爱华网PublishedonWeb12/11/2003
ReversibleSuper-hydrophobicitytoSuper-hydrophilicityTransitionofAligned
ZnONanorodFilms
XinjianFeng,LinFeng,MeihuaJin,JinZhai,LeiJiang,*andDaobenZhu
CenterforMolecularSciences,InstituteofChemistry,ChineseAcademyofSciences,Beijing100080,P.R.China
ReceivedSeptember21,2003;E-mail:jianglei@iccas.ac.cn
Wettabilityisaveryimportantpropertygovernedbyboththechemicalcompositionandthegeometricalstructureofsolidsur-faces.1Super-hydrophobicsurface(withwatercontactangle(CA)largerthan150°)andsuper-hydrophilicsurfaces(CAcloseto0°)havebeenextensivelyinvestigatedduetotheirimportanceforin-dustrialapplications.2-6Recently,withthedevelopmentofsmartdevices,suchasintelligentmicrofluidicswitch,reversiblycontrol-lingthesurfacewettabilityhasarousedgreatinterestandbeenrealizedbymodifyingthesurfacewithstimuli-responsiveorganiccompounds.7However,reversibleswitchingbetweensuper-hydro-phobicityandsuper-hydrophilicityhasneverbeenreported.Long-termdurabilityisalsodifficulttobeexpectedbecauseofpoorchem-ical,thermal,andmechanicalstabilitiesoftheorganiccompounds.Beingawideband-gapsemiconductor,ZnOpossessesdiverserangeofhightechnologicalapplications.8,9However,nearlyalloftheapplicationsarebasedonitsoptical,electronic,andacousticproperties.Inthispaper,controllablewettabilityofalignedZnOnanorodfilmsisreported.Thisinorganicoxidefilmsshowsuper-hydrophobicityandsuper-hydrophilicityatdifferentconditions,andthewettabilitycanbereversiblyswitchedbyalternationofultraviolet(UV)irradiationanddarkstorage.Thiseffectisbelievedtobeduetothecooperationofthesurfacephotosensitivityandthealignednanostructureofthefilms.SuchspecialwettabilitywillgreatlyextendtheapplicationsofZnOfilmstomanyotherimportantfields.
ThealignedZnOnanorodsweresynthesizedviaatwo-stepsolutionapproach.First,ZnOsol,preparedaccordingtothemethodofSakohara,10wasspincastontoaglasswaferseveraltimesandannealedat420°Ctopreparea50-100nmthickfilmofcrystalseeds.Thesol-gelmethodwasadoptedherebecauseofitssimplefacilityandlowcost.Inparticular,itderivesfilmswithpreferentialc-axialorientation,11whichisfavorableforthesubsequentgrowthofZnOnanorods.Thentheas-preparedwaferwassuspendedinanaqueoussolutionofzincnitratehydrate(0.025M)andmethenamine(0.025M)at90°Cfor3h,anditwasremovedfromthesolution,rinsedwithdeionizedwater,dried,andstoredinthedarkundercleanairforseveraldays.Figures1aand1baretypicalfield-emissionscanningelectronmicroscope(FE-SEM)top-imagesoftheaspreparedfilmsatlowandhighmagnifications,respectively,showingahighlyuniformanddenselypackedarrayofnanorods.Thenanorodshaveflathexagonalcrystallographicplanesprojectingoutofthecrystalseedslayer,andtheirdiametersareof50-150nm.Figure1cisacross-sectionalviewofthesample,showingthatthenanorodsgrowalmostperpendicularlyontothesubstrates,andtheirlengthsareabout1.2μm.AsconfirmedbyX-raydiffraction(XRD)witharemarkablyenhanced(002)peak(Figure1d),thesurfaceofthefilmsisthe(001)planeofthenanorods.Thewettabilitywasevaluatedbythewatercontactanglemeasurementoftheas-preparedfilms.Figure2a(right)showsasphericalwaterdropletwithawaterCAof161.2(1.3°.UponUV(obtainedfroma500WHglampwithafiltercenteredat365
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Figure1.(a,b)FE-SEMtop-imagesoftheas-preparedZnOnanorodfilmsatlowandhighmagnifications,respectively.(c)Cross-sectionalviewofthealignedZnOnanorods.(d)XRDpatternoftheas-synthesizednanorod
films.
Figure2.(a)PhotographsofwaterdropletshapeonthealignedZnOnanorodfilmsbefore(left)andafter(right)UVillumination.(b)Reversiblesuper-hydrophobic-super-hydrophilictransitionoftheas-preparedfilmsunderthealternationofUVirradiationanddark
storage.
(10nm)irradiationfor2h,thewaterdropletspreadoutonthefilm,resultinginaCAofabout0°(Figure2a,right).Theseresultsindicatethatthewettabilitychangesfromsuper-hydrophobicitytosuper-hydrophilicity.AftertheUVirradiatedfilmswereplacedinthedarkfor7days,anewwaterdropletwasusedtomeasurethesurfacewettability,andthesuper-hydrophobicityofthenanorodfilmswasobtainedagain.Thisprocesshasbeenrepeatedseveraltimes,andgoodreversibilityofthesurfacewettabilitywasobserved(showninFigure
2b).
10.1021/ja038636oCCC:$27.50?2004AmericanChemicalSociety
Tothoroughlyunderstandthereversiblesuper-hydrophobicitytosuper-hydrophilicitytransitionofthealignedZnOnanorodfilms,thesurfacefreeenergyandthesurfaceroughness,whicharetwomainfactorsgoverningthesurfacewettability,areconsidered.Itiswell-knownthatonlywhenthesurfacefreeenergiesofthevariouscrystallographicplanesdiffersignificantlycouldananisotropicnanorodgrowthberealized,12andafastgrowingplanegenerallytendstodisappearleavingbehindslowergrowingplaneswithlowersurfaceenergy.FortheanisotropicZnOnanorod,thevelocitiesofcrystalgrowthindifferentdirectionswerereportedtobe[-100]>[-101]>[001]≈[00-1].13Figure1showthattheZnOnanorodsarealmostverticallyorientedwiththewell-faceted(001)endprojectingout.ComparedwithotherrandomorientationofZnOnanocrystalfilms,14,15theas-preparedfilmshavethelowestsurfacefreeenergy.Figure1alsoshowsthatthenanorodsgrowseparatelyonthesubstrate,andaircanbepresentinthetroughsbetweenindividualnanorods.Thehydrophobicityofaroughsurfacecanbeintensifiedbyincreasingtheproportionofair/waterinterface,1be.g.,super-hydrophobicsurfaceshavebeensuccessfullyfabricatedbythealignedcarbonnanotubesandpolymernanofibers.4Thenanorodsandaircompositeroughsurfacestructureinthiscasecanalsogreatlyenhancethehydrophobicityofthefilms.Accord-ingly,boththelowersurfacefreeenergyandthehighersurfaceroughnesscontributetothesuper-hydrophobicityoftheas-preparedfilms.
Asreported,15UVirradiationwillgenerateelectron-holepairsintheZnOsurface,andsomeoftheholescanreactwithlatticeoxygentoformsurfaceoxygenvacancies.Meanwhile,waterandoxygenmaycompetetodissociativelyadsorbonthem.Thedefectivesitesarekineticallymorefavorableforhydroxyladsorp-tionthanoxygenadsorption.Asaresult,thesurfacehydrophilicityisimproved,andthewaterCAofarelativeflatZnOsurfacechangesfrom109°to5°.15Foraroughsurface,waterwillenterandfillthegroovesofthefilms,leavingonlytheuppartofthenanorodsnotincontactwiththeliquid,whichisthethree-dimensionalcapillaryeffectofaroughsurface.6ThiseffectresultsinawaterCAofabout0°.Namely,combiningwiththeroughstructure,theUVirradiationcausesthesurfacewettabilityofalignedZnOnanorodfilmstochangefromsuper-hydrophobicitytosuper-hydrophilicity.Ithasbeendemonstratedthatthesurfacebecomesenergeticallyunstableafterthehydroxyladsorption.Whiletheoxygenadsorptionisthermodynamicallyfavored,anditismorestronglybondedonthedefectsitesthanthehydroxylgroup.15AsimilarresulthasalsobeenobservedontheTiO2surface.5b,16Therefore,thehydroxylgroupsadsorbedonthedefectivesitescanbereplacedgraduallybyoxygenatomswhentheUV-irradiatedfilmswereplacedinthedark.Subsequently,thesurfaceevolvesbacktoitsoriginalstate(beforeUVirradiation),andthewettabilityisreconvertedfromsuper-hydrophilicitytosuper-hydrophobicity.
Onthebasisoftheaboveanalysis,itcanbeconcludedthatthereversibleswitchingbetweensuper-hydrophilicityandsuper-hydrophobicityisrelatedtothecooperationofthesurfacechemicalcompositionandthesurfaceroughness.Theformerprovidesaphotosensitivesurface,whichcanbeswitchedbetweenhydrophi-licityandhydrophobicity,andthelatterfurtherenhancesthese
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properties.Inaddition,thereversibleconversionsofthesurfacewettabilityproceedonlybytheadsorptionanddesorptionofsurfacehydroxylgroupsattheoutmostlayerofoxidefilms,5bwhilethestructurebelowtheoutmostlayerremainsstable,freefromchangesinchemicalconditions.Therefore,itisreasonablethatthereversiblewettabilityswitchingpropertiesoftheas-preparedfilmsexhibitlong-termdurability.
Insummary,thewettabilityofalignedZnOnanorodfilmswasinvestigated.Reversiblesuper-hydrophobicitytosuper-hydrophi-licitytransitionwasobservedandintelligentlycontrolledbyalternationofUVilluminationanddarkstorage.Thisreversibletransitionofsurfacewettabilityisacompletelynewconceptforpreparingsmartfilms.Thisstrategycanbeextendedtootherstimuli-responsivesurfaceswithsimilarnanostructureandhigherstability,whichiscertainlysignificantforfutureindustrialapplica-tions.
Acknowledgment.TheauthorsthanktheStateKeyProjectforFundamentalResearch(G1999064504)andtheSpecialResearchFoundationoftheNationalNaturalScienceFoundationofChina(29992530,20/25/02)forcontinuingfinancialsupport.
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