Enhanced electrochromic properties of nanorod based WO3 thin films with inverse opal structure

Highlights: •WO_3, one of electrochromic materials, was synthesized into a needle-shaped colloidal ink. •The polystyrene beads were used as a template to fabricate an inverse opal structure. •As the layer increases, the contact area with the electrolyte increases. •Increasing the area of WO₃ in contact with the electrolyte improves ion transport. •That is, the porous WO₃ film improves the overall electrochromic characteristics. -- Abstract: Tungsten trioxide (WO₃) is a typical electrochromic material that exhibits reversible optical property changes depending on the applied voltage. Previously, studies for its application in smart windows and various technologies have been made. However, improvements to the WO₃-specific low color change rate are needed. The above problem can be solved by fabricating a color change layer with a large specific surface area through physical/chemical modification of the WO₃ layer and structural control for smooth electrolyte injection. In this study, three dimensional porous WO₃ nanostructures were prepared using polystyrene nanospheres. Inverse opal structured WO₃ thin films with high porosity showed excellent electrochromic properties. The change in transmittance was 86% at a wavelength of 700 nm; the response time was 5.55 s; and the coloration efficiency was calculated at 154.94 cm²/C. The response time was about 3 times faster and the coloration efficiency was about 2 times higher than that of the flat thin film. As a result, the WO₃ thin film with an inverted opal structure exhibited excellent electro reversibility and enhanced properties.