3.1 Characterization of the CMNR
3.1.1 Structure of the CMNR
The X-ray diffraction patterns of Ti membrane substrate, ZIF-8/Ti membrane, Cu2+/Zn2+/Ti membrane and Cu/ZnO/Ti CMNR were shown in Figure 3. For the Ti membrane substrate, the diffraction peaks appeared at 2θ=35.1°, 38.4°, 40.2°, 53.0°, 62.9°, 70.7°, 76.2° and 77.4° were corresponded to the (100), (002), (101), (102), (110), (103), (112) and (201) crystal planes of Ti. After assembly of ZIF-8 nanoparticles in Ti membrane, new diffraction peaks appeared at 2=7.3°, 10.4°, 12.7°, 14.7°, 16.5° and 18.0° were corresponded to ZIF-8. In addition to the diffraction peaks of ZIF-8, there were some miscellaneous peaks, which may be attributed to the insufficient purity of the prepared ZIF-8 nanoparticles and the binding or covering of Zn sites on ZIF-8 by APTES26. Part of the diffraction peaks of the ZIF-8 crystal were weakened or even disappeared with the collapse of ZIF-8 structure, after the Cu(NO3)2 solution permeated through the ZIF-8/Ti membrane, since the Cu2+ and Zn2+ were similar in atomic radii and the coordination binding force of Cu2+ and 2-mIm was stronger than that of Zn2+, leading to Zn2+ in ZIF-8 replaced by Cu2+ 17,27-30. For the Cu/ZnO/Ti CMNR, the diffraction peaks at 2θ=31.7°, 34.3°, 36.3°, 47.3°, 56.6°, 67.9° and 69.1° were corresponded to the (100), (002), (101), (102), (110), (112) and (201) crystal planes of ZnO and the diffraction peaks at 2θ=43.3° and 50.4° that could be corresponded to the (111) and (200) crystal planes of Cu0 were also appeared in the Cu/ZnO/Ti CMNR. This phenomenon implied that the Cu/ZnO nanoparticles were successfully immobilized in the Ti membrane substrate by flowing synthesis. The ZnO in Cu/ZnO/Ti CMNR should be derived from Zn(NO3)2 during the process of ion exchange between Cu2+ of Cu(NO3)2 and Zn2+ of ZIF-8, since ZIF-8 can only be thermally decomposed into ZnO in air atmosphere instead of nitrogen atmosphere, but Zn(NO3)2 could be thermally decomposed into ZnO in nitrogen atmosphere, as shown in Figure S6. In addition, the diffraction peaks of oxidized Cu were also found in the Cu/ZnO/Ti CMNR, which might be owing to partial oxidation of Cu by oxygen in the air.