Hierarchically structured, Janus optical nanoengineered wastepaper for switchable radiative cooling/heating

Switchable radiative cooling/heating holds great promise for mitigating the global energy and environmental crisis. Here, we reported a cost-effective, high-strength Janus film through surface optical engineering waste paper with one side decorated by a hydrophobic polymeric cooling coating consisting of micro/nanopore/particle hierarchical structure and the other side coated with hydrophilic MXene nanosheets for heating. The cooling surface demonstrates high solar reflectivity (96.3%) and infrared emissivity (95.5%), resulting in daytime/nighttime sub-ambient radiative cooling of 6°C/8°C with the theoretical cooling power of 100.6 and 138.5 W m −2 , respectively. The heating surface exhibits high solar absorptivity (83.7%) and low infrared emissivity (15.2%), resulting in excellent radiative heating capacity for vehicle charging pile (~6.2°C) and solar heating performance. Impressively, the mechanical strength of Janus film increased greatly by 563% compared with that of pristine waste paper, which is helpful for its practical applications in various scenarios for switchable radiative thermal management through mechanical flipping. Energy-saving simulation results reveal that significant total energy savings of up to 32.4 MJ m −2 can be achieved annually (corresponding to the 12.4% saving ratio), showing the immense importance of reducing carbon footprint and promoting carbon neutrality.