Engineering Surface Structure of Pt Nanoshells on Pd Nanocubes to

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Engineering surface structure of Pt nanoshells on Pd nanocubes to preferentially expose active surfaces for ORR by manipulating the growth kinetics Weicong Wang, Xiang Li, Tianou He, Yaming Liu, and Mingshang Jin Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b04735 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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Engineering surface structure of Pt nanoshells on Pd nanocubes to preferentially expose active surfaces for ORR by manipulating the growth kinetics

Weicong Wang,† Xiang Li,‡ Tianou He,† Yaming Liu,† and Mingshang Jin*,†

†Frontier

Institute of Science and Technology and State Key Laboratory of

Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China. ‡Institute

of Advanced Electrochemical Energy and School of Materials Science and

Engineering, Xi'an University of Technology, Xi'an, Shaanxi 710048, China.

*Address correspondence to [email protected]

KEYWORDS. Pt nanocages, kinetic control, oxygen reduction reaction, Pd nanocubes, surface diffusion

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ABSTRACT. Synthesis of Pt nanoshells on substrates can increase the utilization efficiency of Pt atoms and reduce the amount of Pt used in the applications. However, it is still an enormous challenge in tailoring the required crystal facets of Pt nanoshells on a given substrate. In this work, we demonstrate a facile and convenient approach capable for generating Pt octahedral islands with tunable sizes and densities on Pd nanocubes by manipulating the deposition rate. The key to this synthesis is the fine control over the deposition rate of Pt on Pd seeds. Due to the different reactivities at the surface sites, the deposition of Pt can be controlled at a certain site by carefully tuning the deposition rate. With a low concentration of reductant (8.33 mg/mL of glucose), surface diffusion dominates the process, and thus the Pt cubic shells form on Pd cubic seeds. In contrast, when a higher amount of the reductant (16.67 mg/mL of glucose) is added, the deposition starts to dominate the growth of Pt shells. In this case, the deposition would be controlled at the corners, forming eight large Pt octahedra on a cubic Pd seed. Further increasing the deposition rate can induce much higher deposition rates, in which case, the deposition of Pt would like to take place not only at the corners, but also the edge and surface sites of the seeds. Not surprisingly, this growth habit can result in the formation of high-density octahedral islands on Pd cubic seeds. With the same amount of precursor supply, the higher densities of Pt islands, the smaller the size of the octahedral islands on Pd nanocubes. Unlike other synthetic methods, the size of the octahedral islands on Pd seeds can be even controlled to be smaller than 3 nm by controlling the amount of the Pt precursor. Considering the excellent performance of {111} facets of Pt catalysts toward ORR, 2


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the Pt nanocages with small octahedral islands on the surfaces can exhibit a high activity, with a mass activity 0.68 A/mg, as high as 5.2 times of that of commercial Pt/C.

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Platinum (Pt) is an important catalyst invaluable to both cathode and anode reactions in fuel cells.1-5 Since Pt is extremely expensive, it is necessary to increase the utilization efficiency of Pt atoms by engineering their structure and/or morphology. Recently, Pt nanocages with hollow interior have been demonstrated to present excellent catalytic activity toward ORR due to their high surface area, and a flurry of reports have come out regarding the successful synthesis of Pt nanocages.6-9 A typical research by Xia and co-workers reported Pt nanocages with subnanometer-thick walls can be fabricated by using Pd nanocrystals as sacrificial templates, and these nanocages can dramatically reduce the amount of the costly Pt metal needed to provide catalytic activity in such applications as fuel cells.10 Currently, the synthesis of Pt nanocages mainly relies on the growth of Pt nanoshells on noble metal nanocrystals, which could be etched away to leave behind nanocage later.11 However, this overgrowth still faces challenges in tailoring the exposed crystal facets of Pt nanoshells on a given substrate.12-16 The type of the exposed facets of Pt layers, thus far, is thoroughly determined by those of noble metal nanocrystals served as templates.17-19 Pt(100) surfaces can only be generated on Pd(100) or Ag(100) surfaces, while Pt(111) surfaces on Pd(111) or Ag(111) surfaces.10,20 Previous reports have demonstrated that surface atomic arrangement of a Pt catalyst plays a key role in determining the nature of active sites and thus their catalytic activities.21-32 As a notable example, Pt catalysts terminated in {111} facets are more active than those enclosed by {100} facets, with an enhancement factor as high as

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~4.4.33,34 Kinetic control over the diffusion and deposition rates of Pt on substrates can alter the growth habits of Pt nanoshells.35 It has been reported that a larger diffusion rate, compared with the deposition rate, can appreciably induce the formation of flat surfaces on noble metal substrates. For example, Pt nanoshells can be generated on Pd seeds with different morphologies, including nanocubes, dodecahedra, and octahedra, through kinetically controlling the diffusion and deposition rates of Pt atoms.3,6,11,22,35 In comparison, the growth of Pt would favor the deposition on corners of metal nanocrystals when the deposition rate is higher than the diffusion rate.34 Such growth habits have successfully shown their capability in preparing Pt nanocages and multi-pods nanocrystals in the past few years. However, the effect of the deposition rate of Pt on substrates has not yet been studied systematically thus far, which strongly restricted the fine control of exposed facets of Pt nanoshells on a given substrate. With Pt overgrowth on Pd cubic seeds as a typical example, here we report a facile approach capable for generating Pt octahedral islands with tunable sizes and densities on Pd cubic seeds by manipulating the deposition rate. We chose Pt overgrowth on Pd nanocubes as our initial focus because Pt nanocages via this growth have been proven to possess excellent catalytic properties in fuel cell applications in the past few years. This method uses chloroplatinic acid (H2PtCl6) as a precursor, glucose as a reductant, oleylamine (OAm) as a solvent, and Pd nanocubes as seeds. By carefully controlling the concentration of glucose, the growth kinetic can be finely controlled to achieve a

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proper deposition rate of Pt, thereby obtaining Pt cubic nanoshells and octahedral islands with tunable sizes on Pd seeds, as shown in Figure 1. The key to this synthesis mainly relies on the fine control of the deposition rate of Pt. Theoretically, the deposition rate can be simply tuned by adjusting the concentration of reductant. In this synthesis, glucose served as reductant. Therefore, the deposition rate can be controlled via adjusting the concentration of glucose. When kept the diffusion rate constant, we then finely tune the deposition rate of Pt by slowly increasing the concentration of glucose. When the concentration of glucose was 8.33 mg/mL, the deposition rate would be much smaller than the diffusion rate. In this case, Pt cubic nanoshells can be obtained on Pd cubic seeds (Figure 1, route 1). This result is consistent with previously reported works on the M@Pt (M = Pd or Ag) core-shell nanocrystals obtained by diffusion dominated process.34,35 When 16.67 mg/mL of glucose was used, the deposition rate of Pt will be increased compared with the case of 8.33-mg/mL glucose. Considering the different reactivities of different surface sites (e.g., corner, edge, and surface sites) on Pd nanocrystals, the deposition of Pt would firstly occur at the corners, forming eight large Pt octahedra on a cubic Pd seed as shown in Figure 1, route 2. Interestingly, further increasing the deposition rate can induce much higher deposition rates. In this case, the deposition of Pt would like to take place not only at the corners, but also the edge and surface sites of the seeds. Obviously, this growth habit will result in the formation of high-density octahedral islands on Pd cubic seeds. With the same amount of precursor supply, the higher densities of Pt islands, the smaller the size of the octahedral islands on Pd nanocubes. 6


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Our analyses suggest that by controlling the reduction kinetics, it is not only possible to generate Pt cubic shells on Pd cubic seeds under a diffusion dominated process and eight Pt octahedra on the corners of Pd cubic seeds under a deposition dominated process, but also the size and density of the octahedral islands on Pd seeds can be finely tuned. Unlike other synthetic methods, the size of the octahedral islands on Pd seeds can be even controlled to be smaller than 3 nm by controlling the amount of Pt precursor. Considering the excellent performance of {111} facets of Pt catalysts toward ORR, the Pt nanocages with small octahedral islands on the surfaces can exhibit a high activity, with a mass activity 0.68 A/mg, as high as 5.2 times of that of commercial Pt/C. RESULTS AND DISCUSSION

Figure 1. Schematic illustrating the overgrowth of Pt nanoshells on Pd nanocubes through delicate control over the deposition rate.

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In this synthesis, Pd nanocubes served as seeds were prepared via a protocol reported previously by Jin and Xia.36,37 The typical TEM images of cubic Pd seeds can be found in Figure S1. As can be seen, the average length of Pd nanocubes was 18 nm. These Pd nanocubes were then washed and redispersed in OAm in the presence of glucose and H2PtCl6. Compared with other reductants, such as H2, NaBH4, and citric acid, glucose can provide a proper reducing rate to tune the deposition rate in a much more controllable way.34 The deposition rate of Pt can be easily tuned by finely control the concentration of glucose.

Figure 2. TEM and HAADF-STEM images of Pd@Pt core-shell nanocrystals prepared with different concentrations of glucose: (a, e) 8.33, (b, f) 16.67, (c, g) 20, and (d, h) 31.67 mg/mL. (i-l) The EDX mapping of Pd and Pt, indicating the core-shell structure of the products.

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Figure 2 shows the TEM and high-angle annular dark-field scanning TEM (HAADF-STEM) images of Pt nanoshells grown on Pd nanocubes with different concentrations of glucose, while the diffusion rate was kept at a constant (the reaction temperature was kept at 200 °C). As can be seen in Figure 2a, with a low concentration of glucose (8.33 mg/mL), the obtained Pt nanoshells on Pd cubic seeds would tend to exhibit a cubic shape after the overgrowth since the deposition rate is much smaller than the diffusion rate. Cubic Pt shells can be easily observed in HAADF-STEM image due to the difference in atomic numbers in Pd and Pt, as shown in Figure 2a and 2e. Clearly, the obtained Pt shells are enclosed by {100} facets, thoroughly follow the original shape of the Pd seeds. When the concentration of glucose is increased to 16.67 mg/mL, the deposition of Pt on Pd seeds would favor the deposition on corner sites, forming eight Pt octahedral islands on eight corners of a Pd cubic seed. Figure 2b and 2f show the TEM images of the samples. Clearly, the edge length of each octahedral islands is around 7.8 nm. HRTEM further exhibits the fringe spacing of 2.3 Å, corresponding to the {111} reflections of face-centered cubic (fcc) Pt (Figure S2), confirming an octahedral shape of Pt islands with {111} facets exposed on their surfaces. This growth habit mainly relies on the increase of the deposition rate with a higher concentration of glucose. When the deposition rate is accelerated by further increasing the concentration of glucose to 20 and 31.67 mg/mL, the deposition of Pt would like to take place not only at the corners, but also the edges and faces of Pd seeds, resulting in the formation of high-density nucleation sites on Pd seeds. In this case, smaller, but more Pt octahedral islands would form on the surface 9


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of Pd cubic seeds, as can be seen in Figure 2c, 2g, 2d, and 2f. The sizes of these octahedra are ~4.8 and ~4.4 nm for products obtained with 20 and 31.67 mg/mL glucose, respectively. The formation of Pt octahedral islands on Pd nanocubes could be further confirmed by a set of TEM images of Pd@Pt nanocrystals viewed along different directions (Figure S3 and S4). To further confirm the structure of the products, the elemental distribution of Pd and Pt in the products was characterized by the energy-dispersive X-ray spectroscopy (EDX) mappings (Figure 2i-l). Obviously, Pd only dispersed in the interiors (red color), while Pt could be observed in the shell area (green color), further indicating the core-shell structure of the products. By far, there has been no efficient method that can directly induce the growth of Pt(111) surfaces on Pd(100) substrates. Our results clearly demonstrate that by taking the advantage of the fine control over the deposition and diffusion rates small Pt octahedral islands exposed by active {111} surfaces toward ORR can be prepared on Pd nanocubes directly. In order to better understand the growth process of Pt nanoshells, products taken from the same reaction solution but different time points were collected and characterized by TEM analyses. During this synthesis, we find that the size of Pt octahedral islands on Pd seeds would increase with the reaction time. TEM observations of the octahedral islands produced at different reaction times have been presented in Figure S5, confirming that Pt octahedral islands grown with the reaction time. The average edge length of octahedral islands formed after reacted for 2 h was

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~5.5 nm. In comparison, the edge length of octahedral islands prepared at 3 h was ~7.8 nm, implying the capable of size tuning via varying the reaction time.

Figure 3. (a) ORR polarization curves, (b) and the corresponding Tafel plots of Pd@Pt catalysts in O2-saturated 0.1 M HClO4 solution with a sweep rate of 10 mV·s-1 and a rotation rate of 1600 rpm, normalized to the geometric area of the rotating disk electrode (0.196 cm2). (c) Mass activities and (d) specific activities of Pd@Pt catalysts at 0.9 V versus RHE.

We then used oxygen reduction reaction (ORR) as a probe reaction to demonstrate the ability to engineer the catalytic activity of Pt shells by manipulating the type of the facet exposed on the surface and the size of the octahedra on the shells. To better identify the catalysts, Pd@Pt core-shell nanocrystals prepared with 8.33, 16.67, 20, and 31.67 mg/mL glucose were denoted as Pd@Pt-flat, [email protected] nm, [email protected] nm, and [email protected] nm, respectively. The cyclic voltammograms (CVs) were measured at a sweeping rate of 50 mV s-1 in the potential range of 0.03-1.17 V versus reversible hydrogen electrode (RHE), and in N2-saturated 0.1 M HClO4 solution at room temperature. The specific ECSAs were derived from the charges related to the 11


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desorption of hydrogen and further normalized against the amounts of Pt. The specific ECSAs of the catalysts were measured to be 8, 9.2, 11.4 and 12 m2 g-1Pt, respectively. Clearly, the specific ECSA increases while the size of Pt octahedra grown on shells decrease because smaller particles can provide larger surface area with the same amount of Pt metal (Figure S6). The electrocatalytic performance of the different catalysts toward ORR were then measured. A set of comparative electrochemical tests on ORR performance were recorded at a sweeping rate of 10 mV s-1 using rotating disk electrode (RDE) method, and in an O2-saturated 0.1 M aqueous HClO4 solution. The results were shown in Figure 3. As can be seen, [email protected] nm with abundant 4.4-nm octahedra on shells had a mass activity of 0.164 A mg-1Pt at 0.9 V, which was 4 times greater than Pd@Pt-flat catalyst exposed by {100} facets. This result demonstrates that engineering the exposed surface of Pt shells from {100} to {111} facets could effectively engineer the catalytic performance of a Pt catalyst. The specific activity of Pt octahedral shells, such as in case of [email protected] nm, [email protected] nm, and [email protected] nm, was also 1.5 to 3 times higher than those shells prepared on Pd nanocubes directly, consistent with previous reports on facet-dependent ORR of Pt nanocrystals.33-35

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Figure 4. (a) TEM images of Pt nanoshells with the size of octahedral islands down to 2.7 nm, and (b) the corresponding Pt nanocages prepared through etching. (c) Mass and specific activity for Pt nanocages and commercial Pt/C at 0.9 V versus RHE. (d) Mass activity for Pt nanocages and commercial Pt/C before and after 10,000 cycles. The scale bars in the inset are 5 nm.

Since the overgrowth of Pt octahedral islands was dictated by the deposition rate and the reaction time, the size of the resultant Pt octahedral islands should be easily tuned by adjusting the reaction time and/or the deposition rate. As such, the catalytic performance of Pt nanoshells can be boosted through controlling the size of octahedral islands. The size of Pt octahedral islands could be further tuned to 2.7 nm by reducing the amount of the precursor from 10 mg to 5 mg, while maintaining the concentration of glucose at 31.67 mg/mL (Figure 4a). The samples shown in Figure 4a can be further confirmed by HRTEM images (Figure S7). This result suggests that 13


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we could manipulate the size of octahedral island from 2.7 nm to 7.8 nm based on the approach developed in this work. The Pd core can be further removed, thus leave behind the thin shells with abundant Pt octahedral islands.25 A TEM image of such Pt nanocages derived from [email protected] nm by using an etching procedure is shown in Figure 4b. The ECSA was estimated to be ~40 m2/g (Figure S8), showing a substantial improvement compared with other samples, indicating that more active Pt sites start to expose on the surface through etching Pd@Pt core-shell nanocrystals to hollow Pt nanocages. The ECSA of Pt nanocages is slightly smaller than that of the commercial Pt/C despite the similar size of Pt nanoparticles between them, which may be due to the partial aggregation of the products. Regardless, the mass activity of Pt catalysts can be still improved to as high as 0.68 A mg-1 at 0.9 V, about 5.2 times greater than that of the commercial Pt/C (0.13 A mg-1) (Figure 4c). Tafel plots are shown in Figure S8b. As can be seen, the Tafel slope of Pt nanocages is -68 mV dec-1, which is smaller than that of the commercial Pt/C (-75 mV dec-1), indicating an accelerated charge transfer and thus enhanced reaction kinetics of Pt nanocages. The long-term stability of the catalyst was then evaluated via an accelerated durability test (Figure 4d). The Pt nanocages with octahedral islands showed much better performance, with the ORR mass activity only reduced by 30% after 10,000 cycles. As well, the ECSAs of the Pt nanocages also dropped by ~30% after 10,000 cycles, whereas the specific ECSA of the Pt/C catalyst dropped by 60% (Figure S9). During the durability test, the structure of Pt nanocages can be well maintained (Figure S10). The stability of Pt nanocages obtained in this work is comparable with those Pt 14


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nanocages prepared by other groups. It is believed that the stability of Pt nanocages could be further enhanced by introducing Au cores.38 We think the stability together with the enhanced catalytic activity of our sample will make it an excellent catalyst toward ORR. CONCLUSIONS In summary, we have demonstrated a simple and convenient method capable for generating Pt octahedral islands with tunable sizes and densities on Pd cubic seeds by simply manipulating the deposition rate. We found that the exposed facets of Pt nanoshells would change from {100} to {111} facets through the fine control over the deposition rate of Pt on seeds. Due to the different reactivities at the surface sites, the deposition of Pt atoms can be controlled at a certain site by carefully tune the deposition rate. Increasing the deposition rate can result in the formation of high-density octahedral islands on the surfaces of Pd cubic seeds. With the same amount of precursor supply, the higher densities of Pt islands, the smaller the size of the octahedral islands on Pd nanocubes. Unlike other synthetic methods, the size of the octahedral islands on Pd seeds can be even controlled to be smaller than 3 nm by simply controlling the amount of the Pt precursor. With active {111} facets exposed, these Pt nanoshells can exhibit much enhanced catalytic activity toward ORR than those enclosed by {100} facets. Further etching away Pd seeds produces Pt nanocages occupied by numerous Pt(111) islands, which exhibit superior ORR performance relative to commercial Pt/C catalyst. Catalytic evaluation indicates that Pt nanocages 15


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with small octahedral islands on the surfaces can exhibit high activities toward ORR, with a mass activity as high as 0.68 A/mg, about 5.2 times of that of the commercial Pt/C. These results suggest the capability of the approach reported in providing an opportunity for large-scale synthesis of Pt nanocages exposed by active surfaces for their industrial applications. We expect this method could be further extended to the design and preparation of efficient Pt catalysts with enriched (111) facets exposure for optimal catalytic performance. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions M. J. supervised this study and wrote the paper. W. W. carried out the synthetic experiments, structural characterizations, and data analysis. X. L. contributed to the catalyst preparation and catalytic testing. T. H. and Y. L. offered help in catalytic testing and data analysis. All authors have discussed the results and given the approval to the final version of the manuscript. 16


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ACKNOWLEDGMENT M. J. acknowledgements the supports of National Natural Science Foundation of China (NSFC, No. 21773180 and 21471123), State Key Laboratory for Mechanical Behavior of Materials, “the Fundamental Research Funds for the Central Universities” and “the World-Class Universities (Disciplines) and the Characteristic Development Guidance Funds for the Central Universities” provided by Xi’an Jiaotong University. We thank Miss Jiao Li for her assistance with HRTEM analysis and Mr Guoqing Zhou for his assistance with ICP-MS analysis at Instrument Analysis Center of Xi’an Jiaotong University. REFERENCES (1) Huang, X.; Zhao, Z.; Cao, L.; Chen, Y.; Zhu, E.; Lin, Z.; Li, M.; Yan, A.; Zettl, A.; Wang, Y. M.; Duan, X.; Mueller, T.; Huang, Y. Science 2015, 348, 1230-1234. (2) Strmcnik, D.; Uchimura, M.; Wang, C.; Subbaraman, R.; Danilovic, N.; Vliet, D.; Paulikas, A. P.; Stamenkovic, V. R.; Markovic, N. M. Nat. Chem. 2013, 5, 300-306. (3) Wang, X.; Choi, S.; Roling, L. T.; Luo, M.; Ma, C.; Zhang, L.; Chi, M.; Liu, J.; Xie, Z.; Herron, J. A.; Mavrikakis, M.; Xia, Y. Nat. Commun. 2015, 6, 7594. (4) Sheng, W.; Zhuang, Z.; Gao, M.; Zheng, J.; Chen, J. G.; Yan, Y. Nat. Commun. 2015, 6, 5848. (5) Peng, Z.; Yang, H. J. Am. Chem. Soc. 2009, 131, 7542-7543.

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(24) Tianou, H.; Wang, W.; Yang, X.; Cao, Z.; Kuang, Q.; Wang, Z.; Shan, Z.; Jin, M.; Yin, Y. Nat. Commun. 2017, 8, 1261. (25) Gilroy, K. D.; Yang, X.; Xie, S.; Zhao, M.; Qin, D.; Xia, Y. Adv. Mater. 2018, 30, 1706312. (26) Rong, H.; Mao, J.; Xin, P.; He, D.; Chen, Y.; Wang, D.; Niu, Z.; Wu, Y.; Li, Y. Adv. Mater. 2016, 28, 2540-2546. (27) Jana, S.; Chang, J. W.; Rioux, R. M. Nano Lett. 2013, 13, 3618-3625. (28) Cao, Z.; Chen, Q.; Zhang, J.; Li, H.; Jiang, Y.; Shen, S.; Fu, G.; Lu, B.; Xie, Z.; Zheng, L. Nat. Commun. 2017, 8, 15131. (29) Yang, T.; Zhou, S.; Gilroy, K. D.; Figueroa-Cosme, L.; Lee, Y.; Wu, J.; Xia, Y. Proc. Natl. Acad. Sci. U.S.A. 2017, 114, 13619-13624. (30) Yan, Y.; Du, J. S.; Gilroy, K. D.; Yang, D.; Xia, Y. Zhang, H. Adv. Mater. 2017, 29, 1605997. (31) Sun, Y.; Xia, Y. Science 2002, 298, 2176-2179. (32) Bu, L.; Guo, S.; Zhang, X.; Shen, X,; Su, D.; Lu, G.; Zhu, X.; Yao, J.; Guo, J.; Huang, X. Nat. Commun. 2016, 7, 11850. (33) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493-497.

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(34) Qian, J.; Shen, M.; Zhou, S.; Lee, C.; Zhao, M.; Lyu, Z.; Hood, Z. D.; Vara, M.; Gilroy, K. D.; Wang, K.; Xia, Y. Mater. Today 2018, 21, 834-844. (35) Xie, S.; Choi, S.; Lu, N.; Roling, L. T.; Herron, J. A.; Zhang, L.; Park, J.; Wang, J.; Kim, M. J.; Xie, Z.; Mavrikakis, M.; Xia, Y. Nano Lett. 2014, 14, 3570-3576. (36) Jin, M.; Liu, H.; Zhang, H.; Xie, Z.; Liu, J.; Xia, Y. Nano Res. 2011, 4, 83-91. (37) Jin, M.; Zhang, H.; Xie, Z.; Xia, Y. Energy Environ. Sci. 2012, 5, 6352-6357. (38) Bian, T.; Zhang, H.; Jiang, Y.; Jin, C.; Wu, J.; Yang, H.; Yang, D. Nano Lett. 2015, 15, 7808-7815.

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Table of Contents

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Engineering Surface Structure of Pt Nanoshells on Pd Nanocubes to

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