Effect of calcination temperature on the structural and photocatalytic properties of nickel sulfide nanoparticles for dye degradation and antibacterial applications

The need for multipurpose materials that are capable of degrading dyes and microorganisms is highlighted by the increasing numbers of pollutants contaminating water. This study examines the structural, optical, photocatalytic, and antibacterial characteristics of nickel sulfide (NiS) nanoparticles synthesised with the extract of Sutherlandia frutescens. NiS nanoparticles were synthesized at various calcination temperatures (uncalcined, 300 °C, 500 °C, and 700 °C) and analysed through XRD, FTIR, UV–VIS, BET, TGA, and SEM techniques. FTIR results verified the presence of functional groups derived from the plant extract, with clear peaks confirming the successful formation of nanoparticles, while calcination contributed to eliminating organic residues in some samples. XRD patterns revealed a hexagonal phase of α-NiS with crystallite sizes ranging from 15 nm to 28 nm, depending on calcination temperature. SEM images showed irregular, grain-like morphologies, with higher calcination temperatures resulting in larger and more aggregated particles. Photocatalytic experiments revealed that the material calcined at 300 °C achieved the highest degradation efficiency of Congo red dye (70 %) under UV radiation. Antibacterial assessments showed different inhibition zones against B. subtilis, K. pneumoniae, S. aureus, and Escherichia coli, with NiS synthesized at 300 °C displaying the most potent activity. Trapping experiments verified that holes (h⁺) served as the predominant active species in the photocatalytic reaction. These findings emphasise the potential of green-synthesized NiS nanoparticles as efficient and environmentally friendly materials for wastewater treatment and antibacterial applications.