ARTICLE pubs.acs.org/JPCC
Controllable Deposition of Platinum Nanoparticles on Graphene As an Electrocatalyst for Direct Methanol Fuel Cells Jian-Ding Qiu,*,† Guo-Chong Wang,† Ru-Ping Liang,† Xing-Hua Xia,*,‡ and Hong-Wen Yu*,§ †
Department of Chemistry, Nanchang University, Nanchang 330031, People's Republic of China Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, People's Republic of China § Nano Industrialization Laboratory, Creative Research Institution, Hokkaido University, Sapporo 001-0021, Japan ‡
bS Supporting Information ABSTRACT: Platinum nanoparticles (Pt NPs) with uniform size and high dispersion have been successfully assembled on poly(diallyldimethylammonium chloride) functionalized graphene oxide via a sodium borohydride reduction process. The loading concentration of Pt NPs on graphene can be adjusted in the range of 1878 wt %. The obtained Pt/graphene nanocomposites are characterized by transmission electron microscopy, high resolution transmission electron microscopy, energy dispersive spectrometry, X-ray diffraction, and thermogravimetric analysis. The results show that the Pt NPs with sizes of approximate 4.6 nm uniformly disperse on graphene surface for all Pt loading densities. Electrochemical studies reveal that the Pt/graphene nanocomposites with electrochemically active surface area of 141.6 m2/g show excellent electrocatalytic activity toward methanol oxidation and oxygen reduction. The present method is promising for the synthesis of high performance catalysts for fuel cells, gas phase catalysis, and sensors.
’ INTRODUCTION To satisfy mankind’s ever-increasing energy needs and to tackle daunting environmental issues, one must consider alternative energy sources to replace the currently dominant fossil fuels, petroleum and natural gas. Direct methanol fuel cells (DMFCs), which provide a promising way to convert chemical energy directly into electrical energy, have attracted extensive attention as green power sources for vehicles and portable electronics.15 In this electrochemical cell, methanol is directly oxidized with oxygen from the air to carbon dioxide and water to generate electricity. Platinum-based (Pt-based) electrocatalysts have been widely used as anode and cathode electrocatalysts in DMFCs for the respective methanol oxidation and oxygen reduction reactions. However, the poor utilization coefficient and significantly high costs of Pt catalyst loading per unit area have limited their practical application in DMFCs. To overcome this issue, various types of carbon supports such as single-walled carbon nanotubes,6 multiwalled carbon nanotubes,7 cup-stackedtype carbon nanotubes,8 and graphitic carbon nanofibers9 have been used as highly conductive supports to uniformly disperse platinum nanoparticles (Pt NPs). Of those carbon materials, graphene with interesting 2-dimensional plane structure as carbon support for Pt NPs has shown considerable advantages. It has been extensively studied in recent years10,11 due to its high conductivity (103104 S/m), huge theoretical surface area (calculated value, 2600 m2/g), unique graphitized basal plane structure, and potentially low manufacturing costs.1214 A range of technically important graphene supported Pt NPs r 2011 American Chemical Society
nanocomposites with specific structure features including nanospheres,1517 nanofibers,18 nanowire,19,20 nanotubes,21,22 nanosheets,23 nanowheels,24 nanocages,25 nanodendrites,2631 have been successfully synthesized by various strategies including chemical reduction11,32 and electrochemical reduction.33 For example, on the basis of chemical reduction of H2PtCl6, Seger et al.11 and Li et al.34 synthesized Pt/graphene nanocomposites. Zhou and co-workers33 produced Pt NPs from the reduction of PtCl62- ions via a one step electrochemical reduction at room temperature. Although these reports provided the methods to deposition of Pt NPs on graphene nanosheets, the production of well-controlled dimensions and morphologies and the effective loading of Pt NPs is still challenging. In this work, we report a facile, efficient, and controllable route to disperse Pt NPs on graphene functionalized with poly(diallyldimethyl-ammonium chloride) (PDDA) via a noncovalent strategy. The noncovalent adsorption of PDDA on graphene oxide (GO) not only leads to a highly dense and homogeneous distribution of positive charges on the GO surface, but also preserves the intrinsic properties of the GO without any chemical oxidation treatment. The positive charges were then used as sites for deposition of Pt NPs on graphene. Therefore, the resultant Pt NPs with small size were uniformly dispersed on and both surfaces of graphene, and its loading density could be controlled Received: January 19, 2011 Revised: June 27, 2011 Published: June 30, 2011 15639
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The Journal of Physical Chemistry C by PDDA. The prepared Pt/graphene nanocomposites were characterized using transmission electron microscopy (TEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD) spectroscopy, and thermogravimetry analysis (TGA). The electrocatalytic activity of the Pt/graphene was studied using methanol oxidation and oxygen reduction as the model systems. Results demonstrated that the Pt/graphene nanocatalysts with high electrochemical active surface area (ECSA) showed significantly electrocatalytic activities toward both electrode reactions. The present work promises an interesting strategy to prepare graphene based catalysts for DMFC.
’ EXPERIMENTAL SECTION Materials. Graphite flake (99.8%, 325 mesh) was provided by Alfa. Platinum precursor, hexachloroplatinic (IV) acid hexahydrate (H2PtCl6 3 6H2O), purchased from Sinopharm Chemical Reagent Co. Ltd., was used as received. Poly(diallyldimethylammonium chloride) (PDDA, 20 wt % in water, MW = 200 000350 000) was purchased from Aldrich. High-purity nitrogen, oxygen gas, and distilled water were used in all experiments. NaNO3, H2SO4, and other reagents were of analytical grade. Noncovalent Functionalization of GO by PDDA. GO were synthesized from graphite flake by a modified Hummer’s method.3537 The procedure for the noncovalent functionalization of GO using PDDA is as follows:3840 GO (21 mg) was sonicated in 50 mL of 1 M NaCl solution for 1 h, and then 80 mg of PDDA was added. The solution was sonicated for another 1.5 h. Subsequently, the suspension was centrifuged to isolate the solid product which was then washed with distilled water. This washing process was repeated several times to remove excess PDDA and NaCl. The PDDA-functionalized GO powders were dried in a vacuum oven at 40 °C for 48 h and the final product is denoted as PDDA-GO. Synthesis of Pt/grahene Nanocomposites. PDDA-GO (10.8 mg) was dispersed in 30 mL of distilled water containing 10.8 mg of H2PtCl6, and the solution was sonicated for 0.5 h. An aqueous solution of NaBH4 (30 mL, 0.05 M) was then slowly added dropwise into the above solution under magnetic stirring. After reaction for 2 h, the resulting black solid products were isolated by centrifugation, washed with distilled water and ethanol (V:V = 1:1) to remove the ions possibly remaining in the final products, and then dried in a vacuum oven at 40 °C for 48 h. The obtained products were designated as sample 1. Electrode Preparation and Modification. Prior to use, a glassy carbon electrode (GCE, 3 mm in diameter) was first polished subsequently with 1.0, 0.3, and 0.05 μm alumina slurry and was ultrasonically cleaned with 1:1 nitric acid, ethanol and distilled water, and then dried in a stream of nitrogen gas. The catalyst powder (5.0 mg) was dispersed ultrasonically in 5.0 mL distilled water to form a homogeneous black suspension with 1 mg/mL Pt/graphene, and a 5 μL aliquot of this solution was cast on the pretreated GCE surface. After drying in air, the modified electrode was used as the working electrode for all electrochemical studies. Characterization. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images were obtained on a JEM-2010 transmission electron microscope (JEOL Ltd.). Energy dispersive spectrometry (EDS) analyses were preformed on a FEI QUANTA 200F scanning electron microscope (FEI Co.). X-ray diffraction (XRD, BRUKER) patterns of the nanocomposites were carried
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Scheme 1. Synthesis of Graphene Supported Catalystsa
a
(1) Oxidation of graphite to graphene oxide with larger interlayer distance, (2) exfoliation of graphene oxide by ultrasonication, (3) modification of graphene oxide with PDDA, (4) addition of H2PtCl6, and (5) reduction of Pt precursor to Pt NPs using chemical reducing agent of NaBH4.
out using a Rigaku powder diffractometer equipped with Cu KR1 radiation (λ = 1.5406 Å). Thermogravimetric analysis (TGA) was performed on a simultaneous thermo-gravimetric analyzer (TG/DTA PYRIS DIAMOND). The samples were heated under an air atmosphere from room temperature to 850 °C at a rate of 10 °C/min. Electrochemical measurements were performed on an Autolab PGSTAT30 electrochemical workstation (Eco Chemie). A conventional three-electrode system including an Ag/AgCl reference electrode, a Pt wire counter electrode, and the modified electrode as the working electrode was used. The electrochemical active surface area of Pt NPs was calculated from the hydrogen electrosorption curve, which was recorded between 0.2 and +1.2 V in 0.5 M H2SO4 solution at a scan rate of 50 mV/s. The electrocatalytic activity of the Pt/graphene nanocomposites toward the oxidation of methanol was studied in a 1.0 M H2SO4 solution containing 2.0 M CH3OH at a scan rate of 50 mV/s, and the activity toward the reduction of oxygen was measured in an oxygen-saturated 0.5 M H2SO4 solution at a scan rate of 10 mV/s.
’ RESULTS AND DISCUSSION The synthesis route of the Pt/graphene nanocomposites is illustrated in Scheme 1. The graphene oxide nanosheets functionalized with amine moieties carried PDDA, making the graphene oxide sheets possess large amount of positively charged sites. These positive charges function as active sites attract negatively charged PtCl62- ions via the electrostatic interactions. Since the amine moieties disperse on the graphene oxide nanosheets, reduction of the platinum precursor using NaBH4 results in the formation of Pt NPs with controllable particle diameter, uniform distribution. And the Pt loading on the graphene is determined by the available amine moieties of PDDA. Accompanying the reduction of platinum precursor, the graphene oxide is also reduced to graphene simultaneously. TEM, HRTEM, EDS, XRD, and TGA Characterizations. Morphologies of the Pt NPs deposited on graphene were characterized by TEM and HRTEM. A typical TEM image of the Pt NPs supported on graphene synthesized using a 1:2.5 mass ratio of PDDA-GO to H2PtCl6, is shown in Figure 1a. It clearly shows that Pt NPs with uniform size homogeneously decorated on both sides of the graphene nanosheets as reported previously.32,41 The mean size of the Pt NPs on graphene was estimated to be 4.6 nm. Occasionally, Pt NPs aggregates can be seen only at the graphene 15640
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Figure 1. TEM (a), HRTEM (b), and EDS (c) images of the Pt/graphene catalysts with Pt loading of 50 wt %.
Figure 2. XRD patterns of (a) pristine graphite (b) graphene oxide (c) Pt/graphene nanocomposites synthesized from a 1:3.5 mass ratio of PDDA-GO to H2PtCl6. The insert image is the XRD patterns of Pt/ graphene nanocomposites with the 2θ between 15° and 35°.
edges where abundant amine moieties are available due to the easy adsorption of PDDA at defect sites of the graphene edges. This result also indicates a strong interaction between the functionalized graphene support and the Pt NPs. The Pt NPs decorated on the graphene surfaces can act as “spacers” to prevent the graphene from aggregation and restacking, and both faces of graphene are accessible. Therefore, the resultant Pt/ graphene nanocomposites promise great potential as functional nanomaterials for use in future nanotechnology applications or catalysts.42 The HRTEM image of the Pt/graphene in Figure 1b shows the oriented and ordered lattice fringes for Pt NPs. The d-spacing value of 0.217 nm coincides with that of face centered cubic (fcc) Pt (111).34,43 The formation of the Pt/graphene nanocomposites was further confirmed by energy dispersive spectroscopy (EDS) as shown in Figure 1c. The EDS spectrum shows the peaks corresponding to C, O, and Pt elements, confirming the deposition of Pt NPs on graphene nanosheets. The XRD pattern of (a) pristine graphite, (b)graphene oxide, and (c) Pt/graphene nanocomposites is shown in Figure 2. The X-ray (002) peak44,45 of the pristine graphite is observed at 2θ value of 26.5° (curve a, Figure 2), indicating an interlayer distance of 0.337 nm.4648 This peak shifts to 10.0° (curve b, Figure 2) after oxidation treatment, indicating that the interlayer distance in the graphite oxide increases to 0.709 nm.4648 The interlayer distance in the graphene oxide is expanded due to the presence of epoxide, carboxyl groups, and water molecules
between the graphene oxide layers, which makes the graphene oxide hydrophilic.46,47 These surface functional groups subsequently act as anchoring sites for metal complexes.49 As soon as Pt/graphene is formed, the 2θ value of (002) is shifted to around 23.6° compared with the graphene oxide (see the inset image of Figure 2). It indicates that graphene oxide has been reduced to form graphene after the reduction process via NaBH4.10,50 The XRD pattern of the Pt/graphene composite (curve c, Figure 2) exhibits the characteristic fcc platinum lattice: diffraction peaks at 40.0° for Pt(111), 46.2° for Pt(200), 67.5° for Pt(220), 81.7° for Pt(311) and 85.6° for Pt(322)32 confirming that the platinum precursor has been chemically reduced to Pt NPs by NaBH4. The diffraction peak for Pt(220) is used to estimate the Pt crystallite size since there is no interference from other diffraction peaks. Calculation using the Scherrer equation yields an average crystallite size of Pt (normal to Pt(220)) on graphene of 5.0 nm, which is consistent with the TEM results. In the present method, the Pt loading density can be adjusted by varying the mass ratio of PDDA-GO to H2PtCl6. Figure 3 shows the TEM micrographs of the resultant Pt/graphene nanocomposites with Pt loading ranging from 30 to 78 wt %. It is interesting to observe that the Pt NPs size does not vary with the increase of the Pt loading density. This is very important to compare the catalytic activity of the nanocomposites in latter sections. For the Pt/graphene nanocomposites synthesized from the mass ratio of PDDA-GO to H2PtCl6 ranging from 1:1 to 1:6.0, the deposited Pt NPs uniformly disperse on graphene nanosheets (Figure 3, images ae). The averaged Pt NPs size is ∼4.6 nm. Such excellent dispersion of Pt NPs on graphene nanosheets can be still obtained even with a 1:8.5 mass ratio of PDDA-GO to H2PtCl6 (Figure 3f). We speculate that the PDDA offers large and uniformly distributed active sites for anchoring metal ions, which facilitates the high loading and uniform distribution of Pt NPs on graphene. These results are significant because they show not only the first example of controllable particle diameter and uniformly distributed Pt NPs, but also a very high Pt loading can be obtained on PDDA-GO. The weight loss of the samples (traces a and b in Figure 4) as determined by TGA and the corresponding derivatives of the weight loss with respect to temperature (traces a0 and b0 in Figure 4) shows that the PDDA-modified GO starts to lose mass upon heating below 100 °C due to desorption of adsorbed water.11,35,50,51 There are two significant drops in mass around 220 and 495 °C. The former is assigned to the evolution of CO and CO2 from the PDDA-modified GO caused by the destruction of oxygenated functional groups and the carbonization 15641
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Figure 3. TEM images of Pt/graphene nanocomposites synthesized from precursors with different mass ratios of PDDA-GO to H2PtCl6: (a) 1:1, (b) 1:1.5, (c) 1:2.5, (d) 1:3.5, (e) 1:6.0, and (f) 1:8.5.
Figure 4. Thermogravimetric analysis of PDDA modified GO and Pt/ graphene synthesized from a 1:3.5 mass ratio of PDDA-GO to H2PtCl6. The black lines (a, b) show the change in weight, while the red lines (a0 , b0 ) show the derivatives of the changes in weight with respect to temperature. Both PDDA-modified GO (a, a0 ) and Pt/graphene (b, b0 ) lost water content at temperatures less than 100 °C.
of PDDA.52 The weight loss at ∼390 °C in the Pt/graphene nanocomposites is much lower than that of the PDDA-modified GO, indicating the decrease of the quantity of oxygenated functional groups on the graphene support.50,53 The percentage of Pt NPs by weight is estimated, based on residue, to be 60 wt % for the Pt/graphene nanocomposites. Electrochemically Active Surface Area. The ECSA provides information regarding the number of available electrochemically active sites which is essential for understanding the utility and electrocatalytic activity of platinum in the resultant Pt/graphene nanocomposites. Hydrogen adsorption/desorption in an electrochemical system is commonly used to evaluate the ECSA.11,54,55 As shown in Figure 5, well-defined hydrogen adsorption/desorption characteristics are observed for the Pt/
Figure 5. Cyclic voltammogram of the Pt/graphene (Pt loading 60 wt %) catalysts modified glassy carbon electrode in a N2-saturated 0.5 M H2SO4 solution at a scan rate of 50 mV/s.
graphene (Pt loading 60 wt %) catalysts. A strong adsorption peak in the potential range from +0.05 to 0.05 V and a weak adsorption peak located between 0.05 and 0.2 V are observed during the negative-going potential scan. These two adsorption current peaks can be assigned to the weakly and strongly bonded hydrogen adatoms. The corresponding desorption peaks are observed during the reverse potential scan. The ECSA of Pt is determined to be 141.6 m2/g for the Pt/graphene (Pt loading 60 wt %), which is much larger than the one obtained for Pt/ graphene (44.6 m2/g),34 GO-Pt (16.9 m2/g),11 recent state-ofthe-art Pt based nanomaterials such as PdPt bimetallic nanodendrite (57.1 m2/g),56 (81.6 m2/g),57 carbon nanotube/ionic liquid/Pt NPs hybrids (71.4 m2/g),58 functionalized graphene and functionalized multiwalled carbon nanotubes [Pt/(f-G-fMWNT)]/ Pt NPs hybrids (108 m2/g),59 mesoporous Pt with giant mesocage (74 m2/g),60 and dendritic Pt NPs (56 m2/g).61 15642
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Figure 6. (A) Cyclic voltammograms displaying the catalytic oxidation of methanol on a bare glassy carbon electrode (GCE) (a) and Pt/graphene nanocomposites modified GCE (Pt loading 60 wt %) (b) in 1.0 M H2SO4 solution containing 2.0 M methanol at a scan rate of 50 mV/s. (B) Polts of the are specific activity of methanol oxidation of Pt/graphene against the Pt NPs loading. (C) The forward peak current density of the Pt/graphene catalysts with Pt loading 60 wt % as a function of the cycle number for the methanol oxidation in 1.0 M H2SO4 solution containing 2.0 M CH3OH. (D) Cyclic voltammograms displaying the catalytic reduction of O2 on Pt/graphene nanocomposites modified GCE (Pt loading 60 wt %) in N2-saturated 0.5 M H2SO4 (a) and O2-saturated 0.5 M H2SO4 (b) at a scan rate of 10 mV/s.
This distinguished result should be a result of the small size of Pt NPs dispersed uniformly on graphene nanosheets. This result reveals that the utilization of Pt in the Pt/graphene nanocomposites is very high, which is very important for improving the practical performance of DMFCs and gas phase catalytic reactions. Electrocatalytic Activity of the Pt/graphene Catalysts toward Methanol Oxidation and Dioxygen Reduction Reactions. Methanol offers several advantages over hydrogen as a fuel including the ease of transportation and storage, and high theoretical energy density. The methanol oxidation reaction occurs at the anode in DMFCs. Pt is the most promising candidate among pure metals for application in DMFCs. Pt has the highest activity toward the dissociative adsorption of methanol. However, pure Pt surfaces are usually poisoned by carbon monoxide, an intermediate of methanol oxidation, at room temperature.62 The electrocatalytic performance of Pt/graphene with different Pt lodaing for methanol oxidation was investigated in a 1.0 M H2SO4 solution containing 2.0 M CH3OH solution (Figure S1 in the Supporting Information). As shown in Figure 6A, the bare GCE does not show any electrocatalytic activity toward the oxidation of methanol as expected. However, the Pt/graphene nanocomposites synthesized from a 1:3.5 mass ratio of PDDA-GO to H2PtCl6 show remarkable catalytic activity, displaying prominent current peaks for the oxidation of methanol in both the positive and negative potentials scans. It is interesting to observe that the onset potential for methanol oxidation starts at 0.10 V which is much lower than the ones reported for carbon materials supported catalysts, such as the Pt/ CCG (0.38 V),10 the Pt/RGO (0.5 V),50 and the Pt/graphene
(0.2 V),34 demonstrating the excellent electrochemical catalytic activity of the present Pt/graphene nanocomposites toward the oxidation of methanol. The peak current densities at about 0.68 V (versus Ag/AgCl) in the forward potential scan and 0.47 V (versus Ag/AgCl) in the backward potential scan are 2.53 and 1.31 mA/cm2, respectively. It is reported that the ratio of the forward oxidation current peak (If) to the reverse current peak (Ib), If/Ib, indicates the tolerance of catalysts toward the poisoning species such as adsorbed CO intermediates formed via decomposition of methanol.57,63 A higher ratio implies more effective removal of the poisoning species from the catalysts surface. The If/Ib ratio is about 1.92, which is higher than that of the Pt/chemically converted graphene hybrid (0.83),10 threedimensional Pt-on-Pd bimetallic nanodendrites supported on graphene nanosheets (1.25),57 the Pt-CNT catalysts (1.4),64 and the commercial E-TEK catalyst (0.74),63 showing better catalytic tolerance of the Pt/graphene catalysts. Such a high value indicates that the Pt/graphene hybrid has less carbonaceous accumulation and hence is much more tolerant toward CO poisoning, demonstrating that most of the intermediates were oxidized to CO2 in the forward scan.64 The higher catalytic activity demonstrated by the Pt/graphene nanocomposites can be ascribed to the larger ECSA and uniformed small particle size. The lower onset potential of the methanol oxidation on Pt/ graphene catalysts indicates that the oxidative removal of the intermediates generated from the methanol oxidation can occur easily as compared to that on the conventional Pt/C catalysts (0.44 V).65 Figure 6B shows a plot of the current density for methanol electrooxidation on Pt/graphene as a function of Pt loading. 15643
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The Journal of Physical Chemistry C It shows that this current density increases with the increase of Pt NPs loading on graphene and reaches a plateau at Pt loading g72 wt %. In addition, the long-term stability of Pt/graphene catalysts modified electrodes was also investigated. It is found that the Pt/ graphene nanocomposites with different Pt loading present similar performances. Figure 6C shows the Pt/graphene catalysts with Pt loading of 60 wt % as a function of the potential cycle number for methanol oxidation in 1.0 M H2SO4 solution containing 2.0 M CH3OH. It can be observed that the forward peak current density for methanol oxidation decreases by about 17% within the 3 scans and then keeps stable between the third and fifteenth scans. After the fifteenth scan, the forward peak current density decreases gradually with the successive scans. This performance is better than the one on graphene nanoflake film supported with 85 nm thick Pt nanocluster composite (about 54% loss of current).66 For this reason, all our data for methanol oxidation in the following study are collected from the third potential scan. The decay of current density with the increased cycle numbers can be attributed to the formation of intermediates such as COads, CH3OHads, and CHOads on the catalyst surface during the methanol oxidation reaction,34,50,67 which gradually accumulate to lower the electroactive area of catalysts, significantly poisoning the Pt NPs for methanol oxidation. The electrocatalytic activity of the Pt/graphene nanocomposites toward the electrochemical reduction of oxygen was also studied. Figure 6D shows the typical cyclic voltammograms of oxygen reduction on the Pt/graphene nanocomposites in N2saturated 0.5 M H2SO4 (curve a) and in O2-saturated 0.5 M H2SO4 (curve b) at a scan rate of 10 mV/s. The reduction of oxygen starts at 0.60 V and reaches a peak at 0.41 V. This current peak potential of 0.41 V is more positive as compared to 0.25 V observed on Pt DEN catalysts.68 These voltammetric results clearly demonstrate the excellent electroactivity of Pt/graphene catalysts for oxygen reduction as well.
’ CONCLUSIONS We present a facile, efficient, and controllable route to prepare Pt NPs on graphene nanosheets via one-step chemical reduction process. The Pt NPs with sizes of ca. 4.6 nm uniformly dispersed on both sides of graphene nanosheets. The loading of same sized Pt NPs on graphene can be adjusted in the range from 18 to 78 wt % simply by changing the mass ratio of PDDA-GO to H2PtCl6. The resultant Pt/graphene nanocomposites show higher electrochemical active surface area and better tolerance toward CO, thus, exhibit excellent electrocatalytic activity toward the oxidation of methanol and the reduction of oxygen. These results indicate that graphene nanosheets could be a good candidate as a supporting material of catalysts in fuel cells. The present method is promising for the preparation of high performance catalysts for fuel cells and sensors. ’ ASSOCIATED CONTENT
bS
Supporting Information. Electrocatalytic activity of the Pt/graphene catalysts with different Pt loading toward methanol oxidation and oxygen reduction reactions. Plots of the massspecific peak current density of Pt/graphene for oxygen reduction as a function of Pt NPs loading, and the current density of the Pt/graphene catalysts with Pt loading 60 wt % as a function of the cycle number for the oxygen reduction in O2-saturated 0.5 M
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H2SO4. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel/Fax: +86-791-3969518; E-mail:
[email protected] (J.D.Q.). Tel/Fax: +86-25-83597436; E-mail:
[email protected](X.H.X.). Tel: +81-80-60997889; E-mail:
[email protected] (H.W.Y.).
’ ACKNOWLEDGMENT We greatly appreciate the support of the National Natural Science Foundation of China (Grant Nos. 20865003; 20805023; 21065006), and the Program for Young Scientists of Jiangxi Province (Grant No. 2008222). ’ REFERENCES (1) Wang, H. L.; Turner, J. A. J. Power Sources 2008, 180, 803. (2) Shukla, A. K.; Raman, R. K.; Scott, K. Fuel Cells 2005, 5, 436. (3) Jayaraman, S.; Jaramillo, T. F.; Baeck, S.-H.; McFarland, E. W. J. Phys. Chem. B 2005, 109, 22958. (4) Olah, G. A. Angew. Chem., Int. Ed. 2005, 44, 2636. (5) Winter, M.; Brodd, R. J. Chem. Rev. 2004, 104, 4245. (6) Girishkumar, G.; Vinodgopal, K.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 19960. (7) Wang, C.; Waje, M.; Wang, X.; Tang, J. M.; Haddon, R. C.; Yan, Y. Nano Lett. 2004, 4, 345. (8) Kim, C.; Kim, Y. J.; Kim, Y. A.; Yanagisawa, T.; Park, K. C.; Endo, M.; Dresselhaus, M. S. J. Appl. Phys. 2004, 96, 5903. (9) Tang, H.; Chen, J. H.; Nie, L. H.; Liu, D. Y.; Deng, W.; Kuang, Y. F.; Yao, S. Z. J. Colloid Interface Sci. 2004, 269, 26. (10) Li, Y.; Gao, W.; Ci, L.; Wang, C.; Ajayan, P. M. Carbon 2010, 48, 1124. (11) Seger, B.; Kamat, P. V. J. Phys. Chem. C 2009, 113, 7990. (12) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (13) Bunch, J. S.; van der Zande, A. M.; Verbridge, S. S.; Frank, I. W.; Tanenbaum, D. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L. Science 2007, 315, 490. (14) Park, S.; Ruoff, R. S. Nat. Nanotechnol. 2009, 4, 217. (15) Bigall, N. C.; H€artling, T.; Klose, M.; Simon, P.; Eng, L. M.; Eychm€uller, A. Nano Lett. 2008, 8, 4588. (16) Li, W. B.; Zhai, D. Y.; Qiu, H.; Pang, H. A.; Pan, L. J.; Shi, Y. Chem. Lett. 2011, 40, 104. (17) Nethravathi, C.; Anumol, E. A.; Rajamathi, M.; Ravishankar, N. Nanoscale 2011, 3, 569. (18) Yamauchi, Y.; Takai, A.; Nagaura, T.; Inoue, S.; Kuroda, K. J. Am. Chem. Soc. 2008, 130, 5426. (19) Ye, X. R.; Lin, Y. H.; Wang, C. M.; Wai, C. M. Adv. Mater. 2003, 15, 316. (20) Zhang, J.; Jing, B.; Tokutake, N.; Regen, S. L. J. Am. Chem. Soc. 2004, 126, 10856. (21) Kijima, T.; Yoshimura, T.; Uota, M.; Ikeda, T.; Fujikawa, D.; Mouri, S.; Uoyama, S. Angew. Chem., Int. Ed. 2004, 43, 228. (22) Mayers, B.; Jiang, X.; Sunderland, D.; Cattle, B.; Xia, Y. J. Am. Chem. Soc. 2003, 125, 13364. (23) Song, Y.; Steen, W. A.; Pe~ na, D.; Jiang, Y.-B.; Medforth, C. J.; Huo, Q.; Pincus, J. L.; Qiu, Y.; Sasaki, D. Y.; Miller, J. E.; Shelnutt, J. A. Chem. Mater. 2006, 18, 2335. (24) Song, Y.; Dorin, R. M.; Garcia, R. M.; Jiang, Y.-B.; Wang, H.; Li, P.; Qiu, Y.; Swol, F. v.; Miller, J. E.; Shelnutt, J. A. J. Am. Chem. Soc. 2008, 130, 12602. (25) Song, Y. J.; Garcia, R. M.; Dorin, R. M.; Wang, H. R.; Qiu, Y.; Shelnutt, J. A. Angew. Chem., Int. Ed. 2006, 45, 8126. (26) Wang, H.; Song, Y.; Medforth, C. J.; Shelnutt, J. A. J. Am. Chem. Soc. 2006, 128, 9284. 15644
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