Three-dimensionally printable hollow silica nanoparticles for subambient passive cooling

Solar reflectance and thermal emissivity are critical benchmarks for evaluating the effectiveness of passive cooling strategies. The integration of three-dimensional (3D) printing techniques with passive cooling materials enables local thermal management of multifaceted objects, offering opportunities for unexplored energy-saving applications. For example, conformal printing of cooling materials can mitigate solar absorption caused by the top metal electrodes in solar cells, thereby improving their efficiency and lifetime. In this study, we report the synthesis of 3D printable hollow silica nanoparticles (HSNPs) designed to induce subambient cooling performance under daylight conditions. HSNPs with diameters of 400-700 nm and silica shell thicknesses of approximately 100 nm were synthesized using an in-situ sol-gel emulsion method. Subsequently, these HSNPs were formulated into printable pastes by carefully selecting the mixture concentration and molecular weight of polyvinylpyrrolidone (PVP). The PVP-linked HSNPs exhibited a solar (0.3-2.5 μm) reflectivity of 0.98 and a thermal (8-13 μm) emissivity of 0.93. In contrast to a single silica nanoparticle (NP), the scattering analysis of a single HSNP revealed a distinctive scattering distribution characterized by amplified backward scattering and suppressed forward scattering. In outdoor daytime experiments, the HSNP-printed sample led to the subambient cooling of a dielectric substrate, surpassing the cooling performance of reference materials such as silica NPs, silver pastes, and commercial white plastics and paints.