*2.2.2 Sol-gel synthesis*

It involves the hydrolysis and condensation of metal alkoxides, acetates, nitrates, sulfates, and chlorides. Their hydrolysis in solution results from the dispersion of metal hydroxides, which further undergo condensation leading to the formation of 3-dimensional network namely a gel. The gel is either dried by removing the solvent or treated by chemical reaction to give the condensed MO materials [21]. The solvent used is generally water (aqueous sol-gel). In most non-aqueous synthesis, additives such as surfactant can be required to control the morphology of the particles and most importantly to prevent/reduce their aggregation [22, 23].
