2, University of Campinas (UNICAMP), Campinas, , Brazil
3, Autonomous University of Manizales, Manizales, , Colombia
4, Brazilian Nanotechnology National Laboratory, Campinas, , Brazil
5, The University of Texas at Austin, Austin, Texas, United States
Combinations of iron oxide nanoparticles with metallic nanoparticles have been extensively studied due to their large variety of applications in biomedical and technological fields [Souza, 2017; Moraes Silva, 2016]. Particularly, dual heterostructures composed of gold and iron-oxide have been made by following the so-called seeded-growth reaction sequence, involving the synthesis of gold nanoparticles seeds, which subsequently serve as nucleation and growth sites for the iron oxide phase [Fantechi, 2017]. In this context, to determine the composition and the iron-oxidation state of the iron oxide nanoparticles is of paramount importance to understand their functional properties.
This report focuses on Electron Energy Loss Spectroscopy (EELS) analysis of three sets of Au/FeyOx heterostructures to identify variations in the iron-oxidation state at the core and surface of the iron oxide nanoparticles, as well as at their interface with the gold counterpart. For this purpose, hybrid nanoparticles of Au/FeyOx were produced employing three sets of Au seeds with 3 nm and 7 nm. The use of these seeds leads to hybrid systems with three morphologies; dimers, trimers, and flowers. High angular annular dark field (HAADF) STEM images were acquired on a double corrected FEI Titan Themis operated at 300 keV, while dual EELS was performed at 80 keV.
The results show a predominant magnetite (Fe3O4) phase at the iron oxide core, which is different from the surface layer that exhibits lower electron energy loss. Only for specific nanoparticle morphologies, an additional phase is shown, as a result of the reduction of magnetite to form wustite, which is predominant at the surface. These results may justify some peculiar phenomenologies often observed in this type of hybrid nanoparticles, as for example exchange bias effects or spin polarization transfer [Pineider, 2013; Feygenson, 2015].
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