Controllable microwave and ultrasonic wave combined synthesis of ZnO micro-/nanostructures in HEPES solution and their shape-dependent photocatalytic activities

Size- and morphology-controlled zinc oxide (ZnO) micro-/nanostructures have been successfully synthesized via a facile and rapid microwave and ultrasonic wave combined method in HEPES solution (HEPES   =   2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic acid). The as-prepared ZnO products are characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and UV–vis diffuse reflection spectroscopy (DRS). Various morphologies of ZnO products, including grenade-like, column-like, spindle-like, rod-like, shuttle-like and flower-like micro-/nanostructures are obtained, which are strongly dependent on Zn/HEPES moral ratio, pH value and Zn precursor. It is found that HEPES plays a crucial role in the formation of ZnO micro-/nanostructures with controllable size and morphology. The photocatalytic activities of the prepared ZnO micro-/nanostructures are evaluated by degradation of methylene blue (MB) under UV light irradiation, among which spindle-like ZnO microstructures exhibit superior photocatalytic activity compared with other ZnO products.