Polymeric graphitic carbon nitride (briefly referred to as g-C₃N₄), which reveals a structure resembling graphite and is interconnected by both triazine (C₃N₃) and tris-triazine/heptazine (C₆N₇) rings, is one of the seven types of C₃N₄ (Zheng Y. et al., 2015; Jia et al., 2023). Among the various carbon-nitride allomorphs, the first discovered one is g-C₃N₄, which is the most stable under common atmospheric parameters However, it has not been utilized as a photocatalyst for a very long time. Since 2009, g-C₃N₄ has begun to be favored as a highly promising photocatalyst and its research then experienced explosive growth (Wang et al., 2009b; Cao et al., 2015).

The non-toxic and physicochemical stable polymer semiconductor g-C₃N₄ can be produced via a facile thermal polymerization method by using the low-cost nitrogen-rich and oxygen-free component precursors such as thiourea (Yu et al., 2017), dicyandiamide (Ji et al., 2013), cyanamide (Li et al., 2013), urea (Martin et al., 2014), and melamine (Yan et al., 2009; Mo et al., 2015). Simultaneously, g-C₃N₄ as a graphite-like layered material can be exfoliated to two-dimensional layered material. In recent years, there has also been plenty of research put the spotlight on transforming bulk g-C3N4 into nanosheets through exfoliation, particularly through liquid phase exfoliation (Yang et al., 2013), thermal exfoliation (Xu et al., 2014), ultrasonic exfoliation (Zhao et al., 2014b), chemical exfoliation (Xu et al., 2013), and ultrasonic exfoliation following thermal etching (Zhao et al., 2014a) methods. Furthermore, it is worth noting that controlling the dimension and nanostructure design were also commonly employed research approaches for g-C₃N₄. For the reason that the malleable framework structure of g-C₃N₄, nanorods (Zeng et al., 2018), nanosheets (Yang et al., 2013), nanospheres (Zheng et al., 2015a), and porous (Wu et al., 2020) g-C₃N₄ were satisfactorily obtained. The electronic energy bandgap of g-C₃N₄ is 2.7 eV, allowing it to utilize solar energy at wavelengths shorter than 460 nm (Zheng Y. et al., 2015). This makes it an attractive option for a photocatalyst responsive to visible light. At the same time, the attractive energy levels of the conduction band (CB, −1.1 eV) and valence band (VB, +1.6 eV) in g-C₃N₄ are appropriate for various photocatalytic reactions (Akhundi et al., 2019), such as hydrogen evolution (Kang et al., 2015), CO₂ reduction (Pengfei Xia et al., 2017), disinfection (Ma et al., 2016), and pollutant degradation (Jo and Selvam, 2017).

However, several fundamental disadvantages impede the practical application of g-C₃N₄. First of all, although g-C₃N₄ theoretically possesses a layered two-dimensional arrangement bound by van der Waals forces, its actual specific surface area is quite low. This results in an inability to provide numerous active sites, mitigating the rapid rate of recombination for photoexcited carriers (Niu et al., 2012). Secondly, the effectiveness of photogenerated electron (e⁻) and hole (h⁺) is quite low. Finally, when exposed to visible light, pristine g-C₃N₄ can only harvest light with wavelengths below 460 nm, leading to a relatively inefficient utilization of solar (Prasad et al., 2019). As a consequence, attempts to overcome the disadvantages of g-C₃N₄ and enhance its performance in photocatalysis through various methods have never ceased in recent years.

In the pursuit of advanced photocatalytic systems that effectively harness solar energy and efficiently separate photogenerated electron-hole pairs, various endeavors have been undertaken, heterojunction construction stands out as a promising and attention-grabbing approach. It is noteworthy that among diverse heterojunctions, Z-scheme photocatalytic system, which simulates the photosynthetic mechanisms in nature, has received the greatest attention in recent years (Zhou et al., 2014). Basically, Z-scheme photosynthesis is designed to mimic the process of photosynthesis that occurs in green plants in nature. As shown in Figure 2A, the photocatalysts with semiconductor heterojunctions in the traditional Z-scheme photocatalytic design will produce photoinduced electrons and holes under a light-excited state, with charge carriers remaining in the CB and VB, respectively. Then, an electron acceptor/donor (A/D) serves as an intermediary for electron transfer. Facilitating the transfer of photoinduced electrons is achieved by utilizing the redox mediators of the A/D pair. This enables the electrons to shuttle from the CB of one photocatalyst to the VB of another (Zhou et al., 2014). Therefore, the retained photo-generated e⁻ and h⁺ in each of the two semiconductor photocatalysts engage in distinct reduction and oxidation processes individually. During this process, the photo-generated e⁻ and h⁺ experience improved charge separation and exhibit enhanced redox potentials when involved in redox reactions, thereby enhancing the photocatalytic performance effectively. However, this traditional Z-scheme system that mimics the natural photosynthesis is always a liquid-phase system due to the A/D pairs are often in a liquid-phase environment, such as IO³⁻/I⁻, Fe³⁺/Fe²⁺ (Ge and Li, 2017). Therefore, this type of Z-scheme photocatalytic configuration is usually liquid-phase. This significantly hinders the utilization of photocatalysts, such as in the photocatalytic decomposition of pollutants, where pollutants can retard the redox reaction of the A/D and affect the photocatalytic performance (Zhou et al., 2014). Consequently, the ASSZ and DZ photocatalytic mechanism design have emerged.

Compared to traditional Z-scheme mechanism with a liquid phase, the absence of a liquid phase in ASSZ and DZ systems offers several advantages in terms of preparation and application and has garnered increased attention and research in the current period. The research on Z-scheme photocatalytic systems has entered a completely new stage since Tada et al. (2006) propounded an ASSZ photocatalytic configuration with Au as an electron shuttle between TiO₂ and CdS in 2006. As depicted in Figure 2B, the ASSZ mechanism utilizes solid-state conductors as electronic medium, forming the ohmic junction with minimal contact resistance among the pair of semiconductors, and the path of charge carriers transport is comparable to that of a traditional z-scheme setup (Zhou et al., 2014; Deng et al., 2018). In the advancement of ASSZ photocatalytic mechanism, there has been extensive research on using noble metals (Lu et al., 2017), graphene (Xian et al., 2014), and carbon dots (Asadzadeh-Khaneghah et al., 2021) as solid-state electron conduction mediators. The ASSZ mechanism has effectively addressed the stability issues and has broadened the range of applications in photocatalysis.

As the investigation into Z-scheme photocatalytic configuration advances, another new system, the DZ photocatalytic design, has been suggested. The DZ photocatalytic architecture was originally presented by Yu et al. (2013) in 2013. The operation of DZ was shown in Figure 2C, the photoinduced e⁻ transfer directly from one semiconductor to the another through a tightly surface contact of two semiconductors without the need for an electron medium (Low et al., 2017). The DZ photocatalysis setup retains the benefits of elevated efficiency in charge carrier separation and the optimized oxidation-reduction capacity of photogenerated electron-hole pairs, similar to traditional and ASSZ photocatalytic configurations. And it is worth mentioning that the challenges posed by the shielding effect induced by the carrier transport mediators and photo-corrosion will be effectively resolved in a photocatalytic system with a DZ (Xu Q. et al., 2018).

Composite semiconductor photocatalysts typically possess excellent properties, including a tunable bandgap, increased capture of visible light, and the ability to induce defects to hinder electron-hole pair recombination generated during the assimilation of light (Dahl et al., 2014). Due to its exceptional layered two-dimensional arrangement, g-C₃N₄ is beneficial for hybridization with other components and the construction of composite photocatalysts, such as surface coupling and doping with other semiconductors (Zhao et al., 2015). In recent years, ternary composites with g-C₃N₄ have been a very encouraging method to broaden the applicability of g-C₃N₄. Furthermore, owing to the ability of ternary composite photocatalysts established on g-C₃N₄ form double heterojunction or heterojunction-metal structure, they can optimal performance enhancement of g-C₃N₄ in photocatalysis more effectively comparison with g-C₃N₄-based binary composites (Zhang et al., 2021b). In order to achieve superior performance in terms of visible light reactivity, charge carrier separation, and interfacial charge transfer when compared to binary composite photocatalysts, the g-C3N4 ternary hybrid photocatalysts were designed (Mao et al., 2018; Beyhaqi et al., 2020). In the survey by Xie and Zhang (2018) on ternary Z-scheme photocatalyst based on Ag₃PO₄, it was also found that the ternary system provided enhanced electron transfer channels, accordingly efficiently mitigating the photocorrosion of the photocatalyst. In a similarly constructed ternary nanocomposite Ag₂CrO₄/g-C₃N₄/graphene oxide by Xu D. et al. (2018), it was proposed that the ternary composite system effectively addressed the issue of sluggish electrons and holes transfer and effectively involved photogenerated carriers in the redox reactions, providing more efficient redox reaction sites and making a significant contribution to charge separation. In view of the numerous novel discoveries made in recent years in the field of ternary composite Z-scheme photocatalysts based on g-C₃N₄, we believe it is necessary to conduct a review of this subject to accelerate further advancements in this promising research field.

In the draft and construction of an ASSZ photocatalysis setup, the electron shuttle with good electrical conductivity and electron mobility is a considerable situation that requires careful thought. Traditionally, noble metals like Ag, Au, and Pt have served as carrier transport agents. Simultaneously, the loading of noble metals induces the surface plasmon resonance (SPR) effect, efficiently enhancing the visible light region absorption phenomenon by the photocatalysts (Si et al., 2020; Madhusudan et al., 2021). Furthermore, precious metals on the surface of semiconductor nanostructures form Schottky junctions, optimizing the photocatalytic capabilities (Jiménez-Calvo et al., 2020; Yu et al., 2021). The latest advancements g-C₃N₄-based all-solid-state ternary Z-scheme photocatalysis, employing valuable metals as electron shuttles, are outlined in Table 1.

With the intention of overcoming the hurdles of quick recombination kinetics of light-induced electron-hole pairs and a limited surface-to-mass ratio of pure g-C₃N₄, Lu et al. (2017) proposed a z-scheme ternary photocatalysts g-C₃N₄/Ag/MoS₂ with significantly upgraded visible-light-induced photoactivity. As shown in Figure 3A, the silver (Ag) nanoparticles were decorated on g-C₃N₄/MoS₂ flowerlike microstructure through a photodeposition method, and Ag infiltrated the mesoporous structure within the microspheres composed of g-C₃N₄/MoS₂, positioning itself between g-C₃N₄ and MoS₂ components. The Rhodamine B photocatalytic decolorization and H₂ generation induced by visible light on g-C₃N₄/Ag/MoS₂ composite were 3.56 and 2.08 fold boost, respectively, as opposed to those on g-C₃N₄/MoS₂ (Figure 3B). This notable enhancement in photocatalytic efficiency under visible illumination is largely credited to the collaborative effects arising from the presence of Ag, g-C₃N₄, and MoS₂, operating within a Z-scheme setup as illumination in Figure 3C. In this system, Ag performs the function of a location facilitating charge transport, where photoinduced e⁻ transfer to metallic Ag from the CB of MoS₂ and subsequently passes through Ag to amalgamate with h⁺ situated at VB of g-C₃N₄. At the same time, the absorption and utilization of visible light by the sample are also enhanced owing to the impact of Surface Plasmon Resonance (SPR) caused by the attendance of Ag. The same g-C₃N₄-based ternary structure has also been prepared by other researchers in systems such as g-C₃N₄/Ag/NaTaO₃ (Tang et al., 2018), g-C₃N₄/Ag/BiVO₄ (Chen F. et al., 2017; Deng et al., 2018), gC₃N₄/Ag/Bi₃TaO₇ (Ren et al., 2019), g-C₃N₄/Ag/Ag₃PO₄ (Li G. et al., 2021), g-C₃N₄/Ag/LaFeO₃ (Zhang et al., 2021a), g-C₃N₄/Ag/Ag₂CrO₄(Yu et al., 2021), g-C₃N₄/Ag/AgVO₃ (Liu et al., 2019), g-C₃N₄/Ag/AgCl (Bao and Chen, 2016), and g-C₃N₄/Ag/TiO₂ (Liu et al., 2022). In these systems, the placement of Ag is dispersed at the interface boundary of g-C₃N₄ and another semiconductor, functions as a link for electronic conveyance to expedite charge transfer. In these ternary Z-scheme photocatalytic configurations based on g-C₃N₄, the photocatalytic efficiency when illuminated by visible light has been significantly enhanced as opposed to pristine g-C₃N₄ and binary systems.

Analogous to Ag serving as an electron shuttle, Pt is also a commonly employed noble metal as an electron mediator. For example, Raza et al. (2020) prepared a g-C₃N₄/Pt/Cu₂ZnSnS₄ ternary photocatalyst, which can function as a Z-scheme photocatalyst architecture responsive to visible spectrum for the photochemical reduction of CO₂ into CO and CH₄. The CO and CH₄ yield rates are 3.31 and 5.56 times greater than the values observed for unmodified g-C₃N₄. In this composite ternary photocatalyst based on g-C₃N₄, the existence of Pt and Cu₂ZnSnS₄ results in synergistic effects, including localized surface plasmon resonance, electron sink function, a higher area on the surface, and electron migration in a Z-scheme, all of which collectively enhance the photocatalytic performance.

Additionally, gold (Au) can also act in the capacity of an electron shuttle in a Z-scheme photocatalysis setup. Zhang G. et al. (2020) prepared a Z-scheme photocatalyst, g-C₃N₄/Au/ZnIn₂S₄, which exhibited the highest efficiency in NO removal and a superior CO production rate when illuminated by visible light. Similarly, Madhusudan et al. (2021) also constructed an ASSZ photocatalyst, g-C₃N₄/Zn_0.5Cd_0.5S/Au, decorated with Au. This photocatalyst exhibits a 2.9-fold higher photocatalytic CO₂ reduction efficiency compared to Zn_0.5Cd_0.5S/g-C₃N₄. Worth mentioning is that at this g-C₃N₄/Zn_0.5Cd_0.5S/Au photocatalysts, the chemical bonding among these three components is a cornerstone for enhancing the photocatalytic capability of carbon dioxide reduction.

In those semiconductor-metal-semiconductor heterostructures mentioned above, the presence of precious metals plays the role of an e⁻ transfer bridge in Z-scheme photocatalysis, facilitating effective transmission of photoinduced charges, boosting the e⁻ and h⁺ separation, improving photon absorption in the visible light spectrum, and preserving superior oxidation-reduction capabilities of the composite. Consequently, the effectiveness in photocatalysis of Z-scheme photocatalytic design possesses the ability to be significantly enhanced.

Although the precious metal can be worked as an electronic medium in ASSZ photocatalytic configuration, its high price is a limitation that requires consideration. Carbon nanoparticles, as a nontoxic, inexpensive and environmentally friendly material, possess excellent conductivity that is beneficial for charge transfer. They have attracted considerable interest as key constituents in the preparation of ternary Z-scheme photocatalysis incorporating g-C₃N₄ as shown in Table 1. Wu and Wang (2021) demonstrated that a carbon film loaded on the exterior of WS₂ and g-C₃N₄ composite exhibits superior photocatalytic effectiveness in comparison to g-C₃N₄ and binary g-C₃N₄/WS₂ compound. The photodegradation rate of 2,4-dichlorophenol in the visible spectrum in ternary C@WS₂/g-C₃N₄ composites is about 3.15 and 3.06-fold higher than the values observed for primitive g-C₃N₄ and binary WS₂/g-C₃N₄, respectively. In this system, carbon particles measuring 30–50 nm in size, and having an amorphous structure, are coated onto WS₂, serving as the e⁻ shuttle in Z-scheme photocatalysis. Intriguingly, lacking carbon loading, a type I heterojunction structure forms in binary WS₂/g-C₃N₄, wherein the redox capability of e⁻ and h⁺ is diminished. However, upon the introduction of carbon loading, a Z-scheme structure is established, characterized by electron-hole pairs exhibiting enhanced redox capabilities and superior photocatalytic performance. The incorporation of carbon films between WS₂ and g-C₃N₄ alters the pathway for charge carrier transfer, where carbon acting as an electron transfer shuttle. The CB and VB potentials of g-C₃N₄ and WS₂ were determined by Mott-Schottky experiments, and the photocatalytic mechanism of the photocatalysts was proposed by combining the results of various characterization analyses and photocatalytic performance tests, such as PL, UV-vis DRS, and EPR. Photoinduced e⁻ within the CB of g-C₃N₄ no longer directly transport to the CB of WS₂; instead, photogenerated e⁻ in the CB of WS₂ is shifted via carbon to the VB of g-C₃N₄, which recombines there with h⁺. Consequently, ternary C@WS₂/g-C₃N₄ composites that follow the Z-scheme transport mechanism, attain efficient charge differentiation and demonstrate robust redox capabilities. As a result, photocatalytic efficiency under visible-light is significantly enhanced. Notably, not only can the incorporation of carbon onto the semiconductor surface serve as an e⁻ shuttle to facilitate Z-scheme transport, but as demonstrated in the investigation conducted by Zhao et al. (2021), doping carbon into g-C₃N₄ can also use carbon as an e⁻ shuttle to establish Z-scheme photocatalysis, thereby enhancing photocatalytic performance. In another reaction involving Cu²⁺ reduction to Cu⁺ in a CuO/CDs/g-C₃N₄ ternary photocatalyst employed in a Fenton-like cycle, carbon dots (CDs), serving as dual pathways for charge transfer between CuO/g-C₃N₄ heterojunction interfaces, effectively facilitate charge transfer along such Z-scheme system, yielding a synergistic enhancement in photocatalytic performance (Wu et al., 2023).

In the progression of another metal-free ASSZ photocatalyst, graphene oxide (GO), a category of graphene known for its excellent electron conduction efficiency and remarkable two-dimensional carbon sheet structure, can be a superior choice as an electronic shuttle and listed in Table 1 (Xie and Zhang, 2018). In the research of photocatalytic breakdown of contaminants, the existence of graphene oxide not exclusively effectively boosts charge transfer but furthermore provides an extensive reaction region for the adherence and decomposition of target pollutants (Du et al., 2021). In the ASSZ photocatalyst g-C₃N₄/GO/AgBr, GO serves in the capacity of a conduit for charge transference connecting two semiconductor materials (Miao et al., 2017). In this ternary photocatalyst, the photocatalytic decolorization of Rhodamine B (Rh B) is 7.9 and 2.2 greater than the value exhibited in g-C₃N₄ and binary g-C₃N₄/AgBr, respectively. In another g-C₃N₄/MnO₂/GO Z-scheme heterojunction photocatalyst, GO promotes electron transfer and prevents charge carrier annihilation, effectively mitigating the photocorrosion of g-C₃N₄ (Du et al., 2021). Consequently, in the ternary g-C₃N₄/MnO₂/GO photocatalytic system, a pronounced enhancement is observed in the photolytic breakdown of tetracycline hydrochloride (TC).

Given the excellent electron transfer properties exhibited by GO in ASSZ photocatalysts, it is imperative to mention reduced graphene oxide (RGO), which similarly boasts exceptional two-dimensional layered structure and electron transfer capabilities. In a ternary Z-scheme TiO₂/RGO/g-C₃N₄, as prepared by Wu et al. (2017), RGO acts as a conduction shuttle, effectively suppressing recombination in charge carriers and facilitating the Z-scheme charge separation. The introduction of RGO augments the specific surface area of the sample, leading to an increased number of adsorption and photocatalytic sites. Simultaneously, the strong interaction between RGO and g-C₃N₄/TiO₂ results in a notable narrowing of the bandgap and heightened absorption of visible light. The collaborative impacts of RGO contribute to the enhanced photocatalytic decolorization activity through its multifunctional roles. Ibrahim et al. (2020) utilized pulsed laser ablation in liquids to fabricate a Z-scheme photocatalytic TiO₂/RGO/g-C₃N₄ nanocomposite. The photocatalyst exhibited a hydrogen production that was 93-fold greater than that observed with primitive g-C₃N₄. In this context, RGO itself does not directly contribute to photocatalytic hydrogen production; its primary role is to play the role of an electron transport shuttle from TiO₂ to g-C₃N₄. Thereby, it significantly promotes the constitution of a Z-scheme photocatalytic heterojunction for electron transfer between TiO₂ and g-C₃N₄, simultaneously increasing contact region and enhancing contact tightness between the two semiconductors, leading to improved surface reactions and adsorption kinetics.

When constructing Z-scheme heterojunction photocatalysts through g-C₃N₄ combination with other semiconductors, it is advantageous to utilize another metallic compound as an electron conductor to bridge the two semiconductors and promote the establishment of the heterojunction. In the research aimed at enhancing the photocatalytic performance through establishing a heterojunction regarding both Ag₃PO₄ and g-C₃N₄ composites, Zhu et al. (2020) employed low cost and chemically stable ZnO as a shuttle for electron transfer. In terms of the ternary Z-scheme g-C₃N₄/Ag₃PO₄/ZnO, within a catalyst dosage of 0.6 g/L, a wastewater dosage of 30 mg/L, and a pH level of 6, the TC degradation rate can reach 89.95% under sunlight. It is noteworthy that, during the cyclic experiments for photocatalytic stability assessment, the efficiency of the ternary photocatalyst Ag₃PO₄/g-C₃N₄/ZnO remained almost unchanged after four cycles, while Ag₃PO₄ exhibited a significant decrease in performance after four cycles. This can be attributed to Ag₃PO₄, which, under visible light, generates photogenerated electrons that reduce Ag⁺ to Ag⁰. Furthermore, a portion of Ag₃PO₄ may decompose into Ag₂O, resulting in the loss of Ag₃PO₄. However, in the existence of ZnO as an electron shuttle, it can effectively transfer e⁻ from Ag₃PO₄ CB to g-C₃N₄ VB, establishing a Z-scheme electron transmission. The researchers calculated the VB and CB positions of each semiconductor by combining the results of Mott-Schottky and UV-Vis DRS experiments and found a possible pathway for the generation of •O₂ ⁻ to propose the mechanism mentioned above. This boosts the capability in separating photogenerated charge carriers, consequently strengthening photocatalytic performance and reducing photocorrosion in Ag₃PO₄. Another set of Fe₃N particles possessing metallic transport nature similarly performs the role of pathways for charge transfer in the ternary Z-scheme photocatalyst Fe₃N/Fe₂O₃/g-C₃N₄, promoting electrons migrating between g-C₃N₄ and Fe₂O₃, thereby enhancing the photocatalytic decolorization of Rh B and the ability for photocatalytic reduction of CO₂ (Padervand et al., 2021). The synergistic effects among these three different compounds can effectively improve the efficacy of the photocatalyst.

With the continuous exploration and research of photocatalytic systems with Z-scheme photocatalytic configuration, a DZ photocatalysis setup that does not require electron shuttle has begun to emerge. Since Wang et al. (2009a) stated the discovery that two semiconductor materials in intimate contact can also achieve electron-hole transfer mechanism in a Z-scheme configuration, there has been an increasing focus on DZ photocatalytic configuration. In a DZ photocatalysis setup, the establishment of a single Z-scheme heterojunction requires only a pair of distinct semiconductors featuring harmonized electronic band configurations. In such a ternary system, the component not involved in the advancement of the Z-scheme heterojunction will play additional distinctive roles.

When ternary semiconductor materials possess matching band structures, they can form a dual DZ-scheme structure with extensive capacity for photon capture and promoted electron migration. Furthermore, ternary Z-scheme photocatalysts with appropriate band structures enable more effective charge transfer/separation than binary Z-scheme photocatalysts with single Z-scheme pathway (Liu et al., 2018; Katsumata et al., 2022). In the dual DZ-scheme g-C₃N₄-based ternary photocatalytic configuration, as no noble metal is loaded, there is no light shielding effect on the semiconductors, allowing all semiconductors to effectively absorb light energy and generate charge carriers. Based on distinct pathways of electron and hole recombination in direct dual Z-scheme photocatalytic configuration, they can be classified as three kinds of dual DZs. As shown in Figure 4, the three semiconductors constituting the dual Z-scheme photocatalytic configuration are labeled as A, B, and C. The middle semiconductor situated within the dual Z-scheme photocatalytic configuration, created by the tight interaction of three semiconductors, serves as the basis for categorization. The first type involves the middle semiconductor acting as a shuttle, where e⁻ within the CB of C recombination with the h⁺ within the VB of B, and the e⁻in the CB of B recombination with the h⁺ within the VB of A. The second type involves the h⁺ in the VB of the B recombining with the e⁻ in the CB of A and C. The third type involves e⁻ within the CB of the B transferring and recombining with h⁺ associated with the VB of A and C.

In the past few years, as environmental water contamination has become more and more severe, the use of oxidation advanced treatment has received widespread attention. Among the various effective methods, photocatalysis technology garners significant attention because of its high efficiency, stability, and its efficiency in capturing sunlight effectively (Regmi et al., 2018; Yang et al., 2021b). Ternary Z-scheme photocatalysis technology based on g-C₃N₄ has been extensively utilized for environmental water pollution treatment, with one of its primary applications being the photocatalytic decolorization of organic dyes in sewage. A Z-scheme MoS₂/g-C₃N₄/ZnO (Madhushree et al., 2022) ternary photocatalyst employed for the photocatalytic decolorization of malachite green (MG), and a Z-scheme photocatalytic configuration g-C₃N₄/C/S-g-C₃N₄ (Yang Y. et al., 2021) system for the photocatalytic decolorization of rhodamine-B (RhB), congo red (CR), and methylene blue (MB). Methyl orange (MO) can also be successfully broken down with the attendance of rGO/Fe₂O₃/g-C₃N₄ (Park et al., 2022) as photocatalyst exposed to solar light irradiation. For the purpose of examining the impact of dye density, catalyst amount to be taken, and pH on photocatalytic decolorization of organic dyes, Padervand et al. (2021) researched the decolorization efficiency of RhB using the Fe₃N/Fe₂O₃/g-C₃N₄photocatalyst under various different conditions. The optimum photocatalytic decolorization efficiency was accomplished with a catalyst input of 0.04 g, the pH value was 3.5, and the dye concentration was 5 ppm.

The residues of antibiotics, which are difficult to biodegrade and possess long-term toxicity, persist in water bodies and exert an increasing impact on the environment with the rising usage of antibiotics. The significant existence of antibiotic contamination in the environment possesses the capacity to induce microbial mutation and adaptation, adding to the problem of animal resistance to antibiotics (Huo et al., 2016; Zhao et al., 2021). Consequently, the issue of treating antibiotic residues in water has gained growing attention. Among the various methods available, Z-scheme g-C₃N₄-based photocatalysis stands out as a promising tactic for overcoming this challenge. In the study by Ayappan and Mani (2023), a ternary Z-scheme g-C₃N₄/Bi₂S₃/α-Ag₂WO₄ was employed for the degradation of tetracycline (TC). This ternary Z-scheme photocatalyst achieved a 95.02% degradation of TC within 120 min. Interestingly, they used the solution resulting from the photocatalyst degradation treatment to cultivate mung bean sprouts and test their biotoxicity. The results demonstrated that mung bean sprouts germinated in the photocatalytically degraded TC solution. This effectively proves that, under sunlight irradiation, Bi₂S₃/g-C₃N₄/α-Ag₂WO₄ successfully decomposes TC and reduces its toxicity to organisms. In the investigation of Kang et al. (2020), 97.5% of TC was effectively removed within 50 min when illuminated with light in the perceptible range. The Z-scheme photocatalysis on g-C₃N₄not only effectively degrades TC (Xue et al., 2019; Zheng and Zhang, 2019; Kang et al., 2020; Yu et al., 2020; Du et al., 2021) but has also demonstrated excellent photocatalytic degradation effects on tetracycline hydrochloride (Asadzadeh-Khaneghah et al., 2021), sulfamethoxazole (Ren et al., 2019), metronidazole (Jo and Natarajan, 2016), norfloxacin (Zhang J. et al., 2020), and ciprofloxacin (Deng et al., 2018; Saravanakumar and Park, 2021; Sarkar et al., 2022b).

Furthermore, addressing heavy metal contamination in wastewater is a formidable challenge. For instance, the improper treatment of industrial wastewater can introduce a significant risk to organisms, particularly regarding Cr(VI), necessitating urgent attention. In the study by Ghafoor et al. (2019), Ag/TiO₂/g-C₃N₄ photocatalyst has demonstrated remarkable effectiveness in the photo-reduction of Cr(VI). This photocatalyst is capable of converting all Cr(VI) to Cr(III) in 150 min with solar light irradiation. The ternary g-C₃N₄/Ag/Bi₄O₇ nanosheets prepared by Ye et al. (2019) have also been implemented for photocatalytically reducing aqueous Cr(VI). The specific surface area of g-C₃N₄ was significantly enhanced through thermal spalling, with the distribution of Bi₄O₇ and Ag on the surface of g-C₃N₄ nanosheets. Subjected to illumination within the visible spectrum, the top-performing photocatalyst reached a reduction of almost 98% in Cr(VI) concentration within 60 min. In another ternary Z-scheme photocatalytic configuration, Ag/g-C₃N₄/V₂O₅, the photogenerated reduction of Cr(VI) can also be achieved through photocatalytic reactions (El-Sheshtawy et al., 2019).

Utilizing photocatalysts for overall water cracking is deemed an affordable procedure for sunlight harvesting to yield hydrogen and oxygen. However, the selection of the photocatalyst in this process is limited by the requirement that it must have a suitable bandgap position, be capable of straddling the redox potential of water decomposition into hydrogen and oxygen, and meet the condition of appropriate surface reaction kinetics and good stability (Moniz et al., 2015; Chen S. et al., 2017; Zhao W. et al., 2020). Theoretically, g-C₃N₄ satisfies the aforementioned criteria and represents an ideal photocatalyst for facilitating the water dissociation process for the production of hydrogen and oxygen. Nevertheless, practical uses of pristine g-C₃N₄ encounter issues for instance the high-speed recombination of photoinduced electron-hole pairs, which is related to its inefficiency in the water splitting process. Consequently, research endeavors focused on boosting effectiveness of g-C₃N₄ for water cleavage applications have witnessed continuous and concerted efforts over the past several years (Cao et al., 2015; Gao et al., 2022). To construct Z-scheme heterojunction photocatalysts based on g-C₃N₄ is an effective approach.

The ternary Z-scheme photocatalysis on g-C₃N₄ photocatalysts demonstrated excellent photocatalytic capabilities in water splitting process for hydrogen evolution. Zhang et al. (2023) synthesized a CoS/CdS/g-C₃N₄ Z-scheme configuration, which displayed a significantly advanced hydrogen evolution level of 2,866 μmol⋅g⁻¹⋅h⁻¹. The hollow spherical g-C₃N₄ exhibits a larger specific surface area compared to bulk g-C₃N₄, thereby enhancing the photocatalytic reaction. This rate here is 20-fold increase compared to hollow spherical g-C₃N₄ and 1.4 times more than CdS/g-C₃N₄ in their as-prepared samples. Jo and Selvam (2017) presented findings on Z-scheme photocatalytic configuration g-C₃N₄/CdS with RGO for the photochemical hydrogen evolution from lactic acid-infused water amidst light in the visible spectrum. The rate of hydrogen creation achieved 676.5 μmol⋅g⁻¹⋅h⁻¹, Reflecting a36.5% apparent quantum efficiency (AQE) performance. Furthermore, the system demonstrated robust photostability. Ibrahim et al. (2020) enhanced the success rate of hydrogen yield through water cracking by incorporating TiO₂ nanotubes, RGOand g-C₃N₄ nanosheets to fabricate a ternary Z-scheme photocatalyst g-C₃N₄/RGO/TiO₂. The g-C₃N₄/RGO/TiO₂ system established a notable rate at 32 mmol⋅g⁻¹⋅h⁻¹ which hydrogen is produced, surpassing the rates observed for pristine g-C₃N₄, TiO₂, and TiO₂/RGO by approximately 93, 3.8, and 2.6 times, respectively.

In contrast to photocatalytic water splitting aimed at hydrogen evolution, the procedure for oxygen generation by water cracking presents more significant challenges. The efficiencies of water splitting for oxygen production are comparatively lower than those for hydrogen production, primarily due to the sluggish kinetics that need to be overcome and the substantial overpotential required for the evolution reaction (Li et al., 2022). The dual DZ photocatalytic configuration has witnessed outstanding advances in extremely efficient water splitting for oxygen production. Tian and co-workers (Tian et al., 2019) presented a Z-scheme photocatalytic configuration comprising g-C₃N₄/MoS₂/Ag₃PO₄, achieving the highest oxygen generation rate recorded at 232.1 μmol L⁻¹⋅g⁻¹⋅h⁻¹. In comparison, the other components, Ag₃PO₄, Ag₃PO₄/MoS₂, and Ag₃PO₄/g-C₃N₄, exhibited lower oxygen generation rates of 45.9 μmolL⁻¹⋅g⁻¹⋅h⁻¹, 189.5 μmolL⁻¹⋅g⁻¹⋅h⁻¹, and 198.4 μmolL⁻¹⋅g⁻¹⋅h⁻¹, respectively. Another g-C₃N₄-based dual Z-scheme configuration, ternary g-C₃N₄/Ag₂MoO₄/Ag₃PO₄ composite, also exhibited the highest oxygen production rate recorded at 924.6 μmolL⁻¹⋅g⁻¹⋅h⁻¹, surpassing that of Ag₃PO₄, Ag₂MoO₄ and Ag₂MoO₄/Ag₃PO₄ (Liu et al., 2018). Si et al. (2018) presented a DZ Ag₃PO₄/graphdiyne/g-C₃N₄ component compound, demonstrating improved oxygen formation with a speed of 753.1 μmol⋅g⁻¹⋅h⁻¹. In this photocatalyst, graphdiyne serves as a conductive electron shuttle among Ag₃PO₄ and g-C₃N₄ in the Z-scheme photocatalysis setup. Additionally, graphdiyne also acts as a foundation for maintaining Ag₃PO₄, thereby enhancing O₂ evolution.

In response to the concern of global warming caused by carbon dioxide (CO₂) emissions, a greenhouse effect contributor, the photocatalytic synthesis of high-value chemicals from carbon dioxide, for instance, hydrocarbon fuels, has emerged as an encouraging green technology. This strategy, acting as getting double mileage out of one effort, not only offers a solution to the global warming problem but also addresses the challenges associated with energy and fuel shortages (Kumar et al., 2018; Wang et al., 2020; Jia et al., 2023). However, the fully oxidized CO₂ is extremely stable, the CO₂ reduction needs huge bond dissociation energy for C-O bond dissociation and C-H bond forming. By way of contrast, the mechanism and process of CO₂ photoreduction is complicated, products are diverse. Nevertheless, the fully oxidized CO₂ results in exceptional stability. CO₂ reduction necessitates a substantial bond dissociation energy for the dissociation of C-O bonds and the creation of C-H bonds (Xu D. et al., 2018). Simultaneously, the complexity of CO₂ photoreduction lies in its intricate mechanisms and processes, yielding a diverse array of products (Ong et al., 2016; Tasbihi et al., 2018). Among the numerous photocatalysts employed for CO₂ reduction, the distinctive combination of an applicable bandgap and aligned conduction and valence band positions makes g-C₃N₄ stand out. Consequently, Z-scheme heterojunction photocatalysts on g-C₃N₄ prove highly effective in overcoming challenges associated with CO₂ photoreduction.

Madhusudan et al. (2021) synthesized an outstandingly effective ternary Zn_0.5Cd_0.5S/Au@g-C₃N₄ Z-scheme photocatalysis setup for transforming CO₂ into methanol (CH₃OH), with formaldehyde (HCHO) and methane (CH₄) observed as minor by-products. The rate of photocatalyzed reduction CO₂ into CH₃OH achieved 1.31 μmol ⋅h⁻¹⋅g ⁻¹, demonstrating a 32.7 times increase over g-C₃N₄ (0.04 μmol ⋅h⁻¹⋅g ⁻¹) and 43.6 -fold boost in comparison with Zn_0.5Cd_0.5S (0.03 μmol ⋅h⁻¹⋅ g ⁻¹). Xu D. et al. (2018) stated the evolution of a DZ Ag₂CrO₄/GO/g-C₃N₄ photocatalyst in the field of reduction CO₂ into CH₃OH, with a minor production of CH₄. This ternary photocatalytic configuration based on g-C₃N₄ demonstrated CO₂ conversion efficiency reaching 1.03 μmol g⁻¹, semonstrating a TOF of 0.30 h^–1 during the initial 3 h of illumination across the entire spectrum.

Photocatalyst conversion of CO₂ to renewable carbonaceous fuels can not only produce CH₃OH but also generate carbon monoxide (CO) and methane (CH₄) as a common green fuel product. Raza et al. (2020) prepared a g-C₃N₄/Pt/Cu₂ZnSnS₄ for transforming CO₂ into high-value carbonaceous fuels. In this Z-scheme photocatalyst, the CO production capacity reached 242.3 μmol⋅h⁻¹⋅ g ⁻¹, and the CH₄ yield rate reached 7.961 μmol ⋅h⁻¹⋅ g ⁻¹ with illumination in the visible spectrum. Padervand et al. (2021) also observed the production of 8.03 μmol ⋅h⁻¹⋅ g ⁻¹ and 1.6 μmol ⋅h⁻¹⋅ g ⁻¹ of CO and CH₄, respectively, from the ternary Z-scheme g-C₃N₄/Fe₃N/Fe₂O₃ during CO₂ conversion with H₂O vapor.