Electrochromic properties of nanostructured tungsten trioxide (hydrate) films and their applications in a complementary electrochromic device

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Electrochimica

Acta

j o ur na l h o me p ag e :w w w . e l s e v i e r . c o m / l o c a t e / e l e c t a c t a

Electrochromic

properties

of

nanostructured

tungsten

trioxide

(hydrate)

films

and

their

applications

in

a

complementary

electrochromic

device

Zhihui

Jiao

_,

_Jinmin

_Wang

_,

_Lin

_Ke

_,

_Xuewei

_Liu

_,

_Hilmi

_Volkan

_Demir

_,

_Ming

_Fei

_Yang

_,

Xiao

Wei

Sun

a_{SchoolofElectricalandElectronicEngineering,NanyangTechnologicalUniversity,NanyangAvenue,Singapore639798,Singapore}

b_{InstituteofMaterialResearchandEngineering,A*STAR(AgencyforScience,TechnologyandResearch),ResearchLink,Singapore117602,Singapore} c_{SchoolofPhysicalandMathematicalSciences,NanyangTechnologicalUniversity,NanyangAvenue,Singapore637371,Singapore}

d_{DepartmentofElectricalandElectronicsEngineering,DepartmentofPhysics,UNAM–InstituteofMaterialsScienceandNanotechnology,BilkentUniversity,Bilkent,Ankara06800,} Turkey

e_{SchoolofPhysicalandMathematicalSciences,NanyangTechnologicalUniversity,NanyangAvenue,Singapore639798,Singapore}

f_{DepartmentofAppliedPhysics,CollegeofScience,andTianjinKeyLaboratoryofLow-DimensionalFunctionalMaterialPhysicsandFabricationTechnology,TianjinUniversity,} Tianjin300072,China

a

r

t

i

c

l

e

i

n

f

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Articlehistory: Received31August2011 Receivedinrevisedform 16December2011 Accepted18December2011 Available online 26 December 2011 Keywords: Electrochromism Tungstentrioxide Thinfilm Complementarydevice

a

b

s

t

r

a

c

t

Orthorhombichydratedtungstentrioxide(3WO3·H2O)filmsconsistedofnanosticksand

nanoparti-cleswerepreparedonfluorinedopedtinoxide(FTO)-coatedsubstratebyafacileandtemplate-free hydrothermalmethodusingammoniumacetate(CH3COONH4)asthecappingagent.Irregularnanobrick

filmswereobtainedwithoutcappingagent.Duetothehighlyroughsurface,thenanostick/nanoparticle film depicts faster ion intercalation/deintercalation kinetics and a greater coloration efficiency (45.5cm2_{/C)thanthenanobrickfilm.Acomplementaryelectrochromicdevicebasedonthe}

nanos-tick/nanoparticle3WO3·H2OfilmandPrussianblue(PB)wasassembled.Asaresult,thecomplementary

deviceshowsahigheropticalmodulation(54%at754nm),alargercolorationefficiency(151.9cm2_/C)

andfasterswitchingresponseswithableachingtimeof5.7sandacoloringtimeof1.3sthanasingle 3WO3·H2Olayerdevice,makingitattractiveforapracticalapplication.

© 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Upon electron-transfer or redox reactions, materials that

undergoreversiblecolorchangeswithvariationsintheiroptical

spectraarecalledelectrochromicmaterials[1–3].Electrochromic

devicescomposedofthesematerials,whichallowforcontrolling

colorcycles,have attractedgreat interestthankstotheir

appli-cations important for smart windows[4–6], displays[7,8] and

antiglare mirrors [9]. The tunable light transmittance resulting

fromthecolorchangeoftheelectrochromicfilmsismuchdesired

insmartwindows,which donotonlyincreasetheaestheticsof

traditional windowsbut also save energy by reducing heating

or cooling loads of the building interiors [10]. Among various

electrochromicmaterials,tungstenoxide(WO3)hasbeen

exten-sivelystudiedbecauseof itshighcolorationefficiencyand high

cyclicstabilitycomparedwithothertransitionmetaloxides[11].

∗ Corresponding author at: School of Electrical and Electronic Engineering, NanyangTechnologicalUniversity,NanyangAvenue,Singapore639798,Singapore. Tel.:+6567905369;fax:+6567933318.

E-mailaddress:exwsun@ntu.edu.sg(X.W.Sun).

Moreover,WO3-basedelectrochromicdevicesexhibitlowpower

consumption,agoodmemoryeffectandahighcontrastratio[12],

offeringthedesiredadvantagesinsmartwindowsanddisplays.Itis

widelyrecognizedthatnanostructuredWO3,incomparisontotheir

compactbulkforms,offerpotentialadvantagesinelectrochromic

applicationduetotheirlargesurfaceareathatcouldbothincrease

thecontactareabetweentheelectrodeandelectrolyteandreduce

thediffusionpathofionsthroughtheporousstructures[13].And

the electrochromic efficiency of WO3 can befurther improved

by doping suitable metal ions with higher electronegativity or

loweroxidizingcapabilitythanWions,suchasMoandTi[14,15].

Recently, one-dimensional(1D) WO3 nanostructureswithlarge

surfaceareas,includingnanowires[16,17],nanorods[18,19]and

nanobelts[20],havebeeninvestigated.Forelectrochromic

appli-cations,WO3nanostructuresneedtobeassembledasathinfilm

onconductivesubstratesandthemicrostructuresofthefilm

con-cerning the electrochromic performance largely dependon the

filmassemblingtechniquesandprocessingconditions.Suchthin

filmsofWO3 havebeengrownbyvacuumdeposition[21],

elec-trodeposition[22],sol–gel[23]andhydrothermalmethod[24,25].

Hydrothermal approach is one ofthe mostpromising methods

for fabricatingWO3 film becauseof itsmerits oflow cost, low

0013-4686/$–seefrontmatter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.12.069

154 Z.Jiaoetal./ElectrochimicaActa63 (2012) 153–160

reactiontemperature,flexiblesubstrateselectionandeasy

scaling-upfor production.Moreover,the microstructuresof WO3 films

grown by hydrothermal approach can be precisely tailored by

varyingtheprecursorconcentration,temperature,duration, and

adoptingvarioussurfactantsandcappingagents.

Todate,anumberofhydrothermalmethodshavebeen

devel-opedforpreparing 1DWO3 nanostructures,includingnanorods

[19]andnanoribbons[26] byaddingdifferentinorganicsaltsas

cappingagents.However,previousreportsofdirectlygrownWO3

thinfilmsonsubstrateusingatemplate-freehydrothermalprocess

andtheircorrespondingelectrochromiccharacteristicshavebeen

quitelimited.Forexample,Tu’sgroupreportsahydrothermally

grownWO3nanowirearraysfilmrecently[27].Ahighcoloration

efficiencyof102.8cm2_{/Candfastswitchingresponseof4.2sfor}

col-orationand7.6sforbleachingisachievedforthisfilm.Ourgroup

hasalsofabricatednanobrickWO3 filmontransparent

conduc-tivesubstrateusingacrystal-seed-assistedhydrothermalmethod

previously[24].Thefilmshowsagoodcyclicstabilityand

com-parablecolorationefficiency(38.2cm2_{/C).Althoughconsiderable}

achievementshavebeenmade,theelectrochromicpropertiesof

hydrothermallygrownnanostructuredWO3 filmscanbefurther

improvedbyincreasingtheirsurfacearea.Moreover,thecapping

agenteffectsonthemorphologyofthehydrothermallygrownWO3

filmsandtheirresultantelectrochromiccharacteristicshavenot

beentotallyunderstoodyet.

Comparedwithasinglelayerelectrochromicdevice,itiswell

knownthatacomplementarydevicecontainingtwoproper

elec-trochromiclayerscouldfurtherimprovetheperformance,suchas

theopticalmodulation, cyclicstability andcoloration efficiency

[28,29]. Prussian blue (PB, iron (III) [hexacyanoferrate (II)]), a

coordination-compoundedtransitionmetal hexacyanometallate,

isasuitablecomplementaryelectrochromicmaterialtoWO3due

toitsoutstandingelectrochromicperformanceandproper

oper-ationalvoltage range. Although complementary electrochromic

deviceswithimprovedpropertiesbasedonelectrodepositedWO3

andPBfilmshavebeenreported[30],tothebestofour

knowl-edge,littleworkhasbeendoneonapplyingthehydrothermally

grownnanostructuredhydratedtungstenoxidefilmsinsuchkind

ofdevices.

In this work, we make an attempt to grow nanostructured

WO3filmsdirectlyonFTO-coatedglassbyahydrothermalmethod

usingammoniumacetate(CH3COONH4)asthecappingagent.The

cappingeffectsofCH3COONH4onthestructure,morphologyand

electrochromicpropertyoftheresultingnanostructuredhydrated

tungstenoxidefilmsareinvestigated.Moreover,acomplementary

electrochromicdevicecombiningthehydratedtungstenoxidefilm

withPBfilmisfabricatedand,asaresult,increasedoptical

modu-lationandcolorationefficiencyisdemonstrated.

2. Experimental

2.1. Preparationofcrystalseedlayers,precursorand

hydrothermaltreatment

Thedetailedproceduresforpreparingcrystalseedlayerscan

befoundelsewhere[24].Inatypicalexperimentforpreparingthe

precursor,Na2WO4·2H2O(0.0655g)wasdissolvedinto20mLof

de-ionizedwaterandthen4mLofHClwasaddedintothe

solu-tionuntilnomoreprecipitatewasformed.Theabovesuspension

waskeptin ice bathforabout10min,thenthetop liquidpart

wasremovedandde-ionizedwaterwasaddedtoobtaina20mL

suspension.SubsequentlyH2O2(0.2g)wasaddedintotheabove

suspensionunderintenselystirringandheating.Thewhite

precip-itatewasdissolvedandatransparentsolutionwasobtained.After

stirringfor5min,CH3COONH4(0.1g)wasaddedasthecapping

Fig.1.Schematicofthecomplementarydevice.

agent.ThenNaOH(1mol)solutionwasslowlymixedintothis

solu-tionwhilerigorously stirringuntilthepHvalueof thesolution

reached1.5.For thepurposeofcomparison, asolutionwithout

adding cappingagentwas alsoused.Theas-prepared solutions

weretransferredintoautoclavesasprecursorsforhydrothermal

treatments.TheFTOglassescoatedbyWO3seedlayerswereput

intoautoclavesandthehydrothermalprocesswaskeptat180◦C

for18h.

2.2. ElectrodepositionofPBandpreparationofelectrochromic

device

TheelectrodepositionofPBfilmwascarriedoutbyastandard

three-electrodesystem,wherea cleanFTOservedasthe

work-ingelectrode, a platinum sheetasthecounter electrode, and a

40 35 30

No salt

With CH

COONH

W 4f

Intensity (a.u.)

Binding Energy (eV)

W 4f

(b)

With CH

COONH

(440)

(044)

(400)

(420)

(331)

(113)

(222)

(202)

(220)

(022)

(200)

(002)

(111)

Intensity (a.u.)

2

(deg.)

(020)

Without CH

COONH

(a)

Fig.2.XRDpatterns(a)and(b)tungsten4fregionXPSspectraoftheas-synthesized thinfilmsgrownwithandwithoutCH3COONH4.

Fig.3.FESEMimagesof(a)and(b)thenanobrick3WO3·H2OthinfilmgrownwithoutCH3COONH4;and(c)and(d)nanostick/nanoparticlefilmwithCH3COONH4.Insets: cross-sectionalandHRTEMimage.

Ag/AgCl/sat’dKCl solutionas thereference electrode. The

elec-trodepositionbathofPBcontained10mmolK3Fe(CN)6,10mmol

FeCl3 and 0.1mol KCland theelectrodepositionof PBthin film

wascarriedoutbyapplyingaconstant cathodiccurrentdensity

of50␮A/cm2_{for300s.Thethicknessoftheas-depositedPBfilmis}

about450nm(measuredbyaTENCORP-10SurfaceProfiler).Then

theWO3workingelectrodeandPBcounterelectrodewere

sand-wichedtogetherwithhot-meltSurlynspacers.Aliquidelectrolyte

composedof0.2molLiClO4in␥-butyrolactone(␥-BL)was

intro-ducedbetweenthetwoelectrodesbycapillaryaction.Finallythe

cellwassealedwithepoxy,whichisschematicallyshowninFig.1.

2.3. Characterization

ThephasesofthesynthesisproductswereidentifiedbyX-ray

powderdiffraction(XRD,Siemens),usingCuK␣1(=0.15406nm)

radiation. X-ray photoelectron spectroscopy (XPS) data were

obtained on a Kratos AXIS spectrometer with monochromatic

AlK␣(1486.71eV)X-rayradiation.Themorphologiesofthe

as-prepared thin films were observed by field emission scanning

electron microscope (FESEM, JSM6340). High-resolution

trans-mission electron microscopy (HRTEM) image was obtained by

aJEM-2100 microscopeusinganacceleratingvoltageof200kV.

Theopticalabsorbanceandtransmittancespectraweremeasured

usingaUV/Visspectrophotometer(JESCO V670).

Electrochemi-calmeasurements wereperformedbya three-electrodesystem

(VersaSTAT3FPotentiostat/Galvanostat)withLiClO4 (0.2mol)in

␥-BL as the electrolyte, Pt sheet as the counter electrode and

Ag/AgCl/sat’dKClasthereferenceelectrode.

3. Resultsanddiscussion

3.1. Structuresandmorphologiesofas-preparedfilms

Fig.2(a)showstheX-raydiffraction(XRD)patternsofthe

as-preparedthin filmsgrownwithandwithoutCH3COONH4.Both

filmsshowthesamecrystallinestructureandallpeakscanbewell

indexedtotheorthorhombicphaseof3WO3·H2O(JCPDF87-1203)

withthecorrespondinglatticeconstantsofa=7.345,b=12.547and

c=7.737 ˚A.Thesharppeaksindicatethegoodcrystallinequalityof

156 Z.Jiaoetal./ElectrochimicaActa63 (2012) 153–160 1.5 1.0 0.5 0.0 -0.5 -1.0 -3 -2 -1 0 1 2 2 4 6 8 10 -3 -2 -1 0 1 2 Cur re n t de n si ty (m A/ cm 2) Scan rate 1/2 (V1/2/s1/2)

Current d

e

nsity (mA/cm

)

Potential (V vs. Ag/AgCl)

5mV/s-100mV/s

(b)

With CH

COONH

Without CH

COONH

C

u

rrent density (mA cm

-2

mg

)

Potential (V vs. Ag/AgCl)

(a)

Current density (mA/cm

2)

(c)

5mV/s-100mV/s

C

u

rrent den

sity (mA/cm

)

Potential (V vs. Ag/AgCl)

As prepared

1000th

2000th

C

u

rrent

density (mA/cm

)

Potential (V vs. Ag/AgCl)

(d)

15

(e)

With CH

COONH

Without CH

COONH

Current densit

y

(mA/cm

)

Time (s)

Fig.5. (a)Cyclicvoltammograms(CVs)ofthenanostick/nanoparticleandnanobrick3WO3·H2Ofilmsin␥-butyrolactonewith0.2molLiClO4;(b)and(c)CVsofthe nanos-tick/nanoparticleandnanobrickfilmatthescanratesof5,10,25,50and100mV/s,respectively.Insets:thecathodic/anodicpeakcurrentdensityasafunctionofthesquare rootofthescanrates;(d)CVsofthenanostick/nanoparticlefilmafter1st,1000th,and2000thcycles;and(e)chronoamperometry(CA)curveoftheas-synthesizedfilms recordedat±0.5Vfor40s.

theas-fabricatedfilms.TheelectrochromicperformanceofWO3is

closelyrelatedtoitscrystallinity.CrystallineWO3hasabetter

sta-bilitytoendureacidicelectrolytewithoutdegradationforalonger

cyclictimecomparedtoamorphousone,butatthecostofslower

responsetime andsmallercoloration efficiencyarisingfromits

smallerspecificsurfacearea[31].Byincreasingtheporosityand

preciselycontrollingthecrystalsize,crystallineWO3 filmswith

goodstabilityandfastresponsecansimultaneouslybeachieved.

Fig.2(b)showstheX-rayphotoelectronspectroscopy(XPS)dataof

1000 800 600 400 200 0 20 30 40 50 60

Transmittance (%)

Time (s)

(a)

Optical density

Charge density (C/cm

)

(b)

Transmittance (%)

Time (s)

(c)

Optical density

Charge density (C/cm

)

(d)

Fig.6.Insitutransmittanceresponseandcolorationefficiencyat754nmforthenanobrickfilm(aandb)andthenanostick/nanoparticlefilm(candd).

correctedusingavalueof284.5eVfortheC1speakofcarbon.For

bothfilms,twowell-resolvedpeaksatabout35.5and37.6eVin

thespectraareattributedtothespinorbitsplitdoubletpeaksofW

4f7/2andW4f5/2,respectively.Thesetwopeaksarewellseparated

withoutanyshoulders,indicatingthatallWatomsareinthe+6

oxidizationstates.

Fig.3shows themorphologiesoftheas-synthesizedfilms. It

canbeseenthatthefilmgrownwithoutCH3COONH4consistsof

irregularaggregatednanobrickswithsizesrangingfromtensto

hundredsofnanometers(seeFig.3(a)and(b)),whilethinfilmmade

upofstackednanosticksandnanoparticlesisobtainedbyadding

CH3COONH4 asthecappingagent(Fig.3(c)and(d)).The

nanos-tickswithanaveragelengthof∼50nmarehorizontallystacked

together,leading to a coarsesurface. A lot of small

nanoparti-cleswithsizesoftensofnanometersareaccumulatedunderneath

thesesticks.Clearlatticefringesofthehigh-resolution

transmis-sionelectronmicroscopy(HRTEM)imagefromananostick(inset

ofFig.3(d))indicateitssinglecrystallinequality.Theinset

cross-sectionalimageshowsthatthefilmwithathicknessof∼560nm

hasagood adherencetothesubstrate.Accordingtotherecent

report[32],orthorhombic3WO3·H2Oactuallycontainstwotypeof

corner-sharingWO6octahedras.Onetypeisconductedbya

cen-traltungstenatomthatissurroundedbysixoxygenatoms,while

inthesecondtype, twoof theoxygenatomsarereplaced bya

shorterterminalW ObondandlongerW (OH)2 bond,

respec-tively.Finally,the3WO3·H2Olatticeisformedbystackinguplayers

consistingofthesetwostructuralunits.Herein,theCH3COONH4

hasacappingeffectonthestackingofthesetwostructuralunits,

resultingintheformationofnanosticks.Fig.4schematically

illus-tratestheformationprocessofthefilmsgrownwithandwithout

CH3COONH4.

3.2. Electrochemicalandelectrochromicpropertiesofthe

as-prepared3WO3·H2Ofilms

Cyclicvoltammograms(CVs)wereinvestigatedforbothfilms

andshowninFig.5(a).TheseCVswerenormalizedwithrespect

tothegeometricareaandtotheweightof3WO3·H2Ofilmwithin

thatarea.Duringeachscan,thefilmsundergotypicalreversible

colorchangesfrombluetocolorless.Therecordedcurrentisdue

toLi+_{intercalation/deintercalationandelectrontransferbetween}

W6+_andW5+_{accordingtothefollowingreaction:}

WO3·0.33H2O(bleach)+xLi++xe−↔LixWO3·0.33H2O(blue).

(1)

Theintegratedcathodic/anodiccurrentequatestotheamountofLi+

intercalation/deintercalation.Itcanbeclearlyseenthatthe

nano-stick/nanoparticlefilmgrownwithCH3COONH4 showsahigher

currentdensityforbothintercalationanddeintercalationprocesses

overthesametimeperiodthanthenanobrickone,indicatingfaster

Li+_{intercalation/deintercalationkinetics.Thetotalcathodiccharge}

for the nanostick/nanoparticle film is about 4.2mCcm−2mg−1,

compared to only about 2.0mCcm−2mg−1 for nanobrick

films.

TheCVsofnanostick/nanoparticleandnanobrickfilmsrecorded

between−1.0and1.2VatvariousscanspeedsareshowninFig.5(b)

and(c),respectively.ItcanbeseenfromFig.5(b)thatwhenthe

scanspeedis5mV/s,therearetwooxidationpeaksthatappearat

about−0.83and−0.56V,andtworeductionpeaksthatappearat

−0.45and_−0.85V.Withtheincreasedscan speed,thefirst

oxi-dationpeaksdisappear,whilethesecondpeaksgetbroaderand

158 Z.Jiaoetal./ElectrochimicaActa63 (2012) 153–160 800 700 600 500 400 0.4 0.8 1.2 1.6

PB+WO

(hydrate)

PB

Absorbance (a.u.)

Wavelength (nm)

Fig.7.Opticalabsorptionspectraofacomplementaryelectrochromicdeviceanda singlePBlayerdeviceatthecoloredandbleachedstateunder±0.8V,respectively.

Thereductionpeaksshowasimilartrend.Whenthesecondanodic

peakcurrentdensitiesareplottedagainstthesquarerootofscan

rates,1/2_{,approximatelyalinearrelationshipisobtained(insetof}

Fig.5(b)),whichsignifiesadiffusion-controlledprocess[33].The

effectivediffusioncoefficientDforthediffusingspeciesLi+_canbe

estimatedfromthepeakcurrentdensity(jp)dependenceonthe

1000 800 600 400 200 0 0 20 40 60

Transmittance (%)

Time (s)

(a)

151.9 cm

C

Optical density

Charge density (C/cm

₎

(b)

Fig.8. (a)Switchingtimecharacteristicsbetweenthecoloredandbleachedstates forthecomplementaryelectrochromicdevicemeasuredat±0.8Vfor30sand(b) opticaldensityvariationwithrespecttothechargedensityrecordedat754nm.

squarerootofthepotentialscanrate(1/2_{)assumingasimplesolid}

statediffusioncontrolledprocess[34]:

ıip ı√=2.69×10 5_n3/2_AC√_{D (2)} D=0.1382×10−10n−3_A−2_C−2





wherenisthenumberofelectronstransferredinunitreaction,A

istheeffectivegeometricsurfaceareaofthe3WO3·H2Oelectrode,

andCistheconcentrationofthediffusionspecies(Li+_).The

effec-tivediffusioncoefficientsDLi+havebeencalculatedfromEq.(3)

tobe2.19×10−11and1.39×10−11cm2_{/sfortheintercalationand}

deintercalationprocess,respectively,comparabletothereported

values[35].Fig.5(c)showstheCVsofthenanobrickfilmrecorded

atdifferentscanrates,showingasimilarprofilebutsmallercurrent

density.ThecalculateddiffusioncoefficientsDLi+are9.60×10−12

and6.101× 10−12_cm2_{/sfortheintercalationprocessand}

deinter-calationprocess,respectively,whicharesmallerthanthoseofthe

nanostick/nanoparticlefilm.Thefasteriondiffusionkineticsofthe

nanostick/nanoparticlefilmarisesfromitsmuchroughersurface,

whichincreasesitssurfaceareaandreducestheionsdiffusionpath

length.Thecyclicstabilityofthenanostick/nanoparticlefilmwas

alsomeasuredforthe1st,1000th,and2000thcycleatroom

tem-peratureandtheresultsareshowninFig.5(d).Thecurrentdensity

increasesslightlywithin2000cycles, withoutaclearchangein

theshapeofCVcurve,indicatingexcellentcyclicstabilityofthe

film,whichissimilartothereportedcrystallineWO3nanoparticles

[12].Chronoamperometry(CA)datawererecordedforbothfilms

withthepotentialbeingsteppedfrom−0.5Vto+0.5Vfor40s,as

showninFig.5(e).Duringeachcycle,the3WO3·H2Ofilmschange

frombleachedstatetocoloredstatereversibly.Forbothfilms,the

peakcurrentdensityofbleachingismuchhigherthanthatof

col-oration, and thecurrent densityduring bleaching decaysfaster

thancoloration.ThischaracteristicistypicalforWO3withsmall

ionintercalation/deintercalation[20].Thehigherbleachingcurrent

arisesfromthegoodconductivityoftungstenbronze(LixWO3),

and therapid currentdecay is due totheconductor (LixWO3

)-to-semiconductor(WO3)transition,whilethecoloringkineticsis

alwaysslower than thebleaching one for WO3 filmsowingto

the higher resistance during WO3 toLixWO3 transition.

More-over,thepeakcurrentdensitiesofcoloration/bleaching(−4.9and

14.1mA/cm2_{)ofthenanostick/nanoparticlefilmarehigherthan}

those(−2.4and7.5mA/cm2_{)ofthenanobrickfilm.Theresponse}

time(definedas70%decreaseofcurrentdensity)forcoloration

(tc)andbleaching(tb)arecalculatedfromcurrent–timetransient

datafromFig.5(e).Thenanostick/nanoparticlefilmshowsfaster

coloration/bleachingresponses(tc∼7.0sandtb∼3.0s)thanthose

(tc∼9.0sandtb∼3.5s)ofthenanobrickfilm,inwellagreement

withtheresultsinFig.5(a–c).Thefastresponsesforbothfilmsare

comparabletothespray-pyrolysisdepositedamorphousWO3film

[36].

The in situ transmittance responses of the

nanos-tick/nanoparticle and nanobrick film for a 90% transmittance

changewere alsoinvestigated at 754nm bya ±0.8V bias (see

Fig. 6(a) and (c)). For thenanostick/nanoparticle film, the

col-orationtimetc isfoundtobe7.9s, andthebleachingtime tb is

4.8s. However,for thenanobrick film,thecoloration time tc is

foundtobe7.3s,andthebleachingtimetbis7.1s.Thebleaching

speedofthenanostick/nanoparticlefilmismuchfasterthanthe

nanobrick film, butthe coloration speed is a little slower.It is

worth tomention that a larger optical modulationof ∼45% of

the nanostick/nanoparticle film is achieved, compared to that

only38%ofthenanobrickfilm.Colorationefficiency(CE)values

ofboth filmswerealsostudiedand shownin Fig.6(b)and (d),

Fig.9.Photographsofacomplementaryelectrochromicdeviceatthebleachedstate(a)and(b)thecoloredstateunder±0.8V,respectively.

linearregionofthecurve.It canbeseenthat thecalculated CE

ofthenanostick/nanoparticlefilmis45.5cm2_{/C,largerthanthat}

ofthenanobrickfilm(36.8cm2_{/C).Theimprovedelectrochromic}

performanceofthenanostick/nanoparticlefilmisduetoitslarge

surfacearea.

3.3. Opticalandelectrochromicpropertiesofthecomplementary

device

Tofurtherimprovetheopticalcontrast,colorationefficiencyand

switchingstability,thenanostick/nanoparticlefilmisincorporated

inanelectrochromicdevicewithananodicallycoloredPBfilmas

acomplementaryelectrochromiclayer.ThemorphologyandCV

curveoftheas-depositedPBfilmwerealsoinvestigatedandshown

inFigs.S1andS2,respectively(seesupplementaryinformation).

Fig.7showsopticalabsorptionspectraofthecomplementary

elec-trochromicdeviceandasinglePBlayerdeviceatthebleachedand

coloredstateundera±0.8Vbias.ComparedwiththesinglePBlayer

device,thecomplementarydeviceshowsalargeroptical

modu-lationabove410nm,especially inthenearIRregion,indicating

ahighercolorcontrastandlargerheatregulation.Moreover,the

complementarydevicedepictsahigherabsorptioninthenearUV

regionatbothcoloredandbleachedstatesduetotheabsorptionof

3WO3·H2Ofilms.Theaboveresultishighlydesiredinsmart

win-dowapplicationssincemoreheatcanbeprohibitedfromentering

theinteriorbuildings,resultinginareductionofcoolingloads.

Fig.8(a)showstheinsitucoloration/bleachingtransmittance

responseofthecomplementarydevicemeasuredat754nm.The

maximumtransmittancemodulation(T)ofcoloration/bleaching

wasfoundto beabout 54% afterapplying a ±0.8V voltage for

30s,agreeingwellwiththeabsorbancespectrashowninFig.7.

Obvious color changes can be observed during the switching.

Thecolorationandbleachingtimeextractedas90%transmittance

changesarefoundtobe1.3and5.7s,respectively.Theswitching

responsesofthecomplementarydeviceforboththecolorationand

bleachingprocessesare muchfasterthan thosereportedvalues

[30].For this complementaryelectrochromic device,limitations

of switching response are mainly due to 3WO3·H2O electrode.

The fastcoloration/bleaching kineticsmay beattributed tothe

largeactivespecific areaof theroughsurface, which facilitates

theionsintercalation/deintercalationbyreducingtheirdiffusion

lengths. CEofthe complementarycell wasalsoinvestigated at

its peak absorbance (=754nm) and shown in Fig. 8(b). The

calculated CE value is 151.9cm2_{/C, comparable to that of the}

reported WO3||PB complementary device [30]. And the CE of

thecomplementarydeviceisimprovedbyabout234%compared

witha single3WO3·H2O electrochromic layer(CE=45.5cm2/C).

Photographs ofthe complementarydeviceare shown in Fig.9,

depicting a high contrast between the bleached and colored

states, which leads to the obvious transparence changes. The

deviceshowspromisingapplicationsinenergy-savingsmart

win-dows.

4. Conclusions

Insummary,uniformandwell-adhesive3WO3·H2Ofilms

con-sisted of nanosticks/nanoparticleswere synthesizedvia a facile

andtemplate-freehydrothermalmethodbyaddingCH3COONH4as

thecappingagent.Thinfilmscomposedofaggregatednanobricks

wereobtainedwithoutCH3COONH4.Thenanostick/nanoparticle

film depicts faster charge transfer and greater coloration

effi-ciency (45.5cm2_{/C) than the nanobrick film (36.8cm}2_{/C). A}

complementary electrochromic device based on the

nanos-tick/nanoparticle 3WO3·H2O film and PB film was fabricated

anddemonstrateslargeropticalcontrast(54%at754nm),faster

switchingresponse(tb=1.3sandtc=5.7s)andgreatercoloration

efficiency(151.9cm2_{/C)thanasingle3WO}

3·H2Ofilmdevice.The

complementarydeviceholdsgreatpromiseforpotential

applica-tionsinenergy-savingsmartwindows.

Acknowledgments

The authorswould liketo thank thefinancial support from

the Science and Engineering Research Council, Agency for

Sci-ence,TechnologyandResearch(A*STAR)ofSingapore(projectNos.

0921010057and0921510088),SingaporeNRF-RF-2009-09,and

NationalNaturalScienceFoundationofChina(NSFC)(projectNos.

61006037and61076015).

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in

theonlineversion,atdoi:10.1016/j.electacta.2011.12.069.

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