TiO2nanotubesnanochannelsandmesospongeSelforganize

Pleasecitethisarticleinpressas:D.Kowalski,etal.,NanoToday(2013),(2013)xxx,xxx—xxxAvailableonlineat:Self-organizedformationandapplicationsDamianKowalskia,1,DoohunKimb,2,PatrikSchmukia,c,∗aDepartmentofMaterialsScience,UniversityofErlangen-Nurnberg,Martensstrasse7,D-91058Erlangen,GermanybBatteryResearchCenter,KoreaElectrotechnologyResearchInstitute(KERI),Changwon641-120,SouthKoreacDept.ofChemistry,KingAbdullazizUniversity,Jeddah,Saudi-ArabiaReceived23December2012;receivedinrevisedform26April2013;accepted30April2013KEYWORDSTiO2nanotubes;Nanopores;Non-thickness-limitedoxides;Anodization;Self-organizednanostructures;NanofabricationSummaryAnodizationofvalvemetalsandalloysisapowerfultooltocontrolnanoscalearchitectureformanymetaloxides.Exceptforaluminum,themostexploredsystemisself-organizedoxideformedontitanium,namelyself-organizedTiO2nanotubes,becauseoftheuniquecombinationofgeometrywiththesemiconductivenatureoftitaniathatisapplicableinphotocatalysis,lightharvestingsystems,electrochromicdevices,batteries,matrices,tem-plates,filtrationmembranes,andbio-compatiblematerials.Inthiscontribution,wereviewrecentadvancesintheformationofnanostructuredoxidesintheformofnanotubes,nanoporeswithathrough-holemorphology,mesosponges,nanochannelsandmicroconesgrownonTi,Nb,Ta,Zr,Hf,W,Vandtheiralloys.Wediscussmechanismsoftheirformation,keyfunctionalfeatures,anddescribetheirapplicationsinvariousfieldsofchemistryandelectrochemistry.©2013ElsevierLtd.Allrightsreserved.IntroductionAnodizationofsomemetalsandalloys(suchasTi,Nb,Ta,Zr,Hf,W,Al)leadseithertotheformationofcompactoxidelayers,socalledbarrier-typeanodicoxides,orporousoxidesintheformofnanopores,nanotubes(orderedordisorderedlayers),mainlydependingontheelectrolyteused(Fig.1).Theoxidelayersformedaredielectricsorsemiconductors∗Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](P.Schmuki).1Theseauthorscontributedequallytothemanuscript.2Presentaddress.Tel.:[phone];fax:[phone].ofamorphousorpolycrystallinenature.Anodizationinmanyaqueouselectrolytesleadstotheformationofbarrier-typeanodicoxidefilms.Modificationoftheelectrolytecompo-sitioninsuchawaythatdirectejectionofmetalionsintotheelectrolyte(‘‘dissolution’’)takesplaceduringanodiza-tion(forexampleacidicpHforAlorinthepresenceoffluorideionsformanytransitionmetals)resultsinatran-sitionofbarrier-layerstorandomporousorself-organizednanoporous/nanotubularoxidestructures[1—7].Thefirstworksonvirtuallyperfectself-organizedanodicoxidesaredatedto1995whenMasudaandFukudareportedthatunderoptimizedelectrochemicalconditionsahighdegreeofarrangementofhexagonalaluminananoporescanbeachieved[2].Thesenanostructureshavebeenpaidremarkableattention,inparticularinviewoftheir1748-0132/$—seefrontmatter©2013ElsevierLtd.Allrightsreserved.:D.Kowalski,etal.,NanoToday(2013),:(a)barrier-type-oxidecanbeformedinmostoftheaqueouselectrolytes,(b)mesosponge,microcones,nanochannelsformedinhot-glycerol-phosphateelectrolyte,(c)nanotubesformedinfluorideelectrolytes,and(d)oxidewithopenporouscharacter.applicationsasatemplatefortheformationofnanowiresandnanotubes[8—14].Amongallothermetaloxides,themostinvestigatedisTiO2becauseofitsbio-compatibilityanduniquesemiconductivenaturethelatterusedprimarilyinsolarcellsandphotocatalysis[15—22].Thefirstself-organizedanodicoxidesintheformofporeswereformedonTiandTiAlalloysbyZwillingetal.in1999usingdilutefluoridecontainingelectrolytes[23,24].Thisisthefirstrec-ognizedreportonself-organizedtitaniaandtitania—aluminananostructureswhichwereformedinelectrolytescontain-ingfluorideions.Thelaterworksdemonstratedclearshapeofnanotubeswhichwereclosedatoneendandwereself-organizedontitaniumsubstrate[25].ThesenanotubeswereformedinaqueouselectrolytesandtheirgeometrycouldstronglybeimprovedbymodificationoftheelectrolytecompositionanditspH[26—29].Theirshapewasnot‘‘idealtubular’’andthelengthwaslimitedtoafewmicrometers.Oneofthemostcrucialchangesinimprovingthenanotubesmorphologywastheintroductionoforganicelectrolytes[30].ByanodizationofTiinelectrolytessuchasglycerolorethyleneglycolwithsmallcontentofwaterandfluorideionsnanotubesofaveryuniformshapewereformed.Thesenanotubesweremuchmoreorganizedandtheycouldbegrowntohundredsofmicrometersinlengthreachingtheaspectratioof2000—3000[30,31].Thisdilutefluorideelectrolyteapproach(mostlyinorganicelectrolytes)wassuccessfullytransferredtocreatehighlyorderedoxidestructuresonothermetalssuchastoZr,Hf(tubes),Ta,Nb(pores)orFe,V(tubesandpores)—forrecentoverviewssee[5]andforVsee[32].OvertheyearsitPleasecitethisarticleinpre