Reusable water filtration system: Multifunctional Ag/CeO₂/g-C₃N₄ nanocomposite for photocatalytic dye degradation and solar-driven water desalination

2.1. Production of nanosheets (NSs) (g-C₃N₄)

Melamine was used to create g-C₃N₄; 10 g of melamine was heated to 550°C for 4 h at a rate of 5°C/min in a semi-closed silica crucible. A powder was produced from the resultant g-C₃N₄ after it had cooled to room temperature [25].

2.2. Synthesis of CeO₂ NPs

A hydroxide-mediated technique was used to chemically create CeO₂ NPs [29]. The precursors were sodium hydroxide and cerium nitrate. For 3 h at room temperature, a 0.1 M cerium nitrate solution (60 mL) was stirred, while a 0.3M NaOH solution (20 mL) was added dropwise. For 20 min, the white precipitate was centrifuged at 10,000 rpm. The recovered pellet was cleaned thrice with distilled water and once with ethyl alcohol. It was then dried at 80°C for 1 h and annealed at 300°C.

2.3. Synthesis of Ag/CeO₂ NCs

To produce Ag/CeO₂, 1 g of CeO₂ was added to 8 mL of 0.01 M AgNO₃ solution, and the mixture was stirred for 5 h. Following 16 h of drying at 60°C, the precipitate was calcined for 3 h at 500°C. Lastly, ultrapure water, EtOH, and acetone were used to wash the product [30].

2.4. Synthesis of Ag/CeO₂/g-C₃N₄ NCs

A mixture of silver nitrate (0.157 g), cerium nitrate hexahydrate (2.25 g), and g-C₃N₄ (1 g) was stirred in 50 mL of distilled water for 30 min under ambient conditions. The resulting combination was then calcined at 500°C for 4 h [31].

2.5. Photodegradation of MB dye

For this procedure, 50 mg of the manufactured photocatalyst was used to study the photocatalytic decomposition of MB (10 mg/L, 100 mL) under UV-vis light irradiation using a Xenon lamp 55W (Sinolyn Ballast Fast Bright 55W Reactor with Xenon Ignition Unit AC Digital F5, sold by Sinolyn International Co., Ltd., logistics support by Sinolyn International Co., Ltd. China).

2.6. Seawater desalination experiment

All samples were prepared, vacuum-dried at 70°C, and then placed in a desiccator for 3 h. Then, in a basic distillation system, 50 mg of each sample was added to 100 mL of red seawater (collected from Tiran Island, Saudi Arabia), and the system was exposed to a halogen lamp for 10 h (0 to 8 h), as shown in Scheme 1. The research experiments were conducted at the Natural and Health Sciences Research Center.

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Scheme 1.

Illustrating how a basic distillation system is used to desalinate and purify water.

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3.1. Charcaterization

The TEM image (Figure 1a) reveals the characteristic layered and stacked structure of pure g-C₃N₄. This material, possessing a specific surface area of 21.311 m²/g, exhibits a fluffier texture due to its NS-like morphology. As shown in Figure 1(b), the energy-dispersive X-ray spectrum (EDS) confirms the presence of only carbon (C, 35.27 wt%), nitrogen (N, 52.47 wt%), and oxygen (O, 12.27 wt%). Cerium oxide nanoparticles (CeO₂ NPs) were synthesized using a well-established hydroxide-mediated method. This method involves precipitating a metal salt solution with a base to form a metal hydroxide precipitate, which is then calcined at high temperatures to yield CeO₂ NPs. Transmission electron microscopy (TEM) images (Figure 1c) show that the synthesized CeO₂ NPs are spherical with a uniform size distribution and an average particle size of 24 ± 1.34 nm, confirming successful synthesis within the desired size range with a specific surface area of 34.1 m²/g. The EDS spectrum (Figure 1d) confirms the presence of only cerium (Ce, 77.08 wt%) and oxygen (O, 22.92 wt%), indicating the absence of impurities. The Ag/CeO₂ NCs were observed to be approximately 20 nm in size with a surface area of 73.97 m²/g. The TEM images showed that Ag NPs were present on the CeO₂ NPs’ surface (Figure 1e). As shown in Figure 1(f), the EDS spectrum verified that the NPs contained silver (Ag, 5.25 wt%), cerium (Ce, 54.55 wt%), and oxygen (O, 40.3 wt%). The uniform distribution of Ag around Ce and O further supports the successful synthesis of the Ag/CeO₂ nanohybrid. A TEM image confirms the successful mixing of g-C₃N₄ with ceria NPs, with Ag presumed to be dispersed throughout (Figure 1g) with a specific surface area of 87.49 m²/g. The material’s structure, as expected, exhibits particles with voids and holes. This porosity is a result of gas release during combustion synthesis and contributes to the increased porosity observed within the irregular, layered structure of g-C₃N₄ [32]. The material’s elemental composition consists of 58.68 wt% Ce, 20.82 wt% O, 6.97 wt% C, 9.45 wt% N, and 4.08 wt% Ag (Figure 1h).

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Figure 1.

TEM images and EDS spectra of the prepared samples: (a, b) g-C₃N₄, (c,d) CeO₂ NPs, (e, f) Ag/CeO₂ NCs, and (g, h) Ag/CeO₂/g-C₃N₄ NCs.

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X-ray diffraction (XRD) analysis (Figure 2a) reveals two prominent diffraction peaks for pure g-C₃N₄ at 13.1° and 27.6° due to the (100) and (002) planes, respectively. The (002) band, being more intense, indicates the interlayer spacing of the g-C₃N₄ sheets, while the (100) peak represents the stacking of aromatic rings between these layers. CeO₂ NPs have a well-defined crystalline structure with a hexagonal shape and a lattice spacing of 0.328 nm, which corresponds to the (111) planes of the CeO₂ phase. The diffraction peaks at 28.6°, 33.2°, 47.5°, 56.4°, 59.1°, 69.4°, 77.0°, and 79.1° are assigned to the (111), (200), (220), (311), (222), (400), (331), and (420) planes of CeO₂, respectively. Those bands are reliable with the cubic fluorite structure of CeO₂ (JCPDS No. 34-0394) with a = b = c = 5.411 Å. According to the Scherrer model, the crystal’s size was 25 nm, which is consistent with the size determined by TEM imaging. The absence of other peaks indicates the complete conversion of Ce³⁺ to Ce⁴⁺. This Ce⁴⁺ phase indicates that each cerium atom is surrounded by eight oxygen atoms in a face-centered cubic arrangement, while each oxygen atom is tetrahedrally coordinated by cerium atoms. The (111), (200), and (220) planes of silver are represented by extra bands at 2θ values of 38.2°, 44.3°, and 64.5° (JCPDS 04-0783). The composite was successfully prepared, as evidenced by the presence of the g-C₃N₄ bands at 13.1° and 27.6° (indexed as the (100) and (002) planes, respectively) in the XRD pattern. Several distinctive peaks at 1145, 1213, 1393, 1587, and 1648 cm⁻¹ can be seen in the infrared spectrum of g-C₃N₄ (Figure 2b). The stretching modes of the C-N heterocycles in conjunction with the skeletal vibrations of aromatic rings are represented by these peaks. The triazine units’ breathing mode is responsible for a peak at 810 cm⁻¹. In the CeO₂ spectrum, a broad absorption band between 400 and 700 cm⁻¹ is assigned to the Ce-O stretching vibration [33]. Figure 2(b) displays the FTIR spectrum (400–4000 cm⁻¹) of the synthesized Ag/CeO₂ nanostructured material. A strong peak at 540 cm⁻¹, corresponding to the Ag-O stretching vibration, is observed at a higher wavenumber compared to the previously reported 513 cm⁻¹ [34]. This shift suggests an interaction between the Agⁿ⁺ nanoclusters and oxygen during the redox reaction between Ag and Ce³⁺ and may also indicate a weakening of the Ce-O bond. This weakening is supported by the lower wavenumber of the Ce-O stretching mode (820 cm⁻¹) in Ag/CeO₂ compared to pure CeO₂ (850 cm⁻¹) [34]. The band at 691 cm⁻¹ suggests intermolecular interactions due to the incorporation of Ag into CeO₂. Weak absorption bands at 940, 1530, and 2357 cm⁻¹ are likely due to adsorbed moisture and possibly the large surface area of the prepared material. The peak at 1367.6 cm⁻¹ suggests the presence of trace nitrate due to the N-O stretching vibration [35]. The CeO₂ fundamental vibration, which takes place at about 510 cm⁻¹, is said to have its initial overtone at 1120.4 cm⁻¹. Ceria NPs are characterized by bands at 1590.1, 1367.6, 1060.4, and 850 cm⁻¹ [36]. This spectrum clearly shows the Ce-O bending mode, which has been reported at 820 cm⁻¹ at the same wavenumber. The Ce-O bond stretching vibration is responsible for a faint band at 720 cm⁻¹ [37]. Although Ag-O vibrations should be evident at about 560 cm⁻¹, the infrared spectra show no corresponding peak [38,39].

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Figure 2.

The prepared materials, g-C₃N₄, CeO₂ NPs, Ag/CeO₂ NCs, and Ag/CeO₂/g-C₃N₄ NCs, are shown in their (a) XRD patterns and (b) FTIR spectra.

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The UV-vis diffuse reflectance spectroscopy (DRS) was used to analyse the light absorption features of the produced samples because they significantly affect photocatalytic performance [40,41]. Because of its large band gap, CeO₂ shows very little absorption of visible light (Figure 3a). However, because of surface plasmon resonance (SPR) effects, adding Ag NPs to Ag/CeO₂ microspheres greatly increases visible light absorption, enhancing visible light usage. Light in the visible and ultraviolet spectrums (up to 450 nm) is absorbed by g-C₃N₄. The Ag/CeO₂/g-C₃N₄ composite catalyst exhibits a much greater absorption intensity in the UV and a portion of the visible region when compared to g-C₃N₄ and Ag/CeO₂ separately.

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Figure 3.

(a) Relationship between the UV-Vis absorption spectra and (b) the Kubelka-Munk transformed reflectance spectra of g-C₃N₄, CeO₂, Ag/CeO₂, and Ag/CeO₂/g-C₃N₄

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The Ag/CeO₂/g-C₃N₄ composite does not show the weak SPR peak that is found for Ag/CeO₂ between 400 and 500 nm, possibly because of the low concentration and even dispersion of Ag NPs. Employing the Kubelka-Munk formula (αhv = A(hv-Eg)ⁿ/²), where v is the optical frequency, A is a constant, Eg is the band gap energy, and n is the kind of transition, the band gap energies were calculated. The values of n for CeO₂ and g-C₃N₄ were taken to be 1 and 4, respectively [42,43]. As shown in Figure 3(b), the band gap energy (Eg) of CeO₂ and g-C₃N₄ is approximately 2.69 eV and 3.2 eV, respectively. The composite’s band gap slightly shifts from 2.69 eV to 2.81 eV after particle loading, which may improve photocatalytic efficiency and aid in electron-hole separation. Heterojunctions are created by combining Ag, CeO₂, and g-C₃N₄. The band structure may change due to interactions between several materials, which frequently lowers the composite’s overall E.g. in comparison to its constituent parts. Ag can improve photocatalytic efficiency by facilitating charge separation and transfer. As a result of improved electrical interactions, the bandgap may narrow. The effective bandgap energy can be decreased by introducing Ag into CeO₂, which can produce defect states within the bandgap. Additional electron excitation routes can also be created by the inclusion of vacancies or dopants. Because of quantum confinement, smaller particles frequently have bigger band gaps; however, interactions within a composite can reverse this. Ag/CeO₂/g-C₃N₄ NCs can have improved photocatalytic qualities, exhibiting enhanced changes in Eg values, which makes them appropriate for a range of environmental remediation and energy conversion applications. We may take these factors into account when estimating the conduction band (CB) and valence band (VB) potential positions for the Ag/CeO₂/g-C₃N₄ NC catalyst. CeO₂ possesses VB usually between +2.7 and +3.0 eV (in relation to the normal hydrogen electrode, SHE), and CB at roughly +3.1 eV. g-C₃N₄ possesses a CB of about +2.7 eV and a VB of about +1.5 eV to +1.6 eV (compared to SHE). Unlike semiconductors, silver (Ag) often has a well-defined bandgap, although charge transfer allows it to affect electrical characteristics. Interactions between these materials can affect their possible positions when combined to form a NC. The proportions of CeO₂ and g-C₃N₄: ≈2.1 eV and the CB ≈2.9 eV can be used to approximate the VB location as a weighted average. The Ag/CeO₂/g-C₃N₄ NC predicted VB and CB positions imply that the composite can efficiently promote charge separation and transfer, boosting its photocatalytic activities. For the material’s qualities to be optimized for a variety of applications, the interaction between the various components is essential [22].

3.2. Photocatalytic Activity of MB

As shown in Figure 4, MB dye proved stable under both UV-vis irradiation and dark conditions (Figures 4a and b). The photodegradation of MB in aqueous solution under UV-vis light was investigated using several photocatalysts: CeO₂ NPs, g-C₃N₄ NSs, Ag/CeO₂ NC, and Ag/CeO₂/g-C₃N₄ NCs. UV-visible absorption spectroscopy was used to track the process of degradation. As shown in Figure 4, the degradation efficiency varied considerably among the photocatalysts. CeO₂ NPs degraded MB within 120 min, while g-C₃N₄ NSs achieved similar degradation in 90 min. The Ag/CeO₂ NCs were significantly more effective, achieving complete MB degradation in 60 min. Most notably, the Ag/CeO₂/g-C₃N₄ nanocatalyst demonstrated remarkable performance, completely degrading MB in just 20 min. The overall photodegradation rates under UV-vis irradiation followed this order: Ag/CeO₂/g-C₃N₄ NCs > Ag/CeO₂ NCs > g-C₃N₄ NSs > CeO₂ NPs. When compared to its constituent parts, this work unequivocally shows the Ag/CeO₂/g-C₃N₄ NC’s increased photocatalytic activity.

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Figure 4.

Comparative analysis of MB photodegradation using different (a) photocatalysts under darkness and (b) UV-vis light irradiation. Photocatalytic degradation efficiency of MB over time using (c) CeO₂ NPs, (d) g-C₃N₄ nanosheets, (e) Ag/CeO₂ NCs, (f) Ag/CeO₂/g-C₃N₄ NCs, and (g) the normalized spectra of different nanocatalysts at various exposure times, illustrating the superior activity of the Ag/CeO₂/g-C₃N₄ ternary composite.

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The improved activity can be attributed to several factors. The combined action of CeO₂, g-C₃N₄, and Ag NPs enhances the photocatalytic process. The Ag NPs induce SPR, which improves the absorption of visible light and generates more electron-hole pairs, hence increasing the photocatalytic performance. CeO₂ and g-C₃N₄ produce heterojunctions that make it easier to separate charges, which lowers electron-hole recombination and boosts photocatalytic efficiency. This makes it possible for photogenerated electrons to go from g-C₃N₄ to CeO₂ more effectively, which encourages oxygen reduction and the consequent degradation of MB. The nanostructured materials, particularly the NSs and NCs, likely possess a higher surface area, providing more active sites for MB adsorption and reaction. The significant improvement in MB degradation rate observed with the Ag/CeO₂/g-C₃N₄ nanocatalyst highlights the potential of this material for practical applications in wastewater treatment. The 20-minute degradation time is considerably faster than the other materials tested, suggesting a significant advancement in photocatalytic efficiency. The study effectively demonstrates the benefits of combining multiple components to create a more efficient photocatalyst.

The results suggest that smaller photocatalyst particle size significantly enhances photocatalytic performance, consistent with the principle that NPs with larger surface areas interact more effectively with dye molecules. The Ag/CeO₂/g-C₃N₄ NCs’ significant photocatalytic activity is demonstrated by the fact that, after three hours in dark settings, there is no discernible absorption or degradation (Figure 4a) [44].

Ag/CeO₂/g-C₃N₄ NCs have demonstrated their promise as highly effective catalysts for the degradation of organic pollutants in water by their enhanced photocatalytic performance. MB photocatalytic degradation was carried out in both visible and dim light settings to further assess their efficacy. The following formula was used to get the MB degradation percentage [8]:

Where A₀ is the initial absorbance of the solution and A_t is the absorbance after time t in minutes.

Photocatalytic degradation of MB under 30 min of UV-vis light irradiation was minimal (5.6%) without CeO₂ NPs. However, the addition of various nanoparticles significantly enhanced this process. CeO₂ NPs alone improved degradation to 38.42%, while g-C₃N₄ achieved 55.7%. Further improvements were observed with the combined materials: Ag/CeO₂ NCs reached 76.3%, and the Ag/CeO₂/g-C₃N₄ NCs demonstrated the most significant improvement, achieving an impressive 98.75% degradation.

The straight-line section of the first-order plot was used to calculate the rate constants k (min^–1). About 1.7×10^–2, 1x10^-2, 1.8×10^–3, and 9×10^–4 min^–1 have been obtained for Ag/CeO₂/g-C₃N₄, Ag/CeO₂, g-C₃N₄, and CeO₂, respectively. Of the four catalysts, the Ag/CeO₂/g-C₃N₄ NC catalyst displays the greatest rate constant (1.7×10⁻²min⁻¹), indicating that it is the most efficient for this specific process. The synergistic interaction of g-C₃N₄, cerium oxide (CeO₂), and silver (Ag) NPs greatly accelerates the rate of reaction. The beneficial effects of combining these materials are evident when comparing the rate constant of Ag/CeO₂ (1×10⁻²min⁻¹) with the separate components of g-C₃N₄ (1.8×10−3min⁻¹) and CeO₂ (9×10⁻⁴min⁻¹). A positive interaction between silver and cerium oxide is indicated by the Ag/CeO₂ binary catalyst’s much higher activity compared to either g-C₃N₄ or CeO₂ alone. The catalytic performance of Ag/CeO₂ is further improved by the addition of g-C₃N₄. This implies that g-C₃N₄ is important, perhaps through enhancing the dispersion of Ag and CeO₂ NPs, which raises the active sites’ surface area. improving the catalyst’s redox capabilities by facilitating charge transfer activities. supplying a special electrical environment that encourages reactant adsorption and activation. CeO₂ exhibits the lowest catalytic activity for this reaction at the specified circumstances, as seen by its lowest rate constant (9×10⁻⁴min⁻¹) [45,46]. A total organic content (TOC) analyser (TOC-VCPN, Shimadzu, Japan) was used to measure the total organic content (TOC) both before and after the MB dye was photocatalyzed using the conventional SM 5310B procedure. A significant drop in TOC (from 82 ppm to 4 ppm) was observed, indicating effective MB mineralization using CeO₂/g-C₃N₄/Ag, which equated to a 97% elimination of the MB dye.

The identification of intermediate chemicals produced during MB degradation was done using liquid chromatography-time of flight mass spectrometry (LC-TOF-MS). For the removal of organic pollutants, heterogeneous photocatalysis is a sustainable AOP. The Ag/CeO₂/g-C₃N₄ NC surface is easily adsorbent to the MB molecules during photodegradation, which promotes effective electron transfer between the dye and the photocatalyst. During photocatalytic degradation, demethylation cleavage produced the intermediate products, including A (m/z = 270), B (m/z = 256), and C (m/z = 228), along with phenol (m/z = 94) (Scheme 2 and Figure 5). These findings demonstrate the effectiveness of the Ag/CeO₂/g-C₃N₄ NCs in breaking down complex organic molecules, suggesting their potential for efficient environmental remediation.

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Scheme 2.

Detected intermediates from the degradation process of MB by using Ag/CeO₂/g-C₃N₄ NCs.

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Figure 5.

(a-d) MB photocatalytic degradation over the Ag/CeO₂/g-C₃N₄ NCs, as identified by LCTOF-MS, is suggested to occur via this mechanism. Emphasizing the function of demethylation cleavage, the figure shows the main intermediates and end product generated throughout the degradation process.

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3.3. Desalination of water using the produced NCs

The transformation of optical energy into thermal energy can be conceptualized as the formation of a strong electromagnetic field on the particle surface, which combines all radiative features such as scattering and absorption. The photothermal effect occurs when strongly absorbed light is quickly transformed into heat through a sequence of nonradiative processes. This is made feasible by the produced NPs’ small particle sizes, which fall within the range of cluster sizes with surface plasmon resonance, a special characteristic of plasmonic NPs [47]. Because photons are effectively converted into heat, the resulting nanomaterials have a special photothermal effect. The method known as “water distillation” uses evaporation and condensation to desalinate and purify saltwater. In Figure 6, we present the effects of the prepared nanocatalysts on seawater evaporation for desalination under visible light irradiation using simple distillation. Ag/CeO₂/g-C₃N₄ NCs exhibit a significantly higher temperature rise over time than the other NCs and blank seawater, as illustrated in Figure 6(a). While the temperature for Ag/CeO₂ NCs NPs reaches 98°C after 480 min, the temperature inside the experiment (100 min) hits 99°C. In contrast, the ambient temperature of the blank sample (seawater devoid of NCs) did not rise over 37°C (there was no filtered water for the blank sample after 12 h; Figure 6). Because of their special qualities and promise for effective and sustainable water purification, g-C₃N₄ NSs have become a promising material for photothermal water desalination. Because of its distinct electronic band structure, g-C₃N₄ can absorb light from a wide range of wavelengths, including visible light. This is crucial for photothermal applications, where light energy is converted into heat. The ability to absorb visible light makes g-C₃N₄ particularly attractive for solar-driven desalination, as it can efficiently utilize sunlight, a readily available and renewable energy source [48]. When g-C₃N₄ absorbs light, the energy is converted into heat, causing a rapid increase in temperature. This photothermal conversion efficiency is essential for generating the heat needed for water evaporation in desalination processes. The NS structure of g-C₃N₄ further enhances this photothermal conversion by providing a large surface area for light interaction and heat transfer. g-C₃N₄ exhibits inherent hydrophilicity, meaning it has a strong affinity for water molecules. This property facilitates the efficient transport of water to the surface of the material, where it can be evaporated by the photogenerated heat. The NS structure also aids in water transport by providing pathways and reducing resistance to water flow.

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Figure 6.

Evaluation of NCs for photothermal water desalination. (a) Temperature gain in red seawater treated with different NCs compared to untreated seawater (blank). (b) Temperature increases during three cycles of water distillation using Ag/CeO₂/g-C₃N₄ NCs, demonstrating recyclability. (c) Water evaporation is indicated by the volume reduction of treated saltwater samples following 8 h of exposure to visible light.

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The Ag/CeO₂/g-C₃N₄ NC sample was recycled three times for water desalination and purification, demonstrating high stability upon reuse (Figure 6b). Seawater alone showed no significant temperature increase (remaining below 45°C for 5 h). While NP-treated samples did exhibit some temperature gain, it was less than that of the blank sample, correlating with the observed decrease in seawater volume (Figure 6c). The Ag/CeO₂/g-C₃N₄ NCs have a significantly higher thermal effect on the saltwater, which is probably because of the effective heat transmission between the Ag/CeO₂ NPs and the g-C₃N₄ sheets. These results surpass previously reported findings [49]. The temperature increases with Ag/CeO₂/g-C₃N₄ NCs (reaching 99°C after 100 min) is also faster than that reported for Ag/Au/r-GO NCs (reaching 99°C after 120 min), likely due to their stronger light absorption.

Seawater and tap water were compared with water obtained from the distillation method utilizing plasmonic NPs (Table 1). In contrast to the other water samples, the results show that the distilled water is extremely pure and ion-free.

The cost per unit of performance, degradation efficiency, and solar-thermal conversion efficiency of Ag/CeO₂/g-C₃N₄ are compared to other ternary photocatalysts (Table 2), such as TiO₂/g-C₃N₄/Pt and ZnO/CeO₂/g-C₃N₄.

Table 2 presents a comparative analysis of various ternary photocatalysts, highlighting the superior behavior of the Ag/CeO₂/g-C₃N₄ NC. The Ag/CeO₂/g-C₃N₄ composite demonstrates a degradation efficiency of 98%, outperforming TiO₂/g-C₃N₄/Pt (90%) and ZnO/CeO₂/g-C₃N₄ (85%). This indicates its improved photocatalytic performance, primarily assigned to the incorporation of silver NPs, which increase the charge separation and decrease the electron-hole recombination. This mechanism is crucial for effective photocatalysis. At 30%, the solar-thermal conversion efficiency of Ag/CeO₂/g-C₃N₄ is also the highest among the composites evaluated. This efficiency is significant for applications requiring effective thermal management, such as in photothermal water desalination, where the transformation of solar energy into thermal energy is essential. The composite’s cost of $50/m² positions it competitively against TiO₂/g-C₃N₄/Pt ($105/m²) and is more favorable than ZnO/CeO₂/g-C₃N₄ ($60/m²). This affordability enhances the feasibility of Ag/CeO₂/g-C₃N₄ for large-scale applications, especially in regions where cost is a critical factor in technology adoption. The analysis reveals that the Ag/CeO₂/g-C₃N₄ NC not only excels in photocatalytic and thermal conversion efficiencies but also offers a competitive cost structure. This combination of high performance and affordability makes it a promising candidate for sustainable water purification and environmental remediation applications, suggesting its potential for widespread implementation in real-world scenarios. Further exploration of its long-term stability and performance in varied conditions would provide additional insights into its practical applicability.

3.4. Discussion

The Ag/CeO₂/g-C₃N₄ NC facilitates both photocatalytic dye degradation and photothermal water desalination through distinct yet synergistic mechanisms.

3.4.1. Photocatalytic dye degradation

The combination of Ag, CeO₂, and g-C₃N₄ forms a heterojunction, promoting charge separation and reducing electron-hole recombination [50]. Visible light absorption by g-C₃N₄ (bandgap ∼2.7 eV) generates electron-hole pairs. Ag NPs enhance light absorption via surface plasmon resonance, further exciting electrons and boosting photocatalytic activity [51]. Excited electrons from g-C₃N₄’s VB migrate to CeO₂’s CB, while the resulting holes in g-C₃N₄ oxidize water or hydroxyl ions, producing highly reactive hydroxyl radicals (•OH). Simultaneously, electrons on CeO₂ interact with oxygen to form superoxide radicals (•O₂⁻). These reactive species degrade the MB dye into smaller, less harmful molecules, mineralizing them into CO₂ and water [47]. Additionally, Ag NPs serve as electron sinks, which reduces electron-hole recombination even more and boosts photocatalytic effectiveness overall [50].

3.4.2. Photothermal water desalination

The Ag/CeO₂/g-C₃N₄ NC exhibits strong photothermal properties, particularly in the visible and NIR regions [52]. g-C₃N₄ absorbs visible light, and Ag NPs enhance this absorption through surface plasmon resonance, resulting in localized heating [52]. The absorbed light energy is converted into heat, significantly raising the NC’s temperature. This localized heating drives water evaporation, a key process in desalination [53]. As water evaporates, dissolved salts and impurities are left behind. The NC’s high thermal conductivity facilitates rapid heat transfer to the water, improving evaporation efficiency. In comparison to their individual components, Ag, CeO₂, and g-C₃N₄ work in concert to improve evaporation rates, create more heat, and absorb more light [54].

Silver nanoparticles (AgNPs), despite their considerable interest in applications spanning catalysis, electronics, and biomedicine owing to their unique properties, pose significant environmental and health risks throughout their lifecycle. The synthesis of AgNPs, whether through chemical (often involving toxic substances and generating hazardous waste), physical, or biological (requiring ecological impact assessment), has environmental implications [55]. Exposure to AgNPs can cause cellular and genetic damage due to their ability to penetrate biological tissues, raising serious health concerns [56]. Their release into aquatic environments via wastewater and runoff is toxic to aquatic organisms like fish and invertebrates, even at low concentrations [55], with the potential for bioaccumulation in the food chain threatening both wildlife and human health [56]. In soil, AgNPs can disrupt microbial communities and alter crucial nutrient cycling [54]. The disposal of AgNP-containing products presents a challenge, as conventional waste management may not prevent their release into the environment [57]. Improving methods for silver recovery from waste is essential to mitigate environmental harm, but current technologies need development. Research into biodegradable materials and environmentally sound disposal strategies is critical to lessening the ecological burden of AgNPs [57].

This study presents a novel Ag/CeO₂/g-C₃N₄ nanocatalyst that uniquely integrates the synergistic properties of silver, cerium oxide, and graphitic carbon nitride. The incorporation of Ag nanoparticles enhances light absorption through surface plasmon resonance, allowing for a broader photoresponse in the visible spectrum. This leads to a significant improvement in photocatalytic performance, achieving 99% degradation of MB dye within 20 min, surpassing the effectiveness of individual components. The NC excels in both photodegradation of pollutants and photothermal water desalination. Its efficient photothermal conversion generates localized heat, facilitating the evaporation of water and separation from dissolved salts, resulting in pure desalinated water. The ternary NC is synthesized using a straightforward solution-based impregnation method followed by calcination, making it accessible for large-scale production. The low-cost materials and environmentally friendly synthesis process of the composite make it a sustainable alternative to traditional desalination systems. The unique features of the Ag/CeO₂/g-C₃N₄ NC suggest its potential for broader uses in environmental remediation and sustainable water purification beyond the tested scenarios. Overall, this work highlights the innovative integration of materials and their synergistic effects, paving the way for efficient and sustainable solutions in water treatment technologies.