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Reducing agent free synthesis of graphene from graphene oxideR. Naresh Kumar, P. Shaikshavali, Vadali V. S. S. Srikanth, and K. Bhanu Sankara Rao

Citation: AIP Conf. Proc. 1538, 262 (2013); doi: 10.1063/1.4810070 View online: http://dx.doi.org/10.1063/1.4810070 View Table of Contents: http://proceedings.aip.org/dbt/dbt.jsp?KEY=APCPCS&Volume=1538&Issue=1 Published by the AIP Publishing LLC.

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Reducing Agent Free Synthesis of Graphene from Graphene Oxide

R. Naresh Kumar, P. Shaikshavali, Vadali V. S. S. Srikanth* and K. Bhanu Sankara Rao

School of Engineering Sciences and Technology (SEST), University of Hyderabad, Hyderabad 500046, India E-mail: vvsssse@uohyd.ernet.in

Abstract. Graphene is synthesized by microwave irradiation (MWI) of graphene oxide (GO) and subsequent sonication. MWI of GO is carried in a household microwave oven without using any reducing agents. Sonication of microwave irradiated GO is carried out in distilled water using a probe type sonicator. This method does not evolve any unsafe by-product gases which is otherwise the case when reducing agents are used in the reduction of GO to graphene. Moreover, due to its intrinsic nature, the method is scalable and cost effective. The synthesized product has been characterized as graphene using micro Raman scattering, x-ray diffraction and electron diffraction. Diffraction results show that the synthesized graphene is highly oriented.

Keywords: Graphene Oxide; Graphene; Microwave Irradiation PACS: 61.48.Gh; 63.22.Rc; 78.30.-j; 81.05.ue

INTRODUCTION

Since the advent of graphene as a material with diverse and unique properties, researchers have been trying to synthesize graphene by different methods. Despite involving the use of unsafe reducing reagents to synthesize graphene from graphene oxide (GO), chemical methods have been the most popular ones to produce large amounts of grapheme [1-2]. Of late, microwave aided synthesis routes to obtain large amounts of high quality graphene from GO are also being practiced [3-10]. Even in these cases, use of reagents that are unsafe and/or might produce unsafe by-product gases cannot be avoided. In this work a method that involves microwave irradiation (MWI) of GOwithout using any reducing reagents and subsequent sonication to obtain graphene is presented. By using MWI, reducing atmosphere (like H2 gas) and high temperature requirements which are otherwise required to reduce GO to graphene can be avoided. Moreover, MWI is quick and can be done under room conditions. Due to its intrinsic nature, this method does not evolve any unsafe by-product gases (which is otherwise the case when reducing agents are used to reduce GO to graphene) besides being scalable.

EXPERIMENTAL

Synthesis of GO and Graphene

3g of graphite flakes (flake size 300m) are added to 60ml of concentrated H2SO4 and stirred overnight. To this mixture, 1.5g of NaNO3 was added and stirred for 5 minutes. Next, 60 ml of concentrated H2SO4 was again added to reaction mixture under stirring. All these steps are carried out under room conditions. However, in a subsequentstep, the temperature of the above reaction mixture was maintained at 0 C using an ice bath while 9g of KMnO4 was slowly added to the reaction mixture. 0 C was maintained because the process of KMnO4 addition being highly exothermic. Thereafter, the reaction mixture was stirred at room temperature for 30 minutes. Distilled water (150 ml) was poured slowly into the mixture and temperature of solution was raised to 98 C and reaction mixture was maintained at this temperature for 1h. After cooling the mixture to room temperature, it was further diluted with distilled water (150 ml) and small amount of 30% H2O2 (5 ml) was added. Thus obtained mixture was subjected to multiple washes with distilled water by discarding supernant each time and finally filtrate was collected and kept in a hot oven at 1000 C for overnight to obtain GO (partially oxide graphite) powder. The above discussed method of GO synthesis can be considered as a modified Hummers method [11]. Alternatively, GO powder may be commercially obtained. In the next step, GO powder was subjected to MWI (900W and 2.45GHz) inside a household microwave oven for 3min to obtain puffy graphene worms (GWs). Puffy worm like semblance was Carbon Materials 2012 (CCM12)AIP Conf. Proc. 1538, 262-265 (2013); doi: 10.1063/1.4810070 2013 AIP Publishing LLC 978-0-7354-1162-3/$30.00262

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revealed by morphological analysis. Uniform suspension of graphene sheets (as revealed by structural analysis) are finally obtained in the subsequent step by sonicating GWs in distilled water for 1h.

Characterization of GO and Graphene Scanning and transmission electron microscopes operated at accelerating voltages of 5 and 200kV, respectively

are used to study the morphology of the samples. Electron and x-ray diffraction are used to study crystallinity whilst micro Raman scattering was used to study the phase of the samples. X-ray diffractograms are recorded with Cu K x-ray source (=1.54). Raman spectra are obtained using 532nm green laser excitation source.

RESULTS AND DISCUSSION

Scanning electron micrographs of graphite flakes (starting material), puffy Graphene worms and few-layered graphene are shown in Fig. 1. As-obtained graphite flakes (stacked graphene sheets) are shown in Figs. 1(a) and (b). Morphology of GWs are shown in Figs. 1(c) and (d). It is clear from the micrographs that the MWI was able to fragment bulk GO into many GWs but still keeping the stacking of individual graphene layers along the thickness axis in a single GW intact. A closer observation (Fig. 1(d)) of puffy GWs revealed that even exfoliation along the thickness has taken place to some extent. Sonication of GWs has resulted in further exfoliation leading to the release of graphene sheets as shown in Figs. 1(e) and (f). The graphene sheets have lateral dimensions of at least 1 m2. It can also be observed that some of the regions are folded. These graphene sheets are transparent to the electron beam as observed in transmission electron micrographs (Figs. 2(a) and (b)) indicating the presence of only few sheets (along the c axis) in a single structure.

FIGURE 1. Secondary electron micrographs showing the morphology of graphite flakes ((a) & (b)), graphene worms ((c) &

(d)), and graphene sheets ((e) & (f)). 263Downloaded 20 Jun 2013 to 35.10.236.7. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://proceedings.aip.org/about/rights_permissions

FIGURE 2. (a), (b) Plane view transmission electron micrographs and (c) diffraction pattern of graphene. X-ray diffraction data (Fig. 3) obtained from GWs and graphene showed the typical (002), (100) and (004)

diffraction peaks indicating the formation of graphene through exfoliation. However, it is very clear from the diffractogram obtained from graphene that it is highly oriented. Electron diffraction pattern obtained from graphene showed the typical six fold symmetry expected for graphene. This complemented well with that of x-ray diffraction data.

FIGURE 3. X-ray diffractograms obtained from graphene worms and graphene. A great intensity variation between

graphene worms and graphene (represented by different colors) is evident. Moreover, Raman spectra obtained from GO, GWs and graphene (Fig. 4) showed distinctive D and G bands

corresponding to graphitic materials [12].However, the spectrum obtained from graphene synthesized in this work clearly showed the evolution of the characteristic 2D' Raman band at ~2700cm-1 confirming the formation of grapheme [12].

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FIGURE 4. Raman spectra obtained from graphene oxide, graphene worms and graphene.

CONCLUSION

Graphene was synthesized using a simple and effective way of microwave irradiation of graphene oxide in a house-hold microwave oven. This method is devoid of any reducing agents.

ACKNOWLEDGMENTS

RNK extends his thanks to UGC for the financial support through RGNF. VVSSS thanks SERB, DST, India for providing research funding vide SERB/F/3487/2012-2013 under the Fast Track Scheme for Young Scientist for carrying out this work.

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