From Structure to Optics: The pH-Temperature Interplay in Aqueous Solution CdS Nanoparticles

Cadmium sulfide (CdS) nanoparticles are classified as II-VI semiconductor materials, used in optoelectronic devices due to a band gap (E_g). In this study, CdS nanoparticles were synthesized by the chemical precipitation method, and a systematic evaluation of pH (4.8-10.1) and temperature (50, 75, and 90 °C) was conducted. The effects of these variables were evaluated by UV-VIS spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Results demonstrate that variables determine crystallite sizes (C_s), cluster sizes, and optical properties. CdS crystallization is more affected by pH conditions than by temperature during synthesis; the change in peak intensity in 2θ = 24-29° suggests the formation of a cubic phase (alkaline conditions) and a transition to a hexagonal phase (acidic conditions). Higher temperature improves the quality of the nanoparticles, as evidenced by the reduction in intensity of the peaks associated with secondary materials. The synthesis conditions of CdS nanoparticles significantly affect E_g, widening the range from 2.21 to 2.40 eV. Both temperature and pH conditions change the size of nanoparticles and clusters. Acid conditions promote the formation of rounded and uniform nanoparticles, while alkaline conditions form the largest crystals of CdS. These findings are useful for developing electronic devices that require different semiconductor profiles.