Materials Letters 64 (2010) 357–360

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Materials Letters

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Structures of thermally and chemically reduced graphene

Hye-Mi Ju a, Seung Hun Huh a,⁎, Seong-Ho Choi a, Hong-Lim Lee b

a Nanotechnology Convergence Lab, Korea Institute of Ceramic Engineering and Technology (KICET), 233-5, Gasan-dong, Guemcheon-Gu, Seoul 153-801, Republic of Koreab Advanced Materials Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul 120-749, Republic of Korea

⁎ Corresponding author. Tel.: +82 2 3282 7820; fax:E-mail address: shhuh@kicet.re.kr (S.H. Huh).

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.11.016

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 August 2009Accepted 5 November 2009Available online 12 November 2009

Keywords:GrapheneGraphene oxideReductionStructure

This study investigated the structures and compositions of two types of graphene (GP) produced by thereduction of graphene oxide (GO): GPTR, produced by thermal reduction at 1073 K in N2; and GPCR,chemically reduced with hyrdazine. GPTR and GPCR have a small number of surface oxide groups with thecompositions C100O3±1 and C100O6.5±2 and consist of six layers and three layers, respectively. The interlayerspaces are slightly larger than those in typical graphene produced by “top-down” exfoliation from graphite.These structures and compositions are intrinsic properties of graphene produced by the “bottom-up” layer-by-layer stacking process.

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1. Introduction

Graphene (GP) is typically produced from graphite by a “top-down” process—either physical exfoliation [1] or solvation-assistedexfoliation [2]. The interlayer space of GP (d002) is ∼3.4 Å [3], which isslightly larger than that of the graphite (002) facet (3.348–3.36 Å).This may be due to the turbostratic AB-stacked structure andnanocurvature distortion in GP [4,5], which are different from thewell-developed graphite structures. Furthermore, powdery GP hasbeen produced by a process involving “bottom-up” layer-by-layerstacking of basal graphene sheets, through both representativechemical reduction [3,6] and thermal reduction of graphene oxide(GO). The chemical reduction process is considered to be driven by achemical reaction, whereas the thermal reduction process is thoughtto be driven by thermal energy-induced bond cleavage. Temperature-dependent infrared (IR) spectroscopy [7–9] and mass spectrometry[10] were found to be critical techniques for tracing the temperature-dependent removal of the oxide groups of GO. However, fundamentalstudies of GP produced by either chemical or thermal reduction haverarely been conducted. In this paper, the structures and compositionsof the thermally reduced GP (GPTR) and chemically reduced GP (GPCR)are presented and compared with those of typical GP produced by thetop-down process.

2. Experimental

GO was prepared by the modified Hummers' method [11]. The GOslurry thus produced (Fig. 1a) was dried in a vacuum oven at 60 °C for

24 h (Fig. 1b). The dried GO powders were heated in N2 at 1073 K for1 h. The heating rate and N2 flow rate were 5 °C/min and 50 ml/min,respectively. This process yielded the powdery GPTR. The chemicalreduction of GO using hydrazine is well known [3]. A 4%-GO slurry and1-L deionized water were simply mixed and then sonicated for40 min. Hydrazine monohydrate (10 mL) was added to the GOsolution, which was then heated at 100 °C under a water-cooledcondenser for 24 h. The chemically reduced product (black precipi-tation) was filtered through a Buchner funnel and then washed in amixture of water and methanol. The final product was dried in anoven at 70 °C. This process yielded the powdery GPCR. The producedGO, GPCR, and GPTR were characterized by a field emission scanningelectron microscope (FE-SEM: JEOL, JSM 6700F), a transmissionelectron microscope (FE-TEM, JEOL, JEM-2000EX (200 keV)), a Four-ier Transform Infrared spectroscope (FT-IR, JASCO4100), and an X-raydiffraction (XRD) spectrometer (RIGAKU, D/MAX-2500). X-ray Cu Kα

radiation of 1.54056 Å was used. An X-ray dispersive spectroscope(EDS) with a FE-SEM instrument was used for the elemental analysisof the GO, GPTR, and GPCR.

3. Results and discussion

The surfaces of GO sheets have soft carpet-like morphologies dueto the presence of residual H2Omolecules tightly boundwith carboxylor hydroxyl groups of GO (Fig. 1a), while both the GPTR and GPCR havecrumpled sheet-like morphologies (Fig. 1b, Fig. 1c). FE-TEM images ofthe GO, GPTR, and GPCR show very thin, flat sheets less than ∼2 nmthick (Fig. 1d–f). The XRD pattern of GO shows a large interlayer spaced002=7.941 Å for the (002) peak at 2θ=11.134° because of thepresence of intercalated H2O molecules and various oxide groups(Fig. 2) [7]. The large d002 of GO drastically decreases to d002=3.432 Åfor the GPTR and 3.760 Å for the GPCR after thermal and chemical

Fig. 1. FE-SEM and FE-TEM images of (a, d) GO, (b, e) GPTR, and (c, f) GPCR, respectively.

358 H.-M. Ju et al. / Materials Letters 64 (2010) 357–360

reductions, respectively, due to the removal of the intercalated H2Omolecules and surface oxide groups of GO. The surface of grapheneoxide (GO) is hydrophilic due to oxide groups of –OH and –COOH andinteracts with water molecules via hydrogen bonds. After removingsurface oxide groups through thermal reduction or hydrazine-assisted

Fig. 2. XRD patterns and EDS spectra of (a, d) GO

chemical reduction, the surface of reduced graphene (GP) changes tobe hydrophobic, inter-GP sheets interact with each other via surface πelectrons.

FT-IR spectra indicate that the intercalated H2O molecules andmost of the oxide groups of GO (Fig. 3a) are removed after thermal

, (b, e) GPTM, and (c, f) GPCM, respectively.

Fig. 3. FT-IR spectra of (a) GO, (b) GPTR, and (c) and GPCR.

359H.-M. Ju et al. / Materials Letters 64 (2010) 357–360

reduction (Fig. 3b) or chemical reduction (Fig. 3c). However, a smallnumber of the epoxide groups and the hydroxyl groups remain afterboth types of reductions, as shown in the FT-IR spectra. There arefewer hydroxyl groups than epoxide groups. EDS analysis alsoindicates that the GPTR and GPCR contain smaller amounts of oxygen(Fig. 4) as compared to GO, with the compositions being C100O3±1 forGPTR and C100O6.5±2 for GPCR. The IR and EDS results indicate thatneither chemical reduction using the modified Hummers' method northermal reduction at 1073 K under N2 and ambient light cancompletely remove the oxide groups. The residual epoxide groups,which are at an initial stable form during carbon oxidation and can befurther oxidized into –OH and –COOH, can be explained by high

Fig. 4. Proposed structural mode

thermal stability [12] and low reactivity (hydrazine) compared to –

OH and –COOH [3]. However, we consider that the intercalated H2Omolecules and oxide groups of the GO can be removed by thermal andchemical reduction and that the residual small number of oxygengroups does not exist in the interlayer space, but rather on thesurfaces of the chemically and thermally reduced GP. This suppositionis reasonable because the gap of 0.2–0.4 Å between GP d002=3.432–3.760 and graphite d002=3.348–3.60 Å is too small to contain the∼1.25-Å epoxy group [13,14] or the ∼2.75-Å H2Omolecule [15]. Basedon Sherrer's equation [16], the crystalline thickness of the (002) facetis estimated to be 20.9±0.6 Å for GPTR and 10.7±0.6 Å for GPCR,corresponding to 6.1 and 2.8 layers (thickness/d002), respectively. Inthis estimation, the broadening effects caused by defects are notconsidered. However, the estimated results are in the range of theobserved GP thickness of less than 2–3 nm (Fig. 1). The experimentalresults are summarized in Table 1.

Fig. 4 shows the following two proposed structural models: (1) thesix-layered GPTR with C100O3±1 composition and (2) the three-layered GPCR with C100O6.5±2 composition in which oxygen is presenton the outer surfaces as epoxide and hydroxyl groups. In thesemodels, assuming there are no defects, the sheet areas for both GPTRand GPCR appear to be the same because of the use of the same rawgraphite and GO used for the chemical and thermal reductions. Thefact that the amount of oxygen in GPCR is about twice as large as that inGPCM is further evidence for the proposed structural model because ifoxygen is present at the interlayers or in the interlayer defects, theamount of oxygen should proportionally increase with the number ofGP layers. However, the amount of oxygen in the six-layered GPTRdecreases to ∼50% in comparison with that in the three-layered GPCR.Furthermore, in the structural models, the d002 value of 3.432–3.760 Åfor the reduced GPs is slightly larger than that of ∼3.4 Å for a typicalgraphite produced by the top-down process of physical exfoliation [1]or solvation-assisted exfoliation of graphite [2]. This implies that it isdifficult to match the lattices in the reduced GPs formed from thebottom-up process of layer-by-layer stacking. A combination of the

l for (a) GPTR and (b) GPCR.

Table 1Properties of GPCR and GPTR obtained by XRD patterns.

Graphene GPCR GPTR

Composition C100O6.5±2 C100O3±1a2θ 23.643° 25.940°bd (Å) 3.760 3.432cFWHM 7.6° 3.9°dThickness (Å) 10.7 20.9eNumber of layers(thickness/d)

2.8 6.1

Experimentalconditions

Chemical reduction(modified Hummer) RT

Thermal reduction Arambient 800 °C, 1 h

Error: a(±0.03), b(±0.02), c(±0.5), d(±0.6), and e(±0.3).

360 H.-M. Ju et al. / Materials Letters 64 (2010) 357–360

following effects may be responsible: formation of defects duringreduction, folding of a few layered GPs, and intrinsic nanocurvaturedistortion [4] existing in two-dimensional nanocrystals.

4. Conclusion

Thermal reduction of GO at 1073 K under N2 and ambient light andchemical reduction by the modified Hummers' method result in six-layered GPTR (C100O3±1) and three-layered GPCR (C100O6.5±2),respectively. The interlayer spaces in these reduced GP materials areslightly larger than those in typical graphite, and residual oxygen ispresent on the outer surfaces. These structures and compositions areintrinsic properties of the reduced GP formed by the “bottom-upprocess.” The experimental results and proposed structural models

can be helpful for gaining a deeper understanding of the two-dimensional carbon system and for fabricating a variety of GP or GOnanostructures.

Acknowledgement

This work was supported by the KICET project (KPP08017).

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