Electronic and phononic origins of BaSO4 as an ultra-efficient radiative cooling paint pigment

Abstract

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 BaSO4-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 BaSO4, 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 BaSO4 outperforms the commonly used α-quartz (α-SiO2), and we identify two pertinent characters of BaSO4: i) Although the band gap of BaSO4 is high enough to eliminate solar absorption, it is also moderate enough to enable reasonably high refractive index for strong scattering, and ii) BaSO4 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.