Influence of cobalt (Co) concentration on the structural, optical, and magnetic properties of ball-milled Al₂O₃ nanoparticles

Abstract Cobalt (Co)-doped Al₂O₃ nanoparticles with nominal compositions Al 2−x Co x O₃ (x = 0.06 and 0.08) were prepared by mechanical ball milling followed by annealing at 1100 °C to investigate the influence of Co concentration on their structural, optical and magnetic properties. X-ray diffraction (XRD) patterns confirmed the transformation of nanocrystalline γ-Al₂O₃ into corundum α-Al₂O₃ after annealing, with no detectable Co-related secondary crystalline phases. The shift of the (113) reflection toward lower 2θ values and the increase in lattice parameters, from a = b = 4.76 Å and c = 12.99 Å to a = b = 4.85 Å and c = 13.24 Å, indicated Co-induced lattice expansion. Transmission electron microscopy (TEM) analysis showed that Co addition reduced the average particle size from 37.83 nm for annealed Al₂O₃ to 25.24 and 21.53 nm for Co-0.06 and Co-0.08, respectively. Energy-dispersive X-ray spectroscopy (EDS) analysis detected Al, O and Co, with no additional impurity elements. Fourier-transform infrared (FTIR) spectra retained the characteristic Al-O vibrations, while Co doping caused changes in band intensity and broadening, indicating modification of the local Al-O bonding environment. Ultraviolet–visible (UV-Vis) diffuse reflectance analysis showed that the optical band gap decreased from 4.09 eV for annealed Al₂O₃ to 4.04 and 3.96 eV with increasing Co concentration. Photoluminescence (PL) spectra exhibited mainly UV-blue defect-related emission associated with oxygen-vacancy-related centres. Vibrating sample magnetometry (VSM) measurements revealed a weak magnetic response, with magnetization at 3T increasing from 0.00153 to 0.00269 emu g⁻¹ as the Co concentration increased. These combined structural, optical and weak magnetic responses suggest potential applications of Co-doped Al₂O₃ nanoparticles in UV-blue-emitting materials, optical sensing and dilute magnetic oxide systems.