3.1.1. Chemical Composition

The results obtained by AAS for the determination of Zn and Al content and the content of carbonate and CR in the samples calculated from the determination of the carbon content (see Table 1) showed that the molar ratios Zn/Al and CR/Al were very close to 3/1 and 1/10, respectively (calculated from the amounts introduced in the synthesis mixture) for all the samples besides PZn3Al-CR(Aq) which showed lower values. The higher concentrations of Zn and Al in RZn3Al-CR(Et) are explained by the lower value of the H2O concentration in this sample.

#### 3.1.2. XRD Characterization of CR Functionalized Zn3Al-LDH Samples

The XRD patterns of the CR-loaded powders prepared by both direct precipitation and reconstruction are presented in Figure 4. The XRD patterns of curcumin-loaded powders are compared with those of the curcumin free powders obtained either by precipitation (Zn3Al-LDH) or by reconstruction in water (RZn3Al-LDH). The structural data are gathered in Table 2. The XRD patterns of the powders prepared by coprecipitation reveal that the precipitation in the presence of curcumin generates the formation of a zincite-phase (ZnO, ICDD card no. 36-1451) as by-product alongside with the layered structure. In the pristine Zn3Al-LDH reference sample, the LDH is the dominant phase

and it is similar to the carbonate-intercalated Zn,Al-LDH having a Zn/Al molar ratio of 3 standard, (Zn6Al2(OH)16CO3·4H2O, ICDD card no. 38-0486). Small reflections assignable to hydrozincite, Zn5(CO3)2(OH)2, as a minor phase impurity are also observable (Zn5(CO3)2(OH)2, ICDD card no. 19-1458) (marked by \* in Figure 4a). The lattice parameters are smaller for the PZnAl-CR(Aq) sample, denoting a lower Zn/Al molar ratio due to the formation of the ZnO phase. For the PZn3Al-CR(Et) solid, an extra layered phase with having a larger interlayer space has appeared, thus indicating the intercalation of larger-sized anions. Moreover, the small D003 value obtained for this extra-phase denotes a degree of crystalline disorder along the c-axis, the axis on which the brucite-like layers are stacked.

**Figure 4.** XRD patterns: (**a**) precipitated samples Zn3Al-LDH, PZn3Al-CR(Aq), PZn3Al-CR(Et); (**b**) reconstructed samples RZn3Al-LDH, RZn3Al-CR(Aq), RZn3Al-CR(Et).


**Table 2.** Structural data of the samples obtained from XRD analysis.

1 RZn3Al-LDH is the structure obtained after the rehydration of CZn3Al in water for 24 h at 25 ◦C.

The XRD patterns of the reconstructed samples (Figure 4b) show the partial reconstruction of the layered structure for the sample exposed to an aqueous solution. According to Kooli et al. [38], the hydration of the Zn(Al)O mixed-oxides leads to the formation of LDH with an insignificant amount of a zincite phase only for the calcined sample with a molar ratio Zn/Al=2 while, for higher Zn/Al ratios, residual ZnO is always present. The XRD pattern of the powder reconstructed in a CR ethanolic solution displays only the reflections of a ZnO-phase. The peaks are extremely broad, typically for a ZnO-phase calcined under mild conditions (400 ◦C–500 ◦C) [39]. The amount of ZnO-phase reported

to the layered LDH phases in PZn3Al-CR(Aq), PZn3Al-CR(Et) and RZn3Al-CR(Aq) fresh samples was estimated by considering the integrated intensities of the main single reflections of the ZnO-phase in RZn3Al-CR(Et) as reference. The data are included in the last column of Table 2 and disclosed values between 22 and 45% from the totality of crystalline products. However, it should also be acknowledged that the procedures used for the preparation of these powders, namely precipitation, thermal treatment and rehydration generate also amorphous oxide or hydroxides phases undetectable by XRD [38].

The reflections derived from crystalline CR were not detectable as a separate phase in the di ffraction patterns of any of the CR-loaded samples. This fact may be a consequence of its dispersion as amorphous nano-particles in the inorganic matrix.
