Author’s Accepted Manuscript Optical and magnetic properties of monophasic cadmium ferrite (CdFe2O4) nanostructure prepared by thermal treatment method Mahmoud Naseri www.elsevier.com/locate/jmmm
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S0304-8853(15)30150-5 http://dx.doi.org/10.1016/j.jmmm.2015.05.026 MAGMA60201
To appear in: Journal of Magnetism and Magnetic Materials Received date: 29 October 2014 Revised date: 7 May 2015 Accepted date: 9 May 2015 Cite this article as: Mahmoud Naseri, Optical and magnetic properties of monophasic cadmium ferrite (CdFe2O4) nanostructure prepared by thermal treatment method, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2015.05.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Optical and magnetic properties of monophasic cadmium ferrite (CdFe2O4) nanostructure prepared by thermal treatment method Mahmoud Naseria,∗ a
Department of Physics, Faculty of Science, Malayer University, Malayer, Iran
Abstract This paper reports optical and magnetic properties of CdFe2O4 nanostructure which was prepared by a simple thermal treatment method. Calcination was conducted at temperatures between 673 and 773 K, and final products had different crystallite sizes ranging from 47 to 138 nm. The influence of calcination temperature on the degree of crystallinity, microstructure, and phase composition was investigated by different characterization techniques, i.e., X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), and Fourier transform infrared spectroscopy (FT-IR), respectively. The compositions of the samples were determined by Energy dispersive X-ray analysis (EDXA). The effect of calcination temperature on band gap energy was studied by UV-vis absorption spectra. The formed nanostructures exhibited ferromagnetic behaviors with unpaired electrons spins, which was confirmed by using vibrating sample magnetometer (VSM) and electron paramagnetic resonance (EPR) spectroscopy. Keywords: Nanostructured materials; Optical properties; Magnetic measurements; X-ray diffraction
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1. Introduction Spinel ferrites are one of the most well-known magnetic materials, and they have been investigated intensively in recent years from a purely scientific perspective and because of their unique magnetic, electrical, and optical properties . The spinel structure has the general formula of A2+B23+O4 in which A (the tetrahedral site) represents a divalent metal ion, and B (the octahedral site) represents a trivalent metal ion . Spinel ferrites, which are a subcategory of the spinel structure, consist of a close-packed oxygen arrangement in which 32 oxygen ions form a unit cell. There are several interstices between the layers of oxygen ions that can be separated into two types, i.e., A and B sites, depending on the coordination of the nearest neighboring oxygen ions. In the unit cell, only eight of 64 tetrahedral sites and 16 of 32 octahedral sites are occupied by metal ions . In the case in which B3+ = Fe3+, the resulting spinel ferrites have a general chemical composition of MFe2O4 (e.g., M = Cu, Mn, Zn, Ni, Co) and are used extensively as magnetic materials . Among the spinel ferrite compounds, cadmium ferrite has been studied extensively due to its high electromagnetic performance, excellent chemical stability, mechanical hardness, low coercivity, and moderate saturation magnetization, which make it a good contender for applications as soft magnets, low-loss materials at high frequencies . It has been established that CdFe2O4 has a crystal with cubic symmetry and normal structure . Various methods for synthesizing spinel ferrite nanoparticles have been reported in the literature. Some of the best-known methods are solid state reaction , the sol–gel process , combustion synthesis , the sonochemical technique , the electrochemical method , hydrothermal method , precursor route , co-precipitation , micro-emulsions , reverse micelles , the solvo-thermal method , ball milling , and the microwave processing . Factors and various precipitation agents were used to produce spinel ferrite
nanocrystals of specific sizes and shapes. Examples include the metal hydroxide in coprecipitation method; the surfactant and ammonia in reverse micelles method; the microemulsion method; the organic matrices in the polymeric precursor, sol-gel, and polyol methods, high energy in ball milling; and high-temperature in solid-state reaction. Most of these methods have achieved particles of the required sizes and shapes, but they are difficult to use on a large scale because of the expensive and complicated procedures involved, the high reaction temperatures, the long reaction times, the toxic reagents used, and the by-products produced that have the potential to harm the environment. The thermal treatment method for preparing ferrite nanoparticles is a simple and convenient method that is environmentally friendly in that it does not use or produce toxic substances, and it offers the advantages of simplicity, low cost, low reaction temperatures that make it suitable for use in industrial applications [19-21]. As a result of the present work, the synthesis of CdFe2O4 nanoparticles and the effects of calcination temperature on their properties are reported for the first time, which is a significant extension and enhancement of the thermal treatment method. 2. Experimental 2.1. Materials For the synthesis of CdFe2O4 nanoparticles, metal nitrate reagents were used as precursors, poly (vinylalcohol) (PVA) was used as the capping agent, and deionized water was used as the solvent. Iron nitrate, Fe (NO3)3•9H2O, and Cadmium nitrate, Cd (NO3)2•6H2O, were purchased from Acros Organics with purities exceeding 99%. PVA (MW = 31000 g/mol) was purchased from Sigma Aldrich and was used without further purification.
2.2. Methodology An aqueous solution of PVA was prepared by dissolving 4 g of polymer in 100 ml of deionized water at 353 K, then mixing 0.2 mmol iron nitrate and 0.1 mmol cadmium nitrate (Fe:Cd = 2:1)into the polymer solution and constantly stirring for 2 h using a magnetic stirrer until a colorless solution was obtained. No precipitation of materials was observed before the heat treatment. The mixed solution was poured into a glass Petri dish and heated at 373 K in an oven for 24 h to evaporate the water. The dried solid precursor that remained was crushed and ground in a mortar to form powder. The calcinations of the powders were conducted at 673, 723, and 773 K for 3 h for the decomposition of organic compounds and the crystallization of the nanocrystals. 3. Characterization The nanostructure of the CdFe2O4 nanoparticles was characterized by the XRD technique using a Philips X-pert type instrument with Cu Kα radiation and a wave length λ=1.5405 Å radiation to generate diffraction patterns from powder crystalline samples at ambient temperature in a 2θ range of 10°–80°. The microstructures of the CdFe2O4 nanostructures were studied FESEM using JEOL JSM-6701F type instrument coupled with EDX for elemental analysis. FT-IR spectra were recorded using a PerkinElmer FTIR model 1650 spectrometer. Before recording the spectra, the samples were placed on a Universal ATR Sampling Accessory (diamond coated with CsI) and pressed, and then the spectra were recorded. The band gap energies of the CdFe2O4 nanostructures were characterized using UV-visible absorption spectroscopy (UV1650PCSHIMADZU). Magnetization measurements were conducted using a VSM (Lake Shore 4700) at room temperature with a maximum magnetic field of 10kOe. EPR spectra were recorded on a JEOL JES-FA200 EPR spectrometer (JEOL, Tokyo, Japan) at room temperature.
4. Results and discussion 4.1 Role of PVA in the synthesis of CdFe2O4 nanoparticles To investigate the role of PVA in the synthesis of CdFe2O4 nanoparticles, cadmium ferrite nanoparticles initially synthesized in the absence of PVA which was calcined at 723 K. Fig. 1 shows the microstructures of these cadmium ferrite nanoparticles were obtained by FESEM. Fig.1 confirmed that cadmium ferrite nanoparticles were successfully formed even in the absence of PVA. However, it was observed that the nanoparticles did not have a uniform distribution of shapes as expected, and they were aggregated extensively. Moreover, in some areas, a complete disproportionately distribution was observed. Thus, it can be mentioned that without the use of PVA in the synthesis of nanoparticles, the small nanoparticles aggregate and produce larger nanoparticles , which is due to the existence of the high surface energy for the nanoparticles [23,24]. In addition, PVA plays three crucial roles in synthesizing cadmium ferrite nanoparticles, i.e., (1) controlling of the growth of the nanoparticles; (2) prevention of agglomeration of the nanoparticles; and (3) production of nanoparticles that have a uniform distribution of shapes [22, 25]. Fig.1. 4.2. Crystallinity, microstructure and elemental composition calcined nanoparticles Fig. 2 presents the XRD patterns of the precursor and cadmium ferrite nanoparticles at different temperatures. The XRD results indicate that before the calcination procedure, only a broad reflection of the precursor can be distinguished (Fig.2a). In the present method, cadmium ferrite nanoparticles with cubic symmetry were formed when the precursor was calcined in the temperature range of 673–773 K. After calcinations, all the samples showed the coexistence of the cadmium oxide CdO (ICDD: 001-1049) with the cubic symmetry , and the hematite
phases Fe2O3 (α-Fe2O3) (ICDD: 001-1053) . In addition, the diffraction lines of these spinel cadmium ferrite nanoparticles can be indexed readily to the cubic-type CdFe2O4 (ICDD: 0011087) . The value of the lattice parameter ‘a’ for our cadmium ferrite nanoparticles evaluated from the XRD spectra are recorded in Table 1 in the approximate range of 0.8677 to 0.8693nm which agrees with the bulk sample (0.86970 nm) . Fig.2. and Table.1. The microstructure of the cadmium ferrite nanoparticles was investigated by FESEM, as shown in Fig. 3. The FESEM micrographs show that the microstructures were affected by the calcination temperature. An irregular structure image is revealed in the precursor (as shown in Fig.3a), which his still amorphous. Table 1 demonstrates that the values of the estimated diameters of the cadmium ferrite nanoparticles calcined at 673 and723 were 47, and 68 nm, respectively. The nanoparticles calcined at 773 K created an almost uniform distribution of shapes with extensive agglomeration which had the average size of 138 nm as shown in Fig. 3d. The appearance of some agglomerated areas in the FESEM images was due to the naturallyoccurring interaction between magnetic nanoparticles . In many cases of nanocrystalline magnetic materials, it has been observed that there is a tendency for the nanoparticles to agglomerate . Heat treatment also can result in agglomeration of the nanoparticles as a function of calcining temperature; therefore some degree of agglomeration at the higher calcination temperature appears to be unavoidable , as it has happened for cadmium ferrite nanoparticles well as. Fig.3. The composition of the synthesized nanoparticles was determined by using EDXA, and the pattern that was obtained is shown in Fig. 4. The EDXA spectrum of cadmium ferrite
nanoparticles calcined at 723 K revealed the presence of Cd, Fe and O peaks in the sample, which is in agreement with the XRD results discussed earlier in connection with Fig.2. In this experiment, the Au peak was assigned to the gold substrate that was used to hold the sample. Fig.4. 4.3. Optical, magnetics properties and magnetic resonance study Fig. 5a shows the FT-IR spectrum of the precursor in the wave-number range between 250 and 4000 cm−1. The band with a peak at 1033 cm−1 was assigned to the bands related to C–O–C group . An important absorption peak was verified at 1138 cm−1. According to the literature , this vibrational band is attributed mainly to the crystallinity of the PVA, related to the carboxyl stretching band (C–O). Moreover, the band at 1714 cm−1 was attributed to the stretching vibration of the carbonyl group, C=O, from the aldehyde group . There was a band at 2991 cm−1 that was associated with C–H stretching related to the alkyl groups, and, finally, the large band observed at 3491 cm−1 was linked to the stretching of O–H from the inter-molecular and intra-molecular hydrogen bonds . It can be seen that the FT-IR spectra of the calcined cadmium ferrite nanoparticles (Figs. 5b to 5d) do not indicate the presence of any residual organic compounds and confirm the formation of the organic-free cadmium ferrite nanoparticles. There was still a trace of a broad band absorption peak at temperatures less than 773 K that related to O–H stretching vibration (shown in Fig. 5b and 5c). Therefore, crystallization of cadmium ferrite nanoparticles occurred at temperatures as low as 773 K. The FT-IR spectra of all cadmium ferrite nanoparticles revealed two principle absorption bands in the range of 250-600 cm-1, with the first band (ν1) between 280 to 370 cm-1 and the second band (ν2) between 550 to 585 cm-1. These two, main and broad metal-oxygen bands correspond to the intrinsic stretching vibrations of the metal at the tetrahedral site, Mtetra↔O (observed from 553 to 584 cm-1) and
octahedral-metal stretching, Mocta↔O (observed from 284 to 367 cm-1) (Table 1) .The negligible difference in the frequencies between the characteristic vibrations ν1 and ν2 may be attributed to the long bond length of the oxygen-metal ions in the octahedral sites and the shorter bond length of the oxygen-metal ions in the tetrahedral sites . Fig.5. Fig. 6 shows the Kubelka-Munk remission function (i.e. relationship of [F∞hv]1/2 with the photon energy) corresponding to each spectrum. Fig. 6a indicated the band gap of the precursor was 4.241eV, which was estimated with the linear fit method and showed a dielectric property. Figs. 6b to 6d show that the tendency of band gap energy values to decrease is consistent with the enhancement of calcination temperature in the CdFe2O4 nanostructures that were prepared by the thermal treatment method. The curves disclosed when calcination temperature increased from 673 to 773 K, the estimated band gap energy values of CdFe2O4 nanostructures decreased from 2.0614 to 2.0215eV as listed in Table1. This behavior can be better understood if one considers that the interatomic spacing increases when the amplitude of the atomic vibrations increases due to the increased thermal energy . An increased interatomic spacing decreases the potential seen by the electrons in the material, which in turn reduces the size of the energy band gap . The band gap energies of the CdFe2O4 nanostructures which calcined by thermal treatment method are greater than the theoretical energy required for water splitting (Eg> 1.23 eV); thus, they are suitable candidates for the role of a visible-light photo-catalyst . Fig.6. Fig. 7 shows the magnetic hysteresis loops of the precursor and the nanocrystalline cadmium ferrite calcined at different temperatures, which were measured at room temperature in the range
of approximately −10 to +10 kOe. The insert shows expanded field region around the origin for clear visibility of the readers, in the range of approximately −500 to +500 Oe. Except for the precursor (shown in Fig. 7a), which was a nonmagnetic material, the calcined samples at 673, 723 and 773 K exhibited ferromagnetic behaviors (Figs. 7b-d). Table 1 depicts the values of coercivity field (HC) that are about 153, 141 and 136 Oe for the calcined cadmium ferrite samples at 673,723and 773 K respectively. It was found that the values of HC for the cadmium ferrite samples were observed to decrease with increasing temperature. The variation in the value of the HC with calcination temperature and particle size can be explained on the basis of domain structure, critical size, and the anisotropy of the crystal . Table 2 also provides the values of saturation magnetization (Ms) of the calcined samples, along with calcinations temperatures and particle sizes. These data make it clear that different parameters were responsible for the saturation magnetization decreasing from 23.34 to 12.15 emu/g when the calcination temperature increased from 673 to 773K. Cation inversion is one of the most important parameters that can be effective in the variation of the magnetic properties of cadmium ferrite nanoparticles from the properties of the bulk form of the same material. In bulk form, cadmium ferrite has a normal spinel structure in which all Cd2+ ions are in A sites and Fe3+ ions are distributed in B sites . However, in the bulk form, cadmium ferrite only occurs in intra-sub-lattice (B-B) exchange interactions, and it does not have intra-sub-lattice (A-A) exchange interactions or inter-sublattice (A-B) super-exchange interactions . Inter-sub-lattice (A-B) super-exchange interacvtions of the cations are much stronger than the (A-A) and (B-B) interactions .Due to the cation inversion, which originates from thermal and mechanical treatment (as shown earlier in Fig.6) , the structure of CdFe2O4 transfers from a normal spinel structure to a mixed spinel structure . This cation inversion causes the cadmium ferrite nanoparticles to experience inter-
sub-lattice (A-B) super-exchange interactions and intra-sub-lattice (A-A) exchange interactions in addition to intra-sub-lattice (B-B) exchange interactions. But, due to the degree of inversion, which is large for smaller size particles, inter-sub-lattice (A-B) super-exchange interactions in smaller size particles occur to a greater extent than in larger size particles. Hence, saturation magnetization increases for smaller size particles . M. K. Roy et al. , using Mossbauer’s experiment, showed that the degree of inversion is large in the case of smaller size particles. Also, an impure α-Fe2O3 phase was detected by XRD (Fig. 2), and the surface spin structure can be an influence that increases the saturation magnetization in smaller size particles . Fig. 7 and Table. 2. Fig. 8 shows the EPR spectra of the samples calcined at (a) 673 (b) 723, and (c) 773 K exhibited broad, symmetrical signals. Peak-to-peak line width (ΔHpp), resonant magnetic field (Hr) and gfactor are three parameters that characterize the magnetic properties. It is obvious from Table 2 that the values of ΔHpp increase from 596 to 1443 Oe and the values of g-factor increase from 1.9937 to 2.0315 when the calcination temperature and particle size were increased. In ferrites, variations of ΔHpp and g-factor can be due to dipole-dipole interactions and super-exchange interactions . Table 2 also shows that the value of the resonant magnetic field decreased from 3235 to 2880 Oe as the calcination temperature increased. According to the equation1: g = hv/βH
where h is Planck’s constant, v is the microwave frequency, β is the Bohr magneton (9.274×10-21 erg Oe-1), and H is resonant magnetic field. The resonance magnetic field should decrease when g-factor increases, whereas v is constant in EPR spectroscopy. The addition of Fe3+ ions to an A site, as was discussed in the last part, causes an increase in the super-exchange interactions, contributing to the increase of the internal field and the decrease of the resonance magnetic field
. Increases in ΔHpp and g-factor and decreases in Hr with increasing calcination magnetization values have been reported in ZnFe2O4 nanoparticles which also was a normal spinel ferrite . Fig. 8. 5. Conclusions The nanocrystalline form of CdFe2O4 with a cubic crystal structure was fabricated successfully using a simple, thermal treatment method. The influence of calcination temperature on degree of crystallinity, microstructure, and phase composition was investigated by different characterization techniques, i.e., XRD, FESEM, and FT-IR, respectively. An increase in particle size from 47 to 138 nm was observed when the calcination temperature was increased from 673 to 773 K. PVA was utilized as a capping agent for stabilizing the particles, controlling the growth of the nanoparticles, preventing their agglomeration, and creating a uniform distribution of particle sizes. EDXA was used to characterize the composition of the samples, and it confirmed the presence of Cd, Fe, and O in the sample. Magnetic results were obtained by VSM, and they showed that saturation magnetization and the coercivity field decreased when the calcination temperature increased. Electron paramagnetic resonance (EPR) spectroscopy showed the existence of unpaired electrons and measured peak-to-peak line width (ΔHpp), resonant magnetic field (Hr), and the g-factor values. This simple method, which is cost-effective and environmentally friendly, produces no toxic byproducts and can be used to fabricate pure, crystalline spinel zinc ferrite nanocrystals. Furthermore, this method can be extended to the synthesis of other ferrite nanoparticles of interest. Acknowledgments This work was supported by the Ministry of science research and technology of Iran under the FRGS grant, Malayer University of Iran.
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Figure captions Fig.1. FESEM image of cadmium ferrite nanoparticles in the absence of PVA calcined at 723 K.
Fig.1. Fig.2. XRD patterns of precursor (Fig2a) and cadmium ferrite nanoparticles calcined at (b) 673, (c) 723 and (d) 773 K.
Fig. 2. Fig.3. FESEM microstructures of precursor (Fig2a) and cadmium ferrite nanoparticles calcined at (b) 673, (c) 723 and (d) 773 K.
Fig.3. Fig.4. EDXA spectrum of cadmium ferrite nanoparticles calcined at 723 K.
Fig.4. Fig.5. FT-IR spectra of precursor (Fig6a) and cadmium ferrite nanoparticles calcined at (b) 673, (c) 723 and (d) 773 K.
Fig.6. Tauc plot of band gap energy for (a) precursor and cadmium ferrite nanoparticles calcined at (b) 673, (c) 723 and (d) 773 K. 22
Fig.6. Fig.7. Hysteresis loops of precursor (Fig7a) and cadmium ferrite nanoparticles calcined at (b) 673, (c) 723 and (d) 773 K.
Fig.8. EPR spectra of cadmium ferrite nanoparticles calcined 673, 723 and 773 K. K.
H H H H
H H H
O H OH H O H OH H O H O H
H O H O H O H OH H OH
OH H OH
H OH H O H
OH H OH
H H H
O H O H OH H O H OH HO
O H HO HO H HO H O H
H H O H O H
O H O H OH H O H OH HH
O HO H O HO H HO
H O H
HO H H O H O H O H
H O H
H H H Calcination
H H H H H H
Graphical abstract Table captions Table.1. Average particle sizes (nm) of CdFe2O4 nanoparticles determined from FESEM and wave-number obtained from FT-IR spectroscopy
Table. 1. Specimens CdFe2O4
Calcination temperature (K)
Average particle size FESEM (nm)
Lattice Parameter a (nm)
Wave number (cm−1)
Band gap (ev)
Saturation magnetization Ms (emu/g)
Coercivity Hc (Oe)
Magnetic Gromagnetic resonance ratio Hr(Oe) (g-value)
Table.2. Optical and Magnetic parameters of CdFe2O4 nanoparticles observed by UV-visible, VSM and EPR techniques.
> Metal nitrates were added into an aqueous solution of PVA and the mixed solution was heated at 373 K to evaporate the water >The effect of calcination temperature on crystallinity, morphology and microstructure of cadmium ferrite (CdFe2O4) nanostructures was characterized > Elemental composition, phase composition, optical properties, magnetic properties and magnetic resonance were investigated 27