Synthesis and characterisation of magnesium-doped nanoparticles by the microwave combustion technique for novel applications

The quick oxidation of a material to produce heat is known as combustion. It is an exothermic chemical reaction which employs an oxidising (O _x ) and a reducing (R _x ) agent (Red). A chemical reaction between two or more substances that produces heat and light in the form of a flame is called combustion [16]. This leads to the production of new compounds and heat energy. The oxidising agent is metal nitrate, while the reducing agent is fuel (urea, glycine, citric acid, L-arginine, and so on) [17]. The combustion temperature in an MW-assisted reaction between a nitrate precursor and L-arginine as a fuel typically ranges from 800 to 1,200°C, depending on factors such as the fuel-to-oxidiser ratio, MW power, and the reaction vessel [18]. A stoichiometric or slightly fuel-rich mixture and higher MW power can lead to increased temperatures [19]. The ignition temperature of MW combustion typically depends on the specific reactants and their stoichiometric ratios. For most nitrate-fuel systems, the ignition temperature in MW combustion processes is relatively low, often in the range of 150–250°C [20]. This is because MW energy rapidly heats the mixture, causing localised hotspots and initiating the exothermic reaction even at moderate temperatures. The MW combustion technique process makes use of principles of MWs and thermochemical concepts from propellant chemistry [21]. MWs are electromagnetic waves with frequencies between 300 MHz and 300 GHz [22]. They are located between radio and infrared waves in the electromagnetic spectrum. In this region, MWs with frequencies between 900 MHz and 2.45 GHz can be used for heating. Figure 1 typically shows dipolar polarisation and MW heating in correlation with ionic conduction. (i) Dipolar polarisation is shown in Figure 1a and (ii) ionic conduction is shown in Figure 1b. While charged particles (ionic species or free ions) are essential for comprehending the contribution of these charged particles to the ionic conduction, the reaction mixture of dipoles (such as reagents or polar solvent molecules) is involved in the dipolar process of rotation or polarisation [23]. When exposed to MW frequencies, the dipoles attempt to align themselves along the direction of the applied electric field. The electric field oscillates to generate a dipole, which then tries to track the heat and energy loss from dielectric loss and molecular friction.

Figure 1

MW heating mechanism corresponding to (a) dipolar polarisation and (b) ionic conduction.

The capacity of these “dipoles” to align with the applied MW frequency and field determines how much heat is generated. When the dipole does not have enough time to realign with regard to the field, no MW heating will be produced [24]. A frequency of 2.45 GHz, which is in the middle of these two extremes and gives the molecule dipoles adequate time to align with the field, is used by all commercial MW systems [25]. The softened charged particles in the sample will oscillate below the MW field in the event of an ionic process because the molecular dipoles cannot precisely follow the alternating field. Heat is produced as a result of collisions with nearby molecules during this process [26]. An ac field causes polar molecules to automatically align, which causes friction, rotation, and intermolecular collisions. Therefore, an “inside-out” heat genesis caused by the combined effects of polarisation and ionic forces can lead to rapid MW heating and an increase in the overall temperature. This polarisation process leads to the evolution of structured metal oxides, such as perovskites and spinels. Compared to other metal oxides, perovskites and spinels are now widely used in trans esterification reactions, particularly for the production of biodiesel. Therefore, it is very important to understand the structures of these materials, which are covered in the following sections [27]. Equation (1) gives the chemical reactions during the MW combustion reactions. (1) ( 1 − x ) Ni (NO 3 ) 2 + x Mg(NO 3 ) 2 + 2Fe(NO 3 ) 3 + y C 2 H 5 NO 2 → Ni 1 − x Mg x Fe 2 O 4 + gaseous products . (1-x)\text{Ni}\hspace{.25em}{{\text{(NO}}_{3}\text{)}}_{2}+x{\text{Mg(NO}}_{3}{\text{)}}_{2}+{\text{2Fe(NO}}_{3}{\text{)}}_{3}+y{\text{C}}_{2}{\text{H}}_{5}{\text{NO}}_{2}\to {\text{Ni}}_{1-x}{\text{Mg}}_{x}{\text{Fe}}_{2}{\text{O}}_{4}+\text{gaseous}\hspace{.25em}\text{products}\text{.}

The structures employed in the fabrication of nanoparticles rely on the intended use of nanoparticles, the facilities that are accessible, and the researcher’s preference. The following are some of the structures used in the metal-organic nanoparticles preparation process.

2.2.1

Perovskites

ABO₃ (A = rare earth, alkaline earth, and alkali elements; B = d-block transition metal) is the general formula for perovskite structures. In this case, O is the anion, while A and B are the cations of the modified sites. The coordination number of the A and B atoms is 12 and 6, respectively. Atom A is in the core of the perovskite or at the centre of the body; atom B is in the corner; and the oxygen atom is in the face-centred position. The perfect cubic perovskite structure (ABO₃) is shown in Figure 2. BaTiO₃ is the best-studied perovskite material known, with site A containing divalent anions and site B containing tetravalent ions. Figure 3 depicts the basic structure of BaTiO₃. By maintaining their original crystal structure, chemical substitutions in the A- and/or B-sites of ABO₃-type perovskite can result in altered structural and electronic changes [28]. Applications for these perovskite materials include sensors, capacitors, and piezoelectric displays. BaTiO₃ is the best example of a perovskite structure, with tetravalent ions occupying the “B site” and divalent ions occupying the “A site.”

Figure 2

Ideal cubic perovskite structure (ABO₃).

Figure 3

Perovskite structure.

2.2.2

Spinels

The chemical formula for spinels is AB₂O₄, where O stands for an anion, B for a trivalent metal ion, and A for a divalent metal ion. High electrochemical stability, metal–insulator transition behaviour, superconductivity, ferroelectricity, and ferromagnetism are among the superior qualities of this kind of AB₂O₄ spinels. Spinel-type oxides are employed in many processes, including catalysis, transformers, memory devices, fuel cells, solar cells, sensors, pigments, batteries, and so on (Figure 4).

Figure 4

Image of spinel oxide.

2.2.3

Magnetic spinels

The chemical formula for spinel aluminates is Zn_1–x M _x Al₂O₄ (0 ≤ x ≤ 0.5), where Al is the trivalent metal ion occupying the octahedral sites and M is the divalent metal ion occupying the tetrahedral sites (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Zn²⁺, etc.). An FCC structure with a Fd3m space group makes up the spinel-type aluminates (Figure 4). The metal ions occupy half of the tetrahedral and octahedral places in the cubic, closely packed oxygen lattice that makes up the spinel structure. As a result, the structure has twice as many octahedral sites as tetrahedral sites [29].

A common representation of the chemical formula for the mineral group spinel is AB₂O₄, where “B” stands for trivalent cations, such as aluminium (Al), chromium (Cr), or iron (Fe), and “A” stands for divalent cations, such as magnesium (Mg), zinc (Zn), or iron (Fe). The letter “O” stands for oxygen. The overall formula can therefore be expressed as (A²⁺)(B³⁺)(O²⁻)₄. Figure 5 illustrates the crystal’s spinel structure. Due to the intriguing catalytic interaction between iron and copper sites, CuFe₂O₄ nanoparticles have been employed as a catalyst in numerous applications in recent years. CuFe₂O₄ nanoparticles are therefore magnetically recoverable nanocatalysts in a variety of catalytic applications.

Figure 5

Spinel crystal.

Because of its mild saturation magnetisation, low coercivity, mechanical hardness, and good stability, MnFe₂O₄ nanoparticles are also soft ferrites. Due to their significant atomic-level correlation and reliance on their magnetic properties, MnFe₂O₄ nanoparticles with ferromagnetic characteristics have recently been employed in MRI and other applications. We have selected NiFe₂O₄ as the basis material for our current research project because of its favourable structural, optical, and magnetic characteristics. Furthermore, the size of ferrites affects their structural, optical, and magnetic characteristics. Therefore, we used the MW combustion method to synthesise pure and Mg²⁺ doped NiFe₂O₄ for the current investigation. Hence, the present work is devoted to MW combustion synthesis of Ni_1–x Mg _x Fe₂O₄ (0 ≤ x ≤ 0.5) spinel nanoparticles. The spinels are characterised to determine their structural, morphological, optical, and magnetic properties.

Both pure and metal-doped nanostructured Ni_1–x Mg _x Fe₂O₄ (0 ≤ x ≤ 0.5) were synthesised in various molar ratios with x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5 moles and (M = Mg²⁺). De-ionised water was used to dissolve the raw ingredients. L-Arginine serves as a fuel for MW combustion, whilst nitrate precursors act as oxidisers. The ratio of “fuel to oxidiser (F/O)” was maintained as one by applying the propellant chemistry rule. After that, the solution was moved to a crucible and heated for 10 min in an MW oven. Here, the solution was boiled in a MW oven, which caused it to dehydrate, evaporate, and break down, resulting in the evolution of gas. Ultimately, when the solution achieves spontaneous combustion, ignition occurs, producing a fluffy, white powder. Nanoparticle characterisation is done via X-ray diffraction (XRD) analysis. XRD may be used to examine the unique “fingerprint” of each substance. Utilising a Rigaku (SmartLab) apparatus with a diffraction angle range of 10°–80° and a CuKα radiation source at λ = 0.15406 nm, the structural investigations were conducted.

The following parameters were calculated from XRD studies:

The average crystallite size (L) of pure and doped nickel ferrite was calculated using Scherrer’s formula, which is given in equation (2). (2) L = 0.89 × λ β × cos θ , L=\frac{0.89\times \lambda }{\beta \times \text{cos}\theta }, where θ is the angle of diffraction (half Bragg angle), λ is the incident X-ray wavelength (0.15406 nm), L is the average crystallite size (nm), and β is the full width at half-maximum of the diffraction peak [13].

The prominent diffraction patterns of Mg–NiFe₂O₄ nanoparticles are displayed in Figure 6, which validates their crystal nature. The diffraction peaks correlate with 2θ values of 30.37°, 35.54°, 36.50, 43.38°, 53.83°, 57.06°, 62.74°, and 74.51°.

Figure 6

XRD plot of Ni_1−x Mg _x Fe₂O₄ (0 ≤ x ≤ 0.5) samples.

For the cubic spinel structure with Fd 3m space group symmetry, the results from the entire collection of diffraction peaks match with the standard data quite well (JCPDS card no. 89-4927). For the entire Ni_1−x Mg _x Fe₂O₄ (0 ≤ x ≤ 0.5) nanoparticles, distinctive impurity peaks (Fe₂O₃) were found, indicating a good Mg²⁺ exchange.

The XRD pattern (Figure 7) of Mg–NiFe₂O₄ nanoparticles shows the slight shift of diffraction peaks from the lower to higher angles. The peak position in pure nickel ferrite is at a 2θ value of 35.597°, and it is shifted to 35.678° for magnesium doped nickel ferrite (x = 0.5), and the difference in the 2θ value is 0.81°.

Figure 7

Peak shift of Ni_1−x Cu _x Fe₂O₄ (0 ≤ x ≤ 0.5) samples.

The value of (“a”) for NiFe₂O₄ is found to be consistent with the 8.381 Å value reported by Ghamdi et al. [13]. However, in accordance with Vegard’s law, the lattice constant decreases a little (Table 1) when the concentration of Mg²⁺ increases. The presence of lattice contraction without any violation of lattice symmetry is specified by the increased Mg²⁺ concentration. The reason for this is that Mg²⁺ has a larger ionic radius (1.73 Å) than Ni²⁺ (1.63 Å) [26]. The average size of the crystallites of Mg–NiFe₂O₄ nanoparticles is shown in Table 1. Samples “A, B, C, D, E, and F” had recorded values of 20, 22, 24, 27, 29, and 31 nm, respectively. The observed growth in crystallite size may be attributed to the accumulation of Mg²⁺ ions within the lattice structure (Figure 8).

Figure 8

Lattice parameter variation in Ni_1−x Cu _x Fe₂O₄ (0 ≤ x ≤ 0.5) samples.

The effective crystallite size (D) and strain (k) can be inferred from the slope of the line that was obtained from the analysis. These values were found using the y-axis intercept (D/k). The effective crystallite size (D) and the crystallite size (L) determined by the W-H and Scherrer methods, however, differ slightly. It is found that samples a, b, c, d, e, and f have average crystallite sizes of 20, 21, 23, 25, 27, and 30 nm, respectively.

The FT-IR spectra of Ni_1−x Mg _x Fe₂O₄ nanoparticles, where 0 ≤ x ≤ 0.5, in the wavelength range of 4,000 to 400 cm⁻¹, are depicted in Figure 9. Several peaks were observed, each with their corresponding characteristics.

The presence of absorbed water molecules on the cobalt ferrite samples is indicated by the peaks at 3,438 cm⁻¹ [16].

Figure 9

FTIR spectra of Ni_1−x Mg _x Fe₂O₄ (0 ≤ x ≤ 0.5) samples.

These observations provide valuable information about the molecular vibrations and bonding characteristics present in the Ni_1−x Mg _x Fe₂O₄ nanoparticles.

HR-SEM was used to analyse the microstructural characteristics of Ni_1−x Mg _x Fe₂O₄ nanoparticles. The generated nanoparticles are spherical in shape and appear to be agglomerated and coalescent, as shown in Figure 10a–f. Agglomeration flaws are the result of annealing or interfacial surface tension during the preparation process. Additionally, MW irradiation, which generates enormous thermal energy within the material, causes the agglomeration and coalescence of the nanoparticles [28].

Figure 10

HR-SEM images of (a) Ni₁Mg₀Fe₂O₄, (b) Ni_0.9Mg_0.1Fe₂O₄, (c) Ni_0.8Mg_0.2Fe₂O₄, (d) Ni_0.7Mg_0.3Fe₂O₄, (e) Ni_0.6Mg_0.4Fe₂O₄, and (f) Ni_0.5Mg_0.5Fe₂O₄ samples.

The EDAX spectra of Mg–NiFe₂O₄ nanoparticles are shown in Figure 11a–f. As shown in Figure 11a, the peak verifies the presence of elements such as Ni, Fe, and O. In all the other samples of Mg–NiFe₂O₄ nanoparticles, the Mg peak was observed (Figure 11b–f).

Figure 11

EDX images of (a) Ni₁Mg₀Fe₂O₄, (b) Ni_0.9Mg_0.1Fe₂O₄, (c) Ni_0.8Mg_0.2Fe₂O₄, (d) Ni_0.7Mg_0.3Fe₂O₄, (e) Ni_0.6Mg_0.4Fe₂O₄, and (f) Ni_0.5Mg_0.5Fe₂O₄ samples.

Hysteresis curves were used to calculate magnetic parameters such as Hc, Ms, and Mr, respectively. The magnetic appearances of the samples (Figure 12) were investigated at room temperature under the influence of a stimulated field, with values ranging from −15 kOe to + 15 kOe. As shown in Figure 12a, the coercivity values increase from x = 0 (340.45 Oe) to x = 0.1 (480.14 Oe) and then decrease as the concentration of Mg²⁺ increases until they reach x = 0.5 (188.77 Oe).

Figure 12

Magnetic hysteresis of Ni_1−x Mg _x Fe₂O₄ (0 ≤ x ≤ 0.5) samples.

The magneto crystalli value increases up to x = 0.4 (6.31 emu/g), but then decreases at x = 0.5 (6.12 emu/g). The factors include lattice imperfections, random magnetic spin orientation, and magnetic superexchange interactions due to cation redistributions that correspond to “tetrahedral A” and “octahedral B” sites. As the Mg²⁺ ion fraction increases, the Ms values increase from a (27.15 emu/g) to e (40.20 emu/g) and subsequently falls to f (33.18 emu/g). The value comparison of different samples is displayed in Table 2.

UV-DRS was used to quantify the energy gap of Mg–CuFe₂O₄ nanoparticles. The band gap values were computed using the Tauc equation. According to equation (3), the diffuse reflectance was converted into its appropriate “absorption coefficient” using the “Kubelka–Munk” F(R) function, as shown in Figure 13. The graphs of (F(R)hυ)² vs hυ for samples A and F are shown in Figure 13a and b, respectively. These numbers correspond to the direct band gap measurements and have expanded linear areas to (F(R)hυ)² = 0. The direct band gap values of Mg–NiFe₂O₄ nanoparticles are 3.35, 3.29, 3.25, 2.97, 2.73, and 2.32 eV (Figure 14).

Figure 13

(F(R)hν)² versus hν plots Ni_1–x Mg _x Fe₂O₄ (0 ≤ x ≤ 0.5) for (a) A sample and (b) F sample.

Figure 14

Mg²⁺ fraction vs energy gap of Ni_1−x Mg _x Fe₂O₄ (0 ≤ x ≤ 0.5) samples.

It was found that the band gap of NiFe₂O₄ nanoparticles was 3.37 eV lower than that of the bulk material. The surge in the optical band gap value is caused by the weak quantum confinement effect that occurs at the nanoregime.