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.