Hybrid functional calculated optical and electronic structures of thin anatase TiO2 nanowires with organic dye adsorbates

ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o ur na l ho me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Hybrid

functional

calculated

optical

and

electronic

structures

of

thin

anatase

TiO

2

nanowires

with

organic

dye

adsorbates

Hatice

Ünal

a

_,

_Deniz

_Gunceler

b

_,

_O˘guz

_Gülseren

c

_,

_S¸

_inasi

_{Ellialtıo˘glu}

d

_,

_Ersen

_Mete

a,∗ a_{DepartmentofPhysics,BalıkesirUniversity,Balıkesir10145,Turkey}

b_{DepartmentofPhysics,CornellUniversity,Ithaca,NY14853,USA} c_{DepartmentofPhysics,BilkentUniversity,Ankara06800,Turkey} d_{BasicSciences,TEDUniversity,Ankara06420,Turkey}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received25December2014 Receivedinrevisedform10April2015 Accepted11April2015

Availableonline20April2015

a

b

s

t

r

a

c

t

TheelectronicandopticalpropertiesofthinanataseTiO2(101)and(001)nanowireshavebeen

inves-tigatedusingthescreenedCoulombhybriddensityfunctionalcalculations.Forthebarenanowireswith sub-nanometerdiameters,thecalculatedbandgapsarelargerrelativetothebulkvaluesduetosize effects.Theroleoforganiclightharvestingsensitizersontheabsorptioncharacteristicsoftheanatase nanowireshasbeenexaminedusingthehybriddensityfunctionalmethodincorporatingpartialexact exchangewithrangeseparation.Forthelowestlyingexcitations,directionalchargeredistributionof tetrahydroquinoline(C2-1)dyeshowsaremarkablydifferentprofileincomparisontoasimplemolecule whichischosenasthecoumarinskeleton.ThebindingmodesandtheadsorptionenergiesofC2-1dye andcoumarincoreontheanatasenanowireshavebeenstudiedincludingnon-linearsolvationeffetcs. Thecalculatedopticalandelectronicpropertiesofthenanowireswiththesetwodifferenttypesof sen-sitizershavebeeninterpretedintermsoftheirelectron–holegeneration,chargecarrierinjectionand recombinationcharacteristics.

©2015ElsevierB.V.Allrightsreserved.

1. Introduction

Thewide-bandgapsemiconductortitaniumdioxide(TiO2)has

attractedattentionsincethediscoveryofitsremarkable

photocat-alyticactivityunderUVirradiation[1,2].TiO2canbefunctionalized

foroperationundervisiblelightilluminationbyimpuritydopants

orbysensitizing molecularadsorbates.Thelatterisparticularly

importantindyesensitizedsolarcell(DSSC)applicationswhere

nanoporousTiO2filmisusedastheanodeelectrode[3].Titania

surfacesoffergoodadsorptioncharacteristicsandcaneasilybe

cov-eredwithamonolayerofalightharvestingdye.Theoperational

systemsitsinanorganicsolventelectrolytewitharedoxsystem

likeiodide/triiodidecouple.Photoexcitationofthemoleculeleads

toelectron–holegenerationandsubsequentchargeinjectiontothe

conductionband(CB)oftheoxidewhichactsasacurrent

collec-tor.Then,anelectronisdonatedfromtheelectrolytetorestore

thegroundstateofthedye.Ontheotherhand,thechargecarrier

transferthroughputislimitedduetorecombinationrate.

There-fore,thesurfaceareaoftheoxideandthechoiceofthedyebecomes

importantintheoverallcellefficiency.

∗ Correspondingauthor.Tel.:+905335733595. E-mailaddress:[email protected](E.Mete).

Thecrystalstructuresoftitaniaoccurmainlyinanatase,rutile

andbrookitephasesinnature.Underambientconditionsandin

itsbulkform,rutileisthemoststablepolymorph.Theopticalgaps

aremeasuredas∼3.2eVforanatase[4]andas∼3.0eVforrutile

[5]. Theanatasephase especially its(001)surfaceis knownto

exhibithigherphotocatalyticactivityinmanysituations[6].

More-over,TiO2nanomaterialswithgrainsizessmallerthan14nmare

foundtobeinanataseformratherthaninrutilestructure[7,8].

Electronically, thin anatasenanowiresexhibit differentfeatures

relativetoothernanoparticulatetitaniaelectrodes.Inparticular,

theincrease inthecorrespondingbandgapsand photocatalytic

oxidationpowersbecomemoreapparentasthenanowire

diam-eterstendtobesmallerthan2nmduetotheconfinementeffect

[9–11].

Theprogressinthesynthesisofquasi-one-dimensionaloxide

nanostructures opens up new possibilities to fabricate

techno-logicallyattractiveapplications.Recently, titaniananowiresand

nanorods become candidate building blocks of highly ordered

architecturesforsolarcells[12].Theyofferlargesurface-to-volume

ratioswhichalsoenhancen-typeconductivityproperties[13–15].

In addition, thisis seen asan importantfactor in reducing the

charge carrierrecombination rates.Therefore, theuseof

quasi-one-dimensionaltitaniaasthecurrentcollectorcomponentisa

promisingwaytoimprovecellefficiencies.

http://dx.doi.org/10.1016/j.apsusc.2015.04.086

Organic photosensitizers have become a good alternative to

metaldrivencommercialdyesnotonlybecausetheyarecheap,

environmentallyfriendlyandeasytoisolatebutalsobecausetheir

electronicandanchoringcharacteristicscanbemodifiedthrough

variouslyfunctionalattachablemoieties.Recently,aclassofnovel

organicsensitizersbased ontetrahydroquinoline moiety asthe

electrondonorgrouphavebeenproposedtoachievephotoinduced

intramolecular charge transfer [16]. For instance, -conjugated

electrondonor acceptor(D--A)type C2-1dyehasthehighest

conversionefficiencyof4.5%withinthisfamily[17].

Theoretical prediction of molecular and material properties

havebeenfocusedbymanyresearcherstounderstandandimprove

functionaldyesensitizersadsorbedontheoxide[18–24].

Tetrahy-droquinolinebasedD--AtypedyesonTiO2(101)surfacehave

beenstudiedusingstandarddensityfunctionalslabcalculations

wheretwo-dimensionalperiodicitywasadopted[25,26].In this

work,weexaminedthebandgapfeaturesofthinanatase(101)

and(001)nanowiremodelsusingtherangeseparatedhybrid

den-sityfunctionaltheorycalculations.Then,weconsideredisolated

C2-1(C21H20N2SO2)charge transferdyeonthesenanowiresfor

theiradsorptiongeometriesandtheirbindingenergiesinthe

solu-tionusinganonlinearsolvationmodel.Wehavealsoinvestigated

theelectronicpropertiesoftheresultingcombinedphysicalsystem

suchasthedensitiesofstates,thechargedensitiesofthefrontier

statesandtheabsorptionspectra.Inordertomakeacomparison

withasimpleskeletonmolecule,wehavechosenthecoumarincore

(C9H6O2)andrepeatedthesamecomputationssystematicallyfor

thecoumarinonthethinanatase(001)and(101)nanowiresusing

thehybridmethodbeyondthestandardDFT.Infact,coumarindyes

withvariousanchoringgroupshasbeenextensivelystudiedinthe

literaturebytheoreticalworks[27–39,41,40,42,43].

2. Computationaldetails

Weperformedperiodictotalenergydensityfunctionaltheory

calculationsusingtheimplementationofthescreenedCoulomb

hybridexchange-correlationscheme,HSE[44–46],intheVienna

ab-initio simulation package (VASP) [47]. Single particle states

wereexpandedintermsofplanewavesuptoakineticenergy

cut-offof400eVusingtheprojector-augmentedwaves(PAW)method

[48,49].Brillouinzoneintegrationswerecarriedoutoverak-point

samplingtogetwell-convergedvalues.

ThestandardDFTexchange-correlationfunctionalslikePBE[51]

suffersfromthelackofproperself-interactioncancellation(SIC)as

intheHartree–Fockapproximationtotheinteractingmany

par-ticleproblem.Ingeneral,inthehybridapproaches,thenonlocal

exactexchangeenergyispartiallyadmixedwiththesemilocalPBE

exchangeenergyinordertoimprovethedescriptionofthe

local-izedstatessuchasthed-orbitalsoftheTiO2.Since,thebottomof

theconductionband(CB)oftitaniaiscomposedof3d-stateshaving

t2gsymmetry,hybridDFTmethodssignificantlyhealthebandgap

underestimationofthestandardexchange-correlationschemes.

TherangeseparatedhybridHSEfunctionaltreatstheexchange

energyascomposedoflong-range(LR)andshort-range(SR)parts,

EHSE X =aE HF,SR X (ω)+(1−a)E PBE,SR X (ω)+E PBE,LR X (ω)

whereaisthemixingfactor[50]and ωistherangeseparation

parameter[44–46].Thecorrelationenergyistakenfromstandard

PBEcorrelationenergy[51].Inourcalculationsweusedamixing

factorofa=0.22toreproducetheexperimentalbulkbandgapof

anataseas3.20eV.

Solvation effects have been included at the hybrid HSE

exchange-correlationfunctionallevel forC2-1and coumarinon

theanatase nanowires consideredin this work.We performed

calculations forchloroform and water environmentvia

polariz-ablecontinuummodel(PCM)includingboththenewnon-linear

anditslinearcounterpartasimplementedintheopen-sourcecode

JDFTx[52–55].Thesolventenvironmentisreproducedbya

dielec-tricmediumsurroundingthesolutedyemolecule.Thedielectric

functionofthesoluteturnsonaroundacriticaldensityvaluewhich

Fig.1.Thechargedensitydistribution plotsofthehighestoccupiedstatesof coumarin+001nw(a),C2-1+001nw(c),andofthelowestunoccupiedstatesof coumarin+001nw(b),C2-1+001nw(d)calculatedusingtheHSEfunctional.Inthis ball-stickrepresentation,thered,light-blue,brown,grey,yellowandwhitecolors areusedforO,Ti,C,N,SandHatoms,respectively.(Forinterpretationofthe refer-encestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthe article.)

isusedtomodelthecavityaroundthesoluteandparametrizedto

reproduceexperimentalsolvationenergies[53,54].

Theanatase001nwand101nwmodelstructuresarecleaved

fromthebulkphaseasshown inFigs.1and2.Bothnanowires

withandwithoutC2-1 andcoumarinmoleculesare considered

inlargeperiodictetragonalsupercells.Thenanowireshave

peri-odicityinonedimension.Avacuumseparationofatleast20 ˚Ais

adoptedaround thenanowirein order toavoid anyunphysical

interactionwithitsperiodicimagesinthedirections

perpendic-ulartothenanowireaxis.Wealsousedlargerperiodicityalongthe

nanowireaxisinordertoconsiderthedyemoleculesasisolated

onthe(001)and(101)facets.Weconstructedseveralprobable

initialadsorptiongeometries.Theatomicpositionswerefully

opti-mizedusingtheconjugate-gradientsalgorithmbyminimizingthe

Hellmann–Feynmanforcesoneachatomuntilathresholdvalueof

0.01eV/ ˚Aisreachedtostoptheself-consistentcycles.Therelaxed

nanowiremodelskeeptheanataseformwithoutamajorstructural

distortion[56].

Wecalculatedtheabsorptionspectrabyconsideringthe

tran-sitionsfromtheoccupiedtotheunoccupiedstateswithinthefirst

Brillouinzone[57].Theimaginarypartofthefrequency

depen-dentdielectricfunctionε2(ω)canbeexpandedasasumoversingle

particlestatesdeterminedbytheself-consistentHSEcalculations.

Fig.2.Thechargedensitydistributionplotsofthehighestoccupiedstatesof coumarin+101nw(a),C2-1+101nw(c),andofthelowestunoccupiedstatesof coumarin+101nw(b),C2-1+101nw(d)calculatedusingtheHSEfunctional.

3. Resultsanddiscussion

Thin 001nw and 101nw models have been considered in

periodicsupercellswithlargevacuumseparations.Theiratomic

positionshavebeenrelaxedusingtheHSEexchange-correlation

functional. We didnot fix any of theionic cores to theirbulk

positionsinanyofthegeometryoptimizationcalculationsinthis

work.AlthoughtheHSE-optimizedatomicstructuresofthebare

nanowires are not shown,they didnot haveany major

recon-structionfromtheirinitialgeometriessimilartothoseobtained

previously [56].Thesurface Ti Obond lengthsbecomeslightly

largerthan thebulkvalue of1.95 ˚A.Thisdifferencegets much

smallertowardthecenterofthenanowires.Therefore,both001nw

and101nwmodelsmaintaintheanatasestructure.

We haveconsidereda numberofprobableinitialadsorption

configurationsforC2-1moleculeonbothofthenanowiremodels.

ThetailoxygenandtheOHgrouparefoundtobeactivelyinvolved

intheadsorptionofthedyeontheoxidesurface(seeFigs.1and2).

C2-1moleculecanachieverelativelyhighadsorptionenergiesinits

bidentatemodeaspresentedinTable1.Inthiscase,theOHgroup

losesitshydrogentooneofthenearbysurfaceoxygensandthe

remainingtwotailoxygensformtwoTi Obondswiththeanatase

surface.Inthemonodentatebinding,however,C2-1formsonlyone

Ti Obondthroughitstailoxygen.Thebidentatedye-surfacebond

lengthsare∼2.0 ˚Awhichisclosetothebulkvalue.C2-1molecule

slightlydistortsthenanowirestructureonlylocally.

Asareferenceminimalskeleton,weconsideredthecoumarin

coreonthenanowiresurfaces.Inthiscase,coumarinprefersto

formasinglebondwitha5-foldcoordinatedsurfaceTiatomby

aligningperpendiculartobothofthenanowiresasanenergetically

favorablebindingconfiguration.TheTi Obondsbetweenthedye

andthesurfacebecome2.18 ˚Aand2.21 ˚Aon101nwand001nw,

respectively.Thedistortionofcoumarinonthenanowirestructure

isevensmallerrelativetotheC2-1case.

TheC2-1complexisknowntohavephotoinduced

intramolec-ular charge transferin thegas phase. On theother hand,both

theHOMO andtheLUMOcharge densities remainlocalizedon

theentirecoumarincore.Therefore,theirchargeredistributions

are remarkably different if theyare considered to be isolated.

Theirchargedensityrelocationcharacteristicsuponanexcitation

becomesimportantwhenthesemoleculesareattachedtoanoxide

surface.Inordertointerpretthechargeinjectionfeaturesofthese

twodifferentdyeswecalculatedthechargedensitiesofhighest

occupiedandlowestunoccupiedstatesofdye+nwcombined

sys-tems. Our HSE resultsare depictedin Fig. 1 for 001nwand in

Fig.2for101nwcases.Thedonor-to-acceptorcharacterofC2-1dye,

whichcanbeexplainedastheintramolecularchargetransferfrom

thetetrahydroquinolinemoietytothe-conjugatedacidicpart,is

significantlymodifiedwhenthemoleculeisadsorbedonthe

tita-niasurfaces.Thechargedensityofthehighestoccupiedstateof

thecombinedsystemisdistributedovertheentiremolecule

sim-ilartothecaseofcoumarinonthenanowires.Similarbehavioris

seenonboth001nwand101w.However,thechargedensity

distri-butionsofthelowestlyingunoccupiedstatesofC2-1+001nwand

coumarin+001nwareremarkablydifferent.Thiscanbeclearlyseen

bycomparingthecorrespondingchargedensityplotsinFig.1band

d. Inthecase of101nw,bothdyesexhibitsimilarcharge

redis-tributionfeatures betweenthehighestoccupiedandthelowest

unoccupiedstates.

ThebindingenergiesofC2-1andcoumarinonthe001nwand

101nwformonodentateandbidentateadsorptionmodesare

cal-culated at the HSE level by the standard formulation used in

similarsystemspreviously[18,19].Moreover,weincluded

solva-tioneffectsforthechloroformandwaterenvironmentsusingHSE

functionaland anewnonlinearPCM.OurHSE+PCMresultsare

Table1

Calculatedadsorptionenergiesofdyemoleculesonthinanatase(001)and(101)nanowirestructuresinvacuumandinsolutionusingHSEmethodandnonlinearPCM. EnergyvaluesaregivenineV.

Dye @001nw @101nw

HSE HSE+PCM(CHCl3) HSE+PCM(H2O) HSE HSE+PCM(CHCl3) HSE+PCM(H2O)

Coumarin −0.63 −0.33 −0.07 −0.70 −0.58 −0.44

C2-1(monodentate) −0.73 −0.65 −0.11 −0.62 −0.43 −0.30

C2-1(bidentate) −1.25 −1.18 −0.52 −0.83 −0.70 −0.51

onboth(001and(101)surfacesshowsimilarlymoderate adsorp-tionenergiesinvacuumandalsointhesolution.Singlebondsare foundtobedrasticallyweakenedbythedielectricenvironmentof thestrongsolutionslikewater.Therefore,thesetypeof monoden-tatebindingmodesarenotexpectedtobedurableandstablein theelectrolyteenvironment.Ontheotherhand,bidentatemodeof C2-1ontheoxidesurfacesgivesreasonablystrongbinding ener-gies.Therefore,C2-1dyeformingtwoO-Tibondswiththesurface canevenbestableinanionicsolventlikewater.TheseHSE+PCM resultsshowthesignificanceofanchoringofthelightharvesting sensitizersforareliableDSSCoperation.

TheinclusionofthenonlinearPCMchangesthebindingenergies inthepositivedirection.Themainreasonisthatthedyemolecule andtheadsorptionsitesontheTiO2surfaceinteractwiththe

sol-ventverystrongly.Becausesuchaninteractionisnotpresentin vacuumcalculations,resultsgivemorenegativebindingenergies. Sincewaterisamorepolarsolventthanchloroform,ithasahigher dielectricconstant.Hence,waterinteractsmorestronglywiththe adsorptionsites.ThereforecalculatedbindingenergiesinH2Oare

morepositivethanthoseinCHCl3.

The geometry optimization by minimizing the Hellmann– Feynman forces causes surface ionic cores to relax into their minimumenergypositions.Hence,possiblesurfacestatesare pas-sivatedyieldingacleanbandgap(Fig.3).TheHSEmethodgavethe

bandgapsofbarenanowiresas4.06eVfor001nwand4.01eVfor

001nw.ThesearesignificantlylargerthanbothHSE-calculatedand

experimentallymeasuredbulkvalueof3.20eV.Forthenanowire

diametersaround1nm,HSEcalculationsindicateastrongquantum

confinementeffect[56].Thiswideningofthegap,uponadsorption

ofdyes,resultsinsomemolecularstates tofallintotheenergy

gap,wheremostofthedeeperlyingoccupiedmolecularorbitals

stay in the valence bandas resonant states. Consequently, the

Fermienergyshiftsuptohigherenergiesleadingtoanenergy-gap

narrowingwhichisanimportantfactorforphotovoltaic

proper-ties. Ontheotherhand,thelowestlyingunoccupiedmolecular

levelsofthedyesdelocalizeontheTi3dstatesinsidethe

conduc-tionband(CB)ofthenanowires,aschannelsofexcitedelectron

injectiontothenanowires.Thedensityofstates(DOS)ofthe

com-bineddye+nanowiresystemshavealsobeenpresentedinFig.3

forcoumarin,andC2-1intwomodes,namelymono-andbidentate

forms.Forbothofthenanowires,themolecularstatesofcoumarin

appeararoundtheVBedgewhileoneofthemisisolatedfromthe

rest.ForC2-1monodentatemode,essentiallythreefilledisolated

statesfallintothebandgapofbothnanowiretypes.Ongoingfrom

monodentate tobidentatebonding, anadditional isolated state

appearsabovetheVB.Significantbandgapreductionisobtained

inthecaseof C2-1bridginginbidentateformonbothtypesof

nanowires,whichmakesC2-1dyewithsuchbondingmore

impor-tantforlightharvesting.

Aswediscussedwiththeelectronicstructure, appearanceof

severalnewdyerelatedstateswithinthebandgapofnanowires

might cause a redshift of the optical absorption of these

dye-nanowirecompositesystemsifthetransitionsfromthesestates

aresymmetryallowed,sotheymightbecomeactiveinthe

visi-blepartofthespectrumwhichisverybeneficiaryforphotovoltaic

applications. In order to investigate the optical absorption,we

havecalculatedthedipolematrixelementsbetweenoccupiedand

emptystatesforeachcase,whichisessentiallytheimaginarypart

ofthedielectricfunction,



2(ω).Thecalculatedabsorptionspectra

forcleananatasenanowirealongwiththoseforthe

correspond-ingdye-nanowirecompositesystemsaredepictedinFig.4(a)and

(b)for(001)and(101)nanowires,respectively.Firstofall,when

wecomparethecorrespondingopticalabsorptionspectraofthe

differentnanowireandnw+dyesystems,generalfeaturesarevery

Fig.3. TheHSE-calculatedpartialandtotaldensitiesofstates(DOS)of(C2-1,coumarin)+nanowirecombinedsystemsforlowenergybindingmodes.Theresultsareshown for001nwontheleftandfor101nwontherightpanes.Themolecularcontributionsareindicatedasdark(red)shades.ThedottedlinesdenotetheFermienergiesandset atslightlyabovethehighestoccupiedstates.TheDOSstructuresarealignedwithrespecttothevalencebandmaximumofthebarecasesforeachtypeofnanowires.(For interpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthearticle.)

ε2 (ω) (arb. units)

(a)

001nw coumarin C2-1 mono. C2-1 bi. 0 2 4 6 8 10 ε2 (ω) (arb. units) Energy (eV)

(b)

101nw coumarin C2-1 mono. C2-1 bi.

Fig.4. Absorptionspectraofbareanddyeadsorbed001nw(a)and101nw(b)cases calculatedusingtheHSEhybridDFTmethod.

similarforboth(001)and(101)nanowires.Forbarenanowires,

theabsorptionstartsafter4eV,whichcoincideswithenergyband

gapof the nanowire,therefore wecan saythat the absorption

edgeisfromthevalencebandedgetotheconductionbandedge.

However,whencoumarinisattachedtothenanowire,twonew

states,oneveryclosethevalancebandedgeandtheotherisalmost

0.8eV above theedge, associated with thedye appears within

thebandgapof thenanowire.Theseare thereasonofthetwo

peaksobservedneartheadsorptionedgeabsorptionspectrumof

coumarin+nanowire system,otherwise thespectrum looksvery

similartothebarenanowireone.In thecaseofC2-1dye,there

areseveraldyeoriginatesstateswithinthebandgapforbothof

theadsorptionmodes,monodentateandbidantate.Theabsorption

spectraofbothofthemodesaresimilar.Comparedtothebare

nanowirespectrum,therearetwonewpeaks,onearound2eVat

themiddleofthebandgapandtheotheraround3eValmost1eV

belowtheabsorptionedge(orequivalentlyconductionbandedge).

Formonodentatemode,thepeakat2eVissharpwhilethereare

twopeaksaround3eV.However,forbidentate,wehavetwosplit

peaksaround2eV,andoneverybroadpeakaround3eVinstead

oftwo peaksofmonodentatecase.Comparingthesepeak

posi-tionswithDOSreportedinFig.3,wecanconcludethatthepeak

around2eVisduetoatransitionfromdyeassociatedstateatthe

Fermileveltotheconductionbandedge.Forbothdyetypesthe

LUMOlevelsstronglyresonatewiththeCBwhentheyareattached

tothesurfaceoftheoxidenanowire.Meanwhile,theHOMO-like

levelsappearin thebandgap ofTiO2 nanowiresasisolatedand

well-localizedstatesonthedye.Theassociatedchargedensity

dis-tributionscanalsobeseeninFig.1.Briefly,thelowerlyingpeaks

intheabsorptionspectraforthedye+nwcombinedsystems,are

essentiallyduetothetransitionsfromthedye-relatedHOMO-like

leveltothestatesatthebottomoftheconductionband.

4. Conclusions

We have investigated the band gap related properties and

resultingabsorptionspectraofbarethinanatasenanowireswith

diameters less than 1nm.We used therange separated hybrid

HSEfunctionalwithinDFT.Forthesenanowires,theHSEapproach

estimatesreasonablylargerbandgapsinagreementwiththe

quan-tumconfinementeffect.Thesameleveloftheorywasappliedto

examinetheadsorptionconfigurations,electronicstructuresand

opticalprofilesofD--AtypeorganicC2-1photosensitizeronthe

(001)and(101)facetsoftheoxidenanowires.The

intramolecu-larchargetransfercharacterofC2-1appearstobemodifiedonce

theC2-1formsabidentatebondingwiththesurfaceoxygenson

theanatasenanowires.TheC2-1+nwsystemshowssimilar

spa-tialchargedensityfeatureswiththatofthecoumarin+nwforthe

highest occupiedstatewhich is activelyinvolved inthelowest

lyingphotoexcitation.TheHSE+PCMincludingnonlineardielectric

effectsshowsthatthebindingenergyofC2-1dyeremains

mod-erateinsolutionenvironment.Thismightbeseenasoneofthe

explanationsofwhytheanchoringofsensitizermoleculesonthe

oxidesurfaceisimportant.Thenumberofdyerelatedstatesabove

theVBofthetitaniananowiresisassociatedwiththe

complex-ityofthemolecularstructure.Therefore,functionalmoeitiesare

desirabletoabsorbalargerportionofthevisiblespectrum.

Conse-quently,theuseofrangeseparatedhybriddensityfunctionalsisa

promisingwaytodescribebandgaprelatedelectronicstructures

fordyeandTiO2nanowiresystems.

Acknowledgments.

This work is supported by TÜB˙ITAK, The Scientific and

TechnologicalResearchCouncilofTurkey(Grant#110T394).

Com-putationalresourceswereprovidedbyULAKB˙IM,TurkishAcademic

Network&InformationCenter.

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