Controllable Electrodeposition Adjusts the Electrochromic Properties of Co and Mo Co-Modified WO₃ Films

Abstract

Metal ion modification is considered to be an effective way to construct metal oxides with specific physical and chemical properties. In this paper, we prepare a tungsten oxide (WO₃) film co-modified by Co-ion and Mo-ion to serve as the electrochromic material through a one-step electrodeposition method. The effect of electrodeposition time on film thickness, surface morphology and electrochromic properties is systematically studied as well. The results show that, compared with pure WO₃ film, the surface morphology of the tungsten oxide film modified by Co-ion and Mo-ion (WO₃: Co, Mo) is significantly different. The Co and Mo co-modified film possesses a higher transmission modulation (58.5% at 600 nm) and rapid switching speed (coloring and bleaching time are 2.7 s and 5.6 s, respectively), low impedance value and excellent cycle stability. The performance enhancement is mainly attributed to the coral-like structure of the membrane, which provides a larger specific surface area, more ion adsorption sites and faster ion diffusion. Therefore, this work provides a fast and low-cost method to prepare tungsten oxide electrochromic films co-modified with cobalt and molybdenum ions. At the same time, it also provides an idea to obtain films with different electrochromic properties by adjusting the film thickness.

1. Introduction

The ever-increasing energy demand has become the most severe challenge brought about by the rapid development of world economy. The current energy structure is still dominated by fossil energies (such as coal, oil, and natural gas). These traditional fossil energy reserves are limited and non-renewable, and their excessive consumption causes global environmental problems, such as climate warming [1,2,3,4,5]. Therefore, there is an urgent need to reasonably improve energy efficiency and maintain a sustainable economic development [6,7]. According to statistics, building energy consumption mainly includes heating in winter and cooling in summer, which account for approximately 40% of the total energy consumption of the whole society, so building energy savings are of great significance [8].

Electrochromic technology provides a new solution for energy savings in modern buildings, which can adjust the visible light transmittance of glass and solar radiation energy according to the human will. S. Selkowitz, a senior consultant in architectural science at Lawrence Berkeley National Laboratory, once pointed out that “electrochromic technology is the most promising dimming technology in architectural applications” [9]. Within a certain voltage range, the electrochromic device can obtain different transmittance states by applying different voltages and has a color memory effect. It can still maintain its transmittance state when the voltage is applied under open circuit conditions. This has a wide range of potential applications that are very attractive, such as energy-saving smart windows, large-area information displays and anti-glare car rearview mirrors.

Tungsten oxide-based electrochromic smart glass can not only adjust the transmittance of the visible light region, but also have a blocking effect on infrared light in the colored state [10]. Therefore, electrochromic smart glass can actively block the heat loss caused by the temperature difference between indoor and outdoor environments by controlling the voltage and reduce the large amount of energy consumption caused by building cooling and heating.

In response to the above requirements, scholars have conducted extensive research on the transition metal oxide WO₃. However, the traditional WO₃ material has poor diffusion coefficient, so the light modulation is not ideal, and the area capacitance is poor. The properties of materials are closely related to their structure. Therefore, if we want to change the electrochromic properties of WO₃, we should start with its structure. Metal ion modification is a well-studied strategy that can enhance the electrochromic properties of WO₃ [11]. By introducing appropriate metal ions, the film structure on the surface of the material is changed, and faster reaction kinetics, higher optical modulation efficiency and higher coloring efficiency can be achieved [11,12,13,14,15]. Green et al. used a low-concentration Ni-ion to modify WO₃ films and found that this method can improve the coloring efficiency of the films [16,17]. Zhou et al. modified WO₃ nanofibers with trace Mo-ion, which not only improved the electrochromic performance of the film, but also increased the capacitance of the film [18]. Xu et al. used 1.5% Co-ion to change the growth mode of WO₃ fibers, thereby increasing the specific surface area of the WO₃ film and making the film exhibit a better electrochromic performance in an oily electrolyte solution of lithium salt [19]. However, most of the preparation processes reported above are designed with strict heating, pressurizing and vacuuming processes, which will not be conducive to industrialization. Therefore, the synthesis of high-performance amorphous films on the surface of a substrate through a simple, fast, and low-cost method should be highly valued and further developed. The electrodeposition method is a very demanding method [20]. Because the electrodeposition process does not require high pressure and high temperature, the prepared samples do not require annealing treatment. In addition, considering that single metal ion modification can improve the electrochromic properties of the film, we carry out an exploration of the double metal ion modification film in this paper to further improve the properties.

In this study, we successfully synthesize amorphous WO₃ films modified by Co-ion and Mo-ion on the surface of indium-tin-oxide (ITO) glass by the one-step electrodeposition method. The electrodeposition time was adjusted to control the thickness and morphology of the film, in order to obtain improved electrochromic properties. In addition, compared with the unmodified WO₃ film, the Co-ion and Mo-ion co-modified film with the same electrodeposition time has a lower charge transfer impedance and higher optical modulation rate.

2. Experimental Procedure

2.1. Chemicals

All chemicals were of analytical grade and used as received without further purification. Ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O), cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), sodium tungstate dehydrate (Na₂WO₄·2H₂O), 30% hydrogen peroxide (H₂O₂), 0.5 M sulfuric acid (H₂SO₄) and ethanol were purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China). All aqueous solutions were freshly prepared with deionized (DI) water.

2.2. Preparation of Co and Mo Modified WO₃ Films

Briefly, 0.02 mol Na₂WO₄·2H₂O, 0.15 mmol Co(NO₃)₂·6H₂O and 0.15 mmol (NH₄)₆Mo₇O₂₄·4H₂O were added to 100 mL deionized water, dissolved and mixed by ultrasonication and magnetic stirring. Then, 0.01 mol H₂O₂ was added to the above solution. At this point, many bubbles were generated, so the solution was continuously stirred to make the bubbles escape. After stirring and dissolving, H₂SO₄ was slowly added dropwise to the above solution, and the pH was measured with an accurate pH meter until the pH = 1.0. The purpose of adding acid is to convert Na₂WO₄ into H₂WO₄, waiting for further oxidation. The electrodeposition procedure was carried out through a CHI660B electrochemical workstation. ITO glass was used directly as the working electrode, and an Ag/AgCl electrode and a Pt plate served as the reference electrode and the counter electrode, respectively. The electrodeposition current density is 0.5 mA cm⁻², and the deposition durations were 60, 180 and 300 s. Finally, the prepared film was flushed several times with deionized water and air dried. For comparison, we also prepared a pure WO₃ film.

2.3. Characterization

To analyze the sample’s composition and microstructure, X-ray diffraction (XRD) was conducted on a PANalytical X’Pert Powder (Panalytical B.V., Shanghai, China). XRD data were processed using Jade6.5 software. Field-emission scanning electron microscopy (FESEM) analysis was performed on the JEOL JSM-7800F (JEOL, Tokyo, Japan) to characterize the surface morphologies. High-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and energy-dispersive spectroscopy (EDS) mapping were performed on the Talos F200S (Thermofishier, Waltham, MA, USA). X-ray photoelectron spectroscopy (XPS) experiments were performed on the ESCALAB250Xi (Thermofishier, Waltham, MA, USA), the numerical fitting of XPS was performed using Advantage software, and all of the XPS binding energies were calibrated using the contaminant carbon (C1S) as a reference. All electrochemical measurements were performed on CHI660E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) by using the typical three-electrode set-up at 25 °C in 0.5 M H₂SO₄. The optical properties of the films were studied by a Shimadzu UV-vis–NIR spectrometer. The CV measurements were conducted in a voltage window between −0.4–0.6 V (vs. Ag/AgCl) at 20 mV/s. The electrochemical impedance spectroscopy (EIS) was performed in the frequency range from 100 kHz to 10 mHz at open circuit voltage by applying a 5 mV signal. All these electrochemical measurements were carried out at room temperature.

3. Results and Discussion

As shown in Figure 1a, a certain amount of ammonium molybdate tetrahydrate, cobalt nitrate hexahydrate and sodium tungstate dehydrate were dissolved in deionized water to form a wine-red transparent solution, which was thoroughly mixed by ultrasonic and magnetic stirring. Then, H₂O₂ and H₂SO₄ were added dropwise to the above solution to make the solution Ph = 1, and at this time the solution gradually became a bright yellow transparent solution. When a constant current electrodeposition started, a blue film was quickly deposited on the surface of the ITO glass. Because the ITO glass was the cathode, the film appeared blue. The action of the electric field caused the H⁺ in the electrolyte to be injected into the formed oxide film, causing it to undergo a coloring reaction. When the deposition time increased, metal oxide continued to grow to thicken the film, so that the number of sites in contact with hydrogen ions increased, and the color became darker. During the electrodeposition process under the action of the electric field, the particles in the electrolyte moved in a directional motion and inelastically collided with the substrate, and then the particles stopped moving and were adsorbed on the surface of the substrate. After a certain period of accumulation, a series of groups of a certain size were formed, which are called nuclei. After agglomeration and nucleation, as the particles continued to deposit, the volume of the nuclei continued to increase [21]. After preparing the Co and Mo co-modified WO₃ film, we used XRD characterization to analyze the crystal structure of the film. As shown in Figure 1b, after excluding the diffraction peaks of ITO glass, there is a broad peak near 2θ ≈ 26° representing amorphous materials, indicating the presence of amorphous oxides. This result is consistent with previous reports [22,23]. We also proved that through STEM. The peaks of In₂O₃ in ITO glass are 30°, 35.3°, 37.6°, 50.9°, 55.8° and 60.5°.

SEM analysis was carried out to detect the influence of deposition time on the surface morphology and thickness of the film. Based on Figure 2a–c, we can clearly see that, as the deposition time increases, the surface morphology of the film changes dramatically. As shown in Figure 2a, when the electrodeposition time is 60 s, a dense film is formed on the surface of the ITO glass, and linear objects are formed at the cracks, similar to plants just sprouting on the ground. At this point, the thickness of the oxide film is only 0.42 μm (Figure 2d). When the electrodeposition time increases to 180 s, the cracks in the film increase and split into independent masses, similar to a dry riverbed, as shown in Figure 2b and Figure S1a. At the same time, there are many coral-like pillars in the cracks, which greatly increase the specific surface area of the film in contact with the electrolyte solution. The thickness of the film increases to 1.26 μm (Figure 2e). As the electrodeposition time continues to increase, the lumps on the surface of the ITO glass are gradually covered by coral-like pillars, as shown in Figure 2c and Figure S2. Correspondingly, the thickness of the film is increased to 1.78 μm (Figure 2f). Based on this, we can conclude that the deposition time has a significant effect on the morphology of the film. In addition, to explore the effect of Co-ion and Mo-ion modification on the morphology of the film, we also prepared a pure WO₃ film with an electrodeposition time of 180 s. Additionally, the appearance of surface cracks can be attributed to the tensile stresses induced during drying. Coupling with the rapidly and continuous deposition of particles and uneven nucleation, the irregular formation of the film creates a coral-like surface morphology speeding everywhere as shown in Figure S2. This phenomenon is similar to what was reported in previous studies in [24,25,26]. Comparing Figure 2b,e with Figure S3, it can be clearly seen that, for the same electrodeposition time, the metal ion modification has no significant effect on the film thickness, but has a greater effect on the film’s morphology. In Figure S3, there are almost no coral-like pillars in the cracks, which reduces the specific surface area of the film in contact with the electrolyte solution to a certain extent. As shown in Figure S4, according to previous studies, the Mo ions modified film is microscopically irregular porous [20], while the Co ions modified film is needle-like [19]. In our work, the films co-modified by cobalt and molybdenum ions are coral-like at the microscopic level, which results from the synergistic effect of cobalt and molybdenum ions. In addition, we also used EDS to determine the distribution of elements and atomic ratios (Table S1). Figure 2g shows that the Co, Mo and W elements are distributed very uniformly. In the nano-scale STEM-EDS (Figure S5a), the above three elements are still uniformly distributed. To determine the crystal form of the film and explore the nanoscale morphology of the film, we scraped the film on the surface of the ITO glass and dispersed it in an ethanol solution for TEM detection. The high-resolution TEM image (Figure 2h) and the HAADF-STEM image (Figure S5b) show that the WO₃: Co, Mo film is a stacked layered structure. The selected area electronic diffraction (SAED) (Figure 2h inset) proves that the WO₃: Co, Mo film prepared by electrodeposition has an amorphous structure, which corresponds to the previous XRD test. However, when the scale is enlarged to 20 nm (Figure 2i), we can observe a few nanometer-scale low-range ordered regions (the blue part in the figure). This indicates that some crystalline nanocrystals are randomly dispersed in the amorphous matrix. According to previous studies, the electrochromic properties of inorganic materials have a great relationship with their crystallinity [27,28,29]. Crystalline WO₃ deflects incident light due to the existence of its crystal lattice, so the film often shows a certain haze in the transparent state. At the same time, the ion intercalation channel of crystalline WO₃ is narrow, which is limited to cation intercalation with a smaller radius. Therefore, crystalline WO₃ exhibits poor electrochromic performance. However, the cycle stability of crystalline WO₃ is better than that of amorphous WO₃ [28,29]. Therefore, the formation of WO₃ nanocrystals is expected to be a comprehensive method to improve the electrochromic properties of WO₃ films.

To determine the chemical composition and valence state of metal ion of WO₃: Co, Mo film, XPS was performed. Fitting the XPS peaks of the high-resolution Ti 2p spectrum in the sample (Figure 3a), the peaks at 35.7, 37.8 and 41.2 eV correspond to W 4f _7/2, W 4f _5/2 and W 4f _3/2, respectively [19,30]. For W 4f _7/2 and W 4f _5/2, the higher intensity peak is the W⁶⁺ oxidation state in the film, while the other lower intensity peaks can be assigned to W⁵⁺ [31,32]. In the Mo 3d region (Figure 3b), the two peaks can be assigned to Mo⁶⁺ 3d_5/2 (232.7 eV) and Mo⁶⁺ 3d_3/2 (235.8 eV) [20]. This shows that Mo atoms mainly exist in +6 oxidation states. Figure 3c shows the high-resolution Co 2p spectrum, and the peaks at 781.4 and 797.5 eV belong to the Co 2p_3/2 and Co 2p_1/2 characteristic peaks of cobalt in the +2 valence state, respectively. In addition, the satellite peaks appearing in the cobalt spectrum belong to their corresponding characteristic peaks of oxidized substances [19]. The difference between WO₃: Co, Mo film and pure WO₃ film in the W 4f spectra is shown in Figure 3d. When the Co-ion and Mo-ion are added to the film, the peaks shift slightly to lower binding energy, illustrating that the W⁵⁺/W⁶⁺ proportion increases and the formation of more oxygen vacancy defects [33,34]. This is an important mechanism that electrochromic function can be achieved, while pure WO₃ is a stoichiometric compound whose electronic movement is almost inhibited and thus has no electrochromic function.

The CV test is an effective method to evaluate the ability of the film to insert and extract H-ion. The CV measurements were conducted in a voltage window between −0.4–0.6 V (vs. Ag/AgCl) at 20 mV/s in 0.5 M H₂SO₄ electrolyte. During each scan, when hydrogen ions are inserted, the film appears blue, and when hydrogen ions are extracted, the film becomes transparent. As shown in Figure 4a, the Co-ion and Mo-ion co-modified WO₃ films show higher anode and cathode current densities than pure WO₃ films. In addition, the increase in the CV area means that the ion storage capacity of the film becomes larger, which indicates that the addition of Co and Mo ions improves the electrochemical activity of the film. This is related to the modified nanostructure, which provides more electron transfer sites on the surface of the film. In addition, the Co-ion and Mo-ion co-modified WO₃ films prepared at different electrodeposition time have different thicknesses, so the ion storage capacity has a large difference. As the thickness of the film increases, the ion storage capacity of the film increases, which also leads to differences in optical properties. The shift of the oxidation peak to a high potential indicates that the oxidation reaction on the surface of the film is more intense, which also explains the lower transmittance of the film in the colored state. Additionally, the charge storage capacity is an important indicator to evaluate the electrochromic performance. Therefore, we calculated the capacitance of the film from the CV plot obtained by cyclic voltammetry. Compared with the WO₃ -180s film (119.9 mF), the capacitance of the WO₃: Co, Mo -180s film (131.7 mF) is larger, indicating that it has a greater charge storage capacity, which also explains the lower transmittance of the WO₃: Co, Mo -180s film in the colored state. In addition, the thickness of the film also has a great influence on the capacitance of the film, and the charge storage capacity of the WO₃: Co, Mo -300s film (237.95 mF) is stronger than that of the WO₃: Co, Mo -60s film (38.96 mF), which corresponds to the lower transmittance in the colored state.

By comparing the transmittance modulation in the visible and near-infrared regions, the effects of ion modification and deposition time on the optical properties of WO₃ films were investigated, as shown in Figure 4b. For the Co-ion and Mo-ion co-modified WO₃ films, the optical transmittance of the WO₃: Co, Mo -60s film in the bleached state (T_b) at a wavelength of 600 nm is 83.1%, and the optical transmittance of the colored state (T_c) is 18.8%, which contributes to a transmittance modulation (ΔT = Tb − Tc) of 64.3%. Similarly, at a wavelength of 600 nm, the optical transmittance of the WO₃: Co, Mo -180s film in the bleached state (T_b) is 68.2%, and the colored state (T_c) is 9.7%. Therefore, the transmittance modulation of the WO₃: Co, Mo -180s film is 58.5%, which is larger than that of the pure WO₃-180s film (67.8% of the bleached state and 24.1% of the colored state). The increase in transmittance modulation is attributed to the diffusion nanostructure formed after the addition of cobalt and molybdenum ions, which makes ion diffusion easier and maximizes the use of active areas for charge transfer reactions [19]. For the WO₃: Co, Mo -300s film, the sharp drop in T_b may be due to the over larger thickness of the film, which limits the escape of H-ion. Based on the above discussion, Co-ion and Mo-ion modification can reduce the optical transmittance of the colored state (T_c), and the electrodeposition time also has a great influence on the transmittance modulation of the film.

The electrochromic time is also an important indicator representing the performance of the film. As can be seen from Figure 4c and Figure S6, the coloring time (t_c) of the WO₃: Co, Mo -180s film is 2.7 s, and the bleaching time (t_b) is 5.6 s. These values are slightly higher than those of the pure WO₃ film (t_c = 2.5 s, t_b = 5.3 s). This is because, after the specific surface area increases, the contact area between the film and the electrolyte increases, thereby exposing more active sites. Therefore, more hydrogen ions need to be adsorbed to achieve a lower colored state transmittance. As the film thickness increases, the t_c and t_b of the WO₃: Co, Mo -300s film (t_c = 8.5 s, t_b = 8.6 s) also increase.

We used cyclic voltammetry to test the cyclic stability of the film, as shown in Figure 4d and Figure S6. After 1000 cycles, the WO₃: Co, Mo -180s film still maintains 73.1% of ion storage capacity (96.3 mF), while the pure WO₃-180s film only maintains 62.7% of ion storage capacity (75.2 mF). This shows that cobalt and molybdenum ions can improve the adhesion of the film to the glass substrate. For the WO₃: Co, Mo -60s film, even though it shows the highest transmittance modulation, the thin film is corroded by sulfuric acid during the cycle, which causes its ion storage capacity to drop quickly. For the WO₃: Co, Mo -300s film, due to the thick film, the extraction of H-ion is likely to be restricted, and some H-ion is trapped inside the film in the later stage, resulting in the performance degradation. It can be seen from the performance comparison chart (Figure 4e) that WO₃: Co, Mo -180s film shows the best overall performance. In addition, we found that metal ion modification can reduce the charge transfer barrier of the film, as shown in Figure 4f. This is because the coral-like structure provides a larger specific surface area, more intercalation sites and effective ion transmission. The resistance of the WO₃: Co, Mo -180s film (7.871 Ω) is lower than that of the pure WO₃-180s film (8.282 Ω), which explains why its CV area is larger. Additionally, the thickness of the film also has a significant effect on the impedance; as the thickness of the film increases, its resistance also increases. The resistance of the WO₃: Co, Mo -60s film (4.059 Ω) is significantly lower than that of the WO₃: Co, Mo -300s film (10.331 Ω). The WO₃: Co, Mo -180s film still exhibits good performance compared to other works in Table S2.

4. Conclusions

In summary, we used a one-step electrodeposition method to quickly deposit metal ion-modified WO₃ films with different thicknesses on the surface of the ITO film. The films prepared by the electrodeposition time of 60 s, 180 s and 300 s have thicknesses of 0.42, 1.26 and 1.78 microns, respectively, and completely different morphologies. Both Co and Mo ion modification and electrodeposition time lead to different electrochromic properties of prepared films. Compared with pure tungsten oxide film, the WO₃: Co, Mo -180s film has significantly enhanced electrochromic performance and higher transmission modulation (58.5% at 600 nm) and rapid switching speed (coloring and bleaching time are 2.7 s and 5.6 s, respectively), low impedance value and excellent cycle stability. This is because the coral-like structure formed after the modification of metal ions provides a larger specific surface area, more intercalation sites and effective ion transmission.