Increasing Alkyl Chain Length in a Series of Layered Metal–Organic Frameworks Aids Ultrasonic Exfoliation to Form Nanosheets

Abstract

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.

Blending functionalised ligands to form multivariate metal–organic framework nanosheets (MTV-MONs) with tuneable surface chemistry

“…Previous studies have demonstrated the exfoliation of layered frameworks [Cu(5/5*)(DMF)] n to form MONs. [52][53][54] Incorporation of the 3-methoxypropoxy (5*) or pentoxy (5) fu-BDC ligands imparted relatively hydrophilic or hydrophobic properties, respectively, to the MOF. Exfoliation in water or diethylether (Et 2 O) produced higher concentrations of hydrophilic or hydrophobic MONs respectively, indicating that a good match of solvent and surface polarity may aid exfoliation.…”

“…52 DFT modelling demonstrated H 2 O could coordinate to the axial PW sites, 53 and Pawley and Rietveld refinement of high-resolution Synchrotron powder X-ray diffraction (PXRD) data enabled structural modelling of the frameworks both pre-and postexfoliation in acetonitrile (MeCN). 54 MeCN was found to be a good solvent for exfoliation for these and other related Cu 2 -PW based MOFs 38 and is used throughout this study unless otherwise stated.…”

“…3c). Layer thickness is 1.4 nm according to crystal structures, 52,54 but this is calculated with chains fully extended, which is unlikely when at monolayer thickness. Large, ultrathin monolayer MONs such as these were not previously observed through exfoliation of single-ligand MOFs in Et 2 O.…”

“…We have previously reported an isoreticular series of layered MOFs using fu-BDC with propoxy-pentoxy alkoxy chains (ligands 3-5). 54 We anticipated that there would be a tension between the linkers with longer chain lengths pushing the layers further apart and so weakening inter-layer interactions, and interdigitation of the chains increasing interlayer interactions and making shearing of the layers apart during exfoliation more difficult. Overall, our studies showed that the longer the alkoxy chain the thinner the nanosheets and wider their lateral dimensions.…”

“…1). [52][53][54] These MOFs have the in-plane structure of MOF-2, 55,56 where dinuclear copper PW units are linked in two dimensions through dicarboxylate linkers and functional groups projecting between the layers. Through systematic studies we showed that the small changes in the length and polarity of the functional groups can have significant effect on the dimensions and concentration of the nanosheets formed.…”