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Cite this: DOI: 10.1039/c4cc09146a Received 16th November 2014, Accepted 31st December 2014
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Highly reduced graphene oxide supported Pt nanocomposites as highly efficient catalysts for methanol oxidation† Hongbin Feng,ab Yong Liua and Jinghong Li*a
DOI: 10.1039/c4cc09146a www.rsc.org/chemcomm
The highly reduced graphene oxide using solvated electrons as reductive agents shows low defects and well dispersion that are vital as the support of Pt nanoparticles in direct methanol electrooxidation. The electrochemical experiments demonstrate that the Pt/RGO composites not only greatly enhance the catalytic activity but also dramatically improve the durability of the catalyst.
Platinum (Pt)-based catalysts have attracted much research interest for their unusual catalytic and electrocatalytic properties, which are vital to fuel cells, petroleum and automotive industries.1–4 But the cost of pure Pt catalysts is very high. In order to make the most use of this precious metal and reduce the cost, Pt nanoparticles (NPs) are usually loaded on high surface area supporting materials.5–11 So far, carbon-based materials, such as carbon black, carbon nanotubes, carbon nanofibers, etc., have been extensively used as supporting materials for Pt catalysts.12–16 Recently, graphene (or reduced graphene oxide, RGO) nanosheets have attracted attention as a unique 2D material with higher electrical and thermal conductivities, better transparency and mechanical strength, and larger surface area due to their unique structure composing of sp2-bonded carbon atoms.17–21 Therefore, much effort has been focused on using RGO nanosheets as electrocatalyst support materials for the dispersion of Pt NPs to obtain new functional nanocomposites, and some exciting results have been reported.22–27 However, there are still some problems to hinder the application of RGO as an electrocatalyst support material. Firstly, the RGO sheets tend to stack together through p–p interaction, which sets a higher resistance for the diffusion of reactant molecules and retards the catalytic reaction. In addition, RGO sheets obtained by general chemical synthesis usually contain more defects, a
Department of Chemistry, Beijing Key Laboratory for Microanalytical Methods and Instrumentation, Tsinghua University, Beijing 100084, China. E-mail: [email protected]; Fax: +86 10 6279 5290; Tel: +86 10 6279 5290 b College of Materials Science and Opto-electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China † Electronic supplementary information (ESI) available: Experimental details. See DOI: 10.1039/c4cc09146a
This journal is © The Royal Society of Chemistry 2015
which reduce the electron-transfer rate in and across the interface of the RGO nanosheet. Therefore, it is necessary to prepare the RGO nanosheets not only with low defects but also with well dispersion as the support. In this communication, we report an approach to design highly active and durable electro-catalyst by loading Pt NPs on a support RGO, which is produced by a highly efficient approach for one-pot reduction of GO in solution by using a sodium– ammonia (Na–NH3) as the reducing agent.28 Metallic sodium can be dissolved in liquid ammonia to create a blue solution with strong reducibility due to the generation of ‘‘solvated electron e [NH3]n’’. The composites of RGONa–NH3 nanosheets supported Pt NPs were synthesized in an aqueous solution at 85 1C using RGONa–NH3 nanosheet dispersion in H2PtCl6 solutions with NaBH4 as a reducing agent.23 The use of our RGONaNH3 nanosheets as matrixes for Pt deposition has several advantages. Firstly, they can be easily dispersed in water. Secondly, they can be effectively reduced and have a high conductivity. Thirdly, the composite structure not only enhances the catalytic activity but also dramatically improves the durability of the catalyst. Pt NPs supported on RGONa–NH3 nanosheets were prepared, which were explored as methanol oxidation electrocatalysts. Fig. 1a shows the TEM image that Pt NPs were loaded uniformly on RGONa–NH3 nanosheets without obvious localized aggregation. Pt/RGONa–NH3 composites were prepared using the well-dispersing RGONa–NH3 nanosheets, which were obtained by the Na–NH3 reduction of graphene oxide synthesized by the modified Hummers method.29,30 Dissolution of the sodium metal in the liquid ammonia results in ionization of the metal to form a sodium cation and a solvated electron strongly associated with the solvent ammonia. The sodium–ammonia solution with the solvated electrons can function as a very potent electron source to effectively remove oxygen functionalities with only oxygen content of 5.6 wt% left and restore the planar geometry of the GO sheets.28 Fig. 1b shows the well-dispersed RGO in water. According to Fig. 1a, the typical view under TEM, all Pt-loaded RGO sheets form composites. Besides, most Pt NPs were still attached on the RGO, indicating good contact between Pt NPs
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Fig. 1 (a) TEM images of Pt/RGONa–NH3, (b) the dispersion of Pt/RGONa–NH3 in water, (c) the image shows a HRTEM image of Pt/RGONa–NH3, the scale bar is 3 nm. (d) The EDS spectra of Pt/RGONa–NH3. (e) The XRD pattern of the Pt/RGONa–NH3 composites.
and RGO. The high-resolution image in Fig. 1c shows that the as-synthesized Pt NPs are single-crystalline. Fig. 1d shows the energy dispersive X-ray spectroscopy (EDS) spectra of Pt/RGONa–NH3, indicating the presence of platinum. The peak assigned to Cu in the EDS analysis is attributed to a mesh used for the TEM measurement. The XRD pattern of the Pt/RGONa–NH3 composite is shown in Fig. 1e. The strong diffraction peaks at 2y angles of 39.91, 46.21, 67.91 in the XRD pattern can be assigned to the (111), (200), (220) facets of the face-centered cubic structures of the platinum crystal, which are in good agreement with the standard card of cubic Pt (JCPDS No. 4-802). The resulting Pt/RGONa–NH3 composite shows a wide peak at 23.81, confirming a graphitic nature of RGO sheets. To evaluate the electrochemical activity of Pt/graphene nanocomposites, cyclic voltammogram (CV) experiments were tested within a potential range from 0 to 1 V at a scanning rate of 50 mV s 1 in the solution of nitrogen saturated 0.5 M H2SO4.
Fig. 2 Cyclic voltammograms of Pt/RGONa–NH3 (black line), Pt/Vulcan (red line) electrodes in nitrogen saturated aqueous solution of 0.5 M H2SO4 at a scan rate of 50 mV s 1.
Chem. Commun.
Fig. 3 (a) Cyclic voltammograms at a scan rate of 50 mV s 1, (b) chronoamperometric curves at a fixed potential of 0.6 V vs. Ag/AgCl (KCl sat.) on Pt/RGONa–NH3 and Pt/Vulcan electrodes in nitrogen saturated aqueous solution of 0.5 M H2SO4 containing 0.5 M CH3OH.
As seen from Fig. 2, the Pt/graphene film deposited on a glassy carbon electrode was electrochemically active in which hydrogen adsorption characteristics were presented. The electrochemically active surface area (ECSA) was estimated by integrating the voltammograms corresponding to hydrogen desorption from the electrode surface. The ECSAs for the Pt/graphene and Pt/Vulcan were estimated to be 1585.32 and 13.23 m2 g 1 Pt, respectively, and the high ECSAs were favorable to electrochemical oxidation toward methanol. Fig. 3a shows representative cyclic voltammetry curves of a Pt/RGONa–NH3 composite in the solution of 0.5 M CH3OH in 0.5 M H2SO4. The current density on Pt/RGONa–NH3 was near three times higher than that on Pt/Vulcan, indicating much higher activity toward methanol oxidation on Pt/RGONa–NH3. The ratio of the typical methanol oxidation current density (the forward peak, If) to the residual carbon species oxidation current density (the reverse anodic peak, Ib), If/Ib, can be used to describe the catalyst tolerance to carbonaceous species accumulation.23 The If/Ib ratio was estimated to be 1.1, 1.93 for Pt/Vulcan and Pt/RGONa–NH3 respectively. A low If/Ib ratio indicates poor oxidation of methanol to carbon dioxide during the anodic scan and excessive accumulation of carbonaceous residues on the catalyst surface. We have found that If/Ib of Pt/RGONa–NH3 was much higher than that of the Pt/Vulcan catalysts. Therefore, the performance of Pt/RGONa–NH3 for the methanol electrochemical oxidation can be considered superior to that of Pt/Vulcan. In addition, the amperometric I–t curves (Fig. 3b) also exhibit that Pt/RGONa–NH3 had a higher catalytic stability toward methanol oxidation. The high electrocatalytic activity of Pt/RGONa–NH3 may
This journal is © The Royal Society of Chemistry 2015
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be from the strong electronic interaction between Pt NPs and RGO nanosheets, which suppresses the CO formation during the oxidation of methanol and facilitates the direct oxidation of methanol on the Pt surface. Compared with some other materials, our synthetic sample is shown to exhibit well catalytic performance.31–33 In the electrocatalytic fields, materials with high surface area exhibit substantial advantages concerning mass and charge transport by providing shorter effective lengths for both electronic and ionic transport, a higher electrode/electrolyte contact area, and in some cases also interfacial local effects. On one hand, the highly reduced graphene oxide nanosheets obtained by Na–NH3 solvated electrons show high conductivity with low defects, which may improve the electron-transfer rate in the RGO sheet and across the RGO interface as well. On the other hand, graphene as a two dimensional (2D) nanomaterial has high theoretical specific surface area which might be favorable for the electrochemical reaction. Pt NPs could be easily deposited on the surface of graphene based on 2D flat planes. Especially, our RGONa–NH3 can be easily dispersed in the reaction medium that is favorable to achieve single nanosheet deposition of the Pt NPs. In summary, by using Na–NH3 treatment with active solvated electrons e [NH3]n as the highly reductive agent, we have produced highly reduced graphene oxide with well dispersion in water. We have demonstrated that the supporting material can dramatically enhance the durability of the catalyst and retain the electrochemical surface area (ECSA) of the Pt NPs on the RGO support which is much higher than the commercially available catalyst. We suggest that the well-dispersed and highly reduced unique 2D profiles of the RGO are promising support materials for developing next-generation advanced Pt based fuel cells and their relevant electrodes in the fields of energy and biosensor. This work was financially supported by National Basic Research Program of China (No. 2011CB935704), the National Natural Science Foundation of China (No. 21235004), and Tsinghua University Initiative Scientific Research Program.
Notes and references 1 H. A. Gasteiger and N. M. Markovic, Science, 2009, 324, 48. 2 N. R. Shiju and V. V. Guliants, Appl. Catal., A, 2009, 356, 1. 3 M. Subhramannia and V. K. Pillai, J. Mater. Chem., 2008, 18, 5858.
This journal is © The Royal Society of Chemistry 2015
Communication 4 M. A. Rigsby, W. P. Zhou, A. Lewera, H. T. Duong, P. S. Bagus, W. Jaegermann, R. Hunger and A. Wieckowski, J. Phys. Chem. C, 2008, 112, 15595. 5 L. Wang, C. G. Tian, H. X. Zhang and H. G. Fu, Eur. J. Inorg. Chem., 2012, 961. 6 Z. H. Wen, Q. Wang, Q. Zhang and J. H. Li, Electrochem. Commun., 2007, 9, 1867. 7 S. E. Habas, H. Lee, V. Radmilovic, G. A. Somorjai and P. Yang, Nat. Mater., 2007, 6, 692. 8 Z. Peng and H. J. Yang, J. Am. Chem. Soc., 2009, 131, 7542. 9 J. Zhang, H. Yang, J. Fang and S. Zou, Nano Lett., 2010, 10, 638. 10 L. Wang, C. Tian, H. Wang, Y. Ma, B. Wang and H. Fu, J. Phys. Chem. C, 2010, 114, 8727. 11 J. Yin, L. Wang, C. Tian, T. Tan, G. Mu, L. Zhao and H. Fu, Chem. – Eur. J., 2013, 19, 13979. 12 R. H. Baughman, A. A. Zakhidov and W. A. Heer, Science, 2002, 297, 787. 13 A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183. 14 J. Zhu, Nat. Nanotechnol., 2008, 3, 528. 15 D. Li and R. B. Kaner, Science, 2008, 320, 1170. 16 S. Stankovich, D. A. Dikin, G. H. B. Dommett, K. M. Kohlhaas, E. J. Zimney, E. A. Stach, R. D. Piner, S. T. Nguyen and R. S. Ruoff, Nature, 2006, 442, 282. 17 K. S. Novoselov, A. K. Geim, D. Jiang, V. Morozov, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666. 18 N. Tombros, C. Jozsa, M. Popinciuc, H. T. Jonkman and B. J. van Wees, Nature, 2007, 448, 571. 19 K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos and A. A. Firsov, Nature, 2005, 438, 197. 20 K. S. Novoselov, D. Jiang, Y. Zhang, S. V. Morozov, H. L. Stormer, U. Zeitler, J. C. Maan, G. S. Boebinger, P. Kim and A. K. Geim, Science, 2007, 315, 1379. 21 S. Stankovich, R. D. Piner, X. Q. Chen, N. Q. Wu, S. T. Nguyen and R. S. Ruoff, J. Mater. Chem., 2006, 16, 155. 22 H. Chen, Y. Liang, T. Chen, Y. Tseng, C. Liu, S. Chung, C. Hsieh, C. Lee and K. Wang, Chem. Commun., 2014, 50, 11165. 23 Y. M. Li, L. H. Tang and J. H. Li, Electrochem. Commun., 2009, 11, 846. 24 Y. G. Zhou, J. J. Chen, F. B. Wang, Z. H. Sheng and X. H. Xia, Chem. Commun., 2010, 46, 5951. 25 S. Guo and S. Sun, J. Am. Chem. Soc., 2012, 134, 4. 26 Y. Si and E. T. Samulski, Chem. Mater., 2008, 20, 6792. 27 Y. Li, Y. Li, E. Zhu, T. McLouth, C. Chiu, X. Huang and Y. Huang, J. Am. Chem. Soc., 2012, 134, 12326. 28 H. Feng, R. Cheng, X. Zhao, X. Duan and J. Li, Nat. Commun., 2013, 4, 1539. 29 L. J. Cote, F. Kim and J. Huang, J. Am. Chem. Soc., 2009, 131, 1043. 30 W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339. 31 S. C. Sahu, A. K. Samantara, A. Dash, R. R. Juluri, R. K. Sahu, B. K. Mishra and B. K. Jena, Nano Res., 2013, 6, 635. 32 W. Si, J. Li, H. Li, S. Li, J. Yin, H. Xu, X. Guo, T. Zhang and Y. Song, Nano Res., 2013, 6, 720. 33 D. Huang, Y. Luo, S. Li, B. Zhang, Y. Shen and M. Wang, Nano Res., 2014, 7, 1054.
Chem. Commun.
Published on 02 January 2015. Downloaded by New Mexico State University on 20/01/2015 04:36:26.
COMMUNICATION
Cite this: DOI: 10.1039/c4cc09146a Received 16th November 2014, Accepted 31st December 2014
View Journal
Highly reduced graphene oxide supported Pt nanocomposites as highly efficient catalysts for methanol oxidation† Hongbin Feng,ab Yong Liua and Jinghong Li*a
DOI: 10.1039/c4cc09146a www.rsc.org/chemcomm
The highly reduced graphene oxide using solvated electrons as reductive agents shows low defects and well dispersion that are vital as the support of Pt nanoparticles in direct methanol electrooxidation. The electrochemical experiments demonstrate that the Pt/RGO composites not only greatly enhance the catalytic activity but also dramatically improve the durability of the catalyst.
Platinum (Pt)-based catalysts have attracted much research interest for their unusual catalytic and electrocatalytic properties, which are vital to fuel cells, petroleum and automotive industries.1–4 But the cost of pure Pt catalysts is very high. In order to make the most use of this precious metal and reduce the cost, Pt nanoparticles (NPs) are usually loaded on high surface area supporting materials.5–11 So far, carbon-based materials, such as carbon black, carbon nanotubes, carbon nanofibers, etc., have been extensively used as supporting materials for Pt catalysts.12–16 Recently, graphene (or reduced graphene oxide, RGO) nanosheets have attracted attention as a unique 2D material with higher electrical and thermal conductivities, better transparency and mechanical strength, and larger surface area due to their unique structure composing of sp2-bonded carbon atoms.17–21 Therefore, much effort has been focused on using RGO nanosheets as electrocatalyst support materials for the dispersion of Pt NPs to obtain new functional nanocomposites, and some exciting results have been reported.22–27 However, there are still some problems to hinder the application of RGO as an electrocatalyst support material. Firstly, the RGO sheets tend to stack together through p–p interaction, which sets a higher resistance for the diffusion of reactant molecules and retards the catalytic reaction. In addition, RGO sheets obtained by general chemical synthesis usually contain more defects, a
Department of Chemistry, Beijing Key Laboratory for Microanalytical Methods and Instrumentation, Tsinghua University, Beijing 100084, China. E-mail: [email protected]; Fax: +86 10 6279 5290; Tel: +86 10 6279 5290 b College of Materials Science and Opto-electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China † Electronic supplementary information (ESI) available: Experimental details. See DOI: 10.1039/c4cc09146a
This journal is © The Royal Society of Chemistry 2015
which reduce the electron-transfer rate in and across the interface of the RGO nanosheet. Therefore, it is necessary to prepare the RGO nanosheets not only with low defects but also with well dispersion as the support. In this communication, we report an approach to design highly active and durable electro-catalyst by loading Pt NPs on a support RGO, which is produced by a highly efficient approach for one-pot reduction of GO in solution by using a sodium– ammonia (Na–NH3) as the reducing agent.28 Metallic sodium can be dissolved in liquid ammonia to create a blue solution with strong reducibility due to the generation of ‘‘solvated electron e [NH3]n’’. The composites of RGONa–NH3 nanosheets supported Pt NPs were synthesized in an aqueous solution at 85 1C using RGONa–NH3 nanosheet dispersion in H2PtCl6 solutions with NaBH4 as a reducing agent.23 The use of our RGONaNH3 nanosheets as matrixes for Pt deposition has several advantages. Firstly, they can be easily dispersed in water. Secondly, they can be effectively reduced and have a high conductivity. Thirdly, the composite structure not only enhances the catalytic activity but also dramatically improves the durability of the catalyst. Pt NPs supported on RGONa–NH3 nanosheets were prepared, which were explored as methanol oxidation electrocatalysts. Fig. 1a shows the TEM image that Pt NPs were loaded uniformly on RGONa–NH3 nanosheets without obvious localized aggregation. Pt/RGONa–NH3 composites were prepared using the well-dispersing RGONa–NH3 nanosheets, which were obtained by the Na–NH3 reduction of graphene oxide synthesized by the modified Hummers method.29,30 Dissolution of the sodium metal in the liquid ammonia results in ionization of the metal to form a sodium cation and a solvated electron strongly associated with the solvent ammonia. The sodium–ammonia solution with the solvated electrons can function as a very potent electron source to effectively remove oxygen functionalities with only oxygen content of 5.6 wt% left and restore the planar geometry of the GO sheets.28 Fig. 1b shows the well-dispersed RGO in water. According to Fig. 1a, the typical view under TEM, all Pt-loaded RGO sheets form composites. Besides, most Pt NPs were still attached on the RGO, indicating good contact between Pt NPs
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Fig. 1 (a) TEM images of Pt/RGONa–NH3, (b) the dispersion of Pt/RGONa–NH3 in water, (c) the image shows a HRTEM image of Pt/RGONa–NH3, the scale bar is 3 nm. (d) The EDS spectra of Pt/RGONa–NH3. (e) The XRD pattern of the Pt/RGONa–NH3 composites.
and RGO. The high-resolution image in Fig. 1c shows that the as-synthesized Pt NPs are single-crystalline. Fig. 1d shows the energy dispersive X-ray spectroscopy (EDS) spectra of Pt/RGONa–NH3, indicating the presence of platinum. The peak assigned to Cu in the EDS analysis is attributed to a mesh used for the TEM measurement. The XRD pattern of the Pt/RGONa–NH3 composite is shown in Fig. 1e. The strong diffraction peaks at 2y angles of 39.91, 46.21, 67.91 in the XRD pattern can be assigned to the (111), (200), (220) facets of the face-centered cubic structures of the platinum crystal, which are in good agreement with the standard card of cubic Pt (JCPDS No. 4-802). The resulting Pt/RGONa–NH3 composite shows a wide peak at 23.81, confirming a graphitic nature of RGO sheets. To evaluate the electrochemical activity of Pt/graphene nanocomposites, cyclic voltammogram (CV) experiments were tested within a potential range from 0 to 1 V at a scanning rate of 50 mV s 1 in the solution of nitrogen saturated 0.5 M H2SO4.
Fig. 2 Cyclic voltammograms of Pt/RGONa–NH3 (black line), Pt/Vulcan (red line) electrodes in nitrogen saturated aqueous solution of 0.5 M H2SO4 at a scan rate of 50 mV s 1.
Chem. Commun.
Fig. 3 (a) Cyclic voltammograms at a scan rate of 50 mV s 1, (b) chronoamperometric curves at a fixed potential of 0.6 V vs. Ag/AgCl (KCl sat.) on Pt/RGONa–NH3 and Pt/Vulcan electrodes in nitrogen saturated aqueous solution of 0.5 M H2SO4 containing 0.5 M CH3OH.
As seen from Fig. 2, the Pt/graphene film deposited on a glassy carbon electrode was electrochemically active in which hydrogen adsorption characteristics were presented. The electrochemically active surface area (ECSA) was estimated by integrating the voltammograms corresponding to hydrogen desorption from the electrode surface. The ECSAs for the Pt/graphene and Pt/Vulcan were estimated to be 1585.32 and 13.23 m2 g 1 Pt, respectively, and the high ECSAs were favorable to electrochemical oxidation toward methanol. Fig. 3a shows representative cyclic voltammetry curves of a Pt/RGONa–NH3 composite in the solution of 0.5 M CH3OH in 0.5 M H2SO4. The current density on Pt/RGONa–NH3 was near three times higher than that on Pt/Vulcan, indicating much higher activity toward methanol oxidation on Pt/RGONa–NH3. The ratio of the typical methanol oxidation current density (the forward peak, If) to the residual carbon species oxidation current density (the reverse anodic peak, Ib), If/Ib, can be used to describe the catalyst tolerance to carbonaceous species accumulation.23 The If/Ib ratio was estimated to be 1.1, 1.93 for Pt/Vulcan and Pt/RGONa–NH3 respectively. A low If/Ib ratio indicates poor oxidation of methanol to carbon dioxide during the anodic scan and excessive accumulation of carbonaceous residues on the catalyst surface. We have found that If/Ib of Pt/RGONa–NH3 was much higher than that of the Pt/Vulcan catalysts. Therefore, the performance of Pt/RGONa–NH3 for the methanol electrochemical oxidation can be considered superior to that of Pt/Vulcan. In addition, the amperometric I–t curves (Fig. 3b) also exhibit that Pt/RGONa–NH3 had a higher catalytic stability toward methanol oxidation. The high electrocatalytic activity of Pt/RGONa–NH3 may
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be from the strong electronic interaction between Pt NPs and RGO nanosheets, which suppresses the CO formation during the oxidation of methanol and facilitates the direct oxidation of methanol on the Pt surface. Compared with some other materials, our synthetic sample is shown to exhibit well catalytic performance.31–33 In the electrocatalytic fields, materials with high surface area exhibit substantial advantages concerning mass and charge transport by providing shorter effective lengths for both electronic and ionic transport, a higher electrode/electrolyte contact area, and in some cases also interfacial local effects. On one hand, the highly reduced graphene oxide nanosheets obtained by Na–NH3 solvated electrons show high conductivity with low defects, which may improve the electron-transfer rate in the RGO sheet and across the RGO interface as well. On the other hand, graphene as a two dimensional (2D) nanomaterial has high theoretical specific surface area which might be favorable for the electrochemical reaction. Pt NPs could be easily deposited on the surface of graphene based on 2D flat planes. Especially, our RGONa–NH3 can be easily dispersed in the reaction medium that is favorable to achieve single nanosheet deposition of the Pt NPs. In summary, by using Na–NH3 treatment with active solvated electrons e [NH3]n as the highly reductive agent, we have produced highly reduced graphene oxide with well dispersion in water. We have demonstrated that the supporting material can dramatically enhance the durability of the catalyst and retain the electrochemical surface area (ECSA) of the Pt NPs on the RGO support which is much higher than the commercially available catalyst. We suggest that the well-dispersed and highly reduced unique 2D profiles of the RGO are promising support materials for developing next-generation advanced Pt based fuel cells and their relevant electrodes in the fields of energy and biosensor. This work was financially supported by National Basic Research Program of China (No. 2011CB935704), the National Natural Science Foundation of China (No. 21235004), and Tsinghua University Initiative Scientific Research Program.
Notes and references 1 H. A. Gasteiger and N. M. Markovic, Science, 2009, 324, 48. 2 N. R. Shiju and V. V. Guliants, Appl. Catal., A, 2009, 356, 1. 3 M. Subhramannia and V. K. Pillai, J. Mater. Chem., 2008, 18, 5858.
This journal is © The Royal Society of Chemistry 2015
Communication 4 M. A. Rigsby, W. P. Zhou, A. Lewera, H. T. Duong, P. S. Bagus, W. Jaegermann, R. Hunger and A. Wieckowski, J. Phys. Chem. C, 2008, 112, 15595. 5 L. Wang, C. G. Tian, H. X. Zhang and H. G. Fu, Eur. J. Inorg. Chem., 2012, 961. 6 Z. H. Wen, Q. Wang, Q. Zhang and J. H. Li, Electrochem. Commun., 2007, 9, 1867. 7 S. E. Habas, H. Lee, V. Radmilovic, G. A. Somorjai and P. Yang, Nat. Mater., 2007, 6, 692. 8 Z. Peng and H. J. Yang, J. Am. Chem. Soc., 2009, 131, 7542. 9 J. Zhang, H. Yang, J. Fang and S. Zou, Nano Lett., 2010, 10, 638. 10 L. Wang, C. Tian, H. Wang, Y. Ma, B. Wang and H. Fu, J. Phys. Chem. C, 2010, 114, 8727. 11 J. Yin, L. Wang, C. Tian, T. Tan, G. Mu, L. Zhao and H. Fu, Chem. – Eur. J., 2013, 19, 13979. 12 R. H. Baughman, A. A. Zakhidov and W. A. Heer, Science, 2002, 297, 787. 13 A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183. 14 J. Zhu, Nat. Nanotechnol., 2008, 3, 528. 15 D. Li and R. B. Kaner, Science, 2008, 320, 1170. 16 S. Stankovich, D. A. Dikin, G. H. B. Dommett, K. M. Kohlhaas, E. J. Zimney, E. A. Stach, R. D. Piner, S. T. Nguyen and R. S. Ruoff, Nature, 2006, 442, 282. 17 K. S. Novoselov, A. K. Geim, D. Jiang, V. Morozov, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666. 18 N. Tombros, C. Jozsa, M. Popinciuc, H. T. Jonkman and B. J. van Wees, Nature, 2007, 448, 571. 19 K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos and A. A. Firsov, Nature, 2005, 438, 197. 20 K. S. Novoselov, D. Jiang, Y. Zhang, S. V. Morozov, H. L. Stormer, U. Zeitler, J. C. Maan, G. S. Boebinger, P. Kim and A. K. Geim, Science, 2007, 315, 1379. 21 S. Stankovich, R. D. Piner, X. Q. Chen, N. Q. Wu, S. T. Nguyen and R. S. Ruoff, J. Mater. Chem., 2006, 16, 155. 22 H. Chen, Y. Liang, T. Chen, Y. Tseng, C. Liu, S. Chung, C. Hsieh, C. Lee and K. Wang, Chem. Commun., 2014, 50, 11165. 23 Y. M. Li, L. H. Tang and J. H. Li, Electrochem. Commun., 2009, 11, 846. 24 Y. G. Zhou, J. J. Chen, F. B. Wang, Z. H. Sheng and X. H. Xia, Chem. Commun., 2010, 46, 5951. 25 S. Guo and S. Sun, J. Am. Chem. Soc., 2012, 134, 4. 26 Y. Si and E. T. Samulski, Chem. Mater., 2008, 20, 6792. 27 Y. Li, Y. Li, E. Zhu, T. McLouth, C. Chiu, X. Huang and Y. Huang, J. Am. Chem. Soc., 2012, 134, 12326. 28 H. Feng, R. Cheng, X. Zhao, X. Duan and J. Li, Nat. Commun., 2013, 4, 1539. 29 L. J. Cote, F. Kim and J. Huang, J. Am. Chem. Soc., 2009, 131, 1043. 30 W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339. 31 S. C. Sahu, A. K. Samantara, A. Dash, R. R. Juluri, R. K. Sahu, B. K. Mishra and B. K. Jena, Nano Res., 2013, 6, 635. 32 W. Si, J. Li, H. Li, S. Li, J. Yin, H. Xu, X. Guo, T. Zhang and Y. Song, Nano Res., 2013, 6, 720. 33 D. Huang, Y. Luo, S. Li, B. Zhang, Y. Shen and M. Wang, Nano Res., 2014, 7, 1054.
Chem. Commun.
