FIGURE 5 XPS survey spectra of Ti membrane substrate,ZIF-8/Ti membrane and Cu/ZnO/Ti CMNR (A); High-resolution XPS spectra of Ti 2p (B), Zn 2p (C) and Cu 2p (D)
The loading amount of Cu and ZnO immobilized in Cu/Ti membrane, ZnO/Ti membrane and Cu/ZnO/Ti CMNR were analyzed by ICP and calculation, with the results shown in Table 1. The loading amount of Cu nanoparticles in Cu/Ti membrane was lower than 0.01 mg·g-1 (Ti substrate) due to the inert and poor retention of Ti membrane substrate, if the Cu2+ solution was directly permeated through Ti membrane substrate without ZIF-8 as carrier. Immobilizing ZIF-8 in Ti membrane substrate followed with calcined in air, over 21 mg of ZnO per gram of Ti substrate can be immobilized in membrane pores. Immobilizing ZIF-8 in Ti membrane substrate as a sacrificial intermediate followed with Cu2+ ion exchange and reduction, the loading amount of Cu was about 13 times higher than that in Cu/Ti membrane without ZIF-8 immobilization in advance. Two key issues can be devoted to the loading increase of Cu after immobilization of ZIF-8. The pore size of Ti membrane substrate would be reduced, which can inhibit Cu nanoparticles leaching from the membrane pores during Cu nanoparticles immobilization in membrane pores. Besides, ZIF-8 with high specific area could adsorb Cu2+ via ion-exchange27. High amount of Cu2+adsorption in membrane pores could increase the loading amount of Cu nanoparticles.
TABLE 1 The loading amount of Cu and ZnO nanoparticles in Ti membrane by ICP test and calculation