1-A novel synthesis of graphene by dichromate oxidation

Materials Science and Engineering B 167 (2010) 133–136

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Materials Science and Engineering

B

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /m s e

b

A novel synthesis of graphene by dichromate oxidation

Sourov Chandra,Sumanta Sahu,Panchanan Pramanik ?

Nanomaterials Laboratory,Department of Chemistry,Indian Institute of Technology Kharagpur,Kharagpur,West Bengal 721302,India

a r t i c l e i n f o Article history:

Received 3July 2009

Received in revised form 12October 2009Accepted 17January 2010Keywords:

Graphene sheets Graphene oxide Nano ribbon Zeta potential

a b s t r a c t

A novel synthetic route has been developed to produce very stable aqueous dispersed graphene sheets from the pristine graphite by oxidation with dichromate followed by reduction with hydrazine.The particle size,physical feature and zeta potential of graphene oxide show better than that of the modi?ed Hummer’s method.

? 2010 Elsevier B.V. All rights reserved.

1.Introduction

Graphene and graphene-based nano composites nowadays have much importance due to their exceptional electronic [1]and mechanical properties [2].For those properties graphene-based nano composites are unique materials for microelectrical devices [3],sensors [4],biomedicines [5],mechanic resonators [6],ultra-capacitors [7]etc.Graphene has been synthesized by various methods including scotch tape method [8],electrostatic deposition of graphene [9],thermal or chemical decomposition of graphitic materials [10],thermal [11],chemical [12]and mechanical [8]exfo-liation,chemical vapour deposition [13]etc.In case of chemical exfoliation,at ?rst graphite is converted to its oxide (GO)or epox-ide by modi?ed Hummer’s method [14,15]using KMnO 4or using potassium chlorate [16]or m-CPBA [17](in case of epoxide)as an oxidant.Chemical vapour deposition (CVD)method is the best pro-cess for the synthesis of graphene for microelectronic industry,and the material produced by solution route is not comparable with the material produced by CVD for this class of application.The graphene generated by solution-oxidation (chemical exfoliation)?nds some other areas of applications like sensor,catalyst-support,compos-ite,drug delivery,and gas storage.The graphene sheets produced by chemical oxidation have been used as the electrodes for ultra-capacitors or sensors due to their high surface area,as reported by Ruoff and co-workers [7].Graphene oxide is highly dispersible in water and it could be easily deposited onto SiO 2substrates by spin coating.After reduction it shows reasonable electrical con-

?Corresponding author.Tel.:+919434016995;fax:+913222255303.

E-mail addresses:panchanan 123@871c16828762caaedd33d49c ,pramanik1946@871c16828762caaedd33d49c (P.Pramanik).

ductivity and hence it generates graphene-based semiconductors [3,18].

All the available methods have many serious inconveniences.When KMnO 4is used,the removal of MnO 2generated from the reaction involves a tedious process using H 2O 2.Again when KClO 3is used,it generates a lot of chlorine dioxide gas and the mix-ture is highly hazardous.Similar situation is with m-CPBA as an oxidant.To overcome all these inconveniences,the process that involves a simple oxidation by acidi?ed dichromate has been devel-oped through this investigation.It does not involve the removal of any precipitated product from the reaction mixture.The precip-itate of exfoliated graphite oxide is sonicated to form separated graphene oxide sheet and ?nally graphene sheet is formed by conventional reduction with hydrazine.The quality of chemically produced graphene-oxide and graphene by this process is similar to that of the reported one.So it may ?nd applications in place of GO and graphene (by Hummer’s method)in various ?elds.2.Experimental procedure

In this procedure,at ?rst graphite oxide was prepared from expandable graphite using K 2Cr 2O 7as an ef?cient oxidant.Brie?y,5g of graphite and 3.75g of NaNO 3were mixed in a conical ?ask.Then the mixture was kept in an ice bath and 375ml of conc.H 2SO 4was poured into it with constant stirring.Subsequently 37.6g of K 2Cr 2O 7was slowly added over about 2h with constant stirring.Stirring was again continued for 2h in ice bath.After that mixture was continuously stirred for 5days at room temperature.

The initial colour of the solution became dark yellow,which turned to dark green after 4days.After 5days of stirring 750ml of 5%aqueous H 2SO 4was slowly added to the above mixture over about 1h and during mixing the temperature of the whole system

0921-5107/$–see front matter ? 2010 Elsevier B.V. All rights reserved.doi:

10.1016/j.mseb.2010.01.029

134S.Chandra et al./Materials Science and Engineering B

167 (2010) 133–136Fig.1.Raman spectra of (a)graphene oxide (CGO)and (b)expandable graphite.

was kept at 98?C.After complete addition of water it was again

kept at 98?C for 2h with constant stirring.Then the temperature

was reduced to the room temperature and the whole solution was

stirred vigorously for another 2h.The solid was separated from

the reaction mixture,using centrifuge to remove the water soluble

oxidant and other inorganic salts.

The collected solid was washed 6times with 3%H 2SO 4followed

by 3times with 3%HCl.Each time of washing,solid was suspended

by ultra-sonication and was collected by centrifugation.The resul-

tant graphite oxide was then readily exfoliated to completely

water dispersed graphene oxide (GO)by ultra-sonication.The well-

dispersed graphene oxide was ?nally reduced to graphene sheets

by the reduction with hydrazine.For the stabilization of hydropho-

bic graphene sheets,NH 3[19]or KOH [20]was added to the aqueous

dispersion of graphene oxide (GO)before the hydrazine reduction.

However uses of KOH introduced agglomeration on standing before

addition of hydrazine.This showed that potassium salt of graphene

oxide had poor solubility in water probably due to the same rea-

son as the introduction of divalent ion on the graphite oxide paper,

as reported by Ruoff and co-workers [21].Even it has also been

reported that hydrazine can alone be able to decrease the effec-

tive agglomeration without the addition of any base [22]as usually

appeared for NH 3,too.Stabilizing agents,basically organic or inor-

ganic bases,formed salts with carboxylic acid group of

graphene.

Fig.2.FTIR spectra of (a)expandable graphite and (b)graphite oxide synthesized

by dichromate

oxidation.Fig.3.UV–visible spectra of aqueous dispersed (a)MGO sheets and (b)CGO sheets.The photograph corresponds to the water dispersed MGO and CGO sheets.Without stabilizer an irreversible agglomeration was formed due to the hydrophobic nature of the product.In this investigation tri-ethanolamine (TEA)was used as a stabilizing agent introducing stability of the sol of graphene

inde?nite.Fig.4.(a)FE-SEM image of a single graphene sheet,(b)HR-TEM image of the reduced CGO sheet in the presence of KOH.

S.Chandra et al./Materials Science and Engineering B167 (2010) 133–136

135

Fig.5.HR-TEM images of the reduced CGO sheet(a)in low pH without the presence of any stabilizing base and(b)presence of TEA during the reduction.

3.Results and discussion

Zeta potential of the colloidal graphene oxide was measured at various pH,but for each case the value of zeta potential for the GO produced by K2Cr2O7(CGO)was more negative compared to the GO made by KMnO4(MGO)oxidation[19].At pH≈4,the zeta poten-tial for CGO in water was?38.6mV,whereas in TEA(pH≈12)the value was?48.7mV.Again the zeta potential value of CGO in TEA is more negative than that of the CGO in NH3or KOH.The Raman spectrum(Fig.1)for both the graphite powder and the CGO displays a peak at1582cm?1(G peak)and indicates the two-dimensional hexagonal lattice of the sp2carbons.In addition to this,another peak at1600cm?1(D peak)was also obtained for CGO nanosheets, which was probably due to the defects and disorder in the hexago-nal graphitic 871c16828762caaedd33d49cparison of Raman spectrum of graphite and graphene oxide(not the graphene)has shown the transformation of sp2to sp3hybridization of carbon after oxidation.For this purpose we have used the powder sample of both graphite and graphene oxide instead of the?lm on the silica substrate.However there are reports by Graf et al.[23]and Shen and co-workers[24]to measure the thickness of graphene sheets by Raman re?ection and contrast microscopy.Here we have measured the thickness by AFM.The FTIR spectrum(Fig.2)of the graphite oxide illustrates the presence of C C,C O,C–OH,C–O–C and C–O bonds from the peaks at1584, 1706,3416,1205and1059cm?1respectively.Again the UV–visible spectra(Fig.3)of water dispersed CGO nanosheet show a peak at 266nm whereas MGO nanosheet shows the peak at227nm.The values indicate that in CGO more double bond conjugations are intact than that of MGO sheets[19].The colour of the aq.dispersed MGO sheets is brown;whereas for CGO sheets it is black in nature. Fig.4(a)also shows the FE-SEM image of reduced CGO sheet.The FE-SEM image of the reduced CGO on the silica/SiO2substrate was clearly indicating the thin,individual sheet with the average size of 200nm.The HR-TEM images of the individual graphene layer are shown in Figs.4(b)and5,in which the former shows a graphene nano ribbon,whereas Fig.5(a)and(b)shows a nontransparent and a very good transparent sheet respectively.Figs.5(a),4(b)and5(b) are the TEM images of the graphene sheets made by the

reduction Fig.6.AFM images of the graphene sheets,made by reduction of spin coated graphene oxide(CGO).

136S.Chandra et al./Materials Science and Engineering B

167 (2010) 133–136

Fig.7.TGA curve of(a)graphite and(b)CGO sheet.

of the CGO sheets in acid medium(low pH),without the presence of any stabilizing base,presence of KOH and the presence of TEA respectively.The transparent sheet in the presence of TEA indicates the thin sheet of graphene.Finally,from the AFM image the thick-nesses of the graphene sheets have been measured,which are given in Fig.6.The thicknesses of the graphene sheets are in between 3.9and12nm,which are produced by the deposition and reduc-tion of the high concentrated(≈10mg/ml)CGO sheets on mica.The reported papers of Hou and co-workers[25]and Si and Samulski [26]have agreed that1nm thickness indicates one sheet.Therefore the thicknesses of the graphene sheets indicate that they consist of4–12layers.The thermal stability of this CGO paper has been characterised by thermogravimetric analysis.All the analyses have been conducted under the nitrogen atmosphere over a temperature range of50–800?C with a slow ramp rate of2?C/min.The thermo-gravimetric analysis(TGA)curve(Fig.7)of CGO sheet has shown a minor weight loss at100?C,due to the loss of water.Another sig-ni?cant peak for the weight loss of CGO sheets has been observed at130–280?C(～23%)due to the loss of CO,CO2and steam from the sample.This value is reported as～30%in the case of MGO sheets[15].The quite low value for the CGO sheets is probably due to the decreased amount of oxygen functional groups in them, which also concluded that the CGO sheets are less hydrophilic than the corresponding MGO sheets.Therefore the TGA curve of CGO reveals the percentage of oxygen and water present in CGO, compared with MGO.Again the weight loss at100?C is very low, which signi?es that CGO contain a lesser amount of water as com-pared to MGO.The weight loss in TGA analysis for CGO and the modi?ed graphene oxide made from MGO[20]are very similar indicating that they are chemically close.No signi?cant weight loss was observed for the above temperature range of the expandable graphite.

4.Conclusions

?A simple chemical route has been developed for the prepara-tion of water dispersed nano-sized graphene oxide and graphene

multilayered sheets through oxidation of expandable graphite by acidic dichromate.

?The process is simpler and less hazardous than all existing pro-cesses.

?Here dichromate also acts as a self-indicator during the whole oxidising process as well as separation of the oxidant.

?In this experiment TEA serves as a stabilizing agent as well as base,which decreases the agglomeration of the graphene sheets in water.

Acknowledgements

The authors acknowledge Technology Information and Fore-casting Assessment Council(TIFAC)and Department of Science and Technology,Government of India for funding and authority of Indian Institute of Technology,Khargapur for providing working facilities.

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