2.3.7. Sol-Gel Method

The sol-gel method is a low-cost, simple preparation method and efficient wet-chemical method of high-purity metal oxide materials from LDH precursors through hydrolysis and condensation processes [76]. In this method (Figure 9), the mixed salt solution of Al(NO3)3·9H2O and Mg(NO3)2·6H2O and alkali solution of NH4OH were concurrently added to a beaker and heated under refluxed condition. The pH of the suspension is maintained at 8–10 by adding NH4OH base under magnetic stirring at ambient temperature until the gel formation is achieved. The resultant gel like product is filtered, washed properly with deionized water via re-dispersion/centrifugation and dried overnight. The formed gel was re-dispersed in water by ultra-sonication to produce LDH single layer nanosheets (SLNSs) dispersion. A portion of the LDH SLNS gel was further tried in an oven at 80 ◦C for 24 h to yield a well-crystallized LDH SLNS sol sample. The LDHs synthesized using sol-gel method is thermally very stable, but less crystalline than those synthesized via the co-precipitation method.

**Figure 9.** Schematic representation of sol-gel method of layered double hydroxide (LDH). Reproduced with permission from Reference [77]. Copyrights 2016, Elsevier.

## 2.3.8. Induced Hydrolysis Method

In this method, metal oxides are added dropwise to an acidic solution containing M3<sup>+</sup> metal salts. The metal oxides are dissolved progressively in the acidic solution and precipitated into LDH. The pH is cushioned at 8–10 by the oxide suspension [59]. The obtained solid precipitate is collected, filtered, washed methodically with deionized water and dried at 80 ◦C for 24 h. This method of induced hydrolysis can also be used for synthesis of LDH with di-divalent, di-tetravalent and tri-trivalent systems.
