The objective of the present work is to improve the protection against the oxidation that usually appears in core@shell nanoparticles. Spherical iron nanoparticles coated with a carbon shell were obtained by a modified arc-discharge reactor, which permits controlling the diameter of the iron core and the carbon shell of the particles. Oxidized iron nanoparticles involve a loss of the magnetic characteristics and also changes in the chemical properties. Our nanoparticles show superparamagnetic behavior and high magnetic saturation owing to the high purity
In the recent years the superparamagnetic particles appear to be important agents for a variety of applications in the fields of drug delivery, magnetic resonance imaging (MRI), and cancer treatments like hyperthermia. This is due to the special magnetic characteristic of these particles. That is the reason of our interest in studying the size limits under which the nanoparticles present a superparamagnetic behaviour [
In order to synthesize nanoparticles in the vapor phase, appropriate conditions should be created. Like in the case described in the capillary theory of nucleation and under supersaturation conditions, a vapor phase mixture in thermal plasma (arc-discharge) can become thermodynamically unstable giving place to a nucleation process [
The method varies depending on the phase of the precursor we are interested to use.
Here, the precursor is injected into thermal plasma which provides the necessary conditions to induce reactions that lead to supersaturation and particle nucleation. Under the condition of thermal plasma, the precursor decomposes in radicals, atoms, and ions forming a high temperature ionized gas. The high concentration of species and high temperatures in the plasma arc induce a diffusion process associated with a fast quenching of gas species. During this process, the gas species react and condense to form particles in a similar way to a vaporized material when cooled down by mixing with a cool gas or expanded through a nozzle [
The objective of this work is the control of the synthesis of iron encapsulated in carbon nanoparticles, regarding the size of the Fe core, the diameter of the C shell, and moreover the quantity of the obtained nanoparticles. Fe is one of the most common materials used for the formation of magnetic nanoparticles. It is due to its superparamagnetic behaviour, which is easily controlled by the nanometric size of the magnetic core, and because the Fe is suitable for biomedical applications. The use of a carbon shell is intended to achieve the required biocompatibility and prevents oxidation of the iron core and formation of agglomerations of nanoparticles [
The structural and morphological characteristics like shape and size distribution were studied using transmission electron microscopy (TEM) and selected area electron diffraction (SAED). The absence of oxygen was preliminarily evidenced by the energy-dispersive X-ray analysis (STEM-EDX). The magnetic characteristics were determined using a superconducting quantum interference device (SQUID).
In order to facilitate the study of magnetic behavior and other characteristics of core@shell nanoparticles, we modified an arc-discharge reactor in order to operate for longer time and produce bigger amounts of them (Figure
General picture of the “home made” reactor we used for the synthesis of the iron encapsulated in carbon nanoparticles.
To facilitate the complete collection of the produced nanoparticles we dragged them out of the arc-discharge chamber, by a laminar flow of an inert gas (He) to a flask cooled by liquid nitrogen. The flow of the dragging gas that we used was 3 L/min of volumetric flow of He (at 1 atm), providing a moderated velocity of 65 cm/s in the nozzle outlet. The precursor was in the gas phase. Helium is used to drag microdroplets of ferrocene. The carbon electrodes can be rotated and moved in order to avoid problems that the consumption of carbon could cause. One of the electrodes has a sharp conical tip shape (cathode) and the other has a cylindrical shape with an approximate diameter of 4 cm and can rotate and move backward and forward (anode). The plasma is being generated by a power supply to produce an arc DC current of 40 A.
The pressure was kept stable at near-atmospheric conditions (
We investigated the effects of two technological parameters, the concentration of the Fe precursor into the solvent isooctane and the total gas flow (He).
Both the isooctane and the Fe precursor (ferrocene) were of a purity of 99.9%.
Residence time is the period that the nucleus of the precursor is spending inside the plasma zone. The time that gas atoms and precursor radicals stay inside the plasma zone depends on the gas flow rate. This has an effect on the size of the particles as the longer they stay inside the plasma zone, the larger they grow [
The velocity
By measuring the size of the plasma zone
Table
Flow rate and resulting velocity and residence time for a precursor vapor with 1% of ferrocene concentration.
He flow rate, |
Precursor vapor velocity, |
Residence time, |
---|---|---|
30 ± 2 | 7.07 ± 0.50 | 71 ± 1.5 |
60 ± 2 | 14.13 ± 0.50 | 35 ± 1.5 |
120 ± 2 | 28.28 ± 0.50 | 18 ± 1.5 |
Different concentrations of Fe precursor into the isooctane, from 1% to 4% w/w, were used to determine the effect on the size of the particles and on the total amount of the obtained product.
Images of nanoparticles were observed by TEM (JEOL 2100) in high resolution mode, using 200 kV and a probe size of 0.5 nm [
The iron core diameter average and its standard deviation of all our samples in relation to the two set parameters, He flow rate and ferrocene/isooctane concentration. Samples with 1.1 <
He flow rate |
Iron core size (nm)/geometric standard deviation, | ||
---|---|---|---|
[Ferrocene/isooctane] 1% w/w | [Ferrocene/isooctane] 2% w/w | [Ferrocene/isooctane] 4% w/w | |
30 | 8.18/1.22 |
5.43/1.34 |
5.34/1.33 |
60 | 6.23/1.47 | 6.12/1.46 | 5.22/1.22 |
120 | 5.22/1.25 | — | — |
TEM images of iron encapsulated in carbon nanoparticles produced by arc-discharge (
Another detail in the micrographs of Figure
By treating all the size distribution results with ORIGIN tools,
Histogram of the size distribution for iron nanoparticles grown under precursor concentration of 4% w/w and He flow of 60 mL/min. The iron core is 5.22 nm and the geometric standard deviation σ 1.22.
All the studied samples (Table
Another interesting fact supporting the formation of a carbon shell on the iron nanoparticles is that the use of metals, which are characterized by a higher metal-carbon energy bond, yields nanotubes of smaller radius [
To discuss the size distribution of our samples we can rely on the geometric standard deviation,
The absence of oxygen in the spectrum was preliminarily evidenced by the energy-dispersive X-ray analysis (STEM-EDX). Oxygen peak is not present in the spectrum in the expected energy (O-K
TEM image of a nanoparticle. In the small picture we can see the diffraction pattern from which we obtain the crystalline structure of the iron core. Here it corresponds to the
In addition carbon shell appears surrounding completely the iron cores forming crystalline domains with a structure similar to the fullerene one.
This result suggests that iron core is completely sealed into the cover shell, which is of great importance and interest in biomedical applications [
SQUID measurements were used to investigate the magnetic characteristics of our samples. Two hysteresis loops (Figure
The hysteresis loops measured at 5 K for samples with different precursor concentrations: (a) 1% w/w and (b) 2% w/w. Both were obtained at a precursor gas flow of 30 mL/min.
From the hysteresis loop measured by SQUID at 5 K we have obtained the coercive field
Subsequently we compare the values of the coercive field with those reported in the bibliography for magnetic nanoparticles encapsulated in carbon nanotubes [
In this equation,
SQUID measurements of a sample in concentration of 1%, in a precursor gas flow of 60 mL/min, and in temperatures of 5 and 300 K were taken. In Figure
Graphs of the normalized magnetization M/Ms ratio with the field in 5 K and room temperature.
From the zero field cooled and the field cooled curve it can be seen that the nanoparticles are superparamagnetic in room temperature. The blocking temperature appears near 40 K, where the maximum of the ZFC magnetization is localised. Gittleman’s model describes blocking temperature and the critical volume that separates nanoparticles from the blocked state (
Zero fields cooled and the field cooled magnetization curves for 100 Oe field.
Our proposed synthesis method provides us with the capability of a very good control in the size of the synthesized nanoparticles. Morphological and structural characterization revealed a quite good monodispersion as well as the
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by AGAUR of Generalitat de Catalunya, Project 2009SGR00185, and by MICINN of Spanish Government, Projects CTQ2009-14674-CO2-01 and CSD2006-12. The authors would like to thank the State Scholarships Foundation of Greece (IKY) for the financial support. Also, they would like to thank the people of the Centres Científics i Tecnològics de la UB, CCiTUB, and Nuria Clos for the help with the SQUID measurements.