Optically clear mesoporous silica aerogels for passive daytime radiative cooling

This study demonstrates the use of silica aerogel monoliths as optically transparent radiative cooling materials exhibiting near-unity emissivity in the 8–13 μm atmospheric transparency window. Silica is widely used for radiative cooling applications due to favorable optical properties. However, dense silica exhibits a reflectance peak around 9–9.5 μm due to the Reststrahlen effect, lowering emissivity near the blackbody peak (λ = 9.7 μm) at 300 K which substantially reduces cooling power. Previous approaches used complex photonic structures to enhance emissivity of silica-based radiative cooling materials. Here, a mesoporous microstructure of aerogels is demonstrated to eliminate this characteristic reflectance peak. The nanoparticles (∼4–5 nm radius) and pores (size distribution peak ∼9–20 nm) constituting the aerogel were much smaller than visible wavelengths, minimizing scattering and enabling >90% transmittance. Spectral emissivity and solar absorptance of these aerogels on metallic substrates were determined experimentally and numerically, showing high radiative cooling potential without needing complex nanostructures.