Thermally tuned green-synthesized NiO nanostructures exhibiting enhanced crystallinity, band gap narrowing, and superior photocatalytic activity toward organic dye degradation.

The green synthesis of photocatalytic materials provides a sustainable strategy for environmental remediation. In this study, nickel oxide (NiO) nanoparticles were synthesized using Johrenia aromatica fruit extract as a natural phytochemical-mediated agent, followed by calcination at 500, 600, and 700 °C. Structural characterization confirmed the formation of phase-pure cubic NiO and revealed progressive improvement in crystallinity and reduction in lattice strain with increasing calcination temperature. UV-Vis analysis showed that the optical band gap decreased from 3.24 eV for NiO-500 to 2.63 eV for NiO-700. This band-gap narrowing was associated with improved crystallinity, reduced lattice strain, and calcination-induced modification of the electronic structure, thereby extending the optical response toward the visible region. FESEM analysis revealed temperature-dependent morphological evolution, characterized by particle growth and improved structural uniformity at higher calcination temperatures. Photocatalytic degradation of methylene blue (MB) was strongly dependent on both calcination temperature and catalyst dosage. The optimum performance was achieved using NiO calcined at 700 °C for 3 h and applied at a catalyst dosage of 35 mg, resulting in 96.65 ± 0.04% MB degradation within 65 min (n = 3) under visible-light irradiation. Advanced characterization of the optimized NiO-700 catalyst confirmed high structural purity, favorable surface-charge characteristics, and mesoporous behavior. Reactive-species scavenging experiments indicated that electron-mediated reactive oxygen species (ROS) played a dominant role in the photocatalytic degradation process, while recyclability experiments demonstrated good stability over repeated catalytic cycles. Overall, the combined effects of enhanced crystallinity, reduced optical band gap, favorable charge-carrier processes, and ROS generation contributed to the superior photocatalytic activity of NiO-700, highlighting the potential of J. aromatica-assisted NiO as a sustainable photocatalyst for wastewater treatment.