Green and facile synthesis of hierarchical cocoon shaped CuO hollow architectures

Hierarchical cocoon shaped CuO hollow nanostructures have been synthesized on a large scale by a facile green microwave-assisted aqueous process at reflux in the absence of templates and additives. The as-prepared products were investigated by FESEM, TEM, HRTEM, XRD and UV-vis absorption optical properties. The results reveal that cocoon shaped structures, that is ellipsoids, possess a monoclinc phase CuO with the long and short diameters of about 300 nm and 150-200 nm, respectively. The walls of hollow cocoons with around 50 nm in thickness are self-assembled by nanorods with diameters of 6-8 nm. A possible mechanism for the hollow CuO structures was proposed, in which CO₂ bubbles act as soft-template to aggregate the primary monomers. The band gap energies were estimated to be 2.03 eV, which showed the quantum size effect of the nanosized semiconductors.