J uglans regia L. mediated synthesis of cobalt oxide and zinc-doped cobalt oxide nanoparticles: characterization and evaluation for environmental, antibacterial and cytotoxic potential

Cobalt oxide (Co₃O₄) and novel zinc-doped cobalt oxide (Zn-Co₃O₄) nanoparticles (NPs) were synthesized through green route using bark extract of Juglans regia L. The synthesized NPs were characterized by UV–Vis spectroscopy, X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDX), photo-luminescence analysis, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). The band gaps of Co₃O₄ and Zn-Co₃O₄ NPs were found to be 3.45 and 3.35 eV, respectively. SEM and XRD analysis showed that Co₃O₄ particles are small, regular, spherical and not well dispersed while Zn-Co₃O₄ NPs are large, regular spherical and well dispersed. The TGA analysis revealed that Zn-Co₃O₄ NPs were much stable to temperature and only 20% weight loss occurred during heating range (25–600 °C). The degradation of Malachite green (MG) over Co₃O₄ and Zn-Co₃O₄ NPs followed first-order kinetics. The calculated activation energy for the photo degradation of given dye was found to be 20.2 and 56.8 kJ/mol for Zn-Co₃O₄ and pure Co₃O₄ NPs, respectively. At 100 min, time interval 83 and 96% dye degradation was observed using pure Co₃O₄ and Zn-Co₃O₄, respectively. High percent degradation of dye was found at low concentration (10 ppm) and at optimal dosage (0.05 g) of both of the catalysts. The rate of MG dye degradation was found to increase with increase in temperature and pH of the medium. Antibacterial activity study showed that Zn-Co₃O₄ NPs were more efficient against both gram-negative and gram-positive bacterial strains as compared to pure Co₃O₄. Co₃O₄ NPs at higher concentration were found less toxic as compared to Zn-Co₃O₄ NPs at low concentration. Juglans regia L. was used for the synthesis of cobalt oxide nanoparticles. Zinc oxides have been doped with a number of metals but no study reported doping of Co₃O₄ with Zn. It was also used for the first time for the degradation of Malachite green dye and biological application of the synthesized materials was also investigated.