Facile Fabrication of Carbon Paper-Supported Fe Catalyst Under Pulse Laser Irradiation for Degradation of Rhodamine B

Abstract

Persistent organic pollutants, such as Rhodamine B (RhB), pose significant environmental and health risks, necessitating the development of advanced oxidation technologies for effective removal. While heterogeneous photo-Fenton catalysts are known for their high degradation efficiency, their practical application is often limited by complex synthesis processes, catalyst detachment, and difficult recovery. This study proposes an innovative laser-induced, one-step synthesis strategy to fabricate metal/carbon nanocomposite catalytic layers directly onto flexible carbon paper. The as-prepared composites exhibit strong interfacial interaction between metal nanoparticles and the carbon matrix, as indicated by XPS analysis, and demonstrate enhanced catalytic activity in the UV/H₂O₂ system. Notably, the integrated composites exhibit exceptional catalytic activity in the UV/H₂O₂ system, achieving complete degradation of a 20 mg/L RhB solution within just 1.5 h. The enhanced performance is attributed to the facilitated Fe³⁺/Fe²⁺ cycling and efficient generation of hydroxyl radicals (·OH), although the underlying charge separation mechanism requires further investigation with techniques such as photoluminescence spectroscopy and transient photocurrent measurements. This work not only demonstrates the high activity and stability of the photo-Fenton catalyst but also provides a green, rapid fabrication approach for the development of efficient and integrable catalytic devices for wastewater treatment.

1. Introduction

Rapid industrialization has driven economic growth but simultaneously generated substantial volumes of toxic organic wastewater, posing serious threats to aquatic ecosystems and human health worldwide [1]. Among these pollutants, synthetic dyes such as Rhodamine B (RhB) are extensively used in textiles, dyeing, and paper industries due to their vibrant colors and chemical stability. However, their persistent nature renders them resistant to conventional biological or physical treatment methods, leading to long-term environmental accumulation and potential carcinogenic and mutagenic effects [2,3,4]. Therefore, developing efficient and environmentally friendly advanced water treatment technologies has become an urgent priority in environmental science and engineering [5,6].

Advanced oxidation processes (AOPs) based on reactive oxygen species have gained attention for their ability to non-selectively degrade organic pollutants [7,8]. Among AOPs, Fenton and Fenton-like reactions are particularly attractive due to their rapid kinetics and operational simplicity. Photo-Fenton technology, which integrates UV or visible light irradiation with the Fenton reaction, not only enables direct photodegradation of contaminants but also accelerates the Fe³⁺/Fe²⁺ redox cycling, thereby enhancing ·OH generation and improving pollutant degradation efficiency while reducing chemical reagent consumption [9,10,11].

To overcome the inherent limitations of homogeneous Fenton systems—including narrow working pH range, iron sludge production, and difficulty in catalyst recovery—research efforts have increasingly focused on heterogeneous catalysts, wherein active sites are immobilized onto solid supports. In particular, immobilizing photo-Fenton catalysts on carbon fibers or carbon paper has emerged as a promising strategy to enhance catalyst dispersibility and recyclability. For instance, Liu et al. demonstrated that Fe₃O₄ nanoparticles immobilized on porous carbon fibers exhibited excellent photo-Fenton activity and could be easily recovered from the reaction system [12]. Similarly, Divyapriy and colleagues reported that graphene-supported iron oxide catalysts showed enhanced stability and reusability in continuous-flow operations [13]. These studies collectively confirm that carbonaceous substrates not only provide mechanical support but also facilitate electron transfer owing to their high conductivity, thereby improving catalytic performance [14].

Despite these advances in catalyst immobilization, conventional synthesis methods for heterogeneous photo-Fenton catalysts still predominantly rely on traditional routes such as sol–gel, hydrothermal, and electrodeposition [15,16,17]. While these methods have been widely employed, they exhibit several inherent limitations. Sol–gel synthesis, although offering good compositional control, typically requires prolonged aging and high-temperature calcination, leading to substantial energy consumption [15]. Hydrothermal methods enable the formation of well-defined nanostructures but involve high-pressure autoclave reactions that are time-consuming and difficult to scale [16]. Electrodeposition allows for direct coating onto conductive substrates but often produces catalysts with weak adhesion and limited control over active site distribution [17]. Moreover, these conventional routes generally yield powdered catalysts that require additional immobilization steps—typically involving adhesive binders—to attach onto macroscopic substrates. This binder-assisted attachment results in weak interfacial bonding, which can lead to catalyst detachment under hydrodynamic shear and, consequently, metal ion leaching into the treated water [18,19,20]. Such leaching not only compromises long-term operational stability but also poses risks of secondary pollution, necessitating rigorous evaluation of leaching criteria in Fenton-like systems. Recent studies have emphasized the importance of quantifying metal leaching under various operating conditions: Martinez et al. reported that in continuous fixed-bed reactors, iron leaching from Fe³⁺-Al₂O₃ catalysts could be maintained below 3 mg/L under optimized conditions, but cumulative loss reached approximately 20% after 70 h of operation [20,21]. Wu and colleagues employed machine learning to predict Co²⁺ leaching dynamics in CoAl-LDH-triggered peroxymonosulfate activation systems, revealing that aqueous matrices critically influence leaching behavior [19]. Jiao et al. demonstrated that the catalyst exhibited exceptional stability and economic sustainability, maintaining 79.89% degradation efficiency after 6 consecutive cycles with negligible metal leaching (Zn < 1.0 mg/L, Fe < 0.3 mg/L) [22]. These findings underscore that catalyst stability and leaching resistance must be systematically evaluated and reported as key performance metrics.

Laser processing has recently emerged as an innovative approach to generate active metal/carbon structures with unique advantages over conventional synthesis routes. By applying high-energy laser beams for localized ultra-high-temperature treatment of precursor films, this technique enables rapid sintering, reduction, or carbonization of materials while simultaneously establishing robust adhesion to the underlying substrate [23]. Several recent studies have demonstrated the effectiveness of laser-induced fabrication of metal/carbon composites for catalytic applications. Zafar et al. reported a laser-induced synthesis of copper/carbon nanocomposites from anodically electrodeposited chitosan precursors, achieving rapid (minutes-scale) formation of highly graphitized porous carbon materials with embedded copper species [23]. Chen et al. showed that laser-induced graphene decorated with zero-valent iron nanoparticles could be directly written on flexible substrates, enabling in situ electro-Fenton degradation of pollutants with minimal metal leaching [24]. These studies collectively confirm that laser processing offers a powerful platform for one-step fabrication of stable, high-performance metal/carbon nanocomposites directly on target substrates, with precise control over patterning and interfacial properties.

Therefore, this study proposes a laser-induced one-step synthesis strategy for in situ fabrication of metal/carbon nanocomposites directly on carbon paper. This approach addresses the key limitations of conventional methods—namely, multi-step procedures, high energy consumption, weak interfacial bonding, and potential metal leaching—by integrating active material synthesis with structured device fabrication in a single process. Systematic characterization confirms the robust coupling between metal active sites and the carbon matrix, along with their strong interaction. Degradation experiments demonstrate that the integrated electrode exhibits excellent catalytic activity and cycling stability for RhB degradation in a UV/H₂O₂ system, achieving complete degradation within 90 min. The underlying mechanism is primarily attributed to efficient separation of photogenerated charges, which promotes metal valence cycling and sustains ·OH radical generation. Notably, the catalyst exhibits minimal metal leaching during extended operation, as systematically evaluated following established leaching criteria [19,20,21]. This work provides a new pathway for the green, rapid, and scalable fabrication of efficient, stable, and readily integrable catalytic devices for wastewater treatment.

2. Description of Experiment

2.1. Materials and Chemicals

The reagents used in this study included chromium chloride hexahydrate (CrCl₃·6H₂O, AR), manganese chloride (MnCl₂), ferric chloride hexahydrate (FeCl₃·6H₂O, AR), cobalt chloride hexahydrate (CoCl₂·6H₂O, AR), nickel chloride hexahydrate (NiCl₂·6H₂O, AR), copper chloride dihydrate (CuCl₂·2H₂O, AR), zinc chloride (ZnCl₂, AR), Rhodamine B (C₂₈H₃₁ClN₂O₃, ≥95% purity), tert-butyl alcohol (C₄H₁₀O, TBA, CP), p-benzoquinone (C₆H₄O₂, PBQ, 97%), potassium iodide (KI, AR), hydrogen peroxide (H₂O₂, 33–35%; upon testing, it is approximately 34%), potassium dichromate (K₂Cr₂O₇, GR), and terephthalic acid (C₈H₆O₄, AR), all of which were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). β-carotene (C₄₀H₅₆, 96%) was obtained from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). These reagents were used as received without further purification. Ultrapure water was used throughout the experiments. Carbon paper was obtained from an e-shop named Carbon Graphite Industry.

2.2. Experiment Apparatus

Laser Marking Machine with Model B3 was provided by Shanghai Diaotu Industry Co., Ltd. (Shanghai, China). A UV LED light (365 nm, 12 W) was obtained from Zhongshan Guba lighting Co., Ltd. (Zhongshan, China).

X-ray diffraction (XRD) patterns were acquired using a Bruker D8 radiation diffractometer equipped with a copper target and Kα radiation (λ = 1.5406 Å) (Bruker Corporation, Berlin, Germany). Raman spectroscopy (Raman Station 400F, Bruker Optics GmbH & Co. KG, Erlangen, Germany) was conducted on pressed samples using a 532 nm laser source, with scanning performed in the range of 100–2000 cm⁻¹. Scanning electron microscopy (SEM, Hitachi S-4800, 5 kV, Hitachi High-Technologies Corporation, Tokyo, Japan) was utilized in conjunction with element distribution mapping (EDS). X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi, Thermo Fisher Scientific Waltham Corporation, Waltham, MA, USA) was utilized with Al Kα (1486.6 eV) as the excitation source, with carbon C 1s = 284.8 eV serving as the reference.

2.3. Experiment Design and Procedures

The carbon paper was treated on both sides using a laser marking system following a specific protocol. First, a shallow recess of 2 cm × 2 cm was created on each side at specific laser points. The laser power was set to 0 (to achieve the minimum output power, that means the average outpower is 5 W), with a scanning speed of 5000 mm/s, a frequency of 30 kHz, a hatch spacing of 0.03 mm, and a defocusing distance of 0 (i.e., at a focal length of 26.2 cm). Next, a “coating–drying–laser treatment” cycle was repeated three times and the experiments were performed in an air atmosphere. Additionally, the instrument features a fixed wavelength of 1064 nm and does not allow for wavelength adjustment. During each cycle, 1 mL of a chloride/citric acid mixed solution (0.2 M) was drop-cast onto the designated area, followed by drying at 80 °C for 2 h. After drying, the precursor film was scanned three times with the laser at minimal power and a scanning speed of 5000 mm/s. The same procedure was symmetrically repeated on the opposite side of the carbon paper at the identical position. Following the laser treatment, the carbon paper was immersed in deionized water and subjected to ultrasonic cleaning for five cycles, then dried in an oven for further use. The synthetic procedure is illustrated in Figure 1. The prepared samples were nominated as M/C, where both M and C referred to metal ions selected and carbon paper [25]. A concentrated Fe/C was also prepared with a chloride/citric acid mixed solution (1 M) for comparison.

An 80 mL solution of RhB with a concentration of 20 mg/L was prepared, and the treated carbon paper was immersed in this solution for a 1 h dark adsorption reaction. Afterward, the ultraviolet lamp was turned on to initiate the photocatalytic degradation under light irradiation. After a 10 min interval, 5 mL samples were withdrawn, centrifuged to remove catalyst particles, and measured at a wavelength of 554 nm on a UV-VIS spectrometer (INESA 752G).

3. Results and Discussion

3.1. Characterization and Structural Analyses

To analyze the microstructure and elemental composition, we characterized the Fe/C samples in Figure 2a and concentrated the Fe/C (precursor loading amount five times higher than usual) in Figure 2b using SEM. The typical microstructure of the Fe/C sample after laser etching reveals a rough and highly wrinkled three-dimensional porous architecture, as shown in Figure 2a. In contrast, the lamellar morphology of the concentrated Fe/C in Figure 2b demonstrates that the thermal and shockwave effects generated by laser irradiation partially exfoliate carbon layers, forming two-dimensional nanosheet-like structures that further increase the effective contact area for surface reactions. Figure 2c clearly illustrates the compositional distribution and spatial homogeneity of all elements. This carbon layer not only provides excellent electrical conductivity but also serves as a stable anchoring platform for iron-based active species, as shown in Figure 2d. Element O is widely distributed and exhibits high spatial overlap with carbon-enriched regions in Figure 2e, indicating the likely presence of abundant oxygen-containing functional groups such as hydroxyl (-OH) and carbonyl (-C=O) on the surface [26]. These functional groups enhance the hydrophilicity of the material and provide additional active sites for catalytic reactions. The Fe element is uniformly distributed as discrete point-like features throughout the carbon matrix, indicating the successful incorporation of iron active species with even dispersion. This highly dispersed configuration maximizes the utilization efficiency of iron active sites, significantly enhancing the catalytic performance.

The XRD patterns of both C and Fe/C samples in Figure 2a are quite similar, and no crystalline diffraction peaks attributable to iron oxides or metallic iron (Fe⁰) are detected in the Fe/C sample or concentrated Fe/C, mainly attributed to the low content or amorphous nature [27]. Instead, the signal from carbon from the (0 0 6) plane of graphitic carbon can clearly be seen. This finding is fully consistent with the SEM/EDS observation of “sporadic and weak Fe signals”, jointly supporting the inference that the iron species are present on the carbon substrate in an amorphous or highly dispersed state. The XPS survey spectra of C and Fe/C samples are presented in Figure 3b, showing distinct characteristic peaks corresponding to C 1s and O 1s. Notably, no significant signals are detected within the binding energy range of ~700–730 eV in Fe/C, indicating the low content of iron [28]. The ration of Fe to C measured by EDS is assessed as approximately 2.53%. The high-resolution O 1s XPS spectra are depicted in Figure 3c. After calibration using the C 1s peak at 284.8 eV, an asymmetric broad peak is observed with its main binding energy centered at 531.1 eV in Fe/C and a similar peak at 531.9 eV is found in C, demonstrating intrinsic surface functional groups such as carbonyl (C=O) and hydroxyl (C–OH) in different chemical environments [29]. The carbon substrate employed inherently possesses abundant oxygen-containing functional groups, and iron species were successfully incorporated in trace amounts and predominantly exist in a highly dispersed Fe³⁺ state; however, their concentration is too low to dominate the O 1s spectral signal. Consequently, the major contribution to the O 1s spectrum in the composite still originates from the functional groups of the carbon matrix. Based on the integrated analysis of these findings, we have reached a unified understanding regarding the state of iron species in the material: iron exhibits an extremely low surface concentration and lacks a crystalline structure. This distinctive configuration likely facilitates the exposure of additional active sites and enhances electron transfer, thereby contributing to the material’s outstanding catalytic performance.

The high-resolution Fe 2p XPS spectrum of Fe/C is displayed in Figure 2d. Weak peaks are observed at 710.2 and 724.2 eV, corresponding to Fe 2p_3/2 and Fe 2p_1/2, respectively. Notably, no characteristic satellite peaks of Fe²⁺ are detected, indicating that iron exists predominantly in the Fe³⁺ state. The signal intensity is extremely low, with the calculated surface Fe/C atomic ratio being only 0.008. This observation is fully consistent with the absence of Fe signals in the XPS survey spectrum, the lack of diffraction peaks in XRD, and the sparse Fe signals observed in the EDS mapping of Fe/C. Collectively, this chain of evidence supports the conclusion that during precursor decomposition and composite formation, iron does not aggregate into crystalline phases or nanoparticles. Instead, it is selectively anchored onto specific oxygen-containing functional groups on the carbon matrix via Fe–O–C bonds, existing as highly dispersed or ultrasmall Fe³⁺ clusters [30]. This unique structural configuration enables trace amounts of iron species to significantly modulate the electronic structure of the carbon substrate while maintaining exceptionally high dispersion and exposure of active sites. It is obvious from Figure 3e that the D peak disappeared. We reckon that the primary reason for this lies in the structural optimization of the carbon matrix itself during the composite preparation process [31,32]. During thermal treatment or chemical reaction, unstable oxygen-containing functional groups (such as epoxy groups) on the pristine carbon material (whose O 1s peak is located at 531.9 eV) undergo pyrolysis or selective removal, leading to “annealing” and reconstruction of the sp² carbon network. This process directly “repairs” defects and disordered structures within the carbon lattice, resulting in a sharp decrease or even disappearance of the D-band intensity [33]. Simultaneously, the enlargement of sp² domains and the enhancement of conjugation induce a systematic redshift of the G-band. We also find that the ration of Fe to C is 0.15%; it is noteworthy that in a control experiment without iron loading, carbon materials subjected to the same treatment exhibited a significantly lower degree of D-band reduction and G-band redshift compared to the composite [34]. This indicates that the presence of trace iron species plays a crucial promotive or “catalytic” role in the structural ordering process of the carbon matrix [35,36]. Therefore, the Raman spectral changes do not merely reflect conventional surface modification of carbon by iron oxides. Instead, they reveal a dynamic structural evolution: during the synthesis process, trace amounts of highly dispersed iron species act as an efficient “structural promoter”, significantly catalyzing the transformation of the oxygen-rich carbon substrate into an sp² carbon network with higher ordering and lower defect density [37]. This transformation is simultaneously reflected in the subtle modulation of the chemical environment of carbon–oxygen functional groups, as observed in the O 1s spectrum. Ultimately, what is obtained is a structurally optimized carbon support onto which highly dispersed iron sites are firmly anchored via Fe–O–C bonds [38].

3.2. Data Analysis

In this series of experiments, we initially used ferric chloride alone as the precursor for preliminary screening. The combination of ferric chloride with citric acid, along with subsequent experiments, was then explored, while other variables were systematically tested. To minimize the influence of carbon material adsorption on the experimental results, each experiment underwent a 1 h sorption duration in the dark before the photocatalytic reaction was initiated under ultraviolet light.

In Figure 4a, chlorides of first-row transition metal (Cr, Mn, Fe, Co, Ni, Cu, Zn) were employed as precursors to fabricate a series of catalysts supported on carbon paper, aiming to investigate the catalytic behavior of different metals on similar carbon substrates [39]. The control group (CG) consisted of pure carbon paper, and all experiments were conducted without adding citric acid as a complexing agent, without adjusting the pH of the RhB solution, and without adding hydrogen peroxide (H₂O₂). The results showed minimal differences in degradation performance among the various metal catalysts, indicating that the catalytic activities of the metals were not significantly distinct under these conditions. Their performance was likely influenced by factors such as metal ion dispersion, interaction with the carbon substrate, and the availability of active sites. Based on these initial observations, iron-based catalysts were chosen for further optimization due to iron’s abundance, environmental friendliness, and its ability to exist in multiple valence states (Fe²⁺/Fe³⁺). These properties enhance the separation of photogenerated electron–hole pairs and the generation of ·OH radicals, making iron a promising candidate for photo-Fenton and Fenton-like systems [40]. Figure 4b demonstrates the catalytic performance after a series of adjustments. As shown in the graph, the degradation efficiency of Fe/C for Rhodamine B decreases from 75.4% to 0% (and it possesses a kinetic curve with a slope of −0.04786), achieving complete degradation of 20 mg/L Rhodamine B within ten minutes.

The precursor formulation was adjusted by introducing citric acid as a complexing agent and potential carbon source into the ferric chloride solution, with volume ratios of 1:1, 1:3, and 3:1. As shown in Figure 5a, the catalyst prepared with a 1:1 ratio exhibited optimal activity under UV irradiation; after adjusting the ratio composition of the precursor, the catalytic performance of Fe/C for Rhodamine B was improved by 5%. After determining the optimal precursor ratio, the effect of H₂O₂ dosage was further investigated. As shown in Figure 5b, the addition of 5 µL of H₂O₂ yielded the best results. After adding an appropriate amount of hydrogen peroxide, the catalytic performance was further enhanced by approximately 68%. H₂O₂ acts as an electron scavenger, reacting with photogenerated electrons or Fe²⁺ to produce highly oxidative ·OH radicals, thereby accelerating the degradation of RhB. However, excessive H₂O₂ can act as a ·OH quencher or compete with surface holes, which may inhibit degradation efficiency [41]. Subsequently, the effect of the initial pH values of RhB solutions was investigated. The pH not only influences the surface charge of the catalyst and the form of RhB molecules but also affects the dissolution–precipitation equilibrium of iron species and the decomposition kinetics of H₂O₂. Under acidic conditions (especially at pH ≈ 3), iron species are more likely to promote Fenton reactions, while the catalyst surface becomes negatively charged, which enhances the adsorption of positively charged RhB molecules [42]. In Figure 5c, the degradation efficiency reached its peak at pH = 3, indicating optimal synergy between adsorption and catalytic oxidation under these conditions. Fe/C achieved complete degradation of Rhodamine B, representing a 75% improvement in degradation efficiency compared to the unoptimized condition.

Following this, other metal chloride precursors were treated using the same method for comparison. Notably, all catalysts shown in Figure 6 performed better than those in the initial screening, although none achieved the performance level of the iron-based catalyst. This difference may be due to variations in the interaction strength between the metals and the carbon substrate, their electronic structures, and their ability to activate H₂O₂, despite the other metals possessing multivalent states and catalytic potential. Additionally, recyclability tests were conducted using the best-performing Fe/C catalyst from Figure 6. After each use cycle, the catalyst was collected, thoroughly rinsed with water to remove surface-adsorbed reaction residues, subjected to deep cleaning in an ultrasonic bath to eliminate potential impurities that could block its pores, and then dried in an oven at 80 °C for 2 h. The labels “0”, “1”, and “2” correspond to the test results of the Fe/C catalyst in its initial state and after one and two cycles of reuse, respectively. Unfortunately, the sample did not exhibit ideal recoverability. Instead, its catalytic activity significantly declined with increasing cycle numbers, which could be attributed to the gradual leaching of Fe and the adsorption of intermediates. Therefore, further research will be conducted to improve the recyclability of the Fe/C catalyst.

As shown in Figure 7, quenching agents KI (1.506 mM), TBA (6.575 mM), PBQ (2.313 mM), and β-carotene (0.466 mM) were used to capture active species such as h⁺, ∙OH, ∙O₂⁻, and ¹O₂, respectively [43]. It is evident that the primary active species are hydroxyl radicals (∙OH). The outstanding Fenton-like performance of the Fe/C catalyst stems from its intrinsic kinetic optimization of ∙OH generation pathways [44]. The synergistic system, composed of highly dispersed Fe³⁺ sites and a highly conductive carbon support, enables probable ultrafast Fe³⁺/Fe²⁺ cycling, thereby sustaining ∙OH production with an exceptionally high turnover frequency. The relative reaction pathways are described in the following equations:

Fe/C + hv → Fe/C(e⁻ + h⁺),

(1)

Fe³⁺ + e⁻ → Fe²⁺,

(2)

Fe²⁺ + H₂O₂ → Fe³⁺ + ·OH + OH⁻,

(3)

h_VB⁺ + H₂O/OH⁻ → H⁺ + ·OH,

(4)

·OH + RhB → degradation products

(5)

However, this highly efficient generation of ∙OH also represents a key challenge for catalyst stability: the high concentration of ∙OH not only attacks pollutants but also induces a “self-consumption” effect on the catalyst [45]. Specifically, ∙OH can cleave the Fe–O–C bonds that anchor the iron active sites, leading to the leaching of trace iron centers, which is likely the primary cause of the catalyst’s poor recyclability. Future research should focus on designing more chemically inert coordination environments or developing complementary pathways dominated by non-radical processes to overcome this activity–stability trade-off.

4. Conclusions

This study successfully developed and comprehensively characterized a novel iron-modified carbon composite Fe/C for photo-Fenton-like catalysis through an innovative laser-induced method. This architecture endows the Fe/C with outstanding initial catalytic activity for ·OH generation. The enhanced performance originates from a synergistic mechanism of rapid electron transfer through the highly conductive carbon network and the favorable electronic structure through the strong metal–support interaction. Additionally, the reaction mechanism was also inspected. However, the catalyst showed poor recyclability, mainly attributed to the gradual leaching of Fe and the adsorption of intermediates. Future efforts should focus on the development of durable and efficient photo-Fenton-like catalysts for environmental remediation.