Journal of Colloid and Interface Science 396 (2013) 251–257

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Journal of Colloid and Interface Science

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Preparation and capacitance properties of graphene/NiAl layered double-hydroxidenanocomposite

Zhuo Wang a, Xin Zhang a, Junhui Wang a, Linda Zou b, Zhaotie Liu c, Zhengping Hao a,⇑a Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR Chinab Water Centre for Water Management and Reuse, University of South Australia, SA 5095, Australiac Key Laboratory of Applied Surface and Colloid Chemistry, School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710062, China

a r t i c l e i n f o

Article history:Received 22 November 2012Accepted 9 January 2013Available online 18 January 2013

Keywords:GrapheneLayered double hydroxideNanocomposite electrodeSupercapacitanceElectrochemical property

0021-9797/$ - see front matter Crown Copyright � 2http://dx.doi.org/10.1016/j.jcis.2013.01.013

⇑ Corresponding author. Fax: +86 10 62923564.E-mail address: zpinghao@rcees.ac.cn (Z. Hao).

a b s t r a c t

Graphene/NiAl layered double-hydroxide (LDH) composite with high capacitive properties has been pre-pared in a friendly one-step process. It is found that NiAl–LDH is formed in the addition of precipitatoragent (NaOH and NaNO3) by hydrothermal method, at the same time graphene oxide (GO) is reducedto graphene. The morphology and structure of the obtained material are examined by X-ray diffraction(XRD), Brunauer–Emmett–Teller (BET), transmission electron microscope (TEM), and scanning electronmicroscope (SEM) techniques. It is revealed that the NiAl–LDH disperses well on the surface of grapheneand the formation of NiAl–LDH nanoparticles is beneficial to the peeling of graphene (RGO). More impor-tantly, the addition of NiAl–LDH to graphene endows the materials with desirable specific surface areasand higher porosity. These structural advantages result in higher specific capacitance compared withpristine graphene. Electrochemical property investigations show that the graphene/NiAl–LDH had ahigher specific capacitance than graphene.

Crown Copyright � 2013 Published by Elsevier Inc. All rights reserved.

1. Introduction

Supercapacitors, also known as electrochemical capacitors orultracapacitors, have attracted much attention because of theirintriguing features including high power density and long cycle life[1–3]. The energy is stored in supercapacitors mainly by electricaldouble-layer capacitor (EDLCs) and pseudocapacitors [4]. The elec-trode materials of EDLCs are typically carbon materials with highsurface area, while pseudocapacitors are mostly conducting poly-mers or metal oxides, which transfer the faradic charges betweenelectrolyte and electrode [4]. In order to meet the ever increasingdemand for supercapacitors with high electronic performances, itis essential to develop electrodes which are durable, nontoxicand inexpensive. Numerous materials have been investigated andused as electrodes of supercapacitors; they can mainly be dividedinto three types: (i) carbonaceous materials, (ii) conducting poly-mers, and (iii) transition metal oxides [5–8]. Among the possiblematerials, transition metal oxides were studied extensively, suchas MnO2, Co3O4, ZnO, and so on [9–18]. Layered double hydroxide(LDH) as a kind of heterostructure nanomaterials have shownsuperior properties in applications such as catalysis, sorption, med-icine, and so on [19–21]. However, layered double hydroxide(LDH), as potential electrochemical active materials, its electro-chemical properties is less studied. LDHs are candidates for next-

013 Published by Elsevier Inc. All r

generation electrode materials of supercapacitors because of theirabundant slabs and the electrochemically active sites that led tothe simultaneous formation of the electrical double-layered capac-itance and Faradaic pseudocapacitance. Recently, several groupsdedicated to study the electrochemical properties of CoAl–LDH orNiAl–LDH [22–30]. However, pure LDH usually stacks togetherduring the synthesized process or its low conductivity constrainselectron transfer; these consequently impact on the performanceof electrode materials. So, it is necessary to find an effective meth-od to optimize the electrochemical properties of LDH.

Graphene, a two-dimensional carbon material with unique me-chanic and electronic properties, offers promising properties suchas high surface area, electrical conductivity, high flexibility,mechanical strength and so on and has drawn extensive attentionssince 2004 [31–33]. As a low dimensional material, graphene has ahuge theoretical specific area of 2600 m2/g and presents an excel-lent electrical conductivity of 7200 S/m at room temperature [34–36]. Graphene-based supercapacitors have been reported withhigh specific capacitance in aqueous solution [30]. However, theagglomeration of graphene always limited its capacitances, so itis urgent to explore novel graphene-based materials to overcomethe aggregation and exploit their full potential of electrodes forsupercapacitors. A promising prospective is to use graphene as asubstrate for growth of functional nanomaterials to form newnanocomposites [30]. Lots of nanocomposites were grown on thegraphene substrate such as, NiO, MnO2, ZnO, and so on. These ob-tained hybrid nanocomposites showed enhanced electrochemical

ights reserved.

252 Z. Wang et al. / Journal of Colloid and Interface Science 396 (2013) 251–257

performance, because of the combination of pseudocapacitance ofthe metal oxide and the double-layer capacitance from graphene.Recently, LDH–graphene nanocomposites have attracted intensiveattention, because of their low cost, high redox activity, and envi-ronmentally friendly nature [30]. However, most conventional syn-thesis methods for LDH–graphene nanocomposites are either timeconsuming or involving some toxic precursors, which is not inaccordance with the requirements of energy saving and environ-mental protection. Hence, it is urgent to explore simple and conve-nient method for synthesizing this kind of material.

In this paper, we reported the preparation of graphene/NiAl lay-ered double–hydroxide composite by a one-step hydrothermalmethod for the first time. The NiAl/LDHs were dispersed on thesurface of graphene. LDH formed by co-deposition method usingNaNO3 and NaOH as precipitating agent. The synthesized graph-ene/NiAl–LDH was characteristic by transmission electron micros-copy (TEM), powder X-ray diffraction (XRD), and so on.

2. Experimental section

2.1. Preparation of graphene oxide and graphene/NiAl–LDHnanocomposites

Graphene oxide (GO) was prepared from natural graphite pow-der through a modified Hummers’ method [37]. Graphene/NiAl–LDH nanocomposites were prepared hydrothermally as follows:0.6 g of GO was dispersed in H2O and stirred for 12 h to form ahomogeneous aqueous dispersion. Then, 0.37 g Al(NO3)3 and0.58 g Ni(NO3)2 were added into the GO dispersion followed bystirring at room temperature. NaOH solution (2 M) and NaNO3

(1 M) were added to the solution slowly and simultaneously foradjusting the pH value in rang of 10–11. Then, the mixture wastransferred to a 100 mL Teflon-lined autoclave and maintained at180 �C for 48 h for crystallizing. The products were washed withdeionized water for several times and dried at 50 �C for 24 h.Graphene was synthesized directly by hydrothermal method at180 �C for 24 h.

2.2. Material characterizations

Powder X-ray diffraction (XRD) patterns of all samples werecarried out by a Rigaku powder diffractometer (D/MAX-RB) usingCu Ka radiation (k = 0.15418 nm) at a scanning rate of 4�/min in2h = 10–70�. The structures of the electrode materials were charac-terized by transmission electron microscopy (TEM, JEOL 3010) andscanning electron microscopy (SEM, HitachiS-3000N) techniques.Adsorption and desorption isotherms of nitrogen were measuredwith a NOVA 1200 gas sorption analyser at liquid nitrogen temper-ature (�196 �C). Before the measurements, the samples were de-gassed under vacuum condition at 120 �C for 8 h. The total porevolume was estimated from the amount of nitrogen adsorbed ata relative pressure of 0.99. The Brunauer–Emmett–Teller (BET)method was utilized to calculate the specific surface area usingadsorption data acquired at a relative pressure (P/P0) range of0.05–0.25. The pore size distribution curves were calculated fromthe analysis of the desorption branch of the isotherm based onthe Barrett–Joyner–Halenda (BJH) algorithm. The micropore vol-ume and micropore surface area were estimated by a t-plotmethod.

2.3. Electrochemical measurement

All the electrochemical measurements were carried out in aconventional three-electrode system with 1 M Na2SO4 aqueouselectrolyte at room temperature. The as-synthesized electrode

materials, a platinum wire electrode, and a saturated calomel elec-trode (SCE) were used as working electrode, counter electrode, andreference electrode, respectively. The working electrode was pre-pared by mixing sample (80 wt.%) as active material with poly(tet-rafluoroethene) (PTEF, 20 wt.%) in ethanol to produce ahomogeneous paste. Then, the resulting mixture was coated ontothe Ni foam substrate. The foam was dried at 80 �C in air for 12 hto remove the solvent. The electrochemical performances of theas-prepared material electrodes were tested using a cyclic voltam-metry (CV) method, galvanostatic charge/discharge, and electro-chemical impedance spectroscopy (EIS) on an electrochemicalworkstation (CHI 660D, Shanghai CH Instrument Company, China).The measurement was carried out in 1 M Na2SO4 aqueous electro-lyte at room temperature. CV tests were done between �1.2 V and�0.2 V (vs SCE) at scan rates of 5, 10, 30, 50, and 100 mV s�1. Gal-vanostatic charge/discharge curves were measured in the potentialrange of �1 to 0 V at different current densities, and the EIS mea-surements were carried out in the frequency range from 100 kHzto 0.1 Hz at open circuit potential with an ac perturbation of 5 mV.

3. Results and discussion

3.1. Morphology of GO and NiAl–LDH/RGO

The formation of NiAl–LDH/RGO can be explained as follows(Scheme 1): metal cations were firstly adsorbed on GO surface,using NaOH and NaNO3 as precipitator agent, then the reductionof GO and the crystallization of LDH crystals happened in one-stepprocess. Fig. 1 shows the XRD patterns of the graphite, GO, as-pre-pared RGO, and graphene/NiAl–LDH. The diffraction peak of exfoli-ated GO at 11.17� (001) features a basal spacing of 0.80 nm,showing a well oxidation of graphite to the graphene oxide dueto the introduction of oxygen-containing functional groups onthe graphite sheets [38]. While for graphene, the peak at 11.17�of GO completely disappears and a broad peak at around 25� rep-resenting an interlayer spacing of 0.355 nm is observed. The spaceis a little larger than the natural graphite (0.334 nm), which maybebecause of the partial reduction of graphene oxide. The XRD pat-tern of hybrid graphene/NiAl–LDH material can be indexed to thehexagonal NiAl–LDH (JCPDS No. 22-0452). The d spacing of the(003) diffraction peak of graphene/NiAl–LDH is 0.754 nm. Thisindicates that the graphene sheets are effectively exfoliated bythe addition of NiAl–LDH hybrid. To confirm the reduction of GOto RGO, FT-IR spectra of Go and RGO are given in supplementarymaterials (Fig. S1). Besides, thermal stability of NiAl–LDH/graph-ene has been given in Fig. S2.

Fig. 2 gives the morphology and structure of GO and graphene/LDH (SEM and TEM). As can be seen from Fig. 2A that the as-pre-pared graphene shows a transparent flower-like shape. While forthe structure of graphene/LDH (Fig. 2B), with the addition ofLDH, it is apparent that it exhibits a layer-like structure. TEMimages (Fig. 2C and D) are given to get in-depth study of the struc-ture of the synthesized material. It can be seen that the GO hastransparent character due to the thin layered structure with somewrinkles and folds on the surface. While for graphene/LDH, it isclear that the NiAl/LDH nanoplatelets are distributed on the sur-face of graphene nanosheets and thus effectively prevent therestacking of graphene.

In order to confirm that the formation of graphene/NiAl–LDHprevents the aggregation of graphene nanosheets, nitrogen adsorp-tion and desorption were carried out to obtain the surface area andporous data of the as-synthesized material. The corresponding spe-cific surface area of graphene/NiAl–LDH and graphene is about122.7 m2 g�1 and 56.1 m2 g�1, respectively (Fig. 3). From Table 1,the specific area of graphene/NiAl–LDH was higher than graphene,

Scheme 1. A schematic illustration of the preparation of the nanostructured NiAl–LDH/graphene composite.

20 40 600

20000

40000

60000

2 Theta/Degree

Inte

nsit

y (a

.u.)

LDH-RGO

RGO

GO

Graphite

003

006012

015 018 110 113

Fig. 1. XRD patterns of graphite, GO, RGO, LDH–RGO.

Z. Wang et al. / Journal of Colloid and Interface Science 396 (2013) 251–257 253

and this is confirmed that the deposition of NiAl–LDH helped to re-duce the restacking of the graphene nanosheets. The pore sizeanalysis of graphene/NiAl–LDH indicates that it comprises a largefraction of mesoporous (2–50 nm), which is thought to be the opti-mum pore size for electrosorption. Sample with high specific sur-face area and mesopores is favorable for improving both themain pseudocapacitance of metals and the electric double-layer(EDL) capacitance of graphene because of the easily accessible ofthe hydrated ions in the electrolyte to the exterior and interiorpore surfaces [39,40].

3.2. Electrochemical property

To explore the potential applications of the as-synthesizedgraphene/NiAl–LDH and graphene, the samples were fabricatedas supercapacitor electrodes and characterized by CV, EIS, galvano-static charge/discharge, and stability measurements.

Cyclic voltammetric tests were used to explore the electro-chemical behavior of the obtained graphene/NiAl–LDH and graph-ene nanocomposites in 1 M Na2SO4 electrolyte between �1.2 and�0.2 V at different scan rates and the results are shown in Fig. 4.As presented in Fig. 4, the CV curves for graphene (Fig. 4A) are al-

most ideally rectangular, indicating a typical electrochemical dou-ble-layer (EDL) capacitive behavior with fast current response onthe voltage reversal at each end potential and high reversibility[41]. While for sample graphene/NiAl–LDH, the CV curves areslightly disordered from the curves of rectangular shape of RGO;however, it is obvious that it shows a larger integrated area thangraphene, which results in higher capacitance. The specific capac-itances (Cs) were calculated from the CV curves according to thefollowing equation:

Cs ¼R

IdVtmDV

where Cs is the specific capacitances (F g�1), I is the response cur-rent (A), DV is the potential window (V), t is the potential scan rate(mV s�1), and m is the mass of the electroactive materials in theelectrodes (g). The specific capacitance value calculated from theCV curve at 5 mV s�1 is found to be 203.57 F g�1 and 138.07 F g�1

for graphene/NiAl–LDH and graphene, respectively. The better elec-trical performance of graphene/NiAl–LDH can be attributed to thepseudocapacitance of NiAl/LDH, the EDL capacitance of RGO, theresidual CAO and C@O function groups from the partially reductiveof GO and less aggregation of the graphene nanosheets due to theintroducing of NiAl–LDH nanocomposites.

In order to further investigate the performance of electrodematerials, galvanostatic charge/discharge tests of graphene/NiAl–LDH and graphene were conducted in 1 M Na2SO4 with differentcurrent densities in a voltage range of �1 to 0 V, as shown inFig. 5. The specific capacitances (Cs) of the electrodes can be calcu-lated according to the following equation:

Cs ¼It

ðDVÞm

where m (g) is the mass of the active material in the film electrode, I(A) is the discharge current, DV (V) is the potential window, t is thedischarge time, and Cs (F g�1) is the specific capacitance. Table 2lists the charge/discharge data of the electrode materials calculatedfrom the above equation at different current density. The remark-ably increased capacitance of graphene/NiAl–LDH may be causedby the combination of electric double-layer capacitance of RGOand faradic pseudocapacitance of NiAl–LDH. With the increase inthe current density, the capacitance of electrode is decreased from213.6 F g�1 to 80 F g�1. The decreasing trend of the capacitance

Fig. 2. SEM images of GO (A) and LDH–RGO nanocomposite (B), TEM images of GO (C) and LDH–RGO nanocomposite (D).

0.0 0.2 0.4 0.6 0.8 1.0-100102030405060708090100110

P/P0

Vol

ume

adso

rbed

(

)

cm3

g-1

RGO LDH-RGO

0 10 20 30 40 50 60

0.000

0.005

0.010

0.015

0.020

log rp/nm

dVp

/dlo

g V p

RGOLDH-RGO

(a) (b)

Fig. 3. N2 adsorption–desorption isotherm (a) and pore size distribution (b) of RGO and LDH–RGO samples at 77 K.

Table 1Pore parameters of LDH–RGO and RGO.

Sample SBET (m2 g�1) Vtotal (cm3 g�1) Vmicropore (m3 g�1) Smicropore (m2 g�1)

LDH–RGO 122.7 0.164 0.01375 28.93RGO 56.1 0.101 – –

254 Z. Wang et al. / Journal of Colloid and Interface Science 396 (2013) 251–257

indicates that the partial surface of the electrode is inaccessible atthe high current density. Besides, it is apparent that for the shapesof the discharge curves of the graphene/NiAl–LDH are deviates fromthe ideal voltage–time curves. After the iR drop, by the decrease involtage, the time increasing deviates from a straight line. The phe-nomenon has already reported by Xu et al., Niu et al., and Beidaghiet al. about electrochemically modified graphite electrodes, porous

C-cloth material, and electrochemically activated carbon micro-electrode arrays, respectively [42,43]. The factors contribute to thenon-ideal behavior can be illustrated as follows: (i) the pseudoca-pacitive comes from LDH; (ii) the redistribution of charge withinthe pores of the activated electrodes during charging or discharg-ing; and (iii) the effect of direct equivalent series resistance (ESR).ESR is an important parameter determining the charged/dischargedrate of a supercapacitor, and it can be obtained from the intercept ofthe plot with real impedance axis.

Fig. 6 shows Nyquist plots of graphene and graphene/NiAl–LDH.For the plots of graphene, a semicircle over the high frequencyrange and a linear part in the low frequency region can be ob-served. The semicircle is due to charge-transfer resistance on theelectrode/electrolyte interface, while the low frequency region ismainly characterized by purely capacitive behavior. The region

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2-15

-10

-5

0

5

10

15

20

Voltage (V)

Cur

rent

(

)

A

g-1

Cur

rent

(

)

A

g-1

A

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2-15

-10

-5

0

5

10

15

20 5 m V s-1

10 m V s-1

30 m V s-1

50 m V s-1

100m V s-1

5 m V s-1

10 m V s-1

30 m V s-1

50 m V s-1

100m V s-1

Voltage (V)

B

0 20 40 60 80 10070

80

90

100

110

120

130

140

Sweep rate (mV/s)

Spec

ific

cap

acit

ance

(F

/g)

RGOC

0 20 40 60 80 10080

100

120

140

160

180

200

220

Sweep rate (mV/s)

Spec

ific

cap

acit

ance

(F

/g)

LDH-RGO

D

Fig. 4. CV curves of RGO (A) and LDH–RGO (B) electrodes at different scan rate; relationship between specific capacitance and the scan rate (RGO (C), LDH–RGO (D)).

-20 0 20 40 60 80 100 120 140 160

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Time / sec

Pot

enti

al /

V

1 A g-1

2 A g-1

3 A g-1

4 A g-1

5 A g-1

A

0 100 200 300 400 500 600

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Time / sec

Pot

enti

al /

V

1 A g-1

2 A g-1

3 A g-1

4 A g-1

5 A g-1

B

Fig. 5. Charge–discharge behaviors of RGO (A) and LDH–RGO (B) electrodes at different current density.

Table 2Specific capacitance values of electrodes RGO and LDH–RGO calculated from charge/discharge curves measured at different current densities.

Electrode Current density

1 (A g�1) 2 (A g�1) 3 (A g�1) 4 (A g�1) (A g�1)

LDH–RGO (F g�1) 213.6 167 124.9 95.6 80RGO (F g�1) 135.7 71.9 48.6 38.4 30.4

Z. Wang et al. / Journal of Colloid and Interface Science 396 (2013) 251–257 255

between the high frequency and low frequency regions is called theWarburg region, and this is the combination of both resistive andcapacitive behavior characterized by diffusive resistance. The Ny-quist plots for graphene/NiAl–LDH are different from graphene,which possesses the typical plots of carbon material in the high fre-quency region, and this indicates that the capacitance of graphene/NiAl–LDH nanocomposite is mainly from the pseudocapacitance ofNiAl–LDH. At this region, the equivalent series resistance (ESR,

0 20 40 60 80 100-10

0

10

20

30

40

50

60

70

80

Zre / ohm

Zim

/ oh

m

RGO

LDH-RGO

Fig. 6. The Nyquist plots of RGO and LDH–RGO nanocomposite electrodes with afrequent range of 0.1 Hz–100 kHz in 1 M Na2SO4 electrolyte.

0 200 400 600 800 100075

80

85

90

95

100

105

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2

-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

0.04

0.05

Cur

rent

(A

)

Voltage (V)

Cap

acit

ance

ret

enti

on (

%)

Cycle number

NiAl-LDH/graphene

Fig. 7. Capacitance retention of LDH–RGO at a sweep rate of 50 mV s�1 in1 M Na2SO4 electrolyte. The inset shows the CV curves of LDH–RGO at a sweeprate of 50 mV s�1.

256 Z. Wang et al. / Journal of Colloid and Interface Science 396 (2013) 251–257

obtained from the intersection of the Nyquist plot at the x-axis) ofgraphene is apparently higher than the graphene/NiAl–LDH nano-composite, and this may due to the higher conductivities of graph-ene/NiAl–LDH nanocomposite. The Nyquist plot at high frequencyis the resistance resulting from the frequency dependence of ion dif-fusion/transport. Compared with graphene, the Nyquist plots ofgraphene/NiAl–LDH in low frequency become more vertical imply-ing purely capacitive behavior. These results show that the forma-tion of NiAl–LDH nanoparticles on RGO can significantly improvethe electrical performance due to the intrinsic resistance change ofthe active material and less aggregation of the graphene nanosheets.

Furthermore, the electrochemical stability of the graphene/NiAl–LDH nanocomposite was evaluated by repeating the CV testbetween �0.2 and �1.2 V at a scan rate of 50 mV s�1 for 1000 cy-cles. As shown in Fig. 7, a 9.3% decrease in the specific capacitancewas observed over the first 250 cycles; then, the value of the capac-itance gradually recovered and the electrode thereafter reached arelatively stable state. The incensement is possibly due to the acti-vation process that allows the trapped ions to gradually diffuse out[44]. The capacitance retention was nearly 100% of the initialcapacitance after 1000 cycles, indicating excellent cycle stability,and this is mainly due to the stabilization effect of the grapheneand well disperse of NiAl–LDH on the support.

4. Conclusion

In summary, the graphene/NiAl–LDH was prepared by hydro-thermal method in one step as an electrode material. This materialtakes the advantages both of the electrical double-layer capacitorfrom graphene and of pseudocapacitance of NiAl–LDH nanoparti-cles. Compared with the pristine as-synthesized graphene, thepresent of NiAl–LDH nanocomposites prevents the restacking ofgraphene and decreases the charge-transfer resistance of graph-ene/NiAl–LDH electrode, which makes it showed a specific capaci-tance of 213.57 F g�1 at a current density of 1 A g�1. Additionally,nearly 100% of the original capacitance was retained after 1000 cy-cles, indicating a good cycle stability of composite materials. All ofthe above suggested that the graphene/NiAl–LDH electrode mate-rial was a high promising electrode material for supercapacitorswith excellent performance.

Acknowledgment

This work was financially supported by National Basic ResearchProgram of China (2010CB732300).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcis.2013.01.013.

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