Differential segregation behaviors of Au in Au@Pt and Au@Pd core-shell nanoparticles

When homogenized as core-shell structures, the structural instability of Au cores in bimetallic nanoparticles (NPs) has been well documented in the theoretical calculations, yet there are limited examples in the experiments. It is presented in this study that in the presence of Ag(I), Au cores with the average diameter of 33.0 nm are removed with crystalline Au being hardly found when Au NPs are coated with Pt shells, while the crystallinity and size of Au cores remain intact when they are coated with Pd. Strains follow when heterogeneous metals are embedded around Au cores with relatively large size and relatively large interfacial area, and there are ways for releasing the interface energy for Pd: i) twin boundaries and stacking faults on the surface of Pd shells, and ii) small voids around Au cores. When Au NPs are coated with Pt, however, porous structures rather than crystalline structures are observed with twin defects hardly found, and for releasing high surfaces energies of Pt, Au atoms diffuse actively outward. In the absence of Ag(I), significant alloying progresses with no hollow cores observed. Ag(I) is believed to facilitate the segregation of Au to the surfaces, and during this rapid movement, Pt does not diffuse as quickly as Au and consequently hollow cores are generated in the presence of Ag(I).