Optimizing NiO nanoparticle properties for antibacterial applications via temperature-driven structural modification

Nickel oxide (NiO) nanoparticles are synthesized via a citric acid–assisted sol–gel method and calcined at 300, 400, and 500 °C to investigate the influence of thermal treatment on their structural, chemical, magnetic, and antibacterial properties. X-ray diffraction (XRD) confirmed the formation of single-phase cubic NiO (space group Fm3̅m), with crystallite size increasing from 31.28 nm at 300 °C to 41.70 nm at 500 °C. A corresponding decrease in micro-strain and dislocation density is observed, indicating enhanced crystallinity and reduced lattice defects at higher calcination temperatures. Fourier-transform infrared (FTIR) spectroscopy revealed characteristic Ni–O vibrational bands in the 400–600 cm⁻¹ region, which became sharper and more intense with increasing temperature, while hydroxyl, nitrate, and organic-related bands progressively diminished, confirming effective removal of residual precursors and improved lattice ordering. Magnetic properties of the NiO nanoparticles calcined at 500 °C are examined using vibrating sample magnetometry (VSM), revealing predominantly antiferromagnetic behaviour with a weak ferromagnetic contribution. The saturation magnetization (Ms) is approximately 0.010–0.012 emu g⁻¹, the remanent magnetization (Mr) ~0.002–0.003 emu g⁻¹, and the coercive field (Hc) ~400–700 Oe, attributed to uncompensated surface spins and nanoscale effects. Antibacterial activity evaluated against Staphylococcus aureus and Escherichia coli using agar diffusion and spread plate methods showed superior efficacy for the 500 °C sample, particularly against E. coli. The study demonstrates that optimized calcination enhances structural quality, magnetic response, and antibacterial performance of NiO nanoparticles, highlighting their potential for antimicrobial, biomedical, and environmental applications.