3.1 Phase and morphology identification
The XRD patterns of the samples synthesized at different hydrothermal temperatures (Fig. 1a) and different molar ratios of CoCl2/FeCl3 (Fig. 1b) are shown in Fig. 1. The XRD patterns indicate that cobalt ferrite nanoparticles have been successfully synthesized without unwanted and extra phases and impurities. Seven characteristic peaks could be indexed as a cubic structure of semicrystalline CoFe2O4 nanoparticles. The lattice parameter was a=0.8379nm, corresponding to the JCPDS card no.02-1045. Given that the value of the obtained lattice parameter is close to its theoretical value (\(\approx\)0.8402nm), it could be concluded that nanoparticles have a medium degree of crystallinity.
Figure 1b shows the diffraction peaks of Fe2O3 and Fe3O4 as impurity phases in the CFN 6 sample due to the imbalanced stoichiometric ratios of Co+2 and Fe+3 ions. By increasing the amount of cobalt to iron molar ratio (CFN 4), the unwanted phases of Fe2O3 and Fe3O4 were eliminated. Two XRD peaks were observed at 58.615o and 64.402o for the CFN 8 sample, assigning to the cobalt oxide (Co3O4) crystal phase. The average crystallite size of nanoparticles was calculated by the Scherer equation (Dhkl=Kλ/ßcosθ), where Dhkl is the crystallite size derived from the (400) peaks of the XRD patterns, k the sphere shape factor (0.89), θ the angle of the diffraction, ß the full-width at half-maximum (FWHM) of the (400) peak of the sample.
The data in Table 2 illustrates that an increase of synthesis temperature from 70°C to 200°C leads to a decrease in the crystallite size from 50.49nm to 32.45nm. These results also can be confirmed with the obtained particle size by FESEM. Increasing the hydrothermal synthesis temperature may enhance the nanostructure properties, especially for CoFe2O4 nanoparticles. The bonds will be broken, and the order through the CoFe2O4 nanoparticles will be increased, and consequently, the crystallite size will be decreased. With increasing the hydrothermal temperature, the CoFe2O4 crystallites nucleates and grow, the boundaries between crystallites at which amorphous transforms to crystals decrease in volume. This reduction limits the nucleation and growth of the crystalline phase, as seen in the XRD patterns.
Also, by increasing the hydrothermal temperature over 100°C, the degree of crystallinity increased up to 28%. The highest degree of crystallinity was achieved for CFN 1 sample. The crystallite size was changed from 31.64nm to 37.85nm for CFN 4 and CFN 7 samples by enhancing of CoCl2 content in the reaction medium.
According to X-ray diffraction patterns in Fig. 1c, it becomes clear that by increasing the Gd dopant amount to x = 0.30, the intensity of the peaks increased. Some peaks appeared at 30.9, 286.36, 185.33, 199.49, and 288.60 degrees related to gadolinium oxide (Gd2O3), according to the reference data (JCPDS card no.01-0339). When the dopant value was increased to x= 0.70, the peaks intensity increased once more, and new peaks appeared. A sharp decrease in crystal size was observed by adding gadolinium atoms to the CoFe2O4 compound (x=0.30 and x=0.70), and the degree of crystallinity decreased from 40.585 to 33.902% due to changes in the morphology and crystal structure of the nanoparticles.
The FESEM images of nanoparticles are shown in Fig. 2, Fig. 3, and Fig. 4. The pictures show the effect of hydrothermal temperature on the size and morphology of nanoparticles. As illustrated in the images, increasing the synthesis temperature resulted in a decrease in the size of the nanoparticles. On the other hand, by increasing the cobalt amount in the composition, nanoparticles and agglomeration size increased, which are presented in Table 2. The nanoparticles with irregular morphology and agglomerated state at 70oC changed to spherical shape at 200oC. By increasing the synthesis temperature, the uniform size distribution of the nanoparticles was raised, and the agglomeration of the particles was reduced. It is specified that increasing the cobalt molar ratio in the hydrothermal medium can increase the agglomeration of the particles in the final products. In Figure 4, the effect of the Gd dopant on the size and morphology of nanoparticles is visible, as, in x = 0.30 dopant amount of Gd, the morphology of the nanoparticles changed from 100% quasi-spherical shape to a mixture of quasi-spherical and rod shape. Semi-spherical nanoparticles with an approximate size of 20 to 30 nm and rod shape nanoparticles with a thickness of 20 to 35 nm and length of 200 to 550 nm were estimated. In the CFNG0.70 sample, the morphology of nanoparticles was changed entirely to the unique rod shape. The reason for the nanoparticles' morphology changes is the strong distortion of the crystal lattice and change in the preferential growth directions due to the presence of gadolinium atoms in the crystal structure of nanoparticles[36, 37].
The EDS elemental map of CFN 4 and CFNG0.30 magnetic nanoparticles is shown in Fig. 5 and Fig. 6. The images and results of the EDS elemental map clearly show the uniform intensity of Fe, Co, Gd, and O signals resulting from a uniform distribution of the elements. There was no trace of element segregation and aggregation in the microstructure of the nanoparticles.
The chemical composition of CoFe2O4 nanoparticles synthesized at 200oC (CFN 4) was further investigated by XPS. The XPS peaks of Co, Fe, and O elements can be seen in Fig. 5. The binding energy of the C 1s peak at 285.0 eV was used as a reference for calibration.
The corresponding spectroscopy curve and the concentration table of the existing elements are shown in Fig. 7 and Table.4. The peaks at 709.6 ev, 711.1 eV, and 712.0 eV correspond to Fe+3, and the peaks at 778.5 eV and 780.4 eV were assigned to Co+2. The peak located at 529.9 eV is attributed to the O 1s region. These binding energies for Co and Fe are consistent with Fe 2p and Co 2p binding energies for cobalt ferrite nanoparticles. Also, some small XPS peaks indicate negligible amounts of Ca, Na, and Cl elements as impurities.
3.2 Determination of magnetic properties
Figure 8 and Table.5 show the magnetization value versus the magnetic field at room temperature. In Figure 8.a, the hysteresis loops of the samples synthesized at different temperatures are shown. By increasing the synthesis temperature, nanoparticles' magnetization was increased in large quantities so that the magnetic saturation value (Ms) increased from 17.253 emu/g to 54.438 emu/g for CFN1 and CFN4 samples, respectively.
The coercivity field, Hc, can reflect the ferromagnetism or ferrimagnetism properties. This value describes the force that is necessary to demagnetize a sample completely. By increasing the synthesis temperature, the value of coercivity (Hc) was reduced from 1648.2 Oe at 70°C to 398.83 Oe at 200°C, while the value of retentivity (Mr) was enhanced from 8.360emu/g to 18.245emu/g. The obtained value of Hc for the CFN 4 sample (398.83Oe) is much lower than the reported value by Demortière et al.. Increasing the synthesis temperature, resulting in smaller crystallites and improved crystallinity, a considerable rise in magnetic saturation, and a drop in coercivity and retentivity values could be achieved by increasing the synthesis temperature .
The hysteresis loops of the samples synthesized by different combination ratios are shown in Fig. 8b. As the results show, the highest value of magnetic saturation (MS) is related to the CFN 4 sample with a 1/2 molar ratio. The magnetic saturation value decreases impressively by increasing the cobalt amount in CFN 7 and CFN 8 samples. The CFN 6 sample has less magnetization saturation compared to CFN 4 sample, while it's higher than the value obtained for the CFN 7 and CFN 8 samples. The lowest and highest values of coercivity (Hc) were obtained for CFN 8 (95.28 Oe) and CFN 6 (744.56 Oe) samples, respectively. The excess amount of cobalt in the hydrothermal medium can persuade the formation of the Co3O4 compound in CFN 7 and CFN 8 samples, affecting the obtained value of magnetic saturation. The sharp decrease in magnetic saturation was observed by increasing the molar ratio of CoCl2/ FeCl3 from 1/2 to 1.5/2 and 1/1.
The hysteresis loops of the samples with different amounts of Gd dopant and changes in the magnetic properties are shown in Fig. 8c. CoFe2O4 nanoparticles (CFN 4) showed the highest value of magnetic saturation on a measuring scale of 54.4381 emu/g. In this experiment, the magnetic properties of the nanoparticles were drastically reduced by adding gadolinium so that the value of MS was reduced to 22.947(emu/g) in CFNG0.70. The value of Hc increased from 22.947(Oe) to 1358.800(Oe), which the reason could be the onset of morphological changes from quasi-spherical to rod shape, an increase of nanoparticles size, and the possible presence of gadolinium oxide in the composition.
Inbaraj et al. have synthesized the cobalt ferrite nanoparticles by hydrothermal method at 180°C for 24h. The Ms value of their synthesized nanoparticles was 35 emu/g which had fewer magnetic properties than CFN3 and CFN4 samples (36.136 emu/g and 54.438 emu/g) synthesized at lower temperature and shorter time (150°C and 200°C for 16h) in the present study.