当前位置：文档库 › Simultaneous electrochemical synthesis of few-layer graphene flakes on both electrodes in protic ion

Simultaneous electrochemical synthesis of few-layer graphene flakes on both electrodes in protic ion

This journal is

The Royal Society of Chemistry 2013

https://www.wendangku.net/doc/5f16507020.html,mun.,2013,49,5301--5303

5301

Cite this:https://www.wendangku.net/doc/5f16507020.html,mun.,2013,49,5301

Simultaneous electrochemical synthesis of few-layer graphene flakes on both electrodes in protic ionic liquids ?

Min Mao,a Mengmeng Wang,a Junyan Hu,a Gang Lei,a Shuzhen Chen a and Hongtao Liu*ab

Here we illustrate a simple and moderate electrochemical strategy to simultaneously harvest high-quality few-layer graphene flakes (o 5layers)from both a graphite anode and a graphite cathode in the protic ionic liquids.Specifically,the graphene flakes detached from cathodic graphite receive a defect healing.

Graphene,a one-atom-thick planar sheet of sp 2-bonded carbon with a two-dimensional hexagonal lattice structure,has drawn tremendous attention during recent years.On the basis of its novel properties such as intrinsically superior electrical conductivity,high specific surface area,excellent mechanical strength,and remarkable thermal conductivity,graphene has been viewed as one of the most promising materials in the fields of electronics and photonics,energy storage and conver-sion,biosensing and catalysis,etc.1Accordingly,exploration of the production of graphene in appealing for various applica-tions is significantly important as the first production of graphene by micromechanical cleavage 2is time-consuming and small-scale.To date,there are only two methods potentially capable of mass production of few-layer graphene (FLG)flakes although many other attempts such as direct exfoliation in solvents,3unzipping of carbon nanotubes,4and organic synthetic protocols 5have been carried out.Chemical oxidation–reduction of pristine graphite 6is the most commonly used strategy for preparing reduced graphene oxide flakes on a large scale.However,this approach employs harsh oxidants and corrosive acids,causing heavy defects in the products.Additionally,the operation process is fussy and time-consuming.The other popular method for scalable production of graphene is based on chemical vapor deposition (CVD)and epitaxial

growth techniques.7This approach is used for growing FLG films on appropriate catalyst supports with relatively low levels of defects and impurities.However,the process is very complex and generally involves transfer of graphene to a desired substrate for https://www.wendangku.net/doc/5f16507020.html,pared to other approaches,this method nevertheless requires expensive apparatus.

Recently,electrochemical synthesis of FLG was developed,which is considered as a cheaper and greener strategy.Liu et al.8and Lu et al.9peeled FLGs o?graphite at anodes in alkylimidazolium hexafluorophosphate (RIMPF 6)and alkylimid-azolium tetrafluoroborate (RIMBF 4)ionic liquids,in which large anions PF 6àand/or BF 4àwere intercalated into graphite interlayers by electric field force and split graphite sheets.While Wang et al.10and Zhong and Swager 11harvested high-quality FLG flakes at cathodes in lithium salt (or/and tetra-n -butylammonium (TBA)salt)-dissolved propylene carbonate (PC)electrolytes,in which the intercalation of Li +–PC (or/and TBA +–PC)complexes expanded graphite layers.To produce scalable yields of FLG flakes,the above-stated electrochemical strategies necessarily carried out a post-treatment step by either pro-longed sonication or elevated voltage (>5.0V)to exfoliate most of the expanded graphite.However,the prolonged sonication is supposed to break larger graphene flakes into small pieces,and generate more defects.A higher voltage may intrigue unexpected reactions and introduce more impurities as well as defects.Therefore it is desirable to improve the stated electrochemical synthesis of FLG flakes.

In this contribution,we demonstrate a more e?cient electro-chemical synthesis of FLG flakes (Scheme 1),wherein,the highly expanded graphite at the cathode and the fully oxidized graphite at the anode were acquired in a protic ionic liquid (PIL)at a cell voltage of 3.0V (Fig.S1,ESI?),then they were separately subjected to mechanical grinding in the said PIL to achieve CFLG (cathodic FLG)and AFLG (anodic FLG)flakes.The superiority of the present approach over the reported strategies fundamentally lies in the following four points.Firstly,the method is mild (no harsh reactants)and facile to operate (a 3.0V graphite–graphite cell is shown in Scheme S1,ESI?);secondly,it is more e?cient and productive (both electrodes

Key Laboratory of Resource Chemistry of Nonferrous Metals

(Ministry of Education),College of Chemistry and Chemical Engineering,Central South University,Changsha 410083,China b

State Key Laboratory of Powder Metallurgy,College of Powder Metallurgy,Central South University,Changsha 410083,China.E-mail:liuht@https://www.wendangku.net/doc/5f16507020.html,;Tel:+8673188830886

?Electronic supplementary information (ESI)available:Experimental procedure,diagrammatic drawing of ‘‘graphite–graphite cell’’,digital graphic of samples,AFM,HRTEM and FT-IR spectra,and CV results.See DOI:10.1039/c3cc41909f

Received 14th March 2013,Accepted 22nd April 2013DOI:10.1039/c3cc41909f

https://www.wendangku.net/doc/5f16507020.html,/chemcomm

ChemComm

D o w n l o a d e d b y C e n t r a l S o u t h U n i v e r s i t y o n 14/05/2013 12:13:59. P u b l i s h e d o n 23 A p r i l 2013 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 3C C 41909F

5302

https://www.wendangku.net/doc/5f16507020.html,mun.,2013,49,5301--5303

This journal is

The Royal Society of Chemistry 2013

in a cell simultaneously generate flaky expanded graphite);thirdly,the used IL is not only environmentally friendly,but also e?ective in expanding graphite and preventing graphene restacking;12fourth,mechanical grinding should harvest higher quality FLG flakes since shear forces are more e?cient in detach-ing graphene from graphite sheets and unlikely to generate severe surface defects as in the case of sonicated exfoliation.13

Herein,we select a protic IL 1-butyl-3-methyl imidazolium bisulfate (BMIMHSO 4)rather than any aprotic IL for the purpose of providing equivalent H +cations which can play crucial roles in splitting the cathodic graphite.Lu et al.9carried out the experiments using two graphite electrodes in BMIMBF 4IL and proved that the BMIM +cations were unable to insert into graphite interlayers without water.The dissociation of water at the cathode promoted the initial intercalation.In our work,the extra water was unnecessary since the BMIMHSO 4PIL itself contained equivalent protons that were reduced to H 2gas,and could e?ectively open up the ends of the cathodic graphite layers.The initial expansion at the ends facilitated the larger imidazolium BMIM +cations to intercalate the graphite inter-layers,and with the increasing H +and BMIM +cations inter-calation,the graphite layers were intended to totally expand and even exfoliate.Whereas,the anodic graphite underwent oxidation to generate oxygenated groups (e.g.C–O–C,C–OH,C Q O)that destroyed the surface sp 2conjugative carbon struc-tures.These defects deformed the graphite edges,facilitating the electrochemical intercalation of SO 4à.Su et al.14examined di?erent anions including Br à,Cl à,NO 3à,SO 42àfor the electrochemical exfoliation of graphene flakes,and found that only SO 42àanions exhibited ideal exfoliation e?ciency.To reduce the strong oxidation of graphite by H 2SO 4,KOH was added to make the pH around 1.2,and at a low voltage of 2.5V easily exfoliated SO 42à-intercalated graphite was obtained.We observed that the mild BMIMHSO 4PIL is obviously superior to the electrolyte H 2SO 4+K 2SO 4(pH =1.2)for SO 42àintercalation at a voltage of 3.0V.The low defective flakes were harvested,and the AFLG dispersions were found to be dark in colour instead of yellowish.

To thoroughly exfoliate graphite sheets,we ground the expanded cathodic graphite and the surface-oxidized anodic graphite separately with a small amount of BMIMHSO 4PIL in an agate mortar.Due to the shear force and p –p interaction between the imidazolium ring and the graphene carbon,the graphene layers were e?ectively detached by the imidazolium PIL without restacking.The intercalatants were removed by centrifugation in a 1:1(by volume)dimethyl formamide (DMF)and acetone solution.The collected CFLG and AFLG flakes could be well dispersed in DMF in stock (Fig.S2,ESI?).

The exfoliated AFLG and CFLG flakes (Fig.1a and d)present the folded graphene sheets with crumples.The number of monolayer graphene can be estimated by measuring the thick-ness of the sheets using atomic force microscopy (AFM,Fig.S3and S4,ESI?).Because of crumples and curls on graphene,the calculated layer number is larger than the real layers,especially for less than 5-layer FLG flakes.A direct observation using high-resolution transmission electron microscopy (HRTEM)can clearly reveal the 1–3layer FLG flakes with shallow crumples (Fig.S5,ESI?)and lattice spacing of 0.35nm (Fig.1c).Also the symmetric 2D band (2700cm à1)in Raman spectra for the AFLG and the CFLG identifies the successful fabrication of few-layer graphene (2–3layers).15,16The hexagonal patterns of the electron di?raction indicate the sp 2carbon frameworks with low defects (Fig.1b,c,e,and f).

The carbon defects were further confirmed by the Raman spectra at an excitation laser of 532nm,in which the D peak (B 1350cm à1)and the small D 0shoulder peak (B 1620cm à1)indicate the disorder of the edge carbons whereas the G band (1580cm à1)is related to the ordered in-plane sp 2carbon atoms.Therefore,the intensity ratio (I D /I G )is generally accepted to represent the defective carbon fraction.17,18As shown in Fig.2,the I D /I G value of the pristine graphite (PG),the

AFLG,

Scheme 1Illustration of simultaneous electrochemical synthesis of cathodic few-layer graphene flakes (a)and anodic few-layer graphene flakes

(b).

Fig.1TEM (a and d)and HRTEM (b,c,e and f)of CFLG (a–c)and AFLG (d–f).Insets of (b and c)and (e and f)are electron di?raction patterns of CFLF and AFLG,respectively.

Communication ChemComm

View Article Online

This journal is

The Royal Society of Chemistry https://www.wendangku.net/doc/5f16507020.html,mun.,2013,49,5301--53035303

and the CFLG is 0.11,0.19,and 0.04,respectively.The AFLG originated from the oxidation of the PG actually introduces some oxygenated groups causing partial disorder at the carbon edges.However,the I D /I G value (o 0.2)is the smallest for graphene oxides (chemical and/or electrochemical oxidation of graphite)ever reported.We attribute this low-level defect of the AFLG to the lower operation voltage (3.0V)and the milder electrolyte (BMIMHSO 4PIL),thus resulting in less destruction of sp 2carbon networks.The CFLG presents an I D /I G value of 0.04far less than the parent PG (I D /I G =0.11),implying e?ective defect healing.Bagri et al.19confirmed that the carbonyl and ether groups could not be removed without destroying the parent graphene sheet using thermal reduction methods.This may well account for the fact that the most reduced graphene oxides (rGO)present increased defects compared to the parent graphite carbons.A more e?cient reduction along with healing of the rGO was achieved through hydrogen treatment.In our case,the cathodically produced hydrogen plays dual roles in opening up the interlayers as well as repairing the C–O sp 3defects on the PG.Evidence from the FT-IR spectra (Fig.S6,ESI?)reveals the defect restoration in that the epoxy and alkoxy C–O stretch band (1000–1280cm à1)19of the CFLG is obviously absent.

The voltammetric characteristic curves shown in Fig.S7(ESI?)demonstrate prominent faradic reactions caused by oxygenated groups on the AFLG and the CFLG flakes (whereas the bare GCE shows no faradic reaction).The reductive processes starting from à0.05V (vs.Hg/HgO)make a great contribution to the enhanced electrochemical capacitance in the negative potential https://www.wendangku.net/doc/5f16507020.html,pared with the CFLG-loaded GCE,the AFLG-loaded GCE presents a more reductive capaci-tance because of more defective oxygenated surfaces available.However,the depressed electrical conductivity of the AFLG (0.02s cm à1)intrigues apparent polarization (slopewise contour)that is quite unfavorable for durable applications.The highly conductive CFLG (1.11s cm à1)is evidently more e?cient in practice because of the lower energy loss during charge–discharge cycling.

In conclusion,we have demonstrated an electrochemical graphite–graphite cell method to simultaneously acquire easily exfoliated graphite intercalation on both electrodes,and further harvest the high-quality AFLG and CFLG flakes with the aid of shear grinding.During the whole synthetic processes,the employed BMIMHSO 4PIL plays key but irreplaceable roles in both promoting graphite expansion and preventing graphene restacking.More importantly,the newly reduced hydrogen from the PIL presents a remarkable healing e?ect on graphene defects,which is especially useful to restore com-plete hexagonal carbon networks.Both the AFLG and the CFLG flakes show enhanced capacitive activity in the negative potential regions and are supposed to be better as negative electrode materials for supercapacitor applications.However,from a durable point of view,the higher conductive but lower defective CFLG flakes are more e?cient for electrochemical energy storage.The financial support from the NNSF of China (no.20976189),the Hunan NSF of China (no.10JJ2004),the Specialized Research Fund for the Doctoral Program of Higher Education of China (no.20090162120012),the SRF for the ROCS,SEM,and the Fundamental Research Funds for the Central South Universities (no.2011JQ024)is gratefully acknowledged.

Notes and references

1D.R.Dreyer,R.S.Ruo?and C.W.Bielawski,Angew.Chem.,Int.Ed.,2010,49,9336–9344.

2K.S.Novoselov,A.K.Geim,S.V.Morozov,D.Jiang,Y.Zhang,S.V.Dubonos,I.V.Grigorieva and A.A.Firsov,Science ,2004,306,666–669.

3T.Skaltsas,N.Karousis,H.J.Yan,C.R.Wang,S.Pispas and N.Tagmatarchis,J.Mater.Chem.,2012,22,21507–21512.

4D.V.Kosynkin,A.L.Higginbotham,A.Sinitskii,J.R.Lomeda,A.Dimiev,B.K.Price and J.M.Tour,Nature ,2009,458,U872–U875.5P.Kissel,R.Erni,W. B.Schweizer,M. D.Rossell, B.T.King,T.Bauer,S.Gotzinger,A.D.Schluter and J.Sakamoto,Nat.Chem.,2012,4,287–291.

6S.Park and R.S.Ruo?,Nat.Nanotechnol.,2009,4,217–224.

7S.Grandthyll,S.Gsell,M.Weinl,M.Schreck,S.Hufner and F.Muller,J.Phys.:Condens.Matter ,2012,24,314204.

8N.Liu,F.Luo,H.X.Wu,Y.H.Liu,C.Zhang and J.Chen,Adv.Funct.Mater.,2008,18,1518–1525.

9J.Lu,J.X.Yang,J.Z.Wang,A.L.Lim,S.Wang and K.P.Loh,ACS Nano ,2009,3,2367–2375.

10J.Z.Wang,K.K.Manga,Q.L.Bao and K.P.Loh,J.Am.Chem.Soc.,

2011,133,8888–8891.

11Y.L.Zhong and T.M.Swager,J.Am.Chem.Soc.,2012,134,17896–17899.12J.Lee and T.Aida,https://www.wendangku.net/doc/5f16507020.html,mun.,2011,47,6757–6762.

13N.G.Shang,P.Papakonstantinou,S.Sharma,G.Lubarsky,M.X.Li,

D.W.McNeill,A.J.Quinn,W.Z.Zhou and R.Blackley,https://www.wendangku.net/doc/5f16507020.html,mun.,2012,48,1877–1879.

14C.Y.Su,A.Y.Lu,Y.P.Xu,F.R.Chen,A.N.Khlobystov and L.J.Li,

ACS Nano ,2011,5,2332–2339.

15A.C.Ferrari,J.C.Meyer,V.Scardaci,C.Casiraghi,https://www.wendangku.net/doc/5f16507020.html,zzeri,

F.Mauri,S.Piscanec, D.Jiang,K.S.Novoselov,S.Roth and A.K.Geim,Phys.Rev.Lett.,2006,97,187401.

16D.Graf,F.Molitor,K.Ensslin,C.Stampfer,A.Jungen,C.Hierold

and L.Wirtz,Nano Lett.,2007,7,238–242.

17A.C.Ferrari and J.Robertson,Phys.Rev.B ,2000,61,14095–14107.18L.G.Cancado,A.Jorio,E.H.M.Ferreira,F.Stavale,C.A.Achete,

R.B.Capaz,M.V.O.Moutinho,A.Lombardo,T.S.Kulmala and A.C.Ferrari,Nano Lett.,2011,11,3190–3196.

19A.Bagri,C.Mattevi,M.Acik,Y.J.Chabal,M.Chhowalla and

V.B.Shenoy,Nat.Chem.,2010,2,

581–587.

Fig.2Raman spectra of pristine graphite,AFLG and CFLG at an excitation laser of 532nm.

ChemComm Communication

View Article Online