Evidence for the anisotropic oxidation of gas-phase Al nanoparticles

Transmission electron microscopy was employed to elucidate the morphologies and structures of thermally oxidized gas-phase Al nanoparticles (mean diameter of 10 ± 3 nm) obtained at various temperatures and oxygen pressures. Exposure of the nanoparticles to oxygen at room temperature followed by heating to high temperatures initially generated an amorphous Al2O3 film on the particles that protected the core from further oxidation up to ~1000 °C. However, these particles were completely oxidized to γ-Al2O3 above ~1000 °C. In contrast, exposing bare, preheated Al nanoparticles to oxygen led to direct oxidation to γ-Al2O3 at temperatures as low as ~575 °C. The direct oxidation of Al nanoparticles at 900 °C under low oxygen pressures resulted in the growth of worm-shaped γ-Al2O3 nanoparticles, suggesting anisotropic oxide growth via interfacial oxidation.