*3.1.1 Co-precipitation method*

Co-precipitation [66, 67] is an easy and conventional method to synthesize nanomaterials. The ferrites prepared using this method are of controlled size, highly pure, and have a homogeneous structure. Normally inorganic salts (nitrate, chloride, sulfate, etc.) are used in this method as promoting substance that is dissolved in water or another medium that is suitable to form a homogeneous solution. The pH of the solution is adjusted to 7–9 and the solvent is evaporated to get nanoparticle precipitates. It should be observed that the concentration of salt, temperature, rate of change in pH are the parameters that determine the crystal growth and aggregation of the particles. After precipitation, the solid mass is collected and washed. By heating the residue the resultant product is dried to form hydroxides. The hydroxides are then transformed into crystalline oxides by calcination. Gautam et al. [68] used co-precipitation method to synthesize ZnCu ferrite. In this method, Copper chloride, zinc chloride, iron(III) chloride, and sodium hydroxide are used as raw materials. 3 M solution was prepared in 60 ml of distilled water which is mixed with boiling NaOH and stirred for 60 min between temperature 353–358 K using a magnetic stirrer, maintaining the pH between 11 and 12. The precipitates are allowed to settle down and then the sample was washed many times with distilled water. After washing, the sample was dried in hot air followed by crushing, and finally powdered with a mortar pestle A. C. Prabakar et.al [69] also synthesized ZnCu ferrites by the chemical co-precipitation method by taking a solution of Cu(NO3)2.4H2O, Zn(NO3)2H2O, and Fe2(NO3)3.9H2O as the initial materials. These were mixed to form a homogeneous solution at 358 K. Then, ammonia solution was added dropwise with constant stirring maintaining the pH between 10 and 11. The mixture was heated at 353 K for 1 hr. After washing and drying the ferrite powder was heated at 673 K, 773 K, and 923 K separately and pressed in the form of circular pellets. Precipitates were washed many times and then dried in an oven at 100°C for 8 hr. The co-precipitation method has a unique advantage as it uses the aqueous medium for synthesis. There is an effective grain size control and morphology of the nanoparticles obtained.
