Unraveling the shell growth pathways of Pd-Pt core-shell nanocubes at atomic level by in situ liquid cell electron microscopy

Understanding the formation of core-shell nanomaterials is decisive for controlling their growth, structure, and morphology, which is particularly important in catalysis. As a promising material for photo catalysis application, Pd-Pt core-shell nanoparticles (NPs) have been in the spotlight for many years owing to their catalytic performance typically superior to that of pure Pt nanoparticles. The generation of ultra-thin Pt skins of only a few atomic layers on Pd nanoparticles has turned out to be extremely difficult because Pt tends to form islands during deposition instead of a continuous shell. Therefore, understanding the atomic mechanisms of shell formation is critical for atomic-scale design and control of the platinum shell. Here, by using in situ graphene-based liquid cell scanning transmission electron microscopy (STEM), the growth mechanisms of the Pt shell on Pd nanocubes (NCs) are studied in aqueous solution at the atomic level. Pd-Pt core-shell NPs are formed via two distinct mechanisms: (i) at low concentration of Pt atoms, an ultra-thin skin of only a few atomic layers is formed via atom-by-atom deposition and (ii) at higher concentration of Pt atoms, inhomogeneous islands and thick shells are formed via attachment of Pt clusters. Our study provides a route to control core-shell growth and helps us to understand the exact atomic mechanisms of Pt shell growth on Pd seeds.