Preparation of scalable porous radiation cooling coatings based on liquid-liquid phase separation with a bilayer strategy

Passive daytime radiative cooling (PDRC) offers a zero-energy solution for thermal management by meeting two critical optical conditions: reflecting incoming solar radiation to reduce heat absorption, and efficiently emitting infrared radiation within the 8 to 14μm atmospheric transparency range to dissipate heat into space. However, conventional porous polymer coatings exhibit low refractive index contrast (typically, n ≈ 1.4 for polymers vs. n ≈ 1 for air), which limits their capability for effective light scattering and selective spectral control. In this study, we introduce a scalable bilayer photonic coating fabricated through a liquid–liquid phase separation strategy. By incorporating titanium dioxide (𝑇𝑖𝑂2) and zirconium dioxide (𝑍𝑟𝑂2) nanoparticles into a hierarchically structured matrix of fluoropolymer (Polyvinylidene Fluoride-Hexafluoropropylene, PVDF-HFP), the coating achieves improved solar light scattering and mid-IR emissivity, attributed to tailored refractive index distribution and the presence of infrared-active functional groups. Finite-difference time-domain (FDTD) modeling indicates that appropriately selected particle sizes yield complementary scattering behavior across the solar spectrum. Experimental characterization demonstrates a high solar reflectance of 98.06% and an emissivity in the mid-infrared range of 96.29%. Thermal imaging under sunlight reveals that the surface temperature of the proposed coating is reduced by 5.92°C compared to standard commercial white paints. In addition, the film exhibits excellent mechanical properties, with a tensile strength of 13.19MPa and a fracture elongation of 38.79%. These findings highlight a low-cost and scalable pathway toward high-efficiency PDRC materials, showing significant potential for applications in sustainable building technologies, urban heat mitigation, and passive thermal regulation.