A novel rhombohedron-like nickel ferrite nanostructure: Microwave combustion synthesis, structural characterization and magnetic properties

Research on nickel ferrite nanostructures has drawn great interest because of its inherent chemical, physical and electronic properti es. In this study, we have synthesized rhombohedron like nickel ferrite nanostructure by a rapid microwave assisted combustion method using ethylenediamminetetraacetic acid as a chelating agent. X-ray diffraction, Fourier Transform Infrared spectrometer, transmissi on electron microscope and energy dispersive X-ray microanalyser were used to charact e ize the prepared sample. The magnetic behaviour was analyzed by means of field dependent magnetization measurement which indicates that the prepared sample exhibits a soft ferromagnetic nature with saturation magnetization of 63.034 emu/g. This technique can b e a potential method to synthesize novel nickel ferrite nanostructure with improved magnetic properties.


Introduction
The recent trends in materials research is shifting towards the nanotechnology which offers a unique approach to overcome the shortcomings of their conventional forms due to their large surface to volume ratio and quantum confinement effects [1,2]. Nickel ferrite nanoparticle have received much attention because it is very important group of magnetic nanomaterial due to its extensive applications in high density magnetic storage devices, gas sensors, telecommunication equipments, microwave devices, magnetic guided drug delivery, M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 2 magnetic hyperthermia, magnetic resonance imaging, etc., [3][4][5][6][7][8][9][10]. Nickel ferrite has an inverse spinel structure showing ferrimagnetism that originates from the magnetic moment of anti parallel spins between Fe 3+ ions at tetrahedral sites and Ni 2+ ions at octahedral sites of the cubic structure [3][4][5][6][7][8][9][10]. The particle size and morphology of nickel ferrite nanoparticle plays a vital role on the above mentioned applications. Recently, a number of synthesis methods such as sol-gel, co-precipitation, hydrothermal, microwave irradiation, combustion, etc., have been developed to synthesize NiFe 2 O 4 nanocrystals with various sizes and shapes [3][4][5][6][7][8][9][10][11][12].
Most of these methods have been used to synthesize nanoparticles of the required sizes and shapes, but are difficult to employ on a large scale because of expensive and complicated procedures, high reaction temperatures, long reaction times, toxic regents, removal of byproducts and sophisticated processing [5][6][7][8][9][10]. Among the various methods, microwave synthesise received much attention for the synthesis of nickel ferrite nanoparticles due to several advantages such as shorter time, rapid heating, fast reaction, easy reproducibility, particle size and shape control, high yield, high purity, efficient energy transformation, volume heating, etc., [4,[11][12][13]. Organic modifiers such as oleic acid, urea, citric acid etc., were often used to control the size and shape of the final product in the synthesis process [4,11,14]. To the best of our knowledge, there is no report on the synthesis of nickel ferrite nanoparticles via microwave combustion method using ethylenediamminetetraacetic acid (EDTA) as an organic modifier. Here we report a rapid and simple microwave combustion method to synthesize rhombohedron-like nickel ferrite nanostructure with the aid of ethylenediamminetetraacetic acid (EDTA) as a chelating agent.

Experimental
The chemicals used were nickel nitrate, ferric nitrate, EDTA and NaOH obtained from Merck. All reagents were used without further purification. Distilled water was employed as the solvent.
In a typical synthesis process, nickel nitrate hexahydrate (2.908 g), ferric nitrate nonahydrate (8.08 g) and EDTA (11.167 g) were dissolved in distilled water. The molar ratio of nickel nitrate and ferric nitrate was 1:2 and nitrates to EDTA were 1:1. Then the pH of the obtained mixture was adjusted above 10 by adding 2M of NaOH solution and magnetically stirred for 2 h at 70⁰C. Subsequently, the obtained brown mixture was put in a microwave oven (2.45 GHz, Samsung, India) and irradiated with microwave power of 600W for 30 minutes. The mixture initially boiled then undergoes dehydration followed by combustion with the evolution of large amount of gas and turns into a black colour solid cake. Finally, the obtained solid cakes were crushed into powder.
Crystallographic identification of the phases of the sample was done by X-ray diffraction (XRD) which was carried using Rigaku MiniFlex II powder X-ray diffractometer in the range between 20° ≤ 2θ ≤ 70° with Cu Kα monochromatic radiation (1.5406 Å).
Fourier Transform Infrared (FT-IR) spectrum of the sample was obtained using Perkin Elmer

Results and discussion
EDTA, a member of the polyamino carboxylic acid family, is a complex reagent and it forms metal-EDTA complexes with metal precursors [15]. Hence nickel and iron precursor were mixed with EDTA, a stable Ni-EDTA and Fe-EDTA complexes were formed and it inhibit the reaction between nickel and iron precursor. The microwave heating is emerging technology as an alternative heat source for rapid volumetric heating with shorter reaction time and higher reaction rate. The energy of a microwave photon at a frequency of 2.45 GHz is only 10 -5 eV or about 1 J mol -1 . Upon microwave heating, the microwave energy is transferred to the reaction mixture by interaction of the electromagnetic field at the molecular level resulted in rapid volumetric heating. Due to this rapid volumetric heating, Ni and Fe ions released from their complexes rapidly and caused the burst homogeneous nucleation in a short period and thus crystal grows in anisotropic manner into rhombohedron-like nanostructure as shown in Fig. 1.    octahedral B sites equally. According to the crystal field theory, the magnetic moments arise from the local moments of the Ni 2+ with 3d 8 electrons and Fe 3+ with 3d 5 electrons. The net magnetization comes from the Ni 2+ ions alone (~2µ B ) since Fe 3+ moments ~5µB in both the A and B sites are antiparallel and cancel with each other. This type of ordering results in a saturation magnetization of 2µ B /formula unit (f.u.) or ~50 emu/g at 0K [16][17][18][19]. The value of M S for obtained nickel ferrite rhombohedron-like nanostructure is comparable to that of theoretical saturation magnetization of 50 emu/g calculated using Neel's sublattice theory and to the reported value of 56 emu/g for the bulk sample [16][17][18][19]. M s is the intrinsic property of magnetic materials, but synthesis method and conditions may affect M s of the ferrite nanoparticles [3][4][5][6][7][8][9][10]. Luders et al. have reported a 250% increase in saturation magnetization due to the cationic interchange in NiFe 2 O 4 thin films synthesized by sputtering [19]. It is noteworthy that in comparison to the bulk counterpart, the prepared NiFe 2 O 4 rhombohedronlike nanostructure exhibits high coercivity value than the bulk form [16].

Conclusion
Nickel ferrite nanostructute with rhombohedron shape was synthesized by microwave assisted combustion method using EDTA as a chelating agent. The prepared nickel ferrite exhibits a soft ferromagnetic behaviour with high saturation magnetization which may find novel application in high density magnetic storage devices, gas sensor, microwave devices, magnetic hyperthermia, magnetic resonance imaging, etc.

Research highlights
Rapid and facile method to synthesize nickel ferrite Novel rhombohedron-like nanostructure The obtained product exhibit soft ferromagnetic nature