Thermal Stability of Core–Shell Nanoparticles: A Combined in Situ Study by XPS and TEM

In situ techniques of transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were used to investigate the thermal stability of Ni–Co core–shell nanoparticles (NPs). The morphological, structural, and chemical changes involved in the core–shell reconfiguration were studied during in situ annealing through simultaneous imaging and acquisition of elemental maps in the TEM, and acquisition of O 1s, Ni 3p, and Co 3p XP spectra. The core–shell reconfiguration occurred in a stepwise process of surface oxide removal and metal segregation. Reduction of the stabilizing surface oxide occurred from 320 to 440 °C, initiating the core–shell reconfiguration. Above 440 °C, the core–shell structure was disrupted through Ni migration from the core to the shell. This resulted in the formation of a homogeneous Ni–Co mixed alloy at 600 °C. This study provides a mechanistic description of the alteration in the core–shell structures of NPs under vacuum conditions and increasing annealing temperature, which is crucial for understanding these technologically important materials.