Flower-shaped hierarchical CuO microspheres consisting of interpenetrating nanoslieets were successfully synthesized by reflux condensation method without using any surfactant or templates. X-ray diffraction revealed the formation of single phase CuO exhibiting monoclinic crystal structure. Scanning electron microscopy and transmission electron microscopy confirmed the formation of hierarchical flower-shaped CuO microspheres made up of nanosheets as building blocks. UV-vis diffused reflectance spectroscopy showed a steep absorption between 700 and 950 nm, and the corresponding band gap of CuO nanoparticles was found to be 1.59 eV. To explore the potential capability of flower-shaped CuO microspheres for wastewater treatment, the photocatalytic activity of CuO nanostructures was evaluated by monitoring the photodegradation of methyl orange (MO) and methylene blue (MB) dyes in the presence of hydrogen peroxide under UV light irradiation. Experimental results demonstrated that CuO hierarchical architecture possesses good photocatalytic activity towards MO and MB dyes, indicating the feasibility of using CuO for the effective removal of organic contaminants from wastewater. (C) 2015 Elsevier B.V. All rights reserved.