Physical insights into the high radiative cooling power of cellulose-based materials

As an emerging research direction, radiative cooling attracts extensive research interest because of its zero energy consumption and zero pollution characteristics. In this work, we experimentally find that a cellulose nanocrystal (CNC) film has the reflectivity of 94.6% in the 380-1100 nm range where the sunlight energy is most concentrated and an emissivity of 95.0% in the atmospheric window, which are comparable with those of commonly studied radiative cooling materials, such as Al₂O₃ particles in the same range. Using the radiative cooling theoretical model, the theoretical net radiative cooling power of the CNC film is predicted to be ∼102.91 W m^-2, showing its excellent radiative cooling abilities. Using first-principles calculations with the efficient meta-generalized-gradient-approximation (meta-GGA) functional, we studied the electronic band structure of cellulose-I. The wide band gap and medium dielectric constant of cellulose-I prevent the absorption of sunlight heat and high emissivity in the atmospheric transparent window. This work suggests that cellulose-based materials are promising for radiative cooling applications, and the underlying physical mechanism is also explored.