## 2 Results and Discussion  
The XRD patterns of pristine ZnO, CeO\({}_{2}\)[37], and Ce/ZnO samples (3%, 5%, and 7% of Ce ions) are shown in Figure 1. It can be seen that the XRD pattern of pristine ZnO corresponds to the reflection planes of the wurtzite hexagonal ZnO phase (JCPDS No. 89-13971) [12], while as-synthesized Ce/ZnO samples show additional peaks corresponding to the reflection planes of the cubic-structured CeO\({}_{2}\) (JCPDS No. 34-394) [38]. The lattice constants of ZnO and CeO\({}_{2}\) crystals can be calculated from the X-ray diffraction (XRD) results using a space lattice equation [7; 39]:  
Cubic-structured CeO\({}_{2}\):  
\[\frac{1}{d_{hkl}^{2}}=\frac{1}{a^{2}}(h^{2}+k^{2}+l^{2}). \tag{1}\]  
Wurtzite hexagonal ZnO:  
\[\frac{1}{d_{hkl}^{2}}=[\frac{4}{3}(h^{2}+hk+k^{2})+l^{2}(\frac{a}{c})^{2}] \frac{1}{a^{2}}, \tag{2}\]  
where \(d\) is the spacing between planes of atoms and \(h\), \(k\), and \(l\) are miller indices. Here, \(a\) and \(c\) are the lattice constants of the crystal. The sizes of ZnO and CeO\({}_{2}\) crystals can be calculated by [PERSON]'s equation [40]:  
\[t=(\frac{\lambda K}{\beta cos\theta}), \tag{3}\]  
where \(t\) is average crystal size, \(\lambda\) is the X-ray wavelength, \(K\) is a dimensionless shape factor, \(\beta\) is the broadening at half of the maximum intensity, and \(\theta\) is Bragg's angle. In the XRD results, it is clearly seen that the pristine ZnO sample is more crystalline compared to the as-prepared Ce/ZnO samples. In addition, it can be recognized that the impurity of CeO\({}_{2}\) crystals is exhibited by the broadening peak of CeO\({}_{2}\) crystals. Thus, from the XRD data, we can only determine the crystal size from the ZnO peak (calculated crystal sizes from several reflection planes of pristine ZnO and Ce/ZnO samples are shown in Table S1). The average crystal sizes and lattice constants are summarized in Table 1.  
As Ce ion doping increased, the intensity of the ZnO reflection plane decreased and became broader, while the intensity of the CeO\({}_{2}\) reflection plane increased. Interestingly, even though the amount of zinc is higher than cerium, the intensity of the ZnO was lower than the intensity of the CeO\({}_{2}\). This is due to the scattering factors of zinc and cerium atoms [41]. That is, the Zn\({}^{2+}\) has a lower scattering factor compared with Ce\({}^{4+}\) in Ce/ZnO samples. The scattering factor (f) of an atom is governed by \(\sin(\theta/\lambda)\) (\(\theta\) is the scattering angle and \(\lambda\) is the wavelength of the X-ray) and the atomic number. Therefore, it could be said that heavier atoms can be detected more easily than lighter atoms in the crystal structure. Additionally, it can be seen that the crystal size of Ce/ZnO was reduced compared with that of pristine ZnO. This result agrees with a result obtained by [PERSON] et al. [42]. Although Ce\({}^{4+}\) has a higher ionic radius compared with Zn\({}^{2+}\), Ce\({}^{4+}\) ions possibly move into the ZnO matrix due to the low decomposition temperature of nitrate. Thus, Ce\({}^{4+}\) can enter/intermingle in the ZnO matrix and substitute for the Zn\({}^{2+}\) position to form CeO\({}_{2}\). Figure 2 presents the surface morphology and composition of pristine ZnO and Ce/ZnO samples that were observed by field-emission scanning electron microscopy (FE-SEM). The pristine ZnO has a hexagonal rod-like structure with an average particle size of 432 nm (the Size distribution of pristine ZnO is shown in Figure S1). Ce ion doping had no effect on the shape of the Ce/ZnO. However, the average particle size of the Ce/ZnO showed an increase due to the introduction of Ce into the ZnO structure. HR-TEM was performed to gain deep insight into the crystal structure, growth direction,