## 3 Results and Discussion  
### Characterization of CAL  
To investigate the influence of aging temperature on the crystallizing behaviors of as-synthesized CAL, XRD patterns were obtained as shown in Figure 1. XRD patterns (a) and FT-IR spectra (b) of CaAl-NO\({}_{3-}\) layered double hydroxides (LDHs) prepared at different temperatures (samples were indicated as CAL-x, where x = aging temperature, 60-120 \({}^{\circ}\)C). It was found that no calcium carbonate byproduct formed in the CAL samples synthesized at 60-120 \({}^{\circ}\)C. The CAL samples collected at different temperatures exhibited multiple diffraction peaks at 10.3\({}^{\circ}\), 20.6\({}^{\circ}\), 31.3\({}^{\circ}\), 37.6\({}^{\circ}\), 44.5\({}^{\circ}\) and 46.0\({}^{\circ}\) (20), which was consistent with those reported using the similar method [20]. These peaks correspond to (002), (004), (110), (114), (116) and (018) diffraction planes of the typical CAL structure with nitrate anion intercalated between the main layer (PDF-#54-0849). By comparing (002) and (004) diffraction peak intensities of different samples, it was found that the crystallinity of CAL increased with elevating aging temperature, and the similar results were also reported by [PERSON] [21]. Moreover, the basic layer spacing (distance between two layers + one single layer) can be calculated at (002) diffraction peak by using the Bragg equation. In general, variation of the basic layer spacing at the first diffraction peak indicates clearly that LDHs are completely or partially intercalated by guest anions [22]. All the CAL samples showed the similar spacing d\({}_{002}\) values ranged from 0.858 to 0.876 nm, which was in good agreement with the previously reported values [23; 24]. The FT-IR spectra of CAL samples (see Figure 1. XRD patterns (a) and FT-IR spectra (b) of CaAl-NO\({}^{3-}\) layered double hydroxides (LDHs) prepared at different temperatures (samples were indicated as CAL-x, where x = aging temperature, 60-120 \({}^{\circ}\)C)b) exhibited similar vibration characteristics of LDHs structure, and the aging temperature had little effect on their structure. Broad bands observed at 3636 cm\({}^{-1}\) and 3472 cm\({}^{-1}\) in all CAL were ascribed to the stretching vibration of hydroxyl existed in the hydroxide layer structure, interlayer water and adsorbed water molecules of CAL [25; 26], while the bending vibration of H-O-H existed in some water molecule was observed at 1638 cm\({}^{-1}\)[27]. The overlapping peaks showed at 1300-1500 cm\({}^{-1}\) were ascribed to the antisymmetric stretching vibration of CO\({}_{3}\)\({}^{2-}\) (1430 cm\({}^{-1}\)) [26; 27; 28] and interlayer nitrate groups of CaAl-NO\({}_{3}\)\({}^{-}\) LDHs (1353 cm\({}^{-1}\)) [23], indicating that the as-synthesized CAL samples might be carbonated during their preparation due to CO\({}_{2}\) in the atmosphere. It should be noted that no obvious diffraction peaks of carbonates (e.g., calcite) can be observed in the XRD patterns (Figure 1. XRD patterns (a) and FT-IR spectra (b) of CaAl-NO\({}^{3-}\) layered double hydroxides (LDHs) prepared at different temperatures (samples were indicated as CAL-x, where x = aging temperature, 60-120 \({}^{\circ}\)C)a), which may be related to the stronger diffraction peak of CaAl-NO\({}_{3}\)\({}^{-}\) LDH phase. The other peaks at 500-800 cm\({}^{-1}\) were caused by the lattice vibrations of M-O or M-OH (M represents metal, e.g., Ca or Al). SEM images showed the morphology of CAL samples synthesized at different aging temperatures (Figure 2. SEM images of CaAl-NO\({}^{3-}\) LDH samples synthesized at different temperatures ((a) 60 \({}^{\circ}\)C, (b) 70 \({}^{\circ}\)C, (c) 80 \({}^{\circ}\)C, (d) 100 \({}^{\circ}\)C, and (e) 120 \({}^{\circ}\)C)). It was found that all the CAL samples exhibited regular hexagonal flake-like particle morphology corresponding to hydrocalumite compounds.