Enhanced Visible-Light Photocatalytic Activity of Bi-Doped ZnFe2O4 Thin Films toward Rhodamine B and Metronidazole Degradation

Abstract Bi-doped ZnFe2O4 thin films were fabricated via spray pyrolysis across a systematic doping series (0–0.8 at.% Bi) and characterized for their structural, morphological, optical, and photocatalytic properties. X-ray diffraction confirmed phase-pure ZnFe2O4 with enhanced crystallinity at concentrations of ≤ 0.4 at.% Bi, while a secondary BiFeO3 phase emerged at 0.6% and 0.8% Bi, marking the solubility limit of Bi within the host lattice. The 0.4% Bi-doped film exhibited the most refined microstructure, a homogeneous surface morphology, and uniform elemental distribution of Bi throughout the Bi-ZnFe2O4 matrix. Optical studies revealed a systematic band gap reduction to 2.12 eV at 0.4% Bi, significantly extending visible-light absorption. Under visible-light irradiation, this composition achieved the highest photocatalytic degradation efficiencies across the series, 89% for rhodamine B (RhB) and 75% for metronidazole (MDZ), within 120 min. These results establish Bi doping concentration as the decisive parameter governing structural integrity, optical response, and photocatalytic activity in ZnFe2O4 thin films, positioning the 0.4 at.% Bi composition as a highly promising candidate for efficient visible-light-driven photocatalytic water treatment.