Figure 1. Characterization of MOF nanosheets and membranes. (a) AFM image of MOF-CH3 nanosheets. (b) XRD patterns and (c) pore size distributions of MOF-BDC, MOF-CH3 and MOF-NH2 nanosheets. Cross-sectional (d) SEM and (e) HRTEM images of MOF-CH3@NH2 membrane. (f) XPS depth profiles of MOF-CH3@NH2membrane. Models of the pore structure (left) and the stacking configuration of nanosheets (right) for (g) MOF-CH3@NH2 and (i) MOF-CH3@CH3 membrane surfaces, corresponding to XRD data. (h) XRD patterns of hierarchical MOF lamellar membranes.
Next, hierarchical MOF lamellar membranes were prepared by double-needled electrostatic atomization technology. Concretely, MOF-CH3 nanosheets were sprayed on Nylon substrate to construct a smooth support layer, and the hydrophobic micropores of which would provide low-resistance diffusion paths for molecules that dissolve through the surface layer.[19]Subsequently, MOF nanosheets (MOF-CH3, MOF-BDC or MOF-NH2) were sprayed on support layer to assemble surface layer. The strong electrostatic repulsion among nanosheets under high-voltage field, coupled with the shear force from rotational receiver, contribute to flat deposition of MOF nanosheets on substrate (Scheme 1).[43] The obtained hierarchical lamellar membranes were marked as MOF-CH3@CH3, MOF-CH3@BDC and MOF-CH3@NH2 membranes, respectively.
AFM and SEM images in Figures S8 and S9 show that membrane surface is defect-free and smooth, indicating the ordered stacking of MOF nanosheets. This can be directly confirmed by the cross-sectional SEM and HRTEM images in Figures 1d, e and Figures S10, S11, which also deliver the information that the interlayer space is ~ 0.8 nm and the membrane thickness is around 560 nm. Here, it should be noted that, considering the nanofiltration performance and mechanical stability, membrane thickness of 560 nm is selected. Additionally, the ordered stacking of MOF nanosheets is further identified by the sharp peaks at 2θ = 11.4° on XRD spectra of membranes, which also suggests the interlayer space of 0.76 nm (Figure 1h). And the corresponding structural models are presented in Figures 1g and i.[38] It should be noted that the typical (200) peak for MOF powder is not observed in the spectra of MOF membrane, it is ascribed to the strong diffraction peaks corresponding to nanosheet stacking could mask the weak peaks of MOF powder. This phenomenon is also reported in other literature.[44] Here, MOF-CH3@CH3, MOF-CH3@BDC and MOF-CH3@NH2 membranes are prepared with similar thickness (~ 560 nm) for the following performance testing. Next, MOF-CH3@NH2membranes are employed to XPS compositional depth analysis (Figures 1f and S13b), which shows that the peak of N element (400 eV) disappears abruptly and that of C element (285 eV) enhances at ~ 7 nm. This observation validates that the MOF lamellar membrane contains hierarchical structure, and the thickness of surface layer is around 7 nm. The results are similar for MOF-CH3@BDC membranes as shown in Figures S12 and S13. Furthermore, the manipulated surface layer endows MOF lamellar membranes with distinct wettability. As shown in Figure S14, water contact angles are 38.1°, 71.3°, and 100.6° for MOF-CH3@NH2, MOF-CH3@BDC, and MOF-CH3@CH3 membranes, respectively, indicating the hydrophilicity of MOF-CH3@NH2 and hydrophobicity of MOF-CH3@CH3 membrane surfaces. Such ultrathin and adjustable surface layers with regular pores should provide suitable platform for the investigation of molecular dissolution behaviors.