Synthesis And Properties Of 3-Dimensional Ordered Macroporous Tungsten Oxide Electrochromic Films

As an excellent electrochromic material, tungsten oxide (WO₃) has numerous potential applications, such as smart windows, light switch and rear-riew car mirrors. However, WO₃ still suffers from various shortcomings, for instance, long reponse times on electrochromic switching. In this work, macroporous structure is introduced into WO₃ films to improve their electrochromic peoperties through better penetration of electrolytes into the inorganic framework due to the larger specific surface area. Moreover the character of photonic band gap (PBG) was also studied of 3DOM electrochromic materials.WO₃ films were firstly synthesized by sol-gel techniques. SEM and TEM results showed that WO₃ was dense and had some nanopores. XRD results proved that WO₃ films were amorphous and the chemical composition had been characterized by the XPS technique. It was found that the W6+, W5+ ions co-exist in the colored WO₃ films (W6+ :W5+ = 46%:54% ) , but only W6+ ions were detected in the bleached WO₃ films. The cyclic voltammogram (CV) tests in 1M LiClO4/PC showed that with the increase of voltage and cycle times, the electrochromic reversibility was becoming better. From the different cycle rates of CVs, it is deduced the discoloration process is controlled by the diffusion of Li ion. The diffusion coefficient of intercalation and deintercalation of Li ion were obtained Din=5.345E^-11cm²Â·s^-1, Dde=1.212E^-11cm²Â·s^-1, respectively. The optical density which stands for the optical modulation ability was improved with the increase of voltage. However, the coloring efficiency of WO₃ films was decreased. The switching response time was tested by Potential step method. The coloration and bleaching response time are 5.91s and 10.84s, respectively.Three-dimensional ordered macroporous (3DOM) WO₃ films were synthesized using PS colloid crystal as templates. The morphologies of films were monitored by SEM and TEM. The hexagonal close-packed arrangement of the macropores could be observed in the WO₃ film, indicating the success in preparation of the inverse opals using PS templates. The coloring efficiency of 3DOM WO₃ films reached up to 41.82cm²Â·C^-1 at 1097nm. Moreover, the coloration and bleaching responds time were shortened by 12.18% and 19.19%, respectively. The PBG of 3DOM WO₃ films was especially studied through their reflectance properties. When the Li ions were inserted into the films, the PBG was blue shift, but when the film is bleached, the PBG will red shift to the starting location. Therefore the position of PBG can be controlled by the voltage. Meanwhile, the change of PBG was reversible and recyclable. And the position of PBG was blue shift with the increase of the incident angle, which was consistent with Bragg-Snell's law. When the incidence angle was 55℃, valleys emerged at the peak of reflection because of the Brewster's angle in PS template. But for the 3DOM WO₃ films, the Brewster's angle did not exsit due to the enhanced scattering from the roughness of their surfaces.