Infrared selective emitter with molecular-geometric engineering for effective radiative cooling under extreme heat

Abstract

Passive radiative cooling dissipates heat to outer space through thermal emission within the atmospheric transparency window, enabling cooling without external energy input. However, most existing radiative cooling materials are designed for open-sky conditions and exhibit isotropic, broadband emission on both inner and outer surfaces. This results in significant performance degradation in realistic urban environments, where surrounding structures act as strong thermal radiators. In this study, we propose an anisotropic radiative emitter that decouples inward and outward radiative pathways through a multilayer architecture. The inner surface is designed to exhibit high broadband emissivity of approximately 90% across the 2–14 µm range, whereas the outer surface selectively suppresses mid-infrared absorption outside the atmospheric window while maintaining strong emission within the 8–12 µm range, thereby minimizing parasitic heat gain from the surroundings. The inner layer consists of recycled polypropylene (rPP), an environmentally benign material with strong infrared absorption arising from dye-derived ceramic additives. The outer layer, polyethylene oxide, is molecularly engineered to selectively absorb in the 8–12 μm range, complemented by an intermediate aluminum layer that serves as an optical barrier. This multilayer design establishes a pronounced emissivity asymmetry between the inner and outer surfaces, effectively blocking incoming environmental thermal radiation while preserving efficient heat dissipation toward the sky. The proposed anisotropic multilayer structure provides a scalable strategy for achieving effective radiative cooling in dense and realistic urban environments, where significant thermal back-radiation is unavoidable.