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Electrochimica Acta 111 (2013) 779– 783

Contents lists available at ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

acile synthesis of clean Pt nanoparticles supported on reducedraphene oxide composites: Their growth mechanism and tuning ofheir methanol electro-catalytic oxidation property

enghuang Wua, Huang Huanga, Xiaomei Chenb, Zhixiong Caic, Yaqi Jianga, Xi Chena,∗

Department of Chemistry, College of Chemistry and Chemical Engineering & State Key Laboratory of Marine Environmental Science, Xiamen University,iamen 361005, ChinaCollege of Biological Engineering, Jimei University, Jimei, Xiamen 361021, ChinaKey Laboratory of Analysis and Detection Technology for Food Safety, Ministry of Education, Department of Chemistry, Fuzhou University, Fuzhou 350002,hina

r t i c l e i n f o

rticle history:eceived 25 June 2013eceived in revised form 15 August 2013ccepted 16 August 2013

a b s t r a c t

A facile hydrothermal method has been proposed to prepare reduced graphene oxide (RGO) and enhanceits reducing ability. For the first time, the spontaneous redox reaction between PtCl42− and RGO has beenreported and no additional reductants or surfactants were needed. X-ray photoelectron spectroscopy andtransmission electron microscope images were used to characterize the synthesized Pt nanoparticles sup-

vailable online 28 August 2013

eywords:educed graphene oxidet nanoparticleslectrocatalysis

ported on RGO sheet (PtNPs/RGO) composites. The reaction mechanism has been investigated by changingthe medium pH of the reaction. We further proved that the electro-catalytic property of PtNPs/RGO couldbe tuned and, because of the clean, well-dispersed PtNPs and the synergistic effect between the PtNPsand RGO, the PtNPs/RGO expressed higher electro-catalytic activity and better tolerance for methanoloxidization than a commercial Pt/C catalyst.

uel cells

. Introduction

With the increase of energy demands, the depletion of fossiluel reserves and growing awareness of environmental pollution,ecent research interests have stimulated the development ofenewable energy alternatives worldwide [1,2]. Direct methanoluel cells (DMFCs) are excellent power sources due to their highnergy-conversion efficiency, low pollutant emission, availabilityf methanol fuel, easy distribution and handling [3,4]. However,ommercial applications are still limited by the high cost of thecarce, expensive catalysts such as Pt and Pt-based alloys, currentlyhe most promising anode catalysts [5,6]. In addition, Pt is easilyoisoned by an intermediate, adsorbed CO, in methanol oxidation7]. Therefore, many efforts have been made to enhance their cat-lytic activity and CO tolerance, while reducing the use of Pt-basedatalysts. One of the strategies is to explore novel support materialso effectively disperse the Pt nanoparticles (PtNPs).

Recently, the emergence of graphene has attracted significant

ttention owing to its high surface area, high flexibility and excel-ent conductivity making it an ideal candidate for a new 2D supportn DMFCs [8–10]. The most common way to obtain graphene in

∗ Corresponding author. Tel.: +86 592 2184530; fax: +86 592 2184530.E-mail address: xichen@xmu.edu.cn (X. Chen).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.08.073

© 2013 Elsevier Ltd. All rights reserved.

large quantities employs the chemical reduction of graphene oxide(GO), the oxidized derivative of graphene. GO is highly hydrophilicand water soluble due to carrying abundant oxygen-containingfunctional groups. In addition, the oxygenated groups in GO actas the binding site for PtNPs and improve stability, resulting inincreased electrochemical activity of the PtNPs/GO [11]. So far,there are many reports on the synthesis of Pt/reduced GO (RGO)nanocomposites [12–17] that can usually be obtained from thein situ reduction of Pt salts on the preformed GO in the pres-ence of additional reductants and surfactants. However, the processgenerally involves multi-steps and requires complex manipula-tion before further use. Moreover, commonly used reductantssuch as NaBH4 [13,14] and N2H4 [15] pose environmental andhealth risks while the usage of surfactants such as poly (N-vinyl-2-pyrrolidone)[16,17] are uneconomical and block the active catalyticsites of Pt. Accordingly, developing a facile one-step method toprepare PtNPs/RGO nanocomposites without extra reductants andsurfactants is still a great challenge.

In an earlier study, we successfully prepared clean PdNPs/GOby means of the redox reaction between PdCl42− and GO [18].GO can also act as a reductant in the synthesis of AuNPs/GO

[19] and AgNPs/GO [20]. However no spontaneous redox reac-tion between PtCl42− and GO has been reported. We assumed thatonce the reducing ability of RGO was enhanced, the redox reac-tion between PtCl42− and RGO could be achieved. In this study, a

780 G. Wu et al. / Electrochimica Acta 111 (2013) 779– 783

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Fig. 1. TEM images of

acile hydrothermal method was developed to prepare RGO withnhanced reducing ability in KOH solution. Then the PtNPs/RGOas synthesized in aqueous solution without any additional reduc-

ants or surfactants. For the first time, the spontaneous redoxeaction between PtCl42− and RGO is reported. The result indicatedhat the electric potential of GO could be easily tuned to enhancets reducing ability and, because of the clean, well-dispersed PtNPsnd the synergistic effect between PtNPs and RGO, PtNPs/RGOhowed excellent electro-catalytic activity and better tolerance forethanol oxidization. In addition, the growth mechanism of PtNPsas investigated and we found that the electro-catalytic activity

f PtNPs/RGO could be tuned by modifying the medium pH in theeaction.

. Experimental details

.1. Materials

Graphite powders were purchased from Lvyin Co. (Xiamen,hina); KMnO4, concentrated H2SO4 and NaNO3 from the Chemical

Fig. 2. XPS spectra of (a) Pt 4f and the deconvolution of C 1s of

nthesized PtNPs/RGO.

Reagent Company of Shanghai (China); K2PtCl4 from Wake PureChemicals, Co. Ltd. (Osaka, Japan); 5% Nafion ethanol solution fromthe Aldrich Chem. Co. (USA); Pt/C catalyst from Alfa Aesar (20 wt%);and rod GCEs from BAS Co. Ltd. (Tokyo, Japan). All other reagentswere of analytical grade and used without further purification. Thepure water for solution preparation was from a Millipore AutopureWR600A system (Millipore, Ltd., USA).

2.2. Instruments

The morphology of the PtNPs/RGO was examined using ahigh resolution transmission electron microscope (HRTEM, FEITecnai-F30 FEG). Surface analysis performed using X-ray photo-electron spectroscopy (XPS) was carried out on PHI Quantum-2000XPS equipment. Electrochemical measurements were performedusing a CHI 660B Electrochemical Analyzer (CHI Co. Shanghai,

China) equipped with a conventional three-electrode system:a GCE coated with PtNPs/RGO (5 �L) film, a Pt auxiliary elec-trode and a saturated calomel electrode (SCE) as the referenceelectrode.

(b) the PtNPs/RGO hybrids, (c) the RGO and (d) the GO.

G. Wu et al. / Electrochimica Acta 111 (2013) 779– 783 781

Fig. 3. TEM images of PtNPs/RGO synthesized at pH 7.

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nal (Fig. 2a) and C 1s signal (Fig. 2b) corresponding to the bindingenergy of Pt and RGO, respectively. These results further indicatedthat PtNPs were effectively assembled on the surface of the RGO.

Fig. 4. Illustration of the proposed mec

.3. Synthesis of RGO and PtNPs/GO

GO was prepared from natural graphite using the modifiedummers’ method [21]. 25 mg as-synthesized GO was dispersed

n 50 mL water to obtain a yellow-brown aqueous solution withhe aid of ultrasonication. KOH has previously been used for theeoxygenation of graphite oxide by sonication and heating [22].ere, KOH was simply added to 0.5 mg mL−1 GO solution to reach

concentration of 50 mM, before the solution was heated in anil bath at 100 ◦C for 20 h to obtain RGO. For the preparationf PtNPs/RGO, 0.5 mL RGO and 0.5 mL K2PtCl4 (10 mM) aque-us solution were mixed in a vial under stirring for 80 min at0 ◦C. After reaction, the mixture was washed with 50 mM H2SO4nce and pure water four times and centrifuged to remove theemaining reagents. Then the sediment was redispersed in 0.5 mL2O.

.4. Preparation of the PtNPs/RGO modified GCE

Before modification, a GCE was polished with 1, 0.3, and 0.05 �m-Al2O3, sequentially. After ultrasonic concussion, the polishedCE was dried at room temperature. 5 �L PtNPs/RGO suspensionas then dropped onto the GCE surface and it was air dried at room

emperature.

. Results and discussion

As shown in Fig. 1a, the TEM image reveals the characteristicrinkles on the sheets, indicating the edge of the RGO. A porous

tructure could be observed on the RGO which is attributed to theeaction of GO with KOH [23]. PtNPs with a diameter of about 3 nmFig. 1b) were homogeneously attached and dispersed well on theurface of the RGO at a high density. The interplanar space of the

for PtNP formation onto RGO sheets.

particle lattice from the HRTEM (as shown in Fig. 1c) was 0.230 nm,which agrees with the (1 1 1) lattice spacing of face-centered-cubicPt (0.227 nm). In order to compare the reducing ability of RGO andGO, we also tried to synthesize PtNPs/GO using GO solution. PtNPswere seldom observed on the GO surface (see supplemental mate-rial Fig. S1) and no obvious hydrogen peak could be observed ina cyclic voltammetry (CV) measurement with 0.1 M H2SO4. Thisresult, in turn, indicated that the RGO obtained from the KOHmedium presented an improved reducing ability and the galvaniccell effect took place effectively between RGO and PtCl42−.

The XPS patterns of PtNPs/RGO show the significant Pt 4f sig-

Fig. 5. CVs of PtNPs/RGO (solid line), PtNPs/RGO (pH 7) (dashed line) and commer-cial Pt/C catalyst (dotted line) in an aqueous solution containing 0.1 M H2SO4 at ascan rate of 50 mV s−1.

782 G. Wu et al. / Electrochimica Acta 111 (2013) 779– 783

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Basic Research Program of China (2010CB732402), and the National

ig. 6. (a) CVs and (b) CAs for methanol oxidation reactions catalyzed by PtNPs/RGn an aqueous solution containing 0.1 M H2SO4 and 0.1 M CH3OH.

he XPS results from the RGO and GO (Fig. 2c and d) surface wereelpful in studying the reaction process. The O/C ratio in the RGOecreased remarkably after the GO was treated with KOH, and oxy-enated groups such as epoxide and hydroxyl functional groupsere partially removed, which meant the recovery of the sp2 struc-

ure of RGO. This result revealed that KOH played an importantole in the transformation from GO to RGO, resulting in a highereducing ability of the product. In addition, the remaining oxygen-ontaining functional groups could still act as the binding sites fortNPs. After reaction with PtCl42−, as shown in Fig. 2c, the peakf the C O functional group on the RGO surface at 288.2 eV in thePS spectra almost disappeared. These results indicated that theemaining oxygenated functional groups on the RGO surface playedn important part in the formation of PtNPs, and the RGO was fur-her deoxygenated in the PtNPs/RGO hybrids, which could improvehe conductivity of the PtNPs/RGO in electro-catalysis.

For better understanding the reaction mechanism, and in ordero avoid the effect of KOH, we synthesized PtNPs/RGO in a pH 7olution with the aid of dilute H2SO4, and found that the reactionetween RGO and PtCl42− still happened. TEM (Fig. 3) and XPS datasee supplemental material Fig. S2) showed that the PtNPs/RGOould be synthesized at pH 7 with the same diameter and par-icle lattice of PtNPs and, furthermore, the RGO was also furthereoxygenated in the reaction. These results demonstrated that thepontaneous redox reaction between PtCl42− and RGO, and KOHnly affected the production of RGO.

We further found that no reaction between RGO and PtCl42−

ould be observed when the medium pH was lower than 3, becauset such a pH, the remaining carboxyl group in RGO was protonated.e assumed the reaction mechanism between RGO and PtCl42−

as that: first, after reaction with KOH, GO was transformed to RGO.he sp2 structure of the RGO was recovered with enhanced reduc-ng ability; and second, the deprotonation of the carboxyl group

as essential. Any deprotonated carboxyl group remaining on theGO surface might attack PtCl42− at the same time that the recov-red sp2 structure of RGO offered � electrons to reduced Pt2+ tobtain PtNPs. The proposed mechanism for PtNP formation ontoGO sheets is presented in Fig. 4.

The CVs of GO and RGO modified GCE were also investigated. Asresented in supplemental material Fig. S3, in both 0.1 M H2SO4 and.1 M NaOH solution, the current of RGO was about 10 fold higherhan that of GO, suggesting that the sp2 structure of RGO was recov-red. Furthermore, in 0.1 M H2SO4 solution, the oxidation potentialf RGO was around 0.26 vs SCE/V, much lower than the 0.36 vsCE/V of GO modified GCE, in accordance with the conclusion thathe reducing ability of RGO was enhanced.

The catalytic property of the PtNPs/RGO composite towardethanol oxidation was carried out using a three-electrode system

n 0.1 M H2SO4 solution containing 0.1 M CH3OH at room temper-ture. For comparison, the catalytic performance of 20 wt% Pt/C

id line), PtNPs/RGO (pH 7) (dashed line) and commercial Pt/C catalyst (dotted line)

commercial electrocatalyst (Johnson Matthey) and PtNPs/RGO (pH7) were also measured. All potentials in the CV measurements wereconverted to the standard hydrogen electrode (SHE) values. The CVsof both Pt/C and PtNPs/RGO in 0.1 M H2SO4 solution are presentedin Fig. 5. The electrochemically active surface areas were esti-mated using a calculation of the average of the hydrogen desorptionand absorption area from CVs in 0.1 M H2SO4 solution and were1.12, 1.33 and 1.18 cm2 for PtNPs/RGO, PtNPs/RGO (pH 7) and Pt/C,respectively. As shown in Fig. 6a, the peak current density value(Jf) in a forward (positive) scan for the PtNPs/RGO (1.89 mA cm−2)and the PtNPs/RGO (pH 7) (1.64 mA cm−2) were about 2.15 and 1.86fold higher than that of Pt/C (0.88 mA cm−2), indicating the excel-lent activity of the PtNPs/RGO in electrochemical catalysis towardmethanol electro-oxidation. The difference between PtNPs/RGOand PtNPs/RGO (pH 7) could be attributed to the fact that the KOHcould still act on RGO in the reaction between RGO and PtCl42−,producing a better conductivity of the PtNPs/RGO. Chronoampero-metric (CA) measurement was also used to appraise the durabilityof the catalysts. Fig. 6b demonstrates the CA curves of PtNPs/RGOand Pt/C for methanol oxidation at a fixed potential of 0.77 vs SHE/V.As expected, the current density of PtNPs/RGO was higher thanthat of Pt/C over the entire time range. These results revealed thatthe PtNPs/RGO was of a more durable and higher electro-catalyticactivity than Pt/C systems in the electro-oxidation of methanol.

4. Conclusion

We developed a very simple and green route for the synthe-sis of ultra-small and well-dispersed PtNPs supported on RGO. Forthe first time, the spontaneous redox reaction between PtCl42−

and RGO was reported. The obtained PtNPs/RGO was very “clean”because of the surfactant-free formation process, allowing it toexpress high electro-catalytic ability in methanol oxidation. Thereaction mechanism was also discussed. In addition, consideringthat the reduction potential of AuCl4−, PdCl42− and Ag+ was lowerthan that of PtCl42−, this approach is expected to be a viable andlow-cost strategy to produce RGO functionalized with metal NPssuch as Pd, Au and Ag. It is believed that this kind of nanocatalystwill have great potential in areas such as sensors, fuel cells andcatalyst supports.

Acknowledgements

This research work was financially supported by the National

Nature Scientific Foundation of China (No. 21175112, 21375112)which are gratefully acknowledged. Furthermore, we would like toextend our thanks to Professor John Hodgkiss of The University ofHong Kong for his assistance with English.

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ppendix A. Supplementary data

Supplementary data associated with this article can beound, in the online version, at http://dx.doi.org/10.1016/j.lectacta.2013.08.073.

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