Graphene-Metal Particle Nanocomposites

Transcription

1 J. Phys. Chem. C 2008, 112, Graphene-Metal Particle Nanocomposites Chao Xu, Xin Wang,* and Junwu Zhu Key Laboratory for Soft Chemistry and Functional Materials, Nanjing UniVersity of Science and Technology, Ministry of Education, Nanjing , China ReceiVed: September 9, 2008; ReVised Manuscript ReceiVed: October 26, Introduction Graphene sheets, which possess unique nanostructure and a variety of fascinating properties, can be considered as promising nanoscale building blocks of new composites, for example, a support material for the dispersion of nanoparticles. Here, we present a general approach for the preparation of graphene-metal particle nanocomposites in a water-ethylene glycol system using graphene oxide as a precursor and metal nanoparticles (Au, Pt and Pd) as building blocks. These metal nanoparticles are adsorbed on graphene oxide sheets and play a pivotal role in catalytic reduction of graphene oxide with ethylene glycol, leading to the formation of graphene-metal particle nanocomposites. The typical methanol oxidation of graphene-pt composites in cyclic voltammograms analyses indicated its potential application in direct methanol fuel cells, bringing graphene-particle nanocomposites close to real technological applications. Graphene, a monolayer of carbon atoms packed into a dense honeycomb crystal structure, has attracted tremendous attention from both the experimental and theoretical scientific communities in recent years. 1 Because of its unique nanostructure and extraordinary properties, graphene sheets are attractive as potential nanoscale building blocks for new materials. 2 To explore the application potential of graphene-based materials, an experiment using metal nanoparticles (NPs) to decorate graphene sheets forming graphene-metal particle nanocomposites was conducted. It is well-known that carbon nanotube (CNT) particle nanocomposites owing to their fascinating properties have attracted significant research attention in biomedicine, catalysts, sensors, and so on. 3 In comparison with CNTs, graphene possesses similar stable physical properties but larger surface areas, which can be considered as an unrolled CNT. 4,5 Deposition of inorganic materials, such as metallic, semiconducting, and insulating NP/nanoclusters, onto the graphene nanosheets forms interesting graphene derivatives. Also, integration of graphene and certain functional particles presents special features in the new hybrids, useful, for example, in optical, electrical, catalysis, sensors, and so on. 6 In addition, the production cost of graphene sheets in large quantities is much lower than that of CNTs; 7,8 therefore graphene is a very promising candidate for new carbonaceous supports. Several investigations have been carried out to produce graphite-metallic particles composites; 9-11 for example, electrochemical deposition, metal evaporation, and hydrogen reduction of metallic salts-graphite composite can be used to prepare graphite-metal composites. However, because of the difficulty of exfoliating the graphite flakes, the assynthesized nanocomposites were nonuniform, and it was difficult to obtain the graphene-based composites. Additionally, the difficulty in large-scale synthesis of these composites becomes an obstacle in its application. As recently demonstrated, graphene can be obtained in bulk quantity by chemical reduction of graphene oxide in solution * To whom correspondence should be addressed. Telephone and fax number: wxin@public1.ptt.js.cn. Figure 1. Scheme showing a proposed formation route to anchor metal particles onto exfoliated graphite oxide or graphene sheets. (1) Oxidation of graphite (black lines) to graphite oxide (gray lines) with greater interlayer distance. (2) Exfoliation of graphite oxide by sonication in water solution. (3) Attachment of metal particles on the graphene oxide sheets. (4) Formation of graphene-supported metal particles composites by reduction of the graphene oxide sheets. The distorted carbon sheets are simplified to an idealized planar model. However, because of the van der Waals interactions, the asreduced graphene sheets tend to form irreversible agglomerates and even restack to form graphite. In order to obtain graphene as individual sheets, attaching some molecules or polymers onto the sheets is an approach to reduce the aggregation. 7,15 We surmised that the attachment of inorganic particles, instead of organic materials, onto the graphene may not only prevent the restack of these sheets during the chemical reduction process, but also lead to the formation of a new class of graphene-based nanocomposites (Figure 1). It is possible to use graphene oxide as a precursor to produce graphene-particle composites, 31 and the combination of graphene and inorganic particles may result in some useful properties of carbonaceous material for catalysts and batteries. Noble metal NPs (e.g., Au, Pt, Pd) are of great interest because of their unusual properties and have been widely used to decorate CNTs to form a new class of composites. These composites display interesting properties for gas sensor and catalyts. 3 However, relatively little attention has been paid so /jp807989b CCC: $ American Chemical Society Published on Web 11/19/2008

2 19842 J. Phys. Chem. C, Vol. 112, No. 50, 2008 Xu et al. Figure 2. XRD patterns of composites. (a) Au-C composites. The diffraction peaks of the Au-C composite are similar to that of pure Au (JCPDS No ). (b) Pt-C composites. The inset is a partial amplified image of the Pt-C pattern. The peaks at 2θ ) 39.9, 46.2, 67.9, and 81.4 can be assigned to the (111), (200), (220), and (311) crystalline planes of Pt, respectively, which indicates that the Pt NPs are composed of pure crystalline Pt. far to metal NPs supported on carbon sheets. It has been found that a water-ethylene glycol mixture is a very useful system to synthesize CNT-noble metal particle nanocomposites. Considering the facile exfoliation nature of graphite oxide sheets in water, 16,17 and the mild reducibility of the ethylene glycol 18 for graphene oxide, a water-ethylene glycol system was used to prepare graphene-metal particle nanocomposites. 2. Experimental Section Syntheses of Graphene-Supported Metal Particles Nanocomposites. Graphite oxide was prepared from purified natural graphite bought from Qingdao Zhongtian Company with a mean particle size of 44 um according to the method reported by Hummers and Offeman. 34 A 10 mg portion of graphite oxide powder was dispersed in 10 ml of water by sonication for 1 h, forming stable graphene oxide colloid. 6,7 Then 20 ml of ethylene glycol and 0.5 ml of 0.01 M metal precursor, (water solutions of K 2 PtCl 4,K 2 PdCl 4, and HAuCl 4 3H 2 O) were added to the solution with magnetic stirring for 30 min. Subsequently, the mixture was put in an oil bath and heated at 100 C for6h with magnetic stirring. The graphenes with metal NPs (Au, Pt, and Pd) on them were then separated from the ethylene glycol solution in the centrifuge and washed with deionized water five times. The resulting products were dried in a vacuum oven at 60 C for 12 h. Characterization. Powder the X-ray diffraction (XRD) analyses were performed on a Bruker D8 Advance diffractometer with Cu KR radiation. The diffraction data was recorded for 2θ angles between 2.1 and 90. Samples were prepared in potassium bromide pellets, and Fourier transform infrared spectroscopy (FT-IR) analyses was carried out on a Bruker Vector-22, with X-ray photoelectron spectra (XPS) recorded on a Perkin-Elmer PHI5300 X-ray photoelectron spectrometer, using Mg KR (hυ ) ev) X-ray as the excitation source. Morphology analyses of samples were carried out on JEOL JEM-2100 transmission electron microscope (TEM). Scanning electron microscope (SEM) images and energy dispersive X-ray spectrometer (EDS) analyses were preformed on a JEOL JSM- 6380LV SEM. Raman spectra were recorded from 200 to 2000 cm -1 on a Renishaw Invia Raman Microprobe using a nm argon ion laser. The content of Pt in composite was analyzed on an IRIS Intrepid II XSP. Thermogravimetric analyses (TGA) were preformed on a Mettler TGA/SDTA851 thermogravimetric analyzer from 50 to 700 C at a heating rate of 5 C /min under air flow. Electroactivities of the samples were measured by cyclic voltammetry with use of a three-electrode test cell at room temperature. A thin film electrode technique was used to make the measurements. A glassy carbon disk (3 mm in diameter) held in a Teflon cylinder was used as the working electrode, on which a thin layer of Nafion-impregnated catalyst was cast. A Pt wire served as the counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. The electrolyte for electrochemical measurements was a solution of 2 M methanol in 1MH 2 SO 4. The solution was deaerated with ultrahigh-purity N 2 before scanning. 3. Results and Discussion Noble metal NPs can be easily obtained by reduction of the corresponding metallic salt in a water-ethylene glycol system Figure 2 shows the XRD patterns of the as-synthesized composites (labeled as Au-C, Pt-C, and Pd-C), indicating that the metals Au, Pt, and Pd were formed after being reduced by ethylene glycol. However, no obvious diffraction peaks of graphite oxide or graphite were observed (Figure 1S, Supporting Information). 14,16 Recent studies have shown that, if the regular stacks of graphite oxide or graphite are destroyed, for example, by exfoliation, their diffraction peaks become weak or even disappear. 22,23 It does not matter whether the existence state of the carbon sheets was graphene oxide or graphene in these composites; it was confirmed that the regular layered structure of graphite oxide or graphite was destroyed. Because of the low weight contents of metals in these composites (about 10%, 8%, and 6% for Au-C, Pt-C, and Pd-C, respectively), the influence of the as-reduced metals on the XRD patterns of graphite or graphite oxide is not considerable. 26 During the synthesis process of these composites, we found that the water-ethylene glycol mixture containing graphene oxide sheets without metallic salts was quite stable, and no obvious aggregations or sediments were found under the same conditions. However, when the metallic salts were added to the above system, the carbon sheets easily aggregated and then deposited. A possible explanation is that the adsorption of the as-reduced metallic particles on the surface resulted in the formation of heavier entities, consequently leading to a quick sedimentation (Figure 1). The attached particles may also prevent the restacking of these carbon sheets, and therefore the characteristic diffractions peaks of the layered structure disappeared. The heterostructure of these composites can be verified by the morphological analyses. Figure 4 shows the typical TEM

3 Graphene-Metal Particle Nanocomposites J. Phys. Chem. C, Vol. 112, No. 50, Figure 5. C1s XPS spectra of graphite oxide and Pt-C composite. (a) The C1s XPS spectrum of graphite oxide clearly indicates oxidation with four components that correspond to carbon atoms in different functional groups: the sp 2 -hybridized C-C,theCinC-OH bonds, the epoxy C, and the carboxylate carbon. (b) The intensities of these groups in Pt-C become weak as a result of the deoxygenation, indicating the reduction of graphite oxide in our system. Figure 3. TEM images of Au-C, Pt-C, and Pd-C nanocomposites. (a,b) The bright- and dark-field TEM images of Au-C composite. The inset is an HRTEM image of Au particle. (c,d) TEM images of the Pt-C composite. The inset is an HETEM image of Pt-C. (e,f) TEM images of the Pd-C composite. Figure 4. SEM images and EDS spectra of products. (a) Graphite oxide. The carbon sheets are plane, and the chemical compositions of the sheets mostly consist of C and O. (b) Pt-C composite. The carbon sheets are more corrugated than the starting graphene oxide sheets, and a metal element appears on the surface of such composite, except the initial components. images of as-synthesized nanocomposites. It is clearly seen that the almost transparent carbon sheets were decorated randomly by the nanosized metal particles (Au, Pt, and Pd), and few particles scattered out of the supports, indicating the strong interaction between the particles and supports. 3 Because the monolayer carbon nanosheets were extremely thin, it was hard to make a distinction between them and the carbon-supported films on the copper grid. Comparing the bright- and dark-field TEM images of Au-C composite in Figure 3a,b, the single carbon sheets can be distinguished from the backgrounds. Also, the crumpled silk waves of these carbon sheets leads us to believe that these NPs indeed deposited on some supports: the almost transparent carbon sheets. Such morphology seems like a gem-decorated silk. Of course, we can choose the adjustable silks and diversified diamonds to prepare different products according to the different requirements. The highly dispersed metal NPs on supports with larger surface areas have properties with advantages for catalytic activity and sensor sensitivity. 26 We found that the anchored crystal metal NPs (as the high-resolution TEM (HRTEM) images shown in the insets) distributed uniformly on these single carbon sheets without obvious aggregations. Furthermore, these monolayer sheets possess large surface areas, and particles can deposit on both sides of these sheets. 2,27 Thus, such integration of two-dimensional supports with large surface areas and the highly dispersed NPs can be an exciting material for use in future nanotechnology. As the EDS images show in the insets of Figure 4, the Pt-C composite contains a metal element (Pt), apart from the initial elements C and O. 24,25 Similar results for Au-C and Pd-C are shown in Figure 2S (Supporting Information). Additionally, it is worthwhile to note that the morphology of the monolayer carbon sheets in these as-synthesized composites became more corrugated compared with the starting graphene oxide sheets, especially in Pt-C and Pd-C composites (Figure 3 and 4). Raman analyses revealed that there were structural changes on graphene oxide in these composites. 28,29 These phenomena indicate that some reactions have occurred on graphene oxide with the ethylene glycol. It is known that graphene can be obtained by removal of the oxygen from graphene oxide sheets using physical or chemical reduction. 7,12,14 The results of the XPS implied the decrease tendency of oxygen content in these composites (Table 1S, Supporting Information). As shown in Figure 5, the intensity of some oxygenated functional groups on carbon sheets in the as-synthesized composites was obviously reduced, indicating the deoxygenation of graphene oxide (Figures 3S and 4S, Supporting Information). Specifically, the epoxy groups underwent considerable deoxygenation on the carbon sheets in these composites compared with the starting graphite oxide. 30 The intensity of hydroxyl groups were also reduced, especially in the Pt-C composite (Figure 5). Therefore, it is highly likely that the deoxygenation of graphite oxide is caused by reduction process with ethylene glycol. The FT-IR

4 19844 J. Phys. Chem. C, Vol. 112, No. 50, 2008 Xu et al. species oxidation peaks (I b ) appeared. 33 This example shows that graphene-pt composites have great potential in catalysis area. It is expected to broaden the application areas of graphene-particle composites, for example, the possible workability of Au-C in glucose biosensors. 32 As we know, the CNT-particle nanocomposites have shown excellent benefits in many properties in some applications; however, the high production cost of CNTs became an obstacle in their applications. Compared with CNTs, graphene can be obtained easily by chemical conversion of the inexpensive graphite. Figure 6. Cyclic voltammogram (20 mv/s) of methanol oxidation at Pt-C in 2 mol/l CH 3 OH + 1 mol/l H 2 SO 4 solution at room temperature. The methanol oxidation current peak on Pt catalyst appears at about 0.68 V, and the oxidation peak associated with the removal of the residual carbon species formed in the forward scan appeared around 0.53 V. and Raman spectra analyses included in the Supporting Information can further support the phenomena of reduction (Figures 6S and 7S, respectively, Supporting Information). 6,12 From these results, we believed that the graphene oxide could also be reduced by ethylene glycol in our system. Commonly, the chemical reduction of graphene oxide uses highly toxic reductants, for example, hydrazine. Therefore, such findings may enable us to synthesize graphene using harmless chemical reagents in the future. 6 As mentioned above, the water-ethylene glycol mixture containing graphene oxide sheets was much more stable without metallic salts, and no obvious structural changes were observed on individual graphene oxide sheets, indicating no noticeable reaction had happened on the graphene sheets (Figure 5S, Supporting Information). The addition of metal particles not only broke the stable system, but also changed the nature of graphene oxide. For the metallic (e.g., Au, Pt, Pd) salt/graphene oxide system, metal ions can be easily reduced in the water-ethylene glycol solution to form metallic NPs, the resulting metallic NPs play a pivotal role in catalytic reduction of graphene oxide with ethylene glycol, and the extent of deoxygenation depended on the attached metal (Table 1S and Figure 6S, Supporting Information). Additionally, these NPs attached onto the graphene sheets though van der Waals interactions 3 prevented the aggregation and restacking of the reduced graphene oxide during the reduction process, 15,23 resulting in the formation of graphene--particle composites (Figure 1). CNTs have been used as a support material for the dispersion and stabilization of metal NPs (e.g., Au, Pt, and Pd), and the integration of the two components has displayed interesting properties for gas sensor and catalytic applications. 32 Because of the similarity of structure with CNTs, we also expect that the combination of graphene and NPs would present special features in the new hybrids. Indeed, we found that the composites showed some interesting potential properties. The Pt-C composite was used as a model to test the applicability in real working environments. The electrochemical performance of the Pt-C composite was tested for methanol oxidation, which is closely related to applications in direct methanol fuel cells. Figure 6 shows representative cyclic voltammetry plots of a Pt-C composite in CH 3 OH solution, in which the typical methanol oxidation current peaks (I f ) and the residual carbon 4. Conclusions In summary, we have demonstrated that graphene-metal (Au, Pt and Pd) NPs can be synthesized using graphene oxide sheets as a precursor in the solution approach. In our system, the metal NPs formed first and were adsorbed onto the surface of carbon sheets, which is not only beneficial for the following reduction of graphene oxide by ethylene glycol, but also prevents the restacking of these reduced graphene sheets, resulting in the formation of graphene-supported NP composites. The integration of graphene and the functional particles enables such composites to possess particular properties useful in certain technological applications, for example, in the catalysis area. The finding of ethylene glycol ability to reduce graphene oxide is beneficial because it uses chemical reagent less harmful to humans and the environment. Our studies of the incorporation of inorganic particles and individual graphene sheets opens up the feasibility of synthesis of graphene-particle composites, leading to further development of a broad new class of materials using graphene as supports and moving graphene-based materials much closer to real technological applications. We expect that graphene will become a promising robust support due to its stable properties and larger surface areas. In the future, these composites will also be more competitive than CNTs in terms of production. Acknowledgment. Financial support from the National Natural Science Foundation of China (No ) and the HighTechnologyFoundationofJiangsuProvince(No.BG ) is greatly appreciated. Supporting Information Available: XRD spectra of graphite, graphite oxide, and Pd-C; SEM images and EDS analyses of Au-C and Pd-C; XPS spectra of Au-C and Pd-C; Raman spectra of all samples; FT-IR spectra of Pt-C composite; and TGA curves of graphite oxide, Pt-C, Au-C, and Pd-C. This information is available free of charge via the Internet at pubs.acs.org. References and Notes (1) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (2) Li, D.; Kaner, R. B. Science 2008, 320, (3) Georgakilas, V.; Gournisb, D.; Tzitziosa, V.; Pasquato, L.; Guldie, D. M.; Prato, M. J. Mater. Chem. 2007, 26, (4) Niyogi, S.; Bekyarova, E.; Itkis, M. E.; McWilliams, J. L.; Hamon, M. A.; Haddon, R. C. J. Am. Chem. Soc. 2006, 128, (5) Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. 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