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.