Elsevier

Materials Chemistry and Physics

Volume 213, 1 July 2018, Pages 295-304
Materials Chemistry and Physics

Washing effect on the structural and magnetic properties of NiFe2O4 nanoparticles synthesized by chemical sol-gel method

https://doi.org/10.1016/j.matchemphys.2018.04.022Get rights and content

Highlights

  • Effects of magnetic surface disorder determined in Ni ferrite nanoparticles.

  • Bimodal distribution of particles size induced by a chemical washing process.

  • Redistribution of Ni2+/Fe3+ ions in tetrahedral and octahedral sites provoked by a washing process.

Abstract

NiFe2O4 nanocrystalline samples were synthesized by the sol-gel method. The X-ray diffraction patterns of the as-synthesized samples (S- NiFe2O4) showed the formation of main spinel ferrite structure, with an average crystalline size of ∼50 nm. After a washing process with magnetic separation two sets of NiFe2O4 particles was observed, one with ∼36 nm and other with very small size (∼2–3 nm). X-ray photoelectron spectroscopy (XPS) indicated the presence of Fe3+ and Ni2+ ions on the sample surface. Mössbauer spectra were recorded at 77 K and room temperature and were least square fitted for two sextets which have been assigned to the iron ions occupying the two symmetry sites of the ferrite structure. Magnetic measurements are consistent with the magnetic moment of Ni2+ ions suggesting the formation of an ideal inverse spinel structure in the sample S-NiFe2O4. Moreover, after the washing process with magnetic separation (sample P- NiFe2O4), when the formation of extremely small particles was determined, the Mössbauer and magnetic measurements indicated the reorganization of Ni2+/Fe3+ ions in the tetragonal and octahedral sites of the spinel structure and some features related to thermal relaxations of the small particles are determined.

Introduction

In the last years, magnetic semiconductor nanostructured materials based on ternary transition-metal oxides have been extensively studied, due to the change in physical and chemical properties compared to their bulk samples, thus opening up opportunities for industrial and biomedical applications. Nickel ferrite (NiFe2O4) nanoparticles belong to this class of materials, presenting superior magnetic, thermal and transport properties [[1], [2], [3], [4]], and has been already used in a many of applications, including gas sensing, catalysis, environmental cleanup, magnetic cell separator, magnetic drug carrier, tumor treatment, among others [[5], [6], [7], [8], [9]].

The magnetic properties are strongly affected at the nanoscale dimensions, because when the particle size decreases down to the nano-size, each particle can behave as a single magnetic domain. Besides, the properties which depends on the surface area are also modified because the surface becomes very large in comparison to the bulk material [10]. The electron spins on the surface of a magnetic material are canted (or disordered) because of reduced spin–spin exchange near the surface, reduced coordination number and broken bonds [11]. An extensive amount of research has focused on learning how changes in grain size, cation distribution, and surface properties affect the magnetic properties of ferrite magnetic nanoparticles. Ferrites crystallize in a spinel-type cubic structure, described by the formula AB2O4, where (A) and (B) represent the tetrahedral and octahedral sites, respectively, divided into two different ideal structure types, namely, normal and inverse spinels. In normal spinels, A ions of ferrites AB2O4 are solely located on tetrahedral (8a) sites and Fe ions solely on octahedral (16d) sites within the Fd3m space group. Inverse spinels have half of the Fe ions residing on tetrahedral sites, while the rest of the Fe ions and all the M ions occupy the octahedral sites. Nanocrystalline ferrite systems usually have a mixed spinel structure having the chemical formula (Ni+21-γFe+3γ)[Ni+2γFe+32-γ], where the inversion parameter γ is found to vary, depending on the sample preparation method and the thermal treatment. Since the peculiar properties of ferrites are strictly related to the distribution of cations between octahedral and tetrahedral sites in the spinel structure, the control of cation distribution provides a means to tailor their properties. The changes in the particle size can influence physical properties due to change in cation distribution [12].

The Bulk NiFe2O4 structure presents the inverse spinel structure where Ni2+ ions occupy the octahedral B-sites while Fe3+ ions are half distributed between the tetrahedral A- and octahedral B-sites. The magnetic moments of the ions occupying the tetrahedral and octahedral sub lattices couple together antiparallel and form a collinear ferromagnetic ordering (Néel type) with Curie’s temperature of about 870 K [13,14].

However, nanosized NiFe2O4 presents properties strongly dependent upon the size and shape, stoichiometry, ion site distribution, crystallinity and surface characteristics, which can be controlled by the preparation method [15]. Different preparation techniques have been successfully used to produce nanosized NiFe2O4, including physical (e.g. high-energy ball milling) and chemical (e.g. chemical co-precipitation, sol-gel, and combustion) methods [[16], [17], [18], [19], [20]]. Nevertheless, most of the reported nanosized NiFe2O4 syntheses produced mixed ferrites and/or core-shell morphologies with poor crystalline shell and varying thickness, thus influencing the resulting physical properties remarkably while leading to difficulties in the reproducibility of those properties [[21], [22], [23]].

The physical and chemical characteristics of the materials obtained by the sol-gel method, such as particle size, surface area and mechanical properties, may vary widely depending on the working temperature, operating conditions and the precursor used. However, in general, it can be said that the sol-gel method makes possible to obtain a highly articulated material with a high surface area and higher mechanical properties compared to other synthetic routes [24]. Oxides synthesized by this method, such as MgO, Al2O3, ZnO2, have greater selectivity and activity in catalytic processes, than when synthesized by other methods [25]. One of the difficulties of the sol-gel procedure is that the precursor hydrolysis is very sensitive to water. Even while stirring vigorously, the rate of hydrolysis is so high that the particles precipitate as soon as the water is added. This is not always interesting, especially when looking for greater control over the stages of the reactions developed during the synthesis. A lower rate of hydrolysis can lead to a particle size reduction and to the increase of the surface area of the material which is of great interest in catalytic processes [24,26]. Moreover, Barati et al. [27] reported the formation of NiFe2O4 nanoparticles by a citric acid assisted sol–gel process. The authors reported the formation of NiFe2O4 spinel phase as well as α-Fe2O3 phase at 800 °C and 900 °C. However, by increasing the calcination temperature to 1000 °C, the amount of α-Fe2O3 phase decreases.

In this study we present the chemical synthesis route of nanosized NiFe2O4 particles, which are quasi-spherically shaped, with a highly crystalline core and fully inverse ion site distribution. After washing the as-synthesized samples under magnetic separation we have found a significant change in the particle size and in the distribution of Ni and Fe at the cation sites, unexpectedly. X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), SQUID-magnetometer and Mössbauer spectroscopy were used to characterize the as-synthesized and washed NiFe2O4 nanoparticles.

Section snippets

Experimental

The sol-gel polymerization method using transition-metal nitrates as precursors was employed to synthesize the NiFe2O4 nanoparticles. Stoichiometric ratios of nickel nitrate Ni(NO3)2·6H2O, ferric nitrate Fe(NO3)3·9H2O and citric acid C6H8O7 were dissolved in a small quantity of double distilled water. Aqueous solutions (0.4 and 0.8 mol/L) of nickel and iron nitrates were mixed in stoichiometric proportion (Fe3+:Ni2+/2:1). Subsequently, the aqueous-based mixture was diluted in 50 ml of ethylene

Results and discussion

Fig. 2a shows the room temperature XRD pattern of the as-synthesized sample before (S-NiFe2O4) and after (P-NiFe2O4) the washing process. XRD data analysis of the S-NiFe2O4 sample was carried out by the Rietveld refinement method. Results indicate the formation of the nickel ferrite (FeA3+(FeB3+NiB2+)O42) phase, with almost the same number of Fe-ions at A and B position, in excellent agreement with the data of bulk nickel ferrite (NiFe2O4) (JCPDS card No. 10–0325). A lattice constant of

Conclusion

We report on the synthesis of NiFe2O4 nanoparticles obtained via sol-gel method. The X-ray diffraction pattern of the as-synthesized sample (sample S-NiFe2O4) indicates the formation of a spinel structure with a mean nanoparticles size of ∼50 nm and some extra reflections, which correspond to a secondary phase identified as the NiO phase. Although, the saturation magnetization of ∼42 emu/g is consistent with the presence Ni2+ ions, the lower value can be explained on the basis of the core-shell

Acknowledgments

JAHC acknowledges partial financial support from Brazilian National Research Council (CNPq # 301455/2017-1)) and District Federal foundation for Research (FAPDF # 0193.000823/2015). The authors acknowledge with thanks fruitful discussions with Dr. Zoltan Klencsár.

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