PEG- and PVP-assisted wet-chemical synthesis of ZnS quantum dots via hydrothermal and co-precipitation methods for route-dependent structural and bandgap tuning
Zinc sulphide (ZnS) is a non-toxic, wide-bandgap II-VI semiconductor with well-established optoelectronic properties. This work presents a systematic, concentration-dependent comparison of PEG and PVP across two scalable aqueous routes, linking ligand chemistry to crystallite size and bandgap shifts. ZnS quantum dots (QDs) were synthesized using polymer-assisted wet-chemical methods, hydrothermal processing and room-temperature co-precipitation. Polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG) were used as capping agents to examine their influence on particle growth, dispersion, and optical behaviour. Room-temperature co-precipitation produced QDs with crystallite sizes as small as 2.03 nm, whereas hydrothermal synthesis at elevated temperature yielded larger crystallites exceeding 6 nm. X-ray diffraction confirmed cubic zinc-blende ZnS in all samples, with peak broadening and small lattice-parameter variations consistent with nanoscale dimensions. UV-visible absorption spectra showed systematic shifts of the absorption edge, with optical bandgap values ranging from 3.60 to 3.80 eV, consistent with size-dependent quantum confinement. Fourier-transform infrared spectroscopy and dynamic light scattering verified effective polymer capping and particle size distribution, with PVP providing stronger growth suppression and improved dispersion compared to PEG. Overall, the results highlight how synthesis route and polymer-nanocrystal interactions govern the structural and optical properties of ZnS QDs. The demonstrated bandgap tuning and improved dispersion with PVP are relevant for UV optoelectronic coatings and QD-based layers used in photodetectors and LED interfaces.