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.