ReaxFF studies of surface fluorination of alumina and etching of alumina/aluminum metal heterostructures under gas-phase hydrogen fluoride exposure

The self-limiting surface conversion reactions of alumina and alumina/aluminum metal heterostructures under HF exposure are studied at 1250 and 750 K, respectively, using a newly developed Al/O/H/F ReaxFF force field. Simulations reveal that surface termination and HF concentration significantly influence the fluorination behavior of alumina. The 100% Al-terminated α-Al₂O₃ (0001) exhibits the highest degree of fluorination at equilibrium, while the O-rich surface shows negligible surface conversion. Increasing HF concentration on the Al-rich surface reveals the maximum surface fluorination, resulting in a 1.38 Å decrease in alumina thickness. During the fluorination of Al-terminated alumina, HF dissociates to form hydroxyl groups and fluorides. Subsequently, absorbed HF leads to H₂O formation by transferring hydrogen to hydroxyls, removing surface oxygen and leaving surface Al unsaturated for further fluorination. As the solid AlF_x layer thickens, conversion of the underlying surface relies on hydrogen diffusion from HF adsorbed on AlF_x to the interfacial hydroxyls. This process ceases when the AlF_x layer is thick enough to hinder any effective hydrogen diffusion. In heterostructures, Al atoms from the aluminum layer diffuse into the alumina, forming Al-enriched oxide layers that consume hydrogen and form volatile AlH_x, along with solid AlF_x, which significantly reduces the reliance on hydrogen diffusion for continued reaction. Under rarefied HF exposure, completely different reaction mechanisms emerge for alumina and heterostructures.