Iridescent transparent passive radiative cooling enabled by cellulose nanocrystal assemblies

Recently, passive daytime radiative cooling (PDRC) materials that reflect sunlight and transmit heat through atmospheric transparent windows to outer space without any energy consumption have attracted broad interest. However, most PDRC materials have the complicated structures, white or mirror-like appearance with poor transparency. Here, we reported a transparent iridescent PDRC coating material constructed by the evaporation-induced self-assembly (EISA) of cellulose nanocrystals (CNCs). The iridescent PDRC coating not only shows high light transparency (75 %) because the right-handed circularly polarized light can transmit the unique left-handed chiral nematic liquid crystal structure, but also shows high solar reflection efficiency (95.2 %) and infrared emission efficiency (96.5 %) that endows the coating with high radiative cooling efficiency (119 W/m2). More importantly, the PDRC coating can apply on polymethyl methacrylate (PMMA), polyethylene terephthalate (PET) film and textiles, and the substrate color has no negative impact for cooling. With the excellent radiative cooling efficiency, high transparency, facile preparation process and general applicability, the CNCs-based PDRC coating opens a new pathway toward the potential application of PDRC for environmental sustainability purpose.