Photochromic hierarchical (NH4)xWO3 nanocrystals with bronze tunnel structure for energy-saving windows

Abstract

Understanding photochromism is crucial for developing new ways to modulate the optical responses of materials. Here, we prepared ammonium tungsten bronze (NH4)xWO3, representing a significant advance in photochromism over the rarely chromogenic tungsten bronze. The high color difference, 12.89 and 30.49 for 10 s and 90 s exposure, respectively (AM 1.5G), was due to the large surface area created by the distinctive flower-like hierarchical structure, which exposed more photoactive sites, as well as to the highly efficient adsorption of the proton-coupled electron as a color center on the surface to trapped electrons. The elucidation of NH4+ protonation under the light field and/or chemical interactions with neighboring protonated species was revealed by the ion exchange sites in hexagonal, trigonal, and square sites, suggesting an essential surface structure-breaking behaviors of hydrogen bonds between NH4+ and tunneled WO3 frameworks. In particular, the created energy-saving window, based on hydrogen ammonium tungsten bronze (NH4)yWO3:nH/polyvinyl alcohol/polyvinylpyrrolidone coating, enabled a reduction of indoor air temperature by 6.3 °C in hot weather and reduced temperature difference between various indoor areas. The modification of the chemical environment of WO3 and the identification of the photochromism mechanism hold promise for the development of energy-saving windows using chromogenic nanomaterials.