Studies on role of defects in structural, photoluminescent and photocatalytic behaviours of ZnS nanoparticles

Abstract

This study explores the impact of defects on the structural, photoluminescent, and photocatalytic behaviors of ZnS nanoparticles synthesized by two methods: hydrothermal and microwave-assisted co-precipitation method. The defects are induced by varying stoichiometric ratio and by doping rare earth elements. In the hydrothermal synthesis method, point defects were introduced by varying the S/Zn molar ratio and doping with Ce, La, and Y. These modifications led to the formation of sulphur vacancies and sulphur dangling bonds, which play a crucial role in enhancing photocatalytic activity under visible light. The sulphur defect states and dangling bonds act as trapping sites for charge carriers, facilitating their separation and prolonging their lifetimes, thereby significantly improving photocatalytic efficiency. Photoluminescence (PL) measurements revealed an emission peak around 470 nm, attributed to these sulphur-related defects, further confirming their presence. However, the PL transitions corresponding to La, Ce, and Y dopants were absent in the hydrothermal samples, likely due to the overlapping of emission from the host material, masking the dopant-related emissions but the luminescence intensity tuning is possible by doping. Additionally, the doping process induced a phase transition from cubic to hexagonal structure, which also contributed to the observed changes in material properties. In contrast, ZnS nanoparticles synthesized using the microwave-assisted method did not exhibit photocatalytic activity despite their smaller crystallite size. The absence of sulphur-related PL emission peaks in these samples indicates lack of active sulphur defects, which explains their inactivity in photocatalysis. However, the PL spectra showed transitions corresponding to the dopants Er and Pr, reflecting the influence of the synthesis method on the nature and distribution of defects. This study highlights the importance of synthesis methods and conditions in defect engineering, which enhance the photocatalytic and photoluminescence activity. These findings provide a foundation for optimizing ZnS nanoparticles for targeted applications in photocatalysis and optoelectronics.