Advances in the Synthesis and Long‐Term Protection of Zero‐Valent Iron Nanoparticles

Joshua P Mehta,Emilie Ringe,David Fairen‐Jimenez,Giorgio Divitini,Paul A Midgley,Andrew E H Wheatley,Benjamin R Knappett

doi:10.1002/ppsc.201800120

Joshua P Mehta, Emilie Ringe + Show 5 more

Open Access

https://doi.org/10.1002/ppsc.201800120

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Abstract

AbstractCore@shell Fe@Fe3O4 nanoparticles (NPs) are synthesized via the thermal decomposition of iron pentacarbonyl (Fe(CO)5) in the presence either of oleylamine (OAm) or a mixture of OAm and oleic acid (OA). The heterostructured nanocomposites formed do so by a postsynthetic modification of isolated Fe seeds. This proves the versatility of the coating procedure and represents a significant advantage over previous work with Co seeds owing to the higher magnetic susceptibility, reduced toxicity, and excellent biocompatibility of Fe. Furthermore, the latter system allows the synthetic methodology to be developed from a two‐pot scenario where seeds are isolated then coated, to an easier and more efficient direct one‐pot scenario. The two‐pot method yields proportionately larger cores. However, in both cases, the monodisperse product reveals a carbonaceous interface between the Fe core and oxide shell. Meanwhile for the one‐pot synthesis, the OA:OAm ratio influences both the morphology and dispersity of the product. This is interpreted in terms of competing interactions of the ligands with the iron precursor. Superparamagnetism (SPM) is observed, and microscopic studies reveal oxidative stability of the Fe(0) cores achieved by either method for >6 months. It is proposed that the carbonaceous interface is critical to this sustained oxidative stability.

Highlights

To prove the generality of a recently published procedure for synthesizing Co@Fe3O4 NPs,[25] Co2(CO)8 was replaced by Fe(CO)5 in octadec-1-ene and oleylamine (OAm) in order to furnish OAm-capped Fe seed particles
Transmission electron microscope (TEM) analysis of the product revealed heterostructured NPs based on a core@shell architecture in which the shell constituted a 2–3 nm coating
The preparation by a two-step method of oxidatively stable Fe@ Fe3O4 nanoparticles has established the versatility of a postsynthetic modification (PSM) coating procedure previously limited to protecting toxic NPs

Summary

PSM of Fe Seeds

To prove the generality of a recently published procedure for synthesizing Co@Fe3O4 NPs,[25] Co2(CO) was replaced by Fe(CO) in octadec-1-ene and oleylamine (OAm) in order to furnish OAm-capped Fe seed particles. The polycrystalline shell can be viewed as forming from crystalline OA-Fe NPs that nucleate following the slower solution decomposition of less reactive OA-complexed Fe(CO) and which flocculate to the already formed OAm-Fe seed, trapping capping agent at the resulting interface in doing so To probe this idea, aliquots of the reaction mixture were removed in the course of the direct particle synthesis at time (t) = 1, 5, 15, 30, 45, 60, 75, and 90 min after Fe(CO) injection (Figure S13, Supporting Information). The observation (Figure 3; Figure S14, Supporting Information) of small, crystalline particles outside these core@double-shell NPs, and the confirmed need for an excess of OA to ensure particle heterostructure, is consistent with the solution-nucleation of OA-Fe NPs resulting from the decomposition of OA-coordinated precursor

Conclusions

Experimental Section

Conflict of Interest