Typical SEM images with different magnification of the assynthesized
CeO2 are presented in Fig. 2. The panoramic SEM
image (Fig. 2a) shows that the sample was sphere–shaped with
about 2.5–3.5 μm in diameters. Fig. 2b and c is the higher magnification
SEM image of a unit of CeO2 microsphere. It can be
clearly observed that the micro-sphere was composed of the nanoparticles
about 60–80 nm (in Fig. 2b). A hole about 600 nm in
diameter in Fig. 2c revealed that the microsphere was core shell
structure. And the interior was also composed of the nanoparticles.
Interestingly, in our previous experiments, the micro/
nano dumbbell -shaped CeO2 with hierarchical nanorods was
obtained in the absence of the citric acid while other reaction
parameters was fixed. So the citric acid played a key role in the
formation of micro/nano core shell CeO2. The further studies are
under in progress.