Plasmonic Oxygen‐Deficient TiO_2‐x Nanocrystals for Dual‐Band Electrochromic Smart Windows with Efficient Energy Recycling

Abstract Dual‐band electrochromic smart windows capable of the spectrally selective modulation of visible (VIS) light and near‐infrared (NIR) can regulate solar light and solar heat transmittance to reduce the building energy consumption. The development of these windows is however limited by the number of available dual‐band electrochromic materials. Here, plasmonic oxygen‐deficient TiO_2‐x nanocrystals (NCs) are discovered to be an effective single‐component dual‐band electrochromic material, and that oxygen‐vacancy creation is more effective than aliovalent substitutional doping to introduce dual‐band properties to TiO₂ NCs. Oxygen vacancies not only confer good near‐infrared (NIR)‐selective modulation, but also improve the Li⁺ diffusion in the TiO_2‐x host, circumventing the disadvantage of aliovalent substitutional doping with ion diffusion. Consequently optimized TiO_2‐x NC films are able to modulate the NIR and visible light transmittance independently and effectively in three distinct modes with high optical modulation (95.5% at 633 nm and 90.5% at 1200 nm), fast switching speed, high bistability, and long cycle life. An impressive dual‐band electrochromic performance is also demonstrated in prototype devices. The use of TiO_2‐x NCs enables the assembled windows to recycle a large fraction of energy consumed in the coloration process (“energy recycling”) to reduce the energy consumption in a round‐trip electrochromic operation.