Fuel Cells
One-Pot Synthesis of Platinum Nanocubes on Reduced Graphene Oxide with Enhanced Electrocatalytic Activity Bao Yu Xia, Hao Bin Wu, Ya Yan, Hai Bo Wang, and Xin Wang* Low-temperature fuel cells are promising power sources for portable electronic devices and automotive vehicles.[1] However, slow kinetics and poor stability of commercial Pt-based electrocatalysts towards oxygen reduction reaction (ORR) has limited their practical application.[2] Thus, there is an urgent need to develop highly active and stable catalyst to enable the commercialization of fuel cell devices.[3] Extensive studies show that ORR on Pt electrocatalysts is sensitive to the surface of the metal particles, and control of the shape and size of Pt nanostructures can lead to improved catalytic performance.[4] In particular, Pt nanocrystals enclosed by (100) facets demonstrate superior catalytic activity compared to the Pt (111) ones in H2SO4 electrolytes for ORR.[5] To further reduce the overall use of expensive Pt and improve the stability of Pt nanocrystals, one of the efficient strategies is to load them on various supports such as nanocarbons, metal oxides and metal carbide.[6] Conventionally, supported Pt nanocrystals were obtained using a two-step approach: 1) synthesis of Pt nanocrystals which normally involves the use of polymers, surfactants and biomaterials to facilitate the shape control.[7] and 2) deposition of Pt nanocrystals on supports which is commonly realized by selfassembly techniques combined with noncovalent functionalization and also involves the employment of surfactants or capping agents.[8] For practical applications, it is necessary to remove these bulky molecules. Unfortunately, the subsequent removal of these organic species at high temperature would possibly destroy the initial structure,[9] and the presence of residual capping agents would inevitably cover or block the active sites of the nanocatalysts.[10] Therefore, the development of simple and effective strategy for the preparation of Pt nanocubes with clean surface and simultaneous deposition on support is highly desirable and it is also very challenging due to the strong interaction between the support and newly formed nuclei during the synthesis[6c,11] Among various supports, graphene has been considered as an attractive one owing to its large surface area and high mechanical, thermal, and chemical stability.[12] On the other hand, the abundant functional groups on graphene surface
Dr. B. Y. Xia, H. B. Wu, Y. Yan, H. B. Wang, Prof. X. Wang School of Chemical and Biomedical Engineering Nanyang Technological University 62 Nanyang Drive, Singapore 637459, Singapore E-mail: [email protected] DOI: 10.1002/smll.201302648 small 2014, DOI: 10.1002/smll.201302648
would provide many sites for anchoring Pt nanoparticles.[13] To our best knowledge, reports on the preparation of Pt nanocube/graphene hybrids in one-pot method and the investigation of their electrocatalytic behavior towards oxygen reduction reaction is limited to date.[10a] Herein we demonstrate a facile one-pot approach to achieve the formation of size-controlled Pt nanocubes and simultaneously deposit on reduced graphene oxides (rGO). The resultant Pt nanocubes/ rGO hybrid electrocatalyst exhibits enhanced electrocatalytic performance due to the abundant exposed (100) facets of Pt nanocubes and their uniform distribution on rGO. The as-prepared Pt nanocubes/rGO hybrids were characterized by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) (Figure 1 and S1, Supporting Information). A typical TEM image shows Pt nanocrystals are uniformly distributed on the rGO support (Figure 1a). The sample consists of ∼3.0 nm Pt nanocubes with the high morphological yield of > 90%. The enhanced selected-area electron diffraction (SAED) ring of Pt (200) plane indicates the as-prepared Pt nanocrystals have Pt (100) texture, as shown in the inset of Figure 1a. The high-resolution (HR)TEM image of Pt nanocubes shows clear fringes with the inter-fringe distance of approximately 0.196 nm, which corresponds to the lattice spacing of the Pt (200) planes (Figure 1b). XRD pattern of the as-prepared Pt nanocubes/rGO hybrids (20 wt% of Pt) in Figure 1d displays typical peaks that are in agreement with those of a standard Pt pattern (JCPDS card 04–0802). The relative intensity of the Pt (200) diffraction peak is much stronger than that of the Pt (111) peak with an approximate ratio of 130:100 (Figure 1d). Such enhanced ratio, when compared to the standard facecentered cubic (fcc) Pt pattern with ratio of 52:100, indicates that the Pt nanocrystals on rGO is (100) textured.[5c] When the Pt loading is increased to 50 wt%, FESEM and TEM images show that the particle morphology is still dominated by nanocubes with the increased size of ∼ 6 nm (Figure 1c and S1, Supporting Information). Pt loading ratio on the rGO is confirmed by TGA technique (Figure S2a, see the Supporting Information). The (200)/(111) peak ratio of 65:100 for 50 wt% Pt/rGO hybrids further verifies the dominance of cubic Pt nanocrystals in the product (Figure 1d). During the formation of Pt nanocubes, GO was simultaneously reduced as confirmed by XRD patterns for the appearance of peak at about 26.3° (Figure 1 and S2b, Supporting Information) and Raman analysis for the enhanced G/D peak ratios (Figure S2c, see the Supporting Information), thus achieving the one-pot synthesis of the Pt nanocubes/rGO hybrids.
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Figure 1. TEM images (a, b and c) and XRD patterns (d) of Pt nanocubes/ rGO hybrids: 20% Pt (a, b); 50% Pt (c). Inset of (a) is the selected-area electron diffraction pattern corresponding to Figure 1a.
NaOH appears to be a vital component for the successful formation of Pt nanocubes deposited on rGO. In the absence of NaOH, only small Pt nanoclusters (∼2 nm) are produced, as shown in Figure 2a. In contrast, the addition of 1.8 g NaOH leads to the formation of Pt nanocubes (Figure 1). We propose that amine species produced from the reaction of DMF and NaOH, as suggested by the FT-IR spectrum (Figure S2d, see the Supporting Information), can effectively stabilize the (100)
Figure 2. TEM images of Pt nanocubes/rGO hybrids: a) without the introduction of NaOH, b) 20% and c) 50% Pt nanocubes/rGO with the addition of 3.6 g NaOH. d) Pt nanocubes without the introduction of GO supports. The scale bars in the insets are 5 nm.
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facet of Pt nanocubes. The adsorption of amine groups lowers the total surface energy of the Pt (100) facet and induces the evolution of Pt nanocubes. Such adsorption of amine groups on the specific facets and sites to stabilize the formation of metallic nanocubes has been elucidated previously.[7b,14] If the amount of NaOH further increases to 3.6 g, the final products also exhibit cubic morphologies with a wider size range of 5–10 nm (Figure 2b). Compared with the observations on the products obtained with the addition of 1.8 g NaOH, these observations suggest that excess produced amine species would affect the growth of Pt nanocubes. Therefore an appropriate amount of amine is essential to stabilize the formed Pt nanocubes with small size at the presence of graphene support.[15] On the other hand, the size of the Pt nanocubes deposited on rGO is also strongly affected by the loading amount of Pt (Figure 1 and 2). The size of Pt nanocubes changes from 3 nm to 6 nm when the Pt loading on rGO is increased from 20% to 50%, as shown in Figure 1. Without the introduction of rGO, the resultant aggregation composes of ∼2 nm Pt nanocubes (Figure 2d). We attempt to prepare Pt nanocubes supported on carbon black (CB) and TiO2 nanosheets (NSs). However, Pt nanocubes deposited on carbon black exhibit certain degree of aggregation with lower morphological yield for cube shape (Figure 3a), while some worm-like Pt nanocrystals are obtained in the presence of TiO2 NSs (Figure 3c). These observations were confirmed by the different (200)/(111) peak ratios of 51:100 for Pt/CB and 38:100 for Pt/TiO2 NSs from the XRD patterns (Figure 3b, d). These indicate that GO not only function as the support to deposit and disperse Pt nanoparticles but also take part in the formation of Pt nanocube with the assistance of abundant surface functional groups. It is clear that the formation of Pt nanocubes is particularly sensitive to the nature of supports. Similar to the previous reports on the preparation of Pt nanocrystals with well-defined facets,[6c,16] the supports has a significant impact on the formation of the
Figure 3. TEM images and XRD patterns for Pt/CB (a, b) and Pt/TiO2 NS (c, d).
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small 2014, DOI: 10.1002/smll.201302648
One-Pot Synthesis of Platinum Nanocubes on Reduced Graphene Oxide with Enhanced Electrocatalytic Activity
Pt (100) surface dominated Pt nanocubes which are uniformly distributed on the rGO supports. Furthermore, the accelerated cyclic voltammetry tests are carried out to evaluate the electrochemical stability. After 1000 cycles, the featured CV profiles remain similar for Pt nanocubes/ rGO (Figure 4c). On the contrary, commercial Pt/C shows a remarkable loss in active surface area, although they have similar size in the beginning (Figure 4d). With the further electrochemical durability test up to 10 000 cycles, Pt cube/rGO still retains 51.3% of the electrochemical active surface area and the featured CV characteristics, while Pt/rGO and Pt cube/ CB only retain 24.3% and 7.2% of the electrochemical active surface area respectively. The stability improvement over nanoparticle Pt/rGO may indicate that Pt nanoparticles are more prone to dissolution/aggregation due to the presence Figure 4. a) Cyclic voltammograms (CVs) and (b) the calculated mass activity and specific activity of Pt electrocatalysts. c) Electrochemical accelerated durability test of Pt nanocubes/ of more surface defects. The very low starGO and Pt/CB electrocatalysts recorded in 0.5 M H2SO4 solution before and after 1000 cycles bility of Pt cube/CB is mainly attributed at a scan rate of 20 mV s−1. to the more severe support corrosion of carbon black compared to graphene, as specific morphology because of the different surface chem- can be seen from the significant loss of double layer capaciistries of supports and resulting difference in the interaction tance (Figure 4 and S5, see the Supporting Information).[18] The electrochemical activities towards methanol and formic between support and nuclei. The exposure of cubic surface of Pt nanocubes/rGO acid oxidation reactions of the as-prepared Pt nanocubes/ hybrid is also confirmed by the surface sensitive electrochem- rGO hybrid are also evaluated (Figure S6, see the Supporting ical characterization.[17] Cyclic voltammograms (CVs) of Information), where Pt nanocubes/rGO hybrid electrocatathese nanocatalysts in 0.5 M H2SO4 are shown in Figure 4a. lyst again exhibits better activity than other Pt catalysts. We demonstrate a facile one-pot approach to syntheFor comparison, the results from Pt cube/CB, Pt/rGO and commercial Pt/C catalyst are also included. The commer- size Pt nanocubes/rGO hybrids with controlled size and Pt cial Pt/C catalyst only exhibits a hydrogen desorption peak loading mass. The Pt nanocubes/rGO hybrid electrocatalyst at about –0.1 V. In contrast, there is one additional pair of exhibits superior electrochemical performance for oxygen hydrogen adsorption/desorption peaks at ∼ –0.05 V for cubic reduction reaction, methanol and formic acid oxidation reacPt/rGO and cubic Pt/CB catalysts, which corresponds to Pt tions compared with commercial Pt/C catalyst. The enhanced (100) terrace surface. This prominent current peak reflects electrocatalytic activity and stability is attributed to the unithe fact that substantial Pt (100) planes are present on the form dispersion of Pt nanocubes on rGO supports and its surface of Pt nanocubes/rGO hybrid, in agreement with the dominant Pt (100) facets exposed to the reaction medium. TEM and XRD analysis (Figure 1). The calculated electro- The present method provides a feasible approach to prepare chemical surface areas (ECSA) of Pt nanocubes/rGO and shape and size controlled Pt/supports composites, which holds commercial Pt/C catalysts are 51.0 and 60.8 m2 g−1, respec- great potential for electrochemical and catalytic applications. tively. The catalytic activities of different Pt catalysts are next evaluated in terms of oxygen reduction reaction (ORR) at various rotating rates. (Figure S3 and S4, see the Supporting Information). The number of electron calculated from Koutecky-Levich plots is 3.7 and 3.4 for Pt nanocubes/rGO Supporting Information hybrid and commercial Pt/CB, which is similar to Pt/rGO and Pt cube/CB catalysts (Figure S3 and S4, see the Supporting Supporting Information is available from the Wiley Online Library Information), respectively. It indicates that ORR on Pt nano- or from the author. cubes/rGO favors more on four-electron path. Mass activity and specific activity in Figure 4b for Pt nanocubes/rGO are the highest among the four electrocatalysts (13.9 A g−1 Pt and Acknowledgement 0.28 A m−2 at 0.8 V), which are about 2.1 and 2.5 times of that of commercial Pt/C(6.75 A g−1 and 0.11 A m−2), respectively. We acknowledge financial support from the academic research The apparent catalytic enhancement for ORR is attributed to fund AcRF tier 1 (M4011020, RG8/12 and M4010888, RG20/09) small 2014, DOI: 10.1002/smll.201302648
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Ministry of Education, Singapore and competitive research program (2009 NRF-CRP 001–032), National Research Foundation, Singapore.
[1] R. F. Service, Science 2002, 296, 1222. [2] a) Z. M. Peng, H. Yang, Nano Today 2009, 4, 143; b) M. K. Debe, Nature 2012, 486, 43. [3] a) A. C. Chen, P. Holt-Hindle, Chem. Rev. 2010, 110, 3767; b) S. E. Lohse, C. J. Murphy, J. Am. Chem. Soc. 2012, 134, 15607. [4] a) T. S. Ahmadi, Z. L. Wang, T. C. Green, A. Henglein, M. A. El-Sayed, Science 1996, 272, 1924; b) J. Ren, R. D. Tilley, Small 2007, 3, 1508; c) A. R. Tao, S. Habas, P. D. Yang, Small 2008, 4, 310; d) B. Lim, T. Yu, Y. Xia, Angew. Chem. Int. Ed. 2010, 49, 9819; e) T. K. Sau, A. L. Rogach, F. Jackel, T. A. Klar, J. Feldmann, Adv. Mater. 2010, 22, 1805; f) B. Y. Xia, W. T. Ng, H. B. Wu, X. Wang, X. W. Lou, Angew. Chem. Int. Ed. 2012, 51, 7213; g) B. Y. Xia, H. B. Wu, Y. Yan, X. W. Lou, X. Wang, J. Am. Chem. Soc. 2013, 135, 9480. [5] a) H. Lee, S. E. Habas, S. Kweskin, D. Butcher, G. A. Somorjai, P. Yang, Angew. Chem. Int. Ed. 2006, 45, 7824; b) C. Wang, H. Daimon, Y. Lee, J. Kim, S. Sun, J. Am. Chem. Soc. 2007, 129, 6974; c) C. Wang, H. Daimon, T. Onodera, T. Koda, S. H. Sun, Angew. Chem. Int. Ed. 2008, 47, 3588; d) C. K. Tsung, J. N. Kuhn, W. Y. Huang, C. Aliaga, L. I. Hung, G. A. Somorjai, P. D. Yang, J. Am. Chem. Soc. 2009, 131, 5816; e) D. Xu, Z. Liu, H. Yang, Q. Liu, J. Zhang, J. Fang, S. Zou, K. Sun, Angew. Chem. Int. Ed. 2009, 48, 4217; f) T. Yu, Y. Kim do, H. Zhang, Y. Xia, Angew. Chem. Int. Ed. 2011, 50, 2773; g) Y. W. Lee, S. B. Han, D. Y. Kim, K. W. Park, Chem. Commun. 2011, 47, 6296. [6] a) Y. Yamada, C. K. Tsung, W. Huang, Z. Huo, S. E. Habas, T. Soejima, C. E. Aliaga, G. A. Somorjai, P. Yang, Nat. Chem. 2011, 3, 372; b) G. N. Vayssilov, Y. Lykhach, A. Migani, T. Staudt, G. P. Petrova, N. Tsud, T. Skala, A. Bruix, F. Illas, K. C. Prince, V. Matolin, K. M. Neyman, J. Libuda, Nat. Mater. 2011, 10, 310; c) K. Zhou, Y. Li, Angew. Chem. Int. Ed. 2012, 51, 602; d) Z. Yao, H. Nie, Z. Yang, X. Zhou, Z. Liu, S. Huang, Chem. Commun. 2012, 48, 1027; e) K. H. Park, Y. W. Lee, Y. Kim, S. W. Kang, S. W. Han, Chem. Eur. J. 2013, 19, 8053.
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[7] a) R. Narayanan, M. A. El-Sayed, Nano Lett. 2004, 4, 1343; b) C.-y. Chiu, Y. Li, L. Ruan, X. Ye, C. B. Murray, Y. Huang, Nat. Chem. 2011, 3, 393. [8] a) W. Yang, X. L. Wang, F. Yang, C. Yang, X. R. Yang, Adv. Mater. 2008, 20, 2579; b) S. Zhang, Y. Shao, G. Yin, Y. Lin, J. Mater. Chem. 2010, 20, 2826; c) B. H. Wu, Y. J. Kuang, X. H. Zhang, J. H. Chen, Nano Today 2011, 6, 75; d) S. Wang, X. Wang, S. P. Jiang, Phys. Chem. Chem. Phys. 2011, 13, 6883. [9] S. Z. Wang, L. Kuai, Y. C. Huang, X. Yu, Y. D. Liu, W. Z. Li, L. Chen, B. Y. Geng, Chem. Eur. J. 2013, 19, 240. [10] a) G. Chen, Y. Tan, B. Wu, G. Fu, N. Zheng, Chem. Commun. 2012, 48, 2758; b) H. Vedala, D. C. Sorescu, G. P. Kotchey, A. Star, Nano Lett. 2011, 11, 2342. [11] a) B. Wu, N. Zheng, G. Fu, Chem. Commun. 2011, 47, 1039; b) E. Antolini, J. Perez, J. Mater. Sci. 2011, 46, 4435; c) S. Mao, Z. Wen, H. Kim, G. Lu, P. Hurley, J. Chen, ACS Nano 2012, 6, 7505. [12] a) M. Pumera, Chem. Soc. Rev. 2010, 39, 4146; b) D. Chen, L. Tang, J. Li, Chem. Soc. Rev. 2010, 39, 3157; c) N. G. Sahoo, Y. Pan, L. Li, S. H. Chan, Adv. Mater. 2012, 24, 4203. [13] a) S. Guo, S. Sun, J. Am. Chem. Soc. 2012, 134, 2492; b) Y. L. Li, J. J. Wang, X. F. Li, D. S. Geng, M. N. Banis, Y. J. Tang, D. N. Wang, R. Y. Li, T. K. Sham, X. L. Sun, J. Mater. Chem. 2012, 22, 20170; c) H. Yin, H. Tang, D. Wang, Y. Gao, Z. Tang, ACS Nano 2012, 6, 8288; d) Y. Z. Lu, Y. Y. Jiang, H. B. Wu, W. Chen, J. Phys. Chem. C 2013, 117, 2926. [14] a) M. Chen, J. Kim, J. P. Liu, H. Fan, S. Sun, J. Am. Chem. Soc. 2006, 128, 7132; b) X. Q. Huang, Z. P. Zhao, J. M. Fan, Y. M. Tan, N. F. Zheng, J. Am. Chem. Soc. 2011, 133, 4718. [15] M. R. Axet, K. Philippot, B. Chaudret, M. Cabie, S. Giorgio, C. R. Henry, Small 2011, 7, 235. [16] Y. Xia, Y. Xiong, B. Lim, S. E. Skrabalak, Angew. Chem. Int. Ed. 2009, 48, 60. [17] a) Z. Y. Zhou, Z. Z. Huang, D. J. Chen, Q. Wang, N. Tian, S. G. Sun, Angew. Chem. Int. Ed. 2010, 49, 411; b) B. Y. Xia, H. B. Wu, X. Wang, X. W. Lou, Angew. Chem. Int. Ed. 2013, 52, 12337 [18] X. Wang, W. Z. Li, Z. W. Chen, M. Waje, Y. S. Yan, J. Power Sources 2006, 158, 154.
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Received: August 14, 2013 Revised: September 29, 2013 Published online:
small 2014, DOI: 10.1002/smll.201302648
One-Pot Synthesis of Platinum Nanocubes on Reduced Graphene Oxide with Enhanced Electrocatalytic Activity Bao Yu Xia, Hao Bin Wu, Ya Yan, Hai Bo Wang, and Xin Wang* Low-temperature fuel cells are promising power sources for portable electronic devices and automotive vehicles.[1] However, slow kinetics and poor stability of commercial Pt-based electrocatalysts towards oxygen reduction reaction (ORR) has limited their practical application.[2] Thus, there is an urgent need to develop highly active and stable catalyst to enable the commercialization of fuel cell devices.[3] Extensive studies show that ORR on Pt electrocatalysts is sensitive to the surface of the metal particles, and control of the shape and size of Pt nanostructures can lead to improved catalytic performance.[4] In particular, Pt nanocrystals enclosed by (100) facets demonstrate superior catalytic activity compared to the Pt (111) ones in H2SO4 electrolytes for ORR.[5] To further reduce the overall use of expensive Pt and improve the stability of Pt nanocrystals, one of the efficient strategies is to load them on various supports such as nanocarbons, metal oxides and metal carbide.[6] Conventionally, supported Pt nanocrystals were obtained using a two-step approach: 1) synthesis of Pt nanocrystals which normally involves the use of polymers, surfactants and biomaterials to facilitate the shape control.[7] and 2) deposition of Pt nanocrystals on supports which is commonly realized by selfassembly techniques combined with noncovalent functionalization and also involves the employment of surfactants or capping agents.[8] For practical applications, it is necessary to remove these bulky molecules. Unfortunately, the subsequent removal of these organic species at high temperature would possibly destroy the initial structure,[9] and the presence of residual capping agents would inevitably cover or block the active sites of the nanocatalysts.[10] Therefore, the development of simple and effective strategy for the preparation of Pt nanocubes with clean surface and simultaneous deposition on support is highly desirable and it is also very challenging due to the strong interaction between the support and newly formed nuclei during the synthesis[6c,11] Among various supports, graphene has been considered as an attractive one owing to its large surface area and high mechanical, thermal, and chemical stability.[12] On the other hand, the abundant functional groups on graphene surface
Dr. B. Y. Xia, H. B. Wu, Y. Yan, H. B. Wang, Prof. X. Wang School of Chemical and Biomedical Engineering Nanyang Technological University 62 Nanyang Drive, Singapore 637459, Singapore E-mail: [email protected] DOI: 10.1002/smll.201302648 small 2014, DOI: 10.1002/smll.201302648
would provide many sites for anchoring Pt nanoparticles.[13] To our best knowledge, reports on the preparation of Pt nanocube/graphene hybrids in one-pot method and the investigation of their electrocatalytic behavior towards oxygen reduction reaction is limited to date.[10a] Herein we demonstrate a facile one-pot approach to achieve the formation of size-controlled Pt nanocubes and simultaneously deposit on reduced graphene oxides (rGO). The resultant Pt nanocubes/ rGO hybrid electrocatalyst exhibits enhanced electrocatalytic performance due to the abundant exposed (100) facets of Pt nanocubes and their uniform distribution on rGO. The as-prepared Pt nanocubes/rGO hybrids were characterized by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) (Figure 1 and S1, Supporting Information). A typical TEM image shows Pt nanocrystals are uniformly distributed on the rGO support (Figure 1a). The sample consists of ∼3.0 nm Pt nanocubes with the high morphological yield of > 90%. The enhanced selected-area electron diffraction (SAED) ring of Pt (200) plane indicates the as-prepared Pt nanocrystals have Pt (100) texture, as shown in the inset of Figure 1a. The high-resolution (HR)TEM image of Pt nanocubes shows clear fringes with the inter-fringe distance of approximately 0.196 nm, which corresponds to the lattice spacing of the Pt (200) planes (Figure 1b). XRD pattern of the as-prepared Pt nanocubes/rGO hybrids (20 wt% of Pt) in Figure 1d displays typical peaks that are in agreement with those of a standard Pt pattern (JCPDS card 04–0802). The relative intensity of the Pt (200) diffraction peak is much stronger than that of the Pt (111) peak with an approximate ratio of 130:100 (Figure 1d). Such enhanced ratio, when compared to the standard facecentered cubic (fcc) Pt pattern with ratio of 52:100, indicates that the Pt nanocrystals on rGO is (100) textured.[5c] When the Pt loading is increased to 50 wt%, FESEM and TEM images show that the particle morphology is still dominated by nanocubes with the increased size of ∼ 6 nm (Figure 1c and S1, Supporting Information). Pt loading ratio on the rGO is confirmed by TGA technique (Figure S2a, see the Supporting Information). The (200)/(111) peak ratio of 65:100 for 50 wt% Pt/rGO hybrids further verifies the dominance of cubic Pt nanocrystals in the product (Figure 1d). During the formation of Pt nanocubes, GO was simultaneously reduced as confirmed by XRD patterns for the appearance of peak at about 26.3° (Figure 1 and S2b, Supporting Information) and Raman analysis for the enhanced G/D peak ratios (Figure S2c, see the Supporting Information), thus achieving the one-pot synthesis of the Pt nanocubes/rGO hybrids.
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Figure 1. TEM images (a, b and c) and XRD patterns (d) of Pt nanocubes/ rGO hybrids: 20% Pt (a, b); 50% Pt (c). Inset of (a) is the selected-area electron diffraction pattern corresponding to Figure 1a.
NaOH appears to be a vital component for the successful formation of Pt nanocubes deposited on rGO. In the absence of NaOH, only small Pt nanoclusters (∼2 nm) are produced, as shown in Figure 2a. In contrast, the addition of 1.8 g NaOH leads to the formation of Pt nanocubes (Figure 1). We propose that amine species produced from the reaction of DMF and NaOH, as suggested by the FT-IR spectrum (Figure S2d, see the Supporting Information), can effectively stabilize the (100)
Figure 2. TEM images of Pt nanocubes/rGO hybrids: a) without the introduction of NaOH, b) 20% and c) 50% Pt nanocubes/rGO with the addition of 3.6 g NaOH. d) Pt nanocubes without the introduction of GO supports. The scale bars in the insets are 5 nm.
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facet of Pt nanocubes. The adsorption of amine groups lowers the total surface energy of the Pt (100) facet and induces the evolution of Pt nanocubes. Such adsorption of amine groups on the specific facets and sites to stabilize the formation of metallic nanocubes has been elucidated previously.[7b,14] If the amount of NaOH further increases to 3.6 g, the final products also exhibit cubic morphologies with a wider size range of 5–10 nm (Figure 2b). Compared with the observations on the products obtained with the addition of 1.8 g NaOH, these observations suggest that excess produced amine species would affect the growth of Pt nanocubes. Therefore an appropriate amount of amine is essential to stabilize the formed Pt nanocubes with small size at the presence of graphene support.[15] On the other hand, the size of the Pt nanocubes deposited on rGO is also strongly affected by the loading amount of Pt (Figure 1 and 2). The size of Pt nanocubes changes from 3 nm to 6 nm when the Pt loading on rGO is increased from 20% to 50%, as shown in Figure 1. Without the introduction of rGO, the resultant aggregation composes of ∼2 nm Pt nanocubes (Figure 2d). We attempt to prepare Pt nanocubes supported on carbon black (CB) and TiO2 nanosheets (NSs). However, Pt nanocubes deposited on carbon black exhibit certain degree of aggregation with lower morphological yield for cube shape (Figure 3a), while some worm-like Pt nanocrystals are obtained in the presence of TiO2 NSs (Figure 3c). These observations were confirmed by the different (200)/(111) peak ratios of 51:100 for Pt/CB and 38:100 for Pt/TiO2 NSs from the XRD patterns (Figure 3b, d). These indicate that GO not only function as the support to deposit and disperse Pt nanoparticles but also take part in the formation of Pt nanocube with the assistance of abundant surface functional groups. It is clear that the formation of Pt nanocubes is particularly sensitive to the nature of supports. Similar to the previous reports on the preparation of Pt nanocrystals with well-defined facets,[6c,16] the supports has a significant impact on the formation of the
Figure 3. TEM images and XRD patterns for Pt/CB (a, b) and Pt/TiO2 NS (c, d).
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small 2014, DOI: 10.1002/smll.201302648
One-Pot Synthesis of Platinum Nanocubes on Reduced Graphene Oxide with Enhanced Electrocatalytic Activity
Pt (100) surface dominated Pt nanocubes which are uniformly distributed on the rGO supports. Furthermore, the accelerated cyclic voltammetry tests are carried out to evaluate the electrochemical stability. After 1000 cycles, the featured CV profiles remain similar for Pt nanocubes/ rGO (Figure 4c). On the contrary, commercial Pt/C shows a remarkable loss in active surface area, although they have similar size in the beginning (Figure 4d). With the further electrochemical durability test up to 10 000 cycles, Pt cube/rGO still retains 51.3% of the electrochemical active surface area and the featured CV characteristics, while Pt/rGO and Pt cube/ CB only retain 24.3% and 7.2% of the electrochemical active surface area respectively. The stability improvement over nanoparticle Pt/rGO may indicate that Pt nanoparticles are more prone to dissolution/aggregation due to the presence Figure 4. a) Cyclic voltammograms (CVs) and (b) the calculated mass activity and specific activity of Pt electrocatalysts. c) Electrochemical accelerated durability test of Pt nanocubes/ of more surface defects. The very low starGO and Pt/CB electrocatalysts recorded in 0.5 M H2SO4 solution before and after 1000 cycles bility of Pt cube/CB is mainly attributed at a scan rate of 20 mV s−1. to the more severe support corrosion of carbon black compared to graphene, as specific morphology because of the different surface chem- can be seen from the significant loss of double layer capaciistries of supports and resulting difference in the interaction tance (Figure 4 and S5, see the Supporting Information).[18] The electrochemical activities towards methanol and formic between support and nuclei. The exposure of cubic surface of Pt nanocubes/rGO acid oxidation reactions of the as-prepared Pt nanocubes/ hybrid is also confirmed by the surface sensitive electrochem- rGO hybrid are also evaluated (Figure S6, see the Supporting ical characterization.[17] Cyclic voltammograms (CVs) of Information), where Pt nanocubes/rGO hybrid electrocatathese nanocatalysts in 0.5 M H2SO4 are shown in Figure 4a. lyst again exhibits better activity than other Pt catalysts. We demonstrate a facile one-pot approach to syntheFor comparison, the results from Pt cube/CB, Pt/rGO and commercial Pt/C catalyst are also included. The commer- size Pt nanocubes/rGO hybrids with controlled size and Pt cial Pt/C catalyst only exhibits a hydrogen desorption peak loading mass. The Pt nanocubes/rGO hybrid electrocatalyst at about –0.1 V. In contrast, there is one additional pair of exhibits superior electrochemical performance for oxygen hydrogen adsorption/desorption peaks at ∼ –0.05 V for cubic reduction reaction, methanol and formic acid oxidation reacPt/rGO and cubic Pt/CB catalysts, which corresponds to Pt tions compared with commercial Pt/C catalyst. The enhanced (100) terrace surface. This prominent current peak reflects electrocatalytic activity and stability is attributed to the unithe fact that substantial Pt (100) planes are present on the form dispersion of Pt nanocubes on rGO supports and its surface of Pt nanocubes/rGO hybrid, in agreement with the dominant Pt (100) facets exposed to the reaction medium. TEM and XRD analysis (Figure 1). The calculated electro- The present method provides a feasible approach to prepare chemical surface areas (ECSA) of Pt nanocubes/rGO and shape and size controlled Pt/supports composites, which holds commercial Pt/C catalysts are 51.0 and 60.8 m2 g−1, respec- great potential for electrochemical and catalytic applications. tively. The catalytic activities of different Pt catalysts are next evaluated in terms of oxygen reduction reaction (ORR) at various rotating rates. (Figure S3 and S4, see the Supporting Information). The number of electron calculated from Koutecky-Levich plots is 3.7 and 3.4 for Pt nanocubes/rGO Supporting Information hybrid and commercial Pt/CB, which is similar to Pt/rGO and Pt cube/CB catalysts (Figure S3 and S4, see the Supporting Supporting Information is available from the Wiley Online Library Information), respectively. It indicates that ORR on Pt nano- or from the author. cubes/rGO favors more on four-electron path. Mass activity and specific activity in Figure 4b for Pt nanocubes/rGO are the highest among the four electrocatalysts (13.9 A g−1 Pt and Acknowledgement 0.28 A m−2 at 0.8 V), which are about 2.1 and 2.5 times of that of commercial Pt/C(6.75 A g−1 and 0.11 A m−2), respectively. We acknowledge financial support from the academic research The apparent catalytic enhancement for ORR is attributed to fund AcRF tier 1 (M4011020, RG8/12 and M4010888, RG20/09) small 2014, DOI: 10.1002/smll.201302648
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B. Y. Xia et al.
Ministry of Education, Singapore and competitive research program (2009 NRF-CRP 001–032), National Research Foundation, Singapore.
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Received: August 14, 2013 Revised: September 29, 2013 Published online:
small 2014, DOI: 10.1002/smll.201302648
