Journal of Physics and Chemistry of Solids 74 (2013) 1608–1614

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Journal of Physics and Chemistry of Solids

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journal homepage: www.elsevier.com/locate/jpcs

Electrocatalytic oxidation of methanol on Pt catalyst supportedon nitrogen-doped graphene induced by hydrazine reduction

Yuanyuan Zhao b, Yingke Zhou a,b,n, Ryan O’Hayre c, Zongping Shao b,nn

a State Key Laboratory Breeding Base of Refractories and Ceramics, College of Materials and Metallurgy, Wuhan University of Science and Technology,Wuhan 430081, PR Chinab State Key Laboratory of Materials Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology,Nanjing 210009, PR Chinac Department of Metallurgical & Materials Engineering, Colorado School of Mines, Golden, CO 80401, US

a r t i c l e i n f o

Article history:Received 10 December 2012Received in revised form13 May 2013Accepted 6 June 2013Available online 15 June 2013

Keywords:A. MetalsA. NanostructuresB. Chemical synthesisD. Electrochemical properties

97/$ - see front matter & 2013 Elsevier Ltd. Ax.doi.org/10.1016/j.jpcs.2013.06.004

esponding author at: State Key Laboratory Bamics, College of Materials and Metallurgy,hnology, Wuhan 430081, PR China. Tel./fax: +responding author. Tel./fax: +86 2768 862928ail addresses: zhouyk888@hotmail.com (Y. Zhnjut.edu.cn (Z. Shao).

a b s t r a c t

Hydrazine is often used to reduce graphene oxide (GO) to produce graphene. Recent observationssuggested that when hydrazine is used to reduce GO, the resulting reduced graphene actually containscertain amounts of nitrogen dopants, which may influence the properties of the obtained material, and insome cases may be deployed for beneficial advantage. In this work, we prepared graphene oxide by thechemical oxidation method, then used either hydrazine or sodium borohydride (as a control) to reducethe graphene oxide to graphene and to explore the nature of the nitrogen functionalities introduced byhydrazine reduction. Pt nanoparticles were then deposited on the nitrogen doped (hydrazine-reduced)and undoped (control) graphene substrates, and the morphology, structure, and electrocatalyticmethanol oxidation activity were characterized and evaluated. The results show that the nitrogenfunctional groups introduced into the graphene by hydrazine reduction greatly improve the electro-catalytic activity of the underlying Pt nanoparticles towards the methanol oxidation reaction.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Graphene, a monolayer of carbon atoms tightly-packed into atwo-dimensional (2D) honeycomb lattice, has attracted intenseworldwide interest from both fundamental research and commer-cial application perspectives because of its unique structure andproperties, such as high surface area, high chemical stability,excellent conductivity [1], unique graphitic basal-plane structureand the ease with which it can be produced and modified [2,3].Owing to its unique structure and properties, graphene andchemically-modified graphene have been explored for variousapplications, especially for use in various energy conversion andstorage devices, mechanical resonators, as a substrate for immo-bilizing metal and oxide catalysts, or as metal-free catalysts inoxidation reactions [4–9] The synthesis of graphene has becomeone of the key issues for the commercial development of graphene-based devices and materials. In the past few years, many methods,such as micromechanical exfoliation, chemical vapor disposition,

ll rights reserved.

reeding Base of RefractoriesWuhan University of Science86 2768 862928..ou),

epitaxial growth on electrically insulating surfaces, and the chemi-cal reduction of graphene oxide (GO), have been used to preparesingle-layer or multi-layer graphene [4,5]. Chemical synthesisapproaches can produce graphene in gram-scale quantities, andare thus more practical for large-scale commercial applications. Inparticular, the reduction of GO is the most common method toobtain graphene in large quantities. The graphene oxide to gra-phene route can be accomplished via a number of chemical,thermal, or electrochemical reduction pathways [10–12].

Graphite oxide, mainly produced by the Brodie, Staudenmaierand Hummers methods, consists of a layered structure withnumerous oxygen functional groups (e.g. hydroxyl, epoxide, car-bonyl and carboxyl groups) decorating both the basal planes andedges, which significantly alter the van der Waals interactionsbetween the layers and endow the resulting GO with stronghydrophilicity. The level of the oxidation can be varied on thebasis of the method, the reaction and the graphite precursor used[10–12]. Although GO can be incorporated into homogeneouscolloidal suspensions, it is electrically insulating owing to thedisruption of the graphitic networks by the introduction of variousoxygen functional groups [13]. A wide variety of reducing agents,including hydrazine, hydrazine derivate, vitamin C, bacteria,sodium borohydride, pyrrole, and others have been used to reduceGO to graphene [14–16]. Very recently, it has been reported thatwhen using hydrazine as the reducing agent to reduce GO, the

Y. Zhao et al. / Journal of Physics and Chemistry of Solids 74 (2013) 1608–1614 1609

resulting reduced graphene actually contains a significant numberof nitrogen functional groups both at the platelet edges and on thebasal planes [17,18], which indicates that the incorporated nitro-gen functionalities might have certain effects on the propertiesand applications of thus obtained graphene, and reducing GO withhydrazine may offer a facile method to simultaneously realize thereduction of graphene oxide and doping graphene with nitrogenatoms at low temperature and low cost.

On the other hand, many attempts have been purposely madeto modulate the structural and electrical properties of graphene byintroduction of foreign dopants (e.g. via boron or nitrogen doping),as the doped graphene may have significant application potentialsin a number of areas [19–21]. One particularly promising applica-tion for nitrogen-modified graphene is its potential use as asupport material for direct methanol fuel cell electrocatalysts, anapplication which has only recently begun to be explored [22,23].In direct-methanol fuel cells, insufficient catalytic activity anddurability are key barriers to commercial deployment. In particu-lar, long time operation causes agglomeration and dissolution ofthe Pt nanoparticles decorating the carbon support, which drasti-cally lowers the activity by decreasing the amount of availablesurface area for the methanol electrochemical reaction. Recently,observations suggest that carbon-based catalyst support materialscan be purposely doped with nitrogen functionalities to createstrong, beneficial catalyst-support interactions, thereby improvingdurability and enhancing the catalytic activity of the supported Ptnanoparticles and thus reducing the overall amount of Pt metalrequired [24–28].

In this work, we have prepared graphene oxide by the chemicaloxidation method, and then used hydrazine to simultaneouslyreduce the graphene oxide to graphene and functionalize it withnitrogen groups. Graphene oxide was also reduced by sodiumborohydride (NaBH4) to obtain nitrogen-free undoped graphenefor comparison. Pt nanoparticles were then deposited on thedoped and undoped graphene substrates by NaBH4, and themorphology, structure, and electrocatalytic methanol oxidationactivity of the resulting catalysts were characterized and evalu-ated. Our work further proved the introduced nitrogen function-alities of graphene when reducing GO with hydrazine, and thebeneficial nitrogen-doping effects of graphene substrates on thesupporting Pt nanocatalysts, as the catalysts on doped graphenesubstrates showed greatly improved electrocatalytic activitytowards the methanol oxidation reaction, which demonstratedthat the hydrazine-based reduction of GO may be a promisingroute to prepare nitrogen-doped graphene and composite cata-lysts for fuel cell applications.

2. Experimental

2.1. Preparation of N-doped graphene

The graphene oxide (GO) used in this study was synthesizedfrom crystalline flake graphite by a modified Hummer's method.The as-synthesized GO was purified by a centrifuge at a rotationspeed 5000 rpm to remove unexfoliated GO particles, yielding abrown colloidal solution with a concentration of 7.44 mg/ml. TheN-graphene was synthesized by chemical reduction of the GOusing hydrazine hydrate as the reducing agent in a water solution.In a typical procedure, 13.44 ml of the brown 7.44 mg/ml GOcolloidal solution was mixed with 100 ml of deionized water in a250 ml round flask in an ultra-sonic bath for 1 h to ensure uniformdispersion of the GO. Then, 2 ml of hydrazine hydrate was addedinto the mixture which was magnetically stirred for 1 h. Thesolution was then heated at 100 1C for 24 h using an oil bathequipped with a magnetic stir-bar and a water-cooled condenser.

After that, the mixture was filtered and washed with deionizedwater several times then dried in a vacuum desiccator at 70 1C for24 h to remove residual solvent. The resulting nitrogen-func-tionalized graphene was denoted as N-G. For comparison,undoped graphene, denoted as G, was obtained by reducing thesame GO dispersion with NaBH4 instead of hydrazine.

2.2. Synthesis of Pt/G and Pt/NG catalysts

Pt nanoparticles were deposited on the N-G sheets by thechemical reduction of chloroplatinic acid in water using sodiumborohydride as the reducing agent at room temperature. In atypical experiment, 50 mg of G or N-G was dispersed in 200 ml ofwater in a beaker by ultrasonication for 30 min before 3 mlchloroplatinic acid solution (20 mg/ml H2PtCl6.H2O) was addedto the solution followed by further ultrasonication for an addi-tional 30 min. Excess sodium borohydride with a weight of 1.9 gwas slowly added to the solution with magnetic stirring for 24 h atroom temperature, yielding Pt/G or Pt/N-G decorated compositecatalysts. The Pt/G or Pt/N-G catalysts were separated from thesolution by vacuum filtration using a Buchner funnel and furtherwashed with deionized water. The resulting products were driedin a vacuum desiccator for 24 h at room temperature.

2.3. Characterization of catalysts

Field emission scanning electron microscopy (SEM) and trans-mission electron microscopy (TEM) images were taken on aHitachi S-4800 SEM (Hitachi, Japan) and a JEM-200CX TEM (JEOL,Japan), respectively, to investigate the morphologies of the Pt/Gand Pt/N-G catalysts. X-ray powder diffraction (XRD) was per-formed using a Bruker D8 advance diffractometer with Cu-Kαsource radiation. The diffractometer was operated in the step-scanmode with a 0.02 step between 10 and 901 (2θ). Raman spectrawere recorded from 1000 to 2000 cm−1 on a Horiba HR 800 (JOBINYVON, France) with a 514 nm argon ion laser. X-ray photoelectronspectroscopy (XPS) was performed using a PHI 550 (Perkin-Elmer,US) apparatus using Al Kα radiation (hν¼1486.6 eV). C, H and Ncontent analysis of the samples was also carried out using anElementar Vario EL Ⅲ CHNS elemental analyzer.

2.4. Electrochemical measurements

The electrochemical properties of the samples were assessedvia cyclic voltammetry in a conventional airtight standard three-electrode cell using a Potentiostat/Galvanostat Model 273A elec-trochemical workstation (Princeton Applied Research) at roomtemperature. An Ag/AgCl electrode and a graphite rod electrodewere used as a reference and counter electrode, respectively. 5 mgof catalyst (Pt/G or Pt/NG) was dispersed in a solution consisting of900 μL absolute ethyl alcohol and 100 μL Nafion solution (5 wt% inwater) via ultrasonication for 30 min before 5 μL of this solutionwas dropped onto a glassy carbon electrode to form a thin layerwhich was used as the working electrode. The electrolyte, con-sisting of 0.5 M H2SO4 and 1 M CH3OH, was purged with highpurity argon for 15 min before each test. The cyclic voltammetrydata was obtained by cyclically scanning the potential between 0and 1 V (vs. Ag/AgCl) at scan rates of 10–200 mV/s.

3. Results and discussion

Fig. 1 shows typical SEM and TEM images of Pt/G and Pt/N-G.It is seen that the G and N-G are made up of many stacked layerswith numerous edges. Irregular Pt nanoparticles are dispersed onboth G and N-G sheets, and agglomerations occurred for both

2 3 4 5 60

5

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35

Freq

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y (%

)

Particle size (nm)2 3 4 5 6

0

10

20

30

40

50

Freq

uenc

y (%

)

Particle size (nm)

Fig. 1. SEM images (a) Pt/G and (b) Pt/N-G. TEM images (c) Pt/G and (d) Pt/N-G. Particle size histograms (e) Pt/G and (f) Pt/N-G.

Y. Zhao et al. / Journal of Physics and Chemistry of Solids 74 (2013) 1608–16141610

cases. The Pt nanoparticles could be better resolved using TEM,which enabled a rough particle size distribution analysis to beperformed, the results of which are displayed in Fig. 1e and f. Bothsamples showed similar particle size range, namely, around2–6 nm. For Pt/N-G, the percentage of Pt nanoparticles less than4 nm is around 85%, while for Pt/G sample, such percentage isaround 60%, indicating an increased prevalence of smaller Ptnanoparticles in the Pt/N-G sample. While the differences betweenthe two samples are relatively minor, the increased prevalence ofslightly smaller Pt nanoparticles in the Pt/N-G sample may be dueto the additional nucleation sites associated with the nitrogenfunctionalities that were introduced by the hydrazine-based reduc-tion process.

Fig. 2 shows XRD patterns of the Pt/G (a) and Pt/N-G (b) samples.The characteristic (002) diffraction peak of graphene is observed forboth samples, indicating the successful conversion of GO to gra-phene and the reestablishment of the conjugated graphene net-work (sp2 carbon) due to the reduction process in both cases, andthe broad feature in the patterns also indicates a short-range order

or amorphous nature of the reduced GO. For Pt/G sample, thecharacteristic (002) graphene diffraction peak is located at �251,while for the Pt/N-G sample, the (002) peak is located at �241.Further selected area electronic diffraction experimental results(not shown here) indicate that the Pt/N-G sample has slightly widerd-spacing than Pt/G, for example, 0.252 vs. 0.243 nm for the (101)plane. The slightly wider d-spacing obtained for the Pt/N-G com-pared to the Pt/G is consistent with the fact that the two-dimensional N-G sheets should be more expanded and disordereddue to the effects of the nitrogen incorporation [29]. For bothsamples, the peaks at the 2θ values of �39.6, 46.1, 68.1 and 81.31correspond to the (111), (200), (220) and (311) planes of face-centered cubic (fcc) Pt, respectively [30].

Further insights into the structural properties of the undopedand doped graphene supported Pt samples are obtained fromRaman spectroscopy characterization as shown in Fig. 3. TheRaman spectrum of Pt/G displays two prominent peaks: the Dpeak at 1354 cm−1 corresponding to graphitic step-edge and defectfeatures, and the G peak at 1605 cm−1 corresponding to the

Y. Zhao et al. / Journal of Physics and Chemistry of Solids 74 (2013) 1608–1614 1611

E2g mode of graphite, which arises from the in-phase vibration ofsp2-bonded carbon atoms. In the Raman spectrum of Pt/N-G, the Dband and G band located at 1350 cm−1 and 1596 cm−1, respec-tively, are both shifted to a lower frequency compared to the Pt/Gsample, indicating a reduction in the size of the in-plane sp2

domains and n-type doping caused by the introduced nitrogeninto graphene sheets. The D/G intensity ratio of the Pt/N-G sampleis 1.67 vs. 1.53 for the Pt/G sample, further corroborating adecrease in the average size of the sp2 domains in the Pt/N-Gsystem [31]. By applying peak fitting, the sp3 sub-band of the Gband in Pt/G was obtained at 1559 cm−1, while the sp3 sub-band ofthe Pt/N-G sample was blue-shifted to 1538 cm−1 and the areaunder the sp3 sub-peak was reduced, indicating the decreasedratio of the sp3 carbon [32].

Using XPS analysis, the chemical binding states associated withC, N, and Pt in the Pt/G and Pt/N-G samples were investigated [33].The core level XPS spectra of C1s, Pt4f, and N1s of the Pt/G andPt/N-G samples are shown in Fig. 4, and the relative intensities ofthe different carbon, platinum, and nitrogen species are summar-ized in Table 1. In brief, for Pt/G, the broad C1s spectrum rangingfrom 280 to 294 eV (Fig. 4a) exhibited a considerable degree ofoxidation and could be deconvoluted into three band componentscorresponding to the following functional groups: non-oxygenatedring C (C–C) at 284.6 eV, C in C–O bond at 286.5 eV, andcarboxylate C (O–C¼O) at 289 eV [31,34]. Compared to Pt/G, the

10 20 30 40 50 60 70 80 90

Inte

nsity

(a.u

.)

2θ (degree)

GE(002)

Pt(111)

Pt(200)

Pt(220) Pt(311)

Fig. 2. XRD patterns (a) Pt/G and (b) Pt/N-G.

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Raman Shift (cm-1)

1354

1605

Fig. 3. Raman spectra (a)

C1s spectrum of Pt/N-G (Fig. 4b) displayed an additional band at285.7 eV corresponding to C–N, indicating that the reductionprocess with hydrazine involved both de-oxygenation of thegraphene oxide and nitrogen incorporation into the graphene.The core level spectrum of Pt4f for the Pt/G sample (Fig. 4c) couldbe fitted with four components: Pt0(4f7/2) at 70.9 eV, Pt2+ at72.1 eV, Pt0(4f5/2) at 74.2 eV, and Pt2+ at 76.3 eV [35–37]; whilespectrum of the Pt/N-G sample (Fig. 4d) could be also reasonablydeconvoluted into four similar components. Because the workfunction of graphene at 4.48 eV is smaller than that of Pt at5.56 eV, electron transfer from the graphene sheets to Pt nano-particles likely occurs during the reduction-based synthesis of thePt-graphene hybrid structures [38]. In addition, we observed anoverall increase in the percentage of Pt0 components (Pt0(4f7/2)and Pt0(4f5/2)) in the Pt/N-G sample compared to the Pt/G sample.We hypothesize that these effects could reflect additional electrondonation effects associated with the nitrogen doping, which mightresult in further donation of electrons to the graphene and then toplatinum, thereby leading to an increase in the low binding energycomponents of Pt. Fig. 4e provides the N1s spectrum for the Pt/N-G sample, which could be deconvoluted into three components:pyridine N at 398.6 eV, pyrrole N at 400.5 eV and pyridine N–O at402 eV [39], with relative abundances of 23.6, 57.3 and 19.1%,respectively. In addition, both of the XPS survey scan results (scannot shown) and the elemental analysis results show that the Nconcentration is �2.5 at% for Pt/N-doped graphene, while N ishardly detectable for the undoped sample.

The electrocatalytic behavior of the Pt/N-G, Pt/G catalysts towardsthe oxidation of methanol was examined by cyclic voltammetry (CV)at room temperature in argon-saturated 0.5 M H2SO4+1 M CH3OHsolutionwith a forward potential scan from 0 to 1 V and then reversepotential scan from 1 to 0 V vs. Ag/AgCl, and the results are shown inFig. 5a and b. Both composite electrodes were cycled repeatedly untila steady-state CV curve was obtained. For both samples, the currentpeak around 0.7 V in the forward scan corresponds to electrolyticmethanol oxidation, while the current peak around 0.5 V in thebackward scan corresponds to the removal of the residual carbonac-eous species [40]. The peak current densities of methanol oxidationincreased with the increasing scan rate [41], as shown in Fig. 5c.While the basic CV curve shape is similar for the Pt/G vs. Pt/N-Gcatalysts, the peak current densities for the two catalysts weresignificantly different. For example, at the scan rate of 50 mV/s, thePt/N-G catalyst shows a methanol oxidation peak current densitythat is �6.6X higher than that of the Pt/G catalyst on a Pt-massnormalized basis (218.3 mA/mgPt vs. 33.2 mA/mgPt), demonstratingsignificantly improved electrocatalytic activity for the Pt/N-G catalyst.Based on previous experimental and theoretical studies of undoped

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Pt/G and (b) Pt/N-G.

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pyridine N pyrrole N pyridine N-O

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Pt (4f )

Pt (4f )

Pt Pt

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(a.u

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Binding Energy (eV)

404 402 400 398 396 394

294 292 290 288 286 284 282 280 294 292 290 288 286 284 282 280

82 80 78 76 74 72 70 6882 80 78 76 74 72 70 68

Inte

nsity

(a.u

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Binding Energy (eV)

Fig. 4. Core level XPS spectra: (a) C1s, (c) Pt4f of the Pt/G sample; (b) C1s, (d) Pt4f, (e) N1s of the Pt/N-G sample.

Y. Zhao et al. / Journal of Physics and Chemistry of Solids 74 (2013) 1608–16141612

and N-doped Pt/C model-catalysts, we attribute the improvedelectrocatalytic activity of the Pt/N-G catalyst to a number of factors:(1) as suggested by the TEM and particle size analysis, the Pt/N-Gcatalyst has a greater number of smaller Pt nanoparticles due to theincreased number of nucleation sites provided by the addition of thenitrogen functionalities into this material, which might result in alarger specific surface area and therefore more active sites formethanol oxidation per unit mass of Pt. This point could be furtherconfirmed by comparison of the electrochemically active surface area(ECSA) for both catalysts by measuring the cyclic voltammogramcharge collected in the hydrogen adsorption/desorption region from

the electroactive Pt surface, where ECSA value �54m2/g wasobserved for Pt/N-G, higher than �36m2/g of the Pt/G catalyst.(2) Moreover, we hypothesize that the nitrogen functional groupsintroduced in the hydrazine-treated sample may modulate the elec-tronic structure of the graphene substrate, and subsequently modulatethe electronic properties of the supported Pt catalysts via increasedelectron-transfer between the support and catalyst, as suggested bythe XPS analysis (Fig. 4 and Table 1). In particular, the increase in themetallic (low binding energy) components of Pt seen in the Pt/N-Gcatalyst systemmay lead to an intrinsic enhancement in the methanolelectrocatalytic oxidation activity for this material [24–28].

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4. Conclusion

Graphene oxide has been prepared by the chemical oxidationmethod. Subsequent reduction of the GO by hydrazine leads to theformation of a nitrogen-functionalized graphene product. Com-pared to undoped graphene which is obtained by the reduction ofGO by sodium borohydride, the nitrogen-doped graphene exhibitssignificant chemical and structural changes, which subsequentlymodulate the nucleation and growth behavior of loaded platinumnanopaticles, resulting in more particles with smaller particle size.The hydrazine-induced nitrogen functionalities also modulate the

Table 1Summary of position and assignments of C, Pt, and N components of the Pt/G and Pt/N

Pt/G

Position Attribution Percentage

C 284.6 C–C 87.8286.5 C–O 2.5289 O–C¼O 9.7289 O–C¼O 13.6

Pt 70.9 Pt0(4f7/2) 50.274.2 Pt0(4f5/2)72.1 Pt2+ 49.876.3 Pt2+

N

0

50

100

150

200

250

300

Cur

rent

(mA

/mg P

t)

v1/2

2 4 6 8

0.0 0.2 0.4 0.6 0.8 1.0-20

-10

0

10

20

30

40

50 10mV/s 20mV/s 50mV/s 100mV/s 200mV/s

Cur

rent

(mA

/mg P

t)

Potential (V)

Fig. 5. Room-temperature cyclic voltammograms of methanol oxidation on (a) Pt/G, (b) Pboth catalysts. Electrolyte: 0.5 M H2SO4+1M CH3OH, scan rate: 10–200 mV/s.

electronic properties of the deposited platinum nanoparticles dueto increased electron donation from the support, which wehypothesize may help at least partially explain the significantlyincreased electrolytic activity (�6–8X) for this catalyst towardsmethanol oxidation reaction. These results demonstrate that thehydrazine-based reduction of GO may be a promising route toprepare nitrogen-doped graphene-based composite catalysts forfuel cell applications, and underscore the fact that the effects ofintroduced nitrogen functionalities should not be ignored wheninvestigating the properties and applications of hydrazine-reducedgraphene-based materials.

-G samples.

Pt/NG

Position Attribution Percentage

284.6 C–C 63.2285.7 C–N 3.6286.5 C–O 19.6

70.9 Pt0(4f7/2) 69.974.2 Pt0(4f5/2)72.1 Pt2+ 30.176.3 Pt2+

398.6 Pyridine N 23.6400.5 Pyrrole N 57.3402 Pyridine N-O 19.1

-50

0

50

100

150

200

250

300 10mV/s 20mV/s 50mV/s 100mV/s 200mV/s

Cur

rent

(mA

/mg P

t)

Potential (V)

(mV/S)1/2

Pt/N-G

Pt/G

0.0 0.2 0.4 0.6 0.8 1.0

10 12 14 16

t/N-G and (c) the relationship of peak current density vs. square root of scan rate for

Y. Zhao et al. / Journal of Physics and Chemistry of Solids 74 (2013) 1608–16141614

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (No. 21003075), the Basic Research Program(Natural Science Foundation) of Jiangsu Province of China (No.BK2010558), the National Natural Science Foundation for Distin-guished Young Scholars of China (No. 51025209), the TechnologyFoundation for Selected Overseas Chinese Scholar (MOHRSS), thePre-Research Foundation of WUST (No. 250089), and the U.S. ArmyResearch Office under grant #W911NF-09-1-0528.

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