Photoluminescence of Silica GelWord下载.docx
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Abstract
SilicaGelisacommonmaterialthatiseconomicandeasytoproduce.Itiswidelyusedasdesiccantforitsgreatsurfaceareaandabsorptioncapability.Recentlythephotoluminescenceofsilicagelhasbroughtgreatinterestwithsuccessfulresultsintuningtheemissionoftheluminescence.ThepropertyofthesilicagelandthemechanismofphotoluminescenceareintroducedinthisreviewwithdetailedmodelofODCmodelwhichexplainthephotoluminescenceinsilicagel;
VariousmethodsandfactorstotunethephotoluminescenceareintroducedwithspecificdetailoftheexperimentthatusingAgdopingtoobtainnearwhitelightphotoluminescence.
SilicaGel
Silicagelisagranular,vitreous,porousformofsilicondioxidemadesyntheticallyfromsodiumsilicate.Itwasobtainedasascientificcuriosityasearlyasthe1640sandwasusedinWorldWarIfortheadsorptionofvaporsandgasesingasmaskcanisters.InWorldWarII,silicagelwasindispensableinthewareffortforkeepingpenicillindry,protectingmilitaryequipmentfrommoisturedamage,asafluidcrackingcatalystfortheproductionofhighoctanegasoline,andasacatalystsupportforthemanufactureofbutadienefromethanol,feedstockforthesyntheticrubberprogram.ThesyntheticrouteforproducingsilicagelwaspatentedbychemistryprofessorWalterA.PatrickatJohnsHopkinsUniversityin1918.
Silicagelhasaveryhighspecificsurfaceareaaround800m2/gandanaverageporesizeof2.4nanometersthusithasastrongaffinityforwatermolecules.Itisusefulasadesiccant(dryingagent),oftendescribedas"
absorbing"
moisture,whichmaybeappropriatewhenthegel'
smicroscopicstructureisignored,asinsilicagelpacksorotherproducts.However,materialsilicagelremovesmoisturebyadsorptionontothesurfaceofitsnumerousporesratherthanbyabsorptionintothebulkofthegel.Silicagelisalsousedinfoodadditive,chromatographyandplasticsurgeryimplants.
Recently,thephotoluminescencepropertiesofsilicagelsbroughtupgreatinterestsbecauseitseconomic,efficiencyandtunable.Ithasgreatgreatpotentialinlightingindustry.
MechanismPhotoluminescence
Photoluminescence(PL)islightemissionfromanyformofmatteraftertheabsorptionofphotons.Itisoneofmanyformsofluminescence(lightemission)andisinitiatedbyphoto-excitation[1].Followingexcitationvariousrelaxationprocessestypicallyoccurinwhichotherphotonsarere-radiated.
Fig.1Excitationandrelaxationofphotolumin
Fig.1showsatypicalprocessofexcitationandrelaxationofPL.Whenasemiconductorisexcitedwithalight-sourcethatprovidesphotonswithanenergylargerthanthebandgap[2],theincominglightexcitesapolarizationthatcanbedescribedwiththesemiconductorBlochequations.Oncethephotonsareabsorbed,electronstransittoconductionbandandleaveholesinvalencebandswithfinitemomenta.Theexcitationsthenundergoenergyandmomentumrelaxationtowardsthebandgapminimum.TypicalmechanismsareCoulombscatteringandtheinteractionwithphonons.Finally,theelectronsrecombinewithholesandemitphotonswiththesameenergyasthebandgapasthesemiconductor.
PhotoluminescenceinSilicaGel
Itiswellestablishedthatthephotosensitivityisconnectedtoalight-inducedtransformationofpre-existingpointdefectsinglassmatrixortoacreationofnewones[3].Particularrolesareplayedbyspecificdefects,connectedtopartiallyreducedSiorGespecies,theso-calledoxygendeficiencycenters(ODCs).
Oxygen-DeficiencyCenters
ThestructuralmodelsofSi-andGe-relatedODCsremaincontroversial.Themainreasonforthissituationissimple:
ODCsarediamagneticandthusinvisibletoelectronparamagneticresonance(EPR)techniques.EPRhasprovidedsofarmostofthestructuralinformationondefectsinSiO2-basedglasses.Thethreefundamentalcenterswhichformthebasisofthepresentunderstandingofdefectsinvitreoussilica,theE’-center[4],theperoxyradical(POR)[5]andthenon-bridgingoxygenholecenter(NBOHC)[6]havebeenalldiscoveredandcharacterizedbyEPR.
ThemainopticallyactiveoxygendeficiencyrelateddefectsinsilicaarethefamilyofdifferentparamagneticE’centersandtwokindsofdiamagneticoxygendeficiencycenters,mostcom-monlydenotedasODC(I)andODC(II).
Fig.2showsthetypicalmodelsofoxygendeficiencycenters.
Fig.2.SomevariantsofE’centersinSiO2.[3]
(A)fragmentofaperfectlattice;
(B)asymmetricallyrelaxedoxygenvacancywithanunpairedelectronlocalizedinasp3-likeorbitalofasingleSiatom(the‘classic’modelofE1’centerinα-quartzorEγ’centerinsilica);
(C)refinedmodelforE1’orEγ’centersshowninitslowest-energystatefeaturinganoutwardsrelaxationofthepositivelychargedSiat