*3.3.3 Co-precipitation and thermal evaporation*

Co-precipitation is a simple classical method to synthesize metal oxide nanomaterials. This method is cost-effective, very fast process and useful for larger scale industrial applications [27, 30]. By this method, it is possible to synthesize highly pure nanomaterial through an eco-friendly route. In this typical method, metal salts in the form of nitrate, chloride, or oxychloride as precursor materials are generally

dissolved in aqueous solution and precipitated these into their corresponding hydroxides by addition of a base like sodium hydroxide or ammonium hydroxide. Finally, the precipitates are washed and calcined at high temperature to get metal oxide nanomaterials. Several reports are available on synthesis of indium oxide based nanomaterials by this method [55, 56].

## *3.3.4 Solid state reaction*

Solid-state reaction is a well-known method for the synthesis of polycrystalline material from solid precursor materials. Generally, the reaction occurs at very high temperature. The main advantage of this method is its simplicity and the ability of large scale industrial production. Synthesis of pure In2O3 nanoparticles had been performed via solid state reaction method for the fabrication of optoelectronic devices by Jothibas *et al.* [57]. Moreover, Bykova *et al.* [58] reported Co-In2O3 nanocomposites thin film by solid-state reaction method and investigated its structural and magnetic properties. Recently, Co-In2O3 nanocomposites and cobalt-doped In2O3 have attracted significant attention due to their applications in optoelectronic, spintronic devices and gas sensors [58].
