Free‐Standing Bacterial Cellulose‐Templated Radiative Cooling Liquid Crystal Films with Self‐Adaptive Solar Transmittance Modulation

Abstract Passive radiative coolingmaterials can efficiently reflect solar radiation and spontaneously dissipate heat through the long‐wave infrared (LWIR) transmission window to the coldouter space. They are highly attractive for application in walls and roofs of sustainable energy‐efficient buildings, but it is a challenging task to develop switchable radiative coolers for transparent windows. Herein, we report free‐standing bacterial cellulose‐templated radiative cooling liquid crystal films with high LWIR emissivity and self‐adaptive solar transmittance. Biosynthetic silanized bacterial cellulose with 3D interconnected porous nanostructure is harnessed asa robust template for confining novel liquid crystals with smectic A to chiral nematic phase transition, thereby endowing the film with high solar transmittance modulation ability. The resulting film is found to not only exhibit a remarkable LWIR emissivity, but also adaptively change its solar transmittance between a transparent state and an opaque state according to environmental temperature variation. Outdoor radiative cooling experiments were performed, and the energy‐saving performance was evaluated through a simulation of a 12‐story reference office building with the films applied as radiative cooling windows. This research can shine light on the development of advanced radiative cooling materials with switchable transmittance and their widespread applications in buildings, vehicles, and transparent photovoltaics.