Hybrid emitters with raspberry-like hollow SiO2 spheres for passive daytime radiative cooling

Passive daytime radiative cooling (PDRC) has been vigorously investigated in recent years because it does not require electrical energy, reducing greenhouse gas emissions for a more sustainable society. Representative approaches to improve radiative cooling performance are the design of metamaterials with a long-wave infrared (LWIR) emissivity and porous polymer structures with a high-solar reflectivity. Furthermore, coating materials for a simple process should be researched to improve the applicability of radiative cooling. In this study, we investigate a daytime radiative cooling coating using raspberry-like hollow SiO2 spheres (RHSs) to achieve effective radiative cooling performance and a simple coating process. The raspberry-like morphologies of the synthesized hollow SiO2 spheres contribute to multiple scattering of sunlight, resulting in enhanced performance of the radiative cooling coating. Moreover, the results of an outdoor experiment and energy-saving demonstration suggest that the RHS coating is suitable for effective heat management of the buildings. The RHS coating achieves a cooling temperature of 9.7 °C under solar irradiance of 800 W m−2. The inner temperature of a house coated with PDRC is 6.6 °C lower than that of a bare house under direct sunlight.