This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012 New J. Chem., 2012, 36, 2533–2540 2533

Cite this: New J. Chem., 2012, 36, 2533–2540

Synthetic core–shell Ni@Pd nanoparticles supported on graphene and

used as an advanced nanoelectrocatalyst for methanol oxidation

Mingmei Zhang, Zaoxue Yan, Qian Sun, Jimin Xie* and Junjie Jing

Received (in Montpellier, France) 26th July 2012, Accepted 11th September 2012

DOI: 10.1039/c2nj40651a

In this study, the uniform dispersion of new highly active Ni@Pd core–shell nanoparticle catalysts

supported on graphene (Ni@Pd/graphene) was prepared via a two-step procedure involving a

microwave synthesis method and a replacement method. Several characterization tools, such as

X-ray powder diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive

X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) were employed

to study the phase structures, morphologies and properties of the Ni@Pd/graphene composite.

The results indicated that a uniform dispersion of Ni@Pd core–shell structure nanoparticles on

graphene have an average particle size of 4 nm. The Ni@Pd/graphene composite was used as an

electrocatalyst for alcohol oxidation in alkaline media for fuel cells. The electrocatalytic activity

of Ni@Pd/graphene for ethanol oxidation is 3 times higher than that of the Pd/graphene

electrocatalyst at the same Pd loading. The enhanced electrocatalytic properties could be

attributed not only to the electric synergistic effect between Pd, Ni and graphene, but also the

high use ratio of Pd due to its shell structure.

1. Introduction

The direct methanol fuel cell (DMFC) produces electric power

by the direct conversion of alcohol, and has attracted

considerable attention as a clean power source. However, their

high cost has held back the application of fuel cells for a long

time, especially the high cost of the noble metal supported

electrocatalyst, which is one of the most critical units in fuel

cell systems.1–4 It is necessary to develop new catalyst materials

or improve the efficiency of existing catalysts and supports for

alcohol oxidation. Thus, there is a strong motivation to increase

the utilization of catalysts via their dispersion as small particles

on a support material. Other than high chemical stability and a

large surface area, the support can also play a role in altering

the electronic character and the geometry of the catalyst

particles dispersed on the system.5–6 There have been several

reports on loading metal nanoparticles on carbon supports or

coating carbon nanotubes with metal nanoparticles to improve

the activity and durability of the catalysts.7–11 The combination

of metal and carbon support materials potentially allows

the optimization of both the dispersion and the electrical

conductivity. Graphene, a novel one-atom-thick graphitic

carbon system, is a two-dimensional (2D) macromolecule

exhibiting an extremely high specific surface area (2600 m2 g�1),

and has been actively pursued as a fascinating material with

extraordinary properties. Owing to its good electrical conduc-

tivity and chemical stability, graphene has attracted great

attention as an electrocatalyst support. Chen et al. have

developed a facile method for synthesizing PdNPs supported

on graphene with a very narrow size distribution by the redox

reaction between PdCl42� and GO; the PdNPs-GO is very

‘‘clean’’ because of the surfactant-free formation process,

allowing it to display high electrocatalytic ability in formic

acid and ethanol oxidation.12 Current catalysts in anodes and

cathodes are mainly made of palladium, because they are

highly active for alcohol oxidation in alkaline media, where

many non-noble metals are stable for electrochemical applica-

tions.13,14 A mechanistic study on the electrooxidation of

ethanol on Pd has also been reported based on an in situ

FTIR spectroelectrochemical study.15 However, palladium as

an electrocatalyst makes fuel cells very expensive. Cheaper and

more effective electrocatalysts are therefore needed for the

further development of fuel cell technology, which can

remarkably improve the overall catalytic activity through

synergistic effects between the promoter and the noble metal.

An alloy composed of a noble metal and a transition metal has

been found to have higher electrocatalytic activity than the

simple noble metal electrocatalyst, which is explained by a

synergistic effect between the two metals. Zhang et al. studied

Pt–Ni alloy NPs supported on acid-treated graphene and

found that the alloy catalysts had higher ORR activity than

that of the pure Pt catalysts in both acidic and alkaline

solutions.16 Guo et al. developed a facile wet-chemical proce-

dure to synthesize graphene nanosheet/3D Pt-on-Pd bimetallic

School of Chemistry and Chemical Engineering, Jiangsu University,Zhenjiang 212013, P. R. China. E-mail: Xiejm391@sohu.com;Fax: +86 11 88791800; Tel: +86 11 88791708

NJC Dynamic Article Links

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2534 New J. Chem., 2012, 36, 2533–2540 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

nanodendrite hybrids using a 2D graphene nanosheet as a

support, and found that Pt-on-Pd bimetallic nanodendrites

supported on graphene nanosheets (TP-BNGNs) had a much

higher catalytic activity than conventional E-TEK Pt/C and

PB electrocatalysts for methanol electrooxidation.17 Wang

et al. synthesized Au–Pd core–shell nanocrystals with a tetra-

hexahedral structure by using Au nanocubes as structure-

directing cores, and found high electrocatalytic activity.18

They prepared core–shell electrocatalysts with a noble metal

shell on a noble metal core, however, the core–shell structure

with a noble metal as the shell and a base transition metal as

the core has more advantages. As catalytic reactions occur on

the surface of the nanoparticles, a large fraction of metal in the

core of the nanoparticle is wasted. Consequently, electro-

catalysts with a noble metal at the outer surface and a base

metal as the second atomic layer instead of a noble metal are

of great significance for the use level improvement of noble

metals. Another well known method is the spontaneous

galvanic replacement technique that was proposed by

Adzic and Kokkinidis.19 According to this method, core–shell

bimetallic electrocatalysts with a monolayer noble metal shell on

a noble metal or non-precious metal core can be prepared.20,21

However, most of the synthesized noble metal based core–shell

nanoparticles have either a thicker noble metal shell or an uneven

distribution.

Recently, many reports have used graphene as a substrate

for noble metals and their alloys for electrocatalysts, however,

to the best of our knowledge, few studies involved core–shell

particles supported on graphene as electrocatalysts. The stability

of Pd-based electrocatalysts is extremely important for their

applications in DMFCs. Chronoamperometric experiments

were widely applied to explore catalytic stability and reaction

mechanisms.22 In addition, as a support, graphene successfully

offers new opportunities for core–shell structured designs of

highly efficient electrocatalysts for fuel cells.

Since nickel nanoparticles interact strongly with the graphene

surface,23 they can be highly dispersed and show good stability

with the graphene surface, and the interaction between the Ni

core and the Pd shell which make up Ni@Pd/graphene sets up a

fairly conductive network to facilitate charge-transfer and

mass-transfer processes. The overall effect is the promotion of

the electrochemical activity and stability of Ni@Pd/graphene

compared to Pd/graphene.

Herein, Ni@Pd core–shell nanoparticles supported on graphene

(Ni@Pd/graphene) and their electrocatalytic properties for

alcohol oxidation were studied. Ni was selected as the core

because of the fact that the chemisorption and physisorption

of graphene on Ni surfaces leads to highly dispersed Ni

nanoparticles with good stability on graphene surfaces, and

because of the interactions between the Ni core and the Pd

shell, such as the ligand effect, cause a downshift in d-band

energy center20,21 which is favourable for the promotion of the

electrochemical activity and stability of Ni@Pd/graphene

compared to Pd/graphene. The Ni@Pd/graphene has a

uniform dispersion of Ni@Pd nanoparticles with an average

particle size of 4.0 nm, indicating a low thickness of the Pd

shell. We applied the Ni@Pd/graphene to the catalysis of

alcohol oxidation, and the possible origin of the high activity

of Ni@Pd/graphene was discussed.

2. Experimental details

2.1 Materials

Natural flake graphite was obtained from Qingdao Guyu

Graphite Co., Ltd. with a particle size of 150 nm. Nickel

nitrate (NiNO3), ethylene glycol, and palladium chloride

(PdCl2) were purchased from Sinopharm Chemical Reagent

Co., Ltd., China and used as received without any further

purification. Distilled water was also used throughout

the experiment. All electrochemical measurements were

performed in a three-electrode cell on an IM6e potentiostat

(Zahner-Electrik, Germany) at 30 1C controlled by a water-

bath thermostat. Platinum foil (1.0 cm2) and Hg/HgO (1.0 mol

dm�3 KOH) were used as the counter and reference electrodes,

respectively.

2.2 Preparation of GO aqueous dispersion

Graphite oxide (GO) was prepared from purified natural

graphite using a modified Hummers’ method.24 In this

method, 3.0 g of graphite powder was put into cold (0 1C)

concentrated H2SO4 (150 mL) and NaNO3 (6.0 g) solution in a

500 mL flask. With vigorous stirring, KMnO4 (15.0 g) was

added gradually, and the temperature of the mixture was kept

below 10 1C. The suspension was removed from the ice bath

and was allowed to come to room temperature. The reaction

mixture was stirred at 30 1C for 2 h until it became brown in

colour, and then diluted with distilled water (120 mL). Following

this, the mixture was stirred for 30 min and 25 mL 30 wt%H2O2

was slowly added to the mixture to reduce the residual KMnO4,

after which the color of the mixture changed to brilliant yellow.

The mixture was filtered and washed with 5% HCl aqueous

solution (1000 mL) to remove metal ions, followed by 1.5 L of

distilled water to remove the acid. For further purification, the

as-obtained graphite oxide was re-dispersed in distilled water

and then was dialyzed for one week to remove residual salts

and acids. Finally, the solid was dispersed again in water by

sonication for 30 min and separated by sintered discs. The

final product was dried under vacuum and stored in vacuum

desiccators until further use.

2.3 Preparation of Ni–graphene composite

For synthesizing the Ni–graphene compounds, 30 mg of GO

was dispersed in 100 mL of ethylene glycol (EG) by sonication

for 60 min. In this reaction, ethylene glycol acts as a reducing,

stabilizing, and dispersing agent. Then the solution was com-

bined with 15 mg of nickel nitrate under vigorous stirring, and

then subjected to microwave (MW) heating in a microwave

oven at a temperature of 180 1C, operated at 750W. The pH of

the entire solution was adjusted to 10 by adding NaOH

(2.0 M). At the end of the heating, the solution was cooled

to room temperature, and the product was isolated by several

washes with distilled water to remove the excess EG, and

subsequently separated by sintered discs. Finally the nickel

impregnated graphene (Ni–graphene) was obtained. The Ni

content was analyzed by inductively coupled plasma spectro-

scopy (ICP, Optima2000DV, USA) analysis, which showed

12.8 wt% of Ni in the Ni–graphene composite.

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2.4 Preparation of Ni@Pd/graphene electrocatalyst

Ni@Pd/graphene nanosized electrocatalysts were synthesized

using a replacement method. Typically, 20 mL of 0.093 mol L�1

H2PdCl4 and 1.56 g Ni–graphene were dispersed in 50 mL

distilled water in a beaker. The resulting solution was uniformly

dispersed by sonication for 10 min, and then vigorously stirred

for 24 h at room temperature. The black solid was separated by

sintered discs, washed with deionized water several times, and

finally dried in a vacuum oven at 60 1C. For comparison, Pd

nanoparticles supported on graphene (Pd/graphene) as an

electrocatalyst was also obtained directly by reducing the

H2PdCl4 in graphene suspension using formic acid as the

reducing agent. The theoretical Pd content in both Ni@Pd/

graphene and Pd/graphene were targeted at 12 wt%. The ICP

analysis gave the actual Pd contents as 11.8 wt% for Ni@Pd/

graphene and 11.5 wt% for Pd/grapheme. The amount of

Ni@Pd on the graphene is 18.2 wt% by ICP analysis.

2.5 Preparation of catalyst electrodes

For electrode preparation, 25.1 mg Ni@Pd/graphene or

27.8 mg Pd/graphene were dispersed in 1 mL ethanol and

1 mL 0.5 wt% Nafion suspension (DuPont, USA) under

ultrasonic agitation to form the electrocatalyst ink. The

electrocatalyst ink (40 mL) was then deposited on the surface

of a glassy carbon rod and dried at room temperature over-

night. The total Pd loading was controlled at 0.02 mg cm�2.

All chemicals were of analytical grade and used as received.

2.6 Characterization of the supports and the electrocatalysts

The morphology of the as-prepared samples was examined by

a transmission electron microscope (TEM, JEOL-JEM-2010,

Japan) operating at 120 kV. The structures of the obtained

samples were examined by X-ray diffraction (XRD) using a

D8 Advance X-ray diffractometer (Bruker AXS, Germany)

equipped with Cu-Ka radiation (l = 1.5406 A), employing a

scanning rate of 0.021 s�1 in the 2y range from 101 to 801.

Fourier transform infrared spectra (FTIR) were recorded on a

Nicolet NEXUS470 FTIR spectrometer. Raman scattering

was recorded by a Thermo Electron Corporation DXR

Raman spectrometer (USA) using a 532 nm laser source.

3. Results and discussion

3.1 The formation mechanism of Ni@Pd/graphene

The schematic illustration for the formation of the Ni@Pd/

graphene nanocomposite is shown in Fig. 1. Once the graphite

oxide was synthesized, it was dispersed thoroughly in ethylene

glycol (EG) by sonication for 60 min. Then, the solution was

combined with nickel nitrate under vigorous stirring, and was

then subjected to microwave (MW) heating in a microwave

oven. Finally the compound was isolated by washing several

times with distilled water, and the nickel impregnated graphene

(Ni–graphene) was obtained. After the Ni–graphene was added

to H2PdCl4 solution and continuously stirred for 14 h, well

dispersed Ni@Pd nanoparticles with a core–shell structure on

graphene were obtained.

3.2 FTIR spectra analysis

The Fourier transform infrared spectroscopy (FTIR) data

clearly shows the characteristic features of the oxygen contain-

ing groups present in graphene. The broad intense band

around 3448 cm–1 in Fig. 2(a) can be attributed to the

stretching vibrational mode of the O–H group. The peaks

around 2973 and 2338 cm�1 are due to the asymmetric and

symmetric stretching of –CH2 groups. The bands at 1702 cm�1

might result from CQO stretching vibrations from carbonyl

and carboxylic groups, and the peak at 1540 cm�1 may be

attributed to the skeletal vibrations from unoxidized graphitic

domains. The strong peak at around 1378 cm�1 is due to O–H

bending deformation in –COOH, and the small peak at

1135 cm�1 is attributed to C–O stretching vibrations. The

FTIR spectra clearly confirm that oxygen-containing groups,

such as hydroxyl, epoxy and carboxyl groups, are successfully

bound to the edges of the graphene nanoplatelets through

overoxidation. After the partial surface reduction, the intensity of

the peak at 3448 cm�1 is reduced because of the decrease of –OH,

and other peaks are obviously weaker in Fig. 2(b), which indicates

the attachment of Ni@Pd nanoparticles onto the functional

groups. The results demonstrate that the Ni@Pd nanoparticles

attach to the graphene support via the oxygen functional groups,

which can efficiently disperse metal nanoparticles, and remove

both the poisoned intermediates and accumulated carbonaceous

species, resulting in a higher electrocatalytic activity for ethanol

oxidation.25,26

3.3 Raman spectroscopy analysis

Raman spectroscopy was employed to obtain global structural

information, as opposed to the local structural information

that would be provided by the unreduced and partly chemically

reduced samples. Fig. 3 shows the Raman spectra of (a) GO, (b)

Ni–graphene, and (c) Ni@Pd/graphene. We found that all three

samples gave a similar Raman spectrum in terms of the shapes

and positions of the Raman peaks. They exhibit a D band

around 1347 cm�1, which corresponds to defects in the curved

graphene sheet and staging disorder, and a G band around

1582 cm�1, which should be assigned to the first order scattering

of the E2g phonon of sp2 C atoms. Quite often, the integrated

intensity ratio of the D and G bands (ID/IG) increases with the

amount of disorder for graphitic materials, vanishing for

completely defect-free graphite.27 The ratio of the peak

intensities of the D and G bands (ID/IG) for GO, Ni–graphene,

and Ni@Pd/graphene have been estimated to be around 1.05,

1.04, and 1.01, respectively. It is found that after chemical

reduction on the graphene lattice, a significantly reduced D

band might be anticipated. In the case of Ni@Pd/graphene

(Fig. 3c), similar peaks appear but the intensity of G band is

enhanced, which is primarily due to the metal–carbon inter-

actions.28,29 Wang et al. studied the interaction between nickel

and graphene with Raman spectroscopy to show that a strong

interaction between nickel and graphene exists, which results in

electron transfer between graphene and nickel metal.30 As the

graphene in this study is obtained by the chemical reduction of

GO, there must be many carbon vacancies and defects, which

may enhance the interaction between the metal nanoparticles

and graphene.31–33 Otherwise, the changes in the D/G band

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2536 New J. Chem., 2012, 36, 2533–2540 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

intensity may be caused by the size effect of graphene due to the

intensive sonication.34,35 The results from the Raman spectra

further imply that the structure of the graphene nanosheets has

been maintained after chemical reduction.

3.4 Morphology and elemental analysis

The morphology and element distribution of the Ni@Pd

nanoparticles were examined by TEM and EDS, as shown in

Fig. 4. Fig. 4B shows a large number of extremely small

nanoparticles dispersed homogeneously on the surface of the

graphene, compared with the smooth surface of the graphene

without nanoparticles (Fig. 4A). The HRTEM image describing

the crystalline nature of the Ni@Pd nanoparticles is shown in

Fig. 4C. The single crystalline nature of the Ni@Pd particles is

confirmed; the lattice planes with an interlayer distance of

0.203 nm in the core are indexed to Ni (111) crystal planes,

and the outer layer with a lattice spacing of 0.224 nm corre-

sponds to Pd (111) crystal planes. Clearly, the strong interaction

of the nickel nanoparticles with the graphene surface plays a

key role in the similar size and high dispersity of the Ni@Pd

particles on the supports. It is known that graphene tends to

interact with metal species. Lu et al. predicted that noble metal-

anchored graphene is a highly active catalyst, attributed to the

interaction of the metal atom with graphene resulting from the

partially occupied d-orbital localized in the vicinity of the Fermi

level.36 Zhao et al., using density functional theory (DFT)

calculations, proved that extra Ni–C bonds form at the inter-

face, and more electrons transfer from the interfacial C–C

bonds to the Ni–C bonds.37 Kozlov et al. also showed

that not only does Ni donate electrons to the p-band of

graphene, but graphene also back-donates electrons from its

s-band, thus forming a chemical bond via a donation/

back-donation mechanism.38 Jiang et al. studied the inter-

actions between graphene and mixed Pt–Ni metal nanoparticles

through theoretical computation, and pointed out that the

Pt–Ni nanoparticles can chemically bond with graphene.39 It

can be imagined that the Ni particles were first supported on the

graphene, and then the Pd particles were coated onto the Ni

core. At the same time, Pd particles were fixed onto the

graphene because the bottom of Pd is also in contact with the

graphene. It is possible that the palladium and nickel nano-

particles have strong interactions with the graphene surface,

which restricts the aggregation and growth of the nanoparticles

to form larger particles, resulting in the uniform distribution of

these nanoparticles.

The elemental analysis by EDS proves that Ni@Pd/

graphene is composed of C, Ni and Pd (Fig. 4D, the Cu signal

Fig. 1 Schematic diagram of the synthesis of Ni@Pd nanocatalysts on graphene.

Fig. 2 FTIR spectra of GO (a) and Ni@Pd/graphene (b).

Fig. 3 Raman spectra of (a) GO, (b) Ni–graphene, and (c) Ni@Pd/

graphene.

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comes from the sample holder). Additionally, Pd nanoparticles

supported on graphene with a diameter range of 3–4 nm could

be observed from Fig. 4E. It is obvious that small Pd nano-

particles are uniformly dispersed on graphene, indicating the

success of the microwave-assisted method for the deposition of

monodisperse Pd nanoparticles on graphene sheets in ethylene

glycol (EG) solution. The particle size distribution derived from

the TEM results (Fig. 5a) further illustrates this. The statistical

results of the Ni@Pd/graphene particles derived from the TEM

image show a narrow diameter range from 2 to 6 nm (Fig. 5b).

However, the elemental analysis by EDS proved that the Pd/

graphene has no Ni signal (Fig. 4F).

3.5 XRD analysis

The crystal structures of graphene, Ni–graphene, Pd/graphene and

Ni@Pd/graphene were determined by XRD, as shown in Fig. 6a.

The intense and sharp peak at 2y = 9.71 is associated with the C

(002) planes of the graphite-like structure of GO. After partial

reduction of GO with ethylene glycol, the peak at 2y = 9.71

disappears (Fig. 6b–d). The peaks at 2y = 44.81, 52.01 and 76.61

correspond to the Ni (111), (200) and (220) crystal planes, and the

peaks at 2y = 40.31, 46.81 and 68.31 correspond to the Pd (111),

(200) and (220) crystal planes, respectively. It is important that no

diffraction peaks of the oxides were detectable, and the intense and

sharp peaks clearly indicate the nanostructured nature of the

particles. The particle size of the Pd nanoparticles was estimated

by Scherrer’s equation:

D = Kl/(B cos y)

where D is the average diameter in nm, K is the Scherrer

constant (0.89), l is the X-ray wavelength (l = 0.154056 nm),

B is the corresponding full width at half maximum (FWHM)

Fig. 4 TEM images of GO (A) and Ni@Pd/graphene (B), HRTEM image of Ni@Pd/graphene (C) and EDS spectrum of the Ni@Pd/graphene

composite (D), TEM image of Pd/graphene and the selected area diffraction (SAED) pattern (E), and EDS spectrum of the Pd/graphene composite

(F).

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of the (220) diffraction peak, and y is the Bragg diffraction

angle. The Pd particle size was calculated from all the Pd (111),

(200) and (220) crystal plane parameters, and averaged as

4.0 nm and 3.8 nm for Ni@Pd/graphene and Pd/graphene

respectively, which are very close to the TEM results.

3.6 Ni@Pd/graphene as electrocatalysts for alcohol oxidation

Fig. 7 shows the cyclic voltammograms of ethanol, methanol

and isopropanol oxidation by Ni@Pd/graphene electrodes.

The Ni@Pd/graphene electrocatalyst shows extremely high

activity for ethanol oxidation in terms of the onset potential

and peak current density, in comparison with methanol and

isopropanol in alkaline solution, so ethanol oxidation was

selected as a model reaction for studying the electrocatalytic

performance of Ni@Pd/graphene. Ni@Pd/graphene shows a

peak current density 3 times as high as that of the Pd/graphene

electrocatalyst at the same Pd loading, as shown in Fig. 8A.

It indicated that the mass activity of ethanol oxidation in

1 mol dm�3 ethanol solution on Ni@Pd/graphene was

4428 mA mgPd�1, which is much higher than the value of

1476 mA mgPd�1 for the Pd/graphene electrode. The activity

of the Ni@Pd/graphene electrocatalyst was most likely a result

of the particular structure of the bimetallic nanodendrites and

their good dispersion on the graphene nanosheets with a high

surface area. This result also reveals that the Ni@Pd/graphene

nanoparticles are electrochemically more accessible, which is

very important for electrocatalytic reactions. We found that

Ni–Pd alloy supported electrocatalysts,40 and Pd supported

carbon and metal oxide electrocatalysts at similar electro-

chemical reaction conditions41 display obvious commercial

competition. Moreover, the Ni@Pd/graphene has an approxi-

mately 80 mV more negative onset potential than that of Pd/

graphene. The results indicated that the Ni@Pd core–shell and

alloy structure significantly improved the activity and the

output when used in fuel cells. This is extremely important

for the efficiency of fuel cells. The above results were further

evidenced by comparing the electrochemically active surface

areas (EASAs), as shown in Fig. 8B. The electrochemical

surface areas (ECSAs) of the Pd/graphene and Ni@Pd/

graphene catalysts were studied by CV tests from �0.87 to

0.50 V in 1.0 mol L�1 KOH at a scan rate of 50 mV s�1, and

were calculated based on the PdO reduction peak adapting,

and the assumption of 212 uC cm�2 of the electrode surface.42

They are 108.2 and 77.3 m2 g�1 for Ni@Pd/graphene and

Pd/graphene, respectively. The EASA of Ni@Pd/graphene is

1.4 times higher than that of Pd/graphene, confirming that the

core–shell catalyst could improve the use ratio of the Pd metal.

However, the peak current density of Ni@Pd/graphene is

3.0 times higher than that of Pd/graphene. The improvement

in EASA (1.4-fold) is less than that in the peak current density

(3.0-fold), indicating that the Ni has a synergistic effect on the

Pd electrocatalyst. When Ni was added to form the Ni–Pd

alloy, according to Zhang’s report,43 the Ni would transform

Fig. 5 Size distribution of metal particles in Pd/graphene (a) and Ni@Pd/graphene (b) derived from the TEM images.

Fig. 6 XRD patterns of GO, Ni–graphene, Pd/graphene and

Ni@Pd/graphene.

Fig. 7 Cyclic voltammograms of the oxidation of different alcohols

on the Ni@Pd/graphene electrode, in 1.0 mol dm�3 KOH/1.0 mol

dm�3 alcohol solution at 303 K, scan rate: 50 mV s�1.

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This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012 New J. Chem., 2012, 36, 2533–2540 2539

to Ni(OH)2 in alkaline media at the reaction potential, and

consequently increase the coverage of OHads on the Pd surface,

which would ultimately accelerate the reaction rate through the

equation:

Pd-ðCH3COÞads þOH�����!NiðOHÞ2

Pd-CH3COOHþ e�

The chronopotentiometric testing was carried out as shown

in Fig. 9. The results indicated that the Ni@Pd/graphene

electrocatalyst could sustain larger current densities for stable

ethanol oxidation than the Pd/graphene electrocatalyst. The

electrode potential was polarized to higher potentials at higher

current densities than 10 mA cm�2 on Ni@Pd/graphene due

to the loss of the catalytic activity for ethanol oxidation.

However, the Pd/graphene electrode could not sustain a

constant current density of 6 mA cm�2 due to the lower active

surface area and lower utilization of Pd, even at the same Pd

loading.

All of the above data reveal that the Ni@Pd/graphene

exhibits much enhanced catalytic activity and stability for

ethanol oxidation. There have been several interpretations of

the improved catalytic activity and stability of the alloyed

catalysts. Firstly, it is known that the catalytic activity is

strongly affected by the metal–graphene interaction36,39 and

the core–shell structure of the catalyst, which provides a high

EASA compared to the single metal catalysts, thus leading to

high electrocatalytic activity. Secondly, the downshift in the

d-band energy center of the noble metal induced by the presence of

Fe, Co or Ni has been taken into account to interpret the unusual

catalytic performance of the alloyed catalysts.44–46 The thin layer of

noble metal would become compressive in atom arrangement when

supported on a Fe, Co or Ni substrate. Thus, the anodic catalyst

activity and stability will be improved. To sum up, the metal–

graphene interaction, the ligand effect, the downshift in

d-band energy center19,21 and the core–shell structure may together

promote the electrochemical activity and stability of Ni@Pd/

graphene compared to Pd/graphene.

4. Conclusions

In summary, the Ni@Pd/graphene core–shell nanostructure

was successfully synthesized and uniformly dispersed on graphene

by a microwave-assisted method and a replacement method. For

ethanol oxidation, the Ni@Pd/graphene showed 3 times the

peak current density of the Pd/graphene electrocatalyst at the

same Pd loading. The electrochemical results demonstrated that

the electrocatalytic activity and stability of the Ni@Pd/graphene

composites for alcohol oxidation are significantly enhanced. The

enhanced catalytic activities have been attributed to a number of

factors, including nickel crystals entering into the palladium

crystal lattice leading to a synergistic effect, a d-band center shift,

a Pd skin effect and the interaction between the metal and

graphene.

Our research results could provide new insights into other

graphene-based metallic systems and lead to the preparation

of electrocatalyst materials for alcohol oxidation for fuel cells.

Acknowledgements

This work was supported by the financial support of the Jiangsu

Science and Technology Support Program (BE2010144) and

support from Innovation Funds of Jiangsu University

(CXLX12_0647). Dr Z. X. Yan thanks the support from the

Research Foundation for Talented Scholars of Jiangsu University

(11JDG142).

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Fig. 8 (A) Cyclic voltammograms of ethanol oxidation on Ni@Pd/graphene and Pd/graphene electrodes in 1.0 mol dm�3 KOH/1.0 mol dm�3

ethanol solution at 303 K, scan rate: 50 mV s�1 and (B) cyclic voltammograms of Ni@Pd/graphene and Pd/graphene in 1.0 mol dm�3 KOH

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