Thermal annealing gradient tailoring on ammonium nitrate-capped CdS nanospheres synthesized via green precipitation

Abstract

This study explores the impact of thermal annealing gradients on the physical properties and structural evolution of cadmium sulphide (CdS) nanospheres capped with ammonium nitrate as a modifier, which were fabricated through precipitation and subsequent annealing within 160–480 °C temperature range. The properties were characterized using X-ray diffraction (XRD), ultraviolet–visible (UV–Vis), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques The XRD results show that the present CdS exhibits superior crystallinity compared to pure CdS without capping, transitions from a cubic to a hexagonal phase structure, and increases in crystallite size and crystallinity with increasing temperature. The FTIR spectra postulate that a vibrational band presence evidences ammonium nitrate capping on CdS, with another distinct band that represents CdS in the lower wavenumber region, both intensifying at elevated temperatures. The UV–Vis analysis reveals that CdS exhibits strong ultraviolet (UV) absorption suitable for effective photoreaction under UV light and has a broader band gap compared to bulk CdS. SEM images show an extensive distribution of homogeneous nanospheres over the surface, with increased growth in size when capped with ammonium nitrate and at higher temperatures. As validated by TGA and DSC results, CdS with a smaller crystallite size improves thermal stability and energy transfer, as evidenced by reduced weight loss and a lower endothermic temperature, respectively. Varying the annealing temperature with ammonium nitrate capping can improve the structural and physical properties of CdS, which are beneficial for varied applications such as optoelectronics, energy storage, and photocatalysts.