Development of a hybrid material based on Zn-Mg spinel ferrites covered with polypyrrole: structural, morphological, and electrical characteristics

In this study, we synthesized a range of spinel ferrite ZnxMg 1-x Fe 2 O 4 (x = 0, 0.3) forms using the sol-gel method. Additionally, polypyrrole nanosphere supported by magnesium and zinc ferrites (ZnxMg 1-x Fe 2 O 4 @PPy) were synthesized through in-situ polymerisation. The surfaces of Zn x Mg 1-x Fe 2 O 4 (x = 0, 0.3) and Zn x Mg 1-x Fe 2 O 4 @PPy were subjected to spectroscopic analysis. The data obtained from X-ray diffraction (XRD) confirmed the formation of Zn-Mg spinel ferrite. The electrochemical analysis results demonstrated that the Zn 0.3 Mg 0.7 Fe 2 O 4 @PPy nanocomposites exhibited enhanced redox activity and a higher current intensity. This study highlights the development of hybrid Zn 0.3 Mg 0.7 Fe 2 O 4 @PPy type nanospheres with substantial and effective electrochemical properties, which could be a suitable option for industrial applications.


Introduction
The investigation of hybrid materials has emerged as a prominent area of scientific inquiry, driven by the imperative to identify novel materials with enhanced efficiency.These hybrid materials offer a promising approach, exhibiting superior adjustable properties, including surface modification and enhanced stability, in comparison to their individual components. 1 Spinel ferrites are ferrimagnetic materials that exhibit high resistivity, good magnetization, and excellent chemical stability. 2They also exhibit interesting dielectric and magnetic properties, which render them particularly effective microwave absorbers. 3,4The unit cell of ferrite is constituted by 32 oxygen anions arranged in a cubic pattern, with metal cations distributed in tetrahedral A and octahedral B sites. 5 The properties of these ferrite crystals are dependent upon the types of cations and their distribution in sites A and B. 6 Spinel ferrites represent an excellent option for the syntheses of hybrid materials.The synthesis of ferrite nanoparticles can be achieved through a range of methods, including sol-gel 7 , co-precipitation 8 , sonochemistry 9 , and hydrothermal 10 .Spinel ferrites are particularly well-suited for the elaboration of hybrid materials.However, the electronic conductivity of bare spinel ferrite tends to be relatively low.This efficiency could be enhanced through the introduction of substitutions or combinations.The formation of binary hybrids with improved electrochemical properties is achievable through the combination of spinel ferrites with polymers.This is a principal attribute required for the conduction in polymers.Spinel ferrites have been combined with polypyrrole (PPy) to create nanocomposites.These nanocomposites have a granular structure and exhibit both magnetic and dielectric properties 11 .To illustrate, the in-situ polymerization method has been shown to yield hybrid MgFe₂O₄/polypyrrole materials that consistently demonstrate excellent electrochemical performance. 12Moreover, the study investigates the fabrication of PPy coatings on hollow MgFe₂O₄ double-shell nanospheres with the objective of developing electronic cells and enhancing conductivity and cycling stability.The mixed ferrite spinels of MgFe₂O₄ when combined with PPy display low coercivity and saturation magnetization values, which suggests their potential utilization in transformer cores and switching applications.The objective of the research is to investigate the impact of doping spinel ferrite with zinc (Zn) on its potential for utilization in industrial applications.Nevertheless, the introduction of zinc ions into the spinel ferrite structure does not result in any alteration to the electrochemical behavior.Furthermore, the incorporation of Zn results in the displacement of Fe³⁺ from the tetrahedral sites to the octahedral sites, thereby enhancing the electrical performance of the resulting compounds 12 .Consequently, a variety of ferrite-doped spinel-based polypyrrole nanocomposites are extensively prepared.Nevertheless, research on polypyrrole-coated Zn-Mg ferrite spinel-like composites can be considered to be relatively limited.
This study presents a two-step procedure for synthesising new polymers based on Zn-Mg spinel ferrite (Zn x Mg 1-x Fe 2 O 4 ) coated with polypyrrole.In the initial stage of the procedure, spinel ferrites Zn x Mg 1-x Fe 2 O 4 (x = 0, 0.3) were synthesised using the sol-gel method.In the second step, polypyrrole (PPy) was generated on the surface of the Zn x Mg 1-x Fe 2 O 4 spinel ferrite (Zn x Mg 1- x Fe 2 O 4 @PPy) through in-situ pyrrole polymerization.A comprehensive investigation was conducted using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and Fourier transform infrared (FT-IR) analyses to examine the structure, morphology, and electrochemical behavior of Zn x Mg 1-x Fe 2 O 4 @PPy in order to provide a detailed understanding of these properties.(solid), Pyrrole (98% liquid), Ethanol (99%, liquid) were used.All products were supplied by Sigma Aldrich.Fourier transform infrared spectroscopy (FT-IR) was performed using a Magna 860 FT-IR thermoelectronic spectrometer with dry compound samples in KBr pellets.The FT-IR was used to identify the vibrations of the different bonds as well as the compounds structures.The obtained powder phases were identified using an X-ray diffractometer (D8 Discover diffractometer in Bragg Brentano geometry equipped with a Cu Sealed tube λkα1 = 1.54059Å, a quartz front monochromator, a motorized anti-scatter screen, and an energy-resolved Lynxeye XE-T linear detector).Furthermore, the width of the peaks and the size of the crystallites were determined using the aforementioned apparatus.The morphology of the powders was studied using scanning electron microscopy (SEM) at varying magnifications.The SEM images were recorded using a JOEL 6700F microscope, which features a Field Emission Gun.The images were obtained using an applied voltage of 3 kV.The qualitative composition analysis of the synthesised compounds was performed using energy-dispersive spectroscopy (EDS) characterization.The EDS analysis was conducted at an accelerating voltage of 15 kV using a Bruker Advance apparatus.

General procedure
The stoichiometric proportions of zinc sulfate heptahydrate (ZnSO₄.7H₂O),iron sulfate heptahydrate (FeSO₄.7H₂O),magnesium sulfate heptahydrate (MgSO₄.7H₂O),and citric acid monohydrate (C₆H₈O₇.H₂O) in a 1:2:3 ratio are dissolved in ethanol (150 mL).The homogeneous solution was subjected to thermal agitation at temperatures between 70 and 85 degrees Celsius for a period of approximately four hours, resulting in the formation of a viscous liquid (gel).Subsequently, the gel is subjected to a 16-hour drying process at 100 °C in an oven to eliminate any residual moisture.The resulting precursor is then ground and subjected to calcination in an electric furnace at 500 °C for four hours in the presence of air, with a heating rate of 5 °C/min.(2) Scheme 1 provides a summary of the procedure for synthesising polypyrrole-based nanocomposites and spinel ferrite.This was achieved by separately dissolving 4 g of Zn₀.₃Mg₀.₇Fe₂O₄and 4 g of MgFe₂O₄ in 50 mL of distilled water.Subsequently, the solutions were subjected to magnetic stirring for a period of one hour.Following a 20-minute period of stirring at room temperature, 4 g of FeCl₃ was added to each dispersion.Both solutions were stirred for a period of six hours, after which the resulting black solutions were filtered and then dried in an oven at 100 °C for 12 hours in order to remove any remaining water molecules.The process yielded the final composites, Zn₀.₃Mg₀.₇Fe₂O₄@PPyand MgFe₂O₄@PPy.

Electrochemical characterization of hybrid material
In order to investigate the electrochemical performance, the synthesised materials were deposited onto copper plate electrodes measuring 1 cm x 1 cm.The electroactivity of the synthesised materials was determined through the utilization of cyclic voltammetry.A homogeneous mixture was obtained by grinding 95% of the synthesised compounds, which served as the active material, with 5% Teflon binder in a mortar.The resulting mixture was then dispersed in a solvent (water + ethanol) to form a slurry, which was coated onto a copper plate and dried at 60 °C.Electrochemical studies, including cyclic voltammetry, were conducted using the Metrohm DropSens electrochemical workstation.An electrochemical cell was constructed with a fabricated electrode acting as the working electrode, platinum as the counter electrode, and Ag/AgCl as the reference electrode, with 1M KOH electrolyte.The active surface area of the two electrodes was calculated using the Randles-Sevcik equation (3). 13  = 2.69.
Ip is the peak anodic current, A is the Active surface, D is the diffusion coefficient of the species involved in redox reactions, , Co is the concentration of redox probe, V the Scan rate, N is the number of electrons transferred in the redox reaction , D is the diffusion coefficient (D= 1.3 × 10 −7 cm/s 2 at 25°C).
Ip represents the peak anodic current, A denotes the active surface, D corresponds to the diffusion coefficient of the species engaged in redox reactions (D= 1.3 × 10⁻⁷ cm/s² at 25°C), C 0 is the concentration of the redox probe, V is the scan rate, n is the number of electrons involved in the redox reaction.

FT-IR analysis
FT-IR spectroscopy is used to identify the functional groups present in the ferrite compounds and the nanocomposites formed.The wave number range used is from 4000 to 400 cm - 1 .Figure 1 illustrates the FT-IR spectra of MgFe₂O₄ and Zn₀.₃Mg₀.₇Fe₂O₄,as well as the polymerized forms MgFe₂O₄@PPy and Zn₀.₃Mg₀.₇Fe₂O₄@PPy.The FT-IR spectra display bands below 1000 cm⁻¹ that are attributed to M-O (metal-oxygen) bonds.The presence of elongation vibration peaks at 424 and 570 cm -1 is indicative of the presence of Mg-O and Fe-O bonds, respectively. 14The absorption band observed at approximately 557 cm -1 may be attributed to the stretching vibrations of the Zn-O and Fe-O bonds in a tetrahedral configuration. 15,16The bands observed at 430-470 cm⁻¹ are indicative of the presence of Fe-O bonds in octahedral positions.This evidence corroborates the hypothesis that the spinel structure contains both tetrahedral and octahedral ligand sites. 17,18The broad band absorption peak observed at 3400-3500 cm -1 is attributed to the presence of O-H groups. 19The bands situated between 1630 and 1650 cm -1 are attributed to C-O elongation. 20The peak observed at 1554 cm -1 corresponds to the C-C stretching vibration of the polypyrrole. 21he peaks observed at approximately 1050 cm -1 are attributed to the in-plane deformation of the C-H and N-H bonds of the pyrrole ring.The data corroborate the formation of the synthesised compounds and the reverse or mixed oxide structure, as well as the observations made through the EDS.The presence of characteristic bands of PPy and oxides (MgFe₂O₄, Zn₀.₃Mg₀.₇Fe₂O₄),along with a slight shift in these bands in the composite, provides confirmation of the interfacial interaction between PPy and the oxides that occurs during composite formation.This interaction may be attributed to the binding of oxygen ions from the oxides with nitrogen atoms from the PPy macromolecules.

EDS analysis
The qualitative composition of the samples was analyzed using EDS characterization.Figures 2, 3, 4 and 5 illustrate the EDS spectra of the synthesised compounds.The presented spectra indicate the presence of the various elements that constitute the compounds, along with their respective percentages.The composite EDS analysis revealed the presence of characteristic peaks for the primary elements analysed, including oxygen (O), magnesium (Mg), iron (Fe), zinc (Zn), nitrogen (N), carbon (C), and chlorine (Cl), among others 23 .The presence of polypyrrole in the nanocomposites is corroborated by the N and Cl peaks observed in Figures 5 and 6.Furthermore, the data indicates the formation of both spinel oxides and nanocomposites.

SEM analysis
Scanning electron microscopy (SEM) was used to analyze the surface morphology and elemental composition of the prepared samples.Fig. 7 illustrates the morphology of spinel powders synthesized with and without polymerization.The observation reveals a uniform distribution of grains with varying shapes (spherical and elliptical) and sizes.The presence of agglomerated grains in photographs A and B is attributed to the ferromagnetism of the ferrite MgFe₂O₄ and Zn₀.₃Mg₀.₇Fe₂O₄,,respectively, as well as the existence of porous cavities at the nanometric scale. 30SEM images of MgFe 2 O 4 and Zn 0.3 Mg 0.7 Fe 2 O 4 coated with PPy are shown in photographs C and D, respectively.The coatings were formed through oxidative polymerization.The SEM images provide further evidence that the nanospheres are homogeneous and can be individually identified with a nanoscale width.As illustrated in the figures, the SEM images of PPy on MgFe₂O₄ and on Zn₀.₃Mg₀.₇Fe₂O₄indicate that the arrangement of the ferrite spinels remains unaltered.Instead, the PPy gradually wraps around the spinels in a sequential manner.It is notable that the PPy deposit is confined to the spherical walls, resulting in strong interactions between the PPy and the ferrite spinels.

Electrochemical characterization of the different synthesized compounds
Figure 8 provides an illustration of the electrochemical response (CV) of electrodes prepared with the aforementioned compounds.The cyclic voltammetry (CV) was recorded at a scan rate of 50 mV/s and between the potentials of -1.2 and 0.6 V. Figures 8a and 8b illustrate voltammograms with a cathodic peak at approximately -0.6 V and -0.95 V, respectively.This peak may be associated with the two-step reduction of Fe³⁺ to Fe.Furthermore, an anodic peak at approximately -0.35 V (see Figures 8a and 8b) may be attributed to the reversible oxidation of Fe/Fe 3+ . 31,32 he reduction peak at -0.38 V correlates with the reduction of MgFe₂O₄ to MgO 34 .Additionally, doping Mg with Zn has been observed to enhance its electrical properties in comparison to MgFe₂O₄ (Figure 8b). 34The CV curves display disparate trends when ferrite spinels are fully covered with PPy, as illustrated in Figures 8c and 8d.Furthermore, the current peak values have increased by 0.35.Both the MgFe 2 O 4 @PPy and Zn 0.3 Mg 0.7 Fe 2 O 4 @PPy compounds exhibit two oxidation peaks at approximately -3.24 V and 1.74 V.The shift in oxidation peaks in comparison to the ferrite spinels indicates the presence of a PPy coating on MgFe₂O₄ and Zn₀.₃Mg₀.₇Fe₂O₄ 36.When the ferrite spinels are completely covered with PPy, two reduction peaks emerge at approximately -544 and -1016 mV for MgFe₂O₄@PPy and at approximately -554 and -1098 mV for Zn₀.₃Mg₀.₇Fe₂O₄@PPy,respectively (see Figures 8c and 8d).Furthermore, Table 1 illustrates that the hybrid materials MgFe₂O₄@PPy and Zn₀.₃Mg₀.₇Fe₂O₄@PPyexhibit higher intensities of oxidation and reduction peaks, as well as greater active surface areas.The results indicate that Zn 0.3 Mg 0.7 Fe 2 O 4 @PPy has the most favorable electrochemical properties, which can be attributed to the Zn doping in the ferrite matrix (as illustrated in Figure 8d).This indicates the formation of polypyrrole around the ferrite spinels MgFe₂O₄ and Zn₀.₃Mg₀.₇Fe₂O₄.Furthermore, the deposition of the PPy layer enhanced the conductivity of the ferrite spinels, thereby facilitating the formation of PPy nanospheres.The influence of the scan rate on the 1M KOH electrolyte was examined.Cyclic voltammetry data were collected for a series of electrode samples of varying compounds at scan rates between 30 and 80 mV/s, with an increment of 10 mV/s (see figures A, B, C, and D).The measurements were conducted within the potential range of -1.2 to 0.6 volts with respect to a silver/silver chloride reference electrode.The CV curves indicate a gradual increase in current under the curve with an increase in scan rate, which suggests that the voltammetric current is directly proportional to the scan rate.Furthermore, the peak current is observed to increase with the scan rate, indicating that the rates of electronic and ionic transport are sufficiently rapid at the applied scan rates. 37It can be observed that an increase in the scan rate does not result in a notable alteration to the shape of the cyclic voltammetry curves.This suggests that there is an enhancement in electron conduction within the host materials.Furthermore, elevated scan rates result in a shift of the anodic and cathodic peaks towards higher and lower potentials, respectively. 35In figures 9, 10, 11 and 12, the peaks labelled A', B' and C' represent the three reduction processes.The oxidation peaks A and B are in correspondence with the reduction peaks A' and B'.It can be observed that the oxidation peaks A and B are reversible reaction processes, while the reduction peak C is an irreversible process. 38,33The relationship between the anodic and cathodic peaks demonstrates a linear increase with the square root of the sweep speed, indicating that the redox reactions of different composites are occurring rapidly in the electrodes.Consequently, the electrochemical reactions of the various peaks can be classified as diffusion-controlled reactions. 39

Conclusion
Spinel oxides were synthesised via the sol-gel method and subsequently polymerised through chemical oxidative polymerization, resulting in the formation of nanocomposites.The results of the X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR) spectroscopic methods provided confirmation of the formation of oxides and nanocomposites.The XRD and FT-IR results indicate that all the spinels possess a cubic structure.Furthermore, the interaction between the oxide and pyrrole is based on the formation of hydrogen bonds and electrostatic interactions between the different entities.
The application of scanning electron microscopy (SEM) revealed a uniform distribution of the nanocomposites.The EDS analysis corroborated the presence of the characteristic peaks of these compounds and confirmed the observed 1:2 ratio between Mg and Fe.Voltammograms demonstrated that the polymerised forms exhibited enhanced conductivity compared to the mixed oxides alone.This work illustrates the significance of polymers in enhancing the conductivity of ferrite spinels without modifying their configuration.Furthermore, the possibility of constructing hybrid materials through the interaction between PPy and ferrite spinels is demonstrated.These ferrite spinels can be employed as a model for the in-situ electrodeposition of polypyrrole nanocomposites, which results in the formation of polymers with notable electrochemical properties.

Table 1 .
Kinetic characterization of cyclic voltammetry of different composites at 50 mV/s scan rate.