Mesocrystals—superstructures of crystallographically aligned nanoparticles—are rapidly emerging class of crystalline materials displaying sophisticated morphologies and properties, beyond those originating from size and shape of nanoparticles alone. Due to their organized structures, mesocrystals possess unique characteristic, such as high surface area and ordered porosity, large pore accessibility, and good electronic and ionic conductivities. Thus, mesocrystals are beneficial in many areas of research and applications, including energy storage and conversion, catalysis and photocatalysis, as mechanical reinforcement and in biomedical applications. To date, mesocrystals are reported for various functional nanomaterials. Most common synthesis approaches are based on template-directed synthesis routes, whereby oriented-aggregation of nanoparticles is achieved along an ordered matrix by spatial constraints often supported by the external physical fields. However, such approaches are incompatible with multicomponent systems, thus making the synthesis of multicomponent (MC) mesocrystals still challenging. Nonetheless, MC mesocrystals are particularly interesting since they are combining intimately connected subunits of different functional nanomaterials, thus allowing the utilization of the collective and coupled materials’ functionalities. In this contribution, we will present the novel synthesis of Cu3N mesocrystals, and a new pathway to Cu3N-Cu2O multicomponent mesocrystals. Cu3N mesocrystals are synthesized in one-pot synthesis and their in-depth structural and chemical characterization is done with different TEM techniques (HRTEM, HAADF-STEM-EDS). The analysis of the intermediate products suggests that Cu3N mesocrystals form as a result of non-classical crystallization, and depending of use of different capping agents follow two distinct attachment scenarios of the subunits. Under prolonged heating, Cu3N mesocrystals undergo chemical conversion process. A two-step mechanism is proposed, involving Cu3N decomposition and anion exchange driven by the nanoscale Kirkendall effect, resulting first in multicomponent Cu3N–Cu2O mesocrystals, which subsequently convert into Cu2O nanocages. The insights gained from this work provide a valuable conceptual advances, allowing for expanding the compositional varieties of mesocrystals and their functionalities.

Non-classical crystallization and nanoscale Kirkendall effect in the synthesis of Cu3N and Cu3N-Cu2O multicomponent mesocrystals

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