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1、Chapter2AqueousandNonaqueousSol-GelChemistry2.1IntroductionThemostwidelyusedsynthetictechniqueforbulkmetaloxideshasbeentheceramicmethod,whichisbasedonthedirectreactionofpowdermixtures.Thesereactionsarecompletelycontrolledbythediffusionoftheatomicorionicspeciesthroughthereactantsandpro
2、ducts.Tobringthereactionpart-nerssufficientlyclosetogetherandtoprovidehighmobility,thesesolidstateprocessesrequirehightemperatureandsmallparticlesizes.Althoughtheharshreactionconditionsonlyleadtothermodynamicallystablephases,pre-ventingtheformationofmetastablesolids,theseapproachesgave
3、accesstoalargenumberofnewsolidcompounds,enablingthedevelopmentofstructure-propertiesrelationships.However,incomparisontoorganicchemistry,wherehighlysophisticatedsyntheticpathwaysareemployedtomakeandbreakchemicalbondsinacontrolledway,theceramicmethodisarathercrudeapproach.Itistherefor
4、enosurprisethatforthesize-andshape-controlledsynthesisofnanoparticlesespeciallyliquid-phaseroutesrepresentthemostpromisingalternatives.Incontrasttosolid-stateprocesses,butanalogoustoorganicchemistry,“chimiedouce”approachesofferthepossibilitytocontrolthereactionpathwaysonamolecularleve
5、lduringthetransformationoftheprecursorspeciestothefinalproduct,enablingthesynthesisofnanoparticleswithwell-definedanduniformcrystalmorphologiesandwithsuperiorpurityandhomogeneity[12].Amongthevarioussoft-chemistryroutes,sol-gelpro-cedureswereparticularlysuccessfulinthepreparationofbulkm
6、etaloxides(e.g.,ceramics,glasses,filmsandfibers)[33,20],andthereforetheyhavealsobeenappliedfornanoparticlesynthesis.Butinspiteofgreatefforts,thenumberofoxidicnanoparticlesobtainedbysol-gelchemistryisstillrathersmallcomparedtothevarietyofcompoundsobtainedviapowderroutes.Itturnedoutthatin
7、manycasesasynthesisprotocoldevelopedforabulkmetaloxidecouldnotdirectlybeadaptedtoitscorrespondingcounterpartonthenanoscale.Thereasonsforthisobservationaremanifold.Aqueoussol-gel782AqueousandNonaqueousSol-GelChemistrychemistryisquitecomplex,ontheonehandduetothehighreactivityofthemetal
8、oxideprecurs
