Synthesis and Magnetic Characterization of Graphite-Coated Iron Nanoparticles

1Department of Electrical Engineering, Section of Solid State Electronics, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Avenida IPN No. 2508, 07360 Ciudad de México, Mexico 2PhD Program in Nanoscience and Nanotechnology, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Avenida IPN No. 2508, 07360 Ciudad de México, Mexico 3Departamento de Fı́sica, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Avenida IPN No. 2508, 07360 Ciudad de México, Mexico 4Facultad de Ciencias Fı́sico Matemáticas, Universidad Autónoma de Nuevo León, Avenida Universidad s/n, Ciudad Universitaria, 66455 San Nicolás de los Garza, NL, Mexico

For many applications the physicochemical stability of the nanoparticle is an important issue [12].Because of their chemical activity, nanosized materials easily interact with undesirable molecular species to undergo degradation which means a drawback for some applications [13].Several approaches have been addressed to overcome the stabilization problem, mostly including the organic passivation of the surface nanoparticle [14].In the magnetite nanoparticles case, oleic acid has proven to be an excellent stabilizing agent to prevent its chemical degradation [15].Organic compounds other than oleic acid such as citric acid and polylactic acid [16][17][18] have also successfully used to stabilize and disperse magnetic nanoparticles in aqueous or organic medium [19,20].
In addition to organic molecules, inorganic coatings have successfully been tested as the stabilizing shell of magnetic nanoparticles with inorganic materials; two common examples of inorganic shells are silica [21] and gold [22].In spite of being a biocompatible material and providing a multifunctional platform for surface modification, it was reported that gold shells fail to protect magnetic nanoparticles [23] because they usually have grain boundaries.Therefore, new materials for making more stable and hermetic shells are demanded.
On the other hand, iron nanoparticles are currently studied for environmental remediation and hyperthermia [24].For instance, graphite-coated iron nanoparticles were used to remove heavy metal ions (Cu 2+ , Co 2+ , and Cd 2+ ) from water [25], uranium [26], and arsenic [27] or Co 2+ ions from an effluent [28].In addition, graphite has proven to be an excellent coating to protect the magnetic iron core from physicochemical degradation under corrosive medium [29].
In this paper, we report the synthesis and the structural and magnetic characterization of graphite-coated iron nanoparticles.These particles were obtained from our experiments tending to hermetically coat magnetite nanoparticles with a graphite coating by CVD, CVD being a suitable method to form hard graphite coatings on a variety of metal surfaces.The subsequent characterization indicated that graphite-coated nanoparticles could be obtained, but magnetite transformed into elemental iron due to the reductive atmosphere and methane inside the CVD reactor.Importantly, the prepared particles demonstrated long term stability against oxidation under environmental conditions.

Sample Preparation.
The graphite-coated iron nanoparticles were produced by CVD using magnetite nanoparticles as the starting material.
The magnetite nanoparticles (MNPs) were prepared by coprecipitation of FeCl 2 and FeCl 3 salts in water using NH 4 OH as precipitant [34].The obtained product was thoroughly magnetically assisted water-washed to purify it from byproducts and then dried to obtain a powder comprising magnetite nanoparticles 6 nm in size exhibiting superparamagnetism.
For the CVD process, the as-prepared magnetite nanoparticles powder was placed on a copper foil and introduced in the tube quartz of a homemade CVD system.
The CVD process started by establishing an argon flow (424 sccm) while the temperature was raised up to 800 ∘ C and kept so for 30 min.Afterward, a hydrogen flow (420 sccm) was turned on and the argon flow was turned off while the temperature was raised to 900 ∘ C.After 30 min a methane flow (50 sccm) was added and it is kept for 30 min maintaining the hydrogen flow for additional 30 min.

Phase Composition and Morphology
3.1.1.Morphology.As previously established, the graphitecoated iron nanoparticles (GFeNps) were prepared from magnetite powders comprising particles 6 nm in size.Magnetization experiments indicated that the magnetite sample displays a superparamagnetic behaviour with 50 emu/g saturation magnetization.
After the thermal process, iron-based nanophases other than magnetite developed, depending on the existent atmosphere in the CVD reactor.Under the methane-hydrogen atmosphere, single phase iron nanoparticles along with some carbon-derived nanostructures (graphene sheets, highly oriented graphite, and nanotubes) were obtained.Carbon nanostructures were removed by crushing the as-prepared material in a mortar followed by magnetic separation.As shown in the scanning electron microscopy (SEM) images (Figure 1(a)), the obtained sample consists of core-shell graphite-iron particles 300 and 500 nm in size as the main product.The core-shelled structure for the obtained iron nanoparticles was assessed by HRTEM; Figure 1(b) shows TEM of GFeNps.

Phase Composition
X-Ray Diffraction Characterization. Figure 2 shows the XRD spectra of magnetite nanoparticles (JPCDS-PDF-19-0629) (b) and graphite-encapsulated iron nanoparticles (a).All of the peaks appearing in the CVD treated samples belong to the BCC phase of iron (JPCDS-PDF-6-0696) [35] indicating that magnetite completely transforms into elemental iron.A small peak corresponding to the graphitic material (JPCDS-PDF-56-0159) is also present.If present, iron carbide phases were undetectable for X-ray diffraction.
The crystallite size, as calculated from XRD data using the Scherrer equation, was 11 nm and 23 nm for magnetite and graphite-coated iron, respectively.
Raman Spectroscopy Characterization.Graphitic materials display the D (1350 cm −1 ), G (1580 cm −1 ), and G  (2700 cm −1 ) typical bands in their Raman spectrum, which are ascribed to defects in the crystalline structure, the characteristic sigma bond between two hybrid orbitals sp 2 , and the characteristic overtone of the D band [36][37][38], respectively.In the present case, the representative Raman spectrum of our samples (Figure 3) clearly indicates the high quality for the iron nanoparticle graphite coating, because it resembles to that reported for highly oriented graphite [39,40].
Proposed Formation Mechanism for GFeNps.The above results indicate that graphite-coated iron nanoparticles could be obtained under the previously described experimental conditions.To explain the whole process leading to the GFeNps formation and the graphite coating development (Figure 4), it is noteworthy to mention that preliminary studies on Ar annealed magnetite nanoparticles at 800 ∘ C indicated no phase change in the starting material.Hence, we  have assumed that both the transformation of magnetite to elemental iron and the graphite coating development occur at 900 ∘ C in presence of the methane-hydrogen mixture.At first, the MNPs 6 nm in size agglomerate and then coalesce to develop larger polycrystalline magnetite nanoparticles at 800 ∘ C under Ar atmosphere.Upon establishing the hydrogen-methane atmosphere at 900 ∘ C, carbon and carbon hydride species are produced and then react with iron oxide to reduce it into elemental iron.At 900 ∘ C, the nanoparticle surface is partially melted and the magnetite oxygen reacts with carbon and carbon hydride species to produce elemental iron.The complete transformation of magnetite into iron involves oxygen exodiffusion from the nanoparticle volume.
In regard to the graphite coating development, we believe that it grows following a genuine CVD process.That is, because of the catalytic action of iron widely observed [41] the hydrogenated carbon species catalytically decompose into carbon atoms (black dots in Figure 4) at the partially melted surface of the iron nanoparticle, and then carbon atoms react with iron to form a carbon-rich iron surface.After reaching the carbon supersaturation, graphene nucleates at the grain boundaries and then it conformally grows to completely coat the particle surface.The subsequent graphene overlayers develop on the underneath one through an autocatalytic mechanism involving structural defects in the graphene Carbon layers Iron nanoparticles encapsulated with highly oriented graphite Figure 4: Assumed development of GFeNps: At 800 ∘ C, smaller magnetite nanoparticles coalesce into a larger polycrystalline one under Ar atmosphere.At 900 ∘ C, hydrogen reacts with methane to produce carbon and carbon-hydrogen derived species, which impinge on the partially melted iron surface to form a thin carbon-rich iron surface.After reaching the carbon supersaturation, graphene nucleates at the grain boundaries and then it conformally grows to completely coat the particle surface.
structure.Figure 4 schematizes the proposed formation mechanism for GFeNps.Previous studies have reported similar structures where the graphitization occurred at the big nanoparticles surface, so forming highly oriented graphite which is represented and excellent protection against the oxidation of the magnetic core [42,43].

Magnetic Characterization.
As previously mentioned, the CVD process promotes the iron-oxide-to-elemental-iron conversion.Figure 5 shows the magnetization curves of the material before (a) and after (b) the CVD process.The magnetization curve of MNPs (magnetite) assesses its superparamagnetic character, with saturation magnetization  s = 50 emu/g, which agrees well with that reported for magnetite nanoparticles 6 nm in size [44].
On the other hand, the graphite-coated iron nanoparticles saturation magnetization curve indicates that they are nearly superparamagnetic ones, with coercivity of 17 Oe and  s = 165 emu/g.The superparamagnetic deviation is probably due to the polydisperse nature of the sample.The increment of  s represents 3.3 times the value of the original particles.These striking changes corroborate the notion that magnetite transforms into elemental iron [45], because the measured  s value agrees well with that reported for iron nanoparticles ( s = 178 emu/g) [46].
The superparamagnetic character of magnetite was assessed by fitting the magnetization experimental data to the Langevin equation as proposed by Knobel et al. [47] and Goya et al. [48] (see Figure 6).For this, the lognormal distribution function was used and a spherical shape   fairly well with the magnetic moment for a 6 nm SPION, as estimated by considering 32  B per magnetite unit cell and counting the unit cell number per particle.This suggests that the magnetite sample comprises monodomain nanoparticles.

Conclusions
We reported the preparation and magnetic properties and structural and morphological characterization of graphitecoated iron nanoparticles, which were obtained from CVDprocessed magnetite nanoparticles.The XRD studies indicated the complete transformation of magnetite to iron and the graphite presence.The magnetic characterization revealed a nearly superparamagnetic behaviour for the graphite-coated iron nanoparticles.The relative low coercive field of our sample was attributed to its polydisperse character, which was caused by the process to encapsulate magnetic nanoparticles with graphite using the CVD technique.The Raman spectrum assessed the high quality of the graphitic material coating the iron nanoparticles.

Figure 1 :
Figure 1: (a) SEM image of a GFeNps sample as obtained from the CVD process and (b) HRTEM image of a GFeNp.The inset shows details on the GFeNps core-shell structure.Note that 0.35 nm and 0.206 nm agree well with the interplanar separation of graphite layers and (110) planes of Fe, respectively.

Figure 2 :
Figure 2: X-ray diffraction of graphite-encapsulated iron nanoparticles (a) and bare magnetite (b); the peak marked by * corresponds to the plane (002) of graphite.

Figure 5 :
Figure 5: Magnetization curves at room temperature: magnetite nanoparticles (a) and graphite-coated iron nanoparticles (b).The inset displays the low field magnetization to illustrate the superparamagnetic nature of our samples.