Dynamic Stabilization of Metal Oxide–Water Interfaces

Abstract

ABSTRACT:The interaction of water with metal oxide surfaces plays a crucial role in the catalytic and geochemical behavior of metal oxides. In a vast majority of studies, the interfacial structure is assumed to arise from a relatively static lowest energy configuration of atoms, even at room temperature. Using hematite (-Fe2O3) as a model oxide, we show through a direct comparison of in situ synchrotron X-ray scattering with density functional theorybased molecular dynamics (DFT-MD) simulations that the structure of the (11 ̅ 02) termination is dynamically stabilized by picosecond water exchange. Simulations show frequent exchanges between terminal aquo groups and adsorbed water in locations and with partial residence times consistent with experimentally determined atomic sites and fractional occupancies. Frequent water exchange occurs even for an ultrathin adsorbed water film persisting on the surface under a dry atmosphere. The resulting time-averaged interfacial structure consists of a ridged lateral arrangement of adsorbed water molecules hydrogen bonded to terminal aquo groups. Surface pKa prediction based on bond valence analysis suggests that water exchange will influence the proton transfer reactions underlying the acid/base reactivity at the interface. Our findings provide important new insights for understanding complex interfacial chemical processes at metal oxide-water interfaces.The interfaces between metal oxides and water are among the most important in nature and in emerging energy applications, with wide ranging impacts from photocatalytic water splitting [1][2][3][4] to the geochemical cycling of elements 5,6 . Key chemical processes such as adsorption, electron transfer, growth, and dissolution all depend principally on the atomic structure adopted at these interfaces. For example, dissolution and solute adsorption are regulated by the structure of interfacial water 7,8 . Surface acid/base chemistry 9,10 arises from the types and arrangement of terminal metal-coordinating aquo/hydroxyl groups 3,11-13 .At room temperature an interface is at dynamic equilibrium. In principle, the average interfacial structure depends on the interplay of relatively static atoms at the solid surface with relatively dynamic overlying water molecules. Simulations suggest that movement of overlying water molecules can play an essential role in stabilizing the interface and influencing its chemical behavior 14,15 . However, simulated [14][15][16][17] or spectroscopically probed 18 dynamics are seldom integrated with experimentally derived interface structure models to achieve comprehensive insight into interfacial structure 4 . To understand and predict chemical processes at dynamically active metal oxide-water interfaces, structure and dynamics must be considered as a unified whole.Accurate measurements of interface structure and water ordering rely on interface-sensitive synchrotron X-ray scattering methods 19 . The analysis of multiple crystal truncation rods (CTRs) provides a complete 3-dimensional interface mode...

Acidity Constants of the Hematite–Liquid Water Interface from Ab Initio Molecular Dynamics

“…[46][47][48] This method has been successfully applied to several mineral-water interfaces, 31,[49][50][51][52] but not to the HLWI, which is challenging to simulate due to the open-shell 3d orbital character underlying the antiferromagnetic spin pattern of hematite. We obtain pK a values for the half-layer terminated (012) surface species directly from DFT-MD simulations at GGA+U level with dispersion corrections, that are in good agreement with the recent BVT estimates by McBriarty et al 20 based on X-ray crystal truncation rod (CTR) diffraction data. Both our calculations and recent BVT estimates suggest that the deprotonation of these surfaces require significantly more free energy input than previously thought.…”

Atomic-Scale Structure of the Hematite α-Fe2O3(11̅02) “R-Cut” Surface

“…With different types of preparation, such as chemical etching, hydroxylated surface models with missing cations are also reported to be stable in these conditions. 32,71−73 …”

Enhanced photoelectrochemical water splitting of hematite multilayer nanowire photoanodes by tuning the surface state via bottom-up interfacial engineering

“…2 As a rapid charge transport and transfer between the back substrate, photoactive semiconductor, the catalyst and electrolyte is necessary for efficient STH performance, the electronic and structural properties of back substrate/semiconductor/catalyst/electrolyte interfaces play a vital role in PEC performance. [5][6][7][8][9][10] The electron-hole recombination at the interface of the back substrate and hematite is critical for the electron transport from bulk hematite to the current collector and thus affects the overall PEC photocurrent response. 2,9,[11][12][13][14] For instance, Zheng et al reported an integrated hematite, TiO x and FeOOH photoanode and obtained a photocurrent around 1.5 mA cm -2 , which is still much lower than the theoretical value (12.5 mA cm -2 ) of hematite.…”