Metal–organic framework nanosheets (MONs) are emerging as a novel class of two-dimensional materials. Here we critically review the distinct set of design principles, synthetic approaches, characterisation techniques and applications associated with this exciting new class of materials.

offers advantages over simple inorganic nanosheets in that a diverse range of physical and chemical properties can be programmed into their crystalline structure (Figure 1). This distinct combination of properties makes MONs ideal for a wide range of catalysis, sensing, electronics, and separation applications which will be the focus of this review.Much of the early literature in this field focused on the development of novel MON architectures and new routes towards their synthesis. A diverse range of different metal ions and organic linkers have been used to construct MONs. [4] However the field has rapidly converged on a handful of popular secondary building units (SBUs) which dominate much of the literature thanks to their reliable formation of high aspect ratio nanosheets. Popular classes of MONs that have been widely applied by different groups in applications include those based on the metal paddlewheel (PW) motif, [5][6][7] axially capped Zr and Hf clusters, [8][9][10][11] 2D zeolitic imidazolate frameworks (ZIFs), [12][13][14] and square planar Ni, Co, and Cu SBUs. [15][16][17][18] The modular structure of MONs, ease of functionalization and diverse range of materials they can be combined with means that MONs can be readily "programmed" to provide the desired topology and properties required for a given application.A wide variety of methods have been explored to synthesize MONs, either "top-down" by the exfoliation of layered metalorganic frameworks (MOFs) or "bottom-up" by assembly of molecular building blocks directly into nanosheets. The majority of top-down approaches utilize mechanical energy to overcome inter-layer interactions, such as liquid-assisted ultrasonication, [19] shear-mixing, [20] and grinding/ball-milling, [13] or the less common freeze-thaw [21] and "scotch-tape" methods. [22,23] Other methods involving photochemical, [24] electrochemical, [25] and chemical intercalation, [26] have also been developed but are less widely used. Bottom-up methodologies typically utilize surfactants or modulators to inhibit crystal growth in one dimension, [12,27,28] or layering/interfacial methods to promote anisotropic growth. [15,29,30] The use of sacrificial 2D templates has also recently emerged a promising route. [31,32] Each approach has advantages and disadvantages in terms of the thickness, lateral dimensions, size distribution, quantity and quality of the MONs produced, and the best approach therefore varies depending on the application.

The modular structure of metal-organic framework nanosheets (MONs) provides a convenient route to creating two-dimensional materials with readily tuneable surface properties. Here, the liquid exfoliation of two closely related layered metal-organic frameworks functionalised with either methoxy-propyl (1) or pentyl (2) pendent groups intended to bestow either hydrophilic or hydrophobic character to the resulting nanosheets is reported. Exfoliation of the two materials in a range of different solvents highlighted significant differences in their dispersion properties, as well as their molecular and nanoscopic structures. Exchange or loss of solvent was found to occur at the labile axial position of the paddle-wheel based MONs and DFT calculations indicated that intramolecular coordination by the oxygen of the methoxy-propyl pendant groups may take place. The nanoscopic dimensions of the MONs were further tuned by varying the exfoliation conditions and through "liquid cascade centrifugation". Aqueous suspensions of the nanosheets were used as sensors to detect aromatic heterocycles with clear differences in binding behaviour observed and quantified.

Metal–organic framework nanosheets (MONs) are attracting increasing attention as a diverse class of two-dimensional materials derived from metal–organic frameworks (MOFs). The principles behind the design of layered MOFs that can readily be exfoliated to form nanosheets, however, remain poorly understood. Here we systematically investigate an isoreticular series of layered MOFs functionalized with alkoxy substituents in order to understand the effect of substituent alkyl chain length on the structure and properties of the resulting nanosheets. A series of 2,5-alkoxybenzene-1,4-dicarboxylate ligands (O2CC6H2(OR)2CO2, R = methyl–pentyl, 1–5, respectively) was used to synthesize copper paddle-wheel MOFs. Rietveld and Pawley fitting of powder diffraction patterns for compounds Cu­(3–5)­(DMF) showed they adopt an isoreticular series with two-dimensional connectivity in which the interlayer distance increases from 8.68 Å (R = propyl) to 10.03 Å (R = pentyl). Adsorption of CO2 by the MOFs was found to increase from 27.2 to 40.2 cm3 g–1 with increasing chain length, which we attribute to the increasing accessible volume associated with increasing unit-cell volume. Ultrasound was used to exfoliate the layered MOFs to form MONs, with shorter alkyl chains resulting in higher concentrations of exfoliated material in suspension. The average height of MONs was investigated by AFM and found to decrease from 35 ± 26 to 20 ± 12 nm with increasing chain length, with the thinnest MONs observed being only 5 nm, corresponding to five framework layers. These results indicate that careful choice of ligand functionalities can be used to tune nanosheet structure and properties, enabling optimization for a variety of applications.

We report a new approach to tuning the properties of metal–organic nanosheets (MONs) by blending functionalised ligands to produce multivariate MONs (MTV-MONs).