Tailoring of growth and properties: a benign approach to synthesise ZnO nanostructures without growth-directing agents

Achieving rational control over the growth of nanostructures without employing growth-directing agents has received considerable attention. Such approaches are a key to obtaining nanomaterials pure enough for desired applications. We report on the novel synthesis of nearly monodisperse zinc oxide nanocrystals, randomly oriented nanocrystals in spherical aggregates, and flower-like aggregates of oriented nanocrystals through a benign, ultrasonic assisted approach. It is shown that crystals of defined dimensions and morphology can be obtained by controlling the temperature of the sonicated solution of a well-chosen precursor. The powder samples resulting from sonochemical synthesis are thoroughly characterized by powder x-ray diffraction, transmission electron microscopy, scanning electron microscopy, and by Raman and UV-vis spectroscopies. The results are explained by invoking the phenomena of randomly oriented aggregation, oriented attachment, and ripening, as amply supported by electron microscopy. Photoluminescence spectra of these various ZnO nanostructures reveal a strong dependence on the dimensions and morphology of the nanostructures, indicating that such luminescence can be tailored suitably.