Enhanced Structure Stability of Au Nanobipyramids by an <i>In Situ</i> Customized Silver Armor

Revealing the structure stability and evolution of gold nanocrystals at the atomic scale is crucial to their versatile applications; however, the fundamental mechanism remains elusive due to the lack of <i>in situ</i> characterizations. In this work, the structural evolution of two types of Au nanobipyramids (Au NBPs) at elevated temperatures is monitored through <i>in situ</i> electron microscopy analysis, and there is a sharp distinction between their structure stability despite that they possess the same crystalline structure. Detailed material characterization reveals that the surface alloying of residual Ag with Au (customized Ag armor) can greatly inhibit the Au atom diffusion and contribute remarkably to the stability and surface-enhanced Raman scattering improvement. Moreover, the structure of the Au NBPs is further enhanced when the vulnerable tips are selectively coated with an oxide shell (Cu₂O) to form a dumbbell-shaped nanostructure, which would undergo a completely different structure evolution at elevated temperatures.