Impact of Hydroxylation and Hydration on the Reactivity of α‐Fe₂O₃ (0001) and (102) Surfaces under Environmental and Electrochemical Conditions

Abstract

explored extensively in many industrial applications, e.g., as heterogeneous catalysts, [2] energy storage devices, [3] or biomedical devices. [4] In particular, a major promising application is the development of hematite as a catalytic electrode material for photo-electrochemical (PEC) water splitting systems, [4,5] where photoexcitation leads to the evolution of O 2 and H 2 , thereby directly converting solar energy into chemical energy. In PEC cells, the oxygen evolution reaction (OER) involving (de)hydroxylation and (de)hydration occurs at the hematite/water interface. The local geometric and electronic structures of the hematite surface interacting with water/hydroxyl complexes are expected to play a significant role in determining the material catalytic activity, as these can largely impact the reaction thermodynamics and kinetics for each step of the OER mechanism [6] as well as the charge separation process. [7] Thus, it is critical to gain a detailed understanding of the surface configurations of hematite in contact with water in order to assist the development of efficient hematite-based PEC catalysts. In addition, water being one of the most pervasive molecules in the environment, it is an excellent probe to study substrate properties with regard to redox processes, adsorption capacity, impact of defects, and electronic corrugations in the presence of many other chemical species. [8] Adsorbed water can bind to the oxide surface in a molecular fashion or dissociative fashion (as OH − and H + ), through a variety of mechanisms including electrostatic interactions, charge transfer, or hydrogen bonding. Since a water molecule and its dissociative products possess markedly different chemical natures, the first step to describe many water-associated chemical processes is to establish the fundamental understanding of how water molecules and hydroxyls arrange on a given crystal surface. Despite advanced experimental techniques for surface characterization, it is not a trivial task to unveil the details of the surface configurations; one of the main challenges usually originates in the difficulties to accurately distinguish H 2 O from OH/O on the surface, especially when they are connected in a complex hydrogen-bond network. To shed light on this important issue, we rely here on a theoretical approach Hematite (α-Fe 2 O 3 ) is widely used as a catalytic electrode material in photoelectrochemical water oxidation, where its surface compositions and stabilities can strongly impact the redox reaction process. Here, its surface configurations in environmental or electrochemical conditions are assessed via density functional theory (DFT) calculations conducted at the Perdew, Burke, and Ernzerhof (PBE)+U level. The most energetically favorable surface domains of α-Fe 2 O 3 (0001) and (1102) are predicted by constructing the surface phase diagrams in the framework of first-principle thermodynamics. The relative surface stabilities are investigated as a function of partial pressures of oxygen and water, temperature, ...

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

“…Hence, many chemical processesincluding adsorption, dissolution, and electron transferare determined by the atomic structure and interaction with water . Understanding the reactivity at this metal oxide–water interface begins with understanding how dry surfaces initially interact with water, typically resulting in the dissociation of some fraction of adsorbed water molecules to form a hydroxylated surface. − Because surface hydroxyl groups are known to act as anchor sites for solute binding, their arrangement and density influence a wide range of material properties, including foreign ion incorporation, toxic metal fate and transport, photodegradation of natural organic matter, and so forth. ,− Additionally, the type and distribution of surface hydroxyls strongly impacts the electronic structure of the material and hence plays a critical role in determining water oxidation performance in PEC devices. − …”

Surface Hydration and Hydroxyl Configurations of Gibbsite and Boehmite Nanoplates

“…The chemistry of metal oxide–water interfaces is strongly dependent on the structure and dynamics of its surface atomic groups and molecularly thin layers of adsorbed water molecules. The termination of the otherwise continuous crystalline structure often yields new molecular states at the interface that are different from those in the bulk, due to unsaturated bonding, charge imbalance, a variation of local electric fields, and changes in molecular symmetries and dynamics. − The type and distribution of these new states depends not only on the composition and structure of the solid surface but also on the composition and the pH of the aqueous phase. ,,− The latter is particularly important due to the amphoteric nature of surface hydroxyls and the prospect of pH-dependent hydrolysis reactions causing surface hydroxylation and reconstruction akin to metal hydroxides. ,,, Such behavior is furthermore well-known to be facet-dependent, rendering complex net acid–base chemistry to metal oxide nanoparticles. ,,,, …”

“…2,3,11,13 Such behavior is furthermore well-known to be facetdependent, rendering complex net acid−base chemistry to metal oxide nanoparticles. 1,4,10,16,17 Much of this complexity is embodied in the (oxy)hydroxides of aluminum, which commonly occur in nature as fine-grained minerals such as gibbsite (α-Al(OH) 3 ), bayerite (β-Al(OH) 3 ), diaspore (α-AlO(OH)), and boehmite (γ-AlO(OH)). Because of their typically small particle sizes, as well as their bulk hydroxyl content, selectively probing these critical facetspecific interfacial properties is difficult.…”

Unraveling the Surface Reactivity of Pristine and Ti-Doped Hematite with Water

“…In particular, by modifying the surface acidity and structure, the presence of titanium can influence the interaction between water and hematite. Whereas the latter has been widely investigated on pure hematite both experimentally − and theoretically, − such studies are rather scarce for Ti-doped hematite . In particular, as far as we know the impact of the presence of Ti on the interaction of water and hematite has not been reported.…”

“…The DFT+U approach was adopted for the inclusion of the on-site Coulomb repulsion of both Fe and Ti 3d electrons using the Dudarev approach with the effective parameter U eff (Fe) = 4.3 eV and U eff (Ti) = 3.5 eV. In agreement with previous studies, ,,,− we considered only the Fe-terminated surface modeled by an 18-layer symmetrical slab in a hexagonal cell with a Fe 2 O 3 = 10.133 Å and a vacuum of 20 Å. For doped hematite, two Fe atoms (over the 48 Fe atoms) were substituted by two Ti atomsone close to the surface, the other at the bottom of the cellin order to keep the antiferromagnetism in the whole cell.…”