3.1.3. ATR-FTIR Characterization

The ATR-FTIR spectra of the fresh CR-containing samples are displayed in Figure 5a,b. The spectrum of the reference sample (Zn3Al-LDH) (Figure 5a) presents all the absorption bands specific to carbonate intercalated Zn,Al-LDH at 3428 cm<sup>−</sup><sup>1</sup> (υ-OH), 2981 cm<sup>−</sup><sup>1</sup> (interaction of carbonate and H2O in the interlayer through hydrogen bonds), 1630 cm<sup>−</sup><sup>1</sup> and 772 cm<sup>−</sup><sup>1</sup> (deformation vibrations of interlayer H2O), 1363 cm<sup>−</sup><sup>1</sup> (deformation vibration of carbonate anion), 770 cm<sup>−</sup><sup>1</sup> (Al-OH out-of-plane) 617 cm–1 (deformation of Zn-OH bond), 551 cm–1 and 427 cm–1 (vibrations in Al-O-Al and Zn-O-Zn condensed groups) [40,41].

**Figure 5.** Normalized attenuated total reflectance (ATR)-FTIR spectra: (**a**) precipitated samples Zn3Al-LDH, PZn3Al-CR(Aq), PZn3Al-CR(Et); (**b**) reconstructed samples RZn3Al-CR(Aq), RZn3Al-CR(Et).

The spectra of CR-containing samples were similar to that of Zn3Al-LDH, except the one attributed to RZn3Al-CR(Et) (Figure 5b). In addition, the bands characteristic for neat curcumin could not be delimited from those of the LDH. However, following CR-incorporation by precipitation, the bands of the parent LDH present in the region 4000–2800 cm<sup>−</sup><sup>1</sup> have increased their relative intensity due to the overlapping of the bands attributed to CR (see Figure 6) with those of Zn3Al-LDH (Figure 5a). There is also a noticeable red shifting of the bands at 3428 cm<sup>−</sup>1, 1363 cm<sup>−</sup><sup>1</sup> and 772 cm<sup>−</sup><sup>1</sup> to 3396 cm<sup>−</sup><sup>1</sup>

for PZn3Al-CR(Aq), 3419 cm<sup>−</sup><sup>1</sup> for PZn3Al-CR(Et), 1356 cm<sup>−</sup>1, 759 cm<sup>−</sup><sup>1</sup> for PZn3Al-CR(Aq) and 748 cm<sup>−</sup><sup>1</sup> for PZn3Al-CR(Et), respectively. The presence of the band at 1356 cm<sup>−</sup><sup>1</sup> indicates the contamination of these samples with carbonate most probably caused by the carbonation of NaOH during the manipulation before its utilization in the synthesis. This assumption is sustained by the results obtained in the analysis of carbon content presented in Table 1. In addition to that, the more pronounced asymmetry, the shifting of the bands in the region 3600–3300 cm<sup>−</sup><sup>1</sup> as well as the significant attenuation of the band corresponding to H2O deformation vibrations at 1630 cm<sup>−</sup><sup>1</sup> compared to the reference sample Zn3Al-LDH, emphasizes the contribution of CR interactions with the inorganic matrix through hydrogen bonds. The higher intensity of the band around 550 cm<sup>−</sup><sup>1</sup> compared to the one in the region 780–748 cm<sup>−</sup><sup>1</sup> indicates an increased amount of Zn-O-Zn condensed groups in both CR-functionalized samples obtained by co-precipitation and reconstruction with CR-aqueous solution and it may be correlated with the results obtained from XRD characterization. In the spectrum of PZn3Al-CR(Aq) the band corresponding to water deformation vibrations is red shifted to 1614 cm<sup>−</sup>1, indicating a perturbation in the interlayer region as a consequence of CR-incorporation, while in the spectrum of PZn3Al-CR(Et) the same band is missing. Though, a novel absorption band appears at 1550 cm<sup>−</sup><sup>1</sup> in the spectrum of PZn3Al-CR(Et) indicating the formation of a distorted Zn(II)-CR complex [42]. In the spectrum of RZn3Al-CR(Aq), the band appearing in the hydroxyl vibrations region has a lower relative intensity compared to the one of the reference material indicating a poor reconstruction due to the remaining of a segregate phase of ZnO whose presence was also confirmed by XRD. In addition to that, it was also noticed the absence of the band attributed to water deformation vibrations and the presence of a new band corresponding to Zn(II)-CR complex at 1583 cm<sup>−</sup><sup>1</sup> [42]. The spectrum of RZn3Al-CR(Et) shows five well defined absorption bands at 1505, 1398, 763, 680 and 427 cm<sup>−</sup>1. Among these bands, the one at 1505 cm<sup>−</sup><sup>1</sup> is correlated to the most intense band of curcumin (Figure 6), the band at 1398 cm<sup>−</sup><sup>1</sup> could be assimilated to a red shift of the curcumin band at 1427 cm<sup>−</sup><sup>1</sup> while the band at 680 cm<sup>−</sup><sup>1</sup> could be also due to a red shift of the curcumin band at 808 cm<sup>−</sup>1. The bands at 763 and 427 cm<sup>−</sup><sup>1</sup> are related to M-O vibration modes [40,41]. The absence of the band at ca. 3400 cm<sup>−</sup><sup>1</sup> in this spectrum shows the absence of the reconstruction e ffect.

**Figure 6.** ATR-FTIR spectrum of the neat curcumin (CR) utilized in this study.
