Rational construction of MOF derived α-Fe2O3/g-C3N4 composite for effective photocatalytic degradation of organic pollutants and electrocatalytic oxygen evolution reaction

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 310 (2024) 123972 Available online 1 February 2024 1386-1425/© 2024 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/). Rational construction of MOF derived α -Fe 2 O 3 /g-C 3 N 4 composite for effective photocatalytic degradation of organic pollutants and electrocatalytic oxygen evolution reaction Sakthivel Kumaravel a , * , 1 , Balakrishna Avula b , Chandramoorthy Chandrasatheesh c , 1 , Theophile Niyitanga a , * , Rajasekar Saranya d , Imran Hasan e , T. Abisheik f , Rajakumar S. Rai g , V. Pandiyan f , Krishnakumar Balu h , i , * a School of Materials Science and Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea b Department of Chemistry, Rajeev Gandhi Memorial College of Engineering and Technology (Autonomous), Nandyal, Andhra Pradesh 518501, India c Department of Chemistry, Loyola College, Chennai 600034, Tamil Nadu, India d Department of Biotechnology, SRM Institute of Science and Technology, Ramapuram, Chennai 600089, Tamil Nadu, India e Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia f Department of Physics, Nehru Memorial College (Autonomous), Puthanampatti (Affiliated to Bharathidasan University), Tiruchirappalli 621007, Tamil Nadu, India g Division of Mechanical Engineering, Karunya Institute of Technology and Sciences, Karunya Nagar, Coimbatore 641114, Tamil Nadu, India h Saveetha School of Engineering, Saveetha Institute of Medical and Technical Sciences (SIMATS), Chennai 602105, Tamil Nadu, India i Departamento de Ingeniería y Ciencia de los Materiales y del Transporte, E.T.S. de Ingenieros, Universidad de Sevilla, Avda. Camino de los Descubrimientos s/n., 41092 Sevilla, Spain HIGHLIGHTS GRAPHICAL ABSTRACT •Bare GCN, Fe 2 O 3, and FGCN materials were synthesized by thermal polymerization and hydrothermal methods. •FGCN photocatalyst exhibited excellent performance in RhB and CV degradation under solar irradiation. •FGCN catalyst exhibited excellent catalytic performance in OER. •The composite catalyst showed good stability. ARTICLE INFO Keywords: Fe 2 O 3 /g-C 3 N 4 Photocatalysts Rhodamine B Crystal violet OER ABSTRACT In recent years, researchers have been actively investigating metal oxide-based materials with narrow bandgaps due to their potential applications toward wastewater treatment and oxygen evolution reactions (OER). In this study, we successfully synthesized g-C 3 N 4 (GCN), Fe 2 O 3, and Fe 2 O 3 /g-C 3 N 4 (FGCN) using thermal polymerization and hydrothermal methods. We characterized the physicochemical and structural properties of these materials through various analytical techniques including XRD, FT-IR, UV-DRS, XPS, FE-SEM, and HR-TEM analyses, confirming the effective construction of the FGCN composite catalyst. We evaluated the photocatalytic activity of Fe 2 O 3 , GCN, and FGCN composite catalysts by assessing their ability to degrade rhodamine B (RhB) * Corresponding authors at: Departamento de Ingeniería y Ciencia de los Materiales y del Transporte, E.T.S. de Ingenieros, Universidad de Sevilla, Avda. Camino de los Descubrimientos s/n., 41092 Sevilla, Spain. E-mail addresses: [email protected] (S. Kumaravel), [email protected] (T. Niyitanga), [email protected] (K. Balu). 1 These authors equally contributed. Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.journals.elsevier.com/spectrochimica-acta-part-a- molecular-and-biomolecular-spectroscopy https://doi.org/10.1016/j.saa.2024.123972 Received 24 November 2023; Received in revised form 20 January 2024; Accepted 27 January 2024 Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 310 (2024) 123972 2 and crystal violet (CV) by exposing them to sunlight for 150 min. Among these catalysts, the FGCN composite demonstrated excellent photocatalytic performance, achieving 93 % and 95 % degradation of RhB and CV, respectively, under 150 min of sunlight exposure. The developed Fe 2 O 3 /g-C 3 N 4 @Nickel foam (FGCN@NF) composite catalyst exhibits remarkable OER performance, with a reduced Tafel slope of 64 mV/dec and a low overpotential of 290 mV at a current density of 10 mA/cm 2 and shows excellent durable performance over a long time (15 h). Total Organic Carbon (TOC) analysis confirmed the mineralization of both dyes. The photocatalytic performance remained largely unchanged after five consecutive experiments, demonstrating excellent reusability and photostability. Trapping experiments revealed that O 2 ●– is the main species responsible for the photocatalytic decomposition of various dyes by the FGCN composite catalyst. Therefore, the development of a versatile photo/ electrocatalytic system that can efficiently promote energy conversion in environmental applications has attracted great attention. 1. Introduction Over the past century, the rapid increase of industrial activities worldwide has highlighted the serious impact of environmental pollution on public health. Among these most important issues, water pollution has emerged as a major environmental issue, with a particular focus on the contamination of wastewater by dyes originating from industries such as textiles, printing, dyeing, leather, food, and cosmetics [1,2]. Synthetic dyes are generally not effectively removed by common wastewater treatment processes, leading to their release into the environment [3]. This persistence can have long-term effects on the organisms that inhabit the ecosystem [3]. Among the numerous synthetic dyes used in the textile industry, including methylene blue, congo red, rhodamine, crystal violet, methyl orange, and malachite green, rhodamine B (RhB) and crystal violet (CV) are the most widely used [4]. RhB and CV are widely used in the textile industry and are very popular [5]. The textile industry releases approximately 280,000 tons of dye into the environment worldwide each year, creating significant environmental problems [6]. Therefore, there is a strong need to address this problem by exploring ways to degrade various pollutants, which are highly beneficial to society. One of the most promising approaches to address these issues is the use of semiconductor-based photocatalytic technologies that can harness solar or visible light energy [7]. It is of utmost importance to find reliable ways to effectively remove various environmental pollutants from surface waters using sunlight as a natural energy source [8]. Fossil fuel depletion and associated climate change caused by industrialization and human activities are pressing global issues. As a result, advances in clean energy technology have become important, and water electrolysis for hydrogen production has emerged as a promising green and renewable option [9]. One promising solution is the use of hydrogen production through water electrolysis. This method involves two key reactions: oxygen evolution at the anode and hydrogen evolution at the cathode [10]. Both reactions require the use of cost-effective and efficient electrocatalysts. Nevertheless, the OER exhibits high overpotentials and slow reaction kinetics, highlighting the urgent need to develop electrocatalysts that can significantly increase the reaction rate and overcome the energy barrier. Currently, noble metal oxides such as RuO 2 and IrO 2 serve as high performance for electrocatalysts [11]. However, limited reserves, poor stability, and high costs for largescale applications intended to look at another material [11]. To address this challenge, researchers have focused on designing transition metal oxide-based electrocatalysts that are not only cost-effective and efficient but also stable and readily available [12]. One particularly important aspect is the development of electrocatalysts that exhibit low overpotentials, allowing for more efficient clean energy production. In recent years, extensive research has focused on exploring the potential of transition metal oxides to degrade organic dyes and promote electrocatalytic OER reactions. Various metal oxides have been studied, including ZnO, WO 3 , MoO 4 , MOS 2 , Fe 2 O 3 , Fe 2 O 4 , TiO 2 , Co 3 O 4 , MnO 2 and WS 2 [13–15]. Hematite (Fe 2 O 3 ) is a widely occurring iron oxide that exhibits excellent absorbance in the visible light spectrum, offers various oxidation states and coordination environments, is economically viable, non-toxic, extremely stable, highly reliable, resistant to corrosion, and possesses a narrow band gap width of 2.1 eV [16–18]. However, it has been observed that pure Fe 2 O 3 has poor photocatalytic and electrochemical performance [19]. On the bright side, the researchers were able to increase the catalytic performance of Fe 2 O 3 by combining it with other catalysts to form a heterojunction. In recent years, there has been a growing interest in the non-metallic semiconductor graphitic carbon nitride (GCN) [20]. This material is widely used in photocatalytic and electrochemical OER reactions due to its many advantages such as narrow bandgap, exceptional thermal and chemical stability, non-toxicity, and electron conductivity, making it an excellent material for photocatalysis and electrochemical OER reactions [21]. However, it also has some drawbacks, such as rapid charge recombination rate, limited visible light utilization ability, and reduced charge transfer ability to the catalyst surface, which ultimately limits the photocatalytic and electrocatalytic performance [22]. In contrast, single-phase semiconducting metal oxides face unique challenges, including limited availability of sunlight and visible light, faster charge carrier recombination, and limited charge transfer capacity. To address these limitations, various methods have been explored to improve the performance of photocatalytic and electrochemical OER, including the creation of heterojunctions by combining two or more materials. The extensive utilization of FGCN composites in heterojunction fabrication is due to their exceptional photocatalytic and electrochemical performance, structural controllability, affordability, rapid photoinduced charge transfer, reduced recombination rate, and robust redox ability. Fe-based materials can form heterojunctions with GCN, leading to enhanced photocatalytic and electrochemical OER performance. Notable examples include TiO 2 /Fe 2 O 3 /g-C 3 N 4 [23], Fe 2 O 3 /g- C 3 N 4 /GO [24], α -Fe 2 O 3 /g-C 3 N 4 [17], α -Fe 2 O 3 /CdS/g-C 3 N 4 [25], g- C 3 N 4 /Fe 2 TiO 5 /Fe 2 O 3 [26], and TiO 2 /Fe 2 O 3 [27], RuO 2 –Fe 2 O 3 /HrGO [28], Fe 2 O 3 /FeP [29], Er-MOF/Fe 2 O 3 [30], and, Fe 2 O 3 @CNT [31] composites all exhibit improved photocatalytic and electrochemical OER performance. Among these, composites of g-C 3 N 4 and Fe 2 O 3 stand out for their responsiveness to sunlight, stability, and ease of preparation. A significant method to increase the effectiveness of bare Fe 2 O 3 includes the development of heterostructures with GCN, which enhances the photo/electrocatalytic performances through improvements in parameters such as band structure, electron-hole separation, and surface area. In this study, we used thermal polymerization and hydrothermal methods to synthesize GCN, Fe 2 O 3 , and FGCN composite heterostructures to improve the catalytic performance for both photocatalysis and electrochemical OER. A physicochemical method was utilized to verify the phase formation, surface morphology, and formation of heterostructures. We evaluated the photocatalytic activity for the degradation of organic dyes RhB and CV under sunlight exposure and showed that the optimized FGCN composite photocatalyst has outstanding performance. Moreover, the FGCN showed excellent reusability in multiple cycles of dye removal. For the degradation of RhB and CV, the FGCN heterostructure showed the highest rate constant. The as-developed FGCN@NF composite catalyst reveals remarkable oxygen evolution reaction (OER) performance. Additionally, we also discussed a possible Z- S. Kumaravel et al. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 310 (2024) 123972 3 scheme mechanism to degrade RhB and CV dyes using the fabricated FGCN composite catalyst. 2. Experimental section 2.1. Materials Analytical grade of iron(III) nitrate nonahydrate (Fe(NO 3 ) 3 ⋅9H 2 O), N,N- dimethylformamide (DMF, 99.8 % purity), terephthalic acid (98 % purity), melamine (C 3 H 6 N 6 ), ethanol (EtOH), were acquired from Sigma-Aldrich. Rhodamine B and crystal violet were obtained from Sisco Research Laboratories Pvt. Ltd. (SRL). 2.2. Fabrication of GCN photocatalyst Bare GCN photocatalyst was easily formed through a simple thermal polymerization process [32,33]. The general procedure involves placing 2 g of melamine directly into the furnace and heating it at 550 ◦C for 2 h while keeping it in a covered crucible. After natural cooling to room temperature, the yellow bulk product was collected and ground in a mortar. 2.3. Fabrication of MOF-derived Fe 2 O 3 and FGCN catalysts Fe 2 O 3 derived from MOF precursors was successfully synthesized using a previously reported method [34]. In this process, GCN (approximately 50 mg) was dispersed in DMF (20 mL) by sonication for 30 min to form dispersion A. In the next step, 3 mmol of Fe(NO 3 ) 3 ⋅9H 2 O was added to a beaker containing 30 mL of DMF (solution B). At the same time, 10 mmol terephthalic acid was introduced into another beaker containing 30 mL of DMF (solution C). The sonication process continued until all solids in solutions B and C were completely dissolved. Next, dispersion A, and solutions B, and C were mixed to create mixture D and subjected to sonicated for 60 min. The pH of the reaction mixture was adjusted to a range of 9–10. Finally, solution D was assigned to a 100 mL autoclave and heated at 160 ◦C for 24 h. After cooling to atmospheric temperature, the solid product was collected by centrifugation, followed by multiple washes with ultrapure water and ethanol. The resulting product was then dried at 70 ◦C overnight, followed by calcination at 500 ◦C for 3 h. It is important to note that the preparation of MOF-derived Fe 2 O 3 samples was performed using the same method except for the addition of GCN. 2.4. Characterization of materials The crystalline phase formation and purity of the as-fabricated materials were assessed by X-ray diffraction (XRD) using a (Rigaku Miniflex, Japan). To determine the type of chemical functional groups, present in the materials were assessed by Fourier transform infrared (FT- IR) analysis using a Thermo Fisher iS50 spectrometer. The surface properties of the FGCN most active material were investigated using field emission scanning electron microscopy (FE-SEM/EDS) from (CLARA- Tescan, Czech Republic) and high-resolution transmission electron microscopy (HR-TEM) from (JEOL/JEM-F200). High resolution XPS spectra were measured using (ULVAC–PHI, PHI5000 Version Probe III). To describe the light absorption properties and evaluate the bandgap calculation of the obtained fabricated samples were evaluated by UV–Vis diffuse reflectance spectra (DRS) using a Shimadzu UV-750 spectrophotometer. UV–vis absorption spectra were recorded using a Shimadzu UV-750 spectrophotometer. 2.5. Photocatalytic experiment The photocatalytic performance of Fe 2 O 3 , GCN and FGCN materials was evaluated through the degradation of RhB and CV dyes under 150 min of sunlight exposure. First, an aqueous solution containing RhB (20 ppm) and CV (20 ppm) was prepared. In the standard photocatalytic procedure, 100 mL of RhB and CV dye solutions were placed separately in a 150 mL beaker and mixed with 30 mg of photocatalyst. The mixture was then stirred in the dark for 30 min to establish equilibrium between the catalyst and dye solution. Subsequently, the dye solution was irradiated with direct sunlight while being continuously stirred. The sunlight intensity measured by the TES Datalogging solar power meter reached 98 mW/cm 2 . Approximately 3 mL of dye solution was collected at various time intervals and then subjected to centrifugation at 5000 rpm for 5 min in order to separate the photocatalyst from the dye solution. UV/Vis absorption spectra were recorded at 30-minute intervals during UV irradiation to monitor changes in dye concentration. Calculations presented in previous studies were used to calculate the degradation rate, apparent rate constant (k app ), and half-life period (t 1/2 ) [32,33]. 2.6. Electrochemical measurements Electrochemical measurements for the study of OER were conducted using a Biologic SP-150 workstation. Before fabricating the electrode, the nickel foam (NF) underwent a cleaning process with 3 M HCl, acetone, and deionized water using a sonication bath to clean the nickel. The oxide layer on the foam was removed. Subsequently, the washed NF was dried in an oven at 100 ◦C for 12 h. To fabricate the electrode, 10 mg of the prepared catalyst was dispersed in 480 μ L of ethanol along with 20 μ L of nafion ionomer solution and sonicated for 30 min to produce a uniform ink. This thick gel was then uniformly coated onto NF (1 ×1 cm 2 ) and dried at 80 ◦C for 6 h. The active surface area and mass of the electrodes were 1 cm 2 and 3 mg/cm 2 , respectively. All electrochemical experiments took place in a traditional three-electrode cell containing a 1 M KOH alkaline electrode solution saturated with N 2 . The counter, reference, and working electrodes were a graphite rod, mercury/mercury oxide (Hg/HgO), and the catalyst-modified NF, respectively. The electrochemical calculation has been given in a supplementary file (see Supporting information). 3. Results and discussion 3.1. Characterization studies The X-ray diffraction (XRD) patterns of the synthesized materials, including GCN , Fe 2 O 3 , and FGCN composite, are presented in Fig. 1. The XRD pattern of the synthesized MOF- Fe 2 O 3 sample before calcination is Fig. 1. XRD patterns of the as-synthesized (a) GCN , (b) Fe 2 O 3 , and (c) FGCN composite materials. S. Kumaravel et al. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 310 (2024) 123972 4 shown in Fig. S1 (see Supporting Information). Fig. S1 shows a prominent XRD diffraction peak at 8.88◦for MOF-Fe 2 O 3 . Remarkably, these diffraction peaks are in good agreement with those reported for MOF- Fe 2 O 3 in previous studies [35–38]. The XRD patterns of both Fe 2 O 3 and FGCN materials subjected to calcination at 550 ◦C reveal consistent reflections corresponding to the (012), (104), (110), (113), (024), (116), (018), (214), and (300) planes (Fig. 1). These findings align well with the crystal structure of hematite α -Fe 2 O 3 , as confirmed by the JCPDS number 79-0007 [39]. These findings indicate the successful crystallization of the prepared samples. Furthermore, the absence of the characteristic MOF peak in the XRD pattern suggests that the organic ligands in the MOF structure were completely decomposed. This observation is in good agreement with the findings described in the previous literature [40]. In the XRD analysis, GCN shows distinct peaks at 12.5◦and 27.5◦, corresponding to the (100) and (002) planes of the GCN structure, respectively [41]. Furthermore, there is no evidence of peak shifts, suggesting that the crystal structures of Fe 2 O 3 and GCN are unaffected after composite formation. The intensity of the characteristic Fe 2 O 3 peak decreases slightly with the incorporation of GCN species. However, no clear characteristic peak of GCN is observed in the FGCN composite. This lack can be attributed to the uniform uptake and lowintensity diffraction peaks due to the low concentration of crystalline GCN, which is consistent with previous literature results [32,42,43]. However, the presence of GCN was confirmed with TEM and XPS analyses. Fig. 2 shows the characteristic functional groups and bond vibrations of the prepared catalysts, including Fe 2 O 3 , GCN , and FGCN composite catalysts. In the case of Fe 2 O 3 , two bands are observed at lower wavenumbers, exactly at 426 and 520 cm −1 , which can be attributed to the stretching mode of the Fe-O bond [44]. The pristine GCN exhibits three distinctive vibration regions: the first broad band in the range of 3000–3700 cm −1 corresponds to stretching vibrations of N–H and O–H bonds [45], while the second region spanning 1210–1640 cm −1 is characteristic of the stretching vibrations of C–N and C=N bonds typically found in heterocyclic compounds [46,47]. Furthermore, a sharp peak at 796 cm −1 is indicative of the respiration mode of the triazine structure (C-N-C) [47]. Particularly for the FGCN composite, there is no discernible band in the spectrum when the GCN content is low and homogeneous dispersion. The absorption band of the FGCN composite catalyst showed a slight shift to higher wavenumbers. This observation is consistent with findings in previously published literature [48,49]. Fig. 3 shows the optical properties of the synthesized catalysts Fe 2 O 3 , GCN, and FGCN were examined using DRS-UV Vis spectroscopy. Fig. 3a- c demonstrates that both pristine and composite catalysts exhibit efficient absorption of visible light. The absorption spectrum of GCN shows a clear edge around 400–500 nm, while Fe 2 O 3 shows a broad absorption in the entire visible region, which will be reflected in the FGCN composite. The bandgap energies of the prepared catalysts were determined using the K-M equation, and the corresponding energy spectra are shown in Fig. 3(d-f). After the combination of GCN with Fe 2 O 3 , the bandgap energy decreases from 1.90 eV (Fe 2 O 3 ) to 1.76 eV (FGCN). The optical bandgap energies of Fe 2 O 3 , GCN, and FGCN composite catalysts are found to be 1.90, 2.71, and 1.76 eV, respectively. This change in bandgap energy can be attributed to band bending in the semiconductor heterojunction. The surface morphologies of both Fe 2 O 3 and FGCN composite catalysts were examined by FE-SEM, and the corresponding results are demonstrated in Fig. 4a-d. In Fig. 4a and b, the FE-SEM pictures of Fe 2 O 3 show a spherical morphology consisting of uniform spheres composed of numerous small nanoparticles. Interestingly, the FGCN composite catalyst (Fig. 4c and d) exhibits a morphology similar to that of Fe 2 O 3 . This result clearly suggests that the SEM images cannot reveal GCN sheets, probably due to their homogeneous dispersion or the limited amount of GCN used in the composite. Furthermore, no discernible differences in morphological features are evident between pristine Fe 2 O 3 and FGCN due to their very small sizes. This morphology is in good agreement with previous literature results [44,50,51]. HR-TEM analysis further confirmed the presence of characteristic spherical shapes of Fe 2 O 3 particles uniformly distributed on the GCN surface. The close association of the FGCN composite catalyst can be easily observed in the TEM and HR-TEM images shown in Fig. 5. These images reveal the characteristic spherical morphology of the FGCN nanostructures as shown in Fig. 5a and b. Moreover, the FGCN composite sample clearly shows a uniform distribution of Fe 2 O 3 spheres strongly attached to the GCN surface (marked in red) in Fig. 5 c and g. This results in good agreement with previous literature reports [51–53]. Detailed examination of the surface atomic structure of the as-prepared FGCN nanocomposite samples was carried out using HR-TEM, as evidenced in Fig. 5d and g. As shown in Fig. 5f and k, the well-resolved lattice fringes within the FGCN nanospheres indicate the presence of crystal grains of Fe 2 O 3 . These observations are consistent with the fast Fourier transform (FFT) patterns and corresponding lattice fringes, as shown in Fig. 5e and j, and are further enhanced by inverse FFT (IFFT) analysis. These analyses revealed interplanar spacings of 0.36 nm and 0.27 nm corresponding to the (012) and (104) planes, confirming the crystal structure of Fe 2 O 3 [54]. In Fig. 5g, the enlarged HR-TEM image of FGCN highlights the clear formation of Fe 2 O 3 interfacial bonding on the GCN surface. Furthermore, it is clear from Fig. 5i and l that nanointerfaces are formed between the (104) planes of Fe 2 O 3 , and GCN. The nanointerface junction significantly contributes to catalytic efficiency in both degradation and electrochemical processes. XPS analysis revealed the electronic and oxidation states of the elements within the highly active FGCN photocatalyst. Fig. 6a displays the survey spectrum, highlighting prominent elements such as C (1s), N (1s), O (1s), and Fe (2p). Within the FGCN composite (Fig. 6b), the C1s spectrum shows two distinct peaks at 284.6 eV and 288.2 eV, corresponding to C=C and N-C=N, respectively [55]. The N 1s spectrum (Fig. 6c) can be deconvoluted into three peaks at 398.5 eV, 399.1 eV, and 400.4 eV, representing (C=N–C), N–(C) 3 , and C–N–H, respectively. [56,57]. The high-resolution XPS spectrum of O 1s (Fig. 6d) reveals two peaks at 529.5 eV and 531.3 eV, confirming the presence of high-energy lattice metal–oxygen bonds and surface-adsorbed water molecules [58]. Fig. 6e shows the high-resolution Fe 2p spectrum deconvoluted into two peaks. The binding energy of approximately 710.6 eV and 724.2 eV is attributed to Fe 2p 3/2 and Fe 2p 1/2 , and the two separate peaks at 709.9 & 711.8 eV and 723.6 & 726.1 eV correspond to Fe 2+ and Fe 3+ , respectively [59]. The binding energies of two satellite peaks were Fig. 2. FT-IR spectra of the as-synthesized (a) Fe 2 O 3 , (b) GCN , and (c) FGCN composite materials. S. Kumaravel et al. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 310 (2024) 123972 5 observed at 718.4 and 732.7 eV, respectively, indicating the presence of Fe 3+ oxidation states. These findings provide confirmation of the presence of GCN and Fe 2 O 3 and establish the formation of an FGCN composite catalyst free of appreciable impurities. 3.2. Photocatalytic performance The photocatalytic degradation efficiency of the materials under investigation was evaluated using RhB and CV organic pollutants in the presence of sunlight. To provide a more comprehensive evaluation of Fig. 3. UV–vis DRS spectra (a) GCN , (b) Fe 2 O 3 , (c) FGCN, and (d-f) corresponding KM plots of as-prepared catalysts, respectively. Fig. 4. FE-SEM image of the as-synthesized (a and b) Fe 2 O 3 , and (c and d) FGCN composite materials. S. Kumaravel et al. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 310 (2024) 123972 6 photocatalytic activity, we carried out degradation experiments of RhB and CV dyes with Fe 2 O 3 , GCN, and FGCN composite catalysts. The photocatalytic degradation activity curves of these prepared materials are shown in Fig. 7a–d. First, adsorption experiments were performed by stirring the sample for 30 min in the dark to allow the sample to reach adsorption/desorption equilibrium. Notably, without irradiation, only a small portion (5–10 %) of the dye was adsorbed onto the surface of the materials. This indicates that the prepared material has limited adsorption capacity. Fig. 7b and c shows that the light activation capacity of Fe 2 O 3 is low, and the degradation rate is only 33 % for RhB dye and 27 % for CV dye when exposed to sunlight. On the other hand, the GCN photocatalyst showed a slight increase in photocatalytic activity and degraded 52 % of RhB and 46 % of CV dyes after 150 min of sunlight exposure. Upon introducing Fe 2 O 3 onto the GCN surface, an FGCN composite heterostructure photocatalyst was obtained, which exhibited significantly higher photocatalytic decomposition efficiency (as shown in Fig. 7c and d). This composite catalyst achieved a degradation rate of 93 % for RhB and 95 % for CV organic dyes, outperforming both Fe 2 O 3 and GCN under the same conditions. The order of dye decomposition ability of the synthesized photocatalysts can be summarized as FGCN > GCN >Fe 2 O 3 . The improved photocatalytic efficiency of the FGCN composite catalyst can be attributed to the formation of a heterojunction, which effectively suppresses the rapid recombination of photogenerated electrons and holes. The results clearly demonstrate that the FGCN composite catalyst significantly enhances the decomposition efficiency of both RhB and CV dyes. As shown in Fig. 7e and f, the data fit well with the classical pseudofirst-order kinetic correlation expressed by the equation -ln (C t /C 0 ) =t [60]. For both RhB and CV dyes, the Fe 2 O 3 , GCN, and FGCN photocatalytic degradation rate constants are: 0.00278 min −1 & 0.00207 min −1 , 0.00483 min −1 & 0.00391 min −1 , and 0.01973 min −1 & 0.02076 min −1 , respectively. The k value of RhB dye decomposition in the FGCN composite photocatalytic system (Fig. 7e) is 0.01973 min −1 , which is about 7.0 and 4.0 times better than Fe 2 O 3 and GCN photocatalysts, respectively. Similarly, the k value of CV dye decomposition in the FGCN composite photocatalyst system (Fig. 7f) was 0.02076 min −1 , which was 10.0 and 5.3 times larger than Fe 2 O 3 and GCN photocatalysts, respectively. These results clearly demonstrate that the FGCN composite photocatalytic heterostructure significantly enhances the photocatalytic degradation efficiency of both RhB and CV environmental organic pollutants when exposed to solar irradiation. Evaluation of the photocatalytic efficiency of the fabricated photocatalyst involves tracking the corresponding primary absorption peaks of RhB and CV at 552 nm and 590 nm, respectively. The UV–vis spectra of RhB and MB on the highly effective FGCN composite photocatalyst are shown in Fig. S2a and b, revealing that the maximum absorbance peaks of RhB and MB decrease with time under solar light irradiation. After 150 minutes, both dyes are almost disappearing. A comprehensive analysis of total organic carbon (TOC) was carried out to evaluate the photocatalytic degradation of RhB and CV dyes in the presence of FGCN composite photocatalyst under solar irradiation (Fig. 8). The experiment included various time intervals extending up to 150 min to observe changes in TOC levels. The results shown in Fig. 8 reveal that the TOC of both RhB and CV dye solutions gradually decreased when subjected to treatment with FGCN composite catalyst during solar exposure. Notably, significant dye mineralization was observed after 150 min of illumination. After this period, the final TOC mineralization rates were recorded at 81 % for RhB and 83 % for CV, respectively. Additionally, this study suggests that the dye can transform into various intermediate forms, [61] and complete mineralization can occur after 150 min. The most active FGCN composite catalyst showing the highest photocatalytic activity was selected to evaluate its applicability to various cationic and anionic dyes. Inspection of Table S1 reveals the remarkable effectiveness of the FGCN photocatalyst in degrading both cationic and anionic dyes under 150 min of sunlight exposure. The details of the degradation kinetics of RhB, CV, CR, MB, and MO dyes using the FGCN composite catalyst are shown in Table S1. Assessing the reusability of photocatalysts is of great importance in catalysis, especially when considering commercial applications. To further investigate the reusability of the most active photocatalyst the results are depicted in Fig. 9a and b. For each experiment, the most efficient photocatalysts were separated by centrifugation to remove the suspension and then dried at 373 K for 3 h. The photodegradation efficiency of RhB and CV dyes by FGCN catalyst decreased to 82 % and 80 % compared to the initial performance. However, our results clearly show that our catalysts remain stable and reusable for both dyes for up to five consecutive cycles without major changes, confirming their sustained effectiveness. Table S2 shows the recent literature comparison of photocatalytic performance by various Fe-based catalysts. As shown in Fig.S3, various analytical techniques such as XRD, FESEM, and EDX methods were used to characterize the spent photocatalyst. Fig. S3 shows the high angle XRD patterns of both the fresh and used FGCN catalyst. These figures clearly show that even after the fifth run, the spent catalyst shows no noticeable changes compared to the initial FGCN catalyst. Furthermore, Fig. S4a-c shows the FE-SEM images of the spent catalyst after five runs, which shows that the surface morphology structure remains unchanged. Both analyzes collectively verify that the crystal structure and surface morphology of the spent catalyst remain consistent throughout the photocatalytic reaction. EDS and elemental mapping analysis of the spent catalyst is shown in Fig. S4d-i, and shows no significant evidence of elemental leaching during the reaction. This further supports the high stability and durability of the photocatalyst. In this study, trapping experiments were utilized to investigate the main participants in the photodegradation process when FGCN nanocomposites were exposed to sunlight. The findings are visually represented in Fig. 9c. The introduction of 1 mmol of ammonium oxalate (AO, Fig.5. (a–c) TEM images, (d-l) HR-TEM images of FGCN, and its corresponding FFT and IFFT images, and (i, l) FGCN nanointerfaces edges (crystalline planes) stacked on amorphous GCN in the heterostructure nanocomposites. S. Kumaravel et al. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 310 (2024) 123972 7 h + scavenger), tert-butyl alcohol (t-BuOH, ● OH scavenger), and benzoquinone (BQ, O 2 ●– scavenger) significantly reduced the photocatalysis efficiency of the catalyst. In the absence of scavengers, approximately 93 % degradation of RhB occurred within 150 min. Conversely, when AO, t-BuOH, and BQ were introduced, the degradation rates were 65 %, 47 %, and 33 %, respectively. In particular, BQ had a substantial inhibitory effect on photocatalytic activity, highlighting the importance of O 2 ●– as the main active species in RhB degradation, followed by ● OH and h + , and are in the following order: O 2 ●– > ● OH >h + . According to the results of UV-DRS studies, the FGCN nanocomposite exhibits a reduced bandgap (1.76 eV) compared to GCN (2.71 eV) and Fe 2 O 3 (1.90 eV). Mulliken’s electronegativity theory was adopted to determine the conduction band (CB) and valence band (VB) energies of GCN and Fe 2 O 3 [62]. Fig. 9d proposes a suitable mechanism for the photocatalytic decomposition of both dyes (RhB and CV) over FGCN under sunlight. The band potentials in GCN are −1.125 eV (CB) and + 1.585 eV (VB), while in Fe 2 O 3 they are +0.42 eV (CB) and +2.32 eV (VB), respectively. In normal heterojunction arrangement, both Fig. 6. XPS spectra of the as-synthesized FGCN composite material. S. Kumaravel et al. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 310 (2024) 123972 8 Fig.7. Photo-degradation performance of (a) RhB, (b) CV, (c, d) degradation efficiency of RhB and CV at 150 min, and (e, f) corresponding kinetic curves of asfabricated materials under solar light irradiation. S. Kumaravel et al. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 310 (2024) 123972 9 potentials (CB of Fe 2 O 3 and VB of GCN) are not feasible for the formations of ROS such as superoxide radical anions (O 2 ●– ) and hydroxyl radicals ( ● OH), since CB of Fe 2 O 3 is less negative when compared with standard redox potential of O 2 ●– (O 2 /O 2 ●– = − 0.33 eV vs NHE) [63], and also the VB of GCN is less positive when compared with standard redox potential of ● OH ( – OH/ ● OH = +1.99 eV vs. NHE) [63]. At this condition, it has been proposed that the recombination of e - and h + ions effectively prevents the recombination of the corresponding e - /h + pair through the formation of Z-scheme heterojunction through the coupling of GCN and Fe 2 O 3 [64,65]. In the composite Z-scheme heterojunction Fig. 8. TOC analysis of (a) RhB and (b) CV solution during photodegradation. Fig. 9. (a, b) Reusability test of RhB and CV dyes, (c) Scavenging test for RhB, and (d) Possible photocatalytic degradation mechanism of FGCN composite catalyst. S. Kumaravel et al.