Anomalous coarsening of nanocrystalline zinc oxide particles in humid air

Zinc acetate in combination with water plays a key role during the coarsening of zinc oxide (ZnO) nanocrystals at moderate temperature (85 °C) in air. The growth of ZnO nanocrystals is well known in liquid phase systems, but this work shows that this process is strongly enhanced in powder form by the presence of residual acetate. The growth of the ZnO nanocrystals was documented by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM), showing a preferred growth along [0001] crystallographic direction. An increase of more than 400% in crystal size was observed, which could be related to coarsening but not due to precipitation from solution. In contrast, particle size stayed almost constant if pure zinc oxide powder was used. This growth is expected to slowly occur during storage even under ambient conditions. The limited stability of nanopowders limits their applicability as well as pressing into bulk materials.