2.3.4. Sonochemical Method

In sonochemical method, LDHs are prepared by co-precipitation method followed by sonochemical treatment. In first step, the co-precipitation method is performed to the latter as explained fully under Section 2.3.1 above. In the second step after successful completion of mixing, the resultant solution is subjected to ultrasound irradiation at a given time and temperature. The solid precipitate is filtered, washed thoroughly with deionized water and put in an oven overnight for further drying. This sonochemical method is best described as a synthetic and high intensity ultrasonic/three-fold acoustic cavitation phenomenon which assist in improving the crystallinity of LDH phases [61,65]. When the solution mixture is subjected to ultrasonic irradiation, rapid movement of the fluid leads to three-fold acoustic cavitation phenomenon (Figure 6) in which microbubbles undergo nucleation formation, growth and implosive collapse [65]. The formation of microbubbles produced a distinctive hot spot due to the compressional heating induced by collapsing of bubble and therefore yields the bubble with extremely high temperature, pressure and cooling rates [66].

**Figure 6.** Schematic representation of three-fold acoustic cavitation phenomenon. Reproduced with permission from Reference [67]. Open Access 2020, MDPI Energies.
