Facile synthesis of Pt3Sn/graphene nanocomposites and their catalysisfor electro-oxidation of methanol{

Wei Gao,ab Xiyan Li,a Yunhui Li,b Xiao Wang,a Shuyan Songa and Hongjie Zhang*a

Received 30th April 2012, Accepted 15th August 2012

DOI: 10.1039/c2ce25662b

A novel strategy for obtaining a high dispersion of Pt3Sn

nanoparticles (NPs) on graphene is demonstrated by exploiting

the intermediate phase of Sn/graphene nanocomposite as

template. The results show that the Pt3Sn NPs with sizes of

approximate 3–4 nm uniformly disperse on graphene surface.

Electrochemical studies reveal that the Pt3Sn/graphene (50%)

nanocomposites show excellent electrocatalytic activity toward

methanol oxidation compared with commercial Pt/C catalysts.

In recent years, the tailored design of Pt-based bimetallic alloys has

attracted considerable interest because they are preferable electro-

catalyst candidates for low temperature fuel cells.1,2 In comparison

with monometallic Pt nanostructures, alloying with inexpensive

non-noble metal (M) can reduce the usage of Pt and lower the cost

of the catalyst, and the catalytic activity is often maintained or

even becomes higher depending on the chemical nature of M.3,4

Among various PtM alloys studied thus far, PtSn alloys represent

an interesting group of catalysts not only for hydrogenation5 and

dehydrogenation reactions,6 but also for electro-oxidation of

carbon monoxide (CO),7–9 methanol,10,11 ethanol,12 and ethylene

glycol.13 Previous evaluations on single crystals and thin films of

Pt3Sn for catalytic CO oxidation suggest that Sn as an alloy

component can enhance CO oxidation on Pt by promoting H2O

dissociation on Sn to form Sn–OH and by altering electronic

properties of Pt through its bonding with Pt, weakening the CO

adsorption on Pt.7,14,15 All these studies have indicated that the

PtSn alloys should be a more promising catalyst in methanol

electro-oxidation, in which CO is an intermediate product that

could poison the noble metal catalysts.

As one of the most widely used catalyst supports, the recent

emergence of graphene has provided a high surface area for the

dispersion of catalysts, a porous structure for transferring reactants

and products, and good electrical conductivity required for

electrochemical reactions.16–19 Loading of metal nanocrystals on

graphene sheets can make nanocomposites with highly active

surface areas (calculated value, 2630 m2 g21), high conductivity

(103–104 S m21), and faster carrier mobility (1.20000 cm2 V21

S21), which greatly improved electron transport for enhancing

electrocatalytic reactions.20,21 Moreover, graphene sheets are

hydrophobic and form agglomerates easily and irreversibly in

aqueous solution in the absence of dispersing reagents.22,23 There

has been a great deal of investigation on the Pt-based catalysts

supported on graphene as potential systems for oxidation of

methanol in fuel cell. The catalytic activity is incredibly enhanced

compared with the pure Pt-based nanomaterials.24,25 However, in

most of these cases, the dispersed Pt-based nanoparticles (NPs)

exhibit a wide size range, a non-uniform spatial distribution and

the synthesis methods always need laborious steps.

Herein, we demonstrate a robust strategy for obtaining a high

dispersion of Pt3Sn NPs on graphene by exploiting the

intermediate phase of Sn/graphene nanocomposite as template26

by reduction of Sn2+ and graphene oxide (GO). Electrochemical

analysis results indicate that the as-prepared Pt3Sn/graphene

nanocomposites exhibit exceptional electrocatalytic activity and

stability in methanol electro-oxidation. Our method opens up new

possibilities to engineer graphene-based hybrids for applications in

multifunctional nanoscale devices.

The synthesis of the Pt3Sn/graphene nanocomposites was

carried out in a diethyleneglycol (DEG) medium with the

reduction of NaBH4. In a typical synthesis, 0.1 g of poly(vinyl

pyrrolidone) (PVP, K-30) were dissolved in DEG/GO solution at

room temperature (GO was pre-modified with poly(diallyldi-

methylammonium chloride) (PDDA)). The solution was heated to

160 uC, and then SnCl2?2H2O (0.01 mmol in 2 mL of DEG) was

added. A freshly prepared solution of NaBH4 (4.5 mg in 2 mL of

DEG) was then added dropwise while stirring. After 12–15 min at

160 uC, H2PtCl6 (0.03 mmol in 2 mL of DEG) was added. The

resulting solution was heated to 200 uC for 1 h, resulting in a black

colloidal solution. The products were isolated by centrifugation

and washed with ethanol.

Bright field transmission electron microscopy (TEM) images of

the Pt3Sn/graphene nanocomposites in Fig. 1a and 1b show highly

dispersed metal NPs of 3–4 nm uniformly distributed on the

graphene sheets. The high resolution TEM (HRTEM) of Pt3Sn

alloys in Fig. 1c indicate that the NPs are highly crystalline. The

crystalline fringe is measured to be y0.23 nm, corresponding to

the (111) interplanar spacing of the face centered cubic (fcc) Pt3Sn

aState Key Laboratory of Rare Earth Resource Utilizations, ChangchunInstitute of Applied Chemistry, Chinese Academy of Sciences,Changchun, 130022, Jilin, China. E-mail: hongjie@ciac.jl.cn;Fax: +86 431 8569 8041; Tel: +86 431 8526 2127bSchool of Chemistry and Environmental Engineering, ChangchunUniversity of Science and Technology, Changchun, 130022, P. R. China{ Electronic supplementary information (ESI) available: XRD pattern ofthe Sn/graphene nanocomposites and CVs of the commercial Pt/Ccatalysts and the Pt3Sn/graphene nanocomposites. See DOI: 10.1039/c2ce25662b

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alloy structure (0.231 nm). Fig. 1d is the X-ray diffraction (XRD)

pattern of the as synthesized Pt3Sn/graphene nanocomposites. The

diffraction peak appearing at 2h = 27u can be indexed to the

disorderedly stacked graphene.27 The other diffraction peaks

match well with the fcc Pt3Sn standard, and shift to lower

diffraction angles compared to the fcc Pt peaks, indicating that

alloying Pt with Sn results in a crystal lattice expansion in Pt3Sn

NPs. The composition of the Pt3Sn NPs was measured to be 73/27

(atomic ratio) by both inductively coupled plasma-atomic emission

spectrometry (ICP-AES) and energy dispersive X-ray spectroscopy

(EDS) (inset, Fig. 1a).

In the reaction process, Sn2+ and GO was firstly reduced by

NaBH4 to Sn nanocrystals and graphene to form the intermediate

phase of Sn/graphene nanocomposite, which was confirmed by the

XRD pattern (ESI, Fig. S1{). Then, the intermediate Sn/graphene

nanocomposite acted as the template for in situ reduction of the

Pt4+ to form the Pt3Sn alloys. To confirm this proposition, XPS

was also carried out to investigate the synthesis product at various

stages. At first, the functional groups of GO such as –C–OH

(285.2 eV), –CLO (286.7 eV) and –COOH (288.4 eV) could form

the Sn–O linkages on the GO surface due to the chemical inter-

action (Fig. 2e). Then, on reduction using NaBH4, the –C–C– (sp2)

component corresponding to graphene is restored largely28 and the

binding energy of –CLO component shift to lower values

compared to GO (Fig. 2f). The Sn–O linkages also provided

heterogeneous nucleation sites to form the Sn NPs. As shown in

(Fig. 2b), the major peaks at 485 eV and 493 eV can be assigned to

the 3d5/2 and 3d3/2 states of Sn metal. After adding the Pt4+ to the

Sn/graphene solution, the reducing conditions (such as excess

NaBH4) made the Pt3Sn alloy form (Fig. 2a and 2d).29 As for the

Pt3Sn/graphene nanocomposites, besides the Sn 3d and C signals

(Fig. 2a), the major peaks at 70.51 eV and 73.88 eV can be

assigned to the 4f7/2 and 4f5/2 states of Pt metals (Fig. 2c), which

means that the Pt3Sn nanocrystals have been dispersed on the

graphene nanocomposites.

The electrocatalytic oxidation of methanol using Pt3Sn/graphene

nanocomposites as catalysts was investigated. The thermogravi-

metric analysis (TGA) curves (ESI, Fig. S2{) of the sample showed

that the content of Pt3Sn was 50% in the composites and so

designated as Pt3Sn/graphene (50%) composites. For comparison,

the Pt3Sn/graphene (20%) composites (ESI, Fig. S3{), Pt/graphene

(using the same methods except without adding SnCl2?2H2O in

the reaction solution) and commercial Pt/C catalysts were also

investigated. Fig. S4{ shows the cyclic voltammograms (CVs) of

four catalysts in 0.5 M N2-purged H2SO4 solution at a sweep rate

of 50 mV s21. Well-defined hydrogen desorption/adsorption peaks

and typical Pt oxidation/reduction peaks are observed. The

electrochemically active surface areas (ESA) were calculated by

measuring the coulombic charge of hydrogen adsorption and

Fig. 1 (a),(b) TEM images, inset of Fig. 1a is EDS image, (c) HRTEM

image, and (d) XRD pattern of the as-synthesized Pt3Sn/graphene

nanocomposites. The vertical lines are the standard peaks for Pt3Sn (red

lines, PDF35-1360) and Pt (blue lines, PDF 04-0802).

Fig. 2 XPS curves of (a) the Sn/graphene (black line) and Pt3Sn/

graphene (red line), (b) Sn 3d of the Sn/graphene, (c) Pt 4f of Pt3Sn/

graphene, (d) Sn 3d of the Pt3Sn/graphene, (e) C 1s of GO, and (f) C 1 s

after reduction of GO.

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

and Pt3Sn/grapheme (50%). Inset of (b) is the TEM image of the Pt3Sn/

grapheme (50%) after 1 h of catalyst use.

7138 | CrystEngComm, 2012, 14, 7137–7139 This journal is � The Royal Society of Chemistry 2012

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assuming a value of 210 mC cm22 for the adsorption of a hydrogen

monolayer. The ESA was calculated to be 48.8 m2 g21Pt for Pt3Sn/

graphene (50%), which is smaller than that Pt/C of 55.3 m2 g21Pt,

due likely to the increase of Sn on the alloy Pt3Sn NP surface and

the nonadsorption of hydrogen on Sn. In addition, the ESA was

also calculated for Pt3Sn/graphene (20%) to be 51.8 m2 g21Pt and

58.8 m2 g21Pt for Pt/graphene, respectively. Fig. 3a shows the CV

curves in 0.5 M H2SO4 containing 1 M CH3OH solution. It is

observed that all of the onset potentials of Pt3Sn/graphene (50%),

Pt3Sn/graphene (20%) and Pt/graphene for methanol oxidation

start at about 0.2 V, which are lower than that of Pt/C (0.3 V)

catalysts. In addition, in the potential scans in both positive and

negative directions, the overall current density for methanol

oxidation reaction measured on Pt3Sn/graphene (50%) is the

highest in the four electrodes. The peak current density at the

positive scan on Pt3Sn/graphene (50%) shows a value of

0.16 A mg21Pt, which is almost 2 times than that of Pt/C

(0.07 A mg21Pt). The Pt3Sn/graphene (20%) shows a value of

0.12 A mg21Pt in the positive scan, however, it shows a lower value

of 0.02 A mg21Pt than Pt/C (0.04 A mg21

Pt) in the negative scans,

and as the same results for Pt/graphene, which shows a value of

0.08 A mg21Pt in the positive scan and 0.03 A mg21

Pt in the

negative scans. All these demonstrated that Pt3Sn/graphene (50%)

catalyst has markedly improved activity towards methanol

oxidation reaction than the other catalysts. As is well known,

intermediate carbonaceous species in methanol oxidation, espe-

cially CO, can readily block the Pt catalysts and limit their activity.

The improved activity of methanol oxidation on the Pt3Sn/

graphene (50%) could be attributed to the efficient removal of

poisoning CO species from their surface, which is reflected from

the ratio of the peak current density in a forward sweep (If) to that

in a reverse sweep (Ib), If/Ib.30 As shown in Fig. 3a, If/Ib ratios are

calculated to be 2.8 for the Pt3Sn/graphene (50%), and to be 1.9 for

commercial Pt/C.

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

Pt3Sn/graphene (50%) and commercial Pt/C for methanol oxida-

tion at a fixed potential of 0.70 V/Ag/AgCl. As expected, the

methanol oxidation current of Pt3Sn/graphene (50%) was evidently

higher than Pt/C. The TEM image of the Pt3Sn/graphene (50%)

after 1 h of catalyst use showed that there is no significant

difference in the morphology and distribution after the electro-

chemical experiments. These results indicate that the Pt3Sn/

grapheme (50%) has a durable higher catalytic activity than Pt/C

and stability for the electro-oxidation of methanol.

In conclusion, we demonstrated a robust method for obtaining

a high dispersion of Pt3Sn NPs on graphene by reduction of Sn2+

and GO firstly, then Pt4+ added in the system was in situ reduced

to form the Pt3Sn alloys. As used for the electrocatalytic oxidation

of methanol, the as-prepared Pt3Sn/graphene nanocomposites

exhibited a remarkably enhanced catalytic performance such as the

higher durable catalytic activity compared with commercial Pt/C

catalysts. Additionally, our approach is expected to be a viable

and low-cost strategy to fabricate graphene-based complexes

multi-functional nanomaterials. It is believed that this kind of

nanocatalyst will have a great potential for industrial applications

in future.

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