Versatile and Recyclable Iridescent Cellulose Nanocrystal Composite for Passive Daytime Radiative Cooling

Passive daytime radiative cooling (PDRC) offers a zero-energy cooling strategy by combining strong solar reflectance with efficient thermal radiation through the atmospheric transparency window. However, most existing PDRC materials rely on complex architectures and inherently white or highly reflective surfaces, limiting their visual adaptability and hindering their broader application. Meanwhile, structurally colored photonic cellulose nanocrystal (CNC) materials suffer from irreversible aggregation during drying, resulting in poor recyclability and preventing their practical use as sustainable PDRC coatings. Here, we report a recyclable iridescent PDRC coating constructed through evaporation-induced self-assembly of CNC and polyvinylpyrrolidone (PVP). The resulting photonic coatings exhibit vivid structural colors with a high solar reflectance (82.1%) and excellent infrared emissivity (93%), enabling a maximum temperature reduction of 18.6 °C under simulated solar irradiation. Importantly, the incorporation of PVP imparts outstanding redispersibility to CNC, allowing the dried materials to be uniformly redispersed in water. The regenerated coatings maintain high emissivity (91.6%) and achieve a temperature reduction of 16 °C, demonstrating robust recyclability and a preserved cooling performance. The CNC/PVP coatings can be applied to diverse substrates, including glass, polymer films, and textiles, delivering strong cooling effects under real-world sunlight exposure. Coated textiles demonstrate up to a 15 °C body-cooling effect, highlighting the material's promise for wearable thermal management. Combining renewable composition, iridescent appearance, high radiative cooling efficiency, and recyclability, this work provides a scalable strategy for sustainable photonic PDRC materials and expands their potential applications in energy-efficient buildings, personal cooling, and circular-economy technologies.