Hierarchical Gradient Structural Porous Metamaterial with Selective Spectral Response for Daytime Passive Radiative Cooling

Porous photonic structures have greatly advanced large-scale radiative cooling application owing to its satisfying optical properties, easy-designation, and low cost. However, current reported porous radiative cooling materials mainly focuses on uniform or random pore distribution structure, which failed to achieve precise spectrum control for sunlight and mid-infrared light, thereby weakening the radiative cooling performance. Herein, gradient structural porous metamaterials (GSPMs) are proposed and successfully constructed for maximizing cooling effectiveness based on step-by-step freeze-casting technology. The designed GSPMs with gradient micro-nano porous structure can wide-range scatter entire solar spectrum while possess gradual refractive index transition to increase air-medium interface absorption at mid-infrared regions. Notably, solar reflectivity and mid-infrared emissivity of GSPMs reach 97.3% and 97.6%, achieving maxinum cooling temperature of 8.7 degrees C and net cooling power of 94.1 W m(-2), presenting a higher cooling effect than random porous materials. Moreover, GSPMs enable achieve compatibility with color aesthetic and cooling superiorities due to selective spectra response behavior, as well as can be applied in building materials, realizing efficient energy saving and CO2 emission reduction. This work proposed gradient porous structure can achieve highly efficient daytime radiative cooling, offering new possibilities in the structure design of next-generation porous radiative cooling materials.