Magnetic Spinel-Type CoFe 2 O 4 Nanoparticles : Synthesis and Investigation of Structural , Morphological Properties

Spinel-type metal oxide nanoparticles were synthesized via co-precipitation approach. Mono ethylene glycol (MEG) was used as a capping agent to stabilize the particles and prevent them from agglomeration. The structural, morphological and thermal properties of the calcined sample were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), raman spectroscopy and thermal analysis. Energy-dispersive X-ray analysis (EDX) has also proved that the element composition was composed of pure single phase and contained Co, Fe and O elements. The mean crystallite size of the prepared ferrite nanoparticles was determined to be in the range of 30-345 nm based on the SEM images. The magnetic measurements of the CoFe 2 O 4 nanoparticles were examined with a vibrating sample magnetometer (VSM) at room temperature to determine their magnetic behavior and the magnetic parameters were found.


Introduction
Magnetic metal oxide nanocomposites have attracted a great deal of attention as a composite materials on account of their various applications such as lithium ion batteries [1][2][3], magnetic catalysis [4,5], magnetic resonance imaging (MRI) [6], sensor and actuators, tissue repairing, microwave devices and biotechnology [7].
Among the various spinel ferrites, CoFe2O4, as a wellknown hard magnetic material, has versatile and technologically important materials owing to its high saturation magnetization [8], high coercivity [9], strong anisotropy [10], thermal stability and mechanical hardness.Therefore, CoFe2O4, nano composites are the subject of intense research not only for their fundamental scientific interest, but also for their potential applications in magnetic storage media, bio sensing applications [11], catalytic [4,5,12,13], super paramagnetic materials [14,15] and medical applications.The properties of these nanocomposites mainly depend on their shape, size and structure, which are strongly determined by the preparation methods.Therefore, there are various methods which have been reported previously for the preparation of CoFe2O4 nanocomposites [16][17][18][19][20].
In present work, we were focused on the synthesized of cobalt ferrites using capping agent (MEG) [21] and were fabricated by co-precipitation method.
We proposed that the use of MEG is highly advantageous for the synthesis in order to reduce particle agglomerates as well as to obtain single phase CoFe2O4 nanoparticles.The synthesized CoFe2O4 nanocomposites were examined by XRD, SEM, EDX analysis, Raman spectroscopy and TGA/DTA and VSM in order to determine the phase formation, morphology, elemental analysis, vibrational frequencies, thermal stability and magnetic behaviour respectively.

Material and methods
FeCl3.6H2O, CoCl2.6H2O,Mono ethylene glycol (MEG) (capping agent) and NaOH were purchased from Merck and used without further purification.In addition, ethanol and distilled water as a solvent were used.
The synthesized CoFe2O4 nanocomposites were subjected to X-ray diffraction studies [using a Panalytical diffractometer and a Cu Kα radiation source] to determine the crystal phase composition.The formation and elemental compositions of CoFe2O4 nanoparticles were confirmed by scanning electron microscopy combined with an energy dispersed X-ray analysis which was carried out using FEI Quanta FEG 250.Raman spectra were recorded using a Renishaw Invia model Raman Spectrophotometer.Thermogravimetric analysis (TGA) and differential analysis (DTA) were performed by using a Schimadzu DTG-60H instrument under nitrogen atmosphere with a flow rate of 50 mL min −1 .The heating rate was 10 °C/min.Magnetic measurements were investigated using Lakeshore 7407 model VSM analyzer at room temperature.

Preparation of CoFe2O4 nanocomposite
0.02 mmol of FeCl3.6H2O and 0.01 mmol CoCl2.6H2O were individually dissolved in the 20 mL of MEG and then two solutions were mixed on magnetic stirrer at 70 °C to achieve a homogeneous solution.Then sodium hydroxide solution were added slowly into the solution under magnetic stirring and the mixture immediately turned into a dark brown.The obtained precipitation was filtered, washed several times with distilled water and ethanol to afford the pure product and dried in air.Subsequently, the dried product was calcined in furnace at 700 °C for 6 h in order to remove the organic compounds such as MEG, ethanol leading to the formation of pure cobalt ferrites nanocomposites, as indicated in Figure 1.

Phase identification by XRD
The phases and crystallinity of the calcined specimen were identified by X-ray powder diffraction.As shown in Figure 2, the X-ray pattern of the synthesized CoFe2O4 nanoparticles was depicted.It can be seen from the relative intensities and positions of all the peaks that the crystalline structure of the product was confirm the presence of single-phase CoFe2O4 with a face-centered cubic structure, which was according to JCPDS card no 22-1086.No other characteristic peaks were detected except for CoFe2O4 nanoparticles in XRD patterns.

Morphological characterization
The morphology of the nanoparticles was determined using SEM analysis.It is clear from Figure 3 and 4 that CoFe2O4 have non-uniform morphology with the individual particles have a particles size from 30 to 345 nm but there is agglomeration of particles.We conclude from SEM analysis that non-uniform, heterogeneous morphology and grown CoFe2O4 particles have agglomeration due to magnetic force.
The composition of structure was analyzed by EDX as illustrated in Figure 5.The nanocomposite which consisted of only three kinds of elements, Co, Fe and O were observed as indicated in Table 1.
In the EDX pattern, the presence of Fe, Co and O elements in proper proportions suggested that the expected stoichiometry was maintained in the prepared samples.These results indicated that had spinel form of calcined CoFe2O4 nanoparticles and neither sodium nor carbon signals were detected so, it means that product was pure and had not any impurities.

Spectroscopic characterization
As shown in Figure 6, Raman spectra of the CoFe2O4 nanoparticles (a) were showed six peak maxima at 188, 300, 473, 564, 616 and 689 cm -1 , respectively.Five Raman active modes of cobalt ferrite 3T2g, Eg and A1g were tabulated in Table 2. Eg and T2g (3) modes correspond to the symmetric and antisymmetric bending of oxygen atom in M-O bond at octahedral void.A1g mode were related to the motion of oxygen atom around metal ions (Co 2+ -O, Fe 3+ -O) in the tetrahedral sites at 619 and 689 cm -1 [22].
But, two peaks at 506 and 601 were observed in the raman spectrum taken without calcination.The medium band around 1250 cm -1 was correspond to stretching of oxygen-carbon vibration associated with the MEG as shown in Figure 6.
The thermal behaviours of the as-prepared CoFe2O4 nanoparticles were determined by using TGA/DTA with flow rate N2 of 50 mL per minute as shown in Figure 7. From the TG curve of the calcinated sample no appreciable weight loss to 790 °C was observed.The DTA graph showed an exothermic reaction between 790 and 990 °C, which corresponded, on the TGA graph, to a 10.95 % weight loss.The reason for this weight loss was the presence of hydroxyl groups in the structure and this similar behavior was supported by literatures [23,24].

Magnetic properties
Magnetic measurements of the CoFe2O4 nanoparticles were studied by vibrating sample magnetometer (VSM) at room temperature in order to identify the magnetic states and envisage their behavior.The VSM measurements were recorded in the +20 kOe applied magnetic field.From the obtained hysteresis loops, the saturation magnetization (Ms), remanent magnetization (Mr), Coercivity (Hc) and squareness were determined.The measured values were: Ms=61.05emu/g, Mr= 25.73 emu/g, Hc= 812.47 Oe and Mr/Ms=0.42.According to the results obtained from the magnetic hysteresis loops as shown in Figure 8, as-prepared cobalt ferrite nanoparticles were exhibited ferromagnetic behaviors.Especially with hard magnetic feature observed in CoFe2O4, due to large Hc value.It was observed that if we do not implement calcination process, CoFe2O4 nanoparticles were not attracted by the magnet.In case of implementing calcination process, it was observed that the nanoparticles were attracted by the magnet.The interaction of the as-synthesized CoFe2O4 nanoparticles with magnet were clearly depicted in Figure 9.

Discussion and Conclusion
CoFe2O4 nanoparticles were synthesized successfully via co-precipitation methods assisted to MEG.These method was provided the short reaction times to produce well crystallized nanoparticles and reduce the agglomerations.High purity of the as-prepared nanocrystalline sample was proved by XRD and EDX analyses.SEM image results revealed that calcined sample was showed irregular shape and heterogeneous structure morphology.The CoFe2O4 nanoparticles size were obtained from the analysis of SEM images and were found to be about 30-345 nm.Consequently, when the calcination process was performed, CoFe2O4 nanoparticles were attracted by the magnet and exhibited ferromagnetic behaviors.

Figure 1 .
Figure 1.Schematic representation of the formation of CoFe2O4 nanoparticles by a co-precipitation process.

Figure 2 .
Figure 2. The XRD diffraction patterns of as prepared CoFe2O4 nanoparticles after calcination, before calcination and reference pattern, respectively.

Figure 9 .
Figure 9.The interaction of cobalt ferrite nanoparticles with magnet before and after calcination.

Table 1 .
The elemental composition of CoFe2O4 nanoparticles according to EDX spectrum.