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  • Edge-carboxylated graphene nanosheetsvia ball millingIn-Yup Jeona, Yeon-Ran Shina, Gyung-Joo Sohna, Hyun-Jung Choia, Seo-Yoon Baea, Javeed Mahmooda, Sun-Min Junga,Jeong-Min Seoa, Min-Jung Kima, Dong Wook Changa,b, Liming Daia,c,1, and Jong-Beom Baeka,1

    aInterdisciplinary School of Green Energy/Low-dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), 100Banyeon, Ulsan 689-798, South Korea; bDepartment of Chemical Systematic Engineering, Catholic University of Daegu, 13-13 Hayang, Gyungbuk,712-702, South Korea; and cDepartment of Macromolecular Science and Engineering, Case Western Reserve University, 10900 Euclid Avenue,Cleveland, Ohio 44106

    Edited by Hui-Ming Cheng, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China, and accepted by the Editorial Board February 8, 2012(received for review October 13, 2011)

    Low-cost, high-yield production of graphene nanosheets (GNs) isessential for practical applications. We have achieved high yieldof edge-selectively carboxylated graphite (ECG) by a simple ballmilling of pristine graphite in the presence of dry ice. The resultantECG is highly dispersable in various solvents to self-exfoliate intosingle- and few-layer (5 layers) GNs. These stable ECG (or GN) dis-persions have been used for solution processing, coupled withthermal decarboxylation, to produce large-area GN films for manypotential applications ranging from electronic materials to chemi-cal catalysts. The electrical conductivity of a thermally decarboxy-lated ECG film was found to be as high as 1214 Scm, which issuperior to its GO counterparts. Ball milling can thus provide sim-ple, but efficient and versatile, and eco-friendly (CO2-capturing)approaches to low-cost mass production of high-quality GNs forapplications where GOs have been exploited and beyond.

    carbon dioxide eco-friendly edge-functionalization graphite

    As a building block for carbon nanomaterials of all other di-mensionalities, such as 0D buckyball, 1D nanotubes, and 3Dgraphite, graphene nanosheets (GNs) with carbon atoms denselypacked in a 2D honeycomb crystal lattice have recently attractedtremendous interest for various potential applications (1). Sev-eral techniques, including the peel-off by Scotch tape (2), epitax-ial growth on SiC (3), chemical vapor deposition (CVD) (4, 5),and solution exfoliation of graphite oxide (GO) (6), have beenreported for producing GNs. Although the Scotch tape methodled to the Nobel-Prize-winning discovery of high quality GNs (2),it is unsuitable for large-area preparation of GN films due to tech-nique difficulties. On the other hand, large-area thin GN films upto 30 in. have been prepared by CVD (7). However, the CVDprocess involves extremely careful fabrication processes, whichappears to be too tedious and too expensive for mass production.The widely reported solution exfoliation of graphite into GO,followed by solution reduction (810), allows the mass produc-tion of GNs via an all-solution process. Due to strong interactionsbetween the hexagonally sp2-bonded carbon layers in graphite,however, the solution exfoliation requires the involvement of ha-zardous strong oxidizing reagents (e.g., HNO3, KMnO4, and/orH2SO4) and a tedious multistep process (8, 9, 11, 12). Such a cor-rosive chemical oxidation often causes severe damage to the car-bon basal plane to introduce a large number of chemical andtopological defects (13). As a result, postexfoliation reduction ofGO into reduced graphene oxide (rGO) is essential in order torestore the graphitic basal plane for the resultant GNs (6,1419).To make the matter worse, the reduction reaction also involveshazardous reducing reagents (e.g., hydrazine, NaBH4) with alimited reduction conversion (approximately 70%) (20). The re-duced GO (rGO) still contains considerable oxygenated groupsand structural defects, and thus additional high-temperaturethermal annealing step is required (20).

    To overcome the above-mentioned limitations on the widelystudied GO approach, we report here a method for a simple,

    but effective and eco-friendly, edge-selective functionalizationof graphite without the basal plane oxidation by ball millingin the presence of dry ice (solid phase of carbon dioxide). Highyield of edge-carboxylated graphite (ECG) was produced and theresultant ECG is highly dispersible in various polar solvents toself-exfoliate into GNs useful for solution processing. UnlikeGO, the edge-selective functionalization of the pristine graphitecould preserve the high crystalline graphitic structure on its basalplane. The edge-attached functional groups tend to repel eachother to effectively open up the edges of the ECG, leading toself-exfoliation in solvent for the formation of high-quality GNfilms. Indeed, large-area GN films with an electrical conductivityas high as 1;214 Scm, superior to their GO counterparts, havebeen prepared simply by solution casting the ECG dispersions ona substrate, followed by thermal decarboxylation. In addition, theuse of dry ice (or CO2 gas) as a reagent for carboxylation in theball milling could also facilitate the CO2 capture and storage toreduce their detrimental effects on our planet (21). Therefore,the newly developed ball milling approach, involving neither ha-zardous chemicals nor tedious procedures, outperforms currentprocesses for mass production of high-quality GNs at an unpre-cedented low cost.

    Results and DiscussionIn a typical experiment, ball milling was carried out in a planetaryball-mill machine (Pulverisette 6, Fritsch; Fig. S1A) in the pre-sence of graphite (5.0 g, Fig. 1A), dry ice (100 g, Fig. 1B) andstainless steel balls (Fig. S1B). Detailed experimental conditionsfor the ball milling can be found in the Materials and Methodssection and Supporting Information. Upon opening the stainlesssteel capsule lid (Fig. S1C) in air at the end of ball-milling pro-cess, violent sparkling occurred (Figs. S1D and E and Movies S1,S2, and S3), presumably due to hydration of the ball-milling-in-duced, highly energetic carboxylates (-COO) into carboxylicacids (-COOH) by air moisture (H2O) and residual activated car-bon species (radicals, anions, and cations) into hydroxyl (-OH)and hydroperoxy (-OOH) by air oxygen (O2) and moisture(H2O). The recovered product was further Soxhlet extracted with1 M aqueous HCl solution to completely acidify carboxylates andto remove metallic impurities, if any. The starting graphite(Fig. 1A) gained weight to be 6.28 g of the ECG (Fig. 1C), indi-cating an efficient uptake of CO2 (1.28 g). A high-yield carbox-ylation was thus occurred, as schematically shown in Fig. 1D. The

    Author contributions: J.-B.B. designed research; I.-Y.J., Y.-R.S., G.-J.S., H.-J.C., S.-Y.B., J.M.,S.-M.J., J.-M.S., M.-J.K., and D.W.C. performed research; I.-Y.J., L.D., and J.-B.B. analyzeddata; and L.D. and J.-B.B. wrote the paper.

    The authors declare no conflict of interest.

    This article is a PNAS Direct Submission. H.C. is a guest editor invited by the Editorial Board.1To whom correspondence may be addressed. E-mail: jbbaek@unist.ac.kr or liming.dai@case.edu.

    This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1116897109/-/DCSupplemental.

    55885593 PNAS April 10, 2012 vol. 109 no. 15 www.pnas.org/cgi/doi/10.1073/pnas.1116897109

    http://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1116897109/-/DCSupplemental/Appendix.pdfhttp://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1116897109/-/DCSupplemental/pnas.1116897109_SI.pdf?targetid=SM1http://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1116897109/-/DCSupplemental/pnas.1116897109_SI.pdf?targetid=SM2http://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1116897109/-/DCSupplemental/pnas.1116897109_SI.pdf?targetid=SM3http://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1116897109/-/DCSupplementalhttp://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1116897109/-/DCSupplementalhttp://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1116897109/-/DCSupplementalhttp://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1116897109/-/DCSupplementalhttp://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1116897109/-/DCSupplementalhttp://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1116897109/-/DCSupplemental

  • detailed mechanism of carboxylation via mechano-chemicalprocess by ball milling is proposed in Fig. S2 and confirmedby various spectroscopic measurements (vide infra). Elemental(EA, Table S1) and thermogravimetric analyses (TGA, Fig. S3)revealed that the oxygen content of ECG increased with anincrease in the ball-milling time before leveling off at 48 h.The increase in the ball-milling time also caused a continuousdecrease in the sample grain size until 48 h to reach a steady state,as seen from the scanning electron microscope (SEM, FEINanonova 230) images in Fig. S4. For subsequent investigation,therefore, ECG samples prepared by the ball milling for 48 hwere used unless otherwise stated. As reference, GO was alsoprepared by the modified Hummers process (see, ExperimentalSection in Supporting Information) (12). Figs. S5A and B showtypical SEM and transmission electron microscope [HR-TEM,JEOL JEM-2100F (Cs)] images, respectively, for the GO samplethus prepared. To empathize advantages of the ECG approach,the direct comparison between GO and ECG approaches is pre-sented in Fig. S6.

    Fig. 2A gives a typical SEM image of the pristine graphiteflake, showing micro-scale (100 mesh,

  • a sp3C-H peak at 2;939 cm1 associated with defects (Figs. S2and S6), along with the sp2 peak at 2;919 cm1 (Fig. 3A). Thesespectroscopic results are consistent with the scenario for the re-actions given in Figs. 1D and Fig. S6, which show that the reactivecarbon species (radicals, anions and cations), generated by homo-lytic and heterolytic cleavages of the graphitic CC bonds duringball milling in the presence of dry ice, reacted with carbon