Abstract
The dynamical properties of physically and chemically adsorbed water molecules at pristine hematite-(001) surfaces have been studied by means of nonequilibrium ab initio molecular dynamics (NE-AIMD) in the NVT ensemble at room temperature, in the presence of externally applied, uniform static electric fields of increasing intensity. The dissociation of water molecules to form chemically adsorbed species was scrutinized, in addition to charge redistribution and Grotthus proton hopping between water molecules. Dynamical properties of the adsorbed water molecules and OH– and H3O+ ions were gauged, such as the hydrogen bonds between protons in water molecules and the bridging oxygen atoms at the hematite surface, as well as the interactions between oxygen atoms in adsorbed water molecules and iron atoms at the hematite surface. The development of Helmholtz charge layers via water breakup at Fe2O3–hematite/water interfaces is also an interesting feature, with the development of protonic conduction on the surface and more bulk-like water.
Highlights
The dynamical properties of physically and chemically adsorbed water molecules at pristine hematite-(001) surfaces have been studied by means of nonequilibrium ab initio molecular dynamics (NE-ab initio Molecular dynamics (MD) (AIMD)) in the NVT ensemble at room temperature, in the presence of externally applied, uniform static electric fields of increasing intensity
TiO2 is one of the most studied metal oxides in the literature, α-Fe2O3 has come to be of equal, if not more, interest in recent years, due to its relative ubiquity and low cost.[2−10] despite recent progress in our understanding of titania−water interfaces,[11,12] including via theoretical and molecular simulation methods,[13] the successful modeling of hematite−water interfaces remains rather challenging and elusive. Such metal−oxide/water interfaces provide a rich environment for the study of the dynamical properties of confined water molecules; this is so where hydrogen-bonded molecules play an important role in stabilizing solutes via solvent interactions and in forming “cages”
Very low energy barriers were found for water dissociation on hematite (001) surfaces, confirming experimental reports of widespread room-temperature dissociation of water on hematite (001);[36] this is in some contrast to the debate as to the dominance of physical or chemical water adsorption on rutile (110) at room temperature.[37−39] AIMD studies of the water−hematite interface tend to be reported more rarely than those for water/titania.[2]
Summary
The dynamical properties of physically and chemically adsorbed water molecules at pristine hematite-(001) surfaces have been studied by means of nonequilibrium ab initio molecular dynamics (NE-AIMD) in the NVT ensemble at room temperature, in the presence of externally applied, uniform static electric fields of increasing intensity. The initial structure was prepared in a way that the first solvation layers of the hematite surfaces were only weakly interacting with the oxide, i.e., physi-sorbed, the water molecules from these layers quickly adsorb chemically to the nonsaturated surface iron atoms, as was observed in zero-field simulations in ref 2 for a very similar system.
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