Relating Interfacial Order to Sum Frequency Generation with Ab Initio Simulations of the Aqueous Al₂O₃(0001) and (112̅0) Interfaces

We use density functional theory molecular dynamics simulations to investigate the structure, dynamics, and vibrational sum frequency generation (vSFG) spectra at the Al2O3(0001)–H2O and Al2O3(112̅0)–H2O interfaces. We find that the differences in the vSFG spectra between the two interfaces can be explained by significantly weaker surface–water interactions at the (0001) vs (112̅0) interface. The weaker interactions at the (0001) surface are caused by the flat surface plane and high density of OH groups, leading to a decoupling of the vibrational modes of the surface OH groups and H2O molecules. The (0001) vSFG spectrum thus displays two well-separated peaks at the near-neutral pH, in contrast to the vSFG spectrum of the corrugated (112̅0) interface, which has stronger surface–water interactions and thereby a narrower band in the vSFG spectrum with closely spaced peaks. By simulating the interfaces with both the Perdew–Burke–Ernzerhof (PBE)–Tkatchenko–Scheffler and revised PBE (RPBE) functionals, we find that a proper description of the separation of surface and solution H-bond modes is essential to obtain accurate SFG spectra. The RPBE functional was unable to accurately model the H-bonds of H2O and surface aluminols simultaneously. Finally, we use the H-bond lifetimes and the tetrahedral order parameter for H2O to conclude that water at the (0001) surface is more ordered than at the (112̅0) surface, in contrast to prior interpretations of of X-ray reflectivity and vSFG experiments, highlighting the importance of atomistic models of the H-bond structure and dynamics of the water–oxide interfaces