Synergistic Effects of Fe-Doped ZnO and Graphene Oxide for Enhanced Photocatalytic Performance and Tunable Magnetic Properties

Abstract

This study investigates the synthesis and characterization of Fe–ZnO/GO composites aimed at enhancing magnetic characteristics and improving the photocatalytic degradation of organic contaminants. Reflux condensation was used to synthesize Fe-doped ZnO, which was then mixed with graphene oxide (GO) to form a composite. Energy-dispersive X-ray spectroscopy and electron microscopy demonstrated homogeneous Fe–ZnO dispersion on GO sheets. In contrast, a structural study using X-ray diffraction confirmed the wurtzite phase of ZnO, with peak changes indicative of Fe integration. UV–visible spectroscopy demonstrated an expanded absorption range and a reduced bandgap of 2.76 eV in Fe–ZnO/GO compared to 3.06 eV in ZnO. Photoluminescence studies showed decreased electron–hole recombination due to GO’s electron-accepting properties. The Fe–ZnO/GO composite outperformed Fe–ZnO (97.46%) and ZnO (58.16%) in terms of photocatalytic degradation efficiency for Rhodamine B under UV light, achieving 99.32% degradation within 24 h. A first-order rate constant of 0.2093 h^–1, 5.80 times greater than ZnO, was found via kinetic analysis. The combined effects of GO for improved charge separation and reactive oxygen species generation, and Fe doping for enhanced light absorption, were attributed to the superior performance. Vibrating sample magnetometry confirmed the ferromagnetic behavior of Fe–ZnO/GO, with a saturation magnetization of 7.69 × 10^–4 emu/g. This indicates that the material can be easily separated using a magnetic field, which is beneficial for recycling and reuse. The enhanced photocatalytic activity, structural stability, and reusability make Fe–ZnO/GO a promising candidate for environmental remediation. However, further improvement in its visible-light response and long-term stability is needed for large-scale water treatment applications.

1. Introduction

Water pollution caused by organic contaminants such as dyes, pesticides, and pharmaceutical residues presents a major environmental and public health challenge worldwide. Traditional treatment methods like chemical oxidation, adsorption, and biological processes often fall short due to limited efficiency, high operational costs, and the risk of producing harmful byproducts. In contrast, semiconductor-based photocatalysis has gained attention as an eco-friendly and efficient alternative. − This process harnesses light energy to activate semiconductor materials, leading to the formation of reactive oxygen species (ROS) like hydroxyl (^•OH) and superoxide radicals (^•O₂ ^–), which can break down complex organic pollutants into harmless substances such as carbon dioxide and water, enabling effective and sustainable water purification. ,

Zinc oxide (ZnO) is widely recognized as a promising photocatalyst due to its excellent chemical stability, high electron mobility, and environmental compatibility. Its relatively wide bandgap (∼3.37 eV) allows effective absorption of ultraviolet (UV) light, promoting the generation of electron–hole pairs that drive redox reactions during photocatalysis. However, its practical application is significantly limited by two significant drawbacks: poor absorption in the visible spectrum as UV accounts for only a small fraction of solar energy and a high rate of electron–hole recombination, which restricts photocatalytic efficiency. , To address these challenges, several modification strategies have been developed, including transition metal doping, heterojunction engineering, and integration with conductive carbon-based materials. − Among these, iron (Fe) doping has proven particularly effective in improving ZnO’s photocatalytic performance by narrowing the bandgap for enhanced visible-light absorption and by acting as an electron trap to suppress recombination, thereby extending the lifetime of charge carriers and facilitating the formation of ROS. − Thambiliyagodage and Lokuge (2022) reported that Fe-doped ZnO exhibits improved pollutant degradation efficiency compared to pristine ZnO, underscoring its potential for environmental applications. However, excessive Fe can create recombination centers that hinder activity, making the optimization of dopant concentration essential. In parallel, the incorporation of GO offers an additional pathway to improve ZnO’s photocatalytic behavior. , GO features high electron mobility, a large surface area, and abundant functional groups, enabling it to act as an efficient electron acceptor that facilitates charge transfer, inhibits recombination, and enhances nanoparticle dispersion. − These properties not only increase light absorption and pollutant interaction but also promote photocatalytic stability. Several studies have reported that ZnO-GO composites exhibit superior photocatalytic efficiency due to their enhanced charge carrier dynamics and increased surface interactions with pollutants. − The synergistic integration of ZnO with both Fe dopants and GO has been shown to significantly enhance degradation efficiency under solar irradiation, offering a compelling approach for the development of high-performance photocatalysts for environmental applications.

Recent advancements in photocatalyst engineering have increasingly focused on the rational design of multifunctional composites to simultaneously improve light utilization, charge carrier separation, and recyclability. Among these, Fe–ZnO/GO composites represent a distinctive material platform that integrates optical, electronic, and magnetic functionalities into a single system. The inclusion of Fe not only modifies the band structure but also introduces redox-active sites (Fe³⁺/Fe²⁺) that serve as transient electron sinks, facilitating sequential redox reactions and enabling higher turnover frequencies in pollutant degradation. Unlike pure Fe-doped ZnO, the hybridization with GO provides a two-dimensional conductive interface that accelerates interparticle charge migration and reduces the accumulation of surface-trapped charges, which often lead to undesirable side reactions or deactivation. Additionally, the functional groups in GO contribute to pollutant adsorption, enhancing local concentration near active sites. A key innovation of this composite lies in its magnetic response, arising from Fe incorporation, which enables external-field-assisted recovery and addresses a long-standing challenge in catalyst reuse. The ability to engineer such magnetic-photocatalytic hybrids not only advances the field of water treatment but also reflects a broader shift toward circular material strategies that emphasize efficiency, recoverability, and minimal secondary impact. , Previous reports have shown that Fe–ZnO/GO composites exhibit superior photocatalytic efficiency compared to Fe-doped ZnO or ZnO-GO alone, underscoring the advantages of this hybrid approach. ,

This work aims to develop a multifunctional Fe–ZnO/GO composite that addresses critical limitations of conventional ZnO photocatalysts, including insufficient visible-light absorption, rapid electron–hole recombination, and low reusability. The novelty of this study lies in the synergistic integration of Fe dopants and GO sheets into the ZnO matrix to simultaneously enhance photocatalytic activity, charge separation, and magnetic recoverability. The material was synthesized via a reflux condensation method followed by calcination, producing well-dispersed Fe–ZnO particles anchored onto GO sheets. A comprehensive set of studies was carried out to evaluate structural, optical, charge-transfer, and magnetic properties, as well as photocatalytic degradation efficiency and reusability. Rhodamine B (RhB) was selected as the model pollutant, and its degradation pathway was investigated under UV irradiation. The proposed mechanism involves the generation of ^•OH and ^•O₂ ^– radicals that initiate oxidative attacks on the chromophore structure of RhB, followed by stepwise ring-opening reactions and mineralization into nontoxic end products such as CO₂ and H₂O. The integration of redox-active Fe³⁺/Fe²⁺ centers promotes charge trapping and ROS generation, while GO enhances pollutant adsorption and interfacial charge migration. Together, these features yield a composite with superior degradation rates, magnetic separability, and long-term operational stability, making it a strong candidate for sustainable wastewater treatment applications.

2. Experimental Sections

2.1. Synthesis of ZnO and Fe-Doped ZnO Particles

Reflux condensation was employed to produce the ZnO particles, ensuring high purity and controlled development. First, 100 mL of distilled water was used to dissolve 0.1 M zinc acetate dihydrate (Zn­(CH₃COO)₂·2H₂O) while being constantly stirred to guarantee even dispersion. For 30 min, the solution was agitated with 0.01 M ethylenediaminetetraacetic acid (EDTA) to aid in the complexation process. Once the pH of the solution reached 10, a 10 M sodium hydroxide (NaOH) solution was added dropwise while being stirred to promote uniform nucleation and development of ZnO particles. To encourage the controlled synthesis of ZnO nanoparticles, the resultant mixture was subsequently transferred to a 250 mL round-bottom flask and heated to 90 °C for 6 h while undergoing hydrothermal reaction under reflux conditions. Following the reaction’s conclusion, the solution was allowed to naturally cool to room temperature. To remove any remaining contaminants, the filtered ZnO precipitate was repeatedly washed with distilled water and ethanol. Following a 24 h oven drying process at 60 °C to remove any residual moisture, the purified precipitate was annealed for 2 h at 600 °C to improve phase purity and crystallinity using ferric nitrate nonahydrate (Fe­(NO₃)₃·9H₂O) as the iron precursor, the same process was used to inject 5 mol % Fe into ZnO to create Fe-doped ZnO. This modified the structural and electrical characteristics of the ZnO lattice.

2.2. Synthesis of Fe–ZnO/GO Composite

The Fe–ZnO/GO composite was synthesized using a reflux condensation method to ensure uniform incorporation of Fe–ZnO onto GO sheets. First, 0.1 g of Fe–ZnO powder and a GO dispersion (1 mg/mL) were separately prepared in distilled water under continuous stirring to maintain homogeneity. To achieve better dispersion, the GO solution was sonicated for 30 min before being combined with the Fe–ZnO precursor solution. The resulting mixture was further stirred for 1 h to ensure uniform interaction between the Fe–ZnO particles and the GO sheets. The well-mixed suspension was then transferred into a 250 mL round-bottom flask and subjected to a hydrothermal reaction under reflux conditions at 90 °C for 6 h. This process facilitated the attachment of Fe–ZnO nanoparticles onto the GO sheets, enhancing the composite’s structural integrity and functional properties. After the reaction was completed, the solution was cooled naturally to room temperature. The formed Fe–ZnO/GO composite was collected through filtration and thoroughly washed multiple times with distilled water and ethanol to remove any unreacted residues or impurities. The purified composite was then dried in an oven at 60 °C for 24 h to obtain the final Fe–ZnO/GO powder. The synthesized composite was stored and prepared for further structural, optical, and photocatalytic characterization.

2.3. Photocatalytic Activity Study for Rhodamine B Degradation

The photocatalytic performance of ZnO, Fe–ZnO, and Fe–ZnO/GO composites was evaluated by monitoring the degradation of Rhodamine B (RhB) dye under UV light irradiation. A 10 ppm RhB solution was prepared as the target pollutant, and 50 mg of each synthesized photocatalyst (ZnO, Fe–ZnO, or Fe–ZnO/GO) was dispersed in 100 mL of the dye solution in a beaker. To ensure proper interaction between the catalyst and dye molecules, the suspension was stirred in the dark for 1 h, allowing adsorption–desorption equilibrium to be established. After this pretreatment, the solution was subjected to UV light irradiation within a photocatalytic reactor, where it was continuously stirred to maintain uniform exposure to the catalyst. At predetermined time intervals, small aliquots of the reaction mixture were extracted and centrifuged to separate the photocatalyst particles. The concentration of RhB in the solution was then analyzed by measuring its absorbance at 554 nm using a UV–vis spectrometer. The progressive reduction in absorbance over time indicated the degradation of RhB, allowing the photocatalytic efficiency of each material to be determined based on its ability to break down the dye under UV light exposure. The photocatalytic activity tests were conducted using a black light source (Model: Toshiba FL40T8BL/18W) with an intensity of 2 mW/cm². During the experiments, the distance between the light source and the sample surface was consistently maintained at 60 cm to ensure uniform irradiation of the sample.

Scavenger tests were performed to determine the main reactive species responsible for the degradation of RhB. Different chemical scavengers were used to selectively inhibit specific reactive species: isopropanol (IPA, 10 mM) for hydroxyl radicals (^•OH), ascorbic acid (AA, 10 mM) for superoxide radicals (^•O₂ ^–), silver nitrate (AgNO₃, 10 mM) for electrons (e^–), and sodium sulfite (SS, 10 mM) for holes (h⁺). Each scavenger was added to 100 mL of a 10 ppm RhB solution before the addition of 50 mg of the photocatalyst. The solution was stirred in the dark for 1 h to achieve adsorption–desorption equilibrium, then exposed to UV light. After 24 h of irradiation, samples were collected and analyzed at 554 nm to assess dye degradation. To evaluate catalyst reusability, the photocatalyst was recovered after each cycle, thoroughly washed with distilled water and ethanol, dried at 60 °C overnight, and reused for three cycles under identical conditions to assess its stability and recyclability.

3. Results and Discussion

3.1. XRD Characterization of Crystalline Phases and Lattice Parameters

The X-ray diffraction (XRD) analysis of ZnO, Fe–ZnO, and Fe–ZnO/GO composites provide insight into their structural characteristics. As shown in Figure a, the ZnO sample exhibits diffraction peaks corresponding to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), (202), and (104) planes, confirming the formation of a hexagonal wurtzite structure. These peaks match standard ZnO reference data (JCPDS card no. 2300113), indicating the phase purity of the material. The sharp and intense peaks suggest high crystallinity with well-defined lattice planes. In the Fe–ZnO sample, the same ZnO peaks are present without additional peaks corresponding to iron oxides. The absence of secondary phases suggests that Fe ions successfully substitute for Zn ions within the ZnO lattice, rather than forming separate iron oxide compounds. The incorporation of Fe into the ZnO matrix induces lattice strain, evident from the slight broadening of diffraction peaks compared to pristine ZnO. This broadening indicates a reduction in crystallite size and structural distortion due to Fe incorporation. The substitution of Zn²⁺ ions with Fe²⁺ ions introduces defects and strain within the crystal lattice, influencing the material’s structural properties.

1.

(a) XRD spectra and (b) corresponding peak shifts for ZnO, Fe–ZnO, and Fe–ZnO/GO samples.

For the Fe–ZnO/GO composite, the diffraction peaks remain consistent with the wurtzite ZnO structure, confirming that the ZnO lattice remains intact. However, no distinct peaks for GO are observed around 10°, suggesting that GO sheets are successfully exfoliated and uniformly dispersed within the composite. This dispersion enhances the interaction between ZnO and GO, which can improve the composite’s photocatalytic and electronic properties. The peak broadening in both Fe–ZnO and Fe–ZnO/GO further indicates lattice strain, commonly observed when Zn²⁺ ions are replaced by Fe²⁺ ions due to their comparable ionic radii (Zn²⁺: 0.074 nm, Fe²⁺: 0.078 nm). Figure b illustrates the peak shifts observed in ZnO, Fe–ZnO, and Fe–ZnO/GO composites. The diffraction peaks of Fe–ZnO and Fe–ZnO/GO shift slightly toward lower angles compared to pristine ZnO, suggesting lattice expansion due to Fe incorporation. This trend is consistent with previous studies, which have shown that Fe doping in ZnO leads to peak shifts toward lower angles, indicating lattice distortion. The broadening of diffraction peaks in Fe–ZnO and Fe–ZnO/GO suggests a decrease in crystallite size, supporting the notion that increased Fe content leads to reduced crystallite dimensions. Additionally, the reduced intensity of these peaks implies a decline in crystallinity, potentially due to strain introduced by Fe ions within the ZnO lattice. The slight difference in ionic radii between Fe²⁺ and Zn²⁺ contributes to lattice distortions, which have been reported in Fe-doped ZnO systems as a source of structural perturbations. These structural modifications, induced by Fe doping and GO incorporation, are expected to influence the material’s electronic and photocatalytic properties.

To evaluate the impact of Fe and GO incorporation in ZnO, the average crystallite size (D) was determined using the Debye–Scherrer formula, along with lattice constants (a = b and c) and microstrain, which represents distortion in the crystalline lattice. , The calculated parameters, summarized in Table S1, show that the lattice parameters and c/a ratio of ZnO are consistent with previously reported values for pristine ZnO nanoparticles. However, Fe incorporation led to an increase in these values, suggesting that Fe ions occupy interstitial positions within the ZnO lattice, thereby restricting crystallite growth. A similar trend was observed with GO incorporation, indicating that GO formed chemical bonds with the Fe–ZnO crystal, weakening host atomic bonds and modifying the structural parameters. Additionally, Fe–ZnO and Fe–ZnO/GO composites exhibit higher microstrain than pristine ZnO, implying the formation of defect states within the crystal lattice, which may introduce impurity levels in the energy bands. As a result, the Fe–ZnO/GO composite is expected to show enhanced catalytic activity due to these structural modifications. The structural changes in the material are evident from the variations in crystallite size. Upon doping ZnO with Fe and incorporating GO, a significant reduction in crystallite size was observed. The size decreased from 29.41 nm for pure ZnO to 22.05 nm for Fe–ZnO, and slightly increased to 23.31 nm in the Fe–ZnO/GO composite. This reduction is primarily due to lattice strain caused by the introduction of Fe³⁺ ions, which either substitute for Zn²⁺ in the crystal lattice or occupy interstitial sites, thereby disrupting regular crystal growth. Additionally, GO acts as a two-dimensional support that prevents particle agglomeration and limits crystal expansion, helping to maintain smaller crystallite sizes. Together, the effects of Fe doping and GO incorporation suppress grain growth and increase surface area, which are advantageous for improving photocatalytic performance.

3.2. Surface Morphology and Elemental Composition Study via SEM-EDX

The morphological characteristics of ZnO, Fe–ZnO, and Fe–ZnO/GO composites were analyzed using SEM, revealing distinct structural features. Figure a shows that pristine ZnO consists mainly of nearly spherical particles, indicating uniform nucleation and growth. However, Fe–ZnO, as seen in Figure b, exhibits cone-like structures, suggesting that Fe doping affects the anisotropic growth of ZnO crystals by introducing lattice distortions. The SEM image of Fe–ZnO/GO in Figure c further demonstrates that the cone-like Fe–ZnO particles are uniformly dispersed on GO sheets, indicating strong interfacial interactions that can enhance the composite’s functional properties. Additionally, elemental mapping (Figure d–g) confirms the homogeneous distribution of Zn, O, Fe, and C elements within the Fe–ZnO/GO composite, verifying the successful integration of all components. This uniform dispersion is essential for ensuring consistent photocatalytic performance by providing evenly distributed active sites throughout the composite.

2.

Surface morphology of (a) ZnO, (b) Fe–ZnO, and (c) Fe–ZnO/GO as observed via SEM; (d–g) elemental mapping of Fe–ZnO/GO for Zn, O, Fe, and C.

The elemental composition of ZnO, Fe–ZnO, and Fe–ZnO/GO composites was analyzed using EDX to confirm the successful incorporation of dopants and ZnO nanoparticle formation (Figure S1). The EDX spectrum of pristine ZnO (Figure S1a) displayed strong signals for zinc (Zn) and oxygen (O) with atomic percentages of 53.63 and 46.37%, respectively, confirming the formation of ZnO with a near-stoichiometric ratio. For Fe–ZnO (Figure S1b), the presence of Fe at 3.69%, along with Zn (49.11%) and O (47.19%), indicated successful Fe incorporation into the ZnO lattice. The slight reduction in Zn content compared to pristine ZnO suggests partial substitution of Zn ions by Fe ions. In the Fe–ZnO/GO composite (Figure S1c), the EDX spectrum revealed a high carbon (C) content of 69.98%, along with oxygen (23.41%), zinc (6.28%), and iron (0.32%), confirming the successful integration of GO into the composite. The presence of Fe and Zn further validates the formation of Fe–ZnO on the GO surface, with the high carbon content attributed to the GO sheets, ensuring their effective incorporation within the composite structure.

3.3. Analysis of Surface Area and Porosity via Nitrogen Adsorption

The nitrogen adsorption–desorption isotherms and Brunauer-Emmett-Teller (BET) analysis were used to study the surface properties of ZnO, Fe–ZnO, and Fe–ZnO/GO (Figure S2 and Table S2). All three samples exhibited type IV isotherms with hysteresis loops, a typical feature of mesoporous materials. The pore sizes of these samples fall within the mesoporous range of 2 to 50 nm, confirming their porous nature. Among the materials, Fe–ZnO/GO had the highest BET surface area of 20.87 m²/g, followed by Fe–ZnO at 13.20 m²/g, and ZnO at 7.05 m²/g. This increase in surface area reflects the effects of Fe doping and GO addition on the material’s structure. Fe doping inhibits crystal growth and introduces defects, creating more adsorption sites and increasing the surface area. The addition of GO further boosts surface area due to its sheet-like structure, which enhances porosity and exposes more active sites. The total pore volume also increased from 3.62 × 10^–2 cm³/g for ZnO to 8.50 × 10^–2 cm³/g for Fe–ZnO/GO. Meanwhile, the average pore size decreased from 30.41 nm in ZnO to 18.71 nm in Fe–ZnO/GO, indicating that the pores became more compact and uniform. These improvements in surface area and porosity enhance the materials’ ability to absorb light and provide greater interaction between pollutants and the active surface, which is advantageous for photocatalytic and adsorption applications.

3.4. Absorption and Band Gap Analysis via UV–Vis Spectroscopy

The UV–Vis absorption spectra of ZnO, Fe–ZnO, and Fe–ZnO/GO composites were analyzed to understand their optical properties and bandgap energies. As shown in Figure a, pristine ZnO exhibits a distinct absorption peak at approximately 375 nm, characteristic of its hexagonal wurtzite structure and intrinsic bandgap absorption, resulting from electron transitions from the valence band to the conduction band. , Upon Fe doping, the absorption peak shifts slightly to around 377 nm, indicating modifications in ZnO’s electronic structure due to Fe incorporation. This shift suggests the introduction of additional electronic states by Fe ions, enhancing visible light absorption. Additionally, a slight reduction in absorbance intensity is observed in Fe–ZnO, likely due to changes in crystallinity and defect states within the ZnO lattice. The Fe–ZnO/GO composite exhibits an absorption peak at approximately 378 nm, further red-shifted compared to pristine ZnO and Fe–ZnO, suggesting the formation of localized states within the bandgap due to the interaction between Fe–ZnO and the GO matrix. The integration of GO introduces additional electronic states, promoting enhanced visible light absorption. Similar trends have been observed in other studies, where graphene-based materials combined with metal oxides lead to extended absorption into the visible region, effectively narrowing the bandgap and improving photocatalytic performance.

3.

(a) UV–visible absorbance spectra of ZnO, Fe–ZnO, and Fe–ZnO/GO at room temperature, and (b–d) Tauc plots of ZnO, Fe-ZnO, and Fe-ZnO/GO, respectively.

The optical bandgap (E_g) of ZnO, Fe–ZnO, and Fe–ZnO/GO composites was determined using Tauc plots derived from UV–Vis absorption data, as shown in Figure b–d. Pristine ZnO exhibited a bandgap of approximately 3.06 eV, consistent with reported values for ZnO nanostructures. Upon Fe doping, the bandgap decreased to 2.91 eV, indicating the formation of localized impurity states near the conduction band. These midgap states, introduced by Fe ions, serve as intermediate energy levels that facilitate electron transitions, thereby reducing the excitation energy required. A further bandgap reduction was observed in the Fe–ZnO/GO composite, where E_g dropped to 2.76 eV. The interaction between the Fe–ZnO matrix and the sp² hybridized carbon networks in GO is responsible for this narrowing, which introduces more midgap states. Under visible light irradiation, the addition of GO greatly increases photocatalytic efficiency by modulating the bandgap and enhancing the separation of photogenerated electron–hole pairs. This synergistic interaction between GO and metal oxides has been well-documented, emphasizing GO’s crucial role in bandgap tuning and charge carrier dynamics.

Mott–Schottky analysis was employed to gain a deeper understanding of the photocatalytic mechanism by determining the flat band potentials (Vfb) of ZnO, Fe–ZnO, and Fe–ZnO/GO, as illustrated in Figure a. All samples showed positive slopes in their C^2––V plots, confirming they are n-type semiconductors. The flat band potentials, obtained by extrapolating the x-intercepts, were approximately −0.52 V for ZnO, −0.59 V for Fe–ZnO, and −0.88 V for Fe–ZnO/GO versus Ag/AgCl. These shifts indicate that Fe doping and GO incorporation significantly alter ZnO’s electronic structure. Since the flat band potential in n-type semiconductors approximates the conduction band (CB) edge, the valence band (VB) positions were calculated using the optical bandgap values from UV–vis data, summarized in Figure b. ZnO showed a CB at −0.52 V and VB at +2.54 V (bandgap 3.06 eV), Fe–ZnO had a CB at −0.59 V and VB at +2.32 V (bandgap 2.91 eV), and Fe–ZnO/GO’s CB shifted further negative to −0.88 V with a VB at +1.88 V (bandgap 2.76 eV). To compare these with standard redox potentials, the values were converted from Ag/AgCl to the standard hydrogen electrode (NHE) scale. On this scale, the CB/VB potentials are −0.32 V/+2.74 V for ZnO, −0.39 V/+2.52 V for Fe–ZnO, and −0.68 V/+2.08 V for Fe–ZnO/GO. Importantly, all CB potentials are more negative than the O₂/^•O₂ ^– redox potential (−0.33 V vs NHE), enabling the generation of superoxide radicals essential for photocatalysis. The VB positions of ZnO and Fe–ZnO are sufficiently positive to oxidize water to hydroxyl radicals (^•OH), which require +2.38 V, while Fe–ZnO/GO’s VB is slightly lower but may still support oxidation under UV light. Overall, the band structure of Fe–ZnO/GO is optimized for enhanced charge separation and redox reactions, explaining its superior photocatalytic performance due to Fe doping and GO incorporation.

4.

(a) Mott–Schottky plots of ZnO, Fe–ZnO, and Fe–ZnO/GO electrodes measured in 0.3 M KOH solution at room temperature, showing their semiconductor properties. (b) Schematic energy band diagrams of the samples obtained from Mott–Schottky and UV–vis analysis, illustrating the band edge positions and electronic structure.

3.5. PL Spectroscopy for Band Gap and Defect State Analysis

The photoluminescence (PL) spectra of ZnO, Fe–ZnO, and Fe–ZnO/GO composites, shown in Figure , reveal crucial insights into their electronic structures and defect states. Pristine ZnO exhibits a strong emission peak around 398 nm in the UV region, corresponding to near-band-edge emission, which represents the intrinsic recombination of free excitons. This emission results from the recombination of electrons and holes in the CB and VB. Upon Fe doping, a noticeable quenching of the UV emission is observed, alongside the emergence of visible emission bands between 450 and 600 nm. , The reduction in UV emission intensity suggests that Fe ions introduce nonradiative recombination centers within the ZnO lattice, capturing charge carriers and diminishing near-band-edge emission. The visible emissions are attributed to defect levels introduced by Fe doping, such as oxygen vacancies and interstitial zinc, which create localized states within the bandgap, increasing structural imperfections. The Fe–ZnO/GO composite further alters the PL characteristics, significantly suppressing both UV and visible emissions. The integration of GO, known for its high electron mobility, enables efficient charge transfer from ZnO to GO, reducing the probability of radiative recombination by swiftly transferring excited electrons. This leads to a decrease in near-band-edge and defect-related emissions, indicating enhanced separation of photogenerated charge carriers, which is advantageous for photocatalytic applications. These findings align with previous studies, such as those by Popa et al., which reported that Fe-doped ZnO nanoparticles decorated with carbon nanotubes exhibited reduced PL intensity, demonstrating effective charge separation and lower recombination rates.

5.

Comparison of photoluminescence responses in ZnO, Fe–ZnO, and Fe–ZnO/GO.

3.6. Characterization of Chemical Bonds Using FT-IR Analysis

FT-IR spectroscopy was used to analyze the functional groups and chemical bonding in ZnO, Fe–ZnO, and Fe–ZnO/GO composites across the 400–4000 cm^–1 range (Figure ). This analysis aimed to understand how Fe doping and GO incorporation affect the vibrational structure and surface chemistry of ZnO. A broad absorption band centered around 3444 cm^–1 appeared in all samples, corresponding to O–H stretching vibrations from surface hydroxyl groups and physically adsorbed water molecules. These hydroxyl groups are important because they contribute to surface activity and facilitate charge transfer during photocatalysis. A peak near 2337 cm^–1 was attributed to asymmetric stretching vibrations of atmospheric CO₂, which is commonly present due to environmental exposure during sample handling. The bands at 1514 and 1327 cm^–1 correspond to CO and C–H stretching vibrations, respectively. These likely originate from residual organic groups or surface-bound species left over from the synthesis process or the incomplete removal of byproducts, which is typical in chemically prepared nanomaterials. , In the lower-frequency region (400–600 cm^–1), strong absorption bands correspond to metal–oxygen vibrations, confirming the presence of Zn–O bonds characteristic of ZnO in all samples.

6.

Functional group identification via FT-IR spectra for ZnO, Fe–ZnO, and Fe–ZnO/GO.

In the Fe–ZnO and Fe–ZnO/GO composites, an additional shoulder or broadening near 560 cm^–1 was detected, which is attributed to Fe–O stretching vibrations. This confirms the successful incorporation of Fe ions into the ZnO crystal lattice and indicates lattice distortion caused by Fe doping, consistent with XRD results. Furthermore, a distinct peak around 1580 cm^–1 appeared exclusively in the Fe–ZnO/GO composite, assigned to CC or CO stretching vibrations from the sp²-hybridized carbon domains of GO. This peak confirms that GO was successfully integrated into the composite, suggesting strong interactions between the GO sheets and the metal oxide surface. , Overall, the comparative FT-IR spectra reveal that doping with Fe and adding GO modify not only the surface functional groups but also the vibrational modes of the ZnO lattice, supporting the formation of a chemically bonded composite material. These structural changes are critical because they influence charge carrier behavior and surface reactivity, which in turn affect the photocatalytic efficiency of the materials.

3.7. Surface Characterization Using XPS Analysis

XPS analysis was performed to investigate the chemical composition and oxidation states of ZnO, Fe–ZnO, and Fe–ZnO/GO composites, providing insights into their surface chemistry and electronic interactions. The high-resolution XPS spectra of Zn 2p, O 1s, Fe 2p, and C 1s (Figure ) confirm the elemental composition of the synthesized materials. The Zn 2p spectra for all samples (Figure a–c) exhibit two distinct peaks at approximately 1020.3 eV (Zn 2p_3/2) and 1043.4 eV (Zn 2p_1/2), which are characteristic of Zn²⁺ in the ZnO lattice, consistent with previously reported values. , These peaks confirm that Zn exists predominantly in the +2-oxidation state. The slight shift observed in the Zn 2p peak position for Fe–ZnO, compared to pristine ZnO, indicates that there may be an interaction between Fe³⁺ and Zn²⁺ ions. This shift suggests that Fe ions are either partially incorporated into the ZnO crystal lattice or are coordinated at the surface. When this finding is combined with the XRD results, it reinforces the idea that Fe doping causes structural distortions in the ZnO matrix. These distortions can alter the local electronic environment, which in turn may affect the material’s charge distribution, electronic structure, and overall physicochemical properties. Such changes are essential as they can enhance the photocatalytic behavior and reactivity of the composite.

7.

Deconvoluted XPS spectra of ZnO, Fe–ZnO, and Fe–ZnO/GO: (a–c) Zn 2p, (d–f) O 1s, (g–h) Fe 2p, and (i) C 1s.

The O 1s XPS spectra of ZnO, Fe–ZnO, and Fe–ZnO/GO composites (Figure d–f) reveal two deconvoluted peaks corresponding to different oxygen species within the materials. The first peak at approximately 530.1 eV (O1) is associated with lattice oxygen (O₂ ^–) in ZnO, which is essential for structural stability. The second peak at 531.1 eV (O₂) corresponds to oxygen vacancies, which significantly impact the electronic and catalytic properties of ZnO-based materials. Compared to pristine ZnO, Fe–ZnO exhibits an increased intensity of the oxygen vacancy peak, indicating that Fe doping enhances defect formation, which is beneficial for charge transport and catalytic activity. , The Fe–ZnO/GO composite exhibits a higher concentration of oxygen vacancies, indicating strong chemical interactions between ZnO, Fe, and GO. This observation aligns with previous studies on Fe-doped ZnO, where Fe incorporation promotes structural modifications and defect generation, enhancing the material’s functional performance. −

The Fe 2p and C 1s XPS spectra of ZnO, Fe–ZnO, and Fe–ZnO/GO composites (Figure g–i) provide insights into the oxidation state of Fe and the presence of GO in the composites. The Fe 2p spectrum for Fe–ZnO and Fe–ZnO/GO exhibits characteristic peaks at 711.5 eV (Fe 2p_3/2) and 724.6 eV (Fe 2p_1/2), confirming the presence of Fe³⁺ species, with a satellite peak at 717.3 eV further validating this oxidation state. , In the Fe–ZnO/GO composite, the C 1s spectrum reveals three prominent peaks at 283.3 eV (C–C/C–H), 284.8 eV (C–O), and 286.9 eV (CO), confirming the presence of GO. The increased intensity of oxygen-containing functional groups in the C 1s spectrum indicates strong interfacial interactions between GO and metal oxides, which can facilitate efficient charge transfer, thereby enhancing the composite’s electronic and photocatalytic properties.

3.8. Hysteresis Loop and Magnetic Property Evaluation via VSM

The magnetic properties of ZnO, Fe–ZnO, and Fe–ZnO/GO composites were analyzed using VSM to determine the effects of Fe doping and GO incorporation on their magnetic behavior. The hysteresis loops presented in Figure and the key magnetic parameters summarized in Table S3 provide insights into the magnetic modifications. Pristine ZnO exhibited weak magnetic behavior, with a saturation magnetization (M _s) of 4.09 × 10–4 emu/g, a remanent magnetization (M _r) of 9.62 × 10^–4 emu/g, and a high coercivity (H _c) of 496.50 Oe. These values align with reports on undoped ZnO, which generally exhibits diamagnetic or weak paramagnetic behavior due to its closed-shell electronic configuration and the absence of unpaired electrons. However, some studies suggest that intrinsic defects, such as oxygen vacancies and zinc interstitials, may induce weak room-temperature ferromagnetism in ZnO nanostructures. − The high coercivity observed in pristine ZnO indicates strong resistance to magnetization reversal, which is characteristic of materials with limited exchange interactions.

8.

Magnetic hysteresis loops of ZnO, Fe–ZnO, and Fe–ZnO/GO.

Fe doping significantly enhanced the ferromagnetic behavior of ZnO, with M _s increasing to 5.87 × 10^–4 emu/g, M _r rising to 18.26 × 10^–6 emu/g, and H _c decreasing substantially to 19.53 Oe. Because Fe³⁺ ions replace Zn²⁺ ions in the ZnO lattice, unpaired 3d electrons are introduced, facilitating exchange interactions and promoting ferromagnetic ordering. Previous studies on Fe-doped ZnO systems have reported similar findings, where Fe ions enhance room-temperature ferromagnetism through defect-mediated long-range exchange interactions. , Additionally, research by Madkhali demonstrated that codoping ZnO with Fe and Co further enhanced ferromagnetic properties. The significant reduction in coercivity upon Fe doping indicates decreased magnetic anisotropy, allowing for easier magnetization switching, which is advantageous for spintronic applications. These results highlight the role of Fe doping in tailoring the magnetic properties of ZnO-based materials, making them suitable for advanced technological applications.

The incorporation of GO into Fe–ZnO further enhanced ferromagnetic behavior, increasing the M _s to 7.69 × 10^–4 emu/g while significantly reducing the M _r to 3.45 × 10^–4 emu/g and H _c to 6.25 Oe. The incorporation of GO into Fe–ZnO improved its magnetic response to some extent, indicating successful interaction between the components. However, the measured saturation magnetization of the Fe–ZnO/GO composite is only 7.69 × 10^–4 emu/g, which is relatively low when compared to typical magnetic materials. This low magnetization means that the material does not respond strongly enough to external magnetic fields, making it unsuitable for effective magnetic separation especially in large-scale or practical applications where quick and complete recovery of the photocatalyst is needed. Therefore, while the composite shows some magnetic properties, they are insufficient for practical magnetic-based recovery methods. The decrease in M _r and H _c suggests that GO influences magnetic domain dynamics by introducing additional defect states and reducing magnetic anisotropy. Similar effects have been reported in GO-based magnetic nanocomposites, where GO facilitates electron delocalization and modifies the magnetic coupling between transition metal ions. The squareness ratio (M _r/M _s) provides insight into the nature of magnetic interactions, with values greater than 0.5 indicating strong magnetic coupling and lower values suggesting superparamagnetic-like behavior. ZnO and Fe–ZnO exhibited similar M _r/M _s ratios of 2.35 × 10^–2 and 2.08 × 10^–2, respectively, implying weak magnetic coupling, whereas Fe–ZnO/GO showed a significantly lower ratio of 0.45 × 10^–2, suggesting that GO further reduces remanence by altering interfacial exchange interactions.

These observations align with previous studies on Fe-doped ZnO and ZnO–GO composites, confirming that GO plays a crucial role in tuning the magnetic properties. For instance, Thiyagarajan et al. demonstrated that reduced graphene oxide (rGO)–ZnO composites exhibit room-temperature ferromagnetism, which is attributed to defect states, vacancies, and interfacial effects within rGO and rGO–ZnO structures. Similarly, Ray and Pong (2022) reported that ZnO nanorods decorated with nanocrystalline Au particles enhance ferromagnetism through defect-mediated exchange interactions, while GO incorporation further modifies the material’s electronic and magnetic characteristics. These findings highlight GO’s potential to influence ferromagnetic behavior by modulating electronic interactions, defect structures, and interfacial coupling within nanocomposite systems.

3.9. Photocatalytic Degradation Efficiency Study of Rhodamine B

By detecting RhB degradation under UV light irradiation for 24 h, the photocatalytic activity of ZnO, Fe–ZnO, and Fe–ZnO/GO composites was evaluated. While the absorbance spectra of RhB solutions following degradation employing the various photocatalysts are shown in Figure S3b–d, Figure S3a depicts the characteristic absorbance spectrum of RhB. The absorbance peak at 554 nm was used as the reference for the initial RhB concentration (C ₀), and subsequent absorbance measurements at different times (t) were used to determine the remaining concentration (C). A plot of C/C ₀ vs time (Figure a) visually represents the degradation trend, while the photocatalytic degradation efficiency was calculated using eq . The estimated values, summarized in Table S4 and illustrated in Figure b, provide a comparative analysis of the effectiveness of each photocatalyst in breaking down RhB under UV light exposure.

$$D(\%)=\left(\frac{C_0-C}{C_0}\right)\times 100$$ 1

9.

Kinetic analysis of RhB photocatalytic degradation with ZnO, Fe–ZnO, and Fe–ZnO/GO: (a) C/C₀ vs time, (b) percentage degradation, (c) pseudo-first-order kinetic fitting, and (d) corresponding rate constants.

The degradation vs time plot (Figure b) clearly demonstrates the improved photocatalytic performance resulting from Fe doping and GO incorporation into ZnO. Before UV exposure, the RhB dye solution was kept in the dark for 1 h to assess removal due to physical adsorption alone, which was minimal, 3.76% for ZnO, 4.79% for Fe–ZnO, and 4.12% for Fe–ZnO/GO. This confirms that the majority of dye degradation occurs through photocatalysis rather than simple adsorption. After 24 h of UV irradiation, ZnO achieved 58.17% degradation efficiency, while Fe–ZnO reached 97.46%, indicating that Fe doping significantly enhances photocatalytic activity by improving charge separation and introducing beneficial surface defects. The Fe–ZnO/GO composite further improved degradation efficiency to 99.32%, demonstrating a strong synergistic effect between Fe and GO. The incorporation of GO likely enhances electron mobility, reduces recombination of charge carriers, and facilitates the generation of more reactive oxygen species (ROS), leading to near-complete dye degradation. Figure c presents the kinetic analysis of RhB degradation, modeled using the pseudo-first-order kinetic equation (eq ), providing insights into the reaction rate and catalytic efficiency of the synthesized materials.

$$\ln \left(\frac{C}{C_0}\right)=-\mathit{kt}$$ 2

where C ₀ is the initial concentration before irradiation, C is the concentration at time t, and k is the rate constant.

The photocatalytic degradation rate constants for ZnO, Fe–ZnO, and Fe–ZnO/GO were determined to be 0.0361, 0.1510, and 0.2093 h^–1, respectively, as shown in Figure d. The Fe–ZnO composite exhibits a rate constant that is approximately 4.18 times higher than that of pristine ZnO, demonstrating the significant role of Fe doping in enhancing photocatalytic activity. Furthermore, the Fe–ZnO/GO composite achieves an even greater improvement, with a rate constant 5.80 times higher than ZnO and 1.39 times higher than Fe–ZnO, highlighting the critical contribution of GO in further optimizing photocatalysis. This enhanced performance is attributed to the synergistic effects of Fe and GO, which improve light absorption, reduce the bandgap, and facilitate efficient charge separation. , The presence of GO increases the surface area and provides efficient electron transport pathways, thereby suppressing electron–hole recombination and accelerating photocatalytic degradation. , These findings suggest that Fe–ZnO/GO is an auspicious material for advanced photocatalytic applications.

To evaluate the photocatalytic performance of the synthesized materials, commercial TiO₂ (P25) was used as a benchmark under the same UV irradiation conditions (Figure S4). P25 demonstrated rapid and highly efficient RhB degradation, achieving 99.51% efficiency within just 6 h, with a high-rate constant of 0.8042 h^–1 (Figure S4a–e). In comparison, the Fe–ZnO/GO composite required a longer irradiation time of 24 h to reach a similar degradation level (99.32%). However, it still showed a marked improvement over pristine ZnO and Fe–ZnO, which had lower efficiencies and slower kinetics. The calculated rate constants for ZnO, Fe–ZnO, and Fe–ZnO/GO were 0.0361, 0.1510, and 0.2093 h^–1, respectively, confirming that Fe doping and GO integration significantly enhanced photocatalytic activity. Although Fe–ZnO/GO did not match the fast kinetics of P25, its high final degradation efficiency and improved performance under low-intensity UV light make it a promising alternative. The comparatively lower rate constant is likely due to differences in textural and electronic properties between the synthesized materials and P25. Nonetheless, Fe–ZnO/GO’s photocatalytic behavior is noteworthy, especially considering its potential cost-effectiveness and tunable properties. A detailed comparison with previous literature is provided in Table S5.

The LC–MS (liquid chromatography–mass spectrometry) analysis was performed to understand how RhB is broken down during photocatalysis with different catalysts (Figure ). The untreated RhB solution (Figure a) showed a strong signal at m/z = 443.3, corresponding to the intact RhB molecule, with no other major peaks confirming the dye’s purity. After 24 h of UV exposure with ZnO, Fe–ZnO, and Fe–ZnO/GO catalysts (Figure b–d), the intensity of this RhB peak significantly dropped, and several new peaks appeared at lower m/z values (359.2, 315.3, 214.3, 182.1, 166.0, 132.0, and 74.9). These emerging peaks suggest that RhB undergoes stepwise degradation through processes such as de-ethylation (removal of ethyl groups), deamination (removal of amine groups), and aromatic ring cleavage. Among the samples, Fe–ZnO/GO showed the most effective degradation, its spectrum revealed almost complete disappearance of the RhB peak and the presence of many smaller molecular fragments. This indicates that Fe–ZnO/GO was the most efficient at fully mineralizing the dye into simpler, likely nontoxic molecules such as CO₂ and H₂O. The results confirm that Fe–ZnO/GO enhances the generation of reactive oxygen species (such as ^•OH and ^•O₂ ^–) leading to more complete and efficient dye breakdown.

10.

LC–MS spectra of RhB before and after photocatalytic degradation: (a) Standard RhB solution showing the intact molecular ion at m/z = 443.3, (b) degraded RhB after treatment with ZnO, (c) degraded RhB following treatment with Fe–ZnO, and (d) degraded RhB after treatment with Fe–ZnO/GO. The progressive disappearance of the parent peak and the emergence of lower m/z fragments confirm stepwise degradation and enhanced mineralization, particularly in the presence of Fe–ZnO/GO.

Figure presents the proposed photocatalytic degradation pathway of RhB, which is based on the intermediate products detected by LC–MS analysis. Initially, the molecular ion of RhB appears at m/z = 443, corresponding to its complete chemical structure (C₂₈H₃₁ClN₂O₃). When exposed to UV light, RhB undergoes de-ethylation, resulting in an intermediate fragment at m/z = 359 (C₂₂H₁₉N₂O₃). This compound then experiences deamination, forming another fragment at m/z = 315 (C₂₀H₁₄NO₃). Further degradation leads to aromatic ring cleavage, resulting in a compound with a molecular ion at m/z = 214 (C₁₃H₁₂NO₂). As degradation progresses, smaller carboxylic acids such as C₄H₆O₄ (m/z = 132) and C₃H₆O₂ (m/z = 74) are formed, indicating breakdown into simpler organic acids. Simultaneously, another parallel pathway involves the cleavage of RhB’s xanthene ring, creating an intermediate at m/z = 182 (C₁₃H₁₀O), which further oxidizes to produce C₈H₆O₄ (m/z = 166). These pathways collectively indicate that RhB undergoes multiple stepwise reactions de-ethylation, deamination, and ring-opening leading to the formation of low-molecular-weight compounds. Ultimately, these fragments are mineralized into CO₂ and H₂O, confirming complete degradation. The detection of these intermediates offers direct evidence for the detailed mechanism of RhB decomposition during photocatalysis.

11.

Proposed degradation pathway of RhB under photocatalytic conditions, illustrating the stepwise formation of intermediate products and their corresponding mass-to-charge (m/z) ratios as identified by LC–MS analysis.

The recyclability of the Fe–ZnO/GO photocatalyst was evaluated over three successive RhB degradation cycles under UV irradiation, demonstrating its stability and potential for reuse. Initially, the catalyst exhibited excellent performance with a 99.32% degradation (Figure S5) efficiency in the first cycle. However, a gradual decline was observed in subsequent runs, with efficiencies decreasing to 89.70% in the second cycle and 70.11% in the third. This reduction in activity is likely due to factors such as photocorrosion, surface fouling by residual organic compounds, or deterioration of active sites caused by prolonged exposure to reactive oxygen species. Nonetheless, the catalyst retained over 70% of its original efficiency after three uses, suggesting it possesses a satisfactory level of durability and reusability for practical photocatalytic applications.

XRD analysis was performed on ZnO, Fe–ZnO, and Fe–ZnO/GO samples after 24 h of UV-assisted RhB degradation to assess their structural stability postphotocatalysis. The diffraction patterns, shown in Figure S6, revealed no significant changes compared to the fresh samples, indicating that the materials retained their crystallinity and phase structure. The positions, intensities, and sharpness of the central diffraction peaks remained consistent, with no evidence of peak broadening or phase transformation. For both pristine ZnO and Fe–ZnO, the characteristic peaks corresponding to the hexagonal wurtzite phase remained intact, confirming their structural resilience. Similarly, the Fe–ZnO/GO composite exhibited no signs of additional phases or impurities, indicating that GO incorporation did not negatively impact the crystal structure during extended UV exposure. These findings confirm that all three materials, particularly the Fe–ZnO/GO composite, exhibit good structural stability under photocatalytic operating conditions. To further validate the structural stability of the synthesized materials, SEM images of the Fe–ZnO/GO composites before and after photocatalytic degradation tests were captured and are presented in Figure S7. The initial morphology (Figure S7a) reveals uniformly dispersed Fe–ZnO nanoparticles anchored on the wrinkled and layered GO sheets, indicating successful composite formation with good interfacial contact. After 24 h of UV-induced photocatalytic activity (Figure S7b), the composite maintains its structural features, with no visible aggregation, detachment, or collapse of the nanoparticles. The Fe–ZnO particles remain well-distributed on the GO surface, demonstrating robust interfacial adhesion and resistance to photocorrosion. These observations confirm the excellent photostability and structural integrity of the Fe–ZnO/GO composite, making it a promising candidate for long-term photocatalytic applications.

To understand which reactive species play the key roles in degrading RhB, scavenger experiments were conducted using specific chemical agents that selectively neutralize different radicals or charge carriers. AgNO₃ (Ag⁺) was used to capture electrons (e^–), sodium sulfite (SS) for holes (h⁺), ascorbic acid (AA) for superoxide radicals (^•O₂ ^–), and isopropanol (IPA) for hydroxyl radicals (^•OH). The results shown in Figure reveal that the degradation efficiency of RhB significantly decreased when AA and IPA were added, indicating that superoxide radicals and hydroxyl radicals are the primary reactive species responsible for photocatalytic degradation in the Fe–ZnO/GO system. Meanwhile, the addition of Ag⁺ and SS resulted in only moderate decreases in activity, suggesting that electrons and holes also contribute but are less dominant in the process. These experimental observations align well with the electronic band structure data from Figure b. The CB potentials for ZnO (−0.52 V), Fe–ZnO (−0.59 V), and Fe–ZnO/GO (−0.88 V vs Ag/AgCl) are all more negative than the redox potential needed to reduce oxygen to superoxide radicals (O₂/^•O₂ ^– at −0.33 V vs NHE), confirming the system’s ability to generate ^•O₂ ^– effectively. Additionally, the VB positions of ZnO and Fe–ZnO are more positive than the potential required for producing hydroxyl radicals from water (+2.38 V vs NHE), favoring ^•OH formation. Although the VB of Fe–ZnO/GO (+1.88 V) lies slightly below this threshold, the incorporation of O enhances interfacial charge transfer. It prolongs charge carrier lifetimes, which helps partially generate ^•OH radicals under UV light. Overall, these results demonstrate that the superior photocatalytic performance of Fe–ZnO/GO arises from the efficient production and activity of ^•O₂ ^– and ^•OH radicals, driven by well-aligned band edge positions and improved charge separation due to Fe doping and GO integration.

12.

Effect of various scavengers on RhB degradation efficiency using ZnO, Fe–ZnO, and Fe–ZnO/GO photocatalysts.

The photocatalytic mechanism of the Fe–ZnO/GO composite, as illustrated in Figure , follows a series of key steps involving charge generation, separation, and reactive species formation. ZnO absorbs photons with energy equivalent to or higher than its bandgap when exposed to UV light. This causes e^– in the VB to be excited and move to the CB, forming h⁺ in the VB. When electrons move to the surface and combine with O₂ to form ^•O₂ ^–, while holes oxidize H₂O or OH^– to produce ^•OH, these photogenerated charge carriers are essential for initiating redox processes. The presence of Fe and GO enhances this process by facilitating charge separation, suppressing electron–hole recombination, and accelerating charge transfer. Fe³⁺ acts as an electron trap, thereby improving charge carrier lifetime, while GO serves as an electron acceptor, promoting efficient electron transport. These reactive oxygen species actively degrade organic pollutants, such as RhB, thereby enhancing the photocatalytic performance of the Fe–ZnO/GO composite.

13.

Proposed photocatalytic mechanism of the Fe–ZnO/GO composite under UV irradiation and corresponding time-resolved degradation images of RhB.

$$\mathrm{Z}\mathrm{n}\mathrm{O}+\mathit{hv}\to {\mathrm{e}}^{-}(\mathrm{C}\mathrm{B})+h^{+}(\mathrm{V}\mathrm{B})$$ 3

Fe ions, due to their multiple oxidation states (Fe²⁺/Fe³⁺), act as electron traps by capturing electrons from the CB of ZnO, thereby reducing Fe³⁺ to Fe²⁺:

$$\mathrm{F}{\mathrm{e}}^{3+}+{\mathrm{e}}^{-}\to \mathrm{F}{\mathrm{e}}^{2+}$$ 4

These trapped electrons are subsequently transferred from Fe²⁺ to the GO sheets:

$$\mathrm{F}{\mathrm{e}}^{2+}\to \mathrm{F}{\mathrm{e}}^{3+}+{\mathrm{e}}^{-}(\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{n}\mathrm{s}\mathrm{f}\mathrm{e}\mathrm{r}\mathrm{r}\mathrm{e}\mathrm{d}\mathrm{t}\mathrm{o}\mathrm{G}\mathrm{O})$$ 5

GO sheets function as efficient electron acceptors and conductors, thanks to their high surface area and π-conjugated structure, which facilitate charge separation and minimize electron–hole recombination. Following their transfer from ZnO to GO, photogenerated electrons take part in redox processes on the GO surface. Superoxide radicals, which are highly reactive and aid in the breakdown of organic contaminants, are produced when the electrons specifically engage with adsorbed oxygen molecules. This enhanced electron transfer mechanism not only accelerates the photocatalytic reaction but also prolongs the charge carrier lifetime, making Fe–ZnO/GO an effective photocatalyst for environmental remediation.

$${\mathrm{O}}_2+{\mathrm{e}}^{-}\to {{}^{•}\mathrm{O}}_2^{-}$$ 6

The superoxide radicals formed on the GO surface undergo further reactions with H₂O₂ to generate hydroxyl radicals, which are highly reactive species capable of breaking down organic pollutants.

$${{}^{•}\mathrm{O}}_2^{-}+{\mathrm{H}}_2{\mathrm{O}}_2\to {}^{•}\mathrm{H}\mathrm{O}+\mathrm{O}{\mathrm{H}}^{-}+{\mathrm{O}}_2$$ 7

Concurrently, the photogenerated holes in ZnO’s valence band facilitate oxidation processes by either interacting directly with organic pollutants or indirectly generating more hydroxyl radicals from water.

$$h^{+}+{\mathrm{H}}_2\mathrm{O}\to {}^{•}\mathrm{H}\mathrm{O}+{\mathrm{H}}^{+}$$ 8

These hydroxyl radicals, along with superoxide radicals, play a crucial role in the oxidative degradation of RhB and other organic pollutants, breaking them down into harmless byproducts.

$$\mathrm{RhB}+{}^{•}\mathrm{O}\mathrm{H}/{{}^{•}\mathrm{O}}_2^{-}\to \mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$$ 9

The synergistic combination of Fe, ZnO, and GO in the composite enhances overall photocatalytic efficiency by extending light absorption, improving charge separation, and promoting the continuous generation of reactive oxygen species, making Fe–ZnO/GO a highly effective material for environmental remediation applications.

4. Conclusions

This study successfully developed Fe–ZnO/GO composites that exhibited notable enhancements across multiple properties compared to pure ZnO. The introduction of Fe dopants into ZnO caused lattice strain and reduced crystallite size, which increased defect sites that serve as active centers for photocatalysis. Meanwhile, the incorporation of GO improved charge separation and electron mobility by providing conductive pathways, thus reducing electron–hole recombination. The combined or synergistic effects of Fe doping and GO integration effectively tune the bandgap and enhance charge carrier dynamics, leading to the accelerated generation of reactive oxygen species, key agents in degrading organic pollutants. As a result, the Fe–ZnO/GO composite achieved an impressive 99.32% degradation efficiency of RhB under UV light, alongside a significantly higher reaction rate constant compared to pristine ZnO. Moreover, the composite exhibited enhanced magnetic properties, suggesting potential for easy recovery and reuse via magnetic separation, an essential feature for sustainable photocatalytic applications. Collectively, these findings position Fe–ZnO/GO as a promising photocatalyst material for environmental remediation, especially in water purification. For practical deployment, future research should aim to improve the composite’s long-term stability, optimize synthesis methods for consistency and scalability, and validate photocatalytic performance under natural sunlight and in real wastewater matrices to confirm its effectiveness in real-world scenarios.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c03213.

• Details of chemicals and materials used; characterization equipment; EDX spectra and elemental compositions of ZnO, Fe–ZnO, and Fe–ZnO/GO; nitrogen adsorption–desorption isotherms; time-dependent UV–Vis absorbance spectra of RhB degradation by each catalyst; photocatalytic performance of commercial P25 catalyst under UV irradiation; recyclability performance of Fe–ZnO/GO photocatalyst over three consecutive cycles; XRD patterns of the samples before and after photocatalytic degradation; SEM images of Fe–ZnO/GO composites (a) before and (b) after photocatalytic degradation of Rhodamine B under UV irradiation; lattice parameters, crystallite size (D), and strain (ε); BET surface area, pore volume, and average pore size; magnetic properties including saturation magnetization (M _s), remanent magnetization (M _r), coercivity (H _c), and M _r/M _s ratio; and photocatalytic data including adsorption capacity, degradation efficiency, rate constant (k), and correlation coefficient (R ²). Summary of photocatalytic degradation conditions and efficiencies of various ZnO-based composites reported in different studies, compared with the Fe–ZnO/GO composite in this work (PDF)

The authors declare no competing financial interest.