Synthesis of Spinel Nanocrystalline ZnFe2O4: Structural, Optical, Magnetic and Electrical properties.

The present paper deals with the synthesis of Zinc Ferrite (ZnFe 2 O 4 ) nanoparticles using simple, low cost co - precipitation method and study the structural, optical, magnetic and electrical properties. The samples have been prepared at two different calcination temperatures of 400˚ C and 600˚ C. The XRD studies indicate that the synthesized ZnFe 2 O 4 samples exhibit cubic spinel structure and that the grain size increases with calcination temperature. The FTIR spectra demonstrate bands associated with metal – oxygen bonds in the tetrahedral and octahedral sites. Moreover, no unwanted impurity such as nitrate has been detected in the synthesized samples revealing their high purity. The micro structural analysis shows that though most particles are irregular granular ones, some are spherical. The optical band gap energy is found to increase with increasing calcination temperature. The Vibrating Sample Magnetometer (VSM) studies reveal the super paramagnetic behavior of the synthesized samples, even for a high Fe: Zn ratio of 1:2. The electrical resistivity of the samples decreases with increase in calcination temperature.This phenomenon has been explained on the basis of the Verwey-de Boer mechanism.


Introduction
In the recent years, many research groups are focusing their attention on magnetic oxide nanomaterials because of their innumerable and innovative applications. Ferrites are a class of magnetic oxide materials which have the structure AB 2 O 4 ; in this structure, normally, A occupies tetrahedral, B occupies octahedral cation sites respectively and O occupies the Oxygen anion site [1][2][3]. Transition metal ferrites have the general molecular formula MFe 2 O 4 (where M = Ni, Zn, Mn, Co, Mg etc) and they possess a facecentered cubic (fcc) close packing structure. They are considered as an important group of technological materials because of their properties such as high electrical resistivity, high chemical stability, high saturation magnetization, low eddy current loss, low dielectric loss, ease of preparation, price and performance considerations, good reusability and environmentally friendly nature etc. These properties made them suitable for many applications in magnetic, mechanical, electronic, catalytic and microwave devices [4][5][6][7][8].
Among the transition metal ferrites, the zinc ferrite (ZnFe 2 O 4 ) has been studied extensively due to its different structures composed of normal, inverse and mixed spinel structures and its high electromagnetic performance, excellent chemical stability, mechanical hardness, low coercivity and moderate saturation magnetization, which makes it a good contender for the applications such as soft magnets, low-loss materials at high frequencies, photo catalysis, magnetic resonance imaging(MRI), Li-ion batteries, absorbent materials, biosensors and gas sensors [1,2,[9][10][11][12][13][14][15].The zinc ferrite nanoparticles exhibit unique structural, morphological, opto-electrical, magnetic and photocatalytic activities which are quite different from that of their bulk counterparts, due to their smaller particle size and higher surface area [9].Therefore, controlled synthesis of nanometer sized materials is of great importance for investigating the structure -property relationship of the zinc ferrite nano particles. So far, various methods, such as sol-gel [16,17],co-precipitation [18,19], solution combustion [20,21],thermal decomposition [22,23], solvothermal [24,25],hydrothermal [26,27],ball milling [28] and ceramic route techniques [29] have been used for synthesizing ZnFe 2 O 4 nanoparticles possessing different structures.
Among these methods, the co-precipitation method is simple and straight forward, low coast, non toxic, easy control of p H and capable of yielding crystalline nano materials in short time. Hence this method is chosen for preparing the samples. In the present work, it is envisaged to synthesize ZnFe 2 O 4 nanoparticles through the co-precipitation method and study their structural, morphological, optical, magnetic and electrical properties. The Stoichiometric proportion of zinc nitrate hexahydrate (0.594 g ) and ferric nitrate nonahydrate (1.616 g) have been dissolved in 100 mL of deionized water in two separate beakers. Each solution has been stirred for 30 min at 60˚C using a magnetic stirrer to obtain a homogenous solution. These two solutions are labeled as A and B .The solution B is added drop by drop using a burette to the solution A contained in the beaker and the resultant solution is stirred well for 30 min at 60˚C. This resultant solution is labeled as C. Similarly, 3.644 g of CTAB has been dissolved in 100 mL of deionized water in a beaker and stirred well at 60˚C for 1 h. to get a clear solution. This solution has been labeled as D. The solution D has been added drop by drop using a burette to the solution C contained in a beaker and the resultant solution is stirred well for 30 min at 60˚C and this solution has been labeled as E.0.8M NaOH solution has been prepared using 50 mL deionized water and this solution has been added drop by drop to the solution E simultaneously stirring it until its P H reaches 10. At this stage, brown colored precipitate has been formed in the solution. The solution in the beaker has been left undisturbed until the entire precipitate settles down at the bottom of the beaker. The solution has been drained out and the precipitate is washed 4 times using deionized water. The beaker containing the precipitate has been kept in an oven for 8h. at 80˚C for drying. The dried precipitate has been ground in to a ne powder using an agate mortar and pestle. This powder has been calcined at two different temperatures of 400˚C and 600˚C using a furnace for 2h. in an atmosphere of air.

Materials And
For the purpose of the present study, the ZnFe 2 O 4 samples calcined at 400 ˚C and 600˚C are referred to as ZF1 and ZF2 samples.

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The X-ray diffraction (XRD) patterns of the samples have been obtained using the PANalytical X'PERT-PRO X-ray diffractometer with CuKα radiation in the 2θ angle range of 30˚ to 75˚.The FTIR spectra of the samples have been recorded using the Perkin Elmer SPECTRUM BX II spectrometer in the range of 400 to 4000 cm -1 using potassium bromide (KBr) pellet. The U V -730 JASCO UV-Vis spectrometer has been used to record the UV-Vis spectra of the samples in the wavelength range of 200 to 900 nm. The morphology of the prepared samples has been analyzed by the scanning electron microscope (SEM)-TESCAN-VEGA3 LMU instrument and the elemental composition of the samples has been made by energy dispersive Xray analysis (EDAX) spectra using the Bruker Quantax 200 AS instrument in conjunction with the TESCAN-VEGA3 LMU instrument. The magnetic properties of the samples have been studied using a LDJ9500 vibrating sample magnetometer (VSM) at room temperature with a maximum magnetic eld of 15000 G. A Keithley SCS 4200 source meter has been used for the I-V measurements at room temperature.

Structural characteristics
The crystal structure of the samples and their purity has been analyzed using X-ray diffraction (XRD) studies. The XRD patterns of the samples ZF1 and ZF2 are displayed in nanoparticles using different methods [1-3, 9,10, 16, 21, 30, 31].It can be further observed that, when the calcination temperature is increased to 600˚C, the peaks become more prominent and sharper which demonstrates the sample crystallization becomes better and the mean grain size has become bigger. The lattice parameter 'a' of a cubic crystal can be calculated using the relation where (h k l) are the Miller indices of the re ecting planes and 'd hkl ' is the interplanar spacing. The mean value of the lattice parameter 'a' of ZF1 and ZF2 calculated using the above relation are given in Table  1.The mean grain size (D) of the crystallites has been calculated using the Debye Scherrer formula , where λ is the wavelength of the X ray beam, β is the full width of the diffraction peak at half the maximum intensity (FWHM) measured in radian and θ is the Bragg angle. The mean grain sizes (D) of the samples ZF1 and ZF2 evaluated using the relation (2) is given in Table 1.

FTIR studies
The FTIR spectroscopy is a powerful tool to identify the presence of functional groups in the samples.
The FITR spectra of the samples ZF1 and ZF2 are presented in Fig.2. It is evident from the gure that, both the samples give rise to absorption bands in the range of 4000 to 400 cm -1 and that the samples exhibit bands of the same frequencies.
The band assignments of the FTIR spectra are given below. The broad band centered at 3443 cm -1 can be attributed to the stretching vibrations of O-H functional group. The medium intensity sharp band at 1647 cm -1 can be associated with the bending vibrations of O-H functional group. The existence of these two bands con rms the presence of water molecules adsorbed from the environment on the surface of the nanomaterial due to their high speci c surface areas [21,32].The sharp and medium intensity bands at 2926 cm -1 and 2858 cm -1 can be assigned to C-H stretching bonds. These bands invariably appear in the FTIR spectra of samples and they are seldom considered important in determining the structure [21]. The sharp medium intensity bands occurring at 1745 cm -1 and 1740 cm -1 may be associated with C=O stretching vibrations. These bands may arise due to the presence of traces of adsorbed or atmospheric CO 2 [10]. In cubic spinel ferrites, the metal ions are present in the tetrahedral and octahedral sub lattices and consequently would exhibit two different vibrational bands in the range of 600 -400 cm -1 . The higher frequency band at 556 cm -1 can be assigned to stretching vibrations of the metal -oxygen (M -O) bonds in the tetrahedral site while the lower frequency band at 418 cm -1 can be ascribed to the bending vibrations of metal -oxygen (M -O) bonds in octahedral site [21,[33][34][35].

Morphological characteristics
The surface morphological features of the samples have been assessed using scanning electron microscope (SEM). Fig 3 illustrates the SEM micrographs obtained for the samples ZF1 and ZF2. The micrograph for the sample ZF1 ( g 3a) reveals that, though most particles are irregular granular ones, the presence of spherical particles in a few regions of the micrograph can be noticed and that there are some small pores primarily in the spherical particle regions. The particle distribution is rather non-uniform; the particles are agglomerated in certain locations in the micrograph. The micrograph of ZF2 ( g 3b) shows that the morphological features of this sample is in no way different from the features exhibited by the sample ZF1.
The Energy Dispersive X-ray Analysis (EDAX) has been used to identify the elemental composition of the samples. The EDAX spectra of samples ZF1 and ZF2 are depicted in The band gap energy (E g ) has been calculated using Tauc's formula which shows the relationship between the absorption coe cient (α) and the incident photon energy (hυ) and can be written as: where 'A' is a constant independent of energy and 'n' depends on the type of transition having values 1/2, 2, 3/2 and 3 corresponding to the allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions [36].Since, ZnFe 2 O 4 is a direct band gap material, n takes the value of '½' for the allowed transitions. So, Tauc's formula for this case becomes, The (αhυ) 2 versus (hυ) plots (Tauc plot) for the samples ZF1 and ZF2 are given in Fig 6.Both the plots are found to be linear in the higher energy region. The intercept made by the extrapolated linear portion of the Tauc plot on the 'hυ' axis corresponding to (αhυ) 2 = 0 gives the band gap energy (E g ) of the material.
The band gap energy (E g ) value of the sample ZF1 has been found to be 2.3 eV, which is blue shifted when compared with the bulk ZnFe 2 O 4 (1.9 eV), which may be due to the presence of nano size particles in the study sample. At this juncture, it is pertinent to mention that band gap energy values ranging from 2.1 to 2.4 eV have been reported for ZnFe 2 O 4 nanoparticles by other research groups [9,[37][38][39] ; however, Ahmad Reza Abbalian and MadhiSha ee Afrani [21] have reported a higher E g value of 4.5 eV for ZnFe 2 O 4 nanoparticles prepared by one-step solution compusion method. The band gap energy value of ZF2 sample has been found to be 2.6 eV. This shows that the band gap energy increases with increasing calcination temperature. Similar results have been reported by K. Kombaiah et.al [40] from their studies on CuFe 2 O 4 nanoparticles.

Magnetic characterization
The magnetic properties of the samples ZF1 and ZF2 at room temperature have been investigated using the VSM technique by sweeping the magnetic eld (H) between -15 000 G and + 15 000 G. The samples get magnetized with the applied eld and when the eld is increased the magnetization also increases and vice-versa.  .Sasanka Deka and Joy [45] have also observed similar increase in magnetization when the calcination temperature is increased from 600 K to 873K. The small increase in the magnetization above 400˚C is likely to be due to the growth of the particles during the heating run [45].

Electrical Properties
The DC electrical resistivity is one of the most important properties of ferrites; usually high resistivity is prerequisite for most electronic applications that require negligible eddy currents which degrade the ferrite performance. So, the current -voltage (I-V) characteistics of the pellets prepared using the samples ZF1 and ZF2 have been studied by sweeping the applied voltage between -10V and +10 V .The I -V characteristics obtained for the pellets are presented in g 8.
The g. reveals that,for both the pellets, when the voltage increses, the current also increases proportionally and the I-V plot is a slanting straight line in the rst quadrant , the slope of which gives the conductance of the pellet ; when the applied voltage is reversed , the current also reverses and the I -V plot is a straight line in the third quadrant, having the same slope as that in the rst quadrent; when the applied voltage is swept between -10 V and +10 V, the I -V plot traces the same straight line path. The g. further shows that, the I -V plots of the pellets are silmilar and symmetric and that the pellets exhibit Ohmic behaviour.
The resistivity (ρ) of the material of the pellet (i.e the resistivity of the material of the sample) has been detrmined using the relation Where R is the resistance of the pellet, found from the relation R = V / I ( where V is the applied voltage and I is the measued current ) , 'd' is the thickess of the pellet and A is the area of cross section of the pellet. The resistivities of the samples ZF1 and ZF2 have been evaluated using the relation 5 and the values obtained are 3.96 x 10 6 and 3.13 x 10 6 Ω cm respectively. It can be seen that the electrical resistivity of the samples decreases with increase in calcination temperature indicating the semiconducting nature of the samples.This decrease in resistivity can be explained as a consequence of micro structural and structural changes introduced by the increased calcination temperature. It can be noted that the mean grain size of the samples increases with increase in calcination temperature [section 3.1] which may cause a decrease in resistivity of the sample.The electric conductivity in ferrites can be explained by the Verwey-de Boer mechanism in which electron hopping takes place between ions of the same element present in more than one valence state,which are distributed randomly over crystallographically equivalent lattice sites. In ferrites, the charge carriers are localized at the magnetic ions and the electronic hopping between Fe 2+ and Fe 3+ ions located on octahedral sites is the primary mechanism which allows electrical transport.Obviously, the more the Fe 2+ iron content the higher the conductivity and hence lower the resistivity. The decrease in DC resistivity with the increase in calcination temperature can be attributed to the presence of increase in Fe 2+ content [46][47][48][49][50].So, it can be concluded that, the above mentioned factors may be the cause for the observed drop in resistivity with incresase in calcination temperature .

Conclusions
ZnFe 2 O 4 (Zinc ferrite) nanoparticles, at two different calcination temperatures of 400˚C and 600˚C, have been successfully synthesized using the simple, low cost co -precipitation method. Studies made on the synthesized samples reveal that ZnFe 2 O 4 with cubic spinel structure has been formed, the mean crystallite size increases with calcination temperature, no unwanted impurity such as nitrate has been detected in the synthesized samples indicating their high purity, particles are of irregular granular shape with a sprinkling of spherical particles, the optical band gap energy increases with increasing calcination temperature, the samples exhibit super paramagnetic behaviour and the saturation magnetization of the samples increases with calcination temperature. The studies further reveal that the electrical resistivity of the samples decreases with increasing calcination temperatureand this phenomenon can be explained by Verwey-de Boer mechanism.