Structural transformation from a 3D hydrophilic pillared-layered framework to 2D hydrophobic MOF nanosheets through ligand replacement exfoliation

Abstract

A pillared-layered metal–organic framework (MOF) {[Zn₂(atpt)₂(bpBTD)]·solv}_n (1), where H₂atpt = 2-aminoterephthalic acid and bpBTD = bis(pyridin-4-yl)benzothiadiazole, showing a pcu net topology with double interpenetration was successfully constructed. Thermally activated 1 possessed free voids of 15.3% and repeatedly exhibited two-step CO₂ adsorption isotherms over three runs, with uptakes of 138.8 cm³ g⁻¹ STP at 195 K and P/P₀ = 1. Through a ligand replacement exfoliation approach, metal–organic framework (MOF) nanosheets {[Zn₂(atpt)₂(4-picoline)₂]}_n (1-ns) with a 2D sql net topology were obtained by immersing 1 in 4-picoline. Characterization of 1-ns was achieved using energy-dispersive X-ray spectroscopy (EDS), infrared (IR) spectroscopy, X-ray powder diffraction (XRPD) patterns, and elemental analyses (EA) as well as scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Photoluminescence spectra showed that 1 and 1-ns both emitted blue-light fluorescence in H₂O. In addition, measurements of water contact angles indicated that 1 is highly hydrophilic, while 1-ns shows poor hydrophilicity compared with 1. The double-sided hydrophobic surfaces of 1-ns allowed oil (dichloromethane) but not water to pass through, making it a good candidate for oil/water separation. Electrochemical measurement revealed that the 2D metal–organic nanosheet 1-ns modified glassy carbon electrode (1-ns/GCE) possessed much better electron transfer ability than the 3D pillared-layer framework 1 modified glassy carbon electrode (1/GCE), and thus improves the electrochemical performance and sensitivity.