Iron oxide surfaces

Abstract

The current status of knowledge regarding the surfaces of the iron oxides, magnetite (Fe₃O₄), maghemite (γ-Fe₂O₃), haematite (α-Fe₂O₃), and wüstite (Fe_1−xO) is reviewed. The paper starts with a summary of applications where iron oxide surfaces play a major role, including corrosion, catalysis, spintronics, magnetic nanoparticles (MNPs), biomedicine, photoelectrochemical water splitting and groundwater remediation. The bulk structure and properties are then briefly presented; each compound is based on a close-packed anion lattice, with a different distribution and oxidation state of the Fe cations in interstitial sites. The bulk defect chemistry is dominated by cation vacancies and interstitials (not oxygen vacancies) and this provides the context to understand iron oxide surfaces, which represent the front line in reduction and oxidation processes. Fe diffuses in and out from the bulk in response to the O₂ chemical potential, forming sometimes complex intermediate phases at the surface. For example, α-Fe₂O₃ adopts Fe₃O₄-like surfaces in reducing conditions, and Fe₃O₄ adopts Fe_1−xO-like structures in further reducing conditions still. It is argued that known bulk defect structures are an excellent starting point in building models for iron oxide surfaces.

The atomic-scale structure of the low-index surfaces of iron oxides is the major focus of this review. Fe₃O₄ is the most studied iron oxide in surface science, primarily because its stability range corresponds nicely to the ultra-high vacuum environment. It is also an electrical conductor, which makes it straightforward to study with the most commonly used surface science methods such as photoemission spectroscopies (XPS, UPS) and scanning tunneling microscopy (STM). The impact of the surfaces on the measurement of bulk properties such as magnetism, the Verwey transition and the (predicted) half-metallicity is discussed.

The best understood iron oxide surface at present is probably Fe₃O₄(100); the structure is known with a high degree of precision and the major defects and properties are well characterised. A major factor in this is that a termination at the Fe_oct–O plane can be reproducibly prepared by a variety of methods, as long as the surface is annealed in 10⁻⁷−10⁻⁵ mbar O₂ in the final stage of preparation. Such straightforward preparation of a monophase termination is generally not the case for iron oxide surfaces. All available evidence suggests the oft-studied (√2×√2)R45° reconstruction results from a rearrangement of the cation lattice in the outermost unit cell in which two octahedral cations are replaced by one tetrahedral interstitial, a motif conceptually similar to well-known Koch–Cohen defects in Fe_1−xO. The cation deficiency results in Fe₁₁O₁₆ stoichiometry, which is in line with the chemical potential in ultra-high vacuum (UHV), which is close to the border between the Fe₃O₄ and Fe₂O₃ phases. The Fe₃O₄(111) surface is also much studied, but two different surface terminations exist close in energy and can coexist, which makes sample preparation and data interpretation somewhat tricky. Both the Fe₃O₄(100) and Fe₃O₄(111) surfaces exhibit Fe-rich terminations as the sample selvedge becomes reduced. The Fe₃O₄(110) surface forms a one-dimensional (1×3) reconstruction linked to nanofaceting, which exposes the more stable Fe₃O₄(111) surface. α-Fe₂O₃(0001) is the most studied haematite surface, but difficulties preparing stoichiometric surfaces under UHV conditions have hampered a definitive determination of the structure. There is evidence for at least three terminations: a bulk-like termination at the oxygen plane, a termination with half of the cation layer, and a termination with ferryl groups. When the surface is reduced the so-called “bi-phase” structure is formed, which eventually transforms to a Fe₃O₄(111)-like termination. The structure of the bi-phase surface is controversial; a largely accepted model of coexisting Fe_1−xO and α-Fe₂O₃(0001) islands was recently challenged and a new structure based on a thin film of Fe₃O₄(111) on α-Fe₂O₃(0001) was proposed. The merits of the competing models are discussed. The α-Fe₂O₃(1$\overline{1}$02) “R-cut” surface is recommended as an excellent prospect for future study given its apparent ease of preparation and its prevalence in nanomaterial.

In the latter sections the literature regarding adsorption on iron oxides is reviewed. First, the adsorption of molecules (H₂, H₂O, CO, CO₂, O₂, HCOOH, CH₃OH, CCl₄, CH₃I, C₆H₆, SO₂, H₂S, ethylbenzene, styrene, and Alq₃) is discussed, and an attempt is made to relate this information to the reactions in which iron oxides are utilized as a catalyst (water–gas shift, Fischer–Tropsch, dehydrogenation of ethylbenzene to styrene) or catalyst supports (CO oxidation). The known interactions of iron oxide surfaces with metals are described, and it is shown that the behaviour is determined by whether the metal forms a stable ternary phase with the iron oxide. Those that do not, (e.g. Au, Pt, Ag, Pd) prefer to form three-dimensional particles, while the remainder (Ni, Co, Mn, Cr, V, Cu, Ti, Zr, Sn, Li, K, Na, Ca, Rb, Cs, Mg, Ca) incorporate within the oxide lattice. The incorporation temperature scales with the heat of formation of the most stable metal oxide. A particular effort is made to underline the mechanisms responsible for the extraordinary thermal stability of isolated metal adatoms on Fe₃O₄ surfaces, and the potential application of this model system to understand single atom catalysis and sub-nano cluster catalysis is discussed. The review ends with a brief summary, and a perspective is offered including exciting lines of future research.