XPS-evidence for in-situ electrochemically-generated carbene formation

XPS-evidence

for

in-situ

electrochemically-generated

carbene

formation

Pinar

Aydogan

Gokturk

,

Ulrike

Salzner

,

László

Nyulászi

,

Burak

Ulgut

*

,

Coskun

Kocabas

,

Se

fik

Suzer

*

a_{DepartmentofChemistry,BilkentUniversity,06800Ankara,Turkey} b

DepartmentofPhysics,BilkentUniversity,06800Ankara,Turkey

c

DepartmentofInorganicandAnalyticalChemistry,BudapestUniversityofTechnologyandEconomics,Budapest,Hungary

ARTICLE INFO Articlehistory: Received4January2017

Receivedinrevisedform27February2017 Accepted6March2017

Availableonline14March2017

ABSTRACT

StableN-heterocycliccarbenes(NHC)areaclassofcompoundsthathasattractedahugeamountof interestinthelastdecade.OnewaytoprepareNHCsisthroughchemicalorelectrochemicalreductionof 1,3-disubstitutedimidazoliumcations.Wearepresentinganin-situelectrochemicalX-rayPhotoelectron Spectroscopy(XPS)studywhereelectrochemicallyreducedimidazoliumcationsleadtoproductionof stableNHC.Theelectroactiveimidazoliumspeciesisnotonlythereactant,butalsopartoftheionicliquid whichservesastheelectrolyte,themediumandtheelectroactivematerial.Thisallowsustodirectly probethedifferencebetweentheparentimidazoliumionandtheNHCthroughtheuseofXPS.The interpretation of the results is supported by both observation of reversible redox peaks in the voltammogramandthedensityfunctionaltheorycalculations.

©2017ElsevierLtd.Allrightsreserved.

1.Introduction:

Historically,stablecarbenechemistryhasnotattractedmuch attentionsinceitwasalwaysassumedthatthestabilityofcarbenes weretoo short-lived for any useful study. A seminal paper by Arduengoet.al.[1]resultedinafundamentalshiftinthisfieldby reportingthatstable and “bottle-able”carbenescan be synthe-sized.Thestabilityisachievedstericallybymakinguseofbulky groupsandelectronicallybyusingpi-donating,sigmawithdrawing groups. The molecule reported (1,3-di-l-adamantylimidazol-2-ylidene) contains a 5-membered imidazole ring, which is substitutedwithbulkyadamantylgroups.Thisstudywasfollowed byalargebodyofwork(recentlyreviewedbyHopkinsonet.al.[2]), reporting synthesis of numerous molecules with the generic formula (R2N)2C:, where R can be aryl or alkyl groups. In a

significantnumberofcases,theRgroupscontainnitrogenandare linkedtoformheterocyclicrings.Thisgroupofmoleculesarethus calledN-heterocycliccarbenes(NHC).

Historically, carbenes had been synthesized in a number of differentwaysasreportedbyWanzlick[3].Morerecently,NHCs have most commonly been synthesized beginning with a 1,

3-disubstitutedimidazoliumsaltandtogeneratethecarbenevia deprotonationusingastrongbase(seeforexample[4]and[5]). Thismethodisreportedlyusedinorganicandinorganiccatalysis, wherethecarbenesiteisthecrucialcomponentofanumberof importantcatalyticreactionsasreviewedbyW.A.Hermann[5].It isalsoshownthatelectrochemicalorchemicalreductionofa 1,3-disubstitutedimidazoliumsaltundertherightconditionsleadsto anNHC[6].ChemicallyorelectrochemicallygeneratedNHCshave beenshowntobeactivecatalystsforanumberofreactions(see Ferociet.al.[7]).

One example of an electrochemical preparation of a stable carbenewasreportedbyGorodetskyet.al.[6].A1,3-disubstituted imidazoliumsaltwasshowntoturnintoacarbenebyreduction under electrochemical conditions. The formation of the stable carbeneuponreductionoftheimidazoliumionwasshownbythe emergence of a new electrochemical feature in the cyclic voltammogram.

Roomtemperatureionicliquidsareaclassofcompoundsthat haveappreciableionicconductivitiesandverylowvaporpressures at room temperature. This makes them uniquely suited as electrolytesforin-situexperimentsundervacuum.These proper-ties have been regularly exploited in experiments involving electrochemistryinvacuumsystems[8,9].Alongtheselines,we haverecentlyreportedanotherexperimentstudyingthedetailsof theformationofelectricaldoublelayeronagoldelectrodeusingan

* Correspondingauthor.

E-mailaddresses:ulgut@fen.bilkent.edu.tr(B.Ulgut),suzer@fen.bilkent.edu.tr

(S.Suzer).

http://dx.doi.org/10.1016/j.electacta.2017.03.053

0013-4686/©2017ElsevierLtd.Allrightsreserved.

ContentslistsavailableatScienceDirect

Electrochimica

Acta

ionic liquid [10]. 3,5-Substituted imidazolium derivatives with asymmetricsubstituents areverycommonly usedas cationsof roomtemperatureionicliquids,mostlybecauseoftheirstability andverylowpropensitytocrystallize.Currently,room tempera-tureionicliquidsinvolving1,3-disubstitutedimidazoliumcations are commercially commonplace. Using ionic liquids based on imidazolium derivatives, it was claimed that NHCs can be electrochemically prepared, where it was indirectly shown via NMRspectroscopythatelectrochemicalreduction ofa groupof imidazoliumderivativesyieldedthecorrespondingNHCthrough theNHCs’catalyticactivityin variousorganicreactions[11].By followingtheproductsofthecatalyzedreactions,thepresenceand amountoftheNHCswereinferred.Morerecently,Ferociet.al.[12]

reportedcyclic voltammetric studies of thiazolium salts in the presenceandtheabsenceofacetateions.Undertheconditionsthat wereused,theauthorsshowedthat NHCsarenotstableunless acetate ions are incorporated into the mixture. They further presented voltammetric evidence of NHCs upon reduction of imidazoliumsystems.1-ethyl-3-methylimidazolium acetate has already been extensively reported in the literature to have desirable properties for application in cellulose and chitin processing or carbon dioxide absorption ([13] and references therein).HighbasicityoftheacetateanionhelpsstabilizetheNHC uponreductionoftheimidazoliumcation.

X-ray photoelectron spectroscopy (XPS) is an established analytical tool that is not only used for quantitative chemical analysissince itsinception, but alsohasbeen theplatform for analyzingchargingeffectsof varioussystemsasreportedbyus

[14,15]andothers[16,17].InoneexampleofNHCpreparationusing ionic liquids, Ir nanoparticles were employed to stabilize the carbenesite.Inthatwork,Bernardiet.al.[18]showedthatadding Irtoa sampleof ethyl-3-methylimidazoliumethylsulfate(EMI. EtSO4),resultedinsomeoxidizedIrandformationofanewcarbon specieswithlowerelectronbindingenergy.UsingXPS,theywere abletoassignthenewcarbonsignaltoacarbenespeciesthatis interactingwiththeIrnanoparticlesurfacethatisresultingina partiallyoxidizedIrspecies.Adetailedin-situsynchrotronbased XPSanalysiswasrecentlyreportedbyKruusmaetal.on 1-ethyl-3-methylimidazolium tertrafluoroborate [EMImBF4] deposited on molybdenumcarbidederivedcarbonelectrodewhiletheelectrode wasundervariousnegativebiasestofollowtheelectrochemical reductionproducts[19].InthepristineEMImBF4salt,theN1speak hasonlyonecomponent,correspondingtothetwoequalnitrogen atomsontheimidazoliumring,bearingonlyonepositivecharge (–N=CH-N-)+,withthecorrespondingbindingenergyof402.7eV. Theyreportedthat asecond peakat alower bindingenergyof 400.8eV appeared and grew in intensity as the electrode was polarizedtohighernegativevaluesofupto-2.40V(vs.Ag/AgCl referenceelectrode).In parallel,intensitychangesin the corre-spondingC1s,B1sandF1speakswerereported.Theyattributed thesespectralchangestoelectrochemicalreductionaswellasto decompositionproducts.Inaddition,theyreportedonobservation ofshiftsinthepositionofthebindingenergiesasafunctionofthe potentialsappliedtotheelectrode(s), similartothose reported earlier by Foelske-Schmitz et al., where a potentiostatically controlledDCbiaswasimposedusinganelectrochemicalset-up and recording XPS [20–22]. The unique ability of XPSto yield chemically specific information about local charging allows in depthstudiesofchargeaccumulation.Thishasbeenparticularly tunablewiththeapplication ofexternalelectricalbiasontothe samplewithrespecttotheinstrumentground[23,24].

In thepresentreport,XPShasbeenusedtogatheroxidation stateinformation of individual atoms. We are reporting onan experimentwherewesimultaneouslygeneratecarbenes electro-chemicallyanddetect,withstrongexperimentalandtheoretical evidences,theformationofacarbenespeciesthroughthechanges

ofthechemicalenvironmentofneighboringnitrogenatoms.Toour knowledge,thisis thefirstreportofreversible carbene electro-chemistryasevidencedbysmallpeakseparationinthe voltammo-gramcoupledtothereductionchargebeingequaltotheoxidation charge.

2.ResultsandDiscussion

Fig.1showsthechemicalstructureandasurveyspectrumof the ionic liquid. The XP spectrum of the EMImAc includes photoelectronpeaks for nitrogen 1s, oxygen 1s and carbon 1s electrons.Thenitrogen1sregionisspeci_ficallytheregionwewill be following, as that region is going to develop the readily observable changes due to the carbene formation. The carbon region, which should also develop relevant changes, is much hardertodecodeunequivocallyastherearemorespeciesofcarbon atdifferentbindingenergies.However,aswillshowbelow,the regionalsoprovidesrelevantandsupportinginformation.

AsseenintheN1sregion,aspreparedsampleshowsonlya singlepeakinthespectrumthatcorrespondstothetwoequivalent nitrogenatomsoftheimidazoliumring,carryingoveralloneunit ofpositivecharge,atabindingenergyof402.2eV,whichissimilar withthepreviousstudiesonimidazoliumbasedionicliquids[23– 26].ThefittingofC1sregionisalsoillustratedinFig.1forEMImAc. The contribution to C1s envelope comes both from the EMIm cationandtheacetateions.Thespecificcarbonsarenumberedon thestructurethatisdrawntoillustratethevariouscontributions. For allcarbons a Lorentzian/Gaussianmix(30% Gaussian)peak shapewasused.Wehaveadoptedthebindingenergyvaluesused in Ref. [23] for the C1s peaks of the cation and the guidance outlinedinRef.[27]forothers.Accordingly,C2andC5,aswellasC3

andC4areassignedtobeequivalent.Therefore,theareaforthese

two peaks wereconstrained tobe twiceas much asthe other carbons.C6 isthealiphaticcarbon thatthebindingenergywas

shiftedto285.00eVtocorrectforchargingeffects [17].Thefact that,featuresduetotheunderlyingsiliconisnotobservedonthe surveyspectrum,indicatesthatthethicknessoftheionicliquidon theregionworkedonisthickerthantheaveragescatteringlength ofthephotoelectrons,whichisknowntobe5-10nm.

Fig.1.ASurveyXPspectrumoftheEMImActogetherwiththeN1sandC1slevels recordedatahigherresolution.TheN1sregionconsistsofasinglepeakassignable tothetwoequivalentnitrogenatomsofthecyclicstructure,butthecomplexC1s regionisfittedto8peakstwoofwhichhavebeenfittedtotwoequivalentcarbonsas indicatedintheinset.

2.1.BiasShiftsinXPS

Asthemainemphasisofthisworkistodemonstratethat,by applicationofanexternalvoltagebiaselectrochemicalreductionis achievedtoyieldstableNHC,whereXPSisutilizedtoanalyzeand atthesame time tofollow theprocess,a warning needstobe introducedatthispoint.ApplicationofexternalbiasinXPShasalso beenshowntocausedifferentialcharginginvarioussamplesand devices[14–17].Morerecently,wehavereportedonasymmetric coplanarcapacitorgeometrywithanionicliquidastheconducting mediuminanXPSsetting,wherethedetailsofthesurprising long-rangecharging/dischargingeffectsofthedoublelayer,alongwith its kinetic behavior were explained and modeled [10]. This informationwas harvested throughanalyses of XPSpeak shifts ofcertainelementsonlyafterimposingDC and/orAC electrical biasestothesample.However,aswasalsocautionedinRef.[27], greatcaremustbeexercisedtoextractthecorrectinformation, sincetheverysamebiascanintroducespectralartifactsaswell,as shown in Figure S1in the SupportingInformation (SI)section. Theseshifts,causedbyappliedpotential,caneasilybemistakenfor chemicalshifts,astheappliedpotentialcanshiftdifferentspecies differently, as a result, different peaks may overlap or get separated, depending on induced local charges. Therefore, in ordertojudgetheidentityofdifferentchemicalspecies,itisbestto comparedata thatis takenwith groundedsamplesin order to

mitigate any adverse effects of potentially non-uniform bias effects,asdepictedinFig.2.

ItisclearlyvisibleinFig.2thatevenaftertheelectrodesare grounded,twoseparateN1speaksprevail.Thisproves,without anydoubtthatanewchemicalspeciesisgeneratedandthenew signalisnotduetoanypartialchargingorheterogeneouselectric field effects (see FigureS1in theSIsection).However,the C1s exhibitsslightbroadeninginthehighenergysideunder-3Vdue mostprobablytopartialcharging.

2.2.ElectrochemicalReduction

Onceenough biasisapplied,a reduction isexpectedonthe negativelybiasedelectrodethatwouldleadtocarbeneformation. Underextendedperiodsofappliedbias,thecarbenespeciescanbe showntoformoneitherelectrodedependingonthepolarityofthe appliedpotential.AsFig.3shows,throughout8hourperiods,we canobserveanewN1ssignalformedoneither-3Vor3Vapplied acrosstheionicliquidsample.Thisnewsignalthatformswitha bindingenergythatis
2.5eVlowerthanthebindingenergyofthe N1softheimidazoliumringisattributedtothenitrogenatoms thatarepartoftheoverallneutralNHC.Whenapositivebiasis appliedtothegoldelectrode,thesiliconelectrodeiseffectivelyat -3V vs. the gold wire. This voltage is enough for the silicon electrode to reduce the imidazolium cation, leading to the

formationofthecarbene.Whenanegativebiasisappliedtothe goldelectrode,thereductionhappensonthegoldelectrode,and thecarbeneN1ssignalbecomesmorevisiblenearthegoldwire. Bothoftheseexpectationsareindeedverifiedbyresultsdisplayed inthefigure,inwhichthefirstappearanceofthenewpeakisinthe vicinityoftheSi-electrodeandmovesuptotheAu-electrodein time,andonlyafterswitchingthebiasto-3Vtheintensitygrows nearthatelectrode.InparalleloxidizedSimoieties(SiOx)arealso

producedwithintheILmedium(notshown).

InordertojustifyourargumentthatthenewN1speakisdueto adifferentchemicalspeciesandnotduetoheterogeneouselectric potentialeffects,theXPSmeasurementsarerepeatedattheend whileboththeelectrodesaregroundedaswasshowninFig.2.

Inordertofurthershowthatthesechangescannotbeexplained byexperimental artefacts like X-ray beamdamage, thecontrol experimentofextendedbeamexposurewasperformed.20hours ofX-RayexposureyieldsnonoticeablechangeintheC1s,O1sand N1sregionsofthephotoelectronspectrum.Thisdataisshownin thesupportinginformationasFigureS2.

2.3.ChemicalandElectrochemicalShiftsRevealedbyXPS

The strength of XPS has traditionally been attributed toits abilitytodifferentiatethebonding informationof theanalyzed atomsviatheso-termedchemicalshifts.Equallyimportantisits abilitytoreflecttheelectricalpotentialofthemedium,sincethe measured kinetic energy of the photoelectrons are directly affected. Therefore, it is of utmost importance to disentangle themforcorrectinterpretationoftheresults.WhentheN1sregion of theEMImAc before and after electrochemical reduction and underdifferentbiasapplications(Fig.2)isexaminedthenewpeak at
2.5eVlowerenergycanbeassigned toaneutral–N(imide) withinthestructure.

Uponapplicationof3Vexternalbiasallthepeaksofboththe N1sand C1s regionsundergo a shiftof +2.4and -2.5eVshifts, respectively,duetothevoltagedevelopedwithintheILmedium under the potential stress. The absolute magnitude of 2.5eV indicates that 0.5V drops across the Au electrode/electrolyte interfaceand2.5VontheSielectrode/electrolyteinterface.Since the same absolute amount of excess charge exists on both electrodes, the voltage drop across the interface is inversely proportional tothe double layer capacitance on the respective electrodes Vdoublelayer¼qdoublelayer=Cdoublelayer

 

.Thisindicatesthatthe overalleffectivecapacitance(andthusthecontactarea)ratioofthe

twoelectrodesis1:5.Itcanbeconcludedthattheeffectivecontact areabetween the Au electrode and theionic liquid is 5 times greaterthanthecontactareabetweenSielectrodeandtheionic liquid.However,theuniformityofthisshiftisnotalwaysgranted, sinceitispossiblethatdependingonthetype(twoelectrodeor threeelectrode)andthechemicalnatureoftheelectrochemical set-upused,thesize(smallspotvs.largeone)themake-upofthe analyzedregion, andtheamountanduniformityof thecurrent flow,non-uniformshiftscoulddevelop.

These potential profiles that are prone to non-uniformities could lead to non-uniform shifts in the binding energies. The easiestwaytocircumventthisistorecorddataafterremovalofthe externalpotentialstressesandallowampletimebeforerecording thespectrumtoensurecompletedischarge.Doingtheexperiment withgroundedelectrodesattheendofthepotentialapplication ensures that any newpeaks that areobserved are due tonew chemicalspeciesandnotduetoanynon-uniformpotentialeffects. 2.4.In-situCyclicVoltammogramsandXPSRecording

Forfurtherunderstandingoftheelectrochemistryinthetwo electrode geometry, cyclic voltammetry experiment was per-formedstartingat0Vandsweepingto-2Vandbackatasweep rateof1V/s.ThedataisshowninFig.4(d).Thevoltammogram showsaclearredoxcouplethatweattributetotheredoxreaction that leads toformation of the carbene in a reversible manner. Reductionoftheimidazoliumcationleadstotheformationofthe relevantNHCthatcangetre-oxidizedbacktotheimidazolium. Thissignalischemicallyreversibleasshownbythefactthatthe areaunderneathbothpeaksarethesame.Thisisstrongevidence thatthecarbenethatisformedisstablethroughoutthetimescale oftheexperiment.Thisisinstrongcontrasttothepreviousreports ofreductionofimidazoliumderivativestoproducecarbenespecies ([12])wheretheformedcarbenedoesnotshowanyreversibility. Thisshows that insidethe XPSchamber wherewater vapor or oxygengasisminimized,theelectrochemicallyformedcarbeneis stable. In the two electrode geometry, it is hard to judge electrochemicalreversibilitysince

D

Electrochemical reversibility can be simply measured by an accuratemeasurementof

D

geometry,theresistanceoftheionicliquidisnotcompensatedand thereforecontributessignificantlytothepotentialsandthusthe

D

InordertofollowtheformationofthecarbenespeciesviaXPS inrealtime,amuchslowersweeprate(1mV/s)neededtobeused in order to develop the necessary amount of material to get appreciableXPSsignal.Atthesesweeprates,thefaradaiccurrentis below thenoise level,sothe linearsweepvoltammogram (not shown)doesnotshowanyappreciablefeatures,buttheXPspectra developtheN1speakthatisattributedtotheNHC.Thispeakgrows asthepotentialissweptmorenegative.Thisisnotfollowedonthe reversesweepas thedata getscomplicated witheffects of the stablecarbenespeciesdiffusingawayfromtheelectrode(datanot shown).The XPSresult of this experimentis shown in various formsinFig.4(a,b,c).Fig.4ashowsacontourmapoftheXPspectra atvariouspotentials.Asthepotentialissweptmorenegative,in addition to the overall waveform shifting to lower binding energies, a new peak startsforming at
2.5eVlower binding energies.Thisis moreprominently shownin Fig.4bwhere the green spectrum shows a slice from-2V appliedpotential. This spectrum clearlyshows thepeak that is attributedtothe NHC designatedasN0_{.Thedetailsofthisformationissummarizedin} Fig.4c wherethe ratioof thepeak intensitiesare plotted asa function of the applied bias. It is clear that the NHC amount increasesasthepotentialissweptmorenegative.

Fig.3.XPSpectraoftheN1sandAu4fregionsalongthelinedesignatedunder+3V and-3VBiasandindifferentperiods.

Table1

ExperimentallydeterminedB.E.positionsoftheC1sandN1speakstogetherwithcalculatedshiftsaftercarbeneformation.

C1 C2 C3 C4 C5 C6 N

EMImAcbindingenergies(eV) 288.38 286.58 287.28 287.28 286.58 285.00 402.20 Calculatedshifts(eV) -2.45 -0.72 -1.07 -1.03 -0.35 -0.24 -1.53* *Averagevalueforthetwonitrogenatoms.Detailsaregiveninsupplementaryinformation.

Fig.4.(a)and(b)XPspectraofN1sregionduringElectrochemical-Cycling.(c)Changeintheratiooftheintensityofthereducedneutralnitrogentothatofthecationicone, withtheappliedpotential.(d)Twoelectrodecyclicvoltammagramofthesystem.

Fig.5.XPspectraofC1sregionbefore(a)andafter(c)electrochemicalreductioncomparedtoa76%to24%combinationoftheoreticallypredictedEMImAcandcarbene spectra(b)and(d)showstheoverlayofthebeforeandafterspectra.

3.Discussions

AftertheinsituelectrochemicalreductionofEMImAcionicliquid underexternalbias,notonlytheN1sregionbutalsoC1sregion developedrelevantchanges,i.e.anincreaseintheintensityoflow bindingenergycomponentsoftheC1senvelope,seeFig.2.Similar changeswerealsoobservedinpreviousstudiesandattributedto increasing intensity of aliphatic carbon peaks [C5, C6] [19]. Additionally,fromtheintensityratioofthecationictotheneutral N1scomponents we determinethat 24%of the nitrogenatoms undergoelectrochemicalreductionwithintheanalyzedregion.

InordertounderstandthechangesinC1sspectrumafterthe electrochemicalreaction,wecalculated1sbindingenergiesand thecorresponding shifts. The shifts and experimental EMImAc binding energies are collected in Table 1. The details of the calculationand thecalculatedbindingenergiesareshownin SI alongwithadiscussionofthecalculationresults.

Forallimidazoliumcarbonatomstheorypredictsadecreasein binding energy which is strongest for the carbene carbon and weakens with distance from the carbene center as the overall positivelychargedimidazoliumisnowreducedtoanoverallneutral carbene.Bindingenergiesofthecarbonatomsoftheacetateanion increaseastheprotonisaccepted.Becausetheaceticacidformed duringthereactionprobablydiffusesawayorevaporatesawayfrom thecarbene,theobservedcarbonpeakafterreductionmayconsistof thecarbenecarbonpeaksonly.HighresolutionC1sspectraofIL beforeandaftertheelectrochemicalprocess arecomparedwith theoretical1sshiftsinFig.5.Forthesimulatedshifts(Fig.5b),the EMImAcandcarbenepeaks(withouttheaceticacidcarbons)are combinedwith theweightsof 76% and 24%. Theseweightsare experimentallydeterminedusingtheratioofthepeakareasofthe N1ssignal.Althoughthesimulatedandexperimentalspectraarenot identical,theintensityincreaseinthelowerbindingenergyregionis reproduced.Thissupportstheexperimentalfindingsthatsuggest thattheobservedspectrumisacombinationofEMImAcandcarbene peaks,anditisclearthatacontributionfromahigherbindingenergy carbonisgettingtradedforalowerbindingenergycarbon(Fig.5d). But, it should also be reminded that over the long term XPS measurements,adventitiouscarboncouldhavebeenaccumulated, whichmightalsobecontributingtothelowenergy(
285.0eV) componentofthecompositepeak[27].

ThecyclicvoltammetrydatainsidetheXPSchambershowthat thesubstitutedimidazoliumcationspeciescangetreducedtothe relatedcarbene speciesin a chemicallyreversible manner.This reversibilityshowsthatintheabsenceofanyoxygenandwater, thecarbeneisstablewithinthetime-scalesoftheexperiment. 4.Conclusions

Reversible carbene electrochemistry was observed inside an XPS chamber. The new N1s peak with the binding energy of 399.8eVgeneratedviaelectrochemicalmeansisassignedtothe reducedNatomsoftheNHC.ElectrochemicalandXPSevidences indicatethatreductionofimidazoliumcationsinthepresenceof acetateionsleadstoreversibleformationofNHCs,whichisfurher supportedbycalculations.

5.ExperimentalSection

1-Ethyl-2-methyimidazoliumacetate[EMIMAc]waspurchased fromSigma-Aldrich.Electrochemicalexperiments,alongwiththe

photoelectronspectroscopy werecollectedusing a Si substrate thatwasconnectedtotheinstrumentgroundwithadropofionic liquidformedonthepolishedside.Agoldwire,wasconnectedto the sample holder for electrical connection and was put into physicalcontactwiththeionicliquiddrop.XPSdatawerecollected usingaThermoFisherK-AlphaX-rayPhotoelectronSpectrometer with a monochromatized photon energy of 1486.6eV for all measurements.In-situelectrochemicalmeasurementswere per-formedusingaKeithley2400Source-meterthatisdrivenbyan in-housewrittenLabViewSoftwareinthecontrolledpotentialmode. Thepotentialwasprogrammedwhilethecurrentsweremeasured every1ms.Thegroundofthespectrometerwasconnectedtothe groundoftheKeithleySource-Meter.Thisensuredthatnoground loopsexistedbetweenvariousinstruments.

AppendixA.Supplementarydata

Supplementarymaterialrelatedtothisarticlecanbefound,in the online version, at http://dx.doi.org/10.1016/j. electacta.2017.03.053.

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