TiO2–WO3 core–shell inverse opal structure with enhanced electrochromic performance in NIR region

Abstract

More than 50% of solar energy comes from the infrared region (as radiant heat) of the solar spectrum. Electrochromic (EC) materials, which can dynamically modulate the transmittance of infrared (IR) radiation, can be effectively applied in smart windows for thermal management in buildings. In this work, a core–shell TiO₂–WO₃ inverse opal (IO) structure was fabricated through the electrodeposition of WO₃ onto TiO₂ IO templates. The TiO₂ IO templates were synthesized by introducing TiO₂ into the voids of a polystyrene (PS) colloidal crystal template, followed by calcination to remove the PS microspheres. It was found that the TiO₂–WO₃ IO core–shell structure can modulate NIR transmittance at wavelengths from 700 to 1600 nm in the NIR range when potential is applied in LiClO₄/PC electrolyte. When −0.3 V is applied, up to 60% of NIR radiation in this range can be blocked. The NIR transmittance can be modulated by tuning the applied potential. This study focuses on comparing the novel TiO₂–WO₃ IO structure with electrodeposited WO₃ thin film to fully elucidate the effect of the inverse opal morphology and the TiO₂–WO₃ hybrid system on the optical properties. Results show that the NIR blockage can be sustained up to 90% after 1200 reversible cycles for TiO₂–WO₃ IO structure. The greater surface area of the IO structure increases the number of active sites available for the redox reactions by providing a larger contact area with the electrolyte. The more electroactive area with improved charge transfer enhances the overall NIR transmittance contrast as compared to bulk WO₃ thin film. Furthermore, the addition of WO₃ to TiO₂ to form a composite has been shown to enhance cycling performance and device lifespan.