Electrodeposition of V₂O₅ on TiO₂ nanorod arrays and their electrochromic properties

Abstract

This paper shows a simple approach for the electrodeposition of V₂O₅ nanoparticles on a TiO₂ nanorod arrays substrate by combining hydrothermal and electrochemical deposition methods. The electrochemical and optical properties of the TiO₂/V₂O₅ hybrid film have been investigated and the results show that the hybrid film has obviously better electrochemical and electrochromic properties compared with the single V₂O₅ thin film. The TiO₂/4cir-V₂O₅ hybrid film shows the most improved electrochromic properties with excellent cyclic stability, outstanding transmittance modulation (50% at 780 nm) and high coloration efficiency (28 C⁻¹ cm² at 780 nm). The modified electrochromic properties are mainly due to the TiO₂ nanorod array structure, which contributes to improve the structural stability of the V₂O₅ film and benefits the intercalation/deintercalation process of Li⁺ ions within the V₂O₅ film.

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1. Introduction

Electrochromism is the reversible and visible change in transmittance and/or reflectance of the films induced by an electrochemical oxidation–reduction reaction.¹ The majority of applications of electrochromic devices (ECDs) include: “smart windows” in cars and buildings, reusable price labels and controllable light-reflective or light-transmissive display devices.^2,3 In general, a well performed electrochromic film must possess high optical contrast, good optical memory and cyclic stability.⁴

Many inorganic and organic materials present electrochromism, for inorganic electrochromic materials, especially the transition metal oxides that include oxides of W, V, Mo, Ti (cathodically coloring) and oxides of Ni, Co and Ir (anodically coloring), have attracted a broad interest because of their good performance.^5,6

V₂O₅ has been extensively investigated for its excellent properties like layered structure with high charge capacity,⁷ good Li⁺ ion intercalation ability,⁸ multicolor display⁹ and low valence vanadium state which leads to easy color changes.¹⁰ In addition, V₂O₅ performs exceptional electrochromic behavior owning to its both anodic and cathodic electrochromism properties.^9,11 But there are many disadvantages of using V₂O₅ films for ECDs: poor cycle reversibility,¹² low electrical conductivity and diffusion coefficient of Li⁺ ions,¹³ narrow optical modulation and low coloration efficiency.¹⁴ Extensive efforts have been dedicated to improve the electrochromic properties of V₂O₅, such as stability, coloration efficiency, color contrast and reversibility.

A TiO₂ electrode with a one-dimensional (1-D) nanorod array has attracted a great deal of attention since its transport properties related to electrons and photons are largely affected by thei one-dimensional geometry.¹⁵ Oriented nanorod or nanowire arrays provide a direct pathway for charge transport¹⁶ which leads to increased electron diffusion rate,¹⁷ and they also provide well-aligned characteristics and high specific surface area.¹⁸ Because of these dramatically enhanced properties, nanorod array films of titanium oxide (TiO₂) have been widely used in photocatalysis,^19,20 dye-sensitized solar cells (DSSCs)^17,21 and electrochromic devices (ECDs).^22,23

Titanium–vanadium mixed-oxide thin films have been prepared through several methods, such as sol–gel methods,^24,25 co-sputtering methods²⁶ and anodization.²⁷ However, reports about the fabrication of TiO₂/V₂O₅ core/shell structure hybrid films are rare. In addition, in our previous investigations we realized that the V₂O₅ film can be electrodeposited on a rough substrate (NiO nanoporous substrate) more easily than a smooth substrate (ITO), and the electrochromic properties of V₂O₅ were enhanced effectively. The cycling data showed that NiO/V₂O₅ composite film exhibits a better reversibility than V₂O₅ alone. In our new research, we obtained a simple method for preparing the TiO₂/V₂O₅ core/shell structure hybrid film by electrodepositing the V₂O₅ film on a TiO₂ nanorod array substrate (prepared via a hydrothermal method). Also the effect of the underlying TiO₂ nanorod array substrate on the electrochromic properties of V₂O₅ has been studied.

2. Experimental

2.1 Materials

Titanium n-butoxide, concentrated hydrochloric acid, vanadyl sulfate, sodium dodecyl sulfate, sodium chloride, concentrated sulphuric acid, lithium perchlorate, propylene carbonate (PC) and absolute ethyl alcohol were purchased from commercial sources and used without any further purification. FTO coated glass substrates (7 Ω □⁻¹) were purchased and cut into small pieces (2 × 4 cm² in size) which were cleaned using an ultrasonic method in acetone, ethanol and de-ionized water for 0.5 h, respectively. 1 M nonaqueous solution of LiClO₄ in propylene carbonate (PC) was prepared as the electrolyte.

2.2 Preparation of TiO₂ nanorod array film

TiO₂ nanorod array film has been prepared via hydrothermal method as reported elsewhere.²⁸ The precursor solution was prepared by mixing 15 ml concentrated hydrochloric acid, 15 ml de-ionized water and 0.5 ml titanium n-butoxide in a 100 ml beaker at room temperature. This fresh mixed solution and a clean FTO glass substrate were transferred to a Teflon-lined stainless steel autoclave (45 ml in volume), where FTO substrate was submerged in the solution and placed at an angle against the wall of the Teflon-liner with the conducting side facing down. The hydrothermal synthesis was conducted at 150 °C for 4.5 h in an electric oven. After the synthesis, the FTO substrate was taken out, rinsed extensively with de-ionized water and dried in ambient air. The obtained TiO₂ nanorod array films were dried at 80 °C for 12 h and then annealed at 450 °C in air for 0.5 h.

2.3 Preparation of V₂O₅ film

The V₂O₅ film was electrodeposited on FTO glass by cyclic voltammetry using a three-electrode system, with Ag/AgCl as the reference electrode, a Pt sheet (1 × 4 cm²) as the counter electrode and FTO glass as the working electrode. The precursor solution was obtained by mixing 1.6 g VOSO₄·xH₂O as the source of vanadium, 0.1 g sodium dodecyl sulfate as the surfactant and 0.3 g NaCl in a mixed solution of 22.5 ml ethanol and 17.5 ml de-ionized water. The right amount of H₂SO₄ was added in the solution to keep the pH value in the range of 2–3. Then 4 potential cycle scans were carried out between 0 V and 2.7 V with a rate of 50 mV s⁻¹. After scanning the yellow green film of V₂O₅ was washed with absolute ethyl alcohol to fully clean up the loosely bounded particles on the surface and then it was dried at 80 °C for 12 h.²⁹

2.4 Preparation of hybrid film of TiO₂/V₂O₅

The hybrid film of TiO₂/V₂O₅ was obtained through the electrodeposition of V₂O₅ on to the TiO₂ nanorod array substrate, which served as the working electrode. This required 1, 4 and 8 potential cycles at a potential between 0 V and 2.7 V with a scanning rate of 50 mV s⁻¹ to prepare the hybrid film of TiO₂/V₂O₅. The hybrid films of TiO₂/V₂O₅ conducted with different potential cycles were named as TiO₂/1cir-V₂O₅, TiO₂/4cir-V₂O₅ and TiO₂/8cir-V₂O₅, respectively. After scanning, the TiO₂/V₂O₅ hybrid films were flushed with absolute ethyl alcohol to wipe off the loosely bounded particles from the films and dried at 80 °C for 12 h.

2.5 Characterization

The structures of the films were tested using X-ray diffraction (SHIMADZU XRD-7000) with Cu Kα radiation. The surface morphologies of the samples were obtained through scanning electron microscopy (SEM, JEOL JSM-7600F). The electrochromic spectra properties were observed with UV-visible absorption spectra, measured using a spectrophotometer (UV-2550, SHIMADZU). The chromaticity was acquired with a spectrophotometer (SP60, X-Rite).

The kinetics of coloration and bleach for the films were measured using a spectrophotometer (UV-2550, SHIMADZU) in cooperation with a CHI660C electrochemical workstation. The step voltages for the V₂O₅ film and TiO₂/V₂O₅ hybrid film were set up between −0.8 and 1.5 V, and the time for characterization was 20 s. The homemade three-electrode system (platinum wire as the counter electrode, Ag wire as the reference electrode and the electrochromic film on FTO with the size of 0.9 × 3 cm² as the working electrode) was inserted vertically in the cuvette, which was filled with 1 M LiClO₄ in propylene carbonate (PC) as the electrolyte. To insure the light path was unobstructed, the reference electrode and counter electrode were placed on the side of working electrode.

The characteristics of the electrochromic processes were investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), and the measurements were carried out in a three-electrode system by using a CHI660C electrochemical workstation. A platinum sheet was used as the counter electrode, Ag/AgCl as the reference electrode and 1 M LiClO₄ in propylene carbonate (PC) as the electrolyte.

3. Results and discussion

3.1 Morphology and crystalline structure

The crystal structures of the electrochromic films can be confirmed by the investigations of XRD (X-ray diffraction). Fig. 1 illustrates the XRD patterns of FTO, a TiO₂ nanorod array film annealed at 450 °C for 0.5 h and the TiO₂/V₂O₅ hybrid films. From Fig. 1b, the annealed TiO₂ film reveals diffraction peaks at 36.00°, 41.17°, 50.45°, and 62.66°, which correspond to the (101), (111), (211), and (002) crystal planes of the tetragonal TiO₂ phase (JCPDS 21-1276), respectively. It can be seen clearly that the polycrystalline TiO₂ film has been prepared successfully through the hydrothermal process and heat treatment. At about 8°, a very important peak of V₂O₅ appears, which indicates that the V₂O₅ film has been prepared via electrochemical deposition. As shown in Fig. 1d–f, all the TiO₂/V₂O₅ hybrid films have identical diffraction peaks as shown in Fig. 1c, and this means that V₂O₅ film has an amorphous structure. In addition, there is a gradual reduction of the peak intensity for TiO₂ with the increasing thickness of V₂O₅. It has been reported that the highly-ordered nanoarray structure is helpful for improving the electrochromic performance and long term stability.³⁰ Thus, this nanoarray TiO₂ film can enhance the durability of V₂O₅ when electrodeposited on the surface of polycrystalline TiO₂ nanorod array films.

Fig. 1 XRD pattern of FTO, annealed TiO₂ film, V₂O₅ film and TiO₂/V₂O₅ hybrid films.

Fig. 2 shows the top surface and cross section SEM images of the TiO₂ nanorod array film, the V₂O₅ film and the hybrid films of TiO₂/V₂O₅. As the cross-section morphology shows, the length of the TiO₂ nanorods is about 677.8 nm, the thickness of the pure V₂O₅ film and TiO₂/1cir-V₂O₅, TiO₂/4cir-V₂O₅, and TiO₂/8cir-V₂O₅ hybrid films are 641.3 nm, 816 nm, 998.3 nm and 1.46 μm, respectively. Before the electrodeposition of V₂O₅ in Fig. 2a, the TiO₂ nanorod arrays uniformly cover the surface of the FTO substrate. This intensive film consisted of a large collection of one-dimensional nanorods growing vertically on the substrate. It has been pointed out that one-dimensional nanorod structures generally provide channels for electronic transmission, which are responsible for the diffusion of Li⁺ ions over short lengths.¹⁸ Fig. 2b reveals that V₂O₅ film were electrodeposited directly on the FTO substrate, which is accumulated by massive nanoparticles with lots of pores on the surface. Compared with the crystalline sample, this amorphous and nanoporous structure is beneficial to intercalate larger amounts of Li⁺ ions.³¹ For the TiO₂/V₂O₅ hybrid film (Fig. 2c–e), V₂O₅ nanoparticles cover the top of the TiO₂ nanorod arrays and present an interconnected network structure. With increasing electrodeposition cycles, the porosity of the hybrid film decreased. Clearly, after the TiO₂ nanorod array film was electrodeposited with 8 potential cycles, as shown in Fig. 2e, there are few void spaces on the surface of the hybrid film.

Fig. 2 Surface and cross-section SEM images of (a) V₂O₅, (b) TiO₂, (c) TiO₂/1cir-V₂O₅, (d) TiO₂/4cir-V₂O₅, and (e) TiO₂/8-cirV₂O₅.

3.2 Electrochemical and electrochromic properties

The cyclic voltammograms were recorded between −1.0 V and 1.8 V (versus Ag/AgCl) at a scan speed of 50 mV s⁻¹ in a 1 M LiClO₄/PC electrolyte to study the electrochemical and electrochromic properties of the films. For the TiO₂ nanorod array film (Fig. 3a), there is one pair of redox peaks at −0.46 V and −0.87 V. The cathodic peak is ascribed to Ti⁴⁺ reduction coupled with Li⁺ intercalation, as shown in eqn (1), whereas the anodic peak is assigned to Li⁺ release.³²

Fig. 3 Cyclic voltammograms of (a) TiO₂, (b) V₂O₅, (c) TiO₂/1cir-V₂O₅, (d) TiO₂/4cir-V₂O₅, and (e) TiO₂/8cir-V₂O₅ films measured in 1 M LiClO₄/propylene carbonate.

The transition from TiO₂ to Li_xTiO₂ after the intercalation and deintercalation of Li⁺ ions causes the color change of the film in visible range. During the process of the cathodic scan, due to the insertion of Li⁺ ions, the film becomes colored (light blue). While during the reverse anodic scanning process, the film bleaches (transparent) again following the extraction of Li⁺ ions.

The V₂O₅ film (Fig. 3b) shows a completely invertible CV graph where the cathodic peak occurs at −0.4 V, whereas the anodic peak occurs at 0.7 V. Along with the redox peaks, the color of the film changes obviously, blue-gray (at −0.6 V) to green yellow (at 0.6 V) and then green yellow to orange-yellow (at 1.3 V). This was attributed to the injection/extraction of Li⁺ ions in the V₂O₅ matrix and can be explained by the following equation:^33–35

At the cathodic scanning cycle, a colored vanadium bronze (Li_xV₂O₅) was formed caused by the simultaneous intercalation of electrons and Li⁺ ions into the V₂O₅ film, and this process made the reduction of pentavalent vanadium (V⁵⁺) to its lower valence state (V⁴⁺). During the reverse anodic scan, the electrons and Li⁺ ions were deintercalated out of the V₂O₅ film leading to the oxidation of lower valence vanadium (V⁴⁺) to its pentavalent state (V⁵⁺).

As shown in Fig. 3c–e, the hybrid films show similar reduction peaks and oxidation peaks in contrast with the CV of the V₂O₅ film. This is mainly because the V₂O₅ layer in the hybrid films plays the main role in the intercalation and deintercalation process of Li⁺ ions. While the TiO₂/4cir-V₂O₅ hybrid film in Fig. 3d shows a sharp increase in current density in contrast with that of the single V₂O₅ film, which indicates that the composition of V₂O₅ with TiO₂ nanorod arrays promotes the process of Li⁺ ion insertion into/extraction from the host lattice. Besides, it can be seen that the current of the film shifts to higher values with the increase of electrodeposition time, which indicates that the amount of Li⁺ ions and electrons incorporated into the film increases. This is mainly because the film electrode contains more active material with the duration of electrode position.

The cycle life performance of the V₂O₅ film (Fig. 4a) and TiO₂/V₂O₅ hybrid films (Fig. 4b–d) was studied using 500 cycles with a fixed scan speed of 50 mV s⁻¹. It can be seen in Fig. 4 that the decrease of the current for the TiO₂/1cir-V₂O₅ and TiO₂/4cir-V₂O₅ hybrid films is much smaller than that of the V₂O₅ film. The result shows that the hybrid films have a more stable electrochemical response up to 500 cycles and good cyclic stability, which suggests the improved performance of V₂O₅ as an electrochromic material in a hybrid structure for use in ECDs. However, for the TiO₂/8cir-V₂O₅ hybrid film, the current also has a sharp decrease which is similar to Fig. 4a. The reason is that during the cyclic process of the pure V₂O₅ film, the Li⁺ ion intercalation/deintercalation capacity of V₂O₅ decreases with successive charging and discharging cycles, which may be due to an increase in the electrical resistance produced by Li⁺ ions trapped in structural distortions.³⁶

Fig. 4 Cyclic voltammograms of (a) V₂O₅, (b) TiO₂/1cir-V₂O₅, (c) TiO₂/4cir-V₂O₅, and (d) TiO₂/8cir-V₂O₅ films for 500 cycles.

3.3 UV-visible transmittance spectra

We further investigated the transmittance modulation and switching response of the electrochromic film through optical transmittance measurements. Fig. 5 shows the optical transmittance spectra of the V₂O₅ film and TiO₂/V₂O₅ hybrid films in their colored and bleached states under different voltages between 350 nm and 800 nm in 1 M LiClO₄/PC electrolyte.

Fig. 5 Transmittance spectra of (a) V₂O₅, (b) TiO₂/1cir-V₂O₅, (c) TiO₂/4cir-V₂O₅, (d) TiO₂/8cir-V₂O₅, and (e) TiO₂ films at different voltages.

As can be seen in Fig. 5a, the transmittance modulation (ΔT = T_b − T_c, T_b and T_c denote transmittance in bleached and colored state) in the wavelength range of 600–800 nm of the V₂O₅ film is large, with transmittance modulation of 30% at 780 nm. When the wavelength is below 550 nm, the transmittance modulation becomes small. Compared with the transmittance of the V₂O₅ film, the transmittance of the TiO₂/V₂O₅ hybrid films (Fig. 5b–d) has a similar shift at different voltages. With the increase of the electrodeposition cycle-index of V₂O₅, the hybrid films become thicker, which leads to the lower T_b and T_c for the TiO₂/V₂O₅ films. However, the TiO₂/4cir-V₂O₅ film has the largest transmittance modulation (50% at 780 nm), as compared to 18% for TiO₂/1cir-V₂O₅ and 45% for TiO₂/8cir-V₂O₅.

3.4 UV-visible absorption spectra

Fig. 6 shows the absorption spectra of the V₂O₅ film and TiO₂/V₂O₅ hybrid films in the colored and bleached states. Fig. 6a shows that the absorption of the V₂O₅ film changed obviously depending on whether the film was at a voltage of 1.5 V or −0.8 V. In the bleached state at 1.5 V, a strong absorption occurred in the UV range of 350–400 nm for the V₂O₅ film. However, the absorption peak appeared around 500 nm in the colored state (−0.8 V). This was mainly caused by the Li⁺ ions interacting with the film. In the colored state, the absorption peak at long wavelengths was caused by V⁴⁺. While in the bleached state, V⁵⁺ produced the absorption at short wavelengths. In comparison with the V₂O₅ film absorption spectra, the TiO₂/V₂O₅ hybrid films show a similar trend but the absorption in the range of 450–350 nm is stronger. This is mainly due to the addition of the TiO₂ coated substrate which has a very strong absorption at short wavelengths near the ultraviolet region. Besides, the higher V₂O₅ electrodeposition cycle-index results in the stronger of absorption of TiO₂/V₂O₅ films obviously.

Fig. 6 Absorption spectra of (a) V₂O₅, (b) TiO₂/1cir-V₂O₅, (c) TiO₂/4cir-V₂O₅, (d) TiO₂/8cir-V₂O₅, and (e) TiO₂ films at different voltages.

3.5 Colorimetry

Colorimetry analysis of the samples can quantitatively define and compare the colors of electrochromic materials. Generally, the color for colorimetric analysis can be represented with three parameters: luminance (L*), hue (a*) and saturation (b*). The relative L*, a* and b* values of the V₂O₅ film and TiO₂/V₂O₅ hybrid films at different states were measured (Table 1). At the same time, the photos of the films are provided so that the color changes at different states can be observed visually. At 1.5 V, 0.6 V and −0.8 V, the color of the TiO₂/V₂O₅ hybrid films changes from orange yellow (at 1.5 V) to green yellow (at 0.6 V) and blue gray (at −0.8 V). Because the V₂O₅ layer is the main functional part in the TiO₂/V₂O₅ hybrid films, the hybrid films have no evident change in their color except becoming darker with the thickness of the V₂O₅ layer increasing.

Table 1 The color parameters and optical images of the V₂O₅ and TiO₂/V₂O₅ films

Material	Voltage	L*	a*	b*	Color
V2O5	1.5 V	56.56	14.86	65.84
	0.6 V	52.18	−9.38	38.40
	−0.8 V	38.00	4.51	5.91
TiO2/1cir-V2O5	1.5 V	54.04	10.31	66.16
	0.6 V	50.46	−10.27	35.81
	−0.8 V	33.58	6.54	12.53
TiO2/4cir-V2O5	1.5 V	52.75	20.64	59.34
	0.6 V	40.80	−6.59	33.03
	−0.8 V	24.00	3.36	6.35
TiO2/8cir-V2O5	1.5 V	45.02	27.93	38.16
	0.6 V	29.07	−3.64	10.30
	−0.8 V	21.00	0.07	0.34

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3.6 Switching speed

Switching speed is one of the major parameters in ECDs and a fast switching speed is necessary for a well-performing ECD. In this paper, switching time is calculated for reaching 90% of the maximum transmittance change between bleached (1.5 V) and colored (−0.8 V) states of the electrochromic film at a wavelength of 780 nm. In order to obtain the switching characteristics of the electrochromic films, chronoamperometry (CA) and the corresponding in situ transmittance measurements were employed. Fig. 7 shows the results of T_b and T_c for the electrochromic films and the switching speeds are listed in Table 2. The TiO₂/4cir-V₂O₅ hybrid film exhibits a faster switching time than other reported electrochromic films based on V₂O₅.^37–40 With the increase of V₂O₅ deposition time, the switching speed of the TiO₂/V₂O₅ hybrid films becomes slower. In contrast with the switching speed of the V₂O₅ film, the TiO₂/4cir-V₂O₅ hybrid film has similar coloring and bleaching time. However, the range of change in transmission is much larger for the latter.

Fig. 7 CA and in situ transmittance curves for (a) V₂O₅, (b) TiO₂/1cir-V₂O₅, (c) TiO₂/4cir-V₂O₅, and (d) TiO₂/8cir-V₂O₅ films at 780 nm with a voltage interval between −0.8 V (20 s) and 1.5 V (20 s).

Table 2 Switching speed of the V₂O₅ and TiO₂/V₂O₅ thin films

Samples	Optical modulation range (%)	Switching speed (s)
		Tb	Tc
V2O5 at 780 nm	30%	4	2
TiO2/1cir-V2O5 at 780 nm	18%	1	1
TiO2/4cir-V2O5 at 780 nm	50%	3.8	3.3
TiO2/8cir-V2O5 at 780 nm	45%	7	3.8

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3.7 Coloration efficiency

The coloration efficiency (CE) is defined as the ratio of the change in optical density (ΔOD) at a specific wavelength or integrated over a wavelength range to the amount of charge consumed (Q).³⁷ Apparently excellent ECDs should possess a high CE, which means that the largest transmittance change is caused by the least amount of charge. The change in optical density is calculated by using the following formulas,⁴¹ where T_b is the bleached transmittance and T_c is the colored transmittance:

By combining the chronoamperometry and in situ transmittance versus time test results shown in Fig. 7, the plots of optical density-charge density for the (a) V₂O₅ film and (b–d) TiO₂/V₂O₅ hybrid films were obtained and shown in Fig. 8. The |CE| value for the V₂O₅ thin film is 19.3 C⁻¹ cm² at 780 nm, 21 C⁻¹ cm² for TiO₂/1cir-V₂O₅, 28 C⁻¹ cm² for TiO₂/4cir-V₂O₅ and 22 C⁻¹ cm² for TiO₂/8cir-V₂O₅. The CE of the TiO₂/4cir-V₂O₅ hybrid film is higher than that of the V₂O₅ film and the other TiO₂/V₂O₅ films. In addition, the CE of the TiO₂/4cir-V₂O₅ hybrid film also shows an improvement compared with the reported electrochromic films based on V₂O₅.^42–45

Fig. 8 Variation of the in situ optical density (ΔOD) vs. charge density (Q) for (a) V₂O₅, (b) TiO₂/1cir-V₂O₅, (c) TiO₂/4cir-V₂O₅, and (d) TiO₂/8cir-V₂O₅ films.

The enhancement of color efficiency was mainly due to the TiO₂ nanorod arrays introduced as a substrate, which thus made for a porous network and high specific surface area of the hybrid films. This structure makes it easier for Li⁺ ion intercalation and deintercalation in electrochromic films. Besides, the one dimensional TiO₂ nanorod arrays have the ability to supply superior electron transport properties.²⁸ All of these promote the charge-transfer reactions of the films in electrochromic processes.

3.8 Electrochemical impedance spectroscopy

To better understand the electrochemical behavior of the V₂O₅ film and TiO₂/V₂O₅ hybrid films, electrochemical impedance spectroscopy (EIS) was analyzed using three-electrode systems with an AC voltage of 5 mV in a frequency range of 0.01 Hz to 100 kHz in the bleached state (at a polarization of 1.5 V). Ahead of the EIS test, these electrochromic films needed a sufficient activation process with a cyclic voltammetric test for 10 cycles. The Nyquist plots of the V₂O₅ film and TiO₂/V₂O₅ hybrid films are shown in Fig. 9. The EIS spectra consist of a semicircle which represents the charge-transfer resistance on the interface of the electrode/electrolyte in the high-frequency region and a straight line that attributes to the diffusion of Li⁺ ions into the bulk of the electrode material in the low frequency region. A larger semicircle means a higher charge-transfer resistance and a lower slope indicates a higher ion-diffusion resistance.²²

Fig. 9 Electrochemical impedance spectroscopy (EIS) of TiO₂ film, V₂O₅ film and TiO₂/V₂O₅ hybrid films recorded in the bleached state.

Fig. 9 shows that the semicircle in the high-frequency region for the V₂O₅ film is larger than that of the TiO₂/V₂O₅ hybrid films in their bleached state, and the TiO₂/1cir-V₂O₅ film has the smallest semicircle. This reveals that the migration resistance and the charge-transfer resistance of Li⁺ ions for the V₂O₅ film is larger than that of the TiO₂/V₂O₅ hybrid films. This means the introduction of the TiO₂ nanorod array as the substrate provides a higher porosity in the surface and a larger area of the electrode–electrolyte interface,⁴⁶ leading to an easier migration and charge-transfer process for the Li⁺ ions at the solid oxide/liquid electrolyte interface. Through simulating the EIS curves, the charge-transfer resistance (R_ct) of the pure V₂O₅ film and TiO₂/8cir-V₂O₅ film is 153.7 Ω and 95.6 Ω, respectively. However, in the low-frequency region, it shows that these films have the almost same slope.

4. Conclusions

In summary, we have synthesized a novel TiO₂/V₂O₅ hybrid film by combining the hydrothermal and electrochemical deposition methods. The electrochemical and optical properties of the TiO₂/V₂O₅ hybrid films have been investigated and it shows the improved electrochemical and electrochromic properties compared with the single V₂O₅ thin film. The TiO₂/4cir-V₂O₅ hybrid film shows the most improved electrochromic properties with excellent cyclic stability, outstanding transmittance modulation (50% at 780 nm) and high coloration efficiency (28 C⁻¹ cm² at 780 nm). The modified electrochromic properties are mainly due to the TiO₂ nanorod array structure, which contributes to improve the structural stability of the V₂O₅ film and the intercalation/deintercalation process of Li⁺ ions within the V₂O₅ film.

Acknowledgements

We are grateful to the National Natural Science Foundation of China (Grant no. 21572030, 21272033, 21402023) for financial support.

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