Experimental Optical Assessment of the Photocatalytic Potential of Iron-Based Oxides and Phosphate Compounds

Iron-based oxides and phosphates have attracted considerable attention as low-cost, environmentally friendly materials for solar-energy conversion and environmental remediation. In this work, a comparative experimental investigation of Fe₃O₄, α-Fe₂O₃, α-FeOOH, ε-Fe₂O₃, FeO, and FePO₄ is carried out to establish the relationship between their structural, optical, and photocatalytic-related properties. SEM analysis reveals agglomerated nanosized particles, while optical characterization provides insight into light-harvesting capability and charge-transfer processes.

The investigated compounds exhibit optical band-gap energies within the visible-light region, ranging from 2.16 eV for Fe₃O₄ to 2.34 eV for α-FeOOH. Intermediate values of 2.20, 2.22, 2.30, and 2.32 eV are obtained for FeO, ε-Fe₂O₃, FePO₄, and α-Fe₂O₃, respectively. Significant differences in dielectric behavior are observed, with ε-Fe₂O₃ exhibiting a dielectric constant of approximately 9 compared with about 5 for Fe₃O₄. Moreover, the maximum value of the imaginary dielectric function reaches approximately 4 for ε-Fe₂O₃, significantly higher than those of α-Fe₂O₃ (≈1.6) and Fe₃O₄ (≈1.0), indicating stronger optical transitions and enhanced photon absorption. The third-order nonlinear optical susceptibility varies from 4.6 × 10⁻¹² to 1.1 × 10⁻¹¹ esu, highlighting substantial differences in light–matter interactions among the investigated compounds.

Among the studied materials, ε-Fe₂O₃ exhibits the most favorable combination of optical descriptors, including a visible-light band gap of 2.22 eV, enhanced dielectric response, and strong optical-transition intensity. These characteristics are expected to promote efficient light harvesting and charge separation, which are essential requirements for photocatalytic processes. The obtained structure–property correlations suggest that ε-Fe₂O₃ is a promising candidate for future visible-light-driven photocatalytic applications, including pollutant degradation, wastewater treatment, and solar-energy conversion. The present findings provide valuable guidelines for the rational design of advanced iron-based functional materials for environmental and energy-related technologies.