Improving electrochromic properties of tungsten trioxide by constructing gradient distribution oxygen vacancies through plasma treatment

In recent years, electrochromic materials have been extensively utilized in smart windows, displays, and dimmable devices. WO<sub>3</sub>, as a typical electrochromic material has received significant attention. Existing researches indicate that the concentration and distribution of oxygen vacancies in WO<sub>3</sub> 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 WO<sub>3</sub> films holds important research significance and application prospects. In this work, the electrochromic properties of WO<sub>3</sub> 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 WO<sub>3</sub> 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 WO<sub>3</sub> 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 WO<sub>3</sub> 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 WO<sub>3</sub> film. Overall, these results indicate that plasma treatment is an emerging method of significantly improving the electrochromic properties of WO<sub>3</sub> films.