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加拿大人的 Graphitic C3N4 based noble-metal-free photocatalyst systems_ A review

Applied Catalysis B:Environmental 206(2017)556–588

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Applied Catalysis B:

Environmental

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /a p c a t

Review

Article

Graphitic C 3N 4based noble-metal-free photocatalyst systems:A review

Dilshad Masih,Yuanyu Ma,Sohrab Rohani ?

Department of Chemical and Biochemical Engineering,University of Western Ontario,London,Ontario,Canada

a r t i c l e

i n f o

Article history:

Received 8December 2016

Received in revised form 5January 2017Accepted 10January 2017

Available online 26January 2017

Keywords:g-C 3N 4

Noble-metal-free Nanomaterials Photocatalysis

a b s t r a c t

Many reviews are written on this interesting visible light active polymeric semiconductor material,the graphitic carbon nitride (g-C 3N 4).Yet the ever-expanding volume of the ongoing research on this mate-rials has inspired us to compile this review,especially on its nanoscale architectures of noble-metal-free photocatalyst systems.From the viewpoint of sustainable development,an economical photocatalyst which is made up of abundant elements e.g.C and N has a good prospect for large scale applications.Sta-bility of the photocatalyst material under the experimental conditions is essential for its repeated usage,however,many semiconductors sought for visible-light-driven reaction,particularly sul?des and nitrides are in a compromising situation.However,g-C 3N 4has high chemical-and photo-stability besides its high activity under visible light irradiation.Furthermore,solely semiconductor materials have the intrinsic problem of recombination of photogenerated electron-hole pairs.To overcome this problem,loading of the semiconductor with a co-catalyst,usually a noble metal is a common practice for transfer of electron and thus avoiding the recombination.Development of a noble-metal-free photocatalyst systems is essen-tially important for sustainable applications.Hence,the construction of a hybrid composite structure is interesting in the separation of photogenerated charge carriers.Besides diminishing the rate of recom-bination,the heterostructures are constructed for harnessing a wider spectrum of sunlight.In contrast to bulk semiconductors,their nanoscale counterpart offers a larger number of active sites along with interesting electrical and optical properties.Importantly,construction of extensive junctions between nanomaterials greatly enhance the separation of charges and consequently improve their photocatalytic ef?https://www.wendangku.net/doc/408054558.html,ually,the stability of materials is compromised with the reduction of size to nano level,however,g-C 3N 4and its nanomaterials demonstrate exceptional recycling in photocatalytic testing.One of the most important interests in controlling nanoparticle size,shape and composition is to develop noble-metal-free photocatalyst systems.Here in this review,we have compiled research on all the var-ious applications of noble-metal-free nanoscale photocatalyst systems based on g-C 3N 4.By the end,we conclude the research topic and put forward future perspectives for further developments in designing practicable photocatalyst systems.

Contents 1.Introduction ...........................................................................................................................................5572.

Photocatalysis over noble-metal-free g-C 3N 4based nanomaterials..................................................................................5582.1.Overall water splitting .........................................................................................................................5582.2.H 2evolution reaction .. (558)

2.2.1.Metal-free photocatalyst system ....................................................................................................5592.2.2.Binary hybrid system ................................................................................................................5602.2.3.Ternary and complex hybrid system (563)

2.3.O 2evolution reaction (566)

?Corresponding author.

E-mail addresses:dilshadmasih@https://www.wendangku.net/doc/408054558.html, (D.Masih),srohani@uwo.ca (S.Rohani).

https://www.wendangku.net/doc/408054558.html,/10.1016/j.apcatb.2017.01.061

D.Masih et al./Applied Catalysis B:Environmental206(2017)556–588557

2.4.H2O2generation (567)

2.5.CO2reduction (568)

2.6.N2?xation (569)

https://www.wendangku.net/doc/408054558.html,anic synthesis (570)

2.8.Degradation of organic pollutants (572)

2.8.1.Metal-free photocatalyst system (572)

2.8.2.Transition metal based system (572)

2.8.3.Post-transition metal based system (576)

2.8.4.Semimetal based systems (579)

2.8.5.Rare earth metals based system (579)

2.9.Biocide (580)

2.10.NOx abatement (580)

2.11.Metal ion redox (582)

3.Conclusions and future perspectives (584)

Acknowledgements (584)

References (584)

1.Introduction

Graphitic carbon nitride(g-C3N4)is a polymeric layered mate-rial,structurally analogous to graphene[1].In contrast to the pure C constituent of graphene,g-C3N4is composed of C and N with some impurity of H,which all are abundant raw materials.Semi-conducting properties of g-C3N4also drastically distinguish it from graphene.The band gap of bulk g-C3N4is～2.7eV,and it is a medium band gap semiconductor.Pertaining to its yellow color the optical absorption of g-C3N4lies around460nm making it an interest-ing material for harvesting solar energy.Furthermore,the thermal and chemical stability of g-C3N4in an aqueous suspension phase and under photocatalytic reaction condition makes it an interesting material[2].

This g-C3N4is regarded as the oldest synthetic polymer?rst reported by Berzelius and Liebig in the year1834and named as‘melon’[1].A?ow sheet diagram is provided in Fig.1show-ing a summary of historic developments in understanding g-C3N4 and its application in photocatalysis.In1922,Franklin found the empirical composition of‘melon’to be C3N4.Next,Pauling and Stur-divant derived tri-s-triazine type structure of C3N4in the year1937. By1940it was known that this material‘melon’has a graphite structure as reported by Redemann and Lucas[2].Photocatalysis received enormous attention after Fujishima and Honda reported photolysis of water on TiO2in1972[3].A wide variety of materials, mainly inorganic semiconductors were evaluated for photocat-alytic application.However,nobody paid attention to making use of g-C3N4in photocatalysis until Wang et al.?rst reported in2009 [4].

Since the pioneering photocatalytic studies by Wang et al.in 2009[4,5],g-C3N4has become the focus of research on photocat-alytic https://www.wendangku.net/doc/408054558.html,tely,a comprehensive review article is presented by Ong et al.[2].Besides some other relevant reviews,feature articles and perspectives on g-C3N4are important to read[6–15].

A record of yearly publications on photoactive applications of g-C3N4elucidates the magnitude of research interest in this material (Fig.2).Wang et al.put forward this polymeric material essentially as a metal-free visible-light active photocatalyst[4].An ef?cient overall water splitting ability using solar energy is considered a Holy Grail of photocatalysis.Once this is realized,a commercial scale production of green and renewable chemical fuel will become a viable process.There are several requirements for developing an ef?cient photocatalyst for water splitting under solar irradia-tion:(1)good properties for harvesting solar light,(2)a band gap of suitable energy with valence and conduction band appropri-ately positioned for the desired reactions,and(3)good stability of the photocatalyst under experimental conditions.In principal,this thermally and chemically stable polymeric semiconductor g-C3N4ful?lls the band gap requirement for harvesting solar energy with a band structure suitable for both water oxidation and reduction reactions.At present,there are a lot many ongoing efforts on the development of ef?cient and sustainable noble-metal-free photo-catalyst systems.

Environmental pollution and sustainable supply of greener energy are two of the main global challenges of the current era.Con-sidering the Sun as an almost inexhaustible and primary source of energy,lately,there are many interests in developing semiconduc-tor materials for harvesting solar energy to produce cleaner fuels and resolve the environmental https://www.wendangku.net/doc/408054558.html,tely,visible light active photocatalysts are getting enormous attention for applications to environment and energy sectors.Production of green and renew-able energy carrier,H2from water,reduction of CO2,synthesis of ?ne chemicals and remediation of environmental pollutants are the main explored reactions.

Polymeric g-C3N4is a metal-free medium band gap p-type semiconductor with a reliable chemical and thermal stability.Fur-thermore,its versatile optical and electronic properties with a band gap～2.7eV make it an attractive candidate for harvesting solar energy.TiO2is the most popular photocatalyst material due to its robust reactivity,availability,and chemical stability,however,it absorbs only UV light that limits its application[16].Among vari-ous photoactive applications,g-C3N4have been widely employed as a visible light active photocatalyst for degradation of organic pol-lutants,H2/O2evolution half reactions,complete water splitting, reduction of CO2and organic synthesis(Fig.3).The semiconduc-tor,g-C3N4is composed of earth-abundant elements.However,the noble/precious metals which are mainly loaded as a co-catalyst e.

g.Pt,to avoid the recombination of photogenerated electron-hole pairs are the costly constituents.A photocatalyst system is com-prised of a semiconductor or a junction of semiconductors along with a sensitizer and/or a co-catalyst.Here in this review we will discuss g-C3N4based photocatalyst systems which are altogether free from noble-metals and important for sustainable develop-ment.

Visible-light-driven photocatalyst systems are interesting for ef?cient harvesting of the solar spectrum.The band gap of pure g-C3N4(～2.7eV)corresponds to absorption of blue-light up to 450nm,hence inactive towards major part of the solar radia-tion comping to the earth.Furthermore,this relatively large band gap of the pure g-C3N4over-energizes the ideally required poten-tial(1.23eV)for the most desired water splitting reaction.Wang et al.applied various methods of doping[17,18],copolymerization [19,20],and dye sensitization[21]to modify g-C3N4for an optimum utilization of solar spectrum for a speci?c photoactive reaction. Next,the development of a noble-metal-free or precious-metal-free photocatalyst system is important for making it sustainable.

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556–588

Fig.1.Historic developments on understanding g-C 3N 4and photocatalysis over

it.

Fig.2.Year-by-year publications on photoactive properties of g-C 3N 4.Data from the Web of

Science.

Fig.3.Pie chart distribution of various photoactive applications of g-C 3N 4.

Intrinsically,g-C 3N 4is a layered material in which C 3N 4sheets are connected by Van der Waal forces.Hence single and a few layer sheets of g-C 3N 4are obtained upon the breakdown of these weak forces.Nanoscale materials offer unique regime of catalysis in between homogeneous catalysis and heterogeneous catalysis.However,the stability of nanoscale materials is challenging and needs attention for the important recycling of the material.Fur-thermore,nanomaterials based photocatalysts are more effective with the greatly enhanced availability of active surface sites [22].Wang et al.have done extensive research on the development of g-C 3N 4based nanoscale materials for photocatalytic applications e.g.H 2evolution [22–28],O 2evolution [29,30],and CO 2reduction [31].

In this review,we are summarizing g-C 3N 4based nanomaterials for all the various noble-metal-free applications in photocatalysis.

2.Photocatalysis over noble-metal-free g-C 3N 4based nanomaterials

2.1.Overall water splitting

To mimic the natural photosynthesis,a semiconductor-based photocatalytic water splitting into H 2and O 2has been the focus of many researchers.However,the bottleneck of overall water split-ting is the formation of oxygen–oxygen bond that requires the transfer of four electrons in a single step.The separation of pho-tocatalytically cogenerated H 2and O 2is yet another challenging task.There are only a few studies on overall water splitting over g-C 3N 4based noble-metal-free photocatalyst systems and summa-rized here.However,a vast majority of researchers have studied the half reaction of water splitting,mainly for H 2evolution only.

A composite structure of NiFe-layered double hydroxide and g-C 3N 4was investigated for overall water splitting under visible light irradiation [32].Remarkably,a high rate of production of H 2(744?mol h ?1g ?1)and O 2(443?mol h ?1g ?1)was found over optimized composite.A synergy in the composite of two layered materials as indicated by tuned-in band gap,quenching of pho-toluminescence (PL),and prolonged lifetime of charge carrier was assigned for this enhanced photocatalytic activity.However,the rate of evolution of both H 2and O 2was severely deteriorated with a repeated usage of the photocatalyst.

A composite structure comprising of carbon nanodots and g-C 3N 4was fabricated by Liu et al.as a metal-free photocata-lyst for overall water splitting [33].An optimized nanocomposite demonstrated impressive performance and stability with a quan-tum ef?ciency of 16%under visible-light irradiation.The rate of H 2(8.4?mol h ?1)and O 2(4.1?mol h ?1)evolution was main-tained in 200runs of recycling use of the photocatalyst over 200days.Contrary to the conventional belief of one-step four-electron process for photocatalytic water splitting,this study put forward a new hypothesis of two-step process;photocatalysis (two-electron)and chemical catalysis (two-electron).Carbon nan-odots were recognized for extending the absorption of visible light in the nanocomposite and assigned a center for the chemical catal-ysis step.

2.2.H 2evolution reaction

In contrast to overall water splitting,H 2and/or O 2evolution half reactions at the expense of a sacri?cial reagent are widely studied via photocatalysis.An ef?cient production of H 2gas from water over a suspended photocatalyst material using solar energy is considered an ideal reaction for the supply of green and renew-able energy.Antonietti and Domen groups did the pioneering work on g-C 3N 4towards the development of a stable,inexpensive and abundant photocatalyst material [4].And since then,more than 150papers on H 2and/or O 2evolution half reactions over g-C 3N 4

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based materials have appeared in the journals.Only a couple of publications reported on both of the H2and O2evolution half reac-tions,under varying conditions.In fewer than10papers,research was attempted on O2evolution half reaction only.In comparison to overall water splitting and O2evolution half reaction,exten-sive research(more than130papers)has been carried out on H2 evolution half reaction.

2.2.1.Metal-free photocatalyst system

From the beginning of the research on g-C3N4as a photocat-alyst,it became known that this material has a potential for H2 evolution from water.The rate of H2evolution was varying from batch to batch,in the range of0.1–4?mol h?1under visible-light irradiation[4].Under the experimental conditions,even a meso-porous structure of g-C3N4remained inef?cient for H2evolution from water,the rate of H2evolution reaction drastically increased with the loading of Pt onto g-C3N4.

To extend the research on sustainable metal-free g-C3N4 photocatalyst,Cui et al.synthesized various conjugated nanostruc-tures via solution processing[34].Physicochemical properties and bandgap size of the carbon nitride nanostructures were dependent upon synthesis conditions.Nanobelts with a moderate speci?c sur-face area(30m2g?1)and the band gap of1.78eV demonstrated the highest stability and rate of H2evolution(6.1?mol h?1)under visible light irradiation,amongst all the various conjugated car-bon nitrides.Suryawanshi et al.investigated on the nanocomposite with nanotubes to understand the electronic and morphological changes in g-C3N4[35].Various concentrations of multiwall car-bon nanotubes(MWCNT)were mixed with g-C3N4for the synthesis of an all-carbon photocatalyst.A twofold increase in H2evolution activity was observed under visible-light irradiation for the opti-mized metal-free photocatalyst system,0.5%MWCNT/g-C3N4.

Lately,further progress towards the development of a metal-free photocatalyst was made by functionalization of g-C3N4with electron acceptor carbon nanoparticles derived from zeolitic imi-dazolate framework(ZIF).A metal-free bifunctional catalyst system of carbon nanoparticles decorated on g-C3N4was reported recently by He et al.[36].Precursors for both the carbon nanoparticles and the g-C3N4were mixed together and treated in one-step at 650?C under N2?ow to obtain the composite photocatalyst.Car-bon nanoparticle of about60nm size was in-situ derived from a zeolitic imidazolate framework during the thermal conversion of melamine into g-C3N4.A TEM image of the composite with encir-cled carbon nanoparticles grown on sheets of g-C3N4is provided in Fig.4A.Before photocatalytic testing,the composite was washed with HCl solution to remove the residual Zn,and to make sure it is an all-carbon system.In comparison with the pristine and Pt loaded g-C3N4,all the composites with various concentrations of car-bon nanoparticles displayed better activity towards H2evolution (Fig.4B).The rate of H2evolution over1wt.%carbon nanopar-ticle functionalized g-C3N4(32.6?mol h?1)was36times higher than that of the pure g-C3N4(0.9?mol h?1),under visible-light irradiation.Interestingly,under the experimental conditions this metal-free carbon composite performed2.8times better than3% Pt loaded g-C3N4(11.6?mol h?1).The PL spectra provided in the inset in Fig.4B exhibited a small quenching upon loading of g-C3N4 with Pt metal.However,a signi?cant decrease in the PL intensity was observed for the optimized composite with C nanoparticles, suggesting an improved ef?ciency for separation of charge carri-ers.This trend of the charge carrier dynamics was consistent with the photocatalytic activity.Besides the photocatalytic reduction of water,this g-C3N4composite with C nanoparticles was function-ing as an ef?cient electrocatalyst for hydrogen evolution reaction. Fang et al.prepared carbon dots(C-dots)modi?ed g-C3N4hybrid by a novel strategy using C-dots and dicyandiamide as starting materials[37].The rate of H2evolution over the pure g-C3N4

was Fig.4.(A)TEM image of ZIF derived C nanoparticles grown on g-C3N4sheets,and(B) Photocatalytic H2evolution rate of pure g-C3N4,3wt.%Pt/g-C3N4and g-C3N4com-posites with different amounts of ZIF derived C nanoparticles.Inset in(B)provides PL spectra.

Reproduced with permission from the Ref.[36].Copyright(2016)Royal Society of Chemistry.

91?mol g?1h?1that sharply increased by a factor of2.4times (218?mol g?1h?1)for the sample optimized with C-dots.

A simple process of acid treatment and sonication for the synthesis of few-layer nanosheets of g-C3N4was put forward by Ma et al.[38].In comparison with the bulk g-C3N4,the absorption edge showed a blue shift and the corresponding emis-sion peak also shifted towards lower wavelength.Hence,the band gap was decreased.For the visible-light-induced H2evo-lution reaction,the results showed a comparable activity and rate(110.68?mol g?1h?1)over a few-layer nanosheet of g-C3N4.

A further modi?cation of nanosheets was made by Liang et al. through the development of a holey structure[39].Thermal treat-ment of bulk g-C3N4under NH3atmosphere converted it to holey nanosheets with self-modi?ed carbon vacancies,and an enlarged band gap(2.95eV).As expected for a porous structure,speci?c sur-face area of the holey nanosheets(196m2g?1)was higher than non-porous nanosheets(179m2g?1).In comparison to the photo-catalysis over bulk g-C3N4,a good improvement in photocatalytic ef?ciency was demonstrated by the porous nanosheets.The rate

560 D.Masih et al./Applied Catalysis B:Environmental206(2017)556–588

of H2production from water under visible-light irradiation was 20times faster on the holey nanosheets of g-C3N4(82.9?mol h?1) compared to that of the bulk material(4.2?mol h?1).Furthermore, the holey nanosheets were stable under the experimental condi-tions.

Xia et al.explored the spectral and electronic synergistic effects by integrating g-C3N4nanosheets with carbon quantum dots (CQDs)via a one-step hydrothermal method[40].XRD patterns and TEM images showed the presence of CQDs in the composite struc-ture.DR-UV–vis spectra demonstrated extended absorption in the visible-to-near infrared(NIR)region.Besides H2evolution under UV and UV–vis light irradiations,this half-reaction by a metal-free photocatalyst was also realized under NIR-light for the?rst time. For a composite photocatalyst with optimized(10%)CQDs,the rate of H2production was slow,6.76?mol g?1h?1upon irradiation with a laser beam of NIR-light(808nm).The rate of H2production was increased to50.5?mol g?1h?1under visible-light,and it was found the highest upon illumination with UV–vis,to219.5?mol g?1h?1.

2.2.2.Binary hybrid system

While many researchers were focusing on modi?cations of g-C3N4and loading it with noble metal nanoparticles for H2evolution from water,Hou et al.prepared layered nanojunctions of MoS2and mesoporous g-C3N4for noble-metal-free catalysis[41].The rate of H2production over the nanojunction increased?rst until load-ing0.2wt.%MoS2and then decreased with further loadings.On the other hand,for the Pt-loaded sample,the rate of H2production kept on increasing from0.1wt.%to2.0wt.%.For intermediate loadings of0.5wt.%of MoS2(20.6?mol h?1)and Pt(4.8?mol h?1)loaded samples,the rate of H2evolution on the noble-metal-free system was more than4times higher.The optimized system showed an apparent quantum yield of2.1%at420nm.However,the cyclic runs for H2production over the optimized MoS2system showed deac-tivation.Yu et al.loaded nanoparticles of Ni(OH)2onto g-C3N4by a simple precipitation method[42].TEM analysis showed that some nanoparticles of Ni(OH)2in the range of10–20nm were deposited on the surface of g-C3N4nanosheets.Visible-light-driven photocat-alytic production of H2was enhanced with this low-cost Ni(OH)2 co-catalyst.Among various forms of nickel,Ni(OH)2co-catalyst was found better for H2production,in comparison with pure Ni and NiO.Under the experimental conditions,the rate of H2production over the optimal Ni(OH)2/g-C3N4(7.6?mol h?1)photocatalyst was approaching that of a noble metal loaded g-C3N4(8.2?mol h?1). For repeated use of photocatalyst,a decrease in the activity was noticed after the?rst run and then was maintained for H2pro-duction.The apparent quantum ef?ciency was1.1%and after the second cycle,the H2production remained stable over the Ni(OH)2-modi?ed g-C3N4system.The noble-metal-free system of10–20nm Ni(OH)2precipitated on g-C3N4nanosheets showed comparable photocatalytic activity with Pt/g-C3N4.

In an effort to develop an understanding of a noble-metal-free photocatalyst,Bi et al.studied the changes in the surface band bending upon composite formation of g-C3N4with Ni metal via a solvothermal method[43].About30nm metallic Ni nanopar-ticle were loaded on g-C3N4and characterized by XRD and TEM analysis.Mott-Schottky plots showed a deeper band bending for the optimized Ni/g-C3N4composite,and demonstrated a higher ef?ciency in the separation of photogenerated electron-hole pairs. Under the experimental conditions,pure g-C3N4without Ni co-catalyst was almost inactive.The rate of H2production steadily increased with Ni metal loading onto g-C3N4and reached a max-imum of8.41?mol h?1for about10%Ni/g-C3N4(under full light) which was comparable with that of the one modi?ed with Ni(OH)2, 7.6?mol h?1(under visible light)[42].Many researchers inves-tigated Ni as a noble-metal alternate in learning from nature,as nickel is found in the hydrogenase enzymes that regulate hydrogen in biological systems.

Lately,Kong et al.reported light-assisted rapid preparation of Ni-based robust photocatalyst,a magnetic composite of Ni/g-C3N4 [44].Uniformly dispersed metallic Ni nanoparticles with diameters of30–80nm were photodeposited on the surface of g-C3N4.Under full light,an extremely high rate of H2evolution,4318?mol g?1h?1 was obtained for the optimized system containing7.4%Ni.The apparent quantum yield of H2at400nm was2.01%,and the catalyst was durable after48h of recycling tests.

Another sulfur-based low-cost material,WS2was decorated onto g-C3N4as a noble-metal-free co-catalyst.Akple et al.con-structed a heterojunction between WS2loaded onto g-C3N4 through a gas-solid reaction under an inert atmosphere[45].The composite structure was prepared by a gas-solid reaction under an inert atmosphere.The crystalline structures of both semiconduc-tors in the composite were identi?ed with TEM analysis(Fig.5A& B)and XRD patterns(Fig.5C).Under the experimental conditions, both g-C3N4and WS2were individually inactive for the photocat-alytic H2evolution from water.The rate of H2production over the optimized composite with a very small loading of WS2(nominal 0.01wt.%)onto g-C3N4was101?mol g?1h?1under visible-light irradiation,which was even better than that of the sample with the same amount of Pt co-catalyst(72?mol g?1h?1).As shown in Fig.5D,the cyclic photocatalysis runs demonstrated a stable pro-duction of H2over this noble-metal-free WS2/g-C3N4.Furthermore, the photoelectrochemical measurements con?rmed the stability of the binary hybrid system.

Towards the development of a noble-metal-free photocatalyst for H2generation from water,Chen et al.designed a novel strat-egy to fabricate homogeneously distributed nanoparticles of C and N co-doped TiO2on ultrathin nanosheets of g-C3N4by a simple one-pot solvothermal route[46].As learned from the synergis-tic combination TiO2/g-C3N4,a superior photocatalytic ability was expected for this composite pertaining to its improved optical, electrical and physicochemical properties.Under visible light illu-mination,the nanocomposite exhibited superior H2generation comparing with the individual components of the heterojunction. The rate of H2evolution over the optimized composite with3wt.%C and N co-doped TiO2reached to39.2?mol g?1h?1which was10.9 and21.3times higher than the sole C,N-TiO2nanoparticles,and pure g-C3N4nanosheets,respectively.Furthermore,the optimized catalyst remained stable in recycling tests.Liu et al.hydrother-mally hybridized well dispersed10–20nm CdZnS with g-C3N4 nanosheets for visible-light-driven H2evolution and degradation of organic pollutants[47].Over the optimized composite,the rate of H2evolution from water was208?mol h?1that showed a slight decrease in cyclic runs.Recently,～25nm size CoP nanoparticle modi?ed g-C3N4nanostructures were prepared by Yi et https://www.wendangku.net/doc/408054558.html,ing a simple grinding of the precursors[48].Visible-light-driven rate of H2generation over this noble-metal-free0.25wt.%CoP loaded g-C3N4reached to～475?mol g?1h?1.The rate was～131times higher than that of the pure g-C3N4and even better comparing with the optimized Pt/g-C3N4.The noble-metal-free nanostruc-ture demonstrated no loss of H2production activity in repeated runs.

Recently,Wang et al.prepared heterostructure of?-oxo dimeric iron(III)porphyrin,(FeTPP)2O and g-C3N4through a solution phase chemical reaction[49].Sole(FeTTP)2O and its physical mixture with g-C3N4were not active for photocatalytic H2production. Formation of heterostructure was sought to be important for a photocatalytic activity where(FeTTP)2O acted not just as a photo-sensitizer,but also helped with separation of charges.Full UV–vis light irradiation stimulated the(FeTTP)2O/g-C3N4photocatalyst to produce H2at a rate of about40?mol h?1.While under visible-light irradiation,the rate of H2production was11?mol h?1[49].

D.Masih et al./Applied Catalysis B:Environmental206(2017)556–588561

Fig.5.(A)TEM and(B)HR-TEM image of g-C3N4/WS2(0.01%)sample,(C)XRD patterns of g-C3N4,WS2and various samples of g-C3N4/WS2,and(D)Cyclic runs for the photocatalytic H2production on g-C3N4/WS2(0.01%)sample.

Reproduced with permission from the Ref.[45].Copyright(2015)Elsevier.

Fig.6.(A)Steady-state PL spectra for different components in H2O,the concentrations are the same as those in photocatalytic reaction,(B)Photocatalytic H2evolution amount obtained from water reduction in the presence of different components,(C)Transient?uorescence decay for different components in H2O,the concentrations are the same as those in photocatalytic reaction,and(D)Schematic illustration of the photocatalytic process for H2evolution.

Reproduced with permission from the Ref.[51].Copyright(2013)Royal Society of Chemistry.

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Next,studies on Ni-dimethylglyoxime(dmgH)based molecu-lar catalyst combined with g-C3N4were continued by Cao et al. in order to prepare a noble-metal-free photocatalyst system[50]. The particle size of the sub-microwires of Ni-dmgH was smaller (～230nm)when grown in the presence of g-C3N4.A steady rate of H2generation(1.18?mol h?1)was found for the optimal cou-pling of Ni-dmgH with g-C3N4and maintained in cycling tests. Abnormally a prolonged induction time period was noticed for the onset of the photocatalytic H2evolution reaction over Ni-dmgH/g-C3N4.Cobaloxime was also explored by Cao et al.as an organometallic co-catalyst to fabricate an economical and a noble-metal-free photocatalyst system[51].The importance of the sacri?cial reagent,triethanolamine(TEOA)was established by the spectroscopic investigations and observed in the photocat-alytic production of H2.Quenching of the PL intensity was noted with the construction of a hybrid system of cobaloxime and g-C3N4,and further enhanced in the presence of TEOA(Fig.6A). None of the combinations was active except for g-C3N4loaded with cobaloxime and in the presence of TEOA(Fig.6B).After an early induction period,the steady-state rate of H2production was about2.6?mol h?1.Transient?uorescence studies revealed the separation and transfer of photoexcited electrons from g-C3N4to cobaloxime in the presence of TEOA(Fig.6C).A schematic illus-tration of the role of cobaloxime and TEOA in the separation of photogenerated charges on g-C3N4nanosheet,and thus the pro-duction of H2is given in Fig.6D.

Hong et al.prepared a noble-metal-free,NiS loaded g-C3N4 binary system by a simple hydrothermal method[52].Mesoporous g-C3N4was prepared using SiO2nanosphere(12nm)as a tem-plate and loaded with NiS nanoparticles by hydrothermal method. The optimized composite photocatalyst,ca.1.25wt.%NiS/g-C3N4 showed an ef?cient rate of H2evolution(48.2?mol h?1)upon visible-light irradiation which is around70%that of the Pt/C3N4 under the same experimental conditions.Time course of H2pro-duction over NiS/g-C3N4in the recycle study showed a gradual decrease in the activity.The apparent quantum ef?ciency of1.9% was recorded at440nm.

Recently,Lu et al.also worked on the same system,NiS/g-C3N4, in an effort to develop a visible-light-driven noble-metal-free pho-tocatalyst[53].In contrast to the mesoporous g-C3N4used by Hong et al.[52],a liquid phase exfoliated g-C3N4nanosheets were dec-orated with the co-catalyst NiS by precipitation and hydrothermal method.The thickness of the exfoliated nanosheets of g-C3N4was ～3nm.In the composite structure,nanoparticles of NiS are well anchored onto g-C3N4nanosheets as shown in the TEM images (Fig.7A&B).As noticed in other Ni-based photocatalyst sys-tems,an initial induction time period for the photocatalytic H2 evolution reactions over NiS/g-C3N4under visible-light irradia-tion.The production of H2over nanosheet g-C3N4loaded with noble-metal-free co-catalyst,NiS was25%less ef?cient than that with Pt(5.6?mol h?1).However,under the optimized experimen-tal conditions when equating with Pt loaded g-C3N4photocatalyst, the working ef?ciency of NiS/nanosheet g-C3N4(75%)was better than NiS/mesoporous g-C3N4(70%)composite.The rate of pho-tocatalytic H2production was correlated with the wavelength it was exposed to,and the behavior was parallel to the optical absorbance properties(Fig.7C).Comparing with bulk NiS/bulk g-C3N4(1.6?mol h?1)composite,about2.6times enhancement in the rate of H2evolution was observed over NiS/nanosheet g-C3N4(4.2?mol h?1).A relationship of bulk and nanosheet structure with the photocatalytic performance is illustrated in the schematic (Fig.7D).

Raziq et al.synthesized B-doped g-C3N4nanosheets and fabri-cated their composite with a nanocrystalline anatase TiO2[54].TEM images showed that nanocrystalline TiO2with5nm diameter was well-dispersed on the surface of B-doped g-C3N4.All the various photocatalytic applications of the nanocomposite,TiO2/B-doped g-C3N4in H2evolution,CO2reduction and degradation of envi-ronmental pollutants exhibited rather high activities compared to those of bare g-C3N4,under visible-light irradiation.In this co-catalyst-free system,B-induced surface state near the valence band top was suggested to trap holes and hence enhance the separation of charges.Under the experimental conditions,the photocatalytic H2production over bare g-C3N4(0.52?mol h?1)was negligible. The rate of H2evolution increased21times with B-doping of the g-C3N4(11?mol h?1).Comparing with bare g-C3N4,a very high 29times enhancement of H2production was observed for the optimized nanocomposite of TiO2/B-doped g-C3N4(15?mol h?1). Though the overall rate of H2evolution over TiO2/B-doped g-C3N4 was not good amongst similar TiO2based composite systems,this nanocomposite demonstrated a relatively great increase in ef?-ciency when comparing with pristine g-C3N4.

A visible-light active,highly ef?cient and stable compos-ite of Cd0.5Zn0.5S nanoparticles of smaller than100nm and g-C3N4was prepared by the solvothermal method[55].With-out any co-catalyst,the hybrid heterojunction Cd0.5Zn0.5S/g-C3N4 photocatalyst demonstrated improved rate of H2production of 20.8mL h?1as compared with13.6mL h?1that of the pure Cd0.5Zn0.5S.For the optimized system,the photocatalytic activ-ity was maintained by over95%in the cyclic production of H2. In an effort to prepare a noble-metal-free photocatalyst,Liu et al. fabricated composites of Mn0.8Cd0.2S and g-C3N4nanosheets by a facile hydrothermal method[56].TEM analysis depicted50nm nanoparticles of multi-metal sul?de well-dispersed on the surface of g-C3N4.Under visible-light irradiation,the rate of H2production over pure Mn0.8Cd0.2S was1.2mmol h?1g?1that increased about

3.4times after formation of its composite,Mn0.8Cd0.2S/g-C3N4

(4.0mmol h?1g?1).Cyclic runs for photocatalytic H2production over the optimized composite demonstrated its good stability under experimental conditions.Research on Cd-based sul?de composites of g-C3N4was continued by Liu et al.,and a meso-porous g-C3N4was obtained by using a hard-template loaded with CdLa2S4nanoparticles via a hydrothermal method[57].TEM analy-sis showed an intimate interfacial contact between50nm spherical nanoparticles CdLa2S4and nanosheets of g-C3N4.For the optimized composite of CdLa2S4/g-C3N4,the rate of H2evolution reach to 5.98mmol h?1g?1under visible light irradiation,which was about 7.7times higher than pure CdLa2S4(0.77mmol h?1g?1).The appar-ent quantum ef?ciency of the optimized heterojunction was7.1% at420nm.

Besides Cd-based sul?de materials,Liu et al.also reported on the noble-metal-free fabrication of g-C3N4composite with another bimetallic sul?de via a facile hydrothermal method[58].Hexago-nal ZnIn2S4was chosen for it has a layered structure similar to g-C3N4.Sheet-on-sheet nanocomposites of ZnIn2S4/g-C3N4were prepared with various compositions,and optimal content of g-C3N4 was40wt.%for an ef?cient photocatalytic activity.The thickness of ZnIn2S4sheets was about10nm.A gradual increase in photocat-alytic activity was observed with increasing mass ratio of g-C3N4 onto ZnIn2S4that started to decrease beyond40wt.%loading.An enhanced photocatalytic activity of the optimized nanocompos-ite was ascribed to ef?cient separation and transfer of charges at the interface between ZnIn2S4and g-C3N4rather than physico-chemical properties.Under visible-light irradiation,the rate of H2 production over pure ZnIn2S4was500?mol h?1g?1that increased about1.91times upon formation of its nanocomposite with g-C3N4 (953.5?mol h?1g?1).Cyclic photocatalytic runs demonstrated a stable production of H2over ZnIn2S4/g-C3N4under visible-light illumination.However,comparing with ZnIn2S4/g-C3N4the com-posites of Cd-based S materials with g-C3N4were more ef?cient for the photocatalytic production of H2.Interestingly,around the same time period Zhang et al.also reported on photocatalytic activity

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Fig.7.(A)TEM and(B)HR-TEM image of g-C3N4/NiS sample,(C)Dependence of photocatalytic hydrogen production rate on the wavelengths and optical spectrum of the g-C3N4/NiS-1.0sample,and(D)Schematic illustration of the visible light photocatalytic performance of NiS loaded on to bulk g-C3N4and nanosheets of g-C3N4. Reproduced with permission from the Ref.[53].Copyright(2015)Royal Society of Chemistry.

of sheet-on-sheet heterostructure of ZnIn2S4/g-C3N4with various compositions[59].TEM images of the heterostructure showed that 4–9nm thick sheets of ZnIn2S4were vertically grown onto the nanosheets of g-C3N4and built a hierarchical structure.Similar to the method adopted by Liu et al.[58],ZnIn2S4nanosheets were in-situ grown onto g-C3N4nanosheets through a facile hydrother-mal route.Contrary to the composite compositions studied by Liu et al.[58],when ZnIn2S4was?xed while loading of g-C3N4was varied from0to50wt.%,here in this study g-C3N4was taken as a reference material and modi?ed with various amounts of ZnIn2S4,0–20wt.%.Under visible-light irradiation,the H2pro-duction rate for the optimized photocatalyst system with15wt.% ZnIn2S4loaded onto g-C3N4was14.1?mol h?1which was very low compared to the one reported by Liu et al.(953.5?mol h?1g?1) [58].On the other hand,under the experimental conditions a rela-tive improvement in the optimized photocatalytic system against the reference material ZnIn2S4was reported at nearly4times and1.91times by Zhang et al.[59]and Liu et al.[58],respec-tively.Recycling photocatalytic tests demonstrated good stability for the production of H2over ZnIn2S4/g-C3N4under visible-light irradiation.Regarding the enhancement of photocatalytic activity, in addition to the extended absorption of light by the composite structure of nanosheets,it was mainly attributed to ef?cient sepa-ration and transfer of charges at the interface between ZnIn2S4and g-C3N4.The optimum ZnIn2S4/g-C3N4ratios found for photocat-alytic H2production were quite different for both the research groups,speci?cally at60/40and at15/85by Liu et al.[58]and Zhang et al.[59],respectively.

In the search for an economically viable photocatalyst material, a noble-metal-free heterojunction of ZnS and g-C3N4was con-structed by Suyana et https://www.wendangku.net/doc/408054558.html,ing a one-pot co-pyrolysis synthesis protocol[60].TEM studies indicated4–7nm size ZnS nanoparti-cles in the composite.Transient spectroscopic and photocurrent measurements along with PL spectra revealed a reduction in recombination of charges in the ZnS/g-C3N4composite.Under visible-light irradiation,an overall rate of H2production over the optimized composite14wt.%ZnS/g-C3N4(871?mol g?1h?1)was only about25%higher than pure g-C3N4(670?mol g?1h?1).The heterojunction was active in dye degradation and organic synthesis too.

2.2.

3.Ternary and complex hybrid system

A g-C3N4based complex system comprising of earth-abundant elements,nickel,thiourea,and trimethylamine was in-situ con-structed during the photocatalytic process[197].This complex but low-cost photocatalyst system was highly ef?cient in the produc-tion of H2from water reduction.Under the solar light,the rate of H2production over this complex system was51?mol h?1,and that

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was comparable with Pt co-catalyst loaded g-C3N4(59?mol h?1). Amongst other transition metal(Fe,Cu,and Co)based complex sys-tems,only Co based photocatalyst showed some activity,but less than half that of the Ni comprising catalyst.

A ternary nanocomposite of mesoporous g-C3N4,multi-walled carbon nanotubes(MWCNT),and a metal sul?de co-catalyst was prepared by Zhong et al.via sol-gel followed directing precipita-tion[61].At?rst,a nanocomposite of40–60nm diameter MWCNT was constructed with mesoporous g-C3N4which was prepared with SiO2(12nm)hard template.Three sul?des of different tran-sition metals(Cu,Co,and Ni)were investigated as a co-catalysts to replace the noble metal,Pt.TEM images showed nicely formed ternary hybrid structure,and other characterizations illustrated accordingly favorable optical,and electronic properties.Among the nanocomposites with the same co-catalyst loadings(3wt.%), the one with NiS demonstrated the best photocatalytic activity under visible-light irradiation.TEM studies showed that5–20nm size?ne nanoparticles of NiS were strongly coupled with the sur-face of the?akes of mesoporous g-C3N4.The average rate of H2 production over the nanocomposite with NiS co-catalyst reached 378?mol g?1h?1,and that was1.37times and12.2times higher than with CoS and CuS co-catalysts,respectively.The amount of loading for the best performing NiS co-catalyst was optimized at 1wt.%,and the corresponding rate of H2production reached to 521?mol g?1h?1which was148times higher than that for the mpg-C3N4/MWCNT.A signi?cant loss of H2production activity was observed in the second run.However photocatalytic activity in later cycle did not decrease drastically,and H2production became stable after three cycles.In search of a noble-metal-free photo-catalyst,Yuan et al.loaded NiS co-catalyst onto heterojunction of CdS nanorods and g-C3N4nanosheets via two-step wet chemistry method[62].In-situ grown CdS nanorods of about10nm diame-ter and100nm length were uniformly distributed on the surface of g-C3N4nanosheets.The size of NiS nanoparticles was ranging from10to40nm and HR-TEM exhibited clear lattice fringes of NiS and CdS on g-C3N4nanosheets.The average rate of H2evo-lution under visible-light was increasing with the amount of the co-catalyst,and the optimized loading of NiS was9wt.%.Over the optimized nanocomposite,the rate of H2production reached to 2563?mol g?1h?1that was1582times higher comparing with pure g-C3N4.In the repeated time courses for photocatalytic H2 evolution,the decrease in activity was prominent after the?rst run and maintained in the next cycles.

Instead of a single co-catalyst,Wen et al.loaded dual co-catalyst comprising of NiS and carbon black onto g-C3N4nanosheets to develop a noble-metal-free photocatalyst system[63].A hetero-junction was prepared from30to60nm sized NiS(1.5%)deposited on the surface of g-C3N4and further decorated with20–30nm carbon black(0.5%)nanoparticles.The optimized ternary pho-tocatalyst system exhibited a very high rate of H2evolution, 992?mol g?1h?1which was higher than that of the binary com-posites,and with3wt.%Pt co-catalyst photocatalyst.This ternary composite showed a gradual loss of photoactivity in repeated cycles,typical of sul?de materials.Similarly,acetylene black dec-orated with Ni(OH)2was found a robust dual co-catalyst for an ef?cient photocatalytic activity of g-C3N4under visible-light[64]. The nanoparticle of20–50nm acetylene black formed a chain-like structure on g-C3N4and the composite was decorated with in-situ grown Ni(OH)2nanoparticles.The photocatalytic H2production ef?ciency of pure g-C3N4(0.75?mol g?1h?1)was increased with the loading of acetylene black(2.4?mol g?1h?1).However,with loading a well-known Ni(OH)2co-catalyst for H2evolution reac-tion,a100fold increase in activity was observed comparing with pure g-C3N4.Next,a drastic increase in photocatalytic activity was observed when Ni(OH)2decorated acetylene black was used as a dual co-catalyst.The rate of H2production over this ternary nanocomposite reached to240?mol g?1h?1with a320times increase in comparison with pristine g-C3N4.These studies demon-strated an important strategy of combining nano-carbons with other earth-abundant co-catalysts as replacement of noble metal materials.

For the development of a noble-metal-free photocatalyst,Mori et al.made use of two different types of graphitic carbon nitrides, bulk g-C3N4and delaminated nanostructured g-C3N4in under-standing the effect of the nanostructure[65].They designed a ternary composite with Ni complex and a visible-light-responsive organic dye,thiazole orange(TO;band gap1.84eV).For the nanos-tructured g-C3N4,the BET speci?c surface area(148m2g?1)and pore volume(0.43cm3g?1)were about15and10times,respec-tively large than those of the bulk g-C3N4and found effective in enhancing photocatalytic properties.The amounts of loadings of the active site of molecular level Ni co-catalyst species and the photosensitizer TO were optimized for H2production from aqueous solution and remained stable during the photocatalytic reaction.Ni K-edge X-ray absorption?ne structure spectroscopy and TEM stud-ies on both fresh and used materials demonstrated mononuclear species of Ni co-catalyst that remained stable during photocataly-sis.Hence,molecular species of Ni,rather than the colloidal form, was responsible for the photocatalytic reaction.Over the bulk g-C3N4based optimized system the rate of H2production was about 1.7?mol h?1that increased more than three times upon changing it to nanoporous g-C3N4(about5.8?mol h?1)with a high speci?c surface area and porosity.

A facile thermal process was developed by Pany et al.for one-step N and S incorporation into TiO2and fabrication of its nanocomposite with g-C3N4[66].Nanoparticles of around20nm were observed in the TEM images consistent with the crystal-lite size calculated from XRD pattern by using Scherrer formula. This known synergistic combination of TiO2/g-C3N4was made more effective with the introduction of dopants,N and S.Fur-thermore,small crystallite size,phase pure anatase,and a high speci?c surface area were found effective for enhanced visible light absorption and separation of charges.In this complex system,the leftover of sulfate from the titania precursor was incorporated onto the N,S-TiO2/g-C3N4heterojunction and acted as a co-catalyst. For the optimized N,S-TiO2/g-C3N4system,the rate of photo-catalytic H2production reached to317?mol h?1,which was2.5 times higher than pure g-C3N4.In comparison with simple TiO2/g-C3N4[67](52.71?mol h?1)heterojunction,the overall rate of H2 evolution was higher for this complex system,N,S-TiO2/g-C3N4 (317?mol h?1).Furthermore,the N,S-TiO2/g-C3N4complex sys-tem demonstrated stable performance during repeated usage for H2evolution reaction.

Another TiO2-based complex system was investigated by Jiang et al.towards the development of the noble-metal-free system[68].

A novel ternary composite of TiO2–In2O3with g-C3N4was synthe-sized by a solvothermal method.The nanoparticles of TiO2–In2O3 were densely and uniformly distributed on the surface of g-C3N4. As observed under TEM,the average diameter of the TiO2/In2O3 nanoparticles loaded onto g-C3N4was around12nm(Fig.8A).BET speci?c surface area of the ternary composite reached to90m2g?1. Photocatalytic H2evolution activity of pure and hybrid materials is depicted in Fig.8B.An enhanced photocatalytic activity by the ternary composite was attributed to a low rate of recombination along with a high speci?c surface area.Under visible-light illumi-nation from LED,the rate of H2production was8.6?mol h?1that was higher than binary hybrids,and48times higher than that of the pure g-C3N4.

A complex system of g-C3N4co-sensitized with two dyes namely Eosin Y(EY)and Rose Bengal(RB),and loaded with in-situ grown Co(OH)2nanoparticle was developed for H2evolution under

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Fig.8.(A)TEM image of TiO2-In2O3nanoparticles decorated on g-C3N4,and(B)Pho-tocatalytic H2evolution activity of pure materials,binary composites,and ternary composite.

Reproduced with permission from the Ref.[68].Copyright(2015)Elsevier. visible-light irradiation[69].The average size of Co(OH)2nanopar-ticles in TEM image was3nm and in good agreement with the one calculated from the line width analysis of the diffraction peak using Scherrer equation.Absorption of the visible light was extended from430nm to600nm with the co-sensitization of a binary sys-tem,Co(OH)2/g-C3N4.Under the reaction conditions,only a trace amount of H2was produced over Co(OH)2/g-C3N4.Essentially no H2evolution was observed over the catalyst systems composed of dyes and g-C3N4without Co(OH)2co-catalyst loading.With a synergistic effect of co-catalyst and sensitizers,an optimized rate of H2evolution(143.9?mol h?1)was obtained for the30wt.%Co loaded g-C3N4co-sensitized with EY to RB molar ratio at unity, and operating at pH9.Stable performance in repeated runs of H2 evolution reaction and a pro?le of constant photocurrent ascribed sustainable use of this noble-metal-free system.In contrast to the approach of co-sensitization with two dyes,Hao et https://www.wendangku.net/doc/408054558.html,ed only Eosin Y for photosensitization of g-C3N4and modulated the selec-tive transfer of photogenerated electrons on differently exposed facets of binary metal(Co and Mo)sul?de[70].The Co x Mo1-x S2/g-C3N4composite was prepared via a facile solvothermal method and the dye photosensitizer was added into the aqueous reaction mixture and used for H2evolution reaction.The binary metal sul-?de coupled with g-C3N4comprised of irregular nanoparticles of 50nm to200nm aggregates.In comparison with a30%Co/g-C3N4ratio for an optimum photoactivity,a high amount of co-catalyst at 70%Co x Mo1-x S2/g-C3N4was needed for the best H2production ef?-ciency at pH9and under visible light irradiation.The EY sensitized pure g-C3N4(2.2?mol h?1)showed a slight activity for H2pro-duction from an aqueous solution that increased to29.3?mol h?1 and78.4?mol h?1with loading a single metal sul?de co-catalyst of CoS2and MoS2,respectively.Next,with the co-loading of both the sul?des the complex system Co x Mo1-x S2/g-C3N4demonstrated an enhanced rate of H2production,176.5?mol h?1.Furthermore,the photocatalyst system showed a stable production of H2in repeated cycles.

An arti?cial Z-scheme photocatalytic system of ZnIn2S4/nanocarbon/g-C3N4was developed for visible-light-driven H2production[71].Nanocarbon sandwiched in between the heterojunction forming semiconductors was anticipated to act as a solid electron mediator.The rate of H2production on the nanocomposite photocatalyst system reached to50.32?mol h?1 that was about3.4times higher comparing with ZnIn2S4,and pure g-C3N4was almost inactive.Towards the development of a non-noble metal photocatalyst for H2production from water,Li et al. investigated dual synergetic effects from pyridine modi?cation of g-C3N4and loading with MoS2co-catalyst[72].At?rst,a donor-acceptor system of pyridine modi?ed g-C3N4was synthesized from co-pyrolysis of2,5-dibromopyridine and urea at550?C for 2h.TEM analysis showed nanosized?ower-like MoS2folded from nanosheets of～2nm was loaded onto the donor-acceptor system by a solvothermal method.Under visible-light irradiation,the optimized system with3%MoS2showed an enhanced and stable rate of H2evolution,25?mol h?1.For the MoS2co-catalyst loaded samples,the amount of H2produced over pyridine modi?ed complex photocatalyst was 2.5times higher than the pristine system.

CdS is an important material for visible-active photocataly-sis but,suffers from photo-corrosion,so the construction of its composites is helpful in gaining stability.Recently,Cheng et al. used a low-temperature solid-state method to fabricate ternary nanocomposites of two metal sul?des,CdS and CuS with g-C3N4 [73].The intimate connection between metal sul?des and g-C3N4 nanosheets was clearly observed in the high-resolution TEM to fabricate the triple heterojunction.TEM images of the nanocom-posite showed nanosheets of g-C3N4loaded with about8nm sized CdS nanoparticles closely adjacent to CuS nanoparticles(Fig.9A &B).The size of CuS nanoparticles was smaller with reference to8nm size of CdS.The construction of the triple heterojunc-tion was important for the accelerated separation of photoexcited charge carriers and their respective reactions are illustrated in the schematic diagram,Fig.9C.In comparison with the pure materi-als and binary composites,this ternary composite demonstrated a remarkable enhancement in the photocatalytic activity.The opti-mized nanocomposite exhibited H2production rate of around 57.6?mol h?1under visible light irradiation.The corresponding apparent quantum ef?ciency reached to16.5%at420nm.

Lately,Zhang et al.reported on the design of～7nm Cu-Cu2O well distributed g-C3N4nanocomposite and sensitized with Erythrosin B for visible-light-driven H2production from water[74].TEM analysis showed that～7nm nanoparticles of Cu-Cu2O were well dispersed on the surface of g-C3N4.For the composite with7wt.%copper,the rate of H2reached to 400?mol g?1h?1which was～3times higher compared with pure g-C3N4(140?mol g?1h?1).Next,a signi?cant improvement was observed with the dye sensitization and the rate reached to 5000?mol g?1h?1but showed a slight decrease in recycling tests. Hou et https://www.wendangku.net/doc/408054558.html,ed a combination of foaming-assisted electrospinning process and followed by a solution dipping process for fabrication of ternary hybrid nano?bers of TiO2,WO3,and g-C3N4[75].Thor-oughly mesoporous ternary hybrid nano?bers of around200nm

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Fig.9.(A)TEM and (B)HR-TEM images of the as-prepared ternary composite of CdS/g-C 3N 4/CuS,(C)Inferential schematic mechanism proposed for the photocatalytic production of H 2over the ternary nanocomposite under visible-light irradiation.

Reproduced with permission from the Ref.[73].Copyright (2016)Elsevier.

diameter were synthesized in the process.This noble-metal-free hybrid yielded a high and stable visible-light-driven photocatalytic H 2release at a rate of ～287?mol h ?1compared with the indi-vidual components.In comparison with pristine g-C 3N 4and TiO 2P25,the nano?ber composite showed better stability in consecutive photocatalytic cycles.All of these studies provide important strate-gies towards design and development of noble-metal-free ef?cient photocatalyst systems for sustainable harvesting of solar energy.

2.3.O 2evolution reaction

In natural photosynthesis,water oxidation is a crucial step in a series of reactions required for the sunlight-light-driven conver-sion of CO 2and H 2O into sugar.The water oxidation half-reaction is a key step and major bottleneck to control overall water splitting process as it requires transfer of four electrons [76].In the two half reactions of water splitting,the oxidation is considered a compli-cated process for it involves the sluggish transfer of four electrons.A large scale application of the most active catalyst systems for water oxidation based on Ir and Ru are hindered because of the high cost and environmental issues [77,78].Therefore,the development of low-cost catalyst systems is important for sustainable application of water splitting process.

For water splitting via separate H 2and/or O 2evolution half reac-tions,signi?cant advances have been made with the development of H 2evolution reaction,and only a few researchers have attempted on O 2evolution reaction for it is a dif?cult step requiring transfer for four electrons [13,2].In the photocatalytic reactions,AgNO 3is usu-ally used as a sacri?cial reagent to trap electrons and leave holes for O 2evolution half reaction of water splitting.Photocatalytic O 2evo-lution from water is a key bottleneck for the conversion and storage of solar energy in the chemical form.A sluggish requirement for transfer of four-electrons and high activation energy barrier for oxygen–oxygen bond formation are the main dif?culties with the development of a water oxidation catalyst for photocatalytic O 2evolution half reaction.

The role of band gap engineering towards improved photocat-alytic activity of g-C 3N 4was investigated by its sulfur-mediated solid-state synthesis under N 2?ow [79].Sulfur-doping of g-C 3N 4helped in lowering of the valence band position and was ascribed to the enhanced photocatalytic activity of this metal-free sys-tem for water oxidation reaction.The material produced with sulfur-mediated approach depicted nanopores of 20–30nm and a high speci?c surface area (60m 2g ?1)compared with bulk g-C 3N 4(10m 2g ?1).For the reaction under the UV–vis light,the rate of O 2evolution over the optimized material reached to 20.1?mol h ?1that was 4.2times higher comparing with pure bulk g-C 3N 4.

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Fig.10.(A)HR-TEM images of the Co3O4nanoparticles on sulfur-mediated synthesized g-C3N4(CNS),(B)PL spectra of CNS,Co3O4,and nanohybrids with various amounts of Co3O4loaded on CNS,(C)Visible light active O2evolution by CNS,CNS+3wt.%Co3O4(physical mixture),and3wt.%Co3O4/CNS nanohybrid,and(D)Wavelength dependent O2evolution rate over3wt.%Co3O4/CNS nanohybrid.The inset in(D)is the O2evolution curve under visible light(?>455nm).

Reproduced with permission from the Ref.[80].Copyright(2012)Royal Society of Chemistry.

Next,Zhang et https://www.wendangku.net/doc/408054558.html,bined this g-C3N4photocatalyst prepared via sulfur-mediated synthesis approach with nanoparticles of a water oxidation catalyst,Co3O4[80].As shown in Fig.10A,the composite exhibited a well-constructed nanohybrid structure of g-C3N4with Co3O4nanocrystals of～3nm diameter.A tight electronic and spatial interaction in the nanohybrid structure was observed with an increased quenching of the PL intensity upon loadings of Co3O4(Fig.10B)and further con?rmed by X-ray photoelec-tron spectroscopy studies.A synergistic effect was found upon the construction of a composite of Co3O4nanoparticles and S-doped g-C3N4.The formation of nanojunctions was believed to promote the direct transfer of photogenerated holes to Co3O4.Under visible light,the nanohybrid system with a well-constructed heterojunc-tion demonstrated signi?cantly enhanced O2evolution comparing with pure semiconductor and its physical mixture with Co3O4 (Fig.10C).Loading of Co3O4at3wt.%was found the optimum for water oxidation reaction.In comparison to O2production by the co-catalyst free material(2.8?mol h?1),the rate was signi?cantly enhanced to25.1?mol h?1over this hybrid composite,under vis-ible light irradiation.The rate of O2evolution was dependent on the optical absorption of the organic semiconductor material (Fig.10D).This observation supports that the process of water oxidation was initiated by photo-induced excitation of the poly-meric semiconductor.For the optimized nanohybrid structure,an apparent quantum ef?ciency of1.1%was determined under illumi-nation at420nm.

In search of a metal-free g-C3N4based photooxidation catalyst, Chu et al.tuned the band structure of g-C3N4by incorporat-ing electron-de?cient pyromellitic dianhydride monomer[81]. The TEM analysis showed interconnected particles for the poly-imide composite with sizes of～50nm.Thus obtained polyimide depicted a lowering of valence band position(by0.8V)even more pronounced than that was observed with S-doping(by0.2V), and consequently demonstrated an enhanced capability for water oxidation.Under visible-light irradiation,the initial rate of O2pro-duction on polyimide was estimated to be7.7?mol h?1that was about an order of magnitude larger than that of pristine g-C3N4 (0.8?mol h?1),and almost double comparing with the rate over S-doped g-C3N4(3.6?mol h?1).

2.4.H2O2generation

Photocatalytic generation of H2O2is important for its in-situ use in advanced oxidation process,and potential use as a fuel.

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Furthermore its solubility in water and easy transport compared with H2makes it an interesting candidate as a solar fuel.Kofuji et al.constructed visible light active nanohybrids of g-C3N4with aromatic diimide and graphene for reduction of O2to H2O2[82]. Under visible light irradiation for24h,g-C3N4modi?ed with aro-matic diimide produced14?mol of H2O2.While the optimized catalyst system including appropriate constituent of graphene gen-erated double amount of H2O2(29?mol).Li et al.demonstrated the construction of a solid-gas interface of a Fenton reaction over g-C3N4modi?ed with hydroxyl and Fe3+species[83].These mod-i?cations played signi?cant roles in photoactive production of H+ and Fe2+/Fe3+pair formation,and therefore determined the rate of H2O2generation.Pristine g-C3N4was inactive for producing H2O2 while only hydroxyl modi?ed sample generated about3.4?mol of H2O2.A highly enhanced amount of H2O2generated in1h over the sample modi?ed with hydroxyl and Fe3+species was estimated to be522?mol.

2.5.CO2reduction

Photocatalytic conversion of CO2to value added chemicals is immensely important for that not only reduces the level of green-house gas but also helps meet the demand for the renewable fuels. In the late1970s,Halmann,and Inoue et al.?rst reported the light-induced reduction of CO2[84,85].Arti?cial photosynthesis over semiconductor materials for reduction of CO2into various chem-icals is a highly sought after reaction to ful?ll energy demands and mitigate climate changes.As the production of chemical fuels from CO2has been considered an ideal solution to simultaneously solve the issues with environment and energy.Therefore,many researchers have attempted various semiconductor materials for the photocatalytic conversion of CO2.

Metal-free microstructures of g-C3N4nano-?akes derived from different precursors were evaluated for photoreduction of CO2, under visible-light irradiation.Mao et al.heat treated urea and melamine to modulate microstructure of the produced g-C3N4[86].A mesoporous structure of nano-?akes of around 5nm for the g-C3N4(obtained from urea)produced methanol and ethanol at rates of6.28?mol g?1h?1and4.51?mol g?1h?1, respectively.On the other hand,a non-porous sample of g-C3N4 (obtained from melamine)produced ethanol only and at a lower rate,3.64?mol g?1h?1.Hence,the mesoporous structure from nano-?akes demonstrated higher photoactivity for CO2reduc-tion.Furthermore,alcohols were the only product obtained from the photoreduction of CO2on metal-free g-C3N4.The selectivity of CO2photoreduction products became more diverse with the research work reported by Niu et al.that used bulk g-C3N4and its nanosheets of around2nm size as photocatalysts[87].Under various experimental conditions,methane and acetaldehyde were the only product of CO2photoreduction.Bulk g-C3N4(bandgap ～2.7eV)produced CH3CHO only while conversion over g-C3N4 nanosheets(bandgap～2.9eV)was selective to CH4.

A further development was made by Ong et al.by the construc-tion of hybrid nanostructure via a combined sonication-assisted and surface charge modi?cation strategy[88].A metal-free pho-tocatalyst of2D/2D hybrid heterostructure was prepared from protonated g-C3N4and reduced graphene oxide(rGO).An inti-mate contact across the heterojunction interface was seen by the TEM and spectroscopic analysis.The optimized nanostructure with 15wt.%rGO exhibited a considerably enhanced conversion of CO2 (～14?mol g cat?1)in the presence of water vapors into CH4,under illumination from an energy-saving daylight bulb.The photochem-ical quantum yield was0.56%which was5.4and1.7times higher compared with protonated g-C3N4and15wt.%of rGO on bulk g-C3N4.The enhanced photocatalytic activity of the nanostructure was attributed to the intimately connected rGO for an effective separation of photogenerated charges.Construction of Z-scheme photocatalysts has received much attention for CO2reduction,and an effective indirect Z-scheme of BiOI/g-C3N4was synthesized by Wang et al.via a simple deposition method[89].

Shi et al.decorated nanosheets of g-C3N4onto zirconium metal-organic framework(MOF;UiO-66)via a facile electrostatic self-assembly method[90].TEM analysis of the nanocomposite showed～4nm thick nanosheets decorated onto MOF struc-ture(Fig.11A).BET surface area of the composite was around 1340m2g?1.The nanocomposite was highly active for the pho-tocatalytic reduction of CO2to CO comparing with the g-C3N4 nanosheets alone and the composite prepared from bulk g-C3N4(Fig.11B).Photoluminescence spectra showed a signi?cant quenching of the intensity indicating a better separation of pho-toexcited electron-hole pairs in the nanocomposite(Fig.11C). This phenomenon was further con?rmed with?uorescence life-time measurements.Hence,transfer of photogenerated electrons from the nanosheets to the MOF was substantially suppressing the recombination of electron-hole pairs.Suitable potentials of the nanocomposite for CO2reduction and the proposed mechanism under visible light irradiation are depicted in Fig.11D.Besides a strong adsorption capacity,an ef?cient separation of charge carri-ers was found important for this high activity of the nanocomposite. The rate of CO evolution over the optimized composite containing 10wt.%g-C3N4nanosheet was the highest,and yield reached to more than3times better than that of the nanosheets alone.A slight drop in the photocatalytic activity was seen during the recycling tests.

In an effort to develop noble-metal-free photocatalyst,Zhou et al.developed a series of g-C3N4composites with nitrogen-doped TiO2by a simple pyrolysis process[91].Formation of porous g-C3N4and loading with nanoparticles were evidenced from N2 sorption and microscopy studies.TEM analysis showed nanopar-ticles in the range of20nm to40nm dispersed on the surface of g-C3N4nanosheets.Pertaining to its enhanced absorption of light and ease with the separation of charge carriers,the nanocompos-ite was highly active for CO2reduction selectively into CO.Under 12h irradiation,about15?mol of CO was obtained which was four times higher compared with the commercial TiO2P25.Another similar nanocomposite system for CO2reduction was developed by Raziq et al.by combining nanosize TiO2and B-doping of g-C3N4 nanosheets[54].The coupling of B-doped nanosheets of g-C3N4 with nanocrystalline TiO2for the construction of co-catalyst free nanojunctions was carried out by the facile calcination method. TEM analysis revealed?nite～5nm diameter nanoparticles of TiO2 well-dispersed on the surface of the organic semiconductor.The separation of photogenerated charges was improved after B-doping and further enhanced upon coupling with nanosized TiO2.Upon B-doping of the g-C3N4structure,the amount of CH4in8h pho-tocatalytic reaction increased from～7?mol(pristine g-C3N4)to 68?mol.The highest amount of CH4,106?mol was observed over the nanocomposite.In contrast to the CO2reduction to CO over the nitrogen-doped TiO2composite observed by Zhou et al.[91],here in this study CH4was the main product.The rate of H2evolution was also similarly increased upon B-doping of g-C3N4and further with the construction of heterojunction with TiO2.

A simple impregnation strategy was adopted by He et al. for the synthesis of ZnO nanoparticle-functionalized g-C3N4as shown in Fig.12A[92].TEM analysis showed a close coupling of ZnO nanoparticles onto the surface of g-C3N4.Electrochemical impedance spectra demonstrated ef?cient charge transfer between the components of the optimized nanocomposite(Fig.12B). Comparing with a physical mixture of ZnO and g-C3N4the pres-ence of strong af?nity between the two semiconductors in the composite was further con?rmed by the X-ray photoelectron spec-troscopy.The composite demonstrated a superior performance for

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Fig.11.(A)TEM and HR-TEM images of nanocomposite of Zr-MOF(UiO-66)and g-C3N4nanosheet(CNNS),(B)Time course of CO evolution over bulk g-C3N4(CN),CNNS, UiO-66/bulk CN,and UiO-66/CNNS photocatalysts,(C)PL spectra of CNNS and UiO-66/CNNS,and(D)Schematic of proposed photocatalytic reduction mechanism of CO2by the UiO-66/CNNS heterogeneous photocatalyst under visible light irradiation.

Reproduced with permission from the Ref.[90].Copyright(2015)Wiley.

photocatalytic CO2reduction due to effective separation of charges at the interface(Fig.12C).CO and methanol were the main reaction products from the photocatalytic reduction of CO2.The optimized heterojunction with6wt.%ZnO demonstrated a CO2conversion rate of～46?mol h?1g cat?1that was4.9and6.4times higher than that of the pristine g-C3N4and TiO2P25,respectively.A physical mixture of6wt.%ZnO with g-C3N4(6ZC-PM)showed much less rate of CO2conversion and stressed the importance of good het-erojunction between inorganic-organic semiconductors(Fig.12C). However,the cyclic runs for the optimized catalyst showed a small decrease in the photoactivity.

Besides ZnO,He et al.also used～50nm sized nano-spherical SnO2-X for the construction of a composite with g-C3N4by sim-ple calcination[93].The conduction band position of SnO2-X is closer to the valence band position of g-C3N4,therefore a direct Z-scheme type composite was formed upon coupling these semi-conductors.The nanocomposite was highly ef?cient for reduction of CO2and for the degradation of organic pollutants,under visi-ble light irradiation.Initially,the photocatalytic performance of the composite increased gradually with the SnO2-X concentration and the optimized value was around42wt.%,and with further loading the activity was decreased.The photocatalytic CO2reduction on the optimized composite reached about23?mol h?1g cat?1which was4.3and5times higher than that of the pristine g-C3N4and TiO2P25,respectively.In all the various photocatalyst employed in this study,CO was the main product from the reduction of CO2. The performance of the SnO2-X/g-C3N4nanocomposite was inferior and remained at the half as compared with that of the ZnO/g-C3N4 [92].These studies on noble-metal-free systems present interest-ing strategies towards the photocatalytic production of fuel from CO2and provide important feedback for future developments.

2.6.N2?xation

In nature,after photosynthesis,nitrogen?xation is the second most important chemical process and soil-dwelling bacteria are able to utilize N2from the air.Industrially,Haber–Bosch process for the catalytic synthesis of ammonia from hydrogen and nitrogen under high pressure and the temperature is the main arti?cial pro-cess for nitrogen?xation.The cost of raw material and the process has been pushing for alternate methods for nitrogen?xation.Since the?rst report on photocatalytic conversion of N2,this process has been thought to be the best alternative to traditional techniques. The?rst study on photo?xation was reported by Schrauzer et al.for conversion of N2to NH3over Fe-doped TiO2[94].Recently,many researchers have attempted on the development of a low-cost and

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Fig.12.(A)TEM of a composite of6wt.%ZnO and g-C3N4(6ZC),(B)Electrochemical impedance spectral changes in pure materials,a physical mixture of6wt.%ZnO and g-C3N4(6ZC-PM)and6ZC,and(C)Photocatalytic activities of g-C3N4,ZnO,P25,and composites of2–8wt.%ZnO with g-C3N4(2–8ZC),and6ZC-PM.

Reproduced with permission from the Ref.[92].Copyright(2015)Elsevier. visible-active photocatalyst materials for?xation of N2.Hu et al. constructed a2D/2D heterojunction between protonated g-C3N4 and reduced rGO[95].By acid treatment,the electrostatic surface charge on g-C3N4was modulated to make an effective interfacial contact with rGO,and furthermore,it converted the bulk struc-ture to nanosheets.The rate of visible-light-driven generation of NH4+over the optimized heterojunction with suf?cient interfacial contact was3.7times higher compared with the hybrid struc-ture prepared without surface charge modi?cation.Furthermore, this layered hybrid was highly ef?cient in the photodegradation of organic pollutant and remained stable in repeated runs.

In another study on the arti?cial photo-?xation of N2,Hu et al. prepared a novel heterojunction of ternary metal sul?de,ZnSnCdS with g-C3N4by hydrothermal method[96].The average size of multi-metal sul?de nanoparticles was in the range of30–50nm. An effective separation of photogenerated electron-hole pairs in the strongly coupled heterojunction was evidenced from the imag-ing and various spectroscopy techniques.High-resolution TEM images showed a tight coupling of irregularly shaped ZnSnCdS with sheet-like g-C3N4which is favorable for the transfer of the photogenerated charges(Fig.13).As shown in the optical spec-tra in Fig.13C,the absorption edges of the heterojunctions were located between that of the pristine g-C3N4and ZnSnCdS and fur-ther con?rmed the electronic coupling.Because of the effective interfacial charge transfer,an outstanding N2photo-?xation was observed over the heterojunction,under visible light irradiation (Fig.13D).Besides strong coupling of this ternary sul?de with g-C3N4its de?ciency in sulfur was found synergistic to provide active sites to absorb and activate N2molecules.Under visible light irra-diation,the rate of generation of NH4+(7.543mg L?1h?1)over the optimized composite containing80%ZnSnCdS was33.2-folds and 1.6-folds greater comparing with the pristine g-C3N4and pure ZnSnCdS.The nanocomposite was also ef?cient for photodegra-dation of rhodamine B(RhB),but the optimized concentration of ZnSnCdS was20%only.The same research group also prepared a heterojunction of g-C3N4with another ternary metal sul?de, ZnMoCdS[97].TEM analysis of the composite showed～50nm nanoparticles of ZnMoCdS attached on the surface of g-C3N4.The optimized composite with only20%of ZnMoCdS exhibited the highest rate of generation of NH4+(3.5mg L?1h?1)that was13.5-folds and1.75-fold greater than those of pristine g-C3N4and the ZnMoCdS.In conclusion,compared with the Mo-based ternary metal sul?de composite the one prepared from Sn-based ternary metal sul?de demonstrated superior photocatalytic activity for nitrogen?xation.

https://www.wendangku.net/doc/408054558.html,anic synthesis

Both catalytic oxidation and reduction are industrially impor-tant reactions for all the various organic synthesis.Wang et al. attempted earliest studies on g-C3N4based materials for photo-catalytic organic synthesis reactions[98–100].Performing organic synthesis reactions in a controlled manner and under safe condi-tions is critically desired.Photocatalytic activation of O2or H2O2for oxidation reaction and use of the in-situ generated H2for reduction reaction offer a safe and economical alternative to the industrial processes.Semiconductor materials based on g-C3N4have been evaluated for both oxidation[13,101],and reduction types of reac-tions[60,102].Here in this section,we will summarize some g-C3N4 based noble-metal-free photocatalytic systems employed for the organic synthesis.

Song et https://www.wendangku.net/doc/408054558.html,ed g-C3N4,and further modi?ed it with SnO2 for evaluating photocatalytic esteri?cation of benzaldehyde and various alcohols under visible light irradiation[103].A slice-like morphology for the nanostructure of as-prepared g-C3N4was observed under TEM.Production of various esters in the presence of H2O2was enhanced under irradiation of visible light.Loading of SnO2further signi?cantly promoted the photocatalytic ester for-mation reaction.Dark conversion of benzaldehyde and ethanol to ethyl benzoate was24%that increased upon irradiation with light to32%and further to41%with the promotor SnO2.Zhang et al. activated the metal-free g-C3N4through re?ux in acidic media,

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Fig.13.(A)TEM and(B)HR-TEM images of the ZnSnCdS loaded on g-C3N4(20%),(C)UV–vis spectra of as-prepared materials with Tauc plots in the inset,and(D)Nitrogen photo-?xation performance of the as-prepared catalysts under visible light.The inset in(D)shows the photocatalytic stability of ZnSnCdS-CN(20%).

Reproduced with permission from the Ref.[96].Copyright(2016)American Chemical Society.

essentially changing the bulk to nanostructures[104].The modi?ed polymeric semiconductor was used for the selective aerobic oxida-tion of benzyl alcohol under visible light irradiation.Among all the acidi?ed samples,the one treated with sulfuric acid gave23%yield of benzaldehyde under4h irradiation of visible light which was2.5 times higher comparing with pristine g-C3N4.The enhanced photo-catalytic performance of acidi?ed g-C3N4was attributed to a larger speci?c surface area and enhanced surface chemical property of the nanoporous sheets of g-C3N4.

Shiravand et al.loaded nanoporous silica(LUS-1)with Fe-containing g-C3N4by impregnation followed calcination method [105].From XRD studies they proposed the formation of a nanoscale layer of g-C3N4on the surface of LUS-1.A drastic enhancement in the hydroxylation of benzene to phenol in the presence of H2O2was observed over the optimized nanostructure catalyst under sunlight.

A maximum yield of phenol(16%)was exhibited by g-C3N4with 20%Fe and loaded onto LUS-1.Ye et al.also worked on the selective production of phenol over similar catalyst system;nano-coating of Fe-containing g-C3N4on titanium silicate zeolite(TS-1)[106]. Under visible light irradiation,more than10%yield of phenol was observed over the optimized composite that was higher compar-ing with pure materials.Under visible light,Fe-containing g-C3N4 loaded on LUS-1showed a better yield of phenol～16%comparing with the one loaded on TS-1(10%).

Recently,Nasir Baig et al.worked on magnetic Fe species loaded onto g-C3N4for a sustainable application in photocatalytic hydro-genation,under visible light irradiation[102].TEM analysis showed that aggregates of nano-sized ferrite were loaded onto g-C3N4, Fe@g-C3N4.The nanocomposite systems were screened for the hydrogenation of styrene and with an optimized concentration of Fe(10%)the obtained yield was98%.During the photocatalytic reac-tion at room temperature,hydrazine was used as a source of H2. The magnetically separable nano-ferrite system was ef?cient for the photocatalytic hydrogenation of various alkenes and alkynes. TEM analysis showed that nano-ferrite remained stable during pho-tocatalytic testing.Suyana et al.prepared a composite of g-C3N4 with ZnS by the one-pot synthetic approach[60].TEM analysis showed4–7nm sized nanoparticles of ZnS in the nanocompos-ite.The optimized heterojunction with14wt.%ZnS demonstrated more than90%photocatalytic reduction of p-nitrophenol to p-aminophenol,under240min illumination with sunlight.Sodium

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sul?te was used as a hole scavenger during this photoreduction reaction of p-nitrophenol.

Dai et al.prepared a nanocomposite of CdS with g-C3N4 via a hydrothermal method[107].TEM image depicted～50nm nanoparticles of CdS decorated on the surface of g-C3N4.In com-parison with the individual components,the composite was highly ef?cient for visible light active selective oxidation of benzyl alco-hol by holes to benzaldehyde and reduction of nitrobenzene by electrons to aniline.Under4h of illumination over the optimized system,the yield of benzaldehyde and nitrobenzene was44.6%and 26.0%,respectively.A small decrease in photocatalytic activity was noticed in cycling tests.Zhang et al.prepared a nanocomposite from the decoration of in-situ prepared Ag3PW12O40onto g-C3N4 by stirring with precursors,at room temperature[108].TEM anal-ysis showed that uniform nanospheres(～5nm)of Ag3PW12O40 were anchored on g-C3N4nanosheets.The nanocomposite was highly active for selective photocatalytic conversion of cyclooctene and cyclohexane without adding any oxidant at60?C.The pho-tocatalytic oxidative conversion of cyclohexanone was8.62%with more than99%selectivity to cyclohexanone under near-UV irradia-tion.For cyclooctene,the conversion reached41.2%with selectivity mainly to epoxycyclooctane(77.2%).This noble-metal-free system remained stable in the repeated cycles of photocatalytic oxidation reactions.These studies provide signi?cant guidance in designing sustainable catalyst systems and appropriate reaction conditions for realizing light-induced catalytic oxidation and reduction reac-tions.

2.8.Degradation of organic pollutants

Degradation of organic pollutants in the environment over the semiconductor solids is an extensively explored reaction by het-erogeneous photocatalysis[109].Similarly,most of the studies on photocatalytic properties of g-C3N4and its modi?ed system also focused on environmental remediation reactions,usually using organic dyes as a model pollutant[2,12].From a large volume of research papers on the topic of photocatalytic degradation of organic pollutants(Fig.3),we provide here a summary of few from each of the different categories of materials and modi?cations.In line with the focus of this review,we will mainly discuss some noble-metal-free or precious-metal-free nanoscale photocatalyst systems.

2.8.1.Metal-free photocatalyst system

Development of metal-free photocatalyst materials is important for their sustainable application.Zhang et al.prepared carbon-rich g-C3N4nanosheets by hydrothermal method[110].In comparison with the pristine g-C3N4,the modi?ed semiconductor showed a signi?cantly enhanced photocatalytic degradation of4-nitophenol, under visible light irradiation.Oxygen functionalized g-C3N4and co-doped with S and P was synthesized by hydrothermal post-treatment[111].The modi?ed material was highly active for visible-active photodegradation of RhB,methylene blue(MB), methyl orange(MO)and phenol under anoxic conditions.The high-est RhB degradation constant,0.026min?1was13times higher comparing with the pristine g-C3N4.Dang et al.synthesized a core-shell heterojunction via a self-assembly process[112].A spherical core of?-S was enwrapped with ultra-thin nanosheets of g-C3N4for rapid charge transfer.The composite with35%of g-C3N4 nanosheets demonstrated the highest photocatalytic degradation of RhB with6.7times faster rate compared with?-S.

Modi?cation of g-C3N4with carbon-based nanostructures has been attracting attention for an enhanced absorption of visible-to-IR and fast transfer of photogenerated carriers.Liao et al. fabricated graphene oxide(GO)modi?ed g-C3N4by the sonochem-ical approach[113].This metal-free hybrid structure of nanosheets showed ef?cient photocatalytic degradation of organic pollutants under visible light irradiation.The photodegradation of RhB and 2,4-dichlorophnol over the hybrid structure was 3.8and 2.1 times higher comparing with pristine g-C3N4.Another carbon-based material,multi-walled carbon nanotubes was combined with g-C3N4nanosheets due to electrostatically-driven self-assembly with the hydrothermal method[114].The visible-light-driven pho-todegradation of MB by the composite was8.1times of that of the g-C3N4nanosheets.Bai et al.modi?ed g-C3N4with Fullerene(C60) via a facile thermal treatment[115].Photocatalytic degradation of MB and phenol was carried out under visible light https://www.wendangku.net/doc/408054558.html,-paring with bulk g-C3N4the ef?ciency of the composite was2.9 times and4.0times higher for phenol and MB,respectively.Sun et al.decorated glucose-derived carbon nanospheres onto g-C3N4 via a hydrothermal method[116].For the degradation of phenol under simulated sunlight,the nanocomposite demonstrated4.9 times higher ef?ciency compared with pristine g-C3N4.

2.8.2.Transition metal based system

Photodegradation of organic wastewater over semiconductor materials has received considerable attention to cope up with the environmental challenges.The mainly explored wide band gap semiconductor oxides TiO2and ZnO are not suitable for harvest-ing solar light.Among transition metal based g-C3N4systems, modi?cations with titanium materials are most explored for an understandable combination of these two semiconductor photo-catalysts of main interest.Chang et al.prepared composites of 10–20nm TiO2nanoparticles with exfoliated nanosheets through a facile sol-gel method[117].The hybrid structure was highly active for visible-light-driven degradation of RhB.The apparent reaction rate constant for the optimized composite was2.4and7.0times higher compared with bare TiO2and N-TiO2,respectively.Zhu et https://www.wendangku.net/doc/408054558.html,ed an in-situ calcination method for the synthesis of a composite of homogeneously embedded20–30nm TiO2P25in the sheets of g-C3N4[118].For the optimized nanocomposite,the pho-todegradation ef?ciency was3.3times higher than that of the pure g-C3N4under visible light.Thin?lms of g-C3N4from melamine and ～14nm TiO2nanoparticles were prepared by thermal heating in Ar atmosphere[119].Visible-light-driven degradation of MB over the optimized hybrid was almost twice in comparison with pure TiO2.

Ma et https://www.wendangku.net/doc/408054558.html,ed a hard template,SiO2for preparing mesoporous g-C3N4,and loaded it with5–10nm sized nanoparticles of TiO2 [120].For the degradation of RhB under visible light irradiation,the optimized heterojunction demonstrated1.6times higher rate than that of the pure mesoporous g-C3N4.Zhou et al.prepared an irreg-ularly agglomerated hybrid of20–40nm size from ball milling of TiO2and g-C3N4[121].Photocatalytic activity of the hybrid mate-rial was3.0and1.3times higher than those of the pure g-C3N4 and TiO2,respectively.Li et al.optimized solvothermal conditions for seed-induced growth of50nm to200nm sized TiO2nanopar-ticles loaded onto nanosheets of g-C3N4[122].In comparison with direct-grown composite and pure g-C3N4the seed-grown compos-ites were highly ef?cient for degradation of MO and phenol under visible light irradiation.For the optimized seed-grown compos-ite,the photodegradation rate constant for MO and phenol was30 and14times higher than that of the pristine g-C3N4.Li et https://www.wendangku.net/doc/408054558.html,ed a simple one-step calcination of H2Ti3O7and melamine precur-sors for the synthesis of Ti3+self-doped TiO2heterojunction with g-C3N4[123].The photodegradation of MB was studied under a 30W visible-light-emitting diode.The removal rate constant for the optimized composite reached0.038min?1which was26.7and7.6 times higher compared with pure TiO2and g-C3N4,respectively. Lu et al.prepared a composite of Ti3+containing TiO2rectangu-lar nanosheets of100nm and g-C3N4by hydrothermal-sonication assisted strategy[124].The optimized composite was highly active

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Fig.14.(A)TEM image of TiO2and g-C3N4hybrid structure,(B)HR-TEM of the hybrid,(C)Photocatalytic degradation of MB under UV irradiation,and(D)Photocatalytic degradation of MB under visible-light irradiation.

for visible-light-driven photooxidation of RhB and photoreduction of Cr6+.

Gu et al.developed a facile solvent evaporation method for hybridizing TiO2with dominant{001}facets with g-C3N4[125]. The high-energy{001}facets calculated for the sheet-like anatase nanocrystals(～38X～6nm)was about76%for pure TiO2.The opti-mal amount of g-C3N4in the heterojunction was around5.3%as estimated by the thermogravimetric analysis.Well-de?ned bound-aries of TiO2nanosheets displayed an obscure bulk con?guration upon hybridization with g-C3N4as studied by TEM(Fig.14A).For the hybrid structure,the lattice spacing of0.235nm was seen in the HR-TEM that was assigned to the exposed{001}facet of TiO2 (Fig.14B).Furthermore,an outermost layer of about0.37nm in the HR-TEM was corresponding to a monolayer of g-C3N4(ca.

0.325nm).TEM and spectroscopic studies demonstrated a strong interfacial interaction between TiO2and g-C3N4.The photocatalytic activity of the heterojunction structure was evaluated for the degra-dation of MB,RhB,acridine orange,and2,4-dichlorophenol under both UV and visible light irradiations(Fig.14C&D).The optimized composite showed the highest photocatalytic degradation towards all the organic dyes illustrating the importance of heterojunction.

N-doping in TiO2is an important strategy in making it visible active photocatalyst,and further heterojunction with another suit-able semiconductor is added advantage.Wang et https://www.wendangku.net/doc/408054558.html,ed an in-situ microwave-assisted approach for the synthesis of composites of ～15nm spherical N-TiO2and g-C3N4nanosheets[126].H2TiO3was used as a precursor for TiO2and NH3·H2O as the N-doping source. The heterojunction was highly ef?cient for visible-light-driven photodegradation of RhB and MO.The rate of photodegradation of RhB over the optimize composite was2and2.7times higher than that of the pure g-C3N4and N-TiO2.Li et al.constructed hetero-junction between10and30nm N-TiO2nanoparticles and g-C3N4 nanosheets by direct co-calcination[127].Visible-light-driven pho-todegradation of RhB over the optimized composite was19and 5.3times higher compared with individual N-TiO2and g-C3N4, respectively.Recently,Sun et al.prepared an in-situ composite of N-TiO2nanoparticles of30–50nm and g-C3N4from one-step cal-cination of TiN and melamine[128].Under visible light irradiation, the ef?ciency of optimized composite towards photodegradation of MB was just1.9times greater than compared with N-TiO2.Kumar et al.prepared nanocomposite from N-doped SrTiO3nanoparticles of～50nm and g-C3N4by a polymeric citrate and thermal exfolia-tion method[129].The optimized nanocomposite(0.0558min?1) showed much higher photocatalytic degradation rate for RhB than that of the pure g-C3N4(0.0113min?1),SrTiO3(0.0038min?1),and N-doped SrTiO3(0.0135min?1)under visible light.

There are a few studies on nanocomposites of g-C3N4with Group V metals for the noble-metal-free photodegradation

574 D.Masih et al./Applied Catalysis B:Environmental206(2017)556–588

reactions.Liu et al.prepared a heterojunction of～5nm sized spher-ical nanoparticles of V2O5with lamellar g-C3N4by simple one-pot synthesis[130].Rhodamine B was almost completely removed over the optimized nanocomposite for80min illumination of visible light.And for the same time of illumination,pristine g-C3N4and V2O5showed less than10%photodegradation ef?ciency.Kumar et al.prepared a hybrid nanocomposite of NaTaO3nanoparticles of ～43nm with g-C3N4by a facile ultrasonic dispersion method[131]. Under both UV–vis and visible light,the nanocomposite exhib-ited highly enhanced photodegradation of RhB compared with pure g-C3N4,NaTaO3,and TiO2P25.Photodegradation ef?ciency was almost100%in90min illumination of full light,however,it reached only～90%under visible light only even for twofold time of illumination.Yong et al.also used ultrasonic-assisted method for fabrication of a similar nanocomposite of～30nm sized nanoparti-cles of KTaO3with g-C3N4[132].In comparison with pure materials, the optimized nanocomposite demonstrated enhanced photocat-alytic degradation of RhB,under visible light irradiation.

For the development of a noble-metal-free photocatalyst, MoS2is emerging as an economical alternative co-catalyst.A nanocomposite of MoS2nanoparticles onto g-C3N4nanosheets was fabricated by a low-temperature hydrothermal method[133]. The nanocomposite demonstrated much-enhanced photodegra-dation of MO under simulated solar light.Li et al.coupled nanosheets of MoS2with g-C3N4via a facile ultrasonic method for constructing a visible light active photocatalyst[134].The rate of photodegradation of RhB over the optimized heterostructure reached0.301min?1which was3.6times higher compared with pure g-C3N4.

Li et https://www.wendangku.net/doc/408054558.html,ed solvothermal method for the construction of heterojunction between irregular nanoparticles of Bi2MoO6and g-C3N4[135].In comparison with pure Bi2MoO6and g-C3N4 the optimized composite exhibited enhanced photodegradation ef?ciency for RhB and MB under visible light.After70min of irradiation,optimized heterojunction exhibited about98%pho-todegradation of RhB while pure g-C3N4or Bi2MoO6showed less than60%removal.Around the same time,Tian et al.also prepared heterojunction of Bi2MoO6nanosheets with g-C3N4via a solvother-mal route[136].The optimized composite was highly ef?cient for visible-light-driven degradation of MO and2,4-dichlorophenol. The composite demonstrated more than3times faster photodegra-dation of MO compared with pure g-C3N4or Bi2MoO6.A ternary composite of MoS2nanosheets of～5nm thickness,graphene oxide and g-C3N4was prepared using a facile sonochemical method for visible active photocatalysis[137].The ternary composite exhibited enhanced photodegradation of RhB compared with binary compos-ites and pure g-C3N4.

Visible-light-driven WO3(bandgap 2.7eV)and its modi?ed materials have many potential applications in photocatalysis. Huang et al.prepared composite of100–200nm sized nanoparti-cles of WO3with g-C3N4by a calcination method for the fabrication of a visible light active photocatalyst[138].The heterojunction exhibited an enhanced photodegradation ef?ciency for MB and4-chlorophenol.The optimized composite showed97%degradation of MB within2h and43%degradation of4-chlorophenol within6h. Under the same conditions,pure g-C3N4reached only81%degra-dation of MB and3%degradation of4-chlorophenol.Katsumata et al.prepared composite of WO3particles of100–300nm with g-C3N4by mechanical mixing in an agate mortar[139].The com-posite was used for visible-light-driven degradation of gas-phase acetaldehyde.A complete degradation was achieved on the opti-mized composite with the highest generation of CO2.Contrary to the other studies,Chen et al.illustrated on the formation of Z-scheme between～30nm WO3and g-C3N4via ball milling and heat treatment[140].The optimized Z-scheme photocatalyst was highly ef?cient for visible-light-driven degradation of MB and fuchsin.

The Fig.15.(A)TEM image of the core–shell structure of CuFe2O4@g-C3N4(2:1)hybrid, (B)HR-TEM of the hybrid clearly showing the core–shell structure,and(C)Separa-tion of solid and solvent phases using an external magnet.

rate constants for photodegradation of MB and fuchsin over the optimized composite were4.8times and3.8times higher than that of the pristine g-C3N4.

W-based multi-metal materials are important semiconductors for photocatalysis and their hybridization with g-C3N4also shall be interesting.Wang et al.hybridized ZnWO4nanorods of60×30nm by enwrapping with g-C3N4via a facile chemisorption method

D.Masih et al./Applied Catalysis B:Environmental206(2017)556–588575

[141].The optimized composite was highly ef?cient for the pho-todegradation of MB under both UV and visible light irradiations. Besides a dramatic enhancement of photocatalytic activity under visible light about80%increase was observed under UV irradia-tion.Zhan et al.prepared?lms from the same type of composite and used it for degradation of phenol under UV irradiation[142]. The photodegradation ef?ciency of optimized composite was～2 times of the pure ZnWO4.Tian et al.prepared heterojunction of50–100nm sized CdWO4nanorods with g-C3N4by a simple mixing and calcination method[143].The optimized composite was highly active for photodegradation of RhB under visible light https://www.wendangku.net/doc/408054558.html,paratively,the rate of reaction was1.6and54.6 times higher than that of pure g-C3N4and CdWO4.

Fe-based materials are getting attention for photocatalysis, especially for their magnetic separation in sustainable applications. Vignesh et al.prepared a～19nm ternary magnetic nanocompos-ite from TiO2,MnFe2O4and g-C3N4via chemical impregnation method[144].The optimized ternary composite was highly ef?-cient for photodegradation of MO under simulated solar light.For 3h illumination,MnFe2O4/g-C3N4/TiO2exhibited99.3%degrada-tion while binary composite,MnFe2O4/g-C3N4(4.9%)and pure materials,MnFe2O4(4.2%),g-C3N4(43.7%),and TiO2(85.1%) showed inferior performance.Zhu et al.prepared a nanocom-posite of highly dispersed Fe3O4nanoparticles of5–10nm onto g-C3N4by ultrasonic-assisted wet chemical method[145].Mag-netic conductive imprinted photocatalysts(MCIPs)were prepared from the nanocomposite support and organic template and func-tional monomer.MCIPs were employed for photodegradation of 2-mercaptobenzothiazole(MBT)under visible light irradiation.The optimized MCIP exhibited highly enhanced(～85%)photodegrada-tion of MBT in60min compared with pure g-C3N4(40%).A simple impregnation method was used by Liu et al.for grafting20–50nm nanoclusters of Fe3+species onto the surface of g-C3N4[146].Fur-thermore,a ternary composite was prepared with the addition of graphene for enhanced visible-light-driven degradation of MO.The photodegradation reaction rate constant of the optimized compos-ite was0.0276min?1which was about2and11times higher than that of the binary composite and pure g-C3N4.

A novel core–shell structure of magnetic CuFe2O4@g-C3N4was fabricated through a self-assembly method by Yao et al.[147].TEM image depicted that after hybridization the CuFe2O4nanoparti-cles in the range of50–60nm were well interwoven among the g-C3N4(Fig.15A).Formation of core–shell structure was shown by the HR-TEM analysis,the CuFe2O4nanoparticles were encap-sulated within the?lms of g-C3N4(Fig.15B).The thickness of the g-C3N4shell was in the range of5–7nm.In comparison with the pure semiconductors,the hybrid structure demonstrated an excel-lent photo-Fenton-like catalytic activity for the decolorization of an organic dye,Orange II.Magnetic CuFe2O4@g-C3N4with2:1was found the best photocatalyst which decomposed～98%of organic dye within210min of visible light irradiation.The physical sepa-ration of the composite from the solution with an external magnet is depicted in Fig.15C which illustrated an easy recycling of the catalyst material.

Zhang et al.decorated nanoparticles of spinel ZnFe2O4(ZnFe) on g-C3N4(CN)sheets through a one-step solvothermal route [148].Superior visible active photocatalytic properties of the nanocomposite were ascribed to the effective separation of photo-generated charges and to its high dispersion in water.An optimized heterojunction was obtained for the160CN-ZnFe sample pre-pared from the Fe precursor to g-C3N4ratio of2:1.Fig.16A revealed the distribution of optimized5–6nm nanoparticles of ZnFe2O4attached to g-C3N4sheets.The magnetic properties were effectively controlled by tuning the coverage and size of the spinel oxide(Fig.16B).The optimized composite demonstrated an extremely high visible light photodegradation activity towards MO and phenol comparing with individual semiconductors.The optimized composite demonstrated～98%decomposition of MO in 180min while the photocatalytic activity of pristine g-C3N4(36%) and ZnFe2O4(44%)was very low.Furthermore,a low photocat-alytic activity of a physical mixture indicated the importance of well-connected interface between semiconductors.The curves for visible-light-driven degradation of phenol are depicted in Fig.16C. The enhanced photocatalytic activity of the composite was ascribed to effective separation of electron-hole pairs on nanojunctions. Recyclability of the optimized catalyst for the degradation of MO showed a slight decrease in the activity(Fig.16D).

ZnO is an important semiconductor but poorly responsive to vis-ible light and suffers from high recombination of photogenerated charges.Fabrication of heterojunction between ZnO and g-C3N4is interesting for their appropriate band potentials in enhancing sep-aration of electron-hole pairs.Chen et https://www.wendangku.net/doc/408054558.html,ed a facile solvothermal route for the fabrication of heterojunction between10and30nm sized nanospheres of ZnO and mesoporous g-C3N4[149].The com-posite structure was employed for photodegradation of MB under visible light and simulated solar irradiation and was found ef?cient. In comparison with pure g-C3N4mesoporous the photocatalytic activity was2.3and1.9times higher under visible light and solar irradiation.Vignesh et https://www.wendangku.net/doc/408054558.html,ed mesoporous ZnO nano-triangles of 50–60nm size for construction of a nanocomposite with g-C3N4 nano-foils through a sonochemical impregnation method[150]. The heterojunction structure exhibited highly ef?cient degrada-tion of RhB under solar light irradiation and remained stable in cycling tests.The optimized nanocomposite demonstrated100% removal within60min that was faster than pure g-C3N4and ZnO nano-triangles.In addition to ZnO,nanoparticles of multi-metal oxide,Zn2SnO4was also coupled with g-C3N4for enhanced visible-light-driven photoactivity.Zhang et al.synthesized the composite Zn2SnO4/g-C3N4from heating a mixture of Zn2SnO4and melamine [151].Formation of heterojunction promoted the separation of photogenerated charges and thus enhanced the degradation of RhB under visible light irradiation.The reaction rate constant of 0.038min?1was exhibited by the optimized composite which was 3.2and38times higher than the rates of pure g-C3N4and Zn2SnO4, respectively.

The synthesis of a novel ternary composite of ZnO nanopar-ticles,g-C3N4and GO was systematically studied for enhanced visible-light-driven photocatalysis[152].A schematic of the co-precipitation and calcination process employed for the synthesis of nanoparticles and composites is provided in Fig.17A.The ternary composite with a spherical ZnO nanoparticles of5–10nm size evenly embedded in the porous g-C3N4was obtained by this method as shown in the TEM image(Fig.17B).A drastic quenching of photoluminescence intensity for the ternary composite indi-cated successful hybridization of the components.The optical spectra showed that absorption of the visible-light was signi?-cantly enhanced in the ternary composites(Fig.17C).The optimum content of50%g-C3N4in the binary composite,ZnO/g-C3N4exhib-ited effective hybridization and a high photocatalytic ef?ciency. However,performance of the ternary composite was two times faster than that of the ZnO/g-C3N4.The optimized ternary com-posite exhibited more than99%degradation of MB in just15min under visible-light irradiation,and the photocatalytic activity was maintained in the repeated runs.

CdS and multi-metal sul?des with Cd are fascinating visible-light-driven photocatalyst materials,however their photo-corrosion under working conditions is a serious issue.Cou-pling of Cd-based sul?des with g-C3N4is interesting for enhanced photocatalysis and stability of the system.CdS nanoparticles with10–30nm size were randomly distributed on g-C3N4sheets via an in-situ precipitation-deposition method[153].This novel inorganic-organic composite demonstrated high activity and

加拿大人的礼仪

加拿大人的礼仪加拿大人的礼仪加拿大人在社交场合与客人相见时，一般都行握手礼，亲吻和拥抱礼仅适合熟人、亲友和情人之间。在双方握手以后，他们会说“见到你很高兴”、“幸会”等。加拿大人的姓名同欧美人一样，名在前，姓在后。他们在作介绍时，一般遵循先少后长、先高后低、先宾后主的次序。在朋友众多的场合，他们总是顺着次序介绍，让大家互相认识，有地位较高的人士或辈份较高的长者在场的话，加拿大人总是先把朋友介绍给他们。在隆重的场合，加拿大人总是连名带姓地作介绍。作介绍时，双方都要站起来，友好地正视对方，面带笑容。加拿大人在作自我介绍时，声音适中，一边与别人握手，一边说出自己的姓名。他们对那种扯开嗓门向所有宾客介绍自己的方式很反感。加拿大人餐饮礼仪加拿大人在食俗上与英美人相似。由于气候寒冷的缘故，他们养成了爱吃烤制食品的习惯，这是他们的独特之处。加拿大人用刀叉进食，极爱食用烤牛排，尤其是八成熟的嫩牛排，习惯在用餐后喝咖啡和吃水果。加拿大人在饮食上讲究菜肴的营养质量，偏爱甜味，以面食、大米为主食，副食喜吃牛肉、鸡肉、鸡蛋、沙丁鱼以及西红柿、洋葱、土豆、黄瓜等。调料爱

用番茄酱、黄油等。他们有喝白兰地、香槟酒的嗜好。加拿大人忌食虾酱、鱼露、腐乳以及怪味、腥味的食物和动物内脏。在加拿大，赴宴时最好到花店买一束鲜花送给主人，以表达自己的谢意。在餐桌上，男女主宾一般分别坐在男女主人的右手边。饭前先用餐巾印一印嘴唇，以保持杯口干净。进餐时，左手拿叉，右手拿刀，刀用完后，放在盘子边上。吃东西时不要发出声音，不宜说话，不要当众用牙签剔牙，切忌把自己的餐具摆到他人的位置上。加拿大人认为正确、优雅的吃相是绅士风度的体现。加拿大人喜丧礼仪加拿大基督教徒的婚礼一般都在教堂里举行。在结婚仪式上，牧师要为他们做祷告，祝他们幸福美满，白头偕老。新郎新娘互换戒指，在上帝面前发誓，永远相爱。然后新郎新娘和来宾们一起祷告，一起唱赞美诗。在教堂举行婚礼，新郎和男宾坐在教堂的右手边，新娘和女宾坐在教堂的左手边，新人的亲属和朋友坐在前几排座位上。加拿大人对生孩子比较重视，如父母是基督教徒，要把孩子抱到教堂去，在牧师的主持下给孩子洗礼，孩子的洗礼仪式十分庄重。当加拿大人获悉朋友添了小宝宝，他们会马上打电话或寄张贺卡去恭贺一番。加拿大人去世后，一般都要请牧师做弥撒，使死者的灵魂升入天堂。在葬礼上，亲友要在牧师的祷告声中向墓穴中的灵柩撒下鲜花。参加葬礼的人，见到死者亲属，要和他们握一下手或拥抱一下，轻声地慰问几句，等葬礼仪式完毕以后再离开;如果不举

中国婚姻状况

提要] 日前公布的全国民政事业统计数据显示，今年一季度，我国共有46.5万对夫妻办理了离婚登记，较去年同期增长17.1%，平均每天有5000多个家庭解体。中国离婚率已连续7年递增。“中国式离婚”成为一个令世人关注的现象。国外媒体甚至创造新词“我一代”，分析这个曾号称世界婚姻最稳定的国家离婚率走高的原因…[我来说两句] 一向崇尚“家和万事兴”的古老中国，正遭遇婚姻动荡的冲击。日前公布的全国民政事业统计数据显示，今年一季度，我国共有46.5万对夫妻办理了离婚登记，较去年同期增长17.1%，平均每天有5000多个家庭解体。中国离婚率已连续7年递增。 “中国式离婚”成为一个令世人关注的现象。国外媒体甚至创造新词“我一代”，分析这个曾号称世界婚姻最稳定的国家离婚率走高的原因。中国人的婚姻怎么了？在社会转型期的今天，我们该如何经营维护健康、文明、幸福、安全的婚姻家庭？ ①社会转型期的婚姻震荡 22—35岁人群是离婚主力军，36—50岁年龄段是婚姻平稳期，50岁以上人群离婚率上扬今年29岁的唐娜，2年前结婚，一年后分手。“婚后我才发现找错了人。婚前，他勇于担当的优点，婚后变成了独断专行；婚前，他善交异性朋友，以为他有"女人缘"，谁知婚后半年他便出轨！” “一见钟情，婚了。一怒之下，离了。”北京市朝阳区婚姻登记处一位工作人员介绍，该处近年来登记的离婚夫妻中，35岁以下者超过一半。其中，“80后”又占多数，“闪离”现象日益突出。“有些人办手续时还在吵架，等拿到离婚证后，又抱在一起痛哭。” 不仅“80后”的婚姻多生变故，他们父辈一代的婚姻也亮起红灯。“现在孩子大学毕业可以自立了，家庭责任已经完成，不再彼此凑合了。”年逾50的项先生叹道：“结婚近20年，如人饮水，冷暖自知啊！”北京宏健律师事务所律师郑文旭说，他代理的离婚起诉案中，中老年人约占三成。 “随着社会转型、社会环境变化对婚姻家庭发起的挑战，"新离婚时代"来临。”中国社科院研究员、中国婚姻家庭研究会专家委员会副主任陈一筠说，新中国成立60年来，我国婚姻从前30年的超稳定型阶段，进入后30年尤其是近年来的动荡时期。统计数据显示，从上世纪70年代末开始，我国离婚人数和离婚率持续上升，近5年来增速明显，增幅高达7.65%。去年，全国120多万对夫妻喜结连理的同时，196万多对夫妇劳燕分飞。目前，北京、上海的离婚率已超过1/3。从年龄结构看，22岁—35岁人群是离婚主力军，36岁—50岁婚姻相对平稳，50岁以上离婚率迅速上扬；从教育背景看，学历高低与离婚率高低成反比，学历越低，离婚率越高，学历越高，离婚率越低。陈一筠表示，离婚率的提高，反映了社会发展与时代进步给个人生活带来更多的选择机会与自由空间的同时，也带来了人际矛盾与情感风险。 “我国的婚姻趋势，发生了与西方国家趋同的现象。”陈一筠介绍，婚姻家庭大规模的

加拿大商务习俗

加拿大商务习俗学院: 数学与数量经济学院专业：信息与计算科学专业1班学号：201216341403 姓名：邓正佳课程：下午7、8节

1.国家简介：加拿大为北美洲最北的国家，西抵太平洋，东迄大西洋，北至北冰洋，东北部和丹麦领地格陵兰岛相望，东部和法属圣皮埃尔和密克隆群岛相望，南方与美国国土接壤，西北方与美国阿拉斯加州为邻。领土面积达998万平方公里，为全世界面积第二大的国家。

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的事。 25.50% 41.52% 1.83% 2.10%29.04% 孩子三岁之前主要由谁照顾夫妻双方共同照顾父母长辈照顾其他人员（保姆等）主要由男方照顾主要由女方照顾

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卖妇女罪予以刑罚。第三条禁止包办、买卖婚姻和其他干涉婚姻自由的行为。禁止借婚姻索取财物。第二章结婚第五条结婚必须男女双方完全自愿，不许任何一方对他方加以强迫或任何第三者加以干涉。中华人民共和国刑法(节录) 第二百四十条拐卖妇女、儿童的，处五年以上十年以下有期徒刑，并处罚金;有下列情形之一的，处十年以上有期徒刑或者无期徒刑，并处罚金或者没收财产;情节特别严重的，处死刑，并处没收财产： (一)拐卖妇女、儿童集团的首要分子; (二)拐卖妇女、儿童三人以上的; (三)奸淫被拐卖的妇女的; (四)诱骗、强迫被拐卖的妇女卖淫或者将被拐卖的妇女卖给他人迫使其卖淫的; (五)以出卖为目的，使用暴力、胁迫或者麻醉方法绑架妇女、儿童的; (六)以出卖为目的，偷盗婴幼儿的; (七)造成被拐卖的妇女、儿童或者其亲属重伤、死亡或者其他严重后果的; (八)将妇女、儿童卖往境外的。拐卖妇女、儿童是指以出卖为目的，有拐骗、绑架、收买、贩卖、

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我国离婚率连续7年递增，如何维护现代婚姻家庭关系已经成为一门学问，建立婚姻家庭咨询和社会救疗机制势在必行面对离婚冲击波一向崇尚“家和万事兴”的古老中国，正遭遇婚姻动荡的冲击。日前公布的全国民政事业统计数据显示，今年一季度，我国共有46.5万对夫妻办理了离婚登记，较去年同期增长17.1%，平均每天有5000多个家庭解体。中国离婚率已连续7年递增。 “中国式离婚”成为一个令世人关注的现象。国外媒体甚至创造新词“我一代”，分析这个曾号称世界婚姻最稳定的国家离婚率走高的原因。中国人的婚姻怎么了？在社会转型期的今天，我们该如何经营维护健康、文明、幸福、安全的婚姻家庭？社会转型期的婚姻震荡 22 35岁人群是离婚主力军，36 50岁年龄段是婚姻平稳期，50岁以上人群离婚率上扬今年29岁的唐娜，2年前结婚，一年后分手。“婚后我才发现找错了人。婚前，他勇于担当的优点，婚后变成了独断专行；婚前，他善交异性朋友，以为他有女人缘，谁知婚后半年他便出轨！” “一见钟情，婚了。一怒之下，离了。”北京市朝阳区婚姻登记处一位工作人员介绍，该处近年来登记的离婚夫妻中，35岁以下者超过一半。其中，“80后”又占多数，“闪离”现象日益突出。“有些人办手续时还在吵架，等拿到离婚证后，又抱在一起痛哭。” 不仅“80后”的婚姻多生变故，他们父辈一代的婚姻也亮起红灯。“现在孩子大学毕业可以自立了，家庭责任已经完成，不再彼此凑合了。”年逾50的项先生叹道：“结婚近20年，如人饮水，冷暖自知啊！”北京宏健律师事务所律师郑文旭说，他代理的离婚起诉案中，中老年人约占三成。 “随着社会转型、社会环境变化对婚姻家庭发起的挑战，新离婚时代来临。”中国社科院研究员、中国婚姻家庭研究会专家委员会副主任陈一筠说，新中国成立60年来，我国婚姻从前30年的超稳定型阶段，进入后30年尤其是近年来的动荡时期。统计数据显示，从上世纪70年代末开始，我国离婚人数和离婚率持续上升，近5年来增速明显，增幅高达7.65%。去年，全国120多万对夫妻喜结连理的同时，196万多对夫妇劳燕分飞。目前，北京、上海的离婚率已超过1/3。

2019中国人婚恋状况调查报告

2017中国人婚恋状况调查报告 2018年1月5日

目录【一】调查背景 (2) 【二】调查对象 (4) 1. 性别构成 (5) 2. 年龄构成 (6) 3. 学历构成 (7) 4. 婚姻状况 (8) 【三】调查结果 (9) 1. 《婚姻法司法解释三》改变了单身人士的婚恋观 (9) 2. 90后择偶关键词：独立、享乐、现实 (14) 3. 中国人的初恋大大提早，28%的90后初恋发生在初中或更小 (17) 4. 仅38.9%的单身人士觉得自己很幸福或比较幸福 (18) 5. 家庭、健康和婚姻是中国人幸福的三大支柱 (19) 6. 68%的裸婚人士过得很幸福 (20) 7. 闪婚闪离现象普遍存在 (21) 8. 女性追求经济型婚姻，男性追求传宗接代型婚姻 (23) 9. 46.7%的女性认为应该由男方全款买房或男方付首付 (24) 10. 四成男性不愿在自己出资的房产上加女方名字，四成女性会提出此要求.. 25 11. 近一半的人隐忍家庭暴力的发生，因各种缘故接着并不幸福的生活 (26) 12. 女人因离婚率太高恐婚，男人因可怕失去自由恐婚 (27) 13. 六成的人情愿同意婚前辅导，90后更情愿同意婚前辅导 (28) 14. 男性择偶—感情投入、容貌外表，女性择偶—感情投入、经济条件 (30) 15. 男性28-30岁，女性25-27岁依旧是结婚的最正确年龄 (31) 16. 男性比女性更加认同婚恋网站这一择偶方式 (33) 2017年中国人婚恋调查报告一、调查背景在“后相亲时代”的中国，多元化的相亲形式，折射出国人婚恋观的变化。随着时代的变迁，人们对恋爱、婚姻的态度也有截然不同的改变，2017年8月最高法院颁布《婚姻法

中国婚姻状况调查简析

中国婚姻状况调查简析提要日前公布的全国民政事业统计数据显示，今年一季度，我国共有46.5万对夫妻办理了离婚登记，较去年同期增长17.1%，平均每天有5000多个家庭解体。中国离婚率已连续7年递增。“式离婚”成为一个令世人关注的现象。国外媒体甚至创造新词“我一代”，分析这个曾号称世界婚姻最稳定的国家离婚率走高的原因…[我来说两句] 一向崇尚“家和万事兴”的古老中国，正遭遇婚姻动荡的冲击。日前公布的全国民政事业统计数据显示，今年一季度，我国共有46.5万对夫妻了“中国式离婚”成为一个令世人关注的现象。国外媒体甚至创造新词“我一代”，分析这个曾号称世界婚姻最稳定的国家离婚率走高的原因。中国人的婚姻怎么了？在社会转型期的今天，我们该如何经营维护健康、文明、幸福、安全的婚姻家庭？①社会转型期的婚姻震荡22—35岁人群是离婚主力军，36—50岁年龄段是结婚平稳期，50岁以上人群离婚率上扬今年29岁的唐娜，2年前结婚，一年后分手。“婚后我才发现找错了人。婚前，他勇于担当的优点，婚后变成了独断专行；婚前，他善交异性朋友，以为他有"女人缘"，谁知婚后半年他便你不再彼此凑合了。”年逾50的项先生叹道：“结婚近20年，如人饮水，冷暖自知啊！”北京宏健律师事务所律师郑文旭说，他代理的离婚起诉案中，中老年人约占三成。“随着社会转型、社会环境变化对婚姻家庭发起的挑战，"新离婚时代"来临。”中国社科院研究员、中国婚姻家庭研究会专家委员会主任陈一筠说，新中国成立60年来，我国婚姻从前30年的超稳定后30

统计数据显示，从上世纪70年代末开始，我国离婚人数和离婚率持续上升，近5年来增速明显，增幅高达7.65%。去年，全国120多万对夫妻喜结连理的同时，196万多对夫妇劳燕分飞。目前，北京、上海的离婚率已超过1/3。从年龄结构看，22岁—35岁人群是离婚主力军，36岁—50岁婚姻相对平稳，50岁以上离婚率迅速上扬；从教育背景看，学历高低与离婚率高低成反比，学历越低，离婚率越高，学历越高，离婚率越低。趋势，发生了与西方国家趋同的现象。”陈一筠介绍，婚姻家庭大规模的。

地道的加拿大人会做的36 件事(这才是一般加拿大人的生活)

地道的加拿大人会做的36件事（这才是一般加拿大人的生活） 1、经常忘记锁门（原因有四：治安好；家中没有余钱；床底下没藏赃款；有财产保险）。 2、钱包里的现金不超过50加币（一是人穷；二是电子金融发达，小到杂货店，远到乡村都可以刷卡；三是广泛使用信用卡先消费后付账）。 3、每周买彩票梦想中大奖（因为除了买彩票有机会发财，干什么都不能发财，又无灰色收入，即使当公务员），越是老人越爱买彩票（买个梦，图个乐）。 4、每年至少见会计师两次，第一次是报税纳税，第二次是咨询如何少纳税。 5、每天喝咖啡，最爱Tim Hortons（加国本土品牌咖啡）。 6、如果遇见4个加国人，其中必定1个单身、1个离异有孩、1个有同居恋人（异性，也有的是同性）、1个结婚有两孩子，但他们每个人养至少一条狗或猫。 7、出门不带雨伞、纸巾；夏日炎炎也不打伞遮阳。 8、上扶手电梯永远靠右边站立（左边让给心急的赶时间的人）。 9、住高楼大厦会失眠（加拿大是个大农村，哪里都是矮矮的，不习惯）。

10、排队，从不插队。 11、开车遵守交规，即使深夜无人的时候也要等红绿灯。一上车就找安全带系，无论开车还是坐车。 12、分类投放垃圾（可回收、不可回收、电池、废电器等），无论在公共场所还是在家里。 13、进出商场或办公楼的大门时，回头检查有没有来人，有人就把着门等候。 14、在办公室上班、在教室上课喜欢吃零食。 15、80加币的衣服不等打5折绝对不买，800加币的Iphone刚上市就买。 16、特别小气，哪怕旧餐具也舍不得扔，收拾干净后甩卖或赠送，一天劳累所得还不够买个批萨。 17、每个圣诞节送家人、朋友、同事、助理或雇员礼物或贺卡。 18、每次度假或回母国探亲回来后都给同事或雇员带小纪念品。 19、跟左邻右舍或狭路相逢的人微笑，不管认识不认识。 20、售房搬家的时候，买盆花或小装饰品，写一张充满祝福和温情的贺卡，留给后来的新主人。 21、穿衣注意场合不注意品牌和价钱。日常着装特别老土，也叫朴素吧，进教堂或婚丧嫁娶却男西装女礼裙礼帽，，正式派对绝对考究，老妇少女一律晚礼服露肩挤胸香水首饰的，鬼节（Halloween）派对奇装异服怎么夸张独特怎么来。 22、和朋友外出吃饭AA制；寿星邀请你去餐馆参加生日派对，AA制；同事生日聚餐，AA制，唯一的寿星不出钱，其他人一起分摊。（所以加国的饭局是开心的真情的，绝对不是应酬，能够在一桌吃饭的绝对是情投意合的哥们姐们，不用礼尚往来，不用讲排场、排座次,没有公款消费） 23、邀请最尊贵的客人到家里吃晚餐而不是去餐馆。（除非最亲近的人才被邀请到家里，即使同事几十年也未必知道对方家住哪里） 24、当妈的都会做蛋糕或打理园艺；当爸的都会修房、修车或换水管、整草坪（一是加国人工很贵，二是加国以自己动手为荣）。 25、男人不敢轻易结婚生子。一是婚后必须以家庭和孩子为重心，与妻共同完成一切家务、抚育子女，没有保姆，也不依赖父母；二是离婚一次，男人就破产一次，每次要分一半财产给女方，支付孩子和前妻的生活费，所有的前妻都有份儿继承他的遗产。更不敢养二奶小三，养不起。 26、父母不提供18岁儿女上大学的费用，不给儿女买房子，不带孙子，不跟儿女住，不靠儿女养老。 27、万事预约。看病、修车、理发、旅游，哪怕和老朋友吃饭，甚至打的都要预约。

解读《2010中国人婚恋状况调查报告》(同名20785)

解读《2010中国人婚恋状况调查报告》全国妇联中国婚姻家庭研究会、中国社会工作协会婚介行业委员会和百合网１５日联合发布《２０１０中国人婚恋状况调查报告》。这项历时一年、覆盖全国３１个省份的社会调查，从多元的婚恋文化中提炼出中国当代单身人士婚恋观的现状、存在的问题及其深层原因。 “男公务员”“女教师”成择偶“香饽饽” 报告显示，超过４０％的受访女性希望理想伴侣的职业是公务员，以下依次是企事业管理人员、警察／军人、企业主、医务工作者等。这说明女性在择偶中倾向于寻求稳定；３８．３％的受访男性希望理想伴侣的职业为教师，以下依次是公务员、医务工作者、金融财会人员等。５２．１％的受访男性对女性的职业没有要求。中国社会工作协会婚介行业委员会总干事田范江认为，调查结果说明男性对女性的职业要求不

高，希望伴侣的职业稳定，能够有相对充裕的时间照顾家庭。报告指出，农业户口的城市男性和农村男性差异较大：在受访的农业户口的农村女性中，仅有０．７％选择农民为理想伴侣，因此农村男性择偶非常困难；而农业户口的城市男性选择农民为理想伴侣的比例仅为３．３％。田范江说，这表明长期居住在城市的农村人（如农民工）受城市文化熏陶，其婚恋观已发生变化，不愿与农民结婚。 “男性无房结婚免谈”“婚前财产公证难接受” 在所有备选择偶标准中，除“道德人品”和“脾气性格”外，男性和女性在择偶中最看重“感情投入”。“剩男”和“剩女”最大的不同是：男性在择偶时主要追求女性的外貌，不太看重她们的工作能力；而女性主要追求男性的经济实力和工作能力。

加拿大风土人情简介

o加拿大风土人情简介 o加拿大由於人类进化的历史比较短，所以特别重视已有历史传统的发扬，其他移民的文化也受到政府的保护鼓励和人们的喜爱加拿大第一民族（印第安土着）有图腾崇拜的宗教习裕，在加拿大西部的众多旅游地都有他们的图腾作品，如温哥华斯坦利公园和UBC民俗馆。印第安人认为每个氏族都与某种动植物有亲属关系，这种动植物即为该氏族的图腾。图腾为崇拜偶像，禁杀禁食，并树图腾柱促进繁衍这些图腾形象包括海狸﹑海狮﹑海狼﹑杀人鲸﹑狗鱼﹑三文鱼﹑比目鱼﹑老鹰﹑秃鹰﹑渡鸟﹑乌鸦﹑鸟﹑雷鸟﹑熊﹑狼﹑青蛙﹑蛇﹑铜头蛇等印第安人还崇拜羽毛，羽毛节为传统节日。相传一只山鹰曾解救过他们的祖先，为感谢山鹰的救命之思，祖先将散落在地上的羽毛供奉起来，逐渐形成羽毛节，节日的庆典活动在秋季举行，持继数日。节日里男人们着传统勇士服，头插羽毛，游行狂欢印第安特林吉特人则习惯在身上用颜色绘出各种图腾形象。居住的房屋为巨大的三角顶木极房，上面雕刻﹑彩绘各种图案，房内有很多松木箱用来储存食品和衣物，室内平台既是凳子又当床，房橼的架子和壁龛放置望各种生活用具印第安克里人住在帐篷里，用黑﹑红色在帐篷绘出各种神象及梦中灵魂，将其当作神灵供奉，彩绘帐篷退色後，便将帐篷烧掉以示对神灵的敬仰。克里族男子耳戴巨大耳环，耳环下吊着精工装饰，男子皆梳长辫印第安人的婚礼具有浓郁的民族特色。一般婚礼地点选择在聚集区的公共建筑物中举行，通常是大的木头房屋，亲朋好友﹑左邻右舍和村庄居民都纷纷进入，人们席地而座，互相问候，男女老少都着鲜艳的民族服装。婚礼主持人是酋长和两位长老，当他们来到时全体致意。酋长也身着象徵身份的服饰，此时新郎新娘身穿白色鹿皮服装跪座在酋长对面。仪式开始时酋长点燃蒿草，随着浓香青烟上升，酋长用民族语言向神明祈祷，为新婚夫妇祝福。然後左右的长老边说边唱，酋长点燃长管烟枪，按一定仪式吸完後再传给新郎新娘和所有在场的人，此举象徵和平和友好。吸完烟後有人擡来一大桶汤羹，新郎新娘敬给酋长和长老一大碗後，再分给在场的每一个人。在喜宴结束後，酋长和长老离去，人们则来到一块空地