3.2 Morphology and structure of as-prepared photocatalysts
FESEM image in Fig. 2a shows the delicate ZIF-67 dodecahedron particles. And Fig. 2b manifests that CoP particles with the size of about 800 nm well inherit the polyhedral morphology of their ZIF-67 precursors, but its surface is concave inward and many cracks are formed on the surface of CoP polyhedron due to the porous characteristics of metal organic frameworks (MOFs). ZnCdS nanaoparticles exhibit an average size of approximately 40 nm (Fig. 2c). The FESEM image of ZnCdS/Co3O4QDs heterostructure cages also was investigated as shown in Fig. 2d. However, Co3O4QDs can’t be observed in the FESEM image (Fig. 2e) because its size is only about 4.5 nm as shown in the TEM image (Fig. 2f), but the existence of Co3O4 QDs is proved via EDS and element mapping tests. In Fig. 2g, in addition to Zn (23.52 wt.%), Cd (39.24 wt.%) and S (26.03 wt.%) elements were detected in the micro-scale testing area, Co species (0.94 wt.%) also were identified, which is consistent with the performed results of element mapping as displayed Fig. 2h. And the density of Co element obviously is lower in comparison with that of Zn, Cd, and S elements. These results signify the ZnCdS/Co3O4QDs heterostructure cages can be rationally designed and successfully constructed via fabricating Co3O4 QDs having only the average size of 4.5 nm on the surface of ZnCdS nanoparticles with the average size of 40 nm. Furthermore, it can be seen from Fig. 2i that the architecture of hierarchical CoP/ZnCdS/Co3O4 QDs bi-heterostructure cages was successfully constructed, just as we have wanted. Namely, ZnCdS nanoparticles were decorated on the surface of CoP polyhedron while Co3O4 QDs were integrated into the surface of ZnCdS nanoparticles to form hierarchical CoP/ZnCdS/Co3O4 QDs double cage heterostructures. In Fig. 2l, a set of distinct lattice fringes with spacing of 0.32 nm can be identified as the (002) planes of ZnCdS solid solution, and the lattice fringes of CoP and Co3O4 QDs are not observed because of their low crystallinity and content, consistent with the XRD analysis (Fig. 3a,b). And these elements of Zn, Cd, S, Co and P can be clearly observed in EDS (Fig. 2j) and element mapping images (Fig. 2m), and they are distributed uniformly.