Engineering interfacial charge transfer in a MIL-68(In)/In2S3 heterojunction for highly efficient visible-light-driven Cr(VI) reduction

Photocatalytic reduction represents a promising strategy for remediating toxic hexavalent chromium (Cr(VI)) pollution. MIL-68(In), a typical In-based metal-organic framework, is a potential photocatalyst for this purpose. However, its efficiency is often hampered by the rapid recombination of photogenerated charge carriers and a narrow visible-light response. To address these limitations, we fabricated a MIL-68(In)/In 2 S 3 n-n heterojunction by controllably growing In 2 S 3 on the MOF substrate, aiming to achieve highly efficient Cr(VI) reduction. Under optimized reaction conditions (pH = 4, temperature 25 °C, catalyst dosage 0.2 g·L −1 , initial Cr(VI) concentration 20 mg·L −1 ), the optimized MIL-68(In)/In 2 S 3 composite achieved a Cr(VI) reduction efficiency of 94.44 % within 30 min of visible light irradiation, significantly outperforming its individual components while exhibiting excellent stability and recyclability. A series of photoelectrochemical and radical trapping experiments were conducted to reveal the enhanced mechanism of photocatalytic Cr(VI) reduction. The formed heterojunction interface broadened visible-light absorption and exhibited excellent interfacial charge transfer capability and, driven by a built-in electric field, facilitated the spatial separation of charge carriers. Consequently, photogenerated electrons migrated from MIL-68(In) and accumulate on In 2 S 3 , directly reducing Cr(VI) to Cr(III). Simultaneously, some electrons participated in the generation of superoxide radicals (•O 2 - ), which synergistically contributed to the reduction process. This work provided an effective heterojunction photocatalyst for Cr(VI) wastewater treatment and offered valuable insights for the design of MOF-based photocatalytic systems.