Essay: magnetic nanoparticles of spinel ferrite

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Abstract
In the present work, Zinc ferrite nanostructured materials were synthesized by a microwave combustion method (MCM) using Okra (Abelmoschus esculentus) as a plant extract and compared with the ones synthesized by a conventional combustion method (CCM). Plant extracts play a dual role of both oxidizing and reducing nature. Microwave assisted combustion method is a potential, attractive and energy saving technique when compared to conventional heating, due to the direct heating of the reaction mixtures. They have several advantages, such as, simple, inexpensive, good stability of nanoparticles, less time consumption, and large-scale synthesis. The synthesized products are investigated by the standard characterization techniques, such as, X-ray diffraction (XRD) studies, rietveld analysis, fourier transform infrared spectroscopy (FT-IR) studies, high resolution scanning electron microscopy (HR-SEM), energy dispersive X-ray analysis (EDX), diffuse reflectance spectroscopy (DRS), photoluminescence studies (PL), and vibrating sample magnetometer (VSM) in order to study structural phase, morphology, optical and magnetic properties. The formation of the cubic phase ZnFe2O4 is confirmed by XRD and FT-IR. The morphological studies of the obtained ZnFe2O4 nanoparticles were investigated by the HR-SEM. The optical band gap value was determined by DRS. The change in saturation magnetization (Ms) and coercivity (Hc) was observed by VSM studies.

Introduction
In recent years, magnetic nanoparticles of spinel ferrite attracts great interest, due to their distinctive properties, such as, catalytic, magnetic, optical and electrical properties [1]. They have potential applications in various fields like electromagnetic absorbers, microwave devices, catalysis, sensors, water purification, antibacterial, nanoelectronics, high density storage media, drug delivery, and magnetic resonance imaging [2]. The spinel ferrite is generally described by the formula AB2O4, where A and B represents divalent and trivalent cations respectively. In a normal spinel structure, all divalent cations occupy the tetrahedral sites, whereas the trivalent cations occupy the octahedral interstices [3]. However, in case of inverse spinel structures, trivalent cations occupy all the tetrahedral sites and half of the octahedral sites, whereas the remaining half of the octahedral sites will be occupied by the other divalent cations [4]. ZnFe2O4 is a semiconductor material with a band gap of 1.9 eV. It is a normal spinel with all the Fe3+ ions in the B sites and all the Zn2+ ions in the A sites.
Spinel ferrite nanoparticles can be prepared by a large number of physical, chemical, methods, such as, sol’gel [5], reverse micelle technique [6], hydrothermal [7], and combustion method [8]. The nanoparticles prepared by these methods show different properties. The synthesis of nanoparticles by physical and chemical routes can lead to the production of toxic chemical by-products, require critical conditions of temperature and pressure, expensive and long reflux time of reaction [9, 10]. The combustion synthesis method has attracted many researchers in fabricating the metal oxide nanoparticles, because of its inexpensive precursors, uniform distribution of the metal ions, and it offers excellent chemical stability and modest heating. The combustion method is based on the mixing of metal nitrates, which act as an oxidizing agent and fuels, acting as the reducing agent. Among the several control parameters in a combustion method, fuel plays an important role in determining the morphology and phase formation of the final products of zinc ferrite nanoparticles. Microwave assisted combustion method is a potential technique for the synthesis of zinc ferrite nanoparticles. This method is an attractive and energy saving than the conventional heating, due to the direct heating of the reaction mixtures [11]. Heat will be generated internally within the material, inspite of originating from any external sources by ‘in situ’ mode of energy conversion [12]. In conventional methods, the vessel is heated and then the heat is transferred by convection [10]. Compared to the conventional method, microwave method has advantages, such as, shorter reaction time, lower energy consumption, and better product yield, thus producing small particle size. In addition, it also provides increased reaction kinetics, uniform nucleation, rapid initial heating, growth conditions and enhanced reaction rates [13].
Research has shown the synthesis of nanoparticles using plant extracts to be an attractive choice to promote the synthetic reactions. It provides a facile and ‘green’ method for the synthesis of nanoparticles [14, 15]. Green synthesis procedures have several advantages, such as, simple, inexpensive, good stability of nanoparticles, less time consumption, non-toxic byproducts and large-scale synthesis [16- 21].
In Malvaceae family, Okra (A. esculentus) is the only vegetable crop of significance. The main constituents of A. esculentus are cellulose, hemicelluloses and lignin and the rest are very minor in proportion. Okra (A. esculentus) is a good source of carbohydrates, protein, dietary fiber, calcium, magnesium, potassium and vitamins A and C [22].
The leaf extract of Okra (A. esculentus) has been used as both the reducing agent and the surface stabilizing agent for the synthesis of nanoparticles. Hence, this kindles our interest to make energy efficient, simple and new route towards the preparation of ZnFe2O4 using Okra (A. esculentus) plant extract. To the best of our knowledge, no literature is found on the synthesis of nano-zinc ferrites by a microwave assisted combustion method using metal nitrates and Okra (A. esculentus) extract solution as the precursors. In this present study, we report the nontoxic environmentally friendly synthesis of ZnFe2O4 of an appropriately small size distribution using the okra plant extract utilizing microwave-assisted synthesis. The structure, phase, and morphology of the synthesized products are investigated by the standard characterization techniques in the present study in order to have the comparative investigation with the samples prepared by conventional methods.
2. Experimental
2.1. Preparation of Okra (A. esculentus) plant extract
About 25g of Okra (A. esculentus) leaves collected from local agricultural fields in Chennai, Tamil Nadu were thoroughly washed, ground, and the gel obtained was dissolved in 100 ml of deionized water and stirred for 45 min to obtain a clear solution. The resulting extract was used as the Okra (A. esculentus) plant extract solution.
2.2. Preparation of ZnFe2O4 by conventional (CCM) and microwave heating (MCM) methods
Zinc nitrate (99%, Merck Chemicals, India) and ferric nitrate (98%, Merck Chemicals, India) were dissolved in deionized water and then mixed with the Okra (A. esculentus) extract solution under constant stirring for 5 h at room temperature until a clear solution was obtained. The molar ratio of Zn/Fe was kept at 1:2. The solution was dried in an air oven at 120 ”C for 2 h. The resultant powders were then sintered at 800”C at a heating rate of 5 ”C/min for 3 h in a muffle furnace and the sample obtained was labeled as ZnFe2O4-CCM (prepared by conventional heating).
In the same manner, the clear transparent solution was prepared and placed in a domestic microwave oven (2.45 GHz, 850 W) for 12 min. Initially, the solution boiled, leads to dehydration followed by decomposition with the evolution of gases. This vaporized the solution and it left behind a solid, which was labeled as ZnFe2O4-MCM (prepared by microwave heating).
2.3. Characterization of zinc ferrite samples
The structural characterization of zinc ferrite was investigated using Rigaku Ultima IV high resolution X-ray diffractometer (XRD) for 2” values ranging from 10 to 80′ using Cu K” radiation at ” = 0.154 nm. Structural refinements were studied using the Rietveld method and carried out using the PDXL programme. Both the refined lattice parameters and crystallite size of the ZnFe2O4 powders were obtained. A Perkin Elmer infrared spectrophotometer (model RX l with the working range of 4000cm-1-400cm-1) was used for the determination of the surface functional groups determination. The HR-SEM studies and EDX analysis had been performed using a Joel JSM6360. In order to estimate their band gap energy, the diffuse reflectance UV’visible spectrum of the samples was recorded using Cary100 UV’visible spectrophotometer. The emission properties were investigated using Varian Cary Eclipse Fluorescence spectrophotometer. Magnetic measurements were carried out at room temperature using the PMC MicroMag3900 model vibrating sample magnetometer (VSM) equipped with 1T magnet.
3. Result and discussion
3.1. Powder X-ray Diffraction (XRD) analysis
Fig. 1. shows the powder X-ray diffraction pattern was used to record the crystal information of zinc ferrite nanoparticles synthesized by both conventional and microwave assisted combustion method. The phase identification was carried out by comparing the obtained data with standard diffraction patterns. The diffraction peaks at 2” of 29.90”, 35.22”, 36.84” ,42.80”, 53.09”, 56.59”, 62.13”, 70.47”, 73.48”, 74.48”, and 78.40” can be ascribed to the reflection of (220), (311), (222), (400), (422), (511), (440), (620), (533), (622), and (444) planes of the ZnFe2O4 spinel, respectively. All the detectable peaks in the XRD patterns could be indexed with the standard JCPDS data (82-1042). There is no formation of secondary peaks. It indicates that the synthesized zinc ferrite nanoparticles possess single phase cubic structure.
The average crystallite size of ZnFe2O4 nanoparticles can be calculated from the value of full-width (FWHM) at half-maximum of (311) plane by Scherer’s formula [23]
L=0.89”/(”cos” ) (1)
where L is the crystallite size, ” is the X-ray wavelength, ” is the full width at half maximum (FWHM) and ” is the Bragg’s diffraction angle. The average crystallite size of ZnFe2O4-CCM (sample A) was 74.0 nm. For ZnFe2O4-MCM (sample B) it was found to be 37.0 nm. As the calcinations temperature increases (sample A), the corresponding peaks become stronger in intensity, which implies that the crystallinity is higher and the crystallite size is larger in the sample prepared by conventional method. For sample B, single phase cubic ZnFe2O4 produced within the shorter time which give rise to lower crystallinity and smaller crystallite size.
The lattice parameter has been calculated from X-ray diffraction data using the formula.
sin^2”’=”^2/4 [4/3 ((h^2+hk+k^2)/a^2 )+l^2/c^2 ] (2)
where h is the diffraction angle, ” is the incident wavelength (” = 0.1540 nm), and h, k, and l are Miller’s indices. The crystallite size and lattice parameter for both the samples are listed in Table 1. From the table, it is clear that, the crystallite size along with lattice parameter were increased with annealing temperature. The lattice parameter of the as-prepared samples are found to be 8.4429 ” (sample A), and 8.4410 ” (sample B), which are slightly smaller than the 8.443 ” value reported for bulk zinc ferrite (JCPDS no. 89-4926). The reason for this difference in lattice parameter values was mainly due to the type of preparation method (microwave and conventional method), annealing time, and molar ratio of the starting precursors and the nature of the precursor materials. This increase in the lattice parameter (sample A) can be explained using the surface disorder in the nanoparticles. However, for annealed nanoparticles, the terminated surface unit cells get completed after receiving the atmospheric oxygen through the annealing process and decreases the surface stresses/strains. Therefore the increase in the lattice parameter for annealed nanoparticles may be due to the reduced surface disorder [24].
3.2. Rietveld analysis
The Rietveld refined X-ray diffraction (XRD) pattern for the zinc ferrite samples are shown in Fig. 2. Rietveld refined patterns were continuously done until to get goodness factor was obtained. Fig. 2. illustrates the observed and calculated X-ray pattern and their differences for sample A and sample B. All the reflection peaks are marked and shown at the bottom of the plot. The computed patterns are revealed in the same field as a solid line curve in blue and the difference between the observed and calculated intensities of each fitting is shown in the lower field in red. The values of discrepancy factor (Rwp) and expected values (Rexp) with goodness of fit are listed in Table 1. The value of Rp factor is found to be larger in the conventional combustion method. Similar value of Rp factors have been observed for the nanocrystallite samples [25]. It could be due to the low signal to noise ratio of XRD patterns for nanocrystalline materials. In microwave assisted method, the value of ”2 (goodness of fit) is low, which justifies the goodness of refinement. Whereas in the diffraction pattern of nanocrystalline materials, diffuse scattering is dominant compared to bulk crystalline materials, due to the large ratio of surface to volume atoms, which leads to a decline in crystallinity. Hence, large reliability factors are observed. The lattice parameters increase with the increasing annealing temperature. The increase of annealing temperature gives rise to the volume expansion of the crystal, which in turn increases the volume to surface atoms, thereby leading to enhanced crystallinity and increase in the value of the lattice parameter. Hence, we have observed that the lattice parameter increases with increasing annealing temperature [26].
3.3. Fourier Transform Infrared Spectroscopy (FT-IR)
Fig. 3. Shows the FT-IR spectra of ZnFe2O4 samples. These spectra illustrate the absorption bands below 1000 cm-1, which correspond to Fe-O and Zn-O bonds, respectively. It also shows the characteristic peaks at 552, 554, and 432, 439 cm-1, which can be ascribed to the stretching vibrations of the Zn’O bonds, was found to be in tetrahedral positions and the Fe’O bonds were in octahedral positions [27’29], respectively. These peaks are usually associated with pure ZnFe2O4. The band at 686 cm-1 corresponds to M-OH bending vibrations [30, 31]. The broad peaks around 3400 cm-1 are shows the presence of the hydroxyl group bending vibration, which is related to the vibration of water molecules synchronized to ferrite structure and it may be due to the presence of moisture in the KBr, which is used for making a pellet in an open air [32]. The peak appeared at 1595-1630 cm-1 also confirmed the presence of hydroxyl group in the as-prepared sample [33]. The peaks around 2700- 2950 cm-1 corresponds to C-H stretching vibrations. The most intense band of the spectrum at 1350-1370 cm-1 and 770-880 cm-1 signifies the stretching vibration of Zn-O-Fe bonds of the tetrahedral building units forming the structure.

3.4. High resolution scanning electron microscopy (HR-SEM) analysis
The morphological studies of the obtained ZnFe2O4 nanoparticles were investigated by the HR-SEM analysis. Fig. 4(a-b) represents the HR-SEM images of the agglomerated ZnFe2O4 nanoparticles prepared by CCM (sample A). Fig. 4 (c-d) reveals the formation of ZnFe2O4 nanoparticles prepared by MCM (sample B). All the samples exhibit a compact arrangement of homogeneous nanoparticles. The shape of the ZnFe2O4-MCM particles is basically globular and the particle diameters are in the range of 25-45 nm. The average particle size of the ZnFe2O4-CCM is in the range of 350-800 nm. The micrographs indicate that the particles are nearly spherical with uniform agglomeration to give large and irregular crystals, which may be due to the preparation method, defects, effect of annealing and the presence of magnetic interactions among the particles. It is a well-known fact that the temperature and reaction time are the two essential factors in determining the morphology of the nanomaterials. In contrary to the conventional heating, during the time of microwave heating, the heat is produced internally within the material, rather than from external heating sources, and hence inverted temperature gradient is produced. In microwave method, the nanoparticles have been produced, due to the rapid heating achieved in short time duration, and suppressed diffusion process [34]. Thus, higher temperature of combustion in a furnace caused grain growth compared to the volumetric and rapid microwave combustion, which prevents the grain growth, and thus gives uniform morphology.
3.5. Energy dispersive X-ray (EDX) analysis
Fig. 5 shows the elemental composition and purity of the as-prepared nanoparticles and the analysis is carried out at room temperature. The EDX spectrum confirms both the homogeneity and gradient of the elements Fe, Zn, O present in the sample. The results suggested that the precursors have fully take part in the chemical reaction to form the single phase ZnFe2O4 nanoparticles and it confirms that there is no other impurity present in the samples. It is suggested that the relative atomic mass ratio of the metal ferrites are well matched along with the stoichiometry in preparation. The small peaks at 2.1-2.2 keV in the EDX spectra of both the samples are due to the presence of gold, which is coated on the samples before recording SEM for the enhanced visibility of the surface morphology.
3.6. UV’Vis absorption spectra and optical band gap
The diffuse reflectance spectra of ZnFe2O4 nanoparticles are examined in the range of 200-800 nm at room temperature to examine their optical absorption properties. Fig. 6. shows the band gap energy of the ZnFe2O4 samples analyzed from the reflectance spectra using a modified Kubelka’Munk function F(R) [35],
(3)
F(R) is the Kubelka’Munk function, where R is the reflectance. A graph was plotted against [F(R) h”]2 and h”, and the intercept obtained corresponds to the band gap energy. The predicted band gap values of ZnFe2O4-CCM and ZnFe2O4-MCM are 2.03 eV and 2.08 eV, respectively. It is clear that there is a decrease in the optical band gap of the ZnFe2O4-CCM sample with an increase in the crystallite size. This can be attributed to the increased calcinations temperature. The sample ZnFe2O4’MCM shows higher band gap value with a smaller particle size, due to the chemical defects present in the intergranular region generating a new energy level to reduce the band gap energy [36]. The difference in the band gap has been also been explained on the basis of a variation of the lattice parameters. It has been observed that the band gap energy decreases with an increase in the lattice parameter. The band gab of the ZnFe2O4 nanoparticles with the decreasing crystallite size is the result of quantum confinement effects [37]. But both the nanoparticles have higher energy and lower wavelength when compared to the bulk materials. This might be attributed to the additional sub-band-gap energy levels that are induced by the abundant surface and interface defects in the agglomerated nanoparticles [38]. This deviation may be related through various parameters including the structural parameters, carrier concentrations, and presence of very small amount of impurities, which were not easily detectable by XRD and SEM/EDX techniques.
3.7. Photoluminescence (PL) studies
The PL spectroscopy is a convincing method to examine the existence of defects in semiconductors. Fig. 7 shows the photoluminescence spectra of ZnFe2O4 samples recorded at room temperature employing the excitation wavelength at 412 nm. It may appear from the electron transition from shallow donor levels (Zni) to the valence band (VB). Electrons in the conduction band (CB) first laid-back to a shallow donor levels (Zni) due to the long lifetime of donor levels and then recombined with the holes in the valence band [39]. Since, shallow donor levels (Zni), lies lower the CB, the emission wavelength during the electrons transition from the shallow donor level (Zni) to the VB is somewhat larger than the excitons transition from CB to VB [40].
The PL spectrum for the synthesized samples consists of four bands at 486, 530, 542, and 566 nm respectively. The peak at 486 nm corresponds to the blue emission and it represent a deep level visible emissions associated with the localized levels. The peaks at 530, and 542 nm correspond to the green emissions, which were due to the intra’band gap defects like oxygen vacancies. These defects provide donor levels near the conduction band edge of the zinc ferrites. A yellow emission band located at 565 nm is associated with the interstitial oxygen defects and the energy related to the band is almost the same for the energy band-gap of the as-prepared ZnFe2O4 nanoparticles [41]. Hence, it is assumed that the prepared samples have small particle size, and high surface defects. Surface defects results in uncompensated disorder spin at the surface of the nanoparticles.
3.8. Vibrating sample magnetometer (VSM) studies
The magnetization curves of the investigated ZnFe2O4 samples obtained from room temperature VSM measurements are shown in Fig. 8. The plot for both the samples shows a typical ‘S’ shape hysteresis loops which represent the soft magnetic nature. Variation in saturation magnetization (MS), remanence magnetization (Mr), and coercivity (Hc) value for the samples is shown in Table 2. This vibration attributed to the factors, such as, grain growth, density, anisotropy, A’B exchange interactions and surface spin effect [42, 43]. Lowering of coercivity value in ZnFe2O4-CCM indicates that the prepared sample can be demagnetized easily. Coercivity represents the force required to bring the magnetization to zero and is greatly dependent on the grain size [44]. The magnetic properties obtained by ZnFe2O4-MCM have the prominent advantages of lower temperature, larger coercivity, and higher saturation magnetization when compared to ZnFe2O4-CCM. The magnetic properties of zinc ferrite samples are influenced by the preparation route, cation distribution, grain size and sintering conditions [45]. The increase in saturation magnetization of ZnFe2O4-MCM is due to the decrease in the crystallite size with the subsequent increase in micro-strain as shown in Table 2 [46]. The smaller value of saturation magnetization in ZnFe2O4-CCM is due to the differences in inversion parameter, which indicates the distribution of cations between A-and-B sites of the spinel lattice [47, 48]. Magnetic moment decreases smoothly with the increase of temperature due to, the cationic distribution is dominant in our annealed nanoparticles as compared to surface effects [49]. The decrease in Ms is related to the surface effect of the sample which is sometimes called as the ‘dead’ surface and also inversely proportional to the grain size. The dead surface is associated with the disorder of surface spins [50]. When the crystallite size decreases the number of surface spins in a sample increases. Larger grain size of the sample makes the easy motion of the domain walls and thereby the coercivity decreases.
3.9. Antimicrobial activity of Zinc ferrite nanoparticles.
The antibacterial activities of the zinc ferrite samples were tested against Gram-positive Staphylococcus aureus (96) and Micrococcus luteus (106) and Gram-negative Enterobacter aerogenes (111) and Yersinia enterocolitica (840) bacterial pathogens. For comparison, the antibacterial activity of zinc ferrite prepared by both microwave and conventional method using the concentration of 120 mg of the given sample was dissolved in 300 ”l of DMSO. The 25 ”l of the dissolved sample was loaded to the disc will contain 10 mg/disc concentration by well diffusion method. The zone of inhibition was estimated from mean diameter around the test sample, measured in mm. The samples at 25 ”l exhibited a clear bactericidal zone of inhibition around each specimen, indicating a noticeable effect against bacteria. The detected mean diameter of inhibition zones for both the sample values against given bacteria are shown in Table 4. These results displayed that the zinc ferrites prepared by microwave method have sufficient antibacterial activity for both Gram positive (G+) and Gram negative (G-) bacteria than the ones prepared by the conventional method. The improved bioactivity of the nanostructures is attributed to the higher surface area to volume ratio. The nanostructures need more particles to cover the bacterial colony, which results in the generation of more active oxygen species and it kills the bacteria more successfully. The antibacterial activity of the nanostructures can may either speedily interact with the microbial cells causing the interruption of trans membrane electron transfer, disrupting the cell envelope, oxidizing cell components and producing secondary products, such as reactive oxygen species that can cause damage. In addition, the destruction of the cell membrane might directly lead to the leakage of minerals, proteins and genetic materials causing ultimate cell death [51]. From the antibacterial tests, we confirmed that the zinc ferrite prepared by MCM render the effective antibacterial activity, when compared to CCM.
Conclusions
Zinc ferrite nano particles synthesized by the microwave and conventional combustion method and are characterized by XRD, Rietveld, FT-IR, SEM, EDX, DRS, PL, and VSM. Results of XRD reveals that the single phase crystalline structure of the samples. At the same time, the increase in the intensity of peak, lattice parameter and the crystallite size upon calcinations are also observed. The value of ”2 (goodness of fit) is low, which justifies the goodness of refinement in Rietveld. FT-IR absorption bands confirm the functional groups present in the samples. SEM characterization results show that the obtained samples are nanoparticles with an average size of about 25-45 nm and 350-850 nm respectively for sample A and B. The energy dispersive X-ray spectra con’rm the presence of Zn and Fe in ZnFe2O4 system and the weight percentage is found to be nearly equal to their nominal stoichiometry ratio. DRS measurements show a decrease in the energy gap with increasing crystallite size. The PL results show the optical properties of ZnFe2O4 nanoparticles. VSM measurements show the magnetic properties of zinc ferrite and an increase in saturation magnetization is due to the decrease in the crystallite size with the subsequent increase in micro-strain.

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Figure captions
Fig. 1. XRD pattern of (a) ZnFe2O4-CCM and (b) ZnFe2O4-MCM.

Fig. 2. XRD pattern refinements using the Rietveld method of (a) ZnFe2O4-CCM and
(b) ZnFe2O4- MCM.

Fig. 3. FT-IR spectra of (a) ZnFe2O4-CCM and (b) ZnFe2O4-MCM.

Fig. 4. HR-SEM images of (a-b) ZnFe2O4-CCM and (c-d) ZnFe2O4-MCM.

Fig. 5. EDX spectra of (a) ZnFe2O4-CCM and (b) ZnFe2O4-MCM.

Fig. 6. UV’Visible diffuse reflectance spectra of (a) ZnFe2O4-CCM and (b) ZnFe2O4-MCM.

Fig. 7. Photoluminescence excitation spectra of (a) ZnFe2O4-CCM and (b) ZnFe2O4-MCM.

Fig. 8. Magnetic hysteresis (M’H) loops of (a) ZnFe2O4-CCM and (b) ZnFe2O4-MCM.
Table captions
Table 1. Lattice parameter, crystallite size, and fit parameter values of (a) ZnFe2O4-CCM and
(b) ZnFe2O4-MCM.
Table 2. Magnetization, remanence and coercivity of (a) ZnFe2O4-CCM and (b) ZnFe2O4-MCM.
Table 3. BET surface area and pore volume of (a) ZnFe2O4-CCM and (b)ZnFe2O4-MCM.
Table 4. Antimicrobial activity of (a) ZnFe2O4-CCM and (b)ZnFe2O4-MCM.

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