Graphitic carbon nitride based nanocomposites: a review

Abstract

Graphitic carbon nitride (g-C₃N₄), as an intriguing earth-abundant visible light photocatalyst, possesses a unique two-dimensional structure, excellent chemical stability and tunable electronic structure. Pure g-C₃N₄ suffers from rapid recombination of photo-generated electron–hole pairs resulting in low photocatalytic activity. Because of the unique electronic structure, the g-C₃N₄ could act as an eminent candidate for coupling with various functional materials to enhance the performance. According to the discrepancies in the photocatalytic mechanism and process, six primary systems of g-C₃N₄-based nanocomposites can be classified and summarized: namely, the g-C₃N₄ based metal-free heterojunction, the g-C₃N₄/single metal oxide (metal sulfide) heterojunction, g-C₃N₄/composite oxide, the g-C₃N₄/halide heterojunction, g-C₃N₄/noble metal heterostructures, and the g-C₃N₄ based complex system. Apart from the depiction of the fabrication methods, heterojunction structure and multifunctional application of the g-C₃N₄-based nanocomposites, we emphasize and elaborate on the underlying mechanisms in the photocatalytic activity enhancement of g-C₃N₄-based nanocomposites. The unique functions of the p–n junction (semiconductor/semiconductor heterostructures), the Schottky junction (metal/semiconductor heterostructures), the surface plasmon resonance (SPR) effect, photosensitization, superconductivity, etc. are utilized in the photocatalytic processes. Furthermore, the enhanced performance of g-C₃N₄-based nanocomposites has been widely employed in environmental and energetic applications such as photocatalytic degradation of pollutants, photocatalytic hydrogen generation, carbon dioxide reduction, disinfection, and supercapacitors. This critical review ends with a summary and some perspectives on the challenges and new directions in exploring g-C₃N₄-based advanced nanomaterials.

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Zaiwang Zhao

Zaiwang Zhao obtained his BS degree from Chongqing Technology and Business University (CTBU) in 2012. He is currently a graduate student under the supervision of Prof. Fan Dong at CTBU. His research interests are focused on the design and synthesis of photocatalytic materials for energy and environmental applications.

Yanjuan Sun

Yanjuan Sun received her BS in Environmental Engineering and MS in Chemical Technology from Hubei University of Technology. In 2011, she became an Assistant Professor at Chongqing Technology and Business University. Her research interests include photocatalytic materials for environmental and energetic applications, and separation technology.

Fan Dong

Fan Dong received his PhD in Environmental Engineering in 2010 from Zhejiang University. Currently, he is a Professor at Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of Environmental and Biological Engineering, Chongqing Technology and Business University. He was a visiting scholar from 2009 to 2010 at Hong Kong Polytechnic University. His research interests include semiconductor and plasmonic photocatalysis, CO2 capture and utilization, and air pollution control. He has coauthored more than 50 papers on the topics of his research and has an H index of 20.

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1. Introduction

Over the past few decades, the growing awareness of environmental protection and energy conservation has stimulated intensive research on solar energy utilization.¹ In the domain of pollutant elimination and solar energy conversion, semiconductor photocatalysis has emerged as one of the most fascinating technologies.^2–5 Nevertheless, the wide band gap and the low solar-energy utilization efficiency remain the “bottleneck” of the photocatalysts to satisfy the requirements of applications in a practical way. For instance, the traditional TiO₂ is limited owing to its poor performances associated with visible light application. As a result, it is necessary to seek efficient visible-light-driven (VLD) photocatalysts. For this objective, various modified TiO₂ and TiO₂-alternative photocatalysts have been fabricated.^6–17 Currently, it is still a challenge to design new photocatalysts that are abundant, stable and facile in fabrication besides showing high visible-light performance.

In the search for robust and stable VLD semiconductor photocatalysts, a polymeric semiconductor, graphitic carbon nitride (g-C₃N₄), has recently attracted tremendous attention. The heptazine ring structure and the high condensation degree enable metal-free g-C₃N₄ to possess many advantages such as good physicochemical stability, as well as an appealing electronic structure combined with a medium band gap (2.7 eV).¹⁸ These unique properties make g-C₃N₄ a promising candidate for visible light photocatalytic applications utilizing solar energy. In addition, g-C₃N₄ is abundant and easily-prepared via one-step polymerization of cheap feedstocks like cyanamide,^18,19 urea,^20–22 thiourea,^23,24 melamine^25–27 and dicyandiamide.^28,29 Nevertheless, pure g-C₃N₄ suffers from shortcomings such as rapid recombination of photo-generated electron–hole pairs, a small specific surface area and a low visible light utilization efficiency.^21–29 Consequently, the exploration of facile and dependable strategies to synthesize the modified g-C₃N₄-based photocatalysts with improved physicochemical properties and high photocatalytic activities is of increasing requirement. The g-C₃N₄ has a unique two-dimensional layered structure, which is favorable for hybridizing with other components. Very recently, several approaches have been employed to enhance the visible light photocatalytic performance of g-C₃N₄, for example, formation of surface coupling hybridization utilizing TaON,³⁰ Bi₂WO₆,³¹ or graphene,³² construction of mesoporous structures,³³ doping with metal or nonmetal species Fe,³⁴ Ag,³⁵ Au,³⁶ Pd,³⁷ S,³⁸ B³⁹ and P⁴⁰ and sensitizing with organic dyes.⁴¹ Among these approaches, formation of heterostructures demonstrates a great potential to promote the photocatalytic performance of g-C₃N₄ because the electron–hole pairs can be efficiently separated, and charge carriers could transfer across the interface of the heterostructure to restrain the recombination.

In a coupling process, g-C₃N₄ based heterostructures not only can be formed by combining with visible light excited photocatalytic semiconductor materials with a narrow band gap (such as CdS,⁴² Bi₂WO₃,⁴³ and BiOI⁴⁴), but also can combine with UV excited photocatalysts with large band gaps (such as TiO₂,⁴⁵ ZnO,⁴⁶ and ZnWO₄⁴⁷), which can largely broaden the application of the g-C₃N₄ based nanocomposites. Nevertheless, not all materials can couple with g-C₃N₄ to form a heterostructure. The most important prerequisite condition to form an effective visible light excited g-C₃N₄ based heterostructure is that the candidates should have an appropriate band structure which is beneficial to create a coupling hybridization. Besides, the difference in chemical potential between the coupling semiconductors A and B generates band bending at the interface of junctions. The band bending induces a built-in field, which impels the photogenerated electrons and holes to transfer in opposite directions, resulting in a spatially efficient separation of the electrons and holes pairs on different sides of the heterojunction.^48,49 In addition, the crystal structure in the junction domain of the heterostructure is also important in strengthening the quantum efficiency of the photocatalyst. A distinction in lattice spacing between two semiconductors could probably cause lattice mismatch. The lattice mismatch at the interface may cause defects, which will capture the photo-generated electronic carriers and thus inhibit the diffusion of electrons and holes. Thus, the formation of g-C₃N₄ based heterostructures is an effective approach to enhance charge separation efficiency for an improved photocatalytic performance.

Recently, numerous research studies have been carried out to couple g-C₃N₄ with various semiconductors to enhance the photocatalytic activities.^36,50,51 For instance, Wang and co-workers firstly reported TiN/g-C₃N₄ multi-layer hybridization using a dual-facing-target magnetron sputtering method at room temperature and exhibited enhanced properties in photocatalytic applications in 2008.⁵⁰ This pioneering work has stimulated tremendous interest in the fabrication, modification, and application of g-C₃N₄-based semiconductor photocatalysts. Yan et al. successfully developed a g-C₃N₄/TaON organic–inorganic composite photocatalyst with visible-light response by a milling-heat treatment method that demonstrated an enhanced photocatalytic performance for photodegradation of rhodamine B (RhB) in aqueous solutions.⁵¹ Di et al. prepared the Au/g-C₃N₄ nanocomposite photocatalyst by depositing gold nanoparticles on the surface of a g-C₃N₄ semiconductor to generate metal–semiconductor junctions, which showed well-improved photocatalytic hydrogen evolution with visible light.³⁶

Now that significant advances have been made on g-C₃N₄-based photocatalysts in recent years, we believe that a comprehensive review on this subject is necessary to accelerate further developments in this exciting research domain. This review article is focused on recent progress in the design, fabrication, mechanistic understanding, and potential applications of these g-C₃N₄-based nanocomposites in various realms such as photodegradation of nitrogen oxides, organic contaminant photodegradation, photocatalytic hydrogen evolution, conversion of carbon dioxide to methane fuel, oxygen reduction reaction (ORR), and photoelectrochemical determination of Cu²⁺. Eventually, some concluding remarks and invigorating perspectives on the current situation and further prospects of the g-C₃N₄-related research studies are presented, which may promote the understanding and large scale application of g-C₃N₄-based nanocomposites.

2. g-C₃N₄-based nanocomposites

2.1. Preparation of g-C₃N₄

Generally, carbon nitride materials fabricated by direct condensation of nitrogen-containing organic precursors (for example, urea, thiourea, melamine, dicyandiamide, cyanamide, and guanidine hydrochloride) are bulk materials with a small surface area, normally below 10 m² g⁻¹. For practical applications of the materials in domains such as a single catalyst or a support substrate of co-catalysts (such as heterojunctions), the introduction of well-controlled porosity at the nanoscale in the bulk carbon nitride is mandatory to strengthen its utilization to a large scale. It is worth noting that formation of mesoporous structure and augmentation of specific surface area are crucial to fine-tune the physicochemical properties and improve the photocatalytic performance.⁵² As previously reported, the mesoporous g-C₃N₄ (mpg-C₃N₄) was first obtained by nanocasting/replication of mesoporous silica matrices, which were famous for the generation of the corresponding carbon nanostructures.^53–55 Inspired by this hard template method, tremendous attempts were made to explore new strategies for g-C₃N₄ modification, such as the soft template method,^56–58 the ultrasonic dispersion method,⁵⁹ acidic solution impregnation^60,61 and chemical functionalization.^62–66 As universally applied, the methods mentioned above were good as a proof-of-principle in tailoring the texture and surface chemical properties, as well as the electronic properties.

2.2. Design considerations for g-C₃N₄-based nanocomposites

To develop effective g-C₃N₄-based nanocomposites working for enhanced performance, several pivotal requirements must be considered. First of all, the semiconductor light harvesting antenna must have a narrow band gap to allow for efficient absorption of the solar spectrum. Second, there must be a driving force boosting charge separation and accelerating the transportation process. Furthermore, the semiconductor should possess adequate redox potential for the desired photochemical reactions. Ultimately, a mechanism should be accompanied to guarantee the photochemical stability of the photocatalyst. It is unlikely that a single material system could satisfy all these requirements, while the nanocomposite photocatalysts may have the potential to accomplish these goals. In general, nanocomposite photocatalysts could offer several potential merits: (1) cocatalyst effect: the integration with a proper cocatalyst can lower the redox overpotential at the respective active sites; (2) strengthened light absorption: for instance, semiconductors with small band gaps possessing a high absorption coefficiency can be utilized to functionalize (or sensitize) semiconductor materials with large band gaps; (3) efficient charge separation and transportation: a p–n junction (semiconductor/semiconductor heterostructures) or the Schottky junction (metal/semiconductor heterostructures) with built-in electrical potential can be constructed in heterogeneous photocatalysts to effectively expedite electron–hole pair separation and transportation; (4) decent stability: if the active sites and functional groups on the semiconductor surface can be protected through appropriate surface passivation, it is possible to improve the stability.

In consideration of the diverse mechanisms of different g-C₃N₄ based nanocomposites in photocatalytic reactions, g-C₃N₄ based heterojunctions were summarized and classified into six different main combination systems. Below, we review the recent progress on the g-C₃N₄ based metal-free heterojunction, the g-C₃N₄/metal oxide (metal sulfide) heterojunction, g-C₃N₄/inorganic acid salt composites, the g-C₃N₄/halide heterojunction, g-C₃N₄/noble metal heterostructures, and the g-C₃N₄ based multi-component heterojunction for improved photocatalysis.

3. Categories of g-C₃N₄ based nanocomposites

3.1. g-C₃N₄ based metal-free layered heterojunctions

To our knowledge, metal-free materials, owing to their various advantages such as low cost and environmentally amicable, have attracted multitudinous attention because of their great potential in solving environmental and energy problems. Hence, it is extremely attractive to construct hybridized materials via coupling two metal-free materials. For example, graphene, a two-dimensional macromolecular sheet of carbon atoms, has preferentially attracted much attention due to its outstanding mechanical, thermal, easily-obtained, and electrical properties. It has been widely used in nanoelectronics, biosensing, capacitors, and catalysis domains.^67,68 In particular, it possesses an extremely high specific surface area (∼2600 m² g⁻¹)⁶⁹ and marvelous thermal conductivity (∼5000 W m⁻¹ K⁻¹), guaranteeing a superb mobility of charge carriers (200 000 cm² V⁻¹ s⁻¹). Therefore, owing to the merits mentioned above, graphene is the first metal-free candidate, and is preferentially used to construct layered graphene/g-C₃N₄ nanocomposites. Sun et al. prepared graphene/g-C₃N₄ nanoheterojunctions by the absorption–calculation method, which demonstrated apparently enhanced conductivity and electrocatalytic performance on oxygen reduction reaction (ORR).⁷⁰ Xiang et al. also successfully obtained graphene/g-C₃N₄ nanocomposites by a combined impregnation–chemical reduction strategy, and firstly utilized this graphene/g-C₃N₄ nanoheterojunction in visible-light photocatalytic H₂-production containing Pt as a cocatalyst.³²Fig. 1a and 1b show a typical TEM image of graphene oxide and graphene/g-C₃N₄ nanocomposites. In comparison with graphene oxide, the graphene/g-C₃N₄ composite possessed a more compact structure due to the fact that g-C₃N₄ was sandwiched between graphene sheets through polymerization of melamine molecules pre-adsorbed on the GO sheets. In the aforementioned trials, high H₂-production activity over the graphene/g-C₃N₄ nanocomposites under visible-light irradiation can be achieved. The mechanism of the activity enhancement is illustrated in Fig. 1c.

Fig. 1 (a) TEM images of graphene oxide and (b) the graphene/g-C₃N₄ nanocomposites; (c) proposed mechanism for the enhanced electron transfer in the graphene/g-C₃N₄ composites. (Reprinted with permission from ref. 32. Copyright (2011) American Chemical Society.)

Under visible-light illumination, electrons (e⁻) of g-C₃N₄ are excited from the valence band (VB) to the conduction band (CB), generating holes (h⁺) in the VB. Generally, these charge carriers quickly recombine and only a section of electrons are injected into the Pt nanoparticles due to a Schottky barrier.^71,72 The injected electrons accumulate on the Pt nanoparticles and can effectively reduce H₂O (or H⁺) to produce H₂, while holes accumulate at the valence band of g-C₃N₄ and can react with methanol as a sacrificial reagent. However, when g-C₃N₄ is immobilized on the surface of graphene sheets to form the layered composites, these photogenerated electrons on the CB of g-C₃N₄ could tend to transfer to graphene sheets due to their excellent electronic conductivity, restraining the electron–hole pair recombination.⁶⁸ The transferred electrons will accumulate on the Pt nanoparticles loaded on the graphene sheets via a percolation mechanism⁷³ and then participate in H₂ generation. The major reaction steps in this photocatalytic water-splitting mechanism under visible-light illumination are summarized in eqn (1)–(4).

graphene (e⁻) + Pt→ graphene + Pt (e⁻) (2)

Pt (e⁻) + 2H⁺ → Pt + H₂ (3)

g-C₃N₄ (h⁺) + CH₃OH + 6OH⁻ → g-C₃N₄ + CO₂ + 5H₂O (4)

Cross-linked g-C₃N₄/rGO (reduced graphene oxide) nanocomposites with tunable band structure were synthesized by Li et al. which demonstrated well-enhanced visible light photocatalytic activity.⁷⁴ Later, Wang et al. noticed that a new class of ternary g-C₃N₄/graphene/S nanoheterojunctions was prepared by wrapping reduced graphene oxide and g-C₃N₄ sheets on crystals of cyclooctasulfur (α-S8) in bacterial inactivation under visible light.⁷⁵ Except for the aforementioned achievement, a large number of attempts have been made to explore novel materials coupling with g-C₃N₄ for broadening the applications of g-C₃N₄-based coupling heterojunctions. In summary, these excellent g-C₃N₄ based non-metal nanoheterojunctions are MWNTs (multi-walled carbon nanotubes)/g-C₃N₄,^76,77 polypyrrole/C₃N₄,⁷⁸ P/C₃N₄,⁷⁹ CN/CNS,⁸⁰ g-C₃N₄(from thiourea)/g-C₃N₄(from urea),⁸¹ and C₆₀/g-C₃N₄, as shown in Table 1.⁸²

Table 1 Photocatalytic properties of g-C₃N₄ based metal-free heterojunction composite photocatalysts

Composite photocatalyst	Mass or molar fraction of g-C3N4	Typical parameters of photocatalytic experiments	Photocatalytic activity	Reference photocatalyst; photocatalytic activity	Enhancement factor over the reference photocatalyst	Reference
Graphene/g-C3N4	Mass: 99%	Photocatalytic H2 evolution under visible light; co-catalyst: Pt; sacrificial reagent: methanol	RH2: 451 μmol h^−1 g^−1	g-C3N4: 147 μmol h^−1 g^−1	3.07	32
				Graphene: no data	1.21
Reduced graphene oxide (rGO)/g-C3N4	Mass: 98.4%	Decomposing rhodamine B (RhB) under visible light	84% in 75 min	g-C3N4: 80%	1.05	74
	97.5%		100% in 75 min	rGO: no data	1.25
	94.9%		52% in 75 min		0.65
	80.4%		36% in 75 min		0.45
	Mass: 98.4%	Decomposing 4-nitrophenol under visible light	No data in 150 min	g-C3N4: no data	No data
	97.5%		No data in 150 min	rGO: no data	No data
	94.9%		No data in 150 min
	80.4%		No data in 150 min
Multi-walled carbon nanotubes (MWNTs)/g-C3N4	Mass: 98%	Photocatalytic H2 evolution under visible light; co-catalyst: Pt; sacrificial reagent: methanol	RH2: 7.58 μmol h^−1	g-C3N4: 2.03 μmol h^−1	3.7	76
				MWNTs: no data	No data
Multi-walled carbon nanotubes (CNT)/white C3N4	Mass: no data	Decomposing methyl orange (MO) under visible light	89.7%, in 3 h	White C3N4: no data	No data	77
				CNT: no data	No data
	Mass: no data	Decomposing (RhB) under visible light	85.4%, in 3 h	White C3N4: no data	No data
				CNT: no data	No data
Polypyrrole (PPy)/g-C3N4	Mass: 98.5%	Photocatalytic H2 evolution under visible light; co-catalyst: Pt; sacrificial reagent: no mention	RH2: 15.4 μmol h^−1	g-C3N4: no data	No data	78
	99.5%		No data	PPy: no data	No data
	97.5%		No data
	96%		No data
Red phosphor (r-P)/g-C3N4	Mass: 5%	Photocatalytic H2 evolution under visible light; co-catalyst: Pt; sacrificial reagent: ascorbic acid	RH2: 310 μmol h^−1 g^−1	g-C3N4: 340 μmol h^−1 g^−1	0.91; 14.09	79
	30%		1000 μmol h^−1 g^−1	r-P: 22 μmol h^−1 g^−1	2.94; 45.45
Red phosphor (r-P)/g-C3N4	Mass: 30%	Photocatalytic CO2 conversion into valuable hydrocarbon fuel (CH4) in the presence of water vapor	295 μmol h^−1 g^−1	g-C3N4: 107 μmol h^−1 g^−1	2.76
				r-P: 145 μmol h^−1 g^−1	2.03
RGO/g-C3N4/α-S8	Mass: 30%	Bacterial inactivation (E. coli K-12) under visible-light	No data	g-C3N4: no data	No data	75
g-C3N4/RGO/α-S8				RGO: no data	No data
				α-S8: no data	No data
g-C3N4/C60	Mass: 99.5%	Decomposing (RhB) under visible light	87% in 60 min	g-C3N4: 54%	1.61; 1.80; 1.56	82
	99%		97% in 60 min	C60: no data	No data
	98%		84% in 60 min
CNS/CN-2	Mass: no data	Photocatalytic H2 evolution under visible light; co-catalyst: Pt; sacrificial reagent: no mention	No data	CN: no data	11	80
CN/CNS-2				CNS: no data	2.3
g-C3N4/g-C3N4	Mass: no data	Decomposing	47.6% in 30 min	g-C3N4:(T) 27.3%	1.74	81
		NO under visible light		g-C3N4:(U) 31.7%	1.50
Graphene/g-C3N4	Mass: no data	Conductivity and electrocatalytic performance on oxygen reduction reaction (ORR)	No data	g-C3N4: no data	No data	70
				Graphene: no data	No data

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Significantly, Wang et al. obtained two types of host–guest CN/CNS heterojunctions (Fig. 2) with a facile band alignment by the surface-assisted polymerization: CNS–CN (CN serving as the host) and CN–CNS (CNS serving as the host).⁸⁰ CN/CNS isotype heterojunctions were obtained based on the band alignment between CN and CNS, owing to the slight difference in their electronic band structures. As an amazing result, these two types of CN/CNS heterojunctions mentioned above demonstrated conspicuous enhancement in the photocatalytic activity and stability for hydrogen evolution.

Fig. 2 Schematic illustration of organic heterojunctions formed between CN and CNS. (CN refers to g-C₃N₄, CNS refers to sulfur-mediated CN, and D = donor.)

Inspired by Wang's work, the layered g-C₃N₄/g-C₃N₄ metal-free isotype heterojunction (Fig. 3a) was produced by treating the molecular composite precursors of urea and thiourea under the same thermal conditions. The molecular structure of this CN-T/CN-U heterojunction is illustrated in Fig. 3b. This isotype heterojunction was constructed on the basis that the g-C₃N₄ prepared from separate urea and thiourea have different band structures. Upon visible light irradiation, the photogenerated electrons transfer from g-C₃N₄ (thiourea) to g-C₃N₄ (urea) driven by the conduction band offset of 0.10 eV, whereas the photogenerated holes transfer from g-C₃N₄ (urea) to g-C₃N₄ (thiourea) driven by the valence band offset of 0.40 eV.⁸¹ These two charge transfer processes are beneficial for overcoming the high dissociation barrier of the Frenkel exciton and stabilizing electrons and holes. The redistribution of electrons on the one side of the heterojunction (CN-U) and holes on the opposite side (CN-T) could reduce the electron–hole pair recombination as well as prolonged lifetime of charge carriers, resulting in an exceptionally high photocatalytic capability. These two successful attempts demonstrated that rational design and construction of isotype heterojunctions could open up a new avenue for the development of new efficient visible-light photocatalysts.

Fig. 3 (a) TEM images of CN-TU; (b) schematic illustration of electron–hole separation and transport at the g-C₃N₄–g-C₃N₄ heterojunction interface and in both semiconductors: E_C is the contact electric field for the two components; E_B is the potential barrier in the interfacial depletion layer (E_B < E_C during the photocatalytic reaction); E₁ and E₂ are the internal electric fields induced by the redistribution of the spatial charges in CN-T and CN-U, respectively. CN-T refers to g-C₃N₄ from thiourea and CN-U refers to g-C₃N₄ from urea. (Reprinted with permission from ref. 81. Copyright (2013) American Chemical Society.)

3.2. g-C₃N₄/single metal oxide (metal sulfide)

So far, various single metal oxides (metal sulfides) have been coupled with g-C₃N₄ for enhanced visible light photocatalysis.^83–87 Among abundant metal oxides, TiO₂ (e.g. 3.2 eV), as a widely used photocatalyst, preferentially acted as a candidate for constructing g-C₃N₄ based heterojunctions. The work of Zhou et al., for example, demonstrated that a g-C₃N₄/TiO₂ nanotube array heterojunction was successfully achieved by a simple electrochemical method, showing highly effective visible-light performance with respect to bare g-C₃N₄ and TiO₂ nanotubes.⁴⁵ Zhao et al. obtained g-C₃N₄/TiO₂ hybrid photocatalysts by a facile hydrolysis approach, accompanied by a remarkable enhancement of photocatalytic capability in degradation of phenol both under visible and UV light irradiation.⁸³ Sridharan et al. successfully fabricated a g-C₃N₄/TiO₂ composite with decent photocatalytic performance which was utilized in the treatment of methylene blue (MB) and reduction of hazardous Cr(VI) ions.⁸⁴ Very recently, direct Z-scheme g-C₃N₄/TiO₂ photocatalysts were successfully prepared via simple one-step calcinations with good photocatalytic response for formaldehyde decomposition in air, reported by Yu et al.⁸⁵

WO₃ is another metal oxide that can be used to couple with g-C₃N₄. The band gap of WO₃ is between 2.6 and 2.8 eV.^88–90 For instance, Zang et al. obtained environmentally benign g-C₃N₄/WO₃ composites via a facile mixing–heating procedure, which showed superb performance in methyl orange (MO) degradation.⁸⁸ A WO₃/g-C₃N₄ heterojunction (Fig. 4a and 4b) was prepared by a calcination process accompanied by marvelous visible-light-activity by degradation of MB and 4-chlorophenol, documented by Huang et al.⁸⁹ Apparently, all of the WO₃/g-C₃N₄ photocatalysts demonstrated higher photocatalytic performance than the pristine WO₃, g-C₃N₄ as well as the mechanically blended WO₃ and g-C₃N₄ sample under visible light irradiation (Fig. 4c). Equally important, Katsumata et al. have associated g-C₃N₄ with WO₃ by a physical mixing method to decontaminate the organic gas pollution acetaldehyde (CH₃CHO).⁹⁰

Fig. 4 (a, b) HRTEM images of WO₃/g-C₃N₄ (9.7%) composites; (c) photocatalytic degradation efficiency of MB by g-C₃N₄, WO₃, blend and WO₃/g-C₃N₄ samples with different WO₃ contents under visible light; (d) proposed mechanism for the photodegradation of contaminants on WO₃/g-C₃N₄ composites. (Reproduced from ref. 89 with permission from The Royal Society of Chemistry.)

The remarkably highly increased performance of WO₃/g-C₃N₄ was mainly ascribed to the synergistic effects of the enhanced optical absorption in the visible region, enlarged specific surface areas and suitable band positions. A conceivable degradation mechanism was put forward in Fig. 4d.

As can be seen in Fig. 4d, the top of the VB and the bottom of the CB of WO₃ were calculated to be 3.15 eV and 0.5 eV, respectively.⁸⁹ In the case of g-C₃N₄, the top of the VB is 1.57 V and the bottom of the CB is −1.13 V.⁸⁹ The band gaps of g-C₃N₄ and WO₃ were 2.70 eV and 2.65 eV, respectively. The pristine g-C₃N₄ and WO₃ are two semiconductors which can be both excited to produce photogenerated electron–hole pairs under visible light irradiation. Since the CB potential (−1.13 eV) is lower than the CB edge of WO₃ (0.5 eV), the excited-state electrons on g-C₃N₄ can directly inject into the CB of WO₃. Similarly, the VB position of WO₃ (+3.15 eV) is higher than the VB of g-C₃N₄ (+1.57 eV). The photogenerated holes on the VB of WO₃ can be directly injected into the VB of g-C₃N₄. The redistribution of electrons on the one side of the junction (WO₃) and holes on the opposite side (g-C₃N₄) greatly reduces the electron–hole recombination, which is beneficial to promote the photocatalytic performance. As a result, the photocatalytic activity of WO₃/g-C₃N₄ composites was much higher than that of WO₃ and the pristine g-C₃N₄. Therefore, the enhanced photocatalytic activity was mainly associated with the heterojunction structure and suitable band-edge positions of the two semiconductors, except for enhanced optical absorption in the visible region and enlarged specific surface areas of WO₃/g-C₃N₄ nanojunctions.

Besides, cadmium sulfide (CdS) is a fascinating semiconductor with a relatively low band gap of 2.4 eV, which makes it able to absorb solar light up to 520 nm or even longer.^86,87 Comparing the energy levels of C₃N₄ with CdS, it is fortunate to find that their well-matched band-structures are quite suitable to construct heterostructures that would bring an effective separation and transfer of photogenerated charges. To date, novel CdS/g-C₃N₄ organic–inorganic composites composed of two visible light responsive semiconductors have been fabricated via an “in situ” precipitation–deposition method by Fu et al., exhibiting outstanding visible light photocatalytic activity in 4-aminobenzoic acid removal.⁴²

On account of multiple advantages of combining g-C₃N₄ with oxide (sulfide), a host of attempts have been devoted to exploit novel candidates for coupling with g-C₃N₄.^88–104 They mainly include g-C₃N₄/Fe₂O₃,^91,92 g-C₃N₄/CeO₂,⁹³ g-C₃N₄/MoO₃,⁹⁴ g-C₃N₄/Fe₃O₄,⁹⁵ g-C₃N₄/Ni(OH)₂,⁹⁶ g-C₃N₄/Ag₂O,⁹⁷ g-C₃N₄/MoS₂,⁹⁸ g-C₃N₄/NiS,⁹⁹ g-C₃N₄/TaON,³⁰ g-C₃N₄/ZnO,^100,101 g-C₃N₄/In₂O₃,¹⁰² g-C₃N₄/WO₃,^88,89,103 and SiO₂/g-C₃N₄.¹⁰⁴ These g-C₃N₄ based nanocomposites have become an important family of visible light photocatalysts for contaminant degradation and hydrogen production (Table 2).

Table 2 Photocatalytic properties of g-C₃N₄/single metal oxide (metal sulfide) nanocomposite photocatalysts

Composite photocatalyst	Mass fraction or molar of g-C3N4	Typical parameters of photocatalytic experiments	Photocatalytic activity	Reference photocatalyst; photocatalytic activity	Enhancement factor over the reference photocatalyst	Reference
TiO2/g-C3N4	No data	Decomposing methyl orange (MO) under visible light	100% degradation in 30 min	g-C3N4: no data	No data	45
				TiO2: no data	3
TiO2/g-C3N4	300 °C	Decomposing methylene blue (MB) under visible light	93%	g-C3N4: no data	No data	84
	500 °C		75% degradation in 120 min	TiO2: no data	No data
TiO2/g-C3N4	300 °C	Photocatalytic reduction of Cr(VI) under visible light	72%	g-C3N4: no data	No data
	500 °C		45% in 100 min	TiO2: no data	No data
TiO2/g-C3N4	Mass: 20%	Decomposing phenol under UV light	96.6% degradation in 60 min	g-C3N4: 79.5%	1.2	83
	Mass: 20%	Decomposing phenol under visible light	67.7% degradation in 180 min	TiO2: 65.9%	1.5
TiO2/g-C3N4	Mass: 16.7%	Decomposing phenol under UV light	69.1% in 60 min	g-C3N4: 43.8%	1.5
	50%		82.7% in 60 min	TiO2: no data	No data
	66.7%		96.6% in 60 min	g-C3N4: no data	No data
	83.3%		82.8% in 60 min	TiO2: no data	No data
TiO2/g-C3N4 (Z-scheme)	Mass ratio: 100% (urea: P25)	Decomposing formaldehyde under UV light	94% in 60 min apparent rate constant k: 7.36 × 10^−2 min^−1	g-C3N4: no data	No data	85
				TiO2: no data	No data
				g-C3N4: no data	No data
				P25: 3.53 × 10^−2 min^−1	2.1
WO3/g-C3N4	Mass ratio: no data	Decomposing (MO) under visible light	No data	g-C3N4: no data	No data	88
				WO3: no data	No data
WO3/g-C3N4	Mass: 80%	Decomposing acetaldehyde (CH3CHO) under visible light	No data	g-C3N4: no data	No data	90
	60%		No data	WO3: no data	No data
	40%		No data
	20%		No data
WO3/g-C3N4	Mass: 90.3%	Decomposing MB under visible light	97% degradation in 2 h	g-C3N4: 81%, 3 h	1.2	89
				WO3: 73%, 3 h	1.3
	Mass: 90.3%	Decomposing 4-chlorophenol (4-CP) under visible light	43% degradation in 6 h	g-C3N4: 3%, 6 h	14.3
				WO3: no data
a-Fe2O3/g-C3N4	No data	Electrochemical properties	Supercapacitive performance	g-C3N4: no data	No data	91
				a-Fe2O3: 72 F g^−1	2.3; 1.9
Precursors: [Bmim]FeCl4 anhydrous FeCl			167 F g^−1
			140 F g^−1
Fe2O3/g-C3N4	Mass: 97.2%	Decomposing rhodamine B (RhB) under visible light	100% in 120 min	g-C3N4: 100%, 180 min	No data	92
	95.1%		100% in 160 min	Fe2O3: no data	No data
	93.5%		95.7% in 180 min		0.96
	92%		75.8% in 180 min		0.76
	88.4%		52.5% in 180 min		0.53
ZnO/g-C3N4	Mass: 95.1%	Decomposing rhodamine B (RhB) under visible light	No data	g-C3N4: no data	No data	100
	91.6%		No data	ZnO: no data	No data
	84.4%		97% in 80 min
	41.9%		No data
ZnO/g-C3N4	Mass: 84.4%	Decomposing p-nitrophenol under visible light	No data	g-C3N4: 30%, 5 h	No data
				ZnO: no data	No data
ZnO/g-C3N4	Mass: 5.0%	Decomposing (RhB) under visible light photoreduction of Cr^6+ under visible light irradiation	97.4% in 100 min	g-C3N4: no data	No data	101
				ZnO: no data	No data
	Mass: 5.0%		75.5% in 100 min	g-C3N4: no data	No data
				ZnO: no data	No data
CeO2/g-C3N4	Mass: 87%	Decomposing MB under visible light	95% in 120 min	g-C3N4: 75%, 3 h	1.3	93
	Mass: 94.1%		30% in 5 h	CeO2: 28%, 3 h	3.4
	87.0%	decomposing 4-chlorophenol (4-CP) under visible light	45% in 5 h	g-C3N4: 2.3%	13.0; 2.0
	77.6%		37% in 5 h	CeO2: 15.1%	19.6; 3.0
					16.1; 2.5
MoO3/g-C3N4	Mass: 7.0%	Decomposing MB under visible light	93% in 3 h	g-C3N4: no data	No data	94
				MoO3: no data	No data
Fe3O4/g-C3N4	Mass: 98%	Decomposing MO under visible light	No data	g-C3N4: no data	No data	95
				Fe3O4: no data	No data
Ni(OH)2/g-C3N4	Molar: 99.5%	Photocatalytic H2 evolution under visible light; co-catalyst: no; sacrificial reagent: triethanolamine	RH2: 7.6 mmol h^−1	g-C3N4: no data	No data	96
	99.9%		RH2: 3.3 mmol h^−1	Ni(OH)2: no data	No data
MoS2/g-C3N4	Mass: 99.5%	Photocatalytic H2 evolution under visible light; co-catalyst: no; sacrificial reagent: lactic acid	RH2: 20.6 mmol h^−1	g-C3N4: no data	No data	98
				MoS2: no data	No data
				0.5 wt% Pt/g-C3N4: 4.8 mmol h^−1	4.3
CdS/g-C3N4	Mass: 70%	Decomposing (MO) under visible light	92% in 20 min	g-C3N4: 16%, in 25 min	No data	42
				CdS: no data	No data
				0.3CdS-0.7TiO2: 19%, in 25 min	No data
	Mass: 70%	Decomposing 4-aminobenzoic acid under visible light	73% in 60 min	0.7C3N4-0.3TiO2: 22%, in 25 min	No data
				g-C3N4: 3%	2.7
				CdS: 38%	41.6
NiS/g-C3N4	Mass: 99.5%	Photocatalytic H2 evolution under visible light; co-catalyst: no; sacrificial reagent: triethanolamine	RH2: 9.2 mmol h^−1	g-C3N4: 0.2 mmol h^−1	46.0	99
	98.75%		RH2: 48.2 mmol h^−1	NiS: no data	241.0
	98%		RH2: 32.1 mmol h^−1		160.5
	97.5%		RH2: 13.5 mmol h^−1		67.5
TaON/g-C3N4	Mass: 60%	Decomposing (RhB) under visible light	100% in 50 min	g-C3N4: 100%, in 80 min	No data	30
				TaON: 34%, in 80 min	No data
In2O3/g-C3N4	Mass: 90%	CO2 reduction into hydrocarbon fuels	CH4 production yield: 76.7 ppm	g-C3N4: no data	3.0	102
				In2O3: no data	4.0
WO3/g-C3N4	Mass: 50%	Oxidation of acetaldehyde into CO2	At least 600 ppm of acetaldehyde had been removed in 48 h	g-C3N4: no data	No data	103
				WO3: no data	No data
SiO2/g-C3N4	No data	Decomposing RhB under visible light	Fully degraded in 60 min	g-C3N4: no data	No data	104
				SiO2: no data	No data

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3.3. g-C₃N₄/composite oxide

During the past few years, many groups have reported different types of g-C₃N₄/composite oxide heterojunction photocatalysts. These research studies greatly improve the efficiencies of the photocatalysts and promote their applications in the energy production and environmental remediation.^105,106

Table 3 summarizes and compares the g-C₃N₄/composite oxide heterojunction photocatalysts. Among these photocatalytic systems, Bi₂WO₆/g-C₃N₄ has been the most widely investigated. Ge et al. prepared g-C₃N₄/Bi₂WO₆ heterostructured photocatalysts via mixing and heating methods.³¹ The g-C₃N₄/Bi₂WO₆ photocatalyst had a remarkably enhanced MO photodegradation activity than pure g-C₃N₄ and Bi₂WO₆ under visible light irradiation. This enhancement could be attributed to the enhanced visible-light utilization efficiency and accelerated transfer of photogenerated electron–hole pairs at the intimate interface of heterojunctions, which rationally can be ascribed to the well-aligned overlapping band-structures of g-C₃N₄ and Bi₂WO₆. Subsequently, Wang et al. prepared Bi₂WO₆ hybridized with g-C₃N₄ by facile chemisorptions, which exhibited enhanced photocatalytic performance in MB degradation.¹⁰⁵ Tian et al. hydrothermally synthesized a heterojunction by combining g-C₃N₄ with Bi₂WO₆ with enhanced visible light photocatalytic capability.¹⁰⁶

Table 3 Photocatalytic properties of g-C₃N₄/composite oxide heterojunction photocatalysts

Composite photocatalyst	Mass or molar fraction of g-C3N4	Typical parameters of photocatalytic experiments	Photocatalytic activity	Reference photocatalyst; photocatalytic activity	Enhancement factor over the reference photocatalyst	Reference
a RH2 represents H2 production rate.
BiPO4/g-C3N4	Mass: 4%	Decomposing methyl blue (MB) under UV light	90% degradation in 5 min	BiPO4: no data	2.5	117
				P25(TiO2): no data	4.5
	Mass: 10%	Decomposing MB under visible light	Apparent rate constant k: 0.31 h^−1	g-C3N4: k = 0.06 h^−1	5
Bi2WO6/g-C3N4	Mass: 2%	Decomposing (MB) under simulated solar irradiation	Apparent rate constant: k: 1.0291 h^−1	Bi2WO6; k: 0.6060 h^−1	1.67	105
Bi2WO6/g-C3N4	Mass: 50%	Decomposing methyl orange (MO) under visible light	93% degradation in 120 min	g-C3N4: 31%	3	106
				Bi2WO6: 0.6%	155
Bi2WO6/g-C3N4	Mass: 5%	Decomposing (MO) under visible light	21.1% in 3 h	g-C3N4: 81.4%	0.26	31
	10%		76.0% in 3 h		0.46
	30%		45.2% in 3 h		0.56
	50%		89.6% in 3 h		1.10
	70%		99.9% in 3 h		1.23
	90%		84.4% in 3 h		1.03
Ag3PO4/g-C3N4	Mass: 16.7%	Decomposing RhB under visible light	No data	g-C3N4: no data	3.27	118
				Ag3PO4: no data	1.65
Ag3PO4/g-C3N4	Mass: 25%	Decomposing (MO) under visible light	No data	g-C3N4: no data	5	119
				Ag3PO4: no data	3.5
ZnWO4/g-C3N4	Mass: 5%	Decomposing MB under UV light and visible light	No data	g-C3N4: no data	1.8	114
			No data	g-C3N4: no data	No data
GdVO4/g-C3N4	Mass: 90%	Decomposing (RhB) under visible light	Apparent rate constant: k: 0.0434 min^−1	g-C3N4: no data	3.1	121
				N–TiO2: no data	6.3
				GdVO4: no data	36
NaTaO3/g-C3N4	Mass: 5%	Decomposing (RhB) under UV light and visible light	No data	g-C3N4:	No data	122
	10%			NaTaO3:	No data
				Degussa P25:	No data
SrTiO3:Rh/g-C3N4	Mass: 30%	Photocatalytic H2 evolution under visible light	RH2^a: 81.0 mmol^−1	g-C3N4; RH2: 10.7	7.57, 1.2	123
	20%		223.3	SrTiO3: Rh(0.3 mol%): 68.9	20.9, 3.24
	10%	Co-catalyst: no; sacrificial reagent: not mentioned	92.6		8.7, 1.3
Bi5Nb3O15/g-C3N4	Mass: 50%	Decomposing (MO) under visible light	86% in 3 h	g-C3N4: 80%	1.07; 3.1	124
	70%		94% in 3 h	Bi5Nb3O15: 28%	1.2; 3.4
	90%		93% in 3 h		1.2; 3.3
	Mass: 70%	Decomposing 4-chlorophenol (4-CP) under visible light	100% in 1h	g-C3N4: 72%	1.4
				Bi5Nb3O15: 38%	2.6
Co3(PO4)/g-C3N42	CP-20 (UV irradiation time: 20 min)	Photocatalytic O2 evolution under visible light; co-catalyst: no; sacrificial reagent: AgNO3	No data, 3 h	g-C3N4: 1.28 μmol	6.8	110
Co3(PO4)2/g-C3N4	CP-5	H2 evolution under visible light	No data	g-C3N4: 2.03 μmol h^−1	No data
	CP-10		No data 19.48 μmol h^−1		No data
	CP-20		No data		9.6
	CP-30		No data		No data
	CP-60				No data
N-In2TiO5/g-C3N4	No data	Decomposing (RhB) under visible light	Degraded completely within 20 min	g-C3N4: no data	No data	120
				N-In2TiO5: no data	No data
Ag3VO4/g-C3N4	Mass: 10%	Decomposing basic fuchsin (BF) under visible light Malachite green (MG) Crystal violet (CV)	60% in 2.5 h	g-C3N4: 15%	4.0; 2.0	125
	40%		95% in 2.5 h	Ag3VO4: 30%	6.3; 3.2
	50%		88% in 2.5 h		5.9; 2.9
Ag3VO4/g-C3N4	Mass: 10%		97% in 1 h	g-C3N4: no data	No data
				Ag3VO4: no data	No data
Ag3VO4/g-C3N4	Mass: 10%		75% in 2.5 h	g-C3N4: no data	No data
				Ag3VO4: no data	No data
SmVO4/g-C3N4	Mass: 50.3%	Decomposing (RhB) under visible light	2.07 h^−1	g-C3N4: no data	2.4	104
				SmVO4: no data	6.3

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Li chose SmVO₄ (band gap 2.28 eV) as a promising candidate for constructing g-C₃N₄ coupling heterojunction, and this g-C₃N₄/SmVO₄ composite photocatalyst exhibited high photocatalytic activity and stability for RhB decomposition under visible light irradiation.¹⁰⁷ Moreover, earth abundant cobalt phosphate (Co–Pi) has been demonstrated to work effectively as an oxygen-evolving electrocatalyst^108–110 owing to its attractive characteristics such as the low cost and self-repairing behavior.^111,112 Recently, the Co–Pi species were presented as nanoparticles were well-distributed on the g-C₃N₄ surface, and the Co–Pi/g-C₃N₄ composites exhibit stronger visible light absorption. The Co–Pi/g-C₃N₄ composite samples show significantly enhanced H₂ and O₂ evolution activities.¹¹³

It is well known that ZnWO₄ itself cannot be excited by visible light. In contrast, C₃N₄ can absorb visible light. Wang et al. fabricated C₃N₄/ZnWO₄ coupling composites (Fig. 5a and b), i.e. the ZnWO₄, a semiconductor without visible photocatalytic performance, was successfully photo-sensitized by g-C₃N₄.¹¹⁴ These composites possessed eminent and commendably durable photocatalytic activity in removal of Rh–B, as well as enhanced photocurrent responses under visible light irradiation (Fig. 5c). More importantly, a new mechanism of ZnWO₄ sensitized by carbon nitride is presented in Fig. 5d.

Fig. 5 HRTEM images of ZnWO₄ and C₃N₄/ZnWO₄ photocatalysts (a) ZnWO₄ and (b) C₃N₄/ZnWO₄; (c) photocurrents of ZnWO₄ and C₃N₄/ZnWO₄ electrodes with light on/off cycles under visible light irradiation (λ > 420 nm) ([Na₂SO₄] = 0.1 M); (d) schematic drawing of C₃N₄/ZnWO₄ photocatalyst under visible light. (Reprinted with permission from ref. 114. Copyright (2012) The Royal Society of Chemistry.)

Under visible light illumination, the excited-state electrons of the HOMO of the C₃N₄ would transport to the lower unoccupied molecular orbital (LUMO) of C₃N₄ (π–π* transition). Since the LUMO potential of C₃N₄ (−1.1 eV)¹¹⁵ is lower than the CB edge of ZnWO₄ (−0.8 eV),¹¹⁶ a chemical interaction occurs between them. Hence the excited-state electrons on C₃N₄ can directly inject into the CB of ZnWO₄. The electrons subsequently transfer to the photocatalyst surface to react with water and oxygen to generate hydroxyl and superoxide radicals.¹¹⁴ These reactive radicals are able to oxidize the pollutants. As a result, the CN polymer sensitized ZnWO₄ photocatalyst exhibits enhanced visible light photocatalytic performance. To further broaden the application, many groups have gained success in development of new g-C₃N₄ based heterojunctions with superior performance in photocatalysis, such as C₃N₄/BiPO₄,¹¹⁷ g-C₃N₄/Ag₃PO₄,^118,119 g-C₃N₄/N-In₂TiO₅,¹²⁰ g-C₃N₄/GdVO₄,¹²¹ g-C₃N₄/NaTaO₃,¹²² g-C₃N₄/Rh-SrTiO₃,¹²³ g-C₃N₄/Bi₅Nb₃O₁₅,¹²⁴ and g-C₃N₄/Ag₃VO₄.¹²⁵

3.4. g-C₃N₄/BiOX (AgX)

Recently, as a halide, BiOX (X = Cl, Br, I) with advantages of layered structure and indirect-transition band-gaps were reported as efficient photocatalysts.^126–134 The layered structure can provide large enough space to polarize the related atoms and orbitals, and then induce the presence of internal static electric fields perpendicular to the [Bi₂O₂] slabs and halogen anionic slabs in BiOX.^135–137 It is interesting to couple BiOX with g-C₃N₄ to form layered nanojunctions.

Among the BiOX photocatalysts, BiOBr with an appropriate band-gap (2.75 eV) exhibits high photocatalytic activity. Ye et al. synthesized BiOBr-g-C₃N₄ inorganic–organic composite photocatalysts by a one-step chemical bath method at low temperature.¹³⁸ This heterojunction interacted by facets coupling contributes to the promoted photoinduced charge transfer between BiOBr and g-C₃N₄, and enhances the VLD photocatalytic activity for RhB degradation compared with bare BiOBr and g-C₃N₄. Furthermore, Sun et al. fabricated BiOBr-C₃N₄ heterojunctions (Fig. 6a) by depositing BiOBr nanosheets onto the surface of C₃N₄ nanosheets at room temperature.¹³⁹ The heterojunction possesses intimately contacted interfaces and well-aligned straddling band-structures, which are propitious to the effective separation and transfer of photogenerated charges, bringing an excellent performance. A schematic illustration of the band gap structures for the samples is shown in Fig. 6b.

Fig. 6 TEM of the as-synthesized BiOBr/C₃N₄ nanojunctions (a) and schematic illustration of the band-gap matching and the crystal-plane coupling of C₃N₄/BiOBr nanojunctions (b). (Reproduced with permission from ref. 139, copyright 2014, Elsevier.)

The band structures of the two components are well-matched and aligned with each other (Fig. 6b). Both C₃N₄ and BiOBr can be excited by visible light and then generate photo-induced electrons and holes. The relative CB and VB edge positions of C₃N₄ nanosheets and BiOBr nanosheets imply that the well-matched band energies and crystal planes can form heterojunctions at the nanoscale level. The excited electrons in CB of C₃N₄ can transfer to CB of BiOBr, and excited holes in VB of BiOBr can transfer to VB of C₃N₄, which results in efficient separation and transport of photo-induced electrons and holes. The 2D BiOBr/C₃N₄ layered nanojunctions could effectively strengthen the photocatalytic activity. This work demonstrated that novel 2D nanojunctions, accompanied by high visible light activity, can be constructed by combining two visible-light-active 2D semiconductors with well-coupled crystal planes and well-matched band structures, which could provide a new approach for promoting the activity of current photocatalysts.

BiOI is an attractive p-type semiconductor with a strong photoresponse in the visible light region due to its narrow band gap energy (1.78 eV) and could act as a potential sensitizer to sensitize wide band gap semiconductors. Jiang et al. obtained a novel p–n heterojunction photocatalyst constructed by porous graphite-like C₃N₄ and nanostructured BiOI.¹⁴⁰ In fact, the coupling of p- and n-type semiconductors is believed to be helpful because an internal electric field is built up between them. As a result, the coupling of n-type g-C₃N₄ with a p-type narrow band gap BiOI semiconductor is a good strategy to enhance the visible light absorption capability and the photocatalytic performance of g-C₃N₄ under visible light irradiation. Besides, Wang and his group successfully used BiOCl as an alternative to prepare BiOCl/C₃N₄ heterojunction photocatalysts via an ionic-liquid-assisted solvent-thermal route, and these nanocomposites demonstrated significantly enhanced visible light performance in degradation of MO due to the formation of well-matched heterostructures.¹⁴¹

AgX based photocatalysts have attracted great attention as promising candidates for the development of highly efficient visible light photocatalysts. Some efforts were made to explore a new type of g-C₃N₄ based heterojunction by coupling silver halide. Silver halide AgX (X = Br, I) is a photosensitive material extensively used in the photography field.^142–144 Up to now, Xu et al. have successfully applied AgX (X = Br, I) developing novel visible-light-driven AgX/graphite-like C₃N₄ (X = Br, I) hybrid materials, and the photocatalytic activity dramatically improved using methyl orange (MO) as a target pollutant.¹⁴⁵ The high photocatalytic activity of the hybrid materials could be ascribed to the strong coupling between g-C₃N₄ and AgX, which facilitated interfacial charge transfer and inhibited electron–hole recombination. Table 4 summarizes the photocatalytic properties of different g-C₃N₄/BiOX (AgX) composite photocatalysts.

Table 4 Photocatalytic properties of g-C₃N₄/BiOX (AgX) nanocomposite photocatalysts

Composite photocatalyst	Mass or molar fraction of g-C3N4	Typical parameters of photocatalytic experiments	Photocatalytic activity	Reference photocatalyst; photocatalytic activity	Enhancement factor over the reference photocatalyst	Reference
BiOBr/g-C3N4	Mass: 50%	Decomposing rhodamine B (RhB) under visible light	95% degradation in 30 min	g-C3N4: 15%	6.3	138
				BiOBr: 35%	2.7
BiOBr/g-C3N4	Mass: 50%	NO removal under visible light	32.7% degradation in 30 min	g-C3N4: 22.9%	1.4	139
				BiOBr: 21.2%	1.5
BiOCl/g-C3N4	Molar: 50%	Decomposing methyl orange (MO) under visible light	95% degradation in 80 min	g-C3N4: 11%	8.6	141
				BiOCl: 58%	1.6
BiOI/g-C3N4	Mass: 77.5%	Decomposing methyl blue (MB) under visible light	99% degradation in 3 h	g-C3N4: 64%	1.5	140
				BiOI: 51%	1.9
AgBr/g-C3N4	Molar: 70%	Decomposing MO under visible light	91% degradation in 10 h	g-C3N4: low	No data	145
				AgBr: 23%	4
AgI/g-C3N4	Molar: 97.5%	Decomposing MO under visible light	28% in 3.5 h	g-C3N4: no data	No data
	95%		44% in 3.5 h	AgI: no data	No data
	90%		79% in 3.5 h
	70%		81% in 3.5 h
	50%		62% in 3.5 h
AgBr/g-C3N4	Molar: 70%	Decomposing 4-chlorophenol under visible light	30% in 6 h	g-C3N4: 6.1%	4.9
				AgBr: no data	No data
AgI/g-C3N4	Molar: 70%	Decomposing 4-chlorophenol under visible light	53% in 6 h	g-C3N4: 6.1%	8.7
				AgI: no data	No data

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3.5. g-C₃N₄/noble metal

The semiconductor–metal junction is widely used to create a space–charge separation region (called the Schottky barrier). At the interface of the two components, electrons transfer from one component to the other to align the Fermi energy levels, preventing the charge recombination and enhancing the photocatalytic performance. On the other hand, the surface plasmon resonance (SPR) endowed in noble metals could increase the visible light utilization and probably show special synergistic effects with g-C₃N₄.^146–154 It is therefore feasible to couple noble metals with g-C₃N₄ in order to enhance the photocatalytic activity.^155–161 The work of Di et al., for example, preferentially demonstrated that the deposition–precipitation method was successfully applied to prepare Au(III) nanoparticles on the surface of a structured polymeric g-C₃N₄.¹⁵⁵ The as-synthesized nanoscopic Au–semiconductor heterojunctions effectively accelerate charges transfer on the intimate interface between g-C₃N₄ and Au nanoparticles, enabling high photocatalytic hydrogen production. In addition, given that gold nanoparticles (AuNPs) can serve as photocatalysts for the degradation of dyes under visible-light irradiation through the surface plasmon effect,¹⁵⁶ it is expected that AuNPs/g-C₃N₄ nanohybrids could show improved photocatalytic performance. Cheng et al. proposed that Au nanoparticles (AuNPs) were successfully loaded on graphitic carbon nitride (g-C₃N₄), and the nanohybrids show superior photocatalytic activities for the decomposition of methyl orange under visible-light irradiation due to the facilitated separation of photogenerated electron–hole pairs, and the surface plasmon resonance excitation in AuNPs.¹⁵⁷

It is well-documented that Ag, a famous noble metal, was regarded as a decent co-catalyst in constructing heterojunctions with g-C₃N₄ in photocatalytic utilizations.^158,159 Yang et al. reported that an efficient visible-light plasmonic photocatalyst, Ag/g-C₃N₄ heterostructure, is facilely fabricated by a simple polymerization–photodeposition route.¹⁵⁸ Compared with individual g-C₃N₄, the photocatalytic activity of Ag/g-C₃N₄ was sharply enhanced toward the degradation of MO and p-nitrophenol, which may be ascribed to the enhanced visible-light utilization efficiency due to the SPR absorption of silver nanoparticles as well as fast generation, separation and transportation of the photogenerated carriers. Bai and his group have developed novel core–shell Ag@g-C₃N₄ plasmonic nanocomposites by the facile reflux treating method, exhibiting a superb capability in photodegradation of MB and H₂ evolution.¹⁵⁹ Moreover, by embedding Pd nanoparticles on the g-C₃N₄, a Pd/g-C₃N₄ metal–semiconductor heterojunction was constructed which demonstrated highly strengthened photocatalytic performance in bisphenol removal.¹⁶⁰ Meanwhile, note that Li et al. have successfully obtained g-C₃N₄/noble metal by directly loading with Pt, Au and Pd NPs respectively via a conventional solution impregnation method.¹⁶¹ The functional catalyst/support system can unambiguously act as a tandem catalyst to effectively trigger the water reduction reaction to form H₂ and activation of the as-formed H₂ for further reduction of 4-nitrophenol to 4-aminophenol.

g-C₃N₄ could function as an effective solid stabilizer for chaperoning various ultrafine noble metal NPs.¹⁶¹ It is worthwhile to elucidate the functional mechanisms of g-C₃N₄/noble metal nanohybrids with improved photocatalytic performance. As an example of Au/g-C₃N₄ nanohybrids,¹⁵⁷ a schematic diagram of the photocatalytic mechanism of Au/g-C₃N₄ is presented in Fig. 7a. Owing to surface plasmon resonance of noble metal,⁴ the as-prepared Au/g-C₃N₄ nanocomposites exhibit highly enhanced visible-light photocatalytic activity. On the one hand, the SPR effect of Au nanoparticle causes intense local electromagnetic fields, which can speed up the formation rate of holes and electrons within g-C₃N₄.^158–161 Additionally, the favorable Fermi level of noble metal facilitates the separation of electrons and holes, which in turn enhances the quantum efficiency of g-C₃N₄, due to the intimate combination of noble metal/g-C₃N₄ nanohybrids (Fig. 7b).¹⁶² Moreover, the transfer of electrons shifts the Fermi level to more negative potential, thereby improving the reducibility of electrons in the Fermi level close to the CB of g-C₃N₄.¹⁶³ On the other hand, the efficient utilization of sunlight can be realized due to SPR absorption in the visible-light region as well as UV light response of the interband transition of noble metal nanoparticles. Table 5 shows the photocatalytic properties of different g-C₃N₄/noble metal nanocomposite photocatalysts.

Fig. 7 (a) Schematic diagram illustrating the photocatalytic degradation of organic contaminants over noble metal NPs/g-C₃N₄ hybrid under visible-light irradiation; (b) TEM image of AuNP/g-C₃N₄ nanohybrids. (Reprinted with permission from ref. 157. Copyright (2013) American Chemical Society.)

Table 5 Photocatalytic properties of g-C₃N₄/noble metal nanocomposite photocatalysts

Composite photocatalyst	Mass fraction or mass of g-C3N4	Typical parameters of photocatalytic experiments	Photocatalytic activity	Reference photocatalyst; photocatalytic activity	Enhancement factor over the reference photocatalyst	Reference
Au/g-C3N4	Mass: 4.5%	Decomposing methyl orange (MO) under visible light	92.6% degradation in 150 min	g-C3N4: 28.7%	3.2	157
				Au: no data	No data
Au/g-C3N4	No data	Photocatalytic H2 evolution under visible light; co-catalyst: no; sacrificial reagent: triethanolamine	RH2: no data	Pt/g-C3N4; RH2:	No data	15
Pt-Au/g-C3N4	No data		RH2: no data	No data
Pd-Au/g-C3N4	No data		RH2: no data
Ru-Au/g-C3N4	No data		RH2: no data
Ag-Au/g-C3N4	No data		RH2: no data
m-CNR-Au(2 nm)	No data	Reduction of 4-nitrophenol to 4-aminophenol under visible light	>96% in 5 min	No data	No data	161
m-CNR-Pd(3 nm)	No data
m-CNR-Pt(2 nm)	No data
m-CNR-Pt(2 nm)	No data	Photocatalytic H2 evolution under a 300 W Xe lamp; co-catalyst: no; sacrificial reagent: triethanolamine	>85% H2O conversion	No data	No data
Ag/g-C3N4	Ag: 0.1 g	Decomposing MO under visible light	MO of 74%	g-C3N4: 70%;	1; 1.3	158
	Ag: 0.5 g		MO of 78%	P25 TiO2: 56%	1.1; 1.4
	Ag: 1 g		MO of 86%		1.2; 1.5
	Ag: 2 g		MO of 91%		1.3; 1.6
	Ag: 5 g		MO of 92%		1.3; 1.6
Ag/g-C3N4	Ag: 2 g	Decomposing p-nitrophenol (PNP) under visible light	PNP of 98%	g-C3N4: 83%;	1.2
				P25 TiO2: <10%	>9.8
Ag/g-C3N4	Mass: 99.5%	Decomposing MO methyl blue (MB) phenol under visible light	No data	g-C3N4: no data	2.1	159
			No data	g-C3N4: no data	1.8
				g-C3N4: no data	2.2
Ag/g-C3N4	Mass: 90%	Photocatalytic H2 evolution under visible light; co-catalyst: no; sacrificial reagent: triethanolamine	No data	g-C3N4: no data	30
Pd/g-C3N4	No data	Decomposing bisphenol A under solar light	93.9% degradation in 180 min	Bulk g-C3N4: 6%	15.65	160
				P25 TiO2: 6.7%	14.01
				g-C3N4: 52.1%	1.8

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3.6. g-C₃N₄ based complex system

Although a number of g-C₃N₄-based two-component nanocomposites have been developed for enhanced visible light photocatalysis, there are still some shortages that need to be further addressed. With this aim, multicomponent complex heterojunction systems have been developed, in which two or more visible-light active components are integrated.^164–167 For example, novel Ag/AgBr/g-C₃N₄ ternary composite photocatalysts were successfully constructed via the deposition–precipitation method, presenting a dramatically improved photocatalytic performance for degradation of MO as well as good stability under visible light.¹⁶⁴

The high photocatalytic activity and stability of Ag/AgBr/g-C₃N₄ are mainly attributed to the synergetic effect of the AgBr/g-C₃N₄ interface and metallic Ag (Fig. 8a and 8b). Namely, the matching energy band structure of AgBr/g-C₃N₄ and the excellent electron trapping role of metallic Ag ensured the efficient separation of electron–hole pairs, i.e. metallic Ag quickly evacuated the electrons on the side of AgBr which guaranteed the stability of Ag/AgBr/g-C₃N₄, shown in Fig. 8c. Moreover, Chai et al. designed and prepared g-C₃N₄–Pt-TiO₂ three component nanocomposites via a facile chemical adsorption–calcination process, accompanied by remarkably enhanced performance for hydrogen production under visible light irradiation.¹⁶⁵ The cocatalyst Pt deposited on TiO₂ surfaces in the g-C₃N₄-TiO₂ heterojunction is beneficial for exerting the synergistic effect existing between TiO₂ and g-C₃N₄, which can strengthen the photogenerated carrier separation in space.

Fig. 8 (a) TEM image and (b) HRTEM image of 50% Ag/AgBr/g-C₃N₄; (c) schematic diagram of electron–hole pairs separation and the possible reaction mechanism over Ag/AgBr/g-C₃N₄ photocatalyst under visible light irradiation. (Reproduced with permission from ref. 164, copyright 2013, Elsevier.)

Inspiringly, a new type of Pt-TiO₂/g-C₃N₄–MnOx quaternary composites was also fabricated by an impregnation method with an extraordinarily enhanced high H₂ production capacity, firstly reported by S. Obregón and G. Colón.¹⁶⁶ Such a marked increase in the photoactivity might be related to the incorporation of Pt and g-C₃N₄–MnOx into the charge separation process, which is in favor of retarding the charge recombination and improving the photoactivity for H₂ production. A novel complex g-C₃N₄/CoO/graphene hybrid, with g-C₃N₄ embedded CoO particles covalently supported on a two-dimensional graphene sheet, is synthesized through a facile and scalable method by Jin and his co-workers.¹⁶⁷ Above all, these significant achievements can pave the way for constructing g-C₃N₄-based multi-component heterojunctions and it is highly possible to optimize the visible light absorption, the charge carriers separation and transfer by adequately tailoring the structure of the photocatalysts. Recent progress on g-C₃N₄ based complex nanocomposites is summarized in Table 6.

Table 6 Photocatalytic properties of g-C₃N₄ based complex system

Composite photocatalyst	Mass or molar fraction of g-C3N4	Typical parameters of photocatalytic experiments	Photocatalytic activity	Reference photocatalyst; photocatalytic activity	Enhancement factor over the reference photocatalyst	Reference
Ag/AgBr/g-C3N4	Molar: 50%	Decomposing methyl orange (MO) under visible light	95.3% degradation in 30 min	g-C3N4: 2.5%	38.1	164
				Ag/AgBr: 62.3%	1.5
Ag/AgBr/g-C3N4	Molar: 50%	Decomposing 2-chlorophenol under visible light	70.51% degradation in 4 h	No data	No data
g-C3N4/Pt/TiO2	Mass: 70%	Photocatalytic H2 evolution under visible light; co-catalyst: no; sacrificial reagent: triethanolamine	RH2: 0.178 mmol h^−1	Pt-loaded g-C3N4–TiO2; RH2: 0.124 mmol h^−1	1.4	165
Pt-TiO2/g-C3N4-MnOx	No data	Decomposing phenol under UV light	96.4% degradation	Pt- TiO2: 96.5%	0.9	167
			Time: no data	Pt-TiO2/g-C3N4: 96%	1.0
Pt-TiO2/g-C3N4-MnOx	No data	Photocatalytic H2 evolution under visible light; co-catalyst: no; sacrificial reagent: isopropanol	RH2: 7.5 mmol h^−1 g^−1	Pt- TiO2; RH2: 5.5 mmol h^−1 g^−1	1.36
				Pt- TiO2/g-C3N4; RH2: 6.5 mmol h^−1 g^−1	1.15

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4. Multifunctional applications

Semiconductor-mediated photocatalysis has attracted worldwide attention for its potential in environmental and energy-related applications.^45,117–125 Nevertheless, the rapid recombination rate of photogenerated electron–hole pairs within photocatalytic materials results in low efficiency, thus limiting its practical applications. Therefore, the retardation of recombination of charge carriers is the key for the enhancement of photocatalytic performance of semiconductor photocatalysts. g-C₃N₄-semiconductor hybrid materials as a new class of photocatalysts have recently attracted wide research interest.^138–141 In this regard, nanocomposites that combine g-C₃N₄ and other components could potentially provide desirable efficiency for separating electron–hole pairs. As documented above, the g-C₃N₄-based semiconductor photocatalysts have been widely used for the degradation of pollutants, photocatalytic hydrogen generation and photocatalytic conversion of carbon dioxide to methane fuel, etc. In this section the multiple applications of g-C₃N₄-based nanocomposites are briefly summarized.

4.1. Photocatalytic degradation of pollutants

In recent years, a large quantity of efforts has been devoted to solving the widespread pollution of effluents and gaseous pollutants from urban and agricultural industries. Various catalytic techniques have been applied in environmental conservation. Photocatalysis has been widely used in environmental applications such as air purification,^168–170 water disinfection,^171,172 hazardous waste remediation^173,174 and water decontamination.¹⁷⁵

Graphene carbon nitrogen, as a “rising star” material, has many exceptional properties, such as environmentally benign, stable physicochemical properties, large specific surface area and low bad-gap, etc.^18–20 Hence, g-C₃N₄-based semiconductor photocatalysts have been extensively applied to photocatalytic degradation of environmental pollutants.^89,90 These composites possess a high dye adsorption capacity, extended light absorption boundary, and accelerated charge transportation and separation properties.

For instance, a novel, multi-walled carbon nanotubes (CNT) modified white C₃N₄ composite (CNT/white C₃N₄) was fabricated by Xu et al.⁷⁷ The CNT/C₃N₄ nanocomposite was prepared by the hydrothermal method through electrostatically-driven self-assembly. The CNT/C₃N₄ composite shows a significantly enhanced photocatalytic performance in MB dye removal. A possible photocatalytic mechanism of CNT/white C₃N₄ on the enhancement of visible light performance is proposed (Fig. 9a). Firstly, the electrons are motivated from the VB to the CB of C₃N₄ under visible light irradiation. Secondly, the photo-excited electrons are effectively collected by CNT, hindering the recombination process of the electron–hole pairs. Thirdly, the O₂ adsorbed on the surface of the catalyst can be reduced to active species. The holes and the generated ˙OH can react with the organic dye and generate degradation products. Furthermore, the photocatalytic stability of CNT/white C₃N₄ was investigated through the repeated MB degradation experiments as shown in Fig. 9b. It is clear that the MB dye can be completely bleached after each photocatalytic run, and CNT/C₃N₄ is stable enough during the repeated experiments without significant loss of photocatalytic activity. Therefore, CNT/white C₃N₄ can be used as an effective photocatalyst for organic compound degradation with good stability.

Fig. 9 (a) Proposed photocatalytic mechanism of the CNT/white C₃N₄; (b) recycling in the repeated MB degradation experiments with CNT/white C₃N₄ under visible light irradiation. (Reproduced with permission from ref. 77. Copyright (2013) The Royal Society of Chemistry.)

Moreover, WO₃, a decent visible light catalyst, was also selected to combine with g-C₃N₄.^88–90 Zang et al.⁸⁸ reported that the g-C₃N₄/WO₃ composite exhibited a significant enhancement of photocatalytic degradation of MO in water under visible light irradiation compared to the bare g-C₃N₄ and WO₃. As illustrated in section 3 of g-C₃N₄/metal oxide (metal sulfide), in the g-C₃N₄/WO₃ composite, holes activated on the VB of the WO₃ would transfer to the VB of g-C₃N₄, while the electrons on CB of g-C₃N₄ may transfer to the CB of WO₃ under light irradiation due to the well-matched energy band structure. Therefore, the photo-generated electrons and holes can be separated effectively in this way, and the recombination rate of the photo-generated electrons and holes would be hampered, which contributed to the superior photocatalytic reactivity. Moreover, this coupling heterojunction could also be used to remove the poisonous gaseous pollutant acetaldehyde gas (CH₃CHO), which is a typical volatile organic compound (VOC) and exerts adverse effects on the health of humans. By incorporating g-C₃N₄ with different contents of WO₃, Katsumata et al. studied the photocatalytic performance of this composite in acetaldehyde gas removal.⁹⁰ As a result, they found that as the WO₃ content increases, the photodegradation effect became more conspicuous, and the optimum mass ratio of g-C₃N₄ to WO₃ for photocatalysis is two to eight.⁹⁰ What is more, Huang et al. further utilized this novel composite to treat another two organic pollutants of MB and 4-CP.⁸⁹ The highest degradation efficiency was achieved for the WO₃/g-C₃N₄ (9.7%) sample, which induced 97% degradation of MB within 2 h and 43% degradation of 4-CP within 6 h under visible light irradiation, while pristine g-C₃N₄ only induced 81% degradation of MB within 3 h and 3% degradation of 4-CP within 6 h.⁸⁹

Apart from CNT/C₃N₄ and WO₃/g-C₃N₄ nanocomposites, composites of g-C₃N₄ and other semiconductor photocatalysts such as g-C₃N₄/TiO₂,^83,85 CeO₂/g-C₃N₄,⁹³ g-C₃N₄/CdS,⁴² g-C₃N₄/TaON,³⁰ g-C₃N₄/Bi₂WO₆,^105,106 BiOBr/g-C₃N₄,^138,139 BiOI/g-C₃N₄ ¹⁴⁰ and Pd/g-C₃N₄ ¹⁶⁰ have been reported as efficient photocatalysts for decomposition of pollutants in water. Among these coupling systems, TiO₂ as a famous UV light-driven and g-C₃N₄ as a robust visible-light-driven photocatalysts are favourably selected to be coupled for enhanced photocatalysis. Zhao et al. evaluated both the UV and visible-light photocatalytic activities of g-C₃N₄/TiO₂ for degradation of phenol in water, and found that these composites exhibited excellent photocatalytic performance.⁸³ The pseudo-first-order kinetic constant of phenol degradation on g-C₃N₄/TiO₂ was 2.41 and 3.12 times higher than those on pristine g-C₃N₄ and TiO₂ respectively, which could be ascribed to the wide absorption wavelength range and effective photogenerated charge separation. Moreover, Fu and co-workers reported that the g-C₃N₄ hybridized CdS composite is an efficient photocatalyst.⁴² The optimum activity of 0.7C₃N₄–0.3CdS photocatalyst is almost 20.5 and 3.1 times higher than those of individual C₃N₄ and CdS for methyl orange degradation and 41.6 and 2.7 fold higher for 4-aminobenzoic acid removal, respectively.⁴² Moreover, its activity is also much higher than those of C₃N₄–TiO₂ and CdS–TiO₂ composites, as well as famous N-modified TiO₂. Of particular significance is that the present C₃N₄–CdS composites also demonstrate high stabilities under illumination, in contrast with CdS.⁴² The improvement in both performance and stability should be assigned to the effective separation and transfer of photogenerated charges originating from the well-matched overlapping band-structures and closely contacted interfaces.

Recently, the g-C₃N₄–Bi₂WO₆ composite photocatalysts were studied by Tian and co-workers for photocatalytic degrading MO under visible light irradiation.¹⁰⁶ Encouragingly, the resulting g-C₃N₄–Bi₂WO₆ heterojunctions have a strong absorption in the visible light region and have apparently enhanced photocatalytic performances. This superior activity may be ascribed to the electronic interactions and charge equilibration between g-C₃N₄ and Bi₂WO₆, leading to the shift in the Fermi level and decreasing the conduction band potential of Bi₂WO₆. Simultaneously, the negative shift in the Fermi level of g-C₃N₄–Bi₂WO₆ and the high migration efficiency of photoinduced electrons can suppress the charge recombination effectively, resulting in the enhanced photocatalytic degradation of pollutants. Moreover, with bismuth subcarbonate ((BiO)₂CO₃) as a novel photocatalyst, we achieved new g-C₃N₄/(BiO)₂CO₃ nanojunctions by an in situ strategy of depositing (BiO)₂CO₃ nanoflakes onto the surface of g-C₃N₄ nanosheets through the efficient capture of atmospheric CO₂ (crude material of (BiO)₂CO₃) at room temperature.¹⁷⁶ These nanojunctioned photocatalysts showed excellent visible-light capability by degrading RhB and phenol, accompanied by good stability.

Hitherto, arrays of successful efforts of g-C₃N₄-based heterojunctions have also been made to degrade NOx, which have been considered worldwide as gaseous pollutants. Our groups have investigated the photocatalytic capability of novel 2D BiOBr/C₃N₄ nanojunctions in NOx removal.¹³⁹ The NO removal ratios of BiOBr/C₃N₄ can be achieved to 32.7%, which is much higher than that of the pristine BiOBr (21.2%) and C₃N₄ (21.2%). The enhancement of photocatalytic performance of BiOBr/C₃N₄ could be ascribed to the synergistic effect of well-coupled crystal planes and band structure. To further boost the photocatalytic activities, we constructed the g-C₃N₄/g-C₃N₄ metal-free isotype heterojunction by a facile in situ method.⁸¹ The CN-TU dramatically exhibited an improved NO removal ratio of 47.6%, compared to host substrates of CN-U and CN-T. Besides, this metal-free isotype heterojunction demonstrated good stability as well. The strengthened photocatalytic performance of the g-C₃N₄/g-C₃N₄ isotype heterojunction can be directly ascribed to the efficiently accelerated charge separation across the heterojunction interface as well as prolonged lifetime of charge carriers.

4.2. Photocatalytic hydrogen generation

Hydrogen energy is considered as an ultimate clean fuel in the near future because of its high-energy capacity, environmental benignancy, and recycling utilization.^96,177,178 Photocatalytic splitting of water into hydrogen and oxygen using semiconductor photocatalysts has been considered to be an attractive and promising approach to produce hydrogen energy. A range of semiconductor photocatalysts have been reported to catalyze the evolution of hydrogen from water. However, the practical applications of this strategy are limited on account of the prompt recombination of photogenerated electron–hole pairs within photocatalysts. Considering its visible-light-driven properties and high specific surface area, g-C₃N₄ can be considered as an efficient photocatalyst for enhancing the visible light utilization and accelerating the photo-induced charge transfer hampering the backward reaction, which will highly promote the photocatalytic H₂-production activity.^179,180

With Ni(OH)₂/g-C₃N₄ composites loaded with different contents of Ni(OH)₂, Yu and co-workers⁹⁶ studied their water splitting performance in triethanolamine aqueous solutions. The results demonstrated that Ni(OH)₂ was an efficient co-catalyst for g-C₃N₄ photocatalytically producing H₂. The optimal Ni(OH)₂ loaded on g-C₃N₄ was found to be 0.5 mol%, giving a H₂-production rate of 7.6 mmol h⁻¹ with a quantum efficiency (QE) of 1.1% at 420 nm, depicted in Fig. 10a. The fact that the potential of Ni²⁺/Ni is lower than that of the CB of g-C₃N₄ and more negative than that of H⁺/H₂ helps the electron to transfer from CB of g-C₃N₄ to Ni(OH)₂ and then reduce H⁺ to H₂ with a decent performance (Fig. 10(b)). This work proved that Ni(OH)₂ is a very promising co-catalyst to combine with g-C₃N₄ for visible-light photocatalytic hydrogen production. Very recently, Hong et al.⁹⁹ systematically studied the efficiency of H₂ evolution for the NiS/g-C₃N₄ composite constructed by a simple hydrothermal method. This photocatalyst showed efficient hydrogen evolution (48.2 mmol h⁻¹) under visible light when using triethanolamine as a sacrificial reagent. The optimal ratio was 1.1 wt% of NiS loaded on g-C₃N₄ and the rates of H₂ production can be enhanced by about 250 times compared with the native C₃N₄. The highest apparent quantum efficiency was recorded at 1.9%, induced by 440 nm light irradiation. In order to further explore the possible application of g-C₃N₄/single metal oxide (metal sulfide) composites in hydrogen production, Hou and co-workers⁹⁸ constructed MoS₂/g-C₃N₄ nanojunctions and found that visible-light performance of H₂ production of mpg-CN is significantly improved compared with bare MoS₂ and g-C₃N₄. Inspiringly, the 0.5 wt% MoS₂/mpg-CN performed better than 0.5 wt% Pt/mpg-CN under identical reaction conditions. The geometric similarity (layered morphology) of MoS₂ and g-CN (Fig. 11), accompanied by the mesoporous architectures of mpg-CN, help in forming an intimate planar MoS₂/mpg-CN interface, which can significantly promote the photoactivity of MoS₂/g-C₃N₄. They also discovered that other layered transition metal dichalcogenides (such as WS₂) are also efficient promoters of hydrogen production combined with g-C₃N₄. Herein they have presented not only an example of a catalyst made of abundant C, N, Mo and S elements for efficient H₂ photosynthesis, but also a conceptual advance to rationally design and fabricate thin, effective interfacial 2D junctions between co-catalysts and semiconductors that have similar layered geometric architectures.

Fig. 10 (a) Comparison of the photocatalytic activity of the Ni0, Ni0.1, Ni0.5, Ni1.0, Ni1.6, Ni10, Ni100 and Pt-deposited g-C₃N₄ samples for the photocatalytic H₂ production from triethanolamine aqueous solution; (b) schematic illustration for the charge transfer and separation in the Ni(OH)₂-modified g-C₃N₄ system under visible light irradiation. (Reprinted with permission from ref. 96. Copyright (2013) The Royal Society of Chemistry.)

Fig. 11 Schematic illustration of the idealized structural model of the resulting MoS₂/g-CN layered junctions.

Apart from the g-C₃N₄/single metal oxide (sulfide) composites, research studies have coupled C₃N₄ with other decent photocatalysts to further strengthen the H₂ generation efficiency.^123,155,165 Taking noble metals as an example, Di et al. reported that gold (Au) nanoparticles were successfully deposited on the surface of g-C₃N₄ by the deposition–precipitation method.¹⁵⁵ The as-synthesized nanoscopic Au-semiconductor heterojunctions effectively accelerated the transfer of charges between g-C₃N₄ and Au, which can enable Au-g-C₃N₄ with excellent visible-light activity for hydrogen production. Besides, surface modifying Au/g-C₃N₄ with a second metal Pt (low over-potential for water reduction) further improved the activity of the photocatalytic system. This can be explained by the simultaneous optimization of electron transfer induced by the gold and chemical reactivity afforded by the secondary metal Pt. Moreover, Kang and his group systematically studied the g-C₃N₄–SrTiO₃:Rh photocatalyst for improved H₂ evolution under visible light irradiation.¹²³ A high hydrogen evolution rate of 223.3 mmol h⁻¹ was achieved using 0.1 g of as-prepared photocatalyst powder comprised of 20 wt% g-C₃N₄ and 80% SrTiO₃:Rh (0.3 mol%), measured in aqueous methanol solution. A mechanism for the electron–hole separation of the g-C₃N₄–SrTiO₃:Rh composite under visible light irradiation was proposed. The Rh ions doped into the SrTiO₃ lattice structure formed a donor level from the VB to CB, which eased the transfer of photo-induced holes from the SrTiO₃:Rh surface to the g-C₃N₄ surface and photo-induced electrons from the g-C₃N₄ surface to the SrTiO₃:Rh surface, thereby preventing the recombination of electron–hole pairs. Also, Chai et al. investigated a ternary nanocomposite g-C₃N₄–Pt-TiO₂ in photocatalytic hydrogen evolution.¹⁶⁵ The visible-light-induced photocatalytic hydrogen evolution rate can be remarkably strengthened by coupling TiO₂ with g-C₃N₄, and the g-C₃N₄–(Pt-TiO₂) composite with a mass ratio of 70 : 30 shows the maximum photocatalytic H₂ production rate (178 mmol h⁻¹), as well as excellent photostability. Moreover, the cocatalyst Pt deposited on TiO₂ surfaces is beneficial for exerting the synergistic effect existing between TiO₂ and g-C₃N₄. Thus, the photogenerated electrons of g-C₃N₄ can directionally migrate to Pt–TiO₂ due to the close interfacial connections and the synergistic effect existing between Pt–TiO₂ and g-C₃N₄. The photogenerated electrons and holes can be efficiently separated in space, beneficial for retarding the charge recombination and improving the photoactivity. This study not only shows that g-C₃N₄, as an effective sensitizer of TiO₂, can be widely used to improve photocatalytic H₂ production, but also affords various methods for developing novel, steady and visible-light-responsive photocatalysts.

Also, graphene, a two-dimensional macromolecular sheet of carbon atoms with a honeycomb structure, extremely high specific surface area and thermal conductivity, showed a good photocatalytic performance for hydrogen or oxygen production.¹⁸¹ After the introduction of graphene sheets, g-C₃N₄ is immobilized to form layered composites, which will largely increase the BET and catalytic performance.³² In this system, graphene sheets act as conductive channels efficiently separating the photogenerated charge carriers as well as enhancing the visible-light capability of H₂-production. The optimal graphene content was found to be 1.0 wt%, and the corresponding H₂-production rate was 451 μmol h⁻¹ g⁻¹, which exceeded that of pure g-C₃N₄ more than 3.07 times. Normally, when g-C₃N₄ is immobilized on the surface of graphene sheets to form the layered composites, these photogenerated electrons on the CB of g-C₃N₄ tend to transfer to graphene sheets due to their excellent electronic conductivity, favoring the hole–electron separation. Meanwhile, Sui et al. reported that they have successfully loaded highly-dispersed conductive polymer polypyrrole (PPy) nanoparticles on the surface of g-C₃N₄.⁷⁸ Surprisingly, they found that the H₂ evolution rate of g-C₃N₄ loaded with 1.5 wt% PPy can increase up to 50 times compared with that of pristine g-C₃N₄, and the reaction proceeded in a pure water system excluding the need for sacrificial agents. We believe that this work can provide an effective route for developing efficient photocatalysts for photocatalytic hydrogen evolution from pure water systems.

4.3. Other applications

Solar fuels have been considered to be one of the most important renewable clean energy resources and photocatalytically converting CO₂ into a valuable hydrocarbon fuel (CH₄) is known to be a challenging but promising application for sustainable energy resources.^79,182–186 In g-C₃N₄ based composites, Yuan et al. significantly obtained the red phosphor/g-C₃N₄ heterojunction with enhanced photocatalytic activities for solar fuels production.⁷⁹ The introduction of g-C₃N₄ into the red phosphor surface led to considerable improvement of the photocatalytic activity for CO₂ conversion into CH₄ in the presence of water vapor. The enhancement could be attributed to the effective separation of photogenerated electrons and holes across the red phosphor/g-C₃N₄ heterojunction. Owing to the advantages of non-toxicity, low cost and abundance in nature, this active heterostructural red phosphor/g-C₃N₄ would have great potential for efficient solar fuels production. Very recently, In₂O₃/g-C₃N₄ hybrid photocatalysts were fabricated by Cao and his group, and the resulting In₂O₃–g-C₃N₄ hybrid structures exhibited considerable photocatalytic activities for CO₂ reduction.¹⁰² After 4 h irradiation, the optimal In₂O₃/g-C₃N₄ (10 wt% In₂O₃) exhibited a CH₄ production yield of 76.7 ppm (over 20 mg samples) without any cocatalyst, which is more than three times higher than that of individual g-C₃N₄ and more than four times higher than that of pure In₂O₃. The enhanced activities were attributed to the enhanced interfacial transfer of photogenerated electrons and holes between g-C₃N₄ and In₂O₃, leading to effective charge separation on both parts.

Moreover, g-C₃N₄ based composites were also considered to be a promising candidate in photocatalytic disinfection.⁷⁵ A novel class of metal-free ternary heterojunction photocatalysts was prepared by wrapping reduced graphene oxide and g-C₃N₄ sheets on crystals of cyclooctasulfur (α-S8), reported by Wang et al.⁷⁵ Two distinctive structures were fabricated by wrapping reduced graphene oxide (RGO) and CN sheets in different orders. The first was RGO sheets sandwiched in a heterojunction of CN sheets and α-S₈ (i.e., CNRGOS8), while the second structure was the other way around (i.e., RGOCNS₈). Fascinatingly, both structures exhibited outstanding antibacterial activity under aerobic or anaerobic conditions irradiated with visible light (Fig. 12), which made us choose colibacillus as a representative microorganism to evaluate the photocatalytic water disinfection performances. This work not only provided new inroads into the exploration and understanding of the photocatalytic bacterial inactivation mechanisms, but also afforded to develop suitable protocols for solar-driven photocatalytic water disinfection under different conditions.

Fig. 12 Schematic illustration of the VLD photocatalytic bacterial inactivation mechanisms of (a) CNRGOS₈ and (b) RGOCNS₈ under aerobic conditions, and (c) CNRGOS₈ and (d) RGOCNS₈ under anaerobic conditions. CNRGOS8 was RGO sheets sandwiched in a heterojunction of CN sheets and α-S8, while RGOCNS8 was RGO wrapped in the heterojunction of CN sheets and α-S8. (Reprinted with permission from ref. 75. Copyright (2013) American Chemical Society.)

In addition, g-C₃N₄-based heterojunctions are also utilized to perform as supercapacitors. Xu et al. mentioned that the g-C₃N₄/a-Fe₂O₃ hollow microspheres have been successfully prepared in the presence of a metal ion-containing reactable ionic liquid under the solvothermal conditions.⁹¹ The capacitance of the g-C₃N₄/a-Fe₂O₃ composites as electrode materials was tested by cyclic voltammetry and chronopotentiometry measurements, demonstrating that the g-C₃N₄/a-Fe₂O₃ composites were capable of delivering the largest specific capacitance and the highest coulombic efficiency. It can be assumed that the enhanced specific capacitance of the g-C₃N₄/a-Fe₂O₃ hollow microspheres could be attributed to hollow sphere-like structures, high BET surface area, incorporation of g-C₃N₄, and low electronic resistance. Thus, the g-C₃N₄/a-Fe₂O₃ hollow microspheres are promising candidates for high-performance supercapacitors.

We have made a summary of the design strategy, categories and their multifunctional applications of g-C₃N₄ based nanocomposites as illustrated in Fig. 13.

Fig. 13 Schematic illustration of the g-C₃N₄ based heterojunction/heterostructure and their multifunctional applications.

5. Conclusion and perspectives

In summary, various types of g-C₃N₄-based nanocomposite systems have been designed and constructed. The incorporation of g-C₃N₄ into these nanocomposites can impart them with unique properties and induce enhanced performance, such as a high adsorption capacity, an extended light absorption range, and accelerated charge separation and transportation, which strengthens the overall photocatalytic performance. A wide range of heterostructures, including metal-free, metal/semiconductor, semiconductor/semiconductor, molecule/semiconductor, and multi-component heterostructures, have been explored for the improved photocatalysis by increasing the light absorption, improving the charge separation and transportation, enhancing the catalytic activity and prolonging the charge carrier lifetime. These nanocomposite photocatalysts have been widely used for photocatalytic degradation of pollutants, photocatalytic hydrogen generation, carbon dioxide storage and disinfection. The present review depicts the fabrication, microstructure and photocatalytic performance, as well as the photocatalytic mechanisms when g-C₃N₄ is coupled with an array of materials such as metal-free, single metal oxides (metal sulfides), composite oxides, halides, noble metals, etc. It is anticipated that the g-C₃N₄-based nanocomposites will receive ever increasing research attention in the future.

Although considerable progress has been achieved, the studies in this field are still at the primary stage and further systematic investigations are needed. Rational design of complex heterostructures that could simultaneously facilitate efficient optical absorption, carrier generation, separation, transportation and utilizations is central for a new generation of highly efficient and robust photocatalysts. Significant challenges remain in the synthesis of such complex nanostructures with well-designed architectures and optimized charge cascading processes.

First, a fundamental understanding of the charge transport process in the multi-heterostructure photocatalysts is critical for the optimization of the charge cascading process to maximize the utilization of photogenerated charge carriers for desirable redox chemistry. While it is relatively straightforward to understand electron–hole separation and transportation in a two component heterostructure, the understanding and control of charge transport in multi-component heterostructures have become increasingly more complex, particularly on the nanometer scale. The interface properties of the photocatalysts determine the eventual efficiency of the photocatalytic system. To understand the charge generation, separation and transportation across these nanoscale interfaces is critical. Current investigations mainly focus on the overall apparent efficiency of the entire photocatalytic system. Detailed mechanism studies of the charge transfer process in photocatalysts integrating three or more components are scarce and will be necessary to further advance the field.

Secondly, the stability of g-C₃N₄-based nanocomposites is less addressed and will be the main challenge for photocatalyst development. A photocatalyst with very high efficiency is still useless if the lifetime of the catalyst is too short. For the majority of systems, chemical corrosion and/or photodegradation of photocatalysts are difficult to avoid. The promoted photocatalyst stability has only been achieved in a few examples to date, which usually however require highly complicated synthetic processes. To develop a stable photocatalytic system with high efficiency at low cost will be the central task to realize a practically viable photocatalyst for potential practical applications.

Thirdly, the mechanisms of photocatalytic enhancement by the g-C₃N₄-based semiconductor composite systems are partly unclear. The explanation of photocatalytic activity by the g-C₃N₄ content in the composites is still controversial. Therefore, more studies are required for promoting the general understanding of the enhancement mechanism of g-C₃N₄ based nanocomposites, especially employing advanced in situ techniques. Also, it is necessary to develop a uniform method to assess the photocatalytic performance, as the current evaluation methods are diverse. A photocatalyst working better in water treatment may show poor performance in air purification.

Finally, the rapid development of materials science and nanotechnology in the past few years has resulted in the creation of various advanced photocatalytic materials. Interesting properties may be explored by combining novel photocatalysts with g-C₃N₄. With a better understanding of the fundamental photocatalysis mechanism, assisted by the rapid development of advanced new nanomaterials, the bottleneck of environmental and energy-related global issues could be overcome in the near future. Overall, it is certain that there will be numerous exciting opportunities on g-C₃N₄ based nanocomposites, which require the combined efforts of scientists all over the world.

Acknowledgements

This research was financially supported by the National Natural Science Foundation of China (51478070 and 51108487), the Natural Science Foundation Project of CQ CSTC (cstc2013jcyjA20018), the Science and Technology Project from Chongqing Education Commission (KJ1400617 and KJ130725), and the Innovative Research Team Development Program in University of Chongqing (KJTD201314).

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