Pechini-type sol-gel synthesis of Z-scheme ZnFe2O4/α-Fe2O3 heterojunction nanoparticles for the photocatalytic degradation of methylene blue under natural solar radiation

This work investigates the relationship between the concentrations of Fe and Zn in the synthesis of ZnFe₂O₄/α-Fe₂O₃ heterojunction nanoparticles (NPs) and the photocatalytic performance of the resulting Z-scheme photocatalysts. ZnFe₂O₄/α-Fe₂O₃ NPs were prepared by the Pechini-type sol-gel method using different Fe:Zn ratios of 51:49, 57:43, 63:37, and 69:31 in wt% and calcined at 500 °C for 2 h. FT-IR, X-ray diffraction, and Raman analyses confirmed the presence of a mixture of ZnFe₂O₄ and α-Fe₂O₃ phases in all the samples. Morphology analysis revealed that the samples with Fe:Zn ratios of 51:49 and 57:43 wt% comprised semispherical and icosahedral-shaped NPs. In contrast, the samples with Fe:Zn ratios of 63:37 and 69:31 wt% contained semispherical and elongated icosahedral NPs and disc-shaped particles. The energy band structure and alignment of ZnFe₂O₄ and α-Fe₂O₃ formed a type II heterojunction in all the samples. The sample with a Fe:Zn ratio of 69:31 wt% demonstrated the best photocatalytic performance, achieving 84.3% degradation of methylene blue (MB) after 120 min of exposure to natural solar irradiation and exhibiting a higher first-order kinetics constant of 1.36 × 10⁻² min⁻¹. The superior photocatalytic performance was attributed to the higher relative phase content of α-Fe₂O₃, which acts as an electron mediator in the proposed Z-scheme heterojunction mechanism. The scavenger experiments indicated that the primary species responsible for decomposing MB were ·O₂⁻ and ·OH. Finally, the samples demonstrated excellent recyclability and stability over four cycles.

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