Construction of magnetic S-scheme Bi1.5OI0.5/ZnFe2O4 heterojunction through energy band engineering for efficient Cr(VI) piezo-photoreduction.

Piezo-photocatalytic materials can utilize the piezoelectric effect to generate a polarization electric field, achieving removal rate of pollutants under synergistic light irradiation. In this work, a 3D flower-spherical magnetic S-scheme Bi1.5OI0.5/ZnFe2O4 heterojunction was constructed by an in-situ deposition method and presented superior piezo-photocatalytic activity for Cr(VI) removal. The optimal Bi1.5OI0.5/ZnFe2O4 can remove 100% Cr(VI) within 20 min under simulated sunlight (57 mW/cm2) and ultrasonic (120 W). This excellent activity stems from the synergy between the piezoelectric effect and the photocatalytic effect within S-scheme heterostructure, achieving rapid charge separation. The energy band engineering of Bi1.5OI0.5 is crucial for enhancing piezoelectric catalytic activities (∼1% for Bi1.0OI1.0 vs. 40% for Bi1.5OI0.5) and enabling the construction of an S-scheme heterostructure. Cyclic and magnetic recovery experiments successfully demonstrate the stability and recycling of Bi1.5OI0.5/ZnFe2O4. The mung bean hydroponic experiments show the low phytotoxicity of Cr(VI) wastewater treated by Bi1.5OI0.5/ZnFe2O4.