ReaxFF Molecular Dynamic Simulations of ZnO Nanocluster and Films in H₂ Atmosphere

Reactive molecular dynamics simulations were performed to explore the structural evolution of a ZnO nanocluster and (0001) and (101̅0) surfaces under a H 2 atmosphere at different temperatures. The mechanisms of H 2 dissociation and water formation were analyzed. Our simulations reveal that there are two pathways for H 2 dissociation and three routes for water formation on the surfaces. The nanocluster is more active for H 2 dissociation and water formation than the two surfaces. The gas–solid interactions lead to outward displacement of the substrate O atoms. While the O-terminated surface of the (0001) facet is active for H 2 dissociation and water formation, the Zn-terminated one is inactive for the dissociation. Unlike the (0001) surface which is more easily reduced, the (101̅0) surface is readily hydroxylated. Water formation and desorption results in surface oxygen depletion and Zn aggregation which lead to surface metallization, in accordance with the experimental observations. Our simulations show that Zn sites are not active for H 2 dissociation. By fitting the obtained rate constants at different temperatures, we estimated the activation energy of the H 2 dissociation over ZnO cluster to be 4.06 kcal/mol, in very good agreement with the experimental result of 5.0 kcal/mol.