Bio-inspired dual-mode Janus film with optical adaptation for spatial thermal management and year-round energy saving

Inspired by the Siamese’s seasonal color change, this study presents a dual-mode Janus film with optical adaptation to enable both radiative cooling and solar heating, designed for spatial thermal management (STM) and year-round energy saving. Using a modified nonsolvent-induced phase separation (NIPS) process, polyethylene glycol (PEG) was applied as a buffering and templating agent to create a macro-micro porous thermoplastic polyurethane (PTPU) structure, achieving over 95 % solar reflectance. Incorporating an MXene/waterborne polyurethane (MXPU) layer via dual-casting enhanced photothermal conversion to 87 % at just 0.2 % MXene content due to secondary absorption, while maintaining high overall emissivity (85 %). Serving as a smart curtain, its low thermal conductivity (19.8 mW/m·K) can further improve STM performance. In cooling mode (summer), the Janus film reflects sunlight and radiates heat outward via infrared emission, achieving a temperature reduction of up to 10.3°C and an energy-saving efficiency of 33.1 %. In heating mode (winter), the film absorbs sunlight and efficiently transfers heat indoors via infrared radiation, resulting in a temperature increase of up to 8.0°C and an energy-saving efficiency of 38.0 %. The innovative design offers an attractive, scalable option for next-generation energy-saving systems in indoor environments, supporting the global shift towards more sustainable and efficient energy use.