Panshu
Gui‡
a,
Ziyi
Jin‡
a,
Yufeng
Bai
a,
Zhengqiao
Lv
a,
Jianwei
Mo
a,
Shuai
Chang
b and
Di
Yang
*a
aSchool of Science, Minzu University of China, Beijing 100081, China. E-mail: diyang@muc.edu.cn
bDepartment of Materials Science, Shenzhen MSU-BIT University, Shenzhen, China. E-mail: schang@smbu.edu.cn
First published on 25th July 2023
Herein, using DC magnetron sputtering technology, smooth and dense WO3 films and WO3/TiO2 composite films were successfully prepared under the optimized preparation conditions. Subsequently, electrochromic devices (EC) and photoelectrochemical devices (PEC) were fabricated with a sputtering deposition time of 30 min for the WO3 film and different deposition times for the TiO2 interface layer. The TiO2 interface layers deposited with a sputtering time of 2–3 min were proven to improve the electrochromic properties of the devices. Specifically, compared with the WO3-based EC devices, the WO3/TiO2-based EC devices exhibited a higher transmittance in the bleached sate and much lower transmittance in the coloured state, resulting in a wider transmission modulation range. For instance, the modulation range of the WO3/TiO2-based EC devices reached 73.9% at 600 nm, which is larger than that of the WO3-based EC devices (62.5%). Optical measurements revealed that the TiO2 interface layer reduced the amount of undesirable localized states by protecting WO3, while increasing the amount of Li+ ions embedded in WO3 for device colouration. This was further demonstrated by cyclic voltammetry measurements. The optimized TiO2 interface layer significantly increased the inserted Li+ ion density and extracted Li+ ion density. Interestingly, the difference between the inserted and extracted charge density was obviously reduced after the introduction of the TiO2 interface layer. This indicated that the WO3/TiO2-based EC devices possessed good cycling stability, which is due to the fact that the surface of WO3 was passivated by TiO2, and thus its surface state was not subject to corrosion by the electrolyte and the formation of irreversible interface states was suppressed. Additionally, PEC devices were successfully fabricated based on WO3/TiO2 films to investigate their electrochromic characteristics under a small self-driving power. Consequently, driven by the integrated photovoltaic film, the PEC device exhibited a large transmission modulation in the wavelength range from 500 nm to 1800 nm, and especially in the range from 545 nm to 1077 nm, its transmission modulation range exceeded 80%.
To date, tungsten oxide (WO3) has been intensively studied due to its superior electrochromic properties such as coloration efficiency, stability, low power consumption, good memory effect, and high contrast.3–5 In recent years, the in-depth research on the electrochromic properties of WO3 films has accelerated their commercialization.6–15 Titanium dioxide (TiO2) is commonly used to enhance the electrochromic performance of WO3 and achieve photoelectrochromic properties. Early research mainly focused on TiO2-doped WO3 films. It was shown that the reversibility of tungsten oxide can be improved by adding TiO2 to it and the lifetime of TiO2-doped WO3 thin films can be several times longer than that of pure WO3.16,17 With the development of nanomaterial preparation technology, a variety of WO3–TiO2 composite electrochromic materials has been reported, such as WO3–TiO2 nanoparticles, porous TiO2–WO3 core–shell nanowires, TiO2–WO3 core–shell inverse opal structures, and TiO2/WO3 hierarchical thin films.2,17–20 TiO2–WO3 electrochromic materials prepared using various technologies have their own merits, and once their properties meet the application requirements in the electrochromic field, preparation technology that can achieve large-scale production is particularly favored.
Among the technologies for the preparation of EC devices, the magnetron sputtering method stands out given that the size and composition ratio of the sputtered films can be precisely controlled, the resulting films have high uniformity and good electrochromic performances, and more importantly, it can achieve large-scale production. Several recent reports in the literature showed the indisputable fact that benefiting from the advantages of magnetron sputtering technology, it is expected that ideal EC devices with excellent electrochromic performances and long service lifespans can be prepared.
However, more meticulous and comprehensive research is required to extend the application scope of WO3/TiO2-based EC devices fabricated via magnetron sputtering. In addition, to date, there is no consensus explaining the mechanism of improving the electrochromic properties of WO3/TiO2 using the TiO2 material, which still has to be explored in detail.
Herein, we report the fabrication of high-quality EC devices and PEC devices based on WO3/TiO2 composite films grown via DC magnetron sputtering and investigation of the origin of the improvement in device electrochromic properties using a TiO2 interface layer. In contrast to the WO3 film-based EC devices, the WO3/TiO2-based EC devices possessed a larger transmission modulation range and good cycling stability. Through the combination of quantitative analysis of the localized energy states and driving charge density, we found that the TiO2 interface layer prompted a reduction in the amount of undesirable localized states, an increase in driving ion density, and an enlargement in the ion diffusion coefficient. This was closely related to TiO2 protecting WO3 from electrolyte corrosion, as well as the synergistic effect of energy levels of WO3 and TiO2. The WO3/TiO2 film was optimized according to these understandings, and consequently, the as-prepared WO3/TiO2 devices exhibited a high coloration efficiency of 69 at 600 nm, large-scale transmission modulation range from 500 nm to 1800 nm, and high cycling stability.
In the case of smart windows, EC films are usually required to be highly transparent in their original state. Thus, we measured the transmission spectra of the individual WO3 film and WO3/TiO2 complex films, as shown in Fig. 1(a). The transmittance of all the samples exceeded 90% for a wavelength greater than 417 nm in the visible light range. Moreover, compared with the WO3 film, the transmittance of WO3/TiO2 complex films showed a slight decrease and significant resonance peaks appeared in the spectrum at a wavelength greater than 600 nm. Our analysis showed that these resonance peaks were the result of the mutual enhancement between the interference of the WO3 film and that of the TiO2 film.
Fig. 1 (a) Transmittance spectra of the individual WO3 film and WO3/TiO2 complex films and (b) curve of versus hv. |
To clarify the effect of the TiO2 layer on the optical property of the EC film, we evaluated the bandgap (Eg) of the individual WO3 film and WO3/TiO2 complex films using Tauc's method, as follows:21
(1) |
(2) |
(3) |
For the proposed ECs, LixWO3 was generated in the WO3 film, thereby introducing the LixWO3 energy level in the WO3 energy band, which was reported to be slightly below the conduction band edge (EC) of WO3.23 This means that the localized state just below the EC of WO3 is a crucial parameter for exploring the bleaching-colouring performances. Herein, firstly, we investigated the effect of the TiO2 film on the electrochromic properties through the localized states of the individual WO3 film and WO3/TiO2 composite films.
The localized states below the conduction band (EC) or above the valence band (Ev) participate in light absorption, which can be represented by Urbach energy (EU), as shown below:24,25
([4]) |
The absorption spectra of the WO3 film and WO3/TiO2 composite films in both the bleached and coloured states were measured to explore the changing trend of the localized states (EU) within their bandgap (Fig. S2a, ESI†). Fig. 2 shows the derived relationship between the function ln(A) and the variable hv, where A represents the absorbance. Replacing the absorption coefficient with the absorbance did not change the slope of the curve, and therefore it did not change the value of EU.
Fig. 2 Relationship between the function ln(A) and variable hv from the absorption spectra (a) in the bleached state and (b) in the coloured state. EU was derived within the yellow range. |
For the EC devices based on WO3, WO3/TiO2_1 min, WO3/TiO2_2 min, and WO3/TiO2_3 min, the values of EU were 0.298 eV, 0.167 eV, 0.178 eV, and 0.189 eV when the devices were in the bleached state, whereas they increased to 0.384 eV, 0.892 eV, 1.158 eV, and 1.912 eV, respectively, when the devices were in the colouration state. We considered that the energy level of LixWO3 slightly below the EC of WO313 was the main contributor to the increase in EU when the device transitioned from the bleached state to coloured state. Interestingly, under the bleaching condition, the EU values of all the WO3/TiO2 composite films were smaller than that of the individual WO3 film, whereas under colouration condition, they exhibited a significant increase, even greatly exceeding the EU value of the coloured WO3. This indicates that the TiO2 interface layer reduced the amount of undesirable localized states by protecting WO3 from electrolyte corrosion and increased the amount of Li+ ions embedded in WO3 for device colouration. Undoubtedly, the positive effect of the TiO2 interface layer enlarged the optical transmission modulation range of the EC devices, which was confirmed by the transmission spectrum and cyclic voltammetry measurements.
The transmission spectra of the EC devices under a driving voltage of +0.6 V and −0.6 V indicated that the TiO2 surface layer increased the transmittance in the bleached state and reduced the transmittance in the coloured state, thereby increasing the transmission modulation range (Fig. 3a). Taking 600 nm as an example, the modulation range of the WO3/TiO2-based EC devices reached 73.9%, which was larger than that of 62.5% for the WO3-based EC devices. The bleaching time and coloration time of these EC devices were revealed by bleaching-colouring kinetic measurements (Fig. 3b). As a reference level to calculate the colouring time or the bleaching time of the device, the highest transmittance in the bleaching state as well as the lowest transmittance in the colouring state of the device were measured. The calculation range of the colouring time is the time it takes to decrease from the highest transmittance to 90% of the lowest transmittance. Similarly, the calculation range of the bleaching time is the time it takes to increase from the lowest transmittance to 90% of the highest transmittance. Obviously, the TiO2 interface layer hardly changed the time from bleaching to colouring, which was around 20 s, but it prolonged the time from colouring to bleaching, and the longer the sputtering deposition time of the TiO2 film, the longer the bleaching time of the device, from 12 s to 20 s. The cause for this phenomenon can be explained by the energy levels, as shown in Fig. 3(c).26 The Fermi levels of the WO3 film were represented by two dashed lines to consider the electrochromic state of the film, where Efb and Efc indicate the Fermi level in the bleached and coloured state, respectively.27 The relative position of the Fermi level in the coloured WO3 with respect to the Fermi level of TiO2 plays a pivotal role in determining the responsivity of the bleaching process.26 When the Efc of the coloured WO3 is higher than or close to the Ef of TiO2, electrons would transfer to the TiO2 film and be trapped there, resulting in an extension of the bleaching time of the device.
The impact of the TiO2 interface layer on the electrochromic performance was further explored though cyclic voltammetry (CV) measurements, as shown in Fig. 4(a). By careful observation, it was observed that for the WO3 film, both the anodic current peak and cathodic current peak were located at −0.097 V. However, upon the introduction of a TiO2 layer on the WO3 surface, the anodic peaks shifted towards a positive voltage, while the cathodic peak deviated to a negative voltage. This phenomenon has also been found in a TiO2 nanotube material decorated with WO3 particles and WO3–TiO2 core–shell nanowires, which was considered to be the synergistic effect of these two materials.28,29
The inserted charge density (Qc) and extracted charge density (Qb) were recorded by CV measurements, and based on the Qcs, the coloured efficiencies (CE) of the devices were derived. These physical parameters are summarized in Table 1. Compared with the WO3-based EC device, the introduction of a TiO2 interface layer significantly increased the inserted charge density and the extracted charge density. Furthermore, there was a remarkable decrease in the difference between the inserted charge density and the extracted charge density, indicating the improvement effect of the TiO2 interface layers on the cycle stability of the devices. Combined with the previous analysis of the EU, we believe that the surface of WO3 was passivated by TiO2, and thus its surface state was not subject to electrolyte corrosion, which hindered the generation of irreversible localized states.
Sample name | Q c/Qb (C) | Q b/Qc (%) | CE at wavelength of 600 nm (cm2 C−1) | D (cm2 s−1) |
---|---|---|---|---|
a Q c or Qb was obtained in accordance with the CV measurements by the formula , where v represents the potential scan rate. b CE was obtained from the formula , where Δa stands for the optical absorption difference between the coloration state and bleached state. | ||||
WO3 | 0.0739/0.0669 | 90.5 | 31 | 2.1668 × 10−10 |
WO3/TO2 1 min | 0.1000/0.0977 | 97.9 | 28 | 5.5790 × 10−10 |
WO3/TO2 2 min | 0.1113/0.1066 | 95.8 | 35 | 5.9195 × 10−10 |
WO3/TO2 3 min | 0.0860/0.0816 | 94.9 | 69 | 3.7520 × 10−10 |
WO3/TO2 4 min | 0.1041/0.0889 | 85.4 | — | 2.0079 × 10−10 |
WO3/TO2 5 min | 0.1062/0.0981 | 92.3 | — | 3.1471 × 10−10 |
In accordance with the optical absorption measurements and the derived charge density, the coloured efficiencies of the devices were calculated, as listed in Table 1. There are many physical variables that affect the colouring efficiency, including film thickness, material properties, and driving charge density, and thus it is a multivariable function. In our experiments, the devices based on the WO3/TiO2_3 min film had a high coloration efficiency (CE) of about 69 cm2 C−1.
In addition, based on the CV curves, the diffusion coefficient, D (cm2 s−1), of Li+ was calculated using the peak current, IP, during the anode scans at different scanning rates, as follows:20
IP = 2.71 × 105 × S × n3/2 × D1/2 × C0 × v1/2 | (5) |
Subsequently, 1000 repeated CV measurements were conducted on the WO3 device and the WO3/TiO2_2 min device to investigate the effect of the TiO2 interface layer on the device lifetime. Fig. 5a demonstrates that the closed area of the CV curve of the WO3/TiO2_2 min device was remarkably larger than that of the WO3 device, and furthermore Fig. 5b obviously shows that the difference between the inserted and extracted charge density of the WO3/TiO2_2 min device was smaller than that of the WO3 device. Thus, these results indicate that a TiO2 interface layer with an appropriate thickness is greatly beneficial for improving the lifespan of EC devices.
The self-driving energy of the PEC device was shown by current density–voltage (J–V) measurements (Fig. 6a). The photoelectric conversion efficiency of the PEC device was very low, i.e., only 1.19%, whereas its fill factor (FF) was high, which indicated that the dye-sensitized TiO2 thin film possessed a high capacity for doing work.
The variation in the transmittance spectrum of the PEC devices under self-driving energy was investigated in a wide electromagnetic wave range of 400 nm to 2000 nm, as shown in Fig. 6(b). All the tested devices exhibited a high transmission modulation range under very low self-driving energy. In addition, it can be seen that the TiO2 interface layer increased the transmittance of the PEC devices in the bleached state and enhanced the absorbance of the devices in the coloured state, thereby significantly enlarging the transmission modulation range. Especially for the PEC device based on the WO3/TiO2_3 min composite film, the transmission modulation range ΔT exceeded 80% in the wavelength range of 545 nm to 1077 nm. Simultaneously, the coloured PEC device based on the WO3/TiO2 film had extremely low transmittance in the near-infrared range, which indicates that this device has promising application prospects in smart windows and heat insulation layers.
Photos of the EC device and PEC device in the initial sate, coloured state and bleached state are shown in Fig. S4 and S5 (ESI†), respectively. To elucidate the advantages of the as-prepared devices, we compared their electrochromic properties with that of various TiO2/WO3 composite thin films prepared by representative techniques over the past decade, as listed in Table 2. It is worth noting that due to the lack of research reports on packaged EC or PEC devices based on TiO2/WO3 films, the electrochromic properties of the TiO2/WO3 films listed in Table 2 were all obtained using electrochemical cells with three electrodes.
Preparation method | T b –Tc (%) | Coloration time | CE (cm2 C−1) | Cycling stability | Production scale | Ref. | |
---|---|---|---|---|---|---|---|
WO3/TiO2 core/shell nanowires | The method of template and sputtering | 41.8/650 nm | 30 s | 41.6/650 nm | Peak current decreased after 10 cycles | Poor | 31 |
72.8/950 nm | 67.4/950 nm | ||||||
WO3/TiO2 Honey-comb films | Anodizing co-sputtered W/Ti films | 57.6/550 nm | 41 s 550 nm | 21.8/550 nm | ΔT did not decrease after 500 cycles | Poor | 32 |
71.5/633 nm | 38 s 633 nm | 35.2/633 nm | |||||
68.7/800 nm | 24 s 800 nm | 63.8/800 nm | |||||
TiO2/WO3 core/shell nanoarrays | Hydrothermal method and sol–gel route | 16–73 | — | — | Peak current decreased after 300 cycles | Poor | 33 |
TiO2/WO3 inverse opal structure films | The method of template and dip-coating | 65/1033 nm | 6 s 1033 nm | 111.9/1033 nm | Sustain 90% of the original contrast after 1200 cycles | Poor | 19 |
TiO2 on WO3 nanowire arrays | Hydrothermal method and magnetron sputtering | 87.0/1500 nm | 3 s 633 nm | 102.1/633 nm | 95.6% after 3000 cycles | Poor | 12 |
85.3/633 nm | |||||||
WO3/TiO2 films | A co-solvent method and spin coating method | 62.8/633 nm | 3.3 s | 57.2/633 nm | The peak current ratio compared with the first cycle are ∼80% after CV 1000 cycles | Poor | 18 |
Hierarchical structure WO3/TiO2 films | radio frequency magnetron sputtering | 80/500–900 nm | 30 s | — | Oxidation peak current was no significant change after 1000 cycles | Excellent | 20 |
TiO2-doped WO3 films | DC reactive magnetron sputtering | Low | 0.55 s | 581.39 | — | Excellent | 34 |
Devices based on WO3/TiO2 films | DC reactive magnetron sputtering | >80% 545–1077 nm | 21 s/600 nm | 69/600 nm | Reversibility was no significant change after 1000 cycles | Excellent | This work |
Compared with the WO3-based EC devices, the EC devices based on the optimized WO3/TiO2 film possessed a larger transmission modulation range, higher CE value, superior cyclic stability. Specifically, the WO3/TiO2_2 min-based EC device was capable of thoroughly bleaching or colouring driven by a low voltage of 0.6 V, and the bleaching time and the colouring time were 19 s and 20 s, respectively. After 1000 CV measurements, the reversibility of the device had almost no attenuation, as reflected in the extremely small difference between the inserted charge density and the extracted charge density. Driven by the dye-sensitized TiO2 film, the PEC device based on the optimized WO3/TiO2 composite film exhibited a high transmittance in the electromagnetic wavelength range of 500 nm to 1800 nm under the bleaching condition, whereas it had a large absorbance under the colouring condition. Obviously, it possessed a large transmission modulation range, where especially that in the range of 545 nm to 1077 nm exceeded 80%.
Through the quantitative analysis of important physical quantities such as the bandgap (Eg), localized state (EU), driving charge density, and electrochromic performance, we investigated the mechanism of the TiO2 interface layer affecting the electrochromic performance of the EC devices. The TiO2 interface layer significantly reduced irreversible localization states on the WO3 surface and obviously increased the ion density driving the bleaching-colouring process. These are the main reasons why TiO2 improved the transmission spectra, transmission modulation range and reversible stability of the EC and PEC devices.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ma00272a |
‡ Equal contribution. |
This journal is © The Royal Society of Chemistry 2023 |