Structural, Electrochemical, and Supercapacitor Characterization of Double Metal Oxides Doped Within ZIF-8 Composites

Abstract

This study investigates the application of double metal oxide zinc ferrite (ZnFe₂O₃) doped within zeolitic imidazolate framework-8 (ZIF-8) composites for structural, electrochemical, and supercapacitor characterization. The structural characterization has been carried out using XRD, FTIR, DTA, UV-VIS, and HRTEM. By incorporating ZnFe₂O₃, significant enhancements in the structural integrity and morphology of the ZIF-8 matrices have been achieved, with a decrease in the average crystallite size by about 23%. At 500 °C, the DTA analysis indicated that the weight loss associated with ZnFe₂O₃ decreased by approximately 5%. The estimated E_g values are 3.08 eV and 3.28 eV for ZIF-8 and ZIF-8@ZnFe₂O₃, respectively. Regarding the electrochemical performance of the ZIF-8@ZnFe₂O₃, the anodic peak current density is approximately 0.0025 A (at around 0.5 V), at a scan rate of 10 mV/s. The peak current values increase more rapidly, by about 41%, with increasing scan rate when ZnFe₂O₃ is present, indicating a synergistic effect between the ZIF-8 and ZnFe₂O₃ components. The high observed current density peak at 0.03 V can be attributed to the Fe²⁺/Fe³⁺ redox couple, facilitated by the ZnFe₂O₃ component. The ZnFe₂O₃ addition enhances the electrochemical activity of ZIF-8, leading to increased peak current values at various scan rates. This suggests that the ZnFe₂O₃ may facilitate charge transfer or improve the conductivity of the material.

1. Introduction

Metal–organic frameworks (MOFs) have emerged as a transformative class of materials due to their unique structural characteristics and multifunctionality [1,2]. Comprising metal nodes coordinated by organic linkers, MOFs exhibit exceptional porosity and tunable chemical environments, making them highly attractive for applications in gas storage, catalysis, and drug delivery [3]. Among these, zeolitic imidazolate frameworks (ZIFs) have gained prominence, particularly ZIF-8, known for its enhanced thermal stability and chemical robustness, which enables it to withstand harsh operating conditions while maintaining a framework structure [4]. The ability to systematically modify the composition and architecture of ZIFs opens new avenues for developing advanced material systems tailored for specific technological needs [5].

Incorporating double metal oxides, such as zinc ferrite (ZnFe₂O₃), into MOF matrices can significantly advance structural and functional properties [6,7]. ZnFe₂O₃ is particularly noteworthy due to its favorable electronic and magnetic properties, as well as its capability to absorb light across a broader spectrum, which is critically important for applications in photocatalysis and energy conversion [8]. The strategic doping of ZnFe₂O₃ into ZIF-8 improves its structural integrity and enhances its optical properties, paving the way for superior light-harvesting capabilities and energy transfer mechanisms [9].

The incorporation of double metal oxides into zeolitic imidazolate framework-8 (ZIF-8) matrices has shown significant promise in enhancing electrochemical and supercapacitor properties. Doping ZIF-8 with double metal oxides, such as zinc ferrite (ZnFe₂O₃), not only improves the structural stability and conductivity of the composite but also introduces redox-active sites that facilitate charge transfer processes [10]. This synergy enhances the overall electrochemical performance, resulting in higher specific capacitance and improved energy density [11]. The distinctive porosity of ZIF-8 facilitates efficient ion diffusion, while the inclusion of metal oxides enhances the electroactive surface area, optimizing the mechanisms of charge storage. As a result, these doped composites demonstrate exceptional performance in supercapacitor applications, featuring rapid charge and discharge capabilities along with remarkable cycling stability [12]. With targeted modifications, ZnFe₂O₃-doped ZIF-8 composites can function as advanced and effective energy storage materials, potentially reconciling the need for high energy density with rapid power delivery in electrochemical systems [13].

In this research, the synthesis and characterization of ZnFe₂O₃-doped ZIF-8 composites have been explored at varying concentrations of dopant. By systematically exploring their structural, optical, and electrochemical properties, we aim to unveil the synergistic effects of this dual-component system, ultimately contributing to the advancement of materials designed for enhanced photonic and energy storage applications. This study not only emphasizes the significance of the incorporated metal oxide but also addresses the potential for innovative applications, extending the functional horizons of MOF-based materials.

2. Experimental Techniques

Advanced experimental equipment was used to measure the structural properties of ZnFe₂O₃ and the doped ZIF-8 composites. The crystal structure and size were investigated using X-ray diffraction (XRD, D8 Advance X-ray Diffractometer, Bruker, Billerica, MA, USA). The additional functional group, which appeared due to the double metal oxide doping, was investigated using Fourier Transform Infrared Spectroscopy (FTIR, Vertex 70 FTIR Spectrometer, Bruker, Karlsruhe, Germany). The Quantachrome Instrument provided thermal analysis for the prepared sample, while electrochemical analysis was conducted by a potentiostat Ametek, NOVA, Northern Virginia, VA, USA.

3. Sample Fabrication

The chemical used in this research was purchased from Sigma Aldrich. ZnFe₂O₃-doped ZIF-8 composites can be synthesized using a solvothermal method, which provides a controlled environment for uniform particle formation. The synthesis process was performed as follows: first, zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and 2-methylimidazole (2-MIM) are dissolved in deionized water. The molar ratio of zinc to 2-MIM is maintained at 1:4 to favor ZIF-8 formation. For ZnFe₂O₃ doping, iron (III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O) is added to achieve the desired weight percentage, specifically 5 wt% of ZnFe₂O₃ in the final composite. The precursor solution is transferred into a stainless-steel autoclave and heated at 120 °C for 24 h to promote crystallization. After cooling to room temperature, the resultant solid is filtered, washed with deionized water and ethanol, and then dried in an oven at 60 °C for several hours. Finally, the ZnFe₂O₃-doped ZIF-8 composites are synthesized, and thin films of the material can be developed using a vapor deposition technique. Substrates from glass and quartz wafers are cleaned thoroughly to remove any contaminants that could affect film adhesion and quality. The unit is evacuated to achieve a low-pressure environment, and the material is heated to a temperature that enables sublimation or thermal evaporation of the ZIF-8 powder. As the vapor cools, it condenses onto the substrate, forming a uniform, thin film of ZnFe₂O₃-doped ZIF-8. The thickness of the film can be controlled by adjusting the deposition time and parameters. Scheme 1 shows the graphical abstract of our experiment.

To evaluate the electrochemical performance of the doped ZIF-8 composites, a glassy carbon electrode (GCE) is prepared. The glassy carbon electrode is polished with alumina slurry (1 µm) to achieve a smooth surface, followed by rinsing with deionized water and drying. In total, 5 mg of the synthesized ZnFe₂O₃-doped ZIF-8 powder is mixed with a few drops of polyvinyl chloride (PVC) and a small quantity of ethanol in which the slurry is sonicated to ensure uniform dispersion. The prepared GCE is then used for cyclic voltammetry (CV) measurements. The electrochemical behavior of the ZnFe₂O₃-doped ZIF-8 material can be assessed in suitable electrolyte solutions, allowing for the evaluation of its charge storage capacity and electrochemical kinetics.

4. Results and Discussion

4.1. XRD

The X-ray diffraction (XRD) patterns of ZIF-8 and ZIF-8/ZnFe₂O₃ are presented in Figure 1. The XRD pattern of pure ZIF-8 shows a series of characteristic peaks at 2θ values of around 7.3°, 12.7°, 32.5°, and 36.8°, which correspond to the (011), (002), (222), and (112) planes, respectively [14]. These peaks are in good agreement with the reported pattern for ZIF-8 [10], indicating the successful synthesis of the material. Upon doping with 5 wt% ZnFe₂O₃, the XRD pattern of ZIF-8/ZnFe₂O₃ shows some significant changes. The intensity of the peaks corresponding to the (002) and (112) planes decreases, while the peak corresponding to the (011) plane becomes more pronounced [15]. Additionally, a new peak appears at around 57.5° and 64.3°, which can be attributed to the presence of ZnFe₂O₃ nanoparticles. The broader peaks in the XRD pattern of ZIF-8/ZnFe₂O₃ suggest a smaller crystallite size, which may be due to the incorporation of ZnFe₂O₃ nanoparticles. This is in agreement with previous reports, which have shown that the addition of metal oxide nanoparticles can lead to a decrease in the crystallite size of MOFs [16].

The analysis of the ZIF-8@ZnFe₂O₃ composite reveals several noteworthy characteristics compared to pure ZIF-8. First, the peaks in the ZIF-8@ZnFe₂O₃ spectrum are broader, suggesting either a smaller crystallite size or increased disorder within the composite material. Additionally, the peak positions shift slightly to lower 2θ values, indicating a potential expansion of the unit cell; for instance, the (011) peak appears at approximately 7.4° in ZIF-8@ZnFe₂O₃, compared to 7.6° in ZIF-8. Furthermore, a new peak at around 57.5° in the ZIF-8@ZnFe₂O₃ spectrum, absent in the ZIF-8 spectrum, may indicate the presence of ZnFe₂O₃, reflecting some degree of crystallinity in the composite. The average crystallite size of ZIF-8 is estimated to be about 32 nm, while that of ZIF-8@ZnFe₂O₃ is slightly smaller, around 26 nm.

Table 1. XRD analysis of the synthesized ZIF-8/ZnFe₂O₃ compound.

No.	Compound	2θ°	D-Spacing [Å]	a, b, c	h, k, l	(Crystallite Size) D, (nm)	Lattice Strain (nm)
1	ZIF-8/ZnFe2O3	12.7°	3.70	a = 8.33 b = 7.86 c = 6.91	(002)	17.094	0.00142
2		32.5°	3.66		(222)	32.065	0.00374
3		36.8°	3.95		(112)	29.411	0.00264
4		57.5°	2.83		(440)	21.521	0.00612
5		64.3°	3.91		(442)	41.341	0.00180

shows the XRD analysis of the synthesized ZIF-8/ZnFe₂O₃ compound. As we can see from the table, the average lattice strain of 0.0031 indicates a relatively low level of distortion within the crystal lattice. This low strain could contribute to maintaining the functionality of the ZIF-8 structure, allowing it to retain its characteristic properties while incorporating the metal oxide. Regarding the lattice parameters, with values of a = 16.33 Å, b = 16.86 Å, and c = 14.91 Å, it appears that there may be some expansion in the unit cell dimensions compared to pure ZIF-8. This expansion might be attributed to the incorporation of ZnFe₂O₃, which alters the coordination environment around the ZIF-8 framework.

4.2. Thermal Analysis

The Differential Thermal Analysis (DTA) curve in Figure 2 shows the thermal behavior of ZIF-8 and ZIF-8/ZnFe₂O₃. The DTA curve indicates that the doped sample exhibits improved thermal stability compared to the pristine ZIF-8. The endothermic peak around 500 °C suggests that the doped sample undergoes a phase transition, which might be related to the decomposition of the metal–organic framework. The TGA (Thermogravimetric Analysis) curve (onset Figure 2) shows the weight loss of ZIF-8 and ZIF-8/ZnFe₂O₃ as a function of temperature. The doped sample exhibits a slower weight loss rate, indicating that the incorporation of ZnFe₂O₃ enhances the thermal stability of ZIF-8.

4.3. HRTEM

The Transmission Electron Microscopy (TEM) images in Figure 3a,b reveal the morphology and structure of ZIF-8 and ZIF-8/ZnFe₂O₃. The doped sample shows a more uniform particle size distribution and a higher crystallinity compared to the pristine ZIF-8. The incorporation of ZnFe₂O₃ might have led to the formation of smaller crystallites, which could enhance the surface area and porosity of the material. The TEM images suggest that the incorporation of ZnFe₂O₃ affects the crystallite size and morphology of ZIF-8. This could be related to the nucleation and growth mechanisms of ZIF-8 in the presence of ZnFe₂O₃ nanoparticles. The smaller crystallite size in the doped sample might be beneficial for applications that require high surface areas and porosity, such as catalysis or gas adsorption.

4.4. FTIR

Figure 4 illustrates the Fourier Transform Infrared (FTIR) spectra of ZIF-8 and the composite ZIF-8@ZnFe₂O₃. The FTIR of ZIF-8 and ZIF-8/ZnFe₂O₃ exhibit a prominent peak around 3500 cm⁻¹, which is attributed to the stretching vibrations of the (-OH) functional groups. Notably, the peak observed at 1000 cm⁻¹ in ZIF-8 shifts to approximately 900 cm⁻¹ in the composites, indicating alterations in the framework vibrations [16,17]. The spectra exhibit distinct peaks characteristic of ZIF-8, likely linked to framework vibrations and functional groups. Regarding ZIF-8@ZnFe₂O₃, the first additional peak appears at around 760 cm⁻¹. This peak can be attributed to the Fe-O stretching vibrations in the ZnFe₂O₃ nanoparticles. Incorporating ZnFe₂O₃ into ZIF-8 leads to the formation of metal–oxygen bonds, resulting in this new peak. The second additional peak appears at around 1630 cm⁻¹, which can be assigned to the C=O stretching vibrations of the 2-methylimidazole linkers in ZIF-8. Finally, a peak around 3500 cm⁻¹ may occur because the water molecules can give rise to O-H stretching vibrations [18,19]. The incorporation of ZnFe₂O₃ could improve charge transfer and increase the stability of the composite, making it a promising candidate for advanced materials in energy conversion and storage technologies.

4.5. Energy Gap

The energy gap calculation for ZIF-8 and ZIF-8@ZnFe₂O₃ are introduced in Figure 5. This figure depicts the Tauc’s plots used to estimate the energy gap (E_g) of the materials. As we can see from the figure, the calculated E_g values are 3.08 eV for ZIF-8 and 3.28 eV for the ZIF-8@ZnFe₂O₃ composite. For ZIF-8, the peaks in the UV region can be attributed to the electronic transitions associated with the metal–organic framework’s (MOF) ligands, while additional peaks noted in the ZIF-8@ZnFe₂O₃ composite may result from the presence of ZnFe₂O₃, leading to new energy levels within the bandgap. This incorporation can create defect states, which may contribute to enhanced light absorption and influence carrier dynamics in the material. The observed increase in the energy gap for the composite indicates that the electronic interactions between ZIF-8 and ZnFe₂O₃ modify the electronic structure, potentially leading to improved stability and performance in energy-related applications [20,21]. Indirect transitions can occur when the electron must transition through an intermediate state, leading to a different optical behavior than direct transitions. Previous studies carried out by our team and others have documented similar behaviors in analogous materials [10,16,22]. Research has shown that metal–organic frameworks and their composites frequently display indirect band gaps due to their unique structural and electronic configurations. These findings align with my observation of the indirect absorption mechanism within our studied materials. The absorption spectra for ZIF-8 and ZIF-8@ZnFe₂O₃ suggest a tailing at higher energies, indicating the presence of indirect transitions. Such behavior often necessitates analyzing the data with an exponent of 1/2 to accurately describe the optical properties.

4.6. CV Measurements

The CV curves in Figure 6 show the charging and discharging behavior of ZIF-8 and ZIF-8/ZnFe₂O₃ at different scan rates (10 mV/s, 30 mV/s, and 50 mV/s). The doped sample exhibits a more rectangular shape, indicating a higher electrochemical stability and capacitive behavior. The CV curves in Figure 6 suggest that the doped sample undergoes redox reactions during the charging and discharging process. The presence of ZnFe₂O₃ might promote the formation of redox-active species, which could contribute to improved electrochemical performance. As you can see from the figure, the peak current density for the oxidation reaction (anodic peak) is higher for ZIF-8@ZnFe₂O₃ compared to ZIF-8. For ZIF-8, the anodic peak current density is approximately 0.0012 A (at around 0.5 V) at a scan rate of 10 mV/s. In contrast, for ZIF-8@ZnFe₂O₃, the anodic peak current density is approximately 0.0025 A (at around 0.5 V) at a scan rate of 10 mV/s. This represents a 108% increase in the anodic peak current density. Similarly, the cathodic peak current density is also higher for ZIF-8@ZnFe₂O₃. For ZIF-8, the cathodic peak current density is approximately −0.0010 A (at around 0.2 V) at a scan rate of 10 mV/s. In contrast, for ZIF-8@ZnFe₂O₃, the cathodic peak current density is approximately −0.0018 A (at around 0.2 V) at a scan rate of 10 mV/s. This represents an 80% increase in the cathodic peak current density. These values provide evidence that the introduction of ZnFe₂O₃ has improved the charging and discharging capabilities of the material, as demonstrated by the increased peak current densities.

The peak current density of ZIF-8 and ZIF-8/ZnFe₂O₃ is shown in Figure 7. The doped sample exhibits a higher peak current density, suggesting an improvement in electrochemical activity. The peak current values seem to increase more rapidly, by about 41%, with increasing scan rate when Fe₂O₃ is present, indicating a synergistic effect between the ZIF-8 and Fe₂O₃ components. The Fe₂O₃ addition enhances the electrochemical activity of ZIF-8, leading to increased peak current values at various scan rates. This suggests that the Fe₂O₃ may be facilitating charge transfer or improving the conductivity of the material.

The current density of ZIF-8/ZnFe₂O₃ as a function of potential is shown in Figure 8. The doped sample exhibits a higher current density, indicating a higher electrochemical activity. The curve exhibits characteristic redox behavior, with a distinct peak observed at approximately 0.03 V, indicating the onset of electrochemical activity. Notably, the peak current density reaches a value of around 0.6 mA/cm², signifying a significant enhancement in electrochemical performance compared to pristine ZIF-8. The incorporation of ZnFe₂O₃ into the ZIF-8 framework has resulted in a substantial increase in current density, particularly in the positive potential region. This implies that the modification has led to an improvement in the material’s conductivity and electrochemical reactivity. The high observed peak at 0.03 V can be attributed to the Fe²⁺/Fe³⁺ redox couple, which is facilitated by the ZnFe₂O₃ component. The emergence of this peak suggests that the ZnFe₂O₃ has effectively introduced electroactive sites within the ZIF-8 framework, thereby enhancing the material’s electrochemical performance.

Figure 9 shows the specific capacitance of ZIF-8/ZnFe₂O₃ as a function of voltage rate. The doped sample exhibits a higher specific capacitance, which could be related to ZnFe₂O₃ nanoparticles acting as a conductive pathway, enhancing the electrochemical activity of ZIF-8. The incorporation of ZnFe₂O₃ might increase the surface area and porosity of ZIF-8, allowing for better electrolyte diffusion and ion transport [17]. The ZnFe₂O₃ nanoparticles might promote the formation of redox-active species, contributing to improved electrochemical performance [18,23].

Figure 10 presents the galvanostatic charge–discharge (GCD) characteristics for ZIF-8/glassy carbon electrode (GCE) and ZIF-8/ZnFe₂O₃/GCE composites. The charge and discharge curves for both materials demonstrate a high degree of symmetry and exhibit nearly triangular shapes across various current densities. This symmetry is indicative of the ideal electrochemical behavior associated with capacitive materials, reflecting efficient charge storage and transfer mechanisms. The triangular profile suggests that the composites maintain their capacitive performance even at elevated current densities, which is crucial for practical applications in energy storage devices. These results underscore the commendable capacitive properties of the materials, highlighting their potential for usage in supercapacitors where rapid charge and discharge cycles are essential.

A cycling stability test was performed over 2000 cycles to assess the durability of the ZIF-8/GCE and ZIF-8/ZnFe₂O₃/GCE, as illustrated in Figure 11. The results indicate that after 2000 cycles, the ZIF-8/ZnFe₂O₃/GCE retained approximately 95% of its original capacity, demonstrating impressive stability throughout the cyclic voltammetry (CV) testing. This high retention of capacity reaffirms that the electrode maintains its performance over extended use, which is crucial for practical applications in energy storage devices. Furthermore, the improved electrochemical performance of the ZIF-8/ZnFe₂O₃/GCE can be attributed to the alterations observed in the complex impedance, represented as (Z* = Z′ + jZ″). This change in impedance suggests enhanced conductivity and charge transfer capabilities within the composite, contributing to its superior stability and efficiency during cycling. Overall, these findings highlight the promising potential of the ZIF-8/ZnFe₂O₃ composite for use in long-lasting supercapacitor applications.

Figure 12 illustrates the relationship between the real part of the impedance (Z′) and the imaginary part of the impedance (Z″) for the ZIF-8/GCE and the ZIF-8/ZnFe₂O₃/GCE composites. From the figure, it is evident that the incorporation of ZnFe₂O₃ results in an increase in bulk resistance, rising from 1400 kΩ for the pure ZIF-8/GCE to 1800 kΩ for the ZIF-8/ZnFe₂O₃/GCE. When combined with ZnFe₂O, the overall electrical performance of the composite is adversely affected. This reduction in conductivity leads to an increase in the bulk resistance observed in the ZIF-8/ZnFe₂O₃/GCE compared to the pure activated ZIF-8. The increased bulk resistance may hinder charge transport within the composite, potentially affecting its electrochemical performance. Therefore, while the addition of ZnFe₂O₃ can enhance specific properties, such as electrochemical activity, the overall conductivity balance illustrated in Figure 12 underscores the importance of optimizing composite formulations to achieve desirable performance in electrochemical applications.

In the zinc ferrite (ZnFe₂O₃)-doped zeolitic imidazolate framework-8 (ZIF-8) composites, the charge storage mechanism is enhanced significantly through a dual action: Faradaic redox reactions and electrical double-layer capacitance.

Table 2. Comparison between electrochemical performance for ZIF-based composites.

ZIF-8 Composite	Specific Capacitance F/G	Current Density Peak mA/m2	Ref.
ZIF-8/ZnFe2O3	385	3.5	Present work
ZIF-8@RHBC	452	3.7	[24]
ZIF-7/glucose	228	2.9	[25]
MWCNT/ZIF-8	293.4	3.5	[26]

shows a comparison between electrochemical performance for ZIF-based composites. The studied composite shows a notable increase in peak current density; for instance, the anodic peak current density observed is approximately 0.0025 A at around 0.5 V with a scan rate of 10 mV/s. This value increases by about 41% with rising scan rates when ZnFe₂O₃ is present, illustrating the synergy between ZIF-8 and ZnFe₂O₃ in facilitating charge transfer. Structural characterization indicates a decrease in average crystallite size of about 23% due to the incorporation of ZnFe₂O₃, which enhances the material’s surface area and active sites for charge storage. Furthermore, the estimated electrochemical bandgap values of ZIF-8 and ZIF-8@ZnFe₂O₃ are 3.08 eV and 3.28 eV, respectively, suggesting improved electronic conductivity in the composite. The presence of the Fe²⁺/Fe³⁺ redox couple is crucial, as it can lead to significant capacitive contributions, enabling sustained performance with higher current densities (as evidenced by the observed peak at 0.03 V) and enhancing the overall efficiency of energy storage, making these composites promising candidates for advanced supercapacitor applications.

5. Conclusions

The synthesis and characterization of ZnFe₂O₃-doped ZIF-8 composites have been carried out. Through a systematic examination of their structural, optical, and electrochemical properties, we aspire to reveal the synergistic effects of this dual-component system, thereby contributing to the development of materials intended for improved photonic and energy storage applications. The doped sample exhibits a slower weight loss rate, indicating that the incorporation of ZnFe₂O₃ enhances the thermal stability of ZIF-8. The TEM image for the ZIF-8@ZnFe₂O₃ doped sample shows a more uniform particle size distribution and a higher crystallinity compared to the pristine ZIF-8. The CV curves for the doped sample show a more rectangular shape than pristine ZIF-8, indicating improved electrochemical stability and capacitive behavior. The peak current densities increased by 108% for the anodic peak and 80% for the cathodic peak, demonstrating better charge storage and transfer capabilities of the composite. The addition of ZnFe₂O₃ enhances electron exchange and helps form redox-active species during electrochemical processes. The GCD results support these findings, displaying symmetrical charge and discharge curves across various current densities. ZnFe₂O₃ not only boosts the electrochemical activity of ZIF-8 by providing more active sites but also improves the material’s overall conductivity, which is vital for energy storage. The redox peaks related to Fe²⁺/Fe³⁺ transitions further confirm the significant enhancements achieved with this composite. Overall, these results highlight the ZIF-8/ZnFe₂O₃ composite’s strong potential for advanced energy storage applications, thanks to its rapid charge–discharge capabilities and excellent electrochemical stability, making it suitable for supercapacitors and similar technologies.