Characterization of ZIFs nanoparticles and MMMs
In order to obtain the crystal structure of both ZIFs nanoparticles,
powder X-ray diffraction (PXRD) analysis was performed. Figure 1(a)
shows the PXRD pattern of the simulated ZIF-8, synthesized ZIF-8, and
synthesized Ni-ZIF-8, respectively. It can be seen that the main peaks
of the synthesized ZIF-8 and Ni-ZIF-8 are very clear and identical with
simulated ZIF-8, which proves its structure and phase
purity44. Figure 1 (b) shows the N2adsorption-desorption isotherms at 77 K for Ni-ZIF-8 and ZIF-8. The
overlapping of adsorption-desorption isotherm confirmed its microporous
structure as well. BET analysis provided the specific surface areas of
ZIF-8 (1420 m2/g), and Ni-ZIF-8 (1700
m2/g), while the micropore volumes were 0.58
cm3, and 0.66 cm3 of ZIF-8 and
Ni-ZIF-8, respectively. The specific surface areas of ZIFs depend upon
different synthesis methods, such as solvent used for synthesis, organic
linker, and the synthesis process (solvothermal or hydrothermal). The
BET surface area, pore size, and pore volume of both ZIFs are
well-matched with the reported work37. The SEM images
of both ZIFs are given in Figure S2, which confirms that both ZIFs have
spherical shapes and homogeneous particle sizes. Ni-ZIF-8 has an average
particle size of 40 nm, while ZIF-8 has the 60 nm (Figure S4). The EDX
results (Figure S3) revealed the exact percentage of Zn, Ni, N, and C in
its structure, and it confirms the well-distributed Ni atoms in the
whole structure of Ni-ZIF-8. TGA analysis revealed that Ni-ZIF-8 showed
better thermal stability from ZIF-8, as it was just a 7% decreased in
weight from 30 to 580ºC, which was probably due to the removal of
residual solvent in it (Figure S5). Above 580ºC, there was the breaking
of the imidazole molecule in the Ni-ZIF-8 structure. Furthermore, the
FTIR analysis (Figure 1 (c)) showed that there was no chemical change
occurred in MOF cluster by adding the Ni particles in the ZIF-8
WAXRD analysis was used for testing the microstructure of Ni-ZIF-8
nanoparticles in the MMMs and polymer matrix. As shown in Figure 1(d),
there are five main peaks at 12°, 18.4o,
20.1o, 22.9 o, and
26.8o in the pure PDMS/PVDF membrane. These broad
peaks confirmed the d-spacing of 7.3Å, 4.8 Å, 4.2 Å, 3.9 and 3.3 Å,
respectively, which can be attributed to the average distance between
PDMS and PVDF chains in the membrane. As the Ni-ZIF-8 incorporated with
the polymer, PDMS peaks became weak concerning the filler ratio. At 15%
filling of Ni-ZIF-8, these peaks almost minimized as compared with the
pure membrane peaks. Such findings were related to a disruption in the
chain packing of the polymer due to the addition of Ni-ZIF-8
nanoparticles. In comparison, the major peaks of the Ni-ZIF-8 are
conspicuous at high loading and at the same locations for the pure
Ni-ZIF-8 peaks, which revealed that no structural changes have occurred
in the Ni-ZIF-8 since impregnated with the PDMS.
The FTIR analysis was conducted and shown in Figure 1(e) to check the
chemical structures of pure PDMS and Ni-ZIF-8/PDMS MMMs. The pure PDMS
membrane and all MMMs spectrums were well matched with the reported
data45. For pure PDMS membrane, the characteristics
peaks were at 1256, 1009, and 788, which were accredited to the C-H
symmetric bending vibration, Si-O-Si stretching vibration, and
CH3 rocking vibration in Si–CH3,
respectively. These results confirmed the presence of PDMS in the
membrane. New bands were found in MMMs at 1310, 1145, and 750, which
were attributed to the stretching vibration of C-N, bending vibration of
C–H, and bending vibration of the imidazole ring, respectively. These
new peaks have confirmed the presence of Ni-ZIF-8 in the polymer matrix.
All these peaks confirmed the good affinity of Ni-ZIF-8 nanoparticles
with the PDMS and successful formation of MMMs with different Ni-ZIF-8
loadings.
The thermal stabilities of MMMs were observed by using TGA analysis. The
results are shown in Figure 1 (f), which showed that pure PDMS has good
thermal stability up to 400°C and pure Ni-ZIF-8 up to 580°C. It has been
shown that the incorporation of Ni-ZIF-8 particles into the PDMS matrix
enhanced the thermal stability of MMMs. Throughout the course of weight
loss in the 15% Ni-ZIF-8 MMM, three steps were found. In the primary
step (25-330°C), there was just 4% weight loss was observed, which was
due to the residual solvent removal from the membranes. While in the
second step (330-530°C), the pyrolysis of PDMS branches occurred, which
lost 34.5% weight of membrane, and in the last step (530-800°C) 40%
weight of the membrane was lost due to the degradation of PDMS and
Ni-ZIF-8. Except for observing the thermal stabilities of MMMs, the
exact loading percentages of the Ni-ZIF-8 were also determined using the
yield of the char at the end of this test. The Ni-ZIF-8 loadings from
TGA analysis were observed 4.98%, 9.98%, 15%, and 20% for 5-20%
Ni-ZIF-8 loaded MMMs, respectively, which are consistent with the
casting solution formulation data of the membranes.