Synthesis of Cobalt Oxide Nanoparticles by Atmospheric Pressure Microplasma and Their Structural, Optical, Antifungal and Antibacterial Properties

Cobalt oxide nanoparticles have been synthesized using an easy and innovative atmospheric pressure microplasma technique using different precursor (cobalt nitrate hexahydrate) concentrations. The XRD results show that Co₃O₄ nanoparticles are successfully synthesized without impurity phases, regardless of precursor concentration. The size of nanoparticles increases (33, 38, and 42 nm) when the precursor concentration (10 mM, 15 mM, and 20 mM solution) is increased in solution, exhibiting spherical morphology. In UV-visible spectroscopy, the optical band in the range 400–580 nm and 750–850 nm corresponding to Co₃O₄ nanocrystalline structure is obtained, and the band gap is reduced (3.2, 2.5, and 2 eV) by increasing precursor concentration. In FTIR spectra, a spinal structure of Co₃O₄ is detected consisting of Co-O stretching vibration (580, 560, and 565 cm⁻¹) and O-Co-O (665, 665, and 680 cm⁻¹) bridging vibrations. The obtained nanoparticles exhibit outstanding performance in antibacterial (gram-positive and gram-negative bacteria) and antifungal (Penicillin Digitatum and Rhizopus stolonifer) activities and depend on precursor concentration. In particular, Co₃O₄ synthesized using 20 mM precursor concentration shows the highest antibacterial performance against Staphylococcus aureus (gram-positive) bacteria.