Superior Electrochromic Performance of Tungsten Oxide Embedded with Polypyrrole

IJIRST –International Journal for Innovative Research in Science & Technology| Volume 3 | Issue 04 | September 2016 ISSN (online): 2349-6010

Superior Electrochromic Performance of Tungsten Oxide Embedded with Polypyrrole N. Y. Bhosale Research Scholar Department of Physics D. Y. Patil College of Engineering & Technology, Kasaba Bawada, Kolhapur, Maharashtra-416006, India

A. V. Kadam Assistant Professor Department of Physics D. Y. Patil College of Engineering & Technology, Kasaba Bawada, Kolhapur, Maharashtra-416006, India

Abstract The electrochromic (EC) properties of inorganic-organic hybrids of tungsten oxide/polypyrrole (WO3/PPy) thin films were analyzed. Using electrodeposition, WO3 was coated on a conducting glass substrate (Indium doped tin oxide), followed by thermal treatment. In sequence, the PPy thin film was deposited using chemical bath deposition. The structural, morphological, optical, and EC responses of WO3, PPy, and WO3/PPy films are described. The EC properties indicates considerable enhancement in redox kinetics (response time) and coloration efficiency of WO3/PPy films compared with those of the solitary WO3 and PPy films, with a significant increase in EC stability.

Keywords: Tungsten Oxide, Polypyrrole, Electrochromism, Coloration Efficiency _______________________________________________________________________________________________________ I.

INTRODUCTION

Electrochromism (EC) is a phenomenon where a material undergoes changes in its optical properties when placed in an electric field[1]. Granqvist[2-5] and Deb [6] has reviewed diverse EC materials and their applications. Significantly, inorganic nanomaterials, such as tungsten oxide (WO3) are, extensively used for EC application in smart windows because of their high coloration efficiency, reasonable stability, and relatively low-cost [1] [7] [9]. It is colorless when polarized anodically and dark blue when polarized cathodically. Furthermore, among various conducting polymers, PPy is most studied because of its easy synthesis, cost-effectiveness, high ionic conductivity, environmental stability, and lithium storage ability [10] [11] [12] [13]. It demonstrates EC by changing from orange-yellow color in its reduced state to black˗violet color in its oxidized state. However, polymers have certain disadvantages, such as low mechanical strength, low sensitivity and low EC stability, which decrease their potential for future applications.

Therefore, a scientific focus has emphasized on an inorganic˗organic nanocomposite that concomitantly enhances the properties of their counterparts by modifying each other [14] [15]. Particularly, WO 3/PPy can be deployed as a gas sensor [16] [17] [18] in addition to as an EC device [19] [20] [21], wherein PPy vigorously augments several properties of inorganic materials. Therefore, only few studies reported on EC properties of WO 3/PPy. Rocco et al fabricated an EC device combining dodecyl sulfate doped PPy and the as˗deposited WO 3 [19], wherein they estimated 30% chromatic contrast from light blue to dark blue in the visible and near infrared region, and the electric and optical properties stabilized after approximately 1.5 × 104 c/b chronoamperometric cycles. In 2015, facile approach of polypyrrole/ tungsten oxide composites electrosynthesized in ionic liquids for fabrication of EC device is discussed [20]. The maximum color contrast and maximum coloration efficiency (CE) of the device achieved for PPy-WO3/BMIMTFSI (1-butyl-3-methylimidazolium bis(trifluromethylsulfonyl) imide). Our previous report [21] demonstrated the orthorhombic phase of hybrid WO 3/PPy with highly porous disordered nanoflaky morphology (discussed shortly below) showing excellent EC cycling stability of approximately 20000 c/b cycles.

Here, we report the EC performance of the WO3/PPy film using cell configurations: ITO/ WO3/PPy /LiClO4-PC/SCE. In-situ transmittance was used to determine the response time of coloration/bleaching cycles of the device. Optical modulation was inspected by ex-situ transmittance.

II. EXPERIMENTAL PART The aqueous solutions were prepared using double distilled water (DDW). Pure tungsten (W) powder (99%), hydrogen peroxide (H2O2, 30%), pyrrole (C4H5N), and ammonium persulphate ((NH4)2S2O8) were reagent-grade material from LOBA Cheme (Mumbai, India). The ITO glass plates (25Ω/cm2, 3cm × 0.65 cm) were used as substrates.

The ITO glass substrate was cleaned using an aqueous detergent and ultrasonicated in acetone and ethanol, followed by rinsing with DDW. A 0.5 M solution was prepared by mixing 4.59 g W powder in 30ml H2O2 (constant stirring with DDW), where H2O2 enhances the rate of oxide formation [22]. This solution was stirred for 24 h with mild heating and a platinum foil was dipped to remove excess H2O2 [23] [24]. The second part explains the synthesis of a PPy thin film and a preparation of hybrid WO3/PPy by chemical bath deposition technique using pyrrole (0.03M) chemically polymerized by ammonium persulfate (APS 0.06M). Detailed process for the electrodeposition of WO3, synthesis of PPy and WO3/PPy samples was

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Superior Electrochromic Performance of Tungsten Oxide Embedded with Polypyrrole (IJIRST/ Volume 3 / Issue 04/ 020)

discussed in our previous report [21]. Moreover, WO 3, PPy, and WO3/PPy films were propelled for EC investigations with three standard electrochemical configurations, glass/ITO/WO3/PPy/LiClO4-PC/SCE. X-ray diffraction (XRD, Thermo ARLSCINTAG X'TRA with CuKα irradiation, λ= 0.154056 nm.) was used to analyze crystallinity. The morphologies of films were characterized using scanning electron microscopy (SEM, Hitachi S-4700 II, 25 kV). The EC properties were studied using a three-electrode system (CH instruments, electrochemical analyzer, model-608) with LiClO4 as the electrolyte, graphite as the counter electrode and SCE as the reference electrode. The optical property of the film was monitored using a spectrophotometer having a wavelength range 300-1200 nm. The colored/bleached states switching time characteristics were recorded using a setup consisting of a He-Ne laser (λ = 632.8 nm), Si photo detector and custom made microprocessor˗controlled versatile unit assembled using a computer. The films illuminated using the He-Ne laser beam and a photodiode were used to sense the brightness of the light transmitted through the coating. The unit supplied a square wave potential of ±0.7 V at a frequency of 0.05 Hz, to start the EC electrode. Plots of percentage transmission and a current flowing through the electrode as a function of time yielded the response times and ion storage capacities of the films.

III. RESULTS AND DISCUSSION Structural and Morphological Study

Fig. 1: XRD patterns of WO3 and WO3/PPy inset showing XRD of PPy

Fig.1 shows the XRD patterns of WO3 and WO3/PPy thin films deposited on the ITO substrate. The WO 3 film reveals the crystal phase of monoclinic WO3, semi-crystalline nature of PPy (inset of Fig.1) and an orthorhombic phase of WO 3/PPy [21] (reported elsewhere) with well-indexed diffraction peaks. ITO diffraction peaks indicated using an asterisk (*) [25] [26]. The phase change in WO3 (monoclinic to orthorhombic) may be due to the formation of quasi-particle polarons and bipolarons with improved PPy morphology on a WO3 layer, which is in agreement with that reported in previous studies [27]. The morphological study entails nest like twisted ultrathin ordered nanoflakes for WO 3, however a disordered nanoflakes were observed for WO3/PPy samples (Fig.2 (a-f)) with high porosity reported in our previous article [21].

Fig. 2: Low and high magnification of Scanning electron micrographs for WO3 (a,b), PPy(c,d) and WO3/PPy(e,f) thin films

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Superior Electrochromic Performance of Tungsten Oxide Embedded with Polypyrrole (IJIRST/ Volume 3 / Issue 04/ 020)

Electrochromic and Optical Study Fig. 3 demonstrates the cyclic voltammetry (CV) recorded for WO 3, PPy and WO3/PPy films where inset showing CV of PPy in a 0.5M LiClO4-PC electrolyte at a scan rate of 20 mV/s. The CV of the WO 3 thin film (Fig.3) was recorded at a potential of -1 to +1 V vs SCE that provided a cathodic peak at ˗0.2 V and anodic peak at 0.3 V. The CV of PPy thin films was initiated from ˗1.5 V, reversed at 1.7 V and terminated at ˗1.5 V (inset of Fig.3), wherein a well-defined cathodic peak observed at -0.45V during cathodic scan and an anodic peak at +0.65V during an anodic scan. The PPy film changed color from orange-yellow (reduced state) to black-violet (oxidized state).

Fig. 3: CV curves of the thin films for WO3, PPy and WO3/PPy inset showing CV of PPy

Although the CV recorded for WO3/PPy film (Fig.3) is analogous to that for the WO3 film with a cathodic peak at 0.1V (vs SCE) during a cathodic scan and an anodic peak at 0.3V (vs SCE) during an anodic scan. The current density of the WO3/PPy film was found enhanced with a color changes from brown to green, indicating the chromatic role of PPy in its EC and role of WO3 in its nested, disordered, nanoflaky morphology [21] providing a larger reaction surface area [28]. To study the intercalation/de-intercalation process of Li+ ions with respect to time, chronocoulometry (CC) was performed (Fig.4) at potential steps ±0.7 V vs SCE in the 0.5M LiClO4-PC electrolyte for 10 s, to calculate the amount of charge intercalated. The CC graph of WO3/PPy shows that PPy provides a path for electrical conduction [29] [30].

Fig. 4: Chronocoulometric (CC) plot for thin films WO3, PPy and WO3/PPy recorded in 0.5M LiClO4-PC electrolyte

The switching characteristics of WO3, PPy and WO3/PPy films were studied from in-situ transmittance at 630 nm. Fig.5 shows the transmittance-time response for all the samples upto first five cycles. The studies were performed by switching the samples from an oxidized state to a reduced state by applying alternating square potentials (-1 to +1 V for WO3 and WO3/PPy, 1.5 to +1.7 V for PPy). In this experiment, we have chosen switching time as the times required to switch 95% of the maximum optical contrast at 630 nm wavelength [15]. The WO3 exhibited slower response speed with 5 s for coloration and 4.8 s for bleaching kinetics. The PPy samples showed, faster response time (1.5 s for coloration and 2 s for bleaching). However, the WO3/PPy film showed the fastest response time (0.6 s for coloration and 0.8 s for bleaching) compared with the solitary WO 3 and PPy films. The fastest response time of WO3/PPy was probably because of the acidic nature of PPy that perforated WO 3 during the dip-rise cycle, resulting in faster and easier ion interdiffusion, which means that the WO3 layer is highly permeable to PPy.

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Superior Electrochromic Performance of Tungsten Oxide Embedded with Polypyrrole (IJIRST/ Volume 3 / Issue 04/ 020)

Fig. 5: Variation of transmittance for WO3, PPy and WO3/PPY as a function of time at a wavelength of 630 nm for the first five switching cycles between -1.5 to +1.7 V for PPY and -1 to +1 V for WO3, WO3/PPY in 0.5 M LiClO4-PC;

The optical transmittance of WO3, PPy and WO3/PPy film was analyzed using a spectrophotometer for the colored and bleached state (Fig. 6 (a), (b) and (c)) at a potential of ± 0.7 V vs SCE in the 0.5M LiClO 4-PC electrolyte. The coloration efficiency (CE) is a crucial parameter in EC, which is defined as the change in optical density (ΔOD) at 630 nm per unit charge density (Qi/A), Qi and A indicates charge intercalated and area under electrolyte (1.5 cm × 0.65 cm) respectively. It can be calculated according to CE = ΔOD/(Qi/A) [31], where ΔOD = log (Tb/Tc) [32], in which, Tb and Tc are the transmittances of the thin film in its bleached and colored state, respectively.

(a)

(b)

(c)

Fig. 6: Optical Transmittance spectra of (a)WO3, (b)PPy, (c)WO3/PPy, for as deposited, colored and bleached state at potential ±0.7V in LiCLO4-PC electrolyte.

Table - 1 Evaluation of optical density and coloration efficiency for WO3, PPy, and WO3/PPy sample (tc = response time st colored state, tb = response time at bleached state) Response time (s) Transmittance (%) (at 630 nm) ΔOD Name of sample Qi (C) C.E cm2 /C =log(Tb/Tc) tc tb Tc Tb WO3 5 4.8 23.7 84.31 0.55 0.0039 141.02 PPy 1.5 2 38.3 72.98 0.28 0.012 23.33 WO3/PPy 0.6 0.8 16.65 81.59 0.69 0.0045 153.33

Table 1 shows the ΔOD and CE of WO3, PPy and WO3/PPy thin films. The WO3 film exhibited 0.55 ΔOD at 630 nm (Fig. 6(a)) with a CE of 141.02 cm2/C. The optical behavior of the PPy film (Fig.(6b)) showed 0.28, ΔOD at 630 nm and a CE of the order 23.33 cm2/C, which is low because of its poor cycling stability. In the WO 3/PPy films, ΔOD was 0.69 (Fig.6(c)) at 630 nm and CE was 153.33 cm2/C, exhibiting chromatic contrasts of 62%, which are higher than those of previous studies [19] [20] [33].

The improved CE of WO3/PPy may be due to the disordered nanoflaky morphology and porous structure.

Further the work has been extended to examine the electrochromic cyclic stability reported elsewhere [21], showing an improved EC behavior of hybrid WO3/PPy about 20000 c/b cycles. Therefore, the system (ITO/WO3/PPy) demonstrates inherent superior performance and assists in improving the EC properties.

IV. CONCLUSION An EC WO3/PPy film was assembled with a configuration of ITO/WO 3/PPy and characterized for its EC and optical performance in LiClO4-PC. The response time for the coloring and bleaching processes was enhanced from 5 and 4.8 s to 0.6

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Superior Electrochromic Performance of Tungsten Oxide Embedded with Polypyrrole (IJIRST/ Volume 3 / Issue 04/ 020)

and 0.8 s respectively. The CE for the WO3/PPy hybrid was 153.33 cm2/C, considerably greater than that for their solitary performance (WO3˗141.02 cm2/C, PPy˗23.33 cm2/C), which indicated the faster insertion and deinsertion kinetics in a hybrid WO3/PPy film. The stability of the WO3/PPy film was approximately 20000 c/b cycles. Therefore, WO 3 augments the electrochemical stability of PPy whereas PPy provides its conductivity and faster ion intercalation/deintercalation kinetics to WO3, which heightens the current density and the response time of the WO3 thin films and improves CE. Moreover, the present system complements from brown to green and can be implemented in various applications of the smart window. A study considering the same system in the infrared region with structural changes is progress.

ACKNOWLEDGMENT This work was partially supported by Shivaji University, Kolhapur, MH, India [grant number SU/STAT/VVK/8/3965] and DSTSERB, New Delhi [grant no. PS/030/2013].

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