, *,

(1. College of Chemistry and Chemical Engineering, Anhui University, Hefei 230601, China;2. Department of Chemistry and Molecular Engineering, East China University

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Densityfunctionalcalculationofphysicalpropertiesofg-C3N4/germaneneheterobilayer-theaffectionsofelectricfields

RUANLinwei1,ZHUYujun1*,LUYunxiang2

(1.CollegeofChemistryandChemicalEngineering,AnhuiUniversity,Hefei230601,China;2.DepartmentofChemistryandMolecularEngineering,EastChinaUniversity

ofScienceandTechnology,Shanghai200237,China)

Effectofelectricfieldintensityanddirectiononbindingenergy,densityofstates(DOS),andchargedensityofg-C3N4/germaneneheterobilayerwasinvestigatedbyfirstprinciplecalculations.Thecalculationresultsrevealthelargeimpactofelectricfieldonthephysicalparametersofg-C3N4/germaneneheterobilayer.ApplicationofupwardelectricfieldmovestheDOStowardsleft,whilethedownwardelectricfieldresultsintheright-shiftofDOSing-C3N4/germaneneheterobilayer.Inaddition,nochangesinworkfunctionofheterobilayeroccurunderelectricfields.

g-C3N4;heterobilayer;germanene;electricfields

0　Introduction

Graphiticcarbonnitride(labeledasg-C3N4)hasrecentlyreceivedintensiveattentionsduetoitsgoodphotocatalyticperformanceforwatersplittingandorganicpollutantspurificationundervisiblelightirradiation.Likegraphite,g-C3N4hasatwo-dimensional(2D)planarπconjugationstructure,whichenablestheefficientelectrontransferwithintheπconjugationstructure.However,thephotocatalyticactivityofpristineg-C3N4isstilltoolowtopracticalapplications.Researchershavenowrecognizedthatthesingle-componentg-C3N4cannotachievethehigherphotocatalyticactivityduetotherapidrecombinationofphotogeneratedelectron-holepairs.Tosolvethisissue,formingheterostructuresbycouplingg-C3N4withothermaterialshavebeendemonstratedaseffectivestrategytoimprovethephotocatalyticefficiencyofg-C3N4.Forexample,Daietal.[1]fabricatedg-C3N4/TiO2nanosheethybridsforenhancedphotodegradationoforganiccontaminationsusingvisiblelight.Xingandco-workers[2]usedIn2S3/g-C3N4heterostructurestoimprovethephotocatalyticabilityofg-C3N4.Yangetal.[3]studiedtheinfluenceofpreparationmethodonphotocatalyticperformanceofg-C3N4/WO3compositephotocatalyst,andfoundthatthecompositespreparedthroughhydrothermalmethodexhibitedthehighestphotocatalyticactivity.Zhangetal.[4]designedBi2O3/g-C3N4hybridswithhighvisiblelightactivityformethyleneblueandrhodamineB.Huangetal.[5]studiedeffectofcontactinterfacebetweenTiO2andg-C3N4onthephotoreactivityofg-C3N4/TiO2photocatalyst,anddiscoveredthatthe(101)facethasbetterperformance.

Meanwhile,theoreticinvestigationon2-Dcarbon-basedmaterialhybridshasalsobeenintensivelystudied.Maetal.[6]examinedthebandstructureanddensityofstates(DOS)oftransition-metaldichalcogenideandmxenemonolayer.Medvedevaetal.[7]studiedtheAlN/GaN:Cr(0001)heterostructurebyusingfirstprincipalcalculationandfoundthattheheterobilayerdopedwithCrwidenedthebandgapofg-C3N4.RoomeandCarey[8]calculatedthestructuralstability,electronicandvibrationalpropertiesofdifferentmonolayerconfigurationsofsiliceneandgermaneneheterobilayer.Gaoetal.[9]simulatedthehybridgraphene/anataseTiO2(001)nanocompositesandfoundtheimprovedinterfacialelectrontransferwithgrapheneintroduction.Xuetal.[10]predictedtheimprovedphotocatalyticactivityoverAg3PO4/graphenenanocompositethroughfirstprinciplecalculation.Gengetal.[11]foundthattheelectronicpropertiesofZnOretainedunchangedwhencouplingwithgraphene.Gaoetal.[12]simulatedtheheterobilayersformedbysiliceneandMoS2,anddeducedthatitisacandidatematerialforlogiccircuitsandphotonicdevices.

Itiswellknownthatthephotoelectricpropertiesofsemiconductorareaffectedgreatlybytheappliedexternalelectricfield.Wuetal.[13]foundtheopticalenergygapofg-C3N4bilayercanbeengineeredbytheexternalelectricfield.Zhangetal.[14]calculatedtheeffectsoftransverseelectricfieldonenergygapmodulationofBNribbonsandfoundtheenergygapsnarrowingcausedbythefield-inducedmotionofnearlyfreeelectronstates.Kangetal.[15]investigatedtheaffectionstobandgapsofgraphdiynenanoribbonsfromtransverseelectricfield.Kanetal.[16]verifiedthetransformationofconductivezigzaggraphenenanoribbonintohalfmetalunderelectricfield.Tothebestofourknowledge,thereisnoreportoftheeffectofelectricfieldonthephysicalpropertiesofg-C3N4heterobilayers.

Herein,theauthorsreporttheeffectofappliedelectricfieldontheg-C3N4/Geheterobilayer.Itisrevealedthatthephysicalpropertiesofg-C3N4/Geheterobilayerareaffectedgreatlybytheappliedelectricfield.Theprincipledisclosedbythesimulationcanprovideusefulinformationforthesynthesisofheterobilayers.

1　Methodology and calculation

AllcalculationswereperformedbyDmol3module[17]inMaterialsstudio7.0software.The(001)surfaceofg-C3N4and(111)surfaceofgermaniumwerecleaved,thelatticeparametersofbothsurfacesare19.133 ?and20.155 ?.Wecanassumethatthetwosurfacescangenerateanewheterobilayerbecauseoftheapproximatelatticelength.Theheterobilayerwasbuiltthrough“buildlayer”tabfromthenanosheetsofC3N4andgermaneneobtainedbefore.Inordertogaintheoptimizedstructureofheterobilayer,allstructuresobtainedbeforeusedinthesimulationneedtominimizetheenergy.Generalgradientapproximation(GGA)andPerdew-Burke-Ernzerhof(PBE)[18]functionwereusedinthewholesimulation,andapragmaticmethodtodescribecorrectlyvanderWaalsinteractionsresultingfromdynamicalcorrelationsbetweenfluctuatingchargedistributionshasbeengivenbytheDFT-D2approachofGrimme.DFTsemi-corepseudopotsandDNPbasiswerealsousedinthewholeprocessofsimulation.Thecutoffenergyis900eVandkpointis6×6×3,simultaneously,thenumberofatomsoftotalsystemis96.

TheultimatestructureofheterobilayerwasshowninFig.1,twolayersofstructuremaintainasawholethroughvanderWaalsinteractions.Afterenergyminimization,thegermanenelayercorrugatedcomparedtotheflatC3N4layercankeepthewholesystemstable.Theoriginalandultimatelatticeparametera,b,cofheterobilayeris19.721 7 ?, 19.721 7 ?, 20.000 0 ?and19.722 3 ?, 19.721 6 ?, 20.000 0 ?.Theoriginalandultimateinterlayerdistanceis3.578 ?and3.572 ?.

Fig.1　Schematic diagram of heterobilayer formed by g-C3N4 and germanene

2　Results and discussion

Bindingenergycanbeusedtorevealthedifficultiesofheterobilayerformation.Thereforethebindingenergywasfirstcalculatedtostudytheformationofg-C3N4/Geheterobilayer,asshowninFig.2.Bindingenergyinpresentsystemwasdefinedas

Eb= E(heterobilayer)-E(g-C3N4)-E(germanene).

Itwasfoundthatthebindingenergyofg-C3N4/Geheterobilayervarieswiththedirectionofappliedelectricfield.Theupwardelectricfield(+z)causestheincreasedbindingenergy,whilethedownwardelectricfield(-z)resultsinthedecreasedbindingenergy,asshowninFig.2.Thisisbecausethe+zdirectionisthesameasthedirectionofinduceddipole,onthecontrary,the-zdirectioniscontradicttothedirectionoftheinduceddipole.

Fig.2　Binding energies of heterobilayer as function of electric field

Thebandgapofg-C3N4/Geheterobilayerisalsoaffectedbytheappliedelectricfielddirection,asillustratedinFig.3.Despitetheelectricfielddirection,thebandgapbecomesnarrowedwhenexternalelectricfieldwasapplied.Thisphenomenonisdifferentfromthechangetendencyinbindingenergy,asdiscussedabove.Thedecreaseinbandgapcanbeattributedtothesmallerelectronenergybarriercausedbytheappliedelectricfield[15].Thedifferenceinbandgapchangewiththeelectricfielddirectioncouldbecausedbythedirectionalflowofelectronsfromg-C3N4togermaneneastheupwardelectricfieldcanpromotethemovementofelectronsfromg-C3N4togermanene.Thisresultisverysimilartheauthors’previousresult[19]thatthebandgapdecreaseswithincreasedexternalpressure,butthedeclinedrangewasnarrowercomparedwiththepreviousresult.

Fig.3　The directions of electric fields (a) and the relation between band gap and electric field (b)

ThecalculatedbandgapEgofg-C3N4/germaneneheterobilayeris0.735eV.Suchsmallbandgapindicatesthatg-C3N4/germaneneheterobilayercanabsorbthefullvisiblelightregion[10].Despitetheelectricfielddirection,theconductionbandedgelinearlydecreaseswiththeappliedelectricfield(Fig.4).Notably,thevalencebandedgeincreaseswiththeincreaseofelectricfieldundertheirradiationofupwardelectricfield(+z).Interestingly,adecreasedvalencebandedgeoccurswhenthedownwardelectricfieldgraduallyincreases.

Fig.4　Valence band edge and conduction band edge of heterobilayer

Fig.5shownthedensityofstatesofheterobilayerwithoutexternalelectricfieldapplied.Thedensityofstates(DOS)ofg-C3N4/manganeneheterobilayerwasalsocalculated,asshowninFig.5andFig.6.TheDOSbetween-25and-15eVwasmainlycomposedofC2sorbitals.TheGeporbitalscontributeddominantlytotheDOSfromtheenergyof-12.5eVtotheFermienergy.Inaddition,thetopofthevalencebandwasalsomainlyconstructedbytheGePDOS.

Fig.5　The DOS of heterobilayer without electric field

Fig.6　The s, p, d PDOS and sum DOS of carbon atoms

NotethattheelectricfieldalsoaffectstheDOSoftheg-C3N4/germaneneheterobilayer,asshowninFig.7.TheDOSmovestothelowerenergysidewithincreaseofupwardelectricfieldintensity.Thisresultisconsistentwiththephenomenonthattheelectricfieldcannarrowtheenergyofg-C3N4/germaneneheterobilayer[13].Comparatively,thedownwardelectricfieldpushestheDOStothehighenergylevelwiththeincreaseofelectricfieldintensity.

Fig.7　DOS of heterobilayer with different value of electric field (a) and different direction of electric field (b)

Fig.8　　　　Electron density of heterobilayers without electric field (a), with the value 0.1 and -z direction 　　　electric field (b), with the value 0.1 and z direction electric field (c)

Fig.8showstheelectrondensityofg-C3N4andGelayer,respectively.Electrondensityatg-C3N4layerishigherthanthatofGelayerwithoutexternalelectricfield.Interestingly,thedirectionofelectricfieldaffectsthegapofg-C3N4layerandGelayer.Theupwardelectricfield(+z)widensthegapbetweeng-C3N4andGelayer,whilethedownwardelectricfield(-z)narrowsthegapbetweeng-C3N4andGelayer.Inaddition,Gelayerownshighelectrondensityincomparisontog-C3N4layerwiththevalue0.1of-zelectricfieldshowninFig.8.Notably,thechemicalbondsbetweeng-C3N4andGelayerformedwhendownwardelectricfieldintensityof0.1wasapplied.Ge-C3N4layerhas-2.515emullikenchargeandGelayerhas2.52emullikenchargeatthissituation.Comparatively,g-C3N4layerpossesses0.95emullikenchargeandGermanenelayerowns-0.95emullikenchargewhenupwardelectricfieldintensityof0.1wasaddedtotheg-C3N4/Geheterobilayer.Althoughthemullikenpopulationanalysisistoocoarsetorevealthecharges’spatialdistribution,themullikenchargeing-C3N4andGelayersverifiestheelectrondensityplotandtheasymmetrybetweenthetwolayers.Thechargeredistributionoflayersoccurredinthiskindofhybridheterobilayerwilldemonstratetheconclusionisrightornot.AfurtherchargeanalysisrevealsthateachGeatomtransferes0.05etog-C3N4,whileeachCatomloses0.338eandNatomobtaines0.292einpresentheterobilayer.ThechargeredistributioninpresentheterobilayerisdifferentfromthatinTiO2/GR[20-23],ZnO/GR[24],TiO2/carbonnanotube[25],C60/TiO2[26]etc,inwhichthechargemerelytransfersfromonecomponenttoanother.Thisresultcouldbeascribedtothevariationofelectrondensitycausedbytheelectricfield,asrevealedbyXu’swork[10],hencechemicalbondformedbetweenatomsbelongtodifferentlayers.Thecorrespondingbondlengthofbetweeng-C3N4andGelayerisillustratedinFig.9.Underelectricfieldirradiation,thelengthofN—GeandC—Gebondvariesfrom2.0to2.2 ?,whichislargerthanthatofC—Cbondlength.Thisresultrevealsthattheinteractionbetweeng-C3N4andGelayersisweakerthanthatofcovalentg-C3N4layer.

Fig.9　Lengths of chemical bonds formed in 0.1 electric field of -z direction

WorkfunctionistheenergythatcanbeprovidedfortheelectronwithFermienergyescapingfromtheinnermetaltovacuumlevel.Thesimulatedworkfunctionoftwolayersalmostmaintainsunchangedwhentheelectricfieldintensityincreasesfrom0to0.1,thisresultmaybecausedbythesmallexternalelectricfield,whichcannotaffecttheelectronescape.

For(001)surfaceofg-C3N4,theworkfunctionwascalculatedtobe4.5eV[27]inagreementwiththispaper.Electronscanflowfromthelayerwithhighworkfunctiontothelayerwithlowworkfunctionwhentherewasnoelectricfieldapplied.Hence,electronscanmovetog-C3N4layerbecausetheg-C3N4layerownslowerworkfunction.Theelectronflowdirectioncanbechangedunderelectricfieldirradiation.Fig.10illustratestheworkfunctionofg-C3N4/Geheterobilayerwithelectricfielddirection.Aslightlydecreasedworkfunctionof4.425eVofg-C3N4wasobtainedunderelectricfieldirradiationwhencomparedwiththevalueof4.3eVwithoutelectricfieldirradiation[28].

Fig.10　Work function of heterobilayer when electric field from +z and -z direction with value 0.1

3　Conclusion

Thephysicalpropertiesofg-C3N4/Geheterobilayerwerestudiedbyfirstprinciplecalculations.Itwasfoundthatthebindingenergy,bandgapenergy,anddensityofstatesaregreatlydependentontheappliedelectricfielddirection.Despitetheelectricfielddirection,thebandgapwasdecreasedwiththeincreaseofelectricfieldintensity.TheDOSmovestothelowerenergysideunderupwardelectricfieldirradiation,andmovesbacktothehighenergysideunderdownwardelectricfield.Interestingly,workfunctionofheterobilayerchangedalittleincomparisonwiththepristineg-C3N4.Thepresentresultdemonstratesthegreateffectonexternalelectricfieldonthephysicalpropertiesofg-C3N4heterobilayer.

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(責任編輯于敏)

10.3969/j.issn.1000-2162.2016.02.016

g-C3N4/germanene異質結物理性質的密度泛函計算-電場的影響

阮林偉1，朱玉俊1*，盧運祥2

(1．安徽大學 化學化工學院，安徽 合肥230601；2.華東理工大學 化學與分子工程學院,上海200237)

通過第一性原理計算研究電場強度和方向對于g-C3N4/germanene雙層的結合能、態(tài)密度以及電荷的影響.計算結果顯示，電場對于雙層的物理性質影響很大，方向朝上的電場使得態(tài)密度曲線向左移動，同時方向朝下的電場使得態(tài)密度曲線朝右移動.并且在電場的影響下，功函數(shù)的變化不大.

g-C3N4; 異質結;germanene; 電場

date:2015-03-26

SupportedbytheNationalNaturalScienceFoundationofChina(51002001);theAnhuiUniversityDoctoralScientificResearchFoundation(02303319)

Author’sbrief:RUANLinwei(1990-),male,borninTaihuofAnhuiprovince,masterdegreecandidateofAnhuiUniversity; *ZHUYujun(correspondingauthor):lecturerofAnhuiUniversity,Ph.D,E-mail:zyj8119@sina.cn.

O641Documentcode:AArticleID:1000-2162(2016)02-0093-08