“Pore in Pore” Engineering in Poly(dimethylsiloxane)/Silicon Oxide Foams for Passive Daytime Radiative Cooling

Passive daytime radiative cooling (PDRC), an energy-saving and environmentally friendly strategy, achieves cooling without any power consumption by reflecting sunlight and radiating energy to outer space. Herein, it is shown that a porous-structured poly(dimethylsiloxane)/silicon oxide (PDMS/SiO2) composite foam is fabricated through “pore in pore” engineering, in which a three-dimensional self-supporting silicon dioxide (SiO2) network is constructed in the pores of PDMS foam fabricated via freezing casting. The PDMS/SiO2 foams exhibit a hierarchical porous structure like “SiO2 assembling pores in PDMS micropores” and significantly benefit the reflectivity and infrared emissivity. An average solar reflectivity of ∼90.1% within a full spectral range of 0.3–2.5 μm and an average emissivity of ∼94.1% in the atmospheric window allow for the temperature inside the foam cavity to drop by ∼11.1 °C under direct sunlight with the solar radiation intensity of 871 W m–2. Notably, the hierarchical porous PDMS/SiO2 foams exhibit excellent thermal insulation, antifouling, and hydrophobic performances, weakening the heat flow from the external environment and ensuring long-term outdoor use. Such a powerful radiative cooling performance integrated with thermal insulating and antifouling capability by the foams constructed by “pore in pore” engineering offers a way to efficiently manage sunlight radiation energy to make devices, vehicles, buildings, and other urban objects cooler and helps to save energy in an outdoor sunlight environment.