Understanding Compositional Segregations in Shaped Bimetallic Nanoparticles by Ex-Situ and In-Situ Aberration-Corrected STEM

Compositional segregation is an important physical phenomenon in alloy materials and has significant influences on their physical and chemical properties. In particular, segregation at the surface or subsurface can drastically change the molecular adsorption properties of alloy surfaces and thus becomes a promising way to design highly active catalysts. Understanding segregations in alloy catalyst nanoparticles at atomic level is therefore crucial yet still challenging for future catalyst designs. In this talk, we will highlight some of our recent works on nano segregation effect in shaped Pt alloy catalysts for fuel cell technologies. Particular attentions will be placed on the atomic imaging of nano segregation by using state-of-the-art ex-situ and in-situ aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). In particular, we discovered distinct compositional segregations in octahedral Pt_xNi_1-x nanoparticles, which featured Pt-segregation at the edges/corners and Ni-segregation at the facets (Figure 1) . We explored the physical origin for these distinct segregation behaviors and their impact on the catalytic activities and stability for fuel cell reactions. This will be further complemented by in-situ STEM-EELS experiments to study the structural and compositional evolution of Pt alloy nanoparticles during nanoparticle synthesis, post thermal annealing and solution-phase electrocatalysis, shedding important light on catalyst designs with desired segregation patterns and chemical properties.