High performing smart electrochromic device based on honeycomb nanostructured h-WO3 thin films: hydrothermal assisted synthesis.

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High performing smart electrochromic device based on honeycomb nanostructured h-WO3 thin films: Hydrothermal assisted synthesis Vijay. V. Kondalkara, Sawanta. S. Malib,d, Rohini. R. Kharadea, Kishorkumar. V. Khota, Pallavi. B. Patila, Rahul. M. Manea, Sipra. Choudhuryc, Pramod. S. Patilb, Chang. K. Hongd, Jin. H. Kime, Popatrao. N. a 5 Bhosale * Received (in XXX, XXX) XthXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x

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Herewith, we report the honeycomb nanostructured single crystalline hexagonal WO3 (h-WO3) thin films in order to improve the electrochromic performance. In the present investigation, honeycomb nanostructured WO3 with different unit size and nanowire array with highly nanocrystalline frameworks have been synthesized via hydrothermal technique. The influence of hydrothermal reaction time on honeycomb unit cells, crystallite size, lithium ion diffusion coefficient and switching time for coloration/bleaching were studied systematically. The electrochromic study reveals that honeycomb unit cell size has a significant impact on the electrochromic performance. Small unit cells in honeycomb lead to large optical modulation and fast switching response. A large optical modulation in visible spectral region (60.74% at λ = 630 nm) at potential of -1.2 V with faster switching time (4.29 s for coloration and 3.38 s for bleaching) and high coloration efficiency (87.23 cm2 C-1) is observed in the honeycomb WO3 thin films respect to unit cell diameter of 1.7 µm. The variation in color on reduction of WO3 with applied potential has been plotted on xy-chromaticity diagram and color space coordinate shows the transition from colorless to deep blue state.

1. Introduction

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As an imperative fundamental material, tungsten oxide (WO3) has attracted great interest due to its exciting physicochemical properties and potential wide ranging applications.1 Currently, nanostructured WO3 have been widely used in a variety of fields such as electrochromic,2 photochromic devices,3 4 5 6 thermochromism, photocatalysis, gas sensors, lithium ion batteries7 and solar cells.8 The electrochromic devices (ECD) involves the intercalation/deintercalation of ions in/out of EC materials. ECD is able for reversible and persistently changing their optical properties on applying of external voltage. Moreover, WO3 based ECD exhibits low power consumption, high contrast ratio, good memory effects and chemical stability.9,10 The most essential factor affecting to the ECDs based on transition metal oxides are their coloration efficiency, optical modulation and response time.11 For conventional electrochromic devices, the EC layer is usually fabricated with nanocrystalline dense WO3 thin films in which only surface region of the materials can effectively contact with diffusion ion leading to poor utilization of the EC materials. Moreover, ion diffusion into the interior of dense electrochromic materials is sluggish results in poor EC performance and long response time for coloration/bleaching.12-14To date, number of methods have been launched to fabricate nanostructured WO3 thin films including spray pyrolysis,15 Langmuir-Blodgett,16 sputtering,17 thermal This journal is © The Royal Society of Chemistry [year]

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evaporation,18 hot-wire chemical vapour (HW-CVD),19 template mediated synthesis,20 sol-gel,21 electrodeposition11 and layer by layer deposition etc.22 Moreover, each of these methods has one or more characteristic drawbacks such as required exotic and dangerous reagents, being vacuum dependent, highly energetic and some of these difficult to commercialize. The intension of hydrothermal synthesis is a promising approach for the fabrication of low dimensional hierarchical structured WO3 thin films. The hydrothermal route is facile, has control over the size, requires low reaction temperature, economic for large scale production of large surface area and unique morphology.23,24 The hierarchical structures made up of low dimensional building blocks such as zero dimensional (0D) nanoparticles, one dimensional (1D) nanorods/nanowires and two dimensional (2D) nanoplates/nanosheets offers potential optoelectronic properties to facilitate electrochromic performance. Such architecture exhibits large surface area capillary pathway for ion intercalation and deintercalation.25-27Thus to engineer a nanostructure with unique surface morphology with proper crystal structures is key point to improve electrochromic properties. The large optical modulation and fast switching kinetics could be expected from low dimensional materials.28 By taking into account the practical applications of hierarchical structured WO3 thin films in the EC devices, many researchers have been inspired to synthesize WO3 [journal], [year], [vol], 00–00 |1

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thin films through hydrothermal approach. Yella et al. successfully synthesized three dimensional WO3 brushes and urchin like structures using solvothermal technique.29 A vertically aligned hierarchical WO3 nanosheet array grown via hydrothermal method.30 Zhang et al. investigated EC properties of hydrothermally synthesized WO3 nanotree composed of nanosheet shaped branches.31 However, it is still challenging to synthesis of WO3 thin films into desired hierarchical 1D, 2D and 3D nanostructures with different facets control and proper crystal structure under mild experimental conditions. In this context, we report simple approach to produce (002) facet oriented hexagonal honeycomb like nanostructured WO3 thin films on the seed layer FTO substrate assisted by ammonium sulphate (NH4)2SO4 as capping agent. Formation mechanism of honeycomb unit cell to nanowire WO3 thin films is discussed in details. Open void of honeycomb unit cell contains intertwined nanowires which reduce the diffusion path length of Li+ ions and large tunnels in hexagonal WO3 lead to a large diffusion coefficient. The electrochromic properties were analyzed using cyclic voltammetry and CIE 1931 system for color analysis.

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2. Experimental 2.1 Materials

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All the chemicals wereof analytical reagent (AR) grade and used without further purification; sodium tungstate dihydrate (Na2WO4.2H2O) (99%), hydrochloric acid (HCl) (36%), hydrogen peroxide (H2O2) (30%) and ammonium sulphate ((NH4)2SO4) (99%) purchased from SD-fine chem. Ltd. Lithium perchlorate (LiClO4) (Aldrich, 98%) in propylene carbonate (PC) (Spectrochem, 99%) was used as diffusion ion.

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3. Results and Discussion 85

2.2 Synthesis of h-WO3 honeycomb and nanowire arraythin films WO3 seed layer was deposited on FTO coated glass substrate by spin coating according to our previous report.32Now, hierarchical honeycomb nanostructured WO3 thin film with different unit cell and nanowire arrays were fabricated by ammonium sulphate assisted hydrothermal method. Briefly, sodium tungstate dihydrate (Na2WO4.2H2O, 1.32 g) was dissolved in 40 ml deionised water. The solution was acidified to 2 pH with HCl (3 M). Afterwards, ammonium sulphate (1.0 g) was added to the mixture to control the crystal structure and morphology of the WO3 thin films. The resulting solution was stirred for 15 min then transferred into a 50 ml Teflon lined stainless steel autoclave containing seed layer FTO substrates. The hydrothermal reaction was carried out at 180°C temperature for 4, 6, 8 and 10 h which are designated as H4, H6, H8 and H10 respectively. There after completion of reaction, autoclave was cooled to room temperature. The deposited thin films of WO3 were rinsed with deionised water, dried at room temperature and used as it is for further characterization.

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2.3 Characterizations

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The hydrothermally synthesized WO3 thin films were characterized for their optical, structural, morphological and elemental properties. The optical absorption/transmittance measurement was performed in the wavelength range of 3001100 nm by using a UV-visible spectrophotometer (Shimadzu 2|Journal Name, [year], [vol], 00–00

UV-1800). X-ray diffraction (XRD) patterns were obtained by using X-ray diffractometer (Bruker AXS Model D8 Advance) in the rangeof 20-80° with Cu Kα target having wavelength 1.54 Å. FT-Raman spectra of WO3 thin film were conducted on a Raman spectrophotometer (Bruker MultiRAM, Germany Make) in spectral range of 100-1500 cm-1. Surface morphology of the films was characterized using scanning electron microscopy (JEOLJSM, Japan) at an accelerating voltage 20 kV and transmission electron microscopy (TEM/HRTEM, TECNAI F20 Philips). The particle size determination and lattice structure was investigated by high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) operating at accelerating voltage at 200 kV. The chemical composition and valence states of constituent elements were analyzed by X-ray photoelectron spectroscopy (XPS) (VG Multilab 2000-Thermo Scientific, USA, K-Alpha) with multi-channel detector, which can suffer high photonic energies from 0.1 to 3 KeV. The electrochemical measurements were performed in 0.5 M LiClO4 in PC electrolyte using conventional three electrode system comprising a WO3 as working electrode, Platinum as counter electrode and Ag/AgCl preferred as reference electrode using Autolab PGSTAT100 FRA 32. The device fabrication is given in electronic supplementary information Fig. S1. To obtain the chromaticity coordinate values, colorimetric measurement were carried out by analysing the transmittance spectra using Shimadzu color analysis software equipped with UV-visible spectrophotometer.

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3.1 Formation mechanism of Honeycomband nanowire WO3 thin film Briefly, the formation mechanism and morphological evolution of WO3 thin film with reaction time is illustrated in scheme 1. In early stage of hydrothermal condition, primary WO42nanoparticles were formed through a typical Ostwald ripening process. The tungstate ions have number different species namely WO42-, H2WO42-, H2WO4, W6O46-, W6O216-, under different pH condition where WO42- species are dominant at pH 2.33 At the first, small crystalline nuclei formed and then allowed to grow crystal. The large particles grew at the expense of smaller particles due to solubility of smaller particles. In case, small amount of ammonium sulphate, SO42- play important role. SO42group are preferentially adsorbed on those faces parallel to the caxis of WO3 nanocrystal to minimize the surface energy of that faces.34 The faces along to c-axis have high surface energy and strong tendency to capture monomers from the reaction bath to reduce their surface energy. This cause to growth of nanocrystal along that plane to form honeycomb structure. This nature varies with inherent crystal habit. Thus as the reaction time proceed, growth of nanoparticles assembled gradually evolved honeycomb structure with small unit size. Further prolonged reaction time honeycomb unit cell burst in randomly oriented nanowire array. Phase purity and crystallographic structure of WO3 thin films obtained at different reaction time were determined by X-ray diffraction (XRD) analysis. The XRD pattern of WO3 thin film samples H4, H6, H8 and H10 are shown in Fig. 1, major diffraction peakscan be indexed to (002), (110), (111) (102), (200), (112), (202), (300), (212), (004), (220), (222) (204), (400), (402), (224), This journal is © The Royal Society of Chemistry [year]


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Scheme 1 Schematic illustration of growth mechanism of nanostructured WO3 thin films

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(404) and (116) crystal planes respectively. All the diffraction peaks in the XRD patterns are well indexed to hexagonal crystal structure of WO3 which are in good agreement with standard data for WO3 (JCPDS no. 85-2460). The intensities of the diffraction peaks increased gradually with the increase of hydrothermal time from 4 to 10 h, which indicates improved crystalline structure. The hexagonal nanostructured WO3 give large tunnels for Li+ intercalation/de-intercalation which lead to a faster EC response.35 In addition, the considerable enhancement in intensity of (002) peak suggest that WO3 nanowire arrays grow along the c-axis with (002) direction, so the orientation of as-prepared WO3 nanowires is improved. The crystallite size (D) of all samples was calculated using the (002) diffraction peak of WO3 plane with known Scherrer’s formula (1) D =

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kλ β cos θ

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structure of WO3 was also confirmed by Raman spectra in given in Fig S2.

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(1)

where ‘k’ is dimensionless constant (0.94), ‘λ’ is wavelength of X-rays (1.5406 Å), ‘ߚ’ is corrected broadening of the diffraction line measured at half of its maximum intensity (FWHM), ‘ߠ’ is diffraction angle, ‘D’ is crystallite size. When hydrothermal time was increased from 4 to 8 h the crystallite size increases from 17 to 26 nm and further more or less constant for 10 h. Typically, the increasing crystallinity can noticeably reduce the charge transfer resistance during double injection/ejection of ions and electrons cause to improved electrical conductivity to speed up electrochromic reaction.36 Thus sample H8 can enhance the electrochromic performance by increasing the crystallinity. These results reveal that hydrothermal time was the key factor for controlling crystallite size of WO3. The hexagonal crystal This journal is © The Royal Society of Chemistry [year]

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Fig. 1 X-Ray diffraction patterns of nanostructured WO3thin films 70

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Fig. 2 illustrates surface morphological change of WO3 thin films through a different hydrothermal reaction time examined by Scanning electron microscopy (SEM) micrographs at different magnification. The cross section SEM images of sample H4 to H10 are given in Fig S3. It was observed that for h-WO3 film deposited at 4 h, the periodic honeycomb like nanostructure is formed with nanoscale wall throughout the entire surface (Fig. 2a). The close observation of Fig. 2b clearly reveals that these Journal Name, [year], [vol], 00–00 |3



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Fig. 2 SEM images of WO3 thin films (a, b) H4, (c, d) H6, (e, f) H8 and (g, h) H10 at low and high magnification

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Fig. 3 TEM micrographs of nanostructured WO3 H8 sample (a) (b) TEM images at different magnifications, (c) HRTEM image and (d) SAED pattern. honeycomb unit cell contains smaller aggregated nanowires may honeycomb unit cells having diameter 6 µm possess a 75 be easier to dissolve and disappear due to relatively high specific hierarchical structure with a large number of 1D nanowires. surface energy with increasing reaction time. The aggregation of When the reaction time increased to 6 h, it was observed that nanowires due to irregularly grown nanowires throughout surface diameter of honeycomb unit cells decrease up to 4 µm as shown substrate (Fig. 2h). Active surface area of nanowires may not be in Fig. 2c. Higher magnification image (Fig. 2d) reveals the effectively utilize to reduce polarization and the compact stacking nanowires start to interwoven to minimize honeycombs unit cell. 80 of nanowires which increase the opacity may affect the Li+ When hydrothermal reaction is carried out at 8 h (Fig. 2e), the intercalation/de-intercalation, coloration efficiency and response honeycomb unit cell diameter decreases up to 1.7µm and high time. magnification image (Fig. 2f) shows formation of nanowires More details about the morphological and structural features of intertwined with one another to form a braided structure in WO3 nanowires in honeycomb unit cell were studied by TEM and honeycomb unit cell. The sharp edges of unit cell shrink the 85 HRTEM as shown in Fig. 3.TEM image of H8 sample (Fig. 3a) polarization of electrode. On application of potential, reveals that nanowires intertwined with one another to form a comparatively high electric field occurs at these sharp edges of braided structure which is well consistent with SEM results. The unit cell, resulting in relatively low electric field in the inner single h-WO3 nanowire shown in Fig. 3b. Average length of voids of unit cell and releasing polarization. By taking into nanowire is~600 nm and diameter is 26 nm. HRTEM images of consideration the fact that surface area of the edge is small 90 WO3 nanowires are displayed in Fig. 3c. It demonstrates that portion of total surface area, sample H8 minimize polarization in lattice fringes spacing is about 0.384 nm, which is well indexed electrode and facilitates Li+ insertion/ejection processes.37After to the (002) plane of h-WO3 crystal structure. The HRTEM image prolonged reaction time (10 h), it was observed that honeycomb indicates that growth of primary WO3 nanoparticles along the unit cell collapse and formation of nanowires take place shown in (002) direction of hexagonal WO3 unit cell to form nanowire Fig. 2g.The change in honeycomb to nanowires WO3 may be 95 array. Formation of WO3 nanowires involves the SO42- ion acting attributed due to hydrothermal reaction time which achieves as capping agent which selectively adsorb on parallel c-axis of dissolution-deposition balance beyond the 8 h. Former larger crystallographic facets of WO3 nanocrystal and slow down the nanowire within unit cell grows continuously, where edge of growth rate along that direction resulting in progressive formation


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of c-axis oriented nanowires.38Selected area electron diffraction (SAED) pattern shows (Fig. 3d) that nanowires is a single hexagonal crystal that grows along the (002) direction, which is in good agreement with the results of XRD data. The surface composition and chemical states of WO3 thin films were confirmed by X-ray photoelectron spectroscopy (XPS). The typical survey spectrum of sample H8 is shown in Fig. 4a.

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which was assign to adventitious carbon. Fig. 4b shows the W4f core level XPS spectrum of sample H8.The W4f core level fitted to two spin-orbit doublets at binding energy 35.55 and 37.64 eV for W4f7/2 and W4f5/2, respectively. The splitting between two core levels i.e. W4f7/2 and W4f5/2 is at interval of about 2.09 eV indicates 6+ valence state of W.39High-resolution XPS spectrum of O1s (Fig. 4c) can be deconvoluted to yield three gaussian curves. The O1s spectrum shows a dominant component centred at 530.13 eV, which can be attributed to the crystal lattice oxygen in manner W-O-W. The O1s peak at 531.56 eV is due to small amount hydroxyl group (OH−) on the WO3 surface. The third peak centered at 532.92 eV corresponds to oxygen of atmospheric water molecule adsorbed on the surface.40-41 UV-visible absorption spectra of WO3 samples H4, H6, H8 and H10 at different hydrothermal time were measured in the wavelength range 3001100 shown in Fig. 5a. The absorption edges of these films were found to be 430 nm and the absorption wavelength of samples in range of 350–500 nm increased as the hydrothermal reaction time increased from 4 to 10 h, which was mainly due to the enhancement in crystallinity of samples. The increase in optical absorption was ascribed to great change in surface morphology. In order to confirm band gap energy of WO3 samples, the optical data analyzed using classical absorption equation. (2)

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αhν = a(hν − Eg)n

(2) where, ‘a’ is constant, ‘Eg’ is band gap, ‘h߭’ is photon energy, ‘α’ is absorption coefficientand exponent ‘n’ depends on type of transition, for direct allowed transition, n=1/2 and for indirect allowed transition, n=2. Band gap energy of all samples were estimated from the Tauc plots i.e. variation of (αhυ)1/2 vs photon energy (h߭). The optical band gap value was obtained by extrapolating portion to photon energy axis to zero absorption coefficient (α=0). Band gap value of WO3 sample varies from 2.96, 2.89, 2.85 and 2.81 eV for the sample H4, H6 H8 and H10 respectively. The slight variation in band gap depends on the degree of crystallinity and different orientation of prepared WO3 honeycomb nanostructure on FTO substrates.42 3.2 Electrochemical studies

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Fig. 4 The XPS spectrum of H8 sample (a) Survey spectrum of WO3, (b) W4f core level spectrum and (c) O1s core level spectrum WO3 surface was composed of W, O and trace amount of C 6|Journal Name, [year], [vol], 00–00

The cathodic and anodic behaviour of electrochromic WO3 thin films was evident from CV measurement. Fig. 5b shows CVs of H4, H6, H8 and H10 samples at a sweep rate 50 mV/s in potential range ±1.2 V (vs Ag/AgCl). When potential was stepped towards -1.2 V, the co-intercalation of Li+ ions and equal quantity of electrons into WO3 films leading to formation of blue colored lithium tungsten bronze (LixWO3). Conversely, when polarity of applied voltage is reversed, Li+ ions and electrons de-intercalate at which lithium tungsten bronzes (LixWO3) oxidized into original WO3 form. The Li+ ion intercalation/de-intercalation and redox reaction between W5+ and W6+ is ascribed by the following equation. (3)

WO 3 + xLi

+ xe − ⇔ Li xWO 3

(3) The effective diffusion coefficient (D) of Li+ ions was calculated using well known Randles-Servick’s equation. (4)

D = 115

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2.72×10 × n

p 3/ 2

(4)

× A × C0 ×υ1/ 2



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Fig. 5(a) absorption spectra of WO3 thin films, Inset: plot of (αh߭)1/2 vs. hυ, (b) CVs of WO3 thin films at 50 mV s-1 scan rate. (c) CVs of sample H8 at different scan rate, (d) CVs of sample H8 at different applied voltage. (e) CA response of WO3 thin films, (f) CC trace of WO3 thin films where, ‘D’ is diffusion coefficient, ‘ip’ is peak current density, cathodic peak current increases with acceleration of the sweep rate with slight shift in the anodic peak current. On the other hand ‘n’ is number of electron, ‘A’ is area of the film, ‘Co’ is concentration of active ion in electrolyte, ‘υ’ is scan rate. 65 in case of the sample H4, H6 and H10, there is a large shift in anodic current density shown in Fig S4. Moreover, the ratio Values of diffusion coefficient calculated for the Li+ ion in WO3 between cathodic and anodic peak current is less than unity. The films are listed in Table 1. Diffusion coefficient increases from 1.557×10-9to 3.091×10-9 cm2 s-1 as the size of honeycomb unit cell CV result reveals the honeycomb nanostructure WO3 with unit decreases from the sample H4 to H10. No significant increment in cell 1.7µm (H8) experiences fast insertion kinetics compared to diffusion coefficient of sample H10 is observed as compared to 70 other samples. sample H8. The EC performance depends on the diffusion Furthermore CV recorded for the sample H8 at scan rate 50 mV/s coefficient and diffusion length of Li+ ion in WO3 matrix. The between the different applied potential range 1.2 to -0.2, -0.4, CV plot reveals that the sample H8 shows large amount of Li+ 0.6, -0.8, -1.0, -1.2, -1.4 and -1.6 V are shown in Fig. 5d. The ions intercalation because honeycomb like surface morphology nature of curve is a typical redox reaction in EC WO3 film to with well defined unit cells provide the large surface area for Li+ 75 cause large contrast in coloration and bleaching. When potential ion intercalation. The intertwined nanowires to form the braided is stepped from +1.2 to -0.2 V, the cathodic current density of structure in honeycomb unit cell keeps control over the diffusion WO3 film is 0.194 mA cm-2 and achieved a value as high as 3.629 path length. Onset potential of cathodic peak current for sample mA cm-2 when the potential was reached -1.6 V. The increase in H8 shifted towards the positive potential which indicates reduced cathodic current density reveals complete reduction of W6+ ionic interfacial charge transfer resistance that is insertion of Li+ ions 80 state to W5+ state. Values of diffusion coefficient calculated with can be achieved at a considerable low applied voltage as respect to applied potential are given in Table 2. The diffusion compared to other samples.43,44 During bleaching process, current coefficient found to be increase from 7.344×10-11 to 3.124×10-9 of anodic peak increases steady this reveals higher amount of cm2 s-1. + electron and Li ions de-intercalate during reverse potential The switching kinetics with which ECD can be turn from one sweep. These results reveal that sample H8 is highly capable for 85 state to another state is of important for the practical application. accommodating more charge which is favourable for EC devices. Fig. 5c displays CV curves for H8 sample at different sweep rates from 20 mV/s to 100 mV/s. It was observed that anodic and





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Table1. Electrochromic parameters of nanostructured WO3 thin film

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Transmittance T (%)

Optical density ∆OD

Reversibilit y (%)

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Optical Modulatio n (%) 23.76

0.315

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0.468

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-0.0386

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-0.0435

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0.909

Charge inserted Qi (C cm-2 )

Bleached

Colored

H4

-0.0219

87.81

H6

-0.0311

H8 H10

Additional switching times of coloration/bleaching are related to charge transfer rate at WO3/electrolyte interface, diffusion coefficient of Li+ ion and diffusion path length of active sites. A typical CA trace obtained during coloration and bleaching of WO3 samples are shown in Fig. 5e. The switching time is defined as the time required for 90% change in optical transmittance between colored and bleached state. For sample H4, H6, H8 and H10 calculated switching time from current transient plot for coloration state (tc) and bleaching state (tb) are listed in Table 1. Sample H8 reveals the fastest coloration and bleaching time 4.29 and 3.38 s respectively. The response time of H8 sample is faster than the other samples H4, H6 and H10. The fast switching speed of sample H8 is attributed to the synergistic contribution from the enhanced crystallinity and rough surface formed by the interwoven nanowires is advantageous for Li+ intercalation/deintercalation. This is because they create more coarse surface for lithium to quickly movement and superior electronic conductivity, which is well agree with the results of CV in which cathodic current response for sample H8 shifted towards the positive potential.38 Bleaching kinetics is always faster than coloration kinetics because in bleaching kinetics rapid current decay is due to the conductor (LixWO3) to semiconductor (WO3) transition while during coloring kinetics WO3 films has higher resistance during WO3 to LixWO3 transition.45 In order to investigate the dependence of dynamic range of reversibility of WO3 sample, CC study was carried out at ± 1.2 V for the step of 15 s. Fig. 5f shows the plot of charge vs transient time. From CC curve charge intercalated for all the samples are listed in Table 1. The reversibility of films was calculated as the ratio of charge de-intercalated (Qdi) to charge intercalated (Qi) for coloration/bleaching after 15 s. The electrochromic reversibility for all the samples is listed in Table 1. The sample H8 shows 91.02% reversibility. It is found that the reversibility increase from sample H4 to H8 because it offers well crystalline structure and honeycomb like surface morphology in which unit cell becomes smaller but depth increases from sample H4 to H8 which offers large surface area for controlled Li+ ion diffusion. For sample H10 morphology is turned to nanowires which cannot effectively utilize the surface area and the inter voids in nanowires may not that much control over the diffusion coefficient and path length.


Response Time (s) tc tb

Coloration efficiency( cm2 C-1)

3.82

Diffusion coefficien t(cm2 s-1) 10-9 1.557

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2.945

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3.091

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3.3 Stability study

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Electrochemical stability of all samples was determined by CV between potential ±1.0 V at 100 mV/s from 1 to 1000 cycles as shown in Fig. 6. The cycling stability depends on state of crystallinity and surface morphology. The current density decreases slowly from the first cycles to last cycles for sample H4, H6 and H10. In case of sample H8, there is no change in current density and shape of CVs, only a slight reduction in oxidation end is observed. The significant drop in current density for sample H4, H6 is observed because of low crystallinity and Li+ ion trapping into large size honeycomb unit cell. While in case sample H10, nanowire porous morphology may dip trapping of Li+ ion into the surface but sample H8 has small honeycomb unit cell may be shadow and/or control over the Li+ ion trapping into surface. This indicates the high stability of sample H8.

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Fig. 6 CVs cycling stability of WO3 sample (a) H4, (b) H6, (c) H8 and (d) H10 3.4 Iono-optical studies 75

The change in optical transmittance of WO3 thin films are analyzed by comparing UV-visible transmittance spectra in colored and bleached state in the wavelength range from 300 to 1100 nm at potential ±1.2 V. Fig. 7 reveals that the H4, H6 and This journal is © The Royal Society of Chemistry [year]



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H8samples in bleached state show the significant transmittance with transmittance value larger than 70% at wavelength 630 nm. In contrast the sample H10 has low transmittance (42%) and opacity which limits its use for EC optical modulation. When the WO3 samples cathodically polarized, they show uniform blue color. The changes in optical modulations for allWO3 samples are

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listed in Table 1.The optical modulation increases up to60.74% for sample H8 at wavelength 630 nm. The optical modulation of sample H8 is high as compared to other samples in this work (Table 1). WO3 thin films with honeycomb structure in which unit cells of honeycomb is intertwined with nanowires increase textural boundaries therefore the Li+ ion in the electrolyte can migrate within WO3 host matrix very easily and in controlled manner. The high optical modulation is comparable with that of WO3 nanorods flower, nanowire array obtained by hydrothermal process.35 Fig. 8 shows optical transmittance of H8 thin film at different applied potential of -0.2, -0.4, -0.6, -0.8, -1.0, -1.2, -1.4 and -1.6 V.

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Fig. 7 Optical transmission spectra of colored and bleached states of sample (a) H4, (b) H6, (c) H8 and (d) H10 65

Fig. 8 Optical transmission spectra showing colored and bleached states of sample H8 with respect to applied potential

Table 2. Electrochromic parameters of honeycomb nanostructured WO3 sample H8 at different applied voltage. 70

0.1313

Diffusion Coefficient (cm2 s-1) 7.344×10-11

Coloration efficiency (cm2 C-1) 7.83

63.68

0.221

2.300×10-10

13.185

0.945

42.33

0629

4.655×10-10

37.57

-0.8

1.526

35.39

0.8088

8.714×10-10

48.257

-1.0

2.444

27.85

1.0484

1.361×10-9

62.55

-1.2

3.016

18.95

1.433

2.945×10-9

87.23

-1.4

3.629

16.93

1.546

2.979×10-9

92.24

-1.6

4.218

12.72

1.832

3.124×10-9

109.30

Applied Voltage (V) -0.2

Cathodic Current Density (mA cm-2) 0.1944

Transmittance (%) Colored

Optical density ∆ODi

69.90

-0.4

0.528

-0.6




DOI: 10.1039/C4DT02953D

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The transmittance of WO3 thin film in bleached state is 78%. When the cathodic potential increased, it intensifies uniform blue color and finally changes optical transmittance immensely to 10.4% at -1.6 V. The change in transmittance with respect to applied potential is given in Table 2 and photograph of devices at different applied potential is shown in Fig. 9. The optical transmittance modulation of H8 sample was 68% at -1.6 V. Obvious color changes indicate honeycomb nanostructure of WO3 with 1.7 µm unit cell has potential application.

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Fig. 9 Photographs of electrochromic device of sample H8 at different applied potential. Coloration efficiency (CE) is one of most important parameter for comparing different electrochromic materials. CE represent change in optical density as function of injection/ejection of electronic charge i.e. the amount of charge required for change in optical modulation. It can be calculated from the following equation. (5)

CE = 35

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(∆ODi ) = ln (Tb Qi

Tc ) q A

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(5)

where ‘∆ODi’ is optical density measured at wavelength 630 nm, ‘Qi’ is amount of charge intercalatedinto sample to cause change in optical density. ‘A’ is area of the film, ‘q’ is charge, ‘Tb’ and ‘Tc’ transmittance in bleached and colored state respectively. CE calculated for H4, H6, H8 and H10 samples are given in Table 1. CE value for sample H8 was 87.23 cm2/C, which is much higher than different morphologies; macroporous WO3 array (68 cm2 C-1),46 nanopores (58 cm2 C-1 at 633 nm),47 nanorods (42 cm2 C-1at 633nm),48hierarchical nanotress (43.6 cm2 C-1 at 500 nm)31, solvothermal deposited nanowire (53.5 cm2 C-1)49, electrophoretic synthesized nanowire (70 cm2 C-1)50 and hydrothermal synthesized nanowire (46.2 cm2 C-1)51, solvothermal synthesized nanowire assembly by Langmuir Blodget method(46.2 cm2 C1 52 ). Sothe result reveals that the improvement in the CE value for sample H8 is attributed to more open space among the other honeycomb packed unit cell and nanowires. They have good contact with substrate which reduces interfacial charge transfer resistance. The CE also calculated for sample H8 with respect to different applied potential are given in Table 2. In order to understand the effect of hydrothermal reaction time on electrochemical behaviour of WO3 thin films, Electrochemical Impedance Spectroscopy (EIS) of sample H4 to H10 was

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Fig. 10 EIS spectra of WO3 thin films deposited at different hydrothermal reaction time. Inset shows its fitted equivalent circuit 3.5 Chromaticity

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measured. Fig. 10 shows Nyquist plots of WO3 thin films under ac bias. The Nyquist plot display three region corresponding to high, intermediate and low frequencies. First arc in high frequency region is attributed to impedance related to series resistance (Rs). The intermediate frequency region dominated by charge transfer resistance (Rct) and low frequency region signifies line of Warburg diffusion element (Zw). The magnitude of RS, Rct and Zw were calculated by data fitting into a Randles circuit model presented in inset Fig. 10. The value of RS, Rct and Zw for all samples were listed in Table S1.The Randles circuit model consist of Rs of system (i.e electrolyte/substrate resistance), Rct is (resulted from interfacial redox reaction resistance) connected in parallel with an electrical double layer capacitance (Cdl) at electrolyte/electrode interface and finally Warburg diffusion element (Zw) corresponds to ionic diffusion and charging of film.53 The decreasing value of RS, Rct and Wz were assigned to enhancement in charge transfer and Li+ ion diffusion for the H8 sample than sample H4, H6 and H10. Such enhancement in charge transport for H8 sample initiated from synergistic effect from enhanced crystallinity and intertwined nanowires within honeycomb unit cell keeps control over the diffusion path length. The increasing crystallinity can clearly reduce the charge transfer resistance during double injection/ejection of ions and electrons cause to improved electrical conductivity. Sample H4, H6 has low crystallinity possess poor electronic conductivity compared to sample H8 but even sample H10 has high crystallinity, it cannot reduce polarization at edge of surface.

Colorimetric analysis is a well known characterization technique in the field of electrochromism. It gives the quantitative measurements of the color. The color characteristics of the WO3 films can be described by the two dimensional CIE 1931 color space, created by the International Commission on Illumination (CIE) in 1931. Fig. 11a shows the dynamic changes of the CIE chromaticity curve of sample H8 at different applied potentials. Initially when applied potential (0 V) WO3 is in oxidized state, it is in transparent state and its position on the chromaticity curve is This journal is © The Royal Society of Chemistry [year]


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close to the white point and their a* and b* values are -1.91 and 0.38. When the cathodic potential increased from +1.2 to -1.6 V, the value of a* changes to 1.52 and b* attain more negative value to -38.2 reveals that the color of the WO3 films changes from transparent to uniformly deep blue color. In the CIE 1931 Yxy color space, the tristimulus value Y is a measure of the luminance or brightness of color. Fig. 11b displays the relative luminous transmittance with applied potential for sample H8. When the potential was sweeped from +1.2 to -1.6 V, relative luminance transmittance (%Y) changes in x-y coordinates from 79.46% (bleached) to 12.72% (colored) having luminous transmittance difference(∆Y) is 66.74%

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Shivaji University, Kolhapur, 416004, (M.S.), India b Thin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur, 416004, (M.S.), India c Chemistry Division, Bhabha Atomic Research Centre (BARC), Mumbai, India d School of Applied Chemical Engineering, Chonnam National University, Gwangju, 500-757 (South Korea), e Photonic and Electronic Thin Film Laboratory, Department of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, South Korea *Corresponding author E-mail address: [email protected], Tel.: +91 231 26099338, Fax: +91 231 2692333. Electronic Supplementary Information (ESI) available: EC device fabrication, Raman spectra, cross section SEM, CV at different scan rate and EIS data of WO3 thin films.

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Fig. 11 (a) CIE 1931 chromaticity, (b) luminous transmittance diagram forsample H8 at different applied voltage. 4. Conclusions Honeycomb nanostructured WO3 thin films have been successfully synthesized for high electrochromic performance by sulphate assisted hydrothermal method. Sulphate plays an important role in determining the morphology and control the directional growth. Compared to the nanowires, honeycomb nanostructured WO3 with unit cell 1.7 µm exhibits large optical modulation and high coloration efficiency. The improved electrochromic performance is mainly attributed to honeycomb unit cell composed of intertwined nanowires to form hierarchical braided structure which control ion insertion kinetics leading to enhanced charge transfer reaction. The nanostructured WO3 deposited at 8 h exhibits best electrochromic behaviour with 60.74% optical modulation, fast coloration/bleaching time of 4.29/3.38 s, high coloration efficiency (87.23 cm2 C-1) and stability over 1000 cycles. CIE chromaticity reveals the relatively high luminance difference of 66.74% at -1.6 V. These results will have a large impact on current EC technology and promising for potential application in energy saving smart window. Acknowledgment

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One of the authors Vijay V. Kondalkar wishes to acknowledge the DAE-BRNS Mumbai for financial support through DAEBRNS Major Research Project no.2012/34/51/BRNS/2036.This research was also supported by the basic Science Research program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-20090094055). Notes a Materials Research Laboratory, Department of Chemistry, This journal is © The Royal Society of Chemistry [year]

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