Hydrothermal synthesis of WO3 nanoflowers on etched ITO and their electrochromic properties

Accepted Manuscript Title: Hydrothermal synthesis of WO3 nanoflowers on etched ITO and their electrochromic properties Authors: Nilam Y. Bhosale, Sawanta S. Mali, Chang K. Hong, Anamika V. Kadam PII: DOI: Reference:

S0013-4686(17)31380-4 http://dx.doi.org/doi:10.1016/j.electacta.2017.06.142 EA 29780

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

12-2-2017 21-6-2017 22-6-2017

Please cite this article as: Nilam Y.Bhosale, Sawanta S.Mali, Chang K.Hong, Anamika V.Kadam, Hydrothermal synthesis of WO3 nanoflowers on etched ITO and their electrochromic properties, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.06.142 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Hydrothermal synthesis of WO3 nanoflowers on etched ITO and their electrochromic properties

Nilam Y. Bhosale,1 Sawanta S. Mali2, Chang K. Hong2 and Anamika V. Kadam1 1

D.Y. Patil College of Engineering and Technology, Kasaba Bawada, Kolhapur-416006, Maharashtra, India.

2

Polymer Energy Materials Laboratory, Department of Advanced Chemical Engineering, Chonnam National University, Gwangju, 500-757, South Korea

Graphical Abstract:

Hydrothermal synthesis of HCl etched WO3 thin films

Highlights: 

An electrochromic WO3 film was assembled using HCl wet-etching process.



The hydrothermal method employed to deposit WO3 film without a seed layer.



Etching reduces the thickness of WO3 film with enhanced porosity.



Open tunnel structure of etched WO3 nanoflowers provides faster response time.



Etching improves electrochromic stability and coloration efficiency of the WO3 film.

Herein, we present, for a first time, an electrochromic film of WO3 fabricated on an ITO by wet etching process using hydrochloric acid (HCl).

A low-cost, facile and template-free hydrothermal method

adopted with a simplified architecture where etching process supports WO3 to form an adhesive layer.

The X-ray photoelectron spectroscopy was used to study the surface composition of the films. Electrochemical impedance spectroscopy (EIS) was employed to reveal the charge transfer process at the interface with the help of Randles circuit model. Compared to before etched WO3 configuration, the etched WO3 configuration exhibited a strong enhancement in terms of roughness, porosity, open-tunnel structure, current density, coloration efficiency (about 178.7 cm2C-1) and faster response time. Moreover, electro-optical characterization illustrates high transmittance modulation (about 49% at 630 nm) with excellent stability, making it attractive for a practical application.

Keywords: tungsten oxide, hydrothermal synthesis, etching, electrochromic device

1. Introduction

The interest in electrochromic (EC) devices has increased over the last decade due to their application in solar light control and energy saving in smart windows fabricated using EC materials [1]. These materials can change their optical properties under the applied voltage. Among various EC materials, tungsten oxide (WO3) has been widely studied because of its high coloration efficiency in the visible region of the electromagnetic spectrum, high cyclic stability compared with other transition metals, suitable energy band gap (2.5-2.8 eV), electron mobility (~12 cm2V-1s-1), long holes diffusion length (~150 nm), and stability against photocorrosion in acidic solution [2-3].Three-dimensional assembled structures of WO3 in nanometer or micrometre-scale have a large surface-to-volume ratio because of the porous structure, thereby yielding superior optical modulation with an enhanced chromic response time [4-5]. Among the several WO3 polymorphs, the hexagonal form is of great interest due to its open-tunnel structure [6]. Various methods such as electrodeposition [7-8], sol-gel method [9], atmospheric pressure chemical vapor deposition [10], pulsed laser deposition [11], pulse nanogravimetric technique [12], spin coating [13] have been used to synthesize WO3 thin films. However, these methods require strict reaction conditions such as high-low temperatures, accurate gas concentrations, toxic chemical reagents and expensive complex equipment. Among the various methods, hydrothermal synthesis is a favorable alternative, because of low growth temperature, wherein microstructures of WO3 films can be precisely tailored by varying the precursor concentration, temperature, and duration, by adopting various surfactants and capping agents. Moreover, directly growing WO3 thin films with strong adhesion on

substrates by using the hydrothermal method is difficult. Additional studies must be conducted on the synthesis of WO3 crystals or thin films [3, 14-16]. Nevertheless, the surface roughness, sheet resistance and transmittance of indium tin oxide (ITO) film affect the performance of EC devices [17] and adhesion of material on substrate. Etching plays an inherent role in enhancing these parameters [18-20]. However, very few studies have performed etching by the hydrothermal method.

For instance, researcher studied the electrochromic properties using

cathodic electroetching of electrodeposited WO3 thin film in tetraethyl ammonium chloride solution, resulting in high-quality low-stress films [8]. Other group reported photoelectrochemical properties using wet etching method to the presynthesized WO3 thin film by citric acid [21]. The study reported that the etching process modifies the nanoplates surface with increasing voids. Besides, it enhances the intensity of photocatalytic active crystal planes. Therefore, in this study, we present a novel and a convenient strategy to synthesize WO3 film through wet etching of ITO glass substrates by using HCl followed by the one-step hydrothermal method without a seed layer since it enhances surface area, providing faster charge transfer [22]. Until now, the process of etching by using HCl has not been introduced in the hydrothermal synthesis of WO3 thin films. HCl etching on ITO produces free radicals, consequently increasing the chemical reaction rate. In addition, etching changes the phase from crystalline to amorphous nature of ITO consequently enhancing the adhesion of WO3 films on ITO substrates and markedly improves its EC performance.

2. Experimental 2.1. Materials Sodium tungstate (Na2WO42H2O), oxalic acid (C2H2O42H2O), HCl, lithium perchlorate (LiClO4) and propylene carbonate (PC) were purchased from Loba Chemie and all these analytical reagent-grade chemicals were used without further purification. ITO glass plates (25 ohm cm-2, 3×1 cm2) were used as substrates. All aqueous solutions were prepared using double distilled water (DDW). 2.2. Etching process of ITO glass substrates The glass substrates were cleaned with an aqueous detergent, ultrasonicated in DDW and acetone and rinsed with DDW. For etching the ITO glass substrates, 4M HCl solution was prepared because earlier study reported that ITO dissolves in etchants of halogen acids with a concentration exceeding 4M [23-25]. Furthermore, ITO glass plates were kept in HCl (4M) for 1min. Here, H+ and Clˉ ions collide with ITO and generate indium chloride [InCl; eq. (1)], which is highly reactive [22]. Moreover, it increases the reaction rate by breaking the chemical bonding of ITO [Eq. (1)]. The ITO plates were dried

naturally at room temperature. The bright spots observed on the ITO glass substrates entailed the etching process, represented as etched ITO. In2O3 + 2HCl → 2InCl + H2O + O2

(1)

This reaction attributes the ITO glass substrates facilitating not only the strengthening of the adhesive nature [26] but also the nucleation of WO3. 2.3. Formation of WO3 thin films on etched ITO glass substrate A precursor solution of 0.2M Na2WO42H2O was prepared in DDW; the pH of this solution was 8.5. Acidification was performed by the dropwise addition of HCl to obtain a pH of 1. A white precipitate was formed, which was dissolved by vigorously adding 30mL of oxalic acid (0.1M). After stirring for 15 min, a transparent yellow solution was obtained [27]. These solutions were transferred into a 25 mL Teflon-lined stainless steel autoclave. In the first instance, a bare ITO substrate was dipped vertically in the autoclave, maintained at 100 C and 4 psi (0.2 kg cm-2) for 1h to obtain WO3 deposition by using the hydrothermal method. Further, the as deposited WO3 film on bare ITO glass substrate annealed at 400 °C for 2 h represented as A-WO3. Later, the etched ITO glass substrates were dipped in an autoclave with a previously prepared yellow solution at the aforementioned conditions. Thereafter, the WO3 film gets deposited on the etched ITO which is annealed at 400 °C for 2 h represented as etched AWO3 film. 2.4. Methods The film thickness was measured using a surface profilometer. The thicknesses of A-WO3 and etched A-WO3 films were estimated 274 and 256 nm, respectively. A scanning electron microscope (SEM) was used to characterize morphologies of an as-prepared film (Hitachi S-4700 II, 25 kV). X-ray diffraction (XRD) pattern (Thermo ARLSCINTAG X'TRA with CuKα irradiation, L ¼ 0.154056 nm) was used to analyze the crystallinity. Fourier transform infrared (FTIR) spectroscopic studies were conducted using the Perkin Elmer IR spectrometer (500-4000 cm-1). The optical properties were studied by ultraviolet-visible spectroscopy. The transmission electron microscopy (TEM) images, selected area electron diffraction (SAED) pattern, and high-resolution TEM (HRTEM) images were obtained using a field emission electron microscope (JEOL JEM-2100F) operated at 200 kV. The TEM sample was prepared by drop casting an ethanolic dispersion onto a carbon-coated Cu grid.

The elemental

information regarding the deposited WO3 nanoflower sample was analyzed using an X-ray photoelectron spectrometer (XPS) (VG Multilab 2000-SSK, USA, K-Alpha) with a multi-channel detector, which can endure high photonic energies from 0.1 to 3 keV. An electrochemical study was performed using a three-

electrode system (Electrochemical Analyzer 608, CH Instruments), a graphite rod as the counter electrode, a standard calomel electrode (SCE) as the reference electrode and the A-WO3, etched A-WO3 as a working electrode with an electrolyte solution containing 0.5M LiClO4 dissolved in 1mM propylene carbonate (PC) denoted as LiClO4-PC. Fig.1

3. Results and discussion 3.1 Morphological characterization Fig. 1 (a-l) presents the SEM images for all the samples; the grain and pore sizes are elaborated in Table 1. The bare ITO glass substrates showed agglomerated particle size of 1500 nm [Fig.1 (a and b)]. Moreover, the WO3 film before etching showed a clear, fine nanobrick of thickness 500-700 nm, with fewer changes after annealing [Fig.1 (c-f)] and a porous structure. However, the etched ITO film isolated the agglomerated ITO particles [Fig.1 (g and h)], increasing the film roughness [28]. The etched WO3 films [fig.1 (i and j)] exhibited tiny nanoflowers having a grain size of 80-100 nm, with an incorporated cracky nature, and a pore size of 200 nm. While etched A-WO3 films [fig.1 (k and l)], revealed tiny nanoflowers having a grain size of 50-70 nm, with an integrated cracky nature, and a pore size of 400 nm. As a consequence, the etched A-WO3 film showed a decreased grain size and increased pore size with an open surface, wherein etching plays a key role in the architecture of WO3 thin films. Fig. 1 Table 1 Fig. 2 presents the HRTEM and SAED images of the etched A-WO3 films. Fig. 2a indicates the generation of nanopores on the surface, in agreement with SEM images [fig.1 (k and l)]. The lattice fringes are shown in Fig. 2b illustrates that the WO3 thin films are well crystallized [29]. The diffraction maxima, related to the fringes observed in HRTEM images [fig. 2c] with an interplanar spacing of 0.28 nm oriented along the (001) lattice plane of hexagonal WO3 (JCPDS no. 00-033-1387). Fig. 2 3.2 Structural study The XRD patterns of all the films are shown in Fig.3 (a-f). Fig. 3a exemplifies the crystalline peaks of the bare ITO substrate (indicated by an asterisk) and the XRD patterns of WO3 (Fig. 3b) and AWO3 (Fig. 3c). It possesses a monoclinic phase (JCPDS no. 00-005-0364) of WO3 with well indexed

peaks at 23.14° (002), 23.65 (020), 24.39 (200), 26.11 (011), 8.96 (111), 33.38 (021), 42 (221), 50 (140), 55.54 (212), with less variation in the intensity. The asterisk (*) indicates the ITO peaks (JCPDS no. 00-06-0416) [30]. Fig. 3d represents the amorphous nature of the etched ITO film. The occurrence of amorphous phases is a particularly interesting phenomenon for studying thin film growth [26]. The XRD patterns of the etched WO3 and etched A-WO3 film [Fig. 3 (e and f)] represents the hexagonal phase (JCPDS card no. 00-033-1387) with well-indexed peaks at 13.85 (100), 24.37 (110), 26.84 (101), 28.20 (200), 33.57 (111), 36.6 (201), 42.81 (300), 48.69 (102), 49.87 (220), 55.25 (202), 49.87 (220), with a highly intense peak at 22.69° (001), indicating that most (001) planes are parallel to the substrate [2]. In this case, the ITO peaks are absent due to the amorphous nature, thus increasing the adhesion of WO3 films. The amorphous nature obtained owing to the etching process that detaches a layer of ITO (0.1 % weight of ITO) making it thinner, less dense with enhancement in inter-particle separation possessing short range order of rapidly oscillating atoms about a fixed point. When WO3 bombarded on amorphous ITO (external force) using hydrothermal method, the short range atoms oscillate with larger amplitude that wield stress on WO3 switching monoclinic (a ≠ b ≠ c) to hexagonal (a = b ≠ c) structure. As a result, it removes parametric lattice mismatch preferentially the stress applied on the WO3 particles ensuing in the reduced particle size. The XRD results are analogous with SEM, HRTEM, and SAED. Intense and sharp XRD peaks of the A-WO3 and etched A-WO3 film corroborates a high degree of crystallinity, improving the performance of EC devices. Fig.3 X-ray photoelectron spectroscopy (XPS) is a widely used analytical surface-sensitive technique for investigating the surface composition and chemical states of WO3 and etched WO3 thin film, as shown in Fig. 4. It presents qualitative XPS survey scan analysis obtained in 0-1000 eV binding energy range for A-WO3 and etched A-WO3 samples. The total XPS spectra indicate the presence of tungsten and oxygen. Meanwhile, the carbon peak (C1s) appearing at about 284 eV is due to inadvertent carbon species from the XPS instrument [31]. Moreover, XPS spectra reveal that the WO3 films prepared after etching process on ITO seem to be not contaminated since there is no peak other than characteristic peaks of W and O with additional C peak. This enumerates the compositional purity and quality of deposited films. Fig. 4 Fig. 5 Fig. 6

Fig. 5 and 6 shows the corresponding occurrence of structured tungsten (W 4f) and oxygen (O 1s) peaks for the A-WO3 and etched A-WO3 films under the same conditions as for the XPS survey scan of fig. 4. In Fig. 5(a-b), W4f can be deconvoluted into a doublet with binding energy peaks at 35.7 eV and 37.8 eV, corresponding to the emission of W 4f7/2 and W 4f5/2 core-levels that belong to the six valent tungsten (W6+ ) oxidation state of tungsten atoms [32] with a spin-orbit separation of 2 eV. For etched AWO3 thin films (fig. 5b), the characteristic peaks of W6+ oxidation state of W 4f7/2 and W 4f5/2 are shifted about 0.2 eV to a lower binding energy (35.5 eV and 37.6 eV), inferring that there is no significant variation in the oxidation state of tungsten atoms. Besides, the core-level peaks become sharper and better resolved, indicates an improvement in the crystallinity of film in both the samples. In fig. 6a, the O1s peak was deconvoluted in three components. Component I with a binding energy of 530.35 eV is assigned to the oxygen atoms (O2-) that form the strong W=O bonds [33]. Component II located at 532.3 eV illustrates to water bound at the surface of the samples [34], proving the existence of WO3(H2O)n phases at the surface whereas the component III at the binding energy 535.6 eV corresponds to oxygen originating from water and might be attributed to water adsorbed on W [35]. While etched A-WO3 film signifies Components I and II are present in the film with a shift in the third O 1s peak to the lowest energy, 531.6 eV (fig. 6b) which is ascribed to the W−O peak [36]. This shift may attribute to the shift of WO3 structure from monoclinic (a≠b≠c) to hexagonal (a=b≠c) responsible for removing the parametric lattice mismatch. The ratio of atomic weight % of the tungsten (W4f) and oxygen (O1s) core level is estimated 14.66: 36.78 and 10.55: 32.38 for A-WO3 and etched A-WO3 film which is in good agreement with XRD results. In order to confirm the contribution of C–O bond and the shift occurred in component III of O 1s peak, the FTIR spectra of etched WO3 (Fig. 7a) and etched A-WO3 (Fig. 7b) film studied over the range 500–4000 cm-1 at room temperature. The results provided data on the different modes of vibrations of atoms or molecules with their associated energies in the infra red region. Moreover, the spectra showed how the adsorption bonds of OH groups and water molecules change during thermal condensation. The observed vibration bands are fundamental vibrations of W–O, W=O, and W–O–W chromophores as shown in Table 2. Fig.7 Table 2 For the etched A-WO3 (Fig. 7b) sample, the W–OH group showed a decrease due to the removal of water of WO3H2O [37-39]. Most of the water lost during annealing could be assigned to the bonding state having absorption of approximately 3000-3700 cm-1. Rougier et al and other [40-41] stated the band 1419 cm-1 present in A-WO3 film ascribed to the stretching vibrations of NH4+ due to contamination

with ambient laboratory air which is absent in the etched A-WO3 film indicating removal of NH4+. A broad band shift was observed in etched A-WO3 related to the W–O–W group; this blue shift may be assigned to the bent structure [42] that reduces the grain size, thus increasing the porosity. As a consequence, FTIR does not show any peak of C–O bonding, in complete agreement with XPS measurements. 3.3 Electrochromic and electrochemical properties Fig. 8 The electrochemical study of the A-WO3 and etched A-WO3 films in LiClO4-PC electrolyte was typically characterized through cyclic voltammetry (CV) in the potential range of -0.9 to +1.0 V vs the SCE at a scan rate of 50 mVs-1 [Fig. 8 (a and b)]. A significant shift was observed in the shape of the CV curves. Upon cycling, the A-WO3 film, yellow when deposited, switch from blue to slight transparent in a reversible manner. Contrastingly, the etched A-WO3 film slightly yellow when deposited, switch from being deep blue to considerably transparent in a reversible manner. This coloration and decoloration is attributed to the electrochemical intercalation-deintercalation of Li+ ions and electrons into the WO3 lattice [43] represented by the reaction [Eq. (2)], WO3 + xLi+ + xeBleached

↔

LixWO3

(2)

Colored

For tungsten oxide, ion intercalation is associated with optical coloration whereas deintercalation is associated with bleaching process expressed in terms of diffusion coefficient (D). It is a significant factor in electrochromic studies that depends on the crystallinity, porosity of thin films and diffusing ionic species Li+ [44]. It can be calculated by employing the Randles-Sevcik equation [Eq. (3)], ip= 2.72 ×105 n3/2 D1/2 C0 ν1/2

(3)

Where D is the diffusion coefficient of Li+ ions; ν, scan rate; C0, concentration of active ions in the solution; n, number of electrons (it is assumed to be 1) and ip is the peak current density. The value of anodic peak current density (ipa) and cathodic spike current density (ipc) at a scan rate 50 mVs-1 are 0.08 mAcm-2 and 0.24 mAcm-2 for A-WO3 film respectively. However, an enhancement observed in ipa (0.19 mAcm-2) and ipc (0.53 mAcm-2) at a scan rate of 50 mVs-1 for etched A-WO3 thin film. Eventually, the value of D alters from 3.5×10-11 cm2s-1 (A-WO3) to 7.7×10-11 cm2s-1 (etched A-WO3). In general, electrochromism in WO3 thin films is related to the intervalence charge transfer process between W5+ and W6+ [43, 45]. During the cathodic scan, the reduction of the W6+ to the W5+ leads to blue coloration of the

film (ipc = 0.53 mAcm-2). In the reverse anodic scan, the oxidation of W5+ to W6+ causes bleaching of the film (ipa = 0.19 mAcm-2). The increase in cathodic current density signifies larger concentration gradient of ions into the porous nanoflowers of etched A-WO3 thin films provides high capability of charge intercalation [45]. Fig. 9 Further EIS employed to investigate the inner resistive and capacitive elements using a sine wave of 5 mV amplitude over a frequency range from 100 kHz to 1 Hz. Parts a, b of figure 9 show Nyquist plots of the A-WO3 and etched A-WO3 film. The plots are fitted using Randles circuit model (Fig. 9 in dotted box). RS is the solution resistance, RCT is the charge transfer resistance, CDL is the double-layer capacitance and W is the Warburg impedance due to mass transfer to the electrode. It was found that if an additional resistance RX is included in parallel (fig. 9 outside the dotted box), then the spectra gives good fitting [46]. The observed values of RS, RCT, CDL and RX for A-WO3 film are 86.2 Ω, 114.7 Ω, 35.63 μF and 8768 Ω while for etched A-WO3 film, it is 75.18 Ω, 99.03 Ω, 35.74 μF and 4650 Ω, respectively, which is obtained from the intersection of the curve at the X-axis [47]. Therefore, it can be inferred that etching at the interface between ITO and WO3 imposes decrease in interfacial resistance yielding good contact that contributes excellent facilitation of ion transport [48]. Fig. 10 From chronoamperometric (CA) studies (Fig.10), the response time for A-WO3 and etched AWO3 were recorded for a potential step of  0.7V versus SCE with a residence time of 25 s in the LiClO4-PC electrolyte.

Response time for oxidation/reduction is the time required for the

anodic/cathodic current to achieve a steady state level after the application of reactive voltages. The response time for oxidation and reduction of A-WO3 films (fig. 10 a) was 9.35 and 9.97 s while for etched A-WO3 films; it is 1.98 and 2.5 s respectively (fig. 10 b). The switching speeds are much slower than our earlier report (1 s and 1.5 s for coloration and bleaching) [5], but better than those obtained by J. Zhang et al (7.6 and 4.2 s for coloration and bleaching) [49] and others [14] while comparable to the response time reported by other techniques [8, 50-51]. The slower response time in A-WO3 film is mainly due to the microstructures that exhibits less porosity of pore size about 100 nm. Whereas the nanostructures obtained in etched A-WO3 film is accountable for faster response time that enhances surface to volume ratio and porosity of pore size about 400 nm. The highly porous structure facilitates the intercrossing network on surface supplying more paths for the double insertion (extraction) of ions and electrons into (from) the film.

Fig. 11 (a and b) shows the transmission spectra of the A-WO3 and etched A-WO3 under their colored and bleached states. The transmittance of both films was colored/bleached (c/b) at ±0.7 V vs the SCE for 180 s (Fig. 10a). The coloration efficiency (CE) is an intrinsic property to explore the potential of EC material, enumerated by using Eq. (4). CE depends on the optical density (∆OD) at 630 nm and charge per unit area (Qi/A) intercalated during cycling, as reported in Table 3 where the area of the film is (0.5 × 0.75) cm2. CEλ=630 nm = (∆OD) 630 nm/(Qi/A)

(4)

In this equation (∆OD) 630 nm= log (Tb/Tc) where Tb and Tc represents the transmittance of the film in the bleached and colored states respectively. Fig. 11 Table 3 A CE of 178.7 cm2C-1 at a wavelength of 630 nm was observed in the etched A-WO3 film. This value exceeds the previously reported efficiencies [52, 5] and comparable to the other techniques [50, 53]. Table 3 confirms that optical modulation increased in the etched film (about 49 % at a wavelength of 630 nm) owing to its porous nature that reduces resistance. Consequently, upon cycling, the free electrons of the film acquire a drift velocity with less hindrance and the ion diffusion takes place through the electrolyte more quickly with the increasing CE. The EC stability of the A-WO3 and etched A-WO3 films was probed in the LiClO4-PC electrolyte [Fig. 12 (a and b)]. The A-WO3 film showed poor stability about 2000th c/b cycles with major changes in the CV curve (Fig. 12 a). While the etched A-WO3 film (Fig. 12 b) illustrates the outstanding cyclic stability of approximately 5000 c/b cycles with less modification in the CV curve compared with a previous study [5]. Consequently, the etching process on ITO reveals considerable variations in the EC stability owing to the stiff adhesion of WO3 particles on it. Fig. 12 Eventually, the etching process of ITO provides buckling action (artificial axial load eccentricity) to WO3 films, amending a monoclinic to the hexagonal structure with a propensity to form open channels oriented along the normal to the surface, thus improving EC properties. Therefore, the present etched AWO3 system is far superior to the previously reported systems to synthesize the film rapidly with high adhesion. 4.Conclusions We demonstrated the EC configuration of the A-WO3 and etched A-WO3 films fabricated using a cheap, facile and seed layer-free green technique of the hydrothermal method by adding C2H2O4.2H2O as

the capping agent. The A-WO3 film before etching composed of aggregated microbricks. HCl provides a simplified architecture to WO3 films, forming a uniform and well-adhesive layer with an amendment of the monoclinic to hexagonal phase with nanoflowers having a grain size of 50-70 nm. XPS measurement confirms the stoichiometry of WO3 film and six valent (W6+) oxidation state of tungsten atoms. Moreover, etching reduces the thickness of the etched A-WO3 film with an enhanced pore size, thus offering an open tunnel structure for charges. Additionally, it provides superior optical modulation (49.3 % at 630 nm) with more favorable changes in the current density. The nanoflowery configuration of the etched A-WO3 film depicts faster charge transfer (1.98 s for oxidation and 2.5 s for reduction) and a greater CE (178.7 cm2C-1) with excellent EC stability (5000 c/b cycles) than before etched film. Notably, the etched A-WO3 films are highly promising for potential applications in energy-saving smart windows.

Acknowledgements This work was financially supported by DST-SERB, New Delhi for young scientist fellowship with major sponsoring of the project through grant no. PS/030/2013 and D.Y. Patil College of Engineering & Technology, Kasaba Bawada, MH, India helping to develop a research lab. This work was also supported by Korea Research Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2016H1D3A1909289) for an outstanding overseas young researcher.

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5 (a-b)

Figure 6(a-b)

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11 (a-b)

Figure 12 (a-b)

Figure captions: Figure 1. Scanning electron microscopy images for (a,b) ITO, (c,d) WO3 before etching, (e,f) A-WO3 before etching, (g,h) etched ITO, (i,j) etched WO3 and (k,l) etched A-WO3 samples on conducting glass substrates. All the films are recorded at low and high magnifications. Figure 2. (a,c) HR-TEM image and (b) SAED pattern of the hexagonal crystal structure of etched A-WO3 film. Figure 3. X-ray diffraction patterns for (a) ITO, (b, c) WO3, A-WO3 film before etching process, (d) etched ITO and (e,f) etched WO3, etched A-WO3 film, where asterisk specifies the peaks of ITO. Figure 4. X-ray photoelectron spectroscopy (XPS) spectra of the A-WO3 and etched A-WO3 films for full spectrum. Figure 5 (a-b). X-ray photoelectron spectroscopy (XPS) spectra of the A-WO3 and etched A- WO3 films for W 4f spectrum. Figure 6 (a-b). X-ray photoelectron spectroscopy (XPS) spectra of the A-WO3 and etched A-WO3 films for O 1s spectrum. Figure 7. IR transmittance spectra of (a) etched WO3 and (b) etched A-WO3 sample recorded in the wavenumber of 500−4000 cm-1. Figure 8. CV recorded in 0.5 M LiClO4-PC electrolyte for (a) A-WO3 and (b) etched A-WO3 configuration at the scan rate of 20 mV s-1. Figure 9. Electrochemical impedance spectroscopy (Nyquist plot) of (a) A-WO3 and (b) etched A-WO3 film onto ITO in LiClO4-PC with a frequency loop from 100 kHz to 1 Hz using amplitude of 5mV at the open potential. The experimental data were fit to the equivalent circuit shown. Figure 10. Chronoamperometric curve of the (a) A-WO3 and (b) etched A-WO3 films recorded in LiCLO4-PC electrolyte for 250 s. Figure 11. Optical Transmittance spectra of (a) A-WO3 and (b) etched A-WO3 films in its colored and bleached state at potential ±0.7V in LiCLO4-PC electrolyte recorded in the wavelength range 400-800 nm. Figure 12. EC stability of (a) A-WO3 film for 1st and 2000th and (b) etched A-WO3 film for 1st and 5000th color/bleach cycle. The potential is swept from -1to +1 V (vs SCE) for both configuration at the scan rate of 20 mV s-1.

Table 1 Particulars

WO3

A-WO3

Etched WO3

Etched A-WO3

Grain Size

500-700 nm

400-700 nm

80-100 nm

50-70 nm

Pore Size

100 nm

100 nm

200 nm

400 nm

A represents annealing treatment Table 2:Different vibrational modes as revealed from FT-IR spectra for Etched WO3 and etched A-WO3 samples. Sr.

Wave number (cm-1)

Group

No.

Etched WO3

Assignment

References

etched A-WO3

1

W–OH, H2O

3317

3338

νsym(OH)

38

2

N–H

1419

_

ν (NH)

39-41

3

W–OH

1633

1631

δW-OH

39

4

W=O, W–O

947, 594

948, 599

νW-O

38,39

5

W–O–W

823

876

ν (W-O-W)

38

Stretching vibrations (ν), in-plane bending vibrations (δ).

Table 3:EC properties of A-WO3 and etched A-WO3 (Area of the film is (0.5 × 0.75) cm2) Particular

A-WO3 Etched WO3

Tb (%)

77.7 A- 63.2

Tc (%)

Qi (C)

ΔOD= (Tb/Tc)

62 13.9

0.00138 0.00138

0.0979 0.657

log CE = ΔOD/(Qi /A) (cm2C-1) 26.60 178.7