Integrating passive radiative cooling with robust superhydrophobicity in hierarchically structured PDMS films using synergistic h-BN/SiO2

Passive radiative cooling (RC) has emerged as a promising energy-free cooling strategy to mitigate the adverse energy and environment impacts of conventional mechanical cooling technology. However, practical application of the conventional polydimethylsiloxane (PDMS) based RC film is largely limited by the insufficient cooling performance and limited self-cleaning ability. This paper proposes a PDMS-based RC composite film via tape-casting, incorporating hexagonal boron nitride (h-BN) and silicon dioxide (SiO2) as functional fillers to enhance the RC performance. A rough surface microstructure is further constructed on the film using a spray method to realize superhydrophobicity and self-cleaning functionality. Results indicate that the PDMS RC film exhibits noticeable wide band spectrum modulation: a solar reflectance of 93.2% (95.6% in the visible spectrum) and an atmospheric window emissivity of 92.3%. A sub-ambient temperature reduction of 19.4 °C is determined under 886.0 W/m2 solar irradiance. The enhanced RC performance is attributed to the high solar reflectance enabled by the high refractive index of h-BN and the strong mid-infrared emissivity arising from Sisingle bondO bond vibrations in SiO2. Furthermore, the roughened surface and hydrophobic inorganic particles impart excellent superhydrophobicity to the as-prepared RC film, achieving a water contact angle up to 172.3°. The as-prepared RC film reveals robust performance, maintaining its superhydrophobicity after 10 abrasion cycles and contamination exposure to common liquids, along with excellent mechanical stability during flexibility and load tests. In conclusion, this study designs a PDMS based RC composite film with facile fabrication, mechanical stability and excellent RC performance, offering a feasible solution for energy conservation and climate mitigation.