Graphitic Carbon Nitride-Based S-Scheme Heterojunctions: Recent Advances in Photocatalytic Dye Degradation

Abstract

With the rapid advancement of industrialization, dye-containing wastewater has emerged as one of the primary pollution sources in aquatic environments, posing a significant threat to ecosystems and human health. S-scheme heterojunction photocatalysis technology, known for its high efficiency and environmental compatibility, is considered a strategic solution for addressing environmental pollution challenges. In recent years, significant progress has been made in the development of S-scheme heterojunction photocatalysts based on graphitic carbon nitride (g-C₃N₄). However, systematic summaries and in-depth analyses of these advancements remain limited. This study provides a comprehensive review of the research progress of g-C₃N₄-based S-scheme heterojunction systems in the field of photocatalytic dye degradation. It elaborates on the fundamental concepts, operational mechanisms, and representative applications of these systems while exploring the latest advancements in synthesis strategies, catalytic performance optimization, and the underlying mechanisms. Finally, this review discusses the existing challenges and future prospects of g-C₃N₄-based S-scheme heterojunction photocatalytic materials, aiming to offer valuable insights and guidance for further research in this area.

1. Introduction

In the contemporary era of industrialization, the environmental pollution issues arising from agricultural modernization and large-scale industrial expansion have posed significant challenges to ecological security and public health. Research has demonstrated that organic pollutants, including surfactants, pesticide residues, pharmaceutical intermediates, and synthetic dyes discharged during production processes, pose substantial risks to the environment [1]. Notably, synthetic dyes released by industries such as textile printing and dyeing, food processing, and pulp and paper manufacturing have become critical concerns in water environmental governance due to their high chemical stability, toxicity, and low biodegradability [2]. To address the treatment of these pollutants, researchers have explored various methods, including adsorption [3], membrane separation technology [4], microbial degradation [5], electrochemical oxidation [6], and photocatalytic degradation. Among these approaches, semiconductor photocatalytic systems leverage the photoluminescence effect of photosensitive materials (e.g., TiO₂ and g-C₃N₄) to generate reactive oxygen species, such as hydroxyl radicals (•OH) and superoxide radicals (•O₂⁻), achieving pollutant mineralization and energy transformation [7]. These systems exhibit notable advantages, such as mild reaction conditions, no secondary pollution, and solar-driven operation, offering broad prospects for addressing pollution caused by organic dyes [8]. However, most traditional semiconductor photocatalysts suffer from limitations, including the low efficiency of photogenerated carrier separation, a limited surface area, a narrow absorption spectrum, and poor cycling stability, which significantly impede their practical application [9]. For instance, TiO₂ exhibits a wide band gap (approximately 3.2 eV) and low visible light utilization, restricting its photocatalytic activity primarily to the ultraviolet region, which constitutes only about 5% of solar radiation, thereby greatly limiting its effective utilization of sunlight in photocatalysis [10]. Consequently, this has prompted the academic community to focus research efforts on constructing novel photocatalytic systems, particularly catalytic materials with wide spectral response capabilities, which represent a key breakthrough in achieving effective environmental remediation [11]. This review systematically examines the application of novel g-C₃N₄-based S-scheme heterojunction photocatalytic materials in dye degradation, as illustrated in Figure 1, aiming to provide valuable insights and references for the photocatalytic treatment of organic dyes.

2. S-Scheme Heterojunctions: Proposal and Principles

2.1. Proposal of S-Scheme Heterojunctions

Photocatalytic heterojunction systems are functional structures that effectively integrate two or more semiconductor materials (or semiconductor–metal composites) through heterojunction interface engineering. The primary objective is to significantly enhance photocatalyst performance by leveraging the interfacial synergy between different materials. These heterojunctions, constructed based on energy level alignment principles, are widely recognized in academia as a critical strategy for overcoming the application limitations of photocatalytic technology due to their ability to markedly improve carrier separation efficiency and hole transport capability [12]. They represent an effective and promising approach to enhancing the separation efficiency of photogenerated charges and the oxidation capacity of holes. Current research focuses on developing economically viable heterojunction construction strategies with high carrier separation efficiency, and related advancements have become a major research hotspot in materials science [13]. Based on the relative positions of band gaps, photocatalytic heterojunction systems primarily include type-I, type-II, p-n-type, Z-scheme, and S-scheme configurations [14,15,16]. While traditional type-II heterojunctions achieve spatial carrier separation, they result in a significant attenuation of redox potentials. In contrast, the direct Z-scheme system, inspired by natural photosynthesis, constructs a directional charge transfer channel to effectively maintain the strong redox properties of photogenerated holes and electrons [17]. To address the inherent contradiction between traditional type-II and Z-scheme systems in balancing charge separation and redox capabilities, Yu et al. [17] proposed the stepped (S-scheme) heterojunction theory. This configuration optimizes carrier migration pathways and spatially decouples redox sites by precisely regulating the built-in electric field driven by Fermi level differences, demonstrating superior catalytic performance compared to conventional heterojunction systems [17,18,19]. Studies have confirmed that S-scheme heterojunctions represent a significant breakthrough in overcoming the potential loss of type-II systems and the light absorption limitations of Z-scheme systems [20,21,22].

2.2. Basic Principles of S-Scheme Heterojunctions

S-scheme semiconductor heterostructures are specialized energy band systems formed through the interface coupling of two semiconductor materials with distinct bandgap characteristics, exhibiting an asymmetric S-scheme band-bending distribution at the interface. In contrast to the linear energy band alignment of conventional heterojunctions, this structure generates a dual-bending band configuration during Fermi level equilibration, thereby inducing a unique interfacial charge transfer mechanism. Studies have demonstrated [23] that when high-electron-affinity materials are combined with low-bandgap semiconductor heterostructure components, such S-scheme heterojunctions readily form at the interface. The key advantage of this structure lies in its ability to achieve the efficient separation of photogenerated electron–hole pairs via band gradient engineering while optimizing carrier migration pathways to minimize recombination losses. This dynamic characteristic renders it an ideal platform for constructing advanced optoelectronic devices, particularly in applications such as solar cells, photocatalytic reaction systems, and high-sensitivity photodetectors, showcasing significant application potential [24]. Consequently, the S-scheme charge transfer mechanism represents a rational approach to enhancing photocatalytic performance. Typically, an S-scheme heterojunction can be formed by coupling two semiconductors, where one exhibits a lower Fermi level and functions as an oxidative photocatalyst, while the other possesses a higher Fermi level and serves as a reductive photocatalyst. In S-scheme heterojunctions, the built-in electric field between the semiconductors and the band bending at the interface facilitates interfacial charge transfer and enhances photocatalytic activity [25,26,27]. For instance, when RCN (g-C₃N₄) and VO (V₂O₅) are coupled without light exposure, electrons spontaneously transfer from RCN to VO due to the higher Fermi level (EF) of RCN compared to that of VO, as illustrated in Figure 2a [18]. Furthermore, under illumination, the band bending and resultant built-in electric field interact with Coulombic forces, enabling the rapid transfer of excited electrons from the conduction band (CB) of VO across the interface to recombine with holes (h⁺) in the valence band (VB) of RCN. Consequently, electrons accumulate in the CB of RCN, while holes remain in the VB of VO. These retained carriers enable the reduction of O₂ to •O₂⁻ and the oxidation of adsorbed H₂O to •OH, respectively. Ultimately, these highly reactive radicals contribute to the degradation of tetracycline.

In summary, the S-scheme heterojunction system achieves a synergistic enhancement in the separation efficiency of photogenerated charges and redox potentials through a precise band structure design and carrier pathway optimization, significantly improving photocatalytic performance. Therefore, S-scheme heterojunctions represent one of the most promising candidates for photocatalytic applications.

3. g-C₃N₄’s Role in S-Scheme Heterojunctions and Typical Systems

3.1. g-C₃N₄’s Role in S-Scheme Heterojunctions

Graphitic carbon nitride, as a novel non-metallic polymer semiconductor, features unique two-dimensional conjugated structures that confer it with remarkable photoelectrochemical properties, including a wide spectral response (visible light absorption edge at approximately 460 nm), an appropriate bandgap structure (~2.7 eV), a high specific surface area, and excellent chemical/thermal stability. These advantages have positioned g-C₃N₄ as a material of significant interest in environmental catalysis and clean energy applications [30,31]. Studies have demonstrated its substantial potential in photo-driven pollutant mineralization [32] and photoelectrochemical water splitting for hydrogen production [33,34]. However, practical applications of g-C₃N₄ are still constrained by certain limitations that hinder its photocatalytic efficiency, such as rapid carrier recombination and insufficient hole oxidation potential, which lead to sluggish surface reaction kinetics [35]. To address these challenges, heterojunction construction based on band engineering strategies has emerged as an effective approach to overcoming the performance limitations of single-component materials. By precisely aligning the band structure parameters of the semiconductor components, the interfacial carrier transport dynamics can be optimally regulated [36,37]. Of particular significance is the g-C₃N₄-based S-scheme heterojunction system, which innovatively achieves the spatial separation of photogenerated electron–hole pairs while effectively preserving the system’s high redox potential through cooperative regulation of the interfacial Schottky barrier and built-in electric field. This synergy of charge separation and retention significantly enhances the catalytic efficiency of surface active sites, offering a new paradigm for overcoming the performance bottlenecks of traditional photocatalytic materials [38].

3.2. Typical Systems of g-C₃N₄-Based S-Scheme Heterojunctions

In recent years, innovative research on g-C₃N₄-based S-scheme heterojunction photocatalytic systems has been continuously emerging, with improved optimization strategies and more in-depth exploration of their performance mechanisms. For example, Ha et al. [39] prepared MoS₂/g-C₃N₄ heterojunctions via an in situ synthesis method and constructed an S-scheme charge transfer channel through interface engineering strategies. This catalyst demonstrated excellent photocatalytic performance in Rhodamine B (RhB) degradation, attributed to the directional carrier migration mechanism induced by band alignment. Li et al. [40] reported the main characteristics, design concepts, and photocatalytic applications of g-C₃N₄-based S-scheme heterojunction photocatalysts. Li et al. [41] successfully achieved RhB photocatalytic degradation on the surface of a novel S-scheme 2D/2D Bi₂MoO₆/g-C₃N₄ composite by loading Au as an auxiliary catalyst. Regarding wide-spectrum response regulation, Liu et al. [42] confirmed that the CdS/g-C₃N₄ system achieved effective carrier separation through an S-scheme mechanism while simultaneously expanding the spectral response range. Guo et al. [43] successfully fabricated an Ag-doped g-C₃N₄/iron vanadate (FeVO₄) S-type heterojunction. The incorporation of Ag not only strengthened the interfacial electric field within the S-type heterojunction but also modulated the production of active species in the Ag/g-C₃N₄/FeVO₄/H₂O₂/light system. This non-radical photogenerated hole-based mechanism enables highly efficient degradation of tetracycline (TC), with a notable reduction in the toxicity of the degradation intermediates. Le et al. [28] reported the high efficiency of amoxicillin removal by V₂O₅-g-C₃N₄ S-scheme nanocomposites under sunlight, as shown in Figure 2b. Figure 2c illustrates the preparation process of vanadium dioxide/carbon nanotube network-structured nanocomposites using a high-temperature calcination method. Dai et al. [44] reported a g-C₃N₄/ZnFe₂O₄ S-scheme photocatalyst that was able to remove uranium (VI). Chu et al. [45] constructed a two-dimensional CeO₂/g-C₃N₄ heterointerface via a molten salt template method, which exhibited excellent photocatalytic degradation performance for methylene blue (MB) and tetracycline, attributed to the synergistic effect of the S-scheme mechanism and surface oxygen vacancies. Chen et al. [46] further revealed details of the regulation of methyl orange degradation pathways by the interfacial bonding mode in the C-O chemically bridged CeO₂/g-C₃N₄ S-scheme heterojunction system. In the development of new hybrid systems, Chen et al. [47] designed BiOX/GaMOF heterojunctions, enhancing the removal efficiency of ciprofloxacin to 94.2% through an adsorption–photocatalytic coupling mechanism. The performance advantage originated from the synergy between the porous structure characteristics of metal–organic frameworks and the S-scheme charge transfer mechanism.

4. Application of Graphitic Carbon Nitride-Based S-Scheme Heterojunctions in Dye Degradation

4.1. Application of Graphitic Carbon Nitride-Based S-Scheme Heterojunctions in Cationic Dye Degradation

In recent years, g-C₃N₄-based S-scheme heterojunctions have exhibited remarkable advantages in the degradation of cationic dyes, attributed to their unique charge separation mechanism and controllable band structure. In contrast to traditional heterojunctions, the S-scheme system achieves a significant enhancement in photocatalytic performance by effectively suppressing the recombination of photogenerated electron–hole pairs through the interfacial built-in electric field and directional carrier migration pathways while preserving the high redox capabilities of each component. By studying methylene blue, Rhodamine B, crystal violet (CV), malachite green (MG), and other target pollutants as examples, researchers have developed highly efficient and stable photocatalytic systems via strategies such as multi-component composites (e.g., GO/g-C₃N₄/TiO₂), plasmonic resonance effects (e.g., Ag and Co nanoparticles), and three-dimensional porous structure designs. These systems typically achieve degradation efficiencies exceeding 95.0%, with some even attaining complete degradation at 100.0%. Mechanistic investigations have revealed that interface band engineering and structural synergy effects are the primary drivers of performance optimization. Specifically, S-scheme heterojunctions establish directional charge transfer channels by precisely aligning energy level positions, while three-dimensional porous frameworks and defect engineering significantly enhance light-harvesting capabilities and reaction kinetics efficiency. This section provides a systematic review of the latest advancements in g-C₃N₄-based S-scheme heterojunctions for the degradation of cationic dyes, emphasizing interface construction strategies, charge transfer mechanisms, and multi-component synergistic enhancement principles, thereby offering technical insights for the development of advanced environmental remediation materials.

4.1.1. Methylene Blue

Ren et al. [29] innovatively developed a ternary S-scheme heterojunction interface system composed of graphene oxide/graphitic carbon nitride/titanium dioxide through a multi-stage temperature-controlled self-assembly strategy. This study utilized a gradient calcination process to control the interlayer spacing of graphitic carbon nitride and precisely embedded anatase TiO₂ nanocrystals into the two-dimensional heterojunction interface of graphitic carbon nitride and graphene oxide via an in situ crystallization strategy (Figure 2d). High-resolution transmission electron microscopy (HRTEM) characterization (Figure 2e) revealed that the composite material exhibited a typical sandwich structure: the graphitic carbon nitride crystal planes and the amorphous carbon layers of graphene oxide formed interlayer channels through π-π stacking, while the anatase TiO₂ crystal planes were uniformly distributed on the surface of the two-dimensional substrate (Figure 2f). These results confirmed the successful construction of a ternary heterojunction, indicating that the ternary system was not a simple physical mixture. The GO/g-C₃N₄/TiO₂ composite material achieved a degradation rate of 98.8% for methylene blue after 240 min of visible light irradiation, as shown in Table 1. The performance improvement mechanism can be attributed to the synergistic effect of the electron bridging effect of GO and the precise band alignment of the ternary system, endowing it with excellent photocatalytic activity. Sun et al. [48] developed the Ag/AgI/N-CT-10 plasmonic S-scheme heterojunction system through a multi-step coupling synthesis strategy (hydrothermal–high-temperature calcination–in situ photoreduction). This system utilized nitrogen-vacancy-modified g-C₃N₄ hollow microtubes (N-CT-10), regulated by potassium hydroxide, as the carrier and achieved the precise loading of Ag/AgI nanoparticles via surface functionalization/modification (Figure 3a). Scanning electron microscopy (SEM) characterization revealed that the composite material exhibited a typical hollow tubular morphology with a significantly increased surface roughness of the tube wall, attributed to the uniform distribution of Ag/AgI hetero-particles. HRTEM analysis (Figure 3b,c) confirmed the presence of clear lattice fringes at the interface: the g-C₃N₄ (002) plane and the AgI (002) plane formed an interlaced arrangement, while the metallic characteristics of the Ag (110) plane provided a structural basis for the surface plasmon resonance (SPR) effect [49,50,51]. Electron paramagnetic resonance (EPR) spectra (Figure 3d) revealed a significant signal peak at g = 2.005 and, combined with XPS valence-band spectrum shift analysis, confirmed the existence of nitrogen vacancies [52,53]. Under simulated solar light irradiation for 50 min, this catalyst achieved a 96.9% degradation rate for methylene blue. The performance improvement mechanism was attributed to the synergistic effect of the interface charge trap effect induced by nitrogen vacancies and the SPR effect of Ag nanoparticles. Three-dimensional charge density reconstruction analysis based on density functional theory (Figure 3e) demonstrated that the Ag/AgI40%/N-CT-10 system exhibited a significant charge redistribution along the z-axis direction, confirming the formation of a bipolar charge layer structure at the interface. Differential charge density distribution (Figure 3f) quantitatively demonstrated that electrons accumulated on the surfaces of AgI and Ag. This asymmetric charge distribution formed an interface potential gradient, driving photogenerated electrons to migrate along the S-scheme pathway [54]. Therefore, under the premise of following the S-scheme heterojunction charge transfer mechanism, Ag/AgI40%/N-CT-10 exhibited highly efficient carrier separation efficiency, significantly enhancing photocatalytic degradation efficiency [54].

In the development of multi-component composite systems, Leelavathi et al. [27] constructed a g-C₃N₄/Co/ZnO S-scheme heterostructure via an ultrasonic cavitation-assisted sol–gel method. In this system, Co nanoparticles extended the material’s light response range through the plasmon resonance effect. Under visible light irradiation for 120 min, the g-C₃N₄/Co/ZnO photocatalyst achieved a 96.3% degradation rate for MB. G. Venkatesh et al. [55] successfully fabricated perovskite-type CaSnO₃/g-C₃N₄ S-scheme heterojunction nanocomposites via a straightforward solid-state route. The CaSnO₃/g-C₃N₄ photocatalytic material achieved a 95.0% degradation efficiency for MB under UV–visible light irradiation for 120 min. A robust solid–solid contact interface was formed between g-C₃N₄ and CaSnO₃, providing additional transport pathways and effectively enhancing the separation of photogenerated electrons and holes. Patra et al. [56] synthesized a PbTiO₃/g-C₃N₄ S-scheme heterojunction through an ultrasonic-assisted hydrothermal method. The PbTiO₃/g-C₃N₄ photocatalyst achieved a 99.4% degradation rate for MB under simulated solar irradiation for 90 min. This hybrid heterojunction exhibited an S-scheme charge transfer mechanism. The incorporation of PbTiO₃ nanosheets into the g-C₃N₄ matrix generated a strong electric field, leading to superior photocatalytic activity. This enhancement can be attributed to the effective separation of photogenerated charge carriers. Abedini et al. [57] synthesized Sm₂CeMnO₆/g-C₃N₄ composites via a sol–gel method. At pH 10, the Sm₂CeMnO₆/g-C₃N₄ composite achieved a 96.1% degradation rate for MB under visible light (λ > 365 nm) irradiation for 120 min. In the Sm₂CeMnO₆/g-C₃N₄ (20:80) composite, the two n-type semiconductors jointly formed an S-scheme heterojunction with customized valence- and conduction-band positions, demonstrating excellent performance in generating oxidative radicals. Wang et al. [58] developed a tubular carbon nitride (TCN)/titanium dioxide heterojunction photocatalytic system with S-scheme band alignment using a precursor structure reconstruction and stepwise calcination technique. As shown in Figure 3g, the synthesis process consisted of two critical stages: first, hollow tubular g-C₃N₄ substrate materials were prepared through a molecular precursor morphology control strategy [59], followed by the in situ growth of TiO₂ nanocrystals on the TCN surface via a solvothermal method combined with calcination, ultimately achieving the precise construction of the heterointerface. The hierarchical pore topology structure endowed the material with a large specific surface area, significantly increasing the density of exposed active sites. Under visible light irradiation for 60 min, the tubular g-C₃N₄/TiO₂ catalyst achieved a 96.6% degradation efficiency for MB. The performance improvement can be attributed to the enhanced light capture capability of the tubular structure and the improved charge transfer efficiency at the heterointerface. Barzegar et al. [60] fabricated S-scheme heterojunction g-C₃N₄/TiO₂ composite materials via a straightforward sol–gel method. This composite material was employed in a solar parabolic trough reactor (PTC) to achieve the highly efficient degradation of MB. The g-C₃N₄/TiO₂ heterojunction achieved a 94.9% degradation rate for MB under solar irradiation for 80 min. This outstanding performance was attributed to the S-scheme heterojunction: the conduction-band electrons of g-C₃N₄ and the valence-band holes of TiO₂ were directionally migrated through the built-in electric field, which not only preserved the high redox capability but also effectively suppressed carrier recombination, thereby significantly enhancing photocatalytic activity. Riazati et al. [61] successfully developed a g-C₃N₄/CuO hetero-nanocomposite system using an in situ chemical synthesis strategy. The experimental results demonstrated that the composite catalyst loaded with 5 wt% CuO achieved a degradation efficiency of 93.0% for methylene blue after 180 min of visible light irradiation, with its apparent reaction kinetic constant being approximately six times higher than those of single-component g-C₃N₄ and CuO. Through a mechanism analysis, it was revealed that the enhanced photocatalytic performance of this system was ascribed to the built-in electric field formed by the band-bending effect at the heterointerface, which facilitated the formation of an S-scheme carrier transfer path. This transfer mode effectively inhibited the recombination of photogenerated carriers. In further studies, Ajami et al. [62] constructed a magnetically recyclable CoFe₂O₄/g-C₃N₄ heterostructure using a simple in situ chemical deposition method. This composite material exhibited optimal photocatalytic performance under visible light irradiation for 180 min, achieving 100.0% degradation of MB. The degradation pathway of the CoFe₂O₄/g-C₃N₄ nanocomposite followed an S-scheme mechanism. The heterojunction between cobalt ferrite and graphitic carbon nitride enabled improved light absorption, reduced bandgap energy, and enhanced charge carrier separation. Research on the degradation of MB using S-scheme heterojunction photocatalysts based on g-C₃N₄ has been conducted, with relevant studies reported in [27,29,48,55,56,57,58,60,61,62]. Detailed descriptions of these works are provided in Table 1.

Table 1. The photocatalytic degradation performance of g-C₃N₄-based S-scheme heterojunctions for MB.

Photocatalyst	Synthesis Method	Amount of Catalyst	Light Source	Amount of MB	Time (min)	Efficiency (%)	Ref.
GO/g-C3N4/TiO2	Calcination In situ crystallization	10 mg	350 W Xenon lamp, λ ≥ 420 nm	100 mL, 10 mg/L	240	98.8	[29]
Ag/AgI/g-C3N4	Hydrothermal Calcination Photoreduction	/	Simulated sunlight	/	50	97.0	[48]
g-C3N4/Co/ZnO	Ultrasound Sol–gel	20 mg	300 W Xenon lamp, λ ≥ 420 nm	50 mL, 15 ppm	120	96.3	[27]
CaSnO3/g-C3N4	Solid-state route	50 mg	500 W Halogen lamp	1 g/L	120	95.0	[55]
PbTiO3/g-C3N4	Ultrasound Hydrothermal	50 mg	100 W Halogen lamp	50 mL, 30 mg/L	90	99.4	[56]
Sm2CeMnO6/g-C3N4	Sol–gel	150 mg	9 W three LED lamp, λ > 365 nm	100 mL, 8 mg/L	120	96.1	[57]
TCN/TiO2	Precursor reconstruction, Hydrothermal, Calcination	/	Visible	/	60	96.6	[58]
g-C3N4/TiO2	Sol–gel	20 mg	14.4 W/m LED lamp (SMD 5050 Flexible Strips, Ltd. China), λ = 460 nm	10 mg/L	80	94.9	[60]
g-C3N4/CuO	In situ synthesis	200 mg	150 W visible light source (Osram, Munich, Germany)	50 mL, 2 mg/L	180	93.0	[61]
CoFe2O4/g-C3N4	In situ deposition	200 mg/L	150 W visible light source (Osram, Munich, Germany), λ > 420 nm	50 mL, 2 mg/L	180	100.0	[62]

Table 1. The photocatalytic degradation performance of g-C₃N₄-based S-scheme heterojunctions for MB.

Photocatalyst	Synthesis Method	Amount of Catalyst	Light Source	Amount of MB	Time (min)	Efficiency (%)	Ref.
GO/g-C3N4/TiO2	Calcination In situ crystallization	10 mg	350 W Xenon lamp, λ ≥ 420 nm	100 mL, 10 mg/L	240	98.8	[29]
Ag/AgI/g-C3N4	Hydrothermal Calcination Photoreduction	/	Simulated sunlight	/	50	97.0	[48]
g-C3N4/Co/ZnO	Ultrasound Sol–gel	20 mg	300 W Xenon lamp, λ ≥ 420 nm	50 mL, 15 ppm	120	96.3	[27]
CaSnO3/g-C3N4	Solid-state route	50 mg	500 W Halogen lamp	1 g/L	120	95.0	[55]
PbTiO3/g-C3N4	Ultrasound Hydrothermal	50 mg	100 W Halogen lamp	50 mL, 30 mg/L	90	99.4	[56]
Sm2CeMnO6/g-C3N4	Sol–gel	150 mg	9 W three LED lamp, λ > 365 nm	100 mL, 8 mg/L	120	96.1	[57]
TCN/TiO2	Precursor reconstruction, Hydrothermal, Calcination	/	Visible	/	60	96.6	[58]
g-C3N4/TiO2	Sol–gel	20 mg	14.4 W/m LED lamp (SMD 5050 Flexible Strips, Ltd. China), λ = 460 nm	10 mg/L	80	94.9	[60]
g-C3N4/CuO	In situ synthesis	200 mg	150 W visible light source (Osram, Munich, Germany)	50 mL, 2 mg/L	180	93.0	[61]
CoFe2O4/g-C3N4	In situ deposition	200 mg/L	150 W visible light source (Osram, Munich, Germany), λ > 420 nm	50 mL, 2 mg/L	180	100.0	[62]

Figure 3. (a) An SEM image of Ag/AgI40%/N-CT-10; a TEM image (b) and HRTEM image (c) of Ag/AgI40%/N-CT-10. (d) The ESR spectrum of Ag/AgI40%/N-CT-10. (e) The planar average electron density difference of the Ag/AgI 40%/N-CT-10 interlayer; (f) the charge distribution on Ag/AgI 40%/N-CT-10 (the cyan and yellow areas represent the depletion and accumulation of electrons, respectively) [48]. (g) A schematic diagram of the fabrication process of the TCN/TiO₂ heterojunction structure [58].

Figure 3. (a) An SEM image of Ag/AgI40%/N-CT-10; a TEM image (b) and HRTEM image (c) of Ag/AgI40%/N-CT-10. (d) The ESR spectrum of Ag/AgI40%/N-CT-10. (e) The planar average electron density difference of the Ag/AgI 40%/N-CT-10 interlayer; (f) the charge distribution on Ag/AgI 40%/N-CT-10 (the cyan and yellow areas represent the depletion and accumulation of electrons, respectively) [48]. (g) A schematic diagram of the fabrication process of the TCN/TiO₂ heterojunction structure [58].

4.1.2. Rhodamine B

Leelavathi et al. [27] successfully synthesized g-C₃N₄/Co/ZnO S-scheme heterojunction nanocomposites via a simple ultrasonic-assisted sol–gel method. The g-C₃N₄/Co/ZnO nanocomposite achieved a photocatalytic degradation efficiency of 75.1% for RhB under visible light irradiation for 120 min, and the degradation efficiency increased to 91.6% under solar light irradiation for 120 min, as shown in Table 2. In-depth analysis revealed that the local surface plasmon resonance (LSPR) effect of Co nanoparticles significantly enhanced the local electric field and improved the photocatalyst’s responsiveness to sunlight. Sun et al. [48] developed Ag/AgI/g-C₃N₄ S-scheme heterojunction photocatalysts by integrating hydrothermal synthesis, calcination, and photoreduction techniques. The Ag/AgI40%/N-CT-10 photocatalyst exhibited a degradation efficiency of 95.6% for RhB under simulated solar light irradiation for 50 min. The synergistic enhancement of the nitrogen-vacancy effect and the local surface plasmon resonance effect in Ag/AgI40%/N-CT-10 resulted in superior photocatalytic degradation performance. Alsalme et al. [63] fabricated Zr(HPO₄)₂/g-C₃N₄ S-scheme heterojunctions using an ultrasonic chemical coupling method. The Zr(HPO₄)₂/g-C₃N₄ photocatalyst achieved a degradation rate of 98.0% for RhB under simulated natural solar light irradiation at 1000 W power for 3 h. This excellent performance was attributed to the S-scheme charge transfer mechanism. Chopan et al. [64] successfully prepared an S-scheme photocatalytic system of g-C₃N₄/PPy/ZnO through a multi-step process involving hydrothermal synthesis, calcination, and polymerization. The g-C₃N₄/PPy/ZnO photocatalyst achieved a degradation rate of 99.0% for RhB under visible light irradiation for 60 min. The outstanding photocatalytic activity of g-C₃N₄/PPy/ZnO was ascribed to the oxygen vacancies (OVs) on ZnO, promoting the effective separation of photogenerated carriers and enhancing their redox ability through an S-scheme charge transfer pathway during heterojunction formation. Shoaib et al. [65] successfully fabricated a double S-scheme CdS/TiO₂/g-C₃N₄ heterojunction via a hydrothermal method. This material achieved a degradation efficiency of 99.4% for RhB under visible light irradiation for 180 min, significantly surpassing binary and single-component catalysts. The double S-scheme structure enabled directional charge transfer between CdS and g-C₃N₄ through TiO₂ as a charge bridge, preserving the high-conduction-band electrons of CdS (–0.37 eV) and the strong reduction capability of g-C₃N₄, while the high-valence-band holes of TiO₂ (+2.93 eV) enhanced the oxidation reaction efficiency.

Patra et al. [56] synthesized a PbTiO₃/g-C₃N₄ S-scheme heterojunction using an ultrasound-assisted hydrothermal method. The PbTiO₃/g-C₃N₄ photocatalyst achieved a degradation efficiency of 99.8% for RhB under simulated solar radiation for 60 min. Its superior performance was attributed to the S-scheme charge transfer mechanism. Barzegar et al. [60] prepared an S-scheme heterojunction g-C₃N₄/TiO₂ composite material via an ultrasonic method and applied it for continuous RhB degradation in a solar parabolic trough reactor. The g-C₃N₄/TiO₂ composite material achieved a degradation efficiency of 93.1% for RhB under solar irradiation for 80 min. This outstanding performance was ascribed to the S-scheme heterojunction: the conduction-band electrons of g-C₃N₄ and the valence-band holes of TiO₂ were directionally recombined at the heterojunction interface, retaining the strong reduction capability of g-C₃N₄ and the high oxidation ability of TiO₂. Meanwhile, the solar parabolic trough reactor significantly improved light utilization efficiency by focusing solar energy. Alsalme et al. [66] synthesized an S-scheme heterojunction Ag₂CrO₄/g-C₃N₄ composite material through an acoustic chemical method. The 20 wt% Ag₂CrO₄/g-C₃N₄ composite material degraded 96.0% of RhB under sunlight irradiation for 2 h, outperforming pure g-C₃N₄ (60.0%) and Ag₂CrO₄ (33.0%). Radical-trapping experiments revealed that superoxide radicals and holes were the primary active species. The S-scheme heterojunction preserved the high-valence-band holes of Ag₂CrO₄ (+2.21 eV) and the high-conduction-band electrons of g-C₃N₄ (–1.3 eV) through directional charge recombination at the interface, synergistically enhancing the separation efficiency of photogenerated carriers and redox capability. Alsalme et al. [67] successfully synthesized S-scheme AgI/g-C₃N₄ heterojunctions via a sonochemical method. The 10 wt% AgI/g-C₃N₄ composite material achieved degradation efficiencies of 96.0% for RhB and 86.0% for tetracycline under sunlight irradiation for 2 h, significantly outperforming pure g-C₃N₄ (12.0%) and AgI (40.0%). This enhanced performance was attributed to the S-scheme heterojunction, which facilitated directional charge recombination at the interface, preserving the high-valence-band holes of AgI (+2.28 eV) and the high-conduction-band electrons of g-C₃N₄ (–1.2 eV). Rajendran et al. [68] developed S-scheme g-C₃N₄/TiO₂/CuO ternary heterojunctions using a hydrothermal method. The g-C₃N₄/TiO₂-CuO composite material demonstrated a degradation efficiency of 90.3% for RhB under simulated sunlight irradiation for 120 min, surpassing that of pure g-C₃N₄ (31.8%). The superior performance was ascribed to the S-scheme heterojunction: the conduction-band electrons of g-C₃N₄ and the valence-band holes of TiO₂/CuO were directionally recombined at the interface, retaining the strong reduction ability of g-C₃N₄ and the high oxidation ability of CuO/TiO₂ while simultaneously enhancing the light absorption range and charge separation efficiency. Leelavathi et al. [27] fabricated S-scheme g-C₃N₄/Co/ZnO heterojunctions through an ultrasonic-assisted sol–gel method. This material exhibited degradation efficiencies of 96.3%, 74.5%, and 75.1% for MB, crystal violet, and RhB, respectively, under visible light irradiation for 80 min. Under sunlight irradiation, the degradation efficiency of RhB increased to 91.6%. This outstanding performance was attributed to the S-scheme heterojunction: the surface plasmon resonance effect of Co nanoparticles enhanced light absorption, and the conduction-band electrons of g-C₃N₄ and the valence-band holes of ZnO were directionally recombined at the interface, preserving the strong reduction ability of g-C₃N₄ and the high oxidation ability of ZnO while simultaneously improving the charge separation efficiency. Alsulmi et al. [69] successfully fabricated S-scheme CeO₂/g-C₃N₄ heterojunctions using an ultrasonic-assisted sol–gel method. The 15 wt% CeO₂/g-C₃N₄ composite material achieved a degradation efficiency of 98.9% for RhB under natural light irradiation for 2 h, with a rate constant of 0.0312 min⁻¹, which was 16.4 times higher than that of pure g-C₃N₄. This outstanding performance was attributed to the S-scheme heterojunction: the high-valence-band holes of CeO₂ (+2.37 eV) and the high-conduction-band electrons of g-C₃N₄ (–1.2 eV) were directionally recombined at the interface, preserving the strong oxidation–reduction capabilities of both materials. Additionally, the oxygen vacancies in CeO₂ enhanced light absorption and charge separation efficiency. Alsalme et al. [63] synthesized S-scheme Zr(HPO₄)₂/g-C₃N₄ heterojunctions via an ultrasonic chemical method. The 5 wt% Zr(HPO₄)₂/g-C₃N₄ composite material achieved a degradation efficiency of 98% for RhB under sunlight irradiation for 3 h, with a rate constant of 0.048 min⁻¹. This excellent performance was ascribed to the S-scheme heterojunction: the high-valence-band holes of Zr(HPO₄)₂ (+3.0 eV) and the high-conduction-band electrons of g-C₃N₄ (–1.25 eV) were directionally recombined at the interface, preserving the strong oxidation–reduction capabilities of both materials. Furthermore, ultrasonic treatment promoted the uniform dispersion of nanoparticles, enhancing the light absorption and charge separation efficiency. Alsalme et al. [70] compared the preparation of S-scheme SnS₂/g-C₃N₄ heterojunctions using ultrasonic-assisted and physical mixing methods. The 15 wt% SnS₂/g-C₃N₄ composite material prepared using the ultrasonic method achieved a degradation efficiency of 98.0% for RhB under sunlight irradiation for 2 h, significantly surpassing the physical mixing method (55.0%). The S-scheme structure facilitated the directional recombination of the high-valence-band holes of SnS₂ (+2.15 eV) and the high-conduction-band electrons of g-C₃N₄ (–1.2 eV) at the interface, preserving the strong oxidation–reduction capabilities of both materials. Ultrasonic treatment also promoted the uniform dispersion of SnS₂, suppressing charge recombination and enhancing the light absorption efficiency. Alsulmi et al. [71] developed S-scheme Ag₂CO₃/g-C₃N₄ heterojunctions via an ultrasonic chemical method. The 5 wt% Ag₂CO₃/g-C₃N₄ composite material achieved a degradation efficiency of 95.0% for RhB under natural light irradiation for 2 h, with a rate constant of 0.0141 min⁻¹. This superior performance was attributed to the S-scheme heterojunction: the high-valence-band holes of Ag₂CO₃ (+2.55 eV) and the high-conduction-band electrons of g-C₃N₄ (–1.13 eV) were directionally recombined at the interface, preserving the strong oxidation–reduction capabilities of both materials. Ultrasonic treatment further promoted the uniform dispersion of nanoparticles, enhancing the light absorption and charge separation efficiency. Huang et al. [72] successfully fabricated S-scheme HKUST-1 (copper-based MOFs)/g-C₃N₄ heterojunctions via an ultrasonic-assisted method for the degradation of RhB driven by the synergistic effect of visible light and ultrasound irradiation. The 30% HC (HKUST-1/g-C₃N₄) composite achieved a degradation efficiency of 94.4% for RhB after 120 min of visible light irradiation, with a rate constant of 0.0224 min⁻¹. This outstanding performance was attributed to the S-scheme heterojunction: the high-valence-band holes of HKUST-1 (+2.43 eV) and the high-conduction-band electrons of g-C₃N₄ (–1.21 eV) were directionally recombined at the interface, preserving the strong oxidation–reduction capabilities of both materials. Additionally, the piezoelectric polarization effect induced by ultrasound further enhanced the charge separation efficiency. Basely et al. [73] synthesized S-scheme Bi₂S₃/g-C₃N₄ heterojunctions via an ultrasonic chemical method. The Bi₂S₃/g-C₃N₄ composite material achieved a degradation efficiency of 90.0% for RhB under natural light irradiation for 2 h, with a rate constant of 0.0073 min⁻¹. Its excellent performance was ascribed to the S-scheme heterojunction: the high-valence-band holes of Bi₂S₃ (+2.04 eV) and the high-conduction-band electrons of g-C₃N₄ (–1.34 eV) were directionally recombined at the interface, preserving the strong oxidation–reduction capabilities of both materials. Furthermore, ultrasonic treatment promoted the uniform dispersion of nanoparticles, enhancing light absorption and charge separation efficiency. Alsulmi et al. [67] developed S-scheme AgI/g-C₃N₄ heterojunctions via an ultrasonic chemical method. The 10 wt% AgI/g-C₃N₄ composite material achieved degradation efficiencies of 96.0% for RhB and 86.0% for tetracycline under natural light irradiation for 2 h. Its superior performance was attributed to the S-scheme heterojunction: the high-valence-band holes of AgI (+2.28 eV) and the high-conduction-band electrons of g-C₃N₄ (–1.2 eV) were directionally recombined at the interface, preserving the strong oxidation–reduction capabilities of both materials. Ultrasonic treatment also promoted the uniform dispersion of nanoparticles, enhancing light absorption and charge separation efficiency. Elavarasan et al. [74] successfully synthesized a double S-scheme g-C₃N₄/rGO/ZnO-Ag heterojunction via a hydrothermal method. The HRTEM image in Figure 4a reveals that g-C₃N₄ exhibits a sheet-like structure, rGO displays a layered morphology, and ZnO/Ag presents as spherical agglomerates [75,76]. Additionally, some dark spherical particles can be observed, which are uniformly dispersed within the active ZnO-Ag composite material. The photocatalytic degradation performance of the g-C₃N₄/rGO/ZnO-Ag composite material was evaluated under visible light irradiation for 100 min, with RhB and MB dyes in a mixed aqueous solution serving as the target pollutants. The results demonstrated that the degradation efficiencies of the composite material for RhB and MB were 83.4% and 90.0%, respectively. Figure 4b illustrates the degradation of the RhB/MB mixed dye system by the composite material, with the dye concentration changes monitored at 552 nm (characteristic absorption peak of RhB) and 664 nm (characteristic absorption peak of MB) using spectrophotometry. Figure 4c elucidates the S-scheme charge transfer mechanism. Metal silver nanoparticles (AgNPs) significantly enhance the light energy capture efficiency in the visible light spectrum through the surface plasmon resonance effect. This effect not only induces the formation of a high-intensity localized electromagnetic field but also promotes the excitation kinetics of photogenerated e⁻-h⁺ pairs in semiconductor materials [77]. Upon light irradiation, g-C₃N₄ is excited by photons to generate charge carriers. Notably, the plasmonic resonance characteristics of AgNPs optimize the visible light absorption performance, thereby facilitating the efficient transfer of photogenerated electrons to the conduction band of ZnO and effectively suppressing carrier recombination. The spatial charge separation mechanism at the heterojunction interface regulates the dynamic balance of photogenerated e⁻-h⁺ pairs, significantly reducing their recombination probability and enhancing photocatalytic reaction efficiency. Moreover, the introduction of reduced graphene oxide as an electron transport medium creates a rapid charge transfer channel, optimizing the carrier separation pathway and synergistically enhancing photocatalytic activity.

Sharma et al. [78] successfully fabricated S-scheme α-Fe₂O₃/g-C₃N₄/SiO₂ heterojunctions via hydrothermal and calcination methods for the visible-light-assisted photo-Fenton degradation of RhB. Under the conditions of pH = 3, a 0.6 g/L catalyst loading, and 7 × 10⁻⁴ M H₂O₂, the α-Fe₂O₃/g-C₃N₄/SiO₂ photocatalyst achieved a degradation efficiency of 97% for RhB after 120 min of visible light irradiation. The enhanced photocatalytic performance of the Fe₂O₃/g-C₃N₄/SiO₂ S-scheme heterojunction is attributed to the efficient interface charge separation and band-bending structure between g-C₃N₄ and α-Fe₂O₃, facilitated by the supporting role of SiO₂, which significantly improves the separation efficiency of photogenerated carriers. Bui et al. [79] developed an S-scheme heterojunction composed of oxygen-vacancy-deficient SnO₂₋ₓ nanoparticles and g-C₃N₄ via calcination and hydrothermal methods. Activated by potassium peroxymonosulfate (PMS) in solution, the S-scheme structure of SnO₂₋ₓ/g-C₃N₄ achieved a degradation efficiency of 99.8% for RhB after 180 min of visible light irradiation. The S-scheme heterojunction promotes the directional recombination of the high-valence-band holes of SnO₂₋ₓ (+3.78 eV) and the high-conduction-band electrons of g-C₃N₄ (–1.13 eV) through oxygen vacancies, preserving their strong redox capabilities. Meanwhile, the sulfate radicals (SO₄^•⁻) and •OH generated by PMS activation synergistically enhance the degradation efficiency. Mishra et al. [80] prepared S-scheme g-C₃N₄/NiFe₂O₄ heterojunctions via sol–gel combustion combined with calcination for the visible-light-assisted persulfate (PS) degradation of RhB and tetracycline. The g-C₃N₄/NiFe₂O₄ composite achieved degradation efficiencies of 98.6% and 84.3% for RhB and tetracycline, respectively, after 60 min of visible light irradiation, with rate constants of 0.027 min⁻¹ and 0.018 min⁻¹. The S-scheme heterojunction preserves strong redox capabilities by promoting the directional recombination of the conduction-band electrons of g-C₃N₄ (–1.79 eV) and the valence-band holes of NiFe₂O₄ (+2.22 eV). Additionally, the Fe²⁺/Fe³⁺ redox cycle facilitates PS activation, generating SO₄^•⁻ and •OH, thereby enhancing the overall catalytic efficiency. Liang et al. [81] successfully fabricated an S-scheme heterostructure of potassium-doped carbon nitride (KCN) and ZnO in a KCl/LiCl molten salt system via a high-temperature melting method. Transmission electron microscopy (TEM) analysis (Figure 4d) revealed the disordered assembly morphology of nanorods in the composite, confirming that the molten-salt-induced phase transformation facilitated a topological transition from two-dimensional sheets to one-dimensional nanorods. High-resolution lattice imaging (Figure 4e) revealed a crystal plane spacing of 0.284 nm, which perfectly matched the (100) crystal plane of ZnO [82], while periodic stripes with a spacing of 0.312 nm corresponded to the characteristic parameters of the (100) crystal plane of KCN [83]. X-ray diffraction (XRD) patterns further confirmed the presence of the characteristic diffraction peaks of polytriazine imide (PTI), polyheptazine imide (PHI), and ZnO, verifying the successful synthesis of the ternary composite material [84]. This structural configuration provided a basis for the spatial separation of photogenerated carriers, significantly enhancing the catalytic efficiency of the composite system. Nitrogen adsorption–desorption isotherms (Figure 4f) demonstrated that all samples exhibited type IV isotherms accompanied by H3-type hysteresis loops, indicating that the materials possessed typical mesoporous characteristics [85]. Quantitative analysis (Figure 4g) indicated that the specific surface area and pore volume of KCN/ZnO were notably increased compared to those of the individual components. This hierarchical pore structure not only enhanced light harvesting but also significantly increased the density of active surface sites. Additionally, the average pore diameter of the composite material was intermediate between those of the original components, forming a pore size gradient conducive to reactant diffusion. Photocatalytic evaluation (Figure 4h) showed that KCN/ZnO achieved a degradation efficiency of 92.0% for Rhodamine B under full-spectrum irradiation within 120 min, demonstrating a significant synergistic effect compared to the individual effects of KCN (55.1%) and ZnO (37.2%). Kinetic analysis (Figure 4i) revealed that the degradation process followed a pseudo-first-order reaction model, with the apparent rate constant (k value) of the composite material being 5–8 times higher than those of the individual components, directly validating the mechanism of enhanced catalytic activity [86]. The S-scheme carrier transport channel constructed at the heterointerface not only significantly improved the charge separation efficiency but also retained a high redox potential through band alignment. Sun et al. [87] successfully fabricated a three-dimensional porous sulfur-doped graphitic carbon nitride composite TiO₂/SiO₂/PAN aerogel (S-gTAHP) heterostructure via electrospinning and hydrothermal–freeze-drying techniques. Morphological characterization (Figure 5a) revealed that, after 15 h of hydrothermal treatment (S-gTAHP-15h), the aerogel exhibited a highly ordered pore distribution. This optimized multi-level pore framework not only increased the specific surface area but also provided favorable conditions for the exposure of active sites. Microstructural analysis indicated that TiO₂/PAN short fibers formed a three-dimensional skeleton, while sulfur-doped carbon nitride nanosheets were uniformly embedded in the matrix through SiO₂ crosslinking agents. The UV–Vis absorption spectrum (Figure 5b) demonstrated that TAP-15h (TiO₂/SiO₂/PAN aerogel) had a distinct absorption edge at 390 nm, corresponding to a bandgap value of 3.16 eV (Figure 5c). In contrast, the absorption edge of S-gAP-15h (sulfur-doped g-C₃N₄/SiO₂ aerogel) redshifted to 460 nm, with a narrowed band gap of 2.71 eV. Notably, the ternary composite system S-gTAHP exhibited significantly enhanced absorption characteristics in the visible light region, attributed to the regulation of the band structure via the synergistic effect of sulfur doping and the porous network on light harvesting [88]. As the hydrothermal time increased from 9 to 15 h, the absorption edge of the material continuously redshifted, confirming that the gradual improvement in the three-dimensional porous structure enhanced light energy utilization through multiple light scattering mechanisms. The carrier dynamics were investigated using photoluminescence spectra (Figure 5d) and electrochemical testing systems [89]. S-gTAHP-15h exhibited the lowest fluorescence intensity in the PL spectrum, indicating that the close contact at the heterojunction interface effectively suppressed electron–hole recombination. The transient photocurrent response (Figure 5e) demonstrated that this material had excellent photocurrent response reproducibility, with a photocurrent density value significantly superior to that of the comparison samples. Electrochemical impedance spectroscopy (Figure 5f) further confirmed that S-gTAHP-15h had the smallest capacitive arc diameter, indicating the lowest interfacial charge transfer resistance. These characterization results were consistent with the photocatalytic performance data: under simulated solar light irradiation, this material exhibited excellent degradation efficiency for methylene blue (99.4% degradation rate in 15 min), Rhodamine B (96.1% in 30 min), and tetracycline (84.2% in 40 min). The performance improvement mechanism can be summarized as follows: (1) the three-dimensional multi-level pore structure enhances light energy capture by increasing the density of active sites and extending the light propagation pathway; (2) the efficient carrier migration channels established by the S-scheme heterostructure promote charge separation; (3) the sulfur doping strategy effectively expands the light response range of the material. Pitcheri et al. [90] successfully fabricated an S-scheme heterojunction β-Cu₂V₂O₇/Ni/Pg-C₃N₄ composite photocatalytic system by employing a simple co-precipitation process to directionally load β-Cu₂V₂O₇ nanoparticles onto porous graphitic carbon nitride (Pg-C₃N₄) nanosheets, with the interfacial modification effect of nickel nanoparticles. Photocatalytic performance tests revealed that this composite material achieved a removal efficiency of 98.6% for RhB after 60 min of sunlight exposure, with a reaction rate constant (0.039 min⁻¹) that was 4.5 times higher than that of pure β-Cu₂V₂O₇. The performance enhancement mechanism can be attributed to the following synergistic effects: (1) the built-in electric field formed at the interface between Pg-C₃N₄ nanosheets and β-Cu₂V₂O₇ in the S-scheme heterojunction effectively regulates the carrier migration path; (2) the surface plasmon resonance effect of nickel nanoparticles enhances the excitation and separation of photogenerated electron–hole pairs through a localized electromagnetic field; (3) unique heterointerface engineering significantly prolongs the lifetime of photogenerated charges. Madima et al. [91] developed a direct S-scheme TiO₂/g-C₃N₄ heterojunction by simultaneously calcining TiO₂ precursors and g-C₃N₄. TiO₂ was synthesized using guava leaf extract as a reducing agent via a green synthesis method, while g-C₃N₄ was prepared via the thermal decomposition of melamine. This composite material degraded 96.0% of RhB within 120 min and 95.0% of MB within 150 min under simulated sunlight irradiation. The enhanced photocatalytic activity was attributed to the visible light capture characteristics and the S-scheme heterojunction system formed between the two catalysts, which promoted interfacial charge separation efficiency and prolonged the charge carrier lifetime. Khamesan et al. [92] prepared a 2D/2D S-scheme g-C₃N₄/ZnCr-LDH heterojunction by combining hydrothermal and reflux methods for the degradation of RhB and the in situ production of H₂O₂ under xenon lamp irradiation. The g-C₃N₄/ZnCr-LDH composite achieved a degradation efficiency of 99.8% for RhB and produced 4.75 mmol/L of H₂O₂ after 90 min of simulated sunlight exposure. Comprehensive experimental results, bandgap energy analysis, and scavenging tests confirmed the presence of an S-scheme heterojunction in the synthesized g-C₃N₄/ZnCr-LDH composite material. The proposed S-scheme mechanism accelerates the separation, migration, and utilization of photogenerated charge carriers, thereby enhancing its photocatalytic activity. Xie et al. [93] successfully fabricated an S-scheme g-C₃N₄/Bi/BiVO₄ composite photocatalytic system through a synergistic strategy combining chemical deposition and in situ reduction. High-resolution transmission electron microscopy analysis (Figure 5g) revealed that monoclinic-phase BiVO₄ nanosheets (m-BiVO₄) formed a heterojunction with g-C₃N₄ nanosheets via the (001) crystal plane, with their surfaces being uniformly covered by amorphous Bi nanoparticles. The (121) crystal plane diffraction fringes of m-BiVO₄ were resolved. Energy-dispersive X-ray spectroscopy (EDS) elemental mapping (Figure 5h) further confirmed the homogeneous distribution of C, N, Bi, V, and O elements within the composite system, validating the effective integration of material interfaces. Under visible light irradiation for 70 min, the composite catalyst achieved a degradation efficiency of 99.0% for Rhodamine B, with a reaction rate constant of 0.067 min⁻¹, demonstrating significant kinetic advantages. The performance enhancement mechanism can be attributed to (1) the establishment of a directional charge transfer channel by the S-scheme heterojunction through band alignment, preserving the high redox potentials of the charge carriers; (2) the role of Bi nanoparticles as electron relay stations, enhancing interfacial charge vector migration. Research on the degradation of RhB using S-scheme heterojunction photocatalysts based on g-C₃N₄ has been reported in several studies, including [27,48,56,60,63,64,65,66,67,68,69,70,71,72,73,74,78,79,80,81,90,91,92,93]. Detailed information regarding these works is summarized in Table 2.

Table 2. The photocatalytic degradation performance of g-C₃N₄-based S-scheme heterojunctions for RhB.

Photocatalyst	Synthesis Method	Light Source	Amount of RhB	Amount of Catalyst	Time (min)	Efficiency (%)	Ref.
g-C3N4/Co/ZnO	Ultrasound, sol–gel	300 W Xenon lamp, λ > 420 nm	50 mL, 15 ppm	20 mg	120	75.1	[27]
Ag/AgI/g-C3N4	Hydrothermal, calcination, photoreduction	Simulated sunlight	/	/	50	95.6	[48]
Zr(HPO4)2/g-C3N4	Ultrasonic chemical coupling	1000 W sunlight	100 mL, 2×10−5 M	10 mg	180	98.0	[63]
g-C3N4/PPy/ZnO	Hydrothermal, calcination, polymerization	125 W LED lamp, λ ≥ 420 nm	100 mL, 50 mg/L	100 mg	60	99.0	[64]
CdS/TiO2/g-C3N4	Hydrothermal	50 W LED lamp	50 mL, 25 mg/L	10 mg	180	99.4	[65]
PbTiO3/g-C3N4	Ultrasound, hydrothermal	100 W halogen lamp	50 mL, 10 mg/L	50 mg	60	99.8	[56]
g-C3N4/TiO2	Ultrasound	14.4 W/m LED lamp (SMD 5050 Flexible Strips, Ltd. China), λ = 460 nm	15 mg/L	20 mg	80	93.1	[60]
Ag2CrO4/g-C3N4	Ultrasound	300 W solar simulator	100 mL, 2 × 10−5 M	50 mg	120	96.0	[66]
AgI/g-C3N4	Ultrasound	300 W solar simulator	100 mL, 2 × 10−5 M	100 mg	120	96.0	[67]
g-C3N4/TiO2/CuO	Hydrothermal	500 W Xenon lamp	100 mL, 30 ppm	50 mg	120	90.3	[68]
CeO2/g-C3N4	Ultrasound, sol–gel	110 K to 90 K lumen, natural solar	100 mL, 2 × 10−5 M	20 mg	120	98.9	[69]
SnS2/g-C3N4	Ultrasound	1000 W natural solar	100 mL, 2 × 10−5 M	50 mg	120	98.0	[70]
Ag2CO3/g-C3N4	Ultrasound	450 W natural sunlight	100 mL, 10 mg/L	100 mg	120	95.0	[71]
HKUST-1/g-C3N4	Ultrasound	Ultraviolet–visible light source	50 mL, 10 mg/L	50 mg	120	94.4	[72]
Bi2S3/g-C3N4	Ultrasound	500 W natural sunlight	/	/	120	90.0	[73]
g-C3N4/rGO/ZnO-Ag	Hydrothermal	Visible	100 mL, 40 ppm	60 mg	100	83.4	[74]
α-Fe2O3/g-C3N4/SiO2	Hydrothermal, calcination	100 W LED lamp, λ = 420 nm	100 mL, 10 ppm	60 mg	120	97.0	[78]
SnO2₋ₓ/g-C3N4	Calcination, hydrothermal	Visible light source, λ > 420 nm	20 mg/L	0.33 g/L	150	99.8	[79]
g-C3N4/NiFe2O4	Sol–gel combustion, calcination	Sunlight	50 mL, 20 mg/L	20 mg	60	98.6	[80]
K-g-C3N4/ZnO	High-temperature melting	Visible	/	/	120	92.0	[81]
β-Cu2V2O₇/Ni/Pg-C3N4	Co-precipitation	Sunlight	60 mL, 25 ppm	20 mg	60	98.6	[90]
TiO2/g-C3N4	Calcination	300 W Xenon lamp (Asahi HAL-320)	100 mL, 10 mg/L	50 mg	120	96.0	[91]
g-C3N4/ZnCr-LDH	Hydrothermal, reflux	LED lamp	100 mL, 5 ppm	100 mg	90	99.8	[92]
g-C3N4/Bi/BiVO4	Deposition, in situ reduction	350 W Xenon lamp (Changzhou Siyu, China), λ > 420 nm	50 mL, 10 mg/L	50 mg	70	99.0%	[93]

Table 2. The photocatalytic degradation performance of g-C₃N₄-based S-scheme heterojunctions for RhB.

Photocatalyst	Synthesis Method	Light Source	Amount of RhB	Amount of Catalyst	Time (min)	Efficiency (%)	Ref.
g-C3N4/Co/ZnO	Ultrasound, sol–gel	300 W Xenon lamp, λ > 420 nm	50 mL, 15 ppm	20 mg	120	75.1	[27]
Ag/AgI/g-C3N4	Hydrothermal, calcination, photoreduction	Simulated sunlight	/	/	50	95.6	[48]
Zr(HPO4)2/g-C3N4	Ultrasonic chemical coupling	1000 W sunlight	100 mL, 2×10−5 M	10 mg	180	98.0	[63]
g-C3N4/PPy/ZnO	Hydrothermal, calcination, polymerization	125 W LED lamp, λ ≥ 420 nm	100 mL, 50 mg/L	100 mg	60	99.0	[64]
CdS/TiO2/g-C3N4	Hydrothermal	50 W LED lamp	50 mL, 25 mg/L	10 mg	180	99.4	[65]
PbTiO3/g-C3N4	Ultrasound, hydrothermal	100 W halogen lamp	50 mL, 10 mg/L	50 mg	60	99.8	[56]
g-C3N4/TiO2	Ultrasound	14.4 W/m LED lamp (SMD 5050 Flexible Strips, Ltd. China), λ = 460 nm	15 mg/L	20 mg	80	93.1	[60]
Ag2CrO4/g-C3N4	Ultrasound	300 W solar simulator	100 mL, 2 × 10−5 M	50 mg	120	96.0	[66]
AgI/g-C3N4	Ultrasound	300 W solar simulator	100 mL, 2 × 10−5 M	100 mg	120	96.0	[67]
g-C3N4/TiO2/CuO	Hydrothermal	500 W Xenon lamp	100 mL, 30 ppm	50 mg	120	90.3	[68]
CeO2/g-C3N4	Ultrasound, sol–gel	110 K to 90 K lumen, natural solar	100 mL, 2 × 10−5 M	20 mg	120	98.9	[69]
SnS2/g-C3N4	Ultrasound	1000 W natural solar	100 mL, 2 × 10−5 M	50 mg	120	98.0	[70]
Ag2CO3/g-C3N4	Ultrasound	450 W natural sunlight	100 mL, 10 mg/L	100 mg	120	95.0	[71]
HKUST-1/g-C3N4	Ultrasound	Ultraviolet–visible light source	50 mL, 10 mg/L	50 mg	120	94.4	[72]
Bi2S3/g-C3N4	Ultrasound	500 W natural sunlight	/	/	120	90.0	[73]
g-C3N4/rGO/ZnO-Ag	Hydrothermal	Visible	100 mL, 40 ppm	60 mg	100	83.4	[74]
α-Fe2O3/g-C3N4/SiO2	Hydrothermal, calcination	100 W LED lamp, λ = 420 nm	100 mL, 10 ppm	60 mg	120	97.0	[78]
SnO2₋ₓ/g-C3N4	Calcination, hydrothermal	Visible light source, λ > 420 nm	20 mg/L	0.33 g/L	150	99.8	[79]
g-C3N4/NiFe2O4	Sol–gel combustion, calcination	Sunlight	50 mL, 20 mg/L	20 mg	60	98.6	[80]
K-g-C3N4/ZnO	High-temperature melting	Visible	/	/	120	92.0	[81]
β-Cu2V2O₇/Ni/Pg-C3N4	Co-precipitation	Sunlight	60 mL, 25 ppm	20 mg	60	98.6	[90]
TiO2/g-C3N4	Calcination	300 W Xenon lamp (Asahi HAL-320)	100 mL, 10 mg/L	50 mg	120	96.0	[91]
g-C3N4/ZnCr-LDH	Hydrothermal, reflux	LED lamp	100 mL, 5 ppm	100 mg	90	99.8	[92]
g-C3N4/Bi/BiVO4	Deposition, in situ reduction	350 W Xenon lamp (Changzhou Siyu, China), λ > 420 nm	50 mL, 10 mg/L	50 mg	70	99.0%	[93]

Figure 4. (a) An HRTEM image of the GCRZA S-scheme heterojunction photocatalyst; (b) the photocatalytic activity of the GCRZA nanocomposite toward RhB + MB mixed dyes; (c) a schematic diagram of the photocatalytic degradation process of the GCRZA S-scheme heterojunction photocatalyst [74]; (d) TEM images of the KCN/ZnO nanocomposite; (e) XRD patterns, (f) N₂ adsorption–desorption isotherms, and (g) corresponding pore size distributions of the KCN/ZnO nanocomposite, KCN, and ZnO; the photocatalytic degradation performance of the KCN/ZnO nanocomposite, KCN, and ZnO for RhB and (h) the corresponding photocatalytic degradation kinetics (i) [81].

Figure 4. (a) An HRTEM image of the GCRZA S-scheme heterojunction photocatalyst; (b) the photocatalytic activity of the GCRZA nanocomposite toward RhB + MB mixed dyes; (c) a schematic diagram of the photocatalytic degradation process of the GCRZA S-scheme heterojunction photocatalyst [74]; (d) TEM images of the KCN/ZnO nanocomposite; (e) XRD patterns, (f) N₂ adsorption–desorption isotherms, and (g) corresponding pore size distributions of the KCN/ZnO nanocomposite, KCN, and ZnO; the photocatalytic degradation performance of the KCN/ZnO nanocomposite, KCN, and ZnO for RhB and (h) the corresponding photocatalytic degradation kinetics (i) [81].

4.1.3. Other Cationic Dyes

Leelavathi et al. [27] successfully fabricated a ternary S-scheme heterojunction composite system of g-C₃N₄/Co/ZnO via an ultrasonic-assisted sol–gel method. The experimental results demonstrated that the catalyst achieved a removal efficiency of 74.5% for crystal violet under visible light irradiation for 120 min, as shown in Table 3. The study revealed that the plasmonic effect of cobalt nanoparticles effectively promoted the directional migration of carriers and significantly extended the material’s light response range. Sun et al. [48] developed an Ag/AgI/g-C₃N₄ S-scheme heterojunction photocatalyst by integrating hydrothermal synthesis, calcination, and photoreduction techniques. Under simulated sunlight irradiation for 60 min, Ag/AgI40%/N-CT-10 exhibited a degradation efficiency of 100.0% for crystal violet. The enhanced photocatalytic performance was attributed to the synergistic effects of nitrogen vacancies and localized surface plasmon resonance in Ag/AgI40%/N-CT-10. Mallah et al. [94] synthesized Ca@TiO₂@g-C₃N₄ nanocomposites through a thermal polymerization reaction of non-noble-metal components followed by ultrasonic treatment. Under visible light irradiation for 60 min, the degradation of malachite green reached 99.9%. The high activity of this ternary photocatalyst can be ascribed to the formation of an S-scheme heterojunction at the TiO₂@g-C₃N₄ interface, the generation of defects such as Ti³⁺ and oxygen vacancies (Oᵥₛ) due to the interaction between Ca and TiO₂, and the increased specific surface area of the photocatalyst. Madonna et al. [95] constructed a C-CeO₂/g-C₃N₄ stepwise heterojunction via a hydrothermal method. Under sunlight irradiation for 150 min, the C-CeO₂/g-C₃N₄ nanocomposite degraded 91.9% of malachite green. Comprehensive characterization confirmed that the enhanced photocatalytic activity of this material was primarily attributable to the effective establishment of an S-scheme charge transfer mechanism, which provided a unique carrier separation pathway and significantly suppressed electron–hole recombination.

In summary, g-C₃N₄-based S-scheme heterojunctions have exhibited remarkable performance and hold broad application prospects in the degradation of cationic dyes. By employing various advanced preparation techniques (hydrothermal synthesis, calcination, sol–gel processing, electrospinning, etc.), researchers have successfully developed a range of highly efficient S-scheme heterojunction photocatalytic systems. These systems have achieved significant improvements in the separation efficiency of photogenerated electron–hole pairs, expanded the spectral response range, and enhanced the efficiency of reactive oxygen species generation through the optimization of band structure design and the regulation of carrier migration mechanisms. In degradation experiments involving methylene blue, Rhodamine B, and other cationic dyes, g-C₃N₄-based S-scheme heterojunction composite materials have consistently demonstrated superior photocatalytic performance. Furthermore, studies have elucidated the substantial enhancement effects of various synergistic phenomena (oxygen vacancy engineering, sulfur doping, surface plasmon resonance, etc.) on photocatalytic activity. These findings provide novel insights for designing efficient and stable photocatalytic materials.

Figure 5. (a) An SEM image of the S-gTAHP-15 h aerogel photocatalyst; (b) the UV–Vis absorbance spectra of the photocatalyst and (c) Tauc’s plots of TAP-15 h and S-gAP-15 h photocatalysts; (d) the PL spectra, (e) transient I-t curves, and (f) EIS curves and corresponding equivalent circuit diagrams (illustrations) of different types of aerogel-based photocatalysts [87]. (g) An HRTEM image and (h) the corresponding EDS elemental maps of the g-C₃N₄/Bi/BiVO₄ photocatalyst [93]. (i) The rate constants for the photocatalytic degradation of MO by different photocatalysts [96].

Figure 5. (a) An SEM image of the S-gTAHP-15 h aerogel photocatalyst; (b) the UV–Vis absorbance spectra of the photocatalyst and (c) Tauc’s plots of TAP-15 h and S-gAP-15 h photocatalysts; (d) the PL spectra, (e) transient I-t curves, and (f) EIS curves and corresponding equivalent circuit diagrams (illustrations) of different types of aerogel-based photocatalysts [87]. (g) An HRTEM image and (h) the corresponding EDS elemental maps of the g-C₃N₄/Bi/BiVO₄ photocatalyst [93]. (i) The rate constants for the photocatalytic degradation of MO by different photocatalysts [96].

4.2. The Application of S-Scheme Heterojunction g-C₃N₄ Structures in the Degradation of Anionic Dyes

In recent years, g-C₃N₄-based S-scheme heterojunctions have exhibited remarkable potential in the degradation of anionic dyes, attributed to their distinctive charge separation mechanism and band structure regulation characteristics. In contrast to traditional photocatalytic systems, S-scheme heterojunctions enable the highly efficient spatial separation of photogenerated electron–hole pairs via the built-in electric field and band-bending effect at the interface while preserving the robust oxidation–reduction capabilities of each component. This significantly enhances both photocatalytic activity and the spectral response range. In studies focused on the degradation of anionic dyes such as methyl orange (MO), Congo red (CR), indigo carmine (IC), chrome black T (EBT), alizarin red S (ARS), and erythrosine (ER), researchers have successfully developed composite systems with superior photocatalytic efficiency through strategies including multi-component composites, defect engineering, and three-dimensional structural design. This section systematically reviews the latest advancements in S-scheme heterojunction g-C₃N₄ systems for the degradation of anionic dyes, emphasizing the optimization of band structures, interfacial charge transfer mechanisms, and multi-component synergistic effects, thereby providing valuable technical insights for the rational design of environmental remediation materials.

4.2.1. Methyl Orange

Rana et al. [96] innovatively integrated a cork substrate with the ternary components of g-C₃N₄/ZnO/TiO₂ through a co-precipitation method combined with a physical composite process, thereby constructing a multiphase photocatalytic system with S-scheme heterojunction characteristics. Photocatalytic performance tests demonstrated that the g-C₃N₄/ZnO/TiO₂/cork composite material achieved a removal efficiency of 98.3% for methyl orange within 60 min of visible light irradiation, as shown in Table 3. Kinetic analysis (Figure 5i) revealed a significant cascade effect in the photocatalytic activity of each component: g-C₃N₄/ZnO/TiO₂/cork composite system (0.0524 min⁻¹) > g-C₃N₄/ZnO/TiO₂ (0.0258 min⁻¹) > g-C₃N₄/ZnO (0.0155 min⁻¹) > g-C₃N₄ (0.0121 min⁻¹) > ZnO (0.0107 min⁻¹) > TiO₂ (0.00925 min⁻¹); this fully verified the synergistic enhancement effect of the cork substrate and the multi-component heterojunction. Systematic studies of radical scavenging experiments revealed that when Na₂-EDTA, isopropanol (IPA), and benzoquinone (BQ) were added, the degradation efficiency of MO decreased to 61.3%, 24.5%, and 27.9%, respectively, under the same conditions (Figure 6a). This confirmed the dominant role of h⁺, •O₂⁻, and •OH radicals in the photocatalytic process. Mechanism studies indicated that the special band structure of the double S-scheme heterojunction not only effectively suppressed the recombination of photogenerated electron–hole pairs but also promoted the separation of photogenerated carriers via a unique carrier migration pathway. The multi-component synergy effect further broadened the spectral response range. This interface engineering strategy provides a new design concept for the development of highly efficient and stable composite photocatalysts. Gou et al. [97] successfully fabricated a CoTiO₃/g-C₃N₄ heterojunction photocatalytic system by employing an in situ calcination strategy using ZIF-67@TiO₂ and melamine precursors (synthesis process shown in Figure 6b). The CoTiO₃/g-C₃N₄-2 sample achieved a degradation efficiency of 99.7% for methyl orange after 4 h of visible light irradiation (λ > 420 nm). Its superior performance can be attributed to (1) the directional carrier migration mechanism of the S-scheme heterojunction, which retains strong redox capability while expanding the solar spectral response range; (2) the precise matching of the band structure at the heterointerface, which optimizes the separation efficiency of photogenerated charges.

Onwudiwe et al. [98] constructed a ternary S-scheme heterojunction of g-C₃N₄/Bi₂S₃/CuS via a solvothermal method. This system significantly enhanced carrier separation and suppressed recombination through band engineering. When the loading amount of CuS was optimized to 20%, the removal of methyl orange by the composite material increased to 98.0% within 60 min of visible light irradiation. Experimental characterization and theoretical calculations indicated that the built-in electric field within the heterojunction and the multi-component energy level had a synergistic effect, jointly promoting the effective separation of photogenerated electron–hole pairs. Duan et al. [99] successfully developed a CdS/sulfur-modified graphitic carbon nitride (GCNS) S-scheme heterojunction photocatalytic system based on solid-state diffusion. The experimental results demonstrated that, when the mass ratio of CdS to GCNS was 1:2, the composite catalyst achieved a 100.0% degradation efficiency for MO within 60 min of visible light irradiation. The apparent kinetic constant k was enhanced by factors of 9.67 and 5.39 compared to single-component GCNS and CdS, respectively. Through DFT simulations combined with carrier trajectory analysis, the study revealed the physical mechanism underlying directional electron migration at the heterointerface: unidirectional electron transfer from CdS to GCNS was attributed to the built-in electric field formed by band bending at the interface. This one-dimensional electron transport channel not only broadened the visible light absorption range but also significantly improved the generation efficiency of reactive oxygen species (especially •O₂⁻) via the space charge separation effect. Liu et al. [100] innovatively constructed a g-C₃N₄/TiO₂/ZnIn₂S₄ composite photocatalytic system (CTZA) featuring a double S-scheme heterojunction through an isoelectric-point-controlled calcination strategy, using a three-dimensional graphene aerogel as a structural template. As shown in Figure 6c, compared to traditional composites, CTZ components were uniformly distributed on the surface of graphene sheets, resulting in an ordered three-dimensional interconnected network with multi-level pore characteristics. This unique topological structure originated from the directional self-assembly process induced by π-π stacking interactions between reduced graphene oxide (rGO) layers [101]. This mechanism not only preserved the macroscopic integrity of the aerogel but also enabled the precise spatial arrangement of nanoscale components. High-resolution transmission electron microscopy characterization further revealed that a strong interface coupling effect was established between CTZ nano-units and the graphene aerogel substrate. The characteristic crystal plane spacings of 0.325 nm, 0.352 nm, and 0.191 nm observed in Figure 6d corresponded to the (002) lattice of g-C₃N₄, the (101) plane of anatase TiO₂, and the (110) plane of hexagonal ZnIn₂S₄, respectively. These precise lattice parameter matches confirmed the successful construction of the ternary heterojunction at the atomic level. This ordered assembly ensured the structural stability of each component while constructing an efficient interfacial carrier transport pathway. Through electron spin resonance (ESR) spectroscopy analysis (Figure 6e-f), using DMPO as a radical scavenger, the study demonstrated that the CTZA composite material could simultaneously generate •OH and •O₂⁻ under simulated solar light excitation. Experimental data showed that the characteristic signal peaks of DMPO-•OH and DMPO-•O₂⁻ significantly increased under light conditions. Specifically, the •OH generation intensity of the CTZA system was markedly higher than that of the single TiO₂ system, and the •O₂⁻ generation efficiency exceeded those of single-component g-C₃N₄ and ZnIn₂S₄ systems. Coupling theoretical analysis with experimental data further confirmed that the construction of the double S-scheme heterojunction established a directional charge transport channel in the CTZA composite system. This synergistic mechanism enabled the material to achieve a 97.5% degradation efficiency for MO within 30 min of simulated solar light irradiation and a 98.3% reduction rate for Cr(VI) within 70 min, with the catalytic performance exceeding that of the single-component systems by more than two orders of magnitude. Notably, the three-dimensional porous structure of the material not only enhanced the removal of pollutants through physical adsorption but also enhanced the interfacial kinetics of the photocatalytic reaction by shortening carrier migration paths. This adsorption–catalysis synergy provides a new design concept for developing highly efficient environmental remediation materials. This study elucidated the carrier transport mechanism of the double S-scheme heterojunction within the CTZA composite photocatalytic system. As shown in Figure 6g, under photoexcitation conditions, photogenerated electrons in the CB of TiO₂ recombine with holes in the VBs of g-C₃N₄ and ZnIn₂S₄ via interfacial recombination pathways. This dynamic equilibrium maintains the strong oxidation capability of VB holes in TiO₂ while preserving the high reduction activity of CB electrons in g-C₃N₄ and ZnIn₂S₄. rGO, with its extended specific surface area and π-conjugated framework, plays a multifunctional role as an electron transport medium in the system. On the one hand, it acts as an efficient electron acceptor to promote exciton dissociation and directional charge migration [102,103]; on the other hand, it accelerates the interfacial transfer of CB electrons in g-C₃N₄ and ZnIn₂S₄ through its delocalized π-electron network, thereby significantly enhancing the spatial separation efficiency of photogenerated carriers. Both experiments and theoretical calculations confirm that this charge transport mechanism enables accumulated high-concentration electrons and holes to participate in molecular oxygen activation and water molecule oxidation, respectively, generating highly oxidative •OH and •O₂⁻ radicals [104,105]. These reactive oxygen species effectively degrade organic pollutants such as MO into harmless small-molecule products (e.g., H₂O and CO₂). The core advantage lies in the synergistic enhancement of the double S-scheme heterojunction and the three-dimensional aerogel framework—where the former optimizes band structures to suppress carrier recombination and enhance redox capabilities, while the latter provides an ideal microenvironment and mass transfer channels for interfacial reactions.

Figure 6. (a) The influence of different capture agents on the photocatalytic degradation of MO by g-C₃N₄/ZnO/TiO₂/Cork [96]. (b) The preparation process of the CoTiO₃/g-C₃N₄ nanocomposite [97]. (c) An SEM image of the CTZA photocatalyst; (d) an HRTEM image of the CTZA photocatalyst; ESR spectra of (e) DMPO-•O₂⁻ and (f) DMPO-•OH for different photocatalysts; (g) a schematic diagram of the possible photocatalytic mechanism of the CTZA photocatalyst [100].

Figure 6. (a) The influence of different capture agents on the photocatalytic degradation of MO by g-C₃N₄/ZnO/TiO₂/Cork [96]. (b) The preparation process of the CoTiO₃/g-C₃N₄ nanocomposite [97]. (c) An SEM image of the CTZA photocatalyst; (d) an HRTEM image of the CTZA photocatalyst; ESR spectra of (e) DMPO-•O₂⁻ and (f) DMPO-•OH for different photocatalysts; (g) a schematic diagram of the possible photocatalytic mechanism of the CTZA photocatalyst [100].

Table 3. The photocatalytic dye degradation performance of g-C₃N₄-based S-scheme heterojunctions.

Photocatalyst	Synthesis Method	Amount of Catalyst	Light Source	Dye	Amount of Dye	Time (min)	Efficiency (%)	Ref.
g-C3N4/Co/ZnO	Ultrasound, sol–gel	20 mg	300 W Xenon lamp, λ > 420 nm	CV	50 mL, 15 ppm	120	74.5	[27]
Ag/AgI/g-C3N4	Hydrothermal	/	Simulated sunlight	CV	/	60	100.0	[48]
Ca@TiO2@g-C3N4	Thermal polymerization, Ultrasound	50 mg	500W Xenon lamp	MG	100 mL, 30 mg/L	60	99.9	[94]
C-CeO2/g-C3N4	Hydrothermal	50 mg	solar	MG	100 mL, 30 mg/L	150	91.9	[95]
g-C3N4/ZnO/TiO2/Cork	Co-precipitation	60 mg	250 W halogen lamp	MO	100 mL, 1 × 10−5 M	60	98.3	[96]
CoTiO3/g-C3N4	In situ calcination	/	Visible, λ > 420 nm	MO	/	240	99.7	[97]
g-C3N4/Bi2S3/CuS	Hydrothermal	10 mg	Visible	MO	100 mL, 0.1 mg/L	60	98.0	[98]
CdS/GCNS	Solid-state diffusion	/	Visible	MO	/	60	100.0	[99]
CTZA	Isoelectric point calcination	/	Simulated sunlight	MO	/	30	97.5	[100]
g-C3N4/Ag2WO4/Bi2S3	In situ growth	50 mg	140 W LED lamp	CR	250 mL, 20 mg/L	60	98.0	[106]
R.palustris/RCM@CPU	Hydrothermal	/	100 W light	CR	50 mg/L	480	99.5	[107]
V2O5/Ndef-g-C3N4	Controllable pyrolysis	5 mg	18 W LED lamp	IC	25 mL, 20 ppm	35	98.2	[108]
NiMn2O4/g-C3N4	Ultrasonic co-precipitation	50 mg	400 W lamp (Osram, Munich, Germany)	EBT	50 mL, 10 ppm	120	96.4	[109]
MgO-TiO2@g-C3N4	Ultrasound	50 mg	500 W Xenon lamp, λ > 420 nm	ARS	100 mL, 30 mg/L	60	94.0	[110]
CaSnO3/g-C3N4	Ultrasonic co-precipitation	45 mg	400 W lamp (Osram, Munich, Germany)	ER	45 mL, 10 ppm	90	86.2	[111]

Table 3. The photocatalytic dye degradation performance of g-C₃N₄-based S-scheme heterojunctions.

Photocatalyst	Synthesis Method	Amount of Catalyst	Light Source	Dye	Amount of Dye	Time (min)	Efficiency (%)	Ref.
g-C3N4/Co/ZnO	Ultrasound, sol–gel	20 mg	300 W Xenon lamp, λ > 420 nm	CV	50 mL, 15 ppm	120	74.5	[27]
Ag/AgI/g-C3N4	Hydrothermal	/	Simulated sunlight	CV	/	60	100.0	[48]
Ca@TiO2@g-C3N4	Thermal polymerization, Ultrasound	50 mg	500W Xenon lamp	MG	100 mL, 30 mg/L	60	99.9	[94]
C-CeO2/g-C3N4	Hydrothermal	50 mg	solar	MG	100 mL, 30 mg/L	150	91.9	[95]
g-C3N4/ZnO/TiO2/Cork	Co-precipitation	60 mg	250 W halogen lamp	MO	100 mL, 1 × 10−5 M	60	98.3	[96]
CoTiO3/g-C3N4	In situ calcination	/	Visible, λ > 420 nm	MO	/	240	99.7	[97]
g-C3N4/Bi2S3/CuS	Hydrothermal	10 mg	Visible	MO	100 mL, 0.1 mg/L	60	98.0	[98]
CdS/GCNS	Solid-state diffusion	/	Visible	MO	/	60	100.0	[99]
CTZA	Isoelectric point calcination	/	Simulated sunlight	MO	/	30	97.5	[100]
g-C3N4/Ag2WO4/Bi2S3	In situ growth	50 mg	140 W LED lamp	CR	250 mL, 20 mg/L	60	98.0	[106]
R.palustris/RCM@CPU	Hydrothermal	/	100 W light	CR	50 mg/L	480	99.5	[107]
V2O5/Ndef-g-C3N4	Controllable pyrolysis	5 mg	18 W LED lamp	IC	25 mL, 20 ppm	35	98.2	[108]
NiMn2O4/g-C3N4	Ultrasonic co-precipitation	50 mg	400 W lamp (Osram, Munich, Germany)	EBT	50 mL, 10 ppm	120	96.4	[109]
MgO-TiO2@g-C3N4	Ultrasound	50 mg	500 W Xenon lamp, λ > 420 nm	ARS	100 mL, 30 mg/L	60	94.0	[110]
CaSnO3/g-C3N4	Ultrasonic co-precipitation	45 mg	400 W lamp (Osram, Munich, Germany)	ER	45 mL, 10 ppm	90	86.2	[111]

4.2.2. Congo Red

Jabbar et al. [106] successfully fabricated a novel S-scheme g-C₃N₄/Ag₂WO₄/Bi₂S₃ heterojunction via a multi-step synthesis strategy. The ternary photocatalyst achieved 98.0% degradation of Congo red under visible light irradiation within 60 min, as shown in Table 3. Theoretical analysis revealed that the ternary system realized the directional migration and spatial separation of photogenerated electron–hole pairs by constructing dual S-scheme charge transfer channels. Specifically, the conduction-band electrons of Ag₂WO₄ preferentially recombined with the valence-band holes of Bi₂S₃, while a secondary S-scheme transfer pathway was established between g-C₃N₄ and Bi₂S₃. This synergistic mechanism not only significantly suppressed carrier recombination but also preserved the intrinsic redox potential of each component through optimized band alignment. In the field of bio-photocatalytic synergy technology, Liu et al. [107] innovatively integrated an S-scheme rod-shaped g-C₃N₄/MoS₂ heterojunction (RCM), prepared via hydrothermal synthesis, with a metabolically functional bacterial community. The RCM photocatalyst was immobilized onto the surface of a chitosan-modified polyurethane sponge (CPU) through surface modification techniques, while Rhodopseudomonas palustris, a broad-spectrum metabolic bacterium, was encapsulated within the carrier matrix. The resulting composite system (R. palustris/RCM@CPU) achieved a removal efficiency of 99.5% for Congo red under simulated solar light irradiation over 8 h. Its superior performance can be attributed to a triple synergistic effect: firstly, the S-scheme band structure of RCM enabled efficient charge separation through the interfacial built-in electric field; secondly, R. palustris exhibited excellent environmental adaptability and could mineralize photocatalytic intermediates via enzymatic catalysis pathways; finally, the chitosan-modified CPU carrier demonstrated significantly enhanced surface roughness and functional group density, which not only improved the mechanical stability of the photocatalytic layer but also provided an ideal microecological niche for microbial colonization. This cross-scale “photocatalysis–biodegradation” coupling mechanism offers an innovative solution for the treatment of refractory organic pollutants.

4.2.3. Other Anionic Dyes

Hassan et al. [108] successfully developed a vanadium pentoxide/nitrogen-deficient graphitic carbon nitride (VO/Ndef-CN) composite photocatalytic system via a controllable pyrolysis strategy. The 5VO/Ndef-CN heterostructure achieved a degradation efficiency of 98.2% for indigo carmine dye within 35 min under visible light irradiation, as shown in Table 3. The performance enhancement mechanism primarily originates from two aspects: (1) the S-scheme heterojunction establishes a directional carrier transport channel through band bending, effectively suppressing the recombination of photogenerated electron–hole pairs; (2) nitrogen defect engineering in conjunction with the heterointerface significantly broadens the material’s light response range and enhances its photon capture capability. Yaqoubi et al. [109] synthesized a NiMn₂O₄/g-C₃N₄ heterojunction using an ultrasonic co-precipitation method. In an acidic medium, the NiMn₂O₄/g-C₃N₄ nanocomposite degraded 96.4% of chrome black T within 120 min under visible light irradiation. The S-scheme heterojunction facilitated the efficient separation of photogenerated electron–hole pairs. Alqarni et al. [110] fabricated a ternary MgO-TiO₂@g-C₃N₄ composite through a facile ultrasonic synthesis method. The MgO-TiO₂@g-C₃N₄ heterojunction degraded 94.0% of alizarin red S within 60 min under visible light irradiation. The S-scheme heterojunction spatially separated and retained high concentrations of h⁺ and e⁻ at the VB and CB of g-C₃N₄ and MgO-TiO₂, thereby enabling effective dye degradation. Hosseini et al. [111] prepared a CaSnO₃/g-C₃N₄ nanocomposite via an ultrasonic-assisted co-precipitation method. The CaSnO₃/g-C₃N₄ nanocomposite decomposed 86.2% of erythrosine within 90 min under visible light irradiation. Owing to the efficient utilization of visible light and enhanced charge carrier separation by the S-scheme heterojunction, this nanocomposite demonstrates excellent performance in degrading organic dyes.

In summary, innovative design strategies have been used to develop S-scheme heterojunction g-C₃N₄ structures with remarkable photocatalytic performance in the degradation of anionic dyes. Through various advanced preparation techniques, including co-precipitation, physical blending, in situ calcination, solvothermal synthesis, and solid-state diffusion, researchers have successfully developed a series of g-C₃N₄-based S-scheme heterojunction photocatalytic systems. These systems effectively suppress the recombination of photogenerated electron–hole pairs and enhance the spectral response range through optimized band structure engineering and carrier migration mechanisms while significantly improving the generation efficiency of reactive oxygen species. In degradation experiments involving methyl orange, Congo red, and other anionic dyes, these S-scheme heterojunction composites consistently demonstrated high removal efficiency and superior photocatalytic performance. These research findings not only provide novel insights into the rational design of efficient photocatalytic materials but also offer innovative solutions for addressing refractory organic pollutants in environmental remediation.

5. Conclusions

With the continuous discharge of dye-containing toxic wastewater in modern agricultural and industrial production processes, environmental remediation technologies based on photocatalysis have demonstrated significant application value in industrial wastewater treatment due to their high mineralization efficiency, environmental friendliness, and energy economy. Compared with traditional wide-bandgap semiconductor materials such as TiO₂, which suffer from inherent limitations, including a weak visible light response and low quantum efficiency, S-scheme heterojunction photocatalysts developed in recent years offer an innovative solution to overcome the charge recombination bottleneck of conventional type-II and Z-scheme heterojunctions. By constructing a stepped band structure, these catalysts enable the efficient separation of photogenerated carriers while preserving the redox potential of each component. Graphitic carbon nitride, characterized by its narrow band gap, excellent visible light absorption, and superior physicochemical stability, has emerged as an ideal candidate for constructing S-scheme heterojunctions. Its two-dimensional layered structure facilitates charge transfer and significantly enhances photocatalytic activity. Studies have shown that g-C₃N₄-based S-scheme heterojunctions exhibit remarkable photocatalytic performance in degrading various cationic dyes (e.g., MB, RhB, and crystal violet) and anionic dyes (e.g., Congo red and MO), achieving degradation efficiencies as high as nearly 100.0% in some cases. Researchers have successfully fabricated diverse g-C₃N₄-based S-scheme heterojunction systems using various synthesis methods, including MoS₂/g-C₃N₄, CdS/g-C₃N₄, and PbTiO₃/g-C₃N₄, among others, all of which demonstrate excellent photocatalytic degradation capabilities. Furthermore, this review systematically discusses the performance differences and underlying mechanisms of different catalyst systems in degrading various types of dyes.

6. Prospects

Significant advancements have been achieved in the application of g-C₃N₄-based S-scheme heterojunctions for the photocatalytic degradation of dye wastewater. However, their practical implementation still faces numerous challenges. Future research can focus on the following directions to promote the industrialization and performance optimization of this technology.

6.1. Development and Optimization of Novel Heterojunction Systems

In the field of semiconductor coupling system innovation, more semiconductor materials compatible with g-C₃N₄ (BiOX, MXene, COF, etc.) should be explored to construct advanced S-scheme heterojunctions, addressing the limitations of light absorption and charge separation efficiency in existing systems. For example, introducing materials with broad spectral responses (e.g., black phosphorus) or magnetic components (e.g., Fe₃O₄) can enhance light absorption and enable convenient catalyst recovery. To improve interfacial charge transfer efficiency, surface modification strategies such as doping, defect engineering, and cocatalyst loading can be employed. Specifically, doping with metal single atoms (e.g., Pt or Co) or non-metal elements (e.g., P or B) can increase the density of active sites and extend the lifetime of photogenerated carriers. At the microstructure engineering level, three-dimensional porous g-C₃N₄-based heterojunctions (e.g., core–shell structures, nanoflower-like structures) can be developed to maximize the specific surface area and exposure of reactive sites, thereby enhancing the adsorption and degradation kinetics of dye molecules.

6.2. Coupling of Multiple Technologies and Intelligent Control

In the domain of energy–mass synergy catalysis, a photocatalytic–electrocatalytic coupling system can be constructed to accelerate charge separation via an external electric field while simultaneously generating oxidants such as H₂O₂ through water electrolysis to enhance degradation efficiency. Additionally, in the direction of photothermal regulation, the synergistic mechanism between photocatalysis and thermal catalysis can be investigated. For instance, incorporating photothermal materials (e.g., Au or Cu₂O) can leverage the photothermal effect to elevate reaction temperatures and expedite dye molecule decomposition rates. Regarding intelligent technology integration, machine learning models can be utilized to predict optimal heterojunction compositions and synthesis conditions, reducing experimental cycles. Real-time monitoring of wastewater treatment effects using sensors can enable intelligent control of the catalytic system. In terms of process coupling innovation, the feasibility of integrating photocatalysis with membrane separation, biological degradation, and other technologies can be explored to establish multi-stage treatment processes, improving the overall wastewater treatment efficiency and economic viability.

Future research should prioritize the core objectives of “efficiency” and “intelligence”, driving the transition of g-C₃N₄-based S-scheme heterojunctions from laboratory-scale studies to industrial applications through material innovation and interdisciplinary collaboration. This process not only involves addressing scientific challenges but also requires consideration of environmental safety and economic feasibility, ultimately providing sustainable technical solutions for global dye wastewater management.