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

Abstract

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.

1. 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.

2. Materials and Methods

2.1. 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.

2.2. 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 ESCALAB 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.

3. Results and Discussion

3.1. 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 W4f_7/2 and W4f_5/2, respectively, which corresponded to tungsten atoms in a W⁶⁺ 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 W⁶⁺ spin-orbit doublets existed in the XPS spectra of the W4f core level, proving the two films were both WO₃.

3.2. 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 (∆T_550nm and ∆T_633nm) 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 ∆T_550nm and ∆T_633nm 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 (T_n). Finally, the adhesion factor f was calculated according to the equation f = 1 − (T_n − 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. Comparison between the nanostructured WO₃ film and the ordinary WO₃ film.

Electrochromic Properties	Nanostructured WO3 Film	Ordinary WO3 Film
response time (coloring time/bleaching time)	8.40 s/1.39 s	9.27 s/3.82 s
cycling stability (ion capacity decreasing rate)	53%	81%
optical modulation rates (∆T550nm and ∆T633nm)	43.44%/55.92%	28.64%/40.68%
memory effect (transmittance increasing rate)	21.14%	33.68%
adhesion (value of adhesion factor)	0.99702	0.57148

. 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].

4. 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.