Morphology‐Controlled Silica Nanoparticle Coatings for Transparent Radiative Cooling

In this study, transparent passive radiative cooling coatings are introduced by immobilizing solid and hollow silica (SiO$_2$) 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 (R$_{AW}$) at the expense of higher visible reflectance (R$_{VIS}$). This trade-off is dependent on particle and core diameter. Solid particles with particle diameter >1000 nm can reduce the R$_{AW}$ of glass by up to 65 %, though increase R$_{VIS}$ by 25 %. Meanwhile, the use of hollow-sphere nanoparticles of similar diameters and thin shells (25–50 nm) can reduce the R$_{AW}$ of glass by up to 35 % with minimal changes to R$_{VIS}$. 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.