Surface modification of iron oxides by ion bombardment – comparing depth profiling by HAXPES and Ar ion sputtering

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JournalofElectronSpectroscopyandRelatedPhenomena224(2018)23–26

ContentslistsavailableatScienceDirect

Journal of Electron Spectroscopy and Related Phenomena

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

Surface modiﬁcation of iron oxides by ion bombardment – Comparing depth proﬁling by HAXPES and Ar ion sputtering

M. Fondell

^a,b,∗

, M. Gorgoi

^b

, M. Boman

^a

, A. Lindblad

^a,b

aDiv.InorganicChemistry/Dept.Chemistry(˚Angström),UppsalaUniversity,Box538,SE-75221,Uppsala,Sweden

bHelmholtz-ZentrumBerlinfürMaterialienundEnergieGmbH,Albert-Einstein-Str.15,Berlin12489,Germany

a r t i c l e i n f o

Articlehistory:

Received16December2016

Receivedinrevisedform1August2017 Accepted21September2017 Availableonline28September2017

Keywords:

HardX-rayphotoelectronspectroscopy Depthproﬁling

Ironoxide HAXPES

Synchrotronradiation

a b s t r a c t

Thinfilmsoftheironoxidemaghemite(-Fe2O3)andhematite(˛-Fe2O3)grownonfluorinedopedtin oxide(FTO)withpulsedchemicalvapourdepositionhavebeeninvestigatedwithhardX-rayphotoelectronspectroscopy.Itisfoundthatevenlowenergysputteringinducesareductionofthesurfacelayer intoFeO.SatellitesintheFe2pcorelevelspectraareusedtodeterminetheoxidationstateofiron.Depth profilingwithchangingphotonenergyshowsthattheunsputteredfilmsarehomogeneousandthatthe informationobtainedfromsputteringthus,inthisinstance,representssputterdamagestothesample.

1. Introduction

Therearesixteenoxides andoxyhydroxidesofiron [1],with usesrangingfromfoodcolouring(E172)tocatalysts.Iron(III)oxide (Fe2O3),especiallyinitshematitephase(˛-Fe2O3),exhibitattrac- tivepropertiesforsolarwatersplitting,withaadequatebandgap around2eVcombinedwithafavourablepositionforthevalence band’sedgeandefﬁcientphotonabsorption.Thematerialissta- ble,affordable,abundant,nontoxicandenvironmentallyfriendly.

Saidpropertieshavedirectedsigniﬁcantresearcheffortsregarding hematiteasacomponentinphotoelectrochemicalcell[2–4].

X-rayphotoelectronspectra(XPS)isastandardtechniquefor quantiﬁcationof chemical statesin a materialvia thechemical shiftofcorelevelphotoelectronlines[5,6].Ironoxidespresenta challengingsystemtointerpretphotoelectronspectrafromowing tobackground[7]andsatellitestructures[8,9]aroundtheFe2p region.Asmentioned,thereareanumber ofoxidesandoxyhy- droxides(16)eachhavingdifferentphaseswhichalsocomplicates spectralinterpretation.

Inthispaperwecomparespectraofsputteredandunsputtered surfacesofmaghemiteandhematitetakenwith1487eVphoton energy(i.e.AlK˛).Theunsputteredmaghemitesurfaceandsingle

∗ Correspondingauthorat:Helmholtz-ZentrumBerlinfürMaterialienundEnergie GmbH,Albert-Einstein-str.15,Berlin12489,Germany.

E-mailaddress:mattis.fondell@helmholtz-berlin.de(M.Fondell).

crystalsofFeOand˛-Fe2O₃ arealsomeasuredwithsynchrotron basedhardX-rayphotoelectronspectroscopy(HAXPES)datafor comparison.

AsynchrotronX-raysourceenableustotunetodesiredpho- tonenergies.Herethishavebeenutilisedtoobtainincreasingly largeinformationdepthsinthesamplesatthreephotonenergies aboveAlK˛–withoutmovingthesampleposition.Sincethebind- ingenergy(E_b)forthecorelevelelectronsthatwestudyisconstant, thekineticenergy(E_k)ofthestudiedelectronsincrease instep withthephotonenergy(ω);thekineticenergyisgivenbyEin- stein’srelationforthephotoelectriceffectE_k=ω−E_b−,where

istheworkfunctionofthespectrometer.Forsufﬁcientlyhigh kineticenergiesthemeanfreepath()oftheelectronsgetslarger withincomingkineticenergy.Withthesocalleduniversalcurvefor electronmeanfreepathsinmind,thismeanskineticenergiesabove 50eV[10,11].ByusingsynchrotronradiationtheX-rayenergycan bevariedandthustheavailableinformationdepthdeﬁnedas3, givesriseto99%ofthespectralintensity[6].Thelongermeanfree pathintheHAXPESmeasurementsallowsustodiscernsurfaceoxy- hydridesfromthebulksample,whichbycontrastwasimpossible withtheAlK˛photonenergy.

Wehighlightthat relyingonlyonstandard XPSandsputter- ingcanleadtoerroneousinterpretationofﬁlmcomposition.Itis knownthathydroxidesandoxidesofironcanbereducedbyion bombardment[8,12,13];sputterdamagesintheformofpreferen- tialremovaloflighterspecieshavebeenshowntooccurinphysical vapourdepositedtungstensulphide[14],tinsulphidepowder[15], https://doi.org/10.1016/j.elspec.2017.09.008

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24 M.Fondelletal./JournalofElectronSpectroscopyandRelatedPhenomena224(2018)23–26

andnanocompositecoatingsofTi-Ni-C[16].Herewespecifically usehardX-rayphotoelectronspectroscopywithvariablephoton energytoassessthecompositionofanexsitusampleconsistingof anmaghemitefilmgrownonfluorinedopedtinoxide(FTO).

2. Experimentalsection

PolycrystallinemaghemiteandhematiteweregrownonFTO substratesbypulsedchemicalvapourdeposition.Asprecursors, ironpentacarbonylandO2wereused.Theonlydifferencebetween the two depositions were the use of either N₂ or CO as car- riedgas,resultinginthetwodifferentpolymorphshematiteand maghemite.Theinvestigatedthinﬁlmswerefromthesamedepo- sitionroundbutnottheexactlysameﬁlm.Furtherinformationof thedepositionsandthedepositionparameterscanbefoundinRef.

[17].

Phasedeterminationof theiron oxideﬁlmswereperformed usingRamanspectroscopy.ARenishawmicroRamansystemwas usedwiththe532nmlineofanargonionlaser.X-raydiffraction wasalsousedandisdiscussedwithassociatedreference.

The HAXPES experiments were performed at BESSY II (Helmholtz-ZentrumBerlin),attheKMC-1 dipolemagnetbeam line [18] – using the high kinetic energy photoelectron spec- troscopyendstation(HIKE)[19,20].Thebeamlineisequippedwith aSidoublecrystalmonochromator(DCM)[21]wheretheX-rays arefocussedonthesampleusingaparaboloidglasscapillary.The basepressureinthemeasurementchamberwasinthe10⁻⁹mbar regionthroughouttheexperiment.Thephotoelectronspectrawere recordedusingaVGScientaR4000electronenergyanalyseratnor- malemission(90^◦).FortheHAXPESmeasurementsphotonenergies of2005,3000and6015eVwereused.Allspectrawascalibrated withanAustandardwiththeAu4f_7/2bindingenergytakentobe 84.00eV[22].ThesputteredﬁlmsweremeasuredusingaPhysical InstrumentsQuantum2000ESCAutilizingmonochromatizedAlK˛ radiationataemissionangleof45^◦.Thesputteringofthesamples wasdoneatlowenergyfor8minutesusing200VAr⁺ions.The sputteringwasmadeina4+2+2min.sequenceastoreducethe carbonaccumulatedonthesurface,afterhavingexposedtheiron oxideﬁlmstoair.TheC1sandSn3dcorelevelbindingenergy

regions(Figs.S1andS2inthesupportingmaterial)weremoni- toredtoascertainthatthesputtersequencedidnotsputteraway theironoxide.Tinisonlyispresentwithinthesubstrateandwas notobservedattheinformationdepthusingAlK˛,neitherbefore norafterthesputteringtookplace.Thustheironoxideﬁlmcov- ersthesubstrateentirelyandislandformationmaybeexcluded.

Thereferencemeasurementswereperformedonsinglecrystalsof FeO(100)andFe2O3(0001)purchasedfromMaTeckGmbH.

3. Resultsanddiscussion

TheRamanmeasurementsshowninFig.1areingoodagreement withtheshiftsexpectedfrommaghemiteandhematite[23].The hematitefilmhasalltheexpectedpeaksandalsooneat660cm⁻¹. Thispeakarisefrompossibledislocationsandlackoflongrange orderinthefilm[24].ThemaghemitefilmshowninFig.1consists ofmaghemiteshifts,reportedbyFariaetal.[23].Bothfilmsshowed justonephase,anddidnotexhibitanysignfromotherironoxide phases,likeforinstancemagnetite(Fe₃O₄)orwüstite(FeO),this alsoindicatesthatthelaserpowerusedforthemeasurementswas lowenoughastonotreducethefilms.X-raydiffractionwasalso usedforcharacterisationofthedepositedfilms.FromtheXRDmea- surementsweseeboththesubstrateandthethinfilmontop.Due tothesmallthicknessoftheironoxidethestrongestpeaksderives fromtheFTOsubstratebuttheironoxidepeaksareaswellclearly visible.ThesemeasurementcanbefoundinRef.[17].

Thenanostructureofthefilmscanbeclearlyobservedinthe scanningelectronmicrographsshowninFig.1(b).Thereisadiffer- enceingrainsizecomparinghematiteandmaghemitereflecting differentgrowthconditions(fordetailsseeRef.[17]).Neitheriron oxidehascrystalparametersthatmatchthatoftheFTOsubstrate, thispromotesgrowthofclusterswhichinturncoalesceintoacon- tinuousfilm.Thelargesurfaceareaobtainedisbeneficialforthe intendedphotoelectrochemicalcellapplication.

InFig.1(c)overviewspectraanddetailspectra(Fig.2)taken aroundtheFe2pandO1sregionsaredisplayed.InFig.2foursets ofdataarecomparedinthepanels:(a)untreated(unsputtered) and(b)sputtereddataofmaghemiteandhematite(c)reference datafromsinglecrystalironoxides,FeO(111)and˛-Fe2O3(0001);

Fig.1. (a)Ramanmeasurementsonthehematiteandmaghemitefilms.Panel(b)showsscanningelectronmicrographsofthenanostructuredfilms.(c)HAXPESmeasurements ofthemaghemitefilm(black)andtheFTOusedassubstrate(dashed).

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M.Fondelletal./JournalofElectronSpectroscopyandRelatedPhenomena224(2018)23–26 25

Fig.2. Fe2pandO1scorelevelphotoelectronspectraofmaghemiteandhematite:(a)untreated,(b)sputtered,and(c)references;(d)HAXPESdepthproﬁledataofFe2p andO1sofmaghemite.ValuesineVdesignatephotonenergies.Inthecaseofthesputteredsamples(markedwithanasterisk)thenamingreferstotheinitialsample.

(d)depthproﬁledataofmaghemiterecordedwith2005,3000and 6015eVphotonenergywithoutanysurfacetreatment.

Theoverviewspectra(Fig.1)alsocontainareferencetoFTO (substrate).ComparingthemaghemiteandtheFTOitisclearthat themaghemiteﬁlmisacontinuousﬁlm,sincetherearenotinpeaks visible[15].

The measurements of the single crystalline FeO and Fe2O3

(Fig.2(c))havewellseparatedspinorbitcomponents.InFeOthe positionsfortheFe2p_1/2andFe2p_3/2arelocatedat723.3eVand 707.9eV,whereasforFe2O3theyareshiftedtowardshigherbind- ingenergyat724.3eVand711.0eV.Thisisingoodagreementto reportedvaluesbyGrosvenoretal.[9].Othervalueshavealsobeen reportedfortheFeOandFe2O3butarenotpositionedmorethan 0.2eVfromvaluespresentedhere[25,26].Thethinsolidvertical linesintheFe2pspectraindicatethepositionsoftheFe³⁺andFe²⁺

satellitesat719.3and715.4eVbindingenergyrespectively[9].The referencespectracontainsonlyoneofthosesatellites,thuswecan usethisasaﬁngerprintforthechemicalstateofiron.

Maghemite and hematite have almost identical XPS spectra (Fig.2a),thedifferenceinlatticestructure(cubicvs.hexagonal) havingnoimpactonthechemicalshifts.Intheunsputteredcases both lookakintothehematitereferencespectrum allhavinga prominentFe³⁺satellite[9,26].

Uponsputteringofboththehematiteandmaghemitethereis ashiftoftheFe2pcomponents’bindingenergiestowardlower energiesandthesatellitestructurechangesintothatoftheFeO, visibleinFig.2(b).ThisisaccompaniedbyashiftoftheO1sbinding energytowardhigherbindingenergyandalowerintensityofthe featurearound532eVbindingenergy–aregionassociatedwith surfacehydroxy-groups[26].Sputteringofmaghemitethusacts similarlytoheattreatmentinvacuum,i.e.asareducingatmosphere [17].Thesputteredﬁlmshaveashoulderonthelowerenergyside oftheFe2p_3/2ﬁlmthatcoincideswiththelowenergyshapeofFeO.

Argonionsputteringofironoxideshasbeenobservedtoresultin FeOratherthanmetallicFe,bothwithhematiteandmaghemite (Fe₂O₃)andthemagnetitespinelFe₃O₄[8].

The depth profile obtained by varying the photon energy withoutchangingthesampleposition(Fig.2(d))showsthatthe maghemitespectralstructureoftheFe2ppersistsatleastdown totheinformationdepth(25nmat6015eV,calculatedwithTPP- 2Mmodel[27]),furthermorethehighbindingenergyfeaturein theO1sisindeedemanatingfromthesurfaceasitloosesinten- sitywithincreasingphotonenergy.InFig.1thesurfaceexhibits apronouncedroughness,thisaddsuncertaintytothemagnitude oftheinformationdepth[28]calculatedwiththeTPP-2Mmodel [27].Thecalculateddepthsaresystematicallyoverestimatedsince thesurfacesarenotflat,measurementsongoldsurfaces[29],and SiO₂onSi[30]withvaryingroughnesssuggestthatthedifference ininformationdepthsbetweenthe“flat”andthe“coarse”surface isapproximatelyaconstant,i.e.thedifferentinformationdepths shouldallbemultipliedbyan(unknown)factor(<1).

If,havingdeterminedthesputteringrateandonlytheinforma- tioninthepanels(a)–(c)wereused,anaïveinterpretationwould bethatourdepositedﬁlmconsistedofamaghemite/hematiteover layer(atleast6nmthick,theinformationdepthat1487eV[27]) which is sputtered away, revealing an understoichiometryﬁlm beneath,predominantlyconsistingofFeO.

BesidesashiftintheO1sbindingenergytowardshigherbinding energies(+0.2eV[8,17])forhematite,theFe2p_3/2peak-shapehas alowbindingenergyshoulder,asseeninthereference˛-Fe2O3

spectrum(Fig.2,panel(c).McIntyreetal.havetheseparationof 1.1eV[8]betweentheshoulder’speakandthehighestintensity peak–formaghemitetheydonotresolveanysuchstructure.In panel(d)ofFig.2wealsoﬁndthatsuchatwo-peakstructureis absentformaghemite.

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26 M.Fondelletal./JournalofElectronSpectroscopyandRelatedPhenomena224(2018)23–26

BasedonXRD,RamanspectroscopyandHAXPES,weconclude thattheﬁlmsareeithermaghemiteorhematitebeforethesput- teringtakesplace.ItisunlikelythatFeOweretobedepositedat thechosenprocesstemperatureof300^◦C.FeOismetastablebelow 570^◦CanditsdecompositionleadstometallicironandFe3O4.The presenceofeitherofthoseproductswoulddominatetheRaman spectrum,since they aremuch more Raman-activethan either Fe2O3 phase[23].Furthermore,metalliciron ifpresentinthese kindofﬁlmsisclearlyvisibleinX-raydiffraction[31].Byneither methoddecompositionofFeOwasobservedhere.

UsingonlyXPSwithAlK˛photonenergyitwouldbeimpos- sibletodiscernbetweenmaghemiteandhematiteinthefilm.The electronmeanfreepathistooshorttoprobebeneaththehydroxy- groupsonthesurface,andanionsputteringofthesurfacewould reducethetopmost layer(seenwiththis )to FeO.ThisCatch 22canbeavoidedathigherphotonenergies:theHAXPESdepth profilerevealsthatthefilmishomogenousdownto25nmwith hydroxy-groupsontheimmediatevicinityofthesurface.

4. Conclusion

Weconcludethatgreatcautionmustbeexercisedwhensput- teringisusedtoobtaindepthprofiles,andwiththedatapresented hereaclearcutcaseisdemonstratedwhereionbombardmentsput- teringofthesurfaceprohibitsacorrectinterpretationofthefilm composition.Byavoidingsputteringofthesurfaceandincreasing thephotelectrons’meanfreepathwithincreasedphotonenergywe havebeenabletoassessthecompositionofthefilmoveradepth rangerelevantforapplications,e.g.ironoxidefilmsonFTOforpho- toelectrochemicalwater-splittinghavethicknessesinthevicinity of25nm.

Acknowledgements

WeacknowledgetheHelmholtz-ZentrumBerlinforprovision ofsynchrotronradiationbeamtimeatbeamlineKMC-1ofBESSY II. The research leading to these results has received funding fromtheEuropeanCommunity’sSeventhFrameworkProgramme (FP7/2007-2013)undergrantagreementno.312284.MFthankfully acknowledgetheSwedishRoyalAcademyofSciences(KVA)andAL theSwedishResearchCouncil(No.2014-6463),MarieSklodowska CurieActions,Cofund,ProjectINCA600398foreconomicalsupport.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,athttps://doi.org/10.1016/j.elspec.2017.09.008.

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