Robust Anion Exchange Realized in Crystalline Metal Cyanamide Nanoparticles

Solution-mediated sequential ion exchange has emerged as a powerful yet simple technique to transform nanoparticulates into a complex architecture. However, the state-of-the-art demonstration of such fine-tailored nanostructures greatly relied on cation exchange reaction because it remains to be a great challenge to apply anion exchange without interfering the original morphology and crystallinity of the target particles. Herein, metal cyanamides with a looser cation sublattice enabled by the quasi-linear [NCN](2-) anion units are discovered to be ideal parent compounds to accomplish robust anion exchange reactions. The complete conversion from metal cyanamide nanoparticles to metal chalcogenide nanoparticles (CdNCN to CdS, CdNCN to CdSe, and MnNCN to MnS) is successfully realized by low-temperature reaction in colloidal solution. The nanoparticles retain both the morphology and crystallinity throughout the entire exchange process. In-depth study reveals that the structural similarity in cation packing facilitates the exchange process, and such behavior is independent of the material composition. The quasi-linear structure of the three-atom [NCN](2-) anion results in anisotropic lattice spaciousness, which helps bypass the diffusion rate discrepancy and lattice framework collapse commonly encountered in compounds consisting of single-atom anions. This work sheds light on extending the ion exchange reactions as a synthetic toolbox to a much broader library in nanoparticle synthesis.