Vibrational Density of States of Strongly H-Bonded Interfacial Water: Insights from Inelastic Neutron Scattering and Theory

The molecular scale interaction between water and an oxide surface depends on the strength of the surface hydrogen bonds (H-bonds) through a subtle interplay among surface structure, surface atom polarity, and orientation of sorbed species. Tin oxide (SnO2) in the rutile structure is an important catalytic and gas-sensing material, and its surface properties have been the subject of intense scrutiny. Here we show that the vibrational dynamics of H2O and OH sorbed on SnO2 nanoparticles, probed with inelastic neutron scattering and analyzed with ab initio molecular dynamics, reveals very strong surface H-bonds, with a formation enthalpy twice that of liquid water. This unusually strong interaction results in (i) decoupling of the hydrated surface from additional water layers due to an epitaxial screening layer of H2O and OH species, (ii) high energy of OH wagging modes that provides an experimental indicator of surface H-bond strengths, and (iii) high proton exchange rates at the interface. H-bonding energetics and interfacial structures also control the average degree of dissociation of sorbed water. The close agreement in the vibrational density of states measured experimentally and generated in silico provides validation of the theory, while the atomistic simulations provide atomic/molecular-level details of individual species contributions to the observed spectrum. Together, these integrated studies provide definitive insights into the role of H-bonds in controlling the structure, dynamics, and reactivity of metal oxide/water interfaces.