Direct and Indirect Nucleation of Magnetite Nanoparticles from Solution Revealed by Time-Resolved Raman Spectroscopy

International audienceMagnetite is a widespread inorganic mineral or biomineral with very specific and extraordinary chemical properties in terms of acid−base and oxidation−reduction behavior, thermal stability, and oxygen mobility. Despite the existence of many synthesis methods, the formation mechanisms of this mineral are actively investigated and frequently debated. The coprecipitation reaction (2Fe 3+ + Fe 2+ + 8OH − ⃗ Fe 3 O 4 + 4H 2 O) is the most widespread method to synthesize magnetite under laboratory conditions and at an industrial scale. However, the early stages of magnetite formationnucleation events and precursor/transient phase formationare still questioned and their kinetics is poorly characterized. Here, we perform two series of experiments that differ by how the solutions are mixed: (i) injection of an iron-rich solution into an alkaline aqueous solution, and (ii) injection of an alkaline solution into an iron-rich solution. We show that dynamic in situ Raman spectroscopy provides invaluable information on the direct and indirect nucleation of magnetite nanoparticles (