Revealing the crystal phases of primary particles formed during the coprecipitation of iron oxides

The mechanistic investigation of the coprecipitation formation of iron oxides has been a long-standing challenge due to the rapid reaction kinetics and high complexity of iron hydrolysis reactions. Although a few studies have suggested that the coprecipitation of iron oxide nanoparticles follows a non-classic route through inter-particle attachment, the compositions of the primary particles remain undetermined. Herein, by using a specially designed gas/liquid mixed phase fluidic reactor we controlled the reaction time from 3 s to over 5 min, and successfully identified the concentration of different intermediate phases as a function of time. We suggest that the initial Fe³⁺ ions are hydrolyzed under the alkaline condition to give Fe(OH)₃, which then rapidly dehydrates to yield α-FeOOH. In the presence of Fe²⁺ ions, which could also act as the catalyst, α-FeOOH finally transforms to Fe₃O₄.