Iron oxide surfaces

Abstract

The current status of knowledge regarding the surfaces of the iron oxides, magnetite (Fe 3 O 4 ), maghemite (γ-Fe 2 O 3 ), hematite (α-Fe 2 O 3 ), and wüstite (Fe 1-x O) 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. The atomic-scale structure of the low-index surfaces of iron oxides is the major focus of this review. Fe 3 O 4 is the most studied iron oxide in surface science, primarily because its stability range corresponds nicely to the ultra-high vacuum environment, and because it is 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 Fe 3 O 4 (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 Such straightforward preparation of a monophase termination is generally not the case for other 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 (0001) is the most studied hematite surface, but 2 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 Fe 3 O 4 (111)-like termination. The structure of the bi-phase surface is controversial; a largely accepted model of coexisting Fe 1-x O and α-Fe 2 O 3 (0001) islands was recently challenged and a new structure based on a thin film of Fe 3 O 4 (111) on α-Fe 2 O 3 (0001) was proposed. The merits of the compet...

Importance of the Inter-Electrode Distance for the Electrochemical Synthesis of Magnetite Nanoparticles: Synthesis, Characterization, Computational Modelling, and Cytotoxicity

“…In this larger size cluster, the average Fe oct -Fe oct and Fe oct -O eq bond lengths were found to be 2.94 and 1.91Å, respectively. This is a match with the reported bond distances in the bulk magnetite (2.96 and 2.06 respectively) [67][68][69]. The difference in Fe-O bond length is mostly attributable to the Fe-O-H functions in our cluster rather than the Fe-O-Fe bonds observed in the bulk structure.…”

“…Surface alone vs internal oxidation would also have significant implications for their biomedical application [68], thus a detailed understanding of the presence of different defects in the magnetite (i.e. surface defects and impurities as well as bulk defects) is required [69]. Most of the reported theoretical studies of iron oxides focused on mononuclear and binuclear forms [70][71][72][73][74].…”

A multi-technique study of CO2 adsorption on Fe3O4 magnetite

“…The sample exhibited a sharp Verwey transition at 124 K, an indicator of excellent stoichiometry and purity. 5 The crystal was mounted on a backplate machined from a 1 mm Ta sheet that includes the heating wires ( Fig. 2(a)).…”

“…The O 1s and Fe 2p regions were typical 5 for the clean Fe 3 O 4 (001) surface exhibiting the subsurface cation vacancy (SCV) reconstruction. 31 CO 2 was dosed to a 3.5 mm spot in the centre of the Fe 3 O 4 (001) sample held at 65 K using the MB source described above.…”

“…4 The adsorption and activation of CO 2 are also important in catalysis, including reactions such as CO oxidation, watergas shift, and Fischer-Tropsch synthesis, where iron oxide materials are often used as catalysts or as a support for metal nanoparticles. 5 Finally, CO 2 is often utilized as a probe of the basicity of metal oxide surfaces. 6 Fundamental investigations of CO 2 adsorption on wellcharacterised metal-oxide surfaces are scarce.…”