关于LDHs的专业英语课程论文.docx

关于LDHs的专业英语课程论文.docx

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关于LDHs的专业英语课程论文

ResearchDevelopmentonStructureandmorphologysynthesisoflayereddoublehydroxides

Abstract:

LayeredDoubleHydroxides（LDHs）orhydrotalcite-likecompounds（HTLc）areaclassofanionicclaysmaterialswhosestructureisbasedonbrucite（Mg（OH）2）-likelayersinwhichsomeofthedivalentcationshavebeenreplacedbytrivalentmetalliccationsgivingpositively-chargedsheetsandhydratedinterlayerdomainscontaininganionicspecies.Thischaracteristicresultsinmuchmoredifficultyincontrollingitsstructureandmorphologythansinglemetalhydroxides.ThisreviewprovidesanoverviewoftheLDHsynthesismethods,andtherecentadvancementthathasbeenachievedinthemorphologysynthesisoflayereddoublehydroxides.

Keywords:

Morphologycontrol;Layereddoublehydroxides;Synthesis;Template

1.Introduction

Layereddoublehydroxides（LDHs）arelamellarmixedhydroxidescontainingpositivelychargedmainlayersandundergoinganionexchangechemistry[1].Theexpectedhexagonalplate-orflake-likecrystalliteswithdiametersfromnanometerstomicronsweregrownbyvariousconventionalpreparationmethodssuchasco-precipitation[2],ureahydrolysis[3],separatenucleation[4]andaging,ion-exchange[5],metaloxideprecursor[6]method,reflectingtherigidityandhardnessofLDHs’hostlayer,whichbadlylimitedtheapplicationoflayereddoublehydroxidesincatalysis,adsorptionandotherdomains.Inrecentyears,however,Themorphologyandsizecontrolofinorganicsolidshasachievedgreatdevelopmentinthematerialscience.Anumberofimaginativeroutestosynthesizeinorganicmaterialswithunusualmorphologyhavebeenrecentlyreported[7].manystudieshavebeendevotedtoinvestigatingtheparametersontheStructureandmorphologyofLDHs.Fortunately,withthedevelopmentofbio-mineralization[8],confinedgrowthofnano-particles[9],theinorganic/organicself-assembly[10]andtemplatechemistry[11],Templatechemistryhasbeendevelopedtotailorthematerialformationofminerals[12],poroussolids[13],polymers[14],etc.Sometemplatesthatconsistofbio-macromolecules[15],suchasvesicles[16],micelles[17],micro-emulsions[18],organo-gels[19],andconformationaldefinedpeptides[20],polymers[21],areusedtodirecttheformationofbio-mineralsandhaveledtomaterialswithspecificsize,morphologyandproperties.SeveralnewmethodshavebeenusedtocontrolthemorphologysynthesisofLDHs.

2.CompositionandstructureofLDHs

LDHsareaclassoftwo-dimensionalnano-structuredanionicclays.ThestructureofLDHscanbedescribedasacadmiumiodide-typelayeredhydroxide（e.g.,brucite,Mg（OH）2）,whereafractionofthedivalentcationscoordinatedoctahedrallybyhydroxylgroupshavebeenisomorphouslyreplacedbytrivalentcations,givingpositivelychargedsheets.Thenetpositivechargeiscompensatedbyanionsintheinterlayerregionbetweenthebrucite-likesheets.Somehydrogen-bondedwatermoleculesmayoccupythefreespaceinthisinterlayerregion.ThestructureofLDHsandatypicaloctahedralunitareshowninFig.1[22].Thebasalspacing（c`）isthetotalthicknessofthebrucite-likesheetandtheinterlayerregion.TheoctahedralunitsofM2+orM3+（sixfoldcoordinatedtoOH-）shareedgestoforminfinitesheets.Thesesheetsarestackedontopofeachotherandareheldtogetherbyhydrogenbonding.LDHscanberepresentedbythegeneralformula[M1-x2+Mx3+（OH）2]x+（An-）x/n-mH2O,whereM2+andM3+aredivalentandtrivalentcations,respectively;thevalueofxisequaltothemolarratioofM3+/（M2++M3+）,whereasAistheinterlayeranionofvalencen.TheidentitiesofM2+,M3+,x,andAn-mayvaryoverawiderange,thusgivingrisetoalargeclassofisostructuralmaterialswithvariedphysicochemicalproperties[23].TheparentmaterialoftheseanionicclaysisthenaturallyoccurringmineralhydrotalcitewhichhastheformulaMg6Al2（OH）16CO3-4H2O.

Differentstackingarrangementsofthebrucite-likesheetshavebeenobserved.Thebrucite-likesheetscanbestackedeitherwithtwolayersperunitcellinhexagonalsymmetry（manasseite）orwiththreelayersperunitcellinrhombohedralsymmetry（hydrotalcite）,orinlesssymmetricalarrangements.Ingeneral,LDHsofrhombohedralsymmetryhavemainlybeenfound.M2+（Mg2+,Fe2+,Co2+,etc.）andM3+（Al3+,Cr3+,Fe3+,etc.）ionsthathaveionicradiussimilartothatofMg2+canbeaccommodatedintheholesoftheclose-packedconfigurationofOHgroupsinthebrucite-likesheetstoformLDHs[24].Inaddition,LDHscontainingmorethantwospeciesofthesecondcationhavebeensynthesized.AnotherclassofLDHs,whichcontainsmonovalentandtrivalentmatrixcationssuchas[LiAl2（OH）6]+A--mH2O,hasalsobeenprepared.ThechargedensityandtheanionexchangecapacityoftheLDHsmaybecontrolledbyvaryingtheM2+/M3+ratio.ThemostcommonanionfoundinthenaturallyoccurringLDHsiscarbonate.Inpractice,however,abroadspectrumofcharge-balancinganionsmaybeincorporated,namelyhalides,oxy-anions,silicates,poly-oxometalateanions,complexanions,andorganicanions[25].

Fig.1-SchematicrepresentationoftheLDHstructure.

3.ConventionalpreparationmethodsofLDHs

LDHscanberegardedasaclassofmaterialsthataresimpletosynthesizeinthelaboratory,althoughnotalwaysaspurephases.Ingeneral,thereareseveralapproachestoprepareLDHs.Recently,athoroughreviewhasbeenpreparedbyHeetal[26]onthepreparationofLDHs.

Thesimplestandmostcommonlyusedmethodiscoprecipitation[2].Inthismethod,aqueoussolutionsofM2+andM3+containingtheanionthatistobeincorporatedintotheLDHsareusedasprecursors,ofwhichMgandAlarethemostfrequentlyusedmetalprecursors.Inordertoensuresimultaneousprecipitationoftwoormorecations,itisnecessarytocarryoutthesynthesisunderconditionsofsuper-saturation.Therearegenerallytwotypesofco-precipitationconditions,namelyco-precipitationatlowsuper-saturationandco-precipitationathighsuper-saturation.Co-precipitationathighsuper-saturationgenerallygivesrisetolesscrystallinematerialscomparedtothosewithlowsuper-saturation,duetotheformationofalargenumberofcrystallizationnuclei.Afterprecipitationatlowandhighsuper-saturation,athermaltreatmentprocessisperformedtoincreasetheyieldsandcrystallinityofthematerials.Thisisfollowedbyanagingprocessconductedforaperiodrangingfromafewhourstoseveraldays.InordertoensurethepurityofthesynthesizedLDHs,theuseofthede-carbonatedultra-purewaterandtheapplicationofvigorousstirringincombinationwithnitrogenpurginginthesynthesisprocessarenecessary.

AsynthesismethodofLDHsinvolvingseparatenucleationandagingstepshasbeenproposedbyZhaoeta[3].Thekeyfeaturesofthismethodareaveryrapidmixingandnucleationprocessinacolloidmill,followedbyaseparateagingprocess.Ascomparedtotheconventionalcoprecipitationprocess,thismethodresultsinaslightlyhigherdegreeofcrystallinityoftheLDHmaterials,smallercrystalliteswithahigheraspectratio,andanarrowerdistributionofcrystallitesizes,owingtotheextremeforcestowhichthenucleationmixtureissubjectedinthecolloidmill.

AnumberofstudieshavereportedthesynthesisofLDHsusingtheureahydrolysismethod[4].Ureahasafewuniquepropertiessuchasitsweakbrø´nstedbasecharacteristic,highsolubilityinwater,anditshydrolysisratethatcanbeeasilycontrolled,makingitanattractiveagenttoprecipitateseveralmetalionsashydroxidesorasinsolublesaltswheninthepresenceofasuitableanion.TheoptimumconditionstopreparetheLDHswithagoodcrystalqualityinarelativelyshorttimeusingthismethodhavebeensuggestedtoinvolvedissolvingsolidureaina0.5Msolutionofthechosenmetalchloridestogiveaurea/metalionmolarratioof3.3.Thecompoundspreparedusingthismethoddisplayhomogeneoussizesandplatelet-likeprimaryparticleswithwell-definedhexagonalshapes,whichmaybeveryinterestingfromtheviewpointofnanotechnologysinceLDHsoffernano-sizetwo-dimensionalspacesforthecreationoffunctionalmaterials.

LDHscanbealsopreparedbytheionexchangemethod[5].Thismethodisusefulwhenthecoprecipitationmethodisinapplicable,e.g.whenthedivalentortrivalentmetalcationsortheanionsinvolvedareunstableinthealkalinesolution,orwhenthedirectreactionbetweenthemetalionsandtheguestanionsismorefavorable.Inthismethod,theguestsareexchangedwiththeanionspresentintheinterlayerregionsoftheLDHstoproducespecificanion-pillaredLDHs.

AnothercommonmethodtoproduceLDHsisrehydration/reconstructionusingthestructural‘‘memoryeffect’’[27].ThismethodinvolvescalcinationofLDHstoremovetheinterlayerwater,interlayeranions,andthehydroxylgroups,resultinginmixedmetaloxides.ItisinterestingtonotethatthecalcinedLDHsareabletoregeneratethelayeredstructurewhentheyareexposedtowaterandanions.Inaddition,theanionsincludedneednotbethesamespeciesoriginallypresentintheinterlayeroftheuncalcinedLDHs,andthereforethisisanimportantmethodtosynthesizeLDHswithdesiredinorganicororganicanionstofulfillspecificapplicationrequirements.

Thehydrothermalmethod[28]isusuallyusedwhenorganicguestspecieswithlowaffinityforLDHsarerequiredtobeintercalatedintotheinterlayers,andwhentheionexchangeandcoprecipitationmethodsarenotapplicable.Thismethodhasbeenshowntobeeffectivebecauseonlythedesiredorganicanionscanoccupytheinterlayerspaceunderthehydrothermalconditionsinceinsolublemagnesiumandalumin