Dual-band electrochromism in hydrous tungsten oxide

Abstract

The independent modulation of both visible and near-infrared light by a single material, termed dual-band electrochromism, is highly desirable for smart windows to enhance energy efficiency of buildings. Tungsten oxides are some of the most well studied electrochromic materials due to their reversible, strong coloration when electrochemically reduced in an electrolyte containing small cations or protons. The presence of structural water in tungsten oxides has been associated with faster electrochromic switching speeds. Here, we find that WO₃·H₂O, a crystalline hydrate, exhibits dual-band electrochromism unlike the anhydrous WO₃. Absorption of near-infrared light is achieved at low Li⁺/e^- injection, followed by additional absorption of visible light at higher Li⁺/e^- injection as a result of an electrochemically-induced phase transition. We propose that this dual-band modulation is possible due to the more open structure of WO₃·H₂O as compared to WO₃, which facilitates a larger charge storage capacity in the solid solution regime that benefits the modulation of near-infrared radiation via plasmon absorption, followed by polaronic absorption associated with localized charge storage. The results help inform the structural factors that influence the electrochemically induced spectral response of transition metal oxides.