Effect of calcination temperature on structure and photocatalytic activity of green-synthesized nickel oxide nanoparticles

Nickel oxide (NiO) nanoparticles were synthesized using an aqueous heartwood stem extract of Senegalia catechu as a capping and stabilizing agent, followed by controlled calcination and a systematic investigation of sunlight-driven photocatalytic performance, degradation mechanism, and reusability. The structural and optical properties of NiO nanoparticles were studied using UV–vis spectroscopy, Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). A redshift in the absorption edge and a reduction in band gap from 2.45 eV to 2.19 eV indicate temperature-dependent optical behavior. FTIR and XRD confirmed pure cubic NiO nanoparticles, with crystallite size increasing from 13.13 to 29.76 nm (400–700 °C). SEM showed predominantly spherical morphology. The photocatalytic activity of the NiO nanoparticles was evaluated via the degradation of methylene blue (MB) under natural sunlight. NiO nanoparticles calcined at 700 °C showed the highest degradation efficiency (∼97.93% in 150 min), attributed to reduced band gap, improved crystallinity and calcination-induced morphological evolution. Reactive oxygen species (ROS) scavenging experiments identified h⁺ and •O 2 ⁻ as key active species. These findings highlight the potential of green-synthesized NiO nanoparticles as effective photocatalysts with good reusability over four consecutive cycles for solar-light-driven wastewater treatment.