Interfacial synthesis of hollow γ-Al2O3 microspheres via surfactant-free emulsion templates for passive daytime radiative cooling

Surfactant-free interfacial synthesis of hollow Al 2 O 3 via kinetic modulation • Chelation enables precursor transition from hydrophobic to amphiphilic states. • Simple room-temperature strategy bypasses harsh energy-intensive methods. • PDRC coatings deliver 97.6% solar reflectance and 0.976 mid-IR emissivity. • The scalable coatings realize a remarkable sub-ambient cooling of 9.66 °C. Hollow γ-Al 2 O 3 architectures are highly attractive for passive daytime radiative cooling (PDRC) owing to their low density and unique optical characteristics. However, their fabrication typically relies on sacrificial templates, hydrothermal treatments, or high-temperature processing, while the intrinsically ultrafast hydrolysis of aluminum alkoxides has long prevented their use in hollow-structure engineering. Here, we report a surfactant-free, chelation-enabled interfacial synthesis strategy that overcomes this limitation through kinetic regulation of aluminum sec-butoxide. The coordination process gradually transforms the precursor from hydrophobic to amphiphilic, enabling spontaneous self-emulsification and controlled interfacial assembly. As a result, hierarchical hollow γ-AlO(OH) microspheres are obtained at room temperature and subsequently converted into hollow γ-Al 2 O 3 microspheres by calcination. The resulting hollow γ-Al 2 O 3 microspheres exhibit tunable diameters (3.3–15.8 μm), uniform nanoscale shells (162 ± 60 nm), and an ultralow density of 0.083 g·cm −3 . The process is readily scalable using an industrial homogenizer. When incorporated into coatings, the hollow microspheres provide abundant dielectric interfaces that enhance solar reflection and thermal emission, delivering a solar reflectance of 97.6%, an atmospheric window emissivity of 0.976, and a sub-ambient cooling of 9.66 °C under a solar irradiance of 454.5 W·m −2 . This work provides a scalable and energy-efficient manufacturing strategy for hollow γ-Al 2 O 3 architectures, offering significant potential for large-area radiative cooling coatings and thermal management applications. Download: Download high-res image (108KB) Download: Download full-size image