Bacterial Cellulose‐Based Janus Films with Radiative Cooling and Solar Heating Properties for All‐Season Thermal Management†

Comprehensive Summary Advanced radiative cooling materials with both heating and cooling mode is of pivotal importance for all‐season energy‐saving in buildings. In this work, we report the design and fabrication of bacterial cellulose‐based Janus films (J‐BC) with radiative cooling and solar heating properties, which were developed by two‐step vacuum‐assisted filtration of modified MXene‐doped bacterial cellulose and modified silicon nitride (Si 3 N 4 )‐doped bacterial cellulose, followed by hot‐pressing and drying treatments. The as‐prepared J‐BC films show a unique Janus structure where modified MXene nanosheets and cellulose nanofibers are on the bottom surface, and modified silicon nitride (Si 3 N 4 ) nanoparticles and cellulose nanofibers are on the top surface. The radiative cooling effect of J‐BC films is enabled by the Si 3 N 4 ‐doped bacterial cellulose due to the high mid‐infrared emissivity of Si 3 N 4 nanoparticles, which shows a high solar reflection of ~98.1% and high emissivity of ~93.6% in the atmospheric transparency window (8—13 μm). Thanks to the enhanced photothermal conversion of the modified MXene nanosheets, a reduced solar reflection (6.6%) and relatively low thermal emissivity in the atmospheric window (71.4%) are achieved, making sure the solar heating effect of J‐BC films. In the outdoor tests, J‐BC films achieve a sub‐ambient temperature drop of ~3.8 °C and an above‐ambient temperature rise of ~14.2 °C. Numerical prediction demonstrated that the J‐BC films with dual modes have great potential of all‐season energy saving for buildings and a corresponding energy‐saving map in China is also created. The work disclosed herein can provide an avenue for the shaping of advanced radiative cooling materials for emerging applications of personal thermal management, sustainable energy‐efficient buildings, and beyond.