Ozonolysis–oxidation-driven top-down strategy for the target preparation of ultrathin 2D metal–organic framework monolayers

Two-dimensional metal–organic-framework (2D MOF) nanosheets with a modular nature and tunable structures exhibit a bright future for sensors, separation, and catalysis. Developing ultrathin 2D MOF nanosheets with unique physical and chemical properties is urgently required but very challenging. Although the chemical exfoliation strategy has been regarded as a promising way to exfoliate large amounts of three-dimensional (3D) pillar-layered MOFs, this method still faces many problems and remains insufficient. In this study, a novel chemical exfoliation method is developed for the target preparation of 2D MOF monolayers from the 3D pillar-layered MOFs. The Co/Zn/Cu-MOFs with a pillar ligand of trans-1,2- bis(4-pyridyl)ethylene (bipyen) are subjected to be broken by the cleavage of C=C bonds in the bipyen ligands via an ozone oxidation reaction. As chemical exfoliation is processed via the oxidation of the pillar ligand by ozone, the thickness of the 2D MOFs can be tuned by the control of oxidation time and the obtained 2D Co/Zn/Cu-MOF monolayers are functionalized with a –COOH group. This study provides an effective and general chemical exfoliation method to prepare monolayer MOFs from the 3D pillar-layered MOFs with bipyen as the pillar ligand.