Hierarchical Nanocellulose Photonic Design for Synergistic Colored Radiative Cooling

Daytime radiative cooling (DRC) materials offer a sustainable, pollution-free passive cooling solution. Traditional DRC materials are usually white to maximize solar reflectance, but applications like textiles and buildings need more aesthetic options. Unfortunately, colorizing DRC materials often reduce cooling efficiency due to colorant sunlight absorption. Thus, this study reports a hierarchical photonic structure consisting of photoluminescent scatterer networks and a nanocellulose cholesteric structure. This design effectively addresses the trade-off between cooling efficiency and coloration, achieving enhanced cooling through synergistic all-day coloration. Sustainable nanocellulose with highly ordered cholesteric structures selectively reflects visible wavelengths, generating structural color and high mid-infrared (MIR) emission. The photoluminescent scatterer networks enhance reflectance and convert absorbed light into photoluminescent emission, further promoting cooling. This synergistic photonic interaction results in a significantly enhanced high reflectance of 92%, high MIR emissivity of >90%, stable and tunable structural color appearance, and hours-long afterglow photoluminescence. Consequently, a subambient cooling of up to 11.3 degrees C under sunlight is attainable, accompanied by the ability to produce programmable structural color and photoluminescent patterns for daytime and nighttime visibility. The enhanced cooling efficiency achieved through the synergistic interplay of four optical mechanisms from the UV to MIR region offers a promising design paradigm for other DRC materials.