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One-Pot and Facile Fabrication of Hierarchical Branched Pt−Cu Nanoparticles as Excellent Electrocatalysts for Direct Methanol Fuel Cells Yanqin Cao,†,‡ Yong Yang,*,† Yufeng Shan,†,‡ and Zhengren Huang† †

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, P.R. China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100049, P.R. China S Supporting Information *

ABSTRACT: Hierarchical branched nanoparticles are one promising nanostructure with three-dimensional open porous structure composed of integrated branches for superior catalysis. We have successfully synthesized Pt−Cu hierarchical branched nanoparticles (HBNDs) with small size of about 30 nm and composed of integrated ultrathin branches by using a modified polyol process with introduction of poly(vinylpyrrolidone) and HCl. This strategy is expected to be a general strategy to prepare various metallic nanostructures for catalysis. Because of the special open porous structure, the as-prepared Pt−Cu HBNDs exhibit greatly enhanced specific activity toward the methanol oxidation reaction as much as 2.5 and 1.7 times compared with that of the commercial Pt−Ru and Pt−Ru/C catalysts, respectively. Therefore, they are potentially applicable as electrocatalysts for direct methanol fuel cells. KEYWORDS: Pt−Cu, hierarchical branches, one-pot synthesis, electrocatalysis

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INTRODUCTION In recent years, Pt-based alloy nanostructures have been widely studied as promising candidates of pure Pt for catalysis in various fields, especially in direct fuel cells (DFCs).1−5 By alloying with inexpensive transition metals (Cu, Fe, Co, Ni, etc.),3,6−12 the formation of bimetallic alloys not only decreases the loading amount of Pt and thus reduces the cost as catalysts but also could improve the catalytic properties due to possible synergetic effects.13−18 Compared with the synthesis of singlecomponent metal nanocrystals, the coexistence of two precursors will complicate the synthesis process, especially in the one-pot synthetic strategy.19 Many efforts have been devoted to the morphologycontrolled synthesis of Pt-based bimetallic alloys since this parameter determines the size, facets, and the proportions of active sites, and thus could effectively tailor the catalytic properties.20−22 In particular, nanodendrites have attained extensive interests serving as catalysts owing to their attractive structural features such as porosity, rich edges and corners, and high surface area.23−31 Interestingly, in addition to these advantages, dendritic-like structures with hierarchical branches also possess three-dimensional (3D) interconnected networks composed of ultrathin branches, which will be helpful for electron transfer in the reaction and lead to better catalytic performance.32−34 However, to date, rare reports are available on the bimetallic hierarchical branched nanoparticles with 3D branched networks, especially with small sizes (below 50 nm) © 2016 American Chemical Society

as well as ultrathin branches (below 5 nm), so it remains a great challenge to synthesize this special nanostructure by a facile method. Herein, we provided a facile modified polyol process to successfully synthesize Pt−Cu hierarchical branched nanoparticles (HBNDs) in which poly(vinylpyrrolidone) (PVP) and HCl have been introduced to serve as the shape-control agents. The as-prepared hierarchical Pt−Cu HBNDs have 3D open porous structure and are composed of integrated ultrathin branches, which have an average width of ∼3 nm. The overall size of the HBND is on the order of about 30 nm. The Pt−Cu HBNDs are formed through the anisotropic growth induced by twinned defects in seeds. These nanoparticles exhibit greatly improved specific activity for methanol oxidation reaction (MOR) as much as 2.5 and 1.7 times in comparison with that of commercial Pt−Ru and Pt−Ru/C catalysts.

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RESULTS AND DISCUSSION

Figure 1a,b shows the representative transmission electron microscopy (TEM) images of the as-prepared Pt−Cu HBNDs obtained at 60 min, which are highly dispersive and have hierarchical structures with ultrathin branches of an average Received: November 24, 2015 Accepted: February 17, 2016 Published: February 17, 2016 5998

DOI: 10.1021/acsami.5b11364 ACS Appl. Mater. Interfaces 2016, 8, 5998−6003

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ACS Applied Materials & Interfaces

about 0.22 nm, corresponding to the {111} planes, as shown in HRTEM image in Figure 3c. Then the arms grew along the [110] direction and generated the pentapod-like structure, as shown in Figure 3d. The lattice spacing of parallel fringes was about 0.22 nm, corresponding to the {111} planes, as shown in the high-resolution TEM (HRTEM) image in Figure 3d-1, 3d2, and 3d-3. It is believed that the formation of pentapods is due to the inhomogeneous growth in the defect-enriched region of primary seeds. The arms are single-crystalline according to the fast Fourier transform (FFT) pattern in Figure 3d-1-1. As reaction continues, secondary branches were formed on the backbones along the [110] direction, as shown in the HRTEM image in Figure 3e and the FFT pattern (the inset of Figure 3e-1). The lattice spacing of parallel fringes was about 0.22 nm, corresponding to the {111} planes, as shown in HRTEM images in Figure 3e-1 and 3e-2. It is known that the reduction potential of Cu2+/Cu0 (0.340 V vs RHE) is smaller than that of PtCl4−/Pt0 (0.758 V vs RHE); therefore, some copper ions in the solution may be reduced and reoxidized at varying instances. They may serve as deposition or nucleation sites on the preformed pentapods, and thus lead to the formation of secondary branches, which is consistent with the previously reported Cu2+-assisted synthesis of Pd tripods,35 Pd nanorods,36 and hexoctahedral Au−Pd alloy nanocrystals.37 Similarly, more branches (secondary branches, tertiary branches, etc.) were formed on the backbones and developed outward with further elongation of the backbones, as shown in Figure 3f. According to the HRTEM image of the backbone in Figure 3f-1, the lattice spacing of parallel fringes was about 0.22 nm, corresponding to the {111} planes. Finally, the Pt−Cu HBNDs (Figure 3g) were formed due to the formation and growth of hierarchical branches. In the process, PVP and HCl were investigated as important factors to successfully obtain the hierarchical branched structure because they tune the nucleation and growth process. PVP is a common surfactant used in the preparation of noble-metal

Figure 1. (a) TEM image of Pt−Cu HBNDs obtained at 60 min. (b) Magnified TEM image. The inset in Figure 1b shows the corresponding SAED pattern of a single Pt−Cu HBND.

width of ∼3 nm. The selected area electron diffraction (SAED) pattern in Figure 1b displays that the HBNDs are polycrystals. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM)-energy dispersive spectrum (EDS) elemental mapping images (Figure 2a) demonstrate that these hierarchical nanoparticles are composed of Pt and Cu, and the Pt and Cu atoms are uniformly distributed in the whole nanoparticles. The EDS in Figure 2b reveals that the atomic ratio between Pt and Cu is about 4:1. Moreover, as shown in Figure 2c, the X-ray diffraction (XRD) pattern of the Pt−Cu HBNDs displays characteristic peaks, which can be indexed as a face-centered cubic (fcc) structure. The peaks of Pt−Cu HBNDs were shifted toward the high-angle direction and lie between those of pure fcc Pt (JCPDS 04-0802) and pure fcc Cu (JCPDS 04-0836), confirming the formation of an alloyed structure. To elucidate the formation process of the as-prepared Pt−Cu HBNDs, a series of TEM images have been taken for the samples obtained at different reaction stages, as shown in Figure 3. Primary seeds with multiply twinned defects (mainly fivefold twinned defects) were first formed, and these seeds mainly showed decahedral shape with an average size of 6.8 nm, as shown in Figure 3a. The lattice spacing of parallel fringes was

Figure 2. (a) HAADF-STEM image and the corresponding elemental mapping images showing the distribution of Pt (yellow) and Cu (red) for Pt− Cu HBNDs. (b) EDS elemental analysis. (c) XRD spectrum. 5999


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Figure 3. (a) TEM image and (c−e) HRTEM images of Pt−Cu nanoparticles obtained at 10 min. (d-1, d-2, d-3), (e-1, e-2) Corresponding magnified HRTEM images in the selected sections in parts (d) and (e), respectively. (d-1-1) Corresponding FFT pattern of the selected section in part (d-1). The inset in part (e-1) shows the corresponding FFT pattern. (b, f) TEM images of Pt−Cu nanoparticles obtained at 20 min. (f-1) Corresponding magnified HRTEM image in the selected section in part (f). (g) TEM image of Pt−Cu nanoparticles obtained at 60 min.

growth of the main backbones as well as those branches and thus to obtaining homogeneous nanoparticles. HCl is another important additive to the formation of final hierarchical bimetallic structure. When HCl was excluded from the synthesis, tetrapods without secondary branches were formed, as shown in Figure 5a. If HCl was substituted by HBr, only spherical nanoparticles were obtained (Figure 5b), indicating that Cl− is necessary to the formation of branched nanostructure in this process. In addition, tripods or tetrapods have been obtained by replacing HCl with KCl (Figure 5c), implying that H+ is needed for the growth of different hierarchical branches and thus the final Pt−Cu HBNDs. Therefore, HCl is essential to get the hierarchical branched structure in the process. Furthermore, HCl has been reported to play particular functions in tuning the formation of branches due to oxidative etch,38 which would result in providing active sites for the growth of branches. Branched nanoparticles with fewer branches have been obtained when adding 75 mg of HCl

nanostructures, serving as a stabilizing agent and shape-control agent. In the absence of PVP, hierarchical nanoparticles could not be formed (Figure 4a), indicating that PVP is essential to the formation of the branched structure. When 0.01875 M PVP is added, inhomogeneous nanoparticles with different shapes (small multipods, polyhedrons, and branched nanoparticles) were formed, as shown in Figure 4b. This may be attributed to the PVP not being enough to absorb on the special {100} facets, and thus affect the homogeneous anisotropic growth. When the concentration of PVP is 0.0375 M, homogeneous hierarchical branched nanoparticles were obtained, as shown in Figure 4c. And if the concentration was increased to be 0.075 M, the greater concentration leads to excessive absorption on the special facets in the growth of some primary seeds, and thus results in larger size range than that of products obtained at 0.0375 M, as shown in Figure 4d. Therefore, appropriate concentration of PVP is of great importance to the anisotropic 6000


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Figure 6. CV curves of Pt−Cu HBNDs and commercial Pt−Ru (JM) and Pt−Ru/C (JM) in 0.5 M H2SO4 solution. The scan rate was 50 mV/s. Figure 4. TEM images of Pt−Cu nanoparticles obtained by adding PVP with different concentrations: (a) without PVP, (b) 0.01875 M PVP, (c) 0.0375 M PVP, and (d) 0.075 M PVP.

higher ECSA of Pt−Ru/C may result from their much smaller size of the loaded Pt−Ru nanoparticles. These Pt−Cu HBNDs with low precious metal loading and high surface area should be very desirable for electrocatalysis. We therefore examined their electrocatalytic activities toward methanol electro-oxidation with the aim of evaluating their potential for important energy-conversion technologies, such as direct methanol fuel cells. To compare the activity, the currents were normalized with respect to both the ECSA and the loading amount of the catalysts. Comparison of specific activities and mass activities between Pt−Cu HBNDs and commercial Pt−Ru (JM) and Pt−Ru/C (JM) catalysts are shown in Figure 7a and Figure S3, which show the specific shape of methanol electro-oxidation in the acid solution. For specific activities in the oxidation of methanol, the current density of Pt−Cu HBNDs (1.26 mA/cm2) is 2.5-fold and 1.7fold of that of Pt−Ru (0.50 mA/cm2) and Pt−Ru/C (0.74 mA/ cm2), respectively, which is also much higher than that of the reported Pt−Cu bimetallic nanodendrites (1.2-fold for specific activity,29,31,32 1.2-fold for specific activity,29,31,32 and 1.2-fold for specific activity29,31,32). As shown in Figure S3, the mass activity of Pt−Cu HBNDs (697.58 mA/mgPt) is 5.1 and 1.1 times that of commercial Pt−Ru (135.99 mA/mgPt) and Pt− Ru/C (609.57 mA/mgPt), respectively, indicating that lower cost in practical applications could be achieved by using Pt−Cu HBNDs due to its higher mass activity. The extensively enhanced catalytic activity of the Pt−Cu HBNDs could be ascribed to its unique structure with two points. First, the threedimensional open porous structure together with high ECSA and an ultrathin interconnected network provides excellent electron conductivity and also facilitates mass transfer. Second, the unique hierarchical bimetallic structure may allow greatly enhanced catalytic reactivity as a result of possible electronic, strain, or alloy effects. Chronoamperometric (CA) measurements were performed at 0.62 V (Figure 7b), which confirmed the HBNDs had a higher current density during the whole period. And after 1000s, the current densities of Pt−Cu HBNDs, Pt−Ru, and Pt−Ru/C decreased 84.1%, 96.3%, and 98.6%, respectively, illustrating that Pt−Cu HBNDs possessed better stability than the commercial catalysts. Besides, the durability was further investigated by the potential cycling treatment performed in 0.5 M H2SO4 solution for 500 cycles at room temperature. Calculated from the CV curves in Figure 7c, the ECSA of the Pt−Cu HBNDs (from 51.6 m2/g to 28.5 m2/g) after 500

Figure 5. TEM images of Pt−Cu nanoparticles obtained by adding (a) without HCl, (b) HBr, (c) KCl, and (d) 75 mg of HCl.

(Figure 5d), while hierarchical branched Pt−Cu nanoparticles were obtained when adding 150 mg of HCl (Figure 4c). Interestingly, in addition to the Pt−Cu HBNDs, we have successfully prepared mesoporous Pt and Pd nanoparticles,39 Pt−Pd (Figure S1a of the Supporting Information) and Pt−Ni (Figure S1b) bimetallic nanodendrites by a similar modified polyol process with introduction of PVP and HCl. Therefore, this strategy is expected to be universally used to prepare various Pt-based nanostructures and play an important role in the facial fabrication of excellent practical electrocatalysts. The electrochemically active surface area (ECSA) can reflect the amount of available active sites, and it can be determined from the electric charges of hydrogen adsorption and desorption by cyclic voltammetry (CV) in an aqueous solution of H2SO4 (0.5 M). For comparison, commercial Pt−Ru (JM) and Pt−Ru/C (JM) were measured under the same condition. Calculated from CV curves shown in Figure 6, the ECSAs of the Pt−Cu HBNDs, commercial Pt−Ru (JM), and Pt−Ru/C (JM) were 51.6, 17.3, and 16.0 m2/g, respectively, and were 55.8, 28.8, and 80.0 m2/gPt, respectively, by normalizing in reference to the content of platinum. The Pt−Cu HBNDs possess more active sites than commercial Pt−Ru, while the 6001


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Figure 7. (a) Specific activities of Pt−Cu HBNDs and commercial Pt−Ru (JM) and Pt−Ru/C (JM) normalized by the ECSAs for methanol oxidation reaction (MOR). (b) Chronoamperometric curves at 0.62 V for MOR. (c) CV curves after cycling treatment in 0.5 M H2SO4 solution. (d) Specific activities of Pt−Cu HBNDs after 500 cycles and commercial Pt−Ru (JM) and Pt−Ru/C (JM) for MOR. The scan rate was 50 mV/s.

cycles. Calculated from the CV curves in Figure S2a, the ECSA of Pt−Ru (from 17.3 m2/g to 4.6 m2/g), and from Figure S2b, the ECSA of Pt−Ru/C (from 16.0 m2/g to 7.2 m2/g). Therefore, the as-prepared Pt−Cu HBNDs show better durability. After 500 CV cycles, the Pt−Cu HBNDs still maintain the dendritic structure (Figure S4), and still showed higher specific activity as much as 2.45 and 1.68 times compared with that of commercial Pt−Ru and Pt−Ru/C, respectively, as shown in Figure 7d. Therefore, the as-prepared Pt−Cu hierarchical branched nanoparticles possess greatly enhanced catalytic activity and excellent durability.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by a fund from the National Natural Science Foundation of China (No. 51471182 and No. 51572276).

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CONCLUSION In summary, Pt−Cu hierarchical branched nanoparticles with small size of ∼30 nm have been synthesized by a modified polyol process. In the process, PVP and HCl, serving as the stabilizing agent and shape-control agent, were added to tune the formation of hierarchical branches. Because of the unique 3D open porous structure composed of integrated ultrathin branches, the as-prepared Pt−Cu HBNDs display greatly improved catalytic activity toward methanol oxidation reaction compared with that of commercial Pt−Ru and Pt−Ru/C. Moreover, this work provides a versatile approach for fabricating hierarchical branched bimetallic nanostructures and also demonstrates the importance of designing this special structure for potential applications in the fields of clean energy and green chemistry.

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Experimental details, supplementary TEM figures, CV curves, and electrochemical measurements (PDF)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11364. 6002


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