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Enhanced Electrochromic Properties of Nanostructured WO3 Film by Combination of Chemical and Physical Methods

WO3 films are the most widely used electrochromic functional layers. It is known that WO3 films prepared by pure chemical method generally possess novel nanostructures, but the adhesion between WO3 films and substrates is weak. However, WO3 films prepared by pure physical method usually show relatively dense morphology, which limits their electrochromic properties. In order to break through these bottlenecks and further improve their electrochromic properties, this work first prepared nanostructured WO3 powder by chemical method, and then using this powder as the evaporation source, nanostructured WO3 films were fabricated by vacuum thermal evaporation method. Properties of nanostructured WO3 films were systematically compared with those of ordinary WO3 films. It turned out that the nanostructured WO3 film exhibited better cyclic stability and memory effect, and also the optical modulation rate was 14% higher than that of the ordinary WO3 film. More importantly, the nanostructured WO3 film showed better adhesion with the ITO substrates. These results demonstrate that a combination of chemical and physical methods is an effective preparation method to improve the electrochromic properties of WO3 films.


Introduction
Tungsten oxide (WO3) has been regarded as the most promising electrochromic material due to its wide range of continuously adjustable optical properties, excellent reversibility, low energy consumption, high coloration efficiency, and environmental friendliness [1][2][3]. WO3 films have been used in electrochromic smart windows [4,5], information display [6], photocatalysis [7,8], and energy storage technologies [9,10]. Therefore, further improving the properties of WO3 films has become one of the research hot spots.
As we know, the electrochemical properties of WO3 films depend strongly on their preparation method, morphology and structure. Owing to the high surface activity and loose morphology, nanostructured WO3 can accelerate the electrochemical process and improve the electrochromic properties with respect to the bulk counterparts [11][12][13][14]. So far, various nanostructured WO3, such as nanowires [15], nanorods [16,17], nanocolumnar films [18] and nanoparticles [19] have been prepared and shown good electrochromic properties. Nanostructured WO3 films have been fabricated by various physical and chemical methods, including sol-gel [20], hydrothermal [21], magnetron sputtering [22] and electron-beam evaporation [23], which are just pure chemical or physical methods. Though WO3 films prepared by pure chemical method generally possess novel nanostructures, the adhesion between WO3 films and substrates is weak. On the other hand, WO3 films prepared by pure physical method such as vacuum thermal evaporation and magnetron sputtering have many advantages, but their morphology is relatively dense, which limits the electrochromic properties [24]. Therefore, WO3 films prepared by pure chemical or physical methods lead to the property bottleneck to some extent [25].
Herein, instead of using pure chemical method or pure physical method, nanostructured WO3 films were prepared by combination of chemical and physical methods. First, nanostructured WO3 powder was prepared through chemical synthesis, and then nanostructured WO3 films were fabricated using vacuum thermal evaporation. Their characterization and electrochromic properties were compared with ordinary WO3 films at the same thickness. Particularly, herein the adhesion between the WO3 film and the substrate was discussed, which was a key factor in real application of the electrochromic devices but was rarely present in the literature.

Sample Preparation
The preparation process was shown in Figure 1. The same amount of 0.025 mol Na2WO4•2H2O and CaCl2 were respectively dissolved in 50 mL of deionized water, and 0.5 mol/L Na2WO4 solution and CaCl2 solution were prepared. Then, the two solutions were mixed together and HCl was added to adjust the solution pH to be neutral and left to stand for 72 h. The precipitate was filtered and dried at 70 °C to obtain CaWO4 powder. Next, CaWO4 powder was dissolved in 100 mL 20% HCl. After standing for 24 h, nanostructured WO3 powder was obtained by filtration and drying. In the end, the powder was annealed at 500 °C for 2 h.
The indium tin oxide (ITO) glass substrate was used and first subjected to cleaning of acetone, alcohol and deionized water. Then, the nanostructured WO3 film was prepared by vacuum thermal evaporation (ZHD-300N, Technol, Beijing, China) using the nanostructured WO3 powder as the evaporation source. The distance between substrates and the evaporation source was 11.5 cm. Oxygen and argon were let into the vacuum chamber with the flow ratio of 2:3, where oxygen was introduced to avoid the WO3 vapor from being depleted of oxygen during the long-time evaporation [26], ensuring the WO3 deposition on the substrate. The working pressure was 1.0 Pa, and the temperature of the tungsten boat was kept at 1100 °C for 1.5 h. At this time, the temperature of the substrate varied slightly in the range of 340-350 °C. The substrate rotated at the speed of 10 r/min during the whole process. Finally, nanostructured WO3 films were obtained at a thickness of about 400 nm. In addition, ordinary WO3 films with the same thickness were prepared by extending the evaporation time to 2 h for comparing their electrochromic properties, but the other evaporation parameters were kept the same, but using ordinary WO3 powder as an evaporation source. The deposition rate of the nanostructured WO3 film was faster originating from its large specific surface area. It should be noted that different deposition rates would inevitably cause different microstructures of the deposited film. However, for comparing their electrochromic properties, the thickness of the two WO3 films should be the same, because the optical modulation rate, the memory effect, the adhesion and the cyclic stability all greatly depend on the thickness of the electrochromic film.

Characterization
Morphology of two kinds of WO3 powder and two kinds of WO3 films was observed by scanning electron microscopy (SEM) using a Sigma 500 instrument (Zeiss, Oberkochen, Germany). The structure was examined by X-ray diffraction (XRD) analysis using a Cu Ka radiation (Philips X'Pert diffractometer, Amsterdam, The Netherland). X-ray photoelectron spectroscopy (XPS) analysis was performed by using a Thermo Fisher Scientific ES-CALAB 250 XPS system, Waltham, MA, USA. The cyclic stability of WO3 film was measured by cyclic voltammetry (CV) and the response time was tested by electrochemical double potential step chronograph current test (CA) on an electrochemical workstation (CHI760E). A three-electrode system was formed using WO3 film as the working electrode, Ag/AgCl as the reference electrode, metal Pt plate as the counter electrode, and 1 mol/L LiClO4-PC solution as the electrolyte. CV test was carried out with the voltage ranging from −0.8 V to +0.8 V at a scan rate of 100 mV/s. The CA test was conducted at −0.8 V (coloring) and +0.8 V (bleaching) for 20 s. Optical transmittance spectra of WO3 films at colored state and bleached state were examined at wavelength ranging from 200 to 800 nm using an ultraviolet/visible (UV/VIS) spectrophotometer (Shimadzu UV-2550, Tokyo, Japan). The adhesion between WO3 films and ITO substrates was examined by using Scotch TM adhesive tape (3M, St. Paul, MN, USA) with the adhesion force of 4.7 N/cm.

Morphology and Structure
The morphology of both the nanostructured WO3 powder prepared by the chemical method and the ordinary commercially-bought WO3 powder was analyzed in detail by SEM as shown in Figure 2. The as-prepared nanostructured WO3 powder before annealing showed the shape of nanosheets (Figure 2ai). Its size and distribution were uniform. The WO3 nanosheets were cross-aggregated to form clusters. After annealing at 500 °C, the WO3 powder still showed uniformly nanoclusters (Figure 2aii). For comparison, the ordinary WO3 powder showed a disordered state, among which some clusters were huge and some were tiny (Figure 2b). XRD was employed to analyze the crystal structure and phase properties of the two above WO3 powder. As can be seen in Figure 3a, both the nanostructured WO3 powder ( Figure 3ai) and ordinary WO3 powder (Figure 3aii) were monoclinic and had the three main diffraction peaks at 23.1°, 23.6°and 24.4°, suggesting that these two kinds of WO3 powder had the same crystalline structure. However, the nanostructured WO3 powders showed a bulging peak at 15-23°, suggesting that an amorphous component existed, which demonstrated that the coexistence of crystalline phase and amorphous phase was obtained. Furthermore, the high resolution XPS pattern of element W both in the nanostructured WO3 powder after annealing and ordinary WO3 powder were depicted in Figure 3b, which were the same. The peak energies of 35.3 and 37.5 eV were attributed to W4f7/2 and W4f5/2, respectively, which corresponded to tungsten atoms in a W 6+ formal oxidation state.  Two kinds of WO3 films were prepared by vacuum thermal evaporation respectively using the annealed nanostructured powder and the ordinary powder. It can be seen that the nanostructured WO3 film was composed of uniform nano-sheets, and there was no crack on the surface, as shown in Figure 4a. While the ordinary WO3 film was composed of loose agglomerated clusters, the surface cracked seriously, as illustrated in Figure 4b. Besides the evaporation source, deposition conditions would also be responsible for the different microstructures of the two WO3 films, which were tried to keep the same except the evaporation time. Faster deposition rate of the nanostructured WO3 film originating from its large specific surface area would cause different grain sizes and different morphologies. As can be seen in Figure 5a, XRD spectra of the two WO3 films on the ITO glass substrates showed that except for the diffraction peaks corresponding to the ITO substrate, there were no other obvious diffraction peaks. Both of them were amorphous, mainly because the substrate temperature was not high enough during the evaporation process. As depicted in Figure 5b, only W 6+ spin-orbit doublets existed in the XPS spectra of the W4f core level, proving the two films were both WO3.

Electrochromic Properties
Response time of the WO3 film was recorded by the current-time curve, as shown in Figure 6. The response time was usually taken as 90% of the current change value. The coloring time (t1) and the bleaching time (t2) of the nanostructured WO3 film were 8.40 and 1.39 s, respectively, while those of the ordinary WO3 film were 9.27 and 3.82 s, respectively. CV tests were carried out to record Li + insertion/ extraction processes of the two WO3 films, as shown in Figure 7a. The results showed that the area enclosed by the CV curve of the ordinary WO3 film shrunk rapidly during long-term cycling (Figure 7aii). After 1000 cycles, the ion capacity of the ordinary WO3 film decreased rapidly from 5.69 to 1.08 mC•cm −2 and the decreasing rate was as high as 81%, indicating that its cycling stability was poor. However, the CV curve of the nanostructured WO3 film changed less ( Figure  7ai), and the ion capacity decreased from 6.08 to 2.81 mC•cm −2 after 1000 cycles, which was 53% lower than that of the first cycle. Therefore, the nanostructured WO3 film showed better cycling stability than the ordinary WO3 film. The optical modulation rates at wavelength of 550 and 633 nm (∆T550nm and ∆T633nm) were obtained by comparing the transmittances of the two WO3 films in a fully colored state and a fully-bleached state. The results were represented in Figure 7b. The film was colored or bleached at the potential of 3 V for 2 min. The optical modulation rates of the nanostructured WO3 film were 43.44% at 550 nm and 55.92% at 633 nm (Figure 7bi). However, the optical modulation rates of the ordinary WO3 film were only 28.64% at 550 nm and 40.68% at 633 nm (Figure 7bii). It was known that the ∆T550nm and ∆T633nm of the nanostructured WO3 film were 14% higher than that of the ordinary WO3 film. As to the memory effect, transmittance of the nanostructured WO3 film at 633 nm increased by 21.14% after being fully-colored for 8 h (Figure  7ci), while that of the ordinary WO3 film increased by 33.68% (Figure 7cii), which was close to the transmittance of the bleached state. Hence, the nanostructured WO3 film showed a better memory effect. The adhesion between the two WO3 films and the ITO glass substrate was tested by adhesive tape method and the testing process could be divided into three steps. First, the transmittance of the WO3 film at 550 nm in initial state (T0) was measured by UV/VIS spectrophotometer. Second, the Scotch TM adhesive tape at the force of 4.7 N/cm was leveled and tightly attached to the surface of the WO3 film. Then the tape was torn up and the transmittance of the WO3 film at 550 nm was measured again (Tn). Finally, the adhesion factor f was calculated according to the equation f = 1 − (Tn − T0)/(100 − T0). The closer the value of f was to 1, the greater the adhesion of the WO3 film was. Because at this time almost no film was attached to the adhesive tape. On the contrary, the closer the value of f got to 0, the worse the adhesion of the WO3 film was. At this moment, the whole film was almost adhered to the adhesive tape. The calculated adhesion factor f was shown in Figure 8. It was observed that the adhesion factor f of the nanostructured WO3 film was 0.99702, while that of the ordinary WO3 film was only 0.57148. Consequently, the nanostructured WO3 film with greater adhesion was more reliable in practical applications, which provided a potential solution for the adhesion problem in chemical fabrication of films. Electrochromic properties of the two WO3 films were summarized in Table 1. Faster response time of the nanostructured WO3 film originated from its nano-blade morphology (seen in Figure 4a), which reduced the accumulation and accelerated the ion diffusion rate of Li + [27]. The surface of the nanostructured WO3 film was smooth and uniform, which was conducive to improving the cycling stability. What is more, the nanostructured WO3 film had excellent adhesion with the ITO substrate because of its uniform and continuous structure, while poor adhesion of the ordinary WO3 film probably originated from the cracking morphology (Figure 4b). These results indicate that a combination of chemical and physical methods is effective in improving the properties of electrochromic films. The above findings were verified several times and they showed good reproducibility. However, it should be mentioned that the electrochromic properties of the prepared nanostructured WO3 films were not outstanding when compared with those recently reported, such as the optical modulation of the nanocolumnar structured WO3 film reaching 65% at the wavelength of 600 nm [18]. In Quy's work, the response times of the PVE-WO3 film reached 2.9 and 2.1 s, respectively, for the coloring and bleaching process [19].

Conclusions
In this study, nanostructured WO3 films were prepared by the two-step method. First nanostructured WO3 powder was chemically synthesized, and then nanostructured WO3 films were prepared by vacuum evaporation using the nanostructured powder as the evaporation source. With continuous and uniform nano-blade morphology, the nanostructured WO3 film showed better electrochromic properties than that of the ordinary WO3 film with better cycling stability and memory effect, stronger adhesion to the ITO substrate, a 14% higher modulation rate and a faster response time. Our research provides an idea for preparing films with better properties by combination of different chemical and physical methods.