1. Introduction
Recently, tertiary mixed metal oxide nanoparticles with cubic spinel structures have been identified as the most promising materials for electrochemical energy storage applications, televisions, transformers, magnetic recording media, biomedical applications, etc., because of their exceptional physicochemical, mechanical, magnetic, and dielectric properties.1-6) Extensive efforts have been devoted to developing a modest, stable, and effective synthesis process for nanosized materials, which can control their structural parameters and impart a high specific surface area.7,8) In recent decades, size-tunable ferrite particles with a cubic spinel structure have been identified as remarkable materials because of their exceptional physicochemical properties. The cubic spinel ferrite has received significant research importance because of their outstanding, versatile, and multifield applications. Among cubic spinel ferrite materials, zinc ferrites are unique and technologically important, and find a variety of applications in water splitting, biomedicine, photocatalysts, transformer cores, microwave technology, magnetic recording media, and gas sensors.9-13) The general formula of the cubic spinel structure is AB2O4, where “A” is a bivalent metal ion and “B” is a trivalent metal ion. The spinel ferrite crystal structure consists of a cubic closed arrangement of oxygen ions with 8 tetrahedral (A-site) and 16 octahedral (B-site) interstitial sites in the unit cell.14) Therefore, in spinel ferrites, an enormous amount of vacant A and B positions are available for cations to migrate, amongst the interstitial sites. Nanocrystalline n-type semiconductor ZnFe2O4 has a typical spinel structure with Zn2+ ions at the tetrahedral interstitial sites and Fe3+ ions distributed between the tetrahedral and octahedral interstitial sites.15) The physicochemical, electrical, and magnetic properties of spinel ferrites are noticeably affected by the addition of cations and their preferential distribution among the tetrahedral and octahedral interstitial sites, as well as particle size reduction (i.e., to produce a nanosized material) via manipulation of the preparation method to a great extent. These properties are dependent on the synthesis method, calcination temperature, compound composition, and type and concentration of the dopants.16,17)
Notably, the spinel ferrites nickel ferrite (NiFe2O4) and zinc ferrite (ZnFe2O4) have received extensive research interest because of their excellent chemical stability, excellent catalytic and optical properties, low cost, and facile industrial-scale preparation, and they are favorably used in the microwave industry. ZnFe2O4 is a soft magnetic material with unusual physical, chemical, electrical, and magnetic properties, and these properties are mostly dependent on the Fe2+ to Fe3+ transformation. In addition, the interchange of trivalent metal ions in zinc ferrite considerably affects the physicochemical, magnetic, and electrical properties. Chromium zinc ferrites are extensively studied soft magnetic materials and they can be easily obtained via the introduction of Cr3+ ions into the unit cell of zinc ferrite because their size is analogous to that of Fe3+ ions.9,18-21) Currently, nanoparticles of spinel ferrite are synthesized by different techniques such as chemical co-precipitation,22,23) microwave combustion,24,25) hydrothermal,26) sol-gel,27) and ceramic28) methods.
In the present work, nanoparticles of Cr3+-doped ZnFe2O4 with the general formula CryZnFe2-yO4 (CrZF), where y = 0.0, 0.025, 0.05, 0.075, and 0.1, were prepared by the chemical co-precipitation technique in the presence of a surfactant. This technique is a productive, cheap, and simple surfactant-assisted chemical process that can yield a large quantity of smaller particles with the preferred structure, hierarchy, and desired elemental composition at a lower sintering temperature, compared to the traditional ceramic technique. The effect of Cr doping (low dopant concentration) on the physicochemical properties of undoped zinc ferrite (ZF) is reported here.
2. Experimental Procedure
2.1. Synthesis and characterization
Nanosized particles of spinel ferrite CrZF (y = 0.0, 0.025, 0.05, 0.075 and 0.1) were produced by a surfactant-assisted chemical co-precipitation method. The nitrate salts of metal precursors (analytical reagent) were purchased from Thomas Baker (Chemicals) Pvt. Ltd., Mumbai, India. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), chromium nitrate nonahydrate (Cr(NO3)3·9H2O), iron nitrate nonahydrate (Fe(NO3)3·9H2O), n-hexadecyltrimethylammonium bromide (CTAB), 30% aqueous ammonia (NH3), and lab-made double distilled water were used for the synthesis. All chemicals were used as received without further refinement.
The detailed synthesis procedure of the nanoferrites has been reported in our previous publication.22) To remove organics, the co-precipitated mixture was preheated up to 350°C in a muffle furnace for 2 h. The furnace was allowed to cool down, and then, the powder was removed and milled with acetone to obtain a homogeneous mixture using pestle and mortar. The air-dried samples were heated at 750°C for 4 h in air, as depicted in Fig. 1. Finally, a soft nanocrystalline CrZF powder was obtained by milling with a pestle and mortar. The endothermic and exothermic behaviors and thermal stability of the spinel ferrites were studied by thermogravimetry-differential thermal analysis (TG-DTA, Nietzsche STA 409 TG DSE) in the temperature range from 30°C to 1000°C in synthetic air at a heating rate of 10°C·min−1. The phase purity and structural parameters of ferrites were determined by X-ray diffraction (XRD, PW-1710 Philips) with the Cu Kα radiation (λ = 1.5405 Å) in scanning range of 10-80°. Fourier-transform infrared (FTIR) spectroscopy (Perkin Elmer Spectrum One spectrophotometer equipped with an attenuated total reflectance (ATR) accessory) was performed using KBr pellets in the wave-number range of 350-800 cm−1. The topographical microstructures of the thick film samples were determined by field-emission scanning electron microscopy (FE-SEM) using a SIGMA HV model (in the magnification range from 50x to 100,000x) equipped with an energy-dispersive X-ray (EDS) spectroscopy analyzer (for chemical composition and phase purity determination). The prepared ferrite particle dimensions were determined by a particle size analyzer. The specific surface area and pore size values were calculated using the Brunauer-Emmett-Teller (BET) and Barret-Joyner-Halenda (BJH) techniques by N2 adsorption-desorption measurements (Quantachrome Instruments v10.0). DC electrical resistivity measurements were carried out using a digital picoammeter (DPM-111) to record the current at a constant voltage across the sample by the two-probe method. Room-temperature magnetic measurements up to a maximum field of 15 kOe were carried out using a vibrating sample magnetometer (VSM, PAR EG&G 4500).
3. Results and Discussion
3.1. TG-DTA analysis
The characteristic TG-DTA curve for the chemically co-precipitated CrZF sample (y = 0.05) is displayed in Fig. 2, from which the thermal stability and phase transition temperature of the ferrites were determined. The TG curve of the ZnCr0.05Fe1.95O4 ferrite revealed that the total weight loss was ~ 31.5 %. In addition, the TG curve exhibited minor (~ 9 %) and major (~ 22.5 %) weight loss steps in the temperature range of 25-135°C and 190-320°C, respectively. The minor weight loss was attributed to the loss of moisture and major weight loss was attributed to the decomposition of the carbon-based matrix and conversion of hydroxides into oxides.29) No further weight loss was observed between 414°C and 1000°C. The DTA curve showed a major exothermic peak at 248°C, suggesting the occurrence of combustion of the organic matrix and conversion of the precipitate into metal oxide, which was correlated with the major weight loss step in the TG curve. The plateau observed between 415°C and 1000°C in the TG curve indicated the formation of crystalline ZnCr0.05Fe1.95O4 ferrite without any further weight loss. Thus, a single cubic spinel ferrite phase was formed due to the conversion of the valence state of the metal oxides at high sintering temperatures.
3.2. XRD analysis
Figure 3 presents the XRD patterns of CrZF (y = 0.0, 0.025, 0.05, 0.075, and 0.1) samples annealed at 750°C for 4 h. All XRD patterns were matched with those of ZnFe2O4 (JCPDS card numbers 84-0314 and 89-1012). The major diffraction peaks were assigned to the (111), (220), (311), (222), (400), (422), (511), (440), and (531) planes, and the most intense diffraction peak was assigned to the (311) plane, evidently confirming the formation of a cubic spinel structure. The lattice constant (a) was calculated using equation (1).
where h, k, and l are Miller indices of the crystal planes and d is the interplanar distance for the hkl planes. The value of “a” marginally decreased with an increase in the Cr concentration due to the different sizes of Cr3+ (0.615 Å) and Fe3+ (0.645 Å). Table 1 demonstrates the influence of Cr concentration on the lattice constants of CrZF samples; the results were consistent with Vegard’s law.21,30)
The average crystallite size (DXRD) values for the powder samples were determined using the Debye-Scherrer formula:
Where, λ stands for the incident X-ray wavelength (Cu Kα radiation), β stands for full-width at half maximum (FWHM) in radians in the 2θ scale, θ is the Bragg angle, DXRD is the crystallite size in nm. To obtain β and θ values for all samples, the Gaussian fitting model was adopted. The particle sizes of the samples increased due to the replacement of Fe3+ cations by Cr3+ ions with a smaller radius. DXRD increased from 22.39 nm to 36.13 nm with an increase in the Cr concentration (y = 0.0-0.1). The X-ray densities (dx) of all CrZF samples were determined using the following formula31):
where Z = a constant (Z = 8; the total number of molecules in each unit cell), M = molecular weight (g·mole−1), and N = Avogadro’s number (6.023 × 1023 atoms·mole−1).
The skeletal density (ds) values were obtained by measuring the dry and suspended weights of the samples in xylene according to Archimedes’ principle:
where w is the weight of the sample in air (g), w′ is the weight of the sample in xylene (g), and ρ is the density of xylene (g·cm−3).
From the skeletal and X-ray densities of the samples, the percentage porosity (p) was determined using the following relation32):
The tetrahedral and octahedral site bond lengths (A-O, B-O) and ionic radii (rA, rB) of the cubic spinel structures were calculated using the following formulae33). (6)
where, u and r(O2−) are the oxygen ion parameter and radius of oxygen ion (~ 1.32 Å), respectively. Table 1 clearly demonstrates the effect of Cr doping concentration on the crystallite size, lattice constant, X-ray density, physical density, bond length, ionic radii, etc.
3.3. FT-IR analysis
FT-IR spectroscopy was performed to determine the fundamental changes and tetrahedral and octahedral sites of spinel ferrites. Moreover, from the FTIR spectra, the impurity states and chemical substances present on the particle surface were determined. Fig. 4 shows the transmittance spectra with two main broad metal-oxygen bands in the range of 400-600 cm−1, indicating the presence of a pure cubic CrZF spinel phase in all samples.34) The higher wave number υ1 absorption band, which is generally observed in the range of 544-569 cm−1, was attributed to the vibration of the tetrahedral metal-oxygen and the lower wave number υ2 absorption band observed in the range of 432-439 cm−1 was attributed to octahedral metal-oxygen bond stretching vibration. The absorption bands at 439 cm−1 and 569 cm−1 were assigned to the octahedral and tetrahedral sites of spinel zinc ferrite. The υ1 band observed at around 544-551 cm−1 for the Cr-doped spinel CrZF (y = 0.025, 0.05, 0.075, and 0.1) was assigned to the tetrahedral site and the υ2 band observed at 432-436 cm−1 was assigned to the Fe3+-O2− vibration at the octahedral location; moreover, the shift in the υ1 band location was attributed to the increase in the doping amount of Cr3+ ions in zinc ferrite. As the Cr concentration increased, υ1 decreased (i.e., from 550 cm−1 for Cr0.025ZnFe1.975O4 to 544 cm−1 for Cr0.1ZnFe1.9O4). This indicated that the Cr3+ ions replaced a proportional amount of Fe3+ ions in the octahedral sites, resulting in a reduction in the length of the metal-oxygen bond, which was reflected by an increase in the broadening of the absorption band, as presented in Table 2.21)
3.4. Morphological and elemental analysis
The FE-SEM images of CrZF (y = 0.0, 0.025, 0.075, and 0.1) samples sintered at 750°C are presented in Fig. 5 (a-d). In Fig. 5, nearly uniform and agglomerated grains are observed. The grain size of the sample is usually calculated by the linear intercept method using equation (10).35)
where L, M, and N are the total length of the test line, magnification, and number of intercepts, respectively. The morphology and size of particles were influenced by the Cr-doping content and calcination temperature, and the size ranged from 0.19-0.23 μm. With an increase in the Cr content, the particle size decreased from 0.23 μm to 0.19 μm, which was larger than DXRD of CrZF thick films.
The percentage elemental composition of the prepared CrZF thick film was investigated by the EDS technique. Fig. 5(e-h) present the EDS profiles of the CrZF samples. EDS was performed to verify homogeneity of the prepared samples and examine the presence of impurities originating from the synthesis process. The compositional percentages of Fe, Zn, Cr, and O in the Cr-doped zinc ferrite samples are listed in Table 3 as a function of the Cr doping concentration. The EDS data for the CrZF (y = 0.0, 0.025, 0.075, and 0.1) samples revealed the doping level (wt.%) of Cr in the samples to be 0.69, 1.25, and 1.51 for y = 0.025, 0.075, and 0.1, respectively. However, the wt.% of Fe decreased in the order 50.95, 43.57, 46.60, and 45.07 with increasing y (0.0, 0.025, 0.075, and 0.1, respectively).
3.4. BET analysis
N2 adsorption-desorption measurements were performed to determine the changes in the specific surface area and pore size as a function of the Cr substitution level. Type (IV) hysteresis loops for all CrZF (y = 0.0-0.1) samples obtained at 77 K are depicted in Fig. 6; the Type (IV) hysteresis loop is characteristic to porous materials.36) The surface area values for nanocrystalline CrZF (y = 0.0, 0.025, 0.05, 0.075 and 0.1) samples annealed at 750°C as a function of the Cr doping level are tabulated in Table 3. The specific surface area values for CrZF (y = 0.0, 0.025, 0.05, 0.075, and 0.1) samples are 5.245, 10.411, 5.265, 4.422, and 11.689 m2·g−1, and the cumulative pore volumes are found to be 0.025, 0.033, 0.021, 0.020, and 0.039 cm3·g−1, respectively. The BJH pore volume analysis revealed an average pore diameter of ~ 3-4 nm.
3.5. DC electrical resistivity analysis
Spinel ferrites are well-known high-resistance materials and their conductivity is determined by the drift mobility of electric carriers between ferrous and ferric ions (with +2 and +3 valence states), which are thermally activated. The temperature-dependent electrical resistivity (ρ) of CrZF samples was investigated in the temperature range of 30-600°C. Fig. 7 presents the plot of temperature dependency of the DC electrical resistivity versus 103/T. The electrical resistivity of the synthesized samples exhibited temperature-dependent brakes among linear shape with increasing temperature, suggesting their semi-conductive nature. This change in the resistivity with the Cr content could be explained on the basis of the hopping mechanism. It is well known that the distribution of cations, grain size, and porosity have a strong influence on the resistivity of ferrites. At high temperatures, ferrites acquire thermal energy and oxygen ions escape from CrZF in the form of ZnO, creating oxygen vacancies to maintain charge balance transformation of the Fe3+ to Fe2+ ions, retaining spinel lattice neutrality. In undoped ZF, Zn2+ and Fe3+ ions occupy the tetrahedral (A) and octahedral (B) interstitial sites, respectively, whereas in Cr-doped zinc ferrite, with increasing Cr3+ doping amount, the concentration of Fe3+ ions at the B-sites decreases; consequently, Cr3+ ions at the B-sites hamper electron hopping between Fe ions by blocking the Fe2+ ↔ Fe3+ exchange, which increases the resistivity. Notably, the amount of Fe3+ ions at the octahedral sites decreases as the Cr3+ doping concentration increases and Cr3+ ions do not take part in the hopping mechanism. As observed in Fig. 7, the resistivity increased as the Cr3+ substitution level in Cr-doped zinc ferrite increased. The mesoporous CrZF nanostructures were composed of grains with sizes in the range of 0.19-0.23 μm. The samples contained numerous grain boundaries, which acted as obstacles to the flow of electrons, obstructing hopping of charge carriers; this resulted in increased resistivity.37)
3.6. Magnetic analysis
The room-temperature magnetization vs. applied field curves (M-H loops) for Cr-Zn ferrite samples are shown in Fig. 8. For all samples, the magnetization reached saturation at a magnetic field of 15000 Oe. The magnetic properties were determined from Fig. 8 and all values are listed in Table 4. The saturation magnetization (Ms) significantly increased with the addition of Cr3+. For CrZF samples, when y was increased from 0.025 to 0.1, the Ms value increased from 0.0237 emu·g−1 to 0.1498 emu·g−1. Mostly, the Cr3+ replaced the Fe3+ in octahedral sites, decreasing the magnetic moment of CrZF at low doping concentrations. Hc values were clearly affected by Cr substitution. As observed from Table 4, with an increase in the Cr3+ concentration, the Hc value decreased for samples with y = 0.025 and 0.05, while it increased for samples with y = 0.075 and 0.1. As reported, the Co-doped ZnFe2O4 sample exhibited the highest coercivity value (~ 1834 Oe), which was attributed to the particle size effect, nature of the dopant, and high doping concentration.22)
As the applied field increased, the magnetization increased from zero in both directions, and saturation in the hysteresis loop was not observed, which could be due to the presence of non-magnetic Zn2+ ions and weakly magnetic Cr3+ ions in the spinel ferrites. Usually, three kinds of super-exchange interactions occur in spinel ferrites (A-A, B-B, and A-B), according to Neel’s two-sublattice model. The strength of A-B interaction was higher than those of the other two interactions. ZnFe2O4 showed the lowest magnetization.22) However, doping of Cr into zinc ferrite slightly decreased the magnetization for samples with y = 0.025 and 0.05 due to the substitution of weakly magnetic Cr3+ ions from B-site replaces the comparatively high magnetic Fe3+ ions in B-site was affected due to the A-B interaction. Non-linear magnetization values were obtained because of the presence of Zn2+ ions, which caused spin canting. The Cr0.025ZnFe1.975O4 sample exhibited a low coercivity value of 40 Oe because the replacement of Fe3+ by Cr3+ ions reinforced the sublattice interaction, indicating that the particle was easily magnetized without any flux loss. The changing nature of hysteresis loops with increasing Cr concentration makes this type of ferrites suitable for various applications, such as microwave devices, transformer cores, etc.
4. Conclusions
In the present study, we produced compositionally different nanoparticles of CrZF (0 ≤ y ≤ 0.1) by combining highly soluble nitrate salts of Fe, Cr, and Zn in the presence of CTAB by a chemical co-precipitation technique at a lower calcination temperature compared to that used in the ceramic method. The TG analysis result of the as-prepared sample revealed that above 450°C, the stable phase of chromium zinc ferrite was formed. The XRD results proved that the crystallite size increased from ~ 22 nm to 36 nm with the substitution of Cr3+ into the single cubic spinel structure for all CrZF samples. Furthermore, Cr3+ doping-induced changes were investigated by FTIR spectroscopy on the basis of the bond stretching vibrations of tetrahedral and octahedral metal complexes in the range of 400-600 cm−1. After doping of Cr3+ into zinc ferrite samples, certain deviations in the skeletal and X-ray densities, crystallite size, unit cell volume, and ionic length of the tetrahedral and octahedral sites were observed for ZnFe2O4. The FE-SEM images revealed that all samples were porous and the particles were nearly uniform shaped with an average size of 0.19-0.23 μm; the average size decreased after the incorporation of Cr3+ due to agglomeration at the elevated calcination temperature. The influence of Cr3+ ions caused noticeable variations in the structural, morphological, electrical, and magnetic properties of the CrZF (0 ≤ y ≤ 0.1) spinel ferrite, which strongly depended on the chemical composition, size, and preferential distribution of cations at (A) and (B) sites. Thus, Cr doping in the zinc ferrite increased the resistivity. The incorporation of Cr3+ ions into zinc ferrite led to a decrease in the saturation magnetization, coercivity, and magnetic moment at very low concentrations, and an increase in the magnetic moments of the unit cells at high concentrations. It was confirmed that the Cr-Zn ferrites are soft magnetic materials, which could be used in transformers and motors.
PDF Links
PubReader
ePub Link
Full text via DOI
Download Citation 
Print 
