Probing the Origin of Gold Dissolution and Tunneling Across Ni₂P Shell Using in situ Transmission Electron Microscopy

Hybrid core-shell catalysts possess superior physiochemical properties that are closely related to their atomic structures. Here, we report novel diffusion phenomena in an Au-Ni₂P yolk-shell structure using in situ heating transmission electron microscopy (TEM) analysis. The Au yolks can dissolve completely into and diffuse across the Ni₂P shell at 500 °C, resulting in an inward volume expansion of the Ni₂P shell and shrinkage of the void. The dissolved Au is subsequently segregated, forming a new crystal on the outer layer of the shell. A detailed atomic-scale resolution imaging revealed that the outward Au diffusion and aggregation occur when the Au yolks are epitaxial to the Ni₂P shells. Theoretical analysis shows that the outward diffusion and deposition of Au atoms is primarily driven by the excess epitaxial strain energy.