Structure, optical properties and synthesis of Co-doped ZnO superstructures

Co-doped ZnO nanoparticles were synthesized by a simple chemical method at low temperature with Co:Zn atomic ratio from 0 to 7 %. The synthesis process is based on the hydrolysis of zinc acetate dihydrate and cobalt acetate tetrahydrate heated under reflux to 65 degrees C using methanol as a solvent. X-ray diffraction analysis reveals that the Co-doped ZnO crystallizes in a wurtzite structure with crystal size of 12-5 nm. These nanocrystals self-aggregated themselves in a highly spherical superstructure of broad size distribution. High-resolution transmission electron microscopy image also shows that each sphere is made up of numerous nanocrystals with average interfringe distance of similar to 0.28 nm. The X-ray diffraction patterns, energy dispersive X-ray, scanning electron microscopy and high-resolution transmission electron microscopy micrographs of doping of Co in ZnO confirmed the formation of spherical superstructure and indicated that the Co2+ is successfully substituted into the ZnO host structure of the Zn2+ site. The optical band gap of ZnO nanoparticles was remarkably from 3.32 to 4.12 eV with increase of Co doping levels from 0 to 7 %. Photoluminescence measurements confirm these results.