Synthesis, optical and magnetic properties of pure and Co-doped ZnFe₂O₄ nanoparticles by microwave combustion method

Highlights

• Pure and Co-doped ZnFe₂O₄ were synthesized by a simple microwave combustion method using urea as the fuel.

• The as-synthesized pure and Co-doped ZnFe₂O₄ were found to have good optical as well as magnetic properties.

• An attempt has been made to compare the lattice parameter and the PL intensity.

• The effects of Co-doping on optical and magnetic properties of ZnFe₂O₄ were investigated.

Abstract

Spinel zinc ferrite (Zn_1−xCo_xFe₂O₄) nanoparticles with various particle sizes were prepared by the microwave combustion method using urea as a fuel. The composites were prepared with the addition of cobalt at different molar ratios (x=0.0 to 0.5) to ZnFe₂O₄. The obtained spinel ferrites were characterized by X-ray powder diffraction (XRD) and their mean grain size and morphology were determined by the high resolution scanning electron microscopy (HR-SEM). The magnetic properties of the synthesized ferrites were investigated using room temperature vibrating sample magnetometer (VSM) and their hysteresis loops were obtained. The optical reflectance and photoluminescence (PL) emissions were determined by UV–visible diffuse reflectance spectra (DRS) and PL spectra respectively. The formation of single cubic spinel phase was confirmed by XRD and Rietveld analysis with an average crystallite size is in the range of 43–49 nm. The broadband visible emission band is observed in the entire PL spectrum and the estimated energy band gap is about 2.1 eV. The variation of saturation magnetization (M_s) value of the samples was studied. The prepared lower compositions (0.0, 0.1 and 0.2) show a superparamagnetic behavior and the higher compositions (0.3, 0.4 and 0.5) show a ferromagnetic behavior with hysteresis and that the M_s increases with increasing Co content to reach a maximum value of 65.20 emu/g for Zn_0.5Co_0.5Fe₂O₄. The relatively high M_s of the samples suggests that this method is suitable for preparing high-quality nanocrystalline magnetic ferrites for practical applications. Different mechanisms to explain the obtained results and the correlation between magnetism and structure are discussed.

Introduction

Nanostructured materials are considered very attractive compared to their bulk counterpart, due to their advanced physical and chemical properties, due to the phenomenon called quantum confinement. Therefore, the synthesis of conventional materials as well as new materials at the nanoscale is attracting the attention of scientists. Moreover, the experimental conditions play a very important role in determining the shape, size and purity and hence the drastic modifications of the properties.

Spinel ferrites have been investigated for their usual electrical and magnetic properties as well as for various technological applications including ferrofluids [1], high-density magnetic recording media [2], biomedicine [3] and radar-absorbent materials [4]. Zinc ferrite, ZnFe₂O₄, is one of the most widely studied materials [5], because the high-quality ZnFe₂O₄ nanostructure has generated a lot of interest owing to their potential applications in gas sensing [6], magnetic behavior [7], electrical characteristics [8] and semiconductor photocatalysis [9].

The spinel ferrites can generally be described by the formula AB₂O₄, where A and B denote divalent and trivalent cations, respectively. Zinc ferrite is a normal spinel structure, all of the A (Zn²⁺) sites are tetrahedrally coordinated while the B (Fe³⁺) sites are octahedrally coordinated by oxygen atoms [10].

Several methods have been used for the preparation of nanocrystalline spinel ferrites such as co-precipitation, sol–gel, mechanical alloying, hydrothermal method, ball milling and ultra sonic cavitation [11], [12], [13], [14] methods. However, the above said methods encounter some disadvantages such as, the requirement of complicated equipment, high-energy consuming, higher processing temperature and also require rather long reaction time caused by the multiple steps (calcinations) to complete the crystallization of ferrite nanostructures.

In recent years, the combustion synthesis method has attracted significant attention in fabricating the homogeneous, unagglomerated, multi-component metal oxide ceramic powders, because of its inexpensive precursors, short preparation time, modest heating and relatively simple manipulations [15], [16]. The combustion method is based on the mixing of metal nitrates, which is a oxidizing agent and organic fuels, acting as a reducing agent. It is the organic fuel, which provides a platform for redox reactions during the way of combustion. Among the various control parameters in a combustion method, fuel plays a vital role in determining the morphology and phase formation of the final product. The surface area, size-distribution and agglomeration of the particles in the final product mainly depend on the combustion temperature, which in turn is related to the nature of the fuel and fuel to oxidant ratio.

A variety of combustion methods have been reported to generate the nano-sized ferrite materials using different organic fuels such as glycine, urea, citric acid, sucrose, hydrazine, alanine or carbohydrazide. Farhadi and Zaidi [17] used various organic fuels such as urea, citric acid, glycine and sucrose for the preparation of BiFeO₃. Gabal et al. [18] synthesized nanocrystalline Cr-substituted NiFe₂O₄ via sucrose-assisted combustion route. Deraz [19] has reported the structural, morphological and magnetic properties of cobalt ferrite nanoparticles in relation to the fuel content by the glycine-assisted combustion method.

Costa et al. [20] have prepared the Zn doped NiFe₂O₄ nanoparticles by combustion method using metal nitrates and urea and investigated the effect of Zn doping on the structural and magnetic properties of NiFe₂O₄ nanoparticles. Kambale et al. [21] have also prepared the Ni–Zn ferrite nanoparticles by combustion method using metal nitrates and citric acid, and studied the magnetic and dielectric properties. Alarifi et al. [22] have reported the synthesis of NiFe₂O₄ nanoparticles by the combustion method and studied the effect of glycine/nitrate ratio on the structural, morphological and magnetic properties of the NiFe₂O₄ nanoparticles. Priyadharsini et al. [23] have prepared the Ni–Zn ferrite nanoparticles by the combustion method with metal nitrates and glycine and investigated the structural, spectroscopic and magnetic properties.

Pawar et al. [24] reported the nanocrystalline NiFe₂O₄ by by chemical combustion route using different fuels as polyvinyl alcohol (PVA), glycine and urea and the powder was then annealed at 800 °C for 2 h in air environment in order to remove the impurities. The magnetic measurements showed the ferrimagnetic behavior of all the nanoparticles, while nanoparticles prepared by using urea as a fuel were found to have slightly greater crystallite size and relatively stronger A–O–B interaction, when compared with others (polyvinyl alcohol (PVA) and glycine products). Bamane et al. [25] synthesized Dy doped Co–Zn ferrite nanoparticles by a simple sol–gel auto combustion processs using glycine as a fuel and the size of nanoparticles was found to be 30–40 nm and was obtained by calcinations at 950 °C for 6 h. The above conventional combustion methods have some disadvantageous such as long reaction time, required high temperature for calcinations and low production rates.

Among all the above techniques, microwave combustion method is mostly used to prepare nanocrystalline materials, due to its simplicity, short reaction time from the preparation of reagents to the end product, elimination of intermediary calcinations stages, low cost, energy consuming and its ability to produce large volume products. During the microwave combustion method, the synthesis time is reduced up to 60 times less than the required time in other conventional methods [26]. Sertkol et al. [27] have synthesized Ni–Zn ferrite and Koseoglu et al. [28] have synthesized Co-doped ZnFe₂O₄ using the microwave assisted combustion method. The nanocrystalline core shell Zn_0.7Ni_0.3Fe₂O₄ has been prepared by the microwave assisted synthesis by Sertkol et al. and a superparamagnetic behavior at room temperature was reported [29]. Generally, the combustion processes are depending on the nature of the organic fuel. In this present study we have used urea as a fuel. When compared with other organic fuels, urea is very cheap and easily available. Hence, many researchers have used urea as a fuel in the microwave combustion method [28], [30], [31].

The heating mechanism in microwave combustion processes is fundamentally different from other conventional combustion process. In a microwave oven, due to the interaction of microwaves with the material, microwave radiation is absorbed and converted to thermal energy. In this method, heat is generated from inside the material, in contrast with the conventional heating methods where heat is transferred from the outside. This internal rapid heating allows a reduction of processing time and energy. The use of microwave energy as heating sources for the combustion reaction has many advantages, such as, fast reaction kinetics, cleanness and efficiency. In terms of economical and ecological aspects of the process, the costs reduction with respect to the energy and time, yields high-purity, early phase formation and homogenous fine particles with less energy consuming. We reported earlier, a facile approach to control the size and the magnetic properties of Cu and Sr doped ZnFe₂O₄ nanoparticles by microwave combustion route using urea as a fuel [30], [31].

In this paper, we report a microwave combustion method for a fast and reliable preparation of uniformly distributed pure and Co-doped ZnFe₂O₄ nanoparticles without any additional calcinations and reduction steps. The addition of Co²⁺ ions into ZnFe₂O₄ affects the lattice parameter, crystallite size and influences, magnetic properties as well as its Curie temperature. Therefore, the objective of this study was to investigate the effect of the urea as a fuel on the synthesis of Co-doped Zn ferrites by microwave combustion method, and to evaluate their structural, morphological, optical and magnetic properties.