Ultrathin Pt−Zn Nanowires: High-Performance ... - ACS Publications

Dec 6, 2017 - College of Chemical Engineering, Beijing University of Chemical Technology, 15 BeiSanhuan East Road, ChaoYang District, Beijing. 100029 ...
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Ultrathin Pt-Zn nanowires: high-performance catalysts for electrooxidation of methanol and formic acid jia jing pei, Junjie Mao, Xin Liang, Zhongbin Zhuang, Chen Chen, Qing Peng, Dingsheng Wang, and Yadong Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03234 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 8, 2017

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Ultrathin Pt-Zn nanowires: high-performance catalysts for electrooxidation of methanol and formic acid Jiajing Peia, Junjie Maob, Xin Lianga, Zhongbin Zhuang a, Chen Chenb, Qing Pengb, Dingsheng Wang*b and Yadong Lib a

College of Chemical Engineering, Beijing University of Chemical Technology, 15 BeiSanhuan

East Road, ChaoYang District, Beijing 100029, China b

Department of Chemistry, Tsinghua University, Hai Dian District, Beijing 100084, China

+

J.J.P. and J.J.M. contributed equally to this work.

* E-mail: [email protected] KEYWORDS: Pt-Zn NWs, electrocatalytic performance, MOR, FAOR, trimetallic

ABSTRACT: Herein, we have developed a Zn2+ ion induced reduction strategy (ZIRS) for synthesizing ultrathin Pt-Zn nanowires (NWs) with average size of ~2.2 nm. The precise selection of solvent and the addition of ZnCl2 precursor are of crucial significance to preparing bimetallic NWs. The NWs exhibited electrocatalytic performance compared with commercial

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catalysts, such as electrooxidation of methanol (MOR) and formic acid (FAOR). Moreover, the prepared Pt-Zn NWs can be adopted as a template to prepare Pt-based trimetallic ultrathin NWs.

INTRODUCTION Direct liquid fuel cells (DLFC) is a promising clean energy source, exhibiting high efficiency in energy conversion, prominent sustainability, and low environmental contamination for electronic devices and electrical vehicles.1-4 Intense research efforts concentrated on direct methanol fuel cells (DMFC) and direct formic acid fuel cells (DFAFC) arising from the low-cost and renewable characteristic of methanol and formic acid.5,6 Anodic catalyst with high stability towards long time cell operation is of crucial significance for DLFC.7,8 Thus far, Pt-based electrocatalysts, such as commercial Pt/C, are acknowledged as the optimal electrocatalysts.9-13 Yet, DLFC remains to be a challenge as being commercialized due to the high economic cost, sluggish reaction kinetics and low durability of commercial Pt-based electrocatalysts.14-17 One effective way to decrease the depletion of Pt is to alloy Pt with earth-abundant transition metal. This could reduce the usage of Pt and tune the surface electronic structure of Pt. Pt-based alloys with one-dimensional (1D) nanostructure have aroused extensive and massive concern regarding electrocatalysis because they possess high surface area, high surface electron conductivity, defect-rich surface and distinctive quantum effects compared with bulk materials.18-22 The ultrathin nanowires (NWs) represent the promising candidate catalysts in virtue of their inherent anisotropic morphology. High coherent area between NWs and the carbon substrate not only improves electron transfer between Pt surface and support, but also enhances stability during electrochemical reaction process.23-25 For instance, Guo and co-worker reported trimetallic FePtPd alloy NWs with excellent electrocatalytic performance towards

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electrooxidation of methanol (MOR).26 Huang et al. demonstrated that bimetallic intermetallic Pt-Co NWs with high-index facets enable the marked electrocatalytic activity towards oxygen reduction reaction (ORR).27 Thus, the preparation of Pt-based NWs structure with optimized Pt atom utilization and high electrocatalytic performance shall be urgently required. To date, Pt-based NWs have been achieved by several methods.28,29 For example, Lee and co-worker prepared ultrathin CuPt3 wavy nanowires by oriented attachment of amine-terminated poly(N-isopropylacrylamide) functionalized CuPt3 nanoparticles.30 Yu et al. offered a new tactic to synthesize ultrathin Pd@Pt core-shell NWs originated from Br- ions by galvanic replacement reaction between PtCl62− with pre-prepared Pd NWs. The Br- ions acted as capping and etching agents, which balanced the deposition of Pt and the etching rate of Pd NWs to form ultimate bimetallic core−shell NWs.31 However, Pt-based ultrathin NWs electrocatalysts with high aspect ratio as well as Zn dopant are rarely reported.32-35 Here, we report the synthesis of bimetallic ultrathin Pt-Zn NWs based on a Zn2+ ion induced reduction strategy (ZIRS). The highly uneven Pt-rich surfaces of Pt-Zn NWs are different from the reported Pt-based NWs. Moreover, the as-obtained bimetallic ultrathin Pt-Zn NWs exhibited the enhanced electrocatalytic performance towards the MOR and FAOR in comparison with commercial Pt/C and Pt black catalysts. Furthermore, the as-prepared Pt-Zn NWs can be adopted as a template for preparing Pt-based trimetallic ultrathin NWs. RESULTS AND DISCUSSION The ultrathin Pt-Zn NWs were synthesized based on ZIRS. Typically, K2PtCl4 and ZnCl2 were added into oleylamine (OAm) and ethanol. On that basis, the homogeneous solution was then sealed in a Teflon-lined autoclave and heated at 160 oC for 10 h. Figure 1 shows the structural

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characterization of the as-synthesized Pt-Zn ultrathin NWs. The as-obtained Pt-Zn NWs with length up to several micrometers are shown in the TEM (Figure 1a and Figure S1) and highangle annular dark-field (HAADF) STEM image (Figure 1b). The selected area electron diffraction (SAED) image performs that the NWs are polycrystalline. Figure 1c demonstrates that the average diameter of Pt-Zn NWs is 2.2 nm. The lattice fringes with regular distance of 0.217 nm are well matched with the interplanar distance of (111) facets of Pt-Zn NWs. The mapping profile (Figure 1d) of energy dispersive X-ray spectroscopy (EDS) manifests elements Pt and Zn in bimetallic Pt-Zn NWs with homogeneous distribution. The Pt and Zn atomic content (95.4% of Pt and 4.6% of Zn) determined by EDS spectrum (Figure S2) is consistent with that by ICP-MS (94.2% of Pt and 5.8% Zn). The as-prepared bimetallic Pt-Zn NWs appear an evidently face-centered cubic (fcc) pattern, confirmed by XRD spectrum (Figure S3). The XRD pattern of NWs shows small shift to high angle compared with corresponding Pt standard card, suggesting that small amount of Zn is alloyed with Pt. To further investigate the element valence of Pt-Zn NWs, X-ray photoelectron spectra (XPS) experiments were conducted. The Zn 2p and Pt 4f spectra of Pt-Zn NWs show that Zn on the surface existed in the form of oxidation and Pt maintained metallic state (Figure S4).

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Figure 1. Structural characterization of as-synthesized Pt-Zn ultrathin NWs. (a) TEM image, (inset shows corresponding magnification picture) (b) HAADF-STEM, (inset shows the SAED image) (c) HRTEM image, (d) EDS elemental maps for Pt (orange) and Zn (red). To illuminate the growth mechanism of bimetallic Pt-Zn ultrathin NWs in this work, we have minutely recorded the growth intermediates under different reaction stages. Pt-Zn NWs synthesized under different reaction stages are shown in Figure 2. The starting reaction time was 40 min, and merely a small amount of product could be obtained. The nanoparticle with average size of ~1.6 nm could be expressly observed at Figure 2a. When the reaction time was about 50 min, the emergence of Pt-Zn NWs which were formed by oriented attachment of the aggregates

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was observed at Figure 2b. The average diameter of Pt-Zn NWs is 1.8 nm, which is similar to those of the coexisting aggregates. As the reaction time extend to 1 h, the particle aggregates developed ceaselessly into 1D nanostructures. No obvious structure changes were observed (Figure 2c). With the time further increased to the final reaction time of 10 h, NWs with average diameter of ~2.2 nm and high aspect ratios were achieved (Figure 2d). The diameter distribution of the NWs and the reduction rate of K2PtCl4 during the reaction were shown in Figure 2e and 2f. In accordance with the foregoing experimental results, three clear growth processes can be described in the following: (a) The starting reaction time was 40 min, only particle aggregates with a diameter of ~1.6 nm on the average were found. This is because of persistent reduction of Pt precursors, followed by the growth of initial nuclei. (b) After the reaction proceeded to 50 min, with the continuous consumption of the precursors, the particle oriented-attachment was occurred. (c) With the reaction time constant increasing, the ultrathin NWs were formed ultimately. During the process of oriented-attachment, due to the correspondingly low energy barrier, the Pt or Zn atoms near the joints diffuse along the NWs by means of an Oswald ripening process, resulting in an uneven surface at the high synthesis temperature.30,36

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Figure 2. TEM images of the growth intermediates recorded under different reaction stages: (a) 40 min, (b) 50 min, (c) 1 h, (d) 10 h. (e) The function relationship between the NWs diameter distribution and reaction time. (f) Reduction percentage of K2PtCl4 during the reaction. Previous findings demonstrated that oriented-attachment is possible owing to the capping agent adsorbed to a particular facet.37 In this work, the connection-oriented of Pt-Zn NWs may arise from the prior adsorption of OAm on {100} and {110} planes, resulting in the growth of NWs along the {111} plane. To further prove this assumption, we conducted the controlled experiments below. When we replaced OAm with other solvents (i.e. DMF, benzyl alcohol, octylamine), the 1D nanowire structure could not be found under TEM (Figure S5a, b and c). The above result suggests that OAm was of vital importance in the morphology evolution of NWs. In addition, we proclaim the preparation of ultrathin Pt-Zn NWs by using the ZIRS for the first time. Previous results clearly stated that Zn2+ ion is of crucial significance for forming anisotropic and faceted particles.33,38 Without the addition of ZnCl2, only Pt nanodendrites and many irregular nanocrystals were achieved (Figure S5d). Zn2+ ion is suggested to induce the

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anisotropic growth of Pt-Zn NWs. We also optimized the synthesis of other parameters, to form the Pt-Zn NWs. Through replacing K2PtCl4 with H2PtCl6, mixture of solid nanocrystals and NWs was obtained (Figure S6a). When K2PtCl4 was replaced with Pt(acac)2, no NWs were observed (Figure S6b). In the meantime, the shape selectivity of nanowires was also found to be very sensitive to the substitution of ZnCl2 precursor. When ZnCl2 was instead by Zn(OAc)2 or Zn(NO3)2, a mixture of nanoparticles and NWs was formed (Figure S6c and d). Therefore, the selection of appropriate precursor is of crucial significance for the well-controlled formulation of Pt-Zn ultrathin NWs. To evaluate the electrocatalytic performance of Pt-Zn NWs, the methanol electrooxidation was initially selected as the probe reaction. The MOR measurements were conducted in Arsaturated 0.5 M H2SO4 + 0.5 M CH3OH electrolyte with sweep rate of 50 mV s-1. The region of hydrogen adsorption and desorption in the cyclic voltammetry (CV) curve was explicitly observed (Figure S7). Calculating specific current density must be normalized to the electrochemical active surface area (ECSA). The as-prepared and commercial catalysts presented well-separated proper peaks relative to pure Pt in both forward and reverse sweeps. Specific and mass activity of the Pt-Zn NWs catalysts are obviously preceded the commercial Pt/C and Pt black, suggesting that 1D bimetallic nanostructure significantly enhances the electrochemical performance of the catalyst. The onset potential of Pt-Zn NWs is 290 mV. It is much lower than the equivalent of commercial Pt/C (433 mV), and Pt black (440 mV) catalyst, suggesting better accelerated methanol oxidation in the Pt-Zn NWs. The specific activity of Pt-Zn NWs in MOR reached 3.48 mA cm−2 (Figure 3a). It is 7.8 times and 10.5 times (Figure 3b) higher than Pt/C (0.455 mA cm−2) and Pt black (0.329 mA cm−2). Similarly, the mass activity of Pt-Zn NWs reached 1005.3 mA mg-1Pt, which is 25.4 and 3.15 times higher than comparative Pt black (39.5

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mA mg-1Pt) and Pt/C (318.2 mA mg-1Pt) catalysts, respectively. The Pt-Zn NWs catalysts also performed high stability under the electrochemical measurement condition. To evaluate their MOR durability, chronoamperometry (CA) curves were conducted for 1500s at 0.5 V versus SCE (Figure 3c). Specifically, Pt-Zn NWs exhibited remarkable stability than the corresponding commercial Pt black and Pt/C catalysts.39,40

Figure 3. (a) Specific activities of as-obtained Pt-Zn NWs, Pt black and Pt/C catalysts occurred in 0.5 M H2SO4 and 0.5 M CH3OH electrolyte. (b) Comparison of specific and mass activities of the catalysts. (c) Chronoamperometry curves of the catalysts (0.5 V versus SCE). (d) CO stripping curves for three catalysts. (e) Specific activities of three catalysts carried out in 0.1 M

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HClO4 and 0.5 M HCOOH electrolyte. (f) Comparison of specific and mass activities of the catalysts. To further illuminate the high performance of Pt-Zn NWs, CO stripping experiments were conducted, respectively (Figure 3d). According to the experimental results, we achieve that the peak and onset potentials of Pt-Zn NWs samples shifted left in comparison with commercial Pt/C and Pt black catalysts, demonstrating that CO molecules on Pt-Zn NWs can be availably oxidized and discharged at 0.50 V versus SCE under the electrocatalysis. The Pt-Zn NWs catalysts also performed better activity and stability in FAOR compared with the corresponding counterparts. The CV curves of three electrocatalysts have the same scan rate as MOR (Figure S8). Specific and mass activity of the catalysts were presented in Figure 3e and Figure 3f. It is obvious that the Pt-Zn NWs catalysts indicate higher specific and mass activity than commercial catalysts. The onset potential of Pt-Zn NWs in FAOR is 508 mV versus SCE, which is much lower than corresponding Pt/C (590 mV) and Pt black (581 mV) catalysts. This indicates preferential oxidation of formic acid in the Pt-Zn NWs catalysts. The specific and mass activity of Pt-Zn NWs catalysts in FAOR are achieved to 3.9 mA cm-2 and 1387 mA mg-1Pt, which were 7.5 (4.7)-fold that of Pt black and 6.8 (2)-fold that of the Pt/C catalysts, respectively. In order to evaluate the stability of Pt-Zn NWs toward FAOR, CA curves were tested for 1500s (Figure S9). We also found that the Pt-Zn NWs can display better durability than Pt/C and Pt black toward FAOR. After electrocatalysis, the morphology of Pt-Zn NWs remained unchanged (Figure S10). Compared with previous studies, the electrocatalytic performance of Pt-Zn NWs exceeded most of the reported catalysts.41-48 Moreover, 1D trimetallic NWs can be prepared through employing bimetallic Pt-Zn NWs as the template. Trimetallic Pt-Zn-Au and Pt-Zn-Pd NWs could be obtained simply through adding

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Au and Pd precursor followed by further reduction (see Supporting Information for detail). As shown in HETEM images (Figure 4a), the as-obtained NWs remain unchanged in morphology after introducing of Au. EDS elemental maps (Figure 4b) indicate that Au element could be favorably incorporated into bimetallic NWs system. The HETEM images and EDS elemental maps of Pt-Zn-Pd NWs were shown in Figure 4c and 4d. Using bimetallic Pt-Zn NWs as the template was demonstrated to be a rational approach for preparing trimetallic ultrathin NWs.

Figure 4. (a) HAADF-STEM image of the Pt-Zn-Au trimetallic NWs, (b) EDS maps showing distributions of Pt (yellow), Zn (red) and Au (blue), (c) HAADF-STEM image of the Pt-Zn-Pd trimetallic NWs, (d) EDS elemental maps for Pt (cyan), Zn (red) and Pd (green). CONCLUSIONS

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In summary, we successfully developed ZIRS for synthesizing 1D bimetallic ultrathin Pt-Zn NWs with diameter distribution of ~2.2 nm and up to a few micrometers in length. The use of OAm and appropriate Zn precursor was demonstrated to be the most critic effects for preparing Pt-Zn NWs. Morphology evolution study of product by TEM studies demonstrated the formation of Pt-Zn NWs by oriented attachment mechanism. The as-prepared Pt-Zn NWs exhibited robust electrocatalytic activity and substantially enhanced stability towards MOR and FAOR compared with commercial catalysts. We could further prepare trimetallic NWs (such as Pt-Zn-Au and PtZn-Pd) by using bimetallic NWs as template. This work can provide a new pathway for preparing electrocatalyst with evident electrocatalytic performance, which is promising in related areas and potential applications. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Experimental section, TEM image, XRD pattern, EDS spectrum, XPS spectrum, CV curves, i-t curves. (PDF) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions J.J.P. and J.J.M. contributed equally to this work. Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENT This research was supported by the China Ministry of Science and Technology under Contract of 2016YFA (0202801), the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (201321) and the National Natural Science Foundation of China (21521091, 21390393, 21471089, 21671117, U1463202). REFERENCES (1) Service, R. F. Shrinking Fuel Cells Promise Power in Your Pocket. Science 2002, 296, 1222–1224. (2) Steele, B. C. H.; Heinzel, A. Review article Materials for fuel-cell technologies. Nature 2001, 414, 345–352. (3) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs. Appl. Catal. B. 2005, 56, 9–35. (4) Chen, W.; Chen, S. W. Oxygen Electroreduction Catalyzed by Gold Nanoclusters: Strong Core Size Effects. Angew. Chem., Int. Ed. 2009, 121, 4450–4453. (5) Dillon, R.; Srinivasan, S.; Arico, A. S.; Antonucci, V. J. International activities in DMFC R&D: status of technologies and potential applications. J. Power Sources. 2004, 127, 112–126. (6) Koenigsmann, C.; Wong, S. S. One-dimensional noble metal electrocatalysts: a promising structural paradigm for direct methanol fuel cells. Energy Environ. Sci. 2011, 4, 1161–1176.

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(30) Fu, G.; Yan, X.; Cui, Z.; Sun, D.; Xu L.; Tang, Y.; Goodenough J. B.; Lee, J.-M. Catalytic activities for methanol oxidation on ultrathin CuPt3 wavy nanowires with/without smart polymer. Chemical Science 2016, 7, 5414–5420. (31) Li, H.; S.; Fu, Q.; Liu, X.; Wu, L.; Yu, S. Scalable Bromide-Triggered Synthesis of Pd@Pt Core−Shell Ultrathin Nanowires with Enhanced Electrocatalytic Performance toward Oxygen Reduction Reaction. J. Am. Chem. Soc. 2015, 137, 7862–7868. (32) Simon, J.; Protasenko, V.; Lian, C.; Xing H.; Jena, D. Polarization-Induced Hole Doping in Wide–Band-Gap Uniaxial Semiconductor Heterostructures. Science 2010, 327, 60–64. (33) Chen, Q.; Zhang, J.; Jia, Y.; Jiang, Z.; Xie, Z.; Zheng, L. Wet chemical synthesis of intermetallic Pt3Zn nanocrystals via weak reduction reaction together with UPD process and their excellent electrocatalytic performances. Nanoscale 2014, 6, 7019–7024. (34) Chen, A.; Holt-Hindle, P. Platinum-based nanostructured materials: synthesis, properties, and applications. Chem. Rev. 2010, 110, 3767–3804. (35) Guo, S.; Zhang, S.; Sun, S. Tuning Nanoparticle Catalysis for the Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2013, 52, 8526–8544. (36) Yu, X.; Wang, D.; Peng, Q.; Li Y. PtM (M=Cu, Co, Ni, Fe) Nanocrystals: From Small Nanoparticles to Wormlike Nanowires by Oriented Attachment. Chem. Eur. J. 2013, 19, 233– 239

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TOC

We synthesized ultrathin Pt-Zn nanowires, which exhibit superior performance towards electrocatalysis such as MOR and FAOR.

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