Pt-Frame@Ni quasi Core–Shell Concave Octahedral PtNi3 Bimetallic

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Pt-Frame@Ni quasi Core−Shell Concave Octahedral PtNi3 Bimetallic Nanocrystals for Electrocatalytic Methanol Oxidation and Hydrogen Evolution Shan Wang,† Guang Yang,*,‡ and Shengchun Yang*,†,§ †

School of Science, Key Laboratory of Shaanxi for Advanced Materials and Mesoscopic Physics, State Key Laboratory for Mechanical Behavior of Materials and ‡Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an, 710049, People’s Republic of China § Collaborative Innovation Center of Suzhou Nano Science and Technology, Suzhou Academy of Xi’an Jiaotong University, 215000, Suzhou, People’s Republic of China S Supporting Information *

ABSTRACT: PtNi3 bimetallic concave octahedrons with a majority of platinum atoms deposited on the frames were synthesized in ethylene glycol solution. The high angle annular dark field scanning transmission electron microscopy (HAADSTEM) characterizations and energy-dispersive X-ray spectroscopy (EDS) analysis reveal that the Pt frames have a thickness of less than 2 nm, which surround a nickel core thus forming a quasi core−shell concave octahedral nanoparticle (NP). The element-specific anisotropic growth followed by the nanoscale phase segregation and subsequent oxidation of Ni riched facets are responsible for the formation of the concave nanostructure. The PtNi3 quasi core−shell concave octahedrons exhibit substantially enhanced electrocatalytic properties toward methanol oxidation and hydrogen evolution reaction compared with that of the commercial Pt/C, suggesting that the Ni riched Pt−Ni NPs can be used as a potential candidate for methanol oxidation reaction (MOR) or hydrogen evolution reaction (HER) catalysts with the low utilization of Pt. comprehensive consensus in the field of electrochemical catalysts.23−27 Typically, the PtM bimetallic nanocatalysts with a concave structure have aroused much attention, due to their extraordinary facets and negative curvature which offered a much higher density of low-coordinate atomic steps and kinks, resulting in higher catalytic ability than their counterparts with flat surfaces.28−33 In addition, fabricating the Pt based bimetallic NPs with a very thin Pt surface ( Pt2Ni/C = PtNi3/C. The results indicate that the as-prepared bimetallic PtxNiy/C catalyst do not improve the HER activity on the basis of electrode area, which may be due to the lower loading amount of the bimetallic catalysts (metal loading of ∼10%) deposited on the same area compared to the Pt/C. Interestingly, when the LSV curves was normalized by the mass of Pt, the bimetallic PtxNiy/C catalysts exhibit better catalytic performance for HER. As shown in Figure 8B, the current densities at the same potential are found to be in the order of PtNi3/C > PtNi/C > Pt2Ni/C > Pt/C. The PtNi3/C catalyst shows the highest activity with a current density of 10 A mgpt−1 at the potential of −289 mV (vs Ag/AgCl). Figure 8C presents the geometric area-normalized area current (area activity) and Pt mass-normalized mass current (mass activity) of four catalysts at −0.27 V from which the catalytic activity of four PtxNiy/C for HER reaction can be compared and the trend of catalytic activity agrees well with that shown in Figures 8A and B. The electrochemical durability of the samples is tested by running continuous CVs between −0.52 and −0.02 V (vs

Ag/AgCl) at scan rate of 50 mV/s (Figure 8D). Under acidic electrolyte, dealloying Ni from PtxNiy bimetallic can be conducted.41 As a result, during electrode potential cycling in the range of −0.52 to −0.02 V (vs Ag/AgCl), each PtxNiy sample leaches Ni in its characteristic way with different rates and to different extents, leading the decay of PtxNiy in the catalytic activity for HER. Even though the decay in the catalytic activity of all the samples can be observed between the LSV curves measured at the initial cycle and after 500 CV cycles, the bimetallic NPs still exhibit a higher HER activity than the commercial Pt/C.

4. CONCLUSIONS The concave octahedral PtNi3 NPs with Pt-frame@Ni quasi core−shell structure were synthesized in ethylene glycol solution. We believe that the element-specific anisotropic growth mechanism followed by the nanoscale phase segregation and subsequent oxidation of Ni-enriched facets attributed to the formation of such concave octahedron bimetallic NPs. The nickel-enriched PtNi3 concave octahedral NPs with the Ptenriched frames exhibit enhanced electrocatalytic performance and stability for MOR and HER compared to the commercial Pt/C due to the combination of the composition and facet effects. This work not only provides a method to prepare the bimetallic octahedral NPs with concave surfaces, but also provides the ideas to design advanced catalysts in various fields.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b10083. Molar ratio of Pt to the total Pt and Ni (Pt+Ni) in the Pt23Ni77 was calculated in Calculation details. Scheme S1 illustrates the synthesis process of PtxNiy NPs. Figure S1 shows the photograph of the reaction mixture in different stage; Figure S2 shows the TEM images of Pt−Ni NPs prepared using the standard procedure, except for replacing the air with N2 atmosphere. (PDF) F

DOI: 10.1021/acs.jpcc.5b10083 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Chuansheng Ma from the International Center for Dielectric Research, Xi’an Jiaotong University, for their support with HRTEM. This work is supported by National Natural Science Foundation of China (No. 51271135 and 51202180), the program for New Century Excellent Talents in university (No. NCET-12-0455), the Natural Science Foundation of Shaanxi Province (2015JM5166), the Fundamental Research Funds for the Central Universities, and the project of Innovative Team of Shanxi Province (No. 2013KCT-05).

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DOI: 10.1021/acs.jpcc.5b10083 J. Phys. Chem. C XXXX, XXX, XXX−XXX