ZnO doughnuts: Controlled synthesis, growth mechanism, and optical properties

ZnO doughnutlike cluster particles were prepared by a simple solvothermal route using an ethylene glycol (EG)-water solvent system. It was observed that ZnO doughnuts were obtained when an equal or higher volume fraction of EG was used in the solvent. Microstructural studies revealed that these doughnut-shaped particles were clusters of small hexagonal plates arranged in a regular fashion. Because of the capping property of EG, these constituent hexagonal plates arranged in an angular oriented attachment scheme with a view to minimize their surface energies. The resulting assemblies were concave from one side and convex from the reverse side. With the increasing percentage of EG, the concentration of the capping element increased, resulting in the reduction of the size of the constituent plates, which in turn helps the formation of more and more densely packed ZnO doughnuts. Increases in temperature and pressure also favor the formation of densely packed ZnO doughnuts. These ZnO samples exhibited a strong UV emission, along with a defect-related broad green emission peak.