IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 20 (2009) 455602 (6pp) doi:10.1088/0957-4484/20/45/455602

One-step synthesis of graphene/SnO2nanocomposites and its application inelectrochemical supercapacitorsFenghua Li1,2, Jiangfeng Song1,2, Huafeng Yang1,2, Shiyu Gan1,2,Qixian Zhang1,2, Dongxue Han1,2,3, Ari Ivaska3 and Li Niu1,2,3,4

1 State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of AppliedChemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China2 Graduate University of the Chinese Academy of Sciences, Chinese Academy of Sciences,Changchun 130022, People’s Republic of China3 Laboratory of Analytical Chemistry, Process Chemistry Centre, Abo Akademi University,FI-20500 Abo-Turku, Finland

E-mail: lniu@ciac.jl.cn

Received 27 June 2009, in final form 18 September 2009Published 16 October 2009Online at stacks.iop.org/Nano/20/455602

AbstractA one-step method was developed to fabricate conductive graphene/SnO2 (GS) nanocompositesin acidic solution. Graphite oxides were reduced by SnCl2 to graphene sheets in the presence ofHCl and urea. The reducing process was accompanied by generation of SnO2 nanoparticles.The structure and composition of GS nanocomposites were confirmed by means of transmissionelectron microscopy, x-ray photoelectron and Raman spectroscopy. Moreover, theultracapacitor characteristics of GS nanocomposites were studied by cyclicvoltammograms (CVs) and electrical impedance spectroscopy (EIS). The CVs of GSnanocomposites are nearly rectangular in shape and the specific capacitance degrades slightly asthe voltage scan rate is increased. The EIS of GS nanocomposites presents a phase angle closeto π/2 at low frequency, indicating a good capacitive behavior. In addition, the GSnanocomposites could be promisingly applied in many fields such as nanoelectronics,ultracapacitors, sensors, nanocomposites, batteries and gas storage.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Interest in understanding properties of graphene has led tomany theoretical and experimental efforts worldwide, due to itsprominent properties and broad potential applications in manyfields [1]. However, graphene sheets, unless well separatedfrom each other, tend to form irreversible agglomerates or evenrestack to form graphite through van der Waals interactions.Currently, some chemical [2–5] and physical [6, 7] methodshave been used to produce freestanding graphene sheets.However, all previous graphene sheets were reduced byalkaline reagents in aqueous solution and only dispersed wellin alkaline solution. Aggregation would occur with any changeof the condition of the solution, such as addition of salts or

4 Author to whom any correspondence should be addressed.

acids [2, 8–10]. In contrast, many other materials are unstablein an alkaline environment. This restricts the synthesis of manyhybrid graphene materials and application fields of graphenesheets.

As one kind of graphene hybrid material, graphene/nanoparticles composites have aroused extensive interestas hybridization improves the catalyst performance ofgraphene materials. Recently, platinum particles wereapplied to identify carboxyl groups on graphene edges [11]or separate exfoliated graphene sheets [12]. Gold [13],palladium [14] and silver nanoparticles [15] were alsosuccessfully dispersed on graphene sheets. Similarly,graphene sheets can also serve as support materials to anchorsemiconductor particles and improve the performance ofoptoelectronic and energy conversion devices. Recently,

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Nanotechnology 20 (2009) 455602 F Li et al

TiO2–graphene nanocomposites were synthesized by UV-assisted photocatalytic methodology [16]. SnO2/graphenenanocomposites were prepared by mixing SnO2 hydrosol andgraphene nanosheet dispersions, and its porous electrode wasmade to enhance cyclic performance and lithium storagecapacity [17].

Electrochemical supercapacitors are passive and staticelectrical energy storage devices for applications requiringhigh power density such as electric vehicles, utility load-leveling, heavy-load starting assists for diesel locomotives,military and medical applications, and also low powerapplications such as camera-flash equipment, pulsed-lightgenerators, and as back-up power for computer memory [18].As the energy stored is inversely proportional to thethickness of the double layer, ultracapacitors based onelectrochemical double-layer capacitance (EDLC) have anextremely high energy density compared to conventionaldielectric capacitors [19]. EDLC are similar in constructionto conventional capacitors except that the metal electrodeis replaced by a highly porous electrode [20]. Therefore,porous carbon materials such as activated carbon [21],xerogels [22], carbon nanotubes [23], mesoporous carbon [24]and carbide-derived carbons [25] have been investigated foruse as electrodes in EDLC. Recently, as a new class ofcarbon, graphene materials have been used to constructultracapacitors [19, 26]. However, as far as we know, reportsof the synthesis of graphene sheets in acidic solution andits application in electrochemical supercapacitors are few innumber at present.

In this work, we reduced graphite oxides (GO) by SnCl2

in acidic solution in the presence of HCl and urea to fabricategraphene/SnO2 (GS) nanocomposites. Transmission electronmicroscopy (TEM), x-ray photoelectron spectroscopy (XPS)and Raman spectra were used to confirm the structureand composition of GS nanocomposites. Moreover,the electrochemical supercapacitor characteristics of GSnanocomposites were studied in an aqueous electrolyte.

2. Materials and experiments

2.1. Preparation of GO

GO were synthesized from natural graphite powder (spectralrequirement, Shanghai Chemicals, China) by a modifiedHummers method as originally presented by Kovtyukhova et al[27]. Then GO were subjected to dialysis for 3–4 days tocompletely remove metal ions and acids. Finally, the productwas dried in air at room temperature.

2.2. Synthesis of GS

Dried GO (10 mg) were exfoliated in distilled water (20 ml)with ultrasonic treatment to form a colloidal suspension.Subsequently, 0.15 ml of HCl (36–38%, Beijing Chemicals,China), 0.22 g of SnCl2·2H2O (98.0%, Beijing Chemicals,China) and 0.10 g of urea (99.0%, Beijing Chemicals, China)were added, then the mixture was continually stirred at 60 ◦Cfor 6 h. The product was rinsed completely with distilled waterand dried at room temperature under vacuum.

2.3. Synthesis of chemically converted graphene (CCG)

The CCG composite was prepared by the method presentedby Li et al [2]. Briefly, the purified GO (16.6 mg) weredispersed in distilled water (60 ml) by ultrasonication for30 min. 21.0 μl of hydrazine solution (50 wt% in water,Beijing Yili Chemicals, China) and 235.2 μl of ammoniasolution (25 wt% in water, Beijing Chemicals, China) wereadded to the resulting homogeneous dispersion. After beingvigorously stirred for a few minutes, the vial was put in an oilbath (95 ◦C) for 1 h. The obtained black dispersion was washedseveral times by ultrapure water, collected by centrifugationand dried under vacuum at room temperature.

2.4. Fabrication of GS, CCG and GO films

The glassy carbon (GC, 3 mm in diameter) electrodes werepolished subsequently with 1.0, 0.3 and 0.05 μm aluminaslurry, and then sonicated in water for several times. To prepareGO, CCG and GS-modified GC electrodes, an aliquot of 3 μlof 2.1 mg ml−1 GO, CCG and GS aqueous solution was coatedon the clean GC electrode with a microsyringe, respectively.Then they were dried in air before use.

2.5. Instruments and measurements

TEM image was taken with a JEOL 2000 transmissionelectron microscope operating at 200 kV. XPS analysis wascarried out on an ESCALAB MK II x-ray photoelectronspectrometer. Raman spectra were collected using a Renishaw2000 system with an argon ion laser (514.5 nm) and charge-coupled device detector. CVs were done with a CHI 660electrochemical workstation (CHI, USA) with a conventionalthree-electrode electrochemical cell. EIS was performed usinga Solartron 1255B Frequency Response Analyzer (SolartronInc., UK) with a three-electrode system too. Capacitancevalues were calculated for the CV curves by dividing thecurrent by the voltage scan rate, C = I/(dV/dt). Thespecific capacitance reported is the capacitance for the carbonmaterial of one electrode (specific capacitance = capacitanceof single electrode/weight material of single electrode). Thesquare resistance was measured under ambient conditions by astandard four-probe method. A Keithley 2400 source meter anda Keithley 2000 multimeter (Keithley Instruments, Cleveland,OH) were connected to the sample. Four electrode contactswith an inter-electrode spacing of 1.0 mm were formed on a3 mm thick sample with 12 mm diameter. The sample wasmade by the squash method at 40 MPa pressure.

3. Results and discussion

Figures 1(A) and (B) display the TEM images of GSnanocomposites at low and high magnification. It is evidentthat almost all graphene sheets are separated from each otherand coupled by SnO2 nanoparticles. On comparison withthe TEM image of CCG (figure 1(C)), the surface of GSnanocomposites is much rougher than that of CCG, whichmight be attributed to the growth of SnO2 nanoparticleson graphene sheets. Moreover, the photograph of the GO

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Figure 1. TEM images of the GS nanocomposites at low (A) and high (B) magnification, and the TEM image of the CCG composites (C);photograph of the GO and GS solutions (D).

Figure 2. XPS spectra of the GS nanocomposites (A); insets: the Sn 3d doublet; the C 1s XPS spectrum of the GO (B) and GS (C) compositefilms.

and GS solution was shown in figure 1(D). The color ofthe suspension shifts from brown to black, which furtherindicates the change from GO to GS nanocomposites. SnO2

nanoparticles can interact with the graphene sheets throughphysisorption, electrostatic binding or through charge transferinteractions [16].

Figure 2 shows the XPS spectra of GS nanocomposites.The peaks of tin (Sn 3p, 3d, 4s, 4p, 4d) emerge, which areexpected from SnO2: meanwhile, the peak of C 1s is attributedto graphene sheets. As shown in the inset, the Sn 3d5/2

and Sn 3d3/2 peaks of the SnO2 nanoparticles are at 487.5and 495.9 eV with an 8.4 eV peak-to-peak separation. Itfurther confirms the formation of SnO2 nanoparticles, whichare coupled on graphene sheets [28].

The C 1s XPS spectra of GO and GS nanocompositefilms are shown in figures 2(B) and (C), respectively. Thisregion, as shown in figure 2(B), can be deconvolutedinto four components corresponding to carbon atoms indifferent oxygen-containing functional groups [29]: (a) non-oxygenated C at 284.8 eV, (b) carbon in C–O at 286.2 eV,(c) carbonyl carbon (C=O, 287.9 eV) and (d) carboxylatecarbon (O–C=O, 289.0 eV). The C 1s XPS spectrumof the GS nanocomposite film shows the presence ofthe same functionalities (figure 2(C)) but with a muchsmaller contribution of the oxygenated carbons, indicating thedeoxygenation process.

The significant structural changes occurring duringchemical processing from GO to GS are also reflected in their

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Figure 3. Raman spectra of the GO and GS nanocomposites.

Raman spectra (figure 3). The peaks at about 1358 cm−1

(D band) and 1597 cm−1 (G band) are all observed in the twocomposites, which are ascribed to the graphite substrate [10].However, a decreased D/G intensity ratio in GS can beobserved in comparison with that of GO. This change suggestsa decrease in the average size of the sp2 domains uponreduction of the exfoliated GO [30], and it also illustrates thatnew graphitic domains are created that are smaller in size thanGO before reduction, but more numerous in number. For theGS line, three other obvious peaks are observed at 629, 688and 769 cm−1 corresponding to the A1g, A2u and B2g vibrationmodes of SnO2 nanoparticles [31]. Therefore, the Ramanresults are consistent with the TEM and XPS results, indicatingthe formation of GS nanocomposites.

Figure 4(A) displays CVs of GO, CCG and GS-modifiedGC electrodes in 1 M H2SO4 solution. All CV curves exhibita nearly rectangular shape, indicating good charge propagationwithin the electrodes [23]. The specific capacitance degradesslightly with the scan rate, as shown in figure 4(B), but thecapacitance remains at 34.6 F g−1 at a scan rate of 1.0 V s−1,retaining the box-like characteristics. The CCG and GO

films exhibit a capacitance of 20.7 and 0.62 F g−1 in 1 MH2SO4, respectively, with decreases in the specific capacitanceat higher scan rates. Therefore, the performance of GSnanocomposites with a specific capacitance of 43.4 F g−1 isthe best in the three composites.

The EIS analysis has been recognized as one of theprincipal methods for examining the fundamental behaviorof electrode material for an ultracapacitor. The impedancebehavior might be dominated by three major processesoccurring in the high, medium and low frequency regions,respectively [32]. From figure 5 we can see that the curvesat high frequency are almost parallel to the real axis at theGS-modified electrode. It indicates that the contact resistanceof the GS film is very small. The radius of the semicirclein GO films is the largest, which indicates the polarizationresistance of GO is the biggest among the three composites.Following this impedance arc, all three intermediate regionsare similar, indicating the similar structure of the composites.At low frequency the imaginary part of the impedance curvesapproaches a vertical line in GS nanocomposites, presenting acapacitive-type behavior [33]. The GS nanocomposites presenta phase angle close to π/2, indicating the best capacitivebehavior, in agreement with the CV results.

The good capacitive property of GS nanocompositesmay originate from the following factors. First, the overallhigh electrical conductivity of graphene sheets could bemaintained in GS nanocomposites, which is crucial for asupercapacitor [34]. It can also be concluded from thecomparison between capacitive behaviors of GO and thatof CCG composites. The conductivity of CCG is higherthan that of GO [10] and the capacitive property of CCGis better than that of GO. The GS nanocomposites filmshave a bulk conductivity of 0.023 S cm−1 measured bya standard four-probe method, further indicating its goodconductive property. Second, according to the previous report,the maximum specific surface of a carbon material is alsovery important in constructing a double-layer capacitor [35].The dispersion of SnO2 nanoparticles on graphene sheetsprovides a new way to increase the specific surface ofgraphene sheets. Third, a synergistic effect between SnO2

Figure 4. (A) CVs of a capacitor built from the GO, CCG and GS-modified GC electrodes (0.063 mg each) at a scan rate of 0.1 V s−1 in 1 MH2SO4 solution and (B) specific capacitance as a function of scan rate.

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Nanotechnology 20 (2009) 455602 F Li et al

Figure 5. Nyquist plots for the GO, CCG and GS nanocomposites in1 M H2SO4 aqueous solution electrolytes recorded at roomtemperature with open circuit voltage of 0 V and amplitude of 10 mVover a frequency range of 0.01 Hz–500 kHz.

nanoparticles on graphene sheets improves the capacitiveproperty of GS nanocomposites [36]. The highly dispersedSnO2 nanoparticles onto the graphene sheets might participatein the electrochemical reaction by the following reaction [37]:

SnO2 + H+ + e− ↔ SnOOH. (1)

This synergism is reasonably attributed to the obviousimprovement in the proton diffusion into the nanocompositesduring the charge storage/delivery process. The mesoporousstructure of GS nanocomposites (see figure 1(B)) is helpful tothe H+ ions to diffuse and contact the electroactive surfaceof SnO2 nanoparticles. Therefore, the capacitive propertyof GS nanocomposites is so satisfying. In contrast, theconductivity of GO are bad, as presented in a previousreport [10]. Moreover, in the absence of SnO2 nanoparticles,it is reasonable that the capacitive property of GO and CCG isnot as good as that of GS nanocomposites.

It should be worthwhile noting that our GS nanocompos-ites were prepared in acidic solution, which further broadensthe synthesis methods and application fields of graphene. Thepossible reaction mechanism can be considered as follows:

SnCl2 + graphite oxides + H2O → SnO2 + graphene + 2HCl(2)

CO(NH2)2 + H2O → CO2 + 2NH3 (3)

NH3·H2O + HCl → NH4Cl + H2O. (4)

SnCl2 is used to reduce GO to graphene sheets in the presenceof HCl and urea. Urea is hydrolyzed to CO2 and NH3 (3),then NH3 dissolved in water could neutralize HCl to formNH4Cl (4). Reactions (3) and (4) would be helpful to completereaction (2).

4. Conclusions

In summary, a one-step method has been developed tofabricate GS nanocomposites. GO were reduced by SnCl2

to graphene sheets in the presence of HCl and urea. Thedirect interaction between SnO2 particles and graphene sheetshinders the collapse of exfoliated graphene sheets. The CVand EIS results indicate a good capacitive behavior of GSnanocomposites, which is superior to that of GO and CCG.In addition, the GS nanocomposites could be promisinglyapplied in many fields such as nanoelectronics, ultracapacitors,sensors, nanocomposites, batteries and gas storage.

Acknowledgment

This work was financially supported by the NSFC, China(no. 20673109).

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