Identification of Two-Dimensional FeO₂ Termination of Bulk Hematite α-Fe₂O₃(0001) Surface

Abstract

Iron oxides are among the most abundant compounds on Earth and have consequently been studied and used extensively in industrial processes. Despite these efforts, concrete understanding of some of their surface phase structures has remained elusive, in particular the oxidized α-Fe2O3(0001) hematite surface. We detail an optimized recipe to produce this phase over the entire hematite surface and study the geometrical parameters and composition of its complex structure by means of atomically resolved microscopy, electron diffraction and surface-sensitive spectroscopies. We conclude that the oxidized α-Fe2O3(0001) surface is terminated by a two-dimensional iron oxide with structure, lattice parameters, and orientation different from the bulk substrate. Using total-energy density functional theory for simulation of a large-scale atomic model, we identify the structure of the surface layer as antiferromagnetic, conductive 1T-FeO2 attached on half-metal terminated bulk. The model succeeds in reproducing the characteristic modulations observed in the atomically resolved images and electron diffraction patterns.Mineral iron oxide is known to be available in many stoichiometries, polymorphs, and even mixtures. Hematite (ɑ-Fe2O3), maghemite (γ-Fe2O3), magnetite (Fe3O4), and wüstite (Fe1-XO) are prominent representatives of this class, possessing a wide range of electronic, magnetic and catalytic properties, due to their different oxygen content and characteristic crystal structures 1 .Currently, a significant amount of research is focused on the catalytic processes that occur on the surfaces of iron oxides, such as CO oxidation, wastewater purification, liquid fuel synthesis via Fischer-Tropsch reactions, styrene production or water splitting 2 .The research on iron oxides has turned its focus to nanoparticles 3 and thin films, 4 which are economically advantageous and show versatility beyond that of bulk materials. The critical limit -2D iron oxide films have been achieved in a form of a monolayer of FeO(111) on Pt(111) 5 , Ag (111) 6 , Ru(0001) 7 and Pd(111) 8 . Nevertheless, stability of this monolayer is inherently linked to its strong hybridization with the metal substrate; it remains an open question whether it can exist independently or as termination of an iron oxide crystal 9,10 , in analogy with the recently revisited V2O3(0001) system 11,12 . Another recent survey on new possible candidates for 2D materials suggests that some trilayer structures of metal oxides (e.g. MnO2, CoO2, GeO2) may be stable after exfoliation from a layered bulk 13 .The atomic structures of bulk Fe3O4 and ɑ-Fe2O3 are well-known, but their surface terminations remain elusive 2,14 , owing to their complexity and the fact that their surface stoichiometry can be varied depending on the preparation. Specifically, by removing oxygen from the surface of ɑ-Fe2O3(0001) by selective sputtering, a stoichiometry and structure resembling Fe3O4(111) can be attained 15,16 . Conversely, the surfaces can be partially or fully reoxidized by increasing t...

Gold-Decorated Biphase α-Fe2O3(0001): Activation by CO-Induced Surface Reduction

“…However, we note that there is surface-near oxygen even in the case of the metal-terminated surface. Such an issue would not exist if the surface is oxygen terminated as proposed by Jelinek et al 38…”

“…36 Based on a combination of DFT calculations with STM and LEED Lewandowski et al 37 put forward a model according to which the biphase structure consists of an ordered array of patches of ferryl-, iron-, and oxygen-terminated areas. Very recently still another model was proposed by Jelinek et al, 38 trilayer, i.e., an iron layer sandwiched between two oxygen layers, which would result in an oxygen-terminated surface.…”

Single-Atom Catalysis: Insights from Model Systems

“…94,95 More critically, UHV preparation also often results in a complex superstructure with ≈ 4 nm periodicity, generally referred to as the "biphase" termination. [96][97][98][99] The nature of this biphase structure remains poorly understood at an atomic scale, and is effectively inaccessible to theory due to the large number of atoms per unit cell. DFT modelling of FeOx-based SAC systems generally assumes one of the bulk-truncated (1×1)…”

“…The (0001) facet of hematite has been studied extensively using the surface-science approach, but significant disagreement remains about its possible terminations and especially their respective stability regions . Most studies report iron- or oxygen-terminated bulk truncation models, , but experimental evidence for a ferryl termination has also been reported. , More critically, UHV preparation also often results in a complex superstructure with ∼4 nm periodicity, generally referred to as the “biphase” termination. − The nature of this biphase structure remains poorly understood at an atomic scale and is effectively inaccessible to theory due to the large number of atoms per unit cell. DFT modeling of FeO x -based SAC systems generally assumes one of the bulk-truncated (1 × 1) terminations.…”

Edge-state-induced magnetism in two-dimensional hematene

“…Meanwhile, there are still some debates on the electronic structure of hematene, such as its halfmetallic features, 48,50,51 the magnetic ground state and the geometric structures. [52][53][54] Therefore, deciphering the precise geometric structure and magnetism of hematene is highly desirable and important, not only for hematene itself, but also for the potential applications of hematene-based nanostructures.…”