## 3 Results and Discussion  
### Structural and Microstructural Characterization  
Figure 1 shows the XRD patterns of the prepared ceria oxides in which all the diffraction maxima could be indexed to cubic fluorite-type CeO\({}_{2}\) (PDF 01-078-0694), S.G. _Fm-3m_. Table 1 gathers the cell parameters obtained from Rietveld refinements of these data, by means of the Fullprof program. In the Supplementary Materials file, Figure S1 shows the corresponding fits for representative cases and Table S1 includes the corresponding R-factors. The crystallinity of the samples significantly varies depending on the synthesis route employed. Thus, it is clearly appreciated that the Ce-pH13 and Ce-CMK3 samples present smaller width of the XRD diffraction maxima. The corresponding crystallite sizes in each case as obtained from the Scherrer formula are also included in Table 1.  
Textural features of the ceria compounds were studied by means of nitrogen adsorption measurements. The corresponding specific surface area (S\({}_{\text{BET}}\)) values are included in Table 1. In alignment with the XRD results, the observed different widening of the diffraction peaks is related to the crystallite sizes. Thus, the smaller crystallite sizes fit with the higher surface areas observed. The microstructure of the studied ceria compounds was analyzed by HRTEM. Figure 2 shows representative low magnification images. Relevant differences concerning particle morphology and size can be appreciated. Indeed, whereas Ce-pH7, Ce-PMMA, Ce-ox and Ce-ref show very small particles of predominant mean sizes of about 3 nm, which tend to aggregate, in Ce-pH13 and Ce-CMK3 samples most particles are of about 100 nm. Figure 1: (a) XRD diffraction patters for prepared the ceria materials. The indexation refers to CeO\({}_{2}\) fluorite-type (PDF 01-078-0694); (b) Rietveld profiles and R-factors for the Ce-CMK3 sample. The particle size ranges of the samples are included in Table 1. It can be appreciated that these values are coherent with those obtained from the Scherrer expression. Rigorously, the Scherrer expression gives the coherent diffraction domain size, rather than crystallite size or particle size. Nevertheless, in absence of relevant strain and disorder effects, the order of magnitude of both should be comparable. In our samples, both effects, if present, should be similar. In Figure 3, representative high magnification images of Ce-pH7, Ce-pH13 and Ce-CMK3 are shown. As can be observed, the ED patterns included are coherent with the particle's sizes in each case. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Notation** & **Synthesis** & **Crystallite** & **a (Å)** & **S\({}_{\text{BET}}\)** & **Particle Size** \\  & **Method** & **Size (nm)**b [FOOTNOTE:b]Footnote b: For the first time, the second time  
Morphological differences are also evident among the samples. Thus, pseudo-spherical particles are obtained in the Ce-CMK3 sample, whereas polygonal-rhombic particles are present in Ce-pH13. Moreover, nanovesicles typical of samples obtained by hydrothermal methods are also observed in Ce-pH13. The shape of the tiny, aggregated nanoparticles of Ce-pH7, Ce-PMMA and Ce-ox is pseudo-spherical, whereas they present a bar-like morphology in the Ce-ref sample. Therefore, the CeO\({}_{2}\) compound prepared by different synthesis routes clearly shows different microstructures. In the case of Ce-CMK3, the particle size distribution is somewhat more monodispersive, and the aggregation phenomenon is not appreciated. Figure S2 (in the Supplementary Materials file) includes representative EDX spectra for all the samples. In the case of the Ce-CMK3 anode, small amounts of silicon are detected, probably already present in the carbonaceous material employed in its preparation, which was obtained from a treatment over a CMK3 silica template. FTIR experiments were carried out to analyze the possible different functional groups appearing in the samples because of the different synthesis procedures.