DFT-D study of adsorption of diaminoethane and propylamine molecules on anatase (101) TiO 2 surface

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O

pen

A

rchive

T

OULOUSE

A

rchive

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uverte (

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To cite this version :

Hemeryck, Anne and Motta, Alessandro and Lacaze-Dufaure,

Corinne

and Costa, Dominique and Marcus, Philippe DFT-D

study of adsorption of diaminoethane and propylamine molecules

on anatase (101) TiO 2 surface. (2017) Applied Surface Science,

vol. 426. pp. 107-115. ISSN 0169-4332

Any correspondence concerning this service should be sent to the repository

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DFT-D

study

of

adsorption

of

diaminoethane

and

propylamine

molecules

on

anatase

(101)

TiO

2 surface

A. Hemeryck

a,∗_,1

_,

_A.

_Motta

a_,2

_,

_C.

_{Lacaze-Dufaure}

b

_,

_D.

_Costa

a_,∗

_,

_P.

_Marcus

a

a_{Chimie-ParisTech—CNRS,}_IRCP,_Research_Group_of_{Physico-Chemistry}_of_surfaces,_11,_Rue_Pierre_et_Marie_Curie,₇₅₀₀₅_Paris,_France b_CIRIMAT,_Université_de_Toulouse,_CNRS,_INPT,_UPS,₄_allée_Emile_Monso-BP_44362,₃₁₀₃₀_Toulouse_Cedex_4,_France

Keywords: Adhesion Amine Cohesivefilm DFT-D Diaminoethane Propylamine TiO2

a

b

s

t

r

a

c

t

The adsorption on anatase (101) TiO2 surface of two model amines, diaminoethane (DAE) and

propylamine(PPA),wasinvestigatedusingDensityFunctionalTheory-Dispersionincluded(DFT-D) cal-culations.Theinvestigatedcoverageisrangingfrom0.25monolayertofullcoverage(oneaminemolecule persurfaceTiion).Bothinteractionsoftheadsorbedlayerwiththeanatase(101)TiO2surfaceand

inter-molecularinteractionsaredescribed.Astructuraltransitionfromabridgetoaperpendicularstructure isfoundforDAEwhenevolvingfrom0.25monolayertofullcoverage.Atfullcoverage,adense,ordered adhesivelayerisformed.ForDAE,atintermediatecoverage,differentisoenergeticconfigurationsare foundandstructuraltransitionfromabridgetoaperpendicularstructureisfound.Incontrast,the adsorptionmodeofPPAismoreregularwithonlyperpendicularlyadsorbedmoleculesatall investi-gatedcoverages.Dispersionforcesalreadyaccountfor40%oftheadsorptionenergyatlowcoverage (0.25ML)andarethedrivingforceformonolayerformationwithacontributionof60%upto100%at highcoverage.Asrevealedbymoleculardynamics,themoleculescanchangetheirorientationtowards thesurfaceinaconcertedway.

1. Introduction

Modelaminesareoftenusedtoinvestigatethe epoxyamine-Tiinteractionsatthemolecularlevel(seeRef.[1]andreferences therein).Areliableadsorptionofaminesmoietiesonthesurface isa keypointforadhesion.Thestudyofsuch organic/inorganic interfacesisthuschallengingandmodelingapproachescanhelpto disentanglethehard-softmatterinteractionsattheatomicscale.

Adsorption of several bio- and organic molecules on TiO2,

including amines, has been reviewed [2,3]. Farfan-Arribas and Madix [4]used amineadsorptiontodeterminetheLewis acid-ityofsurface Ti4+_ions_in _rutile_TiO

2 (110)surfaces.Ethylamine

anddiethylaminewerebothfoundtoadsorbonthe stoichiomet-ricsurfacethroughformationofN Tibonds.Ondefectivesurfaces bondingalsooccurredatoxygen-vacancysites.Inbothcasesthe aminesremainedintactbutonthedefectivesurfacelessamines

∗ Correspondingauthors.

E-mailaddresses:anne.hemeryck@laas.fr(A.Hemeryck),

dominique.costa@chimie-paristech.fr(D.Costa).

1_Present_address:_LAAS-CNRS,_Equipe_M3_7,_avenue_du_Colonel_Roche_BP₅₄₂₀₀ 31031Toulousecedex4France.

2_Present_address:_Consorzio_INSTM_UdR_di_Roma_SEDE:_Dipartimento_di_Chimica, UniversitàdiRoma“LaSapienza”,PiazzaleAldoMoro,6-00185Roma,Italy.

wereadsorbed.Theauthorssuggestedthatthismightbedueto adsorptionatthedefectsitesblockingadsorptionatmorethanone neighbouringTi4+_site._The_desorption_activation_energy_decreased

intheorderdiethylamine>ethylamine>ammoniawhichisalsothe orderofdecreasingLewisbasicity.Theauthorssuggestedthatthis wasanevidenceofadsorptiondrivenbyaLewisacid–base interac-tionwiththeTi4+_ions_behaving_as_Lewis_acid_sites_(LAS)._Other_DFT

worksonadsorptionofarginineshowedanadsorptionofthe lat-eralNH2moietyonTiO2witharatherstrongenergyofadsorption

(1.4eVonrutile(110)[5],2.8eVon(100)anatase[6]and1.5eVon (001)anatase[7]).

Recentlyweshowedthatdiaminoethane(DAE),amodelamine molecule,canadsorbonTiO2bysubstitutingwatermoleculesand

formafullmonolayer[1].Our DFTstudyoftheadsorptionofa fulllayerofdiaminoethaneonanatase(101)TiO2concludedtoan

interactionenergyofaround0.5eV(0.7eVwithdispersionforces included)[1].Athermodynamicapproachwasthereforeusedto bridge thegap between calculations at 0K under vacuum and solid/liquidinterface[1].Itwasfoundthatthewatersubstitution byDAEtoformafullDAElayerisfavorableforalargerangeof DAEconcentrationintoluene[1].Thepurposeofthepresentwork istocomplementthepreviousapproachwithmoreinsightinto themechanismsofthemonolayerformation.Indeed,our previ-ousworkwasconductedinconsideringonlyoneadsorptionmode,

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Fig.1. a)Topviewandb)Sideviewoftheperiodicslabusedtomodeltheanatase(101)TiO2surface.Covalenceofthetitaniumandoxygentopmostsurfaceatomsandthe so-called‘shortdirection’[010]and‘longdirection’[11-1]betweenTi4+_atoms_are_shown._Blue_and_red_balls_represent_titanium_and_oxygen_atoms,_{respectively.}_This_color schemeisusedthroughoutthepaper.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

withDAEformingonesinglebondwiththesurface.However,DAE possessestwoNH2functionalgroups,whichcanplayaroleinthe

adhesionwiththesurface.Thiscanbeaproblemforsome applica-tionsbecauseofi)amixtureofdifferentadsorptionmodesthatmay leadtoadisorderedlayer,ii)thehinderingofthereactivemoiety forfurtheradhesiontoasubsequentadsorption,e.g.forpolymer layers.

In the present paper, using first-principles calculations, our first aim is to further explore different adsorption modes of diaminoethane(DAE) molecules on anatase(101)TiO2 surface.

Moreoverwe also investigatethe adsorptionof another model aminemolecule,thepropylamine(PPA),ontheanatase(101)TiO2

surface.PPAexhibitsonesingleactivemoiety,supposedtoreact withsurfaceTi4+_atoms,_whereas_DAE_is_a_bifunctional_molecule

withtwo amine groupterminations, which maybind – or not – through both amines to the Ti4+ _surface _cations. _These _two

moleculescouldthushaveadifferenttopologicalbehaviorwhen interactingwiththesubstrate.Theassemblingofthemoleculesat thesurfaceoftheTiO2substrateisstudiedbyvaryingthecoverage

from0.25monolayer(ML)(oneaminemoleculeeveryfoursurface Tiatoms)to1ML(oneaminemoleculeperTiatom).Theinteraction oftheformedlayerwiththesurfaceis evaluatedthrough Den-sityFunctionalTheory-Dispersionincluded(DFT-D)calculationsto providecluesontheadhesivepropertiesofthefilms.

Inthis paper,wealsofocusand discussdispersion contribu-tions,by performingsystematicDFTand DFT-Dcalculations for alladsorptionmodes.Dispersioninteractionsappeartobecrucial whenoneconsidersself-assembledorganizationofmoleculesonto asurfacethatcouldchangedrasticallyadsorptionmodeinthestudy ofthemolecularassemblywithsurfaceinteraction[8].

2. Method

2.1. Detailsofcalculations

TheperiodiccalculationswereperformedusingtheDFTmethod based on the GGA approximation employing the rPBE [9,10]

exchange-correlation functional as implemented in the plane-waves program VASP [11,12]. The projector-augmented wave (PAW)potentials[13,14]wereusedforthecoreelectron repre-sentationwithaPAWcoreradiusof1.52Åforoxygen.Withplane wavesmethods,thequalityofthebasissetisdeterminedbya sin-gleparameter,thecut-offenergyEcut.Weusedaconvergedvalue

ofEcut=400eV.Theintegrationinreciprocalspacewasperformed

withaMonkhorst-Packgrid[15].

The bulk cell parameters for anatase TiO2 were calculated

asacalc=3.864Å,ccalc=9.707Å,values inagreementwith

exper-imental (aexp=3.776Å and cexp=9.486Å) [16] and other data

determinedwiththesamemethod[17].Theanatase(101)TiO2

surfacewasmodeledbyaperiodicslabdepictedinFig.1.Theunit cellwas(1×2)inthe[010]and[10-1]directionsrespectively.The cellwascomposedbyatotalnumberof72atomswith24titanium atomsand48oxygenatomscorrespondingtoacelldimensionof 10.3×7.6×43Å3withasurfaceareaof78.28Å2.Thecellexposed fourTi4+_cations._We_studied_four_coverages,_adding_one_to_four

moleculesinthecell,thusfromonemoleculeeachfourTi4+_to_one

moleculeperTi4+_cation._A_vacuum_zone_of₃₃_Å_in_the_z_direction

wasusedtocreateasurfaceeffect.12TiandOatomswerefrozen intheirbulkpositionsatthebottomofthecelltosimulateabulk effect.Allotheratomswerefreetorelax.Theperiodicslabwas repeatedinthethreedirections.Ak-pointmeshof(2×2×1)was used.

The anatase(101)TiO2 surface exhibitstwo types of atoms

(Fig. 1a andb): thetopmostatoms arefive-fold coordinatedTi atoms(four atomsTi5c–referred asTi4+_in _the_following)_and

two-foldcoordinatedOatoms(fouratomsO2c).Theotheratoms consideredinthebulkpositionaresix-foldcoordinatedTiatoms (Ti6c)andthree-foldcoordinatedOatoms(O3c).

Adipolarcorrectionwasintroducedalongthezaxis perpen-diculartothesurfacetotakeintoaccountthespuriouspolarity inducedbythenonequivalenceofthetopandbottomsurfaces. Thisprocedurewascarefullyvalidatedinrecentworks[1,18].

The isolated diaminoethane (DAE) and propylamine (PPA) molecules(seeFig.2)wererelaxedinthesamesimulationcellas theanatase(101)TiO2 slab.Becauseoftheexistingfreerotation

aroundtheC2 C3 bondfor both molecules,two representative

geometrieswerechosentostudydifferentconfigurationsofthe adsorbed molecules on thesurface (average orientation of the moleculetothesurface)anddifferentconformationsforeach con-figuration (local orientation of the NH2 ends versus the C2 C3

backbone):oneconformation(referredasE)withthetwo termi-nalgroups(NH2/NH2orNH2/CH3)awayfromeachothers(linear

arrangementoftheC Cbackbone,Fig.2)andtheother conforma-tion(referredasZ)withthetwoterminalgroupsincloserproximity (bentarrangementoftheC Cbackbone,Fig.2).Wehereusethe notationE–Z,asreminiscentofnomenclatureusedtodistinguish structuralcis/transstereoisomers.Sincebothmoleculesgive equiv-alentrelaxedenergiesforEandZconformations,asaconsequence ofthefreerotationaroundtheC2 C3bond,bothconformationsof

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Fig.2. EandZconfigurationsofa)Propylamine(PPA)andb)Diaminoethane(DAE) molecules.Lightbluespheres,darkblueandyellowspheresrepresentcarbon, nitro-genandhydrogenatoms,respectively.Thiscolorschemeisusedthroughoutthe paper.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereader isreferredtothewebversionofthisarticle.)

eachaminemoleculeshavebeenconsideredforadsorptionstudy inthefollowing.

Amineswereadsorbedbyapproachingthemoleculesabovethe LAS,Ti4+_atoms_of_the_anatase₍₁₀₁₎_TiO

2surface,whichhavebeen

demonstratedtobetheadsorptionsitesofdiaminoethane[1].In thestartingconfigurations,aTi Nbondwasformed,respectingthe directionalityoftheTi ObondsinthebulkanataseTiO2structure.

Asfoursurfacefive-foldcoordinatedTi4+_atoms_are_available_in_the

(1×2)simulationcell,itenablesthestudyoftheadsorptionofone tofourmolecules,correspondingtoincreasingcoverage(=0.25, 0.50,0.75and1ML)ofDAEandPPAmolecules,thusexploringthe pathwayfromasingleaminemoleculeadsorptiontowards mono-layerformationabovetheanatase(101)TiO2surface.

ToexploretheinfluenceofvanderWaals(vdW)forcesonthe adsorptiontopologiesandenergetics,weperformedsystematically successivecalculationsin theDFTandDFT-DschemesusingD2 Grimme’sformalismasimplementedinVASP[19,20].Geometries weregenerallyonlyslightlyaffectedbyvdWforces:aslight short-eningof Ti Nbondlength(−2%) wasobserved whenthevdW forceswereincluded.Ifnotstatedotherwise,theenergetic quanti-tiesgiveninthetextaretheonecalculatedwiththeDFT-Dmethod. 2.2. Energeticconsiderations

Inthispaper,theadsorptionenergies1Eadsarecalculatedusing

thefollowingformula: 1Eads=

!

Etotal(mol+surf) −n×Eoptisol(mol) −Eoptisol(surf )

where Etotal(mol+surf) is the total energy of the system with

naminemoleculesadsorbedontheanatase(101)TiO2 surface,

Eoptisol(mol) andEoptisol(surf ) arethetotalenergiesoftherelaxed

isolatedmoleculeandsurface,respectively,andnisthenumberof adsorbedmoleculesonthesurface.Absolutevaluesofadsorption energiesarereportedinthepaper.Inthisway,positivevalues indi-cateexothermicadsorption.Inthefollowing,adsorptionenergies aregivenastotaladsorptionenergypercell,inordertocompare thesurfaceenergygainatdifferentcoverages.Whenadsorption energypermoleculeiscalculated,itwillbementionedspecifically inthetext.1Eadscanbedecomposedintoseveralcontributions:

1Eads=1EadsKS+1Eadsdisp where 1EadsKS is the Kohn-Sham

contribution tothe adsorption energy and 1Eadsdisp is the

dis-persion energy. This dispersive energy can be decomposed

into the molecules/surface interaction and the intermolecular interactions in the organic layer,1Eadsdisp=1Edisp

!

mol/surf

+

1Edisp

!

mol/mol

.Thisallowsusinparticulartoanalyzethe contri-butionofthedispersionforces1Edisp

!

mol/mol

inself-assembling

inthecaseofafullmonolayer.Itisgivenby

1Edisp

!

mol_mol

=1Edisp(layer) −1Edisp(isolated molecule) and

1Edisp(layer) is obtained by a single point calculation of the

dispersive energyin vacuumof themonolayerinthe adsorbed geometryand1Edisp(isolatedmolecule) isthedispersiveenergyof

theisolatedmoleculeinthesamegeometryasinthemonolayer. Notethatallenergiesareevaluatedat0K.Inourpreviousstudy

[1]weproposedawholethermodynamicapproachtopredictthe conditionsofthediaminoethanelayerformationonanataseasa functionofthepartialpressureofwaterinthesystem.The pur-poseofthepresentworkistocomplementtheapproachwithmore insightintothemechanismsofthemonolayerformationonadry anatasesurface.

Weperformedadetailedanalysisofzeropointenergy(ZPE) con-tributionstoadsorptionenergiesinourpreviouswork[1].Thiswas necessarytoprovidethemostpreciseevaluationoffreeenergiesof adsorptionofDAEonanatase.Inthepresentwork,theZPE contri-butionstoadsorptionenergieswerefoundtobe0.10and0.17eV atthelowestandhighestcoverage,respectively.Thesesmall con-tributionsascomparedtothecalculatedenergiesofadsorptionare notincludedintheenergiesofadsorption.

ActivationbarriershavebeendeterminedalongtheMinimum EnergyPathusingadragmethod.Here,thesystemisminimized whileonecoordinateisheldfixedwhileallothercoordinatesof freedominthesystemarerelaxed.Themaximumenergy corre-spondstothetransitionstateanddeterminestheactivationbarrier. Inthefollowinginvestigatedpaths,thedragcoordinateischosen towardz-axisperpendiculartothesurface.

2.3. BornOppenheimermoleculardynamics

DFT-basedBorn-Oppenheimermoleculardynamicssimulations wereperformedwiththeCP2K/Quicksteppackage,usingahybrid Gaussianandplanewavemethodfortheelectronicrepresentation

[21].ThePerdew-Burke-Ernzerhof(PBE)[22]exchange-correlation densityfunctionalwasused.Goedecker-Teter-Hutter pseudopo-tentials(GTH)[23],atriplezetapluspolarizationGaussianbasisset (TZVP)fortheorbitalsandadensitycutoffof400Rywereused.Only fortitaniumatomadoublezetapluspolarizationGaussianbasisset (DZVP)wasemployed.Dispersionforcesweretakenintoaccount withtheGrimmeDFT-D2method[19,20].Theanatase(101)TiO2

surfacewasconstructedusingaslabmodelhavinga(1×4)surface unitcell(10.32Å×15.15Å)witha26Åvacuumregionbetweenthe slabs.Periodicboundaryconditionswereappliedinalldirections ofspace.Itwaslargeenoughinthethreedirectionstoensure con-vergenceofthewavefunctionattheGammapoint.Ononesideof theslabtheanatase(101)TiO2surfacewassaturatedbya

mono-layerofDAEmolecules.Theoverallstoichiometryoftheslabis 48TiO2+8DAE.Dynamicsof10pswereconductedinthe

micro-canonicalNVEensemble(afterequilibrationisachievedthrough canonicalNVTdynamics)withatime-stepof0.5fs.Average tem-peraturewasaround300±11K.

3. Results:amineadsorptionprocess:structuresand energies

3.1. AdsorptionofDAEontheanatase(101)TiO2surface

We considertheadsorptionprocess ofDAEmolecules form-ing zero(physisorption, Ph), one(perpendicular chemisorption, P)or two(bridgechemisorption, Br)N Tibonds.Whenseveral

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Fig.3.a)PossibleorientationsoftheadsorbedDAEmoleculeonthedehydratedanatase(101)TiO2surfaceat0.25ML.Themoleculeliesparalleltothe[010]direction.b–d) moststablestructures.BondlengthsaregiveninÅ.

moleculesareadsorbed,wejustmentiontheiradsorptionmode, forinstanceBrBr,BrP,PPfortwoadsorbedmolecules,BrBrP,BrPP forthreeadsorbedmolecules,etc.ThesubscriptEorZisaddedto indicatetheconformationoftheadsorbedspeciesontheTiO2

sur-face.Finally,whentwomoleculesareadsorbedinthePmode,they maybealignedalongthe[010]orthe[11-1]directionsasdescribed onFig.1,andthiswillbealsonotified.

3.1.1. =0.25ML

Studyingthis coverage(1.28mol/nm2₎_enables_us_to_perform

acomprehensiveand detailedstudyof theinteractionbetween asingleaminemolecule(onemoleculepercell)andtheanatase (101)TiO2surfacewithoutmolecule-moleculelateralinteraction

(wecheckedthatthesizeofoursimulationcellislargeenoughto ensurenointeractionbetweenperiodicimages).Wedetermined severalchemisorbedstates witha combinedformation ofTi N bondsand Hbondsfromboth theNH2 endstosurfaceoxygen

O2catoms.AlltheseadsorbedconfigurationsareshowninFig.3

wheretheadsorptionenergiesandbondlengthsarealsogiven. InthePadsorptionmode, thetwoconfigurationsPE andPZ are

isoenergetic(1Eads=1.06eV). The Ti N averagebond length is

2.32±0.03ÅandtheTi Nbondalwaysfollowsthedirectionality ofthebulkTi Obonds.DependingontheEorZconformerandthe orientationwithrespecttothesurface,theDAEmoleculeexhibits weakHbondswiththesurfacethroughtheadsorbed-amine hydro-genatomspointingtowardssurfaceO2catoms.TheH Odistances are2.43Åand2.48ÅfortheEconformation(Fig.3b),andtheyare 2.35Åand2.78ÅwhentheadsorbedDAEisintheZform(Fig.3c) withonehydrogenatomoftheadsorbedaminegroupinteracting withoneO2catom(H Odistance:2.35Å)andthehydrogenatom ofthenonadsorbedsecondaminegrouppointingtowardsa sur-faceoxygenatomO2c(H Odistance:2.78Å).Otherisoenergetic configurationsstudiedbytiltingthemoleculetowardsthe[010]

or[11–1]directions(seeTableSIintheSupplementarymaterials) suggestthattheadsorbedDAEmoleculeisfreetorotatearoundthe Ti Nbond,highlightingthecapabilitytobendindifferent direc-tionstowardsthesurfaceatlowcoverage,switchingfromEtoZ conformationasschematizedinFig.3a.

Incontrast,intheBradsorptionmode,oneunique configura-tionisstable(Fig.3d)anditisparalleltothe[010]direction,in a ‘bridge’conformationbetween two Tiatoms. TheTi N bond lengthsare2.32Åand2.37Å andtwoweakadditionalNH O2c hydrogenbonds(2.47Åand2.50Å)arepresent.Theadsorption energyis1Eads=1.45eVinthisBrmode,morefavorableby0.39eV

comparedtothePmodes.Thisdifferenceismaintainedwhen con-sideringthedifferencesintherotationalentropycontributionsof thePandBrmodes,whichisoftheorderofRT.IntheBr configu-ration,themoleculeisdeformedingeometryinbetweenEandZ forms(measuredN C C Ndihedralangleof102◦_)._Such_a_bridge

conformationisnotpossiblealongthe[11–1]directionbecauseof sterichindranceduetotheO2csurfaceatoms.Ifwecalculate aver-ageinteractionenergyforoneNH2group,wefindavalueof1.05eV

forthePconfigurationandavalueof0.725eV/NH2groupforthe

Brconfiguration.Inotherwords,whentheDAEmoleculeisbonded totwoTications,thebondwitheachTiisweakerthanwhenitis adsorbedononesingleTi.

WealsoinvestigatedthepathwayfromthePtoBr topology usinga dragmethod.Theassociatedenergetic barrieris ofEact

(DFT)=0.08eVanditdoesnotexistwhendispersionisconsidered duetothemetastabilityofPadsorptionmodeatlowcoverage com-ingfromthehighflexibilityofTi NbondinPmodeasdescribed before.Theback reactionfromBrtoPtopology requireshigher activationenergyof0.39eV.Thisvalueisunder-estimatedwhen dispersionforcesarenotincluded(standardDFTcalculatedvalue: 0.21eV). We can notice herethat van der Waals contributions impactdrasticallycalculationofactivationbarriersandtherefore

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Fig.4.a)andb)Adsorbedconfigurationsat0.5MLforisoenergeticBrBrandBrPZ structures,respectively.c)andd)PossiblearrangementofDAEmoleculesonthe anatase(101)TiO2surfaceatacoverageof0.5MLbyconsideringonlyvertical adsorptionPPmodes(c),andpathfromperpendiculartoparallelconfigurations (d),ModeAandmodeBaresignificantofDAEmoleculesalignedin[010]and[11-1] directionsrespectively.

mustbetakenintoaccount.Consideringboththermodynamicsand kinetics,thedataindicatethatatlowcoverage,theBrmodewillbe favored.

3.1.2. =0.50ML

SimilaranalysisofthepossibleconformationsofDAEonthe sur-facewasdoneforacoverageu₌_0.50_ML_(2.57_DAE_molecules/nm2_). Severaladsorptionmodesforthetwoadsorbedmoleculeswere investigated:BrBr,BrPE,BrPZ,PEPE,PZPZ,PZPE.

In the BrBr mode (Fig. 4a), both DAE molecules form [010] bridgesonthesurface,asatlowercoverage.Atthiscoverageand in this BrBr adsorption mode, allTi4+ _atoms _of _the_surface _are

involved inTi Nbonds.The total adsorptionenergyper cellis 1Eads=2.50eV,i.e.1.25eVpermolecule.Thisadsorptionprocessis

thusexothermic,butslightlylessexothermicthattheadsorption ofasingleDAE(1.45eV).Thisdecreaseofenergygainpermolecule canbeexplainedbystructuralmodifications inthetwoDAEBr adsorptionmode(comparedtotheadsorptionofoneDAEisolated molecule)andTi Nbondlengthsincreasefrom2.32Åto2.35Å andfrom2.37Åto2.40Åand2.42ÅwhentwoDAEmoleculesare adsorbed.

Fig.5.PossiblearrangementsforDAEmoleculesontheanatase(101)TiO2surface atcoverageof0.75ML.

TheBrBradsorptionmode(Fig.4a)isisoenergetictotheBrPZ

configuration(1Eads=2.58eV,seeFig.4b),wheretheDAEmolecule

inPZconfigurationisstabilizedbytheformationofaHbondwitha

surfaceO2catom.ThereisalsoaBrPEconfiguration(notshown)for

thetwoDAEmoleculeswithanadsorptionenergyof2.36eV,less stablethantheBrPZconfiguration.Forthemoleculeevolvingfrom

theBrtoPZtransition,thatisthemorestableBrPconfiguration,the

Ti Nbondattachedtothesurfacechangesfrom2.35Åto2.33Å andtheTi NbondsoftheBr-DAEadsorbedmoleculearereduced from2.40Åto2.38Å.TheassociatedactivationenergyfortheBrto Ptransitionestimatedusingdragmethodis0.27eV(0.10eV with-outvdWcontributions),sothattheBrtoPtransitioncouldoccurto releasestericeffects.ThislatterBrPZconfigurationcanbean

inter-mediateconfigurationadoptedbyDAEstoliberateaTi4+_site_for

theadsorptionofanothermolecule,andthusincreasethesurface coverage.

InthePPmode(someconfigurationsareschematizedinFig.4c andd),duetotheasymmetricenvironmentofeachTi4+_surface_site,

thetwoDAEmoleculescanbealignedeitheralongthe[010]or [11-1]direction(modeA:PP[010]andmodeB:PP[11-1]respectively). ThePEPE[010]andPEPE[11-1]adsorptionmodesarenearly

isoen-ergeticwithanadsorptionenergyofaveragevalue2.01±0.04eV,

Fig.4canddwhereasthePZPZ[11-1]configurationismorestable

thanthePZPZ[010]one(2.08eVvs.1.50eV).Indeed,asthevicinal

Ti–Tidistancesdifferinthe[010]and[11-1]directions(3.78Åand 5.48Årespectively),sterichindranceisresponsibleforthelower adsorptionenergycalculatedfortheZconformationwithrespect toEconformationinthePP[010]topology.Weobservethatinthe [010] direction,Ti N bondsare longercomparedtothe[11–1] direction,evenmorewhenthemoleculeoccupiesmorespacelike inZconformation:Ti Nbondshaveincreasedfrom2.30Åinthe caseofoneadsorbedPE to2.32Åand2.33Åin[11-1]and[010]

directions,respectivelyandfrom2.30Åinthecaseofoneadsorbed PZto2.32Åand2.36Åin[11-1]and[010]directions,respectively.

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Fig.6. PossibleconfigurationsofDAEmoleculesontheanatase(101)TiO2surface atcoverageof1ML.RelaxedconfigurationofthemoststablestructurePEPEPEPEis given.BondlengthsareinÅ.

isoenergeticconfigurations,PEPE[010],PEPE[11-1]andPZPZ[11-1],

whichremainhoweverlessstablethantheBrBrandBrPZones.

WewilldemonstratebelowthattheincreaseoftheTi Nbond lengthswhentwo DAEmoleculesareadsorbed,i.e.asobserved inBrBr,showsadecreaseoftheKohnShamcontributioninthe totaladsorptionenergy.Moredetailsaboutthecontributionsof thedifferentforceswillbeprovidedinthefollowing(comments onFig.9).

3.1.3. =0.75ML

Atthiscoverage,correspondingtothreemoleculespercell,the followingconfigurationswereconsidered,asschematizedinFig.5: i)Thephysisorption(Ph mode)of anadditionalDAEmolecule ontheBrBr[010]configurationobtainedat0.5ML.Itresults in the BrBrPh configuration, with an adsorption energy of 1Eads=2.73eV,

ii)BrPEPh configuration,with1Eads=2.93eV.Interestingly,the

BrPEPhmodeisstabilizedovertheBrBrPhconfigurationanda

Ti4+_atom_is_now_available_for_the_next_step_of_adsorption,

iii)BrPEPE (1Eads=3.07eV) or BrPZPZ (Eads=2.84eV)

configura-tions,and

iv)finallythePPPmode(with1Eads=2.96eVand2.46eVinthePE

andPZmodes,respectively)

Wenotethatthemoststableconfigurations,BrPEPEandPEPEPE

arenearlyisoenergetic,revealingsomedegreesoffreedomofthe moleculeadsorbedinthehalf-filled[010]direction.Moreoverwe showedthatthetransitionfromtheBrtothePmodestartingfrom BrPEPE toreachPEPEPE adsorptionmodeisassociatedtoasmall

activationenergyof0.27eV(DFT-D).InthePEPEPEmode,allthe

Ti NbondsareshortenedcomparedtoBrPEPE.Wealsonoticethat

whenthecoverageincreases,theEformisstabilizedovertheZ formforstericreasons.

3.1.4. =1.00ML

Finally,a fullmonolayerisobtainedbyaddinga fourthDAE molecule on the surface of the slab chosen as a model of the TiO2surface(Fig.6).Obviously,theonlyconfigurationisthePPPP

one.ThePEPEPEPEconfiguration(3.86eV)ismorestablethanthe

PZPZPZPZone(2.93eV)asexpectedduetotheincreasedcrowding

andmoresizeablePZconfiguration.Ifwecomparethechangesin

thecharacteristicsoftheadsorptionmodeinthemonolayer,we firstnotethattheTi Nbondlengthisincreasedfrom2.30Å(0.25 ML)to2.37Åfor1MLPEPEPEPEand2.38–2.39Åfor1MLPZPZPZPZ.

Inaddition,at1ML,thereisaninteractionbetweenoneHatom

Fig.7.Adsorptionenergy(eV)versuscoverageforadsorptionofDAEontheanatase (101)TiO2surfaceasafunctionofthesurfacecoveragewithvariousconfigurations ofadsorbedmolecules(PE,Pz,Br,Ph).

oftheaminegroupandthesurfaceoxygenatomO2cwithabond lengthof2.23Å.Actually,thisNH O2cbondisnotpresentat0.25 MLforthesameadsorptionmode(seeFig.3a).Infact,with increas-ingcoverage,theDAEaminetailsareslightlyreorientedtopoint towardthevicinaladsorbedamineendinthesame[010]row.This rotationisassociatedwithapreferentialNH O2cinteraction.The projecteddensityofstatesanalysis(PDOS)ofthesurfacebeforeand afteradsorptionoftheDAEMLisreportedintheS2section.

Wealsoconsideredtheformationofabilayerstructure consist-ingintheBrBrmodefortwoDAEmoleculesplustwoadditional DAE molecules in a second layer (not shown); the formation ofsuch configuration isexothermic, withanadsorption energy of 1Eads=3.07eV, less stable than the PEPEPEPE configuration

(1Eads=3.86eV,i.e.0.20eVlesspermolecule).

Fig.7 summarizesall the adsorptionenergies as a function of coverage for various configurations. The adsorption energy increaseslinearlywiththesurfacecoverage forthePE

configu-rations(PEtoPEPEPEPE),suggestingthattheenergyofadsorption

permoleculeiscoverageindependent.Thiscouldsuggestthatthe interactionsbetweenthechainsarenegligible.Howeverwewill demonstratebelowthat itisnot thecase.Theincrease ofvdW interactionsbetweenthechainsinthelayeriscompensatedbythe decreaseofthechemicalTi Nbondwiththesurface.Fromthe analysisofallthestructuresforthevariouscoverage,preferred reactionpathsforthemonolayerformationcanbededuced.At =0.25ML,theDAEmolecule(onepercell)chemisorbsinabridge conformationBrwithtwoTi Nbonds.At0.5ML,theBrBr struc-turewhereallTi4+_atoms_are_saturated,_is_isoenergetic_with_the_BrP

Z

configuration.Forcoverageof0.75ML,theBrPEPEandPEPEPEare

alsoisoenergetic.ThefullMListhencharacterizedbyaPEPEPEPE

structure.From0.25 MLto0.75ML,in adsorbedconfigurations whereBrconfigurationsexist,anincreaseoftheTi Nbondlength isobservedfrom2.32Åto2.43Å,associated toincreasingsteric effectscausinganenergypenalty.TheBrtoPtransition compen-satesthisenergydecreasebecausethePadsorptionmodereduces sterichindranceintheadsorbedlayer,allowingforastrengthening ofTi Nbonds,asdeducedfromisoenergeticconfigurationssuchas BrBrandBrPZat0.5MLandBrPEPEandPEPEPEat0.75ML.Wethus

observeagradualBrtoPtransitionwithincreasingcoverage,with intermediatecoveragesituationswhereoneTi4+_atom_is

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unsatu-Fig.8.MoststableconfigurationsobtainedforPPAadsorbedonanatase(101)TiO2surfaceata)0.25MLandb)1ML.BondlengthsaregiveninÅ. rated,enablingtheadsorptionofanadditionalmoleculetooccur

andresultingintheformationofadenselayer.

3.2. AdsorptionofPPAovertheanatase(101)TiO2surface

Herethesituationismuchmoresimpleaseachmoleculecan bindonlytooneTi4+_center_through_the_amine_group._A_similar

cov-erageanalysiswasperformedforthePPAmoleculetoinvestigate theroleoftheterminalgroupininteractionofthemoleculeswith thesurface.Foreachconsideredadsorptionmodeperpendicularto thesurface(EorZconfigurations),itwasfoundforthemoststable configurationsthatthePPAadsorptionenergiesareclose(different bylessthan0.30eV/cell)totheequivalentconfigurationfortheDAE molecule,andthuswhateverthesurfacecoverage.For=0.25,the adsorptionenergiesare1.06eVand1.08eVforthePE

configura-tionsoftheDAEandPPAmoleculesand1.05eVand1.09eVforthe PZconfigurations.Atu=0.5,theadsorptionenergiesare1.98eVand

1.88eV(PEPEmodeA)2.05eVand2.06eV(PEPEmodeB),1.50eV

and1.55eV(PZPZmodeA),2.08eVand2.06eV(PZPZmodeB)forthe

DAEandPPAmolecules,respectively.(FormoredetailsseeTables SIandSIIintheSupplementarymaterials).Moreover,forPPAas forDAE,thePEconformationisfavoredfromintermediate

cover-age(0.75ML,1Eads=2.81eVforPEand1Eads=2.50eVforPZ)to

theMLformation(1Eads=3.55eVforPEand1Eads=2.91eVforPZ

inFig.8).ForthePPAmolecule,fromu₌_0.25_to_1,_the_Ti _N_bond lengthincreasesfrom2.30Åto2.37Å(bothintheEandZ conforma-tions)asinthecaseoftheDAEmolecule,showingthatnoelectronic inductiveeffectstakeplaceintheDAEcomparedtoPPA.At0.25ML, theamineHatompointsinthedirectiontosurfaceO2cforPPAand DAEwhereasat1MLareorientationoftheamineHatomstowards avicinalamineandasurfaceOatomisobservedforPPA.Finally,in themoststableconfigurationofadsorption(PEPEPEPE),the

forma-tionofamonolayerofDAEis0.30eVmoreexothermic/cellthanthe formationofaPPAmonolayer(3.86eVvs.3.55eVforDAEandPPA respectively),duetoastrongerTi NchemicalbondintheDAElayer (29%vs.39%ofthetotalenergyofadsorptionisduetochemical bondingforPPAandDAErespectively).

3.3. Kohn-ShamversusvanderWaalscontributionsinthe monolayerformation

ThefollowingparagraphfocusesonFig.9whereKohn-Sham (Cov)andDispersive(bothintermolecularinteractionsinthelayer (Coh)andinterfacialsubstrate/layeradhesiveenergy(Adh)) contri-butionsofadsorbedconfigurationsofDAEmoleculesasafunction ofcoveragearedetailed.Itisworthtonotethat,forallthe inves-tigatedstructures,dispersionenergy(Coh+Adh)accountsfor42%

(PE)to100%(PZPZPZPZ)ofthetotalstabilizationenergy(Fig.9and

TablesSIIIandSIVintheSupplementarymaterials).

At0.25ML,intheperpendicularPorientationandwhateverthe moleculetopologyonthesurface,acombinationofoneTi Nbond, oneH-bond(NH O2c)and45±2%vanderWaalsinteractionsis obtained.IntheBrstructure,theadsorptionenergyisenhanced, butsurprisingly,withoutanylargeincreaseofthetotalKohn-Sham energy(∼0.07eV).Thiscanbepartlyexplainedbya compensa-tionintheKohn-Shamenergybetweenthecreationofanewbond withthesurfaceandthemoleculeflexibility(seethenextsection AbinitioMolecularDynamics(MD)ofthefullDAElayerinvacuum). Furthermore,inthisconfiguration,Ti Nbondlengthsareslightly increasedcomparedtoPmode.Infact,theenergygaininthebridge structureisbroughtmainlybythemolecule-surfacevanderWaals interactions,as,obviously,theentiremoleculeisclosertothe sur-facethanintheperpendicularorientation,revealingacontribution totheadsorptionenergyupto51%forvdWinteractions.

TheactivationbarriersforthePtoBrtransitionorBrtoP transi-tionarenotlargeevenathighcoverage,whichexplainstheability oftheDAEtoattachandseparateoneofitsextremitiesfromthe surfaceasafunctionofthecoverage.Thisabilitytochange adsorp-tionmodefrom BrtoP andviceversa is intimatelycoupled to thecoveragewheredispersivecontributionswillworkintandem withKohnShamcontributions:Bridgeconfigurationexhibitssteric effectsoncecoveragereaches0.5ML.Itisnoticeablebyanincrease ofthenumberofTi NbondsinbothBrconfigurations.Thesesteric effectscanthenbereducedthroughafavorableBrtoPtransition. Between0.25MLand0.5MLwhenBrconfigurationsareinvolved, weobservethattheKohnShampartofthetotalenergydecreases from49%inBrto43%inBrBr(Fig.9)astheTi Nbondsstretched from2.37Å/2.32ÅinBrto2.40Å/2.35Åand2.42Åand2.35Åin BrBr.FortheBrtoPtransition,notablyinBrPZ,weobservethat

KohnShampartincreasesupto48%,tofinallyprovidean equiv-alenttotaladsorptionenergyofaround2.50eVforBrBrandBrPZ

configurations.Thesameconclusionscanbedrawnfor0.75ML sincethreeisoenergeticadsorbedconfigurationsareidentified,i.e. BrPZPZ,BrPEPE andPEPEPE:inBrPZPZ Ti Nbondsareweakened

(2.43Å,2.37ÅfortheBr-DAE,and2.39Åand2.37Å)resultingin 28%Kohn-Shampartofthetotaladsorptionenergy,whereasthe Kohn-Shamcontributionisenhancedwhenconformationchanges fromBrPZPZtoBrPEPE(28%–47%).Then,theBrtoPtransitionisalso

afavorableprocesstohelpreducingstericeffectsencounteredin BrPZPZ(44%ofKohnShampartinPEPEPE).

AtthefullMLcoverage,theDAE-PEmonolayerformationhas

thestrongestKohn-Shamenergy(1.47eV),representing39%ofthe total(DFT-D)adsorptionenergy(SeeFig.9).Theother61%(2.37eV) comefromthedispersivecontribution,thatcanbedecomposedin 2/3duetotheinteractionbetweenthesurfaceandthemolecules

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Fig.9.KohnShamanddispersivecontributionsobtainedfortheadsorptionofDAE asafunctionofcoverage.Onlyselectedadsorbedconfigurationsaredescribed.The yellowarearepresentstheDFTcontributiondepictedasKohnShampart(Cov). Orangearearepresentsthedispersivecontribution(D)duetotheinteractionenergy betweenthesurfaceandthemolecules(Adh).Greyareaisthecohesiveenergy betweenmoleculescontributingtothedispersiveforces(Coh).Percentofeach contributionofthetotaladsorptionenergyofeachconfigurationisprovided.(For interpretationofthereferencestocolorinthisfigurelegend,thereaderisreferred tothewebversionofthisarticle.)

and1/3due tomolecularinteractions energybetweentheDAE moleculeswithinthemonolayer.Similarratiosarefoundforthe PPA-Emonolayercoverage.

Fig.10.a)PreferentialorientationoftheDAEchainwithrespecttothesurface.b) DistributionofthepreferredorientationsoftheDAEmoleculesplacedinvicinal [010]rows.

AdsorptionsofbothDAEandPPAinZconformationsareless favorablethantheadsorptionoftheEconformationsforthe dens-estmonolayers. TheKohn-Sham energiesarenegligiblewithan athermic valueof−0.06eVand +0.07eVrespectively.For theZ conformations,theadsorptionenergiesarethusonlyduetohigh dispersivecontributions:2.99eVforDAE-Zand2.84eVforPPA-Z. Theseresultshighlightthatmediumscaleandlongscaledispersive forcescanforcetheadsorptionandmonolayerformation,evenin thepresenceofshort-rangesterichindrance.

3.4. AbinitioMolecularDynamics(MD)ofthefullDAElayerin vacuum

FromtheDFTresults,weundertookanabinitioMDstudyofthe moststableMLconfiguration,i.e.thePEPEPEPEDAElayeradsorbed

onTiO2.ItwasfoundthatmostofadsorbedDAEmoleculesretain

a linearPE conformationofthechain alongtheentire

trajecto-ries.Only13%oftheDAEmoleculespresenta stablebendedPZ

conformation.

Inthemonolayer,preferentialorientationsoftheDAEchains withrespecttothesurfacearealsoobserved.Inparticular,DAE chainsaretiltedby+20◦_or₋₂₀◦_with_respect_to_the_plane_normal_to

thesurfacealongthe[10-1]direction(Fig.10a).Thisarrangement allowstheformationofH-bonds(asdetailedbelow)betweenthe aminegroupinvolvedinthecoordinationwiththeTisurfaceatoms andthevicinaloxygensurfaceatoms.Alongthewholetrajectory theDAEmoleculesswitchesfromoneorientationtotheotherone each3ps.AspresentedinFig.10b,thetiltingoftheDAEmolecules placedonthesame[010]rowoccursinaconcertedway.In con-trast,theorientationofvicinalDAEmoleculesadsorbedondifferent [010]rows arenotcorrelated, becausetheyarenot interacting, andthereforetwogeometricalarrangementsarepossible,i.e.DAE moleculesonvicinalrowscanbetiltedinthesameorthe oppo-siteorientations.During50%ofthesimulationtime,DAEchainsof

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Fig.11.HbondsanalysisattheDAE/TiO2interfaceinvacuum.Countsrefertothe percentageoftimeinwhichnH-bondsarecounted.

vicinal[010]rowsliewiththesameorientationwhiletheyliein oppositeorientationfortheother50%oftime(seeFig.10b).

H-bondanalysishasbeenconductedtobetterunderstandthe interactionoftheDAEmoleculeswiththesurfaceandthe inter-actionamongtheDAEmoleculesofthemonolayer(Fig.11a).50% oftheDAEmoleculesareinvolvedinH-bondinteractionwiththe surfaceoxygenatoms,whereasnegligibleH-bondinginteraction occursamongtheterminalaminegroupsofthemonolayer. 4. Conclusion

Theformationofselfassembledmonolayersofdiaminoethane (DAE) and propylamine (PPA) molecules onanatase(101)TiO2

surfacehasbeeninvestigatedfromlowcoveragetoadenseand organizedlayer.Inthecase ofDAE,theresultssuggestthe fol-lowingreactionpathuptomonolayerformation:atlowcoverage, moleculesformbridgebondswithtwosurfaceTi4+_atoms,_then_at

intermediatecoverage,a‘bridgetoperpendicular’transitionofthe adsorbedmoleculesoccurs,liberating successivelyTi4+_atoms,_to

finallyformafullmonolayerwithallmoleculesinaperpendicular orientation,withoneTi Nbondpermolecule.

ForthePPAmolecule,whichexhibitsonesingleNH2

extrem-ity,adsorptionoccurswithmoleculesalwaysperpendiculartothe surface (at all coverages). For both molecules,the full ordered monolayer is the most favored configuration with a formation energyof3.5eV/celland3.8eV/cellforPPAandDAErespectively. Dispersionforcesareessential:VdWinteractionscontributedeeply into themonolayerformation upto40% and 100% ofthe total adsorption energy at low coverage and high coverage, respec-tively.Thedatainthispapershowthattheformationofthedense andwell-organizedSAMinvolvesacompromisebetweenchemical graftingonthesurface,andreductionofstericeffectsallalongthe assemblyprocess.

Acknowledgements

Calculations have been performed at the GENCI (CINES and IDRIS) French supercomputing center under the pro-gramx2011082217. This work was partially supported by the regionIDF(IledeFrance)andVilledeParis(FRESCORTprojectof PôleASTECH).

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,athttp://dx.doi.org/10.1016/j.apsusc.2017.07.

161.

SectionS1providesadditionalenergeticdata,and sectionS2 theelectronicanalysisof1MLofDAEadsorbedonTiO2.SectionS3 therespectivecontributionsofdispersiveandionocovalentforces. SectionS4showssideviewsofadsorbedconfigurationsat0.75ML. References

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