The hydrogen adsorption on Zr-decorated LiB (001)_ A DFT stu(2)

ForH2adsorptionontheZr/LiB(001)system,theaverageadsorptionenergy,Ea(H2),perH2tothesystemwasdeterminedby

EHh

ànEi.ae2T?EtàEmZrtLiBe001TH2n

(2)

wherenindicatesthenumberofH2molecules.Thesubscriptst,mZrtLiB(001)andH2denotethetotalenergyoftheconsideredsystem,thecorrespondingstoragematerialwithnumberofZratomsm,andthefreeH2molecules,respectively.ThenegativevalueofEaindicatesthattheadsorptionisanexothermicreaction,whilethepositivevalueindicatesthattheadsorptionisanendo-thermicreaction.

3.Resultsanddiscussion

3.1.DecorationofZratomsontheLiB(001)surface

TheequilibriumlatticeconstantsofourcalculationandtheexperimentaldataofLiBwerelistedinTable1.Comparingwiththeexperimentaldata[28],theequilibriumlatticeconstantsofaandbwereincreasedby4.05%,whilethecellparametercwasincreasedby0.97%.Inotherpeople'scalculationresults[29],thelatticeofaandbwereincreasedby0.15%,whilethecellparametercwasincreasedby11.09%.Thesewereattributedtothefollowingtworeasons:(i)theGGA'sinherentreason,whichusuallyover-estimatedtheparametersbyafewpercent;(ii)therelativelysmallsizeoftheLiBunitcell,whichcontained4atoms.Thisalsobethereasonoftheobserveddiscrepancybetweentheexperimentalandcalculationresults.Thusitisjusti?edthatthismethodiscredibleandcanbeappliedintothefollowingcalculations.SixperiodicLi(B)layersformedtheslab,whichwasusedtosimulatetheLieB(001)surface.Thelateraldimensionofthesurfacewas8.04?8.04?2.Relaxationinsurfacelayershowedsomeoscillatoryproperty:theoutmostLilayerrelaxedoutwardof1.64?,andsecondrelaxedlayeroutwardof1.18?.TheBatomiclayersrelaxedoutwardof0.65?andsecondlayerrelaxedoutwardof0.39?.Consequently,theslabwasexpandedduetotheserelaxations.

Fig.1showsTiandZrdecoratedLiB(001)surfaces.WehadpreviouslydemonstratedthattheTidopedLiB(001)surfacesef?-cientlyenhancedH2storagecapacityofLiB[21].However,themigratorytendencyofthedopedTiatomsmadeitinappropriateforlong-termH2storagesincetheTiatomsenergeticallypreferredto

Table1

UnitcellparametersofLiB.

Experiments[28]

Calculation[29]ThisworkLiB

a?b?4.022?a?b?4.028?a?b?4.185?c?2.796?c?3.106?c?2.823?a?b?90??,a?b?90??,a?b?90??,g?120??

g?120??

V?45.229?3

V?50.394?3

V?49.442?3

64W.Zhangetal./Vacuum110(2014)62e68

Fig.1.Thetopviewsof(a)Tiand(b)ZratomsdecoratedLiB(001)surface.Thea0,b0arethecorrespondingsideviews.Thered,green,blueandgrayspheresdenoteLi,B,Zr,andTiatoms,respectively.(Forinterpretationofthereferencestocolorinthis?gurelegend,thereaderisreferredtothewebversionofthis

article.)

occupyinterstitialpositionsandtendedtograduallymigrateinto

thebulk(Fig.1(a)).Ontheotherhand,Zratomspreferentially

attachedonthesurfaceandwerefreeofmigrationintothebulk

(Fig.1(b)).Thus,ZrdecoratedLiB(001)isrelativelystableandcan

beregardedasapotentialsystemforH2storage.Weconsideredthe

possibledecorationsitesofZratomsonLiB(001)surface,asshown

inFig.2.TherearepossiblesixdifferentsitesforZrdecorationon

theLiBsurface:thehollow(Hsite),thebridgeoftheLieBbond(B

site),andthetopofBatom(T1site),thetopofLi4atom(T2site,Li4is

Liatomonthesubsurfacelayer),thetopofLi3atom(T4site),thetopofLi5atom(theT3site),respectively,asshowninFig.2(a)and(a0).OurcalculationrevealedthatadsorptionenergiesoftheZratomsadsorbedonH,B,T1,T2,T3andT4sitesareà3.35eV,à3.23eV,à3.46eV,à6.67eV,à6.52eVandà6.55eV,respectively.ThedopingsitesofthedifferentLiatoms(T2~T4)hadasimilaradsorptionenergy,whichmeansthatmorethanoneZratomcouldbedecoratedontheLiB2?2surface.Inourpaper,wehavedis-cussedthesituationsofoneZratom(andtwoZratoms)decoratedonthesurface.However,whenwetriedtodecoratethreeZratomsonthesurface,thedistancebetweentheZreZratomswas

veryFig.2.(a)PossibledecorationsitesforZratomonLiB(001)surface:hollowsite(H),thebridgeoftheLieBbond(B),thetopofB(T1),andthetopofLi(T2,T3,T4),(b)LiB(001)surfaceafterdecoratingoneZratomonT2site.Thered,greenandbluespheresdenoteLi,BandZratoms.The(a0)and(b0)arethecorrespondingsideviews.(Forinterpretationofthereferencestocolorinthis?gurelegend,thereaderisreferredtothewebversionofthisarticle.)

W.Zhangetal./Vacuum110(2014)62e6865

short,makingthesystemunstableanymore.ThemostfavorableadsorptionsitewasfoundtobetheT2site,inwhichaZratomhadthelowestadsorptionenergyofà6.67eV.Fig.2(b)presentedtheatomicstructureofLiB(001)surfaceafteroneZratomisdecoratedontheT2position.WenoticedthatthelithiumatomsaroundtheZratommovedawayfromtheoriginalposition.AsshowninTable2,theZrandLiatomsarepositivecharged,whichresultedinmutualrepelbetweenatoms,andtheatomsaroundZrmovedaway.

AfterconsideringthedecorationsitesofZratoms,wefurtherdeterminethetypeofbondbetweentheZratomandthesurfaceatomsbycalculatingMullikenchargepopulationsofthesurfaceatomsbeforeandafterZrdecoration(Table2).NotethattheMul-likenchargepopulationpresentsthechargetransferbetweenatoms.Thevalueofchargesinbulkare0.99andà0.99forLiandBatoms.TheZrdecorationontheLiBsurfacesigni?cantlyledtomodifyingthechargepopulationofLiandBatomsonthesurfacelayer.ThenegativechargeonBatoms(B1,B2,B3)wasdecreasedabout0.18ewhilethepositivechargeontheLiatoms(Li1,Li2,Li3)wasincreasedabout0.11e.ThechargeontheZratomwas0.06eafterZrdecorating.Asthechargeonthesurfaceatomschanged,moredanglingbondsonthesurfacecouldbeformed.Asaresult,theatomsonthesurfacebecamemoreactive.Moreover,thechargeaccumulationaroundtheZratominducedelectrostatic?eld,whichmayenhancetheadsorptionabilityofH2molecules.Thebondpopulationswerealsoanalyzed,asshowninTable3.Thebondpopulationpresentstheoverlapdegreeoftheelectroncloudoftwoatomsparticipatedinthebonding,whichcanbeusedtoestimatethecovalentorionicnatureofachemicalbond.Theabsolutevalueofthebondpopulationindicatedthecovalentorionicbondinter-action[30e32].IntheZrdecoratedLiB(001)system,thepopula-tionofBeZrbondswereabout1.00e,indicatingthehighcovalentinteraction.OneLiatomwasmainlybondedwithZrandthepop-ulationoftheLi4eZrbondwasaboutà0.57e,whichindicatestheformationofanionicbond.AstheZratomdidnotinteractwithotherLiatoms,therewasnobondpopulationanalysisforLi1,Li2,Li3,andLi5eZrbond.Thus,thecovalentandionicbondswereformedbetweenZratomsandthesurfaceatoms,whichcouldsuppresstheaggregationoftheZratomsresultinginastablestructure.3.2.H2adsorptiononZr-decoratedLiB(001)surface

TheinteractionbetweenH2moleculesandZrdecoratedLiB(001)surfacewasstudied.Firstly,weconsideredtheadsorptionofoneH2moleculeonthedifferentsitesofthesurface.Table4pre-sentstheadsorptionenergyandbondlengthoftheH2moleculeadsorbedonintrinsicLiB(001)andZr-decoratedLiB(001)surface.IntheintrinsicLiB(001)system,LieBbridgesitewastheonlystablesiteamongfourpossiblepositionsforH2moleculeadsorp-tion[20].Incomparison,thereweresevendifferentsitesforH2adsorptionontheZr-decoratedLiB(001)system,amongwhichfoursitescanmeettheadsorptionenergyrequirement(aboutà0.20eV/

Table2

TheMullikenchargepopulationofthesurfacelayerinthe(H2)n/Zr/LiB(001)system,wheretheunitoftheatomchargeisoneelectronchargee.TheatomswerenumberedasshowninFig.2(b).AtomLiB(001)n?0n?1n?2n?3n?4Zre

0.060.260.510.700.96B1à1.10à0.92à0.94à0.97à0.95à0.92B2à1.13à0.94à0.90à0.92à0.89à0.90B3à1.10à0.94à0.95à0.89à0.92à0.91Li10.850.940.930.940.930.94Li20.840.950.960.930.940.95Li30.840.940.940.960.940.94Li4

0.81

1.08

1.09

1.12

Table3

Thebondpopulationofthesurfacelayerinthe(H2)n/Zr/LiB(001)system,wheretheunitoftheatomchargeisoneelectronchargee.Thesubscriptisthenumberingofthesurfaceatoms.Bondn?0n?1n?2n?3n?4H1eZreà0.19à0.17à0.21à0.18H2eZreà0.25à0.25à0.27à0.24H2eB2e0.080.130.050.10B1eZr0.980.970.910.910.81B2eZr0.990.970.890.860.80B3eZr1.000.910.840.860.81Li4eZr

à0.57

à0.66

à0.72

à0.75

à0.86

H2).ThebondlengthsoftheadsorbedH2onthefoursiteswerearound0.81?.Notingtheoriginalbondlengthof0.75?inH2molecule,thebondlengthoftheadsorbedH2becamelonger.ThebondelongationoftheadsorbedH2ontheZr-decoratedLiB(001)surfacecouldbeattributedtothechargetransferfromZratomtoanti-bondingorbitalofH2molecule.ComparingwiththeintrinsicLiBsurface,theZrdecoratedsystemhadmoresitessuitablefortheH2adsorption.

ToverifytheorbitalinteractionbetweenH2moleculesandthesurfaceatoms,theorbitalstructurewereinvestigatedusingtheprojectedelectronicdensityofstate(PDOS).Fig.3displayedthePDOSsofH,Zr,B,andLiatombeforeandafterH2moleculeadsorptiononZr-decoratedLiB(001).BeforeH2moleculeadsorp-tion,thereweretwopeaksinPDOSofZr4dorbitalatà10.0e2.5eV.The?rstpeakaround1.4eVcorrespondedtothehybridizationofZr4dandLi2s,andthelatteronepeakatà1.5eVcorrespondedtothehybridizationofZr4dandB2porbital,whichwereresponsibleforthebondingofZrandLiBsurface.AfterH2adsorbedontheZr-decoratedLiB(001)surface,thePDOSofH1selectronwasobvi-ouslychanged,asshowninFig.3(b).ThepositionofthemainpeakofH1sshiftedfrom0toà8.0eV.AstheZreHbondismorestablethantheHeH,theH1sshiftedtothelowerenergy.TherewerethreepeaksinPDOSofZr4daroundà10e3.0eV.The?rsttwopeaksat1.7andà1.5eVcorrespondedtothehybridizationofZr4dandB2p.Thethirdpeakatà8.6eVwasrelatedtothehybridizationofZr4dorbitalwithB2s,Li2sandH1sorbital.TheseresultssuggestthatZrasa“bridge”interactwithbothH2andLiB,andZrisanimportantatomasadecoratorforhydrogenstoragematerials.AfterelectrostaticadsorptionoftheH2,thechargetransferbe-tweenatomsandtheorbitalhybridizationtakeplacetoformamorestableadsorptionsystem.Inaddition,thishybridizationwassuggestiveofweakchemisorption,notsimplephysisorption,whichisconsistentwiththeresultsdeducedfromadsorptionenergyanalysis.