SiO₂-based all-inorganic daytime radiative cooling coating with dual-window emission

Passive radiative cooling (PRC) coatings play a pivotal role in reducing building energy consumption and mitigating urban heat islands. Unfortunately, the widespread application of most existing PRC coatings is hindered by the environmental concerns associated with organic binders (e.g., VOC emissions) and insufficient mechanical durability against harsh weathering, restricting their practical use in sustainable green buildings. To fill this research gap, a robust all-inorganic and fire-safe PRC coating is proposed via a facile particle-packing strategy comprising alkaline silica sol, spherical amorphous SiO₂ (SA-SiO₂) and α-SiO₂. Unlike organic-based systems, the inorganic framework endows the coating with immunity to UV aging and a superior mechanical hardness of 8H. The optimized ternary packing enables the coating to show a high solar reflectance (92.2%) and a superior broadband mid-infrared emissivity (>90%) spanning both the first (8–13 μm) and second (16–25 μm) atmospheric windows to maximize radiative heat dissipation. The coating achieves continuous sub-ambient cooling in outdoor field tests (reducing indoor temperature by approximately 4–6 °C) and an unparalleled fire resistance with zero toxic emissions. Under extreme flame attack (≈ 1300 °C), it promotes the in-situ formation of a dense ceramic shield, protecting combustible substrates for over 480 s. This work offers a scalable, eco-friendly, and ultra-durable solution for developing intrinsically fire-safe PRC coatings toward next-generation sustainable buildings.