Triple-phase photonic engineering in organic-inorganic hybrid photonic coatings: Multiple scattering interface for stable daytime radiative cooling

Radiative cooling is a sustainable cooling technology, which can show great application in energy saving buildings. Cooling coatings gained more and more interest due to their easy processability and low cost. However, developing scalable, highly solar reflective, outdoor stable and various-substrate-adaptive cooling coatings is still a great challenge. Here, a refractive index mismatch strategy was proposed to develop an organic-inorganic hybrid photonic coating by using nanocellulose-induced self-assembly process in the solution process. By mixing Al₂O₃, Mica and MOF as well as dispersing with cellulose nanofiber, hierarchical nanostructured coating can be obtained. Due to large amount of air pores and multiple scattering interface formation, the resultant cooling coatings exhibited high and broad sunlight reflectance of 96% and high infrared emittance of 93%. Outdoor field test demonstrated OHPC can achieve a daytime subambient cooling of 8.5 ℃ during direct sunlight in Nanjing. Meanwhile, it can be brushed on different kinds of substrate, such as wood, tile, plastic, ceramic, glass, et al, highlighting its universality. Most importantly, OHPC exhibited surface hydrophobicity and UV resistance. Energy simulation indicated over 50% cooling energy can be saved if OHPC is coated on the roofs and walls of buildings in China. This work paves the way for developing scalable, environment-adaptive, and stable daytime cooling coatings for energy savings.