The origin of ZnO twin crystals in bio-inspired synthesis

A fabrication of uniform nacre-like hierarchical nanostructures of faceted ZnO twin-crystals was established by a hydrothermal route using gelatin as the structure-directing agent, zinc nitrate hexahydrate as the Zn source, and hexamethylenetetramine to control alkalinity. Early stage growth of ZnO twin-crystals was investigated by powder X-ray diffraction, scanning electron microscopy and transmission electron microscopy. A new formation mechanism is proposed. In the bio-inspired synthesis, Zn ₅(NO ₃) ₂(OH) ₈·2H ₂O nanoplatelets (10 to 20 nm in size) undergo orientated aggregation with gelatin to form Zn ₅(NO ₃) ₂(OH) ₈·2H ₂O/gelatin mesocrystalline nanoplates (150 to 400 nm in diameter and 20 to 50 nm in thickness). Surface re-crystallization of these nanoplates leads to two thin layers of ZnO separated by gelatin molecules. These double-layer nanoplates, negatively charged on both outer surfaces, are the cores of the twin-crystals. The dipolar Zn ₅(NO ₃) ₂(OH) ₈·2H ₂O nanoplates then stack on both sides of the double-layer nanoplates, followed by a phase transformation to ZnO. Eventually, twin-crystals are constructed in a manner reminiscent to that of an hourglass. The hexagonal morphology of the twin-crystals resulted from a late re-crystallization. The microstructure of the ZnO twin-crystals is very similar to the brick and mortar arrangement found in nacre. The present study is expected to shed light on the formation mechanism of many naturally occurring biominerals, as well as many other synthetic twin-crystals.