Bi-doped ZnS nanoparticles: crystal growth, morphology, and optical properties

The sol-gel method was used to synthesize Zn1−xBixS compounds (x = 0, 0.25, 0.5, 0.75, and 1) with the aim of studying the structural, morphological, and optical effects of bismuth ions within the crystal lattice of zinc sulfide (ZnS). X-ray diffraction results showed that all samples crystallized in the cubic ZnS phase, with the appearance of a bismuth disulfide secondary phase at higher bismuth (Bi) content. The crystallite size increased from 10.5 nm at x = 0 to 26.7 nm at x = 0.75 and reached 31.6 nm at x = 1, accompanied by a sharp decrease in microstrain from 1.48% to about 0.49%. Field emission scanning electron microscopy images revealed a clear morphological transition from spheroidal ZnS particles to layered, sheet-like particles at higher Bi substitution ratios. Ultraviolet–visible spectrum showed a redshift with a band gap decreasing from 3.95 eV for pure ZnS to 3.59 eV at total substitution (x = 1), attributed to bismuth ions’ 6p energy levels near the conduction band edge, creating surface and electron levels that reduce the electron transition energy. These results demonstrate that the sequential replacement of bismuth provides an effective strategy that tailors the structural, morphological, and optical properties of the ZnS compound, making it a promising candidate for thin-film solar cells and photocatalytic applications.