Optimized Synthesis Strategy of Mxene-Loaded Graphitic Carbon Nitride (g-C₃N₄) for Enhanced Photocatalytic Degradation of Rhodamine B

Abstract

Developing efficient photocatalysts is essential for sustainable wastewater treatment and tackling global water pollution. Graphitic carbon nitride (g-C₃N₄) is a promising material because it is active under visible light and chemically stable. However, its practical application is limited by fast recombination of charge carriers and a low surface area. In this study, we report a simple hydrothermal method to synthesize exfoliated porous g-C₃N₄ (E-PGCN) combined with Ti₃C₂ MXene to form a heterojunction composite that addresses these issues. Various characterization techniques (FTIR, XRD, XPS, SEM, BET) confirmed that adding MXene improves light absorption, increases surface area (53.7 m²/g for the composite versus 21.4 m²/g for bulk g-C₃N₄ (BGCN)), and enhances charge separation at the interface. Under UV-visible light irradiation with Rhodamine B (RhB) as the model pollutant, the E-PGCN/Ti₃C₂ MXene composite containing 3 wt% MXene demonstrated an impressive degradation efficiency of 93.2%. This performance is superior to BGCN (66.6%), E-PGCN (82.5%), and E-PGCN/Ti₃C₂ MXene-5 wt% composites (81%). This is due to the excess Mxene which caused agglomeration and reduced activity. Scavenger studies identified electron radicals as the dominant reactive species, with optimal activity at pH ~4.5. This enhanced performance, 1.4 times greater than BGCN and 1.13 times higher than E-PGCN, is ascribed to the synergistic interplay between the excellent electrical conductivity of MXene and the porous structural features of E-PGCN. This work highlights the importance of morphological engineering and heterojunction design for advancing metal-free photocatalysts, offering a scalable strategy for sustainable water purification.

1. Introduction

Rapid industrialization and scientific advancements have a significant impact on the energy crisis and environmental pollution due to the discharge of various substances, including textile dyes, industrial chemicals, pesticides, herbicides, and byproducts [1]. The textile industry—one of the most significant to humanity—generates enormous volumes of wastewater due to the requirement of 50–100 L of water for the fabrication of 1kg finished textile products [2]. Bangladesh is one of the leading textile manufacturing countries where more than 10% of GDP comes from the textile sector but releases between 10% and 15% of the different dyes without the proper physiochemical treatment [3,4,5]. The textile effluents include a variety of chemicals such as dyes, heavy metals, ammonia (NH₃), sulfide (S²⁻), lead (Pb), caustic soda (NaOH), Glauber salt (Na₂SO₄), oil, and grease causing harm to aquatic life biodiversity and human health [6,7]. Despite being recognized as a substantial driver of economic growth, the textile industry stands as a major source of pollution, necessitating the effective and sustainable waste effluent treatment strategies. In this regard, numerous strategies have been developed to treat wastewater, which involves various chemical, physical, and biological approaches, such as advanced oxidation process (AOP), adsorption, reverse osmosis, ion exchange, and ultrafiltration [8,9]. In recent years, photocatalytic wastewater treatment strategies based on advanced oxidation processes (AOPs) have gained significant attention for environmental remediation, owing to their remarkable efficiency, operational simplicity, cost-effectiveness, and eco-friendly nature [10]. AOPs, particularly photocatalysis, have emerged as a transformative technology for waste water treatment for their ability to efficiently degrade a broad range of organic contaminants through the action of highly reactive radicals [11,12]. This approach holds the potential to directly utilize pure, economical, and indestructible solar energy. However, despite the wide spread research, the industrial adoption of solar-driven photocatalysis remains constrained by three fundamental limitations: (1) the absence of scalable synthesis methods for visible-light-active photocatalysts; (2) insufficient understanding of structure–activity relationships; and (3) unresolved trade-offs between charge separation efficiency and long-term material stability [13,14].

Over the past few decades, extensive research has yielded numerous photocatalysts, yet their practical implementation continues to face threefold limitations: wide band gap restricting visible-light absorption, poor stability under prolonged irradiation, and low quantum efficiency [15]. Recently, semiconductor-based photocatalysts have emerged as promising solutions demonstrating high catalytic activity, high turnover number, strong oxidizing ability, tunable electronic structure along with other desirable qualities [16,17]. Among these photocatalysts, graphitic carbon nitride [g-C₃N₄ (GCN)], a visible light active organic polymer semiconductor photocatalyst has triggered a lot of attention for the degradation of organic contaminants because of its suitable and tunable bandgap (~2.7 eV), strong thermal and chemical stability, low cost, non-toxicity, and for the simple and scalable synthesis via thermal polycondensation from the Earth-abundant precursors (urea/melamine) [18,19].

Despite its advantages, bulk g-C₃N₄ (BGCN) suffers from limitations such as rapid charge carrier recombination due to insufficient delocalized π-electrons and low surface area, which hamper its photocatalytic efficiency [20,21]. Also, its lower sunlight absorbance capabilities reduce its photocatalytic efficiency in pollutant degradation [22]. To overcome these challenges, researchers have pioneered strategic structural modifications such as morphology engineering, stoichiometric optimization, and defect and doping strategy [23,24,25]. Hoang et al. demonstrated that g-C₃N₄ nanosheets achieve 98% rhodamine B degradation under visible light, attributed to quantum confinement effects that extend charge carrier lifetimes to ~35 ns and increase surface area to 180 m²/g [26]. In another study, Dang et al. systematically optimized the melamine-to-ammonium chloride precursor ratio, enhancing photocatalytic activity by nearly seven-fold at the optimal 1:3 ratio due to the improved proton adsorption and interlayer charge transport [15]. Recent studies show nitrogen vacancies and phosphorus doping can redshift the absorption edge to 550 nm while creating mid-gap states that suppress electron-hole recombination [23,27]. These modifications collectively address the trilemma of charge dynamics, surface reactivity, and spectral response that plague pristine g-C₃N₄.

Since their first report by Gogotsi and colleagues in 2011, MXenes—an emerging class of two-dimensional (2D) materials composed of transition-metal carbides, nitrides, and carbonitrides—have rapidly captured the attention of the scientific community due to their unique structural and functional attributes [28]. MXene like Ti₃C₂, has immense potential as a cocatalyst to decorate photocatalysts because of its superior light-harvesting ability, excellent metallic conductivity, hydrophilic nature, large specific surface area, high electrical conductivity, and compositional variety [29,30,31,32]. When hybridized with photocatalysts, Ti₃C₂ improves photogenerated electron transfer and minimizes the quick recombination of charge carriers [33,34]. This improvement arises from its capacity to suppress light scattering while simultaneously facilitating the separation and migration of photogenerated charge carriers [35]. Moreover, when Ti₃C₂ with a low conduction band (CB) and g-C₃N₄ with a high valence band (VB) come into contact, the similar Fermi level facilitates the movement of electrons between them. This promotes the formation of a heterojunction and enhances the separation of photoinduced carriers, resulting in improved photocatalytic performance [36]. Recent studies have demonstrated that MXene/g-C₃N₄ composites exhibit exceptional synergistic effects, significantly boosting photocatalytic degradation performance through enhanced charge separation and interfacial reactivity. Nasri et al. [37] reported improved methylene blue degradation using MXene-loaded g-C₃N₄ leveraging conductive network to mitigate charge recombination. Bijari et al. [38] observed a twofold enhancement in Eosin Y dye degradation efficiency for the Mxene/g-C₃N₄ composite. Liu et al. [39] designed a 3D/2D g-C₃N₄/Ti₃C₂ heterojunction, resulting in notable enhancements in dye degradation (percentage) and hydrogen production attributed to effective charge separation. These findings collectively underscore MXene’s dual function as both an electron sink and a cocatalyst, offering a versatile strategy to overcome g-C₃N₄’s intrinsic limitations in photocatalysis.

This study advances the field of photocatalytic materials by addressing three critical limitations of conventional BGCN—limited light absorption, low surface area, and rapid charge recombination—through the rational design of exfoliated porous g-C₃N₄ nanosheets (E-PGCN) integrated with Mxene i.e., E-PGCN/Ti₃C₂ Mxene heterostructures. While prior studies mention hydrothermal synthesis for g-C₃N₄/Ti₃C₂ Mxene composites [40,41,42], this work provides the systematic evidence of its efficacy in fabricating optimized E-PGCN/Ti₃C₂ Mxene heterostructures, bridging a critical knowledge gap. Facile hydrothermal synthesis of E-PGCN/Ti₃C₂ MXene heterostructures provides a practical and scalable strategy for boosting photocatalytic performance toward Rhodamine B (RhB) degradation under visible-light irradiation. Effects of pH have been studied to accurately determine the conditions for the maximum photocatalytic activity as it plays a crucial role in determining degradation efficiency. Moreover, photocatalytic mechanism has been elucidated by scavenger studies as graphitic carbon nitride (g-C₃N₄) photocatalysis proceeds through radical-mediated pathways. The results offer a useful approach for designing high-performance g-C₃N₄-based photocatalysts with potential applications in wastewater treatment and solar energy conversion.

2. Materials and Methods

2.1. Materials

The materials used for the fabrication and characterization are Analytical grade (AG) reagents, including Melamine (C₃H₆N₆) powder (Merck KGaA, Darmstadt, Germany), Ammonium chloride (Research-Lab, Maharashtra, India), Titanium Carbide (Ti₃C₂) powder (Chemazone Inc., Leduc, AB, Canada), Rhodamine B (Loba Chemie Pvt. Ltd., Maharashtra 400005, India), Ethylenediamine tetra-acetic Acid (EDTA), Benzoquinone, Isopropanol, Potassium dichromate and other required chemicals, were obtained from commercial suppliers and utilized as received without additional purification. Deionized (DI) water was employed in the preparation of all aqueous solutions.

2.2. Synthesis of Photocatalysts

2.2.1. Synthesis of BGCN

Bulk g-C₃N₄ (BGCN) was obtained through the thermal polycondensation of melamine at elevated temperatures. In a typical synthesis, 10 g of melamine powder was subjected to calcination at 550 °C for 4 h in ambient air, with the temperature ramped at a rate of 5 °C min⁻¹. Following the thermal treatment, the material was allowed to cool naturally to room temperature, after which the resulting yellow solid was finely ground into powder form. The schematic representation of the synthesis route is provided in Figure 1a.

Figure 1. Schematic presentation for the preparation of (a) BGCN, and (b) E-PGCN.

2.2.2. Synthesis of E-PGCN

The preparation of exfoliated porous g-C₃N₄ nanosheets (E-PGCN) was accomplished using a sequential two-step synthesis approach. At first, 4.0 g melamine and 16 g NH₄Cl (1:4 ratio) were mixed and ground for 15 min and then calcined at 550 °C (2 °C/min ramp) for 4 h to produce yellow porous PGCN powder. In order to fabricate an exfoliated structure with improved porosity and photocatalytic properties, PGCN was thermally exfoliated by reheating at 550 °C (5 °C/min) for 2 h in a covered crucible to prepare the final E-PGCN material. Figure 1b illustrates the synthetic route for E-PGCN.

2.2.3. Synthesis of E-PGCN/Ti₃C₂ Mxene Heterojunction

The synthesis of g-C₃N₄/Ti₃C₂ Mxene heterostructures followed an optimized hydrothermal method as previously reported [43,44], and the method is systematically implemented through four consecutive steps (Figure 2).

Figure 2. Schematic flow diagram illustrating the synthesis of E-PGCN/Ti₃C₂ Mxene heterojunction.

Step 1: Preparation of E-PGCN Suspension (Solution 1)

500 mg of pre-synthesized porous g-C₃N₄ (E-PGCN) powder was suspended in 90 mL of ethanol to prepare Solution-1. The suspension was stirred for 30 min, and thereafter subjected to ultrasonic dispersion for an additional 30 min, and this procedure was reiterated 2 to 3 times to ensure uniform dispersion.

Step 2: Preparation of Ti₃C₂ Mxene Suspension (Solution-2)

At first, as purchased Mxene powder was soaked with NH₃ gas obtained from the evaporation of NH₃ solution for 30 min and then heated at 350 °C for 2 h. Eom et al. [45], reported that annealing at high temperatures (usually 400–800 degree C) using applied NH₃ gas can help to achieve surface nitridation of Ti₃C₂T_x MXene. 15 mg (for 3 wt%) or 25 mg (for 5 wt%) of pretreated Mxene was dispersed in 10 mL of ethanol to obtain Ti₃C₂ Mxene solution (Solution-2). Dispersion process was accomplished by, firstly, stirring for 30 min and then bath sonication for 1 h. This process was repeated 2 to 3 times for uniform dispersion. Due to the NH₃ annealing surface nitrogen content of Mxene was effectively improved from 1.51% to 3.43% (Figure 3a,b). FTIR spectra of Mxene before and after NH₃ annealing (Figure 3c) further verifies the surface nitridation through the presence of N-H peak at 3210 cm⁻¹.

Figure 3. EDX analysis of Mxene to show the effect of NH₃ annealing where (a) Mxene powder before NH₃ annealing and (b) Mxene powder after NH₃ annealing. (c) FTIR spectra of Mxene powder before and after NH₃ annealing.

Step 3: Fabrication of E-PGCN/Ti₃C₂ Mxene Heterojunction

Solution-2 was slowly introduced into Solution-1 under continuous vigorous stirring to enhance interfacial interactions. Following one hour of agitation, the mixture was subjected to bath sonication for an additional hour to ensure uniform dispersion. The homogenized solution was then transferred into a Teflon-lined autoclave and thermally treated at 180 °C for 2 h under controlled conditions.

Step 4: Drying and Crushing

The resulting E-PGCN/Ti₃C₂ Mxene heterojunction was allowed to cool naturally. Then, it was centrifuged and washed with ethanol followed by overnight oven drying at 60 °C. The dried pallets were pulverized into a fine powder, resulting in the final g-C₃N₄/Ti₃C₂ Mxene heterojunction composite. As-synthesized heterostructure powder were designated as E-PGCN/MXene-3 wt% and E-PGCN/Ti₃C₂ Mxene-5 wt%, reflecting 15 mg (3 wt%) and 25 mg (5 wt%) MXene loadings, respectively.

2.3. Characterization

The morphologies and structural features of the synthesized materials were examined using field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectroscopy (EDS) mapping on a Gemini Sigma 300 (ZEISS, Bengaluru, India). Structural and compositional modifications of the materials were further analyzed via Fourier transform infrared spectroscopy (FTIR) using a Spectrum Two FT-IR Spectrometer (PerkinElmer, Shelton, CT, USA) and X-ray photoelectron spectroscopy (XPS), while their crystalline structures were characterized by X-ray diffraction (XRD). To determine the crystalline structure of the as-synthesized materials XRD analysis was conducted on Powder X-Ray Diffractometer, manufacturer: GNR Analytical Instruments Group, Agrate Conturbia, Italy; Model: Explorer; X-Ray Source: Cu K_α, 1.4506 A; X-Ray energy: 40 kV, 30 mA using zero background sample holder in the range of 2θ ~ 5–60 degree. The absorbance of the photocatalytic solutions was measured by UV-vis absorbance spectra analysis. The optical band gap of the synthesized photocatalysts was estimated using UV–visible diffuse reflectance spectroscopy (UV–DRS). Additionally, the specific surface area and porosity were assessed through Brunauer–Emmett–Teller (BET) analysis, performed on a BET Sorptometer: model BET-201-A (PMI, Ithaca, NY, USA) with nitrogen as the adsorbate gas.

2.4. Photocatalytic Tests

The photocatalytic performance of the as-synthesized catalysts (BGCN, E-PGCN, E-PGCN/Ti₃C₂ Mxene-3 wt%, and E-PGCN/Ti₃C₂ Mxene-5 wt%) were determined by testing their ability to degrade rhodamine B (RhB) dye, a model pollutant, in water under UV-visible light. Photocatalytic tests were performed using an irradiation setup consisting of five Philips TL 20W/12 RS SLV/25 lamps (20 W each, Zhuhai, China), emitting across the UV-visible spectrum. A photograph image of the complete experimental apparatus is provided in Figure 4 confirming UV-visible light. For each test, 300 mg of catalyst was added to 50 mL of 20 ppm RhB solution while being constantly agitated through stirring. At first, as-prepared RhB solution with the catalysts were stirred for 20 min in the dark to minimize the effect of external factors such as interference from ambient light and to establish adsorption equilibrium. The contribution due to adsorption of dyes by the photocatalysts was quantified. After the dark period, the stable mixture was exposed to UV-visible light for photocatalytic reactions while maintaining constant magnetic stirring. During the photocatalytic process, 1 mL of solution at intervals of 0, 15, 30, 60, 90, 150, and 165 min were taken from the reaction system which was centrifuged to extract the photocatalyst particles for analysis. This systematic approach allowed us to accurately compare the degradation efficiency of each catalyst under identical conditions while accounting for both adsorption and photocatalytic effects. The residual concentration of the RhB solution was then determined by analyzing the characteristic absorption peak obtained from the UV-visible spectrometer. The relative concentration (C/C₀) of the RhB solution was calculated at 554 nm (λ_max = 554). The ultimate degradation efficiency of Rhodamine B (RhB) was determined using the following equation:

$$\mathrm{Efficiency}=\left\{{\displaystyle \frac{{\mathrm{C}}_0-\mathrm{C}}{{\mathrm{C}}_0}}\right\}\times 100$$

(1)

where C₀ and C are the concentrations of RhB at the beginning of light irradiation and at time t, correspondingly.

Figure 4. A photograph image of the experimental setup used for the photocatalytic activity tests.

To evaluate the influence of pH on the photocatalytic degradation efficiency, four aqueous solutions of Rhodamine B (RhB) were prepared by dissolving 1 mg of the dye in 50 mL of deionized water. The pH of each solution was then adjusted to 4.5, 5.0, 7.0, and 12.0 using hydrochloric acid and ammonium hydroxide solutions. pH measurements were conducted using a calibrated Hanna HI-98107 pH meter. To assess the influence of MXene content on catalytic performance, two different catalyst loadings—300 mg of E-PGCN/MXene-3% and 300 mg of E-PGCN/MXene-5%—were separately introduced into each pH-adjusted solution. The reaction mixtures were then subjected to UV irradiation in a photocatalytic chamber under continuous stirring to ensure homogeneous dispersion of the catalyst and consistent exposure to light. Samples were collected from each reaction vessel at 60, 120, and 180-min intervals to monitor the progression of photocatalytic degradation over time. The collected samples were centrifuged to eliminate residual catalyst particles. The resulting supernatants were then analyzed using a UV-Vis spectrophotometer by measuring absorbance at the characteristic wavelength of Rhodamine B (approximately 554 nm). This analysis enabled the systematic evaluation of both pH effects and MXene loading influence on RhB degradation efficiency and kinetics under UV light irradiation.

The reusability of E-PGCN/MXene-3 wt% was also evaluated in three consecutive reaction cycles. In these experiments, the catalyst was refined from the solution by centrifugation for 1.5 h using DI water and dried at 100 °C for 30 min before reusing in a consecutive cycle. All the measurements were repeated for three times to ensure the reliability of the data.

3. Results and Discussion

Systematic processes have been used for successful synthesis of photocatalysts. BGCN was prepared by a single step thermal polycondensation of melamine powder at 550 °C for 4 h with a controlled heating rate. However, E-PGCN was synthesized via a two-step process involving co-calcination of melamine and NH₄Cl to create porous g-C₃N₄, followed by thermal exfoliation at 550 °C to enhance porosity and surface area. The E-PGCN/Ti₃C₂ MXene heterojunction composites were synthesized via a hydrothermal approach, wherein uniformly dispersed suspensions of E-PGCN and Ti₃C₂ MXene were combined, sonicated, and subsequently subjected to hydrothermal treatment at 180 °C for 2 h. The final composites were washed, dried, and pulverized, with MXene content controlled at 3 wt% and 5 wt% to optimize photocatalytic performance. As-synthesized catalysts were characterized for the photocatalytic application.

3.1. Structural Characterization of As-Synthesized Photocatalysts

The functional groups, chemical bonding, and detailed structure of the BGCN, E-PGCN, E-PGCN/Ti₃C₂ Mxene-3 wt%, and E-PGCN/Ti₃C₂ Mxene-5 wt% were investigated by FT-IR analysis and displayed in Figure 5a. In general, there is no significant difference among the characteristic FT-IR spectra of the as-synthesized photocatalysts. The FT-IR spectrum of g-C₃N₄ exhibits characteristic peaks corresponding to its polymeric heptazine ring structure [46,47]. A broad absorption peak between 3500 and 3000 cm⁻¹ is associated with the O-H and N-H stretching vibration modes indicating incomplete condensation during synthesis [48]. The fingerprint region between 1200 and 1700 cm⁻¹ exhibits multiple peaks corresponding to the C(sp³)–N and C(sp²) = N bonds within the CN heterocyclic structures [49,50]. A distinct peak observed near 810 cm⁻¹ corresponds to the breathing vibration of tri-s-triazine rings, thereby confirming the characteristic g-C₃N₄ framework [51]. These features collectively validate the presence of a polymeric carbon nitride structure with residual amines and π-conjugated systems. The interfacial interactions between g-C₃N₄ and MXene due to the coexistence of Ti₃C₂ and g-C₃N₄ was demonstrated by the distinctive peaks found in the FT-IR spectra of E-PGCN/Ti₃C₂ Mxene. With only minor variations in intensity, the spectral patterns of g-C₃N₄ and E-PGCN/Ti₃C₂ Mxene were almost similar. The tri-s-triazine breathing mode (805 cm⁻¹) persists confirming retained g-C₃N₄ structure after Ti₃C₂ Mxene integration [52].

Figure 5. (a) FTIR spectrum of the BGCN, E-PGCN, E-PGCN/Ti₃C₂ Mxene 3 wt%, and E-PGCN/Ti₃C₂ Mxene-5 wt% captured in pure forms, (b) XRD patterns of Ti₃C₂ Mxene and E-PGCN.

3.2. Crystallinity and Phase Identification

X-ray diffraction (XRD) is a powerful tool for probing the crystalline structure, phase composition, and interlayer spacing of materials, offering critical insights into their functional properties. The X-ray diffraction (XRD) patterns of graphitic carbon nitride (g-C₃N₄) and Ti₃C₂ MXene (Figure 5b) reveal their characteristic layered structures. For g-C₃N₄, diffraction peaks observed at 13.1° (100) and 27.4° (002) are consistent with JCPDS card No. 87-1526 [53], corresponding to the in-plane tri-s-triazine motifs and interlayer stacking, respectively. Specifically, the (100) reflection signifies the planar arrangement of tri-s-triazine units, while the (002) peak indicates π–π stacking between adjacent layers, confirming the graphitic nature of the material [50]. Similarly, MXene’s dominant (002) peak at 7–9° (JCPDS No. 52-0875) reflects its expanded interlayer spacing [50]. Notably, the composite retains sharp, narrow peaks matching these reference patterns, confirming high crystallinity and phase purity. This crystallographic integrity facilitates efficient electron transfer between layers, as evidenced by enhanced photocatalytic activity [52,54]. The (004) peak verifies the periodic organization of Ti₃C₂ layers, whereas the (110) peak denotes the in-plane crystallographic configuration of Ti and C atoms [52].

The XRD analysis of the E-PGCN/MXene-3 wt% composite reveals critical structural modifications that drive enhanced photocatalytic performance. The broadening and increased intensity of g-C₃N₄’s (002) peak at 27.2° indicates partial exfoliation and disrupted interlayer stacking, likely caused by MXene’s interaction, which creates more active sites for charge transfer. Similarly, the significant broadening of the (100) peak at 13.1° suggests the formation of an amorphous interface between g-C₃N₄ and MXene, arising from nanoconfinement effects where g-C₃N₄ layers intercalate into MXene’s expanded interlayer spacing. These structural changes—reduced crystallinity, smaller grain size, and interfacial strain—are directly linked to synergistic interactions in the heterostructure. MXene’s conductive layers facilitate electron transport, while the amorphous interface minimizes recombination losses. Together, these modifications enhance light absorption, charge separation, and photocatalytic efficiency, demonstrating how controlled structural engineering optimizes material performance for environmental and energy applications.

3.3. Morphology and Elemental Analysis

The surface morphology and elemental composition of the synthesized photocatalysts were thoroughly examined using scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) and the results are presented in Figure 6. The results revealed key differences in particle size, shape, porosity, and material composition that are essential for understanding photocatalytic behavior. The SEM image of BGCN (Figure 6a) displays a crumpled, three-dimensional structure, characterized by densely packed and aggregated particles. This morphology suggests a relatively high surface area but limited layer separation, which could hinder electron mobility across the material. In contrast, the E-PGCN sample (Figure 6b) presents a more exfoliated, two-dimensional, flake-like morphology with rough, jagged edges. The high surface roughness and the evidence of layer delamination point to a successful exfoliation process, which enhances surface exposure—an essential trait for catalytic activity. Further exfoliation through heat etching is evident in Figure 6c, where E-PGCN exhibits an ultra-thin, transparent, and glossy sheet-like structure. This morphology indicates a highly separated 2D architecture, ideal for maximizing active sites and facilitating charge separation. Such features are consistent with previously reported morphologies for exfoliated g-C₃N₄ materials [55,56]. The SEM image of Ti₃C₂ MXene (Figure 6d) shows its typical accordion-like, multilayered structure—an architecture known for high conductivity and interlayer ion transport, making it a strong candidate for composite materials. In addition, there are some particle which are mainly oxides of Ti. Finally, the hybrid E-PGCN/Ti₃C₂ MXene-3 wt% composite (Figure 6e) combines the best features of both components. The image reveals an integrated morphology, where wrinkled E-PGCN nanosheets are interspersed with well-ordered MXene layers. This synergistic structure supports efficient charge transfer pathways and enhanced photocatalytic performance.

Figure 6. FESEM images of (a) BGCN, (b) E-PGCN, (c) E-PGCN after heat etching, (d) Ti₃C₂ Mxene, (e) E-PGCN/Ti₃C₂ Mxene-3 wt%, and (f) E-PGCN/Ti₃C₂ Mxene-5 wt% sample, and corresponding EDS spectra of (g) BGCN, (h) E-PGCN, (i) E-PGCN/Ti₃C₂ Mxene-3 wt%, and (j) E-PGCN/Ti₃C₂ Mxene-5 wt%.

In the E-PGCN/Ti₃C₂ MXene composites, the introduction of Ti₃C₂ effectively inhibited the aggregation of g-C₃N₄, leading to a well-dispersed, porous architecture with enhanced surface roughness and an expanded reactive surface area. This structural modification increases interlayer spacing and generates additional channels, thereby promoting light scattering and improving the accessibility of pollutants for photocatalytic reactions [52,57]. This enhances the material’s ability to capture light and absorb contaminants, making it more effective in photocatalysis.

The EDS elemental mapping (Figure 6g–j) further confirms the successful combination of Ti₃C₂ with g-C₃N₄. The BGCN and E-PGCN samples (Figure 6g,h) contain C, N, and O, while the E-PGCN/Ti₃C₂ Mxene composites (Figure 6i,j) contain C, N, O, and Ti, showing a uniform distribution of constituent elements. These results confirm that Ti₃C₂ is evenly spread across the g-C₃N₄ surface, forming an intimate contact between the two materials.

3.4. Elemental Composition and Surface Chemical States

X-ray photoelectron spectroscopy (XPS) was employed to investigate the elemental composition and surface characteristics of the porous g-C₃N₄ nanosheets (E-PGCN). Figure 7 presents the XPS analysis, including the full survey scan and high-resolution spectra for carbon (C 1s), nitrogen (N 1s), and oxygen (O 1s). The survey scan (Figure 7a) confirmed the presence of C, N, and O, consistent with prior EDS findings, thereby verifying the material’s compositional purity. The C 1s spectrum (Figure 7b) was deconvoluted into three distinct peaks at approximately 285.3, 283.2, and 282.04 eV, which correspond to the graphitic sites within the g-C₃N₄ framework: sp²-hybridized carbon atoms bonded to nitrogen in the aromatic N–C=N structure, sp² C=C bonds, and C–NH₂ functionalities, respectively [50,58]. The N 1s spectrum (Figure 7c) exhibited four peaks at 395.75, 397.01, 398.4, and 401.4 eV, which were assigned to the sp²-hybridized C=N–C bonds within the triazine rings, N–(C)₃ bonds, C–NH_x groups, and charge polarization effects, respectively [26,50]. Furthermore, the O 1s spectrum (Figure 7d) displayed binding energy peaks at 531.03 eV and 529.55 eV, which correspond to C–OH groups from adsorbed water molecules and O-N bonds from surface-attached oxygen species [26,59]. These findings confirm the successful synthesis of E-PGCN with a stable carbon-nitrogen framework and surface-active sites, critical for boosting photocatalytic performance. The precise alignment of these bonds with g-C₃N₄’s structure underscores its potential as a robust, efficient material for applications like environmental cleanup or energy conversion.

Figure 7. (a) XPS spectra: full scale, (b) C 1s spectrum, (c) N 1s spectrum (C), and (d) O 1s spectrum of E-PGCN sample.

3.5. Surface Area and Porosity Analysis

Brunauer–Emmett–Teller (BET) analysis was performed to evaluate the porosity and the surface area of the synthesized exfoliated porous g-C₃N₄ (E-PGCN) and its MXene composite (E-PGCN/Ti₃C₂ MXene), with results summarized in Table 1. Figure 8 presents a Type IV N₂ adsorption–desorption isotherm with a pronounced H1-type hysteresis loop, characteristic of uniform mesoporous materials undergoing capillary condensation following IUPAC classification [60]. The plateau at intermediate relative pressures and the well-defined hysteresis confirms a significantly mesoporous structure (Figure 8a–d). The pore size distribution curves (Figure 8e–h) confirmed a hierarchical pore network with the pore sizes range 2 to 50 nm. From Figure 8i, t-plot analysis reveals a linear relationship through the origin, without positive intercept—indicating negligible micropore volume and confirming that porosity is dominated by mesopores rather than micropores [61,62]. BET theory applications require only low-pressure data (P/P₀ < 0.3). Thus, the presence of hysteresis artifacts at high P/P₀ do not invalidate surface area calculations if linear transforms remain robust [62]. From Figure 8a–d, it obvious that the adsorption and desorption branches remain distinct. The slight apparent overlap at P/P₀ ≈ 0.60 (Figure 8a) results from minor shifts in liquid nitrogen levels—not branch crossing. No systematic irregularities are observed within the BET linear range. The BET surface area was calculated over the linear P/P₀ range of 0.04–0.30 (monolayer region), yielding excellent linearity. Consequently, any peripheral irregularities at higher P/P₀ do not impact the derived surface area or pore volume. Figure 8a–d show N₂ isotherms with apparent non-closure due to the equipment constraints. This is due to the lowest target pressure by the equipment which are 30.408 mmHg (P/P₀ ≈ 0.04) for adsorption and 2.22 mm Hg (P/P₀ ≈ 0.0029) for desorption. The quantitative results presented in Table 1 have been established by considering the precision of BET system for the several textural parameters assessed. The reported values of surface area. pore volume, and an average pore size are established based on the specifications of PMI BET 201 A sorptometer. BET surface areas for the BGCN, E-PGCN, E-PGCN/Ti₃C₂ Mxene-3 wt%, and E-PGCN/Ti₃C₂ Mxene-5 wt% samples are 21.3, 47.9, 56.5, and 51.6 m²/g, respectively. Through the formation of the pores and nanostructures, the BET surface area of BGCN was significantly enhanced to 47.9 m²/g for E-PGCN. The heterostructure formation between E-PGCN and Ti₃C₂ Mxene further increased the specific surface area of E-PGCN to 56.5 m²/g for E-PGCN/Ti3C2 Mxene-3 wt%. The BET analysis revealed that E-PGCN/Ti₃C₂ Mxene-3 wt% possesses the highest specific surface area. The BET results also clarified that the specific surface area of E-PGCN/Ti₃C₂ Mxene-5 wt% decreases to 51.6 m²/g due to overloaded Mxene leading to agglomeration.

Table 1. Brunauer–Emmett–Teller (BET) surface areas, total pore volumes, and mean pore diameters of the synthesized catalysts.

Sample	BET Surface Area (m2/g)	Total Pore Volume (cm3/g)	Average Pore Size (nm)
BGCN	21.3	0.11	20.0
E-PGCN	47.9	0.13	10.6
E-PGCN/Ti3C2 Mxene-3 wt%	56.5	0.15	10.6
E-PGCN/Ti3C2 Mxene-5 wt%	51.6	0.14	10.8

Figure 8. Full-range N₂ adsorption–desorption isotherm measured at 77 K for (a) BGCN, (b) E-PGCN, (c) E-PGCN/Ti₃C₂ Mxene-3 wt%, and (d) E-PGCN/Ti₃C₂ Mxene-5 wt% showing a Type IV BJH isotherm with an H₁ hysteresis loop indicative of mesoporosity and the corresponding pore size distribution curves of (e) BGCN, (f) E-PGCN, (g) E-PGCN/Ti₃C₂ Mxene-3 wt%, and (h) E-PGCN/Ti₃C₂ Mxene-5 wt%. The minor apparent overlap near P/P₀ ≈ 0.60 (BGCN) results from liquid N₂ level variation within the dewar and does not reflect experimental error as it was solved instantly leading to stable result after that. And (i) shows the t-plot method for the micropore volume analysis.

Similarly, the pore volumes of BGCN, E-PGCN, E-PGCN/Ti3C2 Mxene-3 wt%, and E-PGCN/Ti₃C₂ Mxene-5 wt% are 0.1067, 0.1264, 0.1499, and 0.1391 cm³/g, respectively. E-PGCN/Ti₃C₂ Mxene-3 wt% again owned the highest pore volume. Large BET surface area and pore volume are desirable because they increase the photocatalytic degradation activity of the photocatalyst. This is explained by the fact that more pollutants can be adsorbed due to the large surface area of the designed heterostructure photocatalyst. As a result, the photocatalytic degradation performance of the E-PGCN/Ti₃C₂ Mxene-3 wt% sample will be superior.

3.6. Optical Properties

The photocatalytic efficiency of semiconductor materials is fundamentally governed by their light absorption capabilities and charge carrier dynamics. Figure 9 presents a comprehensive optical characterization of the synthesized photocatalysts, providing critical insights into their light-harvesting capabilities, charge carrier dynamics, and electronic band structures, which are fundamental to their photocatalytic performance. Figure 9a displays the UV-visible absorption spectrum of the exfoliated and porous graphitic carbon nitride (E-PGCN). The spectrum exhibits a pronounced absorption edge in the visible-light region, centered around 420 nm, which is a characteristic feature of g-C₃N₄ materials. This peak typically appears in the range of 400–420 nm, depending on the degree of exfoliation and the porosity of the structure [63,64]. This enhanced visible light absorption can be attributed to the successful introduction of nitrogen vacancies and a porous morphology during the synthesis process. These defects create mid-gap states below the conduction band minimum, effectively narrowing the bandgap and allowing for the excitation of electrons with lower energy photons [65]. Figure 9b shows the photoluminescence (PL) emission spectrum of the E-PGCN/Ti₃C₂ MXene-3 wt% composite, recorded under an excitation wavelength corresponding to its absorption maximum (550 nm). The PL spectrum is dominated by a strong, broad emission peak centered at approximately 550 nm. This peak is characteristic of the band-to-band recombination of photogenerated electrons and holes in carbon nitride-based materials, where the emitted photon energy corresponds to the material’s bandgap [63,66]. The PL peak of E-PGCN/MXene 3% shows the highest intensity peak between 500 and 600 nm, strongly proving the visible range light absorption ability of E-PGCN-MXene 3 wt%. Crucially, the position of this emission peak (~550 nm/2.25 eV) is red-shifted compared to the absorption onset of pure E-PGCN in Figure 9a (~420 nm/2.95 eV). This Stokes shift is a common phenomenon in semiconductors due to energy relaxation processes between absorption and emission events [67]. For photocatalysis, the intense emission indicates significant charge carrier recombination. However, the key finding is that this recombination occurs at a lower energy (longer wavelength), which is a direct spectroscopic consequence of the bandgap narrowing observed in the Tauc plots (Figure 9c–f). This confirms that the composite system is fundamentally tuned to utilize visible light more effectively than its constituents alone.

Figure 9. Optical characterization of the synthesized photocatalysts where dashed lines indicate the linear extrapolation of the absorption edge used to determine the optical bandgap (Eg) by intersecting the photon energy axis. (a) UV–visible absorption spectrum of E-PGCN; (b) photoluminescence (PL) emission spectrum of E-PGCN/Ti₃C₂ Mxene-3 wt%; (c–f) Tauc plots used to estimate the indirect band gap energies of the as-prepared photocatalysts based on UV–visible diffuse reflectance spectroscopy (UV-DRS): (c) BGCN, (d) E-PGCN, (e) E-PGCN/Ti₃C₂ Mxene-3 wt%, and (f) E-PGCN/Ti₃C₂ Mxene-5 wt%.

Figure 9c–f present the Tauc plots derived from UV-DRS data to quantify the optical bandgap energies (E_g) of all samples, assuming an indirect transition mechanism typical for g-C₃N₃ [68]. In the Tauc plots (Figure 9c–f), the dashed lines are the linear extrapolation of the curve (αhν)^1/2 versus photon energy (hv) that was used to estimate the optical bandgap energy (Eg) of each sample. The straight part of the absorption edge was overlaid with a dashed line and then extrapolated to the point where the photon energy axis. The point of intersection of this dashed line with x-axis is the optical bandgap energy (Eg) of the substance. The results obtained from the Tauc plot analysis are shown in Table 2 and reveals the modifications of the electronic structure of E-PGCN is induced by MXene incorporation. The indirect bandgap energy decreases progressively from BGCN (2.68 eV) to E-PGCN (2.20 eV) and further to the Ti₃C₂ Mxene composites (1.92 eV for 5 wt%, 1.91 eV for 3 wt%). This systematic decrease confirms a strong electronic interaction between E-PGCN and the MXene nanosheets. MXene, with its metallic conductivity, likely forms interfacial Schottky junctions or introduces new energy levels, leading to a further tailoring of the band structure and a significant enhancement of visible-light absorption. However, the marginal difference between 3 wt% and 5 wt% composites suggests that excessive MXene loading (5 wt%) may lead to: (a) shielding of active sites, (b) light scattering effects, and/or (c) formation of recombination centers.

Table 2. Indirect band gap energies of the prepared catalyst materials.

Material	Band Energy, Eg (eV)
BGCN	2.68
E-PGCN	2.20
E-PGCN/Ti3C2 Mxene-3 wt%	1.91
E-PGCN/Ti3C2 Mxene-5 wt%	1.92

3.7. Photocatalytic Performance

The photocatalytic performance of as-synthesized BGCN, E-PGCN, and E-PGCN/MXene composites was systematically evaluated for Rhodamine B (RhB) degradation under both dark and light conditions. To distinguish between adsorption and photocatalytic effects, the process was conducted in two stages: (1) a 20-min dark adsorption phase to assess physical adsorption, followed by (2) 165 min of light irradiation to evaluate photocatalytic activity. Degradation efficiency was quantified using Equation (1) by monitoring absorbance changes at RhB’s characteristic wavelength (554 nm).

Figure 10 presents the adsorption test results, revealing significant differences in dye removal capacity among the catalysts. The adsorption performance correlated strongly with the specific surface area of each material (Table 1), where samples with larger surface areas demonstrated markedly higher adsorption capacities. This relationship stems from the increased availability of active sites for dye molecule attachment in high-surface-area materials [69]. Notably, the E-PGCN/MXene composites exhibited superior adsorption behavior, particularly at higher MXene loadings. This enhancement can be attributed to two key factors: (1) the intrinsic high surface area of MXene contributing additional adsorption sites, and (2) synergistic effects between MXene and the E-PGCN matrix that improve both physical adsorption and catalytic activity. The combined results demonstrate that while surface adsorption plays a crucial initial role in dye removal, the subsequent photocatalytic degradation under light irradiation completes the pollutant elimination process. This dual mechanism highlights the advantage of the MXene-incorporated composites over pure carbon nitride materials.

Figure 10. Adsorption contribution to the degradation process for the catalysts: BGCN, E-PGCN, E-PGCN/Ti₃C₂ MXene-3 wt%, and E-PGCN/Ti₃C₂ MXene-5 wt%.

Figure 11 provides a comprehensive evaluation of the photocatalytic degradation of Rhodamine B (RhB) under visible-light irradiation using different photocatalyst systems: BGCN, E-PGCN, E-PGCN/Ti₃C₂ MXene-3 wt%, and E-PGCN/Ti₃C₂ MXene-5 wt%. Figure 11a shows the UV–Vis absorption spectra of RhB after 165 min of photocatalytic treatment. All samples exhibit the characteristic absorption peak of RhB at 554 nm, attributed to the π–π* electronic transition, which serves as a direct optical indicator of dye degradation. Among the tested photocatalysts, BGCN retains the highest absorbance intensity (>0.4 a.u.), indicating poor degradation efficiency due to its limited photocatalytic activity. This limitation is mainly caused by inefficient charge carrier separation and the high recombination rate of photogenerated electron–hole pairs inherent to pristine g-C₃N₄. By contrast, E-PGCN and E-PGCN/Ti₃C₂ MXene-5 wt% exhibit intermediate residual intensities (0.25–0.35 a.u.), reflecting moderate photocatalytic improvement. Notably, E-PGCN/Ti₃C₂ MXene-3 wt% achieves the lowest absorbance (~0.2 a.u.), signifying the most effective degradation of RhB. This superior activity can be attributed to the optimal incorporation of Ti₃C₂ MXene, which forms a heterojunction that: (i) enhances charge separation via Schottky barrier formation, (ii) increases the availability of active sites, and (iii) minimizes recombination losses by promoting efficient interfacial electron transfer.

Figure 11. (a) UV-visible absorption spectra of Rhodamine B (RhB) after degradation, (b) Time profiles of the degradation of RhB (for triplicate), (c) rate constant for RhB degradation under visible light irradiation, and (d) photocatalytic efficiency of BGCN, E-PGCN, E-PGCN/Ti₃C₂ Mxene-3 wt%, as well as E-PGCN/Ti₃C₂ Mxene-5 wt%.

The time-dependent degradation profiles of RhB are presented in Figure 11b, where the relative concentration (C/C₀) at λmax = 554 nm decreases progressively with irradiation time for all catalysts. Kinetic analysis (Figure 11c) reveals that the degradation follows pseudo-first-order kinetics, with the photocatalytic efficiencies ranked as:

E-PGCN/Ti₃C₂ MXene-3 wt% > E-PGCN > E-PGCN/Ti₃C₂ MXene-5 wt% > BGCN

As shown in Figure 9c,d, E-PGCN/Ti₃C₂ MXene-3 wt% delivers the highest performance, achieving ~93.2% degradation with a rate constant of 0.0145 min⁻¹. In comparison, E-PGCN (~82.5%, k = 0.0095 min⁻¹), E-PGCN/Ti₃C₂ MXene-5 wt% (~81.0%, k = 0.0090 min⁻¹), and BGCN (~66.6%, k = 0.0060 min⁻¹) demonstrate progressively lower activity. The enhanced efficiency of E-PGCN/Ti₃C₂ MXene-3 wt% is linked to its larger BET surface area, higher pore volume, and superior charge carrier separation capability, all of which facilitate the generation of reactive oxygen species (ROS) responsible for RhB mineralization. In addition, the reduced indirect band gap of the E-PGCN/Ti₃C₂ MXene heterostructure (Table 2) allows for extended visible light absorption and greater photocarrier generation, thereby accelerating the degradation process. SEM microstructural analysis (Figure 6) further supports the formation of strong interfacial interactions between g-C₃N₄ and Ti₃C₂ MXene, which promote efficient charge transfer and suppress recombination, leading to a significant enhancement in photocatalytic activity compared to pristine g-C₃N₄.

Interestingly, the photocatalytic performance decreases when the Ti₃C₂ MXene content increases to 5 wt.%. This decline suggests that an optimal loading threshold exists; beyond this, excessive MXene deposition can block active sites, reduce light penetration, and/or promote MXene self-aggregation. These effects collectively impede charge separation and reduce overall photocatalytic efficiency [37].

3.8. Effect of pH of RhB on Degradation Performance

The photocatalytic degradation of Rhodamine B (RhB) using E-PGCN/Ti₃C₂ MXene composites exhibits strong pH dependence, as demonstrated by systematic investigations across acidic to alkaline conditions (pH 4.5–12.0). From Figure 12 and Figure 13, it is observed that both E-PGCN/Ti₃C₂ MXene-3 wt% and E-PGCN/Ti₃C₂ MXene-5 wt% catalysts achieve maximum degradation efficiencies of 96.06% and 83.64%, respectively, at pH 4.5. This enhanced performance under acidic conditions stems from multiple synergistic factors. The protonation of catalyst surface groups generates positive charges that promote strong electrostatic interactions with cationic RhB molecules, facilitating crucial dye adsorption. The improved adsorption and photocatalytic degradation of RhB in acidic conditions result from several combined effects that overcome any possible repulsion from protonated g-C₃N₄. Although g-C₃N₄ becomes slightly positively charged when protonated, the negatively charged surface groups on MXene attract the positively charged RhB molecules [70]. At the same time, strong π–π interactions between the dye’s aromatic rings and the heptazine units of g-C₃N₄ promote firm dye attachment regardless of charge effects. Furthermore, protonation enhances the composite’s dispersibility, inhibiting aggregation and exposing more active sites [71]. Finally, the close contact at the g-C₃N₄/MXene interface enhances charge separation, leading to faster radical generation and higher photocatalytic degradation efficiency [43]. Simultaneously, the acidic environment favors hydroxyl radical (•OH) generation from water molecules due to the enhanced surface charge interactions between the positively charged dye molecules and the catalyst [72,73].

Figure 12. Influence of different pH values on the photocatalytic degradation of RhB dye using E-PGCN/Ti₃C₂ Mxene-3 wt% catalysts.

Figure 13. Influence of different pH values on the photocatalytic degradation of RhB dye using E-PGCN/Ti₃C₂ Mxene-5 wt% catalysts.

As the pH increases to neutral (7.0), degradation efficiency declines by 4.5–6.3% and it can be attributed to weakened electrostatic interactions between the catalyst and dye molecules, partial •OH scavenging by hydroxide ions, and moderated charge separation efficiency. The most pronounced performance reduction occurs under strongly alkaline conditions (pH 12.0), where efficiency decreases by 17–20% relative to pH 4.5. This substantial decline results from catalyst surface deprotonation leading to dye repulsion, •OH radical neutralization through reaction with excess hydroxide ions, and accelerated charge recombination processes. Additionally, the alkaline environment may induce catalyst surface instability, further compromising performance. These observations confirm that mildly acidic environments offer the most favorable conditions for •OH radical stability and photocatalytic efficiency, while extreme alkalinity inhibits the degradation process, underscoring the critical role of pH in optimizing photocatalytic performance [74]. The performance of composite photocatalysts containing MXene is indeed critically dependent on the stability of MXene under different pH conditions. The oxidation behavior of MXenes, such as Ti₃C₂T_x, is highly sensitive to the pH of the surrounding environment, which directly influences their structural integrity, surface chemistry, and functional properties—key factors determining their efficacy in photocatalytic applications [75,76,77].

In alkaline conditions, MXenes undergo rapid degradation due to the deprotonation of surface -OH groups, forming less stable -O⁻ terminations. This process increases the susceptibility of MXene sheets to oxidation, leading to the formation of metal oxide phases (e.g., TiO₂) and a decline in electrical conductivity [78]. Such degradation diminishes the charge separation and transport efficiency in MXene-based photocatalysts, thereby reducing photocatalytic activity. Conversely, in acidic environments, MXenes exhibit greater stability, as the presence of H₃O⁺ ions mitigates oxidation and preserves the conductive and structural properties of MXene [75,79]. However, excessively acidic conditions can still induce defect formation, which may serve as reactive sites for oxidation if not controlled.

Therefore, the photocatalytic performance of MXene composites is directly linked to the pH-dependent stability of MXene. In stable pH regimes (e.g., mildly acidic), MXene maintains its metallic conductivity and facilitates efficient electron-hole separation, enhancing photocatalytic efficiency. In contrast, under unstable pH conditions (e.g., alkaline), the degradation of MXene into insulating oxides compromises its role as a cocatalyst, leading to reduced performance [78,79]. Thus, optimizing pH conditions or employing protective strategies (e.g., antioxidants, surface termination control) is essential to harness the full potential of MXene in photocatalytic systems.

3.9. Catalytic Mechanism

To realize the dominant reactive species involved in RhB degradation over the E-PGCN/Ti₃C₂ Mxene-3 wt% catalysts, a series of radical scavenging experiments were performed using various scavengers to inhibit specific reactive species. K₂Cr₂O₇, IPA, EDTA, and BQ were used as e⁻, •OH, h⁺, as well as •O₂⁻ scavengers [26,80]. In the experiment, 50 mL of a 20ppm dye solution was taken in several beakers. To each beaker, different additives were introduced separately: 14.3 mg of EDTA, 3 mg of benzoquinone (BQ), 14.8 mg of potassium dichromate (K₂Cr₂O₇), and 3 mL of isopropanol (IPA). One beaker containing only the dye solution was maintained as a control. Subsequently, 300 mg of photocatalyst was added to each beaker to initiate the photocatalytic degradation process. After 180 min of irradiation, the solutions were centrifuged and the supernatants were collected for UV-Vis spectrophotometric analysis. As shown in Figure 14a, the blank experiment (without any scavenger) exhibited a high degradation efficiency of 93.3%. Upon introducing specific scavengers, the efficiency decreased significantly, indicating the active species in the photocatalytic process. The addition of potassium dichromate (K₂Cr₂O₇), an electron scavenger, caused a sharp decline to 69.5%, revealing the prominent role of e⁻. Isopropyl alcohol (IPA), a hydroxyl radical (•OH) scavenger, reduced the efficiency to 84.7%, confirming the minimal involvement of •OH radicals. Ethylenediaminetetraacetic Acid (EDTA), a hole (h⁺) scavenger, lowered the efficiency to 83.8%, suggesting a sensible contribution of photogenerated holes. In contrast, benzoquinone (BQ), a superoxide radical (•O₂⁻) scavenger, had also a minimal impact, with the degradation remaining at 87.9%, indicating a limited role for electrons in the direct degradation pathway. These findings suggest that e⁻ radicals is the primary reactive species driving the photocatalytic degradation of RhB [81]. Additionally, the results also evidenced that the removal of RhB particles accomplished by photocatalytic degradation after the chemical adsorption with the help of the radicals that are traced in scavenger experiments. Based on these results, a plausible photocatalytic mechanism is illustrated in Figure 14b.

Figure 14. (a) Effect of different scavengers on the degradation of RhB, (b) Proposed degradation mechanism of RhB over g-C₃N₄/Ti₃C₂ Mxene photocatalyst under UV-visible light.

The photocatalytic process is initiated when a semiconductor absorbs photons with energies equal to or greater than its band gap energy (Eg). This absorption promotes electrons from the valence band (VB) to the conduction band (CB), generating electron-hole pairs. The resulting hole in the VB acts as a positive charge carrier, while the promoted electron acts as a negative charge carrier in the CB. The energetic positions of the conduction band (E_CB) and valence band (E_VB) can be determined empirically using the following formulae [82,83]:

E_CB = X − E_c − 0.5 (E_g)

(2)

E_VB = E_g + E_CB

(3)

where X is the absolute electronegativity, E_c is the energy of free electrons on the hydrogen scale, and E_g is the band gap energy. Using X = 4.64 eV for g-C₃N₄, E_g = 2.20 eV for E-PGCN [37], the conduction and valence band potentials of g-C₃N₄ were calculated to be −0.96 eV and +1.24 eV, respectively (Figure 14b).

Upon UV-visible light excitation of g-C₃N₄/Ti₃C₂ Mxene, electrons in the valence band (VB) of g-C₃N₄ are promoted to its conduction band (CB), leaving behind holes in the VB. Due to the formation of a Schottky junction at the g-C₃N₄/Ti₃C₂ interface, photoexcited electrons are efficiently transferred from the CB of g-C₃N₄ to the Fermi level of metallic Ti₃C₂, where they participate in redox reactions, such as the generation of reactive oxygen species (•O₂⁻) [84]. Meanwhile, holes (h⁺) in the valence band of g-C₃N₄ react with H₂O, producing hydroxyl radicals (•OH). These reactive species use oxidizing agents to break down contaminants like RhB. Ti₃C₂ MXene acts as an electron sink and suppresses charge recombination, thereby enhancing the overall photocatalytic activity. The electron acceptor Mxene increase the electron generation in g-C₃N₄ under sunlight by reducing bandgap. The electrons in g-C₃N₄ are mainly the dominant species in this dye degradation mechanism. The entire mechanism of photocatalytic degradation of organic pollutants (RhB) under visible light on g-C₃N₄ is shown in Figure 14b and in Equations (4)–(8) [37,85,86].

g-C₃N₄ + hν → e⁻ + h⁺

(4)

h⁺ + H₂O → •OH + H⁺

(5)

g-C₃N₄ (e⁻) + Ti₃C₂ → Ti₃C₂ (e⁻) + g-C₃N₄

(6)

O₂ + Ti₃C₂ (e⁻) → Ti₃C₂ + •O₂⁻

(7)

Organic pollutants (RhB) + •O₂⁻/•OH → CO₂ + H₂O

(8)

3.10. Catalyst Reusability

The recyclability study of E-PGCN/MXene-3 wt% for Rhodamine B degradation (Figure 15) reveals a progressive decline in photocatalytic efficiency across successive cycles, decreasing from 91.9% (Cycle 0) to 68.8% (Cycle 1) and further to 48.4% (Cycle 2), despite thorough post-cycle purification involving DI water washing and 1.5 h of centrifugation. This performance deterioration suggests multiple concurrent degradation mechanisms affecting the catalyst’s structural and electronic properties. The initial high activity in Cycle 0 can be attributed to the pristine catalyst’s optimal characteristics: (1) an intact heterojunction interface between E-PGCN and MXene, (2) abundant accessible active sites, and (3) efficient charge separation enabled by the well-defined energy band structure. However, the 25.1% efficiency drop observed in Cycle 1 primarily stems from surface fouling effects, where strongly adsorbed RhB molecules and degradation intermediates persist on the catalyst surface despite purification. These organic residues, potentially bound through π-π stacking interactions with the graphitic carbon nitride matrix, physically block active sites and may also act as recombination centers for photogenerated charge carriers [87]. Moreover, prolonged exposure to the photocatalytic reaction environment can cause partial oxidation or hydrolysis of MXene, leading to surface terminations that alter its electronic structure and reduce charge transfer efficiency. The further 20.4% efficiency decline in Cycle 2 suggests cumulative damage to the hybrid structure, including: (i) irreversible nanoparticle aggregation reducing surface area, (ii) degradation of the polymeric g-C₃N₄ network affecting light absorption, and (iii) disruption of the heterojunction interface impairing charge separation. These structural changes collectively diminish the catalyst’s ability to generate and utilize reactive oxygen species (•OH, O₂•−), which are essential for dye degradation. These findings highlight critical stability challenges that must be addressed to enable practical application of MXene-based photocatalysts. Future research should focus on: (1) developing protective strategies against MXene oxidation, (2) optimizing surface functionalization to resist fouling, and (3) engineering more robust heterojunction architectures. Such improvements could significantly enhance the operational lifetime and economic viability of these promising photocatalytic systems.

Figure 15. Reusability study of E-PGCN/MXene-3 wt% in RhB dye under solar-light irradiation.

4. Conclusions

This study successfully demonstrates the synthesis of g-C₃N₄/Ti₃C₂ MXene heterostructures via a hydrothermal route, achieving remarkable photocatalytic degradation of Rhodamine B under visible light irradiation. The optimized E-PGCN/Ti₃C₂ MXene-3 wt% composite exhibited the highest efficiency (~93.2%), significantly surpassing bulk g-C₃N₄ (66.6%), exfoliated porous g-C₃N₄ (82.5%), and the overloaded MXene-5 wt% variant (81%). The superior activity of the 3 wt% heterostructure arises from the synergistic effects of broadened visible-light absorption, enhanced specific surface area (53.7 m²/g vs. 21.4 m²/g for bulk g-C₃N₄), and more efficient charge carrier separation. However, excessive MXene loading (5 wt%) led to aggregation, hindering light absorption and active site accessibility, thereby reducing photocatalytic performance. These findings emphasize the importance of optimizing MXene content to achieve maximal photocatalytic efficiency.

Although the BET surface area characterization strongly supports the enhanced porosity of the composites, minor irregularities and loop misalignment were observed at higher relative pressures, attributable to known resolution limitations of the PMI BET 201 A instrument. Nevertheless, the BET analysis was performed within the reliable monolayer adsorption domain (P/P₀ = 0.04–0.30), which exhibited excellent linearity and equilibrium. The obtained Type IV isotherms with H1 hysteresis confirm significant mesoporosity with negligible microporosity, as further verified by a linear t-plot. Thus, the reported BET surface area, pore volume, and pore diameter values are robust, reliable, and valid for high-quality porosity characterization.

Mechanistic investigations using radical scavengers confirmed that superoxide radicals (•O₂⁻) are the dominant reactive oxygen species responsible for RhB degradation, highlighting efficient electron transfer from g-C₃N₄/Ti₃C₂ MXene to adsorbed oxygen. Systematic pH-dependent experiments further revealed pH ~4.5 as the optimum condition, where protonation enhances interfacial charge separation and stabilizes reactive oxygen species, collectively maximizing photocatalytic efficiency.

By establishing a hydrothermal strategy for fabricating g-C₃N₄/MXene heterostructures, this work bridges theoretical insight with practical application, validating Ti₃C₂ MXene as a transformative co-catalyst for visible-light-driven photocatalysis. The scalable synthesis, environmental compatibility, and superior efficiency of the optimized heterostructure provide a blueprint for designing next-generation, metal-free photocatalysts for wastewater treatment and sustainable energy solutions. To advance real-world applicability, future research should extend performance evaluations to diverse organic pollutants, explore long-term stability, and assess the techno-economic feasibility of large-scale implementation.

This study lays a strong foundation for the development of cost-effective, environmentally sustainable photocatalytic technologies, contributing meaningfully to global efforts in environmental remediation and clean energy.