Al-Induced Phase and Microstructural Reconstruction in V–Fe–O Oxides: Correlation with Optical Response and Visible-Light-Assisted Dye Degradation

Al-modified V–Fe–O oxides were synthesized by a co-precipitation route followed by calcination to investigate the effect of controlled Al incorporation on phase evolution, microstructure, optical properties, electronic structure, and visible-light-assisted photocatalytic activity. X-ray diffraction revealed a pronounced non-linear evolution of the V–Fe–O framework with increasing Al content, with intermediate Al incorporation promoting enhanced crystallinity and the development of additional diffraction features tentatively associated with a V2O3-related secondary phase. The corresponding microstructural evolution was confirmed by scanning electron microscopy, where the intermediate composition exhibited a distinct morphology characterized by well-developed crystalline domains. This sample also displayed the highest Brunauer–Emmett–Teller (BET) surface area (80 m2 g−1), indicating substantial modification of the textural properties. Diffuse reflectance measurements showed a reduction in the optical band gap from 2.50 eV for the undoped sample to 2.27 eV at the intermediate Al content, extending the optical response toward the visible region. Electron spin resonance (ESR) measurements further revealed pronounced changes in resonance linewidth and line shape, consistent with modifications in the local magnetic environment and the possible stabilization of mixed vanadium oxidation states. Under visible-light irradiation from commercial light-emitting diode (LED) sources, the intermediate Al-modified sample exhibited the highest photocatalytic activity, achieving approximately 95% degradation of Methyl Green within 120 min, compared with about 28% for the undoped oxide. The enhanced photocatalytic response is attributed to the combined effects of improved crystallinity, increased accessible surface area, band-gap narrowing, and Al-induced phase and electronic-structure reconstruction. These findings demonstrate that controlled Al incorporation provides an effective route for tuning the structure–property relationships of V–Fe–O oxides and enhancing their visible-light-assisted photocatalytic performance.