Leaf-vein-inspired robust and UV-resistant nanocomposite paper for passive building cooling and fire protection

Passive daytime radiative cooling (PDRC) technology is a promising technology that can realize zero-energy consumption. The flexible PDRC materials attract much attention owing to their broad applicability. However, the flexible PDRC materials always show unsatisfactory mechanical performance, especially when exposed to prolonged sunlight irradiation. In addition, their fire resistance is also a cause for concern. Herein, inspired by the hierarchical leaf vein structure, a pliable, UV-resistant, nonflammable, robust nanocomposite cooling paper (HTP) composed of hydroxyapatite nanowires (HNWs) and TEMPO-oxidized cellulose nanofibers (TOCNFs) is developed. Through customizing interfacial engineering to introduce physical entanglement and multiple bonding, the leaf-vein-analogous hollow networks formed by HNWs are reinforced by TOCNFs, fully leveraging the advantages of one-dimensional inorganic HNWs and organic TOCNFs at the nanoscale. This enables the HTP to achieve excellent mechanical strength and PDRC performance with a small amount of TOCNFs doping. HTP exhibit a high tensile strength of 48.56 MPa and Young's modulus of 3.07 GPa, surpassing most previously reported flexible PDRC materials. Furthermore, HTP demonstrates outstanding flame resistance and anti-UV aging properties, which benefit from frameworks formed by HNWs and TOCNFs. The exceptional overall performance overcomes the limitations of conventional polymer-based flexible PDRC materials, significantly advancing the practical application of flexible PDRC materials, especially for building cooling.