Synthesis of barbituric acid doped carbon nitride for efficient solar-driven photocatalytic degradation of aniline

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Applied Surface Science

j o ur na l ho me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Full Length Article

Synthesis of barbituric acid doped carbon nitride for efficient solar-driven photocatalytic degradation of aniline

Lin Li

^a

, Qingguo Meng

^b

, Haiqin Lv

^b

, Lingling Shui

^a

, Yongguang Zhang

^c

, Zhang Zhang

^c,d

, Zhihong Chen

^b,c,∗

, Mingzhe Yuan

^b

, Richard Nötzel

^a,c

, Xin Wang

^a,c,∗

, Jun-Ming Liu

^d,e

, Guofu Zhou

^a

aInstituteofElectronicPaperDisplays,SouthChinaAcademyofAdvancedOptoelectronics,SouthChinaNormalUniversity,Guangzhou,Guangdong Province,China

bShenyangInstituteofAutomation,Guangzhou,ChineseAcademyofSciences,Guangzhou511458,China

cInternationalAcademyofOptoelectronicsatZhaoqing,SouthChinaNormalUniversity,GuangdongProvince,China

dInstituteofAdvancedMaterials,SouthChinaAcademyofAdvancedOptoelectronics,SouthChinaNormalUniversity,Guangzhou,GuangdongProvince, China

eLaboratoryofSolidStateMicrostructures,NanjingUniversity,Nanjing210093,China

a r t i c l e i n f o

Articlehistory:

Received11July2017 Receivedinrevisedform 12September2017 Accepted19September2017 Availableonline20September2017

Keywords:

Carbonnitride Copolymerizing Barbituricacid Photodegradation Aniline

a b s t r a c t

Aseriesofbarbituricaciddopedcarbonnitride(CN-BA)photocatalystsweresuccessfullypreparedby copolymerizingdicyandiamidewithbarbituricacid(BA).UnderAM1.5simulatedsunlight,CN-BApho- tocatalystsexhibitenhancedphotocatalyticactivitycomparedtopurecarbonnitrideforthedegradation ofaniline.Thehighestactivityisobtainedwith2%dopedCN-BAphotocatalyst.

Results:onthephotodegradationofanilineindicatethatfortheoptimizedCN-BAphotocatalyst,thecon- centrationofanilinesolutionwasreducedgraduallyfrom16mg/Lto1.354mg/Lin2h.Thiscorresponds toa6timeshigherphotodegradationefficiencythanpurecarbonnitridesamples.Theenhancedpho- tocatalyticactivityofCN-BAreliesontheenhancedsurfacearea,thehigherlightabsorptionandthe reducedrecombinationofthephoto-generatedelectron-holepairs.Thisinterpretationresultsfrommul- tiplecharacterizationswithEPR,BET,N2adsorption,Solid-state¹³CNMR,UV–visDRS,FESEM,andTEM.

Undersimulatedsunlightirradiation,CN-BAisexcitedandgenerateselectron-holepairs.Thephoto- generatedelectronsintheCN-BAconductionbandreactwiththemolecularoxygentoform^•O2−.Part ofthe^•O2−transformsinto^•OH,whichfurtheroxidesaniline.Meanwhile,photo-generatedholesinthe valencebandofCN-BAcanbenefittotheformationof^•OHordirectlyoxideaniline.

©2017PublishedbyElsevierB.V.

1. Introduction

Anilineisanimportantprecursorandintermediatecompound which has been widely applied in the industrial production of polyurethanes, pharmaceuticals, pesticides and rubber addi- tives[1,2].However,anilineisahighlytoxicandcancer-causing compound which is now considered as a major environmen-

∗ Correspondingauthorat:InstituteofElectronicPaperDisplays,SouthChina AcademyofAdvancedoptoelectronics,SouthChinaNormalUniversity,Guangzhou, GuangdongProvince,China;ShenyangInstituteofAutomation,Guangzhou,Chinses AcademyofSciences,Guangzhou,China;InternationalAcademyofOptoelectron- icsatZhaoqing,SouthChinaAcademyofAdvancedoptoelectronics,SouthChina NormalUniversity,GuangdongProvince,China.

E-mailaddresses:chenzhihong1227@sina.com(Z.Chen),wangxin@scnu.edu.cn (X.Wang).

tal pollutant. Nowadays, the major treatment processes for industrial anilinewastewaterare basedonbio-degradationand physical-adsorptionmethods[3].Buttheanilinebio-degradation efficiency is quite low because of its high biological-toxicity.

Physical-absorptionmethodiscostlyand unsuitablefordegrad- inglow-concentrationindustrialanilinewastewater.Therefore,it iscriticaltolookforalow-cost,high-efficientandenvironmental friendlymethodfortreatingindustrialanilinewastewater.

Inrecentyears,semiconductor-basedphotocatalyticdegrada- tionhasbeenregardedasalowcost,efficientandenvironmental friendlystrategytotreatwastewatercontaininganiline[4–8].TiO₂ havebeenwidelyusedasaphotocatalystsforthephotocatalytic degradationofanilineinanaqueoussolutionundervisiblelight irradiation.However,TiO₂canonlyrespondtoUVirradiation,so thereisaweakvisible-lightresponsewhenusingTiO₂-basedpho- tocatalystswhichstronglylimitsitspracticalapplicationsunder

http://dx.doi.org/10.1016/j.apsusc.2017.09.161 0169-4332/©2017PublishedbyElsevierB.V.

solarlight.Therefore,developingnovelhighvisible-light-response photocatalystswithhigh-efficiencyandstabilityhasbecomeahot topicinthefieldofthephotocatalyticdegradationofaniline.

Recently,apolymericsemiconductor photocatalyst,graphitic carbonnitride(CN),hasattractedagreatattentionbecauseofits relativenarrowbandgap,sufficientlynegativeconduction-band positionandhighchemicalstability[9].However,thephotocat- alytic activity of CNversus the degradation organic pollutants isquitelow because ofits relativelyfastcharge recombination andinsufficientabsorptioninthevisible-lightspectralrange.To improvethe photocatalytic performance ofCN, variousmodifi- cationmethods havebeenappliedthroughstructure regulation [10–19],doping[20–27],surfacehetero-junctionandcopolymer- ization[28–37].Amongthose methods,copolymerizationofCN has been regarded as an efficient route, not only for obtain- ing an enhance light-absorption, but also for creating surface hetero-structureswhichcouldreducetherecombinationofphoto- generatedelectron-holes.

In this study, barbituricaciddoped graphitic carbon nitride (CN-BA) photocatalysts were synthesized by copolymerizing dicyandiamide(DCDA)withbarbituricacid(BA)toimprovethe opticalabsorption,electronicandphotoelectricpropertiesofcar- bonnitride.Moreover,thecatalyticactivityofCN-BAversusthe degradationof anilinein aqueoussolutionwasinvestigated for thefirsttime.Allas-preparedCN-BAsamplesexhibitedahigher photocatalyticdegradationofanilineinaqueoussolutionrespect topureCNandtheCN-BA-2.0sample(seedetailsintheexperi- mentalsection)resultedinthehighestphotocatalyticactivity.The concentrationofanilineinaqueoussolutiondegradedfrom16to 1.354mg/LinthepresenceofCN-BA-2.0undersimulatedsunlight irradiationfor2h.Thestructure,surfaceareaandopticalproper- tiesofCN-BAsampleswerecharacterizedbyXRD,SEM,TEM,XPS, DRSandEPRspectra.ThecombinedbenefitsoftheBAdopingin termsofoptical,surfaceandtexturepropertiesleadtoasignificant improvementinthephotocatalyticactivityfordegradationofani- lineinaqueoussolutionundersimulatedsunlightirradiation.The mechanismleadingtothephotocatalyticdegradationofanilinewas alsoinvestigatedsystemically.

2. Experimatalmethods 2.1. Chemicals

Allchemicalreagentswereofanalyticalgrade.Dicyandiamide and barbituric acid (BA) were purchased from Sigma-Aldrich Química.Deionizedwaterwasusedthroughoutinphotocatalytic experiments.

2.2. SynthesisofCNandCN-BA

ForthesynthesisofCNsamples,2.0gofdicyandiamidewere sealedinaquartzboatandheatedinairto550^◦Cfor4hwitha heating-rateof2.3^◦C/min[38].

TheBAdoped CNsampleswere synthesizedfrom dicyandi- amideandBAbyathermalpolymerizationprocess.Inatypical process,2.0gofdicyandiamideand differentqualityofBAwere dispersedin 20mLof deionizedwater. Water wassuccessively removedbyheatingat100^◦Cunderstirring.Thedriedsolidwas grinded,transferredintoaquartzboatand becalcined inairat 550^◦Cfor4hattheheating-rateof2.3^◦C/min.Resultingsamples weredenotedasCN-BA-X,whereX(0.2,0.5,1.0,2.0,3.0)represents themassfractionofBA.

2.3. Characterization

Crystallographic phases of powder were analyzed by X-ray diffraction(XRD)fromX’PertPRO.Fouriertransformedinfrared (FTIR)spectrawereacquiredwithaNicoletMagna-IR170spec- trometer.Thesolid-state[13]CNMRspectrawererecordedusing a Bruker Advance III 500 spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were performed on Thermo ESCALAB250Xiinstrument witha monochromatized AlKaline source(150W).Electronparamagneticresonance(EPR)measure- mentswereperformedonaBrukerER200-SRCspectrometer.The UV–vis diffuse reflectance spectra(UV–vis DRS)were recorded onaU-41000,HITACHIspectrophotometer,Tokyo,Japanbyusing BaSO₄ asareflecting sample.Surfacemorphologyandstructure wereexaminedbyZEISS Ultra55 field-emissionscanningelec- tronmicroscope(FESEM).Transmissionelectronmicroscopy(TEM) imageswerecollectedbyusingaJEM-2100HRfieldemissionelec- tronmicroscope.TheBrunauer–Emmett–Teller(BET)surfacearea wasdeterminedwithaQuantachromeInstrumentsQuadrasorbSI.

2.4. Photocatalytictestfordegradationofaniline

Theconcentrationofanilineintheaqueoussolutionwasmea- suredusinga colorimetricmethod.In atypicalexperiment,the photocatalyticdegradationofanilinewasmeasuredusinga300W Xelamp(AM1.5,outputlightcurrentis15A)assimulatedsun- light.100mg oftheas-preparedphotocatalyst wereaddedinto 150mLofanaqueoussolutioncontainingananilineconcentra- tionof16mg/L.Thesuspensionwasstirredfor30minwithlight irradiationtoreachtheabsorption-desorptionequilibrium.Then simulatedsunlightirradiationwasactivatedbyshiningthesus- pensionwitha300WXelamp(outputlightintensityis1sun,AM 1.5,100mW/cm²).Attimeintervalsof30min,thesolutionwas sampledandcentrifugedtoremovethecatalysts.Theconcentra- tionofanilinewithinthecentrifugedsolutionwasdeterminedby acolorimetricmethod.Thedetectionlimitmethodwasestimated tobe1.6mg/L.

2.5. Thecolorimetricmethod

TheKHSO4(50g/L),NaNO2(50g/L),NH3SO3NH2(25g/L)andN- (1-Naphthyl)ethylenediaminedihydrochloride(20g/L)solutions werepreparedin 50mLconicalflasks, respectively.1mLofthe photodegraded(centrifuged)solutionand9mLofdeionizedwater wereintroducedintothe25mLtesttubewithaglassstopper,then 1mLofKHSO₄ solutionwasaddedtothesolution.Afterthat,a dropofNaNO2solutionwasintroducedintothesolution.Finally theNH₃SO₃NH₂solutionwasusedtowipeouttheNaNO₂excessby addingintotheabovemixtureuntilauniformsolutionwasformed.

Afterbubbles wereentirelywiped out, 1mLof N-(1-Naphthyl) ethylenediaminedihydrochloridechromogenicagentwasadded andthesolutionwasdilutedto25mLwithdeionizedwater.The absorbanceoftheanilinesolutionwasthendetectedby1cmcell at545nmonaUV–visspectroscopyafter30min.

2.6. Photo-electrochemicalexperiments

Topreparetheworkingelectrodes,300mgofthesampleand 20mg2-Naphthol

weregrindedanddispersedinethanoltoformaslurry.Then theslurrywascoatedonFTOconductiveglasssheet.Afterdrying at120^◦Cfor2hinvacuum,theworkingelectrodeswereobtained.

Photocurrentmeasurementswerecarriedout ina conventional three-electrodeelectrochemicalworkstation(CHI660E).Ptsheet andAg/AgClelectrodewereusedasthecounterandreferenceelec- trodes,respectively.A0.5MNa2SO4 aqueoussolutionservedas

Fig.1.XRD(A)patternsofthesamplesofCNandCN-BA-X,andFTIRspectra(B)ofthesamplesofCNandCN-BA-2.0.

theelectrolyte.Theirradiatedareaoftheworkingelectrodewas 1cm².Theintensityoftheincidentlightwas100mW/cm² (AM 1.5G,300-Wxenonlamp).The320speriodon-and-offphotocur- rentresponsesofCNandCN-BA-2.0sampleswereobtainedat0V biasvsAg/AgCl.

3. Resultsanddiscussion 3.1. XRDandFTIRcharacterization

XRDanalysiswasappliedtocheckthecrystalphaseofCNand CN-BAsamples.AsshowninFig.1A,XRDpatternsofpureCNand BA-dopedCNsamplesshowdistinctdiffractionpeaksat13.0^◦and 27.3^◦,suggestingthattheBAdopingdoesnotchangetheCNphase structure.Thediffractionpeakat13.0^◦wasindexedtothe(100) plane,correspondingtotheinterlayerstructuralofCNsheets.The diffractionpeakat27.3^◦wasindexedto(002)plane,thegraphite- likematerial.

FTIRspectra(Fig.1B)wererecordedforCNandCN-BA-2.0sam- ples.Thesimilarstretchpeakswereobserved,suggestingthatthe BAdopingdoesnotalter thecorechemical skeleton integrality ofCN.TypicalstretchmodesofaromaticCNheterocyclesinthe regionof1200–1600cm⁻¹ andbreathingmodeoftheheptazine units(C6N7)at810cm⁻¹wereobservedinbothCNandCN-BA-2.0 samples, confirming the presence of a triazine phase. In addi- tion,wecouldobservethatbroadbandsappear intherangeof 2900–3700cm⁻¹duetotheN HvibrationandH2Omolecules.

3.2. XPScharacterization

XPS measurementswere conducted to get insights intothe chemicalcompositionandchemicalstateoftheelementsinthe BA-dopedCNsamples.AsshowninFig.2A,thesurveyspectraof CN-BA-2.0andCNindicatethepresenceofC,NandOelements withoutothercontaminants, whichconfirmsthattheelemental compositioncorrespondstoCN-BA-2.0andCNsamples.Fig.2B showsthehigh-resolutionC1sspectraofCN-BA-2.0andCNsam- ples.TwopeakswereobservedfortheCNsampleat285.1eVand 288.6eV,correspondingtoC N CandC (N)3,respectively.Com- paredtotheCNsample,thetwopeaksforCN-BA-2.0wereslightly shiftedandlocatedat285.2eVand288.5eV,respectively[39].This slightshiftinthepeaksofCN-BA-2.0isprobablyduetointroduction ofBAintotheframeworkofCN,whichchangesitsbindingenergy.

Fig.2Cpresentsthehigh-resolutionN1sspectraofCN-BA-2.0and CNsamples.Fourpeaksof theCNsamplecan bedistinguished at398.9eV,399.8eV,401.1eVand404.6eV.Thestrongerpeakat 398.9eVwasassignedtotheringnitrogeninthesp2C N Cbonds.

Theweakpeakat399.8eVindicatedthepresenceoftertiarynitro-

genN(C)₃groups.Inaddition,thepeakat401.1eVwasattributed tothepresenceofaminogroups(CNH)andthepeakat404.6eV wasassignedtothechargeeffectsorthepositivechargelocaliza- tioninheterocycles[40].Additionally,thepeaksforC N Cbonds andN(C)₃groupsofCN-BA-2.0samplewerealsoslightlyshifted, likelyduetotheformationoftri-s-triazinebasedcovalentnetworks throughBAdopedCN[41].Fig.2Dpresentsthehigh-resolution O1sXPSspectraofCN-BA-2.0andCNsamples.Asinglepeakwas observedat532.2eV,whichisinagreementwiththeCN-BA-2.0and CNcomposition.ThispeakwasduetotheH₂Ogroupsadsorbedon theCN-BA-2.0surface.AgradualincreaseintheC/Nmolarratio from0.805forpristineCNto0.814forCN-BA-2.0revealsthesuc- cessfulintegrationofBAringsintotheCNnetwork.

3.3. Solid-state¹³CNMRcharacterization

Solid-state¹³CNMRspectroscopywasusedtofurthercharacter- izetheBA-dopedCN,asshowninFig.3.Thespectrumclearlyshows theappearanceofanewbroadpeakat97.9ppmfortheCN-BA-2.0 sample,whichindicatesthatBAwasdopedintheCN-conjugated networkbuttheheptazinc-basedstructureofCNdidnotchange [42,43].

3.4. EPRcharacterization

EPR measurements were performedto investigate the elec- tronicbandstructureofCNandBA-dopedCNsamples.Samplesin darkorvisiblelightirradiationshowthepresenceofonlyonesin- gleLorentzianlinecenteredat3146.3Gwithag-valueof2.0034 (Fig.4).Thissuggeststhepresenceofwell-definedsemiconductor properties.TheEPRsignalintensityofCN-BA-2.0is1354,whichis about6timeshigherthaninCN(212).Itwaspreviouslydemon- stratedthattheefficientgenerationofphotochemicalradicalpairs ishighlybeneficialinphotocatalyticdegradationreactions.Inter- estingly,anenhancedEPRsignalwasobservedwhenCNandCN-BA wereirradiatedwithvisiblelight, indicatingan efficientphoto- chemicalgenerationofradicalpairswithinthesemiconductors.

3.5. N2adsorption–desorptionisothermscharacterization

N₂adsorption-desorptionmeasurementswerecarriedtoinves- tigatetextualpropertiesofCNandCN-BA-2.0samples.Asshowin Fig.5,samplesexhibitasimilarIVbehaviorwithaH1typehys- teresisloop.Inadditional,theinsetindicatesthattheBETsurface areaofCN-BA-2.0(16.251m²/g)ishigherthaninCN(10.469m²/g).

However,theporesizeofCN-BA-2.0(4.450nm)wascomparable withtheundopedCN(4.458nm).ThissuggeststhattheBETsur- faceareaandtheporesizeofpureCNwereamplifiedanddecreased,

Fig.2.High-resolutionXPSspectraoftheaspreparedCN-BA-2.0andCNsamples:Thesurveyspectra(A),C1s(B),N1s(C)andO1s(D)spectraoftheCN-BA-2.0andCN samples.

Fig.3. Solid-state13CNMRspectraforCNandCN-BA-2.0samples.

respectivelyupondoping.Thesestructuralmodificationsremark- ablyimprovedthephotocatalyticactivityrespecttopureCN.

3.6. UV–visDRScharacterization

UV–visDRSspectraofCNandCN-BA-Xsamplesareshownin Fig.6.Interestingly,CN-BAsamplesshowahigherlight-absorption inthevisiblerangerespecttopureCN.Thiscorrespondstoachange ofthepowdercolorfrombrightyellowtodeeporange(seeinset inFig.6),suggestingthatBAdopinginducedmodificationscould enhancethelight-absorptioninvisiblelightregion.

Fig.4.EPRspectrainthedarkandundervisiblelight(␭>420nm)forCNandCN- BA-2.0samples.

3.7. SEMandTEMcharacterization

TEMandSEManalyseswereappliedtostudythemorphology andthemicrostructureofCNandCN-BA-2.0samples.Asshown inFig.7A,theSEMimageofCNclearlyshowdenseclumpsand stackedmassiveparticles,whiletheimageofCN-BA-2.0(Fig.7B) showsthatthesampleisconstitutedofalooserlamellarstructure, resultingintheenlargedsurfacearea.Moreover,itcanbeclearly seenfromTEMimagesofCN(Fig.7C)andCN-BA-2.0(Fig.7D)that densestacksinCNskeletontransformintomuchthinnersheets structuresafterBAdoping.

Fig.5.N2adsorption-desorptionisothermsofCNandCN-BA-2.0,theinsetshows thesurfaceandporesizeofCNandCN-BA-2.0samples.

Fig.6.UV–visDRSspectraofthesamplesofCNandCN-BA-X,theinsetistheoptical pictureofas-preparedCN-BA-Xsamples.

3.8. PhotocatalyticactivityandstabilityofCNandCNdopedofBA ThephotocatalyticpropertiesofCNandCN-BAsampleswere evaluatedbystudyingthedegradationofanilineinaqueoussolu- tion under simulated sunlight irradiation. Experimental results arereported inFig.8.Asshown in Fig.8A,BA-dopedCN sam- plesshowedahigherphotocatalytic activityrespecttoCN;this may be resulted from the enhanced surface area and optical absorbance and the reduction of the recombination of photo- generatedelectron-holepairs.Meanwhile,theCN-BA-2.0shows thehighestphotocatalyticactivityforthedegradationofaniline.

AsthemassfractionofBAwasincreasedover2.0,adecreaseinthe photocatalyticactivityofCN-BAwasobserved.Theprobablereason forthisisthatatoohighBAdopingcouldleadtohigherrecombi- nationofphoto-generatedelectron-holepairs,whichcancelsthe beneficialeffectduetothehighersurfaceareaandwideroptical absorbance.Fig.8Bshowstemporalabsorbancespectraandoptical chromogenicimagesofaqueousanilinewhichgiveadirectestima- tionoftheanilineconcentrationwithinthesolution.Thetypical absorptionpeak ofthechromogenicanilineaqueoussolutionat 545nmgraduallydecreaseswiththesimulatedsunlightirradiation time;thisisassociatedwithagradualcolorchangeofthesolution fromdarkpurpletotransparent.

Tofurtherinvestigatethephotocatalyticreactionofdegrada- tionofaniline,thereactionkineticsofdegradationofanilinewas conducted,andthekineticequationofanilinedegradationshows

afirst-orderkineticmodel(Eq.(1))accordingtothekineticlinear curves.

R=dC dt = kKC

1+KC (1)

WhereRisthedegradationrateofthereactant(mg•L⁻¹•min⁻¹), Candtrepresenttheconcentrationofthereactant(mg•L⁻¹)and theirradiationtime,respectively.Meanwhile,kandKrepresentthe reactionrateconstant(mg•L⁻¹•min⁻¹)andtheadsorptioncoef- ficientof reactant(L•mg⁻¹).WhiletheEq.(1) waschanged the first-orderkineticmodelthroughsimplifiedwhentheinitialcon- centration(C₀)wasverylow.Specificasfollow:

lnC0

C =kKt=Kappt (2)

WhereKapp istheapparentfirst-orderrateconstant(min⁻¹).As showninFig.9Aand9B,theapparentfirst-orderrateconstantorder ofBAdopedCNsampleswasCN-BA-2.0>CN-BA-3.0>CN-BA-1.0>

CN-BA-0.5>CN-BA-0.2>CN.ItwasfoundthatCN-BA-2.0showsthe highestapparentfirst-orderrateconstant(0.02189min⁻¹),which isabout6timeshigherthanthatofpristineCN(0.00293min⁻¹).

Photo-electrochemicalexperimentswereperformedtoinvesti- gatetheabilityoftheas-preparedsamplesforphotoinducedcharge carriersgenerationandseparation. ItcanbeseenfromFig.10A thatpureCNexhibitslowerphotocurrentintensitythanCN-BA- 2.0sample,indicatingthattherewasefficientphotoinducedcharge generation after BAwas doped into pureCN.Additionally, the higherphotocurrentintensityindicatesbetterseparationefficiency ofelectronsandholes[44].Therefore,theresultclearlyrevealed thatBAdopedinCNframeworkcansignificantlyenhancethesep- arationofphoto-generatedelectron-holepair.

Itisveryimportanttoevaluatethestabilityofphotocatalystsfor practicalapplications.Therefore,wecarriedoutrepeatedphotocat- alyticexperimentswithCN-BA-2.0foranilinedegradationunder thesameconditions.AsshownFig.10B,thephotocatalyticactiv- ityofCN-BA-2.0sampleisslightlydeactivatedafterfivesuccessive cyclesofphotodegradationofaniline,whichindicatesthatthepho- tocatalysthasgoodstabilityundersimulatedsunlightirradiation.

3.9. Freeradicalandholescavengingexperiments

Inthephotocatalyticoxygenprocess,aseriesofreactiveoxy- gen species, suchas ^•OH, ^•O₂⁻, h⁺ or H₂O₂ weresupposed to beinvolved.Todistinguishthereactiveoxy-radical,thetrapping experimentsandreactiveradicalqualitativedetectionwerecon- ducted. In the trapping experiment, isopropanol (IPA), N₂ and ammoniumoxalate(AO)wereservedasthehydroxylradical(^•OH) scavenger,superoxideradical(^•O₂⁻)scavengerandhole(h⁺)scav- enger,respectively[45].Itcanbeseen(Fig.11A)thattheaddition ofIPA,N2andAOintheanilinesolutionhasstrongadverseeffect onthephotocatalyticactivityofCN-BAsamples,suggestingthat

•OH, ^•O₂⁻ andh⁺ maybethemain oxygenspeciesinthepho- tocatalyticoxygenprocess.Thestrongesthinderingeffectonthe photocatalyticactivityinthoseoxygenspeciesis^•O2−>^•OH>h⁺, suggestingthatthe^•OHwasgeneratedfromtheindependentreac- tionof^•O₂⁻ orh⁺.Forfurtherunderstandingthephotocatalytic oxygenprocess,theradicalspeciesweredetectedbytheEPRtest, asshowninFig.11B−11D.FromFig.11B,theresultsrevealed thattheintensityof^•O₂⁻ signalsoftheCNandCN-BA-2.0sam- ples increasedundervisiblelight irradiation,whichis indicates that theCNand CN-BA-2.0samples generatethe ^•O₂⁻ radicals undervisiblelightirradiation.FromFig.11C,itcanbeclearlyfound thatthe^•OHsignalscanonlybeobservedintheCN-BA-2.0test- ingsystemsundervisiblelightirradiation.Inadditional,thesinglet oxygenradical(¹O₂)signals(Fig.11D)wasfoundinbothCNand CN-BA-2.0testingsystems undervisiblelightirradiation,which

suggeststhatthe^•O₂⁻radicalswouldtranslatedto[1]O₂andH₂O₂ (2^•O2−+2H⁺→¹O2+H2O2).Basedonthetrappingandradicalqual- itativeexperiments,sometranslatedto^•OHandsomeofthe^•O₂⁻ translatedto[1]O₂+H₂O_2,while theh⁺ maytransferto^•OH or

oxidetheanilinedirectly.Therefore,accordingtotheabovediscus- sion,theproposedphotocatalyticmechanismofCN-BAwasgiven andshown inFig.12.Under visiblelightirradiation,theCN-BA wasexcitedandcouldgenerateelectron-hole pairs.Thephoto-

Fig.7.TheSEMandTEMimagesoftheCNsample(AandC)andtheCN-BA-2.0sample(BandD).

Fig.8.ThephotocatalyticactivitiesofthesamplesofCNandCN-BA-X(A)andtheabsorbancespectrumchangesofanilineinthepresenceofCN-BA-2.0(B),theinsetisthe colorchangeofanilineinthepresenceofCN-BA-2.0.

Fig.9.Kineticstudiesofanilinedegradation(A)andthebargraph(B)oftheapparentrateconstantsofCNandCN-BA-X.

Fig.10.TransientphotocurrentresponseofCN-BA-2.0andCNsamples(A);therepeatedphotocatalyticexperimentsofCN-BA-2.0foranilinedegradationundersimulated sunlightirradiation(B).

Fig.11.Thetrappingexperimentresults(A)ofCN-BA-2.0andtheEPRspectraofthe^•O2−radical(B),the^•OHradical(C),andthesingletoxygenradical(¹O2)(D)ofCNand CN-BA-2.0samples.

generatedelectronswouldreactwiththemolecularoxygentoform

•O2−andthensomeofthe^•O2−translateto^•OH,whichfurther oxideaniline.Thephoto-generatedholecanfurthertransferto^•OH oroxideanilinedirectly.

4. Conclusion

TheBA-dopedCNnanostructurephotocatalystsweresuccess- fullysynthesizedbycopolymerizingdicyandiamidewithbarbituric acid (BA). The BA doped CN samples exhibited the improved morphologyof nanostructure, suppressedrecombination ofthe photo-generatedelectron-holepairsandhighsunlightabsorption.

ThesepropertiesmaketheBA-dopedCNsamplesasahigh-efficient photocatalyticmaterialtodegradeaniline.Theresultofdegrada- tionanilineindicatedthatthebestsample(CN-BA-2.0)shownthe dyedanilinesolutionchangedgraduallyfromdarkpurpletowhite

after2hundersimulatedsunlight (Xelamp)irradiation.Mean- while,thereactionrateconstantofthebestsample(CN-BA-2.0) increased6-foldforphoto-degradationofanilinecomparedtothe pureCNsample.Inphotocatalyticoxidationreactionprocess,the

•O₂⁻/^•OOHradicalplaysakeyroleandisfavorableforthepho- tocatalyticdegradationofaniline.It isensurethatweprovidea simpleandpracticalroutetoimprovethephotocatalyticactivityof pureCNthroughCNdopedBA.

AuthorContributions

L.L., ZH.C. and X.W. designed the experiments. L.L., Z.C., performedthefabricationexperimentsandelectrochemicalmea- surements. ZH.C. and X.W. supervised the project. L.L., QG.M., HQ.L., LL.S., YG.Z., Z.Z.,ZH.C., MZ.Y., R.N.,JM.L., GF.Z., and X.W.

discussedtheresults,performeddataanalysisandcontributedto

Fig.12.ThemechanismoftheBA-dopedCNsamplesphotodegradedaniline.

manuscriptwriting.Alltheauthorsreviewedandcommentedon themanuscript.

Acknowledgment

The authors acknowledge the financial support from the NSFC (Grant No. 51602111), the National Key R&D Pro- gramof China(2016YFB0401502), Guangdong ProvincialGrant (2015A030310196,2014B090915005),GuangdongProvincialSci- enceandTechnologyProject(2017A050506009),thePearlRiver S&T Nova Program of Guangzhou (201506040045), the Hun- dred Talent Programof Chinese Academy Sciences (QG Meng), Guangzhou Post-doctoral Initial Funding and the National 111 Project (ZH Chen), Nansha Science and Technology Project (2016GJ005).

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