Improved electrochromical properties of sol–gel WO 3 thin films by doping gold nanocrystals N. Naseri a , R. Azimirad b , O. Akhavan a , A.Z. Moshfegh a,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 abstractarticle info 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 WO 3 thin films has been studied. The Au–WO 3 thin films 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 the films showed the formation of gold nanoparticles in the films. The optical bandgap energy of Au–WO 3 films 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 WO 3 thin films. 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 WO 3 . The electrochromic properties of the Au–WO 3 samples were also characterized using lithium-based electrolyte. It was found that doping of Au nanocrystals in WO 3 thin films 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–WO 3 thin films. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Research in the field 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, WO 3 has been investigated most extensively [1–6]. This was due integrally to its fast response times, coloration efficiencies, long life times, etc. These properties tendered WO 3 desirable 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 WO 3 film increase with the addition of the porous conducting metallic over layer in a liquid electrolyte cell [11]. Moreover, electrochromic operation of WO 3 films 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 WO 3 thin films [15–17]. He et al. has reported addition of gold over layer on WO 3 thin film 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–WO 3 nanocomposite thin fi lms containing high gold concentration (60 mol% Au) and obtained a shorter response time relative to the pure WO 3 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–WO 3 nanocomposite thin films 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 WO 3 thin film. In this paper, we present data on optical properties, crystalline structure and surface chemical state of Au doped WO 3 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 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.08.001 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf ARTICLE IN PRESS Please cite this article as: N. Naseri, et al., Thin Solid Films (2009), doi:10.1016/j.tsf.2009.08.001 electrochromic behavior of Au–WO 3 thin films under various applied voltages has been also studied. 2. Experimental details Sol– gel Au doped WO 3 thin films were prepared using the following procedure. At first, 6 g of sodium tungsten dehydrate (Na 2 WO 4 ·2H 2 O, 99% Merck) was immersed in 30 ml of nitric acid solution (HNO 3 , 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 H 2 O 2 (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 WO 3 sol was mixed with various amounts of 0.14 molar aqueous solution of HAuCl 4 (99.5% Merck). The molar concentration of Au in the final 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. All films, 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 dried films measured about 200–300 nm. AUV–visible spectrophotometer (Jasco-V530) was used to inves- tigate the optical properties of the films (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–K α radiation source (conventional θ–2θ diffractometer) and step size of 0.05° was used to determine phase form ation, average crystalline size and structure of the layers. X-ray photoelectron spectroscopy (XPS) equipped with an Al–K α X-ray source at an energy of 1486.6 eV was employed to investigate the surface chemical composition of the films. The hemispherical energy analyzer (Specs EA 10 Plus) operating in a vacuum better than 10 −7 Pa was used. All binding energy values were calibrated by fixing 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 WO 3 thin films were investigated on ITO/glass substrates in a 50 ml glassy cell containing two electrodes. The Au–WO 3 films, as working electrodes, were electrochemically cycled in a 1 M LiClO 4 in 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–WO 3 thin films was studied in two states. In the first state, it was measured as a function of time at a fixed wavelength of 500 nm (in which eye has a high sensitivity) at different coloring voltages, and then negative voltages applied for bleaching the films. 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 The b and g ap energy of the deposited thin films h ave b een measured using their linear plots of (αhν) 1/η versus hν. Here, hν is the corresponding incident photon energy and the exponent η depends on the kind of optical transition. η is 1/2, 3/2, 2 and 3 when the transitions are direct allowed, direc t forbidde n, i ndirect allowed a nd in direct forbidden , respectively [1]. Calculation method has been explained in other literatures [1,1 9, 33]. By examining the various values of η,asaresultof line fitting, η=2 was determined for the pure and doped WO 3 films describing thei r i ndirect allowed transitions, respectively (Fig. 2 ). The obtained value of η has been also reported f or other W O 3 compound films including WO 3 –Fe 2 O 3 [33] and WO 3 –SiO 2 [34].AccordingtoFig. 2a, the optical bandgap of the pure W O 3 thin film was measured at about 3.3 eV. This value i s in agreement w ith the reported bandg ap energy of a mor- phous WO 3 thin film by others [1,19,21]. Fig. 2billustrates(αhν) 1/2 as a function of hν for the Au doped WO 3 films measuring optical bandgap energy of 3.1 and 2.8 eV f or th e s ample s con taining 3.2 and 6.4 mol % A u, respectively. T he refore, it i s clear that the bandgap energy of the samples decreased with increasing the Au concentration as a result of the metal doping. Actually, fo r a sem iconductor, t he conduc tion band is curved coloring voltages. By applying the coloring voltage to the electrode at t=10 s, the transmittance of the films 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 the films 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 W V ions were produced, due to the electron transfer from the electrode to W VI ions, and cations were inserted into the films simultaneously so as to maintain electroneutrality. Tungsten bronze (Li x W VI 1 −x W x V O 3 ) 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 WO 3 electrochromic thin films influence on the intensity of color. Low crystallinity [44,45] and high porosity [44,46] of WO 3 film 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 the film, 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 from Fig. 5 that by increasing the coloring voltage, the transmittance of the colored films decreased and the rate of the coloration increased. When the coloring voltage exceeds a certain value, the transmission of the bleached films is markedly lower than its value before coloration. This specific value of coloring voltage called optimum voltage (V opt ) was determined as 2.5 and 3 V for the pure WO 3 and both Au doped WO 3 films, under o ur experimental conditions, respectively (For the pure WO 3 films under applied voltage of 2.5 V, their bleached state requires more than 500 s). In addition, due to measuring equal transmittance for the films, 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 bleached films 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 Li x WO 3 is 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 ARTICLE IN PRESS Please cite this article as: N. Naseri, et al., Thin Solid Films (2009), doi:10.1016/j.tsf.2009.08.001 The derivative of transmittance versus time (dT/dt) under optimum applied voltages has been presented in Fig. 6, for different concentra- tions of Au. This figure has been extracted from Fig. 5 and 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 −1 for t he pure and doped films, respectively as a result of Li + ions migration to the layer. The diffusion of lithium ions occurred rapidly at first and then became slow. So, the coloration rate then decreased and reached to zero. It is obvious that due to higher conductivity of the films containing Au; they were colored faster than the pure WO 3 films. Although the Au doped films had the same coloration rate at first, however the magnitude of |dT/dt| was higher for the film 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 the film 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 = nFACD 1 = 2 π 1 =2 t 1 =2 ð2Þ 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 v ariat ion of co lorin g current a s a fun cti on of ti me for t he Au– WO 3 thin films with different gold c oncentrations at the o ptimum applied voltage.AscanbeseenfromFig. 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 (t 1/2 ) at longer t imes as a r esult of natural convecti on effects [50].Itis clear from the extrapolation of the graphs with current axis that the diffusion coefficient increased with increasing the Au concentration in the thin films. Using Co ttrel equation, the d iffusion coefficie nt of the films was estimated to be 1×10 −8 ,3×10 −8 and 5×10 −8 cm 2 s −1 for the films containing 0, 3.2 and 6.4 mo l% A u, respectively. Therefore, the Au do ping has a positive effect on the cations diffusivity in the WO 3 films. A similar result h as bee n als o rep orted f or th e WO 3 film s dop ed wi th 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. 8 presents the electrochromic charge passed through the Au–WO 3 layers (Q) with the different Au concentrations as a function of applied voltage, in both coloring and bleaching states. As can be seen from the figure, 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 Li x WO 3 and reduction of the transmittance after the coloration process (see also Fig. 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. 5N. Naseri et al. / Thin Solid Films xxx (2009) xxx–xxx ARTICLE IN PRESS Please cite this article as: N. Naseri, et al., Thin Solid Films (2009), doi:10.1016/j.tsf.2009.08.001 The value of x in Li x WO 3 can be estimated using this formula [49]: x = Q Li M = AdeN a ρ ð3Þ where Q Li is 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 N a is the Avogadro number, respectively. Corresponding values of x under different coloring voltages for the pure and Au doped WO 3 films 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 =ln½Tðt 1 ; λÞ = Tðt 2 ; λÞ ð4Þ where T(t 1 ,λ) and T(t 2 ,λ) are transmission of the films a t λ = 500 nm before ( t 1 )andafter(t 2 ) the colo ration process, respectively. For th e abo ve relation, t 1 =10 s, t 2 =500 s and λ=500 nm wer e considered in our experimental conditions. The quantity of ΔOD indicates how much the transmittance o f the electrochromic layer reduces during the coloring process. As listed in Table 2, the change in the optical density w as measured at about 1.6 for the pure WO 3 thin film which decre ased slightly to 1.3 for the doped film containing 3.2 mol% gold. This value declined more strongly to 0.5 f or the sam ple with 6.4 mol% Au. The opti cal density change is attributed to the effective number of W 6+ ↔ W 5+ transitions. Hence, increasing the Au concentration led to the decreasing of active W 6+ sites in the films and the amount of ΔOD decreased. A similar behavio r has be en al so observed fo r Li–Nb 2 O 5 [51] and Fe 2 O 3 – WO 3 [33] electrochromic composite thin films recen tly. Moreover, it is reported that some ions bound with metallic nanoparticles con tained in the electrochromic electrode. Thus, the positive ion transfer takes place competitively, between e lectrochreo mic material (WO 3 )andincorporat- ed metal (Au) [16,17,52,53]. Ther efore, it can be concluded that the reduction in the ΔOD is a logical consequence of decreasing WO 3 content and/or increasing the Au concentration in the layer. A similar to this result has b een also repo rted for vanadium doped WO 3 thin films very r ecentl y [54]. An important parameter for electrochromic films is the coloration efficiency, CE(λ), that is defined as follows [1]: CEðλÞ = ΔODðλÞ ⋅ A = Q ð5Þ where ΔOD(λ) represents change in the optical density at a fixed wavelength (λ), A is the surface area (~2 cm 2 for our samples) and Q is the charge transferred during the coloration process in terms of C. The calculated value of CE(λ) was 35cm 2 C −1 for the pure WO 3 thin film which decreased to 7 cm 2 C −1 by increasing the gold concentra- tion in the film 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 electrochromic films is their response time. It is essential to decrease the time interval needed for coloration process, in practical applications. The colorationtime (t c )canbedefined asthe 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 WO 3 film is 215 s which declined sharply to 26 s for the doped WO 3 thin film containing 3.2 mol% Au. It has been shown that the addition of gold to the WO 3 electrochromic thin film facilitates the electron transfers (more conductivity in the films) 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 the films. On the other hand, as discussed previously, reduction of electrochromic material (WO 3 ) 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 mo l% i n the WO 3 thin film, 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 WO 3 thin films and was determined as 3.2 mol%, in this work. The obtained electrochromic characteristics of the Au–WO 3 thin films under the optimum applied coloring voltage have been summarized in Table 2 for the different Au concentrations. The transm ission spectra of the Au doped WO 3 thin film contain- ing 3.2 mol% Au colored at va rious 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–WO 3 /ITO/glass structure was very similar to the Au–WO 3 /glass structure (see Fig. 1a). It is seen that after applying the voltage, the transmittance of the film decreased (Fig. 5), indica ting the Li + inser- tion to the layer and thus its color changed. This reduction 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) 1.5 2 2.5 3 3.5 4 0 0.12 0.24 0.35 0.92 – 2.04 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) 0 2.5 1.6 90 35 215 3.2 3 1.3 160 19 26 6.4 3 0.5 150 7 50 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. 6 N. Naseri et al. / Thin Solid Films xxx (2009) xxx–xxx ARTICLE IN PRESS Please cite this article as: N. Naseri, et al., Thin Solid Films (2009), doi:10.1016/j.tsf.2009.08.001 of the colored film at the near-IR range was less than the transmittance in the visible range. Actually, the change in optical properties of an electrochromic film 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 (W 6+ and W 5+ ) 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), the films showed more coloration and so, further reduction in transmittance took place. At wavelength of about 400 nm, the maximum transmittance of the colored film was measured, because the color of Li x WO 3 compound 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 Li x WO 3 and the irreversibility of electrochromic reaction occurred at the coloring voltages higher than 3 V. Fig. 9b represents the transmittance of the Au–WO 3 thin films 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–WO 3 films o n ITO coated glass as compared with Au–WO 3 films grown on pure glass can be seen in Figs. 1a and 9b (bleached states). It is clear from the figure that by applying the optimum voltage, the transmittance of the pure WO 3 film 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 doped film 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 for λ > 700 nm. This meant that Au–WO 3 electrochromic 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 WO 3 thin films. 4. Conclusions In summary, Au doped WO 3 thin films 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 WO 3 containing gold nanocrystals has an average crystalline size of about 10 and 60 nm, respectively. In addition, XPS analysis determined the stoichiometric formation of WO 3 and the presence of Au in metallic state. The electrochromic properties of the Au–WO 3 thin 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 WO 3 thin films, 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–WO 3 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 (WO 3 ) and theAu competitive effect. But, interestingly, the Au–WO 3 film 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 WO 3 thin film (215 s). Therefore, an optimum gold concentration was found for the Au–WO 3 thin films with a transmittance reduction close to WO 3 pure film, but with a very fast response time. In other words, an optimum concentration for Au dopant (3.2 mol%) in Au–WO 3 thin films 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 WO 3 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 WO 3 films 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. 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Naseri, et al., Thin Solid Films (2009), doi:10.1016/j.tsf.2009.08.001 . Improved electrochromical properties of sol–gel WO 3 thin films by doping gold nanocrystals N. Naseri a , R. Azimirad b ,. films Optical properties Electrochromic properties Coloration time In this investigation, the effect of gold nanocrystals on the electrochromical properties of sol