Morphology-Controlled Synthesis of Nanocrystalline η-Al2O3 Thin Films, Powders, Microbeads, and Nanofibers with Tunable Pore Sizes from Preformed Oligomeric Oxo-Hydroxo Building Blocks

Abstract

Herein the synthesis and characterization of nanocrystalline η-Al2O3 materials with different morphologies, including dense nanofibers, hierarchically porous microbeads, and highly ordered mesoporous thin films and powders is reported. These materials were fabricated by facile polymer templating, nanocasting, and electrospinning routes using preformed aluminum-oxo-hydroxo species as the building blocks. We show that the oligomeric building blocks used in this work can be formed under nonaqueous conditions with benzyl alcohol as the oxygen donor and further converted to the eta-phase of Al2O3 at temperatures above 700 °C. Electron microscopy, X-ray diffraction, grazing incidence small-angle X-ray scattering, X-ray photoelectron spectroscopy, and physisorption studies collectively verify that the different η-Al2O3 materials are well-defined at the nanoscale and the microscale after annealing in air at temperatures as high as 1000 °C. In addition, data obtained on polymer-templated thin films show that both cubic and 2D-hexagonal structures with large pore sizes between 10 and 30 nm can be achieved. These data also show that the conversion of the initially amorphous frameworks comes at little cost to the ordering of the pore-solid architectures. Overall, we believe that the synthesis parameters described here provide a blueprint for future nanomaterials.