Thermal Annealing ZnO and ZnS Powders as an Alternative to Type II Heterostructure Synthesis

Type II heterostructures of the wide bandgap semiconductors ZnO and ZnS spatially separate holes into ZnS and electrons into ZnO. Charge separation in such core-shell type-II heterostructure powders and nanostructures significantly enhances phosphorescence efficiency and photocatalytic activity. Consequently, there is great interest in developing low cost techniques for reproducibly producing large quantities of these powders. Here we explore a simple thermal annealing approach and examine the effect annealing has on the defect and band edge emissions of four samples: vacuum-annealed ZnO with and without sulfur, and ZnS annealed in vacuum and with oxygen. Scanning electron microscopy and cathodoluminescence spectroscopy revealed aggressive oxygen substitution for sulfur, even creating oxygen vacancy defect emission in ZnS powders, with significant particle-to-particle variability. Evidence suggesting that core-shell structures are formed will be discussed, and the limitations of this approach will be addressed.