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Facilepreparationofyolk–shellstructuredSi/SiC@C@TiO2nanocompositesashighlye?cientphotocatalystsfordegradingorganicdyeinwastewater?
Mei-PinLiu,TingtingSu,LinSunandHong-BinDu*
Theyolk–shellstructuredSi/SiC@C(YSSC)nanosphereswerepreparedbyafacilemagnesiothermicreductionprocess.AftercoatingwithTiO2nanoparticles,theYSSC@TiO2compositespossessaBrunauer–Emmett–Teller(BET)andLangmuirsurfaceareaof423and584m2gà1,respectively,andshowhighadsorptioncapabilityformethylblue(MB)andCongoredinwater.Moreover,thenanocompositesshowastrongphotonabsorbancethroughoutthevisiblelightregion,andexhibitexcellentphotocatalyticperformancefordegradingMBinwater.TheMBdegradationfollowsthepseudo1storderkineticsandthecalculatedrateconstantforYSSC@TiO2underUV(visible)lightirradiationisapproximately4.5(7.9),8(17),and7.6(22.7)timeslargerthanthatofthesynthesizedTiO2,commercialP25andYSSCspheres,respectively.StabilitytestsshowthattheYSSC@TiO2nanospherespossesshighstabilityandmaintaingoodphotocatalyticactivityoverfourrunsofcycleexperiments.TheexcellentphotocatalyticactivityofYSSC@TiO2canbeascribedtothesynergismofhighadsorptionofdyemoleculesonthesurface,extendedlightadsorptionrangeandtheformationofsemiconductorheterostructuresthatallowfore?cientseparationofphotogeneratedelectronsandholesthroughaZ-schemesystemwithinthecatalyst.
Received20thNovember2015Accepted23rdDecember2015DOI:10.1039/c5ra24643am.kkreddy.com
?Electronicsupplementaryinformation(ESI)available:Synthesis,TGA,UV-VisPXRDpatternsandSEMimages.SeeDOI:10.1039/c5ra24643a
utilizationofwide-gapsemiconductorTiO2(Egz3.2eV).8Therefore,muche?orthasbeenmadetoimprovethee?ciencyofthephotocatalyticprocessandextendthelightadsorptionrangeofTiO2.9Thewidely-usedmethodsincludetheformationofsemiconductorheterostructures(alsocalledZ-schemepho-tocatalyst),10doping,11surfacehybridization,12etc.
SiCisawidebandgap(Egz2.4eV)materialwithlight-weight,highchemicalstability,nontoxicityandhighelectronmobility.13Itshowsexcellentpropertiesinareaofenergy,photocatalysis,semiconductorandsupercapacitor.14,15SeveralZ-schemephotocatalystsinvolvingSiChavebeenprepared.16Forexample,SnO2/SiChollowspherenanochainsweresynthe-sizedandshowedtheimprovedcatalyticactivityinH2evolu-tion;17Ag3PO4/Ag/SiCexhibitedenhancedphotocatalyticactivityindegradationofmethylorganic;18b-SiC–TiO2nanocompositesshowede?cientactivityinrhodamineBdegradationundersolarlight.19Nevertheless,thephotocatalyticactivitiesofthesecompositephotocatalystsstillneedtobeenhanced.
Yolkshellisahybridstructureofhollowandcore/shellstructure,withamovablecoreinsideandvoidspace.20Yolk–shellnanomaterialsusuallyshowgoodphotocatalyticactivitiessincethesematerialspossesshighsurfaceareaanddyeadsorptioncapacity,andallowthemultiplereectionsoflightwithintheinteriorvoids.21ThepreparationofSiCwithtailoredmorphologyisnottrivial,despitethatvariousSiC
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nanostructuressuchasnanoparticles,nanorods,nanotubes,nanospheresandmesoporousmaterialshavebeenpreparedbymeansofhigh-temperaturecarbothermalormagnesiothermicreductions,22vapor-solidsynthesis,17,23polymer-assistedsynthesis,14,24etc.Herein,wereportafacilefabricationofanovelyolk–shellstructuredSi/SiC@C(YSSC)nanospheresbyemployingthemagnesiothermicreductionmethodat650??Cfor8h.Theobtainednanospheresnotonlypossesshighsurfaceareaandhighadsorptioncapabilityofdyemolecules,butalsoexhibithighphotocatalyticactivityinthedegradationofmethylblue(MB)whencoatedwithTiO2toformaZ-schemephotocatalyst.TheYSSCandYSSC@TiO2nanosphereshavebeencharacterizedbypowderX-raydi?raction(PXRD),X-ray2.4Characterizationmethods
PXRDpatternswerecollectedinthe2q?5–50??rangewithascanspeedof0.1sdegà1onaBrukerD8AdvanceinstrumentusingaCuKaradiationatRT.SEMimageswereobtainedonaHitachiS-4800eld-emissionscanningelectronmicroscopeatanaccelerationvoltageof5.0kV.TEMimageswereobtainedonaJEM-200CXtransmissionelectronmicroscope.Thermog-ravimetric(TG)analyseswereperformedonasimultaneousSTA449F3thermalanalyserunderaowingoxygen.X-rayphotoelectronspectroscopy(XPS)measurementswererecor-dedwithaPHI5000VersaProbe.AMicromeriticsASAP2020surfaceareaporosimetrysystemwasusedtomeasuregasphotoelectronspectroscopy(XPS),scanningelectronmicro-scope(SEM),transmissionelectronmicroscope(TEM),N2adsorptionandUV-Visspectroscopicanalyses.Theadsorptionandphotocatalyticexperimentsoforganiccompoundshavebeenevaluatedbyusingmethylblue(MB)asamodelcompound.
2.
Experimental
2.1
PreparationofSiO2@RFprecursor
St¨o
berSiO2sphereswererstlysynthesizedaccordingtotheliterature.25Then,theas-preparedSiO2spheresweremixedwith70mLofH2Oand30mLofethanol.2.3gofhexadecyltrimethylammoniumbromide(CTAB)wasaddedintotheabovemixture,followedbyadditionof0.3gofresorcinoland0.1mLofconcentratedNH3solution.Aerstirringforabout30min,0.5mLofformaldehyde(40%aqueoussolution)wasaddeddropwiseintothemixture.Themixturewaskeptstirringfor8h.Theproductwasrecoveredaercentrifugationandwashedwithdistilledwaterseveraltimes.2.2
Preparationofyolk–shellSi/SiC@Cnanospheres(YSSC)
0.3goftheas-preparedSiO2@RFnanosphereswerewellmixedwith0.3gofMgpowders.Themixturewasputintoatubefurnaceandheatedto650??Cfor8hunderaowingArgas(60mLminà1).Theobtainedsolidswerewashedwith2MHClandHF(5%)solutionseveraltimes.Theblackproducts(0.13g)wereobtainedaerdriedundervacuumat70??C.2.3
PreparationofYSSC@TiO2spheres
0.1gof1-hexadecylamineand0.06mLof0.1MKClsolutionweredissolvedin15mLofethanol.Totheabovesolutionwereadded0.04goftheas-preparedYSSCspheres.Aerstirringfor10min,titaniumtetraisopropanolate(1mmol,1.0mL)wasaddeddropwiseintothemixture.Themixturewascontinuallystirredfor1handagedatroomtemperaturefor24h.Thegrayprecipitatewascollectedbycentrifugation,washedwithdeionizedwaterandethanolfor3timesandcalcinedinArat500??Cfor2h.Forcomparison,TiO2waspreparedthroughasimilarmethodwithoutaddingYSSCspheres;andcarbonspheresinC@TiO2wassynthesizedinabsenceofMgduringthepreparationofYSSCspheres.
4064|RSCAdv.,2016,6,4063–4069adsorption.Beforethemeasurements,thesampleswereacti-vatedat200??Cfor5hunderdynamicvacuum.UV-Visdi?usereectancespectrawererecordedonaPerkin-ElmerLambda950UV/VIS/NIRspectrometer.TheUV-VisspectroscopywascarriedonaShimadzuUV-3600spectrophotometer.2.5
Adsorptionofdyes
Congored(CR)andmethylblue(MB)wereselectedasorganicpollutantmodels.Typically,theYSSCspheres(500mgLà1)weremixedwith40mLof200mgLà1CRorMBsolutions.Aerintervalsofstirring,2mLofaliquotswerecollectedandcentrifuged;thesolutionswereanalyzedbyUV-Visspectroscopy.2.6
Photocatalytictest
PhotocatalyticexperimentswerestudiedbydegradationofMBsolution.Ultravioletlamp($365nm,30W)andXelamp(TIPX5002,SOFNInstrumentsCo.Ltd.)wereusedaslightsources.50mLofMBsolution(120mgLà1)and10mgofsolidcatalystweremixed.Beforestartingthephotocatalyticreaction,thesolutionwasstirredunderdarknessfor30mintoreachtheequilibrium.2mLofthesolutionwerecollectedfromthesuspensioninevery5minutesandcentrifuged.Theconcen-trationofMBwasmeasuredonaUV-Visspectrophotometer.
3.
Resultsanddiscussion
3.1
CharacterizationofYSSC
AsshowninFig.1a,theSiO2@RFprecursorsconsistofmono-dispersednanosphereswithadiameterofca.300nm.Thethicknessoftheresinlayerisabout27nm.Aermagnesio-thermicreductionandtreatmentswithdiluteHClandHF,theobtainedSiC/Si@C(YSSC)spheresretainthesphericalmorphology(Fig.1bandc).Thespheresareofyolk–shellstructureasevidencedfromsomebrokenspheresaswellastheTEMimages.Thehollowspheresarecoveredwithaca.21nmthicknessofcarbonlayer.TGAstudiesshowedthattheYSSCspheresconsistofabout8.2wt%ofamorphouscarbon(Fig.S1,ESI?).
AsdepictedinFig.1f,PXRDstudiesshowthattheobtainedyolk–shellYSSCsamplesconsistofcrystallineSiandb-SiCphases.Thethreepeaksat2q?28.4??,47.3??and56.1??canbeassignedtothe(111),(220)and(311)planes,respectively,of
acubicphasesilicon(JCPDS27-1402)witha?5.4306?A.
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Fig.1(a)TEMimageofprecursorSiO2@RFnanospheres;(b)SEMand(c)TEMimagesofYSSCnanospheres(insetsaretheHRTEMimages);(d)SEMand(e)TEMimagesofYSSC@TiO2nanospheres(insetsareHRTEMimages).(f)PXRDofsynthesizedTiO2(i),P25(ii),YSSC@TiO2nanospheres(iii)andYSSCnanospheres(iv);(g)N2adsorption(solidsymbols)anddesorption(emptysymbols)isothermsofYSSC@TiO2nanospheres(squares),YSSCnanospheres(triangles),synthesizedTiO2(circles)andP25(stars).InsetistheBJHporedistributioncurvesofYSSC@TiO2(redsquares)andYSSCnanospheres(bluecircles);(h)adsorptionpro?lesofYSSC(solidsymbols)andYSSC@TiO2(emptysymbols)forMB(circles)andCR(squares)inwater.
peaksat2q?35.6??,41.3??and60.0??arecontributedtothe(111),
?(200)and(220)planesofb-SiC(JCPDS65-0360,a?4.3584A).
Furthermore,twotypesoflatticefringesareclearlyobservedinthehigh-resolutionTEMimages(Fig.1c).Themeasuredd-spacingsare0.252(1)nmand0.310(1)nm,respectively,whichcanbeattributedtothe(111)latticeplaneofb-SiCandSi.TheseresultsareingoodagreementwiththoseofthePXRDstudies,whichgiverisetothe(111)-spacingsof0.2516and0.3130nmforb-SiCandSi,respectively.
Fig.1gshowsN2adsorptionanddesorptionisothermsoftheYSSCspheres.TheisothermsareoftypeIVwithahysteresisring,suggestingthepresenceofmicroporesandmesopores.TheBrunauer–Emmett–Teller(BET)andLangmuirsurfaceareaofYSSCare143and198m2gà1,respectively.ThecalculatedBJHporediameterofYSSCspheresbasedondesorptioncurvesisabout3.6nm.Owingtothehighspecicsurfacearea,largeporesize,goodthermalandchemicalstability,theYSSCspherescouldbeusedase?cientadsorbentsforlargemolecules.Toconrmit,theadsorptionofmethylblue(MB)andCongored(CR)inthewaterovertheYSSCspheresweretested.AsshowninFig.1h,theYSSCspheresdemonstratedhighadsorptioncapacitiesof269and368mggà1towardsCRandMB,respec-tively,muchhigherthanotherporouscore–shellmaterialsre-portedintheliterature,suchascore–shellFe3O4@MIL-100(Fe)(MB,49.4mggà1),26core–shellC@Al2O3(Orange-II,108mggà1),2core–shellg-MnO2/a-MnO2(MB,62mggà1),27andmulti-wallcarbonnanotube(CR,140mggà1).28TheseresultshaveshowntheYSSCspherescouldbepromisingcandidatesforremovingtoxicpollutantsfromwastewater.
3.2CharacterizationofYSSC@TiO2
Fig.1fshowsthePXRDpatternsoftheYSSC@TiO2sample.Besidesthosepeaksofcrystallinesiliconandsiliconcarbide,therestdi?ractionpeakscouldbeindexedtoanataseTiO2(JCPDS21-1272).TheSEMandTEMimagesofYSSC@TiO2inFig.1dandeshowthatthespheresmaintaintheyolk–shellmorphologywithsmallTiO2nanocrystallitesofabout10–15nminsizecoatedonthesurfaceoftheYSSCspheres.LatticefringesofTiO2nanocrystallitescouldbeclearlyseenintheHR-TEMimagesinFig.2e.Thecalculatedd-spacingsisca.0.349(2)nm,whichcorrespondstothe(101)latticeplaneofanataseTiO2showninPXRDstudies(2q?25.28??,d-spacing?0.351(1)nm).TheN2adsorptionanddesorptionisothermsoftheYSSC@TiO2spheresaresimilartothoseoftheYSSCspheres(Fig.1g),butwithlargerN2adsorptioncapacity.TheisothermsarealsooftypeIVwithahysteresisring,suggestingthepres-enceofmicroporesandmesopores.TheBrunauer–Emmett–Teller(BET)andLangmuirsurfaceareasofYSSC@TiO2are423,and584m2gà1,respectively,largerthanthoseoftheYSSCspheres.ThecalculatedBJHporediameteroftheYSSC@TiO2spheresbasedondesorptioncurvesisabout3.7nm,whichissimilartothatoftheuncoatedYSSCspheres.AsshowninFig.1h,theYSSC@TiO2spheresdemonstratedhigheradsorp-tioncapacitiesofdyesinwatercomparedtotheYSSCspheres,takingupca.312and395mggà1ofCRandMBfromwaste-water,respectively.
ThesurveyofXPSspectrawascarriedouttoconrmthecompositionsoftheYSSC@TiO2spheres.FromFig.2a,one
can
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Fig.2XPSspectraofYSSC@TiO2nanospheres:(a)fullrange,(b)C1s,(c)O1s,(d)Si2p,and(e)Ti2p
regions.
ndthestrongsignalsofC1s,Ti2p,O1saswellasthepeakofSi2p.HighresolutionXPSpeaksofC1s(Fig.2b)showthebindingenergypeaksat282.8,284.2and283.8eV,attributabletothesignalsofSi–C,C]CandC–O–Ti,respectively.29–31TheO1sXPSspectruminFig.2crevealsthesignalsofTi–Oat529.0and529.4eV,andtheO–Cpeakat531.2eV,respectively.19,32Thepeaksat99.3and100.8eVinSi2pXPSspectrum(Fig.2d)arehighlyconsistentwiththeSi–SiandSi–Cbonding.29,33Inaddition,theweakpeakat102.4eVcouldbeassignedtotheTi–O–Silinkage.19Twopeaksat458.0and463.6eVinTi2pXPSspectrumareattributedtothoseofTi2p3/2andTi2p1/2,respectively(Fig.2f).30TheseresultsareconsistentwiththePXRDresults,showingthepresenceofTiO2,SiC,SiandCinthespheres.Moreover,theresultsdemonstratetheintimatecontactsbetweenthemviaSi–C,C–O–TiandTi–O–Sibonds,formingsemiconductorheterojunctions.BasedontheXPSspectra,theestimatedmoleratiosofSi,SiC,andTiO2inYSSC@TiO2are1:1.24:4.48.
Thesteady-stateopticalpropertyofYSSC@TiO2wasinves-tigatedbyUV-Visabsorbancespectroscopy.Theneat,as-preparedTiO2andcommercialP25wereusedasblankforcomparisons.AsshowninFig.3,theas-preparedTiO2andcommercialP25exhibitnoabsorptioninvisiblelightregion.Ontheotherhand,theYSSCandYSSC@TiO2spheresshowanenhancedabsorbancethroughoutthevisiblelightregion,whichcouldbeattributedtotheexistenceofSiandSiCwithanarrowerbandgapof1.12eVand2.2eV,respectively.FromtheUV-Visabsorbancedata,thebandgapenergycanbededucedfromplotsof(ahn)1/2versusenergy(hn),whereaisabsorptioncoe?cient,andhnisphotonenergy.34AsshowninFig.S2(ESI?),thecalculatedbandgapenergiesofP25,blankTiO2andC@TiO2are3.11,2.87eVand2.79eV,respectively,whilethatofYSSC@TiO2is1.39eV.
3.3Photocatalyticactivities
Methylblue(MB)wasselectedasapollutantmodelinwatertoevaluatethephotocatalyticactivitiesoftheas-synthesizedYSSC@TiO2underUVandvisiblelights.Forcomparisons,thephotocatalyticdegradationofMBinwaterovercommercialTiO2(DegussaP25,$80nminsize,Fig.S3?),synthesizedanataseTiO2($60nminsize),C@TiO2(hollowspheres),YSSCand5:1(massratio)mixtureofanataseTiO2andYSSC(denotedYSSC–TiO2)werealsoinvestigatedbyusingthesameamountofTiO2orYSSC.TheresultsarepresentedinFig.4.Underdarkness,physicaladsorptionofdyemoleculesoverthecatalystsoccurs.TheadsorptioncapacitiesfollowtheorderofYSSC@TiO2>C@TiO2>YSSC–TiO2zYSSC>synthe-sizedTiO2>P25,whichareinconsistentwiththeorderoftheirspecicBETsurfaceareas,i.e.423,173,151,143,68,
and
UV-Visdi?usere?ectancespectraofP25(i),synthesizedTiO2
(ii),C@TiO2(iii),YSSC(iv)andYSSC@TiO2(v).Fig.3
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environmentaltechnology.Therefore,thecyclingdegradationexperimentsovertheYSSC@TiO2catalystwereevaluatedandtheresultsareshowninFig.4bandd.TheYSSC@TiO2photo-catalystdonotdisplayanobviousdecreaseinphotocatalyticdegradationactivityaer4cycles.PXRDstudiesshowedthatthecrystallinestructureoftheusedcatalystkeptintact(Fig.S4?).TheSEMimage(Fig.S5?)showsthatthemorphologyofthehollowspheresofthecatalystisstillretainedaertherecyclingreactions.TheseresultsindicatethattheYSSC@TiO2photo-catalystpossesseshighstabilityandcanbeeasilyrecoveredforreuseinphotocatalyticdegradationoforganicpollutants.
ThekineticsofMBdegradationwereinvestigatedbyusingtheLangmuir–Hinshelwoodmodel,35,36i.e.ln(C/C)?kt,where on 26/01/2016 06:09:24.
Fig.4
ComparisonofphotocatalyticactivitiestowardsMBdegrada-tionunder(a)UVand(c)visiblelightirradiation:(i)withoutcatalyst,(ii)P25,(iii)YSSCnanospheres,(iv)synthesizedTiO2,(v)YSSC–TiO2,(vi)C@TiO2and(vii)YSSC@TiO2;recyclingtestsofYSSC@TiO2forthedegradationofMBunder(b)UVand(d)visiblelightirradiation.49m2gà1,respectively.TheYSSC@TiO2compositesshowedthelargestadsorptioncapacityof$40%forremovingMBfromwateraer30minofequilibrium,followedbytheC@TiO2spheresandYSSC–TiO2physicalmixtures.Withthelighton,thephotocatalyticdegradationofMBtakesplace.AsshowninFig.4a,thepercentageofMBremovalunderUVlightirradi-ationfollowstheorderofYSSC@TiO2>C@TiO2>YSSC–TiO2>synthesizedTiO2>YSSC>P25.However,takingaccountofphysicaladsorption,thephotocatalyticdegradationofMBwouldfollowtheorderofYSSC@TiO2>C@TiO2>synthe-sizedTiO2>YSSC–TiO2>YSSCzP25.Inotherwords,theYSSC@TiO2isthemoste?cientphotocatalystindegradingdyesinwater,whiletheP25andYSSCspheresaretheleaste?cient.Aerremovalof$40%MBinwaterbyphysicaladsorption,about90%oftherestMBweredegradedoverYSSC@TiO2underUVirradiationwithin30min.Undertheidenticalconditions,however,onlyapproximately42,36,31,24and22%oftherestMBweredegradedunderUVlightirradiationbyC@TiO2,synthesizedTiO2,YSSC–TiO2,YSSCandP25,respectively.SimilartrendshavebeenobservedforphotocatalyticdegradationofMBoverthesecatalystsundervisiblelightirradiation.AsshowninFig.4c,thephotocatalyticdegradationofMBfollowsthesameorderofYSSC@TiO2>C@TiO2>synthesizedTiO2>YSSC–TiO2>YSSCzP25,with82,35,20,16,10,and8%oftherestMBdyedegradedundervisiblelightirradiation,respectively.Itisnotedthatthecatalysts,particularlytheTiO2andYSSC–TiO2,exhibitedlowerphotodegradinge?cienciesundervisiblelightthanUVlight,whichismainlyduetothepoorsolarenergyutilizationofwide-gapsemiconductorTiO2.TheseresultsindicatethatTiO2isthemainactiveingredientforphotocatalyticdegradationofMB,andtheYSSC@TiO2catalystlikelyformssemiconductorheterostructuresthatexhibitthemoste?cientactivityinMBdegradation.
Thestabilityandreusabilityofthephotocatalystsareveryimportantfactorsfortheirutilizationinsustainable
Thisjournalis?TheRoyalSocietyofChemistry2016okisthepseudo-rstorderrateconstant,CandCoareconcen-trationofdyeattimetandt?0,respectively.TheresultsaresummarizedinTable1.ThisequationtsreasonablywellwiththeexperimentaldatawithalltheR2valuesgreaterthan0.9,whichsuggeststhattheMBdegradationsoverthesecatalystsfollowthepseudo1storderkinetics.Thecalculatedrateconstantk1valueforYSSC@TiO2underUVlightis0.0703minà1,approximately3.5,4.5,5.7,7.6and8timeslargerthanthatoftheC@TiO2,synthesizedTiO2,YSSC–TiO2mixtures,YSSCspheresandcommercialP25,respectively.Similarly,thek2valueforYSSC@TiO2undervisiblelightis0.0476minà1,3.8,7.9,9.3,22.7,and17timeslargerthantheC@TiO2,synthesizedTiO2,YSSC–TiO2mixtures,YSSCspheresandcommercialP25,respectively.TheseresultsconrmthatthecompositeYSSC@TiO2nanosphereshavethebestphotocatalyticactivity.TheexcellentphotocatalyticactivityofYSSC@TiO2canbeattributedtoitsuniquestructureandcomposition.Firstly,thecatalystconsistsofyolk–shellstructurednanospherescoatedwithTiO2nanoparticles.ItpossesseslargeBETsurfaceareaof423m2gà1withmesoporesofca.3.7nmindiameter,andcanadsorbupto395mggà1ofMBinwater.ThehighadsorptionabilityoftheYSSC@TiO2spheresisdenitelyapositiveaspectindegradationprocess,37becausethephotocatalyticreactionsonlyoccurontheactivesitesofthesemiconductorsurfaceandthedyesadsorbedonthesurfacecanthusbedegradedinsiturapidlywithoutthepriordi?usion.Theyolk–shellstructureofYSSC@TiO2,ontheotherhand,mayallowthemultiplereec-tionsoflightwithintheinteriorvoidsduringthephotocatalyticreactions,21thusleadingtoimprovedactivities.Secondly,thecatalystismadeupofnarrowerbandgapsemiconductorsSiandSiCbesidesthewidebandgapTiO2nanoparticles.Itshowsastrongphotonabsorbancethroughoutthevisiblelight
region.
Table1
Pseudo-1storderkineticsofMBdegradationovervarious
catalystsunderUVlight(k1)andvisiblelightirradiation(k2)
Catalyst
k1(minà1)R12k2(minà1)R22YSSC@TiO20.07030.910.04760.99C@TiO2
0.01990.950.01250.97SynthesizedTiO20.01570.990.00600.94YSSC–TiO20.01230.980.00510.99YSSC0.00930.980.00210.96P25
0.0089
0.96
0.0028
0.91
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Theextendedlightabsorbancewillimprovethesolarenergyutilizationandthusenhancethee?ciencyofthephotocatalyst.Thirdly,thesemiconductorsSi,SiCandTiO2areintimatelycontactedwitheachotherviaSi–C,C–O–TiandTi–O–Sibonds(Fig.2andS6?),formingsemiconductorheterojunctionsasobservedinmanyotherheterojunctionnanocomposites.17,19,36,38Whenthesemiconductorsareexcitedbylightstogenerateholesandelectronsintheirconductionandvalencebands,respec-tively,theelectronsontheconductionbandminimumofSiandSiCcaneasilydi?useintoTiO2throughtheSchottkybarrier,withholesremainedinthevalencebandofSiCandSi.ThroughthejunctionoftheTiO2andSiC/Siinterface,theelectron–holepairscouldbeeasilyseparatedandthusresultin6A.W.Carpenter,C.F.deLannoyandM.R.Wiesner,Environ.Sci.Technol.,2015,49,5277;T.BoraandJ.Dutta,J.Nanosci.Nanotechnol.,2014,14,613.
7A.A.IsmailandD.W.Bahnemann,J.Mater.Chem.,2011,21,11686;A.KudoandY.Miseki,Chem.Soc.Rev.,2009,38,253;X.Lang,X.ChenandJ.Zhao,Chem.Soc.Rev.,2014,43,473;J.Ran,J.Zhang,J.Yu,M.JaroniecandS.Z.Qiao,Chem.Soc.Rev.,2014,43,7787.
8M.Gao,L.Zhu,W.L.Ong,J.WangandG.W.Ho,Catal.Sci.Technol.,2015,5,4703;M.D.Hernandez-Alonso,F.Fresno,S.SuarezandJ.M.Coronado,EnergyEnviron.Sci.,2009,2,1231.
9H.Xu,S.Ouyang,L.Liu,P.Reunchan,N.UmezawaandJ.Ye,areducedeà–h+recombination.TheelectronsaccumulatedonthesurfaceofTiO2arescavengedbyadsorbedO2togeneratesuperoxideradicals,whichareresponsibleforthedegradationoftheMB.Asresults,YSSC@TiO2isaZ-schemephotocatalystthatexhibitshighcatalyticactivityinthedegradationofMBinwastewater.
4.Conclusions
Insummary,yolk–shellstructuredSi/SiC@C(YSSC)nano-spheresweresuccessfullypreparedthroughasimplemagne-siothermicreductionroute.WhencoatedwithTiO2onthesurface,theformedYSSC@TiO2nanocompositepossesseslargespecicsurfaceareaandexhibitshighadsorptioncapacityfordyemoleculesfromwastewater.Furthermore,itexhibitsastrongphotonabsorbancethroughoutthevisiblelightregion,andishighlyphotocatalyticactivefordegradationoforganicdyeMBinwastewater.Thisrobust,reusable,andenviron-mentalbenignmaterialofYSSC@TiO2maybeofuseinaddressingenvironmentalissuesforsustainabledevelopment,suchaswastewatertreatment.
Acknowledgements
WearegratefulfornancialsupportfromtheNationalNaturalScienceFoundationofChina(21471075)andProgramBforOutstandingPhDCandidateofNanjingUniversity.
Notesandreferences
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