Tuning ZnS nanostructure’s band gap to visible spectrum: experimental and ab-initio studies

ZnS and Zn1-xCdxS were synthesised via thermolysis of the Zinc(II) dithiocarbamate. Upon Cd doping into ZnS, the bandgap shifted from UV to visible region. Both experimental data and ab-initio calculations based on DFT reveal an increase in the lattice constant of ZnS upon Cd doping. The calculated formation energy of Zn1-xCdxS signifies that under Zn-poor conditions, the process is energetically more favorable compared to Zn-rich conditions, with Cd uniformly distributed at the Zn site. Furthermore, the lower bandgap of Zn1-xCdxS than that of ZnS can be attributed to chemical pressure in the structure, which reduces the conduction band toward the Fermi level and elevates the valence band toward the Fermi level, decreasing the band gap value to around 2.5 eV at 319.28 mg Cd concentration (the atomic ratio of Zn/Cd is 0.4:0.6). Solar-assisted degradation of the methylene blue (MB) dye was carried out on both pristine and modified ZnS to elucidate bandgap tuning to visible region upon Cd doping. The highest photodegradation rate (4.18 × 10–2 min−1) of MB dye for Zn0.4Cd0.6S compared to pristine ZnS (1.37 × 10–2 min−1), is probably due to a shift in the bandgap (ZnS) from UV to the visible region (Zn0.4Cd0.6S) upon doping.