溶液稳定、高导电性波纹状石墨烯片

范承伟 张 新 陈 胜 王海芳 曹傲能

(上海大学纳米化学与生物学研究所,上海200444)

溶液稳定、高导电性波纹状石墨烯片

范承伟 张 新 陈 胜 王海芳 曹傲能*

(上海大学纳米化学与生物学研究所,上海200444)

特殊的单原子层二维sp2碳结构给石墨烯带来众多独特的性能和潜在的应用.然而,单层石墨烯容易聚集并会逐渐重新石墨化,这成为其应用的一个重要障碍.本文报道了一种新策略来解决这个问题,即通过在石墨烯表面引入sp2碳纳米结构作为永久的波纹来阻止石墨烯的聚集和石墨化,并使之在溶液中易于分散和稳定.和其他功能化方法不同,该方法没有引入杂原子,不破坏石墨烯的结构和功能.制得的石墨烯具有优异的导电性能(～65000 S·m-1),并具有较好的溶液稳定性.

石墨烯;波纹;碳纳米笼;可膨胀石墨;导电性;溶液稳定性

1 Introduction

Graphene was once regarded unable to exist.1Recent experiments and simulation suggest that it is the transient ripples in three dimension that make the two dimensional(2D)graphene stable.2-4The peculiar 2D structure offers graphene many unique properties and potential applications.5-9Many applications of graphene require solution-processable graphene sheets of high quality in quantity.6,10-20However,one serious obstacle is that graphene is prone to aggregate in suspension and gradually stacks back to graphite.

Currently,most of the solution-processable graphene is the chemically converted graphene or reduced graphene oxide (rGO).13,14,20-25Graphene oxide(GO)is soluble and can be produced cheaply and massively.GO also has many potential applications,26-28but it is electrically insulating and not suitable for many applications.Various methods have been developed to reduce GO and turn it into conductive rGO,10,15but rGO still has substantial content of oxygen,and no longer stable in solution.In some applications,an effective way to avoid the re-stacking of rGO is to synthesize the final graphene-composite material directly from soluble graphene oxide(GO),skipping the graphene-producing step,as we demonstrated previously.15But for many applications,the graphene-producing step is necessary.Chemical exfoliation of graphite or expanded graphite is a promising method to produce solution-processable high-quality graphene,11,12,29-31yet the suspension is not very stable,and the concentration of graphene is usually very low.To stabilize it in suspension,graphene is usually functionalized by either non-covalent or covalent methods.6,16,17,22These methods introduce heteroatoms and molecules onto graphene that usually deteriorate the structure and properties of graphene.Here we report a facile method to produce high-quality solution-processable graphene sheets.The basic idea is to introduce sp2carbon nano-islands on graphene sheet as permanent ripples to prevent the stacking and graphitization of graphene and to make graphene easy to re-suspend.Unlike most functionalization methods,the sp2carbon decoration does not deteriorate the structure and properties of graphene.Therefore,the obtained permanently rippled graphene sheets show significantly increased solubility as well as excellent electronic conductivity.

2 Experimental

2.1 Synthesis

The procedure to prepare the permanently rippled graphene (denoted as GC)is simple,as depicted in Scheme 1.In a typical experiment,0.2 g expandable graphite(EG,chemical grade,～300 μm,from Qingdao BCSM Co.,Ltd.,China)was mixed with 2 mg ferrocene(98%,chemical grade,Sinopharm Chemical Reagent Co.,Ltd.,China),and heated in a microwave oven (800 W,Galanz,G80D23CN2P-T7(B0),China)for 15 s.After microwaving,the volume of the sample expands to more than 200 times of the original.The entirely expanded graphite was then treated with 1.2 mol·L-1hydrochloric acid(Sinopharm Chemical Reagent Co.,Ltd.,China)at 60°C overnight to remove deposited iron particles,followed by washing with deionized water and ethanol.The sample was then dried at 80°C for 12 h.Five milligram of the above sample was sonicated mildly in 50 mLN-methylpyrrolidone(NMP,analytical grade,Sinopharm Chemical Reagent Co.,Ltd.,China)for 75 min under ambient condition.After centrifugation at 3000 r·min-1for 5 min to remove the unexfoliated graphite,the resulted black GCsuspension was stable for several days.EG was also treated following the above procedure without adding ferrocene,and the obtained graphene was denoted as GP.Nature graphite(chemical grade,Sinopharm Chemical Reagent Co.,Ltd.,China)was exfoliated in NMP at the same conditions,too.rGO was prepared for comparison by hydrazine-reduction as reported previously.15

Since the microwave reaction completed too quickly to capture the intermediates of the reaction,we also used a conventional chemical vapor deposition(CVD)method to slow down the process.To achieve good mixing,EG was first expanded alone(without ferrocene)by microwaving,then exfoliated in NMP.The suspension was centrifuged at 3000 r·min-1for 5 min,and the supernatant was filtered and dried.Two mg of the dried powder and 200 mg of ferrocene(because of the inefficiency of CVD method,extra ferrocene has to be used)were put in a quartz boat,and heated with CVD method at 600-950°C for a certain period of time.After cooling down to room temperature,the products were suspended in ethanol and examed by transmission electron microscope(TEM).

2.2 Characterization

Scheme 1 Synthesis process of the permanently rippled graphene

High-resolution TEM images and energy dispersive spectrometer(EDS)spectra were taken on a JEM-2010F microscope(JEOL,Japan).Samples were prepared by drop-casting onto holey carbon grids(300 mesh size)and dried at 80°C for 4 h.Field emission scanning electron microscope(FESEM) images were recorded on a JSM-6700F electron microscope (JEOL,Japan).Raman spectra were collected on a Renishaw invia plus laser Raman spectrometer(UK)with an excitation laser wavelength of 514.5 nm(5 mW)at room temperature. Samples for both FESEM and Raman were prepared by spraying the graphene suspensions on SiO2/Si substrate(SiO2thickness of 300 nm)and annealing under nitrogen gas at 400°C for 4 h.Fourier transform infrared(FTIR)spectra were recorded on a Thermo Nicolet Avatar 370 FTIR spectrometer(USA) with a resolution of 2 cm-1.X-ray photoelectron spectroscopy (XPS)data were collected on an AXIS ultra instrument(Kratos,UK)at 293 K,and a binding energy of 284.6 eV for the C 1s level was used as an internal reference.XPS peaks were deconvoluted using XPS peak 4.1.TGA measurement was performed on a TGA-SDT Q600(TA Instruments,USA)under a nitrogen flow(100 mL·min-1),at a rate of 5°C min-1.Electronic conductivity measurements were carried out on a SB100/A Test Unit(Qianfeng Electronic Instrument,China)using a four-point-probe head with a pin-distance of about 3 mm,samples were prepared by compressing graphene powder at 20 mPa into round disks with a diameter of 1.6 cm and thickness of 0.2 mm.X-ray powder diffraction(XRD)patterns were recorded using a D/MAX-2200 diffractometer(Rigaku,Japan), equipped with a rotating anode and with a Cu Kαradiation source(λ=0.154178 nm).UV-Vis absorption spectra were obtained on a HITACHI U-3010 spectrophotometer.The standard UV absorption at 259 nm versus GCconcentration curve was obtained;the concentrations were determined by weighing the graphene in suspension.

3 Results and discussion

Fig.1 TEM images of graphene(a,b)TEM images of GPand GC,respectively;(c,d)HRTEM images show the folded edges of single-layer and bi-layer GCsheets,respectively. Black arrow points to single-layer nano graphene meshes,and white ones point to carbon nanocages.

During the microwaving process,EG absorbed microwave energy efficiently and turned it into heat.Consequently,the temperature of EG rose quickly in a few seconds,and the interspacing between graphite layers was greatly expanded after microwaving(Scheme 1,bottom).Meanwhile,ferrocene in the mixture evaporated and intercalated into the expanded interspace of EG,where it decomposed and deposited iron nanoparticles(NPs)on the hot surface of EG.The typical HRTEM images of the GCsample are shown in Fig.1.As reported previously,11EG can also be expanded by microwaving in the absence of ferrocene,and then be exfoliated to produce graphene(GP) in organic solvents by sonication.For comparsion,the TEM image of GPis enclosed in Fig.1.GP(Fig.1a)shows featureless regions in the graphene sheet as reported,2while plenty of dark dots are observed on the GCsheets(Fig.1b).These scattered dots are pure carbon structure,since only carbon and copper (copper is from the supporting mesh)have been detected on the sheet by electron disperse X-ray spectroscopy(EDS)(see Fig.2).

The folded edges of GCprovide signatures to indentify the layers of the graphene sheet.2The dark straight lines of the folded graphene in Figs.1c and 1d are signatures of folded single-layer and bi-layer graphene sheets,respectively.Interestingly,the folded edges of GCalso provide a perpendicular view of the decorated carbon islands.Nano graphene meshes(indicated by black arrows in Fig.1)and carbon nanocages(indicated by white arrows in Fig.1)can be identified.All these carbon nano-islands serve as permanent ripples on graphene sheets to stabilize the graphene.These carbon nanocages formed in the presence of small amount of ferrocene(Fig.1d)are single-layer cages.When the amount of ferrocene in the reaction mixture increases,the carbon nanocage grows bigger and consists of multiple layers(Fig.2a).

Raman spectrum is widely used to determine the number of layers of graphene sheets.Fig.3a shows the Raman spectra of GC,GP,EG,and rGO.The IG/I2D(intensity ratio of G peak to 2D peak)of GPis very similar to that of EG,indicating that GPis not well exfoliated and exists mainly as multiple layers or graphite-like.This is consistent with the TEM investigation that thin-layer sheet of GPis rare.The IG/I2Dratio of GCis significantly smaller than that of GP,corresponding to thin-layer graphene sheets.12Compared with rGO,which shows a big D peak,there is a small D peak for GPand no D peak for GC,suggesting much less defect in GC.

Fig.2 HRTEM(a,b)and EDS(c)of carbon nanocage on GC(a)carbon nanocage with iron NP inside(before washing with HCl solution); (b)after washing with HCl solution to remove the deposited iron NP, the white square indicates the EDS(c)detection area.

Fig.3 (a)Raman spectra of expandable graphite(EG),rGO, GP,and GC;and(b)XRD patterns of GPand GC

The decorated carbon nanoislands can increase the yield of suspensible and thin-layer graphene,and make the suspension more stable.In comparison,after one circle sonication and subsequent centrifugation,the yield we obtained is～17%(mass fraction)for suspensible GP,while up to～35%for GC.And 48 h after the suspension,the concentration of GCin NMP is about 0.045 mg·mL-1,which is about 5-fold more than that of GP(see Fig.4).

Thermodynamically,the Gibbs free energy difference(ΔG) between the single-layer graphene sheets and the multilayer graphene/graphite can be calculated by ΔG=ΔH-TΔS.The reported transient ripple2on the single-layer graphene is an entropic term favourable for the single-layer graphene state.This might be one reason that graphite can be exfoliated by heating at above 1000°C.10Enthalpically,due to the unsaturated electronic valence,the single-layer graphene is unfavourable compared to the multiple-layer graphene and graphite.Overall, graphite,not the single-layer graphene,is the most stable state, thus graphene sheets tend to stack up.

Fig.4 UV-Vis absorbance spectra of GCand GPInset shows the photos of the GCand GPsuspension.Curve a corresponds to the 4-fold diluted GPsample of the inset bottle a;Curve b corresponds to the 10-fold diluted GCsample of the inset bottle b.

When the single-layer graphene sheets stack into bi-to multiple-layer sheets,the strong interactions between graphene layers make the stacked layers more and more rigid.This is why the transient ripples on bi-layer graphene sheets become smaller than that on single-layer ones,and disappear eventually on multiple-layer ones,according to the published results.2For the few-layer graphene,although it is enthalpically more favourable than the single-layer one,it loses the flexibility of the single-layer sheet(entropically unfavourable).

For GC,the permanent ripples,namely carbon nanoislands, compensate the enthalpy of the single-layer graphene,and the scatterance feature of the nanoislands keeps the graphene sheet flexible.Therefore,GCis entropically favourable compared to GP.This may explain why GCis more stable than GPin suspension.

It has to be pointed out that,in solution,there is also a big entropical penalty for the single-layer graphene sheet,originated from the ordering of solvent molecules when bounded to graphene sheets.The tighter the binding between the solvent molecule and graphene sheet is,the more ordered the solvent molecules around graphene sheets are,and the larger solvent entropical penalty is.This solvent entropical penalty might be the major driving force for the aggregation of graphene sheets in solution,as the hydrophic effect(solvent entropical penalty originated from the ordering of water molecules around hydrophobic amino acids)is the major driving force for the protein folding.32,33So,currently,there is no solvent that could dissolve graphene into stable solution.Without introducing repulsion force and reducing sticking between graphene sheets by chemical modification,thin-layer graphene sheets will eventually aggregate and precipitate.Therefore,a more realistic approach to prevent the aggregation and stacking of graphene sheet is not trying to find a better solvent,but trying to prevent the graphitization of the aggregates.Adding permanent sp2carbon ripples on graphene sheet is this kind of approach.It is superior to those functionalization methods,because it does not bring in exotic heteroatoms.Moreover,because of the lack of large area of perfect matching surfaces between permanently rippled graphene sheets,the permanent ripples would prevent the graphitization of GCprecipitates.Consequently,GCprecipitates are easily re-suspended by mild sonication,even compressing the GCprecipitates into bulk solid form,as depicted in Scheme 1. XRD results(Fig.3b)show that the area of the(002)peak of solid GCpowder is only about one-sixth of that of GP,indicating much less degree of crystallization(graphitization)for GC.

One of the most amazing properties of graphene is its excellent electronic conductivity.However,the conductivity of graphene produced by the many reported solution-processable methods is very poor.Due to existence of a lot of oxygen content,the electronic conductivity of rGO is not good,usually in the order of 10-103S·m-1,depending on the reduction procedures.13-15,21,34,35Solvent-exfoliation of natural graphite can produce graphene of relatively high quality.Even so,the conductivity of the obtained graphene is just comparable to some rGO films,12much lower than expected for the“pristine graphene”. This is probably due to the fact that there are strong interactions between the graphene layers in graphite,thus the strong sonication treatment is necessary to exfoliate it.The strong sonication would produce many defects on the exfoliated graphene sheets,and then decrease the conductivity of the graphene.

The expanded interlayer spacing of expanded graphite makes it easier to be exfoliated.Therefore,only much milder sonication is required to expand it.In comparison,at our mild sonication condition,the yield of suspensible graphene from the sonication of natural graphite powder is less than 2%,and no few-layer graphene has been identified.

Milder sonication introduces less defect.As expected,GCshows excellent conductivity of～65000 S·m-1,which is about one order of magnitude higher than that of the above reported pristine graphene.12Since the data from different laboratories and different methods might be not comparable,we also prepared the hydrazine-reduced rGO and measured its conductivity as a reference.The measured conductivity for rGO is～140 S·m-1,which is in consistent with the reported values.15,21,34,35Interestingly,the conductivity of GCis also significantly higher than that of GP(～48000 S·m-1),which is produced at the exact sonicating condition.

Fig.5 TEM image capturing the etching paths(outlined by red dotted-lines)on EG sheet by iron NPs and the formation of carbon nanocages around iron NPs(indicated by blue arrows)

Since the microwave reaction completes too quickly to capture the intermediates of the reaction,we used a conventional CVD method to slow down the process and investigate the mechanism of the formation of the permanent rippled graphene structure.As shown by TEM image(Fig.5),the deposited iron NPs can etch the EG sheet and leave nano graphene meshes on the graphene sheets.Along the etching paths,those etched carbon atoms are dissolved in the iron NPs and eventually turned into carbon nanocages.During the producing process of GC, iron NPs might preferably deposit on the defects of EG,which are also the sites where the expansion of EG originates,so the defects might be more likely etched or eliminated.This is likely the mechanism for the decrease of defects in GC.

The decrease of defects has been evidenced by XPS results (Fig.6a).Deconvolution of the C 1s peak shows that there are more than 88%sp2carbon atoms in GC,much higher than that of GP(73%).The rest peaks of GCmainly come from the residual NMP solvent molecules on graphene sample,which is consistent with the literature.12This result indicates little disordered carbon on GC,suggesting that almost all the decorated carbon nanoislands are sp2carbon structures.The defect decrease of GCis also evidenced by the FTIR spectra(Fig.6b) and thermogravimetric analysis(TGA)(Fig.6c).FTIR spectra show that GCis very similar to nature graphite,while GPcontains many oxygen-containing groups such as the peaks around 1090 cm-1(vC―O)and 1260 cm-1(vC―O―C).36TGA shows that GCis the most stable one,with 92%mass remaining after heating to 800°C,while the remained masses for GPand rGO are 86% and 77%,respectively.In addition,the absence of D peak at 1350 cm-1in Raman spectrum of GCindicates the less disordered carbon structure,demonstrating the high-quality of GC.

Fig.6 Valency and stability of graphene prepared by different methods(a)XPS spectra of rGO,GP,and GC;(b)FTIR spectra of natural graphite,rGO,GP,and GC;(c)TGAcurves of rGO,GP,and GC

4 Conclusions

In summary,we report here a facile method to produce solution-processable graphene sheets from EG.The essence of this method is to stabilize graphene sheets and prevent their re-stacking by introducing permanent ripples on the graphene sheet,i.e.,decorating carbon nanoislands on the graphene sheets.Since the decorated nanoislands are sp2carbon structure as that of graphene sheet,they do not deteriorate the graphene and introduce heteroatoms into the system.Thus graphene provides an excellent electronic conductivity up to 65000 S·m-1. The permemantly rippled graphene is readily suspensible in NMP solvent with concentration up to 0.045 mg·mL-1,showing great potential in wide range of solution-processable applications of graphene.

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Solution-Processable,Highly Conductive,Permanently Rippled Graphene Sheets

FAN Cheng-Wei ZHANG Xin CHEN Sheng WANG Hai-Fang CAO Ao-Neng*
(Institute of Nanochemistry and Nanobiology,Shanghai University,Shanghai 200444,P.R.China)

The single atom thick sp2carbon structure of graphene gives rise to its unique properties and potential applications.However,one serious obstacle for its application is that graphene is prone to aggregate in suspension and gradually stack into graphite.Here,we report a novel approach to solve this problem.The basic idea is to introduce sp2carbon nano-islands on the graphene sheets that act as permanent ripples to prevent the stacking and graphitization of graphene and make it easy to re-suspend. Unlike most functionalization methods,this approach avoids the introduction of heteroatoms.Thus,it does not deteriorate the structure and change the properties of graphene.The carbon-rippled graphene has a remarkable electronic conductivity of～65000 S·m-1,and can be readily suspended in solvent.

Graphene;Ripple;Carbon nanocage;Expandable graphite;Conductivity; Solution stability

10.3866/PKU.WHXB201209103

∗Corresponding author.Email:ancao@shu.edu.cn;Tel:+86-21-66135277.

The project was supported by the National Natural Science Foundation of China(21073117),National Key Basic Research Program of China(973) (2009CB930200,2011CB933402),and Shanghai LeadingAcademic Disciplines,China(S30109).

国家自然科学基金(21073117),国家重点基础研究发展规划项目(973)(2009CB930200,2011CB933402)和上海市重点学科(S30109)资助

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