Core–Shell Iron Nanostructures: Structural, Oxidation, and Magnetic Characterization of Self‑Assembled Fe–FeOx Nanocorona Chains

Abstract

Understanding the oxide shell composition of iron core–shell nanostructures is critical for predicting their chemical stability and magnetic performance. We synthesized Fe nanocorona chains (NCs) by reducing FeCl₃ with NaBH₄ in aqueous media, then aged the particles in water for up to 360 min. X‑ray diffraction confirmed a body‑centered cubic Fe core, while transmission electron microscopy revealed a progressively thicker Fe‑oxide shell (2.5–10 nm) as aging time increased. Raman spectroscopy identified hematite (α‑Fe₂O₃) and magnetite (Fe₃O₄) phases; Mössbauer spectroscopy corroborated a mixed Fe₃O₄/α‑Fe₂O₃ shell at short aging, shifting to pure hematite after extended exposure. Vibrating sample magnetometry showed high saturation magnetization (Ms ≈ 1400 emu g⁻¹) for freshly prepared NCs, decreasing with shell growth. These findings suggest that Fe NCs with thin oxide layers are promising r₂ MRI contrast agents, while thicker shells enhance chemical stability for biomedical use.

Introduction

One‑dimensional magnetic nanowires (NWs) exhibit superior shape anisotropy and magnetic moments compared with zero‑dimensional nanoparticles, leading to enhanced performance in magnetic hyperthermia, separation, and imaging. Recent studies report that Fe₃O₄ nanorods and nanowires possess higher transverse relaxivities (r₂) than spherical particles, owing to their larger surface area and higher Ms. Despite extensive research on core–shell iron nanostructures, the exact composition of the oxide shell—whether γ‑Fe₂O₃, Fe₃O₄, α‑Fe₂O₃, or FeO—remains contentious, largely because thin oxides are difficult to resolve. This work addresses that gap by combining XRD, TEM, Raman, and Mössbauer spectroscopy to map the evolution of the oxide shell during aqueous aging.

Materials and Methods

Chemicals

FeCl₃·6H₂O (99 %) and NaBH₄ (98 %) were sourced from National Medicines Corporation Ltd. (China). Argon (99.9 %) was used as the inert atmosphere.

Synthesis of Core–Shell Fe NCs

FeCl₃·6H₂O (3 g) was dissolved in 1 L DI water. NaBH₄ (6 g) was added dropwise (1.5 mL s⁻¹) to the FeCl₃ solution without stirring, generating Fe⁰ nuclei that self‑assemble into chains under magnetic interaction. After synthesis, the precipitates were washed with DI water and ethanol, then dried under Ar. Aging in DI water for 0, 120, 240, and 360 min produced Fe NCs‑0, –2, –4, and –6, respectively.

Characterization

FE‑SEM (NOVA 400 Nano) with EDX was used for morphology and composition. XRD (Cu‑Kα) confirmed crystal phases. Raman spectroscopy (532 nm green laser, 60 mW) identified oxide phases; lower power (6 mW) minimized laser‑induced oxidation. TEM (Tecnai G2 F30) measured core and shell dimensions. Mössbauer spectroscopy (57Co, 320 K) determined iron valence states. Vibrating sample magnetometry (VSM, Lake Shore 7307) measured Ms up to 20 kOe.

Results and Discussion

Morphology and Composition

FE‑SEM images show chain‑like assemblies of Fe nanoparticles separated by thin oxide layers. EDX of Fe NCs‑2 indicates ~77 % Fe and 22 % O by atomic percentage.

XRD

All samples display a sharp peak at 2θ = 44.9°, characteristic of bcc Fe. No oxide peaks appear, indicating the oxide shell is amorphous or below detection limits in XRD.

TEM

Cross‑sectional TEM images reveal a core–shell structure. Shell thickness increases from 2.5 nm (Fe NCs‑0) to 10 nm (Fe NCs‑6) as aging time grows, confirming progressive oxidation.

Raman Spectroscopy

Raman spectra at 60 mW show bands at 217, 275, 386 cm⁻¹ and a broad hump (1200–1300 cm⁻¹), indicative of hematite. At 6 mW, a distinct 670 cm⁻¹ peak appears in Fe NCs‑0 and Fe NCs‑2, confirming magnetite. This peak disappears in Fe NCs‑4 and ‑6, indicating conversion to pure hematite.

Mössbauer Spectroscopy

Both Fe NCs‑0 and ‑6 exhibit sextets and a quadrupole doublet. Fe NCs‑0 shows magnetite (Fe²⁺/Fe³⁺) and hematite contributions; Fe NCs‑6 displays predominantly Fe³⁺, confirming a hematite shell.

Magnetic Properties

Ms values: Fe NCs‑0 ≈ 1400 emu g⁻¹, Fe NCs‑2 ≈ 1420 emu g⁻¹, Fe NCs‑4 ≈ 1200 emu g⁻¹, Fe NCs‑6 ≈ 910 emu g⁻¹. The decrease correlates with reduced magnetic Fe core and increased non‑magnetic hematite shell. The high Ms of Fe NCs‑0 suggests strong r₂ relaxivity (theoretically r₂ ∝ Ms²r²), making them attractive MRI contrast agents.

Conclusion

Fe nanocorona chains exhibit a bcc Fe core and an oxide shell that evolves from a mixed Fe₃O₄/α‑Fe₂O₃ to pure α‑Fe₂O₃ with prolonged water aging. The thin‑shell NCs retain high Ms and are promising r₂ MRI contrast agents, while thicker shells enhance chemical stability for biomedical applications.