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Applied Catalysis B:Environmental 180(2016)663–673

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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

In-situ synthesis of direct solid-state Z-scheme V 2O 5/g-C 3N 4heterojunctions with enhanced visible light ef?ciency in photocatalytic degradation of pollutants

Yuanzhi Hong a ,Yinhua Jiang b ,Changsheng Li a ,Weiqiang Fan b ,Xu Yan c ,Ming Yan b ,Weidong Shi b ,?

School of Materials Science and Engineering,Jiangsu University,Zhenjiang 212013,PR China b

School of Chemistry and Chemical Engineering,Jiangsu University,Zhenjiang 212013,PR China c

School of Energy and Power Engineering,Jiangsu University,Zhenjiang 212013,PR China

a r t i c l e i n f o Article history:

Received 16April 2015

Received in revised form 19June 2015Accepted 25June 2015

Available online 16July 2015Keywords:

Z-scheme heterojunction V 2O 5g-C 3N 4

Photocatalytic activity Visible light

a b s t r a c t

The constructing of direct solid-state Z-scheme heterojunction photocatalytic system has received much attention in environmental puri?cation and hydrogen generation from water.In this study,a novel direct solid-state Z-scheme V 2O 5/g-C 3N 4heterojunctions were synthesized via a facile in-situ growth strategy for the ?rst time.The photocatalytic performance was evaluated by the degradation of rhodamine B (RhB)and tetracycline (TC)under visible light irradiation ( >420nm).Results show that the as-synthesized heterojunctions can signi?cantly enhance photocatalytic activity in comparison with pure g-C 3N 4and V 2O 5.The optimum photocatalytic ef?ciency of VC1.0%sample for the degradation RhB was about 7.3and 13.0times higher than that of individual g-C 3N 4and V 2O 5,respectively.In addition,the VC1.0%sample as well as can ef?ciently degrade methyl orange (MO)and methylene blue (MB)under visible light.By further experimental study,the possible for the enhancing photocatalytic mechanism was found to be a direct solid-state Z-scheme heterojunction system based on the active species trapping and electron spin resonance (ESR)experiments,which not only can improve the photogenerated electron–hole pair’s separation but also exhibit a strong oxidation and reduction ability for ef?ciency degradation of organic pollutants.This work will be useful for the design of other direct solid-state Z-scheme photocatalytic systems for application in energy conversion and environmental remediation.

1.Introduction

Over the past years,semiconductor photocatalysis has been considered as a promising and green technology to resolve the increasing energy and environmental crisis by using of the solar light energy,such as for pollutants degradation and hydrogen generation from water [1–5].As is known,the photogenerated electrons and holes of the single-component photocatalyst can easily recombine each other,which results in poor quantum ef?-ciency and low photocatalytic performance [6].In past years,the use of heterojunction-type photocatalytic system is an impor-tant strategy to overcome this drawback because it can ef?ciently improve the photoexcited electron–hole separation [7–9].Unfor-tunately,the disadvantage of the typical heterojunction system is that the reducibility of photogenerated electrons and the oxidiz-

?Corresponding author.Fax:+8651188791108.E-mail address:swd1978@https://www.sodocs.net/doc/271168424.html, (W.Shi).ability of photogenerated holes usually become weakened after charge transfer [10].That is to say,the high charge-separation ef?ciency and strong redox ability are dif?cult to possess at the same time.Very recently,the construction of arti?cial Z-scheme photocatalytic system is an ideal and effective means because it not only can reduce the bulk electron–hole recom-bination,but also can preserve excellent redox ability [10,11].However,the majority of the synthesized arti?cial Z-scheme pho-tocatalytic systems usually had noble metal (Ag,Ru)[12–14]or redox pair (Fe 3+/Fe 2+,IO 3?/I ?)[15,16],which will bring about great dif?culties to their practical application.Thus,it is neces-sary to construct a novel Z-scheme system including only two visible-light-responsive solid-state photocatalysts.More recently,the synthesis of direct solid-state Z-scheme photocatalytic system has become a research hotspot for application in environmental puri?cation and hydrogen generation from water [17–22].The dif-ferent charge carrier transfer path of typical heterojunction and novel direct solid-state Z-scheme heterojunction are depicted in Fig.1.

https://www.sodocs.net/doc/271168424.html,/10.1016/j.apcatb.2015.06.057

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Fig.1.Schematic illustration of the different charge carrier transfer path of (a)typical heterojunction and (b)novel direct solid-state Z-scheme heterojunction.

Graphitic carbon nitride (g-C 3N 4),a metal-free polymeric semi-conductor,has attracted extensively attention due to its good thermal-chemical stability,electronic and optical characteristics [23].In addition,g-C 3N 4as a novel visible-light-driven photo-catalyst has been reported for hydrogen evolution and pollutant degradation under visible light irradiation [24–29].Nevertheless,the photocatalytic application of individual g-C 3N 4is limited due to the rapid recombination of photogenerated electron–hole pairs,leading to the poor quantum ef?ciency and low photocatalytic activity.Up to now,only a few g-C 3N 4-based direct Z-scheme sys-tems have been successfully prepared for enhancing photocatalytic activity,such as TiO 2/g-C 3N 4[30],BiOCl/g-C 3N 4[31],Bi 2O 3/g-C 3N 4[32],WO 3/g-C 3N 4[33]etc .However,the TiO 2and BiOCl can not excite in visible light because of their larger band gap (E g >3.0eV).On the other hand,although the Bi 2O 3and WO 3can respond to the visible light,they only can respond visible light with the wavelength shorter than 460nm which limit their in solar energy conversion.Therefore,it is a great challenge to construct the g-C 3N 4-based direct solid-state Z-scheme system with a narrow band gap semiconductor photocatalyst for ef?ciently utilizing the solar light.

Vanadium pentoxide (V 2O 5),an important transition metal-oxide semiconductor,has been widely used in lithium-ion batteries,gas sensors and optoelectronic devices [34–36].More-over,V 2O 5is a typical narrow band gap (～2.3eV)semiconductor,which can be a good candidate for capable of capturing visible light [37,38].What’s more,V 2O 5has suitable band edges (E CB =0.47eV,E VB =2.73eV),which can match well with g-C 3N 4(E CB =?1.2eV,E VB =1.5eV)to form a direct solid-state Z-scheme photocatalytic system [27,38].If that can be accomplished,the photoexcited electron in conduction band (CB)of g-C 3N 4shows the strong reducibility and the photoexcited hole on valence band (VB)of V 2O 5exhibits the strong oxidizability,respectively.

In the present study,we have ?rstly prepared the direct solid-state Z-scheme V 2O 5/g-C 3N 4heterojunction photocatalysts by a facile in-situ growth strategy.The photocatalytic performance was evaluated by the degradation of RhB and TC under visible light irradiation ( >420nm).The as-synthesized Z-scheme V 2O 5/g-C 3N 4photocatalysts could signi?cantly enhance photocatalytic activity in comparison with pure g-C 3N 4and V 2O 5.In addition,the VC1.0%sample exhibited the optimum photocatalytic activ-ity as well as possessed excellent photostability after ?ve times cycling photocatalytic reactions.Moreover,the VC1.0%sample also can ef?ciently decompose other pollutant organic dyes such as methyl orange (MO)and methylene blue (MB)under visible light.Furthermore,a novel direct solid-state Z-scheme photocatalytic mechanism for the enhancing photocatalytic activity was also

proposed based on the active species trapping experiments and electron spin resonance (ESR)analysis.2.Experimental 2.1.Materials

Melamine,NH 4VO 3,triethanolamine (TEOA),1,4-benzoquinone (BQ),isopropanol (IPA),and 5,5-dimethyl-1-pyrroline N-oxide (DMPO)were analytical grade agents and purchased from Aladdin (China).Rhodamine B (RhB),tetracycline (TC),methyl orange (MO)and methylene blue (MB)were analytically pure and used without further puri?cation.

2.2.Photocatalysts preparation

The metal-free bulk g-C 3N 4powders were synthesized by the thermal treatment of melamine according to the previous paper [39].Typically,10g melamine was put into a 50mL alumina cru-cible with a cover,then heated in a muf?e furnace at a rate of 2.3?C/min and kept for 4h at 550?C.After being cooled to room temperature,the resulting products were collected and milled into powder in an agate mortar for further use.

The direct solid-state Z-scheme V 2O 5/g-C 3N 4heterojunction photocatalysts were prepared by a facile in-situ growth strategy as illustrated in Fig.2.Speci?cally,a certain amount of bulk g-C 3N 4and different amount of NH 4VO 3were added into an agate mortar,and grounded together.The resultant powders were transferred to a covered alumina crucible,then calcined at 500?C for 1h with a heating rate of 5?C/min.To make clarity,the V 2O 5/g-C 3N 4com-posites with expected V 2O 5contents of 0,0.5,1.0,1.5,2.5and 5.0wt%are referred to as pure g-C 3N 4,VC0.5%,VC1.0%,VC1.5%,VC2.5%and VC5.0%,respectively.For comparison,bare V 2O 5was prepared similarly but without the addition of g-C 3N 4.2.3.Characterization

All of the phase compositions and crystal structures of the prepared samples were determined by powder X-ray diffraction (XRD)method using Cu K ?radiation ( =1.54178?),(D/MAX-2500diffractometer,Rigaku,Japan)with Cu-K ?radiation source (k =1.54056)over the 2?range of 5.0–80?at a scanning rate of 7.0?min ?1.The morphology of as-prepared samples was observed by scanning electronic microscopy (SEM)on an S-4800?eld emission SEM (SEM,Hitachi,Japan).The transmission electron microscopy (TEM),high-resolution TEM (HRTEM)and High angle angular dark ?eld-scanning transmission electron microscopy (HAADF-STEM)

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Fig.2.Schematic illustration of the in-situ growth strategy of V2O5/g-C3N4heterojunction photocatalysts.

were also used to characterization the sample by transmission electron microscopy(Tenai G2F30S-Twin,FEI)using an accel-erating voltage of200kV.The X-ray photoelectron spectroscopy (XPS)was obtained by a Thermo ESCALAB250X(America)elec-tron spectrometer using150W Al K?X-ray sources.The UV–vis diffused re?ectance spectra(DRS)of the samples were obtained from an UV–vis spectrophotometer(UV-2450,Shimadzu,Japan), BaSO4was used as a re?ectance standard.The photoluminescence (PL)spectra of samples were measured on a PerkinElmer LS55 at room temperature using a?uorescence spectrophotometer.The photocurrent measurements were performed by using a CHI660B (Chenhua,China)electrochemical workstation with a standard three-electrode cell at room temperature.The electron spin reso-nance(ESR)analysis was conducted with a Bruker EPR A300-10/12 spectrometer.

2.4.Photocatalytic experiments

The photocatalytic activities of the as-prepared samples were investigated by the degradation of RhB and TC under visible light irradiation.The photochemical reactor was illuminated using a 250W xenon lamp with a420nm cutoff?lter.In each test,50mg of as-prepared samples were added into100mL of10mg/L pollutants aqueous solution.Then the suspension was stirred in the dark for 40min to achieve the adsorption–desorption equilibrium prior to visible light irradiation.The concentration changes of RhB and TC were monitored by measuring the UV–vis absorption of the suspen-sions at15min interval.During irradiation,5mL of the suspension was taken out and centrifuged(10,000rpm,10min)to remove the photocatalysis before measurement.The peak absorbencies of RhB at553nm and TC at357nm were used to determine its con-centration by a TU-1810UV–vis spectrophotometer.In addition, the photocatalytic application of as-synthesized sample was fur-ther estimated by the degradation of MO and MB under visible light.The same operations were carried out except that the pol-lutants aqueous solution was replaced by100mL of10mg/L MO and MB aqueous solution.The concentrations of MO and MB were also determined by a TU-1810UV–vis spectrophotometer at the wavelength of464and664nm,respectively.

2.5.Active species trapping and ESR experiments

It is well known that the holes(h+),superoxide radical(?O2?) and hydroxyl radicals(?OH)are the major reactive species for the photocatalytic oxidation.In order to investigate the main active species responsible for the photocatalytic reaction,TEOA(1mM) [12,40],BQ(1mM)[41,42],and IPA(1mM)[12,43]were respec-tively employed as the scavengers for h+,?O2?and?OH.The method was similar to the former photocatalytic activity test.In addition, the ESR technique was further used to detect the presence of?OH and?O2?radicals in the photocatalytic reaction system under vis-ible light( >420nm).The?OH and?O2?radicals usually can be trapped by the DMPO[44].Before to determine the hydroxyl

rad-

Fig.3.XRD patterns of as-prepared photocatalysts.

icals(DMPO-?OH)and superoxide radicals(DMPO-?O2?),10.0mg samples were dissolved in0.5mL deionized water(DMPO-?OH)or 0.5mL methanol(DMPO-?O2?),and then45?L DMPO was added with ultrasonic dispersion for5min,respectively.

3.Results and discussion

3.1.Characterization of as-prepared samples

The crystalline phases of the as-prepared samples were deter-mined by XRD analysis.As shown in Fig.3,the diffraction peaks of the pure g-C3N4sample appears at13.0?and27.4?,which can be indexed to the(100)and(002)diffraction planes of graphite-like carbon nitride,respectively[41].The bare V2O5sample can be assigned to the orthorhombic phase of V2O5(JCPDS75-0457).The results reveal that as-prepared photocatalysts are well crystallized. For the V2O5/g-C3N4composited samples,the characteristic peaks of g-C3N4are gradually become weak,whereas the peak intensi-ties of V2O5become stronger with the increasing of V2O5content. Moreover,no other impurity phases are discovered,indicating the as-obtained V2O5/g-C3N4heterostructured photocatalysts are two-phase hybrid.

The morphologies of as-synthesized samples were character-ized by SEM.From Fig.4,we can be seen that the pure g-C3N4 was composed of different nanosizes crystals stacking layers with smooth surface.After introducing the V2O5,the V2O5/g-C3N4com-posites samples appeared some agglomeration nanoparticles on the surface of g-C3N4,resulting in the formation of a heterostruc-ture.The morphologies,microstructures and chemical composition of the VC1.0%sample were further carried out by TEM.As shown in Fig.5a,TEM image of VC1.0%sample shows a layered structure, which offers substrate for loading of V2O5.From Fig.5b,the HRTEM

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Fig.4.SEM images of as-prepared samples (a)pure g-C 3N 4;(b)VC1.0%;(c)VC2.5%;(d)VC5.0%.

shows the lattice spaces of VC1.0%sample,which displays the lat-tice spaces of V 2O 5crystallite is determined as 0.287nm,belonging to the (400)crystal plane.However,the lattice fringe of g-C 3N 4is dif?cult to be found.To further observe the combination of g-C 3N 4and V 2O 5in the composited VC1.0%sample,HAADF-STEM is employed.The maps of C,N,V and O are also given in Fig.6,which illustrate that V 2O 5is closely contacted with g-C 3N 4.

Fig.7shows the N 2adsorption–desorption isotherms of as-prepared pure g-C 3N 4and VC1.0%samples.It can be seen that the isotherms are similar and all of them are of classical type IV,sug-gesting the presence of mesopores.The BET surface area (S BET )of the g-C 3N 4was 13.4m 2g ?1,and with increasing the mount of V 2O 5,the S BET of the as-prepared V 2O 5/g-C 3N 4heterojunction photocat-alysts become more and more large (from VC0.5%to VC5.0%,the

S BET were found to be 162.6,163.4,167.3,170.1and 211.8m 2g ?1,respectively).Compared with pure g-C 3N 4,the S BET of V 2O 5/g-C 3N 4composites is increased slightly.In addition,the S BET ,average pore diameter and pore volume of as-prepared samples are listed in Table S1.

The surface chemical compositions and states of as-prepared pure g-C 3N 4and VC1.0%samples were further examined by XPS.As shown in Fig.8,in the C 1s XPS spectrum,the peak at 284.8eV was used as the reference for calibration.The C 1s XPS spectrums peaks of pure g-C 3N 4and VC1.0%samples at 288.1eV and 288.3eV could be assigned to the N C N coordination in graphitic-like carbon nitride [45].In the N 1s XPS spectrums,the main N 1s peaks of pure g-C 3N 4and VC1.0%samples at 398.9eV and 398.6eV are due to the sp 2-hybridized nitrogen (C N C)[27].The C 1s and N

Fig.5.(a)TEM and (b)HRTEM images of as-prepared VC1.0%sample.

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Fig.6.HAADF-STEM images of the VC1.0%sample with maps of C–K,N–K,V–L and O–K.

binding energies change of pure g-C3N4and VC1.0%samples may be attributed to the heterojunction interaction between g-C3N4and V2O5.In the V2p and O1s XPS spectrum of VC1.0%sample,the V 2p peak at517.4eV and524.8eV are ascribed to the V2p3/2and V 2p1/2levels,and the O1s peak at532.2eV corresponds to the V2O5 oxygen atom,respectively[46,47].

The UV–vis diffuse re?ectance spectra of as-prepared samples are shown in Fig9a.As can be seen from the curve,the pure g-C3N4shows absorption wavelengths from the ultraviolet to the visible light region up to460nm,and bare V2O5exhibits a sharp absorption edge at https://www.sodocs.net/doc/271168424.html,pared with that of pure g-C3N4,the absorption edges of the V2O5/g-C3N4heterostructured samples(VC0.5–VC5.0%)show a systematic slight red-shift.It is in good agreement with the colors change of the samples,which from light yellow turns to dark gray with increasing the V2O5content (see insert picture).The results imply that all these as-synthesized photocatalysts possess visible light response.In addition,the opti-cal band gap energy(E g)of a semiconductor photocatalyst can be calculated by the following formula:

?hv=A(hv–E g)n/2(1) where?,hv,A and E g represent the absorption coef?cient,Planck constant,light frequency,proportionality and band gap energy, respectively.The band gap energy for the g-C3N4was determined from a plot of(?hv)1/2vs.hv(n=4for indirect transition)[33],and the V2O5was obtained from a plot of(?hv)2vs.hv(n=1 for direct transition)[48].As shown in Fig.9b,the band gap of g-C3N4and V2O5were estimated to be2.67and2.22eV,which were nearly equal to the previous literatures[27,46].In addition, as for other heterostructured samples(from VC0.5%to VC5.0%),the energy band gaps were found to be2.65,2.63,2.60,2.57and2.54eV, respectively.

The PL analysis was used to estimate the photoinduced charge carriers separation and transfer ability of the as-prepared samples. Before the measurement,samples(10mg)were well-dispersed 5mL of ethanol and the PL emission spectra of the as-prepared samples monitored at an excitation wavelength of350nm.It is well acknowledged that the higher PL intensity indicates the fast recombination of the charge carriers,resulting in lower photocat-alytic activity[49].As shown Fig.10a,it can be seen that pure g-C3N4sample has a main emission peak at about460nm,which can be corresponded to its band gap and ascribed to the band gap recombination of electron–hole pairs[50].After the V2O5was introduced,the heterostructured samples show lower PL emis-sion intensity compared with that of pure g-C3N4,suggesting that the recombination rate of photogenerated charge carriers become lower in V2O5/g-C3N4heterojunction.Moreover,the PL intensity of the VC1.0%sample possesses the lowest PL intensity,suggest-ing its high photocatalytic activity.The PL result suggests that the recombination rate of electron–hole pairs can be restrained by

the Fig.7.N2adsorption–desorption isotherms of as-prepared pure g-C3N4and VC1.0%samples(Closed symbols,desorption;open symbols,adsorption).

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Fig.8.XPS spectra of pure g-C3N4and VC1.0%samples(a)C1s;(b)N1s;(c)V2p;and(d)O1s.

Fig.9.(a)UV–vis diffuse re?ection spectra of as-prepared photocatalysts;(b)Plots of the(?h )1/2vs.photon energy(h )for g-C3N4and other heterojunction samples,plots of the(?h )2vs.photon energy(h )for V2O5.The insert image displays the digital pictures of as-prepared samples.

Fig.10.(a)PL emission spectra and(b)transient photocurrent response of as-prepared samples.

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Fig.11.(a)The photocatalytic activities of as-prepared samples for RhB degradation under visible-light( >420nm);(b)The UV–vis spectral absorption changes of RhB solution photodegraded over VC1.0%sample under visible light irradiation.The inset picture is the color change of RhB aqueous solution.(c)The pseudo-?rst-order reaction kinetics for RhB degradation;(d)The apparent rate constants for RhB degradation.

forming of the novel direct Z-scheme heterojunction system.In addition,the transient photocurrent responses of as-prepared sam-ples electrodes were recorded.The photocurrents were performed for a500s period in an on-and-off cycle mode under visible light. From Fig.10b,it can be seen that pure g-C3N4shows the lower pho-tocurrent intensity than that of other heterostructured samples.In addition,the VC1.0%sample as well as exhibits the highest pho-tocurrent intensity in comparison with other samples.It is believed that the stronger photocurrent intensity reveals the higher elec-trons and holes separation ef?ciency[51].Therefore,the result demonstrates that constructing of direct Z-scheme V2O5/g-C3N4 heterojunctions can improve the photogenerated electron–hole pair’s separation.

3.2.Photocatalytic activity

It is well known that the adsorption property of catalyst is one of the crucial factors to affect the photocatalytic activ-ity.Fig.S1shows the photocatalysts adsorption behaviours over RhB in darkness.It can be seen that the concentration of RhB could reach adsorption–desorption equilibrium after dark reac-tion for40min.In addition,with increasing the V2O5content,the as-prepared V2O5/g-C3N4heterojunction photocatalysts exhibit gradually stronger adsorption properties in compared with that of pure g-C3N4.Moreover,the larger S BET of V2O5/g-C3N4heterojunc-tion samples as well as can facilitate to adsorb more RhB molecules, which are bene?t for improving their visible light activities.

The photocatalytic activities of the as-prepared samples were ?rstly evaluated by the degradation of RhB as a model pollutant under visible light irradiation( >420nm).As shown in Fig.11a,the blank test without the catalyst reveals that the photolysis of RhB molecule is very slowly which can be negligible.Fig.10a indicates that the pure g-C3N4(33.4%)and bare V2O5(21.3%)samples exhibit low photocatalytic performance for degradation of RhB under visi-ble light.However,the as-synthesized V2O5/g-C3N4heterojunction photocatalysts can ef?ciently enhance the photocatalytic activity in compared with the individual g-C3N4and V2O5.The result illus-trates that the V2O5content has a signi?cant in?uence on the photocatalytic activities of V2O5/g-C3N4heterostructured photo-catalysts.When the V2O5content is increased beyond1.0wt%,a decrease in the photocatalytic activity was observed.Among them, the VC1.0%heterostructure shows the highest photocatalytic per-formance(95.5%)compared with the other samples.In addition, Fig.11b displays the UV–vis spectral absorption changes of RhB solution photodegraded over VC1.0%sample as a function of time. Obviously,the main absorption peak of RhB molecule located at 553nm,which decreased rapidly with extension of the exposure time.The results further demonstrate that the RhB molecules are completely degraded after60min visible light irradiation,which is corresponding with the color change of RhB aqueous solution.

The kinetic behaviors of as-prepared samples for photodegra-dation of RhB were investigated further.As illustrated in Fig.11c, all of them?t well with the pseudo-?rst order correlation:

=kt(2)

where C is the concentration of RhB remaining in the solution at irradiation time of t,C0is the initial concentration at t=0,and k is the degradation apparent rate constant.The k values of different samples are shown in Fig.11d,it can be found that when the V2O5 content is below1.0wt%,the photocatalytic activity increased with the increasing of V2O5content.However,when the V2O5content

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Fig.12.The photocatalytic activities of as-prepared samples for the degradation of TC under visible light irradiation.

exceeds1.0wt%,the photocatalytic activity of V2O5/g-C3N4het-erostructured photocatalysts decreased as the increasing of V2O5 content.The optimal loading amount of V2O5on g-C3N4is1.0wt% according to its photocatalytic activity.The k value for RhB degra-dation over VC1.0%(0.0491min?1)sample is about7.3times higher than that of pure g-C3N4(0.00677min?1),and13.0times than that of bare V2O5(0.00379min?1).In addition,the apparent quantum ef?ciency of VC1.0%sample was studied by using a CEL-LED100 light with =420nm.The result was found equal to0.07%(see Supporting information).Thus,the as-obtained photocatalyst can ef?ciently enhance the photocatalytic activity for degradation of RhB under visible light irradiation.

The photocatalytic activities of the as-prepared samples were further studied by the degradation of TC under visible light for evaluating the photocatalytic activity of materials and eliminating the indirect dye photosensitization in the photocatalytic system.As shown in Fig.12,the blank test without the photocatalyst shows that the photolysis of the colorless TC molecule is very slowly which indicating that the TC molecule is very stable in the aqueous solu-tion.Moreover,pure g-C3N4(30.3%)and bare V2O5(19.0%)samples exhibit low photocatalytic performance for degradation of TC under visible light.However,the as-synthesized photocatalysts can also ef?ciently enhance the photocatalytic activity in compared with the individual g-C3N4and V2O5,in which is similar to the result

of Fig.14.The photodegradation performance of VC1.0%photocatalyst for RhB,MO, and MB degradation under visible light irradiation.

the degradation of RhB.In addition,the VC1.0%sample as well as shows the highest photocatalytic performance(75.7%)in compared with the other samples.Therefore,these results suggest that our photocatalyst display excellent visible-light-driven photocatalytic activity and can be a good candidate for application in environmen-tal puri?cation.

The stability of the as-synthesized VC1.0%sample was evaluated by the circulation experiments.After each cycling test,the sam-ple was collected and washed with distilled water and absolute ethanol for three times,then the as-obtained sample were dried in vacuum at80?C for12h for next cycling reused.From Fig.13a,it can be observed that the photocatalytic activity of the VC1.0%sam-ple has no apparent deactivation(90.6%)even after?ve successive recycles for the degradation of RhB under visible light irradiation, which reveals that our photocatalyst possess high stability for its practical application.Moreover,the XRD pattern of the VC1.0%sam-ple after5th run cycle has been researched(Fig.13b).It can be clearly observed that the phase and structure of the VC1.0%sam-ple remained unchanged,which suggests that the sample is stable even after the5th run cycle photocatalytic degradation processes. In addition,in order to further investigate the photocatalytic appli-cation of the as-prepared sample,the photocatalytic performance of the VC1.0%sample was also studied by the degradation of MO and MB under visible light.As shown in Fig.14,the prepared

sam-

Fig.13.(a)The repeated photocatalytic experiments of VC1.0%photocatalyst for degradation of RhB under visible light irradiation;(b)The XRD pattern of the VC1.0%sample after5th run cycle photocatalytic experiments.

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Fig.15.The species trapping experiments for degradation of RhB over pure g-C3N4, V2O5and VC1.0%photocatalysts under visible light irradiation.

ple also can ef?ciently decompose the other colored organic dyes (MO and MB)under visible light.

3.3.Photocatalytic mechanism

The different active species trapping experiments for the degra-dation of RhB over the pure g-C3N4,V2O5and VC1.0%samples were ?rst carried out to explore the enhancing photocatalytic mecha-nism.As shown in Fig.15,for pure g-C3N4(33.4%),when the TEOA was added into reaction solution,the photocatalytic degradation rate is almost invariable(31.1%).However,when the BQ and IPA were added into reaction solution,the degradation rate of RhB inhibited(19.5%and22.8%),respectively.Thus,the?O2?and?OH radicals are the major reactive species in the pure g-C3N4reaction system.On the contrary,for bare V2O5(21.3%),when the TEOA was added into reaction solution,the photocatalytic degradation rate is almost inhibited(9.6%).When the BQ and IPA were added, the degradation rate of RhB almost no changed(20.5%and19.7%), implying that the h+is the major reactive species in the pure V2O5 reaction system.On the other hand,for the VC1.0%sample(95.5%), it can be seen that when TEOA was added,the photocatalytic degra-dation rate is decreased signi?cantly(15.0%),indicating the h+is the predominant active species.When the BQ was added into reac-tion solution,the degradation rate of RhB also inhibited slightly (51.7%),suggesting the?O2?also plays an important role in the pho-tocatalytic process.To our surprise,when the IPA was added into reaction solution,the photocatalytic degradation rate is not obvi-ously changed(94.4%),indicating that the?OH does not contribute to the degradation of RhB.Therefore,the h+and?O2?radicals are the major reactive species in the VC1.0%reaction system.

The ESR technique was further used to detect the presence of

?OH and?O

2?radicals in the pure V

O5,g-C3N4and VC1.0%photo-

catalytic reaction systems under visible light.As shown in Fig.16, for pure g-C3N4and VC1.0%samples,the four characteristic peaks of the DMPO-?OH adducts(Fig.16a)and six characteristic peaks of DMPO-?O2?adducts(Fig.16b)are observed,indicating that the

?OH and?O

2?radicals produced in both g-C

N4and VC1.0%reaction

systems.However,for bare V2O5sample,there are no the charac-teristic peaks of DMPO-?OH adducts or the characteristic peaks of DMPO-?O2?adducts are observed,implying that no?OH or?O2?radicals generated in V2O5reaction system.Moreover,compared with pure g-C3N4and VC1.0%samples,the?O2?and?OH signals intensities of VC1.0%sample is obviously stronger than that of pure g-C3N4,suggesting that the amount of?O2?and?OH radicals gen-erated on the VC1.0%heterostructured surface is more than that of pure g-C3N4.Therefore,according to the above results of the active species trapping experiments and this ESR analysis,it can be inferred that the?O2?played a major role in the g-C3N4and VC1.0% photocatalytic reactions rather than the?OH.

In order to explain the enhanced photocatalytic activity mech-anism,the band edge positions of the valence band(VB)and conduction band(CB)potentials of g-C3N4and V2O5were con-?rmed.For a semiconductor,the VB and CB can be calculated according to the empirical equation:

E CB=X?E e?1

E g(3) E VB=E CB+E g(4)

The X values for g-C3N4and V2O5are4.73and6.10eV,respec-tively[33,52].E e is the energy of free electrons on the hydrogen scale(E e=4.5eV),and E g is the band gap energy of the semiconduc-tor(E g for g-C3N4,V2O5are2.67and2.22eV,respectively).From the calculation,the E CB of g-C3N4and V2O5are about?1.10and0.49V vs.NHE,and the E VB of g-C3N4and V2O5are estimated to be1.57 and2.71V vs.NHE,respectively.Thus,for the pure g-C3N4,the pho-togenerated electrons on the CB of g-C3N4can react with O2to form

?radicals due to the position of CB of g-C

N4is more negative than the potential of O2/?O2?(?0.33V vs.NHE)[53].Whereas,the VB potential of g-C3N4is lower than the standard redox potential of?OH,H+/H2O(2.72V vs.NHE)[54],implies that the photoex-cited holes in the VB of g-C3N4cannot oxidize the adsorbed H2O molecules to product?OH.However,the results of active species trapping and ESR analysis demonstrated that the?OH can be pro-duced in the g-C3N4photocatalytic system.Therefore,the?OH may be generated by further reduction of?O2?,which is an indirect way to form the?OH[32,55]:

e?+O2→?O2?(5)

?+e?+2H+→H

O2(6)

?+H

O→?OH+OH?+O2(7) H2O2→2?OH(8) As for bare V2O5,the photogenerated electrons on the CB of V2O5 cannot react with O2to form?O2?radicals because the position of CB of V2O5is less negative than the potential of O2/?O2?(?0.33V vs.NHE).The VB potential of V2O5is lower than the standard redox potential of?OH,H+/H2O(2.72V vs.NHE),suggests that the pho-toexcited holes in the VB of V2O5also cannot oxidize the adsorbed H2O molecules to product?OH.It is corresponding with the active species trapping experiments and ESR analysis that no?OH or?O2?radicals produced in V2O5system.

On the basic of the above experimental results,a novel direct solid-state Z-scheme mechanism for the enhanced photocatalytic activity of V2O5/g-C3N4heterojunctions was proposed.As illus-trated in Fig.17,both g-C3N4and V2O5can be initiated by the visible-light to yield photogenerated electron–hole pairs.If the charge transfer path of photogenerated electron-hole pairs is like the typical heterojunction system,then the photogenerated elec-trons in the CB of V2O5will product fewer?O2?radicals because of its low reducibility.Thus,the photogenerated electrons in the CB of V2O5tend to transfer and recombine with the photogenerated holes in the VB of g-C3N4.In this way,the more photogenerated electrons accumulated in the CB of g-C3N4can reduce the adsorbed O2to form more?O2?,which is a powerful oxidative specie can break down the chromophores of organic pollutants into small molecules,e.g.,CO2and H2O[56].Meanwhile,the photogenerated holes left behind in the VB of V2O5can directly oxidize organic pollutants.Therefore,it can draw a conclusion that the photocat-alytic reaction of prepared V2O5/g-C3N4heterojunctions followed a direct solid-state Z-scheme mechanism,which could improve

672Y.Hong et al./Applied Catalysis B:Environmental 180(2016)

663–673

Fig.16.DMPO spin-trapping ESR spectra in aqueous dispersion of V 2O 5,g-C 3N 4,and VC1.0%samples for (a)DMPO-?OH and (b)DMPO-?O 2?irradiated for 60

Fig.17.The possible photocatalytic mechanism of V 2O 5/g-C 3N 4heterojunction photocatalysts for degradation of organic pollutants under visible light irradiation.

the photogenerated electron–hole pair’s separation and transfer as well as show a strong oxidation and reduction ability for ef?ciency degradation of organic pollutants.4.Conclusions

In summary,we have ?rstly constructed the novel direct solid-state Z-scheme V 2O 5/g-C 3N 4heterojunction photocatalysts by a facile in-situ growth strategy.The as-prepared photocatalysts could be widely used for environmental puri?cation of organic pollu-tants in aqueous solution because of its highly ef?cient and stable visible-light-driven photocatalytic performance.In addition,the enhancing photocatalytic activity was due to the formation of a direct solid-state Z-scheme heterojunction between the g-C 3N 4and V 2O 5,which resulting in the electrons in the CB of g-C 3N 4exhibits high reducibility and the holes in the VB of V 2O 5shows high oxidizability.Moreover,it should be pointed out that our pho-tocatalysts are cost saving,high ef?ciency,good recyclable and easy for large-scale production.Acknowledgements

The authors are grateful for the National Natural Science Foun-dation of China (21276116,21477050,21301076and 21303074),Excellent Youth Foundation of Jiangsu Scienti?c Committee (BK20140011),Chinese–German Cooperation Research Project (GZ1091).Program for High-Level Innovative and Entrepreneurial Talents in Jiangsu Province,Program for New Century Excellent Talents in University (NCET-13-0835),Henry Fok Education Foun-

dation (141068)and Six Talents Peak Project in Jiangsu Province (XCL-025).

Appendix A.Supplementary data

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N In-situ synthesis of di

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