Polarity and surface structural evolution of iron oxide films

Abstract

Various ordered Fe oxide films were prepared in an ultrahigh vacuum system, and their surface and electronic structure were studied in situ by low-energy electron diffraction and photoelectron spectroscopy. We obtain Fe₂O₃(0001) films with oxygen termination (O–Fe₂O₃(0001)), a polar surface, via FeO(111) films. The O–Fe₂O₃(0001) films can be transformed to a non-polar surface of iron-terminated Fe₂O₃(0001) (Fe–Fe₂O₃(0001)) films. Fe₃O₄(111) films are formed after annealing O–Fe₂O₃(0001) films. A co-existed phase of Fe–Fe₂O₃(0001) and Fe₃O₄(111) is observed via annealing Fe–Fe₂O₃(0001) films. These Fe oxide films with different surface structures and properties can be designedly used as model plates for further studies in surface physics and chemistry.

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Introduction

Iron oxides are used in a wide range of technological, industrial and environmental applications as catalysts, oxidizing agents, electrodes, pigments, and gas sensors.^1–6 Commonly obtained Fe oxides are FeO, Fe₃O₄ and α-Fe₂O₃ with cubic sodium-chloride structure, cubic inverse-spinel structure and corundum structure, respectively.^7–10 Determined by experimental conditions, the surfaces of Fe oxides could be terminated by an O or Fe layer, e.g. O–Fe₂O₃(0001) (oxygen-terminated surface) or Fe–Fe₂O₃(0001) (iron-terminated surface).^11–13 Those surfaces with different surface and electronic structure significantly affect their chemical and physical properties. For instance, previous experimental studies have showed that water is not dissociated on O–FeO(111) films but dissociated on Fe–Fe₃O₄(111) films.¹⁴ Theoretical calculations have suggested that O–Fe₂O₃(0001) is the most stabilized surface after water adsorption, while Fe–Fe₂O₃(0001) surface is unstable at higher water coverage.¹⁵ Recent theoretical studies have also shown that on O–Fe₂O₃(0001), Au donates electrons to the support, while on Fe–Fe₂O₃(0001), Au gains electrons from the support.¹³

Apparently, in iron oxide supported adsorbate systems, the chemical state and surface structure of the support play important roles in the interaction between adsorbate and oxide. Considering the diversity with various chemical states and surface structures in Fe oxides, the different interactions between molecules/atoms and these surfaces are expected. However, the studies of Fe oxide surfaces with different properties are few because those surfaces are very hard to obtain, especially for α-Fe₂O₃(0001). In the present study, various ordered Fe oxide films including FeO(111), O– and Fe–Fe₂O₃(0001), Fe₃O₄(111) as well as co-existence of Fe–Fe₂O₃(0001) and Fe₃O₄(111) films are synthesized and their surface evolutions are investigated.

Experimental section

All experiments were conducted in an ultrahigh vacuum system (VG ESCALAB-5) equipped with low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS).

A single crystal of Mo(110) was used as the substrate to grow Fe oxide films. The temperature of the substrate was monitored by a C-type thermocouple (W-5%Re/W-26%Re) spot welded to the substrate. Before growing Fe oxide films, the substrate was cleaned by heating in ∼10⁻⁷ mbar O₂ at ∼1200 K and flashed to ∼1600 K without O₂. The procedure was repeated until no contaminations (mainly C) were detected except a small amount of residual oxygen, which has no influence on the growth of Fe oxide films.¹⁶

The Fe source consists of high purity Fe wire wrapped around a resistively heated tungsten wire. The deposition rate of Fe is about 0.4–0.5 Å min⁻¹, monitored by a quartz crystal oscillator. Various Fe oxide films with about 40–70 Å thickness were prepared on the Mo substrate by co-deposition of Fe and O₂ and followed annealing at certain temperatures with O₂ (10⁻⁵–10⁻⁷ mbar) or without O₂.

XPS measurements were carried out using Al K_α X-ray line (hv = 1486.6 eV). To avoid any surface change of the films induced by X-ray under a long time irradiation, a pass energy of 50 eV was used to shorten spectra acquisition time, yet the resolution of the spectra is good enough to distinguish Fe chemical states between Fe²⁺ and Fe³⁺. The binding energy (BE) of the XP spectrometer was calibrated with respect to the pure Au (4f_7/2 = 84.0 eV) and Cu (2p_3/2 = 932.7 eV). In data treatment, the BE of 530.1 eV from O 1s was used as an internal reference for any possible charging.^9,17–20 UPS measurements were performed using the He(I) line (hv = 21.2 eV) and a pass energy of 5 eV. The spectra are referenced to the Fermi level (E_F) calibrated by using the Mo substrate, as E_F = 0 eV. All data were collected at room temperature.

Results and discussion

Curve (1) in Fig. 1 presents the XP spectrum of Fe 2p for Fe oxide films which were prepared by depositing Fe on the substrate in ∼10⁻⁷ mbar O₂ at ∼600 K and subsequent annealing at ∼600 K. It is reported that the BEs of Fe 2p_3/2 lines of Fe²⁺ and Fe³⁺ states are at 710.0 and 711.0 eV with their satellite lines at 715.5 and 719.0 eV, respectively.^{7,9,17–19,21–23} The BEs of Fe 2p_3/2 line and its satellite peak of the as grown films are 709.8 and 715.5 eV, which are in a good agreement with the literature values of Fe²⁺,^{7,9,17,18,21,23} indicating a formation of FeO films. The LEED pattern of the FeO films is shown in Fig. 1(a), which exhibits a (1 × 1) pattern with hexagonal symmetry. Based on Mo(110) surface unit cell,²⁴ a two dimensional (2D) lattice constant of ∼3 Å is deduced. Combined with XPS results, we confirm that the FeO films we prepared are FeO(111).^7–10,25 FeO(111) films with similar thickness have been also prepared on Pt(111) recently.²⁶

Fig. 1 Fe 2p XP spectra of the as grown Fe oxide films (Curve 1), after oxidizing in ∼10⁻⁵ mbar O₂ at ∼600 K (Curve 2) and further annealing in ∼10⁻⁵ mbar O₂ at ∼650 K (Curve 3). The shift of the Fe 2p_3/2 peak is indicated by the vertical dash lines. The Fe 2p_3/2 satellite lines of Fe²⁺ and Fe³⁺ are indicated by arrows. (a–c) LEED patterns of the films corresponding to Curve 1–3, respectively. Primary energy is 55 eV for all LEED.

After oxidizing the FeO(111) films in ∼10⁻⁵ mbar O₂ at ∼600 K, the Fe 2p_3/2 line and its satellite peak shift to 711.1 and 719.0 eV, respectively (Curve (2) in Fig. 1). These values are consistent with literature data of Fe³⁺,^{7,9,17–19,21–23} which suggests that Fe²⁺ cations are oxidized to Fe³⁺. This clearly indicates a transformation from FeO to Fe₂O₃ films. The LEED pattern of the Fe₂O₃ films also shows a hexagonal symmetry (Fig. 1(b)). This LEED pattern being similar to that of the FeO(111) films taken with the same incident beam energy infers a 2D lattice constant of ∼3 Å, too. However, the broader spots and the higher background in LEED indicate that the Fe₂O₃ films are rougher than the FeO(111) films.

The BE of the Fe 2p_3/2 peak is almost unchanged after further annealing in ∼10⁻⁵ mbar O₂ at ∼650 K (Curve (3) in Fig. 1). However, the intensity ratio of Fe to O is slightly higher than that at ∼600 K. The corresponding LEED pattern is shown in Fig. 1(c), revealing a hexagonal symmetry but with a different surface structure comparing to Fig. 1(b). On this surface, its 2D unit cell is ∼5 Å by comparing with the LEED pattern of FeO(111) films. The broader spots and higher background in the LEED pattern also indicate a defected surface.

Interestingly, in our experiments, the corresponding LEED pattern of Fe₂O₃ films show two different patterns with the same primary beam energy. Structurally, Fe₂O₃ has four polymorphs with α-Fe₂O₃, β-Fe₂O₃, γ-Fe₂O₃ and ε-Fe₂O₃.²⁷ The α-Fe₂O₃ has corundum structure with lattice parameters a₀ = 5.036 Å, c₀ = 13.749 Å. The β-Fe₂O₃ and γ-Fe₂O₃ have body-centered cubic “bixbyite” and inverse-spinel structures with lattice parameter a₀ = 9.404 and 8.351 Å, respectively. ε-Fe₂O₃ crystallizes in an orthorhombic structure with lattice parameters a₀ = 5.095 Å, b₀ = 8.789 Å and c₀ = 9.437 Å. Based on those structures, ε-Fe₂O₃ cannot give hexagonal LEED patterns, but α-Fe₂O₃(0001), β-Fe₂O₃(111) or γ-Fe₂O₃(111) can. However, the 2D lattice constant of β-Fe₂O₃(111) is 13.299 Å, which is much larger than that we observed. For γ-Fe₂O₃, it is metastable and can be converted to Fe₃O₄ in vacuum at 473 K or transformed into the stable α-Fe₂O₃ in O₂ above 573 K.²⁸ Therefore, the Fe₂O₃ films we prepared are most likely the α-form.

Based on the bulk structure of α-Fe₂O₃, the (0001) face is of hexagonal structure with 2D unit cell of 5.036 Å. This face could be terminated by either an iron or oxygen layer, as shown in Fig. 2(a) and 2(b). It should be noted that the oxygen–oxygen distance in the O layer from bulk structure is different.^12,29 This causes the 2D lattice constant of the O layer to be ∼5 Å, which is the same as the lattice constant of the Fe layer. Previous studies indicated that the surface with the (1 × 1) LEED pattern could be terminated by either Fe or O.^12,29 However, experimental results indicated that surface O atoms are relaxed on the oxygen-terminated surface, which results in a uniform oxygen–oxygen distance of ∼3 Å,^21,30 as shown in Fig. 2(c). Thus, a 2D lattice constant of ∼3 Å should be observed on the oxygen-terminated surface. In the case of the iron-terminated surface, the structure of the O layer is very close to the bulk one^11,12 and the 2D lattice constant remains ∼5 Å. In the present work, the LEED pattern shown in Fig. 1(c), with a 2D lattice constant of ∼5 Å, corresponds to the (1 × 1) structure of α-Fe₂O₃(0001). The LEED pattern with a 2D lattice constant of ∼3 Å in Fig. 1(b) is a structure with respect to the (1 × 1) structure. The films with the LEED pattern should be related to the O termination, because only the oxygen-terminated surface could provide a 2D lattice constant of ∼3 Å.

Fig. 2 Top view of (a) Fe–Fe₂O₃(0001) face in bulk structure, (b) O–Fe₂O₃(0001) face in bulk structure and (c) O–Fe₂O₃(0001) surface after taking surface relaxation into account. The Fe cations are indicated by small balls and the O anions by big balls. The unit cells of the topmost layers are indicated.

The same LEED pattern with surface structure has been also observed from low coverage α-Fe₂O₃ films prepared on α-Al₂O₃(0001) at 523 K.¹⁸ By combining real time reflection high-energy electron diffraction (RHEED) and LEED techniques, the authors suggested that corundum phase in the films is not ordered enough to coherently contribute to the LEED pattern.¹⁸ Generally, the surface of corundum films prepared at lower temperature is rougher and contains defects, but it gets smoother and less defects at higher temperature. For instance, the studies by scanning tunneling microscopy (STM) showed that the α-Fe₂O₃(0001) films prepared at 700 K consist of patches and are covered with defects, while at 1000 K the films are smoother and nearly defect-free.^21,30,31 Therefore surface defects are important to the formation of the LEED pattern, because other 2D periodicities on the surface could be identified without imaging the underlying corundum phase. Recently, a 2D lattice constant of ∼3 Å from oxygen-terminated Fe₂O₃ films prepared on Au(111) was observed by STM.^21,30 In addition, the same growth mode was found on different substrates.^18,21 This indicates that the equilibrium structure of the films may be independent of the substrate nature.¹⁸ We conclude that the LEED pattern shown in Fig. 1(b) corresponds to defected O–Fe₂O₃(0001) films.

To further confirm the origin of our LEED patterns, we conducted Fourier transform on real space models by taking defects into account. This was performed by using the Fourier transform section of scanning probe imaging processor (SPIP) brand software. The surfaces are built by choosing the ionic radii of Fe³⁺ and O²⁻ with 0.64 and 1.32 Å, respectively.²⁹ Based on a previous study, the α-Fe₂O₃ films contain surface defects as depression with an area of about 30–50 Ų.²¹ For oxide films the surface defect extent is generally thought to be in a range from ∼10% to ∼20%. Taking these factors into account, the defects are built by removing Fe and O atoms from the top layers as stoichiometric ratios of Fe₂O₃. Fig. 3(a) and (b) reveal the schematic defected structure for the oxygen- and iron-terminated surface, respectively. Fig. 3(c) and (d) show the details of the defects indicated in (a) and (b), respectively. Fig. 3(e) and (f) give the Fourier transform results for O– and Fe–Fe₂O₃(0001) surfaces with ∼10% defects, respectively. Both Fig. 3(e) and 3(f) show high background intensity and broader spots and Fig. 3(e) reveals a structure compared with Fig. 3(f). These results suggest that the films as shown in Fig. 1(b) and 1(c) correspond with defected O–Fe₂O₃(0001)¹⁶ and Fe–Fe₂O₃(0001) films, respectively. The Fourier transform results for the surfaces with ∼15% and ∼20% defects are similar to that of ∼10%, but only the background noise is increased. The above assignment of termination is further supported by the following studies of valence band structure.

Fig. 3 (a), (b) Schematic structures of the O– and Fe–Fe₂O₃(0001) surfaces with ∼10% defects, respectively. (c), (d) Detailed structures of the defects as indicated in (a) and (b), respectively. The O anions are indicated by big balls and the Fe cations by small balls. (e), (f) Fourier transform results of O– and Fe–Fe₂O₃(0001) surfaces with ∼10% defects, respectively.

Fig. 4 shows UP spectra of the Fe oxide films from 0 and 9 eV. The spectra mainly consist of overlapping and hybridizing of O 2p and Fe 3dⁿL̲ (L̲ denotes a ligand hole) final states.^23,32–34 For FeO(111) films, the spectrum (Curve (a) in Fig. 4) shows a peak at ∼1 eV which originates from Fe 3d⁶L̲ final states, corresponding to Fe²⁺.³² This peak disappears after oxidizing in ∼10⁻⁵ mbar O₂ at ∼600 K (Curve (b) in Fig. 4), implying an oxidation of Fe²⁺ to Fe³⁺. The weak signals around 0–1.5 eV in the Curve (b) may originate from residual Fe²⁺ ions or from surface defect states. After further annealing in ∼10⁻⁵ mbar O₂ at ∼650 K, the intensity of the peak at ∼6 eV is decreased and the intensity of the peak at ∼2.5 eV is increased (Curve (c) in Fig. 4). The emission at ∼6 eV is mainly from O 2p states,^32–34 and the emission at ∼2.5 eV is related to Fe 3d⁵L̲ final states, corresponding to Fe³⁺.^32,34

Fig. 4 UP spectra of the as grown films (a), after annealing in ∼10⁻⁵ mbar O₂ at ∼600 K (b) and further annealing in ∼10⁻⁵ mbar O₂ at ∼650 K (c). The positions related to O 2p, Fe 3d⁵L̲ and Fe 3d⁶L̲ are indicated.

Even though the analytical depth in UPS is higher than that in XPS, the both analytical techniques are surface sensitive and the top atomic layers should contribute to the intensity of the signal most. Therefore, for the oxygen-terminated surface, it should give a higher O 2p intensity and a lower Fe 3d intensity, and it reverses for the iron-terminated surface. In our experiments, the high intensity around 6 eV in the UP spectrum of FeO(111) films (Curve (a) in Fig. 4) is indicative of O termination. The oxygen-terminated FeO(111) films prepared on Pt(111) in ∼10⁻⁶ mbar O₂ has been verified by X-ray photoelectron diffraction measurements.³⁵ For α-Fe₂O₃(0001) films, the higher O 2p intensity and the lower Fe 3d intensity in the Curve (b) in Fig. 4 indicate that the films are of O termination. In contrast, the lower O 2p intensity and the higher Fe 3d intensity in the Curve (c) suggest the films are of Fe termination. These results are consistent with the LEED conclusions.

One may argue that hydroxyl adsorbed on the oxide films can affect the surface structure. Fig. 5 shows the O 1s XP spectra of various Fe oxide films. The symmetric peaks of O 1s exclude the possibility of hydroxyl existing on the surfaces.^36,37

Fig. 5 XP spectra of O 1s from the as grown films (a), after annealing in ∼10⁻⁵ mbar O₂ at ∼600 K (b) and further annealing in ∼10⁻⁵ mbar O₂ at ∼650 K (c). The dash line indicates a BE value of 530.1 eV.

The surfaces of any ionic or partly ionic material can be classified into three types as shown in Fig. 6.³⁸ In type 1 (Fig. 6(a)), each plane consisting of equal anions and cations is electrically neutral and no dipole moment exists perpendicular to the surface (p_z = 0); in type 2 (Fig. 6(b)), each plane is charged but the total dipole moment of the repeat unit vanishes (p_z = 0); and in type 3 (Fig. 6(c)), each plane is also charged and the repeat unit bears a non-zero dipole moment (p_z ≠ 0). For FeO(111) films, along the [111] direction FeO presents a sequence of alternating and equally spaced cation and anion planes, i.e. its (111) surface is of polarity. Note that for a type 3 surface, the electrostatic energy is proportional to the films thickness.³⁹ Therefore, thick FeO(111) films should bear a large electrostatic energy and be unstable. It is known that a significant number of vacancies in Fe sites exist in FeO mineral and the fraction of vacant Fe sites ranges from approximately 5% to 15%.^8,17 In our FeO(111) films, the contained vacancies in Fe sites may reduce the electrostatic energy and stabilize the surface. Normally, the lone electrons at sites surrounding the vacancies will be polarized and these polarized electrons do not occupy the allowed states in the valence band and below, but they introduce defect states in the vicinity of E_F.^40,41 The defect states could be detected by UPS. For FeO, however, it is hard to identify the introduced states because a higher intensity of Fe 3d⁶L̲ emission located around E_F. To study the defect states in detail, more techniques such as scanning probe microscopy are needed.

Fig. 6 Schematic representation of stacking sequence in ionic crystal. (a) With equal anions and cations on each plane and no dipole moment perpendicular to surface. (b) With charged planes but no dipole moment normal to surface. (c) With charged planes and a dipole moment vertical to the surface. p_z is the dipole moment perpendicular to surface. The big and small balls denote anions and cations, respectively. Square brackets indicate the repeat units.

For O–Fe₂O₃(0001) surface, the repeat unit in the [0001] direction is O₃–Fe₂–, being of a type 3 surface,³⁹ which should bear a large electrostatic energy, causing instability. However, the electrostatic energy can be lowered by surface charge modifications.³⁹ The surface of O–Fe₂O₃(0001) films can be modified by defects, impurities and ferryl groups with Fe in the Fe⁵⁺ oxidation state formed on the surface.^42,43 Unfortunately, the latter two factors can not be detected due to limitation of our instrument precision. It is most likely that the polar O–Fe₂O₃(0001) films in our experiments are stabilized by surface defects, which is in evidence from broad spots in the LEED pattern and possible defect states around 0–1.5 eV in the UP spectrum.

For Fe–Fe₂O₃(0001) surface, the repeat unit in the [0001] direction is Fe–O₃–Fe–, i.e. a non-polar surface with a type 2 surface. The transformation from O–Fe₂O₃(0001) to Fe–Fe₂O₃(0001) films we observed suggests that the surface energy of the Fe termination is smaller than that of the O termination. Theoretically, the surface energy of thin oxide films is a function of the chemical potential of O. At thermodynamic equilibrium of the solid–gas interface, the chemical potential of O is given by its value in the molecular gas and written as

where μ_o is the chemical potential of O, T is the temperature, P is the pressure, k is the Boltzmann constant and P⁰ is 1 bar. μ_o (T, P⁰) is taken from experimental data as given in standard thermodynamic tables. For T = 600 K, μ_o (T, P⁰) is −0.61 eV, and with increasing temperature μ_o (T, P⁰) becomes lower.⁴⁴ Thus, considering the pressure we used in preparation of α-Fe₂O₃ films (P is ∼7 × 10⁻⁵ mbar), the chemical potential of O is below −1.03 eV. It is known that the surface energy of Fe–Fe₂O₃(0001) films is lower than that of O–Fe₂O₃(0001) films when the chemical potential of O is below −0.76 eV.¹¹ Our experimental values (<−1.03 eV) are lower than −0.76 eV, which clearly indicates that the surface energy of the Fe termination should be smaller than that of the O termination, i.e. the iron-terminated surface is more stable.

For FeO(111) films, a previous theory showed O termination is more stable than Fe termination when the corresponding chemical potential of O is above −4.35 eV.⁴⁵ Under our experimental conditions (P is ∼7 × 10⁻⁷ mbar), the corresponding chemical potential of O is −1.15 eV. This suggests the prepared FeO(111) films are oxygen-terminated, which is consistent with the UPS results. In an oxidizing process, the oxidation should take place under thermodynamic non-equilibrium conditions. Therefore, O–Fe₂O₃(0001) films are obtained under a lower chemical potential of O in our experiments. However, with annealing, the films approach the thermodynamic equilibrium, i.e. transformation from O–Fe₂O₃(0001) to Fe–Fe₂O₃(0001).

The thermal stability of the α-Fe₂O₃(0001) films with different terminations was studied by annealing without O₂. The results show that the oxygen-terminated surface is stable at temperature <600 K. Fe²⁺ signals can be detected at ∼650 K, and the intensity of Fe²⁺ signals becomes more obvious with increasing temperature. Fig. 7(a) shows the XP spectrum of Fe 2p lines after annealing O–Fe₂O₃(0001) films at ∼700 K. The Fe 2p_3/2 peak shifts to 710.6 eV and no satellite line is observed, which are the typical characteristics of Fe₃O₄.^17,18Fig. 7(b) gives the valence band spectrum. The appearance of the peak at ∼1 eV features Fe²⁺. Fig. 7(c) shows the corresponding LEED pattern, which gives a 2D lattice constant of ∼6 Å as compared with the LEED pattern of FeO(111) films, being consistent with the value of Fe₃O₄(111) (5.92 Å).^7,10,25 These results suggest that O–Fe₂O₃(0001) films are completely reduced to Fe₃O₄(111) films after the annealing.

Fig. 7 (a) Fe 2p XP spectrum after annealing O–Fe₂O₃(0001) films at ∼700 K, the position of Fe 2p_2/3 is indicated by the dashed line. (b) UP spectrum. The position of Fe²⁺ is indicated by the arrow. (c) LEED pattern with primary beam energy of 55 eV.

The Fe–Fe₂O₃(0001) is also thermally stable at temperature <600 K. At ∼650 K, it is slightly changed. At ∼700 K, compared with the Fe–Fe₂O₃(0001) without annealing, its Fe 2p lines become slightly broader and the intensity of Fe³⁺ satellite peak is less visible, as seen in Fig. 8(a). Its valence band spectrum clearly shows a new peak at ∼1 eV (inset of Fig. 8(a)). Those results indicate the part reduction of the Fe³⁺ to Fe²⁺. Fig. 8(b) and (c) show the LEED pattern of the films and the corresponding schematic drawing, respectively. The spots in the LEED pattern are separated into two groups, as indicated by the big and small dots in the schematic drawing, respectively. Both of them are of hexagonal symmetry but with 30° rotation to each other. The corresponding reciprocal lattice lengths of these two set of spots are different, which implies two different structures. In fact, one set of the spots (big dots in the schematic drawing) located at the same positions as that in the LEED pattern of Fe–Fe₂O₃(0001) films and the other set of the spots (small dots in the schematic drawing) located at the same positions of that of Fe₃O₄(111) films, under the same beam energy. Thus it clearly shows that Fe–Fe₂O₃(0001) and Fe₃O₄(111) are co-existing on surface. This result is consistent with the XPS and UPS results that after the annealing, Fe–Fe₂O₃ is not completely reduced to Fe₃O₄, being different from O–Fe₂O₃. Thus, it is confirmed that the non-polar Fe–Fe₂O₃(0001) film is more stable than the polar O–Fe₂O₃(0001) films. In addition, the co-existing surface is different from previous reported biphase surfaces consisting of α-Fe₂O₃(0001) and FeO(111) or Fe₃O₄(111) and FeO(111).^10,25 The co-existed surface may have novel properties, which need more designed experiments to explore.

Fig. 8 (a) XP spectrum of Fe 2p after annealing Fe–Fe₂O₃(0001) films at ∼700 K, the positions of Fe 2p_3/2 and satellite lines for Fe²⁺ and Fe³⁺ are indicated, respectively. The inset of (a) is the UP spectrum and the position of Fe²⁺ is indicated by the arrow. (b) LEED pattern with primary energy of 35 eV, and (c) schematic drawing of (b), the big and small dots are separated into two groups and both are of hexagonal symmetry but with 30° rotation. The thick and thin solid lines represent the reciprocal lattice lengths.

Conclusions

We have prepared and studied in situ ordered Fe oxide films including FeO(111), O–Fe₂O₃(0001), Fe–Fe₂O₃(0001) and Fe₃O₄(111) films by LEED, XPS and UPS. The O–Fe₂O₃(0001) and Fe–Fe₂O₃(0001) films can be obtained by annealing FeO(111) films in ∼10⁻⁵ mbar O₂. Fe–Fe₂O₃(0001) films are more thermally stable than O–Fe₂O₃(0001) films. A co-existence of Fe–Fe₂O₃(0001) and Fe₃O₄(111) films is observed. The present study is helpful to understand surface structural evolution in iron oxides. The Fe oxide films we have provided can be used as model surfaces for investigations into various chemical reactions.

Acknowledgements

We gratefully acknowledge the financial support of this work from the Knowledge Innovation Program of the Chinese Academy of Sciences (KJCX2-EW-W09) and the 973 program of China (2012CB921700).

References

1. A. A. Herzing, C. J. Kiely, A. F. Carley, P. Landon and G. J. Hutchings, Science, 2008, 321, 1331–1335.
2. M. Haruta, T. Kobayashi, H. Sano and N. Yamada, Chem. Lett., 1987, 16, 405–408.
3. B. V. Reddy and S. N. Khanna, Phys. Rev. Lett., 2004, 93, 068301.
4. J. S. Chen, T. Zhu, X. H. Yang, H. G. Yang and X. W. Lou, J. Am. Chem. Soc., 2010, 132, 13162–13164.
5. N. Pailhé, A. Wattiaux, M. Gaudon and A. Demourgues, J. Solid State Chem., 2008, 181, 2697–2704.
6. Y. Wang, S. R. Wang, Y. Q. Zhao, B. L. Zhu, F. H. Kong, D. Wang, S. H. Wu, W. P. Huang and S. M. Zhang, Sens. Actuators B, 2007, 125, 79–84.
7. W. Weiβ, Surf. Sci., 1997, 377–379, 943–947.
8. G. Ketteler, W. Weiss, W. Ranke and R. Schlögl, Phys. Chem. Chem. Phys., 2001, 3, 1114–1122.
9. A. Barbieri, W. Weiss, M. A. Van Hove and G. A. Somorjai, Surf. Sci., 1994, 302, 259–279.
10. N. G. Condon, F. M. Leibsle, T. Parker, A. R. Lennie, D. J. Vaughan and G. Thornton, Phys. Rev. B: Condens. Matter, 1997, 55, 15885–15894.
11. X.-G. Wang, W. Weiss, S. K. Shaikhutdinov, M. Ritter, M. Petersen, F. Wagner, R. Schlögl and M. Scheffler, Phys. Rev. Lett., 1998, 81, 1038–1041.
12. M. Lübbe and W. Moritz, J. Phys.: Condens. Matter, 2009, 21, 134010.
13. A. Kiejna and T. Pabisiak, J. Phys.: Condens. Matter, 2012, 24, 095003.
14. Y. Joseph, W. Ranke and W. Weiss, J. Phys. Chem. B, 2000, 104, 3224–3236.
15. F. Jones, A. L. Rohl, J. B. Farrow and W. van Bronswijk, Phys. Chem. Chem. Phys., 2000, 2, 3209–3216.
16. M. S. Xue, S. Wang, K. H. Wu, J. D. Guo and Q. L. Guo, Langmuir, 2011, 27, 11–14.
17. T. Yamashita and P. Hayes, Appl. Surf. Sci., 2008, 254, 2441–2449.
18. S. Gota, E. Guiot, M. Henriot and M. Gautier-Soyer, Phys. Rev. B: Condens. Matter, 1999, 60, 14387–14395.
19. T. Fujii, F. M. F. de Groot, G. A. Sawatzky, F. C. Voogt, T. Hibma and K. Okada, Phys. Rev. B: Condens. Matter, 1999, 59, 3195–3202.
20. T. Fujii, D. Alders, F. C. Voogt, T. Hibma, B. T. Thole and G. A. Sawatzky, Surf. Sci., 1996, 366, 579–586.
21. X. Y. Deng and C. Matranga, J. Phys. Chem. C, 2009, 113, 11104–11109.
22. A. Barbier, R. Belkhou, P. Ohresser, M. Gautier-Soyer, O. Bezencenet, M. Mulazzi, M.-J. Guittet and J.-B. Moussy, Phys. Rev. B: Condens. Matter Mater. Phys., 2005, 72, 245423.
23. T. Schedel-Niedrig, W. Weiss and R. Schlögl, Phys. Rev. B: Condens. Matter, 1995, 52, 17449–17460.
24. D. H. Guo, M. S. Xue, Q. L. Guo, K. H. Wu, J. D. Guo and E. G. Wang, Appl. Surf. Sci., 2009, 255, 9015–9019.
25. N. G. Condon, F. M. Leibsle, A. R. Lennie, P. W. Murray, D. J. Vaughan and G. Thornton, Phys. Rev. Lett., 1995, 75, 1961–1964.
26. N. Spiridis, D. Wilgocka-Ślęzak, K. Freindl, B. Figarska, T. Giela, E. Młyńczak, B. Strzelczyk, M. Zając and J. Korecki, Phys. Rev. B: Condens. Matter Mater. Phys., 2012, 85, 075436.
27. R. Zboril, M. Mashlan and D. Petridis, Chem. Mater., 2002, 14, 969–982.
28. N. Wiberg, A. F. Holleman, E. Wiberg, Holleman-Wiberg's Inorganic Chemistry, Academic Press, San Diego, 2001.
29. G. Ketteler, W. Weiss and W. Ranke, Surf. Rev. Lett., 2001, 8, 661–683.
30. X. Y. Deng, J. Lee, C. J. Wang, C. Matranga, F. Aksoy and Z. Liu, J. Phys. Chem. C, 2010, 114, 22619–22623.
31. W. Weiss and W. Ranke, Prog. Surf. Sci., 2002, 70, 1–151.
32. A. Fujimori, N. Kimizuka, M. Taniguchi and S. Suga, Phys. Rev. B, 1987, 36, 6691–6694.
33. R. J. Lad and V. E. Henrich, Phys. Rev. B, 1989, 39, 13478–13485.
34. A. Fujimori, M. Saeki, N. Kimizuka, M. Taniguchi and S. Suga, Phys. Rev. B, 1986, 34, 7318–7328.
35. Y. J. Kim, C. Westphal, R. X. Ynzunza, H. C. Galloway, M. Salmeron, M. A. Van Hove and C. S. Fadley, Phys. Rev. B: Condens. Matter, 1997, 55, R13448–R13451.
36. M. A. Brown, E. Carrasco, M. Sterrer and H.-J. Freund, J. Am. Chem. Soc., 2010, 132, 4064–4065.
37. I. Platzman, R. Brener, H. Haick and R. Tannenbaum, J. Phys. Chem. C, 2008, 112, 1101–1108.
38. P. W. Tasker, J. Phys. C: Solid State Phys., 1979, 12, 4977–4984.
39. C. Noguera, Physics and Chemistry at Oxide Surfaces, Cambridge University Press, Cambridge, 1996.
40. C. Q. Sun, Y. G. Nie, J. S. Pan, X. Zhang, S. Z. Ma, Y. Wang and W. T. Zheng, RSC Adv., 2012, 2, 2377–2383.
41. C. Q. Sun, Nanoscale, 2010, 2, 1930–1961.
42. W. Bergermayer, H. Schweiger and E. Wimmer, Phys. Rev. B: Condens. Matter Mater. Phys., 2004, 69, 195409.
43. C. Lemire, S. Bertarione, A. Zecchina, D. Scarano, A. Chaka, S. Shaikhutdinov and H.-J. Freund, Phys. Rev. Lett., 2005, 94, 166101.
44. D. R. Stull and H. Prophet, JANAF Thermochemical Tables, U.S. National Bureau of Standards,Washington DC, 2nd edn, 1971.
45. Y. L. Li, K. L. Yao, Z. L. Liu and G. Y. Gao, Phys. Rev. B: Condens. Matter Mater. Phys., 2005, 72, 155446.

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