Covalently connected core-shell NH₂-MIL-125@COFs-OH hybrid materials for visible-light-driven CO₂ reduction

Herein, the covalently connected core-shell metal-organic frameworks (MOFs)@covalent-organic frameworks (COFs) hybrid materials were successfully constructed by coating the stable COF-OH shell on the NH₂-MIL-125 core. The introduction of the NH₂-MIL-125 core endowed the hybrid materials with high Brunauer-Emmett-Teller (BET) surface area (S_BET) and abundant unsaturated metal sites. And the coating of COF-OH shell endowed the hybrid materials outstanding physicochemical stability and visible-light response, and suitable band gaps. Moreover, the thickness of the COF-OH shell was carefully adjusted according to the feeding amount of NH₂-MIL-125. Impressively, the electron transfer pathway in the formed heterostructure was clarified and it was proven that a type-II heterojunction was generated between the MOFs and the COFs. The formed stable CN covalent bonds in the interfacial layer was beneficial to the photogenerated electron transfer and the electron-hole pairs separation, which greatly enhanced the CO₂ photocatalytic reduction. The product NH₂-MIL-125@COF-3 exhibited the highest CO yield of 22.93 μmol·g^-1·h^-1, about 2 times higher than NH₂-MIL-125 (11.82 μmol·g^-1·h^-1) and 3 times greater than COF-OH (7.26 μmol·g^-1·h^-1). This work can provide helpful ideas for the careful design of the novel MOFs@COFs hybrid materials as well as useful exploration for the CO₂ photocatalytic reduction.