Surface microstructure and its effect on electrode response

249 Pages·1992·8 MB·English

by  Freund, Michael S., 1964-

SURFACEMICROSTRUCTUREANDITSEFFECTONELECTRODERESPONSE
By
MICHAELS.FREUND
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT
OFTHEREQUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
1992
Idedicatethisworkanddissertationtomyfamilywhohavegivenmecontinuous
unconditionalsupport.
ACKNOWLEDGEMENTS
Iwouldliketosincerelythankmychairperson.Dr.AnnaBrajter-Toth,forher
encouragementandsupport. Shehasbeenatremendoussourceofknowledgeandideaswhile
atthesametimeencouragingthedevelopmentofindividualideasandsolutionstoscientific
problems. Shehasalwaysbeentheretoprovidedfocusandguidance.
Iammostappreciativetothemembersofmycommittee.Dr.J.Winefordner,Dr.V.
Young,Dr.D.Talham,andDr.C.Batich. IamindebtedtoDr.T.Cotton,Dr.E.
Hendrickson,Dr.K.Oldham,andDr.M.Wardforprovidingresearchopportunitiesand
support. IwouldalsoliketoexpressmythankstoChen-ChanHsuehforhelpwithcomputer
programming. IwishtoacknowledgefinancialsupportofthisworkthroughtheJosephW.
RichardsFellowshipoftheElectrochemicalSocietyandtheShellFellowship.
Iwouldalsoliketothankallmyfriendsandcolleagues,especiallyCeciliaBasic,
LiakataliBodalbhai,StaceyBoyette,KevinMcKenna,StephanieMyers,KevinVolkand
AllanWitkowskifortheirhelpandsupport.
TABLEOFCONTENTS
Page
ACKNOWLEDGEMENTS iii
LISTOFTABLES vii
LISTOFFIGURES viii
KEYTOSYMBOLS xiii
ABSTRACT xv
CHAPTERS
1 INTRODUCTION 1
1.1 Background 1
1.2 MassTransportatMicroelectrodeArrays 6
1.3 MediationandAdsorptionatElectrodeSurfaces 18
1.4 StructuralandChemicalPropertiesofGraphite 25
1.5 StructuralandChemicalPropertiesofTetrathiafulvalene-
Tetracyanoquinodimethane(TTF-TCNQ) 29
1.6 ScopeoftheDissertation 32
2 EXPERIMENTAL 35
2.1 Materials 35
2.2 ElectrochemicalApparatus 35
2.3 ExperimentalProcedures 36
2.3.1 ElectrodePreparation 36
2.3.1.1 Platinum 36
2.3.1.2 Roughpyrolyticgraphite 36
2.3.1.3 Glassycarbon 37
2.3.1.4 TTF-TCNQ 37
2.3.2 ElectrochemicalTechniques 46
2.3.2.1 Cyclicandlinearsweepvoltammetry 46
2.3.2.2 Rotatingdiskelectrode 47
2.3.2.3 Chronocoulometryandchronoamperometry .... 48
2.3.2.4 Decorationofelectroactiveareasby
electrochemicalcopperdeposits 50
2.3.3 OtherTechniques 50
IV
2.3.3.1 Quartzcrystalmicrobalance 50
2.3.3.2 Scanningtunnelingandatomicforce
microscopy 53
2.4 NumericalMethods 54
2.4.1 SimulationofCurrentResponseatMicroelectrodeArrays 54
2.4.2 SimulationofReversibleandQuasi-ReversibleVoltammetric
Response 62
2.4.3 NumericalSemi-integration 65
3 IDENTIFICATIONANDCONTROLOFMICROELECTRODEARRAY
STRUCTUREATELECTRODESURFACES 69
3.1 EffectsofMicroelectrodeArrayStructureonResponse 69
3.2 MicroelectrodeArrayStructureatTTF-TCNQElectrodes 73
3.2.1 ScanningElectronMicroscopyImagingofTTF-TCNQ
ElectrodeSurfaceStructures 73
3.2.2 EvidenceofMicroelectrodeArrayStructurein
ElectrochemicalResponseatTTF-TCNQElectrodes ... 74
3.2.3 DeterminationofMicroelectrodeArrayDimensions 80
3.2.4 VerificationofMicroelectrodeArrayStructurewith
ElectrochemicalCopperDeposition 91
3.3 MicroelectrodeArrayStructureatGraphiteElectrodes 94
3.3.1 EvidenceforMicroelectrodeArrayStructureatGraphite
Electrodes 95
3.3.2 ElectrochemicalCharacterizationofMicroelectrodeArray
Structure 96
3.3.3 CharacterizationofGraphiteSurfaceStructurewithScanning
TunnelingandAtomicForceMicroscopy 96
3.4 Conclusions 110
4 ROLEOFSURFACE/SUBSTRATEINTERACTIONSINTHERESPONSEOF
ELECTRODES 112
4.1 Theory 112
4.1.1 MultipleStepElectronTransferReactions 113
4.1.2 MediatedElectronTransferReactions 116
4.1.3 AdsorptionatElectrodeSurfaces 119
4.2 SurfaceChemistryofTTF-TCNQElectrodes 120
4.2.1 StabilityofTTF-TCNQElectrodesUnderElectrochemical
Conditions 122
4.2.2 MechanismofElectronTransferatTTF-TCNQElectrodes .... 129
4.3 CharacterizationofWeakSurface/SubstrateInteractionsWhichResult
inIncreasedSurfaceConcentrations 145
4.3.1 IdentificationandQuantitationofWeakAdsorptionwithSemi-
integralAnalysis 147
4.3.2 WeakAdsorptionofp-BenzoquinoneonGraphiteElectrodes ... 155
4.3.3 Thin-layerModelofWeakAdsorption 158
V
4.3.4 Semi-integralAnalysisinthePresenceofThin-layerBehavior
andDiffusion 167
4.4 Conclusions 173
5 SUMMARYANDFUTUREWORK 175
APPENDICES
A ACQUISITION,MANIPULATION,ANDSEMI-INTEGRAL
ANALYSISPROGRAMSOURCECODE 181
B CHRONOAMPEROMETRICSIMULATIONANDOPTIMIZATION
PROGRAMSOURCECODE 213
C CYCLICVOLTAMMETRICSIMULATIONANDTHIN-LAYER
MODELPROGRAMSOURCECODE 218
REFERENCES 223
BIOGRAPHICALSKETCH 231
VI
LISTOFTABLES
Table Page
Table3.1 ElectrochemicalresponseofFe(CN)e^'^atTTF-TCNQElectrodes 79
Table3.2 Calculatedvaluesofdiffusionlayerthicknessanddimensionlesstime
for10and119fimmicroelectrodes 85
Table4.1 Predictedtransfercoefficientsforaconsecutivefourstep(ECEC)
electrodereactionwithdifferentrate-determiningsteps 117
Table4.2 Chargeandcalculatedsurfaceexcessofbothstrongandweaklyadsorbed
p-benzoquinoneonRPG;n=2andA=0.047+0.005cm^ 163
vii
LISTOFFIGURES
Figure Page
Figure1.1 Freeenergyversusreactioncoordinateforasingle-electronreduction
ofOxtoRedatpotentialE. Thepathwayincludesaprecursorstate,
P,atransitionstate,A,andasuccessorstate,S. AG*isactivation
freeenergyforoverallreduction,AG’isactivationfreeenergyforthe
elementaryelectrontransferstep,AG"isthefreeenergydrivingforce
fortheoverallreduction,Wpistheworkassociatedwiththeformation
ofaprecursorstatefrombulkreactant(Ox)priortoelectrontransfer,
w,istheworkassociatedwiththeformationofbulkproduct(Red)
fromthesuccessorstatefollowingelectrontransfer 3
Figure1.2 Schematicofadiffusionlayeratapolarizeddiskelectrodewithradius
r. (a)Growthofadiffusionlayerasafunctionoftime(t,<t2<t3<tj
wherelinesrepresentacrosssectionoftheinterfacebetweenthe
diffusionlayerandthebulksolution;and(b)directionofdiffusion,
representedbyarrows,asafunctionoftime,wherediffusion
perpendiculartotheelectrodesurfaceisapproximatedbysemi-infinite
lineardiffusionanddiffusionextendingoutradiallyisapproximated
byradialdiffusion 9
Figure1.3 Schematicofanelectrodearraywheredotsrepresentareasof
electroactivityseparatedbyinactiveregions:(a)distributionof
electrodesacrossinertsurface;(b)hexagonalarraymodel;(c)
hexagonalarraymodelunitcellwhichcanbeapproximatedbya
circularunitcellofequalcrosssectionalarea. Dimensionsofcircular
unitcellaregivenbyr(radiusofactivearea)andR„(radiusof
geometricareaincludingactivearea) 14
Figure1.4 Schematicofdiffusionlayergrowthat(a)apolarizedarrayofdisk
electrodes;and(b)atasingleunitcellofthearray,whererandR„
aretheradiioftheactiveandtotalgeometricareasoftheunitcell,
respectively 16
Figure1.5 Freeenergyversusreactioncoordinateforasingleelectronreduction
ofOxtoRedatpotentialE. Threedifferentpathwaysareshown
wherePASrepresentsapathwaywithenergeticallyfavorable
precursorandsuccessorstates;P’A’S’representsapathwaywherethe
precursorandsuccessorstatesarefurtherstabilizedviasurface
viii
interactionsalthoughthetransitionstateremainsunaffected(weak
overlap);andP’A”S’representsasimilarpathwaywiththeexception
ofthetransitionstatewhichisaffectedbystrongsurfaceinteractions
(strongoverlap). DefinitionsofAG*,AG*,andAG“arethesameas
inFigure1.1 20
Figure1.6 Schematicfor(a)homogeneousmediatedelectrocatalysis;(b)
heterogeneousmediatedelectrocatalysis;and(c)unmediated
heterogeneouselectrocatalysis. A/Brepresentstheredoxactive
mediator 22
Figure1.7 Schematicofmediated(a)reduction(ofOxtoRed)and(b)oxidation
(ofRedtoOx)wheretheA/Bredoxcoupleactsasamediatorwitha
formalpotential(E®)negativeoftheOx/Redcouplefora
reductionorpositiveforanoxidation 24
Figure1.8 Structuresof(a)singlecrystalgraphite,wheretheinterplanardistance
{CJT)is3.354A,and(b)glassycarbonwheretheribbondiameter
andheightisca.20and10A,respectively 28
Figure1.9 (a)Molecularstructuresoftetrathiafulvalene(TTF)and
tetracyanoquinodimethane(TCNQ);and(b)thecrystalstructureTTF-
TCNQmolecularsolidwhichhastheformoffacetofacesegregated
stacks 31
Figure2.1 Scanningelectronmicrographsof(a)unpolished13:1TTF-
TCNQ/PVCpolymerpasteelectrodesurface,200X;(b)polished13:1
TTF-TCNQ/PVCpolymerpasteelectrodesurface,200X. Bar
represents100/im 40
Figure2.1 (continued)(c)highpressurepressedpelletsurface5400X. Bar
representsl/xm 42
Figure2.2 ScaimingelectronmicrographsofaTTF-TCNQ/PVCspincoatedfilm
a(af)te3r0c0opXpe(rBadrepo=sit1i0o0n/axtm)-;0.(1b)V1v0e0r0suXs(SBCaEr=in5100;mtmM)CuS04-5H20: 45
Figure2.3 Schematicof(a)quartzcrystalmicrobalanceandelectrochemical
apparatus;and(b)goldelectrodepatternsdepositedonATcutquartz
crystal 52
Figure2.4 Chronoamperometricsimulationandoptimizationprogramalgorithm.
Experimentallydeterminedelectrodegeometricarea.A,n,D,C*,and
chronoamperometricresponse,i(t),areacceptedasinput 57
Figure2.5 (a)Microelectrodearraymodelshowingunevendistributionof
microelectrodes(dotsindicateactiveareas);(b)aggregatesofsmaller
IX
microelectrodes;(c)approximationofmicroelectrodeaggregatesbya
singlelargermicroelectrode 60
Figure2.6 Cyclicvoltammetricsimulationforthin-layermodelprogram
algorithm. Equilibriumconstantsacceptedasinput,K^,o*andK^.Red,
describeconcentrationequilibriumofOxandRedbetweenthefirst
andsecondvolumeelementadjacenttotheelectrodesurfaceinafinite
elementrepresentationofdiffusion 65
Figure2.7 Numericalsemi-integrationprogramalgorithm. Experimental
voltammetricdata,i(E),andscanrateareacceptedasinput. Step(2)
performsthenumericalsemi-integrationbasedonequations2.24and
2.25 67
Figure3.1 Zonediagramofcyclicvoltammetriccharacteristics(shownasinlays)
asafunctionof andA(l-0),andasafunctionofexperimental
parameters(unitcellradius,R„,standardheterogeneousrateconstant,
k",fractionofactivearea, 1-6,andscanrate,v)fordiskelectrode
array 72
Figure3.2 EmxMperFiem(eCnNt)asl^'(^in1).0anMdcKaCllculaatthedig(hO-)prceyscsluircevpoelllteatr(mAn,ogCr)amasndof153:1
unpolishedpolymerpaste(B,D)TTF-TCNQelectrodes. Scanrates
were100mV-s’(C,D)and5000mV-s‘(A,B) 77
Figure3.3 (a)Plotof(Ee;^)'versusR„forunpolished13:1TTF-TCNQ/PVC
polymerpasteelectrodeafterbackgroundsubtraction,whereCAwas
performedin2mMFe(CN)e^'^/lMKClandpotentialwasstepped
from-1-0.400to0.000VversusSCEwithapulsewidthof1000ms.
(b)Experimental(O)andsimulated(solidanddashedlines)
chronoamperometricresponsebasedonmicroelectrodedimensions
determinedfromtheleast-squaresoptimizationprogram;firstiteration
r=10,R„=12nm( );seconditerationr=119,R„= 121fim
(---) 82
Figure3.4 Plotoflogip_eversuslogvfor2mMFe(CN)e^'VlMKClatan
unpolishedTTT-TCNQ/PVCelectrode. Slopeofthesolidlineis0.5.... 88
Figure3.5 PlotofQversust*'^for2mMFe(CN)ft^'^/lMKClatanunpolished
TTF-TCNQ/PVCelectrode. Potentialstep-t-0.400to0.000V
versusSCE 90
Figure3.6 ScanningelectronmicrographsofTTF-TCNQ/PVCelectrodesafter
copperdepositionat-0.155VversusSCEin50mMCUSO4, lOOOX
magnification, (a)unpolished;(b)polished 93
Figure3.7 Plotof(Eej^'versusR„for( )untreatedGCand( )ECGC
electrodeafterbackgroundsubtraction. Experimentalconditionsare
thesameasthoseinFigure3.3 98
X