Fossil energy shortage and environmental pollution are two global challenges that have risen great concerns. Photocatalysis with solar light can address these concerns. Photocatalysis by semiconductors can produce fuels such as H2 by photoelectrochemical (PEC) water splitting and can degrade environmental pollutants . Among the semiconductors that have been used for these purposes, graphitic carbon nitride (g-C3N4) has received great attention in recent years.
The g-C3N4 is a non-toxic, low-cost, and environment-friendly material that has been used as a photocatalyst in recent years due to its visible light absorption, tunable bandgap, and easy synthesis methods. The g-C3N4 can be synthesized through thermal condensation of nitrogen-rich precursors, such as melamine, cyanamide, dicyandiamide, urea, and thiourea. The bandgap of g-C3N4 is≃2.7eV, the valence band (VB) maximum is at ≃1.6eV and the conduction band (CB) minimum is at ≃1.1eV, vs. normal hydrogen electrode (NHE). This band structure makes the g-C3N4 capable of absorbing visible light .
To date, several modifications have been made to increase the photocatalytic activity of g-C3N4. Increasing the number of active facets has been focused on by changing the morphology and increasing the surface area. By controlling the growth process, nanoparticles of g-C3N4 with a large surface area can be obtained. Doping with elements such as N and B has also been used to tune the electric properties of g-C3N4 and increase its photocatalytic performance. Among several modification routes, constructing heterojunctions with other noble metals and semiconductors is advantageous since can result in enhancing the charge separation and light-harvesting from a larger part of the spectrum. The band positions of the heterojunction components should be able to accumulate the space charge at the interfaces of two components to facilitate the charge carrier separation .
Titanium dioxide (TiO2) has dominated the research field in photocatalysis over the few past decades and has found several applications in solar cells , hydrogen production from PEC water splitting , environmental pollution mitigation, etc [6,7]. However, the high recombination rate and inherent wide band gap have limited the wide-scale practical application of this material. Therefore, numerous research has been devoted to strategies for charge separation and band gap engineering of TiO2 to increase its photocatalytic efficiency and increase light-harvesting . Simultaneously, efforts have been devoted to searching for alternative materials to harvest sunlight and perform photocatalytic activity. The g-C3N4 is one of the widely applied alternatives. Intensive work on carbon nitride has arisen since the prediction made by Liu and Cohen in 1989 that covalent solids formed between nitrogen and carbon are superior candidates for reaching ultra-hard materials . Several types of C3N4 exist including cubic, pseudocubic, α-, β-, and graphitic which the latter is the most stable form at ambient conditions .
Graphitic carbon nitride (g-C3N4) has a similar skeleton to graphite with C–N covalent bonds instead of C–C bonds in the graphite structure. The layers in g-C3N4 are linked to each other by weak van der Waals forces . The graphitic phase allotrope of carbon nitride is the most stable in the ambient environment. The basic building blocks of g-C3N4 have been proposed to be triazine units (C3N3). Tri-s-triazine (heptazine, C6N7) rings which compared to triazine, are more energetically favored building blocks of g-C3N4 is the other structural model. Fig.1 shows the triazine or tri-s-triazine rings forming g-C3N4 by trigonal nitrogen atoms cross-linking the building blocks .
The large band gap and charge recombination are the bottlenecks in the development of semiconductors for solar energy conversion with high efficiency. Thus, the search for a semiconductor photocatalyst with a suitable band gap resulted in the development of g-C3N4 as a polymeric semiconductor that provoked a new wave of excitement in the research in this field . The g-C3N4 is promising for future solar-based fuel production and environmental pollution degradation. The g-C3N4 can also be used in heterojunctions to construct g–C3N4–based photocatalysts in Fenton-like catalysis for the degradation of pollutants . Furthermore, compared with the bulk phase, the g-C3N4 with other dimensionalities may have a different band gap and also are easier for some modifications such as interface engineering and elemental doping .
This review aims to give a brief discussion on the mechanism of photocatalysis and clarify why g-C3N4 is preferred to be used in heterostructure constructions with other suitable semiconductors. Different types of heterostructures that are made with g-C3N4 are introduced and recent advances have been discussed. The g-C3N4 can be synthesized in the form of conventional 3D, and also 2D, 1D, and 0D structures. Therefore, the review is followed by a discussion of the synthesis methods of g-C3N4 with different dimensionalities. In recent years, some other modifications on g-C3N4 have been applied to increase their photocatalytic efficiency. These approaches such as elemental doping, surface engineering of g-C3N4, application of co-catalyst, and loading a single atom in the basal plane have been addressed. Finally, the challenges and prospects of the g-C3N4 heterostructures are explained.
Mechanism of heterogeneous photo-catalysis
The mechanism of heterogeneous photo-catalysis is represented in Fig.2. The heterogeneous photo-catalysis includes seven stages or four major steps: 1- light harvesting; 2- excitation; 3- charge separation and transfer (also known as stages 3, 4, and 5); electrocatalytic reactions (also known as stages 6 and 7) [17,18]. The first stage is strongly dependent on the morphology and surface structure of the photocatalyst [18,19]. Light harvesting can be enhanced for mesoporous  and
When a semiconductor is in its ground state, has outstanding chemical stability. However, when the semiconductor is irradiated by light, the electrons in the VB will be excited to CB, if the photon energy is not less than the band gap energy of the semiconductor. Simultaneously, a hole is produced in the VB of the semiconductor. If an electric field exists, the electron-hole pairs can travel to the surface of the semiconductor resulting in highly active charge carriers on the surface.
Fig.3 shows different types of g–C3N4–based heterojunctions. In type II heterojunctions based on g-C3N4 photocatalyst, all photocatalyst is illuminated by light radiation, and the CB of photocatalyst II (PCII) receives the electrons from the CB of photocatalyst I (PCI) due to the potential difference which exists in this type of heterojunctions. Simultaneously, the holes in these two semiconductors may move in opposite directions to electrons. Therefore, the electrons are assembled in the CB
Conventional type II heterojunction systems
The main problem in photocatalysts is the recombination of the photogenerated electron-hole pairs. The photogenerated electrons in the CB tend to return to VB with subsequent recombination which is unfavored for a photocatalyst . However, when a semiconductor with a suitable band structure lays in intimate contact with the g-C3N4, the charge transfer across the interface of g-C3N4/semiconductor will promote the electron-hole pair separation. g-C3N4 can be used as a photocatalyst due to its
The term ‘Z Scheme’ is a name that is adopted from the natural photosynthesis process. The traditional Z scheme was initially explained by Bard  in which, a liquid phase Z scheme was designed with two semiconductors attached with a shuttle redox mediator (electron acceptor/donor pair). The shuttle redox mediator facilitates electron transportation from the CB of photocatalyst I to the VB of photocatalyst II (Fig.7a–c). The limitation of the traditional Z scheme photocatalysts is the
The photocatalysts can be classified as oxidation photocatalysts (OP) and reduction photocatalysts (RP), depending on their band structures. Ops are mainly used in environmental degradation. In OPs, photogenerated electrons are fruitless, while photogenerated holes are the contributive factors (Fig.8a). In comparison, RPs, which have the characteristic of a high CB, are mainly used for the production of solar fuels. In RPs, photogenerated holes are useless and should be removed by sacrificial
Synthesis of g-C3N4
Reactive oxygen-free and nitrogen-rich compounds containing pre-bonded C–N core structures are the common precursors for the synthesis of g-C3N4. Such materials (e.g. heptazine and triazine derivatives) are unstable and/or highly explosive. The synthesis of g-C3N4 with defects is preferred when it is going to be used in catalysis. For the production of defective polymeric species, the condensation pathways from cyanamide to dicyandiamide, melamine, and all other C/N materials can be applied.
A brief comparison between different g-C3N4 heterojunctions
The most simple types of heterojunctions are type I, type II, and type III with a straddling gap, staggered, and broken gap, respectively. In the type I heterojunctions (Fig.11a), the CB and VB of one semiconductor (A) are higher and lower than the other semiconductor (B) bands, respectively. Consequently, when the heterojunction is irradiated, the holes and electrons will be accumulated at the VB and CB of the semiconductor B, respectively. Therefore, the charge carriers accumulation happens
Challenges and future prospective
The g-C3N4 is one of the emerging photocatalysts that has several advantages over traditional TiO2 photocatalysts including visible light response and easy synthesis. However, the high rate of charge carrier recombination in g-C3N4 has hindered its wide application in its pristine form. Several strategies have been applied to overcome the g-C3N4 shortages, including the application of a co-catalyst, surface engineering, crystalline manipulation, elemental doping, and construction of a
Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.
Declaration of competing 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.
The author would like to appreciate the Shahrood University of Technology and the Iranian Nanotechnology Initiative Council for their financial support of this project.
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