Nano Kirkendall Effect Related to Nanocrystallinity of Metal Nanocrystals: Influence of the Outward and Inward Atomic Diffusion on the Final Nanoparticle Structure

The Kirkendall effect is a classical phenomenon in materials science, and it is referred to as a nonreciprocal interdiffusion process through an interface of two metals with strikingly different atomic diffusivities, leading to a formation of vacancies called Kirkendall voids. The nanoscale Kirkendall effect has been vastly applied in the fabrication of hollow nariostructures after the first report on the synthesis of Co-based hollow nanocrystals. In this Feature Article, we briefly start with an introduction on the Kirkendall effect concept, followed by the general synthetic strategy toward the production of hollow Kirkendall voids. The overall synthetic strategies are based on the design of diffusion couples at the nanoscale, and then, we discuss the factors that govern the formation of Kirkendall voids at the nanoscale, from the viewpoint of the nanoparticle size, nanoparticle crystallinity, and nanoparticle environment. We conclude with a summary and perspectives on the design of hollow nanostructures governed by the Kirkendall effect.