Effect of pH Value on Synthesis and Properties of Zinc Cobalt Ferrite Nano Powders Prepared via Co-Precipitation Method

The effect of pH value on the synthesis of zinc cobalt ferrite nano powder via co-precipitation method was investigated. Zinc cobalt ferrite (Co 0.6 Zn 0.4 Fe 2 O 4 ) precipitated using a solution of nitrate raw materials under different pHs (pH=8, 10, 11, 12 and 14) and calcined at 750 °C for 2 hours. The obtained nano-powders were characterized by X-ray diffraction (XRD), Williamson-Hall relations and Extrapolate functions, field emission scanning electron microscopy (FESEM) and vibrating sample magnetometer (VSM) measurements. The amount of pH had a significant effect on the structural and magnetic properties of the synthesized powders. At pHs 8 and 10, a tensile strain and above that a compressive strain was created in the system. By changing the tensile strain to compression the crystallite diameter increased, so that the crystallite diameter increased from 19.3 nm at pH 8 to 47.8 nm for pH =11. The H C of the all samples was in the range of 140-150 Oe. The saturation magnetization, anisotropy and Bohr magneton constants were 100.38 emu/gr, 15357.70 erg/Oe and 4.27, respectively for the synthesized sample at pH=11, which were higher than the other samples. These changes are due to the effect that pH has on the parameters of the crystallite diameter, particle size and also on the cation distribution of the ions in tetrahedral/octahedral spaces.


Introduction
Spinel ferrites with general formula MFe2O4 or MO. Cation distribution can effect electron mutation, magnetic exchange interactions, modify magnetic properties, and consequently the use of ferrites [4][5][6][7]. Due to this and achieving various properties such as magnetic permeability, coercive force, anisotropy constant, saturation magnetization (Ms), mechanical stiffness, physical and chemical stability, the range of use of these materials is varied. Accordingly, much research has been done on the doping effect of different 2/3 valent elements inside the ferrite structures on the synthesis and magnetic properties [5][6][7][8][9][10][11][12][13][14][15]. Among them, cobalt ferrite is interested due to properties such as high magnetic permeability, high coercive force (HC) (≥50 KOe), good saturation magnetization (MS) (80 emu/gr), mechanical hardness and excellent physical and chemical stability. Hence, it is a good candidate for high-density magnetic recording, ferrofluid technology, drug delivery, magnetic resonance imaging (MRI), magnetic hyperthermia (MH) for cancerous tissues and radar absorption materials (RAM) [13].
Cobalt ferrite (CoFe2O4) has an inverse spinel structure. This means that 2-valent cobalt ions are in the octahedral (B) positions and half of the 3-valent iron ions are in the tetrahedral (A) spaces and the other half are in the octahedral sites [14,15]. On the other hand, the zinc element is a low-cost material and it's doping into cobalt ferrite to achieve the desired magnetic properties, especially for hyperthermia applications, is of interest to many researchers. Zinc ferrite is a normal spinel with 2-valent ions in the tetrahedral positions and all 3-valent ions in the octahedral spaces [2]. As a result, zinc doped cobalt ferrite may have a modified spinel structure (Zn 2+ [16]. Therefore, the synthesis of zinc cobalt ferrite is of great interest due to its potential application in multiple domains including magnetic hyperthermia. The properties of synthesized ferrites depend on the particles size, morphology and composition [17]. There are a number of methods for preparing magnetic nanoparticles (gaseou, solidous and liquidous phase synthesis). Although uniform nanoparticles have been synthesized using some of these methods, precise control of their size, shape and surface is generally challenging [8,18,19].
Among the various methods, Coprecipitation method is an easy and conventional way for the synthesis of magnetic nanoparticles from aqueous salt solutions. Ferrites are usually synthesized in an aqueous medium by chemical reaction according to following equations [17,19]: And as a result: Where M can be Fe 2+ , Mn 2+ , Co 2+ , Cu 2+ , Mg 2+ , Zn 2+ , Ni 2+ .
In powder synthesis by coprecipitation method, it is first necessary to precipitate various elements as hydroxide in the reaction system (Equations 1-2 and 1-4), then react with each other to form the desired product phasse after calcination [17]. The pH value required for the formation of different elements as hydroxide varies. This means that the iron ion may precipitate at a pH value. This number is another value for the cobalt precipitate and for the zinc ion on another [7]. In general, complete precipitation at pHs between 8 and 14 is expected [7,19]. The size, morphology and composition of magnetic nanoparticles largely depend on the parameters affecting the synthesis process such as the type of salt used (eg chlorides, sulfates, nitrates), type of elements, M 2+ /Fe 3+ ratio, reaction temperature and pH value [19]. Therefore, in order to achieve the optimum properties, it is important to investigate the effective parameters on the formation of synthesized particles and how they affect them.
Although the pH parameter is one of the most effective factors in precipitation, and in particular the final properties of the powders, but so far, little research has been done on how it affects microstructure and magnetic properties of cobalt ferrite. However, Xiaogu Huang [7] has studied the effect of this factor on the properties of electromagnetic loss. Therefore, in this work, the link between pH parameter and cationic distribution, strain created in the structure, morphology and size of the crystallites have been investigated. Then, the role of these changes on magnetic properties of synthesized nanopowders has been discussed.

Materials
In oredr to synthesis of zinc cobalt ferrite via co-precipitation method iron nitrate (Fe(NO3)3.

Synthesis method
To determine the effect of pH value on the process of zinc cobalt ferrite synthesis, at a constant molar ratio of zinc (xzn = 0.4), (Co0.6Zn0.4Fe2O4), 0.5 M solutions of each of the starting materials were first prepared on a heater stirrer at 400 rpm and 50 °C for 10 minutes.
The prepared solutions were then mixed with each other and the pH of the resulting system was adjusted to 8, 10, 11, 12 and 14 with NaOH solution. After the solution was brought to the desired pH, the resulting solution was stirred at 80 °C for 5 hours at 700 rpm. The precipitates were poured onto filter paper and washed several times with deionized water. It is noteworthy that with the increase of the first NaOH droplets, the sheet-shaped precipitates were formed in the solution and with the further increase of NaOH the volume of the precipitates increased and the color of the solution changed from brown-cherry to dark

Analysis techniques
Thermogravimetric analysis (TA Instruments, D-32609 Hullhorst, STA 504, Bahr Co. Germany) was taken to determine the calcination temperature. Phase compositions, strain analysis and lattice parameters of samples were analyzed by X-ray diffraction (XRD) (D8 ADVANCE, BRUKER, Germany) analysis. FESEM analysis (Mira 3-XMU) was performed to investigate the microstructural changes, size and morphology of the nanoparticles. The magnetic properties of the samples were also evaluated by VSM (Danesh Pajouh Co. Iran) technique.

Thermogravimetric analysis
To determine the calcination temperature, thermogravimetric analysis was taken from the Z4P11 sample which is shown in Figure 1. By examining the TG/dTG diagram ( Fig. 1-a), it is observed an endothermic peak at a temperature of about 750 °C, which represents about 18.28% of the weight loss ( Fig. 1- hence this temperature was selected as calcination temperature for prepared precipitates. The appearance of endothermic peaks around 100 °C and in the range of 220 to 460 °C are attributed to the evaporation of water present in the system as well as the nitrate and NaOH compounds, respectively [13]. The reaction system of the raw materials can be written as follows: Also the weight changes of the calcined sample at 750 °C for 2 hours (20.94% weight loss) showed good agreement with the TG analysis. Higher weight loss in calcination is probably due to longer time.

XRD analysis
The XRD patterns of the synthesize samples at 750 °C for 2 hours are shown in Figure 2:  [8] who reported that at pH=8 and at pHs above 11, some Fe2O3 and Zn (OH)2 impurities are present in the product, respectively. However, the appearance of higher backgrounds at pHs 8 and 14 than other samples may be attributed to the presence of glass or crystalline phases that were not observed by XRD analysis.
It is well known that the intensity of X-rays scattered from the planes is related to their atomic density. In addition, it has been reported that in solid solutions, including spinel ferrites the intensities depend on the cationic distribution of the elements in different positions [3]. Therefore, for each XRD spectrum, the peak intensity ratio of plane (440) to plane (220) ‫؛‬ I(440) / I(220) is calculated for each prepared sample that will be listed in Table 1.
The corresponding diagram is given in Figure 3.  [20,21]. Thus, by doping zinc into the cobalt ferrite structure, the Zn 2+ ions are in the tetrahedron position [21]. This placement of ions means the force applied to the 3 valent iron ions in the tetrahedral space to move to the octahedral position. Assuming, that distribution of elements within the structure of zinc cobalt ferrite can be considered as (Zn 2+ [3 , 14, 20,21].
The Williamson-Hall relation was also used to determine the exact crystalline diameter as well as the strain within them: In this equation, D is the grain size on an angstrom scale, λ is the wavelength of Cukα (1.54056 Å̇), B is the broadening line at half the peak height in radians, ε is the lattice strain and θ is the bragg angle of the scattered peak. The line slope is the lattice strain and the point that the line intersects the y axis is equal to 0.9.λ/ D [22].
The results of the Williamson-Hall equation for synthesized powders prepared at different pHs are given in Table 1 and Figure 4.  So the trend of changing the crystallite diameters by pH is shown in Figure 5:  Table 1). Such that, at pHs 8 and 10, the slope is negative, indicating the type of tensile strain in the system, and the strain slope has decreased from -7*10 -4 to -3.5*10 -4 . That is, the strain decreased with increasing pH from 8 to 10. The crystallite diameter has shown an increasing trend in the pH range from 19.2 nm to 30.1 nm (Table 1, Fig. 5). As the pH increases to 11 and higher, the slope of the line changes, meaning that the type of strain in the system is changed to pressure (Fig. 4).
It is necessary to explain that in this state the amount of compressive strain in the system decreases with increasing pH from 11 to 12 and then, it increases again with increasing further pH to 14. This trend of strain change is also observed in the amount of crystallite diameters. That is, at pH=11 the crystallite diameter from 47.8 nm decreased to 27.2 nm, then increased to 51.4 nm (Figures 4 and 5, Table 1). In other words, the change in the tensile strain to compressive strain increases the diameter of the crystallites, and the more this strain increases, the larger the crystallite diameter and vice versa.
Another point to consider is that the strain values in the different planes also differ for a sample. This behavioral difference is also seen in the intensity of the scattered peaks in these centers. As noted, the I(440)/I(200) ratio, that is, the intensity of the peak scattered on a plane passing through the tetrahedron site with respect to the peak intensity obtained from the plane in octahedral space, has changed for different samples (Fig. 3). The presence of lattaice strain and these differences in the structure are due to the effect that pH has on the movement of ions in network spaces [22].
To determine the lattice parameter varoius methods can be used. The most accurate way is to plot the curve of the lattice parameter in terms of the extrapolate function cos 2 θ/sinθ [23].
Therefore, the graph of extrapolate functions for the synthesized powders was drawn ( Figure   6). Theoretical density (X-ray density) and specific surface area of the powders were also calculated using the following equations [10,24]: Which, ρx is theoretical density (gr/cm 3 ), M is molecular weight of the sample (gr/mole), N is the Avogadro number, a is lattice parameter and 8 denotes the number of unit cells.
Therefore, the results of the extrapolate functions for the lattice parameter, as well as the theoretical density and specific surface area of the powders are presented in Table 2.   The effect of pH on the lattice parameter of synthesized powders is also illustrated in Figure   7. Although in all the powders mentioned, the composition and other synthesis conditions were constant except pH, but the lattice parameter was changed with pH alteration. However, these changes are very small. This is attributed to changes in the distribution of ions within the structure by the pH value, just as the amount of strain within the structure also altered ( Figure 4). In fact, the relationship between the transition metals and the lattice parameter is justified by the Vegard's law. This rule applies to solid solutions of cobalt ferrite nanoparticles substituted with transition metals [13,15,25]. On the other hand, according to  (Table 2) for different samples is consistent with this statement. The theoretical density of the samples is also inversely correlated with the lattice parameter according to Equation 3-6, which means that as the lattice parameter increases, the density decreases and vice versa.

FESEM/EDX analysis
FESEM analysis was performed to compare the microstructure of powders prepared under pH 8, 11 and 14 (Fig. 8). The micrographs of the synthesized powders show that in all samples the particle size range is less than 60 nm and is relatively uniform. This uniformity is most evident in synthesized powsers at pH=11 (Fig. 8-b). The particle size in the samples prepared at pH=11 is somewhat grater than the other samples. In the powders prepared at pH 8 and especially 11, the cubic shape of the formed particles is quite evident. By increasing the pH to 14 (Fig. 8-c) the particles appear to be finer, and although the particle size distribution is uniform, the shape of the particles is slightly out of cubic state and look partially spherical. The particles are also axial and the agglomeration of the particles is seen in this sample. Particle agglomeration is due to the higher surface-to-volume ratio of the nanoparticles, which causes high interfacial tension and the particles adhere to each other. In addition, magnetic nanoparticles tend to cluster and stick together due to the interaction of magnetic dipoles [26].
It seems that by increasing the pH and increasing the concentration of OHions in the system up to pH=11, a supersaturated lattice is created in the medium and slows down the nucleation. As a result, the rate of grain growth is greater than that of nucleation in this range, which results in the growth of grains and particles, according to Xiaogu Huang et al. [7]. As the pH increased, especially at pH=14, the particles became smaller (Fig. 8-c). This means that the rate of nucleation is higher than the grain growth rate and the reverse effect of hydroxide concentration on the system, as growth has occurred less, particle's surfaces have increased and agglomeration has taken place. In addition, quantitative EDS analysis for Z4P11 specimen can be seen in Figure 8-d. Z4P11 is approximately the same composition as Co0.2Zn0.4Fe2O4. However, very minor amounts of sulfur and aluminum are present (less than 1%), which are very likely to be related to the raw materials consumed.

Magnetic properties
To investigate the effect of pH value on the magnetic properties of the prepared samples, vibrating sample magnetometer (VSM) analaysis of synthesized powders were performed which are given in Figure 9. The extracted results from these graphs are also presented in Table 3.  The amount of pH value had a significant effect on the saturation magnetization as it can be seen in Figure 10. As soon as pH changed from 8 to 11, Ms increased from 95.48 emu/gr to 100.38 emu/gr, and then decreased at pHs 12 and 14 to 85.68 emu/gr and 81.98 emu/gr, respectively. Since the composition is the same in these samples, the changes in Ms are attributed to the two factors of crystallite diameter and particle size, as well as to the different presence of elements in tetrahedral/octahedral sites. Larger grains tend to have larger magnetic domains. It is easy to move the domain wall, magnetize or non-magnetize with greater grains [15,27]. In this way, specimens with larger grains have less HC and higher MS.
On the other hand, FESEM analysis (Fig. 8) and Fig. 5 showed that as the pH changed to 11, the particle size and crystallite diameter increased. Since the existence spins at the particles surfaces are highly disordered, they have decreased with increasing particle size [27,28]. As a result, the amount of Ms in the Z4P11 sample increased. This increase in MS, as previously mentioned, is attributed to the migration of Fe 3+ ions from the tetrahedral to the octahedral space and the change in interaction by intermediating oxygen ions, known as superexchange interactions, between sublattices A and B [3, 26 and 29]. As the pH increases above 11, the crystallite diameter first decreases and then increases (Table 1). Therefore, the saturation magnetization is expected to decrease and then increase. However, this trend only decreases with increasing pH (Fig. 10). The micrograph image of the synthesized particles shows that at pH=14 (Fig. 8-c), the synthesized powders have become agglomerated because of small size.
So, it is expected that the magnetization of the sample reduced by the same extent. Also, the amount of coercive force for samples was 140-150 Oe (Fig. 10). These values show significant decrease compared to that obtained from the synthesis of zinc cobalt ferrite by Mohammad Sajjad Hossain et al. [16] (HC=726.13 Oe). This reduction is attributed to the movement of Co 2+ ions to the tetrahedral position [3]. For comparison, the amount of saturation magnetization obtained by other researchers synthesizing ferrite nanoparticles is presented in Table 4:  6 Topkaya et al. [38] The saturation magnetization of the present work (100.38 emu/gr) is almost greater than the Ms of most ferrite nanoparticles synthesized by others (Table 4). In addition, according to Wuhlfarth's theory, anisotropic constant (K) and magnetic moment (nB) for the formula unit are obtained by Equations (3)(4)(5)(6)(7)(8) and ( Where Mw is molecular weight. Therefore, the values of K and nB are given in Table 5, and their changes vs. pH parameter are shown in Figure 11.  spaces. Therefore, based on the changes seen in Figure 11, it can be deduced as pH amount rises, some Fe 3+ ions moves to B sites from A sites, while Co 2+ would be forced to go tetrahedral positions. This means reduction of cobalt ion concentration in octahedral space, and consequently decreases in anisotropic constant which enhances magnetic moment.

Conclusions
Single phase zinc-doped cobalt ferrite nanoparticles prepared under various pHs via co-