Hydrothermal Synthesis of Heterostructured g-C₃N₄/Ag–TiO₂ Nanocomposites for Enhanced Photocatalytic Degradation of Organic Pollutants

Abstract

In this study, heterostructured g-C₃N₄/Ag–TiO₂ nanocomposites were successfully fabricated using an easily accessible hydrothermal route. Various analytical tools were employed to investigate the surface morphology, crystal structure, specific surface area, and optical properties of as-synthesized samples. XRD and TEM characterization results provided evidence of the successful fabrication of the ternary g-C₃N₄/Ag–TiO₂ heterostructured nanocomposite. The heterostructured g-C₃N₄/Ag–TiO₂ nanocomposite exhibited the best degradation efficiency of 98.04% against rhodamine B (RhB) within 180 min under visible LED light irradiation. The g-C₃N₄/Ag–TiO₂ nanocomposite exhibited an apparent reaction rate constant 13.16, 4.7, and 1.33 times higher than that of TiO₂, Ag–TiO₂, and g-C₃N₄, respectively. The g-C₃N₄/Ag–TiO₂ ternary composite demonstrated higher photocatalytic activity than pristine TiO₂ and binary Ag–TiO₂ photocatalysts for the degradation of RhB under visible LED light irradiation. The improved photocatalytic performance of the g-C₃N₄/Ag–TiO₂ nanocomposite can be attributed to the formation of an excellent heterostructure between TiO₂ and g-C₃N₄ as well as the incorporation of Ag nanoparticles, which promoted efficient charge carrier separation and transfer and suppressed the rate of recombination. Therefore, this study presents the development of heterostructured g-C₃N₄/Ag–TiO₂ nanocomposites that exhibit excellent photocatalytic performance for the efficient degradation of harmful organic pollutants in wastewater, making them promising candidates for environmental remediation.

1. Introduction

Nowadays, organic contaminants have become major sources of water pollution worldwide due to population growth and rapid agricultural and industrial development [1]. Modern commercial dyes are a major category of organic pollutants known for their robust structural and color stability owing to their high degree of aromaticity and extensively conjugated chromophores. With widespread use in diverse industries, such as food, cosmetics, leather, plastics, printing, and textiles, these dyes pose a health risk to humans and ecological systems if unintentionally released into the environment [2,3]. Furthermore, the discharge of these toxic and carcinogenic synthetic colors into water bodies could obstruct sunlight penetration, which is harmful to natural aquatic activities that involve photosynthesis and other biodegradation processes. These hazardous pollutants seriously harm human health and the environment if they are not effectively removed [1,4]. Therefore, various conventional methods have been used for wastewater treatment, such as filtration, adsorption, advanced oxidation, coagulation, sedimentation, disinfection, reverse osmosis, and biological processes [1,5]. However, these conventional methods have several limitations, including low efficiency, high chemical costs, difficult preparation, time consumption, a limited visible light absorption range, and the generation of secondary waste [1,6]. Therefore, it is imperative to develop efficient, cost-effective, and sustainable wastewater treatment technologies to eliminate hazardous organic pollutants [5]. Since the pioneering work of Fujishima and Honda in 1972, heterogeneous photocatalysis has attracted tremendous attention as an advanced oxidation process for the elimination of pollutants in wastewater [7]. Therefore, photocatalysis is a sustainable, cost-effective, green, and clean technology for the decomposition of organic pollutants in wastewater into H₂O, CO₂, and inorganic minerals [8]. Photocatalysts use solar energy to promote electrons from the valence band to the conduction band, leading to the generation of electron–hole pairs, which then initiate reactions with water and oxygen molecules or hydroxyl groups. Therefore, superoxide anions (^•O₂⁻) and hydroxyl radicals (^•OH) are produced, and these reactive oxygen species (ROS) actively participate in photochemical redox reactions throughout the photocatalytic process [9]. Moreover, holes may react with hydroxyl ions (OH⁻) or water molecules to produce hydroxyl radicals (^•OH), and they may directly participate in the oxidative decomposition of pollutants [8].

TiO₂ is the most widely used oxide-based semiconductor photocatalyst for the elimination of harmful organic pollutants because of its intriguing properties such as low cost, high stability, and excellent optical properties [10,11]. However, owing to its high energy bandgap (3.2 eV), TiO₂ can only be active in the UV light region, which accounts for less than 5% of the total solar energy, leading to limited utilization of solar energy. Additionally, the poor photocatalytic efficacy of TiO₂ can be attributed to the fast recombination of photoinduced charge carriers [12]. Many researchers have explored various strategies to overcome the limitations of TiO₂. Among them, many studies have focused on noble metal deposition and semiconductor coupling for the formation of heterojunctions [9]. Recently, noble metal deposition on TiO₂ photocatalysts has attracted extensive research attention [10,12,13]. The surface plasmon resonance effect of noble metals, such as Ag, Au, Pt, and Pd, can improve charge carrier separation and transfer and visible light absorption, leading to enhanced photocatalytic efficiency [5,14]. Moreover, the migration of electrons from the conduction band of TiO₂ to noble metals is facilitated by the creation of a Schottky barrier at the interface, promoting the separation of photoinduced charge carriers and retarding the recombination of electron–hole pairs, all of which contribute to the overall enhancement of the photocatalytic performance [11,15]. Among the different noble metals, silver (Ag) remains the primary choice because of its high stability, low cost, and facile preparation [16]. In addition, Ag possesses vacant orbitals that can serve as active sites and electron acceptors, facilitating the movement of electrons in composite materials, and its surface plasmon resonance effect can significantly broaden the visible light absorption range [15,17]. Therefore, researchers have shown significant interest in Ag deposition on TiO₂ because of its potential to enhance the photoactivity of TiO₂ [13,18,19,20]. The key role of Ag in Ag-doped TiO₂ nanoparticles is to enhance their photocatalytic activity through mechanisms such as electron trapping, improving the visible light absorption range, and modifying the surface properties, leading to improved electron–hole separation and greater surface electron excitation [13]. Although the deposition of Ag can decrease the rate of photoinduced charge carrier recombination, it is not sufficient to improve TiO₂ photocatalytic performance at a broad range of visible-light absorption. Therefore, the deposition of plasmonic Ag metal, in tandem with coupling with small-bandgap semiconductors having suitable energy bandgap positions, can create a heterojunction that further boosts the photocatalytic efficiency of TiO₂ [5]. In recent years, the coupling of TiO₂ and g-C₃N₄ has gained considerable attention owing to their highly compatible band positions [21,22,23,24,25,26]. A heterojunction formed between TiO₂ and g-C₃N₄ reduces the recombination rate of charge carriers and improves photocatalytic activity [1]. Therefore, g-C₃N₄ is considered to be the best candidate for coupling with TiO₂ to construct heterojunctions because of its suitable band position, chemical stability, and low cost [27]. Graphitic carbon nitride (g-C₃N₄) is an n-type polymeric semiconductor that has been extensively studied owing to its potential use in the elimination of organic pollutants. It is cost-effective, non-toxic, eco-friendly, easy to synthesize, and has a mid-energy bandgap (2.7 eV) as well as strong physicochemical stability [6]. Despite its many advantages, bulk g-C₃N₄ exhibits weak photocatalytic performance owing to factors such as fast recombination of photoexcited charge carriers, limited visible light absorption range, and low specific surface area [28]. Researchers have found that modifying the photocatalytic properties of g-C₃N₄ is as important as modifying TiO₂, and the synergistic effect of Ag deposition on TiO₂ and coupling with g-C₃N₄ can lead to enhanced photocatalytic activity of the ternary composites [29,30,31,32,33,34,35,36]. In particular, heterostructured g-C₃N₄/Ag–TiO₂ nanocomposites have emerged as a promising area of research in the field of photocatalytic degradation of organic pollutants and have been extensively studied due to their remarkable visible-light-driven photocatalytic performance. Therefore, many strategies have been used to fabricate heterostructured g-C₃N₄/Ag–TiO₂ nanocomposites, including the microwave-assisted approach [37], freeze-drying route [9], hydrothermal method [36,38], calcination [39,40], physical mixing-calcination method [41], physical mixing [42,43], chemical reduction methods, and so on [11].

Most recent research findings on g-C₃N₄/Ag–TiO₂ nanocomposites have used xenon lamps with cutoff filters [11,14,15,17,42,43,44] as visible light sources. However, they have various drawbacks, such as being hazardous, having a short lifespan, being overheated quickly, being expensive, and posing difficulties in handling [6]. However, LED lamps are non-toxic and energy-efficient alternatives to conventional lamps because they convert more energy into light and do not generate excessive heat. Furthermore, LED lamps are cost-effective, durable, and emit only the wavelengths required to save energy [45]. As part of green and sustainable chemistry, this study used an energy-saving 50 W LED lamp to measure the photocatalytic degradation efficiency of the as-synthesized samples.

In this study, a heterostructured g-C₃N₄/Ag–TiO₂ nanocomposite was successfully fabricated using an easily accessible hydrothermal technique, followed by calcination treatment. First, a simple sol-gel synthesis route was used to prepare pristine TiO₂ and 3 mol% Ag-doped TiO₂ nanoparticles. Second, g-C₃N₄ nanosheets were synthesized using the single-step calcination of a mixture of urea and thiourea at 600 °C in an air medium. Finally, a facile one-step hydrothermal technique and calcination were used to prepare a g-C₃N₄/Ag–TiO₂ nanocomposite using a desired weight ratio of 3:1 from the fabricated g-C₃N₄ nanosheet to 3 mol% Ag–TiO₂ nanoparticles. The structural, morphological, and optical properties and specific surface area of the prepared materials were characterized by field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller (BET) nitrogen adsorption/desorption technique (BET), photoluminescence (PL), and UV/vis diffuse reflectance spectroscopy (DRS). In addition, the photocatalytic performance of the prepared pristine TiO₂, binary Ag–TiO₂ nanoparticles, g-C₃N₄ nanosheets_, and g-C₃N₄/Ag–TiO₂ nanocomposites was examined by assessing their ability to degrade rhodamine B (RhB) upon illumination by energy-saving visible LED light.

2. Materials and Methods

2.1. Materials

In this study, Ag-doped TiO₂ nanoparticles were prepared using several reagents, including titanium tetra isopropoxide (C₁₂H₂₈O₄Ti, 97%, Sigma-Aldrich, St. Louis, MO, USA), silver nitrate (AgNO₃, 99%, Merck, Boston, MA, USA), glacial acetic acid (CH₃COOH, 99%, Merck), nitric acid (HNO₃, 69%, Merck), and absolute ethanol (C₂H₅OH, Sigma-Aldrich). Moreover, analytical-grade urea (CH₄N₂O, 99.5%, Merck) and thiourea (CH₄N₂S, 99%, Merck) were used to synthesize g-C₃N₄ nanosheets. Fabricated Ag–TiO₂ nanoparticles and g-C₃N₄ nanosheets were used to synthesize heterostructured g-C₃N₄/Ag–TiO₂ nanocomposites. All the chemicals and reagents were used as received without further purification. Distilled water was used in all experiments.

2.2. Synthesis

2.2.1. Preparation of g-C₃N₄ Samples

g-C₃N₄ nanosheets were prepared by one-step calcination of a mixture of urea and thiourea in a muffle furnace under an air atmosphere. Typically, specific amounts of urea and thiourea were carefully weighed and placed into a 50 mL ceramic crucible with a lid, which was then wrapped with aluminum foil paper and subjected to calcination at 600 °C for 3 h. The resulting product was cooled to room temperature and carefully ground into a fine powder.

2.2.2. Preparation of Ag–TiO₂ Nanoparticles

Ag-doped TiO₂ (Ag–TiO₂) nanoparticles were prepared using a sol-gel method. First, solution A was obtained by dissolving 22 mL of titanium isopropoxide (TTIP) in 150 mL of ethanol, followed by constant stirring for 45 min. In solution B, 3 mol% of silver nitrate was dissolved in 40 mL of distilled water with the addition of 5 mL of glacial acetic acid. Solution B was then slowly dripped into solution A and vigorously stirred. The resulting milky sol was constantly stirred for 2 h while carefully adjusting the pH value to 2–3 by adding 2 mL of nitric acid. Subsequently, the white precipitate obtained from the sol was allowed to settle for 4 days at room temperature, after which the solid gel was collected and washed several times with distilled water by centrifugation at 4000 rpm for 10 min. The washed samples were dried at 80 °C for 16 h in a vacuum oven. The resulting products were ground and calcined at 400 °C for 4 h. Finally, the samples were ground again and used for characterization. Similarly, pristine TiO₂ nanoparticles were synthesized under the same experimental conditions without the addition of AgNO₃ as a precursor of Ag.

2.2.3. Fabrication of g-C₃N₄/Ag–TiO₂ Nanocomposite

A facile hydrothermal method followed by calcination was used to fabricate a heterostructured g-C₃N₄/Ag–TiO₂ nanocomposite with a weight ratio of 3:1 of fabricated g-C₃N₄ nanosheets and 3% Ag–TiO₂ nanoparticles, respectively. The process involves dissolving 0.3 g of g-C₃N₄ nanosheets in 100 mL of distilled water by ultrasonication for 1 h and then adding 0.1 g of 3% Ag–TiO₂ nanoparticles to the solution, followed by vigorous stirring for 2 h. In addition, a small amount of hydrofluoric acid was added continuously during stirring to control nanocrystal formation. The resulting solution was transferred to a 100 mL stainless-steel autoclave and maintained at 180 °C for 6 h. Following hydrothermal treatment, the mixture was cooled to room temperature and subsequently washed several times with distilled water. Finally, the resulting sample was dried at 80 °C for 15 h and then ground into an ultrafine powder.

2.2.4. Characterization

The crystal structures of the as-synthesized samples were analyzed using X-ray powder diffraction (PanAlyticals, Almelo, The Netherlands) with Cu-Kα radiation (λ = 1.5406 Å) operating at 40 kV and 15 mA. The diffraction patterns were measured over a range of 10–70°, with a step width of 0.01° and a scanning rate of 10°/min. The functional groups and chemical structures of the samples were analyzed using FTIR spectroscopy (IR Tracer-100, Shimadzu, Kyoto, Japan) within the wavenumber range of 600–3600 cm⁻¹. The bandgap energies of the samples were analyzed using UV/vis diffuse reflectance spectroscopy (UV-2600, Shimadzu, Kyoto, Japan) with barium sulfate as the background substance within the wavelength range of 300–700 nm. The separation of photoinduced electron–hole pairs in the as-prepared samples was examined using PL spectrophotometer (RF-6000, Shimadzu, Kyoto, Japan) with an excitation wavelength of 360 nm. Field-emission scanning electron microscopy (FE-SEM, MAIA3 XMH, TESCAN BRONO s.r.o, Brono, Czech) and high-resolution transmission electron microscopy (HR-TEM, JEM-2100F, JEOL, Tokyo, Japan) were used to investigate the morphology of the samples at an accelerating voltage of 200 kV. The N₂ adsorption–desorption isotherms and Brunauer–Emmett–Teller specific surface area (S_BET) analyses were performed using a Micromeritics ASAP 3000 (Micromeritics, Norcross, GA, USA) equipped with a nitrogen adsorption device. Additionally, the Barrett–Joyner–Halenda (BJH) method was used to determine the pore size distribution and volume curves of the as-synthesized samples.

2.2.5. Photocatalytic Activity Testing

The photocatalytic activities of as-prepared photocatalysts were determined by assessing their ability to degrade rhodamine B (RhB) using a 50 W crystal white LED lamp (5000 lm, 6500 k, 220–240 V) as a visible light source (Phillips, Kolkata, India). A photolysis experiment was conducted on an aqueous RhB solution under visible LED light illumination for 180 min. Adsorption and photocatalytic experiments were performed by dispersing 50 mg of each photocatalyst in 100 mL of a 10 mg/L RhB aqueous solution. A dark adsorption experiment was conducted for 60 min to establish the adsorption–desorption equilibrium between RhB and the photocatalysts. Throughout the experiments, vigorous magnetic stirring was applied. At specific time intervals during each experiment, 3 mL aliquots were withdrawn from the solution and subjected to centrifugation to separate the photocatalyst. The absorbance of RhB was measured at its maximum absorption wavelength (λ = 554 nm) using a UV/vis spectrophotometer (2450, Shimadzu, Kyoto, Japan). The rate of RhB dye degradation (η) in aqueous solution was calculated using the following equation [46,47]:

$$\mathsf{\eta }=\frac{{\mathrm{A}}_0-\mathrm{A}}{{\mathrm{A}}_0}\times 100\mathrm{\%}$$

(1)

where A₀ and A refer to the initial absorbance and absorbance at a specific irradiation time t, respectively.

A pseudo-first-order kinetic model was utilized to determine the apparent degradation rate constant (k_app) of RhB dye over g-C₃N₄/Ag–TiO₂ composites [11,47]:

$$\mathrm{l}\mathrm{n}\frac{{\mathrm{A}}_0}{\mathrm{A}}={\mathrm{k}}_{\mathrm{a}\mathrm{p}\mathrm{p}}\mathrm{t}$$

(2)

where k_app (min⁻¹) is the rate constant and t (min) is the specific time interval of irradiation.

3. Results and Discussion

3.1. XRD Analysis

The crystal structures of the as-prepared samples, including pristine TiO₂, Ag–TiO₂, g-C₃N₄, and g-C₃N₄/Ag–TiO₂, were investigated using X-ray powder diffraction (XRD), and the results are shown in Figure 1. The XRD patterns of g-C₃N₄ revealed the presence of two characteristic peaks at around 13.1° (100) and 27.2° (002), which correspond to in-plane repeating tri-s-triazine units and interplanar stacking of conjugated aromatic units, respectively [6,48]. The absence of a characteristic Ag peak in the composites, as confirmed by the XRD spectra, can be attributed to the low amount of Ag or its high dispersion [38,41,42]. The diffraction peaks of pristine TiO₂ and Ag–TiO₂ nanoparticles are predominantly in the anatase phase (ICSD File No. 076173) and located at 2θ positions of around 25.356° (101), 37.847° (004), 48.145° (200), 53.974° (105), 55.186° (211), 62.812° (204), and 68.879° (116). In addition to the characteristic anatase phase peaks, the Ag–TiO₂ nanoparticles also exhibited a rutile phase, with diffraction peaks located at 2θ of 27.4392° (110), 306.0847° (101), and 3541.2744° (111) (ICSD File No. 088627). The pristine TiO₂ sample was primarily composed of the anatase phase, with a minor brookite phase at 2θ of 30.68°, corresponding to the (121) crystal plane, which indicated consistency with previous findings [49]. The XRD patterns of the g-C₃N₄/Ag–TiO₂ nanocomposite contained diffraction peaks arising from both Ag–TiO₂ and g-C₃N₄, thereby confirming the successful formation of the composite. Moreover, a higher g-C₃N₄ content was found to be capable of intensifying the recognizable peak at approximately 27.6°, resulting in a minor peak position shift in the composite [11]. Both anatase and rutile TiO₂ diffraction peaks were observed for the g-C₃N₄/Ag–TiO₂ composite [42,50].

The average crystallite size (D) of the as-synthesized photocatalysts was estimated using the Scherrer equation [21]:

$$\mathrm{D}=\frac{\mathrm{K}\mathrm{\lambda }}{\mathsf{\beta }\mathrm{c}\mathrm{o}\mathrm{s}\mathsf{\theta }}$$

(3)

where K is a constant (0.94), λ is the X-ray wavelength (0.15406 nm), β is the full width at half maximum (FWHM), and θ is the diffraction angle. As calculated from the Scherrer equation, the average crystallite sizes of the as-synthesized photocatalysts, including pure TiO₂, Ag–TiO₂, g-C₃N₄, and g-C₃N₄/Ag–TiO₂, were approximately 5.2, 9.1, 4.3, and 7.6 nm, respectively.

3.2. Optical Properties Analysis

3.2.1. UV/vis DRS Analysis

The light absorption properties and bandgap energies of all synthesized samples, including TiO₂, g-C₃N₄, Ag–TiO₂, and g-C₃N₄/Ag–TiO₂, were analyzed using UV/vis diffuse reflectance spectroscopy (DRS), as shown in Figure 2.

As shown in Figure 2b, the bandgap energies of the Ag–TiO₂ and g-C₃N₄/Ag–TiO₂ composites are narrower than those of the pristine TiO₂ and g-C₃N₄ samples. The absorption edges of the g-C₃N₄/Ag–TiO₂ nanocomposite exhibited a considerable redshift toward higher wavelengths, indicating a significant improvement in the visible light absorption of the composites, as shown in Figure 2a [11,50]. This may be due to the surface plasmon resonance effect of Ag, which is deposited on the surface of the TiO₂ nanoparticles, and the formation of heterostructures in the composites [17]. The bandgap energy (E_g) of the as-synthesized samples was calculated using a modified Tauc equation with the Kubelka–Munk function:

$$(\mathrm{F}(\mathrm{R}){\mathrm{h}\mathrm{\upsilon })}^{\frac{1}{2}}=\mathrm{A}(\mathrm{h}\mathrm{\upsilon }-{\mathrm{E}}_{\mathrm{g}})$$

(4)

where F(R) = (1 − R)²/2R, F(R) is the Kubelka–Munk function, R is the reflectance, hυ is the photon energy, and A is a constant [11]. The bandgap energy can be estimated by determining the point at which the linear trendline on the plot of (F(R)hυ)^1/2 versus (hυ) intersects the hυ-axis [11,42]. The bandgap energies of TiO₂, Ag–TiO₂, g-C₃N₄, and g-C₃N_4/Ag–TiO₂ photocatalysts were calculated to be 3.04, 2.89, 2.94, and 2.73 eV, respectively. Compared to pristine TiO₂ and g-C₃N₄, Ag–TiO₂ shows a slight red shift in the absorption edge [9]. Furthermore, coupling g-C₃N₄ with Ag–TiO₂ shows a significant red shift compared to TiO₂, Ag–TiO₂, and g-C₃N₄, which allows for enhanced visible-light absorption [48]. Therefore, the synergetic effect of coupling g-C₃N₄ and loading Ag on TiO₂ ameliorated the separation and transportation of photogenerated charge carriers, leading to an overall increase in photocatalytic performance [9,51].

3.2.2. PL Analysis

Photoluminescence (PL) analysis was used to study the separation of photoinduced electron–hole pairs in the as-synthesized samples [43]. Figure 3 shows the PL spectra of pristine TiO₂, Ag–TiO₂, g-C₃N₄, and g-C₃N₄/Ag–TiO₂ samples obtained at an excitation wavelength of 360 nm. Both g-C₃N₄ and g-C₃N₄/Ag–TiO₂ displayed main emission peaks at approximately 440 nm, as shown in Figure 3. A higher PL intensity was observed for g-C₃N₄ compared to g-C₃N₄/Ag–TiO₂, indicating faster recombination of photoinduced electron–hole pairs in g-C₃N₄ [36]. The PL spectra of pristine TiO₂ and Ag–TiO₂ nanoparticles did not exhibit any significant emission peaks, indicating that they had a lower rate of recombination of photogenerated electron–hole pairs [50]. The reduced PL emission peak of the g-C₃N₄/Ag–TiO₂ composite compared to g-C₃N₄ can be attributed to a lower rate of recombination of photogenerated electron–hole pairs, which may be due to the formation of a heterostructure between g-C₃N₄ and Ag–TiO₂ [42,43].

3.2.3. FTIR Analysis

Figure 4 shows the results of the FTIR analysis conducted to identify the functional groups and chemical structures of all-prepared photocatalysts.

The stretching modes of C–N and C=N heterocycles were observed at 1200–1740 cm⁻¹ in the g-C₃N₄ samples [6,52]. In the g-C₃N₄ samples, peaks observed at around 1458, 1560, and 1630 cm⁻¹ can be attributed to C=N heterocycles, and the stretching mode of C=O was detected at 1740 cm⁻¹. Additionally, the FTIR peaks at 1230, 1319, and 1400 cm⁻¹ represent the aromatic C–N stretching mode, whereas the peaks at 805 and 885 cm⁻¹ indicate the breathing mode of the tri-s-triazine ring units. The presence of broad bands from 3040–3300 cm⁻¹ shows N–H and O–H stretching modes, which can be attributed to absorbed H₂O molecules [6,21,47,53].

The broad peaks appearing at 3300–3000 cm⁻¹ are due to the stretching vibrations of the O–H group on the TiO₂ surface [19,54]. The peak located around 1626 cm⁻¹ is due to bending vibrations resulting from chemically adsorbed H₂O molecules [18]. The stretching vibrations of Ti–O and Ti–O–Ti exhibited by the TiO₂ nanoparticles were observed at around 850 cm⁻¹ [11,19,55]. The peaks at 1216, 1366, 1438, and 1738 cm⁻¹ correspond to the carbonyl (C=O) mode. The peak observed at 1180 cm⁻¹ corresponds to the stretching vibrations of the Ti–OH molecule [13]. In the g-C₃N₄/Ag–TiO₂ sample, characteristic peaks attributed to both TiO₂ and g-C₃N₄ were observed, confirming the presence of both materials in the composite sample [11,21].

3.3. Morphological Analysis

Figure 5 shows the FE-SEM and HR-TEM morphologies of g-C₃N_4, Ag–TiO₂, and g-C₃N₄/Ag–TiO₂ samples.

The SEM micrograph of pristine TiO₂ nanoparticles exhibits irregularly shaped agglomerates, as shown in Figure 5a. The SEM and TEM images of Ag-doped TiO₂ nanoparticles are shown in Figure 5b,e, respectively. In Figure 5b, the SEM image depicts the agglomeration-induced formation of a cluster of Ag–TiO₂ nanoparticles and the homogeneous distribution of spherical Ag–TiO₂ nanoparticles [56,57]. The presence of small Ag⁰ islands on the surface of Ag-doped TiO₂ nanoparticles is clearly observed in the TEM image, as shown in Figure 5e [58]. The SEM image shows significant adhesion between the Ag–TiO₂ nanoparticles and the g-C₃N₄ nanosheet, indicating a strong interface between the two constituents of the composite, as shown in Figure 5c. Notably, the presence of g-C₃N₄ improves the dispersion of TiO₂ nanoparticles by preventing their agglomeration [11].

Based on the TEM image shown in Figure 5d, the g-C₃N₄ nanosheet exhibited a transparent appearance, which can be attributed to its thin and two-dimensional structure. These results also indicate that g-C₃N₄ has a fluffy structure [59].

The TEM image in Figure 5f depicts a strong interfacial region between g-C₃N₄ and Ag–TiO₂. As can be seen in Figure 5f, the layered structure with the lightest color was identified as g-C₃N₄, whereas the stacked structure comprising small particles was inferred to be TiO₂. The black particles situated between TiO₂ and g-C₃N₄ were identified as Ag nanoparticles [38]. The lattice fringe spacing of TiO₂ was 0.377 nm, which corresponds to the (101) crystal plane of TiO₂ [42]. Therefore, it can be concluded that the TEM images of the g-C₃N₄/Ag–TiO₂ nanocomposite represent the microstructure derived from its constituents, such as TiO₂, g-C₃N₄, and Ag nanoparticles, thereby demonstrating the successful formation of the composite. Moreover, the results indicated that the agglomerated Ag–TiO₂ nanoparticles were well dispersed on the porous architecture of g-C₃N₄. In addition, creating a close interfacial contact between g-C₃N₄ and Ag–TiO₂ heterostructures may effectively promote the transfer of photoinduced electrons. This could potentially suppress the rate of charge–carrier recombination and enhance the overall photocatalytic activity [41,42].

3.4. Nitrogen Adsorption–Desorption Analysis

Figure 6 depicts the nitrogen adsorption–desorption isotherms and pore size distribution of the as-synthesized samples.

Based on the N₂ adsorption–desorption isotherms, all synthesized photocatalysts exhibited typical IV isotherms of mesoporous materials with H1, H2, and H3 hysteresis loops for g-C₃N₄, Ag–TiO₂, and g-C₃N₄/Ag–TiO₂, respectively, as shown in Figure 6a [17,21,43]. The specific surface area of Ag–TiO₂ nanoparticles increased slightly because of the incorporation of Ag nanoparticles. However, the pore volume of Ag–TiO₂ is not as high as its BET surface area because the Ag nanoparticles tend to block the pores in TiO₂. Moreover, the surface area of the composite slightly decreased with the coupling of Ag–TiO₂ to g-C₃N₄, potentially owing to the covering and blocking of the surface-active sites of Ag–TiO₂ by g-C₃N₄ [21]. However, the excellent photocatalytic degradation efficiency of the g-C₃N₄/Ag–TiO₂ nanocomposite might not be underestimated owing to the small specific surface area. In addition, the excellent photocatalytic performance of the composite is a consequence of a combination of factors such as the extended visible light absorption range, enhanced crystallite size, and reduced recombination of charge carriers owing to its narrowed bandgap energy [15]. Previous studies have reported that g-C₃N₄/Ag–TiO₂ nanocomposites exhibit excellent photocatalytic performance despite having a smaller surface area than their constituent components [60].

Figure 6b shows the pore size distribution of the as-synthesized samples obtained using the Barrett–Joyner–Halenda method. The average pore sizes of the Ag–TiO₂, g-C₃N₄, and g-C₃N₄/Ag–TiO₂ samples were 6.5, 25.3, and 10.5 nm, with the corresponding total pore volumes of 0.15, 0.52, and 0.06 cm³·g⁻¹, respectively. The g-C₃N₄ nanosheet showed the highest values for both pore size and total pore volume, which are essential factors in enhancing photocatalytic activity. The average crystallite size, energy bandgap, specific BET surface area (S_BET), average pore size, and pore volume of the as-synthesized samples, including Ag–TiO₂, g-C₃N₄, and g-C₃N₄/Ag–TiO₂ nanocomposites, are summarized in

Table 1. Crystallite size, energy bandgap (E_g), specific surface area (S_BET), pore size, and pore volume of the as-synthesized samples.

Photocatalysts	Crystallite Size (nm)	Eg (eV)	SBET (m2·g−1)	Pore Size (nm)	Pore Volume (cm3·g−1)
Ag–TiO2	9.1	2.89	90.8	6.5	0.15
g-C3N4	4.3	2.94	79.5	25.3	0.52
g-C3N4/Ag–TiO2	7.6	2.73	22.5	10.5	0.06

.

3.5. Evaluation of Photocatalytic Activity

Figure 7 shows the photocatalytic degradation efficiency of RhB in the presence of photocatalysts under visible LED light irradiation.

The photolysis experiments were conducted in the absence of as-prepared photocatalysts, and almost no degradation of RhB dye was observed. The adsorption experiments were performed using the as-synthesized samples in the dark for 60 min. The g-C₃N₄/Ag–TiO₂ nanocomposite exhibited an adsorption rate of 10%. In contrast, TiO₂ exhibited the lowest adsorption rate of only 1.15%, whereas Ag–TiO₂ and g-C₃N₄ had adsorption rates of 7% and 3.5%, respectively. The photocatalytic activities of all the prepared photocatalysts were evaluated by studying the photocatalytic decomposition efficiency of RhB dye under visible LED light irradiation. After 180 min of irradiation with visible LED light, g-C₃N₄ and g-C₃N₄/Ag–TiO₂ photocatalysts exhibited remarkably higher photodegradation efficiencies of 94.83% and 98.04%, respectively, whereas pure TiO₂ showed the lowest photodegradation efficiency of 25.9%. The binary Ag–TiO₂ nanoparticles exhibited an intermediate photodegradation efficiency of 56.73%. The rate of degradation of binary Ag–TiO₂ photocatalysts is higher than that of pristine TiO₂ but lower than that of g-C₃N₄ and g-C₃N₄/Ag–TiO₂ nanocomposites [48].

The enhanced photocatalytic activity of the g-C₃N₄/Ag–TiO₂ composite was due to the synergetic effect of the localized surface plasmon resonance (LSPR) impact of Ag and its role as an electron-conduction bridge, as well as the existence of an excellent heterostructure between Ag–TiO₂ and g-C₃N₄. These factors ameliorate the charge carrier separation efficiency and inhibit electron–hole pair recombination, which boosts the overall photocatalytic performance [38]. In terms of photogenerated charge separation, g-C₃N₄ has a more negative conduction band potential than that of TiO₂. Meanwhile, the Ag nanoparticles serve as charge separation centers for photogenerated electrons from the conduction band of TiO₂ owing to their lower Fermi level energy. Consequently, the photogenerated electrons and holes can be easily transported to the adjacent semiconductor surface, thus suppressing the rate of recombination of photoinduced electron–hole pairs, leading to improvements in photocatalytic efficiency [38,42].

Figure 7b shows the linear pseudo-first-order kinetics of the RhB dye degradation. The apparent reaction rate constants (k_app) of TiO₂, Ag–TiO₂, g-C₃N₄, and g-C₃N₄/Ag–TiO₂ were directly calculated by the equation ln(C/C₀) = k_appt, as shown in Figure 7b. The apparent rate constant (k_app) of TiO₂ nanoparticles is 1.66 × 10⁻³ min⁻¹ because the catalytic activity of TiO₂ is lower under visible light. The k_app of Ag–TiO₂ is 4.65 × 10⁻³ min⁻¹, which is mainly due to the plasmon resonance effect of the Ag nanoparticles on the surface of the TiO₂ nanoparticles. The k_app of g-C₃N₄ is 16.46 × 10⁻³ min⁻¹ and that of the g-C₃N₄/Ag–TiO₂ nanocomposite is increased to 21.84 × 10⁻³ min⁻¹. Therefore, the g-C₃N₄/Ag–TiO₂ nanocomposite exhibited a reaction rate constant 13.16, 4.7, and 1.33 times higher than that of TiO₂, Ag–TiO_2, and g-C₃N₄, respectively. The synergistic effect of the heterostructure comprising g-C₃N₄ nanosheets, TiO₂, and Ag nanoparticles leads to enhanced photodegradation efficiency by effectively accelerating the transfer of photogenerated electrons and inhibiting the fast recombination of electron–hole pairs [17]. The photocatalytic degradation efficiency of g-C₃N₄/Ag–TiO₂ nanocomposites was compared with that of various previous studies in

Table 2. Comparison of photocatalytic performance of g-C₃N₄/Ag–TiO₂ nanocomposites for degrading RhB dye obtained from the current study versus previous studies.

Photocatalyst	Catalyst Dosage	Pollutant Concentration	Light Source (λ ≥ 420 nm)	Irradiation Time	Degradation Efficiency	Refs.
g-C3N4/Ag–TiO2	50 mg	RhB, 10 mg/L	300 W Xe lamp	30 min	98.13%	[42]
g-C3N4/Ag–TMCs	20 mg	RhB, 20 mg/L	300 W Xe lamp	15 min	100%	[15]
g-C3N4/Ag–TiO2	50 mg	RhB, 5 mg/L	500 W Xe lamp	105 min	92.7%	[17]
g-C3N4/Ag–TiO2	50 mg	RhB,10 mg/L	300 W Xe lamp	60 min	100%	[43]
g-C3N4/Ag–TiO2	50 mg	RhB, 5 mg/L	300 W Xe lamp	30 min	96%	[41]
Ag/g-C3N4/TiO2	20 mg	RhB, 10 mg/L	300 W Xe lamp	120 min	100%	[38]
g-C3N4/Ag–TiO2	50 mg	RhB, 10 mg/L	50 W LED lamp	180 min	98.04%	This work

. We found that the g-C₃N₄/Ag–TiO₂ nanocomposite fabricated in this study demonstrated excellent photocatalytic activity compared to those in previous works.

3.6. Mechanism of Photocatalytic Degradation

Based on the results and discussions presented, including XRD, UV/vis DRS, and PL analyses, as well as previous reports [11,15,17,38,41], a plausible photodegradation mechanism for ternary g-C₃N₄/Ag–TiO₂ photocatalysts is proposed in Figure 8. The conduction band (E_CB) and valence band (E_VB) potentials relative to the normal hydrogen electrode (NHE) of the samples were determined using the following equations:

$${\mathrm{E}}_{\mathrm{V}\mathrm{B}}=\mathrm{\chi }-{\mathrm{E}}^{\mathrm{e}}+{0.5\mathrm{E}}_{\mathrm{g}}$$

(5)

$${\mathrm{E}}_{\mathrm{C}\mathrm{B}}={\mathrm{E}}_{\mathrm{V}\mathrm{B}}-{\mathrm{E}}_{\mathrm{g}}$$

(6)

where χ is the absolute electronegativity of the semiconductor; TiO₂ = 5.81 eV and g-C₃N₄ = 4.73 eV. The energy of free electrons on the hydrogen scale is denoted as E^e and has a value of 4.5 eV, and E_g is the bandgap energy [42,43]. From UV/vis DRS analysis results, the valence band and conduction band potentials of g-C₃N₄ were 1.7 eV and −1.24 eV, while those of TiO₂ were 2.83 eV and −0.21 eV, respectively.

Figure 8 shows that under visible LED light irradiation, electrons were excited from the valence band to the conduction band (CB) of TiO₂ and g-C₃N₄. Electrons could easily migrate from g-C₃N₄ to TiO₂ because of the more negative conduction band potential of g-C₃N₄ (E_VB = −1.24 eV) than that of TiO₂ (E_VB = −0.21 eV). These electrons combine with the adsorbed O₂ on the TiO₂ surface to form ^•O₂⁻, which is then used for the decolorization of RhB. Similarly, it appears that g-C₃N₄ (+1.7 V vs. NHE) has a lower valence band potential than H₂O/^•OH (+2.38 V vs. NHE), indicating that holes are incapable of oxidizing H₂O into ^•OH radical species; however, they can directly participate in RhB oxidation. Therefore, holes and ^•O₂⁻ radicals may play a major role in the decolorization of RhB into harmless byproducts such as H₂O and CO₂ [17,42,43,61].

Ag deposited on TiO₂ in g-C₃N₄/Ag–TiO₂ nanocomposites acts as a bridge for electron conduction, resulting in improved separation of electron–hole pairs on g-C₃N₄ through the generation of a Schottky barrier between Ag nanoparticles and TiO₂ [17,38]. In addition, due to the surface plasma resonance effect, Ag nanoparticles in heterostructured g-C₃N₄/Ag–TiO₂ nanocomposites can be photoexcited, producing electrons that migrate to the conduction band of TiO₂ [48]. The transfer of excess electrons from the conduction band of TiO₂ to Ag nanoparticles in heterostructured g-C₃N₄/Ag–TiO₂ nanocomposites reduces the recombination of photoinduced charge carriers [15]. The incorporation of Ag nanoparticles, in addition to the excellent heterostructure formation between TiO₂ and g-C₃N_4, results in excellent photocatalytic degradation efficiency of RhB [17,38]. Moreover, the close interfacial connections between Ag–TiO₂ and g-C₃N₄ allow for efficient electron migration and spatial separation of electrons and holes, retarding their recombination rate and thereby improving photoactivity [60].

4. Conclusions

A simple hydrothermal technique followed by calcination was used to fabricate g-C₃N₄/Ag–TiO₂ composites with high photocatalytic performance for RhB dye degradation under visible light irradiation. The heterostructured g-C₃N₄/Ag–TiO₂ composite was synthesized under optimal conditions at 180 °C for 6 h with a 3:1 weight ratio of g-C₃N₄ to Ag–TiO₂. Various analytical tools were employed to characterize the crystal structures, morphologies, microstructures, chemical structures, and optical and physicochemical properties of the as-synthesized samples. The experimental results demonstrate that the heterostructured g-C₃N₄/Ag–TiO₂ composite is outstanding for the decomposition of RhB dye under visible LED light irradiation and exhibits a degradation efficiency of 98.04%. The high photocatalytic activity observed for the g-C₃N₄/Ag–TiO₂ nanocomposite may be due to the synergetic effect of the LSPR effect of Ag and its role as an electron-conduction bridge, as well as the formation of a strong heterostructure between Ag–TiO₂ and g-C₃N₄. These factors significantly enhanced the efficiency of photogenerated charge carrier separation and transfer, improved visible light absorption, increased crystallite size, and inhibited charge carrier recombination, thereby improving the overall photocatalytic performance. Therefore, the present study shows that g-C₃N₄/Ag–TiO₂ nanocomposites possess enormous potential for the effective elimination of hazardous organic pollutants in wastewater.