Nanofibrous Biomaterial-Based Passive Cooling Paint Structurally Linked by Alkane-Oleate Interactions.

Passive radiative cooling materials, which provide cooling without consuming electricity, are widely recognized as an important technology for reducing greenhouse gas emissions and delivering thermal comfort to less industrialized communities. Optimizing thermal and optical properties is of primary importance for these materials, but for real-world utilization, ease of application and scalability also require significant emphasis. In this work, we embed the biomaterial hydroxyapatite, in the form of nanoscale fibers, within an oil-based medium to achieve passive cooling from an easy-to-apply paint-like solution. The chemical structure and bonding behaviors of this mixture are studied in detail using FTIR, providing transferable conclusions for pigment-like passive cooling solutions. By reflecting 95% of solar energy and emitting 92% of its radiative output through the atmospheric transparency window, this composite material realizes an average subambient cooling performance of 3.7 °C in outdoor conditions under a mean solar irradiance of 800 W m -2 . The inflammability of the material provides enhanced durability as well as unique opportunities for recycling which promote circular economic practices. Finally, the surface structure can be easily altered to tune bonding behaviors and hydrophobicity, making it an ideal passive cooling coating candidate for outdoor applications.