The ATR-FTIR spectra (Figure 5a) confirmed the successful embedding of
the MOF nanoparticles into the TFN membranes. The
presence of the characteristic
bands of C=O (amide I) and N–H (amide II) at 1645 and 1550
cm−1, respectively, in the ATR-FTIR spectra of the
membranes confirmed the successful formation of a polyamide layer on the
HPAN substrate. The increase in sharpness of the 1435 and 1260
cm-1 peaks confirmed the successful
UiO-66(Zr)-NH2or
UiO-66(Zr/Ti)-NH2 loading, respectively, during the IP
reaction (Figure 5b). Furthermore, the additional peaks at 1255 and
~800 cm−1, which were ascribed to the
C–O bonds39 and Zr–O clusters40,
respectively, further confirmed the formation of bonds between the
UiO-66-NH2 nanoparticles and polyamide layer on the
surface of the HPAN substrate during the IP reaction. The fabricated
TFC, TFN-(Zr)-2, and TFN-(Zr/Ti)-2 membranes were further analyzed using
XPS, and the results confirmed the presence of
UiO-66(Zr)-NH2 or UiO-66(Zr/Ti)-NH2nanoparticles in the fabricated TFN membranes (Supporting Information,
Figure S11). In addition, the zeta
potentials of the TFN-(Zr)-1, TFN-(Zr)-2, TFN-(Zr/Ti)-1, and
TFN-(Zr/Ti)-2 membranes at various pH values revealed that the surfaces
of the TFN membranes were negatively charged at neutral pH, which is a
requirement for the electrochemical cation permselective separation of
both the Na+/Mg2+ and
Li+/Mg2+ systems (Supporting
Information, Figure S12).