This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 2885–2887 2885

Cite this: Chem. Commun., 2012, 48, 2885–2887

Green synthesis of Pt/CeO2/graphene hybrid nanomaterials with

remarkably enhanced electrocatalytic propertiesw

Xiao Wang,ab

Xiyan Li,ab

Dapeng Liu,aShuyan Song

aand Hongjie Zhang*

a

Received 28th November 2011, Accepted 17th January 2012

DOI: 10.1039/c2cc17409j

We developed a facile strategy for clean synthesis of Pt/CeO2/

graphene nanomaterials with remarkably enhanced catalytic

properties. The graphene oxide (GO) could be used as an

oxidant to oxidize Ce3+

into CeO2 NPs, and L-lysine was used

as a linker to realize the in situ growth of Pt NPs around CeO2

NPs dispersed on graphene.

Owing to its merits like low cost, less environmental pollution,

and quick start at low temperature, etc., the direct methanol

fuel cell (DMFC) has been considered the most potential

energy carrier to meet the continuously increasing demand

for portable electronic devices.1,2 However, problems such as

the low methanol electro-oxidation kinetics and methanol

permeation across the proton exchange membrane obstruct

its commercialization.3 To prevent the noble metal catalysts

from being poisoned by CO that is an intermediate product of

anodic methanol oxidation, efforts have been devoted to

developing Pt-based alloys such as Pt–Ru, Pt–Ni, etc.4,5 On

the other hand, the recently developed strategy is to load Pt

nanoparticles (NPs) on high-surface-area metal oxides as

supporting materials. Research on the metal oxides-based

composites such as Pt/CeO2, Pt/TiO2 and Pt/SnO2, etc.6–8

has confirmed that such a kind of hybrid nanomaterials could

exhibit higher catalytic activities and stabilities during the

methanol oxidation. Especially, rare earth oxide CeO2 is of

particular interest due to the high oxygen transfer ability, high

efficiency for gaseous CO oxidation and much lower price,

which might significantly promote methanol oxidation and

reduce the preparation cost of the catalyst.9 However, due to

the low electron conductivity of CeO2 at the cost of catalytic

performance, it still necessarily deserves the investigation of

the structural design of Pt/CeO2 based catalysts to weaken the

side effects resulting from the low electron conductivity and

the lack of attachment of Pt and CeO2 NPs.

In recent years, the emergence of graphene with its unique

properties, such as high surface area and high electrical

conductivity, has opened a new avenue for utilizing two-

dimensional carbon material as a support in DMFCs.10–12

Actually, graphene supported Pt catalysts have been extensively

investigated and commonly employed as electrocatalysts for

methanol electro-oxidation.11,12 However, some serious problems

including stability and poisoning of Pt/graphene catalysts by

intermediate species were observed during the electrocatalytic

processes, which limited their further applications. In addition,

irreversible agglomeration of graphene was inevitable, which

made it difficult to provide large surface areas for good

adherence and homogeneous distribution of Pt NPs. In this

case, the combination of CeO2, Pt, and graphene may lead to

the materials with enhanced electrocatalytic activity for

methanol oxidization as well as stability and dispersibility in

the catalytic process.

Herein, we design a green and facile method to synthesize the

novel Pt/CeO2/graphene composite, which could present several

important benefits: (a) CeO2 added in the composites has high

efficiency for CO oxidation derived from its own oxygen defects

in the methanol oxidation.13 (b) Graphene could improve the

conductivity required for electrochemical reactions and also

provide a large scaffold for anchoring Pt and CeO2 NPs owing

to its large specific surface area and two-dimensional planar

conjugation structure. (c) Pt NPs grown around CeO2 NPs in situ

on the graphene nanosheet surface with the exposed clean active

surface will enhance the utilization efficiency of Pt catalysts and

the electrocatalytic activity for methanol oxidization.

As described in Scheme 1, the composites were firstly

prepared by dispersing the CeO2 NPs on graphene, and then

Scheme 1 Schematic representation of the synthesis of Pt/CeO2/

graphene composites.

a State Key Laboratory of Rare Earth Resource Utilization,Changchun Institute of Applied Chemistry, Chinese Academy ofSciences, Changchun 130022, P. R. China.E-mail: hongjie@ciac.jl.cn; Fax: +86-431-85698041;Tel: +86-431-85262127

bGraduate University of the Chinese Academy of Sciences,Beijing 100049, P. R. China

w Electronic supplementary information (ESI) available. See DOI:10.1039/c2cc17409j

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2886 Chem. Commun., 2012, 48, 2885–2887 This journal is c The Royal Society of Chemistry 2012

Pt NPs were selectively grown around CeO2 NPs supported on

graphene. Since there is no reducer such as ethylene glycol,14

hydrazine15 or NaBH4,16 etc. employed in the process of

preparation of CeO2/graphene, graphite oxide with a large

amount of oxygen-containing groups (e.g., carboxyl, hydroxyl,

and epoxy groups) could serve as a strong oxidant to oxidize

Ce3+ into CeO2.17–19 The obtained graphene plays the role of

stabilizer for its surface anchored CeO2 NPs and at the same

time CeO2 NPs serve as spacers to prevent the graphene from

aggregating and restacking after removal of solvents, finally

leading to the separated manolayer CeO2/graphene hybrid

materials. This has been firmly proved by atomic force micro-

scopy (AFM) analysis (Fig. S3, ESIw). The powder X-ray

diffraction (XRD) patterns of GO and the synthesized CeO2/

graphene are shown in Fig. 1a and b. Compared with the

characteristic diffraction peak at 2y = 10.61 in the XRD

pattern of disordered GO, the XRD pattern of the CeO2/

graphene shows a broad, low intensity peak at 2y = 22.51,

indicating the reduction of GO, and all the other peaks can be

indexed to the fluorite structured CeO2 in good consistence

with the standard data of CeO2 (JCPDS No. 34-0394). Raman

spectra also proved directly the reduction of GO (Fig. 1c and

d). As shown in Fig. 1c, GO displays two prominent peaks at

B1585 andB1351 cm�1 corresponding to the G and D bands,

respectively. After redox process, the D/G intensity ratio

increased slightly from GO to graphene, suggesting a decrease

of sp2-domain induced by the Ce3+ reduction.

The morphology of the CeO2/graphene hybrid was investi-

gated by transmission electron microscopy (TEM) and AFM

analysis. As shown in Fig. 2a and b, the as-obtained CeO2/

graphene hybrid was well dispersed on graphene nanosheets

with a uniform size of about 3 nm. The AFM analysis (Fig. S3,

ESIw) reveals that the average thickness of CeO2/graphene

hybrids obtained in this work was ca. 5 nm. When compared

with well exfoliated GO sheets, with a spacing of ca. 0.7 nm

(Fig. S1, ESIw), the greater thickness of CeO2/graphene hybrids

further suggested that the CeO2 was completely covered on

surfaces of the graphene. The energy-dispersive X-ray spectrum

(EDX) of CeO2/graphene showed the peaks of C, O, Ce and

Cu elements (from the Cu grid, Fig. S2a, ESIw).After the formation of CeO2/graphene nanocomposites,

bio-molecular L-lysine was introduced to further modify the

surface of the CeO2/graphene. Then Pt NPs were introduced

which are supposed to be preferably adsorbed on the hydro-

philic surface of CeO2, rather than on the hydrophobic surface

of graphene sheets. Fig. 2c and d show that the majority of the

Pt NPs are formed around CeO2 NPs, which can be seen from

the adjacent NPs with different lattice spacing, B0.2295 and

B0.3143 nm, corresponding to Pt (111) and CeO2 (111)

planes,20 respectively. The EDX spectrum showed the peaks

of Pt, C, O, Ce and Cu elements, further confirming the

formation of Pt/CeO2/graphene composites (Fig. S2b, ESIw).It is surmised that the L-lysine was preferred to adsorb on the

hydrophilic surface of CeO2 NPs which were full of –NH2

groups to induce the in situ growth of Pt NPs around CeO2

NPs.21 Controlled experiments were conducted without the

modification of L-lysine on CeO2/graphene, and the result

showed that the Pt NPs were aggregated together with a bad

dispersivity on the graphene (Fig. S5, ESIw). It may be due to

the presence of residual oxygenate groups on the reduced

graphene (Fig. 2b) which could bind with the Pt NPs.

However, the Pt NPs showed significant mobility on graphene

surfaces and hence tend to agglomerate,22,23 which also

indicated the important role of L-lysine in the formation of

Pt/CeO2/graphene composites.

Fig. S4 (ESIw) shows the survey of XPS spectra of CeO2/

graphene and Pt/CeO2/graphene composites. After reduction

of GO (Fig. S4a and b, ESIw), the peaks associated with C–C

(284.6 eV) became predominant, while the peaks related to

the oxidized carbon species such as C–OH (285.2 eV), C–O

(286.7 eV) and O–CQO (288.4 eV) were greatly weakened.24

These results indicate that GO has been well deoxygenated to

form graphene, which is very important for improving its

conductivity. The XPS patterns of the resulting CeO2/graphene

composite show significant Ce 3d signals corresponding to the

binding energy of CeO2 (Fig. S4c, ESIw). The Ce 3d binding

energy peaks, such as those at 883.7, 889.6, 899.6 and 918.3 eV,

for the graphene composite are consistent with a previous

report on Ce4+.25 As for the Pt/CeO2/graphene composites,

besides the Ce 3d and C signals, the major peaks at 70.6 eV

and 73.9 eV can be assigned to the 4f7/2 and 4f5/2 states of Pt

metals (Fig. S4d, ESIw), which means that the Pt metals have

been dispersed on the CeO2/graphene nanocomposites.

Fig. 1 XRD patterns of (a) GO and (b) CeO2/graphene composites.

Raman spectra of (c) GO and (d) CeO2/graphene composites.

Fig. 2 (a, b) TEM images of CeO2/graphene. (c, d) TEM images of

Pt/CeO2/graphene composites. The scale bar in (d) is 1 nm.

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This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 2885–2887 2887

The electrocatalytic oxidation of methanol using Pt/CeO2/

graphene composites as catalysts was investigated. The

Pt/graphene hybrids and commercial Pt/C catalysts were also

investigated for comparison. Fig. 3a shows the cyclic voltam-

metry (CV) curves in 0.5 M H2SO4 containing 1 M CH3OH

solution. The onset potential, peak potential, current density

and the tolerance (If/Ib) are listed in Table S1 (ESIw). It is

interesting to observe that the onset potential of Pt/CeO2/

graphene for methanol oxidation starts at 0.15 V, which is

lower than that of CeO2/graphene (0.23 V) and Pt/C (0.20 V)

catalysts, and is also much lower than the ones reported for

carbon materials supported catalysts,10,26 demonstrating the

excellent electrochemical catalytic activity of the present

Pt/CeO2/graphene nanocomposites toward the oxidation of

methanol. In addition, considering the electrochemical active

surface area (ECSA) of the Pt/CeO2/graphene did not change

much compared to Pt/graphene (Fig. S6, ESIw), it was furtherproved that the Pt NPs are selectively deposited on CeO2/

graphene and methanol is much more easily electro-oxidized

on Pt/CeO2/graphene hybrids. As listed in Table S1 (ESIw),the Pt/CeO2/graphene catalyst exhibits especially higher peak

current densities of 366 A g�1Pt at about 0.69 V (versusAg/AgCl)

in the forward potential scan (If) and 230 A g�1Pt at 0.50 V

(versus Ag/AgCl) in the backward potential scan (Ib), respec-

tively. Correspondingly, the higher If/Ib ratio of Pt/CeO2/

graphene indicates that methanol molecules are more effec-

tively oxidized on Pt/CeO2/graphene during the forward

potential scan, and also it is speculated that the added ceria

may assist the removal of the carbonaceous intermediate.

Chronoamperometric (CA) measurement was also used to

appraise the durability of catalysts. The CA technique is an

effective method to evaluate the electrocatalytic activity and

stability of catalyst material. Fig. 3b demonstrates CA curves

of Pt/CeO2/graphene, Pt/graphene and Pt/C for methanol

oxidation at a fixed potential of 0.742 V/Ag/AgCl. As

expected, the methanol oxidation current of Pt/CeO2/graphene

was evidently higher than Pt/graphene and Pt/C systems.

These results indicate that the Pt/CeO2/graphene has a durable

higher catalytic activity than Pt/graphene and Pt/C systems for

the electro-oxidation of methanol.

In conclusion, we demonstrated a facile and green method

to synthesize the Pt/CeO2/graphene nanocomposites. The GO

could be used as a green and efficient oxidant to oxidize Ce3+

cations into CeO2 NPs, leading to the in situ formation of

CeO2/graphene hybrids. L-Lysine serves as linkers to connect

Pt and CeO2 NPs together on graphene. As used for the electro-

catalytic oxidation of methanol, the obtained Pt/CeO2/graphene

composites exhibited a remarkably enhanced catalytic performance

such as the higher catalytic activity compared with simple

Pt/graphene and commercial Pt/C catalysts. Additionally, our

approach is expected to be a viable and low-cost strategy to

fabricate graphene-based complexes multi-functional nano-

materials. It is believed that this kind of nanocatalysts will

have a great potential for industrial applications in future.

The authors are grateful for the financial aid from the

National Natural Science Foundation of China Major Project

(Grant No. 91122030), ‘863’-National High Technology

Research and Development Program of China (Grant No.

2011AA03A407) and National Natural Science Foundation

for Creative Research Group (Grant No. 20921002).

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Fig. 3 (a) CV and (b) CA curves of (a) Pt/C catalysts, (b) Pt/graphene

hybrids and (c) Pt/CeO2/graphene composites.

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