Tunable morphologies of ultrathin ZnO nanostructures synthesized by a plasma afterglow-assisted oxidation process and their photocatalytic properties

Abstract

The fast growth of ultrathin zinc oxide nanostructures is reported in this study, by using a flowing microwave plasma oxidation method: zinc films coated by a thin copper buffer layer are oxidized in low-pressure afterglow. This work highlights the versatility of plasma afterglow treatments and demonstrates that the morphology of ZnO nanostructures (nanowires, nanoribbons and nanocombs) can easily be controlled with good reproducibility by adjusting experimental parameters such as the working pressure during the oxidation process. The rapid growth of ultrathin nanostructures (∼1 min) is due to the afterglow area which is a very oxidizing medium since it contains high concentrations of oxygen atoms and excited oxygen molecules. The surface morphology and structural and optical properties of the as-synthesized nanostructures are studied by means of SEM, HRTEM and UV–visible absorption, respectively. Ultrathin ZnO nanowires and nanoribbons with high aspect ratios exhibit excellent photocatalytic activity for the degradation of a model organic dye: methyl orange (MO), by comparison with pure ZnO thin films without nanostructures and thick ZnO nanowires synthesized by thermal oxidation. These results are supported by photocurrent measurements. The enhancement in photocatalytic performance could be attributed to higher surface area and higher transfer efficiency of photoinduced charge carriers to the surface of ultrathin nanowires and nanoribbons. The reusability of ZnO nanowire samples is also investigated by recording the photoactivity after several cycles of photodegradation.