Speaker: Chanbum Park, Ruhr-Universität Bochum

Metal-water interfaces play a crucial role in electrochemical reaction mechanisms relevant to energy storage and conversion devices. However, since these interfaces are buried between solid and liquid phases, it remains challenging to experimentally resolve the intrinsic structural and electronic properties of such electrified interfaces, in particular those of the interfacial water molecules. As a result, their fundamental nature is still far from being fully understood up to being controversial. In this work, we quantify the structural and electronic interactions at the gold-water interface. Our findings reveal that the dipole moments and electronic properties of water molecules near the metal surface can vary significantly depending on their orientation and hydrogen-bonding states. By employing a novel efficient implementation for localizing molecular orbitals in hybrid systems such as water interacting with a metallic surface via partially occupied Wannier functions---which allows us to compute the contributions of individual water molecules to the total interfacial electronic dipole---we uncover that water molecules in close proximity to the gold surface exhibit significantly increased dipole moments. We disclose that this is due to distinct polarization of those electron lone pairs which point toward the metal surface, thus belonging to water molecules with their O-H bonds pointing toward the water layer.

Additionally, we present evidence of electronic polarization and charge transfer effects in vibrational spectra through peak broadening and frequency shifts of dangling O-H bonds near the metal surface, as observed in both sum-frequency generation (SFG) spectroscopy and fully atomistic ab initio molecular dynamics simulations.

Overall, the strong electronic interactions between metal and water that we reveal give rise to orientation-dependent electronic heterogeneity. Our findings provide new insights into the fundamental properties of metal-aqueous interfaces with broad implications for electrochemistry, catalysis, and energy-related applications.