The coprecipitation formation study of iron oxide nanoparticles with the assist of a gas/liquid mixed phase fluidic reactor

Coprecipitation is the most commonly used method to synthesize iron oxide nanoparticles (IONPs) due to its low cost and environmentally friendly synthesis process. Recently, several studies have proposed that the coprecipitation formation of IONPs is through the aggregation of initially formed primary particles. However, there are still lack of systematic researches about the room temperature coprecipitation formation process of IONPs under quick mixing mode. Here, a gas/liquid mixed phase fluidic reactor was specially designed, and the influences of reaction time, reaction pH and valence of iron ions on the coprecipitation formation of IONPs in quick mixing mode were studied. The NH3 gas was used as the alkaline reactant. At pH above 10.0, the iron ions promptly hydrolyzed and precipitated to form primary nanoparticles, and then the primary nanoparticles accreted to form Fe3O4 nanoparticles in 2 min 30 s. As the reaction pH increased from 2.5 to 9.5, extremely small particles gradually aggregated to form Fe3O4 nanoparticles. Large Fe3O4 nanoparticles with a mean diameter of ~ 8.1 nm were completely formed at pH 9.5. And a small amount of poorly crystalline intermediate phases including akageneite, ferrihydrite and goethite were observed during this process. When Fe2+ ions were absent during the reaction process, the final products had poor crystallinity and weak magnetic properties. While if only Fe2+ ions were used in the precursor, the sheet-like green rusts generated firstly, and Fe3O4 nanoparticles would gradually form on the surface of green rusts with the partial oxidation of Fe2+ ions. The results of regulating the reaction conditions especially reaction pH indicate that this reaction design can effectively reduce the formation of intermediate crystal phases.