Tunable Optical Bandgap and Enhanced Visible Light Photocatalytic Activity of ZnFe₂O₃-Doped ZIF-8 Composites for Sustainable Environmental Remediation

Abstract

Metal–organic frameworks (MOFs), particularly ZIF-8, have emerged as promising materials due to their high porosity, tunability, and chemical stability. In this study, we report the synthesis of ZnFe₂O₃-doped ZIF-8 composites with 10 wt% loading via a solvothermal method to enhance their optical and photocatalytic performance. Structural analyses confirmed the successful incorporation of ZnFe₂O₃ without disrupting the ZIF-8 framework. Optical studies revealed enhanced absorption in the visible range, a narrowed bandgap (4.26 eV vs. 4.37 eV for pristine ZIF-8), and an increased extinction coefficient, indicating superior light-harvesting potential. The photocatalytic activity was evaluated by methylene blue (MB) degradation under visible light, where the 10 wt% ZnFe₂O₃-ZIF-8 composite achieved 90% degradation efficiency, outperforming pristine ZIF-8 (67.8%). The catalyst also demonstrated excellent recyclability over five cycles and a proposed degradation mechanism involving ·OH and ·O₂⁻ radical formation. These findings demonstrate the potential of highly doped ZnFe₂O₃@ZIF-8 composites for environmental remediation and photonic applications.

1. Introduction

Metal–organic frameworks (MOFs) have garnered significant attention recently due to their remarkable structural properties, tunable porosity, and extensive surface areas [1]. These properties make MOFs suitable for various applications in gas storage [2], separation [3], catalysis [4], and sensing [5,6]. Among the diverse types of MOFs, zeolitic imidazolate frameworks (ZIFs) stand out due to their unique high stability and versatility [7]. The prototypical member of this family is ZIF-8, which is particularly notable for its excellent thermal and chemical stability, as well as its intrinsic luminescent properties [8]. These characteristics position ZIF-8 as a promising candidate for innovative applications in optoelectronics [9,10], photonics [11], energy storage [12], and wastewater pollutant removal [13,14,15,16].

Despite its advantageous properties, pristine ZIF-8 often exhibits limited photocatalytic activity due to rapid electron–hole recombination and insufficient light absorption in the visible spectrum. Recent studies have explored the incorporation of metal oxides or nanoparticles into metal–organic frameworks (MOFs) to address these limitations and enhance their optical and photocatalytic properties. Zinc ferrite (ZnFe₂O₃), a mixed-metal oxide, has attracted considerable interest due to its high thermal stability, superior light absorption in the visible region, and ability to facilitate charge separation. Introducing ZnFe₂O₃ into MOFs can further enhance their properties, leading to composite materials with superior functionality [17]. ZnFe₂O₃ has attracted considerable interest due to its favorable physicochemical attributes, including high stability, effective light absorption, and magnetic properties [18]. The incorporation of ZnFe₂O₃ into ZIF-8 matrices not only fortifies the structural integrity but also synergistically augments the optical and electrochemical performance of the composites. This interplay between organic and inorganic components can unlock novel energy conversion and storage applications, particularly in supercapacitors and photonic devices [19].

Recent literature reveals a growing body of research focused on the optical properties and laser-limiting capabilities of various metal–organic frameworks (MOFs) and their composites. Studies indicate that incorporating metal oxides can significantly modify the absorption spectrum and enhance nonlinear optical properties, providing pathways for developing advanced materials that can mitigate laser damage and improve performance in photonic applications [20]. Furthermore, investigations into the energy gap of these composite materials have shown that doping can lead to a shift in electronic transitions, promoting enhanced performance in light-harvesting and storage applications [21].

Previous studies, including our recent work on 5 wt% ZnFe₂O₃-doped ZIF-8, have demonstrated improved electrochemical and supercapacitor behavior of ZIF-8 composites [22]. In this work, we advance the investigation by exploring a higher doping level (10 wt%) of ZnFe₂O₃ in ZIF-8, specifically targeting optical tuning and enhanced photocatalytic degradation of methylene blue (MB) under visible light. The emphasis shifts from electrochemical storage to photocatalysis and photonic applications, with a detailed assessment of band structure, extinction coefficient, and degradation kinetics. This study offers new insights into the structure–property relationships at elevated dopant levels, positioning ZnFe₂O₃@ZIF-8 as a promising platform for environmental remediation technologies.

2. Results and Discussion

2.1. Structural Properties

XRD is a crucial tool for examining the internal structure of novel composites. To look more closely at how the structure changes with more ZnFe₂O₃, Figure 1 shows the XRD patterns for ZIF-8, ZnFe₂O₃@ZIF-8 (5 wt%) [22], and ZnFe₂O₃@ZIF-8 (10 wt%). All samples display reflections linked to the ZIF-8 framework; however, the diffraction patterns of the doped composites, especially the 10 wt% sample, show notable deviations from the standard ZIF-8 patterns documented in the literature [23,24] and PDF card No. 00-062-1030. Most notably, the sharp and intense peaks at 2θ ≈ 31.7° and 36.9° are not present in standard ZIF-8 but are consistent with the crystalline phases of ZnFe₂O₃ or associated oxides, as indexed in PDF card No. 00-022-1012. This confirms the formation of secondary oxide phases and suggests partial structural transformation or surface crystallite formation at higher dopant concentrations. While some characteristic ZIF-8 peaks remain (e.g., at ~7.3° and 12.8°), the overall pattern indicates that the introduction of ZnFe₂O₃ at elevated levels leads to the coexistence of multiple crystalline phases and partial distortion of the ZIF-8 lattice.

However, the 10 wt% sample shows broader and weaker peaks, especially at the (002) and (112) spots, and new peaks related to ZnFe₂O₃ appear around 35.5° [25]. Compared to the 5 wt% sample, the difference suggests a higher degree of lattice distortion and secondary phase formation at higher dopant loading. These structural changes indicate that more ZnFe₂O₃ nanoparticles are being added, which correlates with the improved photocatalytic performance observed in the 10 wt% sample. In our previous work [22], we reported the structural behavior of ZIF-8 doped with 5 wt% ZnFe₂O₃, where the composite maintained good crystallinity with moderate peak broadening and minor phase impurity. In the current study, a higher ZnFe₂O₃ loading was used, resulting in further suppression of the ZIF-8 peak intensities and more pronounced secondary oxide peaks. This indicates increased structural distortion and partial framework substitution, possibly due to the excess oxide interacting with the coordination environment of zinc centers. The broader diffraction peaks also suggest reduced crystallite size or increased lattice strain, which are common outcomes of metal oxide doping in MOFs at higher concentrations. Overall, the XRD results confirm that while the ZIF-8 framework remains intact, higher ZnFe₂O₃ doping levels induce additional phase signatures and structural perturbations, effects that were much less pronounced at the lower 5 wt% level [22]. These changes may enhance catalytic surface activity and facilitate electron transfer by introducing more defect sites and heterojunction interfaces, which are beneficial for photocatalytic applications.

The thermal stability and decomposition behavior of the ZIF-8@ZnFe₂O₃ composite were investigated using thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTA). Figure 2 presents the simultaneous TGA (black curve) and DTA (blue curve) results for the ZnFe₂O₃-doped ZIF-8 composite. The TGA curve exhibits a three-step weight loss profile: an initial minor mass loss (~5%) below 150 °C is attributed to the evaporation of physically adsorbed water and residual solvents, as indicated by a shallow endothermic peak in the DTA curve. A more pronounced weight loss (~20%) between 250 and 350 °C corresponds to the partial decomposition of the organic 2-methylimidazole linker, accompanied by a broad exothermic peak, suggesting catalytic degradation induced by ZnFe₂O₃ nanoparticles. At temperatures above 500 °C, a significant exothermic peak and a sharp mass loss (~35–40%) are observed, marking the collapse of the ZIF-8 framework and the transformation of the hybrid structure into stable metal oxide residues, such as ZnO, Fe₂O₃, or ZnFe₂O₄ spinel. The final residual mass beyond 600 °C confirms the presence of thermally stable inorganic components. Compared to pristine ZIF-8, the composite exhibits enhanced thermal stability, likely due to interfacial interactions between the ZIF-8 framework and the ZnFe₂O₃ dopant. These results are in strong agreement with previous studies on similar MOF-based hybrids [22,26], reinforcing the suitability of the ZnFe₂O₃@ZIF-8 composite for applications involving thermal and photocatalytic processes.

On the other hand, the DTA thermogram exhibits distinct endothermic and exothermic transitions, providing insight into the thermal stability and decomposition behavior of the composite. A small endothermic peak observed around 100–150 °C corresponds to the removal of physically adsorbed moisture and residual solvents (e.g., DMF or methanol) retained during the synthesis process. This feature is typical for metal–organic frameworks and confirms the presence of weakly bound surface molecules [27,28]. In the intermediate temperature range (250–350 °C), a broad exothermic peak is apparent, which is associated with the partial decomposition of the organic 2-methylimidazole linker. The shift in this peak relative to pristine ZIF-8 suggests a catalytic effect introduced by ZnFe₂O₃, which facilitates the thermal degradation process. This behavior indicates the successful incorporation of ZnFe₂O₃ nanoparticles into the framework and their interaction with the host matrix. A more prominent exothermic peak appears at 510 °C, which is attributed to the complete collapse of the ZIF-8 framework and the formation of thermally stable metal oxide residues. The evolution of this peak at a slightly elevated temperature compared to pure ZIF-8 reflects an enhancement in thermal stability due to the presence of ZnFe₂O₃, potentially leading to the formation of spine-type ZnFe₂O₃ structures [27]. In our previous study [22], ZnFe₂O₃ was incorporated into ZIF-8 at a doping ratio of 5 wt%, demonstrating noticeable improvements in thermal behavior.

In contrast, the current study employs a higher doping level, which further enhances the thermal stability of the composite. The observed delay in decomposition temperature and slower mass loss rate confirms that increasing the ZnFe₂O₃ content strengthens the composite’s resistance to thermal degradation. This enhancement is likely due to intensified interactions between the metal oxide and the ZIF-8 matrix, which act as thermal buffers and stabilize the structure. These results align with previous reports [22,29] and reinforce the conclusion that the ZnFe₂O₃@ZIF-8 system, particularly at higher doping levels, is highly suitable for thermally demanding applications, such as photocatalysis under visible light.

The SEM image of pristine ZIF-8 (Figure 3a) reveals a typical morphology of rhombic dodecahedra, which is consistent with the reported crystal structure of ZIF-8 [30]. The surface of the ZIF-8 crystal is smooth and free of any impurities, indicating high crystallinity. The particle size distribution is uniform, with most particles falling in the range of 15 μm. In contrast, the SEM image of ZIF-8@ZnFe₂O₃ (Figure 3b) shows a significant change in the material’s morphology and surface characteristics. The introduction of ZnFe₂O₃ has led to the formation of a rough and porous surface, indicating the successful deposition of ZnFe₂O₃ on the ZIF-8 surface.

In contrast, the SEM image of ZIF-8@ZnFe₂O₃ (10 wt%) shows a noticeable change in morphology. The surface becomes rougher and more porous, indicating successful integration of ZnFe₂O₃ into the ZIF-8 structure. The particle size distribution becomes more heterogeneous, with some degree of aggregation observed. Notably, small spherical nanoparticles (average diameter ~43 nm), corresponding to ZnFe₂O₃, are visible and uniformly distributed on the ZIF-8 crystal surfaces, further supporting successful doping. These features confirm successful incorporation and suggest the creation of porous, high-surface-area sites that are ideal for photocatalysis.

Functional groups stacking on molecules have distinctive infrared spectra, and this property has made infrared spectroscopy the gold standard for physical investigations into molecular structure and functional group identification [31]. The Fourier transform infrared (FTIR) spectra of ZIF-8 and ZnFe₂O₃@ZIF-8 are presented in Figure 4. The FTIR spectrum confirms the successful synthesis and doping of ZIF-8 with ZnFe₂O₃. The key peaks at approximately 1580 cm⁻¹ and 1140 cm⁻¹ are related to the vibrations of the imidazole ring, indicating that the ZIF-8 structure remains intact after doping. Additionally, the peak at approximately 420 cm⁻¹, attributed to Zn-N stretching, remains prominent in both pristine and doped samples, indicating that the primary framework is retained. Although we expected to see Fe-O or Zn-O stretching in the fingerprint region between 500 and 600 cm⁻¹, the current spectra do not show any clear new peaks in that area. The broadband near ~3400 cm⁻¹ observed in both spectra is attributed to O-H stretching vibrations of surface hydroxyl groups or physisorbed water molecules. These -OH groups may participate in photocatalytic processes by forming ·OH radicals, as previously reported [32]. The slight shift in some peaks (e.g., imidazole ring vibrations) suggests interactions between the ZnFe₂O₃ dopant and the ZIF-8 framework, potentially due to coordination or hydrogen bonding. The FTIR results demonstrate the successful doping of ZIF-8 with ZnFe₂O₃ while retaining the MOF’s essential framework structure. Incorporating ZnFe₂O₃ introduces metal–oxide-specific vibrations without disrupting the primary framework, thereby enhancing the composite’s potential for photocatalytic and energy storage applications.

2.2. Optical Properties

The optical transmission and reflection spectra of ZIF-8 and ZIF-8@ZnFe₂O₃ films are presented in Figure 5. The ZIF-8 film exhibits a smooth decline in transmission with increasing wavelength, indicative of its broad optical absorption. In contrast, the ZIF-8@ZnFe₂O₃ composite exhibits a more complex transmission profile, featuring distinct modulations and dips that signify the introduction of additional absorption bands resulting from the doping of ZnFe₂O₃. These features highlight the enhanced optical activity of the composite, which is attributed to the synergistic interaction between the ZIF-8 framework and ZnFe₂O₃ nanoparticles. The reflection spectra reveal low reflectivity for both films, consistent with the inherent properties of ZIF-8. However, the ZIF-8@ ZnFe₂O₃ composite demonstrates slightly increased reflectivity in specific regions due to surface morphology alterations or refractive index changes arising from the integration of ZnFe₂O₃. ZnFe₂O₃ is known for its strong visible-light absorption and charge-transfer properties, making it an excellent candidate for enhancing the photocatalytic efficiency of hybrid systems. The observed dips in the transmission spectrum indicate the formation of new energy states in ZnFe₂O₃, which may facilitate light harvesting and improve charge carrier separation within the composite. The results demonstrate that integrating ZnFe₂O₃ into the ZIF-8 framework effectively tunes its optical properties, making it a promising material for applications in photocatalysis and energy storage technologies.

The extinction coefficient (k) measures how much light is absorbed or scattered by a material at a specific wavelength. The absorption coefficient (α) and the extinction coefficient (k) can be determined by [33]

$$\alpha ={\displaystyle \frac{1}{x}}\ln \left({\displaystyle \frac{1-R^2}{2T}}+\sqrt{R^2+{\displaystyle \frac{1-R^2}{4T^2}}}\right)$$

(1)

k = λ × α/4π.

(2)

where x is the sample thickness, and R and T are the measured reflectance and transmittance, respectively.

The extinction coefficient spectra of ZIF-8 and ZIF-8@ZnFe₂O₃ films are shown in Figure 6. The extinction coefficient provides critical insights into the light absorption capability of the material and energy dissipation processes. For the ZIF-8 film, the extinction coefficient exhibits one prominent peak at shorter wavelengths, corresponding to electronic transitions within the ZIF-8 framework. This peak is followed by a gradual decline at higher wavelengths, indicating reduced optical absorption in the visible and near-infrared regions. In the case of the ZIF-8@ZnFe₂O₃ composite, the spectra show enhanced extinction in the shorter wavelength region, with a broader and more intense peak compared to pristine ZIF-8. This enhancement suggests that ZnFe₂O₃ doping introduces additional electronic states or transitions due to the formation of heterojunctions between the ZIF-8 framework and ZnFe₂O₃ nanoparticles. The extended tail in the visible region for ZIF-8@ZnFe₂O₃ further highlights its improved light-harvesting capabilities, making it a promising material for photocatalytic applications. The significant increase in k for ZIF-8@ZnFe₂O₃ suggests enhanced interfacial interaction between the two components, facilitating the absorption of high-energy photons and potentially improving photocatalytic performance. This behavior confirms that integrating ZnFe₂O₃ into the ZIF-8 framework is an effective strategy for tailoring its optical properties in advanced energy and photocatalytic applications.

The indirect energy bandgap of ZIF-8 and ZIF-8@ZnFe₂O₃ films was determined using Tauc plots, as shown in Figure 7. The Tauc relation was used to calculate the bandgap energy, which is given by [34]

$${\left(\alpha h\upsilon \right)}^{{\displaystyle \frac{1}{2}}}=Q\left(h\upsilon -E_g\right)$$

(3)

where Q is a constant. A bandgap of approximately 4.37 eV was found for ZIF-8. This value aligns with previous reports on ZIF-8 materials [35], confirming its wide bandgap semiconducting nature. In comparison, the ZIF-8@ZnFe₂O₃ composite exhibits a reduced bandgap of 4.26 eV, indicating the influence of ZnFe₂O₃ doping. The reduced E_g can be attributed to the introduction of mid-gap states and improved charge carrier transfer at the ZIF-8/ZnFe₂O₃ interface. This behavior aligns with studies on metal oxide-doped MOFs, where bandgap narrowing enhances light absorption and photocatalytic activity [36]. The narrowed bandgap of the composite is advantageous for photocatalysis under visible light, as it extends the absorption range of the material and promotes efficient charge separation. These findings further demonstrate the potential of ZIF-8@ ZnFe₂O₃ as a tailored material for advanced photocatalytic and energy applications.

The refractive index, n, can be calculated using [33]

$$n=\left({\displaystyle \frac{1+R}{1-R}}\right)+\sqrt{{\displaystyle \frac{4R}{{\left(1-R\right)}^2}}-k^2}$$

(4)

The refractive index spectra of ZIF-8 and ZnFe₂O₃-doped ZIF-8 are presented in Figure 8. Both samples exhibit a prominent peak at shorter wavelengths, which decreases smoothly as the wavelength increases. For pristine ZIF-8, the refractive index exhibits a sharp peak and then gradually diminishes, demonstrating typical dielectric behavior. In contrast, the ZnFe₂O₃-doped sample shows consistently higher refractive index values across the entire wavelength range.

The enhanced refractive index in ZIF-8@ZnFe₂O₃ can be attributed to the incorporation of ZnFe₂O₃ nanoparticles. These nanoparticles increase the material’s density and polarizability, thereby improving its optical response. The sharp peak observed at shorter wavelengths corresponds to electronic transitions and resonance effects, indicating enhanced light–matter interaction due to doping. Furthermore, the broader optical response observed in the doped sample suggests enhanced light-harvesting capabilities, which are beneficial for photocatalytic applications. The observed enhancement in the optical response of ZIF-8@ZnFe₂O₃, compared to pristine ZIF-8, highlights the synergistic effect of combining ZnFe₂O₃ with the porous ZIF-8 framework. This improved optical performance underscores the potential of such composites in advanced photocatalytic and energy storage applications.

The dielectric constant (ε₁ = n² − k²) and dielectric loss (ε₂ = 2 nk) spectra for ZIF-8 and ZnFe₂O₃-doped ZIF-8 are depicted in Figure 9. Both ε₁ and ε₂ exhibit distinct behavior as a function of hν, reflecting the impact of ZnFe₂O₃ doping on the dielectric properties of the composite. For the dielectric constant (ε₁), ZIF-8@ZnFeO₃ exhibits higher values across the measured wavelength range than pristine ZIF-8. This enhancement can be attributed to the introduction of ZnFe₂O₃ nanoparticles, which increase the polarization within the composite. The peak observed in the higher wavelength region for ZIF-8@ZnFe₂O₃ signifies improved charge storage capability due to the synergistic effect between the ZnFe₂O₃ and the ZIF-8 framework. The dielectric loss (ε₂) spectra reveal the energy dissipation associated with the charge migration and relaxation processes. ZIF-8@ZnFe₂O₃ demonstrates a sharper peak at shorter wavelengths, indicating a higher degree of energy absorption, which is characteristic of materials with enhanced dipole relaxation mechanisms. The smoother behavior of ε₂ at longer wavelengths reflects reduced loss, aligning with improved dielectric performance.

The enhanced dielectric behavior of ZIF-8@ZnFe₂O₃ is consistent with findings reported in the literature for metal oxide-doped MOF composites. Studies have shown that the incorporation of metal oxides such as ZnFe₂O₃ increases the dielectric constant and loss due to improved electronic coupling and the formation of heterogeneous interfaces, which facilitate charge polarization [37]. Additionally, the synergistic effects between metal oxides and MOFs have been highlighted as a key factor in achieving enhanced dielectric performance [38]. Overall, the improved dielectric constant and reduced energy loss of ZIF-8@ZnFe₂O₃ underscore its potential for energy storage and electronic applications. These enhancements are indicative of the structural and compositional advantages introduced by ZnFe₂O₃ doping, making this composite material a promising candidate for next-generation dielectric and optoelectronic devices.

2.3. Photocatalytic Study

To better understand how different amounts of dopant affect photocatalytic efficiency, we compared 5 wt% and 10 wt% ZnFe₂O₃-doped ZIF-8 composites directly. As illustrated in the newly added Figure 10, all samples exhibited a significant decrease in MB absorbance after 60 min under visible light, confirming photocatalytic activity. The ZnFe₂O₃@ZIF-8 (10 wt%) composite exhibited the most significant reduction in MB absorbance, outperforming both the 5 wt% sample and pristine ZIF-8. This improvement is attributed to having more catalytic sites and better charge transfer with increased doping. The data clearly show that the 10 wt% ZnFe₂O₃-doped ZIF-8 composite works better as a photocatalyst than the 5 wt% version.

The photocatalytic efficacy of ZIF-8 and ZIF-8@ZnFe₂O₃ was assessed by observing the degradation of a methylene blue (MB) solution under visible light at different time intervals, as shown in Figure 11. Figure 11a shows the absorbance of MB in the presence of ZIF-8. The initial absorbance of MB is significant, but it decreases steadily as visible light increases. At 0 min, the absorbance is approximately 0.9, which reduces to around 0.6 after 30 min of exposure to visible light. The absorbance decreases, reaching around 0.4 after 60 min and 0.2 after 120 min. This indicates that the ZIF-8 catalyst effectively degrades MB over time. In Figure 11b, the absorbance of MB is shown in the presence of ZIF-8@ZnFe₂O₃. The trend is like that observed in Figure 11a, but the degradation of MB is more pronounced in this case. The initial absorbance is similar but rapidly drops with increasing visible-light time. After 30 min, the absorbance decreases to approximately 0.4; after 60 min, it reaches around 0.2.

The absorbance continues to decline, reaching around 0.1 after 120 min. This suggests that the ZIF-8@ZnFe₂O₃ catalyst is more effective at degrading MB than the ZIF-8 catalyst. Overall, these results demonstrate the potential of both ZIF-8 and ZIF-8@ZnFe₂O₃ catalysts for the degradation of methylene blue under visible-light conditions, with the latter showing improved performance. The superior photocatalytic performance of ZIF-8@ZnFe₂O₃ can be attributed to the synergistic effects between ZnFe₂O₃ and the ZIF-8 framework. The incorporation of ZnFe₂O₃ enhances the light absorption capacity of the composite, as demonstrated by its higher refractive index and broader optical response. Additionally, ZnFe₂O₃ facilitates improved charge separation and transfer at the interface, thereby reducing electron–hole recombination and increasing the availability of reactive oxygen species for the degradation of MB. The faster degradation of MB in the presence of ZIF-8@ZnFe₂O₃ highlights its potential as a highly efficient photocatalyst, making this composite material a promising candidate for wastewater treatment and environmental remediation applications.

Figure 12 illustrates the photocatalytic degradation of MB under visible light in the presence of different catalysts: pure MB solution (without catalyst), ZIF-8, and ZIF-8@ZnFe₂O₃ composites. The MB concentration exhibited only a minimal decrease in the absence of any catalyst, indicating negligible photolysis under visible light and underscoring the necessity of a photocatalyst for effective degradation. In contrast, the inclusion of ZIF-8 led to a significant reduction in MB concentration over time, demonstrating its inherent photocatalytic activity. Notably, ZIF-8@ZnFe₂O₃ exhibited superior performance, with the steepest decline in MB concentration, particularly during the initial stages of visible light.

The enhanced performance of the ZIF-8@ZnFe₂O₃ composite can be attributed to the synergistic interaction between ZIF-8 and ZnFe₂O₃. ZIF-8 provides a high surface area and serves as a light-harvesting framework, while ZnFe₂O₃ contributes to improved charge carrier dynamics. Specifically, the incorporation of ZnFe₂O₃ enhances charge separation and light absorption, likely contributing to the improved photocatalytic efficiency observed in the composite material. Similar findings have been reported in studies where doping metal–organic frameworks (MOFs) with metal oxides, such as ZnO, enhances photocatalytic activity due to improved charge separation and transfer [39]. Furthermore, combining ZIF-8 with metal oxides results in molecule size selectivity and excellent photocatalytic efficiency [40]. This observation aligns with studies demonstrating that coupling metal oxides with MOFs enhances photocatalytic efficiency under both UV and visible light. For instance, a novel nanocomposite of ZIF-8 and MoSe₂ has been shown to exhibit enhanced photocatalytic activity in degrading organic pollutants [41]. The lack of substantial degradation in the absence of a catalyst underscores the necessity of a photocatalyst for efficient MB degradation. While ZIF-8 alone shows moderate activity due to its structural and optical properties, the integration of ZnFe₂O₃ significantly amplifies its photocatalytic potential, making the ZIF-8@ZnFe₂O₃ composite a promising candidate for environmental remediation applications.

Photocatalytic efficiency (PCE%) was calculated using the formula

$$PCE\%=\left({\displaystyle \frac{C_o-C_t}{C_o}}\right)\times 100=\left(1-{\displaystyle \frac{A_t}{A_o}}\right)\times 100$$

(5)

where A_o is the initial absorbance and A_t is the absorbance at time t. The PCE% of ZIF-8 and ZIF-8@ZnFe₂O₃ catalysts in degrading methylene blue (MB) under visible light is presented in Figure 13. The plot shows the PCE% of each catalyst against UV light irradiation time (in minutes). The results demonstrate that ZIF-8 and ZIF-8@ZnFe₂O₃ catalysts exhibit significant photocatalytic activity towards MB degradation under UV light. However, the ZIF-8@ ZnFe₂O₃ catalyst displays a noticeably higher PCE than ZIF-8 throughout the visible-light period. The PCE% of ZIF-8 increases steadily with increasing visible-light time, reaching a maximum efficiency of around 60% after 120 min.

In contrast, the PCE% of ZIF-8@ZnFe₂O₃ increases more rapidly, achieving maximum efficiency of approximately 90% after 120 min of visible light exposure. The enhanced photocatalytic performance of ZIF-8@ZnFe₂O₃ can be attributed to the synergistic effect of the ZnFe₂O₃ component, which improves the separation and transfer of photogenerated electrons and holes, thereby increasing the catalytic activity. The higher PCE% of ZIF-8@ZnFe₂O₃ also suggests that the ZnFe₂O₃ component may have increased the surface area, pore volume, and/or adsorption capacity of the catalyst, allowing for more efficient degradation of MB. These findings are consistent with previous studies. For instance, Jing et al. [42] demonstrated that ZIF-8 exhibits efficient photocatalytic activity for MB degradation under visible light, which is attributed to its ability to generate hydroxyl radicals. The enhanced photocatalytic efficiency observed in the ZIF-8@ZnFe₂O₃ composite highlights the potential of integrating metal–organic frameworks (MOFs) with metal oxides to develop effective photocatalysts for environmental remediation applications.

The disintegration rate of a pseudo-first-order response obeyed the kinetic expression [43]:

$$ln\left({\displaystyle \frac{C_t}{C_o}}\right)=-kKt=-k_{app}t$$

(6)

where k is the degradation rate constant, K is the adsorption equilibrium constant, and k_app is the apparent kinetic rate constant. The C_t/C_o can be replaced by A_t/A_o because the concentricity is specific to the absorbance. Figure 14 illustrates the plot of a linear correlation between ln(C_t/C_o) and visible-light time, indicating that the degradation of MB follows a pseudo-first-order kinetic model. The linear fitting of the data suggests that the degradation rate of MB is directly proportional to the concentration of MB. The slope of the linear fit represents the apparent rate constant (k_app) of the degradation reaction.

Comparing the two catalysts, the slope of the linear fit for ZIF-8@ZnFe₂O₃ is steeper than that of ZIF-8, indicating a higher apparent rate constant and a faster degradation rate of MB in the presence of ZIF-8@ZnFe₂O₃. This finding is consistent with previous results, which demonstrated that ZIF-8@ZnFe₂O₃ exhibits higher photocatalytic efficiency and degradation rates compared to ZIF-8. The higher degradation rate of MB in the presence of ZIF-8@ZnFe₂O₃ can be attributed to the enhanced photocatalytic activity of the ZIF-8@ZnFe₂O₃ catalyst, which is due to the synergistic effect of the ZnFe₂O₃ component. Incorporating ZnFe₂O₃ into the ZIF-8 framework may have increased the surface area, pore volume, and/or adsorption capacity of the catalyst, leading to more efficient degradation of MB.

Finally, the photocatalytic performance of various catalysts for the degradation of methylene blue (MB) under different conditions has been extensively reported in the literature.

Table 1. Photocatalytic efficiency (PCE%) and apparent rate kinetic constant (k_app) were compared to literature values for MB removal.

Catalyst	Light Source	PCE (%)	kₐₚₚ (min−1)	Reference
ZIF-8	Visible	67.8	7.52 × 10−3	Present work
ZIF-8@ZnFe2O	Visible	90	12.51 × 10−3	Present work
Fe/ZnO/SiO2	Visible	100	Not reported	[44]
MXene	Visible	90.2	30 × 10−3	[45]
NiO-ZnONCs	UV	72	1.50 × 10–2	[46]
Cr2O3-CNT NPs	Visible	68	9.80 × 10–4	[47]

summarizes the photocatalytic efficiency (PCE%) and apparent rate constants (kₐₚₚ) for ZnFe₂O₃-doped ZIF-8 and other catalysts, highlighting their strengths and limitations. Pristine ZIF-8 exhibits moderate photocatalytic activity among the studied catalysts, attributed to its structural and optical properties. However, its performance is limited by rapid electron–hole recombination and a wide bandgap. Integrating ZnFe₂O₃ into the ZIF-8 framework significantly improves its photocatalytic efficiency, achieving a degradation efficiency of 90% under UV light with an apparent rate constant of 12.51 × 10⁻³ min⁻¹. This enhancement can be attributed to the synergistic effects between ZnFe₂O₃ and ZIF-8, which improve charge separation, reduce recombination rates, and enhance light absorption.

In comparison, other catalysts [39,40,41,42], such as Fe/ZnO/SiO₂ nanoparticles, have demonstrated 100% MB degradation efficiency under visible light. These nanoparticles benefit from their ability to utilize a broader light spectrum, but their synthesis and stability can present challenges for large-scale applications. Similarly, MXene-based photocatalysts have shown high efficiencies, reaching 90.2%, due to their excellent electrical conductivity and surface activity. However, their stability and long-term performance under various environmental conditions remain to be investigated. The results from this study position ZnFe₂O₃-doped ZIF-8 as a competitive and versatile photocatalyst for environmental remediation, particularly under visible-light conditions. The improved performance underscores the benefits of integrating metal–organic frameworks (MOFs) with metal oxides, offering a pathway for developing advanced materials for wastewater treatment and other photocatalytic applications.

2.4. Photocatalytic Reusability and Stability

The long-term stability and reusability of photocatalysts are crucial parameters for evaluating their feasibility in practical environmental remediation applications. To assess the photocatalytic durability of the prepared materials, cyclic degradation experiments were conducted for five consecutive runs using methylene blue (MB) under visible light, as shown in Figure 15.

The ZnFe₂O₃@ZIF-8 composite maintained a high photocatalytic efficiency throughout the five cycles, with only a minor reduction from ~90% in the first cycle to ~86% in the fifth. This minimal decrease (~4%) indicates strong photostability, robust structural integrity, and negligible catalyst leaching or deactivation. In contrast, pristine ZIF-8 exhibited a more noticeable decline, with the photocatalytic efficiency dropping from ~68% to ~63% by the fifth cycle. The superior recyclability of ZnFe₂O₃@ZIF-8 is attributed to several factors: (i) the strong interaction between ZnFe₂O₃ nanoparticles and the ZIF-8 framework, which inhibits photocorrosion and enhances charge separation; (ii) the structural stability of the MOF during light-induced catalytic reactions; and (iii) the retention of high surface area and active sites after repeated use. These findings align with prior studies, which demonstrate that MOF/metal oxide hybrids often exhibit better cyclic performance compared to pristine MOFs due to synergistic interfacial effects [41,42]. Overall, the excellent recyclability of ZnFe₂O₃@ZIF-8 under visible-light illumination emphasizes its potential as a robust and reusable photocatalyst for practical wastewater treatment systems.

2.5. Mechanism of Photocatalytic Degradation

The enhanced photocatalytic activity of ZnFe₂O₃@ZIF-8 under visible light is primarily attributed to the synergistic interaction between ZnFe₂O₃ nanoparticles and the ZIF-8 framework, which promotes efficient charge separation, reduced electron-hole recombination, and generation of reactive species (Figure 16). Upon visible-light irradiation (λ ≥ 420 nm), both ZIF-8 and ZnFe₂O₃ absorb photons and generate electron–hole pairs (e⁻/h⁺):

$$Z\mathrm{n}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3+\mathrm{h}\mathsf{\nu }\to {\mathrm{e}}^{-}+{\mathrm{h}}^{+}$$

Due to the heterojunction formed between ZIF-8 and ZnFe₂O₃, photogenerated electrons from ZnFe₂O₃ can transfer to the conduction band of ZIF-8, while holes remain in the valence band of ZnFe₂O₃. This spatial separation suppresses recombination and allows the chargers to participate in redox reactions at the catalyst surface:

Oxygen reduction: Electrons reduce dissolved oxygen into superoxide radicals:

$${\mathrm{O}}_2+{\mathrm{e}}^{-}\to {\cdot \mathrm{O}}_2^{-}$$

Hydroxyl radical formation: Superoxide radicals and holes further generate hydroxyl radicals:

$${\mathrm{O}}_2^{-}+{\mathrm{H}}^{+}\to \cdot \mathrm{H}{\mathrm{O}}_2\to \cdot \mathrm{O}\mathrm{H}$$

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{h}}^{+}\to \cdot \mathrm{O}\mathrm{H}+{\mathrm{H}}^{+}$$

Organic dye degradation: These reactive oxygen species (⋅OH,⋅O₂⁻) attack the methylene blue dye molecules and oxidize them into harmless products:

$$\mathrm{M}\mathrm{B}+{\mathrm{O}}_2^{-}\to \mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+\mathrm{o}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{r} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{s}$$

Thus, the heterojunction structure facilitates enhanced separation and migration of charge carriers, improving the production of active species for dye degradation.

3. Experimental Technique

3.1. Sample Fabrication

The synthesis of ZnFe₂O₃-doped ZIF-8 is achieved through a solvothermal method. First, we dissolve 2.97 g (10 mmol) of zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and 3.28 g (40 mmol) of 2-methylimidazole (2-MIM) in 50 mL of deionized water, stirring at 500 rpm for 30 min. The obtained solution showed ZIF-8. This method of making ZIF-8 aligns with earlier techniques that also employ room-temperature precipitation, as described by Park et al. [48]. Separately, ZnFe₂O₃ nanoparticles were prepared via a thermal decomposition method [22]. The Zn/MIM molar ratio was selected as 1:4 to ensure optimal nucleation and crystal growth, consistent with similar excess-ligand approaches reported in the literature for improved ZIF-8 morphology. The ZnFe₂O₃ nanoparticles (10 wt%) were synthesized by thermally decomposing a mixed nitrate solution containing Zn(NO₃)₂·6H₂O and Fe(NO₃)₃·9H₂O in a 1:2 molar ratio at 450 °C, following the protocol from our earlier work [22]. The mixture was transferred to a Teflon-lined autoclave and heated at 120 °C for 24 h, allowing the nucleation and growth of the ZIF-8 framework while incorporating ZnFe₂O₃. The autoclave was allowed to cool to room temperature. The resultant product was collected by centrifugation at 5000 rpm, washed three times with ethanol and deionized water, and then dried in a vacuum oven at 60 °C for 12 h to obtain the ZnFe₂O₃-doped ZIF-8 powders. For comparison, pristine ZIF-8 was synthesized under identical conditions without the addition of ZnFe₂O₃.

Thin films of ZIF-8 and ZnFe₂O₃-doped ZIF-8 can be prepared via a spin-coating method, which provides a uniform layer suitable for optical characterization. Glass slides were used as substrates. The cleaning process involved ultrasonic cleaning in acetone and ethanol (each for 10 min), followed by rinsing with deionized water and blow-drying with nitrogen to ensure surface cleanliness and uniformity. The synthesized ZIF-8 or ZnFe₂O₃-doped ZIF-8 powder (20 mg) was dispersed in a mixture of polyvinyl alcohol (1 wt.%) and ethanol (10 mL) to form a slurry. The mixture was sonicated to obtain homogeneous dispersion. The resulting slurry was deposited onto the prepared substrates using a spin coater. The spin-coating process was performed at 2000 rpm for 30 s, ensuring a uniform film thickness. After deposition, the coated film was allowed to dry at room temperature for 12 h to remove residual solvent, followed by thermal curing at 60 °C for 2 h to enhance adherence and stability.

3.2. Measurement Tools

X-ray diffraction (XRD) patterns were recorded using a PANalytical Empyrean X-ray Diffractometer (Almelo, The Netherlands) with CuKα radiation (λ = 1.5406 Å). The data were collected over a 2θ range of 5–80° at a scan rate of 0.02 °/s. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed using a TGA 5500 Thermogravimetric Analyzer (TA Instruments, New Castle, DE, USA) under a nitrogen atmosphere at a heating rate of 10 °C/min, from room temperature to 800 °C. Scanning electron microscopy (SEM) images were obtained using a JEOL JSM-7600F field-emission SEM (Tokyo, Japan) at an accelerating voltage of 5 kV. Fourier transform infrared (FTIR) spectroscopy was performed using a Bruker Tensor 27 spectrometer (Bremen, Germany) over a wavenumber range of 4000–400 cm⁻¹ to analyze functional groups and bonding interactions. A Shimadzu UV-3600 UV–visible spectrophotometer (Kyoto, Japan) was used.

3.3. Photocatalytic Degradation

The photocatalytic activity of ZIF-8 and ZIF-8 (10 wt%) incorporated with ZnFe₂O₃ (ZIF-8@ZnFe₂O₃) was evaluated using methylene blue (MB) dye degradation under visible light. A 25 mL solution of MB (10 mg/L) was prepared in deionized water and used as the model pollutant. For the experiments, 30 mg of the catalyst was dispersed into the MB solution and stirred in the dark for 30 min to achieve adsorption–desorption equilibrium. The solution was then exposed to visible light from a 300 W xenon lamp (λ ≥ 420 nm, and an AM 1.5G optical filter. Aliquots were withdrawn at 10 min intervals, and the remaining MB concentration was determined using a UV–vis spectrophotometer (Jenway, 6800, Staffordshire, UK) at 664 nm.

4. Conclusions

This study successfully synthesized 10 wt% ZnFe₂O₃-doped ZIF-8 composites using a solvothermal method, significantly enhancing their structural, optical, and photocatalytic properties. The incorporation of ZnFe₂O₃ nanoparticles into the ZIF-8 framework was confirmed through XRD, FTIR, and SEM analyses, which revealed the preservation of the crystalline structure and the formation of a porous composite with well-distributed ZnFe₂O₃ nanoparticles. However, it is essential to note that the structural identification of ZIF-8 in the doped composites is not yet complete. The XRD patterns, especially at high dopant loading (10 wt%), are very different from those of regular ZIF-8. For example, there are sharp diffraction peaks that come from crystalline ZnFe₂O₃. These data imply that the structure is partially distorted and the phases are separated, which means that ZIF-8’s phase purity and crystallographic integrity are not satisfactory at high dopant concentrations. The optical studies demonstrated a reduction in the bandgap from 4.37 eV (pristine ZIF-8) to 4.26 eV for the doped composite, along with improved light absorption and extinction coefficients, highlighting the material’s enhanced light-harvesting capabilities. The photocatalytic performance of the composite was significantly superior, achieving a 90% degradation efficiency of methylene blue under visible light compared to 67.8% for pristine ZIF-8. This enhancement is attributed to the synergistic effects between ZnFe₂O₃ and ZIF-8, which improve charge carrier separation, reduce electron–hole recombination, and facilitate the generation of reactive oxygen species. These findings demonstrate the potential of ZnFe₂O₃-doped ZIF-8 composites as highly efficient photocatalysts for environmental remediation, particularly in wastewater treatment. This study provides a foundation for further exploration of ZnFe₂O₃-doped ZIF-8 composites in advanced energy and environmental applications. Future work could optimize the synthesis process, explore visible-light-driven photocatalytic performance, and scale up the material for industrial applications. Additionally, integrating this composite into multifunctional energy storage systems and its potential for broader environmental applications merits further investigation.