Challenges and Opportunities for g-C₃N₄-Based Heterostructures in the Photodegradation of Environmental Pollutants

Abstract

Graphitic carbon nitride (g-C₃N₄) is emerging as one of the most promising non-metallic semiconductors for the degradation of pollutants in water by photocatalytic processes. Its exceptional reduction–oxidation (redox) potentials and adequate band gap of approximately 2.7 eV give it the ability to absorb in the visible light range. However, the characteristic sensitivity to light absorption is limited, leading to rapid recombination of electron–hole pairs. Therefore, different strategies have been explored to optimize this charge separation, among which the formation of heterostructures based on g-C₃N₄ is highlighted. This review addresses recent advances in photocatalysis mediated by g-C₃N₄ heterostructures, considering the synthesis methods enabling the optimization of the morphology and active interface of these materials. Next, the mechanisms of charge transfer are discussed in detail, with special emphasis on type II, type S, and type Z classifications and their influence on the efficiency of photodegradation. Subsequently, the progress in the application of these photocatalysts for the degradation of water pollutants, such as toxic organic dyes, pharmaceutical pollutants, pesticides, and per- and polyfluoroalkyl substances (PFAS), are analyzed, highlighting both experimental advances and remaining challenges. Finally, future perspectives oriented towards the optimization of heterostructures, the efficiency of synthesis methods, and the practical application of these in photocatalytic processes for environmental remediation.

1. Introduction

The inefficient and improper management of the final disposition of organic pollutants coming from domestic and industrial activities leads to their accumulation in various aquatic ecosystems, making this type of pollution one of the most serious environmental issues and/or waste problems that humanity currently faces [1]. In this regard, water contamination by organic pollutants such as toxic industrial dyes [2], pharmaceutical residues [3,4], agrochemical pesticides [5] and perfluoroalkyl substances (PFAS) [6,7], rank high among the most worrisome emerging pollutants. Moreover, the relevant toxicity, persistence, and bioaccumulative potential can have toxic, carcinogenic, or endocrine-disrupting effects, even at trace concentrations [8]. The identification and classification of these organic pollutants reveal that they possess remarkable chemical stability and resistance to biodegradation. Furthermore, they can even remain intact for prolonged periods, posing a serious risk to aquatic biodiversity [9] and human health [10,11]. Considering this context, in recent years, a large number of technologies have been developed for the treatment of contaminated water, such as membrane filtration [12,13], degradation using biological agents [14,15], physical and chemical absorption processes [16,17], advanced oxidation processes (AOPs) [18,19,20], among others [21].

Heterogeneous photocatalysis is an advanced oxidation method that converts solar energy into chemical energy using semiconductor materials. Thus, this is a promising approach for environmental remediation because it permits the degradation and mineralization of organic pollutants using light-driven photochemical reactions [22,23]. Through photocatalysis, photon energy generates electron–hole pairs (e⁻/h⁺) in the bandgap of a semiconductor, which can trigger redox reactions that convert complex pollutants into harmless molecules (CO₂, H₂O, mineral salts, etc.) [24]. Amongst the most investigated photocatalysts, graphitic carbon nitride (g-C₃N₄) is emerging as one of the most promising materials for the degradation of organic pollutants in water by photocatalytic processes. This polymeric and metal-free material, with a bandgap of ~2.7 eV, can be activated with visible light, unlike traditional photocatalysts such as TiO₂ that require UV light [25,26,27]. In addition, g-C₃N₄ is chemically stable, inexpensive, and easily synthesized from abundant precursors (urea, melamine, and more.). These qualities highlight g-C₃N₄ as a sustainable candidate for environmental remediation. However, g-C₃N₄ presents limitations such as moderate surface area, reduced absorption of specific wavelengths, and especially rapid recombination of the photogenerated electron–hole pairs (e⁻/h⁺), which is reflected in the decrease in the quantum efficiency of the catalyst [28]. To overcome these adverse effects, a key strategy has been the development of heterostructures based on g-C₃N₄, which is coupled with other semiconductors or co-catalysts to expand spectral absorption and facilitate charge separation [24]. These heterostructures that include a wide range of combinations (including metal oxides, sulfides, nitrides, carbon materials, and more) have been shown to significantly improve the photocatalytic activity of g-C₃N₄ [29,30]. Interestingly, multiple studies reported substantial increases in pollutant degradation by coupling g-C₃N₄ with appropriate semiconductor oxides [31,32]. This optimization is critical, given that the pollutants show resistance to conventional degradation methods. Moreover, recent studies have examined the efficiency of g-C₃N₄ and its heterostructures in depredating environmental pollutants. For example, Chen et al. demonstrated that temperature alteration in a BiFeO₃/g-C₃N₄ heterostructure system achieves a significant enhancement in dye decomposition, highlighting the versatility of g-C₃N₄ in photocatalytic applications [33]. Likewise, Rubab et al. showed the effectiveness of novel g-C₃N₄/Mg-ZnFe₂O₄ composites in dye degradation, suggesting significant potential for applications in water purification [34]. Furthermore, the study by Sert, B. et al. highlights the efficiency of the g-C₃N₄/ZnO heterojunction system, in the photocatalytic organic pollutant, emphasizing the adaptability of g-C₃N₄ to several heterostructure configurations to improve its photocatalytic performance [35]. On the other hand, the investigation of heterojunction mechanisms has progressed, with three main charge transfer schemes being recognized: the type II heterojunction, Z-scheme-type heterostructure, and the emerging S-scheme-type heterostructure [28]. Each mechanism entails a distinct band arrangement and electron and hole migration pathways, with implications for charge separation efficiency and the redox power available for reactions. Understanding and optimizing these mechanisms in g-C₃N₄ heterostructures is crucial to maximize their performance in environmental photocatalysis.

Despite remarkable advances regarding g-C₃N₄ heterostructures, important knowledge gaps remain in the current literature regarding the preparation, modification, and application of g-C₃N₄-based heterostructures for the photodegradation of environmental pollutants, with a special emphasis on charge transfer mechanisms and photocatalytic performance. Initially, the synthesis methods of g-C₃N₄ and the preparation of the heterostructures are discussed, relating to the strategies employed for tuning the final properties of the photocatalysts. Then, we explore three types of charge separation mechanisms (type II, Z, and S), providing theoretical foundations, experimental examples, and a comparative analysis of their efficiency, stability, and applicability. Next, we reviewed the specific applications of g-C₃N₄ heterostructures in the photocatalytic degradation of four classes of organic pollutants: dyes, pharmaceuticals, pesticides, and PFASs. Moreover, each subsection discusses emblematic studies, highlighting that photocatalyst morphology, doping, and choice of second semiconductor influence the removal of each pollutant. Finally, conclusions and future perspectives are provided, examining the current advantages and limitations of these heterostructures compared to other technologies and proposing research directions to improve the stability and scalability of the photocatalysts, which could lead to environmental applications. Overall, this review serves as a comprehensive and up-to-date reference for the scientific community, facilitating the informed design of more efficient and sustainable heterostructure-based g-C₃N₄ photocatalytic g-C₃N₄ systems (Figure 1).

2. Preparation Methods

The g-C₃N₄ is a two-dimensional organic semiconductor material showing a bandgap of approximately 2.7 eV and high chemical stability. The g-C₃N₄ presents a nitrogen-rich tri-s-triazine structure that provides optical and electronic properties for photocatalysis performance and, under thermal condensation of economic nitrogenous precursors, can be easily synthesized. Under visible light conditions, the g-C₃N₄ generates reactive radical species (•O₂⁻, •OH, holes h⁺) capable of oxidizing complex organic compounds [36]. Therefore, it has been proposed as a highly promising environmental photocatalyst for degrading different organic pollutants such as dyes, pharmaceuticals, pesticides, PFASs, etc., that are found in wastewater [24,36]. In this context, synthetic routes that maximize surface area, active sites, and visible light absorption are key parameters to improve g-C₃N₄ performance. The photocatalytic effectiveness of g-C₃N₄ and its derivative heterostructures is highly dependent on the synthesis method. Interestingly, these critical conditions determine the materials morphology, surface area, structural defects, active site distribution, and the photogenerated charge separation efficiency. In the following, we first review methods for the synthesis of g-C₃N₄ with high photocatalytic activity and, subsequently, strategies for the preparation of g-C₃N₄-based heterostructures, highlighting the influence of the synthesis route on the optimization of the optoelectronic properties of the material and, hence, the photocatalytic activity.

2.1. Synthesis Methods for g-C₃N₄

The synthesis of g-C₃N₄ generally involves the thermal polymerization of nitrogen-rich precursors between 500 °C and 600 °C. Moreover, the thermal polymerization strategy is the most widely appreciated technique due to the simple required parameters, low cost, scalability, and the application of common nitrogen-rich precursors such as melamine, dicyandiamide, cyanamide, urea, thiourea, and ammonium thiocyanate [37,38]. Upon heating these precursors in the presence or absence of an inert atmosphere, they condense into a structure in the form of conjugated tri-s-triazine sheets stacked by Van der Waals forces [39], which form the graphitic carbon nitride g-C₃N₄ (Figure 2). This material has a moderate surface area, basal, and surface-active sites that are suitable for catalysis applications. However, the thermal process has the disadvantage of finely controlling the morphology or crystalline order of the material [40].

The thermal condensation process for synthesizing g-C₃N₄ from the aforementioned nitrogen-rich precursors involves a progressive thermal polymerization, during which nitrogen-rich functional groups reorganize and condense, releasing ammonia (NH₃) and other volatile molecules. As illustrated in Figure 3, compounds such as cyanamide, urea, and thiourea act as starting molecules, while dicyandiamide may form as a transient intermediate from cyanamide. All these precursors ultimately converge at a common stage with the formation of melamine, which serves as a key intermediate precursor in the formation of g-C₃N₄. Upon heating, these precursors undergo a series of structural rearrangements and condensation reactions that lead to the formation of heptazine (C₆N₇) and triazine (C₃N₃) monomeric units, also referred to as tri-s-triazine and s-triazine, respectively. The formation of these monomeric units involves the deamination and partial decomposition of the nitrogen functional groups, generating gases such as NH₃, CO₂, CS₂, and H₂S, whose release depends on the precursor and the calcination temperature. The tri-s-triazine monomeric unit is thermodynamically more stable than triazine (C₃N₃) under ambient conditions and constitutes the predominant structural core in the polymeric networks of g-C₃N₄ [41,42,43,44,45].

Figure 4 shows the reaction pathway followed by urea, which is a typical precursor for the synthesis of g-C₃N₄. Three urea molecules are required to obtain one melamine molecule through consecutive conversion to biuret, cyanuric acid, ammelide, ammeline, and ultimately melamine. This is followed by the subsequent condensation of heptazine units at temperatures close to 550 °C, leading to a conjugated polymeric network with C–N and C=N bonds, which is characteristic of g-C₃N₄. Although thermally straightforward, this process involves a delicate balance between structural condensation and thermal stability, highlighting the importance of strictly controlling the temperature during synthesis to preserve the structural integrity of g-C₃N₄ and optimize its photoelectrocatalytic properties [41].

In order to overcome the limitations of the thermal polymerization technique, alternative methodologies have emerged, such as chemical vapor deposition (CVD). CVD conducts the formation of volatile precursors, using cyanamide and melamine, along with others, which are decomposed on a hot substrate, subsequently promoting the formation or deposition of g-C₃N₄ layers. Although CVD offers g-C₃N₄ thin films or nanostructures showing better control, purity, thickness, and morphology, it requires specialized equipment [24,46]. Moreover, CVD facilitates precise control for the generation of coatings with g-C₃N₄ material, obtaining flat and homogeneous layers (instead of granular powder) [47,48]. Interestingly, Chubenko et al. reported an innovative method for the synthesis of g-C₃N₄ thin films by CVD in a two-zone tube furnace, using melamine as the solid precursor and dry argon as the carrier gas, the temperatures were independently controlled for evaporation (350 °C) and substrate annealing (500–650 °C). Then, the films were deposited on flat glass and single-crystal silicon substrates, reaching maximum thicknesses of up to 1.2 μm at 600 °C. The authors applied scanning electron microscopy (SEM) analyses, revealing uniform surfaces with a well-defined layered structure. Meanwhile, X-ray diffraction (XRD) showed a preferential orientation along the (002) plane, indicative of robust crystallinity. In addition, depending on the growth conditions, the films showed good transparency in the visible region and tunable refractive index [49]. Another study by Chubenko et al. reported that g-C₃N₄ thin films with a thickness of approximately 200–1200 nm could be obtained in just 3–5 min (at 500–620 °C) by CVD, exhibiting a highly crystalline two-dimensional structure. Furthermore, the X-ray diffraction (XRD) examination illustrated the presence of a single interplanar g-C₃N₄ (002) peak and the absence of the g-C₃N₄ (210) diffraction peak (around 2θ = 13 degrees) that characterizes the in-plane distance between the heptazine units in the melon chains. This information indicated that the layers of the material are perfectly crystallized and parallel aligned to the surface substrate [48]. Far more important is the fact that the CVD process allows controlling the structure and optical properties of g-C₃N₄, making it suitable for advanced optoelectronic applications. Lu Chen et al. using melamine as a precursor heated to 300 °C in the evaporation zone and a polymerization temperature of 550 °C, the authors achieved g-C₃N₄ thin films approximately 10 nm thick deposited on substrates such as silicon, glass, and anodic aluminum oxide (AAO). The resulting g-C₃N₄ structure exhibited an effective distribution of C–N–C bonds and thiazide groups. Figure 5 schematically illustrates the growth mechanism of ultrathin g-C₃N₄ films by CVD. In this process, solid melamine is used as the precursor and is thermally heated in a tubular furnace to generate vapor, which is transported toward a receiving substrate (e.g., glass, quartz, Cu, or AAO) through a controlled flow of inert gas (N₂ or Ar). The vaporized melamine deposits on the substrate and, as the temperature increases (typically between 500 and 600 °C), in situ thermal polymerization takes place. During this thermal condensation, melamine molecules undergo progressive deamination and reorganize into more condensed structures such as melem and melon, releasing ammonia (NH₃) as a byproduct. These intermediates link through C–N–C bonds to form an ordered conjugated network of graphitic g-C₃N₄, composed of tri-s-triazine (heptazine) units arranged in stacked layers via π–π interactions [50]. This method allows precise control over film thickness and uniformity, due to the regulated deposition of the precursor and the self-limiting nature of the surface condensation reaction. The combination of vapor transport, directed deposition, and thermal condensation enables the direct formation of thin g-C₃N₄ films with good adhesion, high lateral crystallinity, and optimal structural continuity on various surfaces.

The hydrothermal and solvothermal methodologies involve reacting to a solution of nitrogen precursors, such as melamine, urea, or cyanamide solution, in an autoclave reactor with a Teflon internal lining. In these methodologies, it is important to have precise control of the temperature and, if possible, the pressure. These variables have a direct and significant effect on the polymerization, crystallization, morphology, and optoelectronic properties of the synthesized g-C₃N₄ [51,52,53]. The main advantage of the thermal route is the generation of porous or lamellar g-C₃N₄ nanostructures with a high surface area, which increases the number of active sites and facilitates the adsorption and degradation of pollutants. Moreover, the use of solvents (water in hydrothermal or organic solvents such as acetonitrile in solvothermal) modulates the dissolution of precursors and crystal growth, enabling the incorporation of dopants or the formation of heterostructures in a single step [36,52,54]. Compared to the conventional calcination process, the hydrothermal/solvothermal methods operate at lower temperatures and offer greater control over the composition of the resulting material. For instance, Abdullahi et al., reported that they were successfully synthesized g-C₃N₄ using a solvothermal methodology with acetonitrile as the solvent and temperatures of 160, 180, and 200 °C. They detected that the material synthesized at 200 °C exhibited a photocurrent of approximately 25 μA cm⁻² when a potential of 1.7 V vs. RHE was applied, demonstrating enhanced performance in methylene blue degradation compared to the samples prepared at lower temperatures. The improved photoelectrochemical and photocatalytic activity of g-C₃N₄ synthesized at 200 °C is attributed to the higher synthesis temperature improving the crystalline and favorably modifying the electronic structure of the material [52] (Figure 6).

The sol–gel method is a mild chemical process that involves the dispersion of metallic or molecular precursors and the subsequent formation of a “sol” which is gelled and then calcined to give a solid material, as illustrated in Figure 7 [55,56]. For the synthesis of g-C₃N₄, this method allows for dissolving nitrogenous precursors, such as melamine, dicyandiamide, cyanamide, or urea, in liquid phases, together with sol–gel silica templates, surfactants, or another precursor to organize the polymeric network. Then, the resultant gel is heated between 500 and 600 °C, thus these precursors condense to form the g-C₃N₄ structure, releasing gases (NH₃, CO₂, etc.) and generating a laminar graphitic phase. The advantage of the sol–gel route is that it provides control over particle size, porosity, and g-C₃N₄ composition, enabling a homogeneous and modifiable network with doping or vacancies [36]. In practice, non-hydrolytic routes with solvents such as benzyl alcohol have been used for nitrogenous and related materials, although many recent studies employ mesoporous templates (SBA-15, silica in sol–gel) to increase the surface area. In general, the sol–gel stage performs an organic xerogel, which is then calcined to crystalline g-C₃N₄ [57].

2.2. Preparation of g-C₃N₄-Based Heterostructures

The assembling of g-C₃N₄/semiconductor heterostructures is an essential strategy to enhance photoactivity, as it combines the photoelectrochemical properties of both components and facilitates the spatial separation of charge carriers [58]. There are numerous approaches to prepare these nano-heterostructures, ranging from in situ methods during g-C₃N₄ synthesis to post-synthesis procedures that couple the performed components. In the following, some of the commonly used strategies are discussed, accompanied by examples of heterostructures reported in the recent literature, along with their advantages, limitations, and impact on photocatalytic activity.

One of the methodologies implemented in the preparation of heterostructures is the obtainment by coupling during the calcination of g-C₃N_4. This is a simple route that involves combining precursors of another semiconductor (oxides, sulfides, and more) with the precursor of g-C₃N₄ in the thermal polymerization. Thus, during the calcination process, the g-C₃N₄ is simultaneously formed, and the semiconductor material is deposited and/or grown over the substrate surface, achieving well-integrated heterostructures. In this regard, we can discuss the work reported by Puyang Zhou et al., suggesting that a g-PCN/BFO (g-C₃N₄/BaFe₁₂O₁₉) (Figure 8) heterostructure was synthesized via an in situ thermal polymerization approach and was subsequently employed as a productive photo-Fenton catalyst to degrade antibiotics [59]. On the other hand, Tan et al. reported a one-step nanostructured g-C₃N₄/TiO₂ heterojunction synthesized by the co-pyrolysis of urea with commercial TiO₂. The resulting material (CN/TiO₂-24) demonstrated 10.8-fold higher H₂-evolving activity than pure g-C₃N₄, attributed to the intimate g-C₃N₄–TiO₂ interaction that facilitates electron transfer between the two phases [60]. Similarly, g-C₃N₄/ZnO [32] and g-C₃N₄/SnO₂ [61] have been prepared by mixing Zn or Sn oxides with melamine before calcination, obtaining type II heterostructures with robust interfacial contact. The advantage of the thermal method is the simplicity and the good heterointerfacial contact achieved. However, it can be challenging to control the proportion and distribution of the second phase during the calcination process.

Another strategy for developing the heterostructure involves growing the second semiconductor on preformed g-C₃N₄, a process known as in situ hydrothermal synthesis, which is illustrated in Figure 9. Interestingly, Haohao Huo et al. obtained a highly efficient photocatalytic Zn₃In₂S₆/gC₃N₄ binary heterostructure for tetracycline (TC) degradation [62]. For example, using thermally obtained g-C₃N₄ as surface substrate, metal oxide nanoparticles can be deposited following a hydrothermal reaction. In this sense, Yuanyuan Li et al. synthesized Mn₃O₄/g-C₃N₄ p-n heterostructures through the reaction of Mn(II) salts in the presence of g-C₃N₄ films under solvothermal conditions. The Mn₃O₄ component grew anchored on the surface of g-C₃N₄, forming a p-n contact that enhanced charge separation without Pt co-catalyst [63]. Similarly, the solvothermal methods have been employed to assemble g-C₃N₄/Bi₂WO₆, g-C₃N₄/BiVO₄, and other combinations in which the bismuth component crystallizes on the g-C₃N₄ matrix [64,65]. These routes serve as a platform for creating an intimate interconnection between phases and directing the formation of 2D/2D heterostructures (sheet-sheet), which is beneficial for charge conduction at the planar interface.

The method of wetting deposition or impregnation consists of dispersing g-C₃N₄ in a precursor solution of the second material, followed by drying and soft calcination to form the second semiconductor in situ. For instance, the g-C₃N₄/BiOX heterostructure is obtained by impregnating g-C₃N₄ in Bi³⁺ and X-solution (where X = Cl, Br, I), then drying and calcining to crystallize BiOX films on the surface of g-C₃N₄. This impregnation–calcination technique is widely used for heterostructures containing bismuth oxyhalides (BiOCl, BiOBr, BiOI) that, after coupling with g-C₃N₄, have been shown to extend the response to visible and enhance dye degradation [28,66]. The advantage of these films is their simplicity and versatility to deposit layers or nanoparticles of many compounds on g-C₃N₄. Nonetheless, an important limitation is the adhesion force resulting from the coupling reaction, which may not achieve the physical requirements like hydrothermal in situ synthesis, sometimes requiring a coupling agent or additional treatment to reinforce the bond.

An efficient and effective method is the exfoliation and solution assembly of bulk g-C₃N₄, which can be exfoliated into ultrathin nanosheets (approximately 2–10 nm thickness) by ultrasound or chemical processes [67,68]. These g-C₃N₄ nanosheets offer a larger surface area and an enhanced number of exposed active sites. Moreover, metallic or even semiconducting nanoparticles can be deposited and anchored by chemical reductions or precursor decomposition. Cheng et al. exfoliated g-C₃N₄ in sheets and then reduced HAuCl₄ to deposit Au nanoparticles (approximately 5–20 nm diameter) on the surface. The resulting heterostructure exhibited high efficiency in the photodegradation of orange low-visibility methyl because AuNPs act as electron scavengers (forming a Schottky junction) and promote charge–hole separation [28]. Similarly, g-C₃N₄/Ti₃C₂ (MXene) and g-C₃N₄/reduced graphene have been assembled following this technique, taking advantage of the solution-dispersed on the g-C₃N₄ films to couple them with conductive 2D materials. These 2D/2D configurations with conductive carbon enhance electronic mobility and conductivity, thereby increasing the photocatalytic activity, as observed in previous work on herbicide degradation [31]. The disadvantage of this procedure may be the need for exfoliation steps and the possible re-aggregation of films; however, by controlling the conditions (using appropriate solvents, sonication, and stabilizing agents), surface homogeneous compositions can be achieved.

Synthesis by photocatalytic or chemical deposition allows some heterostructures to be fabricated by taking advantage of photocatalysis itself. For example, by photodepositing an oxide on g-C₃N₄ by illuminating a suspension of g-C₃N₄ in the presence of metal precursors, the photogenerated g-C₃N₄ reduces the metal, which then oxidizes water, forming a bonded oxide. This method has been useful for depositing metal co-catalysts (Pt, Ag) or semiconductors such as Ag/AgBr (from Ag⁺ and Br⁻), generating active heterostructures for H₂ production [69]. The advantage is in situ formation under mild conditions, although it is limited to materials whose precursor can be photocatalytically reduced/oxidized by g-C₃N₄.

In the recent literature, numerous combinations of g-C₃N₄ with other materials have been explored, each with specific objectives. Among them, we can highlight:

• g-C₃N₄/TiO₂: A widely studied type II heterojunction that leverages the robustness of TiO₂ and extends its visible-light response thanks to g-C₃N₄ [28].
• g-C₃N₄/ZnO: A type II or S-scheme system (depending on doping) that combines ZnO, is high conductivity with g-C₃N₄ is visible-light absorption. It shows improved dye degradation (97% degradation of crystal violet by g-C₃N₄/ZnO in 150 min). ZnO must be stabilized against photocorrosion through coupling [32].
• g-C₃N₄/BiVO₄: A prototype direct Z-scheme heterostructure, where g-C₃N₄ (strong reducer) couples with BiVO₄ (strong oxidizer). It retains high redox-potential carriers and has demonstrated effective degradation of pesticides, dyes, and pharmaceuticals under visible light [70].
• g-C₃N₄/graphene (or rGO): A conductor-dielectric nanocomposite. Graphene does not generate carriers, but it enhances conductivity by acting as a bridge and adsorbent. A g-C₃N₄/graphene composite achieved 100% atrazine degradation in 5 h vs. 40% with g-C₃N₄ alone. It improves electron transfer speed and stability [31].
• g-C₃N₄/MXenes (Ti₃C₂): Similar to graphene, MXenes are 2D conductors that, when coupled with g-C₃N₄, enhance charge separation and provide adsorption sites for charged pollutants. Notable improvements have been observed in anionic dye degradation [71].
• g-C₃N₄/MOFs: Incorporating g-C₃N₄ into porous metal–organic frameworks combines photoactivity with high adsorption. For example, ZIF-67/MIL-100(Fe)@g-C₃N₄ showed accelerated PFOA removal by combining MOFs’ high PFAS adsorption affinity with g-C₃N₄ photocatalytic activation [72]. The challenge lies in achieving good photoelectric contact between organic and inorganic phases.
• g-C₃N₄/Noble metals: Although not a semiconductor-semiconductor heterojunction, loading g-C₃N₄ with Au, Ag, or Pt creates Schottky heterostructures. The metallic nanoparticles trap electrons, prolonging hole lifetimes in g-C₃N₄, which has been shown to improve dye degradation [28].

Each of these heterostructures offers opportunities to expand spectral absorption, accelerate e⁻/h⁺ pair separation, increase pollutant adsorption, or add additional reactive pathways. However, they also present challenges to overcome. In general, controlled preparation is crucial to achieving intimate and well-distributed contact between g-C₃N₄ and the second material, as this directly influences good pollutant degradation performance, whereas agglomerations or poorly integrated phases will reduce effectiveness.

The particle size of g-C₃N₄ plays a critical role in determining its physical and photoelectrocatalytic properties, particularly when forming heterojunctions with other semiconductors in applications involving the oxidation of organic pollutants. Smaller particles significantly enhance the efficiency of photogenerated charge separation by reducing diffusion distances and improving interfacial electron transfer, thereby minimizing internal charge recombination and substantially increasing photocatalytic activity [73,74]. Moreover, nanoscale size greatly improves the quality of the heterojunction interface, ensuring more intimate and continuous contact between semiconductors, which is essential for efficient charge transfer and structural stability. Particle size reduction also increases the specific surface area and the density of exposed active sites, thus accelerating surface oxidation reactions and maximizing interaction with organic pollutants [75,76,77]. Additionally, in particles smaller than 10 nm, quantum confinement effects emerge, tuning the electronic and optical properties of the material by modifying its bandgap in a favorable manner, thereby enhancing the utilization of visible solar light in photoelectrocatalytic processes [78,79]. Consequently, precise control over particle size emerges as an essential strategy for the rational design of highly efficient photocatalytic heterostructures for the degradation of organic contaminants, fully leveraging the synergistic potential of nanostructured g-C₃N₄ in combination with complementary semiconductors.

3. Charge Transfer Mechanism

The efficiency of any photocatalyst is strongly dependent on the proper separation and transfer of photogenerated charges. In the context of coupled semiconductors (heterostructures), the band alignment between interface phases is a key determinant of the path that electrons and holes will follow once excited. This leads to the emergence of a variety of different heterojunction mechanisms, each with its own unique characteristics. The main proposed schemes for semiconductor heterostructures are Type II, Z-scheme, and S-scheme. In the following sections, we explain the fundamentals of each mechanism and their application to g-C₃N₄-based heterostructures. We also present illustrative examples from the literature that highlight the advantages and limitations of these mechanisms in terms of charge separation and photocatalytic activity are explained below.

3.1. g-C₃N₄ Type II Heterostructures

In a type II heterostructure, the valence bands (VBs) and conduction bands (CBs) of the two semiconductors are staggered such that the CBs and VBs of one lie above or below those of the other [80]. This condition generates a step-like arrangement where under light excitation, electrons (e⁻) tend to flow toward the semiconductor with the lower (less energetic) CBs, while holes (h⁺) migrate to the semiconductor with the higher (more positive) VBs [81]. In other words, both photocatalysts generate e⁻/h⁺ pairs, and then the e⁻ from one “falls” into the CB of the other, while the h⁺ from the second “rises” to the VB of the first, spatially segregating the charges [82]. The band potential difference, a crucial factor, drives this flow and can be favored by band bending and internal electric fields at the material interface. Figure 10 illustrates the architecture proposed by Kazi M. Alam et al. for the type II heterostructures g-C₃N₄/BiOI and g-C₃N₄-S/BiOI, which exhibit visible light absorption up to 650 nm and efficient charge separation. The g-C₃N₄-S/BiOI configuration achieved a photocurrent density of 0.70 mA cm⁻², reduced charge transfer resistance to 100 Ω, and showed a carrier lifetime of 1.75 ns, supporting a charge transfer mechanism with high operational stability. The STEM elemental mapping (Figure 10III(e–i)) confirms the homogeneous distribution of Bi, O, I, C, and N across the g-C₃N₄-S/BiOI interface. This spatial uniformity indicates a well-integrated heterojunction with minimal phase separation, which is crucial for facilitating directional charge migration and reducing recombination. These mappings corroborate the proposed Type II charge transfer mechanism by demonstrating intimate interfacial contact necessary for efficient electron and hole separation [83].

A typical example of a type II heterostructure is g-C₃N₄/TiO₂ material, where g-C₃N₄ plays a pivotal role. It has a CB at approximately −1.1 eV (vs. NHE), which is more negative (higher in energy) than TiO₂’s CB (approximately −0.5 eV). After coupling–transposing to visible light, the g-C₃N₄ is photoexcited (TiO₂ only absorbs UV), generating electrons in its CB that quickly transfer to TiO₂, while the holes remaining in g-C₃N₄ are VBs [60,84]. This charge separation, largely facilitated by g-C₃N₄, results in e⁻ accumulation in TiO₂ and h⁺ accumulation in g-C₃N₄, reducing recombination and increasing carrier lifetimes, which allows more electrons and holes to participate in surface reactions such as O₂ reduction and organic pollutant oxidation. The practical outcome is a substantial improvement in the photocatalytic activity of the g-C₃N₄/TiO₂ heterostructure, indeed attributed to this charge separation and increased number of active sites [85]. Other type II heterostructures studied with g-C₃N₄ include g-C₃N₄/ZnO, g-C₃N₄/SnO₂, and g-C₃N₄/CeO₂, among others, which share the feature that g-C₃N₄ has a more negative CB. For example, in g-C₃N₄/ZnO, the composite nearly degrades crystal violet dye (97% in 150 min) under visible light. Far outperforming g-C₃N₄ or ZnO alone due to e⁻ from g-C₃N₄ transferring to ZnO, where they effectively reduce O₂ to superoxide radicals, while h⁺ in g-C₃N₄ oxidizes the dye [32].

The main advantage of the type II mechanism is thus the efficient spatial separation of electrons and holes (e⁻/h⁺), minimizing the probability of direct recombination [86]. Additionally, combining two semiconductors can expand the system’s spectral absorption range if one covers UV and the other visible light or different visible ranges [87]. For example, in g-C₃N₄/TiO₂, the system can utilize UV (activating TiO₂) and visible light (activating g-C₃N₄). However, an inherent drawback of type II heterojunctions is that relocating carriers sacrifices some of their redox power. The accumulated electrons end up in the less energetic (less reducing) CBs and the holes in the less positive (less oxidizing) VBs. In g-C₃N₄/TiO₂, the electrons migrating to TiO₂ show a more modest reduction potential (that of TiO₂, sufficient for O₂/•O₂⁻ but perhaps not for H⁺/H₂), while the holes in g-C₃N₄ are less oxidizing than those of TiO₂ would be if they were photoexcited. This means that while separation increases, the reactions that they can drive may be more energetically limited. Thus, type II heterostructures sometimes cannot degrade highly stable pollutants requiring strong oxidants (such as PFAS) because their holes are not oxidatively potent enough. Nevertheless, for many purposes (mineralization of dyes, common pharmaceuticals), type II heterostructures have proven effective and simpler to design.

In summary, in g-C₃N₄ type II heterostructures, g-C₃N₄ typically acts as the high-CB component, transferring electrons to the second semiconductor and retaining holes. This improves e⁻/h⁺ separation but reduces the redox strength of each carrier a trade-off that has worked well in practice to enhance moderate photocatalytic activities.

3.2. g-C₃N₄ Z-Scheme Heterostructures

The Z-scheme mechanism was inspired by the natural photosynthesis “Z”-scheme and represents a configuration where two photocatalysts each retain one of the most energetic carriers (one retains high-energy e⁻, the other highly oxidizing h⁺). In contrast, the lower-energy carriers recombine with each other. Unlike type II, here, the electrons from the CB of one semiconductor recombine with the holes from the VB of the other across the heterointerface rather than migrating to opposite bands [88,89]. As a result, the remaining carriers are electrons in the CB of the semiconductor with the more negative CB (higher reducing power) and holes in the VB of the semiconductor with the more positive VB (higher oxidizing power). Thus, the Z-scheme achieves the best of both semiconductors, it maximizes available redox power while keeping useful carriers separated [86,87,88]. Moreover, photocatalytic Z-scheme heterostructures are classified into two main categories: (i) Mediated Z-scheme, which requires a redox or conductive intermediary (e.g., I⁻/IO₃⁻, Fe³⁺/Fe²⁺, or metallic nanoparticles) to facilitate interfacial e⁻/h⁺ recombination, and (ii) a Direct Z-scheme, where the selective recombination occurs at the semiconductor/semiconductor interface without external mediators, through intimate and efficient contact [90]. Most g-C₃N₄-based heterostructures reported in the literature correspond to direct Z-scheme configurations, leveraging their lamellar morphology to maximize the interfacial contact area. A representative example is the g-C₃N₄/BiVO₄ heterojunction, which combines the reducing properties of g-C₃N₄ with the strong oxidizing capacity of BiVO₄. In this system, g-C₃N₄ has a CB at approximately −1.1 eV (vs. NHE), while BiVO₄ VB is around +2.4 eV (vs. NHE) [91]. Thus, under visible-light irradiation, both semiconductors are photoexcited, generating electron–hole pairs in their respective bands. Subsequently, the photoinduced electrons in the CB of BiVO₄ (which has a lower reducing potential) recombine with the holes in the VB of g-C₃N₄ (which has a lower oxidizing potential) across the solid interface. Therefore, the most energetic charge carriers are retained, electrons in the CB of g-C₃N₄ (with high reducing power) and holes in the VB of BiVO₄ (with high oxidizing power) [92]. This arrangement maximizes the system’s redox efficiency, and the accumulated electrons in g-C₃N₄ can reduce O₂ to generate superoxide radicals (•O₂⁻) or reduce high-activation-energy species, whilst the holes in BiVO₄ can oxidize H₂O or OH⁻ to form hydroxyl radicals (•OH), facilitating the degradation of recalcitrant pollutants. This direct Z-scheme architecture not only enhances charge separation efficiency but also broadens the spectrum of applications in environmental decontamination and hydrogen production [89]. Boyuan Wu et al. [93] have made a significant scientific contribution focused on the interfacial modulation of Z-scheme heterostructures, by coupling ultrathin nanosheets of carbon nitride C₃N₅ (N-CN) with the organometallic compound Zr₆O₄(OH)₄(2-amino-1,4-benzenedicarboxylate)₆ (NH₂-UiO-66). The optimized configuration, NH₂-UiO-66/N-CN-2, achieved an outstanding hydrogen evolution rate of 3936 µmol h⁻¹ g⁻¹ under visible light irradiation, representing an enhancement of up to 48.6 times compared to pristine NH₂-UiO-66. This remarkable performance was primarily attributed to the efficient charge separation and transfer, facilitated by the formation of a stable and intimate Z-scheme interface, as illustrated in (Figure 11). This study provides valuable insights into the design of advanced MOF-based photocatalysts, emphasizing the critical importance of precise interface modulation in Z-scheme heterostructures to maximize efficiency in sustainable energy applications [93].

In a previous work, Vinesh et al. [94] presented the synthesis and characterization of a CuWO₄/g-C₃N₄ nanocomposite designed as a Z-scheme system for the photodegradation of tetracycline (TC) under visible light irradiation. The combination with g-C₃N₄ stabilized the structure of CuWO₄, mitigating its propensity for photocorrosion and improving the separation of e⁻/h⁺ pairs. This information has significant practical implications in the design of heterogenous structures, as it enhances the efficiency of the photocatalyst. Moreover, the direct Z-scheme heterojunction favored the internal recombination of low-energy carriers, retaining electrons in the conduction band of g-C₃N₄ and holes in the valence band of CuWO₄, thereby maximizing the redox potential. Furthermore, the CuWO₄/g-C₃N₄ achieved a 100% degradation efficiency for tetracycline 7.4 times higher than that of pure g-C₃N₄, which degraded only 13% of tetracycline in 120 min. On the other hand, the DFT and Bader studies corroborated the interfacial charge transfer, while trapping experiments confirmed the predominant involvement of •OH radicals, followed by •O₂⁻ and h⁺ in the photocatalytic mechanism [94].

In this regard, Li et al. [95] developed a novel ternary heterostructure of g-C₃N₄/α-Fe₂O₃/Bi₃TaO₇ (CN/FO/BTO) via an ultrasound-assisted calcination method, designed as a dual Z-scheme system for the photocatalytic degradation of ciprofloxacin under visible light. Interestingly, the synergy between the redox properties of Bi₂TaO₇ (high oxidizing capacity), the magnetic and oxidative function of α-Fe₂O₃, and the high reduction capability of g-C₃N₄ enabled efficient charge carrier separation and enhanced visible light absorption. Moreover, the dual Z-scheme system facilitated the selective recombination of low-energy carriers, retaining electrons in the CB of g-C₃N₄ and holes in the VB of BTO and FO, thereby maximizing the redox power. The optimized sample (CN/FO/BTO-30) achieved a ciprofloxacin degradation efficiency of 95.6% in 120 min, significantly outperforming individual and binary systems (up to 15.9 times more efficient than α-Fe₂O₃ alone). In this work, the dominant reactive species identified were •OH and •O₂⁻ radicals. Additionally, the inclusion of α-Fe₂O₃ provided magnetic properties that facilitated the separation and reuse of the photocatalyst without a significant loss of activity after multiple cycles [95].

One of the main challenges in constructing heterostructures with a Z-scheme mechanism is the need to achieve efficient interfacial contact between the coupled semiconductors. This interface plays a critical role in the selective recombination of low-energy charge carriers (particularly electrons in the CB of the second semiconductor and holes in the VB of the first). If the heterointerface quality is poor, this desired recombination does not occur efficiently, and the system may behave electronically similar to a type II heterojunction, where spatial charge separation follows a different pathway to the Z-scheme, compromising the available redox potential [96]. Thus, to maximize charge transfer efficiency and ensure the selective recombination inherent to the Z-scheme, the formation of interfaces with extensive contact areas has been proposed. In this context, 2D/2D contact configurations have proven highly effective, as they enable intimate and extended interaction between the semiconductor phases. A representative example is the direct heterojunction between g-C₃N₄ and Bi₂O₃ nanosheets, where the bidimensional arrangement of both structures favors charge transfer and enhances photocatalytic activity [97].

Another widely used strategy to facilitate electron transfer in Z-scheme systems is the introduction of conductive mediators, such as metallic nanoparticles (e.g., Au, Ag) or carbon-derived materials (e.g., graphene, rGO). These mediators act as efficient bridges for electron transport, permitting their transfer from the CB of the first semiconductor to the VB of the second, where they recombine with low-energy holes. A paradigmatic example is the g-C₃N₄/Ag/Ag₃PO₄ system, in which metallic silver nanoparticles not only serve as a bridge for electronic transfer but also play a crucial role in consolidating the Z-scheme architecture, thereby ensuring the integrity of the system and helping improve the stability of Ag₃PO₄ by mitigating its photocorrosion [98].

The use of conductive mediators in Z-scheme heterostructures, while effective in facilitating interfacial charge transfer, introduces challenges. These mediators, such as metallic nanoparticles or carbon materials, can increase synthetic complexity and, in some cases, act as excessive charge trapping centers, favoring undesired recombination of electrons and holes (quenching effect), thereby reducing the overall photocatalytic efficiency of the system. Despite these limitations, g-C₃N₄-based Z-scheme heterostructures have proven to be an effective strategy for optimizing spatial charge separation and retaining carriers with the highest redox potential. Moreover, the high-energy electrons in the conduction band and the strong oxidizing holes in the valence band thus maximize the surface redox reactions ideal for mineralizing persistent pollutants and water splitting, although their synthesis may require greater control [99,100].

3.3. g-C₃N₄ S-Scheme Heterostructures

The S-scheme mechanism (short for “step-scheme”) is an emerging concept introduced in 2019 and represents a significant evolution of traditional heterostructures. It integrates the advantages of type II and Z-scheme configurations, incorporating internal electric fields as a crucial element in charge separation [101]. In contrast to the conventional Z-scheme, the S-scheme operates without the need for external mediators or forced recombination. Instead, it relies on band engineering and the spontaneous formation of an interfacial electric field. In a typical S-scheme, both semiconductors are of n-type, with one serving as a highly reducing photocatalyst (very negative CB) and the other as a strongly oxidizing one (very positive VB) [102]. Once these semiconductors interact, their Fermi level differences cause electrons to flow from the semiconductor with the higher CB to the one with the lower CB until equilibrium is reached. This process creates a space charge region with an internal electric field (E) directed from the semiconductor with the lower CB to the one with the higher CB [103,104].

The interfacial electric field plays a crucial role in the dynamics of charge separation. Under irradiation, photogenerated electrons in the CB of the oxidizing photocatalyst tend to recombine with holes in the VB of the reducing photocatalyst at the interface, driven by the direction of the field. This results in the retention of the more energetic carriers, electrons in the CB of the reducer, and holes in the VB of the oxidizer, thus conserving a strong redox potential [104,105]. The main difference from the conventional Z-scheme lies in the fact that, in the S-scheme, the internal field guides and reinforces this selective recombination, thereby improving overall efficiency in charge separation and photocatalytic activity [106]. Therefore, this mechanism represents a key factor in the advancement of the field of photocatalysis and materials science. A representative example is shown in Figure 12, which highlights the work of Qiaoya Tang et al., who developed an S-scheme heterojunction based on two n-type semiconductors, Bi₂WO₆ and g-C₃N₄. This configuration exhibits a staggered band alignment, with a CB of −0.75 eV for g-C₃N₄ and a VB of +2.72 V for Bi₂WO₆. Under visible light irradiation (λ > 420 nm), the system achieved CO and CH₄ production rates of 5.44 μmol g⁻¹ h⁻¹ and 0.91 μmol g⁻¹ h⁻¹, respectively, demonstrating efficient charge separation and strong reducing capability [102].

There are other S-scheme heterostructures with g-C₃N₄ is g-C₃N₄/ZnO, in which both are n-type. ZnO has a slightly less negative CB than g-C₃N₄ and a slightly less positive VB. When joined, a field forms that favor electrons from ZnO flowing toward g-C₃N₄. Under visible light illumination, g-C₃N₄ is excited; ZnO can also be excited via dopant states), and has benn is established that electrons from ZnO recombine with holes from g-C₃N₄, leaving electrons in g-C₃N₄ and holes in ZnO. In fact, Zhang et al. obtained an optimized ZnO/g-C₃N₄ S-scheme (10% g-C₃N₄) that achieved 95% paracetamol degradation in 60 min under visible light, much superior to non-optimized versions. This outstanding performance was attributed to the S-scheme efficiently leveraging the electrons in g-C₃N₄ (reducing O₂ to ROS species) and the strongly oxidizing holes in ZnO (directly oxidizing paracetamol), with an internal field preventing undesired recombination. The authors also observed high stability over cycles, indicating that ZnO did not suffer photocorrosion due to the rapid consumption of its electrons by g-C₃N₄ [107]. Interestingly, the S-scheme can be considered a conceptual evolution of the direct Z-scheme, providing a theoretical framework to understand n-n heterojunctions with internal fields. Its advantages include (i) very efficient charge separation driven by both the band difference and the internal electric field of the p-n or n-n junction; (ii) retention of the high redox power of carriers (as in the Z-scheme); and (iii) in some circumstances, greater stability because the interfacial recombination of “waste” carriers preferentially occurs in the junction region, minimizing damage from the accumulation of highly reactive charges in a single component. Different S-schemes report better stability than type II analogs. Furthermore, Li et al. argued that S-scheme photocatalysts exhibit highly improved and stable performances in H₂ production and pollutant degradation, surpassing conventional II and Z schemes [108]. Therefore, implementing S-scheme structures is a challenging strategy that requires a good understanding of each semiconductor is properties (types, band positions, and more.) to properly combine them. Moreover, most S-schemes are binary n-n systems; if one of the materials were p-type, a classic p-n junction would typically form, described by conventional p-n theory rather than strict S-scheme behavior (although sometimes a similar effect can be achieved). For example, g-C₃N₄ (n) combined with another n-type material is ideal for an S-scheme; with a p-type such as CuO, it would typically form a p-n heterojunction not described by the strict S-scheme but by the traditional p-n theory (sometimes a similar effect can be achieved).

The g-C₃N₄ S-scheme heterostructures constitute an advanced strategy in photocatalyst design, integrating the efficient charge separation of type II heterojunctions with the high redox potential of Z-schemes, achieved through band engineering and the formation of internal electric fields. Recent studies have demonstrated their competitive efficacy in degrading recalcitrant pollutants and driving high-energy-demand reactions, in addition to their notable operational stability. These promising initial results suggest significant future research and application expansion of g-C₃N₄-based S-scheme heterostructures for environmental and energy photocatalysis [109,110].

Figure 13 summarizes the fundamental charge transfer mechanisms in photocatalytic heterostructures with type II, Z-scheme, and S-scheme configurations. In the type II heterojunction (Figure 13I), the semiconductors exhibit staggered band alignments that promote spatial charge separation, with electrons migrating to the conduction band (CB) of the lower-energy semiconductor and holes to the valence band (VB) of the higher-energy one. Although this configuration suppresses charge recombination, it also reduces the redox potential due to charge accumulation at intermediate energy levels. In contrast, Z-scheme heterostructures (Figure 13II) retain the most energetic charge carriers through selective recombination. In the traditional variant, this recombination occurs via a liquid redox mediator (Figure 13II(a)); in the solid-state version, a solid conductor is used (Figure 13II(b)); and in the direct Z-scheme, recombination takes place at the interface without mediators (Figure 13II(c)). These configurations optimize the redox potential, particularly in the direct Z-scheme. Finally, the S-scheme heterojunction (Figure 13III) is based on band bending induced by electron redistribution upon Fermi level equilibration, which generates an internal electric field. This field drives the recombination of the less energetic carriers, retaining only the electrons with higher reducing power and the holes with stronger oxidizing ability, thereby enhancing photocatalytic efficiency without the need for external mediators. This comparative overview highlights how interfacial design and band alignment dictate the efficiency of charge separation and the redox potential of photogenerated carriers.

4. Photodegradation of Environmental Pollutants

The following section reviews specific applications of g-C₃N₄-based heterostructures in the photocatalytic degradation of organic pollutants. Four representative categories of emerging pollutants have been selected: toxic organic dyes, pharmaceutical pollutants, pesticides, and per- and polyfluoroalkyl substances (PFAS). For each category, research studies employing g-C₃N₄ heterostructures are discussed, highlighting the following:

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The effect of the synthesis method of g-C₃N₄-based heterostructures on the photocatalytic degradation performance of organic pollutants.

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The effect of the morphology and design of g-C₃N₄-based heterostructures (nanosheets vs. bulk, mesoporosity, etc.) on the photocatalytic degradation performance of organic pollutants.

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The effect of g-C₃N₄ doping in the coupled heterostructures and its influence on the photocatalytic degradation performance of organic pollutants.

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The influence of the type of coupled semiconductor and the involved charge transfer mechanism (type II, Z, and S) on the photocatalytic degradation performance of organic pollutants.

4.1. g-C₃N₄ Organic Toxic Dyes

Organic dyes are common pollutants found in industrial effluents (e.g., textiles, leather tanning, paper, and more), characterized by their high visibility and toxicity to aquatic life and humans, and some are even potentially carcinogenic. The numerous investigations being carried out to unveil an effective method for the degradation of organic pollutants employed broad family models of pollutants which we can classify into Cationic dyes such as methylene blue (MB), malachite green (MG), crystal violet, rhodamine B, and rhodamine 6G that stand out in addition to azo dyes, including methyl orange (MO), Congo red, and toxic yellow. Other dye groups such as thionine, erythrosine, tartrazine yellow, and xanthene are also used [111]. In general, their sulfonated aromatic ring usually confers stability against environmental degradation. In this sense, heterogeneous photocatalysis has proven to be an effective strategy to decolorize and mineralize dyes in water under solar or visible light. For instance, doped g-C₃N₄ in different heterostructures has been extensively studied in the degradation of dyes due to its visible activity. Interestingly, g-C₃N₄ can degrade some dyes under visible light, typically removing 50–70% in several hours. Nano-heterostructures based on g-C₃N₄ show substantial improvements, the photodegradation studies report very high degradation percentages (>80%), requiring from 0 to 90 min for the degradation of model dyes under visible or UV light, although the activity varies according to the catalytic structure [32]. Considering the second semiconductor coupled to g-C₃N₄, numerous materials have been tested, such as metal oxides (TiO₂, ZnO, CuO, Fe₂O₃, WO₃, Bi₂O₃, SnO₂, CeO₂, NiO_x, etc.), bismuth oxysalts (BiVO₄, Bi₂WO₆, BiOX), sulfides (CdS, MoS₂, ZnS), phosphates (Ag₃PO₄), and even carbon-derived materials (graphene, CNTs) as co-catalysts rather than active semiconductors. The results indicate that type II heterostructures of g-C₃N₄ conjugated with TiO₂, ZnO, and WO₃ improve the initial dye decolorization efficiency; however, sometimes they show an incomplete degradation efficiency due to lower residual oxidizing power. Nevertheless, g-C₃N₄/WO₃ has been shown to degrade ~90% of methylene blue in 2 h (visible), versus 50% with g-C₃N₄ alone [24]. Ternary combinations have also shown promising results (e.g., AgBr/Ag/g-C₃N₄, and Ag-mediated Z-scheme) demonstrating significantly higher degradation kinetics for MO and MB compared to the corresponding binary systems. Thus, researchers pay special attention to the dual function of Ag as both plasmon and Z-scheme bridge [69].

An optimal charge transport mechanism that preserves the highly reactive carriers can be achieved through Z-scheme heterostructures, as demonstrated by Li et al. the authors synthesized a BiVO₄/g-C₃N₄ system via impregnation, achieving a 98.3% degradation of malachite green (MG) within 60 min in the presence of H₂O₂ and showing a kinetic rate constant of 0.05391 min⁻¹. Moreover, the addition of H₂O₂ (20 mM) further enhanced the photocatalytic activity, resulting in complete degradation within 40 min and an increased rate constant of 0.19484 min⁻¹. Interestingly, the trap experiments and electron spin resonance (ESR) spectroscopy analysis confirmed that the Z-scheme mechanism promotes the retention of electrons in the conduction band (CB) of g-C₃N₄ and holes in the valence band (VB) of BiVO₄, generating •O₂⁻ (superoxide radicals) and •OH (hydroxyl radicals) responsible for dye oxidation. The system exhibited excellent stability over five consecutive cycles [112]. Previously, Qiu et al. synthesized a ternary Ag/ZnO/g-C₃N₄ photocatalyst with 1D-0D-2D hierarchical morphology through electrostatic self-assembly, integrating Ag nanowires (1D), ZnO nanoparticles (0D), and porous g-C₃N₄ nanosheets (2D). This configuration formed a direct Z-scheme heterojunction that enables efficient spatial charge separation, characterized by electrons in the CB of ZnO recombined with holes in the VB of g-C₃N₄, residual electrons in the CB of g-C₃N₄, and holes in the VB of ZnO remain available for redox reactions. In addition, the localized surface plasmon resonance (LSPR) effect of Ag nanoparticles extends light absorption into the visible region, enhancing the generation of reactive species such as •O₂⁻ (superoxide radicals) and h⁺ (holes). The Ag-ZnO/g-C₃N₄ ternary system exhibited a 98.3% degradation efficiency for methylene blue (20 mg/L) within 30 min under UV-visible irradiation, significantly outperforming its individual precursors. Similarly, the photocurrent analysis and electrochemical impedance spectroscopy (EIS) demonstrated enhanced charge carrier mobility and suppressed electron–hole recombination [113]. Complementarily, Hamza et al. engineered a g-C₃N₄/TiO₂/CuCo₂O₄ ternary heterostructure that achieved 99.9% rhodamine B (RhB) degradation within 60 min under simulated solar irradiation (AM 1.5G, 100 mW/cm²). They attributed the efficiency to the Z-scheme charge transfer mechanism between the three components, as it maintains high redox potentials [114].

In contrast, nanoheterostructures based on g-C₃N₄ materials show substantial improvements in degrading several dyes. An outstanding case is reported by R. Manimozhi et al. who synthesized a g-C₃N₄/ZnO heterostructure by a hydrothermal method, using g-C₃N₄ obtained by pyrolysis of melamine at 520 °C and ZnO prepared from zinc nitrate and hexamine. The photocatalytic activity was evaluated by the degradation of a 20 ppm solution of the crystal violet dye under visible irradiation. The results indicated that g-C₃N₄/ZnO nanocomposite achieved a degradation efficiency of 97% in 3 h, outperforming the individual precursor materials (88% each). Moreover, the degradation kinetics followed a pseudo-first-order model, with a rate constant of 0.014 min⁻¹ for g-C₃N₄/ZnO, higher than those of separate g-C₃N₄ and ZnO (0.010 min⁻¹ each) precursors. Trapping experiments indicated that holes (h⁺) are the predominant reactive species in the oxidative process. The proposed mechanism corresponds to a type II heterojunction, where excited electrons in the CB of g-C₃N₄ (−1.13 eV) transfer to the CB of ZnO (−0.28 eV), while holes remain in the VB of g-C₃N₄ (1.57 eV), promoting charge separation and the generation of •O₂⁻ and •OH radicals. After five cycles, the efficiency slightly decreased to 94%, confirming good operational stability [32]. This study demonstrates that band alignment between g-C₃N₄ and the coupled semiconductor enhances visible light absorption, charge separation and dye degradation efficiency. These parameters position these heterostructures as robust candidates for environmental photocatalysis applications. The work presented by Kocijan et al. who synthesized a g-C₃N₄@TiO₂ heterostructure through a simple hydrothermal method using titanium isopropoxide and urea as precursors, is an example of these aplications. The authors reported the photocatalytic activity through methylene blue (MB) degradation under simulated solar irradiation, where the nanocomposite with 32 wt% g-C₃N₄ achieved 99% degradation in 60 min, with a kinetic constant of 54.9 × 10⁻³ min⁻¹, superior to TiO₂ (8.2 × 10⁻³ min⁻¹) and g-C₃N₄ (9.8 × 10⁻³ min⁻¹) materials. The scavenger studied revealed that holes (h⁺) and electrons (e⁻) are the key reactive species, while hydroxyl radicals (•OH) have a secondary contribution. This supports a mechanism where photoexcited electrons in the conduction band of g-C₃N₄ (−1.13 eV) transfer to that of TiO₂ (−0.5 eV), while holes remain in the valence band of g-C₃N₄ (+1.57 eV), enabling efficient redox reactions and confirming a type II charge transfer mechanism [115].

Recent studies have reported cases of S-type heterostructures, which can be considered analogous to direct Z-scheme, but where charges are not completely neutralized, generating a strong internal field at the interface. In this scheme, the bands tilt forming an internal potential barrier that retains lower-energy photogenerated carriers, while others recombine. Yaqoubi et al. synthesized a CuMn₂O₄/g-C₃N₄ heterojunction through an ultrasound-assisted and coprecipitation method, achieving an S-scheme architecture with high performance in dye photodegradation under visible light. The exfoliated g-C₃N₄ maintains its 2D layered morphology, acting as a support for homogeneously distributed CuMn₂O₄ nanoparticles, which facilitates effective interfacial contact. The CuMn₂O₄/g-C₃N₄ nanocomposite achieved a 91% degradation efficiency of prepared erythrosine at 10 ppm in 90 min. The morphological properties, such as high surface area (18.6 m²/g) and mesostructured porosity, along with superior stability in successive cycles, reinforce its potential for environmental applications [116].

4.2. Pharmaceutical Pollutants

Pharmaceutical products that end up in water bodies through human/animal excretion or industrial disposal constitute pollutants of significant concern. These include antibiotics, analgesics, anti-inflammatories, synthetic hormones, antiepileptics, antidepressants, and metabolites like caffeine. Many pharmaceutical pollutants are stable (multiple rings, halogenated groups) and persist at low concentrations (ng–μg/L) after conventional treatments [117]. Photocatalysis using g-C₃N₄ heterostructures has emerged as a promising approach for degrading these pollutants effectively.

4.2.1. Antibiotics

Antibiotics such as tetracycline (TC), ciprofloxacin, chloramphenicol, and levofloxacin are frequently detected in water bodies. An emblematic example is tetracycline, widely studied for degradation. Vinesh et al. reported a CuWO₄/g-C₃N₄ Z-scheme heterostructure achieving 100% degradation of tetracycline (20 mg/L) within 120 min under visible light, outperforming pure g-C₃N₄ by approximately 7.4 times. This enhanced performance was attributed to highly positive VB (+2.7 eV) of CuWO₄ generating oxidizing holes and g-C₃N₄ supplying high-energy electrons, effectively forming radicals (•OH and •O₂⁻) that mineralize tetracycline. This system showed excellent stability over multiple cycles [94]. Additionally, g-C₃N₄/BiOCl heterostructures synthesized by ultrasonication and stirring achieved degradation of antibiotics like levofloxacin under solar irradiation [118]. Microwave-assisted synthesized BiOCl/g-C₃N₄ heterostructures degraded 96% nizatidine in just 30 min [119]. Similarly, calcination-based synthesis produced intimate interfaces in g-C₃N₄/TiO₂ heterostructures, significantly enhancing tetracycline degradation (90.1% in 30 min) by extending absorption to the visible spectrum [120].

4.2.2. Analgesics and Anti-Inflammatories

Analgesics and anti-inflammatories such as paracetamol, diclofenac, and ibuprofen are commonly found in aquatic environments. Hassan et al. developed an S-scheme ZnO/g-C₃N₄ heterostructure that efficiently degraded 95% of paracetamol within 60 min, achieving 93% total organic carbon mineralization. The optimal performance (with 10% g-C₃N₄) was due to a band gap reduction (2.39 eV), increased surface area (11.3 m²/g), morphology favorable for active site exposure, and effective ROS generation, primarily hydroxyl radicals (•OH) [107].

Other drug pollutants include antiepileptics (carbamazepine), antidepressants, and caffeine, which similarly exhibit stability and persistence. Although detailed case studies for these specific compounds were not highlighted explicitly in the reviewed literature, they remain important targets for g-C₃N₄-based photocatalytic systems, as these compounds generally share chemical resilience features that photocatalytic approaches effectively address.

To address the challenges posed by these diverse pharmaceutical pollutants effectively, considerable attention has been given to optimizing synthesis techniques and material morphology. Methods like hydrothermal treatment, microwave-assisted synthesis, ultrasonication, and one-step synthesis optimize interface quality and morphological control, crucial for enhanced charge separation. Particularly, 2D/2D heterostructures, nanosheets, and hierarchical three-dimensional architectures (mesoporous, laminated, nanoflowers) improve photocatalytic performance by increasing surface area, active sites, and charge transfer efficiency. In contrast, pure g-C₃N₄ demonstrates lower efficiency, underscoring the necessity of controlled morphology for improved performance.

Furthermore, doping g-C₃N₄ with metals (Cu, NiFe) or non-metals (P, S) markedly enhances conductivity, broadens absorption spectra, and improves photocatalytic activity. CuO/g-C₃N₄ showed significantly increased tetracycline degradation due to enhanced surface area, photo-voltage response, and reduced recombination [121]. Magnetic NiFe₂O₄ doping enabled a combined photo-Fenton effect, achieving 94.5% degradation of tetracycline within 80 min due to the generation of reactive •OH radicals via Fe²⁺ ions [122]. Non-metal doping (e.g., P-doped g-C₃N₄ with Bi₂WO₆ and S-doped CdS/g-C₃N₄) similarly enhanced charge separation, significantly increasing degradation efficiency [123,124].

Additionally, electron transfer mechanisms (type II, Z-scheme, S-scheme) significantly determine photocatalytic efficacy. Z-scheme heterostructures, like Bi₂WO₆/g-C₃N₄ with persulfate addition, enhanced tetracycline degradation to 98% in 60 min, outperforming pure g-C₃N₄ by a factor of 3.05 due to efficient electron transfer and enhanced radical formation (•SO₄⁻, •O₂⁻, and h⁺) [125]. Similarly, LaMnO₃/g-C₃N₄ heterojunctions achieved efficient electronic separation and maintained high redox potentials for effective pollutant removal [125]. These mechanisms underline the strategic importance of heterostructure configuration in improving charge migration and pollutant degradation efficiency.

Overall, advances in g-C₃N₄-based heterostructures indicate significant potential for efficient pharmaceutical pollutant degradation, employing optimized synthesis methods, controlled morphologies, strategic doping, and tailored electron transfer mechanisms.

4.3. Pesticides

Pesticides, including herbicides, insecticides and fungicides, used in agriculture can contaminate water sources through runoff and leaching. Many are organochlorine, organophosphorus or nitroaromatic molecules that are persistent and bioaccumulative. Their degradation using advanced oxidation methods such as photocatalysis is challenging, but it is crucial to remediate waters affected by agricultural, industrial, and domestic activities. The persistent presence of pesticides in water bodies represents a serious environmental and public health problem. In this context, heterogeneous photocatalysis with visible semiconductors is emerging as a viable solution to mineralize these resistant pollutants. Among them, g-C₃N₄ heterostructures have been studied in the degradation of pesticides such as imidacloprid (neonicotinoid insecticide), atrazine (triazine herbicide), paraquat (herbicide), methomyl, dichlorvos, and others [126,127]. Among the most important advances in the photocatalysis of g-C₃N₄ for pesticide degradation, doping, defect generation in g-C₃N₄, and heterostructure formation are highlighted, and these have been key strategies in overcoming the high charge recombination presented by g-C₃N₄ [128].

Several studies have been conducted exploring the application of g-C₃N₄ heterostructures for the effective removal of pesticides via photocatalysis, with a focus on aspects such as synthesis methods, morphology control, doping, and charge transfer. A remarkable and pioneering study in this field was reported by Xue Liu et al., who used g-C₃N₄ synthesized through the thermal condensation of melamine to photodegrade imidacloprid in water, a highly toxic insecticide. The photocatalytic assays were performed under visible-light irradiation (λ > 400 nm), achieving approximately 90% degradation of imidacloprid within 5 h. The degradation byproducts were identified using liquid chromatography coupled with mass spectrometry (LC-MS), revealing cleavage pathways of the imidazole ring and the nitroimine group in imidacloprid. The g-C₃N₄ followed the typical photoexcitation mechanism, in which the electrons of the conduction band reduced O₂ to superoxide radicals (•O₂⁻), and the valence band holes oxidized water to generate hydroxyl radicals (•OH), collectively driving pesticide degradation [129].To improve efficiency in the photoexcitation of imidacloprid pesticide under visible-light irradiation, many researchers had reported different uses of heterostructures based on g-C₃N₄, which is the case for Takumi Inoue et al. who developed an innovative type Z heterostructure based on graphitic carbon nitride (g-C₃N₄) and converter slag (CS), which is a metallurgical byproduct containing significant amounts of metallic oxides such as Fe₂O₃. Through calcination at 550 °C, an effective bond formed between g-C₃N₄ and CS, promoting photoinduced charge separation under visible-light irradiation (λ > 420 nm), allowing the converter slag’s electrons on the conduction band (CB) to transfer their charge to the valence band (VB) of g-C₃N₄, retaining reducing electrons in the latter and oxidizing holes in CS, generating reactive species such as superoxide radicals (•O₂⁻) and hydroxyl radicals (•OH). This setup improved photocatalytic activity, reaching an imidacloprid pesticide degradation efficiency of 93.8% in 60 min, with a pseudo-first-order kinetic constant of 0.054 min⁻¹. The effective union between g-C₃N₄ and CS favors the separation of photoinduced charges under visible light irradiation (λ > 420 nm), allowing the allowing electron transfer from the conduction band of the slag and hole transfer to the valence band of g-C₃N₄, retaining reducing electrons in the latter and oxidative holes in the CS. This configuration enhanced the photocatalytic activity, achieving an imidacloprid pesticide degradation efficiency of 93.8% in 60 min, with a pseudo-first-order kinetic constant of 0.054 min⁻¹ [130]. Additionally, Baneesh Patial et al. reported the hydrothermal synthesis of a Z-type heterostructure (m-t)BiVO₄/g-C₃N₄, containing 10 wt% g-C₃N₄. In this system, photoexcited electrons from BiVO₄ migrate to the valence band of g-C₃N₄, leaving active holes in BiVO₄ and high-energy electrons in g-C₃N₄, promoting the formation of reactive radicals. This study achieved a 94.2% degradation of imidacloprid (20 mg·L⁻¹) in just 30 min at neutral pH under UV-C light (15 W·m⁻²), outperforming pure BiVO₄ (83.2%). The process followed a first-order kinetic model (k = 0.11 min⁻¹), and scavenging experiments confirmed that hydroxyl radicals (•OH) were the main active species. The catalyst maintained 92% of its efficiency after five photocatalytic cycles [70].

Shiqiao Zhou et al. synthesized a Z-type heterostructure of g-C₃N₄/WO₃, which enabled the efficient separation of electron–hole pairs, where the photoinduced electrons in g-C₃N₄ remained active in its conduction band, and the holes in the valence band of WO₃ participated in oxidative reactions, favoring the generation of •O₂⁻ and •OH radicals. The WO₃/g-C₃N₄ photocatalyst with 30 wt% WO₃ achieved 93.8% degradation of imidacloprid in 60 min, with a kinetic constant of 0.054 min⁻¹. It also demonstrated good stability after five cycles of pesticide photodegradation [131].

Beyond imidacloprid, heterostructures based on g-C₃N₄ have achieved the photodegradation of various pesticides. For instance, atrazine, a widely used herbicide, was degraded using a ZnO/g-C₃N₄ heterostructure synthesized by Nguyen Thi Thanh Truc et al. This system allows the migration of holes from the valence band of ZnO to that of g-C₃N₄, while the electrons in the conduction band of g-C₃N₄ remain active, promoting the generation of •O₂⁻ and •OH radicals. A photodegradation efficiency of 92.3% for atrazine was achieved in 120 min, with a pseudo-first-order kinetic constant of 0.0183 min⁻¹, maintaining this performance after five catalytic cycles [132].

Xiongfang An et al. synthesized a three-component heterostructure photocatalyst based on biochar–g-C₃N₄-MgO through a hydrothermal approach and calcination in an inert atmosphere. This photocatalyst showed 80.1% degradation of the pesticide dinotefuran in 260 min under simulated light, surpassing the system without MgO (56.6%). The electron transfer from the conduction band of MgO to the valence band of g-C₃N₄ promoted the simultaneous generation of •OH and •O₂⁻ radicals, which were responsible for the oxidation of dinotefuran. In this heterostructure, biochar acts as an efficient conductor and bridge between g-C₃N₄ and MgO [133].

Sahima Tabasum et al. synthesized a ternary nanocomposite g-C₃N₄/GO/V₂O₅ via a hydrothermal method, achieving remarkable efficiency in the photodegradation of the organophosphorus pesticide chlorpyrifos under visible light. The integration of graphene oxide (GO) and V₂O₅ with g-C₃N₄ enabled the effective separation of photogenerated charges, with GO acting as an electron bridge and V₂O₅ as a hole trap, supporting an indirect Z-scheme mechanism. Estimated band levels suggest that the excited electrons from V₂O₅ migrate to GO, while the holes remain in the valence band of V₂O₅, facilitating the generation of hydroxyl radicals (•OH), identified as the key reactive species. At pH 4, the nanocomposite achieved 88.97% degradation in 120 min, outperforming pure g-C₃N₄ (67.47%) and the binary composites. With 0.1% H₂O₂, the efficiency increased to 90.5%. The system demonstrated high stability and sustained activity over five cycles, underscoring its potential for the environmental remediation of organophosphorus pesticides [134].

Nekooie et al. developed a g-C₃N₄/(Cu-TiO₂) nanocomposite by depositing Cu-doped TiO₂ onto g-C₃N₄. Copper acts as a visible-light dopant in TiO₂ and a charge separation promoter. Under visible light, the g-C₃N₄/Cu-TiO₂ degraded approximately 60% of the insecticide endosulfan in 80 min, tripling the efficiency of pure g-C₃N₄ under the same conditions. The improvement is attributed to the electron flow from g-C₃N₄ (with a more negative conduction band) to TiO₂, and the hole transfer from TiO₂ to g-C₃N₄ (with a more positive valence band), which reduces recombination and extends the visible light response due to Cu doping [135].

Another significant study was conducted by Sivakumar Vigneshwaran et al., who developed a CS/g-C₃N₄ heterostructured photocatalyst based on chitosan (CS) and g-C₃N₄ via a sol–gel method, aimed at the remediation of water contaminated with chlorpyrifos, a highly toxic organophosphorus pesticide. The incorporation of CS notably improved the efficiency of g-C₃N₄ by acting as an electron transporter and promoting the separation of photogenerated charges. The photocatalyst achieved 85% degradation in 50 min, compared to 72% with pure g-C₃N₄, under visible light irradiation from a 300 W xenon lamp. The •OH radicals and h⁺ were the main active species, and the kinetics followed a pseudo-first-order model. Furthermore, CS/g-C₃N₄ showed high stability after five reuse cycles [136].

In conclusion, g-C₃N₄-based heterostructures have also proven effective in degrading a wide variety of pesticides, achieving the mineralization of highly persistent compounds such as atrazine, when combined with suitable materials and under efficient charge transfer schemes. A common finding is that the initial adsorption of the pesticide onto the photocatalyst is crucial: many pesticides are hydrophobic, so adding components that increase surface hydrophobicity can enhance the degradation rate. Additionally, the generation of •OH radicals remains key to breaking down stable structures; photocatalysts that promote •OH formation generally perform better. g-C₃N₄ heterostructures, with their design versatility, allow for the incorporation of such characteristics, hence their strong performance. Further studies at the pilot scale using real agroindustrial wastewater are needed to confirm their practical viability in pesticide remediation.

4.4. Per- and Polyfluoroalkyl Substances (PFAS)

Per- and polyfluoroalkyl substances (PFASs), such as perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), are considered highly stable emerging organic pollutants. They possess strong C─F bonds and high chemical stability, making them resistant to degradation by conventional methods. Since PFAS generally do not degrade or naturally dissipate in aquatic environments, there is an urgent need to develop efficient and effective treatment technologies to remove these substances from water.

Advanced photocatalysis has recently been explored for PFAS degradation, often combined with oxidative processes. g-C₃N₄ alone exhibits limited efficacy for the photocatalytic degradation of PFAS. However, heterostructures and coupled systems have been designed to enhance degradation. One approach involves improving PFAS adsorption onto the photocatalytic surface, as PFASs exhibit low reactivity but may become more reactive in the presence of radical sites. For example, halogen (F)-doped g-C₃N₄ has been shown to increase surface hydrophobicity and generate nitrogen vacancies, enabling the capture of molecules such as PFOA [137,138].

Beyond doping, numerous studies have developed g-C₃N₄-based heterostructures coupled with other semiconductors to enhance photoinduced charge separation and improve solar spectrum utilization. For highly recalcitrant pollutants like PFAS, maximum redox potential is required to break C─F bonds, which has driven interest in Z-scheme and S-scheme architectures. In a Z-scheme, coupled photocatalysts selectively recombine specific charge carriers, retaining the most energetic CB electrons and the most potent VB holes from each material, mimicking the natural Z-scheme of photosynthesis. The S-scheme (step-scheme) is a specialized case of direct p–n heterojunction or band bending, where preferential charge migration and interfacial recombination leave a “strong” electron in the CB of one semiconductor and a “strong” hole in the VB of the other, simultaneously reducing recombination and achieving a high redox power [139].

A Z-scheme heterostructured photocatalyst applied to PFAS degradation was reported by Peidong Su et al., who developed indirect Z-scheme heterostructures of g-C₃N₄/ZIF67 and g-C₃N₄/MIL-100(Fe) via in situ hydrothermal synthesis. This highly adsorbent and sunlight-active photocatalyst was optimized for the photodegradation of PFOA. The incorporation of ZIF67 and MIL-100(Fe)—two metal–organic frameworks (MOFs) with high surface areas (177.9 and 161.5 m²·g⁻¹, respectively)—provided pores for PFAS trapping and metal sites capable of activating oxidants, while g-C₃N₄ generated photoinduced e⁻/h⁺ pairs. The bandgaps were reduced to 2.14 eV (ZIF67@C₃N₄-24.4%) and 2.23 eV (MIL-100(Fe)@C₃N₄-70%), enhancing the visible-light response. ZIF67@C₃N₄-24.4% achieved 79.2% PFOA photodegradation in 8 h, while MIL-100(Fe)@C₃N₄-70% reached 60.5%. Holes (h⁺) and •O₂⁻ radicals were identified as the dominant reactive species. These heterostructures demonstrated outstanding synergy between adsorption and oxidation [72]. Juying Li et al. developed an advanced photocatalytic system based on exfoliated g-C₃N₄ (CN) and Fe³⁺ for the degradation of PFOA under visible light, highlighting the effectiveness of the CN/Fe heterostructure, which achieved degradation and defluoridation rates exceeding 95% within 70 h, even for concentrations as low as 20 μg/L. LC-MS analysis revealed the formation of short-chain PFASs (PFHpA, PFHxA, PFPeA, PFBA), demonstrating a stepwise degradation pathway. This study illustrates the potential of g-C₃N₄ in sustainable environmental remediation technologies [140]. Navidpour, A.H. et al. synthesized a citric acid-modified ZnO@g-C₃N₄ composite via mechanical milling and heat treatment for the removal of perfluorooctanoic acid (PFOA). The incorporation of 5% by weight of modified g-C₃N₄ reduced the band gap of ZnO from 3.23 to 2.89 eV, improving visible light absorption and facilitating the formation of an S-scheme heterostructure that promotes effective separation of photoinduced charges. Under UV irradiation, the nanocomposite achieved an apparent removal rate of 0.468 h⁻¹, compared to 0.097 h⁻¹ for ZnO alone, which increased to 0.868 h⁻¹ upon the incorporation of peroxymonosulfate (PMS), demonstrating the key role of •SO₄⁻ and •OH radicals. The photodegradation was accompanied by progressive mineralization of PFOA, mediated by superoxide radicals (•O₂⁻) and photogenerated holes. This hybrid strategy demonstrates high efficiency, low cost, and feasibility for the remediation of fluorinated pollutants under visible and UV light [139].

On the other hand, Jianhua Yang et al. designed a heterostructure based on Fe-g-C₃N₄/BiVO₄ through hydrothermal synthesis, aimed at the efficient degradation of perfluorooctanoic acid (PFOA). Iron doping of g-C₃N₄ significantly improved visible light absorption and electron–hole pair separation, facilitating the formation of a Z-scheme charge transfer mechanism. This mechanism allows electrons in the CB of BiVO₄ to transfer to the VB of Fe-g-C₃N₄, while preserving the holes in BiVO₄ and electrons in Fe-g-C₃N₄, generating reactive species such as •OH and •O₂⁻. Under visible light (500 W) and pH 3.0, the system achieved 88.6% PFOA degradation (5 mg·L⁻¹) in 12 h, with pseudo-first-order kinetics (k = 0.168 h⁻¹). DFT studies and EPR confirmed interfacial charge distribution and the participation of Fe²⁺/Fe³⁺ as active centers. The system showed high stability (73.7% efficiency after 5 cycles), standing out as a promising strategy for PFAS remediation in aqueous media [141].

Wenli Yao et al. developed an innovative heterostructure based on tourmaline-doped g-C₃N₄ with trace cobalt (Co/TM/g-C₃N₄), which exhibited high photocatalytic efficiency in the degradation of perfluorooctanoic acid (PFOA) through synergistic activation of peroxymonosulfate (PMS) under visible light. This structure favored efficient electron–hole pair separation, as evidenced by the decrease in photoluminescence intensity. The system showed 81.11% PFOA removal in 4 h (at pH 3, 0.5 g/L catalyst, 2.5 g/L PMS) and a defluorination rate of 31.12%. The mechanism involved multiple reactive species (•SO₄⁻, •OH, and •O₂⁻), with •O₂⁻ and h⁺ being predominant. The Co–P coupling and the presence of tourmaline optimized band alignment and charge transfer, maximizing radical generation. This strategy highlights the role of g-C₃N₄-based heterostructures in the advanced remediation of resistant PFAS in real aqueous matrices [142].

A highly relevant and important study was conducted by Cuizhu Sun et al., who developed a reusable photocatalytic sponge based on chitin/polyethyleneimine/O-g-C₃N₄ (ChPCN), which demonstrated exceptional efficiency for PFAS removal, achieving photodegradation rates of 97.9% for PFOA and 99.7% for perfluorooctane sulfonates (PFOSs) under visible irradiation. The O-g-C₃N₄ material was synthesized via hydrogen peroxide oxidation, reducing the bandgap energy from 2.3 eV (g-C₃N₄) to 1.7 eV, significantly improving light absorption and electron–hole pair separation. The ChPCN sponges exhibited a well-interconnected porous morphology and homogeneous photocatalyst distribution, favoring both pre-adsorption and accessibility to active sites. In the ChPCN heterostructure, the conduction band (−0.67 eV) and valence band (+1.82 eV) of O-g-C₃N₄ were properly aligned to enable the generation of reactive species such as •O₂⁻, ·OH, and holes (h⁺), which synergistically participated in C–F bond cleavage through S-scheme mechanisms [143].

It is clear that, although research focused on the photocatalytic degradation of PFAS through g-C₃N₄-based heterostructures is increasing daily, studies reported in the literature remain scarce. While multiple investigations have explored Z-scheme, S-scheme, and other heterostructures for other pollutants, only a few have systematically addressed PFAS defluorination, despite this approach demonstrating notable efficiency improvements, with degradation rates exceeding 90% and relevant defluorination due to better charge separation, increased generation of reactive species and preserved redox potential. However, these advances represent isolated cases, and the lack of a systematic foundation of studies on modified or coupled g-C₃N₄ for PFAS limits a comprehensive understanding of the mechanisms involved and their scalability in complex real matrices.

Therefore, it is essential to promote research that rationally designs functional g-C₃N₄ heterostructures (

Table 1. g-C₃N₄-based heterostructures in the photodegradation of environmental pollutants.

Heterostructure	Synthesis Method	Environmental Pollutants	Degradation (%)	Time (min)	Source Light	Ref.
g-C3N4/grafeno	Ensemble solution	Atrazine	100	300	Visible	[31]
g-C3N4/ZnO	Hydrothermal	Crystal violet	97	180	Visible	[32]
BiFeO3/g-C3N4	Calcination	Rhodamine B	~94	-	Visible	[33]
g-C3N4/Mg-ZnFe2O4	Hydrothermal	Methylene blue	High	-	Visible	[34]
g-C3N4/ZnO	Calcination	Crystal violet	95.9	120	UV	[35]
g-C3N4/SnO2	Calcination	nitric oxide (NO)	32	30	Visible	[61]
Zn3In2S6/g-C3N4	solvothermal	Tetracycline	80	150	Visible	[62]
g-C3N4/BiOCl, BiOBr, BiOI	Impregnation	Dye	High	-	Visible	[66]
MIL-100(Fe)@g-C3N4	Calcination	PFOA	70	-	Visible	[72]
CuWO4/g-C3N4	Calcination	Tetracycline	88	120	Visible	[94]
ZnO/g-C3N4	Hydrothermal calcination	Paracetamol	95	60	Visible	[107]
g-C3N4/BiVO4	Solvothermal	Malachite green	98	60	Visible	[112]
Ag/ZnO/g-C3N4	Autoensemble	Methyl blue	98	30	UV-Visible	[113]
g-C3N4/TiO2/CuCo2O4	Solvothermal	Rhodamine B	99	60	Solar simulated	[114]
g-C3N4@TiO2	Hydrothermal	Methyl blue	99	60	UV-A and simulated solar irradiation	[115]
CuMn2O4/g-C3N4	Coprecipitation—ultrasonic	Erythrosine	91	90	Visible	[116]
g-C3N4/TiO2	Calcination	Tetracycline	90	30	Solar simulated	[120]
NiFe2O4/g-C3N4	Sol–gel	Tetracycline	94.5	80	Visible	[122]
Bi2WO6/P-g-C3N4	Solvothermal	Tetracycline	High	-	Visible	[123]
CdS/S-g-C3N4	Difusión en estado sólido (SSD)	Tetracycline	91	60	Visible	[124]
		Methyl orange	~100

) specifically aimed at PFAS degradation, considering key aspects such as band-level alignment, structural stability in aqueous media, efficient radical generation, defluorination performance, and applicability in real water samples. These studies will help bridge the critical gap between material design and the effective remediation of highly persistent emerging contaminants.

5. Conclusions and Future Perspectives

Heterogeneous photocatalysis based on graphitic carbon nitride (g-C₃N₄) and its heterostructures represents a highly effective and promising strategy to address the growing environmental challenges associated with persistent organic pollutants in effluents and water bodies. This metal-free semiconductor stands out due to its chemical stability, low cost, and ability to be activated under visible light irradiation, offering a sustainable and efficient framework for environmental applications.

The development of g-C₃N₄-based heterostructures has demonstrated significant improvements in photocatalytic efficiency, primarily by optimizing the separation of electron–hole pairs generated through photoexcitation. The different charge transfer mechanisms (type II, Z-scheme, and S-scheme) possess unique characteristics that determine the degradation efficiency of specific pollutants. In particular, Z-scheme and S-scheme heterostructures emerge as the most promising due to their ability to preserve the redox potential of the generated charge carriers, achieving high mineralization rates even for highly persistent pollutants such as PFAS.

Despite these advances, major challenges remain in the controlled and reproducible synthesis of heterostructures with optimal interfaces, which is crucial to ensuring efficient charge transport. Additionally, the long-term stability and scalability of these systems continue to be key limitations for their practical application in real wastewater treatment processes.

As future perspectives, it is critically important to advance in the following key areas:

✓

Developing innovative and sustainable synthesis techniques that enable precise control over the morphology, composition, and interfacial structure of g-C₃N₄ heterostructures.

✓

Deepening the fundamental understanding of interfacial charge transfer mechanisms through advanced techniques such as transient spectroscopy and computational modeling, facilitating the rational design of photocatalysts with optimized performance.

✓

Exploring ternary and quaternary combinations with advanced materials (MXenes, MOFs, metallic nanoparticles) to further expand spectral absorption and enhance charge separation, promoting synergies that significantly increase photocatalytic efficiency.

✓

Carrying out a comprehensive evaluation of economic and environmental feasibility for scaling these processes to an industrial level, including life cycle assessments (LCA) and pilot studies validating their performance under real operational conditions.

In summary, g-C₃N₄-based heterostructures offer a broad horizon for research and technological development in the application of photocatalysis to environmental remediation. Continued progress in this advanced oxidation technology will enable the realization of viable and sustainable solutions to address the environmental crisis caused by water pollution from toxic and persistent substances.