Elemental Segregation in Multimetallic Core-Shell Nanoplates

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Elemental Segregation in Multimetallic Core-Shell Nanoplates Faisal Saleem, Zhicheng Zhang, Xiaoya Cui, Yue Gong, Bo Chen, Zhuangchai Lai, Qinbai Yun, Lin Gu, and Hua Zhang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b05197 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Journal of the American Chemical Society

Elemental Segregation in Multimetallic Core-Shell Nanoplates Faisal Saleem‡,†, Zhicheng Zhang‡,†, Xiaoya Cui‡,†, Yue Gong∆,#,†, Bo Chen‡, Zhuangchai Lai‡, Qinbai Yun‡, Lin Gu∆,#,*, Hua Zhang‡,§,* ‡Center

for Programmable Materials, School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore. §Department

of Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China. National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190 China. ∆Beijing #School

of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China.

Supporting Information ABSTRACT: In this work, we report an element segregation

phenomenon in two-dimensional (2D) core-shell nanoplates, subsequently resulting in the formation of yolk-cage nanostructures after selective electrochemical etching. By using PtCu nanoplates as templates, PtCu@Pd core-shell nanoplates are formed. Interestingly, during the growth of Ru on the PtCu@Pd core-shell nanoplates, due to the selective element diffusion, PtCuPd@PdCu@Ru nanoplates are obtained. After selectively etching of PdCu in PtCuPd@PdCu@Ru using electrochemical method, the PtCuPd@Ru yolk-cage nanostructures are obtained. As a proof-of-concept application, this unique nanostructure shows superior electrocatalytic activity and stability towards the methanol oxidation reaction (MOR) as compared to the PtCu nanoplates and commercial Pt/C catalyst.

Multimetallic nanostructures have received increasing interests due to their widespread utilization in catalytic reactions.1-10 Recently, elemental segregation has emerged as an efficient strategy to precisely tune the elemental distribution in metallic alloy nanostructures,11-19 resulting in enhanced catalytic properties. Until now, the segregation of elements has been realized by the electrochemical approach,12,20 heating,17,18,21,22 and adsorption of capping agents.19,23,24 For example, Shao-Horn and coworkers reported that the surface composition of AuPt alloy nanoparticles can be tuned by segregation through the heating treatment, and the best electrocatalytic performance of AuPt alloy nanoparticles towards the methanol oxidation reaction (MOR) was obtained when the atomic percentage of Pt on the surface was tuned from 90% to 68%.21 Recently, Yang and coworkers reported the anisotropic segregation of Pt an Ni elements in PtNi alloy nanoparticles to form Pt-rich nanoframes, which greatly improved the electrocatalytic performance towards the oxygen reduction reaction.17,18 However, to date, it still remains great challenge to achieve the segregation of elements in multimetallic nanostructures

for specific catalytic reactions, which is significant not only in fundamental studies but also in practical applications. In this work, we report the selective element segregation and etching in multimetallic core-shell nanoplates. As shown in Scheme 1, by using the PtCu nanoplateas template, the PtCu@Pd core-shell nanoplate is prepared (Step 1). Then, the selective element diffusion process, i.e., segregation, occurs when Ru is coated on the surface of PtCu@Pd nanoplate, in which the core of PtCu is transformed into PtCuPd and Pd is transferred into PdCu, to form PtCuPd@PdCu@Ru nanoplate (Step 2). The PdCu between the PtCuPd core and Ru shell can be selectively etched by an electrochemical method25,26 to obtain PtCuPd nanoplate encapsulated in Ru nanocage, referred to as PtCuPd@Ru yolk-cage nanostructure (Step 3), which exhibits high activity and durability towards the MOR.

Scheme 1. Schematic illustration of the synthesis of PtCuPd@Ru yolk-cage nanostructure from the PtCu nanoplate. The transmission electron microscopy (TEM) images (Figure S1a,b) show that the average lateral size of PtCu nanoplates is 10.8±1.4 nm. The TEM images of PtCu@Pd coreshell nanoplates (Figure 1a) and PtCuPd@PdCu@Ru nanoplates (Figure 1b, see Experimental Section for details in Supporting Information) vertically oriented on the TEM molybdenum grids indicate their average thicknesses of 3.4±0.8 nm and 5.5±0.9 nm, respectively (Figure S2). Aberration-corrected high-angle annular dark-field scanning

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TEM (HAADF-STEM) images (Figure 1c,d) confirm that the core only contains a few atomic layers (less than 4 layers). This can be easily distinguished by the Z-contrast, i.e., the brighter

atom

Figure 1. (a) TEM image of PtCu@Pd core-shell nanoplates. (b) TEM image of PtCuPd@PdCu@Ru nanoplates. (c) Aberrationcorrected HAADF-STEM image of PtCu@Pd core-shell nanoplates. (d) Aberration-corrected HAADF-STEM image of PtCuPd@PdCu@Ru nanoplates. The red arrow indicates the Ru island. (e) Magnified aberration-corrected HAADF-STEM image of PtCu@Pd core-shell nanoplates taken from the white dotted square in (c). (f) The FFT patterns taken from the corresponding white dotted squares in (e). (g) Magnified aberration-corrected HAADF-STEM image of PtCuPd@PdCu@Ru nanoplates taken from the white dotted square in (d). (h,i) The FFT patterns taken from the corresponding white dotted squares in (g). column inside the nanostructure indicates the Pt-based alloy. The aberration-corrected HAADF-STEM image shows that the lattice distance of PtCu@Pd core-shell nanoplates is about 0.229 nm (Figure 1e) when viewed from the [101] direction (Figure 1f), which is close to that of pure Pd. In comparison, the lattice distance of PtCuPd@PdCu@Ru nanoplates is about 0.223 nm (Figure 1g) when viewed from the [101] direction (Figure 1h), indicating that the lattice is slightly compressed after loading the Ru. This is further confirmed by their X-ray diffraction (XRD) patterns, in which the peak ascribed to (111) plane of PtCuPd@PdCu@Ru nanoplates is slightly shifted to higher angle as compared to that of PtCu@Pd core-shell nanoplates (Figure S3). Owing to large amount of Ru precursor used in the synthesis of PtCuPd@PdCu@Ru

nanoplates, some Ru overgrew to form island-shell on the surface, as indicated by the arrows in Figure 1d and Figure S4. More importantly, the atom stacking mode, i.e., “ABCABC”, confirms the face-centered cubic (fcc) structure of PtCuPd@PdCu@Ru nanoplates (Figure 1g). Due to the epitaxial growth, the Ru islands also possess an unusual fcc phase (bottom-right corner in Figure 1g), as further evidenced by the FFT pattern (Figure 1i). Figure 2a shows the HAADF-STEM and the corresponding EDS elemental mapping images of PtCu@Pd core-shell nanoplates. It is clearly seen that Cu and Pt are in the center, and there is no diffusion of Cu to the shell of Pd. After the growth of Ru, the selective segregation happened, i.e., Cu diffused out of the center and Pd diffused towards the center


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Journal of the American Chemical Society as shown by arrows in the EDS elemental mapping images of PtCuPd@PdCu@Ru nanoplates (Figure 2b), resulting in the formation of PtCuPd@PdCu@Ru nanoplates. As reported previously, heating and capping agents can affect the segregation behavior. For example, Au could be enriched at the surface of AuPt bimetallic alloy nanoparticles under heating at 250 °C,21 and PtCu alloy nanoparticles showed Cu-rich surface when exposed to CO.24 In this work, the synthesis was carried out at 150 oC for 18 h. This experimental condition can easily decompose formaldehyde to CO.27 The resulting CO could act as the capping agent to induce the segregation process.28,29 In a control experiment, we performed the reaction without addition of Ru precursor but keeping the other conditions unchanged. After the reaction, no obvious change in morphology was observed in the PtCu@Pd core-shell nanoplates (Figure S5a), and the EDS elemental mapping images confirm that no segregation phenomenon happened in the PtCu@Pd core-shell nanoplates (Figure S5b). Therefore, in our experiment, the selective element diffusion process only occurs when Ru is coated on the surface of PtCu@Pd core-shell nanoplates (Figure 2). Moreover, previous reports indicate that the elemental segregation in metallic nanostructures may arise from the strain.30-33 Therefore, in this work, the coating of Ru on the surface of PtCu@Pd core-shell nanoplates might induce the formation of strain, leading to the elemental segregation.

can be identified (Figure 3c), indicating that Ru is still fcc phase after the etching process. The EDS results (Figure S7 and S8) show that the atomic ratios before and after etching are Pt7.0Cu7.5Pd31.0Ru54.5 and Pt15.1Cu1.9Pd15.3Ru67.7, respectively, indicating the selective etching of Pd and Cu in PtCuPd@PdCu@Ru nanoplates to form PtCuPd@Ru yolkcage nanostructures.

Figure 3. (a) TEM image of PtCuPd@Ru yolk-cage nanostructures. Inset: high-magnification TEM image of PtCuPd@Ru yolk-cage nanostructures. (b) Aberrationcorrected HAADF-STEM image of a typical PtCuPd@Ru yolkcage nanostructure. Inset: the corresponding FFT pattern. (c) Magnified aberration-corrected HAADF-STEM image taken from the yellow rectangle in (b). (d) HAADF-STEM and the corresponding EDS elemental mapping images of a typical PtCuPd@Ru yolk-cage nanostructure.

Figure 2. HAADF-STEM and the corresponding EDS elemental mapping images of (a) PtCu@Pd core-shell nanoplates and (b) PtCuPd@PdCu@Ru nanoplates. The arrows in white and yellow in (b) indicate the segregated Pd and Cu, respectively. By using electrochemical method, after 75 cycles of cyclic voltammetry (CV) in 0.5 M H2SO4 aqueous solution at scan rate of 50 mV s-1 (Figure S6), the PdCu between the PtCuPd core and the Ru shell in PtCuPd@PdCu@Ru nanoplates can be selectively etched, leading to the formation of PtCuPd@Ru yolk-cage nanostructures (Figure 3a-c), which is also confirmed by the EDS elemental mapping images (Figure 3d). The aberration-corrected HAADF-STEM image (Figure 3b) shows a typical PtCuPd@Ru yolk-cage nanostructure along the [110] zone axis. The atom stacking mode, i.e., “ABCABC”,

As a proof-of-concept application, the PtCuPd@Ru yolkcage nanostructures were used as catalyst towards the MOR. The activities of PtCuPd@Ru yolk-cage nanostructures, PtCuPd@PdCu@Ru nanoplates, PtCu nanoplates, commercial Pt/C catalyst and commercial Ru/C catalyst were investigated in an aqueous solution containing 1 M KOH and 2 M methanol. As shown in Figure 4a,b and Figure S9, the PtCuPd@Ru yolk-cage nanostructures showed much higher mass activity (2.48 A mgPt+Pd-1) as compared to PtCu nanoplates (1.44 A mgPt-1) and commercial Pt/C catalyst (1.22 A mgPt-1). It is worth mentioning that the PtCuPd@PdCu@Ru nanoplates and the commercial Ru/C catalyst exhibited almost no activity towards the MOR (Figure S9). Impressively, the electrocatalytic activity of PtCuPd@Ru yolk-cage nanostructures is comparable with and even better than most of previously reported MOR electrocatalysts (Table S1). Chronoamperometric measurement at -0.3 V



Figure 4. (a) CV curves of PtCuPd@Ru yolk-cage nanostructures, PtCu nanoplates and Pt/C recorded in an aqueous solution containing 1 M KOH and 2 M methanol at scan rate of 50 mV s-1. (b) Comparison of the mass activity of PtCuPd@Ru yolk-cage nanostructures, PtCu nanoplates and Pt/C. (c) Chronoamperometric measurement at -0.3 V (vs. SCE) in an aqueous solution containing 1 M KOH and 2 M methanol. (vs. SCE) in the N2-saturated 1 M KOH aqueous solution containing 2 M methanol (Figure 4c) was used to investigate the stability of these PtCuPd@Ru yolk-cage nanostructures, which show much higher durability after 6,000 s as compared to PtCu nanoplates and Pt/C. Specifically, after the electrocatalytic stability measurement for 1,000 s, the PtCuPd@Ru yolk-cage nanostructure, PtCu and Pt/C catalysts keep about 55%, 24% and 11% of the initial response currents, respectively, towards the MOR (Figure 4c). Furthermore, after 6,000 s, the PtCuPd@Ru yolk-cage nanostructure, PtCu and Pt/C catalysts still keep about 28%, 2% and 5% of the initial response currents, respectively, towards the MOR. TEM image after stability test (Figure S10) showed that there is no obvious morphology change in PtCuPd@Ru yolk-cage nanostructures. The high mass activity and stability could be attributed to the following reasons. First, incorporation of Pd into PtCu nanoplate to form PtCuPd nanoplate is beneficial to enhance the electrocatalytic activity of PtCuPd@Ru yolk-cage nanostructure. Second, the top and bottom surfaces of PtCuPd nanoplates are directly exposed to reactants inside the Ru nanocages, and the Ru nanocages can prevent the aggregation and stacking of PtCuPd nanoplates.34,35 Third, after the electrochemical etching, the obtained ultrathin PtCuPd nanoplates possess clean surface, which is favorable for the MOR. In summary, by using PtCu nanoplates as templates, PtCu@Pd core-shell nanoplates are synthesized. Interestingly, during the growth of Ru on the PtCu@Pd core-shell nanoplates, due to the selective element diffusion, the PtCuPd@PdCu@Ru nanoplates are obtained. After selectively electrochemical etching of PdCu in PtCuPd@PdCu@Ru nanoplates, the PtCuPd@Ru yolk-cage nanostructures are formed. Impressively, the obtained PtCuPd@Ru yolk-cage nanostructures exhibited superior activity and stability. The selective segregation and electrochemical etching strategy presented in this work might provide a new approach to the construction of other multimetallic core-shell nanostructures, which could have unique physicochemical properties and various promising applications.

AUTHOR INFORMATION Corresponding Author *[email protected]; [email protected]; [email protected].

Author Contributions †These

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by MOE under AcRF Tier 2 (MOE2015-T2-2-057; MOE2016-T2-2-103; MOE2017-T2-1-162) and AcRF Tier 1 (2016-T1-002-051; 2017-T1-001-150; 2017-T1002-119), and NTU under Start-Up Grant (M4081296.070.500000) in Singapore. L. Gu acknowledges the Key Research Program of Frontier Sciences, CAS (No. QYZDBSSW-JSC035) and National Natural Science Foundation of China (51672307, 51421002). We would like to acknowledge the Facility for Analysis, Characterization, Testing and Simulation, Nanyang Technological University, Singapore, for use of their electron microscopy (and/or X-ray) facilities. H.Z. thanks the support from ITC via Hong Kong Branch of National Precious Metals Material Engineering Research Center, and the Start-Up Grant from City University of Hong Kong. We also thank Prof. Jun Luo, Ms. Jing Liu, Mr. Tong Zhou and Mr. Kai Wang at the Center for Electron Microscopy in Tianjin University of Technology for supporting this project.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/xxxx. This file includes Figures S1-S10 and Table S1.

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