Green synthesis of cobalt oxide nanoparticles and the effect of annealing temperature on their physiochemical and biological properties

The cobalt oxide nanoparticles (Co₃O₄ NPs) were synthesized via the green method using Piper nigrum (P. nigrum) leaves extract and were calcined at different temperatures. The crystalline nature was studied through x-ray diffractometer (XRD). With increasing calcination, the amorphous phase transitioned to the crystalline phase. Diffuse reflectance spectroscopy (DRS) was used to observe UV-Visible light absorbance phenomenon, and Tauc's plot was used to calculate the band gap energy, which was found to decrease with calcination. Scanning electron microscopy (SEM) was used to analyze the surface morphology, and Fourier transform infrared (FTIR) spectroscopy was used to classify the surface functional groups. The antibacterial efficacy of the Co₃O₄ NPs was examined against Gram-positive bacteria (GPB) and Gram-negative bacteria (GNB), whereas its antioxidant potential was explored against ABTS free radicals. Antibacterial and antioxidant potentials of Co₃O₄ NPs were found to decrease with increasing calcination. (paper)