Corchorus Olitorius-Mediated Green Synthesis and Characterization of Nickel and Manganese Ferrite Nanoparticles

Abstract

Developing a method for preparing Ni and Mn ferrites was the main objective of this study due to the importance of these materials in high-frequency applications. These ferrites were made by assisting combustion with dried leaves of Corchorus olitorius and then heating them to 700 °C. Several methods, including FTIR, XRD, TEM, and SEM/EDX, were used to characterize these ferrites. The thermal behavior, surface and magnetic properties of the as-prepared materials were determined. The results revealed that the method used is cheap, economical, environmentally friendly and makes it easy to produce the studied ferrites. FTIR, XRD, TEM, and SEM/EDX analyses show the formation of nanocrystalline ferrites with brittle, spongy and spinel-type structures, having two main vibration bands located around 400 cm⁻¹ and 600 cm⁻¹. However, TG-DTG results display the thermal behavior of different materials which consisted of unreacted oxides, carbon and the corresponding ferrites in the range of 300 °C to 600 °C. Moreover, complete conversion of the unreacted oxides to the equivalent ferrite was achieved by increasing heat treatment from 600 °C to 1000 °C. Ferrites are heated at 700 °C, which reduces their surface area. The magnetic properties of different ferrites calcined at 700 °C were estimated using the VSM technique. The magnetism of Fe-based materials containing Ni and Mn is 12.189 emu/g and 25.988 emu/g, respectively. Moreover, the squareness and coercivity of Ni ferrite are greater than for Mn ferrite.

1. Introduction

Transition element-based ferrites have gained great importance, especially with the recent technological advances. These ferrites are a well-known and significant family of iron oxide materials which are thermally and chemically stable [1]. The spontaneous magnetic moment of most ferrites, which is below the Curie temperature, has led to ferrites gaining great importance in engineering and technology [1]. Among these ferrites are copper, manganese, nickel and cobalt ferrites, which have applications in many fields. Thermal sensor switches depend on ferrite materials, so they are included in the manufacture of many devices such as refrigerators, air conditioners, and electronic ovens. Due to the distinctive characteristics of ferrites, such as the magnetostrictive property, ultrasonic waves can be produced by applying an alternating magnetic field on a ferrite rod [1]. In addition, the distinctive insulation properties of most ferrites make them suitable for use as ferromagnetic insulators in electric motors, as flat rings for loud speakers, in windscreen wiper motors and as correction magnets for TVs. The high rectangular hysteresis loop of some ferrites led it to be used in the manufacture of computer memory systems for rapid storage, with subsequent retrieval of digital information. It is known by necessity that ferrites do not form except in the presence of iron oxide, and there are many types to give spinel, hexagonal, garnet and ortho ferrites according to their structural classifications [2]. According to the magnetic properties, the ferrites are also divided into soft and hard ferrites: hard ferrites (permanent magnets) retain their magnetism after being magnetized, while soft ferrites (temporary magnets) do not retain this magnetism [3].

The tetrahedral (A) and octahedral (B) sites were interstitial regions that constituted the whole crystal structure of the spinel ferrite, MFe₂O₄ (M = Ni²⁺, and Mn²⁺). [4]. The occupation of the divalent metal ions in these sites resulted in classification of the spinel ferrites into normal, inverse and mixed/random types [5]. The framework of Mn and Ni ferrites is considered to be inverse spinel [6,7,8,9]. Ni, Mn and Fe ions could be positioned in different crystallographic positions (A and B) surrounded by oxygen atoms in a cubic close-packed structure. Ferrites have unique and distinctive properties, including a strong magnetic property with high permeability, relatively low conductivity, and low eddy current and dielectric losses. The driving force of the previous properties of ferrites is the distribution of different cations between A and B sites [10]. This cation distribution in ferrites is greatly affected by many factors, including: the history of precursors, the preparation route, the calcination temperature, doping process and the configuration of ferrite which is a single- or multi-ferrite. Both the crystal structure and the microstructure of ferrite will change and be a mirror of all the changes that result from all the previous factors. Moreover, the arrangement of atoms in the crystal structures shows a pronounced tendency towards the highest possible symmetry. Sometimes, an ideal symmetry may not occur in the crystal structures as a result of some adverse factors that derive from certain properties of the atoms. This deviation from the highest possible symmetry is often small, and can be called pseudo symmetry. The symmetry reduction was observed during phase transitions and solid state reactions. In other words, the cation distribution in the crystal structure resulted in a small deviation from the highest possible symmetry. In fact, we see that the most important factor influencing the cation distribution which is affected by each of the crystal and microstructures is the preparation method [11]. In addition, nickel and manganese ferrites can be considered as soft magnetic materials [12,13,14].

These kinds of the spinel ferrites have received great care and attention due to their unique properties in various applications such as removal of pollutant ions from aqueous systems, magnetic recording media, electronic devices, drug delivery systems, cancer therapy, sensors, actuators and catalysis [15,16,17]. Indeed, a better understanding of the magnetism of the ferrites is crucial not only for basic physics but also because of the great technological importance of ferromagnetism in information storage, color imaging, bio-processing and ferro-fluids [17]. The magnetic characteristics of ferrite-based nanoparticles are sensitive to synthesis issues such as the method and conditions of preparation [6,7,8,11,12,13]. These concerns could be caused by chemical and physical processes used to make ferrite-based nanoparticles, including chemical co-precipitation, sol-gel auto combustion, reverse microcell, microwave hydrothermal, sonochemical, forced hydrolysis, one-step, high-energy ball milling, solvothermal, and microemulsion methods. [6,7,8,18,19,20,21]. In comparison to conventional methods, the novel green synthesis for different ferrites utilizing fuel derived from natural materials has significant advantages, including simple and affordable preparation with composition flexibility and large-scale manufacture of homogeneous nanoparticles in one step [11].

In this manuscript, nano crystalline NiFe₂O₄ and MnFe₂O₄ were successfully synthesized through the direct reaction between metal nitrates and dried leaves of Corchorus olitorius-based powder. Several methods were used to determine the ferrites’ thermal, structural, surface, morphological, and magnetic properties as synthesized.

2. Materials and Methods

2.1. Materials

Ferrite nitrate hydrate, manganese (II) nitrate, and nickel (II) nitrate all have linear formulas. Fe (NO₃)₃·9H₂O, Mn (NO₃)₂·4H₂O, and Ni (NO₃)₂·6H₂O respectively, were the chemical components used. The Sigma-Aldrich Company provided these materials (Darmstadt, Taufkirchen, Germany). These reagents were used quantitatively and did not need to be processed further. Corchorus olitorius dried leaves were used to make powder, which was obtained from Egyptian agriculture.

2.2. Preparation Method

Two samples of NiFe₂O₄ (S1) and MnFe₂O₄ (S2) were manufactured using the dried leaves of Corchorus olitorius-based powder-mediated combustion route. The investigated sample was created by thoroughly mixing an equimolar mixture of ferric nitrate hydrate and metal nitrate with 0.5 g powdered dried leaves of Corchorus olitorius in a Pyrex beaker, taking into account the stoichiometric ratio of Fe/M = 2 (M = Ni, and Mn). The resulting materials were first swirled at 70 °C in order to allow the water to evaporate, to boost the viscosity of the materials being studied. The temperature was then increased to 120 °C, causing the combination to become a gel. The produced precursor gel was calcined for 15 min at 300 °C to bring the temperature to container level. When a spark ignited in one corner and instantly spread throughout the created mass with the appearance of an incandescent combustion, a significant amount of foam had already begun to form; the result was a dense, fluffy solid. Two other specimens were prepared by burning part of the S1 and S2 samples at 700 °C for two hours to obtain S3 (calcined NiFe₂O₄) and S4 (calcined MnFe₂O₄), respectively.

2.3. Characterization Systems

Thermogravimetry (TG) and differential derivative thermogravimetry analyses were carried out using a computerized Shimadzu thermal analyzer (TGA 60 Japan) (Shimadzu, Osaka, Japan) (DTG). Under air at a flow rate of 30 mL min⁻¹, the investigated solids were evaluated. To eliminate the peak shape and temperature effects of sample weight, persistent weights of the as-prepared samples (nearly 48 mg) were employed. Alumina (α-Al₂O₃) has been employed as the reference source for this approach. Heating rate was set at 10 °C min⁻¹.

A BRUKER D8 advance diffractometer (Karlsruhe, Germany) and X-ray diffraction technology were used to evaluate the structural characteristics of several nanoparticles. Cu Kα radiation was used to run the patterns at 40 kV, 40 mA, at a scanning speed of 2° per minute. Equations (1)–(3) have been used to obtain the mean crystallite size (d), stress (ε), and dislocation density (δ) of nickel and manganese ferrites present in the analyzed product based on X-ray diffraction line broadening and Scherrer equation calculations as follows [11,20,21,22]:

d = B λ/β cos θ

(1)

δ = 1/d²

(2)

ε = β cos θ/4

(3)

where d is the average crystallite size of the phase under investigation, B is the Scherrer constant (0.89), λ is the used X-ray beam’s wavelength, β is the full-width half maximum (FWHM) of diffraction, and θ is the Bragg angle.

Using a Perkin-Elmer spectrophotometer, different materials’ Fourier-transform infrared (FTIR) spectra were measured (type 1430). We combined 200 mg of vacuum-dried IR-grade KBr with 2 milligrams of each solid sample. The FTIR spectra were measured over a range of 1000–4000 cm⁻¹. Once the mixture had been treated in a vibrating ball mill for three minutes, it was dispersed using a steel die with a 13 mm diameter. The FTIR spectrophotometer’s double-grating spectrophotometer container was filled with the identical disks.

In order to distribute individual particles over mount setup and copper grids, the samples were disseminated in ethanol and then treated ultrasonically for a short period of time. These procedures were used with the JEOL JAX-840A scanning electron microscopy (SEM) (JEOL, Tokyo, Japan) and the JEOL Model 1230 transmittance electron micrograph (TEM) (JEOL, Tokyo, Japan) cameras, respectively.

A Delta Kevex device was connected to an electron microscope, JED-2200 Series, to perform energy dispersive X-ray analysis (EDS with Mapping) (JEOL, Tokyo, Japan). A 20 kV accelerating voltage, 120 s accumulation duration, and 6 mm window width were the variables used. The surface molar composition was determined using the Asa technique, Zaf-correction, and Gaussian approximation.

Several materials’ specific surface area (S_BET), total pore volume (V_P), monolayer adsorption volume (V_m), and mean pore radius (ȓ) were calculated using nitrogen adsorption isotherms at 77 K using a conventional volumetric equipment (Brunauer–Emmett–Teller method) and surface area analyzers from Micrometrics’ Gemini VII 2390 V1.03 series (Microtrac, Alpharetta, GA, USA). Prior to the measurements, each sample was out-gassed for 2 h at 200 °C and a lower pressure of 10⁻⁵ Torr.

The magnetic characteristics of the under-recognized ferrites were tested using a vibrating sample magnetometer (VSM; 9600-1 LDJ, Lowell, MA, USA), which was utilized to examine the magnetic properties of the examined solids in a maximum applied field of 20 kG.

3. Results

3.1. Thermal Behavior of the Investigated Solids

The TG and DTG curves of the S1 and S2 samples were identified, as seen in Figure 1. The analysis of this figure showed that: (i) the heat treatment of these samples in the range from room temperature to 1000 °C led to their passage through various physical and chemical processes, including dehydration, decomposition, and solid state reaction, which finish at approximately 1000 °C. (ii) The course of the dehydration process changed according to the investigated samples containing different metal cations. Various dehydration processes were achieved in the range of 25–350 °C with subsequent release of some gases, moisture and hydration water from different samples. In this temperature range, we saw charring (carbonization) of Corchorus olitorius powder because it contains organic materials. These stages were confirmed by DTG peaks located at 225–275 °C with weight loss of 9.79%, and 10.67% for the samples containing Ni and Mn, respectively. (iii) Partially solid state reaction between the resulting oxides (NiO, or Mn₂O₃ and Fe₂O₃) can occur in the temperature range of 300–600 °C yielding the corresponding ferrites, namely NiFe₂O₄, and MnFe₂O₄. This reaction is accompanied by weight loss of 2.3%, and 3.2% for the samples containing Ni, and Mn, respectively. In addition, the decarbonization reduced to ash, which was removed by heating above 600 °C. This observation was also confirmed by DTG peaks located at 625–700 °C for different samples. (iv) The ash in the S1 and S2 samples was eliminated as the heat treatment for the samples was increased, stimulating the solid–solid contact between unreacted oxides and producing the corresponding ferrites with weight losses of 1% and 5.6% for the samples containing Ni and Mn, respectively.

3.2. Investigation of the Structural Properties of the Investigated Materials

Figure 2 shows the XRD patterns of the NiFe₂O₄ and MnFe₂O₄ nanoparticles as they were manufactured. At 2θ = 18.08°, 30.08°, 35.10°, 44.10°, 57.09°, and 62.07°, respectively, for the S1 sample, the diffraction peaks with the following planes at (111), (220), (311), (400), (511) and (440) are found. These peaks match the NiFe₂O₄ standard diffraction of the cubic spinel structure and Fd3m space group (PDF no. 44–1485). The calcination temperature at 700 °C resulted in a slight shift in the position of these peaks with a slight increase in the peak heights, as shown in the XRD pattern of the S1 sample. Other diffraction peaks at the planes (111), (220), (311), (222), (400), (422), (511), (440) and (620) were detected at 2θ = 18.20°, 30.06°, 35.60°, 37.10°, 43.26°, 53.60°, 57.19°, 62.85° and 72.31°, respectively, for the S2 sample. These peaks are consistent with the standard spectra of cubic spinel structure and Fd3m space group for MnFe₂O₄ (PDF no. 074–2403). The calcination temperature at 700 °C resulted in a slight shift in the position of these peaks with a slight increase in the peaks height as shown in the XRD pattern of the S2 sample.

In fact, XRD analysis (not given) of the S1 and S2 samples without the presence of dried leaves of Corchorus olitorius did not contain any diffraction lines related to any crystalline phase. In other words, the products of these treated mixtures were amorphous. On the other hand, the presence of dried leaves of Corchorus olitorius with the previous mixtures, followed by heating at 300 °C for 15 min, led to formation of moderate crystalline spinel phases for the corresponding ferrites. The crystallinity of the as-prepared ferrites was enhanced by additive heating at 700 °C for 2 h, as shown in XRD of the S3 and S4 samples. These observations prove the aim of this study, which is summed up in the use of dried leaves of Corchorus olitorius and its effect as fuel in obtaining various ferrites, in addition to the effect of heat treatment at 700 °C on these ferrites.

The XRD data were used to determine a number of structural characteristics, including the crystallite size (d), lattice constant (a), unit cell volume (V), X-ray density (Dx), stress (ε), and dislocation density (δ). Our ability to calculate additional parameters, such as the separation between magnetic ions (L_A and L_B), ionic radii (r_A, r_B), and bond lengths (A–O and B–O) on tetrahedral (A) and octahedral (B) sites was made possible by these results.

Table 1. Lattice parameters for NiFe₂O₄ in the S1 and S3 samples, and for MnFe₂O₄ in the S2 and S4 samples.

Parameters	S1	S2	S3	S4
d, nm	15	20	13	45
a, nm	0.8387	0.8468	0.8474	0.8363
V, nm3	0.5881	0.6072	0.6085	0.5849
Dx, g/cm3	5.2931	5.0441	5.1157	5.2364
LA, nm	0.3628	0.3667	0.3669	0.3621
LB, nm	0.2957	0.2989	0.2995	0.2952
A-O, nm	0.1915	0.1936	0.1937	0.1912
B-O, nm	0.2162	0.2185	0.2186	0.2158
rA, nm	0.0595	0.0616	0.0617	0.0592
rB, nm	0.0842	0.0865	0.0866	0.0838
ε	0.0044	0.0204	0.1111	2.2676
δ, Lines/nm	1.19 × 10−3	2.50 × 10−3	5.92 × 10−3	0.63 × 10−3

has a list of all parameters. As can be observed from the table, heating nickel ferrite to 700 °C causes an increase in the values of: δ, ε, a, L_A, L_B, r_A, r_B, A–O, and B–O, followed by a drop in the d and D_x values. The manganese ferrite example revealed the opposite tendency.

3.3. Investigation of the Chemical Functional Groups of Materials Studied

In order to identify the chemical functional groups in the investigated samples, Fourier-transform infrared spectroscopy (FTIR) is used. This technique brought about an infrared (IR) spectrum of the solids studied depending on how the different functional groups absorb characteristic frequencies of IR radiation. Since the studied samples were for the purpose of obtaining different ferrites, we must follow Waldron’s report related to infrared absorption in ferrites [23]. According to Waldron, the cation distribution at the tetrahedral (A-) and octahedral (B-) sites is related to the two fundamental vibration modes at 600 cm⁻¹ and 400 cm⁻¹, respectively, which make up the entirety of the spinel ferrites. Figure 3 displays the FTIR spectra for the calcined, S1, S2, S3, and S4 samples in the 4000–400 cm⁻¹ area at room temperature. The two primary absorption bands (ν₁ and ν₂) for the various samples were located at 567–517 cm⁻¹ and 437–407 cm⁻¹, respectively. The presence of these bands is consistent with Waldron’s report and confirms the formation of ferrites with the spinel-type structure. In addition, depending on the type of spinel ferrite being analyzed, these bands could be attributed to the M-O bond-stretching vibrations of Fe-O, Ni-O, and Mn-O at both A- and B-sites. Based on our prior research, Ni, Mn, and Fe cations favor the B-site whereas Fe cations can be spread between the A and B sites, resulting in the development of inverse spinel ferrites [6,7,8,9]. On the other hand, Waldron also indicated the possibility presence of low- as well as high-frequency shoulders around the two main bands. In this regard, the authors observed small bands at shoulder, ν^1* and ν^2*, in all samples located at 677–620 cm⁻¹ and 440–416 cm⁻¹, respectively. They assumed the emergence of these shoulders at 677–620 cm⁻¹ as a result of the vibration of divalent cations for Ni, and Mn [6,7,8,9,11,24,25]. In addition, they assumed that the other shoulders or splitting of absorption bands (ν₂), including few small subsidiary bands at 440–416 cm⁻¹, are due to presence of ferrous cations [11,26]. These observations, therefore, confirm that all prepared ferrites have random spinel structures.

Moreover, the vibrations of the hydroxyl groups (O-H) that adsorb water molecules on the surface of uncalcined materials can be linked to the bands at 3248–3048 cm⁻¹ for uncalcined samples. However, depending on the presence of carbon traces that resulted from an internal combustion process of Corchorus olitorius powder, the FTIR spectra of these samples show bands in the region of 1356–1348 cm⁻¹ associated with the stretching vibrations of hydrogen–carbon and hydroxyl–carbon (C-OH and C-H) [11]. On the contrary, it was found that the roasted samples did not contain the bands of both C-OH, C-H and O-H groups. Finally, the heat treatment of the as-prepared samples at 700 °C resulted in a shift in the positions of all bands with a subsequent increase in the intensities of these bands. On other words, the heat treatment at 700 °C brought about elimination of both moisture water and carbon residue resulting from the combustion process with a subsequent increase in the solid state reaction between the reacting oxides, yielding the corresponding ferrites.

3.4. Morphological Studies of the Investigated Materials

Surface morphology of the S1 and S2 samples is represented by SEM analysis in Figure 4. The S1 and S2 samples’ SEM image showed brittle, spongy structures with pores and voids. In accordance with the XRD data, the particle size for the S2 sample is smaller than that for the S1 sample. The SEM pictures for the S1 and S2 samples showed various aggregates.

The illustration in Figure 4 shows the S1 and S2 samples’ EDS patterns. This figure illustrates the presence of the signal-defining elements of nickel (Ni), iron (Fe), and oxygen (O) in the S1 sample, as well as the presence of manganese (Mn), iron (Fe), and oxygen in the second sample (O). The acquired FTIR analysis confirms that the signal at 0.27 Kev in the patterns is caused by the presence of carbon (C) as an impurity. Yet, as demonstrated in Figure 5, the S1 sample’s EDS mapping results show that Ni, Fe, and O atoms are fairly evenly distributed.

As illustrated in Figure 6 and Figure 7 TEM analysis was used to carry out the surface morphological analyses of the S3 and S4 samples. The creation of particles with homogeneous, uniform size and diameters ranging from 9 nm to 32 nm was achieved during the preparation of Ni ferrite (S3 sample) utilizing dried leaves of Corchorus olitorius as an assisted combustion method, according to TEM pictures. The average particle size in the S4 sample is 67 nm, however, some clumps of particles can be visible. In order to further investigate the crystallinity of the developed nanoparticles, the S3 and S4 samples underwent high resolution transmission electron microscopy (HRTEM) and continuously selected area electron diffraction (SAED), which is apparent in Figure 6. Based on the fast Fourier transform (FFT) images, the lattice fringe was found at 0.252 nm, which related to the crystal plane of (311) indicating the successful synthesis of cubic spinel nickel and manganese ferrites. On the other hand, the SAED analysis shows small bright spots as rings around a central spot having different diameters. This result confirms the successful synthesis of polycrystalline nickel and manganese ferrites.

3.5. Surface Properties of the as-Prepared Materials

For the study of the textural characteristics of as-synthesized materials, N₂ adsorption/desorption isotherms carried out at 77 K were used. The surface features of the S1, S2, S3, and S4 samples are depicted as isotherms in Figure 8.

Table 2. Surface properties of the S1, S2, S3 and S4 samples.

Samples	SBET (m2/g)	Vm (cc/g)	Vp (cc/g)	ȓ (nm)	BET-C (Constant)
S1	57	13	0.112	8	9
S2	103	24	0.243	10	11
S3	47	11	0.1909	16	13
S4	24	6	0.0378	6	8

provides the values of several surface attributes for these samples. Analysis of these isotherms reveals type-IV based isotherms with a type H3 hysteresis loop as the formation type. The networking effects of the as-prepared samples on a material’s pore structure were observed in the S1 and S2 samples. Examination of Table 2 revealed that: (i) the calcination at 700 °C led to a decrease in the values of V_m and S_BET as shown in these values for the S3 and S4 samples, which are lower than that of the S1 and S2 samples. (ii) The values of V_p and ȓ changed according to the nature and the morphological behavior of the as-prepared ferrite. One cannot ignore the effect of the calcination temperature on the V_p and ȓ values. On the other hand, non-local density functional theory (NLDFT)-based adsorption of nitrogen led to the characterization of the porosity or pore size distribution of the investigated solids shown in Figure 8. This figure shows that most of the pores for the S1, S2, S3 and S4 samples have an average size located at 5.85 nm, 19.80 nm, 13.83 nm and 5.07 nm, respectively.

3.6. Magnetic Behavior of the Materials Studied

The hysteresis loop was in fact utilized to calculate the magnetic properties for the calcined sample, including the coercive field (H_c), remanent magnetization (M_r), saturation magnetization (M_s), squareness (M_r/M_s), anisotropy constant (K_a), and experimental magnetic moment (μ_m). Ka and μ_m can be calculated according to the next expression [19]:

μ_m = M_wM_s/5588

(4)

μ_i = M²_S d/K

(5)

K_a = H_cM_s/0.96

(6)

where M_w is the molecular weight of the investigated ferrites. K_a is the expressive term for anisotropy, which is the ability of the crystalline material to change values of physical characteristics when measured in different directions. μ_m is the magnetic moment that is calculated based on the practical results of each of H_c and M_s. In

Table 3. The magnetic properties of the S3 and S4 samples.

Samples	Ms (emu/g)	Mr (emu/g)	Mr/Ms (emu/g)	Hc (Oe)	μm	Ka (erg/cm3)
S3	7.374	0.933	0.1265	171.17	0.3093	1287.97
S4	15.60	1.310	0.0840	92.84	0.6442	1477.86

, the values of H_c, M_r, M_s, M_r/M_s, K_a, and m are reported. These parameters were extrapolated from the hysteresis loop which is included in the magnetic curves illustrated in Figure 9. These graphs were determined using the VSM technique with the application of a magnetic field in the range from −20 to +20 kOe at room temperature. The values of these parameters were greatly affected by the difference of the divalent cation (Ni²⁺ and Mn²⁺) involved in the formation of the corresponding ferrites. The value order of M_s, M_r, and K_a for the investigated ferrites was as follows: MnFe₂O₄ ˃ NiFe₂O₄. Moreover, the value order of H_c and M_r/M_s for these ferrites was as follows: NiFe₂O₄ ˃ MnFe₂O₄. In addition, the value order of μm for the investigated ferrites was as follows: MnFe₂O₄ ˃ NiFe₂O₄.

4. Discussion

XRD analysis (not given) of the S1 and S2 samples without the presence of dried leaves of Corchorus olitorius did not contain any diffraction lines related to any crystalline phase. In other words, the products of the treated mixtures were amorphous. On the other hand, the presence of dried leaves of Corchorus olitorius with the previous mixtures followed by heating at 300 °C for 15 min led to the formation of moderate crystalline spinel phases for the corresponding ferrites. The crystallinity of the as-prepared ferrites was enhanced by additive heating at 700 °C for 2 h as shown in XRD of the S3 and S4 samples. These observations prove the aim of this study, which is summed up in the use of dried leaves of Corchorus olitorius and its effect as fuel in obtaining various ferrites, in addition to the effect of heat treatment at 700 °C on these ferrites.

It is well known that the different ferrites are obtained through the solid-state reaction between the ferric oxide (Fe₂O₃) and divalent metal oxides (MO = NiO, CoO, ZnO and CuO…..etc) at certain preparation conditions. In order for the solid state reaction to continue, both ferric and ferrous ions must be available, depending on the fact that the trivalent iron is the one that initiates the reaction and the divalent iron is the one that works to continue this reaction yielding the corresponding ferrites [25]. Hence, the presence of a small amount of carbon that results from the combustion process will stimulate this reaction, depending on its work on bringing in the divalent iron [25]. In this regard, we find that the green synthesis of ferrites, which depends on the use of natural materials, will provide us with this small amount of carbon, which will enhance the solid state reaction between the constituents involved in the formation of the desired ferrites.

We can express the following for all of the above and formation of different ferrites (MFe₂O₄) through our perception of the reaction mechanism in the solid state between the stoichiometric oxides of both Fe and various elements (M = Ni, and Mn) as follows: (i) At the Fe₂O₃ interface: two processes take place, the first involving the interaction of metal ions (M²⁺) with iron oxide, surrounding it with a thin layer of matching ferrite which forms a barrier that prevents the progress of this reaction between them. Moreover, this process also leads to the production of Fe²⁺ cations. The second process involved the reaction of carbon resulting from the combustion process with Fe₂O₃ and the formation of additional Fe²⁺ cations. (ii) At the divalent metal oxide (MO) interfaces: another reaction between the resulting Fe²⁺ cations and MO to produce an excess of the corresponding ferrites with subsequent formation of metal ions (M²⁺) which return to complete the solid state reaction and so on.

The XRD results revealed that (i) MnFe₂O₄ and NiFe₂O₄ samples, respectively, showed single spinel-type structures based on Bragg reflection characteristics in the S1, S2, S3, and S4 samples. (ii) The S1 and S2 samples’ relative intensities of various reflections arising from various crystal plane reflections are, respectively, lower than those of the S3 and S4 samples. (ii) The difference in the responding cations’ ionic radii causes the lattice parameters to vary when the temperature is raised to 700 °C or the divalent element (Ni or Mn) is changed, but the distribution of the cations between the A and B sites also changes.

In this study, the IR and XRD analyses confirmed the success of the combustion process using dried leaves of Corchorus olitorius in the preparation of various ferrites, whose crystallinity increased by roasting at 700 °C. XRD measurements displayed the diffraction lines related to the spinel-type structure with a shift and an increase in the peaks height of these lines by heat treatment at 700 °C. Similar behavior was observed in the IR analysis which confirmed formation of spinel ferrite due to presence of two main vibration band around 400 cm⁻¹ and 600 cm⁻¹. IR results show that the uncalcined samples (S1 and S2) contain trace amounts of carbon which stimulate the solid state reaction yielding the corresponding ferrites. However, these results also confirm the disappearance of this carbon from the calcined samples depending on the absence of the vibration band located at 1356–1348 cm⁻¹ which is related to a carbon atom.

SEM, EDX, TEM, FFT and SAED analyses confirm that the as-prepared materials consisted entirely of spinel nickel and manganese ferrites with particle sizes ranged between 3 nm and 25 nm. In addition, these investigations display the polycrystalline nature of the formed solids.

Complex pore structures of the S1, S2, S3 and S4 samples were observed depending on the shape and type of their isotherms. Adsorption/desorption isotherms show adsorption of a particular adsorptive on each sample with pores of differing widths depending on each given pore shape. Type IV isotherms of the investigated samples show a number of features typically associated with micro-/mesoporous characteristics. At low pressures, a steep rise in the adsorption branch was observed due to micro-pores filling. In a higher pressure range, the adsorption isotherms exhibit a multilayer adsorption region due to capillary condensation in meso-pores with various slopes having moderate value in the case of the S1 and S2 samples and low values with the S3 and S4 samples. Additional delayed pore condensation in in-particle voids was observed in the case of the S1 and S2 samples due to the presence of different phenomena (networking effects) such as pore blocking, cavitation and metastability of the adsorbed multilayer. The capillary condensation is accompanied by an apparent type H3 hysteresis. The different shapes of the H3 loop are also linked with non-rigid aggregates of plate-like particles (slit-like pores). In addition, the desorption branch has a narrow hysteresis loop and is parallel to the adsorption branch. These findings confirm a narrow distribution of uniform meso-pores and limited networking effects. NLDFT-based pore size distribution patterns show one peak for all manufactured samples located in the meso-pores scale. Average pore size of the samples containing nickel ferrite increases from 5.85 nm to 13.83 nm with a subsequent increase in the mean pore radius from 7.85 nm to 13.23 nm by heating at 700 °C. This indicates that the heat treatment resulted in pore wedding in the pore structure of the nickel ferrite-based sample leading a decrease in the surface area. The opposite behavior was observed in the case of a sample containing manganese ferrite, but with its presence came a progressive decrease in the surface area.

The effects of an applied magnetic field on different magnetic properties of the investigated ferrites were determined from a study of the hysteresis cycle. At room temperature, the resulting hysteresis loops display the typical of soft magnetic behavior for Ni and Mn ferrite. These ferrites show room temperature ferromagnetism behavior as shown from their hysteresis loops. Among the different factors that affect the magnetic behavior of these ferrites is the cation distribution at the tetrahedral (A)/octahedral (B) sites of the spinel structure. Tunability of the cation distribution of spinel ferrites can be achieved using some variables such as the ionic radii, crystal lattice, precursors, fabrication route and heat treatment indicating the deviation from the high possible symmetry of the ferrites obtained.

In this study, the difference of the divalent metal (Ni and Mn) which was used in the preparation of the investigated ferrites, led to the difference in the net magnetism of these ferrites, depending on the difference in its magnetic moment which arises from the difference between the magnetic moments of the different elements in A- and B-sites [24]. The order of magnetism for calcined sample was as follows: MnFe₂O₄ ˃ NiFe₂O₄. The substitution of Ni cations by Mn cations resulted in an increase in the saturation magnetization due to spin canting and surface spin disorder [26]. However, the magnetic moment of Mn atoms (5 μB) is greater than that of Ni atoms (2 μB). So, the experimental net magnetic moment of the S4 sample is higher than that of the S3 sample. Indeed, the origin of the interactions between the A and B sublattices in the spinel lattice system (AB₂O₄) consist of three categories as follows: (i) the intersublattice (A-B) super-exchange interactions that are much stronger than other interactions. (ii) the intra sublattice (A-A) and (B-B) exchange interactions [27]. As Fe³⁺ ions transferred from the B site to the A-site, the accumulation of Fe³⁺ ions increased in the A site with an increase in the super-exchange interactions of FeA³⁺–FeB³⁺ ions [27,28,29,30,31]. This observation is more pronounced in case of Mn ferrite compared to Ni ferrite. This behavior could be attributed to nature, orientation and concentrations of Mn and Ni cations in A- and B-sites.

Estimation of the domain state which leads to the magnetization changes of the investigated ferrites can be determined from the squareness (Mr/Ms) of these ferrites. Different ferrites in this work are multi-domain type because their squareness is smaller than 0.5 [25]. Because the squareness and coercivity can also be linked to the magnetic anisotropy and super-exchange interaction between the cations in A- and B-sites, MnFe₂O₄ has the largest anisotropy constant compared to NiFe₂O₄. The magnetic anisotropy of materials is expressed by the term “anisotropy constant” which is the directional dependence of the material’s magnetic moment with preferred orientations of this moment in the easy magnetization axes. Moreover, the magnetic anisotropy is the utmost importance parameter for soft and especially hard magnetic materials because it is linked to their coercivities. In cubic systems, the anisotropy belongs to magnetocrystalline or crystal-type anisotropy, which is an intrinsic property of the material, and originates from the spin–orbit interaction of the electrons. In other words, the spatial arrangement of the electron orbitals is strongly linked to the crystallographic structure. Consequently, when the electrons interact via their spins, they force the latter to align along well-defined crystallographic axes.

One cannot ignore the surface anisotropy in the as-prepared samples as well. The surface anisotropy is expected to interact with the atoms on the surface’s specimens, while the dipolar interactions are considered to act in the bulk of the specimens depending on the difference in the co-ordination number between both bulk and surface atoms [32]. In addition, the dipolar interaction or magnetocrystalline anisotropy favors the parallel in-plane magnetization, while magnetic surface anisotropy favors perpendicular out-of-plane magnetization. The physical competition between magnetocrystalline and surface anisotropies depends on the size of particles. According to size of particles, the magnetic surface anisotropy dominates the contributions with low size as observed in the S3 sample. On the other hand, the magnetocrystalline anisotropy dominates the competition in the S4 sample due to high particle size and the spins lie on the plane of the crystal. These findings are consistent with the change in the values of ε, δ and d for the as-prepared samples.

Finally, the authors believe that this method is considered an advanced self-combustion method because it is cheap, economical, easy and fast in preparing nickel and manganese ferrites. Most importantly, this method is considered to have a glowing self-combustion, meaning that the combustion speed is not instantaneous and fast, but is somewhat moderate. This moderation in the combustion process leads to control of the nucleation and crystallization process of the particles.

5. Conclusions

This study confirms that combustion assisted by the dried leaves of Corchorus olitorius resulted in the formation of nanosized Ni and Mn ferrites. From this study, we can conclude the following observations:

• The preparation method led to the formation of nanocrystalline Ni and Mn ferrites at low temperature. The structural properties of these ferrites increases by increasing heat treatment to 700 °C for 2 h.
• The thermal analysis showed that the thermal treatment of the as-prepared materials from room temperature to 1000 °C resulted in chemically stable Ni and Mn ferrites.
• All the uncalcined and calcined materials in this study, have spinel-type ferrites with two main vibration bands located at 400 cm⁻¹ and 600 cm⁻¹.
• The heat treatment resulted in a decrease in the values of S_BET and V_m for the investigated materials. In addition, this treatment affects the material’s pore structure.
• The magnetism of the as-prepared NiFe₂O₄ and MnFe₂O₄ nanoparticles are 7.374 emu/g and 15.60 emu/g, respectively.