steam[28]. In contrast, the thermal stability of graphene is better than thatof GO. TGA traces of graphene shows a steady weight loss upon theincrease in temperature, which is about 20% of its total weight, indi-cating the removal of oxygen-containing functional groups afterchemical reduction.

The morphologies of the prepared graphene samples dispersedin water were obtained using high resolution TEM images as shownin Fig. 5. The large graphene sheet with dimension of hundreds ofnanometers was observed on the top of copper grid. Interestingly, inour sample, we found the presence of a mixture of graphene layers.They were very stable under electron beam, resembling rippled silkwaves. The inset shows the measured selected area electron diffrac-tion pattern (SAED), which is typical for few-layered graphenesheets. In order to further investigate the surface morphology of the

AFM image of graphene nanosheets after their coating on a freshlycleaved mica substrate by spin-coating of their aqueous solution.The size of graphene sheet is around a micrometer and the average

ts with another glutathione to form GSSG and (b) the one-step reduction of GO to

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T.A. Pham et al. / Colloids and Surfaces A:

hickness of graphene sheet was measured about 8 nm from theeight profile of AFM image. It is interesting to note that the thick-ess of graphene nanosheet is larger than that of previous studies29]. The reason for this could be that the capping reagent playsn important role in the thickness of prepared graphene althoughost of oxygen functional groups were removed after the reduction

30].To examine the dispersion stability of the prepared graphene

n various solvents, the samples were dispersed in distilled waternd several polar aprotic solvents, including THF, DMF, DMSO at

typical concentration of 0.5 mg mL−1 followed by ultrasonica-ion for 30 min. The digital images provide a vivid observationn the water-dispersible properties of the samples after sonica-ion and after two days as shown in Fig. 7. For the just sonicatedamples, it can be clearly seen that prepared graphene could beispersed in both distilled water and polar aprotic solvents as evi-enced by the existence of black homogeneous solutions. Afterwo days, a few aggregation was observed in the case of THF andMF whereas the graphene still represents stable dispersions inMSO and distilled water. These results confirm that graphene

uspension could prepare not only in water but also in commonrganic solvents, thus generating further processing and manipu-ation of prepared graphene for practical applications in the nearuture. The reason for this could be due to the graphene may haveome residual oxygen functional groups, such as carboxylic groups.hus, oxidized product of GSH could form hydrogen bondingith the residual oxygen functionalities on the graphene surface.

his interaction may disrupt the �–� stacking between individ-al graphene nanosheets and thus, preventing their aggregation31].

Finally, similar to the case of reduction of GO by vitamin C,he mechanism for reduction of GO via GSH under the employedonditions are currently unclear. However, the stabilization mech-nism of the graphene nanosheets suspension may originate fromhe oxidized product of GSH. In the reduced state, each GSHs able to release a proton and then reacts with another reac-ive glutathione to form glutathione disulfide (GSSG) as shownn Fig. 8a. It is well-known that GO contain mainly two typesf reactive oxygen species, including hydroxyl and epoxy func-ional groups on the basal plane. The protons have commonlyigh binding to these oxygen-containing groups, yielding waterolecules and the yellow–brown GO aqueous dispersion has been

hanged into black homogenous solution after reduction as shownn Fig. 8b.

. Conclusion

In the present study, a green and facile reduction route ofO to produce graphene has been reported. The main advan-

age of this approach is that GSH itself and oxidized productsre environmentally friendly. The graphene could be dispersed inoth distilled water and some polar aprotic solvents with sim-le ultrasonication due to oxidized product of GSH could playn important role as a capping agent in stabilization grapheneimultaneously. Moreover, our method obtained graphene sheetsia the chemical reduction of GO in a relatively short timeeaction and mild experimental conditions. These results demon-trated that this approach could be provided a facile and lowost solution-phase processing techniques to produce grapheneanosheet in a bulk quantity for generating graphene-basedybrid materials. Because this reduction method avoids the use

f toxic reagents, the prepared graphene nanosheets are expectedo offer a great deal of flexibility for various potential applica-ions not only in electronic devices, but also in biocompatible

aterials.

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ochem. Eng. Aspects 384 (2011) 543– 548 547

Acknowledgement

This research was supported by Corporate-affiliated ResearchInstitute of Academic Industrial–Institutional CooperationImprovement. No. S7080008110.

References

[1] C.G. Navarro, M. Burghard, K. Kern, Elastic properties of chemically derivedsingle graphene sheets, Nano Lett. 8 (2008) 2045–2049.

[2] J. Wu, W. Pisula, K. Mullen, Graphene as potential material for electronics,Chem. Rev. 10 (7) (2007) 718–747.

[3] J.M. Pereira, J.P. Vasilopoulos, F.M. Peeters, Tunable quantum dots in bilayergraphene, Nano Lett. 7 (2007) 946–949.

[4] S. Park, K.S. Lee, G. Bozoklu, W. Cai, S.T. Nguyen, R.S. Ruoff, Graphene oxidepapers modified by divalent-enhancing mechanical properties via chemicalcross-linking, ACS Nano 2 (2008) 572–578.

[5] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V.Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon film, Sci-ence 306 (2004) 666–669.

[6] Y. Lu, B.R. Goldsmith, N.J. Kybert, A.T.C. Johnson, DNA-decorated graphenechemical sensors, Appl. Phys. Lett. 97 (2010) 083107.

[7] J.D. Fowler, M.J. Allen, V.C. Tung, Y. Yang, R.B. Kaner, B.H. Weiller, Practi-cal chemical sensors from chemically derived graphene, ACS Nano 3 (2009)301–306.

[8] Y.H. Lu, M. Zhou, C. Zhang, Y.P. Feng, Metal-embedded graphene: a possiblecatalyst with high activity, J. Phys. Chem. C 113 (2009) 20156–20160.

[9] R. Kou, Y. Shao, D. Wang, M.H. Engelhard, J.H. Kwak, J. Wang, V.V. Viswanathan,C. Wang, Y. Lin, Y. Wang, I.A. Aksay, J. Liu, Enhanced activity and stabilityof Pt catalysts on functionalized graphene sheets for electrocatalytic oxygenreduction, Electrochem. Commun. 1 (1) (2009) 954–957.

10] Z.S. Wu, W. Ren, L. Wen, L. Gao, J. Zhao, Z. Chen, G. Zhou, F. Li, H.M. Cheng,Graphene anchored with Co3O4 nanoparticles as anode of Lithium Ion batterieswith enhanced reversible capacity and cyclic performance, ACS Nano 4 (2010)3187–3194.

11] G. Wang, X. Shen, J. Yao, J. Park, Graphene nanosheets for enhanced lithiumstorage in lithium ion batteries, Carbon 47 (2009) 2049–2053.

12] D. Wei, Y. Liu, Y. Wang, H. Zhang, L. Huang, G. Yu, Synthesis of N-doped grapheneby chemical vapor deposition and its electrical properties, Nano Lett. 9 (2009)1752–1758.

13] X. Reina, J. Jia, D. Ho, H. Nezich, V. Son, M.S. Bulovic, J. Dresselhaus, Kong,Large area, few-layer graphene films on arbitrary ssubstrates by chemical vapordeposition, Nano Lett. 9 (2009) 30–35.

14] S. Stankovich, R. Piner, X. Chen, N. Wu, S.T. Nguyen, R.S. Ruoff, Stable aqueousdispersions of graphitic nanoplatelets via the reduction of exfoliated graphiteoxide in the presence o poly(sodium 4-styrensulfonate), J. Mater. Chem. 16(2006) 155–158.

15] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu,S.T. Nguyen, R.S. Ruoff, Synthesis of graphene-based nanosheets via chemicalreduction of exfoliated graphite oxide, Carbon 45 (2007) 1558–1565.

16] S.M. Kang, S. Park, D. Kim, S.Y. Park, R.S. Ruoff, H. Lee, Simultaneous reductionand surface functionalization of graphene oxide by mussel-inspired chemistry,Adv. Funct. Mater. 2 (1) (2011) 108–112.

17] T. Kim, H. Lee, J. Kim, K.S. Suh, Synthesis of phase transferable graphene sheetsusing ionic liquid polymers, ACS Nano 4 (2010) 1612–1618.

18] M. Lotya, Y. Hernandez, P.J. King, R.J. Smith, V. Nicolosi, L.S. Karlsson, F.M. Blighe,S. De, Z. Wang, I.T. McGovern, G.S. Duesberg, J.N. Coleman, Liquid phase pro-duction of graphene by exfoliation of graphite in surfactant/water solutions, J.Am. Chem. Soc. 131 (2009) 3611–3620.

19] J. Park, W.C. Mitchel, L. Grazulis, H.E. Smith, K.G. Eyink, J.J. Boeckl, D.H. Tomich,S.D. Pacley, J.E. Hoelscher, Epitaxial graphene growth by carbon molecularbeam epitaxy (CMBE), Adv. Mater. 2 (2) (2010) 4140–4145.

20] Y. Si, E.T. Samulski, Synthesis of water soluble graphene, Nano Lett. 8 (2008)1679–1682.

21] G. Wang, X. Shen, B. Wang, J. Yao, J. Park, Synthesis and characterizationof hydrophilic and organophilic graphene nanosheets, Carbon 47 (2009)1359–1364.

22] M.J. Fernanderz-Merino, L. Guardia, J.I. Paredes, S.V. Rodil, P.S. Fernanderz,A.M. Alonso, J.M.D. Tascon, Vitamin C is an ideal substitute for hydrazine inthe reduction of graphene oxide supspensions, J. Phys. Chem. C 114 (2010)6424–6432.

23] C. Zhu, S. Guo, Y. Fang, S. Dong, Reducing sugar: new functional molecules forthe green synthesis of graphene nanosheets, ACS Nano 4 (2010) 2429–2437.

24] G.A. Bagiyan, I.K. Koroleva, N.V. Soroka, A.V. Ufimtsev, Oxidation of thiol com-pounds by molecular oxygen in aqueous solutions, Russ. Chem. Bull. Int. Ed. 5(2003) 1134–1141.

25] T.A. Pham, B.C. Choi, Y.T. Jeong, Facile covalent immobilization of cadmiumsulfide quantum dos on graphene oxide nanosheets: preparation, characteri-zation, and optical properties, Nanotechnology 21 (2010) 465603.

26] D.R. Dreyer, C.W. Sungjin Park, R.S. Bielawski, Ruoff, The chemistry of grapheneoxide, Chem. Soc. Rev. 3 (9) (2010) 228–240.

27] T.A. Pham, N.A. Kumar, Y.T. Jeong, Covalent functionalization of graphene oxidewith polyglycerol and their use as templates for anchoring magnetic nanopar-ticles, Synth. Met. 16 (0) (2010) 2028–2036.

5 Physic

[

[

48 T.A. Pham et al. / Colloids and Surfaces A:

28] T.A. Pham, B.C. Choi, K.T. Lim, Y.T. Jeong, A simple approach for immobilizationof gold nanoparticles on graphene oxide sheets by covalent bonding, Appl. Surf.Sci. 25 (7) (2011) 3350–3357.

29] D. Li, M.B. Muller, S. Gilje, R.B. Kaner, G.G. Wallace, Processable aqueous dis-persions of graphene nanosheets, Nat. Nantechnol. 3 (2008) 101–105.

[

[

ochem. Eng. Aspects 384 (2011) 543– 548

30] Q. Su, S. Pang, V. Alijani, C. Li, X. Feng, K. Mullen, Composites ofgraphene with large aromatic molecules, Adv. Mater. 2 (1) (2009)3191–3195.

31] J. Zhang, H. Yang, G. Shen, P. Cheng, J. Zhang, S. Guo, Reduction of grapheneoxide via l-ascorbic acid, Chem. Commun. 4 (6) (2010) 1112–1114.