Preparation of nano-sized ZnO powder by freeze-drying

Abstract

Zinc oxide(ZnO) has been widely applied in the fields of cosmetics, paint, coating and gas sensor. Size down of ZnO is essential for the preparation of sol or film of ZnO in the application. The size and shape of ZnO particles are controlled by varying drying method. The ZnO was prepared by mixing of zinc acetate and sodium hydroxide at 0°C. Then it was freeze dried or 20 °C vacuum dried. After washing the residual ions, it was further compared by drying at 20 °C vacuum drying or 200 °C oven drying. The ZnO powder was characterized by XRD, Brunauer-Emmet-Teller analysis(BET) and transmission electron microscope(TEM). The ZnO have wurzite structure with ∼10 nm particle size when it was prepared by freeze drying and then 20 °C vacuum drying after washing the residual ions. On the other hand, without freeze drying, the ZnO particles observed to be aggregated to 200-400 nm with primary particle size of∼20 nm. The smaller size of ZnO particle in freezing drying compared to 20 °C vacuum drying may be explained by frozen water preventing the growth of ZnO particles.