Mechanistic Study on Magnetite Nanoparticle Formation by Thermal Decomposition and Coprecipitation Routes

The synthesis of poly (acrylic acid) [PAA]-coated superparamagnetic Fe 3 O 4 nanoparticles was carried out in two ways, either by thermal decomposition of an organometallic iron precursor or by coprecipitation of aqueous solutions of ferrous and ferric chloride salts. PAA-coated particles obtained in the former route were spherical, isolated, and nearly monodisperse, whereas aggregates of coated particles formed in the latter route. A nanoparticle formation mechanism is proposed to explain such contrasting morphology. It is based on estimated time scales of different individual steps in the synthesis routes, diffusion of Fe 3 O 4 nanoparticle over a PAA polymer chain, Brownian collision, and coagulation of particles and experimental aging time. We find that the coagulation time scale is smaller in the thermal decomposition route and is hence essentially complete within the experimental aging time. This results in formation of PAA-coated, isolated Fe 3 O 4 nanoparticles. In contrast, the coagulation time scale in coprecipitation is very large as compared to the experimental aging time, resulting in incomplete coagulation, and hence formation of an aggregated morphology of particles.