Synergistic interface engineering in n-p-n type heterojunction Co₃O₄/MIL/Mn-STO with dual S-scheme multi-charge migration to enhance visible-light photocatalytic degradation of antibiotics

Constructing an effective multi-heterojunction photocatalyst with maximum charge carrier separation remains challenging. Herein, a high-efficient Co₃O₄/MIL-88A/Mn–SrTiO₃ (Co₃O₄/MIL/Mn-STO) n-p-n heterojunction photocatalyst was successfully prepared by a simple hydrothermal method for the photodegradation of sulfamethoxazole (SMX). The combination of MIL and Co₃O₄/Mn-STO established an internal electric field and heterojunction, accelerating the separation of carriers, and thus improved photocatalytic performance. In the Co₃O₄/MIL/Mn-STO photocatalytic system, 95.5 % of SMX was degraded in 90 min. The photocatalytic kinetic removal rate of Co₃O₄/MIL/Mn-STO reached 0.0337 min⁻¹, 8 times of Co₃O₄ (0.0041 min⁻¹), 5.2 times of Mn-STO (0.0062 min⁻¹), 4.6 times of MIL (0.0078 min⁻¹), and 3.6 times of MIL/Mn-STO (0.0095 min⁻¹). Remarkably, superoxide radicals (^•O₂⁻) and holes (h⁺) have been recognized as the main active species in the degradation process through reactive species elimination experiments and electron spin resonance (ESR) tests. The experimental and theoretical proved the in-built interfacial contact and synergistic effect between the photocatalyst accomplished with low bandgaps, high specific surface area, more reaction sites, high electron-hole pair separation, and maximum solar-light utilization. The molecular structure and possible degradation routes with intermediate products in the photocatalytic system were investigated using a liquid chromatography-mass spectrometer (LC-MS) and DFT calculations. This work provided new insight into the guidelines of rational design/growth of new multicomponent photocatalysts to remove antibiotics and other emerging contaminants in wastewater.