Fast response of complementary electrochromic device based on WO3/NiO electrodes

Nanoporous structures have proven as an effective way for enhanced electrochromic performance by providing a large surface area can get fast ion/electron transfer path, leading to larger optical modulation and fast response time. Herein, for the first time, application of vacuum cathodic arc plasma (CAP) deposition technology to the synthesis of WO3/NiO electrode films on ITO glass for use in fabricating complementary electrochromic devices (ECDs) with a ITO/WO3/LiClO4-Perchlorate solution/NiO/ITO structure. Our objective was to optimize electrochromic performance through the creation of electrodes with a nanoporous structure. We also examined the influence of WO3 film thickness on the electrochemical and optical characteristics in terms of surface charge capacity and diffusion coefficients. The resulting 200-nm-thick WO3 films achieved ion diffusion coefficients of (7.35 × 10−10 (oxidation) and 4.92 × 10−10 cm2/s (reduction)). The complementary charge capacity ratio of WO3 (200 nm thickness)/NiO (60 nm thickness) has impressive reversibility of 98%. A demonstration ECD device (3 × 4 cm2) achieved optical modulation (ΔT) of 46% and switching times of 3.1 sec (coloration) and 4.6 sec (bleaching) at a wavelength of 633 nm. In terms of durability, the proposed ECD achieved ΔT of 43% after 2500 cycles; i.e., 93% of the initial device.


Results
electrochromic mechanism for nio-Wo 3 system. As shown in Fig. 1(a-c), the application of voltage (an electric field) to the device causes positive ions to move toward the electric field, while the electrons move in the opposite direction. The movement of ions (electrons) into the electrochromic (ion storage) layers is responsible for the coloration (bleaching) of the ECDs. The underlying physics involved in the electrochromic reactions can be represented using the following the redox equations: x 3 3 where M indicates the lithium ions (Li + ) or hydrogen ions (H + ) ions. The WO 3 thin film changes from transparent to deep blue under the effects of electron insertion (i.e., photo-effected intervalence electron transfer from W 6+ to W 5+ sites). The electrochromic mechanism governing the behavior of Li + ions against ion insertion/ extraction in the NiO electrode can be represented using the following the redox equations: NiO , NiO (coloration) NiO (bleaching) ( e ) (2) x y x y x y z x The reduction of Ni 3+ to Ni 2+ leads to the bleaching of the NiO film (during the cathodic scan), and coloration of NiO film via the oxidation of Ni 2+ to Ni 3+ (in the reverse process). Continuously applying negative voltage to the NiO electrode (ion storage layer) causes the insertion of electrons and Li + ions leading to the oxidation of Ni 2+ to Ni 3+ , with the result that the coloration state is dominant. In Fig. 1(c), for test step, to understand surface charge capacity for WO 3 /NiO electrode films, which were integrated CA curves defined as both intercalation surface charges (Q in ) and extraction surface charges (Q out ). Here, the complementary surface charge capacity ratio R is defined as the insertion WO 3 electrode divided by the extraction NiO electrode. Wo 3 /ITO and NiO/ITO films: surface charge capacity ratio. The complementary ECD in the current study included two electrochromic electrodes, as in thin-film batteries. Thus, we calculated the surface charge capacity ratio of the electrodes in both directions. We first sought to elucidate the electrochemical and energy storage properties of the WO 3 /ITO/glass by constructing three electrode cells, which comprised a working electrode (WO 3 film on ITO/glass), a counter-electrode (Pt mesh) and a reference electrode (Ag/AgCl) in 0.5 M LiClO 4 /Perchlorate (LiClO 4 /PC) solution.
The surface charge capacity of the WO 3 layers was based on integral to CA curves was carried with from −1.5 to 0.3 V versus AgCl/Ag in intercalation surface charges (Q in ) and extraction surface charges (Q out ). As shown in Fig. 2(a), the 200-nm-thick WO 3 electrode returned the following values: Q in (7.38 mC cm −2 ), Q out (8.4 mC cm −2 ), and reversibility, Q in /Q out is about 87% better than the other samples. Here, complementary charge capacity ratio R is defined as the intercalation WO 3 electrode divided by the extraction NiO electrode according to following equation: in out 3 where Q in (WO 3 ) is the surface charge capacity of the WO 3 electrode during intercalation and Q out (NiO) is the surface charge capacity of the NiO electrode during the extraction process. Q out (NiO) was carried with from 0.7 V versus Ag/AgCl and observed Q out (NiO) value is 7.45 mC cm −2 . As shown in Fig. 2(b), we also assessed the degree to which the thickness of the WO 3 layer (175 nm, 200 nm, 225 nm and 250 nm) affected the charge capacity ratio when using an NiO/ITO/glass electrode of 60 nm in thickness. With the charge capacity of the NiO Diffusion coefficient and transmittance optical modulation as a function of WO 3 /ITO film thickness. Figure 3(a 1 ,b 1 ,c 1 ,d 1 ) showed cycle voltammetry (CV) curve of WO 3 /ITO films at four different thicknesses and applied the potential voltage from −1.5 V (coloration) to 0.3 V (bleaching) at a scan rate of 200 mV/s. In Fig. 3, the CV curve of WO 3 /ITO films were measured at 25 th -cycle for four different thicknesses 175 nm, 200 nm, 225 nm, and 250 nm respectively. Furthermore, the diffusion coefficient D of Li + ions during injection/ extraction of ions into/out of the WO 3 film can be calculated using the Randles-Servick Eq. (4) 19 . ; v is the scan rate (V• s −1 ); D is the diffusion coefficient (cm 2 s −1 ) and J p is the peak current. Table 1 lists the values for J pc (cathodic peak current density), J pa (anodic peak current density), and diffusion coefficient D (working area 3 × 4 cm 2 ). Sample 2 (with 200 nm-thick) of WO 3 /ITO films in Table 1 presented the highest ion diffusion coefficients of 7.35 × 10 −10 cm 2 /s (oxidation) and 4.92 × 10 −10 cm 2 /s (reduction). From Table 1, the higher diffusion coefficient indicates a larger contact area and greater porosity resulting in faster ion insertion/extraction. www.nature.com/scientificreports www.nature.com/scientificreports/ Figure 3(a 2 ,b 2 ,c 2 ,d 2 ) presents the optical transmittance spectra of WO 3 /ITO/glass between bleached and colored states at different various thicknesses (175 nm, 200 nm, 225 nm, and 250 nm). At a fixed wavelength of 633 nm, optical transmittance varied as a function of thicknesses from 38.35% to 52.68%. Note that the modulation of optical transmittance of 200-nm-thick WO 3 film (ΔT = 52.68%) was higher than that of the other samples, as indicated by the larger enveloped area in the CV curve. Actually, the area of the CV curve is deeply related to the charge stored (capacity) at porous WO 3 film 20 indicates that more charges are taking part in redox reactions. Fig. 4 shows the hemispherical surface structures of the WO 3 film deposited using CAP. Top-view SEM image of WO 3 pattern inset of Fig. 4. Figure 5(a) presents the X-ray diffraction (XRD) patterns of the WO 3 film deposited on a glass substrate. The porous WO 3 film presented only one broad peak at ~23°, indicating an amorphous structure, as described in previous studies 36 . X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical composition of the WO 3 film surface. The electrochemical testing of Li x WO 3 (WO 3 :Li) samples in 0.5 M liquid-electrolyte of LiClO 4 /PC solution was performed using a three-electrode cell, comprising a working electrode (WO 3 electrode film on ITO/glass), a counter-electrode (Pt mesh) and a reference electrode (Ag/AgCl). The color of the WO 3 /ITO/glass changed from deep blue (colored state; −1.35 V) to transparent (bleached state; 0.2 V), is accordance with the intercalation/ extraction of ions (Li + ) into/out of the WO 3 electrode. The thickness of the WO 3 film was shown to have considerable influence on the electrochromic properties by XPS analsis. Figure 5(c,e) show XPS spectra of W 4 f in tungsten oxide films (200 nm) and (250 nm) in coloration states. In Fig. 5(c,e), the peaks W 4f 7/2 and W 4f 5/2 of W 6+ and W 5+ that are located at binding energies of 35.17 and 37.24 eV corresponding to W4 f 5/2 and W4 f 7/2 of W 5+ in the Li x WO 3 . The coloration process indicates the movement of Li + ions and electrons into the WO 3 films, such that the W 6+ obtained an e − to become W 5+ , resulting in a corresponding shift in the peak to a lower energy level. The content of W 5+ in Li x WO 3 film (i.e., η(W 5+ )) can be calculated using the following equation:   www.nature.com/scientificreports www.nature.com/scientificreports/ The fitted spectrum can be separated into two gaussian doublets shown Fig. 5(c-f). In this redox reaction Eq.(5), we evaluated ions transformed from the W 6+ to the W 5+ state. In Fig. 5(c-e), we calculated that approximately 30% (200 nm) and 35% (250 nm) of the ions transformed from the W 6+ to the W 5+ state, see Table 2. This is an indication that a larger number of W 6+ (bleaching) Li + ions took part in the reduction reaction to become W 5+ (blue). As shown in Fig. 3(b 2 ,d 2 ), we found that as the thickness of the film increased, Q in (WO 3 ) curve gradually increased as did the power of lithium-ion injection, optical transmittance of 200-nm-thick WO 3 film (T coloration = 25.12%) and 250-nm-thick WO 3 film (T coloration = 18.13%) at a fixed wavelength of 633 nm. Figure 5(df) presents high-resolution XPS spectra of W 4 f in tungsten oxide films (200 nm) and (250 nm) in bleaching states. In the W 4 f spectrum in the beached state ( Fig. 5(d)), the peaks at binding energies of 35.6 and 37.7 eV correspond to W4 f 5/2 and W4 f 7/2 of W 6+ . Thus, we can deduce that only W 6+ ions were present in the WO 3 thin films in the bleached state. But when the film thickness reaches 250 nm, the film can't completely fade, which is still light blue, which means that W 5+ (blue) can't completely convert to W 6+ (bleaching). This is confirmed by the detection of W 5+ in a later XPS analysis of the 250 nm bleached film. This shows that there are residual W 5+ ions in the process of fading oxidation when lithium ion is injected into the coloring reduction reaction Therefore, appropriate film thickness, will help improve the electrochromic optical modulation performance. The 200-nm-thick film contained only W 6+ ions; however, some of the W 5+ ions in the 250-nm-thick film did not convert into W 6+ ions, thereby decreasing the penetration of the bleaching state, as shown in Fig. 3(b 2 ,d 2 ), we found optical transmittance of 200-nm-thick WO 3 film (T bleaching = 78.85%) and 250-nm-thick WO 3 film (T bleaching = 70.05%) at a fixed wavelength of 633 nm. In Fig. 5(f), we calculated that approximately 80% (250 nm) of the ions transformed from the W 5+ to the W 6+ state, see Table 2. Ning et al. 42 claimed that lattice strain can affect the diffusion and migration of lithium ions, such that the coefficient of lithium ion diffusion decreases under the effects of pressure-induced strain. Thus, the failure of the 250-nm-thick film to recolor may be due to the thickness of the film, which extended the lithium ion migration path, such that the remaining stress hindered the transfer of lithium ions and the conversion of W 5+ to W 6+ . Therefore, tungsten oxide film as a cathodic electrochromic layer, the film thickness should be selected appropriately, can improve modulation optical transmittance to optimization conditions.

Coloration efficiency of WO 3 /ITO films as a function of thickness. Coloration efficiency (CE) is
an important criterion in the evaluation of electrochromic materials. CE is defined as the optical density charge (ΔOD) per unit of inserted charge Q in (Q in = Q/A, where A is the 20 : where T bleaching and T coloration refer to the transmittance of bleaching and coloration state. Generally, a high CE value is an indication of large optical modulation under small charge insertion. Figure 6 Figure 7(a) presents the in-situ transmittance of WO 3 /NiO ECD at 633 nm, as analyzed during a continuous potential cycle from −1.5 V (colored potential, V c ) to 0.8 V (bleached potential, V b ). In Fig. 7(b,c) show that the coloration state (charge process) and bleaching state (discharge process) of ECDs were measured by CA curves and in-situ optical response of transmittance at fixed 633 nm. The coloration and bleaching of switching times or speed was a prominent characteristic for ECD system, which was defined as the time required for a 90% change in the full transmittance modulation [43][44][45] . As shown in Fig. 7(c), ECD achieved a maximum optical modulation reached 46% and the switching times at a wavelength of 633 nm were obtained coloration (3.1 sec) and bleaching (4.6 sec) (see supplementary video). The electrochromic and optical properties of our work compared with other authors researches are detailed in www.nature.com/scientificreports www.nature.com/scientificreports/  Figure 8 illustrates the durability of the ECDs in terms of transmittance optical modulation measured in intervals of 15 s. As shown in Fig. 8, even after 1000 cycles (approximately 10 hours), there was no significant degradation in the optical modulation performance of the ECD. As shown in Fig. 8, transitions between bleached and colored states remained steady until 1000 cycles, at which point switching performance degraded gradually, dropping to 93% of the as-synthesized samples by 2500 cycles.

Methods
Cathodic arcs can be used for the reactive deposition of various nitrides and oxides. Nonetheless, CAP technology has not been widely adopted, due to violent plasma-liquid pool interactions at cathode spots, which can cause the emission of macro-particles (MPs) that degrade the quality of the resulting film. This can largely be overcome by steering the arc rapidly across the surface of the cathode under high working pressure to reduce the spot residence www.nature.com/scientificreports www.nature.com/scientificreports/ time and limit the formation of erosion craters 46 . In recent years, researchers have shifted emphasis from monolithic coatings to higher performing multilayers and nano-composites. As shown in Fig. 9(a), the proposed arc gun set up relies on the flow of argon (for insertion) and oxygen (reaction) to control the formation of the electrode structure. It is difficult to measure the dynamics of a cathode spot; therefore, we employed a high-speed   www.nature.com/scientificreports www.nature.com/scientificreports/ video camera to capture images of light emission from various spots across the target plane in sequences of 1 sec, as shown in Fig. 9(b,c). The deposition parameters are shown in Table 2 and 3, and the schematic drawing of the cathodic arc deposition is presented in Fig. 9(a). Figure 10 presents a schematic diagram showing the process of ECD fabrication. Figure 10(a): Step 1 and 2 involved respectively depositing an EC film/ITO on one glass substrate and a counter film/ITO on another glass substrate.
Step 3 involved fitting the two components together and sealing them with epoxy adhesive. Note glass beads were used as spacers to maintain a cavity between the EC film and the counter film to hold liquid electrolyte (<100 μm). Note also that a small gap was created in the epoxy for use as an inlet into the space.
Step 5 showed ECD consisted of seven layers: A central ionic conduction layer (electrolyte) sealed between an electrochromic (EC) layer and an ion storage (complementary) layer, which were sandwiched between two transparent conducting layers, which were in turn sandwiched between two glass substrates. Figure 10(b) mainly describes the process of component packaging. Figure 10(b) Step 1: Dispensing for one side pre gluing.
Step 2: UV glue curing. Step 3      www.nature.com/scientificreports www.nature.com/scientificreports/ temperature, as shown Fig. 10(a). The base chamber pressure was maintained at less than 2 × 10 −5 Torr using a turbo pump. Tables 4 and 5 list the fabrication parameters. CAP was used to deposit WO 3 /NiO as working/ counter electrodes. As shown in the schematic in Fig. 10(a), the active area of the ECD (3 × 4 cm 2 ) possessed the following structure: glass/ITO/WO 3 /liquid electrolyte/NiO/ITO/glass. Our primary focus in the current study was the analysis of WO 3 films in terms of charge capacity and diffusion coefficient. The WO 3 /NiO series were fabricated on ITO glasses as electrochromic layers, which are listed in Tables 4 and 5. Measurements. Electrochemical characterization was performed using cycle voltammetric (CV) and chronoamperometric (CA) (Autolab, model PGSTAT 30) measurements. In-situ UV-Vis measurements were obtained using a spectrophotometer (Ocean Optics, DH-4000-BAL) in conjunction with CA analysis.

conclusions
In conclusion, we have developed a CAP deposition as an alternative to sputtering in order to achieve high deposition rates with low-cost method of producing EC film based on WO 3 for ECD applications. The proposed deposition scheme was applied to the synthesis of WO 3 films with nanostructured surface features in various thicknesses (175 nm, 200 nm, 225 nm, and 250 nm). The complementary WO 3 (200 nm)/NiO (60 nm) ECD exhibits higher optical modulation (46% at 633 nm), faster response times (t b = 4.6 s, t c = 3.1 s) and higher CE (90 cm 2 /C). During the durability test, the transmittance change of ECD remained 43% after 2500 cycles, which was about 93% of original state.   Table 5. More details of ECD deposition parameters.