Multiscale scattering design of bio-based phosphorylated nanocellulose aerogels for flame-retardant radiative cooling

Optimizing radiative cooling aerogels requires understanding how thermal insulation and optical properties jointly influence the temperature of the space beneath them. However, concurrent changes in filler composition and structure complicate this relationship. Here, we investigate a comparative design using hollow fillers at different characteristic length scales: microscale hollow glass spheres (micro-GM) and nanoscale hollow SiO₂ spheres (nano-HS), separately incorporated into a phosphorylated cellulose nanofiber (P-CNF)/BaSO₄ matrix. The resulting aerogels exhibit comparable solar reflectance (93.8-94.0%) and atmospheric-window emittance (96.5-96.8%), but different thermal conductivities and outdoor chamber temperature responses. Although the micro-GM composite has lower thermal conductivity, a lower chamber temperature is recorded beneath the nano-HS composite. The latter achieves a maximum chamber temperature reduction of 12.4 °C relative to the chamber covered with pristine P-CNF. This contrast indicates that thermal conductivity alone is insufficient to predict cooling of the covered space under the tested conditions. The composite aerogels also exhibit suppressed heat release, while silanization of the nano-HS composite improves surface hydrophobicity and retains comparable outdoor chamber temperature profiles. These findings provide a comparative basis for evaluating hollow-filler architectures through both optical and thermal characterization and the temperature response of the covered space, without attributing the observed differences solely to particle size.