Freeze-drying is a convenient cryochemical powder processing method. In this paper, Co-doped ZnO nanomaterials (CZN) with Co content from 2 mol% to 10 mol% have been successfully synthesized via a novel freeze-drying route. X-ray diffraction (XRD), transmission electronic microscopy (TEM) and high-resolution TEM (HRTEM) were used to characterize the structure and morphology of the as-synthesized samples. All samples have a similar hexagonal wurtzite phase without metal cobalt, cobalt oxides or composites in the samples with Co concentration below 6 mol%. TEM image shows that CZN (4 mol% Co) is nanometer scale with average crystal size of 40 nm. Samples have a spherical morphology with moderate aggregation. The band gap energy of CZN from ultraviolet-visible (UV-vis) spectroscopy decreases with increasing the Co doping concentration. (c) 2008 Elsevier B.V. All rights reserved.