improved electrochromical properties of sol–gel wo3 thin films by doping

7 re-presents the variation of coloring current as a function of time for the Au– WO3thinﬁlms with different gold concentrations at the optimum applied voltage.. As can be seen from theﬁ

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Improved electrochromical properties of sol –gel WO 3 thin ﬁlms by doping

gold nanocrystals

N Naseria, R Azimiradb, O Akhavana, A.Z Moshfegha,c,⁎

a

Department of Physics, Sharif University of Technology, P.O Box 11155-9161, Tehran, Iran

b Institute of Physics, Malek Ashtar University of Technology, Tehran, Iran

c

Institute for Nanoscience and Nanotechnology, Sharif University of Technology, P.O Box 14588-89694, Tehran, Iran

a b s t r a c t

a r t i c l e i n f o

Article history:

Received 30 April 2008

Received in revised form 25 July 2009

Accepted 4 August 2009

Available online xxxx

PACS:

81.20.Fw

82.47.Jk

Keywords:

Au nanocrystals

Tungsten oxide ﬁlms

Optical properties

Electrochromic properties

Coloration time

In this investigation, the effect of gold nanocrystals on the electrochromical properties of sol–gel Au doped

WO3thinﬁlms has been studied The Au–WO3thinﬁlms were dip-coated on both glass and indium tin oxide coated conducting glass substrates with various gold concentrations of 0, 3.2 and 6.4 mol% Optical properties

of the samples were studied by UV–visible spectrophotometry in a range of 300–1100 nm The optical density spectra of thefilms showed the formation of gold nanoparticles in the films The optical bandgap energy of Au–WO3films decreased with increasing the Au concentration Crystalline structure of the doped films was investigated by X-ray diffractometry, which indicated formation of gold nanocrystals in amorphous WO3thinfilms X-ray photoelectron spectroscopy (XPS) was used to study the surface chemical composition of the samples XPS analysis indicated the presence of gold in metallic state and the formation of stoichiometric WO3 The electrochromic properties of the Au–WO3samples were also characterized using lithium-based electrolyte It was found that doping of Au nanocrystals in WO3thinfilms improved the coloration time of the layer In addition, it was shown that variation of Au concentration led to color change

in the colored state of the Au–WO3thinﬁlms

1 Introduction

Research in theﬁeld of electrochromic transition metal oxide ﬁlms

has gained a lot of attention in the past several decades In an

electrochromic device, its transmittance and reﬂectance change in a

reversible manner under the application of an external voltage[1,2]

Since an electrochromic reaction involves electron conduction and ion

diffusion, the electronic conductivity and ionic diffusivity in

electro-chromic materials are clearly critical factors[3]

Among the transition metal oxides that exhibit electrochromic

properties, WO3has been investigated most extensively[1–6] This

was due integrally to its fast response times, coloration efﬁciencies,

long life times, etc These properties tendered WO3desirable for use in

information displays[7], anti-glare rear view mirrors of automobiles

[8,9]and the so-called “smart windows” [10] However, there are

many efforts to improve its coloration performance (lower coloration

time, higher color intensity and reversibility) for practical

applica-tions For this purpose, Haranahalli and Holloway have found that the

coloration and bleaching rates of WO3ﬁlm increase with the addition

of the porous conducting metallic over layer in a liquid electrolyte cell

[11] Moreover, electrochromic operation of WO3ﬁlms doped with Co,

Cr and Ni has been reported[12,13]

It has been known that gold nanoparticles have excellent inertness

as well as excellent stability, and increase metallic properties and also conductivity of a layer resulting in the enhancement of electrochromic performance[12,14] Concerning these advantages, gold nanoparticles have been recently used to modify electrochromic properties of WO3

thinﬁlms[15–17] He et al has reported addition of gold over layer on

WO3thinfilm formed by physical vapor deposition resulted in a better change of optical density in electrochromic process[15] In the other works[16,17], some researchers have studied electrochromic proper-ties of co-sputtered Au–WO3 nanocomposite thin films containing high gold concentration (60 mol% Au) and obtained a shorter response time relative to the pure WO3 thin film However, in their films containing high Au concentrations, the transfer of positive ions occurs competitively between gold and tungsten trioxide resulting in the formation of ion-metal compounds which are inactive in the coloration process In these conditions, they have observed reduction

of optical density change in the Au–WO3nanocomposite thinfilms as compared to the pure thin films Therefore, it is expected that an optimum doping of gold metallic nanophases can increase conductiv-ity and improve electrochromic performance of WO3thinfilm

In this paper, we present data on optical properties, crystalline structure and surface chemical state of Au doped WO3 thin ﬁlms synthesized by easy and low cost sol–gel method In addition,

Thin Solid Films xxx (2009) xxx–xxx

⁎ Corresponding author Department of Physics, Sharif University of Technology, P.O.

Box 11155-9161, Tehran, Iran Tel.: +98 21 6616 4516; fax: +98 21 6601 2983.

E-mail address: moshfegh@sharif.edu (A.Z Moshfegh).

TSF-26480; No of Pages 8

doi: 10.1016/j.tsf.2009.08.001

Contents lists available atScienceDirect

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electrochromic behavior of Au–WO3thinﬁlms under various applied voltages has been also studied

2 Experimental details

Sol–gel Au doped WO3 thin ﬁlms were prepared using the following procedure At ﬁrst, 6 g of sodium tungsten dehydrate (Na2WO4·2H2O, 99% Merck) was immersed in 30 ml of nitric acid solution (HNO3, 65% Merck) for 15 min to exchange the Na+ions with

H+ After washing three times with distilled water, the obtained yellow–greenish precipitate was dissolved in 10 ml of H2O2 (30% Merck), and then 1 ml of ethanol was added to the solution After 24 h,

it was exposed to light using a commercial 100 W lamp for 2 h in order

to concentrate the solution The color of the solution changed from colorless to light yellow, and it was stable for a long time Then, after another 24 h aging, the prepared WO3sol was mixed with various amounts of 0.14 molar aqueous solution of HAuCl4(99.5% Merck) The molar concentration of Au in theﬁnal sols was varied by the amounts

of 0, 3.2 and 6.4 mol% The blended sols were stirred magnetically for

2 h, and subsequently the deposition process was performed by dipping the cleaned microscope slide glass and indium tin oxide (ITO) coated glass (commercial ITO; ~1 µm thickness and electrical sheet resistance ~ 100Ω/□) into the solution for 60 s and pulling them out at

a rate of 1 mm/s Allfilms, at first, were dried at 100 °C and then annealed at 200 °C in air for 1 h Using the optical method[18], the thickness of the driedfilms measured about 200–300 nm

A UV–visible spectrophotometer (Jasco-V530) was used to inves-tigate the optical properties of theﬁlms (without eliminating the substrate effect) in the wavelength range of 300–1100 nm with 1 nm resolution Philips PW 3710 proﬁle X-ray diffractometry (XRD) with

Cu–Kαradiation source (conventionalθ–2θ diffractometer) and step size of 0.05° was used to determine phase formation, average crystalline size and structure of the layers X-ray photoelectron spectroscopy (XPS) equipped with an Al–KαX-ray source at an energy

of 1486.6 eV was employed to investigate the surface chemical composition of theﬁlms The hemispherical energy analyzer (Specs

EA 10 Plus) operating in a vacuum better than 10− 7Pa was used All binding energy values were calibrated byfixing the C(1s) core level to the 285.0 eV All of the peaks were deconvoluted using SDP software (version 4.1) with 80% Gaussian–20% Lorentzian peak fitting The electrochromic properties of the Au doped WO3thinfilms were investigated on ITO/glass substrates in a 50 ml glassy cell containing two electrodes The Au–WO3 films, as working electrodes, were electrochemically cycled in a 1 M LiClO4in propylene carbonate (PC) electrolyte in a glass test vessel, using pure graphite as the counter electrode All measurements have been performed at room temper-ature (~25 °C) and in air Experimental details of the electrochromic test have been reported elsewhere[19] The coloration transmittance

of the Au–WO3thinfilms was studied in two states In the first state, it was measured as a function of time at afixed wavelength of 500 nm (in which eye has a high sensitivity) at different coloring voltages, and then negative voltages applied for bleaching thefilms Furthermore, during the electrochromic process, the magnitude of the current between the two electrodes was recorded In the second state, the transmittance was measured by the spectrophotometer in a range of

300–1100 nm wavelength for the ﬁlms colored after a constant time

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The band gap energy of the deposited thinfilms have been measured using their linear plots of (αhν)1/η versus hν Here, hν is the corresponding incident photon energy and the exponentη depends on the kind of optical transition.η is 1/2, 3/2, 2 and 3 when the transitions are direct allowed, direct forbidden, indirect allowed and indirect forbidden, respectively [1] Calculation method has been explained in other literatures[1,19,33] By examining the various values ofη, as a result of linefitting, η=2 was determined for the pure and doped WO3films describing their indirect allowed transitions, respectively (Fig 2) The obtained value ofη has been also reported for other WO3compoundfilms including WO3–Fe2O3[33]and WO3–SiO2[34] According toFig 2a, the optical bandgap of the pure WO3thinfilm was measured at about 3.3 eV This value is in agreement with the reported bandgap energy of amor-phous WO3thinfilm by others[1,19,21].Fig 2b illustrates (αhν)1/2as a function of hν for the Au doped WO3films measuring optical bandgap energy of 3.1 and 2.8 eV for the samples containing 3.2 and 6.4 mol% Au, respectively Therefore, it is clear that the bandgap energy of the samples decreased with increasing the Au concentration as a result of the metal doping Actually, for a semiconductor, the conduction band is curved

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coloring voltages By applying the coloring voltage to the electrode at

t = 10 s, the transmittance of theﬁlms continuously decreased and

then they were colored For all the samples, the applied coloring

voltages were disconnected at t = 500 s, and then, the polarity of the

applied voltages was inverted 10 s later By changing the polarity, the

transmittance of theﬁlms was increased and they were bleached All

the samples were almost colorless before cathodic polarization,

whereas they turned colored after the polarization When a negative

bias was applied, some WVions were produced, due to the electron

transfer from the electrode to WVIions, and cations were inserted into

theﬁlms simultaneously so as to maintain electroneutrality Tungsten

bronze (LixWVI

1 − xWxVO3) was thus produced and a color appeared due

to the optical intervalence charge transfer between tungsten atoms

having different valence[3,43] The degree of coloration depends on

the value of x, which changes from grey to gold by increasing the x in tungsten bronze But, only for small values of x (x≤0.5) it leads to blue color and the coloration reactions are reversible[3] On the other hand, the preparative conditions of the WO3 electrochromic thin ﬁlms

inﬂuence on the intensity of color Low crystallinity[44,45]and high porosity[44,46]of WO3ﬁlm and addition of conducting materials to it

[47]facilitate the diffusion of Li+to the layer Thus, more penetrative ions cause stronger change in the color of theﬁlm, during coloration process Moreover, other properties of the tungsten oxide layer such as its stoichiometry[48,49]and thickness[19]can also affect its electro-chromic performance

It is clear fromFig 5that by increasing the coloring voltage, the transmittance of the coloredfilms decreased and the rate of the coloration increased When the coloring voltage exceeds a certain value, the transmission of the bleachedfilms is markedly lower than its value before coloration This specific value of coloring voltage called optimum voltage (Vopt) was determined as 2.5 and 3 V for the pure

WO3 and both Au doped WO3 ﬁlms, under our experimental conditions, respectively (For the pure WO3ﬁlms under applied voltage

of 2.5 V, their bleached state requires more than 500 s) In addition, due to measuring equal transmittance for theﬁlms, before coloration and after bleaching, it was found that the electrochromic reaction was reversible for voltages lower than the optimum value But after applying a higher voltage, the transmittance of the bleachedﬁlms further decreased and the process was irreversible This is due to an effect called“site saturation” in which reversibility of optical effects at high Li+intercalations in the LixWO3is not occurring[1,43]

Fig 5 The variation of optical transmission of the Au doped WO 3 thin ﬁlms containing: (a) 0 (the pure WO 3 ), (b) 3.2 and (c) 6.4 mol% Au, in 1 M LiClO 4 + PC electrolyte as a function of time for various coloring voltages at λ=500 nm.

Fig 4 XPS spectra of W(4f) for (a) 0, (b) 3.2, (c) 6.4 mol% Au doped WO 3 ﬁlms and Au

(4f) for the ﬁlms containing (d) 3.2 and (e) 6.4 mol% Au, respectively.

4 N Naseri et al / Thin Solid Films xxx (2009) xxx–xxx

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The derivative of transmittance versus time (dT/dt) under optimum

applied voltages has been presented inFig 6, for different

concentra-tions of Au Thisﬁgure has been extracted fromFig 5and it has been

shown for the time interval between 0 and 200 s for more clarity As can

be seen, by applying the coloring voltage, the coloration rate increased

rapidly, from 0 to 1 and 6 s−1for the pure and dopedﬁlms, respectively

as a result of Li+ions migration to the layer The diffusion of lithium ions

occurred rapidly atﬁrst and then became slow So, the coloration rate

then decreased and reached to zero It is obvious that due to higher

conductivity of theﬁlms containing Au; they were colored faster than

the pure WO3 ﬁlms Although the Au doped ﬁlms had the same

coloration rate atﬁrst, however the magnitude of |dT/dt| was higher for

theﬁlm containing 3.2 mol% Au during the t≥14 s It means that for the

ﬁlm with 3.2 mol% Au, diffusion of the ions took place easier and the

total color change of theﬁlm was more than the ﬁlm with 6.4 mol% Au,

during the coloration process

According to the Cottrell equation, coloration current decays with

time under a constant voltage by

iðtÞ = nFACD1= 2

where n, F, A, C and D are ionic valence number, Faraday constant,

electrode area, concentration and diffusion coefﬁcient of cations in the

ﬁlm, respectively [50] Therefore, the diffusion coefﬁcient can be

calculated from logarithmic graph of current versus time[50].Fig 7

re-presents the variation of coloring current as a function of time for the Au–

WO3thinﬁlms with different gold concentrations at the optimum applied

voltage As can be seen fromFig 7, a linear behavior is observed for the

current–time diagram in small values of t with approximately the same

slopes for the different samples But, there is a deviation from the power

law (t1/2) at longer times as a result of natural convection effects[50] It is

clear from the extrapolation of the graphs with current axis that the

diffusion coefﬁcient increased with increasing the Au concentration in the

thinfilms Using Cottrel equation, the diffusion coefficient of the films was estimated to be 1×10−8, 3×10−8and 5×10−8cm2s−1for thefilms containing 0, 3.2 and 6.4 mol% Au, respectively Therefore, the Au doping has a positive effect on the cations diffusivity in the WO3films A similar result has been also reported for the WO3films doped with ITO conductive nanopowder[47]

During the electrochromic process, the total charge transferred through the electrodes can be evaluated using the Q =∫idt equa-tion[19].Fig 8presents the electrochromic charge passed through the

Au–WO3layers (Q) with the different Au concentrations as a function of applied voltage, in both coloring and bleaching states As can be seen from theﬁgure, by increasing the applied voltage, the charge that passed through the electrochromic layers during the coloration was increased, corresponding to an increase of the x value in the LixWO3and reduction

of the transmittance after the coloration process (see alsoFig 5) Although the bleaching charges also increased by increasing the applied voltage, their values for the voltages less than or equal to the optimum voltage are the same as the amount of corresponding coloring charges which is consistent with the reversibility of electrochromic reaction For the higher voltages, the bleaching charges are lower than the amount of the corresponding coloring charges due to“site saturation” effect

Fig 6 The absolute value of the transmittance derivative curves versus time under

optimum applied coloring voltage for the ﬁlms containing different Au concentrations.

Fig 7 The log–log plot of the electrochromic current versus time for the Au doped WO 3

thin ﬁlms containing different Au concentrations, under optimum applied coloring

voltage.

Fig 8 The variation of the transferred charge versus the applied voltage for the Au doped WO 3 thin ﬁlms containing: (a) 0 (the pure WO 3 ), (b) 3.2 and (c) 6.4 mol% Au.

5

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The value of x in LixWO3can be estimated using this formula[49]:

where QLiis the total charge of the intercalated lithium ions, A is the

area, d is the thickness, M is the molar mass,ρ is the density of the

oxide ﬁlms, e is the elementary charge and Na is the Avogadro

number, respectively Corresponding values of x under different

coloring voltages for the pure and Au doped WO3ﬁlms have been

presented in Table 1 It is clear that the value of x increases by

increasing the applied voltage and it is less than 0.5 for the voltages

lower than optimum coloring voltages, which leads to reversibility of

the reactions[3]

A measurable parameter in electrochromic reactions is the change

of the optical density (ΔOD) which can be calculated using the

following relation[19]

ΔODðλÞ = ln½Tðt1; λÞ = Tðt2; λÞ ð4Þ

where T(t1,λ) and T(t2,λ) are transmission of the ﬁlms at λ=500 nm

before (t1) and after (t2) the coloration process, respectively For the above

relation, t1=10 s, t2=500 s andλ=500 nm were considered in our

experimental conditions The quantity ofΔOD indicates how much the

transmittance of the electrochromic layer reduces during the coloring

process As listed inTable 2, the change in the optical density was

measured at about 1.6 for the pure WO3thin ﬁlm which decreased

slightly to 1.3 for the dopedﬁlm containing 3.2 mol% gold This value

declined more strongly to 0.5 for the sample with 6.4 mol% Au The optical

density change is attributed to the effective number of W6+↔W5+

transitions Hence, increasing the Au concentration led to the decreasing

of active W6+sites in theﬁlms and the amount of ΔOD decreased A

similar behavior has been also observed for Li–Nb2O5[51]and Fe2O3–

WO3[33]electrochromic composite thinﬁlms recently Moreover, it is

reported that some ions bound with metallic nanoparticles contained in

the electrochromic electrode Thus, the positive ion transfer takes place

competitively, between electrochreomic material (WO3) and

incorporat-ed metal (Au)[16,17,52,53] Therefore, it can be concluded that the

reduction in theΔOD is a logical consequence of decreasing WO3content

and/or increasing the Au concentration in the layer A similar to this result

has been also reported for vanadium doped WO3thinﬁlms very recently

[54]

An important parameter for electrochromicﬁlms is the coloration

efﬁciency, CE(λ), that is deﬁned as follows[1]:

whereΔOD(λ) represents change in the optical density at a ﬁxed

wavelength (λ), A is the surface area (~2 cm2for our samples) and Q

is the charge transferred during the coloration process in terms of C

The calculated value of CE(λ) was 35cm2C− 1for the pure WO3thin film which decreased to 7cm2C− 1by increasing the gold concentra-tion in thefilm to 6.4 mol% This reduction agrees with the decrease of ΔOD, as the Au concentration increased in the films

Another determinant factor that must be considered in the fabri-cation of electrochromicfilms is their response time It is essential to decrease the time interval needed for coloration process, in practical applications The coloration time (tc) can be defined as the requisite time for reduction of the transmittance of the layer from 10% to 90% of the final reduction[55,56] In our study, the coloration time evaluated for the pure WO3film is 215 s which declined sharply to 26 s for the doped

WO3 thinfilm containing 3.2 mol% Au It has been shown that the addition of gold to the WO3 electrochromic thinfilm facilitates the electron transfers (more conductivity in thefilms) and decreases the coloration time[15] The coloration time is also affected by diffusion coefficient of cations[1,57] InFig 7, we have also shown that increasing the Au concentration improved the cations diffusivity in thefilms On the other hand, as discussed previously, reduction of electrochromic material (WO3) in the layer and the Au competitive effect decreased the change in optical density and influenced on coloration process negatively Hence, as we observed, by increasing the Au concentration

to 6.4 mol% in the WO3thinﬁlm, the coloration time of the doped layer increased slightly to 50 s This is because the negative effects became stronger than the Au positive effects (including higher conductivity and increase in cations diffusivity) Therefore, there is an optimum gold concentration that minimizes the coloration time of the Au doped WO3

thinﬁlms and was determined as 3.2 mol%, in this work The obtained electrochromic characteristics of the Au–WO3 thin ﬁlms under the optimum applied coloring voltage have been summarized inTable 2for the different Au concentrations

The transmission spectra of the Au doped WO3thinﬁlm contain-ing 3.2 mol% Au colored at various voltages after the constant time of

500 s in the range of 300–1100 nm wavelength have been shown in

Fig 9a Before applying the coloring voltage, the transmittance of the

Au–WO3/ITO/glass structure was very similar to the Au–WO3/glass structure (seeFig 1a) It is seen that after applying the voltage, the transmittance of theﬁlm decreased (Fig 5), indicating the Li+ inser-tion to the layer and thus its color changed This reducinser-tion of trans-mission was stronger at the longer wavelengths So, the transmittance

Table 1

The estimated values of x in Li x WO 3 for pure and Au doped WO 3 ﬁlms.

Au mol % Voltage (V)

3.2 0.13 0.21 0.35 0.46 1.26 1.78

6.4 0.13 0.21 0.34 0.47 1.19 1.7

Table 2

Some important electrochromic characteristics of the Au doped WO 3 thin ﬁlms.

Au (mol%) V opt (V) ΔOD Q (mC) CE (cm 2 C − 1 ) t c (s)

Fig 9 Transmission spectra versus wavelength for (a) the Au doped WO 3 thin ﬁlm containing 3.2 mol% Au at different applied coloring voltages and (b) the Au doped WO 3

thin ﬁlms with different Au concentrations in both bleached (B) and colored (C) states.

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of the coloredﬁlm at the near-IR range was less than the transmittance

in the visible range Actually, the change in optical properties of an

electrochromicﬁlm under applying a coloring voltage can be described

by small polaron effects Small polarons are formed when excess

electrons polarize their surrounding lattice so that localization of the

wave function takes place essentially on lattice site[1] A small overlap

between wave functions corresponding to adjacent, as well as strong

disorder, is conducive to polaron formation The absorption process is

connected with polaron transfer by hopping between neighboring W

sites (W6+and W5+) sites The excess energy during photon assisted

hopping is given off as phonons[1] By increasing the coloring voltage up

to the optimum value (3 V), theﬁlms showed more coloration and so,

further reduction in transmittance took place At wavelength of about

400 nm, the maximum transmittance of the coloredﬁlm was measured,

because the color of LixWO3compound is blue for 0 <x < 0.5 But, by

increasing the coloring voltage to higher values, this peak in

transmittance curve disappeared and transmittance increased in

near-IR range This is due to the increase of the x value in the LixWO3and the

irreversibility of electrochromic reaction occurred at the coloring

voltages higher than 3 V

Fig 9b represents the transmittance of the Au–WO3thinﬁlms grown

on ITO coated glass with different Au concentrations as a function of

wavelength in both bleached (B) and colored (C) states at the optimum

coloring voltage Difference in transmittance of the Au–WO3ﬁlms on

ITO coated glass as compared with Au–WO3ﬁlms grown on pure glass

can be seen inFigs 1a and 9b(bleached states) It is clear from the

ﬁgure that by applying the optimum voltage, the transmittance of the

pure WO3ﬁlm decreased with a maximum value at about 400 nm of

wavelength This peak in transmittance spectrum was similar for the

ﬁlms containing 3.2 mol% Au with a broader peak around the same

wavelength But, for the dopedﬁlm with 6.4 mol% Au, the transmittance

of colored state curve differed signiﬁcantly In this spectrum, the

trans-mittance peak disappeared and the curve showed an increasing

behavior with increasing the wavelength and it reached at a constant

value forλ>700 nm This meant that Au–WO3electrochromic layer

with 6.4 mol% Au showed the reddish-brown color instead of blue in the

colored state Consequently, it can be concluded that various

concen-trations of Au can also result in different coloring states for the Au doped

WO3thinﬁlms

4 Conclusions

In summary, Au doped WO3thinﬁlms with different Au

concentra-tions of 0, 3.2 and 6.4 mol% have been prepared using sol–gel method

UV–visible spectrophotometry showed that the deposited ﬁlms are

highly transparent and plasmon absorption peaks indicated that partial

formation of gold nanoparticles occurred in the range of 510–550 nm

wavelengths Using XRD analysis, it was found that the 3.2 and 6.4 mol%

Au doped WO3containing gold nanocrystals has an average crystalline

size of about 10 and 60 nm, respectively In addition, XPS analysis

determined the stoichiometric formation of WO3and the presence of Au

in metallic state The electrochromic properties of the Au–WO3thin

ﬁlms grown with three different Au contents were studied in 1 M

lithium-based electrolyte An optimum coloring voltage was 2.5 and 3 V

for pure and both Au doped WO3thinﬁlms, respectively that minimized

the transmittance of the sample Moreover, the change of optical

density, total charge passed through the electrodes, coloration efﬁciency

and coloration time of Au–WO3 thin ﬁlms were measured at their

optimum voltages Comparing the obtained results, it was clariﬁed that

the change of optical density and coloration efﬁciency of the ﬁlms

decreased with increasing the gold concentration, due to the reduction

of the electrochromic material (WO3) and the Au competitive effect But,

interestingly, the Au–WO3ﬁlm containing 3.2 mol% Au showed a very

fast coloration time (26 s) which was ten times shorter as compared

with the coloration time of the pure WO3thinﬁlm (215 s) Therefore, an

optimum gold concentration was found for the Au–WO3thinﬁlms with

a transmittance reduction close to WO3pureﬁlm, but with a very fast response time In other words, an optimum concentration for Au dopant (3.2 mol%) in Au–WO3thinﬁlms has been found in which the reduction

of optical density change was just 18% that is negligible compared with the reduction of response time (87%) By studying the variation of transmittance versus wavelength for the samples colored at the optimum voltages, it was shown that the color of these electrochromic layers turned to dark blue for the pure as well as for the samples doped with 3.2 mol% Au in WO3 ﬁlms, and turn to reddish-brown for the 6.4 mol% Au doped samples Thus, it is suggested that the variation of Au nanocrystal content in WO3ﬁlms can also lead to various colors in the colored state

Acknowledgments The authors wish to thank the Research Council of Sharif University

of Technology for the ﬁnancial support of the project The partial support of the High Technology Organization (HTO) of the Ministry of Industries and Mines is appreciated The assistance of Mr S Raﬁei for XPS measurements is greatly acknowledged

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