Cellulose nanocrystal composite films with bioinspired structural color for mechanically robust passive daytime radiative cooling.

Passive daytime radiative cooling (PDRC) offers a sustainable solution to global energy challenges by dissipating heat without energy input. However, conventional PDRC materials face trade-offs between biodegradability, color integration, optical transparency, and mechanical robustness. Herein, a biomimetic, structurally colored PDRC film fabricated via evaporation-induced self-assembly of cellulose nanocrystals (CNCs), betaine, and polyvinyl alcohol was developed. By controlling the chiral nematic nanostructure of CNCs, simultaneous transparency and broadband reflectance (71 % across 0.2-2.5 μm) are realized, while maintaining >73 % thermal emittance in the 8-13 μm atmospheric window. The films exhibit exceptional mechanical properties (22 MPa strength, 35 % strain) and tunable structural color, addressing the brittleness and aesthetic limitations of existing PDRC systems. Notably, the resulting film exhibits remarkable passive cooling effects: a 5.9 °C reduction and a 20 °C drop on exposed surfaces compared to the ambient temperature. For the first time, the coordinated optimization of visible light transmittance, solar reflectance and mechanical properties has been achieved. This work not only demonstrates transparent and flexible CNC-based PDRC films with cooling functionality but also addresses a critical gap in sustainable cooling technologies, paving the way for next-generation sustainable cooling technologies in smart buildings and adaptive packaging.