, *,

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



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 Åand20.155 Å.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 Å, 19.721 7 Å, 20.000 0 Åand19.722 3 Å, 19.721 6 Å, 20.000 0 Å.Theoriginalandultimateinterlayerdistanceis3.578 Åand3.572 Å.

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 Å,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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10.3969/j.issn.1000-2162.2016.02.016

g-C3N4/germanene异质结物理性质的密度泛函计算-电场的影响

阮林伟1，朱玉俊1*，卢运祥2

(1．安徽大学 化学化工学院，安徽 合肥230601；2.华东理工大学 化学与分子工程学院,上海200237)

通过第一性原理计算研究电场强度和方向对于g-C3N4/germanene双层的结合能、态密度以及电荷的影响.计算结果显示，电场对于双层的物理性质影响很大，方向朝上的电场使得态密度曲线向左移动，同时方向朝下的电场使得态密度曲线朝右移动.并且在电场的影响下，功函数的变化不大.

g-C3N4; 异质结;germanene; 电场

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