Cobalt Ferrites: Formation from Nitrate Solutions under the Action of DC Discharge

A new method for obtaining ultra�ne particles of cobalt ferrites is proposed. This synthesis is a two-step process: the �rst step is the synthesis of ultra�ne particles from aqueous solutions of nitrates under the action of non-equilibrium low-temperature plasma. The second stage is high-temperature treatment of the resulting powders. The action of plasma on solutions of iron and cobalt nitrates leads to the formation of a colloidal suspension at the plasma-solution interface in the liquid anode. The kinetics of co-precipitation from solutions under the action of plasma has been studied. It is shown that the process of formation is complex, includes several stages. The rate of formation of particles directly depends on the concentration of iron nitrate in the initial mixture. An increase in the discharge current leads to an increase in the rate of particle formation. The obtained substances were studied immediately after the plasma-solution interaction, after centrifugation, and after high-temperature treatment. X-ray diffraction analysis showed that the resulting ultra�ne particles are a mixture of hydroxonitrites and hydroxides of cobalt and iron. The data of thermogravimetric analysis con�rm the data of X-ray diffraction analysis. The surface morphology was studied using a scanning electron microscope; the resulting powders have a well-developed surface. The resulting particles are characterized by two sizes, 92 nm and 1.46 µm. The magnetic characteristics of the particles were studied using a vibrating magnetometer at room temperature with a maximum applied �eld of up to 30 kOe. The coercive force of the obtained particles was 210 Oe. The saturation magnetization (M S ) obtained at room temperature was found to be 65 emu/g and remanent magnetization (M r ) was 22 emu/g.


Introduction
Over the past few decades, a new branch has emerged in the development of plasma technologies and research on the synthesis of micro-and nanomaterials obtained through the interaction of plasma with liquids [1,2].Studies of the formation of particles in solution using various types of gas discharges are actively conducted all over the world.However, the amount of information on the formation of insoluble oxygen-containing compounds, in particular iron and cobalt, when exposed to discharges on their aqueous solutions is extremely limited.There are even fewer studies on the production of oxygencontaining particles containing two or more metal cations.
Cobalt ferrites with a spinel type structure are attractive to many researchers due to their unique properties, including chemical resistance, high mechanical strength, electrical, magnetic and catalytic properties.Cobalt and iron oxides (like all transition metal oxides) have a variable oxidation state, which allows them to participate in redox reactions and allows them to be used to create e cient and highquality energy storage and conversion systems [3].However, individual metal oxides typically have low conductive properties and unfavorable stability, which limits their use.At the same time, binary oxide systems are free from this drawback.Depending on the type of synthesis and the applied conditions, CoFe2O4 can form a ferrite spinel of the reverse or mixed type.Among ferrites, much attention in various applications is paid to spinel ferrites.
The active use of spinels in various elds of science and technology leads to an increase in the number of publications devoted to the study of their properties and applications [4][5].It is known to use spineltype cobalt ferrites in biomedical applications, for example, for drug delivery, diagnostics, magnetic resonance imaging and radiation therapy [6], as well as in non-invasive imaging of tumors and early detection of malignant growth due to their optical and magnetic properties.[7].They also work well as photocatalysts for photo degradation of organic contaminants and dyes such as ethylene blue, rhodamine B, methyl orange, methyl red, crystal violet, etc. and other pollutants such as 4-chlorophenol, and 2-dichlorophenol.Additional doping of ferrites is possible, which leads to an improvement in photocatalytic activity [8].Ferrites are also used as photocathodes for hydrogen evolution reactions [9].
The review article [10] describes the use of not only the advantages of using cobalt spinels, but also ferrite ones.It should be understood that ferrites with a spinel type structure not only have excellent electrochemical properties and high performance, but are less toxic for use as electrodes in supercapacitors.
It is well known that the structure and properties of cobalt ferrites directly depend on the methods and methods of production.A distinction is made between top-down and bottom-up methods.In turn, the former include obtaining with the help of mechanochemical -synthesis [11], hydrothermal [12].To the second, sol-gel [13], green synthesis, co-precipitation [14], micro-emulsion [15], CVD and methods using low-temperature gas-discharge plasma [16,17,18].These methods have their own advantages and disadvantages.For example, the hydrothermal method is easily scalable, has size control and gives a fairly large yield.This requires a su ciently long time for the formation of particles, the use of high pressure, which implies the presence of technically complex and expensive equipment.In the case of the sol-gel method, a fairly simple technique is used, with high size control and scalability.But long reaction times are needed, a large number of different components, and there is no good control of crystallinity.Mechanochemical methods give small particle sizes and a narrow size distribution.However, organic solvents are used, preparation times are long, impurities are often present in such particles and additional puri cation is required.Co precipitation is also easy to scale, with a fairly simple technique and a high degree of morphology control.But the formation reaction takes a long time, careful selection of conditions and pH control are required, while the product contains impurities that are di cult to clean.The latter, a method based on the interaction of plasma with solutions, is little studied and promising, since the implementation does not require a large amount of complex equipment, many different chemical reagents, and solar energy can be used to generate electricity, which also further reduces the cost of the process.The use of a glow discharge for the synthesis of ferrite has not been su ciently studied.Depending on the parameters of the glow discharge, it is possible to change the rate of processes occurring between several inhomogeneous media and thereby changing the phase and morphological composition of the products obtained.For example, in [19], the authors showed that a signi cant variation in some processing conditions leads only to heating the solution, without the formation of the necessary powder.
In this paper, we consider the production of cobalt ferrites from a mixture of aqueous solutions of cobalt and iron nitrates under the action of low-temperature gas-discharge plasma of atmospheric pressure.This method, despite the presence of plasma, is distinguished by its ease of implementation, the absence of a large number of reagents and a fairly short reaction time.Conventionally, the process of formation of ferrites can be divided into two stages.The rst is the production of hydroxonitrates in solutions under the action of a gas discharge, the second is the high-temperature treatment of the resulting powders, with the formation of cobalt ferrites with a spinel-type structure.

Experimental
A DC glow discharge was generated between two titanium electrodes located 5 mm above the surface of an aqueous solution of a mixture of iron and cobalt nitrates in an H-shaped glass cell.The cell scheme is shown in Fig. 1.The cell parts are separated by a cellophane membrane.The solution under the metal cathode was the liquid anode and the solution under the metal anode was the liquid cathode.The solution was prepared by dissolving cobalt nitrate II and iron nitrate III (analytical grade) in distilled water.
The concentration ratio of iron and cobalt nitrates in the solution was varied in such a way as to eventually obtain a solid phase in which Co:Fe = 1:2.The initial concentrations of nitrates are shown in Table 1.During the burning of the discharge, the kinetics of the formation of particles in solution was studied.The kinetics of the colloidal particle formation process was investigated using the method of turbidimetry.The intensity of the light passing through the layer (1 mm below the surface) of the solution was measured with an AvaSpec-2048 FT-2 spectrometer (Avantes, Netherlands).The optical length was 45 mm.The light source was a He-Ne laser (λ = 632.8nm).
Immediately after the end of the plasma-solution interaction, the solution was centrifuged, the mother liquor was drained, and the solid precipitate was dried at a temperature of 50°C in air until the liquid completely evaporated.The morphology and composition of the resulting powders were studied.After that, they were subjected to high-temperature treatment at a temperature of 1000°C for 1 hour.Powders after high-temperature treatment were also studied.
The method of dynamic light scattering (DLS: Photocor Compact -Z, Russia) was used to determine the size and zeta potential of the resulting particles in solution.Scanning electron microscopy (SEM: Tescan Vega 3SBH, Czech Republic) was used to observe the microstructure of the coatings surface and the cross-section.The composition of the powders was studied by energy dispersive spectroscopy (EDS: Aztec EDS, Oxford Instruments Ltd., England).Thermogravimetric analysis and differential scanning calorimetry instrument were carried out using STA 449 F1 Jupiter thermal analysis instrument, Netzsch, Germany.The phase structures of the ferrite powders was characterized by X-ray diffractometer (XRD: DRON 3 M, Burevestnik, Russia) with Cu Kα radiation of 0.154 nm wavelength.The diffraction patterns were analyzed using QualX2 software [20] and the open crystallographic COD database [21].The pH of the solution before and after solution treatment was measured using a PHT-028 multivariable water quality monitor (Kelilong, China).The hysteresis loops of powders were measured by vibrating sample magnetometer (VSM, Cryogenic Limited, England) with an applied eld up to 30 kOe at room temperature.

Phenomenology of observed phenomena
Typical visible changes occurring in the cathode and anode cells were as follows (Fig. 2).Immediately after the discharge was ignited, the formation of rather dark brown agglomerates was visually observed at the plasma-solution contact point.The color of the resulting colloidal particles is typical of hydroxo compounds of iron and cobalt.After 60 s, a colloidal suspension of rufous color began to form at a depth of 5 mm from the surface of the solution.At the same time, the formation of agglomerates continued.At 90 s, the area of the colloidal solution increased into the depth of the cell, and a precipitate began to form.By 5 min, the nal formation of a brown-red colloidal region occurred 1.5 cm from the surface of the solution deep into the cell, while active precipitation of large agglomerates continued.After 10 minutes of plasma interaction with the solution in the anode cell, a stable colloid suspension was formed (1-1.5 cm deep from the surface) and a huge amount of agglomerates uniformly distributed over the entire volume of the cell (almost completely falling to the bottom of the cell after 30 minutes after the end of the plasma processing, however, the entire solution changed color to rufous).The main "accumulation" of the substance occurred after 10 minutes.To accumulate the powder, the solutions were subjected to plasma treatment for about 25-30 minutes.

Kinetic regularities of precipitation
The kinetics of the formation of a solid phase in solutions under the action of a discharge was studied, and an exponential decrease in the intensity of light transmitted through the solution was observed.A typical dependence of the light transmitted through the solution vs. time is shown in Fig. 3.The showed behaviour is typical for all concentrations and discharge currents.The kinetic curve is well described by an exponential function (determination coe cient R 2 > 0.9).Such processing makes it possible to determine the effective rate constants or characteristic times.The results of this processing are shown in Fig. 4.Each point is the result of averaging ve measurements.The obtained dependences of the change in the rate constants of formation of the solid phase on time are shown in Figs. 4. As can be seen from the gure, the rate of formation of the solid phase in the solution under the action of the discharge increases with current.This was directly related to the accumulation of active particles in the solution under the action of the discharge.It is noteworthy that the rate of formation of the solid phase in solutions of iron nitrate was three times higher than in solutions of cobalt nitrate.One of the reasons for the differences in the behaviour of the solid phase formation constants in solutions of iron and cobalt nitrates depending on the discharge current can be associated with a large difference in the solubility product of hydroxides (К(Fe(OH) 3 ) = 3.8⋅10-38; К(Co(OH) 2 ) = 1.6⋅10-18).That is, thermodynamically, the formation of iron hydroxide Fe(OH) 3 is possible already at pH = 2.3 and above, and the formation of cobalt hydroxide Co(OH) 2 is possible at pH = 5.8 and above.For this reason, at equal concentrations of Fe 3+ and Co 2+ ions, the formation of hydroxocompounds of iron is more probable than that of cobalt.And to obtain a stoichiometric compound, an excess concentration of cobalt ions is required compared to iron (See Table 1).
Note that no visible formation of a colloidal solution in the cathode part was observed.Apparently, this is due to the fact that the pH of the solution in the anode part is signi cantly higher than in the cathode part [24].

Characterization of sediment
Since the main goal of our research was to obtain cobalt ferrite with a spinel-type structure and the formula CoFe 2 O 4 , then we will mainly consider the sample 5, which at the output gives the closest result to CoFe 2 O 4 .Table 2 shows the results of energy dispersive analysis of the obtained powders after hightemperature treatment.As can be seen, a change in the concentration of the initial components leads to a change in the stoichiometric ratio of iron and cobalt in CoFe 2 O 4 .
Table 2 The result of the EDS of powders after high-temperature treatment.
Sample number The content of the resulting oxides in % X-ray diffraction analysis of sample 2 powder shown in Fig. 5.According to [22], the obtained powders after plasma treatment are Fe 2 O 3 .X-ray diffraction analysis did not show the presence of cobalt in the sample 2, while energy-dispersive X-ray spectroscopy showed the presence of ~ 1% cobalt.This could be due to the well-developed surface of the resulting powder in the pores of which traces of a cobalt compound remained.However, it is interesting that the sample was an oxide without additional hightemperature treatment which was also con rmed by the EDS data.This is due to the process of formation of particles in solution.The concentration of iron nitrate is 3 times greater than the concentration of cobalt nitrate, while the pH of the solution is low and cobalt particles simply cannot be formed.What we see on the EDS is most likely not washed nitrate trapped by iron particles.X-ray diffraction analysis of the sample 5 powders showed that the powders obtained as a result of plasmasolution treatment are X-ray amorphous.EDS analysis of the sample showed that the product resulting from the interaction of the plasma with the solution was a mixture of cobalt and iron hydroxonitrates.An indirect con rmation that the micelles contain nitrate ions is that the measured zeta potential is negative (~-20 mV).In this case, the negative potential-forming ions can be OH -and NO 3 -ions.But at realized solution pH (2.5-3), the concentration of OH -ions is several orders of magnitude lower than the concentration of nitrate ions.Therefore, most likely nitrate ions are potential forming ions.Further heat treatment of the powders gives us the presence of a large number of re ections corresponding to spinel ferrites CoFe 2 O 4 (Fig. 6) [23].(hexahydrate cobalt nitrate begins to decompose at a temperature of 74°C and 35°C in iron nitrate hexohydrate.).The second endothermic peak corresponds to the decomposition of cobalt hydroxide to oxide (160°C).In this case, there is no peak on 500°C, which corresponding to the decomposition of iron hydroxide to oxide.This is most likely due to the fact that a complex chemical compound is formed, the decomposition temperature of which is lower.
The study of solutions by dynamic light scattering showed that particles of two characteristic sizes were formed: 97 nm, 35%, and 1492.2 nm, 65%.In this case, the zeta potential was − 24.76 mV.The studies were carried out in the mother solution and repeated after a month.After a month of storage of the sample in the mother solution, a color change to russet and the dissolution of the precipitate were observed.At the same time, the DLS data showed that one fraction with a size of 278 nm and a zeta potential of -21.63 mV remained in the solution.Most likely, this was due to the acidity of the solution, which after 10 minutes of plasma treatment was pH = 2.5.
The room temperature magnetizations of sample 5 for applied magnetic eld are shown in Fig. 9.The saturation magnetization (M S ) obtained at room temperature was found to be ~ 65 emu/g and remanent magnetization (M r ) was 22 emu/g.The saturation magnetization for single-crystal cobalt ferrite is 80 emu/g.Magnetic coercivity, valued as ~ 485 Oe, which is also lower than that of bulk cobalt ferrite, known as 980 Oe [25].The magnitude of the magnetic properties depends on the size of the particles, so that a decrease in the size of the particles leads to a decrease in the saturation magnetization and coercive force.

Conclusion
The possibility of synthesizing cobalt ferrites from solutions of iron and cobalt nitrates under the action of a DC glow discharge at atmospheric pressure in air is shown.Using turbidimetry, the kinetics of the formation of a solid phase in a solution under the action of plasma was studied.The settling rate of ironcontaining particles is three times higher than that of cobalt-containing particles for solutions containing only one type of nitrate.Based on the data obtained, it can be concluded that the main contribution to the mechanisms of formation and precipitation is made by Fe 3+ ions in solution.After high-temperature treatment of the resulting composites, pure oxides with a given stoichiometric ratio of iron and cobalt obtained.

Declarations Ethical Approval Not applicable
Competing interests The authors have declared no con ict of interest for this article.

Figure 4 The
Figure 4

Table 1
Initial concentrations of solutions.