Graphitic Carbon Nitride/CeO₂ Nanocomposite for Photocatalytic Degradation of Methyl Red

Abstract

Nanosized ceria (CeO₂) and a graphitic carbon nitride-loaded ceria (CeO₂/GCN) nanocomposite were synthesized using a straightforward and efficient method and characterized by XRD, FTIR, SEM, TEM, TGA, and BET analyses. These techniques confirmed that CeO₂ was effectively supported on the surface of GCN, with particle sizes of the CeO₂/GCN composite in the range of 10–15 nm and a pore size of 3.33 nm. The photocatalytic activity of the CeO₂/GCN nanocomposite and CeO₂ NPs in the degradation of methyl red dye under sunlight radiation was studied using UV–visible spectroscopy. A noticeable red shift in the CeO₂/GCN nanocomposite compared to pure CeO₂ NPs suggests a reduction in its band gap energy, calculated at 3.90 eV for CeO₂ NPs and 2.97 eV for the CeO₂/GCN nanocomposite. This band gap reduction enhances the photocatalytic degradation process, achieving a removal efficiency of 99.92% within a short irradiation time of 40 min for the CeO₂/GCN nanocomposite, compared to 69.47% for CeO₂ NPs. These findings indicate that graphitic carbon nitride significantly enhances the photocatalytic properties of CeO₂ NPs.

1. Introduction

Azo dyes exhibit at least one R–N = N–R′ functional group, which is also the chromophore group responsible for the coloring of the dye [1]. They generally absorb in the visible range (350–650 nm). They are widely used in food, textile, cosmetics, inks, and pharmaceutical industries [2].

All azo dyes are generally water-soluble and can be either natural or synthetic. Approximately 10–15% of the dye remains in the solution and generates colored wastewater which may be released into water bodies without proper treatment [3].

Thus, industrial effluents discharged with azo dye residuals become a threat to human health and aquatic life, as they are toxic to fish, algae, aquatic plants, and other organisms in the water.

Moreover, these dyes break down during use and release highly toxic and carcinogenic chemicals known as aromatic amines [4]. Benzidine and its derivatives are major azo reduction products of the azo dyes, and these are potentially highly toxic and carcinogenic [5]. Therefore, treating wastewater containing dye is necessary.

Several studies have focused on physical, chemical, and biological treatments [6]. Physical methods such as flocculation, membrane filtration, and adsorption are non-destructive and commonly transfer the pollutants to other systems, causing secondary pollution [7]. Chemical methods are high-cost due to the production of large amounts of by-products which require proper disposal or further management [8]. On the other hand, biological treatment is ineffective due to azo dyes’ resistance to aerobic degradation. Additionally, anaerobic degradation may generate carcinogenic aromatic amines [9,10].

Methyl red (CI Acid Red 2) is a well-known dye extensively used in textile dyeing and paper printing [11]. It causes eye and skin irritation, headache, dizziness, and irritation of the respiratory and digestive tract if inhaled or swallowed. It is a suspected carcinogen and mutagen [12].

Recently, using photocatalysis to degrade azo dyes has been recommended due to its non-toxic, insoluble, low-cost, and highly efficient properties [13].

Ceria (CeO₂), one of the reactive rare earth oxides, can change its oxidation state between trivalent (+3) and tetravalent (+4). This ability allows it to store and release oxygen. The +4 oxidation state is more stable than the +3 oxidation state due to CeO₂’s electronic structure. Because of this valence state shift, CeO₂ is an important material with a wide range of applications, including semiconductors [14], catalysts, vehicle gas engines (as three-way catalytic converters) [15], adsorbents [16], water−gas shift reactions [17], electrolytes [18], fuel cells [19], and exhaust purification [20].

Due to its extensive redox chemistry, versatile acid and base properties, and high oxygen storage capacity [21], CeO₂ has also emerged as an excellent photocatalyst with a narrow band gap (Eg = 2.92–3.2 eV) [22] that absorbs light in the near UV and slightly in the visible light region.

Furthermore, the crystal planes of CeO₂ play a crucial role in photocatalysis [23]. It has been reported that more oxygen vacancies and hydroxyl groups are beneficial on the CeO₂ {100} plane compared to the {111} plane [24]. Additionally, the migration abilities of electrons and holes on the {100} and {111} facets differ. Holes are encouraged to move toward the CeO₂ {100} plane, while electrons transfer toward the {111} plane, which results in increased photocatalytic activity [25].

To increase the photoactivity of CeO₂ and decrease its band gap, it is beneficial to extend its light absorption into the visible light range, which represents ~45% of solar light [26]. To achieve visible light-driven catalysis, various strategies have been studied, such as doping or decorating CeO₂ with metals and forming heterojunctions between CeO₂ and other semiconductors, including TiO₂ [27], CuO [28], Mn₃O₄ [29], and Bi₂WO₆ [30]. Additionally, several CeO₂-based materials (e.g., CeO₂/graphene [31] and CeO₂/g-C₃N₄ [32]) have been explored for their applications in the photodegradation of organic contaminants. Among these, CeO₂/graphitic carbon nitride (g-C₃N₄) has attracted significant attention [33].

Graphitic carbon nitride (GCN) is a metal-free photocatalyst known for its good chemical and thermal stability under ambient conditions; it is also considered an active photocatalyst due to its ease of synthesis, low cost, eco-friendliness, and well-maintained band gap energy in the visible region, which is attributed to the strong covalent bond between the nitrogen and carbon [34]. Furthermore, the high surface area of GCN is due to the 2D stacking of π conjugated planes, similar to graphite [35].

The developed photocatalysts were used for wastewater treatment, highlighting the unique aspects of using coupled CeO₂/GCN composites for the photocatalytic degradation of organic dye pollutants under visible light, a relatively rare and unexplored approach in the current literature. Based on the concepts and ideas mentioned above, this study synthesized cubic CeO₂ nanoparticles and investigated their potential as a photocatalyst for the degradation of MR dye in aqueous solutions under visible light. Furthermore, cerium oxide nanoparticles were modified using an innovative strategy with graphitic carbon nitride to enhance their photocatalytic performance, which represents a significant contribution to advancing photocatalytic technology for wastewater treatment.

The dye pollutants’ degradation is investigated under different time periods. The photocatalytic activity of the prepared cerium oxide nanoparticles decorated with graphitic carbon nitride was explored and compared with that of untreated cerium oxide nanoparticles.

2. Materials and Methods

2.1. Materials

Cerium sulfate (Ce₂(SO₄)₂), oxalic acid (H₂C₂O₄), and melamine (C₃H₆N₆) were obtained from Sigma-Aldrich Corp. (St. Louis, MO, USA), while a 25% ammonia solution (NH₄OH) was purchased from Oxford Lab Fine Chem LLP (Navghar, India). Methyl red dye was obtained from LOBA Chemie (Mumbai, India), and ultrapure double-distilled water was used for all solution preparations. All the plots presented were generated using OriginPro 2025 software, www.OriginLab.com, (28 November 2024).

2.2. Synthesis of Cerium Oxide Nanoparticles

The cerium oxide nanoparticles (CeO₂ NPs) were formed by the coprecipitation of cerium sulfate and oxalic acid [36]. In a typical synthesis, approximately 1 g ± 0.001 g of cerium sulfate and 1 g ± 0.001 g of oxalic acid were dispersed in 100 mL of ultrapure water inside an opaque and wrapped Erlenmeyer flask (secured from light) and stirred vigorously at room temperature for 5 h to obtain a clear solution. The next day, the solution mixture (approximately 100 mL) was added dropwise to 10 mL of 25% ammonia solution at a rate of 830 µL per minute and stirred for an additional 2 h at room temperature. The resulting white-colored precipitate was separated by simple filtration, washed with ultrapure water 3–4 times (using about 200 mL of ultrapure water), and dried at 70 °C for 24 h to obtain a fine powder of CeO₂ NPs.

2.3. Synthesis of Graphitic Carbon Nitride

For the synthesis of graphitic carbon nitride (GCN), a thermal polycondensation process was utilized. Approximately 5 g ± 0.001 g of white powder melamine was placed in a crucible with a cover and heated in a muffle furnace at a temperature of 550 °C (ramp rate 5 °C/min) for 4 h. After the heating period, cooling to room temperature resulted in yellow-colored powder, confirming the formation of GCN. This was stored in a brown bottle for further use.

2.4. Synthesis of Graphitic Carbon Nitride-Loaded CeO₂ Nanocomposite

The typical formation of the graphitic carbon nitride-loaded CeO₂ nanocomposite (CeO₂/GCN) proceeded as follows: A suitable amount of CeO₂ was mixed with a corresponding amount of GCN in a ratio of 2 parts CeO₂ to 1 part C₃N₄ (weight/weight) and ground using a mortar and pestle for 25 min. The resulting mixed powder was transferred into a crucible with a cover and heated at 400 °C in a muffle furnace for 4 h with a heating rate of 30 °C/min. The obtained CeO₂/GCN nanocomposites were excellent talc-like and pale yellow in color. After cooling to room temperature, they were stored for further use.

2.5. Graphitic Carbon Nitride-Loaded CeO₂ Nanocomposite Characterizations

The chemical, physical, and morphological properties of the synthesized CeO₂/GCN nanocomposite were studied using various techniques. Functional group analysis was carried out using a single-beam Fourier transform infrared (FTIR) spectrometer (Bruker Vertex 80 and Hyperion 2000 microscope, Germany) with the KBr pellet method, covering the wavelength range of 4000 to 500 cm⁻¹.

The crystallinity and crystal structure of the CeO₂/GCN nanocomposite were analyzed using X-ray diffraction (XRD) with Cukα radiation (λ = 1.540 Å) in the 2θ range of 10–80°, performed on a Rigaku Ultima IV X-ray diffractometer.

The thermal stability of the CeO₂/GCN nanocomposite was investigated by thermogravimetric analysis (TGA) using a Netzsch TG 209 F1 Iris (Germany) under a dry nitrogen atmosphere.

The BET surface area and pore parameters of the CeO₂/GCN nanocomposite were determined from low-pressure gas adsorption isotherm measurements, performed using a Micromeritics Gemini VII sorption system equipped with a liquid nitrogen bath at 77 K.

The structure of the samples was explored using Transmission Electron Microscopy (TEM) analysis, conducted with a Morgagni 268 (FEI, Holland) under a 60 kV accelerating voltage.

The morphology and elemental distribution on the surface of the CeO₂/GCN nanocomposite were captured by scanning electron microscopy (SEM) using the Bruker Vertex 80 and Hyperion 2000 microscope, operating at an accelerating voltage of 10 kV.

Optical absorption studies were performed using a UV-Vis spectrophotometer (Shimadzu-1800), measuring in the wavelength range of 200–900 nm at room temperature. Measurements were conducted using a quartz cell with distilled water as a blank solution.

2.6. Photocatalytic Degradation of Methyl Red Dye

The photocatalytic activity of the CeO₂/GCN nanocomposite was evaluated using a UV/Vis spectrophotometer to monitor the degradation of methyl red (MR) dye solution under sunlight exposure. The experiment was conducted as follows: Eight samples, each containing 10 mL of 25 ppm MR solution, were prepared and adjusted to pH 7 using 0.1 M HCl/NaOH. At room temperature, 15.0± 0.1 mg of the CeO₂/GCN nanocomposite was added to each sample. The suspensions were stirred in the dark for at least 30 min to establish an equilibrium between adsorption and desorption. Afterward, while continuously stirring, the samples were exposed to sunlight. At intervals of 0, 10, 15, 30, 60, 90, 120, and 180 min, aliquots were taken, filtered through 0.2 µm syringe filters, and stored in brown bottles. The absorbance of each sample at 425 nm was immediately measured using a UV-Vis spectrophotometer to determine the rate of dye degradation.

To achieve reliable results with good reproducibility and accuracy, each measurement was run three times, and then the average was considered.

The % degradation was determined using the following formula:

$$\% \mathrm{o}\mathrm{f} \mathrm{d}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=\left[{\displaystyle \frac{\mathrm{C}\mathrm{i}-\mathrm{C}\mathrm{t}}{\mathrm{C}\mathrm{t}}}\right]\times 100$$

(1)

where $\mathrm{C}\mathrm{i}$ and $\mathrm{C}\mathrm{t}$ (mg/L) are the initial concentration and the concentration of MR at time t, respectively.

3. Results and Discussion

3.1. Characterization of CeO₂/GCN Nanocomposite

3.1.1. X-Ray Diffraction (XRD) Analysis

The X-ray diffraction (XRD) pattern in Figure 1 reveals key features of the CeO₂/GCN nanocomposite. The distinct diffraction peaks at 13.1° and 27.4° correspond to the (100) and (002) planes of pure GCN, respectively, which align well with the JCPDS reference card No (087-1526). The sharp and narrow diffraction peaks of CeO₂ NPs indicate good crystallinity, and these peaks are associated with the cubic crystal phase (JCPDS 034-0394). The most distinct peak is at 28.5°, corresponding to the (111) plane of CeO₂, which typically appears in the same region. Moreover, the absence of additional peaks suggests that no impurities are present, confirming the purity of the CeO₂ NPs. Furthermore, the CeO₂/GCN nanocomposite exhibits peaks at identical positions to those of pure CeO₂ NPs, with a minor increase in peak intensities. This indicates the preservation of the cubic structure of CeO₂ NPs within the CeO₂/GCN nanocomposite. These findings strongly confirm the successful formation of the CeO₂/GCN nanocomposite, characterized by the cubic phase of CeO₂ and the crystalline phase of GCN.

3.1.2. Fourier Transform Infrared (FTIR) Spectrometer

Figure 2 presents the FTIR spectrum of CeO₂ NPs and the CeO₂/GCN nanocomposite. The peaks observed in the range of 700–880 cm⁻¹ are attributed to the stretching vibrations of the Ce–O bond [37]. The bands at 800 cm⁻¹ and 880 cm⁻¹ correspond to the out-of-plane bending vibrations of the C–N heterocycle. Additionally, the bands in the 1130–1700 cm⁻¹ range are associated with the C–N stretching vibrations within the aromatic ring, while the peak around 1635 cm⁻¹ is linked to another C–N stretching vibration. A broad absorption band located around 3300 cm⁻¹ corresponds to O–H stretching vibrations from residual water and hydroxyl groups [38].

In the CeO₂/GCN nanocomposite spectrum, the peaks in the 1130–1700 cm⁻¹ region become more intense compared to the same region in the CeO₂ NP spectrum. Conversely, the intensity of CeO₂ NP-related bands is reduced in the CeO₂/GCN nanocomposite, likely due to the influence of GCN. This FTIR analysis confirms the successful synthesis of the CeO₂/GCN nanocomposite.

3.1.3. Scanning Electron Microscope (SEM)

The surface morphology and chemical composition of the CeO₂/GCN nanocomposite are shown in Figure 3. SEM images of the CeO₂/GCN nanocomposite were captured at different magnifications (10,000×, 40,000×, and 80,000×). The images of the CeO₂/GCN nanocomposite reveal a nanosheet-stacked structure with good crystallinity. Its surface appears smooth, with a thickness of just a few nanometers. The SEM images of the CeO₂/GCN nanocomposite clearly indicate that the ceria grains are randomly distributed across the GCN sheets. These SEM images are well in accordance with the XRD results shown in Figure 1.

Relatively weak van der Waals forces dominate the interlayers of GCN [39]. However, the interaction between the CeO₂ nanoparticles and the GCN support overcomes the van der Waals forces between the carbon–carbon layers. As a result, the van der Waals interaction force between the layers is significantly weakened. This alteration in the CeO₂-GCN nanocomposite surface is beneficial for separating photogenerated electrons and holes, providing a shorter electron–hole (e⁻/h⁺) diffusion and transfer distance compared to bulk material.

3.1.4. Transmission Electron Microscopy (TEM)

The CeO₂/GCN nanocomposite was also examined via TEM analysis, as shown in Figure 4, which supports the SEM and XRD analysis.

The images reveal that the CeO₂/GCN nanocomposite exhibits a uniform, well-defined, and pure structure, with a high degree of crystallinity and small particle sizes averaging between 10 and 15 nm. They also confirm that the CeO₂ is uniformly distributed and interacts with the GCN, leading to the formation of a stable composite material.

3.1.5. Thermogravimetric Analysis (TGA)

To investigate the thermal stability of the CeO₂/GCN nanocomposite, thermogravimetric analysis was performed from room temperature to 950 °C at a heating rate of 10 °C/min under air conditions. As shown in Figure 5, both CeO₂ NPs and the CeO₂/GCN nanocomposite demonstrate excellent thermal stability, with minimal weight loss (3.5% for CeO₂ NPs and 4.51% for the CeO₂/GCN nanocomposite) observed between 110 °C and 530 °C. This stability is attributed to the robust cubic structure of ceria. In the temperature range of 350–900 °C, ceria shows very high thermal stability, stabilizing at approximately 7.5%. These weight losses are due to the removal of adsorbed water during the initial phase, Subsequently, CeO₂ NPs can undergo oxidation–reduction processes, especially in the presence of oxygen. A mass loss may also be associated with the re-oxidation of Ce³⁺ to Ce⁴⁺. This indicates that CeO₂ NPs are thermally stable at higher temperatures, typically above 500 °C. A sharp weight reduction of around 530 °C in the CeO₂/GCN nanocomposite indicates the combustion of GCN, which occurs at a lower temperature compared to pure GCN. This behavior is due to the catalytic role of CeO₂, which facilitates GCN oxidation by absorbing and supporting reactive oxygen from the air [40].

At temperatures above 670 °C, the CeO₂/GCN nanocomposite exhibits a small weight loss, stabilizing at approximately 29.18%. This indicates that the CeO₂/GCN nanocomposite shows a greater weight loss compared to pure CeO₂ NPs, due to the presence of GCN. At the same time, it maintains high-temperature structural stability with no phase transitions observed.

Based on the combined findings from XRD, FTIR, SEM, TEM, and TGA analyses, it can be concluded that the CeO₂/GCN nanocomposite was successfully synthesized.

3.1.6. BET Surface Area Analysis

The BET surface area of the CeO₂/GCN nanocomposite was found to be 68.50 m²g⁻¹ (

Table 1. BET surface area, total pore volume, and particle size of CeO₂/GCN nanocomposite.

BET Surface Area (m2g−1)	Langmuir Surface Area (m2g−1)	Single Point Surface Area at P/P° (m2g−1)	Total Pore Volume (cm3g−1)	Pore Size (nm)
68.50	105.49	67.99	0.057	3.33

). The average pore diameter is 3.33 nm, which indicates the presence of mesopores. This pore size, within the range of 2–50 nm, confirms that the CeO₂/GCN nanocomposite belongs to mesoporous materials [41], a characteristic that can be easily verified by TEM images. The surface area value further supports the significant photocatalytic activity of the CeO₂/GCN nanocomposite.

3.2. Photocatalytic Degradation Analysis

The photocatalytic degradation ability of the catalyst can be assessed by calculating the band gap energy values using the absorption spectra for both direct and indirect transitions. Tauc’s equation (Equation (2)) was used to obtain the optical band gap (Eg) of CeO₂ NPs and the CeO₂/GCN nanocomposite.

$$\left(\mathsf{\alpha }\mathrm{h}\mathsf{\nu }\right)=\mathrm{A}(\mathrm{h}\mathsf{\nu }-\mathrm{E}\mathrm{g}\mathrm{o}\mathrm{p}\mathrm{t}{)}^n$$

(2)

According to Lambert–Beer–Bouguer Law, the absorption coefficient (Equation (3)) is given by the following:

$$\mathsf{\alpha }=\left[{\displaystyle \frac{\mathrm{A}}{\mathrm{t}}}\right]\times 2.303$$

(3)

where (A) represents the absorbance and (t) is the width of the quartz cuvette (1 cm).

For the direct transition, Tauc plots were generated by plotting (αhν)² against (hν). Meanwhile, (αhν)^1∕2 was plotted against (hν) to obtain the indirect transition graph for each sample. Here, hν, h, ν, and α represent the photon energy, Planck’s constant, the photon frequency, and the absorption coefficient, respectively [42].

As is well known, photodegradation of the dye is achieved when UV light interacts with the photocatalyst. Photons having energy equal to or greater than the band gap of the photocatalyst excite the electrons from their valence band (VB) to the conduction band (CB) and produce positive holes (h+) in the VB.

In the case of this nanocomposite, the transfer of photoexcited electrons and holes between CeO₂ and GCN suppresses the recombination of photogenerated h+/e⁻ pairs. This makes the CeO₂/GCN nanocomposites more efficient photocatalysts than pure CeO₂ NPs.

As shown in Figure 6, both photocatalysts exhibit absorption spectra in the UV-A to visible range (320–450 nm). The photocatalysts display absorption bands at 320 nm and 420 nm for CeO₂ NPs and the CeO₂/GCN nanocomposite, respectively. This indicates a noticeable red shift in the CeO₂/GCN nanocomposite compared to pure CeO₂ NPs, suggesting a reduction in its band gap value [43]. This is further supported by the data in

Table 2. Direct and indirect optical band gaps and Urbach energies of CeO₂ NPs and CeO₂/GCN nanocomposite.

Sample	Property	Value (eV)	Standard Deviation (eV)	Standard Error (eV)
CeO2 NPs	Direct optical band	3.91	0.66	0.26
	Indirect optical band	1.61	0.47	0.19
	Urbach energy	4.09	0.44	0.18
CeO2/GCN Nanocomposite	Direct optical band	2.97	0.51	0.20
	Indirect optical band	0.12	0.58	0.24
	Urbach energy	0.68	0.89	0.36

, which shows the band gap energies for CeO₂ NPs and the CeO₂/GCN nanocomposite as 3.90 eV and 2.97 eV, respectively.

The x-axis intersection points of the extrapolated dashed lines in Figure 6b–d were determined based on the linear fit of the data points in the region where a clear linear trend is observed, in order to estimate the band gap energy. A decrease in the band gap enhances the photocatalytic degradation process. Furthermore, linear regression analysis was performed, and the results demonstrated a high regression coefficient (R² greater than 0.99), indicating low variability of the data points around the trend, with good accuracy, reliability, and reproducibility. In addition, the statistical analysis reported small values for both the standard deviation and the standard error of the predicted intercepts, indicating low variability and high precision in the predicted intercepts of the regression model.

During irradiation, the photogenerated excited electrons on the CB in GCN are quickly transferred to CB in CeO₂, react with O₂, and generate superoxide anion radicals (O₂•⁻), while photogenerated holes on VB in CeO₂ are transferred to the VB in GCN, react with H₂O, and produce hydroxyl radicals (OH•) according to the following reactions:

CeO₂/GCN + h𝜐 → CeO₂ (h⁺ + e⁻)/GCN (h⁺ + e⁻)

(4)

CeO₂ (h⁺ + e⁻)/GCN (h⁺ + e⁻) → CeO₂ (e⁻ + e⁻)/GCN (h⁺ + h⁺)

(5)

O₂ + e⁻ → O₂⁻

(6)

H₂O + h⁺ → h⁺ + OH⁻

(7)

OH⁻ + h⁺ → OH

(8)

MR (dye) + OH → CO₂ + H₂O⁺

(9)

MR (dye) + O₂⁻ → CO₂ + H₂O

(10)

Production of highly reactive oxide species, such as the superoxide anion radical (O₂•⁻) and hydroxyl radical (OH•), can allow them to participate effectively in the degradation of MR dye molecules into simple and small species such as H₂O and CO₂ [44,45].

3.2.1. Effect of Irradiation Time

The photodegradation reaction is influenced by irradiation time. The optical properties of both synthesized photocatalysts were investigated using UV–vis spectroscopy, and the results are presented in Figure 7. A sharp peak around 300 nm, corresponding to CeO₂ NPs, confirms the success of the preparation method and aligns well with the XRD and FTIR results. In addition, the spectra clearly show a distinct maximum absorption peak of MR in the visible range at around 420 nm in both the CeO₂ NPs and CeO₂/GCN nanocomposite spectra. The intensity of the MR peaks decreases over time, indicating that photocatalytic efficiency is directly proportional to the exposure duration.

Furthermore, the photocatalytic efficiency of the CeO₂/GCN nanocomposite is superior to that of the pure CeO₂ NPs, as shown in Figure 7a,b. For instance, the absorbance values at 420 nm of the remaining MR in solution after 120 min of irradiation are 0.140 for the CeO₂/GCN nanocomposite and 0.242 for CeO₂ NPs. Notably, the peak at 300 nm is completely absent in the CeO₂/GCN nanocomposite spectrum in Figure 7b. This may be due to the interaction between CeO₂ and GCN, which could alter the electronic structure and absorption characteristics. A similar phenomenon is observed in Figure 6a, where the peak at 300 nm is almost completely absent. This observation supports the consistency of the obtained results and reflects the current state of the art in this field.

The photocatalytic behavior of CeO₂ NPs and the CeO₂/GCN nanocomposite was compared under a fixed mass, as shown in Figure 7c,d. The CeO₂/GCN nanocomposite significantly enhanced the photocatalytic efficiency for MR dye removal, achieving a maximum removal efficiency of approximately 99.92% within a short irradiation time of 40 min, at which point it reached a plateau.

In contrast, under the same conditions, CeO₂ NPs showed a lower removal efficiency, reaching around 69.47%. These findings suggest that the CeO₂/GCN nanocomposite exhibits a superior degradation capability for MR dye compared to CeO₂ NPs alone. Thus, the CeO₂/GCN nanocomposite is more effective for the removal of organic dyes.

3.2.2. Effect of MR Dye Initial Concentration

The photocatalytic activity is influenced by the initial concentration of MR dye. Therefore, two MR initial concentrations (25 and 50 ppm) were tested with different intervals of irradiation time (0, 10, 15, 30, 60, 90, 120, and 180 min), using 10 mL of dye and 15 mg of the photocatalyst. The results for the degradation process, as monitored by UV-Vis spectroscopy, are shown in Figure 8a,b for CeO₂ NPs and the CeO₂/GCN nanocomposite, respectively. The % degradation efficiency of each catalyst was calculated.

For both catalysts, the % degradation efficiency increased consistently with longer irradiation times. Additionally, as the initial dye concentration decreased, the % degradation efficiency became greater.

The maximum photodegradation was achieved at 180 min: 35.61% and 69.47% for 25 and 50 ppm, respectively, for CeO₂ NPs, and 95.26% and 99.93% for 25 and 50 ppm, respectively, for the CeO₂/GCN nanocomposite. This is because, at lower concentrations, the active sites of the catalyst remain available, whereas, at higher concentrations, the active sites become saturated. This performance can be attributed to the enhanced driving force for mass transfer, which increases with the initial dye concentration, resulting in deeper penetration of the dye into the internal pores of the catalyst in the liquid phase.

It can be concluded that the optimum condition for achieving 100% photodegradation efficiency is easily reached. These results are also fully aligned with the other findings presented in Figure 7a,b.

4. Conclusions

A novel photocatalytic CeO₂/GCN nanocomposite can be synthesized using simple and low-cost methods. This material shows great potential in artificial photosynthesis and environmental remediation. Its application in the photocatalytic treatment of industrial wastewater is highly recommended due to its thermal stability, high catalytic activity, efficiency, sunlight responsiveness, abundance, low cost, non-toxicity, proper band gap structure, and excellent photostability during prolonged use, making commercial implementation feasible.

The CeO₂/GCN nanocomposite exhibits a lower band gap energy compared to CeO₂ NPs (2.97 eV vs. 3.90 eV), significantly enhancing its removal efficiency to 99.92% within a short irradiation time of 40 min, compared to 69.47% for CeO₂ NPs alone.

This highly reactive CeO₂/GCN nanocomposite represents a promising photocatalytic material, capable of effectively degrading pollutants in industrial wastewater.