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.