A facile approach for obtaining NiFe2O4@C core-shell nanoparticles and their magnetic properties assessment

https://doi.org/10.1016/j.diamond.2020.108159Get rights and content

Highlights

  • NiFe2O4@C core-shell nanoparticles are successfully obtained via TCVD method.

  • Ordering in the graphite shell has been enhanced as the coating time increased.

  • Samples revealed the ferrimagnetic feature after coating.

Abstract

NiFe2O4/C core-shell nanoparticles were prepared using the two-step process, including reverse microemulsion followed by the thermochemical vapor deposition (TCVD). Nanoparticles with 5 and 30 nm size, in average, were obtained respectively after each step. Micro-Raman spectra for the coated samples were recorded and results confirmed the persistence of the ordered and disordered carbon-based materials in products. It is, also, found out that the defect densities were declined by increasing coating time from 1 h to 2 h. Magnetic assessment revealed that the coercivity (Hc) raised from 0.6 to a about maximum of 62.7 Oe at room temperature for the as-prepared sample to the one coated. Hc-value, for the sample coated for an hour, was lower than the sample coated for 2 h confirming the effect of the crystallite size on the coercivity in a single domain substance. On the other hand, saturation magnetization (Ms) for samples coated for 1 and 2 h was recorded, respectively, as 46.52 and 27.62 emu/g. Lower amount of Ms for the sample coated for 2 h was attributed to the higher thickness of the carbon shell around the NiFe2O4 cores.

Introduction

Having and working on a multifunctional matter is hot and attracts an attention of numerous research groups all around the globe. Not only spinel ferrite/carbon nanocomposite is not an exception, but also they are considering these days more and more due to their potential application in biomedicine [1], lithium-ion batteries [2], catalysts [3], sensors [4] and microwave absorptions [5].

A well-known spinel ferrite structure is chemically formulated as MFe2O4 in which M refers to a divalent cation, usually Zn2+, Ni2+, Co2+, Mn2+, Cu2+ and so on. In the mentioned formula, iron is normally trivalent, Fe3+; however, the magnitude of its distribution in the interstitial positions depends on both the processing atmosphere and persisting cations in the formula. This gives rise to changes in the magnetic feature of the spinel and also the charge on Fe itself. Cubic structure of spinel ferrite crystallizes in Fd3¯m space group. There are two renowned interstitial positions in the spinel ferrite structure termed tetrahedral and octahedral. Each spinel ferrite structure has 16 octahedral and 8 tetrahedral positions which are occupied by di- and trivalent cations. Other positions of the structure occupy by the oxygen anions to make a full structure of a spinel ferrite. Spinel ferrites, based on the di- and trivalent cations distribution in interstitial positions, are categorized into three types named the normal spinel, the reverse spinel, and the mixed spinel ferrite. NiFe2O4 is one of the most applicable spinel ferrites and has a reverse spinel structure in which Ni2+ cations occupy half of the octahedral positions and Fe3+ cations reside into the other half of octahedral and all the tetrahedral positions. The proposed formula for nickel ferrite is Fe3+[Ni2+Fe3+]O42− [6,7]. The reason behind this is that the crystal field splitting energy (CFSE) for divalent Ni2+ (d8) cation is more than CFSE for trivalent Fe3+ (d5+) cation. Unpaired electrons in the 3d-orbital of Ni2+ and Fe3+ cations and their contributions with oxygen through the super exchange mechanism leads to coupling and results in emerging ferrimagnetism in NiFe2O4 compound [8].

Carbon based compounds such as graphite, graphene, carbon nanotubes and other derivatives are widely under investigation to find out their potential application. By now, a wide range of applications for them have been discovered and made these compounds interesting for researcher to employ them for having unique features [[9], [10], [11]]. Carbon, by itself, behaves like a non-magnet and dielectric which makes it potentially applicable in various range of compounds and devices [12].

Various reports on composites of ferrites with either carbon-based compounds or non‑carbonic ones have been proposed and the related samples were extensively characterized. This compromises various types of ferrite such as cobalt ferrite, manganese ferrite and so on [[13], [14], [15]]. Zhang et al. [16] prepared a mixture of graphene/NiFe2O4 and tried to modify it with multi-wall carbon nanotubes and gather the magnetic features and report 20 emu/g as saturation magnetization. Wang et al. [5], also, decorated the carbon base with NiFe2O4 particles and reported 54.7 and 37.8 emu/g, respectively, for NiFe2O4 and decorated carbon with NiFe2O4. Although reports [[17], [18], [19]] have been proposed on the same compounds, the common mismatch in all reports is the aggregation of the magnetic particles/nanoparticles which potentially can affect the magnetic features by cooperation among NiFe2O4 accumulations.

In the current study, reverse microemulsion and thermo chemical vapor deposition (TCVD) as the two-step process was employed for composite preparation. In this way, the size of magnetic nanoparticles will be controlled and a complete coating owing high quality will be made. Although there are enough number of reports on NiFe2O4/C composites; however, there is rare work specifically on NiFe2O4@C core–shell nanoparticles, to our knowledge. Overall, in this work, not only new core–shell nanoparticles were synthesized. We, also, also, employed CTAB as a surfactant to prohibit the aggregation of magnetic nanoparticles and keep them as a single domain magnet. Employing these two techniques leads to achieve ultra-small nanoparticles with a narrow size distribution which make the products potentially applicable.

Section snippets

Experimental procedure

A schematic of the steps of the process including the reverse microemulsion followed by the CVD has been illustrated in Fig. 1. Experimentally, Iron (III) chloride hexahydrate (FeCl3·6H2O (+%99)) and Nickel (II) chloride hexahydrate (NiCl2·6H2O (+%99)) were purchased from MP Biomedicals. Isooctane, 1-butanol, sodium hydroxide (NaOH (%99)), and cetyl trimethyl ammonium bromide (CTAB) purchased from MERCK chemicals, and were used without further purification. The reverse microemulsion was

Structural and microstructural analysis

As mentioned earlier, there are 16 and 8 interstitial octahedral and tetrahedral positions in spinel structure, respectively, and one of the reliable methods to find out and confirm the cations placement in these positions is FTIR spectroscopy. Fig. 2 presents the metal-oxide bonding in octahedral and tetrahedral positions. Metal-oxide bonding is generally in the range of 400 to 700 cm−1. In this study, octahedral and tetrahedral vibrations have been respectively detected at around 471 cm−1 and

Conclusion

In this study, NiFe2O4/C core-shell nanoparticles were successfully prepared through reverse microemulsion and TCVD approach. Their structural and magnetic properties have been characterized. TEM images were revealed that the as-prepared nanoparticles are spherical-like and are smaller than 10 nm which were increased then by the calcination process and their morphology has been changed as well. XRD patterns are also revealed that the crystallization of the NiFe2O4 has not been accomplished

Author statement

Contribution of each author is provided as below:

Hamed Bakhshi: contribution in the experimental and the characterization sections, writing and editing the initial draft.

Maryam Mohamad Azari: writing the initial draft and contribution in the experimental section.

Ali Shokuhfar: Presenting the idea of the study, supervision, and editing the initial and the final draft.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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