1 Introduction

Thin films of niobium pentoxide (Nb2O5) are promising candidates for electrochromic device applications such as smart windows, electronic displays and electro-optic devices [1,2,3]. Among the different stoichiometries of niobium oxide, namely NbO, Nb2O3, NbO2 and Nb2O5, niobium pentoxide is thermodynamically stable [4, 5] and has been the focus of much attention to researchers owing to its unique electrochromic properties. It has been found that niobium pentoxide films reversibly and persistently change their colours on applying an external electric potential to them [6, 7]. Colour change arises due to insertion and removal of ions into and out the material during electrochemical redox reactions. In the case of crystalline Nb2O5 films, the colour changes from transparent to blue under the influence of applied electric potential, whereas brown colouring effect is observed in the amorphous Nb2O5 films [8].

The electrochromic properties of Nb2O5 thin films are strongly dependent upon the microstructural features of the films, which can be modified by the addition of appropriate dopants in the host lattice network. Schmitt and Aegerter [9] have studied Zr, Sn, Li, Ti and Mo- doped Nb2O5 films prepared by the dip-coating technique. Heusing et al. [10] have examined the influence of Mo and Li doping independently in the electrochromic performance of Nb2O5 films and found that the film undergoes a change in colour from transparent to brown, grey or blue depending on the crystallinity of the film. Pehlivan et al. [6] have investigated tantalum pentoxide-doped Nb2O5 films prepared by sol–gel spin-coating method. Microstructural and electrochromic characteristics of WO3-doped Nb2O5 thin films have been studied by Pehlivan et al. [11].

The microstructural characteristics of the films are influenced greatly by the film preparation technique as well as preparation conditions including precursors, temperature, etc. Various deposition techniques have been used to prepare Nb2O5 films, which include sol–gel spin-coating [6], dip-coating [9,10,11], RF sputtering [12], spray pyrolysis [13], chemical vapour deposition [14], hydrothermal process [15], etc. Pehlivan et al. [6] have reported amorphous nature of as-deposited 15% Ta2O5-doped Nb2O5 films employing sol–gel spin-coating technique using niobium (V) ethoxide and tantalum (V) ethoxide as precursors. Schmitt and Aegerter [9] have reported four different structures for the sol–gel derived Nb2O5:Ti and Nb2O5:Mo films, which depend upon the sintering-temperature as well as the doping level. Al-Baradi et al. [12] have found an orthorhombic phase for the rf sputtered Nb2O5 films. Mujawar et al. [13] have reported a monoclinic phase of Nb2O5 for the spray-deposited films. Amorphous nature of the Nb2O5 films prepared by the chemical vapour deposition at 350 °C has been reported by Maruyama and Kanagawa [14]. Wen et al. [15] have reported an orthorhombic structure for the hydrothermally synthesized Nb2O5 nanorod array films.

Our intent in the present investigation is to study the effect of W doping on the microstructural, optical and electrochromic properties of Nb2O5 films prepared by the dip-coating method using sols obtained by the chloroalkoxide route, i.e. by dissolving niobium pentachloride in ethanol and acetic acid. To the best of our knowledge, such study on the influence of W doping in the Nb2O5 films has not yet been reported in literatures.

2 Experimental details

All the chemicals used here were of high purity. Firstly, niobium chloroethoxide solution of 0.2 M concentration was prepared by dissolving required amount of NbCl5 in ethanol. Next, required amount of acetic acid (Ac) was added to the solution to get molar ratio Nb:Ac = 1:2. The mixture was then ultrasonicated at room temperature for 30 min. Ultrasonification process accelerates the chemical reaction of NbCl5 with ethanol and acetic acid. The sol of Nb2O5 was thus prepared. For W doping, required amount of (NH4)2WO4 was first dissolved in a little amount of warm water, and then after cooling it to room temperature, it was added to the above-prepared sol. The thin films of undoped and W-doped Nb2O5 were deposited onto the glass and fluorine doped tin oxide (FTO) coated glass substrates by the dip-coating process. Prior to deposition, the glass substrates were thoroughly cleaned, first by labolene detergent and then rinsed with deionized water, and again ultrasonicated in trichloroethylene for 15 min and further rinsed with deionized water several times. Finally, the substrates were dried in air at room temperature. After dipping the cleaned substrates into sols for a while, the substrates were gently taken out the sols. The films thus deposited on the substrates were first dried in air at room temperature followed by thermal treatment at 550 °C for 40 min in air. The as-prepared film in each case has the electrode area of 1.5 cm2.

The post-heat treated films were subjected to further characterizations of microstructural, optical and electrochromical properties. The crystallographic phases of the films were characterized by X-ray diffraction (XRD) using a Rigaku X-ray diffractometer with CuKα1 (λ = 1.5406 Å) radiation within the range of 10–80° (2θ) in glancing angle mode. The surface morphologies of the films were examined using a field-emission scanning electron microscope [Model: Nova NanoSEM 450] and a scanning probe microscope [Model: NT-MDT NTEGRA Prima]. Elemental analyses of the films were conducted with an EDS (Energy dispersive X-ray spectroscopy) detector attached to the scanning electron microscope. The UV–visible transmittance spectra of the films were obtained from a Shimadzu UV–Vis–NIR spectrometer [Model: UV 3600 Plus] in the wavelength range 300–800 nm at normal incidence. The electrochromic properties of the films were analysed using an Electrochemical Analyser (Model: Autolab PGSTAT101) controlled by Nova software and a conventional three-electrode electrochemical cell containing a 0.1 M solution of LiClO4 dissolved in propylene carbonate as the electrolyte, Nb2O5 film deposited on the FTO glass substrate as the working electrode, a platinum wire as the counter electrode, and Ag/AgCl as the reference-electrode.

3 Results and discussion

3.1 Film formation

Dip-coating process used in the present investigation is a facile and economical technique to deposit thin films on a substrate. The technique involves immersing a substrate into the precursor solution at a constant speed followed by holding it for some time and finally withdrawing the substrate from the solution at a constant speed. The microstructural features of the films are influenced greatly by various process parameters such as immersion time, withdrawal speed, precursor solution concentration, dip-coating cycles, etc. [16, 17]. To prepare Nb2O5 thin films, sols were obtained by the chloroalkoxide route. First, on dissolving NbCl5 in ethanol, niobium chloroethoxide is formed by partial alcoholysis [17, 18]:

$${\text{NbCl}}_{ 5} + {\text{ xC}}_{ 2} {\text{H}}_{ 5} {\text{OH }} \to {\text{ NbCl}}_{{( 5- {\text{x}})}} \left( {{\text{OC}}_{ 2} {\text{H}}_{ 5} } \right)_{\text{x}} + {\text{ xHCl}}$$
(1)

When acetic acid is added to the solution, it reacts with the excess ethanol forming ethyl acetate and water:

$${\text{CH}}_{ 3} {\text{COOH }} + {\text{ C}}_{ 2} {\text{H}}_{ 5} {\text{OH }} \to {\text{ CH}}_{ 3} {\text{COOC}}_{ 2} {\text{H}}_{ 5} + {\text{ H}}_{ 2} {\text{O}}$$
(2)

Niobium chloroethoxide already formed in the solution then undergoes both the hydrolysis and condensation reactions, resulting in the formation of Nb2O5 sol [19]:

$${\text{NbCl}}_{{( 5- {\text{x}})}} \left( {{\text{OC}}_{ 2} {\text{H}}_{ 5} } \right)_{\text{x}} + {\text{ 5H}}_{ 2} {\text{O }} \to {\text{ xC}}_{ 2} {\text{H}}_{ 5} {\text{OH }} + \, \left( { 5- {\text{x}}} \right){\text{HCl }} + {\text{ Nb}}\left( {\text{OH}} \right)_{ 5}$$
(3)
$$2 {\text{Nb}}\left( {\text{OH}} \right)_{ 5} \to {\text{ Nb}}_{ 2} {\text{O}}_{ 5} + {\text{ 5H}}_{ 2} {\text{O}}$$
(4)

3.2 Microstructural studies

X-ray diffraction profiles of the undoped and W-doped Nb2O5 thin films deposited on FTO coated glass substrates are depicted in Fig. 1. Analyses of the XRD results using X’Pert HighScore Plus software reveal that the diffraction profiles are in good agreement with the JCPDS card no. 30-0873 and therefore all the films are polycrystalline having an orthorhombic phase of niobium pentoxide. In addition to XRD peaks of FTO glass, prominent peaks corresponding to (0 0 1), (1 8 0), (1 8 1), and (1 14 1) planes of Nb2O5 are observed in the patterns. In 5 at.% W-doped film, the most intense peak is (1 8 0) followed by the peak (0 0 1), whereas in all other films, the peak (0 0 1) is the most prominent one followed by the peak (1 8 0). Figure 2 presents a change in the preferential orientation I(0 0 1)/I(1 8 0) of the lattice planes with increase in W dopant concentration. The orthorhombic phase of Nb2O5 has been reported by other researchers also [6, 8, 20]. Pehlivan et al. [6] have obtained the orthorhombic phase of sol–gel spin-coated Nb2O5 thin films when the films were calcined at 550 °C in air for 10 h. Pawlicka et al. [8] have reported the orthorhombic phase of Nb2O5 films prepared by the sol–gel method on calcination of the films at around 600 °C. Raba et al. [20] have reported the formation of orthorhombic Nb2O5 phase upon heat-treatment of the sol–gel synthesized niobium pentaoxide powders at 750 °C in air.

Fig. 1
figure 1

XRD profiles of the W-doped Nb2O5 thin films. Vertical lines correspond to the peaks of FTO glass

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Fig. 2
figure 2

Intensity ratio I(0 0 1)/I(1 8 0) as a function of W dopant concentration

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The unit cell parameters have been determined from the XRD peaks and are listed in Table 1. It has been found that the unit cell parameters for the undoped Nb2O5 thin films are very close to the values listed in JCPDS card no. 30-0873 (a = 6.175 Ǻ, b = 29.17 Ǻ, c = 3.930 Ǻ, and angles α = β = γ = 90°). Being the different ionic radii of W6+ and Nb5+ the incorporation of tungsten ions into Nb2O5 lattice system results in a little change in the lattice parameters of the unit cell in the W-doped films. The crystallite size and the lattice strain of the films have been estimated by the Scherrer equation and the tangent formula, respectively.

Table 1 Structural parameters of undoped and W-doped Nb2O5 films
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The two-dimensional (2D) high magnification surface morphological study of the Nb2O5 films has been carried out using SEM micrographs, shown in Fig. 3a–d. The SEM micrograph of the undoped film reveals that the film has the porous and interlinked nano-fibrous morphology. Agglomeration of nano-fibres at a few places is also observed. Upon 1 at.% W doping, the nano-fibrous morphology changes to a morphology comprising of irregular shaped large grains with agglomeration of grains at a few places. On increasing W doping to 3 at.%, the morphology appears to be denser with agglomeration of grains at relatively more places. Finally, with 5 at.% W doping, the morphology changes again to the short nano-fibrous morphology including the irregular shaped large grains at a few places. The EDS spectra of elemental analyses of the films are shown in Fig. 3e–h which confirms the presence of Nb and O in the undoped film, and W, Nb and O in the W-doped films.

Fig. 3
figure 3

a–h FESEM micrographs and the corresponding EDS profiles of the Nb2O5 films

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The AFM images of the Nb2O5 films were recorded in non-contact mode, and the two-dimensional (2D) and three-dimensional (3D) topographic images of scan area (3 µm × 3 µm) are shown in Fig. 4a–h. The surface topographical parameters of the films have been determined using Nova AFM software and are listed in Table 2. Compared with the undoped film, the image surface area has been found to enhance in the 1 and 3 at.% W-doped films, but it decreases in the 5 at.% W-doped film. It is obvious from Table 2 that the 3 at.% W-doped film has the maximum root mean square (rms) roughness. Usually, higher is the rms roughness of the film, larger would be the image surface area of the film and consequently the film would be more conducive to exhibit improved electrochromic performance.

Fig. 4
figure 4figure 4

a–h Two-dimensional (2D) and three-dimensional (3D) AFM images of the Nb2O5 films

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Table 2 Topographical parameters of undoped and W-doped Nb2O5 films
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3.3 Optical studies

Figure 5 presents the transmission spectra of the undoped and W-doped Nb2O5 thin films deposited on plane glass substrates. Undoped Nb2O5 film exhibits optical transmittance varying between 60% and 65% in the visible range. Upon W doping, the transmittance deceases first for the 1 at.% doping and then increases again for the 3 at.% and 5 at.% doped films relative to the undoped film. Variation in transmittance is attributed to the scattering of light owing to different film morphologies (surface roughness) [21, 22].

Fig. 5
figure 5

Transmission spectra of the Nb2O5 films

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Both direct and indirect energy band gap transitions in Nb2O5 have been reported in literatures [6, 7, 13, 23]. The energy band gaps for direct and indirect transitions can be obtained employing the Tauc method [22] by extrapolating the linear portion of a plot of (Ahν)2 versus hν for direct transition, or (Ahν)1/2 versus hν for indirect transition, to the abscissa, where A is the optical absorbance, and hν is the photon energy. Figure 6 presents the required plot for direct transition in Nb2O5 films. The energy bandgap values thus obtained are found to be ~ 3.60 eV. Upon 1 at.% W doping, a little change in the energy bandgap value is observed, which may be attributed to the density of localized energy states in the bandgap. It has been reported that the energy bandgap values vary in the range of 3.2–3.9 eV for Nb2O5 films prepared by different techniques [24] (and references therein).

Fig. 6
figure 6

Plot of (Ahν)2 versus hν

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3.4 Electrochromic studies

3.4.1 Cyclic voltammetry

Cyclic voltammetry (CV) measurements were performed by applying dc potential sweep in the range of ± 1.5 V (vs. Ag/AgCl reference electrode) at four different potential scan rates 10, 20, 50 and 100 mV/s in the 0.1 M LiClO4/propylene carbonate electrolyte. The cyclic voltammograms of the films comprising of a single oxidation–reduction cycle are depicted in Fig. 7a–d. It has been found that during cathodic scan the Nb2O5 film electrodes undergo a progressive increase in the blue coloration under the application of increasing negative potential, whereas during anodic scan the blue film electrodes become bleached and colourless when they are subjected to positive potential. During cathodic scan, on the application of increasing negative potential, the negative current density begins to increase, attaining a highest value when the film gets maximum blue coloration. Blue colouration to the film arises due to insertion of electrons and Li+ ions into the Nb2O5 films, which causes reduction of Nb5+ ions to lower valence state Nb4+ ions. For anodic scan, when the potential changes from − 1.5 to + 1.5 V, the current density begins to change from negative to positive value, attaining a highest value and then decreases during which the film is bleached. This phenomenon occurs due to intercalated charge extraction from the film structure; consequently, Nb4+ ions get oxidised to Nb5+ ions, resulting in the bleaching of film. Coloration and bleaching phenomena in Nb2O5 films can be represented by the following electrochemical redox reaction [13]:

Fig. 7
figure 7

a–d Cyclic voltammograms of the Nb2O5 thin films at different scan rates. Arrow indicates the scan direction

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$${\text{Nb}}_{ 2} {\text{O}}_{ 5} \left( {\text{colourless}} \right) + x{\text{Li}}^{+} + x{\text{e}}^{-} \leftrightarrow {\text{ Li}}_{{\text{x}}} {{\text{Nb}}}_{ 2} {\text{O}}_{ 5} \left( {\text{blue}} \right)$$
(5)

LixNb2O5 is, in general, called as the lithium-niobium bronze. The cathodic and anodic peak current densities and the corresponding cathodic and anodic potentials at different scan rates for all the films are listed in Table 3. Charges intercalated and deintercalated during cathodic scan and anodic scan, respectively have been obtained from the integration of respective parts of cyclic voltammograms, and are listed in Table 3. As evident from Table 3, the net charge density for the films improves upon W doping. In the case of 10 mV/s scan rate, the net charge density for the undoped Nb2O5 thin film has been found to be 8.4 C/g, whereas for the same scan rate, the net charge densities have been obtained to be 27.7 C/g, 32.9 C/g, and 19.7 C/g for the 1, 3 and 5 at.% W-doped films, respectively. Further, in each film, the net charge density has been observed to decrease with the increasing scan rate. This may be attributed to the fact that at higher scan rates the Li+ ions do not get enough time to intercalate/deintercalate into and out the film.

Table 3 Analyses of cyclic voltammetry measurements of the undoped and W-doped Nb2O5 thin films
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All the cyclic voltammograms exhibit a spike at the potential of − 1.5 V during cathodic scan and a peak during anodic scan. Non-appearance of a well-defined peak in cathodic region is a consequence of the formation of back emf within the niobium bronze during ion insertion. Figure 8a–d shows the linear variation of cathodic/anodic peak current densities with the square root of the potential scan rates, which confirms a diffusion controlled mechanism in the redox reaction process.

Fig. 8
figure 8

a–d Plots of peak current densities versus the square root of potential scan rates

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In each film, the cathodic and anodic peak current densities have been found to enhance with the increase in scan rates. This is attributed to the fact that on increasing the potential scan rate, the diffusion layer at the film electrode surface gets shortened, and consequently an increase in the current density is observed. It is also noticed that the anodic peak current position shifts with the varying scan rate. Changing of anodic peak position towards higher potential implies that the change from coloured to bleached state is slower [25]. This may be attributed to decrease in the rate of bleaching kinetics and residual coloration. Further, relative to the undoped Nb2O5 film, the anodic peak current position shifts towards the lower potential in the W-doped films. This implies that the rate of bleaching kinetics is faster in the W-doped Nb2O5 films, which is closely linked to the microstructural characteristics of the films.

The diffusion coefficient of Li+ ions into the Nb2O5 host lattice has been determined using the Randles–Sevcik equation [26]:

$$D^{1/2} = \frac{{J_{p} }}{{2.72 \times 10^{5} n^{{{\raise0.7ex\hbox{$3$} \!\mathord{\left/ {\vphantom {3 2}}\right.\kern-0pt} \!\lower0.7ex\hbox{$2$}}}} C\nu^{1/2} }}$$
(6)

where D is the diffusion coefficient (cm2/s), Jp is the peak current density (A/cm2), n is the number of electrons involved in the redox process (it is assumed to be 1), C is the concentration of active ions in the electrolyte solution (mol/cm3), and ν is the scan rate (V/s). A change in the diffusion coefficient value upon W doping is attributed to varying mobility of the diffusing ionic species (Li+) in the redox reaction. In fact, microstructural characteristics of the film affect mobility of the diffusing ionic species to a great extent.

Colouration efficiency (η) is a wavelength-dependent electrochromic parameter correlating the optical contrast with the charges intercalated per unit electrode area, defined by the relation [27]:

$$\eta (\lambda ) = \frac{\Delta OD}{Q/A} = \frac{{\ln \left( {T_{bleached} /T_{coloured} } \right)}}{Q/A}$$
(7)

where ΔOD is the change in optical density, Tcoloured and Tbleached are the transmittances of the film in coloured and bleached states, respectively at a certain wavelength, Q is the intercalated charge, and A is the film electrode area. Figure 9 presents the transmittance spectra of the films in coloured and bleached states. It is evident from Fig. 9 that the 3 at.% W-doped film has the maximum optical density. The colouration efficiency of the Nb2O5 film has been found to improve upon W doping with the optimum value 68.7 cm2/C at 600 nm for the 3 at.% W-doped Nb2O5 film.

Fig. 9
figure 9

Transmittance spectra of the Nb2O5 films in coloured and bleached states

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To examine the electrochemical stability of the Nb2O5 films, reduction–oxidation cycles were repeated several times by applying a dc potential sweep in the range of ± 1.5 V (vs. Ag/AgCl reference electrode) (Fig. 10a–d). It is clear from Fig. 10a–d that in each case the cyclovoltammetric curve remains almost unchanged even after 100 reduction–oxidation cycles, which establishes excellent electrochemical stability of the films.

Fig. 10
figure 10

a–d Electrochemical stability of the Nb2O5 films

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3.4.2 Chronoamperometry

Figure 11a–d shows chronoamperometric plots of the Nb2O5 films, which were recorded for 30 s by applying a dc potential stepped between –1.5 V and +1.5 V (vs. Ag/AgCl reference electrode) for 5 cycles. During the time interval 0–30 s, the film was in colored state on stepping the voltage from its initial potential of 0.0 to − 1.5 V for 30 s, while during the next time interval 30–60 s the film was in bleached state on reversing the voltage to +1.5 V. As evident from Fig. 11a–d, both coloration current density and bleaching current density following the potential step are smooth and decrease continuously with time. It is obvious from Table 4 that the reversibility of the film decreases upon W doping. The colouration and bleaching times have been estimated from the current versus time transients. It has been found that the colouration and bleaching times of the film upon W doping become short relative to the undoped film, and therefore the switching response of the film improves upon W doping. In general, the coloration process is slower than the bleaching in both undoped and W-doped Nb2O5 films due to faster bleaching kinetics [28].

Fig. 11
figure 11

a–d Chronoamperometry curves of the undoped and W-doped Nb2O5 thin films

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Table 4 Analyses of chronoamperometry measurements of the undoped and W-doped Nb2O5 thin films
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3.4.3 Electrical impedance spectroscopy

Figure 12 presents the Bode plots representing the impedance (Z) and the phase angle of the Nb2O5 films as a function of frequency. It shows the frequency domain behavior of the system. As evident from Fig. 12, the phase angle at a certain frequency remains almost the same for all the films, but relative to the undoped film, the impedance shows a little variation upon W doping. For all the films, the impedance has been found to be maximum at the lowest frequency and decreases exponentially with the increase of frequency of ac signal. This is attributed to the fact that the interface energy states near the grain boundaries in the film response effectively at lower frequencies of ac signal owing to their large relaxation times.

Fig. 12
figure 12

Bode magnitude/phase angle plots of the Nb2O5 films

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Plots of the real versus imaginary components of the impedance of the Nb2O5 films measured in the frequency range of 100 kHz–24 Hz are depicted in Fig. 13, and are referred to as the Nyquist impedance plots. These plots have been characterized using Nova software by means of fit and simulation analysis. The equivalent electrical circuit, shown in Fig. 14, has been used for fitting the EIS data, and the circuit fitting parameters are listed in Table 5. The lower values of goodness of fit, χ2, for the fitting curves indicate that the proposed EEC model is in good agreement with the experimental data. In the EEC, Rs is the resistance of LiClO4/propylene carbonate electrolyte; Rfilm and Cfilm are the Nb2O5 film resistance and capacitance, respectively; Rct is the charge transfer resistance (interfacial reaction resistance); W is the Warburg diffusion impedance (ionic diffusion in the film electrode), and Cdl is the double-layer capacitance of the Nb2O5/electrolyte interface [29]. Mathematically, Warburg impedance is described as: W = 1/[Y0 (jω)0.5], where Y0 is the admittance (1/|Z|) at ω = 1 rad/s. It is inferred from Table 5 that the circuit elements are strongly dependent upon the microstructural features of the films. The varying charge transfer resistances are attributed to the fact that effective surface areas of the films in contact with the electrolyte and the redox reaction rates at different locations over the film surface are different.

Fig. 13
figure 13

Nyquist impedance plots of the Nb2O5 films

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Fig. 14
figure 14

Equivalent electrical circuit used for fitting the EIS experimental data

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Table 5 Electrical parameters resulting from the fitting of the experimental data to the EEC
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A comparison of electrochromic performance of W-doped Nb2O5 films (this work) with those of other Nb2O5-based films reported earlier in literatures is presented in Table 6.

Table 6 Electrochromic performance of Nb2O5-based thin films prepared by various methods
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4 Conclusions

  1. (a)

    Nb2O5 thin films (undoped and W-doped) prepared by the dip-coating process using sols obtained by the chloroalkoxide route are polycrystalline possessing an orthorhombic phase of niobium pentaoxide.

  2. (b)

    Undoped and the film doped with 5 at.% W have the nano-fibrous morphology, whereas 1 at.% and 3 at.% W-doped films have a morphology comprising of irregular shaped large grains.

  3. (c)

    Roughness factor defined as the ratio of the image surface area to the image projected surface area is in the range 1.32–4.31 for these films.

  4. (d)

    The 3 at.% W-doped film exhibits the best coloration efficiency 68.7 cm2/C at 600 nm.

  5. (e)

    All the films show excellent electrochemical stability, and the switching response of the film improves upon W doping.