Bioinspired All‐Cellulose‐Based Highly Scattering Films for Sustainable Radiative Cooling

Abstract Passive radiative cooling is a promising technology for mitigating global warming by reflecting sunlight and radiating heat into the supercooled outer space. This approach has attracted increasing attention. However, achieving efficient light scattering typically depends on high‐refractive‐index inorganic materials, such as titanium dioxide. Despite its widespread application, recent research reveals that titanium dioxide may pose a potential carcinogenic risk. As a sustainable alternative, cellulose offers renewability, biodegradability, and biocompatibility. Inspired by the scale structure of the Calothyrza margaritifera beetles, all‐cellulose‐based highly scattering films are developed to overcome the intrinsic low refractive index of cellulose. These films consist of ethyl cellulose microspheres with optimized size and filling fraction as scattering centers and cellulose nanofibers as a supporting network to anchor these microspheres. Despite their ultrathin thickness (10 µm), these films achieved a reflectivity of 70%. When applied for passive radiative cooling, 300‐µm‐thick all‐cellulose‐based films reduced temperature by 8 °C during the day and 2 °C at night. The use of cellulose to achieve thinner, more efficient scattering materials while minimizing material usage, offering a sustainable and safer alternative to titanium dioxide as a scattering material. Such all‐cellulose‐based highly scattering films hold great promise for the fields of functional coatings, foods, and personal care products.