Accepted Manuscript Enhanced electrochromic performance of 2D grid-structured WO3 thin films
Zhiqiang Xie, Qianqian Zhang, Qingqing Liu, Jin Zhai, Xungang Diao PII: DOI: Reference:
S0040-6090(18)30188-3 doi:10.1016/j.tsf.2018.03.044 TSF 36548
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
28 September 2017 6 January 2018 15 March 2018
Please cite this article as: Zhiqiang Xie, Qianqian Zhang, Qingqing Liu, Jin Zhai, Xungang Diao , Enhanced electrochromic performance of 2D grid-structured WO3 thin films. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Tsf(2017), doi:10.1016/j.tsf.2018.03.044
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ACCEPTED MANUSCRIPT Enhanced electrochromic performance of 2D grid-structured WO3 thin films
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[Title Page]
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Enhanced electrochromic performance of 2D grid-structured WO3 thin films
Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry
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Zhiqiang Xiea,b, Qianqian Zhanga,b, Qingqing Liua, Jin Zhaia,* , Xungang Diaob,*
b
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of Education, School of Chemistry, Beihang University, Beijing 100191, PR China School of Physics and Nuclear Energy Engineering, Beihang University, Beijing
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100191, PR China
Correspondence information:
Jin Zhai; School of Chemistry, Beihang University, Beijing 100191, China. E-mail:
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[email protected]
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Xungang Diao; School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, PR China. E-mail:
[email protected]
*
Corresponding author. Jin Zhai; E-mail address:
[email protected] Xungang Diao; E-mail address:
[email protected]
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ACCEPTED MANUSCRIPT Enhanced electrochromic performance of 2D grid-structured WO3 thin films
Enhanced electrochromic performance of 2D grid-structured WO3 thin films
a
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Zhiqiang Xiea,b, Qianqian Zhanga,b, Qingqing Liua, Jin Zhaia,* , Xungang Diaob,*
Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry
School of Physics and Nuclear Energy Engineering, Beihang University, Beijing
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b
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of Education, School of Chemistry, Beihang University, Beijing 100191, PR China
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100191, PR China
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Abstract
Controlling the microstructure of Tungsten trioxide (WO3) film is believed to be an
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efficient strategy to enhance its electrochromic (EC) performance in recent years. Here,
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WO3 thin films with two-dimensional (2D) grid structure are fabricated through electrodeposition using polystyrene (PS) nanofibers as sacrificial templates. The surface of WO3 thin films have grooves formed by the removal of PS nanofibers and thus the
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films are divided into grid structure. Due to the larger specific surface area and lower
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diffusion impedance induced by the grid structure, these WO3 films demonstrate an obvious enhancement of EC performance compared with the plane WO3 thin films obtained using the same electrodeposition parameters in the absent of PS nanofibers templates. The transmittance modulation rate (ΔT) of the grid-structured WO3 thin films is over 50% larger than that of the plane WO3 films at the wavelength of 550 nm. Consequently, the grid-structured WO3 films have a higher coloration efficiency (71.8 *
Corresponding author. Jin Zhai; E-mail address:
[email protected] Xungang Diao; E-mail address:
[email protected]
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cm2/C) compared with the plane WO3 films (58.8 cm2/C). Furthermore, the coloration/bleaching time of grid-structured WO3 films are 1.27/0.93 s, which is much shorter than the plane WO3 films (2.34/2.18 s). The grid-structured WO3 films in this work show potential applications in the construction of highly performance and rapidly
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responsive EC devices.
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Keywords: 2D grid structure; WO3 thin films; electrochromic performance; controlling
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microstructure
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1. Introduction
Electrochromic (EC) materials are a sort of materials that have reversible changes in optical properties under voltage modulation, which show potential applications in smart
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windows, anti-dazzle car mirrors and information displays [1-4]. Among various EC materials, inorganic EC materials have many advantages such as fast response time,
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excellent chemical stability, low manufacture cost and easy industrialization, which are
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widely used in the development of high performance EC devices [5]. As the most widely used inorganic EC materials, Tungsten trioxide (WO3) have attracted significant
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research interest owing to the excellent color switching and good chemical stability [6,
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7]. Despite of this, WO3 thin films obtained using the classical electrodeposition method still exist many deficiencies that could limit their practical applications, such as long
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color/bleaching time and low coloration efficiency.
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In order to enhance the EC performance of WO3 films, many strategies were developed over recent years, such as compositing with other organic EC materials [810], adjusting the crystalline state [11], doping with other elements [12, 13] and
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controlling the micro- or nano-structure. Among these methods, controlling the micro-
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and nano-structure of EC thin films is an efficient way to enhance the EC performance mainly because larger specific surface areas could benefit for ions injection and extraction during EC reactions. In recent years, various kinds of micro- and nanostructure have been created using WO3 thin films to improve the EC performance. For instance, Simona and coworkers prepared the 3D network structure WO3 thin films using polystyrene spheres as templates to ahcieve an enhancement of the coloration efficiency [14]. Torsten et al. reported WO3 thin films with highly ordered three-
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demission mesoporous sturcture that show an improvement in coloration efficiencies, cycle stability and response time [15]. Therefore, constructing surface microstructure on WO3 thin films show great potentials in optimizing EC perforamce and thus developing high-performance EC devices.
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Here, we demonstrate grid-structured WO3 EC films exhibiting an significantly enhanced EC performance compared with classical plane films. This 2D grid structure
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is achieved by electrochemical deposition using PS nanofibers as sacrificial templates.
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Compared to the classical plane WO3 films, the grid-structured WO3 films showed an obvious enhancement in the optical modulation rate and CE value of ~ 50% and 20%,
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respectively, at the wavelength of 550 nm. Furthermore, the response time of the grid-
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structured WO3 films is shortened by over 50% than that of the plane ones. Compared with other nanostructurd WO3 films that have been reported previously, the grid-
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structured WO3 film had reasonable electrochromic performance too. For instance, the
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optical modulation rate of honeycomb nanostructured h-WO3 fabricated by Kondalkar et al. was about 60.74% at the wavelength of 630 nm under the voltage of ±1.2 V[16]. At the voltage of ±1 V, the grid-structured WO3 films have the optical modulation of
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64.1% at the wavelength of 550 nm. Based on excellent EC performance, this grid-
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structured WO3 films can be further used to construct highly efficient and rapidly responsive EC devices.
2. Experimental
2.1 Preparation of PS nanofibers
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Fluorine-Tin Oxide (FTO) coated glass substrates (2 cm × 3 cm) were ultrasonically cleaned in acetone, ethanol and distilled water for 10 mins respectively, followed by drying with the flowing nitrogen gas. The PS nanofibers were synthesized using the electrospinning method [17]. The typical procedure was described as follows: First, 0.5
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g of PS was dissolved into 5 g N, N-dimethylformamide (DMF) under vigorous stirring for 4 hours to get the transparent solution. Subsequently, the above solution was drawn
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into a 10 mL plastic syringe with a 30 gauge capillary needle. The needle was connected
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to the positive electrode of a DC-power supply electrospinning system. Then, the FTO glass substrates were placed on a grounded stainless steel foil located at a distance of
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150 mm away from the needle tip. The PS nanofibers were generated at the voltage of
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30 KV, the flow rate of the solution was between the ranges of 0.5-2 μL/min. Finally, a
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thin layer of nanofibers were deposited on the FTO glass substrates after a few minutes.
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2.2 Electrochemical deposition of grid-structured and plane WO3 thin films The WO3 thin films were prepared by classical electrochemical deposition [18]. Briefly, 1.6 g tungsten powder (99%) was dissolved into 15 ml 30 wt% H2O2 under ice-
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water bath for one hour, after filtration and reflux to remove unreacted W power and
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H2O2, peroxotugnstic acid solution (PTA) was got. With 15 ml ethanol added, the final pale yellow color stable sol used for electrodeposition was got. The electrochemical deposition was performed in a standard 3-electrode cell with PS nanofibers covered FTO glass substrate as working electrode, Ag/AgCl electrode as reference electrode and Pt foil as counter electrode. The electrodeposition experiment was carried out at a constant voltage of -0.50 V for seveal minutes. Then, the obtained FTO glasses covered by WO3 and PS nanofibers were dipped into tetrahydrofuran for 12 hours to remove PS
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nanofibers. Finally, the grooves were formed where the PS nanofibers were located, and 2D grid-structured WO3 thin film was obtained. For comparison, a plane WO3 film was deposited on FTO glass using the same parameters in the absent of PS nanofibers.
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2.3 Characterization The morphologies of PS nanofibers and WO3 thin films were characterized by
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scanning electron microscopy (Quanta 250 FEG, FEI, Czech); structure and
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composition of WO3 thin films was determined by X-ray diffraction spectrometer (XRD-6000, Shimadzu, Japan) and X-ray photoelectron spectroscopy (250Xi,
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Thermofisher, Ameirca). The optical transmittance was measured using a UV-vis-NIR
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spectrophotometer (U-3010, hitachi, Japan) with blanking for FTO glass and the power source was electrochemical workstation in Chronoamperometry mode. Cyclic
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voltammetry (CV), chronoamperometry (CA) and electrochemical impedance
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spectroscopy measurements of WO3 thin films were performed in a three-electrode cell system assembled with electrochemical workstation, 1.0 M LiClO4 Propylene carbonate
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(PC) as electrolyte, Ag/AgCl as reference electrode and Pt foil as counter electrode.
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3. Results and discussion
The fabrication process of grid-structured and plane WO3 films are shown in Fig. 1. From the SEM images it can be seen that the PS nanofibers with an average diameter of around 150 nm and a length over dozens of micrometers are randomly oriented on the FTO glass substrate. The prepared PS nanofibers are embedded into WO3 thin films during electrodeposition. After dissolution of the nanofibers, the grooves were formed
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at the location of the nanofibers and thus led to the generation of the grid-structured thin films. To assess the performance of the grid-structured WO3 film, blank experiment was performed in plane WO3 film obtained through electrodeposition using the same parameters in the absent of PS nanofibers. Fig. 1c and Fig. 1d show sketch maps and
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SEM images of grid-structured and plane WO3 thin films. The as-prepared gridstructured films have grooves with a width ranging from 100-300 nm depending on the
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diameter of PS nanofibers. SEM images of plane WO3 thin film reveal that the film is
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well covered and presents compact surface. The two kinds of WO3 films have similar thickness of ~ 600 nm. The thickness of WO3 in grooves varies from zero to several
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hundred nanometers, also, there were channels inside the WO3 films,shown as Fig. 2.
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The crystalline state of grid-structured and plane WO3 thin films were studied by XRD analysis. There is no resolved diffraction peaks shown in Fig. 3a, indicating a
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typical characteristic of amorphous WO3. In general, WO3 with amorphous structure is
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more suitable for EC applications than crystallized structure because of high coloration efficiency and fast color/bleach time [19]. XPS measurements were performed to further investigate the chemical composition and oxidation state of WO3 thin films (Fig. 3b).
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There are spin-orbit doublets in this spectrum corresponding to W 4f7/2, W 4f5/2 and W
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5p3/2 peaks which are located at 35.9, 38.2 and 41.8 eV, respectively. These values are consistent with the reported WO3 [20, 21]. Note that the grid-structured WO3 film and plane WO3 film have the same amorphous structure, chemical composition and oxidation state (Fig. 3a, b), which guaranteed the following studies on the changes of EC performance when introducing the grid structure into the WO3 films. Fig. 4 shows the transmittance spectra of grid-structured WO3 thin films and plane WO3 thin films in their colored and bleached state at the wavelength from 350 to 850
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nm. The oxidation and reduction voltages applied on the films was provided by electrochemical workstation in Chronoamperometry mode, the high voltage was +1 V and the low voltage was -1 V, the electrolyte was 1.0 M LiClO4 in propylene carbonate (PC) solution. When applied +1.0 V voltage, the WO3 thin film was transparent (inset of
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Fig. 4a). Then, the color of the film was switched to be blue with a lower transmittance when the applied voltage converted to -1.0 V (inset of Fig. 4a). From the transmittance
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spectra it can be seen that the grid-structured films have lower transmittance at both
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bleached and colored state but a larger transmittance modulation rate (ΔT) at the wavelength range from 350 to 850 nm. The lower transmittance rate at the bleached
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state of grid structure WO3 film mainly ascribed to the light refraction in the void
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structure [22].
We further investigated the light switching transmittance changes in situ under
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several cycles of alternating redox potentials at the wavelength of 550 nm. Fig. 4b
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shows the transmittance modulation rate of grid-structured and plane WO3 thin films at the time ranging from 400 to 520 s. The time interval was chosen according to the time of ΔT reaching stable. For grid-structured and plane WO3 thin films, the ΔT were
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calculated as ca. 64.3% and 40.3%, respectively. The former one is more than 50%
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larger than the latter one, indicating that the grid structure in WO3 film is beneficial to enlarge the ΔT value. Furthermore, cyclic stability of the prepared plane and gridstructured WO3 thin films are evaluated. As shown in Fig. 4c and Fig. 4d, the ΔT value of both plane and grid-structured WO3 films decreased as the redox reaction went on. For grid-structured WO3 film, the ΔT value decreased slightly from 0 s to 9000 s, as the cycles went on, ΔT value decreased rapidly until vanished. For plane WO3 film, the ΔT value was smaller but more stable than the grid-structured WO3 film. These results
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indicated that the grid structure in WO3 film enlarged the ΔT value but reduced the cycling stability of electrochromic performance. Also, higher voltages (±1.5 V) were applied during redox reaction to evaluate the cycling stability. At higher voltage, ΔT values of the two sorts WO3 films enlarged, but the cycling stability decreased more
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obviously too, and the grid-structured WO3 films had more significant decreased tendency than the plane one, as shown in Fig 4 (e,f). The improved electrochromic
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performance of grid-structured WO3 films is mainly attributed to the grooves and
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channels inside the WO3 films controls the ions insertion and extraction kinetics leading to an enhanced charge transfer reaction, also, the enhanced charge transfer reaction
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accelerated the degradation of WO3. Meanwhile, the thinner WO3 in grooves were more
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easily to short out, this may be another reason why the grid-structured WO3 films were more unstable.
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Coloration efficiency is an important factor to evaluate the EC performance. Higher
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CE value gives a larger optical modulation at smaller charge insertion or extraction, and thus can induce long-term cycling stability of EC materials [11]. The CE value can be
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calculated from the following formulas: CE = ΔOD / ΔQ = log(Tb/Tc) /ΔQ
(1)
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In order to calculate the CE value of two WO3 films, Cyclic Voltammetry (CV) and in situ transmittance rate tests at the wavelength of 550 nm were performed. Fig. 5 shows the CV curves of WO3 thin films measured in 1.0 M LiClO4 PC solution at a sweep rate of 100 mV·s-1 and time-transmittance curves in situ recorded under the CV tested at 550 nm. There are two anodic waves presented in both CV curves, these can be attributed to the insertion of two distinct proton species: hydrated Li+ ions absorbed onto WO3 surface or enclosed into pores of the oxide, and hydrated Li+ ions from the
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bulk solution. The grid-structured WO3 film had a higher peak current, indicating a lower internal resistance and a higher electrochemical activity. These results are similar with that obtained in the reported porous WO3 films [20]. The charge (∆Q) inserted into (or extracted from) the grid-structured and plane films were 12.39 and 9.82 mC/cm2
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calculated from the CV curves. Combining the CV curves with in situ transmittance spectra, the CE value of grid-structured WO3 film was calculated to be 71.8 cm2/C,
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which is higher than that of the plane WO3 film (58.8 cm2/C).
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Coloration and bleaching response times (tb and tc) of EC film are of great importance to determine its potential applications. Switching times can be defined to be the time
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required for 90% change in full current change in CA curves [23] The switching
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characteristics of the grid-structured and plane WO3 thin films were investigated through the CA measurements with three electrodes system: WO3 thin films as working
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electrode, Pt foil as counter electrode and Ag/AgCl electrode as reference electrode, the
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voltage applied was ±1 V. The results were shown in Fig. 6a. For grid-structured WO3 thin film, the response time of bleached and colored state were 0.93 s and 1.27 s (Fig. 6b), which were much faster than those of the plane film (2.18 s and 2.34 s). Compared
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to the plane film, the accelerate of response time are attributed to the larger EC
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material/electrolyte interface and shorter ion diffusion path of grid-structured WO3 film
The reason for the enhanced EC performance of grid-structured WO3 films is that the grooves in the WO3 film enlarged the contact area between WO3 and the liquid electrolyte, which in turn shortened diffusion path of Li+ ions in and out the film (Fig. 7a). The EC reaction of WO3 film in LiClO4 PC electrolyte can be indicated as double injection of ions and electrons shown as equation 2 [24].
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WO3 + xLi+ + xe- ↔ LixWO3
(2)
To clarify this mechanism, we further studied the electrochemical impedance spectroscopy spectra (EIS) of grid-structured and plane WO3 films (Fig. 7b). The EIS measurements were performed at the frequency ranging from 10-2 to 105 HZ. Generally,
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EIS plot is composed of a semicircle in the high frequency region and a sloping line in the low frequency region. The high frequency region represents the charge-transfer
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resistance and the low frequency region is ascribed to the Warburg diffusion impedance
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which represents the diffusion of ions into the film of electrode [25-28]. The corresponding equivalent circuit for impedance analysis was displayed in the inset of
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Fig. 7a. In the equivalent circuit, Rs, Rct, Cdl, W and Cp represent the solution resistance,
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charge transfer resistance, double layer capacitance, Warburg impedance and pseudo capacitance, respectively. It can be seen the semicircle in high frequency was not
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obvious because this step was not the control step. In the low frequency region, the grid-
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structured WO3 film had lower line slope than the plane film, which indicated that the grid-structured film has a lower ion diffusion resistance [29]. Therefore, it was easier for hydrogen ions diffusion in and out from the WO3 film during EC process, which
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contributed to a faster EC response time and a higher coloration efficiency of grid-
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structured WO3 film [30].
4. Conclusion In
summary,
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demonstrated
grid-structured
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thin
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through
electrodeposition with the assistance of PS nanofibers templates. Compared with classical plane WO3 thin films, the grid-structure ones exhibited an enhanced EC performance with large transmittance modulation rate, high coloration efficiency and
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fast switching speed. The enhancement of EC properties were mainly ascribed to the grid structure of WO3 thin films. This structure not only made the charge-transfer and the electrolyte penetration become easier in the film, but also promoted the infiltration of electrolyte and reduced the diffusion path. The grid-structured WO3 thin films in this
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work can be further used to develop highly efficient and rapidly responsive EC devices.
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Acknowledgements
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Z.X. and Q.Z. contributed equally to this work. This work was supported by the National Natural Science Foundation (21701003, 21641006), China Postdoctoral
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Science Foundation Grant (2015M580035, 2017T100022), Fundamental Research
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Funds for the Central Universities (YWF-16-JCTD-B-03), National Program on Key Research Project of China (2016YFB0303901) and the Beijing Municipal Natural
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Science Foundation (2161001).
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6866-6872.
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efficient electrochromic and electrocatalytic applications, Crystengcomm, 16 (2014)
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ACCEPTED MANUSCRIPT Enhanced electrochromic performance of 2D grid-structured WO3 thin films
Figure Captions Fig. 1. The fabrication process of grid-structured and plane WO3 films and the corresponding SEM images. (a) SEM image of PS nanofibers through electrospinning. (b) SEM image of WO3 film embedded with PS nanofibers. (c) Top and side views of
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Figure. 2. The sectional view of grid-structured WO3 films.
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grid-structured WO3 films. (d) Top and side views of plane WO3 films.
Fig. 3. X-Ray diffraction patterns (a) and XPS survey spectra of the W 4f peak (b) of
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the grid-structured and plane WO3 thin films.
Fig. 4. Optical transmittance spectra of the grid-structured and plane WO3 thin films. (a) Wavelength versus transmittance spectra of WO3 thin films. Ⅰ, Ⅲ: Transmittance spectra
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of plane WO3 film. Ⅱ, Ⅳ: Transmittance spectra of grid-structured WO3 film; Inset: digital photos of the grid-structured WO3 at bleached and colored state (b) Comparison of the grid-structured and plane WO3 thin films at the wavelength of 550 nm. (c)
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Transmittance of plane WO3 film at the cycle time from 0 to 12000 s at the wavelength
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of 550 nm., (d) Transmittance of grid-structured WO3 film at the cycle time from 0 to 12000 s at the wavelength of 550 nm.
Fig. 5. Cyclic Voltammetry (a) and in situ transmittance of the grid-structured and plane WO3 thin films recorded at the wavelength of 550 nm (b).
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ACCEPTED MANUSCRIPT Enhanced electrochromic performance of 2D grid-structured WO3 thin films
Fig. 6. (a) Chronoamperometry measurements of the grid-structured and plane WO3 thin films. (b) Histogram of coloration and bleaching time.
Fig. 7. (a) Mechanism diagram for EC reactions grid-structured of the grid-structured
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and plane WO3 films in LiClO4-PC electrolyte. (b) Electrochemical impedance
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spectroscopy spectra of the grid-structured and plane WO3 thin films.
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ACCEPTED MANUSCRIPT Enhanced electrochromic performance of 2D grid-structured WO3 thin films
Highlights 1. Grid-structured tungsten trioxide (WO3) electrochromic thin film was developed. 2. The grid structure enlarged transmittance modulation rate in redox reactions.
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3. The grid structure also enhanced color efficiency and shortened response time.
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