Doped Nano Cobalt Oxides: Improvement of Photophysical Properties and Recycling Process via Photodegradation of Congo Red Dye

Objectives This study aimed to synthesize pure and metal-doped Co₃O₄ nanoparticles and evaluate the influence of Ni mono-doping and Cu/Ni or Cd/Ni co-doping on their structural, morphological, optical, magnetic, surface, and visible-light photocatalytic properties toward Congo Red (CR) removal. Material and Methods Pure Co₃O₄ and Ni-, Cu/Ni-, and Cd/Ni-doped Co₃O₄ nanoparticles were synthesized using an oxalate-assisted co-precipitation method. The prepared materials were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), optical spectroscopy, surface-area analysis, and magnetic measurements. Their photocatalytic performance was evaluated through CR decolorization under visible-light irradiation, and the reaction kinetics and catalyst reusability were assessed. Results XRD and FTIR analyses confirmed preservation of the cubic spinel Co₃O₄ structure after doping. The crystallite size decreased from 18.20 nm for pure Co₃O₄ to 16.75, 13.90, and 14.35 nm for the Ni-, Cu/Ni-, and Cd/Ni-doped samples, respectively. The highest microstrain, 3.92 × 10 -4 , was observed for Cu/Ni co-doping. SEM and EDX analyses revealed dopant-dependent morphological changes and confirmed the presence of the intended elements. The apparent optical-transition energies ranged from 1.392 to 1.438 eV, while the specific surface areas varied between 53.2 and 73.1 m 2 g - 1 . All samples exhibited soft magnetic behavior, with saturation magnetization values of 1.133 -1.662 emu g - 1 . After 80 min of visible-light irradiation, CR decolorization increased from 53% for pure Co₃O₄ to 67%, 81%, and 94% for the Ni-, Cu/Ni-, and Cd/Ni-doped samples, respectively. The corresponding pseudo-first-order rate constants were 10.15 × 10 -3 , 16.25 × 10 -3 , 20.55 × 10 -3 , and 22.65 × 10 -3 min -1 . The Cd/Ni-doped catalyst retained approximately 81% of its initial activity after seven cycles. Conclusion Metal doping substantially enhanced the visible-light photocatalytic activity of Co₃O₄, with Cd/Ni co-doping producing the highest activity and good recyclability. The improvement resulted from the combined effects of crystallite refinement, surface accessibility, and defect-induced lattice perturbations rather than surface area alone.