methods on boron Rh(1 1 nitride 1) by of electronspectroscopic Investigation adsorption of properties the of borazine andcharacterisation Applied Surface Science

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ContentslistsavailableatScienceDirect

Applied Surface Science

j o ur na l ho me pa g 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

Investigation of the adsorption properties of borazine and characterisation of boron nitride on Rh(1 1 1) by electron spectroscopic methods

A.P. Farkas

^a,∗

, P. Török

^a

, F. Solymosi

^c

, J. Kiss

^a,c

, Z. Kónya

^b,c

aDepartmentofPhysicalChemistryandMaterialsScience,UniversityofSzeged,Szeged,Hungary

bDepartmentofAppliedandEnvironmentalChemistry,UniversityofSzeged,Szeged,Hungary

cMTA–SZTEReactionKineticsandSurfaceChemistryResearchGroup,Szeged,RerrichB.tér1,H-6720Szeged,Hungary

a r t i c l e i n f o

Articlehistory:

Received22December2014 Receivedinrevisedform2April2015 Accepted9May2015

Availableonline18May2015

Keywords:

Boronnitride h-BN Rh(111) Borazine Methanol HREELS

a b s t r a c t

TheadsorptionanddissociationofborazinewereinvestigatedonRh(111)singlecrystalsurfacebyAuger electronspectroscopy(AES),highresolutionelectronenergylossspectroscopy(HREELS)andtempera- tureprogrammeddesorption(TPD)methods.Borazineisoneofthemostfrequentlyappliedprecursor moleculesinthepreparationprocessofboronnitrideoverlayeronmetalsinglecrystalsurfaces.On Rh(111)surfaceitadsorbsmolecularlyat140K.Wedidnotfindanypreferredorientation,although thereisevidenceof“flat”andperpendicularmoleculargeometry,too.Dehydrogenationstartsevenbelow 200Kandfinishesuntil∼7–800K.NootherboronornitrogencontainingproductswereobservedinTPD beyondmolecularborazine.Throughthehydrogenlossofmoleculeshexagonalboronnitridelayerforms inthe600–1100KtemperaturerangeasitwasindicatedbyAESandthecharacteristicopticalphonon HREELlossesofh-BNoverlayer.Theadsorptionbehaviouroftheboronnitridecoveredsurfacewas alsostudiedthroughtheadsorptionofmethanolat140K.HREELSandTPDmeasurementsshowedthat methanoladsorbedmolecularlyandafractionofitdissociatedtoformsurfacemethoxyandgasphase hydrogenontheh-BN/Rh(111)surface.

1. Introduction

Theprocedureofmodiﬁcationandself-assemblyofnanostruc- turesonsurfacesisatpresentinthefocusofsurfacescience[1–10].

Inthiscontext,theepitaxialgrowthofultra-thinlayersofhexago- nalboronnitrideontransitionmetalsurfaceshasattractedalotof attention.Inspiredbytherichfunctionalitiesofgraphene,scientists haveakeeninterestinthetwo-dimensionalofhexagonalboron- nitridecrystals(so-called“white”graphene).Theh-BNpossesses anumberofadvantageouspropertiesoveritscarboncounterpart, includingaconstantwidebandgap(5–6eV)higherchemicalinert- nessandthermalstability,enhancedoxidationresistanceandgood opticalproperties.Thesedistinctionsmakeh-BNuniquelyattrac- tiveforapplicationsinelectronics,photonics,andnanocomposites [1].Furthermoreitisalsoausefulcandidateincatalyticinvesti- gationsasawellknownchemicallyinertsupportmaterialwhich doesnotinteractelectronicallywiththesupportedmoleculesor

∗Correspondingauthor.Tel.:+3662343682;fax:+3662544106.

E-mailaddress:arnold.farkas@chem.u-szeged.hu(A.P.Farkas).

metalclusters[2].Particularlyinterestingistheh-BN/Rh(111)system,especiallysincethediscoveryofaself-organizedboronnitride superstructureonaRh(111)surfacebyCorsoetal.[3].Itdisplaysa h-BNnanomesh,intowhichmoleculesandnanoparticlescanbe readilyadsorbedandthus arrangedontheatomiclengthscale.

Itisalsoaninterestingfeaturethataconsiderablestructuraldif- ferencedoesexistbetweenh-BNoverlayersformedonPt(111) andRh(111)despitethesamesymmetryandcloselatticeparam- eters[4].OnPt(111)h-BNformsaratherﬂatsinglemonolayer, whileonRh(111),it growsin ananomeshformbecauseofthe strongerinterfacialchemicalbondingasaconsequenceofbetter orbitaloverlap.

Therearetwomainwaystoproduceh-BNontheRhsurface.The ﬁrstisthethermaldecompositionofboronandnitrogencontaining molecules(i.e.:borazine),however,itsformationisalsopossible bytheinteractionofsegregatedboron[5]ordehydrogenationof decaborane[6]withNcontainingadsorbants,likeNO.Following thelatterroutea cleansingleBNlayerwaspreparedinmono- layerorclosetomonolayercoveragethroughtheinteractionofNO withboron-containingpolycrystallineRhsurfaceabove900K[5].

ThiswasprovedbytheappearanceofintenseAugertransitions http://dx.doi.org/10.1016/j.apsusc.2015.05.060

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368 A.P.Farkasetal./AppliedSurfaceScience354(2015)367–372

at176eV(B)and 384eV(N)above 400K,which aretypicalfor B–Nspecies. The formation of boron nitridewas alsodetected at9.0–9.5eVbyUPSafterthispreparation.Theobservedphoto- emissioncanbeattributedtotheformationofbondbetween Band N. On the other handin a wealthof preliminary works thepreparationofh-BNwasinvestigatedondifferentsubstrates includinge.g.Pd(111)[7],Cr(110)[8],Ni(111)[9][[1–10]and Refs.therein].However,veryfewworksdealwiththechemistryof precursormoleculesduringthepreparationstepsofboronnitride overlayer[7,10,11].Koelandco-workersinvestigatedtheadsorp- tionofborazineonAu(111)andPt(111)singlecrystalsurfaces byelectronspectroscopicmethods[11],neverthelesstoourbest knowledgethereisnosimilarworkonRh(111)surfaceuntilnow.

Ourmainpurposewastorevealtheadsorptionpropertiesof borazineontheRh(111)singlecrystalsurfacebyelectronspec- troscopicmethods;furthermorewe investigatedtheadsorption propertiesof theprepared boron nitrideoverlayer throughthe adsorptionofmethanolatlowtemperature.Thesurfacestability ofmethanolonaninactiveBNlayerpreparedonanactiveRh(111) offersapossibilityofcomparisonwithdataobtainedonatomically cleanRh(111).

2. Experimental

TheRh(111)crystalusedinthisworkwascutfromasingle crystalbouleandwasaproductoftheMaterialResearchCorpora- tion(purity99.99%).Itwasmountedbetweentwotantalumwires, whichwereconnectedviaacopperblockdirectlytoaliquidnitro- genreservoir.Initiallythesamplewascleanedbyrepeatedcycles ofAr⁺sputtering(typically1kV,1×10⁻⁷mbarAr,300K,2␮Afor 10–30min)andannealingto900–1100Kuntilnocontaminations weredetectedbyAESandXPS.Thebasepressureinthechamber wasaround1×10⁻⁹Torr.Thesamplewasheatedresistivelyfrom 140to1200K.Itstemperaturewasmonitoredbyachromel–alumel thermocouplespotweldedintothesideofthecrystalandwascon- trolledwithafeedbackcircuittoprovidealinearheatingrateofca.

10K/s.Gasesweredosedthrougha0.1mmdiametercapillarythat terminated∼3cmfromthesample.Thelocalpressureatthesample wasabout10⁻⁷mbarduringdosing.Thedosingtemperaturewas

∼140Kunlessotherwisenoted.Theexperimentalworkwasper- formedinatwo-levelUHVchamberwitharoutinebasepressure of5×10⁻¹⁰mbarproducedbyturbomolecularpump.Thecham- berwasequippedwithfacilitiesforAES,XPS,HREELSandTPD.

TheHREELspectrometer(LK,ELS3000) issituatedinthelower levelofthechamberandhasaresolutionof20–40cm⁻¹(FWHM).

Thecountratesintheelasticpeakweretypicallyintherangeof 1×10⁴−1×10⁵counts-per-second(cps).Allspectrareportedwere recordedwithaprimarybeamenergyof6.5eVandatanincident angleof60^◦withrespecttothesurfacenormalinthespeculardirec- tion.Borazineof>99.8%puritywastheproductofKatchemLtd.

Borazinewasstoredat254Katalltimesexceptwhenchargingthe doser.Allgasdosinglineswerepassivatedandevacuatedpriorto borazineintroduction.

3. Resultsanddiscussion

3.1. Preparationoftheh-BNlayer

3.1.1. BorazineadsorptiononRh(111)byAESandTPD

ForthepreparationofBNspeciestheadsorptionanddecom- positionofborazineonRh(111)surfacewasinvestigatedﬁrstby Auger-electronspectroscopy(Fig.1).Weappliedtwomainprepa- rationmethods.Weadsorbedborazineatlowtemperature(140K) followedbyannealingthesampleto900K.TheAESpeakscharac- teristicofB–Nspeciesappearedonthespectraat175and384eV

Fig.1. AESspectratakenonthecleanandborazinecoveredRh(111)surfaceafter

∼24Lborazineadsorptionatdifferenttemperatures.AESKVVlinesofboronand nitrogenoncleansurface(a)afterborazineadsorptionat140K(b),annealingthe adsorbedlayerto900K(c)andadsorptionofborazineat900K(d).

kineticenergy[5].Whentheadsorptionofborazinewasperformed athighertemperature(900K)theintensityofthesepeaksincreased further.Dongandco-workersusingasimilarmethod(lowdose adsorptionatRTfollowed byhightemperature annealing)con- cluded thatthedensityof h-BNislandswashigherthan before whentheypreparedthesurfacelayeronlybyhightemperature adsorption[12].Inthenewpreparationprocess∼250islands/␮m appearedbecauseofthe2Dnucleationandgrowth.Augerelectrons alsocarryinformationabouttheenvironmentoftheirparentatoms.

Theenergyseparationofthemultipletlinesischaracteristicfor thebondingpartnerandinthiswaythisdemonstratessomekind ofﬁngerprintofthechemicalenvironmentoftheexcitedatoms.

HencewealsoinvestigatedtheformationoftheBNlayerduring annealingthesurfaceafterborazineadsorptionat140K.Thefine structureoftheB(KVV)Augerseriesafterborazineadsorptionat 140Kandthatafterhightemperaturedepositiondifferin some distinctivefeatures.InthelattercaseathreelineAugerfinestruc- tureappearedonthespectrawithcharacteristicenergyseparation markingtheformationofaBNoverlayer,whichisdifferssignifi- cantlyfromtheAESfinestructureofBorB₂O₃ [13].Incontrast, whenweadsorbedborazineatlowtemperatureandinvestigated thecondensedmolecularlayerwefoundjustaverytinythirdAES peakat153eV(seeFig1a–d).Althoughtheintensityandtheresolu- tionoftheN(KVV)Augerlinesaresmallerwithcarefulevaluation ofthedataweareabletoobservethesameenhancementofthe characteristicAugerlinesinthecaseofBN.Thissmalldifferencein thefinestructureoftheAESspectracouldbeasignofthecomple- tionofthedehydrogenationofborazineontherhodiumsurface.

NotonlytheﬁnestructureoftheB(KVV)AESlinesbutalsothe B(175)/Rh(306)peaktopeak AESintensityratiochangeddur- ingannealing.Aftermultilayeradsorptionofborazinewereached

∼0.20B/Rhatomicratio,whichdecreasedimmediatelyafteraslight annealingcausedbythelowtemperaturedesorptionofcondensed borazinelayer(Fig.2A).Above300Ktheratioachievedaconstant valueat0.12.Withcontinuingexposureofthesurfacewithincreas- ingamountofborazineat900Kwewereabletoattainamaximum B/RhAESratioof∼0.18–0.21.However,evenraisingtheadsorp- tiontemperatureto1000Kdidnotresultinahigheratomicratio;

insteaditdecreasedslightlyto∼0.165.Thereasonofthisatten- uationcouldbeamildalterationofthesurfacelayer;STMresults indicatedthatatthistemperature(∼1000K)thenarrow,elongated h-BNislandstransformedintomorecompacth-BNislandswith adefective nanomeshsuperstructure[12].We alsocheckedthe continuityoftheBNoverlayerbyscanningthesamplesurfacein consecutiveAugermeasurementsandwedidn’tﬁndanysigniﬁcant alterationoftheAESB/Rhpeaktopeakintensityratio(notshown)

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Fig.2.EffectofannealingontheB(175)/Rh(306)peaktopeakAESratioafteradsorptionofborazine(∼10L)oncleanRh(111)surface(A)andborazine(M80)andhydrogen (M2)TPDspectra(B)following0.15LborazineadsorptiononRh(111)surfaceat140K.

whichsuggestsahomogeneousdispersionoftheBNoverlayeron theRh(111)surfaceatleastasdetectedbyAES.

Connectingto theAES resultswe studied theadsorption of borazine at lower exposures (0.15L) at 140K on Rh(111) and wefollowedthedissociationanddesorptionofpossiblereaction productsof borazinewithtemperatureprogrammeddesorption techniques,TPD(Fig.2B).Inthetemperaturerangeof100–1000K theonlydesorptionproductswereH₂andB₃N₃H₆.TPDspectrafor thesetwodesorptionproductsaredisplayedinFig.2B.Borazine desorbed from the surface with one sharp peak at Tp=176K, whichisdeﬁnitelyconnectedtothedesorptionofthecondensed adsorbateoverlayerwithzeroorderdesorptionkinetics(exposure dependencespectranotshown).Koelandco-workersfoundthat onPt(111)surfacetheexposureof0.03Lborazinesaturatedthe ﬁrstchemisorbedlayerandatlargerdosesaborazinemultilayer

wasformed[11].InAESmeasurementsweobservedthat0.15L exposureofborazinegivesa∼0.12B/Rhpeaktopeakatomicratio at300K(atsaturation)whichisingoodagreementwiththevalue observedafterannealingthecondensedlayerofborazinetoRT.

Aftermultilayerdesorptionat∼200Kdehydrogenationreactions occurred.Thesupposeddehydrogenationreactionmechanismis thefollowing:

B₃N₃H_6(a)→ B₃N₃H₆-n(a)+nH_(a) (1)

B₃N₃H₆-n(a)→BN+H_(a) (2)

H_(a)+H_(a)→H_2(g) (3)

TPDresultsproved thathydrogendesorptionalreadystarted slightlybelow200Kandcontinued–inaverybroadtemperature

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Table1

Characteristicvibrationsofborazineondifferentsinglecrystalsurfaces.

Vibrationmode B3N3H6

gas-phaseD3h

[11]

B3N3H6on Pt(111)at 110K[11]

B3N3H6on Pt(111)at 170K[11]

B3N3H6on Au(111)at 110K[11]

B3N3H6on Au(111)at 180K[11]

B3N3H6on Rh(111)at 140K[this work]

B3N3H6on Rh(111)at 300K[this work]

A2 ␯8,␥-BH 918 915 910 910 920 920

␯9,␥-NH 719 710 710 710 720 730

␯10,␥-BN 394 400 400 400 410 405

E ␯11,␯as-NH 3486 3485 3485 3460 3480 3480

␯12,␯as-BH 2520 2535 2535 2490 2510 2530

␯13,␯as-BN 1465 1465 1465 1460 1460 1450

␯14,␯as-BN 1460

␯15,␦-BH 1096

␯16,␦-NH 990 955

␯17,␦-BN 518 560(?)

range–upto∼800K(Fig.2B).TheTPDpeakmaximumatm/e=2 (H2)wasobservedaround310K.

3.1.2. HREELSresults

Afteraveryhighexposureofborazine(∼45L)onRh(111)sur- faceat140K wefollowedtheeffectofannealingontheHREEL spectraoftheadsorbedlayer(seeFig.3A).At140Ktherewereloss peaksat410,720,920,1050,1460,2510,2980andat3480cm⁻¹ whichcorrespondwelltothegasphaseIRspectraofborazine[11].

Thevibrationalmodesofborazineandtheirassignationsarecol- lectedinTable1.Thepeakat1460cm⁻¹(Erepresentation/in-plane motions)belongstotheB–Nasymmetricvibration,andthepeaks observedwithsmallerintensityat2510and3480cm⁻¹ arecon- nectedwiththeEB HandN Hasymmetricmodes,respectively.

At200Kall peaksdecreased in intensitydue tothemultilayer desorption.Furtherheatingoftheadsorbedlayerto300Kledtothe signiﬁcantattenuationoftheA₂modesat410,720and920cm⁻¹ andatthesametimethe1460cm⁻¹peakremainedintense.Con- sequentlytherelativeintensitiesofthepeakschangedandatroom temperaturethelatterpeakdominatedthespectra.Atandabove 500Kthespectrashowedacompletelydifferentpicture.Thepeaks fromtheB HandN Hregionsdisappearedverylikelyduetothe almostcompletedehydrogenationreactionsofborazine.Although thebroadened peakat∼720–750cm⁻¹ strengthened andabove thistemperatureitwasthemostintenseoneonthespectra.We supposethatthispeakbelongstoapartiallydehydrogenatedfrag- mentanditisnotthereappearanceofthe␥-N Hpeak,namely alltheotherpeakscorrespondingtotheA2representationsare absent.

This idea is supportedby the TPD measurements in which hydrogen desorption was observed in this temperature range (Fig. 2B). Although the loss at the N H stretching region at 3480cm⁻¹ wasstilldetectable,itdisappearedwhenwereached thetemperatureatwhichthedehydrogenationprocesscompleted (∼700–800K).Above900Kthepositionofthepeakat720cm⁻¹- shiftedtohigherwavenumbers(to790cm⁻¹).Thispeakbelongs tothetransverseoptical(TO)phononwithout-of-planepolarization[14].Theremainingtwomainlosspeaksareattributedtothe phononswithin-planepolarization;thehigherenergypeakori- ginatesfromthelongitudinaloptical(LO)phonon,andthelower energy one from the transverse optical phonon (see Table 2).

The observed HREEL spectraat 1100K correspond wellto the literaturedataonhexagonalboronnitridelayer(h-BN)[11].Tak- ingintoaccountthatinspeculargeometryonlymodeshavinga dynamicdipolecomponentperpendiculartothesubstratesurface areallowedwecanconcludeinthiscasethatduetoaverystrong contributionof thein-planemodes (E)a signiﬁcantpartofthe moleculesareorientedonthesurface perpendicularor atleast inslightlytiltedposition.Thisbehaviourissimilartotheresults detectedonPt(111)surface,atthesametimeitisincontrastwith

theresultsobservedonAu(111)surface,where“ﬂat”orientation geometryoccurred[11].

Toproveourpresumptionwefollowedtheeffectofexposure ontheHREELspectraafteradsorptionofborazineat140K(Fig.3B).

Atlowexposure(0.05L)weobservedonlyalittlesignalshowing thatborazineadsorbedonthesurfaceanduntil0.1LtheA₂modes dominatedthespectra.Thissuggeststhatsimilarlytobenzeneon theRh(111)surface[15]atlowexposuresandatlowtemperature borazineadsorbsinaplanargeometryi.e.withthearomaticring planeparalleltothemetalsurface.Neverthelessathighercover- agethereisamixedstateofmoleculegeometriesandwedidn’t perceiveanystronglypreferredorientationofthemoleculeswith respecttothesurfaceplane.Theﬁrstcaserequiresamoresubstrate speciﬁcchemicalbondingmechanismbetweenthemoleculesand thesurfacewhichisresponsiblefortheperpendicularorientation.

Wesupposethat(similarlytothepreviousresultsonPt(111)surface[11])a-typeelectrondonationandinteractionoccursfrom theringNatomtotheemptydstatesofrhodium.Forsuchaninter- actionadehydrogenatednitrogenatomisneededintheborazine molecules.OurTPDresultsprovedthatdehydrogenationreactions havealreadystartedbelow200K,inthiswaytheexplainedmech- anismcouldbepossibleontheRh(111)surface.

3.2. Methanoladsorptiononh-BNcoveredRh(111)

Theeffects of a smallamountof promotersincludingboron havebeenrecognizedalongtimeago.Earlierweinvestigatedthe effectsofsegregatedboronontheadsorptionbehaviourofvarious molecules(e.g.:CO2,C2N2,O2)onRhsurfaces[16–18].Similarlyto thecaseofadsorbedNOwealsoobservedtheformationofastable boronnitridespeciesfromtheinteractionofCN_(a)withthesegre- gatedborononthesurface[17].TheadsorbedCO₂andO₂onboron containingRhsurfacesshowedhighafﬁnityfortheformationof stableboronoxide[16,18].

Inthepresentworkweinvestigatetheadsorptionproperties and the reactivity of a relatively inerth-BN layeron Rh(111) surface.Methanoladsorptionwaschosentotestthecatalyticprop- ertiesoftheh-BNlayerandinthiswaywealsousedmethanolto characterizethepreparedBNlayer.Weadsorbed∼6Lofmethanol

Table2

CharacteristicvibrationsofBNspeciesondifferentsinglecrystalsurfaces.

System ^aTO_⊥(cm⁻¹) ^aTO_||(cm⁻¹) LO(cm⁻¹)

h-BN/Ni(111)[22] 728 1360 1360

h-BN/Pd(111)[22] 784 1384 1432

h-BN/Pt(111)[22] 792 1384 1464

h-BN/Rh(111)[thiswork] 790 1360 1460,1510

Bulkh-BN[22] 776,824 1352,1360 1600

aTO_⊥transverseopticalphononswithout-of-plane,LO(longitudinaloptical)and TO_||phononswiththeinplanepolarization.

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Figure3. (A)EffectofannealingontheHREELspectraafter∼10LborazineadsorptiononcleanRh(111)surfaceand(B)effectofexposureafteradsorptionofborazineat 140K.

onthecleanandBNcoveredsurfaceat140Kandfollowedtheeffect ofannealingontheHREELspectra(Fig.4AandB).Theappearanceof thelossfeaturesfromthevariousvibrationmodesofmethanolon thesurfacelayerat710,1090,1140,1465,2980and3260cm⁻¹sug- geststhatmethanoladsorbsmolecularlyonthecleanandalsoon theh-BNcoveredRh(111)at140K.Theappearanceoftheselosses agreeswellwiththegasphasevibrationaldataofmethanol[19].An interestingfeatureisthatthetwomainlossescorrespondingtothe BNlayerarealsoobservableonthespectratakenat140K(Fig.4B), however,theintensityratioofthetwopeakschangedatthistem- perature.ThemaindifferencebetweentheHREELSresultsobserved onthecleanandBNcoveredlayerthatonthecleansurfacewe observeasigniﬁcantlossat400KduetotheC Ostretchingvibra- tionbelongstothedecompositionproductofmethanolbondedto asurfacerhodiumatom.

AstheTPDresultsshowedtheweaklybondedmoleculesdes- orbedfromthesurfaceat∼160K(Fig.4C).Thiscausedasigniﬁcant attenuationofthelosspeaksofmethanolbelow200KintheHREEL spectra,but it is also clearthat a small part of the molecules remainedonthesurface.Thesemoleculeseitherstayedinmolec- ular formor decomposed—probablyto methoxy and hydrogen suggested by the disappearance of the ␦(OH) vibration loss at 3350cm⁻¹andthepresenceofthefeaturesat710,1085,1140,1465

and2940cm⁻¹belongingtomethoxyspecies[20].Inaformerstudy oncleanRh(111)wegotsimilarresults,namelythatmethanolthat wasadsorbedat100Konthecleansurfacedesorbedbelow300K infourdifferentpeaksat∼135,148, 200and255Kasasignof multilayerandrecombinativedesorptionprocesses.Themoststa- blepartofthemoleculesdecomposedathighertemperatureand providedH₂andCOasdesorptionproductsat360and490K[21].

OnBNcoveredRh(111)weobservedtheabovementionednar- rowlosses(connectingtomethoxyspecies)upto400K(Fig.4B), whichsuggestedaconsiderablestabilityofthemethoxyspecies onthisinertsurface.TPDspectraalsocorroboratedthatmethanol desorptioncompletedviarecombinativedesorptiononlyaround 400K.Althoughwepointedoutthepresenceofasmallfractionof H₂(Tp∼350K)andCO(Tp∼490K)intheTPDspectraclosetothe detectionlimitofourMS(notshown)–incontrasttothecleansur- face–nosignalsofcarbon-monoxidewereobservedintheHREEL measurementsontheboronnitridecoveredRh(111)surface.In thelightofourpreviousresultsobservedoncleansurface,thelack ofCOlossesintheHREELSsuggeststhattheRh(111)surfacefully coveredbyboronnitridelayer.Above400Kallofthelossesbelong- ingtohydrocarbonspeciesdisappearedfromtheHREELspectra andonlythevibrationscharacteristicoftheh-BNoverlayerwere observable.

Figure4.EffectofannealingontheHREELspectraafter∼6Lmethanoladsorptionontheclean(A)andh-BNcovered(B)Rh(111)at140K.TPDspectraofmethanol(M31) onh-BNcoveredRh(111)at140K(C).

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4. Conclusions

•WeprovidedﬁngerprintdatabyAugerspectroscopythatmakes itpossibletodifferentiatebetweenadsorbedborazinemultilayer andh-BNoverlayer.

•Borazineadsorbedmolecularlyat140KonthecleanRh(111)surface,anddehydrogenationreactionsstartedevenbelow200K.

Hydrogendesorptiontookplace ina widetemperature range from190Kto800K.Nootherboronornitrogencontainingprod- uctswereobservedintheTPDspectra.

•Boronnitridelayerformationbegan above600Kas indicated bytheAESandHREELSmeasurements.Thestrongcharacteris- ticlossesofwelldeﬁnedh-BNappearedat∼1000KintheHREEL spectra.

•Borazineadsorbsina“ﬂat”positionontheRh(111)surfaceatlow exposuresbutathighercoverageperpendicularorslightlytilted positionsdominatedthegeometryofadsorbedmolecules.Nev- erthelessatmultilayercoveragewedidn’tperceiveanystrongly preferredorientationofthemoleculeswithrespecttothesurface plane.

•MethanoladsorbedmolecularlyonBNcoveredRh(111)at140K.

Asmallpartofthemoleculeswerestableupto400K onthe surface.Wedidn’tﬁndanysignofdecompositionproductsin contrastwiththecleansurfacewhereCOproducedanddesorbed above400K.

Acknowledgments

The authors wish to thank Dr. Albert Oszkófor the careful revisionofthemanuscript. Thisresearchwassupportedbythe EuropeanUnion and the State of Hungary, co-ﬁnanced by the EuropeanSocialFundintheframeworkofTÁMOP-4.2.4.A/2-11/1- 2012-0001‘NationalExcellenceProgram’.

References

[1]Z.Liu,Q.Xue,T.Zhang,Y.Tao,C.Ling,M.Shan,Carbondopingofhexagonal boronnitridebyusingCOmolecules,J.Phys.Chem.C117(2013)9332–9339.

[2]M.Turner,V.B.Golovko,O.P.H.Vaughan,P.Abdulkin,A.Berenguer-Murcia, M.S.Tikhov,B.F.G.Johnson,R.M.Lambert,Selectiveoxidationwithdioxygenby goldnanoparticlecatalystsderivedfrom55-atomclusters,Nature454(2008) 981–983.

[3]M.Corso,W.Auwärter,M.Muntwiler,A.Tamai,T.Greber,J.Osterwalder,Boron nitridenanomesh,Science303(2004)217–220.

[4]A.B.Preobrajenski,A.S.Vinogradov,MayLingNg,E. ´Cavar,R.Westerström, A.Mikkelsen,E.Lundgren,N.Mårtensson,Inﬂuenceofchemicalinteraction atthelattice-mismatchedh-BN/Rh(111)andh-BN/Pt(111)interfacesonthe overlayermorphology,Phys.Rev.B:Condens.Matter75(2007)245412.

[5]J.Kiss,K.Révész,G.Klivényi,F.Solymosi,Preparationofaboronnitridesingle layeronapolycrystallineRhsurface,Appl.Surf.Sci.264(2013)838–844.

[6]A.Tillekaratne,M.Trenary,Adsorptionanddehydrogenationofdecaboraneon thePt(111)surface,J.Phys.Chem.C113(2009)13847–13854.

[7]M.Morscher,M.Corso,T.Greber,J.Osterwalder,Formationofsinglelayerh-BN onPd(111),Surf.Sci.600(2006)3280–3284.

[8]F.Müller,S.Hüfner,H.Sachdev,One-dimensionalstructureofboronnitrideon chromium(110)—astudyofthegrowthofboronnitridebychemicalvapour depositionofborazine,Surf.Sci.602(2008)3467–3476.

[9]I.Shimoyama,Y.Baba,T.Sekiguchi,K.G.Nath,NEXAFSspectraofanepitaxial boronnitrideﬁlmonNi(111),J.Electron.Spectrosc.Relat.Phenom.137–140 (2004)573–578.

[10]P.J.Chen,M.L.Colaianni,J.T.YatesJr.,Thethermaldissociationofdecaborane onSi(111)-(7×7)anddopingeffectsinthenearsurfaceregion,J.Appl.Phys.

72(1992)3155.

[11]R.J.Simonson,M.T.Paffett,M.E.Jones,B.E.Koel,Avibrationalstudyofborazine adsorbedonPt(111)andAu(111)surfaces,Surf.Sci.254(1991)29–44.

[12]G.Dong,E.B.Fourre,F.C.Tabak,J.W.M.Frenken,Howboronnitrideformsa regularnanomeshonRh(111),Phys.Rev.Lett.104(2010)096102.

[13]G.Hanke,K.Müller,AcomparisonoflowenergyAugerspectraofthenitrides andoxidesofthelightelementslithium,berylliumandboron,Surf.Sci.152/153 (1985)902–910.

[14]E.Rokuta,Y.Hasegawa,K.Suzuki,Y.Gamou,C.Oshima,Phonondispersionof anepitaxialmonolayerﬁlmofhexagonalboronnitrideonNi(111),Phys.Rev.

Lett.79(1997)4609–4612.

[15]M.Neumann,J.U.Mack,E.Bertel,F.P.Netzer,Themolecularstructureofben- zeneonRh(111),Surf.Sci.155(1985)629–638.

[16]F.Solymosi,J.Kiss,Theeffectofboronimpurityontheadsorptionanddissoci- ationofCO2onRhsurfaces,Chem.Phys.Lett.110(1984)639–642.

[17]F.Solymosi,L.Bugyi,EffectsofboronimpurityonthesurfacereactionsofC2N2

onRh(111)andRhfoil,Appl.Surf.Sci.21(1985)125–138.

[18]J.Kiss,K.Révész,F.Solymosi,Segregationofboronanditsreactionwithoxygen onRh,Appl.Surf.Sci.37(1989)95–110.

[19]A.P.Farkas,F.Solymosi,Effectsofpotassiumontheadsorptionanddissociation pathwaysofmethanolandethanolonMo2C/Mo(100),Surf.Sci.602(2008) 1475–1485.

[20]J.L.Davis,M.A.Barteau,Spectroscopicidentiﬁcationofalkoxide,aldehyde,and acylintermediatesinalcoholdecompositiononPd(111),SurfSci.235(1990) 235.

[21]F.Solymosi,A.Berkó,T.I.Tarnóczi,Adsorptionanddecompositionofmethanol onRh(111)studiedbyelectronenergylossandthermaldesorptionspec- troscopy,Surf.Sci.141(1984)533–548.

[22]A.Nagashima,N.Tejima,Y.Gamou,T.Kawai,C.Oshima,Electronicstatesof monolayerhexagonalboronnitrideformedonthemetalsurfaces,Surf.Sci.

357–358(1996)307–311.