KINETICS AND THERMODYNAMICS IN THE SYNTHESIS OF ZnO NANOWIRES AND NANOSHEETS BY VARIOUS SYNTHETIC ROUTES

1-D nanostructures, such as nanowires and nanotubes, have been extensively studied due to their potentials as building blocks for fabricating electronic, magnetic, electrooptic, and electrochemical nanodevices. Especially, wide-bandgap semiconductor nanowires and nanorods including ZnO and SnO2 were reported significantly. As for their synthetic routes, a carbothermal reduction process has been most widely reported while the direct evaporation of Zn/ZnO and MOCVD process were also investigated. Although a number of reports on the synthesis of ZnO nanowires by the carbothermal reduction process, there have been no comprehensive investigations on the kinetics and thermodynamics regarding on the source vapors of Zn and oxygen in this process. In the present work, we studied the effect of oxygen based on the considerations of the related thermodynamics and kinetics. According to our repeated experiments, it is believed that ZnO nanowires could not be synthesized with Ar carrier gas only and there should be a critical limit of additional oxygen for the synthesis of ZnO nanostructures in a carbothermal reduction process. An oxygenassisted carbothermal reduction process enables an evolution of not only ZnO nanowires but also interesting nanorods and nanosheets in series with controlling oxygen partial pressure in the range of 10 to 10 atm (Fig. 1). The sufficient level of reactant vapors could be established in not only an O2-assisted carbothermal reduction procces but also a MOCVD process, where we could also confirm 1.5 and 2D ZnO nanostructures. We will also discuss on some typical processes reported for ZnO nanowire synthesis as shown in Fig. 2.