An S-Scheme MIL-101(Fe)-on-BiOCl Heterostructure with Oxygen Vacancies for Boosting Photocatalytic Removal of Cr(VI)

Abstract

Hexavalent chromium (Cr(VI)) may be a hazardous and nonbiodegradable waste matter which will cause substantial environmental damage. Fabricating powerful photosystems to achieve efficacious elimination of Cr(VI) holds eminent promise in solving environmental issues. Thanks to their outstanding photo/electrical properties, large surface area , and customizable structure, metal-organic framework (MOF) catalysts have attracted widespread attention within the field of pollutant degradation and reduction. Nevertheless, due to the recombination of photo-generated charge carriers, pristine semiconductor MOFs’ photocatalytic performance is inadequate. To overcome this challenge, one of the most typical and effective strategies is to create heterojunctions by combining MOFs with another semiconductor. Among these strategies, the innovative step-scheme (S-scheme) heterojunction has gained increasing prominence. Unlike traditional type II and Z-scheme heterojunctions, the built-in electric field at the S-scheme heterojunction boundary enhances spatial charge separation and boosts redox capacity, thereby improving photocatalytic performance. In this study, a creative MOF-based S-scheme architecture with oxygen vacancies (OV) was built via in situ growth of MIL-101(Fe) crystals on the surface of OV-rich BiOCl microspheres . The optimized MIL-101(Fe)/BiOCl heterojunction exhibited exceptional photocatalytic performance in photo-reducing high concentrations of Cr(VI) and 88.5% of Cr(VI) solution (10 mg·L −1 , 100 mL) can be removed within 60 min, which is about 4.4 and 9.0 times that of BiOCl and MIL-101(Fe). Besides, the MIL-101(Fe)/BiOCl manifests impressive practical implementation prospect due to its high antiinterference property, robustness and reusability. Photoelectron spectroscopy results validated that built-in electric field, bending band, and Coulomb attraction facilitated the transition of photoelectrons from the conduction band (CB) of BiOCl to the valence band (VB) of MIL-101(Fe), where they recombined with the photo-created holes. This suggests an S-scheme interfacial photo-carrier detachment mechanism at the MIL-101(Fe)/BiOCl interface. In addition, BET measurements indicated a notable increase in surface area with the introduction of MIL-101(Fe). The OV-rich S-scheme MIL-101(Fe)/BiOCl heterostructure boasts more reactive sites, enhanced interfacial charge separation, and optimal redox ability of photo-carriers, leading to enhanced photocatalytic properties. Measurements of active radical scavenging and electron spin resonance (ESR) confirm that e − and ·O - 2 are the primary active species during photocatalysis . These discoveries would open up new avenues for developing defective semiconductor/MOF S-scheme photocatalyst for environmental purification . Download: Download high-res image (240KB) Download: Download full-size image