Study on synthesis and blue emission mechanism of ZnO tetrapodlike nanostructures

Zinc oxide (ZnO) tetrapodlike nanostructures were synthesized by thermal evaporation of Zn powder at different flow rates of argon. Scanning electron microscopy, x-ray diffraction, transmission electron microscopy, and photoluminescence were employed to study the structural features and optical properties of the product. Results show that the flow rate of argon gas has a comparative great influence on the morphology of ZnO nanostructures. As the flow rate increases, nano-ZnO morphology changes from an initial mixture of tetrapod, nanowire, nanosheet, and nanodendritic forms to a uniform tetrapodlike morphology. The fact that the ultraviolet emission weakens and the blue emission strengthens with increase of the argon flow rate suggests that the oxygen vacancy is most likely responsible for the blue emission. This was confirmed by the experimental results: the blue light emission disappears after the products are oxidized in air at 700°C, and then reappears after the oxidized products are deoxidized in H2 at 700°C. The growth mechanism of the ZnO tetrapods is also discussed.