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Fabricationoftitaniumoxidenanotubearraysbyanodicoxidation
JianlingZhao,XiaohuiWang*,RenzhengChen,LongtuLi
DepartmentofMaterialsScienceandEngineering,StateKeyLaboratoryofNewCeramicsandFineProcessing,TsinghuaUniversity,
Beijing100084,People’sRepublicChina
Received9July2004;receivedinrevisedform31January2005;accepted16February2005byC.E.T.Gonc¸alvesdaSilva
Availableonline3March2005
Abstract
TheformationoftitaniumoxidenanotubearraysontitaniumsubstrateswasinvestigatedinHFelectrolytes.Underoptimized
electrolyteandoxidationconditions,well-orderednanotubesoftitaniawerefabricated.Topologiesoftheanodizedtitaniumchangeremarkablyalongwiththechangingofappliedvoltages,electrolyteconcentrationandoxidationtime.Electrochemicaldeterminationandscanningelectronmicroscopeindicatethenanotubesareformedduetothecompetitionoftitaniaformationanddissolutionundertheassistanceofelectricfield.Apossiblegrowthmechanismhasalsobeenpresented.q2005ElsevierLtd.Allrightsreserved.
PACS:81.05;82.45
Keywords:A.Nano-tubes;A.Titaniumoxide;D.Anodicoxidation
1.Introduction
Titaniumdioxideisoneofthemostwidelystudiedchemicalsubstancesbecauseofitswidelyapplicationincatalytic,gas-sensingandcorrosion-resistancematerials.Moreover,itisgainingconsiderableinterestduetouniqueandexcellentpropertiesinoptics,electronics,photo-chemistryandbiology[1–3].Anodicoxidationisacommonlyusedsurfacetreatmentmethodespeciallyinformingporousaluminastructure.Formationofporousaluminastructurehasbeenwidelystudied.Manykindsofnanostructuresincludingnanowires,nanorodsandnano-tubeshavebeenfabricatedbytakingnanoporousaluminaastemplates[4–7].Therewereafewattemptstocompareanodicoxidegrowthonothervaluemetalswithporousalumina.Electrochemicaloxidizationoftitaniumhasbeenstudiedinsulfuricacid,phosphoricacid,aceticacidandchromicacidwithorwithoutHFsolution[2,8–10].
*Correspondingauthor.Tel.:C861062784579;fax:C861062771160.
E-mailaddress:wxh@mail.tsinghua.edu.cn(X.Wang).
Differentmorphologieswereobtainedinoxidizedtitaniumcomparedwithalumina[1,2,11].Amirrorimageofthebehaviorofporousaluminacannotbecopiedintheformingofporoustitania.
ThispaperinvestigatestheoxidationprocessoftitaniuminHFaqueoussolutionviaconstant-voltageexperiments.Underoptimizedelectrolyteandoxidationconditions,well-orderednanotubesoftitaniawerefabricated.Topologiesoftheanodizedtitaniumchangeremarkablyalongwiththechangingofappliedvoltages,electrolyteconcentrationandoxidationtime.Apossiblegrowthmechanismispresented.
2.Experimental
Thehighpuritytitaniumfoils(10!10!0.5mm3)usedinthisstudywereobtainedfromGeneralResearchInstituteforNonferrousMetals(Beijing,China).Theywerepolishedmechanicallytoamirrorimageandwashedintwice-distilledwaterandacetonebyultra-sonicwashingbeforeuse.Electrochemicalexperimentswerecarriedoutusingadirectcurrent(dc)voltagesource(DH1722,DahuaCoop.,Beijing,China).
0038-1098/$-seefrontmatterq2005ElsevierLtd.Allrightsreserved.doi:10.1016/j.ssc.2005.02.028
706J.Zhaoetal./SolidStateCommunications134(2005)705–710
Fig.1.Titaniummicrostructureafterdifferenttreatments(a)titaniummetalsafterbeingpolishedmechanically,(b)titaniummetalsafterbeinganodizedat20VabovetheHFsolution,(c)titaniummetalsafterbeinganodizedat20Vandthenetchedin1wt%HF,(d)titaniummetalsafterbeinganodizedat20Vfor30minin1wt%HFsolution.
Titaniumfoilswereusedasanodicelectrodewhileplatinum(20!20!0.1mm3)wasusedascathodicelectrode.Thedistancebetweenanodicandcathodicelectrodeswas20mm.Electrolyteinthisprocesswas0.1w5.0wt%HFsolution.Allsolutionswerepreparedfromreagentgradechemicalsanddeionizedwater.Allanodizationexperimentswerecarriedoutatroomtemperature.Duringtheexperiments,thesolutionswerestirredusingamagneticstirrer.Aftertheanodization,thesampleswererinsedindeionizedwater,driedandcharacterized.
ElectrochemicalmeasurementswereconductedusingCHI660aelectrochemicalanalyzer(CHInstrumentsInc.,Shanghai,China).RamanspectrawereobtainedonaRM1000RamanSpectrometer(RenishawPLC).X-raydiffractionmeasurementswereperformedonD/max-RBdiffractometer(Rigaku,Rotafles)usingCuKaradiation(0.15416nm).Themicrostructureswereobservedonfieldemissionscanningelectronmicroscopes(JSM-6301,JEOLInc.,Japan).Thecross-sectionphotographswereobtainedbyobservingmechanicallyfracturedsample.Thetrans-missionelectronmicroscopystructuresandselectedareaelectrondiffractionwereobtainedinTEM-200CXelectronmicroscope(JEOL,Japan).
3.Resultsanddiscussion
3.1.Structuresoftitaniummetalssurfaceafterdifferenttreatments
WhentitaniummetalswereanodizedinHFsolution,structuresoftheanodizedtitaniumchangeremarkablyalongwiththechangingofappliedvoltages,electrolyteconcen-trationandoxidationtime.Fig.1showstitaniummicro-structureafterdifferenttreatments.Thesurfaceoftitaniummetalsisevenandsmoothafterbeingpolishedcarefully.However,thesurfaceiscoveredwithacompactthinoxidefilmduetotheintrinsicpropertyofvalvemetal[2].Fig.1(c)indicatesthecompactoxidebeginstodissolveanddissolutiontakesplaceonlyinselectivearea.Inaword,compactoxidefilmatthetitaniumsurfaceexistedwithorwithouttheassistanceofelectricfieldandHFsolutioncanetchtheoxidequickly.Buttheetchingspeedwasdifferentatdifferentareaoftheoxideduetothedifferentstressonthesurfaceofoxidefilmwhichwascalledselectiveetching.Fig.1(d)showsthatthenanotubestructurewasformedafteranodizingtitaniummetalsat20Vfor30minin1wt%HFsolution.
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Fig.2.I–tcurvesatconstantvoltagesduringtheanodizingprocessin1wt%HFsolution.
3.2.Current/timetransientatconstantvoltages
Fig.2showsthecurrenttransients(I–tcurves)recordedduringholdingthesamplesat5and10Vin1wt%HFsolution.
Theinitialdrasticcurrentdropisduetotheformationofcompactoxidefilmatthebeginningstagewhichelevatestheresistanceandreducescurrentdensities.
Duringanodizationthecolorofthetitaniumoxidelayerchangesfromdarkpurpletoblue,yellowandgreenswiftly.Thecolorchangingisduetotheincreasingthicknessoftitaniumoxidethroughaninterferencephenomenonbetweenthereflectedbeamfromtheoxidesurfaceandthebeamwhichpenetratesthesurfaceoxideandthenisreflectedfromtheinterfaceofthesurfaceoxideandtitaniumsubstrate[12].ThenduetothesolubilityofthetitaniumoxideinHF-containingsolutions,thecurrentdensitiesstarttoincreaseinthenextstageandporesstarttogrowrandomly.Thisisfollowedbyacompetitionbetweenthe
Fig.3.Ramanspectraofanodizedtitanium,anataseandrutile(a)anatase,(b)rutile,(c)anodizedtitanium.
growingpores,andthecurrentdensitiesstarttodecreaseagain,untilastable,orderedporegrowthhasbeenestablished.Thecurrentdensitiesshowperiodicalfluctuations.Thefluctuationsarecloselyrelatedtothegrowthanddissolutionoftheoxidefilms.Thismaybeascribedtoapassivationandadepassivationreactionthatarecompeting[13].
3.3.Ramanspectraandselectedareaelectrondiffractionofanodizedfilm
Ithasbeenreportedthatinthecaseoftheanodizationoftitanium,theoxideexhibitsaphasetransitionwithincreasingpotentialfromamorphousphaseintoacrystallinephasesuchasanataseorrutile(brookitesometimes)[1,14].Thiscrystallographictransformationiscloselyrelatedtotheelectricalbreakdown,whichisdependentontheelectro-chemicalparameterssuchastheelectrolyteconcentrationandthecurrentdensity[10].Thecrystallizationdevelopsafterbreakdown.However,thereexistssomediscrepancyaboutthevaluesofthevoltageatwhichthebeginningofthebreakdownprocessandtheevolutionofcrystallizationbegins.Disagreementsbetweenvariousstudiesmayarisefromvaryingsurfacepreparationandgrowthconditionsexemplifiedbythecurrentdensity,thechemicalcompo-sition(concentration),andtheelectrolytetemperature.InHFsolution,previousauthorsshowedtheporoustitaniawasamorphousandnobreakdownwasmentioned[1,11].Butinourstudy,Ramanspectraofnanotubetitaniafilms(Fig.3)showedaweakRamanscattering.ComparingtheRamanspectrumoftitaniumafteranodizedin1wt%HFsolutionat20Vfor30minwiththatofrutileandanatase,onecanseethatthisoxidefilmwasconstitutedwithmixtureofamorphousandcrystallineoxides.Selectedareaelectrondiffractionwasconductedforthemechanicallyremovedtitaniananotubes.Theresultshowsthatthenanotubestructuresarepartlycrystalline(Fig.4).Thisindicatesthat
Fig.4.Selectedareaelectrondiffractionofthetitaniananotubes.
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thereisanevolutionofcrystallizationduringtheanodizingprocess.AtthispointitisinagreementwithRefs.[13,15]whichreportedananatasepeakinXRDdiagramsofporoustitaniaformedin0.15wt%HF-containingsolutionandbreakdownvoltageliesbelow5V.
3.4.Structuresoftitaniummetalssurfaceunderdifferentanodizingconditions
Differentanodizingconditionsleadtodifferentsurfacestructuresoftitania.Fig.5(a)–(d)givestheFE-SEMimagesofthesurfacestructuresobtainedwithdifferentvoltages,3,20,30,and40Vin1wt%HFsolutionfor30min.
Atlowanodizingvoltage(Fig.5(a))onlypartofthesurfacewerecoveredwithporoustitaniaandnonanotubesstructureformed.Asthevoltageincreasedto20V,nanotubesstructureformedwithaninnerdiameterofw100nm(Fig.5(b)).Afterthatfurtherraisingtheappliedvoltageleadstorapiddissolutionoftitaniawhichresultsinthinningandbreakageoftheformednanotubes(Fig.5(c)and(d)).Thedissolutionoftitaniaathighervoltageindicatesfield-assistantchemicaldissolutionoftheoxideattheoxide–electrolyteinterface.Duetotheappliedelectricfield,theTi–Obondundergoespolarizationandisweakened,promotingdissolutionofthemetaloxide[10,11].Fig.6givestheFE-SEMimagesofthesurfacestructuresobtainedwithdifferenttime,5,10,30and60minat20Vin1wt%HFsolution.At5minthetitaniananotubeshave
alreadybeenformed.Increasingtheanodizingtimehaslittleeffectontheinnerdiametersofthenanotubes(Fig.6(a)–(c)).InFig.6(d)obviousdissolutionofthesurfaceoxidetakesplaceandsomenanotubesoverlapduetolongeranodizingtime.Inaddition,itisobservedthatnewporesinsideexistingporesarecreatedalongwithincreasingtheanodizingtimewhichisinagreementwiththeresultsofJinsubChoi[2](Fig.6(d)).
Fig.7givesthecross-sectionimagesofanodizedtitaniumfoilsat20Vfor15minin1wt%HFsolution.Amoreclearstructurewithcolumnarshapeisrevealedfromthesephotographs(Fig.7(a)and(b)).Cross-sectionphotographsshowthefilmthicknessofnanotubesisabout200nmunderthiscondition(Fig.7(a)).Thebottomofthenanotube(Fig.7(b))isidenticalwiththeso-calledbarrierlayer,athinoxidizedlayerseparatingtheporouslayerfromthemetalsubstrates.Someofthenanotubeshavebeendestroyedduetothemechanicalfracturing.Thedestroyedpartsshowthehollowinteriorstructure(showninFig.7(b)witharrow).Thenanotubemorphologyisalsoverifiedbytransmissionelectronmicroscopyofthemechanicallyremovedfilm(Fig.8).3.5.Growthmodels
ElectrochemistryofgrowthofporoustitaniumoxideinHFsolutionisquitedifferentfromthatofporousaluminumoxide[1,2,15].Thedevelopmentoftitaniananotubearrays
Fig.5.FE-SEMimagesofthesurfacestructuresobtainedwithdifferentvoltagesin1wt%HFsolutionfor30min(a)3V,(b)20V,(c)30V,(d)40V.
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Fig.6.FE-SEMimagesofthesurfacestructuresobtainedwithdifferenttimeat20Vin1wt%HFsolution.(a)5min,(b)10min,(c)30min,(d)60min.
onthesurfaceoftitaniummaybedescribedasfollows.Initially,athinlayerofoxideformsonthetitaniumsurface(Fig.9(a))whichcorrespondstothedrasticallydropofcurrentdensityatthefirststageofoxidation(Fig.2).Stressesexistintheoxideduetothevolumechangeaccompanyingtheoxidationoftitanium.Thestressesencouragetheformationofcrystallineoxide[16].Whenitistransformedtoadensecrystallinestructuresuchasanataseorrutile,thestressintheoxidedecreasesobviously.Thiscrystallographictransformationiscloselyrelatedtothebreakdown[12](Fig.9(b)).Selectivedissolutiontakesplaceduetodifferentstressesandcrystallinestate.Theinitiallyformedporesareirregularduetothenonuniformcorrosion.Speedofdissolutionandoxidizationisdifferentinporesofdifferentdiameters.Alongwiththeanodization,thepores’diametersturntobemoreuniform(Fig.9(c)).
Thethicknessoftheoxidefilmisthickeratthewalloftheporethanatthebottom.Consequently,theelectricalfieldintensityattheporebottomismuchhigherthanthatatthewall,titaniawillbeconsumedathigherratenearthebottomofthepore,whichresultsinfurtherporegrowthtowardtheTisubstrate.(Fig.9(b)–(e))[1,10].AstheTi–Obondenergyishigh(323kJ/mol),inthecaseoftitaniaitisreasonabletoassumethatonlyporeshavingthintitaniawallswillbeformedduetorelativelylowionmobilityandrelativelyhighchemicalsolubilityoftheoxideintheelectrolyte,henceunanodizedmetallicportionscaninitiallyexistbetweenthepores.Astheporesbecomedeeper,the
electricfieldinthesemetallicregionsincreases,enhancingthefield-assistedoxidegrowthandoxidedissolution,andtheinterporesvoidsstartforming(Fig.9(e)).Therefore,bothvoidsandtubesgrowinequilibriumtofinallyyieldatubularstructure(Fig.9(f)).Ifthetitaniumoxideeitherinthewallorattheporebottomdissolvesatabalancerate,theporedepthkeepsconstantanddoesnotchangewithanodizingtime.Inaddition,breakdownsometimesoccursagaininsidetherepassivatedpores(Fig.9(g)and(h));itlookslikethereistheformationofsmallporesinsidetheexistingpores(Fig.6(d)).Thiskindofstructureoftentakesplacewhenincreasinganodizingtimeandusingstrongacid(forexampleHNO3)aswellasHFacid.Thiscanbeexplainedthatlongeranodizingtimeandstrongeracidwillfacilitatecrystallizationandbreakdownoftitania[12].
4.Conclusions
ThispaperinvestigatestheoxidationprocessoftitaniuminHFaqueoussolutionthroughconstant-voltageexper-iments.Underoptimizedelectrolyteandoxidationcon-ditions,wellorderednanotubesoftitaniawerefabricated.Topologiesoftheanodizedtitaniumchangeremarkablyalongwiththechangingofappliedvoltages,electrolyteconcentrationandoxidationtime.Incontrasttoporousaluminamembranes,adifferentmorphologywasobtainedinthecurrentstudy.Thenanotubearraysoftitaniaarevery
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Fig.7.Fe-SEMimagesofanodizedtitaniumfoilsat20Vfor15minin1wt%HFsolution(a)cross-section(b)bottomandcross-section.
interestingandmayhavemanypotentialapplicationsincatalytic,gas-sensing,optic,electronic,photochemicalandbiomedicalmaterials.Apossiblegrowthmechanismhasalsobeenpresented.
Fig.8.TEMimagesofanodizedtitaniumfoilsat20Vfor30minin1wt%HFsolution.
Fig.9.Schematicdiagramofformationoftitaniananotubearrays(a)oxidelayerformation,(b)burstofoxidebytheformationofcrystallites(poreformation),(c)growthoftheporesduetofield-assistantdissolutionoftitania,(d)immediaterepassivationofporetips,(e)voidsformationinthemetallicpartbetweenthepores,(f)formationofnanotubesoftitania,(g)burstofrepassivatedoxide,(h)formationofnewporesinsideexistingpores.
Acknowledgements
ThisworkissupportedbytheHighTechnologyResearchandDevelopmentProject,ChinaunderGrantNo.863-2001AA325010andtheMinistryofSciencesandTechnologyofChinathrough973-ProjectunderGrantNo.2002CB613301.
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