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

Abstract

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.