Surface Compositional Change of Iron Oxide Porous Nanorods: A Route for Tuning their Magnetic Properties.

Alberto Casu,Danilo Loche,Andrea Falqui,Sergio Lentijo-Mozo

doi:10.3390/molecules25051234

Alberto Casu, Danilo Loche + Show 2 more

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https://doi.org/10.3390/molecules25051234

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Abstract

The capability of synthesizing specific nanoparticles (NPs) by varying their shape, size and composition in a controlled fashion represents a typical set of engineering tools that tune the NPs magnetic response via their anisotropy. In particular, variations in NP composition mainly affect the magnetocrystalline anisotropy component, while the different magnetic responses of NPs with isotropic (i.e., spherical) or elongated shapes are mainly caused by changes in their shape anisotropy. In this context, we propose a novel route to obtain monodispersed, partially hollow magnetite nanorods (NRs) by colloidal synthesis, in order to exploit their shape anisotropy to increase the related coercivity; we then modify their composition via a cation exchange (CE) approach. The combination of a synthetic and post-synthetic approach on NRs gave rise to dramatic variations in their magnetic features, with the pores causing an initial magnetic hardening that was further enhanced by the post-synthetic introduction of a manganese oxide shell. Indeed, the coupling of the core and shell ferrimagnetic phases led to even harder magnetic NRs.

Highlights

Iron oxides attract considerable attention due to their many physical properties, which vary greatly among different phases and represent the key to their successful use in a wide array of applications, from hyperthermia, magnetic resonance imaging and drug delivery, to magnetic separation, catalysis and magnetic data storage [1,2,3,4,5,6,7]
We showed that the molecular weight (MW ) of PEI is a key parameter that modifies the shape of the NPs during both their synthesis and reduction
By comparing NPs obtained with and without PEI addition, we observed that the introduction of PEI determines a consistent variation in the shape and size of the resulting β-FeOOH NPs and, the use of PEI polymers with different MW gave rise to dramatic variations in the shape and size of the final products obtained by reduction of β-FeOOH

Summary

Introduction

Iron oxides attract considerable attention due to their many physical properties, which vary greatly among different phases and represent the key to their successful use in a wide array of applications, from hyperthermia, magnetic resonance imaging and drug delivery, to magnetic separation, catalysis and magnetic data storage [1,2,3,4,5,6,7]. When considering the possible applications that revolve around the magnetic response of iron oxides, modifying the magnetic hardness and/or saturation magnetization of the nanoparticles (NPs) up to values equal or higher than bulk ones represents one of the biggest existing challenges, since it aims at obtaining nanosized objects capable of equaling or overcoming the current limits of a given material To reach these goals, different interconnected aspects, such as the structural order and the shape of the NPs, become fundamental in determining the final features of magnetic NPs. the choice of an anisotropic shape over the spherical isotropic shape increases the shape component of the effective magnetic anisotropy constant, while any variation in the NP composition affects the magnetocrystalline anisotropy, and both have an impact on the net magnetic hardness. Both kinds of modification can be dramatically affected by Molecules 2020, 25, 1234; doi:10.3390/molecules25051234 www.mdpi.com/journal/molecules

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