Spontaneous oxidation of disordered fcc FePt nanoparticles

Abstract

In this work we present new results on spontaneous oxidation of disordered fcc FePt nanoparticles. The "as-made" oleic acid and oleylamine coated FePt nanoparticles of average size 4 nm synthesized by a high-boiling coordinating solvent method were exposed to air over a period of days and characterized structurally and magnetically by means of different techniques such as XANES, XPS, EXAFS, and SQUID magnetometry. The "as-made" FePt nanoparticles stabilize in the disordered fcc structure and have a very low magnetic saturation ͑M s = 11 emu/ g͒ and a huge coercive field ͑H c = 1800 Oe͒ compared to the low temperature bulk values of the disordered fcc FePt. We observed that the coercive field and the magnetic saturation change with the time the sample is exposed to air and these changes are associated with the oxidation or passivation of the nanoparticle surface that gives place to a core-shell structure. Indeed, the study on the electronic properties of the nanoparticles confirms the magnetic results and indicates that when the nanoparticles are exposed to air, changes in the oxidation state of both Fe and Pt occur, the oxidation state of Fe coming close to hematite. The formation of hematite tends to soften the "as-made" FePt nanoparticles as observed by the reduction of the coercive field to almost one third of the original value. Although the hematite softens the FePt nanoparticles, there is an exchange coupling at the interface of the core-shell characterized by the increase of the coercive field from 300 to 900 Oe when the sample is cooled in an applied field of 50 kOe.

Ab Initio Studies of Segregation, Ordering, and Magnetic Behavior in (Fe–Pt)n, n = 55 and 147: Design of Fe75Pt72 Nanoparticle

“…This shows significant variation in the bond lengths, and these have been also given in Table . As we mentioned above for the 55-atom nanoparticles, these bond lengths are comparable with the experimental values − on larger nanoparticles.…”

Edge-Enhanced Oxygen Evolution Reactivity at Ultrathin, Au-Supported Fe2O3 Electrocatalysts

“…Catalyst morphologies were varied between discrete, 5−6 nm diameter nanoparticles to near monolayer coverages by controlling the XPSdetermined Fe loadings between 2.0 × 10 −10 and 2.9 × 10 −9 mol Fe /cm 2 (Table S1). 20 The Fe 2 O 3 composition was confirmed with both XPS 30 and X-ray absorption spectroscopy (Figures S2 and S3), 31,32 and STM imaging showed that all structures formed with a consistent apparent height of ∼0.5 nm (Figure S4) that corresponds to a bilayer stacking of the O−Fe−Fe−O unit in the 2L-Fe 2 O 3 /Au catalysts. 20 The 2L-Fe 2 O 3 /Au catalysts were transferred out of the UHV chamber for electrochemical testing under ambient conditions in 0.1 M KOH electrolyte.…”

Preparation and characterization of supported magnetic nanoparticles prepared by reverse micelles

“…For naked 3d elemental nanomagnets (Fe, Co, Ni) as well as for alloys (e.g. FePt or CoPt) one can expect a strong deterioration of the targeted magnetic properties under ambient conditions [ 67 – 68 ]. For FePt alloy NPs, we recently investigated the oxidation behavior in more detail by XPS and signal modeling taking into account the spherical shape of NPs [ 66 ].…”