Hierarchical Hollow Spheres of ZnO and Zn_1−xCo_xO: Directed Assembly and Room-Temperature Ferromagnetism

A novel water-bubble template process has been developed to synthesize uniform hierarchical hollow spheres of ZnO and transition metal doped ZnO in ethylene glycol. For undoped ZnO, nanorods are formed due to the preferred c-axial growth and assembled into highly regular hollow spheres. Co-doped ZnO, in contrast, favors lateral growth into nanodisks because of the deterred c-axial extension caused by the Co dopants. Nevertheless, the Zn_1−xCo_xO (x = ∼0.59%, ∼1.62%, ∼5.21%) nanodisks, albeit with a very different morphology from the nanorods, are also assembled cleanly into high-quality hollow spheres. The synthesis conditions have been carefully studied and the products have been extensively characterized by a variety of techniques. In particular, Raman scattering and photoluminescence spectra of the Zn_1−xCo_xO hollow spheres prove that the Co atoms occupy the Zn sites of wurtzite ZnO. Also, room temperature magnetic measurements show robust coercivities, signifying the ferromagnetism that is characteristic of dilute magnetic semiconductors (DMS).