Facile Hydrothermal Synthesis and Characterization of Novel Co-Doped Iron Oxides Nanoparticles with Tunable Structural, Optical, and Magnetic Properties

Fe 1-x Co x O nanoparticles (x = 0.00-0.20 at.%) were synthesized via a hydrothermal method to systematically investigate the effect of cobalt incorporation on their structural, optical, surface, and magnetic properties. X-ray diffraction confirmed the formation of phase-pure rhombohedral Fe 2 O 3 with no detectable secondary cobalt phases, while progressive peak shifts indicated successful substitution of Co ions into the iron oxide lattice. Cobalt doping led to a controlled increase in crystallite size accompanied by reduced lattice strain and dislocation density, reflecting improved structural ordering. FTIR spectra revealed modifications in metal oxygen bonding consistent with dopant-induced lattice distortion. Morphological analysis showed aggregated near-spherical nanoparticles, with enhanced grain growth at higher cobalt concentrations. Nitrogen adsorption-desorption measurements demonstrated mesoporous characteristics, with a maximum BET surface area of 39.54 m 2 g -1 at (x = 0.12). X-ray photoelectron spectroscopy identified mixed Fe 2+ /Fe 3+ and Co 2+ /Co 3+ oxidation states, indicating strong Fe–O–Co interactions and increased surface defect density. Optical studies revealed a systematic blue shift of the absorption edge and a widening of the optical band gap from 2.57 to 3.23 eV with increasing cobalt content. Magnetic measurements showed enhanced saturation magnetization and coercivity, reaching 9.1 emu g -1 and 275 Oe at (x = 0.12), followed by a decline at higher doping levels due to defect-mediated spin canting. These results demonstrate that controlled cobalt doping enables effective tuning of Fe 2 O 3 nanoparticle properties, highlighting their potential for applications in spintronics, magnetic sensing, and photocatalysis.