The hydrogen adsorption on Zr-decorated LiB (001)_ A DFT stu(3)

Theinteractionbetweentheatomscanalsobeintuitivelyveri-?edbyanalysisoftheelectrondensitydistributionbetweentheatoms.Fig.4illustratestheelectrondensitydistributionoftheZr-decoratedLiB(001)systemsbeforeandafterH2adsorption.The

Table4

Summaryofadsorptionenergy(Ea)andbondlength(rHeH)ofH2adsorbedonintrinsicLiBandZrdopedLiBondifferentadsorptionsites.Adsorptionsite

H2/LiB(001)H2/Zr/LiB(001)Ea(eV)

rHeH(?)Ea(eV)rHeH(?)Li-topà0.080.76à0.080.78B-top

à0.020.76à0.190.81Hollowverticalà0.060.76à0.190.81LieBbridgeà2.192.00à0.190.82BeZrbridgeeeà0.180.80LieZrbridgeeeà0.090.78Zr-top

e

e

à0.10

0.78

66W.Zhangetal./Vacuum110(2014)62e68

(a)

3.61.80.02.41.20.01.40.70.00.10.0

-8

DOS(State/eV)

Zr_d Zr_s B_s-4

Energy(eV)

04

(b)1.5

0.021

01.60.80.00.10.0

-8

H_s

DOS(State/eV)

Zr_d Zr_s B_p B_s-4

Energy(eV)

04

Fig.3.ThePDOSofZr/LiB(001)surface:(a)beforeH2adsorptionontheZr/LiB(001)surface,(b)afterH2adsorption.(ThePDOSofZratà8.6eVisampli?edandpresentedintheinsetforclearerpresentation).

blueregionsneartheatomsrepresenttheelectrondeletion,whiletheredregionsrepresenttheelectronenrichment.Thewhitere-gionsmeanthattheelectrondensityobviouslydoesnotchange.BeforeH2adsorption,theelectronmainlyexistedintheregion

betweenBandZratoms,asshowninFig.4(a).ItmeansthattheZratomsmainlyinteractwiththeBatoms,whichconsistedwiththefactthattheBeZratomshadahighcovalentinteractionobtainedfromthebondpopulationcalculation(Table3).AfterH2adsorptionontheZr-decoratedLiB(001)system,someelectronswerefoundintheregionbetweenHandZratoms(Fig.4(b)),indicatingthattheadsorptionofH2ontheZr/LiB(001)wasweakchemisorption.ItwasworthwhiletonotethattheinteractionbetweenH2andthesurfaceatomsmainlytookplacebetweentheH2andZratoms.ThissuggeststhattheH2adsorptionabilityofZr-decoratedLiB(001)systemmaybefurtherenhancedifmoreZratomscanbedecoratedontheLiB(001)surface.

Secondly,weinvestigatedhowmanyH2moleculescanbeadsorbedontheZr-decoratedLiB(001)surface.Weconstructedanadsorptioncon?gurationwithmorethanoneH2moleculesadsor-bedonthedifferentadsorptionpositions.Table5liststheadsorptionenergiesandchargepopulationofH2moleculewithdifferentnumberofadsorbedH2molecules.AsthenumberoftheadsorbedH2moleculesincreased,theadsorptionenergychangedslightly,rangingfromà0.16toà0.30eV/H2.ItwasnotedthatasingleZratomdecoratedonLiB(001)surfacecanadsorbupto4H2moleculeswithanaverageEazà0.20eV/H2whichisintherangeofidealadsorptionenergylevels.Inaddition,itwasobservedthatthepositivechargesonZrincreasedfrom0.06to0.96easthenumberofadsorbedH2moleculesincreased,whilealltheH2hadanegativechargesofaroundà0.13e.TheincreasedpositivechargesoftheZrwillenhancetheelectric?eldaroundZratom.The?eldenhancementmightinducethepolarizationofH2moleculeswithstrongerelectrostaticinteraction.Asaresult,thebindingofZreH2canbestrongenoughtoadsorbH2moleculeswithoutdissociation.

ToexaminebondcharacteristicoftheatomsformorethanoneH2moleculesadsorptionontheZr-decoratedLiB(001)system,weconductedthecalculationofMullikenchargeandbondpopulationofthesurfacelayer,astabulatedinTables1and2AsthenumberoftheadsorbedH2moleculesincreased,thechargesoftheLiB(001)surfaceatomshadnoobviouschange,whilethepositivechargeon

Fig.4.Theillustrationsofelectrondensitydistributionforthe(a)Zr/LiB(001)and(b)H2/Zr/LiB(001)systems.Thea0andb0representthecorrespondingsideviews.Thered,green,whiteandbluespheresdenoteLi,B,HandZratoms.Thelightbluecirclesin(a)and(b)representtheelectronenrichmentbetweenZreB,HeZratoms,respectively.(Forinter-pretationofthereferencestocolorinthis?gurelegend,thereaderisreferredtothewebversionofthisarticle.)

W.Zhangetal./Vacuum110(2014)62e68

Table5

TheadsorptionenergiesEaandchargepopulationformorethanoneH2adsorbedonZr/LiB(001)system,wheretheunitoftheatomchargeisoneelectronchargeeModel

Zr/LiB(001)H2/Zr/LiB(001)(H2)2/Zr/LiB(001)(H2)3/Zr/LiB(001)(H2)4/Zr/LiB(001)

ChargeChargeChargeChargeChargeEa

onZron1stH2on2ndH2on3rdH2on4thH2(eV/H2)0.060.260.510.700.96

à0.12à0.12à0.11à0.12

à0.21à0.30à0.16à0.20

67

à0.13à0.12à0.13

à0.13à0.09

à0.13

theZratomincreasedfrom0.06to0.96e(Table2).ItsuggeststhattheH2mainlyinteractedwiththeZratomratherthantheothersurfaceatoms.Table3alsopresentedthebondpopulationsofH2moleculeadsorbedonZr/LiB(001)system.WiththeincreaseofadsorbedH2molecules,thebondpopulationofBeZrbondsdecreasedfrom1.00to0.80e,indicatingthatthecovalentbondofBeZrwasweakened.ThebondpopulationofHeZrbondwasaroundà0.17toà0.25,indicatingweakionicbond.ThebondpopulationofLieZrbondsdecreasedfromà0.57toà0.86e,whichrevealsthattheoverlapdegreeofthetwoatomsincreased.WenoticethatthechargeonZrandLi4are0.96and1.12eVafter4H2adsorption,whichmeanstheywillrepeleachotherasthecoulombinteraction.So,thedistancebetweenZrandLi4changedfrom2.68to2.96?.

AfterthoroughlyunderstandingtheadsorptionbehaviorofH2moleculesontheZrdecoratedLiB(001)2?2surface,weturnedtoadjustthenumberofdecoratedZratomstoevaluatethemaximumH2storageabilityofthissystem.EventhoughwefoundthattheadsorptionenergiesofZratomsonthetopofdifferentLiatoms(T2,T3,andT4)oftheLiBweresimilar,weshouldconsidertheminimumdistancebetweenadsorbedatomstoavoidtheatomicinteractions.Theminimumdistance,lZreZr,betweentwodecoratedZratomsonthesurfaceshouldbelargerthan5.32?,whichwasabouttwiceofthatinbulkZr(lZreZr?3.17?).SimilarcriteriahadbeenappliedintheCedecoratedboronnitridenanotubes(BNNT)system[13]andAldecoratedgraphenesystem[33],wheretheminimumatomicdistancewastwiceofthatinthebulk.Thus,anotherZratomcanbedecoratedatT3site(onthetopofLi5)becausethedistancebetweentheZratomsislargerthan5.32?.Consequently,oursimulationrevealedthattwoZratomscouldbedecoratedontheLiB(001)2?2surface.TheincreasednumberofdecoratedZratomswouldenhancetheelectric?eldneartheZratomsandpolarizationofmoreH2molecules.Thiswill

allowmoreH2moleculestobeadsorbedonthesystem.OurcalculationdemonstratedthatthetwoZratomsdecoratedLiB(001)surfacecouldadsorbupto8H2molecules.FromtheEqua-tion(2),theaverageadsorptionenergyof8H2wasobtainedtobeà0.20eV/H2.Notethattheadsorptionenergiesforthecaseof4H2and8H2adsorptionsweresimilarbecausethedistancebe-tweenadsorbedZratomswaslargeenoughtoavoidtheinterac-tionbetweentwoZrdecoratedsystems.Fig.5showedthetopandsideviewsoftheequilibriumcon?gurationsoftheZratomsdecoratedLiB(001)surfaceafteradsorbingdifferentnumbersofH2molecules.TheH2moleculesafteradsorptiontendedtotilttowardZratomsbecauseofincreasedpositivechargeonZratoms.ThepositivelychargedZrandnegativelychargedH2interactedthroughattractiveCoulombforce.ItwasimportanttonotethatthechargesoftheadsorbedH2weretransferrednotonlyfromZratoms,butalsofromtheLiBsurfaceatomsthroughthebackdonationprocesses.4.Conclusions

Inconclusion,theadsorptionbehaviorofH2moleculesontheZrdecoratedLiB(001)2?2surfaceisstudiedusingdensityfunctionaltheory.We?ndthattheZrdecorationonLiB(001)systemisdemonstratedtobestablewiththeadsorptionenergyofà6.67eV.IntheZr-decoratedLiB(001),ahighcovalentinteractionexistsforBeZrbondwhileionicnatureshowsforZreLibond.Furthermore,theadsorptionofH2wasgreatlyenhancedbydecorationofZronLiB(001).TheZratom-decoratedLiB(001)canadsorbupto4H2moleculeswiththeadsorptionenergiesofà0.16toà0.30eV/H2.TheorbitalinteractionbetweenH2,ZrandBisduetotheH1s,Zr4dandB2sorbitalhybridization.Thepopulationanalysisshowsthatthereischargetransferbetweenatoms,andtheredistributedchargeinduceselectric?eldaroundtheZratomwhichhelppolarizeH2.ThepolarizedH2isthenbondedtopositivechargedZrbyCoulombforce.TheZratomactsasa“bridge”betweenH2andsurfaceatomsofLiB.TheH2adsorptionontheZrdecoratedLiB(001)surfaceisweakchemisorption,whichhasbeendemonstratedbytheelectrondensitydistribution.FurtherinvestigationrevealsthattwoZratomscanbedecoratedonthe2?2super-latticeLiBsurface.Thus,Zr-decoratedLiB(001)surfacecanadsorbupto8H2moleculeswithaverageadsorptionenergyofà0.20eV/H2.OurresultssuggestthattheZr-decoratedLiB(001)systemwillbeapromisinghydrogenstoragematerialsatisfyingtherequirementoftheDOEforcommercial

applications.

Fig.5.TherelaxedhydrogenadsorptionbasedontheZr-decoratedLiB(001)surface.Theplotsshowtherelaxedmodelswiththenumberofhydrogenmoleculesof(a)1,(b)4,and(c)8,respectively.Thea0,b0andc0representthecorrespondingsideviews.Thered,green,blueandwhitespheresdenoteLi,B,Zr,andHatoms.(Forinterpretationofthereferencestocolorinthis?gurelegend,thereaderisreferredtothewebversionofthisarticle.)

bbs.99jianzhu.com内容：建筑图纸、PDF/word 流程，表格，案例，最新,施工方案、工程书籍、建筑论文、合同表格、标准规范、CAD图纸等内容。