Shape Evolution of Biconcave Djurleite Cu_1.94S Nanoplatelets Produced from CuInS₂ Nanoplatelets by Cation Exchange

Development of nanomaterials of previously unavailable shapes and compositions continues to be a key need and interest in nanotechnology. Here, we report the preparation of unique biconcave djurleite Cu_1.94S nanoplatelets (NPls) from template CuInS₂ (copper indium sulfide, CIS) NPls via a cation exchange (CE) reaction. Upon initiation of the CE reaction, the In³⁺ ions diffuse out of the CIS crystal lattice, and the remaining copper sulfide adopts the djurleite phase almost instantly. This rapid phase transition produces numerous vacancies and defects before Cu⁺ ions can diffuse into the nanostructures. The formation of a biconcave shape is attributed to the assembly and migration of these defects. The flat surfaces of the NPls are ultimately restored through a ripening process that produces single-crystalline NPls much thicker than the starting templates. Striped NPls were observed in the final products, due to stacking faults at the boundary between newly deposited and residual layers of djurleite. These studies not only provide a better understanding of the relationships among composition, morphology, and crystal structure for copper sulfide-based nanocrystals, but also provide a pathway to a previously inaccessible morphology.