Electrochromic behavior of WO3 nanoplate thin films in acid aqueous solution and a protic ionic liquid

    Electrochromic behavior of WO 3 nanoplates thin film in acid aqueous solution and a protic ionic liquid Jos´e de Ribamar Martins Neto, Roberto M. Torresi, Susana I. Cordoba de Torresi PII: DOI: Reference:

S1572-6657(15)30090-4 doi: 10.1016/j.jelechem.2015.08.032 JEAC 2257

To appear in:

Journal of Electroanalytical Chemistry

Received date: Revised date: Accepted date:

20 May 2015 23 July 2015 25 August 2015

Please cite this article as: Jos´e de Ribamar Martins Neto, Roberto M. Torresi, Susana I. Cordoba de Torresi, Electrochromic behavior of WO3 nanoplates thin film in acid aqueous solution and a protic ionic liquid, Journal of Electroanalytical Chemistry (2015), doi: 10.1016/j.jelechem.2015.08.032

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Electrochromic behavior of WO3 nanoplates thin film in acid aqueous solution and a

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protic ionic liquid

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José de Ribamar Martins Neto , Roberto M. Torresi and Susana I. Cordoba de Torresi* Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo.

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Av. Lineu Prestes, 784. P.O.box 26077, 05513-970 São Paulo-SP, Brazil *Corresponding author. Tel.: +55 11 3091 2350; fax: +55 11 3815 5557.

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E-mail address: [email protected] (S.I. Cordoba de Torresi)

Abstract

ACCEPTED MANUSCRIPT WO3 nanoplates were synthesized in non-aqueous solvent using a one-step process ultrasonic irradiation. The nanostructured morphology was maintained in the films prepared

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by Electrophoretic Deposition (EPD) process onto ITO substrates. Spectroelectrochemical

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experiments were carried out by using both, H2SO4 and a Protic Ionic Liquid (PIL) Nmethyl-pyrrolidinium tetrafluoroborate, electrolytes. Cyclic voltammograms (CVs) were

change

from

colorless

to

blue.

The

integrated

cathodic

current

of

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color

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combined with transmittance measurements for all films that undergo typical reversible

intercalation/deintercalation was used to calculate the coloration efficiency (CE) of films;

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the value obtained being in good agreement with nanostructured WO3 films. The coloration/bleaching processes showed good color efficiency, fast coloration time and

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optical contrast in acid electrolytes. The results using PIL as electrolyte, instead of H2SO4,

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show that all electrochromic parameters were improved, including, the cyclic durability that

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was higher in this medium. Therefore, the use of a PIL enables proton intercalation but no dissolution suggesting its use as a suitable electrolyte for electrochromic devices.

Keywords: Electrochromism, Tungsten oxide, EPD, Protic Ionic Liquid.

Introduction

ACCEPTED MANUSCRIPT Electrochromic materials are capable of changing their optical properties, reversibly or persistently when an electrical voltage is applied [1,2,3]. Electrochromism of tungsten

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oxide (WO3) nanostructures has been widely studied in the last years. From all these studies

summarized by the overall reaction:

(Eq. 1)

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WO3 + xM+ + xe- ↔ MxWO3

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a large majority of authors agree in assuming that the electrochromic effect in WO 3 can be

(Colorless)

(Blue)

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where M+ is generally H+, Li+ or Na+. Several physical studies of the reaction product, i.e.

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the "bronze" MxWO3, have been performed [4]. It is widely accepted that the mechanism for coloration is related to the intervalence charge transfer (IVCT) model for WO3, first

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proposed by Faughnan et al. [5]; it was based on the assumption that the injected electrons

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are localized onW6+ ions to form W5+, the coloration being due to transferring electrons

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from W5+ to the adjacent W6+ sites. However, unless these two W sites are non-equivalent, the two configurations are energetically equivalent. An alternative model is based on the concept of polaron formation, that displacing atoms or ions in a material from their carrier free equilibrium position produces a potential that will bind a charge carrier by self-trapping [6]. It has been suggested that the coloration in amorphous WO3 is due to small polaron formation [7]. Electrochromic tungsten oxide is potentially applied in energy-efficient smart windows, display devices, switchable mirrors and photoelectrochromic devices. The performance of electrochromic devices in visible wavelength range for civil applications has been extensively studied [8,9]. However, diffusion of positive ions into the tungsten oxide layer is often slow, sometimes taking minutes to complete. Many efforts have been made in this area to understand the intercalation phenomena in WO3 thin films using

ACCEPTED MANUSCRIPT electrogravimetric studies [10,11,12]. Since the chemical diffusion coefficient of protons (DH+) is one order of magnitude larger than that of lithium ions (DLi+), electrochromic

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systems based on proton electrolytes (e.g., aqueous H2SO4) are mandatory for display

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applications and preferred for other applications. Unfortunately, proton insertion from aqueous H2SO4 solution currently results in rapid degradation of the electrochromic films

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[13].

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There are two important criteria for selecting an electrochromic material. The first one is the time constant of the intercalation reaction, which is limited both by the diffusion

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coefficient and by the length of the diffusion path. While the former depends on the chemical and crystal structure of the metal oxide, the latter one is determined by the

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material’s microstructure [14]. In the case of nanoparticles, the smallest dimension is

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related to the diminution of the diffusion path length phenomena. Thus, designing a

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nanostructure with a small size, while maintaining the proper crystal structure, is a key to obtaining a material with fast insertion kinetics, enhanced durability, and superior performance.

The second important criterion is coloration efficiency (CE), the change in absorbance (A) per inserted charge unit (Q), that is, CE = ΔA/Q [4]. A high CE provides large optical modulation with a small charge insertion or extraction. This is a crucial parameter for electrochromic devices, since a lower charge-insertion or extraction rate enhances the long-term cycling stability. Different methods to obtain tungsten oxide nanoparticles has been reported, including sol-gel process [15], chemical vapor deposition [16], solvothermal [17] and electrochemical [18]. Sonochemical synthesis was used to prepare tungsten oxide nanomaterials using different precursors such as tungsten hexacarbonyl and tungsten

ACCEPTED MANUSCRIPT chloride [19,20] and different solvents such as ethanol [21] and benzyl alcohol [22]. Electrophoretic deposition is an important method of film formation, where charged

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nanostructures dispersed in an appropriate solvent can be deposited onto transparent

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substrates under influence of an electric field. There are some articles reporting electrochromic tungsten oxide films obtained by immobilization of nanoparticles using

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EPD technique [23,24].

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As stated, tungsten oxides have been most extensively studied but because of their high dissolution rate in acidic electrolyte solutions, these films can only be used in lithium-

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based organic electrolytes, resulting in slower response times. Furthermore, extended durability, even in Li+ systems, has not yet been demonstrated.

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Ionic liquids (ILs) are a class of solvents which are increasingly being used in a

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variety of applications due to a number of desirable properties [15,16]. ILs are defined as

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those fused salts which have a melting point below 100 °C, while those with higher melting points are frequently referred to as molten salts. ILs can be divided into two broad categories: aprotic ionic liquids and protic ionic liquids (PILs). PILs are produced through proton transfer from a Brønsted acid to a Brønsted base. They are an interesting subset of ILs, with some distinguishing features when compared to aprotic ionic liquids. The majority presents no negligible vapor pressures, but their beneficial properties for certain applications outweigh this potentially negative property. The protic nature of PILs is a crucial feature in a number of applications, including biological applications [27], organic synthesis [28], proton conducting electrolytes for polymer membrane fuel cells and catalysts [29]. PILs have attracted attention as electrolytes for their suitable electrochemical properties, including high electrochemical and dimensional stability, good ionic conductivity, and long-term durability. Some systems

ACCEPTED MANUSCRIPT have been investigated and applied in batteries , double-layer supercapacitors , fuel cells and chemical sensors [30].

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For the electrolyte application in electrochromic devices (ECDs), the properties

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mentioned above have to include also the excellent optical transparency and high photostability of the electrolyte. PILs systems based on pyrrolidinium meet these

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requirements. Moreover, they are environmentally friendly and exhibit only a low toxicity

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when comparable to other electrolytes like propylene carbonate (PC). In this article we report our strategy for building up electrochromic nanostructured

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films with tungsten oxide nanoplates synthesized by sonochemical method and deposited using electrophoretic deposition (EPD) technique onto a transparent conductor substrate

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PIL electrolytes.

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(ITO) and proton intercalation/deintercalation of the as-prepared films in both aqueous and

Experimental methods

Materials: Tungsten (VI) chloride (99.99%) was purchased from Aldrich. Benzyl alcohol (anhydrous, 99.8%), ethanol, Tetra hydrofurane (THF) and acetonitrile were purchased from Synth. All chemicals were used without further purification.

Synthesis of tungsten oxide nanoplates

WO3 nanoparticles were prepared by modification of previous method described in literature [20]. First, 100 mg of WCl6 (0.25 mmol), was slowly added to 10 mL of benzyl alcohol (86.50 mmol) under vigorous stirring at room temperature. The solution was

ACCEPTED MANUSCRIPT introduced into the sonication cell with a probe from Sonics-Vibracell. The reaction mixture was sonicated during 9 min at 60% amplitude to fully react. The sonication was

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carried out without cooling, so that, a temperature of 92°C was reached at the end of the

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reaction. The resulting suspension was centrifuged and the precipitate was thoroughly washed several times with ethanol and THF. The collected material was left to dry in air

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and finally ground into a powder. The as-prepared powder was green-yellow due to the

after annealing in air at 300°C for 2 h.

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sub-stoichiometric state of the as-synthesized WO3 nanoplates, but became slightly yellow

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For Raman and XRD characterization, powder of nanoplates were obtained by centrifugation (Eppendorf centrifuge, 13,400 rpm) during 1 h followed by vigorous

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washing with deionized water in order to remove superficially adsorbed ions, and further

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centrifugation for one additional hour. The residual solution was removed and the solid

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obtained was dried at low pressure and room temperature in a desiccator. The phases of the synthesis products were identified by powder X-ray diffraction (XRD, Rigaku Miniflex 30 kV/15 mA, λ = 1.54056 Å). Average crystallite/grain size was calculated using the Debye–Scherrer equation after carrying out background subtraction. Crystallite size values calculated using the XRD results obtained are only approximate since the effect of instrumental line broadening on these was not taken into account. Raman spectra were obtained in a Renishaw Raman Imaging Microscope (System 3000), connected to a CCD detector (Wright, 600 x 400 pixels), using the 514 nm excitation radiation (He-Ne laser Spectra Physics, model 127). The Transmission Electron Microscopy images were collected using Jeol 1200 EXII Electron Microscope. The samples were dispersed in acetonitrile and a drop of the suspension was air dried onto carbon-coated copper grids.

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Film formation

The substrates, indium-doped tin oxide substrates (ITO, Delta Technologies, sheet

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resistance 15-25 Ω cm-1, surface area 0.5 cm2) (0.7 cm × 3 cm in size), were cleaned by

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ultrasonication in acetone, ethanol and water for 15 min, respectively. Then the substrates

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were further washed thoroughly with water.

The nanoplates suspension was prepared by dispersing the WO3 nanoplates in

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acetonitrile with 2 mg cm-3 concentration using an ultrasound probe from Sonics-Vibracell. Films were made by using the EPD technique with nanoparticles immobilized onto indium-

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doped tin oxide substrates used as the anode and a platinum foil as the cathode by applying

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potentials at different range from 100 to 300 V, during 60s between the two parallel

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electrodes placed 1.0 cm apart in an electrochemical cell containing the nanoparticle suspension. The coated

substrates were overnight air-dried before morphological and

electrochromic studies.

The morphologies of the as-prepared thin films were observed by scanning electron microscopy (SEM) with a Jeol microscope, model JSM-7401F. Thin film growth was monitored by UV-vis spectroscopy with an HP 8452A spectrophotometer.

PIL Electrolyte Preparation. Equimolar amounts of the tetrafluoroboric acid solution were slowly added while stirring to the N-methylpyrrolidine contained in a roundbottom flask over ice. The reaction with strong inorganic acids is extremely aggressive and care should be taken. The temperature during reaction was maintained below 15 °C. Excess water was removed by drying under vacuum at 60 °C. The final product was N-

ACCEPTED MANUSCRIPT methylpyrrolide tetrafluoborate ([C3mpyr][BF4])

which is typically hygroscopic and

required vacuum freeze-drying prior to use [31].

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The amount of water contained in the PILs was determined using a coulometric

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Karl-Fisher titrator from Metrohm (model 831). Samples of PIL were injected in the titrator using a syringe with the sample to analyze.

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Electrochemical and electrochromic properties of the EPD films were performed in

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a traditional three-electrode cell system. The WO3 EPD film coated glass served as the working electrode, and a Pt wire were used as the counter electrode. For aqueous

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electrolyte an Ag/AgCl (KCl-saturated) electrode was used as reference electrode, whereas in PIL electrolyte a Ag quasi-reference electrode was used. H2SO4 1.0 mol L-1 was

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employed as aqueous electrolyte whilst [C3mpyr][BF4] was used as PIL electrolyte in the

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electrochemical measurements. Spectroelectrochemical experiments were performed with

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an Autolab PGSTAT 30 (Ecochemie) potentiostat/galvanostat. The electrochromic analyses were obtained simultaneously with electrochemical ones by using a solid-state light source (WPI Inc.). Plastic-optic fibre cables of 1 mm diameter were used for transporting the light from the electrochemical cell to a photodiode amplifier PDA 1 (WPI Inc.), connected to the potentiostat obtaining, in this way, in situ current/transmittance profiles as a function of potential.

Results and Discussion

Figure 1 shows the TEM images of the synthesized tungsten oxide nanoplates. As can be seen from the images, the tungsten oxide forms nearly square, two-dimensional nanoplates. These nanoparticles have facets ranging from 40 to 70 nm. The low

ACCEPTED MANUSCRIPT magnification TEM image indicates that the WO3 nanoplates have their size in good

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agreement with the size calculated from the powder XRD patterns.

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Fig. 1

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Figure 2 displays XRD patterns of the as-synthesized and the annealed WO3 at

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various temperatures. All the peaks can be indexed to those of the cubic structure of WO3.H2O patterns (JCPDS 041-0905). The intense and sharp diffraction peaks indicates

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that the as-synthesized platelets are well-crystallized. Average crystallite size for these nanoparticles was calculated approximately to be 51 nm using the Debye–Scherrer formula,

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d = 0.9 λ / β cosθβ where d is the particle size and λ, β, and θβ are respectively the X-ray

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the Bragg peaks [32].

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wavelength, the Bragg diffraction angle and the full width at half maximum (in radians) of

Figure 2

The infrared spectra of the tungsten nanoparticles are shown in Figure 3. The results are comparable with those obtained by Daniel [33] showing spectra for tungsten oxide powder before and after annealing process. The stretching and bending bands in the region between 500 and 1000 cm-1 (maximum at 640 cm-1) are a clear finger print region of the metal oxide framework O-W-O and W-O-W vibrational modes. The slightly and large broad peak observed at ~3424 cm−1 of the IR spectrum can be assigned to O-H stretching bonds attributed to the presence of adsorbed water or hidroxil groups on the surface of the WO3 nanoparticles before the calcination process. Stretching and bending of O-H take

ACCEPTED MANUSCRIPT place in W-OH terminal bonds that are formed when O-H bonds strongly attached to surface oxygen atoms or surface adsorbed water molecules [9] and [34]. The low intensity

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of this peak would suggest that the presence of H2O may only be a surface phenomenon and not the inclusion of H2O molecules into the bulk/ bond structure of the film [33]. On

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the other hand, the presence of a complimentary characteristic peak at1622 cm−1 due to H-

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nanoparticles produced by ultrasound process.

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O-H stretching suggests that there is water incorporated into the bulk of the WO3

Figure 3

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The Raman spectrum of the powder sample after annealing is shown in Figure 4. It is important to note that Raman is very sensitive to the vibration modes associated with the

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oxide. The three strongest bands in the spectrum at 271, 706 and 808 cm-1 are due to the γ-

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phase [35]. Peaks associated with the ε-phase would be expected to appear at 640 and 679

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wavenumbers. It is difficult to detect these features in the sample, which may reflect the poor sensitivity of Raman to this phase.

Figure 4

Thin films of tungsten oxide nanoplates were deposited by a unique EPD process onto ITO glass substrates. Fig. 5 shows the SEM image of WO3 thin film onto ITO. It is possible to see clearly that stacked structure of WO3 nanoplates were preserved during the film formation. It is also evident that these films are highly porous with large active surface areas. As illustrated by cyclic voltammetry measurements, this large surface area results in very significant improvements in the electrochromic properties.

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Fig. 5.

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In Figure 6, the cyclic voltammograms (CV) obtained with films deposited by EPD technique in different electric fields are shown; the deposition time of 60 seconds was

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maintained constant in all these cases. As a rule, tungsten oxide films show uniform blue

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color when cathodically polarized and transparent appearance when anodically polarized. The integrated cathodic-current density equates to the amount of protons intercalated to

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form the tungsten bronze. When compared with the cathodic charge quantities of amorphous and crystalline tungsten oxide films, the nanoparticle films show a higher

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charge-insertion density over the same time period, probably due to larger surface area in

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nanostructured films. Both electric charge and chromatic contrast (%T) increase with the

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electric field, indicating that the amount of nanoparticles deposited depends on the voltage applied. It is clear that, by increasing the deposition time, greater amounts of nanoparticles are deposited. The electrodes obtained by EPD show rough surface indicating that the nanosized features of the particles is maintained during the deposition. The CV was measured with the cycling of 20 mV s-1 sweep rate for each of the four films deposited by EPD in 1.0 mol L-1 H2SO4 at room temperature. As the potential becomes more negative, the current density increases corresponding to a intercalation process where electrons from the electrode and H+ ions from the acid solution are co-inserted into the tungsten oxide film, which involves the formation of a hydrogen tungsten bronze (HxWOy) and consequently changes in optical properties. We can explain this process in terms of double charge injection model and the absorbance change due to intervalence transition between WVI and WV sites.

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Figure 6:

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In order to investigate the optical properties, the EPD film deposited at 200 V cm-1

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was used in colored and bleached states. It was chosen due to good coloration efficiency showed in figure 6. Investigation was carried out by applying different bias potentials range

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voltage of -0.1 to -0.4 V vs. Ag/AgCl in H2SO4 electrolyte. For each applied potential time

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of 60s was applied before each spectrum. Absorbance spectra of the film in bleached and various colored states are displayed in Figure 7. The absorbance spectrum in colored state

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displays an obvious band beginning at 520 nm at more positive potential and it moves to lower values with decreasing the potential until -0.4 V, which is consistent with the blue color observed at range voltage of 0.1 to -0.4 V in H2SO4 electrolyte. The film exhibits a change in transmittance that covers both visible light and near-infrared region, which would be important for smart windows applications due to the thermal absorption in the near infrared region.

ACCEPTED MANUSCRIPT Figure 7: The EPD thin film showed a good contrast and optical modulation in an acidic

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electrolyte. Thus, some electrochromic parameters such as response time and coloration

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efficiency are dependent on the length of diffusion path and the diffusion coefficient of the electrons and cations, indicating that phase and morphology control of the nanoparticles are

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very important to achieve fast response times, good coloration efficiency and durability.

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The commercial use of electrochromic films has been limited by weaknesses like long switching time, insufficient cyclability and long-term stability. To avoid some of these

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problems, a significant part of the research need to be focused on the electrolytes. Aqueous electrolytes are not always good candidates because of their narrow potential stability

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window and, in the specific case of the tungsten oxide, poor stability in the presence of

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water.

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In this work we report a new approach that is based on the use of a protic ionic liquid (PIL), which is consistent with the strategy of replacing aqueous electrolytes by nonaqueous ones, such as PILs, in electrochromic systems. This approach consists of primary amine cation N-methylpirrolidinium combined with inorganic anion tetrafluorborate. Scheme 1 illustrates the Bronsted acid/base pairs used in this study. CH3

H CH3

N

+

HBF4

Scheme 1: Synthesis of the PIL ([C3mpyr][BF4]).

N

BF4-

ACCEPTED MANUSCRIPT The PIL obtained in this work presents a slightly pale yellow appearance, indicating that it contains colored impurities since most pure PILs are colorless. However, albeit of

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this coloration, their physicochemical properties and transmittance results were found to be

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appropriated, which shows that the presence of such impurities has no effect on their properties. The PIL were therefore used without any further purification. The water content

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also was monitored by Karl Fisher method after and before the measurements and it was

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about 8000 ppm.

Electrochemical and electrochromic performance of the PIL [C3mpyr][BF4] were

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compared with a typical aqueous electrolyte such as H2SO4. The coloration and bleaching processes were due to intercalation and deintercalation of H+ from the electrolyte to the

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oxide films. Cyclic voltammograms were performed in PIL electrolyte in the 1.0 to -0.5 V

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vs. Ag/Ag+ potential range; protons were inserted into the lattice of WO3 nanostructured

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film, while the potential sweep towards the opposite direction was accompanied with the extraction of the protons.

The electrochromic reaction is related to the double intercalation/deintercalation of species in the nanostructured film matrix. In this case, we can associate the absorbance (A) to the inserted charge per unit area, as was already done for inorganic and organic materials [36] and [37]. On the other hand, current can be related to the derivate of the absorbance related to time: (2) Where ε is the molar absorptivity of the film, b is the film thickness, c is the concentration of optically active sites on the electrode, j is the current density, z is the number of electrons participating in the redox reaction and F is the Faraday constant.

ACCEPTED MANUSCRIPT If the reaction responsible for the coloration change of the material were the same producing a redox wave during the voltammetry and all the current is consumed in the

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electrochromic process, the dA/dt versus E profile must coincide with the j/E profile

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obtained under the same experimental conditions. This statement is correct if all current related to the electrochromic process has a faradaic nature, separating, in this way, faradaic

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from capacitive currents.

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Figure 8 shows the behavior of WO3 film deposited at 200V cm-1 during cyclic voltammetric measurement at 5 mV s-1 sweep rate. Figure 8a shows the classical current-

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voltage curve. Fig. 8b shows the corresponding transmittance variation at 633 nm wavelengths while Fig. 8c shows dA/dt curves calculated from the absorbance variation and

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the current-voltage curve. The dA/dt curve in figure 8(c) represents the rate of the

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coloration and the bleaching states of the film and it is obtained by taking the derivate of

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absorbance versus potential plot. Comparison between CV and dA/dt curves provides information that both electrochemical and optical reaction occurs in the same region of potential. During cathodic reaction, a peak can be observed at 0.2 V vs. Ag/Ag+. This electrochemical reaction may be attributed to the same reaction as observed in H2SO4 electrolyte, involving the intercalation of H+ in the oxide. At potentials more negative than 0.4 V vs. Ag/Ag+ the current in excess observed probably has a capacitive nature and is not related at any coloration process as shown in dA/dt curve. During anodic polarization, the bleaching process also occurs in accordance with deintercalation process.

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Figure 8.

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The electrochromic reaction with intercalation/deintercalation of protons also occur using PIL. Protons are available in the electrolyte and it involves the formation of hydrogen

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tungsten bronze. All these observations provide sufficient evidence to demonstrate that

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protons intercalation in WO3 film in PIL comes up in a manner similar to that in aqueous solutions. The proton transfer and ionicity of PIL has been estimated for Mayrand-

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Provencher using the relation of ΔpKa [38], which is defined as the pKa difference between the base and acid in a defined solvent and can provide a way to quantify the proton transfer

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capability between the acid and the base. Indeed, a large ΔpKa indicates a strong proton

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transfer and a good ionicity, whereas if the ΔpKa is small, a poor ionicity is obtained. In the

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case of [C3mpyr][BF4] the ΔpKa value was estimated at 9.82 [39], which can be considered as a high value and an indication of a good conductivity of the PIL. This would be an explanation for the obtained electrochromic results that indicate the formation of the tungsten bronze in the same way that it was observed in sulfuric acid solutions The electrochemical stability of the WO3 film during the electrochromic process was investigated by cyclic voltammetry measuring the inserted charge and the extracted charge after 500 cycles of long-term voltammetric sweeping. The inserted charge and absorbance change are almost at a constant value during the electrochromic process, indicating the good stability of the nanostructured film in PIL electrolytes. During an electrochromic process, the switching time can be defined as the time required for obtain partial or total change in absorbance and can be observed within a few seconds after application of the bias voltages [40]. The coloration and bleaching times were

ACCEPTED MANUSCRIPT calculated as the time required to reach 90% of total absorbance change. The coloration time was 2.8 s, while the bleaching time was 23.0 s. The coloration and bleaching process

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of the WO3 film in PILs electrolyte were similar to those achieved in H2SO4 electrolyte,

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with a smaller decrease of both values. It is worth to mention that a larger optical modulation of ~70% in PIL electrolyte is achieved compared to that ~65% of acid

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electrolyte. These values are summarized on table 1.

Table 1: Electrochromic parameters from tungsten oxide film.

3.0 s

[C3mpyr][BF4]

2.8 s

Tbleaching

%T

Coloration Efficiency(cm2 C-1)

24.0 s

65.6

43.4

23.0 s

70.3

52.4

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H2SO4

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Tcoloration

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Electrolyte

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The coloration efficiency (CE) correlates the change in absorbance with the charge intercalated. CE values of the WO3 EPD 200V cm-1 film were calculated for PIL and acid solution electrolytes. Figure 9 shows the plot of absorbance at a wavelength of 633 nm versus the charge density by using a constant discharge-current of 0.25 mA cm-2. The CE is extracted from the slope of the line fits to the linear region of the curve. It is possible to see that the calculated CE of WO3 film in PIL electrolyte was 52.4 cm2 C-1, just a slightly larger than that for WO3 film in H2SO4 solution (44.4 cm2 C-1).

ACCEPTED MANUSCRIPT Figure 9:

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Figure 10 shows the optical switching behavior under cronoamperometric

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conditions of two different WO3 nanostructured films in both 1.0 mol L-1 H2SO4 and PIL electrolyte [C3mpyr][BF4]. It is evident the larger durability when PIL electrolyte is used

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instead H2SO4. This result suggests that loss of crystallinity with successive proton

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intercalation/deintercalation in the film structure is greater when the acid solution is used instead of PIL electrolyte. In the system containing the aqueous acid solution electrolyte,

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decreasing of optical contrast values is caused by irreversible intercalation and film dissolution. The dissolution rate of WO3 follows the general dissolution mechanism for

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metal oxides and it does not depend on the oxide thickness. Dissolution of WO3 films

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includes the existence of both hydration process step and generation of porosity/roughness

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[41]. WO3(s) + H2O

[WO3.H2O] + H+

[WO3.H3O+] + H+

[WO3.H2O] [WO3.H3O+] WO22+ + 2 H2O

(3) (4) (5)

Supplementary material (Figure S1) shows Uv-vis spectra taken in the acid electrolyte at different times: before, during and after consecutives electrochromic cycling. It can be seen that together with the monotonic decrease of the chromatic contrast of the film during cycling, and absorption band increases in the ~230 nm region of the solution spectrum. It was already reported the absorption corresponding to tungstate species in this ultraviolet region [42]. The dissolution is kinetically controlled by the pH, indicating that is also promoted by H+ concentration at the oxide/electrolyte interface. The specific nature of electrolyte anion seems to play no kinetic role, except in promoting porosity/roughness

ACCEPTED MANUSCRIPT [43]. In our work, WO3 EPD films are more quickly dissolved in aqueous electrolyte than in PIL. Using PIL electrolyte, the WO3 dissolution is significantly slower once the hydration

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process (eq.(3)) is limited by the smaller amount of water than in the case when aqueous

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electrolyte is used. The amount of water in the nanostructured film was estimated using WO3 density [44] and TGA results (Figure S2, supplementary material) as being 0.5 mol of that means about 11.41 mol L-1.

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water per mol of tungsten oxide (WO3.0.5H2O);

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Considering the amount of water in PIL (8000 ppm or 0.44 mol L-1) , it is much lower than the amount in the oxide; as a result, the small amount of water available in the

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oxide/PIL interface must be responsible for delaying the hydration process of the oxide film and preventing premature film dissolution. In this way,WO3 film stability in PIL is

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achieved for long period of time increasing the long-term cyclability.

Figure 10

Conclusions In conclusion, the crystalline tungsten oxide nanoparticles were synthesized by sonochemical method and post-annealing treatment. XRD and TEM results revealed that WO3 nanoplates present a nanosized dimension. Meanwhile SEM results showed that EPD technique can be used to achieve dense nanostructured films. The films demonstrated good electrochromic performance with fast response time, good color efficiency and different durability in each electrolyte used. Furthermore, the PIL electrolyte exhibits an excellent

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process of the oxide.

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Acknowledgements

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Financial support was provided by FAPESP (2009/53199-3 and 2010/08043-8), CNPq and CAPES. Authors acknowledge Prof. Dr.Marcio Vidotti (UFPR) for TEM images and useful

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Highlights

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Graphical abstract

Tungsten oxide nanoplates were obtained by one spot sonochemical method. Electrochromic films were prepared by electrophoretic deposition. Electrochromic performance in acid and protic ionic liquid was investigated. Limited amount of water in PIL avoids film dissolution.