2.3.5. Hydrothermal/Solvothermal Method

The hydrothermal synthesis method illustrated in Figure 7a,b is similar to the co-precipitation method. In this method, two solutions containing M2<sup>+</sup> and M3<sup>+</sup> metal salts are added dropwise to another solution containing base under vigorous stirring at room temperature. Thus, the suspension is transferred into a Teflon-lined autoclave and heated at higher temperature (100–180 ◦C) for many hours (10–48 h) based on the metal ions [68]. The pH of the supernatant solution is in the threshold range of 8–10. The solid precipitate is collected by centrifugation washed thoroughly with deionized water and ethanol and dried in an oven overnight. The hydrothermal method is useful for synthesis of highly crystalline LDHs with uniform morphology compared to co-precipitation technique [69]. Solvothermal method is a synthesis method where a chemical reaction takes place in a closed solvent system at elevated temperatures above the boiling point and standard pressures. In a typical solvothermal synthesis, the amount of organic solvent such as glycerol or alcohol is used in a non-aqueous solution at somewhat high temperatures, while hydrothermal method refers to synthesis via chemical reactions in aqueous solution just above boiling point of water in a closed vessel. Many scientists realized the importance of preparing inorganic nanomaterials using hydrothermal and solvothermal reactions, upon which e ffective syntheses of novel high-technology and green materials would be established. The main reason for this remarkable and milestone achievement in preparing nanomaterials is the easy of processing which include low temperature process, low energy consumption, no harm to the environment and more importantly high degree of crystallinity of the material can easily be produced [68,69].

**Figure 7.** Schematic representation of (**a**) hydrothermal method and (**b**) solvothermal treatment of layered double hydroxide (LDH). Reproduced with permission from Reference [70,71]. Copyrights 2017 and 2019, RSC and Elsevier.

#### 2.3.6. Adsorption and Layer-by-Layer Method

In this case, adsorption (Figure 8a) can be referred to as the adhesion of divalent and trivalent ions (M<sup>2</sup>+ and M3<sup>+</sup>) from a liquid or dissolved solid to surface of the LDH adsorbent. This creates a film of the adsorbate over the surface in many processes such as chemical, physical, biological and natural systems and widely used in various industrial applications [72]. The adsorption process may occur through weak van der Waals forces (physisorption) or covalent bonding (chemisorption) and also may occur due to electrostatic attraction between the adsorbate and surface of the adsorbent. It is a surface phenomenon most widely adopted in wastewater treatment for removal of various organic contaminants from aqueous solution.

**Figure 8.** Schematic representation of (**a**) adsorption method, (**b**) layer-by-layer deposition and (**c**) direct co-precipitation method of layered double hydroxide (LDH). Reproduced with permission from Reference [73]. Copyrights 2018, John Wiley and Sons.

Layer-by-layer (LBL) assembly (Figure 8b) is a universal method for coating substrates with polymers, colloids, biomolecules, and even cells. This presents superior control and versatility when compared to other thin film deposition techniques in certain research and industrial applications. The LBL technique is known to support electrostatic interactions between positively charged layers and negatively-charged molecules and leads to nanostructured thin films [74]. This LBL deposition technique has three types of methods known as (i) the dipping layer-by-layer deposition technique (dipping-LBL); (ii) spray layer-by-layer deposition method (spray-LBL) and (iii) spin layer-by-layer deposition method (spin-LBL) method. Dipping-LBL is executed by chronologically adsorbing opposite charged materials onto a substrate via enthalpic and entropic driving forces [75]. In this method, the time depends on both the diffusion and adsorption of molecules, solutions or suspensions. Spray-LBL is a deposition technique where divalent and trivalent solutions are sprayed onto a vertical substrate, and the layer is formed after completion of drying in an oven overnight [58]. In spin-LBL method, the solutions or suspensions are deposited on a substrate attached to a spin coater, and the rotation speed generates a high centrifugal force. Thus, high rotational speed with high airflow rate at the surface leads to fast drying times of the liquid which in turn quickly and easily produce very uniform layers or thin films. In both spray-LBL and spin-LBL methods, the total time does not depend on the diffusion of molecular species. The co-precipitation method shown in Figure 8c has already been previously explained in Section 2.3.1.
