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

Abstract

The molecular scale interaction between water and an oxide surface depends on the strength of the surface hydrogen bonds (Hbonds) through a subtle interplay among surface structure, surface atom polarity, and orientation of sorbed species. Tin oxide (SnO 2 ) 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 H 2 O and OH sorbed on SnO 2 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 H 2 O 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. O xide−water interfaces are ubiquitous in heterogeneous catalysis, 1−6 protein folding, 7 environmental remediation, 1 mineral growth and dissolution, 8,9 and light−energy conversion in solar cells. 3−5,10 In all these systems, the structure and properties of water at the interface determine and control the specific chemical processes. 6 The (110) surfaces of cassiterite (SnO 2 ) and isostructural rutile (α-TiO 2 ) are prototypical oxide surfaces that have been intensely studied. These materials are widely used in catalysis, gas sensing, pigments, and many other applications. 1,3−6,8−13 However, the interpretation at the molecular level of the interfacial structures and dynamics remains highly controversial. Particularly the sorption energy, the strength of hydrogen bonds (H-bonds), and the degree of water dissociation upon adsorption are among the most discussed quantities. For instance, thermochemistry 14−17 and corresponding inelastic neutron scattering (INS) analyses on hydrated α-TiO 2 and SnO 2 nanoparticles 18−20 suggest that the higher surface energy of α-TiO 2 leads to stronger surface water interactions as compared to SnO 2 . On the other hand, theoretical predictions indicate that SnO 2 (110) interacts more strongly with water molecules than α-TiO 2 (110), with dissociative adsorption being thermodynamically favored on SnO 2 (110). 3,4,13,21−29 The degree of dissociation depends on the strength of the Hbond network at the interface 20−24,27 as determined by t...

X-ray and Neutron Scattering of Water

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Understanding the interface between silicon-based materials and water: Molecular-dynamics exploration of infrared spectra

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