Enhanced radiative cooling and flame retardancy through phosphate-linked hollow metal-organic framework spheres

Introducing porous structure to bring more reflective interfaces has become a universal approach in designing polymer-based radiative cooling materials. However, the formation of porous structure during the construction process will produce a lot of hazardous substances that directly enter the environment, especially volatile organic compounds. Besides, the fire safety of radiative cooling coatings is a crucial consideration, preferentially applied to building surfaces. Herein, in terms of safety and sustainability, phosphorized metal–organic framework hollow spheres (P-ZIF) are synthesized using coordination bond effects and the sacrificial template method, to avoid the traditional pore-creating process and improve the flame retardancy. Leveraging the principle of backscattering enhancement in hollow structures, the traditional highly-pollutional fore-creating process is successfully avoided. Besides, the particles-based composite coating produces a sub-ambient temperature of 1.6 °C with a cooling efficiency of 63.3 W/m², demonstrating a huge radiative cooling potential. Meanwhile, the composite coating exhibits superior self-extinguishing properties to aggressive flame, with a 40.4 % reduction in maximum heat release rate. In summary, metal–organic frameworks, prepared through controlled composition and structural adjustment, tactfully avoid the high-polluted fore-creating process and effectively improve the fire safety of radiative cooling materials, developing the potential application of radiative cooling technology in terms of safety and sustainability.