Solid-state diffusion in Cu core/Ni shell nanoparticles*

Cu core/Ni shell nanoparticles with nominal Cu/Ni atomic ratios of 2/1, 1/1,1/2, and 1/3 have been prepared in a microwowave reactor by the sequential reduction of Cu acetate (at 100 ^○C) and Ni acetate (at 175 ^○C) in ethylene glycol using sodium hypophosphite as the reducing agent. The measured room temperature lattice parameters of the as-prepared samples are slightly smaller/larger than the corresponding bulk Cu and Ni lattice parameters indicating a small amount of diffusional mixing at the Cu/Ni interface during sample preparation. Ex-situ vacuum annealing experiments followed by room temperature x-ray diffraction measurements and in-situ high temperature x-ray diffraction measurements have been used to follow the conversion of the as-prepared core/shell nanoparticles to a homogeneous Cu-Ni alloy as a function of both the temperature and the time. These measurements reveal that significant diffusional mixing occurs starting at a temperature between 300 and 400 ^○C and complete mixing with the formation of a homogeneous Cu-Ni alloy occurs at a temperature of 600 ^○C.