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A photoemission spectroscopy study of the initial oxidation of epitaxial fcc and bcc Fe films grown on Cu(100)
Accepted Manuscript A photoemission spectroscopy study of the initial oxidation of epitaxial fcc and bcc Fe films grown on Cu(100)
Renato de Mendonça, Maximiliano D. Martins, Mathieu Silly, Fausto Sirotti, Waldemar A.A. Macedo PII: DOI: Reference:
S0040-6090(17)30484-4 doi: 10.1016/j.tsf.2017.06.049 TSF 36056
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
31 January 2017 14 June 2017 25 June 2017
Please cite this article as: Renato de Mendonça, Maximiliano D. Martins, Mathieu Silly, Fausto Sirotti, Waldemar A.A. Macedo , A photoemission spectroscopy study of the initial oxidation of epitaxial fcc and bcc Fe films grown on Cu(100), Thin Solid Films (2017), doi: 10.1016/j.tsf.2017.06.049
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ACCEPTED MANUSCRIPT A photoemission spectroscopy study of the initial oxidation of epitaxial fcc and bcc Fe films grown on Cu(100) Renato de Mendonçaa,b*, Maximiliano D. Martinsa, Mathieu Sillyc, Fausto Sirottic,
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Centro de Desenvolvimento da Tecnologia Nuclear – CDTN, Belo Horizonte 31270-901,
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a
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Waldemar A. A. Macedoa
REDEMAT, Escola de Minas, Universidade Federal de Ouro Preto – UFOP, 35400-000,
Ouro Preto, MG, Brazil. c
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b
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MG, Brazil.
TEMPO Beamline, Synchroton SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48,
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*Corresponding author
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91192 Gif-sur-Yvette Cedex – France
E-mail address: [email protected]
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Phone: +55 31 30693210
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ACCEPTED MANUSCRIPT A photoemission spectroscopy study of the initial oxidation of epitaxial fcc and bcc Fe films grown on Cu(100)
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Abstract
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The initial oxidation of Fe is a fundamental issue due to its importance for corrosion and
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catalysis. In this study, we investigate the initial oxidation of epitaxial fcc and bcc Fe thin films grown on a Cu(100) substrate at room temperature. Fe thin films deposited with two
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different thicknesses on Cu(100) were prepared by molecular beam epitaxy in order to
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obtain the fcc Fe(100) and bcc Fe(110) surfaces. The structures and the chemical composition of the Fe films were evaluated by LEED and photoelectron spectroscopy,
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using the facilities at the TEMPO beamline of the SOLEIL Synchrotron. The Fe 2p and O
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1s high-resolution spectra were collected during oxidation at a constant O2 pressure in order to build the kinetic curves, which are discussed taking into account the Fromhold-Cook
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theory. Our result revealed the different initial oxidation kinetics of the films and showed
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that bcc Fe(110) surface has a much higher reactivity with O2 than the fcc Fe(100). The Fe 2p peak analysis suggests the formation of Fe1-xO and an Fe3+-rich oxide on the bcc
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Fe(110) surface and Fe1-xO on the fcc Fe(100) surface.
Keywords: Fe oxidation, Fe/Cu(100), fcc Fe(100), bcc Fe(110), Photoelectron Spectroscopy
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ACCEPTED MANUSCRIPT 1. Introduction
The initial oxidation of metals has been continuously revisited due to new tools that have deepened our knowledge in this fundamental area for corrosion and catalysis [1-10]. The
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initial oxidation process can be remarkably affected by the electronic and structural
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properties of the metal’s surface [1, 2]. Parameters such as the bonding energy of the atoms
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on the surface, the availability of possible bonds, the relaxation effects of the surface, the concentration of defects, and the atomic density are notable issues that can play an
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important role in this phenomenon [1-12].
In this context, iron is one of the most complex systems for surface oxidation studies,
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basically because different oxides (Fe1-xO, Fe3O4, -Fe2O3 and -Fe2O3) can be formed due
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to several thermodynamic conditions and surface structures [2, 5, 9, 11, 12]. The initial oxide formed at room temperature is often Fe1-xO, which is not appropriate for the
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passivation because it has a relative high concentration of defects [12-19]. This oxide has
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the same crystalline structure of NiO and CoO, although it is highly deficient in cations and therefore rich in vacancies [2]. In this structure, each vacancy provides an available site for
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the diffusion of Fe2+, and consequently, for relatively quick oxide growth.
What concerns the initial oxidation of iron, an additional understanding of its behavior on the different surfaces can be useful to design higher performance materials. In addition, the large number of parameters considered in the oxidation process [14, 16, 18-22] has made it more difficult to provide a clear understanding of the resulting compounds formed. Aiming
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ACCEPTED MANUSCRIPT to extend our knowledge on this issue, we studied the effect of the O2 exposure on Fe films surfaces deposited on Cu(100). The Fe films were prepared with two specific thicknesses in order to obtain different structures at the surface. Experimentally, Fe on Cu(100) is the typical system chosen to stabilize fcc Fe as epitaxial ultrathin films at room temperature.
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Due to the small lattice misfit between Cu (3.615 Å at 20ºC) and fcc Fe (lattice parameter
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3.59 Å at 20ºC, as extrapolated from the high temperature -Fe phase), the epitaxy of fcc Fe
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on Cu is favored. In fact, this system was extensively studied and, for thermally deposited
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films, Fe grows epitaxialy on Cu(100) presenting a fcc phase below a critical thickness of 10-12 atomic monolayers (ML), above which the structure relaxes into the bcc Fe(110)
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phase [23-25].
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Here, the oxidation process of the Fe films was studied by real time photoemission
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spectroscopy using the facilities of the TEMPO beamline at SOLEIL Synchroton [26]. Our results showed that, compared to the bcc-Fe film, the fcc-Fe film is considerably more
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resistant to the initial oxidation. Furthermore, the analysis of the Fe 2p photoemission peak
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indicates the formation of a Fe1-xO layer in both surfaces and suggests an additional growth
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of a Fe3+-rich layer on the bcc-Fe surface film.
2. Experimental
The Cu(100) surface was prepared by Ar ion sputtering and annealing (773 K for 10 minutes) cycles in an ultrahigh vacuum chamber (base pressure of 3 × 10-10 Torr). The well-ordered surface was evaluated by Low Energy Electron Diffraction (LEED), see
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ACCEPTED MANUSCRIPT Figure 1 (a). The cleanliness of the surface after the sputtering was checked by photoemission spectroscopy.
The Fe films were deposited at room temperature (RT) by means of an electron beam
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evaporator. The Fe deposition rate was estimated with a quartz microbalance and fixed at
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0.8 Å/min. Films with two thicknesses were deposited, namely 20 Å and 56 Å thick
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(around 11 and 28 monolayers, respectively). The LEED patterns obtained, shown in Figure 1(b) and 1(c), correspond to a fcc Fe(100) and a bcc Fe(110) surfaces, as already
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reported in the literature [23, 27]. The absence of spurious elements was confirmed by
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photoemission spectroscopy.
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Figure 1
Systematic surface exposure of the surfaces to oxygen (99.999%) was carried out at 5.7 ×
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10-9 Torr and room temperature. The total O2 exposure was 50 Langmuir (1 L = 1 × 10-6
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Torr·s). The kinetic curves were obtained by analyzing the high-resolution Fe 2p and O 1s photoemission (PE) peaks collected every 18 seconds during O2 exposure. The spectra were
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fitted using the Spectral Data Processor (SDP) software [XPS International, Mountain View, USA, www.xpsdata.com] with Voigt (Gaussian + Lorentzian) curves and Shirley background subtraction. In particular, Fe 2p3/2 peaks were fitted with three components Fe0, Fe2+ and Fe3+. Due to the complex Fe 2p3/2 structure for Fe2+ and Fe3+ components, these ones were fitted with broader peaks as pointed by Bhargava [5] and Aronniemi [28].
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ACCEPTED MANUSCRIPT Photoelectron spectroscopy experiments were carry out at TEMPO beamline (SOLEIL Synchrotron), using a SCIENTA SES2002 electron energy analyzer equipped with a 2D line detector for time-resolved experiments [29]. Photoemission spectra were obtained at
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normal electron emission using synchrotron radiation of 840 eV.
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3. Results and Discussion
The widescan PE spectra collected on the fcc Fe(100) and bcc Fe(110) surfaces before and
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after the O2 exposure are presented in Figure 2. The chemical composition of the surfaces
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after Fe film deposition reveals no significant oxygen and carbon contamination. The lines corresponding to the Fe 2p, Fe 3s and Fe 3p, and O 1s photoemission peaks along with the
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O and Fe Auger lines were identified, as shown in Figure 2(d). The higher O 1s intensity
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observed in the wide spectrum for the bcc Fe(110) surface already shows the higher oxygen
Figure 2
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amount measured on this surface after oxygen exposure (Figure 2(d)).
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High-resolution Fe 2p spectra are presented in Figure 3. The binding energy (B.E.) of the Fe 2p3/2 peaks are centered at 706.5 eV and 706.4 eV for the fcc Fe(100) and bcc Fe(110) surfaces, respectively. These values are in agreement with the metallic Fe reported in the literature [5]. The Fe 2p spectrum for the fcc Fe(100) surface, after exposure, shows a small component around the main peak that is indicative of oxide formation (Figure 3(c)). A more significant oxide component is observed on the bcc Fe(110) surface (Figure 3(d)). The Fe 2p3/2 peak exhibits different components after exposure. The main oxide component 6
ACCEPTED MANUSCRIPT measured for the oxidized fcc Fe(100) surface is situated at 709.5 eV, whereas the one present in the bcc Fe(110) surface is located at 710.1 eV. This suggests that different kinds of oxides are formed on the Fe film surfaces. Fe 2p3/2 binding energies of the main oxides formed on the Fe surfaces are already attested in the literature [5, 30-33]. The peak situated
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at 709.0 eV typically indicates the formation of Fe1-xO, which is in agreement with our fcc
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Fe(100) data. For the Fe2O3 and Fe3O4 compounds, no significant difference is observed at
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Figure 3
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the Fe 2p3/2 binding energies, which are situated between 710.6 and 711.2 eV [33].
To clear up the oxidation process of the Fe on the surfaces, we de-convoluted the Fe 2p3/2
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spectra measured during the oxygen exposure. Full Width Half Maximum (FWHM) and
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binding energies of the peaks were fixed during the fitting procedure. As presented in Figure 3(e, f), the Fe 2p3/2 signal was decomposed into three components: a first
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contribution at 706.3 eV, another at 709.2 eV, and a third one at 711.2 eV, corresponding to
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Fe0, Fe2+, and Fe3+, respectively [30-33]. Table 1 presents the fitting parameters (B.E., FWHM and peak area) after 50 L of O2 exposure. Different ratios of Fe2+/Fe3+ are observed
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on the surfaces after exposure: 3.1 and 2.5 for the fcc Fe(100) and bcc Fe(110) surfaces, respectively. Due to the B.E. values and high Fe2+ concentration observed on the surfaces, it can be pointed out that Fe1-xO grows on both surfaces. In addition, after exposure, the bcc Fe(110) surface presents a higher concentration of Fe3+, which indicates the formation of Fe3+-rich oxide.
Table 1 7
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The analysis of the O 1s high-resolution spectra also give useful information about the oxides formed on the surfaces (see Figure 4). The main peak observed at 530.0 eV corresponds to oxygen atoms bonded to Fe [5, 31, 33]. The second small component at
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531.2 eV is difficult to determine. It is typically related to OH- groups [5, 31] due to
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reaction with spurious water in the chamber atmosphere. However, this one can be also
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associated with defective oxygen (oxygen atoms associated with defects) and adsorbed
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molecular oxygen as well [34].
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Figure 4
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The peak intensities resulting from the deconvolution procedure are used to calculate the
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oxide layer thickness using the equation [31]
nox.ox. , Iox is the spectral intensity of the Fe 2p from the oxide, IFe is the n Fe Fe
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where K
(1)
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I ox. d 1 K exp I Fe cos ox.
spectral intensity of the Fe 2p from the substrate, and ox is the electron attenuation length of the Fe 2p photoelectrons for the oxide, which is assumed to be the same for the Fe substrate in this study. is the take off angle, measured with respect to the surface normal ( = 90 - ), d is the thickness of the native thin film, K represents the normalization parameter, nox the atomic density of the film, and nFe the atomic density of the substrate.
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ACCEPTED MANUSCRIPT Table 2 presents the estimated thickness of the oxide layer grown on each surface. The calculated values also demonstrate the remarkable oxidation of the bcc Fe (110).
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Table 2
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The Fromhold-Cook (FC) theory for initial oxidation considers the influence of electronic
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structure in the process [1, 15]. According to this theory, the electrons from the metal are captured by oxygen molecules that dissociate in order to bind to surface sites. Once the
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surface is covered by oxygen, an electron current must tunnel the initial oxide formed on
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the surface in order to maintain the oxygen ionization [1, 15, 18]. The results suggest that
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the electron’s availability is higher for the bcc Fe(110) film.
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Figure 5 presents O2- adsorption curves, obtained by photoemission spectroscopy, during the exposure of the fcc Fe and bcc Fe films. These adsorption curves are typical of the
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oxide growth on metal at low temperatures, where the diffusion of the ions through the
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oxide film formed under the electro(chemical) potential became low and the rate of electron transport by thermal emission is zero. According to FC theory, at the beginning, a relatively
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fast linear growth is observed, which corresponds to the chemisorption stage on a clean surface. During this period, the molecular oxygen moves on the surface and it is dissociated into ions. This process evolves into the formation of an initial oxide layer. The high initial oxidation rates are induced by the presence of an electrostatic potential through the oxide film grown.
Figure 5
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The potential is the result of the equilibrium setup between the electronic states of the metal and the acceptor levels provided by oxygen molecules, atoms and ions adsorbed on the oxide surface. This potential is maintained by the tunneling current across the film that
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keeps the oxide/gas interface negative and oxide/metal interface positive. While the
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tunneling current decreases exponentially with the increase of the oxide film thickness, a
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significant drop in the oxide film growth rate occurs. After that, the oxidation process changes, which, basically, depends on two mechanisms: the slow diffusion of the ions,
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which could be modified by the increase of the temperature, and the electron current
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through the oxide layer for the oxygen reactions, such as:
½ O2 (gas) + 2e- (metal) O2- (surface)
(2)
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or
(3)
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½ O2 (gas) + e- (metal) O- (surface)
where molecular oxygen from the gas phase could be dissociated by the electrons from the
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metal surface in order to form different oxygen valence states as O2- and O- [2]. In our case,
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the O2- adsorption curves reveal that the bcc Fe(110) and fcc Fe(100) surfaces provide different electronic conditions for the initial growth of oxide layer. Based on FC theory, the
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results indicate that the high electron availability of bcc film produces a faster initial oxidation.
The deconvolution procedure applied to the Fe 2p3/2 high-resolution spectra results in the kinetic oxidation curves shown in Figure 6, where the intensity of the iron oxidation states are presented as a function of the oxygen dose. A comparative analysis of these data also
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ACCEPTED MANUSCRIPT demonstrates that the oxides formed on bcc Fe(110) and fcc Fe(100) surfaces are distinct. The corresponding Fe kinetic oxidation on the fcc Fe(100) surface is slower than that presented by bcc Fe(110) surface for the same exposure. As presented in Figure 6(a), a decrease in Fe0 was observed on the surface at the initial exposure time and the appearance
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of Fe2+ and Fe3+, whose intensities gradually increased during exposure. The Fe2+ and Fe3+
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species are probably associated with the Fe1-xO formation. During the oxidation process,
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when the formation of Fe1-xO is significant, the Fe2+ formation rate is higher than the Fe3+ one. The formation of a thinner oxide layer, as presented in Table 2, allows the lower
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tunneling current and the gradual growth of the oxide formed on the fcc Fe(100) until 50 L
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of exposure.
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In contrast, the bcc Fe(110) surface shows a fast decrease in the Fe0 component replaced by
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an increase in the Fe2+ oxidation state, Figure 6(b). This indicates the formation of an initial Fe1-xO layer. After a dose of 10L, the Fe2+ amount remains almost constant and Fe3+
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appears and starts to increase. Initially, the formed Fe3+ is associated with the Fe1-xO
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structure. The additional Fe3+ increase is observed while Fe2+ is constant, this excessive Fe3+ is related to the decrease of the initial oxide layer growth as well the formation of
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more oxidized compounds.
The surface change could be attributed to a surface reaction with oxygen, forming a Fe3+ oxide rich layer created by the increase of the oxygen concentration due to the higher exposure time. Theoretically, the followed reactions are described below [9]
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ACCEPTED MANUSCRIPT Fe + ½ O2 FeO
(4)
3FeO + ½ O2 Fe3O4
(5)
2Fe3O4 + ½ O2 3Fe2O3.
(6)
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The presented results indicate that the regime observed in the initial growth on the surfaces
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is associated with different mechanisms. On the fcc Fe(100) surface, the almost saturation
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regime is reached quickly while the Fe1-xO is formed. This result agrees with Zaman et al. data that have observed an oxygen layer on the fcc Fe (100) surface. This layer should be
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able to block the adsorption and dissociation of CO in oxidizing atmosphere [35].
Under FC theory point of view, it seems that the availability of electrons from the metal to
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reactions should have limited the growth on the fcc surface due to the electronic metal
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characteristics. For the bcc Fe(110) surface, the saturation regime is observed when the trivalent iron concentration increases. This could be related to different mechanisms. The
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first one is associated to the thicker oxide film, which is a barrier that decreases the
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tunneling current for oxidation process; and the second one is the formation of a Fe3+-rich
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oxide that reduces Fe cations diffusion in the oxide film [14-15].
Figure 6
4. Conclusions
The initial oxidation of Fe films formed on Cu(100) was investigated by real time photoemission measurements in the TEMPO beamline at the SOLEIL Synchroton. The
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ACCEPTED MANUSCRIPT experimental correlations between the surface structures and the initial kinetics of oxidation were demonstrated. At low O2 exposure, the two surfaces were covered with different oxide layers, i.e. with distinct thicknesses, composition and the growth mechanisms. The oxidation process of the fcc Fe(100) surface presented a slower kinetic than bcc Fe(110)
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surface. The oxygen adsorption curve presented by the fcc Fe(100) surface showed two
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distinct stages: an initial stage for lower coverage with faster kinetic and a final slow linear
xO
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stage. On the other hand, the bcc Fe(110) surface indicated the growth of two oxides, Fe1and Fe3+-rich oxide. The oxygen adsorption curve presented an initial faster stage, but
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with higher coverage and a final constant stage corresponding to the oxygen saturation. The
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role of the Fe3+-rich layer in the saturation stage for bcc Fe(110) is clear, because it
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decreases the oxide layer growth.
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Acknowledgements
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The authors are grateful to Brazilian agencies CNPq, FAPEMIG, and CNEN for financial
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support and Synchrotron SOLEIL for the use of facilities.
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Figure Captions
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Figure 1 – LEED patterns of (a) Cu(100) surface, (b) 20 Å of Fe on Cu(100) and (c) and 56
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Å of Fe on Cu(100). Both LEED patterns were taken at 122 eV. The similar LEED patterns
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for the Cu (a) and Fe (b) confirm the epitaxial growth of the fcc Fe for low coverage. The
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particular superstructures on the LEED pattern (c) reveal a bcc Fe(110) on Cu(100) [25].
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Figure 2 – PE widescans: (a) fcc Fe(100) and (b) bcc Fe(110) as deposited on Cu(100); (c) fcc Fe(100) and (d) bcc Fe(110) after O2 exposure (50 L). Fe 3s, Fe 3p, Fe 2p, and O 1s
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photoemission peaks and O(KLL) and Fe(LMM) Auger lines were identified.
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Figure 3 – High-resolution Fe 2p spectra from (a) fcc Fe(100) and (b) bcc Fe(110) as deposited; (c) fcc Fe(100) and (d) bcc Fe(110) after O2 exposure (50 L). The guidelines
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show the changing of Fe 2p binding energy. The Fe 2p3/2 peak (experimental and curve
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fitting) are presented on the right side (e, f) with fitting components: Fe0, Fe2+ and Fe3+.
Figure 4 – High-resolution O 1s spectra from fcc Fe(100) (a) and bcc Fe(110) (b) surface after O2 exposure (50 L). The inset (c) shows the O 1s bcc Fe(110) peak fitting with O-2 and O- components.
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Figure 6 – Evolution of Fe 2p3/2 peak components during the oxygen exposures for (a) fcc
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ACCEPTED MANUSCRIPT Table 1
FWHM
%
Fe0
706.3
1.3
18.1
Fe2+
709.3
3.3
58.5
Fe3+
711.2
3.2
23.4
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FWHM
%
706.5
1.3
78.7
Fe2+
709.2
3.2
16.2
Fe3+
711.2
3.2
5.2
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Fe0
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B.E. (eV)
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fcc Fe(100)
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B.E. (eV)
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B.E. - Binding Energy
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FWHM - Full Width Half- Maximum
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bcc Fe(110)
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Exposure
Oxide thickness
fcc Fe(100)
50 L
2.9 Å
bcc Fe(110)
50 L
15.3 Å
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ACCEPTED MANUSCRIPT HIGHLIGHTS:
Initial oxidation of Fe films are studied by real time photoemission measurements
fcc Fe (100) surface presented slower initial oxidation than bcc Fe (110) surface
Analysis showed the formation of Fe1-xO on fcc Fe (100) and bcc Fe (110) surfaces
The oxidation of bcc Fe surface is related to the formation of Fe 3+ rich layer
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Renato de Mendonça, Maximiliano D. Martins, Mathieu Silly, Fausto Sirotti, Waldemar A.A. Macedo PII: DOI: Reference:
S0040-6090(17)30484-4 doi: 10.1016/j.tsf.2017.06.049 TSF 36056
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
31 January 2017 14 June 2017 25 June 2017
Please cite this article as: Renato de Mendonça, Maximiliano D. Martins, Mathieu Silly, Fausto Sirotti, Waldemar A.A. Macedo , A photoemission spectroscopy study of the initial oxidation of epitaxial fcc and bcc Fe films grown on Cu(100), Thin Solid Films (2017), doi: 10.1016/j.tsf.2017.06.049
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ACCEPTED MANUSCRIPT A photoemission spectroscopy study of the initial oxidation of epitaxial fcc and bcc Fe films grown on Cu(100) Renato de Mendonçaa,b*, Maximiliano D. Martinsa, Mathieu Sillyc, Fausto Sirottic,
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Centro de Desenvolvimento da Tecnologia Nuclear – CDTN, Belo Horizonte 31270-901,
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a
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Waldemar A. A. Macedoa
REDEMAT, Escola de Minas, Universidade Federal de Ouro Preto – UFOP, 35400-000,
Ouro Preto, MG, Brazil. c
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b
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MG, Brazil.
TEMPO Beamline, Synchroton SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48,
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*Corresponding author
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91192 Gif-sur-Yvette Cedex – France
E-mail address: [email protected]
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Phone: +55 31 30693210
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ACCEPTED MANUSCRIPT A photoemission spectroscopy study of the initial oxidation of epitaxial fcc and bcc Fe films grown on Cu(100)
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Abstract
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The initial oxidation of Fe is a fundamental issue due to its importance for corrosion and
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catalysis. In this study, we investigate the initial oxidation of epitaxial fcc and bcc Fe thin films grown on a Cu(100) substrate at room temperature. Fe thin films deposited with two
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different thicknesses on Cu(100) were prepared by molecular beam epitaxy in order to
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obtain the fcc Fe(100) and bcc Fe(110) surfaces. The structures and the chemical composition of the Fe films were evaluated by LEED and photoelectron spectroscopy,
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using the facilities at the TEMPO beamline of the SOLEIL Synchrotron. The Fe 2p and O
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1s high-resolution spectra were collected during oxidation at a constant O2 pressure in order to build the kinetic curves, which are discussed taking into account the Fromhold-Cook
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theory. Our result revealed the different initial oxidation kinetics of the films and showed
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that bcc Fe(110) surface has a much higher reactivity with O2 than the fcc Fe(100). The Fe 2p peak analysis suggests the formation of Fe1-xO and an Fe3+-rich oxide on the bcc
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Fe(110) surface and Fe1-xO on the fcc Fe(100) surface.
Keywords: Fe oxidation, Fe/Cu(100), fcc Fe(100), bcc Fe(110), Photoelectron Spectroscopy
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ACCEPTED MANUSCRIPT 1. Introduction
The initial oxidation of metals has been continuously revisited due to new tools that have deepened our knowledge in this fundamental area for corrosion and catalysis [1-10]. The
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initial oxidation process can be remarkably affected by the electronic and structural
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properties of the metal’s surface [1, 2]. Parameters such as the bonding energy of the atoms
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on the surface, the availability of possible bonds, the relaxation effects of the surface, the concentration of defects, and the atomic density are notable issues that can play an
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important role in this phenomenon [1-12].
In this context, iron is one of the most complex systems for surface oxidation studies,
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basically because different oxides (Fe1-xO, Fe3O4, -Fe2O3 and -Fe2O3) can be formed due
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to several thermodynamic conditions and surface structures [2, 5, 9, 11, 12]. The initial oxide formed at room temperature is often Fe1-xO, which is not appropriate for the
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passivation because it has a relative high concentration of defects [12-19]. This oxide has
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the same crystalline structure of NiO and CoO, although it is highly deficient in cations and therefore rich in vacancies [2]. In this structure, each vacancy provides an available site for
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the diffusion of Fe2+, and consequently, for relatively quick oxide growth.
What concerns the initial oxidation of iron, an additional understanding of its behavior on the different surfaces can be useful to design higher performance materials. In addition, the large number of parameters considered in the oxidation process [14, 16, 18-22] has made it more difficult to provide a clear understanding of the resulting compounds formed. Aiming
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ACCEPTED MANUSCRIPT to extend our knowledge on this issue, we studied the effect of the O2 exposure on Fe films surfaces deposited on Cu(100). The Fe films were prepared with two specific thicknesses in order to obtain different structures at the surface. Experimentally, Fe on Cu(100) is the typical system chosen to stabilize fcc Fe as epitaxial ultrathin films at room temperature.
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Due to the small lattice misfit between Cu (3.615 Å at 20ºC) and fcc Fe (lattice parameter
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3.59 Å at 20ºC, as extrapolated from the high temperature -Fe phase), the epitaxy of fcc Fe
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on Cu is favored. In fact, this system was extensively studied and, for thermally deposited
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films, Fe grows epitaxialy on Cu(100) presenting a fcc phase below a critical thickness of 10-12 atomic monolayers (ML), above which the structure relaxes into the bcc Fe(110)
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phase [23-25].
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Here, the oxidation process of the Fe films was studied by real time photoemission
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spectroscopy using the facilities of the TEMPO beamline at SOLEIL Synchroton [26]. Our results showed that, compared to the bcc-Fe film, the fcc-Fe film is considerably more
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resistant to the initial oxidation. Furthermore, the analysis of the Fe 2p photoemission peak
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indicates the formation of a Fe1-xO layer in both surfaces and suggests an additional growth
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of a Fe3+-rich layer on the bcc-Fe surface film.
2. Experimental
The Cu(100) surface was prepared by Ar ion sputtering and annealing (773 K for 10 minutes) cycles in an ultrahigh vacuum chamber (base pressure of 3 × 10-10 Torr). The well-ordered surface was evaluated by Low Energy Electron Diffraction (LEED), see
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ACCEPTED MANUSCRIPT Figure 1 (a). The cleanliness of the surface after the sputtering was checked by photoemission spectroscopy.
The Fe films were deposited at room temperature (RT) by means of an electron beam
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evaporator. The Fe deposition rate was estimated with a quartz microbalance and fixed at
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0.8 Å/min. Films with two thicknesses were deposited, namely 20 Å and 56 Å thick
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(around 11 and 28 monolayers, respectively). The LEED patterns obtained, shown in Figure 1(b) and 1(c), correspond to a fcc Fe(100) and a bcc Fe(110) surfaces, as already
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reported in the literature [23, 27]. The absence of spurious elements was confirmed by
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photoemission spectroscopy.
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Figure 1
Systematic surface exposure of the surfaces to oxygen (99.999%) was carried out at 5.7 ×
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10-9 Torr and room temperature. The total O2 exposure was 50 Langmuir (1 L = 1 × 10-6
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Torr·s). The kinetic curves were obtained by analyzing the high-resolution Fe 2p and O 1s photoemission (PE) peaks collected every 18 seconds during O2 exposure. The spectra were
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fitted using the Spectral Data Processor (SDP) software [XPS International, Mountain View, USA, www.xpsdata.com] with Voigt (Gaussian + Lorentzian) curves and Shirley background subtraction. In particular, Fe 2p3/2 peaks were fitted with three components Fe0, Fe2+ and Fe3+. Due to the complex Fe 2p3/2 structure for Fe2+ and Fe3+ components, these ones were fitted with broader peaks as pointed by Bhargava [5] and Aronniemi [28].
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ACCEPTED MANUSCRIPT Photoelectron spectroscopy experiments were carry out at TEMPO beamline (SOLEIL Synchrotron), using a SCIENTA SES2002 electron energy analyzer equipped with a 2D line detector for time-resolved experiments [29]. Photoemission spectra were obtained at
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normal electron emission using synchrotron radiation of 840 eV.
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3. Results and Discussion
The widescan PE spectra collected on the fcc Fe(100) and bcc Fe(110) surfaces before and
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after the O2 exposure are presented in Figure 2. The chemical composition of the surfaces
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after Fe film deposition reveals no significant oxygen and carbon contamination. The lines corresponding to the Fe 2p, Fe 3s and Fe 3p, and O 1s photoemission peaks along with the
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O and Fe Auger lines were identified, as shown in Figure 2(d). The higher O 1s intensity
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observed in the wide spectrum for the bcc Fe(110) surface already shows the higher oxygen
Figure 2
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amount measured on this surface after oxygen exposure (Figure 2(d)).
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High-resolution Fe 2p spectra are presented in Figure 3. The binding energy (B.E.) of the Fe 2p3/2 peaks are centered at 706.5 eV and 706.4 eV for the fcc Fe(100) and bcc Fe(110) surfaces, respectively. These values are in agreement with the metallic Fe reported in the literature [5]. The Fe 2p spectrum for the fcc Fe(100) surface, after exposure, shows a small component around the main peak that is indicative of oxide formation (Figure 3(c)). A more significant oxide component is observed on the bcc Fe(110) surface (Figure 3(d)). The Fe 2p3/2 peak exhibits different components after exposure. The main oxide component 6
ACCEPTED MANUSCRIPT measured for the oxidized fcc Fe(100) surface is situated at 709.5 eV, whereas the one present in the bcc Fe(110) surface is located at 710.1 eV. This suggests that different kinds of oxides are formed on the Fe film surfaces. Fe 2p3/2 binding energies of the main oxides formed on the Fe surfaces are already attested in the literature [5, 30-33]. The peak situated
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at 709.0 eV typically indicates the formation of Fe1-xO, which is in agreement with our fcc
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Fe(100) data. For the Fe2O3 and Fe3O4 compounds, no significant difference is observed at
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Figure 3
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the Fe 2p3/2 binding energies, which are situated between 710.6 and 711.2 eV [33].
To clear up the oxidation process of the Fe on the surfaces, we de-convoluted the Fe 2p3/2
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spectra measured during the oxygen exposure. Full Width Half Maximum (FWHM) and
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binding energies of the peaks were fixed during the fitting procedure. As presented in Figure 3(e, f), the Fe 2p3/2 signal was decomposed into three components: a first
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contribution at 706.3 eV, another at 709.2 eV, and a third one at 711.2 eV, corresponding to
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Fe0, Fe2+, and Fe3+, respectively [30-33]. Table 1 presents the fitting parameters (B.E., FWHM and peak area) after 50 L of O2 exposure. Different ratios of Fe2+/Fe3+ are observed
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on the surfaces after exposure: 3.1 and 2.5 for the fcc Fe(100) and bcc Fe(110) surfaces, respectively. Due to the B.E. values and high Fe2+ concentration observed on the surfaces, it can be pointed out that Fe1-xO grows on both surfaces. In addition, after exposure, the bcc Fe(110) surface presents a higher concentration of Fe3+, which indicates the formation of Fe3+-rich oxide.
Table 1 7
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The analysis of the O 1s high-resolution spectra also give useful information about the oxides formed on the surfaces (see Figure 4). The main peak observed at 530.0 eV corresponds to oxygen atoms bonded to Fe [5, 31, 33]. The second small component at
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531.2 eV is difficult to determine. It is typically related to OH- groups [5, 31] due to
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reaction with spurious water in the chamber atmosphere. However, this one can be also
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associated with defective oxygen (oxygen atoms associated with defects) and adsorbed
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molecular oxygen as well [34].
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Figure 4
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The peak intensities resulting from the deconvolution procedure are used to calculate the
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oxide layer thickness using the equation [31]
nox.ox. , Iox is the spectral intensity of the Fe 2p from the oxide, IFe is the n Fe Fe
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where K
(1)
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I ox. d 1 K exp I Fe cos ox.
spectral intensity of the Fe 2p from the substrate, and ox is the electron attenuation length of the Fe 2p photoelectrons for the oxide, which is assumed to be the same for the Fe substrate in this study. is the take off angle, measured with respect to the surface normal ( = 90 - ), d is the thickness of the native thin film, K represents the normalization parameter, nox the atomic density of the film, and nFe the atomic density of the substrate.
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ACCEPTED MANUSCRIPT Table 2 presents the estimated thickness of the oxide layer grown on each surface. The calculated values also demonstrate the remarkable oxidation of the bcc Fe (110).
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Table 2
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The Fromhold-Cook (FC) theory for initial oxidation considers the influence of electronic
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structure in the process [1, 15]. According to this theory, the electrons from the metal are captured by oxygen molecules that dissociate in order to bind to surface sites. Once the
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surface is covered by oxygen, an electron current must tunnel the initial oxide formed on
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the surface in order to maintain the oxygen ionization [1, 15, 18]. The results suggest that
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the electron’s availability is higher for the bcc Fe(110) film.
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Figure 5 presents O2- adsorption curves, obtained by photoemission spectroscopy, during the exposure of the fcc Fe and bcc Fe films. These adsorption curves are typical of the
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oxide growth on metal at low temperatures, where the diffusion of the ions through the
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oxide film formed under the electro(chemical) potential became low and the rate of electron transport by thermal emission is zero. According to FC theory, at the beginning, a relatively
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fast linear growth is observed, which corresponds to the chemisorption stage on a clean surface. During this period, the molecular oxygen moves on the surface and it is dissociated into ions. This process evolves into the formation of an initial oxide layer. The high initial oxidation rates are induced by the presence of an electrostatic potential through the oxide film grown.
Figure 5
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The potential is the result of the equilibrium setup between the electronic states of the metal and the acceptor levels provided by oxygen molecules, atoms and ions adsorbed on the oxide surface. This potential is maintained by the tunneling current across the film that
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keeps the oxide/gas interface negative and oxide/metal interface positive. While the
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tunneling current decreases exponentially with the increase of the oxide film thickness, a
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significant drop in the oxide film growth rate occurs. After that, the oxidation process changes, which, basically, depends on two mechanisms: the slow diffusion of the ions,
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which could be modified by the increase of the temperature, and the electron current
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through the oxide layer for the oxygen reactions, such as:
½ O2 (gas) + 2e- (metal) O2- (surface)
(2)
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or
(3)
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½ O2 (gas) + e- (metal) O- (surface)
where molecular oxygen from the gas phase could be dissociated by the electrons from the
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metal surface in order to form different oxygen valence states as O2- and O- [2]. In our case,
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the O2- adsorption curves reveal that the bcc Fe(110) and fcc Fe(100) surfaces provide different electronic conditions for the initial growth of oxide layer. Based on FC theory, the
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results indicate that the high electron availability of bcc film produces a faster initial oxidation.
The deconvolution procedure applied to the Fe 2p3/2 high-resolution spectra results in the kinetic oxidation curves shown in Figure 6, where the intensity of the iron oxidation states are presented as a function of the oxygen dose. A comparative analysis of these data also
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ACCEPTED MANUSCRIPT demonstrates that the oxides formed on bcc Fe(110) and fcc Fe(100) surfaces are distinct. The corresponding Fe kinetic oxidation on the fcc Fe(100) surface is slower than that presented by bcc Fe(110) surface for the same exposure. As presented in Figure 6(a), a decrease in Fe0 was observed on the surface at the initial exposure time and the appearance
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of Fe2+ and Fe3+, whose intensities gradually increased during exposure. The Fe2+ and Fe3+
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species are probably associated with the Fe1-xO formation. During the oxidation process,
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when the formation of Fe1-xO is significant, the Fe2+ formation rate is higher than the Fe3+ one. The formation of a thinner oxide layer, as presented in Table 2, allows the lower
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tunneling current and the gradual growth of the oxide formed on the fcc Fe(100) until 50 L
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of exposure.
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In contrast, the bcc Fe(110) surface shows a fast decrease in the Fe0 component replaced by
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an increase in the Fe2+ oxidation state, Figure 6(b). This indicates the formation of an initial Fe1-xO layer. After a dose of 10L, the Fe2+ amount remains almost constant and Fe3+
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appears and starts to increase. Initially, the formed Fe3+ is associated with the Fe1-xO
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structure. The additional Fe3+ increase is observed while Fe2+ is constant, this excessive Fe3+ is related to the decrease of the initial oxide layer growth as well the formation of
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more oxidized compounds.
The surface change could be attributed to a surface reaction with oxygen, forming a Fe3+ oxide rich layer created by the increase of the oxygen concentration due to the higher exposure time. Theoretically, the followed reactions are described below [9]
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ACCEPTED MANUSCRIPT Fe + ½ O2 FeO
(4)
3FeO + ½ O2 Fe3O4
(5)
2Fe3O4 + ½ O2 3Fe2O3.
(6)
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The presented results indicate that the regime observed in the initial growth on the surfaces
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is associated with different mechanisms. On the fcc Fe(100) surface, the almost saturation
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regime is reached quickly while the Fe1-xO is formed. This result agrees with Zaman et al. data that have observed an oxygen layer on the fcc Fe (100) surface. This layer should be
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able to block the adsorption and dissociation of CO in oxidizing atmosphere [35].
Under FC theory point of view, it seems that the availability of electrons from the metal to
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reactions should have limited the growth on the fcc surface due to the electronic metal
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characteristics. For the bcc Fe(110) surface, the saturation regime is observed when the trivalent iron concentration increases. This could be related to different mechanisms. The
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first one is associated to the thicker oxide film, which is a barrier that decreases the
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tunneling current for oxidation process; and the second one is the formation of a Fe3+-rich
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oxide that reduces Fe cations diffusion in the oxide film [14-15].
Figure 6
4. Conclusions
The initial oxidation of Fe films formed on Cu(100) was investigated by real time photoemission measurements in the TEMPO beamline at the SOLEIL Synchroton. The
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ACCEPTED MANUSCRIPT experimental correlations between the surface structures and the initial kinetics of oxidation were demonstrated. At low O2 exposure, the two surfaces were covered with different oxide layers, i.e. with distinct thicknesses, composition and the growth mechanisms. The oxidation process of the fcc Fe(100) surface presented a slower kinetic than bcc Fe(110)
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surface. The oxygen adsorption curve presented by the fcc Fe(100) surface showed two
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distinct stages: an initial stage for lower coverage with faster kinetic and a final slow linear
xO
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stage. On the other hand, the bcc Fe(110) surface indicated the growth of two oxides, Fe1and Fe3+-rich oxide. The oxygen adsorption curve presented an initial faster stage, but
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with higher coverage and a final constant stage corresponding to the oxygen saturation. The
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role of the Fe3+-rich layer in the saturation stage for bcc Fe(110) is clear, because it
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decreases the oxide layer growth.
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Acknowledgements
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The authors are grateful to Brazilian agencies CNPq, FAPEMIG, and CNEN for financial
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support and Synchrotron SOLEIL for the use of facilities.
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[26] F. Polack, M. Silly, C. Chauvet, B. Agarde, N. Bergeard, M. Izquierdo, O. Chubar, D. Krizmancic, M. Ribbens, J.-P. Duval, C. Basset, S. Kubsky, F. Sirotti, F. TEMPO: a New Insertion Device Beamline at SOLEIL for Time Resolved Photoelectron Spectroscopy Experiments on Solids and Interfaces, AIP Conference Procedure 1234 (2010) 185-188.
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ACCEPTED MANUSCRIPT [27] P. S. Schmailzl, K. Schmidt, P. Bayer, R. Döll, K. Heinz, The structure of thin epitaxial Fe films on Cu (100) in the transition range fcc→bcc, Surf. Sci. 312 (1994) 73-81. [28] M. Aronniemi, J. Sainio, J. Lahtinen, Chemical state quantification of iron and
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chromium oxides using XPS: the effect of the background subtraction method, Surf.
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[29] N. Bergeard, M. G. Silly, D. Krizmancic, C. Chauvet, M Guzzo, J. P. Ricaud, M.
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Izquierdo, L. Stebel, P. Pittana, R. Sergo, G. Cautero, G. Dufour, F. Rochet, F. Sirotti, Time-resolved photoelectron spectroscopy using synchrotron radiation time structure,
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J. Synchrotron Radiat. 18 (2011) 245-250.
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[30] T. Yamashita, P. Hayes, Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide
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materials, Appl. Surf. Sci. 254 (2008) 2441-2449. [31] T.-C. Lin, G. Seshadri, J. Kelber, A consistent method for quantitative XPS peak
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analysis of thin oxide films on the clean polycrystalline iron surfaces, Appl. Surf. Sci.
[32] C. Donik, A. Kocijan, D. Mandrino, I. Paulin, M. Jenko, B. Pihlar, Initial oxidation of
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duplex stainless steel, Appl. Surf. Sci. 255 (2009) 7056-7061. [33] T. Yamashita, P. Hayes, Effect of curve fitting on quantitative analysis of Fe0.94O and Fe2O3 using XPS, J. Electron Spectrosc. 152 (2006) 6-11. [34] B. P. Payne, M. C. Biesinger, N.S. McIntyre, The study of polycrystalline nickel metal oxidation by water vapour, J. Electron Spectrosc. 175 (2009) 55-65.
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ACCEPTED MANUSCRIPT [35] S. S. Zaman, H. Omer, J. Jonner, Z. Novotný, A. Buchsbaum, M. Schmid and P.
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Figure Captions
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Figure 1 – LEED patterns of (a) Cu(100) surface, (b) 20 Å of Fe on Cu(100) and (c) and 56
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Å of Fe on Cu(100). Both LEED patterns were taken at 122 eV. The similar LEED patterns
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for the Cu (a) and Fe (b) confirm the epitaxial growth of the fcc Fe for low coverage. The
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particular superstructures on the LEED pattern (c) reveal a bcc Fe(110) on Cu(100) [25].
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Figure 2 – PE widescans: (a) fcc Fe(100) and (b) bcc Fe(110) as deposited on Cu(100); (c) fcc Fe(100) and (d) bcc Fe(110) after O2 exposure (50 L). Fe 3s, Fe 3p, Fe 2p, and O 1s
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photoemission peaks and O(KLL) and Fe(LMM) Auger lines were identified.
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Figure 3 – High-resolution Fe 2p spectra from (a) fcc Fe(100) and (b) bcc Fe(110) as deposited; (c) fcc Fe(100) and (d) bcc Fe(110) after O2 exposure (50 L). The guidelines
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show the changing of Fe 2p binding energy. The Fe 2p3/2 peak (experimental and curve
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fitting) are presented on the right side (e, f) with fitting components: Fe0, Fe2+ and Fe3+.
Figure 4 – High-resolution O 1s spectra from fcc Fe(100) (a) and bcc Fe(110) (b) surface after O2 exposure (50 L). The inset (c) shows the O 1s bcc Fe(110) peak fitting with O-2 and O- components.
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ACCEPTED MANUSCRIPT Figure 5 – Oxygen adsorption curves obtained by photoemission spectroscopy (O1s peak intensity), during the O2 exposure of the (a) fcc Fe(100) and (b) bcc Fe(110) surfaces.
Figure 6 – Evolution of Fe 2p3/2 peak components during the oxygen exposures for (a) fcc
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Fe(100) and (b) bcc Fe(100) surfaces.
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FWHM
%
Fe0
706.3
1.3
18.1
Fe2+
709.3
3.3
58.5
Fe3+
711.2
3.2
23.4
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FWHM
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706.5
1.3
78.7
Fe2+
709.2
3.2
16.2
Fe3+
711.2
3.2
5.2
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Fe0
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B.E. (eV)
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fcc Fe(100)
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B.E. (eV)
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B.E. - Binding Energy
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FWHM - Full Width Half- Maximum
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bcc Fe(110)
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Exposure
Oxide thickness
fcc Fe(100)
50 L
2.9 Å
bcc Fe(110)
50 L
15.3 Å
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 5
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Fig. 6
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ACCEPTED MANUSCRIPT HIGHLIGHTS:
Initial oxidation of Fe films are studied by real time photoemission measurements
fcc Fe (100) surface presented slower initial oxidation than bcc Fe (110) surface
Analysis showed the formation of Fe1-xO on fcc Fe (100) and bcc Fe (110) surfaces
The oxidation of bcc Fe surface is related to the formation of Fe 3+ rich layer
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