Modification in the microstructural and electrochromic properties of spray-pyrolysed WO₃ thin films upon Mo doping

Abstract

WO₃ thin films were grown onto the glass and fluorine-doped tin oxide-coated glass substrates using chemical spray pyrolysis technique. X-ray diffraction analyses reveal that all the films possess orthorhombic phase of WO₃. Morphologies of the films have been found to vary with Mo-doping concentrations. Three-dimensional atomic force micrographs reveal that the 5 at% Mo-doped film has the maximum image surface area and is optimal for improved electrochromic performance. Cyclic voltammetry studies show that the cathodic and anodic peak current densities have the highest values for the 5 at% Mo-doped WO₃ film. Relative to the undoped film, the change from coloured to bleached state is faster for the 5 at% Mo-doped WO₃ film. Considering the reversibility and the switching response from chronoamperometry and the coloration efficiency from cyclic voltammetry, it has been concluded that the 5 at% Mo-doped WO₃ film has an optimal electrochromic response.

Highlights

• As-deposited films possess orthorhombic phase of WO₃.

• 5 at% Mo-doped WO₃ films have the maximum image surface area.

• WO₃ film doped with 5 at% Mo exhibits fastest switching response.

• 5 at% Mo-doped film exhibits optimal electrochromic performance.

1 Introduction

Electrochromism is the phenomenon exhibited by certain materials, where the materials reversibly change their colours when they are subjected to an electric field. Reversible change in colour arises when the material undergoes different electronic states during electrochemical redox reactions. The most important feature of electrochromism is that an electric field is required only during the change (colour or bleach process), which can be controlled at any intermediate stage. Owing to persistence and reversible nature of colour change, such materials, called electrochromic materials, are used in smart windows, which control the amount of light and heat passing through them. In addition, electrochromic materials are used to automatically dimming rear-view mirrors in various ambient light conditions, and more recently, in electrochromic displays.

Electrochromic materials belong to both inorganic and organic chemical systems. Inorganic systems include transition metal oxides, phthalocyanine compounds, Prussian blue, etc., whereas organic systems are bipyridilium systems, conducting polymers, etc. [1,2,3,4,5,6]. Among transition metal oxides, tungsten oxide (WO₃) is the most promising one for electrochromic device applications, and therefore has been widely studied. Kostis et al. [7] have reported the improved electrochromic behaviour of highly porous WO₃ films deposited by the hot wire method combined with rapid injection of oxygen pulses. Bhosale et al. [8] have studied the electrochromic properties of hydrothermally synthesised WO₃ nanoflowers on etched ITO substrates. Li et al. [9] have investigated the electrochromic performance of amorphous tungsten oxide films deposited by DC sputtering. Karuppasamy and Subrahmanyam [10] have studied electrochromic behaviour of sputter-deposited vanadium doped WO₃ thin films.

Various techniques such as pulsed laser deposition [11], spin coating [12], sol–gel [13], electrodeposition [14], sputtering [15], etc. have been employed to prepare WO₃ thin films. In the present work, chemical spray pyrolysis technique being simple in operation and involving low-cost equipment, has been used to grow WO₃ thin films. It is a well established fact that electrochromic performance of the films is closely linked to the microstructural features of the films, which are greatly influenced by the deposition technique, the precursor and the solvent used. Another effective way to alter the microstructural features of the film is the inlay of appropriate dopants in the host lattice network. Although addition of dopants such as V, Ti, Zn, Nb, Ni, Ru, etc. in WO₃ lattice network has been studied for improving the electrochromic behaviour of the films [10, 16,17,18,19], but exploration of new dopants as well as less-investigated dopants such as Mo, Gd, etc. in WO₃ lattice network to improve electrochromic performance of the films always remains the subject of great interest to the materials scientists, which is behind the motivation and the driving force for this investigation. Yin et al. [15] have studied the influence of Gd doping on electrochromic behaviour of sputter-deposited WO₃ films and found high coloration efficiency and fast switching response for the Gd-doped film. Karuppasamy and Subrahmanyam [10] have reported that the addition of vanadium to WO₃ thin films decreases coloration efficiency of the films. Paipitak et al. [16] have investigated the influence of Ti and Zn doping on the structural and electrochromic performance of sol–gel-derived WO₃ thin films and found that small doping concentration of Ti improves the electrochromic performance of the films. Influence of Nb doping on the electrochromic properties of spray-deposited WO₃ thin films have been studied by Bathe and Patil [17]. They have reported that the coloration efficiency is decreased upon Nb doping, but the charge storage capacity, reversibility and cycle stability improve for the doped films. Green et al. [18] have studied electrochromism in sputter-deposited Ni_xW_1−x oxide films and reported that Coloration efficiency improves for films with 0.10 < x < 0.15. Cazzanelli et al. [19] have examined the influence of Ru doping on the optical and electrochromic properties of sputter-deposited WO₃ thin films, and found that the presence of Ru ions affects these properties of the films. To the best of our knowledge, there are only a very few investigations on the effect of Mo doping on the electrochromic behaviour of tungsten oxide thin films [20, 21]. León et al. [20] have reported the improved electrochromic behaviour of sprayed WO₃ thin films by Mo doping. Madhavi et al. [21] have found that 1.3 at% Mo doping to sputter-deposited WO₃ thin films improves optical modulation and coloration efficiency of the films. In this paper, we have reported the results on the electrochromic performance of spray-pyrolysed WO₃ thin films upon Mo doping. The novelty of this study is that the host and dopant precursors used here are quite different from those used by earlier researchers [20], resulting in modification in the microstructural and electrochromic properties of the films.

2 Experimental details

Undoped and Mo-doped WO₃ thin films were grown onto the glass and fluorine doped tin oxide (FTO)-coated glass substrates, employing the chemical spray pyrolysis technique. Spray pyrolysis involves the pyrolytic decomposition of a precursor solution when sprayed onto the hot substrate, resulting in the formation of thin films. The host precursor solution used here was ammonium tungstate ((NH₄)₂WO₄) dissolved in warm water (60–70 °C). Ammonium tungstate as the host precursor was chosen in the present investigation because first, the decomposition reaction was thermodynamically favourable and no residue of the reactant was left behind in the deposited films; second, the precursor salt was completely dissolved in the warm water without any further chemical treatment; and third, the precursor salt was commercially available and cost-effective. The dopant salt used was molybdenum (V) chloride (MoCl₅). All the chemicals were of high purity obtained from Alfa Aesar (U.K.). Prior to deposition, the glass and FTO-coated glass substrates were cleaned thoroughly. The schematic representation of home-made spray pyrolysis set-up has been described elsewhere [22]. The deposition parameters were optimised to grow good quality thin films by controlling the process parameters after several depositions. It has been found that substrate temperature is the most crucial parameter in spray pyrolysis, which was kept at ~390 ± 10 °C. The concentration of host precursor solution was 0.1 m. The electrode area of as-deposited film in each case was 2.5 cm². The amounts of tungsten oxide deposited on the substrates were determined with the help of an electronic precision balance (Citizen make; Model CX165). The reproducibility of the films was confirmed by analysing the experimental results of several samples under identical conditions. The formation of WO₃ film can be described by the following chemical reaction [23]:

$$\left( {{\mathrm{NH}}_{\mathrm{4}}} \right)_{\mathrm{2}}{\mathrm{WO}}_{\mathrm{4}}{\mathrm{ = WO}}_{\mathrm{3}}{\mathrm{ + 2NH}}_{\mathrm{3}} \uparrow {\mathrm{ + H}}_{\mathrm{2}}{\mathrm{O}} \uparrow$$

(1)

The as-deposited films were subjected to further characterisations of different physical properties. The crystallographic phases of the as-deposited films were determined by X-ray diffraction (XRD) analyses using a Rigaku X-ray diffractometer with CuKα₁ (λ = 1.5406 Å) radiation. The surface morphologies of the films were evaluated using a field-emission scanning electron microscope and an atomic force microscope (AFM). Compositional analyses of the films were carried out by the energy-dispersive X-ray (EDX). The optical transmittance spectra of the films were recorded by employing a Shimadzu UV–Vis–NIR spectrometer (Model: UV 3600 Plus) in the wavelength range of 300–800 nm. The electrochromic behaviour of the films deposited on FTO-coated glass substrates were studied using a three-electrode electrochemical cell configuration with the film as a working electrode, platinum wire as a counter electrode, and Ag/AgCl as a reference electrode. All measurements were taken by means of an Electrochemical Analyser (Model: Autolab PGSTAT101) in the electrolyte of 0.01 m H₂SO₄.

3 Results and discussion

3.1 Structural and morphological studies

3.1.1 XRD analyses

The XRD profiles of the as-deposited films onto FTO-coated glass substrates are shown in Fig. 1. All the films are polycrystalline in nature and possess orthorhombic phase of WO₃ (JCPDS Card No. 71-0131). No phase corresponding to molybdenum or other molybdenum compound is observed in the pattern, indicating that molybdenum gets incorporated into the tungsten oxide lattice. Orthorhombic phase of WO₃ has also been reported by Madhavi et al. [20] in the sputtered tungsten oxide films. Rajagopal et al. [24] have observed orthorhombic phase of WO₃ in hydrothermally synthesised WO₃ nanostructures. León et al. [21] have reported the monoclinic phase of WO₃ in spray-pyrolysed tungsten oxide films, where the spray solution used was tungsten chloride (WCl₆) dissolved in dimethylformamide (HCON(CH₃)₂). It has been established that WO₃ shows the polymorphism behaviour, i.e., it crystallises in a variety of crystal lattice systems. The basic structure of tungsten oxide consists of WO₆ octahedra shared among several arrangements (edges, corners, and planes) and connected with one another by W–O–W bonds. The structural transformation occurs due to the shifts in the position of W atom in the octahedra, and the changes in the W–O bond lengths [2, 25]. In fact, the polymorphism of WO₃ is influenced considerably by the growth technique as well as process parameters including precursors and solvents used in the technique.

Fig. 1

XRD profiles of WO₃ films

It has been observed that the preferential growth of the film along different lattice planes is closely related to the varying Mo-dopant concentration. The low intensity peaks (132) and (420) observed in the XRD pattern of undoped WO₃ disappear with the addition of 1 at% Mo doping, whereas additional peaks in the XRD pattern are observed upon higher Mo doping. The diffraction peaks corresponding to FTO glass substrate are not seen in the XRD profile of 5 at% Mo-doped WO₃ film. It is inferred that nucleation and growth kinetics of the films are dependent upon Mo doping concentration. The d-spacings corresponding to three most intense peaks observed in the XRD profiles of the films, and the average crystallite size estimated from these peaks using the Scherrer Eq. 2 given below [22] are listed in Table 1. The unit cell parameters (a, b, and c) and the lattice strain (ε) have been determined using the Eq. 3 and the tangent formula (4) given below [22, 26], respectively, and are listed in Table 1.

$$D = \frac{{0.9\lambda }}{{\beta \prime \cos \theta }}$$

(2)

$$\frac{1}{{d^2}} = \frac{{h^2}}{{a^2}} + \frac{{k^2}}{{b^2}} + \frac{{l^2}}{{c^2}}$$

(3)

$$\varepsilon = \frac{{\beta \prime }}{{4\tan \theta }}$$

(4)

where λ, β′ and θ represent the X-ray wavelength, the full width at half maximum (FWHM) of the diffraction peak, and the Bragg’s diffraction angle, respectively. Variation observed in the crystallite sizes of different films is attributed to the density of nucleation sites during the film growth, which, in turn, is influenced by the Mo doping concentration. The unit cell parameters listed in Table 1 are found to be in close agreement with those described in JCPDS Card No. 71-0131 (a = 7.341 Ǻ, b = 7.570 Ǻ, and c = 7.754 Ǻ). A little variation observed in the unit cell parameters upon Mo doping is attributed to incorporation of molybdenum into WO₃ lattice network as the ionic radii of W⁶⁺ and Mo⁶⁺ are different.

Table 1 Structural parameters of undoped and Mo-doped o-WO₃ films

3.1.2 SEM analyses

Figure 2a–d presents the SEM micrographs and the corresponding EDX profiles of the WO₃ films. The undoped WO₃ film exhibits island-like morphology comprising of tiny grains. With the addition of molybdenum in WO₃, the morphology of film is changed. Upon 1 at% Mo doping, the film presents interweaved nest-like morphology. With the increase of doping to 2 at%, the morphology changes to root-like architecture. On further enhancement to 5 at% Mo doping, the film presents densely interweaved nest-like morphology. The enhanced interweaved-channels in 5 at% Mo-doped film provide sufficient pathways for easy diffusion of ions into the film network and faster kinetics which improve the electrochromic performance of the films. The EDX profiles confirm the presence of elements W and O in the undoped film, and W, Mo, and O in the doped films.

Fig. 2

a–d SEM images and the corresponding EDX profiles of the WO₃ films

3.1.3 AFM analyses

Two- and three-dimensional AFM images, depicted in Fig. 3a–d, of the WO₃ films have been analysed using the Nanoscope Analysis software. The topographical parameters, viz., image surface area, image projected surface area, root mean square roughness (R_q) and average roughness (R_a) of the films thus obtained are listed in Table 2. As evident from the Table 2, among all the films, the image surface area is maximum for the 5 at% Mo-doped film, implying that the topography of the film is optimal for improved electrochromic performance. Further, the values of R_q and R_a are maximum for the 1 at% Mo-doped film followed by those of 5 at% Mo-doped film. This may arise due to more number of nucleation sites available during the time of film growth in the case of 5 at% Mo-doped film relative to that available in the 1 at % doped film; which is in agreement with the topographies of these films [Fig. 3(b, d)].

Fig. 3

a–d Two-dimensional and three-dimensional AFM images of the WO₃ films

Table 2 Topographical parameters of undoped and Mo-doped o-WO₃ films

3.2 UV–visible spectroscopy studies

Optical transmission spectra of the WO₃ films deposited onto glass substrates are shown in Fig. 4. It has been found that all the films exhibit high transmittance varying between 65% and 75% in the visible range. Because of different film morphologies (surface roughness), there is a variation in transmittance arising out of the scattering of light [27]. Shifting of the absorption edge towards longer wavelength upon Mo doping implies decrease in the band gap energy. The optical band gap energies have been determined using the well-known Tauc’s equation [28]:

$$\alpha h\nu = K\left( {h\nu - E_g} \right)^p$$

(5)

where K is a constant, E_g is the band gap energy of the semiconductor, α is the absorbance coefficient, and the values of exponent p depend upon the nature of band transition (p = ½ for direct transition, and p= 2 for indirect transition).

Fig. 4

Optical transmission spectra of the WO₃ films

Having the reports of both direct and indirect transitions occurred in WO₃ [22, 29], the band gap energy value has been determined for both types of transitions. The direct band gap energy has been determined by extrapolating the linear portion of the (Ahν)² versus hν plots to the energy-axis, shown in Fig. 5. Inset of Fig. 5 presents the plot of (Ahν)² versus hν of 1 at% Mo-doped film in the extended ordinate. The direct band gap energies thus obtained are in the range of 3.50–3.56 eV. The direct band gap energy values of WO₃ films depend upon the crystallinity of the films, and can be found in the range of 3.27–3.65 eV [30]. The indirect band gap energy has been estimated from the linear extrapolations of the plots of (Ahν)^1/2 versus hν to the energy-axis, shown in Fig. 6. Variation in the indirect band gap energies arises owing to the formation of allowed energy states in the band gap region of WO₃. In general, semiconducting materials may have direct or indirect band gap depending upon their crystal structures which are influenced by the preparative conditions. But in some cases, a material may have the both if it has a partial crystalline nature or is under the application of strain. Sharon et al. [31] have also reported both direct and indirect band gap energies for orthorhombic WO₃ film prepared by the vacuum evaporation technique.

Fig. 5

Plot of (Ahν)² versus hν

Fig. 6

Plot of (Ahν)^1/2 versus hν

3.3 Electrochromic studies

The basic mechanism involved in electrochromism is the phenomenon of charge transport in the crystal lattice. WO₃ thin film with tungsten sites as oxidation state W⁺⁶ is colourless in nature. Owing to insertion of charges during electrochemical reaction, W⁺⁶ are reduced to W⁺⁵ states, giving blue coloration to the film. On electrochemical oxidation, W⁺⁶ sites are generated, changing blue coloration in the film to colourless. It has been widely accepted that insertion and extraction of electrons and metal ions (H⁺, Li⁺, etc.) in the crystal lattice play an important role in electrochromism. The electrochemical redox reaction involving coloration and bleaching phenomena in WO₃ thin films can be written as [32]:

$${\mathrm{WO}}_{\mathrm{3}}\left( {{\mathrm{colourless}}} \right){\mathrm{ + }}{\it{x}}{\mathrm{M}}^{\mathrm{ + }}{\mathrm{ + }}{\it{x}}{\mathrm{e}}^{\mathrm{ - }} \leftrightarrow {\mathrm{M}}_{\it{x}}{\mathrm{WO}}_{\mathrm{3}}\left( {{\mathrm{blue}}} \right)$$

(6)

where M⁺ = H⁺, Li⁺, etc. M_xWO₃ is, in general, called as tungsten bronze. The subscript x in the formula M_xWO₃ represents the fractional number of sites, called the insertion coefficient, which are filled in the WO₃ lattice. At low x, intervalence charge transfer between W⁺⁵ and W⁺⁶ adjacent states results in blue coloration to the film. At higher x, the inserted ions expand the lattice of WO₃, whereas the compensating electrons enter the extended states of conduction band and thus alter its electronic structure which, in turn, affects its optical properties [1]. The electrochromic behaviour of the WO₃ films has been investigated by analysing the results of cyclic voltammetry (CV), Chronoamperometry, and impedance spectroscopy measurements.

3.3.1 Cyclic voltammetry

To investigate the cathodic/anodic behaviour of the WO₃ films, CV measurements were performed in the potential region of + 0.7 V to − 0.7 V at four different scan rates viz. 10, 20, 50, and 100 mV/s in the electrolyte of 0.1 m H₂SO₄. The cyclic voltammograms of the films thus measured are depicted in Fig. 7. Usually, when the WO₃ film electrodes are cathodically polarised, they have a progressive increase in the blue coloration with increasing cathodic potential. When the blue film electrodes are anodically polarised, they are bleached and almost colourless. During cathodic scan, as the potential is decreased, the current density goes on decreasing and attains a maximum negative value at the potential of − 0.7 V, corresponding to deep blue coloration to the film. This process is associated with the insertion of electrons and H⁺ ions into the WO₃ films, resulting in the reduction of W⁺⁶ to W⁺⁵ states. During anodic scan, with the increase of potential, the current density goes from negative value to a maximum positive value and then decreases, corresponding to bleaching of the films. This is associated with the extraction of intercalated charges from the film structure, leading to the oxidation of W⁺⁵ → W⁺⁶. As evident from the cyclic voltammograms, all the films show a well-defined broad anodic peak during anodic scan, whereas a spike instead of a peak is observed at the potential of − 0.7 V during cathodic scan. Non-appearance of a well-defined peak in cathodic region is attributed to the back electromotive force formed within the tungsten bronze during ion insertion [33]. As shown in Fig. 7, the cathodic and anodic peak current densities are found to have the highest values for the WO₃ film doped with Mo concentration of 5 at%.

Fig. 7

Cyclic voltammograms of the WO₃ films. Arrow indicates the scan direction

It has been found that for each film the anodic peak current density increases with the increasing scan rate and also shifts towards the higher potential. Shifting of anode peak position towards higher potential indicates that the change from coloured to bleached state is slower at higher scan rates [21]. This may be attributed to slower electron exchange kinetics at higher scan rates. The cathodic peak (spike) current density also rises with the increase of scan rate. The increase in cathodic and anodic peak current densities at higher scan rates is due to enhanced extent of H⁺ ions insertion and extraction in the WO₃ film structure [34, 35]. Furthermore, the increased area of cyclovoltammetric curve with the increasing scan rate indicates larger amount of charge exchange during redox reaction at higher scan rates.

Figure 8 presents the cyclic voltammograms of all the films recorded at a particular scan rate of 10 mV/s. Except for the case of 2 at% Mo doping, the cathodic and anodic peak current density increases for the WO₃ films doped with 1 at and 5 at% Mo. This indicates high capability of charge insertion and extraction for the film structure [36]. Also the anodic peak potential shifts towards the lower potential relative to that observed in the undoped WO₃ film. This indicates that the change from coloured to bleached state is faster for the WO₃ films doped with Mo. This is closely related to the microstructural features of the films. Among all the films, the area of cyclovoltammetric curve of the 5 at% Mo-doped WO₃ film has been found to be maximum [Fig. 8], signifying the greatest amount of charge exchange taking place in the film during electrochemical redox reaction. This reveals that insertion and extraction of H⁺ ions are enhanced in WO₃ film doped with 5 at% Mo.

Fig. 8

Comparison of cyclic voltammograms of the WO₃ films at the scan rate of 10 mV/s. Arrow indicates the scan direction

The extent of insertion and extraction of H⁺ ions into the WO₃ host lattice can be studied by calculating the diffusion coefficient using Randles–Sevcik equation [37, 38]:

$$J_p = 2.72 \times 10^5C\sqrt {n^3D\nu }$$

(7)

where J_p is the peak current density in A/cm², n is the number of electrons involved in the redox process (it is assumed to be 1), D is the diffusion coefficient in cm²/s, C is the concentration of active ions in the electrolyte solution in mol/cm³, and ν is the scan rate in V/s. The diffusion coefficients thus obtained are listed in Table 3. It has been found that there is a variation in the diffusion constant values upon Mo doping, which could be due to variation in the mobility of the diffusing ionic species (H⁺) in the redox reaction. The mobility of the H⁺ ions in the film is influenced greatly by the microstructural features of the film.

Table 3 Electrochromic properties of undoped and Mo-doped o-WO₃ thin films

To calculate the coloration efficiency of the WO₃ films, the optical transmittance spectra of the coloured (with the application of cathodic potential of − 0.7 V) and bleached (with the potential of + 0.7 V) states of the films were recorded, and are shown in Fig. 9. The change in optical density (∆ OD) has been determined from the optical transmittances of coloured (T_coloured) and bleached (T_bleached) films using the relation:

$$\Delta \,{\mathrm{OD}}\left( \lambda \right) = {\mathrm{ln}}\left( {\frac{{{\mathrm{T}}_{{\mathrm{bleached}}}}}{{{\mathrm{T}}_{{\mathrm{coloured}}}}}} \right)$$

(8)

Fig. 9

Optical transmittance spectra of the coloured and bleached states of the WO₃ films. Solid and dashed lines represent the coloured and bleached states, respectively

Coloration efficiency (η) defined as the ratio of the change in optical density to the amount of charge intercalated, has been determined by the relation [39]:

$$\eta \left( \lambda \right) = \frac{{\Delta OD\left( \lambda \right)}}{{Q{\mathrm{/}}A}}$$

(9)

where A is the electrode area of the film and Q is the amount of charge intercalated which is derived from the CV measurements [9, 38]. Figure 10 presents the plot of change in optical density, ΔOD, versus wavelength, λ, of the WO₃ films, whereas plot of the amount of charge intercalated per unit area, Q/A, versus Mo-dopant concentration is depicted in Fig. 11. It has been found that the coloration efficiency of the WO₃ film improves upon Mo doping, having the highest value 43.3 for the doping of 5 at% Mo. This evinces that the structure of 5 at% Mo-doped WO₃ film provides an easy way to diffusion and charge transfer process of ions.

Fig. 10

Plot of change in optical density, ΔOD, versus wavelength, λ, of the WO₃ films

Fig. 11

Plot of the amount of charge intercalated per unit area, Q/A, versus Mo dopant concentration

The electrochemical stability of the WO₃ films has been examined by repeating reduction–oxidation cycles between the sweep potential range from + 0.7 V to − 0.7 V several times at the scan rate of 50 mV/s [Fig. 12]. In all the films, the nature of the cyclovoltammetric curves remains almost unchanged even after 100 reduction–oxidation cycles, thus establishing good electrochemical stability of the films.

Fig. 12

Electrochemical stability of the WO₃ films

3.3.2 Chronoamperometry

Chronoamperometry measurements were carried out for 10 cycles with the potential being stepped between − 0.7 V and + 0.7 V vs. Ag/AgCl for 25 s, and are shown in Fig. 13. Here, we only present the first three cycles to avoid clustering of data in the figure. In the time interval 0–25 s, the film was in coloured state while in the next time interval 25–50 s the film was in bleached state. As evident from the Fig. 11, in 5 at% Mo-doped film, on applying the potential of − 0.7 V, the current density remains constant for a considerable time and then begins to decrease. This leads to infer that in the beginning for a considerable time, there is no potential barrier formed at the WO₃-electrolyte interface; as a result the current density arising due to insertion of H⁺ ions into the film remains almost constant. Later on, a progressive potential barrier at the WO₃-electrolyte interface is formed, resulting in a decrease in current. Further, during oxidation cycle, on applying the potential of + 0.7 V, the current density remains constant for some time and thereafter begins to fall. Therefore, it is inferred that during bleaching, in the beginning for some time, the extraction of H⁺ ions from the bulk of the film is not governed by the space-charge. The WO₃ film doped with 5 at% Mo exhibits fastest switching response. In general, the coloration process is slower than the bleaching in WO₃ films owing to faster bleaching kinetics [17].

Fig. 13

Chronoamperometry curves of the WO₃ films

The reversibility of the films, defined as the ratio of the charge extracted to the charge intercalated in the film, is listed in Table 3. The values of charge intercalated and charge extracted are obtained from the integration of the cathodic and anodic current densities. It has been found that the reversibility is optimal for the film with 5 at% Mo doping. Considering the reversibility and the switching response from chronoamperometry and the coloration efficiency from cyclic voltammetry it has been concluded that the 5 at% Mo-doped WO₃ film has an optimal electrochromic response.

3.3.3 Electrical impedance spectroscopy

Figure 14 presents the Nyquist plots in the frequency range of 10 kHz–1 Hz, comprising of the real versus the imaginary component of the electrical impedance of the WO₃ films. The Bode plots comprising of the logarithm of the measured impedance (Z) and the phase angle, versus the logarithm of the frequency are depicted in Fig. 15. The Bode plots provide a good comparison of total impedance values for WO₃ films. The Nyquist impedance plots have been analysed using Nova software, taking into consideration the equivalent circuit shown in the inset of Fig. 12, and the fitted parameters are listed in Table 4. In the equivalent circuit, R_s corresponds to the electrolyte resistance; R_film and C_film are the resistance and capacitance of the WO₃ film, respectively; R_ct is the charge transfer resistance at the electrolyte/electrode interface, signifying how beneficial is the film morphology for proton transfer; W is the Warburg diffusion impedance (ionic diffusion in the film electrode), and C_dl is the capacitance of WO₃/electrolyte interface [40]. Mathematically, Warburg impedance is expressed as: W = 1/[Y_o (jω)^0.5], where Y_o is the admittance (1/ |Z|) at ω = 1 rad/s. Among all the circuit elements, the charge transfer resistance is the most important element of the circuit, closely linked to the electrochromic behaviour of the films. The charge transfer resistance is the resistance offered to the process of electrons from the film electrode to the H⁺ ions in the liquid phase, and therefore it should have low value for films showing good electrochromic properties [41, 42]. Goodness of fit, χ², in impedance spectroscopy is also included in Table 4. The lower values of chi-square support the best fit of Nyquist plots. As evident from Table 4, the charge transfer resistance is least for the 5 at% Mo-doped WO₃ film. That is why during chronoamperometry measurements [Fig. 13], on applying the cathodic potential, the current density remains constant for a considerable time in the case of 5 at% Mo-doped film.

Table 4 Equivalent circuit fitting parameters of the Nyquist impedance plots of the o-WO₃ films

A comparison of this work with those for other dopants in WO₃-based films/devices reported earlier in literature is presented in Table 5.

Table 5 Electrochromic response of different dopants in WO₃-based films/devices

4 Conclusions

All the as-prepared films possess orthorhombic phase of WO₃. With varying Mo-doping concentrations, the morphologies of films change. Three-dimensional AFM image analyses show that the 5 at% Mo-doped film has the maximum image surface area. The optical direct band gap energies of WO₃ films have been found to be in the range of 3.50–3.56 eV. Among all the films, WO₃ doped with 5 at% Mo exhibits fastest switching response and highest colouration efficiency. It has been concluded that the spray-pyrolysed 5 at% Mo-doped WO₃ films have an optimal electrochromic performance, and therefore have the potential to be used as an effective electrochromic material for smart window applications.

Fig. 14

Nyquist impedance plots of the WO₃ films. Solid and dashed lines represent the experimental and fitted data, respectively. Inset of Fig. 12 represents the equivalent circuit fitting

Fig. 15

Bode plots of the WO₃ films. Solid and dashed lines represent the experimental and fitted data, respectively