Eco-friendly one-pot synthesis of highly dispersible functionalized graphene nanosheets with

free amino groups

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Nanotechnology 24 (2013) 045609 (6pp) doi:10.1088/0957-4484/24/4/045609

Eco-friendly one-pot synthesis of highlydispersible functionalized graphenenanosheets with free amino groups

Zhiting Liu, Xuezhi Duan, Gang Qian, Xinggui Zhou and Weikang Yuan

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, 130Meilong Road, Shanghai 200237, People’s Republic of China

E-mail: xgzhou@ecust.edu.cn

Received 13 October 2012Published 8 January 2013Online at stacks.iop.org/Nano/24/045609

AbstractAn eco-friendly, facile and scalable hydrothermal approach, in which the reduction andfunctionalization of graphite oxide (GO) are completed in one pot, is proposed for thesynthesis of monolayer 3-aminopropyltriethoxysilane (APTES)-functionalized graphenes(A-FGs). Atomic force microscopy, transmission electron microscopy and x-ray diffractionanalyses indicate that the as-synthesized A-FGs consist of only one or a few layeredgraphenes, while x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopyand thermogravimetric analysis reveal that APTES is bonded to graphene by the dehydrationreaction between the Si–OH (produced by APTES hydration) and the –OH on the GO surface.As a result, free amino groups are left on the A-FGs. Moreover, A-FGs are highly dispersiblein dimethylsulfoxide, APTES and ethylene glycol, and their solubilities are up to 0.89, 4.03and 0.90 mg ml−1, respectively.

S Online supplementary data available from stacks.iop.org/Nano/24/045609/mmedia

(Some figures may appear in colour only in the online journal)

1. Introduction

Graphene is the two-dimensional form of nano carbon, andhas remarkable properties including extraordinary mechanicalstrength, great thermal conductivity and excellent electricalproperties [1]. This makes graphene very promising for widepotential applications such as microelectronics, electrodes,sensors and composite materials [2–8]. Many efforts havebeen made to develop low-cost and large-scale methodsfor graphene production. Compared to micromechanicalexfoliation [9], epitaxial growth [10], chemical vapordeposition [11] or bottom-up approach based on chemicalsynthesis [12], reduction of graphite oxide (GO) is morewidely used, not only because of the low cost and high yieldof the production, but also because of the chemical versatilityof GO for further functionalization [13–15].

In fact, the as-synthesized chemically reduced grapheneshave a strong tendency to stack, and usually undergo irre-versible aggregation. As a result, they are poorly dispersible

in most solvents and polymer matrices, leading to the poorprocessibility of chemically reduced graphenes. To solve theseproblems, the graphene is usually functionalized with aryldiazonium salts [16, 17], isocyanates [18], amines [19–22]and silanes [23], among which the functionalization withsilanes is more attractive when both the process safetyand the production cost are taken into consideration.Recently, Niu et al succeeded in functionalizing graphenenanosheets with 3-aminopropyltriethoxysilane (APTES) [24].The reaction, catalyzed by dicyclohexylcarbodiimide, wasbetween the epoxy groups of GO and the amino moietiesof APTES. Therefore, free Si–O–R groups were exposedon the as-prepared graphenes. However, the Si–O–R groupsare relatively inactive and hence are difficult to graft ontomost organic materials, such as polymers, or to anchor guestmolecules, such as biomolecules and metal ions of catalystprecursors. Compared to the Si–O–R groups, the aminogroups on APTES, which are nucleophilic, are much morereactive with acyl chlorides, acid anhydrides and metal ions

10957-4484/13/045609+06$33.00 c© 2013 IOP Publishing Ltd Printed in the UK & the USA

Nanotechnology 24 (2013) 045609 Z Liu et al

or nanoparticles [25], etc. If the amino groups are left onthe graphene nanosheets, the graphene will be more activeand find wider application as a catalyst, biosensor and inconducting polymer materials.

The traditional process for the synthesis of functionalizedgraphene through the reduction of GO not only involvesseveral steps of dispersion, filtration, washing and drying,which increases the cost of production, but also useshazardous materials as reducing agents or solvents suchas hydrazine and toluene, which makes the processenvironmentally unfriendly. Hence, there is a need to developa facile and eco-friendly method to prepare functionalizedgraphene.

Herein, we report a one-pot hydrothermal method toprepare APTES-functionalized graphenes (A-FGs) with freeamino groups. The whole synthesis process was performedin an autoclave, and only water, which is cheap and safe,was used as the reductant and solvent. The as-preparedA-FG nanosheets were characterized by transmission electronmicroscopy (TEM), atomic force microscopy (AFM) andx-ray diffraction (XRD). By combining x-ray photoelectronspectroscopy (XPS) measurement, Fourier transform infraredspectroscopy (FTIR) and thermogravimetric analysis (TGA),we confirmed that the GO had been reduced in the hy-drothermal process and the graphenes had been successfullyfunctionalized by APTES. Moreover, the dispersibility ofA-FGs in commonly used solvents is also studied.

2. Experimental details

2.1. Preparation of GO

Graphite oxide was synthesized from natural graphite bya modified Hummers’ method as originally presented byKovtyukhova and his co-workers [26]. Briefly, 12 g ofgraphite was first pretreated with a mixture of concentratedH2SO4 (50 ml), K2S2O8 (10 g) and P2O5 (10 g), followed bywashing and drying. For the oxidation step of the synthesis,KMnO4 (60 g) as the oxidant was added to a H2SO4 solution(460 ml) of the pretreated graphite and stirred at a temperaturebelow 10 ◦C. The mixture was then allowed to react at 35 ◦Cfor 2 h after which distilled water (920 ml) with a temperaturenot above 50 ◦C was carefully added. The mixture was stirredfor 2 h during which time 2.8 l of distilled water wasadded. Then 50 ml of 30% H2O2 was put into the mixture,producing a brilliant yellow color and bubbling suspension.The suspension was then centrifuged and washed with a totalof 5 l of 10% HCl solution followed by 5 l deionized waterto remove the acid. Finally the GO was dried in the oven at45 ◦C.

2.2. Synthesis of A-FGs and hydrothermal reducedgraphenes (HRGs)

A GO dispersion was prepared by dispersing 64 mg GOinto 64 ml distilled water under ultrasonication for 1 h. Itwas mixed with 16 ml 99% APTES and charged into a100 ml Teflon-lined autoclave, the temperature of which was

held at 60 ◦C for 6 h and then at 200 ◦C for 12 h. Theintermediates and final products are denoted as A-FGO andA-FGs, respectively. After the hydrothermal reaction, A-FGOand A-FGs were washed with abundant ethanol and waterto remove the residual APTES, and then dried at 100 ◦Cin a vacuum oven. To compare with the A-FGs, the aboveprocedure was exactly followed but without using APTES,and the final product is denoted as HRGs.

2.3. Characterizations

The microstructures and morphologies of the samples werecharacterized by TEM (JEOL JSM 2010, Japan). The numberof the graphene layers was determined by AFM (NanoScopeIIIa MultiMode, USA) and the crystal phase was characterizedby XRD (Rigaku D/Max2550VB/PC, Cu Kα radiation).The chemical states and the elemental compositions of GO,A-FGO and A-FGs surfaces were studied by XPS (KratosXSAM-800) using Al Kα (hυ1486.6 eV) x-ray as theexcitation source. The type of functional surface group wasstudied by FTIR on a Magna-IR 550 (Nicolet, USA), usingsamples prepared in potassium bromide pellets. To indicatethe thermal stability of the surface groups, TGA experimentswere carried out on a TA SDT-Q600 thermobalance in anitrogen flow and at a heating rate of 10 ◦C min−1 to 800 ◦C.

Moreover, A-FGs were sonicated in the solvents in-cluding dimethylsulfoxide (DMSO), tetrahydrofuran (THF),APTES, acetone, dimethylformamide (DMF), ethylene glycol(EG) and tetrachloromethane (CCl4) for 60 min to determinetheir solubilities. The resulting suspensions were thencentrifuged using a Techcomp CT14D centrifuge at 600 rpmfor 90 min. After centrifugation, the A-FG suspensions werefiltered through polyvinylidene fluoride (PVDF) or nylonfilters. Careful measurements of the filtered mass, accountingfor residual solvent, gave the concentration of the dispersedphase after centrifugation.

3. Results and discussion

Scheme 1 shows the possible mechanism of the synthesisof A-FGs with amino groups. The formation of amide isexcluded, because it is often the case that even simpleamides resist formation [27]. Furthermore, the reaction ofan amino group and a carboxyl group to form an amideusually requires activation of a carboxylic acid moiety inthe presence of thionyl chloride or coupling reagents suchas dicyclohexylcarbodiimide, 4-dimethylaminopyridine and1-hydroxybenzotriazole [28], which are not used in ourexperiment. Therefore it is believed that APTES is bondedto GO by the dehydration reaction of the Si–OH and the–OH of GO. The detailed process is described as follows:water, the solvent, first reacts with the ethoxy groups ofAPTES to form APTES silanols. The silanols then react withthe hydroxyl groups of GO by dehydration to form A-FGO,leaving free amino groups pointing outward. During thisprocess, polycondensation of APTES silanols also takes place.Though some oxygen-containing groups, e.g. epoxy, may stillremain on the surface of the A-FGO, they are further reduced

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Scheme 1. Illustration of the synthesis route of A-FGs. For clarity,minor functional groups, e.g. carboxyl groups and carbonyl groups,have been omitted at the edges.

Table 1. Elementary compositions of GO, A-FGO and A-FGs.

Samples

Atomic composition (at.%)

C O N Si

GO 70.19 29.81 — —A-FGO 63.53 23.58 6.67 6.22A-FGs 68.65 18.65 6.58 6.13

by water at high temperature (200 ◦C). At this temperature,polycondensation of APTES silanols also takes place, whichform a lateral siloxane network with aminopropyl groups toprevent the graphene sheets from aggregating.

Figure 1(a) shows the XRD patterns of natural graphite,GO and A-FGs. For natural graphite, there is a sharp peakcentered at 26.50◦, which is the characteristic peak (002)of graphite. As for GO, the (002) graphite peak almostdisappears and a peak appears centered at 8.96◦, whichis the characteristic peak (001) of GO. However, thesecharacteristic peaks are not observed on A-FGs, indicatingthat GO has been reduced and the multilayers have beenefficiently exfoliated [29]. Observation by AFM (figures 1(b)and (c)) reveals the presence of monolayered graphene, andthe thickness of monolayered A-FGs is about 1.0 nm, which isbetween 0.6 nm, as reported for the monolayered chemicallyconverted graphene [14], and 1.2 nm for the monolayeredwater soluble sulfonated graphene [30]. It is greater than thethickness of pristine graphene (0.335 nm), owing to the lateralsiloxane network on the A-FG sheets. Therefore, it is believedthat A-FGs are dominated by single- or few-layer graphene.

Under the TEM, the graphene with APTES functional-ization looks transparent and resembles crumpled silk muslin(A-FGs, figure 1(d)), while the graphene without APTESfunctionalization stacks to form large aggregates (HRGs,figure 1(e)).

In the XPS spectrum of the GO surface, only C 1s andO 1s peaks are observed (figure 2(a)), while in the XPSspectrum of the A-FGs surface, N 1s, Si 2s and Si 2p peaks,which originate from APTES, appear (figure 2(b)). This is

Figure 1. XRD patterns (a) of graphite GO and A-FGs. AFMimage of A-FGs (c) and its corresponding line profile (b). TEMimages of A-FGs (d) and HRGs (e).

a strong indication of the successful bonding of APTES tothe graphene. Table 1 summarizes the normalized elementalcomposition of GO, A-FGO (see figure S1 available at stacks.iop.org/Nano/24/045609/mmedia) and A-FGs, as determinedby the corresponding XPS peak areas and corrected bytheir respective atomic sensitivity factors. The C/O atomicratios of GO, A-FGO and A-FGs are 2.35, 2.69 and 3.68,respectively. A-FGs have a lower oxygen content than GO,which is because of the reduction of GO by the hydrothermaltreatment. However, the oxygen content of A-FGs is higherthan that of the reported graphene produced by the reductionof GO [13, 15], because APTES is linked to the grapheneby the C–O–Si bonds resulting in more oxygen left after thereduction.

The C 1s XPS spectra of GO and A-FGs are fitted withGaussian curves to elucidate their surface states. For GO,there exist five distinct Gaussian peaks centered at 284.6,285.6, 286.6, 287.8 and 289.1 eV, respectively, as shown infigure 2(c). These five peaks correspond to C–C/C–H, C–OH,C–O, C=O and O=C–O, respectively [24, 31]. After GO istreated by the hydrothermal process, the peak intensities of theoxygen groups of HRGs (see figure S2 available at stacks.iop.org/Nano/24/045609/mmedia) significantly decrease. Thisindicates that GO has been reduced by water which acts asa source of H+ to catalyze the dehydration of alcohol analogsby the protonation of hydroxyl groups [32, 33]. For A-FGs,the intensities of the C–O, C=O and O=C–O peaks (seefigure 2(d)) are much lower than those of GO, indicating thatA-FGs have a lower oxygen content. Moreover, for A-FGs,the peak centered at 283.7 eV is ascribed to C–Si [31]. TheA-FG peak centered at 285.6 eV has a higher intensity thanthat of GO from the C–OH peak, but this is because of thecontribution of C–O–Si and C–N which have peak intensityalso at 285.6 eV [31, 34].

Fitting the N 1s XPS spectrum of A-FGs with Gaussiancurves produces two distinct peaks centered at 399.4 and401.9 eV, which are assigned to the free amino group (–NH2)and the protonated amine (–NH+3 ), respectively [35]. The arearatio of the two peaks is about 6.5, indicating that most of the

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Figure 2. XPS spectra of GO (a) and A-FGs (b). C1s XPS spectra of GO (c) and A-FGs (d). N 1s XPS spectra of A-FGs (e). FTIR spectraof GO and A-FGs (f).

amino groups are unprotonated and active for reaction withother chemicals such as acyl chloride.

As seen in figure 2(f), on the FTIR spectrum of GO,the following characteristic absorption peaks exist: a broadshoulder at 3410 cm−1 for the O–H stretching vibrations,a peak at 1735 cm−1 for the C=O stretching vibrations, apeak at 1622 cm−1 for the vibrations of the adsorbed watermolecules, a peak at 1350 cm−1 for the O–H deformationvibrations, a peak at 1261 cm−1 for the C–OH stretchingvibrations, and a peak at 1096 cm−1 for the C–O of epoxystretching vibrations [30, 36–38]. However, on the spectrumof A-FGs, these characteristic peaks of GO disappear, whilesome new peaks identifying A-FGs appear. The broadshoulder at 3449 cm−1 is indicative of the N–H of aminogroups, the Si–OH and/or the O–H stretching vibrations.The peaks at 1653, 1570 and 750 cm−1 are ascribed tothe –NH2 asymmetric, symmetric deformation vibration andwagging and twisting, respectively. The peaks at 1193, 1119and 929 cm−1 are ascribed to the stretching vibrations ofSi–C, Si–O–Si and Si–OH, respectively, among which theSi–O–Si is formed by condensation among the silanols. The

peaks at 1032 and 691 cm−1 are due to the Si–O–C stretchingvibrations [23, 24, 39–41], which are contributed by thecondensation of the Si–OH and –OH groups on GO, but notby the Si–O–C2H5 in APTES because it is unstable in waterand can be easily hydrolyzed to Si–OH.

The TGA curves of GO and A-FGs are displayed infigure 3. For GO, there are three main weight loss steps.The first weight loss of ∼13% around 100 ◦C is due tothe evaporation of water. The second weight loss of ∼32%between 100 and 210 ◦C is due to the decomposition ofthe labile oxygen groups (such as carboxylic, anhydrideand lactone groups) and the removal of residual water. Thelast weight loss occurs between 210 and 300 ◦C, accountingfor ∼8% of initial mass, is due to the pyrolysis of thestable oxygen groups (such as phenol and carbonyl) [42–44].As for A-FGs, the weight loss is reduced to only ∼10%below 300 ◦C, which suggests that most oxygen groups onA-FGs have been reduced by the hydrothermal treatment.The reduction degree of A-FGs measured by TGA is in goodagreement with the reported results [24, 44]. The significantweight loss (∼10%) of A-FGs between 440 and 560 ◦C is

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Figure 3. TGA curves of GO and A-FGs.

Figure 4. Photographs of the dispersion of A-FGs in ethanol, H2O,DMSO, THF, APTES, DCM, acetone, DMF, NMP, EG and CCl4.

due to the decomposition of APTES; this again implies thatAPTES has been successfully covalently bonded to graphene.

To further show the surface property of the functionalizedgraphene, the dispersibility of A-FGs in different solventsis tested. The dispersions are prepared by dispersingA-FGs in the various solvents via ultrasonication at aconcentration of 1 mg ml−1, and photographs of them areshown in figure 4. Clearly, A-FGs are dispersed in DMSO,THF, APTES, acetone, DMF, EG and CCl4, but poorlydispersed in water, ethanol, dichloromethane (DCM) andN-methyl-2-pyrrolidone (NMP). In addition, the solubilityand stability of A-FGs in DMSO, THF, APTES, acetone,DMF, EG and CCl4 are studied. After centrifugation at 600rpm for 90 min, the dispersions of A-FGs in DMSO, APTESand EG remain homogeneous, and their solubilities are upto 0.89, 4.03 and 0.90 mg ml−1, respectively, which aremuch higher than the solubilities of the reported graphenesin solvents [45–48]. It must be noted that the dispersionsof A-FGs in APTES and EG are found to be stable formore than 2 months. In view of the good dispersibility in anumber solvents and the active amino groups of the surface,A-FGs may find a number of practical applications in sensors,composite materials and catalyst supports.

4. Conclusion

In summary, we report an eco-friendly, scalable and facilehydrothermal approach to synthesize functionalized graphenenanosheets with free amino groups. In this process, APTESis grafted onto the surface of graphene by the dehydrationreaction between the Si–OH and the –OH of GO, thusleaving the amino groups on the surface of the graphenelayers. The final product is dominated with monolayer or few

layered graphene nanosheets. Moreover, A-FGs are highlydispersible in DMSO, APTES and EG, and their solubilitiesare up to 0.89, 4.03 and 0.90 mg ml−1, respectively.Especially for A-FGs in APTES and EG, the dispersions arestable for more than 2 months. The results reported hereprovide a framework for the synthesis of highly dispersiblefunctionalized graphenes with free amino groups, which mayhave potential applications in anchoring metal nanoparticles,grafting organic materials, etc.

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