Hollow skeletal macroporous aluminum hydroxide monoliths and their calcined derivatives with high diffuse reflectance

Abstract Macroporous aluminum hydroxide monoliths with continuous hollow skeletons were fabricated through a scalable, template‐free sol‒gel synthesis from metal‒salt precursors. Tuning the concentration of propylene oxide, which serves as a proton scavenger, yielded crack‐free monoliths several centimeters in diameter. Scanning electron microscopy revealed a three‐dimensional porous network composed of an interconnected nanoplatelet shell. The as‐prepared sample exhibited a Brunauer–Emmett–Teller‐specific surface area of 429 m 2 g ‒1 . Calcination at 500°C converted the framework into γ‐alumina while preserving the hollow architecture and a high specific surface area of 314 m 2 g ‒1 . Subsequent heating to 1250°C produced α‐alumina while preserving the monolithic shape and continuity of the skeletal framework; however, the hollow structure collapsed, and the surface area decreased significantly. Optical measurements showed that aluminum hydroxide and α‐alumina monoliths both exhibited high total diffuse reflectance across the ultraviolet–visible–near‐infrared (UV–Vis–NIR) range (400–1200 nm), comparable to Spectralon. The α‐alumina retained over 90% reflectance up to 2500 nm and demonstrated excellent thermal stability. These results suggest that the fabricated monoliths could be used as diffuse reflectance reference materials.