Direct Visualization of Temperature-Induced Phase Separation of Completely Miscible Au─Pd Alloy by In Situ TEM.

In situ transmission electron microscopy (TEM) studies reveal key insights into the structural and chemical evolution of nanoparticles (NPs) under external stimuli like heating and biasing, which is critical for evaluating their suitability in chemical reactions and their tendency toward forming novel NP systems. In this study, starting from a core@shell Au nanotriangle (AuNT)@Pd nanostructure, the formation of a phase-separated bi-metallic Au─Pd NP system at high temperature is reported, despite the fact that Au and Pd are miscible in the entire composition and temperature range. In situ TEM heating of bare AuNT@Pd core@shell structures up to 1000 °C is performed. Between 400 and 800 °C, an initial alloy formation is observed. Notably, higher initial loading of Pd increases the melting temperature of the bi-metallic system. However, the most important observation is the separation of the nanostructure into Au and Pd phases at temperatures above 850 °C for high Pd doping. The extent of Pd separation depends on the amount of initial Pd loading. A Janus Au─Pd nanostructure is formed at the end of the thermal treatments at 1000 °C. The phase-separated NP is observed to be highly stable and could be clearly beneficial for various applications, particularly in catalytic processes.