Elsevier

Surface Science

Volume 606, Issues 21–22, November 2012, Pages 1594-1599
Surface Science

Surface structure of γ-Fe2O3(111)

https://doi.org/10.1016/j.susc.2012.06.010Get rights and content

Abstract

The surface structure of γ-Fe2O3(111) has been investigated with a range of surface techniques. Two different surface structures were discovered depending upon surface preparation techniques. Sputtering followed by annealing in vacuum produced a reduced surface characterised by a (2 × 2) LEED pattern, whereas sputtering followed by annealing in 1 × 10 6 mbar oxygen produced a surface characterised by a (√3 × √3)-R30° LEED pattern. The latter appears to be a very low conductivity surface, whereas the former has the band gap expected for maghemite (~ 2.0 eV). We propose that the reduced surface is a magnetite-like layer, whereas the oxidised surface is an Fe2O3-like layer.

Graphical abstract

Highlights

► γ-Fe2O3 presents two different surface structures, depending on treatment conditions. ► These structures are (2 × 2) under vacuum annealing and (√3 × √3)R30 under oxidative annealing. ► We believe that these are due to magnetite(111)-like and hematite(0001)-like terminations.

Introduction

Iron oxides are an important class of materials with applications in fields ranging from construction to magnetic devices from sensors to heterogeneous catalysis. A number of important reactions take place in the presence of an iron oxide-based catalyst. These include the production of styrene from ethyl benzene [1] gold supported on iron oxide for the oxidation of CO at low temperature [2] and mixed oxide catalysts with Mo used for the selective oxidation of methanol to formaldehyde [3], [4], [5], [6]. Notwithstanding the technological importance of iron oxide, there is relatively little work in the literature on bulk iron oxide single crystal surfaces, and in this paper we report the first study of the surface structure of maghemite, γ-Fe2O3.

Iron oxide exists in several phases, with haematite (α-Fe2O3) and magnetite (Fe3O4) being by far the most widely-reported as these are the principle ores of iron. Here, however, we are interested in the behaviour of maghemite, γ-Fe2O3, a much more rarely-investigated material in surface science, but nonetheless an important technological material widely used in magnetic storage devices. Hence it is important to begin to investigate the surface properties of this material.

Studies of the surface structures of various iron oxide phases are not trivial because multiple stable surface structures may exist [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Early LEED and XPS investigations of α-Fe2O3(0001) relate the surface structure to the annealing temperature. An initial LEED pattern corresponding to a p(2 × 2) was found upon vacuum annealing to 700 °C, but this changes to a (√3 × √3)-R30° structure after further annealing to 820 °C [7], [8]. The subsequent study adapted an iron oxide phase diagram to rationalise the transformations observed on an α-Fe2O3(0001) single crystal [8]. LEED patterns similar to those seen in other work were observed by Condon et al. [11], with a more complex surface structure becoming apparent at higher temperature and when annealing in partial pressures of O2. Initially, sputtering left the surface disordered and reduced, requiring long annealing periods in the presence of O2 to achieve a bulk oxide-like stoichiometry. The first LEED-characterised layer to form was Fe3O4(111) after annealing up to 775 °C, followed by a Fe1  xO(111)/α-Fe2O3(0001) interface with a complex LEED pattern.

Some studies have shown the significant benefits of using STM to probe iron oxide surfaces, notably Wiesendanger et al. who achieved atomic level imaging on Fe3O4 [10]. STM of various terminating structures on an α-Fe2O3(0001) single crystal was first reported by Condon et al. Annealing their sample to 1000 K produced an epitaxial Fe3O4(111) layer comprising two coexisting terminations separated by steps. One termination was identified as a ¼ monolayer of O atoms on top of ¾ monolayer of Fe atoms, whereas the second consisted of a ½ monolayer of Fe atoms overlaying a close-packed O layer in agreement with previous work [11], [12], [13]. However, upon annealing to 1100 K in 1 × 10 6 mbar O2 a biphase ordering [13] was observed. STM revealed two different structures coexisting as islands on the surface, identified as α-Fe2O3(0001) and Fe1  xO(111). The long range order of the islands resulted in a superlattice believed to be responsible for the floretted LEED pattern which was seen.

A study on the (110) face of an Fe3O4 single crystal revealed a well ordered structure upon sputtering and annealing to 1200 K. Subsequently the surface underwent a reconstruction showing no comparable resemblance to known iron oxide phases [14].

In parallel with the work above there has been considerable effort devoted to making thin layer model iron oxide surfaces, and this has been thoroughly reviewed by Weiss and Ranke [16]. In general thin layer oxides display similar structural behaviour to the bulk crystals described above, but give the capability of switching surface structures/phases between FeO-like, Fe2O3-like and Fe3O4-like forms, depending on the exact details of oxidation treatment. However, it may be that the electronic structure of very thin films is different from that of the bulk crystal, and any exposed underlying Pt (used as the support) may have significant effects on, for instance, their reactive behaviour. Thus it is important to compare such materials with bulk oxides, such as the single crystal iron oxide work reported here and in the papers cited above, especially so since it is not clear that γ-Fe2O3 has actually been produced in thin film form [16].

In our laboratories we are engaged in studies of the structure and reactivity of both thin film iron oxides and bulk iron oxide single crystals [17], [18]. In this case we are dealing with an example of the latter. As noted above, there is relatively little surface science work in the literature on bulk iron oxide single crystals, and nothing on γ-Fe2O3. We are particularly interested in the inter-relationships between the surface structure of the various iron oxides, and in how the surface structure relates to surface reactivity. The iron oxide phase maghemite (γ-Fe2O3), which is the subject of this investigation, has similarities to both magnetite and haematite—it is isostructural with the former, that is, it is a spinel ferrite, and exhibits ferrimagnetism, and yet is fully oxidised like the latter and so contains no Fe2 +. Thus it is an inherently interesting material, but to our knowledge there have been no surface science studies of bulk single crystal maghemite, and hence the main aim of this work was to investigate the surface structure of this material in a well-defined, single crystal form.

Section snippets

Experimental

The experiments were carried out on two pieces of equipment: a small ultra high vacuum (UHV I), and a custom-designed Omicron Multiprobe UHV system (UHV II). UHV I comprises a stainless steel chamber maintaining a base pressure of ~ 5 × 10 10 mbar equipped with an ISIS ion gun for sample cleaning, an OCI system for low energy electron diffraction (LEED) and Auger electron spectroscopy (AES), and a Hiden quadrupole mass spectrometer. The naturally grown γ-Fe2O3(111) crystal (from Surface Preparation

LEED and Auger

Two different surface terminations of the crystal were obtained, depending on the annealing conditions. When cleaned by Ar ion bombardment and annealed in vacuum for 20 min at 873 K, a hexagonal (2 × 2) structure was found (Fig. 1a) which gave an AES OKLL/FeL3M23V peak to peak ratio of 4.1(Fig. 2). Note that in our notation, the (1 × 1) pattern refers to the O-sub lattice forming close-packed layers common to the iron oxide (111) surfaces. However, when the sample was annealed in oxygen (1 × 10 6 mbar)

Discussion

Two different surface terminations have been found on the γ-Fe2O3 single crystal, which can be obtained by simple treatments under mild conditions in UHV. One, the (2 × 2), is a somewhat reduced surface prepared by sputtering the sample and annealing in vacuum; the other, the (√3 × √3)R30° is a more oxidised surface prepared by sputtering the sample and annealing in oxygen.

The results accumulated from AES indicate that the difference in oxygen concentration in the surface region between the vacuum

Conclusions

We have investigated the surface properties of γ-Fe2O3 and have identified two different surface terminations. One of the them is formed by vacuum annealing and has a (2 × 2) surface mesh, while the other, formed by oxidative treatment, has a (√3 × √3)R30° mesh. The former appears to be a somewhat reduced surface and, by comparison with work on other iron oxide surfaces, seems to be magnetite-like. The (√3 × √3) is more oxidised and is α-Fe2O3-like. There are significant differences in the surface

Acknowledgments

We are grateful to EPSRC for funding for D.E. (EP/E03974X/1) and to the Welsh Livery Fund for support for a visit to FHI by R.D.

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