Visible-light-driven CaFe2o4 photocatalysis: mechanistic insights from experimental and DFT studies
Abstract
Understanding the origin of reactive oxygen species (ROS) in ferrite-based photocatalysts remains a major challenge when experimentally observed reactivity appears inconsistent with band-edge energetics. In this work, the mechanistic origin of superoxide radical generation in visible-light-driven CaFe2O4 photocatalysis was investigated through combined experimental and density functional theory (DFT) analyses. CaFe2O4 nanoparticles were synthesized via a sol-gel auto-combustion route. They were characterized by x-ray diffraction, Scanning electron microscopy/energy-dispersive x-ray spectroscopy, Fourier transform infrared, Raman spectroscopy and UV–vis diffuse reflectance spectroscopy. The results confirm the formation of a pure orthorhombic phase with a narrow band gap of 1.87 eV, suitable for visible-light absorption. Using Acid Green 25 as a model pollutant, rapid degradation was observed under low-power visible LED light. This was especially true in acidic conditions, where stronger adsorption favours interfacial charge transfer. The kinetics follow the Langmuir–Hinshelwood model, confirming a tight coupling between surface adsorption and photocatalytic reaction pathways. Radical scavenging experiments identify superoxide radicals (O2•−) as the dominant reactive species, with photogenerated holes also playing a significant role in oxidation. Interestingly, band-edge analysis suggests that direct reduction of dissolved oxygen by conduction-band electrons is thermodynamically unfavourable. To resolve this apparent inconsistency, a surface-mediated electron-transfer mechanism is proposed. It involves Fe3+/Fe2+ redox cycling and defect-associated electronic states. DFT calculations further reveal that the G-type antiferromagnetic configuration stabilises the electronic structure. It also promotes charge separation, in line with the experimentally observed photocatalytic activity. Together, these findings offer fresh insight into defect-assisted ROS generation and surface-controlled charge-transfer pathways in visible-light ferrite photocatalysts.