Impact of grain size and structural changes on magnetic, dielectric, electrical, impedance and modulus spectroscopic characteristics of CoFe2O4 nanoparticles synthesized by honey mediated sol-gel combustion method

Advances in Natural Sciences: Nanoscience and Nanotechnology PAPER • OPEN ACCESS Impact of grain size and structural changes on magnetic, dielectric, electrical, impedance and modulus spectroscopic characteristics of CoFe2O4 nanoparticles synthesized by honey mediated solgel combustion method To cite this article: Raghvendra Singh Yadav et al 2017 Adv. Nat. Sci: Nanosci. Nanotechnol. 8 045002 View the article online for updates and enhancements. Related content Studies on structural, optical and magnetic properties of cobalt substituted magnetite fluids (CoxFe1xFe2O4) Blessy Babukutty, Nandakumar Kalarikkal and Swapna S Nair - Effect of Mn2+ doping and SiO2 coating on magneto-optical properties of CoFe2O4 nano-particles Kamal R Awad, M M S Wahsh, A G M Othman et al. - Influence of Rare Earth (Gd3+) on Structural, Gigahertz Dielectric and Magnetic Studies of Cobalt ferrite Erum Pervaiz and I H Gul - This content was downloaded from IP address 195.178.95.132 on 29/06/2018 at 13:36 1 © 2017 Vietnam Academy of Science & Technology RaghvendraSinghYadav1, IvoKuřitka1, JarmilaVilcakova1, JaromirHavlica2, JiriMasilko2, LukasKalina2, JakubTkacz2, JiříŠvec2, VojtěchEnev2 and MiroslavaHajdúchová2 1 Centre of Polymer Systems, University Institute, Tomas Bata University in Zlín, Trida Tomase Bati 5678, 760 01 Zlín, Czechia 2 Materials Research Centre, Brno University of Technology, Purkyňova 464/118, 61200 Brno, Czechia E-mail: yada[email protected].cz Received 9 February 2017 Accepted for publication 26 July 2017 Published 29 August 2017 Abstract In this work CoFe2O4 spinel ferrite nanoparticles were synthesized by honey mediated sol-gel combustion method and further annealed at higher temperature 500 °C, 700 °C, 900 °C and 1100 °C. The synthesized spinel ferrite nanoparticles is investigated by x-ray diffraction, Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis/differential scanning calorimetry (TGA/DSC), field emission scanning electron microscopy, x-ray photoelectron spectroscopy and vibrating sample magnetometer. The x-ray diffraction study reveals face-centered cubic spinel cobalt ferrite crystal phase formation. The crystallite size and lattice parameter are increased with annealing temperature. Raman and Fourier transform infrared spectra also confirm spinel ferrite crystal structure of synthesized nanoparticles. The existence of cation at octahedral and tetrahedral site in cobalt ferrite nanoparticles is confirmed by x-ray photoelectron spectroscopy. Magnetic measurement shows increased saturation magnetization 74.4 emu g−1 at higher annealing temperature 1100 °C, high coercivity 1347.3 Oe at lower annealing temperature 500 °C, and high remanent magnetization 32.3 emu g−1 at 900 °C annealing temperature. The magnetic properties of synthesized ferrite nanoparticles can be tuned by adjusting sizes through annealing temperature. Furthermore, the dielectric constant and ac conductivity shows variation with frequency (1–107 Hz), grain size and cation redistribution. The modulus spectroscopy study reveals the role of bulk grain and grain boundary towards the resistance and capacitance. The cole-cole plots in modulus formalism also well support the electrical response of nanoparticles originated from both grain and grain boundaries. The dielectric, electrical, magnetic, impedance and modulus spectroscopic characteristics of synthesized CoFe2O4 spinel ferrite Advances in Natural Sciences: Nanoscience and Nanotechnology Impact of grain size and structural changes on magnetic, dielectric, electrical, impedance and modulus spectroscopic characteristics of CoFe2O4 nanoparticles synthesized by honey mediated sol-gel combustion method R S Yadav etal 045002 ANSNCK © 2017 Vietnam Academy of Science & Technology 8 Adv. Nat. Sci.: Nanosci. Nanotechnol. ANSN 2043-6254 10.1088/2043-6254/aa853a Paper 4 Advances in Natural Sciences: Nanoscience and Nanotechnology IOP Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 2017 2043-6254/17/045002+14$33.00 https://doi.org/10.1088/2043-6254/aa853a Adv. Nat. Sci.: Nanosci. Nanotechnol. 8 (2017) 045002 (14pp) R S Yadav etal 2 nanoparticles demonstrate the applicability of these nanoparticles for magnetic recording, memory devices and for microwave applications. Keywords: spinel ferrite, nanoparticles, green synthesis, magnetic property, dielectric property Classification numbers: 2.03, 4.02, 5.02 1. Introduction Recently, spinel ferrite nanoparticles have been extensively studied due to its interesting properties for various applications [1–3]. Among the spinel ferrites, CoFe2O4 is an attractive material due to its appreciated properties such as high coercivity, moderate saturation magnetization, large magnetocrystalline anisotropy, high electrical resistivity, high magnetostrictive coefficient, good mechanical hardness and chemical stability [4, 5]. It has technological potential applications in high density recording systems, microwave absorbers, targeted drug delivery systems, magnetic resonance imaging contrast agent, high value of the specific absorption rate for hyperthermia treatment, cell separation and detection, catalyst for water splitting, removal of heavy metals from waste water, magnetic switches, chemical sensors, stress and non-contact torque sensors, etc [6–13]. The properties of spinel ferrite nanoparticles are highly sensitive to the various factors such as preparation techniques, particle size, morphology, annealing temperature and cation distribution at tetrahedral and octahedral sites. Bulk CoFe2O4 possesses an inverse spinel structure with Co2+ ions in octahedral site and Fe3+ ions equally distributed between tetrahedral and octahedral sites. However, nanosized CoFe2O4 ferrite nanoparticles exhibit cationic inversion and percentage of presence of Co2+ and Fe3+ ions at both sites depends on the method of preparation of spinel ferrite nanoparticles [14]. The predominant superexchange interaction between the cations at tetrahedral and octahedral sites via oxygen ions influences the magnetic properties of spinel ferrite [15]. In the last decade, spinel ferrite nanostructures have been synthesized by various chemical synthesis methods such as sol-gel, co-precipitation, hydrothermal, solvothermal, sonochemical method, electrochemical, reverse micelles [16–21]. Nowadays, there is a great need to develop simple, costeffective and environment friendly methods for production of spinel ferrite nanoparticles. Recently, biosynthesis is an alternative synthesis technique to synthesize spinel ferrite nanoparticles. Laokul etal [22] demonstrated the use of a simple synthetic method using cheap precursors of aloe vera plant extract which provides high-yield nanosized ferrites with well crystalline structure and acceptable magnetic properties, and the method is expected to prepare nanocrystalline oxides of other interesting materials. Phumying etal [23] reported synthesis of ferrite nanocrystalline powders using aloe vera plant extract solution in hydrothermal synthesis technique. Moreover, Manikandan et al [24] synthesized ferrite nanoparticles via aloe vera plant extract using microwave combustion method. Wongpratat etal [25] investigated the effect of cations distribution on magnetic properties of Co1−xNixFe2O4 (x = 0, 0.25,0.50, 0.75 and 1.0) nanoparticles synthesized by aloe vera extract solution assisted hydrothermal method. Kombaiah etal [26] studied the microwave assisted and conventional combustion synthesis of hibiscus rosa-sinensis plant extract based ZnFe2O4 nanoparticles and their optical and magnetic properties. The hibiscus rosa-sinensis is an herbaceous plant and the bio-chemical constituents of this plants are taraxerol acetate, β-sitosterol, campesterol, stigmasterol, cholesterol, ergosterol, lipids, citric, tartaric and oxalic acids, fructose, glucose, sucrose, flavonoids and flavonoid glycosides. Sun et al [27] reported magnetic properties of the nanocrystalline CoFe2O4 ferrite thin films prepared by a novel sol–gel method using glucose as an additional agent. Further, Tong et al [28] developed a versatile glucose-engineered precipitation–sintering process for the selective and mass preparation of sponge-like ferrite (M = Fe, Co, Zn, Ni, Mn) micro-polyhedra. In this research group’s reported work, glucose acted as a reductant, protecting agent, structure-directing agent and sacrificial template. Furthermore, our research group also investigated the effect of structural changes on magnetic properties of ferrite nanoparticles obtained using starch in sol-gel combustion synthesis [29–31]. It is interesting to study formation and property of spinel ferrite nanoparticles synthesized via a method mediated through natural source of glucose and fructose, i.e. honey. Herein, we used a green synthesis route via honey mediated sol-gel auto-combustion method to prepare CoFe2O4 spinel ferrite nanoparticles. This is a novel way with a unique combination of biological species with an aqueous solution containing salts of desired metals for sol-gel and self-igniting combustion process. The term green synthesis is used since natural honey is used for synthesis of ferrite nanoparticles which act as gelating and reducing agent or fuel for sol-gel auto-combustion synthesis. Honey is sweet viscous fluid made by bees and the major constituents of honey are glucose and fructose [32]. The presence of glucose and fructose in natural honey obviously plays a role on the formation of nanoparticles as viscous medium, protecting agent and natural reductant. The synthesis of ferrite nanoparticles using honey avoids the usage of harmful and toxic reducing agent in combustion method. The use of honey is an environmently friendly, simple and efficient route for formation of cobalt ferrite nanoparticles. In the present work honey is used for synthesis of CoFe2O4 spinel ferrite nanoparticles. Further, the effect of grain size and structural changes on magnetic, dielectric and electrical properties are investigated. Furthermore, the detailed study of modulus and impedance spectroscopy is carried out to reveal the contribution of grain and grain boundary on electrical transport mechanism and relaxation process. Adv. Nat. Sci.: Nanosci. Nanotechnol. 8 (2017) 045002 R S Yadav etal 3 2. Experimental 2.1. Materials and preparation method The nanoparticles of CoFe2O4 spinel ferrite were synthesized using honey mediated self-propagating sol-gel auto-combustion method. All the chemicals used in our experiments, Fe(NO3)3·9H2O, Co(NO3)2·6H2O were of analytical grade and were purchased from Alfa Aesar GmbH & Co KG, Germany. Honey was product of Australia, made by bees that forage on eucalyptus trees, packed in the UK for Tesco Stores Ltd, Cheshunt, EN8 9SL, UK. In a typical synthesis, Co(NO3)2·6H2O (4.16 g) and Fe(NO3)3·9H2O (11.6 g) were dissolved in distilled water (150 ml) to obtain a mixed solution. An aqueous solution (150 ml) of honey (10 g) was mixed with the metal-nitrate solution. The mixed solution was placed on a hot plate with continuous stirring at 100 °C. During evaporation, the solution formed a very viscous gel. Then the gel was heated to 350 °C to initiate a self-sustaining combustion reaction and produce as-burnt ferrite powder. The main constituents of honey, i.e. glucose and fructose are ‘single’ sugars or monosaccharides. These monosaccharides have the same molecular formula (C6H12O6), however, the arrangement of atoms are different in each case. When the metal-honey ferrite gel precursor was placed at temperature 350 °C, the decomposition of glucose and fructose with release of gases nitrogen and carbon dioxide occurred and then final product cobalt ferrite powders formed. The following equation represents the decomposition of metal-honey ferrite gel precursors into cobalt ferrite: Co(NO 3 ) 2 ·6H 2 O+2Fe(NO 3 ) 3 ·9H 2 O+2C 6 H 12 O 6 +2O2 ∆ −→ CoFe2O4+4N2↑+27H2O↑+12CO2↑. Finally, the as-prepared cobalt ferrite nanoparticles powders were additionally annealed in a furnace in air atmosphere at 500 °C, 700 °C, 900 °C and 1100 °C, for 2 h, to achieve different nanosized ferrite samples. The samples as prepared and annealed at 500 °C, 700 °C, 900 °C, 1100 °C were designated as CoH, CoH5, CoH7, CoH9, CoH11, respectively. 2.2. Characterization techniques The structural characteristics of synthesized cobalt ferrite nanoparticles were investigated using PANalytical Empyrean x-ray diffractometer with Cu-Kα radiation (λ = 1.5406 Å). Raman measurements were performed using Nanofinder-S, SOLAR TII, Ltd, at 488 nm excitation with laser power 13 mW with Ar+ laser excitation source over the range of 150–850 cm−1. Fourier transform infrared (FTIR) spectroscopy of ferrite nanoparticles were recorded using Nicolet iS 50 FTIR spectrometer. The morphology of the synthesized spinel ferrite nanoparticles were studied using a field emission scanning electron microscope (FESEM) model JEOL JSM-7600F, equipped with an energy dispersive spectroscopy (EDS) system. Magnetic hysteresis loops were measured using a vibrating sample magnetometer (VSM 7407, Lake Shore) at room temperature with maximum applied magnetic field of 10 kOe. The thermal analysis was carried out using a simultaneous thermogravimetric and differential thermal analysis (TG-DTA) system using TA Instruments Q600. Cation distribution at octahedral and tetrahedral sites and valence states in samples were investigated by x-ray photoelectron spectroscopy (XPS) using Kratos Analytical Axis Ultra DLD. The dielectric constant and dielectric loss tangent were measured in the frequency range 1–107 Hz at room temperature using a Broadband Dielectric Impedance Analyzer Concept 40 (Novocontrol, Germany). The sample dimension of pellet of ferrite nanoparticles was 20 mm in diameter and 0.5 mm in thickness. The complex impedance measurement was carried out with a standard sample cell BDS 1200 employing RC model. 3. Result and discussion 3.1. Thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) study Thermal analysis was carried out for cobalt ferrite precursor gel to investigate the honey-mediated sol-gel auto-combustion process for formation of cobalt ferrite nanoparticles. Figure1 shows TGA/DSC curves of the dried gel of ferrite precursors. The TGA curve exhibits a multi-step weight loss. The weight loss from room temperature to 110 °C is due to loss of residual water in the ferrite precursor gel, which appears on the DSC curve as an exothermic peak at 90 °C [33]. Thereafter, a continuous three-step weight loss is noticed at temperature ranging from 148 °C to 350 °C. This also appears as three broad exothermic peaks at 154 °C, 252 °C, 350 °C on the DSC curve. The first weight loss is attributed to the decomposition of the honey (i.e. fructose and glucose). The second exothermic peak with large weight loss is attributed to the decomposition of the dried ferrite precursor gels which was initiated by oxidation–reduction reaction between the nitrate and honey and evo lution of CO2 and NOx gases [34]. The third exothermic peak is attributed to decomposition of the organic residual combustion. There is no further weight loss after 350 °C, therefore, it indicated pure cubic cobalt ferrite 0 200 400 600 800 1000 0 20 40 60 80 100 Temperature ( o C) Weight (%) 0 10 20 30 40 50 60 Heat flow (w/g) Figure 1. TGA/DSC study of sol-gel ferrite precursors for formation of cobalt ferrite nanoparticles by honey mediated sol-gel auto-combustion method. Adv. Nat. Sci.: Nanosci. Nanotechnol. 8 (2017) 045002 R S Yadav etal 4 spinel phase formation at this temperature via honey mediated sol-gel auto-combustion method. 3.2. Structural study The phase formation of synthesized cobalt ferrite nanoparticles was investigated by powder x-ray diffraction. Figure2 shows x-ray diffraction (XRD) patterns of CoFe2O4 spinel ferrite nanoparticles synthesized by honey mediated autocombustion synthesis and further annealed at 500 °C, 700 °C, 900 °C, and 1100 °C. The presence of diffraction planes (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0) in the diffraction patterns confirms that all the samples exhibit cubic spinel structure having space group Fd3m. All the diffraction peaks are well matched with the standard JCPDS data for CoFe2O4 (22-1086) [35]. The unindexed diffraction peak in CoH7 sample is due to presence of α-Fe2O3 phase [36]. The broad diffraction peak indicated that particles are of nanosize range. It is also noticeable that the diffraction peaks are sharper and narrower with an increase of annealing temperature. This indicated the improvement of crystallinity with increase of annealing temperature 500 °C, 700 °C, 900 °C and 1100 °C. The crystallite size and micro-strain analysis were evaluated using Williamson–Hall method [37]. According to this method, x-ray diffraction peak broadening βhkl =βsize +βstrain . The actual peak broadening ( β ) is obtained by correcting the experimental peak broadening ( βex ) and the instrumental broadening ( βin ) as β2=β2 ex −β2 in . Hence, XRD peak broadening βhkl =βsize +βstrain equationcan be in modified form as: β hkl = 0.94λ Dcosθ +4εtanθ , (1) β hklcosθ= 0.94λ D +4εsinθ , (2) where β is full width at half maximum, λ is wavelength of Cu-Kα radiation (λ = 1.5406 Å), ε is strain, and D is crystallite size. The Williamson Hall plots of βcosθ versus 4sinθ for cobalt ferrite nanoparticles are shown in figure3. A linear plot provides the intercept as the crystallite size and the slope as the strain. The obtained crystallite size and strain of cobalt ferrite nanoparticles are tabulated in table1. It can be observed that Figure 2. X-ray diffraction pattern of cobalt ferrite nanoparticles synthesized by honey mediated sol-gel auto-combustion synthesis and further annealed at 500 °C, 700 °C, 900 °C, and 1100 °C. Figure 3. Williamson–Hall plots for cobalt ferrite nanoparticles synthesized by honey mediated sol-gel combustion method. Adv. Nat. Sci.: Nanosci. Nanotechnol. 8 (2017) 045002 R S Yadav etal 5 the crystallite size increases with increase of annealing temperature. The size of CoH sample is 20 nm and increases to 88 nm for CoH11 sample. The strain value varies from 1.7 × 10−3 for CoH sample to −3.7 × 10−5 for CoH11 sample. The lattice constant of CoFe2O4 spinel ferrite nanoparticles has been calculated by d-spacing using the relation [38] a=dhklh 2 +k 2 +l 2 (3) where (h, k, l) are the Miller indices. The lattice constant was found to increase with increase of annealing temper ature, as shown in table 1. The increase in lattice constant with annealing obeys Vegard’s law [39]. The increase in lattice constant with increase of annealing temperature is associated with variation in microstructure during annealing of mat erial and thermal activated ordering or reordering of cations in cubic spinel structure. X-ray density (dx) of CoFe2O4 spinel ferrite nanoparticles can be calculated by considering that a basic unit cell of the cubic spinel structure contains 8 ions [40] as follows dx= 8M Naa 3 , (4) where M is the molecular weight of sample, Na is Avogadro’s number (6.0225 × 1023 particles/mole), and a is lattice parameter converted into cm units. The x-ray density (dx) values of CoFe2O4 spinel ferrite nanoparticles are tabulated in table1. It is found that the density is influenced by the annealing temper ature. Density of cobalt ferrite nanoparticles with high values for the small sized particle was observed. Further, the tetrahedral (A)-site radii (rA), octahedral (B)-site radii (rB), hopping length for tetrahedral site (dA), hopping length for octahedral site (dB), tetrahedral and octahedral bond length (dA× and dB×), tetrahedral edge (dA×E), shared and unshared octa hedral edge (dB×E and dB×EU) for synthesized cubic CoFe2O4 spinel ferrite nanoparticles are evaluated with lattice constant ‘a’, oxygen positional parameter ‘u’ (0.381 Å) and oxygen ion radius Ro from the following equations[41]: rA =( u−0.25 ) a√3−Ro,rB =( 0.625 −u ) a−Ro, dA=0.25a√3, dB=0.25a√2, dA×=a√3(u−1/4),dB×=a[3u 2 −(11/4)u+(43/64)]1/2, (5) dA×E=a√2(2u−1/2),dB×E=a√2(1−2u), dB×EU =a[4u 2 −3u+(11/16)]1/2. The evaluated values of these structural parameters are tabulated in tables 1 and 2. It is noticeable that the evaluated value of dA and dB increases with increase of grain size of ferrite nanoparticles. The increase in the value of dA and dB with increase grain size is associated with increase in distance between magnetic ions with increase of grain size. It can be also observed that dA ˃ dB; which indicates that the electron hopping between ions at tetrahedral A site and octahedral B site is less probable than that between octahedral B site and octahedral B sites. Furthermore, the evaluated value of rA, rB, dA×, dB×, dA×E, dB×E, dB×EU increases with increase of grain size. It is also associated with cation redistribution on grain size growth. 3.3. FESEM study Figure 4 shows the field emission scanning electron microscopy (FESEM) micrographs of cobalt ferrite nanoparticles. It can be observed from FESEM image that CoH sample nanoparticles were spherical with particle size range 10–25 nm, as shown in figure4(a). Further, it is also evident that particle size increases with annealing temperature. The CoH7 sample nanoparticles were with grain size 20–120 nm, whereas CoH11 sample nanoparticles were in the range of 70–150 nm, as ahown in figures 4(b) and (c). Figure 4(d) shows the typical EDX spectrum of the representative cobalt ferrite sample. This spectrum marks the presence of Co, Fe Table 1. Crystallite size, strain, lattice parameter, x-ray density (dx), radii of tetrahedral and octahedral sites rA and rB, respectively, for cobalt ferrite nanoparticles. Sample Crystallite size (nm) Strain Lattice parameter (Å) dx (gm cm−3)rA (Å)rB (Å) CoH 20 1.7 × 10−38.3817 5.292 0.5217 0.6651 CoH5 26 8.7 × 10−48.3844 5.287 0.5223 0.6657 CoH7 40 5.6 × 10−48.3862 5.284 0.5227 0.6662 CoH9 75 7.3 × 10−58.3875 5.282 0.5230 0.6666 CoH11 88 −3.7 × 10−58.3900 5.277 0.5236 0.6672 Table 2. Hopping length (dA) and (dB), tetrahedral bond length (dA×), octahedral bond length (dB×), tetrahedral edge (dA×E), shared (dB×E) and unshared octahedral edge (dB×EU) for cobalt ferrite nanoparticles. Sample dA (Å)dB (Å)dA× (Å)dB× (Å)dA×E (Å)dB×E (Å)dB×EU (Å) CoH 3.6292 2.9629 1.9017 2.0462 3.1051 2.8207 2.9650 CoH5 3.6304 2.9639 1.9023 2.0469 3.1061 2.8216 2.9660 CoH7 3.6312 2.9645 1.9027 2.0473 3.1068 2.8222 2.9666 CoH9 3.6318 2.9650 1.9030 2.0477 3.1073 2.8226 2.9671 CoH11 3.6328 2.9658 1.9036 2.0483 3.1082 2.8235 2.9680 Adv. Nat. Sci.: Nanosci. Nanotechnol. 8 (2017) 045002 R S Yadav etal 6 and O which further confirms the formation of pure CoFe2O4 spinel ferrite. 3.4. Raman spectroscopy Raman spectroscopy is a powerful technique for investigating the atomic structure of nanoparticles [42]. Figure 5 shows the Raman spectra of CoFe2O4 spinel ferrite nanoparticles. Group theory analysis predicts the following optical phonon distribution: 5T1u + A1g + Eg + 3T2g, in which, the 5T1u modes are IR active, whereas the other five (A1g + Eg + 3T2g) modes are Raman active composed to the motion of O ions and both A-site and B-site ions in the spinel structure [43]. Furthermore, the A1g mode is associated to symmetric stretching of the oxygen anion, the Eg mode is associated to symmetric bending of the oxygen anion, and the T2g mode is due to asymmetric stretching of the oxygen anion with respect to the tetrahedral and octahedral cations [44]. It can be seen from figure5 and table3 that as synthesized CoFe2O4 spinel ferrite nanoparticles shows Raman modes at ~198, ~297, ~461, ~558, ~604 and 680 cm−1. Raman modes at around 680 cm−1 shows a shoulder like feature at the lower wavenumber side (~604 cm−1). These bands were assigned to A1g(1) and A1g(2) modes, demonstrating the stretching vibrations of the Fe–O amd M–O bonds in tetrahedral sites. The lower frequency Raman modes (~198, ~297, ~461, and ~558 cm−1) were assigned to the T2g and Eg Raman modes, demonstrating the vibration of the spinel structure. It is also noticeable from figure 5 and table 3 that there were a little shift in Raman modes with increase of annealing temperature. It was associated with increase of particle size and cation redistribution in CoFe2O4 spinel ferrite annealed samples [42–44]. The cation redistribution can be noticed by comparing the relative intensities of the A1g(1) and A1g(2) modes. The change in ratio of intensity with increase of annealing temperature is observed and it is tabulated in table3. 3.5. FTIR spectroscopy The FTIR spectra of CoFe2O4 spinel ferrite nanoparticles were recorded in 75–900 cm−1 range, are as shown in figure6. The positions of the vibrational bands of CoFe2O4 spinel ferrite are tabulated in table4. In spinel ferrite structure, the vibrational band around 550 cm−1 corresponds to the stretching vibration of tetrahedral group and the vibrational band around 350 cm−1 corresponds to the stretching vibration of octahedral groups [45]. From figure6 and table4, it can be seen that the FTIR spectra consist of two major absorption bands, the first at about 550 cm−1 (ν1) and the second one at about Figure 4. Cobalt ferrite nanoparticles with FESEM images (a) as prepared, (b) annealed at 700 °C, (c) annealed at 1100 °C, and EDX pattern (d). Figure 5. Raman spectra of cobalt ferrite nanoparticles at different annealing temperatures. Adv. Nat. Sci.: Nanosci. Nanotechnol. 8 (2017) 045002 R S Yadav etal 7 350 cm−1 (ν2). These absorption bands further confirm the formation of CoFe2O4 spinel ferrite structure. A small shift in the absorption bands of the tetrahedral and octahedral sites can be observed from figure6. The force constant, which is proportional to atomic number of metal ions, atomic number of oxygen ions and metal-oxygen bond length, is associated with the shift of absorption band position due to tetrahedral (A) sites and octahedral (B) sites, respectively [46]. Further, the value of force constant, namely, FT and FB for the tetrahedral (A) and the octahedral (B) sites, respectively, are evaluated from the following equation[47] F=4π2c2ν2µ, (6) where F is the force constant, c is the light velocity 2.99 × 108 m s−1, ν is the vibration frequency of the A and B sites, µ is the reduced mass for the Fe3+ ions and the O2− ions (~2.065 × 10−26 kg mol−1). The evaluated value of force constant is tabulated in table4. The value of force constant for tetrahedral site was in the range of 2.08 × 102 N m−1–1.97 × 102 N m−1, whereas it was 0.82 × 102 N m−1 − 0.66 × 102 N m−1 for octahedral site. The variation of force constant is associated with cations redistribution among tetrahedral and octahedral sites with variation of grain size [48]. 3.6. XPS study XPS measurements were carried out to investigate electronic state and cation distribution. It can be observed from figures 7(a) and (c), that the high resolution Co 2p XPS spectra of CoH5 and CoH11 sample is associated with two spin–orbit doublets characteristics of Co 2p3/2 and Co 2p1/2 and two shakeup satellites [49]. Figure7(a) shows two peaks situated at ~780 eV and ~795 eV, corresponding to the Co 2p3/2 and Co 2p1/2 for Co2+ state, which confirm that the valence state of cobalt is 2. Further, for CoH5 sample, the Co 2p3/2 peak is deconvoluted into two peaks situated at 780.2 and 782.6 eV (figure 7(a)), corresponding to Co in two crystallographic environment, i.e. octahedral and tetrahedral sites in spinel ferrite. The deconvoluted two peaks have binding energies at 780.2 and 782.6 eV, which is associated with Co2+ ions at octahedral sites and tetrahedral sites, respectively. In accordance with integrated intensity of deconvoluted peaks, the distribution of Co2+ ions is 70% at octahedral sites and 30% at tetrahedral sites. Similarly, for CoH11 sample, the Co 2p3/2 spectra is also deconvoluted into two peaks situated at 780.1 and 782.0 eV (figure 7(c)). The binding energies of deconvoluted peaks at 780.1 and 782.0 eV are assigned to Co2+ ions at octahedral sites and tetrahedral sites, respectively. In CoH11 sample, the distribution of Co2+ ions is 66% at octahedral sites and 34% at tetrahedral sites. Further, from figures7(b) and (d) it can also be observed that the high resolution Fe 2p spectra of CoH5 and CoH11 sample, is associated with two spin–orbit doublets characteristics of Fe 2p3/2 and Fe 2p1/2 and two shakeup satellites [50]. Two peak situated at ~711 eV and ~725 eV represent the Fe 2p3/2 and Fe 2p1/2 for Fe3+ state, confirming that the valence state of iron is 3. Furthermore, with the integrated intensity of deconvoluted peaks, the distribution of Fe3+ ions is 76% at octahedral sites and 24% at tetrahedral sites in CoH5 sample, whereas, it is 77% at octahedral sites and 23% at tetrahedral sites in CoH11 sample. Therefore, the occupation formula for CoFe2O4 nanoparticles can be described as follows Co+2 0.30Fe+3 0.48Co+2 0.70Fe+3 1.52O 4 Table 3. Raman modes for cobalt ferrite nanoparticles. Sample Raman peak (cm−1)Ratio of intensity IA1g(1)/IA1g(2) A1g(1) A1g(2) T2g(1) T2g(2) EgT2g(3) CoH 680 604 558 461 297 198 1.388 CoH5 680 603 561 463 287 221 1.433 CoH7 672 587 562 464 276 214 1.173 CoH9 678 599 559 460 287 217 1.447 CoH11 685 603 563 463 301 204 2.420 Figure 6. FTIR spectra of cobalt ferrite nanoparticles at different crystallite sizes. Table 4. The frequency ν1 and ν2 and force constant FT and FB for the tetrahedral and octahedral sites, respectively, for cobalt ferrite nanoparticles. Sample ν1 (cm−1)ν2 (cm−1) FT × 102 (N m−1) FB × 102 (N m−1) CoH 533 334 2.08 0.82 CoH5 529 333 2.05 0.81 CoH7 523 327 2.00 0.78 CoH9 521 314 1.99 0.72 CoH11 519 301 1.97 0.66 Adv. Nat. Sci.: Nanosci. Nanotechnol. 8 (2017) 045002 R S Yadav etal 8 for CoH5 sample and Co+2 0.34Fe+3 0.46Co+2 0.66Fe+3 1.54O 4 for CoH11 sample. 3.7. Magnetic properties Figure 8 shows the variation of magnetization as a function of applied magnetic field for cobalt ferrite nanoparticles at room temperature. The hysteresis curves show the variation of magnetization with variation of grain size of ferrite nanoparticles. The evaluated value of variation in magnetic parameters such as saturation magnetization (Ms), remanence magnetization (Mr) and coercivity (Hc) are tabulated in table5. Further, the experimental magnetic moment (ηB) was evaluated in accordance with the following relation [41]: η B= MM S 5585 , (7) where M is the molecular weight. Moreover, the anisotropy constant is evaluated using following relation [51] Hc= 0.96 K MS , (8) where K is anisotropy constant. In ferromagnetic materials, the magnetization will not pass through origin, however, a magnetization versus applied magnetic field (M–H) loop, is formed. Figure8 indicates ferromagnetic behaviour of CoFe2O4 nanoparticles. Since, the M–H loops do not pass through the origin, therefore, at zero applied magnetic field, it retains a magnetization value, and it is the remnent magnetization (Mr). Coercivity is a measure of Figure 7. XPS spectra of cobalt ferrite nanoparticles: (a, b) annealed at 500 °C, and (c,d) annealed at 1100 °C. Figure 8. Hysteresis curve of cobalt ferrite nanoparticles at different crystallite sizes. Adv. Nat. Sci.: Nanosci. Nanotechnol. 8 (2017) 045002