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.