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

We developed an Al/O/H ReaxFF force field to explore chemical reactions on alpha-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 alpha-Al2O3 (0001), consistent with quantum chemical studies. Molecular dynamics (MD) simulations of H2O/alpha-Al2O3 (0001) reveal that water autocatalysis plays a significant role in accelerating H2O dissociations on Al-terminated alpha-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 x H 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 alpha-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/alpha-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 alpha-Al2O3 (0001) in H2 gas exposure, we reduced the H-H sigma bond energy parameter, equivalent to lowering the H2 dissociation barrier by similar to 19.4 kcal/mol during the simulation. After similar to 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 alpha-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.