One-step-processed bilayer ethyl cellulose for full-colour sub-ambient daytime radiative cooling

Passive radiative cooling can cool objects exposed to sunlight without requiring additional energy input. However, it remains challenging to develop aesthetically pleasing coloured materials without sacrificing solar reflectance. Here we present a biomass-derived, bilayer and coloured ethyl cellulose (BCEC) coating fabricated in a single casting step through controlled drying-induced self-stratification. By adjusting the precursor concentration, we tune the thickness of the top layer to produce different colours through thin-film interference without introducing solar absorption. The hierarchically porous bottom layer provides high solar reflectance and long-wave infrared emission. The BCEC coating achieves solar reflectance of 97.0% and sub-ambient daytime radiative cooling of up to 9 °C under a solar intensity of 800 W m−2. In field tests conducted in the humid subtropical climate of Hong Kong, the BCEC coating outperforms commercially available coloured paints and fluorescence-based coloured coatings. Our one-step phase-separation approach can simplify fabrication, facilitating the practical deployment of this technology. Fabricating coloured radiative cooling coatings that are easy to produce and do not compromise cooling performance is challenging. Ying Liu et al. develop a one-step fabrication method that yields coloured ethyl cellulose coatings while preserving high solar reflectance.