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metal oxide nanoparticles on graphene and their electrochemical applications
Electrochimica Acta 56 (2011) 3338–3344
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Microwave-assisted one-pot synthesis of metal/metal oxide nanoparticles on graphene and their electrochemical applications Shuangyin Wang a , San Ping Jiang b,∗ , Xin Wang a,∗ a b
School of Chemical and Biomedical Engineering, Nanyang Technological University, 50 Nanyang Drive, 639798 Singapore, Singapore Curtin Centre for Advanced Energy Science and Engineering, Department of Chemical Engineering, Curtin University of Technology, WA 6845, Australia
a r t i c l e
i n f o
Article history: Received 8 November 2010 Received in revised form 4 January 2011 Accepted 4 January 2011 Available online 21 January 2011 Keywords: Graphene PtRu electrocatalysts Tin oxide Methanol oxidation Supercapacitor
a b s t r a c t An effective synthesis strategy of hybrid metal (PtRu)/metal oxide (SnO2 ) nanoparticles on graphene nanocomposites is developed using a microwave-assisted one-pot reaction process. The mixture of ethylene glycol (EG) and water is used as both solvent and reactant. In the reaction system for the synthesis of SnO2 /graphene nanocomposite, EG not only reduces graphene oxide (GO) to graphene, but also results in the formation of SnO2 facilitated by the presence of a small amount of water. On the other hand, in the reaction system for preparation of PtRu/graphene nanocomposites, EG acts as solvent and reducing agent for reduction of PtRu nanoparticles from their precursors and reduction of graphene from graphene oxide. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) characterizations confirm the feasibility of the microwave-assisted reaction system to simultaneously reduce graphene oxide and to form SnO2 or PtRu nanoparticles. The as-synthesized SnO2 /graphene hybrid composites show a much higher supercapacitance than the pure graphene, and the as-prepared PtRu/graphene show much better electrocatalytic activity for methanol oxidation compared to the commercial E-TEK PtRu/C electrocatalysts. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Supercapacitors, as energy storage devices based on simple charge-separation at an electrochemical surface between electrode and electrolyte, has attracted more and more attentions due to their higher power density and longer life cycle compared to batteries [1,2]. Materials that could be used as supercapacitor electrode materials include metal oxide, conducting polymer, and carbon materials [3,4]. Of particular interest are carbon-based electrode materials because of their lost cost and potential to realize both high specific capacitance and high power density as a result of their high surface area [5]. More recently, graphene, a novel unique carbon material with one-atom thick layer 2D structure, has been used as supercapacitor electrode material due to its excellent stability, good conductivity, and high surface area [6–10]. Chen et al. synthesized graphene materials using the gas–solid reduction process and achieved high specific capacitance of 205 F g−1 in aqueous electrolyte with power density of 10 kW kg−1 [1]. On the other hand, in order to further improve the interfacial capacitance, hybrid graphene-based composites have also been developed by incorpo-
∗ Corresponding authors. E-mail addresses: [email protected] (S.P. Jiang), [email protected] (X. Wang). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.01.016
rating pseudo-capacitive materials, such as polyaniline [11,12] or metal oxide [13,14]. Pseudo-capacitive materials normally show low conductivity and pseudocapacitors solely based on them cannot support fast electron transport required at high rate. However, in such hybrid material where these pseudo-capacitive materials are decorated on the continuous graphene network, highly conductive graphene network ensures fast electron transport through the supercapacitor electrode and improvement can be expected with the introduction of pseudo-capacitance. For example, Wu et al. prepared graphene/polyaniline nanofibre composite as supercapacitor electrode, which showed improved performance compared to individual graphene or polyaniline materials [12]. Niu et al. synthesized graphene/SnO2 nano-composite in acid solution, achieving higher specific capacitance compared to graphene and graphene oxide [13]. On the other hand, direct methanol fuel cells (DMFCs) are promising power sources for various applications, due to their low operating temperatures, relatively quick start-up, and high power density [15–22]. Methanol oxidation reaction (MOR) are the key anodic reactions in DMFCs. However, even with the stateof-the-art electrocatalysts such as the nanostructured platinum or platinum-based alloy materials supported on high surface area carbon support, it is still a challenge to reduce the polarization loss and to enhance the efficiency of the Pt-based electrocatalysts due to the sluggish MOR [19,21–24]. In the recent years, various strate-
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gies have been developed to improve the electrocatalytic activity for methanol electrooxidation, e.g., the addition or incorporation of a second element in Pt electrocatalysts, or the proper morphological (shape and size) control. Recently, graphene has been extensively used as electrocatalyst support for methanol electroxidation. E. Yoo et al. [25] deposited Pt subnanoclusters on graphene nanosheet, giving rise to an extraordinary modification to the properties of Pt nanocluster electrocatalysts on which the unusually high activity for methanol oxidation reaction was observed, reflecting the advantage of graphene as catalyst support. On the other hand, the role of 2-D structure of graphene in the electrocatalyst application of graphene–Pt nanoparticles was demonstrated by Seger and Kamat [26]. These examples indicate the potential and extensive application of graphene as electrocatalyst support. In this work, we developed a novel one-pot synthesis strategy to prepare both SnO2 /graphene nanocomposites and PtRu/graphene with the microwave-assisted polyol process and studied their electrochemical performance. The synthesis approach is effective and fast, since the microwave heating provides sufficient heat needed for the reaction within very short time [27,28]; and the reduction of graphene oxide to graphene and the conversion of SnO2 nanoparticles from SnCl2 precursors and the reduction of PtRu nanoparticles from their metal precursors are realized simultaneously in this one-pot approach. The electrochemical investigation for supercapacitor and fuel cell application indicates that the as-prepared electrode materials show advantageous performance, which could be attributed to the good disperse of metal or metal oxide nanoparticles on graphene and their synergetic interaction with graphene support.
2. Experimental GO was synthesized from natural graphite powder by a modified Hummers method [29]. Briefly, 0.9 g of graphite powder was added into a mixture of 7.2 mL of 98% H2 SO4 , 1.5 g K2 S2 O8 , and 1.5 g of P2 O5 . The solution was kept at 80 ◦ C for 4.5 h followed by thorough washing with water and drying. Subsequently, the as-treated graphite was put into a 250 mL beaker, into which 0.5 g of NaNO3 and 23 mL of H2 SO4 were then added while keeping the beaker in the ice bath. 3 g of KMnO4 was then added slowly. After 5 min, the ice bath was removed and the solution was heated up to and kept at 35 ◦ C under vigorous stirring for 2 h, followed by the slow addition of 46 mL of water. Finally, 40 mL of water and 5 mL H2 O2 was added, followed by water washing and filtration. The exfoliation of graphene oxide was performed by ultrasonication in a bath sonicator for 1 h. To prepare the hybrid SnO2 /graphene nanocomposites (denoted as SnO2 /G-MW-EG), the microwave-assisted one-pot synthesis method was developed. Briefly, 100 mg GO and appropriate amount of SnCl2 precursors (100 mM in ethylene glycol) for designed SnO2 /GO weight ratios were added into the mixture (200 mL) of ethylene glycol (EG) and water with the volume ratio of H2 O/EG = 0.1. The solution was then placed in a house-hold microwave oven (700 W) and heated for 2 min, and then the microwave was turned off and rested for 2 min, followed by turning on the microwave oven to heat the solution for another 2 min. The as-synthesized products were filtrated and washed, followed by the drying in vacuum oven at 70 ◦ C for 24 h. To prepare the hybrid PtRu/graphene nano-electrocatalysts, the similar microwave-assisted one-pot synthesis method was developed. Briefly, 80 mg GO was dispersed well in 200 mL of EG–water mixture (H2 O/EG = 0.1) under sonication, followed by the addition of appropriate amount of RuCl3 (10 mM) and H2 PtCl6 (10 mM) for a designed weight ratios between PtRu and GO. The pH value of the
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Fig. 1. FTIR spectra of GO and G-MW-EG.
reaction system was adjusted to 11. The suspension was placed in a house-hold microwave oven (700 W) and heated for 2 min, and then the microwave was turned off and rested for 2 min, followed by turning on the microwave oven to heat the solution for another 2 min. The as-synthesized products were filtrated and washed, followed by the drying in vacuum oven at 70 ◦ C for 24 h. The X-ray photoelectronic spectroscopy (XPS) characterization was carried out using Kratos, AXIS Ultra system with a 150 W, 15 kV Monochromatic Alumina K␣ source. The crystalline properties of as-prepared SnO2 and PtRu nanoparticles on graphene were characterized by X-ray diffraction (XRD). The morphology of SnO2 /graphene and PtRu/graphene was examined by the TEM (Transmission Electron Microscopy, JEOL 2010) at an acceleration voltage of 200 kV. The electrochemical characterizations were carried out in a three electrode glass cell in 1 M H2 SO4 electrolyte for supercapacitor characterization and in 0.5 M + 0.5 M MeOH for electrocatalysis characterization; and Pt wire and Ag/AgCl were used as counter electrode and reference electrode, respectively. The working electrodes were made according to the following procedure: the as-obtained powder after drying were re-dispersed in ethanol with a concentration of 1 mg ml−1 , followed by dropping 10 l of the suspension onto the glassy carbon electrode (GCE). 3. Results and discussion In order to confirm the feasibility of microwave-assisted polyol method for the reduction of graphene oxide (GO) to graphene in the one-pot synthesis process of SnO2 /graphene and PtRu/graphene, we first reduced GO in the ethylene glycol/water mixture in the absence of metal precursors under the microwave heating condition (denoted as G-MW-EG). Fig. 1 shows the FTIR spectra of GO and microwave-assisted ethylene glycol reduced graphene (GMW-EG). The FTIR spectra of GO shows a strong peak around 1630 cm−1 due to aromatic C C. It also exhibits peak at 1721 cm−1 due to the C O stretching as well as peaks due to carboxy C–O (1416 cm−1 ), epoxy C–O (1227 cm−1 ), and alkoxy C–O (1050 cm−1 ) groups situated at the edges of GO. After the microwave-assisted EG reduction, the peaks at 1721 cm−1 , 1416 cm−1 decrease significantly, suggesting the reduction of GO. The FTIR data are similar to the data reported in the literature for samples obtained by the classical hydrazine reduction method [30]. Besides, the photograph of GO and G-MW-EG solution was shown in the inset of Fig. 1. As observed, the color of the solution shifts from brown to black after
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S. Wang et al. / Electrochimica Acta 56 (2011) 3338–3344
Fig. 2. XPS spectra of C 1s in the samples of GO (a) and G-MW-EG (b).
the MW-EG reduction, which confirms the successful reduction of GO. The C 1s XPS spectra of GO and G-MW-EG samples are shown in Fig. 2. The spectra can be deconvoluted into four components corresponding to carbon species in different oxygen-containing functional groups: (a) non-oxygenated C at 284.6 eV, (b) carbon in C–O at 285.6 eV, (c) epoxy carbon (286.7 eV) and (d) carbonyl carbon (C O, 288.2 eV). Compared with that of GO, the XPS spectrum of graphene obtained after the MW-EG reduction process shows greatly suppressed peaks corresponding to the oxygen-containing groups, indicating the success of the reduction process. Nevertheless, the sp2 carbon network is retained in reduced graphene. The hybrid SnO2 /graphene composite was synthesized in ethylene glycol in the presence of a small amount of water using SnCl2 as precursors. Xin et al. [31] reported the formation mechanism of SnO2 nanoparticles in the EG/water system. In the EG-rich solution (H2 O/EG = 0.1), SnCl2 was surrounded and protected by EG molecules and Cl− was replaced by EG molecules. Once the solution was heated in air, the water molecules might attack the Sn–OCH2 CH2 OH bonds, leading to the formation of Sn(OH)2 , subsequent decomposition into SnO, and further oxidation by oxygen in air into SnO2 . The TEM images of the hybrid nanocomposite as given in Fig. 3 clearly show the successful formation of SnO2 /graphene with SnO2 nanoparticles of 2–3 nm. Further evidence for the formation of SnO2 can be seen from the XPS spectrum of Sn in the SnO2 /graphene sample (the inset of Fig. 3), where
Fig. 3. TEM image of SnO2 /G-MW-EG. The inset is XPS spectrum of Sn 3d in the SnO2 /G-MW-EG sample.
the Sn 3d5/2 and Sn 3d3/2 peaks are observed at 487.3 eV and 495.7 eV, respectively. These characterization results indicate that the microwave-assisted EG system not only leads to the reduction of graphene oxide to reduced graphene, but also simultaneously results in the formation of SnO2 nanoparticles. As a result, hybrid SnO2 /graphene nanocomposites can be synthesized by this facile, fast, and effective one-pot synthesis strategy. The XRD patterns of the as-obtained SnO2 nanoparticles on graphene were given in Fig. 4. The diffraction peaks at around 26.6◦ , 33.8◦ , 51.9◦ , and 61.8◦ are due to the diffraction at the (1 1 0), (1 0 1), (2 1 1), and (3 0 1) planes of SnO2 , respectively, confirming that the as-prepared SnO2 nanoparticles are well-crystallized. The electrochemical properties of SnO2 /G-MW-EG were investigated by cyclic voltammetry (CV) technique. For comparison, G-MW-EG without SnO2 was also examined. Fig. 5a displays the CV curves of the SnO2 /G-MW-EG and G-MW-EG supercapacitor electrodes in 1 M H2 SO4 , and the curves were collected at a scan rate of 50 mV s−1 . It could be observed that both CV curves have similar and nearly rectangular shape, indicating good charge propagation within the electrode. However, higher capacitive current and redox peaks are observed in the CV for the SnO2 /G-MW-EG, which are attributed to redox reactions on tin oxide. The specific capacitance of the electrodes was calculated by integrating the cyclic voltammogram curve to obtain the charge (Q), and subsequently dividing this charge by the mass of the active electrode materials (m) and
S. Wang et al. / Electrochimica Acta 56 (2011) 3338–3344
Fig. 4. XRD pattern of SnO2 /graphene.
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Fig. 6. The dependence of the specific capacitance of SnO2 /G-MW-EG on the SnO2 /GO weight ratios.
the width of the potential window (E) [32]. C=
Fig. 5. (a) CV plots of G-MW-EG and SnO2 /G-MW-EG in 1.0 M H2 SO4 at a scan rate of 50 mV s−1 and (b) the dependence of specific capacitance on the scan rate.
Q E × m
(1)
According to Eq. (1), the specific capacitance of SnO2 /G-MW-EG and G-MW-EG is 99.7 F g−1 and 52.7 F g−1 , respectively. The enhanced specific capacitance of SnO2 /G-MW-EG electrode is related to the pseudocapacitance that originates from the uniformly distributed tin oxide nanoparticles on graphene. The effect of potential scan rates on the capacitance of the two electrode materials was also investigated. At a slow scan rate, the profiles of the two electrodes both have rectangular-like shapes. As the scan rate increases, however, the profiles of the two electrodes become gradually deformed, indicating the capacitance value decreases as the scan rate increases, as shown in Fig. 5b. This result demonstrates that the as-prepared SnO2 /graphene hybrid nanocomposites may be used as active electrode materials for supercapacitors. Since both graphene and SnO2 nanoparticles contributed to the specific capacitance of the nanocompostes, it will be of great interest to investigate the dependence of the capacitance on the weight ratio between SnO2 and graphene oxide. As shown in Fig. 6, with the increase of the SnO2 fraction relative to graphene oxide, the specific capacitance increased due to the contribution of more efficient SnO2 species. On the other hand, as the ratio increased above 1:1, the specific capacitance commenced to decrease probably due to the aggregation of SnO2 nanoparticles at high SnO2 /GO ratio, indicating the advantageous role of graphene with high surface area and excellent electron conductivity as the SnO2 nanoparticle support. Previously, we have developed a microwave-assisted polyol reduction method to reduce and deposit PtRu nanoparticles on functionalized carbon nanotube and investigated their electrocatalytic activity for methanol oxidation in acid media [21]. In that work, the pre-functionalization of carbon nanotube, which introduced functional groups on surface of carbon nanotube, was performed prior to the deposition of PtRu nanoparticles due to the inert nature of carbon nanotube surface. In the present study, graphene oxide was used as precursor of graphene support. The fact is that there are amounts of functional groups (e.g., –COOH, –OH, and –COH) in graphene oxide, so it is believed that these functional groups on graphene oxide may function as active sites to anchor metal precursors or nanoparticles. Meanwhile, the EG reduction also works well to reduce graphene oxide to graphene as evidence above. Therefore, it is possible to realize the one-
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Fig. 7. TEM images of PtRu/graphene: (a) low magnification and (b) high magnification.
pot synthesis/deposition of metal nanoparticles and reduction of graphene oxide in such a system. The TEM images in Fig. 7 confirm the successful preparation of PtRu/graphene nanocomposites with the as-developed microwave-assisted one-pot synthesis approach. It could be observed that well-dispersed PtRu nanoparticles with average particle size of 2 nm were deposited onto graphene 2-D structure. The distribution of PtRu nanoparticles is very uniform (Fig. 7c), indicating the microwave-assisted one-pot method is very effective to prepare and deposit metal nanoparticles on the surface of graphene. It should be pointed out that the approach developed here is ideal to deposit high-loading metal nanoparticles on graphene (PtRu/GO ratio is 1:1 in the current case) without aggregates, which would significantly favor the electrocatalytic activity toward methanol oxidation [33]. The XRD patterns of the as-obtained PtRu nanoparticles on graphene were shown in Fig. 8. The diffraction peaks at around 26.6◦ , 39.6◦ , 46.3◦ , and 67.8◦ are due to the diffraction at Pt(1 1 1), Pt(2 0 0), and Pt(2 2 0), consistent with the face-centered cubic (fcc) structure of platinum (PtRu alloys would take the fcc structure of Pt if the Ru content is below 60%), confirming that the as-prepared PtRu nanoparticles are wellcrystallized.
Fig. 8. XRD pattern of PtRu/graphene.
S. Wang et al. / Electrochimica Acta 56 (2011) 3338–3344
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Fig. 10. The dependence of the current density of MOR on PtRu/graphene electrocatalysts on the PtRu/GO weight ratios.
catalytic activity of PtRu/graphene toward MOR is not efficient. However, with the increase of PtRu/graphene ratio, the current density increased significantly. But the decrease of the current density was observed with the further increase of the PtRu/GO ratios, due to the reduced surface area of PtRu nanoparticles caused by the nanoparticle aggregation. This phenomenon is consistent with our previous research work on the relationship between electrocatalytic activity and interconnectivity of Pt nanoparticles on carbon nanotube supports [34].
Fig. 9. (a) CV curves of methanol oxidation on PtRu/graphene and E-TEK PtRu/C and (b) chronoamperometric curves of methanol oxidation at 0.6 V vs. SCE.
The electrocatalytic activity of the PtRu/graphene as potential electrocatalysts for methanol oxidation was examined in acid medium. Fig. 9a shows the cyclic voltammograms (CVs) curves of PtRu/graphene measured in nitrogen purged 0.5 M MeOH + 0.5 M H2 SO4 solutions. For a comparison, the electrocatalytic activity of the commercial electrocatalysts toward methanol oxidation was also evaluated. As shown in Fig. 9a, the current exhibits the well-known features of methanol oxidation on Pt-based electrocatalysts. As a kinetically controlled reaction, the activity of methanol oxidation on Pt can be represented by the magnitude of the anodic peak. The anodic peak current density of methanol oxidation on PtRu/graphene electrode is 218.1 mA mg−1 , which is almost four times higher than that on the commercial ETEK PtRu/C electrocatalysts (53.3 mA mg−1 ). On the other hand, the onset potential in the CV curves of methanol oxidation on PtRu/graphene is also more negative than that on E-TEK PtRu/C (0.15 V vs. 0.2 V). The higher anodic current density and more negative onset potential for methanol oxidation indicate a higher electro-catalytic activity on PtRu/graphene than commercial E-TEK PtRu/C. As shown in Fig. 9b, the chronoamperometric experiments at 0.6 V in 0.5 M MeOH + 0.5 M H2 SO4 solution demonstrated that PtRu/graphene also showed higher steady current density than the commercial electrocatalyst, PtRu/C. Therefore, the assynthesized PtRu/graphene could act as efficient electrocatalysts for methanol oxidation in direct methanol fuel cells. Similar as the SnO2 /graphene study as supercapacitor electrode materials, the dependence of the electrocatalytic activity of PtRu/graphene toward MOR on the PtRu/GO ratio is also of great importance. As shown in Fig. 10, at relatively low PtRu/GO ratios, the electro-
4. Conclusion In conclusion, we successfully developed an effective microwave-assisted reaction system to prepare SnO2 /graphene and PtRu/graphene hybrid nanocomposites. The reduction of graphene oxide to graphene and the formation of tin oxide or PtRu alloy nanoparticles are realized simultaneously. The existence of both ethylene glycol and water are critical for the preparation of the SnO2 /graphene composites. Ethylene glycol is an effective reducing agent to reduce graphene oxide to graphene under the microwave heating condition. On the other hand, water is involved in the formation of tin hydroxide which results in the formation of SnO2 nanoparticles deposited on graphene after further decomposition and oxidation in air. The as-synthesized hybrid SnO2 /graphene composite showed a much higher supercapacitance and could be considered for potential supercapacitor application. In the meantime, the microwave-assisted one-pot synthesis approach could lead to well-dispersed PtRu nanoparticles on graphene, which showed much higher electrocatalytic activity for methanol compared to the commercial electrocatalyst, E-TEK PtRu/C. It is believed that the advantageous properties of graphene including high surface area and excellent electron conductivity do a great favor to enhance the supercapacitor performance of SnO2 nanoparticles and electrocatalytic activity toward MOR of PtRu nanoparticles. The as-developed microwave-assisted polyol method is efficient to prepare metal and/or metal oxide nanoparticles on graphene. On the other hand, it is well-known that PtSnO2 nanocomposites could be used as efficient electrocatalysts for ethanol electrooxidation [35]. The microwave-assisted polyol method developed in this work can not only reduce Pt precursors to Pt nanoparticles but also oxidize tin precursors to SnO2 , which would provide a one-pot approach to prepare PtSnO2 on graphene. Combining the advantages of graphene support and the synergetic
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effect of PtSnO2 , it is believed that PtSnO2 /graphene would be able to efficiently catalyze ethanol electrooxidation. Acknowledgements This work is supported by the academic research fund AcRF tier 2 (MOE2009-T2-2-024), Ministry of Education, Singapore and competitive research program (2009 NRF-CRP 001-032), National Research Foundation, Singapore, and by the Agency for Science, technology and Research (A*Star), Singapore under SERC Grant No. 072 134 0054. References [1] Y. Wang, Z.Q. Shi, Y. Huang, Y.F. Ma, C.Y. Wang, M.M. Chen, Y.S. Chen, Journal of Physical Chemistry C 113 (2009) 13103. [2] K.T. Lee, C.B. Tsai, W.H. Ho, N.L. Wu, Electrochemistry Communications 12 (2010) 886. [3] J.F. Zang, S.J. Bao, C.M. Li, H.J. Bian, X.Q. Cui, Q.L. Bao, C.Q. Sun, K.R. Lian, J. Guo, Journal of Physical Chemistry C 112 (2008) 14843. [4] G.M. Suppes, B.A. Deore, M.S. Freund, Langmuir 24 (2008) 1064. [5] D.W. Wang, F. Li, Z.G. Chen, G.Q. Lu, H.M. Cheng, Chemistry of Materials 20 (2008) 7195. [6] M.S. Goh, M. Pumera, Electrochemistry Communications 12 (2010) 1375. [7] H.L. Wang, Q.L. Hao, X.J. Yang, L.D. Lu, X. Wang, Electrochemistry Communications 11 (2009) 1158. [8] K. Zhang, L.L. Zhang, X.S. Zhao, J.S. Wu, Chemistry of Materials 22 (2010) 1392. [9] D.S. Yu, L.M. Dai, Journal of Physical Chemistry Letters 1 (2010) 467. [10] D.W. Wang, F. Li, Z. Wu, W. Ren, H.M. Cheng, Electrochemistry Communications 11 (2009) 1729. [11] J. Yan, T. Wei, B. Shao, Z. Fan, W. Qian, M. Zhang, F. Wei, Carbon 48 (2010) 487. [12] K. Zhang, L.L. Zhang, X.S. Zhao, J.S. Wu, Chemistry of Materials 22 (2010) 1392.
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Microwave-assisted one-pot synthesis of metal/metal oxide nanoparticles on graphene and their electrochemical applications Shuangyin Wang a , San Ping Jiang b,∗ , Xin Wang a,∗ a b
School of Chemical and Biomedical Engineering, Nanyang Technological University, 50 Nanyang Drive, 639798 Singapore, Singapore Curtin Centre for Advanced Energy Science and Engineering, Department of Chemical Engineering, Curtin University of Technology, WA 6845, Australia
a r t i c l e
i n f o
Article history: Received 8 November 2010 Received in revised form 4 January 2011 Accepted 4 January 2011 Available online 21 January 2011 Keywords: Graphene PtRu electrocatalysts Tin oxide Methanol oxidation Supercapacitor
a b s t r a c t An effective synthesis strategy of hybrid metal (PtRu)/metal oxide (SnO2 ) nanoparticles on graphene nanocomposites is developed using a microwave-assisted one-pot reaction process. The mixture of ethylene glycol (EG) and water is used as both solvent and reactant. In the reaction system for the synthesis of SnO2 /graphene nanocomposite, EG not only reduces graphene oxide (GO) to graphene, but also results in the formation of SnO2 facilitated by the presence of a small amount of water. On the other hand, in the reaction system for preparation of PtRu/graphene nanocomposites, EG acts as solvent and reducing agent for reduction of PtRu nanoparticles from their precursors and reduction of graphene from graphene oxide. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) characterizations confirm the feasibility of the microwave-assisted reaction system to simultaneously reduce graphene oxide and to form SnO2 or PtRu nanoparticles. The as-synthesized SnO2 /graphene hybrid composites show a much higher supercapacitance than the pure graphene, and the as-prepared PtRu/graphene show much better electrocatalytic activity for methanol oxidation compared to the commercial E-TEK PtRu/C electrocatalysts. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Supercapacitors, as energy storage devices based on simple charge-separation at an electrochemical surface between electrode and electrolyte, has attracted more and more attentions due to their higher power density and longer life cycle compared to batteries [1,2]. Materials that could be used as supercapacitor electrode materials include metal oxide, conducting polymer, and carbon materials [3,4]. Of particular interest are carbon-based electrode materials because of their lost cost and potential to realize both high specific capacitance and high power density as a result of their high surface area [5]. More recently, graphene, a novel unique carbon material with one-atom thick layer 2D structure, has been used as supercapacitor electrode material due to its excellent stability, good conductivity, and high surface area [6–10]. Chen et al. synthesized graphene materials using the gas–solid reduction process and achieved high specific capacitance of 205 F g−1 in aqueous electrolyte with power density of 10 kW kg−1 [1]. On the other hand, in order to further improve the interfacial capacitance, hybrid graphene-based composites have also been developed by incorpo-
∗ Corresponding authors. E-mail addresses: [email protected] (S.P. Jiang), [email protected] (X. Wang). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.01.016
rating pseudo-capacitive materials, such as polyaniline [11,12] or metal oxide [13,14]. Pseudo-capacitive materials normally show low conductivity and pseudocapacitors solely based on them cannot support fast electron transport required at high rate. However, in such hybrid material where these pseudo-capacitive materials are decorated on the continuous graphene network, highly conductive graphene network ensures fast electron transport through the supercapacitor electrode and improvement can be expected with the introduction of pseudo-capacitance. For example, Wu et al. prepared graphene/polyaniline nanofibre composite as supercapacitor electrode, which showed improved performance compared to individual graphene or polyaniline materials [12]. Niu et al. synthesized graphene/SnO2 nano-composite in acid solution, achieving higher specific capacitance compared to graphene and graphene oxide [13]. On the other hand, direct methanol fuel cells (DMFCs) are promising power sources for various applications, due to their low operating temperatures, relatively quick start-up, and high power density [15–22]. Methanol oxidation reaction (MOR) are the key anodic reactions in DMFCs. However, even with the stateof-the-art electrocatalysts such as the nanostructured platinum or platinum-based alloy materials supported on high surface area carbon support, it is still a challenge to reduce the polarization loss and to enhance the efficiency of the Pt-based electrocatalysts due to the sluggish MOR [19,21–24]. In the recent years, various strate-
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gies have been developed to improve the electrocatalytic activity for methanol electrooxidation, e.g., the addition or incorporation of a second element in Pt electrocatalysts, or the proper morphological (shape and size) control. Recently, graphene has been extensively used as electrocatalyst support for methanol electroxidation. E. Yoo et al. [25] deposited Pt subnanoclusters on graphene nanosheet, giving rise to an extraordinary modification to the properties of Pt nanocluster electrocatalysts on which the unusually high activity for methanol oxidation reaction was observed, reflecting the advantage of graphene as catalyst support. On the other hand, the role of 2-D structure of graphene in the electrocatalyst application of graphene–Pt nanoparticles was demonstrated by Seger and Kamat [26]. These examples indicate the potential and extensive application of graphene as electrocatalyst support. In this work, we developed a novel one-pot synthesis strategy to prepare both SnO2 /graphene nanocomposites and PtRu/graphene with the microwave-assisted polyol process and studied their electrochemical performance. The synthesis approach is effective and fast, since the microwave heating provides sufficient heat needed for the reaction within very short time [27,28]; and the reduction of graphene oxide to graphene and the conversion of SnO2 nanoparticles from SnCl2 precursors and the reduction of PtRu nanoparticles from their metal precursors are realized simultaneously in this one-pot approach. The electrochemical investigation for supercapacitor and fuel cell application indicates that the as-prepared electrode materials show advantageous performance, which could be attributed to the good disperse of metal or metal oxide nanoparticles on graphene and their synergetic interaction with graphene support.
2. Experimental GO was synthesized from natural graphite powder by a modified Hummers method [29]. Briefly, 0.9 g of graphite powder was added into a mixture of 7.2 mL of 98% H2 SO4 , 1.5 g K2 S2 O8 , and 1.5 g of P2 O5 . The solution was kept at 80 ◦ C for 4.5 h followed by thorough washing with water and drying. Subsequently, the as-treated graphite was put into a 250 mL beaker, into which 0.5 g of NaNO3 and 23 mL of H2 SO4 were then added while keeping the beaker in the ice bath. 3 g of KMnO4 was then added slowly. After 5 min, the ice bath was removed and the solution was heated up to and kept at 35 ◦ C under vigorous stirring for 2 h, followed by the slow addition of 46 mL of water. Finally, 40 mL of water and 5 mL H2 O2 was added, followed by water washing and filtration. The exfoliation of graphene oxide was performed by ultrasonication in a bath sonicator for 1 h. To prepare the hybrid SnO2 /graphene nanocomposites (denoted as SnO2 /G-MW-EG), the microwave-assisted one-pot synthesis method was developed. Briefly, 100 mg GO and appropriate amount of SnCl2 precursors (100 mM in ethylene glycol) for designed SnO2 /GO weight ratios were added into the mixture (200 mL) of ethylene glycol (EG) and water with the volume ratio of H2 O/EG = 0.1. The solution was then placed in a house-hold microwave oven (700 W) and heated for 2 min, and then the microwave was turned off and rested for 2 min, followed by turning on the microwave oven to heat the solution for another 2 min. The as-synthesized products were filtrated and washed, followed by the drying in vacuum oven at 70 ◦ C for 24 h. To prepare the hybrid PtRu/graphene nano-electrocatalysts, the similar microwave-assisted one-pot synthesis method was developed. Briefly, 80 mg GO was dispersed well in 200 mL of EG–water mixture (H2 O/EG = 0.1) under sonication, followed by the addition of appropriate amount of RuCl3 (10 mM) and H2 PtCl6 (10 mM) for a designed weight ratios between PtRu and GO. The pH value of the
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Fig. 1. FTIR spectra of GO and G-MW-EG.
reaction system was adjusted to 11. The suspension was placed in a house-hold microwave oven (700 W) and heated for 2 min, and then the microwave was turned off and rested for 2 min, followed by turning on the microwave oven to heat the solution for another 2 min. The as-synthesized products were filtrated and washed, followed by the drying in vacuum oven at 70 ◦ C for 24 h. The X-ray photoelectronic spectroscopy (XPS) characterization was carried out using Kratos, AXIS Ultra system with a 150 W, 15 kV Monochromatic Alumina K␣ source. The crystalline properties of as-prepared SnO2 and PtRu nanoparticles on graphene were characterized by X-ray diffraction (XRD). The morphology of SnO2 /graphene and PtRu/graphene was examined by the TEM (Transmission Electron Microscopy, JEOL 2010) at an acceleration voltage of 200 kV. The electrochemical characterizations were carried out in a three electrode glass cell in 1 M H2 SO4 electrolyte for supercapacitor characterization and in 0.5 M + 0.5 M MeOH for electrocatalysis characterization; and Pt wire and Ag/AgCl were used as counter electrode and reference electrode, respectively. The working electrodes were made according to the following procedure: the as-obtained powder after drying were re-dispersed in ethanol with a concentration of 1 mg ml−1 , followed by dropping 10 l of the suspension onto the glassy carbon electrode (GCE). 3. Results and discussion In order to confirm the feasibility of microwave-assisted polyol method for the reduction of graphene oxide (GO) to graphene in the one-pot synthesis process of SnO2 /graphene and PtRu/graphene, we first reduced GO in the ethylene glycol/water mixture in the absence of metal precursors under the microwave heating condition (denoted as G-MW-EG). Fig. 1 shows the FTIR spectra of GO and microwave-assisted ethylene glycol reduced graphene (GMW-EG). The FTIR spectra of GO shows a strong peak around 1630 cm−1 due to aromatic C C. It also exhibits peak at 1721 cm−1 due to the C O stretching as well as peaks due to carboxy C–O (1416 cm−1 ), epoxy C–O (1227 cm−1 ), and alkoxy C–O (1050 cm−1 ) groups situated at the edges of GO. After the microwave-assisted EG reduction, the peaks at 1721 cm−1 , 1416 cm−1 decrease significantly, suggesting the reduction of GO. The FTIR data are similar to the data reported in the literature for samples obtained by the classical hydrazine reduction method [30]. Besides, the photograph of GO and G-MW-EG solution was shown in the inset of Fig. 1. As observed, the color of the solution shifts from brown to black after
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Fig. 2. XPS spectra of C 1s in the samples of GO (a) and G-MW-EG (b).
the MW-EG reduction, which confirms the successful reduction of GO. The C 1s XPS spectra of GO and G-MW-EG samples are shown in Fig. 2. The spectra can be deconvoluted into four components corresponding to carbon species in different oxygen-containing functional groups: (a) non-oxygenated C at 284.6 eV, (b) carbon in C–O at 285.6 eV, (c) epoxy carbon (286.7 eV) and (d) carbonyl carbon (C O, 288.2 eV). Compared with that of GO, the XPS spectrum of graphene obtained after the MW-EG reduction process shows greatly suppressed peaks corresponding to the oxygen-containing groups, indicating the success of the reduction process. Nevertheless, the sp2 carbon network is retained in reduced graphene. The hybrid SnO2 /graphene composite was synthesized in ethylene glycol in the presence of a small amount of water using SnCl2 as precursors. Xin et al. [31] reported the formation mechanism of SnO2 nanoparticles in the EG/water system. In the EG-rich solution (H2 O/EG = 0.1), SnCl2 was surrounded and protected by EG molecules and Cl− was replaced by EG molecules. Once the solution was heated in air, the water molecules might attack the Sn–OCH2 CH2 OH bonds, leading to the formation of Sn(OH)2 , subsequent decomposition into SnO, and further oxidation by oxygen in air into SnO2 . The TEM images of the hybrid nanocomposite as given in Fig. 3 clearly show the successful formation of SnO2 /graphene with SnO2 nanoparticles of 2–3 nm. Further evidence for the formation of SnO2 can be seen from the XPS spectrum of Sn in the SnO2 /graphene sample (the inset of Fig. 3), where
Fig. 3. TEM image of SnO2 /G-MW-EG. The inset is XPS spectrum of Sn 3d in the SnO2 /G-MW-EG sample.
the Sn 3d5/2 and Sn 3d3/2 peaks are observed at 487.3 eV and 495.7 eV, respectively. These characterization results indicate that the microwave-assisted EG system not only leads to the reduction of graphene oxide to reduced graphene, but also simultaneously results in the formation of SnO2 nanoparticles. As a result, hybrid SnO2 /graphene nanocomposites can be synthesized by this facile, fast, and effective one-pot synthesis strategy. The XRD patterns of the as-obtained SnO2 nanoparticles on graphene were given in Fig. 4. The diffraction peaks at around 26.6◦ , 33.8◦ , 51.9◦ , and 61.8◦ are due to the diffraction at the (1 1 0), (1 0 1), (2 1 1), and (3 0 1) planes of SnO2 , respectively, confirming that the as-prepared SnO2 nanoparticles are well-crystallized. The electrochemical properties of SnO2 /G-MW-EG were investigated by cyclic voltammetry (CV) technique. For comparison, G-MW-EG without SnO2 was also examined. Fig. 5a displays the CV curves of the SnO2 /G-MW-EG and G-MW-EG supercapacitor electrodes in 1 M H2 SO4 , and the curves were collected at a scan rate of 50 mV s−1 . It could be observed that both CV curves have similar and nearly rectangular shape, indicating good charge propagation within the electrode. However, higher capacitive current and redox peaks are observed in the CV for the SnO2 /G-MW-EG, which are attributed to redox reactions on tin oxide. The specific capacitance of the electrodes was calculated by integrating the cyclic voltammogram curve to obtain the charge (Q), and subsequently dividing this charge by the mass of the active electrode materials (m) and
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Fig. 4. XRD pattern of SnO2 /graphene.
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Fig. 6. The dependence of the specific capacitance of SnO2 /G-MW-EG on the SnO2 /GO weight ratios.
the width of the potential window (E) [32]. C=
Fig. 5. (a) CV plots of G-MW-EG and SnO2 /G-MW-EG in 1.0 M H2 SO4 at a scan rate of 50 mV s−1 and (b) the dependence of specific capacitance on the scan rate.
Q E × m
(1)
According to Eq. (1), the specific capacitance of SnO2 /G-MW-EG and G-MW-EG is 99.7 F g−1 and 52.7 F g−1 , respectively. The enhanced specific capacitance of SnO2 /G-MW-EG electrode is related to the pseudocapacitance that originates from the uniformly distributed tin oxide nanoparticles on graphene. The effect of potential scan rates on the capacitance of the two electrode materials was also investigated. At a slow scan rate, the profiles of the two electrodes both have rectangular-like shapes. As the scan rate increases, however, the profiles of the two electrodes become gradually deformed, indicating the capacitance value decreases as the scan rate increases, as shown in Fig. 5b. This result demonstrates that the as-prepared SnO2 /graphene hybrid nanocomposites may be used as active electrode materials for supercapacitors. Since both graphene and SnO2 nanoparticles contributed to the specific capacitance of the nanocompostes, it will be of great interest to investigate the dependence of the capacitance on the weight ratio between SnO2 and graphene oxide. As shown in Fig. 6, with the increase of the SnO2 fraction relative to graphene oxide, the specific capacitance increased due to the contribution of more efficient SnO2 species. On the other hand, as the ratio increased above 1:1, the specific capacitance commenced to decrease probably due to the aggregation of SnO2 nanoparticles at high SnO2 /GO ratio, indicating the advantageous role of graphene with high surface area and excellent electron conductivity as the SnO2 nanoparticle support. Previously, we have developed a microwave-assisted polyol reduction method to reduce and deposit PtRu nanoparticles on functionalized carbon nanotube and investigated their electrocatalytic activity for methanol oxidation in acid media [21]. In that work, the pre-functionalization of carbon nanotube, which introduced functional groups on surface of carbon nanotube, was performed prior to the deposition of PtRu nanoparticles due to the inert nature of carbon nanotube surface. In the present study, graphene oxide was used as precursor of graphene support. The fact is that there are amounts of functional groups (e.g., –COOH, –OH, and –COH) in graphene oxide, so it is believed that these functional groups on graphene oxide may function as active sites to anchor metal precursors or nanoparticles. Meanwhile, the EG reduction also works well to reduce graphene oxide to graphene as evidence above. Therefore, it is possible to realize the one-
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Fig. 7. TEM images of PtRu/graphene: (a) low magnification and (b) high magnification.
pot synthesis/deposition of metal nanoparticles and reduction of graphene oxide in such a system. The TEM images in Fig. 7 confirm the successful preparation of PtRu/graphene nanocomposites with the as-developed microwave-assisted one-pot synthesis approach. It could be observed that well-dispersed PtRu nanoparticles with average particle size of 2 nm were deposited onto graphene 2-D structure. The distribution of PtRu nanoparticles is very uniform (Fig. 7c), indicating the microwave-assisted one-pot method is very effective to prepare and deposit metal nanoparticles on the surface of graphene. It should be pointed out that the approach developed here is ideal to deposit high-loading metal nanoparticles on graphene (PtRu/GO ratio is 1:1 in the current case) without aggregates, which would significantly favor the electrocatalytic activity toward methanol oxidation [33]. The XRD patterns of the as-obtained PtRu nanoparticles on graphene were shown in Fig. 8. The diffraction peaks at around 26.6◦ , 39.6◦ , 46.3◦ , and 67.8◦ are due to the diffraction at Pt(1 1 1), Pt(2 0 0), and Pt(2 2 0), consistent with the face-centered cubic (fcc) structure of platinum (PtRu alloys would take the fcc structure of Pt if the Ru content is below 60%), confirming that the as-prepared PtRu nanoparticles are wellcrystallized.
Fig. 8. XRD pattern of PtRu/graphene.
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Fig. 10. The dependence of the current density of MOR on PtRu/graphene electrocatalysts on the PtRu/GO weight ratios.
catalytic activity of PtRu/graphene toward MOR is not efficient. However, with the increase of PtRu/graphene ratio, the current density increased significantly. But the decrease of the current density was observed with the further increase of the PtRu/GO ratios, due to the reduced surface area of PtRu nanoparticles caused by the nanoparticle aggregation. This phenomenon is consistent with our previous research work on the relationship between electrocatalytic activity and interconnectivity of Pt nanoparticles on carbon nanotube supports [34].
Fig. 9. (a) CV curves of methanol oxidation on PtRu/graphene and E-TEK PtRu/C and (b) chronoamperometric curves of methanol oxidation at 0.6 V vs. SCE.
The electrocatalytic activity of the PtRu/graphene as potential electrocatalysts for methanol oxidation was examined in acid medium. Fig. 9a shows the cyclic voltammograms (CVs) curves of PtRu/graphene measured in nitrogen purged 0.5 M MeOH + 0.5 M H2 SO4 solutions. For a comparison, the electrocatalytic activity of the commercial electrocatalysts toward methanol oxidation was also evaluated. As shown in Fig. 9a, the current exhibits the well-known features of methanol oxidation on Pt-based electrocatalysts. As a kinetically controlled reaction, the activity of methanol oxidation on Pt can be represented by the magnitude of the anodic peak. The anodic peak current density of methanol oxidation on PtRu/graphene electrode is 218.1 mA mg−1 , which is almost four times higher than that on the commercial ETEK PtRu/C electrocatalysts (53.3 mA mg−1 ). On the other hand, the onset potential in the CV curves of methanol oxidation on PtRu/graphene is also more negative than that on E-TEK PtRu/C (0.15 V vs. 0.2 V). The higher anodic current density and more negative onset potential for methanol oxidation indicate a higher electro-catalytic activity on PtRu/graphene than commercial E-TEK PtRu/C. As shown in Fig. 9b, the chronoamperometric experiments at 0.6 V in 0.5 M MeOH + 0.5 M H2 SO4 solution demonstrated that PtRu/graphene also showed higher steady current density than the commercial electrocatalyst, PtRu/C. Therefore, the assynthesized PtRu/graphene could act as efficient electrocatalysts for methanol oxidation in direct methanol fuel cells. Similar as the SnO2 /graphene study as supercapacitor electrode materials, the dependence of the electrocatalytic activity of PtRu/graphene toward MOR on the PtRu/GO ratio is also of great importance. As shown in Fig. 10, at relatively low PtRu/GO ratios, the electro-
4. Conclusion In conclusion, we successfully developed an effective microwave-assisted reaction system to prepare SnO2 /graphene and PtRu/graphene hybrid nanocomposites. The reduction of graphene oxide to graphene and the formation of tin oxide or PtRu alloy nanoparticles are realized simultaneously. The existence of both ethylene glycol and water are critical for the preparation of the SnO2 /graphene composites. Ethylene glycol is an effective reducing agent to reduce graphene oxide to graphene under the microwave heating condition. On the other hand, water is involved in the formation of tin hydroxide which results in the formation of SnO2 nanoparticles deposited on graphene after further decomposition and oxidation in air. The as-synthesized hybrid SnO2 /graphene composite showed a much higher supercapacitance and could be considered for potential supercapacitor application. In the meantime, the microwave-assisted one-pot synthesis approach could lead to well-dispersed PtRu nanoparticles on graphene, which showed much higher electrocatalytic activity for methanol compared to the commercial electrocatalyst, E-TEK PtRu/C. It is believed that the advantageous properties of graphene including high surface area and excellent electron conductivity do a great favor to enhance the supercapacitor performance of SnO2 nanoparticles and electrocatalytic activity toward MOR of PtRu nanoparticles. The as-developed microwave-assisted polyol method is efficient to prepare metal and/or metal oxide nanoparticles on graphene. On the other hand, it is well-known that PtSnO2 nanocomposites could be used as efficient electrocatalysts for ethanol electrooxidation [35]. The microwave-assisted polyol method developed in this work can not only reduce Pt precursors to Pt nanoparticles but also oxidize tin precursors to SnO2 , which would provide a one-pot approach to prepare PtSnO2 on graphene. Combining the advantages of graphene support and the synergetic
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effect of PtSnO2 , it is believed that PtSnO2 /graphene would be able to efficiently catalyze ethanol electrooxidation. Acknowledgements This work is supported by the academic research fund AcRF tier 2 (MOE2009-T2-2-024), Ministry of Education, Singapore and competitive research program (2009 NRF-CRP 001-032), National Research Foundation, Singapore, and by the Agency for Science, technology and Research (A*Star), Singapore under SERC Grant No. 072 134 0054. References [1] Y. Wang, Z.Q. Shi, Y. Huang, Y.F. Ma, C.Y. Wang, M.M. Chen, Y.S. Chen, Journal of Physical Chemistry C 113 (2009) 13103. [2] K.T. Lee, C.B. Tsai, W.H. Ho, N.L. Wu, Electrochemistry Communications 12 (2010) 886. [3] J.F. Zang, S.J. Bao, C.M. Li, H.J. Bian, X.Q. Cui, Q.L. Bao, C.Q. Sun, K.R. Lian, J. Guo, Journal of Physical Chemistry C 112 (2008) 14843. [4] G.M. Suppes, B.A. Deore, M.S. Freund, Langmuir 24 (2008) 1064. [5] D.W. Wang, F. Li, Z.G. Chen, G.Q. Lu, H.M. Cheng, Chemistry of Materials 20 (2008) 7195. [6] M.S. Goh, M. Pumera, Electrochemistry Communications 12 (2010) 1375. [7] H.L. Wang, Q.L. Hao, X.J. Yang, L.D. Lu, X. Wang, Electrochemistry Communications 11 (2009) 1158. [8] K. Zhang, L.L. Zhang, X.S. Zhao, J.S. Wu, Chemistry of Materials 22 (2010) 1392. [9] D.S. Yu, L.M. Dai, Journal of Physical Chemistry Letters 1 (2010) 467. [10] D.W. Wang, F. Li, Z. Wu, W. Ren, H.M. Cheng, Electrochemistry Communications 11 (2009) 1729. [11] J. Yan, T. Wei, B. Shao, Z. Fan, W. Qian, M. Zhang, F. Wei, Carbon 48 (2010) 487. [12] K. Zhang, L.L. Zhang, X.S. Zhao, J.S. Wu, Chemistry of Materials 22 (2010) 1392.
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