金纳米粒子的电化学制备和AFM表征

电　子　显　微　学　报

JournalofChineseElectronMicroscopySociety

Vol220,No152001210

文章编号:100026281(2001)0520599204

ElectrochemicalpreparationandAFMcharacterization3

ofgoldnanoparticlesonHOPGTANGJing,WUJian2ming,XIEZhao2xiong,MAOBing2wei(StateKeyLab.forPhys.Chem.ofSolidSurfaces,Dept.ofChem,XiamenUniv,Xiamen361005,China.)Abstract:WepresentastudyontheelectrochemicalpreparationofAunanoparticlesonHOPGfromHAuCl4solutioncharacterizedbytappingmodeAFM.IthasbeenshownthatdepositiononactivatedHOPGandnon2activatedHOPGfollowsprogressiveandinstantaneousnucleationandgrowthmode,respectively.Thesizeandmorphologyofthenanoparticlesdependsonthedepositioncondition.Aunanoparticlesofnarrowsizedistributionof

～50nmcanbepreparedwhichhasatendencytoarrangeintoatwo2dimensionalorderonhomogeneousHOPG

(non2activated).TheelectrocatalyticpropertiesandSERSpropertiesoftheAunanoparticlesarebrieflydiscussed.Keywords:AFM;electrodeposition;nanoparticle;Au

中图分类号:TB383;O621125;TN16　　文献标识码:A

Nanosizedmetalparticles(andthinfilm)showsignificantlydifferentphysicalandchemicalpropertiesfromthoseoftheirbulk

[1]

andgreateffortshavebeendevotedtopreparationandcharacterizationofmetal

nanoparticles.Forexample,massiveAuismuchlessreactivethannoblemetalsuchasPt,RhandPdinhomogeneouscatalysisandelectrocatalysis.However,enhancedreactivityforCOoxidationfromasupportedAunanoparticlesonTiO2

[2]

hasbeenobservedduetotheelectronicandstructuraleffectofthecomposite.

[3]

Duetoitsuniqueproperties,goldnanoparticlesarealsothecandidateforsingle2electrontunnelingexperiment.Tofacilitateinvestigationandapplicationoftheproperties,preparationofnanoparticlesonsurfacebecomesoneofthefocusesofthefield.Asthepreparationmethodbecomesincreasinglysophisticated

[4,5]

,electrodepositionremainsasaneasy,efficientandcontrollablewayfortheformationof

[6]

metalnanoparticlesonelectrodesurfaces.Withelectrochemicalcontrol,propercompositionofelectrolytesaswellassuitablesurfacepretreatment,nucleationandgrowthbehaviorforthenanoparticleformationmaybeadjusted.

Inthispaper,wereportelectrochemicalpreparationandAFMcharacterizationofgoldnanoparticlesonhighlyorientedpyrolyticgraphite(HOPG)ofdifferentsurfaceconditions.Resultsarepresentedtoshowtheinfluenceofelectrodepotential,depositiontimeandsurfacepretreatment(activatedandnon2activated).ThesurfacemorphologyofthedepositsisinvestigatedbytappingmodeAFMthatisusefulinthestudyofweaklyattacheddepositonsubstratesurfaces.Theelectrocatalyticpropertiesandopticalpropertieswillalsobediscussed.

Experimental

Electrochemicalexperimentswerecarriedoutinaconventionalthree2electrodecell.Asaturatedcalomelelectrode(SCE)andaplatinumwirewereusedasthereferenceelectrodeandthecounterelectrode,respectively.TheworkingelectrodewasafreshlycleavedHOPG.Dependingondemand,the

　　基金项目:自然科学基金资助项目(No.29833060)

　　ThisprojectisfinancedbyNationalNaturalScienceFoundationofChina(No.29833060)

© 1994-2007 China Academic Journal Electronic Publishing House. All rights reserved. http://www.cnki.net

　6　00电子显微学报　J.Chin.Electr.Microsc.Soc.第20卷

HOPGmaybeelectrochemicallyactivatedpriortoelectrodepositioninphosphorousbyanodicoxidationina

[7]

011molΠLphosphatebuffer(pH=710)solutions.Electrodepositionwascarriedoutinafreshlyprepared1mmolΠLHAuCl414H2Oand011molΠLHClO4solutionfollowingapresetpotentialandtimecontrolscheme.Electrocatalyticactivitiesofthenanoparticlesweremeasuredinasolutioncontaining012molΠLethyleneglycoland011molΠLNaOH.Allreagentswereofanalyticalgradeandsolutionsweremadeofmilliporewater.TappingmodeAFMcharacterizationofthedepositwasperformedex2situonaDINanoscopeIIIa.

Resultsanddiscussion

Thedepositionstate

[8]

initialstagemetaldependsbehaviorstronglyonthesubstratesurface.Defectsofthesurfacesuchassteps,kinksoftenserveasthenucleationsites.Therefore,westartedwiththeactivatedHOPGsurfacefromwhichdenseactivesitesfornucleationareexpected.AFMcharacterizationshowsthatHOPGafterexperiencing60min

oxidationat112Vin011molΠLphosphoricacidisatomicallyroughbutmaintainsitssmoothnesswithinthescaleofabout10nminareasonablylargearea,whichprovidesasuitablesurfaceforelectrodepositionandsubsequentcharacterizationbyAFM.Fig.1(a)istheAFMimagesshowingtheelectrodepositedAunanoparticlesontheactivatedHOPGsurfacefollowingapotentialstepsequence.Notethattheopencircuitpotentialofthesystemisaround+019Vandthereisnoparticularreasonforthefirstnegativepotentialstepto0Vinthissetofexperiments.Whilesteppingbackfrom-013Vto0Vallowsfurthergrowthofthenucleiinaslowerspeed.Denselydistributedbutisolatednanoparticlesof70～80nmwereobservedpresumablyonthedefectsitesoftheHOPGsurface.ThetimeinfluenceonthenucleationandgrowthisshownbyFig.1(b).Sizesofthelargeandthesmallislandareabout50nmand400nm,respectively.FromthesizedistributionshowninFig.1(a)and(b),aprogressivenucleationandgrowthmodeat-013Vissuggested.ParticleaggregationinFig.1(b)islikelyduetotheoverlapofparticlesafterprolongedgrowthat0V.

FormationofmoreuniformAunanoparticlesisdesirableonnon2activatedHOPG.Instantaneousnucleationandgrowthmodeisexpectedonthealmostdefect2freesurfaceofHOPG.AsisshowninFig.2(a)and(b),onlypartoftheHOPGsurfacewerecoveredwithnanoparticlesof～75nm.Itshouldbenotedthatsteppingintoeither+013Vor-013Valreadyfallintothediffusion2controlledregimeofpotentialwherethegrowthrateofthenucleimaintainsthesame.Thus,thedepositionpotentialinfluencesmainlythedensityoftheparticles.TheformednanoparticleshaveasimilarnarrowsizedistributionforbothFig.2(a)and(b)comparedwiththoseonactivatedHOPGsurfacesshowninFig.1(a)and(b).SomebrighterorbiggerparticleswereseenontopofthedenselypackedfirstlayeroftheAunanoparticles.Thiswasbecause

© 1994-2007 China Academic Journal Electronic Publishing House. All rights reserved. http://www.cnki.net

Fig.1　AFMimagesofelectrodepositedgoldnanoparticlesonactivatedHOPG

surfacepotentialchangefollowsthesequenceof(a)0V10s,-013V60s,0V～30sand(b)0V10s,-013V10s,0V～100s.Scansize

μμ3m×3m.

第5期TANGJingetal.:ElectrochemicalpreparationandAFMcharacterizationofgoldnanoparticlesonHOPG601　　

asthetimeincreasedthepartiallycoveredsurfaceprovidedanewstartingpointforfurthernucleationofAuontheasformedAunanoparticles.Ineithercase,therewasatendencyfortheparticlestoarrangeintoorder.Interestingly,particlescanbearrangedinalonglineonsomepartsofthesurfaceasshowninFig.2(b).Thereevidences

areandexperimentaltheoretical

[9]

Fig.2　AFMimagesofelectrodepositedgoldnanoparticlesonnon2activatedHOPG

surfaceat(a)013Vfor10sand(b)-013Vfor60s.

μμScansize3m×3m.

calculationsthatsmallaswellaslargemetalclusterscanrearrangeonthesurfacethroughdiffusionofmetalatomsattheperipheralsoftheclusters.Forthepresentwork,wehavenodefiniteexplanationforthemorphologyofthenanoparticlesonsurfaceyet,butwespeculatethatthedrivingforceforthistendencyislikelyduetotheweakadhesionenergyofthegolddepositandfastdiffusionofthegoldatomswithintheparticlesonhomogeneousHOPGsurface.Arviaandcoworkersreportedinarecentpaperthattheshapeofthegoldislandsweredeterminedbytheanisotropicdiffusion,whichisofthree2dimensionalcentralcoreandlargequasitwo2dimensionalbranching.Thisisobviouslynotthepresentcase.Diffusioninthepresentworkisisotropicandasaresult,roundshapednanoparticlesareformed.Thediffusionismorefavorableatmorepositivepotentialbecauseofthehighertendencyforatomstomoveforthandbackandthushigherdegreeoftwo2dimensionalaggregationat013V,Fig.2(a).Itis,however,hardtoexplainwhyAunanoparticlespersisttheirshapeevenwhentheyareveryclosetoeachother.Itseemstheactivationenergyfortheinterdiffusionofatomsfromdifferentnanoparticlesareveryhigh.ItshouldbenotedthatadhesionoftheAudepositissoweakthatitisonlypossibletocharacterizethenanoparticlesusingtappingmodeAFMwhichloosesthechanceofin2situmonitoring.

Amongthefoursurfacemorphologies,onlytheoneshowninFig.1(b)showssurfaceenhancedRamansignal(SERS).Themainfeatureofthemorphologyisthecoexistenceoflarge(400nm)andsmall(50nm)Auislandsthatareincontactwitheachothersomehow.ItisexpectedthatsuchamorphologywouldgeneratestrongelectromagneticcouplingandthussurfaceenhancedRamansignal.

[10]

[6]

TostudytheelectrocatalyticbehaviorofAunanoparticlesonHOPG,cyclicvoltammmetricmeasurementswereperformedusingtheelectrooxidationofethyleneglycolastheprobe.EthyleneglycolisapotentialcandidateforfuelcellofcommoninterestwhoseelectrooxidationbehavioronAu(hkl)hasbeenstudied

[11]

.Inthesolutionof012molΠL(CH2OH)2+011molΠLNaOH,theonsetoftheethyleneglycol

electro2oxidationwasfoundtobeat0127VonamassiveAuelectrode,Fig.3(solidline1).WhereasontheAunanoparticlemodifiedHOPGelectrode(Fig.1(b)),thereisabout50mVloweringoftheoxidationpotentialandmorethan20timesenhancementinelectrocatalyticcurrentdensity,Fig.2(dottedline2).TheenhancedelectrocatalyticpropertyoftheAunanopoarticlesisbelievedtoberelatedtothesurfaceeffectandsmallsizeeffectofthenano2sizedgoldparticle

[12]

.

© 1994-2007 China Academic Journal Electronic Publishing House. All rights reserved. http://www.cnki.net

　6　02电子显微学报　J.Chin.Electr.Microsc.Soc.第20卷

Conclusion

We

have

shownthatAu

nanoparticlesonHOPGcanbepreparedbyelectrodeposition.Thesize

and

morphology

of

the

nanoparticlesonHOPGstronglydependsontheoriginalHOPGsurface

condition

and

the

Fig.3　CyclicvoltammogramofmassiveAuelectrode(solidline1)andgold

nanoparticleselectrodepositedonnon2activatedHOPGelectrode(dottedline2)in012molΠLethyleneglycoland011molΠLNaOHsolution.Thecurrentdensityisconversedwithrespecttothegeometricareaoftheelectrodesurface.Sweeprate:50mVΠs.

depositioncondition.FastdiffusiononthehomogeneousHOPGsurfaceprovides

atendencyfortheinto

nanoparticlesto

rearrange

order,yetthehighactivationenergyfortheinterdiffusionof

atomsbetweendifferentnanoparticlespreventthenanoparticlefrombeingcoalesced.Thepresentstudydemonstratesthatpreparationofmetalnanoparticlesofdifferentsizeandmorphologyonsurfaceisdesirablebyelectrodeposition.Systematicstudiesareunderprogress.References

[1]RaoCNR,KulkarniGU,GovindarajA,SatishkumarBC,ThomasPJ.PureApplChem,2000,72:21.[2]ValdenM,LaiX,GoodmanDW.Science,1998,281:1647.

[3]ClausP,BrucknerA,MohrC,HofmeisterH.JAmChemSoc,2000,122:11430.[4]MartinJE,WilcoxonJP,OdinekJ,ProvencioP.JPhysChemB,2000,104:9475.[5]ZhengJW,ZhuZH,ChenHF,LiuZF.Langmuir,2000,16:4409.

[6]MartinH,CarroP,CreusAH,ConzalezS,AndreasenG,SalvarezzaRC,ArviaAJ.Langmuir,2000,16:2915.[7]ZhangJD,WangEK.JElectroanalChem,1995,399:83.

[8]周绍民.金属电沉积2原理与研究方法.上海:上海科学技术出版社,1987.P197,[9]WenJM,ChangSL,BurnettJW,EvansJW,ThielPA.PhysRevLett,1994,73:2691.

[10]KottmannJP,MartinOJF.In:TianZQ,RenBeds.Progressinsurfaceramanspectroscopy———Theory,techniques

andapplications.Xiamen:XiamenUniversityPress,2000.p45.[11]AdzicRR,Avramov2IvicM.JCata,1986,101:532.[12]MukerjeeM.JApplElectrochem,1990,20:537.

金纳米粒子的电化学制备和AFM表征

汤　儆,吴剑鸣,谢兆雄,毛秉伟

3

(固体表面物理化学国家重点实验室,厦门大学化学系,厦门361005)

摘　要:本文以高序石墨为基底,氯金酸溶液为支持电解质,以对表面破坏力小的轻敲模式原子力显微镜为观察手段,研究基底的表面状态及电沉积条件对形成的纳米颗粒尺寸和形态的影响,并简要讨论所形成的金纳米粒子的表面Raman增强效应和对乙二醇氧化的电催化效应。关键词:AFM;电沉积;纳米粒子;Au

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