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. 2023 Aug 3;8(33):29959–29965. doi: 10.1021/acsomega.3c00541

Impact of Cobalt Doping on the Structural, Optical, and Dielectric Properties of MgAl2O4 Spinel Material

Kamran Ullah †, Shoaib Ali Khan †, Abid Zaman ‡,*, Mahidur R Sarker §, Asad Ali ‡,∥,*, Vineet Tirth ⊥,#, Amnah Mohammed Alsuhaibani , Ali Algahtani ⊥,#, Tawfiq Al-Mughanam ∇, Moamen S Refat ○, Shafrida Sahrani §,*, Muhammad Abrar †
PMCID: PMC10448643  PMID: 37636967

Abstract

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Nanomaterials (NMs) with structural, optical, and dielectric properties are called functional or smart materials and have favorable applications in various fields of material science and nanotechnology. Pure and Co-doped MgAl2O4 were synthesized by using the sol–gel combustion method. A systematic investigation was carried out to understand the effects of the Co concentration on the crystalline phase, morphology, and optical and dielectric properties of Co-doped MgAl2O4. X-ray diffraction confirmed the cubic spinel structure with the Fd3̅m space group, and there was no impurity phase, while the surface morphology of the samples was investigated by scanning electron microscopy. The dielectric properties of the synthesized material are investigated using an LCR meter with respect to the variation in frequency (1–2 GHz), and their elemental composition has been examined through the energy-dispersive X-ray technique. The existence of the metal–oxygen Mg–Al–O bond has been confirmed by Fourier transform infrared spectroscopy. The value of the dielectric constant decreases with the increasing frequency and Co concentration. The optical behaviors of the Co2+-doped MgAl2O4 reveal that the optical properties were enhanced by increasing the cobalt concentration, which ultimately led to a narrower band gap, which make them exquisite and suitable for energy storage applications, especially for super capacitors. This work aims to focus on the effect of cobalt ions in different concentrations on structural, optical, and dielectric properties.

Introduction

Oxides having spinel-like structures are widely used in modern technology. Such functional materials are used for many purposes like humidity sensor, catalysts, optical windows, and so forth.1−4 The spinel magnesium aluminate (MgAl2O4) is one of the most well-known structural materials having a large number of applications in various fields.5,6 The spinel-like materials are used extensively in metallurgical, electrochemical, and chemical industries due to their outstanding optical and dielectric properties.7−9 The spinel-type oxides have the general formula AB2O4, where A and B refer to two distinct cations of equal ionic sizes and are chemically and thermally stable.10 The MgAl2O4 spinel crystal exhibits a cubic structure with space group (Fd3m).11 The oxygen sublattice’s spatial coordination has pseudo-cubical close packing. The cubic unit cell has 64 tetrahedral and 32 octahedral interstices between oxygen atoms. Al3+ inhabit 16 of the 32 octahedral interstices and Mg2+ fill 8 of the 64 tetrahedral interstices.12

The temperature affects the permittivity and ac conductivity of the nanoscale MgAl2O4 with frequency, and as a result, the real part of the dielectric constant and loss factor both increase with the temperature and decrease with frequency. Additionally, ac conductivity exhibits an opposite trend with frequency, although it continues to rise with a rise in temperature.13,14 The valence electrons of the doped elements are impacted by transition metals of (3d) levels, which in turn affect the defect levels of the host material MgAl2O4. As a result of this interaction, the A- or B-site cations were insistently replaced with transition metal ions to attain the desired band structure tuning for certain applications. For instance, Fe2+ impurities dominate at the A-site of MgAl2O4 and subjugate their magnetic properties.15 The optical behaviors’ of the transition metal-doped MgAl2O4 depend on these two factors, valence and site type of the doped transition-metal ions.

Several researchers have investigated the dielectric behaviors of manganese-, chromium-, and iron-doped MgAl2O4 fabricated through the co-precipitation method and found that the modified value of relative permittivity was increased.16 The continuous isomorphic between magnesium oxide and cobalt oxide is the foundation for the production of cobalt-doped MgAl2O4 samples; this allows for Co2+ ions to replace Mg2+ ions, which is attributed to their quite close ionic radii, and yields solid solutions of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09).17,18 Several researchers had adopted various techniques to fabricate the cobalt-doped MgAl2O4 such as sol–gel, microwave combustion, solid-state reactions, co-precipitation method, and so forth.19 Only some researchers like Ullah et al. used the sol–gel combustion approach to prepare Co2+-doped MgAl2O4.4

In this research work, an effort is made to synthesize the solid solution of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel via a sol–gel combustion method. Due to the importance of this method, we studied the effect of Co2+ on the relationship between the structural, microstructural, vibrational, and dielectric properties (dielectric constant and tangent loss) of the Mg1–xCoxAl2O4 materials, which, by varying of the frequency, are improved.

Results and Discussion

X-ray Diffraction Analysis

Figure 1 shows the X-ray diffraction (XRD) pattern of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel solid solution. High intensity, sharp peaks with hkl values (220), (311), (400), (511), and (440) identified the samples’ crystalline structure. The spinel cubic structure was confirmed by indexing of peaks, whose lattice constant is 8.08 Å, which matches exactly with PDF card # 01-075-1800 along with space group (Fd3̅m). The spinel’s single-phase cubic structure is confirmed by the absences of an impurity peak. For each composition, the XRD patterns have a high intensity peak at Braggs angle (2θ = 36°), which correlate to the plane of (311).20 It has been observed that the lattice constant of MgAl2O4 spinel increases with Co2+ contents. The variation in the lattice parameters is attributed to the slight difference in the ionic radii of cobalt ions (0.74 Å) and magnesium ions (0.65 Å). This increase in the lattice constant reveals that Mg2+ ions have been substituted by Co2+ ions in the crystal structure.12,21 The volume of the unit cell increases with Co2+ contents and has been reported in this work. According to Vegard’s law, the variations in the lattice parameters may be due to the ionic radius of the dopant element. However, most of cobalt–magnesium exchange occurs at the spinel’s tetrahedral site.22Figure 1b shows the peaks shifting toward lower Bragg’s angle because the ionic radius of the dopant material (Co2+) is larger than that of the host material (Mg2+).23 The Sheerer equation is used to determine the average crystallite size of samples, as shown in eq 1.24,25

graphic file with name ao3c00541_m001.jpg 1

Figure 1.

Figure 1

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(a) XRD pattern of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel calcined at 800 °C and (b) zoomed view of the (3 1 1) peak shifting toward the lower angle.

The Williamson Hall analysis is used to calculate the lattice strain in the structure, as reported by Mote et al.26 The dislocation density (δ) is determined by using this eq 2

graphic file with name ao3c00541_m002.jpg 2

The average crystallite size, dislocation density, lattice strain, and micro-strain of the synthesized samples are summarized in Table 1.

Table 1. Calculated Average Crystallite Size (D), Dislocation Density (δ), Lattice Strain (η), and Micro Strain (ε) of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) Spinel.

parameters X = 0.00 X = 0.03 X = 0.06 X = 0.09
average crystallite size “D” (nm) 34.241 39.125 51.570 54.135
dislocation density “δ” (×10–3nm–2) 8.5300 6.7900 3.7600 3.6900
lattice strain “η” (×10–2) 0.1013 0.0886 0.0672 0.0451
micro strain “ε” (×10–2) 1.2367 1.0692 0.8531 0.7797
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The XRD theoretical density (ρth) was determined from the relation

graphic file with name ao3c00541_m003.jpg 3

where M is relative molecular mass, Z is that the number of atoms per unit cell, NA is Avogadro’s number, and a is the lattice parameter of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel. Table 2 shows the physical properties like structural, lattice parameters, volume, density, and porosity of the solid solution of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel. It has been observed that the porosity increases with the decreasing relative densities, as Co2+ was increased.27 Porous materials are a class of materials with low density, large specific surface, and a range of novel properties in the electrical, mechanical, thermal, and acoustical fields.28,29 The porosity was calculated by using eq 4.30

graphic file with name ao3c00541_m004.jpg 4

where ρexp is experimental and ρth is theoretical density.

Table 2. Variation of Physical Properties of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) Spinel with Co2+ Contents.

contents (x) structure lattice parameters (nm) volume (nm3) ρtheoretical (gm/cm3) ρexperimental (gm/cm3) ρrelativedensity (%) porosity (%)
X = 0.00 cubic 0.808 0.528 3.58 3.56 99.6% 0.55%
X = 0.03 cubic 0.797 0.506 3.34 3.30 98.7% 1.19%
X = 0.06 cubic 0.793 0.499 3.10 3.01 97.2% 2.90%
X = 0.09 cubic 0.781 0.476 2.86 2.73 95.34 4.54%
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Surface Morphological Studies

Figure 2a–d shows the scanning electron microscopy (SEM) micrograph of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel calcined at 800 °C for 4 h in air. The size, shape, and grain boundary of all the samples are analyzed by using SEM. Figure 2a shows that the base sample exhibited a plate-like structure. The structure of the base sample is in accordance with the previous literature.9,31 It was also found that the grain size decreases with the increasing Co2+ content. The lowest average grain size and maximum homogeneity were observed at x = 0.06. The morphology of the sample changes to that of nanotubes with Co2+ contents (at 3%), as shown in Figure 2b. Increasing the cobalt content to 6% revealed the shifting morphology from nanotubes to the irregular shape, as shown in Figure 2c.

Figure 2.

Figure 2

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SEM micrograph of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel (a) x = 0.00, (b) x = 0.03, (c) x = 0.06, and (d) x = 0.09.

However, some large grains may be grown due to the agglomeration of the smaller grains.24,32 The shape of the cobalt-doped magnesium aluminate changes into a disc with increasing Co2+ contents from 0.06 to 0.09, as shown in Figure 2d.33

The elemental composition of each sample is analyzed by using the energy-dispersive X-ray spectroscopy (EDX) technique. It confirms the presence of certain elements in the samples under investigation. Moreover, EDX analysis revealed the replacement of Mg2+ ions by Co2+ ions. Figure 3a–d shows the EDX pattern of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel.

Figure 3.

Figure 3

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EDX spectra of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel: (a) x = 0.00, (b) x = 0.03, (c) x = 0.06, and (d) x = 0.09.

EDX patterns confirmed the presence of O, Mg, Al, and Co elements in the solid solution of MgAl2O4. The elemental composition of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel is shown in Table 3.

Table 3. Elemental Composition of the Various Elements Present in the Synthesized Samples.

elements X = 0.00 X = 0.03 X = 0.06 X = 0.09
Mg 19.20 20.02 20.62 18.67
Al 44.80 41.70 40.91 42.08
O 36.00 34.41 35.45 31.80
Co   3.87 3.02 7.45
total (in %) 100 100 100 100
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Figure 4a–d shows the transmission electron microscopy (TEM) micrographs of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel. The internal morphology and size of the synthesized pure and different cobalt-doped MgAl2O4 spinel are examined by using TEM. From the morphologies, it is clearly visible that highly crystalline, nanometer-sized ternary oxides were obtained. The doped Co displayed well-dispersed metal nanoparticles on the spinel support, as shown in Figure 4d.

Figure 4.

Figure 4

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TEM micrographs of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel: (a) x = 0.00, (b) x = 0.03, (c) x = 0.06, and (d) x = 0.09.

Optical Properties

UV–Visible Spectroscopy

The optical properties of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel were examined by using UV–visible spectroscopy. The energy band gaps for all samples have been determined by using the TAUC, as shown in eq 5

graphic file with name ao3c00541_m005.jpg 5

Figure 5a–d shows the UV spectra of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel and reports that the band gap energy decreases from 4.09 to 2.79 eV with increasing Co2+ contents. This is due to the minor difference of ionic radii of Co2+ (0.67 Å) and Mg2+ (0.65 Å).34

Figure 5.

Figure 5

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UV spectra of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel: (a) x = 0.00, (b) x = 0.03, (c) x = 0.06, and (d) x = 0.09.

The high band gap values of MgAl2O4 may be attributed to the s–p hybridization between the 3s and 3p of Mg and Al, respectively, which causes the lowest energy levels in the conduction band to be filled.35 On the basis of two distinct cation symmetries, Mg2+ (A) and Al+3 (B) are used to reveal the optical behavior of 3d transition metals (Co, Ni, and Mn)-doped MgAl2O4 (AB2O4) spinel. The Co2+ (3d7) ions at the A-site with Td symmetry exhibit a high band gap energy due to their high excitation energy (over 3.30 eV).36

FTIR Study

The Fourier transform infrared (FTIR) spectroscopic analysis revealed information about bonding and phase composition of the samples recorded in the range of wave number 400–4000 cm–1, as shown in Figure 6.

Figure 6.

Figure 6

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FTIR spectrum of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel.

The frequency range 500–900 cm–1 shows the stretching vibrational mode of aluminum–oxygen (Al–O), metal–oxygen (M–O), and metal–oxygen–aluminum (M–O–Al) for the spinel structure.37 The existence of the nitrate group is confirmed by the absorption at about 1380 cm–1.38 Also, the absorption band in the 1200–1600 cm–1 of the absorption spectra reveals that the Mg2+ is substituted by Co2+ ions and occupy the tetrahedral sites of the spinel MgAl2O4 nanocrystallites in all spectra. The band about 3480 cm–1 is attributed to the (O–H) vibrations of the water molecules, which are absorbed by the samples.39

Microwave Dielectric Properties

In the current research work, the dielectric properties, that is, relative permittivity and tangent loss, have been measured by using impedance spectroscopy at the frequency range (from 1 to 2 GHz). The dopant concentration affected the dielectric permittivity and loss of the Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel.40 The value of relative permittivity of the base sample decreases with the increasing Co2+ contents, as shown in Figure 7.

Figure 7.

Figure 7

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Variation of the dielectric constant with Co concentration and frequency of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel.

The obtained values of relative permittivity for pure and doped samples are 44, 35, 27, and 20 at 1 GHz frequency. Figure 6 also shows that the value of relative permittivity decreases with the increasing operating frequency. The value of relative permittivity could be explained easily according to the relative dipole moments and lattice structure of the sample.41 In the structure of MgAl2O4 spinel, the permanent dipole moment attained the Centro symmetric position on c-axis, which modifies the value of relative permittivity. It is concluded that the smaller the c/a ratio, the lower will be the value of relative permittivity.42 A Co-doped MgAl2O4 compound is suitable for the application of the humidity sensor, data storage devices, and communication technology.14,18,29Figure 8 shows the plot of tangent loss (tan δ) versus operating frequency for the pure and doped MgAl2O4 sample.

Figure 8.

Figure 8

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Variation of dissipation factor of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel with frequency.

Figure 8 shows that the tangent loss decreases with the increasing operating frequency and Co2+ contents, which may be due to the accumulation of charge carriers and thermal activation energy.43 The tangent loss values show fluctuations with frequency, which may be due to the substitution of a lower ionic cation (Mg2+) by a higher ionic cation (Co2+). The frequency-dependent tangent loss is a dimensionless quantity, and it is good for base station applications.44

Conclusions

The solid solution of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) spinel has been synthesized by the sol–gel combustion method. The XRD studies reveal that Co2+-doped MgAl2O4 exhibits a cubic structure. The base and doped samples have same lattice parameters. The spinel structure was unchanged by the substitution of the Co2+ cation. The SEM micrograph shows that the surface morphologies change with Co2+ contents from a plate-like shape to irregular nanotubes. The EDX studies confirm the existence of Al, Mg, O, and Co elements. The FTIR analysis suggested the existence of the M-Al-O bond. The improved optical properties have been reported by doping of Co2+ concentrations. The band gap energy and dielectric constant decrease with the increasing doping concentrations without the structure distortion. The overall findings may help in the application of smart technology.

Materials and Experimental Procedure

The solid solution of Mg1–xCoxAl2O4 (0.00 ≤ x ≤ 0.09) is synthesized by using the sol–gel combustion method. The chemical precursors used to synthesize the samples are aluminum nitrate (Al(NO3)3·9H2O), magnesium nitrate (Mg(NO3)2·6H2O), cobalt nitrate (Co(NO3)2·6H2O), and citric acid (C6H8O7·H2O), and they are mixed with an appropriate stoichiometric ratio. All the precursor chemicals were purchased from Sigma-Aldrich with a minimal purity research rating of 99.96%. The solution of all the raw materials is blended in a beaker at a 1:2 molar ratio. The anhydrous citric acid was used as a combustible fuel catalyst. The citric acid and metal nitrates are both taken in an equal stoichiometric ratio. In order to obtain a homogeneous and thick gel of solution, all the chemicals are poured into a beaker that contained 25 mL of deionized water and then heated continuously and stirred for an hour at 120 °C on a hot plate. The whole procedure is carried out in a ventilated enclosure. The specimens were then inserted in a muffle furnace at 400 °C for combustion by a fierce exothermic reaction. To achieve the desired phase of spinel aluminates, the fine powder of each composition was calcined at 800 °C for 4 h. The Co2+ ions are substituted on the A-site of AB2O4 i–e Mg-site cation.

Characterizations

The absolute information of the cell volume, crystallite size, structural lattice strain, lattice constants, and the crystal structure was obtained by crystallographic studies using Panalytical X’Pert software. The grain size was determined using ImageJ software. The microstructural and phase analysis was performed using SEM (JSM-5910, JEOL Japan) and XRD (JDX-3532, JEOL, Japan) with Cu Kα radiation (λ = 0.154 nm). The elemental composition of each sample was analyzed by (EDX) spectroscopy using the Oxford Inca X-Act to verify the existence of the probable elemental contents. The absorption spectra of the (FTIR) were obtained on a Perkin-Elmer GX FTIR system with a resolution of 10 cm–1 in the 400–4000 cm–1 range to study the bond and functional group attached to the samples. The optical properties were studied by using ultraviolet–visible spectroscopy. The absorption spectra of the samples were investigated using a Perkin-Elmer UV–vis Spectrometer, λ = 25, in the visible range. An LCR meter was employed to measure the dielectric properties of the sintered sample (1–2 GHz)..

Acknowledgments

This research was funded by Universiti Kebangsaan Malaysia under Grant Code GGPM-2021-050 and GGPM-2022-064. The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University Abha 61421, Asir, Kingdom of Saudi Arabia for funding this work through the Large Groups Project under grant number RGP.2/142/44. Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2023R65), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

The authors declare no competing financial interest.

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