Accepted Manuscript Influence of disordered morphology on electrochromic stability of WO3/PPy Digambar K. Gaikwad, Anamika V. Kadam, Sawanta S. Mali, Chang K. Hong PII:
S0925-8388(16)30227-4
DOI:
10.1016/j.jallcom.2016.01.226
Reference:
JALCOM 36577
To appear in:
Journal of Alloys and Compounds
Received Date: 3 December 2015 Revised Date:
18 January 2016
Accepted Date: 28 January 2016
Please cite this article as: D.K. Gaikwad, A.V. Kadam, S.S. Mali, C.K. Hong, Influence of disordered morphology on electrochromic stability of WO3/PPy, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.01.226. 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.
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Mechanism:
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A schematic of the mechanism is proposed in above fig. The mechanism is based on three step process: (i) WO3 coated on ITO by electrodeposition followed by thermal treatment. It produces ordered nanoarrayed morphology. (ii) A second step involving deposition of PPy
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by chemical bath deposition on ITO. It possesses globular morphology. (iii) When PPy coated on WO3, PPy applies shearing force on WO3 and produces disordered nanoarrayed
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morphology.
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Influence of disordered morphology on electrochromic stability of WO3/PPy Digambar K. Gaikwad1, Anamika V. Kadam1,2, Sawanta S. Mali3, Chang K. Hong3 D. Y. Patil college of Engineering & Technology, Kasaba Bawada, Kolhapur-416006
Maharashtra, India
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1
2
D.Y. Patil Medical University, Kasaba Bawada, Kolhapur-416006 Maharashtra, India
3
Polymer Energy Materials Laboratory, Department of Advanced Chemical Engineering,
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Chonnam National University, Gwangju, Korea 500-757 Email:
[email protected]
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Abstract:
Tungsten oxide (WO3) films are critical for smart windows because of their capacity of varying the throughput of visible light and solar energy. This study highlights the evolution of structural and morphological changes of electrodeposited WO3 thin films coated with polypyrrole (PPy) by
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using chemical bath deposition. The structural and surface properties of WO3 thin films were studied using X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy. The electrochemical stability was inspected
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using repetitive cyclic voltammetry (CV) cycles for each sample in LiClO4-PC electrolyte for
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prolonged periods. The results showed an improvement in the electrochemical stability after the CV study.
Keywords: Tungsten oxide, Polypyrrole, Electrodeposition, Chemical Bath Deposition, Cyclic Voltammetry
1. Introduction:
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Electrochromism, a reversible phenomenon of changing color by using charge transfer under the action of electric field [1,2], was discovered 45 years ago [3]. Since then, considerable advancements have been made in the syntheses of electrochromic (EC) materials, manufacturing
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of EC devices, in EC properties, and in applications of EC materials that have been extended to smart windows, sensors, displays, radar-absorbing surfaces, and active camouflages [4-8]. In most applications, EC stability has become increasingly significant for enhancing a device’s life.
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Recently, various studies have been published on EC stability [9-12] with a majority of them focusing on the development of nanostructured organic - inorganic hybrid material [13-15].
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Despite the great potential in EC stability, many fundamental questions regarding structural and optical properties remain unsolved. Therefore, this paper focuses on the synthesis of an inorganic - organic hybrid material to improve structural, optical, and EC properties. For the synthesis, tungsten oxide (WO3) is selected as an inorganic material because of its
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fast color switching, long term stability, and durability [13], whereas polypyrrole (PPy) is chosen as an organic material because of its easy processibility and high coloration tolerability [16]. However, narrow color variation, low coloration efficiencies and need of high electrical power
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input coloration of WO3 alongwith low mechanical strength and stability drawbacks of PPy limit its use in electrochromic applications [13]. Consequently, amalgamation of PPy on WO3
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advances various properties, increasing its potential for various applications. Therefore, we synthesized a hybrid film consisting of WO3 on ITO by using a simple electrodeposition technique, followed by deposition of a polypyrrole (PPy) layer by using chemical bath deposition (CBD) technique. Structural and morphological analysis of the prepared samples (WO3, PPy, WO3/PPy) was performed using techniques including scanning electron microscopy (SEM), X-ray diffraction (XRD), high-resolution transmission electron
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microscopy (HRTEM), Fourier transform infrared (FT-IR), and ultraviolet-visible (UV-VIS) spectrophotometer. 2. Experimental section:
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2.1. Materials:
Pure tungsten (W) powder (99%), hydrogen peroxide (H2O2, 30%), pyrrole (C4H5N) and ammonium persulphate (APS; (NH4)2S2O8) were purchased from Loba Chem Chemicals and all
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these analytical reagent grade chemicals were used without any further purification. ITO glass
double distilled water (DDW). 2.2. Methodology:
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plates (25Ω/cm2, 3×1 cm2) were used as substrates. All aqueous solutions were prepared using
For the preparation of WO3 thin films by electrodeposition, an electrolyte solution was prepared using 4.59 g of W powder dissolved in 30 ml H2O2 (30%) with constant stirring, and the
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concentration of the solution was reduced up to 0.5M by an addition of DDW. With mild heating and stirring, the solution was kept overnight, and excess H2O2 was removed by dipping a platinum foil [17, 18]. The films were deposited on ITO glass substrates, cleaned with aqueous
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detergent, ultrasonicated in acetone, and ethanol followed by rinsing with DDW. A typical threeelectrode system was used for deposition which includes an ITO as the working electrode,
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graphite as a counter electrode, and a saturated calomel electrode (SCE; CHI150) was used as a reference electrode. The reaction was carried out in an electrochemical workstation at potential 0.7 V for 15 min at room temperature. The deposited films (A-WO3) were annealed at 400°C for 3 h to obtain the final WO3 thin films (B-WO3). In the subsequent part, PPy thin films were prepared using CBD technique, in which the chemical polymerization of 0.03M pyrrole (in 100 ml DDW) with APS solution (0.06M in 50 ml
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DDW) was conducted. When the mixture is stirred continuously at a constant low temperature (5 °C), it turns to a characteristic black color, indicating an immediate beginning of organic polymerization reaction [19]. The ITO substrate was dipped vertically in this PPy solution for 10
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min, raised up, and rinsed with DDW. This dip-rise cycle was repeated twice to obtain a PPy film of moderate thickness. For the synthesis of hybrid WO3/PPy films, B-WO3 films were dipped vertically in the prepared PPy solution for 10 min, raised up, and rinsed with DDW. This
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dip-rise cycle was repeated twice to obtain a B-WO3/PPy film of moderate thickness. 2.3. Characterization techniques:
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Morphologies of an as-prepared film were characterized using SEM (Hitachi S-4700 II, 25 kV). XRD (Thermo ARLSCINTAG X'TRA with CuKα irradiation, λ= 0.154056 nm) was used to analyze the crystallinity. The film thickness was measured using a surface profilometer. The thicknesses of B-WO3, PPy, and B-WO3/PPy were observed to be 220, 160, and 420 nm,
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respectively. FT-IR studies were conducted using the Perkin Elmer IR spectrometer in the range of 500 to 4000 cm−1. The optical properties were studied using the UV-VIS spectrophotometer. The TEM, selected area electron diffraction (SAED) pattern, and HRTEM images were obtained
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using a field emission electron microscope JEOL JEM-2100F operated at 200kV. The TEM sample was prepared by drop casting of ethanolic dispersion onto a carbon-coated Cu grid. An
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electrochemical study was performed using a three-electrode system (CH instruments, Electrochemical analyzer, and model-608) with LiClO4 electrolyte, graphite as the counter electrode, and SCE as the reference electrode. 3. Results and discussion: 3.1. Structural and morphological properties: Fig. A.1 (a, b)
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Fig. A.1 shows the XRD patterns of A-WO3, B-WO3, PPy, and B-WO3/PPy deposited on ITO substrates. The XRD pattern of a deposited A-WO3 thin film, before annealing illustrates (Fig. A.1a), that there is no peak of the WO3 phase; however, all observed peaks were well-
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indexed with cubic W (JCPDS card 01-088-2339). Peaks indicated by an asterisk (*) represent diffraction peaks of ITO [20, 21], and are observed in all four samples. The B-WO3 film reveals
0364). No peaks of other impurities were observed.
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monoclinic crystal phase (Fig. A.1a) with well-indexed diffraction peaks (JCPDS card 00-005-
The PPy XRD analysis shows (inset in Fig. A.1b) a broad peak shouldered in a region
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between 2θ= 13°- 21°, 28°- 34°, 39°- 43° corresponds to semicrystallinity (which is also reported in previous studies [22,23]). The shouldered peak between 2θ= 13°- 21° is assumed to be closed packing of benzene rings and points crystalline domain in amorphous PPy [24]. In general, the XRD patterns of the conducting polymers show broad peaks riding over a broad hump, which
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indicates their semicrystalline nature [25]. The XRD pattern of B-WO3/PPy composite films exhibited (Fig. A.1b) diffraction peaks of orthorhombic WO3 (JCPDS-35-0270) at 2θ = 14°, 23.92°, 28.75°, 37.7°, and 40.17° [26]. In addition, broad peaks riding over a broad hump were
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observed. The presence of a broad, small, and well-distinct peak indicates the nanocrystalline nature of the B-WO3/PPy film [27]. A phase change in WO3 from monoclinic to orthorhombic is
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in agreement with reports by Suri et.al, in which they studied the phase change induced by PPy in iron-oxide/PPy composite [28]. Moreover, it may be by virtue of parametric angular change as PPy truncates WO3 at a lower temperature (i.e., in monoclinic α = γ = 90°, β ≠90° however in orthorhombic α = β = γ = 90°). The crystallite size of the samples was evaluated using the Debye - Scherer formula [29]. The average crystallite size of the B-WO3, and B-WO3/PPy samples was calculated as 50 and 15 nm, respectively.
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Fig. A.2 (a, b, c, d) Fig. A.2 expresses the SEM images of the annealed B-WO3 (Fig. A.2a and 2b). Fig. A.2a
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shows that the B-WO3 film is uniform and consists of microflowery morphology with high porosity. Particle size distributions of micro-flowers vary from 10 to 20 µm, with the majority in the range between 15 and 20 µm. The high magnification SEM images (Fig. A.2b) disclose that
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each flower incorporated by a nest-like twist and ultra-thin ordered nanoflakes, nearly 0.5 – 1 µm in length and 50 nm in thickness, make the product a hierarchical microstructure [26]. Here, the
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word ordered nanoflakes indicate that nanoflakes are present within the microflower. The SAED pattern of an annealed WO3 film shows (Fig. A.2c) WO3 to have a monoclinic crystalline structure, and the film is oriented along the (200) zone axis. The spotty pattern clearly demonstrates that the nano-scale crystals of the monoclinic lattice structure coexist in certain
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localized regions of the film. Fig. A.2d represents high-resolution TEM image of the annealed WO3 sample. The clear lattice spacing of 0.25 nm corresponding to the d-spacing of the (200) plane, indicates the growth of as- synthesized WO3 ordered nanoarrays along the (200) direction.
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These results coincide with the XRD, SEM and SAED analysis results.
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Fig. A.3 (a, b, c, d)
The surface morphology of the PPy film (Fig. A.3a and 3b) on the ITO substrate is
compact, and dense with circular micrograins of size ranging from 1-3 µm. Fig. A.3c describes the SAED pattern of the PPy sample in which Debye ring confirms the semicrystalline nature of the sample. In addition, the HR-TEM image (Fig. A.3d) of the sample confirms the semicrystalline nature of PPy. Fig. A.4 (a, b, c, d)
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Fig. A.4a demonstrates that when PPy is deposited on WO3, it stamps out the microflowers of WO3 and creates disordered nanoflakes with an increase in porosity, as shown in the magnified image (Fig.A.4b). The SAED pattern of the composite WO3/PPy sample shows
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(Fig. A.4c) an orthorhombic crystalline structure of WO3 and the film is oriented along (111) zone axis. The Debye rings along with the spotty pattern clearly corroborate that the nano-scale crystals of the orthorhombic lattice structure coexist in certain localized regions of the film. Fig.
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A.4d presents an HRTEM image of a composite WO3/PPy sample. The interplanar spacing (0.39 nm), as shown by the arrows, corresponds to the (111) lattice plane of orthorhombic WO3-
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disordered nanoarrays. These results concur with the XRD, SEM, and SAED analysis results. Fig. A.5
The FTIR spectra of the three samples (WO3, PPy, and WO3/PPy) are represented in Fig. A.5. The FTIR spectra stratify a different mode of vibrations of atoms or molecules with
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associated energies in the infrared region. The stretching, bonding, and bending can be studied with FT–IR measurements. The crystalline structure of WO3 formed of WO6 octahedra, exaggerates oxygen connectivity as W–O–W [30]. The observed vibration bands are
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fundamental vibrations of W - O, W = O and W–O–W chromophores. For molecular orientation and structure determination, most pertinent modes are in-plane bending vibrations (δ), stretching
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vibrations (ν) and out of plane deformation vibrations (γ), as enlisted in Table A.1. The shift in the stretching vibration of hybrid WO3/PPy endorses homogeneous distribution of polymeric chain in WO3 owing to the Vander Walls interaction [31-33]. The disappearance of the bands at 1360 and 1098 cm-1 signifies a decrease in the degree of azide bondage and carbonyl groups [37]. Table A.1
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Fig. A.6 Band gap values (Fig. A.6) were calculated using equation αhν=A(hν−EG)2, where EG (eV) is the band gap energy of the semiconductor, A is a constant, h is Planck's constant, and ν is
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the frequency of light [38]. For B-WO3 and PPy (Fig. A. 6a and 6b) samples, we observed band gaps in the range 3.01 and 2.79 eV, respectively [39]. However, for B-WO3/PPy composite (Fig. A. 6c), the band gap (2.88 eV) is intermediary with WO3 and PPy. A change in the optical band
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gap of the WO3/PPy sample compared with that of WO3 can be directly linked to the transition in the symmetrical phase [40]. In addition, the measurement of the band gap shows the quantum
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confinement effect owing to change in the optical band gap by reducing the grain size [41]. In this case, in comparison with the quantum confinement effect, the phase transition was a more dominant factor in varying the optical band gap [42]. 3.2 Electrochromic Study
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Fig.A.7
Coloration and bleaching of B-WO3, PPy and B-WO3/PPy thin films was carried out by applying a potential step of ±1 V for a fixed time. The transmittance spectra were recorded for all
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samples in their colored and bleached states in the wavelength range of 350 to 800 nm. The optical absorption of the thin layer is described by a dimensionless quantity, called optical
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density (OD). The change in OD (∆OD) of B-WO3, PPy, and B-WO3/PPy thin films at different wavelengths, in their colored and bleached states, was determined using the following equation [44];
∆OD = log (Tb/Tc), ………..(1)
where, Tb and Tc are the transmittances of thin films in bleached and colored states, respectively. Fig. A7 elucidates the graph of OD variation for different wavelengths, demonstrating a boost in ∆OD of WO3/PPy hybrid than their sole performance.
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Fig. A.8 (a, b, c) Furthermore, ECS of B-WO3, PPy, and B-WO3/PPy was monitored in LiClO4-PC electrolyte, as displayed in Figs. A- 8, respectively. Although the B-WO3 film evinces excellent
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cyclic stability of approximately 20,000 c/b cycles without any clear amendment in the shape of the CV curve (Fig.A.8a), after the 500th c/b cycle, the PPy film showed poor EC stability with decrease in CV current density (Fig. A.8b). Subsequently, the PPy film etched away from ITO
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and did not show any electrochemical property. Fig. A.8c reveals the EC stability for BWO3/PPy that summarizes the stability of approximately 20,000th cycles without considerable
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variation in the shape of the CV curve. This indicates that PPy is not electroactive in the hybrid configuration, considering that WO3 is an N-type semiconductor, and behaves as an insulator in the anodic potential regime. The voltammograms are almost identical to those shown for the bare WO3, and remains an unaltered EC stability of WO3. The improved electrochromic properties
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may be due to stiff adhesion of the WO3 layer filled with pores of ITO nanoparticles and the intimate contact between ITO/B-WO3/PPy films that reduces the ohmic drop of the PPy film [43]. This result concurs with XRD results, in which the size of particulate that decreases
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intermolecular distance, and ultimately increases the tunneling current is of importance. This is because of the dominant role of WO3 in PPy. Thus, excellent cycling stability and improved
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electrochromic performance made WO3/PPy composite an efficient candidate for electrochromic devices.
Conclusion:
The WO3/PPy nanocomposite films were prepared by electrodeposition of WO3 coated by PPy onto an ITO glass substrate. The structural and morphological study entails a decrease in the crystallite size and increase in the interplanar spacing of WO3/PPy with disordered morphology.
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The disordered morphology of WO3/PPy offers excellent electrochemical stability as high as 20000 c/b cycles with improved OD. Thus, the WO3/PPy nanocomposite film demonstrates a promising potential for electrochromic devices.
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Acknowledgement:
The author acknowledges thanks to financial support from DST-SERB, New Delhi for major sponsoring of the project through grant no. PS/030/2013 and D.Y. Patil College of Engineering
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Table A.1 Different vibrational modes as revealed from FT–IR spectra for WO3, PPy and WO3/PPy samples Wave numbers (cm-1)
Group
No.
Assignment
WO3
PPy
WO3/PPy
W-OH, H2O,
3459
-------
3566
2.
N-H
--------
3487
3566
3.
C-H
---------
2980,
2935
2810
2690
W-OH
1589, 1049
υN-H
35
υC-H
36
1550,
δW-OH
34
1010
1403
υC-C, υC-N
19
-------
1403
υOH, δOH
34
1360
-------
γN-H
36
1194
1215
υN–C
36
1098
-------
γN+H2
19
942
-------
884
υ W-O
34
---------
970, 834,
884, 734,
γC–C, γC–H
36
715, 590
570
-------
734,
υ W–O–W
34
--------
6.
OH, W–O
1440
7.
N-H
--------
8.
N–C
--------
9.
N-H
--------
9.
W-O
10.
C–C, C–H
TE D
C-C, C-N
786,
EP
W–O–W
34
1460
5.
670
AC C
11.
-------
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4.
υOH
SC
1.
Ref.
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Sr.
681
AC C
EP
TE D
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SC
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Fig. A.1
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SC
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AC C
EP
TE D
Fig. A.2
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SC
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AC C
EP
TE D
Fig. A.3
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Fig. A.4
AC C
EP
TE D
Fig. A.5
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SC
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AC C
EP
TE D
Fig. A.6
AC C
EP
TE D
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Fig. A.7
AC C
EP
Fig. A.8
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Figure captions: Fig. A.1 XRD patterns for (a) as deposited WO3 film before annealing and after annealing, (b)
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PPy and WO3.1/3H2O/PPy composite
Fig. A.2 (a,b) Scanning Electron Microscope (SEM) images of WO3 (after annealing), (c) SAED
SC
pattern of the monoclinic crystal structure., (d) HR-TEM image of the annealed WO3 sample.
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Fig. A.3 (a,b) Scanning Electron Microscope (SEM) images of PPy sample, (c) SAED pattern of the PPy., (d) HR-TEM image of the PPy sample.
Fig. A.4 (a,b) Scanning Electron Microscope (SEM) images, (c) SAED pattern of the
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orthorhombic crystal structure., (d) HR-TEM image of the WO3/PPy composite.
EP
Fig. A.5 FT-IR transmittance spectra for (a)WO3, (b) PPy, (c) WO3/PPy
Fig. A.6 (αhυ)1/2 is plotted as a function of hυ from which band gap energy is obtained for
AC C
(a)WO3, (b) PPy, (c) WO3/PPy
Fig. A.7 (a) stability of B-WO3 for 1st and 20000th cycle, (b) stability of PPy for 1st and 500th cycle, (c) stability of B-WO3/PPy for 1st and 20000th cycle, measured in 0.5M LiClO4-PC electrolyte
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Fig.A.8 Variation of change in optical density (∆OD) vs wavelength for B-WO3, PPy, B-
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WO3/PPy samples
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Highlights 1. Nanoarrayed WO3/PPy composite was synthesized.
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2. Interplanar spacing enhances due to PPy coating. 3. PPy applies shearing force on WO3 produces disordered morphology.
AC C
EP
TE D
M AN U
SC
4. Nanocomposite showed high stability in LiClO4-PC.
