Hollow and Concave Nanoparticles via Preferential Oxidation of the Core in Colloidal Core/Shell Nanocrystals

Abstract

Hollow and concave nanocrystals find applications in many fields, and their fabrication can follow different possible mechanisms. We report a new route to these nanostructures that exploits the oxidation of Cu_2–<i>x</i>Se/Cu_2–<i>x</i>S core/shell nanocrystals with various etchants. Even though the Cu_2–<i>x</i>Se core is encased in a thick Cu_2–<i>x</i>S shell, the initial effect of oxidation is the creation of a void in the core. This is rationalized in terms of diffusion of Cu⁺ ions and electrons from the core to the shell (and from there to the solution). Differently from the classical Kirkendall effect, which entails an imbalance between in-diffusion and out-diffusion of two different species across an interface, the present mechanism can be considered as a limiting case of such effect and is triggered by the stronger tendency of Cu_2–<i>x</i>Se over Cu_2–<i>x</i>S toward oxidation and by fast Cu⁺ diffusion in copper chalcogenides. As the oxidation progresses, expansion of the inner void erodes the entire Cu_2–<i>x</i>Se core, accompanied by etching and partial collapse of the shell, yielding Cu_2–<i>x</i>S_<i>y</i>Se_1–<i>y</i> concave particles