the reduction and oxidation of fe2o3(0001) surface...
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Surface Science 609 (2013) 67–72
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Surface Science
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The reduction and oxidation of Fe2O3(0001) surface investigated by scanningtunneling microscopy
Yuanyuan Tang, Huajun Qin, Kehui Wu, Qinlin Guo, Jiandong Guo ⁎Beijing National Laboratory for Condensed-Matter Physics & Institute of Physics, Chinese Academy of Sciences, Beijing 100190, PR China
⁎ Corresponding author. Tel.: +86 10 82648131.E-mail address: [email protected] (J. Guo).
0039-6028/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.susc.2012.11.005
a b s t r a c t
a r t i c l e i n f oArticle history:Received 15 September 2012Accepted 14 November 2012Available online 24 November 2012
Keywords:Oxide surfaceScanning tunneling microscopyIron oxidesSurface phasesAnnealing treatment
By using scanning tunneling microscopy (STM), we study the structure of the (0001) surface of hematite(α-Fe2O3) pre.pared by ultra-high vacuum treatment. The surface is reduced into a Fe3O4(111) phase by Arion sputtering followed by annealing in vacuum, while it is oxidized to a honeycomb superstructure byannealing in O2. High-resolution STM images reveal that the O-terminated FeO(111), Fe- and O-terminatedFe2O3(0001) domains coexist with each other in the superstructure. Unlike the reduction of Fe2O3(0001)whose depth increases with repeated annealing in vacuum, the oxidation of Fe3O4(111) occurs only partiallyon the top layers by annealing in O2.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Iron oxides have been studied intensively in the forms of singlecrystals and epitaxial films, not only because of their vital roles inheterogeneous catalysis reactions [1–6], but also for their potentialapplications in spin electronics and others [7,8]. Hematite (Fe2O3) hasbeen applied in chemical engineering, such as degradation of chlorinat-ed compounds, styrene synthesis, and many other important industrialprocesses [9,10]. Studies of the reaction mechanisms at atomic scale[11,12] show that the catalytic reactivity is related to Fe cations andnearby vacancies on the polar surface. Magnetite (Fe3O4) has been con-sidered as a half-metal ferromagnet theoretically [13,14]. Experimentalresults also show that the polarization at Fermi level (EF) is sensitive tothe surface stoichiometry [15,16]. Since the chemical and physical prop-erties are crucially affected by themicrostructures of iron oxide surfaces,to understand the corresponding reconstruction, atomic termination,defects, as well as lattice relaxation is indispensable to obtain controlla-ble functionalities of the material.
However, the variable valence states of iron cation cause a rathercomplicated phase diagram of iron oxides with several easily inter-changeable phases [1,17,18]. Especially on the surfaces obtained withultra-high vacuum (UHV) treatment, even subtle variance of the prepa-ration parameter results in distinct stoichiometry and different micro-scopic structures. Stoichiometric α-Fe2O3(0001) surfaces can be formedby oxidizing Fe3O4(111) or FeO(111) films epitaxially grown on metalsubstrates [19–22], exposing either Fe or O atomic layer according to
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the initial surface termination (see Fig. 1). The charge transfer betweenthe film and substrate plays an important role in the stabilization ofthe polar surface [23]. Besides, the existence of ferryl (Fe=O) groupshave also been reported to stabilize the surface of hematite thin films[24,25]. A well-defined stoichiometric surface is hard to obtain onsingle-crystallineα-Fe2O3 inUHV. Instead, a non-stoichiometric “biphase”surface is commonly observed on α-Fe2O3(0001) prepared by annealingin oxygen [26]. It consists of islands of Fe2O3 and Fe1−xO arranged in a su-perstructure with the periodicity of ~40 Å due to themismatch of the ox-ygen layers of Fe2O3(0001) and Fe1−xO(111). The UHV treatment alsoresults in the non-stoichiometric surface on Fe3O4(111) with Fe3O4(111)and Fe1−xO coexisting with each other [27]. But a stoichiometric surfaceon single-crystalline Fe3O4(111) can be recovered by annealing thesample in 10−6 mbar O2 [27–33].
In addition to the deviation of stoichiometry and valance from thebulk crystals, relaxation may also occur on the surface of iron oxides.By ab initio calculations and scanning tunneling microscopy (STM)observations, Wang et al. found 57% and 79% inward relaxation onthe top layers of Fe and O terminations onα-Fe2O3(0001), respectively[20]. Low energy electron diffraction (LEED) studies revealed that therelaxation effect even involved several atomic layers beneath the sur-face [34–36]. In brief, the correlation between the stability of differentphases and the preparation conditions needs to be clarified in regardto the complicated iron oxide surface. In this paper, we use STM tostudy the structural transformations on the α-Fe2O3(0001) surfacecontrolled by Ar ion sputtering followed by annealing. Both the sur-face chemical composition and the reconstruction are sensitive tothe oxygen ambience during annealing. The UHV annealing leads tothe formation of Fe3O4(111) layers. And repeated treatment cyclesresult in the reduction of Fe2O3 penetrating deep into the bulk. By
a
b
c
Fig. 1. Structural models of (a) α-Fe2O3(0001), (b) Fe3O4(111) and (c) FeO(111). The left panel shows the side views and the right panel shows the top views. The unit cells of theFe- and O-terminated surfaces are labeled by the dark gray (red) and light gray (yellow) rhombuses, respectively. Note that there are two types of Fe termination on Fe2O3(0001) orFe3O4(111) surface — the Feoct1 (Feoct2) termination shows a hexagonal network, while the Feoct2 (Fetet1) termination shows a honeycomb network on Fe2O3(0001) [Fe3O4(111)],respectively, although the unit cells of the surface lattices are identical.
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annealing the sample in O2 (1×10−6 mbar), three types of domainsare formed and arranged in a honeycomb-like superstructure on thesurface. Detailed observation shows that these domains correspondto different atomic layers in different phases of iron oxides, indicatingthe oxidation of the surface. Furthermore the oxidation occurs onlypartially on the surface, while the buried Fe3O4(111) layers cannotbe fully oxidized to Fe2O3(0001) by repeated annealing in O2.
Experiments were performed in an UHV STM system (Omicron)with the base pressure of low 1×10−10 mbar. A naturally occurringhematite (α-Fe2O3) was cut into a 1.5-mm-thick wafer with its (0001)surface polished. The sample was mounted on a molybdenum plateand loaded into the UHV chamber. Then the sample was sputteredby Ar ion with the beam energy of 0.5 keV for 10 min followed byannealing. Direct current was applied to a tungsten wire to resistivelyheat the sample, whose temperature was monitored by an infraredpyrometer. The annealing was in UHV or O2 ambience at temperaturesbetween 600–850 °C, as specified in the following for each case. Allthe STM measurements were carried out at room temperature inthe constant-current mode. The ex situ X-ray diffraction (XRD) wasmeasured by Rigaku UltimalV diffractometer with Cu Kα radiation(1.54 Å).
2. Results and discussion
2.1. Structures of iron oxides and their surfaces
Due to the similar stability of different iron oxides, UHV treatmentnormally forms mixed phases on the α-Fe2O3(0001) surface. Domainsrelated to Fe3O4(111) and FeO(111) also exist since they show similaranion frameworks on the close-packed oxygen layers, as illustrated inFig. 1. The Fe2O3 single crystal exhibits the corundum structure. Along[0001], oxygen anions form an hcp sublattice with slightly distortedhexagonal planes stacked in ABAB sequence with interspacing of2.29 Å. Iron cations occupy the centers of the distorted octahedra
formed by O anions. So the Fe2O3 lattice can be viewed as an alternativestack of one O layer and two Fe layers along [0001] [Fig. 1(a)]. There arethree possible types of bulk truncated surface on Fe2O3(0001). The Olayer shows a hexagonal lattice with the periodicity of 2.91 Å. Boththe Feoct1 and Feoct2 layers have the same periodicity of 5.03 Å, whilethe Feoct2-terminated surface exposes the single Fe layer in a hexagonallattice on theO layer and the Feoct1-terminated surface exposes the dou-ble Fe layers forming a honeycomb lattice. Taking anO layer as the basalplane, the height is 0.85 Å for Feoct2 and 1.45 Å for Feoct1 [37].
The Fe3O4 single crystal has the inverse spinel structure. Oxygenanions form a close-packed fcc sublattice with slightly distortedhexagonal planes stacked in ABCABC sequence along [111]. The tetra-hedrally coordinated sites formed by oxygen ions are occupied byFe3+ cations. Other Fe cations occupy the octahedrally coordinatedinterstices, among which a half exhibits +3 valance state while theother half exhibits +2. This lattice can be described as a stack ofO1-Feoct1-O2-Fetet2-Feoct2-Fetet1-O1 layers along [111] where the sub-scripts “tet” and “oct” refer to the tetrahedrally and octahedrally coordi-nated sites, respectively [Fig. 1(b)]. The O-terminated surface showsa hexagonal lattice with the periodicity of 2.97 Å [37]. The situationof Fe termination is rather complicated. Ideally there could have beenseveral types of Fe layer exposing on Fe3O4(111) surface. But theoreticalcalculations [13,38,39] have suggested only two possible energeticallyfavored types, i.e., Feoct2 and Fetet1. The Fetet1-terminated surface exposesthe single Fe layer in a hexagonal lattice on the O layer, while theFeoct2-terminated surface exposes the double Fe layers forming ahoneycomb lattice. Both of the favored Fe terminations have thesame surface unit cell with periodicity of 5.92 Å.
The structure of FeO single crystal is relatively simple, showing theNaCl lattice. Along the [111] direction FeO is stacked with hexagonalFe and O layers alternatively in ABCABC sequence, respectively, withthe interspacing of 1.25 Å [Fig. 1(c)]. Both of the two possible termi-nations, i.e., the Fe and O layers show hexagonal lattices with thesame periodicity of 3.04 Å [37].
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In brief, the lattice constants of the bulk-truncated hexagonalFe2O3(0001), Fe3O4(111) and FeO(111) surfaces are 5.03 Å, 5.92 Åand 3.04 Å on the Fe terminations, and 2.91 Å, 2.97 Å and 3.04 Å onthe O terminations, respectively [37]. As discussed in the following,we observe the periodicities of ~5 Å and ~6 Å in STM images on thesample surface, which are the characteristics of the Fe2O3(0001)and Fe3O4(111) surfaces terminated by the single Fe layers, respec-tively. Therefore the domains formed by these two layers can bedistinctly identified. The periodicity of ~3 Å is also observed in someareas, possibly related to the O-terminated Fe2O3(0001) or Fe3O4(111),or even FeO(111) with either Fe or O termination. To distinguish them,we notice that the unit cell of Fe termination layer rotates by 30° withrespect to that of the O termination on Fe2O3(0001) surface, while theorientations are aligned with each other on Fe3O4(111) and FeO(111).This fact has been used as the main evidence to distinguish the Fe2O3
phase by LEED and STM observations [26]. Further evidence for theidentification of the type of surface termination layer can be obtainedby analyzing the height of step between different domains.
2.2. The surface in “regular” phase
Treated with Ar ion sputtering followed by annealing at ~700 °Cfor 30 min in UHV, the α-Fe2O3(0001) surface shows a hexagonalstructure with the periodicity of 6.0±0.5 Å, as shown in Fig. 2. It isthe characteristic of the Fe-terminated Fe3O4(111)-(1×1) latticeand has been commonly observed on the surface of single crystallineFe3O4(111) [7,29–33,40–43]. In the following, we refer to this surface
a
c
Fig. 2. (a) STM image (45 nm×45 nm,−2 V/20 pA) of the R surface. The minimum step heig−1.5 V/20 pA) of the R surface. (c) XRD of the initial α-Fe2O3(0001) sample and after repe
as the “regular” (R) phase. The minimal step height on the surfaceis measured as ~4.8 Å in our STM image [44], consistent with theinterspacing of two adjacent equivalent layers in Fe3O4(111). Furtherevidence is provided by the XRD analysis [Fig. 2(c)]. Compared with theoriginal Fe2O3(0001), the sample shows additional diffraction peakscorresponding to Fe3O4(111) {h,h,h} planes after repeated sputteringand annealing in UHV. Due to the preferential sputtering of oxygen,Fe2O3(0001) is reduced. And with the sputtering dose increasing, thereduction penetrates deep into the bulk, forming a relatively thicklayer of Fe3O4(111) that can be detected by XRD.
Among the two possible types of Fe termination layers onFe3O4(111) [13,38,39], i.e., Feoct2 and Fetet1 (see Fig. 1 and the corre-sponding text), the Feoct2 termination typically shows honeycombstructure in unoccupied-state STM images [32] that is different fromour observations. Additionally, the Feoct2 termination is usually obtainedon the Fe3O4(111) surface in an oxygen-poor condition [6,32]. In thecurrent case, due to the presence of the underneath bulk Fe2O3, theoxygen-poor condition is difficult to achieve on the surface since oxygenmay move towards the surface at high temperature during annealing.Therefore the R surface is attributed to the Fe3O4(111) surface terminatedby the single layer of Fetet1.
2.3. The surface in “honeycomb” phase
By further annealing the samplewith R-phase surface in O2 ambience(1×10−6 mbar) and subsequently cooling it to room temperature inthe same oxygen ambience, terraces with different long-range ordering
b
ht is measured as 4.8 Å along the line. (b) High-resolution STM image (12 nm×12 nm,ated sputtering and annealing in UHV.
70 Y. Tang et al. / Surface Science 609 (2013) 67–72
are formed on the surface, as labeled as H in Fig. 3(a). The honey-comb superstructure on H terrace (referred to as “H” phase) hasthe periodicity of 40±3 Å. Each honeycomb shows a patch at thecenter (the γ domain) surrounded by six small patches, which areactually two types of domains adjacent to each other [labeled as αand β in Fig. 3(b), respectively]. The α domain is ~0.8 Å higherthan β, while β and γ are almost at the same height, as shown in theunoccupied-state STM image Fig. 3(c). Small irregular protrusions in γdomains are visible by STM. As compared with the high-resolutionimages presented in the following, the origin of these protrusions arepurely electronic.
Honeycomb superstructure has been observed on Fe3O4(111) sur-face prepared by UHV annealing [27,30]. The “S3” phase reported byPaul et al. [30] coexisted with R phase. But the step height betweenR and S3 was ~1.5 Å, different from our measurement that shows astep of 1.8 Å between α on the H terrace and R2 on R (or a step of3.0 Å between α and R1) as compared with the step of 4.8 Å withinR phase (R1 and R2) [44]. Additionally, the periodicity of the honey-comb “S3” phase was typically around 50 Å [27,30], also inconsistentwith our observation on the H superstructure (40 Å). Actually in thecurrent work, due to the oxidation environments provided duringannealing, the Fe3O4(111) surface is oxidized and the observed Hsuperstructure can be attributed to the “biphase” that was obtainedon Fe2O3(0001) [26]. As shown in Fig. 4(a), α and β domains are ar-ranged alternatively on the corners of the honeycomb (the γ domain)[45]. Both α and β domains show hexagonal features, with the period-icities of 5.0 Å and 3.0 Å, respectively. And the orientation of theβ-type sublattice rotates by 30° with respect to the α-type. All these
a
c
Fig. 3. (a) STM image (30 nm×30 nm,−2.5 V/10 pA) of the surface with R and H coexistingcell of the honeycomb superstructure is marked by the rhombus with the α, β and γ doma
STM observations are in good agreement with the previously reported“biphase” [26] where Fe2O3(0001) and FeO(111) domains coexist onthe surface.
Moreover, with the high-resolution STM images of both unoccupied-and occupied-states, we can identify the atomic layer in the γ-typesublattice. Fig. 4(a) shows a honeycomb structure with periodicity of5.0 Å in γ, characteristic of the Fe2O3(0001) surface where the Feoct1and Feoct2 layers are visible (see Fig. 1). On the other hand, in theoccupied-state image that is mainly contributed from O 2p states, ahexagonal structure with periodicity of 3.0 Å is observed. And the unitcell orientation rotates by 30° from that in the unoccupied-state image,consistent with the lattice of O layer on Fe2O3(0001) surface. Since pre-vious calculations [20,46,47] suggested that the double-layer Fe termina-tion on Fe2O3(0001) is strongly disfavored under oxidizing conditions,we concluded that the γ domain corresponds to the O-terminatedFe2O3(0001) surface.
Different from the γ domain, the α domain shows no atomic reso-lution in the occupied-state image that represents the density ofstates (DOS) of O sites on the Fe2O3(0001) surface [Fig. 4(b)]. Thissuggests that the charges on O sites are screened by a surface layer,i.e., they correspond to the terminations other than O layer. Consider-ing the characteristic periodicity and unit cell orientation observedin the unoccupied-state image [Fig. 4(a)], we attribute the α domainto Fe2O3(0001) terminated by Feoct2. Although the β domain showsno atomic resolution in the occupied-state image as well, it cannotbe identified as the Fe termination since the image contrast is alsocontributed from the partially filled Fe 3d states, in combinationwith that from O 2p. Considering the α and β sublattices share the
b
with each other. (b) STM image (17 nm×17 nm, 3 V/50 pA) of the H terrace. The unitins labeled. (c) The line profile along AB in (b).
Fig. 4. (a) The unoccupied- (+3 V/30 pA) and (b) occupied-state (−2 V/30 pA) STMimages (17 nm×7 nm) of the H phase.
71Y. Tang et al. / Surface Science 609 (2013) 67–72
same O layer as the basal plane, their height difference (0.8 Å) sug-gests that the β domain corresponds to the FeO(111) surface termi-nated by O. Strictly speaking, the H superstructure is a “triphase”lattice consisting of O-terminated FeO(111), Fe- and O-terminatedFe2O3(0001) sublattices.
Fig. 5 illustrates the structural model of the H phase surface withthe top three O layers included. Since the surface is prepared by oxi-dizing Fe3O4(111), we consider the O layers as in Fe2O3(0001) config-uration and allow only the top one to be relaxed laterally [48]. Thesuperstructure with the periodicity of 40 Å can be obtained only byexpanding the top layer lattice by 7.6% (the O–O distance is 3.13 Åin the top layer compared with 2.91 Å in the two bottom layers).This results in three types of regions with different stacking sequenceof the O layers. The first type is in ABA sequence, corresponding tothat in Fe2O3(0001) lattice. A single layer of Fe sits atop the topmostO in Feoct2 configuration with 7.6% lateral expansion, forming the α
ββ: ABC α: ABA
γ γ :ABB 1st layer(topmost)
2nd layer
3rd layer
Fig. 5. Illustration of the H superstructure with the top three close-packed oxygen layersincluded. Stacked in AB sequence, the second and third layers have the periodicity of2.91 Å, while the topmost (first) layer is expanded by ~7.6% (3.13 Å). The resulting threetypes of stacking sequence in different areas are shown in the inset.
domain. The second type is in ABC sequence, with the topmost Olayer expanded by ~3% in relative to the bulk truncated FeO(111)lattice. Such that the β domain is formed. The third type is in theABB sequence, inducing the instability in the region. To response,the periodicity of the topmost O layer shrinks to ~3 Å as we observein the STM image within the γ domain. Occasionally the γ area ap-pears lower than it normally is by ~2.0 Å (not shown). Such a heightcorresponds to the interspacing of adjacent O layers in Fe2O3(0001)lattice, suggesting that the top O and Fe layers are missing. This isalso a possible channel to reduce the instability on γ surface.
It was reported that on the reconstructed FeO and Al2O3 surfaces[49–51], the topmost layer could rotate and result in the superstruc-tures. In fact, another consequence of the top layer rotation is theorientation shift between the oxygen sublattice in each domain andthat of the superstructure. This is not observed in our current work,suggesting that the lattice expansion on the top layer is responsiblefor the polar compensation on the Fe2O3(0001) surface. For verifica-tion, further ab initio calculations would be important.
Annealing the sample in O2 repeatedly, the monophased H super-structure is formed on the surface of the Fe3O4(111) layers. However,the H phase is not a fully-oxidized phase since Fe2+ ions exist in the βsublattice. And the XRD of the sample with H surface always showsthe diffractions from Fe3O4(111) no matter how much it is annealedin oxygen. As proposed by Kim et al. [52], once a Fe2O3 layer is formedon the sample surface it may act as a barrier that prevents the oxygenfrom penetrating into the underneath lattice. Within the O2 pressurerange in the current work (less than 1×10−6 mbar), the iron oxidecan never be fully oxidized back to Fe2O3.
3. Conclusions
In summary, we study the surface structure of α -Fe2O3(0001) pre-pared by Ar ion sputtering followed by annealing in UHV or in O2 ambi-ence with the partial pressure of 1×10−6 mbar. The UHV annealingresults in the reduction of the surface, forming a layer of Fe3O4 (111).And the surface can be partially oxidized by annealing in O2. TheFe-terminated FeO(111), Fe- and O-terminated Fe2O3(0001) sublatticescoexistwith each other and are arranged in a honeycombsuperstructurewith the periodicity of ~40 Å. Unlike the reduction of Fe2O3(0001) thatcan penetrate deep into the bulk upon repeated UHV annealing, theoxidation of the Fe3O4(111) occurs only partially on the top layer(s). Itis revealed that the surface structure of iron oxide is not only sensitiveto the O2 pressure during annealing, but also determined by the treat-ment history. The Fe2O3(0001) surface can always be reduced butthe formed Fe3O4(111) layers cannot be easily oxidized back to Fe2O3
reversibly.
Acknowledgments
The authors thank Dr. Xiaoguang Meng of Stevens Institute ofTechnology for providing the Fe2O3 sample, and Dr. Xuetao Zhu forhelpful discussions. This work was supported by the “973” Programof China (2012CB921700), NSFC Project 11027406 and SpecificFunding of the Discipline and Graduate Education Project of BeijingMunicipal Commission of Education.
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similar superstructure as discussed below. However, the periodicity of the Feoct2termination in the α domain would have responded to the change, which is notobserved experimentally.
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