StabilityoftheSilicaSolsPreparedby

StabilityoftheSilicaSolsPreparedbyAcid/BaseTwo-StepCatalyticSol-GelProcessDongDong,LiuXiaobo,HuWencheng(StateKeyLaboratoryofElectronicThinFilmsandIntegratedDevices,UniversityofElectronicScience&TechnologyofChina,Chengdu610054,China)Abstract:Aacid/basetwo-stepcatalyticsol-gelprocesswasemployedtofabricateSiO2films.ThesilicasolsforSiO2filmswereobtainedbyadmixingaacid-solandabase-sol,andthestabilityofthemixedsolswasmainlyinvestigated.TheSEMimageshowedporousandcrack-freefilms.Theeffectsofthemolarratioofbase-sol/acid-solandtheagingtimeofacid-solandbase-solonthegelationtimeofthemixedsolswerediscussed.Thestabilityofthesilicasolswasakeyfactorforthemultiple-coatingprocess.Keywords:silicasols;two-stepcatalyticprocess;stability;gelationtimeCLCnumber:TQ127.2Documentcode:AArticleID:1002-185X(2010)S2-061-04Thesol-gelmethodhasbeenprovedtobeaquiteversatiletechnologywhichisemployedtoproduceawidevarietyofmaterials,suchasmetallic,inorganic,organicandhybridmaterialswithdifferentfunctions[1].PorousSiO2filmisagreatlyattractivematerialforitsapplicationinthefieldsofmicroelectronicsandoptoelectronicsduetoitsexcellentthermal-insulatingproperty,lowdielectricconstantandcontrollablerefractiveindex[2-5].Theapplicationofthesol-gelmethodinfabricatingSiO2filmshasbeenahottopicandattractedmuchattentionfordecades.SiO2filmsareusuallypreparedinsol-gelprocessthroughthehydrolysisandcondensationofTEOSunderacidicoralkalinecondi-tions.Theacid-catalyzedprocessresultsinasilicasolwiththestructureoflinearlinkthatoccasionallycross-linksandthebase-catalyzedprocessformsasolconsistingofhighlybranchedclusters[6].Sothefilmbyacid-catalyticprocessisdenserandthinnerandtendstocrack,whilethedepositedfilmisporousandnoteasytocrack,butithasarelativelypooradhesionandtendstoformsilicapowders.Therefore,atwo-stepcatalyzedprocesswasemployedinpreviousreferences[7-9].Usually,thedilutedammoniaisaddedtotheacid-catalyzedsolsolutiontopromotethecondensationreaction.Anotherwayistomixtheacid-catalyzedsolwithbase-catalyzedsol.Wuetall[10]usedthismethodtopreparethenano-porousSiO2filmswithcontrollablerefractiveindex.Duringtheprocessoffabricatingfilms,thepropertiesofthesolsplayanimportantroleinthestructuresandpropertiesofSiO2films.Muchworkhasbeencarriedouttostudythegrowthofthesilicaparticlesandtheinfluenceoftheprecursorconcentration,temperature,pHvalueandtheadditivesonthepropertiesofsilicasols[11-15].Thetwo-stepcatalyzedprocessacceleratesthereactiontoformsilicanetwork,butitisadversetothestabilityofthesols.WepreparedporousSiO2filmsviaacid/basetwo-stepcatalyticsol-gelprocessandthispaperfocusedonthestabilityofthesilicasolsobtainedbymixingacid-catalyzedsolandbase-catalyzedsol.1ExperimentalTEOSwasusedasprecursor,andthesolventwasamixtureof2-methoxyethanol,isopropanolandethanoltoformavolatilizationgradientduringtheheatingstageandtoreducethecrackformation.Inthefirststep,TEOS,waterandthesolventweremixed,andthenstronglystirredunderammonia(NH4OH)catalysis(pH~6.4).After6h,thesolsolution(referredasbase-sol)wasagedatroomtemperatureforaweekatleast.Atthesametime,TEOSwasdilutedinthemixturesolventandpartiallyhydrolyzedwithwaterunderacidicconditionsofnitricacid(HNO3)(pH~3.5).Themixedsolutionwasstirredat60°Cfor90mintopromotethehydrolysisreaction.Aftercontinuouslystirredforhoursatroomtemperature,thesolwasformed(referredasacid-sol).BothsolswerepreparedwiththeTEOSconcentrationof40wt%andthemolarratioofH2O/TEOSwasabout2.Inthefollowingstep,twotypesofsolsweremixedbyvaryingthemolarratiosofbase-sol/acid-solfrom0to10,andstirredfor6-10hatroomtemperature.Priortocoating,theobtainedsolsolutionswerefilteredby0.22µmmilliporefiltertoeliminatethecontaminations.TheSiO2filmsweredepositedbyspinning-coatingat2500r/minfor30s.Theresultedwetfilmswerethermallytreatedupto650°Cinair,andthethickfilmswerepreparedbymultiple-coatingprocedure.Fouriertransforminfraredspectroscopy(FT-IR)spectrawereobtainedwiththeFT-IRinstrument(ShimadzuFTIR8400)inthe400-4000cm-1wavenumberrangetodeterminethepropertiesofthesilicasols.ThemorphologiesofSiO2filmswereinvestigatedbyscanningelectronmicroscopy(SEM,JSM-5900LV,Japan).2ResultsandDiscussions2.1FT-IRspectraThechemicalspeciesandchemicalbondingstatewereinvestigatedbytheFT-IRspectra.Fig.1showstheFT-IRspectraoftheacid-sol,thebase-solandthemixedsolinamolarratioofacid-sol/base-sol~1:1.Comparingthosecurves,itisfoundthattheacid-solhasabroadandstrongabsorptionpeakrelatedtotheSi-Ostretchingvibrationcenteredat~1100cm-1duetotherelativelycompletehydrolysisofTEOS,whilethebroadpeakbecomesnarrowandisdividedtotwopeaksat1060and1122cm-1forthebase-sol.Itindicatesthatthestructureofsilicacolloidsunderbasicconditionisdifferentfromthefibriformstructureunderacidiccondition.Thebase-solmayformtwostructures:cagestructureandnetworkstructurecorrespondingtotheSi-Ovibrationat1122and1060cm-1[16],respectively.Bymixingthetwosamples,theabsorptionpeakat~1100cm-1waspresented,whichistheintervenientlocationbetweentheabovetwocases.Fig.1FT-IRspectraofacid-sol(a),mixedsolinamolarratioofacid-sol/base-sol1:1(b),andthebase-sol(c)2.2Effectofbase-sol/