Anisotropic phase segregation and migration of Pt in nanocrystals en route to nanoframe catalysts

Abstract

Compositional heterogeneity in shaped, bimetallic nanocrystals offers additional variables to manoeuvre the functionality of the nanocrystal. However, understanding how to manipulate anisotropic elemental distributions in a nanocrystal is a great challenge in reaching higher tiers of nanocatalyst design. Here, we present the evolutionary trajectory of phase segregation in Pt–Ni rhombic dodecahedra. The anisotropic growth of a Pt-rich phase along the 〈111〉 and 〈200〉 directions at the initial growth stage results in Pt segregation to the 14 axes of a rhombic dodecahedron, forming a highly branched, Pt-rich tetradecapod structure embedded in a Ni-rich shell. With longer growth time, the Pt-rich phase selectively migrates outwards through the 14 axes to the 24 edges such that the rhombic dodecahedron becomes a Pt-rich frame enclosing a Ni-rich interior phase. The revealed anisotropic phase segregation and migration mechanism offers a radically different approach to fabrication of nanocatalysts with desired compositional distributions and performance.

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Figure 1: Formation process of the rhombic dodecahedra.
Figure 2: Development of rhombic dodecahedra and corrosion to three-dimensional nanostructures.
Figure 3: STEM–EDS analysis of segregation and migration of Pt in Pt–Ni rhombic dodecahedra.
Figure 4: Structural evolution over time in Pt–Ni rhombic dodecahedra.
Figure 5: Summary of the complete growth process of a Pt–Ni rhombic dodecahedron.

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Acknowledgements

The research conducted at Lawrence Berkeley National Laboratory was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, under Contract No. DE-AC02-05CH11231 (surface). Z.N. gratefully acknowledges support from the International Postdoctoral Exchange Fellowship Program 2014. D.K. acknowledges support from Samsung Scholarship. All HRTEM, HAADF-STEM, and EDS mapping made use of the National Center for Electron Microscopy at the Molecular Foundry. XPS data was collected at the Molecular Foundry. We acknowledge M. Marcus and the use of Beamline 10.3.2 at the Advanced Light Source for collection of EXAFS data. The Molecular Foundry and the Advanced Light Source are supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under Contract No. DE-AC02-05CH11231. We acknowledge P. Alivisatos for access to the Bruker D-8 XRD and E. Kreimer of the Microanalytical Facility in the College of Chemistry, UC Berkeley for access to ICP analysis.

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Z.N., N.B. and P.Y. designed the experiments and wrote the paper. Z.N. and N.B. performed the experiments and contributed equally to the work. Y.Y. and N.K. performed HRTEM and HAADF-STEM analysis. D.K. and Z.N. completed XPS analysis. C.C. initiated the work. G.A.S. and P.Y. guided the work. All authors commented on the manuscript.

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Correspondence to Peidong Yang.

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Niu, Z., Becknell, N., Yu, Y. et al. Anisotropic phase segregation and migration of Pt in nanocrystals en route to nanoframe catalysts. Nature Mater 15, 1188–1194 (2016). https://doi.org/10.1038/nmat4724

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