Scheme 1. Various Stages Involved in the Formation of Hollow/Concave NCs.

(a) Beginning of the oxidative process: electrons and Cu⁺ ions start being extracted from the Cu_2–xS shell of an initial Cu_2–xSe/Cu_2–xS core/shell NC. The loss of Cu⁺ ions from the shell is counterbalanced by the diffusion of Cu⁺ ions from the Cu_2–xSe core to the shell. (b) Early stage of etching: vacancy coalescence creates a void in the core. (c) Later stage: the core has been dismantled, etching of the shell begins and part of the shell collapses. A ternary alloy is being formed at the core/shell interface. (d) Eventually, a porous Cu_2–xS_ySe_1–y nanocage remains.