Structurally Colored Photonic Janus Films for Switchable Radiative Cooling and Solar Heating

Personal thermal management (PTM) requires materials that can adapt to dynamically changing environments, yet most existing systems are limited to single-mode cooling or heating and lack tunable optical appearance. Here, we report a structurally colored photonic Janus film that enables switchable radiative cooling and solar heating within a single material platform. The cooling side is based on cellulose nanocrystal (CNC) photonic structures, providing high mid-infrared emissivity (up to 0.91) while enabling tunable structural color through pitch modulation, thereby achieving radiative cooling without sacrificing visual appearance. The heating side consists of a carbon nanotube (CNT) layer with high solar absorptivity (~92%), enabling efficient photothermal conversion, further enhanced by low-voltage Joule heating. By simply flipping the film, reversible switching between cooling and heating modes is achieved, allowing adaptation to varying thermal conditions. In addition, the system is compatible with continuous fabrication, enabling scalable production. This work demonstrates a dual-mode photonic thermal management system that integrates optical tunability with switchable thermal regulation, providing a viable pathway toward wearable microclimate control.