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

Magnetite 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+) + Fe2+ + 8OH(-->) Fe3O4 + 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 formation-nucleation events and precursor/transient phase formation-are 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 (<15 nm) from aqueous solution. When a mixed-valent iron solution (0.5 M Fe2+ + 0.5 M Fe3+) is injected (2.3 or 12 mL/min) into an alkaline solution (4 M NaOH), dark colloidal particles form instantaneously and the magnetite signal is rapidly detected in Raman spectra after 3 or 7 min, depending on the injection rate. This result demonstrates that the mixed-valent iron is instantaneously dehydrated leading to the formation of magnetite-like colloidal (or primary) particles peaking in the range of 674-678 cm(-1) in the Raman spectra, with the peak position stabilizing rapidly at 673 cm(-1). Conversely, when alkaline solution is added into the mixed-valent iron solution, Raman spectroscopy reveals a complex reaction mechanism and kinetics. First, iron dehydration (315 cm(-1)) and formation of green rust (500-503 cm(-1)) as the transient phase related to the olation process are detected and interpreted by the formation of hydroxo bridges accompanied with expelling of molecular water. Second, the green rust and available ferric iron (ions or colloids) react to nucleate magnetite nanoparticles via an oxolation process related to the formation of oxo bridges accompanied with the expelling of hydroxylated water. We also quantified the nucleation time of magnetite and the hydrophilic-to-hydrophobic change in the suspension by the temporal behavior of the bending mode of molecular water. Our results show that, under our experimental conditions, amorphous transient phases during direct or indirect magnetite formation from ionic solutions do not exist or that such phases do not show a specific Raman signature.

Automated summary

Magnetite is an inorganic mineral with unique chemical properties, and the method of synthesis is still debated. Dynamic in-situ Raman spectroscopy provides valuable information on the formation of magnetite nanoparticles.