Tunable Coatings on Various Substrates for Self‐Adaptive Energy Harvesting with Daytime Solar Heating and Nighttime Radiative Cooling

Abstract In this work, tunable vanadium dioxide (VO 2 ) metafilms on different substrate materials fabricated via low‐oxygen furnace oxidation are demonstrated for self‐adaptive daytime solar heating and nighttime radiative cooling. Because of its thermally‐driven insulator‐to‐metal phase transition behavior, the VO 2 metafilms work as a spectrally‐selective solar absorber with a high solar absorptance of 0.86 and a low infrared emissivity of ≈0.2 at daytime, while they behave as a selective cooler at nighttime to dissipate heat effectively through the atmospheric transparency window with a high emissivity of ≈0.76 to cold outer space. From the outdoor vacuum tests, a significant temperature rise up to 169 K upon solar heating and a temperature drop of 17 K at night are experimentally observed from these tunable VO 2 metafilms. With effective atmospheric temperature fitted in situ, the accurate heat transfer model shows excellent agreement with the stagnation temperature measurement, indicating a high heating power of ≈400 W m −2 at 80 °C sample temperature in the middle of the day, and a cooling power of ≈60 W m −2 at 30 °C in equilibrium with the ambient at night. This work would facilitate the development of self‐adaptive coatings with cost‐effective and scalable fabrication approaches for all‐day energy harvesting.