ReaxFF Study of Surface Chemical Reactions between α-Al₂O₃ Substrates and H₂O/H₂ Gas-Phase Molecules

We developed an Al/O/H ReaxFF force field to explore chemical reactions on α-Al₂O₃ surfaces in H₂O/H₂ gas-phase environments. This force field generates surface energy profiles of A-, C-, R-, and M-planes with various terminations (Al- or O-) and predicts the thermodynamic and kinetic behaviors of hydrolysis on Al-terminated α-Al₂O₃ (0001), consistent with quantum chemical studies. Molecular dynamics (MD) simulations of H₂O/α-Al₂O₃ (0001) reveal that water autocatalysis plays a significant role in accelerating H₂O dissociations on Al-terminated α-Al₂O₃ (0001). Compared with the 50% Al-terminated surface, the 100% Al-terminated surface becomes more easily hydroxylated at temperatures as low as 350 K, relying more on an O_xH_y clustering mechanism than complete H₂O dissociations, and desorbs significantly more H₂O molecules once heated up to 500 K or higher. But heating cannot eliminate surface hydroxyls for either case, and achieving a Gibbsite-like surface by H₂O exposure is unlikely. H₂O dissociations on α-Al₂O₃ (0001) terminated with randomly distributed surface Al species deviate from 1-2 and 1-4 pathways due to irregular vacancy defects, and a random surface appears to be more reactive to H₂O than the ordered one with the same surface Al coverage. Simulations of H₂/α-Al₂O₃ suggest that the combination of a dense surface O coverage and a low thermodynamic surface stability leads to elevated H₂ dissociation kinetics. To accelerate the surface O removals of 100% O-terminated α-Al₂O₃ (0001) in H₂ gas exposure, we reduced the H-H σ bond energy parameter, equivalent to lowering the H₂ dissociation barrier by ∼ 19.4 kcal/mol during the simulation. After ∼ 1.5 ns, the surface termination became comparable to the 100% Al-terminated one but retained a small quantity of hydroxyls. This force field reveals how the α-Al₂O₃ crystallographic plane and the surface termination influence the dissociation behaviors of H₂O/H₂ gas molecules and lays the foundation for future force field developments targeted at thin film epitaxy on sapphire.