Electrodeposited gold nanoparticles on glassy carbon: Correlation between nanoparticles characteristics and oxygen reduction kinetics in neutral media

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Identification number: DOI: 10.1016/j.electacta.2013.10.172

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http://dx.doi.org/10.1016/j.electacta.2013.10.172

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Eprints ID: 11357

To cite this version:

Gotti, Guillaume and Fajerwerg, Katia and Evrard, David and Gros, Pierre

Electrodeposited gold nanoparticles on glassy carbon: Correlation between

nanoparticles characteristics and oxygen reduction kinetics in neutral media.

(2014) Electrochimica Acta, vol. 128 . pp. 412-419. ISSN 0013-4686

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T

oulouse

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OATAO

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OATAO is an open access repository that collects the work of Toulouse researchers and

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Electrodeposited

gold

nanoparticles

on

glassy

carbon:

Correlation

between

nanoparticles

characteristics

and

oxygen

reduction

kinetics

in

neutral

media

Guillaume

Gotti

_,

_Katia

_Fajerwerg

_,

_David

_Evrard

_,

_Pierre

_Gros

b_{CNRS,LaboratoiredeGénieChimique,F-31062Toulouse,France}

c_{CNRS,LCC(LaboratoiredeChimiedeCoordination),205routedeNarbonne,F-31077Toulouse,France} d_{UniversitédeToulouse,UPS,INPT,LCC,F-31077Toulouse,France}

Keywords:

Goldnanoparticleselectrodeposition Oxygenreductionreaction Neutralmedia

Koutecky–Levichanalysis Electrochemicalkinetics

a

b

s

t

r

a

c

t

Goldnanoparticles(AuNPs)weredepositedontoglassycarbon(GC)byconstantpotentialelectrolysis (CPE)usingvarioussetsofpotentialanddurationfrom−0.3to0.7V/SHEand10to1800s,respectively. Thephysico-chemicalcharacteristicsoftheas-obtaineddepositswereinvestigatedbycyclic voltam-metry(CV)inH2SO4,fieldemissiongunscanningelectronmicroscopy(FEG-SEM),andPbunderpotential

deposition(UPD).Theirperformancestowardtheoxygenreductionreaction(ORR)inaNaCl–NaHCO3

(0.15M/0.028M,pH7.4)neutralsolutionwereexaminedandcorrelatedtoAuNPssizeanddensity.The bestresultswereobtainedusingthedepositswhichexhibitedahighdensity(555±49mm−2)of rela-tivelysmallAuNPs(25±12nm).TheKoutecky–Levichtreatmentwassystematicallyappliedtoallthe depositsinordertodeterminethenumberofelectronsnexchangedfortheORRinthepotentialrange from0.1to−1.0V/SHE.Thevaluesofthecathodictransfercoefficientsˇnwerealsoextractedand com-paredtothevaluesreportedforunmodifiedGCandbulkAu.Amapoftheˇnvaluesasafunctionof AuNPselectrodepositionpotentialanddurationwasalsoprovided.

1. Introduction

Theoxygen(O2)reduction reaction(ORR) isprobablyoneof themoststudiedredoxprocessesintheliterature,bothfromthe mechanistic[1–3]andkinetic[4–8]pointsofview.Amongstthe numerous works dealing withthis electrochemical system, the DamjanovicmodelisofparticularinterestwhichdescribestheORR asamulti-electronreaction[9].Thus,O2reductionmaybeachieved byafour-electron“direct”pathway(Eq.(1))orviatwo succes-sivebielectronicstepsinvolvinghydrogenperoxide(H2O2)asan intermediatespecies(Eqs.(2)and(3))[10–12]:

O2+4H++4e−→ 2H2O E◦ =1.23V/SHE (1)

∗ Correspondingauthorat:CNRS,LCC(LaboratoiredeChimiedeCoordination), 205routedeNarbonne,F-31077Toulouse,France.Tel.:+33561333130; fax:+33561553003.

∗∗ Correspondingauthorat:UniversitédeToulouse,UPS,INPT,Laboratoirede GénieChimique,118routedeNarbonne,F-31062Toulouse,France.

Tel.:+33561556073;fax:+33561556139.

E-mailaddresses:Katia.Fajerwerg@lcc-toulouse.fr(K.Fajerwerg),

evrard@chimie.ups-tlse.fr(D.Evrard). 1 _ISEMember.

O2+2H+_+2e−_{→ H}

2O2 E◦ =0.70V/SHE (2) H2O2+2H++2e−→H2O E◦ = 1.78V/SHE (3) GnanamuthuandPetrocellihavealsoanalyzedaseriesofmore complexreductionpathsforO2reductiononnoblemetals(Pt,Pd, Au)andalloys(Pd–AuandPt–Au)[13].

ThisstandinginterestinORRismainlyduetothewiderangeof applicationdomainsitisinvolvedin,fromchemicalorbiochemical analysis[14,15]toindustrialfieldssuchaspharmaceutics[16], cor-rosion[17]orenergystorageandconversion[18,19].Forinstance, O2maybeuseddirectlyasoxidationagentinfoodprocess[20].It maybealsoreducedintoH2O2whichcanbefurtherusedasa ster-ilizingagent[16].Morerecentlythedevelopmentoffuelcellsdrove aparticularemphasisonORRkineticstudyinacidicorbasicmedia onvariouscatalystsincludingcarbon-basedmaterials,enzymes, macrocyclesorcoordinationcomplexesoftransitionmetals[21]. Amongstthenumerousmaterialsused,nanoparticles(NPs),and especiallygoldnanoparticles(AuNPs)havebeenextensively stud-ied[22,23].Inacidicmedia,kineticdatahavebeenobtainedby electrochemicalimpedancespectroscopy[24],rotatingringdisk electrode[25]andscanningelectrochemicalmicroscopy[26].In manystudiestheAuNPselectrocatalyticactivitytowardORRhas beenfoundto besize [25–27]and crystal structure-dependent

[26].Indeed,thebestresultshavebeenobservedonsmall(around 20nmdiameter)[28]andverysmall(lessthan10–15nm)AuNPs

[26,27],andforhighAu(110)/Au(111)ratio,Au(100)facesbeing notpresentinsmallNPs[24].WainhaspostulatedthatsmallNPs possesshighfractionsof highindex steps,edgesand kinksites whichmayfavorelectrocatalysis[26].InalkalinesolutionORRis knowntobefasterthaninacidicmedia[3].Asimilartrendhasbeen reportedwithrespecttoAuNPsstructure,theactivitytowardO2 reductiondecreasingintheorderAu(100)≫Au(110)>Au(111)

[29].However,thesizeeffectisnotasclearasinthecaseofacidic media:Tangetal.havenoticedORRkineticstobe2.5timeshigher on3nmdiameterAuNPssupportedoncarbonthanon7nmones

[30],whereasLeeetal.haveobservedbetterresultson8nmNPs comparedto3and6nmones[31].El-Deabetal.havealsoreported aninterestingstudyinwhichrelativelybigAuNPs(ca.50–300nm) wereselectivelyenrichedinthemostactivecrystallographicfaces, namelyAu(100)andAu(110),andexhibitedbetterkineticactivity towardtheORRthansmaller(ca.10–40nm)NPs[32].AuNPsshape hasalsobeenreportedtohaveaninfluenceontheirelectrocatalytic response[33,34].Forinstance,Kuaietal.haveshownthat icosa-hedralAuNPs exhibitedenhanced electrocatalyticperformances towardORRandhaveproposedtheirmultiple-twinnedstructure tobemorechemicallyactivebecauseoftheirhighsurface-defect density[34].

Surprisingly,thereareveryfewreportsdealingwithORRon AuNPsinneutralmedia,althoughsuchstudieswouldbeofgreat interest for biological or medical topics [35,36]. Rajet al. have highlightedtheelectrocatalyticeffectofAuNPsanchoredby dif-ferent organiclayers onbare Auelectrodes toward theORR in phosphatebuffersolution(pH7.2)[37].InthecaseofAuNPs sta-bilized by cystamine,the2-electron reduction of O2 into H2O2 hasbeen foundtooccur ata potential130mV higher than on polycrystallineAuelectrode.Althoughtheresultingpeakcurrent wasenhanced 1.2 timescompared toan unmodified electrode, nokinetic feature hasbeenprovided inthis study.Shim et al. have reported the comparison of the ORR kinetics onbulk Au and AuNPs in phosphate buffer (pH 7.4) and have shown the influenceoftheelectrodepretreatment,i.e.200successive poten-tial scans in 1MH2SO4 [38]. A Koutecky–Levich mathematical treatmentofthesteady-statevoltammetriccurveshasbeen per-formedbutitsexploitationhasbeenlimitedtothedetermination ofthenumberofelectronsnexchanged.OnlyEl-Deabetal.have conducted a more detailed kinetic study upon ORR in neutral phosphatebuffersolutionbycombiningKoutecky–Levich analy-siswithrotatingPtringGCdiskelectrodemeasurements[39].The valuesofnhavebeenobtainedatvariouspotentialsandan inter-estingestimationoftheenergysavingsduringH2O2 production hasbeenproposed.Howeverthislatterworkdidnotgoasfaras thedeterminationofkineticparameterssuchascathodic trans-fercoefficients ˇn sothat there is alack for kineticdata upon ORR in neutralmedia. Veryrecently,we havereported a com-pletekineticstudyofORRinseveralneutralsolutionsonglassy carbon(GC)andbulk Au[40].Inthepresentwork,wedescribe theperformancesofAuNPsdepositedonGCbyconstantpotential electrolysis(CPE)usingvariousdepositiondurationsand poten-tialswithrespecttoORRinaNaCl–NaHCO3(0.15M/0.028M,pH 7.4)neutralmedium.Thedifferentdepositswerecharacterizedby cyclicvoltammetry(CV)inH2SO4andfieldemissiongunscanning electronmicroscopy(FEG-SEM).Furthermore,Pbunderpotential deposition(UPD) experimentswereconductedin order togain information on the structure of the AuNPs. The data obtained uponstructure,sizeanddensityofthedepositswerecorrelated to the cathodic transfer coefficients ˇn determined using the Koutecky–Levichmethod.Finally,theseˇnvalueswerecompared tothosepreviouslyobtainedinthesameneutralmediaonbulk materials.

2. Experimental

2.1. Chemicalsandapparatus

Allthesolutionswerepreparedusingultrapurewater(Milli-Q Millipore,18.2Mcm).HAuCl4·3H2O(proanalysisgrade)was pur-chasedfromAcrosOrganics.NaNO3(suprapurgrade)andPb(NO3)2 (analyticalgrade)wereobtainedfromMerck.95%H2SO4 (norma-purgrade)and37%HClweresuppliedbyVWRProlabo.70%HClO4 waspurchasedfromAldrich.NaClandNaHCO3(analyticalgrade) werefromFisherScientific.

Excepttheelectrodeactivationwhichwasperformedatroom temperature,all theexperimentswere performedat controlled temperatureusingaFisherScientificIsotempthermoregulator.

All the electrochemical experiments were carried out in a standardthree-electrodewater-jacketedcellusingam-AutolabII potentiostat(Metrohm) interfacedtoa personal computer con-trolledwithNOVA1.9softwarepackage(Metrohm).AMetrohm Ag/AgCl/KCl 3Melectrode, separated from the electrochemical cellbyaTeflonPTFEcapillarycontainingthesupportelectrolyte solutionand terminated bya ceramic diaphragm(Dtype), and aMetrohmglassycarbon(GC)wirewereusedasreferenceand counter electrodes, respectively. Allthepotentials given in the textand thefigures arereferred toSHE(EAg/AgCl/KCl3M=0.199V vs.SHE).WorkingelectrodeswereGC(d=3mm,A=7.069mm2₎ and Au (d=2mm, A=3.142mm2_{) rotatingdisk electrodesfrom} Radiometer. Gold nanoparticles modified GC (AuNPs-GC) elec-trodeswerepreparedusingGCplatesfromOrigaLysElectroChem SAS(d=5.5mm,A=23.758mm2_{).Thecorrespondingvalueof} geo-metricalsurfaceareaAwasusedtocalculatecurrentdensitiesfrom currents.Whennecessary,workingelectrodeswererotatedusing arotatingsystemModelEDI101interfacedwithaCTV101speed controlunitfromRadiometer.

Whenindicated,thesolutionsweredeaeratedusingaN2stream for15min.AN2atmospherewasalsomaintainedoverthesolution duringthecorrespondingexperiments.

2.2. Electrodepreparationandmodification

Alltheworkingelectrodeswerecarefullypolishedpriortouse. Allelectrodeswerefirstpolishedbysiliconcarbidegrindingpaper (grit1200)for10s.GCsurfaceswerepolishedsuccessivelybya 9mm,3mm,1mmand0.25mmdiamondsuspension(Presi)ona clothpolishingpadduring2minforeachsize.Auelectrodeswere polishedby5mm,1mmand0.3mmaluminaslurry(Presi)onacloth polishingpadduring2minforeachsize.Betweeneachpolishing step,thesurfaceswerecleanedwithMilliQwater.Finally,all elec-trodeswererinsedinanultrasonic96%ethanolbath(threetimes for10min)andcleanedwithMilliQwater.Afterdrying,the qual-ityofthepolishingstepwasverifiedbycheckingthesurfacestate usingaNikonEclipseLV150opticalmicroscope.

AuNPswereelectrodepositedat10◦_{ContoGCplatesusing} con-stantpotentialelectrolysis(CPE)atagivenpotential(from−0.3V to0.7V)forvarioustimes(between10sand1800s)froma deae-rated0.1MNaNO3 solutioncontaining0.25mMHAuCl4(pH=3). TheresultingAuNPs-GCelectrodeswerethencarefullyrinsedusing MilliQwater andactivatedina deaerated0.5MH2SO4 solution by running30 scans between 0.2V and 1.6V at a scan rateof 100mVs−1_.

2.3. AuNPscharacterization

TheAuNPs-GCsurfacewascharacterizedbyfieldemissiongun scanningelectronmicroscopy(FEG-SEM)usingQuanta250FEG FEIequipmentwithanacceleratingvoltageof5kVanda work-ingdistancebetween3and8mmdependingonthesample.Image

Fig.1.CVsrecordedonaGCelectrodeinadeaerated0.1MNaNO3solution con-taining0.25mMHAuCl4:(a)firstand(b)secondscan;(c)CVrecordedinthesame conditionsintheabsenceofHAuCl4.Scanrate:50mVs−1.

analysiswascarriedoutusingahomemadeprogramforparticles counting(densityestimation)andaveragediametermeasurement developedusingMatLabimageprocessingtoolboxsoftware.The densityandaveragesizeofAuNPswereevaluatedfroma85.3mm2 GCsurfaceanalysiscountingaminimumof40and4000particles (dependingonthechargeusedduringtheelectrodepositionstep). Foreachdeposit,theerrorwascalculatedfromtheanalysisofat leastthreedifferentSEMimagesusingtheadequatemagnification. Pb underpotential deposition (UPD) experiments were con-ductedina0.01MHClO4solutioncontaining10−3MPb(NO3)2.A potentialstepof−0.2Vwasappliedfor10sbeforestrippingfrom −0.2Vto0.6Vat50mVs−1_.

2.4. Oxygenreduction

ElectrochemicalreductionofO2onunmodifiedandmodified electrodes wasperformed at 20◦_{C in an aerated NaCl/NaHCO}

3 (0.15M/0.028M– pH7.4) solution by using linear sweep vol-tammetryanddifferentelectroderotationrates(from600rpmto 2600rpm).Theelectrodepotentialwassweptfromtheopen cir-cuitpotential(ocp)tothatcorrespondingtohydrogenevolutionat apotentialscanrateof1mVs−1_.

3. Resultsanddiscussion

3.1. AuNPselectrodeposition

Thecyclicvoltammogram(CV)ofadeaerated0.1MNaNO3 solu-tioncontaining0.25mMHAuCl4 wasrecordedonGCinorderto choosetheparametersforAuNPselectrodeposition(Fig.1).NaNO3 wasusedassupportingelectrolyteinordertoavoidcoalescence phenomena,aspreviouslyreportedintheliterature[41].Onthe firstforward scan (Fig.1, solid line),one singlereduction peak wasnoticedat 0.63Vwhich was absentwhen recordingCV in Au(III)-freeNaNO3solution(Fig.1,dottedline).Itisrelatedtothe well-knownthree-electronreductionprocessofAu(III)whichleads totheformationofmetallicAuNPsontheelectrodesurface(Eq.(4))

[42–45]:

AuCl4−+3e−→ Au+4Cl− E◦ =1.00V/SHE (4) Onthesecondforwardscan(dashedline),thispeakwasshifted 110mV tohigher potentialvalues, in accordancewith thermo-dynamicswhichpredictsaneasiergrowthofpreviouslyformed AuNPsthannucleationofnewAuNPsonGCelectrode[44–47].It

Fig.2.Lastofthe30consecutivescansrecordedbyCVina0.5MH2SO4 solu-tiononAuNPs-GCelectrodespreparedbyCPEusingthefollowingconditions:(a) −0.3V/1800s;(b)−0.3V/10s;(c)0.7V/1800s;(d)0.7V/10s.Scanrate:100mVs−1_.

hastobenotedthatthisphenomenonwasnotaccompaniedbya currentcrossoveronthefirstbackwardscan,contrarytowhathas beendescribedinmanyreportsincludingourownworks[42–45]. Thismaybeexplainedtakingintoaccountfirst,therelativelylow differenceofenergyrequiredforthereductionofAuonGCandon Au(asshownbythelittlepotentialshiftofAu(III)reductionpeak betweenfirstandsecondforwardscans,respectively)andsecond, theverycathodicinversionpotential(−0.3V)whichinducedan importantcontributionofthediffusionaccordingtoCottrell’slaw (currentproportionaltot−1/2_).

In this work, AuNPs were electrosynthesized using CPE, consideringourpreviousworkshowingthatitisthebest electrode-positionmodetogetNPswithcontrolledsizeanddensity[45].To getdifferentAuNPsdeposits,bothelectrolysispotentialand dura-tionwerevaried.FromtheanalysisofFig.1,CPEwereperformedat two“extreme”potentials,i.e.−0.3Vand0.7Vfor10sand1800s.

3.2. AuNPscharacterization

The four different AuNPs deposits were first activated and characterized by recording CV in 0.5M H2SO4 [43,48]. Fig. 2 presentsthelastof the30 consecutivescans recordedfor each deposit.Atthisstageoftheexperiment,noevolutionwasnoticed betweenoneandfollowingscan.Whatevertheelectrodeposition conditions,two phenomenawereobserved: first,theformation of Au“oxides” between 1.3 and 1.6V during the forwardscan andsecond,thereductionoftheselatteraround1.13Vduringthe backwardscan. These globalconsiderationsareconsistentwith previouslyreportedresults[44,45,48].Thechargescorresponding toAuNPs formation (QAu(III)), electrodepositedAuNPs oxidation (QAu(0))andAuoxide-likereduction(Qoxides)aresummarizedin

Table1.Fromthesevalues,itclearlyappearsthatthemostefficient depositionisCPEat−0.3Vfor1800sconsideringtheamountof depositedAu(QAu(III)=9.7mC).Moreover,foragivenCPEduration, themostefficientdepositionoccurredat−0.3V.Inotherwords,the mostefficientdepositionmoderequiresthehighestcathodic over-potentialwiththelongesttime.Thishasalreadybeenobserved by several authors [43,48,49] and is consistent with literature datawhichreportsthathighoverpotentialsfavorinstantaneous nucleation[42].Thus,CPEat−0.3Vallowstheformationofmore numerous nuclei than when performed at 0.7V whatever the electrolysisduration.Asaconsequence,theAuactivesurfacearea ismuchmoreimportantwhendepositingat−0.3V,ascanbeseen bycomparingcurves(a)and(c),and(b)and(d)fromFig.2andthe

Table1

CharacteristicofAuNPsonGCandcathodictransfercoefficientˇnfortheORRforeachelectrodepositionconditions.

Depositno. Electrodeposition parameters

QAu(III)(mC)a QAu(0)(mC)b Qoxides(mC)c Density(mm−2)d Averagediameter (nm)d ˇn 1 0.7V/10s 0.07 0.7 0.2 9±2 86±20 0.45±0.03 2 0.7V/1800s 2.2 1.1 0.7 0.5±0.1 315±89 0.29±0.03 3 −0.3V/10s 0.2 0.7 1.0 465±30 10±5 0.38±0.19 4 −0.3V/1800s 9.7 7.3 8.3 555±49 25±12 0.61±0.03 a_Q

Au(III)isthechargeconsumedduringtheelectroreductionstepbyCPEin0.1MNaNO3containing0.25mMHAuCl4. b_Q

Au(0)isthechargeconsumedinthepotentialrangefrom1.35and1.6duringtheformationofAuoxide-likeonAuNPsdepositsin0.5MH2SO4. c _Q

oxidesisthechargecorrespondingtothereductionofAuoxide-likein0.5MH2SO4. d_{SeeSection2fordetailsonNPsdensityandaveragediameterestimation.}

correspondingQoxidesvaluesfromTable1.Itisnoteworthythatthe amountsofchargeconsumedforAuoxide-likeformationandtheir reductionexhibitedcomparablevaluesforeachdepositexceptthe oneperformedat0.7Vfor10s.Inthislattercase,theQoxidesvalue wasfoundtobe3.5timeslowerthanthecorrespondingQAu(0), thussuggestinganinstabilityofthesenuclei.Itmaybeassumed thatthesenucleirearrangewhileoxidizedorevendissolvebefore theirreductiontakesplaceduringthebackwardscan.

AlltheseresultswereconfirmedbyFEG-SEManalyses(Fig.3). ThefourmicrographsshowedthatCPEat−0.3Vaffordedmuch moredensedepositsofsmallerAuNPsthanCPEat0.7V(seeTable1

forquantitativefeaturesuponsizeanddensity).Intheformercase, theAuNPswererelativelysmallandalmosthemispherical-shaped, and homogeneously distributed on the GC surface. Whatever theelectrolysistimeat−0.3V,thedepositswereverydense,in accordancewiththefactthathighcathodicoverpotentialsfavor nucleationprocesses [42]. TheAuNPs density forboth deposits wasinthesameorderof magnitude(465and555mm−2 for10 and1800s,respectively).Ithastobenoticedthatthelittle differ-enceindensitybetweenthe10sand1800sdepositsmaynotbe duetolongerreductiondurationbuttotheimpossibilityforthe countingsoftwaretodetectverysmallnuclei.Anyway, increas-ingtheelectrodeposition timeledtoslightly biggerAuNPs and stillhomogeneousdeposits.Thisisconsistentwithourwork[45]

andFinotetal.’sone[49]whoshowedtheinfluenceofthe over-potential on nucleation/growth processes. In the former work, lowcathodicoverpotentialassociatedtolongelectrolysisduration ledto“popcorn-like”aggregates,whereasinthelatteronehigh cathodicoverpotentialaffordedmorehomogeneousdepositsthan lowone[49].Onthecontrary,performingelectrodepositionat0.7V ledtoverysparsedepositsofbigAuNPs,evenforlow electroly-sistime.Increasingtheelectrolysisdurationinducedasignificant increaseinparticlesdiameter,leadingtotheformationofverybig, raspberry-likeAuNPs(Fig.3b,inset).

InordertogetmoreinformationuponAuNPsstructuration,Pb underpotentialdeposition(UPD)experimentswereperformedin HClO4[50–52].Thistechniqueconsistsinthedepositionofa mono-layerorsubmonolayerofPbonanothermetalatapotentialmore positivethanthatofbulkPbdeposition.Itisstructuresensitiveand allowscrystallographicfacesinformationtobeobtained[53].Inour case,adepositionpotentialof−0.2Vwaschosen.Thistechnique hasbeenrecentlyusedbyseveralauthorstoidentifythe crystal-lographicfacesofAuNPs.Thestrippingpeakscanbeassignedby comparisonwiththeliteraturedataforsinglecrystalsand polycrys-tallinegold[33,54–56].ThestrippingstepsforeachAuNPsdeposit aresummarized inFig.4. Theveryweakresponseobtainedfor theAuNPsdepositedbyCPEat0.7Vfor10sisconsistentwiththe

QAu(III)valuecorrespondingtoelectrodeposition(Table1)anddid

Fig.3. FEG-SEMimagesofAuNPs-GCelectrodespreparedfromadeaerated0.1MNaNO3solutioncontaining0.25mMHAuCl4byCPEat0.7Vfor(a)10sand(b)1800sand at−0.3Vfor(c)10sand(d)1800s.

Fig.4.StrippingstepofPbunderpotentialdepositionrecordedina0.01MHClO4 solutioncontaining10−3_MPb(NO

3)2onAuNPs-GCelectrodespreparedbyCPEat −0.3Vfor(a)1800sand(b)10sandat0.7Vfor(c)1800sand(d)10s.Pbdeposition conditions:−0.2Vfor10s;scanrate:50mVs−1_.

notallowanyinformationtobeextracted.TheAuNPsdeposited at−0.3Vforthesametimeexhibitedawell-definedpeakat0V typicalof(111)-orientedterraces[51,52].Althoughtheglobal elec-trochemicalresponse islittle,theobservationofthis peakis in accordancewiththelargeamountdepositionofPbonAu(111) facesreportedbyseveralauthors[50,56].TheAuNPs deposited at0.7Vfor1800sexhibitedthesamepeakat0Vassociatedtoa smallshoulderat0.02V.Theselatterwerepoorlydefineddueto thelowsizeofthesurfacedomains[55].Howeverthesplitofthe peakat0Visindicativeofdifferentsizedomains,thepeakitself beingassociatedtowideAu(111)faceswhereastheshoulderis ratherrelatedtonarrowerAu(111)domains[55].Duetoamuch greateramountofdepositedAu(seetheQ_Au(III)valuesinTable1), theAuNPsdepositedat−0.3Vfor1800sexhibitedawell-defined electrochemicalresponse,theglobalshapeofwhichbeing consis-tentwithapolyorientedAudeposit[57].Aclear,splitpeakat0V wasnoticedfortheAu(111)facestogetherwithtwobroad sig-nalscenteredat−0.15Vand0.23V,respectively.Bycomparison withliteraturedata,thissignalmaybeattributedtoAu(110)faces

[54,55].Thebroadsignalaround−0.15Vhasbeenobserved sev-eraltimesforpolyorientedAu[33,54,57]buthasneverbeenclearly attributed.Interestingly,nocharacteristicsignalofAu(100)faces around0.05V[52]wasobservedonthevoltammograms,whatever thedepositconsidered.Finally,itwasverifiedthatunmodifiedGC surfacesdidnotundergoanyelectrochemicalresponsetowardPb UPD(notshown).

3.3. KineticsstudyofORRonAuNPs-GCelectrodes

Fig.5comparesthesteady-statevoltammogramsrecordedin anaeratedNaCl–NaHCO3 (0.15M/0.028M–pH7.4)solutionon bulkAu,unmodifiedGC,andAuNPs-GCelectrodespreparedbyCPE at−0.3Vand0.7Vfor1800s.Forthesakeofclarity,the voltam-mogramsobtainedusingAuNPsdepositswithshorterelectrolysis durationwerenotprovided.Ithastobenoticedthattheselatter depositsexhibitedpoorrepeatability.Intheneutralmediumused thetwoconsecutivetwo-electronreductionstepsofO2occurred atveryclosehalf-wavepotentialsonbulkAu(Fig.5a),ca.−0.1V and−0.4VforO2andH2O2reduction,respectively.Inparticular, thesecondwaveassociatedtoH2O2 evolutionwasrelatively ill-defined,startingfrom−0.2V.Onthecontrary,onunmodifiedGC electrodeO2 and H2O2 reductionreactionswerenoticedat dis-tinctpotentials,ca.−0.6Vand−1.2V,respectively(Fig.5b).The

Fig. 5. Steady-state voltammograms recorded in an aerated NaCl–NaHCO3 (0.15M/0.028M,pH7.4)solutionon:(a)bulkAu;(b)unmodifiedGC;AuNPs-GC electrodespreparedbyCPEfor1800sat:(c)−0.3V;(d)0.7V.Scanrate:1mVs−1_.

largecathodicshiftobservedclearlyillustratedtheslower hetero-geneouselectronictransferkineticsofORRonGC.Theseresults havebeenextensivelydiscussedinourrecentwork[40]. AuNPs-GCelectrodesexhibitedsignificantlyenhancedkineticsfortheORR comparedtounmodifiedGC(Fig.5candd),inaccordancewith theresultspreviouslyreportedbyEl-Deabetal.[39].Itis note-worthythatthekineticspropertiesofthefunctionalizedelectrodes werefoundtobestronglydependentontheAuNPssizeand den-sity.TheAuNPs-GCelectrodepreparedbyCPEat0.7Vexhibited onesingle4-electronwaveforO2 reductioncenteredat−0.55V (Fig.5d)whereasontheAuNPs-GCelectrodepreparedbyCPEat −0.3Vtwobielectronicwavesat−0.1Vand−0.6Vwerenoticed, respectively(Fig.5c).Thislatterelectrodeaffordedfasterkinetics thantheformeroneandpropertiesveryclosetobulkAu(Fig.5a). ThisisconsistentwiththeobservationthatdecreasingAuNPssize inducesadecreaseincathodicoverpotential[27,38]andabetter selectivityforthetwoconsecutivebielectronicoxygenreduction waves.Indeed,comparedtobulkmaterial,thetwowavesofORR appearedinourcaseslightlybetterseparated.

To go into further detail steady-state voltammograms were recordedonbothAuNPs-GCelectrodespreparedbyCPEat−0.3V and0.7Vfor1800satvariouselectroderotationratesω(Fig.6Aand Brespectively).Fromtheocppotentialtoaroundtheearlystageof theO2reductionwavethecurrentwasfoundtobeindependent onωwhatevertheelectrodeconsidered,indicatingacharge trans-ferlimitedreactionrate.Forlowerpotentialsthecathodiccurrent increasedwhileincreasingωinaccordancewiththeLevich equa-tion(proportionaltoω1/2_{).Forbothelectrodes(Fig.6AandB)the} currentplateauoftheO2reductionwavecorrespondingtomass transferlimitationwaspoorlydefinedasanevidenceforamixed kinetic-diffusioncontrolmechanismfortheORR.Allthese obser-vationscomparefavorablywithourpreviousresultsobtainedon bulkAu[40].Theill-defineplateaumayalsobeduetoavariation ofthenumberofelectronsnconsumedbythereductionreaction whiledecreasing thepotential,assuggested byseveralauthors

[28,58].ThedataobtainedfromFig.6AandBwereanalyzedusing theKoutecky–Levichrelation[59]:

1 j = 1 jk + 1 jd =− 1 nFkC− 1 0.62nFD2/3_ω1/2

v

where jk and jd are the kinetic limited and mass trans-fer controlled current densities, respectively, F is the Faraday constant (96,500Cmol−1_{), k is the potential-dependent charge} transfer rate constant, C is the dioxygen bulk concentration (0.24×10−6_molcm−3 _{underatmosphericpressure)[60],Disthe}

Fig.6. (A)Steady-statevoltammogramsrecordedinanaeratedNaCl–NaHCO3(0.15M/0.028M,pH7.4)solutionusingdifferentelectroderotationratesωonAuNPs-GC electrodespreparedbyCPEfor1800sat−0.3V.Scanrate:1mVs−1_{.(B)Steady-statevoltammogramsrecordedinanaeratedNaCl–NaHCO}

3(0.15M/0.028M,pH7.4)solution usingdifferentelectroderotationratesωonAuNPs-GCelectrodespreparedbyCPEfor1800sat0.7V.Scanrate:1mVs−1_{.(C)Variationofthenumberofelectronsexchanged} duringtheORRasafunctionoftheelectrodepotential.ValuesextractedfromtheKoutecky–Levichtreatmentappliedtothesteady-statevoltammogramsrecordedinan aeratedNaCl–NaHCO3(0.15M/0.028M,pH7.4)solution(Fig.5)onAuNPs-GCelectrodespreparedbyCPEfor1800sat:(a)−0.3V;(b)0.7V.(D)Variationofthe kinetic-limitedcurrentdensityLnjkoftheORRasafunctionofthepotentialappliedtotheelectrode.Valuesdeducedfromthesteady-statevoltammogramsrecordedinanaerated NaCl–NaHCO3(0.15M/0.028M,pH7.4)solutiononAuNPs-GCelectrodespreparedbyCPEfor1800sat:(a)−0.3V;(b)0.7V.

diffusion coefficient (1.96×10−5_cm2_s−1_{) [61,62], and  is the} kinematic viscosityof theaqueous solution(0.01cm2_s−1_)[63]. Accordingtothislatterequationtheinverseofthecurrent den-sityj−1_{wasplottedasafunctionofω}−1/2_{fordifferentpotentialsin} therangefrom0.05Vto−1V(notshown).Allthesecurves exhib-itedalineartrend,theslopeofwhichallowedthetotalnumber ofelectronstobedeterminedforeachpotential(Fig.6C).Thetwo AuNPs-GCelectrodesclearlypresenteddifferentbehaviortoward theORR.ForthedepositpreparedbyCPEat−0.3V,thenumberof electronsconsumedreached2asearlyas−0.05V,indicatingthat theH2O2productionstepwaslimitedbymasstransferfromthis potential(Fig.6A).From−0.2Vthenumberofelectronsincreased upto4atpotentialaround−0.5Vasaconsequenceofthe comple-tionofthereductionprocessleadingtotheformationofH2O.The valuesofthenumberofelectronsslightlyhigherthan4whichwere observedfrom−0.5Vto−0.65Vmaybeassumedtoresultfrom waterreductionwhichstartsinthispotentialrange.Onthe con-trary,usingthedepositpreparedbyCPEat0.7V,theORRstartedat morecathodicpotential(ca.−0.05V)andthenumberofelectrons directlyincreasedupto4forpotentialaround−0.25V,confirming thatnointermediarytwo-electronstepoccurredonthiselectrode (Fig.6B).

FromtheinterceptsoftheKoutecky–Levichplotsthekinetic limitedcurrent densitiescontributionjkwereobtainedforboth AuNPs-GCelectrodes.ThecorrespondingplotsofLnjkasafunction ofthepotential(Fig.6D)allowedthecathodictransfercoefficient ˇntobecalculatedfromthechargetransferrateconstantk:

k=_k◦_exp





wherek◦_(cms−1_{)istheintrinsicchargetransferrateconstant,Ris} thegasconstant(8.31Jmol−1_K−1_),andE◦ ′_{istheapparentstandard}

potential.Thevaluesfoundfortheˇnwere0.61and0.29forthe AuNPsdepositspreparedbyCPEat−0.3Vand0.7Vrespectively. Consideringthenumberofelectronsexchangedineachcase(as shownin Fig.6C)theeffectiveˇvalues are0.30and 0.07.This makesevidenceforthebetterkineticsresultsobtainedfortheORR usingthemostcathodicdeposits.Theˇnvaluescalculatedforthe shortelectrolysisdurationdepositsarealsopresentedinTable1, althoughthepoorrepeatabilityofthevoltammogramsmadethem hardtoobtain.Thislackofrepeatabilitymaybeattributedto rear-rangementsoftheAuNPsduringthevoltammetricmeasurements duetopotentiodynamicperturbation,oreventodissolutionofthe smallestAuNPs[45,64,65].Thecomparisonofallfourdeposits rein-forcesthecorrelationbetweenlow-sizeAuNPsobtainedbyvery lowdepositionpotentialsandbetterkineticsresults(seeˇnvalues inTable1).Thevaluesobtainedfortheˇnalsocomparefavorably withthosewereportedinthesameneutralmediumonbulkAu andunmodifiedGCelectrodes,ca.0.39and0.24respectively[40].

FinallyamorecompletesetofAuNPsdepositswaspreparedby varyingthepotentialappliedforCPEevery0.25Vfrom−0.3Vto 0.7Vandusingelectrolysisdurationof10,60,300,600and1800s. Foreachexperimentalconditionthecorrespondingˇnvaluewas calculatedaspreviouslydescribed.Fig.7depictsagraphical repre-sentationofthesevaluesasafunctionofbothelectrolysispotential andduration.Fromthisrepresentation,a well-definedareawas noticedwhichcorrespondstothehigherˇn values.Thisareais clearlypositionedinthepartofthegraphcorrespondingtothe lowestelectrolysispotentialsand thelongestelectrolysistimes, confirmingthetrendobservedwiththefirsttwodeposits.Thus, from the kinetic point of view the best results were obtained forelectrodeswhichexhibiteddensedepositsofrelativelysmall AuNPs.Thisis inaccordancewithourpreviousworksin which weassumedthatinsuchacasethedepositmaybeviewedasan arrayofspherical-shapedAu“nanoelectrodes”whichacttogether

Fig.7.EvolutionofthecathodictransfercoefficientˇncorrespondingtotheORRin anaeratedNaCl–NaHCO3(0.15M/0.028M,pH7.4)solutiononAuNPs-GCelectrodes asafunctionofthepotential(xaxis)andduration(yaxis)usedtopreparethe depositsbyCPE.

[44,45].ThisisalsoconsistentwithWain’sworkwhoshowedthat thebestresultswithrespecttoORRwereobtainedonthesmallest AuNPs,whichexhibitedthehighestAu(110)/Au(111)ratio[26]. Inourcase,thepresenceofverysmallAuNPsorevennanoclusters

[66]whichwouldstronglycontributetotheglobalresponseofthe modifiedelectrodescannotbeexcluded[67],althoughthe magni-ficationoftheSEG-FEMmicrographswasnothighenoughtoallow theirobservation.

4. Conclusions

Inthiswork,AuNPswereelectrodepositedontoGCelectrodesby constantpotentialelectrolysis.BothAuNPssizeanddensitywere foundtobestrongly dependentonthe electrolysisparameters, thesmallestNPs(25nm)withhighestdensity(555mm−2)being obtainedwiththehighestcathodicoverpotential(E=−0.3V)and thelongestelectrolysistime(1800s).Theresponseofthedeposits towardtheORRwasexaminedinaNaCl–NaHCO3(0.15M/0.028M, pH7.4)neutralmedium.FromtheKoutecky–Levichtreatmentof thecurrent–potentialcurves,thevalueofthenumberofelectronsn

exchangedduringthereactionwasextracted.Theresultsrevealeda direct4-electronreductionpathwayforO2onAuNPs electrogener-atedbyCPEat0.7V,whereastwosuccessivebielectronicstepswere evidencedonAuNPsobtainedusinghighercathodicoverpotential. Inthislast casetheAuNPs-GCelectrodesexhibitedsignificantly enhancedkineticsforORRwithmuchhigherˇnvalues(0.61) com-paredtobulkAu(0.39)andunmodifiedGC(0.24).Thesevalues werefoundtobestronglycorrelatedtothephysico-chemical char-acteristicsoftheAuNPsdeposits.Thebestresultsareobtainedusing thedepositsexhibitingthesmallestNPsandthehighestdensity.On thebasisofthesedata,worksareinprogresswhichaimat develop-ingnewnanoparticle-basedsensorsforO2determinationinneutral media.

Acknowledgements

TheauthorsthankthePôledeRechercheetdel’Enseignement Supérieur (PRES) Toulouse and the Région Midi-Pyrénées for financialsupportandYannickHallezforhishelpinMATLAB pro-gramming.

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