Harnessing optimized SiO₂ particles for enhanced passive daytime radiative cooling in thin composite coatings

Passive daytime radiative cooling (PDRC) technologies can achieve sub-ambient temperatures by reflecting sunlight and radiating heat to outer space without energy consumption. Conventional PDRC structures, however, suffer from high costs, limited scalability, and poor resistance to pollution. This study presents an optimized Stöber method combined with cetyltrimethylammonium bromide (CTAB) modification to synthesize SiO₂ particles (SiP-C) with nano-synapses surface morphology (30–50 nm) and a broad particle size distribution (100–800 nm). These features facilitate both Rayleigh and Mie scattering, resulting in excellent solar reflectance (97 %) and selective emissivity (0.95). By mixing SiP-C with polyvinylidene fluoride (PVDF), a PDRC coating was fabricated using a simple blade-coating method. The coating, with a thickness of only 143 μm, demonstrated a solar reflectance of 95 % and emissivity of 0.98, achieving a cooling temperature of 10.5 °C below the shielded air temperature under high solar irradiance (981 W/m²) and relative humidity (56.4 %). The SiP-C nano-synapses also imparted hydrophobic properties (contact angle of 142°), enhancing resistance to environmental pollution. The composite PDRC coating thus offers scalability, low cost, high performance, and compatibility for various energy-saving cooling applications.