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In recent years, electrochromic materials have been extensively utilized in smart windows, displays, and dimmable devices. WO3, as a typical electrochromic material has received significant attention. Existing researches indicate that the concentration and distribution of oxygen vacancies in WO3 are both important in determining electrochromic effect. However, it has been reported that traditional preparation methods such as annealing can significantly reduce the ability to modulate the crystallinity and optical performance. Hence, proposing a novel approach to enhance the electrochromic properties of WO3 films holds important research significance and application prospects. In this work, the electrochromic properties of WO3 thin films are enhanced by increasing the oxygen vacancy concentration and forming its gradient distribution on the surface through plasma treatment. Firstly, the oxygen vacancy concentration and distribution of the film are optimized by regulating the power and duration of the plasma treatment. Secondly, the structure and optical properties of the plasma treated WO3 films are analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV-Vis spectroscopy. Finally, the stability and response speed of each film during the electrochromic cycle are evaluated via electrochemical tests. Through plasma treatment, the concentrations of oxygen vacancies on the surfaces of WO3 films are all significantly increased, and a gradient distribution is formed, which is conducive to enhancing the ability to inject and extract electrons. The treated WO3 films demonstrate better electrochemical stability and chromic stability during the electrochromic cycle, and their transparencies and electrochromic response speeds are also significantly enhanced. Additionally, by increasing the concentration of oxygen vacancies through plasma treatment, the band gap of the film decreases and the electrical conductivity increases, which further validates the effectiveness of modulating concentration of oxygen vacancies on the electrical conductivity of WO3 film. Overall, these results indicate that plasma treatment is an emerging method of significantly improving the electrochromic properties of WO3 films.
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Keywords:
- WO3 /
- oxygen vacancy /
- plasma /
- electrochromism
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图 1 (a)WO3和(b)缺氧WO3的晶胞结构
Figure 1. Cell structures of (a) WO3 and (b) anoxic WO3.
图 2 (a) WO3和(b)缺氧WO3的能带结构以及(c)总态密度
Figure 2. (a) Band structure of WO3 and (b) anoxic WO3 and (c) total state density.
图 3 等离子体改性装置系统示意图
Figure 3. System diagram of plasma modification device.
图 4 (a)不同处理前后的WO3薄膜XRD测试结果; (b)不同等离子体处理时长前后的WO3薄膜XRD测试结果
Figure 4. (a) XRD results of WO3 films before and after different treatments; (b) XRD results of WO3 films before and after different plasma treatment times.
图 5 不同处理前后的WO3薄膜拉曼光谱测试结果
Figure 5. Raman spectrum test results of WO3 films before and after different treatments.
图 6 不同处理前后的SEM图像
Figure 6. SEM images before and after different processing.
图 7 不同处理前后的 (a) W 4f和(b) O 1s高分辨率XPS光谱; (c)不同处理前后W 4f高分辨率XPS光谱的拟合分峰结果
Figure 7. (a) W 4f and (b) O 1s high-resolution XPS spectra before and after different treatments; (c) fitting peak-splitting results of W 4f high-resolution XPS spectra before and after different treatments.
图 8 不同处理前后的WO3薄膜 (a) XPS和表面EDS 原子比; (b) 薄膜深度方向原子比
Figure 8. WO3 films before and after different treatment: (a) XPS and surface EDS atomic ratio; (b) film depth direction atomic ratio.
图 9 等离子体处理前后的WO3薄膜的(a)透过率光谱和(b)带隙能量图
Figure 9. (a) Transmittance spectrum and (b) bandgap energy diagram of WO3 thin films before and after plasma treatment.
图 10 (a)原始状态、(b)退火处理后和(c)等离子体处理后WO3薄膜不同电位范围的CV测试和注入和抽出电荷数量
Figure 10. CV tests and charge quantities inserted and extracted for (a) initial preparation, (b) after annealing and (c) after plasma treatment of WO3 films at different potential ranges.
图 11 (a)原始状态、(b)退火处理后和(c)等离子体处理后WO3薄膜的CV测试和实时光谱透过率以及施加不同电压后的薄膜照片; (d)原始状态、(e)退火处理后和(f)等离子体处理后WO3薄膜在–1.0和+2.0 V电势下30s的电流密度变化(黑色曲线)、透光率变化(红色曲线)以及在有色和漂白状态之间的切换时间特性
Figure 11. CV test and real-time spectral transmittance of WO3 films at (a) initial preparation, (b) annealing and (c) plasma treatment, and photographs of films at different voltages; (d) changes in current density (black curve), transmittance (red curve) and switching time between colored and bleached WO3 films at -1.0 V and + 2.0 V potentials for 30s after initial preparation, (e) annealing and (f) plasma treatment.
图 12 (a)原始状态、(b)退火处理后和(c)等离子体处理后WO3薄膜的长时CV测试; (d)原始状态、(e)退火处理后和(f)等离子体处理后WO3薄膜的长时透过率测试
Figure 12. Long-term CV tests of WO3 films (a) in their original state, (b) after annealing and (c) after plasma treatment; long term transmittance tests of WO3 films (d) in their original state, (e) after annealing, and (f) after plasma treatment.
表 1 不同处理工艺方案的四探针电导率测试结果
Table 1. Results of four-probe conductivity tests for various treatment schemes.
样品处理方式 电导率/(10–3 S·m–1) 原始状态 8.27 退火处理 25.30 等离子体处理 10.80 
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表 2 不同等离子体处理工艺方案设计及测试结果
Table 2. Design and test results of different plasma treatment processes.
等离子体放
电功率/W改性时间/min O/W原子比 光学透过率/% 5 10 2.86 84.5 10 20 2.75 81.1 20 30 2.58 74.2 5 20 2.83 82.8 10 30 2.71 78.4 20 10 2.68 79.7 5 30 2.80 80.6 10 10 2.79 83.2 20 20 2.63 76.3
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