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

Abstract

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 mesoporous-silica-coated Au-M (Ag, Pd, Pt) core-shell nanoplatelets (NPLs) with a triangular shape to alloyed platelets at temperatures at least 300 °C below the lowest melting point of the metals while still retaining the out-of-equilibrium 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 core-shell 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 °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 °C while both systems still had an anisotropic plate-like shape, which resulted in Janus-like 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.