C A R B O N 4 8 ( 2 0 1 0 ) 1 1 2 4 – 1 1 3 0

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Catalytic performance of Pt nanoparticles on reducedgraphene oxide for methanol electro-oxidation

Yongjie Li a,b, Wei Gao b, Lijie Ci b, Chunming Wang a,*, Pulickel M. Ajayan b,1

a Department of Chemistry, Lanzhou University, Lanzhou 730000, PR Chinab Department of Mechanical Engineering and Materials Science, Rice University, Houston 77005, USA

A R T I C L E I N F O

Article history:

Received 30 August 2009

Accepted 19 November 2009

Available online 24 November 2009

0008-6223/$ - see front matter � 2009 Elsevidoi:10.1016/j.carbon.2009.11.034

* Corresponding author: Tel.: +86 931 8911895E-mail addresses: wangcm@lzu.edu.cn (C

1 Tel.: +1 713 348 5904; fax: +1 713 348 5423

A B S T R A C T

We have investigated a simple approach for the deposition of platinum (Pt) nanoparticles

onto surfaces of graphite oxide (GO) nanosheets with particle size in the range of 1–5 nm

by ethylene glycol reduction. During Pt deposition, a majority of oxygenated functional

groups on GO was removed, which resulted in a Pt/chemically converted graphene

(Pt/CCG) hybrid. The electrochemically active surface areas of Pt/CCG and a comparative

sample of Pt/multi-walled carbon nanotubes (Pt/MWCNT) are 36.27 and 33.43 m2/g, respec-

tively. The Pt/CCG hybrid shows better tolerance to CO for electro-oxidation of methanol

compared to the Pt/MWCNT catalyst. Our study demonstrates that CCG can be an alterna-

tive two-dimensional support for Pt in direct methanol fuel cells.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Graphene, one-atom thick planar sheet of hexagonally ar-

rayed sp2 carbon atoms, has attracted tremendous scientific

attention in recent years [1–4]. This two-dimensional (2D)

material exhibits excellent physical and chemical properties,

which makes it promising for potential applications in many

technological fields, such as nanoelectronics, sensors, nano-

composites, batteries, supercapacitors and hydrogen storage

[1]. Especially, graphene has potential application as a hetero-

geneous catalyst support in direct methanol fuel cells [5–8]. It

is well known that carbon nanotubes (CNTs), having excellent

electrical, mechanical and structural properties [9], and fasci-

nating performances in biomedicine, catalysts, sensors, and

so on [10], have been extensively studied as supports for the

dispersion of precious metal nanoparticles to enhance elec-

trocatalytic activity in fuel cells [11–13]. The most studied

bimetallic PtRu alloy catalysts have been demonstrated to

be much more effective in the electro-oxidation of methanol

[14–17]. In comparison with CNTs, graphene not only

er Ltd. All rights reserved

; fax: +86 931 8912582.. Wang), ajayan@rice.edu

.

possesses similar stable physical properties but also larger

surface areas [4,18,19]. Additionally, production cost of CCG

sheets in large quantities is much lower than that of CNTs

[4,20]. Several investigations have been carried out to produce

graphene nanosheets in bulk quantity by chemical reduction

of graphite oxide (GO) in solution [21–24]. Since abundant

functional groups on the surfaces of GO can be used as

anchoring sites for metal nanoparticles [25], it is possible to

use them as a support to produce graphene-nanoparticle hy-

brids. Combination of graphene and functional nanoparticles

may lead to materials with interesting properties for a variety

of applications, and they are specifically expected to have en-

hanced electrocatalytic activity.

In this work, a mild and environmental friendly reductant,

ethylene glycol, was used as both reductive and dispersing

agent for deposition of Pt nanoparticles on sheets of GO.

The as-prepared Pt/CCG hybrids show higher electrocatalytic

activity than Pt/MWCNT catalyst. Our study opens a novel,

and insightful way to develop electrocatalyst for fuel cells

application.

.

(P.M. Ajayan).

C A R B O N 4 8 ( 2 0 1 0 ) 1 1 2 4 – 1 1 3 0 1125

2. Experimental section

2.1. Synthesis of graphite oxide (GO)

GO was synthesized from graphite powder by a modified

Hummers method [26] as originally presented by Kovtyukh-

ova et al. [27,28].

2.2. Functionalization of multi-walled CNTs (MWCNTs)

The MWCNTs (MWCNTs > 95 wt.% purity from CheapTubes)

have outer diameters in the range of 10–20 nm with the

length of 10–30 lm. The surface treatment process of

MWCNTs is according to the method reported by Xing [11].

In a typical experiment, MWCNTs (100 mg) were mixed with

94 mL of HNO3 (69%), 80 mL of H2SO4 (98%), and 6 mL of

deionized water (DI water) in a 250 mL Pyrex glass flask.

The flask was then put in the ultrasonic bath and treated

for 3 h. After that, the mixture was filtered and washed with

DI water several times. The sonochemically treated

MWCNTs were dried in a vacuum desiccator at room

temperature.

2.3. Synthesis of Pt/CCG, Pt/MWCNT and reduction of GOby ethylene glycol (EG)

Pt nanoparticles were deposited on GO sheets by a chemical

reduction of chloroplatinic acid (H2PtCl6) in ethylene glycol–

water solution. In a typical procedure, 1.16 ml aqueous solu-

tion of 0.0443 M H2PtCl6, 8.84 ml of DI water and 90 mg of

GO were added into 40 ml ethylene glycol in a 100 ml flask.

The mixture was first ultrasonically treated for 4 h to ensure

GO being uniformly dispersed in ethylene glycol–water solu-

tion. The reduction reaction was then performed at 120 �Cfor 24 h under constant stirring. The Pt/CCG hybrids were fi-

nally separated by filtration and washed with deionized water

several times. The resulting product was dried in a vacuum

desiccator at room temperature (Fig. 1). For comparison,

deposition of Pt nanoparticles on MWCNTs was also achieved

by the same procedure. In order to understand the chemical

reduction of GO, GO was treated by ethylene glycol–water

solution without metallic salts in the same condition (labeled

as GO/EG).

2.4. Characterization

X-ray powder diffraction (XRD) was carried out using a Rigaku

D/Max Ultima II diffractometer with Cu Ka radiation

(k = 0.15418 nm). The diffraction data was recorded for 2h an-

gles between 3� and 90�. The morphologies of Pt nanoparticles

supported on CCG and MWCNTs were characterized using

JEOL 2100 field emission gun transmission electron micros-

copy (TEM). X-ray photoelectron spectroscopy (XPS) was per-

formed on a PHI Quantera SXM Scanning X-ray Microprobe

with an Al cathode (hm = 1486.6 eV) as the X-ray source set at

100 W and a pass energy of 26.00 eV. The content of Pt in com-

posite was analyzed by a Q-600 simultaneous thermo-gravi-

metric analyzer (TGA) from room temperature to 1000 �C at

a heating rate of 15 �C/min under air flow.

2.5. Electrochemical measurements

The electrochemical properties of the samples were mea-

sured by cyclic voltammetry in a standard three-electrode cell

using PSGSTAT-30 (Autolab) electrochemical workstation at

room temperature. A glassy carbon electrode (3 mm in diam-

eter) was used as the working electrode, on which 20 lL of a

paste of the catalyst was applied. The paste was a dimethyl-

formamide-nafion (49:1 volume ratio) solution with catalyst’s

concentration approximately 2.5 mg ml�1. A Pt wire was used

as the counter electrode and an Ag/AgCl electrode was used

as the reference electrode. Electrochemically active surface

area (ECSA) of Pt nanoparticles was calculated from hydrogen

electrosorption curve, which was recorded between �0.2 and

+1.2 V in 0.5 M H2SO4 solution. To measure methanol electro-

oxidation reaction activity, cyclic voltammetry was per-

formed between �0.2 and +1.0 V in a mixing solution contain-

ing 1 M CH3OH and 0.5 M H2SO4. The scan rate was set at

50 mV s�1 and the electrolyte solutions were deaerated with

ultrahigh purity argon prior to any measurements.

3. Results and discussion

Pt/CNTs hybrids have shown excellent properties in some

applications [11–13]; however, the high production cost of

CNTs has placed an obstacle in their applications. Meanwhile,

in order to deposit metal nanoparticles on CNTs surface,

functionalization needs to be done using nitric and sulfuric

acids. Although CNTs have large surface areas, nanotube

bundling and incomplete functionalization makes the acces-

sible areas very limited. Compared to CNTs, CCG nanosheets

can be easily obtained by chemical conversion of inexpensive

graphite oxide. Furthermore, using GO as a starting material

for deposition of metal nanoparticles has several advantages.

Primarily, GO nanosheets have large surface areas and both

sides of nanosheets are accessible. Secondly, the abundance

of functional groups (such as carboxyl, carbonyl, hydroxyl,

and epoxide) on the surfaces and edges of GO nanosheets al-

lows for expedient synthesis of hybrids. Finally, GO can be

easily converted by chemical reduction methods to graphene,

which offers better electrical conductivity.

3.1. Morphologies and size distributions

Morphologies of Pt nanoparticles deposited on CCG nano-

sheets and MWCNTs have been characterized by TEM. Fig. 2

shows the typical TEM images of as-synthesized hybrids. It

is clearly seen in Fig. 2a that all the transparent CCG nano-

sheets are uniformly decorated by the nanosized Pt particles

with very few aggregations, indicating a strong interaction be-

tween graphene support and particles. Furthermore, these

monolayer sheets possess large surface areas, and particles

can be deposited on both sides of these sheets [5,6,29–31].

Additionally, by functioning as a ‘‘spacer’’, these Pt nanoparti-

cles attached onto the graphene surface can prevent the re-

duced GO (as discussed later) from aggregation and

restacking, and both the faces of graphene are accessible in

their applications. Highly dispersed metal nanoparticles on

Fig. 1 – Scheme shows a formation route to anchor platinum (Pt) nanoparticles onto chemically converted graphene (CCG)

nanosheets. (1) Oxidation of pure graphite powder to graphite oxide. (2) Formation of Pt/CCG hybrids.

Fig. 2 – Transmission electron microscopy (TEM) images of (a) Pt/CCG and (b) Pt/MWCNT hybrids. The insert (c) is high

resolution TEM (HRTEM) image of Pt nanoparticles in Pt/CCG hybrids. The highly crystalline Pt nanoparticles showing fcc

crystal structure are very evident in the Pt/CCG hybrids.

1126 C A R B O N 4 8 ( 2 0 1 0 ) 1 1 2 4 – 1 1 3 0

supports with larger surface areas have advantages in cata-

lytic activity and sensor sensitivity [11]. Therefore, Pt/CCG hy-

brids should be a potential material for use in future

nanotechnology. The high resolution image of Pt/CCG hybrid

in Fig. 2c shows the oriented and ordered lattice fringes for

Pt nanoparticles. The d-spacing value of 0.227 nm coincides

with that of fcc Pt (1 1 1).

In Fig. 2b, it can be seen that Pt nanoparticles are also

deposited on MWCNTs, however, their dispersion is not as

uniform as that on CCG surfaces and some Pt nanoparticles

are agglomerated. The mean Pt nanoparticles sizes estimated

from TEM images are 2.75 and 3.5 nm on CCG sheets (shown

in Table 1) and MWNTs, respectively.

3.2. X-ray diffraction

Fig. 3 shows XRD patterns of GO, GO/EG, Pt/CCG and Pt/

MWCNT. In Fig. 3a, the characteristic diffraction peak (0 0 2)

of GO [32] at 2h = 10.6� (corresponding to a d-spacing of

0.749 nm) is ascribed to the introduction of oxygenated

functional groups, such as epoxy, hydroxyl (–OH), carboxyl

(–COOH) and carbonyl (–C@O) groups attached on both sides

and edges of carbon sheets. These surface functional groups

will subsequently act as anchoring sites for metal complexes

[25]. The diffraction peak at around 43� is associated with the

(1 0 0) plane of the hexagonal structure of carbon [33]. It can

be seen in Fig. 3b that the typical diffraction peak (0 0 2) of

Table 1 – Comparison of different parameters between Pt/CCG and Pt/MWCNT hybrids.

Samples Average Pt size (nm) ECSA (m2/g) If/Ib

Pt/CCG 2.75 36.27 0.83Pt/MWCNT 3.5 33.43 0.72

*If/Ib represents ratio of the forward and backward anodic peak currents.

Fig. 3 – XRD patterns: (a) graphite oxide (GO); (b) graphite

oxide after reduction by ethylene glycol (GO/EG); (c) Pt/CCG

and (d) Pt/MWCNT hybrids.

Fig. 4 – High-resolution C1s XPS spectra of GO, GO after

reduction by ethylene glycol (GO/EG), Pt/CCG hybrids and

pure graphite powder (PG). Each curve was obtained by 25

scans at high sensitivity. XPS characterization indicates

better restoration of the graphene structure after reduction

by ethylene glycol with chloroplatinic acid than without it.

C A R B O N 4 8 ( 2 0 1 0 ) 1 1 2 4 – 1 1 3 0 1127

GO shifts to higher angle after reduction by ethylene glycol.

This could be attributed to fact that GO nanosheets are par-

tially reduced to graphene and restacked into an ordered crys-

talline structure.

Fig. 3c and d show X-ray diffractions of Pt/CCG and Pt/

MWCNT, respectively. It is obvious that the position of the

(0 0 2) diffraction peak (d-space 0.34 nm at 26.23�) moves

slightly to higher angle after deposition of Pt nanoparticles

on CCG nanosheets, which indicates that GO is further con-

verted to the crystalline graphene, and the conjugated graph-

ene network (sp2 carbon) has been reestablished due to the

reduction process. Pt nanoparticles are suggested to play a

crucial role in the catalytic reduction of GO when using ethyl-

ene glycol as a reducing agent [5]. The strong diffraction peaks

at 2h = 39.6�, 46.2�, 67.5� and 81.4� can be assigned to the char-

acteristic (1 1 1), (2 0 0), (2 2 0) and (3 1 1) crystalline planes of

Pt, respectively, which possesses face-centered-cubic (fcc)

structure. The diffraction peak for Pt (2 2 0) is used to estimate

the Pt particle size by the Scherrer’s equation: D = 0.89k/(B cos

h) [33]. Here, the wavelength k is equal to 0.15418 nm, and B is

the full width at half-maximum (FWHM). The calculated aver-

age particle size of Pt on CCG sheets and MWCNTs are 2.96

and 3.38 nm, respectively, which are consistent with the

TEM results.

3.3. X-ray photoelectron spectra

XPS was performed on samples containing reduced GO with

and without Pt nanoparticles, which were obtained by chemi-

cal reduction reactions carried out in ethylene glycol–water in

order to understand the individual chemical reduction reac-

tions involved. The C1s peak for sp2-hybridized carbon usually

appears near 284.5 eV for pure graphite powder. After oxida-

tion, the intensity of sp2-hybridized C1s peak is significantly re-

duced, and additional peak can be identified as hydroxyl,

epoxide and carboxyl groups, which indicates that graphite is

fully oxidized into GO by introducing these oxygenated func-

tional groups (Fig. 4).

Table 2 summarizes the deconvoluted peak positions and

the areas relative to C1s sp2 peak (expressed as a percentage).

It can be seen that after reducing GO by ethylene glycol with-

out chloroplatinic acid, the carbon–carbon skeleton is rees-

tablished and the intensity of additional peak of oxygenated

functional groups is substantially reduced. On the other hand,

by depositing Pt onto the surfaces of GO nanosheets, the XPS

spectra of Pt/CCG hybrids is much more smooth compared to

original GO and reduced GO by ethylene glycol. The intensity

of some oxygenated functional groups on graphene sheets

after Pt deposition further decreases and carbon–carbon skel-

eton is recovered to a higher extent. Specifically, the hydroxyl

group signal underwent considerable decrease in this hybrids

compared with the starting GO, which could be assumed that

hydroxyl groups act as anchoring sites for Pt nanoparticles.

Meanwhile, the intensity of epoxy and carboxylic groups

was also reduced, which may be because of the reduction

by ethylene glycol and anchoring Pt nanoparticles. It is as-

sumed that Pt nanoparticles play a reductive or catalytic role

during the reduction process [5]. It is well known that graph-

ene can be chemically converted from GO by removing the

surface functional groups using physical or chemical reduc-

Table 2 – XPS data of C1s of GO, GO/EG and Pt/CCG hybrids deconvoluted into four peaks, binding energies and relative areapercentages with respect to C–C bonds in parentheses.

Samples C–C C–OH C–O–C HO–C@O

GO 284.49 (100) 286.40 (175) 287.70 (30) 288.88 (11)GO/EG 284.48 (100) 286.27 (24) 287.8 (8) 288.93 (3)Pt/CCG 284.49 (100) 286.12 (8) 287.2 (5) 288.27 (2)

Fig. 5 – Electrochemical catalytic performances of Pt/CCG

and Pt/MWCNT hybrids. Measurements were performed in

(a) 0.5 M H2SO4 and (b) 0.5 M H2SO4 + 1 M CH3OH solutions

with scan rate of 50 mV s�1. The electrochemically active

surface areas measured from (a) for Pt/CCG and Pt/MWCNT

are 36.27 and 33.43 m2/g, respectively. The If/Ib ratios

measured from (b) for Pt/CCG and Pt/MWCNT are 0.83 and

0.72, respectively.

1128 C A R B O N 4 8 ( 2 0 1 0 ) 1 1 2 4 – 1 1 3 0

tion [4,34,35]. Usually, chemicals used for reduction of GO are

very toxic, such as hydrazine [21,22,24]. Ethylene glycol is

much less harmful and serves as an alternative for GO reduc-

tion (seen from the data above).

3.4. Electrochemically active surface area

Pt/CNTs hybrids have been widely used in direct methanol

fuel cells [12,13]. It is expected that our as-prepared Pt/CCG

hybrids would present even better performance in this appli-

cation because of their structural advantage. Cyclic voltam-

metry (CV) is a convenient and efficient tool used to

estimate the ECSA of Pt catalyst on an electrode. The ECSA

of an electrocatalyst not only provides important information

regarding the number of electrochemically active sites per

gram of the catalyst, but also is a crucial parameter to com-

pare different electrocatalytic supports. Hydrogen adsorp-

tion/desorption peaks are usually used to evaluate ECSA of

the catalyst. The CV curves for different electrocatalysts,

namely Pt/CCG and Pt/MWCNT, in 0.5 M H2SO4 solution at a

scan rate of 50 mV s�1 are shown in Fig. 5a.

It shows characteristic peaks for the formation and reduc-

tion of Pt oxide. In the potential region of �0.2 to 0 V (vs. Ag/

AgCl), typical hydrogen adsorption and desorption peaks

from polycrystalline Pt were observed using CCG nanosheets

and MWCNTs as supporting materials. The integrated area

under the adsorption peak in the CV curves represents the to-

tal charge concerning H+ adsorption, QH, and has been used to

determine ECSA by employing the equation [36]

ECSA ½cm2=g of Pt�

¼ charge ½QH;lC=cm2�210 ½lC=cm2� � electrode loading ½g of Pt=cm2� :

Our calculation indicates that Pt/CCG has higher ECSA va-

lue (36.27 m2/g) than Pt/MWCNT (33.43 m2/g) (Table 1). It is

due to the fact that Pt nanoparticles are smaller and more

uniformly dispersed on the surfaces of CCG sheets, when

compared with MWCNTs. It is well known that smaller cata-

lyst particles show higher catalytic activity. Furthermore, Pt

nanoparticles dispersed on CCG surface will allow for uni-

form dispersion of the graphene nanosheets by reducing

aggregation of the sheets, thereby producing much more

accessible Pt sites for efficient catalytic activity in comparison

with MWCNTs used as catalyst support.

3.5. Methanol electro-oxidation

Cyclic voltammograms (CV) for electro-oxidation of methanol

measured at 0.5 M H2SO4 containing 1 M CH3OH solution are

shown in Fig. 5b. Both CV curves show a similar methanol oxi-

dation current peak in the forward scan and an oxidation peak

in the backward scan corresponding to the removal of the

residual carbonaceous species formed in the forward scan. It

shows that the current density of Pt/CCG is higher than that

of the Pt/MWCNT. This indicates that the electrocatalytic

activity of Pt/CCG is obviously higher than that of Pt/MWCNT,

which is consistent with the result of the ECSA. The ratio of

the forward anodic peak current (If) to the backward anodic

peak current (Ib) can be used to evaluate the catalyst tolerance

to the intermediate carbonaceous species accumulated on

electrode surface [37]. The higher If/Ib value indicates higher

C A R B O N 4 8 ( 2 0 1 0 ) 1 1 2 4 – 1 1 3 0 1129

tolerance to intermediate carbon species, which means meth-

anol can be oxidized to carbon dioxide much more efficiently.

The ratios for Pt/CCG and Pt/MWCNT are calculated to be 0.83,

0.72, respectively (Table 1), which suggests that Pt/CCG has

less carbonaceous accumulation and hence is much more tol-

erant toward CO poisoning. This indicates that Pt/CCG pos-

sesses a higher catalytic efficiency for methanol oxidation

than Pt/MWCNT. A possible bifunctional effect [38] between

Pt nanoparticles and remaining oxygenated groups is similar

with the commonly accepted bifunctional mechanism of

methanol electro-oxidation between Pt and Ru [39]. Recently,

it has been reported that oxygen-containing groups can im-

prove the electrocatalytic activity of Pt to some extent by play-

ing the role of ruthenium in the case of PtRu catalyst [38,40] to

remove those intermediate carbonaceous species and contrib-

ute to the low poisoning. In the forward scan (Fig. 5b) when the

potential arrives at�0.30 V, methanol molecules adsorb on the

surface of Pt and Pt–CO will form after a dehydrogenation pro-

cess. Meanwhile, oxidation of H2O on CCG defects generates

CCG–OH, and then the adsorption and oxidation will be en-

hanced gradually with the increased potential until the poten-

tial reaches the highest value of �0.65 V. The remaining

functional groups like –COOH and –OH groups in GO nano-

sheets (incomplete reduction in presence of Pt, as shown

above) may be responsible for improved tolerance to CO ob-

served in Pt/CCG hybrids. Thus, by choosing ethylene glycol

as both reductive and dispersing agent, we can not only con-

trol the reduction effectiveness of graphite oxide, but also

modify CCG sheets by Pt nanoparticles, and hence offer en-

hanced performance in methanol electro-oxidation. There-

fore, our simple process for preparation of Pt/CCG hybrids

will have promising applications in the future.

4. Conclusions

Inconclusion,wepresentafacile,economicandenvironmental

benign strategy to deposit Pt nanoparticles on CCG nanosheets

using an ethylene glycol–water system. We have demonstrated

that GO nanosheets were chemically converted to graphene

during Pt deposition and the concomitant reduction by ethyl-

ene glycol. Moreover, because of the higher ECSA and better tol-

erance towards CO, the Pt/CCG hybrids show much more

enhanced catalytic activity when compared with Pt/MWCNT.

The CCG nanosheet–nanoparticles hybrids could be a promis-

ing system for other heterogeneous catalysts as well.

Acknowledgements

This work was supported by the National Natural Science

Foundation of China (Grant No. 20775030). The authors Pulic-

kel M. Ajayan, Lijie Ci and Wei Gao acknowledge support from

Rice University faculty startup funds.

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