Magnetite nanoparticles obtained by solution combustion synthesis

This research presents a comprehensive investigation into the synthesis and characterization of magnetite nanoparticles through solution combustion reactions ignited by conventional means. In addition to the structural and compositional ﬁ ndings, this study's main investigation results include the speciﬁ c surface area measurements conducted using the BET method. The analysis revealed speciﬁ c surface area values for the synthesized magnetite nanoparticles at varying propellant-to-oxidant ratios, demonstrating a substantial decrease in speciﬁ c surface area as the ratio increased. Speciﬁ cally, speciﬁ c surface areas of 72.203 m 2 /g for the 1:1 ratio, 22.240 m 2 /g for the 1:1.5 ratio, and 9.204 m 2 /g for the 1:2 ratio were determined. Furthermore, calculations based on the BET results and assuming spherical magnetite nanoparticles provided average particle sizes of 16±1 nm for the 1:1 ratio, 51±2 nm for the 1:1.5 ratio, and 125±4 nm for the 1:2 ratio. These ﬁ ndings highlight the impact of synthesis parameters on the nanoparticles' surface area and size, shedding light on their potential applications in various ﬁ elds, including nanomedicine and magnetic diagnostics. Overall, this research contributes valuable insights into the synthesis, characterization, and tunable properties of magnetite nanoparticles, oﬀ ering potential avenues for their utilization across diverse industries.


Introduction
Nanotechnology, situated at the convergence of science, engineering, and materials science, has ushered in revolutionary breakthroughs across various industrial sectors.Notably, the magnetic properties of materials fi nd extensive utility in science and technology, contributing to advancements in acoustic systems [1], proximity sensors [2], electrical machinery [3], magnetic separators [4], as well as everyday consumer electronics like mobile phones and cameras [5].The shift towards the nanoscale has catalyzed the emergence of a novel realm within magnetic materials science: magnetic nanomaterials.The composition, structure, and properties of magnetic nanomaterials exhibit wide-ranging diversity, contingent upon the starting materials, synthesis techniques, and the practical challenges they are designed to address.Predominantly, magnetic nanomaterials are derived from elements such as Ni, Co, and Fe [6].Within this domain, Febased compounds, particularly Fe 3 O 4 magnetite, assume a prominent role.
Magnetite nanoparticles, composed of iron oxide (Fe 3 O 4 ) and characterized by their diminutive size on the nanoscale, showcase a plethora of intriguing properties that set them apart from their macroscopic counterparts.Notably, they play a pivotal role in the realm of nanomedicine, contributing to the development of innovative magnetic diagnostic methodologies such as magnetoresistance and microHall (μHall) biosensors [7,8].
In line with the classifi cation of nanomaterials based on their geometric and structural attributes, magnetic nanomaterials can assume zero-dimensional (0D) structures, represented by magnetic nanoparticles and nanopowders.Additionally, one-dimensional (1D) confi gurations encompass magnetic nanofi bers, nanorods, nanotubes, and other elongated forms [9].The synthesis of magnetite nanoparticles has garnered substantial interest among researchers due to their vast potential applications.The table below provides a concise summary of the primary methods employed for the synthesis of magnetite nanoparticles.
While magnetite nanoparticles hold immense potential across diverse industries, they are https://doi.org/10.18321/cpc21(3)147-157[16] accompanied by a set of challenges that warrant careful consideration.The synthesis of uniform nanoparticles with precise control over size and shape remains a complex and cost-intensive endeavor.
Researchers must navigate the delicate balance between achieving desired properties and ensuring scalability for industrial utilization.Furthermore, the seamless integration of magnetite nanoparticles into various industries necessitates rigorous safety assessments, particularly in biomedical and environmental contexts.Prioritizing biocompatibility and conducting thorough toxicity evaluations are paramount, as any adverse eff ects could compromise the advantages of these nanoparticles [17].
In the realm of environmental applications, continued research is imperative to assess the cost-eff ectiveness and long-term sustainability of magnetite nanoparticle-based remediation systems.Factors such as maintenance, remediation effi ciency, and recycling strategies must be taken into account to determine their practical viability.
Among the myriad methods available for nanoparticle production, the solution combustion method emerges as an appealing and effi cient approach.This technique leverages the exothermic nature of chemical reactions to swiftly generate nanoparticles under controlled conditions, rendering it an attractive choice for producing magnetite nanoparticles with meticulous control over size, morphology, and magnetic properties [18].
Consequently, this study endeavors to expand the existing knowledge base on magnetite nanoparticles, elucidate the synthesis process of solution combustion, characterize the resulting nanoparticles comprehensively, and shed light on their potential applications.

Method for obtaining of magnetite (Fe 3 O 4 ) nanoparticles
Magnetic Fe 3 O 4 magnetite nanoparticles were prepared by solution combustion at diff erent oxidant ratios.Iron nitrate (Fe(NO 3 ) 3 ‧9H 2 O) and citric acid (C 6 H 8 O 7 ‧6H 2 O) of analytical purity, without further purifi cation, were used as starting components.
Varying the ratio of fuel to oxidant changes the pH of the initial solution and aff ects the dispersibility of the fi nal product obtained.The synthesis of magnetite nanoparticles was carried out at the following iron nitrate to citric acid ratios: 1:1, 1:1.5 and 1:2.
The process of solution combustion occurs as a result of the chemical interaction between citric acid and iron nitrate according to the reaction below: Magnetite nanoparticles were collected by magnetic fi eld, washed several times with water, and then dried.

X-ray phase analysis
The MiniFlex X-ray diff ractometer, a cuttingedge 5th generation instrument, was employed for the comprehensive qualitative and quantitative analysis of polycrystalline materials.Operating under precise imaging settings, including adjustable X-ray tube voltage (up to 40 kV), a constant tube current (15 mA), goniometer step movements as fi ne as 0.02 degrees 2θ for spot intensity measurements, a consistent sample rotation speed of 10 rpm within its own plane, and a scanning range spanning from 3 to 90 degrees 2θ, this instrument ensured meticulous data collection.The use of a Kβ fi lter enabled monochromatization of X-ray radiation, enhancing measurement accuracy.Subsequent data processing and analysis were conducted with the PDXL database, off ering advanced capabilities for interpreting the acquired data.This holistic approach facilitated a comprehensive and precise evaluation of the crystalline characteristics of the materials, aff ording valuable qualitative and quantitative insights into their composition and structure for diverse scientifi c and industrial applications.

Electron and optical microscopy
The investigation into the structure, dimensions, and morphology of the acquired samples was conducted utilizing a Quanta 200i 3D scanning electron microscope (FEI, USA) with an accelerating voltage of 30 kV.This microscopy analysis was performed at the "National Nanotechnology Laboratory of Open Type," (al-Farabi Kazakh National University, Almaty, Kazakhstan).The utilization of this method enabled the determination of surface structures in volumetric imagery, facilitating the evaluation of both structural features and the sizing of individual particles.It is worth noting that the Quanta 200i 3D SEM is equipped with an energy dispersive X-ray system (EDS), allowing for the analysis of a wide range of chemical elements spanning from B to U. The energy resolution of this system is 132 eV (Mn Kα).EDS analysis was employed to ascertain the elemental composition of magnetite nanoparticles.
For the examination of microcracks in fi bers and the determination of mesophase centers in carbon pitches, a LeciaDM 600 M automated digital optical microscope, located at the "National Nanotechnology Laboratory of Open Type," (al-Farabi Kazakh National University, Almaty, Kazakhstan), was employed.This optical microscope facilitates work with magnifi cations ranging from 150x to 1500x.

Measurement of specifi c surface area
The specifi c surface area of the samples was determined utilizing the BET (Brunauer-Emmett-Teller) method through thermal desorption of inert gases, employing the SORBTOMETR-M instrument.This selection of methodology was driven by several advantageous features compared to static techniques.Notably, it obviates the need for a vacuum apparatus, off ers straightforward installation, and facilitates automation.The core principle involves the dynamic desorption of gas-adsorbate from the surfaces of the test materials.In the BET method, a heliumnitrogen mixture with predefi ned composition is directed through an adsorber housing the sample.Following degassing, achieved by elevating the sample's temperature within a stationary gas stream at a specifi ed level, previously adsorbed gases are desorbed from the sample surface.Furthermore, specifi c surface area measurements were conducted employing helium gas as the carrier, blended with argon at a fl ow rate of 48-50 ml/min, and the argon concentration in the mixture ranged from 3% to 6%.The sample suspension quantity ranged from 0.03 to 0.15 g.The associated measurement error was limited to 5% [19].

Measurement of magnetic properties of samples
Magnetic moment values of the studied material samples were assessed using a 14T Cryogenic vibrating sample magnetometer (VSM), which comprises the following components: 1) a mechanical vibration generator; 2) a rod affi xed with a quartz sample holder containing the material under examination; 3) an electronic measurement unit.Before conducting measurements, the sample of the material being investigated was enclosed within a plastic tube and securely fastened between the clamps of the rod.Mechanical vibrations were generated using an electrodynamic loudspeaker, with a rod serving to transmit these vibrations from the generator to the sample.The vibration amplitude of the sample was maintained at 0.03 meters, with a frequency of 200 Hz.A capacitive sensor was employed to stabilize the vibration amplitude.Following phase detection and amplifi cation, the signal from the measuring coils was directed to the input of the Hall sensor.The data from the measurement of signal parameters, including the magnetic moment of the investigated material sample and the magnetic fi eld induction within it, were documented using a personal computer [20][21][22].

Results and discussion
This research is centered on the synthesis of magnetic iron oxides through solution synthesis combustion reactions initiated by conventional ignition.Initially, a thorough examination of the characteristics of iron oxide nanoparticles generated through conventional ignition combustion reactions was conducted.
In our study, we conducted a comprehensive investigation of magnetite nanoparticles using advanced techniques such as XRF, optical microscopy and SEM with EDS-analysis.
To ascertain the crystal structure of the magnetite nanoparticles synthesized through solution combustion, we conducted X-ray diff raction analysis using a MiniFlex 300/600 diff ractometer, as illustrated in Fig. 1 (g-k).This analysis aimed to explore the impact of varying the concentration ratio between the propellant (citric acid) and the oxidant (ferric nitrate) on both the crystal structure and the crystallite sizes of the nanoparticles.
Crystallite sizes were determined using the Williamson-Hall method with the instrument's standard software.For the 1:1 ratio, the crystallite size was calculated to be 178 angstroms (Å), while for the 1:1.5 ratio, it was found to be 100 Å, and for the 1:2 ratio, the crystallite size measured 143 Å.
Based on the outcomes of X-ray diff raction analysis carried out utilizing the MiniFlex 300/600 diff ractometer, the crystalline phase identifi ed within the samples was confi rmed to be magnetite (Fe 3 O 4 ).Notably, variations in the concentration ratio of the fuel to oxidant were observed to exert a direct infl uence on the size of the crystallites.
Alterations in the ratio of the initial constituents, namely iron nitrate and citric acid, as well as adjustments in the synthesis parameters, result in noteworthy modifi cations in the structure and composition of the ultimate product.Comprehensive elemental analysis (Fig. 1d-f) was conducted on the iron oxide samples across all three ratios (1:1, 1:1.5, 1:2).The investigations revealed a notable trend wherein the percentage of carbon content in the fi nal product exhibited an increase proportionate to the higher concentration of the oxidant, indicating a direct correlation between oxidant concentration and carbon content in the synthesized materials.
Figure 1a-c depict optical images of magnetite nanoparticles obtained under varying ratios of fuel to oxidant concentration.Optical microscopy investigations of these magnetite nanoparticle samples have revealed the presence of carbon.These studies have unveiled a signifi cant phenomenon in the synthesis process by the solution combustion method, where magnetite nanoparticles undergo sintering with concurrently formed carbon.The resultant material exhibits a profusion of macropores and channels, a consequence of the release of substantial quantities of gaseous products stemming from the reaction between iron nitrate and citric acid.
To delve deeper into the morphology and structure of the magnetite nanoparticles synthesized through the solution combustion method, scanning electron microscopy was employed.SEM images of the acquired samples are depicted in Fig. 1d-f.These examinations have elucidated that during the synthesis process, magnetite nanoparticles agglomerate and coalesce into conglomerates characterized by a layered structure.Particularly noteworthy is the prevalence of the amorphous phase, especially within agglomerates surpassing the 100 nm scale.To ensure uniform dispersion of magnetite nanoparticles in the shielding material, separation of these agglomerates was carried out through ultrasonic pretreatment of nanomagnetite nanoparticle powders in aqueous suspension.
Furthermore, the outcomes of the EDS analysis have unveiled a correlation between the fuel-tooxidant concentration ratio in the solution combustion reaction and the resultant composition and structure of the samples.An increase in fuel concentration is associated with an augmented predominance of amorphous carbon in the samples, as evidenced by carbon content percentages of 24.37% at the stoichiometric ratio (1:1), 32.7% at the 1:1.5 ratio, and 38.06% at the 1:2 ratio.
The specifi c surface area of all synthesized samples of magnetite nanoparticle powders was determined using the BET-analysis.The specifi c surface area analysis results for magnetite nanoparticles, along with calculations of the average size of the products obtained at varying stoichiometric ratios of propellant to oxidant, are presented in Table 2 below.The BET method revealed specifi c surface area values of 72.203 m 2 /g for the 1:1 ratio, 22.240 m 2 /g for the 1:1.5 ratio, and 9.204 m 2 /g for the 1:2 ratio.
The structural characteristics of the synthesized nanoparticles were found to be signifi cantly infl uenced by the ratio of propellant to oxidant.Specifi cally, three distinct ratios of iron nitrate to citric acid (1:1, 1:1.5, 1:2) were systematically investigated.X-ray phase analysis underscored the pivotal role played by the initial component ratio in the synthesis of magnetite (Fe 3 O 4 ) nanoparticles.
A comprehensive evaluation of the structure and morphology was undertaken through optical and scanning electron microscopy (SEM).The optical microscopy results unveiled the presence of carbon in the synthesized magnetite nanoparticle samples, with the synthesis process involving sintering of magnetite nanoparticles alongside the formation of carbon.SEM studies corroborated this observation by revealing the agglomeration of magnetite nanoparticles into conglomerates characterized by a layered structure.Moreover, the predominant presence of an amorphous phase, particularly evident in clusters exceeding 100 nm in size, was noted.Additionally, the specifi c surface area analysis conducted using the BET method elucidated specifi c surface area values of 72.203 m 2 /g for the 1:1 ratio, 22.240 m 2 /g for the 1:1.5 ratio, and 9.204 m 2 /g for the 1:2 ratio.Based on the obtained data, calculations were performed to deduce the average sizes of magnetite nanoparticles synthesized via the solution combustion method.These calculations, under the assumption of spherical magnetite nanoparticles, yielded an average particle size of 16±1 nm for the 1:1 ratio, 51±2 nm for the 1:1.5 ratio, and 125±4 nm for the 1:2 ratio.The magnetic properties of nanoparticles are intricately infl uenced by a multitude of factors, including their chemical composition, crystal lattice type, structural defects, particle size, shape, and morphology.Consequently, through careful material selection and the manipulation of synthesis conditions, it is possible to fi nely tune the magnetic properties of the resulting nanomaterials, albeit within certain constraints.The ferromagnetic attributes of materials exhibit a strong size dependency, with individual particles encountering thermal fl uctuations as they diminish in size, ultimately leading to a nullifi cation of the collective magnetization of the particle ensemble.However, when nano-sized particles are exposed to a suffi ciently robust magnetic fi eld, their potential energy of magnetization surpasses the energy associated with thermal fl uctuations.This phenomenon causes them to behave as ferromagnets while in the magnetic fi eld, giving rise to their superparamagnetic properties.
The magnetic moment values of the magnetite nanoparticles obtained in this study were assessed using a Cryogenic 14T vibrating sample (b) (a) magnetometer.The results of these measurements for a 1:1 sample are graphically presented in Fig. 2a, 2b.The conducted investigations have revealed that the magnetite nanoparticles transition to a superparamagnetic state, as evidenced by the absence of hysteresis in the fi eld magnetization curve.This transition to the superparamagnetic state can be attributed to the nanoparticles' transition into a singledomain state, where uniform magnetization prevails throughout the volume.

Outlook
Nanosized magnetite nanoparticles and composites based on them are a potential material for the future with a wide range of applications.This material is also unique due to its high electromagnetic characteristics.Table 3 shows the main parameters and possibilities for using magnetite nanoparticles.
Thus, according to the table, it can be seen that magnetite nanoparticles obtained by the solution combustion method can play an important role in the production and use of composite materials.This once again proves a number of advantages of the method of obtaining magnetic nanoparticles and their future application.*rGO -Reduced graphene oxide

Conclusion
In this study, we explored the synthesis and characterization of magnetite nanoparticles through solution synthesis combustion reactions initiated by conventional ignition.Our investigation encompassed a thorough analysis of these nanoparticles, employing advanced techniques such as XRF, optical microscopy, SEM with EDSanalysis, and X-ray diff raction.Notably, our fi ndings elucidated the crucial role played by the propellantto-oxidant concentration ratio in infl uencing the crystal structure and crystallite size of the synthesized magnetite nanoparticles.Elemental analysis revealed a direct correlation between oxidant concentration and carbon content in the fi nal product.Optical microscopy and SEM investigations unveiled the presence of carbon and the sintering phenomenon, leading to the formation of macropores and channels.Additionally, BET analysis provided specifi c surface area values that varied with the concentration ratio.Magnetic property assessments demonstrated the transition of magnetite nanoparticles to a superparamagnetic state, indicative of their potential utility in diverse applications.This comprehensive exploration contributes to the understanding of magnetite nanoparticles and their prospects across various fi elds, off ering valuable insights for future nanomaterial research and development.

Table 1 .
Methods of production of magnetite nanoparticles.

Table 2 .
Particle size as a function of fuel/oxidiser ratio

Table 3 .
Summary of applications of composite material based on magnetite nanoparticles