A structural investigation of the interaction of oxalic acid with Cu(110)

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warwick.ac.uk/lib-publications

Original citation:

White, T. W., Duncan, D. A., Fortuna, S., Wang, Y.-L., Moreton, Ben, Lee, T.-L., Blowey, P.,

Costantini, G. and Woodruff, D. P.. (2017) A structural investigation of the interaction of

oxalic acid with Cu(110). Surface Science.

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Surface Science 668 (2018) 134–143

ContentslistsavailableatScienceDirect

Surface

Science

journalhomepage:www.elsevier.com/locate/susc

A

structural

investigation

of

the

interaction

of

oxalic

acid

with

Cu(110)

T.W.

White

a

_,

_D.A.

_Duncan

b,c

_,

_S.

_Fortuna

d

_,

_Y.-L.

_Wang

e

_,

_B.

_Moreton

a

_,

_T.-L.

_Lee

c

_,

_P.

_Blowey

f

_,

G. Costantini

a

_,

_D.P.

_Woodruﬀ

f,∗

a_Chemistry_Department,_University_of_Warwick,_Coventry,_CV4_7AL,_UK

b_Technische_Universität_München,_Physik_Department_E20,_Garching,_D-85748,_Germany c_Diamond_Light_Source,_Didcot,_OX11_0DE,_UK

d_SISSA,_Via_Bonomea_265,_Trieste,_Italy

e_Institute_of_Physics_&_University_of_Chinese_Academy_of_Sciences,_Chinese_Academy_of_Sciences,_Beijing_100190,_China f_Physics_Department,_University_of_Warwick,_Coventry,_CV4_7AL,_UK

a

b

s

t

r

a

c

t

TheinteractionofoxalicacidwiththeCu(110)surfacehasbeeninvestigatedbyacombinationofscanningtunnellingmicroscopy(STM),lowenergyelectron diffraction(LEED),softX-rayphotoelectronspectroscopy(SXPS),near-edgeX-rayabsorptionfinestructure(NEXAFS)andscanned-energymodephotoelectron diffraction(PhD),anddensityfunctionaltheory(DFT).O1sSXPSandOK-edgeNEXAFSshowthatathighcoveragesasinglydeprotonatedmonooxalateisformed withitsmolecularplaneperpendiculartothesurfaceandlyinginthe[110]azimuth,whileatlowcoverageadoubly-deprotonateddioxalateisformedwithits molecularplaneparalleltothesurface.STM,LEEDandSXPSshowthedioxalatetoforma(3×2)orderedphasewithacoverageof1/6ML.O1sPhDmodulation spectraforthemonooxalatephasearefoundtobesimulatedbyageometryinwhichthecarboxylateOatomsoccupynear-atopsitesonnearest-neighboursurfaceCu atomsin[110]rows,withaCu–Obondlengthof2.00±0.04Å.STMimagesofthe(3×2)phaseshowsomecentredmoleculesattributedtoadsorptiononsecond-layer Cuatomsbelowmissing[001]rowsofsurfaceCuatoms,whileDFTcalculationsshowadsorptionona(3×2)missingrowsurface(witheverythird[001]Cusurface rowremoved)isfavouredoveradsorptionontheunreconstructedsurface.O1sPhDdatafromdioxalateisbestfittedbyastructuresimilartothatfoundbyDFTto havethelowestenergy,althoughtherearesomesignificantdifferencesinintramolecularbondlengths.

1. Introduction

It is well-establishedthat carboxylic acidsdeprotonate when ad-sorbedon many metalsurfaces,andnotably onCu(110), whichhas provedtobethemodelsurfaceusedinmanyofthesestudies.Indeed, moregenerally,carboxylate-substratebondingiswidelyusedtotether arangeofmoleculestosurfaces(e.g.[1]).Thepresenceoftheformate speciesHCOO,thedeprotonatedformofthesimplestcarboxylicacid, formicacid(HCOOH,FoA– seeFig.1),onCu(110)wasﬁrstinferred byWachsandMadixintemperature-programmedreactionspectroscopy (TPRS)fromapre-oxidisedCu(110)surfaceexposedtomethanol[2]. SubsequentlyYingandMadix[3] deducedfromTPRSthatsurface for-mateisformedbyexposureofCu(110)toformicacid.Spectroscopic ob-servationoftheformatespeciesonCu(110)wassubsequentlyachieved withbothultravioletandX-rayphotoelectronspectroscopy(UPSand XPS)[4],high-resolutionelectronenergylossspectroscopy(HREELS)

[5]andreﬂection-absorptioninfraredspectroscopy(RAIRS)[6].While thesespectroscopicstudiesclearlyindicatedthatformateisbidentate bondedtothesurfacethroughthetwo(equivalent)Oatoms,withthe molecularplaneperpendiculartothesurface,itwasultimately scanned-energymodephotoelectrondiﬀraction (PhD[7])thatprovideda de-tailed quantitative description of the local bonding geometry [8,9].

∗_{Corresponding}_author.

E-mailaddress:d.p.woodruﬀ@warwick.ac.uk (D.P.Woodruﬀ).

Speciﬁcally,themoleculeisadsorbedinashortbridgesiteonCu(110), themolecularplanelyingina[110]close-packeddirectiononthe sur-face,withthetwoOatomsalmostexactlyatoptwonearest-neighbour surfaceCuatoms(Fig.2).

Subsequently,asurfaceacetatespecies,formedbydeprotonationof aceticacid(AA,Fig.1)exposedtotheCu(110)surface,wasidentified spectroscopically[10–12]andshowntohaveitscarboxylateplanelying ina⟨110⟩azimuthwithitsC–Caxisperpendiculartothesurface,while aPhDinvestigation[13]identifiedthelocalbondingsitetobethesame asfortheformatespecies.Athirdsimplecarboxylate,namelybenzoate formedbythedeprotonationofbenzoicacid(BA,Fig.1)onCu(110), wascharacterisedbyHREELS,RAIRS,lowenergyelectrondiffraction (LEED) andscanningtunnellingmicroscopy (STM) [14–16] showing thatatlowcoveragesthephenylringofthebenzoateisessentially par-alleltothesurface,butathighcoveragethemoleculestandsup.APhD investigationofthisstanding-upphase[17]foundthelocalbonding ge-ometryonCu(110)tobeidenticaltothatofformateandacetate.The molecularorientationwas alsoconfirmedbyelectron-stimulated des-orptionion-angulardistributions(ESDIAD)[18].STMstudiesindicate thatCuadatomsmaybeassociatedwiththebenzoatespecies,atleastat low-coverages[19–21],formingbenzoatedimerswithanintermediate CuadatomorclustersoffourbenzoatemoleculesaroundapairofCu

https://doi.org/10.1016/j.susc.2017.10.025

Received19September2017;Receivedinrevisedform23October2017;Accepted26October2017 Availableonline28October2017

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Fig.1.Schematicdiagramsofthecarboxylicacidsdiscussedinthetext,namely:formicacid(FoA),aceticacid(AA),benzoicacid(BA),oxalicacid(OA),succinicacid(SA),malicacid (MalA),tartaricacid(TA),fumaricacid(FuA),maleicacid(MaleA)andterephthalicacid(TPA).

adatoms.However,atleastforthehigh-coveragephases,thelocal bond-inggeometriesofallthesesimpledeprotonatedsingle-carboxylicacid groupmoleculesareidenticalandshownoevidenceforanyinﬂuence ofCuadatoms.

Interestingquestionsarise,however,regardingthebehaviourof di-carboxylicacids.Aretheysinglyordoublydeprotonated,andwhatit isthelocalbondinggeometry?Hereweseektoanswerthesequestions forthesimplestdicarboxylicacid,oxalicacid.Interestingly,therehave alreadybeenanumberofinvestigationsofseveralmorecomplex di-carboxylicacids,shownin Fig.1,motivatedin largepartbyinterest intheinteractionofchiralmolecules(andrelatednon-chiralspecies) onsurfaces[22].Forseveralofthesemolecules,namelytartaricacid, TA[23,24],succinicacid,SA[25],andmalicacid,MalA[26],the re-sultsofmainlySTMandRAIRSstudiesindicatethatthemoleculesare doubly-deprotonatedandadoptalying-downconﬁgurationatlow cov-erage,butaresinglydeprotonatedandstandup athighercoverages. Moreover,lowcoverageSTMandXPSstudiesofterephthalicacid,TPA, onCu(110)clearlyindicateadoubly-deprotonatedlying-downspecies

inseveraldiﬀerentorderedphases[27],whileaRAIRSstudyindicates thatathighercoverageasingly-deprotonatedstanding-upphaseoccurs

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T.W.Whiteetal. SurfaceScience668(2018)134–143

Fig.2. Schematicdiagramsshowingthelocaladsorptiongeometriespreviouslyreported fortheformate,monotartrateandbitartratespeciesonCu(110)basedonPhD investiga-tions.NotethatthePhDtechniqueisinsensitivetothelocationofHatoms,sotheseare omittedfromthediagrams.OHspeciesarerepresentedbytheassociatedOatomswith adiﬀerentcolouring.(Forinterpretationofthereferencestocolourinthisﬁgurelegend, thereaderisreferredtothewebversionofthisarticle.)

havingbeenseenforthesimpledeprotonatedaminoacidsglycinateand alanate,inwhichbondingtothesurfaceisnotonlybythecarboxylate OatomsbutalsotheaminoNatom[30,31].

Ratherdiﬀerentbehaviourhasbeenreportedfortheinteractionof maleicacid(MaleA)andfumaricacid(FuA),withtheapplication of STM, LEEDandRAIRS together withC 1s XPS, leading tothe con-clusionthat maleic acid is always doubly deprotonatedon Cu(110)

[32] whereasfumaricacidis onlydetected(withRAIRS) asa singly deprotonatedspecieson thissurface[33].These twoacidmolecules arethecis andtrans formofbutenedioicacid,andwhiletheconclusion regardingmaleicacidmaybe attributabletothe cis conformation of themoleculeitisnotclearwhythe trans conformationoffumaricacid shouldprecludethepossibilityofitbeingdoublydeprotonated.

TheonlypreviousstudyofoxalicacidonCu(110)appearstobethat ofMartin,ColeandHaq[34]whoconcluded,onthebasisofRAIRSdata, thatthemoleculestandsupandissinglydeprotonated(monooxalate). Atlowexposuresnoabsorptionbandscouldbedetected;however,this couldbeconsistentwiththepresenceofalying-down(presumably dou-blydeprotonated-dioxalate)speciesinwhichthemolecularvibrational modeswouldnotbedipoleactiveandthuscannotbedetectedinRAIRS. AninvestigationofoxalicacidinteractionwithCu(111),usingSTMand high-resolutionsoft(synchrotronradiation)XPS(SXPS),indicatedthat bothmono-anddi-oxalatespeciesoccurred,primarilyatlowandhigh coveragerespectively[35].

Here wereport the results ofexperiments conducted toobtaina rathercompleteunderstandingofthestructuralandchemical proper-tiesofthesurfacespeciesresultingfromexposureofCu(110)tooxalic acid.Basiccharacterisationofthesystemwasachievedthroughtheuse ofSTM,LEED,andSXPS,particularlyfromtheO1sstate.Quantitative structuralinformationhasbeenobtainedfromO1sPhDcombinedwith OK-edgeNEXAFS(near-edgeX-rayabsorptionfinestructure).Wefind thatoxalicacidfollowsthepatternofmostother(morecomplex) di-carboxylicacidsinadoptingadoublydeprotonatedlying-downoxalate atlowcoveragebutasinglydeprotonatedstanding-upspeciesathigh coverage.ThePhDdataprovideacleardeterminationofthelocal ad-sorptiongeometryforthemonooxalate,butidentifyingtheexact geom-etryofthedioxalateprovedmorechallenging.However,STMimagesof the(3×2)phaseledindirectlytothepossibilitythatthisphaseinvolves [001]missingrows,andDFTcalculationsconfirmthatthedioxalateis

morestronglyadsorbedonsuchareconstructedsurface.O1sPhDdata areconsistentwiththequalitativemodelfavouredbytheDFT calcula-tions.

2. Experimentaldetails

InitialcharacterisationexperimentswereperformedinaUHV sur-facesciencechamberbasedattheUniversityofWarwick,ﬁttedwitha LEEDoptics,alow-temperatureSTM,andtheusualfacilitiesfor sam-plehandlingandcleaning.TheCu(110)samplewaspreparedinsituby cyclesof1keVAr+_ion_bombardment_and_annealing_to₈₇₀_K_for₅_min untilacleanwell-orderedsurfacewasobtainedasindicatedbyLEED andSTM.Oxalicaciddepositionwasbyroomtemperatureevaporation fromakovarglasstube attachedtotheUHVsystem;duringvacuum bakeofthemetalliccomponents(toonly373K)thistubewaskeptclose toroomtemperaturetoavoidexcessmateriallossduetothehighvapour pressure.Thetubewasthenoutgassedwithaheatgun,purgedintothe pumpinglineofthesampleloadlock.

SXPS,NEXAFSandPhDmeasurementsweremadeintheUHV sur-facescienceend-stationofbeamlineI09atDiamondLightSource.This beamlineisﬁttedwithapairofcantedundulators,eachfeedinga dif-ferentmonochromator,butbothbringingthefocussedmonochromated beamstothesamespotonthesample.Thelower-energyundulator radi-ationbranchisﬁttedwithagrazing-incidenceplane-grating monochro-matorcapableofprovidingphotonsatthesampleintheenergyrange 100–1100eV.Thehigher-energybranch,usingadouble-crystalSi(111) monochromator,wasnotusedintheexperimentsreportedhere.Sample cleaninganddosingusedthesamemethodsasintheWarwick cham-ber,althoughsampleorderingandcleannessinthiscasewereassessed usingalow-currentLEEDopticsandSXPspectraobtainedusingaVG ScientaEW4000electronspectrometerwitha±30° acceptanceanglein theplaneofincidenceandpolarisationoftheradiation,mountedwithin thispolarisationplaneatanangleof60° betweentheincidentradiation andthedirection fromthesampletothecentreofthedetector.This analyserwasalsousedtocollectthephotoemissiondataforthePhD experimentsandtheAugerelectronemissionintheNEXAFSstudies.

3. Surfacecharacterisationresults

3.1. STM and LEED

InitialcharacterisationofthesystemusingSTMandLEEDshowed thatanominalexposureatroomtemperatureofoxalicacidof5×10−6 mbars,asmeasuredbyaniongaugewithinthemainchamber,ledtoa disorderedsurfacewithnoorderedmolecularimagingdetectablewith STM(Fig.3(a)).However,afterannealingto398KbothSTMandLEED indicatedthepresenceofa(3×2)orderedphase,asshowninFig.3(b andd).Thiswastheonlyorderedphaseseen byeithertechniqueat anysurfacecoverage.WhiletheprotrusionsseeninSTMimageofthe (3×2)phaselackanyvisibleinternalstructurethesearetentatively as-sumedtobeduetoindividualadsorbedmolecules.Thisattributionis supportedbySXPSdataobtainedlater(seebelow),whichprovideda coverageestimateofaphasegivinga(3×2)LEEDpatternof approxi-mately0.2ML,broadlyconsistentwithonemoleculeper(3×2)surface unitmesh(0.17ML).Furthersupportforthisinterpretationisprovided bytheSTMimageofFig.3(c),obtainedafterfurtherannealingto423K, whichledtoalossofmoleculesfromthesurfaceandproduceda dis-orderedsurfacethatdoesshowindividualad-species.Inparticular,a featureinthecentreofthesuperimposedgreencircleappearstobean isolatedadsorbedmoleculeinanotherwisecleanareaofthesurface,and indeedtheshapeofthisfeaturecouldbeconsistentwithalying-down fullydeprotonateddioxalatemoleculethatSXPSandNEXAFSdata(see below)indicatetobepresentonthesurfaceinthe(3×2)phase.

TheSTMimageofthe(3×2)phase(Fig.3(b))showsaparticularly surprisingsetoflocaldefects,namelyprotrusionsatthecentreofthe (3×2)orderedfeaturesthatappeartobequitesimilartothoseofthe

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2 nm

[001] _ [110]

(0,1) (1,0)

2 nm

b

c

5 nm

a

_ [110] [001]

d

Fig.3. (a)STMimageofthedisorderedunannealedsurface.(b)STMimageand(d) LEEDpatternrecordedat148eVfromtheordered(3×2)phaseformedbyoxalicacidon Cu(110)followingannealingofahigher-coveragedisorderedphaseto398K.(c)shows anSTMimageobtainedafterfurtherheatingto423K.Tunnellingconditions:(a)120pA,

−1.1V;(b)180pA,−1.1V;(c)100pA,1.1V.

2 nm

+1.1 V

x

(nm)

1 2

60

40

20

0

z

(pm)

2 nm

-1.1 V

a

b

c

Fig.4. (a)and(b)showSTMimagesofthe(defected)(3×2)phaseobtainedatopposite biasvoltages(butthesametunnellingcurrentof180pA).(c)showstheline-scanproﬁles alongtheequivalentlocationsinthetwoimages,colour-codedingreenandblue.(For interpretationofthereferencestocolourinthisﬁgurelegend,thereaderisreferredtothe webversionofthisarticle.)

un-defectedregions.Atﬁrstsightthesecouldbeinterpretedas addi-tionalidenticalmolecules,butthecentreofa(3×2)mesh(oranymesh involvinganodd-numberedperiodicityinonedirection)cannot corre-spondtoanidenticallocalsite.Infacttheapparentheightofthecentred defectprotrusions,relativetothoseoftheregular(3×2)protrusions, doesdependonthetunnellingconditions(seeFig.4),withtheratioof theseheightsbeingcloseto1.0inpositivesamplebias(emptysurface

Fig.5. SXPspectraaroundtheO1sandC1semissionpeaksrecordedfromthetwo diﬀerentcoveragesofoxalicacidspeciesonCu(110).

stateimaging),butcloseto1.25innegativesamplebias(occupied sur-facestateimaging).Thisobservationisconsistentwiththeviewthatthe moleculesgivingrisetothesetwofeaturesdodiﬀer.

NoticethattheSTMimageofFig.3(b)alsoshowsanotherdefect structure,namelyanantiphasedomainboundarycloseto,andrunning parallelto,theright-handedgeoftheimage,markedwithan arrow-head.Images(notshown)fromlesswell-orderedpreparationsshowa muchhigherdensityoftheseantiphasedomainboundariestoproducea ‘striped’ structurethatledtoLEEDpatternswithspotsplittingof(n ×2) character where n issigniﬁcantly largerthan3.Notice,though,that thewidthoftheantiphasedomainboundariesappearstocorrespondto twicetheCu–Cuatomspacingalong[110]soallmoleculesonbothsides oftheseboundariesoccupyidenticallocalsites.

3.2. SXPS and NEXAFS

SamplepreparationfortheexperimentsperformedatDiamond fol-lowedessentiallythesamerecipesusedinWarwick,butcomparisonof theO1sandCu2pSXPspectraprovidedestimatesoftheassociated cov-erages.Fortheinitially-depositedhigh-coveragephaseandthe lower-coverageannealedphaseSXPSledtocoverageestimatesof0.35MLand 0.20ML,respectively.Asremarked above,thelowercoveragephase yieldeda(3×2)LEEDpatternthat,foronemoleculepersurfaceunit mesh(asimpliedbytheSTMimages)wouldcorrespondtoacoverage of0.17ML,consistentwiththeSXPSestimate.NoorderedphaseLEED patternwasobtainedfromthehigher-coveragepreparation;published densityfunctionaltheory(DFT)calculationsforamono-oxalatephase werebasedon(2×2)orderingwithacoverageof0.25ML[36] buta c(2×2)orderingwithacoverageof0.5MLwouldalsoappeartobe possible (andwas reportedbyMartinetal.[34]).TheSXPS-derived coverageliesbetweenthesetwovalues.

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bindingenergy,maybeattributedtoemissionfromacarboxylicmoiety thatisstillprotonated;wemaythereforeinferthatthisspectrumisfrom amonooxalatespecies.Suchaspeciesmustgiverisetothree chemically-distinctO1scomponentscorrespondingtotheOatomsofdeprotonated COOandtheOHandC=Ocomponentsoftheintactacidgroup.Aunique unconstrainedthree-componentfittothisspectrumisnotpossible,and thefitshowninFig.5 (usingDoniach-Sunjiclineshapes)is basedon theassumptionthattheCOOcomponenthasexactlythesamebinding energyandspectralwidthasthatseeninthespectrumfromthe low-coveragedi-oxalate spectrum,while theothertwocomponents,each haveanintegratedintensitythatis approximatelyonehalfofthatof theCOOcomponent.Theseconstraintswouldbeconsistentwitha sur-facebeingcoveredbyastanding-upmonooxalatespecies.Noticethat withtheseconstraintsappliedtheresultingfitshowninFig.5 hasthe OHandC=OGaussianpeakwidthsbeingapproximatelytwicethatof theCOOpeak;theoriginofthiseffectisunclearalthoughtheOatoms intheOHandC=Ocomponentsdohaveaverydifferentelectronic en-vironmentawayfromthesurfacetothatoftheOatomsintheCOOthat arebondedtothesurface.

Asafurtheraidtotheidentificationofthethreecomponents,DFT calculationsofthechemicalshiftstobeexpectedfromthismono-oxalate specieswereperformedusingtheCASTEPcode[37],assumingac(2×2) orderingofthe(structurally-optimised)mono-oxalatespecies,butwith asinglecoreholeinac(6×6)unitmeshtominimisehole-hole inter-actionsinthecalculation.Thesecalculationsledtoestimatedchemical shiftsrelativetotheCOOcomponentof0.02eVand1.9eVfortheC=O andOHcomponents,respectively.Thisclearlyindicatesthatthe compo-nentwiththehighestbindingenergyisfromtheOHoxygenatom,while theintermediatecomponent,withonlyasmallenergyshiftfromthe COOcomponent,correspondstotheC=Ocomponent.The correspond-ingchemicalshiftvalues(fortheC=OandOHcomponents)usedinthe experimentalfitofFig.5 are0.6eVand2.1eVwithrelativeintensities ofCOO:C=O:OHof1:0.58:0.64.However,fitstothisspectrumproved particularlyinsensitivetotherelativebindingenergyoftheC=O com-ponentandreasonablefitscouldalsobeobtainedwithamuchsmaller chemicalshiftoftheC=Ocomponentof0.2eV,muchclosertothe DFT-predictedvalue.Aconsistenttrendofallofthesefitswasthatthe rela-tiveintensityoftheCOOcomponentwaslowerthanthattobeexpected fromthemolecularstoichiometry;thisisconsistentwithattenuationof thiscomponentduetoinelasticscatteringofthephotoelectronspassing throughtheupperpartofthemolecularlayer.

Thisapparentlystraight-forwardinterpretationoftheO1sspectrain termsofmonooxalateanddioxalatespeciesathighandlowcoverages wouldleadonetoexpectasingleC1scomponentfromthelow cover-agephase,whilefromthehighcoveragephasetheremaybetwo com-ponentswithasplittingduetodifferencesbetweentheCOOandCOOH carbonatomenvironments(informateandformicacidonCu(110)a differenceof∼2eVhasbeenreportedwithindividualvaluesof287.5– 288.0eVand289.6–290.0eV,respectively[38,39]).Infactthe high-coveragephaseshowstwodominantpeaksatphotoelectronbinding en-ergiesof284.1eVand288.5eV,whilethelow-coveragespectrumhas threecomponentsatenergiesof284.1eV,287.7eVand289.9eV.This behaviourwithsimple‘clean’ O1sspectraandcomplexmulti-peakC1s spectrahasalsobeenreportedforoxalicacidinteractionwithCu(111) byFaraggietal.[35]. Thepeakat284.1eVlabelled#1inFig.5 is consistentwiththepresenceofgraphiticcarbon.Inotherexperiments wehaveconductedofoxalicacidonCu(111)thispeakcandominate thespectrumwithagraphiticcarboncoverageestimatesignificantlyin excessof1ML,onlyconsistent(inthepresenceofcoadsorbedoxalate peaks)withthepresenceofthree-dimensionalislandsofgraphitic ma-terial.Asthispeakonlyappearedafteroxalicaciddepositionitmust clearlyresultfrom some kindofdecomposition reaction on the sur-face.However,perhapssurprisingly,wefoundnoevidenceforthispeak toincreasewithcontinuedexposuretotheincidentsynchrotron radia-tion,indicatingthatitisnotduetobeam-induceddamage.Thepeaks at287.7eVand288.5eV(#2and#2′)mustbeattributedtoCatoms

Fig.6. OK-edgeNEXAFSspectrarecordedfromthelowandhighcoveragephasesformed byoxalicreactionwithCu(110),recordedforincidenceintwoorthogonalazimuthal di-rections,andattwodiﬀerentincidenceangles,deﬁnedastheanglebetweenthesurface normalandtheA-vectoroftheincidentradiation.Normalincidenceisthuslabelled90°.

withintheadsorbedoxalatespecies.Thisenergydifferencebetweenthe twophasescannotbeattributedtothelowcoveragepeakbeingdue toonlyCOOwiththehigh-coveragepeakbeingduetobothCOOand COOHspecies,becausetheenergyshiftisofoppositesigntothatforthe HCOOandHCOOHspeciesreportedabove.Suchanenergyshiftshould alsobeaccompaniedbyabroadeningofthepeakfromthehigh cover-agephaseduetotheunderlyingdoublet,butthewidthsofthetwofitted peaksinFig.5areidentical.Theonlyplausibleexplanationforthe dif-ferenceseemstobetheverydifferentelectronicscreeningenvironment ofCatomsinalying-downandstanding-upmolecule.Theoriginofthe C1speak#3at289.9eVisevenlessclear.Faraggietal.intheirstudy ofoxalicacidonCu(111)havesuggestedthatthispeakmaybedueto somekindofCO₂orCO₃speciesbuttheseseemunlikelytobestableat roomtemperatureandevenlesslikelytosurvivethesampleannealing. Theenergyofthisfeaturecouldcorrespondtoanintactcarboxylicacid (asinthevaluereportedforformicacid),butthistoo,isunlikelytobe stableafterannealing,whiletheO1sspectrumshowsnohintofsucha speciesbeingpresentfromthelow-coveragesurface.Amoreprobable explanationisthatthisfeatureisashake-upsatelliteofthecarboxylate Cpeak#2;thisassignmentofasimilarfeatureinthecaseoftartaricacid onCu(110)wasfoundtoaccountforanotherwisenon-stoichiometryof themoleculeimpliedbytheC1sXPS[30].

Supportingevidencefortheassignmentofthelow-coveragephaseto alying-downmoleculeandofthehigh-coveragephasetoastanding-up moleculeisprovidedbytheOK-edgeNEXAFSdataofFig.6.These ori-entationsaredemonstratedbythedependenceoftherelativeintensity ofthesharppeakataphotonenergyofapproximately535eV,dueto

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citationtoa𝜋∗_-resonance,_on_the_polarisation_direction_of_the_incident radiation.Thispeakisexpectedtohaveits maximumintensitywhen theA-vectoroftheincidentradiationisperpendiculartothemolecular plane.Thusinthelowcoveragephasethisintensityislargerat graz-ingincidencethanatnormalincidence,consistentwithalying-down conﬁguration.Forthehighcoveragephasetheintensityofthisfeature isgreatestwhentheA-vectorliesinthesurfaceplaneinthe[001] az-imuth,indicatingnot onlythat themoleculestandsup,butthatthe molecularplaneliesinthe[110]azimuth.Notice,though,thatthe𝜋∗ -resonancepeakdoesnotcompletelyvanishatnormalincidencefrom thelowcoveragephase,noratnormalincidenceinthe[110]azimuth fromthehighcoveragephase,implyingasmallamountofdisorderor tiltingandtwisting.

4. Structuredetermination:PhDdata,simulations,andDFT calculations

4.1. Introduction

ThePhDtechniqueexploitsthecoherentinterferenceofthedirectly emittedcomponentof aphotoelectronwavefield fromanear-surface atomwiththosecomponentsscatteredbyatomsinthelocal environ-mentoftheemitter.Byvaryingtheincidentphotonenergy,the pho-toelectronkineticenergy(andhencethephotoelectronwavelength)is varied,causingthescatteredcomponentsofthephotoelectron wave-fieldtoswitchinandoutofphasewiththedirectlyemittedcomponent. Theresultingmodulationsinthephotoemissionintensityinaspecific direction,asafunctionofphotonenergy,providestructural informa-tiononthelocalenvironmentoftheemitter.InthepresentcasePhD modulationspectrawereobtainedbymeasuringphotoelectronenergy distributioncurves(EDCs)oftheO1speak(s),at4eVstepsinphoton energy,overthephotoelectronkineticenergyrangeof50–300eV,fora rangeofdifferentpolaremissionanglesinthe[110]azimuth.The2-D detectorfittedtothelargeacceptance-angleelectronspectrometer al-lowedseparatespectratobeextractedat5° intervalsovera50° polar emissionanglerangeforanyspecificsampleorientation.Thesespectra werethenprocessedfollowingourgeneralPhDmethodology(e.g.[6]) inwhichtheindividualEDCsarefittedbyoneormoreGaussianpeaks, aGausserrorfunction(step),andatemplatebackgroundobtainedfrom thewingsoftheEDCs.Theintegratedareasofeachoftheindividual peakswerethenplottedasafunctionofphotoelectronkineticenergy,

I (E ),andusedtodefineastiff spline,I ₀(E ),throughI (E ),thatrepresents thenon-diffractive intensityandinstrumentalfactors.Thespline was thensubtractedfrom,andusedtonormalise,theintegratedareas,to providethefinalPhDmodulationspectrum,𝜒(E )=(I (E )− I 0(E ))/I 0(E ). Theresults of DFTcalculationsfor theadsorbedmonooxalate on Cu(110)havealreadybeenpublished[37].Thesecalculations,assuming thatthePBEexchangecorrelationsufficestodescribethechemisorbed configurations,haveshownthatverticallystandingmoleculesare ener-geticallyfavouredallowingforhighpackingdensities.

However,forflatlyinggeometriesthedispersionforcesshouldalso betakenintoaccountforanaccurateestimationoftheadsorption en-ergyandthecomputationalapproachusedforthenewcalculations re-portedherediffersfromthatreportedinref.[37]ashereLondonforces arealsoaccountedfor[40].Specificallycalculationswereperformed withthe Perdew–Burke–Ernzerhofgeneralised gradient-corrected ap-proximation(PBE-GGA)fortheexchangeandcorrelationenergy func-tional[41]withaddedsemi-empiricalGrimme’sDFT-D2vanderWaals interactioncorrection.Thespin-polarizedKohn-Shamequationswere solvedintheplanewavepseudopotentialframework,asimplemented inthePWscfcodeoftheQuantumESPRESSOdistribution[42,43] The surfacewasmodelledwithaslabconsistingoffiveatomiclayers, sepa-ratedinthezdirectionbymorethan13Å,withthelatticeparameterset totheequilibriumvaluecalculatedforthebulk(3.66Å).Thelower-most twolayersconstrainedtotheirbulk-likecoordinates,andenergieswere

Fig.7. ExperimentalPhDmodulationspectraatdiﬀerentpolaremissionanglesinthe

[110]azimuthrecordedfromthetwoﬁttedcomponentsoftheO1semissionfromthe high-coveragephaseproducedbyoxalicacidreactionwithCu(110).

calculatedusingaregulargridofk-pointscomparabletothe12×14×1 gridusedforthecleanCu(111)(1×1)surface.

Ingeneral,DFTcalculationsshowthatﬂat-lyingfullydeprotonated conﬁgurations,likelytodecomposeintoCO₂onthecleancopper sur-face,canbestabilisedbythepresenceofadatomsorsurface reconstruc-tions.TheformationenergyΔE associatedwiththeprocessofadsorbing

m oxalicacidmoleculesonacleancoppersurfacewiththereleaseofn

hydrogenmoleculesandtheremovalof k Cuatomscanbecalculated as:

Δ𝐸=𝐸tot+𝑛𝐸H2+𝑘𝐸Cu−𝑚𝐸COOH2−𝐸surf

where E _totistheenergyoftheassembledsystem,𝐸H2, E Cu ,𝐸COOH2are theenergiesofanisolatedmolecularhydrogen,abulkcopperatom,and anisolatedoxalicacidrespectively,whileE _surfistheenergyoftheclean unreconstructedsurface.

4.2. High-coverage phase

AsdiscussedinSection3.2,theO1sSXPspectrafromthehigh cover-agephaseactuallycomprisethreechemically-shiftedcomponents,but there issome uncertaintyintheexactvalueofthechemical shiftof theC=OcomponentrelativetothatfromtheCOOoxygenatoms, al-though itis clearlysmall. Inevitably,thesignal-to-noiseratiosof the angle-resolvedspectra, extractedby averagingover only a5° range, areworsethanthoseofthewide-angleintegratedspectraofFig.5,so itprovedunrealistictoseparateallthreecomponentsinameaningful fashion,andﬁtstothesePhDEDCswereconductedwithonlytwo com-ponents,ahighbindingenergypeakcorrespondingtoemissionfromthe OHoxygenatom,andasinglelowbindingenergypeak,corresponding toemissionfromallthreeoftheCOOandC=Ooxygenatoms.Asetof theresultingPhDmodulationspectrarecordedat10° intervalsin emis-sionangleisshowninFig.7.

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Fig.8. Comparisonofexperimental1sPhDspectrafromthelowbindingenergy(COO+C=O)peakfromthehighcoveragephaseformedbyoxalicacidreactionwithCu(110)withthe resultsofmultiplescatteringsimulationsforthebest-ﬁtstructure.Thisstructureisshownschematicallyontheright(includinganarbitrarily-placedHatom)togetherwiththestructural parametervalues,includingsmallperpendicularandparallelshiftsoftheCuatoms,bondedtothecarboxylate,relativetotheideally-locatedCu(110)surfaceatoms.

oftheCOOandC=Ooxygenatomsarealsoratherweak,butparticularly nearnormalemissionthereisclearlyanunderlyinglong-period modula-tion.Thislongperiodindicatesthatthescatteringpath-lengthdiﬀerence isreasonablyshort,whilethefactthatthemodulationsarestrongestat normalemissionsuggeststhattheOatomsgivingrisetothese modu-lationslieclosetoatopCusurfaceatoms,thusgivingrisetoscattering inthegenerally favoured180° backscatteringangle. However,while theCOOoxygenatomscontributetoaclearlong-periodmodulation, theemissionfromtheC=Ooxygenatomisexpectedtocontributeonly weakmodulations,likethatobservedfromtheOHspecies.The conse-quentialdampingofthePhDmodulationswill,atleastinpart,account fortherelativelyweakamplitudeofthemodulations(lessthan±10%) observedevenatnormalemission.

Thisinterpretationisconfirmedandquantifiedbytheresultsof mul-tiplescatteringsimulationsforarangeofpossibleadsorptionsites,Cu–O bond-lengths,moleculartwistsandtilts,localsurfacerelaxations,and vibrationalamplitudesoftheemitterandscattereratoms.Identifying thebest-fitstructureoversuchasearchisaidedbytheuseofan objec-tivereliability-or R -factordefinedasanormalisedsumofthesquared differencesbetweenthetheoreticalandexperimentalmodulation am-plitudesR =Σ(𝜒th−𝜒ex)2 /Σ(𝜒th2 +𝜒ex2 ).Thestructuralmodelyielding thelowestvalueofR isidentifiedasthebest-fitstructure.Fig.8showsa comparisonofasubsetoftheexperimentalCOO+C=OO1sPhDspectra (takenfromthecompletesetshowninFig.7,mostlyforsmallpolar an-glesforwhichthemodulationsarestrongest)withtheresultsofmultiple scatteringsimulationsforthebest-fitstructuralmodel,withan associ-atedvalueof R of0.40.Thisrelativelyhighvalue(R valuesof0.2or lesscanbeachievedforsomeadsorbatesystems)reflectsthefactthat allmodulationsareratherweakandsotherelativelevelofthenoise isquitehigh.ThisstructureisalsoshownschematicallyinFig.8 to-getherwiththebest-fitstructuralparametervalues(theOHhydrogen atomisshownforcompletenessbutitslocationcannotbeobtainedfrom aPhDstudyduetoitsweakscatteringcross-section).Thelocal bond-inggeometryisequivalenttothatoftheformate,acetate,benzoateand mono-tartratespeciesontheCu(110)surface;themoleculebridgestwo nearest-neighbourCuatomsalongthe[001]azimuthwiththebonding Oatomsslightlyoff atopsites.Thismodelisalsoconsistentwiththe re-sultsofDFTcalculationsthatfindstanding-upmonooxalatespeciestobe

energeticallyfavouredoverawiderangeofcoverages[36].TheCu–O nearest-neighbourbondlengthofthebest-ﬁtstructureshowninFig.8is 2.00±0.04Å,tobecomparedwiththevaluefoundintheDFT-D2 cal-culationsof2.00Å.Moreprecisely,whenintermolecularH-bondingis presenttheCu–OdistanceofanOalsoengagedintheH-bondis2.01Å andshortensto1.99Å fortheotherO.Thesecalculationsindicatethat theheightdiﬀerenceofthecarboxylateOandCatomsis0.59Å;O1s PhDisrelativelyinsensitivetothisparameter,sothediscrepancywith thetheoreticalvalueisnotunreasonable.

4.3. Low-coverage phase

As the SXPS from the low-coverage lying-down dioxalate phase showsonlyasingleO1s peak,extractingtheassociatedPhDspectra wasstraightforward.Unfortunately,identifyingastructuralmodelfor whichmultiplescatteringsimulationsgaveasatisfactoryﬁttothesedata provedtobeconsiderablymorechallengingdespiterunningavast num-berofcalculationsbasedonthree-dimensionalgridsearchesarounda numberofplausiblestartingmodels,butalsorunningautomatedsearch routinesfromdiﬀerentstartingpointsusingtheparticle-swarm optimi-sation(PSO)method [44].These proceduresidentifystructureswith thelowest R -factorsinamultidimensionalparameterhyperspace. Un-fortunately,althoughthisprocessdidyieldsomestructuralmodelswith valuesof R downto∼0.40,thesemodelsinvolvedstructural parame-tervaluesthatwerephysicallyimplausible-forexampleCu–Onearest neighbourdistancesof∼2.30Å ormore,orof1.6Å orless.

Ofcourse,theinformationfromNEXAFSandSXPSprovidessome clearconstraintsonthelikelystructure.ThesingleO1speaksinSXPS clearlyindicatesthespeciesisfullydeprotonatedandNEXAFSshowsthe molecularplaneisnear-paralleltothesurface,soitseemslikelythatall fourOatomsbondtotheCusurface.Ontheotherhand,theO1sPhD modulationsatnormalemissionareextremelyweak(seeFig.9),anddo notshowanycleardominantlong-periodmodulations.Thisseemsto in-dicatethattheOatomsare not directlyatopsurfaceCuatoms,asinthe monooxalatespecies,andindeedasinallothercarboxylatespreviously studiedbyPhDonCu(110).Ofcourse,theabilityofadicarboxylateto haveallOatomsclosetoatoponCu(110)dependsonthematchingof theO–OdistancesinthemoleculeandtheCu–Cudistancesonthe

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Fig.9.Comparisonof4individualO1sPhDspectrarecordedfromthelowcoverage oxalatephaseonCu(110)withtherepresentativespectruminthesamegeometryfrom thehighcoveragephase(asshowninFigs. 7 and8 ).Theindividuallow-coveragespectra arerecordedfromdiﬀerentpreparationsandindicatethedegreeofreproducibilityofthe weakmodulations.

face.Inthisregard,dioxalatelackstheﬂexibilityofboththeditartrate speciesandtheaminoacids.

WeakmodulationsmayinpartarisefromthefactthatthePhD mod-ulationscomefromanincoherentsumofthemodulationsfromfour differentOatomsinthedioxalate;thesefourOatomscouldhave dif-ferentlocaladsorptionsites,althoughthetwo-foldsymmetry ofboth themoleculeandthesubstratemightreasonablybeexpectedtoleadto mostOatomsoccupyingsymmetrically-equivalentsites.Nevertheless, findingamodelstructurethatshowssuchextremelyweakmodulations atnormalemissionproveddifficult,whilethegenerallyweak modula-tionsatallangles(andthushigherlevelsofnoiserelativetothe mod-ulations)necessarilyleadstohighervaluesofthe R -factorevenforthe bestmodels.Thisalsoresultsinweakvariationsof R inmanyregions ofstructuralparameterhyperspace,renderingsearchesbasedonlyon identifyingthelowest R valueslesseffective.

Thereis,however,onefurtherpieceofinformationemergingfrom theSTMdatathatmayhelptoidentifyoneimportantingredientofthe correctstructuralmodel.Speciﬁcally,theSTMimagesofFigs.3 and

4provideevidenceforapossiblesurfacereconstructionassociatedwith thelow-coverage(3×2)phase.Theseimagesappeartoshowthatsome ofthe(3×2) unitmesheshave moleculesat theircentresaswellas attheircorners, despitethefactthatthecentreandcornersitesare inequivalentwithrespecttoCu(110)surfacestructure.Inviewofthisit isunsurprisingthattheSTMlinescansatdifferentbiaspotentialsalso indicatethatthesecentredmoleculesdifferinsomewayfromthoseat thecorners.Fig.10 showsamodelthatcouldaccountforthiseffect. Specifically,ifthe(3×2)phasehaseverythird[001]outermostlayer Curowmissing,astandingupmonooxalatespeciescouldbondtothe exposedsecondlayerCuatomsinasitethatliesatthecentreofthemesh formedbythelying-downdioxalatespeciesoccupyinghigh-symmetry sitesonthedouble-Cuatomrowsthatarenotmissing.Ofcourse,the O1sSXPspectrafromthelow-coveragephaseindicatethepresenceof verylittle,ifany,standing-upmonooxalateinthesurfacepreparations fromwhichPhDdataweremeasured.However,theSTMimagesofthe defected(3×2)phasemaybetakentoprovideuswithevidencethat thesemissing rowsareacharacteristic ofa perfectlyordered (3×2) phase.

Toexplorethisideaofamissingrowmodelofthe(3×2)dioxalate phase,furthersearchesof possible structuresbasedon thissubstrate model,startingfromdiﬀerentlocaladsorption sitesandorientations, wereundertakenusingthePSOroutine.AsforthePSOsearchesof mod-elsontheunreconstructedsurface,severaloptimisedstructureswithlow

R -factorswerefoundwithCu–Obondlengthsthatwerephysically

unrea-Fig.10. Schematicmodelofamissing-row(3×2)phaseformedbylying-downdioxalate species,showingthatadditionaladsorptionofstanding-upmonooxalatespecieswould occupysitesatthecentreofthismesh,potentiallyaccountingforthe’defect’sitesseenin theSTMimagesofFigs. 3 and4 .ForclaritytheoutermostCulayeratomsareshownin adiﬀerentcolourfromthoseoftheunderlyingbulk.(Forinterpretationofthereferences tocolourinthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

sonablylongorshort,butonesolutionemergedthathadamore plausi-bleCu–Onearest-neighbourbondingdistanceof1.86Å.Thismodel,and acomparisonofasubsetoftheexperimentalO1sPhDspectrawith sim-ulationsbasedonthesamemodel,areshowninFig.11.Noticethatthe subsetofspectrachosenforthesecalculationsaremostlythosewiththe strongestmodulations,althoughthenormalemissionspectrumisalso included;clearlythecorrectmodelshouldatleastreproducethelackof strongmodulations(±5%orless)inthishigh-symmetrydirection.The valueofthe R -factorobtainedfromthestructureshowninFig.11 is 0.50,whichiscertainlyratherhigh.Notsurprisinglyinviewofthis, vi-sualinspectionofthespectrashowsclearlythatthesimulatedspectra includeafewmodulationsthataremuchlargerthanthoseofthe experi-mentaldata.Ontheotherhand,mostoftheexperimentalpeakenergies arereproducedquitewell,andthesimulationsforthenormalemission spectradoshowextremelyweakmodulationsovermostoftheenergy range,akeyfeatureoftheexperimentaldata.

Ofcourse,anothersourceofpossiblequantitativestructuralmodels isenergyminimisationcalculationsusingDFT.InfactthepublishedDFT studyofoxalicacidderivativesonCu(110)[37] didconsiderpossible structuresformedbyaﬂat-lyingdioxalateonthissurfacebutconcluded thattheadsorptionenergywasverysigniﬁcantlylowerthanthatofthe standing-upmonooxalateandindeedthatmostpossibleadsorptionsites didnotleadtostableadsorptionbutratherdissociationintotwoCO₂ molecules.

NewDFT-D2calculationswereperformedforthemissingrow struc-tureleadingtotwopossiblestableadsorptionstructuresfordioxalate, oneofwhichhasasigniﬁcantlylowerenergythananyadsorption struc-tureontheunreconstructedsurface.TheseareshowninFig.12 along withthevaluesoftheassociatedadsorptionenergy.Speciﬁcally,with themoleculelyingabovethedoublerowsbetweenthemissingrows, butcentredaboveasecondlayerCuatomwiththeC–Cmolecularaxis alignedalongthe[110]azimuth,theenergywasfoundtobe171meV lowerthantheequivalentsiteontheunreconstructuredsurface.

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vari-T.W.Whiteetal. SurfaceScience668(2018)134–143

Fig.11. ComparisonofexperimentalandsimulatedO1sPhDspectrarecordedfromtheCu(110)(3×2)dioxalatephaseforthestructuralmodelillustratedontheright.

Fig.12. Schematictopviewsofthetwostabledioxalateadsorptiongeometriesfoundin DFTcalculationsforthe(3×2)missingrowstructureofCu(110).Forclaritytheoutermost Culayeratomsareshowninadiﬀerentcolourfromthoseoftheunderlyingbulk.Also indicatedarethecalculatedadsorptionenergiespermoleculeΔE.(Forinterpretationof thereferencestocolourinthisﬁgurelegend,thereaderisreferredtothewebversionof thisarticle.)

ationofthe R -factorwithchangestoeachstructuralparameter[6],we findthevaluesofinteratomicdistancestobe:Cu–O,1.86±0.06Å;C–C, 1.15±0.10Å;C–O,1.12(+0.21/−0.09)Å.Forcomparisonthevaluesof theseparametersfoundintheDFTcalculationsare:Cu–O,1.94Å;C–C, 1.58Å;C–O,1.27Å.Evenbearinginmindthelargeerrorestimatesin theprecisionofthesePhD-derivedvalues,therearesignificant quanti-tativedifferencesbetweentheoryandexperiment,despitethesimilarity inthequalitativestructureintermsoflocaladsorbate-substrateregistry andazimuthalorientation.Perhapsthemostobviousproblemwiththe experimentally-determinedstructuralparametervaluesistheC–C dis-tancethatisunphysicallyshort,eventakingaccountofthelimited

pre-cision.Ofcourse,theO1sPhDmeasurementsareprimarilysensitiveto therelativelocationofthenearest-neighbourCuatoms,whicharemore stronglyscatteringthanCatomsandarealsolocatedatmorefavourable locationsforbackscattering.InthisregarditisnotablethattheDFT cal-culationsindicatesigniﬁcantlateralrelaxationsoftheoutermostlayer Cuatoms,notonlyinthe[110]azimuth(intothespaceleftbythe miss-ingrowby0.08Å),butalsoalong[001]byalargervalueof0.20Å. TestsimulationsofthePhDspectraincorporatingthislarge[001]shift (thatwasnottestedintheoriginalPSOsearch)bymovingtheOatoms toretainthesameO–Cunearest-neighbourregistryledtoanincreasein theR -factor,andthusalesssatisfactoryagreementwiththePhD exper-imentalspectra

WethereforeconcludethatthePhDbest-fitstructureofFig.11 does representaclear(local) R -factorminimuminparameterspacethatis qualitativelysimilartothelowestenergystructure foundin theDFT calculationsbutdoesinvolvesomesignificantlydifferentstructural pa-rametervalues,atleastoneofwhich(theC–Cdistance)isunphysical. TherearealmostcertainlyotherlocalR -factorminimaatother param-etervalues,butwehavebeenunabletofindonewithrealisticvaluesof theparametertowhichtheOPhDismostsensitive,namelytheCu–O bondlength.DespitetheunphysicalC–Cbondlengthvalue,itseems ex-tremelylikelythatthisisthequalitativelycorrectstructuralmodel.The reasonsunderlyingthefailureofthePhDtechniquetoprovideatotally convincingfinalquantitativestructuralsolutiontothisparticular prob-lemareunclear.Theweakmodulationsandthuspoorersignal-to-noise ratioarecertainlyafactor, andcanbe attributedtothefactthatall theOatomsareinlowsymmetrysites,displacedbyalmost1Å from theidealatopsitesrelativetosurfaceCuatoms.Thiscontrastswiththe situationforthestanding-upphasesofothercarboxylicacidsinwhich theOatomsaredisplacedfromatopsitesbyonly∼0.1Å.Notethatin searching foralternativestructuresthatmightprovide abetterfitto thePhDdataarangeofCuadatomstructureswerealsoinvestigated; Cuadatomshavebeensuggestedtoplayaroleinanumberof molecu-laradsorbatephasesoncoppersurfacesincludingbenzoateonCu(110)

[45].Althoughsomeofthesemodelswerefoundtohavetotalenergies in DFTcalculationscomparabletothatofthemissing-row structure, noneofthesemodelsledtoanimprovedﬁttotheexperimentaldata.

Ofcourse,wecannotexcludethepossibilitythattwoormore co-existentadsorption geometriesarepresent.OurSTMimagesindicate thatcoadsorptionofstanding-upmono-oxalatespeciesadsorbedonto thesecondlayeratthemissingrowlocations,canoccur,butPhD

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ulationsbasedonsuchaco-occupationmodelledtothebestﬁtbeing foundwithzerooccupationofthisspecies.Moreover,theO1sXPSdata excludeanysigniﬁcantco-occupationofthisstanding-upspecies.Itis alsonotablethattheDFTcalculationsstronglysuggestalternative ad-sorptionsitesofthedioxalatewouldbeunstable.

5. Conclusions

Thecombination of(particularly O1s)SXPSandOK-edge NEX-AFShaveprovidedclearevidencefortwodiﬀerent adsorbedspecies onCu(110)resultingfromexposure tooxalic acid.Atlow coverages adoubly-deprotonateddioxalatespecieslieswithitsmolecularplane approximatelyparalleltothesurface,whileathighcoveragesthe ad-sorbateisasingly-deprotonatedmonooxalatewithitsmolecularplane perpendiculartothesurfacelyinginthe[110]azimuth.STMandLEED showthatthehighcoveragephaseformsanordered(3×2)structure, theimpliedcoverageof1/6MLbeingconsistentwithSXPSdata.STM imagesalsoshowthepresenceof additionalfeatures,assumed tobe extraoxalicacid-derivedmolecules,thatoccupysitesofthecentreof the(3×2)unitmeshformedbythedioxalatespecies.Symmetry argu-mentsclearlyexcludethepossibilitythattheseadditionalmoleculesare dioxalatespeciesatequivalentlocalsites,buttheimagescouldbe con-sistentwiththepresenceofmissing[001]outermostlayerCurowswith monooxalatespeciesbondedtosecondlayerCuatomsinthelocation ofthemissingrows.

O1sPhDdatafromthehigh-coveragephasewasfoundtobe con-sistentwiththemonooxalatespeciesbondedthroughthecarboxylateO atomsinnear-atopsitesrelativetonearest-neighboursurfaceatomsin the[110]rows.Thislocalbondinggeometryisequivalenttothoseof for-mate,acetate,benzoateandsingly-deprotonatedtartaricacid.Finding amodelstructureforthedioxalateontheunreconstructedCu(110) sur-facefromtheO1sPhDdataprovedtobemorechallenging,atleastin partduetotheweakmodulations,particularlynearnormalemission. However,DFTcalculations,based onareconstructed(3×2) surface, witheverythird[001]Cusurfacelayerrowmissing,assuggestedby thepresenceof thecentredunit meshSTMimages, showedthatthe dioxalatebonds significantlymorestronglytothissurface.O1s PhD simulationsbasedonadsorptiononthis reconstructedsurface identi-fieda best-fitstructural modelqualitativelysimilar tothatfoundto havethelowestenergyinDFTcalculations.Specifically,themolecule isfoundtooccupyahollowsiteontheCu[001]doublerows,directly aboveasecondlayerCu atom,withtheC–C axisalongthe[110] az-imuth.However,somesignificantdifferencesinstructural parameter valuesarefoundbetween theDFT-andPhD-derivedmodels,withat leastoneintramolecularbondlength(C–C)beingunreasonablyshortin thePhD-derivedmodel.

All data presented in this paper are available at http://wrap. warwick.ac.uk/94089.

Acknowledgements

Theauthors thank Diamond Light Sourcefor accesstobeamline I09(proposalnumberSI8436)thatcontributedtotheresultspresented here. DAD would like to acknowledge funding from the Alexander von Humboldt Foundation. S.F. acknowledges the CINECA Award N.HP10CJCRO0,2011fortheavailabilityofhighperformance comput-ingresourcesandsupport.G.C.acknowledgesﬁnancialsupportfromthe EUthroughtheEuropeanResearchCouncil(ERC)Grant“VISUAL-MS” andfromtheEPSRC (EP/G043647/1).

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