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

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<ul><li><p>nl</p><p>g</p><p>R C</p><p>Un</p><p>a s</p><p>hite</p><p>duc</p><p>(Pt/CCG) hybrid. The electrochemically active surface areas of Pt/CCG and a comparative</p><p>sample of Pt/multi-walled carbon nanotubes (Pt/MWCNT) are 36.27 and 33.43 m2/g, respec-</p><p>sheet</p><p>ted tre</p><p>attention in recent years [14]. This tw</p><p>geneous catalyst support in direct methanol fuel cells [58]. It</p><p>trocatalytic activity in fuel cells [1113]. The most studied</p><p>bimetallic PtRu alloy catalysts have been demonstrated to</p><p>be much more effective in the electro-oxidation of methanol</p><p>[1417]. In comparison with CNTs, graphene not only</p><p>of graphite oxide (GO) in solution [2124]. Since abundant</p><p>may lead to materials with interesting properties for a variety</p><p>The as-prepared Pt/CCG hybrids show higher electrocatalytic</p><p>activity than Pt/MWCNT catalyst. Our study opens a novel,</p><p>and insightful way to develop electrocatalyst for fuel cells</p><p>application.</p><p>* Corresponding author: Tel.: +86 931 8911895; fax: +86 931 8912582.E-mail addresses: wangcm@lzu.edu.cn (C. Wang), ajayan@rice.edu (P.M. Ajayan).</p><p>C A R B O N 4 8 ( 2 0 1 0 ) 1 1 2 4 1 1 3 0</p><p>avai lab le at www.sc iencedi rec t .com</p><p>elsis well known that carbon nanotubes (CNTs), having excellent</p><p>electrical, mechanical and structural properties [9], and fasci-</p><p>nating performances in biomedicine, catalysts, sensors, and</p><p>so on [10], have been extensively studied as supports for the</p><p>dispersion of precious metal nanoparticles to enhance elec-</p><p>of applications, and they are specifically expected to have en-</p><p>hanced electrocatalytic activity.</p><p>In this work, a mild and environmental friendly reductant,</p><p>ethylene glycol, was used as both reductive and dispersing</p><p>agent for deposition of Pt nanoparticles on sheets of GO.which makes it promising for potential applications in many</p><p>technological fields, such as nanoelectronics, sensors, nano-</p><p>composites, batteries, supercapacitors and hydrogen storage</p><p>[1]. Especially, graphene has potential application as a hetero-</p><p>functional groups on the surfaces of GO can be used as</p><p>anchoring sites for metal nanoparticles [25], it is possible to</p><p>use them as a support to produce graphene-nanoparticle hy-</p><p>brids. Combination of graphene and functional nanoparticlesmaterial exhibits excellent physical and chemical properties,1. Introduction</p><p>Graphene, one-atom thick planar</p><p>rayed sp2 carbon atoms, has attrac0008-6223/$ - see front matter 2009 Elsevidoi:10.1016/j.carbon.2009.11.034</p><p>1 Tel.: +1 713 348 5904; fax: +1 713 348 5423tively. The Pt/CCG hybrid shows better tolerance to CO for electro-oxidation of methanol</p><p>compared to the Pt/MWCNT catalyst. Our study demonstrates that CCG can be an alterna-</p><p>tive two-dimensional support for Pt in direct methanol fuel cells.</p><p> 2009 Elsevier Ltd. All rights reserved.</p><p>of hexagonally ar-</p><p>mendous scientific</p><p>o-dimensional (2D)</p><p>possesses similar stable physical properties but also larger</p><p>surface areas [4,18,19]. Additionally, production cost of CCG</p><p>sheets in large quantities is much lower than that of CNTs</p><p>[4,20]. Several investigations have been carried out to produce</p><p>graphene nanosheets in bulk quantity by chemical reductionAvailable online 24 November 2009 groups on GO was removed, which resulted in a Pt/chemically converted grapheneCatalytic performance of Pt nagraphene oxide for methanol e</p><p>Yongjie Li a,b, Wei Gao b, Lijie Ci b, Chunmina Department of Chemistry, Lanzhou University, Lanzhou 730000, Pb Department of Mechanical Engineering and Materials Science, Rice</p><p>A R T I C L E I N F O</p><p>Article history:</p><p>Received 30 August 2009</p><p>Accepted 19 November 2009</p><p>A B S T R A C T</p><p>We have investigated</p><p>onto surfaces of grap</p><p>by ethylene glycol re</p><p>journal homepage: www.er Ltd. All rights reserved.oparticles on reducedectro-oxidation</p><p>Wang a,*, Pulickel M. Ajayan b,1</p><p>hina</p><p>iversity, Houston 77005, USA</p><p>imple approach for the deposition of platinum (Pt) nanoparticles</p><p>oxide (GO) nanosheets with particle size in the range of 15 nm</p><p>tion. During Pt deposition, a majority of oxygenated functional</p><p>evier .com/ locate /carbon.</p></li><li><p>sides of nanosheets are accessible. Secondly, the abundance</p><p>C A R B O N 4 8 ( 2 0 1 0 ) 1 1 2 4 1 1 3 0 11252. Experimental section</p><p>2.1. Synthesis of graphite oxide (GO)</p><p>GO was synthesized from graphite powder by a modified</p><p>Hummers method [26] as originally presented by Kovtyukh-</p><p>ova et al. [27,28].</p><p>2.2. Functionalization of multi-walled CNTs (MWCNTs)</p><p>The MWCNTs (MWCNTs &gt; 95 wt.% purity from CheapTubes)</p><p>have outer diameters in the range of 1020 nm with the</p><p>length of 1030 lm. The surface treatment process of</p><p>MWCNTs is according to the method reported by Xing [11].</p><p>In a typical experiment, MWCNTs (100 mg) were mixed with</p><p>94 mL of HNO3 (69%), 80 mL of H2SO4 (98%), and 6 mL of</p><p>deionized water (DI water) in a 250 mL Pyrex glass flask.</p><p>The flask was then put in the ultrasonic bath and treated</p><p>for 3 h. After that, the mixture was filtered and washed with</p><p>DI water several times. The sonochemically treated</p><p>MWCNTs were dried in a vacuum desiccator at room</p><p>temperature.</p><p>2.3. Synthesis of Pt/CCG, Pt/MWCNT and reduction of GOby ethylene glycol (EG)</p><p>Pt nanoparticles were deposited on GO sheets by a chemical</p><p>reduction of chloroplatinic acid (H2PtCl6) in ethylene glycol</p><p>water solution. In a typical procedure, 1.16 ml aqueous solu-</p><p>tion of 0.0443 M H2PtCl6, 8.84 ml of DI water and 90 mg of</p><p>GO were added into 40 ml ethylene glycol in a 100 ml flask.</p><p>The mixture was first ultrasonically treated for 4 h to ensure</p><p>GO being uniformly dispersed in ethylene glycolwater solu-</p><p>tion. The reduction reaction was then performed at 120 Cfor 24 h under constant stirring. The Pt/CCG hybrids were fi-</p><p>nally separated by filtration and washed with deionized water</p><p>several times. The resulting product was dried in a vacuum</p><p>desiccator at room temperature (Fig. 1). For comparison,</p><p>deposition of Pt nanoparticles on MWCNTs was also achieved</p><p>by the same procedure. In order to understand the chemical</p><p>reduction of GO, GO was treated by ethylene glycolwater</p><p>solution without metallic salts in the same condition (labeled</p><p>as GO/EG).</p><p>2.4. Characterization</p><p>X-ray powder diffraction (XRD) was carried out using a Rigaku</p><p>D/Max Ultima II diffractometer with Cu Ka radiation</p><p>(k = 0.15418 nm). The diffraction data was recorded for 2h an-</p><p>gles between 3 and 90. The morphologies of Pt nanoparticlessupported on CCG and MWCNTs were characterized using</p><p>JEOL 2100 field emission gun transmission electron micros-</p><p>copy (TEM). X-ray photoelectron spectroscopy (XPS) was per-</p><p>formed on a PHI Quantera SXM Scanning X-ray Microprobe</p><p>with an Al cathode (hm = 1486.6 eV) as the X-ray source set at</p><p>100 W and a pass energy of 26.00 eV. The content of Pt in com-</p><p>posite was analyzed by a Q-600 simultaneous thermo-gravi-</p><p>metric analyzer (TGA) from room temperature to 1000 C at</p><p>a heating rate of 15 C/min under air flow.of functional groups (such as carboxyl, carbonyl, hydroxyl,</p><p>and epoxide) on the surfaces and edges of GO nanosheets al-</p><p>lows for expedient synthesis of hybrids. Finally, GO can be</p><p>easily converted by chemical reduction methods to graphene,</p><p>which offers better electrical conductivity.</p><p>3.1. Morphologies and size distributions</p><p>Morphologies of Pt nanoparticles deposited on CCG nano-</p><p>sheets and MWCNTs have been characterized by TEM. Fig. 2</p><p>shows the typical TEM images of as-synthesized hybrids. It</p><p>is clearly seen in Fig. 2a that all the transparent CCG nano-</p><p>sheets are uniformly decorated by the nanosized Pt particles</p><p>with very few aggregations, indicating a strong interaction be-</p><p>tween graphene support and particles. Furthermore, these</p><p>monolayer sheets possess large surface areas, and particles</p><p>can be deposited on both sides of these sheets [5,6,2931].</p><p>Additionally, by functioning as a spacer, these Pt nanoparti-</p><p>cles attached onto the graphene surface can prevent the re-</p><p>duced GO (as discussed later) from aggregation and</p><p>restacking, and both the faces of graphene are accessible inoxidation reaction activity, cyclic voltammetry was per-</p><p>formed between 0.2 and +1.0 V in a mixing solution contain-ing 1 M CH3OH and 0.5 M H2SO4. The scan rate was set at</p><p>50 mV s1 and the electrolyte solutions were deaerated withultrahigh purity argon prior to any measurements.</p><p>3. Results and discussion</p><p>Pt/CNTs hybrids have shown excellent properties in some</p><p>applications [1113]; however, the high production cost of</p><p>CNTs has placed an obstacle in their applications. Meanwhile,</p><p>in order to deposit metal nanoparticles on CNTs surface,</p><p>functionalization needs to be done using nitric and sulfuric</p><p>acids. Although CNTs have large surface areas, nanotube</p><p>bundling and incomplete functionalization makes the acces-</p><p>sible areas very limited. Compared to CNTs, CCG nanosheets</p><p>can be easily obtained by chemical conversion of inexpensive</p><p>graphite oxide. Furthermore, using GO as a starting material</p><p>for deposition of metal nanoparticles has several advantages.</p><p>Primarily, GO nanosheets have large surface areas and both2.5. Electrochemical measurements</p><p>The electrochemical properties of the samples were mea-</p><p>sured by cyclic voltammetry in a standard three-electrode cell</p><p>using PSGSTAT-30 (Autolab) electrochemical workstation at</p><p>room temperature. A glassy carbon electrode (3 mm in diam-</p><p>eter) was used as the working electrode, on which 20 lL of a</p><p>paste of the catalyst was applied. The paste was a dimethyl-</p><p>formamide-nafion (49:1 volume ratio) solution with catalysts</p><p>concentration approximately 2.5 mg ml1. A Pt wire was usedas the counter electrode and an Ag/AgCl electrode was used</p><p>as the reference electrode. Electrochemically active surface</p><p>area (ECSA) of Pt nanoparticles was calculated from hydrogen</p><p>electrosorption curve, which was recorded between 0.2 and+1.2 V in 0.5 M H2SO4 solution. To measure methanol electro-their applications. Highly dispersed metal nanoparticles on</p></li><li><p>1 01126 C A R B O N 4 8 ( 2 0supports with larger surface areas have advantages in cata-</p><p>lytic activity and sensor sensitivity [11]. Therefore, Pt/CCG hy-</p><p>brids should be a potential material for use in future</p><p>nanotechnology. The high resolution image of Pt/CCG hybrid</p><p>in Fig. 2c shows the oriented and ordered lattice fringes for</p><p>Pt nanoparticles. The d-spacing value of 0.227 nm coincides</p><p>with that of fcc Pt (1 1 1).</p><p>In Fig. 2b, it can be seen that Pt nanoparticles are also</p><p>deposited on MWCNTs, however, their dispersion is not as</p><p>uniform as that on CCG surfaces and some Pt nanoparticles</p><p>are agglomerated. The mean Pt nanoparticles sizes estimated</p><p>from TEM images are 2.75 and 3.5 nm on CCG sheets (shown</p><p>in Table 1) and MWNTs, respectively.</p><p>Fig. 1 Scheme shows a formation route to anchor platinum (P</p><p>nanosheets. (1) Oxidation of pure graphite powder to graphite o</p><p>Fig. 2 Transmission electron microscopy (TEM) images of (a) P</p><p>resolution TEM (HRTEM) image of Pt nanoparticles in Pt/CCG hy</p><p>crystal structure are very evident in the Pt/CCG hybrids.) 1 1 2 4 1 1 3 03.2. X-ray diffraction</p><p>Fig. 3 shows XRD patterns of GO, GO/EG, Pt/CCG and Pt/</p><p>MWCNT. In Fig. 3a, the characteristic diffraction peak (0 0 2)</p><p>of GO [32] at 2h = 10.6 (corresponding to a d-spacing of0.749 nm) is ascribed to the introduction of oxygenated</p><p>functional groups, such as epoxy, hydroxyl (OH), carboxyl</p><p>(COOH) and carbonyl (C@O) groups attached on both sidesand edges of carbon sheets. These surface functional groups</p><p>will subsequently act as anchoring sites for metal complexes</p><p>[25]. The diffraction peak at around 43 is associated with the(1 0 0) plane of the hexagonal structure of carbon [33]. It can</p><p>be seen in Fig. 3b that the typical diffraction peak (0 0 2) of</p><p>t) nanoparticles onto chemically converted graphene (CCG)</p><p>xide. (2) Formation of Pt/CCG hybrids.</p><p>t/CCG and (b) Pt/MWCNT hybrids. The insert (c) is high</p><p>brids. The highly crystalline Pt nanoparticles showing fcc</p></li><li><p>tions involved. The C1s peak for sp2-hybridized carbon usually</p><p>appears near 284.5 eV for pure graphite powder. After oxida-</p><p>tion, the intensity of sp2-hybridized C1s peak is significantly re-</p><p>duced, and additional peak can be identified as hydroxyl,</p><p>epoxide and carboxyl groups, which indicates that graphite is</p><p>fully oxidized into GO by introducing these oxygenated func-</p><p>tional groups (Fig. 4).</p><p>Table 2 summarizes the deconvoluted peak positions and</p><p>the areas relative to C1s sp2 peak (expressed as a percentage).</p><p>It can be seen that after reducing GO by ethylene glycol with-</p><p>out chloroplatinic acid, the carboncarbon skeleton is rees-</p><p>tablished and the intensity of additional peak of oxygenated</p><p>G and Pt/MWCNT hybrids.</p><p>ECSA (m2/g) If/Ib</p><p>36.27 0.8333.43 0.72</p><p>nts.</p><p>1 0 ) 1 1 2 4 1 1 3 0 1127Table 1 Comparison of different parameters between Pt/CC</p><p>Samples Average Pt size (nm)</p><p>Pt/CCG 2.75Pt/MWCNT 3.5</p><p>*If/Ib represents ratio of the forward and backward anodic peak curre</p><p>Fig. 3 XRD patterns: (a) graphite oxide (GO); (b) graphite</p><p>oxide after reduction by ethylene glycol (GO/EG); (c) Pt/CCG</p><p>and (d) Pt/MWCNT hybrids.</p><p>C A R B O N 4 8 ( 2 0GO shifts to higher angle after reduction by ethylene glycol.</p><p>This could be attributed to fact that GO nanosheets are par-</p><p>tially reduced to graphene and restacked into an ordered crys-</p><p>talline structure.</p><p>Fig. 3c and d show X-ray diffractions of Pt/CCG and Pt/</p><p>MWCNT, respectively. It is obvious that the position of the</p><p>(0 0 2) diffraction peak (d-space 0.34 nm at 26.23) movesslightly to higher angle after deposition of Pt nanoparticles</p><p>on CCG nanosheets, which indicates that GO is further con-</p><p>verted to the crystalline graphene, and the conjugated graph-</p><p>ene network (sp2 carbon) has been reestablished due to the</p><p>reduction process. Pt nanoparticles are suggested to play a</p><p>crucial role in the catalytic reduction of GO when using ethyl-</p><p>ene glycol as a reducing agent [5]. The strong diffraction peaks</p><p>at 2h = 39.6, 46.2, 67.5 and 81.4 can be assigned to the char-acteristic (1 1 1), (2 0 0), (2 2 0) and (3 1 1) crystalline planes of</p><p>Pt, respectively, which possesses face-centered-cubic (fcc)</p><p>structure. The diffraction peak for Pt (2 2 0) is used to estimate</p><p>the Pt particle size by the Scherrers equation: D = 0.89k/(B cos</p><p>h) [33]. Here, the wavelength k is equal to 0.15418 nm, and B is</p><p>the full width at half-maximum (FWHM). The calculated aver-</p><p>age particle size of Pt on CCG sheets and MWCNTs are 2.96</p><p>and 3.38 nm, respectively, which are consistent with the</p><p>TEM results.</p><p>3.3. X-ray photoelectron spectra</p><p>XPS was performed on samples containing reduced GO with</p><p>and without Pt nanoparticles, which were obtained by chemi-</p><p>cal reduction reactions carried out in ethylene glycolwater in</p><p>order to understand the individual chemical reduction reac-functional groups is substantially reduced. On the other hand,</p><p>by depositing Pt onto the surfaces of GO nanosheets, the XPS</p><p>spectra of Pt/CCG hybrids is much more smooth compared to</p><p>original GO and reduced GO by ethylene glycol. The intensity</p><p>of some oxygenated functional groups on graphene sheets</p><p>after Pt deposition further decreases and carboncarbon skel-</p><p>eton is recovered to a higher extent. Specifically, the hydroxyl</p><p>group signal underwent considerable decrease in this hybrids</p><p>compared with the startin...</p></li></ul>

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