Synthesis of nitrogen-doped graphene supported Pt nanoparticles catalysts and their catalytic activity for fuel cells

Accepted Manuscript Title: Synthesis of nitrogen-doped graphene supported Pt nanoparticles catalysts and their catalytic activity for fuel cells Author: Qizhong Sun Seok Kim PII: DOI: Reference:

S0013-4686(14)02285-3 http://dx.doi.org/doi:10.1016/j.electacta.2014.11.077 EA 23749

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

21-8-2014 29-10-2014 12-11-2014

Please cite this article as: Qizhong Sun, Seok Kim, Synthesis of nitrogen-doped graphene supported Pt nanoparticles catalysts and their catalytic activity for fuel cells, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2014.11.077 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.

Synthesis of nitrogen-doped graphene supported Pt nanoparticles catalysts and their catalytic activity for fuel cells

Qizhong Sun1, Seok Kim1*[email protected] 1

Dept. of Chemical and Biochemical Engineering, Pusan National University, San 30, Jangjeon-

dong, Geumjeong-gu, Busan 609-735, Korea

Dept. of Chemical and Biomolecular Engineering, Pusan National University, San 30, Jangjeon-

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dong, Gemjeong-gu, Busan 609-735, Korea. Tel.: +82-51-510-3874; Fax: +82-51-512-8563

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Highlights

► Nitrogen-doped graphene was prepared by pyrolysis of graphene oxide, with cyanamide

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as the nitrogen-containing precursor, at various temperatures,

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► Pt nanoparticles were well dispersed on the N-G surfaces, with small particle sizes and

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narrow size distributions, ranging from 1 to 5 nm.

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particle sizes.

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► Improved electrocatalytic activity can be ascribed to the optimized dispersion and smaller

Graphical abstract

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Pt/N-G nanocatalysts were successfully prepared, and investigated using various techniques.

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The results showed that the Pt nanoparticles were well dispersed on the N-G surfaces, with small particle sizes and narrow size distributions, ranging from 1 to 5 nm. Enhancements of

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the reactive surface area and specific peak current densities in methanol oxidation for the

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Pt/N-G catalysts, compared with the Pt/G catalyst, were obtained. The improved

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electrocatalytic activity can be ascribed to the optimized dispersion and smaller particle sizes

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induced by nitrogen doping.

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ABSTRACT

In this work, we have developed an efficient approach to prepare the nitrogen-doped

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graphene supported Pt nanocomposites (Pt/N-G). Nitrogen-doped graphene (N-G) was

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prepared by pyrolysis of graphene oxide with cyanamide as nitrogen containing precursor at different temperatures, which led to high and controllable nitrogen contents. Subsequently,

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the Pt nanoparticles were dispersed over N-G surface by modified chemical polyol reduction process. The morphology and nanostructure of as-prepared catalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD), which verified that the Pt nanoparticles were uniformly dispersed over NG surface with a narrow size distribution. The functional groups on the surface of the

catalysts were investigated by the Fourier transform infrared spectra (FT-IR), while the elemental composition and nitrogen bonding configurations in N-G were further evaluated by X-ray photoelectron spectroscopy (XPS). Furthermore, electrochemical properties are studied by cyclic voltammetry (CV). The Pt/N-G catalysts showed the superior electrocatalytic activity toward methanol oxidation compared to that of Pt loaded on undoped graphene (Pt/G). The results suggested that N-G could be used as an effective catalyst support for fuel

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cell application.

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Keywords: nitrogen-doped graphene, catalytic activity, Pt nanoparticles, methanol oxidation,

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fuel cell

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1. Introduction

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The ease of handling, high energy density, low to zero emissions and low operating

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temperature has made direct methanol fuel cells (DMFCs) a promising, clean-energy

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technology with the potential application for portable electronic devices and vehicles [1-4].

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Nevertheless, challenging issues including methanol crossover through the proton exchange membrane and the insufficient activity of the anode catalysts, are the critical obstacles

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hindering commercial viability of DMFCs [5,6]. To date, Pt-based catalysts have long been

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regarded as the most promising and effective electrocatalysts used for both the methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR). But, under a long term electro-catalytic operating condition, Pt-based catalysts generally suffer from dissolution, Ostwald ripening and aggregation of Pt nanoparticles as well as corrosion of support materials, which will lead to degrade the overall fuel cell performance [7-9].

To overcome these drawbacks, numerous efforts have been devoted in designing and developing the new advanced carbon support materials with high surface area, high electrical conductivity, strong affinity and durability, such as carbon black (CB) [10,11], carbon nanofibers (CNFs) [12], carbon nanotubes (CNTs) [13,14] etc. Recently, graphene, a twodimensional carbon nanostructure consisting of single-layer of sp2 hybridized carbon atoms, attracted tremendous scientific and technological interests as an ideal carbon support due to

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its exceptional properties such as high specific surface area, excellent electrical conductivity, high thermal and chemical stability and potentially low cost [15-19]. In addition, graphene

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can not only be used effectively to reduce the Pt loading and in turn reduce the cost but also

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be less susceptible to poisoning species than the aforementioned carbon materials [20,21]. However, due to fewer active sites for the inert graphitized surface of graphene, the efficient

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utilization of Pt nanoparticles over graphene with an optimized dispersion and size

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distribution still is a key issue need to be addressed. Therefore, various strategies have been

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developed to further tailor the properties of graphene via chemical functionalization and chemical doping with foreign atoms. At present, many previous studies have proved that

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chemical doping of heteroatoms such as boron, nitrogen, phosphorus, sulfur into the carbon framework is an effective strategy to achieve this ultimate goal [22,23]. Notably, nitrogen

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doping can effectively alter the physicochemical properties of carbon support, including

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enhancing the electrical conductivity, and strengthening the interaction between Pt nanoparticles and delocalized π bond of nitrogen doped support, resulting in higher binding

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energy and stronger attachment. More importantly, nitrogen doping can provide abundant anchoring sites for the immobilization of Pt nanoparticles that lead to uniform dispersion and narrow size distribution [24-26]. Up to now, general methods for nitrogen-doped graphene synthesis mainly include chemical vapor depositon (CVD) [27], thermal annealing of graphene oxide (GO) with various N-containing precursors [28,29], arc discharge of a

graphite electrode in the presence of pyridine vapor [30], nitrogen plasma treatment [31] and solvothermal reduction of GO in N-methyl-2-pyrrolidone (NMP) [32]. Although nitrogendoped graphene have been studied frequently, but we aim to simplify the synthetic route and obtain the higher electrocatalytic performance in a DMFC environment by improving the dispersion state of metal particles and surface property of support. To the best of our knowledge, it was not fully understood and systematically studied on the influence of

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nitrogen-doped graphene on loaded Pt nanoparticles under fuel cell operation based on cyanamide as a nitrogen sources.

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In this study, we reported a facile approach for the synthesis of nitrogen-doped graphene

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supported Pt nanocomposites (Pt/N-G) with having enhanced activity for a methanol oxidation. The synthesis process is illustrated in Fig 1. At first, nitrogen-doped graphene is

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successfully prepared by pyrolysis of GO in the presence of cyanamide at different

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temperature. Then, the resultant N-G was acted as a support for Pt nanoparticles deposition

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by chemical polyol reduction process. In addition, with the aid of anionic surfactant sodium dodecyl sulfate (SDS), cyanamide can be well adsorbed on both sides of GO with highly

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ordered and homogeneous distribution by electrostatic interaction, leading to uniform

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nitrogen doping in graphene which will be beneficial to Pt dispersion.

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2. Experimental 2.1. Materials

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Natural graphite powder (99.9%, 1 ppm mesh) was obtained from Bay carbon incorporated USA. Platinum precursor, chloroplatinic acid hexahydrate (H2PtCl6-6H2O) and sodium dodecyl sulfate (SDS) was purchased from Sigma-Aldrich. Cyanamide was provided by Alfa Aesar. All other chemicals used in this study were of analytical grade and used as received without further purification. Deionized (DI) water was used in all experiments.

2.2. Preparation of nitrogen doped graphene (N-G) Graphite oxide was prepared from natural graphite by using modified Hummers method [33,34]. In a typical procedure, sodium dodecyl sulfonate (0.1g) was first added into GO dispersion solution (100mL, 1mg/mL) and ultrasonicated for 30 min. Subsequently, 8 mL cyanamide solution (50%) was added gradually under magnetic stirring, forming a uniform

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mixture. The mixed solution was then heated at 90 ℃ until completely dry. After that, the obtained product was placed into the center of a tubular furnace and annealed at 500 ℃ to

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induce the polymerization of cyanamide to form C3N4 polymer layer on the surface of the GO.

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To synthesis NG, the resulting C3N4-G composites were further pyrolyzed at the designed

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reaction temperature (800, 900, 1000℃) and kept for an hour. For simplicity, the final

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samples are denoted as NG-800, NG-900, and NG-1000, respectively. All the annealing and

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cooling processes were carried out under an Ar ambient.

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2.3. Synthesis of N-G supported Pt nanoparticles (Pt/N-G)

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Initially, H2PtCl6-6H2O was added in the EG (60ml) with magnetic stirring for 15 min until

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complete dissolution. The pH value of this mixture was adjusted to 11 with 1M NaOH aqueous solution. Then as-obtained N-G (0.1 g) was dispersed into the above solution by

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ultrasonic treatment for 1 h. Afterwards, the suspension solution was heated under refluxing

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condition at 130℃ for 5 h to ensure that H2PtCl6 was completely reduced. Finally, the product was collected by centrifugation, washed, and freeze dried for 24 h. For comparison,

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Pt loaded on undoped graphene (Pt/G) was also prepared through the same procedure.

2.4. Electrode Preparation and Modification To prepare the nanocatalysts-loaded working electrode, a glassy carbon electrode was first polished to a mirror finish with alumina slurry, then rinsed thoroughly with ethanol and

distilled water in an ultrasonic bath to remove any alumina residues. The catalyst powders (5 mg) were dispersed in a mixed solution consisting of 0.5 ml of water and 0.1 ml of 5 wt% Nafion solution via ultrasonicated for 1 h. The catalyst ink was cast dropwise onto the pretreated bare GCE surface to form a thin layer, dried at 50 ℃ for 0.5 h.

2.5. Structural characterization

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Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were taken on a S3500N (HITACHI) instrument and a JEM-2010 microscope, respectively.

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Fourier transform infrared (FT-IR) spectra were recorded at ambient temperature using an

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FTIR spectrometer (PerkinElmer). X-ray diffraction (XRD) measurements were carried out

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with a X’Pert-MPD system (PHILIPS) using sample suspension deposited on 300 mesh of Cu

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Kα (λ= 1.5406Å) with scattering angles (2θ) of 10º-75º. X-ray photoelectron spectroscopy

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(XPS) analysis was performed on an ESCALAB 250 (Thermo Fisher Scientific) equipped

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with an Al Kα radiation source (1486.6eV) and X-ray energy of 15 kV.

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2.6. Electrochemical measurements All electrochemical measurements were performed on an Galvanostat/Potentiostat

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(IVIUMSTAT, Netherland) and a standard three-electrode cell. A glassy carbon

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electrode(GCE) was used as the working electrode, a platinum wire was used as the counter electrode and Ag/AgCl was used as the reference electrode. Cyclic voltammetry

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measurements were studied in 1 M H2SO4 solution or a mixture of 1.0 M H2SO4 and 2.0 M CH3OH solution with a potential range from -0.2 and 1.0V at a scan rate of 50 mV/s.

3. Results and discussion Typical XRD patterns of (a) Pt/G, (b) Pt/N-G-800, (c) Pt/N-G-900 and (d) Pt/N-G-1000

samples were presented in Fig. 2. The characteristic diffraction peak at 2θ of 24.3º is assigned to the (002) plane of G or N-G, implying the successful conversion of the initial GO. Moreover, the strong diffraction peaks observed at 39.8º, 46.5º, and 67.8º can be indexed to the (111), (200) and (220) planes of the face-centered cubic (fcc) structure of Pt, respectively, revealing that a crystalline structure of the loaded Pt nanoparticles. The average Pt crystallite size can be estimated by the following Scherrer equation from the (220) peak [35]:

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0.9λ B2θ cos θ max

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δ=

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where d is average particle size (nm), B2θ is full width of the diffraction peak at half maximum in radians (FWHM), λ is wave length (1.5406Å) and θmax is the angle at the

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position of the peak maximum. By this equation, the average crystallite size of Pt/G, Pt/N-G-

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800, Pt/N-G-900 and Pt/N-G-1000 were estimated to be 3.9, 2.6, 2.5 and 2.8 nm, respectively,

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which suggesting that nitrogen doping can be efficient and favorable for distribution of Pt

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nanoparticles.

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Figure 3 showed the FT-IR spectra of (a) GO, (b) Pt/G, (c) Pt/N-G-800, (d) Pt/N-G-900 and

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(e) Pt/N-G-1000 samples. In the FT-IR spectrum of GO, various characteristic peaks

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including the stretching absorption peaks of C=O, deformation vibration peaks of O-H, stretching vibration peaks of C-OH and C-O are observed at 1738 cm-1, 1420 cm-1, 1228 cm-1,

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and 1051 cm-1, respectively and the peak at 1626 cm-1 could be ascribed to the skeletal vibrations of unoxidized graphitic domains. However, in the case of Pt/G and Pt/N-G samples, the intensity of these characteristic peaks significantly decreases and a sharp peak at around 1560 cm-1 appeared, which can be assigned to the skeletal vibration of the graphene sheets, indicating that GO has been successfully reduced to graphene. Furthermore, for the spectra of

Pt/N-G samples, the broad peak at about 1100 cm-1 could be observed, resulting from the formation of C-N bonds and the residual C-O functional groups [36].

The morphology and nanostructure of the Pt/G and Pt/N-G samples were first characterized by SEM with different magnifications. Compared with Pt/G (Fig. 4a-b), the Pt/N-G samples display a distinct crumpled and porous structure while maintaining the typical sheet-like

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morphology, resulting from the successful decomposition of adsorbed cyanamide over GO surface via pyrolysis treatment (Fig. 2c-h). However, as a result of the lower contrast, it is

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hard to distinguish the difference between the Pt/N-G-800, Pt/N-G-900 and Pt/N-G-1000

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according to the general SEM observations.

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Pt nanoparticles dispersion properties and size distributions of (a, b) Pt/G, (c, d) Pt/N-G-800,

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(e, f) Pt/N-G-900 and (g, h) Pt/N-G-1000 samples were further investigated by TEM. As

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shown in Fig. 5a and b, it can be clearly seen that the Pt nanoparticles were poor dispersed and seriously aggregated on the undoped graphene. In contrast, Pt nanoparticles were

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uniformly and densely loaded on the N-doped graphene. However, for the N-G-1000, the big agglomerations were also observed and might be due to the randomly deeper crumples

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closely associated with each other and overlap, resulting in forming a rougher surface under

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1000 ℃ ultrahigh temperature condition. Meanwhile, Fig. 6. displays the histograms of Pt particle size distributions obtained by measuring the sizes of about 100 randomly selected

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particles. On the undoped graphene, a broad particle size distribution of Pt nanoparticles is found with diameters was in the range of 2.0-7.0 nm, while that was 1.0-4.5 nm, 1.0-4.5 nm and 1-5 nm with a slightly narrower size distribution on the N-G-800, N-G-900, N-G-1000, respectively. According to the statistical survey results, the average diameters of Pt nanoparticles were about 4.0 nm, 2.6 nm, 2.5 nm, and 2.9 nm for Pt/G, Pt/N-G-800, Pt/N-G-

900 and Pt/N-G-1000, respectively, consisting with the previous results based on XRD. These results can confirm that incorporation of nitrogen doping into the carbon lattice is responsible for the nucleation and growth mechanism of Pt nanoparticles, leading to smaller particle size, better dispersion and narrower size distribution.

The XPS measurements were conducted to analyze elemental composition and functional

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groups of the samples. Fig. 7 shows the typical XPS spectra of N-G-900 and the deconvolution analysis of the C1s, N1s and O1s core level spectra. The elemental analysis of

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G, N-G-800, N-G-900 and N-G-1000 samples were summarized in Table 2. The XPS survey

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spectrum of N-G-900 in Fig. 7a presents three predominant peaks at around 284.6 eV, 398.1 eV and 531.7 eV that correspond with C1s, N1s and O1s spectrum, respectively, clearly

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indicating the existence of carbon atoms connected to nitrogen and oxygen heteroatoms.

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Furthermore, a small peak located at around 497.1 eV was ascribed to Na element from the

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anionic surfactant (SDS). After the nitrogen doping process, the nitrogen content was found to be 8.83 wt%, while the oxygen content decreases to 7.45 wt%, demonstrating the effective

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reduction under the thermal treatment. As shown in Fig. 7b, the high resolution XPS of C 1s spectrum could be deconvoluted into five different peaks, the most intense one at 284.6 eV

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corresponding to C-C, while the other four peaks at 285.9, 286.5, 287.9 and 288.9 eV are

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assigned to C-N, C-O, C=O and O-C=O, respectively. For the high resolution O 1s spectrum could be fitted with three peaks: O-C at 531.5 eV, O=C at 532.5 eV and O=C-O at 533.7 eV

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is presented in Figure 7c. Finally, Fig. 7d shows the N 1s spectrum which be fitted to four different types of doped nitrogen including pyridinic-N at 398.1 eV, pyrrolic-N at 399.9 eV, graphitic-N at 401.1 eV and pyridinic N-oxide at 403.2 eV, and verifying that the pyridinic-N was dominant in the formed nitrogen functional groups. Many previous researches have reported that the pyridinic-N usually play a crucial role in the stabilization of Pt particles and

electrocatalytic performance [37,38]. In addition, as shown in Table 2, the increase of pyrolysis temperature was accompanied by a decrease of nitrogen content. The nitrogen content was 12.13% at 800

and slowly decreased to 8.83% at 900 ℃, 7.68% at 1000 ℃,

indicating that the incorporated nitrogen concentration can be changed by adjusting the pyrolysis temperatures.

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The cyclic vlotammograms of as–prepared samples were shown in Fig. 8. The CV measurements on the samples were performed in a N2 saturated 1 M H2SO4 aqueous solution

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with a potential range from -0.2 and 1.0V at a scan rate of 50 mV/s. As shown in 8a, the

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electrochemical active surface area (ECSA) values can be determined from the coulombic

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charge for the hydrogen adsorption/desorption (QH) in the anodic scan (-0.2 to 0V vs.

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Ag/AgCl) according to the following equation [39]:

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QH [Pt] × 0.21 is the coulombic charge for hydrogen adsorption/desorption (mC/cm2), [Pt]

Where QH

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ECSA(m 2 /g) =

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represents the platinum loading (g/cm2) in the electrode and 0.21 represents the charge

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required to oxidize a monolayer of H2 on bright Pt surface (mC/cm2). The calculated ECSA

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value of Pt/N-G-900 was 76.9 m2/g, which is much higher than one obtained for the Pt/G

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(31.2 m2/g), Pt/N-G-800 (57.8 m2/g) and Pt/N-G-1000 (43.5 m2/g). The ECSA analysis indicated that the Pt/N-G catalysts have a larger number of available electrochemical active

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sites which are electrochemically more accessible.

The electrocatalytic activities toward methanol oxidation of the as-prepared samples were measured in a N2 saturated 1.0 M H2SO4 and 2.0 M CH3OH aqueous solution. The electrocatalytic activities of the catalysts toward methanol oxidation are compared with

regard to forward peak current density and onset potential. The relevant data are listed in Table 1. As shown in Fig. 8b, it can be seen that the onset potential for methanol oxidation of Pt/N-G-900 is more negative than that of the other samples. The low onset potential presents clear evidence for excellent electrocatalytic activity toward methanol oxidation. Additionally, the forward peak current density for catalysts showed the following order: Pt/N-G-900 (13.0 mA/cm2) > Pt/N-G-800 (10.9 mA/cm2) > Pt/N-G-1000 (9.2 mA/cm2) > Pt/G (6.1 mA/cm2). It

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indicated that the Pt/N-G catalysts possess much better electrocatalytic activity compared to that of undoped catalysts. These improvements might be ascribed to the following two

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aspects. Firstly, the optimized dispersion and smaller size of the loaded Pt nanoparticles was

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achieved on N-G support, therefore provide the larger specific surface area and more active sites available for a methanol oxidation reaction. Secondly, the incorporation of nitrogen can

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enhance the conductivity, and modulate the electronic structures of the graphene, thus

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changing the metal-support interaction or stability. Moreover, the Pt/N-G-900 exhibited the

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highest electrochemically active surface area and forward peak current density among a set of Pt/N-G samples. It is related to the content of nitrogen and degree of graphitization can be

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adjusted at various pyrolysis temperatures and in turn influence the dispersion ability of Pt nanoparticles. Consequently, it can be concluded that the reasonable temperature for supports

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preparation plays an important role in the synergistic effect between Pt nanoparticles and N-G

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support, which will lead to the better electrocatalytic performance.

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4. Conclusions In summary, the Pt/N-G nanocatalysts have been successfully prepared and investigated by

various techniques. The TEM and XRD results revealed that Pt nanoparticles were well dispersed on the N-G surface with small particles size and a narrow size distribution ranging from 1-5 nm. The XPS analysis clearly verified that the incorporation of nitrogen atoms

within the graphene and the pyridinic-N was dominant in the formed four types of nitrogen species. As compared with Pt/G, remarkable enhancements in electrochemical active surface area and specific peak current density for methanol oxidation are obtained in the Pt/N-G catalysts. The improved electrocatalytic activity can be ascribed to the optimized dispersion ability, smaller particle size and higher conductivity induced by the incorporated nitrogen doping. These results suggested that nitrogen-doped graphene could serve a favorable

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candidate as a supporting material of catalysts for fuel cells.

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Acknowledgments

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This research was supported by the Basic Science Research Program through the National

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Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT, and Future

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Planning, Korea (Grant No.: NRF-2011-0009007). This research was also supported by the

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BK21 PLUS Centre for Advanced Chemical Technology (Korea) (21A20131800002).

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References

TE

[1] C.M. Zhou, H.J. Wang, F. Peng, J. H. Liang, H. Yu, J. Yang,

EP

7717.

[2] M. Winter, R.J. Brodd,

Langmuir 25, (2009) 7711-

Chemical Reviews 104 (2004) 4245-4269.

CC

[3] J.D. Qiu, G.C. Wang, R.P. Liang, X.H. Xia, H. W. Yu,

The Journal of Physical

A

Chemistry C 115 (2011) 15639-15645. [4] L.F. Dong, R.R.S. Gari, Z. Li, M.M. Graig, S.F. Hou,

Carbon 48 (2010) 781-787.

[5] C.L. Fang, D. Julius, S.W. Tay, L. Hong, J.Y. Lee,

The of Physical Chemistry B 116

(2012) 6416-6424. [6] V. Selvaraj, M. Alagar,

Electrochemistry Communications 9 (2007) 1145-1153.

[7] B.M. Luo, X.B. Yan, J.T. Chen, S. Xu, Q.J. Xue,

International Journal of Hydrogen

Energy 38 (2013) 13011-13016. [8] S. Kim, H.J. Sohn, S.J. Park,

Current Applied Physics 10 (2010) 1142-1147.

[9] Y. Liu, D.G. Li, V.R. Stamenkovic, S. Soled, J.D. Henao, J.D. Henao, S.H. Sun,

ACS

Catalysis 1 (2011) 1719-1723. Journal of Power Sources 159 (2006) 42-45.

[11] G.X. Wang, G.Q. Sun, Q. Wang, S.L. Wang, H. Sun, Q. Xin,

International Journal of

Hydrogen Energy 35 (2010) 11245-11253.

PT

[10] S. Kim, S.J. Park,

Journal of Power Sources 185 (2008) 711-716.

RI

[12] M. Okada, Y. Konta, N. Nakagawa,

Electrochimica Acta 53 (2008) 4082-4088.

[14] S. Kim, S.J. Park,

Analytica Chimica Acta 619 (2008) 43-48.

SC

[13] S. Kim, S.J. Park,

Chemical Reviews 112 (2012) 6027-6053.

N

U

[15] D. Chen, H.B. Feng, J.H. Li,

[17] Y.T. Liang, M.C. Hersam,

M

Materials 19 (2009) 2782-2789.

Journal of the American Chemical Society 132 (2010)

TE

D

17661-17663.

Advanced Functional

A

[16] L.H. Tang, Y. Wang, Y.M. Li, H.B. Feng, J. Lu, J.H. Li,

[18] H.B. Wang, B.Y. Xia, Y. Yan, N. Li, J.Y. Wang, X. Wang,

The Journal of Physical

EP

Chemistry B 117 (2013) 5606-5613.

CC

[19] S.K. Park, S. Kim,

Electrochimica Acta 89 (2013) 516-522.

[20] L.F. Dong, R.R.S. Gari, Z. Li, M.M. Craig, S.F. Hou,

A

[21] Y. Li, L.H. Tang, J.H. Li,

Carbon 48 (2010) 781-787.

Electrochemistry Communications 11 (2009) 846-849.

[22] C.H. Choi, S.H. Park, S.I. Woo,

ACS Nano 8 (2012) 7084-7091.

[23] D.S. Geng, S.L. Yang, Y. Zhang, J.L. Yang, J. Liu, R.Y. Li, T.K. Sham, X.L. Sun, Applied Surface Science 257 (2011) 9193-9198. [24] S.J. Jiang, L. Zhu, Y.W. Ma, X.Z. Wang, J.G. Liu, J.M. Zhu, Y.N. Fan, Z.G. Zou, Z. Hu,

Journal of Power Sources 195 (2010) 7578-7582. [25] R. Chetty, S.H. Jundu, W. Xia, M. Bron, W.G. Schuhmann,

Electrochimica Acta 54

(2009) 4208-4215. [26] A.R. Corpuz, T.S. Olson, P. Joghee, S. Pylypenko, A.A. Dameron, H.N. Dinh,

Journal

of Power Sources 217 (2012) 142-151. [27] D.C. Wei, Y.Q. Liu, Y. Wang, H.L. Zhang, L.P. Huang, G. Yu,

Nano Letters 9 (2009)

PT

1752-1758. [28] Z.H. Sheng, L. Shao, J.J. Chen, W.J. Bao, F.B. Wang, X.H. Xia,

884-888.

U

ACS Catalysis 2 (2012) 781-794.

N

[30] H.B. Wang, T. Maiyalagan, X. Wang,

Advanced Energy Materials 2 (2012)

SC

[29] Z.Y. Lin, G. Waller, Y. Liu, M.L. Liu, C.P. Wong,

RI

4350-4358.

ACS Nano 5 (2011)

Nano

A

[31] H.M. Jeong, J.W. Lee, W.H. Shin, Y.J. Choi, H.J. Shin, J.K. Kang, J.W. Choi,

M

Letters 11 (2011) 2472-2477.

[32] V.H. Pham, T.V. Cuong, S.H. Hur, E. Oh, E.J. Kim, E.W. Shin, J.S. Chung,

Journal of

TE

D

Materials Chemistry 21 (2011) 3371-3377. [33] W.S. Hummers, R.E. Offeman,

EP

1339-1339..

CC

[34] J.Y. Park, S. Kim,

[35] X.J. Bo, L.P. Guo,

Journal of the American Chemistry Society 80 (1958)

International Journal of Hydrogen Energy 38 (2013) 6275-6282. Electrochimica Acta 90 (2013) 283-290.

A

[36] Z.Y. Lin, G. Waller, Y. Liu, M.L. Liu, C.P. Wong,

Advanced Energy Materials 2 (2012)

884-888. [37] Y.C. Xin, J.G. Liu, X. Jie, W.M. Liu, F.Q. Liu, Y. Yin, J. Gu,

Electrochimica Acta 60

(2012) 354-358. [38] W.Y. Wong, W.R. W. Daud, A.B. Mohamad, A.A.H. Kadhum, K.S. Loh, E.H. Majlan,

International Journal of Hydrogen Energy 38 (2013) 9370-9386. Electrochemistry Communications 11 (2009) 846-849.

PT

[39] Y.M. Li, L.H. Tang, J.H. Li,

Figure captions

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Figure 1 : Shematic illustration of the preparation for Pt/N-G samples.

SC

Figure 2 : XRD patterns of (a) Pt/G, (b) Pt/N-G-800, (c ) Pt/N-G-900 and (d) Pt/N-G-1000. Figure 3 : FT-IR spectra of (a) GO, (b) Pt/G, (c) Pt/N-G-800, (d) Pt/N-G-900 and (e) Pt/N-G-

N

U

1000.

A

Figure 4 : SEM images of (a, b) Pt/G, (c, d) Pt/N-G-800, (e,f ) Pt/N-G-900 and (g, h) Pt/N-G-

M

1000.

D

Figure 5 : TEM images of (a, b) Pt/G, (c, d) Pt/N-G-800, (e,f ) Pt/N-G-900 and (g, h) Pt/N-G-

TE

1000.

Figure 6 : Histograms of Pt particle size distributions of (a) Pt/G, (b) Pt/N-G-800, (c ) Pt/N-

EP

G-900 and (d) Pt/N-G-1000.

CC

Figure 7 : XPS survey spectrum of N-G-900 (a), and core level spectra: (b) C 1s: (c) N 1s; (d) O 1s.

A

Figure 8 : CV curves of Pt/G, Pt/N-G-800, Pt/N-G-900 and Pt/N-G-1000 at a scan rate of 50 mV/s between -0.2 and 1.0 V vs. Ag/AgCl in (a) 1.0 M H2SO4 solution and (b) 1.0 M H2SO4 + 2.0 M CH3OH solution. Table 1 : Average crystalline size of particles, ECSA values, and IF, IR values of Pt/G, Pt/NG-800, Pt/N-G-900 and Pt/N-G-1000.

Table 2 : The elemental analysis of G, N-G-800, N-G-900 and N-G-1000 obtained from XPS results.

D

M

A

N

U

SC

RI

PT

Figure 1

A

CC

EP

TE

Figure 2

TE

EP

CC

A D

Figure 3

PT

RI

SC

U

N

A

M

TE

EP

CC

A Figure 4

D

PT

RI

SC

U

N

A

M

Figure 5

TE

EP

CC

A D

PT

RI

SC

U

N

A

M

Figure 6

TE

EP

CC

A D

PT

RI

SC

U

N

A

M

Figure 7

TE

EP

CC

A D

PT

RI

SC

U

N

A

M

TE

EP

CC

A

Figure 8

D

PT

RI

SC

U

N

A

M

Table 1

TE

EP

CC

A D

PT

RI

SC

U

N

A

M

ECSA(m2/g)b

IF(mA/cm2)b

IR(mA/cm2)b

Pt/G

Average crystalline size (nm)a 3.9

31.2

6.1

7.0

Pt/N-G-800

2.6

76.9

10.9

9.7

Pt/N-G-900

2.5

57.8

13.0

10.5

Pt/N-G-1000

2.8

43.5

9.2

9.7

Samples

From XRD results, bFrom CV results.

N

U

SC

RI

PT

a

M

A

Table 2

C (wt%)

EP

N-G-900

TE

N-G-800

A

CC

N-G-1000

O (wt%)

83.06

-

16.94

75.91

12.13

11.97

83.72

8.83

7.45

85.61

7.68

6.71

D

Samples G

N (wt%)