Synthesis of CuN and CuN-CuO multicomponent mesocrystals: non-classical crystallization and nanoscale Kirkendall effect.

Mesocrystals are superstructures of crystallographically aligned nanoparticles and are a rapidly emerging class of crystalline materials displaying sophisticated morphologies and properties, beyond those originating from size and shape of nanoparticles alone. This study reports the first synthesis of CuN mesocrystals employing structure-directing agents with a subtle tuning of the reaction parameters. Detailed structural characterizations carried out with a combination of transmission electron microscopy techniques (HRTEM, HAADF-STEM-EXDS) reveal that CuN mesocrystals form by non-classical crystallization, and variations in their sizes and morphologies are traced back to distinct attachment scenarios of corresponding mesocrystal subunits. In the presence of oleylamine, the mesocrystal subunits in the early reaction stages prealign in a crystallographic fashion and afterwards grow into the final mesocrystals, while in the presence of hexadecylamine the subunits come into contact through misaligned attachment, and subsequently, to some degree, realign in crystallographic register. Upon prolonged heating both types of mesocrystals undergo chemical conversion processes resulting in structural and morphological changes. A two-step mechanism of chemical conversion is proposed, involving CuN decomposition and anion exchange driven by the nanoscale Kirkendall effect, resulting first in multicomponent/heterostructured CuN-CuO mesocrystals, which subsequently convert into CuO nanocages. It is anticipated that combining nanostructured CuN and CuO in a mesocrystalline and hollow morphology will provide a platform to expand their application potential.