Accuracy in Resolving the First Hydration Layer on a Transition-Metal Oxide Surface: Experiment (AP-XPS) and Theory

Abstract

Understanding the equilibrium conditions at the metal oxide/ aqueous interface is a key component toward visualizing the structure of water in confined environments and differentiating the catalytic activity of transition-metal oxides. While ambient pressure X-ray photoelectron spectroscopy (AP-XPS) has been the primary technique to investigate the formation of a hydration layer on many surfaces, results over the extended relative humidity (RH) range accessible experimentally have not been compared quantitatively to theoretical predictions. With the use of first-principles theoretical methods and accumulated knowledge of AP-XPS spectral analysis, we do so here for a model surface, TiO 2 -terminated undoped SrTiO 3 (100) (STO). The measured distribution of OH and H 2 O coverages from vacuum up to the first hydration layer is in good agreement with a static density functional theory (DFT) configuration involving partial dissociation of H 2 O per Ti-atom mediated by H-bonding. Furthermore, ab initio molecular dynamics (AIMD) simulations at 300 K for select coverages (1/4, 1/2, and 1 ML) test the role of fluctuations and entropy in the competition between adsorption and dissociation with coverage. This comparison between theory and experiment for OH and H 2 O coverages on STO provides a foundation for a more quantitative assessment of the first hydration layer and associated competition between adsorption, dissociation, and H-bonding on transition-metal oxide surfaces.

Moderate Electron Doping Assists in Dissociating Water on a Transition Metal Oxide Surface (n-SrTiO3)

“…MgO exhibits nearly full hydroxylation, which is also consistent with calculations . The coordination geometry also matters: unlike its rutile TiO 2 (110) counterpart, the TiO 2 -terminated perovskite SrTiO 3 is consistent with partial water dissociation by both experiment and theory …”

“…Therefore, the data suggests that the extracted monolayer of pure water is atop a hydroxylated layer for 0.7% Nb STO while intermixed between the two layers for 0.1% Nb STO. The intermixing for 0.1% Nb STO does assume that there are no empty metal sites in the first hydration layer, which is corroborated by a number of DFT calculations 49−51 and AIMD calculations 7,13 for hydrated STO that show the lowest energy configurations to have both metal sites coordinated by either H 2 O or OH.…”

Interplay between Facets and Defects during the Dissociative and Molecular Adsorption of Water on Metal Oxide Surfaces

“…The synchronous rise of both surface hydroxyls and adsorbed water highlights a mixed adsorption mode for hydroxylation of the (001) surface, where cooperative effects between physiosorbed water molecules and surface hydroxyls facilitate further hydroxylation. In addition to α-Fe 2 O 3 (001), such a phenomenon has been observed repeatedly on metal oxide and perovskite oxide surfaces including Fe 3 O 4 (001), MgO(100), , TiO 2 (110), LaFeO 3 , and SrTiO 3 , to name a few. Together, these suggest that strong H-bonding interactions between an adsorbed water molecule and a surface hydroxyl stabilizes a partially dissociated final state that promotes further surface water dissociation. , …”

The Sensitivity of Metal Oxide Electrocatalysis to Bulk Hydrogen Intercalation: Hydrogen Evolution on Tungsten Oxide

“…Metal oxides are known to interact strongly with water, resulting in adsorbed hydroxyls, protons, and associated water molecules that can block surface active sites. , Thus, despite the agreement between experimental and computational results, we remained concerned that the aqueous electrochemical environment could facilitate strong surface hydration and limit H adsorption at active W sites. Since metal site blocking via hydration would inhibit the HER mechanism we proposed in Figure , we further examined the free energies of water interacting with various surface sites.…”