Thermal-induced structural evolution of mesoporous oxides Fe–Co–Ni for enhanced visible-light dye degradation

This study investigates the influence of calcination temperature on the physicochemical characteristics and photocatalytic performance of mesoporous trimetallic Fe–Co–Ni oxides for methylene blue (MB) degradation. Materials were calcined at 300 °C, 400 °C, 550 °C, and 700 °C and systematically characterized.BET analysis revealed distinct differences in porosity, which FeNiCo-550 exhibiting the highest surface area of 130.43 m²/g and a narrow average pore diameter of 3.61 nm indicating an optimal mesoporous framework. FeNiCo-300 displayed high porosity (128.68 m²/g, 3.61 nm, 0.2668 cm³/g), while thermal sintering at 700 °C significantly reduced surface area to 79.60 m²/g and pore volume to 0.1499 cm³ /g. A drastic increase in pore diameter at 400 °C to 7.99 nm suggests transitional pore restructuring. XRD analysis revealed increasing crystallinity with temperature, with Fe₂O₃ as the dominant phase at 300 °C (phase ratio 3:1:1, Fe:Co:Ni), transitioning to CoO- and NiO-rich phases at 550 °C (1:3:4) and 700 °C (1:4:5), indicating thermal phase reorganization. TEM and selected area electron diffraction (SAED) patterns confirmed the formation of polycrystalline oxide domains with uniform mesoporous morphology, particularly for FeNiCo-550. EDX elemental mapping indicated homogeneous spatial distribution of Fe, Co, and Ni, with the 2:1:1 at. ratio best preserved at 300 °C and 550 °C,suggesting efficient metal incorporation under these conditions. FTIR spectra showed loss of surface hydroxyl groups depending on the temperatures. UV–Vis diffuse reflectance spectroscopy revealed slight band gap shifts from 1.582 eV (FeNiCo-300) to 1.587 eV (FeNiCo-550) which attributed to lattice ordering and particle growth. Photocatalytic tests showed FeNiCo-550 achieved the highest MB degradation (>90 % within 90 min), due to its synergistic phase composition, porosity, crystallinity, and electronic structure. These results highlight calcination temperature as a decisive factor in tuning multimetallic photocatalysts for enhanced environmental remediation.