Structural transformation from a 3D hydrophilic pillared-layered framework to 2D hydrophobic MOF nanosheets through the 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 pcu net topology with double interpenetration was successfully constructed. Thermally activated 1 possessed free voids of 15.3% and repeatedly exhibited a 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 ligand replacement exfoliation approach, metal–organic framework (MOF) nanosheets {[Zn2(atpt)2(4-picoline)2]}n (1-ns), with 2D sql net topology, were obtained by immersing 1 in 4-picoline. Characterizations of 1-ns were achieved by energy-dispersive X-ray spectroscopy (EDS), infrared (IR) spectroscopy, X-ray powder diffraction (XRPD) patterns, and elementary 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 on water contact angle indicated that 1 is highly hydrophilic while 1-ns shows poorer hydrophilicity compared with 1. The double-sided hydrophobic surfaces of 1-ns allowed the oil (dichloromethane) but protected water to pass through, being good candidate for oil/water separation. Electrochemical measurement revealed that the 2D metal–organic nanosheet 1-ns possesses modified glassy carbon electrode (1-ns/GCE) much better electron transfer ability compared with the 3D pillared-layer framework 1 modified glassy carbon electrode (1/GCE), and thus improves the electrochemical performance and sensitivity.