ZnO-Fe ₂ O ₃ Heterojunctions with Enhanced Charge Carrier Separation via a Type II Mechanism for Photocatalytic Degradation

ZnO-Fe2O3 heterojunction composites were synthesized via a facile coprecipitation method, followed by annealing at 450 °C, and evaluated as visible-light-active photocatalysts for the degradation of Acid Red 114 (AR114), a persistent azo dye. Structural and surface characterizations by XRD, FT-IR, UV–Vis DRS, FE-SEM, and XPS confirmed the formation of distinct crystalline phases and strong interfacial coupling between ZnO and Fe2O3. The optimized ZnO-Fe2O3 (95:5) composite exhibited the highest photocatalytic activity, achieving approximately 90% degradation of AR114 within 90 min under visible light, outperforming pristine ZnO (79%) and Fe2O3 (12%). UV–Vis DRS revealed a notable red shift and band gap narrowing from 3.14 eV (ZnO) to 2.45 eV, while XPS evidenced charge redistribution between Zn2+ and Fe3+ ions, supporting the formation of a type-II heterojunction that effectively promoted charge separation and suppressed electron−hole recombination. The effect of calcination temperature, catalyst dosage, and dye concentration demonstrated that optimized crystallinity, surface hydroxyl density, and photon utilization are key to achieving superior activity. Radical scavenging experiments identified hydroxyl radicals (·OH) as the primary oxidative species responsible for dye mineralization. The composite maintained 87% of its efficiency after five consecutive cycles with minimal metal leaching (<0.2%), confirming its structural stability and environmental safety. The enhanced performance arises from synergistic interfacial charge transfer, a narrowed band gap, and abundant surface-active sites. Overall, the study establishes ZnO-Fe2O3 as a cost-effective, durable, and visible-light-driven photocatalyst with significant potential for sustainable treatment of dye-contaminated wastewater.