Liquid exfoliation of a series of expanded layered Cu(ii)-paddlewheel metal–organic frameworks to form nanosheets

Abstract

Ultrasonic liquid exfoliation provides a convenient route for converting layered materials into nanosheets. However, the relationship between the structure and morphology of the bulk materials and the properties of the resulting nanosheets remains poorly understood. In this work, we prepare an isoreticular series of layered metal–organic frameworks (MOFs) based on linear aromatic dicarboxylate derivatives (L1, L2, L3) with three different linker lengths (L3 > L2 > L1) and using copper(II) nitrate and acetate as metal ion sources. Liquid exfoliation of large crystals of all three MOFs [Cu₂(L)₂](solvent)₂, synthesised from Cu(NO₃)₂, produced monolayer nanosheets with longer linkers leading to larger lateral dimensions. Exfoliation of smaller MOF crystals, formed using the copper(II) acetate salt under identical conditions, produced a much higher concentration of multi-layer nanosheets with smaller lateral dimensions. These results indicate that the initial crystal size plays an important role in determining both the lateral dimensions and the thicknesses of nanosheets. Such insights contribute to a deeper understanding of the design principles governing metal–organic framework nanosheets (MONs) and other two-dimensional materials.