Formation of Spherical Calcium Sulfate Mesocrystals: Orientation Controlled by Subunit Growth
The design and synthesis of highly ordered and oriented hierarchical structure such as mesocrystal is of great interests due to its superior performance. However the orientation mechanism is not yet fully understood. Here we report a subcrystallite growth controlled orientation process to synthesize the anisotropic α-calcium sulfate hemihydrate (α-HH) spherical mesocrystals in a Na2EDTA-contained 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 the growth of subcrystallites after self-assembly is crucial for mesocrystal formation, which turns the subunits from irregular shape to uniform rod-like shape and synchronously aligns them in the same direction. The crystalline growth of subunits is mainly resulted from the supply of lattice ions released from Ca-EDTA complexes. The cross-section observation reveals that the growth and orientation of subcrystallites start exteriorly and proceed inwardly step by step. The work deepens the understanding of mesocrystal formation and opens up a potential way for design and synthesis of the complex and highly ordered structure.