Zinc oxide (ZnO) nanostructures have been synthesized by a simple controlled precipitation method to study the effects of the type solvent – including water, acetic acid and ethylene glycol – on the formation of ZnO and on its photocatalytic activity for the decomposition of H 2 O 2 in aqueous solution. Scanning electron microscopy (SEM) and synchrotron-based X-ray diffraction (XRD) were used to determine the morphology, crystallinity, and chemical composition of the ZnO structures. Raman spectroscopy and photoluminescence measurements were carried out to determine the nature of the defects present in the different ZnO nanostructures and how they affect the photocatalytic activity. Based on the results, we propose plausible growth mechanisms underlying the formation of ZnO with different morphology, according to the solvent used during the synthesis. A direct relation between the photocatalytic activity and the defects type was established, suggesting that defects play a vital role in modulating the photocatalytic response.