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

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.


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 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].

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.  The g-C 3 N 4 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-C 3 N 4 -based semiconductor heterostructures with appropriate photocatalytic properties could be an effective approach for enhancing the PEC water splitting efficiency [40,41].

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 H 2 and O 2 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 sta- Processes 2021, 9, 1959 4 of 26 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-C 3 N 4 , 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-C 3 N 4 -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-C 3 N 4 -based binary and ternary heterostructured photoelectrodes for the PEC water splitting reaction are still lacking. Therefore, this review summarizes the different types of g-C 3 N 4 -based binary and ternary heterostructured photoelectrodes used for PEC water splitting reaction, along with their advantages and limitations.

Importance of g-C 3 N 4 -Based Hybrid Heterostructured Electrodes
Generally, when the pristine g-C 3 N 4 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-C 3 N 4 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-C 3 N 4 materials. For instance, the construction of heterostructured electrodes with g-C 3 N 4 and other semiconductors, doping with metals and non-metals, and the fabrication of mesoporous heterostructures [52][53][54]. Among them, the construction of g-C 3 N 4 -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-C 3 N 4 -based heterostructured electrodes, g-C 3 N 4 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].

g-C 3 N 4 -Based Binary Heterostructured Photoelectrodes for PEC Water Splitting
The pristine g-C 3 N 4 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-C 3 N 4 -based binary heterostructures have been constructed, and some of them are reported here. Binary heterostructures of g-C 3 N 4 , 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-C 3 N 4 , 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-C 3 N 4 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-C 3 N 4 heterostructured electrode exhibited the lowest charge transfer resistance (R ct ) compared to the remaining samples. Further, the 10% MnO x /g-C 3 N 4 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-C 3 N 4 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-C 3 N 4 @ZnO (CN/ZN) heterostructured electrode with different amounts of g-C 3 N 4 (0.01, 0.05, 0.10, 0.15, 0.20 g) using a facile reflux technique [60]. The XRD results (Figure 3a (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 zerodimensional (0D) g-C 3 N 4 NPs covered over the one-dimensional (1D) TiO 2 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 ZnWO 4 nanorods decorated over the 2D g-C 3 N 4 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 ZnWO 4 nanorods on the surface of g-C 3 N 4 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 ZnWO 4 and g-C 3 N 4 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 A novel hybrid carbon-doped CuO/g-C 3 N 4 (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 SnO 2 NPs (various loading of 0.175, 0.6, and 1 g) decorated over the surface of porous g-C 3 N 4 nanosheets (CN/SN) using a facile microwave synthesis method [63]. The XRD results demonstrated that the CN/SN heterostructured material exhibited both the g-C 3 N 4 and SnO 2 phases. The morphology results showed the highly porous structure of CN nanosheets and the irregular shape of SnO 2 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 m 2 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 Na 2 SO 4 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-C 3 N 4 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 × 10 5 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-Fe 2 O 3 nanotubes combined with various concentrations (1, 2, and 3.75 wt.%) of ruptured tubular g-C 3 N 4 (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-Fe 2 O 3 nanotubes were anchored over the tubular g-C 3 N 4 , 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 Na 2 SO 4 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-C 3 N 4 (BI/CN-M) and few-layered g-C 3 N 4 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 Na 2 SO 4 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 TiO 2 nanotube arrays (TNTAs) and dispersed the g-C 3 N 4 nanoparticles over TNTAs using anodic oxidation followed by calci-Processes 2021, 9,1959 8 of 26 nation methods [66]. The formation mechanism of g-C 3 N 4 /TNTAs (CN-TN) heterojunction was schematically represented in Figure 4a. The crystallographic results showed the anatase phase of TiO 2 and interlayer stacking of g-C 3 N 4 , which significantly confirmed the formation of heterojunctions. The morphology results from Figure    Liu and co-workers effectively fabricated the g-C 3 N 4 -coated Fe 2 O 3 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-C 3 N 4 nanoparticles over the surface of Fe 2 O 3 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-C 3 N 4 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 Na 2 SO 4 electrolyte under visiblelight 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 (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 Bi 2 WO 6 quantum dots (QDs) dispersed on the g-C 3 N 4 (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-C 3 N 4 -based various binary heterostructured electrodes is presented in Table 1.
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.

g-C 3 N 4 -Based Ternary Heterostructured Photoelectrodes for PEC Water Splitting
Numerous efforts have been executed to fabricate g-C 3 N 4 -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-C 3 N 4 -based complex compounds containing three components have been introduced, and some of the results are discussed here. A novel hybrid ternary anatase TiO 2 /rutile TiO 2 /g-C 3 N 4 (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 TiO 2 and the interlayer stacking of the CN units. The electron microscopy results validated that the TiO 2 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 Fe 3 O 4 nanocubes dispersed over the g-C 3 N 4 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 Bhandary and colleagues fabricated a bimetallic AgNi alloy surrounded by g-C 3 N 4 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-C 3 N 4 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 Ag 3 PO 4 /Ag 2 MoO 4 materials combined with various concentrations (0.4, 0.8, and 1.2 wt.%) of multilayered g-C 3 N 4 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 Ag 3 PO 4 and Ag 2 MoO 4 , 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-C 3 N 4 /Au-SnO 2 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 SnO 2 , 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 Na 2 SO 3 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 nanorods that were distributed over the surface of g-C3N4, which formed the CN/B ternary heterostructured electrode using the facile hydrothermal method followed b cination process [97]. The XRD results demonstrated that the ternary CN/BO/BP h structured electrode exhibited g-C3N4, Bi2O3, and BiPO4 phases, confirming the pre of individual components. The morphology results showed that the Bi2O3 NPs we chored over BiPO4 nanorods, and the binary BO/BP heterostructured material was d uted over the surface of the g-C3N4 nanosheets. When compared to pure samples, the absorption efficiency of the CN/BO/BP heterostructured electrode enhanced toward visible region.  Li and co-workers systematically fabricated 0D Bi 2 O 3 NPs encapsulated with BiPO 4 nanorods that were distributed over the surface of g-C 3 N 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-C 3 N 4 , Bi 2 O 3 , and BiPO 4 phases, confirming the presence of individual components. The morphology results showed that the Bi 2 O 3 NPs were anchored over BiPO 4 nanorods, and the binary BO/BP heterostructured material was distributed over the surface of the g-C 3 N 4 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 Na 2 SO 4 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 TiO 2 NPs with carbon nanotubes that dispersed over the g-C 3 N 4 nanosheets (TO/CN/CNT) using a facile hydrothermal method [98]. The XRD results (Figure 8a) confirmed the anatase phase of TiO 2 , the graphitic nature of CNT, and the interlayer stacking of CN. The morphology results (Figure 8b) displayed that the TiO 2 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 Na 2 SO 4 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-C 3 N 4 /NiFelayered 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 m 2 g −1 than the pure CN (126.32 m 2 g −1 ), GDY (95.03 m 2 g −1 ), and NF (42.63 m 2 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  pared 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. To enhance the utility of visible-light and diminish the reunion of charge carriers, a novel fluorine-doped, chlorine-intercalated g-C 3 N 4 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 watersplitting performance.
Wu et al. systematically fabricated a novel Bi 2 MoO 6 -Ru binary heterostructured material anchored on the surface of g-C 3 N 4 (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 MoO 3 /g-C 3 N 4 (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 MoO 3 , and the heterostructure formation did not influence the individual phase. The morphological investigation inferred that the irregular-shaped MoO 3 nanocrystals and Cu nanoparticles dispersed on the g-C 3 N 4 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 Na 2 SO 4 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-C 3 N 4 -based various ternary heterostructured electrodes is presented in Table 2. 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    [112] NiFe/N-rGO/g-C 3 N 4 Calcinations, electrostatic self-assembly, and hydrothermal 0.8 vs. Ag/AgCl 0.1 M Na 2 SO 4 Visible −0.97 [113]

Future Perspectives
The experimental evidence revealed that g-C 3 N 4 -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-C 3 N 4 with a second carbon component could further enhance the overall conductivity of the heterostructured electrodes and the durability of g-C 3 N 4 catalysts [40,52,115]. Thus far, experimental investigations indicated that g-C 3 N 4 -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-C 3 N 4 -based heterostructured electrodes with various hierarchical and porous morphologies [70,91,116]. (ii) Most of the reported heterostructured catalysts have been investigated using g-C 3 N 4 nanosheet morphologies only. It is worth exploring other morphologies of g-C 3 N 4 such as nanorods, nanoparticles, nanotubes, porous nanosheets, etc. [61,64]. (iii) The simultaneous modification of g-C 3 N 4 -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-C 3 N 4based 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.

Conclusions
This review summarized and discussed the recent advances in g-C 3 N 4 -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-C 3 N 4 -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-C 3 N 4 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-C 3 N 4 -based heterostructured electrodes exhibited superior photocurrent densities with lower R ct value and enlarged visible-light absorption capability when compared to the binary g-C 3 N 4 -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-C 3 N 4 -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-C 3 N 4 -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-C 3 N 4 for the PEC water splitting reactions, which needs further elaborative research on this fascinating material and the other g-C 3 N 4 -based heterostructured electrode designs.

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.