Abstract
At elevated temperatures, bimetallic nanomaterials change their morphologies because of the interdiffusion of atomic species, which also alters their properties. The Kirkendall effect (KE) is a well-known phenomenon associated with such interdiffusion. Here, we show how KE can manifest in bimetallic nanoparticles (NPs) by following core–shell NPs of Au and Pd during heat treatment with in situ transmission electron microscopy. Unlike monometallic NPs, these core–shell NPs did not evolve into hollow core NPs. Instead, nanoscale voids formed at the bimetallic interface and then, migrated to the NP surface. Our results show that: (1) the direction of vacancy flow during interdiffusion reverses due to the higher vacancy formation energy of Pd compared to Au, and (2) nanoscale voids migrate during heating, contrary to conventional assumptions of immobile voids and void shrinkage through vacancy emission. Our results illustrate how void behavior in bimetallic NPs can differ from an idealized picture based on atomic fluxes and have important implications for the design of these materials for high-temperature applications.
Highlights
At elevated temperatures, bimetallic nanomaterials change their morphologies because of the interdiffusion of atomic species, which alters their properties
Au appears brighter in scanning transmission electron microscopy (STEM) images because it has a higher Z of 79 versus 46 for Pd
There are small voids on the Pd side of the assynthesized samples, heating led to additional voids that did not appear to be associated with the original ones
Summary
Bimetallic nanomaterials change their morphologies because of the interdiffusion of atomic species, which alters their properties. The STEM image sequences from these experiments suggest that the nanoscale voids were mobile during heating and that they moved towards the NP surface, contrary to earlier microscopic studies that indicated the formation of stable voids in bimetallic NPs15,16.
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