Facile Hydrothermal Synthesis of a Graphene Oxide–Cerium Oxide Nanocomposite: A Highly Efficient Catalyst for Azo Dye Degradation

Abstract

The pervasive discharge of synthetic dyes into aquatic ecosystems poses a significant threat due to their chemical stability, low biodegradability, and carcinogenicity. Conventional dye remediation methods—such as biological treatments, coagulation, and adsorption—have demonstrated limited efficiency and poor reusability, particularly against resilient azo dyes. Cerium oxide (CeO₂) nanoparticles have gained traction as photocatalysts owing to their redox-active surfaces and oxygen storage capabilities; however, issues like particle agglomeration and rapid charge recombination restrict their catalytic performance. To address these challenges, this study presents the novel synthesis of a graphene oxide–cerium oxide (GO-CeO₂) nanocomposite via a facile in situ hydrothermal approach, using graphite from lead pencils as a sustainable precursor. The composite was structurally characterized using UV–visible spectroscopy, XRD, FTIR, and TEM. The GO matrix not only facilitates uniform dispersion of CeO₂ nanoparticles but also enhances interfacial electron mobility and active site availability. The nanocomposite demonstrated exceptional photocatalytic degradation efficiencies for methyl orange (94%), methyl red (98%), congo red (96%), and 4-nitrophenol (85.6%) under sunlight irradiation, with first-order rate constants significantly exceeding those of pure CeO₂. Notably, GO–CeO₂ retained strong catalytic activity over four degradation cycles, confirming its recyclability and structural stability. Total organic carbon (TOC) analysis revealed 79% mineralization of methyl orange, outperforming CeO₂ (45%), indicating near-complete conversion into benign byproducts. This work contributes a scalable, low-cost, and highly active heterogeneous photocatalyst for wastewater treatment, combining green synthesis principles with improved photodegradation kinetics. Its modular architecture and reusability make it a promising candidate for future environmental remediation technologies and integrated photocatalytic systems.

1. Introduction

Synthetic dyes are among the most prevalent and persistent pollutants in industrial wastewater due to their complex chemical structures, stability under various environmental conditions, and resistance to biological degradation. The textile sector alone contributes approximately 20% of global industrial water pollution, with azo dyes making up over 70% of total dye usage by weight [1,2]. These dyes and their breakdown products, such as aromatic amines, pose significant risks to human health due to their mutagenic and carcinogenic nature. In aquatic systems, their strong coloration reduces light penetration and disrupts photosynthetic activity, threatening both marine biodiversity and ecological balance [3,4]. It is estimated that around 10–15% of dyes used in textile processing are discharged directly into effluents. Their highly variable molecular configurations, intense color, and low biodegradability make effective removal through conventional treatments particularly challenging. Commonly employed methods—adsorption [5,6,7], chemical oxidation, membrane separation, biological treatment, and electro-coagulation—often suffer from high operational costs, generation of secondary waste, low selectivity, and limited recyclability [8,9].

Recently, heterogeneous photocatalysis has emerged as a promising strategy for dye remediation, offering the ability to mineralize organic pollutants into benign products such as CO₂, H₂O, and inorganic acids [10]. Cerium oxide (CeO₂), an n-type semiconductor with a band gap of ~2.89 eV, has garnered attention for its photocatalytic capabilities, driven by its redox-active surface and oxygen storage/release capacity [11]. Under UV or visible light irradiation, CeO₂ generates electron–hole pairs that facilitate oxidation reactions, but its performance is hindered by poor charge separation, limited surface area, and nanoparticle agglomeration. To enhance CeO₂’s catalytic efficacy, researchers have explored hybrid composites with carbon-based materials such as carbon black, carbon nanotubes, and, more recently, graphene oxide (GO). GO provides a high surface area, excellent thermal and electrical conductivity, and a rich surface chemistry comprising hydroxyl, carboxyl, and epoxy groups. These functional moieties not only promote strong interaction with metal oxide nanoparticles but also facilitate pollutant adsorption and electron transfer [12,13,14,15]. The negatively charged GO sheets offer electrostatic stabilization and dispersion platforms for uniformly anchoring metal species, enhancing overall photocatalytic kinetics.

Moreover, the pursuit of green synthesis strategies for nanocatalysts has led to innovative developments using biopolymers, natural carbon sources, and minimal chemical processing. Khan et al. recently designed ZnO–chitosan textile composites templated with metal nanoparticles for dye degradation, showcasing an efficient and recyclable catalytic system [6]. Albukhari and coworkers developed silver nanoparticle-loaded cellulose paper with strong catalytic reduction capabilities against nitrophenols and dyes [16]. Further, Khan et al. introduced quaternary core–shell sulfide nanocomposites for simultaneous organic dye degradation and hydrogen production, demonstrating multifunctionality in sustainable water treatment [17].

Despite these advances, major gaps remain in terms of scalable synthesis, catalyst recovery, structural stability, and multi-cycle recyclability in real wastewater conditions. Most existing systems rely on commercial graphite sources and multi-step synthesis routes, which may limit practical applications. In response, this study presents a facile, eco-conscious approach for synthesizing GO–CeO₂ nanocomposites via hydrothermal integration using graphite derived from lead pencils. Tour’s method was applied for GO oxidation, followed by ultrasonic-assisted incorporation of cerium nitrate to form the nanocomposite. Structural, optical, and morphological properties were characterized using UV–Visible spectroscopy, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM). The composite’s photocatalytic performance was evaluated against representative azo dyes—methyl orange (MO), methyl red (MR), and congo red (CR)—as well as 4-nitrophenol (4-NP), with further validation through recyclability, mineralization studies, and mechanistic heterogeneity analysis. This work contributes to a cost-effective, structurally stable, and high-performance heterogeneous photocatalyst for environmental remediation, paving the way for sustainable wastewater treatment technologies.

2. Results and Discussion

2.1. Synthesis Mechanism of GO–CeO₂ Nanocomposite

Figure 1 illustrates the synthetic route for the GO–CeO₂ nanocomposite. GO was prepared from lead pencil graphite following Tour’s method [18], yielding a brown powder that signaled successful oxidation, corroborated by UV-Visible spectroscopy. The nanocomposite was fabricated via ultrasonication of GO and Ce(NO₃)₃ aqueous dispersions. Gradual mixing (30 min stirring at 25 °C) and acidic adjustment (pH = 3 using dilute HCl) facilitated electrostatic interaction between Ce³⁺ ions and GO surfaces [19]. Oxidation of Ce³⁺ to Ce⁴⁺ was induced by 50 mg of 33 wt% H₂O₂, and subsequent pH elevation to 9 initiated hydroxide-nitrate exchange. Hydrothermal treatment at 180 °C for 2 h promoted CeO₂ nucleation. The reaction pathway is captured by Equations (1)–(3), confirming CeO₂ formation via stepwise hydroxylation and oxidation of cerium species:

2Ce³⁺ + H₂O₂ + 2H₂O → 2Ce(OH)₂²⁺ + 2H⁺

(1)

This reaction represents the oxidation of Ce³⁺ to hydrolyzed Ce⁴⁺ species in aqueous phase—a transient step before CeO₂ nucleation. The process does not imply a strongly acidic medium; rather, it indicates that H₂O₂ acts as an oxidant, and minor proton generation accompanies hydrolysis.

Subsequent condensation and dehydration steps lead to CeO₂ formation as shown in Reactions (2) and (3):

4Ce³⁺ + O₂ + 4OH⁻ + 2H₂O → 4Ce(OH)₂²⁺

(2)

Ce(OH)₂²⁺ + 2OH⁻ → CeO₂ + 2H₂O

(3)

At neutral or alkaline pH, cerium ions (Ce³⁺ or Ce⁴⁺) typically generate insoluble Ce(OH)₄ precipitates, resulting in unregulated agglomeration and inadequate dispersion on GO. Cerium ions remain soluble and stable when the pH is kept at an acidic level (~3), which guarantees homogenous nucleation of CeO₂ nanoparticles on the GO surface and regulated hydrolysis. Maintaining an acidic pH (~3) keeps cerium ions in a soluble and stable form, ensuring controlled hydrolysis and uniform nucleation of CeO₂ nanoparticles on the GO surface [20,21,22].

The produced sample was collected and analyzed using a UV-visible spectrophotometer to confirm the GO-CeO₂ nanocomposite formation (absorbance peak). After the completion of the reaction, the product was centrifuged at 4000 rpm to obtain the pellets of the nanocomposite.

2.2. Optical, Structural, and Morphological Characterization

2.2.1. UV–Visible Absorption and Band Gap Analysis

UV–Vis spectroscopy confirmed the formation of both GO and GO–CeO₂ nanocomposites (Figure 2a,b). GO displayed a characteristic peak at 235nm (π–π* transition of C=C bonds) and a shoulder at 300nm (n–π* transition of C=O groups) [23]. The GO-CeO₂ nanocomposite showed more intense and sharper band gaps at 217 nm and 315 nm, as shown in Figure 2b. The absorption peak at 315 nm is attributed to surface plasmon resonance (SPR), arising from charge transfer between Ce³⁺ ions and oxygen vacancies within the CeO₂ lattice. This localized SPR effect enhances light–matter interaction and is consistent with previously reported CeO₂-based plasmonic systems [24,25]. To clarify this mechanism, the UV–Vis spectrum of pure CeO₂, inset of Figure 2b, was compared with that of GO–CeO₂. Pure CeO₂ typically exhibits a broad absorption edge below 300 nm due to its intrinsic band gap (~3.2 eV), whereas the GO–CeO₂ composite shows a distinct SPR peak at 315 nm, confirming the role of oxygen vacancy–mediated charge oscillations.

Tauc’s equation, αhv = (hv−E_g)^1/2, has been used to find the direct optical band gap, E_g, for GO nanoparticles and GO-CeO₂ nanocomposite, whereas hv is the incident photon energy and α is the absorption coefficient [26]. Using the density (β) and concentration (C) of the material, α = 2.303 × 10³ Aβ/lC, where L is the light path length (1 cm) [26]. Figure 2c illustrates the extrapolated linear region of the Tauc plots, from which band gap energies were identified at the x-axis intercept. GO exhibited a direct band gap of 3.62 ± 0.05 eV, while the GO–CeO₂ composite showed a slightly reduced value of 3.52 ± 0.04 eV, suggesting enhanced electron delocalization in the hybrid structure. Additionally, two higher-energy transitions were observed at 5.58 ± 0.07 eV and 4.79 ± 0.06 eV for GO and GO–CeO₂, respectively. These secondary gaps may stem from intrinsic defects or electronic transitions associated with CeO₂ states within the composite. The slight band gap narrowing in GO–CeO₂ is attributed to structural modifications induced by CeO₂ nanoparticle integration. As confirmed by XRD analysis, reduced crystallite size enhances orbital overlap, thereby decreasing the energy required for charge excitation. Conversely, the presence of CeO₂ aggregates and a broad size distribution—evident from TEM imaging—could lead to localized band edge distortions, contributing to the observed multi-band behavior and optical gap broadening [26].

2.2.2. XRD Analysis and Structural Evaluation

The crystallographic features and phase composition of GO and the GO–CeO₂ nanocomposite were elucidated using XRD, as presented in Figure 2d. The diffractogram of GO exhibited a prominent peak at 2θ ≈ 11.3°, indexed to the (001) lattice plane, which is characteristic of oxidized and exfoliated graphene layers. A secondary, low-intensity signal at approximately 26.4°, corresponding to the (002) reflection, was attributed to residual unoxidized graphite domains [27]. The increased interlayer spacing relative to pristine graphite indicates successful functionalization with oxygen-containing groups—such as hydroxyl, epoxy, and carboxyl—which allow water and ions to intercalate between the sheets.

Upon integration with cerium oxide, the composite revealed well-defined peaks at 2θ values of 28.5°, 33.1°, 47.5°, and 56.3°, corresponding to the (111), (200), (220), and (311) planes of fluorite-structured CeO₂ (JCPDS No. 34-0394) [28]. The observed shift in GO’s main peak from 11.3° to 9.2° and its broadening post-compositing suggest expansion of interlayer distance and increased structural disorder due to CeO₂ intercalation [29].

The structural parameters were estimated from XRD charts and presented in

Table 1. The XRD parameters for the GO and GO-CeO₂ samples.

XRD Parameters	GO-CeO2 Sample					GO Sample
	(001) GO	(111) CeO2	(200) CeO2	(220) CeO2	(311) CeO2	(001) GO	(002) G
2$\theta$ (°)	8.58	29.48	33.20	46.52	56.50	9.20	26.46
d-value (Å)	10.31	3.03	2.70	1.95	1.63	9.61	3.37
Rel. Intensity (%)	26.3	100	64.1	74.3	68.3	100	24.7
Measured Intensity	44	167	107	124	114	115.7	28.6
β (°)	3.20	0.80	0.79	1.34	0.67	1.08	0.54
TC	-	1.31	0.84	0.97	0.89	-	-
D (nm)	2.60	10.73	10.97	6.75	14.07	7.71	15.80
δ (nm−2) × 10−5	148.0	8.7	8.3	21.9	5.1	16.8	4.0
Microstrain (%) × 10−2	18.61	1.33	1.16	1.36	0.54	5.86	1.00
Crystallite per surface area (m−2) × 1014	584.74	2.45	2.04	6.36	0.58	20.94	0.85
SSA (m2g−1)	12.33					9.69

. The values of crystallite size of the samples were estimated using Debye-Scherrer’s equation, D = 0.94 λ/β cos θ, utilizing the wavelength of XRD (λ), full-width at half maximum (β), and the diffraction angle (θ), considering the observed peaks in Figure 2d [30]. The D value for GO (001) is 7.71 nm and reduced to 2.60 nm in the composite, whereas the D value for CeO₂ (111) is much higher (10.73 nm). The decrease in crystallite size upon compositing with CeO₂ leads to a decrease in the number of graphene layers in each domain and increases the defect density. This may be attributed to the intercalation of CeO₂nanoparticles between the GO layer to cause more exfoliation of GO. This nanoscale intercalation can significantly improve the amount and quality of active contact sites between the CeO₂ nanoparticles and GO layers. This was also related to the increase in the defect density, which was confirmed by the calculation of the minimum dislocation density (δ) from the reciprocal of D², as shown in Table 1 [31]. The value of δ for GO increases from 16.8 × 10⁻⁵ to 148 × 10⁻⁵ nm⁻², whereas CeO₂ showed lower δ values. Also, the texture coefficient (TC) was calculated from the values of the relative intensities for CeO₂ planes, which indicate the professional growth of CeO₂ crystallites along the (111) plane with TC >1 [32,33]. The microstrain is also resulting from imperfections and can be estimated from the broadening of XRD peaks using the Williamson–Hall method, β/4 tan θ [34,35]. The value of GO (001) microstrain for GO-CeO₂nanocomposite is almost three times its value for GO nanopowder (Table 1). The crystallite per unit surface area (N) and the specific surface area (SSA, area per unit mass) of the materials are important structural parameters. These parameters are estimated from N = d/D³ and SSA = 6000/(D ρ), where D is the crystallite size, d is the inter-planar distance, and ρ is the density of nanoparticles [36,37]. The values of N and SSA are presented in Table 1. The N values for CeO₂ are less than their values for the GO XRD peak. The highest value of N (584.74 × 10¹⁴ m⁻²) is observed for GO (001) in the nanocomposite, indicating the high reduction that happened to GO crystallites during the preparation of the nanocomposite. Additionally, the GO-CeO₂nanocomposite showed a higher SSA value (12.33 m²/g) than GO powder. Therefore, the nanocomposite is supposed to be more catalytically effective than pure GO.

2.2.3. FTIR Spectroscopic Analysis

The functional groups and surface chemistry of the synthesized GO and GO–CeO₂ nanocomposite were explored using FTIR spectroscopy, with the spectra depicted in Figure 3. Both samples exhibited broad absorption bands around 3500 cm⁻¹ and 1636 cm⁻¹, attributed to O–H stretching vibrations and adsorbed water molecules, respectively—signals that reflect the hydrophilic nature of the materials [38]. In the case of GO, Figure 3a, several characteristic peaks affirm the successful oxidation of graphite. These include a notable band at 2931 cm⁻¹ corresponding to the C–H stretching vibration of methylene groups, and a weaker signal at 2887 cm⁻¹ linked to alkyl C–H stretching. A prominent absorption at 1739 cm⁻¹ is assigned to the C=O stretching of carboxylic acid functionalities, while the 1642 cm⁻¹ peak indicates the presence of C=C bonds within the sp²-hybridized carbon domains. Additionally, the band near 1432 cm⁻¹ corresponds to O–H bending, and the signal at 1097 cm⁻¹ arises from C–OH stretching associated with alkoxy groups [39,40]. These features collectively confirm the introduction of oxygen-rich functional groups onto the graphene sheets, indicative of a high degree of oxidation and strong hydrophilic behavior.

The spectrum of the GO–CeO₂ nanocomposite (Figure 3b) revealed further structural insights. In addition to retaining the functional bands of GO, a distinct absorption band near 480 cm⁻¹ appeared, corresponding to Ce–O–Ce vibrations—a direct signature of cerium oxide formation. Moreover, slight shifts in oxygen-containing functional group bands compared to pristine GO suggest chemical interaction rather than simple physical mixing. These observations indicate strong interfacial bonding, likely involving Ce–O–C linkages formed during hydrothermal synthesis. Another absorption at 1642 cm⁻¹ is associated with physisorbed O–H groups [41]. The preservation of GO’s functional profile alongside the introduction of CeO₂ features confirms a chemically integrated and structurally stable composite, ideal for enhanced photocatalytic applications.

The structural and chemical evidence from XRD and FTIR strongly supports the partial reduction of GO and the formation of strong interfacial interactions with CeO₂, consistent with previous reports [42,43,44]. Raman analysis (Figure S1, Supplementary Materials) further complements these findings, showing characteristic D (~1350 cm⁻¹) and G (~1580 cm⁻¹) bands. The ID/IG ratio decreased from 0.98 for GO to 0.85 after composite formation, indicating a reduction in defect density and partial restoration of sp² domains. Additionally, the Ce–O–Ce vibration observed in FTIR and the fluorite-phase peaks in XRD confirm CeO₂ formation, where cerium predominantly exists in the +4 oxidation state. However, the coexistence of Ce³⁺ cannot be excluded, as oxygen vacancies commonly occur in ceria-based systems. These vacancies enhance charge separation and electron trapping, which are beneficial for photocatalytic performance.

2.2.4. TEM Study of GO and GO-CeO₂ Nanocomposite

The morphological features of the synthesized GO and GO–CeO₂ nanocomposites were thoroughly investigated using TEM, as displayed in Figure 4a–h. This analysis provided direct visualization of the catalysts’ structure, particle distribution, and surface texture—essential attributes for photocatalytic performance. The TEM images of GO, shown in Figure 4a–d, reveal a distinctly flaky, multilayered architecture composed of ultra-thin sheets with uneven folding and wrinkling. These sheets exhibit lateral dimensions spanning several micrometers, indicating effective exfoliation and extension of the basal plane. The layered configuration is a hallmark of oxidized graphite and is consistent with the presence of oxygen-containing functional groups identified in FTIR analysis. This broad surface area and hydrophilic nature of GO not only facilitate high nanoparticle loading but also enhance the material’s ability to adsorb dye molecules during photocatalytic degradation.

Upon cerium oxide incorporation, the TEM micrographs in Figure 4e–h clearly show the successful anchoring of CeO₂ nanoparticles on GO sheets. The images confirm that CeO₂ particles formed with consistent morphology and size, predominantly distributed homogeneously across the GO surface. In some regions, aggregates of CeO₂ were observed along with a few free-floating nanoparticles, likely due to localized nucleation fluctuations. The composite architecture suggests that cerium ions preferentially nucleate on GO nanosheets rather than in the bulk solution—a behavior driven by electrostatic interactions between negatively charged GO functional groups and cerium precursors. This structured integration is influenced by synthesis conditions such as ultrasonic dispersion, reaction temperature, solubility parameters, and the inherent wrinkled morphology of GO, all of which contribute to guided self-assembly of CeO₂ nanoparticles [45]. A histogram inset in Figure 4h, generated using ImageJ software (version 2018), quantifies the particle size distribution, indicating a narrow range centered around 2.4 ± 0.5 nm. This fine-scale dispersion is critical, as smaller and uniformly distributed CeO₂ particles promote effective light absorption and electron–hole separation-core mechanisms in photocatalysis.

TEM observations further confirm that CeO₂ nanoparticles are anchored on the external surfaces of GO sheets rather than confined within interlayer spaces. This conclusion is supported by the absence of overlapping lattice fringes and the clear visibility of particle boundaries on sheet edges and basal planes. The hydrothermal synthesis route plays a key role in achieving this configuration by promoting in situ nucleation and growth of CeO₂ on oxygenated functional groups of GO, ensuring strong interfacial bonding and homogeneous dispersion.

Overall, the TEM results corroborate the structural findings from XRD and FTIR, reinforcing that GO acts as a robust support for CeO₂ nanoparticle growth. The synergistic nanostructure ensures high surface accessibility, uniform active site exposure, and enhanced interfacial charge transfer—features that collectively improve the efficacy of the composite in photodegradation applications for azo dyes and nitrophenolic compounds.

2.3. Photocatalytic Degradation of Azo Dyes

Azo dyes are a noteworthy family of water pollutants that are widely utilized in a variety of industrial sectors, such as printing, textiles, paper, and pharmaceuticals. Due to their extensive use, water-soluble, chemically stable dye molecules are released into aquatic habitats, endangering human health as well as marine biodiversity [16,46]. Common examples of refractory azo dyes that withstand natural degradation and endure in effluents are MO, CR, and MR. Table S1 (Supplementary Materials) displays their general physical characteristics. There are still few efficient photocatalysts with high activity, structural stability, and reusability, despite the fact that a variety of nanomaterials have demonstrated encouraging results in breaking down such dyes [47,48,49,50].

According to the Mars–van Krevelen (MvK) model, which stresses surface lattice oxygen atoms functioning as active species during redox reactions, the degradation mechanism on CeO₂-based catalysts is consistent [51,52]. In addition to increasing electron mobility and surface reactivity, oxygen vacancies in CeO₂ also aid in the dissociation processes necessary for full mineralization and promote dye molecule adsorption [53]. With the extra advantage of removing contaminants rather than transmitting them, photocatalysis offers an environmentally beneficial and economically feasible alternative to physical or biological treatment approaches, which are sometimes limited by selectivity and waste creation [54].

During photocatalytic degradation, azo dyes such as MO, CR, and MR are broken down into low-molecular-weight organic acids (e.g., acetic acid, formic acid) and carbon dioxide (CO₂). These products result from oxidative cleavage of azo bonds and subsequent mineralization, consistent with reported degradation pathways in CeO₂-based systems [55,56,57].

The photocatalytic efficiency of the synthesized GO–CeO₂ nanocomposite was evaluated in comparison with bare CeO₂ across multiple dye systems under sunlight irradiation. The degradation progress was tracked via UV–visible absorption spectroscopy. For each target dye, a 3 mM aqueous solution (30 mL) was prepared and mixed with 30 mg of catalyst. To ensure adsorption–desorption equilibrium, the mixtures were stirred in the dark for one hour before initiating light exposure, after which the initial absorbance spectrum was recorded.

Photodegradation kinetics were quantified based on a pseudo-first-order model, using the following logarithmic expression [58]:

r = ln (C_t/C_o) = ln(A_t/A_o) = −k_obst

(4)

Here, C₀ and C_t represent the initial and time-dependent dye concentrations, respectively, while A_o and A_t correspond to their absorbance values. The apparent rate constant k_obs was determined from the slope of the linear plot of ln(A_t/A_o) versus time.

2.3.1. Methyl Orange Degradation

MO was selected for photocatalytic evaluation due to its ecological relevance and typical azo structure. A 3 mM aqueous solution of MO (30 mL) was treated with 30 mg of catalyst, and following adsorption–desorption equilibrium, the reaction mixture was irradiated. Figure 5a,b displays the UV/Vis absorption spectra of MO photodegradation by GO/CeO₂ at various time intervals (0 to 150 min) and CeO₂ at various time intervals (0 to 5 h). Figure 5c,d shows the corresponding degradation percentages and kinetic plots.

The GO–CeO₂ nanocomposite achieved 90.48% degradation of MO within 150 min, whereas pure CeO₂ required 300 min to reach 89.53% removal under identical conditions, Figure 5c. This enhanced activity is attributed to several structural advantages of the composite: the uniform dispersion of CeO₂ on GO sheets ensures higher exposure of active sites, and the reduced band gap energy (3.52eV vs. 3.62eV for GO alone) promotes broader light absorption. Additionally, the improved surface area (12.33 m²/g), elevated defect density, and smaller crystallite domains observed in XRD and TEM analyses facilitate effective charge separation. Kinetic analysis based on the pseudo-first-order model [58], Figure 5d and Table S2 (Supplementary Materials), showed a rate constant (k_obs) of 0.0121 ± 0.0003 min⁻¹ for GO–CeO₂ and 0.00578 ± 0.0002 min⁻¹ for CeO₂, confirming a 2.1-fold increase in reaction rate.

2.3.2. Congo Red Degradation

CR is another widely used azo dye known for its persistent coloration and carcinogenic potential. The aqueous solution of CR gives an intense peak at 493 nm. A similar experimental protocol was followed using 3 mM CR solution. The UV/Vis absorption spectra of CR degradation by GO/CeO₂ (0 to 120 min) and CeO₂(0 to 4 h) are depicted in Figure 6a,b. As shown in Figure 6c, GO–CeO₂ accomplished 92.79% degradation in 120 min, outperforming CeO₂, which achieved 89.18% over 240 min.

The faster decomposition kinetics are indicative of stronger dye–catalyst interactions in the nanocomposite system. The oxygenated functional groups on GO contribute to effective dye adsorption, while the CeO₂ nanoparticles act as active sites for radical generation. From Figure 6d and Table S2 (Supplementary Materials), the pseudo-first-order rate constants derived from linear plots were 0.01812 ± 0.0004 min⁻¹ for GO–CeO₂ and 0.00495 ± 0.0002 min⁻¹ for CeO₂, demonstrating a 3.7-fold enhancement in catalytic efficiency. Such performance surpasses many previously reported CeO₂ composites and further underscores the synergy between GO and CeO₂ in facilitating pollutant breakdown [16,41,43,44,45].

2.3.3. Methyl Red Degradation

MR, an azo dye known for its toxic effects upon exposure, was selected for the final degradation test. The aqueous MR solution gives an absorption peak at 425 nm. The UV–Vis spectra, degradation percentages, and kinetic plots are summarized in Figure 7a–d. The UV-Vis absorption spectra of the MR photodegradation using GO/CeO₂ at different time intervals (0 to 140 min) and CeO₂ at different time intervals (0 to 270 min) are shown in Figure 7a,b. From Figure 7c, the GO–CeO₂ nanocomposite showed superior degradation (98% in 140 min) compared to CeO₂ (96% in 270 min), reinforcing the consistent catalytic advantage of the hybrid system.

The rate constant for MR degradation using GO–CeO₂ was calculated from Figure 7d and Table S2 (Supplementary Materials) as 0.01016 ± 0.0003 min⁻¹, nearly double that of CeO₂ (0.005094 ± 0.0002 min⁻¹). These results correlate well with the high surface area, defect-rich structure, and well-dispersed CeO₂ nanoparticles observed in morphological and crystallographic analyses. The small particle size (~2.4 ± 5 nm), as confirmed by TEM, supports enhanced interfacial contact and minimizes electron–hole recombination.

2.3.4. Comparative Analysis of Photocatalytic Efficiency

The data outlined in

Table 2. Comparison between the catalytic performance of the presented CeO₂ and GO/CeO₂ nanocomposite catalysts and previously reported benchmark CeO₂-based catalysts in the degradation of different dyes.

Catalyst	Dyes	Degradation%	Time (min)	Rate	Ref
CeO2	CR	82.4	120	0.0560	[59]
bare-CeO2	CR	28.2	150	0.0020	[60]
CeO2	Acid orange 7	~50	360	0.1420	[61]
ZnO2/CeO2	RhB	98	120	0.1143	[62]
CeO2 NPs	MO	98.6	50	0.0117	[63]
CeO2/rGO	CR	~75	140	0.0110	[64]
CeO2	MB	10	120	0.0080	[65]
CeO2	AO7	81.7	180	0.0067	[66]
CoFe2O4/CeO2 (1:4 ratio)	Rose Bengal	98%	90	0.021	[67]
Alg–Asc@CeO2	CV	99.5	73	0.013	[68]
g-C3N4/CeO2 nanocomposite	MR	94%	120	0.017	[69]
CeO2–GO–PAMcomposite	MB	99%	45	0.031	[70]
GO-CeO2	MO	94	150	0.0121	This study
	MR	98	140	0.0102
	CR	96	120	0.0181
CeO2	MO	90	300	0.0058
	MR	94	270	0.0051
	CR	85	240	0.0050

presents a comprehensive comparison between the GO–CeO₂ nanocomposite and various benchmark CeO₂-based catalysts—for the degradation of a wide range of organic dyes [59,60,61,62,63,64,65,66,67,68,69,70]. Notably, the GO-CeO₂ system demonstrates consistently high degradation percentages across all tested dyes (MO, MR, CR), with rapid reaction times and favorable rate constants.

For instance, the GO–CeO₂ composite achieved a 94% degradation of MO in 150 min, exceeding the performance of ZnO₂/CeO₂ [62] and CeO₂/rGO [64], which required comparable or longer times for similar dyes with lower rate constants. In the case of methyl red, the nanocomposite reached 98% removal in 140 min—demonstrating efficiency superior to other systems such as CeO₂/MB [65], and even outperforming more complex constructs like g-C₃N₄/CeO₂ nanocomposite [69] for MR. The rapid and near-complete degradation of CR (96% in 120 min) also stands out, especially when compared to bare CeO₂ (85% in 240 min) and previously reported nanosystems with slower kinetics [59,60]. These results reflect the distinct structural advantages of the GO–CeO₂ hybrid, stemming from the uniform dispersion of CeO₂ nanoparticles on GO’s surface, as confirmed by TEM imaging. The nanoscale particle distribution (~2.4 ± 5 nm), reduced crystallite size, and increased defect density enhance active site exposure and promote efficient light absorption and charge separation—key factors in photocatalytic activity. Additionally, the elevated specific surface area (12.33 m²/g) enables better interaction with pollutant molecules and facilitates rapid photochemical reactions.

Furthermore, the present GO–CeO₂ system offers significant advantages in terms of cost-effectiveness and scalability. Unlike multi-component or heavily doped catalysts that require expensive precursors and complex synthesis routes, the GO–CeO₂ composite is prepared using readily available materials through a simple hydrothermal process. This approach minimizes production costs and facilitates large-scale implementation for wastewater treatment, making it a practical and sustainable alternative to more elaborate catalytic architectures. Unlike other systems that rely on dopants or elaborate architectures to achieve high performance, the GO–CeO₂ nanocomposite combines simplicity in synthesis with exceptional catalytic capability. Its improved kinetics, structural integrity, and demonstrated recyclability align well with the criteria for sustainable environmental remediation technologies. This underscores the novelty and relevance of your material, especially when placed alongside previously published designs [59,60,61,62,63,64,65,66,67,68,69,70].

2.3.5. Photocatalytic Mechanism for MO Degradation Using GO–CeO₂ Nanocomposite

The photodegradation mechanism of MO over the GO–CeO₂ nanocomposite is schematically illustrated in Scheme S1 (Supplementary Materials). Upon exposure to visible light, the nanocomposite undergoes photoexcitation wherein photons with energies equal to or greater than its band gap (E_g) induce electron transition from the valence band (VB) to the conduction band (CB), leaving behind positively charged holes. This generation of electron–hole pairs constitutes the primary activation step, described by Equation (5):

GO-CeO₂ +hν → CeO₂/GO (e⁻(CB) + h⁺(VB))

(5)

The photogenerated holes in the VB interact with water molecules adsorbed on the catalyst surface, leading to the formation of highly reactive hydroxyl radicals (•OH), which play a key role in oxidizing dye molecules. This step is represented by Equation (6) [71]:

H₂O + h⁺(VB) →•OH(ads) + H⁺(ads)

(6)

The extent to which dye molecules undergo mineralization on the GO–CeO₂ surface by hydroxyl radicals (•OH) largely depends on their molecular structure and inherent stability. These radicals, known for their strong oxidative potential, not only facilitate the breakdown of complex organic compounds but can also effectively eliminate microbial contaminants present in the solution [72]. Simultaneously, the excited electrons in the CB reduce dissolved oxygen to form superoxide radicals ( O₂⁻), as expressed in Equation (7) [72]:

O₂ + e⁻(CB) → O₂⁻(ads)

(7)

These reactive oxygen species (•OH and O₂⁻) initiate the degradation of MO and other dyes such as MR, CR, and 4-NP by attacking the azo (–N=N–) and aromatic bonds. This leads to the formation of intermediate compounds like aromatic amines and phenolic derivatives. These intermediates are further oxidized into low-molecular-weight organic acids (e.g., acetic acid, formic acid), which are eventually mineralized into carbon dioxide (CO₂) and water (H₂O). Additionally, the process releases inorganic ions such as nitrate (NO₃⁻) and sulfate (SO₄²⁻), depending on the dye structure. The transformation pathway is consistent with previously reported mechanisms in similar photocatalytic systems [73,74]. The superoxide radicals also help maintain charge neutrality and suppress electron–hole recombination, which is essential for sustaining photocatalytic activity. Protonation of superoxide radicals can lead to the formation of hydrogen peroxide (H₂O₂), which further decomposes into additional hydroxyl radicals, enhancing the oxidative environment.

The superior activity of the GO–CeO₂ composite can be directly attributed to its structural synergy: the uniformly dispersed CeO₂ nanoparticles provide abundant oxygen vacancy sites that enhance radical generation, while the GO scaffold promotes charge transport and pollutant adsorption. The mechanism integrates both redox reactions and surface-mediated radical pathways, confirming the heterogeneous nature of the photocatalytic process.

The enhanced photocatalytic activity of the GO–CeO₂ composite is attributed to (i) photoexcitation of CeO₂ generating electron–hole pairs, (ii) rapid electron transfer from CeO₂ to the conductive GO sheets (which act as an electron acceptor/transport pathway) that suppresses e⁻/h⁺ recombination, and (iii) subsequent formation of reactive oxygen species ( O₂⁻,•OH) at the surface (assisted by oxygen vacancies in CeO₂), which oxidize the target dye—a mechanism consistent with recent experimental and EPR/scavenger studies [75,76,77]. Although direct scavenger or EPR analyses were not performed in this work, similar systems have demonstrated the roles of •OH, h⁺, and O₂⁻ through isopropanol (IPA), ethylenediaminetetraacetic acid (EDTA), and benzoquinone (BQ) quenching experiments, as well as EPR detection of radical species under visible light irradiation [75,76,77].

2.4. Photocatalytic Reduction of 4-Nitrophenol by GO–CeO₂ Nanocomposite

4-NP is a prevalent industrial pollutant used in the synthesis of pharmaceuticals, dyes, fungicides, and insecticides. Its persistence in aqueous environments poses a serious health risk due to its toxicity; acute exposure can result in neurological symptoms and cyanosis, while direct contact causes mucosal and ocular irritation. Owing to its chemical stability and environmental relevance, 4-NP serves as a representative probe molecule for evaluating the catalytic activity of nanomaterials.

In this study, a 3 mM aqueous solution of 4-NP (30 mL) was combined with 30 mg of GO–CeO₂ catalyst and stirred in the dark for 20 min to establish adsorption–desorption equilibrium. Following spectral baseline acquisition, 5 mL of freshly prepared 15 mM ice-cold NaBH₄ was introduced to initiate the reduction reaction. A distinct color change from pale yellow to greenish-yellow indicated the formation of the intermediate 4-nitrophenolate ion. The reaction was then exposed to simulated solar irradiation, and the degradation process was monitored via UV–Vis spectroscopy using a Shimadzu UV-1800 spectrophotometer(Kyoto, Japan) over the 200–800 nm range. A strong absorbance peak at 402 nm, characteristic of 4-nitrophenolate ions, gradually diminished over 110 min, Figure 8a, confirming progressive reduction. The degradation efficiency was quantified using Equation (8):

Reduction (%) = 100 × (1 − A_t/A₀)

(8)

The GO–CeO₂ nanocomposite achieved a reduction efficiency of 85.6% within 110 min, Figure 8b. For kinetic evaluation, the reaction followed a pseudo-first-order model described by Equation (4). The linear fit of ln (A_t/A₀) versus time yielded a rate constant k_app = 2.243 × 10⁻⁴ s⁻¹, as shown in the inset of Figure 8b. This result corroborates the high catalytic activity of the nanocomposite, which significantly exceeds that of many previously reported CeO₂-based catalysts (see

Table 3. Catalytic performance of the GO-CeO₂ nanocatalyst for the degradation of 4-NP in comparison with the previously reported catalysts.

Catalyst	Time (min)	k (min−1)	Amount of Catalyst (mg)	Literature
GO-CeO2	120	0.949	5	[23]
Cu/RGO/Fe3O4	3.15	1.02	5	[78]
CuO–rGO	18	0.1291	1	[79]
Cd/Ce/RGO	1.0	4.9	120	[80]
CeO2-TiO2	80	0.42	30	[81]
Au/Graphene	12	0.003	13	[82]
C, N-TiO2	420	.005	10	[83]
G/TiO2	160	0.021	10	[84]
Fe3O4/C/Ag	26	0.044	20	[85]
CeO2 microspheres	120	---	40	[86]
GO-CeO2	110	0.224	5	This work

) [23,78,79,80,81,82,83,84,85,86]. Comparative data demonstrate that while systems such as CuO–rGO [79] and Au/graphene [82] require lower reaction times or catalyst amounts, the GO–CeO₂ structure delivers a balanced combination of efficiency, reaction speed, and environmental compatibility.

Mechanistically, the reaction proceeds via the photo-assisted reduction of 4-NP to 4-aminophenol (4-AP) in the presence of NaBH₄ and GO–CeO₂. Upon dissolution, NaBH₄ ionizes to produce BH₄⁻ ions, which act as surface hydrogen donors [87,88,89]. The GO–CeO₂ surface mediates electron transfer from BH₄⁻ to the nitro group of 4-NP, enabling its conversion to an amino group, as outlined in Equations (9) and (10):

P–NO₂ + 6H⁺ + 6e⁻→ P–NH₂ + 2H₂O

(9)

BH₄⁻ + 2H₂O → BO₂⁻ + 4H₂

(10)

The reaction pathway illustrated in Figure 8c confirms that the catalyst surface plays an essential role in facilitating electron transport and hydrogen transfer. The superior performance of GO–CeO₂ can be attributed to its structural features discussed earlier: high surface area, defect-rich graphene matrix, and uniform CeO₂ nanoparticle dispersion with abundant oxygen vacancies—all contributing to enhanced reduction kinetics and molecular interaction.

2.5. Reusability and Stability of the GO–CeO₂ Photocatalyst

In heterogeneous photocatalysis, catalyst reusability is a critical factor that directly impacts both environmental sustainability and operational cost-effectiveness. Many nanoparticle-based catalysts demonstrate strong initial activity but suffer from rapid performance loss in subsequent cycles due to agglomeration, surface deactivation, or leaching. The challenge is further compounded by difficulties in catalyst recovery, especially with colloidal systems that remain dispersed in the reaction medium. Therefore, developing structurally stable and easily recoverable catalysts with consistent activity across multiple cycles is essential for practical applications.

To assess the recyclability of the GO–CeO₂ nanocomposite, a series of reuse experiments was conducted targeting three model azo dyes-MR, MO, and CR-chosen for their structural compatibility and known susceptibility to oxidative degradation. For each dye system, the composite was tested over four consecutive degradation cycles. Between each run, the catalyst was carefully rinsed three times with deionized water to remove residual adsorbates and reaction intermediates, then reused under identical conditions. The degradation performance over repeated cycles is illustrated in Figure 9a–c. For MR, the initial degradation time of 140 min showed only minor increases in subsequent runs, reaching 160 min by the fourth cycle. In the case of MO, complete degradation extended from 150 min in the first run to 178 min in the fourth cycle. Similarly, CR degradation times increased modestly from 120 min initially to 138 min after four cycles. These gradual shifts in reaction time are attributed to partial loss of catalyst mass during washing and possible surface fouling by intermediate products, which may transiently block active sites.

After each cycle, the catalyst was recovered by centrifugation, thoroughly washed with ethanol and deionized water, and dried for reuse. The total mass loss after four cycles was approximately 3%, indicating excellent recovery efficiency and minimal leaching of active components. Although post-catalytic XRD or FTIR characterization was not performed in this study, such analyses will be included in future work to provide direct evidence of structural stability.

Despite these effects, the GO–CeO₂ nanocomposite retained substantial photocatalytic activity across all test systems. This robust recyclability is closely linked to its microstructural advantages observed in earlier characterizations. TEM analysis confirmed the uniform and stable dispersion of CeO₂ nanoparticles across GO sheets, which helps maintain surface accessibility and prevents aggregation during repeated use. The presence of oxygenated functional groups on GO facilitates strong anchoring of CeO₂ particles, while the high surface area (12.33 m²/g) and elevated defect density contribute to consistent radical generation throughout the cycles. Furthermore, XRD analysis revealed enhanced crystallite stability, and the reduction in domain size upon compositing—alongside increased microstrain and dislocation density—supports resilient electron–hole separation and minimal recombination losses over time. The synergistic interplay between the GO scaffold and CeO₂ nanoparticles ensures sustained light absorption, pollutant adsorption, and catalytic turnover during repeated operations. In summary, the GO–CeO₂ nanocomposite demonstrates excellent recyclability and long-term stability, making it a promising candidate for scalable and repeatable wastewater remediation applications.

2.6. Mineralization Assessment of MO Dye

To evaluate the extent of complete organic degradation beyond mere decolorization, total organic carbon (TOC) analysis was conducted on MO dye solutions treated with GO–CeO₂ and CeO₂ photocatalysts, Figure 9d. TOC measurement provides a quantitative estimate of carbon-containing organic residues, offering a direct indication of the degree of mineralization achieved during the photodegradation process [90]. The initial TOC concentration, determined after 30 min of dark-phase adsorption equilibrium, was found to be 18.06 mg·L⁻¹. Following 150 min of sunlight irradiation, TOC levels decreased to 3.792 mg·L⁻¹ in the presence of GO–CeO₂ and to 9.927 mg·L⁻¹ with CeO₂ alone. The mineralization efficiency was calculated using Equation (11):

R(%) = 100 × (TOC_0e−TOC_t)/TOC_0e

(11)

where TOC_0e is the value of total organic carbon at the time of sorption equilibrium, and TOC_t is the total organic carbon value at analysis time (t) after irradiation.

Based on this equation, GO–CeO₂ achieved a mineralization efficiency of 79.00%, substantially higher than the 45.03% obtained with CeO₂. This significant improvement reflects the composite’s enhanced ability to transform complex dye molecules into inorganic end products such as CO₂ and H₂O. The superior mineralization performance of GO–CeO₂ can be attributed to the optimized structural and functional properties revealed in previous characterizations. The uniformly distributed CeO₂ nanoparticles on the GO surface, confirmed by TEM and XRD analyses, provide abundant reactive sites and facilitate the formation of reactive oxygen species (ROS), including hydroxyl and superoxide radicals. The increased surface area, higher defect density, and presence of oxygenated functional groups on GO promote effective pollutant adsorption and interfacial charge transfer. Moreover, GO’s contribution to electron mobility and stabilization of CeO₂ nanoparticles mitigates recombination of photoinduced carriers, prolonging radical activity and enhancing oxidative degradation. The high degree of mineralization observed here demonstrates not only efficient photocatalytic performance but also the potential of GO–CeO₂ as a sustainable material for advanced water treatment applications.

2.7. Heterogeneous Catalytic Behavior of GO–CeO₂ Nanocomposite

To determine whether the degradation of MR by the GO–CeO₂ nanocomposite proceeds via a homogeneous or heterogeneous mechanism, two established analytical techniques were employed: the hot filtration test and the mercury poisoning assay. Both assessments confirmed the heterogeneous nature of the catalytic process and the surface-based activity of the GO–CeO₂ material under irradiation.

2.7.1. Hot Filtration Test

The hot filtration test was carried out under standard photocatalytic conditions until approximately 30% of the MR dye had been degraded. At this point, the catalyst was separated from the reaction mixture via centrifugation, and the supernatant was continuously irradiated for an additional 24 h. UV–Vis spectral analysis revealed no further reduction in MR concentration following catalyst removal, indicating that active species were not leached into solution and that dye degradation was strictly surface-mediated. This result confirms the catalytic behavior of GO–CeO₂ as heterogeneous in nature, with reaction confined to the solid–liquid interface [91].

2.7.2. Mercury Poisoning Test

To further corroborate catalyst heterogeneity, the mercury poisoning test was applied. Elemental mercury (Hg⁰) is known to form amalgams with catalytically active metal surfaces, effectively inhibiting their function. Upon addition of a small quantity of Hg⁰ to the reaction mixture under constant stirring, immediate cessation of the photocatalytic activity was observed. A subsequent UV–Vis scan confirmed the absence of additional MR degradation, implying that catalyst deactivation occurred due to surface amalgamation rather than solution-phase interactions. This outcome reinforces that the GO–CeO₂ nanocomposite functions as a truly heterogeneous catalyst in the photocatalytic process [92]. The GO–CeO₂ system’s heterogeneity is consistent with its morphological and structural features. TEM analysis showed strong anchoring of CeO₂ nanoparticles on GO sheets, ensuring that catalytic sites remained surface-confined and were not prone to leaching. The composite’s layered structure and extensive surface functionalization, validated through FTIR and XRD, support the retention of active sites and robust interfacial interactions during repeated catalytic cycles.

Nevertheless, the mercury poisoning test should be interpreted cautiously. Mercury can alter surface properties, block active sites nonspecifically, and affect optical absorption by reflecting incident light. Additionally, changes in temperature or pH upon mercury addition may influence photocatalytic activity independently of electron trapping. Therefore, this test provides only qualitative evidence of surface photoactivity. In future work, more definitive mechanistic approaches such as radical scavenger experiments and electron paramagnetic resonance (EPR) analysis will be employed to validate charge-transfer processes and identify active species.

3. Experimental Details

3.1. Materials

All chemicals were used as received without further purification. Cerium nitrate (Ce(NO₃)₃), MO, MR, CR, and analytical-grade reagents including sulfuric acid (H₂SO₄), phosphoric acid (H₃PO₄), hydrochloric acid (HCl), and hydrogen peroxide (H₂O₂) were procured from Daejung Chemicals, Siheung-si, Republic of Korea. Lead pencils (graphite source) were purchased from local suppliers (Kohat, Pakistan). Deionized (DI) water was used throughout for solution preparation.

3.2. Synthesis of Graphene Oxide (GO)

Graphene oxide was synthesized using a modified Hummers/Tour method starting from ground graphite powder [18]. Briefly, 1.0 g of graphite was dispersed in a mixture of concentrated sulfuric acid (160 mL) and phosphoric acid (17.5 mL) in a 9:1 ratio (v/v) and stirred for 24 h at 35 °C to allow acid intercalation. After oxidation, the mixture was diluted with 140 mL of deionized water, followed by the slow addition of 2 mL of 30% hydrogen peroxide under continuous stirring. The appearance of effervescence and a brown coloration indicated oxidative conversion of graphite to graphite oxide/GO. The suspension was centrifuged (4000 rpm, 10 min) and washed repeatedly with deionized water until the supernatant reached near-neutral pH. The product was dried at room temperature. All reactions were performed in triplicate.

The oxidation process can be represented by a simplified, non-stoichiometric equation (indicative only):

C(graphite) + αH₂SO₄ + βH₃PO₄+ γH₂O₂+ δH₂O ⟶ graphite oxide (C_xO_yH_z) + PO₄³⁻+ SO₄²⁻+ O₂ + H₂O

(12)

This equation is illustrative rather than fully balanced because the composition of GO (C_xO_γH_z) varies with the degree of oxidation. The reaction involves oxidative cleavage of C–C bonds and introduction of oxygen-containing functional groups (hydroxyl, epoxy, and carboxyl) on the graphite layers, which imparts hydrophilicity and facilitates exfoliation into GO sheets.

3.3. Preparation of GO–CeO₂ Nanocomposite

The composite was prepared via a hydrothermal process, enabling the direct growth of CeO₂ nanoparticles on graphene oxide sheets under controlled temperature and pressure. This method promotes uniform dispersion, strong interfacial contact, and defect formation, which are essential for catalytic enhancement. Unlike physical mixing, hydrothermal synthesis yields a structurally integrated material with improved functional properties.

The GO–CeO₂ hybrid material was synthesized using a combined ultrasonic and hydrothermal approach. Initially, 50 mg of GO was dispersed in 60 mL of deionized water and subjected to ultrasonic treatment for 30 min to ensure uniform dispersion. In parallel, 100 mg of cerium nitrate was dissolved in an equal volume of deionized water and sonicated under identical conditions. The two solutions were then slowly merged under continuous ultrasonication, and the pH was carefully adjusted to 3 using diluted hydrochloric acid to stabilize cerium ions in solution. Ultrasonication was continued for an additional 30 min to facilitate interaction between GO sheets and cerium species. To initiate the oxidation of Ce³⁺ to Ce⁴⁺, hydrogen peroxide (H₂O₂) was introduced into the mixture. Subsequently, the pH was raised to 9 by adding 0.5 wt% sodium hydroxide, promoting ion exchange between hydroxide and nitrate species and aiding precursor transformation.

The resulting suspension was transferred into a Teflon-lined stainless steel autoclave and subjected to hydrothermal treatment at 180 °C for 2 h to induce nucleation and growth of CeO₂ nanoparticles on the GO surface. Formation of the nanocomposite was verified by UV–Vis spectroscopy, which revealed characteristic absorbance features. The final product was recovered by centrifugation at 4000 rpm, yielding a solid pellet that was washed, dried, and analyzed using comprehensive characterization techniques. To assess reproducibility, the synthesis procedure was performed in triplicate under identical conditions.

3.4. Catalytic Photodegradation of Azo Dyes

Photocatalytic activity was investigated using MO, MR, and CR as model dyes (anionic dyes). All photocatalytic experiments were conducted under natural sunlight irradiation between 11:30 am and 2:00 pm on 14 June 2022 at the Chemistry Department, Kohat University of Science and Technology, Pakistan. This time window was selected to ensure consistent solar intensity and minimize variability in spectral exposure. In each trial, 30 mg of GO–CeO₂ nanocatalyst was dispersed into 100 mL of 3 mM dye solution and stirred under solar irradiation, i.e., photocatalytic tests were performed under natural sunlight at ambient conditions (25 ± 2 °C). Then, the nanoparticles were entirely removed from the solution using centrifugation. At set intervals, 3 mL aliquots were extracted, and UV-visible spectra were recorded to monitor the degradation of the dye. The photocatalytic measurements were conducted in triplicate for the representative sample, and the reported data represent the average values. Control experiments were performed under identical conditions without a catalyst to account for potential self-degradation. All degradation experiments were conducted using dye solutions at their natural pH, typically around 5–6, without external adjustment. This approach ensured consistent conditions across all tests, allowing for a direct comparison of the intrinsic photocatalytic performance of the CeO₂–GO composites.

3.5. Catalytic Reduction of 4-Nitrophenol

Catalytic activity was further examined via the reduction of 4-NPto 4-aminophenol. A 365 nm UV light photoreactor was used for irradiation. A solution of 10 mL of 1 mM 4-NP was mixed with 10 mg of GO–CeO₂ nanocomposite, along with sodium borohydride as the reducing agent. Progress of the reduction reaction was tracked using UV-visible spectroscopy. Triplicate measurements were obtained for each measurement, and the corresponding average values are reported.

3.6. Samples Characterization

The optical properties of the synthesized GO and GO–CeO₂ nanocomposite were examined using a Shimadzu UV-1800 spectrophotometer. To identify functional groups and verify surface chemistry, FTIR was conducted in ATR mode over the spectral window of 4000–400 cm⁻¹, utilizing a Thermo Scientific Nicolet iS50 system (Waltham, MA, USA). Morphological assessment was performed using a high-resolution transmission electron microscope (JEM-2100, JEOL, Tokyo, Japan), which provided detailed images of particle distribution and nanostructure. Crystallinity and phase composition were evaluated using XRD with a JDX-3532 diffractometer (JEOL, Tokyo, Japan), equipped with a CuKα radiation source (λ = 1.5418 Å). Measurements were collected across a 2θ range of 10° to 80°, applying a step interval of 0.02° and a dwell time of 0.5 s per step.

4. Conclusions

In this work, GO was successfully derived from graphite via a modified Tour method and incorporated with CeO₂ nanoparticles through hydrothermal synthesis to construct a GO–CeO₂ nanocomposite. Characterization techniques—UV/Vis spectroscopy, XRD, FTIR, and TEM—confirmed the formation of a structurally uniform and defect-rich hybrid material. The integration of GO with CeO₂ notably improved the dispersion of active sites and promoted efficient charge carrier separation, resulting in enhanced photocatalytic properties. The nanocomposite demonstrated outstanding performance in degrading a range of organic pollutants. It achieved 98% removal of methyl red within 140 min, 96% of CR in 120 min, and 94% of methyl orange in 150 min—significantly surpassing the efficacy of CeO₂ alone. Kinetic modeling revealed accelerated degradation rates, while TOC analysis confirmed a high degree of mineralization, with GO–CeO₂ eliminating 79% of organic carbon from methyl orange compared to just 45% with pure CeO₂. These results affirm the composite’s ability to completely oxidize dye molecules into environmentally benign end products. Further investigations revealed the catalyst’s durability, with stable activity sustained across four reuse cycles. Hot filtration and mercury drop tests substantiated the heterogeneous nature of the reaction, confirming that dye degradation occurred solely at the catalyst surface without contribution from dissolved species. Overall, the GO–CeO₂ nanocomposite exhibits considerable potential as a practical and efficient photocatalyst for wastewater treatment. Its synergistic structural design, cost-effective preparation, and recyclability offer a promising pathway for scalable environmental remediation. Looking ahead, future work will focus on optimizing photocatalytic performance under varying pH and temperature conditions, applying advanced spectroscopic tools to trace degradation pathways, and conducting post-reaction characterizations to assess structural integrity. Mechanistic validation using radical scavenger and EPR techniques will be pursued, alongside exploring the composite’s applicability to emerging contaminants and its integration into immobilized formats for continuous-flow water treatment systems.