Effect of ultrasonic irradiation on γ-Fe2O3 formation by co-precipitation method with Fe3+ salt and alkaline solution

The effect of ultrasonic irradiation on direct maghemite (γ-Fe2O3) preparation by a co-precipitation method with Fe3+ salt (Fe(NO3)3) and an excess amount of alkaline (KOH) solution without going through the conventional magnetite (Fe3O4) formation route was explored in comparison with impeller stirring. The preparation procedure for obtaining iron oxide nanoparticles was designed using the sequential processes of precipitation, decantation, drying and thermal dehydration, and ultrasonic irradiation or impeller stirring was done during the precipitation process. γ-ferric oxyhydroxide (γ-FeOOH) was partially formed in addition to α-ferric oxyhydroxide (α-FeOOH) and thermally dehydrated to γ-Fe2O3 and hematite (α-Fe2O3) by ultrasonic-assisted co-precipitation of Fe3+ salt and the excess KOH solution, whereas only α-FeOOH and α-Fe2O3 were synthesized by impeller stirring. The difference between the products of the two methods was explained by the Lamer model associated with the nucleation and growth of FeOOH. Magnetization increased as the crystallite diameter decreased, which is estimated to facilitate partial formation of magnetic γ-Fe2O3. Magnetization was enhanced by a lower ultrasonic frequency due to the stronger shock wave induced by the cavitation effect