Thermally stimulated structural evolution of bimetallic nanoplatelets - Changing from core-shell to alloyed to Janus nanoplatelets

Gold-based bimetallic nanostructures exhibit unique optical and catalytic properties that are strongly dependent on their composition and nanoscale geometry. Here we show the nano -structural transformation of mesoporoussilica-coated Au -M (Ag, Pd, Pt) core-shell nanoplatelets (NPLs) with a triangular shape to alloyed platelets at temperatures at least 300 degrees C below the lowest melting point of the metals while still retaining the out-ofequilibrium triangular shape and intact mesoporous shell. Before the alloying started the rough core-shell morphology of the Au-Pd and Au-Pt NPL systems were first observed to relax into a much smoother coreshell morphology. The alloying temperature was found to be related to the melting points and atom fractions of the shell metals; the higher the melting point and atomic fraction of the shell metal, the higher the temperature required for alloying. The highest alloying temperature was found for the Au-Pt system (650 degrees C), which is still hundreds of degrees below the bulk melting points. Surprisingly, a phase separation of Au and Pt, and of Au and Pd, was observed at 1100 degrees C while both systems still had an anisotropic plate -like shape, which resulted in Januslike morphologies where the pure Pt and pure Pd ended up on the tips of the NPLs as revealed via in -situ heating in the scanning transmission electron microscope (STEM). The Janus -type morphologies obtained at elevated temperatures for the NPLs composed of combinations of Au-Pt and Au-Pd, and the smooth core-shell morphologies before alloying, are very interesting for investigating how differences in the bi-metallic morphology affect plasmonic, catalytic and other properties.