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ContentslistsavailableatScienceDirectAppliedCatalysisB:Environmentaljournalhomepage:www.
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com/locate/apcatbBondingCdS-Sn2S3eutecticclustersongraphenenanosheetswithunusuallyphotoreaction-drivenstructuralrecongurationeectforexcellentH2evolutionandCr(VI)reductionChaoXue,XiaoqingYan,HuaAn,HeLi,JinjiaWei,GuidongYangXJTU-OxfordInternationalJointLaboratoryforCatalysis,SchoolofChemicalEngineeringandTechnology,Xi'anJiaotongUniversity,Xi'an,710049,ChinaARTICLEINFOKeywords:GrapheneCdS-Sn2S3RecongurationActivesitesPhotocatalysisABSTRACTInthiswork,anovelCdS-Sn2S3@reducedgrapheneoxide(rGO)eutecticclusterheterostructuresweresynthe-sizedviaafacileone-pothydrothermalmethod,whichcanmakeCdS-Sn2S3uniformlydispersedontotherGOnanosheetssurfaceandthusformaspeciallyactivesitessustained-releasetablets.
Theoptimizedsampledis-playedcontinuouslyincreasedvisible-light-drivenphotocatalyticactivityandstrongdurabilitybothinphoto-catalytichydrogenevolutionandhexavalentchromium(Cr(VI))reduction.
Withtheseresults,weforthersttimendthepeculiarlystructuralrecongurationeectproducedintheCdS-Sn2S3@rGOcomposite,andthereaction-drivenreconguringofgraphene-basedCdS-Sn2S3eutecticscanactassustained-releasesystemofac-tivesites,whichtriggersincomparablemerits,suchasmoreactivesites,shortercharge-transferdistanceandtheeectiveinterfacialcontact.
Inaddition,therGOnanosheetsservedaselectronshuttletoecientlyacceleratetheseparationandtransferofphotogeneratedelectron-holepairs.
Thesecombinedeectsendowedthehybridssystemwiththeever-increasinghydrogen-generatingeciencyandenhancedstabilityinreductionreaction.
1.
IntroductionInpastdecades,a"bottom-up"approachisfrequentlyemployedtoprovideinsightsintosurfacedynamics,adsorption-desorptionprocessesandthepossiblereactionpathwaysduringtheheterogeneouscatalyticprocesseswithaxedsolidcatalystsurfacestructure[1–4].
However,inmanycases,thegeometricstructureandcompositionofsurfacesareeasilytobealteredduringtherealisticcatalyticreaction,whichresultsintheconventionalanalysismodelsareusuallysimplied.
Recentstudiesdemonstratethatstructuralreconstructionsofnanostructuredmaterialsnotonlytailorthesizeandmorphologyofthecatalyticpar-ticles,butalsoreconstructthesurfaceeventsorevensubsurfaceregions[3].
Thisrecongurationintheirspatialarchitecturecantriggersomeofadvancedfunctionsofcatalysts.
Forexample,(i)Thesizeandmor-phologyofthenano-scaledmaterialscanchangedrasticallyinresponsetochangingreactionenvironment[5].
Especially,thelagerparticlescanbreakupintothesmalleronesaccompanyingwithinternalre-congurations,whichnotonlychangetheelectronicpropertiesofthesurface,butalsosignicantlyincreasethespecicsurfaceareaofcat-alysts,thusexposingalargenumberofsurfaceactivesites.
(ii)Thesurfacesreconstructionswillaltersubsurfaceofmaterialstoreachthethermodynamicequilibriumunderreactionconditions[4].
Meanwhile,somecompositionoftheconstituentelementsmaymigratefrombulkordeeplayersofsubsurfaceregionstothetopsurfacelayerandcon-tinuouslycreatenewactivesitesforparticipatinginvarioushetero-geneouscatalyticreactions[6].
Giventhefavorableattributes,thestructuralrecongurationofnano-architecturesisaversatileimplementationstrategytodevelopvisible-light-drivenphotocatalystsforsolar-inducedphotoelectricorphotochemicalconversionofhydrogen,whichisalsoconsideredtobeapromisingrouteinresponsetochallengingenergycrisesandenviron-mentalissues[7–11].
Therefore,tremendouseortshavebeendevotedtopreparehighly-ecientphotocatalyticsystemsanduncoverthein-volvedreactionmechanism.
However,asfarasweknow,almostalltheexistingresearchfocusesontheimprovementofthephotocatalyticperformancebybandgapengineering[12],surfaceandinterfacedesign[13],dyesensitization[14],orexploringnovelsemiconductorswithnarrowbandgap[15].
Oncethesearchitectedsemiconductorsweresynthesized,theystillpossessaxedgeometricmorphologythatcannotbereconguredleadingtoalimitedpracticabilityinphotocatalysis.
Pioneeringattemptshavedemonstratedthattwodimensional(2D)grapheneisanattractivehostmatriceswhichcanprovidesanidealplatformforthedesignofrecongurablesystems,owingtoitsstruc-turalexibility,largespecicsurfaceareaandoutstandingconductivityhttp://dx.
doi.
org/10.
1016/j.
apcatb.
2017.
10.
008Received16June2017;Receivedinrevisedform18September2017;Accepted6October2017Correspondingauthor.
E-mailaddress:guidongyang@xjtu.
edu.
cn(G.
Yang).
AppliedCatalysisB:Environmental222(2018)157–166Availableonline07October20170926-3373/2017ElsevierB.
V.
Allrightsreserved.
MARKandelectronmobility[16–18].
Inspiredbythemeritsofrecongur-ability,inthiswork,weforthersttimefabricatedanovelCdS-Sn2S3@rGOeutecticclusterheterostructuresviathefacilehydrothermalmethod.
Comparedtothetraditionalgraphene-basedhybrids,thisin-geniousCdS-Sn2S3@rGOheterostructurescouldin-situreconguretheirspatialarchitectureandinthiswaytosignicantlyimprovethevisiblelightphotocatalyticactivitiesanddurabilityforH2evolutionandCr(VI)reduction.
Furthermore,ourndingsprovideanovelinsightintotheinterplayofstructureandphotocatalyticpropertiesofCdS-Sn2S3@rGOphotocatalyticsystem.
Webelievethatthesearchitecturestrategiesprovidethepossibilitytoresolvethecurrentbottlenecksaboutactivityanddurabilityreportedinpreviousgraphene-basedmetalsuldephotocatalysts.
2.
Experimentalsection2.
1.
MaterialsGlucose(C6H12O6),cadmiumchloridehemi(pentahydrate)(CdCl2·2.
5H2O),tintetrachloridepentahydrate(SnCl4·5H2O),L-cysteine(C3H7NO2S)werepurchasedfromSinopharmChemicalReagentCo.
,Ltd.
Allthechemicalreagentswereanalyticalgradeandusedas-re-ceived.
2.
2.
Theexfoliationofgrapheneoxide(GO)TheGOwassynthesizedbythemodiedHummers'method.
Inordertoobtainthehighest-qualitymonolayergraphenenanosheets,theas-preparedGOwerefurthertreatedbytheultrasonic-assistedex-foliationprocedure.
Inbrief,60mgofGOwasaddedto20mLofdeionizedwater.
Themixturewastransferredintoanincubatorat0°Candtreatedbysonicationforabout6h,formingadarkbrownsus-pensionwithaconcentrationof3mgmL1.
2.
3.
SynthesisofhybridCdS-Sn2S3@rGOeutecticclusterheterostructureThehybridCdS-Sn2S3@rGOeutecticclusterheterostructuresweresynthesizedusingafacialone-pothydrothermalmethod.
Inatypicalprocedure,4mLoftheas-obtainedGO(3mgmL1)suspensionwasdispersedin30mLofdeionizedwaterundertheultrasonicationfor5min.
Then0.
2gofglucose,0.
2307gofCdCl2·2.
5H2Oand0.
1052gSnCl4·5H2Owereaddedintotheaforementionedmixtureundervig-orousstirringfor4h,followedbytheadditionof0.
59gofL-cysteine.
Afterbeingstirredforextra1h,thehomogeneoussuspensionwastransferredintoa100mLTeon-linedstainlessautoclaveandmain-tainedat180°Cfor2hbeforecoolingdowntoroomtemperature.
Thenalproductswerecollectedbycentrifugation,washedwithdistilledwaterandabsoluteethanolseveraltimes,andvacuum-driedfor12hat60°Cforfurthercharacterization.
Aseriesofgraphene-basedmetalsulphidesheterostructurewiththedierentmolarratioofCdtoSnwerefabricatedviatheaboveprocedurebycontrollingtheconcentra-tionofCdCl2·2.
5H2OandSnCl4·5H2O.
Forcomparison,thepristineCdSandSn2S3weresynthesizedunderthesameconditionswithoutaddingGO,respectively.
2.
4.
CharacterizationThecrystalstructureofsampleswasinvestigatedusingX-raydif-fraction(XRD;SHIMADZU,LabXXRD-6100)withCu-Karadiationatascanrateof10°min1.
X-rayphotoelectronspectra(XPS)werecarriedoutonAXISULtrabldapparatustoexaminetheelementalcompositionsandchemicalvalencestateoftheas-preparedsamplesinthenear-surfacerange.
Themorphologiesandnanostructuresofallthesampleswereaccomplishedbyeld-emissionscanningelectronmicroscopy(FE-SEM;FEIQuantaF250,200kV)equippedenergydispersiveX-rayspectroscopy(EDX)andelectronicenergylossspectroscopy(EELS),aswellastransmissionelectronmicroscopy(TEM;JEOL,JEM-2100).
Thetime-resolvedphotoluminescencespectrawererecordedonaPTIQuantaMaster400uorescencespectrometer(HORIBAScientic)withanexcitationwavelengthof402nm.
Athermogravimetricanalysis(TGA;BeijingHenvenScienticInstrumentFactory;HTC-3)wasper-formedtoquantitativelyinvestigatethethermalpropertiesoftheas-preparedsamples.
Andthesampleswereheatedfromambienttem-peratureto800°Cataheatingrateof10°Cmin1underaN2atmo-sphere.
TheconcentrationofCd2+insolutionwasmeasuredbyaninductivelycoupledplasmaatomicemissionspectroscopy(ICP;SHIMADZUCorp.
,Japan;ICPE-9000).
A75WXenonlampisusedastheexcitationsourceforsteadystateexperiments.
Thephotoluminescence(PL)spectraoftheas-synthesizedsamplesinthepowderformwereinvestigatedatroomtemperatureonFLS980uorescencespectrometer(EdinburghInstruments)withanexcitationwavelengthof325nm.
2.
5.
PhotoelectrochemicalperformancemeasurementThephotoelectrochemicalmeasurementswererecordedonanelectrochemicalworkstation(CHI760DShanghaiChenhua)inathree-electrodesystem.
Theirradiationlightsourcewasa300WXenonlamp(Nbet,HSX-F/UV300)equippedwithanultravioletcut-oglasslter(λ>420nm).
AnAg/AgClelectrodeandaplatinumwirewerere-spectivelyusedasthereferenceelectrode,thecounterelectrode.
Aglassycarbonelectrodetip(eectivearea:0.
076cm2)modiedwiththepreparedsampleswereservedastheworkingelectrode.
Allafore-mentionedelectrodeswereworkingina0.
5MNa2SO4aqueoussolu-tionastheelectrolyte.
2.
6.
PhotocatalyticactivitymeasurementThephotocatalytictestsofthepreparedcatalystwereperformedbythereductiontheofK2Cr2O7aqueoussolution(100mg/L)underthevisiblelightirradiationofa300WXenonlamp(Nbet,HSX-F/UV300).
Typically,0.
05gphotocatalystand0.
03gtartaricacidasholesacriceagentwereaddinginto50mL100mg·L1K2Cr2O7aqueoussolutionandthesuspensionwasmagneticallystirredinthedarkfor60mintoobtaintheadsorption-desorptionequilibriumbeforethevisiblelightillumination.
Duringthereactionprocess,3mLofthesuspensionwastakenoutandcentrifugatedtoisolatethephotocatalystatacertaintimeintervalof3min,thentheresidualconcentrationofhexavalentchromiumCr(VI)wasanalyzedonaUV1900PPCspectrophotometer.
ThedetectionwavelengthforthecharacteristicabsorptionpeakofCr(VI)is350nm.
Thecyclingexperimentswereexecutedwiththesimilarconditionsexceptthattheresidualsolutionwasanalyzedaftervisible-lightirradiationfor15min.
2.
7.
PhotocatalyticH2evolutionThephotocatalyticH2evolutionexperimentwasperformedinaPyrexreactioncellwitha300WXenonlamp(Nbet,HSX-F/UV300)asthevisiblelightsource.
Typically,0.
5gphotocatalystwasdispersedinmixedsolutioncontaining10mLoflacticacid,3wt.
%H2PtCl6and50mLofwaterwithconstantmagneticallystirring.
Priortovisibleil-lumination,thesystemwasdegassedwithnitrogenfor30mintoguaranteethatthereactionsystemwasunderanaerobicconditions.
TheamountofH2generatedwasanalyzedbygaschromatography(BFRL,SP-2100A,Beijing,China).
3.
ResultsanddiscussionAsiconicallyshowninScheme1,theternaryultraneCdS-Sn2S3@rGOeutecticclusterheterostructuresweresynthesizeddirectlybyfacileone-pothydrothermalprocess.
Assistedbythemechanicalexfoliation,GOnanosheetswithmultifunctionaloxygenicgroupsalmostuniformlydispersedinthemixedaqueoussolutionscontainingdierentC.
Xueetal.
AppliedCatalysisB:Environmental222(2018)157–166158concentrationsoftinions(Sn4+)andcadmiumions(Cd2+)precursors.
Meanwhile,plentifulpositivelychargedmetallicionscanbeeasilyxedbyanchoringsitesonthesurfaceofelectronegativeGOnanosheets,andsubsequentlyreactedwithS2releasedfromL-cysteine,duringthehydrothermalprocess.
Thenucleationandself-growthofCdSandSn2S3weresimultaneouslyoccurredalongwiththereductionofGO.
It'sworthnotingthattheintroductionofglucosenotonlyeectivelypre-venttheagglomerationofCdS-Sn2S3eutecticclustersbutalsoactasasuitablereductantforGOreduction.
Experimentaldetailsareeluci-datedintheexperimentalsection.
Inthiswork,themolarratiosofCdStococatalystSn2S3werexedat4:3and8:3,andtheresultedhybridnanostructureswerelabeledasCGS-1andCGS-2,respectively.
X-raydiraction(XRD)givesthedirectinformationaboutthecrystalphaseofas-synthesizedpureandhybridsystems.
AsshowninFig.
1a,theXRDpatternsofCGS-1andCGS-2togetherwiththoseofCdS/rGO(denotedasCG),Sn2S3/rGO(denotedasGS)aswellasthepristineCdSandSn2S3demonstratedthehighcrystallinityandallofthedistinctdiractionpeakscanbeindexedtothehexagonalCdS(JCPDS41-1049),andorthorhombicSn2S3(JCPDS14-0619),respectively.
Additionally,thestructurewasconrmedbyFouriertransforminfrared(FT-IR)spectroscopy,whichwouldbediscussedlater.
DirectevidencefortheimmobilizationofCdSandSn2S3nano-crystalsonrGOnanosheetscanbeobservedfromtheeldemissionscanningelectronmicroscopy(FESEM)andtransmissionelectronmi-croscopy(TEM).
AsindicatedwitharrowheadsinFig.
S1(Supportinginformation),theultrathinGOnanosheetsretainwrinkleshapesafterultrasonicexfoliation,whichcannotonlypreventGOnanosheetsfrombeingrestackedduringthehydrothermalprocess,butalsoprovideahighsurfaceareafornucleating[16,17].
SEMimagesinFigs.
S2,S3(Supportinginformation)andFig.
1clearlyshowthatthein-situgrowthofsingleorbinarymetalsuldesonrGOnanosheetscanecientlypreventtheaggregationofbothrGOnanosheetsandloadedmetalsuldes,comparingtothepristineSn2S3nanoparticlesandCdSmi-crospheres.
AsforCGS-2composites(Fig.
1bandc),it'sobviousthatnumerousisland-likeCdS-Sn2S3nanocrystalsarehomogeneouslydis-persedandtightlyattachedonrGOsurfacewithaparticlesizeof16.
7-27.
8nm.
ThisstructuralcharacteristicofeutecticclustersgivesrisetothepossibilityofrecongurationonrGOsurface.
TheedgesofrGOScheme1.
SchematicillustrationoftheintegrationofCdS-Sn2S3eutecticclusterswithGOnanosheets.
Fig.
1.
(a)XRDpattensofallsamples.
(b,c)SEMimages;(d,e)TEMimages;(f)SAEDpatternand(g)HRTEMimagesofCGS-2composites.
C.
Xueetal.
AppliedCatalysisB:Environmental222(2018)157–166159nanosheetsareclearinTEMimages(Fig.
1dande).
ItfurtherconrmedthatadensemassofuniformandultraneCdS-Sn2S3eutecticclustersuniformlydistributedonbothsidesoftherGOnanosheets,whichcouldsignicantlyaugmenttheexposureareaofactivesites[18].
These-lectedareaelectrondiraction(SAED)pattern(Fig.
1f)indicatesthepolycrystallinenatureoftheseeutecticclusters.
Additionally,thedis-tinctbrightspotsformingmultiplesetsofhexagonsdemonstratedarelativelyhighlyorderedgraphenestructureandaregularstackingoffew-layerrGOnanosheetsinhybridsystem[19,20].
Atypicalhigh-re-solutionTEM(HRTEM)imageinFig.
1gclearlyrevealthatCdS-Sn2S3eutecticclusterstightlyanchoredontorGOsurfacestoformthehet-erostructureswithwell-denedphaseinterfaces.
Thelatticefringeswithdspacingof0.
335and0.
265nmcanbeassignedtothe(111)and(221)planesofthecubicorthorhombicSn2S3,respectively.
Italsopresentswell-denedhexagonalCdSnanocrystalswithhighlyexposed(002)facets.
Remarkably,thebinarynanocrystalsareintertwinedto-getherandestablishedsucientandstableinterfacialconnectionswithrGOnanosheets,whichendowtheseuniqueultrathin2Dnanosheetshybridswithsuperiorityinshorteningdistanceofchargetransferdif-fusion,improvinginterfacialchargetransfer,suppressingchargere-combination,andexposingmoreactivesitesforphotoredoxreaction.
X-rayphotoelectronspectroscopy(XPS)spectra(Fig.
S4,Supportinginformation)andelementalmappingimages(Fig.
S5,Supportingin-formation)furtherevidencedtheco-existenceoftheC,Cd,S,andSnelementsintheobtainedCGS-2sample.
Thehigh-resolutionXPSspectrumoftheO1s(Fig.
S4c,SupportingInformation)impliesthepartialresidualofoxygen-involvedfunctionalgroupsandadditionaldefectsinsurfaceofrGO.
AsmanifestedinFig.
S5d–f(SupportingInformation),itisdiculttoidentifytheclearboundariesofCdSandSn2S3nanocrystals,attributingtotheirquitesmallersizedarchitecture.
Instead,wecanreasonablydrawaconclusionthattheCdS-Sn2S3eu-tecticclusterswerehomogenouslydistributedonthesurfaceoftherGOnanosheets,resultinginanintimatelyinterfacialcontact.
Fig.
2ashowstheRamanspectraofGO,CGS-1andCGS-2nano-composite.
Twodistinctpeakslocatedat1345cm1and1590cm1canbeclearlyseeninRamanspectrumofGO,whichcorrespondtothecharacteristicDandGbands,respectively[21,22].
Bycontrast,bothCGS-1andCGS-2displayedDbandat1333cm1andGbandat1577cm1,respectively.
Theslightbandsshifttowardshigherwave-lengthnumberindicatedasignicantelectrontransferbetweentheCdS-Sn2S3eutecticclustersandrGOnanosheets[23,24].
Inaddition,therelativeintensityratioofDtoGband(ID/IG)forCGS-2compositewas1.
23,whichwashigherthanthoseofCGS-1(1.
18)andGO(1.
03).
ThehigherintensityratiosindicatethereductionofGOandthehigherdefectdensityinCGS-2composite[25,26].
Notably,theexisteddefectscanprovidemoreactivesites,whichplayavitalroleforimprovingthephotocatalyticactivity[27].
Furthermore,signicantchangesalsopresentedinFT-IRspectra(Fig.
2b).
AscomparedtooriginalGO,CdS-Sn2S3@rGOheterostructuresexhibitedmuchlowerabsorptionintensityatthepeaksassignedtotheoxygen-containingfunctionalgroups,suggestingthesuccessfulreductionofGOtorGOviahydrothermalreductionprocess[20,26–28].
TheUV–visdiusereectionspectra(DRS)(Fig.
2c)reectthatallas-preparedsamplesexhibitedwideabsorptionintheultravioletandvisiblelightregion.
Obviously,thecombinationofgraphenehassig-nicantlyexpandedtheabsorptionrangeoftheternaryCdS-Sn2S3@rGOnanocomposites.
Especially,theCGS-2compositeshowedstrongervisiblelightabsorptionthanthoseofCG,CGS-1compositeandCdS.
ThisenhancedlightabsorptionpropertywillleadtomoreecientutilizationofthesolarenergyfortheCdS-Sn2S3@rGOcompositesystem.
AlthoughSn2S3exhibitedthesimilarstrongabsorptionin-tensityinvisiblelightregion,thephotocurrenttransientresponseofpureSn2S3wasnegligible(Fig.
2d),implyingtherapidrecombinationrateofphoto-excitedelectron-holepairs.
ItisnoticeablethattheinitialvalueofphotocurrentdensityforCGwasashighas58.
5μAcm2,whichwas21.
2timeshigherthanthatofsingleCdS.
ItindicatesthatthepresenceofrGOcanfavorthephoto-inducedelectronstransferfromCdStorGOnanosheets,thusimprovingthechargeseparatione-ciency.
However,anobviouslyphotocurrentdecaycanbeobservedinCGwiththeincreaseofperiodicilluminationtime,owingtotheFig.
2.
(a)RamanspectraofGO,CGS-1andCGS-2excitatedat633nm;(b)FT-IRspectra,(c)UV–visDRSspectraand(d)Transientphotocurrentresponsesofas-fab-ricatedsamples.
C.
Xueetal.
AppliedCatalysisB:Environmental222(2018)157–166160photocorrosionofCdS.
Incontrast,CdS-Sn2S3@rGOheterostructuresexhibitedbetterphotocurrentstabilitythanCG.
ThephotocurrentdensityforCGS-2andCGS-1werehighlyreproducibleforseveralon-ocyclesandremainedat26.
1μAcm2and10.
01μAcm2,respectively.
ThispersistentstabilityfurtherindicatesthattheintegrationoftheconstructedCdS-Sn2S3eutecticclustersheterojunctionandrGOwithsuperiorelectricalconductivityresultsinthelowerrecombinationrateandthehigherseparationeciencyofphoto-generatedelectron-holepairs.
AsignicantdecreaseofthePLintensitywasobservedinCGS-2composites(Fig.
S6,Supportinginformation),indicatingslowerre-combinationrateofphoto-inducedelectron-holepairs[21].
Thephotocatalyticpropertiesoftheas-synthesizedphotocatalystswerethoroughlyinvestigatedbyphotocatalytichydrogenproductionundervisible-lightillumination(λ≥420nm)using10vol.
%lacticacidaqueoussolutionasscavengerand3wt.
%Ptasaco-catalyst.
AsdepictedinFig.
3a,thepureSn2S3andGSshowedtheminimumpho-tocatalyticactivitybecauseoftherapidrecombinationofphoto-gen-eratedelectron-holepairsindirectbandgapsemiconductorSn2S3.
Undervisiblelightirradiationfor6h,theinitialH2evolutionrateofCGS-2compositewasreachedupto165.
7μmolh1g1,whichwas1.
1and1.
0timeshigherthanthatofCGS-1andCGcomposites,re-spectively.
ItissurprisingthattheyieldofH2overCGS-2compositeundergoesacontinuousincrease(Fig.
3b),andtheH2evolutionrateachievedthehighestvalueof1719.
2μmolh1g1afterninerecycles.
Evenaftercyclesrecycles,thenanohybridsdidnotshowsignicantlossofactivity,andtheH2evolutionrateretainsat1671.
0μmolh1g1.
ItindicatesthattheoptimalratioofSn2S3imposedasignicantinuenceonH2productionactivityoftheCdS-Sn2S3@rGOeutecticclusterhet-erostructures[29,30].
AndtheoptimizedCGS-2sampledisplayedgoodactivityandstabilityinconsecutivephotocatalyticH2evolutionpro-cess.
ItisamazingthattheresultantCGS-2compositesalsodisplayedabetterphotocatalyticactivityandexcellentstabilityforCr(VI)reduc-tion.
ThedetectionwavelengthforthecharacteristicabsorptionpeakofCr(VI)is350nm.
AsillustratedinFig.
4a,thereductionofCr(VI)hardlyoccurredintheabsenceofphotocatalystsorinthepresenceoftartaricacidundervisiblelightirradiationfor15min.
ItismoreconvincingthatCGS-2displayedthehighestphotocatalyticactivitywithareduc-tionrateof98.
5%.
WhiletheremovalecienciesofCr(VI)areonly90%,76.
5%and28.
9%overCGS-1,CG,andGS,respectively.
Notably,boththepristineCdSandSn2S3exhibitedasimilarlowreductionratioofCr(VI)(approximately50.
7%)underthesameconditions,duetotherapidrecombinationofphoto-generatedelectron-holepairs.
AscanbeseenfromFig.
S7,allthecharacteristicabsorptionpeakofCr(VI)overCGS-2appearat350nm,andthecharacteristicabsorptionpeakin-tensityofCr(VI)wasgraduallydecreasedwiththeincreaseofirradia-tiontime.
Undervisiblelightirradiationfor15min,thecharacteristicabsorptionpeaklocatedat350nmdisappeared,indicatingthattheheavymetalionshavebeencompletelyreducedduringthephoto-reductionprocess.
AsshowninFig.
4b,theaforesaidphotoreactionsfollowedapseudo-rstorderdynamicsmodel[31–34].
Signicantly,theresultantCGS-2possessedexcellentstabilityandrecyclabilityforphotocatalyticreductionofCr(VI).
TheremovalecienciesofCr(VI)remainat95.
8%eveninthetenthreusecycle(Fig.
4d).
ThisternaryCdS-Sn2S3@rGOeutecticclusterheterostructuresshowgoodpotentialinpracticalwastewaterpurication.
ItisbelievedthatthestructuralreorganizationofCdS-Sn2S3eutecticclustersoccurredongraphenenanosheetsplayavitalrolefortheever-increasingphotocatalyticH2evolutionoverCGS-2composite.
ComprehensiveanalysisofTEMandXRDconvincinglyveriedtheinterplaybetweenstructureandphotocatalyticactivities.
TEMimagesinFig.
5andFig.
S8(Supportinginformation)revealedthatdramaticchangeoccurredinthewholemorphologyofCGS-2compositesatdierentreactiontime.
ComparedwiththeoriginalCGS-2sample(Fig.
5a),theisland-likeCdS-Sn2S3eutecticclusterstendedtoself-dis-perseandconvertedintonumerousultranemetalsuldesnanocrystalsonrGOplatformunderthevisiblelightirradiation.
Particlesizesdis-tributionsdeterminedbyTEMpresentthatthenumberofthelargeparticlesrangingfrom10to30nmdecreasedgraduallywiththein-creaseofreactiontime.
Onthecontrary,thetotalnumberofsmallernanoparticles(concentratedon2–3nmindiameter)increasedsyn-chronously.
FurtherobservationrevealedthatthevisualizedCdS-Sn2S3eutecticclusterswerecompletelycollapsedafterphotocatalyticH2evolutionfor6h(Fig.
5m)ormore(60h,Fig.
5n).
Andnewlyformednanocrystalsweresotinythatitcannotbeidentiedautomaticallybythesoftware.
HRTEMimagesinFig.
S8bandd(Supportinginforma-tion)depictthatthedissolvedmetalsuldecrystallitesandin-siture-ducedPtnanoparticlesdenselyanchoredonthegraphenenanosheetsindicatinganirreversiblestructuralreconstructionofCdS-Sn2S3@rGOheterostructuresduringthephotocatalysisprocess.
TheresultofEDXspectrum(Fig.
S8e;Supportinginformation)furtherconrmedthatthemetallicPt0wassuccessfullyphoto-depositedonthesurfaceofCdS-Sn2S3@rGOnanocompositeduringthephoto-reductionprocess.
ThesurfaceelementalcompositionsandchemicalstatesofthefreshCGS-2sampleandtheusedCGS-2sampleobtainedafterphotocatalytichydrogenevolutionfor60hwereinvestigatedbyX-rayphotoelectronspectroscopy(XPS)analysis.
AsdemonstratedinFigs.
S4aandS9a(SupportingInformation),thewholeXPSsurveyspectrashowedthatthereareC,O,S,CdandSnelementscoexistinthetwosamples.
Asshowninboththehigh-resolutionC1sandO1sXPSspectraofthefreshCGS-2sampleandtheusedCGS-2sampleobtainedafterphotocatalytichydrogenevolutionfor60h(Fig.
S4b,candS9b,c),therearestillsomeoxygen-containingfunctionalgroupsintheCdS-Sn2S3@rGOhetero-structures,indicatingthattheGOnanosheetswasnotcompletelyre-ducedtotherGOnanosheets.
Additionally,thetwoasymmetricalpeakslocatedat161.
5and163.
7eVintheS2pspectracanbeascribedtotheS2p3/2andS2p1/2(Figs.
S4dandS9d;Supportinginformation).
AsshowninFig.
S4eandFig.
S9e(SupportingInformation),Cd3dlocatedataround405.
1and411.
9eVcanbeassignedtoCd3d5/2andCd3d3/2,respectively.
TheSn3dspectraofthefreshCGS-2sample(Fig.
S4f;Fig.
3.
(a)Time-resolvedH2evolutionovervariousphotocatalystsundervisiblelightirradiationand(b)CyclingtestofH2evo-lutionoverCGS-2composite(λ>420nm).
C.
Xueetal.
AppliedCatalysisB:Environmental222(2018)157–166161Supportinginformation)andtheusedCGS-2sampleobtainedafterphotocatalytichydrogenevolutionfor60h(Fig.
S9f;SupportingInformation)demonstrateasimilardoubletatbindingenergiesof486.
8and495.
2eV,correspondingtoSn3d5/2andSn3d3/2,respectively.
ComparedwiththefreshCGS-2sample,therearetwoextrapeakslo-catedat72.
2and75.
4eVinPt4fXPSspectrumoftheusedCGS-2sampleobtainedafterphotocatalytichydrogenevolutionfor60h(Fig.
S9f;SupportingInformation),whichcanbeassignedtoPt4f7/2andPt4f5/2,respectively,attributingtotheformationofmetallicPt0duringthephoto-reductionprocess.
Asisknowntoall,XPSisausefulsurface-sensitivetechnique,whichcanbringussomeimportantsurfacein-formationtriggeredbythestructuralreconguration.
MostnoteworthyisthatthecalculatedatomicconcentrationofSandSnelementsinfreshCGS-2sampleare1.
08%and0.
56%,respectively,whichareslightlylargerthanthoseoftheusedCGS-2compositesobtainedafterphoto-catalytichydrogenevolutionfor60h(thecalculatedatomiccon-centrationofSandSnelementsareonly0.
80%and0.
17%,respec-tively).
Conversely,therelativelyatomicconcentrationofCdelementinfreshCGS-2samplewas0.
26%,whilethecalculatedvalueofCdelementisasahighas0.
54%fortheusedCGS-2compositesobtainedafterphotocatalytichydrogenevolutionfor60h.
TheseresultsshowthatthecontentofCdSonthesurfaceoftheusedCGS-2compositesobtainedafterphotocatalytichydrogenevolutionfor60hishigherthanthatofthefreshCGS-2sample.
Onthecontrary,thecontentofSn2S3onthesurfaceoftheCdS-Sn2S3@rGOheterostructuresdecreasedafterphotocatalytichydrogenevolutionfor60h.
Thesephenomenacanbeattributedtothefactthat,benetingfromthephotoreaction-drivenstructuralrestructuringbehavior,amassofSn2S3nanocrystalscoveredonthesurfaceofCdStendedtobreakup,thusresultinginthehighlydispersedSn2S3nanocrystalsandmoreexposedCdSnanocrystalsonthesurfaceofCdS-Sn2S3@rGOheterostructuresafterphotocatalytichy-drogenevolutionreaction.
TheXPSanalysisresultsfurtherconrmthepeculiarlystructuralrecongurationeectoccurredontheCdS-Sn2S3@rGOnanocomposite.
Wealsocarriedouttheadditionalthermogravimetricanalysis(TGA)andinductivelycoupledplasmaatomicemissionspectroscopy(ICP)measurementtoinvestigatethestabilityofCdS-Sn2S3@rGOhy-bridizedphotocatalyst.
AsshowninFig.
6a,TGAanalyseswereper-formedonboththefreshCGS-2compositesandtheusedCGS-2com-positesafterphotocatalytichydrogenevolutionfor6hfromambienttemperatureto800°Cataheatingrateof10°Cmin1.
Duetothein-uenceofnitrogenowdisturbance,wecanobservethephenomenonofoverweightintheinitialheatingstage(30–150°C)forthetwosamples.
Obviously,theweightlossoffreshCGS-2compositesiscal-culatedtobe24.
2wt.
%at700°Cduetotheremovalofthecrystalwaterandthenumerousoxygen-containingfunctionalgroupsinvolvedingraphene.
However,theTGAcurveoftheusedCGS-2compositesobtainedafterphotocatalytichydrogenevolutionfor60hexhibitsaninterestingthree-stepprocess:therstdegradationstepisfrom150to447°C,andtheweightlossofsampleisabout7.
8wt%at447°C,duetotheremovalofthecrystalwaterandtheresidualscavengerlacticacidaswellasthepartofoxygen-containingfunctionalgroups.
Next,theTGAcurveshowsagradualincreaseinmassduringperiodofheat-upandtheweightlossofsampleisonlyabout3.
5wt%at550°C,whichisascribedtothefactthatthemetallicPt0wasoxidizedbytheoxygen-containingfunctionalgroupsandnallyconvertedintoplatinumoxide(PtO2).
Inthethirddegradationstep,thegradualdecayoftheTGAcurvefromaround550°Cto700°CrepresentsthatSn2S3wasgraduallyoxidizedbythereleasedoxygen-containingfunctionalandtransformedintothetindioxide,thusleadingtoaweightlossofnomorethan5.
7wt.
%at700°C.
Furthermore,theconcentrationofCd2+inaqueoussolutionwasmeasuredbyICP.
Beforethephotocatalyticreaction,66mgofCGS-2composites(thecontentofCdSis42.
24mginthissample)wasdis-persedin60mLdeionizedwaterwhichcontaining10mLlacticacidaselectrondonors.
AsdemonstratedinFig.
6b,afterphotocatalytichy-drogenevolutionfor2h,theconcentrationofCd2+intheaqueoussolutionweremeasuredtobe4.
20mgL1,andthecorrespondingFig.
4.
(a)Visiblelightphotoreductionof100mgL1K2Cr2O7aqueoussolution;(b)thereactionkineticsand(c)theapparentrateconstantsforphotoreductionofaqu-eousCr(VI)overvarioussamplesinthepresenceof0.
4moLL1tartaricacid;(d)CyclingphotoreductioncurvesofCGS-2sampleundervisible-lightillumination(λ>420nm).
C.
Xueetal.
AppliedCatalysisB:Environmental222(2018)157–166162derivedpercentageofthedissolvedCdSisonly0.
76wt.
%.
Underthevisiblelightirradiationfor4h,theconcentrationofCd2+intheaqu-eoussolutionslightlyincreasedto8.
31mgL1.
Itindicatesthatonly1.
51wt.
%ofCdSintheCdS-Sn2S3@rGOheterostructureswasdissolvedintowaterduringthephotocatalyticreductionprocess.
Whentheir-radiationtimeisextendedto6h,thecalculatedvalueofCd2+is13.
53mgL1,correspondingto2.
46wt.
%ofthedissolvedCdS.
Basedontheabove-mentioneddata,itcanbeconcludedthatalthoughtheFig.
5.
TEMimagesofCGS-2compositesunderphotocatalyticreactionfor(a)0h,(b)1h,(c)2h,(g)3h,(h)4h,(i)5h,(m)6h,(n)60h.
ParticlesizesdistributionsdeterminedbyTEM:(d)0h,(e)1h,(f)2h,(j)3h,(k)4h,(l)5h.
C.
Xueetal.
AppliedCatalysisB:Environmental222(2018)157–166163drasticstructuralreconstructionwashappenedovertheas-preparedCdS-Sn2S3@rGOheterostructuresduringthephotocatalysisprocess,onlyatinyfractionofCdSwasdissolvedinaqueoussolutionsafterphotocatalytichydrogenevolutionfor6h,indicatingthattheCdS-Sn2S3eutecticclusterbondingonthesurfaceofrGOnanosheetspos-sessedexcellentchemicalstability,andthephotocorrosionofCdScanbesignicantlyinhibitedinthiskindsspecialgraphene-basedna-nosheetphotocatalysts.
Moreover,furtherevidencefortheunusuallyphotoreaction-drivenstructuralreconstructionprocessofCdS-Sn2S3eutecticclusterscomesfromXRDpatterns.
AsshowninFig.
7a,thecrystalstructureofhex-agonalCdSandorthorhombicSn2S3inCGS-2compositehasnochangewiththephotoreactionongoing.
However,thecalculatedcrystalsizesoftheorthorhombicSn2S3showedobviouschange,whichdecreasedwiththeincreaseofreactiontime.
Incontrast,thecrystalsizesofthehexagonalCdSalmostwereunchanged(TableS1,SupportingIn-formation).
Thisphenomenonimpliesthatthecontinuouslyre-structuringwasoccurredinSn2S3nanocluster,andthustheCdS-Sn2S3@rGOcompositecanserveassustained-releasetabletstoinducetheexplosionofnumerousactivesitesduringthehydrogenevolutionreactionprocess.
Fig.
7schematicallyillustratedthetentativeme-chanismofchargetransferandseparationonCGS-2compositebeforeandafterreconstruction.
AsshowninFig.
7c,beforethephotoreduc-tion,thefreshCGS-2compositeshowsatypicalgraphenenanosheetstructure,andamassofSn2S3nanocrystalsintertwinedandcoveredonthesurfaceofCdStoself-assemblymacroaggregatedCdS-Sn2S3eutecticclusterswithalargediameterrangingfrom16.
7nmto27.
8nm,whichthusseriouslyhinderedthelightabsorptionandgenerationofphoto-inducedelectronsandholesinsideCdS.
Furthermore,theaggregationeectofthelargeCdS-Sn2S3particlesnotonlyleadedtoalongishgeometricalchargetransferpathwaysinCdSparticles,butalsoresultedinlowinterfacialcontacteciencywith2Dsurface.
Itisbelievedthatalltheabove-mentioneddisadvantagesseverelyhamperedthechargecarriermigrationandseparationinCGS-2composites,andthusleadingtoarelativelylowphotocatalyticactivityintheearlystageofH2generationfromwatersplitting.
However,undercontinuousillumina-tion,thelocalphotothermaleectofgraphenetriggeredosig-nicantlystructuralrestructuringofCdS-Sn2S3eutecticclusterslo-catingontherGOnanosheets.
AccordingtotheaforementionedTEM(Figs.
5b–nand7d)andXRDanalyses,thecocatalystSn2S3nanocrystalsunderwentdramaticandirreversiblestructuralrestructuring.
Bene-tingfromitsintriguingstructuralrestructuringbehavior,theisolatedCdS-Sn2S3eutecticclusterstendedtobreakup.
Thus,agrowingnumberofcocatalystsSn2S3withsmallercrystalsizeandtinyCdSnanocrystalseventuallyscatteredthroughoutthewholerGOplatformtoformtheeectivemetalsuldes-grapheneinterfacialcontact,respectively,re-sultinginmoreexposedCdSlight-adsorptionsitesandSn2S3reactivesites[7].
Attributingtotheshortchargecarrierdiusionlength(below10nm)derivedfromsmallerparticlesize,thephoto-inducedelectronsandholescaneasilymigratetothesurfaceofCdS,thusdecreasingtheprobabilityofchargecarrierrecombinationinsideCdS[35].
What'smore,theintroducedfew-layerrGOnanosheetsnotonlyareanidealsupportforsemiconductorloading,butalsoactasanecientelectronacceptorandchargetransferbridge,owingtothelowerFermilevelofrGO(-0.
08eVvs.
NHE)[36–41].
Therefore,theaccumulatedelectronsattheCdSedgecanvectoriallytransferfromCdStotherGOframeworkandultimatelymigratetothedualcocatalystsofSn2S3andPtnano-particlesviatheconstructedinterfacialheterojunctions(reddashedlineinFig.
7d),accompaniedwiththequenchingofthephoto-inducedholesbythescavengeroflacticacid.
Andtheprolongedlifetimeofphoto-inducedchargecarrierscanbefurtherconrmedbytime-resolvedphotoluminescencespectra(Fig.
7b)[42–46].
Overall,itcanbecon-cludedthattheunusuallyphotoreaction-drivenstructuralrecongura-tionofCdS-Sn2S3eutecticclustersonrGOnanosheetshasasignicantimpactinengineeringCdS-Sn2S3@rGOcompositeswithhighly-ecientinterfacialchargesseparationandtransfer,therebyleadingtoasub-stantiallyenhancedphotocatalyticperformance,aswellasthestability.
Thispositiveeecthasalreadybeenrobustlydemonstratedbythevisible-lightphotocatalyticH2evolutionoverCGS-2composites,whichdisplaysmuchhigherH2evolutionrate(1671μmolh1g1)after10cyclesofintensiveaccelerateddurabilitytesting.
4.
ConclusionsInsummary,wehavereportedafacilesynthesisstrategyforthelarge-scalefabricationoftheternaryCdS-Sn2S3@rGOeutecticclusterheterostructures.
NumerousCdS-Sn2S3eutecticclustersanchoredontorGOsurfacescanbereconguredduringthephotocatalyticreactionprocess,whichactedassustained-releasesystemofactivesites.
TheresultantCdS-Sn2S3@rGOeutecticclusterheterostructuresexhibitedhighactivityandstabilitybothinphotocatalyticH2evolutionandCr(VI)reductionundervisiblelightirradiation.
Thecombinationofshortercharge-transferdistance,theeectiveinterfacialcontact,su-periorelectricalconductivityofrGOandever-increasingsurfacere-activesitesecientlypromotetheseparationandtransferofphoto-generatedelectron-holepairs,thusendowCdS-Sn2S3@rGOheterostructureswiththeformidablysuperiordurabilityinphoto-reductionreactionprocess.
Weenvisionthatourresearchwillprovidenewinsightintothedesignofnanoarchitectureswithhighactivityforenvironmentalremediationandsolarhydrogengeneration.
AcknowledgmentsThisworkwasnanciallysupportedbytheNaturalScienceBasicResearchPlaninShaanxiProvinceofChina(GrantNo.
2017JZ001),theNationalNaturalScienceFoundationofChina(GrantNo.
21303130),StateKeyLaboratoryofHeavyOilProcessing(GrantNo.
SKLOP201602001),theFundamentalResearchFundsfortheCentralUniversities(GrantNo.
cxtd2017004)andtheOpenFundoftheStateKeyLaboratoryofLuminescentMaterialsandDevices(SouthChinaFig.
6.
(a)TGAcurvesofthefreshCGS-2compositesandtheusedCGS-2compositesafterphotocatalytichydrogenevolutionfor6h;(b)Time-dependentconcentrationofCd2+intheaqueoussolutionoverCGS-2nanocompositesunderirradiationwithvisiblelight(λ>420nm).
C.
Xueetal.
AppliedCatalysisB:Environmental222(2018)157–166164UniversityofTechnology,GrantNo.
2016-skllmd-04).
ThanksforthetechnicalsupportfromInternationalCenterforDielectricResearch(ICDR),Xi'anJiaotongUniversity,Xi'an,China;theauthorsalsoap-preciateMs.
DaiandMr.
MafortheirhelpinusingSEM,EDXandTEM,respectively.
AppendixA.
SupplementarydataSupplementarydataassociatedwiththisarticlecanbefound,intheonlineversion,athttp://dx.
doi.
org/10.
1016/j.
apcatb.
2017.
10.
008.
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