The semiconductor material excited by light irradiation of more intensity or band gap equivalent intensity drives the photocatalytic H₂O splitting. During this scenario, electron flow takes place from the VB to the CB, whereas the hole (h⁺) remains in the VB of the material. The photo-generated electrons (e⁻) and holes (h⁺) potentially reduce H⁺ and oxidize the H₂O. In this case, if the bottom of CB is more negative relative to the H⁺ ion reduction potential, it can be a suitable candidate for water reduction. As illustrated in Figure 2A, the VB value should be greater than the H₂O molecule’s oxidation potential to generate an O₂ molecule (1.23 eV vs. NHE at pH = 0). Furthermore, the semiconductor band gap value should be greater than the thermodynamic requirement of 1.23 eV, and it must span the redox potential of H₂O in order to be a viable candidate for one-step excitation water splitting (Hisatomi et al., 2014). Recombining the photoinduced electronic pair clouds can happen in bulk and during a photocatalytic H₂O response to the bulk catalyst surface. Recombination of electron and hole pairs may reduce the photocatalyst performance.

In 1979, the traditional Z-scheme photocatalyst was proposed (Bard, 1979). The components of this photocatalytic system are two photocatalysts and a redox couple. The redox couple consists of an electron acceptor (A) and an electron donor (D), including I₃ ⁻/I⁻¹ and Fe^+3/Fe⁺². In the conventional Z-scheme mechanism (Figure 2B), photogenerated electrons in the CB of 2D g-C₃N₄ react with A to form D, while photogenerated holes in the VB of other 2D catalysts react with D to form A. As a result, the electrons in other 2D catalysts CB and the holes in 2D g-C₃N₄ VB are preserved. However, traditional Z-scheme mechanisms have limitations and drawbacks. Due to the necessity of redox ion pairs, traditional Z-scheme photocatalysts are only applicable in the solution phase.

The S-scheme heterojunction, which is comprised of an oxidation photocatalyst (OP) and a reduction photocatalyst, was proposed to overcome the inadequacy of traditional type-II heterojunction and Z-scheme heterojunction (RP) (Xu et al., 2020). Overall, the S-scheme mechanism vividly describes the charge transfer pathway in heterojunction photocatalysts, but it is also consistent with the scientific principles and experimental phenomena. The S-scheme heterojunction photocatalyst has both high charge separation efficiency and potent redox capability. As anticipated, it has received a great deal of attention since its proposal. Numerous sources discuss the fabrication and photocatalytic performance of 2D/2D g-C₃N₄-based S-scheme heterojunction photocatalysts (g-C₃N₄/BiOBr) (Zhang et al., 2021).

This mini-review focuses on significant and advanced phenomena in engineering 2D g-C₃N₄-based heterostructure photocatalysts, particularly for hydrogen production. The main aspects of 2D g-C₃N₄ heterogeneous photocatalysts often provide some rising strategies for contriving various 2D heterostructure photocatalysts. Further, it also provides an understanding of the designs of g-C₃N₄-based heterogeneous catalysts, along with special attention to the underlying mechanism of photocatalyzed recombination of electron–hole pairs. Moreover, the recent advancement and challenges of g-C₃N₄-based heterostructure photocatalysts for H₂ production have subsequently been highlighted. Finally, a brief overview of 2D heterogeneous photocatalysts relevant to water reduction (H₂ evolution) or water oxidation (O₂ evolution) Z-scheme is described, and the current state of science and key questions are addressed.

Melamine, melon, melam, and melem are recognized as heptazine and triazine-based molecular compounds, whereas the coplanar tri-s-triazine unit is regarded as the fundamental structural motif required to produce g-C₃N₄. As shown in Figure 3, the basic tectonic units of g-C₃N₄ are triazine (C₃N₃) and tri-s-triazine/heptazine (C₆N₇) rings. Unlike conventional organic semiconductor materials, g-C₃N₄ cases have large stability, including resistance to heat and chemicals. Thermal gravimetric analysis (TGA) and thermal gravimetric analysis (TG) on g-C₃N₄ indicate that it is non-volatile up to 600°C and will be nearly completely decomposed at 700°C. The flake-like structure of g-C₃N₄ is very similar to that of graphite, as is well known.

Wang et al., for the first time, used g-C₃N₄ as visible light photocatalysts for hydrogen production (Wang et al., 2009). Graphitic carbon nitride was later discovered to absorb visible light owing to its 2.7 eV band gap value, which is consistent with a wavelength of 460 nm. Moreover, the CB of g-C₃N₄ in its bottommost is more negative relative to the H₂O reduction potential of the H₂ molecule. The valence band uppermost region is more positive as compared to the oxidation potential of water to O₂ molecule. So, graphitic carbon nitride applied as a photocatalyst could be a suitable candidate for H₂O splitting in visible light (Yan et al., 2016). It is very imperative to understand the band distance of g-C₃N₄ upsurges as the bulk graphitic carbon nitride flake off to monolayer by quantum confinement effect.

The strategies to fabricate the 2D–2D g-C₃N₄-based hetero-structured photocatalysts Interfaces play a crucial role in the photocatalytic performance of the 2D photocatalysts, as the quality of the interface is determined by the construction strategy. To date, numerous effective fabrication techniques for the synthesis of 2D–2D g-C₃N₄-based heterostructure photocatalysts, such as ultrasonic absorption (Zhang et al., 2018; Ayodhya and Veerabhadram, 2020), hydrothermal method (Tian et al., 2013), electrostatic self-assembly (Ma et al., 2017; Wang et al., 2022), and chemical vapor deposition (Zhang and Fu, 2018), have been extensively studied. Nonetheless, one of the simplest ways to construct the 2D–2D g-C₃N₄-based heterostructure is to disperse the two distinct 2D components in the solution via stirring or sonication to form a mixture. After drying the mixture in an oven to evaporate the solvents, the 2D–2D photocatalyst can be obtained. Using ultrasonic dispersion and drying, 2D–2D g-C₃N₄/N-doped La₂Ti₂O₇ layered heterostructures were fabricated. Nevertheless, these 2D–2D interfaces were successfully fabricated by a weak interaction between two 2D components, and the 2D components were easily separated during the photocatalysis process (Cai et al., 2017b). However, using a hydrothermal method, this issue can be resolved. For instance, g-C₃N₄/ZnIn₂S₄ (Manchala et al., 2019) and TiO₂-g-C₃N₄ (Zhang et al., 2017) heterostructure photocatalysts, etc., have been prepared by the hydrothermal method, and numerous intimate interfaces were formed between the g-C₃N₄ and the second counterparts. The heterointerface junctions not only enhance the stability but also enhance the generation of electron–hole pairs and inhibit their recombination. Overall, the hydrothermal method is an energy-efficient and environmentally friendly method because the reaction occurs under closed system conditions. Furthermore, the hydrothermal method is kinetically slow at all temperatures, making it easy to control. To further enhance the kinetics of crystallization, a microwave–hydrothermal method was also developed. For instance, TiO₂/g-C₃N₄ heterostructures were created by a simple microwave–hydrothermal process, which demonstrated enhanced photocatalytic H₂ production activity compared to TiO₂ (Girish et al., 2022). Additionally, electrostatic self-assembly is a viable technique for fabricating intimate 2D–2D interfaces. Notably, surface charge modification plays a significant role in the engineering of 2D–2D photocatalysts with intimate interfacial contact using this method (Ma et al., 2017; Wang et al., 2022). To achieve electrostatic self-assembly, the surface charges on various 2D photocatalysts must be modified to obtain opposing charges (i.e., positive and negative charges). Notably, the zeta potential value can be used to calculate the photocatalyst’s charge. For instance, to form the 2D/2D g-C₃N₄/rGO by electrostatic self-assembly, the g-C₃N₄ was protonated by concentrated H₂SO₄ and HNO₃ under mild ultrasonication to obtain the positively charged surface; the measured zeta potential value was +37.2 mV (Vinesh et al., 2020). With the aid of ultrasonication and agitation, the g-C₃N₄/rGO intimate interface was obtained. In addition, the stacking interactions between the sp² lattices of g-C₃N₄ and the sp² graphene lattices, as well as the hydrogen-bonding interactions between the nitrogen-containing groups in g-C₃N₄, are advantageous for electrostatic self-assembly. Due to the intimate interface and the introduction of rGO, the hydrogen production rate of g-C₃N₄/rGO (557 mol g⁻¹ h⁻¹) was significantly higher than that of g-C₃N₄ (158 mol g⁻¹ h⁻¹). Furthermore, the construction of the 2D/2D g-C₃N₄/rGO by electrostatic self-assembly facilitates the photocatalytic reduction of carbon dioxide to methane. Chemical vapor deposition (CVD) is also an efficient method for constructing 2D/2D heterostructures with intimate interfaces, such as intraplane and interplane interfaces (Zhang and Fu, 2018). Typically, in the CVD method, gas molecules are injected into a reaction chamber that has been heated to a specific temperature. For instance, the CVD-fabricated intraplane Fe₂O₃/g-C₃N₄, type-II InSe/g-C₃N₄ heterostructure, and g-C₃N₄/TiO₂ exhibited excellent optoelectronic and photovoltaic performance (Wang et al., 2014b). Yuanzhi Hong produced Ta₂O₅/g-C₃N₄ heterojunctions using a straightforward, one-step heating procedure. Under visible-light irradiation (>420 nm) (Hong et al., 2017), the photocatalytic activity of as-prepared photocatalysts was determined by splitting water for hydrogen evolution under visible-light irradiation. Compared to pure g-C₃N₄, the obtained heterojunctions demonstrated significantly enhanced hydrogen production. The heterojunction of 7.5% TO/CN exhibited the highest photocatalytic hydrogen evolution efficiency, which was approximately 4.2 times that of pure g-C₃N₄. In addition, the 7.5% TO/CN sample exhibited excellent photochemical stability even after 20 h of photocatalytic testing. Although CVD is a powerful technique for the synthesis of 2D/2D materials, the gaseous byproducts of the process are typically extremely toxic. Moreover, using the CVD method to synthesize 2D materials on a large scale remains a formidable challenge.

As described earlier, in 2009, Wang et al. first discovered that 2D layered g-C₃N₄, along with featuring a 2.7 eV band gap value, is a favorable photocatalyst utilizing visible light for H₂ production. After that, many researchers have devoted their attention to the synthesis of 2D heterostructure photocatalysts with 2D g-C₃N₄, as proper band construction is an important deliberation in electing the second semiconductor for engineering g-C₃N₄-based heterostructure photocatalysts. For example, Di et al. (2014) investigated g-C₃N₄/BiOI heterostructure photocatalysts through a simple hydrothermal approach; schematic representation is illustrated in Figure 4A. Transmission electron microscopy demonstrates the 2D morphology of C₃N₄/BiOI heterostructure (Figure 4B), and HRTEM images show the different crystal fringes patterns, which corresponds to the g-C₃N₄ layered structure and BiOI structure (Figure 4C). A clear interface between g-C₃N₄ and BiOI could be seen in Figure 4C. The intimate interface between the two constituents is helpful to transfer the charge along the interfaces. BiOI constitutes the band gap (1.94 eV), which displays a decent photo response in the visible spectrum of light (Figure 4D). Owing to the appropriate band placement of g-C₃N₄ and BiOI, the photo-induced hole–electron pair may be powerfully separated to operate the photocatalytic reaction.

Later on, Xu et al. (2018) employed a facile electrostatic self-assembly method to synthesize a 2D Fe₂O₃/g-C₃N₄ heterostructure photocatalyst. A robust interaction was observed among Fe₂O₃ and g-C₃N₄, originating from the transition and separation of electron and hole pair charges.

Interestingly, the movement of charges through the 2D Fe₂O₃/g-C₃N₄ interface was found suitable for the Z-scheme. Therefore, Fe₂O₃/g-C₃N₄ photocatalysts were applied for direct photocatalytic water splitting through Z-scheme with visible-light irradiation using Pt as a co-catalyst. The morphology of the 2D Fe₂O₃/g-C₃N₄ heterostructure is depicted in Figure 5. As can be studied by the TEM in Figures 5A,B, the Fe₂O₃/g-C₃N₄ heterostructure exhibited 2D morphology. The exfoliated g-C₃N₄ reveals a morphology similar to curled-veil, stated as typical flexible nanosheets (Figure 5C). The FFT profile reveals the presence of six identical (1 2 0) spots, corresponding to the (0 0 1) basal plane up and the (0 0 1) basal plane down, implying that a crystal fringe distance of 0.25 nm represents (1 2 0) planes (Figure 5D). Consequently, the lower right corners of Figures 5E,F display the typical structure of a hexagonal nanoplate made of Fe₂O₃ with exposed facets. Based on the Fe₂O₃ and g-C₃N₄ morphologies, the TEM images with 10% Fe₂O₃/g-C₃N₄ show hexagonal nanoplate (red and blue circles in Figures 5E,F), which represents Fe₂O₃, while curled nanosheets were identified as g-C₃N₄. The nanoplates of Fe₂O₃ are predominantly accumulated on the edges of g-C₃N₄, promoting the establishment of an interface of the heterostructure. The effectiveness of the photocatalysts was assessed by their H₂O splitting ability to produce hydrogen using visible light as an irradiation source. Triethanolamine (TEOA) was utilized to scavenge the holes. The nanoparticles of Pt played the role of co-catalysts, which were accumulated on the photocatalyst surface through in situ photoreduction. Figure 5G demonstrates that pristine Fe₂O₃ performs a very poor H₂ generation performance, whereas g-C₃N₄ exhibited mild photocatalytic hydrogen generation at a 30.1 mmol h⁻¹ g⁻¹ rate. Interestingly, the photocatalytic performance of the Fe₂O₃/g-C₃N₄ heterostructure for the H₂ evolution was found 398.0 mmol h⁻¹ g⁻¹, almost 13-times that of the pristine g-C₃N₄. A detailed photocatalytic hydrogen generation mechanism on Fe₂O₃/g-C₃N₄ heterostructure is illustrated in Figure 5I. As shown in Figure 5I, photocatalytic systems primarily consider two possible pathways: the conventional type-II heterojunction and the direct Z-scheme system. As seen in Figure 5I (type-II), the CB and VB energies of g-C₃N₄ are 1.1 and 1.7 eV, respectively. While the CB and VB values of Fe₂O₃ are 0.3 and 2.4 eV, to be obtained from the empirical formula (Wang et al., 2015). However, due to the low CB value of Fe₂O₃, electrons cannot participate thermodynamically in the photocatalytic hydrogen evolution reaction. As shown in 4i (type-II), if the composite followed the traditional type-II mechanism, g-C₃N₄ would transfer to the CB of Fe₂O₃, while photogenerated holes would transfer from the VB of Fe₂O₃ to the VB of g-C₃N₄ when exposed to visible light. In this case, the photocatalytic activity of the composite should be less than that of g-C₃N₄. However, the actual experimental results showed that the photocatalytic activity of the composite Fe₂O₃/g-C₃N₄ is higher than that of g-C₃N₄. On the basis of the preceding results and data analysis, it is proposed that a direct Z-scheme charge transfer route can occur over Fe₂O₃/g-C₃N₄ composites, thereby enhancing the photocatalytic performance in H₂ production. In particular, when both Fe₂O₃ and g-C₃N₄ absorb photons with sufficient energy, electrons are excited from their respective VB to CB. As a result, the Fe₂O₃/g-C₃N₄ composites retain both the high oxidation ability of Fe₂O₃ and the high reduction ability of g-C₃N₄, thereby providing a substantial driving force for the water reduction reaction. The photogenerated electrons formed on the g-C₃N₄ surface would transfer to Pt NPs in order to participate in the surface water reduction for H₂ evolution, whereas the photogenerated holes collected on the Fe₂O₃ surface could be consumed in TEOA oxidation. This direct Z-scheme charge transfer process significantly improves charge separation efficiency and provides a large driving force for the photocatalytic water reduction reaction, thereby enhancing the performance of photocatalytic water splitting.

Recently, Zhong et al. (2018) developed a self-assembled 2D O-g-C₃N₄/TiO₂ heterostructure photocatalyst by single-pot solvothermal method for the H₂ evolution reaction (HER) with visible light photocatalytic radiations. The two-dimensional existence of each component of the heterostructure itself gives rise to broad, unique surface areas, a marked quantum containment effect, and exposed active sites. The 2D photos of O-g-C₃N₄/TiO₂ 1:1 taken by HAADF-STEM reveal a fine heterostructure formation (Figure 6A). Element mapping performed with EDX reveals that the larger nanosheet is O-g-C₃N₄, and the small nanosheets around its border are TiO₂ (Figures 6B–F). In order to analyze the interface among the two elements, the electron energy loss spectra (EELS) were collected in separate contact regions 1 and 2, as well as in the virgin TiO₂ leaf areas, as shown in Figure 6G. Figure 6H displays the photocatalytic activity for HER on the 2D O-g-C₃N₄/TiO₂ with visible light irradiation. Electrons in the VB of O-g-C₃N₄ are excited to the CB by the incident photons with appropriate energy. TEOA trapped the photo-induced holes produced in the valence bands of O-g-C₃N₄, whereas photo-induced electrons passed from the heterojunction of covalent NeOeTi into the valence band of TiO₂ nanosheet.

Photo-induced electrons eventually enter into and evenly accumulate on the surface of TiO₂, a Pt co-catalyst, and the water splitting happens through electrons into H₂ gas. The heterojunction identified through NeOeTi linkage caused effective charge separation at the interface and the effect of band bending, which expanded the absorption range and enhanced the photocatalytic activity of 2D O-g-C₃N₄/TiO₂ heterostructure photocatalyst, as shown in Figure 6I.

Very recently, Jia et al. (2020b) reported that 2D 15% FeSe₂/CNNS heterostructure exhibits superior photocatalytic hydrogen generation performance (1655.6 μmol h⁻¹ g⁻¹) with no co-catalyst in Na₂S/Na₂SO₃ aqueous medium and excellent stability for 12 h. In addition, it also demonstrated the simultaneous elimination of chromium (VI) and methylene blue (MB) using sunlight irradiation. Most notably, relative to conventional single-step four-electron reaction, FeSe₂/CNNS can trigger the photocatalytic H₂O splitting to hydrogen generation through a sequential two-electron, two-step reduction reaction based on capturing of active free radical and H₂O₂ sensing investigation. The efficiency is simultaneously realized by such 2D/2D inter-plane hetero-structures, as seen in Figure 7.

Here, we summarize 2D g-C₃N₄-based heterostructure photocatalysts and compounds that contain 2D g-C₃N₄. Table 1 lists the experimental conditions for photocatalytic water splitting and their photocatalytic performances by using different sacrificial reagents.

Overall, photocatalytic H₂ production from water using either UV-light-responsive photocatalysts or 2D g-C₃N₄-based visible-light-responsive photocatalysts is a low-efficiency process (Chen and Mao, 2007; Hong et al., 2013). This is primarily due to the high rates of electron and hole recombination induced by photoexcitation. To increase the efficiency of H₂ production from water splitting, electron donors are typically required to act as a sacrificial agent, consuming holes and preventing the recombination of photoinduced electrons and holes on the semiconductor surface. Common sacrificial electron donors include Na₂S–Na₂SO₃, methanol, triethanolamine, lactic acid, etc., and several studies have compared the H₂ production rates of photocatalysts using various sacrificial agents. For instance, Hong et al. examined the H₂ production performance of 2D g-C₃N₄/NiS (g-C₃N₄ as the photocatalyst and NiS as the co-catalyst) in solutions of triethanolamine, lactic acid, oxalic acid, and ascorbic acid. According to the results, the H₂ evolution rate for C₃N₄/NiS in triethanolamine was 48.2 mol h⁻¹ g⁻¹, whereas the other three sacrificial agents did not support photocatalytic water splitting. These findings and other reports on photocatalysts indicate that sacrificial agents are indispensable for achieving high H₂ evolution rates. Some studies show that sacrificial agents play a role in the dispersion of noble metal nanoparticles (co-catalysts). It has also been determined that the sacrificial agents have a significant effect on the loading amount, particle size, and distribution of various metals on the surface of g-C₃N₄. For instance, in methanol solution, the actual loading amount of Pt and Au is greater than in triethanolamine solution. In the presence of methanol, the distribution and size of Pt nanoparticles are improved, whereas the distribution and size of Au nanoparticles are improved in the presence of triethanolamine. As a result, the Pt- and Au-decorated g-C₃N₄ photocatalysts synthesized exhibit notably distinct charge transfer properties, resulting in enhanced photocatalytic activities of the same g-C₃N₄ photocatalyst under diverse conditions (Cao et al., 2018).

Similarly, the recombination of photogenerated electron–hole pairs of g-C₃N₄ can be inhibited by loading the co-catalyst on the surface of g-C₃N₄. Briefly, co-catalysts can be categorized as singly loaded (noble-metal, metal with high abundance, and non-metallic) and co-loaded hybrid co-catalysts, which effectively promote the separation of photogenerated electron–hole and subsequently improve photocatalytic performance. Therefore, bimetallic, noble-metal co-catalysts, co-loading with different co-catalysts or one co-catalyst with various components, and the potential for solar-driven hydrogen evolution appear more promising.

Inspired by the rule of C and N in g-C₃N₄, also overall photocatalytic water splitting enhanced by the use of 2D g-C₃N₄-based hetero-structured photo-catalysts, as abundant interfaces between different components have gained attention for enhanced light absorption and facilitated photogenerated charge separation in the photo-catalysis. Recently, successful photocatalytic systems have been one of two approaches to the splitting of H₂O into H₂ and O₂. A single particulate photo-catalyst is used to split water via one-stage excitation. A robust, reproducible particulate matter photo-catalyst that can be used under visible light satisfies various requirements. These include the best band gap and band position with enough driving potential. These materials also explain efficient charge-separation and conversion of electron–hole pairs, catalytic surface reduction, oxidation of water, and low corrosion. The preferred approach to achieving the two-stage excitation is well known as a Z-scheme process that is used to combine two photocatalytic with an electron transfer mediator.