*3.2. X-ray Fluorescence*

The elemental composition and metal ratios of the samples were obtained via XRF analysis and listed in Table 3. All samples were found to have a small amount of zirconium, yttrium, and iron contamination from the milling media and the milling chamber. Samples S1 and S2 contained SiO2 introduced by the selected commercial grade MgO reagent. XRF analysis was conducted to ensure that the correct metal ratios were applied to the raw materials added to the system and are therefore not an indication of the composition of the LDH phases present within each sample. They are an indication of the metal ratios within the overall sample obtained. Calculated metal ratios were observed to correlate with those adapted from literature.


**Table 3.** Calculated MII:MIII ratios of each sample, after wet milling for 1 h with a Netzsch LME 1 horizontal bead mill, as obtained through XRF analysis.

## *3.3. X-ray Di*ff*raction Analysis*

Samples analysed were observed to have minor unindexed peaks present. This could be attributed to unidentified phases present or impurities due to mill degradation. Future studies should be conducted in attempt to adequately investigate all phases present within samples obtained.

**Mg-Al LDH.** Figures 2 and 3 show the XRD spectra obtained for samples S1 and S2. The reaction had not ye<sup>t</sup> reached completion at the selected synthesis conditions prior to ageing. The sample synthesised with a 2:1 metal ratio (S1) exhibited no clear LDH peaks with no crystalline LDH phase detected within the sample. Ageing of the sample resulted in a clear LDH pattern with primary and secondary peaks located at 2θ values of 13.49◦ and 27.22◦, respectively. This correlated with a basal spacing of 0.759 nm. Comparatively, S2 exhibited primary and secondary LDH peaks prior to ageing at 2θ values of 13.37◦ and 26.99◦, respectively. This correlated with a basal spacing of 0.767 nm. Ageing of the sample resulted in more complete conversion of raw materials to LDH product, with clear peaks observed at 2θ values of 13.48◦ and 27.10◦. The basal spacing was calculated to be 0.760 nm. Basal spacing has been found to be influenced by numerous factors including milling time, the amount of water present and possible carbonate contamination [15]. Lattice imperfections as a result of mechanically induced amorphitisation could further contribute to larger basal spacing values [5]. The addition of water has been known to decrease the degree of supersaturation, which could negatively impact morphology and crystallinity of the synthesised LDHs. It has further been observed that the crystallinity of LDHs can pass through a maximum, with lattice imperfections increasing with an increase in milling time [26]. The calculated basal spacing values obtained for both S1 and S2, after ageing, were found to be similar with those reported in literature [15]. Similarly, spectra for the aged samples obtained further correlated with existing literature [9,15,27].

**Figure 2.** (**a**) X-ray di ffraction analysis of the Mg-Al LDH sample (S1), synthesised with a 2:1 MII:MIII ratio, after ageing for 24 h at 80 ◦C.; (**b**) X-ray di ffraction analysis of sample S1 prior to ageing after 1 h of wet milling at 2000 rpm.

**Figure 3.** (**a**) X-ray diffraction analysis of Mg-Al LDH sample (S2), synthesised with a 3:1 MII:MIII ratio after ageing for 24 h at 80 ◦C; (**b**) X-ray diffraction analysis of sample S2 prior to ageing after 1 h of wet milling at 2000 rpm.

**Ca-Al LDH.** XRD reflection patterns for samples S3 and S4 are as depicted in Figures 4 and 5, respectively. The presence of LDH was observed prior to ageing for S3 with a primary peak at a 2θ of 13.50◦. This correlated to a basal spacing of 0.759 nm. Ageing of the sample improved conversion of raw materials to LDH product, with a decrease in Al(OH)3 and Ca(OH)2 peak intensity observed. Twinning primary peaks were observed at 2θ values of 13.54◦ and 13.20◦ corresponding to basal spacing's of 0.757 nm and 0.776 nm, respectively. Comparatively, the XRD spectra for S4 depicted the presence of LDH within the sample despite the lack of a direct carbonate source. The primary peak was observed to occur at 2θ of 13.47◦, with a basal spacing of 0.761 nm. The presence of CaCO3 was further noted and likely due to atmospheric carbonate contamination. Ageing of the sample resulted in the formation of katoite (Ca3Al2(OH)12) as well as twinning primary LDH peaks. These were observed to occur at 2θ of 13.27◦ and 13.58◦, corresponding to basal spacing of 0.772 nm and 0.755 nm. Twinning peaks could be attributed to the presence of different LDH phases within the sample and differ through carbonate content [22,27,28]. Basal spacing values obtained for both samples, before and after ageing, sugges<sup>t</sup> the presence of either or a mixture of calcium monocarboaluminate and a dehydrated polymorph of calcium hemicarboaluminate that forms upon ageing. Basal spacing reported for each of these were 0.750 nm [22,27] and 0.760–0.770 nm [22,28], respectively. Ca-Al LDH synthesised in the presence of a carbonate source formed more readily when compared to that synthesised with no CaCO3. Previous studies have shown that Ca-Al LDH and katoite (tricalcium aluminate) can result when reacting Ca(OH)2 and Al(OH)3 in the absence of an additional phase or carbonate source, with little to no LDH formation occurring at times [11,21,29]. Studies have also shown that, upon the addition of a third phase such as CaCO3 or CaCl2·2H2O LDH, formation occurs more readily with little to no katoite formation. This suggests a complex relationship between the formation of Ca-Al LDH and katoite. It has been suggested that the presence of pillared anions such as Cl− and CO3<sup>2</sup>− assist in the formation of the layered structure of Ca-Al LDH. The third phase therefore stabilises the Ca-Al LDH structure allowing for formation to occur more readily [21].

**Figure 4.** (**a**) X-ray diffraction analysis of sample Ca-Al LDH sample (S3), synthesised with a 2:1 MII:MIII ratio, in the presence of a carbonate source, after ageing for 24 h at 80 ◦C.; (**b**) X-ray diffraction analysis of sample S3 prior to ageing, after 1 h of wet milling at 2000 rpm.

**Figure 5.** (**a**) X-ray diffraction analysis of sample Ca-Al LDH sample (S4), synthesised with a 2:1 MII:MIII ratio, without the presence of a carbonate source, after ageing for 24 h at 80 ◦C.; (**b**) X-ray diffraction analysis of sample S4 prior to ageing after 1 h of wet milling at 2000 rpm.

**Zn-Al LDH.** The XRD patterns of samples S5 are depicted in Figure 6. The primary LDH peak was observed to occur at a 2θ value of 13.86◦, corresponding to a basal-spacing of 0.749 nm. Conversion prior to ageing was observed to be incomplete with Zn5(CO3)2(OH)6 and Al(OH)3 peaks observed at 2θ values of 15.08◦ and 21.31◦, respectively. Ageing of the sample resulted in the increase in LDH peak intensity with a primary peak at 2θ of 13.70 which corresponds to a basal spacing of 0.748 nm. A decrease in raw material peaks were observed with ageing; however, conversion remained incomplete for the selected synthesis conditions. Metal ratios (MII:MIII) typically employed for the synthesis of Zn-Al LDH are between 2:1 and 4:1 for conventional methods such as co-precipitation [18]. It has recently been suggested that molar ratios suitable for mechanochemical synthesis range between 1:1 and 2:1, with nearly pure phase Zn-Al LDH as the result [18]. Slight differences were observed for basal spacing values obtained. These were observed to differ from those reported in literature (0.758 nm, Zn4CO3(OH)6·H2O as starting material) with the Zn content influencing the basal spacing obtained [18].

**Figure 6.** (**a**) X-ray diffraction analysis of Zn-Al LDH sample (S5), synthesised with a 1:1 MII:MIII ratio, after ageing for 24 h at 80 ◦C.; (**b**) X-ray diffraction analysis of sample S5 prior to ageing after 1 h of wet milling at 2000 rpm.

**Cu-Al LDH.** The XRD spectra obtained for samples S6 and Cu2(OH)2CO3 can be seen in Figure 7. The results for S6 prior to ageing were considered to be inconclusive as no obvious LDH peaks were identified. A decrease in Cu2(OH)2CO3 peak intensities were, however, observed to occur after 1 h of milling activity. Ageing of the sample resulted in the formation of a peak at 13.74◦ with a second peak present at approximately 27.76◦. Identification of the LDH peaks were difficult due to prominent and overlapping Cu2(OH)2CO3 peaks. Basal spacing associated with the observed primary peak was determined to be 0.746 nm. This was observed to be smaller than that reported in literature (0.754 nm) [17]. The formation of Cu-Al LDH was reported to be dependent on the selected rotational speed and therefore the degree of amorphitization [17].

**Figure 7.** (**a**) X-ray diffraction analysis of Cu-Al LDH sample (S6), synthesised with a 2:1 MII:MIII ratio, after ageing for 24 h at 80 ◦C.; (**b**) X-ray diffraction analysis of sample S6 prior to ageing after 1 h of wet milling at 2000 rpm.

#### *3.4. Fourier Transform Infrared Spectroscopy*

The main purpose of the FT-IR data was to support the notion that LDH was present within each sample. This was due to the fact that not all LDH peaks were easily identifiable when conducting XRD analysis.

**Mg-Al LDH.** The FT-IR spectra for S1 and S2, before and after ageing, are depicted in Figures 8 and 9. Prior to ageing peaks were observed to occur between 3500 cm<sup>−</sup><sup>1</sup> and 3700 cm<sup>−</sup><sup>1</sup> for both samples and could be attributed to the stretching vibrations of free –OH groups [15]. Similarly, peaks located between 3250 cm<sup>−</sup><sup>1</sup> and 3600 cm<sup>−</sup><sup>1</sup> are likely due to bonded –OH within both samples [15]. Peaks located at 1367 cm<sup>−</sup><sup>1</sup> (S1) and 1365 cm<sup>−</sup><sup>1</sup> (S2) could be attributed to carbonate interactions (CO3<sup>2</sup>− *v3* vibrations) [9,15,20]Ageing of the samples at 80 ◦C for 24 h, resulted in the intensification of these peaks. A broad peak, from 3250–3700 cm<sup>−</sup>1, specifically 3425 cm<sup>−</sup><sup>1</sup> (S1) and 3460 cm<sup>−</sup><sup>1</sup> (S2), was observed to form upon ageing of both samples. This could be assigned to the –OH stretching vibrations that occur within the layered brucite like structure of the LDH as well as interlayer water molecules [9,15,20]. Peaks observed between 500 and 900 cm<sup>−</sup><sup>1</sup> could be attributed to M-O and MO-H (M = Mg, Al) vibrations [30]. Peaks located from 1100–900 cm<sup>−</sup><sup>1</sup> are typical of Si-O interactions from SiO2 impurities in the MgO raw material [30]. The FT-IR spectra for both S1 and S2 after ageing coincide with spectra observed in literature [9,15,20,27].

**Figure 8.** (**a**) FT-IR analysis of the Mg-Al LDH sample (S1) prior to ageing, synthesised with a 2:1 MII:MIII ratio, after 1 h of wet milling at 2000 rpm.; (**b**) FT-IR analysis of sample S1 after ageing for 24 h at 80 ◦C.

**Figure 9.** (**a**) FT-IR analysis of the Mg-Al LDH sample (S2) prior to ageing, synthesised with a 3:1 MII:MIII ratio, after 1 h of wet milling at 2000 rpm.; (**b**) FT-IR analysis of sample S2 after ageing for 24 h at 80 ◦C.

**Ca-Al LDH.** FT-IR spectra for S3 and S4, before and after, ageing are depicted in Figures 10 and 11. Peaks observed between 3700–3300 cm<sup>−</sup><sup>1</sup> could be due to MO-H vibrations within each sample [5,21] as well as the vibration of –OH (*v2*) within the inorganic main layers of the LDH structure [5]. Prior to ageing, S3 depicted peaks at 1418 cm<sup>−</sup><sup>1</sup> and 876 cm<sup>−</sup>1, which could be due to carbonate vibrations on the surface of the LDH structure present [5,21]. Similarly peaks at 1370 cm<sup>−</sup><sup>1</sup> could be attributed to CO3<sup>2</sup>− *v3* vibrations within the interlayer of the LDH structure [21]. Ageing of the sample resulted in similar spectra to that obtained prior to ageing. The twinning carbonate interactions observed near 1366 cm<sup>−</sup><sup>1</sup> have been suggested further to be the result of two different environments for carbonate present, likely due to different Ca-Al LDH phases present [27]. The spectra for S4 prior to ageing were observed to be similar to that of S3. Peaks observed at 1414 cm<sup>−</sup><sup>1</sup> could be the result of carbonate within the system [5,21]. Synthesis, drying, and ageing were conducted, without the use of an inert gas, under air atmosphere. Carbonate contamination was therefore possible. Ageing of the sample resulted in the formation of twinning peaks at 1366 cm<sup>−</sup><sup>1</sup> and 1415 cm<sup>−</sup>1. These could once again be attributed to interlayer and surface carbonate interactions of LDH formed within the system [5,21].

**Figure 10.** (**a**) FT-IR analysis of the Ca-Al LDH sample (S3) prior to ageing, synthesised with a 2:1 MII:MIII ratio in the presence of a Carbonate source, after 1 h of wet milling at 2000 rpm.; (**b**) FT-IR analysis of sample S3 after ageing for 24 h at 80◦.

**Figure 11.** (**a**) FT-IR analysis of the Ca-Al LDH sample (S4) prior to ageing, synthesised with a 2:1 MII:MIII ratio in the absence of a Carbonate source, after 1 h of wet milling at 2000 rpm.; (**b**) FT-IR analysis of sample S4 after ageing for 24 h at 80 ◦C.

**Zn-Al LDH.** The FT-IR spectra for S5, as well as that of Zn5(CO3)2(OH)6, is depicted in Figure 12. The peaks at and before 3468 cm<sup>−</sup><sup>1</sup> could be attributed to the stretching vibrations of –OH groups within the sample [18]. Spectra of the sample was observed to resemble that of the Zn5(CO3)2(OH)6 raw material before implantation of the ageing step. Ageing of the sample resulted in more complete conversion of raw materials to LDH product. Broadening of peaks between 3000 cm<sup>−</sup><sup>1</sup> and 3700 cm<sup>−</sup><sup>1</sup> were noted and are due to O-H stretching of hydroxyl groups [31]. Peak formation at approximately 1356 cm<sup>1</sup> and 1620 cm<sup>−</sup><sup>1</sup> was observed and is likely due to the asymmetric stretching vibrations of CO3<sup>2</sup>− within the interlayer of the LDH [18,30,31]. Peaks below 1000 cm<sup>−</sup><sup>1</sup> could further be attributed to M-O vibrations (M = Zn, Al) [18,31].

**Figure 12.** (**a**) FT-IR analysis of the Zn-Al LDH sample (S5) prior to ageing, synthesised with a 1:1 MII:MIII ratio, after 1 h of wet milling at 2000 rpm; (**b**) FT-IR analysis of sample S5 after ageing for 24 h at 80 ◦C.

**Cu-Al LDH.** The FT-IR spectra for S6 and Cu2(OH)2CO3, before and after ageing, are depicted in Figure 13. Samples obtained depicted similar spectra to that of the Cu2(OH)2CO3. Identification of bond interactions associated specifically with LDH was therefore difficult and inconclusive. It was noted, however, that additional and broadening of peaks occurred between 3300 cm<sup>−</sup><sup>1</sup> and 3700 cm<sup>−</sup><sup>1</sup> and was consistent with Cu-Al LDH spectra reported in literature [17,32]. Additional peaks could further be attributed to bonded and free –OH groups within the sample [30]. Ageing resulted in the formation of a minor peak at 1632 cm<sup>−</sup><sup>1</sup> which could be due to the vibrations of water molecules [17].

**Figure 13.** (**a**) FT-IR analysis of the Cu-Al LDH sample (S6) prior to ageing, synthesised with a 2:1 MII:MIII ratio, after 1 h of wet milling at 2000 rpm.; (**b**) FT-IR analysis of sample S6 after ageing for 24 h at 80 ◦C.
