FIGURE 4 TG and DTG curves of Ti membrane substrate, ZIF-8/Ti membrane and Cu/ZnO/Ti CMNR
The TG and DTG curves of Ti membrane substrate, ZIF-8/Ti membrane and Cu/ZnO/Ti CMNR were shown in Figure 4. It was found that the mass of Ti membrane substrate was hardly changed in the range of 35 ℃ and 650 ℃, which implied the good thermal stability of Ti membrane substrate. Two obvious mass degradation phenomena were observed in the two intervals of 100 ℃~150 ℃ and 250 ℃~400 ℃ for the ZIF-8/Ti membrane, as shown in the DTG curve. The two steps can be attributed to the decomposition of guest molecules (such as uncoordinated 2-mIm and solvent molecules) and the gradual collapse of ZIF-8, respectively. No significant step was observed in the TG curve of Cu/ZnO/Ti CMNR, which indicated the Cu/ZnO nanoparticles that immobilized in the pores of Ti membrane substrate having good thermal stability. A gradual mass decrease can be observed for ZIF-8/Ti membrane and Cu/ZnO/Ti CMNR from 400 ℃~650 ℃, which could be due to the decomposition of organic matter after Ti membrane substrate was modified by silanization31. After being heated at 650 ℃, the residual mass percentages were in the order: Ti membrane substrate > Cu/ZnO/Ti CMNR > ZIF-8/Ti membrane.
In order to analyze the elements and valence states of the nanoparticles immobilized in membrane, the Ti membrane substrate, ZIF-8/Ti membrane and Cu/ZnO/Ti CMNR were scanned by X-ray electrons, and the results were shown in Figure 5A. It can be seen that Ti element, O element due to the surface passivation and C element due to the carbon adsorption could be detected for Ti membrane substrate32. Zn and N elements were found for ZIF-8/Ti membrane in addition to C and O, which were the components of ZIF-8 exactly. For Cu/ZnO/Ti CMNR, the Cu, Zn, O, Ti and C elements were found without N element detected, which indicated the ZIF-8 was collapsed and the enhancement of O element peak intensity could ascribe to the Zn2+ derived into ZnO. It should be noted that Ti element was disappeared in ZIF-8/Ti membrane and only a weak signal of Ti element was found in Cu/ZnO/Ti CMNR, which could be clearly observed from the high-resolution scan spectrum of Ti element (Figure 5B). The peaks at the binding energies of 464.5 eV and 458.65 eV were corresponded to the 2p1/2 and 2p3/2 orbitals of Ti4+, respectively. This indicated that the Ti element in Ti membrane substrate existed in the form of TiO2, which was contradict with XRD results. This contradiction phenomenon may owe to the difference in the detection depth between XRD and XPS scanning, since the XPS could only reach the TiO2 film on the surface of Ti membrane substrate. Besides, the disappearance or weakness of Ti element in ZIF-8/Ti membrane and Cu/ZnO/Ti CMNR can be also explained by the low detection depth of XPS, since the signal of Ti element from XPS was hidden due to ZIF-8 or Cu/ZnO immobilized in the pores of the Ti membrane substrate. The high-resolution scanning energy spectrums of Zn in ZIF-8/Ti membrane and Cu/ZnO/Ti CMNR were shown in Figure 5C. The characteristic peaks at 1044.6 eV and 1021.5 eV were corresponded to 2p1/2 and 2p3/2 orbital of Zn2+, indicating that the Zn element existed in ZIF-8/Ti membrane was in the form of a divalent ion. The valent state of Cu element in the Cu/ZnO/Ti CMNR could be identified in Figure 5D. Two characteristic peaks at 952.05 eV and 932.1 eV were attributed to 2p1/2 and 2p3/2 orbital of Cu0, which proved most of the Cu elements were existed in the form of zero valence in Ti membrane substrate. In addition, the characteristic peak at 934.5 eV could be corresponded to the 2p3/2 orbital of Cu2+, indicating that a small amount of Cu element was oxidized by air, which was consistent with the results of XRD.