Morphological features of Co 3 O 4 nanoparticles obtained by solution combustion method

The global environmental crisis has made it imperative to enhance tools and techniques for monitoring and analyzing environmental parameters. Gas sensors, crucial for air quality assessment, continually undergo technological advancements to enhance accuracy and eﬃ ciency in detecting harmful substances. They play an essential role in ensuring safety in workplaces, urban areas, and industries, aiding pollution control eﬀ orts.Enhanced gas sensor performance hinges on careful selection and control of gas-sensitive materials and their structure. This involves optimizing gas-sensitive compounds, employing advanced materials, and developing technologies for sensitive and rapid substance detection. One promising compound for this purpose is Co 3 O 4 oxide, synthesized eﬃ ciently using the solution combustion method. This method oﬀ ers simplicity and allows for precise control over product structures and properties, enabling customization for speciﬁ c requirements and ensuring high detection eﬃ ciency and accuracy.In this study, Co 3 O 4 particles were synthesized from a mixture of cobalt nitrate and glycine with the addition of nitric acid using the solution combustion method. The inﬂ uence of nitric acid addition and the fuel-to-oxidizer ratio on the morphological characteristics of the cobalt oxide was investigated. The results from SEM, TEM, XRD, and SAXS analyses conﬁ rmed that the addition of nitric acid and a fuel-rich mixture lead to nanoparticles with smaller diameter spread and more stable characteristics.


Introduction
Nowadays, gas sensors are attracting considerable attention from researchers due to the aggravated environmental problems.Gas sensors are of paramount signifi cance in determining the type and concentration of pollutants in the air.There are diff erent types of gas sensors such as semiconductor, photoionization, electrochemical, etc. [1].
Conductometric semiconducting metal oxide gas sensors, also, is one of the groups of gas sensors suitable for conducting gas measurements under atmospheric conditions.They have several advantages including fl exibility in manufacturing, ease of use, low cost, and, most importantly, the ability to detect a wide range of gases [2].
An essential characteristic of conductometric semiconducting metal oxide gas sensors is the reversible interaction of gas with the material surface, which can be aff ected by the natural properties of the basic components, microstructure of the sensing layers, surface area, and external factors such as temperature and humidity [2][3][4][5][6][7].The sensitivity of gas sensors plays an important role in their operation.However, there is currently no single and universal gas sensor for the detection of diff erent gases.Therefore, various transition metal oxides like Fe 2 O 3 [8], Cr 2 O 3 [9], NiO [10], and nontransition metal oxides including pre-transition metals like ZnO [11], Al 2 O 3 [12], SnO 2 [13], and so on are used in conductometric meters for the detection of combustible, oxidizing and reducing https://doi.org/10.18321/cpc21(3)159-171gases.Inertness, structure stability, and the easiness of measuring the electrical conductivity, which depends on the electronic confi guration, are the main parameters in the selection of oxides for gas sensors.Despite the wide range of oxides, transition metal oxides with d 0 and d 10 electron confi gurations have a practical application in gas sensors.Such oxides include TiO 2 [14], WO 3 [15], and Co 3 O 4 [16].
In addition, cobalt oxide nanoparticles can be used in magnetic materials [21], ceramic pigmentation [22], as a catalyst [23], electrochromic devices [24], and rechargeable batteries [19].Therefore, the synthesis of cobalt oxide powder with improved characteristics has attracted huge interest recently.
There are numerous methods for the synthesis of cobalt oxide.However, each of them has its own advantages and disadvantages, and the choice of the method depends on the area and purpose of the material application.[29] As can be seen from Table 1, despite the various methods for the synthesis of cobalt oxide, obtaining homogeneous nanoparticles remains a costly method because controlling the size, shape, and morphology of the product, as well as the valence of cobalt ions like Co 3+ and Co 2+ , is a labor-intensive process.Moreover, for extensive applications of Co 3 O 4 , including in gas sensors, the oxide must have high stability and dispersion.Also, it is necessary to properly approach the issue of synthesis waste disposal, for example, in the synthesis of nanoparticles by co-precipitation, in general, besides metal oxide, many compounds are formed, which are diffi cult to utilize.Therefore, a more ecological method of synthesis of Co 3 O 4 should be preferred.
Among a variety of methods for the synthesis of cobalt oxide, the solution combustion method is highlighted.It is based on the self-propagating combustion of a mixture of fuel and oxidizer in the liquid phase.Gradual heating of reagents dissolved in water leads to an exothermic redox reaction resulting in the formation of metal oxide [17,30].Compared to other methods, this method is characterized by simplicity, practicality, and rapidity.
Furthermore, by changing the ratio of oxidizer and fuel, it is possible to obtain cobalt oxide nanoparticles with specifi ed properties and structure, which expands the scope of the product application.

Materials
The following materials and equipment were used in this work: cobalt nitrate hexahydrate (Co(NO 3 ) 2 •6H 2 O), glycine (C 2 H 5 NO 2 ) and nitric acid (all reagents were chemically or analytically grade); magnetic stirrer (MS-H340-S4); and laboratory plate.

Synthesis of Co 3 O 4 by solution combustion method
The synthesis of ultra-disperse metal oxide particles is based on the exothermic process of interaction between the components of the solution system: fuel and oxidizer.Co 3 O 4 nanoparticles were synthesized by the solution combustion method.Cobalt nitrate hexahydrate (Co(NO 3 ) 2 • 6H 2 O) as oxidizer and glycine (C 2 H 5 NO 2 ) as fuel were used to produce cobalt oxide.The reaction equation of glycine-nitrate synthesis of cobalt oxide is as follows: The infl uence of the ratio of fuel and oxidizer on the morphology of the obtained nanoparticles was investigated.For this purpose, synthesis was carried out from a mixture of cobalt nitrate hexahydrate and glycine in the stoichiometric ratio φ=1 (3 moles of cobalt nitrate to 6 moles of glycine) and under the condition of a fuel-rich mixture φ=1.5 (for 3 moles of cobalt nitrate -9 moles of glycine).The eff ect of nitric acid addition on the dispersion of cobalt oxide was investigated.Nitric acid was added in an amount of 10% by weight of cobalt nitrate.
The initial reagents were completely dissolved in 50 ml distilled water in a heat-resistant beaker and then evaporated to a volume of 5-7 ml.After evaporation, the reaction mixture was heated to 260 °C, whereupon spontaneous ignition of the solution was observed.The decomposition temperature of glycine was taken into account while selecting the self-ignition temperature.The ignition of the fuel mixture in solution leads to a temperature increase to 1200 °C and precipitation of the fi nal product.The product of synthesis was washed with distilled water, and then dried at 80 °C for 24 h.

Methods of Co 3 O 4 characterization
Currently, microscopic techniques are widely used to analyze the structure of various materials, including nanoparticles, and have an essential role for their characterization.These techniques include visible spectrum imaging, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).One of the signifi cant advantages of visual methods is the ability to represent structure in diff erent regions of the samples.Thus, the images obtained provide information useful for comparing localized structures throughout the sample.Despite these advantages, optical methods do not provide the quantitative data necessary for comparative analysis of diff erent nanoparticles.Therefore, samples were examined by energy-dispersive X-ray spectroscopy (EDX) to determine the elemental composition.

Transmission electron microscopy
Transmission electron microscope (TEM) JEM-1011 in the Kazakhstan-Japan Innovation Center of the Kazakh National Agrarian University from JEOL company from Japan was used to study the structure of cobalt oxide.This microscope is equipped with a Morada digital camera (Olympus, Japan).Its main specifi cations include an accelerating voltage that can be adjusted from 40 to 100 kV, an accurate resolution of 0.3 nm, a linear resolution of 0.14 nm, a LaB6 electron gun, and a magnifi cation range from 100 to 1,000,000.This technique plays an important role in the development and characterization of various nanomaterials, including nanoparticles, and is an integral part of modern nanotechnology and materials science.

Scanning electron microscopy
Quanta 200i 3D scanning electron microscope (FEI, USA) with an accelerating voltage of 30 kV was used to study the structure, size, and morphology of samples ("National Nanotechnology Laboratory of Open Type" Al-Farabi KazNU, Almaty, Kazakhstan).This method allows imaging of the surface, which makes it possible to determine both the structure and size of individual particles.This microscope also has an energy-dispersive X-ray analysis (EDX) system that can determine the chemical composition in the B to U range, with a resolution of 132 eV (Mn Kα).EDX analysis was used to determine the chemical composition of cobalt oxide nanoparticles and quantitative data.

X-ray diff raction analysis
The crystal structure of the nanomaterial samples was investigated through X-ray diff raction analysis (XRD) using a DRON-3M multipurpose X-ray diff ractometer with copper radiation, which has an IBMPC-based control and recording system in digital form.XRD was used to obtain data on the lattice parameters of the substances, to determine their phase composition and the degree of amorphousness of the sample.The samples were examined under the following imaging settings: the X-ray tube voltage reached 30 kV, the tube current was 30 mA, and the goniometer was moved with an angular step of 0.05° 2θ while measuring the intensity up to 1.0.The sample was rotated in its plane at a speed of 60 rpm.Analysis of the X-ray data to determine the angle and intensity of refl ection was performed using the program "Fpeak".Phase analysis was performed using the program "PCPDFWIN" and the diff raction data database.The obtained spectra were identifi ed using the JCPDS X-ray database.The apparatus error of X-ray pulse measurement was less than 0.4%.

Small-angle X-ray scattering method
To investigate the nanoscale catalyst particles, small-angle X-ray scattering was applied.SAXS curves were analyzed using a Hecus S3-MICRO diff ractometer with a Cu-C radiation fi lter.The value of the scattering vector modulus q=4‧π‧sinΘ ⁄λ, where 2Θ is the scattering angle, λ is the wavelength of the radiation used, thus λ was equal to 1.54 A and 2Θ was equal to 0.008÷8) was used as the scattering coordinate.The scattering intensity was recorded in the range q from 0.006 to 0.6 Å-1; q is linearly related to the correlation L~2π/q.The small-angle curves for glycerol and several samples of nanoparticles dissolved in glycerol and obtained by the SAXS method were used to determine the size distribution (radius histogram) in the spherical approximation for nanoparticles.

Results and discussion
To establish the chemical and phase composition of the synthesized cobalt oxide nanoparticles, X-ray diff raction analysis was carried out, which showed that in all cases monophasic cobalt oxide with the formula Co 3 O 4 was obtained (Fig. 1).The diff ractogram shows a characteristic diff raction pattern with a series of distinct peaks corresponding to planes ( 111), ( 220), ( 311), ( 222), (400), ( 511) and ( 440).The diff raction peaks and refl ections are consistent with JCPDS card: 00-042-1467 [31].All refl ections are attributed to the typical Co 3 O 4 phase.The monophasic cobalt oxide Co 3 O 4 has a crystal symmetry corresponding to the crystal structure of spinel.The spinel structure is a type of cubic crystal structure with a space group known as Fd3m (face-centered cubic structure), which is also referred to as a "cubic densely packed" structure.In the spinel structure, Co 3 O 4 consists of two diff erent types of cations (Co 2+ and Co 3+ cobalt ions) distributed in a specifi c order within the crystal lattice.This arrangement results in the characteristic symmetry of the Fd3m spinel structure.In the Co 3 O 4 spinel structure, oxygen ions (O 2-) form a tightly packed face-centered cubic (FCC) lattice, while cobalt ions occupy both octahedral and tetrahedral positions within this oxygen lattice.The arrangement of cobalt ions within these positions contributes to the unique symmetry of Co 3 O 4 as a spinel.To establish the chemical and phase composition of the synthesized cobalt oxide nanoparticles, X-ray diff raction analysis was carried out, which showed that in all cases monophasic cobalt oxide with the formula Co 3 O 4 was obtained (Fig. 1).
To determine the eff ect of nitric acid, experiments were carried out without and with the addition of nitric acid to the initial mixture.The obtained samples were investigated by small-angle X-ray scattering.Glycerol was used as a matrix.Small-angle curves for glycerol and Co 3 O 4 nanoparticles (φ=1, without the addition of nitric acid) and Co 3 O 4 (φ=1, with the addition of nitric acid) (Fig. 2a, 2b).The contribution of small-angle scattering of glycerol was subtracted from the curve to determine the size distribution of  nanoparticles (the sphere radius is indicated in the histogram) in the spherical approximation.
As can be seen from the graph of the distribution of cobalt oxide nanoparticles by diameter, 8% of particles have a diameter close to 5 nm, about 9-10% of nanoparticles have diameters up to 4 nm, a diameter of 5-6 nm corresponds to 1.5% of the studied nanoparticles, the contribution of the remaining nanoparticles is less 1%.The remaining fraction of nanoparticles has dimensions much larger than the maximum permissible diameter, so their contribution is not taken into account and is not displayed on the graph.
As can be seen from the distribution plot of Co 3 O 4 nanoparticles obtained without the addition of nitric acid (Fig. 3), the main fraction of particles has a diameter up to 6 nm, but particles with diameters up to 25 nm are also present.As can be seen from the distribution graph of Co 3 O 4 nanoparticles obtained with the addition of nitric acid (Fig. 4), the bulk of the particles have diameters up to 8 nm.In this case, the nanoparticles have diameters up to 10 nm and there are no particles with larger diameters.The results obtained showed that the addition of nitric acid allows obtaining more monodisperse particles with a small spread.
As can be seen from the obtained scanning and transmission electron microscope images (Fig. 5), for cobalt oxide particles at stoichiometric fuel-tooxidizer ratio φ=1, the particle size range is from 23 to 60 nm, and agglomerates larger than 500 nm are also present.The formation of agglomerates can be attributed to high-temperature fl uctuations during the self-ignition of the mixture.For the cobalt oxide nanoparticles at a ratio of φ=1.5, the particle size ranges from 20 to 65 nm, without large agglomerates (Fig. 6).A comparison of the two samples based on SEM and TEM images illustrates the positive eff ect of fuel addition above the stoichiometric ratio.The reaction of the fuel with the oxidizer leads to the decomposition of the initial components with the formation of gaseous products that lead to further dispersion of the fi nal product.Thus, the obtained results confi rm the effi ciency of the synthesis of Co 3 O 4 nanoparticles by the solution combustion method.Changing the composition of the initial mixture can signifi cantly change the morphology of the obtained product and clearly illustrates the possibility of controlled synthesis.The synthesized Co 3 O 4 nanoparticles are perspective materials for application in gas sensors.
Co 3 O 4 as a transition metal oxide has chemical, phase, and structural stability, which allows increasing the temperature if necessary, and high electrical conductivity, which allows recording the chemoresistive response that occurs during the redox reactions of metal oxide with the detected gas [2,32].For this purpose, in metal-oxide gas sensors, the sensitive material Co 3 O 4 is heated at a certain temperature.The fl ow of electricity within this material depends on the number of free electrons.When the sensing material is in clean air, the oxygen (O 2 ) in the atmosphere adsorbs on the surface of the sensing material, attracts free electrons, and keeps the electrons on the surface as ions.This leads to an increase in the resistance of the sensor, resulting in a decrease in the fl ow of electrons within the sensing material.In the presence of reducing gases such as methane or propane, these gases interact with the adsorbed oxygen, releasing bound electrons within the sensing material.This results in a decrease in the resistance of the sensor, allowing more electrical current to fl ow.As the concentration of reducing gases increases, the resistance of the sensor decreases further, allowing even more electrical current to fl ow.According to the sensitivity characteristics of Co 3 O 4 , there is a certain relationship between the sensor resistance and the gas concentration in the atmosphere, which provides information about the concentration of pollutants in the air [33,34].
In Table 2, examples of Co 3 O 4 applications in gas sensors for the determination of a wide range of gases are shown.The main parameters of Co 3 O 4 -based gas sensors are given.
Successful results of Co 3 O 4 application in gas sensors show the prospect and relevance of the development and optimization of methods for obtaining cobalt oxide nanoparticles with controlled parameters of morphology and structure to obtain stable and repeatable results.

Conclusion
Co 3 O 4 nanoparticles were obtained by solution combustion method as a result of the exothermic redox reaction of cobalt nitrate hexahydrate

Fig. 4 .
Fig. 4. Size distribution of Co3O4 nanoparticles in the spherical approximation with the addition of nitric acid (φ=1).

Table 1 .
Methods of cobalt oxide synthesis

Table 2 .
Key parameters of Co 3 O 4 -based gas sensors (Co(NO 3 ) 2 •6H 2 O) and glycine (C 2 H 5 NO 2 ).The eff ect of the addition of nitric acid and the fuel: oxidizer ratio on the structure and dispersibility of cobalt oxide nanoparticles was investigated.The positive eff ect of the addition of nitric acid was established.