Optical properties of zinc sulfide nanopowders and ZnS/Ag$_{2}$S heteronanostructures

Nanopowders of zinc sulfide (ZnS) and ZnS/Ag$_{2}$S heteronanostructures were synthesized by chemical deposition from aqueous solutions. A change in the ratio between the concentrations of reagents allowed us to obtain ZnS nanopowders with a particle mean size from 2 to 10 nm. The Ag$_{2}$S nanoparticle size in the resulting heteronanostructures was 9–30 nm and the ZnS surface layer thickness was equal to 4–5 nm. The diffuse reflectance spectra for nanostructured ZnS and ZnS/Ag$_{2}$S heteronanostructures were measured. Based on the analysis of the obtained spectra, the bandgap energy $E_g$ in the studied sulfide nanostructures was estimated. Upon a decrease in the nanoparticle size from 10 to 2 nm, the $E_g$ value in ZnS nanopowders increased in a range of 3.17–3.36 eV. An increase in the content of Ag$_{2}$S in the ZnS/Ag$_{2}$S heteronanostructures led to a decrease in the bandgap energy and a decrease in the luminescence intensity.