Observing Growth of Nanostructured ZnO in Liquid

Hydrothermal synthesis is commonly used to produce a large area of ZnO nanowires because of its simple and inexpensive process. However, the mechanism of hydrothermal synthesis remains unknown. In this work, zinc acetate and HMTA dissolved in deionized water as a precursor solution were sealed in a liquid cell for observation by in situ transmission electron microscopy. The growth of ZnO nanowires was classified into two steps. The first step was the nucleation and growth of ZnO nanoparticles. The ZnO nanoparticles grew as a result of either isotropic monomer attachment on the {21̅1̅0} and {01̅10} surfaces or coalescence of nanoparticles in the same crystal arrangement. The second step was the anisotropic growth of ZnO nanoparticles into nanowires on the (0001) surface. Because the (0001) surface is Zn-terminated with positive charges that can attract the negatively charged monomers, i.e., [Zn(OH) 4 ] 2–,the monomers tended to deposit on the (0001) surface, resulting in ZnO nanowires growing along the [0001] direction. Moreover, the growth of ZnO nanowires was identified to be a reaction-controlled system. The direct observation of the dynamic process sheds light on the hydrothermal synthesis method.