Synthesis Factors Dependence of Magnetic Properties of CoFe2O4 and CoFe2-xGdxO4 Semicrystalline Nanoparticles with desired Morphology through Hydrothermal Procedure


 In the present study, CoFe2O4 and CoFe2-xGdxO4 nanoparticles were synthesized by the hydrothermal process. The CoFe2O4 nanoparticles were synthesized at different temperatures (70oC, 100oC, 150oC, and 200oC), molar ratio of CoCl2/ FeCl3 (0/2, 0.75/2, 1/2, 1.5/2, and 2/2). Gadolinium-doped cobalt ferrite (CoFe2-xGdxO4) nanoparticles have also been synthesized with Gd/Fe molar ratios of 0.18 and 0.53. The XRD patterns indicate that cobalt ferrite and Gadolinium-doped cobalt ferrite nanoparticles have been successfully synthesized without impurities with a medium degree of crystallinity. The XRD patterns show that by increasing the synthesis temperature from 70oC to 200oC, the size of the nanoparticles decreased from 50.49nm to 32.45nm while the morphology of the nanoparticles also changed from a shapeless and agglomerated state to a spherical shape. The XPS curve illustrated several peaks corresponding to Fe+3, Co+2, and O 1s. The binding energies for Co and Fe were consistent with Fe 2p and Co 2p binding energies for cobalt ferrite nanoparticles. The magnetic saturation value (Ms) increased from 17.253 emu/g to 54.438 emu/g with a rise in the synthesis temperature. The effects of FeCl3/CoCl2 molar ratio on the magnetic properties showed the highest value of Ms (54.438 emu/g) and the coercivity (HC) of 744.56 Oe for a 2/1 molar ratio. The addition of gadolinium to the composition resulted in a reducing of the magnetic properties of nanoparticles; accordingly, the amount of saturated magnetization was reduced to 22.469 emu/g. Another effect of gadolinium dopant in the composition was a change in nanoparticle morphology from spherical to rod shape. The final aim of this study was to investigate the possible utilization of CoFe2O4 and CoFe2-xGdxO4 nanoparticles in medical treatment in the near future.


Introduction
Magnetic nanoparticles are one of the most important and most used types of nanoscale materials which their unique features create speci c applications for them compared with other nanostructures [1,2] .
Magnetic nanoparticles are emerging as essential biomedical functional nanomaterials in areas such as drug release, tissue engineering, theranostics, and lab-on-a-chip due to their exclusive chemical and physical properties. Iron oxide-based nanoparticles have wide applications in various elds, such as ZnFe 2 O 4 nanoparticles in petrochemical [3] , nickel ferrite nanocomposites as catalysts, CoFe 2 O 4 core/shell nanoparticles for advanced hyperthermia application [4] , etc. The ease of collecting and retrieving magnetic nanoparticles in their application as a catalyst is one of the advantages of these nanoparticles, which is the reason for their ever-increasing usage in this category [5] .
However, the magnetic properties of iron oxide nanoparticles are widely in uenced by their physicochemical properties (morphology, size distribution, and surface nature) [6] . On the other hand, the doped element has a great deal of application for iron oxide nanoparticles in various industries, with its favorable magnetic properties.
The formation of the cobalt ferrite phase can be changed the initial properties of iron oxide nanoparticles which can improve the bioapplications of these nanoparticles. Biocompatibility, thermal and chemical stability are the unique properties of spinel ferrite nanoparticles with MFe 2 O 4 (M = Cr, Mn, Co, Ni, Cu, and Zn) the general formula, which has led to abundant applications in the diagnosis and medical sciences [4], [5], [7][8][9][10][11] . The crystal structure of CoFe 2 O 4 is an inverse spinel-type structure where oxygen atoms constitute a face center cubic (FCC) lattice. In this regard, divalent Co 2+ ions are shifted to octahedral "B" sites, and trivalent Fe 3+ ions are occupied by both tetrahedral "A" and octahedral "B" sub-lattice sites [7] . This is a good indication that the CoFe 2 O 4 nanoparticles have been suitable for a wide variety of technological and medical applications [12][13][14][15] .
The substitution of rare-earth cations such as gadolinium into the inverse spinal lattice leads to structural disorder and lattice strain which could bring signi cant changes in the morphological and magnetic properties. Also, several other parameters can affect the nature of the CoFe 2 O 4 magnetic nanoparticles.
Composition ratio, morphology, degree of crystallinity, particle size distribution, doping, and synthesis temperature in uence these materials' magnetic behavior and application. However, the synthesis temperature, dopant, and molar ratio of FeCl 3 /CoCl 2 are the three crucial parameters on the magnetic properties and microstructure of CoFe 2 O 4 nanoparticles sciences [4], [5,7,9,11] .
This study aimed to introduce a possible synthetic pathway for synthesizing semicrystalline cobalt ferrite nanoparticles at low temperatures with the desired morphology. Also, the effects of dopant, hydrothermal temperature, and the composition ratio of raw materials on the formation and magnetic properties were investigated to achieve the non-agglomerated nanoparticles with spherical morphology and high magnetic saturation value (M S ) value, and low coercivity (H c

2-3-Synthesis of Gd doped cobalt ferrite (CoFe 2-x Gd x O 4 ) nanoparticles
The stoichiometric amounts of FeCl 3 .9H 2 O and CoCl 2 .6H 2 O were dissolved in 100ml distilled water and stirred for 15 minutes at room temperature. Afterward, a speci c quantity of gadolinium was dissolved in 3ml hydrochloric acid (35%wt) to dissolve pure gadolinium. Upon dissolution of gadolinium, the gadolinium solution was added to the initial solution and stirred for an additional 30 minutes. The atomic ratios of gadolinium (x) doped in the composition were X: 0.00, 0.30, and 0.70. Reducing the amount of gadolinium chloride (GdCl 3 ) from the amount of iron chloride will ensure that the ratio of iron chloride and cobalt chloride remains constant at 2:1. NaOH solution was added drop-by-drop into the nal solution until the pH was adjusted to 10. At last, a solution was placed in a 150 ml Te on-lined stainless steel autoclave, sealed, and kept at 200°C for 16 hours. The black precipitates were collected and washed several times with distilled water and ethanol and dried in the oven at 50 o C for 3h.

Materials characterization
The X-ray diffraction (XRD) patterns of the particles were collected on a Philips X'pert pw3040/60 X-ray diffractometer with Cu kα radiation (λ = 0.15147nm). Field emission scanning electron microscope (FESEM) images and energy-dispersive X-ray spectroscopy (EDS) were obtained by Tescan BRONO-mira3 LMU. A magnetic hysteresis loop of samples was measured at room temperature using a vibrating sample magnetometer (VSM) model 7407 Lakeshore cryotronics. X-ray photoelectron spectroscopy (XPS) was performed at room temperature using a JPS-9010TR (JEOL) instrument with an Mg Kα X-ray source. All binding energies were calibrated by referencing C 1s (285.0 eV).

Phase and morphology identi cation
The XRD patterns of the samples synthesized at different hydrothermal temperatures (Fig. 1a) and different molar ratios of CoCl 2 /FeCl 3 (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 CoFe 2 O 4 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 ( ≈ 0.8402nm) [32] , it could be concluded that nanoparticles have a medium degree of crystallinity. Figure 1b shows the diffraction peaks of Fe 2 O 3 and Fe 3 O 4 as impurity phases in the CFN 6 sample due to the imbalanced stoichiometric ratios of Co +2 and Fe +3 ions [33] . By increasing the amount of cobalt to iron molar ratio (CFN 4), the unwanted phases of 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 con rmed with the obtained particle size by FESEM. Increasing the hydrothermal synthesis temperature may enhance the nanostructure properties, especially for CoFe 2 O 4 nanoparticles. The bonds will be broken, and the order through the CoFe 2 O 4 nanoparticles will be increased, and consequently, the crystallite size will be decreased. With increasing the hydrothermal temperature, the CoFe 2 O 4 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 [34] .
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 CoCl 2 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 (Gd 2 O 3 ), 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 CoFe 2 O 4 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 [35] .
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 70 o C changed to spherical shape at 200 o C. By increasing the synthesis temperature, the uniform size distribution of the nanoparticles was raised, and the agglomeration of the particles was reduced. It is speci ed that increasing the cobalt molar ratio in the hydrothermal medium can increase the agglomeration of the particles in the nal products [17] . 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 CFNG 0.70 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 CFNG 0.30 magnetic nanoparticles is shown in Fig. 5 and Fig. 6 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 [38] . Also, some small XPS peaks indicate negligible amounts of Ca, Na, and Cl elements as impurities. Figure 8 and Table.5 show the magnetization value versus the magnetic eld 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 (M s ) increased from 17.253 emu/g to 54.438 emu/g for CFN1 and CFN4 samples, respectively.

Determination of magnetic properties
The coercivity eld, Hc, can re ect 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 H c for the CFN 4 sample (398.83Oe) is much lower than the reported value by Demortière et al. [39] . 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 [40] .
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 (M S ) 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 (H c ) 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 Co 3 O 4 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 CoCl 2 / FeCl 3 from 1/2 to 1.5/2 and 1/1 [41] .
The hysteresis loops of the samples with different amounts of Gd dopant and changes in the magnetic properties are shown in Fig. 8c. CoFe 2 O 4 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 M S was reduced to 22.947(emu/g) in CFNG 0.70 . The value of H c 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 [37] .
Inbaraj et al. [42] have synthesized the cobalt ferrite nanoparticles by hydrothermal method at 180°C for 24h. The M s 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.

Conclusions
CoFe 2 O 4 nanoparticles with spherical morphology were synthesized by the hydrothermal process at low temperatures. The results clearly showed that by increasing the synthesis temperature, the size of nanoparticles was changed in the range of 50.49nm to 32.45nm, and the morphology of nanoparticles changed from irregular morphology and agglomerated state at 70°C to semi-spherical shape at 200 o C. The magnetic saturation increased from 17.253emu/g to 54.438emu/g by increasing the synthesis temperature from 70°C to 200 o C. The most suitable combination ratio for the highest magnetic saturation was CoCl 2 /FeCl 3 :1/2, in which the entire phase structure was composed entirely of cobalt ferrite spinel, and there was no trace of impurities. The results of different combination ratios determined that the effect of the presence of cobalt oxide (Co 3 O 4 ) on the reduction of magnetic properties of synthesized nanoparticles was more than iron oxide phases (Fe 2 O 3, Fe 3 O 4 ). Accordingly, it can be concluded that cobalt ferrite nanoparticles synthesized at 200 o C with a composition ratio of CoCl 2 /FeCl 3 :1/2 has nanometer size, regular spherical morphology, and higher superparamagnetic property. Finally, the results of Gadolinium dopant showed a sharp decrease in magnetic properties and changes in the morphology of nanoparticles from quasi-spherical to rod shape, which these changed shape nanoparticles by Considering their dimensions and magnetic properties can be used in various elds [43][44][45] .

Declarations
Funding: N/A Con icts of interest/Competing interests: The authors certify that they have NO a liations with or involvement in any organization or entity with any nancial interest or non-nancial interest in the subject matter or materials discussed in this manuscript.) Availability of data and material: The raw/processed data required to reproduce these ndings can be shared upon request.