Zinc oxide (ZnO) was synthesized using a microwave assisted hydrothermal (MAH) process based on chloride/urea/water solution and under 800 W irradiation for 5 min. In the bath, Zn(2+) ions reacted with the complex carbonate and hydroxide ions to form zinc carbonate hydroxide hydrate (Zn(4)CO(3)(OH)(6)center dot H(2)O), and the conversion from Zn(4)CO(3)(OH)(6)center dot H(2)O to ZnO was synchronously achieved by a MAH process. The as-prepared ZnO has a sponge-like morphology. However, the initial sponge-like morphology of ZnO could change to a net-like structure after thermal treatment, and compact nano-scale ZnO particles were finally obtained when the period of thermal treatment increased to 30 min. Pure ZnO nanoparticles was obtained from calcination of loose sponge-like ZnO particles at 500 degrees C. The analysis of optical properties of these ZnO nanoparticles showed that the intensity of 393 nm emission increased with the calcination temperature because the defects were reduced and the crystallinity was improved. (C) 2011 Elsevier Ltd. All rights reserved.