*3.3.1 Sol-gel method*

Sol-gel is a wet chemical method for synthesis of various nanomaterials from sols. Generally, this method involves controlled hydrolysis of metal alkoxides like zirconium propoxide or metal salts such as metal nitrates or chlorides in an aqueous or organic solvent medium. It is to be noted that at the initial step of this method the hydrolysis and polycondensation reactions occur that lead to the generation of polymeric/colloidal sol with particles of nano dimension [5, 18]. By increasing mass of the desired material into sol or other significant changes in sol like solvent substitution, pH variation, solvent evaporation, etc. results the formation of gel with three-dimensional network of porous nature. The solvent is enclosed inside the porous gel structure [5, 18]. Heat treatment of the gel structure produces dense ceramics. It is well-known that the extremely pure and homogeneous multicomponent oxide can be synthesized by this method. Metal ions doped indium oxide and indium oxide based nanomaterials in the form of thin films or bulk nanocomposites can be fabricated/synthesized by adopting the sol-gel method [4–6, 12, 13, 18].
