A bionic green roof enabled by a TiO2–LiCl/PVA composite film on a reflective aluminum substrate

Urban heat islands and the rising energy consumption of dense cities call for scalable passive cooling solutions. Conventional green roofs offer ecological and thermal benefits but are constrained by the optical and physiological limits of natural leaves. Here, we report a bionic green roof material that replicates key leaf-inspired functions while enabling effective thermal regulation through engineered optical and hygroscopic properties. The material features a TiO 2 –LiCl/PVA composite film on a reflective aluminum substrate, combining high solar reflectance, strong sky-facing emissivity, and low downward thermal emission for efficient radiative cooling. The LiCl-doped PVA film is capable of reversible moisture absorption–desorption, emulating transpiration-driven evaporative cooling without irrigation or stomatal limitation. Outdoor experiments confirm significant reductions in roof and indoor temperatures under both sunny and cloudy conditions, while simulations predict 12.7%–23.1% reductions in annual cooling energy consumption in typical hot urban climates. The bionic green roof also exhibits excellent durability and self-cleaning capability, demonstrating a robust, maintenance-free approach for climate-adaptive and energy-efficient architecture. These results highlight a scalable material-based strategy that addresses key limitations of traditional vegetated roofs by integrating radiative and evaporative cooling, offering a practical route toward low-carbon and climate-adaptive architecture.