Figure 3. (a)-(d) Scanning electron microscopy (SEM) surface
images, (e)-(h) magnified SEM surface images, and (i)-(l)
cross-sectional SEM images of the metal–organic framework membranes in
this study. Here TFN-(Zr)-1 and TFN-(Zr)-2 denote thin-film
nanocomposite membranes with UiO-66(Zr)-NH2 loadings of
0.01% and 0.03% (w/v) in 50 mL of n-hexane solution,
respectively, and TFN-(Zr/Ti)-1 and TFN-(Zr/Ti)-2 denote thin-film
nanocomposite membranes with UiO-66(Zr/Ti)-NH2 loadings
of 0.01% and 0.03% (w/v) in 50 mL of n-hexane solution,
respectively.
In addition, the AFM morphology images of the aforementioned TFC and TFN
membranes were obtained to visualize more topographic details and the
measured surface roughness values, viz. their mean (Ra) and root mean
square (RMS, Rq) roughness (Figure 4, Table 1, and Supporting
Information, Figure S10). We determined that the Ra values of the TFN
membranes were higher than those of the pristine TFC membrane (Ra =
±25.4).35 Moreover, as the MOF nanoparticle loading
increased, the Ra and Rq values of the TFN membranes decreased
significantly, and that was attributed to the increase in the number of
MOF nanoparticles that filled the ridge-and-valley structures.
Furthermore, for the same MOF loading, the Ra and Rq values of the
TFN-(Zr) membranes were relatively higher than those of the TFN-(Zr/Ti)
membranes. That could be attributed to the surface of the
UiO-66(Zr/Ti)-NH2 nanoparticles being relatively rougher
than that of the UiO-66(Zr)-NH2 nanoparticles (Figure
1b). The increase in surface roughness after the incorporation of the
MOF nanoparticles into membranes was highly beneficial and increased the
exposed surface area for effective ion
permeation.36-38 Thus, the precisely controlled
symmetrical ridge-and-valley morphology and high surface roughness of
the fabricated MOF membranes confirmed the merits of the adopted
membrane fabrication strategy.