Superhydrophobic nanoparticle mixture coating for highly efficient all-day radiative cooling

Abstract

Radiative cooling without energy consumption is an ideal environmentally friendly cooling method. Despite the remarkable progress made so far, the majority of radiative cooling materials still suffer from high costs and vulnerability to outdoor contamination challenges. Especially in humid environments, dew attaches to the surface of radiators at night, leading to decreased performance. Here, we present a new radiative cooling structure that consists of a superhydrophobic SiO2 nanosphere top layer, a polyvinyl fluoride (PVF) middle layer, and a metallic Ag bottom layer. The SiO2 nanosphere layer not only induces efficient light scattering and helps to obtain ultrahigh solar reflectivity (94.2%) but also provides a low-surface-energy nanoscale textured surface for self-cleaning, with a maximum surface contact angle of 154°. Meanwhile, infrared phonon-polarization resonances, such as SiO2′s Si-O-Si bond and PVF’s -C-F group, result in a high average thermal emissivity (>90%) in the atmospheric transparency window (ATW, 8–13 μm). Outdoor experiments in humid Xiamen showed promising sub-ambient cooling of ∼ 3.3 °C for surface cooling and ∼ 2.4 °C for space cooling in direct sunlight. Additionally, the tilt experiment demonstrated that the cooling performance is further enhanced by a tilted radiator surface on highly humid nights. More importantly, the superhydrophobic self-cleaning properties of this coating prevented outdoor contamination or wetting of its surface, making it a promising option for building cooling.