Biomimetic Alumina Film for Passive Daytime Radiative Cooling

Passive daytime radiative cooling is receiving more and more attention as a cooling method that does not consume energy to cool objects. However, most radiative cooling materials require the mixing of multiple particles, which increases the manufacturing process requirements. Most radiative cooling materials are susceptible to outdoor abrasion, pollution, and UV exposure, which leads to decreased performance. In this paper, the biomimetic film with both radiative cooling and self-cleaning functions was prepared by a simple evaporation-induced self-assembly process and spraying process using Al₂O₃ as the main radiatively cooling raw material, poly(vinylidene fluoride-<i>co</i>-hexafluoropropylene) [P(VDF-HFP)] as the additive, and polydimethylsiloxane as the binder. Therefore, we can get a biomimetic-Al₂O₃-P(VDF-HFP)-PDMS film (BAPPF). The BAPPF was spectrally selective. Specifically, the reflectance of BAPPF in the solar spectrum (0.45-0.89 μm) exceeded 100%, and the average reflectance in the main thermal effect of the solar spectrum (0.78-1.10 μm) was 99.92%. In addition, the BAPPF had 96.97% reflectance in the solar spectrum (0.3-2.5 μm) and 90.55% mid-infrared emissivity in the atmospheric window (8-13 μm). The combined performance enabled BAPPF to reduce temperature by up to 8.4 °C over air temperature in outdoor tests. What's more, BAPPF had excellent superhydrophobic properties, with a water contact angle of up to 159°. The self-cleaning of the BAPPF prevented contamination and maintained cooling even when working outdoors for long periods of time. Furthermore, the BAPPF maintained high performance after mechanical friction, chemical corrosion, UV aging, and water impact, which has broad reference value and application prospects.