## 3 Results and Discussion
### Characterization of LDH Powder
#### 3.1.3 Composition  
The elemental information of the prepared samples characterized by EDS is shown in Figure 7, and the atomic percentage of various elements is displayed in Table 1. According to this table, C element accounted for an obviously higher percentage in C-low than that in C-high, which was caused by the intercalation of a larger number of carbonates in C-low. This result was in good accordance with the XRD result above. As for the O element, the atomic percentage was 68.40% in C-high and 62.59% in C-low, respectively. It can be seen that the atomic percentage of O in these two samples was close to each other, and remarkable differences cannot be observed. It can be seen that the Mg/Al ratio differed greatly between the C-high and C-low samples. This value was 1.88 \(\pm\) 0.07 for C-high, and it was close to the ratio of Mg/Al in the reactant. However, this value increased to 2.79 \(\pm\) 0.16 when the reactant concentration was reduced. This result indicates that different MgAl-LDH was formed when the reactant concentration was changed. As explained above, the difference in pH value in the synthesis process of the two samples may result in this observation. According to the literature, the change of M\({}^{2+}\)/M\({}^{3+}\) ratio did not have an influence on the position of the (003) plane, and thus the difference in C-high and C-low in terms of the (003) plane position can only be attributed to the difference in intercalated anions [19]. In addition, the elemental distribution and the atomic percentage of different elements in the samples of C-high and C-low are shown in Figure 8, and it can be seen that the various elements including C, O, Mg, and Al were distributed uniformly, verifying the formation of MgAl-LDH instead of a mixture of oxides or hydroxides of magnesium and aluminum. The FTIR spectra are shown in Figure 9. The broad peak occurring in the range of 3000-3700 cm\({}^{-1}\) of the two samples can be attributed to the hydroxyl group in LDH and the adsorbed water molecules. The two peaks around 2300 cm\({}^{-1}\) were related to the asymmetric stretching vibration of CO\({}_{2}\) in the atmosphere. The peak at 1645 cm\({}^{-1}\) was due to the bending vibration of water molecules. The adsorption peaks below 1000 cm\({}^{-1}\) can be assigned to the lattice vibrations and stretching modes of M-O and M-OH (M refers to Mg\({}^{2+}\) and Al\({}^{3+}\)) [15]. For the sample of C-high, the peak at 1355 cm\({}^{-1}\) with a shoulder peak at higher wavenumber can be attributed to theintercalated nitrates and carbonates. The peak at 1362 cm\({}^{-1}\) can be assigned to the intercalated carbonates in the sample of C-low. Figure 6: The XRD result of the synthesized samples. Furthermore, an XPS test was carried out to demonstrate more detailed information related to chemical bonds. As can be seen from Figure 10a, the survey spectra of the two samples were similar to each other and indicated the presence of Mg, Al, C, N, and O. The high-resolution XPS spectra of N at 407.2 eV for C-high and 407.0 eV for C-low can be attributed to nitrates, indicating that nitrates existed in both samples. It should be noted that the signal of N element of C-high was obviously much stronger with less noise in comparison with that of C-low. The C 1s XPS spectrum for C-high consisted of two components including ubiquitous contaminant carbon at 284.8 eV and C=O group of CO\({}_{3}\)\({}^{2-}\) at 288.6 eV [15]. The C 1s spectrum for C-low also contained two peaks appearing at 248.8 eV and 287.7 eV. The slight shift of C=O may be related to a different environment caused by a different pH in the synthesis. The O 1s XPS spectrum of C-high included three peaks, corresponding to H\({}_{2}\)O at 533.1 eV, NO\({}_{3}\)\({}^{-}\) at 532.4 eV, and CO\({}_{3}\)\({}^{2-}\) or Al(OH)\({}_{3}\) at 531.8 eV [20]. For the C-low sample, the peak at 532.3 eV can be attributed to NO\({}_{3}\)\({}^{-}\); the peak at 531.6 eV can be attributed to CO\({}_{3}\)\({}^{2-}\), and the peak at 530.9 eV was due to Mg(OH)\({}_{2}\)[21].