Mesostructured alumina as powders and thin films

Abstract

Mesoporous aluminas offer a wide variety of applications, including catalysis, gas sensing, optics, photovoltaics, and ion exchange. However, the pores are commonly unordered and show a wide pore size distribution, creating a need for porous aluminas with well defined, tailored pore systems. Here we report the synthesis and detailed structural studies of mesoporous alumina as thin films and as powders. The synthesis was based on evaporation-induced self-assembly (EISA) of triblock copolymers Pluronic P-123 and Pluronic F-127 as structure directing agents. Structural studies of the thin films were done by grazing incidence small-angle X-ray scattering (GISAXS) supported by numerical simulations, and X-ray reflectivity. From the quantitative GISAXS studies of the as-cast films, the unit cell dimensions, space groups and also certain details of the regular micellar shape could be retrieved, indicating that the micelles were deformed by compression along the sample normal. As-cast thin films prepared from F-127 were found to give 110-oriented body-centered cubic micellar structures, while films prepared from P-123 had hexagonally packed tubular micelles. Mesoporous powder materials with narrow pore size distribution and high surface area, as characterized by N₂ physisorption, were synthesized using the same strategy. The atomic-scale phase transformation of the powder from amorphous to crystalline takes place above 800 °C, as shown by in situ X-ray diffraction. Both the thin films and the powders were tailored to give mesoporous alumina with a pore size of about 5 nm.