Photoluminescence of Silica Gel.docx

Photoluminescence of Silica Gel.docx

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PhotoluminescenceofSilicaGel

PhotoluminescenceofSilicaGel

MokeMao1568610

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