Cellulose-based porous polymer film with auto-deposited TiO2 as spectrally selective materials for passive daytime radiative cooling

Passive daytime radiative cooling materials significantly alleviate the current energy shortage and global warming without consuming any energy. However, conventional inorganic nanoparticles can hardly achieve excellent cooling effect during daytime due to their low reflectivity. Broadband super white nanoparticle-based radiators demonstrate excellent solar reflectivity under direct sunlight and low absorption. Accordingly, we introduced TiO2 nanoparticles with the advantages of high refractive index, high infrared emissivity and low solar absorption into the porous cellulose-based film via a facile phase separation strategy. The cavities were distributed around 5 μm in size due to the limited evaporation of solvents, while the introduction of TiO2 nanoparticles effectively enhanced the solar reflectance to 0.97. Moreover, the scattering model demonstrated that the high solar reflectivity was mainly produced by the randomly agglomerate effect of nanoparticles and these nano-clusters also contribute to a high mid-infrared emissivity. As a result, the photonic film presented about 10 °C cooling capacity even under the high-density solar radiation (~897 W/m2) according to the out-door measurement. The random nano-clusters filling strategy avoids the complicate process of specific size particles and has a significant potential for propel the polymer-based cooling technology to satisfy the need of large-scale thermal management.