2.3.1. Co-Precipitation Method

Co-precipitation method is also referred to as a large-scale and direct technique typically utilized for the synthesis of LDH platelets with different divalent and trivalent cations (M<sup>2</sup>+ and M3<sup>+</sup>) coupled with many inorganic anions (Cl<sup>−</sup>, NO<sup>−</sup>3 , CO2<sup>−</sup>3 ) and organic molecules/outsized biomolecules [57]. In this co-precipitation method, a dropwise addition of alkali solution into the divalent and trivalent layer cations/mixed metal salts containing solution in a proper ratio resulted in formation of LDH as shown in Figure 5. During this method, a pH emerged as a very crucial factor which negatively influences both the structural and chemical properties of LDH component to a larger extent. In a dropwise addition, the pH of the reaction mixture is maintained constantly at the range of 8–10 and purged at N2 atmosphere in an attempt to achieve high chemical homogeneity in LDH [61]. The resulting solution mixture is allowed to stay for a long period of time in order to obtain a reproducible and well-crystallized LDH structural material. The obtained precipitate is collected by filtration, washed thoroughly with deionized water and dried in an oven overnight. The underpinning principle of co-precipitation is based on a simple, economical and industrially feasible technique utilized for the synthesis of metal oxide materials in solution. This led to the brucite-like layers' formation, which uniformly dispersed metallic cations and inorganic anions.

**Figure 5.** Schematic representation of co-precipitation, ion exchange and reconstruction methods LDH. Reproduced with permission from Reference [59]. Copyrights 2018, Elsevier Science Ltd.

## 2.3.2. Ion Exchange Method

Anion-exchange method is better for the incorporation of layered silicates into a solution containing anions species ready for exchange as compared to other methods. The ion exchange method (Figure 5) is based on the exchange of anions in interlayer space with other anionic species. In this method, the precursor LDH is suspended in an aqueous solution containing the anionic species to be exchanged. The suspension is then stirred constantly for several hours at room temperature. The solid precipitate is then collected by filtration, washed several times with deionized water and dried in an oven overnight. Furthermore, the interlamellar region can also contain water molecules and is referred to as internal water dominant region where water molecules are organized by the inorganic layers via hydrogen bonding. The high anion exchange capacity of LDH matrix-like compounds produces their interlayer ion exchange by outgoing anions and incoming useful anions, easily accomplished and reflected in LDH precursor product formation [62] (Figure 5).
