Quasi spherical and capsule like ZnO nanostructures have been successfully synthesized via a simple precipitation route without the assistance of external capping agents. The effect of annealing temperature on the properties of ZnO was investigated. In all cases, hexagonal wurtzite crystalline structure of phase pure ZnO was obtained. The crystallinity was found to be gradually increasing with annealing temperature. At low annealing temperatures, more or less spherical ZnO nanoparticles were clearly observed, whereas they tend to grow as nanocapsules with increasing the annealing temperature. The formation of single crystalline nanocapsules was observed at 600 degrees C. The photoluminescence spectra indicated the annealing dependent emission features, especially in the spectral intensity. The dye pollutant methylene blue was found to be completely degraded under UV light irradiation over the ZnO nano photocatalysts. The highest photoactivity was shown by nanocapsules obtained at 600 degrees C and was found to be highly reusable. (C) 2014 Elsevier B.V. All rights reserved.