Sunlight-Driven Photocatalysis in Hydrothermally Coupled ZnO/Fe3O4 Heterostructures from Bioengineered Nanoparticles

We report a fully biogenic route to ZnO, Fe3O4, and their hydrothermally coupled ZnO/Fe3O4 heterostructure and establish a synthesis–structure–function link. Phase-pure, quasi-spherical wurtzite ZnO and finer inverse-spinel Fe3O4 nanoparticles assemble into a biphasic interface without forming a solid solution; optical analysis yields Eg = 2.36 eV (ZnO), 1.46 eV (Fe3O4), and 1.45 eV (ZnO/Fe3O4), while PL shows near-band-edge quenching and green–yellow defect reweighting at 490–560 nm, consistent with interfacial band bending. Magnetically, ZnO/Fe3O4 is soft-ferrimagnetic with MS/MR/HC = 226 emu g−1/17 emu g−1/0.010 T (at 300 K), enabling rapid magnetic recovery. Under natural sunlight (572.6 ± 32 W m−2), adsorption-corrected methylene blue removal (10 mg L−1; 10 mg in 50 mL) gives real degradation rates RDR = 90% (ZnO), 65% (ZnO/Fe3O4), and 30% (Fe3O4) at 180 min, with pseudo–first-order constants k = 1.9 × 10−2, 0.7 × 10−2, and 0.4 × 10−2 min−1, respectively; dark adsorption baselines are 10%, 14%, and 49%. Reusability over four cycles preserves pseudo-first-order kinetics (ZnO/Fe3O4: 65% → 50%). Scavenger tests implicate OH• as the dominant oxidant in ZnO and ZnO/Fe3O4, and O2−• in Fe3O4. Taken together, the band alignment, photoluminescence quenching, radical-scavenger profiles, and kinetic synergy are consistent with a defect-rich S/Z-scheme-like ZnO/Fe3O4 heterojunction, delivering a green, sunlight-operable, and recyclable platform for affordable wastewater remediation.