Microwave synthesis of ZnO microcrystals with novel asymmetric morphology

This paper reports a discovery of novel morphological features of ZnO asymmetric micron-sized single crystals synthesized by microwave irradiation of W/O emulsions containing a Zn source in the inner aqueous phase. The influence of the zinc ion concentration on the size and shape is studied in detail. The morphological and structural properties show that the obtained ZnO powders are almost mono-dispersed ZnO single crystals with a novel morphology of truncated hexagonal pyramids ca. 1 μm in diameter. A scanning precession electron diffraction analysis reveals that the asymmetrically shaped ZnO single crystals thus obtained have a crystallographic c-axis perpendicular to the bottom hexagonal flat surfaces, and annular bright-field imaging indicates that the bottom surfaces are Zn-terminated ZnO(0001)-Zn polar faces. These new findings are led from a combination of well-controllable liquid-phase emulsion synthesis of the oxide powders and highly advanced atomic-resolution TEM imaging technique, suggesting for the first time a highly probable relationship between the polarity of the wurtzite-type crystal structure and the novel asymmetric morphology of the ZnO microcrystals obtained in this study.