Article pubs.acs.org/JPCC
Oxygen Adsorption on the Fe(110) Surface: The Old System − New Structures † ́ Kinga Freindl,*,† Tomasz Ossowski,‡ Marcin Zając,§,∥,⊥ Nika Spiridis,† Dorota Wilgocka-Slęzak, † † ‡ †,∥ Ewa Madej, Tomasz Giela, Adam Kiejna, and Józef Korecki †
Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek 8, 30-239 Kraków, Poland Institute of Experimental Physics, University of Wrocław, Plac M. Borna 9, 50-204 Wrocław, Poland § European Synchrotron Radiation Facility (ESRF), P.O. Box 220, F-38043 Grenoble, France ∥ Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Kraków, Poland ‡
S Supporting Information *
ABSTRACT: Adsorption of oxygen on the (110) surface of epitaxial iron films on tungsten (110) was studied using lowenergy electron diffraction (LEED), low-energy electron microscopy (LEEM), and Auger electron spectroscopy within an exposure range of 0−300 Langmuir (L). Selected oxygen adsorption structures on Fe(110) reported in the literature were critically compared and revised in reference to the present study. The initial adsorption of 1/4 oxygen monolayer resulting in the commonly observed (2 × 2) structure was followed by a structure that was frequently termed as (3 × 1). Its complex LEED pattern was ultimately resolved and interpreted as originating from two structural domains of a large oblique unit cell (eight times larger than the substrate unit cell) and 3/8 oxygen coverage. A new (3 × 2) structure was identified at a coverage of 2/3. The domain interpretation of last two structures was verified by LEEM and confirmed by density functional theory calculations. The onset of oxygen−iron bonding formation was recognized by the change in the symmetry of the LEED pattern and the shape of the iron AES signal. Finally, the formation of an iron oxide FeO(111) monolayer was evidenced at the oxygen exposure of ∼300 L.
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exposure range between 0.5 Langmuir (L)3 and 3.5 L.7 The (2 × 2) superstructure corresponds to 0.25 coverage. The next ordered structure that appears with increasing exposure is, in our opinion, not fully recognized. It is referenced in the literature as (3 × 1),9 c(3 × 1),3,4,6 or “split (3 × 1)”3,4 and was observed by different authors in a broad exposure range (from 33 to 60 L14). The coverage corresponding to these structures is usually presumed to be approximately 0.33 to 0.4.7 A comparison of the literature data3,15 indicates that the structure termed as (3 × 1) or similar can be in fact much more complex and has never been exactly resolved. With further exposure resulting in coverage above 0.4, no ordered structure was reported7 until a diffused hexagonal LEED pattern attributed to the onset of oxide formation3,4,6 was observed. Depending on the source, the formation of an FeO(111) layer is reported to start from 10 L8 to 30 L.16 Further oxidation (which is beyond the scope of the present work), including oxygen penetration below the surface, requires
INTRODUCTION Iron and its oxides have been intensively investigated both in the past and recently. Originally, oxygen adsorption studies were motivated by their importance for catalysis (e.g., industrial processes such as Haber−Bosch and Fischer−Tropsch reactions1). Recently, the (1 × 1) oxygen structure on Fe(001) has become interesting for spin-polarized applications.2 Special attention has been paid to the Fe(110) surface because it is the closest packed among the low indexed surfaces. This stable and reproducible surface is suitable for model investigations and has been exploited in numerous experimental3−10 and theoretical adsorption studies.11−13 Despite the long history, the broad literature data are fragmentary or even contradictory regarding some points. The present paper sheds new light on the oxygen adsorption on Fe(110) by giving a novel explanation of a structure observed for over 40 years and reporting a new one not observed before. Adsorption of molecular oxygen at room temperature was intensively studied in the past using low-energy electron diffraction (LEED). At the initial adsorption stage, a (2 × 2) superstructure (relative to the primitive rhombic unit cell of the Fe(110) substrate) is reported in a relatively narrow oxygen © 2016 American Chemical Society
Received: November 15, 2015 Revised: December 30, 2015 Published: January 24, 2016 3807
DOI: 10.1021/acs.jpcc.5b11177 J. Phys. Chem. C 2016, 120, 3807−3813
Article
The Journal of Physical Chemistry C
and 2/3, respectively. For these two structures, we proposed atomistic models supported by DFT calculations. Further stages of adsorption are characterized by changes in the symmetry of LEED patterns and the shape of iron AES signals. These changes are attributed to the onset of an oxygen−iron bond formation, finally leading to the FeO(111)-like structure observed at an oxygen exposure of ∼300 L.
higher exposures or adsorption temperatures, and the stoichiometry and structure of oxide strongly depends of the preparation conditions and spreads from the FeO,7,16−18 through a mixture of FeO and Fe3O4,19,20 to Fe2O3.21 A comment is needed on the oxygen coverage corresponding to the different structures identified mainly by LEED. Different methods of the coverage calibration are used,5,7,15 and one of them is direct comparison of LEED patterns and spectroscopic data.4,6 There is a consensus that molecular oxygen is dissociatively adsorbed from a precursor diffusing on the iron substrate.5,7 In agreement with the previous theoretical calculations11 based on the density functional theory (DFT), most experimental papers have assumed that oxygen adsorbs at the long-bridge sites (lb)5,6 (refer to Figure 1 for surface geometry and adsorption
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EXPERIMENTAL METHODS AND THEORETICAL BASIS The experiment was performed in an ultrahigh-vacuum (UHV) system (base pressure 1 × 10−10 mbar) equipped with molecular beam epitaxy (MBE) deposition system and a LEED/AES spectrometer. For the oxygen adsorption experiment, we used the (110) surface of epitaxial Fe films grown on W(110). The use of the epitaxial Fe films on W(110) facilitates the preparation of a clean surface. The W(110) substrate was prepared through cycles of annealing at 1600 K under an oxygen pressure of 5 × 10−8 mbar, followed by a flash heating at 2100 K under UHV conditions until an atomically clean surface, as confirmed by AES and LEED, was obtained. Iron was deposited by MBE from an effusion cell (BeO crucible). The deposition was monitored with a quartz crystal, which allowed the estimation of the coverage with an accuracy of 0.1 ML. The typical deposition rate was 1 Å/min. By optimized Fe growth, an atomically flat Fe(110) surface was obtained with terraces from several tens to hundreds of nanometers. As shown by the quantitative I(V)-LEED analysis, for the currently used 4 nm thick iron films, a 1%-inward relaxation of the outermost atomic layer was observed as the only difference in the surface structure compared with bulk iron23 On the contrary, it should be noted that in this thickness range the film magnetization points not along the [001] direction, as expected for the bulk crystal, but along [1−10].24 Adsorption experiments were performed at room temperature using molecular oxygen with a purity of 5.0 N that was dosed with a precise leak valve. Depending on the total exposure, a partial pressure between 1 × 10−8 mbar and 1 × 10−7 mbar was applied. To estimate the coverage corresponding to a given exposure, we used the intensity of the KLL oxygen Auger peak at 510 eV, with the (2 × 2) structure and the corresponding 0.25 coverage taken as a reference. The selected structures were analyzed in a LEEM system consisting of an Elmitec LEEM microscope and a preparation chamber equipped with deposition, adsorption, and LEED facilities.
Figure 1. (110) surface of BCC iron. The surface adsorption sites, including the long-bridge (lb), short-bridge (sb), 3-fold hollow (th), and on top (ot) sites are indicated.
sites), although Erley and Ibach5 suggested oxygen adsorption at the 3-fold hollow sites (th) at higher coverages. Recently, the oxygen adsorption on Fe(110) was theoretically revisited,22 and it was concluded that for low coverages the adsorption at lb and th sites is almost degenerated in energy, whereas at higher coverages the th adsorption site is the most favorable. We report the LEED and AES study of oxygen adsorption at room temperature on a (110) surface of epitaxial Fe films on W(110) within the exposure range of 0−300 L. The work is focused on two structures of oxygen: the socalled (3 × 1) and a new structure that has not been reported in the literature, (3 × 2). LEED and low-energy electron microscopy (LEEM) patterns of both structures are shown to originate from two structural domains of a large oblique unit cell, and coverages for these structures are determined as 3/8
Figure 2. LEED patterns for structures of oxygen adsorbed at room temperature on 20 ML Fe(110)/W(110) as a function of increasing exposure at an electron energy 55 eV: (a) (2 × 2) structure at 0.6 L (arrows indicate the unit cell of the (1 × 1) structure), (b) “A” structure at 4.7 L, (c) (3 × 2) structure at 17.2 L, and (d) FeO oxide at 244 L. 3808
DOI: 10.1021/acs.jpcc.5b11177 J. Phys. Chem. C 2016, 120, 3807−3813
Article
The Journal of Physical Chemistry C DFT calculations were performed in the Vienna ab initio simulation package (VASP)25,26 using the generalized gradient approximation and Perdew−Burke−Ernzerhof exchange−correlation functional.27 The O/Fe(110) systems were approximated by a slab of nine Fe atomic layers with oxygen atoms adsorbed on either slab surfaces and a surface supercell corresponding to a symmetry resulting from the LEED experiment. The slabs were separated by a vacuum region of ∼16 Å. The positions of all atoms in the supercells were fully relaxed and optimized until the forces on each atom were
