Dual-Band Electrochromism\nin Hydrous Tungsten Oxide

Abstract

The\nindependent modulation of visible and near-infrared light by\na single material, termed dual-band electrochromism, is highly desirable\nfor smart windows to enhance the energy efficiency of buildings. Tungsten\noxides are commercially important electrochromic materials, exhibiting\nreversible visible and near-infrared absorption when electrochemically\nreduced in an electrolyte containing small cations or protons. The\npresence of structural water in tungsten oxides has been associated\nwith faster electrochromic switching speeds. Here, we find that WO₃·H₂O, a crystalline hydrate, exhibits dual-band\nelectrochromism unlike the anhydrous WO₃. This provides\na heretofore unexplored route to tune the electrochromic response\nof tungsten oxides. Absorption of near-infrared light is achieved\nat low Li⁺/e^– injection, followed by\nthe absorption of visible light at higher Li⁺/e^– injection as a result of an electrochemically induced phase transition.\nWe propose that the dual-band modulation is possible due to the more\nopen structure of WO₃·H₂O as compared to\nWO₃. This facilitates a more extended solid-solution Li⁺ insertion regime that benefits the modulation of near-infrared\nradiation via plasmon absorption. Higher degrees of Li⁺/e^– insertion lead to polaronic absorption associated\nwith localized charge storage. These results inform how structural\nfactors influence the electrochemically induced spectral response\nof transition-metal oxides and the important role of structural water\nbeyond optical switching speed.