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 E_g = 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 M_S/M_R/H_C = 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.