Abstract
The effects of cobalt incorporation in spherical heterostructured iron oxide nanocrystals (NCs) of sub-critical size have been explored by colloidal chemistry methods. Synchrotron X-ray total scattering methods suggest that cobalt (Co) substitution in rock salt iron oxide NCs tends to remedy their vacant iron sites, offering a higher degree of resistance to oxidative conversion. Self-passivation still creates a spinel-like shell, but with a higher volume fraction of the rock salt Co-containing phase in the core. The higher divalent metal stoichiometry in the rock salt phase, with increasing Co content, results in a population of unoccupied tetrahedral metal sites in the spinel part, likely through oxidative shell creation, involving an ordered defect-clustering mechanism, directly correlated to core stabilization. To shed light on the effects of Co-substitution and atomic-scale defects (vacant sites), Monte Carlo simulations suggest that the designed NCs, with desirable, enhanced magnetic properties (cf. exchange bias and coercivity), are developed with magnetocrystalline anisotropy which increases due to a relatively low content of Co ions in the lattice. The growth of optimally performing candidates combines also a strongly exchange-coupled system, secured through a high volumetric ratio rock salt phase, interfaced by a not so defective spinel shell. In view of these requirements, specific absorption rate (SAR) calculations demonstrate that the rock salt core sufficiently protected from oxidation and the heterostructure preserved over time, play a key role in magnetically mediated heating efficacies, for potential use of such NCs in magnetic hyperthermia applications.
Highlights
From the standpoint of the present work, MNPs offer highly exploitable capabilities in the rapidly developing field of nanobiotechnology.[8]
The growth of optimally performing candidates combines a strongly exchangecoupled system, secured through a high volumetric ratio rock salt phase, interfaced by a not so defective spinel shell. In view of these requirements, specific absorption rate (SAR) calculations demonstrate that the rock salt core sufficiently protected from oxidation and the heterostructure preserved over time, play a key role in magnetically mediated heating efficacies, for potential use of such NCs in magnetic hyperthermia applications
The trend becomes more prominent at elevated Co levels that appear to impede the mechanism for the oxidative conversion of rock salt to spinel
Summary
From the standpoint of the present work, MNPs offer highly exploitable capabilities in the rapidly developing field of nanobiotechnology.[8]. Modification of morphological features (size-shape tuned; d ∼ 20–40 nm),[31] growth of shells (for interfacial interactions; d ∼ 15 nm)[12,32] and alteration of the chemical composition (e.g. Zn-substituted ferrites; d ∼ 15 nm).[33] The efficiency of the first two strategies, together with favourable nano-structural effects due to emerging Fe-site vacancies, mediating the composition, have been recently discussed for FexO@Fe3−δO4 bimagnetic NCs (d < 25 nm).[34] The beneficial influence of defective structures with subcritical nanocrystal sizes (d ∼ 8–10 nm) has been demonstrated with doped ferrites In such cases, partial substitution of Fe by transition metals in the nanostructure optimizes the effective magnetic anisotropy of iron oxide nanocrystals (IONCs).[19] While this pathway maintains the desired morphology/size, it provides improved heating generation (cf specific absorption rate, SAR), at least up to a certain level of substitution (e.g. at x ∼ 0.6, in CoxFe3−xO4).[35,36]. The experimental PDF, G(r), can subsequently be modelled by calculating the following quantity directly from a presumed structural model:
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