Evaluating the Critical Roles of Precursor Nature and Water Content When Tailoring Magnetic Nanoparticles for Specific Applications

Abstract

Because of the broad range of application of iron oxide nanoparticles (NPs), the control of their size and shape on demand remains a great challenge, as these parameters are of upmost importance to provide NPs with magnetic properties tailored to the targeted application. One promising synthesis process to tune their size and shape is the thermal decomposition one, for which a lot of parameters were investigated. But two crucial issues were scarcely addressed: the precursor's nature and water content. Two in house iron stearates with two or three stearate chains were synthesized, dehydrated, and then tested in standard synthesis conditions of spherical and cubic NPs. Investigations combined with modeling showed that the precursor's nature and hydration rate strongly affect the thermal decomposition kinetics and yields, which, in turn, influence the NP size. The cubic shape depends on the decomposition kinetics but also crucially on the water content. A microscopic insight was provided by first-principles simulation showing an iron reduction along the reaction pathway and a participation of water molecules to the building unit formation.

Unravelling the Thermal Decomposition Parameters for The Synthesis of Anisotropic Iron Oxide Nanoparticles

“…Synthesis of optimized cubes id adapted from [ 45 , 48 ]. Iron oxide nanocubes (NC) were synthesized from iron stearate (III).…”

“…Currently, most published results on the synthesis of nanocubes are based on the Kovalenko’s method using different combination of ligands (NaOl/OA [ 22 , 38 , 40 , 41 , 42 ]–OA/Oleylamine [ 43 ]—OA/4-Bisphenyl carboxilic acid [ 39 , 44 ]) and iron precursors are mainly iron oleate or iron acetylacetonate. We recently demonstrated that the water content was an important parameter to control the cubic shape as it induces the formation of NaOl inverse micelles affecting thus the amount of NaOl available to drive a specific growth [ 45 ]. Few papers reported the synthesis of iron oxide nanoplates by the thermal decomposition methods.…”

“…In our group, core-shell Fe 1−x O@Fe 3−x O 4 nanocubes [ 48 , 49 ] were previously synthesized by adapting the Kovalenko’s method [ 28 ] (sodium oleate and oleic acid ratio 3.6:1) using an in house iron oleate precursor. Nevertheless, the shape quality of nanocubes was highly dependent on the iron oleate batch and several iron oleate batches were often synthesized without succeeding in obtaining homogeneous nanocubes [ 45 ]. Replacing iron oleate by a commercial iron stearate led to shape heterogeneity.…”

“…Replacing iron oleate by a commercial iron stearate led to shape heterogeneity. By synthesizing in house iron stearates bearing either two (FeSt 2 ) or three (FeSt 3 ) stearate chains and either hydrated or dehydrated, we showed that the hydration rate of iron stearates was decisive [ 45 ]. Nanocubes with straight faces presenting a core-shell composition Fe 1−x O@Fe 3−x O 4 were only obtained with dehydrated FeSt 3 .…”

“…Considering that the synthesis of anisotropic shaped NPs is very important, due to their enhanced properties, the understanding of their synthesis mechanisms is still challenging. In that context and based on our previous investigations [ 45 , 48 ], we propose to investigate the effect of the structure of the precursor, which has been poorly studied up to now, and of the amount of sodium oleate on the nanoparticle shape. That’s why in this work, four precursors, hydrated and dehydrated FeSt 2 and FeSt 3 , have been decomposed in anisotropic conditions from an adapted Kovalenko’s method [ 48 ].…”