A nanoporous gold-polypyrrole hybrid nanomaterial for actuation Sensors and Actuators B: Chemical

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Sensors and Actuators B: Chemical

j ourn a l h o m e pa g e :w w w . e l s e v i e r . c o m / l o c a t e / s n b

Research Paper

A nanoporous gold-polypyrrole hybrid nanomaterial for actuation

Ke Wang

^a,∗

, Charlotte Stenner

^a

, Jörg Weissmüller

^a,b

aInstituteofMaterialsPhysicsandTechnology,HamburgUniversityofTechnology,Hamburg,Germany

bInstituteofMaterialsResearch,MaterialsMechanics,Helmholtz-ZentrumGeesthacht,Geesthacht,Germany

a r t i c l e i n f o

Articlehistory:

Received26October2016

Receivedinrevisedform17March2017 Accepted5April2017

Availableonline6April2017

Keywords:

Actuation Hybrid Nanoporousgold Polypyrrole

a b s t r a c t

Wediscussactuationwithahybridnanomaterialthatismadebyelectro-polymerizingpyrroleonthe internalsurfacesofdealloying-derivednanoporousgoldandthenlettingaqueouselectrolytebeimbibed intheremainingporespace.Inthisway,activepolypyrroleﬁlmsarecontactedbytwoseparatebut individuallycontiguousconductionpaths,providingefﬁcienttransportofionsintheelectrolytechannels andofelectronsinthemetalskeleton.Themetalskeletonalsoservestoenhancethemechanicalbehavior oftheactuator.Actuationexploitsthedimensionchangesofthepolymerwhenionsareexchangedwith theelectrolyteinapseudo-capacitiveway,atpotentialsnegativeoftheclassicoxidation/reductionof polypyrrole.Ourexperimentswithmillimeter-sizebulksamplesindicatefastswitchingandsubstantially largerstrainamplitudethannanoporousmetalactuators.

1. Introduction

Among materials for actuation, low-voltage electrochemical systemsutilizingconductingpolymersreceiveconsiderableatten- tion[1,2].Thelowvoltageissafe,compatiblewithbatterysources, andpowerinputispotentially low[3].Conductingpolymersin contactwithelectrolytecanbeoxidizedorreducedbyanodicor cathodiccurrent.Ionsandsolventareexchangedduringthesereac- tion,sothatcharge andosmoticpressureremainbalanced.The compositionvariationisnotonlycoupledtochargestorage,but alsotoavolumechange[4,5].Thus,conductingpolymeractuators exploitthecouplingbetweentheelectricchargeandmechanical strainduringelectrochemicalprocessesthatrequirelowapplied potentials[2,4,6,7].Asadrawback,ratesofactuationtendtobe low becauseoftherelatively slowtransport of ionswithinthe polymer[6,8].Thinﬁlmgeometries,forinstancein“ionic poly- mermetalcomposites”improvethekinetics,yetattheexpense oflargecompliance[6].Furthermore,theperformanceofconduct- ingpolymeractuatorscanberestrictedbycreep[3,9]andbythe lowstressthatissustainedduringactuation[3,10–12].Here,we explorepolypyrrole(PPy),awellinvestigatedelectroactivepoly- mersystem[13].Exploitingastrategythathasbeendemonstrated forapplicationas supercapacitors[14,15],we structurethePPy sothatitcontainsseparateconductionchannelsforionsaswell electrons,interspersedatsubmicronscale.Atthesametime,we

∗Correspondingauthor.

E-mailaddress:k.wang@tu-harburg.de(K.Wang).

incorporateamechanicallyrobustmetallicskeletonphasewhich enhancesstrengthandstiffness.

Nanoporousmetalsmadebydealloying[16–18]taketheformof monolithic,millimetersizedbodiesthatconsistofahomogenously interconnectednetworkofnanoscale‘ligaments’andporechannels withacharacteristicsizethatcanbecontrolleddowntowellbelow 10nm[19–21].Nanoporousgold(NPG)providesamodelsystem fordealloying-mademetalnanostructuresbecauseofthemate- rial’s particularlyreproducible synthesis and itshighly uniform microstructure.Duetosize-effects,theligamentsthatmakeupthe metalskeletonarelocallyverystrong.Thus,eventhoughNPGis highlyporous,signiﬁcantvaluesofthestiffnessandoftheeffec- tivemacroscopicyieldstrengthhavebeenreported[22–26].The mechanicalperformanceisgreatlyenhancedwhentheporespace isﬁlledwithpolymer,suchasepoxyresin[27,28].Thisobserva- tionsuggeststhatNPGdecoratedwithPPyastheactivecomponent mightprovideahybridnanomaterialthatcompromisesbetween thelargeactuationamplitudeofPPyandthegoodmechanicalprop- ertiesofmetalnanomaterials.

Actuationiswelldocumentedfornanoporousmetalswithno PPycomponent[29–33].Whentheporespaceisﬁlledwithaque- ouselectrolyte,thematerialsactuateunderelectrochemicalcontrol of theirsurface stress[34,35]. Thishas been demonstrated for nanoporousgold[20,36],platinum[29,37],silver[32],nickel[33], palladium[38]andnanoporousAu-Ptalloy[21].Thematerialsare distinguishedbytheirgood electronicconductivityandbytheir strengthandstiffness.On theotherhand,strainamplitudesare considerablylessthaninpolymeractuatorsandmightbeneﬁtfrom enhancement.

http://dx.doi.org/10.1016/j.snb.2017.04.025

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Here, we explorethe NPG-polypyrrolesystemfor actuation, witha speciﬁcfocusonmakingnotthinﬁlmsbut macroscopic, monolithic actuatormaterials.We showthat byfunctionalizing thegold-electrolyte interfacewiththinPPylayerstheactuation amplitudeofthematerialcanbestronglyenhancedcomparedto actuationwithpureNPG.Atthesametime,switchingresponse timesarefoundsubstantially fasterthaninbulkactuatorsfrom conductivepolymerswithouttheinterpenetratingmetalskeleton.

2. Materialandmethods

CylindricallyshapedNPGsampleswithdiameter0.9mmand lengthof1.5mmwerepreparedbyfollowingtheproceduresin Ref.[28].TheAu₂₅Ag₇₅masteralloywaspreparedbyarc-melting, homogenizedbyannealingfor100hat850^◦C,shapedtoacylinder bywiredrawingandﬁnallyannealedinvacuumduring3hat650^◦C forrecovery.

Themasteralloysamplesweredealloyedin1MHClO₄prepared fromHClO4(SuprapurR,Merck)andultrapurewater(18.2Mcm) atdealloyingpotential0.75Vandambienttemperature.Whenthe currentfelltobelow10␮A,thepotentialwassteppedto0.85Vand heldfor3h,whichcompletedthedealloying.Inordertoremove surfaceandsubsurfaceoxides,thesampleswerethensubjectedto 40potentialcyclesintheinterval0–1.0Vatscanrateof10mVs⁻¹ [20].Immersioninultrapurewateranddryinginvacuumfor3days followed.Allsampleswerethenannealedat300^◦Cinairfor1hto increasetheligamentsize.Energydispersivex-rayspectroscopyin thescanningelectronmicroscope(SEM)suggeststhattheresult- ingnanoporousmaterialisessentiallypureAu,witharesidualAg contentof≤2at.-%[27].

Withtheexceptionoftheelectropolymerization(seebelow),all electrochemicalexperimentsinthisworkusedAg/AgCl pseudo- reference electrodes,and all respectivepotentials are speciﬁed versusthis electrode. The referencepotential wasmeasuredas +0.202Vvs.versusanAg/AgClelectrodeinsaturatedKClsolution (WorldPrecisionInstruments,Inc.,whichisitself+200mVvs.the standardhydrogenelectrode,SHE).Thus,ourpotentialsareshifted by+0.402VversusSHE.

Bulksamplesofnanoporousgold-polypyrrole(NPG-PPy)hybrid materialweremadebyelectrochemicalpolymerizationofpyrrole in NPG.Thecommercialpyrrolemonomer (Sigma-Aldrich)was puriﬁedbypassingthroughacolumnofbasicaluminaandstored inadarkbottleat4^◦C.CommercialanhydrousLiClO₄(Merck)and HPLCgradeacetonitrile(Sigma-Aldrich)wasusedasreceived.The electrolytesolutionwas0.1Mpyrroleand0.1MLiClO4inacetoni- trilecontaining2%water.Inathree-electrodesetupaPtwireserved ascounterelectrodeandasaturatedcalomelelectrode(SCE,World PrecisionInstruments)asreferenceelectrode.BulkNPGsamples wereusedastheworkingelectrodeandﬁrstlyimmersedintothe electrolytesolutionfor½h.ThepolymerizationofpyrroletoPPy usedconsecutivesquarewavesofpotentialbetween−500mV(2s) and+800mV(8s)for450,900and2200cycles,whichneededin totalaround1.25, 2.5and6h,respectively.Thehybridmaterial werethenrinsedthoroughlywithultrapurewater.

composition.CompressiontestsusedaZwick1484testingmachine withtheelongationmeasuredbetweentheloadsurfaces.Afeed- back loop controlled and progressively reduced the crosshead speedsoastomaintainconstanttruestrainrateat10⁻⁴s⁻¹while thesamplelengthvaried.

3. Results 3.1. Microstructure

SEMimagespriortopolymerdecoration(Fig.2A)illustratethe bicontinuousstructureofNPGandtheligamentsizeinoursamples, L=250±50nm.Inthisﬁrststudyweselectedthecomparatively largeLinordertoassuresufﬁcienttransportchannelcross-section foreasymonomertransportduringpolymerizationandforfastion transportduringactuation.

The success of electrochemical polymerization of pyrrole is demonstratedinFig.2(B),whichshowsthefracturecross-section SEMmicrographofNPG-PPyhybridmaterial.Aconformalcoating oftheligamentswithPPy,80nmthick,isapparent.Theelectropoly- merizationexploitsthegoodadsorptionofpyrrolemonomerson goldsurfacesandthehighelectricalconductivityofNPGelectrodes [14].ThehomogeneousPPylayerthickness,throughoutthebulk nanoporoussample,suggeststhatattachmentkineticsmaylimit thegrowthrate.Thethicknessof PPylayerfor NPG-PPyhybrid nanomaterialcanbetunedbyadjustingthetime,t_P,ofthepoten- tiodynamicpolymerization.HybridnanomaterialwithPPylayersof thickness15±5nm(t_P=1.25h),50±10nm(2.5h)and80±20nm (6h)areshowninFig.2(C)–(E),respectively.

3.2. Electrochemicalcharacterization

Fig.3summarizeselectrochemicalpropertiesofpureNPGand NPG-PPycompositecharacterizedbycyclicvoltammogramsin1M HClO4.Thedashedgraphsinpart(A)oftheﬁgurerefertopureNPG;

notethedistinct oxygenadsorptionand desorption peakswith theironsetsataround0.8V.Theoxygen-on-goldelectrosorption featuresaremissingintheCVsoftheNPG-PPyhybridnanomaterial (solidlines).Thisshowsthatthegoldsurfaceisindeedcompletely coatedbyPPy.TheprevailingfeatureoftheCVsonthehybridnano- materialsisanapparentadsorptionpeakwithanonsetat0.91V, andacorrespondingdesorptionpeakthatisshiftedsubstantiallyto negativepotentials,withanonsetat0.75V.Thisbehaviorissimilar toearlierreportsonvoltammetryforPPy[41].Ashiftoftheelec- trosorptionpeakinourscanstopositivepotential,ascomparedto publisheddata,mayberationalizedastheconsequenceofthepH- dependentpotential[42]alongwiththeacidicnature(pH∼1)of ourelectrolyte.Theﬁgureshowsthatrepeatedcyclingtoanupper potentialvertexof1.2VleadstoaslightdecreaseofthePPy-related peaks,whilenooxygen-on-goldelectrosorptionfeatureappears.

Part Bof Fig. 3shows CVs in a largerpotential interval. As themostobviousfeature,thecurrentincreasesdrasticallyatthe upperendofthepotentialscale,andthevoltammogramdegrades severelyduringthe10cyclesoftheﬁgure.Mostremarkably,the

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Fig.1.(A)Schematicillustrationofactuationmeasurementsperformedinsituinelectrochemicalcellarrangedindynamicmechanicalanalyzer(DMA)combinedwith potentiostat.(B)PhotographofelectrochemicalcellmountedinDMA.Glasscuvetteservedasinsituelectrochemicalcellandfusedsilicapushrodtransfersloadtothe sample.RE,CE,andWEdenotereference,counter,andworkingelectrode,respectively.

Fig.2. (A)ScanningelectronmicrographsofbicontinuousopenporosityofNPGwithligamentdiameterofabout250±50nm.(B)Scanningelectronmicrographsofcross- sectionalsurfacesofNPG-PPycomposites,whichclearlyshowsaconformalcoatingofPPy.Itdemonstratesthattheelectropolymerizationissuccessful.Notethatthewater channelstillremains.LargemagniﬁcationSEMimagesofNPG-PPycompositeswithanapproximateaveragePPythicknessof(C)15±5nm,(D)50±10nmand(E)80±20nm, whichissynthesizedbyelectropolymerizationforaround1.25h,2.5hand6h.

ﬁnal cycle appearsconsistent withthe CVof gold in Fig.3(A).

Thissuggeststhatscansto>1.2VleadtotheremovalofthePPy layer.ThisisconsistentwithreportsofoveroxidationofPPyathigh potential[43–45].Thefeaturenearthelowerpotentialvertex(see insetinFig.3(B))indicatesanirreversiblereactionthatrestrictsthe potentialrangeatthisendofthevoltammogram.

Fig.3(C)showsCVsinasmallerpotentialintervalthatavoids allelectrosorptionpeaksandirreversiblefeatures,−0.1–0.4V.In this regime the processes on pureNPG (black line) are essen- tiallycapacitive.Inthehybridsamples,thecharging/discharging currentsincreasewiththelayerthickness.Indeed, thegraph of (pseudo-)capacitance,C,versusPPylayerthicknessinFig.3(D) ishighlylinear.ThesubstantiallylargerCoftheNPG-PPyhybrid nanomaterialsconﬁrmsthealready establishedfact[14,15]that muchlargeramountsofchargecanbestoredinapseudo-capacitive

wayinthehybridmaterial.WealsofoundthatCVswherehighly reproducibleovermanypotentialcyclesinthispotentialinterval, indicatingagoodstabilityofthePPylayer.

3.3. Actuation

Weﬁrstinspectactuationexperimentswhichwereperformed, forthesameNPG-PPysample(50nmlayerthickness),intwodif- ferentpotentialranges:−0.1to0.4Vand0.5–1V.Thescanrate was2mVs⁻¹.Wemeasurethedeformationduringactuationby amacroscopiceffectivestrain,␧,whichisdeﬁnedastherelative changeinsample length,l.Thatis,we take␧=ıl/l0 withl0 the initiallength.Ineachactuationexperiment,␧wasmeasureddur- ing8subsequentpotentialscans.Therepresentativeresultswere showninFig.4(A)and(B).Itisseenthatthesampleexpandswith

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Fig.3. Electrochemicalcharacterizationrepresentedbycyclicvoltammograms(CVs)ofcurrentIversuselectrodepotentialEin1MHClO4.(A)Fivesuccessivecyclicscans between−0.25Vand1Vfornanoporousgold(dashedline)andbetween0.1and1.2VforNPG-PPycomposite(solidline)atscanrateof10mVs⁻¹.(B)Tensuccessivecyclic scansbetween−0.25and1.5VforNPG-PPycompositeatscanrateof10mVs⁻¹.(C)EightsuccessiveCVscansbetween−0.1to0.4Vat2mVs⁻¹forpureNPGandNPG-PPy compositeswithdifferentthicknessofPPy.(D)Pseudocapacitance,C,versusPPylayerthicknessforallgraphsof(C).TheCwascalculatedfromtheaverageoftheabsolute currentdividedbyscanrate.

Fig.4.ActuationmeasurementsforNPG-PPycompositewithPPylayerof50nmin1MHClO4atscanrateof2mVs⁻¹andintwodifferentpotentialranges:(A)−0.1to0.4V and(B)0.5–1V.Sixsweepsofpotential,E,(blueline,rightordinate)versustime,t,areshownexemplarily.Strain,␧,(darkredline,leftordinate)increaseswithincreasing E.(Forinterpretationofthereferencestocolourinthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

increasingpotential.Themeanpeak-to-peakamplitudes,0.075%

and0.073%,respectively,arenearlythesameforthemorenega- tiveandmorepositivepotentialintervals.Thus,theactuationin thepotentialregime−0.1to0.4Vachievestheidenticalstrainasin thehigherpotentialregime,butwithlesschargetransportandless hysteresis.Therefore,allfurtherinvestigationsfocusedon−0.1to 0.4Vpotentialinterval.

Fig.5explorestheactuationofsampleswithdifferentPPylayer thickness.Transientsofstrainandpotential(range−0.1to0.4V, scanrate2mVs⁻¹)areshowninpart(A).AllPPy-coveredsamples expandduringpositive-goingscans,consistentwithswellingdue totheuptakeofanions.Fig.5(B)showsthepeak-to-peakactuation amplitudeversusthePPylayerthickness.Similartothecapacity, thestrainamplitudeincreaseslinearlywiththelayerthickness.The

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Fig.5.Actuationmeasurementsin1MHClO4.(A)MechanicalcharacterizationrecordedsimultaneouslywithcyclicvoltamogramsshowninFig.3(C):variationofpotential, E,(rightordinate)withtime,t,indicatedbygreenlineandcorrespondingvaluesofeffectivestrain,␧(leftordinate)indicatedbyblank,red,blueanddarkredlinefor purenanoporousgold,andNPG-PPycompositewithPPylayerof15nm,50nmand80nm,respectively.(B)ActuationamplitudeversusPPylayerthicknessforthedatain (A).TheseemphasizetheenhancementofthepropertiesoverNPG.(C)Effectivestrain␧versusvolume-specificcharge,q^v,correspondingtothemeasurementshownin(A) forNPG-PPywithPPythicknessof80nm.Datawasaveragedover3actuationcycles,separatelyforpositiveandnegativegoingsweeps.Wecanseereversiblemacroscopic straininresponsetovariationofvolume-specificcharge.(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtothewebversionofthis article.)

Fig.6.ActuationmeasurementsforNPG-PPycompositewithPPythicknessof50nm in1MHClO4respondstostepchangeofpotentialbetween−0.1Vand5V.Variation ofpotentialE(rightordinate)withtime,t,isindicatedbygreenlineandcorre- spondingchangeofeffectivestrain,␧(leftordinate)withtimeisindicatedbydark redline.(Forinterpretationofthereferencestocolourinthisﬁgurelegend,the readerisreferredtothewebversionofthisarticle.)

strainamplitude,∼0.19%,ofthe80nmPPylayersampleismore than12timeshigherthanthe∼0.015%ofpureNPG.

Itisofinteresttoquantifytheactuationbehaviorintermsofthe charge-responseofameasurefordeformation.Proposedresponse coefficientscan bespecificfora deviceconfiguration –suchas bimorphcantilevers[4]–ortheycanbematerials-specific[46,47].

Sincethedeformationofourmaterialmaybeassumeduniformina coarse-grainedpicturethatignoresthenanoscalemicrostructure, theexperimentsaffordadirectcharacterizationofthematerials behavior.Intheirphenomenologicalthermodynamicsanalysisof actuationandsensingwithhybridnanomaterials,Stenneretal.[47]

havedeﬁnedaneffectivestrain-chargecoefﬁcient,A^∗=d␧

⁄

dq^V|_T, astheresponseoftheeffectivemacroscopicstrain␧tochangesof thevolume-specificcharge,q^v,measuredatconstantloadT.Here, q^V isdefinedasnetelectriccharge,Q,overtotalsamplevolume, V(solidandpores),withVmeasuredinthestrain-freereference state.ViaA*onecanassesstheinducedstrainpercharge,standard- izedtothesamplevolume,andthereforequantitativelyevaluate amaterial-specificactuationability.Fig.5Bshowstheaveraged strainplottedversusq^vfortheNPG-PPysamplewithPPythickness of80nm.Thedatawasaveragedfrom3actuationcycles,separately forpositiveandnegativegoingsweeps.Linearregressionprovides A*=0.0675±0.0004mm³C⁻¹.

Theactuationresponseratemaybeparameterizedbythetime, t_½,requiredtoreachhalfofthesaturationstrainafterapotential jump.Fig.6showsresultsforjumpsbetween−0.1Vand0.5V.For

Fig.7.Variationoftruestress,␴T,withengineeringstrain,␧E,incompressiontests withpurenanoporousgold(NPG)andinananomaterialsfromNPGandpolypyrrole (PPy).Ligamentsizesinbothsamplesare250nm,PPylayerthicknessis50nm.

theexampleofthe50nmlayerthicknesssampleweﬁndt_½≈1sfor both,expansionandcontractionreactions.

3.4. Compressiontests

Fig. 7 shows compressive stress-strain curves for mm-sized samplesofNPGandNPG-PPyhybridmaterialwithPPylayerthick- ness50nmandligamentsize250nm.Excellentdeformabilitywas foundfor bothmaterials.Uponloading,NPGexhibitsanimme- diateyieldonsetwithnoelasticregimeandastress-straingraph witheverincreasingworkhardening. Thisagreeswithprevious resultsinRef.[28,48].AscomparedwiththepureNPG,theNPG-PPy hybridmaterialsamplespresentasubstantially differentstress- strainbehavior.Aninitialelasticregimeismoreclearlyexpressed, even thoughtheslope indicatesa low Young’s modulus,about 0.1GPa.Yieldingsetsinataround6MPa;itisfollowedbyashort plateauofconstantﬂowstressandﬁnallyagainaregimeofstrong workhardening.

4. Discussion

Thediscussionofourobservationstartsoutwithremarkson theactuationmechanism.ForreferenceweﬁrstfocusonNPG,a metallicstructurewithoutPPycoatingandincontactwithaque- ouselectrolyte.Here,polarizingtheelectrodesurfacesimpactsthe interatomicbondforcesbetweenthemetalatomsin theouter- mostinteratomicplane[49,50].Inordertopreservethemechanical

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goldskeletonand,thereby,oftheentiresample.

Thenatureoftheionexchangeandthemagnitudeofthevol- umechangeofPPydepends onsynthesis conditionand onthe electrolyte usedfor actuation.Smelaet al.[2] emphasizedthat changingthesynthesisconditions,suchastheanion,solvent,or currentdensity,yieldswhatareessentiallydifferentmaterials.PPy is polymerizedin thefully oxidizedstate, which hasoneposi- tivecharge along the backbonefor every three tofourpyrrole monomerunitsandanequalnumberdopantanions.Thenatureof thedopantimpactsthepropertiesofthepolymer[13].Inourwork, thePPy(ClO₄⁻)systemwaschosenduringpolymerizationbecause thissystemshowscomparativelylargeactuationstrain,seeFig.8 inRef.[51].Theelectrolyteinouractuationexperimentusedthe sameanion,sothatvolumechangesconnecttotheexchangeof ClO₄⁻[52].

While the above behavior is well-established, the potential rangein whichit isexploitedinourexperiment isnot.Studies ofboth,ionexchange andactuationwithPPyfocus,tothebest ofourknowledge,invariablyonthe“oxidation/reduction”process thatcanbeassociatedwiththepronouncedelectrosorptionpeaks incyclicvoltammograms;ourFig.3Adisplaysthesepeaksingood agreementwiththeliterature.Yet,ourinvestigationofactuation pointstowardsarangeofmorenegativepotentials,inwhichthe CVsexhibitasmallerandnearlyconstantcurrent,seeFig.3C.The electrochemicalsignatureis herephenomenologicallysimilarto thecapacitiveregimeofidealizedmetalelectrodes.

Ourresultscharacterizetheabove,pseudo-capacitiveregime asmostappropriateforactuationbecauseofitslackofhystere- sisandthehighercyclestability.Itisparticularlyremarkablethat thestrainperchargeinourhybridmaterialislargerforpseudo- capacitivechargingascompared towhatwe ﬁndintheclassic oxidation/reductionregime. Furthermore,theapparentcapacity andtheactuationamplitudevarylinearlywiththePPylayerthick- nessinthisregime.Obviously,pseudo-capacitivecharginginvolves theexchangeofanionsbetweentheaqueousmediumandthePPy.

Yet,thelinearcharge-potential relationasopposed totheelec- trosorptionpeak ofthe oxidationreduction reaction suggestsa nonspecificinteractionoftheanionswiththepolymer.Pseudo- capacitivechargingofPPymaythenarisefromthebuildupofa space-chargelayer,similartothediffuselayeratachargedmetal electrodesurface.Thescreeningofelectricfieldsbyspacecharge impliesthatsuchlayerscanonlybeobservedin theimmediate vicinityofasurface.Thephenomenonwouldthenbespecificto thinPPylayers,suchasthosecoatingournanoporouselectrode.

Martinezet al. [53,54]investigated actuation in a geometry somewhatsimilartoours,yetwithmuchlargerdimensions.Gold orPlatinumwires,500␮mindiameter,werecoatedwithseveral

␮mthickPPylayersandthethicknessvariationofthePPy–i.e., theradialstrainofthecoatedwire−duringelectrochemicalcycles studied.Focusingontheoxidation/reductionprocess, thiswork foundalinearcharge-straincorrelation,asinthepresentstudy.

WhileMartinezetal.advertisesubstantialparasiticlosscurrents intheirAu/PPyexperiments,thedataofourFigs.3and5Ctes- tifytoanalmostidealpseudo-capacitivebehavior,inwhichcharge ispracticallycompletelyrecoveredafterpotentialcycles.Further-

hybridmaterialsarefoundsubstantiallyfasterthaninbulkactua- torsfromconductivepolymerswithouttheinterpenetratingmetal skeleton.EarlyworkonPPyactuatorsshowedlargestrain,butlong responsetimes.ForPPyﬁlms,Bayetal.[55]reportedanactuation strainratenear0.06%/swhileSpinksetal.[56]found0.01%/s.Even thoughourNPG-PPyhybridmaterialwasinvestigatedintheform ofbulksamplesofmillimeterdimension,itsstrainrateof0.1%/s exceedsthoseearliervalues.

Besidesrevealingaductile-brittletransitionwhengoingfrom thereducedtotheoxidizedstate,insitumechanicaltestsonPPy ﬁlmsindicate yield stressesaround 3–5MPa [57]. Compression testsonourNPG-PPysuggestsimilaryieldstrength,around6MPa, andextendeddeformabilityincompression.

Inrelationtothemechanicalperformanceitissigniﬁcantthat ourﬁrstexplorationofthehybridnanomaterialsworkedwithcom- paratively largeligament size, L=250nm. The strength of bulk samplesofNPGhasbeenshowntovaryastheinverseofL,down toligamentsizestenfoldlessthanthoseofthepresentsamples [26].Furthermore,theeffectiveYoung’smodulusofNPGincreases tenfoldwhengoingfromL=150nmto20nm[26].Thus,reducing Lprovidesopportunitiesforsubstantiallyenhancingstrengthand stiffnessofourmaterialinfutureexperiments.Studiesofthiseffect areinpreparationinourlaboratory.ReducingLalsohasimportant repercussionsfortheactuationbehaviorthatwillrequireexplo- ration.ThePPylayerswillneedtobethinner,whichreducesthe anionexchangeperinternalsurfacearea.Ontheotherhand,the netsurfaceareaincreasesas1/L.Themaximumtotalamountof PPythatcanbeaccommodatedinthehybridmaterialscaleswith theporevolumeoftheNPGmatrix;thisvolumeremainsinvari- antduringthedownscaling ofL.Thus, itmaybeexpectedthat theoverallactuationamplitudewillbemaintainedatsmallerliga- mentsize,whereasstrengthandstiffnessareexpectedtoincrease substantially.

5. Conclusions

Wehaveexploredanelectricallytunablenanoporousgoldbased hybrid nanomaterial for actuation. In ouractuation concept, a nanoscalehimcoatingofpolypyrroleisgrownontotheinternal metalsurfacesbyelectrochemicalpolymerization,andtheremain- ingporespacefilledwithaqueouselectrolyte.Inthisway,theactive polymerfilmsarecontactedbytwoseparatebutindividuallycon- tiguousconductionpaths,providingefficienttransportofionsin theelectrolytechannelsandofelectronsinthemetalskeleton.The metalskeletonalsocarriesloadandthusdefinesthemechanical behavioroftheactuator.

Contrary to earlier studies, our actuation does not exploit the classic oxidation/reduction of polypyrrole, but instead the exchangeofanionswiththepolymeratmorenegativepotentials.

Theexchangeappearstoinvolvethebuildupofaspacechargelayer through unspeciﬁc ion accommodation within the polypyrrole layer.Weﬁndthisprocesstoprovidesomewhatlargeractuation perchargetransferalongwithabetterstabilityofthepolymer.

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Thelinearstrainamplitudeofourmaterialreaches0.19%,which is12timeshigherthanthatofpurenanoporousgoldundersimi- larconditions.Furthermore,switchingtimesareattractiveinview ofthemacroscopicdimensionsofouractuators.Thelargestrain amplitude,fastswitching,andtheavailabilityofcentimetersize monolithicsamplesdistinguishthematerialasanactuator.

Acknowledgment

Thisworkwasfunded byDFGthroughSFB986“Tailor-Made Multi-ScaleMaterialsSystems–M³”,sub-projectB2.

References

[1]T.F.Otero,J.M.Sansinena,ArtiﬁcialMusclesBasedonConductingPolymers, Bioelectrochem.Bioenergy38(1995)411–414.

[2]E.Smela,Conjugatedpolymeractuatorsforbiomedicalapplications,Adv.

Mater.15(2003)481–494.

[3]G.M.Spinks,V.Mottaghitalab,M.Bahrami-Saniani,P.G.Whitten,G.G.

Wallace,Carbon-nanotube-reinforcedpolyanilineﬁbersforhigh-strength artiﬁcialmuscles,Adv.Mater.18(2006)637–640.

[4]T.F.Otero,J.G.Martinez,J.Arias-Pardilla,Biomimeticelectrochemistryfrom conductingpolymers.Areview:artiﬁcialmuscles,smartmembranes,smart drugdeliveryandcomputer/neuroninterfaces,Electrochim.Acta84(2012) 112–128.

[5]T.F.Otero,Biomimeticconductingpolymers:synthesis,materials,properties, functions,anddevices,Polym.Rev.53(2013)311–351.

[6]T.Mirfakhrai,J.D.W.Madden,R.H.Baughman,Polymerartiﬁcialmuscles, Mater.Today10(2007)30–38.

[7]T.F.Otero,Coulovoltammetricanddynamovoltammetricresponsesfrom conductingpolymersandbilayermusclesastoolstoidentifyreaction-driven structuralchanges.areview,Electrochim.Acta212(2016)440–457.

[8]J.D.Madden,R.A.Cush,T.S.Kanigan,I.W.Hunter,Fastcontractingpolypyrrole actuators,Synth.Met.113(2000)185–192.

[9]E.Smela,W.Lu,B.R.Mattes,Polyanilineactuators–part1.PANI(AMPS)in HCl,Synth.Met.151(2005)25–42.

[10]Y.Sonoda,W.Takashima,K.Kaneto,Characteristicsofsoftactuatorsbasedon polypyrroleﬁlms,Synth.Met.119(2001)267–268.

[11]G.M.Spinks,L.Liu,G.G.Wallace,D.Z.Zhou,Strainresponsefrompolypyrrole actuatorsunderload,Adv.Funct.Mater.12(2002)437–440.

[12]S.Hara,T.Zama,W.Takashima,K.Kaneto,Artiﬁcialmusclesbasedon polypyrroleactuatorswithlargestrainandstressinducedelectrically,Polym.

J.36(2004)151–161.

[13]G.G.Wallace,G.M.Spinks,L.A.P.Kane-Maguire,P.R.Teasdale,Conductive ElectroactivePolymers:IntelligentMaterialsSystems,Seconded.,CRCPress, BocaRaton,FL,USA,2003.

[14]F.H.Meng,Y.Ding,Sub-micrometer-thickall-solid-statesupercapacitorswith highpowerandenergydensities,Adv.Mater.23(2011)4098–4102.

[15]X.Y.Lang,L.Zhang,T.Fujita,Y.Ding,M.W.Chen,Three-dimensional bicontinuousnanoporousAu/polyanilinehybridﬁlmsforhigh-performance electrochemicalsupercapacitors,J.PowerSources197(2012)325–329.

[16]R.Li,K.Sieradzki,Ductile-BrittletransitioninrandomporousAu,Phys.Rev.

Lett.68(1992)1168–1171.

[17]J.Erlebacher,M.J.Aziz,A.Karma,N.Dimitrov,K.Sieradzki,Evolutionof nanoporosityindealloying,Nature410(2001)450–453.

[18]J.Weissmüller,R.C.Newman,H.J.Jin,A.M.Hodge,J.W.Kysar,Nanoporous metalsbyalloycorrosion:formationandmechanicalproperties,MRSBull.34 (2009)577–586.

[19]J.Snyder,P.Asanithi,A.B.Dalton,J.Erlebacher,Stabilizednanoporousmetals bydealloyingternaryalloyprecursors,Adv.Mater.20(2008)4883–4886.

[20]H.J.Jin,S.Parida,D.Kramer,J.Weissmüller,Sign-invertedsurface stress-chargeresponseinnanoporousgold,Surf.Sci.Rep.602(2008) 3588–3594.

[21]H.J.Jin,X.L.Wang,S.Parida,K.Wang,M.Seo,J.Weissmüller,Nanoporous Au-PtalloysAslargestrainelectrochemicalactuators,NanoLett.10(2010) 187–194.

[22]C.A.Volkert,E.T.Lilleodden,Sizeeffectsinthedeformationofsub-micronAu columns,Philos.Mag.86(2006)5567.

[23]J.Biener,A.M.Hodge,J.R.Hayes,C.A.Volkert,L.A.Zepeda-Ruiz,A.V.Hamza, etal.,SizeeffectsonthemechanicalbehaviorofnanoporousAu,NanoLett.6 (2006)2379–2382.

[24]H.J.Jin,L.Kurmanaeva,J.Schmauch,H.Rosner,Y.Ivanisenko,J.Weissmüller, Deformingnanoporousmetal:roleoflatticecoherency,ActaMater.57(2009) 2665–2672.

[25]A.Mathur,J.Erlebacher,SizedependenceofeffectiveYoung’smodulusof nanoporousgold,Appl.Phys.Lett.90(2007).

[26]N.Mameka,K.Wang,J.Markmann,E.T.Lilleodden,J.Weissmüller, Nanoporousgold−Testingmacro-scalesamplestoprobesmall-scale mechanicalbehavior,MaterResLett4(2016)27–36.

[27]K.Wang,J.Weissmüller,Compositesofnanoporousgoldandpolymer,Adv.

Mater.25(2013)1280–1284.

[28]K.Wang,A.Kobler,C.Kubel,H.Jelitto,G.Schneider,J.Weissmüller, Nanoporous-gold-basedcomposites:towardtensileductility,NPGAsia Mater.7(2015)e187.

[29]J.Weissmüller,R.N.Viswanath,D.Kramer,P.Zimmer,R.Würschum,H.

Gleiter,Charge-inducedreversiblestraininametal,Science300(2003) 312–315.

[30]J.Biener,A.Wittstock,L.A.Zepeda-Ruiz,M.M.Biener,V.Zielasek,D.Kramer, etal.,Surface-chemistry-drivenactuationinnanoporousgold,Nat.Mater.8 (2009)47–51.

[31]H.-J.Jin,J.Weissmüller,Bulknanoporousmetalforactuation,Adv.Eng.Mater.

12(2010)714–723.

[32]E.Detsi,M.S.Sellès,P.R.Onck,J.T.M.DeHosson,Nanoporoussilveras electrochemicalactuator,ScriptaMater.69(2013)195–198.

[33]Q.Bai,Y.Wang,J.Zhang,Y.Ding,Z.Peng,Z.Zhang,Hierarchicallynanoporous nickel-basedactuatorswithgiantreversiblestrainandultrahighwork density,J.Mater.Chem.C4(2016)45–52.

[34]J.Weissmüller,J.W.Cahn,Meanstressesinmicrostructuresduetointerface stresses:ageneralizationofacapillaryequationforsolids,ActaMater.45 (1997)1899–1906.

[35]W.Haiss,Surfacestressofcleanandadsorbate-coveredsolids,Rep.Prog.

Phys.64(2001)591–648.

[36]D.Kramer,R.N.Viswanath,J.Weissmüller,Surface-stressinduced macroscopicbendingofnanoporousgoldcantilevers,NanoLett.4(2004) 793–796.

[37]R.N.Viswanath,D.Kramer,J.Weissmüller,Adsorbateeffectsonthesurface stress-chargeresponseofplatinumelectrodes,Electrochim.Acta53(2008) 2757–2767.

[38]J.Zhang,Q.Bai,Z.Zhang,Dealloying-drivennanoporouspalladiumwith superiorelectrochemicalactuationperformance,Nanoscale8(2016) 7287–7295.

[39]X.Xiao,M.e.Wang,H.Li,P.Si,One-stepfabricationofbio-functionalized nanoporousgold/poly(3,4-ethylenedioxythiophene)hybridelectrodesfor amperometricglucosesensing,Talanta116(2013)1054–1059.

[40]E.Detsi,P.Onck,J.T.M.DeHosson,Metallicmusclesatwork:highrate actuationinnanoporousGold/Polyanilinecomposites,AcsNano7(2013) 4299–4306.

[41]B.Z.Yan,J.Yang,Y.F.Li,R.Y.Qian,Cyclicvoltammetryofpolypyrrolein anhydrousacetonitrile,Synth.Met.58(1993)17–27.

[42]S.Shimoda,E.Smela,TheeffectofpHonpolymerizationandvolumechange inPPy(DBS),Electrochim.Acta44(1998)219–238.

[43]J.B.Schlenoff,H.Xu,Evolutionofphysicalandelectrochemicalpropertiesof polypyrroleduringextendedoxidation,J.Electrochem.Soc.139(1992) 2397–2401.

[44]T.Osaka,T.Momma,S.-i.Komaba,H.Kanagawa,S.Nakamura, Electrochemicalprocessofformationofaninsulatingpolypyrroleﬁlm,J.

Electroanal.Chem.372(1994)201–207.

[45]T.W.Lewis,G.G.Wallace,C.Y.Kim,D.Y.Kim,Studiesoftheoveroxidationof polypyrrole,Synth.Met.84(1997)403–404.

[46]T.Shoa,J.D.W.Madden,T.Mirfakhrai,G.Alici,G.M.Spinks,G.G.Wallace, Electromechanicalcouplinginpolypyrrolesensorsandactuators,Sensor Actuat.A:Phys.161(2010)127–133.

[47]C.Stenner,L.-H.Shao,N.Mameka,J.Weissmüller,Piezoelectricgoldstrong charge-loadresponseinametal-basedhybridnanomaterial,Adv.Funct.

Mater.26(2016)5174–5181.

[48]N.Mameka,J.Markmann,H.-J.Jin,J.Weissmüller,Electricalstiffness modulation–conﬁrmingtheimpactofsurfaceexcesselasticityonthe mechanicsofnanomaterials,ActaMater.76(2014)272–280.

[49]F.Weigend,F.Evers,J.Weissmüller,Structuralrelaxationinchargedmetal surfacesandclusterions,Small2(2006)1497–1503.

[50]J.M.Albina,C.Elsässer,J.Weissmüller,P.Gumbsch,Y.Umeno,Abinitio investigationofsurfacestressresponsetochargingoftransitionandnoble metals,Phys.Rev.Bk85(2012)125118.

[51]K.Kaneto,Y.Sonoda,W.Takashima,Directmeasurementandmechanismof electro-chemomechanicalexpansionandcontractioninpolypyrroleﬁlms, Jpn.J.Appl.Phys.Part1-Regul.PapersShortNotesRev.Pap.39(2000) 5918–5922.

[52]N.Alizadeh,F.Tavoli,Enhancingelectrochromiccontrastandredoxstability ofnanostructurepolypyrroleﬁlmdopedbyheparinaspolyanionindifferent solvents,J.Polym.Sci.APolym.Chem.52(2014)3365–3371.

[53]J.G.Martinez,T.F.Otero,E.W.Jager,Effectoftheelectrolyteconcentrationand substrateonconductingpolymeractuators,Langmuir30(2014)3894–3904.

[54]J.G.Martinez,T.F.Otero,E.W.Jager,Electrochemo-dynamicalcharacterization ofpolypyrroleactuatorscoatedongoldelectrodes,Phys.Chem.Chem.Phys.

18(2016)827.

[55]L.Bay,K.West,P.Sommer-Larsen,S.Skaarup,M.Benslimane,Aconducting polymerartiﬁcialmusclewith12%linearstrain,Adv.Mater.15(2003) 310–313.

[56]G.M.Spinks,G.G.Wallace,L.Liu,D.Zhou,Conductingpolymers

electromechanicalactuatorsandstrainsensors,Macromol.Symp.192(2003) 161–170.

[57]P.Murray,G.M.Spinks,G.G.Wallace,R.P.Burford,Electrochemicalinduced ductile—brittletransitionintosylate-doped(pTS)polypyrrole,Synth.Met.97 (1998)117–121.

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JörgWeißmüllergraduatedasDiplom-IngenieurinMaterialsScienceafterstudies attheUniversitätdesSaarlandesandtheUniversityofDundee.HereceivedhisDoc- torateinEngineeringfromUniversitätdesSaarlandesbasedonstudiesonmetallic

nanoporousmetalsbydealloyingasamodelsystemforinterfaceeffectsonplasticity, elasticityandfunctionofnanomaterials.