Recent Trends in Graphitic Carbon Nitride-Based Binary and Ternary Heterostructured Electrodes for Photoelectrochemical Water Splitting

Abstract

The graphitic carbon nitride (g-C₃N₄) is a class of two-dimensional layered material. The ever-growing research on this fascinating material is due to its unique visible light absorption, surface, electrocatalytic, and other physicochemical properties that can be useful to different energy conversion and storage applications. Photoelectrochemical (PEC) water splitting reaction is one of the promising applications of g-C₃N₄, wherein it acts as a durable catalyst support material. Very recently, the construction of g-C₃N₄-based binary and ternary heterostructures exhibited superior PEC water splitting performance owing to its reduced reunion of e-/h+ pairs and the fast transfer of charge carriers at the heterostructure interface. This review compiles the recent advances and challenges on g-C₃N₄-based heterostructured photocatalysts for the PEC water splitting reaction. After an overview of the available literature, we presume that g-C₃N₄-based photocatalysts showed enhanced PEC water splitting performance. Therefore, it is believed that these materials have tremendous opportunities to act as durable catalyst support for energy-related applications. However, researchers also considered several limitations and challenges for using C₃N₄ as an efficient catalyst support material that must be addressed. This review article provides an overview of the fundamental principles of PEC water splitting, the current PEC water splitting research trends on g-C₃N₄-based binary and ternary heterostructured electrodes with respect to different electrolytes, and the other key factors influencing their photoelectrochemical performance. Finally, the future research direction with several recommendations to improve the photocatalytic efficiency of these materials is also provided at the end.

1. Introduction

With the rapid industrial developments and the expanding global population, the energy demand is constantly increasing, which has posed a significant threat to human life [1,2]. Furthermore, the extensive use of fossil fuels and industrial waste has led to a tremendous increase of greenhouse gases in the atmosphere [3,4]. On the other hand, renewable energy technologies such as wind, solar, and tidal energy have great potential for future generations [5]. Among these, solar energy is considered the most promising one, due to its earth abundance and non-hazardous nature [6]. However, the wavelength of solar illumination reaching the earth is greater than 340 nm, containing only a small portion of UV light (<3%) and a large amount of visible light in the range of 400–800 nm. Therefore, the design and synthesis of suitable visible-light active semiconductor materials having a small bandgap are essential to harvest the abundant solar energy for photocatalytic and other renewable energy-related applications [6,7].

Hydrogen is an attractive green fuel that can be used for many different applications, such as heating, driving vehicles, and material refining [8,9]. Hydrogen has the potential to replace fossil fuels due to its highest energy density and the possibility to use in internal combustion engines and fuel cells, which will be the future for emission-free transportation [10,11,12]. Moreover, different water splitting techniques have been introduced for hydrogen production, such as electrochemical, thermochemical, photocatalytic, photoelectrochemical (PEC), and biological hydrogen production [13,14]. Among them, PEC water splitting is one of the ways to utilize solar energy for hydrogen generation, which can be further used for many other energy-related applications. [15,16]. Nevertheless, hydrogen production via PEC water splitting is one of the promising methods due to its safety factors, non-toxicity, and the final reaction that produces pure hydrogen and oxygen without any byproducts [17,18].

1.1. Fundamental Concepts of Semiconductor-Mediated Heterogeneous Photocatalysis

Semiconductor-based nanostructures have received significant attention in recent years for energy and environmental applications due to their cost-effective and straightforward synthesis process, and mainly they have nanograins of different sizes and different kinds of heterostructured materials [19,20]. Further, the pristine semiconductor photocatalysts have exhibited minimal PEC water splitting abilities due to their rapid reunion of charge carriers and having a lower surface area [21,22]. Thus, a combination of various semiconductor materials with different morphologies can increase the performance in the visible-light range of the solar spectrum. Moreover, the coupled heterostructured materials with two or more components should have different energy levels that can play an essential role in the interfacial charge transfer of the electron (e⁻) and holes (h⁺) for the reduction and oxidation of water molecules on the surface of heterostructured electrodes to achieve a higher PEC water splitting activity [23,24].

In recent years, the graphitic carbon nitride (g-C₃N₄) semiconductor-based binary and ternary heterostructured materials have acted as promising photocatalysts for the PEC water splitting and photocatalytic pollutant removal applications [25,26,27]. Therefore, several investigations have been done on g-C₃N₄-based heterostructured photocatalysts for the PEC water splitting application. Moreover, the heterostructured electrodes are found as an attractive candidate due to their enhanced properties (such as larger surface area, active reactive sites, and higher light absorption capacity) when compared to their bulk counterparts [28,29]. In the case of a g-C₃N₄-based heterostructured electrode, the suitable band edge potentials of the conduction band (CB) and valence band (VB) of different nanostructured semiconductor materials with various morphologies are combined with g-C₃N₄, which could facilitate the charge transfer kinetics of photogenerated e⁻/h⁺ through the interfacial contact. Moreover, the solar-light active nanostructured materials upon the irradiation generate reactive oxygen species, and it also assists in suppressing the recombination rate of photogenerated e⁻/h⁺ pairs [30,31]. Therefore, in heterostructured photocatalysts, the assembly of heterojunction could improve the photocatalytic reactions due to its extended absorption of light, synergistic interactions with facile charge transportation, and the reduced recombination rate of charge carriers [32,33].

1.2. The Graphitic Carbon Nitride (g-C₃N₄) Material

The g-C₃N₄ is a kind of strong allotrope of carbon nitride family having a bandgap of around 2.7 eV, and typically it is in the form of two-dimensional (2D) nanosheets comprising of tri-s-triazines interlayers [34]. The g-C₃N₄ is one of the graphite-like layered structure materials (shown in Figure 1) with superior prospects due to their ease of availability, low price, and environmental friendliness [35].

The g-C₃N₄ molecular surface contains a large number of π-conjugated planar layers, which have an excellent complexing effect with high stability, so the material has been used for photocatalytic and PEC applications in recent years [37]. However, because of the utilization of only visible light, fast recombination of e⁻/h⁺ pairs, low charge separation efficiency, and the moderate specific surface area, it can provide fewer active sites, which reduces its effective usage as a pristine photocatalyst [38,39]. Thus, in order to avoid these disadvantages, the design and synthesis of g-C₃N₄-based semiconductor heterostructures with appropriate photocatalytic properties could be an effective approach for enhancing the PEC water splitting efficiency [40,41].

1.3. Basic Principles of PEC Water Splitting

The increased accumulation of greenhouse gases has resulted in a constant increase in the global average temperature, enabling a shift in perspective and redirecting the focus from carbon-based energy resources to carbon-free renewable energy technologies [42,43]. In this context, PEC water splitting has foremost importance, in that it generates only H₂ and O₂ at two electrodes without any other polluting gases [5,44]. The schematic representation of the PEC water splitting mechanism is presented in Figure 2.

Moreover, the PEC-based experiments are performed at room temperature, and the semiconductor electrodes are fabricated using inorganic compounds, which are stable in aqueous electrolytes. In contrast, organic-based semiconductor electrodes show poor stability in PEC water splitting reactions [45,46]. The PEC water splitting device employs quantum solar energy conversion materials in which the semiconductor absorbs incoming photons, resulting in the electron–hole pairs generation process. The incident sunlight on the photoanode excites electrons from the valence band to the conduction band, leaving behind holes in the valence band of semiconductor material [47]. Further, the charge separation typically occurs by the migration of carriers under the influence of the electric field, and the charge transfer occurs at the semiconductor/electrolyte interface. In a photoelectrode, the minority carriers are transported to the electrochemical interface in order to drive the reaction. For a photoanode, holes oxidize water molecules, and for a photocathode, electrons reduce the H⁺ ions at the electrode surface [48].

For the past few years, solar-driven visible-light active semiconductor materials were combined with g-C₃N₄, which showed a better PEC water splitting performance due to their enhanced light absorption ability, robust synergistic interface, the enlarged surface area, and the effective separation of e⁻/h⁺ pairs [49]. Furthermore, these g-C₃N₄-based binary and ternary heterostructured electrodes with various morphologies can significantly progress the movement of charge carriers at the electrode/electrolyte interface and, in turn, enhance the PEC water splitting efficiency. However, despite these recent signs of progress, critical reviews on g-C₃N₄-based binary and ternary heterostructured photoelectrodes for the PEC water splitting reaction are still lacking. Therefore, this review summarizes the different types of g-C₃N₄-based binary and ternary heterostructured photoelectrodes used for PEC water splitting reaction, along with their advantages and limitations.

2. Importance of g-C₃N₄-Based Hybrid Heterostructured Electrodes

Generally, when the pristine g-C₃N₄ is subjected to light, it absorbs photons and subsequently generates the photo-induced e⁻/h⁺ pairs. However, the excited electrons in the CB are in a most unstable state and will tend to occupy the stable state, thus coming back to the VB and recombining with photogenerated holes [50]. Therefore, to suppress the recombination of charge carriers and achieve maximum efficiency, g-C₃N₄ should combine with other semiconductor photocatalysts through the formation of heterostructured electrodes [51]. Recently, some approaches have been introduced to enhance the solar-driven visible-light active photocatalytic performance of g-C₃N₄ materials. For instance, the construction of heterostructured electrodes with g-C₃N₄ and other semiconductors, doping with metals and non-metals, and the fabrication of mesoporous heterostructures [52,53,54]. Among them, the construction of g-C₃N₄-based binary and ternary heterostructured electrodes exhibited a superior PEC activity due to its reduced reunion of electron–hole pairs and the quick transfer of charge carriers at the interface of the heterostructured electrodes. Furthermore, while developing g-C₃N₄-based heterostructured electrodes, g-C₃N₄ should associate with both the visible and UV light-based semiconductor materials, which could enlarge the absorption of the solar spectrum to the maximum range [55,56].

3. g-C₃N₄-Based Binary Heterostructured Photoelectrodes for PEC Water Splitting

The pristine g-C₃N₄ material has limitations such as fast e⁻/h⁺ reunion rate, restricted to visible-light absorption range, and the moderate surface area, which urges the development of new approaches to enhance its applications [57,58]. To overcome the above limitations and achieve greater efficiencies, g-C₃N₄-based binary heterostructures have been constructed, and some of them are reported here. Binary heterostructures of g-C₃N₄, along with various concentrations of MnO_x (2, 5, 7, 10, and 15%), were synthesized via in-situ thermal decomposition method to enhance the PEC activity [59]. The XRD result showed that the hexagonal phase of g-C₃N₄, the monoclinic phase of MnO_x, and the heterostructure formation did not affect the individual components. The morphology results established that the irregular-shaped MnO_x nanoparticles (NPs) dispersed over the g-C₃N₄ surface. The visible-light-harvesting ability of an electrode was enhanced by the formation of heterostructure when compared to pristine samples. From the electrochemical impedance spectroscopy (EIS) results, the 10% MnO_x/g-C₃N₄ heterostructured electrode exhibited the lowest charge transfer resistance (R_ct) compared to the remaining samples. Further, the 10% MnO_x/g-C₃N₄ heterostructured electrode showed a higher photocurrent density (0.003 A cm⁻² at 1.25 V vs. Ag/AgCl) than the remaining photoanodes. Thus, the synergistic interactions among g-C₃N₄ and MnO_x can effectively progress the mobility of charge carriers and diminish the reunion of e⁻/h⁺ pairs, which enhances the PEC performance. Wang et al. successfully constructed a core-shell g-C₃N₄@ZnO (CN/ZN) heterostructured electrode with different amounts of g-C₃N₄ (0.01, 0.05, 0.10, 0.15, 0.20 g) using a facile reflux technique [60]. The XRD results (Figure 3a) confirmed the hexagonal phase of ZnO and the interlayer stacking of CN units with no impurity phases. The morphology studies (Figure 3b) showed the heterostructure formation with an effective distribution of ZnO NPs over the CN surface. When compared to the pristine ZnO, the visible-light absorption ability of the CN/ZN heterostructured electrode (Figure 3c) is enhanced through the formation of heterostructures. The EIS results (Figure 3d) demonstrated that the CN/ZN-0.15 heterostructured electrode exhibited the lowest charge-transfer resistance (R_ct) among all samples. From the transient photocurrent (I-t) results (Figure 3e), the CN/ZN-0.15 photoelectrode exhibited a sharp increase of photocurrent density with a quick photoresponse than the other samples. Furthermore, the CN/ZN-0.15 photoelectrode provided a highest photocurrent density at 1.5 V vs. SCE in 0.1 M Na₂SO₄ electrolyte under visible-light illumination. This improvement in the PEC water splitting activity (Figure 3f) is owed to the core-shell morphology, higher light absorption capability, formation of synergistic interfaces, and the facile charge carrier transport.

Xiao et al. successfully constructed a binary heterostructured electrode in which zero-dimensional (0D) g-C₃N₄ NPs covered over the one-dimensional (1D) TiO₂ nanotube arrays with an interfacial oxygen vacancy layer (CN/OV-TO). This heterostructured electrode was synthesized using anodic oxidation followed by the reduction and vapor deposition processes [61]. The visible-light-harvesting ability is improved substantially after forming the heterostructure and oxygen vacancies (OV) associated with CN, TO, and CN/TO. The structural and morphology investigations confirmed the formation of 0D/1D heterostructured electrodes. Further, the EIS results demonstrated that the constructed CN/OV-TO heterostructured electrode exhibited the smallest R_ct value among all prepared samples. In addition, the photocurrent density of CN/OV-TO photoanode is around 0.72 mA cm⁻² at 1.23 V Vs. RHE under visible-light illumination. This value is almost eight times higher than the CN/TO heterostructured electrode without an interfacial OV layer. This enhanced water splitting ability was attributed to the Z-scheme mechanism along with the interfacial OV layer, which delays the reunion of e⁻/h⁺ pairs and endorses the facile charge carrier transport mechanism. Reddy et al. constructed 1D ZnWO₄ nanorods decorated over the 2D g-C₃N₄ nanosheets using a facile hydrothermal synthesis method to enhance the PEC water splitting reaction under solar light [62]. By the formation of the heterostructured electrode, the solar light-harvesting capability was remarkably improved. The crystal phase and morphology analysis confirmed the formation of the heterostructured electrode by the successful decoration of ZnWO₄ nanorods on the surface of g-C₃N₄ nanosheets. Further, the EIS results validated that the constructed 2D/1D heterostructured electrodes exhibited the lowest R_ct value when compared to pristine samples. Moreover, the photocurrent density of the 2D/1D heterostructured electrode is almost 6.75 times and 3.31 times higher than the pristine ZnWO₄ and g-C₃N₄ electrodes, respectively. Nevertheless, the 2D/1D heterostructured electrode exhibited a very good stability and thus the formation of a binary heterostructured electrode could effectively improve the migration of charge carriers with a lower charge transfer resistance and suppress the reunion of e⁻/h⁺ charge carriers.

A novel hybrid carbon-doped CuO/g-C₃N₄ (C-CuO/CN) heterostructured electrode was prepared using a facile one-step microwave-supported approach to improve the PEC performance under visible source [40]. The crystal phase results confirmed the formation of the monoclinic phase of CuO and the interlayer stacking of the CN units. The morphology results demonstrated that the CuO seems to be an evenly dispersed micro-flowers consisting of intermixed ultrathin nanosheets and distributed over the surface of CN. The photoluminescence spectra of C-CuO/CN heterostructured material showed a very weak intensity compared to the CuO, CN, and CuO/CN materials, and this enhanced PL activity can be ascribed to the significance of carbon dopant, which influences lattice defects. The C-CuO/CN heterostructured electrode exhibited the lowest charge transfer resistance of 3.48 kΩ cm⁻² than the CuO (18.53 kΩ cm⁻²) and CuO/CN (7.69 kΩ cm⁻²) electrodes. Furthermore, the photocurrent density of the C-CuO/CN electrode is around −2.85 mA cm⁻² at 0 V vs. RHE under visible source, and it is around 227% and 38% higher than the CuO and CuO/CN electrodes, respectively. Thus, the carbon-doping, influence of lattice defects, and the formation of interfacial interactions significantly enhanced the PEC performance.

Seza and co-workers successfully constructed novel photoelectrodes made of SnO₂ NPs (various loading of 0.175, 0.6, and 1 g) decorated over the surface of porous g-C₃N₄ nanosheets (CN/SN) using a facile microwave synthesis method [63]. The XRD results demonstrated that the CN/SN heterostructured material exhibited both the g-C₃N₄ and SnO₂ phases. The morphology results showed the highly porous structure of CN nanosheets and the irregular shape of SnO₂ NPs. The light absorption capacity of heterostructured CN/SN electrode enhanced towards the visible-active region compared to the pristine samples. The CN/SN-0.175 heterostructured electrode exhibited the highest specific surface area of 195 m² g⁻¹ compared to the other samples. From the linear sweep voltammetry (LSV) results, the CN/SN-0.175 heterostructured electrode showed a photocurrent density of 0.033 mA cm⁻² at 1.2 V vs. Ag/AgCl in 0.2 M Na₂SO₄ electrolyte under visible-light irradiation, and this photocurrent value is superior to the remaining photoelectrodes. This enhanced PEC performance can be ascribed to the formation of porous nanosheets, enhanced visible-light absorption ability, and the formation of the interfacial heterostructure.

Novel binary g-C₃N₄ combined with carbon nanotubes (CN/CT) heterostructured film was systematically fabricated on the FTO substrate using a facile polycondensation method to enhance PEC water splitting ability [52]. The structural and morphological analysis evidence the formation of CN/CT heterostructured electrodes with synergistic interfaces. As a result, the light-harvesting capacity systematically improved towards the visible-active region by the development of heterostructure. Further, the EIS results validated that the constructed CN/CT heterostructured electrode showed the lowest charge transfer resistance of 6.5 × 10⁵ Ω, and it is around 9.3 times higher than the pristine CN electrode. The photocurrent density of the CN/CT heterostructured electrode showed 0.075 mA cm⁻² at 1 V vs. Ag/AgCl under visible light, and it is around 11 times superior to the pure CN photoanode. Thus, combing CN with CT significantly enhanced the electron conductivity, light-harvesting ability, and the separation of charge carriers, which further improved the PEC water splitting efficiency. Bakr and co-workers successfully constructed a binary Z-scheme heterostructured electrode using α-Fe₂O₃ nanotubes combined with various concentrations (1, 2, and 3.75 wt.%) of ruptured tubular g-C₃N₄ (FO/CN) using an electrostatic self-assembly technique [64]. The XRD results revealed that both crystal phases were presented in the FO/CN heterostructured electrode. The morphology results showed that the α-Fe₂O₃ nanotubes were anchored over the tubular g-C₃N₄, and the bandgap modification was detected for the FO/CN heterostructured material. The EIS results exhibited the lowest charge transfer resistance value for FO/CN-1 sample. Furthermore, the FO/CN-1 heterostructured electrode showed the highest photocurrent density of 0.0055 mA cm⁻² at 1 V vs. SCE in 0.1 M Na₂SO₄ electrolyte under sunlight. This improvement in the PEC performance was due to the morphological interfaces, heterostructure formation, and quick transportation of charge carriers.

Novel binary heterostructured electrodes made of BiOI nanoplates combined with multilayered g-C₃N₄ (BI/CN-M) and few-layered g-C₃N₄ nanosheets (BI/CN-S) were systematically synthesized using a simple solvothermal technique for the enhancement of PEC water splitting reaction [65]. The XRD results showed the formation of the tetragonal phase of BiOI and the interlayer stacking of CN, and both crystal phases have appeared in the BI/CN-S heterostructured electrode. The morphology results demonstrated that the BI nanoplates were decorated over the CN-S nanosheets, which confirmed the establishment of the heterostructured material. Further, the FTIR and Raman spectra results evidently showed the synergistic interactions among BI and CN materials. From the EIS results, the BI/CN-S heterostructured electrode showed the lowest charge transfer resistance than the pure BI, CN, and BI/CN-M electrodes. Moreover, the observed photocurrent density of BI/CN-S heterostructured electrode was around 0.20 mA cm⁻² at −0.5 V vs. Ag/AgCl in 0.1 M Na₂SO₄ under visible-light irradiation, and the current density value was superior to BI/CN-M (0.16 mA cm⁻²), BI (0.09 mA cm⁻²), CN-M (0.05 mA cm⁻²), and CN-S (0.04 mA cm⁻²) electrodes. The enhanced water-splitting ability of the heterostructure material was attributed to the enlarged light absorption ability, the diminished reunion of charge carriers, and the synergistic trap passivation in the heterostructured electrode.

Liu and colleagues constructed the well-designed TiO₂ nanotube arrays (TNTAs) and dispersed the g-C₃N₄ nanoparticles over TNTAs using anodic oxidation followed by calcination methods [66]. The formation mechanism of g-C₃N₄/TNTAs (CN-TN) heterojunction was schematically represented in Figure 4a. The crystallographic results showed the anatase phase of TiO₂ and interlayer stacking of g-C₃N₄, which significantly confirmed the formation of heterojunctions. The morphology results from Figure 4b–d showed that the g-C₃N₄ nanosheets covered the walls of the TiO₂ nanotubes. From the LSV results (Figure 4e), the photocurrent density of CN-TN electrode was observed around 0.86 mA cm⁻² and is nearly double that of pure TNTAs electrode. The transient photocurrent (I-t) results (Figure 4f) demonstrated that the CN-TN heterojunction electrode exhibited a sharp increase of photocurrent density with a quick photoresponse compared to the as-prepared pristine samples. The charge transfer resistance of the CN-TN heterojunction electrode (Figure 4g) was significantly reduced among the other samples under light illumination. Thus, the formation of binary heterojunction between g-C₃N₄ and TNTAs (Figure 4h) could significantly enhance the transportation of charge carriers and reduce the reunion of photo-induced e⁻/h⁺ pairs, which in turn enhanced the PEC water splitting efficiency.

Liu and co-workers effectively fabricated the g-C₃N₄-coated Fe₂O₃ nanoplatelets (CN/FO) electrodes using the facile electrodeposition followed by chemical vapor deposition methods [67]. The morphology and structural results demonstrated the distribution of g-C₃N₄ nanoparticles over the surface of Fe₂O₃ nanoplatelets. Further, the light-harvesting ability was significantly enhanced after the formation of CN/FO heterostructuredelectrode, and the PL intensity was diminished when compared to the pure CN and FO electrodes. From the EIS analysis, the CN/FO heterostructured photoanode showed the lowest R_ct value compared to the remaining photoanodes. The photocurrent density of the CN/FO heterostructured electrode showed around 0.78 mA cm⁻² at 0.4 V vs. Ag/AgCl in 1.0 M NaOH under visible light, and it is around 70 times higher than the pure FO photoanode. Therefore, the introduction of CN over the surface of FO considerably enriched the light-harvesting ability, separation of e⁻/h⁺ pairs, and quicker interfacial charge transfer, which improved the water-splitting ability. Novel binary 2D/2D heterostructured electrode composed of porous g-C₃N₄ and BCN nanosheets (CN/BCN) was successfully fabricated using a facile thermal polymerization technique [68]. The XRD results demonstrated that both the crystal phases of CN and BCN were presented in the CN/BCN heterostructured electrode. The morphology results showed that the porous CN nanosheets were anchored over the BCN nanosheets. From differential reflectance spectroscopy (DRS) results, the loading of CN towards BCN could substantially enhance the light-harvesting capability. The charge transfer resistance of CN/BCN electrode was around 2.8 kΩ, which is 11 times and five times lesser than CN and BCN electrodes, respectively. Further, the transient photocurrent density of the CN/BCN electrode is higher than the individual CN and BCN electrodes. Moreover, the CN/BCN heterostructured electrodes showed a photocurrent density of 0.62 mA cm⁻² at 0.62 V vs. Ag/AgCl in 0.1 M Na₂SO₄ electrolyte under visible-light illumination, which was around 8 and 3 times higher than the CN (0.08 mA cm⁻²) and BCN (0.21 mA cm⁻²) electrodes, respectively. Thus, the morphological 2D interfaces, superior migration of charge carriers, and the separation of e⁻/h⁺ pairs significantly enhanced the PEC water splitting performance.

Murugan and colleagues systematically fabricated a binary g-C₃N₄/TiO₂ (CN/TO) heterostructured electrode with different concentrations of CN (5, 10, 15, 20, and 25 wt.%) using a facile in-situ soft template technique (Figure 5a) for the improvement of PEC water splitting performance [69]. The XRD results (Figure 5b) confirmed the anatase phase of TiO₂, the interlayer stacking of CN, and both the phases have appeared in the CN/TO heterostructured electrode. The morphology results (Figure 5c,d) demonstrated that the TO NPs were decorated over the CN nanosheets. From the EIS analysis (Figure 5e), the CN/TO-20 photoanode showed the lowest R_ct value than the remaining photoanodes. Furthermore, the photocurrent density (Figure 5f–g) of CN/TO-20 heterostructured electrode was around 0.0723 mA cm⁻² at 1.23 V vs. RHE in 1 M KOH electrolyte under visible-light and the photocurrent density value was superior than the CN (0.00082 mA cm⁻²), TO (0.0366 mA cm⁻²), CN/TO-5 (0.0472 mA cm⁻²), CN/TO-10 (0.0544 mA cm⁻²), CN/TO-15 (0.0632 mA cm⁻²), and CN/TO-25 (0.0658 mA cm⁻²) electrodes. The enhanced water splitting ability was accounted for their enlarged light absorption ability, the diminished reunion of charge carriers, and synergistic interfacial effect in the heterostructured electrode. The same group, Murugan and co-workers, successfully fabricated a binary combination of hierarchical Bi₂MoO₆ and various amounts (1, 3, 5, 7, 10, and 12 wt.%) of g-C₃N₄ (BMO/CN) using a solvothermal technique for the enhancement of PEC water splitting ability [70]. The XRD results revealed the formation of the orthorhombic phase of BMO, the interlayer stacking of CN, and both the phases have appeared in the BMO/CN heterostructured electrode. The regulation of the bandgap was detected for the BMO/CN heterostructured electrode compared to the pure samples, which enlarged the visible-light-harvesting capability. From the EIS analysis, the BMO/CN-10 heterostructured electrode showed the lowest R_ct value, the photocurrent density of the BMO/CN-10 electrode was around 0.0212 mA cm⁻² at 1.23 V vs. RHE in 1 M KOH electrolyte under visible-light illumination, and the photocurrent density value was about 28 and 7 times greater than the pristine CN and BMO electrodes, respectively. Thus, heterostructure formation significantly enhanced the light-harvesting ability, separation of e⁻/h⁺ pairs, and quicker interfacial charge transfer, which progressed the water-splitting ability.

Mohammad et al. successfully fabricated the novel heterostructured electrode using different concentrations (2 and 4 mM) of SnO₂ NPs decorated over the surface of g-C₃N₄ nanosheets (CN/SN) via a simple hydrothermal method [71]. The XRD results confirmed that both the crystal phases of SN (tetragonal rutile phase) and CN were presented in the CN/SN heterostructured electrode without any impurity phases. The FTIR and XPS results established the formation of CN/SN heterostructured electrodes. The visible-light-harvesting ability was significantly enhanced after forming the CN/SN heterostructured electrode, and the PL intensity was diminished compared to the pure CN and SN materials. The CN/SN-4 mM heterostructured electrode demonstrated a higher specific surface area of 181.54 m² g⁻¹ than the pure SN (181.13 m² g⁻¹) and CN/SN-2 mM (178.69 m² g⁻¹) electrodes. From the EIS results, the CN/SN-4 mM heterostructured electrode showed the lowest charge transfer resistance when compared to the pure CN, SN, and CN/SN-2 mM electrodes. Furthermore, the photocurrent density of CN/SN-4 mM photoelectrode was around 2.1 mA cm⁻² at 1 V vs. Ag/AgCl in 0.2 M Na₂SO₄ under visible-light and it was around eight times higher than the pure SN electrode. The improved water splitting performance was owing to the enhanced surface area, light absorption ability, diminished reunion of charge carriers, and the synergistic effect in the heterostructured interface.

Kumar et al. successfully constructed heterostructured electrodes made of NaNbO₃ nanofibers along with different concentrations (2, 4, and 8 wt.%) of g-C₃N₄ nanosheets (NNO/CN) using facile hydrothermal followed by chemisorption techniques (Figure 6a) for better PEC water splitting performance [31]. The XRD results (Figure 6b) demonstrated that both the crystal phases of CN (interlayer stacking) and NNO (orthorhombic phase) were presented in the NNO/CN heterostructured electrode. The surface morphology images (Figure 6c) confirmed the effective distribution of NNO nanofibers on the surface of CN nanosheets. The DRS analysis (Figure 6d) showed that the visible-light-harvesting ability was significantly enhanced in the NNO/CN heterostructure electrode. The EIS results (Figure 6e) demonstrated that the NNO/CN-4 photoelectrode exhibited the lowest R_ct value, and the I-t results (Figure 6f) indicated that the NNO/CN-4 photoelectrode showed a sharp increase of photocurrent density with a quicker photoresponse than the other photoelectrodes. Moreover, the NNO/CN-4 photoelectrode showed a photocurrent density of 12.5 mA cm⁻² at 1 V vs. Ag/AgCl in 0.5 M NaOH electrolyte under visible-light, which was around three-times higher than the pristine NNO electrode (4.3 mA cm⁻²). Thus, the formation of heterostructured electrode (Figure 6g) significantly enhanced the light-harvesting ability, separation of e⁻/h⁺ pairs, and quicker interfacial charge transfer, which significantly improved the water-splitting ability.

A novel binary heterostructured electrode made of Bi₂WO₆ quantum dots (QDs) dispersed on the g-C₃N₄ (CN/BWQ) was prepared by a simplistic in-situ hydrothermal method [72]. The X-ray diffraction results confirmed the interlayer stacking of the CN units and the orthorhombic phase of BW, even after the formation of a heterostructured electrode. Furthermore, the morphological examination revealed that the BW QDs were uniformly distributed over the surface of CN. When compared to the pristine samples, bandgap tuning was observed for the CN/BWQ heterostructured electrode, which significantly enhanced the visible-light-harvesting capability. Moreover, the CN/BWQ heterostructured electrode demonstrated the lowest R_ct value, the photocurrent density of CN/BWQ photoanode was observed around 0.39 μA cm⁻² at 1.23 V vs. RHE under visible-light illumination, and the current density value was about 2.4 and 1.8 times higher than the pristine CN and BWQ electrodes, respectively. Thus, forming a synergistic interfacial heterostructure and the Z-scheme mechanism could significantly enhance the water-splitting activity. The PEC performance of g-C₃N₄-based various binary heterostructured electrodes is presented in Table 1.

4. g-C₃N₄-Based Ternary Heterostructured Photoelectrodes for PEC Water Splitting

Numerous efforts have been executed to fabricate g-C₃N₄-based binary heterostructured electrodes to overcome the constraints and improve the PEC water splitting ability. However, the PEC performance is still limited and not up to the target of practical applications. To overcome these issues, g-C₃N₄-based complex compounds containing three components have been introduced, and some of the results are discussed here. A novel hybrid ternary anatase TiO₂/rutile TiO₂/g-C₃N₄ (A/R/CN) heterostructured electrode was prepared with different mass ratios of CN (20, 30, 40, 50, and 60 mg) using a facile thermoset hybrid technique for the enhancement of water splitting performance [93]. The XRD result confirmed the anatase and rutile phases of TiO₂ and the interlayer stacking of the CN units. The electron microscopy results validated that the TiO₂ NPs were distributed over the surface of CN, which signifies the formation of a heterostructured electrode. The light-harvesting capacity systematically improved towards the visible region in the case of the ternary heterostructured electrode. From the EIS results, the A/R/CN40 heterostructured electrode showed the lowest charge transfer resistance value than the pure P25, CN, and A/R/CN20, A/R/CN30, A/R/CN50, and A/R/CN60 heterostructured electrodes. Further, the photocurrent density of the A/R/CN40 photoanode was superior to the other electrodes under solar light illumination. Thus, the formation of interfacial interactions among the three components could improve the migration of charge carriers with more separation, which further enhanced the PEC water splitting capability.

Reddy and co-workers have developed a novel ternary electrode made of CdS NPs and Fe₃O₄ nanocubes dispersed over the g-C₃N₄ nanosheets (CS/FO/CN) using two-step solvothermal techniques for the enhanced PEC water splitting performance [37]. The morphology results demonstrated that the CdS NPs entered into the Fe₃O₄ lattices and were distributed on the surface of CN nanosheets. The XRD results confirmed the presence of the hexagonal phase of CdS, the magnetite phase of Fe₃O₄, the interlayer stacking of CN in CS/FO/CN heterostructured electrode, and the heterostructure formation did not affect the individual phases. The CS/FO/CN electrode showed a superior specific surface area of 40.022 m² g⁻¹. Further, the CS/FO/CN ternary heterostructured electrode showed enlarged light-harvesting performance. The charge transfer resistance of CS/FO/CN ternary heterostructured electrode was significantly reduced when compared to CdS, CN@CS, and CS@FO electrodes upon visible-light irradiation. The photocurrent density of the CS/FO/CN ternary heterostructured electrode showed around 0.0238 mA cm⁻² at 0.2 V vs. Ag/AgCl under visible-light irradiation, and it was around 5.95, 4.76, and 7.93 times higher than CdS, CN@CS, and CS@FO electrodes, respectively. This improvement in the PEC performance was ascribed to the ternary interfacial charge transport, separation efficiency of e⁻/h⁺ pairs, and the facile transportation of charge carriers.

Bhandary and colleagues fabricated a bimetallic AgNi alloy surrounded by g-C₃N₄ nanosheets (CN/AgNi) electrodes using a facile one-pot solid-state heat treatment method to improve the PEC water splitting capability [94]. The structural and morphological studies confirmed the heterostructure formation with a successful decoration of bimetallic AgNi alloy over the surface of g-C₃N₄ nanosheets. After the formation of the heterostructure, the light absorption capability remarkably improved. Further, the EIS results confirmed that the constructed CN/AgNi heterostructured electrode showed the lowest R_ct value among the pure samples. Furthermore, the CN/AgNi photoelectrode showed the highest photocurrent density of 1.29 mA cm⁻² at 1.0 V vs. Ag/AgCl in 1.0 M NaOH electrolyte under visible light, and the value was more significant than the CN, CN/Ag, and CN/Ni electrodes. Nevertheless, the CN/AgNi heterostructured electrode exhibited excellent stability, and thus the formation of a heterostructure could effectively improve the migration of charge carriers with lower charge transfer resistance and suppress the reunion of e⁻/h⁺ pairs.

The novel ternary heterostructured electrodes of Ag₃PO₄/Ag₂MoO₄ materials combined with various concentrations (0.4, 0.8, and 1.2 wt.%) of multilayered g-C₃N₄ nanosheets (AP/AM/CN) were systematically fabricated using the in-situ co-precipitation technique by Liu and colleagues [95]. The XRD results showed the formation of the cubic phase of Ag₃PO₄ and Ag₂MoO₄, the interlayer stacking of CN, and the ternary AP/AM/CN heterostructured electrode contained all three phases. The morphology results demonstrated that both the AP and AM NPs were distributed over the surface of CN nanosheets, which confirmed the establishment of the ternary heterostructured electrode. Further, the FTIR and Raman spectra results evidently showed synergistic interactions among AP/AM and CN interfaces. The visible-light-harvesting ability was considerably enhanced through the formation of ternary AP/AM/CN heterostructured electrodes when compared to the CN, AP, AM, and AP/AM electrodes. From the EIS results, the ternary AP/AM/CN photoanode showed the lowest charge transfer resistance value when compared to the CN, AP, AM, and AP/AM electrodes. Moreover, the photocurrent density of the ternary AP/AM/CN photoelectrode was superior to the other samples under solar light illumination. Thus, the enlarged solar light absorption ability could effectively diminish the reunion of charge carriers and the dual Z-scheme charge carrier pathway that eventually enhanced the PEC water splitting ability.

A novel hybrid ternary g-C₃N₄/Au-SnO₂ QDs (CN/Au/SQD) heterostructured electrode was prepared with different mass ratios of Au-SQD (10, 20, and 30 mg), using calcination and sonication methods (Figure 7a) for the enhancement of PEC water splitting performance [96]. The XRD results revealed the presence of the tetragonal phase of SnO₂, interlayer stacking of the CN units, and the metallic phase of Au. The electron microscopy results (Figure 7b) confirmed that the Au-SQDs were distributed over the surface of CN, which signifies the heterostructure formation. The light-harvesting efficiency (Figure 7c) was systematically improved towards the visible region through the heterostructure formation. The PL spectra (Figure 7d) of CN/Au/SQD heterostructured electrode showed a very weak intensity compared to the SQD, CN, and Au-SQD samples. From the EIS results, the CN/Au/SQD ternary electrode showed the lowest R_ct value than the SQD, CN, and Au-SQD electrodes. Furthermore, the CN/Au/SQD photoelectrode displayed the superior photocurrent density (Figure 7e) of 0.0402 mA cm⁻² at 1.0 V vs. Ag/AgCl in 0.1 M Na₂SO₃ electrolyte under visible-light irradiation, and the value was significantly greater than the SQD, CN, and Au-SQD electrodes. Thus, the fabrication of metal-semiconductor combined with CN ternary heterostructured electrodes (Figure 7f) could improve the migration of charge carriers with more separation efficiency, which further enhanced the PEC water splitting performance.

Li and co-workers systematically fabricated 0D Bi₂O₃ NPs encapsulated with BiPO₄ nanorods that were distributed over the surface of g-C₃N_4, which formed the CN/BO/BP ternary heterostructured electrode using the facile hydrothermal method followed by calcination process [97]. The XRD results demonstrated that the ternary CN/BO/BP heterostructured electrode exhibited g-C₃N₄, Bi₂O₃, and BiPO₄ phases, confirming the presence of individual components. The morphology results showed that the Bi₂O₃ NPs were anchored over BiPO₄ nanorods, and the binary BO/BP heterostructured material was distributed over the surface of the g-C₃N₄ nanosheets. When compared to pure samples, the light absorption efficiency of the CN/BO/BP heterostructured electrode enhanced towards the visible region. From the EIS analysis, the ternary CN/BO/BP photoanode exhibited the lowest R_ct value compared to the pure CN, BO, BP, CN/BP, BO/BP electrodes. From the LSV results, the CN/BO/BP photoanode showed a photocurrent density of 0.095 mA cm⁻² at 1.25 V vs. Ag/AgCl in 0.1 M Na₂SO₄ electrolyte under solar light irradiation, and the value was 2.4 and 1.3 times superior to the BP and BO/BP electrodes, respectively. This enhanced PEC performance could be ascribed to the formation of the interfacial heterostructure with the Z-scheme process, which hinders the recombination of charge carriers and higher solar-light absorption ability.

Chaudhary et al. have developed a novel ternary heterostructured electrode consisting of the TiO₂ NPs with carbon nanotubes that dispersed over the g-C₃N₄ nanosheets (TO/CN/CNT) using a facile hydrothermal method [98]. The XRD results (Figure 8a) confirmed the anatase phase of TiO₂, the graphitic nature of CNT, and the interlayer stacking of CN. The morphology results (Figure 8b) displayed that the TiO₂ NPs and CNTs were decorated on the surface of CN nanosheets. Further, the TO/CN/CNT ternary heterostructured electrode showed an enlarged light-harvesting performance. The R_ct value (Figure 8c) of the TO/CN/CNT ternary heterostructured electrode was significantly reduced compared to the TO, CN, and TO/CN electrodes under visible-light illumination. The photocurrent density (Figure 8d) of the TO/CN/CNT ternary electrode showed 2.43 mA cm⁻² at 0.6 V vs. Ag/AgCl in 0.5 M Na₂SO₄ electrolyte under visible-light, and the current density value was around 6.4, and 3.2 times higher than the CN, TO/CN photoelectrodes, respectively. Thus, the formation of interfacial interactions among the three components (Figure 8e) could effectively improve the migration of charge carriers with more separation, which in turn enhanced the PEC water splitting efficiency.

Si and co-workers systematically fabricated graphdiyne (GDY)-anchored g-C₃N₄/NiFe-layered double hydroxide (CN/GDY/NF) electrode via the facile hydrothermal method [99]. The XRD results revealed that CN, GDY, and NF crystal phases were presented in the CN/GDY/NF heterostructured electrode without any impurity phases. Further, the FTIR and XPS results confirmed that the heterostructured material consisted of individual compounds. The CN/GDY/NF heterostructured material presented a higher specific surface area of 208.75 m² g⁻¹ than the pure CN (126.32 m² g⁻¹), GDY (95.03 m² g⁻¹), and NF (42.63 m² g⁻¹) materials. The visible-light-harvesting ability was significantly enhanced after forming the CN/GDY/NF heterostructured electrode, and the PL intensity was diminished compared to the pristine CN, GDY, and NF samples. From the EIS results, the CN/GDY/NF photoelectrode showed the lowest R_ct value than the pure CN, GDY, and NF electrodes. Furthermore, the photocurrent density of ternary CN/GDY/NF photoelectrode was observed around 0.178 mA cm⁻² at 1.4 V vs. RHE under visible light, and the photocurrent density value was around 45 times higher than the pristine CN electrode. The enhanced water splitting activity was ascribed to the formation of synergistic interactions among the ternary components, which emphasized the visible-light absorption ability and quick transfer of charge carriers with lower charge-transfer resistance.

Gopalakrishnan et al. adopted the spin-coating technique and successfully fabricated the heterostructured electrode in which SrTiO₃ NPs covered on the surface of g-C₃N₄ nanosheets and coupled with silicon nanowires (ST/CN/Si) [100]. The crystallographic analysis confirmed the formation of ST, CN, and Si phases in the ST/CN/Si heterostructured electrode without any impurity phases. The morphology results demonstrated the uniform distribution of ST nanoparticles and Si nanowires on the surface of g-C₃N₄, which confirmed the formation of a synergistic interfacial heterostructure. Further, the light-harvesting ability was significantly enhanced towards the visible region through the formation of a ternary ST/CN/Si heterostructured electrode. From the EIS analysis, the ST/CN/Si photocathode exhibited the lowest charge transfer resistance than the pure CN, ST, Si, and CN@Si electrodes. The photocurrent density of the ST/CN/Si heterostructured electrode showed around 28 mA cm⁻² at 0.8 V vs. RHE in 0.5 M Na₂SO₄ electrolyte under visible light, and this current density value was superior to the other electrodes. Therefore, the introduction of Si nanowires and ST nanoparticles on the surface of CN considerably enriched the light-harvesting ability, separation of e⁻/h⁺ pairs, and quicker interfacial charge transfer, which significantly progressed the water-splitting ability.

To enhance the utility of visible-light and diminish the reunion of charge carriers, a novel fluorine-doped, chlorine-intercalated g-C₃N₄ combined with various concentrations of BiOI (10, 25, 57, and 75%)-based CNF-Cl/BI heterostructured electrodes was prepared using a facile in-situ hydrothermal technique by Alam and co-workers [101]. The XRD results confirmed the interlayer stacking of the CN units and the tetragonal phase of BI even after the formation of a CNF-Cl/BI heterostructure. The morphology results demonstrated that the BI nanoplates were uniformly covered on the surface of the CNF-Cl electrode. The bandgap tuning was observed for the CNF-Cl/BI heterostructured electrode when compared to pure samples, which significantly enhanced the visible-light absorption ability. From the EIS results, the CNF-Cl/BI50% heterostructured electrode exhibited the lowest R_ct value than the remaining samples. Moreover, the photocurrent density of CNF-Cl/BI50% photoanode was observed around 1.28 mA cm⁻² at 1.23 V vs. RHE under visible-light illumination, and the value was around 4.6 and 3.2 times higher than the CNF-Cl and BI electrodes, respectively. Thus, the formation of synergistic interfaces, improved charge separation, and the enlarged light-harvesting ability significantly enhanced the PEC water-splitting performance.

Wu et al. systematically fabricated a novel Bi₂MoO₆-Ru binary heterostructured material anchored on the surface of g-C₃N₄ (BMO/Ru/CN) using a facile hydrothermal method (Figure 9a) [102]. The XRD results (Figure 9b) demonstrated that all the crystal phases of BMO, Ru, and CN were presented in the BMO/Ru/CN heterostructured electrode without any impurity phases. Further, the peak shift of XPS confirmed the formation of ternary BMO/Ru/CN heterostructured material. The visible-light-harvesting ability (Figure 9c) was significantly enhanced after the BMO/Ru/CN heterostructure formation, and the PL intensity was diminished compared to the BMO, CN, BMO/Ru, Ru/CN, and BMO/CN samples. The morphology results (Figure 9d and e) revealed the uniform distribution of BMO microspheres and Ru NPs on the surface of CN. From the EIS results (Figure 9f), the BMO/Ru/CN heterostructured electrode showed the lowest R_ct value than the BMO, CN, BMO/Ru, Ru/CN, and BMO/CN electrodes. Furthermore, the photocurrent density of ternary BMO/Ru/CN photoelectrode was around 1.0 mA cm⁻² at 1.4 V vs. Ag/AgCl under visible light, and the obtained photocurrent density was approximately 25 times higher than the BMO/CN electrode. The enhanced water splitting performance (Figure 9g) was ascribed to the formation of synergistic coupling among the ternary components, which emphasized the visible-light absorption ability and facile transportation of charge carriers with a lower R_ct value.

Patnaik and colleagues successfully synthesized the various concentrations (2, 4, 6, and 8 wt.%) of Cu-promoted MoO₃/g-C₃N₄ (Cu/MO/CN) hybrid heterostructured electrode using the in-situ pyrolysis followed by the impregnation techniques [103]. The XRD results confirmed that the interlayer stacking of the CN units, the orthorhombic phase of MoO₃, and the heterostructure formation did not influence the individual phase. The morphological investigation inferred that the irregular-shaped MoO₃ nanocrystals and Cu nanoparticles dispersed on the g-C₃N₄ surface. Further, the visible-light absorption ability was improved by forming the heterostructure, and the distribution of Cu NPs was confirmed by the occurrence of a surface plasmon resonance band. From the EIS results, the 2% Cu/MO/CN heterostructured electrode exhibited the lowest charge-transfer resistance value among other samples. Further, the 2% Cu/MO/CN heterostructured electrode showed a photocurrent density of around 5.1 mA cm⁻² using 0.5 M Na₂SO₄ electrolyte under visible-light illumination, which was higher than the other electrodes. Thus, the synergistic interactions having rich-oxygen vacancies could effectively improve the mobility of charge carriers and diminish the reunion of electron/hole pairs, which enhanced the PEC performance. The PEC performance of g-C₃N₄-based various ternary heterostructured electrodes is presented in Table 2.

5. Future Perspectives

The experimental evidence revealed that g-C₃N₄-could be used as a photocatalyst and decent catalyst support material for various metal oxide-based heterostructured electrodes fabrication [29,114]. However, hybridizing the g-C₃N₄ with a second carbon component could further enhance the overall conductivity of the heterostructured electrodes and the durability of g-C₃N₄ catalysts [40,52,115]. Thus far, experimental investigations indicated that g-C₃N₄-carbon hybrid catalysts could improve the overall PEC performance.

• (i). To further improve the photocatalytic activity and water splitting efficiency, innovative strategies are required to synthesize g-C₃N₄-based heterostructured electrodes with various hierarchical and porous morphologies [70,91,116].

• (ii). Most of the reported heterostructured catalysts have been investigated using g-C₃N₄ nanosheet morphologies only. It is worth exploring other morphologies of g-C₃N₄ such as nanorods, nanoparticles, nanotubes, porous nanosheets, etc. [61,64].

• (iii). The simultaneous modification of g-C₃N₄-based heterostructured electrodes through one or more of the following methods: (i) the rational design of nanoarrays architectures, (ii) doping of atoms or molecules, and (iii) loading of various co-catalysts. These strategies can also offer extraordinary improvement of PEC performance due to the synergetic effect of the different catalytic mechanisms [63,117].

• (iv). Moreover, the theoretical and computational screening investigations must be performed to elucidate the charge transfer mechanism, PEC water splitting efficiency, stability, and strategies for improving the overall photocatalytic activity of g-C₃N₄-based catalysts.

• (v). In addition, these computational studies can serve as a guide for experimentalists, especially for selecting the optimal materials and making their hybrid compositions. Therefore, understanding the effect of materials matrices on water splitting performance is needed. It will also help to develop the other requirements of catalysts, such as stability and cost consumption for the real-time application of the PEC water splitting process.

• (vi). Furthermore, electrolytes used in the PEC water splitting have significant influences on the overall PEC performance. Besides, it is also essential to maintain the system at a constant temperature to get accurate PEC measurements. Therefore, it is suggested to utilize water baths or thermostatic systems during the PEC measurements. In addition, a light source should also be equipped with infrared cut-off filters to block the unnecessary heating effect and the evaporation of electrolytes.

6. Conclusions

This review summarized and discussed the recent advances in g-C₃N₄-based binary and ternary heterostructured electrodes for PEC water splitting. The PEC water splitting is the most straightforward and promising way to produce solar fuels, considering the safety factor, environmental friendliness, and the abundance of solar energy. The process is usually carried out at room temperature and the usage of inorganic materials, lending the system robustness, which is difficult to achieve with organic-based photocatalytic materials. The g-C₃N₄-based binary and ternary heterostructured electrodes exhibit excellent water splitting performance and stability due to the synergistic interactions with large surface area and facile transportation of charge carriers. There are certain drawbacks associated with PEC water splitting; however, they can be overcome by combining it with the biological processes that are capable of producing a large quantity of hydrogen. The heterostructured electrodes made of g-C₃N₄ with various binary and ternary semiconductor nanostructured materials are promising photocatalysts in terms of efficient formation of reactive oxygen species and lower recombination of electrons and holes. From the results mentioned above, the ternary g-C₃N₄-based heterostructured electrodes exhibited superior photocurrent densities with lower R_ct value and enlarged visible-light absorption capability when compared to the binary g-C₃N₄-based heterostructured electrodes. This phenomenon could be attributed to the large surface area, synergistic interfaces among the ternary components with a quick charge carrier transportation, thus eventually reducing the reunion of charge carriers. Apart from the experiments discussed here, the g-C₃N₄-based heterostructured electrodes can also be used in various concentrations of different electrolytes with a wide range of operating potentials. Experimental studies have revealed that g-C₃N₄-based heterostructured electrodes could act as efficient photocatalyst support materials for PEC water splitting. However, several bottlenecks still exist that can hinder the use of g-C₃N₄ for the PEC water splitting reactions, which needs further elaborative research on this fascinating material and the other g-C₃N₄-based heterostructured electrode designs.

Author Contributions

Conceptualization and methodology, writing—original draft preparation, R.K.; writing—original draft preparation, S.G.P.; writing—editing, T.G.L.; writing—editing, C.R.M.; writing—editing, D.-Y.L.; resources, J.S.; funding acquisition, writing—review and editing, S.K.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Research Foundation of Korea (NRF) funded by the Korean Government (2020R1A2C1012439; 2021R1F1A1046648 (MSIT)), Republic of Korea.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Two fundamental structural units of g-C3N4. Reprinted with permission from reference [36], Copyright 2018, Royal Society of Chemistry.

Figure 2. Schematic representation of PEC water splitting mechanism.

Figure 3. (a) XRD patterns, (b) HRTEM image of 15%CN@ZO material, (c) optical absorption spectra, (d) Nyquist plots, (e) I-t curves, and (f) charge transfer mechanism of the CN@ZO heterostructured electrode. Reprinted with permission from reference [60], Copyright 2017, Elsevier.

Figure 4. (a) Formation mechanism of CN-TN heterojunction, SEM images of (b) TNTAs, (c) CN-TN heterojunction, (d) TEM image of CN-TN heterojunction, (e) LSV curves, (f) I-t curves, (g) Nyquist plots, and (h) charge transfer mechanism of the CN-TN heterojunction. Reprinted with permission from reference [66], Copyright 2018, Springer.

Figure 5. (a) Formation mechanism of CN@TO heterostructured material, (b) XRD patterns, (c) SEM image of 20-CN@TO material, (d) TEM image of 20-CN@TO material, (e) Nyquist plots, (f) LSV curves, (g) I-t curves, and (h) charge transfer mechanism of the CN@TO heterostructured electrodes. Reprinted with permission from reference [69], Copyright 2020, Elsevier.

Figure 6. (a) Formation mechanism of NNO/CN heterostructured material, (b) XRD patterns, (c) TEM image of NNO/CN heterostructured material, (d) optical absorption spectra, (e) Nyquist plots, (f) I-t curves, and (g) charge transfer mechanism of the NNO/CN heterostructured electrode. Reprinted with permission from reference [31], Copyright 2020, IOP.

Figure 7. (a) Formation mechanism of CN/Au-SQD ternary heterostructured material, (b) TEM image of CN/Au-SQD heterostructured material, (c) optical absorption spectra, (d) PL spectra, (e) LSV curves, and (f) charge transfer mechanism of the CN/Au-SQD ternary heterostructured electrodes. Reprinted with permission from reference [96], Copyright 2020, Elsevier.

Figure 8. (a) XRD patterns, (b) TEM image of TO/CN/CNT heterostructured material, (c) Nyquist plots, (d) LSV curves, and (e) charge transfer mechanism of the TO/CN/CNT ternary heterostructured electrode. Reprinted with permission from reference [98], Copyright 2020, Elsevier.

Figure 9. (a) Formation mechanism of the BO/Ru/CN ternary heterostructured material, (b) XRD patterns, (c) optical absorption spectra, (d) TEM image, (e) HRTEM image of BO/Ru/CN heterostructured material, (f) Nyquist plots, and (g) charge transfer mechanism of the BO/Ru/CN ternary heterostructured electrode. Reprinted with permission from reference [102], Copyright 2019, American Chemical Society.

Table 1

The PEC performance of g-C₃N₄-based binary heterostructured photoelectrodes.

Photoelectrodes	SynthesisMethods	Potential (V)	Electrolytes	Light Sources	Photocurrent Density (mA cm−2)	Ref.
g-C3N4/ZnO	Dip coating	0.5994 vs. RHE	0.5 M Na2SO4	Visible	0.952	[73]
Pt/g-C3N4	Dip coating	0 vs. RHE	0.1 M Na2SO4	Solar	−2.5	[74]
g-C3N4/CoP	Impregnation	0.4 vs. Ag/AgCl	0.05 M Na2SO4	Solar	0.15	[75]
g-C3N4/BiVO4	Hydrothermal	1.23 vs. RHE	1 M KOH	visible	0.021	[76]
g-C3N4/CdS	Electrophoretic and chemical bath deposition	0 vs. Ag/AgCl	0.5 M Na2SO3	Visible	5.4	[77]
g-C3N4/CNTs	Liquid exfoliation	0 vs. SCE	0.5 M Na2SO4	Visible	0.0018	[78]
g-C3N4/SnO2	Pulsing electrophoresis	0.2 vs. Ag/AgCl	0.1 M NaOH	Solar	1.8	[79]
g-C3N4/BiVO4	Sol-gel	-- vs. Ag/AgCl	0.1 M Na2SO4	Solar	0.00046	[80]
g-C3N4/InVO4	Mixing calcination	0.9 vs. Ag/AgCl	0.1 M Na2SO4	Solar	0.013	[81]
g-C3N4/In2S3	Wet chemical	1.5 vs. Ag/AgCl	0.1 M Na2SO3	Visible	0.005	[82]
g-C3N4/Cu2O	Electrophoretic deposition	−0.4 vs. Ag/AgCl	0.1 M NaNO3	Visible	−1.38	[83]
Phosphorus doped g-C3N4	Sintering	1.2 vs. Ag/AgCl	0.5 M Na2SO4	Near-infrared	0.0014	[84]
g-C3N4/BiVO4	Electrodeposition	1.23 vs. RHE	0.5 M Na2SO4	Solar	0.42	[85]
g-C3N4/CuO	Co-precipitation	1.2 vs. Ag/AgCl	--	Visible	0.68	[86]
g-C3N4/MoS2	Solvothermal	−0.4 vs. Ag/AgCl	0.1 M PBS	Solar	0.037	[87]
g-C3N4@CuTi	Hydrothermal	0.65 vs. Ag/AgCl	1 M KOH	Visible	0.014	[88]
g-C3N4/LaFeO3	Magnetron sputtering	−0.3 vs. Ag/AgCl	0.1 M Na2SO4	Solar	0.004	[89]
g-C3N4@Si NWs	Metal-assisted etching, and liquid atomic layer deposition	1 vs. Pt	0.5 M Na2SO4	Visible	0.316	[90]
Cobalt doped g-C3N4	Calcination	Ag/AgCl	0.1 M Na2SO4	Solar	0.058	[91]
g-C3N4@CoFe	Solvothermal	0.2 vs. Ag/AgCl	1 M KOH	Visible	0.196	[92]

Table 2

The PEC performance of g-C₃N₄-based ternary heterostructured photoelectrodes.

Photoelectrodes	SynthesisMethods	Potential (V)	Electrolytes	Light Sources	Photocurrent Density (mA cm−2)	Ref.
ZnO/Au/g-C3N4	Thermal vapor condensation	−0.5 vs. RHE	0.2 M Na2SO4	Solar	−0.29	[104]
g-C3N4/Pt/ZnO	Spin coating	0.5 vs. Ag/AgCl	0.5 M Na2SO4	Solar	0.12	[105]
g-C3N4/Ti-Fe2O3	Electrodeposition	0.23 vs. Ag/AgCl	1.0 M NaOH	Visible	2.55	[106]
g-C3N4/TiN-NFG	Ultra-sonication	0.11 vs. RHE	0.5 M Na2SO4	Solar	−0.196	[107]
TiO2@g-C3N4/Co-Pi	Hydrothermal, chemical bath deposition, electrodeposition	1.23 vs. RHE	0.1 M Na2SO4	Solar	1.6	[108]
Ag@g-C3N4/ZnS	electrophoretic deposition	1.23 vs. RHE	0.3 M KCl	Visible	0.1	[109]
Fluorine-dopedg-C3N4/TiO2	In-situ growth	0.6 vs. Ag/AgCl	0.1 M KOH	Visible	0.18	[110]
Ti3+-TiO2/O-g-C3N4	Hydrothermal	0.1 vs. Ag/AgCl	0.1 M Na2SO4	Visible	0.0034	[111]
TiO2@P-C3N4/Co3O4	Chemical bath deposition	1.23 vs. RHE	0.5 M Na2SO4	Visible	1.58	[112]
NiFe/N-rGO/g-C3N4	Calcinations, electrostatic self-assembly, and hydrothermal	0.8 vs. Ag/AgCl	0.1 M Na2SO4	Visible	−0.97	[113]

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