Structural characterization of self-assembled chain like Fe-FeOx Core shell nanostructure

Abstract

One of the big challenge of studying the core-shell iron nanostructures is to know the nature of oxide shell, i.e., whether it is γ-Fe₂O₃ (Maghemite), Fe₃O₄ (Magnetite), α-Fe₂O₃ (Hematite), or FeO (Wustite). By knowing the nature of iron oxide shell with zero valent iron core, one can determine the chemical or physical behavior of core-shell nanostructures. Fe core-shell nanochains (NCs) were prepared through the reduction of Fe³⁺ ions by sodium boro-hydride in aqueous solution at room atmosphere, and Fe NCs were further aged in water up to 240 min. XRD was used to study the structure of Fe NCs. Further analysis of core-shell nature of Fe NCs was done by TEM, results showed increase in thickness of oxide shell (from 2.5, 4, 6 to 10 nm) as water aging time increases (from 0 min, 120 min, 240 min to 360 min). The Raman spectroscopy was employed to study the oxide nature of Fe NCs. To further confirm the magnetite phase in Fe NCs, the Mössbauer spectroscopy was done on Fe NCs-0 and Fe NCs-6. Result shows the presence of magnetite in the sample before aging in water, and the sample after prolonged aging contains pure Hematite phase. It shows that prolonged water oxidation transforms the structure of shell of Fe NCs from mixture of Hematite and Magnetite in to pure hematite shell. The Magnetic properties of the Fe NCs were measured by VSM at 320 K. Because of high saturation magnetization (Ms) values, Fe NCs could be used as r₂ contrasts agents for magnetic resonance imaging (MRI) in near future.

Keywords: 1-D nanostructures; Fe nanochains; Raman spectroscopy; Transmission electron microscopy.

Fig. 1

Highly shape anisotropic magnetic nanowires prepared through direct synthesis or assembly methods and their biomedical applications

Fig. 2

Schematic illustration of synthesis and formation of core-shell Fe NCs

Fig. 3

a–e FE-SEM image of (a) FeNCs–0, (b) FeNCs–2, (c) FeNCs–4, (d) FeNCs–6, (e) EDX pattern of Fe NCs–2, inset table show the atomic and weight percentage of iron and oxygen element

Fig. 4

XRD pattern of (a) Fe NCs-0, (b) Fe NCs-2, (c) Fe NCs-4, and (d) Fe NCs-6

Fig. 5

TEM image of a Fe NCs-0, b Fe NCs-2, c Fe NCs-4, and d Fe NCs-(6) showing core-shell nature of Fe NCs with an increase in oxide thickness of 2.5 nm, 4 nm, 6 nm and 10 nm

Fig. 6

Raman Spectra of (i) FeNCs-0, (ii) FeNCs-2, (iii) FeNCs-4, (IV) FeNCs-6 at 60 mW laser power with Green laser

Fig. 7

Raman Spectra of a Fe NCs-0 (I), Fe NCs-2 (II), Fe NCs-4 (III), and Fe NCs-6 (IV) collected at 6 mW laser power with Green laser and b Fe NCs-0 (I), Fe NCs-2 (II), Fe NCs-4 (III), and Fe NCs-6 (IV) collected at 6 mW laser power with He-Ne laser

Fig. 8

Raman Spectra of Fe NCs-0 (I), Fe NCs-2 (II), Fe NCs-4 (III), Fe NCs-6 (IV) at 3 mW laser power with He-Ne laser

Fig. 9

Mössbauer spectra of Fe NCs-0 at 320 K

Fig. 10

Mössbauer spectra of Fe NCs-6 at 320 K

Scheme 1

a Formation of Fe NCs prepared through 120, 240 and 360 min of water aging, b mechanism of O₂ activation route over the core-shell Fe NCs

Fig. 11

Magnetic hysteresis measured at room temperature, inset shows the enlarged curve between – 2.0 and 2.0 kOe

Fig. 12

Ms Vs R_core /R_shell curve, taking the average diameter of Fe NCs 96 nm