3.3. Fabrication of the polyamide layer on the HPAN substrate
The surface and cross-sectional SEM images of the pristine HPAN porous
substrate (Supporting Information, Figures S8a and b, respectively)
revealed that the pores were uniformly distributed on the flat HPAN
surface. In contrast, a continuous
polyamide layer fully covered the HPAN substrate after the IP reaction
(Supporting Information, Figures S8c and d) and formed TFC membranes.
The SEM images of the
TFN-(Zr)-1,
TFN-(Zr/Ti)-1, TFN-(Zr)-2, and TFN-(Zr/Ti)-2 membrane surfaces (Figures
3a, b, c, and d, respectively) revealed their uniform morphology over
large surface areas. Furthermore, the magnified SEM images of the
membrane surfaces illustrated the uniform distribution of the
ridge-and-valley structures of the nanocomposite membranes (Figures
3e-h). In addition, the formation of edges and corners within the
surface MOF layers, which were caused by the crystal structure of the
MOF particles, could be observed on the surfaces of the aforementioned
membranes. The SEM cross-sectional images further illustrated the
inverse relationship between the MOF loading and surface layer
thickness, and revealed that the 100-200 nm MOF-loaded nanocomposite
layers (Figures 3i-l) were much thinner than the 250 nm pristine
polyamide surface layers of the TFC membranes (Supporting Information,
Figure S5d). The thickness of the surface layer decreased with
increasing MOF loading (approximately 200 nm for the TFN-(Zr)-1 and
TFN-(Zr/Ti)-1 membranes and approximately 100 nm for the TFN-(Zr)-2 and
TFN-(Zr/Ti)-2 membranes). This suggested that the presence of
the UiO-66(Zr)-NH2nanoparticles in the structure of the membranes hindered the diffusion
of aqueous DETA toward the substrate, and thus, decreased the rate of
the IP reaction.1 The structure of the TFN-(Zr/Ti)-2
membrane with optimized
UiO-66(Zr/Ti)-NH2loading was further analyzed using TEM. The results were in agreement
with the cross-sectional SEM image in Figure 3l and further confirmed
the presence of the UiO-66(Zr/Ti)-NH2 nanoparticles
(Supporting Information, dark regions in Figure S9) in the structure of
the membrane.