Unveiling the formation mechanism of polydisperse iron oxide nanoparticles in coprecipitation route

Abstract

Coprecipitation is the simplest, fastest, and most inexpensive method for preparing metal oxide nanoparticles (NPs). It is considered one of the most convenient methods for simultaneously precipitating two or more aqueous metal salt precursors (such as chlorides, nitrates, oxychloride, etc.) into NPs in a basic solvent, yielding uniform composition. However, the main drawback of this route is the production of NPs with a broad size distribution (polydispersed particle formation), which limits their use in various applications. The present work addresses the experiments and formation mechanism of polydispersed iron oxide magnetite (Fe3O4) NPs using this route. Experiments involving the drop-wise addition of mixed ferric (Fe3+) and ferrous (Fe2+) chloride salt solutions to NaOH base solution at room temperature resulted in a polydispersed Fe3O4 NP system containing both large and small particles at the end of 30 min of the reaction. A mechanism is proposed to explain the formation of these polydispersed Fe3O4 NPs based on the time scales of different individual particle formation events and the experimental aging time. The mechanism elucidates that the time scales of events such as Fe3O4 nucleation, diffusional-limited and Ostwald ripening growth of small and large particles, and the coagulation of two small particles or a small particle with a large particle are smaller, and completed within the experimentally allowed aging time. Conversely, the coagulation time for two large particles exceeds the experimental time, preventing complete coagulation. As a result, incomplete coagulation of large particles and the completion of all other events lead to the formation of a polydispersed Fe3O4 NP system at the end of the synthesis. The combined experiments and time scale-based mechanism explain the polydispersed NP formation in the coprecipitation route, and the insights gained from this understanding can be beneficial for optimizing synthesis conditions to achieve a controlled size distribution of Fe3O4 and other NP systems.