Synthesis of Nickel Zinc Iron Nanoparticles by Coprecipitation Technique

Nanoparticles nickel ferrite of size 6 – 8 nm, synthesized by the coprecipitation method with x varying from 0 to 1.0. The powder samples were characterized by XRD, VSM and FTIR. The average crystallite sizes of the particles were determined from X-ray diffraction. X-ray analysis showed that the samples were cubic spinel. The lattice constant (a o ) increased with the increase in zinc substitution. The specific saturation magnetization (M S ) of the particles was measured at room temperature. The magnetic parameter of M S , was found to decrease with the increase in zinc substitution. Fourier transform infrared spectroscopy (FTIR) spectra of the Ni 1-x Zn x Fe 2 O 4 with x = 0, 0.5, 1 in the range 400 – 4000 cm were reported. The spinel structure and the crystalline water adsorption of Ni 1-x Zn x Fe 2 O 4 nanoparticles were studied by using FTIR.


Introduction
Magnetic properties of nanometer sized particles have attracted considerable attention in recent years because of their unique properties. For the preparation of magnetic fluids, nanoparticles with a particle size of order of 10 nm and with a narrow size distribution were required 1 . However, the size of the magnetic particle was decreased below a critical length, domain formation was no longer energetically favored and the particles existed as a single domain 2 . Magnetic nanoparticles have aroused increasing interest among researchers of various fields due to their extensive applications such as information storage system, medical diagnostics, ferrofluid technology, etc., [3][4][5][6][7] . This is mainly because the properties of nanoparticles differ from those of the corresponding bulk material [8][9] . Zinc substituted mixed ferrites (Ni-Zn) chosen here is highly sensitive to temperature. Ferrofluids constituted by these ferrites may be good candidates to be used as liquid carriers in heat exchangers using magneto caloric energy conversion 10,11 . It is well known that the magnetic property can be altered by the addition of zinc. Addition of zinc also affects the lattice parameter (a 0 ). Various preparation techniques, such as sol-gel pyrolysis method 12 hydrothermal technique 13 and mechanical alloying 14 has been used to prepare ferrite nanoparticles. But coprecipitation method is considered to be an economical way of producing fine particles 15,16. The physical properties of nanoparticles are of current interest due to the size-dependent behavior observed in the nanometer length scale and high crystallinity. Preparation and properties of Ni-Zn ferrites have been reported only for a particular value of x or limited values. Jeyadevan et al. 17 successfully prepared nearly monodispersed single-domain Nickel ferrite particles, which could be used for the high-density recording media. Morais et al. 18 reported the possibility of controlling the size of nanoparticles using different stirring speeds. Temperature sensitive magnetic fluid having Ni 0.3 Zn 0.7 Fe 2 O 4 particles was used for the study of thermal convection 10 . Hydrocarbon based Nickel ferrofluid was also reported 19 . A.kale et al. 20 synthesized Ni 1-x Zn x Fe 2 O 4 nanoparticles with x varying from 0.1 to 0.5. To the best of our knowledge complete range of Ni 1-x Zn x Fe 2 O 4 with x varying from 0 to 1.0 have been reported for nano particles. In the present work, we study the Ni 1-x Zn x Fe 2 O 4 with x varying from 0 to 1.0. The influence of zinc substitution on the crystallites size and magnetic properties depends on the preparation condition also. Here we report the modified preparation condition for the production of fine particles of Ni 1-x Zn x Fe 2 O 4 with x varying from 0 to 1.0. Structural and magnetic properties of Ni-Zn ferrite with x varying from 0 to 1.0 have also been discussed.

Synthesis of Ni 1-x Zn x Fe 2 O 4 nanoparticles
The magnetization of substituted ferrite nano particles synthesized by coprecipitation depends mostly on parameters such as reaction temperature, pH of the suspension, initial molar concentration etc. 21 . Ultra fine particles of Ni 1-x Zn x Fe 2 O 4 with x varying from 0 to 1.0 were prepared by co-precipitating aqueous solutions of NiCl 2 , ZnCl 2 and FeCl 3 mixtures respectively in alkaline medium. The mixed solution of NiCl 2 , ZnCl 2 and FeCl 3 in their respective stoichiometry (100 mL of 0.5 M NiCl 2 , 100 mL of 0.5 M ZnCl 2 and 100 mL of 2 M FeCl 3 in the case of Ni 0.5 Zn 0.5 Fe 2 O 4 and similarly for the other values of x) was prepared and kept at 60 °C. This mixture was added to the boiling solution of NaOH (0.63 M dissolved in 1200 mL of distilled water) within 10 seconds under constant stirring. Nano ferrites are formed by conversion of metal salts into hydroxides, which take place immediately, followed by transformation of hydroxides into ferrites. At first solid hydroxides of metals in the form of fine particles were obtained by the coprecipitation of metal cations in alkaline medium (coprecipitation step) (Equations 1 and 2).
. 3 The solid solution of metal hydroxides was transformed to complex zinc substituted ferrites when subjected to heating in the alkaline medium (ferritization step).
The solutions were maintained at 85 °C for one hour. This duration was sufficient for the transformation of hydroxides into spinel ferrite (dehydration and atomic rearrangement involved in the conversion of intermediate hydroxide phase into ferrite) 21 . Sufficient amount of fine particles were collected at this stage by using magnetic separation. These particles were washed several times with distilled water followed by acetone and dried at room temperature.

Particle characterization
The X-ray diffraction (XRD) patterns of the samples were recorded on a Philips ® PANALYTICAL X' PERT PRO X-ray powder diffractometer using Cu K∝(λ = 1.54060 Å) radiation. Slow scans of the selected diffraction peaks were carried out in step mode (step size 0.05°, measurement time 5 seconds, measurement temperature 25 °C, standard: Si powder). The crystallite size of the nanocrystalline samples was measured from the X-ray line broadening analyses using Debye-Scherrer formula after accounting for instrumental broadening (Equation 3): where λ -wavelength of X-ray used in Å, b -FWHM in radians in the 2θ scale, θ -the Bragg angle, D XRD -crystallite size in nm 22 . The lattice constant (a o ) was determined for various values of zinc content. Ni-Zn ferrite has a spinel structure 20 .

Magnetic measurements
Room temperature magnetic measurements with a maximum magnetic field of 1194.15 kA.m -1 were carried out using a Lakeshore vibrating sample magnetometer (VSM) (model 7404) and parameters like specific saturation magnetization (M s ), coercive force (H c ) and remanence (M r ) were evaluated.

Spectral measurements
FTIR spectra were recorded for the dried samples of Ni 1-x Zn x Fe 2 O 4 with x varying from 0 to 1.0 with an ABB BOMEM 104 FTIR (range 400 -4000 cm -1 ) spectrometer. The dried samples were in KBr matrix, and spectra were measured according to transmittance method. The spectra were resolved with a resolution of 4 cm -1 .

Physical characterization
Generally, XRD can be used to characterize the crystallinity of nanoparticles, and it gives theaverage diameters of all the nanoparticles. The precipitated fine particles were characterized by XRD for structural determination and estimation of crystallite size. XRD patterns were analyzed and indexed using powder X software 23 . All experimental peaks were matched with the theoretically generated one and indexed. The lattice constant (a o ) was computed using the 'd' value and with their respective (h k l) parameters. Analysis of the diffraction pattern confirms the formation of cubic spinel structure for all the samples. The strongest reflection comes from the (311) plane, which denotes the spinel phase. All the compositions had a spinel structure. The peaks indexed to (220), (311), (400), (422), (511) and (440) planes of a cubic unit cell, correspond to cubic spinel structure. The calculated lattice constant (a o ), identified the samples to be cubic spinel and was refined using Powder X. The XRD pattern for Ni 1-x Zn x Fe 2 O 4 with x = 0, 0.2, 0.4, 0.6, 0.8 and 1 is shown in Figure 1.The peaks of (220), (311), (400), (422), (511) and (440) have been deconvoluted to Lorentzian curves, using Peak Fit software for the determination of FWHM value of the indexed peaks 24 . The lattice constant was found to increase from 8.212 to 8.462 Å with the increase in zinc concentration. The lattice constant (a o ) increased with the increase in Zn content, which suggested the formation of a compositionally homogeneous solid solution and was found to be within the range of the lattice constants of ZnFe 2 O 4 and NiFe 2 O 4 . This increase could be also attributed to the substitution of the large sized Zn cation for the small sized Ni cation. The lattice constant obtained for NiFe 2 O 4 (8.384 Å) is close to that known of bulk NiFe 2 O 4 (8.395 Å) [3] . The lattice constant for Ni-Zn ferrites reported by R. Anantharaman et al. 25 is very close to the present experimental values. Figure 2 shows that the lattice constant increases with increase in zinc concentration. The crystallite size (D XRD ) was estimated bythe Debye -Scherrer formula 23 using the full width at half maximum value of the respective indexed peaks. The average crystallite size (D aveXR ) decreases from 8.95 to 6.92 nm when the partial substitution of zinc increases (x = 0 to x = 1.0). This value is close to the crystallite size reported by A. Kale et al. 20 . Though all the samples were prepared under identical condition, the crystallite size was not the same for all Zn concentrations. This was probably due to the preparation condition followed here which gave rise to different rate of ferrite formation for different concentrations of zinc, favoring the variation of crystallite size. The variation of average crystallite size with the zinc concentration is given in Figure 3. Ferrofluids can be conveniently prepared by making use of particles in this size range.

Magnetic measurements
In the cubic system of ferrimagnetic spinels, the magnetic order is mainly due to a super exchange interaction mechanism occurring between the metal ions in the A and B sublattices. The substitution of nonmagnetic ion such as Zn, which has a preferential A site occupancy results in the reduction of the exchange interaction between A and B sites. Hence, by varying the degree of zinc substitution, it is possible to vary magnetic properties of the fine particles. Figure 4 shows the room temperature hysteresis loop of the powder samples for various zinc substitutions. From Figure 4 it can be seen that the variation pattern of specific saturation magnetization (M s ) as a function of Zn content shows an increase for Zn substitutionsand reaches a maximum value of 48.72 Am 2 .Kg -1 at 1194.15 kA.m -1 for x = 0. The changes in the specific saturation magnetization with the degree of zinc substitution  are given in Figure 5. It is clear from the Figure 4 that the particles do not show any saturation for x = 0.9 and 1.0.even at 1194.15 kA.m -1 and it almost behaves linear. The changes in magnetic property of M s , is due to the influence of the cationic stoichiometry and their occupancy in the specific sites. In addition, formation of dead layer on the surface, existence of random canting of particle surface spins 24,26 , non saturation effects due to random distribution of particle size, deviation from the normal cation distribution, presence of adsorbed water etc., 11 might be the cause for the reduction of magnetic properties of nanoparticles. Reaction temperature of 85 °C and 12.0 pH were used to synthesis fine particles Ni 1.0 Fe 2 O 4 (8.95 nm in size) having highest specific saturation magnetization of 48.72 Am 2 .Kg -1 . The deviation of cation distribution in nanoparticles from the bulk, influences the temperature dependence of magnetization. The hysteresis curve ( Figure 4) recorded at room temperature shows very low remanence, coercivity for large zinc concentration proves that the particles are super paramagnetic at room temperature.

Spectral measurements
The FTIR spectra for Fe 3 O 4 and for Ni 1-x Zn x Fe 2 O 4 with x = 0, 0.5 and 1 are shown in Figure 6. By overlaying the FTIR spectra for Fe 3 O 4 and for Ni 1-x Zn x Fe 2 O 4 with x = 0, 0.5 and 1.0, the spectral similarities are observed. The broad feature between 3441.43 -3219.90 cm -1 is due to O-H stretch which corresponds to the hydroxyl groups attached by the hydrogen bonds to the iron oxide surface and the water molecules chemically adsorbed to the magnetic particle surface (associated water content) 27 . From these results, it appears that the hydroxyl groups are retained in the samples during the preparation of the uncoated Ni 1-x Zn x Fe 2 O 4 spinel ferrites prepared by coprecipitation method. Ghose et al. 28

Conclusion
Preparation technique suitable for preparation of Ni-Zn substituted ferrite nanoparticles is reported. Ni 1-x Zn x Fe 2 O 4 nanoparticles can be prepared by the coprecipitation method for the full range of composition with x varying from x = 0 to1. The formation of Ni 1-x Zn x Fe 2 O 4 was confirmed by the X-ray diffraction. The lattice constant was found to increase with the increase in zinc concentration. The average crystallite size (D aveXR ) decreased when the partial substitution of zinc increased (x = 0 to x = 1.0). The specific saturation magnetization was found to decrease with the increase in zinc substitution. Low coercivity was observed for the high Zn concentration for the Ni-Zn ferrite nanoparticles. In the present work, the samples prepared by coprecipitation method showed superparamagnetic behavior, which was documented by the hysteresis loop, measured at room temperature. Here the NiFe 2 O 4 prepared by coprecipitation method shows that it is a not very hard magnetic material since the hysteresis loop is very small and it is known that ZnFe 2 O 4 is a soft magnetic material. This soft-magnetic nanoparticles have interesting applications, in magnetic coatings and in the preparation of Ferrofluids. FTIR was used to confirm the formation of Fe-O bonds and presence of the associated water content in the samples.