Dual MOFs-Derived CdS/In₂S₃ heterojunction photocatalyst for fast and efficient reduction of Cr(VI)

Traditional methods for treating Cr(VI)-containing wastewater often encounter issues such as high energy consumption, susceptibility to secondary pollution and limited treatment efficiency. In contrast, photocatalytic technology harnesses solar energy to efficiently reduce Cr(VI) to less toxic Cr(III) under mild conditions, offering advantages including operational simplicity and environmental friendliness, thereby providing a superior alternative. This study proposes a dual MOF precursor strategy to synthesise Cd-ZIF@NH₂-MIL-68 composite materials, which undergo further sulphurisation to successfully construct a CdS/In₂S₃ type-II heterojunction photocatalyst with a hollow structure. This material exhibits a larger specific surface area, increased exposure of active sites and abundant heterojunction interfaces. This heterojunction achieves an ultrafast, 99.26% removal of Cr(VI) within only 18 min under LED illumination, exhibiting outstanding stability with 88.10% activity retention after five cycles. Systematic characterization, photoelectrochemical tests and density functional theory (DFT) calculations collectively confirm that the successful band engineering in CdS/In₂S₃ induces the formation of a type-II heterojunction, thereby significantly facilitating the efficient separation and migration of photogenerated charges. The combination of radical trapping experiments and electron paramagnetic resonance (EPR) spectroscopy identifies photogenerated electrons (e^-) and superoxide radicals (·O₂^-) as the predominant reactive species. This work provides new insights for designing highly efficient MOFs-derived heterojunction photocatalysts for the rapid remediation of Cr(VI) pollution.