In-situ amination and polymerization of SiO2 microspheres grants high infrared emissivity for daytime radiative cooling film

Radiative cooling technology offers a green idea for addressing energy crisis. SiO 2 is widely employed in preparation of radiative cooling films due to its excellent spectral selectivity within the atmospheric window. Based on the functional group matching principle, connecting C-N bonds onto the Si-O-Si framework is beneficial for enhancing the mid-infrared thermal radiation capability. However, achieving robust covalent integration of these functional groups into the Si-O-Si skeleton of silica and establishing chemically cross-linked composite films with polymeric matrices remains a significant challenge. In this study, amino-modified SiO 2 microspheres(NSMs)were synthesized by in-situ amination and incorporated into a polyurethane (PU) prepolymer, followed by in-situ polymerization to prepare the composite film. On the one hand, amino groups grafted in-situ on the NSMs can react covalently with isocyanate groups in PU prepolymer to establish a chemically cross-linked network. On the other hand, urea groups generated during in-situ polymerization facilitate covalent connection of C-N bonds into the Si-O-Si skeleton. Besides, the in-situ reaction promotes homogeneous dispersion of the NSMs within the PU matrix and enhances the mechanical properties of the composite film. As a result, the as-prepared NSMs-PU film demonstrated an average infrared emittance of ∼ 94.8%, yielding an average ambient cooling temperature drop of ∼ 4.20 ℃ under low-intensity natural sunlight irradiation (< 600 W m −2 ). Additionally, the film exhibits excellent mechanical toughness of 16.03 MJ m −3 and its intrinsic hydrophobicity endows self-cleaning functionality. This work offers an in-situ amination and polymerization strategy to contribute to the practical application of daytime radiative cooling.