Caffeine-assisted facile synthesis of platinum@palladium core-shell nanoparticles supported on reduced graphene oxide with enhanced electrocatalytic activity for methanol oxidation

Accepted Manuscript Title: Caffeine-assisted facile synthesis of platinum@palladium core-shell nanoparticles supported on reduced graphene oxide with enhanced electrocatalytic activity for methanol oxidation Author: Jin-Xia Feng Qian-Li Zhang Ai-Jun Wang Jie Wei Jian-Rong Chen Jiu-Ju Feng PII: DOI: Reference:

S0013-4686(14)01589-8 http://dx.doi.org/doi:10.1016/j.electacta.2014.07.152 EA 23197

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

14-6-2014 25-7-2014 27-7-2014

Please cite this article as: J.-X. Feng, Q.-L. Zhang, A.-J. Wang, J. Wei, J.R. Chen, J.-J. Feng, Caffeine-assisted facile synthesis of platinum@palladium core-shell nanoparticles supported on reduced graphene oxide with enhanced electrocatalytic activity for methanol oxidation, Electrochimica Acta (2014), http://dx.doi.org/10.1016/j.electacta.2014.07.152 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Caffeine-assisted facile synthesis of platinum@palladium core-shell nanoparticles supported on reduced graphene oxide with enhanced electrocatalytic activity for methanol oxidation

a

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Jin-Xia Feng,a b Qian-Li Zhang,b Ai-Jun Wang,a* Jie Wei,b Jian-Rong Chen,a Jiu-Ju Feng a *

College of Geography and Environmental Science, College of Chemistry and Life Science,

b

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Zhejiang Normal University, Jinhua 321004, China

School of Chemistry and Biological Engineering, Suzhou University of Science and Technology,

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Suzhou, 215009, China

*Corresponding author: [email protected] (JJF), [email protected] (AJW); Tel./Fax: +86 579

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82282269.

Abstract

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A facile, rapid, and wet-chemical co-reduction method is developed for synthesis of

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platinum@palladium core-shell nanoparticles supported on reduced graphene oxide

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(denoted as Pt@Pd/RGO) with the assistance of caffeine, without any seed or template. Caffeine is used here as a structure-directing agent and a capping agent,

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which is critical to the formation of Pt@Pd core-shell nanoparticles. Furthermore, the as-synthesized Pt@Pd/RGO shows the enlarged electrochemically active surface area, remarkably enhanced catalytic activity and improved stability for methanol oxidation reaction (MOR), compared to Pt/RGO, Pd/RGO, commercial Pt black and Pd black catalysts.

Keywords: Reduced graphene oxide; core-shell nanoparticles; Electrocatalysis; Methanol oxidation reaction

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1. Introduction Direct methanol fuel cells (DMFCs) have attracted increasing interest because of their potential applications in portable electronic devices with high efficiency, low emission,

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and simple system [1, 2]. It is known that platinum (Pt) is widely used as catalysts in

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DMFCs [3]. However, the sky-rocketing price and the scarce resources of Pt have

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hindered their further commercial applications. Therefore, it is urgent to develop substitutes for Pt catalysts to reduce the overall cost and enhance poisoning resistance

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simultaneously.

One strategy to solve these problems is the design of Pt-based bimetallic

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catalysts such as Pt-Pd [4], Pt-Cu [3], Pt-Ru [5], Pt-Au [6], Pt-Ni [7], Pt-Ag [8], and Pt-Co [9]. Among the Pt-based bimetallic catalysts, Pt-Pd catalyst is a representative

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candidate with better catalytic performance in alkaline DMFCs [4, 10, 11], because the existence of Pd can improve the catalytic activity of methanol and resistance to

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CO-poisoning for the variation of Pt electronic structure [12, 13]. Additionally, the introduction of Pd can reduce the cost due to its relatively low price and high abundance on earth [14].

To date, a variety of Pt-Pd alloy nanostructures have been synthesized, such as

cages [15], wires [16], tetrahedra [17], dendrites [18, 19], clusters [20], octahedra [21], cubes [22, 23], and core-shell nanostructures [24]. Particularly, core-shell nanostructures have received increasing attention for their unique catalytic properties [11, 25]. For instance, Zhang et al. synthesized Pd@Pt core-shell nanostructures, which exhibited the improved catalytic activity for methanol oxidation reaction (MOR)

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than those of individual Pd and Pt counterparts [26]. In another example, Long et al. prepared Pt@Pd core-shell nanoparticles with the enhanced catalytic activity towards MOR [27].

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Another strategy is to load Pt catalysts on a support to maximize the catalytic

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activity and minimize the use of Pt [19]. Earlier studies have demonstrated that an

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ideal support should have low cost, large surface area, good electrical conductivity to promote electron transfer in redox reactions, strong catalyst-support interactions to

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improve catalyst efficiency and facilitate electron transfer, and easy recovery of the loaded catalyst [23, 28]. As manifested, graphene is an appropriate candidate for its

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large specific surface area (~2600 m2 g–1), two-dimensional structure, superior electrical conductivity, and potentially low-cost manufacturing [29, 30]. Furthermore,

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graphene oxide (GO), derived from graphene, has many oxygen-containing functional groups on its surface, including hydroxyl, epoxide, carbonyl, and carboxyl groups.

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These features make GO good solubility in aqueous media and even some organic solvents, providing more active sites for supporting metallic catalysts [31-33]. Therefore, GO and reduced GO (RGO) used as supports have demonstrated the improved performances towards MOR [34-36]. Herein, a facile, rapid, and straightforward co-chemical reduction method was

developed for synthesis of Pt@Pd core-shell nanoparticles uniformly supported on RGO nanosheet (denoted as Pt@Pd/RGO) in the absence of any seed or template, using caffeine as a structure-directing agent and a capping agent. The electrocatalytic performances of the as-prepared Pt@Pd/RGO were investigated in some detail, using

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MOR as a bench model system.

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2. Experimental 2.1. Materials

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Graphite powder (99.95%, 8000 mesh), chloroplatinic acid (H2PtCl6·6H2O),

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palladium chloride (PdCl2), caffeine, hydrazine hydrate (N2H4·H2O), methanol, commercial Pt black and Pd black were purchased from Shanghai Aladdin Chemical

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Reagent Company (Shanghai, China). All the other chemicals were of analytical grade and used without further purification.

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1.0 g H2PtCl6·6H2O was directly dissolved with 50 mL water (38.62 mM, H2PtCl6 solution). 1.77 g PdCl2 was dissolved with 0.4 mL HCl (37%), and then

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diluted to 100 mL with water (100 mM H2PdCl4). All of the aqueous solutions were

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prepared with twice-distilled water throughout the whole experiments.

2.2. Preparation of Pt@Pd/RGO Firstly, GO was synthesized by the modified Hummer’s method [37, 38].

Typically, 1.0 g graphite powder was mixed with 1.0 g NaNO3 and 33 mL H2SO4 (98%) in a water-ice bath. Next, 6.0 g KMnO4 was slowly added to the above mixture

at 35 °C for 1.5 h. After that, 40 mL water was gradually put into the mixture, followed by maintaining the temperature at 95 °C for 35 min. Nest, the mixture was diluted with 100 mL water, and then 6 mL H2O2 (30%) was added to the mixture. The mixture was purified by centrifugation and thoroughly washed with water. After the

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complete removal of remaining salts and acid, the resulting GO suspension was further treated by ultrasonication to obtain exfoliated GO. For typical synthesis of Pt@Pd/RGO, 0.5 mL GO (0.5 mg mL–1) was put into 10

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mL caffeine solution (25 mM) in a water-ice bath. After thoroughly mixing, 0.5 mL of

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100 mM H2PdCl4 and 1.29 mL of 38.62 mM H2PtCl6 were gradually put into the

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above mixture under stirring. Then, 0.2 mL of hydrazine hydrate (80%) was drop-wisely added into the mixed solution, followed by the solution color change

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from yellow to black. Next, the black reaction solution was further stirred for 15 min. The final product was collected by centrifugation and thoroughly washed with water,

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and then dried at 60 °C in a vacuum for further characterization. For comparison, Pt/RGO and Pd/RGO were prepared in a similar way, except only using individual

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2.3. Characterization

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H2PtCl6 and H2PdCl4 as the precursor, respectively.

The as-prepared samples were characterized by transmission electron

microscopy (TEM) and high-resolution TEM (HRTEM) using a JEM-2100F HR operated at 200 kV, equipped with an energy-dispersive X-ray spectrometer (EDS). The samples were dispersed in ethanol by ultrasonication, followed by depositing a drop of the suspension on a Cu grid. X-ray diffraction (XRD) measurements were performed on a Bruker-D8-AXS diffractometer system equipped with Cu Kα radiation (Bruker Co., Germany). Raman spectra were recorded from 100 to 2500 cm–1 by a Renishaw Raman system model 1000 spectrometer equipped with a

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charge-coupled-device

(CCD)

detector,

using

a

633

nm

He/Ne

laser.

Fourier-transform infrared (FT-IR) measurements were acquired on a Nicolet NEXUS-670 Fourier transform infrared spectrometer with KBr beam splitter in the

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wave-number range from 500 to 4000 cm–1. X-ray photoelectron spectroscopy (XPS)

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measurements were conducted on a K-Alpha XPS spectrometer (ThermoFisher, E.

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Grinstead, UK), using Al Kα X-ray radiation (һν = 1486.6 eV), and the binding energy was calibrated by means of C 1s peak energy of 284.6 eV. Thermogravimetric

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analysis (TGA, NETZSCH STA 449C) were performed in air. The samples were

2.4. Electrochemical measurements

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heated from room temperature to 800 °C at a heating rate of 10 °C min–1.

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All electrochemical experiments were conducted on a CHI 660D electrochemical workstation (CH Instruments, Chenhua Co., Shanghai, China). And a standard

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three-electrode cell was used, containing a platinum wire, a saturated calomel electrode (SCE), and a bare or modified glassy carbon electrode (GCE, 3.0 mm in diameter) as the counter electrode, reference electrode, and working electrode, respectively.

For preparation of Pt@Pd/RGO modified electrodes, 2 mg of Pt@Pd/RGO was

firstly put into 1 mL water under ultrasonication to obtain a homogeneous suspension. Next, 8 µL of the suspension was casted on the clean GCE and dried in air. Afterward, 4 uL of Nafion (0.05%) was placed onto the electrode surface to seal the sample in place. For comparison, Pt/RGO, Pd/RGO, Pt black and Pd black

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modified electrodes were fabricated under the same conditions. The loading of each catalyst was 0.226 mg cm–2. For comparison, the catalytic currents of the catalysts were normalized with the associated electrochemically active surface area (ECSA)

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and mass of the loading catalyst on the electrode surface to calculate the specific

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activity and mass activity.

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The ECSA of the catalyst modified electrode was calculated by CO stripping experiments in 0.5 M H2SO4. Specifically, CO was firstly bubbled through 0.5 M

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H2SO4 for 20 min to form CO adlayer on the surface of the catalyst at 0.1 V (vs. SCE). Then, excess CO in the solution was purged with high-purity N2 for 30 min. The

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amount of adsorbed CO (COads) was evaluated by cyclic voltammetry at a scan rate of 50 mV s–1. The first cycle was conducted to electro-oxidize the COads, while the

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second cycle was used as background to confirm whether excess COads was removed from the catalyst layer. The catalytic performances of the catalyst modified electrodes

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were determined by cyclic voltammetry in 1.0 M KOH containing 1.0 M methanol. Chronoamperometric experiments were carried out at –0.25 V in 1.0 M KOH containing 1.0 M methanol.

3. Results and Discussion

3.1. Characterization of Pt@Pd/RGO The morphologies of the typical products were characterized by TEM experiments. As displayed in Fig. 1A, the product contains many well-defined Pt@Pd core-shell particles evenly distributed on RGO, without any obvious aggregation.

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Representative high-magnification TEM images demonstrated good crystallinity of Pt@Pd core-shell particles (Fig. 1C, D, and E). The adjacent fringes show the d-spacing distance of 0.227 nm (Fig. 1B, c and d), which agrees well with the (111)

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planes of face-centered cubic (fcc) Pt [39]. Meanwhile, the lattice fringes with the

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spacing of 0.196 nm and 0.225 nm (Fig. 1B, a, b and e) correspond to the (200) and

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(111) planes of fcc Pd, respectively [40]. These results strongly demonstrate that the as-prepared Pt@Pd core-shell particle is composed by a Pt core inside and a complete

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shell of Pd outside, without any alloy formed, as strongly supported by HAADF-STEM-EDS mapping images and EDS line scanning profiles of Pt and Pd

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(Fig. 2), where Pd is mainly found in the branch regions, while Pt is only detected in the core region.

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The amount of caffeine is important for synthesis of Pt@Pd core-shell nanostructures. The absence of caffeine induces the formation of non-uniform

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aggregated Pt@Pd nanoparticles on the surface of RGO (Fig. S1 A, Supporting Information). This is due to the absence of stabilizing or capping agent when a large numbers of Pt and Pd atoms are quickly produced. Alternatively, excessive caffeine yields well-dispersed dendritic nanoparticles on the surface of RGO (e.g. 50 mM, Fig. S1 B), implying the essential role of caffeine as a structure-directing agent for preparation of core-shell nanostructures. These observations strongly demonstrate caffeine as a capping agent and a structure-directing agent in the formation of Pt@Pd core-shell nanoparticles on the surface of RGO [41]. Moreover, less (e.g. 1 mM) and excess (e.g. 7.5 mM) amounts of the precursors (i.e. H2PtCl6 and H2PdCl4) produce

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Pt-Pd products with partial agglomeration (Fig. S2, Supporting Information), indicating the essential role of the total amount of the precursors. It is known that the standard reduction potential of PtCl62–/Pt (0.73 V) is higher

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than that of PdCl42–/Pd (0.591 V). Thus, Pt nanocrystals are initially formed at the

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very early stage (Fig. S3 A, Supporting Information). Next, Pd shell can grow

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epitaxially on Pt core to form Pt@Pd core-shell nanostructures (Fig. S3 B, Supporting Information), owing to the fcc structure of both Pt and Pd. That is, Pd atoms on shell

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coherently match well with the lattice structure of Pt core [27, 42], again showing caffeine as a structure-directing agent for synthesis of Pt@Pd core-shell nanoparticels

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[41].

The composition and crystal structures of Pt@Pd/RGO were confirmed by EDS

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and XRD measurements. As depicted in Fig. S4 (Supporting Information), EDS spectrum shows the peaks corresponding to C, Pd, and Pt elements, revealing the

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coexistence of Pt and Pd on the surface of RGO. Furthermore, the atomic ratio of Pd to Pt is about 2.0.

Fig. 3 provides XRD spectrum of Pt@Pd/RGO, in which four representative

diffraction peaks can be assigned to the (111), (200), (220), and (311) planes of fcc metallic structure. However, the peak positions of Pt are very close to those of Pd in the XRD spectrum, making it difficult to resolve the peaks of Pt and Pd in the present synthesis, owing to the similarity of bulk Pd (JCPDS-46-1043 Pd) and Pt (JCPDS-04-0802 Pt) crystalline structures [43]. In addition, a broad peak is observed at about 23° for Pt@Pd/RGO, different from that of GO only with a strong peak

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centered at 11°. This observation confirms that GO is efficiently reduced to RGO during the one-pot reaction process [44]. XPS is a powerful tool to determine surface chemical state and composition of

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Pt@Pd/RGO. As illustrated in Fig. 4A, the peaks corresponding to Pd 3d and Pt 4f are

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observed at 337.51 and 76.36 eV, respectively, which indicate the coexistence of Pd

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and Pt in Pt@Pd/RGO. In the high-resolution Pt 4f XPS region (Fig. 4B), two pairs of doubles located at Pt 4f7/2 (71.35 eV), Pt 4f5/2 (74.62 eV), and Pt 4f7/2 (72.78 eV), Pt

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4f5/2 (76.22 eV) are assigned to Pt0 and PtII species [45], respectively. Similarly, for the Pd 3d region, the doublet at 335.69 and 341.09 eV (Fig. 4C) are attributed to Pd0,

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while the doublet at 337.46 and 342.65 eV is come from PdII [46]. Taken together, Pt0 and Pd0 are the predominant species by evaluating their peak intensities, indicating

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efficient reduction of PtCl62– and PdCl42– ions in the present synthesis. Furthermore, the peak at 284.81 eV is attributed to the binding energy of C 1s,

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which can be deconvoluted into four sublevels centered at 284.81, 286.94, 288.16, and 289.58 eV, corresponding to the C–C, C–O, C=O, and O–C=O groups (Fig. 4D), respectively [47]. Specifically, the prominent sp2-hybridized C–C peaks and the relatively weaker peak intensities of oxygen-containing groups indicate the efficient reduction of GO [48], which agrees well with XRD analysis of Pt@Pd/RGO. As shown in Fig. 5 A, there are two distinct bands detected at around 1337 and 1594 cm−1 for Pt@Pd/RGO (Fig. 5A, curve a) and GO (Fig. 5A, curve b), which can be designated as D and G bands of graphene, respectively [49]. The D band

corresponds to the defects in the curved graphene sheet and staging disorder, while

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the G band is related to the stretching mode of crystal graphite [50]. Meanwhile, the intensity of the D band increases after the reduction of GO, and the intensity ratio of D to G band (ID/IG) is obviously increased, compared with that of GO. It means the

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effective reduction of GO to RGO [51], as strongly demonstrated by the XPS and

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XRD analysis.

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As shown in Fig. 5B, it can be clearly discerned four types of carbons on the FT-IR spectrum of GO (Fig. 5B, curve b): the stretching vibration of C–O bond at

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1056 cm−1, stretching vibration of C–OH band at 1226 cm−1, skeletal vibration of C=C band at 1625 cm−1, and stretching vibration of C=O band at 1739 cm−1. In

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addition to the mentioned bands, there is a strong band at 3421 cm−1, which is ascribed to the O–H stretching vibration [52]. Most importantly, the characteristic

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peak of C–OH stretching at 1226 cm−1 and C=O stretching at 1739 cm−1 are disappeared for Pt@Pd/RGO (Fig. 5B, curve a), which manifest the complete

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reduction of GO to RGO [53], as supported by the XPS and XRD results. The thermal stability of Pt@Pd/RGO was studied by TGA analysis (Fig. 6, curve

a), using Pt/RGO (Fig. 6, curve b), Pd/RGO (Fig. 6, curve c), and GO (Fig. 6, curve d) as references. Clearly, TGA curves of Pt@Pd/RGO, Pt/RGO, and Pd/RGO display much smaller drops in mass than those of GO under the identical conditions. Specifically, the weight loss below 200 °C is attributed to the evaporation of the absorbed water molecules on the surface and the removal of some oxygen-containing functional groups. The subsequent gradual weight loss from 200 to 250 °C mainly comes from the decomposition of the oxygen-containing functional groups. And the

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weight loss above 550 °C ascribes carbon oxidation [54]. These results further confirm the efficient reduction of GO to RGO. The amount of metal loading is

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89.39% for Pt@Pd/RGO, 95.85% for Pt/RGO, 92.65% for Pd/RGO, respectively.

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3.2. Electrochemical measurements

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The ECSA provides important information regarding the number of electrochemically active sites of a catalyst. In this work, CO stripping experiments

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were performed to evaluate the ECSA of the as-prepared catalysts (Fig. 7), assuming a value of 0.42 mC cm–2 for the oxidation of a CO monolayer [55] Q m  0.42

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ECSA 

where Q is the charge for CO desorption electrooxidation in micro-coulomb (mC),

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m is the loading amount of Pt and Pd (g), and 0.42 is the charge required to oxidize a

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monolayer of CO on metal (mC cm–2). As a result, the ECSA of Pt@Pd/RGO is

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calculated to be 15.11 m2 g–1. This value is larger than those of Pt/RGO (10.42 m2 g–1),

Pd/RGO (10.59 m2 g–1), commercial Pt black (5.91 m2 g–1) and Pd black (1.72 m2 g–1).

It demonstrates that Pt@Pd/RGO has the enhanced catalytic activity and is expected to have better performances in fuel cells. Fig. 8A displays the electrocatalytic performances of Pt@Pd/RGO (curve a),

Pt/RGO (curve b), Pd/RGO (curve c), commercial Pt black (curve d) and Pd black (curve e) modified electrodes towards MOR. The methanol oxidation current density are 130.37, 16.53, 96.11, 30.46 and 8.39 mA cm–2 for Pt@Pd/RGO, Pt/RGO, Pd/RGO, Pt black and Pd black, respectively. It indicates that Pt@Pd/RGO possesses enhanced

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catalytic efficiency for MOR, compared with the other catalysts. Meanwhile, the mass activity of Pt@Pd/RGO is about 4.6, 1.4, 4.8 and 17.4 times higher than those of Pt/RGO, Pd/RGO, Pt black and Pd black (Fig. 8B),

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respectively. This value is higher than those of Pd/graphene (0.36 A mg−1) [14],

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Pt/graphene (0.21 A mg−1) [56], and Pt/MWCNT (0.16 A mg−1) [57]. More

importantly, the specific activity (Fig. 8B) of Pt@Pd/RGO is 42.63 A m−2, which is

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also higher than those of Pt/RGO (13.45 A m−2), Pd/RGO (40.28 A m−2), Pt black

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(22.76 A m−2) and Pd black (30.50 A m−2). These results reveal the enhanced catalytic activity of Pt@Pd/RGO, which might be ascribed to the unique nanostructures.

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Furthermore, the stability of Pt@Pd/RGO modified electrode was investigated by chronoamperometry in 1.0 M KOH containing 1.0 M methanol at a potential of

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–0.25 V for 6000 s (Fig. 9), using Pt/RGO, Pd/RGO, Pt black and Pd black as standards. The rapid current decay can be explained in terms of the poisoning of the

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intermediate species during MOR [58]. However, the Pt@Pd/RGO modified electrode shows a slower current decay during the measured time, compared with the other catalysts. These results indicate that Pt@Pd/RGO is stable and occupied with robust poisoning tolerance in alkaline media. The improved catalytic performances of Pt@Pd/RGO are ascribed to the below

reasons: (I) The synergetic effects of Pt and Pd [59, 60]. (II) Pt@Pd core-shell nanostructures contribute the larger specific surface area and provide more active sites available for MOR [61]. (III) The high loading and good dispersion of Pt@Pd core-shell nanostructures on RGO that enlarges the surface area and facilitates

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electron transfer [19, 62]. (IV) Excellent electrical conductivity of graphene [30].

4. Conclusion

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In summary, a facile, rapid, and straightforward wet-chemical co-reduction

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method was developed for synthesis of well-dispersed Pt@Pd core-shell nanoparticles

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on RGO without any seed or template, using caffeine as a structure-directing agent and a capping agent. The resulting Pt@Pd/RGO shows the enhanced catalytic activity

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and better stability towards MOR, compared with Pt/RGO, Pd/RGO, commercial Pt black and Pd black. The as-prepared Pt@Pd/RGO might be a promising candidate of

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Acknowledgments

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effective cathode catalysts in fuel cells.

This work was financially supported by the Nation Natural Science foundation of

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China (21175218, 21275130, 21275131, and 51178283), Zhejiang province university young academic leaders of academic climbing project (pd2013055).

References

[1] C.-Y. Wang, Fundamental models for fuel cell engineering, Chemical Reviews

104 (2004) 4727. [2] R. Service, Fuel cells. Shrinking fuel cells promise power in your pocket, Science (New York, NY) 296 (2002) 1222. [3] B.Y. Xia, H.B. Wu, X. Wang, X.W. Lou, One-pot synthesis of cubic PtCu3

14

Page 14 of 36

nanocages with enhanced electrocatalytic activity for the methanol oxidation reaction, Journal of the American Chemical Society 134 (2012) 13934. [4] J.-J. Lv, J.-N. Zheng, S.-S. Li, L.-L. Chen, A.-J. Wang, J.-J. Feng, Facile synthesis

ip t

of Pt-Pd nanodendrites and their superior electrocatalytic activity, Journal of

cr

Materials Chemistry A 2 (2014) 4384.

us

[5] L. Gan, R. Lv, H. Du, B. Li, F. Kang, High loading of Pt-Ru nanocatalysts by pentagon defects introduced in a bamboo-shaped carbon nanotube support for high

an

performance anode of direct methanol fuel cells, Electrochemistry Communications 11 (2009) 355.

M

[6] Y. Hu, H. Zhang, P. Wu, H. Zhang, B. Zhou, C. Cai, Bimetallic Pt-Au nanocatalysts electrochemically deposited on graphene and their electrocatalytic

te

d

characteristics towards oxygen reduction and methanol oxidation, Physical Chemistry Chemical Physics 13 (2011) 4083.

Ac ce p

[7] Y. Hu, P. Wu, Y. Yin, H. Zhang, C. Cai, Effects of structure, composition, and carbon support properties on the electrocatalytic activity of Pt-Ni-graphene nanocatalysts for the methanol oxidation, Applied Catalysis B: Environmental 111 (2012) 208.

[8] J.-N. Zheng, J.-J. Lv, S.-S. Li, M.-W. Xue, A.-J. Wang, J.-J. Feng, One-pot synthesis of reduced graphene oxide supported hollow Ag@ Pt core-shell nanospheres with enhanced electrocatalytic activity for ethylene glycol oxidation, Journal of

Materials Chemistry A 2 (2014) 3445. [9] Y. Yu, H.L. Xin, R. Hovden, D. Wang, E.D. Rus, J.A. Mundy, D.A. Muller, H.D.

15

Page 15 of 36

Abruna, Three-dimensional tracking and visualization of hundreds of Pt-Co fuel cell nanocatalysts during electrochemical aging, Nano letters 12 (2012) 4417. [10]W. He, H. Jiang, Y. Zhou, S. Yang, X. Xue, Z. Zou, X. Zhang, D.L. Akins, H.

ip t

Yang, An efficient reduction route for the production of Pd-Pt nanoparticles anchored

cr

on graphene nanosheets for use as durable oxygen reduction electrocatalysts, Carbon

us

50 (2012) 265.

[11] Y. Lei, B. Liu, J. Lu, R.J. Lobo-Lapidus, T. Wu, H. Feng, X. Xia, A.U. Mane, J.A.

an

Libera, J.P. Greeley, Synthesis of Pt-Pd core-shell nanostructures by atomic layer deposition: application in propane oxidative dehydrogenation to propylene, Chemistry

M

of Materials 24 (2012) 3525.

[12]G. Hu, F. Nitze, H.R. Barzegar, T. Sharifi, A. Mikołajczuk, C.-W. Tai, A.

te

d

Borodzinski, T. Wågberg, Palladium nanocrystals supported on helical carbon nanofibers for highly efficient electro-oxidation of formic acid, methanol and ethanol

Ac ce p

in alkaline electrolytes, Journal of Power Sources 209 (2012) 236. [13]C. Xu, L. Cheng, P. Shen, Y. Liu, Methanol and ethanol electrooxidation on Pt and Pd supported on carbon microspheres in alkaline media, Electrochemistry

Communications 9 (2007) 997. [14]Y. Zhao, L. Zhan, J. Tian, S. Nie, Z. Ning, Enhanced electrocatalytic oxidation of methanol on Pd/polypyrrole-graphene in alkaline medium, Electrochimica Acta 56 (2011) 1967. [15]H. Zhang, M. Jin, Y. Xia, Enhancing the catalytic and electrocatalytic properties of Pt-based catalysts by forming bimetallic nanocrystals with Pd, Chemical Society

16

Page 16 of 36

Reviews 41 (2012) 8035. [16]Q. Yuan, J. Zhuang, X. Wang, Single-phase aqueous approach toward Pd sub-10 nm

nanocubes

and

Pd-Pt

heterostructured

ultrathin

nanowires,

ip t

Communications (2009) 6613.

Chemical

cr

[17]A.-X. Yin, X.-Q. Min, Y.-W. Zhang, C.-H. Yan, Shape-selective synthesis and

us

facet-dependent enhanced electrocatalytic activity and durability of monodisperse sub-10 nm Pt-Pd tetrahedrons and cubes, Journal of the American Chemical Society

an

133 (2011) 3816.

[18]B. Lim, M. Jiang, P.H. Camargo, E.C. Cho, J. Tao, X. Lu, Y. Zhu, Y. Xia, Pd-Pt

M

bimetallic nanodendrites with high activity for oxygen reduction, Science 324 (2009) 1302.

te

d

[19]S. Guo, S. Dong, E. Wang, Three-dimensional Pt-on-Pd bimetallic nanodendrites supported on graphene nanosheet: facile synthesis and used as an advanced

Ac ce p

nanoelectrocatalyst for methanol oxidation, Acs Nano 4 (2009) 547.

[20]N. Toshima, M. Harada, T. Yonezawa, K. Kushihashi, K. Asakura, Structural analysis of polymer-protected palladium/platinum bimetallic clusters as dispersed catalysts by using extended X-ray absorption fine structure spectroscopy, The Journal of Physical Chemistry 95 (1991) 7448. [21]Y.-W. Lee, A.-R. Ko, D.-Y. Kim, S.-B. Han, K.-W. Park, Octahedral Pt-Pd alloy catalysts with enhanced oxygen reduction activity and stability in proton exchange membrane fuel cells, RSC Advances 2 (2012) 1119. [22]B.-S. Choi, Y.W. Lee, S.W. Kang, J.W. Hong, J. Kim, I. Park, S.W. Han,

17

Page 17 of 36

Multimetallic alloy nanotubes with nanoporous framework, Acs Nano 6 (2012) 5659. [23]Y. Lu, Y. Jiang, H. Wu, W. Chen, Nano-PtPd cubes on graphene exhibit enhanced activity and durability in methanol electrooxidation after CO stripping-cleaning, The

ip t

Journal of Physical Chemistry C 117 (2013) 2926.

cr

[24]S.E. Habas, H. Lee, V. Radmilovic, G.A. Somorjai, P. Yang, Shaping binary metal

us

nanocrystals through epitaxial seeded growth, Nature Materials 6 (2007) 692.

[25]G. Fu, K. Wu, J. Lin, Y. Tang, Y. Chen, Y. Zhou, T. Lu, One-pot water-based

an

synthesis of Pt-Pd alloy nanoflowers and their superior electrocatalytic activity for the oxygen reduction reaction and remarkable methanol-tolerant ability in acid media,

M

The Journal of Physical Chemistry C 117 (2013) 9826.

[26]H. Zhang, Y. Yin, Y. Hu, C. Li, P. Wu, S. Wei, C. Cai, Pd@ Pt core-shell

te

d

nanostructures with controllable composition synthesized by a microwave method and their enhanced electrocatalytic activity toward oxygen reduction and methanol

Ac ce p

oxidation, The Journal of Physical Chemistry C 114 (2010) 11861. [27]N.V. Long, T. Asaka, T. Matsubara, M. Nogami, Shape-controlled synthesis of Pt-Pd core-shell nanoparticles exhibiting polyhedral morphologies by modified polyol method, Acta Materialia 59 (2011) 2901.

[28]X. Yu, L. Kuai, B. Geng, CeO2/rGO/Pt sandwich nanostructure: rGO-enhanced

electron transmission between metal oxide and metal nanoparticles for anodic methanol oxidation of direct methanol fuel cells, Nanoscale 4 (2012) 5738. [29]R. Carrera-Cerritos, V. Baglio, A.S. Aricò, J. Ledesma-García, M.F. Sgroi, D. Pullini, A.J. Pruna, D.B. Mataix, R. Fuentes-Ramírez, L.G. Arriaga, Improved Pd

18

Page 18 of 36

electro-catalysis for oxygen reduction reaction in direct methanol fuel cell by reduced graphene oxide, Applied Catalysis B: Environmental 144 (2014) 554. [30]I.V. Lightcap, T.H. Kosel, P.V. Kamat, Anchoring semiconductor and metal

ip t

nanoparticles on a two-dimensional catalyst mat. storing and shuttling electrons with

cr

reduced graphene oxide, Nano letters 10 (2010) 577.

us

[31]T.-Q. Xu, Q.-L. Zhang, J.-N. Zheng, Z.-Y. Lv, J. Wei, A.-J. Wang, J.-J. Feng, Simultaneous determination of dopamine and uric acid in the presence of ascorbic

an

acid using Pt nanoparticles supported on reduced graphene oxide, Electrochimica Acta 115 (2014) 109.

M

[32]S.I. Shin, A. Go, I.Y. Kim, J.M. Lee, Y. Lee, S.-J. Hwang, A beneficial role of exfoliated layered metal oxide nanosheets in optimizing the electrocatalytic activity

te

d

and pore structure of Pt-reduced graphene oxide nanocomposites, Energy & Environmental Science 6 (2013) 608.

Ac ce p

[33]C. Nethravathi, E. Anumol, M. Rajamathi, N. Ravishankar, Highly dispersed ultrafine Pt and PtRu nanoparticles on graphene: formation mechanism and electrocatalytic activity, Nanoscale 3 (2011) 569. [34]F. Li, Y. Guo, Y. Liu, H. Qiu, X. Sun, W. Wang, Y. Liu, J. Gao, Fabrication of Pt-Cu/RGO hybrids and their electrochemical performance for the oxidation of methanol and formic acid in acid media, Carbon 64 (2013) 11. [35]H. Wang, J. Du, Z. Yao, R. Yue, C. Zhai, F. Jiang, Y. Du, C. Wang, P. Yang, Facile fabrication, characterization of Pt-Ru nanoparticles modified reduced graphene oxide and its high electrocatalytic activity for methanol electro-oxidation, Colloids and

19

Page 19 of 36

Surfaces A: Physicochemical and Engineering Aspects 436 (2013) 57. [36]J.-J. Lv, S.-S. Li, J.-N. Zheng, A.-J. Wang, J.-R. Chen, J.-J. Feng, Facile synthesis of reduced graphene oxide supported PtAg nanoflowers and their enhanced

ip t

electrocatalytic activity, International Journal of Hydrogen Energy 39 (2014) 3211.

cr

[37]W.S. Hummers Jr, R.E. Offeman, Preparation of graphitic oxide, Journal of the

us

American Chemical Society 80 (1958) 1339.

[38]Y. Xu, H. Bai, G. Lu, C. Li, G. Shi, Flexible graphene films via the filtration of

an

water-soluble noncovalent functionalized graphene sheets, Journal of the American Chemical Society 130 (2008) 5856.

M

[39]L. Wang, Y. Yamauchi, Strategic synthesis of trimetallic Au@ Pd@ Pt core-shell nanoparticles from poly (vinylpyrrolidone)-based aqueous solution toward highly

te

d

active electrocatalysts, Chemistry of Materials 23 (2011) 2457. [40]T. Sun, Z. Zhang, J. Xiao, C. Chen, F. Xiao, S. Wang, Y. Liu, Facile and green

Ac ce p

synthesis of palladium nanoparticles-graphene-carbon nanotube material with high catalytic activity, Scientific reports 3 (2013) 1.

[41]M.N. Nadagouda, R.S. Varma, Green synthesis of silver and palladium nanoparticles at room temperature using coffee and tea extract, Green Chemistry 10

(2008) 859.

[42]S.I. Sanchez, M.W. Small, J.-M. Zuo, R.G. Nuzzo, Structural characterization of Pt-Pd and Pd-Pt core-shell nanoclusters at atomic resolution, Journal of the American Chemical Society 131 (2009) 8683. [43]X. Yang, Q. Yang, J. Xu, C.-S. Lee, Bimetallic PtPd nanoparticles on

20

Page 20 of 36

nafion-graphene film as catalyst for ethanol electro-oxidation, Journal of Materials Chemistry 22 (2012) 8057. [44]S. Pei, J. Zhao, J. Du, W. Ren, H.-M. Cheng, Direct reduction of graphene oxide

ip t

films into highly conductive and flexible graphene films by hydrohalic acids, Carbon

cr

48 (2010) 4466.

us

[45]J.J. Wang, G.P. Yin, J. Zhang, Z.B. Wang, Y.Z. Gao, High utilization platinum deposition on single-walled carbon nanotubes as catalysts for direct methanol fuel cell,

an

Electrochimica Acta 52 (2007) 7042.

[46]M. Liu, Y. Lu, W. Chen, PdAg nanorings supported on graphene nanosheets:

Functional Materials 23 (2013) 1289.

M

highly methanol-tolerant cathode electrocatalyst for alkaline fuel cells, Advanced

te

d

[47]S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Synthesis of graphene-based nanosheets via chemical

Ac ce p

reduction of exfoliated graphite oxide, Carbon 45 (2007) 1558. [48]J.-H. Yang, N. Myoung, H.-G. Hong, Facile and controllable synthesis of prussian blue on chitosan-functionalized graphene nanosheets for the electrochemical detection of hydrogen peroxide, Electrochimica Acta 81 (2012) 37. [49]A. Ferrari, J. Robertson, Interpretation of Raman spectra of disordered and amorphous carbon, Physical review B 61 (2000) 14095. [50]F. Tuinstra, J.L. Koenig, Raman spectrum of graphite, The Journal of Chemical Physics 53 (2003) 1126. [51]F. Jiang, Z. Yao, R. Yue, Y. Du, J. Xu, P. Yang, C. Wang, Electrochemical

21

Page 21 of 36

fabrication of long-term stable Pt-loaded PEDOT/graphene composites for ethanol electrooxidation, International Journal of Hydrogen Energy 37 (2012) 14085. [52]W. Yue, Z. Lin, S. Jiang, X. Yang, Preparation of graphene-encapsulated

cr

batteries, Journal of Materials Chemistry 22 (2012) 16318.

ip t

mesoporous metal oxides and their application as anode materials for lithium-ion

us

[53]K.-H. Liao, A. Mittal, S. Bose, C. Leighton, K.A. Mkhoyan, C.W. Macosko, Aqueous only route toward graphene from graphite oxide, Acs Nano 5 (2011) 1253.

an

[54]G. Wang, J. Yang, J. Park, X. Gou, B. Wang, H. Liu, J. Yao, Facile synthesis and characterization of graphene nanosheets, The Journal of Physical Chemistry C 112

M

(2008) 8192.

[55]F. Maillard, S. Schreier, M. Hanzlik, E.R. Savinova, S. Weinkauf, U. Stimming,

te

d

Influence of particle agglomeration on the catalytic activity of carbon-supported Pt nanoparticles in CO monolayer oxidation, Physical Chemistry Chemical Physics 7

Ac ce p

(2005) 385.

[56]Y. Li, L. Tang, J. Li, Preparation and electrochemical performance for methanol oxidation of Pt/graphene nanocomposites, Electrochemistry Communications 11

(2009) 846.

[57]S.-F. Zheng, J.-S. Hu, L.-S. Zhong, L.-J. Wan, W.-G. Song, In situ one-step method for preparing carbon nanotubes and Pt composite catalysts and their performance for methanol oxidation, The Journal of Physical Chemistry C 111 (2007)

11174. [58]B. Liu, J. Chen, C. Xiao, K. Cui, L. Yang, H. Pang, Y. Kuang, Preparation of

22

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Pt/MgO/CNT hybrid catalysts and their electrocatalytic properties for ethanol electrooxidation, Energy & fuels 21 (2007) 1365. [59]Q. Yuan, Z. Zhou, J. Zhuang, X. Wang, Pd-Pt random alloy nanocubes with

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tunable compositions and their enhanced electrocatalytic activities, Chemical

Shi,

G.-H.

Yang,

J.-J.

Zhu,

Sonoelectrochemical

fabrication

of

us

[60]J.-J.

cr

Communications 46 (2010) 1491.

PDDA-RGO-PdPt nanocomposites as electrocatalyst for DAFCs, Journal of Materials

an

Chemistry 21 (2011) 7343.

[61]V.L. Nguyen, M. Ohtaki, T. Matsubara, M.T. Cao, M. Nogami, New experimental

M

evidences of Pt-Pd bimetallic nanoparticles with core-shell configuration and highly fine-ordered nanostructures by high-resolution electron transmission microscopy, The

te

d

Journal of Physical Chemistry C 116 (2012) 12265. [62]C. Zhu, S. Dong, Synthesis of graphene-supported noble metal hybrid

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nanostructures and their applications as advanced electrocatalysts for fuel cells, Nanoscale 5 (2013) 10765.

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Captions

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Fig. 1 (A, B) TEM and (C, D, E) HRTEM images of Pt@Pd/RGO.

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Fig. 2 HAADF-STEM-EDS mapping images of Pd (A), Pt (B), and over-lapping

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Pt-Pd elements (C), respectively. EDS line scanning profiles of Pt@Pd/RGO (D).

an

Fig. 3 XRD patterns of Pt@Pd/RGO (curve a), and GO (curve b).

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Fig. 4 Survey (A), and high-resolution Pt 4f (B), Pd 3d (C), and C 1s (D) XPS spectra

te

d

of Pt@Pd/RGO, respectively.

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Fig. 5 FT-IR (A) and Raman (B) spectra of Pt@Pd/RGO (curve a) and GO (curve b).

Fig. 6 TGA curves of Pt@Pd/RGO (curve a), Pt/RGO (curve b), Pd/RGO (curve c)

and GO (curve d).

Fig. 7 CO-stripping voltammograms of Pt@Pd/RGO (A), Pt/RGO (B), Pd/RGO (C),

Pt black (D), and Pd black (E) modified electrodes in 0.5 M H2SO4 at a scan rate of 50 mV s–1. The corresponding ECSA (F).

Fig. 8 (A) CVs of the Pt@Pd/RGO (curve a), Pt/RGO (curve b), Pd/RGO (curve c),

24

Page 24 of 36

Pt black (curve d) and Pd black (curve e) modified electrodes in 1.0 M KOH containing 1.0 M methanol at a scan rate of 50 mV s–1. The corresponding specific

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activity and mass activity (B).

cr

Fig. 9 Chronoamperometric curves of the Pt@Pd/RGO (curve a), Pt/RGO (curve b),

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Pd/RGO (curve c), Pt-black (curve d) and Pd-black (curve e) modified electrodes in

Ac ce p

te

d

M

an

1.0 M KOH containing 1.0 M methanol.

25

Page 25 of 36

Figures

a b

an

c

te

d

0.225 nm 0.196 nm

D

e 0.225 nm

2 nm

d

2 nm

E 2 nm

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b

10 nm

0.227 nm

M

a

d

e

100 nm

0.227 nm

B

us

RGO

C

cr

c

A

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Fig. 1

26

Page 26 of 36

A

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Fig. 2

B

Pt-M

us

cr

Pd-L

100 nm

an

100 nm

300

C

Overlay

Pd Pt

te

d

Counts

M

200

D

100 0

-100 0

50 100 150 200 250

Ac ce p

Distance / nm

27

Page 27 of 36

(111)

(200)

us

cr

b Intensity / a.u.

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Fig. 3

(220) (311)

an

a Pt Pd

45 60 2 / degree

M

30

75

90

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te

d

15

28

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B

Pt (2+)

D

M

Pd (0)

80 76 72 68 Binding Energy / eV

an

84

Pd 3d5/2 Pd 3d3/2

Pt (0)

te

d

Pd (2+)

Intensity / a.u

C

0

Pt 4f7/2 Pt 4f5/2

cr

Pt4f C1s

1200 1000 800 600 400 200 Binding Energy / eV

Intensity / a.u

Intensity / a.u

Intensity / a.u

Pd3d

us

A

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Fig. 4

348 346 344 342 340 338 336 334 Binding Energy / eV

C-O C=O O-C=O

292 288 284 Binding Energy / eV

280

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296

C-C

29

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cr

b

1200

1800 -1 Wavenumber / cm

B

a

1600

2000

Ac ce p

O-H

te

C-O C-OH O-H C=C C=O

d

b

1400

an

us

a

1000

Transmission / a.u.

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A

M

Raman Intensity / a.u.

Fig. 5

3600 3000 2400 1800 1200 -1 Wavenumber / cm

600

30

Page 30 of 36

80

c a

60

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b

cr

100

us

Mass / %

Fig. 6

40

an

20

d

0 200

400 600 o Temperature / C

800

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te

d

M

0

31

Page 31 of 36

Fig. 7

2n

-2

d cycle

-4

0.2 0.4 0.6 E / V (vs. SCE)

0

cle 2nd cy

-2

0.8

-4 -0.2

1.0

0.0

0.2

an

-0.2 0.0

cr

0

2

0.562

2

B

le

4

1s tc yc

e

us

cl

-2

1s

y tc

j / mA cm

A

0.543

j / mA cm

-2

4

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6

0.4

0.6

0.8

1.0

E / V (vs. SCE)

3 2

-2

3 0

-0.2

0.0

0.2 0.4 0.6 E / V (vs. SCE)

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2

te

-3

0.8

E

1

1s

tc

-1

-0.2

0.0

0.2 0.4 0.6 E / V (vs. SCE)

c 2n d cy

0.8

-1

20

0

le

y tc

cl

e

0

-2

le yc

1s

1

1.0

0.765

-2 j / mA cm

cle

d

2n d c y

D

15 10

0.498

e

M

cl

0.637

j / mA cm

-2

1s

y tc

j / mA cm

C

2 -1 ECSAs / m g

6

-0.2

0.0

0.2 0.4 0.6 E / V (vs. SCE)

c le 2n d cy

0.8

1.0

F Pt@Pd/RGO Pt/RGO

Pd/RGO Pt-black

5 Pd-black

1.0

0

32

Page 32 of 36

A

cr

a

50

d

us

c

100

b

an

j / mA cm

-2

150

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Fig. 8

e

0 -0.6

0.2

te

d

B

0.0

Specific activity Mass activity

40

1.0 0.8

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0.6 0.4

20

0.2

0

Pt@Pd/RGO Pt/RGO Pd/RGO Pt-black Pd-black

Mass activity / A mg-1

-2 Specific activity / A m

60

-0.4 -0.2 E / V (vs.SCE)

M

-0.8

0.0

33

Page 33 of 36

ip t

Fig. 9

cr us

20

a

10

an

j / mA cm

-2

30

d

b c

M

0

e

1500

3000

4500

6000

d

0

Ac ce p

te

Time / s

34

Page 34 of 36

Graphical Abstract

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B

Pt-M

cr

Pd-L

100 nm

100 nm

300

C

Overlay

D

Pd Pt

an

Counts

200

us

A

100 0

0

50 100 150 200 250 Distance / nm

te

d

M

-100

A facile, rapid, and wet-chemical co-reduction method is developed for synthesis

Ac ce p

of Pt@Pd/RGO with the help of caffeine, without any seed or template. The as-prepared nanocomposite exhibits the enhanced electrocatalytic activity and improved stability for methanol oxidation reaction, compared with Pt/RGO, Pd/RGO, commercial Pt black and Pd black catalysts.

35

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Research Highlights

Research Highlights

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► Pt@Pd/RGO is synthesized by a wet-chemical co-reduction method. ► This method is simple and facile, without any seed, template, or organic solvent.

cr

► Caffeine is served herein as a structure-directing agent and a capping agent.

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te

d

M

an

us

► The as-prepared Pt@Pd/RGO shows the enhanced catalytic activity for MOR.

Page 36 of 36