Effect of pulse voltage and aluminum purity on the characteristics of anodic

ThinSolidFilmsxxx(2011)xxx–xxx

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ThinSolidFilms

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Effectofpulsevoltageandaluminumpurityonthecharacteristicsofanodicaluminumoxideusinghybridpulseanodizationatroomtemperature

C.K.Chung⁎,W.T.Chang,M.W.Liao,H.C.Chang

DepartmentofMechanicalEngineering,andCenterforMicro/NanoScienceandTechnology,NationalChengKungUniversity,Tainan,Taiwan701,ROC

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infoabstract

Mostoftheanodicaluminumoxide(AAO)templatesareperformedbypotentiostaticalmethodat0–10°CtoinhibittheJoule'sheatenhanceddissolutioninaluminumoxideforwell-orderedcellconfiguration.Inthisarticle,weproposethehybridpulseanodization(HPA)methodwitheffectivesuppressionofJoule'sheatgenerationtofabricateAAOatroomtemperature.Effectsofpurityofaluminum(Al)foilsandpulsevoltageontheevolutionofporecharacteristicshavebeeninvestigated.TheAAOmorphologyiscapturedbyscanningelectronmicroscopeandanalyzedviagray-scaleimaginginordertoidentifytheporesizedistribution.Theincreasedappliedpotentialresultsinthewidenedporesandnon-uniformcellarrangementduetotheincreasedcurrentdensityandvariation.Moreover,low-purityAlfoilsleadtothereducedAAOdistributionuniformityowingtotheunevenelectricfieldinducedpitsontheAlsurfaceforinferiorporearrangement.Extendingboththepositiveandnegativepulseperiodfrom1sto5sduringHPAcanenhancethedistributionuniformityofAAOfromhigh-purityAlbyuptoabout95%.Inaddition,therelationshipbetweenAAOconfigurationandAlpurityandpulsevoltageisfurtherdiscussedandestablished.

©2011ElsevierB.V.Allrightsreserved.

Keyword:AAO

Porousmaterial

HybridpulsevoltageDistributionuniformityRoomtemperature

1.Introduction

Theanodicaluminumoxide(AAO)filmisoneofthemostwell-knowntemplatesforgrowingthenano-structuredmaterialsbecauseitnotonlyoffersself-orderednanostructuresbutalsocanbeobtainedbytheeffectiveelectrochemicalmethod.EarlierstudiesshowedthatthewellorderedconfigurationofAAOwascausedbymechanicalstresswhichwasproducedfromvolumeexpansion[1,2].Exceptforthemechanicalstress,theelectricfieldstrengthatbarrierlayerwasalsorelatedtowellorderingconfigurationbecausetheelectricfieldstrengthisanimpressedvoltageperunitthicknessoftheoxidelayer[3,4].However,thedrivingforcefortheself-organizationoftheporesstillremainsunclear.SinceMasudaandFukuda[5]proposedatwo-stepanodizationmethodandobtainedthewell-orderedAAOconfigurationsuccessfullyin1995,theformingmechanismandpotentialapplicationsofAAOhavebeenwidelystudied.AAOhasbeenappliedsuccessfullytoavarietyofapplicationssuchasoptics[6],nanocompositematerial[7],bio-technology[8],supercapacitor[9],magneticrecording[10],surface-enhancedRamanscattering[11],humiditysensors[12],light-emittingdiodes[13],photoniccrystal[14],nanopatternstransferring[15],nanomaterialssynthesis[16],p–njunctions[17]andLCDdisplay[18].ManyAAOtemplateswereperformedbydirectcurrentanodization(DCA)atlowtemperaturesof0–10°C.Recently,cyclicpulseanodizationwasappliedforAAO

fabricationtoobtainthemodulatedperiodicarchitectures[19].Moreover,Chungetal.[20]haddemonstratedthatthehybridpulseanodization(HPA)couldovercomethelimitationofconventionallow-temperatureAAOprocesstoformnanoporesatroomtemperature(RT).PulsevoltagecouldsuppressthegenerationofJoule'sheatinelectronicscircuiteffectively[21].TheheatgenerationisharmfultonanostructuresbecauseitrandomlyaccumulatesallovertheAAOstructureespeciallythediscontinuousgeometrytoenhancethedissolutioneffect.Then,anon-uniformAAOdistributionoccurs.Inthisarticle,atwo-stepHPAhasbeeninvestigatedbymeansofbothhigh-purityandlow-purityaluminumandthevariedpulsepotentialstogetherwithpositive-negativepulseperiod.Thegray-scaleimagingprocessisappliedforanalyzingtheAAOporedistributionforevaluatingtheevolutionofAAOdistributionuniformitywithAlpurityandpulsevoltage.2.Experimentalprocedures

EffectofpulsevoltageandAlfoilpurityontheevolutionofAAOcharacteristicsbyHPAwascomparativelystudiedatroomtemper-aturearound25°C.TheAAOwasanodizedin0.3MoxalicacidandtwokindsofAlfoilpurityof99.997%(highpurity,H.P.)and99%(lowpurity,L.P.)(AlfaAesar,AJohnsonMattheyCompany,USA)wereused.BothH.P.andL.P.Alfoilswereanodizedatafixed30VforpurityeffectstudyandtheL.P.AlwasconductedforAAOevolutionatvariousappliedpotentialsforpulsevoltageeffectstudyaswellthattheH.P.AlwasexaminedforgoodqualityofAAOevolutionatalteredpositive-negativepulseperiod.Theappliedhybridpulsewasconstructedfromapositivesquarewavefollowedbyanothernegativesquarewave.The

⁎Correspondingauthor.Tel.:+88662757575x62111;fax:+88662352973.E-mailaddress:ckchung@mail.ncku.edu.tw(C.K.Chung).0040-6090/$–seefrontmatter©2011ElsevierB.V.Allrightsreserved.doi:10.1016/j.tsf.2011.01.029

Pleasecitethisarticleas:C.K.Chung,etal.,ThinSolidFilms(2011),doi:10.1016/j.tsf.2011.01.029

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positivepulsevoltagesvariedfrom20V,30Vto40Vandthenegativeonewasfixedat−2V.Thesmallamountofnegativevoltagewasusedtoinhibitthecathodiccurrentduringt−period.FormationofAAOwasperformedbymeansofthepotentiostat(Jiehan5000,Taiwan)andthethree-electrodeelectrochemicalcellwiththeplatinummeshasthecounterelectrode,withthegoldcoatedsiliconastheworkingelectrode,andwithAg/AgClasreferenceelectrode.Inordertoachieveasmoothsubstratepriortoanodization,theAlfoilspecimenswereelectrochemicallypolishedinamixtureofHClO4andC2H5OHwithaweightratioof1:5for30s.Atthefirst-stepanodization,theAlfoilwasperformedbyHPA(V+=20,30,40V,V−=−-2V;t+=1and5s,t−=1and5s)for1h.Thentheanodicoxidelayerwasremovedinamixtureofphosphoricacid(8wt.%)andchromicacid(3.6wt.%)atRTfor1htoleavethepretexturedAl2O3onAlsurfaceasaself-assembledmask.Thesecondstepofanodizationwassubsequentlyconductedbythesamepulseconditionasthefirststepofanodization,andthedurationswas3h.ThemorphologyandporecharacteristicsofAAOfilmswerethenexaminedbyHighResolutionFieldEmissionScanningElectronMicroscope(HR-FESEM,JEOLJSM-7000F,Japan).Inordertoillustratetheporedistributionsindetails,SEMmicrographsoftheAAOfilmsatdifferentparameterswerefurtheranalyzedbygray-scaleimaginganalysis.Fig.1ashowstheSEMmicrographofAAOconfigurationwhichwasanodizedfromhigh-purityAlbyHPAat30V.Thediameterandamountofporescanbeestimatedusingcommercialsoftware(Image-Proplus,MediaCyberneticsInc.)bycollectingtheadequaterangeingrayscaleofnanoporeswhichareshownincirculardarkregion.Thehistogramofgraylevelsisdividedfrom0(fullyblack)to255(fullywhite)asshowninFig.1b.Finally,theamountofporesperμm2versuscorrespondingporediametercanbeobtainedandshowninFig.1c.3.Resultsanddiscussion

Fig.2showstheSEMmicrographsandtherelationshipbetweentheamountofporesperμm2andporediametersofAAOfromL.P.Al(emptysquaresymbol,blackline)andH.P.Al(solidtrianglesymbol,redline).BothH.P.AlandL.P.Alwereallanodizedat30VbyHPAatroomtemperature.InthecaseofL.P.Al,therangeof55±5nmoccupiesthemaindistributionand62.6%poresformedwithinthisrange.Ontheotherhand,85.2%poresformedwithintherangeof45±5nminH.P.Al.ItrevealsthatthemainporessizeofAAOformedinH.P.AlissmallerthanthatinL.P.Al(45±5nmb55±5nm)andhigherconcentrativeporedistributionuniformityisformedinH.P.Al(85.2%)thanthatinL.P.Al(62.6%).Thedistributionuniformityisdefinedastheratiooftheporeslocatedatthesepeakstothewholeamountofpores.Asaresult,AAOobtainedfromthecostlyH.P.AlshowsbetterdistributionuniformityandsmallerporesizethanthosefromcheapL.P.Albecauseofthehigh-purityAlwithinhibitedimpurity-enhanceddissolution.TheL.P.AlfoilcontainshighercontentsofimpuritiesincludingtheprimarySiandFewhicharesummedabout0.6%andothersofZn,Cu,Mg,MnandTielementswhicharesummedabout0.3%–0.4%.Eachelementhastheindividualoxidativerate,andisunfavorabletothedistributionuniformityofAAOconfiguration.TheimpuritiesarenotonlyharmfultothedistributionuniformityofAAOconfigurationbutalsoenhancingtheporewideningduetoimpurities-enhanceddissolu-tionduringAlanodization.Moreover,earlierpublicationreportedthatcellconfigurationwasinitiatedfromthepitsonAlsurface[22].ThedissolutionrateallovertheAlsurfaceisunevenespeciallyonimperfectionsofgrainboundaries,impurities,dislocationsandnonmetallicinclusionswhichmaycauseafasterdissolution.Accordingly,theimpurity-inducedunevenelectricfieldforirregularpitsontheAlsurfaceleadstoinferiorporesandcellsconfiguration.

Fig.3ashowstherelationshipbetweentheamountofporesperμm2anddiametersofporesofAAOfromL.P.AlbyHPAatroomtemperatureattheanodizedpotentialat20V(emptysquaresymbol,

Fig.1.Thegray-scaleimagingprocessforanalyzingthedistributionuniformityofAAO:(a)theSEMmicrographofAAOconfigurationwhichisanodizedfromhigh-purityAlbyHPAat30V,(b)thehistogramofgraylevelsinthecapturedareaisdividedfrom0(fullyblack)to255(fullywhite),(c)theconvertedlinegraphoftheamountofporesperμm2versusthecorrespondingporediameter.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

blackline),30V(solidtrianglesymbol,redline)and40V(solidcircularsymbol,greenline).Thepeaksinabovethreecurvesarelocatedat30±5nm(20V),55±5nm(30V)and75±5nm(40V),respectively,forthemainporedistribution.Consequently,thedistributionuniformitiesare74.9%,62.6%and55.1%for20V,30Vand40V,respectively.Thedistributionuniformitydecreaseswithincreasingappliedpotentials.Fig.3bshowstherelationshipbetweentheporesizeandappliedpotentialbystatisticallytakingtheporedensitylargerthan5poresperμm2forthemeanvalueanddeviation.ThefittinglineofexperimentaldataisgovernedasY=−15.48+2.23X,thatis,theporesizeofAAOincreasesaround2.2nmwhentheappliedpotentialincreases1V.Thisphenomenonissimilartothevoltage-enhancedporewideningbyconventionaldirectcurrentanodization(DCA)process[1,3,4,23].Referringtopriorpublications,theypresentedthataverageporediameterof65nmwasformedbyDCAat25V[3],100nmat40V[4,5],300nmat120V[3],400nmat

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Fig.2.TheSEMmicrographsandtherelationshipbetweentheamountofporesperμm2andporediametersofAAOfromL.P.Al(emptysquaresymbol,blackline)andH.P.Al(solidtrianglesymbol,redline)byHPAat30Vatroomtemperature.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

160V[3],500nmat195V[3]and600nmat240V[4].ThedifferencebetweenHPAandDCAisrelatedtothediminishedJoule-heat-dissolutioneffectinHPA.Inpulsecurrent,theaveragecurrentdensitycanberegardedasIavg=peakcurrent×dutycycle[24].So,thecurrentefficiencyinHPAdependsstronglyonthedutycycleandissmallerthanthatinDCA.Therefore,lowerJoule'sheat(P=I2×R,P:power,I:current,R:resistance)isgeneratedinHPAcomparedtoDCA.Also,theJoule'sheatinHPAcanbedissipatedduringthenegativepulseperiodwithnearlyzerocathodiccurrentandliquidelectrolytecooling.Inaluminumanodization,bothformationanddissolutionofaluminumoxideoccurredsimultaneouslyandcompetedwitheachother.TheJoule'sheatcausedinanodizationenhancesthedissolutionofAlandresultsinporewidenespeciallyinDCA.ThatisoneofthereasonsformostofconventionalAAOsbeingperformedatlowtemperature(0–10°C).ThepulsevoltageofHPAbenefitsfromthesuppressionofJoule'sheatingwithtimewhichiseffectivelyreducingthechemicaldissolutionreactionandresultedinbetteruniformityinporedistributionofAAO.Fig.4aandbshowstheschematicappliedhybridpulsepotentialandthecorrespondingalternatingcurrent,respectively.TheHPAmethodcanexhibiteffectivecoolingfromthenearlyzerocathodiccurrentandhigh-thermalconductivityliquidelectrolytetosuppressJouleheatdissolutionreactionforpromotingAAOporesdistributionuniformityatsevereconditionsofthecheaplow-purityAlfoilswithoutanyheattreatmentandtherelativelyhighroomtemperaturewithoutcoolingequipment.Moreover,thecurrentatpositivepulsedurationfunctionsaschargingcurrentandlargercurrentfromhighervoltageisdisadvantageoustothedistributionuniformity.Themeanpositivecurrentatourvariousappliedpotentialsof20,30and40Vare93.31,123.38and132.68mA,respectively.ItincreaseswithgrowingappliedpotentialandexhibitslargercurrentwhichmaycausethereduceddistributionuniformityofAAOasshowninFig.3a.

Fig.3.(a)Therelationshipbetweentheamountofporesperμm2andporediameterswithSEMinsetsofAAOfromL.P.Albyanodizedpotentialat20V(emptysquaresymbol,blackline),30V(solidtrianglesymbol,redline)and40V(solidcircularsymbol,greenline)atroomtemperature;(b)TheexperimentaldatafromHPAinthisstudyandthefittinglinecanbegovernedasY=−15.48+2.23X.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

WithregardtothecellarrangementofAAOconfiguration,Onoetal.[3]reportedthatelectricfieldstrengthwasacrucialfactoraffectingtheself-ordering.Thehighelectricfieldisrelatedtohighcurrentdensityfromhighconcentration[3,4,22].Onoetal.[3]fabricatedtheAAOsfromvariousveryhighelectrolyteconcentrationsatthesameappliedvoltageinordertocomparativelystudythecellarrangementandreportedthatwell-orderedconfigurationcouldbeobtainedfromhighelectrolyteconcentration.Inourviewpoint,thecellconfigurationfromhighelectrolyteconcentrationseemsmoreclosetoideallypackedhexagonalarrayratherthanthesameporesize.Fig.5showstheSEMmicrographsofAAOinlocalareafromvariouselectrolyteconcentrationsinOno'sstudy[3]andthegray-scaleimagingtechnologyisalsoappliedtoanalysisofporedistribution.AllthecurvesofAAOinthehighmolarconcentrationof2,3,4and5mol/laremainlylocatedintherangeof250±25nmandthedistributionuniformitiescalculatedare68.0%,68.2%,58.5%and59.8%,respectively.Thedistributionuniformityisnothighanddistinctlydecreasingfrom3to4mol/lwithincreasingelectrolyteconcentrationorcurrentdensity.Therefore,itisarguedthathighelectricfieldishelpfulinformingideallyhexagonalcellconfiguration,butisunfavorabletothedistributionuniformityduetothecorrespondinghighcurrentforhighJoule'sheatgeneration.

Fig.6showstheSEMmicrographandtherelationshipbetweentheamountofporesperμm2anddiametersofporeofAAOfromH.P.Alby

Pleasecitethisarticleas:C.K.Chung,etal.,ThinSolidFilms(2011),doi:10.1016/j.tsf.2011.01.029

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Fig.6.TheSEMmicrographandtherelationshipbetweentheamountofporesperμm2andporediametersofAAOfromH.P.AlbyHPAat30Vatt+=5s,t−=5satroomtemperature.

Fig.4.Theschematicdiagramsof:(a)appliedhybridpulsepotentialand(b)correspondingalternatingcurrent.

4.Conclusions

HPAat30Vwiththeprolongedbothpositiveandnegativepulseperiodof5s.Themainporesizeislocatedat35–40nmwithahighdistributionuniformityofabout95.0%.ComparedtotheH.P.AlinFig.2,extendingbothpositiveandnegativepulseperiodfrom1sto5sduringHPAcanenhancedistributionuniformityofAAOuptoabout95.0%togetherwithinterporewallthickness.TheprolongednegativepulseperiodismorefavorabletoreducetheJoule'sheatinthedistributionofuniformitytogetherwithlowerchemicaldissolu-tionforthesmallerporesizeandthickerwall.

WehavesuccessfullyinvestigatedthecellarrangementofAAOfromdifferent-purityAlfoilsatvariousappliedvoltagesbyHPAatroomtemperature.ThecellarrangementwasevaluatedthroughSEMmicrographsbycommercialsoftwareofImage-Proplusforidentifyingtheaverageporediameteranduniformity.TheaverageporessizeofAAOformedinH.P.AlissmallerthanthatinL.P.Al(45±5nmb55±5nm).Also,betteruniformitywasobtainedinH.P.Al(85.2%)thanthatinL.P.Al(62.6%)becauseH.P.Alcontainslittleimpurities.L.P.Alcontainsmanyimpuritieswhichmayhaveindividualoxidativerateandcontributesto

Fig.5.Citeddatainreference[3]:therelationshipbetweentheamountofporesandporediameterswithSEMinsetsofAAObyvariouselectrolyteconcentrations.

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theporewideningandnon-uniformityduetounevenelectricfieldfollowedbyunevenpitsontheAlsurface.Theuniformitydecreaseswithincreasingappliedpotential(74.9%for20V,62.6%for30Vand55.1%for40V)becauseoftheincreasedcurrentdensityandvariationathighvoltage.TheporesizeofAAOincreaseswithincreasingappliedpotentialwhichisaroundlinearly2.2nmpervolt.HPAmeritssuppressioninJoule'sheatingtoreducechemicaldissolutionreactionandresultsinbetteruniformityofAAOporedistribution.Theprolongedpositiveandnegativepulseperiodpromotesthedistributionuniformityuptoabout95%togetherwiththesmallerporesizeandthickerwallduetothediminishedJoule'sheatandchemicaldissolution.Acknowledgements

ThisworkispartiallysponsoredbyNationalScienceCouncilundercontractNo99-2221-E-006-032-MY3.WewouldliketothankCenterforMicro/NanoScienceandTechnologyinNationalChengKungUniversityfortheaccessofanalysisequipments.References

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Pleasecitethisarticleas:C.K.Chung,etal.,ThinSolidFilms(2011),doi:10.1016/j.tsf.2011.01.029