Bioinspired Superwhite Cellulose Photonic Materials with Anisotropic Diffusion Scattering for Passive Radiative Cooling

Abstract Light is a powerful and sustainable resource; materials with unique light interaction ability, i.e., superwhite materials, are sought to improve the performance of several light‐centered technologies. However, achieving such materials with extreme optical performance is still a great challenge due to the lack of an effective but scalable micro/nano structure manipulation approach. Here, a simple top‐down strategy is reported, programmable ball milling strategy, to develop scalable superwhite photonic materials following a solvent‐free structure reconstruction process based on sustainable cellulose. With the help of different kinds of surfactants, the cellulose will undergo shear/impact deformation and reassembly to form a series of unusual (crystal‐like structure, particle‐like structure, pellet‐like structure, and sheet‐like structure) structured cellulosic photonic materials (HCPM) with ultrahigh solar reflectivity (as high as 99.3% in visible light region), which has never been realized in materials with a low refractive index. Based on experimental and simulation results, an anisotropic scattering mechanism is proposed to clarify this phenomenon. Combined with high emission (0.978) in the mid‐infrared region, HCPM can achieve a daytime radiative cooling efficiency of 6 °C. Moreover, it can serve as a modifier to tailor the optical structure of fabrics, aerogels, and paints. This work offered a general strategy for the scalable production of extreme optical metamaterials for the requirements of energy and environmental fields.