Pathways for Electron Transfer at MgO-Water Interfaces from Ab Initio Molecular Dynamics

The nature of electron transfer across metal oxide-water interfaces depends significantly on the band gap of the oxide and its band edge energies relative to the potentials of relevant aqueous redox couples. Here we focus on the water interface with MgO, a prototypical wide band gap oxide whose conduction band edge is close in energy to that of water. We investigate the behavior of an excess electron at and out of equilibrium near the interface using ab initio molecular dynamics based on hybrid density functional theory. Our simulations show that under equilibrium conditions the excess electron (donated by an Al impurity in MgO) localizes to a midgap defect state comparable in energy and shape to a hydrated electron in bulk water. To characterize the electron transfer from the conduction band of MgO to interfacial product states, we dope near-equilibrium configurations of the pristine MgO-water system with Al and run short trajectories of these instantaneously out-of-equilibrium systems. We observe two distinct products associated with the excess electron: a surface-localized electron (esurf) and an aqueous hydrogen radical (H•). The H• pathway exhibits a much higher activation barrier despite being more exoergic, making esurf the kinetic product. Our characterization of the pathways on the basis of Marcus theory is consistent with the poor observed utility of MgO for water radiolysis. Moreover, we anticipate that the computational framework employed here will be broadly applicable to assessing electron transfer mechanisms at aqueous, photocatalytic interfaces.