TiO₂ nanocrystalline pigmented polyethylene foils for radiative cooling applications: Synthesis and characterization

Radiative cooling devices should ideally operate with a shield substrate that completely blocks solar radiation but is transparent in the "atmospheric window" (8-13 μm) region. In this paper, we introduce a new type of shield for radiative cooling applications based on the use of polyethylene foils pigmented with nanocrystalline TiO₂. Homogeneous shields were prepared by spreading colloidal suspension of TiO₂ (rutile ca. 60 nm) between two polyethylene sheets followed by hot pressing. Optical and structural properties of the films were investigated using X-ray diffraction, TEM, and UV/vis/NIR and FTIR spectroscopy. The shields show high IR transmittance and high solar reflectance, which are favorable characteristics for solar radiation shields in radiative cooling devices. The mechanism for obtaining improved optical properties of nanocrystalline foils (in comparison with submicrometer bulk foils) is discussed with respect to both the materials and the shield preparation method. The simplicity and low-cost preparation of nanocrystalline TiO₂ films, with their superior optical characteristics, could find widespread use in radiative cooling and other environmental applications.