Observation of the structural, optical and magnetic properties during the transformation from hexagonal NiS nano-compounds to cubic NiO nanostructures due to thermal oxidation

Abstract

Single phase α-NiS nano-compounds with uniformly distributed hierarchical networks were synthesized by a microwave-assisted hydrothermal technique. The materials were evaluated for thermal stability under an oxidative environment and at temperatures between 150°C and 600°C. NiS materials showed stability at 300°C and NiO formation was observed from 350°C to 600°C. The annealing effect on the crystalline size and IR absorption of the annealed samples is reported by XRD and FTIR studied. The EPR properties of the annealed materials were studied and compared to the oxidized materials. The transition temperature of 350°C for the formation of NiO from NiS oxidation was confirmed by calculating the number of spins obtained from the EPR data. Vibrating sample magnetometry (VSM) shows that this pure NiS has both ferromagnetic ordering and paramagnetic domains. Further, the transition temperature of −9°C of the pure α-NiS nano-compounds was confirmed by performing electrical measurements on the as-synthesized material.