AnovelTiO2nanotubearrays/MgTixOymultiphaseheterojunctionfilmwithhighefficiencyfor photoelectrochemicalcathodicprotectionChang Feng&ZhuoyuanChen&JiangpingJing&Mengmeng Sun&GuiyingLu&JingTian&JianHou https://ebookmass.com/product/a-novel-tio2-nanotube-arraysmgtixoy-multiphase-heterojunction-film-with-high-efficiencyfor-photoelectrochemical-cathodic-protection-chang-fengzhuoyuan-chen-jiangping-jing-mengmeng-sun-guiyi/
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CorrosionScience
journalhomepage: www.elsevier.com/locate/corsci
AnovelTiO2 nanotubearrays/MgTixOy multiphase-heterojunctionfilmwith highefficiencyforphotoelectrochemicalcathodicprotection
ChangFenga,b,c,d,e,ZhuoyuanChena,c,d,e,*,JiangpingJinga,d,e,MengmengSuna,d,e, GuiyingLua,b,d,e,JingTiana,b,d,e,JianHouc
a KeyLaboratoryofMarineEnvironmentalCorrosionandBio-fouling,InstituteofOceanology,ChineseAcademyofSciences,7NanhaiRoad,Qingdao266071,China
b UniversityofChineseAcademyofSciences,19(Jia)YuquanRoad,Beijing100049,China
c StateKeyLaboratoryforMarineCorrosionandProtection,LuoyangShipMaterialResearchInstitute,WenhaiRoad,Qingdao266237,China
d CenterforOceanMega-Science,ChineseAcademyofSciences,7NanhaiRoad,Qingdao266071,China
e OpenStudioforMarineCorrosionandProtection,PilotNationalLaboratoryforMarineScienceandTechnology(Qingdao),No.1WenhaiRoad,Qingdao,266237,China
ARTICLEINFO
Keywords:
A.TiO2/MgTixOy
A.Multiphase-heterojunctionfilm
B.SKP
C.Photoelectrochemicalcathodicprotection
C.Stability
ABSTRACT
Heterojunctionengineering,asarisingstarinthephotoelectrochemicalcathodicprotection(PECCP)field, contributestopromotetheseparationofthephotoinducedelectronsandholes.Inthispaper,theTiO2/MgTixOy multiphase-heterojunctionfilmwaspreparedanditsPECCPperformancewasstudied.Duetoagoodenergy bandalignmentgradientformedbetweenthemultiphases,theseparationefficiencyofthephotoinducedelectronsandholesgeneratedbyTiO2/MgTixOy aredramaticallyenhanced,leadingtoitsgoodPECCPperformance andhighstability.SKPis,forthefirsttime,usedtostudythesurfaceworkfunctionoftheTiO2/MgTixOy multiphase-heterojunctionfilmforcharacterizingitsPECCPperformance.
1.Introduction
Asapromisinggreentechnology,photocatalysisandphotoelectrochemistryareextensivelystudiedandusedinthefieldsofenvironmentandenergy[1–6].Asanimportantbranchofphotocatalysisand photoelectrochemistry,thephotoelectrochemicalcathodicprotection (PECCP)technologyusestheseparatedphotoinducedelectronsgeneratedbysemiconductormaterialsandtransfersthemtothecoupled metaltoprovidecathodicprotection.Thisisaneffectivewaytoprotect metallicmaterialsusingsolarenergy.DuringthePECCPprocess,the photoelectricconversionsemiconductormaterialwillnotbeconsumed andthusthistechnologywillnotpollutetheenvironment.Meanwhile, thecontrolsynthesisofthephotoelectrodeisrelativelysimpleandthe costislow[7,8].Therefore,thePECCPtechnologyisapromising,green andenvironmentallyfriendlycorrosionprotectiontechnologywitha greatapplicationpotential[9,10].TiO2 [11,12],ZnO[13,14],g-C3N4 [15],SrTiO3 [16,17]etc.arecommonsemiconductormaterialswith goodPECCPperformance.However,asinglephotoelectricconversion semiconductormaterialtendstohaveafastrecombinationrateofthe photogeneratedelectronsandholes,enablinglessphotoinducedelectronstobeeffectivelyutilizedforPECCP.Therefore,furthermodificationsofthephotoelectricconversionsemiconductormaterialisofgreat significancetorealizethesustainableandeffectiveutilizationofthe
⁎ Correspondingauthor.
E-mailaddress: zychen@qdio.ac.cn (Z.Chen).
https://doi.org/10.1016/j.corsci.2020.108441
excitedphotogeneratedelectronsforPECCP.
Asaneffectivewaytosignificantlyinhibittherecombinationofthe photogeneratedelectronsandholes,heterojunctionengineeringhas beenreportedtoachieveeffectiveseparationofthephotogenerated electronsandholes[18–20],andtoefficientlytransferthephotogeneratedelectronstothecoupledmetals,therebyenhancingthe PECCPperformanceofsemiconductormaterials.Thereportedheterojunctionsystems,suchasSrTiO3/TiO2 [21,22],Bi2X3/TiO2 (XisSorO) [23,24],In2O3/TiO2 [25],SnO2/TiO2 [26],WO3/TiO2 [27,28],Ag2S/ TiO2 [29],Ni2S3/TiO2 [30],N-dopedTiO2/TiO2 [31],BiVO4/TiO2 [32],ZnInS/TiO2 [33],Co3O4/ZnO[34],ZnxMg1-xO/ZnO[35],g-C3N4/ ZnO[36],TiO2/ZnO[37],g-C3N4/In2O3 [38],etc.,showsignificant improvementsinthePECCPperformance.Althoughtheestablishment ofheterojunctionscansignificantlyimprovethePECCPperformanceof compositeheterojunctionmaterials,itslong-termstabilityisstillabig challenge.Buetal.havesystematicallyreportedthePECCPmechanism forsteelusingtheSrTiO3/TiO2 compositephotoelectrodes[21].However,thestabilityofthephotoelectrodesappliedforthePECCPshowed obviousshortcomings[21].Sunetal.haveinvestigatedtheenhanced PECCPperformanceoftheIn2O3/TiO2 composite[25].Thestability testingwasstillanimportantparttobefurtherverified.Kuangetal. havedesignedadual-functionalZnxMg1-xOsolidsolutionnanolayer modifiedZnOtussock-likenanorodstoapplyforthePECCP[35].The
Received18October2019;Receivedinrevisedform31December2019;Accepted4January2020
Availableonline08January2020
0010-938X/©2020ElsevierLtd.Allrightsreserved.
stabilityoftheperformancewasanimportantfactorthatbothersthem inchoosingthebestphotoelectrode.Accordingly,howtodesigna photoelectrodewithhighPECCPactivityandstabilityisstillanurgent issuetobesolved.Ascanbewellknown,theestablishmentofmultiphaseheterojunctionscaneffectivelyacceleratetheseparationofthe photogeneratedchargecarriers[39–43].Awellbandalignmentformed amongthemulti-heterojunctionscanprovidealargerchargecarrier transfergradient[39],whichcansignificantlyinhibitthesecondary recombinationofthephotogeneratedelectronsandholes.And,this providesanideatodesignanefficientandstablephotoelectrodeappliedforPECCP.
Inaddition,thescanningkelvinprobe(SKP)testingsystemisconsideredtobeanovelmicro-electrochemicaltechnologywhichcan measurethesurfaceworkfunction(WF)ofamaterial.Ithasbeen widelyusedinthefieldsofmetalliccorrosion,coatings,solarcellsand photocatalysis[44–51].Ononehand,SKPtechnologycanbeusedto recordthesurfacepotentialofthematerialandobservethepotential distributionofdifferentcomponentsinthemicro-region.Ontheother hand,thesurfacepotentialdistributionmeasuredbySKPtechnology canbetransformedintosurfaceWF,whichcanbeusedtoanalyzethe capabilityofthesurfaceelectronsescapingfromthematerialandstudy theapplicationinelectrochemistryandphotoelectrochemistry.Hua etal.haveusedtheSKPtechnologytoprovethatthediffusionrateof hydrogenin(001)and(101)grainsof304SSisfasterthanthatin(111) grains,andhydrogenisprovedtobetrappedatthephaseboundary betweenausteniteandmartensite[52,53].Lietal.exploredthereasons fortheimprovementofthephotocatalyticperformanceofAg-modified TiO2 throughSKPtechnology[44].TheirresultsshowedthatlowWF madetheAg-modifiedTiO2 beeasiertoescapeelectrons,andthus acceleratedtheseparationofphotogeneratedchargecarriers,leadingto theenhancementofthephotocatalyticperformanceoftheAg-modified TiO2.AlthoughSKPtechnologyhasbeenappliedinvariousresearch fields,thereisnoreportconcerningabouttheSKPanalysisinthefield ofthePECCP.Inviewofthis,theintroductionoftheSKPtechnology intotheareaofthePECCPwillplayapositiveroleforcomprehensively andprofoundlyunderstandingtheeffectofthephotoelectrodesonthe PECCPprocess.
MgTixOy isconsideredtobeawidebandgapsemiconductormaterialwithgoodphotocatalyticperformance[40,54].Thewideband gapcanmakethephotoinducedelectronsandholesgeneratedby MgTixOy bedifficulttorecombine,therefore,thephotogenerated chargecarrierscanefficientlytransferintheprocessofthephotocatalyticreactions.Furthermore,MgTixOy hasamorenegativeconductionbandpotentialthanTiO2,whichisbeneficialtoPECCP. Meanwhile,awellbandalignmentcanbeformedbetweenTiO2 and MgTixOy,therefore,theTiO2/MgTixOy multiphaseheterojunctionsenablethephotogeneratedchargecarrierstomigratedirectionally.Inthis way,morephotogeneratedelectronscanbeseparatedandparticipate inthePECCPreactions,therebygreatlyimprovingthePECCPperformanceandstabilityoftheTiO2/MgTixOy multiphaseheterojunction system.
Inthepresentpaper,anovelTiO2/MgTixOy multiphase-heterojunctionfilmwaspreparedandreportedforthefirsttime,andits PECCPperformanceandstabilityfor304SSwerestudied.SKPtechniquewasusedforthefirsttimetostudythesignificantlyenhanced PECCPperformanceofthepreparedTiO2/MgTixOy multiphase-heterojunctionfilm.ThelowestsurfaceWFoftheTiO2/MgTixOy multiphase-heterojunctionfilmmakesitbeeasiertoescapetheelectrons, andhigheramountofphotogeneratedelectronscanbeproducedfor protecting304SSundersimulatedsolarlightillumination.Inaddition, theestablishmentofmultiphaseheterojunctionsmakestheTiO2/ MgTixOy filmshavedurablestabilityforthePECCP.Thisstudyfurther enrichesthemeansofcharacterizingthephotoelectrodes,andprovides animportanttheoreticalbasisforunderstandingthepromotionofthe PECCPperformanceandthestabilityofthephotoelectrodes.
2.Experimentalsection
2.1.Preparationofthephotoelectrodes
Thereagentsusedintheexperimentswereallpurchasedfrom SinopharmChemicalReagentCo.,Ltdwithoutfurtherpurification.The anataseTiO2 nanotubearrays(NTAs)(TiO2(A))photoelectrodewas preparedbyatwo-stepanodizationmethodaccordingtotheprevious reports[55,56].Acleanedtitanium(Ti)sheetwasusedastheanode andaPtelectrodewasusedasthecathode,whichwasplacedinparallel intheelectrolyteofethyleneglycol(0.35wt%NH4F,10wt%H2O).A constantpotential(60V)wasappliedforanodicoxidationfor1hat roomtemperature.After10minofultrasoniccleaningin10wt%HCl solution,theobtainedTisheetwasrinsedwithdeionizedwaterfor severaltimes.Theabove-mentionedprocessofanodicoxidationwas repeatedonemoretime.Afterthat,thesamplewasputintodeionized waterandultrasonicallycleanedfor30S.Thesamplewasdried,and thenwasannealedat450°Cfor3.5htopreparetheTiO2(A)photoelectrode.
TheTiO2/MgTixOy photoelectrodewaspreparedasfollows.The magnesiumhydroxidewasfirstlydepositedontothesurfaceofthe TiO2(A)photoelectrodebyamulti-potentialstepmethodusinga CHI660Delectrochemicalworkstation(ShanghaiChenhuaInstrument Co.,Ltd.,Shanghai,China).Athree-electrodeconfigurationwasapplied inthedepositionprocess,inwhichthepreparedTiO2(A)wasusedas theworkingelectrode,thePtelectrodeworkedasthecounterelectrode, andthesaturatedcalomelelectrode(SCE)actedasthereferenceelectrode,respectively.0.1MMg(NO3)2 solutionwasusedastheelectrolyte.Duringthedepositionprocess,thesteppotentialswere−1.5and −1.2V,respectively,andthecorrespondingtimewas5sand0.5s. After8cyclesofdeposition,theobtainedsamplewaswashedwith deionizedwaterandalcohol.Subsequently,thepreparedsamplewas thermallytreatedinamufflefurnaceat700°Cfor2htoobtainthe TiO2/MgTixOy photoelectrode.Ascanbewellknown,thephase transformationofTiO2 occursduringhightemperaturetreatment(≥ 550°C),resultinginthecoexistenceofanataseandrutileTiO2 (TiO2(R))[57–59].Therefore,asacomparison,thepreparedTiO2(A) photoelectrodewasalsothermallytreatedinamufflefurnaceat700°C for2htoobtaintheTiO2(A)/TiO2(R)photoelectrode.
2.2.Characterizationofthepreparedphotoelectrodes
X-raydiffractometer(XRD,D/max-500,RigakuCo.,Tokyo,Japan) wasusedtocharacterizethecrystalstructuresofthepreparedphotoelectrode.Fouriertransforminfrared(FT-IR)spectraweretestedusing aFouriertransforminfraredspectroscopy(FT-IR,Thermo-Nicolet8700, ThermoElectronScientificInc.,USA)atroomtemperature.UV–vis diffusereflectancespectrophotometer(U-41000;HITACHI,Tokyo, Japan)wasusedtoanalyzetheopticalabsorptionpropertiesofthe preparedTiO2(A),TiO2(A)/TiO2(R)andTiO2/MgTixOy photoelectrodes.Photoluminescence(PL)spectraofthepreparedphotoelectrodes weremeasuredwithafluorescencespectrometer(PL,Microconfocal RamanSpectrometer,HoribaJobinYvonLabRAMHR800,325nm, France).Fieldemissionscanningelectronmicroscopy(FE-SEM,ZEISS, ULTRA55,Germany)wasusedtoanalyzethemicromorphologiesof thepreparedTiO2(A),TiO2(A)/TiO2(R)andTiO2/MgTixOy photoelectrodes.TheelementalcompositionandmappingofthepreparedphotoelectrodewereanalyzedbyanX-rayenergydispersivespectrometer (EDS,Oxford,UK).ThemicrostructuresofthepreparedTiO2/MgTixOy photoelectrodeandinterfacialinformationofdifferentphasecomponentswereobservedbyfieldemissiontransmissionelectronmicroscope (FE-TEM,TecnaiG2F20,FEICompany,USA).
2.3.Photoelectrochemical(PEC)performance,PECCPperformanceand SKPmeasurements
ThelightsourceusedforthePECandPECCPperformancemeasurementsis150-WXelamp(PLSSXE300,ChangtuoCo.Ltd.,Beijing, China).AsimulatedsolarilluminationisobtainedbyaddinganAM1.5 Gfilterandadjustingthelightintensityofthislightsourceto100mW cm−2.ThePECperformancewastestedinathree-electrodecellsystem usingtheCHI660Delectrochemicalworkstation.Thevariationsofthe currentdensitiesoftheTiO2(A),TiO2(A)/TiO2(R)andTiO2/MgTixOy photoelectrodesweremeasuredusingthepreparedphotoelectrodeas theworkingelectrode,theplatinumelectrodeasthecounterelectrode, andAg/AgCl(saturatedKCl)asthereferenceelectrode.Thebiasvoltagewassetas0V(vsAg/AgCl)andtheelectrolytewas0.1MNa2SO4 solution.Thevariationsinthecurrentdensitiesandthemixedpotentials,andthepolarizationcurvesofthegalvaniccoupleofthe304SS electrodeandthepreparedphotoelectrodesweremeasuredtocharacterizethePECCPperformance.Thegalvaniccoupleofthe304SS electrodeandthepreparedphotoelectrodeswasusedastheworking electrode.TheAg/AgCl(saturatedKCl)electrodeandtheplatinum electrodeservedasthereferenceandcounterelectrode,respectively. Boththe304SSelectrodeandthepreparedphotoelectrodesareplaced in3.5wt%NaClsolutionandthebiasvoltagewassetas0V(vsAg/ AgCl)duringthetestingprocess.Intermittentsimulatedsolarlight(100 mW,AM1.5G)wasilluminatedonthesurfaceofthephotoelectrodes. Electrochemicalimpedancespectroscopy(EIS)testswereperformedat opencircuitpotentialoverthefrequencyrangebetween105 and10-1 Hz,withanACvoltagemagnitudeof5mV.Thepolarizationcurves weremeasuredusingtheCHI660Delectrochemicalworkstationwitha scanrateof1mV s-1 from-400to400mV(vsopencircuitpotential).
ThesurfaceWFsofthepreparedphotoelectrodeswereanalyzed usingSKP(VersaSCAN,Ametek).Thetungstenprobewiththediameter of250μmwasactedasthereferenceanddetectionprobe.Thetesting areaonthesurfaceofthepreparedphotoelectrodeswas1×1mm2 and thescanningratewassetas50μm s−1 withsensitivityof500μV.The surfaceWFsofthephotoelectrodescanbecalculatedbasedonthe followingformula[60–62]:WF(Sample)=WF(Tungsten)+ΔW (photoelectrode)/1000.Amongthem,WF(Tungsten)istheWFofthe referenceelectrodewithstandardvalueof4.55eVfortungsten;WF (Sample)isthesurfaceWFforthepreparedphotoelectrode,ΔW (photoelectrode)isthesurfacepotentialofthephotoelectrodeobtained fromtheSKPmeasurement.
3.Resultsanddiscussion
Fig.1AshowstheXRDpatternsofthepreparedTiO2(A),TiO2(A)/ TiO2(R)andTiO2/MgTixOy.FortheXRDpatternofTiO2(A),thediffractionpeaksat25.5°,37.9°,48.4°,54.2°and55.3°areobserved,correspondingtothe(101),(004),(200),(105)and(211)crystalplanesof standardanataseTiO2 (JCPDSNo.21-1272),respectively.Theother
diffractionpeaksintheXRDpatternofTiO2(A)areattributedtothe characteristiconesoftheTisubstrate[55,56].FortheXRDpatternof TiO2(A)/TiO2(R),thecoexistenceofanataseandrutilephasesofTiO2 canbeclearlyobservedafterhightemperaturetreatmentat700°C.In additiontothediffractionpeaksofTisubstrateandanataseTiO2,the correspondingrutilephasesofTiO2 (JCPDSNo.21-1276)areobserved at27.6°,36.1°,41.2°,44.1,54.3°,56.7°,64.1°and68.9°,whichareassignedtothe(110),(101),(111),(210),(211),(220),(310)and(301) crystalplanes,respectively.FortheXRDpatternofTiO2/MgTixOy,the weakcharacteristicpeaksofMgTiO3 andMgTi2O5 canbeobserved.It maybeduetotherelativelysmallamountofMgTixOy depositedonthe surfaceofTiO2 tubeorifice.Althoughtheintensitiesofthediffraction peaksareweak,itcanbeclearlyseenthatthediffractionpeaksare locatedat19.1°,21.2°,23.9°,35.5°,40.6°and63.8°,respectively.These diffractionpeakscorrespondwelltothe(003),(101),(012),(104), (110)and(214)crystalplanesofrhombohedralMgTiO3 (JCPDSno.060494).Inaddition,thediffractionpeaksat32.7°and48.6°areobserved,whichareassignedtothe(230)and(331)crystalplanesof orthorhombicstructureofMgTi2O5 (JCPDSno.35-0792).TheXRD resultssuggestthatMgTixOy wassuccessfullymodifiedonthesurfaceof TiO2 Fig.1BdisplaystheFT-IRspectraofthepreparedTiO2(A), TiO2(A)/TiO2(R)andTiO2/MgTixOy.TheFT-IRspectraofTiO2(A)and TiO2(A)/TiO2(R)showahighdegreeofconsistencyandthebroadband below750cm−1 ismainlyfromthestretchingvibrationmodeofTi-OTiinTiO2 [63,64].Noothercharacteristicpeakswereobserved,indicatingthehighpurityofTiO2(A)andTiO2(A)/TiO2(R).FortheFT-IR spectrumofTiO2/MgTixOy,anotherbroadregionatapproximately 1400−1550cm−1 canbeobserved,whichcomesfromtheTi–carboxyliccomplexesandhydroxylgroup[65–67].TheFT-IRresults indicatethatthecarboxylandhydroxylgroupscanbeeasilyadsorbed onthesurfaceofTiO2(A)/TiO2(R),whichfacilitatesthecontactbetweenthephotoelectrodeandtheelectrolytesolution.
Fig.2 showstheSEMimagesofTiO2(A),TiO2(A)/TiO2(R)and TiO2/MgTixOy,respectively.Asshownin Fig.2A,TiO2(A)exhibitsa goodNTAstructure.Theorificesarecloselyarrangedwithadiameter ofapproximately80nm. Fig.2BshowstheSEMimageoftheTiO2(A)/ TiO2(R)photoelectrode.ThesurfaceofTiO2(A)/TiO2(R)ismuch smootherthanthatofTiO2(A),whichmaybeduetotheformationof anataseandrutileTiO2 afterthehightemperaturetreatment.Forthe SEMimageofTiO2/MgTixOy displayedin Fig.2C,somenanoparticles obviouslyappearattheorificesandarewellcoatedonthesurfaceof theorificeofTiO2 NTAs.Thismaybethesurface-modifiedMgTixOy nanoparticles. Fig.2DshowstheSEMimageofthecrosssectionof TiO2/MgTixOy.Theheightofthenanotubescanbemeasuredasapproximately4μm.Thebasicstructureofthenanotubeshasnotbeen destroyedafterhightemperaturetreatmentat700°C,andthetubular structureremainsintact.
Fig.3AshowstheSEMimageandthecorrespondingEDSspectrum ofTiO2(A).InadditiontoTiandOelements,nootherimpurityelementsareobserved.TheatomicmasspercentagesofTiandOwere
33.19%and66.81%,whichfitswellwiththeelementcompositionof TiO2. Fig.3BshowstheSEMimageandthecorrespondingEDSresults ofTiO2/MgTixOy.Ti,OandMgelementsareclearlyobservedwiththe atomicpercentagesof24.13%,67.91%and7.96%,respectively.As showninthecorrespondingEDSmappingresults,Tielementcoversthe wholescanningareaduetotheTisubstrate.While,OandMgelements correspondwelltotheSEMimageshownin Fig.3B.Thetubeorificeis clearlyobservedintheEDSMgandOmappings,indicatingthatMgand OelementsarewelldispersedonthesurfaceoftheTiO2 nanotubeorifices.ThedifferencebetweentheSEMimagesin Fig.3 andthosein Fig.2 ismainlyduetothefactthatEDSistestedunderhighexcitation
voltages,whichwillbreakdownthesurfacecomponentsandmakethe surfacemorphologybedifficulttobeobserved.
ThecompositionandinterfacialinformationoftheTiO2/MgTixOy multiphaseheterojunctionsarefurtherstudiedbyHRTEM. Fig.4A presentstheTEMimageobservedatlowmagnification.Thestructureof nanotubescanbeclearlyobserved,whichcorrespondswelltotheSEM imageofTiO2/MgTixOy shownin Fig.2.Theedgeofthenanotubeswas furtherenlarged,asshownin Fig.4B.Thenanostructureswithdifferent latticefringesarefoundandformtheheterojunctionsonthesurfaceof thenanotube.Theobservationresultsarefurthermagnifiedattheselectedtworectangularareas. Fig.4Ccorrespondstotherectangular
areaI.ThelatticefringesofTiO2(R),MgTiO3 andMgTi2O5 canbe observedwiththevaluesof0.29,0.27and0.33nm,respectively. Furthermore,thelatticefringesofTiO2(A),TiO2(R),MgTiO3 and MgTi2O5 canbeclearlyobservedcorrespondingtotherectangulararea II,asshownin Fig.4D.Combiningwiththephysicalcharacterization resultsshownin Figs.1–4,itcanbeprovedthatMgTixOy issuccessfully coatedonthesurfaceofTiO2 NTAs,andthemultiphaseheterojunctions areformed.
Fig.5 showstheUV–visabsorptionspectraoftheTiO2(A),TiO2(A)/ TiO2(R)andTiO2/MgTixOy photoelectrodes.Ascanbefound,thelight absorptionthresholdofTiO2(A)isobservedat382nm.Forthe TiO2(A)/TiO2(R)photoelectrode,thelightabsorptionthresholdhasa slightredshiftcomparedwiththatofTiO2(A),whichmaybecausedby theformationofTiO2(R)withanarrowerbandgap.Inadditionto havingthesameabsorptionthresholdasTiO2(A)/TiO2(R),theTiO2/ MgTixOy photoelectrodehasanadditionalabsorptionthresholdatapproximately330nm,indicatingthesuccessfulsynthesisofMgTixOy on thesurfaceofTiO2 NTAs.
ThePECCPperformanceofthepreparedphotoelectrodesarecharacterizedbymeasuringthephotoinducedcurrentdensitiesandthe photoinducedpotentialdrops,andtheresultsareshownin Fig.6 Fig.6Ashowsthevariationsinthegalvaniccurrentdensitiesbetween the304SSelectrodeandthepreparedphotoelectrodesin3.5wt%NaCl solutionunderintermittentsimulatedsolarlightillumination.Positive excitationcurrentdensitiesareobtainedunderlightillumination,indicatingthatthephotoinducedelectronsgeneratedbythephotoelectrodestransfertothecoupled304SSelectrodeandprovidethePECCP forit.Thephotoinducedcurrentdensityofapreparedphotoelectrode, whichisprovidedforprotectingthecoupledmetallicelectrode,isobtainedbysubtractingthestablegalvaniccurrentdensitybetweenthe protectedmetalelectrodeandthephotoelectrodeinthedarkfromthat underlightillumination.Asshownin Fig.6A,thephotoinducedcurrent densityofTiO2(A)is9μA·cm−2 andthatofTiO2(A)/TiO2(R)is23 μA·cm−2.While,thatofTiO2/MgTixOy is49μA·cm−2,whichis5.4and 2.1timesofthatofTiO2(A)andTiO2(A)/TiO2(R).Theresultsshownin Fig.6AindicatethatTiO2/MgTixOy canproducethemostphotogeneratedelectronsandcanprovidethemostelectronsneededforthe cathodicprotectionofthecoupled304SS. Fig.6Bshowsthevariations ofthemixedpotentialofthe304SSelectrodecoupledwiththepreparedphotoelectrodesin3.5wt%NaClsolutionunderintermittentsimulatedsolarlightillumination.Themixedpotentialsofthe304SS electrodecoupledwiththepreparedphotoelectrodesimmediatelyshift tonegativedirectiononcethelightisswitchedon,demonstratingthat thepreparedphotoelectrodescanprovidethePECCPforthecoupled 304SSelectrode.Thephotoinducedmixedpotentialdropisthemixed potentialofthe304SSelectrodecoupledwiththepreparedphotoelectrodeunderlightilluminationminusthatinthedark.Asshownin Fig.6B,thephotoinducedmixedpotentialdropsofTiO2(A),TiO2(A)/ TiO2(R)andTiO2/MgTixOy are-230,-250and−320mV,respectively. TheTiO2/MgTixOy exhibitsthemaximumphotoinducedmixedpotentialdrop,indicatingitsexcellentPECCPperformance.Thephotoinducedmixedpotentialdropresultsshownin Fig.6Baresimilartothe photoinducedcurrentdensityresultsshownin Fig.6A,andbothof themprovethatTiO2/MgTixOy hasthebestPECCPperformance. Meanwhile,in Fig.6B,thestabilityofthepreparedphotoelectrodesis evaluatedbymeasuringthepotentialvariationsduringthelongdurationoflightillumination.Asshownin Fig.6B,themixedpotentialof the304SSelectrodecoupledwiththepreparedphotoelectrodesis
Fig.6. (A)Thevariationsinthegalvaniccurrentdensitiesbetweenthe304SSelectrodeandthepreparedphotoelectrodesand(B)thevariationofthemixed potentialsofthegalvaniccoupleofthe304SSelectrodeandthepreparedphotoelectrodesin3.5wt%NaClsolutionunderintermittentsimulatedsolarillumination.
stabilizedfor900sinthedark.Subsequently,thelightisswitchedon andilluminatedfor2honthephotoelectrodes.Themixedpotentialsof the304SSelectrodecoupledwiththeTiO2(A)andTiO2(A)/TiO2(R) photoelectrodeschangefrom-0.37and-0.42Vto-0.33and-0.39V, showingaslightdecreaseintheirPECCPperformanceduringthelight illumination.However,forTiO2/MgTixOy,themixedpotentialofthe 304SSelectrodecoupledwiththeTiO2/MgTixOy photoelectrodedoes nothavesignificantchangeduringthe2-hlightillumination.Theresultsshownin Fig.6Bindicatethatthereisnoobviousattenuationof thePECCPperformanceoftheTiO2/MgTixOy photoelectrodeduringthe longdurationoflightillumination,demonstratingthattheTiO2/ MgTixOy photoelectrodepossesseshighstabilityandhasgoodapplicationprospectsinthefieldofPECCP.
Thepolarizationcurvesof304SSelectrodeand304SSelectrode coupledwiththeTiO2,TiO2(A)/TiO2 andTiO2/MgTixOy photoelectrodesintheabsenceandpresenceofsimulatedsolarlightillumination areshownin Fig.7.Thecorrosionpotentialofthe304SSwasabout −154mV(vs.Ag/AgCl)beforecouplingwiththepreparedphotoelectrodesin3.5wt%NaClsolution,however,thoseof304SSpositivelyshiftto-102,-101and−113mVaftercouplingwithTiO2, TiO2(A)/TiO2 andTiO2/MgTixOy photoelectrodesinthedark,respectively.Undersimulatedsolarlightillumination,significantnegative potentialshiftsareobtainedfor304SScouplingwithdifferentphotoelectrodes,indicatingthegoodPECCPperformanceoftheprepared photoelectrodes.Underthesimulatedsolarlightillumination,thecorrosionpotentialsof304SScouplingwithTiO2,TiO2(A)/TiO2 andTiO2/ MgTixOy are-353,-426and−486mV,respectively.Theseresultsarein
Fig.7. Thepolarizationcurvesofthe304SSelectrodeandthe304SSelectrode coupledwiththeTiO2,TiO2(A)/TiO2 andTiO2/MgTixOy photoelectrodesinthe absenceandpresenceofsimulatedsolarlightillumination.
goodagreementwiththepreviousPECCPtestsshownin Fig.6,which furtherindicatestheexcellentPECCPperformanceoftheTiO2/MgTixOy photoelectrode.
SKPtechnologyisusedtofurthercharacterizethecapabilityofthe electronsescapingfromthesurfaceofthepreparedphotoelectrodes. ThesurfaceWF,whichreferstotheminimumenergyrequiredtoescape anelectronfromthesurfaceofthephotoelectrode,canbeeasilycalculatedfromthesurfacepotentialofthephotoelectrodesmeasuredby SKP[68,69].ThesmallertheWFis,theeasieritisforelectrontoescape,i.e.forelectronstoflowoutofthephotoelectrodeandparticipate inthereactions[70]. Fig.8Adisplaysthesurfacepotentialdistributions ofdifferentphotoelectrodesmeasuredbySKP.Ascanbefoundin Fig.8A,thepotentialfluctuationofthepreparedindividualphotoelectrodeissmallerthan±50mVaroundthemedianvalue,suggesting thatthesurfaceofwhichisfairlyflatanduniform.Bycomparingthe surfacepotentialsofTiO2(A),TiO2(A)/TiO2(R)andTiO2/MgTixOy photoelectrodes,TiO2(A)hasthemostpositivesurfacepotentialdistributionwithanaveragevalueofapproximately990mV.TiO2(A)/ TiO2(R)hasasurfacepotentialofapproximately830mV.While,TiO2/ MgTixOy hasthemostnegativepotentialofapproximately690mV, whichis300and140mVlowerthanthoseofTiO2(A)andTiO2(A)/ TiO2(R),respectively. Fig.8BshowsthesurfaceWFsoftheprepared photoelectrodesobtainedfromtheresultsshownin Fig.8A.Thesurface WFsofTiO2(A),TiO2(A)/TiO2(R)andTiO2/MgTixOy photoelectrodes are5.54,5.38and5.24eV,respectively.Thelowestsurfacepotential distributionandsurfaceWFofTiO2/MgTixOy indicatethatthemultiphaseheterojunctionscaneasilyexcitetheelectronsandtransferthem tothesurfaceof304SStoprovideeffectiveprotectionforit.
Fig.9AshowsthevariationsinthecurrentdensitiesoftheTiO2(A), TiO2(A)/TiO2(R)andTiO2/MgTixOy photoelectrodesin0.1MNa2SO4 solutionunderintermittentsimulatedsolarillumination.Asshownin Fig.9A,allofthepreparedphotoelectrodesshowspositiveexcitation currentdensitiesunderlightillumination.Thephotoinducedcurrent densityoftheTiO2(A)photoelectrodeis0.12mA cm−2,andthatofthe TiO2(A)/TiO2(R)is0.16mA cm−2.However,thephotoinducedcurrent densityoftheTiO2/MgTixOy photoelectrodeis0.29mA⋅ cm−2,whichis approximately2.4timesand1.8timesoftheTiO2(A)andTiO2(A)/ TiO2(R)photoelectrodes,respectively.Thegenerationofphotoinduced currentisakeyfactorthataffectsthePECCPperformanceofthephotoelectrode.TheTiO2/MgTixOy photoelectrodehasthehighestphotoinducedcurrentdensity,indicatingthatithasthebestPECCPperformance.
Theelectrochemicalimpedancespectroscopyandthephotoluminescencespectroscopyofthepreparedphotoelectrodeswere measuredtoevaluatethephotogeneratedchargecarriermigration abilityandtherecombinationabilityofthephotogeneratedelectrons andholes.Therelevantresultsareshownin Figs.9Band9C.Asshown in Fig.9B,theTiO2/MgTixOy photoelectrodehasthesmallestsurface resistancethanTiO2(A)andTiO2(A)/TiO2(R),indicatingthattheTiO2/ MgTixOy hasthefastestelectronmobility.Inaddition,asshownin
Fig.8. (A)ThesurfacepotentialdistributionsofTiO2(A),TiO2(A)/TiO2(R)andTiO2/MgTixOy photoelectrodesmeasuredbySKPtechnique;and(B)thesurfaceWFs ofthepreparedphotoelectrodestransformedfromtheSKPresultsin Fig.8A.
Fig.9. (A)ThevariationsinthecurrentdensitiesoftheTiO2(A),TiO2(A)/TiO2(R)andTiO2/MgTixOy photoelectrodesunderintermittentlightillumination,(B) Electrochemicalimpedancespectra(EIS)and(C)photoluminescencespectraoftheTiO2(A),TiO2(A)/TiO2(R)andTiO2/MgTixOy photoelectrodes.
Fig.10. SEMimageoftheTiO2/MgTixOy photoelectrodeafterthePECCPtestandtheXRDandFT-IRspectraoftheTiO2/MgTixOy photoelectrodebeforeandafter thePECCPtests.
Fig.11. TheproposedtransfermechanismofthephotogeneratedchargecarriersandtheschematicdiagramofthePECCPoftheTiO2/MgTixOy multiphaseheterojunctions.
Fig.9C,thephotoluminescenceintensityofTiO2/MgTixOy hasasignificantdecreasecomparedwiththatofTiO2(A)andTiO2(A)/TiO2(R), indicatingthatthesecondaryrecombinationofthephotogenerated electronsandholeissignificantlyinhibitedfortheTiO2/MgTixOy photoelectrode.TheseresultsdisplaythattheTiO2/MgTixOy photoelectrodecanrealizethefasttransmissionofthephotogeneratedcharge carriers,caneffectivelyinhibitthesecondaryrecombinationofthe photogeneratedelectronsandholes,andcangeneratethehighest photoinducedcurrentcomparedwiththeTiO2(A)andTiO2(A)/TiO2(R) photoelectrodes,therefore,achievetheexcellentPECCPperformanceof theTiO2/MgTixOy photoelectrode.
TheSEMimageofTiO2/MgTixOy afterthePECCPtestsandtheXRD andFT-IRanalysesoftheTiO2/MgTixOy photoelectrodebeforeand afterthePECCPtestsareshownin Fig.10.Asshownin Fig.10A,the micromorphologyofTiO2/MgTixOy doesnothavesignificantchanges afterthePECCPtests.Furthermore,theXRDandFT-IRcurvesofthe TiO2/MgTixOy photoelectrodeafterPECCPtestsarehighlyconsistent withthosebeforePECCPtests,asshownin Fig.10BandC.Theseresults demonstratethatTiO2/MgTixOy photoelectrodehasahighstabilityin theprocessofPECCP.
Fig.11 showstheproposedtransferprocessesofthephotogenerated chargecarriersintheTiO2/MgTixOy multiphaseheterojunctionsandits PECCPmechanism.TheTiO2/MgTixOy photoelectrodehassignificantly enhancedphotogeneratedcurrentdensityandphotoinducedmixedpotentialdrop,indicatingthattheenergybandstructuresofTiO2 and MgTixOy matchwellandtheTiO2/MgTixOy multiphaseheterojunctions withagoodenergybandalignmentgradientareformed.Thephotogeneratedchargecarrierscanmigratedirectionallyundertheactionofthe multiphaseheterojunctions,makingthephotogeneratedchargecarriersbe welltransferred.ItwasreportedthatthebandgapsofMgTiO3,MgTi2O5, TiO2(R)andTiO2(A)are3.7eV,3.4eV,3.0eVand3.2eV,respectively, andtheconductionbandpotentialsofthemarearrangedinthefollowing order:MgTiO3 <MgTi2O5 <TiO2(R)<TiO2(A)[40,54,71,72].ThephotogeneratedelectronscanbetransferredtoTiO2(A)throughtheconductionbandgradientformedbytheTiO2/MgTixOy multiphaseheterojunctions,andeventuallybetransferredto304SSthroughtheTisubstrateto provideeffectiveprotectionforit.Meanwhile,thephotogeneratedholes willbefurthertransferredtotheTiO2/MgTixOy multiphaseheterojunctionsundertheactionoftheheterojunctionelectricfield,andthecorrespondingoxidationreactionswilloccurwiththeelectrolyte.Furthermore, thefabricationoftheTiO2/MgTixOy multiphaseheterojunctionsgreatly increasestheseparationefficiencyofthephotogeneratedchargecarriers andeffectivelysuppressestherecombinationofthephotogeneratedelectronsandholes.Inthisway,alargenumberofphotogeneratedelectrons willcontinuouslyflowfromtheTiO2/MgTixOy photoelectrodetothe coupled304SSelectrodeunderlightillumination,thussignificantlyenhancingthelong-termPECCPperformance.
4.Conclusions
Inthepresentpaper,ahighlyefficientandstableTiO2/MgTixOy multiphase-heterojunctionfilmwaspreparedanditsPECCPperformancefor304SSwasstudied.MgTixOy areuniformlydispersedonthe surfaceofTiO2 NTAs.MgTiO3,MgTi2O5,TiO2(R)andTiO2(A)contact wellandformamultiphase-heterojunctionsystem.Thephotoinduced currentdensityoftheTiO2/MgTixOy multiphase-heterojunctionfilmis 49μA·cm−2 underthesimulatedsolarlightillumination,whichis5.4 and2.1timesofthatofTiO2(A)andTiO2(A)/TiO2(R).ThephotoinducedmixedpotentialdropsoftheTiO2/MgTixOy multiphase-heterojunctionfilmis−320mV,while,thoseofTiO2(A)andTiO2(A)/ TiO2(R)are-230and-250mV,respectively.Meanwhile,theTiO2/ MgTixOy multiphase-heterojunctionfilmexhibitsextremelyhigh PECCPstability.Duringthe2hoflightillumination,thePECCPofthe TiO2/MgTixOy multiphase-heterojunctionfilmshowsnoweakening trend.ThelowersurfaceWFmakestheTiO2/MgTixOy multiphaseheterojunctionfilmbemucheasiertoescapeelectrons,whichisofgreat
benefittotheimprovementofthePECCPperformanceoftheTiO2/ MgTixOy multiphaseheterojunctions.Furthermore,thegoodPECCP performanceoftheTiO2/MgTixOy multiphaseheterojunctionsisattributedtotheestablishmentofmultiphaseheterojunctionsystem, whicheffectivelyinhibitsthesecondaryrecombinationofthephotogeneratedelectronsandholes,andacceleratesthemigrationofthe photogeneratedchargecarriers.SKPtechnologyhasbeenusedforthe firsttimetostudythePECCPperformanceoftheTiO2/MgTixOy multiphase-heterojunctionfilm.TheintroductionofSKPtechnologyinto thePECCPresearchhasgreatlyenrichedthetestmethodsinthefieldof PECCP.Atthesametime,theestablishmentofmultiphaseheterojunctionsprovidesanewideaforthedesignofthePECCPfilm.
Dataavailability
Alldataincludedinthisstudyareavailableuponrequestbycontact withthecorrespondingauthors.
DeclarationofCompetingInterest
None.
Acknowledgements
ThisworkwasfinanciallysupportedbytheNationalNaturalScience FoundationofChina(GrantNos.41576114,41676069),KeyResearch andDevelopmentProgramofShandongProvince(GrantNos. 2019GHY112066,2019GHY112085),StateKeyLaboratoryforMarine CorrosionandProtection,LuoyangShipMaterialResearchInstitute, China(ProjectNo.614290101011703),andQingdaoInnovative LeadingTalentFoundation(GrantNo.15-10-3-15-(39)-zch).
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