Hollow and concave nanoparticles via preferential oxidation of the core in colloidal core/shell nanocrystals

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-x)Se/Cu(2-x)S core/shell nanocrystals with various etchants. Even though the Cu(2-x)Se core is encased in a thick Cu(2-x)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-x)Se over Cu(2-x)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-x)Se core, accompanied by etching and partial collapse of the shell, yielding Cu(2-x)S(y)Se(1-y) concave particles.