Fig.2 Characterization of the synthesized NH2-ZIF-8 samples. (a) XRD patterns. (b-d) SEM images of NH2-ZIF-8 crystals with mean size of (b) 82 nm, (c) 133 nm and (d) 245 nm, scale bar=500 nm, (e) N2 adsorption isotherms at 77 K, (f) FTIR spectrum of ZIF-8 and NH2-ZIF-8.
Fabrication and characterization of MMCMs
Highly permeable porous substrates are critical to fabricating MMCMs, and the gutter layer on the substrates has been regarded to provide a smooth surface and avoid the penetration of the dilute solution into the pores of substrates.4,42 In this work, we employed crosslinked polydimethylsiloxane (PDMS) as the gutter layer and precoated it onto the PAN substrates via spin-coating. As shown in Fig. S3, we successfully fabricated modified substrates with a gutter layer of ~100 nm thick and the CO2 permeance is ~10000 GPU, wherein the trans-membrane resistance is negligible.
We fabricated the MMCMs through spin-coating, and the thickness of top selective layer could be controlled by adjusting the spinning speed. Herein, we employed NH2-ZIF-8 nanoparticles as MOF fillers, distributed in 6FDA-DAM or bromo-functionalized polyimide (PI-Br), wherein the MMMs are denoted as MMCM-A and MMCM-B, respectively. To further reduce the thickness of the top layer, we increase the spin-coating speed and employed NH2-ZIF-8 with smaller size.
We employed SEM to observe the cross-sectional and surface morphologies of MMCMs. As shown in Fig.3a-c, the MOF-protruding structure was formed in MMCM-A. The cross-sectional images show that the selective layer is ~280 nm thick (Fig. 3a-b), and MOF crystals could protrude the polyimide matrix. The surface SEM image (Fig. 3c) shows the clear rhombic facets of NH2-ZIF-8 crystals appearing on the thin film and no defects or pinholes were generated during the membrane fabrication. The cross-sectional images of the MMCM-A with a thinner selective layer is shown in Fig 3d-e. With a MOF loading of 20 wt%, the selective layer of MMCM-A is ~150 nm thick, while NH2-ZIF-8 crystals could also nearly penetrate the whole thickness. However, it is noteworthy that lots of pinholes appearing on the surface of the film, resulting in non-selective defects which could cause a significant decrease in gas selectivity (Fig. 3f).