Effect of Cu Concentration on Optimizing the Optical, Morphological, and Structural Characteristics of ZnIn2S4 Nanosheets for Photoresponse Applications

The ZnIn2S4 ternary compound has attracted considerable interest because of its suitability in various photocatalysis and optoelectronic sectors. In this study, ZnIn2S4 nanoparticles were synthesized by using a straightforward colloidal method with varying amounts of Cu doping. Structural analysis indicated that the material was polycrystalline, with the presence of a primary ZnIn2S4 rhombohedral phase with the coformation of secondary ZnS phases. The selected area electron diffraction (SAED) pattern clearly showed the corresponding planes of the sample. The introduction of doping caused the broadening of the crystalline peaks and decreased their intensity. The changes in the peak intensity in the X-ray diffraction (XRD) and Raman spectra at different doping levels suggested structural modifications. Morphological studies showed the formation of nanosheets with a marigold flower-like structure, which did not vary significantly with doping. The presence of broad reflectance in the vis-near-infrared (NIR) region with a reduced bandgap is beneficial for photocatalytic and sensing applications. Higher doping levels in the ZnIn2S4 lattice introduced intermediate states inside the energy gap, thereby altering the material's optical properties. The redshift in the absorption edge due to doping led to a decreased optical bandgap, which was associated with increased disorder and defects. Refractive indices, calculated by using various theoretical models, increased as the bandgap decreased. Photoluminescence studies at room temperature showed a redshift in the peak position and broadening of the peak width. Thermal studies revealed the existence of both endothermic and exothermic peaks. Additionally, the photoresponse investigation showed changes in the rise and decay times of the material in addition to an increase in the current value. The observed changes in the studied materials are useful for optoelectronic and photoresponsive applications.