Elsevier

Applied Surface Science

Volume 257, Issue 12, 1 April 2011, Pages 5265-5269
Applied Surface Science

Parametric studies on iron–carbon composite nanoparticles synthesized by laser pyrolysis for increased passivation and high iron content

https://doi.org/10.1016/j.apsusc.2010.11.069Get rights and content

Abstract

Iron/iron carbide core and carbon shell nanoparticles with improved magnetic properties were successfully synthesized by laser pyrolysis. As iron and carbon precursors, iron pentacarbonyl and pure or argon-diluted acetylene/ethylene mixtures, respectively, were used. The aim of the present optimization is the improvement of the magnetic properties of the nanomaterials by the increase of the iron percent in powders simultaneously to the maintaining of the protective character of the carbon coverage of nanoparticles. The chemical content and the crystalline structure were monitored by EDX, XRD and TEM techniques. In the first study, the content of acetylene as carbon source was diminished from 75% to 0%. Consequently the percent iron increased from 10 at.% to 28 at.% while oxygen remained relatively constant (around 5 at.%). In the second step, only diluted ethylene was used (maximum 87.5 vol.% Ar). In this case, an increase of iron to 46 at.% is observed. An optimum 50% carbon source dilution was found. Above this value, the carbon content increases and below it, superficial oxidation increases through the diminishing of the carbon shell. The magnetic properties and the Fe phase composition of the Fe–C samples were analyzed by temperature dependent Mössbauer spectroscopy.

Research highlights

▶ Fe-based/C core–shell particles at high Fe content were prepared by laser pyrolysis. ▶ Improved magnetic properties by the increase of the core iron content were obtained. ▶ High Fe (in at.%) was obtained by the diminishing the acetylene precursor.

Introduction

Iron based nanoparticles have been of outstanding interest emerging from their size-dependent physical and chemical properties. In particular, metallic iron cores with 15–20 nm have enhanced coercivity as compared to bulk iron [1] while around 4 nm or less iron nanoparticles become superparamagnetic at room temperature [2]. Dispersions with functionalized iron based nanoparticles have biomedical applications as magnetic carriers for drug delivery and contrast agent [3], [4]. They are also used as catalysts for Fisher–Tropsch coal liquefaction [5] and carbon nanotubes synthesis [2], [6]. Zero-valent iron based nanoparticles have been used for water and soil decontamination [7], [8]. Unfortunately, iron nanoparticles are unstable in the environmental conditions, iron reactivity towards oxygen-based species increasing with specific surface area increasing. The iron nanoparticles encapsulation seems to be the only solution to preserve its metallic (zero-valent) state and, consequently, their magnetic behaviour. The iron based cores with carbon shells fulfil this condition. Furthermore, the nonmagnetic carbon shell minimizes the magnetic interactions between the iron cores and thus the nanocomposite material acts as a group of isolated non-interacting single magnetic domains.

We have previously reported on core–shell Fe–C nanocomposite particles with approximately 3–7-nm sizes that were synthesized by the CO2 laser pyrolysis of volatile iron and carbon precursors [6]. The nanoparticles presented superparamagnetic properties and the Fe content was <15 at.%. The aim of the present work is the optimization of the process in order to improve the magnetic properties of the nanomaterials by increasing the iron percent in powders simultaneously with the maintaining of the protective character of the carbon shell. Vapors of Fe(CO)5 as iron precursor and different mixtures of C2H2 and C2H4 or simply C2H4 diluted with Ar in different proportions were used as carbon source. Different analytical methods were used in order to monitor the atomic percentage of Fe, C and O, the structural features and the nanopowders morphologies.

Section snippets

Experimental data

The laser pyrolysis technique adapted to the synthesis of nanoparticles with iron based cores embedded in carbon shells has been described in detail earlier [6], [9]. The admission of the gases through the reaction chamber implies a nozzle with three concentric tubes. The reactive gas mixtures interact orthogonally with a focused CW CO2 laser beam. Fe(CO)5 vapors entrained by 10 standard cubic centimeters per minutes (sccm) C2H4 flow were admitted through the central tube, while various

Results and discussions

Fe(CO)5 easily decomposes at temperatures higher than 200 °C, releasing iron based particles [9]. The decomposition process of Fe(CO)5 via the laser pyrolysis technique and the synthesis of iron based nanoparticles were analyzed elsewhere [5], [10]. Laser intensities giving rise to reaction flame temperatures <750 °C do not allow the independent hydrocarbon decomposition (with the formation of the iron-free carbon clusters) [11]. Within the laser flame, the hot freshly formed iron based clusters

Conclusion

The synthesis of composite nanoparticles with iron-core/carbon shell structure may be performed by imposing specific conditions to the laser pyrolysis technique. Nanoparticles containing as much as 40 at.% Fe and <6 at.% oxygen are reported. Optimum conditions are found in which ethylene diluted in Ar is the carbon source (50/50 vol.% dilution). The structural analysis identifies α-Fe and Fe3C as the main crystalline phases of the iron-based cores wrapped in the carbon sheets. Magnetic

Acknowledgements

This research was partially supported by the Project 71/083/2007 – PARTENERIATE – PNCD2 Program of the Romanian Ministry of Education and Research.

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