Surface Science 609 (2013) 67–72
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Surface Science
journal homepage: www.elsevier.com/locate/susc
The reduction and oxidation of Fe2O3(0001) surface investigated by scanning
tunneling 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
a r t i c l e
i n f o
Article history:
Received 15 September 2012
Accepted 14 November 2012
Available online 24 November 2012
Keywords:
Oxide surface
Scanning tunneling microscopy
Iron oxides
Surface phases
Annealing treatment
a b s t r a c t
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 Ar
ion sputtering followed by annealing in vacuum, while it is oxidized to a honeycomb superstructure by
annealing in O2. High-resolution STM images reveal that the O-terminated FeO(111), Fe- and O-terminated
Fe2O3(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 partially
on 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 single
crystals and epitaxial films, not only because of their vital roles in
heterogeneous catalysis reactions [1–6], but also for their potential
applications in spin electronics and others [7,8]. Hematite (Fe2O3) has
been applied in chemical engineering, such as degradation of chlorinated compounds, styrene synthesis, and many other important industrial
processes [9,10]. Studies of the reaction mechanisms at atomic scale
[11,12] show that the catalytic reactivity is related to Fe cations and
nearby vacancies on the polar surface. Magnetite (Fe3O4) has been considered as a half-metal ferromagnet theoretically [13,14]. Experimental
results also show that the polarization at Fermi level (EF) is sensitive to
the surface stoichiometry [15,16]. Since the chemical and physical properties are crucially affected by the microstructures of iron oxide surfaces,
to understand the corresponding reconstruction, atomic termination,
defects, as well as lattice relaxation is indispensable to obtain controllable functionalities of the material.
However, the variable valence states of iron cation cause a rather
complicated phase diagram of iron oxides with several easily interchangeable phases [1,17,18]. Especially on the surfaces obtained with
ultra-high vacuum (UHV) treatment, even subtle variance of the preparation parameter results in distinct stoichiometry and different microscopic structures. Stoichiometric α-Fe2O3(0001) surfaces can be formed
by oxidizing Fe3O4(111) or FeO(111) films epitaxially grown on metal
substrates [19–22], exposing either Fe or O atomic layer according to
⁎ Corresponding author. Tel.: +86 10 82648131.
E-mail address: [email protected] (J. Guo).
0039-6028/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.susc.2012.11.005
the initial surface termination (see Fig. 1). The charge transfer between
the film and substrate plays an important role in the stabilization of
the polar surface [23]. Besides, the existence of ferryl (Fe=O) groups
have also been reported to stabilize the surface of hematite thin films
[24,25]. A well-defined stoichiometric surface is hard to obtain on
single-crystalline α-Fe2O3 in UHV. Instead, a non-stoichiometric “biphase”
surface is commonly observed on α-Fe2O3(0001) prepared by annealing
in oxygen [26]. It consists of islands of Fe2O3 and Fe1−xO arranged in a superstructure with the periodicity of ~40 Å due to the mismatch of the oxygen layers of Fe2O3(0001) and Fe1−xO(111). The UHV treatment also
results in the non-stoichiometric surface on Fe3O4(111) with Fe3O4(111)
and Fe1−xO coexisting with each other [27]. But a stoichiometric surface
on single-crystalline Fe3O4(111) can be recovered by annealing the
sample in 10 − 6 mbar O2 [27–33].
In addition to the deviation of stoichiometry and valance from the
bulk 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 on
the top layers of Fe and O terminations on α-Fe2O3(0001), respectively
[20]. Low energy electron diffraction (LEED) studies revealed that the
relaxation effect even involved several atomic layers beneath the surface [34–36]. In brief, the correlation between the stability of different
phases and the preparation conditions needs to be clarified in regard
to the complicated iron oxide surface. In this paper, we use STM to
study the structural transformations on the α-Fe2O3(0001) surface
controlled by Ar ion sputtering followed by annealing. Both the surface chemical composition and the reconstruction are sensitive to
the oxygen ambience during annealing. The UHV annealing leads to
the formation of Fe3O4(111) layers. And repeated treatment cycles
result in the reduction of Fe2O3 penetrating deep into the bulk. By
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Y. Tang et al. / Surface Science 609 (2013) 67–72
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 the
Fe- 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) or
Fe3O4(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.
annealing the sample in O2 (1 × 10 −6 mbar), three types of domains
are formed and arranged in a honeycomb-like superstructure on the
surface. Detailed observation shows that these domains correspond
to different atomic layers in different phases of iron oxides, indicating
the oxidation of the surface. Furthermore the oxidation occurs only
partially on the surface, while the buried Fe3O4(111) layers cannot
be 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 occurring
hematite (α-Fe2O3) was cut into a 1.5-mm-thick wafer with its (0001)
surface polished. The sample was mounted on a molybdenum plate
and loaded into the UHV chamber. Then the sample was sputtered
by Ar ion with the beam energy of 0.5 keV for 10 min followed by
annealing. Direct current was applied to a tungsten wire to resistively
heat the sample, whose temperature was monitored by an infrared
pyrometer. The annealing was in UHV or O2 ambience at temperatures
between 600–850 °C, as specified in the following for each case. All
the STM measurements were carried out at room temperature in
the constant-current mode. The ex situ X-ray diffraction (XRD) was
measured 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 treatment
normally forms mixed phases on the α-Fe2O3(0001) surface. Domains
related to Fe3O4(111) and FeO(111) also exist since they show similar
anion frameworks on the close-packed oxygen layers, as illustrated in
Fig. 1. The Fe2O3 single crystal exhibits the corundum structure. Along
[0001], oxygen anions form an hcp sublattice with slightly distorted
hexagonal planes stacked in ABAB sequence with interspacing of
2.29 Å. Iron cations occupy the centers of the distorted octahedra
formed by O anions. So the Fe2O3 lattice can be viewed as an alternative
stack of one O layer and two Fe layers along [0001] [Fig. 1(a)]. There are
three possible types of bulk truncated surface on Fe2O3(0001). The O
layer shows a hexagonal lattice with the periodicity of 2.91 Å. Both
the Feoct1 and Feoct2 layers have the same periodicity of 5.03 Å, while
the Feoct2-terminated surface exposes the single Fe layer in a hexagonal
lattice on the O layer and the Feoct1-terminated surface exposes the double Fe layers forming a honeycomb lattice. Taking an O layer as the basal
plane, the height is 0.85 Å for Feoct2 and 1.45 Å for Feoct1 [37].
The Fe3O4 single crystal has the inverse spinel structure. Oxygen
anions form a close-packed fcc sublattice with slightly distorted
hexagonal planes stacked in ABCABC sequence along [111]. The tetrahedrally coordinated sites formed by oxygen ions are occupied by
Fe 3+ cations. Other Fe cations occupy the octahedrally coordinated
interstices, among which a half exhibits + 3 valance state while the
other half exhibits + 2. This lattice can be described as a stack of
O1-Feoct1-O2-Fetet2-Feoct2-Fetet1-O1 layers along [111] where the subscripts “tet” and “oct” refer to the tetrahedrally and octahedrally coordinated sites, respectively [Fig. 1(b)]. The O-terminated surface shows
a hexagonal lattice with the periodicity of 2.97 Å [37]. The situation
of Fe termination is rather complicated. Ideally there could have been
several types of Fe layer exposing on Fe3O4(111) surface. But theoretical
calculations [13,38,39] have suggested only two possible energetically
favored types, i.e., Feoct2 and Fetet1. The Fetet1-terminated surface exposes
the single Fe layer in a hexagonal lattice on the O layer, while the
Feoct2-terminated surface exposes the double Fe layers forming a
honeycomb lattice. Both of the favored Fe terminations have the
same surface unit cell with periodicity of 5.92 Å.
The structure of FeO single crystal is relatively simple, showing the
NaCl lattice. Along the [111] direction FeO is stacked with hexagonal
Fe and O layers alternatively in ABCABC sequence, respectively, with
the interspacing of 1.25 Å [Fig. 1(c)]. Both of the two possible terminations, i.e., the Fe and O layers show hexagonal lattices with the
same periodicity of 3.04 Å [37].
Y. Tang et al. / Surface Science 609 (2013) 67–72
In brief, the lattice constants of the bulk-truncated hexagonal
Fe2O3(0001), Fe3O4(111) and FeO(111) surfaces are 5.03 Å, 5.92 Å
and 3.04 Å on the Fe terminations, and 2.91 Å, 2.97 Å and 3.04 Å on
the O terminations, respectively [37]. As discussed in the following,
we observe the periodicities of ~ 5 Å and ~ 6 Å in STM images on the
sample surface, which are the characteristics of the Fe2O3(0001)
and Fe3O4(111) surfaces terminated by the single Fe layers, respectively. Therefore the domains formed by these two layers can be
distinctly identified. The periodicity of ~3 Å is also observed in some
areas, 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° with
respect to that of the O termination on Fe2O3(0001) surface, while the
orientations 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 the
identification of the type of surface termination layer can be obtained
by 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 °C
for 30 min in UHV, the α-Fe2O3(0001) surface shows a hexagonal
structure with the periodicity of 6.0 ± 0.5 Å, as shown in Fig. 2. It is
the characteristic of the Fe-terminated Fe3O4(111)-(1 × 1) lattice
and has been commonly observed on the surface of single crystalline
Fe3O4(111) [7,29–33,40–43]. In the following, we refer to this surface
a
69
as the “regular” (R) phase. The minimal step height on the surface
is measured as ~ 4.8 Å in our STM image [44], consistent with the
interspacing of two adjacent equivalent layers in Fe3O4(111). Further
evidence is provided by the XRD analysis [Fig. 2(c)]. Compared with the
original Fe2O3(0001), the sample shows additional diffraction peaks
corresponding to Fe3O4(111) {h,h,h} planes after repeated sputtering
and annealing in UHV. Due to the preferential sputtering of oxygen,
Fe2O3(0001) is reduced. And with the sputtering dose increasing, the
reduction penetrates deep into the bulk, forming a relatively thick
layer of Fe3O4(111) that can be detected by XRD.
Among the two possible types of Fe termination layers on
Fe3O4(111) [13,38,39], i.e., Feoct2 and Fetet1 (see Fig. 1 and the corresponding text), the Feoct2 termination typically shows honeycomb
structure in unoccupied-state STM images [32] that is different from
our observations. Additionally, the Feoct2 termination is usually obtained
on the Fe3O4(111) surface in an oxygen-poor condition [6,32]. In the
current case, due to the presence of the underneath bulk Fe2O3, the
oxygen-poor condition is difficult to achieve on the surface since oxygen
may move towards the surface at high temperature during annealing.
Therefore the R surface is attributed to the Fe3O4(111) surface terminated
by the single layer of Fetet1.
2.3. The surface in “honeycomb” phase
By further annealing the sample with R-phase surface in O2 ambience
(1×10−6 mbar) and subsequently cooling it to room temperature in
the same oxygen ambience, terraces with different long-range ordering
b
c
Fig. 2. (a) STM image (45 nm × 45 nm, −2 V/20 pA) of the R surface. The minimum step height is measured as 4.8 Å along the line. (b) High-resolution STM image (12 nm × 12 nm,
−1.5 V/20 pA) of the R surface. (c) XRD of the initial α-Fe2O3(0001) sample and after repeated 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 honeycomb superstructure on H terrace (referred to as “H” phase) has
the periodicity of 40 ± 3 Å. Each honeycomb shows a patch at the
center (the γ domain) surrounded by six small patches, which are
actually two types of domains adjacent to each other [labeled as α
and β in Fig. 3(b), respectively]. The α domain is ~ 0.8 Å higher
than β, while β and γ are almost at the same height, as shown in the
unoccupied-state STM image Fig. 3(c). Small irregular protrusions in γ
domains are visible by STM. As compared with the high-resolution
images presented in the following, the origin of these protrusions are
purely electronic.
Honeycomb superstructure has been observed on Fe3O4(111) surface prepared by UHV annealing [27,30]. The “S3” phase reported by
Paul et al. [30] coexisted with R phase. But the step height between
R and S3 was ~ 1.5 Å, different from our measurement that shows a
step of 1.8 Å between α on the H terrace and R2 on R (or a step of
3.0 Å between α and R1) as compared with the step of 4.8 Å within
R phase (R1 and R2) [44]. Additionally, the periodicity of the honeycomb “S3” phase was typically around 50 Å [27,30], also inconsistent
with our observation on the H superstructure (40 Å). Actually in the
current work, due to the oxidation environments provided during
annealing, the Fe3O4(111) surface is oxidized and the observed H
superstructure can be attributed to the “biphase” that was obtained
on Fe2O3(0001) [26]. As shown in Fig. 4(a), α and β domains are arranged alternatively on the corners of the honeycomb (the γ domain)
[45]. Both α and β domains show hexagonal features, with the periodicities of 5.0 Å and 3.0 Å, respectively. And the orientation of the
β-type sublattice rotates by 30° with respect to the α-type. All these
a
STM observations are in good agreement with the previously reported
“biphase” [26] where Fe2O3(0001) and FeO(111) domains coexist on
the surface.
Moreover, with the high-resolution STM images of both unoccupiedand occupied-states, we can identify the atomic layer in the γ-type
sublattice. Fig. 4(a) shows a honeycomb structure with periodicity of
5.0 Å in γ, characteristic of the Fe2O3(0001) surface where the Feoct1
and Feoct2 layers are visible (see Fig. 1). On the other hand, in the
occupied-state image that is mainly contributed from O 2p states, a
hexagonal structure with periodicity of 3.0 Å is observed. And the unit
cell orientation rotates by 30° from that in the unoccupied-state image,
consistent with the lattice of O layer on Fe2O3(0001) surface. Since previous calculations [20,46,47] suggested that the double-layer Fe termination on Fe2O3(0001) is strongly disfavored under oxidizing conditions,
we concluded that the γ domain corresponds to the O-terminated
Fe2O3(0001) surface.
Different from the γ domain, the α domain shows no atomic resolution in the occupied-state image that represents the density of
states (DOS) of O sites on the Fe2O3(0001) surface [Fig. 4(b)]. This
suggests that the charges on O sites are screened by a surface layer,
i.e., they correspond to the terminations other than O layer. Considering the characteristic periodicity and unit cell orientation observed
in the unoccupied-state image [Fig. 4(a)], we attribute the α domain
to Fe2O3(0001) terminated by Feoct2. Although the β domain shows
no atomic resolution in the occupied-state image as well, it cannot
be identified as the Fe termination since the image contrast is also
contributed from the partially filled Fe 3d states, in combination
with that from O 2p. Considering the α and β sublattices share the
b
c
Fig. 3. (a) STM image (30 nm × 30 nm, −2.5 V/10 pA) of the surface with R and H coexisting with each other. (b) STM image (17 nm × 17 nm, 3 V/50 pA) of the H terrace. The unit
cell of the honeycomb superstructure is marked by the rhombus with the α, β and γ domains labeled. (c) The line profile along AB in (b).
Y. Tang et al. / Surface Science 609 (2013) 67–72
Fig. 4. (a) The unoccupied- (+3 V/30 pA) and (b) occupied-state (−2 V/30 pA) STM
images (17 nm × 7 nm) of the H phase.
same O layer as the basal plane, their height difference (0.8 Å) suggests that the β domain corresponds to the FeO(111) surface terminated by O. Strictly speaking, the H superstructure is a “triphase”
lattice consisting of O-terminated FeO(111), Fe- and O-terminated
Fe2O3(0001) sublattices.
Fig. 5 illustrates the structural model of the H phase surface with
the top three O layers included. Since the surface is prepared by oxidizing Fe3O4(111), we consider the O layers as in Fe2O3(0001) configuration and allow only the top one to be relaxed laterally [48]. The
superstructure with the periodicity of 40 Å can be obtained only by
expanding the top layer lattice by 7.6% (the O–O distance is 3.13 Å
in the top layer compared with 2.91 Å in the two bottom layers).
This results in three types of regions with different stacking sequence
of the O layers. The first type is in ABA sequence, corresponding to
that in Fe2O3(0001) lattice. A single layer of Fe sits atop the topmost
O in Feoct2 configuration with 7.6% lateral expansion, forming the α
β: ABC
α: ABA
71
domain. The second type is in ABC sequence, with the topmost O
layer expanded by ~ 3% in relative to the bulk truncated FeO(111)
lattice. Such that the β domain is formed. The third type is in the
ABB sequence, inducing the instability in the region. To response,
the periodicity of the topmost O layer shrinks to ~ 3 Å as we observe
in the STM image within the γ domain. Occasionally the γ area appears lower than it normally is by ~ 2.0 Å (not shown). Such a height
corresponds to the interspacing of adjacent O layers in Fe2O3(0001)
lattice, suggesting that the top O and Fe layers are missing. This is
also 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 superstructures. In fact, another consequence of the top layer rotation is the
orientation shift between the oxygen sublattice in each domain and
that of the superstructure. This is not observed in our current work,
suggesting that the lattice expansion on the top layer is responsible
for the polar compensation on the Fe2O3(0001) surface. For verification, further ab initio calculations would be important.
Annealing the sample in O2 repeatedly, the monophased H superstructure is formed on the surface of the Fe3O4(111) layers. However,
the H phase is not a fully-oxidized phase since Fe 2+ ions exist in the β
sublattice. And the XRD of the sample with H surface always shows
the diffractions from Fe3O4(111) no matter how much it is annealed
in oxygen. As proposed by Kim et al. [52], once a Fe2O3 layer is formed
on the sample surface it may act as a barrier that prevents the oxygen
from penetrating into the underneath lattice. Within the O2 pressure
range in the current work (less than 1 × 10 −6 mbar), the iron oxide
can never be fully oxidized back to Fe2O3.
3. Conclusions
In summary, we study the surface structure of α -Fe2O3(0001) prepared by Ar ion sputtering followed by annealing in UHV or in O2 ambience with the partial pressure of 1 ×10−6 mbar. The UHV annealing
results in the reduction of the surface, forming a layer of Fe3O4 (111).
And the surface can be partially oxidized by annealing in O2. The
Fe-terminated FeO(111), Fe- and O-terminated Fe2O3(0001) sublattices
coexist with each other and are arranged in a honeycomb superstructure
with the periodicity of ~40 Å. Unlike the reduction of Fe2O3(0001) that
can penetrate deep into the bulk upon repeated UHV annealing, the
oxidation of the Fe3O4(111) occurs only partially on the top layer(s). It
is revealed that the surface structure of iron oxide is not only sensitive
to the O2 pressure during annealing, but also determined by the treatment history. The Fe2O3(0001) surface can always be reduced but
the formed Fe3O4(111) layers cannot be easily oxidized back to Fe2O3
reversibly.
Acknowledgments
The authors thank Dr. Xiaoguang Meng of Stevens Institute of
Technology for providing the Fe2O3 sample, and Dr. Xuetao Zhu for
helpful discussions. This work was supported by the “973” Program
of China (2012CB921700), NSFC Project 11027406 and Specific
Funding of the Discipline and Graduate Education Project of Beijing
Municipal Commission of Education.
γ :ABB
1st layer
(topmost)
2nd layer
3rd layer
Fig. 5. Illustration of the H superstructure with the top three close-packed oxygen layers
included. Stacked in AB sequence, the second and third layers have the periodicity of
2.91 Å, while the topmost (first) layer is expanded by ~7.6% (3.13 Å). The resulting three
types of stacking sequence in different areas are shown in the inset.
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