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.