Ultrasonic Exfoliation of Hydrophobic and Hydrophilic Metal–Organic Frameworks To Form Nanosheets

Abstract

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.

2D framework materials for energy applications

“…with alkoxy substituents), or the medium in which exfoliation takes place, can lead to improved exfoliation result. [106][107][108][109][110] In this way, Liu et al were able to achieve excellent exfoliation yields of Zn 2 (bim) 4 (bim À ¼ benzimidazolate) of up to 47% (4-6 nm thickness) by using ionic liquids instead of typical delamination solvents such as pure water, methanol, ethanol, acetone, THF or DMF. 108 Besides the ultrasonic induced delamination of rather simple layered MOFs, nanosheets of larger layered structures can also be exfoliated through mechanical exfoliation methods (i.e.…”

Ultrasonic‐assisted exfoliation for 2D Zn(Bim)(OAc) nanosheets used as an oil‐soluble additive in lubricants

“…This can be ascribed to the reduction in the number of layers and accounts for the appearance of O‐containing groups on the surface of the nanosheets. [ 27 ] The yield of NS‐Zn(Bim)(OAc)‐MeOH is up to 21%, which demonstrates highly efficient exfoliation in ultrasonication. The HR‐TEM and AFM characterizations (Figure 2a,c and Supporting Information Figure S4) confirm that NS‐Zn(Bim)(OAc)‐MeOH possesses a sheet‐like structure and thickness of approximately 3–4 nm (about three to five layers).…”

Monolayer nanosheets formed by liquid exfoliation of charge-assisted hydrogen-bonded frameworks

“…Exfoliation experiments were performed using a previously reported bath sonicator setup which we have used to exfoliate layered MOFs to form MONs. 54,55 Here, "soer" high frequency (80 kHz) ultrasound is used to minimise fragmentation and samples are stirred in a temperature-controlled bath to minimise hot-spots and ensure reproducibility. 54 During ultrasonication, shear forces and cavitation exert force on the bulk material and induce exfoliation, aer which the solvent-nanosheet interaction must overcome inter-sheet attraction to prevent reaggregation.…”

“…54,55 Here, "soer" high frequency (80 kHz) ultrasound is used to minimise fragmentation and samples are stirred in a temperature-controlled bath to minimise hot-spots and ensure reproducibility. 54 During ultrasonication, shear forces and cavitation exert force on the bulk material and induce exfoliation, aer which the solvent-nanosheet interaction must overcome inter-sheet attraction to prevent reaggregation. As such, a range of polar solvents (acetone, acetonitrile, DMF, DMSO, and water) were initially investigated for the exfoliation of both 1 and 2$TP, as it was hypothesized that these would best penetrate between the layers of each system, leading to enhanced exfoliation and colloidal stabilization.…”