Metal-Free Graphitic Carbon Nitride Photocatalyst Goes Into Two-Dimensional Time

Graphitic carbon nitride (g-C3N4) is always a research hotspot as a metal-free visible-light-responsive photocatalyst, in the field of solar energy conversion (hydrogen-production by water splitting). This critical review summarizes the recent progress in the design and syntheses of two-dimensional (2D) g-C3N4 and g-C3N4-based nanocomposites, covering (1) the modifications of organic carbon nitrogen precursors, such as by heat treatment, metal or metal-free atoms doping, and modifications with organic functional groups, (2) the influencing factors for the formation of 2D g-C3N4 process, including the calcination temperature and protective atmosphere, etc. (3) newly 2D g-C3N4 nanosheets prepared from pristine raw materials and bulk g-C3N4, and the combination of 2D g-C3N4 with other 2D semiconductors or metal atoms as a cocatalyst, and (4) the structures and characteristics of each type of 2D g-C3N4 systems, together with their optical absorption band structures and interfacial charge transfers. In addition, the first-principles density functional theory (DFT) calculation of the g-C3N4 system has been summarized, and this review provides an insightful outlook on the development of 2D g-C3N4 photocatalysts. The comprehensive review is concluded with a summary and future perspective. Moreover, some exciting viewpoints on the challenges, and future directions of 2D g-C3N4 photocatalysts are discussed and highlighted in this review. This review can open a new research avenue for the preparation of 2D g-C3N4 photocatalysts with good performances.


INTRODUCTION
The energy crisis has become a growing concern as society continues to develop, which further necessitates the development of sustainable energy sources to supersede traditional fossil fuels (Chang et al., 2017;He et al., 2017aHe et al., ,b, 2018Wang et al., 2018;Zhang G. G. et al., 2018). The hydrogen produced by the photocatalytic water splitting reaction under sunlight, resulting in solar-to-chemical energy conversion, has been deemed to play a key role in resolving the solar-to-chemical energy conversion Zhang G. G. et al., 2018;Zhang S. W. et al., 2018). As a half reaction of the hydrogen production via water splitting, the reaction progress is the decrease of protons/water to hydrogen (Bard and Fox, 1995;Zou et al., 2001). Although the produced hydrogen process refers to the simple reactants, demanding only two electrons to generate a hydrogen molecule, the reaction kinetics is slow due to the large energy barriers in the multiple reaction steps (Tu et al., 2013;Wondraczek et al., 2015;Zhang N. et al., 2015;Zhang et al., 2017a). At the same time, the reduction of water to hydrogen requires many photo-induced holes with oxidant properties . Therefore, photocatalysts are indispensable for these reactions, which can generate photo-induced electrons and holes under sunlight. So far, the most effective photocatalysts are still metal-based materials Ai et al., 2018). However, the high cost and heavy-metal-toxicity of these photocatalysts limit their usage.
In recent decades, abundant non-metal photocatalysts, mainly based on earth-abundant non-metals elements (P, S, N, and C), have been explored. Among them is graphitic carbon nitride (g-C 3 N 4 ), which has recently been widely used in the field of photocatalytic water splitting, as a metal-free and environmentally friendly photocatalytic material, Low et al., 2014;Dong and Cheng, 2015), of which the bulk and granulated g-C 3 N 4 are the most widely used electrocatalysts for hydrogen production. This seriously inhibits the efficiency of photocatalytic water splitting . Moreover, the application of g-C 3 N 4 is restricted in the reaction because of its frequent photo-corrosion under sunlight. Therefore, continuous efforts have been made to develop more stable and efficient g-C 3 N 4 -based heterogeneous photocatalysts in recent years (Wang et al., 2011;Shi et al., 2015;Yang et al., 2016). Additionally, as a new type of two-dimensional (2D) material, 2D g-C 3 N 4 FIGURE 1 | (A) XRD patterns of samples. (B) UV-Vis spectra of g-C 3 N 4 and CNF-x (inset shows optical band gaps(E) g of g-C 3 N 4 and sample-2.0). (C) XPS spectrum of sample-2.0. (D) Solid-state MAS-NMR spectrum of sample-2.0 (Wang et al., 2010). Copyright 2010, American Chemical Society. has been utilized as a photocatalyst in solar-driven water splitting. The progress in this research field is discussed in this review.
This critical review summarizes the recent progress made in the formation of 2D g-C 3 N 4 (g-C 3 N 4 -based nanocomposites) for hydrogen production, and further elucidates the modifications of functional groups, the influencing factors of the formation process, new methods, heterojunction nanostructures, and so on. In addition, the DFT calculations for the g-C 3 N 4 systems are also summarized to provide an insightful outlook. Finally, this review is concluded with a summary and future perspective.

MODIFICATIONS OF CARBON NITRIDE
As a fascinating material, 2D g-C 3 N 4 has attracted worldwide attention (Ma et al., 2014;Liang et al., 2015), and promises access to a wide field of applications compared with other photocatalytic materials, due to its outstanding features, such as its non-metal and non-toxicity (Liu G. et al., 2015;Zhang G. G. et al., 2015;Zhang et al., 2017b). Furthermore, g-C 3 N 4 is a wide-band gap indirect semiconductor (Schwinghammer et al., 2013) with an appealing electronic structure. This allows its direct use as a heterogeneous photocatalyst. However, the photocatalytic effect of pure g-C 3 N 4 is inferior to those of metal semiconductor photocatalysts. Therefore, some modifications, such as metal-free or metal atom doping, are necessary to improve the photocatalytic effects of g-C 3 N 4 . For the doping of g-C 3 N 4 with metal-free atoms, halogen elements are very important and effective (Groenewolt and Antonietti, 2005;Chang et al., 2015;Han et al., 2016;Ye et al., 2016;Ma et al., 2017), The ionic radii of the incorporated guests of halogen elements decrease in the order F<Cl<Br (Chong et al., 2013). Generally, by using a heating treatment (dicyandiamide) in eutectic melting salt, such as LiY and KY (Y=F, Cl or Br), bulk g-C 3 N 4 can chemically and physically be exfoliated into thin layers (Li + , K + or X − ) (Bojdys et al., 2013;Ma et al., 2017). As early as 2010, Wang et al. reported the synthesis of a fluorinated polymeric carbon nitride, which was employed as a heterogeneous catalyst for hydrogen generation from water. In addition, it was also used for the oxygenation of benzene into phenol under visible light (Figure 1, Wang et al., 2010). Other metal-free atoms are also used to dope g-C 3 N 4 , such as O, C, N, P, S, and B (Ran et al., 2015;Feng et al., 2016;Lu et al., 2017;Zhu et al., 2017). Zhang et al. used S8 (elemental sulfur) and melamine as the raw materials to obtain g-C 3 N 4 -S x , where x refers to the quality of S8 , in which the absorption edges of CN-S x samples became marginally redshifted with adding S8 contents (Figure 2), thus decreasing the corresponding band gaps (E g ) from 2.76 to 2.58 eV .
FIGURE 5 | Fabrication of 2D g-C 3 N 4 nanosheets using a simple method from bulk g-C 3 N 4 powders for hydrogen evolution under visible light . Copyright 2013, Wiley-VCH.
the first time, which could improve the visible-light absorption, decrease the band gap (E g ), expedite the charge mobility and extend the lifetime of charge carriers. All these characteristics are necessary to improve photocatalytic activity Wang X. C. et al., 2009).
Since Wang et al. proposed the preparation of g-C 3 N 4 as an efficient photocatalyst (Wang X. C. et al., 2009), g-C 3 N 4 materials have gradually become a hot topic in the field of energy and catalysis research, due to advantages such as its low-cost, sustainability and visible-light response (Martin et al., 2014a; Xu et al., 2015;Zheng et al., 2015;Kang et al., 2016;Li J. et al., 2016). In recent years, high-efficiency 2D g-C 3 N 4 nanosheet photocatalysts have been prepared by an organic reaction. For example, phenylene groups can be part of carbon nitrides through the copolymerization of 2-aminobenzonitrile (CN-ABN 0.5 ) with dicyandiamide (Zhang et al., 2010;. The optical absorption edge of carbon nitride red-shifted to 700 nm from that of the pristine carbon nitride (460 nm), as the 2-aminobenzonitrile content increased. The sample (CN-ABN 0.05 with a platinum co-catalyst) showed the topmost photocatalytic evolution of hydrogen (147 µmol h −1 ) compared with pristine carbon nitride (18 µmol h −1 at λ > 420 nm) (Zhang et al., 2010;. Zhao et al. designed a 2D g-C 3 N 4 organic material (with a thickness of about 1.5 nm), which was successfully synthesized from melamine raw materials for the first time. The synthetic method for the 2D g-C 3 N 4 organic material was simple and efficient. Based on the organic synthesis theory, the synthetic mechanism was theoretically explored (Figure 4, Zhao G. et al., 2018). These photocatalysts have good photocatalytic hydrogen  production compared to common bulk g-C 3 N 4 .

THE G-C 3 N 4 AND G-C 3 N 4 -BASED NANOCOMPOSITES
Two-dimensional g-C 3 N 4 with atomic thickness has become a fascinating material in photocatalysis, because of the large specific surface area and efficiently photoexcited carriers, which can decrease the possibility of electron-hole recombination (Zhu et al., 2010;Shiraishi et al., 2014Shiraishi et al., , 2015Liu et al., 2015a,b;Shi et al., 2015). However, the synthesis or exfoliation of ultrathin (monolayer or bilayer) 2D g-C 3 N 4 nanosheets with a homogeneous thickness, continues to be a large-scale challenge.
It is known that g-C 3 N 4 has a two-dimensional laminated structure parallel to graphene and the theoretical specific surface area of the ideal monolayer g-C 3 N 4 can reach up to 2,500 m 2 g −1 . Inspired by the formation of graphene from graphite exfoliation, many effective ways have been explored for the exfoliation of raw FIGURE 10 | Photocatalytic mechanism of g-C 3 N 4 photocatalystt . Copyright 2012, American Chemical Society.
bulk g-C 3 N 4 to obtain a 2D ultrathin structure, such as ultrasonic liquid exfoliation, chemical exfoliation, and thermal oxidation exfoliation as well as other methods (Niu et al., 2012;Yang et al., 2013;Feng et al., 2016). For example,  N 4 nanosheets from bulk g-C 3 N 4 powders with a simple and cost-effective liquid exfoliation method (Figure 5, Yang et al., 2013). These nanosheets possess the structural features of homogeneous decentralized carbon and nitrogen atoms, an infinitesimal thickness, a large specific surface area (BET) and an optimal bandgap, which can bring about good photocatalytic activity with regards to the hydrogen evolution in visible light .
Although these methods can effectively synthesize some g-C 3 N 4 nanosheets and improve the photocatalytic property, the recombination of the electron-hole on the surfaces of the 2D materials, remains a key issue for most singlephase photocatalysts (Dong et al., 2013;Martin et al., 2014b;Ye et al., 2015). Therefore, the concept of 2D g-C 3 N 4based nanocomposites was proposed. Theoretical models have predicted that the restoration of photo-generated electrons/holes could be pounding down because of their effective spatial isolation on the heterojunction interface (Dong et al., 2013). Additionally, other advantages of photocatalytic reactions can also be achieved such a: good visible-light absorption and outstanding surface reaction activity. Herein, the design of 2D g-C 3 N 4 -based nanocomposites has become a research hotspot to improve the photocatalytic performance (Iwase et al., 2011;FIGURE 11 | (A,B) TEM images, and (C) EDS mapping images of P-C 3 N 4 /ZnIn 2 S 4 nanocomposites . Copyright 2016, Royal Society of Chemistry.
Frontiers in Chemistry | www.frontiersin.org FIGURE 12 | Water photolysis mechanism of P-C3N4/ZnIn2S4 photocatalyst under simulated solar irradiation . Copyright 2016, Royal Society of Chemistry. Lin and Wang, 2014;Chen et al., 2015;Han et al., 2016;She et al., 2016She et al., , 2017. For example, She et al reported that small amounts of α-Fe 2 O 3 nanosheets could actively promote the exfoliation of g-C 3 N 4 , preparing a 2D hybrid structure that exhibited an effective Z-scheme junction (She et al., 2017). The nanostructured hybrids presented a high H 2 evolution rate >3 × 10 4 µmol g −1 h −1 and the quantum efficiency was about 44.35% at 420 nm, which is the uppermost value reported so far for g-C 3 N 4 photocatalysts (Figure 6, She et al., 2017).
A layered-structure, MoS 2, is also a candidate for incorporation with g-C 3 N 4 to construct 2D/2D nanocomposites (Hou et al., 2013;Li X. G. et al., 2016). For example, Li et al. designed a 2D g-C 3 N 4 and MoS 2 heterojunction via means of the self-assembly of 2D g-C 3 N 4 with MoS 2 nanosheets (Li X. G. et al., 2016). As shown in Figure 7, the 2D g-C 3 N 4 and MoS 2 nanosheets were prepared from the exfoliation of bulk g-C 3 N 4 and MoS 2 raw materials, through ultrasonication (Li X. G. et al., 2016). Thin g-C 3 N 4 and MoS 2 2D nanosheets were observed in 2D g-C 3 N 4 /MoS 2 nanocomposites by TEM images (Figure 7). This type of g-C 3 N 4 / MoS 2 photocatalysts also showed a good photocatalytic effect.

MECHANISM OF A 2D G-C 3 N 4 PHOTOCATALYST SYSTEM
In a single 2D g-C 3 N 4 system, the photo-excited electrons of the conduction band (CB) generally return to the valence band (VB) (Tian et al., 2014), while the unpopular recovery of photo-generated electrons and holes are a great disadvantage of photocatalytic reactions . The photocatalyst is used as a semiconductor, to intimately constitute with g-C 3 N 4, to create a suitable band structure. The spatial isolation of photogenerated electrons and holes can be realized through an effective charge transfer on the two semiconductor interfaces (Figure 8, Jiang et al., 2013;Liu et al., 2016;Zhang X. J. et al., 2016;Fu et al., 2017). Commonly, the bandgap of pristine g-C 3 N 4 bandgap is about 2.7 eV and their CB and VB are situated at −1.1 eV and +1.6 eV, respectively (Cao et al., 2015). g-C 3 N 4 is therefore used as a photocatalyst for photo-reduction reactions, because of its sufficiently negative conduction band position in Figure 8. Generally, 2D g-C 3 N 4 -based hetero-junction systems are very effective in separating photo-generated electron/hole pairs, because of the component photocatalyst has this kind of Z-Z band structures (Cao et al., 2015). Therefore, an appropriate band-structure is important to consider when choosing the component photocatalyst for the structuring of 2D g-C 3 N 4 -based heterojunction photocatalysts (Fu et al., 2017).
Additionally, the band gap requires that the oxidation of the photo-generated hole has enough strength, in order to obtain oxygen from the oxidation of water, and the photogenerated electron must restore enough, to reduce the water, in order to yield H 2 (Li et al., 2012). In other words, the location of the HOMO-LUMO band must consume the water oxidation-reduction potential . As illustrated in Figure 9, it is able to run half of two independent reactions, by calculating the carbon nitride band positions (Thomas et al., 2008;Maeda et al., 2014). The type of containment in an organic semiconductor is a rare condition in Figure 10  .
In 2016, Chen et al. fabricated a 2D/2D P-doped g-C 3 N 4 /ZnIn 2 S 4 photocatalyst by an in situ loading method, wherein ZnIn 2 S 4 nanosheets where grown on the P-doped mesoporous g-C 3 N 4 nanosheet surface . As shown in Figure 11. the 2D nanosheet structure can clearly be observed for the P-C 3 N 4 /ZnIn 2 S 4 nanocomposites. Moreover, the EDS mapping images of the P-C 3 N 4 /ZnIn 2 S 4 show that all the elements (Zn, In, S, C, N, and P) are evenly dispersed on the surface of the photocatalyst . This type of a special 2D/2D surface contact can provide more contact areas between P-C 3 N 4 and ZnIn 2 S 4 , which is conducive to an effective charge carrier separation. Under light irradiation, the photo-generated electrons can transfer from the CB of ZnIn 2 S 4 to the CB of P-C 3 N 4 . Similarly, the photo-generated holes can shift from the VB of P-C 3 N 4 to the VB of ZnIn 2 S 4 , as shown in Figure 12. The spatial isolation of photo-generated charge carriers can vastly optimize the catalytic performance of the P-C 3 N 4 /ZnIn 2 S 4 photocatalyst .

SUMMARY AND OUTLOOK
Currently, two-dimensional g-C 3 N 4, a metal-free and visible-light-responsive photocatalyst, in the field of hydrogenproduction through water splitting, is a hot topic in research. This critical review summarizes the ultramodern progress in the design and preparation of 2D g-C 3 N 4 and g-C 3 N 4 -based composites. Although significant advances in 2D g-C 3 N 4based photocatalysts have been made, photocatalytic efficiency remains too low. However, its wide application proves that 2D g-C 3 N 4 -based photocatalysts are prospective materials in the practical application of efficient sun-energy conversion in the future.

AUTHOR CONTRIBUTIONS
All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.