Two-dimensional g-C3N4 nanosheets-based photo-catalysts for typical sustainable processes

Xiodong Zho,Qin Liu,Xiolei Li,?,Huiming Ji,Zhurui Shen,?

a Key Laboratory of Advanced Ceramics and Machining Technology,Ministry of Education,School of Materials Science and Engineering,Tianjin University,Tianjin 300072,China

b School of Materials Science and Engineering,Nankai University,Tianjin 300350,China

Keywords:Graphitic carbon nitride (g-C3N4)nanosheets Exfoliation approaches Hetero-junction Z-scheme mechanism Photo-catalytic applications

ABSTRACT Graphitic carbon nitride (g-C3N4) has been widely studied as a visible light responsive photocatalyst in recent years,due to its facile synthesis,low cost,high stability,and appropriate bandgap/band positions.In this review,we firstly introduce and compare various exfoliation approaches of bulk g-C3N4 into ultrathin g-C3N4 nanosheets.Then,many modification strategies of g-C3N4 nanosheets are also reviewed,including heterojunction construction,doping,defect control,and structure design.Thereafter,the charge transfer mechanism in g-C3N4 nanosheets based heterojunctions is present,e.g.,Z-scheme,S-scheme and other forms.Besides,the photocatalytic applications of g-C3N4 nanosheets based photocatalysts are summarized including environmental remediation,energy generation and storage,organic synthesis,and disinfection.This review ends with a summary and some perspectives on the challenges and new directions in exploring g-C3N4 nanosheets-based photocatalysts.

1.Introduction

All the time severe environment pollution and latent source crisis have drawn increasingly attention towards solar energy utilization by reason of the merits including environmentally friendly and inexhaustible supply.As a consequence,semiconductor photocatalysts are considered to be a potential strategies for pollutant elimination and clean energy acquisition.Research on semiconductor photo-catalysis was initiated in 1972 with the application towards photo-chemical hydrogen generation by TiO2electrodes [1],however,the large bandwidth leaded to low spectral utilization rate,numerous approaches have been applied to absorb added solar irradiation,improving the photo-catalytic ability subsequently[2–6].Besides,many other semiconductors such as ZnO,WO3,SnO2[7–16] have also been identified as potential photo-catalysts.However,the low sunlight absorption as well as poor separation ability of photo-generated charges still block the development of photo-catalytic materials towards higher catalytic activity [17–19],a huge number of research articles,including coupling with heterogeneous semiconductors,element doping,and defect controlling,has been published on the modification of photo-catalysts for better performance [20–27].

Recently,graphite-like carbon nitride (g-C3N4) has drawn plenty of study on account of simple preparation,low price,high tolerance against temperature,acid or alkali solutions,and narrow band-gap with appropriate band position (ca.-1.3 eV andca.+1.4 eV) [28–30],which can be widely used in water splitting,CO2reduction,dye-stuff decomposition,disinfection and so on [31–39].The history of g-C3N4for photo-catalysis can be traced back to 2009 [40],Wangetal.firstly utilized this tri-s-triazines material for photo-catalytic water splitting.The g-C3N4nanosheets was synthesized by Niuetal.for the first time in 2012 [41],the bulk g-C3N4was thinned to 2 nm thickviathermal treatment,the photo-catalytic activities have been remarkably improved due to increased specific area,the enhanced charge transport ability and the improved lifetime of photo-induced charges compared with bulk g-C3N4.Nevertheless,the original g-C3N4is limited in photocatalytic reaction because of the disadvantages including poor sunlight spectral utilization and quick annihilation rate of photogenerated charges [42–46],in recent years,many attempts,such as construction of hetero-junctions,doping,dye sensitization,and structure design,have been made in towards modification of g-C3N4nanosheets [47–58].Among those methods,hetero-structures construction towards g-C3N4enhanced the photo-catalytic capacity due to the effective separation performance as well as fast migrate ability of photo-induced charges.Meanwhile,the carries transfer mechanism in hetero-junctions has been studied by theoretical calculation and characterization for the deeply understanding about the improvement of photo-catalytic performance[59–64].

This review concentrates on numerous research towards the exfoliation methods,modification strategies,mechanistic understanding and adhibition of g-C3N4nanosheets-based nanocomposites.Primarily,the main preparation approaches of ultra-thin g-C3N4nanosheets are presented and compared.Secondly,several methods for elevating the performance of g-C3N4nanosheets,such as construction of hetero-junctions,elements doping,defect controlling,and building nanostructures,are discussed.Next,the type-II system,Z-scheme as well as S-scheme mechanism were presented,demonstrating the charge migrate process in g-C3N4nanosheetsbased hetero-junctions.Another section of this review summarizes the photo-catalytic adhibition of g-C3N4nanocomposites applying to pollution decomposition,hydrogen production,nitrogen fixation,disinfection and so on.Finally,this paper gives some conclusions and further perspectives about above mentioned researches,in turn,promoting more effective application towards this photocatalytic material.

2.Exfoliation of g-C3N4 into 2D nanosheets

g-C3N4is a layered material which consists of the multilayer two-dimensional conjugated structures formedviatri-s-triazines cells [65],the acting force between different layers is weaker than the interlayer binding force [66,67].Since two-dimensional nanosheets have many excellent advantages,including huge specific area,ultrahigh electron mobility,as well as plentiful surface reactive sites [68–70],it is highly rational to thinning bulk layered g-C3N4to attain less layers nanosheets for much superior photocatalytic performance.

The thinning for multilayer matter was applied in clay materials,which could spontaneously swell and delaminate with the destroy of interlayer interaction in water [71,72].In 2004,K.Novoselov and co-workers [73] attained graphene with many intriguing properties by the mechanical exfoliation towards graphite,since then,great efforts have been made to exfoliate layered materials into nanosheets [74–78].Because of the similarities between graphite and g-C3N4,the exfoliation of the latter into nanosheets has been widely studied [79–84],the synthetic approaches which have been established included the temperature,ultrasonic and chemical exfoliation.

2.1.Thermal oxidation exfoliation

Niuetal.prepared thinned g-C3N4nanosheets for the first time[41].Based on the knowledge the force between layers including van der Waals’force as well as the hydrogen bond are not strong enough to withstand thermal energy,leading to the separation between different layers [85],as a consequence,a decrease of thickness to the desired nanoscale happened,the g-C3N4nanosheets with the approximate 2 nm thick (6～7 layers) was obtained after thermal exfoliation from bulk materials in air at high temperature (Fig.1a).The as-prepared nanosheets exhibited huge specific area (306 m2/g),enhanced charge mobility,and prolonged life-span for photo-excited charges,the photo-catalytic activities have been remarkably enhanced as a consequence.The preparation of nanosheetsviatemperature treatment depended on many factors,such as etching time,temperature,precursor masses,and atmosphere [86–90].Graphene-like g-C3N4nanosheets with promoting photo-catalytic capacity was synthesized by Dong and his researchers [91]viaa thermal exfoliation approach under different temperatures from 450 °C to 550 °C,with the elevation of temperature,the thickness of resultant nanosheets gradually decreased,the surface area increased at the meantime.Until now,the thermal oxidation strategy for g-C3N4nanosheets with improved photo-catalytic ability has been widely applied because of its lowcost and easy operation [92–99].

Fig.1.(a) Schematic of the formation process of the g-C3N4 nanosheets by thermal oxidation etching of bulk g-C3N4 at 500 °C in air.Reproduced with permission[41].Copyright 2012,Wiley.(b) Schematic illustration of the liquid-exfoliation process from bulk g-C3N4 to ultra-thin nanosheets.Reproduced with permission [100].Copyright 2012,American Chemical Society.

2.2.Ultrasonic exfoliation

Liquid exfoliation of layered materialsviaultrasonic process is considered to be a effective way for preparing twodimensional nanosheets [75].Synthesizing ultra-thin g-C3N4nanosheets through ultrasonic treatment route from bulk material in solvent is environmentally friendly and can result in a relatively high product yield compared to thermal oxidation etching approach [79].During the exfoliation process,solvent molecules would intercalate into layered materials,resulting in enlarging of lamellar spacing as well as damage of van der Waals force among different layers,at the meantime,the ultrasonic-produced bubbles would gradually oscillate and crack,leading to the liquid jets which could shear the layered materials into ultra-thin nanosheets.It is known that the similar surface energy between the bulk g-C3N4as well as reagent molecules would bring efficient exfoliation for high-yield g-C3N4nanosheets [74],besides,the polarity solvent could be more effective for intercalation and swelling due to the presence of dangling hydrogens among multilayer g-C3N4material.Zhang and co-workers [100] firstly delaminated bulk g-C3N4in water to form nanosheets with an approximate 2.5 nm thick,layered g-C3N4was swelled and exfoliated after the intercalation of water molecules under ultrasonic conditions (Fig.1b).To date,ultrasonic exfoliation became a commonly strategy for preparing g-C3N4nanosheets in water and other reagents,such as propanol,ethanol,N,N-dimethylformamide [101–106].Zhaoet al.[107] synthesized ultra-thin g-C3N4nanosheets possessing 1 nm thick through ultrasonic exfoliation combined with thermal oxidation of bulk g-C3N4with comparatively high yield (～24%).The resultant nanosheets showed significantly improved photo-catalytic ability for hydrogen evolution.

2.3.Chemical exfoliation

Since the exfoliation efficiency of both thermal oxidation exfoliation and ultrasonic exfoliation is unsatisfactory,the chemical exfoliation method with high product field has been applied to synthesize ultra-thin g-C3N4nanosheets [81,108-113].The bulk g-C3N4could be exfoliatedviaredox reactions and intercalation processviachemical exfoliation,ions such as H+,Li+,and Br-could be effectively intercalated between the layers of bulk materials at the liquid environment under the assistance of strand nitrogens as acceptor sites,swelling the crystal,increasing the interlamellar distance,decreasing the sheets attraction,as well as bringing down the barrier potential of desquamation in a consequence,leading to the high-yield g-C3N4nanosheets with a micrometers wide but few-layer thick [76].The product yield and the thickness can be influenced by ionic species,amount of oxidant and exfoliation time.Xu and other researchers [114] firstly synthesized the unilamellar g-C3N4nanosheets with the high product field up to 60% by chemical exfoliation methodviathe addition of H2SO4,the average thickness of as-synthesized nanosheets is approximately 0.4 nm thick,that is similar to theoretical value for unilamellar nanosheets (0.325 nm).The photo-catalytic performance of g-C3N4nanosheets remarkably improved on account of increasing surface area and charge mobility.Porous g-C3N4nanosheets with 1 nm thick have been prepared by Shietal.[115] using phosphoric acid.Besides the intercalation effect of H+(Fig.2a),the interaction between OH-and terminal or bridged–NH(2)group of the triazine units in bulk material would also decrease the interlayer force and accelerate the thinning process (Fig.2b).Mono-layered g-C3N4has been obtained using KOH with a large transverse dimension with micron-grade but only 0.4 nm thick by Li and co-workers[116].

Fig.2.Schematic illustrations of chemical exfoliation strategy for preparing g-C3N4 nanosheets by (a) H3PO4 (Reproduced with permission [115].Copyright 2016,Wiley) and (b) KOH (Reproduced with permission [116].Copyright 2017,Elsevier).

In summary,thermal oxidation exfoliation is a simple and effective approach to get a thinner nanosheet,but it should be pointed out that hazardous gas will release and abundant interface defects are generated due to the destroy of hydrogen bridge of conjugated rings as well as the polymeric chainviaoxidation process at high temperature,besides,the break of tri-s-triazines cells framework could also lead to an extremely low product yield.The ultrasonic exfoliation strategy has many advantages comparing with thermal oxidation method,including a relatively high product yield,environment friendly,and the reservation of crystal texture as well as stoichiometric proportion of original g-C3N4.However,that is still limited by time-consuming and the destroying of the lateral size towards g-C3N4vialong-time ultrasonication,which will lead to low aspect ratio and decreased photocatalytic activities of resultant g-C3N4nanosheets.Chemical exfoliation strategy can simultaneously reserve the chemical stoichiometric ratio and lateral size of the original bulk g-C3N4in comparison with thermal exfoliation and ultrasonic exfoliation,besides,it also possesses other considerable superiorities,such as functionalization on the nanosheet surface,high exfoliation efficiency,as well as the probability of exfoliation into monolayer structure[72].However,some functional groups such as amino and carboxyl will partially remain in g-C3N4through chemical delamination processing,resulting in an extra amphoteric behavior of the nanosheets.

3.Modification of g-C3N4 nanosheets

Pristine g-C3N4still suffer from unsatisfactory photo-catalytic efficiency due to the poor visible-irradiation absorption range,ready annihilation of photo-induced charges,and relatively low surface area.Many attempts have been made to deal with those problems,including construction of hetero-junctions,elements doping,defects controlling,and building nanostructures.

3.1.Hetero-junction construction

Constructing hetero-junctionviag-C3N4and other semiconductors could be regarded as a splendid method for dealing with the above problems,the annihilation of photo-induced electron-hole charges could be partly avoided in the resultant by the strong interaction in the interfaces of hetero-junctions,so far,many species of g-C3N4based hetero-structures have been deeply researched[108,117-125].

3.1.1.g-C3N4/inorganicsemiconductorhetero-structure

Zhang and other researchers [126] constructed a BiOCl/g-C3N4hybridviathe two-step calcination-hydrothermal method and applied it as a photo-catalyst towards the decomposition of Rhodamine B as well as hydrogen generation.Their experimental results revealed that the photo-catalytic performance of as-prepared composite was improved by the successfully formation of heterojunctions.Wang and co-workers [127] constructed a ultra-thin g-C3N4/Bi2WO6nuclear-hull structure,which showed the best photocatalytic degradation activity towards phenol as closely 5.7-fold high as which of original g-C3N4,in addition,1.9-fold compare to Bi2WO6nanosheetviavisible light irradiation,at the meantime,phenol degradation performance under full spectrum by using g-C3N4/Bi2WO6photo-catalyst possessed 3.3 times than which of pure g-C3N4,in addition,1.5-fold towards Bi2WO6nanostructure by reason of the enlarged irradiation absorption range as well as separation for photo-induced chargesviaforming heterostructure.CoWO4/g-C3N4hetero-structures were attained by Wangetal.[128],the improved photo-catalytic activity for H2generation could be attributed to fast photoinduced charge separation rate and transport ability benefiting from the band offset structure(Fig.3a).Apart from TiO2and Bi2WO6,other inorganic semiconductor materials including ZnO [129],NiS [130],CdS [131],Cu2O[132] and Mn0.2Cd0.8S [133] also have been reported for the improved photo-catalytic behaviors of pure g-C3N4nanosheets.

Fig.3.(a) Schematic illustration for photocatalytic mechanism of CoWO4/g-C3N4 photocatalysts.Reproduced with permission [128].Copyright 2017,Springer.(b) The suggested photocatalytic RhB degradation mechanism over 30PCN@UiO-66 system under visible-light illumination.Reproduced with permission [134].Copyright 2021,Elsevier.(c) Mechanism of the photo-generated charge transfer and induced photocatalytic processes for H2 evolution in the presence of methanol under visible-light irradiation on g-C3N4/metal hetero-structure.Reproduced with permission [144].Copyright 2018,Elsevier.

3.1.2.g-C3N4/organicmacromolecularmaterialhetero-structure

Some organic macromolecular material such as porphyrin and metal organic frameworks also can be used to form heterostructures with g-C3N4nanosheets.Because of some advantages of high specific area and tunable pore size,the macromolecular materials can exhibit excellent solar harvesting performance,besides,the conjugatedπbond could lead to a stable combination between organic materials and g-C3N4through the generation ofπ-πbond,which accordingly resulted in stable structure and outstanding photo-catalytic activity.UiO-66 coupled with g-C3N4nanosheets has been constructed by Mardiroosietal.[134],the enhanced photo-catalytic degradation capacity putted down to formation of hetero-junctionviaUiO-66 and g-C3N4,nevertheless,electrons could effectively transfer to g-C3N4under the force of built-in potential produced through hetero-junction structure,giving rise to a enhanced response towards light range and the decreasing of annihilation for electron/hole pairs (Fig.3b).Xuetal.[135] utilized another metal-organic framework BIF-20 to constructed heterostructure with g-C3N4nanosheets,leading to a significantly enhanced CO2photo-reduction rate.Other organic macromolecular materials including tetraphenylporphyrin [136],ZIF-8 [64],and Ni-MOF [137] also been widely studied for the elevation of photocatalytic activity towards g-C3N4nanosheets.

3.1.3.g-C3N4/metalhetero-structure

Recently,forming a hetero-junction between metal nanoparticles and g-C3N4nanosheets has been studied as an effective method to improve the photo-catalytic performance of g-C3N4[101,138-141].On account of the surface plasmon resonance effect,the metal nano-particles could be the high-efficiency electron acceptors [142],thus accelerating the separation rate of photogenerated charges in g-C3N4nanosheets,in the meantime,increasing the light absorption.Wang [143] synthesized a g-C3N4/Pt(111)hero-structure with improved photo-reaction activity of CO oxidation.Zhang and co-workers [144] enhanced the photo-catalytic hydrogen evolution ability of g-C3N4nanosheets facing visible-light irradiationviadecorating Ag,Au and Pt,compared with original g-C3N4,the hydrogen evolution ability of the optimal resultant could exhibit about a 14-fold increase,because of the effective separation of the photo-induced charge pairs as well as the co-catalytic action of noble metal nanoparticles (Fig.3c).

3.1.4.g-C3N4/carbonmaterialhetero-structure

Carbon quantum dots,reduced graphene oxide and multiwall carbon nanotube brought many notices because of the extraordinary chemical as well as physical characters [145–148],in recent years,it have been deeply studied and subsequently proved that decorating g-C3N4nanosheets with carbon quantum dots would accelerate the migration for photo-induced charges effectively byπ-πbond force generatedviaCQDs and g-C3N4nanosheets,which could inhibit photo-induced carries annihilation partlyviathe formation of intermolecular forces [149].A C3N4/rGO/C3N5sandwich heterojunction was synthesizedviaa post-annealing process by Liuetal.[150],the as-prepared hybrid photo-catalyst exhibited much superior H2evolution activity compared with pure g-C3N4nanosheets because of high interfacial carries separation migration ability.

What is more,the materials mentioned above also been synergistically utilized to form ternary or quaternary structure for enhancing the photo-catalytic behavior of g-C3N4nanosheets[119,151-154].Dengetal.[95] constructed ternary Ag/N-GQDs/g-C3N4hybrids,such as-synthesized ternary structure exhibited 92.8% as well as 31.3% degradation of tetracycline through simulated sunlight and near-infrared illumination,respectively,providing a novel strategy towards effective photo-catalytic reaction under full-spectrum illumination.Shietal.[155] synthesized the ternary Ag3PO4/Co3(PO4)2/g-C3N4hetero-junction with enhanced photo-catalytic degradation efficiency,the as-prepared hybrid photo-catalyst could reach a superb removal of tetracycline(88%) after 120 min facing visible irradiation.

With such construction of hetero-junctions,the specific area enlarged,at the same time,the band position changed,leading to a higher solar harvesting efficiency and separation rate of photoinduced charges,which can effectively elevate the photo-catalytic behavior of g-C3N4nanosheets.

3.2.Homo-junction construction

Apart from forming hetero-structure,using g-C3N4itself for constructing the g-C3N4/g-C3N4homo-junction also attracted attentions in recent years.A novel g-C3N4nanosheets recombined with C3N4QDs nanocomposites was synthesized by Xuetal.[156],the resultant exhibited more stable and excellent behavior in comparison with unitary g-C3N4nanosheets towards the detection of PDGF-BB.Qiao with co-workers [157] obtained the g-C3N4/g-C3N4Z-scheme homo-junction with improved rhodamine degradation rate,which was about 15.4-fold compared with which of bulk g-C3N4.The increasing photo-catalytic capacity could put down to huge exposed specific area,efficient separation and excellent redox abilities of the photo-induced charges with a formation for the homo-junction.

3.3.Doping

Another method to improve photo-catalytic behavior of g-C3N4nanosheets is doping,typically,metal with nonmetal element doping for g-C3N4have been studied for the reason of partly adjusting crystallographic,electronic as well as band positions to enhance photo-catalytic capacityviabroadening irradiation response range,improving photo-generated charges migration,and lengthening life-span of photo-induced carriers.Doping with nonmetal species including B,N,Cl and so on has been widely reported[158–161].Jiang and other researchers [47] fabricated N doped g-C3N4nanosheets with a midgap state created by N doping,as a consequence,the as-synthesized g-C3N4nanosheets showed higher photo-catalytic capacity towards tetracycline degradation than original g-C3N4due to the easy generation and transport of electronsviathe midgap state.A K,P,O,S-doped C3N4has been constructedviaDengetal.[162],such co-doping of above elements towards g-C3N4broadened the light absorption,besides quickened the migration for photoinduced carries,leading to the superior photo-catalytic behavior for antibiotics destroying in natural water facing visible light irradiation.Yangetal.[163] constructed Odoped g-C3N4composites,leading to the elevated CB edge as well as the detachment and movement rate of photoinduced charges,the as-prepared samples showed highly improved photo-catalytic hydrogen evolution ability,which was approximated 5.2-fold compared with which of original g-C3N4facing ultraviolet-visible light.The nanostructure for g-C3N4catalyst doped with metal elements like Ni,Co,Na,K,Sr and Fe [164–166] also been widely studied for achieving superior photo-catalytic performance of g-C3N4.An Al-doped g-C3N4hybrid photo-catalyst was constructed by Choi and co-workers [167].The introduction of Al atom doping sites could effectively modify the band structure of pure g-C3N4with a midgap state,leading to the increased photo generated charges of the as-prepared photo-catalyst.

3.4.Defect control

Since some researchers have proved that defects are efficient captures for electrons [168,169],defect controlling could be an effective way for dealing with the ready recombination of photoinduced charges of g-C3N4nanosheets.Dong and co-workers[170] demonstrated that photo-excited charges would separate more efficiently in carbon vacancy-modified g-C3N4(Ns-g-C3N4)than in pure one due to the capture ability of vacancy,at the same time,the carbon vacancy-induced defect states in the band gap extended the response spectrum for g-C3N4to the whole visible area,leading to the efficient photo-reduction towards NO under visible light.Li and co-workers [171] studied a high-temperature polymerization strategy to obtain N vacancy-modified g-C3N4with improved photo-catalytic hydrogen evolution activity,the band structures were modified due to the variation of C/N ratio,favoring the separation speed of photo-excited charges with the light response range.

With doping or defect control,a midgap could be introduced into bandgap of g-C3N4nanosheet,which would be an effective electron capture center,leading to the decreased recombination rate of photo-induced charges.Besides,the bandgap of g-C3N4nanosheet would narrow after manufacturing defects,consequently,the light absorption range would broaden,resulting in the better photo-catalytic performance (Fig.4).

Fig.4.The charge transfer mechanism towards doping and defect control.

3.5.Structure design

Moreover,structure design of g-C3N4nanosheet has been researched for improving those photo-catalytic performanceviaenlarged surface area and shortened migration distance of photoinduced charge carriers.Constructing of porous structure and ultra-thin structure were two mainly strategies in this area[34,172-174].Lietal.[175] prepared holey g-C3N4nanosheet (hCNNS) photo-catalysts with a ultra-thin thickness and high pore volumeviaa two-step calcination,leading to a large surface area,the as-synthesized hCNNS possessed more surface reactive sites,strengthened photoredox ability,shortened the migration distance of photo-induced charges,and widened the irradiation response range,as a consequence,the visible-light-driven photocatalytic hydrogen generation speed was significantly elevated in contrast with original g-C3N4.Liu and co-workers [176] constructed mesoporous g-C3N4nanosheetsviahydrothermal treatment,it was found that the mesoporous nanostructure achieved a significantly enhanced hydrogen producing speed of 136.9 μmol/h,which was 15 times higher than pristine g-C3N4.Ultrathin g-C3N4nanosheets were attainedviarapid high-temperature treatment by Zhangetal.[96],numerous active sites for photocatalytic repercussion with enhanced solar harvesting performance were obtained through the remarkable enlarged specific region,besides,stronger driving force was acquired through the enlarged bandgap structure because of quantum confinement effect in thinner g-C3N4nanosheets,realizing a 25.5-folds improved visible-light photocatalytic H2-producing behavior compared with pure g-C3N4.

As a summary,since pristine g-C3N4has the disadvantages such as poor irradiation absorption range,ready annihilation of photoinduced charges,and relatively low surface area,many modification approaches have been studied.The construction of heterostructure and homo-structure effectively promoted charges separationviathe matched energy level,doping and defect control of g-C3N4nanosheet could narrow the pristine bandgap as well as introduce a midgap,extending the light utilization range as a consequence,the structure design could increase specific surface area of g-C3N4nanosheet to provide more active sites for the photocatalytic reaction.However,there are still some questions such as the more suitable band match for hetero-junctions,the optimum concentration of doping as well as defects,and the less damage of original triazine ring during structure design,which should be further studied in the future.

4.Mechanism of g-C3N4-based hetero-structures

Among the strategies which were introduced above for the improvement towards the photo-catalytic performance of g-C3N4nanosheet,a construction for hetero-structure could be the most general one due to many merits,including effective separation of photo-induced charge carries,high harvesting of solar light,and the relative low cost.As a consequence,the photo-generated charge transfer mechanism of hetero-junctions has been widely studied over the years.

4.1.Type-II hetero-structure

The traditional type-II hetero-junction is formed by g-C3N4and other semiconductors.The difference of band positions between the multiple semiconductors leading to the generation of band bending in the hetero-junction.When exposed to light,the photogenerated electrons would migrate from the higher CB level to the lower one because of the formation of built-in electric field generated from the band bending,while the photo-induced holes migrate in the opposite direction.Specially,the wavelength of light irradiation at which the photo-reaction could occur corresponds to a narrower band gap of those semiconductors.Therefore,the formation of type II hetero-junction would extend the light response range and improve the photo-excited charge separation,which could in turn enhance the photo-catalytic performance [177].

Yi and co-workers [178] synthesized Nb2O5/g-C3N4nanosheets hybrid,the g-C3N4nanosheets have more negative CB band positions than those of Nb2O5,as a consequence,the photo-induced electrons could migrate from g-C3N4to Nb2O5(Fig.5a).Since the CB level of Nb2O5is higher than the H+/H2potential,the photogenerated electrons in Nb2O5could split water to produce H2directly.Similarly,Majumdaretal.[179] constructed Bi4NbO8Cl/g-C3N4hetero-structure,according to calculation results,the CB and VB band positions of Bi4NbO8Cl are about -0.54 and 2.07 eV,while those of g-C3N4are about -1.20 and 1.84 eV,respectively,because of the formation of type-II hetero-junction,the photo-excited electrons in the CB of g-C3N4could inject into that of Bi4NbO8Cl,while opposite transfer occurring in the photo-induced holes in the VB of Bi4NbO8Cl,the effective separation of photo-induced charges could improve the activity of photo-catalytic degradation of oxytetracycline significantly as a consequence.Other type-II hetero-junction hybrid photo-catalysts such as NiS/g-C3N4[130],Bi2WO6/g-C3N4[180],BiOCl/g-C3N4[104] and g-C3N4nanosheet/ZnIn2S4nanoleaf[48] have also been reported as efficient photo-catalysts for pollution degradation,water splitting and organic synthesis on account of the effective separation of photo-excited carriers and enlarged light response range.

Fig.5.(a) Schematic of electron transfer in Nb2O5/g-C3N4 nanosheets hybrid photo-catalyst.Reproduced with permission [178].Copyright 2021,Elsevier.The charge transfer mechanism of SCS (b) and SS (c) Z-scheme.

4.2.Z-scheme hetero-structure

The type-II hetero-junctions provided effective charge separation,however,the lower CB level and the higher VB level of which limited the redox ability of as-prepared photo-catalysts,leading to a weakened performance for photo-catalytic reaction.In such case,a new type of charge transfer mechanism has been studied,which was called Z-scheme hetero-structure,simultaneously achieve effective separation of photo-excited charges as well as strong redox ability.The Z-scheme hetero-structure could mainly be divided into two types: Ionic-state and all-solid-state,the latter can be further divided into semiconductor-conductor-semiconductor (SCS) Zscheme hetero-junctions and semiconductor-semiconductor (SS) Zscheme hetero-junctions.

In the SCS Z-scheme hetero-junctions,appropriate conductors such as noble metal particles,graphene and many other conductive materials can be inserted between semiconductors to provide the pathway for electron migration to improve the efficiency of charge transport and separation [181,182].The electrons generated in the CB of one semiconductor,migrated to noble metal particles which has high work function,and subsequently recombined with holes induced in the other semiconductor (Fig.5b),simultaneously,the remaining photo-generated electrons in higher CB and holes in lower VB of two semiconductors could be applied to the photocatalytic reduction and oxidation reactions,respectively.

Wu and co-workers [183] employed many approaches to study Z-scheme mechanism towards BaTiO3/Au/g-C3N4hybrid composites.The radical scavengers experiment indicated O2-radicals as an active species in photo-catalytic degradation,VB XPS and Mott-Schottky plots experiments calculated the band positions of both BaTiO3and g-C3N4,considering that the CB level of BaTiO3was less-negative comparing with H+/H2potential,photo-induced electrons prefer to accumulate in g-C3N4rather than BaTiO3,as a consequence,the photo-excited charges transfer pathway followed Z-scheme,the H2evolution and photo-degradation performance of Rhodamine B improved due to high separation rate of the photo-excited charges.Other SCS Z-scheme system such as Ag/AgVO3/carbon-rich g-C3N4[184],W18O19/Au/g-C3N4[185] and ZnO/Au/g-C3N4[186] have also been widely constructed for the improving photo-catalytic performance in water splitting,pollution mitigation,and CO2reduction of g-C3N4nanosheets.

However,high-cost and shielding effect limited the widely adhibition of noble metals such conductor-assistant Z-scheme heterostructure [63],different from SCS Z-scheme system,the photoexcited charges could directly transfer between two semiconductors in the SS Z-scheme photo-catalyst.In this manner,the photocatalytic efficiency can be further increased because of the shortened migration distance of the photo-excited charges (Fig.5c)[187–194].A TiO2/g-C3N4composite which followed Z-scheme mechanism was fabricated by Liuetal.[195].Mott-Schottky plots were tested to calculate the band positions of both TiO2and g-C3N4,electron spin resonance experiment indicated O2-radicals as an active species,on account of the less-negative CB level towards TiO2compared of O2-/O2potential,the as-prepared composite showed decreased photo-excited charges recombination rate and prolonged lifetimeviathe formation of Z-scheme hetero-junction,leading to the enhanced photo-catalytic degradation performance.Zhang and co-workers [196] employed many approaches to study Z-scheme mechanism towards V2O5/P-g-C3N4hybrid composites.Both trapping agent and electron spin resonance testing indicated h+and O2-radicals as an active species in photo-catalytic degradation,considering that the CB level of V2O5was less-negative comparing with O2-/O2potential,the O2-radicals were generated by the photo-excited electrons in g-C3N4rather than V2O5,the photo-degradation performance of methyl orange improved due to high separation rate of the photo-excited charges.

4.3.S-scheme hetero-structure

Another hetero-structure which is called step-scheme (Sscheme) hetero-structure has also been discussed by Fuetal.viathe ultra-thin WO3/g-C3N4hetero-junction photo-catalysts [61] to remedy the drawbacks of type-II hetero-junction as well as further improve photo-catalytic performance.Under irradiation,the photoinduced electrons in WO3and the holes in g-C3N4were recombined under the effect of internal electric field between two semiconductors (Fig.6),remaining holes in WO3and electrons in g-C3N4to achieve photo-catalytic oxidation and reduction reactions,respectively.This S-scheme charge carrier transfer mechanism ensures the as-constructed WO3/g-C3N4hybrid composites with enhanced electron–hole pair separation ability and elevated redox capacity,leading to the fast photo-catalytic water splitting rate.As a result,the H2evolution rate of as-prepared composite was improved,which was 1.7-fold compared with pristine g-C3N4.

Fig.6.The S-scheme charge transfer mechanism between WO3 and g-C3N4 under light irradiation.Reproduced with permission [61].Copyright 2019,Elsevier.

Fig.7.(a) Absorptive,photo-catalytic removal of aqueous MO over graphene-carbon quantum dots/g-C3N4 nanosheets composite under visible light (λ >420 nm),and (b)corresponding kinetic simulation curves.(a,b) Reproduced with permission [149].Copyright 2018,Elsevier.(c) Cr(VI) reduction efficiency by different photocatalysts and(d) schematic illustration of the reduction of Cr(VI) by g-C3N4/C/Fe2O3 under light irradiation.(c,d) Reproduced with permission [205].Copyright 2021,American Chemical Society.

5.Applications of g-C3N4-based photo-catalysts

The g-C3N4-based photo-catalysts possess a large surface area,which can contribute high light harvesting and numerous active sites to accelerate the photo-catalytic reaction,and an appropriate band structure for light absorption and photo-catalytic redox reaction,besides,the good accessibility and high stability against temperature,acid or alkali solutions also make the g-C3N4nanosheetsbased photo-catalysts become the extensive applications such as pollution degradation,energy generation,nitrogen fixation.Herein,some remarkable applications will be listed as follows.

5.1.Photo-degradation of environmental pollutants

Semiconductor photo-catalysis is an excellent material to solve the environment questions.On account of the appropriate band structure and good stability,the g-C3N4nanosheets has widespread application towards the photo-catalytic reaction of numerous water pollutants,such as organic dye-stuffs[108,129,134,197-201],aromatic compounds [202,203],antibiotic[47,95,120,204],heavy metal [60].The removal of air contaminants such as NO [34,170,173] and CO [143] has also been investigated.

5.1.1.Waterpollutants

Amorphous Cu-doped FeOOH nanoclusters/ultra-thin g-C3N4nanosheets hybrid photo-catalyst was synthesized by Zhangetal.[94],when exposed to visible light,the resultant could decolorize～98.7% MB within 40 min,the highly enhanced photo-catalytic ability (more than 8.1 times) in comparison with original g-C3N4nanosheets endowed the resultant photo-catalyst with prospective industrialization applicability.Heetal.[149] obtained graphene–carbon quantum dots/g-C3N4nanosheets compositeviathe hydrothermal way,the MO removal ratio of the as-prepared photo-catalyst was up to 91.1% under 4 h-visible light irradiation,which achieved approximately 7.6-fold improvement of original g-C3N4(Figs.7a and b).Dong and co-workers [168] utilized hydrochloric acid to gain ultra-thin g-C3N4nanosheet for elevated photo-catalytic degradation rate of RhB.As a result,the RhB could be decomposed completely after 75 min-exposure of the visible light.Yangetal.[200] constructed Nb2O5/g-C3N4photo-catalystviathe one-step heating approach,realizing a better photocatalytic performance,95% of RhB,15% of MO and 87% of MB were degraded after one hour facing visible irradiation by as-prepared materials,respectively.Zhang and co-workers [204] prepared NaNbO3/g-C3N4composites to deal with the ofloxacin degradation,the as-prepared samples exhibited increased photo-catalytic ability under simulated sunlight,99.5% of ofloxacin was removed within 30 min by the optimum sample.The effective removal of highly toxic Cr5+to Cr3+using visible irradiation as driving force was successfully achievedviag-C3N4nanosheets-based photo-catalysts such as g-C3N4/C/Fe2O3[205] and ternary Au/g-C3N4/W18O49hybrid composite [60],which could reduce more than 80% of Cr(VI) within 30 min (Figs.7c and d).The photo-degradation of aromatic compounds such as phenol has been studied by Wangetal.[127],the results illustrated the constructed g-C3N4/Bi2WO6core-shell structure possessed superior photo-catalytic phenol degradation activity towards simulated sunlight as well as visible light due to the strong oxide ability and decreased recombination of photo-generated charges.

5.1.2.Gaspollutants

Wangetal.[143] prepared the g-C3N4/Pt (111) nanostructure for the effective photo-catalytic oxidation of CO.Huangetal.[173] fabricated porous g-C3N4nanosheets,on account of the merits such as huge surface area as well as excellent charge carriers separation ability,ultra-thin g-C3N4has efficient application in visible-light-driven photo-catalytic NO mitigation,achieving a 5.5-fold increasing in comparison with original g-C3N4.Dong[166] prepared Sr-modified g-C3N4nanosheets for the elevated photo-catalytic NO reduction capacity.A redshift of absorption edge occurred after doping,resulting in the extended light adsorption (Fig.8a),at the meantime,the intercalation of Sr enlarged the bond length of g-C3N4,resulting in the weakened Coulomb interaction for gas molecules with easy activation of O2molecules on the catalyst surface.The composites have increased NO removal rate compared with original g-C3N4nanosheets (Fig.8b).Moreover,Jiang and co-workers [39] also studied the photo-oxidation behavior of NO to nitrateviaN-doped TiO2/g-C3N4hetero-structure,the removal ratio of resultant photo-catalyst towards ppb-level NO could reach 46.1% within 30 min under visible light illumination.

Fig.8.The UV–vis spectra (a) and photocatalytic activity for NO removal (b) of as-prepared samples.(a,b) Reproduced with permission [166].Copyright 2018,Elsevier.(c)Production rate of CO and (d) Production rate of CH4 for C-In2O3/g-C3N4 composite under ultraviolet light.(c,d) Reproduced with permission [206].Copyright 2021,Elsevier.

5.2.Energy generation

5.2.1.CO2reduction

The hydrocarbon fuel has many merits including renewable and clean,according to the ability for the coverting of CO2into CH4,the applications of CO2reduction of photo-catalysts have been widely studied.

A C-In2O3/g-C3N4composite was fabricated by Xuetal.[206]viathe solvent method for the photo-reduction towards CO2,the CO and CH4evolution rate of 153.42 μmol g-1h-1and 110.31 μmol g-1h-1were achieved under ultraviolet light irradiation,Especially,the large surface area towards irradiation as well as decreased recombination of photo-excited charges endowed the resultant with significantly enhanced photo-catalytic performance(Figs.8c and d).Liu and co-workers [136] obtained cobalt meso–tetra-phenylporphyrin/g-C3N4hetero-structure,the high specific surface area and porous structure with suitable hole size favored CO2adsorption,at the meantime,the promoted separation ability of photo-induced electrons and holes increased electron utilization efficiency,leading to the superior CO2catalytic conversion performance.Nieetal.[194] fabricated the g-C3N4/ZnO composite with a 4.1 times CH3OH production rate compared to pristine g-C3N4.The PL measurement was carried out to certify the existence of·OH in photo-catalytic reaction.Xia and co-researchers [52] constructed g-C3N4nanosheets with a thickness ～3 nmviathermal exfoliation approach under the NH3atmosphere,the obtained nanosheets exhibited remarkably enhanced photo-catalytic CO2reduction performance.Other g-C3N4nanosheets-based photocatalysts such as BIF-20@g-C3N4[135],g-C3N4/WO3[207],and g-C3N4/CdS [208] also be constructed for the effective CO2photocatalytic reduction.

5.2.2.Watersplitting

Apart from hydrocarbon fuel,hydrogen energy has also been regarded as a potential and promising fuel for the substitution of traditional fossil energy due to the renewability and high energy storage [209].Photo-catalytic decomposition of water by photocatalysts could be a effective method for generating hydrogen.Numerous photo-catalysts have been studied to achieve the photocatalytic hydrogen generation,nonetheless,low solar energy utilization ratio and poor redox capacity due to improper band structure limited the practical applications of those photo-catalysts.Considering the g-C3N4nanosheets-based materials with suitable band position and large surface,the hydrogen evolution of which has been discussed as a consequence [158,163,210,211].

Heetal.[130] constructed the g-C3N4/NiS hybrid heterostructure with an excellent photo-catalytic H2evolution ability of 29.68 μmol/h under visible light illumination,which could be comparable to some samples using Pt as cocatalyst.The CdS/g-C3N4nanocomposite was prepared [131] for the effective photocatalytic H2production performance (2120 μmol g-1h-1) due to the enhanced charges separation and transfer rate by the formation of hetero-junction (Fig.9).In addition to high H2generation rate,the recyclability and durability also made as-prepared photocatalysts eligible for widespread practical application.A ternary CdS/RGO/g-C3N4composite was constructed by Joetal.[182],the resultant showed enhanced photo-catalytic hydrogen generation performance compared with CdS/g-C3N4photo-catalysts,proving the RGO as an efficient electron mediator.PL emission spectra and transient photo-current measurements were applied to further demonstrate the charge separation and migration,the as-prepared composite showed decreased photo-excited charges recombination rate and prolonged lifetimeviathe formation of Z-scheme heterojunction,leading to the increased photo-catalytic hydrogen generation ability.Tahiretal.[212] fabricated g-C3N4/TiH1.92photocatalyst with a highly effective H2evolution ability of 75.55 μmol/h in visible light region.What is more,Zhangetal.[144] utilized noble metals (Ag,Au and Pt) to decorate g-C3N4/TiO2nanocomposite for the further improving of photo-catalytic H2-production activity to 519.73 μmol g-1h-1.Other g-C3N4nanosheets-based heterostructures such as SnS2/g-C3N4nanosheet [213],NiCo2O4/g-C3N4[214],ruthenium phosphide/g-C3N4[117],WS2/g-C3N4[133],g-C3N4/rGO/PDIP [152],FeSe2/g-C3N4[215] also have been reported as efficient photo-catalysts for the photo-catalytic hydrogen evolution.

Fig.9.Photocatalytic activity in H2 production.(a) The time-dependent H2 production curves of different photocatalysts.(b) H2 generation rate of different photocatalysts;All the photocatalytic H2 generation tests were measured under visible-light irradiation (λ>400 nm) and in 80 mL aqueous solution containing 10 vol% sacrificial reagents without cocatalysts.Reproduced with permission [131].Copyright 2022,Elsevier.

Besides constructing g-C3N4-based hetero-structures,controlling of morphology and defects could also been widely applied in photo-catalytic H2-production of g-C3N4to achieve much better performance [171,176,216].Zhangetal.[96] fabricated ultra-thin g-C3N4nanosheets with abundant mesopores and macropores structure,leading to excellent increased (25.5 times) light-driven photocatalytic H2generation ability.Defects rich g-C3N4nanosheets was constructed [172],the resultant exhibited significantly improved photo-catalytic H2evolution rate of 761.8±4.3 μmol g-1h-1under simulated sunlight illumination.

5.3.Other applications

Furthermore,g-C3N4nanosheets hybrids were also applied in photo-catalytic disinfection [154],N2fixation [169],organic synthesis [217],and so on.Lietal.[218] synthesized Ag2WO4/g-C3N4photo-catalyst with significantly enhanced photo-catalytic disinfection performance,as a result,Escherichiacolicells have been distorted and destroyed totally by as-prepared hybrid photo-catalyst within 90 min.Cao and co-workers [169] fabricated ultra-thin sulfur-doping g-C3N4(SCNNSs) possessing carbon vacancies,due to the superior N2molecules adsorption and activation ability,as well as the effective photo-generated charge carries separation,the N2fixation rate could reach 5.99 mmol L-1h-1gcat-1after 4-h simulated sunlight illuminationviathe obtained SCNNSs.

Apart from photo-catalytic field,g-C3N4nanosheets based composites also utilized to perform as lithium-ion batteries [219],photoelectrochemical devices [220],supercapacitor [221],fluorescence sensor [222,223],and bio-imaging and bio-sensor [156,224] and so on.With many advantages,such as low cost,good temperature and chemical stability,tunable electronic structure,appropriate band position,and acceptable redox potential,various application possibilities of g-C3N4nanosheets-based photo-catalysts will be explored in the near future.

6.Summary and perspective

This review summarizes the exfoliation methods,modification strategies,photo-catalytic mechanism and applications of g-C3N4nanosheets,numerous progresses have been achieved in the fabrication and functionalization of g-C3N4nanosheets for efficient water splitting,pollution mitigation,CO2photo-reduction,and so on in the past three years.To conclude,the g-C3N4nanosheets can be attainedviathree approaches: thermal oxidation,ultrasonic,and chemical exfoliation.In order to deal with poor light absorption range,high recombination rate of photo-induced charges,and low surface area of g-C3N4,many modification strategies such as construction of hetero-junctions,elements doping,defect control,and structure design have been widely studied and reported,the synthesis of those g-C3N4-based photo-catalysts improved the production ability and separation rate of photo-induced chargesviathe formation of type-II hetero-junctions,Z-scheme and S-scheme hetero-junctions,in addition,the redox ability can be further enhanced in the Z-scheme and S-scheme system,leading to a better photo-catalytic performance in environmental pollution degradation,energy production,nitrogen fixation,disinfection and so on.

In spite of the great achievements of g-C3N4nanosheets over the years,there are still many challenges for this material in photocatalysis.Firstly,although many exfoliation ways have been studied for attaining ultra-thin g-C3N4nanosheets,the decrease of lateral size,relative low product yield,and secondary pollution still exist.We could boost pressure during the thermal exfoliation process,cutting down the reaction temperature and time,leading to the less destroy of lateral size.We could also combine those exfoliation methods to elevate the product yield.Secondly,since the reports focus more on the improvement of photo-catalytic efficiency,actual mechanisms toward photo-catalytic reaction especially the water splitting and CO2reduction are still up in the air yet until now,we could use the theoretical computational simulations to analyze the photo-induced charge carrier transfer pathway and dynamics to further study the mechanism.Besides,the potential applications of g-C3N4nanosheets could further extend.In view of the facile synthesis,low price,non-toxicity,high stability against temperature and chemicals,and appropriate band gap and band position,the g-C3N4-based photo-catalysis could be widely applied in the near future.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors gratefully acknowledge the financial support by the Natural Science Foundation of China (Nos.21872102 and 22172080).