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CorrosionScience

journalhomepage: www.elsevier.com/locate/corsci

High-eﬃciencyphotoelectrochemicalcathodicprotectionperformanceof theTiO2/AgInSe2/In2Se3 multijunctionnanosheetarray

XuhongJianga,b,d,1,MengmengSuna,b,c,1,ZhuoyuanChena,b,c,*,JiangpingJinga,b,c, ChangFenga,b,d

a KeyLaboratoryofMarineEnvironmentalCorrosionandBio-fouling,InstituteofOceanology,ChineseAcademyofSciences,7NanhaiRoad,Qingdao,266071,China

b CenterforOceanMega-Science,ChineseAcademyofSciences,7NanhaiRoad,Qingdao,266071,China

c OpenStudioforMarineCorrosionandProtection,PilotNationalLaboratoryforMarineScienceandTechnology(Qingdao),No.1WenhaiRoad,Qingdao,266237,China

d UniversityofChineseAcademyofSciences,19(Jia)YuquanRoad,Beijing,100049,China

ARTICLEINFO

Keywords:

A.AgInSe2/In2Se3 nanoparticles

A.Multijunctions

A.TiO2 nanosheetarray

C.Photoelectrochemicalconversion performance

C.Photoelectrochemicalcathodicprotection performance

1.Introduction

ABSTRACT

Optimizationofmultijunctionphotoelectricconversionmaterialswithamuchnegativebandpotentialisvery importantforimprovingthephotoelectricconversionandphotoelectrochemicalcathodicprotectionperformancebecausethewell-matchedmultijunctioncanassistthefasttransportofphotogeneratedelectrons.Inthis paper,a “green” AgInSe2/In2Se3 sensitizedTiO2 nanosheetarray(NSA)photoanodewasprepared.Verticallygrowntwo-dimensionalTiO2 NSAwiththemultijunctionofTiO2 NSA/AgInSe2(7)/In2Se3(3)improvestheseparationeﬃciencyandthetransferofphotoinducedchargecarrierscomparedwithsingleAgInSe2 sensitized TiO2 NSA.TheTiO2 NSA/AgInSe2/In2Se3 photoanodeachieveshighlyeﬃcientphotoelectrochemicalcathodic protectionperformancefor316LSSinNaClsolutionunderAM1.5lightillumination.

Itisvitaltocontrolcorrosionforthebenefitofmankindsince corrosioncancausehugeeconomiclossesandcatastrophicaccidents [1].Amongallofthecorrosionprotectionmethods,thephotoelectrochemicalcathodicprotection(PECCP)hasattractedwideattentions owingtoitsenergy-saving,environmentallyfriendly,andeconomical virtues,whichmainlyutilizesthephotoelectricconversioneffectof semiconductorstoconvertsolarenergyintoelectricalenergyandthen transportsthephotogeneratedelectronstothecoupledmetals,thus achievingthecathodicprotectioneffect[2,3].Thekeyofthistechnologyistochoosesuitablesemiconductormaterialsandtakeadvantageoftheirexcellentphotoelectrochemical(PEC)conversion properties.TiO2 hasbeenprovedtobethemostpromisingsemiconductormaterialsduetoitsappropriateelectronicbandstructure, environmentalfriendliness,highphotostabilityandlowcost,playinga centralrolein fieldsofphotovoltaiccells[4,5]aswellasphotocatalytic pollutantsdegradation[6].However,thebandgapofTiO2 is3.2eV, whichlimitsitslightabsorptiononlytotheultravioletregion[7],and alsothephotogeneratedcarriersareeasytoberecombined.Recent researcheshavebeenfocusingonovercomingtheabovedefects.Forthe

highcarrierrecombinationrate,aneffectivemethodistoconstruct directelectricalpathwaysinordertotransportthephotoinduced electronsrapidlyandreducethegrainboundariessoastoenhancethe photogeneratedcarriercollectionefficiency.Differentmorphologiesof well-organizedTiO2 nanostructures,suchasone-dimensional(1D)nanorods[8,9]andnanotubes[10–14],two-dimensional(2D)nanosheet [15,16]andthree-dimensional(3D)nanotree,nanoforestandnanolawn [17–21]etc.havebeendeveloped,andtheelectron-transportefficiency andPECconversionperformancehavebeenimprovedtodifferentextents.Especially,the2DTiO2 nanosheetarray(NSA)isofsignificance forimprovingtheperformanceofthePECsystem[22–26],whichcan providesufficientnanoparticles(NPs)loadingandlightharvesting area,improvetheelectrontransportandchargeseparationefficiency, thusresultinachievingexcellentPECproperties.

Apartfrommorphologymodiﬁcation,theconstructionofsuitable semiconductorheterojunctionalsoplaysasubstantialroleonreducing therecombinationrateofthephotogeneratedelectron-holepairsby eﬀectivelyseparatingthem.Andmoreimportantly,couplingTiO2 with visiblelightresponsivenarrowbandgapsemiconductormaterialscan widenthelightresponsetovisiblelightregion,andgreatlyimprovethe PECconversionperformance[27–31].Comparedwithbulk

⁎ Correspondingauthorat:KeyLaboratoryofMarineEnvironmentalCorrosionandBio-fouling,InstituteofOceanology,ChineseAcademyofSciences,7Nanhai Road,Qingdao,266071,China.

E-mailaddress: zychen@qdio.ac.cn (Z.Chen).

1 XuhongJiangandMengmengSunaretheco-ﬁrstauthors.

https://doi.org/10.1016/j.corsci.2020.108901

Received28April2020;Receivedinrevisedform15July2020;Accepted26July2020

Availableonline29July2020

semiconductormaterials,NPsaremoresufficientlyinheterojunction withmatchedenergybandsandenergylevels,andhasbeenextensively investigated[32–35].And,unliketheCdandPbelementswhose toxicityhashinderedtheirfurtherapplications,theI–III–VI2-group(I: Cu/Ag,III:Ga/In,VI:S/Se/Te)ternarychalcogenideshaveattracted moreattentionbecauseoftheirnarrowbandgaps,solarenergy-absorbingability,lightstabilityandlowtoxicity.CuInS2 [36,37],AgInS2 [38],CuInSe2 [39,40],AgInSe2 [41–43]etc.arebecomingalternatives ofthePb-andCd-basedsemiconductorsforPECconversionapplicationslyingintheirhighabsorptioncoefficientandabilitytoincrease photoconversionefficiency[44].Recently,novelnano-photocatalystof TiO2-decoratedAgInSe2 hasbeensynthesizedandwasappliedtodye degradation,whichshowedeffectivedegradationefficiencyandstability[41].ThebandgapofAgInSe2 isaround1.20eV[41],closetothe optimalbandgapforabsorbingthesolarspectrum.Thiscouldbean idealPECmaterialtosubstitutecontaminatedsubstancesintheareasof PECconversionandPECCP.

Besides,forheterojunctionstructure,attheinterfaceofsemiconductorcomposite,unmatchedenergybandarrangementandhigh surfacestatedensitymayexist,whichresultsinachanceofcharge recombinationattheinterface[45].Therefore,interfacialoptimization oftheheterojunctionisalsonecessarytofurtherimprovethePEC conversionefficiency[46].Tosolvethisproblem,amultijunction heterostructurecompositewithwell-matchedbandpotentialscanbe constructed,thenthephotogeneratedelectron-holepairswillbeseparatedandtransferredmoreefficiently[47–49].Changetal.fabricateda seriesofTiO2/CuInS2,TiO2/Cu2S/CuInS2,TiO2/Cu2Se/CuInS2,TiO2/ In2S3/CuInS2,TiO2/In2Se3/CuInS2 configurations,whichuseCu2S, Cu2Se,In2S3,In2Se3 asthebufferlayersofthebulkmaterial,andthe PECconversionefficiencieswere0.58%,1.06%,1.22%,0.89%and 1.35%,respectively.Thisresultandotherreportshavedemonstrated thatamongallthecurrentlystudied “green” co-sensitizers,suchas Cu2S,Cu2Se,In2S3,In2Se3,InP,ZnS,ZnSeandZnTe,In2Se3 hasbeen provedtobethemosteffectiveonetoactasacompositelayerofthe multijunctionowingtoitsbetterPECperformance[50–54].Therefore, thestudiesfordesigningeffective,energybandwell-matchedmultijunctionsystems,especiallycombiningwithassistlayers,arecriticalto furtherpromotethecarriertransmissionefficiencyattheheterointerfaceandthusenhancethePECactivityofphotoanodes.

Inthepresentwork,anAgInSe2/In2Se3 co-sensitizedTiO2 NSA multijunctionphotoanodewasconstructed,andthePECconversionand PECCPperformancewereexploredinsimulatedseawater(3.5wt% NaClsolution)undersimulatedsunlight(AM1.5light)illumination. Thisconditiondoesnotcontainanyaddedman-madehole-scavengers andismorecloselytotherealisticmarineenvironment.ThePECperformanceofthepreparedTiO2 NSA/AgInSe2/In2Se3 wasinvestigated underintermittentAM1.5lightillumination.Besides,theenergyband potentialofthen-typesemiconductorofTiO2/AgInSe2/In2Se3 multijunctioniscomparativelynegative,therefore,itsapplicationinthe PECCPfor316Lstainlesssteel(SS)wasalsoexplored.And,ahighly eﬃcientPECCPperformanceinNaClsolutionunderAM1.5lightilluminationwasobtained.Simultaneously,therelationshipbetweenthe multijunctionstructureandthePECandPECCPperformanceaswellas thecorrespondingmechanismwasanalyzed.Theresultsobtainedin

thisworkwillcontributefordesigningeﬃcientphoto-functional coatingtodeveloptheexpandedPECapplicationsinthePECCP ﬁeld.

2.Experimental

2.1.PreparationofTiO2 NSAandTiO2 NSA/AgInSe2/In2Se3 photoanodes Allreagentsusedinthisstudywereofanalyticalgradesandwere directlyusedwithoutfurthertreatments.A fluorine-dopedtinoxide (FTO)glass(1×2cm2)wasultrasonicallycleanedwithacetoneand ethanolfor10min,respectively,andthendriedinairbeforegrowing TiO2 NSAonit.The2DTiO2 NSA filmwaspreparedonthecleanedFTO conductiveglassbyasimpleone-stephydrothermalmethod.Inbrief, 15mLconcentratedhydrochloricacid(massfraction36.5–38%)and 15mLdeionizedwaterweremixedtogetherandstirredfor5min.Then, 0.5mLtitaniumbutoxide(TBT)wasaddedtothepreparedHClsolution.Afterstirringforanother5min,0.25gammoniumhexafluorotitanate((NH4)2TiF6)wasaddedandfurtherstirredfor5min. Then,themixturewastransferredtoa50mLTeflon-linedstainlesssteel autoclave.Finally,theFTOsubstratewasplacedatanangleagainstthe walloftheTeflonlinerwiththeconductivesidefacingdown.The hydrothermalsynthesiswasconductedat170,180and200°Cfor12h inanelectricoven,respectively.Afterthat,theautoclavewascooled down,andtheFTOsubstratewastakenoutandrinsedwithdeionized waterthoroughlyanddriedintheair.

TheAgInSe2 NPsweredepositedontotheTiO2 substratebysuccessiveionlayerabsorptionandreaction(SILAR)technique.Typically, theobtainedTiO2 NSA filmsubstratewassuccessivelyimmersedinto threedifferentaqueoussolutionsfor5minforfourtimes, firstlyina 0.02MAgNO3 aqueoussolution(cationprecursor)andthenina0.02M Na2SeSO3 aqueoussolution(anionprecursor),thirdlyina0.02M In2(SO4)3 aqueoussolution(cationprecursor),and finallyina0.02M Na2SeSO3 aqueoussolutionagain.Betweeneachimmersionstep,the sampleswererinsedthoroughlywithdeionizedwatertoremovethe excessweakly-boundedions.TheNa2SeSO3 aqueoussolutionwas synthesizedbydissolvingelementalseleniumpowder(99.99%)inan aqueoussodiumsulfitesolutionat90°C,adjustingpHto12–14with sodiumhydroxide,asadoptedfrompreviousliterature[55,56].The Se2+ solutionpreparationmethodinthepresentpaperismorestable thanthosepreparedbyreducingthealcoholsolutionofseleniumdioxidewithsodiumborohydride[50,57].Such4-timesimmersionprocedureistermedasonecycleofthedepositionprocess,andseveral timesofthisimmersioncyclewererepeateduntilspecifiedamountof AgInSe2 NPswascompounded.Byadjustingthenumberofdeposition cycles,theamountofdepositedAgInSe2 NPscanbecontrolled.

Afterthat,theIn2Se3 layerwasincorporatedontheTiO2 NSA/ AgInSe2 photoanodebyanotherSILARdeposition.TheIn2Se3 depositioncycleissimilartotheAgInSe2 depositionprocedurebutwithout immersingintotheAgNO3 aqueoussolution.Finally,theobtained samplewasdenotedasTiO2 NSA/AgInSe2(m)/In2Se3(n),wheremand nrepresentthenumberofthedepositioncyclesofAgInSe2 andIn2Se3 NPs,respectively.ThepreparationprocedureoftheTiO2/AgInSe2/ In2Se3 multijunctionnanosheetarrayphotoanodeisschematicallyillustratedin Scheme1

The316LSSelectrodewaspreparedbyembeddinga316LSS squareinepoxyresin,exposinganareaof10×10mm2 fortesting. Afterthat,the316LSSelectrodewassuccessivelywetgroundwithSiC paperto2000grits,andthenultrasonicallycleanedinanalyticalgrade ethanolfor5minanddried.

2.2.Characterizations

Thesurfacemorphologyandthemicrostructureoftheprepared photoanodeswereexaminedbya field-emissionscanningelectronmicroscope(SEM)(ULTRA55,ZeissCompany,Germany)andahighresolutiontransmissionelectronmicroscope(HRTEM,TecnaiG20,FEI Company,USA).Thecrystallinestructuresofthepreparedmaterials werecharacterizedusingUltimaIVX-RayDiffractometer(XRD) (RigakuCo.,Tokyo,Japan)withCuKα radiation.Fouriertransform infrared(FTIR)spectraweretestedusingaFouriertransforminfrared spectroscopy(FTIR,Thermo-Nicolet8700,ThermoElectronScientific Inc.,USA)atroomtemperature.Theelementalcompositionsandthe bondinginformationwereidentifiedusingX-rayphotoelectronspectroscopy(XPS)onanX-rayspectrometer(ModelThermoESCALAB25 XI,AlKα,hν =1486.6eV,MonoX-raysource).Theopticalabsorption propertiesweremeasuredonaUV/Visspectrophotometer(SHIMADZU UV-2600,Japan).Thephotoluminescence(PL)emissionintensitiesof thepreparedmaterialswererecordedbya fluorescencespectrometer (FluoroMax-4,HORIBAJobinYvon,France).

2.3.ElectrochemicalandPECperformancemeasurements

ThePECperformanceofthepreparedmaterialswasdeterminedby i-Vcurvesandopencircuitpotential(OCP)variationsaswellasthe photoinducedcurrentdensitiesandpotentiodynamicpolarization curvesunderintermittentsimulatedsolarlight(AM1.5)illumination. Thei-Vcurveswereconductedatthepotentialrangeof 0.8Vto 0.4V.Thepotentiodynamicpolarizationcurvesweremeasuredfrom 250to250mV(vs.OCP).Theelectrochemicalimpedancespectroscopy(EIS)testswereperformedinthedarkatopencircuitpotential overthefrequencyrangebetween105 and10 2 Hz,withanACvoltage magnitudeof5mV.Mott-Schottkyplotswereconductedinthedark withthepotentialrangeof-0.8Vto0.4Vandthefrequencyof1000Hz andtheACvoltagemagnitudeof10mV.Theabovementionedi-V,EIS, andMott-Schottkymeasurementswereallconductedin0.1MNa2SO4, whiletheOCP,photoinducedcurrentdensityandpolarizationcurves weremeasuredin3.5wt%NaClsolution.Allofthemeasurementswere performedusingaCHI660Delectrochemicalworkstation(Shanghai ChenhuaInstrumentCo.,Ltd.),byemployingaPtelectrodeasthe counterelectrodeandanAg/AgCl(saturatedKCl)asthereference electrode,respectively.

2.4.PECCPperformancemeasurements

Thephotoinducedvariationsofthecurrentdensitiesofthegalvanic couplingbetweenthe316LSSelectrodeandthephotoanodesandthe mixedpotentialsofthe316LSSelectrodecoupledwiththephotoanodeswereexaminedtostudythePECCPperformanceoftheprepared samples.Allmeasurementswereperformedin3.5wt%NaClsolution underintermittentsimulatedAM1.5solarlightilluminationusingthe CHI660Delectrochemicalworkstation(ShanghaiChenhuaInstrument Co.,Ltd.).Theexperimentalarrangementisschematicallyillustratedin Fig.1,whichissimilartothoselistedinpreviousreports.Byusingthis experimentalsetup,thecurrentdensityandpotentialcanbemeasured simultaneously[2,58].Thegalvaniccurrentdensitiesbetweenthe photoanodeandthe316LSSelectrodeweremeasuredwithoutany appliedpolarization.A300-WXearclamp(PLS-SXE300,BeijingPerfectLightCo.Ltd.,China)wasusedasthelightsourcetogenerate AM1.5lightwiththepowerenergydensityof100mWcm 2

3.Resultsanddiscussion

3.1.MorphologyandstructureanalysesofthepreparedAgInSe2/In2Se3 decoratedTiO2 NSA

Inordertopreparethephotoanodeswithbestperformance,the influenceofhydrothermaltemperatureonthemorphologiesandperformanceofthepreparedphotoanodeswasperformedandtheresults areshownin Fig.2.Withtheincreaseofhydrothermaltemperature from170°Cto200°C,theshapeofeachnanosheetisbarelychanged, however,thedensity,sizeandthicknessoftheTiO2 NSAincreasewith thehydrothermaltemperature(Fig.2a–c).HigherhydrothermaltemperaturecanpromotethegrowthoftheTiO2 NSA.Meanwhile,the nanosheetswhicharetoosparseortoocrowdedwillnotbenefitthe lightharvestingandtheelectrontransport.Asconfirmedby Fig.2d,the photoinducedcurrentdensityoftheTiO2 NSAsubstratepreparedat 180°Cachievedthehighestvalue,indicatingthattheTiO2 NSApreparedat180°CpossessesthebestPECconversionperformance.As shownin Fig.2b,thepreparedplainTiO2 NSAat180°Cconsistsofa seriesofverticallyarrangedandstaggerednanosheets.Theinsetin Fig.2bshowsthepartialenlargedviewoftheformednanosheets.A well-facetedcrystalstructurecanbeseen,andthejunctionedgeofthe adjacentdifferentcrystallographicplanescanalsobeclearlyseen.The nanosheetsareinterlacedandinterconnectedwitheachother.Theside lengthofthesenanosheetsisapproximately2 μm,andthethicknessofa singlenanosheetisapproximately200nm.Thisorderedstructurewill facilitatethetransportofthephotogeneratedcarriers.Subsequent analyseswerebasedonthephotoanodespreparedundertheoptimal hydrothermaltemperatureof180°C.

XRDpatternswererecordedtoinvestigatethecrystalphasesofthe preparedAgInSe2/In2Se3,AgInSe2,In2Se3 decoratedTiO2 NSAandpure TiO2 NSAsamples,andtheresultsareshownin Fig.3.Thediffraction peakscorrespondingtoanataseTiO2 andFTOglassareclearlyshownin Fig.3.Thediffractionpeaksat2θ =25.3°,37.8°,48.0°,55.1°and62.7° areattributedto(101),(004),(200),(211)and(204)crystalplanesof anataseTiO2 (JPCDSNo.21-1272)[59,60],respectively.Asshownin Fig.3,muchhigherintensityratiosofthediffractionpeakof(004) crystalplanetothoseofotherdiffractionpeaksareobservedincomparisonwiththoseinthestandardpatternofanataseTiO2,demonstratingthatthesynthesizedTiO2 hasahighlyexposed(004)crystal plane.However,nodiffractionpeaksfromAgInSe2 andIn2Se3 aredetectedin Fig.3,indicatingthelowamount,welldispersionand/orlow degreeofcrystallinityofthedepositedAgInSe2 andIn2Se3 inthesynthesizedphotoanodes.SimilarresultsofloadingNPsusingthesame SILARmethodwerealsoreportedinpreviousliterature[38,55,61].

Themorphologiesofthepreparedcompositephotoanodesandthe combinationstatesofAgInSe2 andIn2Se3 ontheTiO2 NSAwereanalyzed,and Fig.4a–eshowtop-viewSEMimagesofthepreparedTiO2 NSA/AgInSe2(7),TiO2 NSA/AgInSe2(7)/In2Se3(3),TiO2 NSA/ AgInSe2(3)/In2Se3(3)andTiO2 NSA/AgInSe2(11)/In2Se3(3)photoanodesandcross-sectional-viewimageofTiO2 NSA/AgInSe2(7)/ In2Se3(3)photoanode,respectively.Asshownin Fig.4a,forTiO2 NSA/ AgInSe2(7),plentyof ﬁneNPsaredistributeduniformlyonthesurface oftheTiO2 NSA.While,withthefurtherdepositionofIn2Se3 onTiO2 NSA/AgInSe2(7),theformedNPsonTiO2 NSA/AgInSe2(7)/In2Se3(3) becomelargerandeasiertobeobserved,asshownin Fig.4b.Thelarger NPsofAgInSe2(7)/In2Se3(3)distributeuniformlyonthesurfaceofTiO2 NSA,andexhibitawellcombinationontothesheets.Thisensuresthe largelight-harvestingofboththenanosheetsandthelargeamountof AgInSe2/In2Se3 NPs.While,in Fig.4c,forTiO2 NSA/AgInSe2(3)/ In2Se3(3),severalscatteredNPsdepositedonthesurfaceoftheTiO2 NSAcanbeseen.ForTiO2 NSA/AgInSe2(11)/In2Se3(3)photoanode (Fig.4d),alotofNPsaredepositedontoTiO2 NSA,someofwhich aggregatetogethertoformlargerparticles.Thiswillhindertheelectron transportprocess.TheamountoftheNPsontheTiO2 NSA/AgInSe2(7)/ In2Se3(3)photoanodeisbetweenthoseoftheTiO2 NSA/AgInSe2(3)/

Fig.1. SchematicillustrationsoftheexperimentalsetupforthePECCPmeasurments:(a)thephotoinducedcurrentdensitybetweenthepreparedphotoanodeandthe 316LSSelectrodeand(b)thephotoinducedmixedpotentialsofthecoupled316LSSelectrodeandthepreparedphotoanode.

Fig.2. SEMimagesoftheTiO2 NSAsubstratespreparedunderdiﬀerenthydrothermalreactiontemperatures:(a)170°C,(b)180°Cand(c)200°C;and(d)the correspondingphotoinducedcurrentdensitiesoftheTiO2 NSAphotoanodespreparedatdiﬀerenttemperatureunderintermittentAM1.5lightillumination.

In2Se3(3)andTiO2 NSA/AgInSe2(11)/In2Se3(3)photoanodes.Onone hand,properamountofNPscanguaranteeenoughlightabsorptionand generatealargestamountofelectrons,ontheotherhand,comparativelysmallsizeofNPscanmakesurethegeneratedelectronstomigraterapidlyattheinterfacesofNPsandnanosheets.Thecross-sectionalviewSEMimageoftheTiO2 NSA/AgInSe2(7)/In2Se3(3)isshown in Fig.4e,thethicknessofthewholeTiO2 NSA ﬁlmisapproximately 2 μm.And,theTiO2 NSA ﬁlmisnearlyperpendiculartotheFTOsubstrate,whichisconsistentwiththetop-viewSEMimageshownin Fig.4b.

Fig.5 showstheSEMimageandthecorrespondingEDSelemental mappingofTiO2 NSA/AgInSe2(7)/In2Se3(3),fromwhichtheTi,O,Ag,

InandSeelementsareclearlyobserved.ThedistributionofTiandO elementsintheEDSmappingisingoodagreementwiththecorrespondingSEMimage.While,theelementsofAg,InandSeareevenly distributed,indicatingthattheAgInSe2(7)/In2Se3(3)NPsaredispersed evenlyonthesurfaceoftheTiO2 NSA.

ThepreparedTiO2 NSA/AgInSe2(7)andTiO2 NSA/AgInSe2(7)/ In2Se3(3)photocatalystswerefurtherinvestigatedbyTEMandHRTEM toconﬁrmthedepositedNPs,andtheresultsareshownin Fig.6 Fig.6a andbaretheTEMimagesofTiO2 NSA/AgInSe2(7)andTiO2 NSA/ AgInSe2(7)/In2Se3(3),respectively.BothimagesillustratetheNPsin thesizeof20 50nmonasheet-likestructure.Thediﬀerencebetween Fig.6aandbisthatsomeclustersareobservedtosurroundtheNPsin

Fig.3. XRDpatternsofthepreparedsamples.

Fig.6b,while,in Fig.6a,theboundariesofnanosheetsandNPsare clearandobvious.FurtherHRTEManalyseshelptodistinguishthe diﬀerentlatticesandtheresultsareshownin Fig.6a1,a2,b1andb2. Fig.6a1anda2in Fig.6 arethecorrespondingHRTEMimagesofTiO2

NSA/AgInSe2(7)sampleinthesquareareasof Fig.6a,whileFigures 6b1and6b2in Fig.6 correspondtoHRTEMimagesofTiO2 NSA/ AgInSe2(7)/In2Se3(3)sampleinthesquareareasof Fig.6b.Thelattices spacingof0.352nmand0.189nmcorrespondtothe(101)and(200) planesofanataseTiO2 (JCPDS12-1272)[62,63],implyingtheformationofanatasecrystalofTiO2.Theobservedfringespacingof0.211nm and0.327nmareconsistentwiththe(220)and(111)latticedistances incubicAgInSe2 (JCPDS65-7084)[42,64,65],conﬁrmingtheprepared crystallinestateoftheAgInSe2 NPs.TheHRTEMresultsshownin Fig.6 conﬁrmthesuccessfuldepositionofAgInSe2 NPsontothesurfaceof TiO2 nanosheets.

ApartfromtheclearlyseenAgInSe2 NPs,theweakcrystallinity layeraroundtheAgInSe2 NPsmightbetheIn2Se3 phase,whichisattributedtothelowcrystallinityofIn2Se3 obtainedviatheSILARdepositionprocess[50,61,66].Moreover,theTEMandHRTEManalyses ofthepreparedTiO2 NSA/In2Se3(10)wasconductedtostudythestate ofIn2Se3 depositedonTiO2 NSAbytheSILARdepositionmethod.High concentrationofIn3+ andSe2 precursorsolutions(tentimes)were employedtodepositedsuﬃcientamountofIn2Se3 ontoTiO2 NSA. Fig.7a,candearetheTEMimagesoftheTiO2 NSA/In2Se3 atlow magniﬁcations, Fig.7b,dandfaretheHRTEMimagesinthecorrespondingsquareareasshownin Fig.7a,cande.FromtheTEMimageof TiO2 NSA/In2Se3(10),theTiO2 nanosheetsseemtobecoatedby something.FromalloftheHRTEMimages,anataseTiO2 nanosheetsare detected,eachnanosheetiscoatedwithalayerontheedge,which

Fig.4. Top-viewSEMimagesof(a)TiO2 NSA/AgInSe2(7),(b)TiO2 NSA/AgInSe2(7)/In2Se3(3),(c)TiO2 NSA/AgInSe2(3)/In2Se3(3)and(d)TiO2 NSA/AgInSe2(11)/ In2Se3(3)photoanodes;Cross-sectional-viewSEMimage(e)ofTiO2 NSA/AgInSe2(7)/In2Se3(3)photoanode.

Fig.5. SEMimageandthecorrespondingEDSelementalmappingresultsoftheTiO2 NSA/AgInSe2(7)/In2Se3(3)photoanode.

couldbeIn2Se3 withalowcrystallinityandobscuredmorphology [38,50,55,61,67].ThisresultisalsoagreedwiththeSEMimageofTiO2 NSA/AgInSe2(7)/In2Se3(3)shownin Fig.4e,inwhichtheedgeofthe TiO2 nanosheetsbecomeobscuredcomparedwiththoseofpureTiO2 NSAin Fig.2b.AstheTEMandHRTEMimagesofTiO2 NSA/

AgInSe2(7)/In2Se3(3)shownin Fig.6b,b1,b2,alargeamountofobscuredparticlesareobserved,whichcouldbetheIn2Se3 particlesdepositedonthesurfaceoftheTiO2 NSAbySILARprocess.TheseIn2Se3 layersactasthecompositelayeroftheAgInSe2 NPssensitizedTiO2 NSA,andtheformedmultijunctionstructurecanhelptofurther

Fig.6. TEMandHRTEMimagesoftheprepared(a)TiO2 NSA/AgInSe2(7)and(b)TiO2 NSA/AgInSe2(7)/In2Se3(3)photoanodes.a1anda2showthecorresponding HRTEMimagesofTiO2 NSA/AgInSe2(7)inthesquareareaof(a);b1andb2showthecorrespondingHRTEMimagesofTiO2 NSA/AgInSe2(7)/In2Se3(3)inthesquare areaof(b).

Fig.7. TEMandHRTEMimagesofthepreparedTiO2 NSA/In2Se3 photoanode;a,c,e:TEMimages;b,d,f:thecorrespondingHRTEMimagesinthesquareareas.

separatethephotoinducedelectronsandholeseﬃciently,andalsothe NSAarchitecturecanpromotethetransferofthephotoinducedelectronsasadirectelectrontransmissionpath.

XPSspectrawereusedtoanalyzethestatesofAg,InandSeinthe preparedTiO2 NSA/AgInSe2(7)/In2Se3(3)photoanode,andtheresults areshownin Fig.8 Fig.8aillustratesthetotalsurveyspectrum,which revealsthepresenceofTi,O,Ag,In,SeaswellasCimpurityfromthe absorptionofCO2 gaseousmoleculesandSifromtheFTOglass.HighresolutionXPSspectraofAg,In,Secoreregionsaregivenin Fig.8b–d. ThebindingenergiesforAg3d5/2 and3d3/2 areobservedat368.3and 374.3eV,respectively,whichareattributedtothemonovalentstateof Ag(Ag+)[68].IntheXPScorelevelspectraofIn3d,twopeaksatthe bindingenergiesof445.3eVand452.9eVareobserved,whichcorrespondtotheIn3d5/2 andIn3d3/2 statesandconﬁrmthepresenceofthe trivalentnatureofIn(In3+)inpreparedTiO2 NSA/AgInSe2(7)/ In2Se3(3)[41].InSeXPScorelevelspectra,abroadpeakat54.3eV correspondstotheSe3dstateandconﬁrmstheexistenceofSe2 inthe preparedTiO2 NSA/AgInSe2(7)/In2Se3(3)[69–73].Allofthedetected Ag+,In3+ andSe2 areinagreementwiththepreviousreportsof AgInSe2 andIn2Se3 nanocrystals[69–73],demonstratingthatAgInSe2 andIn2Se3 weresuccessfullypreparedinTiO2 NSA/AgInSe2(7)/ In2Se3(3).

3.2.Analysesoftheopticalpropertiesofthepreparedphotoanodes

TheopticalpropertiesofthepreparedAgInSe2/In2Se3,AgInSe2 and In2Se3 NPssensitizedTiO2 NSAandpureTiO2 NSAnanostructureswere studiedusingUV–visdiﬀusereﬂectionspectroscopy,asshownin Fig.9a.Owingtothewidebandgapof3.2eV,theTiO2 NSAshowsits fundamentalabsorptionsharpedgerisingat387nmintheUVlight region.ForIn2Se3 sensitizedTiO2 NSAphotoanode,theadsorptionregionisconsistentwiththatofTiO2 NSA.However,fortheAgInSe2 and

AgInSe2/In2Se3 NPssensitizedTiO2 NSA,theabsorptionpropertiesin thevisible-lightregionof400 800nmareextremelyenhanced,which isduetothesensitizationofAgInSe2 andAgInSe2/In2Se3 NPs.And,the absorptionintensityofTiO2 NSA/AgInSe2(7)/In2Se3(3)ishigherthan thatofTiO2 NSA/AgInSe2(7),whichisrelevanttotheadditionalIn2Se3 compositelayer.DuetothenarrowbandgapofAgInSe2/In2Se3 NPs withvisiblelightresponsecapability[41],thelightabsorptionregion ofTiO2 NSA/AgInSe2(7)/In2Se3(3)isbroadenedtovisiblelightregion, whichovercomestheshortcomingofnarrowphotoresponserangeof TiO2

ThePLanalysesoftheTiO2 NSA/AgInSe2(7)/In2Se3(3),TiO2 NSA/ AgInSe2(7)andTiO2 NSAphotoanodeswerealsoconductedtoreveal theefficiencyofchargecarriertrapping,transferandseparationin semiconductors,andtheirPLemissionspectraareshownin Fig.9b.The lowerPLintensitydemonstratesthelowerrecombinationrateofthe photoinducedelectronsandholes,indicatingahigherPECconversion activity[74].ForTiO2 NSA/AgInSe2(7)sample,thePLemissionpeaks at614,688,712,732,764nmarelowerthanthoseofpureTiO2 NSA photoanode.ForTiO2 NSA/AgInSe2(7)/In2Se3(3),thePLintensitiesare furtherreducedcomparedwiththoseofTiO2 NSA/AgInSe2(7).Therefore,theheterojunctionofTiO2 NSA/AgInSe2(7)willimprovetheseparationofphotogeneratedelectronsandholescomparedwithpure TiO2 NSA.And,themultijunctionofTiO2 NSA/AgInSe2(7)/In2Se3(3) willfurthersignificantlyseparateandtransferthephotoinducedelectronsandholesmoreefficientlycomparedwithTiO2 NSA/AgInSe2(7). ThePLresultsconfirmtheimportanceofthemultijunctionofTiO2 NSA/AgInSe2/In2Se3 inpreventingtherecombinationofthephotoinducedcarriers.

3.3.PECconversionperformanceofthepreparedphotoanodes

Thephotoinducedi-VcurvesoftheTiO2 NSA/AgInSe2(7)/In2Se3(3),

Fig.8. (a)XPSsurveyspectrumandhigh-resolutionXPSspectraof(b)Ag3d,(c)In3dand(d)Se3dofthepreparedTiO2 NSA/AgInSe2(7)/In2Se3(3)photoanode.

TiO2 NSA/AgInSe2(7),TiO2 NSA/In2Se3(3)andTiO2 NSAphotoanodes, togetherwiththeTiO2 NSA/AgInSe2/In2Se3 photoanodeswithdiﬀerent AgInSe2 depositioncyclesaredepictedin Fig.10aandb.Amongallof thesamplesinvestigated,theTiO2 NSA/AgInSe2(7)/In2Se3(3)photoanodeexhibitsthehighestphotoinducedcurrentdensity.ThisisattributedtothedepositedIn2Se3 layeraroundAgInSe2 NPsthatfurther improvesthephoto-to-currentconversionthroughthewell-matched energybandandacceleratesthetransferofphotogeneratedelectronsas wellastheseparationofphotoinducedelectron-holesinthesystem. Themultijunctionelectric ﬁeldattheinterfaceofTiO2 NSA/AgInSe2/ In2Se3 facilitatestheseparationofthephotogeneratedcarriersand henceimprovesthePECperformanceofTiO2 NSA/AgInSe2/In2Se3 Fig.10cpresentsthephotoinducedOCPvariationsoftheTiO2 NSA, TiO2 NSA/In2Se3(3),TiO2 NSA/AgInSe2(7),TiO2 NSA/AgInSe2(7)/ In2Se3(3)photoanodesconductedin3.5wt%NaClsolutioninthedark andunderAM1.5lightillumination.Assoonasswitchingonthelight,

theOCPsofthesephotoanodesshiftnegativelyasaresultofthegenerationandaccumulationofthephotoinducedelectronsontheelectrodesurface.Afterswitchingoﬀ thelight,theOCPsofthesephotoanodesshowpositiveshifts.TheTiO2 NSA/AgInSe2(7)/In2Se3(3) photoanodeexhibitsthemaximumphotoinducedpotentialdropof approximately280mV,indicatingthattheTiO2/AgInSe2/In2Se3 multijunctiongreatlyenhancesthePECperformanceunderAM1.5light illumination.ThemorenegativethephotoinducedOCPis,themore negativethequasi-Fermilevelofthephotoanodeis,andthebetter PECCPperformanceis.

ThepotentiodynamicpolarizationcurvesoftheTiO2 NSA,TiO2 NSA/AgInSe2(7),TiO2 NSA/In2Se3(3)andTiO2 NSA/AgInSe2(7)/ In2Se3(3)photoanodesweremeasuredin3.5wt%NaClsolutioninthe absenceandpresenceofAM1.5lightillumination,andtheresultsare shownin Fig.10d.Underlightillumination,theOCPsofthesephotoanodesshifttomorenegativevalues,whichareagreedwiththose

Fig.9. (a)UV/Visdiﬀusereﬂectancespectraofthepreparedsamples,(b)PhotoluminescencespectraofpureTiO2 NSA,AgInSe2 andAgInSe2/In2Se3 NPssensitized TiO2 NSA.

Fig.10. Thephotoinducedvariationsofthei-Vcurvesof(a)theAgInSe2/In2Se3,AgInSe2,In2Se3 NPssensitizedTiO2 NSAphotoanodesandpureTiO2 NSA photoanodeand(b)TiO2 NSA/AgInSe2/In2SephotoanodeswithdiﬀerentAgInSe2 depositioncyclesunderswitchingonandoﬀ theAM1.5lightin0.1MNa2SO4 solution;(c)ThephotoinducedvariationsoftheOCPsoftheAgInSe2/In2Se3,AgInSe2,In2Se3 NPssensitizedTiO2 NSAphotoanodesandpureTiO2 NSAphotoanodein 3.5%NaClsolutionunderAM1.5lightillumination;(d)ThepotentiodynamicpolarizationcurvesoftheTiO2 NSA,TiO2 NSA/AgInSe2(7),TiO2 NSA/In2Se3(3)and TiO2 NSA/AgInSe2(7)/In2Se3(3)photoanodesmeasuredin3.5wt%NaClsolutionintheabsenceandpresenceofAM1.5lightillumination.

observedin Fig.10candareduetothegenerationandaccumulationof photoinducedelectronsonthephotoanodes.Boththeanodicand cathodicpolarizationcurrentdensitiesofthesephotoanodesmeasured underlightilluminationshowsigniﬁcantincreasescomparedwith thoseobtainedinthedark,whicharecausedbytheparticipationofthe photogeneratedholesandelectronsintheanodicandcathodicreactionsoftheelectrochemicalpolarizationprocess.Amongwhich,the TiO2 NSA/AgInSe2(7)/In2Se3(3)photoanodepossessesthehighest anodicpolarizationcurrentdensityandthemostnegativeOCPamong allthepreparedphotoanodes.ThisresultindicatesthatthephotoinducedelectronsandholesgeneratedbytheTiO2 NSA/AgInSe2(7)/ In2Se3(3)photoanodeareseparatedtothegreatestextent,whichmakes ithavethemostphotogeneratedholestoparticipateintheanodicwater oxidationreactionsoftheelectrochemicalpolarizationprocessandthus makesitsanodicpolarizationcurrentdensitybeinggreatlyenhanced. Meanwhile,thelargestnumberofthephotoinducedelectronsaccumulateontheTiO2 NSA/AgInSe2(7)/In2Se3(3)photoanodeduetothe maximumseparationofthephotoinducedcarriersgeneratedbyit.This resultsintheobservationofthemostnegativeOCPofthisphotoanode underlightillumination.Theelectrochemicalpolarizationresults shownin Fig.10dfurtherdemonstratethattheTiO2 NSA/AgInSe2(7)/ In2Se3(3)photoanodepossessesthelargestseparationeﬃciencyofthe photogeneratedelectronsandholesamongallofthepreparedphotoanodes,whichmakesithavethebestphotoelectrochemicalcathodic protectionperformance.Moreover,forthecoupledphotoanodes/316L SSelectrodes,therearemajorissuesforthepotentiodynamicpolarizationcurvesmeasuredonthembecausetheyaregalvaniccorrosion systems.Whencouplingthephotoanodewiththe316SSelectrode,it becomesimpossibletosortoutwhichelectrodethecurrentiscoming

from.Therefore,thepotentiodynamicpolarizationcurvesofthecoupledphotoanodes/316SSelectrodewerenotperformedinthepresent work.

3.4.PECCPperformanceofthepreparedphotoanodes

InordertocharacterizethePECCPperformanceoftheprepared photoanodes,thephotoinducedvariationsofthecurrentdensitiesof thegalvaniccouplingbetweenthe316LSSelectrodeandthephotoanodeswithoutanyappliedbiaspotentialandthephotoinducedvariationsofthepotentialsofthe316LSSelectrodecoupledwiththe photoanodesunderintermittentsimulatedsunlight(AM1.5)illuminationweremeasured.Theresultsareshownin Fig.11.Duringthe measurements,boththephotoanodeandthe316LSSelectrodewere immersedin3.5wt%NaClsolution. Fig.11ashowsthevariationsin thecurrentdensitiesbetweenthe316LSSelectrodeandtheprepared photoanodes.Positiveexcitationcurrentdensitiesareobtainedunder lightillumination,signifyingthatthephotoinducedelectronsgenerated bythesemiconductortransferfromthephotoanodetothecoupled 316LSSelectrodeandthusprovidecathodicprotectionforit.The variationsofthecurrentdensitiesintheabsenceandpresenceoflight illuminationarethephotoinducedcurrentdensities.Inthiswork,the maximumphotogeneratedcurrentdensityofapproximately7 μAcm 2 belongstotheTiO2 NSA/AgInSe2(7)/In2Se3(3)photoanode,whichis muchhigherthanthoseoftheTiO2 NSA/AgInSe2(7)andTiO2 NSA/ In2Se3(3).

Asshownin Fig.11b,thevariationsofthepotentialsofthe316LSS electrodecoupledwiththephotoanodesexhibitnegativeshiftswhen thelightisswitchedon.Withswitchingoﬀ thelight,thepotentials

Fig.11. (a)Thephotoinducedvariationsofthecurrentdensitiesbetweenthepreparedphotoanodesandthe316LSSelectrode,(b)thephotoinducedvariationsof themixedpotentialsofthe316LSSelectrodecoupledwiththephotoanodes,(c)thephotoinducedvariationsofthecurrentdensitiesbetweenthe316LSSelectrode andtheAgInSe2/In2Se3 NPssensitizedTiO2 NSAphotoanodespreparedbydepositingdiﬀerentamountofAgInSe2 NPs,and(d)thephotoinducedvariationsofthe mixedpotentialsofthe316LSSelectrodecoupledwiththeAgInSe2/In2Se3 NPssensitizedTiO2 NSAphotoanodespreparedbydepositingdiﬀerentamountofAgInSe2 NPsunderintermittentAM1.5lightilluminationin3.5wt%NaClsolution.

immediatelyshifttowardspositivedirectionandthengoslowlybackto theirinitialpotentials.Thephotoinducedpotentialdropisthedifferenceofthepotentialsintheabsenceandpresenceoflightillumination. Thenegativelyshiftofthepotentialsdemonstratesthatthephotoinducedelectronsgeneratedbythephotoanodearetransferredtothe coupled316LSSelectrode,therebyprovidingcathodicprotectionfor 316LSS.Theseresultsareconsistentwiththoseofthephotoinduced currentdensitiesshownin Fig.11a.Thephotoinducedpotentialdrops oftheTiO2 NSA/AgInSe2(7)/In2Se3(3)-316LSS(approximately 236mV)arealsomuchhigherthanthoseofotherphotoanodes.The multijunctionTiO2 NSA/AgInSe2/In2Se3 furtherenhancestheseparationefficiencyofthephotoinducedelectronsandholesbythemultijunctioneffect,andthusimprovesthePECCPperformanceunder AM1.5lightillumination.BecauseofthedifferenceintheFermilevels andenergybandstructuresofAgInSe2,In2Se3 andTiO2,numerousinternalmultijunctionelectrostatic fieldscanbebuiltattheinterfacesof thepreparedTiO2 NSA/AgInSe2(7)/In2Se3(3),thustheseparationof thephotoinducedelectronsandholeswillbegreatlypromotedunder theexcitationofthesimulatedsunlight.Asaconsequence,theTiO2 NSA/AgInSe2(7)/In2Se3(3)photoanodeexhibitsthehighestPECCP performance.

Moreover,theeﬀectofthedepositionamountofAgInSe2 NPsonthe PECCPperformanceofthepreparedTiO2 NSA/AgInSe2/In2Se3 photoanodefor316LSSunderintermittentAM1.5lightilluminationhasalso beenstudiedbychangingthedepositioncyclesofAgInSe2,andthe resultsareshownin Fig.11candd.ForTiO2 NSA/AgInSe2(3)/In2Se3(3) photoanode,thephotogeneratedcathodicprotectioncurrentdensity andthephotoinducedpotentialdropare2 μAcm 2 and160mV,

respectively,demonstratingalowPECCPefficiencyduetotheinsufficientloadingamountofAgInSe2 NPs.Withtheincreaseofthe loadingamountofAgInSe2 NPs,thePECCPperformanceisenhanced, andtheTiO2 NSA/AgInSe 2(7)/In2Se3(3)photoanodeexhibitsthelargestphotoinducedcathodicprotectioncurrentdensityof7 μAcm 2 in NaClsolutionandthephotoinducedpotentialdropof236mV.Withthe furtherincreaseoftheloadingamountofAgInSe2 NPs,thedecreased PECCPperformanceisobserved.TheexcessivedepositionofAgInSe2 NPswillleadtotheagglomerationofthedepositedNPs.Thisreduces theeffectiveheterojunctionarea,therebyreducingthemultijunction effectamongAgInSe2,In2Se3 andTiO2 NSA,andcausingtherecombinationofthephotoinducedelectronsandholes.Theexcessive depositionofAgInSe2 NPsresultsintheagglomerationofAgInSe2 NPs canalsobeprovedbytheSEMimages(Fig.3d),fromwhichtheNPs loadedonTiO2 NSAbecomemoreandthenclustertogetherwiththe increasedloadingamountofAgInSe2 NPs.

Inordertocharacterizethestabilityofpreparedphotoanodes,the SEMimageoftheTiO2 NSA/AgInSe2(7)/In2Se3(3)photoanodeafterthe PECCPtestsandtheXRDpatternsandtheFTIRspectraoftheTiO2 NSA/AgInSe2(7)/In2Se3(3)photoanodebeforeandafterthePECCP testswererecorded,andtheresultsareshownin Fig.12.Asshownin Fig.12a,themicromorphologyofTiO2 NSA/AgInSe2(7)/In2Se3(3) photoanodeafterPECCPdoesnotchangesigniﬁcantly,maintainingthe samemorphologyofthatbeforethePECCPtests(Fig.4b).Besides,the XRDpatternafterPECCPtestsshownin Fig.12bwasbasicallyconsistentwiththatbeforethePECCPtests.Furthermore,theFTIRspectrumoftheTiO2 NSA/AgInSe2(7)/In2Se3(3)photoanodeafterPECCP testsarehighlyconsistentwiththosebeforePECCPtests,asshownin

Fig.12. (a)SEMimageoftheTiO2 NSA/AgInSe2(7)/In2Se3(3)photoanodeafterPECCPtests,(b)XRDpatternsand(c)FTIRspectraoftheTiO2 NSA/AgInSe2(7)/ In2Se3(3)photoanodebeforeandafterPECCPtests.

Fig.12c.Theresultsshownin Fig.12 indicatethattheTiO2 NSA/ AgInSe2(7)/In2Se3(3)photoanodehasgoodPECCPstability.

3.5.Chargetransferpropertyand ﬂatbandpotentialoftheprepared photoanodes

EISanalyseswereperformedtostudythechargetransferproperty ofthepreparedphotoanodes,andtheresultsareshownin Fig.13a.In general,smallerdiameterofthesemicirclearcoftheNyquistplotindicatesfasterinterfacialchargetransfercapability.Asshownin Fig.13a,thediametersofthesemicirclearcdecreaseintheorderof TiO2 NSA>TiO2/In2Se3(3)>TiO2/AgInSe2(7)>TiO2/AgInSe2(7)/ In2Se3(3),indicatingthehighestinterfacechargetransferefficiencyof TiO2/AgInSe2(7)/In2Se3(3).TheNyquistplotswere fittedusingthe equivalentcircuitshownintheinsetin Fig.13a.Inthisequivalent electricalcircuit,Rs representsthesolutionresistance;Qmeansthe constantphaseangleelement,whoseimpedanceisequalto (Y0(jω)n) 1,and ω istheac-voltageangularfrequency(rads 1), Y0 and narethefrequency-independentparameters.Rf andQc representthe resistanceandcapacitanceofthesurface film,respectively.Rct andQdl representtheresistanceandcapacitanceofthedoublelayer,respectively,and W representstheWarburgresistance.In Fig.13a,themeasureddataarethedotswithdifferentsymbols,andthe fittedresultsare thesolidlines.Themeasureddataare fittedverywell.The fitted parametersarelistedin Table1.Asshownin Table1,Rf ofTiO2/ AgInSe2(7)/In2Se3(3)andTiO2/AgInSe2(7)aresmallerthantheother twophotoanodes,suggestingthattheresistancesoftheTiO2/ AgInSe2(7)/In2Se3(3)andTiO2/AgInSe2(7)aredecreasedwiththedepositionofAgInSe2 NPs.AndtheTiO2 NSA/AgInSe2(7)/In2Se3(3) photoanodeshowsthesmallest Rf value,revealingthatTiO2 NSA/ AgInSe2(7)/In2Se3(3)photoanodepossessesthesmallestresistanceand

thehighestelectrontransmissionperformance.Thisalsoindicatesthat theelectrontransferisfurtheracceleratedandtheinterfacialcharge transferbarrierisfurtherreducedafterdepositingtheIn2Se3 layer, leadingtothepromotionofthePECandPECCPperformanceofthe TiO2 NSA/AgInSe2(7)/In2Se3(3)photoanode.

Mott-Schottkyplotreportsontherelationbetweenthecapacitance ofthespacechargeregionandtheappliedpotentialwiththespeciﬁc formulaforann-typesemiconductorlistedasfollows[3]:

(1) where Csc isthecapacitanceofthespacechargeregioninthesemiconductor, ε istherelativepermittivityofthesemiconductor, ε0 isthe vacuumpermittivity(8.854×10 14 Fcm 1), e istheelectroncharge (1.602189×10 19 C), ND isthechargecarrierdensity, E istheapplied potential, Efb isthe ﬂatbandpotential, k istheBoltzmannconstant (1.38066×10 23 JK 1)and T istheabsolutetemperature(298K). Mott-Schottkyplotsofthepreparedphotoanodesareshownin Fig.13b. TheslopesoftheMott-Schottkyplotsofthepreparedphotoanodesare positive,indicatingthen-typesemiconductorcharacteristicsofthe preparedsamples.AccordingtoEq. (1),the Efb ofthesemiconductors canbeobtainedfromthehorizontalintercept.Asshownin Fig.13b,the Efb ofTiO2 NSAisapproximately-0.46V(vs.Ag/AgCl),whichequalsto -0.26V(vs.SHE).Consideringthen-typesemiconductorcharacteristics ofthepreparedmaterials,itcanbeconcludedthattheCBpotentialof pureTiO2 locatesatapproximately-0.26V(vs.SHE),whichiscloseto thereported-0.29V(vs.SHE)oftheCBofTiO2 [17].Afterdecorated withAgInSe2 NPs,the Efb negativelyshiftsto-0.57V(vs.Ag/AgCl), revealingthattheFermilevel(Ef)ofTiO2 waspulledtoamorenegative valueandformamorenegative Eco-f duetothedepositedAgInSe2 NPs withmorenegativeenergyband.Furthermore,afterIn2Se3 decoration,

Fig.13. (a)EISplotsoftheAgInSe2/In2Se3,AgInSe2,In2Se3 NPssensitizedTiO2 NSAphotoanodes,andpureTiO2 NSAphotoanodein0.1MNa2SO4 solutioninthe dark.(b)Mott-SchottkyplotsoftheAgInSe2/In2Se3,AgInSe2,In2Se3 NPssensitizedTiO2 NSAphotoanodes,andpureTiO2 NSAphotoanodein0.1MNa2SO4 solution inthedark.

Table1

FittedparametersoftheEISequivalentcircuitdatashownin Fig.13a.

theTiO2/AgInSe2(7)/In2Se3(3)photoanodeshowsthemostnegative Efb of-0.63V(vs.Ag/AgCl).ThecomparativelynegativeEfb denoteshigher activityandutilizationrateofthephotogeneratedelectronsforreductionreactions,suchasthephotocatalytichydrogenevolutionfrom watersplitting,aswellasthePECCPformetallicmaterials.

Furthermore,the ND ofthesemiconductorsisinverselyproportional totheslopeoftheMott-Schottkyplots.Thesmallerslopecontributesto thelarger ND anddenotesthehigherconcentrationofchargecarriers [3].From Fig.13b,theobtainedthechargecarrierdensitycanbe rankedas ND (TiO2 NSA/AgInSe2(7)/In2Se3(3))> ND (TiO2 NSA/ AgInSe2(7))> ND (TiO2 NSA/In2Se3(3))> ND (TiO2 NSA).TheTiO2 NSA/AgInSe2(7)/In2Se3(3)hasthesmallestslope,correspondingtothe largest ND andthehighestchargecarrierdensity.Thiswillbeneﬁtthe generationandtransferofphotoinducedelectronsinann-typesemiconductorphotoanode.Therefore,theaforementionedresultsreveal thattheTiO2 NSA/AgInSe2(7)/In2Se3(3)withcomparativelynegative Efb andhighchargecarrierdensitypossessessuperiorPECconversion capabilityintheutilizationofelectrons.

3.6.PromotionmechanismofthePECandPECCPperformanceofTiO2 NSA/AgInSe2/In2Se3 photoanode

Fig.14 schematicallydescribesthemechanismfortheimproved PECconversionandPECCPperformanceofTiO2 NSA/AgInSe2(7)/ In2Se3(3)underAM1.5lightinNaClsolution.InthepreparedTiO2 NSA/AgInSe2/In2Se3 multijunction,AgInSe2 andIn2Se3 arevisiblelight-responsivesemiconductors.Thebandgap(Eg)ofAgInSe2 is 1.57eVwiththeCBandVBpotentialsof-1.64Vand-0.075V(vs.SHE), respectively[41].Besides,the Eg ofIn2Se3 is1.35eV,whoseCBandVB potentialslocateat-0.83Vand0.95V(vs.SHE),respectively[70,75].

The Eg,CBandVBofTiO2 are3.2eV,-0.29Vand2.91V(vs.SHE), respectively[17].TheCBofIn2Se3 islocatedbetweenTiO2 and

AgInSe2,makingitbemoreefficientintransferringphotoinduced electronsbetweentheAgInSe2/TiO2 heterojunction.OncetheTiO2 NSA/AgInSe2(7)/In2Se3(3)isexcitedbysimulatedsunlight,theelectronsintheVBsofAgInSe2,In2Se3 andTiO2 areexcitedtotheirCBsto producethephotogeneratedelectrons.Duetothedifferenceinenergy bandpotentialsofAgInSe2,In2Se3 andTiO2,whoseCBpotentialsarein theorderof ECB(AgInSe2)< ECB(In2Se3)< ECB(TiO2),thephotoinduced electronsgeneratedontheCBofAgInSe2 willtransfertotheCBof In2Se3 andthenfurthertotheCBofTiO2 todecreasetheenergyofthe system.Simultaneously,withtheVBpotentialsintheorderof EVB (AgInSe2)< EVB(In2Se3)< EVB(TiO2),thephotoinducedholesgeneratedontheVBofTiO2 canbetransferredtotheVBofIn2Se3 andfurther totheVBofAgInSe2 and finallyparticipateintheoxidationreaction withtheambientNaClsolution.Therefore,constructinganIn2Se3 compositelayeraroundAgInSe2 helpstheformationofTiO2 NSA/ AgInSe2/In2Se3 multijunctionwithwell-matchedenergybandstructure.TheTiO2 NSA/AgInSe2/In2Se3 multijunctionwillfurtherfacilitate thetransferofthephotogeneratedelectronsandholesbetweenTiO2 andAgInSe2,asdescribedin Fig.14.Besides,theNSAstructurecan offeralargelight-harvestingareaandfastelectrontransmission channel,andthusincreaselightabsorptionareaandpromotethe transferofphotogeneratedelectronstowardssubstrate.Therefore,the TiO2 NSA/AgInSe2/In2Se3 photoanodeexhibitsthehighestphoto-tocurrentconversionefficiency.

SincetheFermilevelsofAgInSe2 andIn2Se3 arerelativelynegative, theTiO2 NSA/AgInSe2/In2Se3 multijunctionwillpulltheFermilevelof TiO2 NSAtoanegativedirection,asillustratedintheMott-Schottky plots.Whenann-typesemiconductorisexposedtosimulatedsunlight, thephotoinducedelectronswillaccumulateonitsCB,inducinganegativeshiftofthequasi-Fermilevelofthephotogeneratedelectrons. Then,thepotentialofthesystemwillnegativelyshift.Hence,forthe TiO2 NSA/AgInSe2(7)/In2Se3(3),alargeamountofthephotoinduced

Fig.14. ProposedmechanismsfortheenhancedPECaswellastheenhancedPECCPperformanceofTiO2

electronswillpushthequasi-Fermilevelofthephotogeneratedelectronstoamorenegativelevelthanthosegeneratedbyotherphotoanodes.WhencouplingtheTiO2 NSA/AgInSe2(7)/In2Se3(3)photoanodewiththe316LSSelectrode,thephotogeneratedelectronswillbe transferredtothecoupled316LSSelectrodeandachievethePECCP effectforit.Therefore,theTiO2 NSA/AgInSe2(7)/In2Se3(3)exhibitsan excellentPECCPpropertyfor316LSSundersimulatedsunlightilluminationandshowsgreatapplicationpotentialsinthe fieldofthe PECCPformetals.Theoptimizationofmultijunctionphotoanodewitha muchnegativebandpotentialcanbebeneficialforthePECCPapplicationofmetalsinmarineenvironment.

4.Conclusions

Theenvironment-friendlyAgInSe2/In2Se3 NPsdecoratedTiO2 NSA photoanodewithamultijunctionstructurewasfabricatedinthispaper. TheTiO2 NSAwithnumerousvertically-growntwo-dimensionalnanosheetscanofferalargelightharvestarea,andthedirectelectron transferpathways,whichbenefitstheseparationofthephotogenerated electronsandholes.Meanwhile,theconstructedTiO2 NSA/AgInSe2/ In2Se3 multijunctionavoidstheunmatchedenergybandarrangement betweenAgInSe2 andTiO2 comparedwithsingleAgInSe2 NPsdecoratedTiO2 NSA,andcanleadtothefurtherpromotionofthecharge generation/separationefficiencyundersimulatedsunlightillumination. FortheTiO2 NSA/AgInSe2(7)/In2Se3(3)photoanode,AgInSe2 and In2Se3 arepreparedatoptimalquantity,anddistributedevenlyonthe surfaceofTiO2 NSA,formingcloseinterfacialadhesiontoTiO2 NSA. Thelightresponserangeisbroadenedduetothevisiblelightresponse ofnarrowbandgapofAgInSe2/In2Se3 NPs.ThePLresultsreflectthat theTiO2 NSA/AgInSe2/In2Se3 multijunctionphotoanodeleadstothe swiftseparationofthephotogeneratedelectronsandholescompared withthebi-junctionofTiO2 NSA/AgInSe2.Moreover,fortheTiO2 NSA/ AgInSe2(7)/In2Se3(3)photoanode,EISandMott-Schottkyanalysesindicatethatthereducedchargetransferbarrier,themorenegativeEfb as wellasthehighestchargecarrierdensitybenefittothetransmissionof thephotogeneratedelectrons.Undersimulatedsunlightillumination, themultijunctionsystemexhibitsenhancedthephoton-to-current conversionactivity.ThisstudyconfirmsthatthecomplexofAgInSe2 sensitizerwithIn2Se3 assistlayercanoptimizetheinterfacialmicrostructureofthephotoanode,therebyeffectivelyimprovingthePEC activity.Whencouplingthemultijunctionphotoanodewith316LSS, thephotogeneratedelectronsonthephotoanodecanbetransportedto themetallicmaterialandprovidecathodicprotectionforit.Finally,the TiO2 NSA/AgInSe2(7)/In2Se3(3)photoanodewithamuchmorenegativebandpotentialexhibitsafurtherenhancedPECCPperformancefor 316LSSbyprovidingthephotogeneratedcathodicprotectioncurrent densityof7 μAcm 2 andthephotoinducedpotentialdropof236mV undersimulatedsunlightilluminationinNaClsolution.

Dataavailability

Theraw/processeddatarequiredtoreproducethe ﬁndingscannot besharedatthistimeasthedataalsoformspartofanongoingstudy.

CRediTauthorshipcontributionstatement

XuhongJiang: Investigation,Datacuration,Formalanalysis, Validation,Visualization,Methodology,Writing-originaldraft, Writing-review&editing. MengmengSun: Fundingacquisition, Resources,Investigation,Datacuration,Formalanalysis,Validation, Visualization,Methodology,Writing-originaldraft,Writing-review& editing. ZhuoyuanChen: Fundingacquisition,Resources,Projectadministration,Supervision,Methodology,Validation,Conceptualization, Datacuration,Writing-originaldraft,Writing-review&editing. JiangpingJing: Fundingacquisition,Investigation,Datacuration, Formalanalysis,Validation,Visualization,Methodology. ChangFeng:

Investigation,Datacuration,Formalanalysis,Validation,Visualization, Methodology.

DeclarationofCompetingInterest

Theauthorsdeclarethattheyhavenoknowncompeting ﬁnancial interestsorpersonalrelationshipsthatcouldhaveappearedtoinﬂuencetheworkreportedinthispaper.

Acknowledgements

Thisworkwas ﬁnanciallysupportedbytheNationalNaturalScience FoundationofChina(GrantNos.41676069,41976036,41906034), KeyResearchandDevelopmentProgramofShandongProvince(Grant No.2019GHY112085,2019GHY112066),andQingdaoAppliedBasic ResearchPlanProgram(GrantNo.19-6-2-79-cg).

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CHAPTER XVIII STRANGE QUARTERS

THE approach to the castle was by a series of terraces connected by a narrow zig-zag road. It stood on a small plateau formed in the wooded hill which rose with almost perpendicular abruptness behind it. Its aspect was curious enough, but the most astounding thing about it was its position, its unexpectedness, and the contrast with its wild surroundings.

As Ludovic and his companions made their way up the winding road their curiosity grew at every step. And the curiosity was not altogether without apprehension.

“The last thing I looked to find in these wilds,” Ompertz observed with a puzzled look at the grey silent building. “It is like a fairy tale.”

Ludovic was a little anxious, having his responsibility in mind, as to the outcome of the adventure. But such ideas did not seem to trouble Ompertz.

“Our greatest piece of luck,” he said, “is that the palace is inhabited. There are plenty of old castles about in these parts, but they have been handed over long ago to the bats and owls. Now, that lighted window bodes a more comfortable reception than a screech and a flutter.”

“No doubt it is a shooting-lodge,” Ludovic suggested.

“It can be nothing else,” Ompertz agreed. “We may look for a good supper and a night’s rest, if not for a carriage.”

They had now reached the gateway which led to the entrance door. Here the horses were made fast, and then Ompertz pulled the iron bell-handle that hung in the porch. Scarcely had his hand left it, when the door was thrown open, sending a blinding flood of brightness into the black night, and disclosing a great square hall,

hung with trophies and implements of the chase. Two men in quaint liveries stood at the door. As it opened they made way for a third with white hair and beard, who came forward and, with a bow, motioned the travellers to enter. Ludovic in a few words gave the reason of their seeking shelter. Taking it as a matter of course, the old man listened gravely, and then ushering them into a room off the hall, asked them to wait there.

“I will at once inform my master of your arrival,” he said deferentially, and so left them.

The four looked at one another in astonishment.

“Well, if this is not an extraordinary place to light upon in the mountains,” Ompertz exclaimed, accepting his good fortune with a laugh.

To Ruperta alone, since her experience was narrowed to one phase of life, did their reception seem short of wonderful.

“Everything now,” said Ludovic, “depends upon our host; who he is, and whether he is likely to recognize us. Supposing that he does not, you and I, Princess, must pass as brother and sister; Countess Minna and Captain von Ompertz are our friends and travelling companions. Let us hope our incognito may not be suspected.”

As he spoke, the door was opened by the old steward, who, with a bow—for those were days of ceremonial—ushered their host into the room. A man as singular as was his dwelling. He seemed the very incarnation of power, with his broad chest, massive throat and strongly marked features. His hair and beard were black, his complexion swarthy, but his eyes, curiously, were light blue. He was plainly dressed, but a certain dignity of look and movement gave him an air of distinction. He bowed, and greeted the travellers with almost an excess of welcome.

“I should be very sorry to hear of your mishap,” he said, “were it not for the pleasure it gives me to be your host to-night.”

His voice, Ludovic thought, was the deepest he had ever heard. There was, too, a peculiar sustained vibration in it, like the deep pedal notes of an organ.

“We must consider ourselves very fortunate,” Ludovic added, after a word of thanks, “to have found a shelter so splendid and unexpected in this place.”

Their host laughed, showing, in contrast to his black beard, a row of dead white teeth. “I do not wonder at your surprise,” he said. “But I love a mountain life, its wildness and its sport. At the same time, sense of comfort and luxury in one’s home enhances by contrast one’s enjoyment of these surroundings.”

“Naturally,” Ludovic agreed, his opinion of their singular host still hanging in doubt.

“Many people pretend to love a mountain life,” the other continued, “but they make themselves woefully uncomfortable, and soon fly back to towns and civilization. I may, perhaps, claim to have the courage of my fancy.”

The man’s manner was perfect, far more refined than his appearance would have suggested, yet to Ludovic’s keen perception there was something about him which made him doubt the depth, the reality of his frankness.

“My servants have probably told you my name. No? It is Irromar, Count Irromar, and this, my principal place of residence, is called the Schloss Teufelswald.”

Ludovic accepted the information with a bow, and some inward congratulation that their identity was not likely to be known to this secluded nobleman. Irromar? The name, though, seemed not unfamiliar.

The Count’s deep voice interrupted his attempt to recall it.

“And now, may I know whom I have the honour of entertaining?”

Ludovic gave the name he had assumed during his incognito, presenting Ruperta as his sister, changing Minna’s title to simple Fräulein, and giving Ompertz alone his actual designation. During the introduction the Count’s eyes rested rather longer on Ruperta’s face than Ludovic liked, and their expression seemed to have something in it which exceeded greeting; but then that was natural.

She was a queen among women, and might have come, no doubt, as a revelation to this mountain-dweller.

“We are in haste to push on with our journey,” Ludovic said. “If we might beg the loan of a carriage, our horses are still fresh, and——”

The Count made a quick gesture of protest. “It is not to be thought of, my dear sir. As to the carriage, why, the whole of my stable would be at your service, were I cruel enough to allow you to leave my roof this wild night.”

“Nevertheless, I should be glad if you would permit us to continue on our way,” Ludovic persisted. “We have lost too much time already.”

The Count smiled. “Which you will certainly not recover by starting before morning. What, Lieutenant,” he added in an easy tone of masterful remonstrance, “it would be nothing less than an outrage to drag these ladies out again into the storm and darkness They are fatigued enough already, one can see.”

Ruperta spoke a word to second Ludovic’s urging; but their host would not hear of their departure.

“I am an inexorable host,” he laughed. “If you come to my inn, the reckoning I charge is that you make wise use of the hospitality it affords. Now—ah, Gomer,” he said as the old steward entered, “you have come to tell us that supper is ready. Come, my friends; I shall give myself the pleasure of joining you. The wild weather has given me a second appetite.”

With a deferential bow, he offered his arm to Ruperta. She hesitatingly took it and he led her from the room. The masterful peremptoriness of his insistence was so coated with the good humour of a frank hospitality, that it could not without ungraciousness be withstood, so Ludovic, comforting himself with the reflection that Ruperta and Minna would have a much-needed rest, was forced to accept the delay and submit to his host’s decree.

The Count led the way to a fine square dining hall, where a luxurious supper table had been prepared. The room curiously reflected its owner. In spite of its air of great refinement, there yet seemed flung over it a subtle suggestion of brute strength, almost savagery. Upon

the solid oaken floor were strewn rugs made of the skins of bears and wolves. The walls were hung with vivid tapestries on which were worked flamboyant pictures of war and sport almost brutal in their realism. Antlers and swords, armour and sporting weapons were the ornaments of the room; it was essentially the dwelling-place of a strong adventurous personality. But there was the touch of scarcely restrained savagery which seemed, to delicate minds at least, to make the tone of the place repulsive. And, over all, the note of strength; fierce, dominant strength.

The good fare and sparkling wine after the hardships of the long journey soon made the travellers take a more cheerful view of the situation, and put them in a frame of mind to accept with thankfulness the shelter, and with resignation the delay, which this accident had provided. Even Ruperta began to take a manifest interest in her unusual surroundings and could join almost animatedly in conversation with her host. With a tact, which had in it something of suspicion, the Count forebore to question closely any of the party as to the purpose and extent of their journey, accepting a nebulous explanation on Ompertz’s part, who airily accounted for their presence in those mountain wilds by their having missed the high road, with amused toleration of an obvious fiction. Then he adroitly turned the conversation to general topics, talking of war and campaigning to the captain, of sport to Ludovic, of lighter social matters to the ladies. Although he was found keeping his state in that wild spot, the Count soon proved that he was far from being exclusively a dweller with nature. He was familiar with many capitals and their society, and was by no means ignorant of what was going on in the more civilised world beyond his mountain fastness. He happened to mention Rollmar.

“You know the Chancellor?” Ludovic asked.

“Not personally; well enough by reputation, though, and we have corresponded, not too amicably, more than once. Yes, we are well known to one another,” the Count laughed grimly. “It is well for one of us, perhaps, that I stand some leagues outside his jurisdiction.”

“You would try a fall with him?” Ompertz suggested.

“We should hardly be likely to leave one another in peace. Chancellor Rollmar loves coercion, not to say tyranny, and I—well, I brook no interference with my liberty of will.”

There was scarcely need for the statement; the man’s determined nature was obvious.

“I am just now amused,” he continued, “in watching a little scheme of the old fox’s where chance is trying a fall with him. I allude to a matter which must be, at least partially, known to you; the projected marriage between Princess Ruperta and Prince Ludwig of DraxBeroldstein.”

“Ah!” Ludovic bent forward with assumed interest in order to direct the Count’s notice from Ruperta. “I suppose not even a possible mutual dislike between the parties will avail against Rollmar’s intention there.”

The Count laughed. “No. I must give our friend the Chancellor credit for strength of purpose to brush aside such a harmless fly as that. But now he is faced by something more like a difficulty. You have not heard the latest news? No? It is scarcely likely; but I make a point of being well posted. Yes; within the last few days a change has come over the situation which may prove an awkward blow to the old schemer. King Josef has died suddenly from an accident.”

“So Prince Ludwig is King?” Ompertz observed.

With a knowing shake of the head the Count drew back his black fringed lips in a smile. “Prince Ludwig, as most of the world knows to its great amusement, has run away and hidden himself to escape the bride Rollmar has ready for him. Why, is his affair; for report speaks of her as a beauty. However, perhaps he did not consider the sugar sufficient to disguise the medicine. Well, the extraordinary part of the affair is this. Uncle Josef dies. Nephew Ludwig, the Unready, is not to be found, consequently Nephew Ferdinand, the Alert, springs up, and, seizing the opportunity, coolly seats himself upon the vacant throne.”

A long, low whistle sounded through the room. Ompertz’s lips were pursed; he was staring at Ludovic in bewildered suspicion.

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