Atomistic characteristics of ultra-efficient radiative cooling paint pigments: the case study of BaSO4

Radiative cooling has recently revived because of its significant potential for saving energy and combating climate change. Several ultra-efficient particle-matrix cooling nanocomposites such as BaSO 4 -acrylic paints have been demonstrated via trial-and-error approaches, but the atomistic characteristics of their pigments remain elusive. In this work, we use first-principles calculations to predict the full-spectrum optical constants of BaSO 4 , and successfully explain the ultra-high reflectance in the solar spectrum (0.28–2.5 μm) and simultaneously high normal emittance in the sky window (8–13 μm) observed in previous experiments. Efficient radiative cooling pigments require high refractive index n and low extinction coefficient κ in the solar spectrum. However, our results show that they cannot be tuned independently, but are both tied to the electronic band gap. Eliminating κ would require a high band gap, which would yield low n , creating a dilemma to address for radiative cooling. By systematic comparison, we show that BaSO 4 outperforms the commonly used α -quartz ( α -SiO 2 ), and we identify two pertinent characters of BaSO 4 : i) Although the band gap of BaSO 4 is high enough to eliminate solar absorption, it is also moderate enough to enable reasonably high refractive index for strong scattering, and ii) BaSO 4 has complex crystal structure and appropriate bond strength that yield a high number of infrared-active zone-center optical phonon modes in the Reststrahlen bands, and these modes show strong four-phonon scattering which is a previously unknown mechanism that contributes to the high emissivity in the sky window. Our first-principles approach and physical insights pave the way for further search of efficient radiative cooling materials. In general, the pursuit of efficient particle-matrix radiative cooling paints has been via trial-and-error process. The atomistic characteristics that lead to ultra-efficient radiative cooling remain elusive. Using the state-of-the-art first principles calculations, we identify two pertinent characters of BaSO 4 : i) it has a moderate enough band gap to enable reasonably high refractive index for strong scattering, and ii) its complex crystal structure and appropriate bond strength yield numerous infrared phonon modes in the Reststrahlen bands, and these modes show strong four-phonon scattering which is a previously unknown mechanism that contributes to the high emissivity in the sky window. Our work demonstrates the atomistic design for ultra-efficient radiative cooling materials and will impact other radiative materials as well. • The work demonstrates the efficiency and reliability of first-principles predictive design for radiative cooling materials, which is beyond the traditionally tedious trial-and-error approaches. • BaSO 4 -acrylic nanocomposites not only show ultrahigh solar reflectance but also high average emittance in the sky window. • BaSO 4 has a high bandgap to eliminate solar absorption and resulting in a reasonably high refractive index for strong scattering. • BaSO 4 has numerous strongly four-phonon scattering IR phonon modes in the Reststrahlen bands, which is a previously unknown mechanism that contributes to the high emissivity in the sky window. • This work paves the way for the atomistic design of ultra-efficient radiative cooling materials and will impact other radiative materials as well.