Charge-density analysis of magnesium-doped bentonite: Enhanced structural and electrical properties with tight-binding calculations

2.1 Materials and methods

This investigation used the materials as received. All reagent operations were performed in a fume chamber.

Granular Bento (CAS# 1302-78-9) from Sigma-Aldrich has a nearly homogeneous particle size and a surface area of around 1000 m²/g. Sigma-Aldrich (USA) supplied magnesium nitrate hexahydrate (CAS# 13446-18-9). Our theoretical investigation used XTB tight-binding semi-empirical quantum chemistry software for geometry optimization, nonbonded interaction site identification, and electrochemical calculations (Bannwarth et al., 2019). Bentonite’s 3-D structural file was retrieved from the Crystallography Open Database (code #1100106) (Gražulis et al., 2012). The XTB software was used to optimize the geometry of a Mg nanocluster at the bentonite structure’s center.

2.2 Apparatus and measurements

EDX analyses were done using the JSM 6510A scanning electron micrograph module. XRD patterns were taken using a Philips X’Pert-Pro MPD diffractometer at 40 kV, 40 mA, and Cu Kα 1.5418 Å radiation from 5° to 60°. We measured dielectric characteristics and electrical conductivity from 25°C to 120°C and 100 Hz to 1 MHz using the E4990A Impedance Analyzer. No D.C. bias was used, and A.C. amplitude was 0.5 V. We constructed 1 mm-thick, 0.5 cm-radius circular disk (or pellet) samples for impedance measurement. We measured the sample surface temperature with a mercury bulb thermometer on a temperature-controlled hot plate. Estimated dielectric permittivity (ϵ$`$) comes from:

wherein C represents the capacitance, d denotes the sample thickness, ${\epsilon }_o$ is the free space permittivity, and A represents the area of the measuring electrode on the sample’s surface.

The estimated electric modulus is given by M* $=M^{\prime }+i{M}^{″}$ which has a real $M^{\prime }=\frac{{\epsilon }^{\prime }}{{{\epsilon }^{\prime }}^2+{{\epsilon }^{″}}^2}$ and imaginary $M^{″}=\frac{{\epsilon }^{″}}{{{\epsilon }^{\prime }}^2+ {{\epsilon }^{″}}^2}$ components that are based on the real, ${\epsilon }^{\prime }$, and imaginary, ${\epsilon }^{″}$, permittivity. To determine AC conductivity, use the formula ${\sigma }_{ac}= \omega {\epsilon }_0{\epsilon }^{\prime }tan\delta$, where $tan\delta$ is the loss tangent, and $\omega$ is the angular frequency of the applied A.C. signal from the impedance analyzer. To avoid impedance interference, copper tape electrodes on samples with thin adhesive coverings received the A.C. signal. This method avoided conductive pastes like silver paste, which could cause impedance signals.

2.3 Composite preparation

The Mg/Bento was prepared using the wet impregnation method. Initially, Bento was stirred while adding H₂O. We added a predetermined amount of metal salt to the blend and rapidly swirled it for three hours at room temperature. A powdered composite was obtained by drying the solution at 393 K for 12 hours. Finely grinding the powder produced a smooth texture. A compressor formed circular, disk-like pellets from 0.30 g of fine powder for dielectric measurements. The manuscript uses “Bento” to refer to undoped bentonite, whereas “Mg/Bento” refers to Bento doped with <1% Mg for clarity and consistency.

3.1 Composites characterizations

3.1.1 X-ray diffraction (XRD)

The study examined pure bento with 1% Mg using XRD. Fig. 1 shows pure Bento and Mg/Bento spectra. The Bento contains SiO₂ (S), quartz (Q), and montmorillonite (M) (Salah et al., 2019), with quartz being the most prominent component based on its XRD peaks. Both samples have a SiO₂ diffraction peak at 2θ = 21.52° and montmorillonite peaks at 2θ = 23.6°, 28.16°, and 61.94°. Quartz diffraction peaks were identified at 2θ = 26.72°, 50.11°, 51.16°, 54.17°, and 68.12°. Comparable patterns were found by Salah et al. (2019). The Mg-NPs’ XRD pattern is shown in Fig. 1. The XRD spectrum has multiple distinct peaks. Five characteristic peaks were seen in Mg/Bento at 2θ = 30.16°, 32.56°, 36.11°, 43.42°, 57.11°, and 64.34° corresponding to the crystallographic indices (100), (002), (101), (102), (110), and (103), respectively.

Close

Fig. 1.

XRD intensities of bentonite (Bento) and Mg/Bento composites with SiO₂ (S), Quartz (Q), Magnesium (Mg), Montmorillonite (M), and impurities (Im).

Export to PPT

The observed peaks align with the data provided in the database (JCPDS 04-0770), verifying that the resulting microspheres consisted of Mg. Furthermore, five peaks were observed at 2θ = 39.76°, 47.39°, 48.67°, 54.97°, and 56.30°, were observed as impurity phase. Their peaks might be related to the Magnesium disilicide (Mg₂Si) as addressed by Xiao et al. (2020) and Yi et al. (2023). Based on the hexagonal Mg structure (Walker and Marezio, 1959), the lattice parameters (a, c) of Mg nanoparticles were computed (Karyaoui et al., 2015). Where ‘d’ is interplanar distance and ‘h’, ‘k’, and ‘l’ are Miller indices.

(2)

$\frac{1}{{\text{d}}_{\text{hkl}}^2}=\frac{4}{3}\left[\frac{{\text{h}}^2+\text{hk}+{\text{k}}^2}{{\text{a}}^2}\right]+\frac{{\text{l}}^2}{{\text{c}}^2}$

The values obtained for a, and c were 2.71 Å and 5.50 Å, respectively. This calculation uses first order Brag reflections at 2θ = 30.16°, and 36.12° for the Mg XRD intensities with (100) and (101) indices shown in Fig. 1. The average grain size (D) was calculated to be approximately 7 nm and 15 nm for Bento and Mg/Bento respectively, from the Debye - Scherrer’s equation (Patterson, 1939).

(3)

$D=0.94\lambda \text{/}\beta \cos \theta$

where λ is the incident CuKα radiation wavelength, β is full width at half maximum (FWHM) of the respective peak and θ represents the diffraction peak angle. Thus, the XRD profile in Fig. 1 confirmed the existence of Mg nanocrystals, demonstrating that the technique utilized in this work successfully prepared the Mg nanoparticles.

3.2 SEM results

The surface morphology of uncoated and magnesium-coated bentonite adsorbents was examined using SEM. Fig. 2 illustrates the SEM images of uncoated bentonite (a) and Mg/Bento (b). The uncoated bentonite displays large pores with varied sizes, shapes, and distributions across its surface. These pores provide insight into bentonite’s natural texture and structure, highlighting its high surface area and irregular pore distribution. In contrast, the Mg/Bento composite shows a significantly altered surface morphology. The SEM images reveal a flaky configuration with noticeable irregular agglomerations within the bentonite matrix. The Mg/Bento particles are substantially less agglomerated compared to the uncoated bentonite sample. Some larger particles are present, seemingly formed by the aggregation or overlapping of smaller ones.

Close

Fig. 2.

SEM micrographs of (a) Bento and (b) Mg/Bento. SEM: Scanning electron microscope.

Export to PPT

This observation suggests that the magnesium coating process affects the particle size distribution within the composite, leading to a mixture of particle sizes and shapes. The irregular morphology and varied particle sizes in the Mg/Bento composite complicate the determination of precise particle dimensions in the SEM images. The agglomerated nature of the composite makes it challenging to measure individual particle sizes accurately.

3.3 EDX results

An XRD spectrometer analyzed pure bentonite and Mg/bento composite element compositions (Fig. 3). The figure demonstrates that pure bentonite has a lot of O, Si, C, Al, and little Fe, Mg, N, Na, and Ca. Salah et al., (2019) and Özcan et al. (2009) reported similar findings. It contained O, Si, C, Al, Ca, Fe, Mn, Na, S, N, Cl, and Mg. Mg/Bento’s EDX spectra show a peak for Mg, among other elements, indicating good growth.

Close

Fig. 3.

The EDX results for undoped (a)Bento and (b) Mg/Bento. EDX: Energy-Dispersive X-ray.

Export to PPT

3.4 Dielectric properties

We investigated the dielectric properties of Bento and Mg/Bento composites over a frequency range of up to 1 MHz and temperatures from 25°C to 120°C.

The frequency-dependent permittivity (E’) of both Bento and Mg/Bento was measured at various temperatures. In this frequency range, both materials exhibited a monotonic decrease in permittivity with increasing frequency at all temperatures, as illustrated in Fig. 4. The inability of the material’s dipoles to quickly realign with the alternating electric field as the frequency increases causes this decrease in permittivity with rising frequency. As a result, the overall polarization of the material diminishes, leading to a reduction in dielectric permittivity, particularly at lower frequencies. This behavior is consistent in both pure bentonite and Mg-incorporated bentonite samples. We observe that the permittivity of Mg/Bento is lower than that of pure bentonite at lower temperatures (below 50°C), suggesting that the incorporation of Mg nanoparticles initially reduces polarization. However, as the temperature increases, the dielectric constant of Mg/Bento shows a pronounced rise, particularly at higher frequencies.

Close

Fig. 4.

Variations in the permittivity of Bento and Mg/Bento samples over different frequencies at (a) 25°C, (b) 50°C, (c) 75°C, (d) 100°C, and (e) 120°C, (f) illustrates the sample pellet equipped with copper connectors utilized for data collection.

Export to PPT

Temperature significantly influences the dielectric properties of Mg/Bento, causing Mg/Bento to exhibit more substantial changes in permittivity compared to pure bentonite as the temperature increases. The Maxwell-Wagner interfacial polarization effect, prominent in heterogeneous materials like Mg/Bento, explains this behavior. At lower frequencies, charge carriers accumulate at the interfaces between the Mg nanoparticles and the bentonite matrix, enhancing polarization.

As temperature increases, the mobility of these charge carriers also increases, leading to a more significant polarization effect at the interfaces, resulting in a higher dielectric constant. The data demonstrate that at elevated temperatures, the permittivity of Mg/Bento surpasses that of pure bentonite, highlighting the enhanced Maxwell-Wagner interfacial polarization due to the presence of Mg nanoparticles, which introduce additional interfaces that amplify this effect (Ellison et al., 2018). On the other hand, Fig. 5 shows the dielectric loss (E”) of Bento and Mg nanoparticles supported on bentonite (Mg/Bento) as a function of frequency at various temperatures. Fig. 5 presents the dielectric loss (E’’) of Bento and Mg/Bento as a function of frequency at various temperatures. At 25°C, Mg/Bento exhibits a higher dielectric loss than pure bentonite, indicating that incorporating Mg nanoparticles enhances energy dissipation mechanisms, potentially due to increased electrical conductivity or more active dipole relaxation processes.

Close

Fig. 5.

Dielectric loss changes of pure Bento and Mg-incorporated Bento at different frequencies at (a) 25°C, (b) 50°C, (c) 75°C, (d) 100°C, and (e) 120°C.

Export to PPT

As temperature increases, the dielectric loss of bentonite exceeds that of Mg/Bento, with this difference becoming more pronounced at higher temperatures, except in certain low-frequency regions. In other words, Mg/Bento exhibits a lower dielectric loss at high temperatures due to the stabilizing influence of Mg nanoparticles, which improve charge transport while reducing energy dissipation.

This enhancement arises from the interfacial polarization facilitated by Mg, which allows for more efficient charge alignment, thus improving dielectric stability at high temperatures. The variation in dielectric loss with temperature can better be understood with the permittivity data (E’) presented in Fig. 4. The monotonic decrease in permittivity (E’) with increasing frequencies for both samples suggests reduced dipolar alignment at higher frequencies. While Mg/Bento exhibits lower permittivity than bentonite at low temperatures, the permittivity of Mg/Bento significantly increases with temperature, especially at higher frequencies, due to enhanced charge carrier mobility and Maxwell-Wagner interfacial polarization. We emphasize that the observed increase in permittivity for Mg/Bentonite at elevated frequencies can be attributed primarily to Maxwell-Wagner polarization. Nevertheless, the dispersion of magnesium nanoparticles within the bentonite matrix could act as an additional contributing factor, as it introduces extra charge-trapping sites that enhance the overall dielectric response. This increase in polarization at elevated temperatures corresponds to the observed decrease in dielectric loss for Mg/Bento, indicating that although permittivity increases due to improved dipole alignment or charge accumulation, energy dissipation (E’’) is more controlled in Mg/Bento compared to Bento. This relationship between E’ and E’’ highlights the impact of temperature and frequency on the dielectric behavior of these materials, with Mg/Bento exhibiting distinct characteristics due to the presence of Mg nanoparticles. Therefore, Mg doping significantly enhances bentonite’s dielectric properties by promoting interfacial polarization and establishing conductive pathways within the composite structure. The integration of Mg nanoparticles enhances permittivity by increasing the mobility of charge carriers, especially at higher temperatures, thus strengthening the dielectric response.

3.4.1 Ac conductivity (σ_ac)

Understanding the conductivity of materials is crucial to investigating the impact of Mg/Bento. In Fig. 6, the electrical conductivity as a function of frequency is illustrated for both Bento and Mg/Bento across various temperatures. In general, the rate of electrical conductivity increases exponentially with frequencies, while the conductivity generally increases with increasing temperature (although there is a slight mismatch at 120°C) (Essaleh et al., 2023).

Close

Fig. 6.

Conductivity variation of Bento and Mg-incorporated Bento samples as a function of frequency at (a) 25, (b) 50, (c) 75, (d) 100, and (e) 120°C.

Export to PPT

Fig. 6 illustrates the exponential increase in electrical conductivity with frequency across various temperatures for both bentonite and Mg/Bento. The powers law dependence of conductivity describes how the frequency of the applied electric field influences the movement of charge carriers and explains this behavior. The charge carriers gain more energy at higher frequencies, allowing them to move more freely and contribute to higher conductivity. The power law dependence of the conductivity on frequency typically represents this relationship, with the frequency exponent reflecting the material’s response. In our case, the exponent at high frequency is around one, suggesting some variable range hopping conduction between localized sites such as the Mg nanoparticles. Incorporating Mg nanoparticles establishes new conductive pathways within the bentonite matrix, facilitating charge carrier flow. These pathways reduce the resistance to charge movement, as the Mg nanoparticles act as conductive fillers that bridge charge gaps in the matrix, enhancing overall conductivity. Furthermore, Mg nanoparticles create more interfaces within the bentonite matrix, serving as sites for improved charge carrier movement. These interfaces decrease barriers to charge mobility, facilitating faster charge migration. Conductivity typically increases with temperature, as elevated temperatures supply additional energy to charge carriers, thereby improving their mobility. The activation energy for charge carriers in Mg/Bento composites is lower than that in pure bentonite. Incorporating magnesium nanoparticles reduces the energy barrier for charge mobility by establishing conductive pathways. This enhancement in conductivity is temperature-dependent, thereby improving overall electrical performance. Fig. 6 depicts the conductivity, while Fig. 5 illustrates the dielectric loss. The Mg/Bento composite and pure bentonite demonstrate a frequency-dependent dielectric loss. However, Mg/Bento exhibits lower dielectric loss at elevated temperatures than pure bentonite. This is likely due to the presence of magnesium, which enhances charge transport, resulting in higher conductivity with reduced energy loss. Additionally, magnesium nanoparticles may decrease resistive impedance by improving charge mobility. However, the data exhibits some irregularity at 120°C, which could be attributed to structural changes or other temperature-dependent factors that influence the charge carrier dynamics differently at this temperature.

It is worth stressing that the incorporation of magnesium nanoparticles significantly enhances the temperature-dependent conductivity of the Mg/bentonite composite compared to pure bentonite. This improvement stems from better charge transport mechanisms and reduced activation energy for carrier mobility. The Mg nanoparticles create additional pathways for charge conduction, lowering energy barriers for hopping or tunneling between localized states. Moreover, they likely increase polarization at the composite interfaces, aligning with the Maxwell-Wagner polarization model, which suggests that charge accumulation occurs at these interfaces. As thermal energy rises, charge carrier mobility improves, enhancing conductivity. In contrast, pure bentonite has limited conductivity due to fewer conduction pathways and higher activation energy for charge transport.

3.4.2 The electric modulus

The electric modulus behavior (real, M′, and imaginary, M′′) offers insights into the relaxation dynamics and conduction mechanisms of materials. The real component of the electric modulus, M′, typically exhibits an increase in frequency for both Bento and Mg/Bento. Fig. 7 illustrates that an increase in frequency results in a decrease in dielectric permittivity, which corresponds to the observed rise in M′.

Close

Fig. 7.

Depicts the variations in the real component of the electric modulus for both Bento and Mg-incorporated Bento samples over various frequencies and temperatures: (a) 25°C, (b) 50°C, (c) 75°C, (d) 100°C, and (e) 120°C.

Export to PPT

Fig. 8 shows the imaginary electric modulus M′′ versus frequency for Mg/Bento samples. The data reveals a trend where M′′ decreases as temperature rises, with a notable convergence toward zero at lower frequencies. This trend signifies the elimination of electrode polarization in the samples due to the thermally activated nature of the dielectric constant. Moreover, M’’ increases with frequency for both samples, likely forming a broad peak at higher frequencies. This behavior suggests a relaxation process associated with charge movement, particularly at the interface between the Mg nanoparticles and the bentonite. This interface polarization is more significant for Mg/Bento samples, making the peak structure more noticeable compared to pure Bento. The broad peak observed in the imaginary modulus of Mg/Bento at higher frequencies may be linked to a relaxation mechanism, particularly charge carrier hopping. Mg nanoparticles enhance this hopping mechanism by generating conductive sites that facilitate the movement of charges. As a result, this relaxation process becomes more pronounced compared to pure bentonite. As the temperature rises, M’’ values for both samples become more spread out, suggesting that the relaxation processes are more temperature-dependent, causing increased variation in the charge dynamics (Alotaibi et al., 2020).

Close

Fig. 8.

Illustrates the changes in the imaginary component of the electric modulus for pure Bento and Mg/Bento specimens across various frequencies and temperatures: (a) 25°C, (b) 50°C, (c) 75°C, (d) 100°C, and (e) 120°C.

Export to PPT

3.5 Tight binding calculation

The evaluation of the energies associated with HOMO and LUMO enables accurate predictions in the examination of charge transfer, chemical reactivity, bioactivity, and compound stability (Abdel-Baset and Hassen 2016). Fig. 9 presents the iso-surfaces of the HOMO and LUMO for both the Bento and the Mg/Bento compounds, derived from their geometrically optimized structures. The localization of LUMO within the nanoclusters of the Mg/Bento structure could suggest its chemical stability. According to Table 1, the calculated HOMO energy level for pure bentonite is -10.4376 eV. The HOMO energy level for Mg/Bento exhibits a modest increase, reaching -9.1189 eV.

Close

Fig. 9.

The geometrically optimized Bento (left panels) and Mg/Bento (right panels) structures. The HOMO and LUMO iso-surfaces (yellow and blue iso-surfaces with -0.01 and 0.01 au levels, respectively). HOMO: Highest occupied molecular orbital, LUMO: Lowest unoccupied molecular orbital.

Export to PPT

In assessing kinetic stability, the HOMO-LUMO energy gap (E_LUMO – E_HOMO) serves as a valuable metric, with E_HOMO and E_LUMO denoting the energies of the highest occupied molecular orbital and the lowest unoccupied molecular orbital, respectively.

An increased energy gap signifies that a greater amount of energy is necessary for the excitation of electrons. The comparison of energy gaps between Bento and Mg/Bento reveals that the introduction of Mg has resulted in a reduction of the band gap of Bento (1.6260 eV), which may have implications for its conductivity and electronic characteristics. Significantly, the Mg/Bento exhibits an energy gap of 0.4312 eV, indicating its promising potential for applications in conductivity. Chemical hardness, defined as η = – (E_HOMO – E_LUMO)/2, serves as a significant parameter that indicates a structure’s resilience against deformation. An increased HOMO-LUMO energy gap signifies enhanced hardness, reflecting superior stability and greater resistance to alterations in electronic distribution or polarization. Table 1 indicates a reduction in the chemical hardness of Mg/Bento to 0.2156 eV in comparison to that of Bento, implying diminished stability and an increased tendency to accept electrons. Moreover, the negative binding energy observed in the Mg/Bento samples substantiates the stability of the Mg nanocomposites on the Bento surface.

3.6 Charge-density analysis

The charge density difference method is a crucial tool for elucidating the electronic interactions at the interface of nanocluster-supported systems. By providing detailed insights into the redistribution of electronic charge, this method helps understand how interfacial phenomena influence the overall properties of the composite material. This approach is particularly relevant in the context of Mg nanoclusters supported on Bento, where nonbonding interactions play a significant role. The Maxwell–Wagner effect effectively explains the pronounced dependence of dielectric permittivity on frequency, as revealed by experimental dielectric measurements of the Mg/Bento system (Barsoukov and MacDonald 2018). The charge density difference analysis provides a deeper understanding of this phenomenon. The calculated charge density difference reveals a significant accumulation of negative charge at the interface between the Mg nanocluster and the Bento, as illustrated in Fig. 10. The Maxwell–Wagner effect explains this concentration of negative charge density, which is consistent with the experimental findings. The nonbonding nature of the interaction between the Mg nanocluster and the Bento surface suggests that the charge redistribution is primarily due to electrostatic interactions and polarization effects rather than chemical bonding. The Bento surface, composed of Si-O tetrahedral and Al-O octahedral sheets, interacts with the Mg nanocluster through van der Waals forces and electrostatic interactions.

Close

Fig. 10.

(a) Calculated relaxed Mg/Bento structure. Charge density 2D plots across the (b) Mg nanocluster region, (c) the Bento region, and (d) the Mg-Bento interface region.

Export to PPT

These nonbonding interactions lead to electronic charge redistribution, with negative charge density concentrating at the interface. This interfacial charge accumulation enhances the interfacial polarization, contributing significantly to the dielectric relaxation observed experimentally.The agreement between the theoretical charge density distribution and the experimental dielectric measurements underscores the importance of considering interfacial effects in understanding the dielectric properties of nanocluster-supported systems. As shown in Fig. 8, the charge density difference method highlights the concentration of negative charge density at the Mg nanocluster-Bento interface, consistent with the experimental dielectric data explained by the Maxwell–Wagner effect. This finding underscores the importance of interfacial charge in determining the dielectric properties.

It suggests that nonbonding interactions play a crucial role in the electronic behavior of the Mg/Bento system. Understanding these interfacial phenomena is essential for designing and applying nanocomposites with tailored electronic and dielectric properties.