In this study, transparent passive radiative cooling coatings are introduced by immobilizing solid and hollow silica (SiO₂) spheres on glass substrates. It is showcased that particle morphology within a sub-monolayer coating strongly influences visible and atmospheric window reflectance of glass. Solid and hollow-sphere particles of total diameter within the Mie regime reduce atmospheric window reflectance (R₀ₖ) at the expense of higher visible reflectance (Rₕ₈ₒ). This trade-off is dependent on particle and core diameter. Solid particles with particle diameter >1000 nm can reduce the R₀ₖ of glass by up to 65 %, though increase Rₕ₈ₒ by 25 %. Meanwhile, the use of hollow-sphere nanoparticles of similar diameters and thin shells (25–50 nm) can reduce the R₀ₖ of glass by up to 35 % with minimal changes to Rₕ₈ₒ. These spectroscopic trends are validated numerically via both Mie theory and effective medium theory. The work demonstrates that hollow-sphere morphology is a valuable lever to control passive radiative cooling for various solar applications requiring transparency, such as coatings for windows or photovoltaic devices.
Lam et al. (Thu,) studied this question.