Decorating Natural Silk Nanofiber Aerogel with a Hierarchical Structure via TiO₂ for Improved UV Protection and Radiation Cooling

Daytime radiant cooling achieves a sustainable cooling effect by reflecting sunlight and radiant heat. However, absorption of sunlight by emitters and parasitic heat gain can significantly reduce radiative cooling temperatures. To improve the light reflectivity and emissivity in the mid-infrared band, SNF@TiO₂ aerogel with high stability and efficient radiative cooling effect was constructed using nanosilk and titanium dioxide. The hierarchical structure of the aerogel stores more air, which reduces the thermal conductivity (0.0333 W·m^-1·K^-1) and parasitic heat gain. TiO₂ provides excellent UV resistance while increasing solar reflectance and atmospheric window emissivity. The average solar reflectance and average IR emissivity of SNF@TiO₂ were 89.4 and 92.3%, respectively. Compared with the subambient (<i>I</i>: 800 W·m^-2, PE-covered air) temperature, the average cooling temperature of SNF@TiO₂ under direct sunlight reached 11.5 °C. Meanwhile, the outdoor subambient (<i>I</i>: 900 W·m^-2, PE-covered air) average temperature drop of SNF@TiO₂ reached 12.1 °C after UV (40 mW·cm^-2) continuous radiation for 10 days (6 h per day), displaying highly stable radiative cooling properties. In addition, the SNF@TiO₂ aerogel has good mechanical elasticity and thermal insulation properties. This study offers great potential for silk fiber materials for outdoor radiant heat management.