Morphological and Magnetic Properties of Superparamagnetic Carbon-Coated Fe Nanoparticles Produced by Arc Discharge

Spherical carbon coated iron particles of nanometric diameter in the 5–10 nm range have been produced by arc discharge at near-atmospheric pressure conditions (using 5–8 ·104 Pa of He). The particles exhibit a crystalline dense iron core with an average diameter 7 4±2 0 nm surrounded by a sealed carbon shell, shown by transmission electron microscopy (TEM), selected-area diffraction (SAED), energy-dispersive X-ray analysis (STEM-EDX) and electron energy loss spectroscopy (EELS). The SAED, EDX and EELS results indicate a lack of traces of core oxidized phases showing an efficient protection role of the carbon shell. The magnetic properties of the nanoparticles have been investigated in the 5–300 K temperature range using a superconducting quantum interference device (SQUID). The results reveal a superparamagnetic behaviour with an average monodomain diameter of 7.6 nm of the nanoparticles. The zero field cooled and field cooled (ZFC-FC) magnetization curves show a blocking temperature (TB) at room temperature very suitable for biomedical applications (drug delivery, magnetic resonance imaging—MRI–, hyperthermia).


INTRODUCTION
In the recent years, superparamagnetic particles have had an important role due to their unique magnetic properties in the modern medical field for several applications, such as drug delivery, magnetic resonance imaging-MRI-and hyperthermia among other cancer curing. 1 The superparamagnetic behaviour is revealed under a critical size for a monodomain nanoparticle, which depends on the saturation magnetization and the magneto-crystalline anisotropy of the material.In addition, nanoparticles with spherical shape are more suitable for applications in the human body. 2 In case of carbon coated iron nanoparticles, the carbon shell protects nanoparticles from oxidation which could help to maintain higher magnetic responses and making the system biocompatible with the human body.
In this article, the structure and morphologic characteristics about shape and size distribution of carbon-coated iron-nanoparticles (Fe@C) have been studied using transmission electron microscopy (TEM) and selected area electron diffraction (SAED).Besides, the absence of oxygen in the nanoparticles has been determined by energy-dispersive * Author to whom correspondence should be addressed.
X-ray analysis (STEM-EDX) and electron energy loss spectroscopy (EELS).Furthermore, in order to establish relationships between magnetic behaviour and morphological and structure characteristics, the superparamagnetic characteristics of Fe@C nanoparticles have been investigated.

EXPERIMENTAL DETAILS
The arc discharge apparatus used in this experiment is based on a vertical carbon rod (50 mm in length and 12 mm in diameter) as the cathode (99.9% purity).Multiple foils of iron (purity 99.99%) as raw material were tightly fixed in a hole of the graphite column which is grounded and used as the anode.A modification of the arc discharge reactor allows a particle manipulation procedure at pristine conditions.The nanoparticles can be directly collected and dispersed in deionised water without exposition to the atmosphere.Pure He was used as the source of plasma.The He pressure of the reaction chamber was set in the range of 5-8 • 10 4 Pa, near-atmospheric pressure conditions.The current applied on the electrode was 40 A, the voltage was about 125 V and the discharge process was maintained for 5-10 minutes.

Morphological and Magnetic Properties of Superparamagnetic Carbon-Coated Fe Nanoparticles
Aguiló-Aguayo et al.
A Philips CM30 operating at 300 kV was used for TEM and SAED structural characterization.STEM-EDX and EELS analysis were obtained by a TEM operating at 200 kV, which is equipped with a JEOL JEM 2010F field emission gun.A superconducting-quantuminterference device (SQUID) magnetometer was used to study magnetic behaviour of carbon coated iron nanoparticles in the temperature range 5-300 K and using fields up to 55 kOe.

RESULTS AND DISCUSSION
The TEM images of carbon coated iron nanoparticles (Fe@C) indicate that the presence of nanoparticles with a spherical shape and carbon encapsulation of 3-5 nm is confirmed (Fig. 1).The SAED pattern referring to Fe@C nanoparticles (inset Fig. 1) demonstrates their crystalline structure.The electron diffraction pattern is identified as -Fe in the [11-1] zone axis, which shows that the core nanophases are in the form of -Fe (bcc).
The size distribution of the iron core of nanoparticles is determined from multiple TEM images (248 total population).The diameters are sorting into a histogram and data histogram are satisfactorily distributed following a function f D which is described by a logarithmic-lineal distribution 3 where D 0 is the most probable particle diameter and is the standard deviation of ln D .The average iron core diameter is 7.4 nm with a standard deviation of 2.0 nm (Fig. 2) in agreement with the following magnetic studies.In order to obtain more information about the elements present into nanoparticles, the electron probe was scanned along a region of the specimen and the intensity of the transmission electron signal was measured using detectors (STEM-EDX energy-dispersive X-ray analysis).The spectrum profile obtained from STEM-EDX analysis informs us about the presence of iron and carbon in the nanoparticles among other elements (Si, Cu, Cl, S) coming from the TEM grid, detectors or contamination (Fig. 3).Oxygen contamination is not observed in the spectrum profile which is in agreement with the no presence of iron oxides phases in the SAED pattern.This fact means iron core is completely sealed by carbon shell which protects nanoparticles from oxidation.In comparison with other techniques to produce core/shell nanoparticles, arc discharge technique can efficiently avoid oxidation of iron nanoparticles, which is a persistent problem to all forms of production of nanoscale iron particles. 4he composition map obtained by overlapping the C and Fe maps from electron energy loss spectroscopy (EELS) is shown in Figure 4.The green and red areas represent the Fig. 3. STEM-EDX analysis of Fe@C nanoparticles.The absence of oxygen in the spectrum demonstrates the protection of the nanoparticles from oxidation due to the sealed carbon shell.
Morphological and Magnetic Properties of Superparamagnetic Carbon-Coated Fe Nanoparticles carbon and iron distributions, respectively.In accordance with SAED pattern and STEM-EDX analysis, there are no traces of oxygen in the elemental mapping.Magnetic properties of Fe@C nanoparticles have been studied using (SQUID).The total mass of our sample was about 119 g, this value was obtained oxidizing in air at 600 C for 30 min, converting all the carbon material into CO 2 .The remaining iron oxide was weighted and the iron content was extracted by considering all the material as Fe 2 O 3 .The corresponding saturation magnetization is 67 emu/g at 5 K, which is lower than the saturation magnetization of -Fe bulk (212 emu/g) at 0 K, however, is still higher than saturation magnetization of iron oxides (10-50 emu/g). 5The loss of magnetization as the particle size decrease depends largely on the crystalline magnetic anisotropy energy constant K 6 as well as, the coating  of particles with non-magnetic materials smay result in decrease in M S values. 7he magnetization (M) versus the applied field (H ) at different temperatures 5, 75, 150 and 300 K has been represented in Figure 5.The ratio of remnant to saturation magnetization at room temperature is M r /M s ∼ 0 08, which clearly indicates that nanoparticles are in a superparamagnetic state.In addition, zero field cooled and field cooled (ZFC-FC) magnetization curves (Fig. 6) show a maximum at the blocking temperature (T B ), above this temperature which is around 300 K superparamagnetic behaviour is observed. 8Therefore, magnetization curve at room temperature can be fitted to a Langevin function 9 (Fig. 7):

Fig. 1 .Fig. 2 .
Fig. 1.TEM image of spherical iron nanoparticles encapsulated with a carbon shell of 3-5 nm thick.The SAED pattern in the inset of the figure reveals the crystalline structure of nanoparticles.The diffraction pattern corresponds to -Fe in the [11-1] zone axis.

Fig. 4 .
Fig. 4. Chemical map from Fe@C nanoparticles (red corresponds to the iron core distribution and green to the carbon distribution).

Fig. 6 .
Fig. 6.Zero field cooled and field cooled magnetization curves (ZFC-FC) show the superparamagnetic behaviour of our nanoparticles above room temperature.

Fig. 7 .
Fig.7.Normalized magnetization versus H/T at 300 K fitted to a Langevin function.The effective moment found was = 9 66 • 10 −17 emu corresponding to a domain size of spherical nanoparticles with a diameter about 7.6 nm in accordance to TEM observations.