Hematite, its stable surface terminations and their

Hematite, its stable surface terminations and their reactivity towards water Elena Voloshina Humboldt-Universität zu Berlin, Institut für Chemie, Unter den Linden 6, 10099 Berlin, Germany [email protected] Abstract Iron oxides are abundant minerals on earth and play an important role in a variety of applications ranging from geochemistry, weathering, corrosion science, biomedicine, magnetic devices to heterogeneous catalysis and photocatalysis. The surface chemistry of these oxides is dominated by interactions with water and solvated ions. Thus, a detailed understanding of the interaction between metal oxides and water, which determines oxide formation and dissolution, is indispensable. This article is devoted to one of the most common and stable iron oxide phases, hematite (a-Fe2O3) and adsorption of water on its stable surface terminations. Particular attention is given to the role of surface defects, which significantly influence the interaction of water with the metal-oxide surface. Keywords Density functional theory; Hematite; Hubbard U; Hydration; Hydroxilation; Iron oxide surface; Reducible oxide; Surface defect; Water adsorption; Water dissociation Glossary DFT density functional theory DFT+U density functional theory with additional Hubbard U Feoct octahedrally coordinated Fe cation Fetet tetrahedrally coordinated Fe cation + H2Oa, H a, OH a adsorbed water molecule and water dissociation residues (H+ and OH-), respectively HREELS high-resolution electron energy loss spectroscopy HSE Heyd-Scuseria-Ernzerhof functional LEED low energy electron diffraction ML monolayer PBE Perdew-Burke-Ernzerhof functional PBE+U(+D2) Perdew-Burke-Ernzerhof functional with additional Hubbard U (augmented by the Grimme ‘D2’ term) STM scanning tunnelling microscopy TPD temperature programmed desorption UHV ultra-high vacuum V O XPS

oxygen vacancy X-ray photoelectron spectroscopy

Nomenclature Lattice constants (a,c) Distance (d) Magnetic moment Band gap (Eg)

pm pm µ B eV

Oxygen chemical potential Surface free energy Adsoprption energy (Eads) Adsorption pressure

eV meV . Å-2 kJ . mol-1 Torr (1 Torr = 133.3224 Pa)

Hematite - bulk structure and properties Hematite (a-Fe2O3, the ‘a-‘ is omitted henceforth) has the corundum crystal structure, with lattice constants of a = 504 pm and c = 1375 pm [1]. In this structure, layers of distorted hexagonally closepacked oxygen ions are separated by an iron double layer with Fe3+ occupying two-thirds of the octahedral sites with a -(Fe-O3-Fe)- stacking sequence along the c axis [Figure 1 (a)]. Below the Néel temperature (TN = 963 K), Fe2O3 is antiferromagnetic with weak ferromagnetism. The high-spin d5 Fe3+ cations within one bilayer in the (0001) planes are ferromagnetically coupled to each other while antiferromagnetically coupled to the adjacent Fe bilayers. The magnetic moment is determined to be 4.6 µB per atom [2].

Hematite is an indirect band gap semiconductor with Eg » 2.1 eV [3]. As the upper edge of the valence band is dominated by oxygen p states, hematite is generally considered to be a charge-transfer rather than a Mott-Hubbard insulator [4]. Standard Kohn-Sham density functional theory (DFT) incorrectly predicts narrow band gaps for bulk hematite [for example, the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional gives 0.4 eV] and a magnetic moment of just 3.4 µB per atom and overestimates the interlayer spacings [5]. The reason is in self-interaction errors inherent in the standard DFT, which are large for localized Fe 3d electrons in Fe2O3. The DFT+U method, which includes exact intra-atomic exchange energy, yields lattice constants a = 507 pm and c = 1387 pm, an Fe magnetic moment of 4.2 µB, and a band gap of 2.1 eV for the bulk hematite, in good agreement with the most widely accepted experimental values. A further practical way to correct the above deficiency of a standard DFT is based on use of hybrid functionals, which contain a fractional amount of exact HF exchange. We note, however, that the band gap of bulk hematite increases to (unphysical) 3.4 eV when the screened Heyd-Scuseria-Ernzerhof (HSE) hybrid functional is used. As suggested in literature [6], the agreement with experiment can be significantly improved when using a reduced Fock exchange contributing in the HSE functional, i.e. 12 % instead of the standard HSE value of 25 %. Note, the HSE (12 %) and PBE+U results are shown to be almost identical to each other [5]. Hematite surfaces The two natural growth faces of hematite are the (001) and the (012) [or the (0001) and (1102) surfaces, respectively, within the four-index scheme]. Fe2O3(001). Due to this layered structure of Fe2O3 along the [001] direction different (001) surface terminations are, in principle, possible [Figure 1 (b)]. They fall into two classes: oxygen termination and iron termination. One usually considers four possible pristine terminations. Three of them are obtained by simple bulk cleavage above the oxygen layer (O3-Fe-Fe-R), above the Fe double layer (FeFe-O3-R), and between the Fe layers (Fe-O3-Fe-R). One further possible termination, is a single-metal termination capped with O ions atop the metal ions to produce a layer of ferryl (Fe=O) species.

Experimental investigations of the surfaces of both bulk Fe2O3(001) and epitaxially grown thin films showed significant variations in the relative stabilities of different surface terminations depending on the method of surface preparation. When growing Fe2O3 as films on Pt(111), the coexistence of two different domains (a single-metal termination and a ferryl termination) was observed at intermediate oxygen pressures, while higher and lower oxygen pressures led to one or the other of these domains becoming dominant [7]. Sometimes, even formation of different phases has been observed. For example, some studies indicate the coexistence of Fe2O3(001) and FeO(111) phases (see e.g. Ref. [4]). Recently, it was shown that the so-called bi-phase termination of Fe2O3 is related to a thin overlayer of Fe3O4 [8]. Contrary to the observations made when growing Fe2O3 films on metal surfaces, the clean Fe2O3(001) surface grown epitaxially (ca. 35 nm thick) on a-Al2O3(001) is single-Fe-terminated [Figure 2 (a)] and, in this case, the surface structure of Fe2O3(001) is similar to that of a-Al2O3(001) (1 ´ 1) [9]. There was no evidence for a stable O-terminated surface in this x-ray photoelectron diffraction experiments. The same conclusions were reached when studying the surface of a bulk Fe2O3(001) crystal by scanning tunneling microscopy (STM) [10] and low-energy electron diffraction (LEED) [11].

When considering theoretical results published so far one may find that they strongly disagree with each other. Some of them suggest the Fe-terminated surface of hematite is preferred at low oxygen pressures and that O-terminated surfaces should occur at increasing oxygen chemical potentials, leaving a small stability domain to ferryl-terminated surfaces (see e. g. Refs. [7] and [12]). According to the others, the O-terminated surfaces are out of the physically meaningful range of oxygen chemical potentials (see e. g. Ref. [5]). A closer look reveals that the overall conclusion depends on whether calculations are performed in a standard DFT or DFT+U. In view of the improvements seen for bulk hematite properties calculated with DFT+U rather than with pure DFT, it is widely accepted to adopt the same approach for hematite surface studies. Figure 1 (c) shows the surface phase diagram for Fe2O3(001) calculated using PBE+U [5]. Note that hybrid functionals yield qualitatively similar results for the relative surface stabilities as compared to PBE+U [5]. Upon systematization of the above experimental and theoretical results, the next step is to figure out to which extend they agree with each other. This would be very difficult task unless we call to mind that most of the theoretical studies published so far model the Fe2O3 bulk-like thick film and, if proper computational settings are utilized, good agreement between calculations and experiments performed for thick films or single crystals is achieved, giving preference to so-called half-metal termination [Fe-O3-Fe-R, see Figure 2 (a)]. The overall stability of the Fe-O3-Fe-R structure may have a very simple explanation as it is the only structure without a notable surface dipole. The most important geometric property of any surface is the coordinative unsaturation of its surface atoms and ions. In the case of Fe-O3-Fe-R, the surface cations are only 3-fold coordinated with oxygen ligands. This low coordination is not energetically preferred, and both experimental measurements [11] and theoretical calculations [5] agree that the surface cations partially relax into the underlying plane of oxygen ions. This inward relaxation is accompanied by charge redistribution in the surface layer and appearance of the surface state which has an Fe-3dz2 character [5]. Fe2O3(012). The Fe2O3 (012) surface is prevalent on nano-hematite [13]. Still, a few studies exist of this surface on single crystals. Figure 3 shows a schematic model of the bulk-terminated (1 ´ 1) surface of Fe2O3(012). This surface is known to reduce in vacuum leading to a (2 × 1) surface reconstruction, that

possesses surface Fe2+ sites. It is relatively easy to regenerate to the (1 × 1) surface, which possesses only Fe3+ sites, in ultra-high vacuum (UHV) through high temperature O2 treatment [14].

Molecular water adsorption on hematite surfaces An essential step in determining how bulk water reacts with a metal oxide surface at the molecular level is to study the initial interaction of water molecules with a clean surface. This is best accomplished in a UHV environment in which a well-characterized clean, single-crystal metal oxide is exposed to water vapor at very low pressures, followed by photoemission spectroscopy or another UHV method that is sensitive to the electronic structure of the surface and the adsorbed water. The studies of water adsorption on Fe2O3(001) by means of ultraviolet photoemission spectroscopy (UPS) and synchrotron-based X-ray photoelectron spectroscopy (XPS) find evidence for dissociative chemisorption of water on this surface, with an observed ‘threshold’ pressure of ca. 10-4 Torr for the dissociative chemisorption of water vapor on terrace sites on this surface [15,16]. Note, however, in these experiments the metal-oxide surfaces are exposed to water in a preparation chamber, which is pumped down to a base pressure of