Formation of spherical calcium sulfate mesocrystals: orientation controlled by subunit growth†
The design and synthesis of highly ordered and oriented hierarchical structures such as mesocrystals is of great interest due to their superior performance. However, the orientation mechanism is not yet fully understood. Here, we report a subcrystallite growth controlled orientation process to synthesize anisotropic α-calcium sulfate hemihydrate (α-HH) spherical mesocrystals in a Na2EDTA-containing ethylene glycol–water system. The spherical mesocrystals consist of rod-like nanoparticles with parallel crystallographic alignment along the c-axis. The time-dependent experiments indicate that the growth of subcrystallites after self-assembly is crucial for mesocrystal formation, which changes the subunits from an irregular shape to a uniform rod-like shape and synchronously aligns them in the same direction. The crystalline growth of subunits mainly results from the supply of lattice ions released from Ca–EDTA complexes. The cross-section observation reveals that the growth and orientation of the subcrystallites start exteriorly and proceed inward step by step. This work deepens the understanding of mesocrystal formation and opens up a potential way for the design and synthesis of complex and highly ordered structures.