Synthesis and characterization of zinc doped nickel nanoferrites via sol-gel auto-combustion technique

Over the last few decades, numerous researchers have made significant contributions to the development of spinel ferrite owing to its unique and versatile structural, spectroscopic, and magnetic characteristics. These ferrites are extensively used in various applications such as microwaves, drug delivery, gas sensors, and electronic devices. Their technological applications require low porosity, high density, and specified microstructure. In this work, we prepared Zinc-doped Nickel nanoferrites Ni1-xZnxFe2O4 (x= 0, 0.1, 0.2, 0.3) using the sol-gel auto-combustion technique. Structural analysis of prepared samples was done using X-ray diffraction and Scanning Electron Microscopy, the elemental composition was examined using EDAX and optical studies using UV-visible spectroscopy. The average crystallite size was found in the 40.6 to 64.92 nm range. EDAX analysis confirmed the presence of expected elements in the samples. From UV-visible spectroscopy characterization studies it was observed that as particle size increases, there will be an increase in the band gap energy of ferrite nanoparticles. The Nickel-Zinc nanoferrite absorbs energy in the range of 233nm to 237nm.


Introduction
In recent decades, spinel ferrites have been widely studied by scientists and researchers owing to their exceptional structural, magnetic and spectroscopic characteristics [1].These ferrites have significant implications in numerous fields, such as microwaves [2], drug delivery [3], gas sensors [4], and electronic devices [5].To be helpful in these applications, they require specific microstructures, low porosity and high densities [6].The examined nanoferrite generates spinel crystals that belong to the Fd3m space group.The octahedral and tetrahedral positions in their lattice surroundings are occupied by divalent A and trivalent B cations, respectively [7].Based on the characteristics of spinel structures, it is possible to distinguish between nickel zinc ferrites, zinc and nickel ferrites, with each having inverse spinel, tetrahedral and normal spinel positions correspondingly.In large nickel-zinc ferrite systems, zinc ions are more likely to occupy tetrahedral positions, while nickel ions tend to occupy the octahedral position.However, recent studies validate that even in nanocrystalline form, a minor proportion of nickel and zinc ions can interchange their positions, resulting in the opposite occupation of octahedral and tetrahedral positions [8].Nickel-zinc spinel ferrites have exceptional performance at high frequencies, exhibit superior magnetic permeability, and have low electrical conductivity [9].Various techniques have been employed to prepare nickel-zinc nanoferrite, including sol-gel, hydrothermal approach, microwave combustion, etc., [10].Among these methods, the sol-gel technique stands out as a remarkably convenient and efficient method, having demonstrated a proven ability to achieve small and uniformly-sized particles, while maintaining high energy efficiency and purity.Moreover, it offers a high degree of composition, making it an ideal choice for preparing nickel-zinc ferrite nano-particles in a cost-effective manner, as we have adopted to do in this work.

Materials and Methods
The chemical precursors used in the reaction were Ferric nitrate nonahydrate (Fe(NO3)3).9H2O),Nickel nitrate hexahydrate (Ni(NO3)2.6H2O),Zinc nitrate hexahydrate (Zn(NO3)2.6H2O),Glycine (C2H5NO2) and distilled water.The sol-gel auto-combustion method was employed to prepare Zinc doped Nickel Nanoferrites Ni1-xZnxFe2O4 (x= 0, 0.1, 0.2, 0.3).The stoichiometric ratio of Nickel nitrate, Ferric nitrate, Zinc nitrate, and Glycine were weighed with a 1:3 molar ratio (Metal nitrates: Fuel) and all the nitrates and fuel were mixed in a certain quantity of deionised water and stirred for 30 minutes to get a homogeneous solution.The pH of 7 was maintained by adding ammonia dropwise.Upon raising the temperature to 90°C, the brown viscous gel began to develop.The gel undergoes auto-combustion and forms a foam-like structure.Further, the foam is converted into ash, and it is cooled then poured into a mortar and grinded.The obtained powder was sintered at 600 0 C for about 6 hrs using a muffle furnace [11].

Characterization
The samples were analysed by a Panalytical X-PERT Pro MPD instrument to record their X-ray diffractograms.The samples were scanned from 0 to 80 degrees with a scanning speed of 5 degrees/min and a step size of 0.02 degrees.The morphology of the samples were studied by a scanning electron microscope (SEM) model S-3400.UV-Vis absorbance investigations were examined in the 200-800 nm range using a carry 5000 UV-Vis-NIR spectrophotometer.

Scanning Electron Microscopy
The Scanning electron micrographs of the zinc-doped nickel ferrite composites are displayed in Figure 3.It indicates that the surface of the ferrite sample has several tiny pores, which are due to the numerous gases released during the combustion process, and it is a characteristic of the combustion synthesis.Due to high surface energy and tendency to aggregate, pore-free crystallites form on the surface [12].

Elemental Analysis
The EDAX spectra of zinc doped nickel ferrite composites are shown in Figure 4.It explains the elemental results for a sample containing significant amounts of Iron (Fe), Nickel (Ni), Zinc (Zn) and oxygen.Although these primary magnetic elements are present in the sample, their weight percent varies in concentration [13].The elemental weight percent's for each sample is also recorded.The individual elements present in a sample can be identified from their scan position and intensity in the spectra.The higher the intensity, the more will be concentration of the element in the sample.The data obtained from these test results are used to identify and illustrate differences in material composition, as displayed in Table 1.

UV Spectroscopy
Figure 5 shows UV-Visible absorption spectra of zinc doped nickel ferrite as a function of wavelength.The band gap energy was estimated using Tauc's plot (Figure 6).For a direct bandgap, Tauc's equation states that the absorption coefficient close to the band edge is [14]: where, depending on the kind of transition, A is constant, Eg is the band gap energy, hυ is the photon energy, α is the absorption coefficient.
For each sample, the band gap energy has been computed from the straight line intercept at α = 0 and is given in table 2. From the Figure 5 it was observed that the Zinc doped Nickel nanoferrites exhibits energy absorption in the range of 233-237 nm.

Conclusions
Zinc doped nickel ferrite nanoparticles have been prepared by employing sol gel auto combustion technique with glycine as fuel, and the samples were annealed at 600 0 C for 6 hours.XRD confirmed the development of spinel structure and single phase of the nickel-zinc ferrite nanoparticle samples.The average crystallite sizes of the prepared samples varies from 40.6 to 64.92 nm.SEM images shows a number of fine pores which are attributed to the large number of gases liberated during the combustion process.The presence of expected elements was confirmed by EDAX analysis.Additionally, UV-Visible spectroscopy demonstrated that the increase in particle size directly corresponds to an increase in band gap energy.Interestingly, the Nickel-zinc nanoferrite exhibits energy absorption in the range of 233-237 nm.

Figure 1 .
Figure 1.Flowchart of synthesis procedure Ni1-xZnxFe2O4 ferrite nanocomposites.3.CharacterizationThe samples were analysed by a Panalytical X-PERT Pro MPD instrument to record their X-ray diffractograms.The samples were scanned from 0 to 80 degrees with a scanning speed of 5 degrees/min and a step size of 0.02 degrees.The morphology of the samples were studied by a scanning electron microscope (SEM) model S-3400.UV-Vis absorbance investigations were examined in the 200-800 nm range using a carry 5000 UV-Vis-NIR spectrophotometer.

Table . 1
. Atomic weight percent of Zinc doped nickel ferrite nanocomposites.

Table 2 .
Band gap energies of Zinc doped nickel nanoferrites