Synthesis and characterization of NiFe2 O4 nanoparticles by tartaric acid assisted sol-gel auto-combustion method

Nickel ferrite nanoparticles were synthesized by modified sol-gel auto-combustion method. The final product obtained was annealed at 600 °C. The thermal, structural, morphological, vibrational, optical and magnetic properties were studied using TGA, XRD, SEM, FTIR, UV-DRS and VSM techniques, respectively. The thermo gravimetric analysis shows three step decomposition of residual nitrate and tartrate complexes with a total weight loss of 6.69% below 600 °C. Further, there was no significant weight loss above 600 °C. XRD confirmed the spinel cubic phase with Fd3m space group. The average crystallite size was 14 nm. An agglomerated random morphology with porous flake like microstructure was observed. The vibrational analysis confirmed the presence of Ni-O, Fe-O stretching vibrational modes in the wave number range 400-600 cm−1. Tetrahedral site (kTd) and octahedral site (kOh) force constant values were 2.20×105 and 1.31×105 dynes/cm, respectively. From UV-DRS spectra, the Kubelka-Munk function was used to calculate the optical band gap and it was 1.71 eV. The magnetic saturation, retentivity and coercive field values: Ms = 22.35 emu/gm, Mr = 5.20 emu/gm, Hc = 165 Gauss were obtained from VSM analysis. The calculated magnetic moment value was 1.115 μB. The law of approach to saturation was employed to analyze the VSM data in saturation region. The calculated magneto-crystalline anisotropy constant, K1 value was 0.857×105 erg/cc.


Introduction
Ferrites have received considerable attention among the Scientists and Researchers.They continue to be the subject of technical research due to their remarkable chemical and thermal stability, as well as their structural, magnetic, electrical, optical, and catalytic properties.They find several applications in technological fields: magnetic hyperthermia, catalysis, permanent magnets, EMI shielding, sensors, waste water treatment, energy storage and microwave devices.The type of the ions, their charges, and the distribution of cations among tetrahedral (T d ) and octahedral (O h ) sites significantly impact the characteristics of ferrites nanoparticles.
Nickel ferrite is technologically valuable soft ferrite material with an inverse spinel structure, and it has n-type semiconducting nature.In NiFe 2 O 4 , Ni 2+ ions will be distributed among available O h -sites and Fe 3+ ions will be evenly distributed among both T d -and O h -sites.In recent years, preparation of ferrite nanoparticles with different preparation conditions and their effects on structure and morphology has been explored [1][2][3][4].In order to produce stoichiometric and chemically pure spinel ferrite nanoparticles, widely used chemical methods, such as hydrothermal [5], microwave combustion [2], combustion [3], co-precipitation [1] and sol-gel technique [6] have been developed.Because of its simple preparation procedure, easy to manage stoichiometry, good chemical homogeneity, high purity and crystallinity with minimal processing time, the sol-gel combustion method is one of the most often utilized synthesis approaches [7].The process of synthesis is easy, involving an exothermic reaction between metal salts and fuels.Appropriate fuels like citric acid [8], ethylene glycol, urea, glycine [6], aloe-vera [9] and honey [10] have been used.

Synthesis
A modified sol-gel auto combustion method was used for synthesizing NiFe 2 O 4 nanoparticles.The procured chemicals are of analytical grade.The stoichiometric proportion of Nickel nitrate hexahydrate, Iron nitrate nonahydrate and L-tartaric acid (TA) in the molar ratio 1:2:4 were taken.In 100 ml of de-ionized water, Ni(NO 3 ) 2 .6H 2 O and Fe(NO 3 ) 3 .9H 2 O were dissolved.Then, aqueous solution of tartaric acid was added to nitrate solution.EG was added in 70:30 (TA:EG) ratio.The solution was stirred for 1 hr at 70 °C.The mixture was next neutralized with aqueous Ammonia and heated to 100 °C until the solution turned into a gel.The gel underwent an auto-combustion reaction after which fluffy powder was produced.It was then dried for 1 hr at 200 °C in a Muffle furnace.The obtained yield was pulverized in an Agate Mortar and Pestle.Then, the powder was annealed for 2 hr at 600 °C to obtain Nickel ferrite nanoparticles.It was designated as NF-600.

Characterization
Thermal analysis was carried out to understand the thermal stability of the sample using Thermo Gravimetric Analyser (TGA Q50 Series).PANalytical X'Pert MPD X-ray diffractometer with CuK α radiation (λ=1.5406Å) was used to record XRD (X-day diffraction) data for identifying the structure and phase.To investigate the surface morphology, a ZIESS Scanning Electron Microscope (SEM) was employed.Fourier Transform Infrared (FTIR -Agilent Technologies Cary 600 Series) spectra was recorded to analyze the vibrational bands.UV-DRS data was recorded in reflectance mode to study the optical properties using Perkin Elmer UV-Vis Spectrometer-Lambda 750.To examine magnetic characteristics, A Lakeshore 665 Vibrating Sample Magnetometer (VSM) was used to record the M-H curve.

Thermo gravimetric analysis
As-prepared Nickel ferrite powder was subjected to thermo gravimetric analysis between 27 ˚C and 850˚C with a ramping rate of 10 ˚C/minute.Figure 1(a) shows the TGA curve of noncalcined Nickel ferrite.The weight loss was noticed up to 600 ˚C in three steps.In the first step, the decomposition continued upto 120 ˚C due to the adsorbed water content amounting to 3.04% weight loss.In the second step, the decomposition continued upto 300 ˚C corresponding to residual nitrates and ethylene glycol precursors with 1.84% weight loss.In the third stage, 1.81% weight loss, observed at higher temperature from 300˚C to 600 ˚C was due to the decomposition of residual tartrate precursors.Therefore, the total weight loss observed was ~ 6.69%.Further, the as-prepared Nickel ferrite sample was calcined at 600 ˚C for 2 hr.Thus, the weight loss up to 600 ˚C indicates oxidation of metal ions and formation of ferrite phase [1,3], which was confirmed by XRD, FTIR analysis and are shown in figures 1(b) and (d), respectively.

XRD analysis
Figure 1(b) depicts an XRD pattern of Nickel ferrite sample annealed at 600 °C.The observed Bragg peaks match well with the standard crystal structure ICSD data (00-010-0325).A cubic spinel phase (Fd3m space group) was observed.The high crystallinity of the sample is shown by the strong and distinct reflection peaks.The absence of secondary peaks represent the sample's phase purity.The average crystallite size (D) was calculated using the Debye-Scherrer formula and the structural parameters such as lattice constant (a), interplanar spacing (d hkl ), cell volume (V), dislocation density (δ), and x-ray density (ρ x ) were determined [11].Table 1 summarizes the computed structural parameters.The reported results agree well with the literature values [1,12].14.00 δ (nm -2 ) 0.0051 ρ x (gm/cc) 5.420

Morphological analysis
The scanning electron micrograph of NF-600 is shown in figure 1 (c).An agglomerated random morphology with porous flake like microstructure was observed.The synthesis conditions may have an impact on the microstructure of the nanoparticles.In addition, pores and voids that have been formed because of the emission of gases that were released during combustion reaction may be noticed [13].

FTIR analysis
The FTIR spectra of spinel ferrite show two primary absorption bands in the 400-600 cm -1 region.The lower wavenumber band (v 2 ~400 cm -1 ) is assigned to intrinsic stretching vibrations of metal ions present at the O h -site.The higher wavenumber band (v 1 ~600 cm -1 ) is related to the intrinsic stretching vibrations of the metal ions at the T d -site [2].The absorption occurs at different frequencies depending on the mass of metal cations and strength of the bond between metal cations and oxygen [14].The recorded infrared spectra of NF-600 sample in the 400-4000 cm -1 wave number range is shown in figure 1(d).Inset in figure 1(d) displays magnified IR spectra in the 400-700 cm -1 range.The absorption bands (table 2(a)) at v 1 -550 cm -1 , v 2 (1)-408 cm -1 , v 2 (2)-423 cm -1 , and v 2 (3)-470 cm -1 are assigned to the intrinsic stretching vibrational modes of metal cations with oxygen ions ((Fe 3+ -O 2-) A , (Fe 3+ -O 2-) B and (Ni 2+ -O 2-) B ) present at T d (A) and O h (B) -sites, respectively.The related bands at 1363 cm -1 and 3349 cm -1 were also seen due to the stretching vibrations of the C-O bond and O-H group due to the residual traces and water molecules adsorbed on the surface of the sample.The vibrational bands that have been detected agree well with the values that have been reported [9].The values of the force constants were calculated using the formula: k=4π 2 cν 2 µ, where, c is the speed of light, ν is the wavenumber (frequency of ions at the T d and O h -sites) and µ is the reduced mass for (M-O) Metal-Oxygen bonds [15].Tetrahedral site (k Td ) and octahedral site (k Oh ) force constants (table 2 (b)) were found to be 2.20 ×10 5 and 1.31 ×10 5 dynes/cm, respectively.

Optical analysis
The optical characteristics of the NF-600 sample was studied using UV-Visible diffused reflectance spectra.Figure 2 (a) depicts the reflectance spectra of NF-600 from 200 to 850 nm.Maximum reflectance was detected in the 200-300 nm wavelength region, with a significant rise in 700-850 nm wavelength region.As shown in figure 2 (b), the optical energy band gap (E g ) was determined by plotting the Kubelka Munk function (F(r) hʋ) 2 v/s Energy and extrapolating the linear portion of the curve to F(r) 2 =0.Typically, ferrite structures have two bands: an O-2p orbital at the valance band and a Fe-3d orbital at the conduction band [18].The obtained direct energy band gap value of 1.71 eV is substantially within the range of previously reported values [8,19,20].As a result, it can be concluded that NiFe 2 O 4 nanoparticles are optically active in the visible region.

VSM analysis
The magnetic properties of NF-600 sample was studied by VSM at RT at 1.5 Tesla applied magnetic field.Based on the hysteresis curve shown in figure 3

c)
magnetization, N A = 6.022×10 23 mol -1 (Avagadro's number) , β= 9.27×10 -21 (conversion factor) and the magnetic moment was estimated to be 1.115µ B [8,21].The magnetic measurements data at higher magnetic field (~above 1 Tesla) are fitted to the law of approach in order to comprehend the magnetic anisotropy of NF-600 sample.The law of approach method (LA) was introduced to analyze the data in the saturation part.LA denotes the dependency of magnetization M on the applied field for H>>H c .The higher magnetic field parts (above 1 Tesla), the M-H curve is fitted to the following equation : , where, (cgs system), M-magnetization, H-applied field, M s -magnetic saturation, K 1 -magneto-crystalline cubic anisotropy constant and the term kH-forced magnetization.8/105-the numerical co-efficient is used to describe the cubic anisotropy of random polycrystalline samples.Typical curve fit to LA is shown in figure 3 (b).Using the M s and b values obtained from LA fitting, the K 1 value was 0.857×10 5 erg/cm 3 , is in accordance with the inverse spinel ferrites [15,[22][23][24].

Conclusion
NiFe 2 O 4 nanoparticles were successfully prepared using a modified sol-gel autocombustion method with tartaric acid and ethylene glycol.TGA investigation revealed that the remaining precursors decomposed up to 600 °C, implying that the sol-gel auto-combustion process occur at lower temperatures to produce spinel ferrite phase.XRD investigation revealed the formation of a single phase cubic spinel structure with a lattice constant of 8.312 Å and average crystallilte size, 14 nm.An agglomerated random morphology with a porous flake-like microstructure was found.The observed FTIR characteristic absorption bands of the Nickel ferrite phase were in the 400-600cm -1 range.Kubelka-Munk function was used to estimate the optical energy band gap and was found to be 1.71 eV.The VSM study of produced Nickel ferrite reveals ferrimagnetic behavior with a saturation 1.122×10 6 K 1 (erg/cm 3 ) 0.857×10 5 magnetization of 22.35 emu/gm.The law of approach method was used for analyzing VSM data in the saturation region.The calculated magneto-crystalline anisotropy constant, K 1 value was 0.857×10 5 erg/cc.