Preparation of zinc oxide nanopowder by freeze-drying

The preparation of the acetate precursor of ZnO by freeze-drying was studied and compared with the powder obtained when the same starting acetate salt was thermally decomposed. Nanosized precursor particles of zinc acetate were formed when the quantity of solvent, the temperature and the vacuum during freeze-drying were all controlled. The subsequent calcinations of the acetate precursors at 300, 400 and 500degreesC for 1h resulted in the formation of nanosized zinc oxide powders. The nature of these nanopowders has been characterized using X-ray diffraction, differential thermal analysis and both scanning and transmission electron microscopy. It is shown that freeze-drying leads to the production of homogeneous precursors at low temperature, consistent with the molecular dispersion of the starting solution. Decomposition upon heating the precursors showed the opposite characteristics, i.e., they are inhomogeneous, exhibiting more acetate at 300degreesC and bimodal sizes at 400degreesC. The average size of the uniform particles of ZnO obtained by freeze-drying was approximately 100 nm. This result opened up an opportunity for producing uniform ZnO nanopowders from acetate salts by freeze-drying.