Powerful combination of 2D g-C3N4 and 2D nanomaterials for photocatalysis: Recent advances

Journal Pre-proofs Review Powerful combination of 2D g-C3N4 and 2D nanomaterials for photocatalysis: Recent advances Xin Zhang, Xingzhong Yuan, Longbo Jiang, Jin Zhang, Hanbo Yu, Hou Wang, Guangming Zeng PII: DOI: Reference:

S1385-8947(20)30466-6 https://doi.org/10.1016/j.cej.2020.124475 CEJ 124475

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Chemical Engineering Journal

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20 September 2019 11 February 2020 15 February 2020

Please cite this article as: X. Zhang, X. Yuan, L. Jiang, J. Zhang, H. Yu, H. Wang, G. Zeng, Powerful combination of 2D g-C3N4 and 2D nanomaterials for photocatalysis: Recent advances, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124475

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Powerful combination of 2D g-C3N4 and 2D nanomaterials for photocatalysis: Recent advances

Xin Zhang a, b, Xingzhong Yuan a, b*, Longbo Jiang a, b*, Jin Zhang a, b, Hanbo Yu a, b, Hou Wang a, b, Guangming Zeng a, b a. College of Environmental Science and Engineering, Hunan University, Changsha 410082, P.R. China b. Key Laboratory of Environment Biology and Pollution Control, Hunan University, Ministry of Education, Changsha 410082, P.R. China

* Corresponding author at: College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China. Tel.: +86 731 88821413; fax: +86 731 88823701. E-mail address: [email protected] (X. Yuan); [email protected] (L. Jiang);

Contents 1. Introduction ................................................................................................................4 2. Fabrication of g-C3N4 based 2D/2D composite .........................................................7 2.1 Hydrothermal/solvothermal method ..........................................................................8 2.2 Self-assembly method ...................................................................................................9 2.3 In-situ growth method ................................................................................................11 2.4 Microwave irradiation method .................................................................................12 2.5 Mechanical grinding method ....................................................................................13 2.6 Other synthetic methods ............................................................................................14

3. Categories of g-C3N4 based 2D/2D heterojunctions ................................................15 3.1 Hybridization with 2D graphene materials ............................................................15 3.2 Hybridization with 2D transition metal dichalcogenides ....................................19 3.3 Hybridization with 2D metal oxides........................................................................23 3.4 Hybridization with 2D bismuth-based semiconductors .......................................28 3.5 Hybridization with 2D phosphorus ..........................................................................32 4. Basic principles of g-C3N4 based 2D/2D heterojunction .........................................34 4.1 Interfacial characteristic and types of 2D/2D heterojunction .............................34 4.2 Charge transfer path ....................................................................................................36 4.2.1 Co-catalyst .........................................................................................................36 4.2.2 Traditional type-II heterojunction.................................................................37 4.2.3 All-solid-state Z-scheme heterojunction .....................................................38 4.3 Mechanism insight of 2D/2D heterojunction.........................................................40 5. Applications .............................................................................................................41 5.1 Hydrogen generation ..................................................................................................41 5.2 Carbon dioxide reduction ..........................................................................................43 5.3 Degradation of organic pollutants ............................................................................45 5.4 Photocatalytic disinfection ........................................................................................47 6. Conclusions and perspectives ..................................................................................49 Conflicts of interest ......................................................................................................53 Acknowledgments........................................................................................................53 References ....................................................................................................................53

Abstract As a fascinating metal-free conjugated polymer semiconductor, 2D graphitic carbon nitride (g-C3N4) has been widely used as a visible-lightresponsive photocatalyst. Generally, g-C3N4 exhibits some appealing properties, such as exceptional thermal and chemical stability, appropriate band structure, as well as low cost. However, the pristine g-C3N4 is limited by the fast recombination of the photoinduced electron-hole pairs, limited surface area, and insufficient sunlight absorption. Very recently, the construction of 2D/2D heterojunctions has attracted widespread attention owing to their distinct advantages, including the intimate interface, high surface area, suitable band potential and so forth. Therefore, this review summarized the latest progress in the coupling g-C3N4 with other 2D nanomaterial systems, including 2D graphene-like materials, 2D transition metal dichalcogenides, 2D metal oxides, 2D bismuth-based semiconductors and 2D phosphorous. Furthermore, the photocatalytic mechanism of g-C3N4 based 2D/2D heterojunctions was systematically studied and their corresponding charge-transfer pathways on the interface surface were elaborated. In addition, the photocatalytic applications of the g-C3N4 based 2D/2D composites in the realm of hydrogen generation, carbon dioxide reduction, pollutant degradation, and photocatalytic disinfection were also reviewed. Finally, the current challenges faced by the formation of g-C3N4 based 2D/2D composite and the opportunities for further development are also proposed. Key words: 2D g-C3N4; 2D nanomaterials; Photocatalysis; Heterojunction; Interface charge transfer.

1. Introduction

Semiconductor-based photocatalysis, as a renewable, promising and environmentally friendly technology, has been implemented in the comprehensive applications around the worldwide, including energy conversion and environmental restoration [1-5]. At present, traditional TiO2 has been widely used because of its outstanding thermodynamic stability, superior photocatalytic performance, as well as relative nontoxicity [6, 7]. However, TiO2 can only respond in the ultraviolet light because of the broad bandgap (3.2 eV). Unfortunately, the energy distribution of ultraviolet light in solar spectrum energy is less than 4%~5%, which considerably restricts the solar energy utilization efficiency of TiO2 [8-11]. Therefore, it is urgent to develop innovative photocatalysts with superior visible-light response [12]. In the last decade, graphitic carbon nitride (g-C3N4) has been an encouraging candidate for superior visible-light response photocatalyst due to the proper band structure, high stability as well as low cost (Fig. 1a). The "real-life" g-C3N4 is a polymeric layered nanomaterial with a defectrich N-bridged tri-s-triazine or s-triazine (Fig. 1b) [13, 14]. Generally, g-C3N4 is endowed the unique optical properties, superior thermal endurance, physicochemical stability and appealing electronic structure [15]. These features allow it to be an efficient metal-free polymeric semiconductor. However, the practical applications are always constrained by certain deficiencies of pristine g-C3N4, including low specific surface area, the inadequate utilization rate of the sunlight, poor electrical conductivity and fast recombination of photoinduced electron-hole pairs [9, 16, 17].

In order to address these challenges, many modification methods have been applied to enhance the photocatalytic activity of pristine g-C3N4, including elements/heteroatoms doping, heterojunction construction, nanostructure design, copolymerization, dye sensitization, etc. [14, 18-24]. Among the current progress of the g-C3N4 modification strategies, integrating with other semiconductors to construct heterojunction has been regarded as a reliable and valid means. Generally, the g-C3N4-based heterojunction could not only inhibit the recombination of photoinduced charge carriers via the formation of the intimate interface but also endow the photocatalysts with some unique features. Therefore, the reasonable construction of g-C3N4-based heterostructure provides a feasible means of developing the highly efficient visible-light-response photocatalysts [25]. The contact categories between nanomaterials of varying dimensions were compared (Fig. 1c). It can be seen that the 2D/2D heterojunction displays several excellent characteristics. For example, the large contact interface creates more transfer channels for photoinduced charges and reduces the internal resistance. Furthermore, the ultrathin 2D/2D structure also achieves abundant catalytic active sites and shortened transfer distance, as well as enhanced light absorption ability (Fig. 1d). Meanwhile, the 2D/2D heterojunction photocatalysts exhibits excellent stability. Therefore, coupling different 2D materials to construct 2D/2D heterojunction composite has attracted enormous attention [3, 25-27]. Given the advantages of 2D g-C3N4 as well as 2D/2D intimate interface, g-C3N4 based 2D/2D heterojunction composites have been widely fabricated to

enhance the photocatalytic performance. Typically, hybridization with 2D graphene-like materials, 2D transition metal dichalcogenides, 2D metal oxides, 2D bismuth-based semiconductors and 2D phosphorous have been explored. For example, Yuan et al. proved that the hydrogen evolution rate of the 2D/2D MoS2/g-C3N4 is far beyond the 0D/2D Pt/g-C3N4 photocatalyst [28]. The outstanding photocatalytic activity can be attributed to the formed 2D interfaces and the large contact area between MoS2 and g-C3N4 nanosheet. Similarly, Yu et al. successfully constructed 2D/2D rGO/g-C3N4 nanocomposite through a simple solvothermal strategy [29]. They found that rGO/g-C3N4 nanocomposite exhibited fast separation efficiency of charge carriers and improved visible-light absorption due to abundant coupling interfaces and strong interfacial interaction between rGO and g-C3N4. Recently, our research group discovered plenty of g-C3N4 based heterojunction such as h-BN/g-C3N4, P doped g-C3N4/g-C3N4, and g-C3N4/MOFs, etc [12, 24, 30-35]. Very interestingly, the recent outcome reveals that g-C3N4 base 2D/2D heterojunction can greatly improve the photocatalytic performance. In the past few years, several excellent reviews have focused on the realms of fabrication strategies, properties, and related applications of gC3N4 in solving the energy and environmental challenges [36-42]. Subsequently, some comprehensive reviews reported the modification of gC3N4, including heterojunction construction, elemental doping, functionalization, and copolymerization [15, 43-54]. Moreover, recently some prominent reviews focus on the structure tuning and rational nanostructure design of g-C3N4 photocatalysts for applications in emerging fields,

including hydrogen evolution, CO2 conversion, biomedicine, nanosensing, and photocatalytic disinfection [55-64]. Although, many reviews on the photocatalytic activity of g-C3N4 are available in the prior literature, but most of these are not completely emphasized upon the interface mechanism of composite materials. In addition, Review related the g-C3N4 based 2D photocatalyst interface electron transfer is rarely reported. Therefore, it is necessary to systematically disclose that the interactive nature between 2D g-C3N4 and other 2D semiconductors and explain the interface mechanism of the g-C3N4 based 2D/2D heterojunction systems. In this review, the synthesis approaches are introduced first by classification. Then a series of representative g-C3N4 based 2D/2D heterojunction photocatalysts are exemplified. The insight mechanism and interface electron transfer of the g-C3N4 based 2D/2D heterojunction systems are discussed in detail. Later, the basic principles of the 2D/2D heterojunction interface are elaborated. Moreover, the applications of g-C3N4 based 2D/2D nanostructures are explained. Finally, some summative remarks and perspectives of g-C3N4 based 2D/2D heterojunction photocatalysts are concisely presented.

2. Fabrication of g-C3N4 based 2D/2D composite

Since 2D nanomaterials firstly prepared through mechanical exfoliation, a large number of synthetic strategies are beginning to emerge to obtain the 2D nanomaterials with different nanostructures. It mainly can be classified into two categories: the top-down approach and bottom-up

fabrication. Similarly, the preparation of 2D/2D composites and control of their microstructures are very crucial for modulating the photocatalytic performance. The synthesis of g-C3N4 based 2D/2D composite mainly involves self-assembly of some smaller structure units into relatively complex systems through the weak interactions. Up until now, many approaches have been proposed to effectively prepare the 2D/2D heterojunction composite photocatalyst with an intimate interface. The common synthetic methods are described in detail below. The advantages and disadvantages of different methods have been discussed. 2.1 Hydrothermal/solvothermal method The hydrothermal method refers to the use of an aqueous solution as a reaction system in a specially closed reactor (autoclave), and the inorganic system is produced by heating the reaction system to a critical temperature. However, there are obvious deficiencies in the hydrothermal process, which are often limited to the preparation of oxide materials or a few water-insensitive sulfides. Hence, the solvothermal method is proposed to expand the application range. Hydrothermal/solvothermal methods have become more and more important approaches to synthesize highly active nanocomposites with special metastable structures and special condensed states. Furthermore, it also become a powerful and hopeful method in crystal engineering of coordination complexes, particularly for the complexes that are not directly reactive with metal ions and ligands [65]. Kumar et al. prepared 2D/2D nanocomposites of g-C3N4 loaded with N-ZnO nanosheets through hydrothermal synthesis strategy [66].

Typically, N-doped ZnO was added into the solution of g-C3N4 and stirred for 48 h, followed by evaporation of the water, and then a powder composite of N-doped ZnO/g-C3N4 nanoplates was prepared after drying at 100 ℃ for 1 h. Similarly, MoS2/g-C3N4 [67], BiOCl/g-C3N4 [68], ZnIn2S4/g-C3N4 [69], et al. were also successfully obtained via this method. Hydrothermal/solvothermal method has been widely applied because of its advantages of facile, energy-saving, low-cost, high reactivity, environmentally friendly, as well as the ease of controlling of the aqueous solution. Furthermore, the as-obtained nanomaterial exhibits the characteristic of high purity, good crystal structure, shape, and controllable size [70]. However, unique reaction process determines that hydrothermal/solvothermal method requires special equipment and the waste liquid produced in the reaction process needs to be further treated. 2.2 Self-assembly method The “self-assembly method” is largely used to pre-fabricate the 2D/2D heterojunction composite, they are mechanically/chemically exfoliated from bulk samples or separated from the present 2D nanosheets. Subsequently, the 2D/2D heterojunction is constructed by a manual stack method, in which the 2D layers are integrated by weak van der Waals (vdW) interlayer interaction [27]. Furthermore, studies showed that the interlayer distance and the superposition lattice orientation of 2D/2D heterojunction, are crucial to their photocatalytic performance. The interlayer interactions in the heterojunction can be successfully modulated by altering the layer number of nanomaterials. Moreover, the superposition lattice

orientation can greatly affect the properties of the heterojunction, which mainly can be ascribed to its major influence on interface contact [27, 71]. Ran et al. firstly fabricated a metal-free 2D/2D heterojunction of phosphorene/g-C3N4 nanosheet (CNS) through a simple self-assembly method [72]. Meanwhile, another efficient 2D/2D composite was prepared by hybridizing oxygen doped graphitic carbon nitride (O-gC3N4) with borondoped reduced graphene oxide (B-rGO) via a combined sonication-assisted electrostatic self-assembly strategy [73]. In detail, O-g-C3N4 was dispersed in deionized (DI) water and followed by the addition of B-rGO aqueous solution, with ultrasonication for 30 min, following by continuous stir for 2 h in the room temperature. Then a composite with a greyish hue was obtained after drying overnight in the oven at 60 ℃ (Fig. 2). In summary, the self-assembly method is the common combination strategy of the 2D/2D heterojunction composites, in which molecules are arranged into stable aggregates via non-covalent bond interactions under equilibrium conditions. The as-obtained composite possesses a relatively ordered structure and controllable morphology. While the solid-state reaction limits the flexibility of the reaction intermediates and causes an incomplete polycondensation with relatively low crystallinity. Therefore, it is suitable for the preparation of nanomaterials with poor stability, such as integrating g-C3N4 and BP by self-assembly. 2.3 In-situ growth method There are still some shortcomings of the self-assembly method in the aspects of interface pollutants and weak interfacial interaction force.

Therefore, the “in-situ growth method” as the substitute approach has been proposed to fabricate the perspective 2D/2D heterojunction. The insitu growth method is derived from the concept of in-situ crystallization and in-situ polymerization. It involves the direct growth of the 2D/2D heterojunction component, or the second phase is formed on the surface of another 2D nanomaterial by one-step growth or multi-step transformation. Chemical vapor deposition (CVD) and wet-chemical synthesis are the two main approaches. Studies showed that different synthetic strategies may result in distinct heterojunction interfaces and latent applications. Xia et al. fabricated g-C3N4-based 2D/2D heterostructure photocatalyst through in-situ growth of MnO2 on the surface of g-C3N4 nanosheets [74]. Specifically, g-C3N4 nanosheets were dispersed into the DI water under sonication. Subsequently, the MnCl2 ·4H2O was slowly added to prepare the g-C3N4/MnO2 nanocomposite. Moreover, Fu et al. successfully synthesized 2D/2D MoS2/g-C3N4 heterojunction through an in-situ interfacial engineering technology [67]. The in-situ growth method maintains the integrity and stability of the nanomaterials and makes them less prone to agglomeration. The outstanding characteristic of the as-prepared 2D/2D heterojunction is largely owed to the good lattice orientation and strong interface coupling of the heterojunction components [27]. 2.4 Microwave irradiation method As an efficient synthesis method with high speed, low energy consumption, microwave irradiation exhibits excellent product performance.

The heating principle of microwave radiation is outlined as follows. First, the microwave synthesizer generates an alternating electric field. When the electric field of the microwave radiation changes, the polar small molecules collide and oscillate, and a thermal effect occurs. Second, due to the friction-like effect between molecules, some of the energy is converted into heat to exacerbate the molecular motion. Finally, the metastable molecules are further ionized, so that the heating rate can increase rapidly in a short time. Liu et al. fabricated 2D/2D SnS2/g-C3N4 heterojunction composite via heating the uniform dispersion of g-C3N4 and SnS2 monolayers utilizing a microwave muffle furnace [75]. g-C3N4 nanosheets were dissolved in the mixed solutions of water and ethanol and ultrasonically dispersed about 1 h, the as-obtained SnS2 was transferred into the solution of g-C3N4 and followed by stirring continuously for 12 h. Finally, the sample was thermally treated in the microwave muffle furnace at 300 ℃ for 2 h and then the product was prepared. Besides that, Ding et al. fabricated a series of 2D/2D ZnIn2S4/g-C3N4 heterojunction composites by a microwave-assisted method [76]. The primary advantages of the microwave-irradiation method are facile synthesis, fast reaction rate, as well as high reaction selectivity. In addition, different synthetic processes can be controlled by programming the microwave settings. High-yield composites can be obtained within short minutes. Thus, this method exhibits promising potential in large-scale applications [37, 77, 78]. Furthermore, because microwave radiation is more stable and efficient in liquid media than the common solid-state reaction system [55]. Thus, multifaceted liquid phases such as aqueous

solutions, polyols, and ionic liquids have been considered as reaction media during the microwave-assisted reaction to improve the stability of microwave radiation. 2.5 Mechanical grinding method Mechanical grinding involves the physical mixing of the semiconductor precursors with g-C3N4 precursors [37]. By the action of external mechanical forces, that is, by frequent collisions of the grinding balls, grinding jars and particles, the particles are repeatedly pressed, deformed, broken, and welded during the ball milling process. With the continuation of the ball milling process, the grain gradually refines, and the defect density of the particle surface increases. For instance, Wang et al. fabricated g-C3N4/TiO2 composite through milling melamine and commercial TiO2 as the precursors [79]. Then the mixture was collected after the ball milling process and calcined at 600 ℃ for 4 h in a muffle furnace, a 2D/2D interface was produced in the heterojunction. Similarly, Yu et al. prepared 2D/2D g-C3N4/WO3 heterojunction composite via the same synthetic route [80]. This method receives extensive attention due to its advantages, including facile, high mass production and reliable operation. Moreover, this method is an alternative to improve the contact interface between different nanomaterials without adding an agent. However, the method also suffered from crystal defects and uneven distribution of nanoparticles on the g-C3N4 surface [81].

2.6 Other synthetic methods In addition to the several common synthesis strategies mentioned above, a variety of synthesis methods, like one-pot thermal condensation method, impregnation strategy, deposition strategy, ionic liquid strategy, adsorption-in situ transformation method and so forth, have emerged. And different synthetic routes can greatly affect the photocatalytic performance to some extent. Ai et al. successfully prepared g-C3N4/rGO hybrid photocatalysts via one-pot thermal condensation of melamine with rGO, the composite exhibited excellent photocatalytic activity [82]. He et al. synthesized the ZnO functionalized g-C3N4 composites by an impregnation strategy, the composite displayed a higher CO2 conversion rate, which was almost 4.9 and 6.4 times over the g-C3N4 and P25, respectively [83]. Wang et al. prepared a series of g-C3N4/BiOI composites by depositing the BiOI on the surface of g-C3N4, and the obtained hybrid displayed more excellent photocatalytic performance than pristine single components [84]. Li et al. proposed an ionic liquid system containing Fe3+ for the preparation of g-C3N4/α-Fe2O3 hollow microspheres via a green solvothermal strategy [85]. Furthermore, Yu et al. successfully fabricated a-MoSx/g-C3N4 photocatalysts by direct loading the a-MoSx nanomaterials on the gC3N4 surface via an adsorption-in situ transformation method. The a-MoSx/g-C3N4 (3wt%) hybrid photocatalyst showed improved visible-light photoactivity, which was almost 90.03 times compared to that of pristine g-C3N4 [86]. All in all, every method has its advantages and disadvantages, and thus choosing a suitable approach or combining multiple methods is particularly significant.

3. Categories of g-C3N4 based 2D/2D heterojunctions In addition to the well-known g-C3N4, various 2D materials are also proposed [87]. The 2D materials refer to low dimensional materials that possess sheet-like structures and the thickness ranging from single layer to a few nanometers with the basal plane dominating the total surface area. Because of their unique shapes, the 2D structure possesses exotic electronic properties, large specific surface area, and abundant exposed active sites [88]. As shown in Fig. 3, graphene-like materials, transition metal dichalcogenides, typical 2D metal oxides, bismuth-based semiconductors, and phosphorus are generally coupled with g-C3N4 to form the 2D/2D heterojunction composite with an intimate interface. The main advantages of g-C3N4 based 2D/2D heterojunction are concentrated on extended light absorption, enlarged specific contact area and prolonged charge carriers lifetimes [89]. Hence, the construction of a suitable 2D/2D heterojunction is regarded as the most promising means to improve the photocatalytic activity. 3.1 Hybridization with 2D graphene materials Graphene materials, as a metal-free photocatalytic material, have attracted numerous attentions due to their remarkable advantages. Such as outstanding mechanicals, excellent thermal conductivity, uniquely electrical properties as well as facile obtained. Graphene, a macromolecular sheet of two-dimensional of sp2-bonded carbon atoms has a honeycomb lattice [90, 91]. It has been widely

served as an electron acceptor to facilitate the transfer of photoinduced electron-hole pairs because of their excellent electron conductivity and intriguing electronic properties [22]. Since graphene and g-C3N4 exhibit the identical sp2-bonded π structure as well as carbon network, it is conducive to develop new compatible photocatalytic materials for nanostructure [92, 93]. Recently, 2D/2D hybrid graphene/g-C3N4 heterojunction was obtained through a combined impregnation chemical reduction method [94]. This study demonstrated that the graphene sheets could perform as the electronic conductive channels to effectively transfer the photoinduced electron-hole pairs. The edge of the absorption band showed a slight redshift due to the development of C-O-C bonds as the covalent cross-linker between graphene and g-C3N4. To further study the internal mechanism and the interfacial electronic structure, Du et al. carried out theoretical calculations based on the hybrid density functional theory (DFT), and further studied the interface between the g-C3N4 and graphene nanosheet [95]. On the one hand, studies showed the optimized geometry structure and their corresponding band dispersions of the graphene (Fig. 4a, 4c) and g-C3N4 (Fig. 4b, 4d). Besides, to deeply explore the interface between graphene and g-C3N4, the stacking patterns were investigated. The results indicated that part of the C atoms of graphene were placed on the N atoms of g-C3N4 surface, while the other atoms were placed above the holes of g-C3N4 (Fig. 4e~f) [95]. Moreover, Ong et al. successfully deposited the g-C3N4 on the surface of the graphene nanosheet (Fig. 4g) [93]. The experiment further precisely predicted that the graphene was intercalated into the g-C3N4 framework by the π-π type interaction force. On the other hand, the charge

density at the interface of graphene/g-C3N4 heterojunction was redistributed by the formation of triangular-shaped electron-hole rich regions (Fig. 4h) [22]. The formation of electron-hole puddles was largely attributed to the built-in electric field in the heterojunction interface, which facilitated the charge transport. Thus, the formation of the 2D/2D graphene/g-C3N4 heterojunction may trigger an original optical transition and improve the electron conductivity. Besides, the composite exhibited enhanced absorbance in the visible-light region upon the hybridization with the graphene. This could be attributed to the surface charge improvement of pristine g-C3N4, thereby resulting in an efficient electronic transition in the 2D/2D heterojunction interface. Reduced graphene oxide (rGO), as a compromise to the harsh synthetic process of pure graphene has been widely used in photocatalysis. It also possesses remarkable integrative properties, such as theoretically high surface area, as well as the minimal optical adsorption [73, 93]. Ai et al. successfully fabricated the hybrid photocatalyst of g-C3N4/rGO through a one-pot thermal condensation method. The 2D/2D close interfacial was formed by the strong interaction between rGO and g-C3N4, which exhibited superior visible-light photoactivity [82]. To a certain extent, rGO can improve the adsorption ability and increase the visible-light absorption of the 2D/2D rGO/g-C3N4 heterojunction. However, some studies demonstrated that the rGO and the unmodified g-C3N4 (Fig. 5a) all showed negative polarity [96-98]. Owing to the presence of strong electrostatic repulsion, they may not be successfully coupled together, resulting in the decline of photocatalytic performance. Fortunately, HNO3 and H2SO4

pretreatment can protonate the g-C3N4 easily (Fig. 5b), and then the rGO/protonated g-C3N4 was successfully fabricated via sonication-assisted electrostatic self-assembly strategy [99]. The 2D/2D heterointerface was constructed via the π-π stacking interactions as well as electrostatic attraction. Meanwhile, the band-gap of the rGO/protonated g-C3N4 layer can be reduced by the formation of the cross-linked C-O-C covalent bonding [92, 100, 101]. Furthermore, although a similar synthetic method was employed, it is apparent that the surface morphologies of rGO/gC3N4 (Fig. 5f) and rGO/protonated g-C3N4 (Fig. 5c~e) were different [99]. The unmodified g-C3N4 were prone to aggregate each other rather than coupling with rGO tightly due to the weak van der van force. This phenomenon could be attributed to the mutual electrostatic repulsion between the negatively charged unmodified g-C3N4 and rGO, which greatly restrained the effective hybridization of each other. As a result, interface contact was insufficient to construct the heterojunction with coherent and discerned, which further impeded the separation of photoinduced electrons and holes [99]. Thus, protonation played a critical role in the interface charge transport process of photocatalysis. Besides, the protonated g-C3N4 could significantly enhance light-harvesting ability owing to the sufficient defects on the porous and protonated surfaces as well as the multiple scattering effects. The protonated g-C3N4/rGO photocatalyst was expected to cover the full visible spectrum [96]. 3.2 Hybridization with 2D transition metal dichalcogenides Belonging to the class of two-dimensional materials, transition material dichalcogenides (TMDs) has attracted a wide range of attention,

owing to their diversity of chemical composition, intriguing chemical and thermal properties, as well as unique crystal structures. TMDs exhibits great potential in the field of optoelectronic catalysis owing to their intrinsic band gaps (1~2.5 eV). Moreover, it possesses strong light-matter interaction, high carrier mobility, and spin characteristics, and the 2D layer material is bonded by the weak van der Waals force. Therefore, it is beneficial to construct the 2D/2D homogeneous or heterojunctions with high quality and not limited to the lattice match condition of traditional heterojunctions. Bulk MoS2 is a common 2D material with an indirect bandgap. However, it will turn to be a direct bandgap when it was thinned to a monolayer with a similar structure to graphene [102]. Reports suggested that the layered crystal structure was composed of "S-Mo-S" covalently bonded and connected by weak van der Waals force. MoS2 has attracted considerable attention recently, owing to its intriguing electronic, optical, as well as catalytic properties [103]. MoS2, as the co-catalyst, exhibits great potential in g-C3N4 based photocatalysts due to its catalytic activity (the strained 1T-MoS2 shows catalytic activity on both and edge and basal sites, and for stable 2H-MoS2, only the edge S sites are active). [104]. For example, Ge et al. reported that the 2D/2D MoS2/g-C3N4 composite exhibited a higher hydrogen production rate, which was up to 11.3 times than the pure g-C3N4 [105]. According to the analysis of the experiment results, it is considered that the enhanced photocatalytic performance was mainly originated from the two aspects. On the one hand, MoS2 can act as the electron sinks, which accepted electrons and further constrained the

photogenerated carrier recombination as well as prolonged the life of electron-hole pairs [106]. On the other hand, the synergistic effect of the strong interfacial interaction between two different 2D semiconductors also played an essential role. However, the interfacial composition, atomic details, and charge transfer mechanism of the interface between the g-C3N4 and MoS2 sheets were still unclear. In another study, Fu et al. synthesized 2D/2D MoS2/g-C3N4 van der Waals (VDW) heterojunction via a facile, and in situ interfacial engineering strategy [67]. A series of investigations of the geometries and elemental analysis illustrated that the as-obtained composite nanosheets displayed high homogeneity and quality (Fig. 6a~b), and this view was further supported by the element mapping. Besides, the X-ray photoelectron spectroscopy (XPS) showed a new peak corresponding to the Mo-N bond, which further demonstrated the strong interaction between two different 2D components. To further study the 2D/2D g-C3N4/MoS2 heterojunction, Hu et al. proposed the theoretical studies on the structural, electronic, electrical and optical properties of the 2D van der Waals heterojunction using DFT calculations [107]. The g-C3N4 showed obvious geometric distortion due to the presence of the MoS2 and exhibited a typical VDW equilibrium spacing between the MoS2 sheet and g-C3N4. Moreover, it is further predicted that the heterojunction structure was relatively stable by calculating the interfacial adhesion energy. To clearly describe the g-C3N4/MoS2 interface interaction, an in-depth analysis was carried out. According to their geometric structures and the differential charge density of g-C3N4/MoS2 heterojunction (Fig. 6c), nitrogen atoms nearby the g-C3N4 were attracted to the MoS2 layer, suggesting that the delocalized nitrogen atoms were

highly activated. According to the DFT calculations, the charge density differences were caused by the orientation mobile of electron holes, it is obvious that the charge redistribution mainly occurred in the 2D/2D interface area. Therefore, a polarization field was generated, which could promote the effective transfer of photoinduced carriers [107]. In addition, due to the synergistic effect of the g-C3N4 layer and MoS2 sheet, the obtained layer nanocomposite exhibited superior optical absorption ability. It can be verified by the absorption coefficients of the g-C3N4/MoS2 heterojunction composite (Fig. 6d). Furthermore, based on the presented density of states (DOS) (Fig. 6e), the suitable band alignment between two different 2D layer materials may lead to the formation of a type-II heterojunction in the 2D/2D g-C3N4/MoS2 nanocomposite, thereby facilitating the efficient transfer of photoinduced carriers. Finally, the carrier transfer mechanism of the 2D/2D g-C3N4/MoS2 heterojunction interface was proposed (Fig. 6f). ZrS2 is another ideal substrate to couple with g-C3N4, which can construct the 2D/2D VDW heterostructure with a suitable band structure (Fig. 7a). Previous studies showed that the fast recombination of photoinduced charge carriers can greatly decrease the photocatalytic activity [108]. As a result, the 2D/2D g-C3N4/ZrS2 heterostructure has aroused wide attention because of the efficient charge transfer across the interface. According to the Density of states (DOS) analysis (Fig. 7b), the VB of g-C3N4/ZrS2 was chiefly contributed by the N_2p orbitals of g-C3N4, whereas the CB of g-C3N4/ZrS2 mainly originated from the Zr_4d orbital of ZrS2 [109]. Owing to the construction of the internal polarized field

at the 2D/2D interface region, the electron-rich center was largely around the Zr region, while the hole-rich was around the region of N (Fig. 7c). The effective separation of the photoinduced carries could remarkably improve the photocatalytic performance. Furthermore, from the calculated optical absorption spectra (Fig. 7d), we could see that the optical absorption edge of the 2D/2D heterojunction photocatalyst was extended to the visible-light region. On the other hand, the high binding energies demonstrated that g-C3N4/ZrS2 was stable under light irradiation and mild chemical conditions. Therefore, it can be served as a promising photocatalyst. HfS2, as an ionic crystal with the moderate band gap, is connected by weak van der Waals forces, which is like ZrS2. HfS2 is also expected to be a potential candidate for the photocatalysts due to their high photosensitivity and superior field-effect response. At present, the 2D/2D HfS2/gC3N4 heterostructure was studied, and the equilibrium distance of the heterojunction further demonstrated that it belongs to the typical VDW heterostructure (Fig. 8a~b). Besides, the binding energy confirmed that the 2D/2D HfS2/g-C3N4 VDW heterostructure exhibited excellent stability. According to the total density of states (TDOs) and partial density of states (PDOs) (Fig. 8c), the CB of the heterojunction was composed of Hf 5d and S 3p orbitals located in the HfS2 layer. Whereas the VB was comprised of the C 2p and N 2p states that originated from other layers [110]. Because of the existence of a transitional orbit from N 2p to Hf 5d, the HfS2/g-C3N4 nanocomposite could make full use of the visible-light, it was consistent with the redshift of the optical absorption edge of the heterojunction (Fig. 8d). The enhancement of photocatalytic performance can also

be attributed to the efficient transport of the photoinduced carriers. Therefore, the charge density differences at the 2D/2D interface were described in detail (Fig. 8e). Many electrons originating from the built-in electric field redistributed at the interface and promoted the separation of photoinduced electron-hole pairs. It is easy to find that the atoms of Hf, B, and C were tended to deplete electrons according to the Bader charge analysis, while the atoms of S, N were inclined to aggregate electrons. Therefore, plenty of charges accumulated on the lower layer of the interface, and the charge depletion occurred in the upper layer. Based on the above mentioned, which indicated that the HfS2/g-C3N4 composite photocatalysts can enhance the photocatalytic performance by facilitating the efficient separation of the electron-hole pairs in the 2D/2D heterojunction interface. 3.3 Hybridization with 2D metal oxides A majority of 2D metal oxides exhibit exotic electronic properties and large specific surface areas, which is generally derived from the layer materials. Coupling the 2D metal oxides with g-C3N4 can fabricate promising composite photocatalysts. The as-obtained hybrid possesses several merits, including the adjustable bandgap, excellent light absorption, low-cost, and the fast transfer of photoinduced electron-hole pairs. In summary, integrating 2D metal oxides into g-C3N4 nanosheet to construct a 2D/2D heterojunction can effectively improve the photocatalytic performance. Therefore, it is of great significance to study the interface electron transfer mechanism as well as the role of the 2D/2D heterojunction. TiO2 has been preferentially used as a photocatalyst material for constructing g-C3N4 based 2D/2D heterojunction to solve an environmental

problem, because of their strong oxidizing power, low cost, and high chemical stability [111-118]. For example, Li et al. fabricated a series of 2D/2D g-C3N4/TiO2 heterojunction composites via a simple calcination method [119]. Meanwhile, research showed that the difference calcination temperatures always have a great influence on the photocatalytic performance of the composites, which was chiefly attributed to the effect of calcination temperature on the structure of g-C3N4. Similarly, Sridharan et al. prepared a 2D/2D g-C3N4/TiO2 heterojunction composite via a thermal transformation methodology [120], which significantly enhanced the photocatalytic efficiency and performance stability. According to the above experiments, the enhanced photocatalytic performance was largely attributed to the following two aspects: (i) synergistic effect; (ii) strong light absorption. However, the internal mechanism of the 2D/2D composite interface interaction was still unclear. Zhong et al. fabricated a 2D/2D O-g-C3N4/TiO2 heterostructure composite through an in-situ solvothermal synthetic method [121]. The as-obtained composite can remarkably boost the visible-light photocatalytic activity for H2 evolution by 6.1 times compared with the mixture of O-g-C3N4 and TiO2 nanosheet. A series of the electron microscopy data showed the fine 2D/2D interfacial structure of the composite (Fig. 9a), it is further demonstrated that the small ultrathin leaf-shaped TiO2 nanosheet grew on the edge of larger g-C3N4 nanosheet (Fig. 9f). Additionally, according to EDS elemental maps of the composite, it can be seen that the corresponding areas of the C, N, O, Ti elements were clearly at the heterojunction interface (Fig. 9b~e). It further indicated the formation of the 2D/2D O-g-C3N4/TiO2 heterojunction [121]. Moreover, DFT computations suggested a strong affinity

between TiO2 and O-g-C3N4 through N-O-Ti covalent bonding, which drastically enhanced the synergetic effect of the light absorption of O-gC3N4 and high surface area of TiO2. Therefore, it can greatly boost the visible-light photocatalytic activity for H2 generation. Finally, the mechanism for photocatalytic hydrogen evolution on the O-g-C3N4/TiO2 composite was proposed (Fig. 9g). WO3 is considered as a pioneering visible-light-driven nanomaterial because of their distinct properties of photocatalytic and electrochromic [19, 122-125]. Compared with TiO2, WO3 has a narrow optical band gap (2.7 eV), which endows it with excellent energy conversion ability [126, 127]. For instance, Jin et al. fabricated a 2D/2D g-C3N4/WO3 heterojunction composite and it exhibited excellent photocatalytic performance for the degradation of organic pollutants under visible-light illumination [128]. Fu et al. fabricated ultrathin 2D/2D g-C3N4/WO3 heterojunction photocatalysts by electrostatic self-assembly of WO3 layers and g-C3N4 nanosheets. The H2 evolution activity of the composite was greatly enhanced, which was about 1.7 times higher than pure g-C3N4 [129]. Based on the analysis of the zeta potentials of bulk WO3, WO3 nanosheets and g-C3N4 nanosheets (Fig. 10a), the opposite surface potentials led to the strong electrostatic attraction between WO3 and g-C3N4 nanosheets, which was conducive to the interfacial charge transfer. Therefore, stable 2D/2D g-C3N4/WO3 heterojunction was informed via the Coulomb electrostatic interaction (Fig. 10b). In addition, a series of morphological analysis further described the interface characteristics of 2D/2D heterojunction (Fig. 10c-h). It is not difficult to find that the presence of an intimate contact interface between two heterojunction components

which can be ascribed to the strong interfacial electrostatic adherence force. The study demonstrated that the appropriate interfacial contact of the 2D/2D heterojunction composite played an irreplaceable role in promoting the interfacial charge transfer. Besides, ZnO is a fascinating 2D photocatalyst for the photodegradation of organic and inorganic compounds. However, its relatively wide bandgap of 3.3 eV limits their photocatalytic application under the visible near-infrared (NIR) light irradiation. Fortunately, the well-matched band structure and large exciton binding energy determine that the ZnO is particularly suitable for coupling with g-C3N4 to construct 2D/2D heterostructures [66, 130-132]. To date, Liu et al. successfully prepared the 2D/2D g-C3N4/ZnO heterojunction photocatalyst via the depositionprecipitation method. It can significantly improve the properties of visible light photooxidation and photoreduction [133]. Thereafter, they further studied the possible mechanism of g-C3N4 via ZnO modification. The outstanding visible-light photocatalytic performance was derived from the improved electron transportability. Specifically, the accumulated electrons on the ZnO were transferred to the adsorbed O2 on the heterojunction photocatalysts surface to yield ·O2−. Meanwhile, rich holes in the g-C3N4 exhibited strong oxidizing ability, which could directly degrade the RhB. In addition, Kumar et al. synthesized 2D/2D N-ZnO/g-C3N4 heterojunction composite via the method of thermal polymerization combining with the hydrothermal synthesis [134]. According to the TEM and HRTEM images, we could see that the pristine g-C3N4 showed winkled (Fig. 11a~c), thin as well as folded sheets with relatively smooth surfaces. Thus, the N-ZnO exhibited crumpled sheet-like structures (Fig. 11d~f). Otherwise,

the N-ZnO nanosheets were anchored perfectly over the g-C3N4 surface. These images mentioned above distinctly showed the formation of the 2D/2D heterostructure with the intimate face-to-face contact (Fig. 11g~h). In addition, the Energy-dispersive x-ray spectra (EDAX) demonstrated the presence of all the constituent elements in the composite (Fig. 11i), and further confirmed the formation of 2D/2D heterojunction interface. The formed 2D/2D heterojunction possessed strong interfacial interactions and large contact area, which not only provided plentiful reactive sites but also prolonged the lifetime of photoinduced carriers. 3.4 Hybridization with 2D bismuth-based semiconductors The two-dimensional bismuth-based semiconductors are mainly comprised of bismuth oxyhalides and partial other bismuth-based materials. As a novel layered ternary oxide semiconductor, it draws great attention owing to its potential photocatalytic performance and the excellent crystalline structure. The specific layered structure of bismuth oxyhalides, which comprises [Bi2O2]2+ layers sandwiched between two slabs of halogen ions via the van der Waals interaction [135]. It provides adequate space to polarize the relevant atoms and orbitals and subsequently induces the appearance of built-in electric field perpendicular to the [Bi2O2] slabs and halogen anionic. Thereby the excellent visible-light-induced photoactivity was produced [49, 136]. Moreover, other 2D bismuth-based nanomaterials were also regarded as potential photocatalysts owing to their unique electronic properties. However, the current limitations of the 2D Bi-based heterostructure materials were proposed, such as poor

reduction and low separated rate of photoinduced carriers. Hence, it is of great meaning to couple them with g-C3N4 to construct the 2D/2D nano junctions. BiOI exhibits a narrow bandgap (1.78 eV) and long-wavelength-absorption. In addition, it also could perform as a potential sensitizer for the wide bandgap semiconductors. Hu et al. prepared a new 2D/2D g-C3N4/BiOI composite through a simple ultrasonication strategy [137]. According to the SEM images (Fig. 12a~b), the surface of the BiOI was uniformly covered by g-C3N4 to fabricate the 2D/2D composite photocatalyst. The as-prepared 2D/2D g-C3N4/BiOI hybrid photocatalysts exhibited a wide light-harvesting capability in the visible region, so the utilization of solar energy was greatly improved. Besides, Dai et al. detailed analysis of charge transfer, interface interaction as well as band offsets by DFT calculations. The calculation results demonstrated that the interaction between the two heterojunction components was really weak, thus further illustrated that the g-C3N4/BiOI was VDW heterostructure (Fig. 12c) [138]. Furthermore, a novel 2D/2D BiOI/g-C3N4 heterojunction photocatalyst was synthesized by Jiang and co-workers [139]. The coupling of n-type g-C3N4 and the p-type BiOI nanomaterial was regarded as an alternative approach, which can improve the visible light adsorption capability and the photocatalytic activity under visible-light illumination. The 2D/2D heterojunction with a tight contact interface and the well-aligned straddling band-structure was conducive to the effective transport of the photoinduced charge carriers. Therefore, the excellent photocatalytic performance was obtained. A schematic illustrated the bandgap matching of

BiOI and g-C3N4 before contact and the charge transfer mechanism of g-C3N4/BiOI heterojunction after contact (Fig. 12d). The photoinduced electrons could be transferred from the g-C3N4 to the BiOI, while the excited holes could migrate from the BiOI to the g-C3N4. Thus, the effective transfer of the photoinduced electron-hole pairs was achieved, and the optimum photocatalytic activity of the composite was nearly 4.2 and 5.3 folds over the pristine g-C3N4 and BiOI, respectively. Bismuth oxybromide (BiOBr) is a significant V-VI-VII ternary compound with crystal in the tetragonal matlockite structure [140]. Recently, because of their appropriate bandgap (2.75 eV) and the unique optical properties, BiOBr based 2D/2D heterojunction draws extensive attention and can be regarded as a reliable candidate for photocatalysis under visible-light illumination. Yu et al. fabricated BiOBr/g-C3N4 inorganic-organic composite via a one-step chemical bath method [141]. The 2D/2D BiOBr/g-C3N4 heterojunction interacted by facets coupled BiOBr with g-C3N4, which was beneficial to photoinduced charges transfer, and improved the visible-light photocatalytic activity. Besides, Sun et al. constructed a novel 2D/2D BiOBr/g-C3N4 nano junction composite by the growth of BiOBr on the surface of g-C3N4 nanosheet in room temperature [142]. According to the images of TEM (Fig. 13a) and HRTEM (Fig. 13b), the BiOBr/g-C3N4 heterojunction was successfully constructed. The asobtained composite with 2D/2D closely contacted interface, unique optical property, and the well-aligned straddling band structure greatly constrained the recombination of photoinduced electron-hole pairs and enhanced photocatalytic activity toward degradation of organic. In addition,

Liu et al. further studied the reaction mechanism of the g-C3N4/BiOBr composite [143]. Considering the different work functions, interfacial binding energies, and Fermi level at the interface, two possible photocatalytic mechanisms were proposed. It included type-II heterojunction (Fig. 13c) and Z-scheme mechanism (Fig. 13d), in which the photoinduced carriers possessed different migrate directions [144, 145]. Therefore, the efficient transfer of electron-hole pairs on the 2D/2D heterojunction interface was achieved, and the recombination rate was greatly reduced. Furthermore, Ge et al. synthesized a new visible-light-induced 2D/2D g-C3N4/Bi2WO6 heterostructure composite through mixing and heating methods [146]. The composite photocatalyst exhibited outstanding photocatalytic performance and excellent stability in methyl orange degradation under the condition of visible-light illumination. The performance enhancement was derived from the improved visible-light utilization efficiency and the accelerated transmission of photoinduced electron-hole pairs in the 2D/2D heterojunction intimate interface, whereas all of them could be rationally attributed to the well-aligned overlapping band-structure of the 2D heterojunction component. Subsequently, the atomic scale 2D/2D gC3N4/Bi2WO6 heterojunction composite was constructed by Wang and co-workers [3]. The TEM (Fig. 13e) and HR-TEM (Fig. 13f) images of the g-C3N4/Bi2WO6 indicated that an intimate interface was formed in the ultrathin heterojunction between ug-C3N4 and m-Bi2WO6. According to the effective charge transfer in the heterojunction interface, the formation of the 2D/2D ultrathin heterojunction was the major factor to improve the photocatalytic activity of the composite. Finally, the charge separation process of the g-C3N4/Bi2WO6 heterojunction was proposed (Fig. 13g~h).

This study supplied a new insight and clearer mechanism for the application of the atomic scale 2D/2D heterojunction photocatalysts. Sun et al. chose BiVO4 as a potential candidate material to combine with g-C3N4 and successfully constructed a 2D/2D BiVO4/g-C3N4 heterostructure nanocomposite with interfacial interaction. The as-obtained BiVO4/g-C3N4 hybrid displayed excellent photocatalytic activity and stability for RhB decomposition than pure g-C3N4 and BiVO4 [26]. On the one hand, the enhanced photocatalytic performance can be attributed to the formation of a 2D/2D heterojunction interface in the hybrid. Therefore, the effective separation of electron-holes was achieved. The formation of the 2D/2D heterojunction interface was further verified by the SEM images (Fig. 14a-c). On the other hand, the photocatalytic enhancement mechanism of the 2D/2D heterostructure also could be ascribed to the great adsorption rate, improved heterostructure stability, and the appropriate energy level alignment. Finally, the electron transfer mechanism and the photodegradation process of RhB over the BiVO4/g-C3N4 nanosheet were investigated (Fig.14d). 3.5 Hybridization with 2D phosphorus Phosphorene, an emerging anisotropic two-dimensional layered structure, has attracted tremendous attention in recent years. Because of its unique band structure, thickness-dependent tunable gap energy, excellent electronic properties, and fast charge carrier transport, which endows the promising application of phosphorene in energy conversion/storage [147]. Based on the existing research, many reports were successfully

constructed metal-free 2D/2D phosphorene/g-C3N4 heterojunction [148-154]. For example, Ran et al. firstly constructed 2D/2D phosphorene/gC3N4 composite through a facile self-assembly approach by physical mixing, and the composite displayed outstanding visible-light photocatalytic activity [72]. An investigation of the TEM (Fig. 15a) and HRTEM (Fig. 15b) image for the phosphorene/g-C3N4 composite showed that the gC3N4 was tightly attached to the surface of few-layer phosphorene nanosheets (FP). In addition, the DFT calculations further confirmed the strong electron coupling between the phosphorene and g-C3N4 bonded by the van der Waals force. Researchers considered that the formation of a 2D/2D intimate interface electronic interaction between the phosphorene and g-C3N4 interface was the main reason for the improvement of photocatalytic activity. Accordingly, not only the effective interfacial charge transfer was achieved, but also the properties of g-C3N4 were significantly improved. Subsequently, this hypothesis was affirmed by advanced characterization techniques. The differential charge density of g-C3N4 and phosphorene was calculated [72], it is apparent that the electron density of the phosphorene surface was extracted from the adjacent g-C3N4 (Fig. 15c). On the other hand, the research indicated the construction of a 2D/2D type I (straddling type) heterojunction between phosphorene and g-C3N4 nanosheet, and the charge transfer mechanism of the 2D/2D interface was proposed (Fig. 15d). To be specific, the photoinduced electrons were transferred from the CB of g-C3N4 to the phosphorene, while the photoinduced holes also transferred from the VB of g-C3N4 to the phosphorene. Therefore, the adjacent phosphorene acted as the electron acceptor to inhibit charge

recombination of the photoinduced carriers and the electrons on the CB of phosphorene were trapped by the interfacial P-N bond for the subsequent H2 generation, while holes in phosphorene were rapidly quenched by methanol. Based on the above discussion, the H2 evolution of 2D/2D phosphorene/g-C3N4 originated from facilitated charge separation and efficient catalytic sites for the surface reaction. Besides, the composite stability experiment showed that the performance of photocatalytic hydrogen production still retained 57.4% of their original photocatalytic activity after a 14 h visible-light illumination, indicating its reasonable photocatalytic stability. Furthermore, Zhang et al. developed a 2D/2D heterojunction of black phosphorus (BP)/g-C3N4 for photocatalytic H2 generation. The efficient charge transfer between BP and g-C3N4 (likely due to formed N-P bonds) and broadened photon absorption (supported both experimentally and theoretically) can contribute to the enhancement of photocatalytic performance [155]. As a result, we can conclude that the introduction of phosphorene into the g-C3N4 structure for constructing the 2D/2D heterojunction was conducive to improve the photocatalytic hydrogen generation activity.

4. Basic principles of g-C3N4 based 2D/2D heterojunction

4.1 Interfacial characteristic and types of 2D/2D heterojunction The construction of 2D/2D heterostructure can be employed as a suitable modification strategy and has received exponentially increased

attention in recent years. Generally, good interfacial bonding is the prerequisites for the successful construction of the 2D/2D interface [156]. Therefore, it is necessary to explore and optimize the interaction forces in order to achieve strong interfacial bonding in the composite. The bonding strength of the interface is affected by various factors, such as the chemical bonds, π-π Conjugation, Coulomb force, or hydrogen bond, etc. [157]. While in the absence of the chemical bonding between the two different semiconductor materials, the non-bond interactions have a great influence on the interface bonding strength, including the electrostatic interaction and the van der Waals forces [158]. For example, the GO and protonated g-C3N4 were mixed to obtain a 2D/2D heterojunction composite through the interaction of π-π stacking and the electrostatic attraction [47]. Based on the previous studies [159, 160], the integration of 2D g-C3N4 with other 2D layer structure to construct the 2D/2D heterojunction system. It provided a tighter physical coupling, thus the contact area between the different 2D materials was increased. Besides that, the as-obtained composite possessed a stronger optoelectronic structure coupling effect, it was conducive to shorten the diffusion distance and improve the transmission rate of the charge carriers on the contact interface. Accordingly, the excellent photocatalytic performance was achieved. Additionally, the previous works have supplied a variety of building blocks and fabrication ways to construct the 2D/2D heterostructures. It can be engineered with different contact interfaces in the directions of vertical and lateral [161-163]. In a vertical direction, two (Fig. 16a) or multiple (Fig. 16b) monolayer sheets of divergent nanomaterials are stacked to form 2D/2D heterojunction with face-to-face interface contact. By

regulating the comparative orientation between those single components, the heterojunction can be designed with atomic precision [164, 165]. In the lateral direction, 2D/2D heterojunction with cross-section interface contact was successfully formed via epitaxial growth strategy [164, 166]. It can not only realize the paralleled contacts but also achieve the patterned contacts (Fig. 16c-d). Different categories of heterojunction contact interface have a different effect on catalytic active sites and photocatalytic performance. 4.2 Charge transfer path 4.2.1 Co-catalyst Each photocatalytic reaction generally required to meet three fundamental requirements, including the light adsorption and separation of charge carriers, and a catalytic surface reaction [36]. Previous reports suggested that there are many effective methods to optimize the light adsorption and surface area among the photocatalysis [167], but few breakthroughs have been made in the charge transfer of the co-catalysts. The loaded co-catalysts usually have two major functions, the one is regarded as an electron sink, thereby realizing the effectively separate of the photoinduced charge carriers. Another function is considered as the reduction or oxidation active sites and then decreases the overpotential as well as the charge transfer resistance [40]. Molybdenum disulfide has been regarded as promising candidates to replace Pt for HER in recent years. Their unique structural and electronic properties allow them to have many opportunities to be designed as highly efficient co-catalysts over various

photo harvesting semiconductors [168]. For example, the MoS2/g-C3N4 hybrid photocatalyst was proposed by Zhao and co-workers. The photoinduced electrons in CB of g-C3N4 can be easily transferred to the MoS2. The unsaturated active S atoms existed on exposed edges of MoS2 and had strong bonds to H+ in the solution. The trapped electrons at MoS2 can easily reduce the bonded H+, resulting in H2 generation [169]. In another study, Yu et al. also indicated that the amorphous MoSx nanoparticles loaded on the g-C3N4 surface by the method of adsorption-in situ transformation. The amorphous MoSx nanoparticles can provide more unsaturated active S atoms as the efficient active sites to rapidly capture protons from solution, and then promote the direct reduction of H+ to H2 by photogenerated electrons. The result suggested the superior H2generation activity, and the content of co-catalyst could greatly affect the photocatalytic performance [86]. In a word, as a co-catalyst, the amorphous MoSx provided substantial unsaturated S atoms, which can work as the effective active sites, thereby greatly enhancing the photocatalytic activity of H2 generation. 4.2.2 Traditional type-II heterojunction Conventional g-C3N4-based type-II heterojunction system extremely facilitates the separation of electron-hole pairs, which is mainly ascribed to the band structure. Among them, both the CB and the VB position of g-C3N4 are higher or lower than another 2D semiconductor (Fig. 16e) [36]. When the nanomaterial is irradiated by photons with energy over than or equal to the bandgaps of the two semiconductors, both components of

the heterojunction can be excited simultaneously. Subsequently, the difference of chemical potential between g-C3N4 and another heterojunction component gives rise to band bending at the interface of the heterojunctions. Thereby inducing the generation of an inherent electronic field can trigger the adverse direction migration of photoinduced electron-hole pairs. In another case, the semiconductor can act as an electron/hole collector when the photon energy can only irritate one semiconductor. The different spatial aggregation of charge carriers can achieve in both situations. Thus, the type-II heterojunction extremely improves the separation efficiency of the photoinduced charge carriers across the interface. For instance, Zhang et al. constructed a 2D/2D isotype type-II heterojunction between two different crystal phases of the g-C3N4 substance. The separation rate of photoinduced charge carriers was greatly improved [170]. 4.2.3 All-solid-state Z-scheme heterojunction The existing studies showed that the 2D/2D type-II heterojunction exhibited an intrinsic deficiency. It is strenuous to realize remarkable charge-separation efficiency and the powerful redox capability simultaneously. Inspired by the photosynthesis process of green plants [25, 171], all-solid-state-Z-scheme heterojunction (2D/2D) was proposed. It is quite different from the traditional type-II heterojunction in the photogenerated carriers transfer mechanism between different heterojunction components. Besides, the holes and electrons with more efficient redox ability can be retained in the g-C3N4-based direct Z-scheme system, resulting in greatly enhanced photocatalytic efficiency compared with the traditional

type-II heterojunction. Furthermore, according to the existence or absence of the electron mediators, the all-solid-state Z-scheme heterojunction can be classified into two types [31, 36]: g-C3N4-based direct Z-scheme systems (Fig. 16f) as well as g-C3N4-based Z-scheme systems with a conductor (Fig. 16g). Generally, the former, the photoinduced electrons directly transfer from the 2D semiconductor with less negative CB to recombine with the holes in another 2D semiconductor via the contact interface. For example, Wang et al. successfully fabricated a series of 2D/2D BiOI/g-C3N4 composite via a facile deposition strategy [84]. The photocatalytic activity was increased obviously in comparison with the mechanically mixed samples. The shift of the binding energy in the XPS spectrum indicated that there was intense interaction between the intimately contacted phase of the g-C3N4 and BiOI. While the latter, the conductor material can be regarded as the charge transmission bridge, which further formed the known ohmic contact, and showed a low contact resistance between two 2D semiconductors. For instance, Chen et al. successfully prepared the WO3/Ag/g-C3N4 ternary composite through the method both of solvent evaporation and in situ calcination to further improve the photocatalytic performance [172]. Furthermore, a more in-depth research was proposed by Liu and co-workers. a Z-scheme 2D/2D hybrid photocatalyst was successfully fabricated by post-annealing atomically pt-g-C3N4 and hydrogen-treated WO3. It can not only maintain the redox capability of the heterojunction components but also reduce the recombination of photoinduced carriers, leading to excellent photoactivity for H2 generation [173]. Therefore, the examples mentioned above showed that the effective separation of photoinduced electron-hole pairs was

achieved in the Z-scheme heterojunction system. Meanwhile, a higher redox capability can also be retained in the photocatalytic reaction [31, 174]. 4.3 Mechanism insight of 2D/2D heterojunction In addition to the impact of compositions as well as dimensionality of 2D nanomaterials, the intimate integrated interface continuously plays a critical role in enhancing the photocatalytic activity of 2D/2D heterojunction. It mainly through charge carriers delocalization induced, thereby altering its physicochemical properties and functions [163, 164, 175]. The photocatalytic performance improvement mechanism of the 2D/2D heterojunction composite can be divided into the following three parts. (i) superfine nanostructure as well as interfacial defects to create new activity centers; (ii) the band can be altered to enhance the redox capability of the active centers; (iii) rapid electronic transmission route at the 2D/2D heterojunction interface. Existing researches show that engineering structures is an effective way to improve the photocatalytic activity and generate new active sites, such as modulating the size, surface area, crystal facet, porosity, and defects [15, 47, 176]. Therefore, on the one hand, the appropriate heterojunction is designed by the interfacial effects of 2D/2D heterostructure, which provides many active sites for photocatalytic reactions. On the other hand, considering the existence of the electrostatic field as well as the bending of bands, the rectified contact of the heterojunction

interface exerts a significant influence on the redox capability of heterojunction components. Furthermore, besides the direct influence of structure or defects on the reaction active centers, the transportable and separation of the photoinduced charges also play a significant influence on the photocatalytic performance. Hence, the formation of the 2D/2D interface with strong interaction, is a practicable method to promote the rapid charge transfer, thereby enhancing the catalytic performance in the photocatalytic system.

5. Applications

g-C3N4-based 2D/2D hybrid photocatalysts have been applied in the realm of energy conversion as well as environmental remediation, including hydrogen generation, carbon dioxide reduction, pollutant degradation, and photocatalytic disinfection. The following section provides a brief overview of the applications of g-C3N4 based 2D/2D composite photocatalysts. 5.1 Hydrogen generation As is known to all, hydrogen energy is considered as promising renewable energy in the future, due to its properties of high-energy-density, environmental benignancy as well as recycling utilization [49, 177]. Wang et al. firstly applied g-C3N4 in photocatalytic hydrogen production under visible-light illumination in the existence of a sacrificial agent [178], which opened up a new way for the production of hydrogen. While the

photocatalytic activity for the H2 yield of the pristine g-C3N4 is quite low. In order to enhance hydrogen production, a series of g-C3N4-based 2D/2D heterojunction photocatalysts were proposed. The H2 yields of different 2D/2D heterojunction photocatalysts are summarized in Table 1. According to the basic principle of photocatalytic overall water splitting, the excited electrons can induce the H+ reduction to produce H2, while the holes can lead the H2O oxidation to form O2 [22, 179, 180]. Recently, Liu et al. prepared 2D/2D SnS2/g-C3N4 composite via heating the uniform dispersion of g-C3N4 and SnS2 by a microwave muffle furnace [75]. A series of SEM and HR-TEM investigations revealed that the SnS2 was successfully deposited onto the surface of g-C3N4 nanosheet (Fig. 17a~g). The obtained 2D/2D heterojunction composite showed the outstanding photocatalytic activity for H2 generation. The photocatalytic H2 yield of the optimized SnS2/g-C3N4 composite (972.6 μmol h-1 g-1) was 2.9-folds and 25.6-folds over the pristine g-C3N4 and SnS2, respectively. The reusability experiments also indicated that hybrid possessed excellent stability in photocatalytic hydrogen production, and the slight decline may be due to the loss of the sample by centrifugation. Besides, according to the electrochemical impedance spectroscopy (EIS) spectra as well as the UV-vis diffuse reflection absorption of the dissimilar samples. It is further demonstrated that the improved photocatalytic activity was chiefly attributed to the effective charge transport of the 2D/2D heterojunction interface. Finally, a synergistic mechanism for photocatalytic reduction of H2 over g-C3N4/SnS2 heterojunction was put forward as shown in Fig. 17h, which was consistent with the aforementioned experimental results elaborated. Furthermore, Lin et al. developed a novel 2D/2D

g-C3N4/ZnIn2S4 composite via a simple one-step surfactant-assisted solvothermal method for photocatalytic H2 evolution [69]. The type-I binary heterojunction interfaces were formed, thus providing the prerequisite for the generation of the 2D/2D heterojunction with high-speed charge transfer nanochannels. The photoinduced electrons and holes of g-C3N4 all transferred to the ZnIn2S4 nanosheet via the 2D/2D heterojunction interface. Then the accumulated electrons on the CB of ZnIn2S4 can reduce the hydrogen ions in aqueous solution to generate hydrogen, while these holes on the VB of ZnIn2S4 were quickly quenched by the sacrificial electron donor of TEOA. This process contributed to a high photoinduced charge separation and migration efficiency, ultimately leading to a remarkable visible-light-driven HER activity and almost 69.5, 15.4 times higher than that of pure g-C3N4 nanosheet, ZnIn2S4 nanoleaf, respectively. 5.2 Carbon dioxide reduction Photocatalytic carbon dioxide reduction is an ideal solution, which can not only reduce greenhouse gas emission but also meet the requirement for renewable fuels simultaneously. Therefore, in recent years, many researchers have committed to study different semiconductor composites for the photocatalytic conversion of CO2. The CO2 reduction of the different g-C3N4 based 2D/2D heterojunction composites are summarized in Table 2. CO2 can be converted into HCOOH, CH3OH, CO, CH4, and HCHO during the photocatalytic process. Previous studies showed that photocatalytic carbon dioxide reduction is a multielectron transfer process. The probable reactions processes

are listed below, as well as the corresponding redox potentials [25, 36]. CO2+2H++2e-→HCOOH

E0redox=-0.61V (vs. NHE at pH 7)

CO2+2H++2e-→CO+H2O

E0redox=-0.53V (vs. NHE at pH 7)

CO2+4H++4e-→HCHO+H2O

E0redox=-0.48V (vs. NHE at pH 7)

CO2+6H++6e-→CH3OH+H2O

E0redox=-0.38V (vs. NHE at pH 7)

CO2+8H++8e-→CH4+2H2O

E0redox=-0.24V (vs. NHE at pH 7)

In addition, the adsorption of CO2 by the photocatalyst is an essential step and prerequisites for the CO2 reduction. It is well known that gC3N4 presents a two-dimensional π-conjugated structure on its surface, it can participate in π-π interaction with CO2 molecules due to the carbon dioxide also contains delocalized π-conjugated electrons. Thereby CO2 can be absorbed on the surface of photocatalyst by the π-π interactions [47], which causes the destabilization of CO2 molecules during the photocatalytic reaction. For example, in order to enhance the conversion efficiency of CO2 to fuels under visible-light illumination. He et al. synthesized a 2D/2D ZnO/g-C3N4 composite photocatalyst through a facile impregnation strategy [83]. The cycling experiments indicated that a slight decline of the CO2 conversion rate can be observed in the first three cycling runs, and then the rate changed few. The slight decline in activity was typically ascribed to either the inactivation of some active sites or

the structural change of the catalyst in the photocatalytic reaction. Furthermore, the structure of g-C3N4 (Fig. 18a~b), ZnO (Fig. 18c~d), and gC3N4/ZnO composites were investigated by SEM and TEM, respectively (Fig. 18e~f). The intimate interface between ZnO and g-C3N4 nanosheet was clearly displayed. The ZnO/g-C3N4 heterojunction composites exhibited outstanding photocatalytic CO2 reduction activity, which was higher 4.9 and 6.4 times than pristine g-C3N4 and P25, respectively. This was chiefly attributed to the improvement of CO2 adsorption ability, the enlargement of the special surface area as well as the increased charge separation efficiency, etc. Among them, the formation of a 2D/2D heterojunction intimate interface played a critical role, which achieved the effective separation of photoinduced charge carriers. Meanwhile, the photocatalytic mechanism for CO2 reduction on the ZnO/g-C3N4 composite was illustrated (Fig. 18g). Furthermore, Xu et al. fabricated a 2D/2D Bi4NbO8Cl/g-C3N4 heterojunction photocatalysts via high-energy ball-milling and post-sintering to realize intimate interfacial interaction [190]. It presented enhanced photocatalytic CO2 reduction activity for CO production. The enhancement on photocatalytic activity of Bi4NbO8Cl/g-C3N4 composites was large owing to the synergistic effect of favorable 2D/2D structure and construction of type-II heterojunction with intimate interfacial interaction, thus boosting the charge separation. The aforementioned work demonstrated that photocatalytic efficiency and the sensitivity of CO2 reduction were significantly improved by forming a 2D/2D heterojunction. 5.3 Degradation of organic pollutants

Semiconductor photocatalysis is regarded as an efficient and economical strategy for the elimination of organic pollutants in the environment. Owing to its unique electronic structure, cost-saving, and excellent physicochemical properties, g-C3N4-based 2D/2D heterojunction photocatalysis has been extensively used to photodegrade various environmental pollutants. A more comprehensive list of organic pollutants treated with the 2D/2D composite photocatalyst is summarized in Table 3. g-C3N4 based 2D/2D heterojunction composite exhibits eminent advantages for the degradation of various organic pollutants, which is mainly ascribed to the several reasons listed below. The 2D semiconductor material can be served as the cocatalysts to supply adequate catalytic sites for the photocatalytic degradation. In addition, the 2D semiconductor material with conductivity can act as the efficient electro-transfer channels to improve the transfer of the photoinduced electron-hole pairs. Last but not least, the formation of a 2D/2D heterojunction between two 2D semiconductors with the different Fermi levels, which can modulate the bandgap and result in redshift of the optical absorption edge. Thus, the absorption of visible light is enhanced, and the conversion rate of solar energy is increased. Noteworthy, under visible-light illumination, the electrons (e-) are trapped by O2 to form the ·O2-. While the h+ can directly oxidize the organic contaminants or catch OH- or H2O to form the highly strong, non-selective ·OH [36, 42, 44]. These active species mentioned above (h+, ·O2-, ·OH) can oxidize the contaminants into carbon dioxide, water, inorganic ions and smaller organic molecular [201]. For example, Li et al. successfully fabricated 2D/2D MoS2/g-C3N4 heterojunction

photocatalyst via a facile impregnation and calcination method [202]. The photocatalytic degradation activity and stability were greatly improved. A TEM image of the composite showed that the MoS2 nanosheets were uniformly attached on the surface of the g-C3N4 (Fig. 19a). The enlarged images of the HR-TEM (Fig. 19b~c) indicated the formation of the 2D/2D close interface in the heterojunction, which was beneficial to the charge separation. The as-obtained 2D/2D MoS2/g-C3N4 composite displayed significantly enhanced photocatalytic performance under visible-light illumination and possessed considerable stability. The photocatalytic mechanism of the 2D/2D MoS2/g-C3N4 heterostructure was discussed (Fig. 19d), which effectively separated the photoinduced charge carriers and greatly improved the photocatalytic performance. Furthermore, an ultrathin 2D/2D BiOCl/g-C3N4 nanocomposite was successfully synthesized through the in situ one-pot solvothermal method by Zhang and co-workers [203]. The obtained composite significantly enhanced the photodegradation of RhB dye under visible-light irradiation owing to the synergistic effect in BiOCl/g-C3N4 nanosheet, which had 3.9 and 12.8 times higher photocatalytic efficiency compared to pure BiOCl and g-C3N4, respectively. The synergism reflected in the widened optical window for effective light absorption, enlarged interfacial area for more efficient electron-hole separation, and a shorter diffusion distance for facilitated charge transfer. 5.4 Photocatalytic disinfection g-C3N4 based 2D/2D heterojunction also exhibits great application prospects in photocatalytic disinfection. The latest researches illustrate

that the g-C3N4-based 2D/2D heterojunction composites displayed excellent antibacterial activity under visible-light illumination. For instance, Li et al. successfully fabricated 2D/2D Bi2MoO6/g-C3N4 heterojunction through an in-situ solvothermal strategy. The photocatalytic activity for the bacteria disinfection was greatly improved [191], and the 20%-BM/CNNs exhibited the optimal photocatalytic activity. The enhanced photocatalytic activity was attributed to the increased contact area and the improved charge separation efficiency at the 2D/2D heterojunction interface. Furthermore, the recycling experiment showed that a slight decline of the photocatalytic antibacterial activity could be found after four cycles of repeated experiments. The slight decline may be attributed to the loss of the sample by centrifugalization. Therefore, g-C3N4 based 2D/2D photocatalysts can be considered as a stable photocatalyst. In addition, the mechanism analysis revealed that the h+ was the chief reaction species which could directly inactivate bacterial cells in the photocatalytic disinfection process (Fig. 18h). Since then, this view was verified again. Xia et al. indicated that a novel 2D/2D Z-scheme g-C3N4/m-Bi2O4 heterojunction composite enhanced photocatalytic inactivation of Escherichia coli [224]. Under the optimal conditions of g-C3N4/m-Bi2O4 (1:0.5), E. coli K-12 was fully inactivated within 1.5 h, which was 1.9 and 5 times over the Bi2O4 and g-C3N4, respectively. Furthermore, a more in-depth research was proposed by Sun and co-workers. The nano-composite of GO/gC3N4 was successfully synthesized through a facile sonochemical method, and its antibacterial activity against Escherichia coli (E. coli) was investigated [225]. The photoinduced holes were confirmed to be the major active species for photocatalytic sterilization and might cause the

distortion and rupture of the cell membrane. The analysis further indicated that the introduction of GO not only enhanced light absorption but also largely improved the photoinduced electron-hole separation to generate more holes, thus directly improving the bactericidal ability of GO/g-C3N4. The above examples revealed the exceptional antibacterial activity of the g-C3N4-based 2D/2D composite under visible-light illumination. While of note, research in this area is still in infancy, and the photocatalytic disinfection mechanisms of the various g-C3N4-based 2D/2D heterojunction photocatalysts are needed further investigation.

6. Conclusions and perspectives

In summary, we reviewed various categories of g-C3N4-based 2D/2D heterojunction composites and focused on the description of the interface electron-hole transfer. Numerous studies illustrated that the appropriate band structure is the most important prerequisite for the construction of an efficient 2D/2D heterojunction interface. The integrating 2D g-C3N4 with another 2D nanomaterial creates a large intimate interface. In detail, the chemical potential difference of the coupled 2D semiconductors with another one results in band bending at the interface of the heterojunction. Photoinduced charge carriers migrate to contrary directions by the induced built-in electric field. Furthermore, the formation of a 2D/2D heterostructure can enhance the stability of the photocatalysts either, which is mainly ascribed to the alleviation of the photo-corrosion and

agglomeration. More importantly, the existence of a synergistic effect between the different 2D heterojunction components broadens the absorption region and improves the utilization/conversion rate of sunlight. Last but not least, the crystal structure of the 2D/2D heterojunction composite is also critical to the high quantum efficiency of photocatalyst, which can be explained by the followed theory: (1) the difference of lattice spacing between the two 2D semiconductors is likely to result in a lattice mismatch at the interface; (2) the defects can trap the electronic carriers generated by the light excitation, thereby inhibiting the recombination of charge carriers. Substantial examples about the g-C3N4-based nanostructure indicated that the construction of the 2D/2D close interface is an effective way to significantly improve the photocatalytic performance. Although the promising advances of the g-C3N4 based 2D/2D heterojunction photocatalysts was achieved in recent years. There are still several major challenges waiting to be addressed. (i) On the one hand, the practical application of g-C3N4 based 2D/2D heterojunction system requires massive production of the high-quality photocatalysts g-C3N4. However, conventional synthetic methods are not feasible for mass production. As a result, the effective synthetic route is an urgent needed to be explored to meet the large-scale production of the photocatalysts with high efficiency and stability. On the other hand, how to improve the application of the 2D/2D heterojunction composite in industrialization and commercialization is also a through-provoking question. (ii) Numerous techniques have been proposed to synthesize the 2D/2D nanocomposite, and the different strategies have displayed dissimilar

applications’ scope, advantages, and disadvantages. We should choose the appropriate preparation method according to the characteristics of the nanocomposite. Hitherto, the 2D/2D composite has been successfully obtained, while the formation mechanism of the 2D/2D interface is not thorough enough and the interfacial force of 2D/2D heterojunction is still unclear. Therefore, to further improve the photocatalytic performance, more attention should be paid to the formation of the 2D/2D heterojunction interface and the role of interfaces, which requires more advanced technical support. (iii) At present, most styles of the 2D g-C3N4 have been suffered from the fast recombination of photoinduced charge carriers, which greatly restricts the photocatalytic performance of g-C3N4. Controllable synthesis of nano-sized morphologies is served as one of the most efficient approaches to improve its performance. Therefore, semiconductor nanosheets describe a splendid future owing to their unique characteristics, including extremely thin thickness, high aspect-ratio, large surface area, plentiful surface groups to anchored co-catalyst, as well as porosity generated in the unique structures. 2D g-C3N4 nanosheets can be prepared by various exfoliation strategies, such as thermal oxidation exfoliation, ultrasonic exfoliation, as well as chemical exfoliation, which can significantly improve the photocatalytic performance. (iv) The photocatalytic performance enhancement mechanism of the 2D/2D heterojunction interface remains to be explored. A partial explanation concerning the improved performance of 2D/2D photocatalysts was given, while others are still open to discussion. In previous

researches, the enhancement mechanism is almost the same, that is, the effective separation of the photoinduced carriers. Although this is correct, it is not in-depth and specific enough. Some physical and theoretically calculated technologies are encouraged to be used in this field. The insight mechanism of g-C3N4-based photocatalysts can be explored through theoretical calculations and model simulations, which can further guide us to rationally design efficient photocatalysts. (v) Furthermore, besides the widely accepted charge carriers transfer mechanism, the thermodynamics and kinetics of surface catalytic reactions, and the interactive nature between different heterojunction components are also needed more comprehensive investigations. Some advanced characterization techniques may be very useful tools for this purpose, which can greatly deepen our understanding on the structureactivity and interface coupling of the g-C3N4-based photocatalysts. For example, operand TEM, synchrotron-based X-ray absorption near edge structure (XANES) and transient-state surface photovoltage (SPV) spectroscopy. These state-of-art characterization approaches can be used to disclose the interactive nature between heterojunction components. (vi) Several 2D nanomaterials have been integrated to construct 2D/2D VDW heterojunctions for energy conversion in recent years, because of the segregated 2D material that can be stacked into different VDW heterostructures without considering lattice matching. However, VDW heterojunction possesses the complex, less constrained, as well as the more vulnerable interface, which is vastly different from the traditional and

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Fig. 1 (a) Electron configuration of aggregate melon. (b) S-triazine (upper) and tris-s-triazine (lower) as tectons of g-C3N4. Reprinted with permission from ref. [46] Copyright 2012, Wiley-VCH. (c) Schematic illustration of interface contacts between nanomaterials with different dimensions. Reproduced with permission from ref. [25] Copyright 2014, Royal Society of Chemistry. (d) Schematic structures of bulk heterojunction and 2D/2D heterojunction with coupled interface. Reprinted with permission from ref. [27] Copyright 2019, WILEY-VCH.

Fig. 2 Schematic illustration of the fabrication process of O-g-C3N4/B-rGO by ultrasonic assisted electrostatic self-assembly. Reprinted with permission from ref. [73] Copyright 2018 The Royal Society of Chemistry.

Fig. 3 Current 2D library. Monolayers proved to be stable under ambient conditions (room temperature in air) are shaded blue; those probably stable in the air are shaded green, and those unstable in the air but that may be stable in an inert atmosphere are shaded pink. Grey shading indicates 3D compounds that have been successfully exfoliated down to monolayers. Reprinted with permission from ref. [87] Copyright 2013 Macmillan Publishers.

Fig. 4 Optimized geometry for a graphene (a) and graphitic carbon nitride (g-C3N4) monolayer (b). Band dispersions calculated by the HSE06 functional for graphene (c) and g-C3N4 (d) Green and blue spheres represent the C and N atoms, respectively. Optimized graphene/g-C3N4 interface in an (e) top view and (f) side view. Note, the band gap opening. White, green, and blue balls represent C atoms on graphene and C and N atoms on a g-C3N4 monolayer, respectively. Reprinted with permission from ref. [95] Copyright 2012 American Chemical Society. (g) A TEM image of the graphene/g-C3N4 composite photocatalysts interface. Reprinted with permission from ref. [93] Copyright 2015 The Royal Society of Chemistry; (h) Charge transfer at the graphene/g-C3N4 interface: top and side view of the three-dimensional charge density difference plots. Green and blue balls represent C and N atoms, respectively. Yellow and light blue isosurfaces represent, respectively, charge accumulation and depletion in the space with respect to isolated graphene and g-C3N4. Note the triangular-shaped electron-hole puddle. Reprinted with permission from ref. [95] Copyright 2012 American Chemical Society.

Fig. 5 TEM images of (A) Bulk g-C3N4 (white dotted circles indicate pores) and (B) Exfoliated protonated g-C3N4. (C) TEM, (D) High resolution transmission electronmicroscopy (HRTEM) and (E) FESEM image of 15rGO/ protonated g-C3N4 nanocomposites. (F) A TEM image of 15rGO/g-C3N4 nanocomposites. Blue and red boundaries denote g-C3N4 and rGO, respectively. Insets of (B) and (C) show the SAED patterns of protonated g-C3N4 and 15rGO/ protonated g-C3N4 samples. Inset of (E) shows the enlarged FESEM image corresponding to the green rectangle from the image shown in panel (E). Reprinted with permission from ref. [99] Copyright 2015 Elsevier.

Fig. 6 Geometries and elemental analysis of free-standing MoS2/g-C3N4 vdW nanosheets. (a) The Z-contrast high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imaging of a single MoS2/g-C3N4 nanosheet. (b) HRTEM imaging of the as-prepared MoS2 layers. Reprinted with permission from ref. [67] Copyright 2016 Elsevier. (c) Geometric structures and differential charge density of g-C3N4/MoS2 heterojunction. The gray, blue, yellow and blue balls denote carbon, nitrogen, sulfur and molybdenum atoms, respectively. The red and blue regions indicate electron increase and decrease, respectively. (d) Absorption coefficients of g-C3N4 monolayer and a g-C3N4/MoS2 heterojunction. (e) The density of states of g-C3N4/MoS2 heterojunction, g-C3N4, and MoS2 monolayers. The Fermi level is set to zero and marked by green dotted lines. (f) Schematic illustration of carrier transfer and separation in g-C3N4/MoS2 heterojunction. Reprinted with permission from ref. [107] Copyright 2014 The Royal Society of Chemistry.

Fig. 7 Electronic and optical properties of the g-C3N4/ZrS2 heterostructure. (a) Calculated band structure with respect to the vacuum level for the g-C3N4/ZrS2 heterostructure. The position of the reduction level for H+ to H2 is indicated by the red line and the oxidation potential of H2O to O2 is the blue line. (b) Calculated local DOS and partial DOS of the g-C3N4/ZrS2 heterostructure. (c) The charge densities of the VB (blue) and CB (red) for g-C3N4/ZrS2. (d) Calculated light absorption

properties of the g-C3N4 monolayer, g-C3N4/ZrS2 heterostructure and g-C3N4/ZrS2 heterostructure by applying 3% strain. Reprinted with permission from ref. [109] Copyright 2016 The Royal Society of Chemistry.

Fig. 8 The optimized geometry of the HfS2/g-C3N4 vdW heterojunctions. The pink, blue, gray, yellow and sky-blue balls indicate B, N, C, S and Hf atoms, respectively. (a) top view of the heterojunctions, (b) side view of the heterojunctions. (c) TDOS and PDOS for HfS2/g-C3N4. (d) Optical properties of HfS2/g-C3N4 heterojunctions.

(e) 3D charge density differences in the HfS2/g-C3N4 heterojunction. The yellow and sea-green isosurfaces represent charge aggregation and depletion, respectively. Reprinted with permission from ref. [110] Copyright 2018 The Royal Society of Chemistry.

Fig. 9 (a) Representative HAADF-STEM image of O-g-C3N4/TiO2 and (b) C, (c) N, (d) O, and (e) Ti EDS elemental maps of the corresponding area. (f) Superposed N, Ti and HAADF maps showing the interface regions and TiO2 leaves. (g) The mechanism for photocatalytic hydrogen evolution on O-g-C3N4/TiO2 composite under visible-light irradiation. Reprinted with permission from ref. [121] Copyright 2018 Elsevier.

Fig. 10 (a) Zeta potentials of bulk WO3, WO3 nanosheets and g-C3N4 at pH=4. (b) The formation schematic diagram of 2D/2D WO3/g-C3N4 heterojunctions by Coulomb electrostatic interaction. TEM and HRTEM images of (a,b) WO3 nanosheets, (c,d) g-C3N4 nanosheets and (e,f) WO3/g-C3N4 samples. Reprinted with permission from ref. [129] Copyright 2018 Elsevier.

Fig. 11 TEM and HRTEM images of (a, b, c) g-C3N4 nanosheets, (d, e, f) N-ZnO nanosheets, (g, h) N-ZnO/g-C3N4 heterojunction composite and (i) EDAX of 2D/2D N-ZnO/g-C3N4 heterojunction. Reprinted with permission from ref. [134] Copyright 2017 Elsevier.

Fig. 12 SEM images of (a) and (b) g-C3N4/BiOI; Reprinted with permission from ref. [137] Copyright 2018 The Royal Society of Chemistry. (c) The structure of the g-C3N4/BiOI heterostructure. Reprinted with permission from ref. [138] Copyright 2017 Elsevier. (d) Diagram of the band energy of BiOI and g-C3N4 before contact and formation of a p–n heterojunction and the proposed charge separation process of BiOI/g-C3N4 heterostructures under visible-light illumination. Reprinted with permission from ref. [139] Copyright 2013 The Royal Society of Chemistry.

Fig. 13 (a) TEM (b) HRTEM images of the as-synthesized BiOBr/g-C3N4 nanojunctions. Reprinted with permission from ref [142] Copyright 2013 Elsevier. Schematic illustration of two possible mechanisms for charge-transfer and photocatalysis in BiOBr coupled with g-C3N4 under visible-light. (c) Type II heterojunction (d) Zscheme system. Reprinted with permission from ref. [143] Copyright 2017 Elsevier. (e) TEM and (f) HR-TEM images of the g-C3N4/Bi2WO6 heterojunction. (g) The LUMO (top) and HOMO (bottom) states of the monolayer Bi2WO6 nanosheets; (h) Photocatalytic mechanism scheme of g-C3N4/Bi2WO6 heterojunctions under visible light irradiation (>420 nm). Reprinted with permission from ref. [3] Copyright 2017 Elsevier.

Fig. 14 SEM images of as-prepared (a) BiVO4, (b) g-C3N4, and (c) BiVO4/g-C3N4. (d) Schematic of the mechanism of electron transfer and the photodegradation process of RhB over the BiVO4/g-C3N4 Nanosheet. Reprinted with permission from ref. [26] Copyright 2018 Elsevier.

Fig. 15 A typical (a) TEM image, (b) HRTEM image of phosphorene/g-C3N4 heterojunction. (c) Side-view differential charge density map of g-C3N4 and

phosphorene. The yellow and blue regions represent net electron accumulation and depletion, respectively. (d) The charge separation and transfer in the FP/g-C3N4 system under visible-light illum ination (λ > 400 nm). Reprinted with permission from ref. [72] Copyright 2018 WILEY-VCH.

Fig. 16 (a–d) Schematic illustration of typical 2D/2D heterostructures with different contact interfaces. Reprinted with permission from ref. [27] Copyright 2019, Wiley-VCH. Schematic illustration of different semiconductor heterojunctions for the photocatalytic reaction: (e) Traditional type II heterojunction; (f) All solid-state S-S Z-scheme heterojunction; (g) All-solid-state S–C–S Z-scheme heterojunction. S I, S II, A, and D stand for semiconductor I, semiconductor II, electron acceptor, and an electron donor, respectively. Reprinted with permission from ref. [36] Copyright 2014, Wiley-VCH.

Fig. 17 TEM images of (a) g-C3N4, (b) and (c) g-C3N4/SnS2; HR-TEM image of (d) g-C3N4/SnS2; (e-g) are magnified views of corresponding areas in Fig. 18d; (h) The synergistic mechanism for photocatalytic reduction of H2 over SnS2/g-C3N4 heterojunction. Reprinted with permission from ref. [75] Copyright 2018 Elsevier.

Fig. 18 SEM and TEM images of (a, b) g-C3N4, (c, d) ZnO, and (e, f) ZnO/g-C3N4 heterojunction composite. (g) Schematic illustration of the photocatalytic mechanism for CO2 reduction on the ZnO/g-C3N4 heterojunction composite. Reprinted with permission from ref. [83] Copyright 2014 Elsevier. (h) Schematic diagram of the photocatalytic mechanism of the Bi2MoO6/g-C3N4 heterojunction composites. Reprinted with permission from ref. [191] Copyright 2016 Elsevier.

Fig. 19 (a) TEM image of MoS2/g-C3N4 heterojunction. (b, c) HR-TEM image of MoS2/g-C3N4 heterojunction. (d) Photocatalytic mechanism of the 2D/2D MoS2/gC3N4 heterostructures. Reprinted with permission from ref. [202] Copyright 2014 American Chemical Society.

Table 1 The H2 yields of different 2D/2D heterojunction photocatalysts photocatalyst

synthesis method

Types of heterojunctions

light source

H2 evolution rate

times

ref

g-C3N4/Ca2Nb2TaO10 g-C3N4/ZnIn2S4 g-C3N4/WO3 g-C3N4/MoS2 Cu-MoO3/g-C3N4 Au-C3N4-MoS2 SnS2/g-C3N4 graphene/g-C3N4

Exfoliation reassembly In-situ growing Partially embedding Hydrothermal Impregnation Ultrasonic chemical Microwave muffle heating Impregnation chemical

Type-II Type-I Z-scheme VDW Z-scheme Type-II Type-II Type-II

300 W Xe lamp 300 W Xe lamp 300 W Xe lamp No date 150 W Xen lamp 300 W Xe lamp 300 W Xe lamp 350 W Xe lamp

43.54 μmol h−1 g−1 2.78 μmol h−1 g−1 3.12 μmol h−1 g−1 No date 652 μmol h−1 g−1 52.5 μmol h−1 g−1 972.6 μmol h−1 g−1 451 μmol h−1 g−1

2.8 69.5 7 / / 2.08 2.9 3.07

[181] [69] [80] [182] [183] [184] [75] [94]

g-C3N4/MoS2 g-C3N4/rGO g-C3N4/ZnCr LDH g-C3N4/TiO2 g-C3N4/WO3 g-C3N4/TiO2 g-C3N4/CdS g-C3N4/MoS2

Pyrolysis Dissolution-precipitation Self-assembly Impregnation method Hydrothermal Ball-milling/Calcination No date Impregnation method

VDW Type-II Type-II Type-II Z-scheme Type-II Type-Ⅱ Type-II

350 W Xe lamp 300 W Xe lamp 150 W Al-Ka X-ray source 500 W Xe lamp 300 W Xe lamp 450 W mercurylamp No date Xe arc lamp

No date 715 μmol h−1 g−1 186.97 μmol h−1 g−1 52.71 μmol h−1 g−1 1853 μmol h−1 g−1 22.4 μmol h−1 g−1 No date 23.1 μmol h−1 g−1

8.7 13 2.8 5 6.5 2 / 11.3

[185] [186] [187] [117] [122] [188] [189] [105]

Table 2 The CO2 reduction of the different g-C3N4 based 2D/2D heterojunction composite photocatalyst

charge-transfer mechanism

fabrication strategy

light source

g-C3N4/CeO2

Hard-template route

Xe lamp

g-C3N4/rGO

Self-assembly

15 W daylight lamp

Co-catalyst

g-C3N4/SBA-15

Vapor condensation

/

/

g-C3N4/ZnO

Impregnation method

500 W Xe lamp

Type-II

g-C3N4/ZnO

One-step calcination

300 W Xe lamp

Z-scheme

g-C3N4/Bi2WO6

In-situ hydrothermal

300 W Xe lamp

Z-scheme

g-C3N4/NiAl-LDH

In-situ hydrothermal

300 W Xe lamp

Type-II

g-C3N4/MnO2

In-situ redox

300 W Xe lamp

Z-scheme

g-C3N4/SnS2

One-step hydrothermal

300 W Xe light

Z-scheme

/

Co-catalyst

300 W Xe lamp

Type-II

g-C3N4/graphene g-C3N4/N-TiO2

Impregnation-thermal reduction In-situ synthesis

Type-II

production rate (μmol h-1 g-1)

The maxium CO and CH4 yields reaching 0.590 and 0.694 μmol h-1 g-1 The highest CH4 evolution rate reaching the 13.93 μmol h-1 g-1 / The optimal CO2 conversion rate is 45.6 μmol h-1 g-1 The highest CH3OH production rate is 0.6 μmol h-1 g-1 The optimized CO production rate is 5.19 μmol h-1 g-1 The highest CO evolution rate reaching the 8.2 μmol h-1 g-1 The highest CO production rate reaching the 9.6 μmol h-1 g-1 The highest CH3OH yield reaching the 2.3 μmol h-1 g-1 The highest CH4 production rate reaching the 5.87 μmol h-1 g-1 The highest CO production rate reaching the 14.73 μmol h-1 g-1

ref [192] [99] [193] [83] [194] [195] [196] [197] [198] [93] [199]

g-C3N4/BiOI

Simple deposition approach

300 W Xe lamp

Z-scheme

The maximal yield rates of CO, CH4, and O2 attained 4.86, 0.18 and 2.78 μmol h-1 g-1

[200]

Table 3 Treatment of the various pollutants by g-C3N4 based 2D/2D heterojunction photocatalysts preparation of g-C3N4 (precursor)

charge-transfer mechanism

photocatalytic system

fabrication strategy

light source

g-C3N4/N-KTiNbO5 g-C3N4/Fe2O3

One-step calcination Thermal heat treatment

A 300 W Xe lamp A 500 W Xe lamp

Melamine Melamine

Type-II Co-catalyst

RhB/BPA RhB

[204] [205]

p-g-C3N4/GO

Self-assembly

A 500W Xe lamp

Urea

Co-catalyst

RhB/CIP

[206]

g-C3N4/Bi3O4Cl

Solid phase calcination

A 250 W Xe lamp

Urea

Z-scheme

Antibiotic

[207]

g-C3N4/ZnO

Ball Milling

A 500W Xe lamp

Melamine

Co-catalyst

RhB

[208]

g-C3N4/BN

Hydrothermal

A 300 W Xe lamp

Dicyandiamide

Z-scheme

RhB

[209]

g-C3N4/WO3

Calcination

A 300 W Xe lamp

Dicyandiamide

Type-II

MB/4-CP

[19]

g-C3N4/rGO

Thermal heat treatment

A 350 W Xe Lamp

Melamine

Co-catalyst

RhB

[210]

g-C3N4/g-C3N4

Bottom-up approach

A 50 W fluorescent lamp

RhB

[211]

g-C3N4/Bi4O5I2

Mixed calcination

Visible light (λ > 420 nm)

Melamine

Type-II

RhB/NO

[212]

g-C3N4/CeO2

Mixing-calcination

A 300 W Xe lamp

Dicyandiamide

Type-II

MB/4-CP

[213]

g-C3N4/Bi2O2CO3

Mixed calcination

A 500 W Xe lamp

Melamine

Type-II

RhB

[214]

g-C3N4/rGO g-C3N4/SnS2 g-C3N4/BiOCl

Thermal heat treatment Ultrasonic dispersion Solvothermal

A 1000 W Xe lamp A 300 W Xe lamp A 300 W Xe lamp

Cyanamide Urea Urea

/ Type-II Type-II

RhB/4-nitrophenol RhB 4-CP

[92] [215] [68]

g-C3N4/MnO2

Wet-chemical

/

Urea

Z-scheme

Phenol

[74]

g-C3N4/rGO

Thermal condensation

A metal halide lamp

Melamine

/

MB/Phenol

[82]

g-C3N4/TiO2

Hydrolysis approach

A Xe short arc lamp

Melamine

Type-II

Phenol

[216]

g-C3N4/TiO2 g-C3N4/BiOI

Thermal heat treatment EG-assisted solvothermal

A 50 W Halogen lamp A 300 W Xe lamp

Melamine Urea

Type-II Type-II

MB RhB/MB/MO

[111] [217]

Melamine/thiourea

Z-scheme

Applications

ref

g-C3N4/SnO2

Two-step process

A 300 W Xe lamp

Melamine

Co-catalyst

MO

[218]

g-C3N4/BiOCl

Solvent-thermal

A 300 W Xe lamp

Melamine

Co-catalyst

MO

[219]

g-C3N4/MoS2

Impregnation and calcination

A 300 W Xe lamp

Dicyandiamide

Type-II

RhB/MO

[220]

g-C3N4/rGO

Ball Milling

A 300 W Xe lamp

Melamine

Co-catalyst

OA

[221]

g-C3N4/TiO2

Calcination

A 300 W Xe lamp

Melamine

Z-scheme

propylene

[119]

g-C3N4/WO3

Planetary mill

A light emitting diode

Melamine

Z-scheme

CH3CHO

[128]

CH3CHO

[127]

TC

[222]

HCHO

[223]

g-C3N4/WO3

/

A fluorescent light lamp

g-C3N4/K+Ca2Nb3O10−

Hydrothermal

A 500W tungsten lamp

g-C3N4/TiO2

Calcination route

A 15 W, 365 nm UV lamp

Melamine/Dicyandiamide Urea P25 and urea

Type-II Co-catalyst Z-scheme

RhB: rhodamine B; BPA: bisphenol A; CIP: ciprofloxacin; MB: methylene blue; 4-CP: 4-chlorophenol; MO: methyl orange; OA: oxalic acid; TC: tetracycline

107

Highlights 

Fabrication of the 2D g-C3N4 based 2D heterojunction composites



g-C3N4 based various 2D composite for improved photocatalytic activity is summarized.



Interface charge transfer about g-C3N4 based 2D/2D heterojunction systems is explicated.



Increased exposed active sites and improved charge separation rate are obtained.



Perspectives and challenges by g-C3N4 based 2D/2D heterojunction are presented.

108

109