ZnS nanocrystals were prepared by the chemical co-precipitation method. The effect of annealing temperature
(Ta) on the morphological, structural and optical absorption behavior was investigated using x-ray diffraction
(XRD), high resolution transmission electron microscope (HRTEM), UV-vis spectroscopy, selected area electron
diffraction (SAED) and Fourier transform infrared (FTIR) spectroscopy. Analysis of XRD patterns for as prepared
and annealed samples showed that, increasing Ta leads to an increase in the crystallite size (D) from 2.67 to
19.6 nm. It is noticed that the obtained values of lattice parameters from HRTEM Images and SAED patterns are
in good agreement with that deduced from XRD analysis. Furthermore, annealing process at 600 oC and 700 oC
results, in complete phase transformation from as prepared ZnS cubic structure to ZnO hexagonal structure.
Analysis of the XRD patterns, SAED, HRTEM and FTIR confirm this phase transition. Analysis of the optical
absorption spectra indicates noticeable decrease in the direct band gap from 4.70 to 3.22 eV with
increasing Ta. This behavior is attributed to the enhancement in crystallinity and the increase in particle size of ZnS nanoparticles. Moreover, UV photo-induced effect on the optical absorption edge was studied.