art%3A10.1007%2Fs11837-014-1236-0.pdf

Progress in Large-Scale Production of Graphene.Part 1: Chemical Methods

YUAN LI1 and NITIN CHOPRA1,2,3

1.Department of Metallurgical and Materials Engineering, Center for Materials for Infor-mation Technology (MINT), The University of Alabama, Box 870202, Tuscaloosa, AL 35401,USA. 2.Department of Biological Sciences, The University of Alabama, Box 870344, Tuscaloosa,AL 35401, USA. 3.e-mail: [email protected]

Graphene is a two-dimensional nanomaterial that has unique electrical,mechanical, thermal, and optical properties. For realizing the practicalapplications of graphene, one of the major challenges lies in cost-effectiveproduction of graphene-based nanomaterials at a large scale. Significantresearch efforts have been demonstrated in regard to scalable manufacturingof graphene and show strong potential for their commercialization andindustrialization. Here, we review the state-of-the-art techniques developedfor the scalable production of graphene. This review mainly discusses the top-down techniques including exfoliation of bulk graphite and chemical reductionof graphene oxide. Critical comparison for graphene quality, structure, andyields for different techniques is discussed and specific examples are describedin detail.

INTRODUCTION

Two-dimensional (2-D) nanomaterials are ofincreasing importance due to their ability to confine orquantize electrons, phonons, or photons within theplane of the material.1,2 This is especially moreintriguing for single-layered (one atom thick layers)and atomically-flat 2-D nanomaterials such asgraphene.3 Graphene is a truly 2-D carbon sheet withone atomic thickness comprised of sp2-hydridizedcarbon atoms arranged into a two-dimensionalhexagonal network.4 Long-range p-conjugation ingraphene yields them extraordinary and unique elec-trical, mechanical, and thermal properties.5 Forexample, graphene has charge carrier mobilityof 200,000 cm2 V1 s1,6 Youngs modulus of1,100 GPa,7 fracture strength of 125 GPa,7 andthermal conductivity that can go as high as 5,0008,000 W m1 K1.8 Other advantages of graphene9

include its chemical stability, mechanical flexibility,high specific surface area (2630 m2 g1),10 and hightransparency across the entire visible spectrum with a97.7% optical transmittance.11 In addition, graphenewhen surface oxidized can allow for diverse chemicalfunctionalization possibilities to result in functionaland engineered surface, devices, and heterostruc-tures.1214 Thus, graphene and graphene-based

composite nanostructures have been intensivelystudied since its free-standing form was first discov-ered at 2004 by Geim et al. at Manchester University.4

Witha unique set ofpropertiesandabilities toresult innovel physical phenomena, graphene has penetratedin a variety of application areas including energy,15,16

display,17 thermal management,18,19 nanoelectron-ics,20 optoelectronics,21 sensors,22,23 antennas,24 beamcontrol and optics,25 spintronics,26 plasmonics,27

electro-optical devices,28 and composites.29 However,to raise the technology readiness level of graphene-based devices and nanosystems, it is extremely criticalto develop scalable routes to fabricate or producegraphene. At the same time, maintaining an opti-mized cost is a daunting task for any designed fabri-cation route for this novel nanomaterial. Thus,scalable manufacturing of graphene with reliablequality and properties has emerged as a topic of graveinterest for the scientific and industrial community.

In regard to graphene fabrication and growth,various approaches has been developed such aschemical or mechanical exfoliation, reduction ofgraphene oxide, epitaxial growth, and chemicalvapor deposition (CVD) approaches.3033 Methodsusing chemical and mechanical exfoliation ofgraphite were among the first to beinvented andwere one of the easiest ways to get micron-scale

JOM, Vol. 67, No. 1, 2015

DOI: 10.1007/s11837-014-1236-0 2014 The Minerals, Metals & Materials Society

34 (Published online December 9, 2014)

graphene flakes for laboratory research but not yeta scalable approach.4 On the other hand, the epi-taxial method requires ultrahigh vacuum condi-tions30 while graphene produced by reductionmethods shows largely degraded electrical/optical/thermal properties.5 Moreover, several groupsreported the synthesis of single-layer, bi-layer, andfew-layer graphene (referred to as SLG, BLG, andFLG hereafter, respectively) with area of squarecentimeters via a CVD approach.34,35 Such CVD-grown graphene shows excellent electronic andoptical properties such as low sheet resistance, highcarrier mobility and good optical transparency.30 Inaddition, CVD methods are device- or CMOS-com-patible approaches36 and allow for easy integrationof graphene production in industrial productionlines.

Recently, many research groups have demon-strated their intensive efforts in exploring advancedmethods used for scalable production of graphene,and considerable progress has been achieved.37

These include either the further improvement of theexisting methods or the unique invention of novelsynthesis techniques. One can classify these well-studied approaches into low-temperature techniquesincluding modified exfoliation, reduction of grapheneoxide, etc. (all regarded as top-down approaches) andhigh-temperature techniques including epitaxialgrowth, CVD, etc. (all regarded as bottom-upapproaches). It is also important to note that therehas been a progressive/exponential increase ingraphene-based research articles and patents sincethe discovery of graphene in 2004.38 Many of theseapproaches have been well developed and evenmature for industrial production while some meth-ods are still under study and need further improve-ment. Thus, it is critical to review the large-scalegraphene production techniques. In this part of thereview article, we discuss the recent progress in themass production of graphene via the top-downapproaches including exfoliation and reductionmethods. Elegant and widely accepted representa-tive examples are further explained in detail.

EXFOLIATION OF GRAPHITE INTOGRAPHENE

The exfoliation method to produce grapheneinvolves the splitting of layered graphite materialsinto 2-D sheets with atomic thickness and flatness.The first successful attempt at producing single- andfew-layer graphene was based on the exfoliation ofhighly-oriented pyrolytic graphite by scotch tape.4 Toachieve the large-scale production of graphene,intensive energy has to be provided via mechanical,ultrasonication, electrochemical, and other methodssuch as microwave, laser, and arc discharge.39 Thesescalable graphene synthesis studies are furtherclassified and summarized in Table I.

For the mechanical exfoliation of graphite, theenergy required to split stacked carbon layers is

provided by shear stresses, ball milling, and/orgrinding.72 For instance, recently, Paton et al.40

reported the scalable production of large quantities ofdefect-free few-layer graphene by shear exfoliation inN-methyl-2-pyrrolidone (NMP) solvent. As shown inFig. 1a, a Silverson model L5M mixer equipped witha closely spaced rotor/stator (Fig. 1b, c) was used togenerate high shear for exfoliation of graphite. Theresulting graphene nanosheets in NMP are shown inFig. 1d. The morphology and structure of thesegraphene sheets were characterized with TEM(Fig. 1eh), XPS (Fig. 1i) and Raman (Fig. 1j). Theresults indicated that graphene with high quality(well-exfoliated, non-oxidized, defect-free) can beproduced using a broad range of mixing conditions.These authors further conducted large-scale exfolia-tion (Fig. 1km) trials in water surfactant solution,yielded similar high-quality graphene nanosheetswith a production rates exceeding 100 g h1 on scale-up to V = 10 m3. Similar exfoliation studies con-ducted using ball milling,41 planetary milling,43 andgrinding,56 either in dry or wet environments, werealso reported to be feasible for producing graphenenanostructures at a large scale (Table I). The mainadvantages of such mechanical exfoliation methodslie in their simplicity, high yield and low cost, whilethey were also reported to be time-consuming and tohave relatively low yield efficiency33 due to loss ofmaterial during the attrition process.

Splitting of graphite layers by sonication is also acommon solution-based approach and effective forproducing graphene with large yields. The fre-quently used solvent mediums include organics withsurface tension of 40 mJ m2 (viz. the property ofthe surface of a liquid that allows it to resist anexternal force, due to the cohesive nature of itsmolecules) such as NMP,47 N,N-dimethylformamide(DMF),73 ortho-dichlorobenzene (o-DCB).74 In addi-tion, aqueous solutions containing ionic or non-ionicsolutes such as (NH4)2CO3,

48 1-pyrenesulfonic acidsodium salt (Py-1SO3),

49 Gum Arabic,53 NMP,75 andpyrene51 have been also used. Sonication-assistedexfoliation has been successfully used for large-scalesynthesis of mono- and few-layer graphene withsatisfactory quality, but was still found to be time-consuming and to have low yield efficiency. Toovercome this problem, various catalytic additivessuch as FeCl3,

45 chlorosulfonic acid and H2O246

were further used to improve the splitting efficiency(referred as interlayer catalytic exfoliation). As anexample, Geng et al.45 reported the scalable pro-duction of large-size pristine few-layer graphene viaa FeCl3-assisted interlayer catalytic exfoliationprocess. As schematically shown in Fig. 2a, naturalgraphite flakes were first reacted with FeCl3through a conventional two-zone vapor transporttechnique form graphite intercalated compounds(GIC). The following layer splitting was accom-plished by the interlayer catalytic reaction of FeCl3-GIC with H2O2, in which interlayer FeCl3 serves aseffective catalyst and H2O2 both as reductant and

Progress in Large-Scale Production of Graphene. Part 1: Chemical Methods 35

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(600 mAh g1) and remarkable lithium storagestability (>1000 cycles).

Electrochemical-based exfoliation was reported asanother feasible approach for large-scale productionof graphene.5558 Such exfoliation takes place in amixture of solvents containing liquid with a narrowelectrochemical window (e.g., water) and liquid witha large electrochemical window, such as room-tem-perature ionic liquid (RTIL).56 In the typical process,natural graphite large flakes or rods were used asthe anode materials and the splitting of graphitelayers was achieved according to the following the-ory:55 (1) electrolysis of water at the electrode pro-duces hydroxyl and oxygen radicals; (2) the oxygenradicals start corroding the graphite anode on edgesites, grain boundaries and defect sites, whichresults in the opening up of edge sheets; (3) the RTILanions intercalate within the edge sheets and initiatethe electrode expansion; (4) the precipitation of somesheets results in the creation of graphene sheets insolution. Electrochemical method is simple, and it iseasy to realize the large-scale exfoliation of graphitewith low cost, while the main disadvantage isthe difficulty in controlling the uniformity or

homogeneity in the graphene structure in the largesample-set. By-products such as carbon ribbons and/or nanoparticles were also produced and at the sametime were difficult to separate.

Besides these commonly used exfoliation methods,additional methods5966 have been reported to in-crease the scalability, improve the yield, or obtaingraphene products in a special form (nanosheets,falkes, graphene quantum dots). For instance, arcdischarge-based exfoliation65,66 was reported to bescalable, cost-effective, and easy to realize the dopingof product (N-doped graphene). Exfoliation assistedby microwave illumination59,60 or laser ablation61

was also reported for the production of graphenequantum dots (GQDs). Moreover, research reportedby Qian et al.62 and Liu et al.63 show that the qualityof graphene and process yield can be significantlyimproved by conducting the exfoliation in a solvo-thermal or hydrothermal approach. In addition,Hummers method6771 has been rigorously used forthe scalable production of graphene oxide andgraphene. This method involves series of stepsincluding harsh chemical treatment and processingof exfoliated and oxidized graphene in a reducingenvironment. More details on this process are dis-cussed in a later section of this review.

REDUCTION OF GRAPHENE OXIDE

In a typical chemical reduction process, naturalgraphite was first exfoliated into graphene oxidevia a (modified) Hummers method as describedpreviously, followed by reduction of oxygen-con-

Fig. 1. Production of graphene by shear exfoliation. (a) Silverson model L5M high-shear mixer with mixing head in a 5-L beaker of graphenedispersion. (b, c) Close-up view of the mixing heads. (d) Graphene product dispersed in NMP. (eh) TEM images of the graphene nanosheets. (i)XPS and (j) Raman spectra of the exfoliated graphene. (k, l) Rotor and stator for large-scale trials. (m) Shear exfoliation of graphite in 100-Lwater-surfactant solution. (Reprinted with permission from Ref. 40. Copyright 2014, Nature Publishing Group).


taining functionalities such as hydroxyl (COH)and carboxyl (COOH) using various reducingagents.5 The effective production of graphenethrough this method was first demonstrated byRuoffs group.76,77 Hydrazine hydrate was used asreduction agent and directly added to an aqueousdispersion of graphene oxide (GO) produced via amechanical energy-assisted Hummers method.One of the advantages of such a chemical reductionmethod lies in its low cost and massive scalability.The starting material is simple graphite, and thetechnique can easily be scaled-up to produce gramquantities or larger of reduced graphene oxide(rGO). Thus, this method has since been inten-sively studied by many groups. Besides hydrazinehydrate, the frequently used reduction agents alsoinclude NaBH4,

78 (NaCl, AgNO3, MgCl2, FeSO4,CuCl2 or AlCl3), sugars (glucose, fructose and su-crose), hydriodic acid/acetic acid, Fe powder, andpyrrole (Table II). In addition, an electrochemical

method87 was also used for the reduction ofgraphene oxide. However, one problem with thisoriginal aqueous reduction of graphene oxide wasthat the removal of oxygen groups caused thereduced sheets to become less hydrophilic and toquickly aggregate in solution. Several researchershave tried to raise the pH during reduction toresult in charge-stabilized colloidal dispersions.Meanwhile, other reports have focused on con-ducting the reduction reaction in a non-aqueoussolution containing anhydrous hydrazine, sodiumammonia (Na-NH3), and thiophene, or in a dryenvironment by using solid hydrazine, urea, H2,HCl, thermal reduction, or laser irradiation(Table II).

As a representative example, we presentthe chemical reduction conducted by Feng et al.79

(Figure 3). The natural graphite was used as thestarting material, which was further exfoliated tographene oxide via a modified Hummers method.

Fig. 2. Production of a few-layer graphene (FLG) by interlayer catalytic exfoliation. (a) Schematic illustration for the interlayer catalytic exfoliationfrom FeCl3 intercalated graphite (FeCl3GIC) to graphene layers. (b) Large volume of FLG suspension (right) and graphite flakes (bottom left). (c)A bottle with10 g FLG powder. (d) SEM and TEM (inset) images of the FLG showing thin and soft morphologies. (e) The highly cyclic stability ofthe FLG in lithium storage. The inset is a photograph of a lithium-ion coin cell. (Reprinted with permission from Ref. 45. Copyright 2013, NaturePublishing Group).

Li and Chopra38

The reduction of the graphene oxide was completedby sodium-ammonia (Na-NH3) liquid settled in adry ice-acetone bath. Dissolution of the sodiummetal in liquid ammonia resulted in ionization ofthe metal to form a sodium cation and a solvatedelectron that strongly associated with the solventammonia (Fig. 3a, b). These solvated electronsfunctioned as a potent electron source to effectivelyremove oxygen functionalities and restore the pla-nar geometry of the GO sheets. Figure 3c, d showsthe GO powders dispersed in liquid ammonia (c) andstirred for 20 min (d). Then, sodium metal wasadded and the reduced graphene oxide (rGO) solu-tion is shown in Fig. 3e. Several characterizationswere conducted. Atomic force microscopy (AFM) inFig. 3f, g shows a thickness of 0.91 nm for singlerGO. The TEM image in Fig. 3h shows sheet-likegraphene, of which the crystal structure was furtherconfirmed by the selected area electron diffraction(see inset). Figure 3i shows a large single sheet(>10 lm). XPS spectra for C 1s (Fig. 3j) show theelimination of oxygen-containing group after thechemical reduction, which is consistent with theTGA curve in Fig. 3k, as the mass drop 200C(corresponding to removing of COH/COOH groups)was not observed on graphite and rGO. Figure 3lshows the Raman spectra of GO and rGO film. Anobvious D-band was observed for all samples. This isbecause a lot of defects were created by the strongacid during the Hummers process, and thusgraphene produced by chemical reduction normallyshows relatively low quality as compared with epi-taxial methods.

An advantage of chemical reduction method isthat it can be modified and combined with othertechniques for developing novel scalable compositegraphene materials with specific shape and ad-vanced electronic, optical and thermal properties.For instance, Yu et al.96 reported a scalable methodto continuously produce hierarchically structuredcarbon micorfibres comprised of an interconnectednetwork of single-walled carbon nanotubes(SWCNTs) and nitrogen-doped rGO (Fig. 4af). Thefabrication was based on a hydrothermal processconducted in a fused-silica capillary column(Fig. 4a). SWCNTs were treated with nitric acid andwere further hybridized with GO in the presence ofethylenediamine (EDA) as nitrogen dopant to dopeGO with a concomitant reduction during thehydrothermal process. Figure 4b shows a photo-graph of the carbon fibers, of which the morphologywas further characterized by SEM (Fig. 4ce).These hybrid carbon fibres present large specificarea (396 m2 g1) and high conductivity(102 S cm1), and were further used to make high-volumetric performance micro-supercapacitors(Fig. 4f), which shows a maximum power density of1085 mW cm3. Another example83 in Fig. 4gnshows the mass production of high-quality novelerythrocyte-like graphene microspheres (ELGMs).This was achieved by electrospraying a GOT

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suspension (Fig. 4h) at the Taylor cone, where theGO sheets (Fig. 4i) were negatively charged anderupted into droplets (Fig. 4g). These GO dropletswere collected into a coagulation solution and fur-ther reacted with positively charged cetyltrimeth-ylammonium (CTAB), forming erythrocyte-like GOmicrospheres (Fig. 4jm). In order to obtain ELGMs(Fig. 4n), these GO microspheres were chemicallyreduced by using hydrazine hydrate. The as-pre-pared porous ELGMs exhibit excellent capability forfast and recyclable removal of oil and toxic organicsolvents from water, reaching up to 216 times of itsweight in absorption efficiency, which is tens oftimes higher than that of conventional sorbentmaterials.

OUTLOOK AND FUTURE PROSPECTS

The worldwide supply of natural graphite hasbeen estimated at 800,000,000 tonnes, and thusdevelopment of a top-down method such as exfolia-tion and reduction are of great promise and impor-tance. Considerable progress has been made on thelarge-scale production of graphene from the labo-ratory-scale exfoliation and chemical reduction

approaches. The main advantages of exfoliation andreduction method are simplicity, low productioncost, and good feasibility for large-scale production.Meanwhile, shortcomings include time the con-sumption, size limit, impurities, and low quality(especially for rGO) of graphene. For instance, thesolvents and stabilizers utilized in chemical meth-ods have to be safe and impart greater stability tothe graphene suspensions. In addition, thesemethods are challenging when it comes to achievinga well-dispersed graphene suspension with uniformphysical characteristics and, thus, making graph-ene separation a daunting task. With respect to thequality of graphene, both exfoliation and reductionmethods inevitably introduce a significant amountof oxygen and other defects into the product. And,thus, finding routes for complete restoration of thesp2 carbon network of pristine graphene is ofinterest. Thermal annealing of reduced grapheneoxide sheets, for instance, has produced improvedresults. Overall, these approaches resulting indefective graphene have led to applications that donot require high purity material such as pasteelectrodes, sensors, printable devices, and compos-ites. For a high-quality and large-area graphene

Fig. 3. Mass production of graphene by reducing graphene oxide in a Na-NH3 system. (a) The generation of solvated electrons by dissolution ofthe sodium in liquid ammonia. (b) Solvated electrons containing anhydrous liquid ammonia. (c) Liquid ammonia kept in dry ice-acetone bath. (d)GO powder dispersed in liquid ammonia. (e) Black RGO solution was obtained after the reduction of GO with solvated electrons. (f, g) AFMimage and its height profile of rGO. (h) TEM image and SAED pattern of rGO nanosheet. (i) SEM image of large RGO sheet. (j) XPS spectra ofgraphite, GO and rGO. (k) TGA thermograms for GO, and rGO. (l) Raman spectra of GO film and RGO films by dipping in the Na-NH3 solution atdifferent time. (Reprinted with permission from Ref. 79. Copyright 2013, Nature Publishing Group).

Li and Chopra40

product that is required for electronics, optoelec-tronics, and thermal management, bottom-upapproaches such as epitaxial growth and CVD

method hold high potential for large-scale singlecrystalline graphene production, which is a focus ofpart II of this review.

Fig. 4. Novel graphene materials produced via chemical reduction of GO. (af) Synthesis of CNT/graphen hybrid fibres: (a) process schematic,(b) a dry fibre with diameter of 50 mm and length of 0.5 m. (ce) SEM shows the front view and cross-section view of CNT/graphene fibres, (f)photograph of a bent micro-supercapacitor. (Reprinted with permission from Ref. 96. Copyright 2014, Nature Publishing Group.) (gn) Synthesisof erythrocyte-like graphene microspheres: (g) experimental set-up, (h) GO suspension, (i) SEM of GO sheets, (j) a general optical microscopeview of the erythrocyte-like GO microspheres, (k) Size distribution of erythrocyte-like GO microspheres, (l) SEM image of the erythrocyte-like GOmicrospheres, (m) optical image of erythrocyte-like GO microspheres after drying, (n) SEM image of erythrocyte-like graphene microspheresreduced by hydrazine hydrate. (Reprinted with permission from Ref. 83. Copyright 2013 Nature Publishing Group).


ACKNOWLEDGEMENTS

This work was made possible by National ScienceFoundation (Award #: 0925445) and NSF-EPSCoR-RII award. The authors thank the University ofAlabamas Office of sponsored programs and Re-search Grant Committee Award for additional sup-port. The authors thank Dr. S. Kapoor for proofreading the manuscript.

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Progress in Large-Scale Production of Graphene. Part 1: Chemical MethodsAbstractIntroductionExfoliation of Graphite into GrapheneReduction of Graphene OxideOutlook and Future ProspectsAcknowledgementsReferences

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