Electrochromic and Capacitive Properties of WO₃ Nanowires Prepared by One-Step Water Bath Method

Abstract

In this paper, WO₃ nanowires were successfully synthesized via a one-step water bath method at an appropriate temperature. The XRD (Energy Dispersive Spectrometer), SEM (Scanning electron microscope), TEM (Transmission Electron Microscope) and other characterization methods proved that the synthesized product was WO₃, and the product of water bath reaction for 9 h showed the nanowires’ structure. The nanowires were evenly distributed, and the length ranged from 2 μm to 4 μm. The results showed that the nanowires had excellent light transmittance (66%), a very short response time (1.2 s, 2 s) and excellent color rendering efficiency (115.2 cm² C⁻¹) at 650 nm. The electrochemical performance test showed that the specific capacity of the WO₃ nanowires was up to 565 F/g at 1 A/g. Change the different current densities and cycle 100 times, then return to the initial current density, accounting for 99% of the initial specific capacity of 565 F/g. We used this method for the first time to prepare tungsten oxide nanowires and investigated the bifunctional properties of the material, namely the electrochromic and capacitive properties. All of these data indicate that WO₃ nanorods have excellent electrochromic and electrochromic properties and have potential market prospects in the fields of electrochromic glass, variable glasses, advertising, and supercapacitors.

1. Introduction

The rapid development of population and industrialization have caused serious environmental pollution and energy consumption, which has led to great inconvenience to human production and life [1]. The goal of current development is to develop an energy form that is economical and clean, easy to control and convert, and can meet the needs of different occasions [2]. Transition metal oxides are widely used in charging and discharging processes, which can realize the intercalation and transition of ions and electrons. Among many transition metal oxides, WO₃ has the advantages of diverse valence states and unique configuration, as well as being low cost, non-toxic, and stable in acid and oxidation conditions [3]. This makes it widely used in electrochromic and other related technical fields and is considered to be one of the most promising electrochromic materials, attracting the interest of many researchers [4]. WO₃ has a special crystal structure and an appropriate bandgap (2.5–3.5 eV) and has been extensively studied in the fields of electrochromic [5], photocatalysis [3], sensors [6], supercapacitors [7], and solar cells [8], making it one of the most popular transition metal oxides. In the field of electrochromism, WO₃ has the characteristics of a fast response speed, a high coloring efficiency, a low activation potential gradient, and a small color change [9]. Compared with other transition metal oxides, the color state of WO₃ has stronger and more uniform light absorption, higher contrast, a better memory effect, and longer-lasting stability [10]. Klinke [11] et al. investigated the formation of tungsten oxide nanowires under chemical vapor deposition (CVD). When WO₃ is exposed to hydrogen and methane at 900 °C, dense arrays of nanowires with diameters of about 10 nm are formed. Spanu [12] et al. successfully prepared WO₃ thin films by argon plasma sputtering, which is milder and more direct, with particles of about 500 nm in length and diameter. There is no need for harsh conditions, such as high temperature (above 600), vacuum conditions, or catalysts. Hyungjoo [13] et al. fabricated single crystal tungsten oxide (WO_x) nanowires based on local stress induction. This method has significant advantages. Compared with other methods, it is simpler and enhances the compatibility of the process by controlling the temperature. Zheng [14] et al. synthesized WO₃ nanofibers on a Cu conducting fluid collector via a simple hydrothermal method. This material has a good cycle reversibility (98% at 1.0 A/g), a good cycle stability (93% after 5000 cycles), a high specific capacitance (436 F/g at 1.0 A/g), and a low charge transfer resistance (29 Ω). Assembled into an asymmetric supercapacitor, the device has a power density of 450 W kg⁻¹, an effective working range of 0–1.8 V, and an energy density of 99.0 W h kg⁻¹. There are many preparation methods for WO₃ nanomaterials, such as the hydrothermal method, which is simple and convenient, and the synthesized film, which is stable and has good electrochemical performance.

In this paper, a novel WO₃ nanowire was synthesized by one-step hydrothermal method. The XRD analysis shows that the triclinic WO₃ has been successfully prepared. The phase structure of WO₃ is analyzed using Raman spectroscopy. The test results are consistent with the XRD results. The performance of the cyclic voltammetry and the response time test of the electrochromic properties of WO₃ nanowires are excellent. It has great light transmittance (66%), a very short response time (2 s, 5 s) and excellent color rendering efficiency (50 cm² C⁻¹). WO₃ nanowires have excellent electrochemical properties, with a specific capacity up to 565 F/g at 1 A/g. Under the condition of 2 A/g, the specific capacity retention rate is 98.4% after 10,000 cycles. This study will contribute to the realization of the multifunctional applications of the material, and WO₃ nanowires have potential applications in future smart windows and energy storage devices, electrochromic glass, variable glasses, and advertising.

2. Materials and Methods

All reagents used in the experiments were not further purified. Prior to the start of the experiments, foam Ni (size: 1 × 1 × 0.1 cm³) was ultrasonically washed in ethanol and deionized water for 0.5 h, respectively.

2.1. Synthesis and Deposition of WO₃ Nanowires

Materials used in this experiment are Na₂WO₄·2H₂O and HCl. The Na₂MoO₄·2H₂O was produced by Shanghai Siyu Chemical Technology Corporation. The purity of the sample was 99%. HCl is produced by Shandong Yanhe Chemical Corporation. The purity of the product is 99.5%. The synthesis process of WO₃ nanorods is as follows: Dissolve 100 g sodium tungstate dihydrate in 500 mL deionized water, heat the prepared solution (0.5 M Na₂WO₄) at 80 °C, and then, add 3 M hydrochloric acid drops until white precipitate (pH~1) is formed. Transfer the mixture to a water bath. Cooled the samples to room temperature, remove, clean three times with ethanol and deionized water, dried at 50 °C for 8 h, and, finally, anneal in air at 350 °C for 2 h. Ultrasonically clean the conductive substrate in deionized water, acetone, and dilute hydrochloric acid for several times in turn. Immerse the cleaned transparent conductive substrate in tungsten oxide mixed solution and react at 90 °C for 2 h, and deposit tungsten trioxide on the nickel foam conductive substrate.

2.2. Microstructure and Phase Characterization

Scanning electron microscopy (SEM, JEOL-6360Lv, Tokyo, Japan) and transmission electron microscopy (TEM, JEOL JEM-201, Tokyo, Japan) were used to analyze the microstructure of the prepared materials. The phase structure of the as-prepared materials was analyzed by X-ray diffraction (Aolong Y2000, Liaoning, China, Cu Kα, λ = 0.15405 nm). The as-prepared samples were analyzed by energy dispersive spectroscopy (EDS, Horiba 7021-H, Shanghai, China). micro-Raman analysis (Renisha Raman Imaging Microscope inVia, Shanghai, China. the spatial resolution is 0.5 μm horizontally and 2μm vertically, and the spectral resolution is 1 cm⁻¹. The test wavelength was 532 nm, and the laser power was 6.0 mW).

2.3. Electrochemical Test

The CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) was used for electrochemical measurement. The electrolyte was 3 M KOH aqueous solution. In the three-electrode system, WO₃ nanomaterial was used as the working electrode, a platinum plate was used as the counter electrode, and a saturated calomel electrode (SCE, Shanghai Analysis domain Instrument Equipment Co., Ltd., Shanghai, China) was used as the reference electrode. Electrochemical impedance spectroscopy (EIS, Beijing Yicheng Hengda Technology Co., Ltd., Beijing, China) was performed by applying an AC voltage of 8 mV in the frequency range of 0.01 Hz to 5 kHz. Specific capacity and charge are calculated by the following formula:

Cs = It/mΔV

(1)

where Cs (F/g) is the specific capacity, i(A) is the current density, Δt (s) is the discharge time, and ΔV (V) is the pressure drop during the discharge process.

3. Results

The XRD and Raman of the products prepared in the experiment are shown in Figure 1. It can be seen from Figure 1a that the five diffraction peaks between 20° and 70° are representative peaks of WO₃ (JCPDS No. 83-0949) of the trioblique phase, which are (002), (020), (202), (400), (214), respectively. Other peaks also fit the standard card. There are no other diffraction peaks, indicating very high purity of the sample. The diffraction peak is narrow and sharp, indicating that the crystallization degree of the prepared product is high. Figure 1b shows the Raman spectrum of WO₃ thin film. There are clear strong spectra at 314, 645, 796, and 943 cm⁻¹. The strong frequency band of 796 cm⁻¹ corresponds to a symmetric tensile vibration [15], and the strong frequency band of 645 cm⁻¹ corresponds to the asymmetric tensile vibration of the W-O bond. The band in the range of 300~600 cm is caused by the bending vibration of the O-W-O bond [16]. Concerning the basal peak at 943 cm⁻¹, in the literature [17], this band is usually assigned to the stretching mode of the W = O double bond. The results are consistent with the XRD analysis and confirm the formation of WO₃. In order to observe the morphological features of WO₃ nanowires, we carried out SEM tests on the material, as shown in Figure 2. Figure 2a is the SEM picture of the nanowires. It can be seen that a large number of nanowire structures are formed on the foam nickel conductive substrate by the WO₃ nanomaterials.

The XRD and Raman of the products prepared in the experiment are shown in Figure 1. It can be seen from Figure 1a that the five diffraction peaks between 20° and 70° are representative peaks of WO₃ (JCPDS No. 83-0949) of the trioblique phase, which are (002), (020), (202), (400), (214), respectively. Other peaks also fit the standard card. There are no other diffraction peaks, indicating very high purity of the sample. The diffraction peak is narrow and sharp, indicating that the crystallization degree of the prepared product is high. Figure 1b shows the Raman spectrum of WO₃ thin film. There are clear strong spectra at 314, 645, 796, and 943 cm⁻¹. The strong frequency band of 796 cm⁻¹ corresponds to a symmetric tensile vibration [15], and the strong frequency band of 645 cm⁻¹ corresponds to the asymmetric tensile vibration of the W-O bond. The band in the range of 300~600 cm is caused by the bending vibration of the O-W-O bond [16]. Concerning the basal peak at 943 cm⁻¹, in the literature [17], this band is usually assigned to the stretching mode of the W = O double bond. The results are consistent with the XRD analysis and confirm the formation of WO₃. In order to observe the morphological features of WO₃ nanowires, we carried out SEM tests on the material, as shown in Figure 2. Figure 2a is the SEM picture of the nanowires. It can be seen that a large number of nanowire structures are formed on the foam nickel conductive substrate by the WO₃ nanomaterials.

In order to observe the structure of the nanowire more carefully, we conducted a high-power SEM test on the structure of the nanowire, as shown in Figure 2b. It can be observed that these nanowires are evenly distributed with a length between 2 and 4 μm. The thin WO₃ nanowires are evenly arranged and sufficiently separated to form a unique array of nanowires. Figure 2c,d are the SEM mapping tests. It can be seen that the material only contains W and O elements, indicating that WO₃ nanowires do not contain other impurities. In order to analyze the effect of time on the synthetic products, we tested SEM images of reaction time 1, 4, 9, and 18 h under the same growth parameters, as shown in Figure 3.

Figure 3a is the SEM image of the hydrothermal reaction for 1 h. We can find from this figure that a large number of particles are closely arranged and combined with each other to form clusters of particles. Figure 3b shows the SEM image of the hydrothermal reaction for 4 h. It can be found that there are many linear nanostructures interlaced with each other, but a small number of cluster particles can still be found. When the reaction time reaches 9 h, we can see a large number of nanowires, and the presence of cluster particles is almost invisible. When the reaction time is 18 h, it can be seen from Figure 3d that the nanowires obtained will become larger and aggregate or fragment. The nanowires become larger in size, and the linear structures collapse and pile on top of each other. Through the above analysis, we obtained the schematic diagram of the formation of WO₃ nanowires, as shown in Figure 4.

There is a large amount of WO4²⁺ and Na⁺ in Na₂WO₄ solution. When the appropriate HCl is added, the WO₄²⁻ and H⁺ in the solution will react to generate a WO₃ precursor [7]. These precursors have very high surface energy, and they are attracted to each other. As the reaction time increases, these materials aggregate and grow, forming clusters of particles. During the growth process, the cluster particles will determine their own growth direction and further grow into nucleation sites, and the cluster particles gradually form the structure of the WO₃ nanowires. Finally, when the reaction time reaches 18 h, the diameter of the nanowires will increase, the nanowires will collapse, and the structure will be completely changed. The chemical reaction equation for the substance is as follows:

Na₂WO₄ → 2Na⁺ + WO₄²⁻

(2)

WO₄²⁻ + 2H⁺ → H₂WO₄

(3)

H₂WO₄ → WO₃ + H₂O

(4)

In order to understand the electrochromic properties of WO₃ more accurately, we tested the response times of substances reacting 1 h, 4 h, 9 h, and 18 h under the same conditions, as shown in Figure 5.

The results showed that the response time after dyeing and bleaching were WO₃ 9 h (2 s, 5 s) < WO₃ 4 h (3 s, 6 s) < WO₃ 18 h (4 s, 7 s) < WO₃ 1 h (5 s, 9 s), respectively. This indicates that when the reaction time is 9 h, the generated WO₃ nanowires have the fastest response time. The absorbable light sensitivity of WO₃ is 450–1000 nm, which can be widely used in various electrochromic materials. The transmission spectra of WO₃ nanowire films prepared in the experiment at different voltages (−2 to +2 V) are shown in the Figure 6a. At 650 nm, the transmittance difference between the colored state and the bleached state is 66%. In the voltage range (+2 to −2), a color change occurs only at a negative voltage and dark blue. After moving to positive voltage, the film is bleached and returned to its original transparent state, as shown in the illustrations in Figure 6a.

The current-time response is shown in Figure 6b. The dyeing reaction time was about 1.2 s, and the bleaching reaction time was about 2 s. In order to study the influencing factors of a WO₃ electrochromism, we analyzed the electrochromism mechanism of WO₃ in the lithium-ion electrolyte. The principle can be explained as follows: electron insertion/stratification and the charge balance of lithium ions, which determine the coloration/bleaching process of WO₃ [18]. In addition, coloration efficiency (CE) is one of the most commonly used methods to determine the effective optical transmission with the change of the applied voltage. Coloring (T_c) or bleaching (T_b) occurs at certain wavelengths when the film is exposed to sunlight. Color efficiency (CE) is the ratio of the optical density (OD) to the charge density (Q) per unit area [19]. The relevant formula is as follows:

$$\mathrm{CE}=\frac{\Delta \mathrm{OD}}{\Delta \mathrm{Q}}=\frac{\log \left(\mathrm{Tb}/\mathrm{Tc}\right)}{\Delta \mathrm{Q}}$$

(5)

At 650 nm, the color efficiency (CE) of WO₃ nanowires is 115.2 cm² C⁻¹.

In order to accurately understand the WO₃ transmittance at different hydrothermal reaction times, we analyzed the transmittance at 1 h, 4 h, 9 h, and 18 h under the same conditions, as shown in Figure 7. As can be seen from Figure 7, the transmittance of WO₃ 9 h (60%) > WO₃ 4 h (50%) > WO₃ 18 h (40%) > WO₃ 1 h (37%). The change in light transmittance of the hydrothermal reaction for 9 h is the largest, indicating that the electrochromic performance is the best, which is consistent with the above analysis. The excellent electrochromic properties of WO₃ nanowires are attributed to the excellent WO3 material and nanowire array, which effectively improve the porosity of the material, improve the specific surface area and active site of the material, provide more channels, accelerate the rate of ion implantation and extraction, reduce the accumulation of ions, and accelerate the diffusion rate. We compare our research work with other literature, such as

Table 1. Comparison of Electrochromic Properties.

Material	Wavelength (nm)	Transmittance (%)	Coloration Efficiency (cm2 C−1)	Response Time (s)	Cyclic Stability	Ref.
NiO@C		35%	113.5	4.5, 8.5	90.1% (20,000)	[20]
WO3 nanoparticles	650	54.1%	83.87	1.1, 1.2	80.3% (10,000)	[21]
Porous WO3 amorphous	633		56.8	3.2, 5.6	95.7% (2000)	[22]
WO3	633		121	16, 13	94.5% (2000)	[23]
NiMoO4 nanoflake	630	57%	31.44		65% (2500)	[24]
Ag/NiO nanowires	550	65.7%	51.9		80.7% (10,000)	[25]
WOx nanorod	800	67.7%	101	15, 21	91.8% (2000)	[26]
WO3 platelets	500		67	2.28, 1.98	87% (1000)	[27]
WO3 Nanowires	650	66%	115.2	1.2, 2	98.4% (10,000)	This paper

.

In order to explore whether the reaction product has a potential application value for electrode materials, we carried out electrochemical tests on the material. Figure 8a shows the cyclic voltammetry curves of WO₃ nanowire film at a voltage window of 0.8 V at 5, 20, 40, 70, and 100 mV/s. It can be seen from the figure that the cyclic voltammetry curves of different current densities are similar to rectangles, and the curve increases proportionally with the increase of sweep. The curve area and peak current both increase continuously, indicating that the material has good electrochemical behavior [28]. Figure 8b shows the charge and discharge test of the material at the current density of 1, 3, 5, 8, and 10 A/g. According to Formula 1, we calculate the specific capacities at different current densities. As shown in Figure 8c, the specific capacities at 1, 2, 3, 5, and 8 A/g are 565, 530, 505, 470 and 455 F/g, respectively. Figure 8d shows the specific capacity retention of the material after 100 cycles of alternating current densities of 1, 5, 3, 10, 8 and 1 A/g. When the current density is 1 A/g, the specific capacity of the material is 565 F/g. Change the different current density, cycle 100 times each time, and then return to the original current density of 1 A/g, the specific capacity is 560 F/g, which is 99% of the original specific capacity of 565 F/g. It can be found that when the current density is changed, the specific capacity of the material attenuated is very small, indicating that the material has good rate performance. Cyclic stability is also an important factor for evaluating the performance of the material. In the experiment, we tested the specific capacity retention rate of the material after 10,000 cycles at 2 A/g, as shown in Figure 8e. It can be seen that after 10,000 cycles, the specific capacity of the material reaches 98.4% of the initial specific capacity, indicating that the device has a good cycle stability. Figure 8f is the Nyquist diagram of the WO₃ nanowire film after the first and 10,000th cycles. In the high-frequency region, there is no significant difference in the curve, and the structure remains good, indicating that there is basically no deformation of the active substance, and the conductivity is basically unchanged. The slope in the low-frequency region decreased slightly, indicating a slight increase in the diffusion resistance, mainly because the electrode material may fall off during the multiple charge-discharge processes [29].

Table 2. Comparison of electrochemical properties.

Material	Current (A/g)	Specific Capacity (F/g)	Cyclic Stability	Ref.
hierarchical porous carbon/WO3	0.5	429.6	94.3% (5000)	[30]
WO3·2H2O/bamboo charcoal	0.5	391	82% (10,000)	[31]
WO3	0.5	432	86.6% (10,000)	[16]
WO3@CuO	1	284	85.2% (1500)	[32]
graphene−WO3 nanowire	1	465	97.7% (2000)	[33]
WO3	1	508.9	91.42% (2000)	[34]
WO3·H2O	2	154.0	91.5% (1000)	[35]
Amorphous WO3	1	436	93% (5000)	[14]
WO3 nanowires	1	565	98.4% (10,000)	This paper

shows a comparison of WO₃ performance with references.

The results of the above experimental data show that the prepared WO₃ nanowire material has a high electrochemical performance. The main reason is that the nanowires are evenly distributed and have a large specific surface area, which makes the ion diffusion distance shorter and exposes many active sites, so that they have a faster kinetic performance and high-speed spontaneous energy and improve the electrochemical performance of the material. Through the above analysis, it can be concluded that WO₃ nanowires have excellent electrochromic and electrochemical properties and have a broad application market in the fields of electrochromic glass, variable glasses, advertising, and supercapacitors.

4. Conclusions

In this paper, WO₃ nanowires were successfully synthesized via a one-step hydrothermal method at a suitable temperature. The nanowires were uniformly distributed, and the length was between 2 and 4 μm. According to the experimental results, we obtained the growth mechanism of WO₃ nanorods. The electrochromic properties and capacitors of the synthesized product were studied. The data show that the material has excellent light transmittance (66%), a very short response time (1.2 s, 2 s) and excellent color rendering efficiency (115.2 cm² C⁻¹), as well as excellent electrochemical properties. Under the condition of 1A/g, the specific capacity is up to 565 F/g, and, under the condition of 2 A/g, the specific capacity retention rate after 10,000 cycles is 98.4%. The excellent properties of WO₃ nanowires are promising in the fields of electrochromic glass, variable glasses, advertising, and electrode materials for supercapacitors.