Waning-and-waxing shape changes in ionic nanoplates upon cation exchange

Flexible control of the composition and morphology of nanocrystals (NCs) over a wide range is an essential technology for the creation of functional nanomaterials. Cation exchange (CE) is a facile method by which to finely tune the compositions of ionic NCs, providing an opportunity to obtain complex nanostructures that are difficult to form using conventional chemical synthesis procedures. However, due to their robust anion frameworks, CE cannot typically be used to modify the original morphology of the host NCs. In this study, we report an anisotropic morphological transformation of Cu_1.8S NCs during CE. Upon partial CE of Cu_1.8S nanoplates (NPLs) with Mn²⁺, the hexagonal NPLs are transformed into crescent-shaped Cu_1.8S-MnS NPLs. Upon further CE, these crescent-shaped NPLs evolve back into completely hexagonal MnS NPLs. Comprehensive characterization of the intermediates reveals that this waxing-and-waning shape-evolution process is due to dissolution, redeposition, and intraparticle migration of Cu⁺ and S^2-. Furthermore, in addition to Mn²⁺, this CE-induced transformation process occurs with Zn²⁺, Cd²⁺ and Fe³⁺. This finding presents a strategy by which to create heterostructured NCs with various morphologies and compositions under mild conditions.