Thorium effect on the oxidation of uranium: Photoelectron spectroscopy (XPS/UPS) and cyclic voltammetry (CV) investigation on (U1 − xThx)O2 (x = 0 to 1) thin films

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Thorium effect on the oxidation of uranium: Photoelectron spectroscopy (XPS/UPS) and

cyclic voltammetry (CV) investigation on (U1 − xThx)O2 (x = 0 to 1) thin films

Cakir, P.; Eloirdi, R; Huber, F.; Konings, R. J.M.; Gouder, T

DOI

10.1016/j.apsusc.2016.10.010

Publication date

2017

Document Version

Final published version

Published in

Applied Surface Science

Citation (APA)

Cakir, P., Eloirdi, R., Huber, F., Konings, R. J. M., & Gouder, T. (2017). Thorium effect on the oxidation of

uranium: Photoelectron spectroscopy (XPS/UPS) and cyclic voltammetry (CV) investigation on (U1 −

xThx)O2 (x = 0 to 1) thin films. Applied Surface Science, 393, 204-211.

https://doi.org/10.1016/j.apsusc.2016.10.010

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ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o u r n a l ho me p ag e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Full

Length

Article

Thorium

effect

on

the

oxidation

of

uranium:

Photoelectron

spectroscopy

(XPS/UPS)

and

cyclic

voltammetry

(CV)

investigation

on

(U

1 −

x

Th

x

)O

2

(x

=

0 to

1)

thin

ﬁlms

P. Cakir

a,b,∗

_,

_R.

_Eloirdi

a

_,

_F.

_Huber

a

_,

_R.J.M.

_Konings

a,b

_,

_T.

_Gouder

a

a_European_Commission,_Joint_Research_Centre,_P.O._Box_2340,_D-76125,_Karlsruhe,_Germany

b_Department_of_Radiation_Science_and_Technology,_Delft_University_of_Technology,_Mekelweg_15,_2629,_JB_Delft,_The_Netherlands

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received17August2016 Receivedinrevisedform 27September2016 Accepted3October2016 Availableonline4October2016 Keywords: Thinﬁlm Mixedoxide Electrochemistry Corrosion Actinideoxide

a

b

s

t

r

a

c

t

ThinﬁlmsofU1−xThxO2(x=0to1)havebeendepositedviareactiveDCsputtertechniqueand

charac-terizedbyX-ray/Ultra-violetPhotoelectronSpectroscopy(XPS/UPS),X-rayPowderDiffractometer(XRD) andCyclicVoltammetry(CV)inordertounderstandtheeffectofThoriumontheoxidationmechanism. Duringthedeposition,thecompetitionbetweenuraniumandthoriumforoxidationshowedthat tho-riumhasamuchhigheraffinityforoxygen.Depositionconditions,timeandtemperaturewerealsothe subjectofthisstudy,tolookatthehomogeneityandthestabilityofthefilms.Whilecoreleveland valencebandspectrawerenotalteredbythetimeofdeposition,temperaturewasaffectingtheoxidation stateofuraniumandthevalencebandduetothemobilityincreaseofoxygenthroughthefilm.X-ray diffractionpatterns,corelevelspectraobtainedforU1−xThxO2versusthecompositionshowedthat

lat-ticeparametersfollowtheVegard’slawandtogetherwiththebindingenergiesofU-4fandTh-4fare ingoodagreementwithliteraturedataobtainedonbulkcompounds.Tostudytheeffectofthoriumon theoxidationofU1−xThxO2ﬁlms,weusedCVexperimentsatneutralpHofaNaClsolutionincontact

withair.Theresultsindicatedthatthoriumhasaneffectontheuraniumoxidationasdemonstratedby thedecreaseofthecurrentoftheoxidationpeakofuranium.XPSmeasurementsmadebeforeandafter theCV,showedarelativeenrichmentofthoriumattheextentofuraniumatthesurfacesupportingthe formationatalongertermofathoriumprotectivelayeratthesurfaceofuranium-thoriummixedoxide. ©2016TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Thorium-Uranium mixed oxides are interesting nuclear fuel materials. Compared to uranium- plutonium mixed oxide, the higherthermalstabilityandmeltingtemperatureresultsinalarger margintomelting[1].TheuseofThorium-Uraniummixedoxide resultsin theproduction ofsmallerquantitiesofTransuranium elements[2].

Duringthegeologicalstorageofusednuclearfuel,the radionu-clidesembeddedintheuraniumfuelmatrix,whichareproduced duringthereactorirradiation,canbereleasedviathedissolutionof thefuelmatrix.Uraniumhastwostableoxidationstates,(IV)and (VI),andseveralmixedvalencephases(i.e.U3O7,U4O9,U3O8).The

solubilityofuraniumincreasesseveralmagnitudesasthe oxida-tionstateincreasesfromU(IV)toU(VI)inthematrix[3].Onthe

∗ Correspondingauthorat:EuropeanCommission,JointResearchCentre,P.O.Box 2340,D-76125,Karlsruhe,Germany.

E-mailaddress:pelincakir@outlook.com(P.Cakir).

otherhand,ThO2ischemicallystable,havingoneoxidationstate

(IV),anditsdissolutionisreportedtobeextremelydifficult[4]. Sincethefirst contactof thematerialwiththeenvironment happensonthesurface,ourinterestistoobservechangesandthe evolutionoftheoxidelayerformingattheinterface.Inthiswork, thinfilmsof(U, Th)mixedoxidesformedbysputter deposition technique[5]areusedinsteadofusingbulkmaterial[6–9].The useofthinfilmstosimulatethesurfaceofbulkcompoundsresults inahighflexibilityforcompositionalchanges(O/(U+Th)orU/Th ratios).Moreover,itallowsdepositionoflayersofdifferent thick-nessontovariablesubstrates,withdifferentmicrostructurewhen changingthetemperatureandgaspressureduringthedeposition. Materialssuchas uranium− thoriumoxides,aredifficultto studybyphotoelectronspectroscopy[10],which isduetotheir semiconductorpropertiesasaresultofwhichtheflowofcurrent cannotbeachievedproperlyalongthebulksamplethickness.This aspectcanbelimitedoravoidedbytheuseofthinfilmsbecause thelowthicknessresultsinalowresistanceandthevoltagedrop canbeneglected[11,12].

http://dx.doi.org/10.1016/j.apsusc.2016.10.010

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Themaingoalofthisstudyistounderstandtheeffectofthe sta-bletetravalentactinideTh(IV)ontouraniumdioxideandtofollow theelectronicstructure,oxidationstateandredoxreactionsonthe surface.

Thepaperisdividedintothreesections.Theﬁrstpart investi-gatestherelativeoxygenafﬁnitybetweenuraniumandthorium, bybringingthemintocompetition.Thesecondpartexaminesthe effectofhightemperatureandlowtemperaturedepositiononthe surfacepropertiessuchasoxygendiffusionandatomic segrega-tion.Alsoin this part,we compare U1-xThxO2 (x=0 to1) thin

filmstobulkmaterialstoconfirmtheiruseasmodel,byanalysing theirelectronicstructureandlatticeparametersversustheir com-position. The third part consists of electrochemical studies on U1-xThxO2 films(X=0 to 1). Electrochemistry, especially cyclic

voltammetry(CV)ofUO2sampleshasbeenintensivelyemployed

[13–18]howevertothebestofourknowledge,therehasbeenno CVrecordonuranium-thoriummixedoxides,probablyduetothe semiconductorproperties.TheobjectiveofthecurrentCVstudies istoexaminetheoxidationofU1-xThxO2beforeandaftertheCV

usingXPS,lookingatthecompositionandtheoxidationstate.

2. Experimental

ThethinﬁlmsofU1−xThxO2(x=0to1)werepreparedin-situ

bydirectcurrentreactiveco-sputteringfromthoriumanduranium metaltargetsinagasmixtureofAr(6N)andO2(6N).Theoxygen

concentrationintheﬁlmswasadjustedbychangingtheO2partial

pressure(10−8mbar–6×10−6mbar),whiletheArpartialpressure wasmaintainedat5×10−7mbar.Thecompositionofthefilmsis controlledbychangingtherespectivetarget voltagesfor Uand Thtarget.Thethinfilmsweredepositedontosiliconwafer(111) substrates,whichwerecleanedbyAr ionsputtering (4keV) for 1min.Theplasmainthediodesourcewasmaintainedbyinjection ofelectronsof50–100eVenergy(triodesetup),allowingworking atlowArpressureinabsenceofstabilizingmagneticfields.After deposition,thethinfilmsweretransferredtotheXPS-UPSanalysis chamberwithoutexposingthemtoair.

Photoelectronspectroscopydatawererecordedusinga hemi-sphericalanalyserfromOmicron(EA125U5).Thespectrawere taken using Mg K␣ (1253.6eV) radiation with an approximate energyresolutionof1eV.UPSmeasurementsweremadeusingHeII (40.81eV)excitationradiationproducedbyahighintensity win-dowlessUVraregasdischargesource(SPECSUVS300).Thetotal resolutioninUPSwas0.1–0.05eVforthehighresolutionscans.The backgroundpressureintheanalysischamberwas2×10−10mbar. ThespectrometerwascalibratedbyusingAu-4f7/2lineofmetalto

giveavalueat83.9eVBEandCu-2p3/2lineofmetalat932.7eV

BEforXPS,andonHeIandHeIIFermi-edgesforUPS. Photoemis-sionspectraweretakenatroomtemperature.Quantiﬁcationofthe spectrawasdoneusingCasaXPSsoftware(version2.3.13Dev50).As RelativeSensitivityFactors,Scoﬁeldcross-sectionsforMg-K␣ radi-ation[19]weretaken.Anexampleofpeaksdeconvolutionwiththe CasaXPSsoftwareisreportedinFig.1.

Fortheelectrochemicalstudy,astandard3-electrodesetupwas usedwithaworkingelectrodecomposedofU1−xThxO2 (x=0.00,

0.10,0.44,0.84,1.00)thinﬁlmsdepositedontogoldfoilsurface; thereferenceelectrodewasanAg/AgCl(3MKCl)electrode and aPtwireascounterelectrode.Goldfoilswereﬁrstcleanedwith Ethanol/1NH2SO4/H2Othen heatedtill300◦C underultra-high

vacuum(UHV).Asadhesionlayer,aninterfacecomposedofa(U,Th) metallayerwasdepositedat300◦Cbetweenthegoldfoilandthe U1-xThxO2ﬁlm.AllpotentialvaluesinthispaperareversusAg/AgCl.

Themeasurementswerecarriedoutwithastationaryelectrodein anunstirredsolution.Theelectrolytewasa0.01MNaClsolution atneutralpHincontactwithair.Experimentswerecarriedout atroomtemperature(22±3◦C)inaclosedTeﬂonelectrochemical

Table1

Bindingenergyof4f5/2corelevelpeakforThmetal,Umetal,ThO2andUO2.

Substance 4f5/2(eV) satellite

Thmetal 342.3[20,43] –

Umetal 388.40[44] –

ThO2 346.8[20,45] 7.3

UO2 390.95[11,46] 6.7

cellwithanelectrolytevolumeof3ml.Appliedpotentialswerenot correctedforvoltagedropbecauseofthenegligibleelectrode resis-tanceoftheﬁlmelectrodes[11].Beforethescans,theelectrodes werepreconditionedatthemostcathodicpotentialfor5minto reduceanyhigheroxidesformedduringthetransportation.The cyclovoltammetry(CV)measurementswererecordedinpotential sweepcyclesinaﬁrstseries(15cycles)from−1.000VAg/AgClupto

+0.600VAg/AgCl,andbackto−1.000VAg/AgClandtheninasecond

series(15cycles)from−1VAg/AgClto0.8VAg/AgClatascanrateof

0.010Vs−1.UltrapurewaterfromaMilliQ-system(>18M)was used.Chemicalswereallp.a.grade(Merck,Darmstadt).

TheX-raydiffractionanalysesweremadeona conventional Phillips PW3830 powder diffractometer with a Cu X-ray tube (40kV,30mA,K␣1=0.1540560nm).Filmsofabout360nm(1Å/s) thicknessesweredepositedat100◦ConaSi(111)wafer.The pat-ternswererecordedatroomtemperatureinastepscanmodeover a2rangeof[10–100]◦,withastepsizeof0.01◦andacounttime of5sperstep.

3. Resultsanddiscussion

3.1. Relativeoxygenafﬁnity

Tomeasuretherelativeoxygenafﬁnityofthoriumanduranium, aseriesofthinﬁlmsweredepositedsuccessivelybyincreasingthe oxygenpartialpressurewithalowincrementandanalysingthem in-situbyXPS.TheU-4fandTh-4fcorelevelspectraenableto inves-tigatetheoxidationofuraniumandthoriumthroughtheirbinding energy(BE)peak,theirshapeandtheirsatellites.Asreference val-ues,Table1reportsthe4f5/2BEofthoriumanduraniumpresentin

themetalandinthedioxide,aswellasthecorrespondingsatellite. Fig.2reportsU-4f5/2andTh-4f5/2corelevelspectraof(U,Th)Ox

(x<2) thin ﬁlms obtained successively by co-deposition under slightincrease ofoxygenpartialpressureand (U,Th)metalﬁlm spectraareusedasreference(redplots).Itshouldbenotedthatthe oxygenpartialpressuresusedinthisexperimentarenotuniversal values,butvaryaccordingtotheexperimentalset-up.TheBEand thepeakshapesobtainedfor(U,Th)metalareinagreementwith thosereportedinliteratureforsingleandbulkelementofuranium andthorium[20,21].

Theinitialaddingofoxygenduringdepositionaffectsfirstthe thoriumasshownbytherelativeincreaseofthed-screenedpeak whereastheuraniumpeakkeepsconstantinshapeandinbinding energy.Thequickeroxidation ofthoriumrelativetouraniumis confirmedbythefurtherandnearlycompleteoxidationofthorium (greencurves)whileforuraniumthef-screenedpeakisstillthe mainpeak.Thissimpleexperimentdemonstratesanobviousand much strongeraffinityofoxygenforthoriumthanforuranium, asshownbytheoxidationofuraniumstartingonlyoncethorium isnearlycompletelyoxidised.Thisisinagreementwiththehigher stability(lowerGibbsenergyofformation)ofTh4+_relatively_to_U4+_.

TheshifttolowerbindingenergyofTh-4fandU-4fpeaksistaking placeduetothedecreaseofFermi-energylinkedtochargecarrier depletion[22,23]inthesample,asreportedinapreviousstudyon ThO2[20].Itisacoherentshift,occurringforallphotoemissionlines

(includingO-1s).Thethicknessoftheoxidelayerissmallenough toallowelectronstotunnelthrough.Thisavoidsthechargingupon photoemissionandstillpermitsawell-deﬁnedFermi-level[24].

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Fig.1. PeaksDeconvolutionofTh-4f5/2andU-4f5/2,withpeakspositions,FWHM,peaksareaandquantiﬁcationobtainedwithCasaXPSsoftware.

Fig.2. Th-4f7/2line(left)U-4f7/2line(right)corelevelspectraforco-deposited(U,Th)ﬁlmsversustherelativeincreaseofpartialpressureofoxygen(PO2).

3.2. Inﬂuenceofdepositionconditionsandcomparisonwithbulk data

3.2.1. Inﬂuenceofdepositiontimeandtemperatureon U0.50Th0.50O2thinﬁlms

Inthefollowingsection,wecomparetheeffectofdeposition timeoffilmsatroomtemperatureonthecorelevelandvalence bandspectra.Theideabehindistoinvestigatethereproducibility andthehomogeneityofthefilmsurfaceasafunctionofthefilm

thickness,goingfromatomictobulkproperties.WhileXPSprobes adepthofabout100Å,XRDislookingintoasampledepthofthe orderofa␮m.Sinceweshowedthatthecompositionalongthe thicknessoftheﬁlm(i.e.depositiontime)isconstant,weconsider thatthecompositionofthesurfaceisrepresentativeofthatofthe bulk.

AﬁlmcompositionofU0.50Th0.50O2 hasbeenchosenforthis

experimentsseries.Thedepositionrateisabout1Å/s.Fig.3shows thecorelevelspectraU-4fandTh-4f(A),thevalenceband(B)and

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Fig.3. TheinﬂuenceofthedepositiontimeontheU-4fandTh-4fcorelevelspectra(A)andonHeIIvalencebandspectra(B)forU0.50Th0.50O2.

Fig.4. TheinﬂuenceofthedepositiontemperatureontheU-4fandTh-4fcorelevelspectra(A)andontheHeIIvalencebandspectra(B)foraU0.50Th0.50O2.

thecorrespondingO–1s(middle)ofU0.50Th0.50O2ﬁlmdeposited

during5minand25mincorrespondingto30and150nm, respec-tively.Thedepositionconditionswerekeptrigorously thesame despite a small differencefor theoxygen partialpressure. The Fig.3Ashowsthespin-orbitsplitsofuraniumandthorium4f5/2

and4f7/2.Thebindingenergiesof4f7/2 inuraniumandthorium

are380.3eVand334.3eVrespectively,correspondingtouranium (IV)andthorium(IV)oxidationstates.Thesatellitepeaksandthe peakpositionshavebeenextensivelystudiedinpreviouspapers bothonThO2[20]andUO2[11].Thepositionandtheintensityof

thesatellitepeaksarecharacteristicfortheoxidationstatesofthe materialsandarelinkedtotheﬁnalstateoccupation.Thesatellite peakpositionsforU-Thmixedoxides(6.7eVforU-4f_7/2and7.3eV forTh-4f_7/2)arealsothoseexpectedforthe+IVoxidationstate[25]. Withintheuncertaintythequantiﬁcationofthespectrallines,using CasaXPSsoftware,doesnotindicateanyatomicsegregationatthe surfaceforbothdepositions,andthisisalsoemphasizedbythe

con-stantfullwidthathalfmaximum(FWHM)andbindingenergies. Thisshowsthestabilityofthedepositiontechnique.

TheHeIIvalencebandspectra(Fig.3B)aremoresensitiveto5f statescomparedtoHeI(notreportedhere)andduetotheshort rangeoftheemittedphotoelectrons,UPSismoresensitivetothe surfacethanXPS.Asthoriumdoesnothavea 5fstate,thepeak at1.3eVbelowEFis duetotheU-5f2.Thepeakbetween3and

9eVisattributedtoO-2pbandemission.WhileXPSspectradid notshowquantitativedifferences,UPSshowsdifferentO-2pband intensityandthusdifferentratioO-2p/U-5f,goingfrom5.7to7.0 for5.1×10−6and6.1×10−6mbarpartialoxygenpressure, respec-tively.

Withthesamegoalasthepreviousexperiments,wecompared theelectronicstructureofU0.50Th0.50O2ﬁlmspreparedwiththe

sametimeofdepositionatroomtemperatureandat390◦C.Fig.4A reportstheU-4fandTh-4fcorelevelstatesandtheO–1sspectra (insetFig.4),whileFig.4BshowsthecorrespondingHeIIvalence bandofU0.50Th0.50O2.Firstweobservedaclearsuperpositionofthe

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Fig.5.LatticeparameterofU1-xThxO2ﬁlmsversusxobtainedinthisworkand

comparedtoliteraturedataobtainedonbulkcompounds(A).Th-4f7/2andU-4f7/2

BindingenergyinU1-xThxO2ﬁlmsobtainedinthisworkandcomparedtoliterature

obtainedonbulkcompounds(B).

Th-4fcorelevelpeaksforbothtemperatures,whileasmall differ-enceappearsfortheintensityoftheU-4fpeaksandfortheshapeof theirsatellites.Thisshowsthattemperaturedoesnothaveaneffect onthethoriumstatesoncethisiscompletelyoxidised,whereasit influencestheuraniumstatesduetothediffusionoftheoxygenin thefilm,leadingtofurtheroxidationofU.Thisisalsoshowninthe magnifiedsatelliteintensities(insetFig.4A)andbytheslightshift towardshigherBEobservedininsetO–1sfigure.Thecomposition ofthefilmdepositedat390◦CcouldbequantifiedasU0.51Th0.49O2

whichiswithintheuncertaintyofthetechniquesimilartothe com-positionoftheﬁlmdepositedatroomtemperature.InFig.4B,the HeIIvalencespectrashowamaineffectofthetemperatureonthe O-2pbandwhoseintensityandFWHMdecrease,whilethe5f2_peak

shiftsslightlytohigherBE.Thediffusionoftheoxygenfromthetop surfacetotheinneroftheﬁlmcanexplainthiseffect.

3.2.2. Electronicstructureandlatticeparameterversus compositionofU1-xThxO2ﬁlmsandbulkmaterials

Tovalidatetheuseofthinﬁlmsasmodel,weproceededwiththe depositionofaseriesofU1−xThxO2(x=0to1)ﬁlmsmonitoringthe

U-4fandTh-4fbindingenergiesin-situandalsothelattice param-eterversusthecompositioninex-situbyXRD.Thecorresponding datareportedinFig.5Aarecomparedtothedataobtainedonbulk samples[25–29].Thelatticeparametersobservedforourthinﬁlms areclosetotheonesreportedonbulksample,followingVegard’s lawexpectedforthissolidsolution.Theinterceptofthelinearﬁt ofourworkandofAnthonysamyetal.[25]studyforUO2are5.448

and5.465Å,respectively.Thesmallerlatticeparameterfoundfor ourfilmcanbeexplainedbythepresenceofthestressinthefilms andwiththesmallcrystallitesizeasshownbythebroadeningof theXRDpeaks[30].Anotherparameterwhichmayinfluencethe evolutionofthelatticeparameteristheoxygencontentwhich com-paredtobulkcompoundsmightbeslightlydifferentfromourfilms producedin-situ.However,XPSresultsshowedthatthefilmsare stoichiometric.

Fig.5BshowstheBEofU-4f_7/2andtheTh-4f_7/2inU1−xThxO2

(x=0to1)versusthecompositionsandcompareswiththedata obtainedon bulk samples [25,31]. Vael et al. [32] pointed out thatthebindingenergyofU-4f7/2forU(IV)rangesfrom379.9to

380.9eV.Ourresultsstayinthereportedrangewhichshowsthat UandThmixedoxidearestoichiometric.Bindingenergiesof Th-4f7/2arealsostableina0.6eVrangeandinagoodagreementwith

Fig.6. CyclicvoltammetryonU1-xThxO2(x=0,0.10,0.44,0.84,1)ﬁlmsforthefırst

twocycles.

dataobtainedbyAllenetal.[31]butdifferentby1eVrelativelyto theonereportedbyAnthonysamyetal.[25].

Tosummarize,U1−xThxO2(x=0to1)mixedoxidesﬁlmsfollow

Vegard’slawandthebindingenergiesareingoodrelationwiththe onesobtainedforbulkmaterials.Itisapparentfromtheseresults thatourthinﬁlmscanbeusedasmodelforactinideoxidebulk samples,despitethemicrostructurewhichcanbedifferent(stress, preferentialorientation,...).

3.3. Electrochemicalstudies

Manystudieshavebeenreportedonchemical,physical proper-tiesandleachingexperimentsof(U,Th)mixedoxides[6,9,33,34]. Sunderetal.[9]showedthattheoxidationprocesstakingplace atthesurfaceof(U,Th)mixedoxidesamplesaresimilartopure UO2.However,comparedtoUO2theleachingexperimentsshowed

thatthedecreaseoftheuraniumdissolutionrateandthishasbeen linkedtotheloweruraniumcontentpresentinmixedoxideandin contactwithsolution.Thisstatementhasbeensupportedby Heis-bourgetal.[6,33]whostudiedthekineticsofdissolutionof(U,Th) mixedoxidesboththorium-anduranium-richsamples.Also,XPS analysesofsamplesobtainedafterleachingexperiment demon-strateasurface enrichedin thorium,formingaprotective layer disablingfurtherdissolutionoftheuranium[6].Demkowiczetal. [34]alsoreportedthattheuraniumdissolutionrateinfresh sam-plesof(U,Th)O2 was10to40timeslowerthanforconventional

UO2fuel.

Thuscompared topureUO2,thereisa clearindicationfor a

lowerdissolutionrateofuraniumin(U,Th)mixedoxide,however theoxidationandreductionprocessisnotclearyet.Sunderetal. [9]pointedoutthatduetothehighelectricalresistivity,working withelectrochemicaltechniquesonsuchasystemasbulk mate-rialwasnotpossible.Toovercomethisdifficulty,thinfilmscanbe agoodalternativeasdemonstratedbyMiserqueetal.[11],who reportedcyclicvoltammetryonUO2thinfilms.BycomparingCV

measurementsonUO2thinﬁlm(1␮m)andbulkUO2(1mm),itwas

conﬁrmedthattheﬁlmsmadebyDCsputteringtechniquehave muchlowerresistancethanthebulkelectrodesandtheIRdropfor cyclicvoltammetryisnegligible.

Fig.6showstheﬁrsttwocyclesofU1−xThxO2 (x=0.10,0.44,

0.84,1.00)electrodesscannedbetween−1Vto0.6V(vs.Ag/AgCl) in0.01MNaCl.Theﬁrstcyclesareindicatedwithsolidlines,the secondonesareindicatedwithdashedlines.Romannumberson thegraphsindicatethepeakpositionsofthesuggestedreactions basedonliterature[17,35],Duringtheﬁrstcycle,theUO2electrode

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win-Fig.7. CyclicvoltammetryforU1-xThxO2(x=0,0.10,0.44,0.84,1)ﬁlmsfromthe3rduptothe15thcycleintherangeof−1Vto0.6V(A).CyclicvoltammetryforU1-xThxO2

(x=0,0.10,0.44,0.84,1)ﬁlmsforhigherpotentialwindowupto0.8V(vs.Ag/AgCl)andgoldsubstratecycles(yellowlines)(B).(Forinterpretationofthereferencestocolour inthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

dowitisthermodynamicallyimpossibletooxidisehomogeneous UO2andthepreviousstudiesattributedthecurrentchangetothe

differentenergysitesorinhomogeneitysuchasgrainboundaries andhyperstoichiometry(e.gUO2+x)onthesurfaceoftheelectrode

[17,35].TheﬁrstcyclefortheUO2,U0.90Th0.10O2,U0.56Th0.44O2and

U0.16Th0.84O2electrodesdoesnotshowanysigniﬁcantpeaks,

how-ever,thesecondcyclesofUO2andU0.90Th0.10O2electrodesshowa

slightcurrentincrease.Ontheotherhand,atlowercontentof ura-nium(i.e.U0.56Th0.44O2andU0.16Th0.84O2 electrodes)wedonot

observetheincreaseofthecurrent,whichcanbeexplainedbya loweroxidationonthesurfaceintheﬁrstcycles.

InregionII,theoxidationofUO2toUO2+xstartsandinthe

sub-sequentregionIII,UO2+xincreasestoUO2.33byO2−incorporation

intothelattice.Athigherpotentials,theoxidationprocessmight leadeithertoitsdissolutionasUO22+orrecrystallizationasUO2.5

andUO2.66 (duetotheadsorbedUO22+).However,inneutralto

slightlyalkalineelectrolytes,UO22+insolutionmightre-precipitate

ontheelectrodeeitherasschoepite(UO3·H2O)orasmetaschoepite

(UO3·2H2O)[17,35].

InregionIII,afastincreaseofthecurrentisobservedforboth UO2andU0.90Th0.10O2electrodesindicatingtheonsetof

dissolu-tion,whichisnot tosuchextentthecase forU0.56Th0.44O2 and

U0.16Th0.84O2 ﬁlms. Thisprocesswasstudiedbymonitoringthe

masslossfromtheUO2electrodeinsolutionsofpH=5topH=8

usingEQCMbySeibertetal.[12].

Region IV and V are thereduction peaks of oxidised layers observed on cathodic potentials. These peaks are usually cou-pledwiththeanodicoxidationpeaks.Thepotentialsofthepeaks arerelatedtothethicknessoftheoxidelayerformedduringthe anodicscansatthesurface[36].Inneutralelectrolytes,regionIV is observedand attributed toreduction of UO3·nH2O toUO2+x.

UO3·nH2O phases are insulators and thought to precipitate as

porouslayeranddonotinterferewiththereductionofthe under-lyingoxides [37,38]. RegionV is associated tothereduction of underlyingoxidessuchasUO2.33/UO2+xorUO2.5,UO2.67 created

inregionIII,asstatedabove[35].StartingfromregionIVandgoing toregionV,UO2andU0.90Th0.10O2electrodesshowhighercathodic

currentsindicatingthereductionofU(VI)toU(IV).Thisisnotthe case for U0.56Th0.44O2 and U0.16Th0.84O2. Thisbehaviouris also

reﬂectedinregionVwhileUO2 andU90Th10O2 electrodesshow

reductiontostoichiometricUO2,theothertwoelectrodesdoesnot

indicateanycompellingcurrentactivity.Thelowcurrentobserved atthispotentialwindowontheU0.56Th0.44O2 and U0.16Th0.84O2

electrodescanbeattributedtothelowercontentofuraniumin

con-tactwiththesolution.Alsothesubstitutionofuraniumbythorium inUO2latticeleadstothealterationoftheelectricproperties(from

semi-conductorUO2toinsulatorThO2)ofmixedoxidesamples.It

decreasestheelectricalconductivityandthusthedissolutionrate ofuraniumasreportedinliterature[9,39].

Fig.7Arepresentsthecyclesfrom3rd_till₁₅th_CV_in_the_range

of−1Vto0.6VAgCl/Ag.InFig.7A,inregionV,wedoobservethat

thecathodiccurrentincreaseswiththeuraniumcontent.While U0.90Th0.10O2and,toalesserextent,U0.56Th0.44O2behaveina

simi-larwayaspureUO2,showingashiftingtohighercathodicpotential

alongthesuccessivecycle,U0.16Th0.84O2remainsconstantcurrent

inthis region.Theshiftingtolowerpotentialseemstoindicate theformationofathickeroxidelayerincreasingalongthe succes-sivecycles(indicatedwiththearrows).Uncompletedreductionin domainVleadstoanincreaseofoxidationonregionIofthecurrent gettinghigherandhigherineachcycle.Thesamephenomenonis notobservedforU0.56Th0.44O2andU0.16Th0.84O2electrodesdueto

theloweramountofuranium.InregionIIandIII,whereoxidation ofUO2 andUO2+x istakingplace,thecurrentincreaseswiththe

contentoftheuraniumpresentintheelectrodes.

Whenthe scans continued upto 0.8VAg/AgCl for another 15

cycles,differenceswereobservedasshowninFig.7B.Lookingat thepeaksinregionVandregionI,wedoobserveanoppositetrend comparedtoFig.7A.Thebroaderscanwindowtohigheranodic potentialleadsashiftingtolowercathodicpotentialofregionV andtoadecreaseofcurrentalongthecyclesinregionsVandI. Thismightbeduetofactthathigherdissolutionratesareachieved athigheranodicpotentials.Thisdecreaseinintensityofpeaksin regionVandImayberelatedtothedecreaseofthethicknessof thehyperstoichiometricoxidelayers(regionV),thusleadingtoless differentenergysitesorgrainboundariesonthesurface(regionI). Thereasonwhythisishappeninginthisbiggerpotentialwindow isnottotallyclear.

Goldusedassubstrateandconsideredasnoblemetaldoesnot interferestronglyintheworkingpotentialwindowchoseninthis study,asshowninFig.7Binyellowlines.Inthepotentialwindows upto0.6V(notreportedhere)ithasnoeffectasdemonstrated bythelowcurrentvalue,butwhenmeasuredupto0.8Vapeak markedasX onthecathodicpotentials isobserved.Thismight beattributedtotheelectrolytereduction.Theﬁlmshaveacertain porosityenablingthecontactofthesolutionwithgoldsubstrate. TheintensitypeakXincreasealongthesuccessivecyclesandcan berelatedtotheoxidelayergettingmoreandmoreporouswhile thegoldsurfaceincreasestogetherwithitsrelatedcurrent.

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Table2

CompositionandU-4f5/2,Th-4f5/2andO–1sofﬁlmsbeforeandafter30cyclesofCVin[NaCl]=0.01M.

Sample BeforeCV AfterCV

Composition BindingEnergy,eV Composition BindingEnergy,eV

U-4f5/2 Th-4f5/2 O-1s U-4f5/2 Th-4f5/2 O-1s

1 U0.67Th0.33O2 380.5 334.0 529.8 U0.57Th0.43O2+x 381.3 334.1 529.7/531.2

2 U0.56Th0.44O2 380.5 333.9 529.8 U0.40Th0.60O2+x 381 333.9 529.8/531.2

3 U0.16Th0.84O2 380.5 333.9 529.8 U0.14Th0.86O2+x 380.9 333.8 529.7/531.3

Fig.8. U-4fandTh-4fofU0.67Th0.33oxideﬁlmbeforeandafter15cycles,insetgraph

representsthecorrespondingO–1sspectra.

ThecurrentcountsfortheU0.56Th0.44O2andU0.16Th0.84O2

elec-trodes throughout their consecutive CVs have about the same values,whichcanbeexplainedbytheloweramountofuranium atthesurface.TheU0.16Th0.84O2electrodeisratherbehavingasthe

goldsubstrateitself.Ithasahighercurrentthangoldsubstratedue tothesmallamountofuraniumonthesurface.Nonethelessthe currentactivityshouldberelatedtotheelectrolyteinterference becauseThoriumisnotexpectedtoshowanyoxidationonthese potentialwindow[40].

Fig.8comparestheU-4fandTh-4fspectraofU0.67Th0.33O2ﬁlm

obtainedbeforeandafterCVasanexamplethegeneral observa-tions.FirsttheU-4fpeaksshifttohigherbindingenergyindicating furtheroxidationasUO2+x,unliketheTh-4fpeaksthatkeepa

con-stantbindingenergybecausenofurtheroxidationthanTh(IV)can takeplace.We alsoobservedthechangeintheintensitypeaks. WhenthetwospectrabeforeandaftertheCVexperimentare nor-malizedtoTh-4f,theU-4fintensitypeaksdecreasedaftertheCV cycles,indicatingaloweruraniumcontentatthesurfacecompared totheinitial,asdepositedﬁlmcomposition.AlsotheshiftofU-4f tohigherbindingenergyafterCVexperimentindicatesahigher oxidationstateforuraniumatthesurface.Thequantiﬁcationusing CasaXPSshowsthatthecompositionchangesfromU0.67Th0.33O2

toU0.57Th0.43O2+x.IntheinsetofFig.8weobservethebroadening

ofthepeakO–1swhichcanbedeconvolutedintwocomponents, atlowBEandoneathigherBE,correspondingtoO2−andtoOH− respectively[33,41].TheO–1sshiftismorepronouncedontheﬁlms thatcontainhigheramountofuraniumonthecomposition.

TosummarizetheCVexperiment onthemixed oxideﬁlms, Table2reportsthecompositionsandtheBEofU-4f_5/2,Th-4f_5/2and O–1scorelevelpeaksforthedifferentsamplesbeforeandafterCV cycles.Theresultsshowastrongdecreaseofuraniumcontent rela-tivetothorium,decreasingbyabout30at%(sample2)comparedto theinitialcomposition.Thepreferentialdissolutionofuraniumat thesurfaceenablestoexplainthisresultleadingtoanenrichment ofthoriumatthesurfacewhichalongoxidationanddissolutionof U(VI)providesaprotectionlayerinhibitingthefurtheroxidation oftheuraniumpresentdeeperintheﬁlm.Thethoriumeffecthas

beendiscussedinliterature[6,9,33]reportingitsroletopassivate thesurface,limitingfurtheroxidationofuraniumanddecreasing thedissolutionrateofuranium.Ourresultsenabletoconﬁrmthis processtakingplaceatthesurfaceofthesampleincontactwitha neutralsolution.

4. Conclusionandsummary

ThinﬁlmsofU1−xThxO2(x=0to1)mixedoxideswere

investi-gatedbyXPS/UPS,XRDandCVtoestablishthepossibleinﬂuence ofthoriumontheoxidation/dissolutionprocess.

WeﬁrstinvestigatedtherelativeoxygenafﬁnityofThandUand oxidationofuraniumstartedonlyoncethoriumwascompletely oxidized.Thisobservationisconsistentwiththehigherstability ofThO2 (fG0 (298K):−1170kJmol−1)comparedtoUO2 (fG0

(298K):−1031kJmol−1[42]).

Basedoncorelevelandvalencebandspectra,homogeneityof theﬁlmscouldbeshowedalongthedeposition.Alsodeposition temperature(from25◦Ctoabout400◦C)hadnoinﬂuenceon tho-riumoxidationstatewhileuraniumundergoesfurtheroxidation, seenbyashiftoftheU-4fsatelliteandtheU-5f2 _peak,_and_by_a

changeoftheO-2p/U-5f2_intensity_ratio.

Todeterminethesuitabilityofthinﬁlmsasmodelsystemfor nuclearfuelwecomparedthelatticeparametersandtheU-4fand Th-4fcorelevelbindingenergyofﬁlmstobulkcompounds,fora seriesofcompositionsofU1−xThxO2(x=0to1).Thelattice

param-etersfollowedtheVegard´ıslaw,withaslightdeviationattributed tostresspresentintheﬁlms.AlsothebindingenergiesofU-4fand Th-4fcorelevelswereinagreementwiththosereportedonbulk compounds.

Cyclicvoltammetrywasusedtofollowthesurfaceredox reac-tionsfordifferentcompositions.Inthepotentialwindowof[−1 to0.6]V(vsAg/AgCl),oxidativedissolutionofuraniuminneutral pHsolutionsuggeststheformationofa layerofhigheroxideat thesurface.Shiftofthepeakstothehighernegativepotentialsare observedinthecathodicregionVatabout−0.9V(vsAg/AgCl), aswell asin current intensityincrease in theanodic regionat about−0.5V(vsAg/AgCl)inregionI,indicatingformationofthicker layershyperstoichiometricoxidesoneachcycleperformed.The intensity and the positionof the peaks showed a proportional relationwiththethoriumcontentinUO2 matrix.However,ona

largerpotentialwindow[-1to0.8]V (vsAg/AgCl),an opposite behaviourisobserved,suchaslowerintensityandadverse direc-tionshiftsoneachcycles.Thischangeinbehaviourshowedthat thesuccessivecyclesresultinthinnerlayersof hyperstoichiomet-ricoxidesonthesurface.Ontheotherhand,inthehigherpotential windowweobservedhighercurrentcountsforbothanodicand cathodicpotentialsforﬁlmswithhigherthoriumcontent(going fromU0.56Th0.44O2andU0.16Th0.84O2).Thiscanbeexplainedbythe

factthatahigherthoriumconcentrationrequireshigherpotentials tooxidiseuranium.

TheXPSspectraobtainedonsamples,beforeandafterCV exper-iments,indicatedclearlyenrichmentinthoriumatthesurfaceand ahigheroxidationstateofuranium.Theresultsalsoindicatedthat ahigherinitialuraniumcontentonthesurfaceleadstoahigher

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shiftofU-4fbindingenergies,suggestingahigheroxidationstate ofuranium.Thiswassupportedbytheshapeofthecorresponding O–1sspectrumshowinghighercontributionofoxygenfromOH− groups.

Acknowledgments

WethankD.WegenandA.Seibertforfruitfuldiscussionsand alsotoS.StumpfandZ.BaofortheXRDmeasurements.P.Cakir acknowledgestheEuropeanCommissionandJointResearch Cen-tre.

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