Design, Fabrication, and Characterization of N-Doped Decorated CeO2 Composite for Degradation of Malachite Green

INTRODUCTION
When the poisonous materials enter water (i.e. oceans, seas, rivers, and lakes), water contamination occurs. These materials can dissolve in water or remain suspended on the surface of the water. The result of this issue is poor water quality. Water contamination occurs in various ways. In recent decades, water contamination is one of the most important industrial and environmental hazards and can be a concern for urbanization. Different sources, for instance, plastics, papers, and textile industries are generated these pollutants.  [1-4]. Methylthioninium chloride, commonly named Methylene blue (MB), is an heteroaromatic dye that used more in industry. Because of the high toxicity and accumulations of this chemical in the environment, it has caused serious environmental problems. The recent studies presented that many researches have been performed to eliminate or separate of MB from water [5].
Graphene quantum dots have been introduced as a new member of nanocarbon structures. [6]. Based on recent reports, these nanocarbon structures have an excellent fluorescence [7, 8]. Besides, they reveal great biocompatibility [9], high cytotoxicity [10], high solubility in different solvents [11], can be graft with other structures via H-bonding [12]. Due to the wide availability of carbon sources, these properties hold promise for development in various applications: Fuel Cell, energy, sensors, and photocatalysts [13-15]. Doping is a smart approach to change optical and electrical properties of bulk semiconductor materials. The efficiency of GQD scaffolds can be easily optimized by doping agents: sulfur, nitrogen, and sulfur-nitrogen. Reports refer that the GQDs scaffolds doped with nitrogen have not large band gap. Furthermore, these structures show an absorption in the visible zone [16, 17]. Therefore, the range application of these scaffolds can be expanded in various fields: biosensors, photocatalyst, energy conversion, and biological imaging. [18, 19]. 
Among researchers, cerium(IV) oxide (CeO2) is an appropriate case for photocatalytic performance, since it has unique 4f electrons, rich defect structures, great chemical stability, high oxygen storage capacity, and inexpensive price [20-22]. To the best of our knowledge, there are not many scientific papers available on GQDs-cerium oxide hybrids and their photocatalytic performance. Based on the mentioned background, it is a type of novel nanocarbon-based materials that contain oxygen, which can be beneficial for photocatalytic performance.
In this current paper, we introduced a facile and effective hydrothermal route to prepare nitrogen-doped graphene quantum dots by using citric acid 2-Hydroxypropane-1,2,3-tricarboxylic acid and ethylenediamine as carbon and nitrogen sources, respectively. In following, CeO2/N-doped GQDs nanocomposites was prepared. The obtained compound has been confirmed by various spectroscopy techniques: PL, XRD, UV-Vis, FT-IR, and FE-SEM. The photocatalytic activity of products was finally investigated against malachite green under UV-Vis irradiation.

MATERIALS AND METHODS
Preparation of CeO2 nanoparticles 
According to previously method [23], Ce(SO4)2.4H2O was firstly dissolved in pure water (40 mL). Then, the alkaline solution (KOH, 5 M) was added. Also, the pH of medium was controlled by pH-meter set and reached 11. Next, the whole mixture was put to the autoclave system and kept under hydrothermal conditions (15h/ 140 °C). At completion, after filtering and washing, the obtained solid was dried at 50 °C. To give CeO2 nanoparticles, the solid was finally calcined (120 minutes/ 600 °C).

General procedure of N-doped decorated CeO2 nanocomposites preparation
Citric acid (1 g) was completely dissolved in 40 mL of pure H2O and stirred for 3 min to form the transparent solution. After that, ethylenediamine (0.5 mL) was added and stirred again for 60 s at room temperature. Next, 1.5 g of as-prepared Ce2O nanoparticles was added and stirred again for 5-6 min at room temperature. The mixture was then moved to autoclave and heated in an oven (12h/ 180 °C). The as-prepared CeO2/N-doped GQDs nanocomposite was obtained, washed two times with dry C2H5OH, and dried in an oven.

Photocatalyst test 
Under UV light waves, the photocatalytic performance of as-prepared N-doped decorated CeO2 nanocomposites was studied by checking the degradation of methyl blue (MB) in a water media. Test condition included the use of UV lamp (400 W, Orsam model) located at a determined distance 0.2 meter away from the reaction medium. The as-synthesized dye solution (100 mL solution including the mixture of CeO2 nanoparticles (0.1 g) or as-prepared N-doped decorated CeO2 nanocomposites (0.1g), MB solution (10 ppm), and double-distilled water in neutral pH) was stirred (0.5 h) in a dark medium to get suitable balance of adsorption and desorption of the dye molecules on the surface of N-doped decorated CeO2 nanocomposites, which is necessary for them to function effectively as a photocatalyst. The air was pumped into reaction medium to remain the solution oxygen-saturated all over the reaction. Next, the CeO2 nanoparticles and N-doped decorated CeO2 nanocomposites were separated from samples (6 mL), collected from the degraded solution at different times, and centrifuged at 12000 rpm (5 min). The concentration of dye was investigated by UV-Vis strategy. 

RESULTS AND DISCUSSION
Ce2O nanoparticles was prepared by hydrothermal process. The hydrothermal process was also applied for the synthesis of the dye molecules on the N-doped decorated CeO2 nanocomposites [6]. The characterization of pure Ce2O nanoparticles and the N-doped decorated CeO2 nanocomposites were characterized by XRD method. The XRD results of pure Ce2O nanoparticles and N-doped decorated CeO2 nanocomposites were displayed in Fig. 1. From Fig. 1a, the all diffraction peaks were observed and this pattern matched with standard card (JCPDS Card No. 00-034-0394) [24]. Moreover, a broad peak is located at 2Theta= 22° relating to the [002] plane cab corresponded to disordered structure of N-doped graphene quantum dots (Fig. 2a) [1]. The functional groups of surface were studied through FT-IR method. The FT-IR curve of pure Ce2O nanoparticles and N-doped decorated CeO2 nanocomposites were displayed in Fig. 2. According to Fig. 2a, the absorption bond at wavenumber of 3280 cm-1 is corresponded to the stretching absorption of hydroxyl group. The band is located at wavenumber of 665 cm-1 is related to Ce-O band [25, 26]. The presence of O-H (strong and broad peak), C=O, C=C, and C-N bands are seen at 3430, 1665, 1475-1575, and 1215 cm-1, respectively [27]. These results confirmed the generation of the N-doped GQDs. In addition, data from FT-IR data display that the obtained compound includes N and COOH containing functional groups.
To study of the morphology and particles size of nanocomposites, FE-SEM analysis was used. The FE-SEM pictures of Ce2O nanoparticles and N-doped decorated CeO2 nanocomposites are shown in Fig. 3. Based on FE-SEM images, the average particles size of obtained nanostructures was reported 434 nm. In addition to FE-SEM analysis, EDX analysis was used for investigation of chemical composition. The EDX pattern confirmed the presence on cerium, oxygen, nitrogen, and carbon elements in the CeO2/N-doped GQDs nanocomposites. The results of EDX analysis were displayed in Fig. 4. 
The photocatalytic efficiency of a material is closely linked to its light absorption capability. Fig. 5a and Fig. 5c presents the ultraviolet-visible diffuse reflectance spectra (DRS) for CeO2 and the CeO2/N-GQDs nanocomposite. To determine the most effective light source for initiating photocatalytic dye degradation, an understanding of the band gap is essential. The proposed band gap for pure CeO2 and the N-doped decorated CeO2 nanocomposites was assessed by comparing the (ahυ)ⁿ curve to zero hυ (Fig. 5b and Fig. 5d). Based on this tests, the estimated band gap of pure CeO2 nanostructure was determined to be 2.63 eV. In contrast, the N-doped decorated CeO2 nanocomposites exhibited a lower band gap of 2.47 eV, indicating that UV-Vis light irradiation is the most suitable source for photocatalytic activity. These findings confirm that N-GQDs and CeO2 have been appropriately hybridized to establish a Z-scheme heterojunction, which enhances charge separation and improves photocatalytic efficiency.
Malachite green has been found to be moderately toxic and an extreme irritant. Studies indicate that it can intercalate with DNA, potentially leading to mutagenic effects. The photocatalytic degradation of malachite green was conducted to evaluate the functionality of the as-prepared photocatalysts. The initial tests were performed under controlled conditions, including a pH solution=7, a primary of malachite green: 6 mg/L, and a photocatalyst loading: 50 mg. The mixture was stirred in the dark medium for 0.5 h to achieve a balance of adsorption and desorption, with samples collected every 15 minutes to monitor the adsorption of malachite green before light exposure. Subsequently, the photoreactor was illuminated with UV-vis light. The photocatalytic performance of CeO2 and the CeO2/N-GQDs nanocomposite in terms of malachite green removal efficiency is depicted in Fig. 6. N-doped decorated CeO2 nanocomposites achieved 94% removal (Fig. 6b), meanwhile, CeO2 alone achieved 63% removal within the same timeframe (Fig. 6a). The improved photocatalytic performance of N-doped decorated CeO2 nanocomposites compared to CeO2 was attributed to band gap reduction and the establishment of a N-doped decorated CeO2 nanocomposites.
The catalyst dosage has a crucial role in determining the rate of organic dye degradation in photocatalytic reactions. While increasing the catalyst quantity generally enhances degradation by providing more active regions for light absorption and the generation of reactive species, for instance: hydroxyl radicals (•OH). The catalyst acts as a reaction surface, facilitating the generation of reactive oxygen species (ROS) that break down malachite green [28]. A higher catalyst concentration accelerates degradation, but practical limitations must be considered when optimizing its amount. Excessive catalyst usage may lead to higher costs and complex recovery processes, while insufficient catalyst can result in partial oxidation of the dye or extended reaction times. Striking the right balance requires experimental validation to determine the most effective material concentration. In the conducted tests, photocatalytic efficiencies of 82%, 94%, and 89% were achieved with catalyst dosages of 3, 6 g, and 9 mg/L, respectively, for N-doped decorated CeO2 nanocomposites (Fig. 7). These findings underscore the importance of optimal catalyst loading, ensuring maximum dye degradation efficiency while maintaining economic feasibility and effective reaction control.

CONCLUSION
In this study, Ce₂O nanoparticles and N-doped decorated CeO2 nanostructures were successfully fabricated through the hydrothermal route and comprehensively confirmed applying XRD, DRS, FT-IR, FE-SEM, and EDX analyses. Structural and functional investigations confirmed the incorporation of N-doped GQDs, leading to the formation of a Z-scheme heterojunction, which effectively enhances charge separation and photocatalytic activity. Band gap analysis revealed that the N-doped decorated CeO2 nanostructures exhibited a lower band gap (2.47 eV) compared to pure CeO₂ (2.63 eV), indicating improved light absorption under UV-Vis irradiation. These findings validate the efficient hybridization of CeO₂ with N-GQDs, which extends the material’s photocatalytic responsiveness and enhances its degradation capability. The photocatalytic functionality of the synthesized nanocatalyst was further assessed through the degradation of malachite green, a toxic organic dye with potential mutagenic effects. Under optimized conditions (pH 7, the primary concentration of dye: 6 mg/L, and 50 mg catalyst loading), the N-doped decorated CeO2 nanocomposites achieved a superior degradation efficiency of 94%, significantly surpassing pure CeO₂ (63%). The enhanced performance was attributed to band gap reduction, increased charge separation yield, and the generation of reactive oxygen species (ROS). Furthermore, catalyst dosage optimization was explored to determine the most effective concentration for maximum dye degradation. Experimental results demonstrated that a balanced catalyst loading is essential to avoid excessive reagent costs and unwanted charge recombination effects. The N-doped decorated CeO2 nanostructures exhibited photocatalytic efficiencies of 82%, 94%, and 89% at catalyst dosages of 3, 6, and 9 mg/L, respectively, confirming the importance of dosage regulation in achieving optimal degradation performance.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.