Structure of Stoichiometric and Oxygen-Rich Ultrathin FeO (111) Films

Article pubs.acs.org/JPCC

Structure of Stoichiometric and Oxygen-Rich Ultrathin FeO(111) Films Grown on Pd(111) Helene Zeuthen,† Wilhelmine Kudernatsch,† Guowen Peng,‡ Lindsay R. Merte,† Luis K. Ono,† Lutz Lammich,† Yunhai Bai,‡ Lars C. Grabow,‡,§ Manos Mavrikakis,‡ Stefan Wendt,*,† and Flemming Besenbacher*,† †

Interdisciplinary Nanoscience Center (iNANO), Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark ‡ Department of Chemical and Biological Engineering, University of Wisconsin-Madison, Madison, Wisconsin, 53706, United States § Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas 77204, United States S Supporting Information *

ABSTRACT: Monolayer thin FeO(111) films were grown on Pd(111) and oxidized by atomic oxygen (O). The stoichiometric and oxidized films were studied in detail by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Compared to the previously studied FeO(111)/Pt(111) system, small structural differences were observed for stoichiometric FeO monolayer films. Upon O exposure, the stoichiometric FeO film reconstructs, leading to the formation of new O-rich structures incorporating increasing amounts of additional O atoms. At low O exposures, the STM images exhibit bright features of regularly sized triangular structures assigned to O-adatom dislocation loops. A model of this O-rich structure composed of four-fold O-coordinated Fe atoms is proposed and confirmed by DFT calculations. Furthermore, these O dislocation loops induce the inversion of the FeO film and enclose portions of the film in which the order of the high-symmetry domains is inverted. For higher O exposures, the formation of FeO2−x islands coexisting with O-adatom dislocations and stoichiometric FeO patches was observed. These FeO2−x islands are reminiscent of the O-rich structures previously reported for FeO supported on Pt(111) and are catalytically active toward CO oxidation.

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However, for oxidation temperatures of ∼1000 K, exclusively the single FeO(111) bilayer with a misfit angle of ∼0.6° was found to grow on Pt(111).4,18,19,25 The hexagonal (1 × 1) unit cell of Pt(111) has a smaller lattice constant (2.77 Å) than the Fe−O bilayer (∼3.1 Å), creating a ∼10% lattice mismatch between the FeO monolayer and the underlying Pt(111) substrate. This mismatch is the origin of a characteristic moiré modulation with a large periodicity of ∼25 Å. In the following discussion, a perfect single FeO(111) bilayer is denoted as an “FeO monolayer”. The moiré pattern gives rise to a modulated, periodic surface potential, providing a templating effect for the self-organization of adsorbates on FeO.4,28−30 Defects containing undercoordinated Fe atoms such as O vacancies and step sites in (sub)monolayer FeO films readily dissociate H2O and alcohols and even oxidize CO under ultrahigh vacuum (UHV).7,25,31−33 Under reaction conditions with CO and O2 pressures in the millibar range and sample temperatures around 450 K, the FeO monolayer on Pt(111) reconstructs into a trilayer O−Fe−O film that reacts with CO through a Mars−van Krevelen-type

INTRODUCTION In recent years, ultrathin oxide films have attracted significant attention in both science and technology because of their unique physicochemical properties often differing from those of their bulk counterparts.1−4 Well-ordered, thin oxide films on planar metal substrates have been used, for example, in the modeling of supported, dispersed metal catalysts at the atomic scale3−5 and in reaction studies on inverse model catalysts with a focus on the properties of the metal-oxide interface.6−8 Iron oxide-based films are of particular interest because Fe constitutes the second most abundant metal in Earth’s crust,9,10 resulting in low material costs. Additionally, bulk iron oxides have industrial relevance in several catalytic reactions such as the water−gas shift reaction,11 NO abatement,12−14 and the Fischer−Tropsch reaction.15 FeO(111) thin films were first grown on Pt(111) by Vurens et al.16 in 1988, and through the continuous interest in the following two decades, the FeO monolayer was particularly well studied and characterized in great detail.2,4,17−28 Typical FeO film preparations consist of consecutive cycles of Fe deposition and postoxidation in an O2 atmosphere (10−7−10−6 mbar) at elevated temperatures (830−1000 K), resulting in hexagonal Fe−O bilayer structures terminated by close-packed O layers.2,17,20−23 For oxidation temperatures lower than 1000 K, FeO structures with different thicknesses have been reported.2,21 © 2013 American Chemical Society

Received: April 30, 2013 Revised: June 7, 2013 Published: July 2, 2013 15155

dx.doi.org/10.1021/jp4042638 | J. Phys. Chem. C 2013, 117, 15155−15163

The Journal of Physical Chemistry C

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reaction.26,34,35 The system exhibits enhanced catalytic CO oxidation activity exceeding that of the bare Pt(111) substrate.36 Previously, ultrathin FeO(111) films have been grown not only on Pt(111) and Pt(100) single crystals,16−18,20,37 but also on other common metal supports such as Ag(111),38 Au(111),39 Mo(100),40 and Ru(0001),41 inducing unique properties in the FeO films. For example, the atomic and moiré periodicities of the FeO monolayer films are influenced by the lattice constant of the substrate. Even though the different metal supports generate only subtle differences in the structure of the overlaying FeO films, the various FeO−metal systems can exhibit pronounced differences in their physicochemical properties. In the present work, we used Pd(111) as a substrate for the FeO monolayer film, because Pd is a well-known noble-metal catalyst42−44 that is of fundamental importance in three-way catalytic converters.45,46 In addition, Pd is widely used in a number of hydrogenation/dehydrogenation reactions47,48 and for the synthesis of organic fine chemicals,49 and Fe or FeOx in combination with Pd has been shown to improve the catalytic abilities of Pd.47,50,51 The structures of both stoichiometric and oxidized FeO(111) films were studied. We report on the formation of O-adatom dislocation loops for low O exposures onto the stoichiometric FeO(111) film. For higher O exposures, the formation of FeO2−x trilayers was observed, which is reminiscent of the O-rich structures previously reported for FeO supported on Pt(111).

Computational Details. Spin-polarized density functional theory (DFT) calculations using the DFT + U approach of Dudarev et al.54 were performed using the Vienna Ab Initio Simulation Package (VASP) code.55,56 Projector augmented wave potentials57,58 were used for describing the electron−ion interactions, and the generalized gradient approximation (GGAPW91)59 was used to describe the exchange-correlation functional. The electron wave function was expanded with plane waves with an energy cutoff of 400 eV. The parameters describing the on-site Coulomb interaction between Fe 3d orbitals were chosen as U = 4 eV and J = 1 eV (Ueff = U − J = 3 eV). To model monolayer FeO(111) films supported on Pd(111), the experimentally observed FeO(111)−(7 × 7)/ Pd(111)−(8 × 8) unit cell was used, consisting of one layer of Fe atoms and one layer of O atoms supported on a three-layer-thick Pd(111) slab. The experimental lattice parameter of FeO(111) was used (3.1 Å), corresponding to a Pd−Pd spacing of 2.71 Å in the (111) plane. The periodically repeated slabs in the z direction were separated by vacuum layers of ∼15 Å. The FeO monolayer film and the top Pd layer were fully relaxed, whereas the two Pd layers at the bottom were kept fixed. All nonfixed atoms were allowed to fully relax until the Hellmann−Feynman forces were smaller than 0.02 eV/Å. The Brillouin zone was sampled with the Γ point only.

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RESULTS AND DISCUSSION Structure of Stoichiometric Monolayer FeO Films Grown on Pd(111). A typical STM image of a monolayer FeO film grown on Pd(111) is shown in Figure 1a, along with the

EXPERIMENTAL AND THEORETICAL METHODS Experimental Details. Monolayer-thick FeO films were grown on Pd(111) using a procedure similar to that previously described for the FeO/Pt(111) system.22,24,25,28 The Pd(111) crystals were cleaned by cycles of Ar+ sputtering [p(Ar) = 1 × 10−6 mbar, 1 keV, 20 min] at room temperature and then subjected to annealing in ultrahigh vacuum (UHV) at 950 K. If necessary, the Pd(111) crystals were annealed in oxygen [p(O2) = 5 × 10−8 mbar] at ∼750 K to remove C impurities. The cleanness of the Pd(111) surface was judged from atomically resolved scanning tunneling microscopy (STM) images. Submonolayer quantities of Fe (Goodfellow, 99.99%) were deposited on Pd(111) at room temperature using an electronbeam evaporator (Oxford Applied Research, EGCO4). Subsequently, the Fe was oxidized in 1 × 10−6 mbar O2 at ∼950 K (10 min). The STM experiments were performed in a UHV chamber with a base pressure of ∼1 × 10−10 mbar equipped with a homebuilt, variable-temperature Aarhus STM instrument.52,53 The STM measurements were carried out with mechanically cut Pt/Ir tips in constant-current mode at room temperature. STM was used to monitor the film growth, as an FeO monolayer film with a characteristic moiré pattern can easily be distinguished from the bare Pd(111) surface. One monolayer (1 ML) of FeO is defined as Fe and O coverage in a perfect FeO bilayer film grown on Pd(111), corresponding to an FeO coverage of 0.82 ML with respect to the atomic density of Pd(111).24 Atomic oxygen (O) was generated with a commercial gas cracker (Oxford Applied Research, TC-50) located approximately 10 cm from the sample. The cracker was degassed for 3 min prior to O exposure, and the power of the cracker was set to 54 W (corresponding to the cracking threshold of O2) for every experiment. The actual exposure of atomic oxygen on the samples is unknown, but it is proportional to the O2 pressure and the exposure time. Throughout the text, O exposures are given in terms of O2 background pressures and exposure times.

Figure 1. (a) High-resolution STM image (150 Å × 150 Å, +65 mV, 3.3 nA) of the close-packed monolayer FeO film grown on Pd(111). The scale bar is 25 Å. (b) Corresponding LEED pattern obtained at 105 eV. (c) Top-view ball model of the proposed atomic structure with the three high-symmetry domains. Moiré unit cells are indicated in panels a−c. (d) Side-view ball model of the FeO monolayer consisting of two atomic layers, Fe and O, and thus comprising a bilayer film. 15156



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corresponding low-energy electron diffraction (LEED) pattern (Figure 1b) and the atomic structural model (Figure 1c,d). Similarities to the FeO/Pt(111) system4,22,24,25 allowed the structural features observed on FeO/Pd(111) to be identified by direct comparison of the STM images. This new FeO monolayer film grown on Pd(111) is composed of a hexagonal surface structure (Fe−O). Based on our STM and LEED results, we found that the periodicity of this hexagonal surface structure is ∼3.1 Å. In addition, we found a moiré superstructure with a periodicity of ∼22 Å, resulting from the lattice mismatch between the hexagonal Fe/O lattice (∼3.1 Å) and the Pd(111) substrate (2.75 Å).60 Because of the differences in the local density of states, the moiré superstructure appears in the STM images as characteristic bright and dark regions.22 Similarly as observed for FeO/Pt(111),22 the tip state and the scanning parameters strongly influence the appearance of the Pd(111)-supported FeO monolayer. The FeO/Pd(111) system exhibits the same three high-symmetry domains as previously observed for FeO/ Pt(111).4,24,25 The TOP, face-centered cubic (FCC), and hexagonal close-packed (HCP) domains, marked in Figure 1c, are assigned according to the local stacking order of the atoms.22 The TOP domains are characterized by Fe atoms located atop Pd atoms and O atoms located at Pd hollow sites. The FCC domains are composed of both Fe and O atoms located in the Pd hollow sites. Finally, the HCP domains are characterized by Fe atoms located at the Pd hollow sites and O atoms located atop Pd atoms. Figure 1b shows a typical LEED pattern observed for FeO/ Pd(111) at a kinetic energy of 105 eV, which is similar to the LEED pattern observed for FeO/Pt(111).20,24 The substrate unit cell is shown using light blue lines, and the overlayer unit cell is drawn in black. The floret pattern around each Pd reflection arises from the moiré superstructure. Orange lines indicate the spacing between the moiré reflections, and in real space, a p(8 × 8) structure of the moiré pattern with respect to the Pd surface can be inferred. Thus, the LEED pattern suggests that eight Pd lattice spacings correspond to one moiré spacing of ∼22 Å, and the STM images suggest that seven Fe/O lattice spacings, each of ∼3.1 Å, correspond to one moiré spacing. The smaller nearestneighbor distance of Pd(111) (2.75 Å) compared to that of Pt(111) (2.77 Å)60 affects the size of the overlayer FeO unit cell. At ∼22 Å, the periodicity of the moiré pattern for FeO/Pd(111) is smaller than the ∼25 Å previously reported for FeO/ Pt(111).25 In addition, for FeO/Pd(111), the rotation of the FeO atomic unit cell with respect to the substrate is less than 0.6°. For comparison, the rotation is known to be ∼0.6° for FeO/ Pt(111).18,19 Top and side views of the optimized structure of an FeO monolayer supported on Pd(111) are shown in Figure 2, and extracted structural and electronic properties of the modeled film are summarized in Table 1. To model the FeO/Pd(111) system, we used the experimentally observed FeO(111)−(7 × 7)/ Pd(111)−(8 × 8) unit cell (Figure 1). The Fe atoms shown as light blue balls have opposite spin compared to the Fe atoms shown in purple. Thus, a row-wise antiferromagnetic (RWAFM) structure was assumed in our calculations. The distance between the FeO layer and the topmost Pd layer (ZFeO−Pd) is largest in the TOP domain (2.79 Å) and smaller in the FCC and HCP domains (2.59 Å), indicating a weaker FeO− substrate interaction in the TOP domain and a stronger interaction in the FCC and HCP domains. This is similar to the situation found for FeO/Pt(111).22,61 The Fe−Fe lattice parameters were calculated for each domain and found to be 3.15

Figure 2. DFT-based atomic structure of stoichiometric FeO(111)−(7 × 7)/Pd(111)−(8 × 8) shown in top view (upper panel) and side view (lower panel). Fe atoms in light blue and purple have opposite spins. TOP, HCP, and FCC domains are indicated.

Table 1. Structural and Electronic Propertiesa of 1 ML FeO/ Pd(111) Calculated Using the FeO(111)−(7 × 7)/Pd(111)− (8 × 8) Unit Cell domain

L (Å)

ZFeO−Pd

δZ (Å)

ΔV (eV)

TOP FCC HCP average

3.15 3.01 3.08 3.1

2.79 2.59 2.59 2.65

0.47 0.74 0.64 0.64

−0.32 0.31 −0.04 0

a L is the Fe−Fe distance; ZFeO−Pd is the distance between the FeO layer and the topmost Pd layer, defined as (ZFe + ZO)/2 − ZPd; δZ is the rumpling of the FeO layer, defined as δZ = ZO − ZFe; ΔV is the variation of the electrostatic potential between the local value (V) and the cell average (⟨V⟩) estimated at a distance of 5 Å from the Pd(111) surface. The calculated value of ⟨V⟩ is 5.41 eV for FeO/Pd(111).

Å (TOP), 3.08 Å (HCP), and 3.01 Å (FCC). The lateral lattice parameters were found to expand (contract) in the TOP (FCC) domain. With the definition given in Table 1, the average rumpling of the FeO film on Pd(111) is 0.64 Å, slightly smaller than that of FeO on Pt(111) (0.68 Å). The local rumpling is largest in the FCC domain (0.74 Å) and smallest in the TOP domain (0.47 Å). The calculated electrostatic potential variations with respect to the cell average (ΔV) for different domains are listed in the last column of Table 1. ΔV values were calculated to be −0.32, −0.04, and 0.31 eV for the TOP, HCP, and FCC domains, respectively. Overview of the Structural Features Observed upon Oxidation of FeO/Pd(111). To mimic high O2 pressures in UHV, we exposed the freshly prepared FeO films to different amounts of atomic O at room temperature and at 500 K. Here, only the results obtained after O exposures at 500 K are presented, but identical features were observed in the STM images upon O exposure at 300 K. Figure 3 displays STM images revealing sequential structural changes in the oxidation process of the FeO film as the exposure time was increased from 30 to 90 s at p(O2) = 1 × 10−8 mbar (Figure 3a−c) and after oxidation for 2 min at 1 × 10−7 mbar (Figure 3d). O exposure at 1 × 10−8 mbar O2 for 30 s led to the formation of large triangular and zigzag15157



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Figure 3. STM images (500 Å × 500 Å) showing the structural changes of FeO/Pd(111) upon O exposure at 500 K. Images were acquired after O exposure in a background of 1 × 10−8 mbar O2 for (a) 30 s (1.14 nA, 550 mV), (b) 90 s (0.79 nA, 398 mV), and (c) 180 s (0.43 nA, 216 mV). The STM image in panel d was acquired after a 120-s O exposure at an O2 background pressure of 1 × 10−7 mbar using It = 0.26 nA and a bias of 541 mV. Scale bars are 100 Å throughout.

Figure 4. (a−c) STM images showing O-adatom dislocation loops. Scale bars are 25 Å. (a) 150 Å × 150 Å large area, scanned after low O exposure [p(O2) = 1 × 10−8 mbar for 30 s at room temperature and 30 s at 500 K] using It = 2.67 nA and Vt = 250 mV. A large O-adatom dislocation loop is seen. Two moiré unit cells outside the loop, labeled “1” and “3”, and one inside, labeled “2”, are indicated. (b) 150 Å × 130 Å STM image (1.32 nA, 1846 mV) acquired after 300 s of O exposure in a background of 1 × 10−8 mbar O2. A long zigzag-shaped O dislocation separates two areas with upward pointing (left) and downward pointing (right) small triangular O dislocations. (c) 50 Å × 50 Å high-resolution STM image (−2.08 nA, −127 mV) showing an O-adatom dislocation loop obtained after O exposure in a background of 1 × 10−8 mbar O2 for 300 s. Guidelines along the Fe atoms help to visualize the inversion of the atomic unit cell. (d) Proposed atomic structure of the O-adatom dislocation loop shown in image c.

shaped features (Figure 3a). The edges of the largest triangular loops stretched over approximately five moiré unit cells. Further O exposure for 30 s (Figure 3b) led to the formation of smaller closed triangular loops arranged in a regular hexagonal pattern. The triangles were uniformly sized, the majority having side lengths corresponding to nine bright protrusions. In the following section, we present further examples of the triangular loops and explain them as O-adatom dislocations. Even further O exposure of an FeO/Pd(111) surface characterized by triangular loops, such as the one depicted in Figure 3b, led to the appearance of bright circular patches, as shown in Figure 3c. These bright patches were observed to form preferentially within the closed triangles close to the corners or at kink sites. After a total O exposure of 2 min at p(O2) = 1 × 10−7 mbar (Figure 3d), the bright patches were observed to cover the whole Pd(111) surface, forming a regular pattern. Eventually, the surface was fully saturated with the bright patches; see, for example, Figure 3d. In the following, we assign these bright patches to O−Fe−O trilayer islands, based on a comparison to FeO/Pt(111). Structure of O-Adatom Dislocation Loops. Figure 4a shows a high-resolution STM image of an FeO film after a low O exposure [p(O2) = 1 × 10−8 mbar for 30 s at room temperature and 30 s at 500 K] that led to the formation of a large O dislocation loop. When this STM image was acquired, the STM tip was in a tip mode that allowed the three high-symmetry domains to be distinguished based on their different apparent heights. In Figure 4a, the contrast was chosen to highlight these differences in the apparent heights within the moiré pattern at the cost of saturating the color scale at the dislocation. Three moiré unit cells are marked with their corners positioned at the darkest domains. In the unit cells outside the boundaries of the dislocation loop labeled “1” and “3”, the color changes from

dark to medium-bright and then to bright and back to dark (black → red → yellow → black) passing along the long diagonal of the unit cell from left to right. However, inside the dislocation loop, the unit cell labeled “2” shows an inverted order, with the color shifting from dark to bright and then to medium-bright and dark (black → yellow → red → black) along the long unit cell diagonal from left to right. This observed domain reversal between the FeO film regions enclosed by and excluded by the triangle is caused by the excess O atoms in the film. To accommodate these excess O atoms, the entire O lattice inside the dislocation loop is shifted. Restructuring of the film is induced by minimization of the surface energy, and eventually, the energetically most favorable configuration can be reached.22,25 The STM image in Figure 4b shows that a very high density of the triangular O dislocation loops formed on the FeO/Pd(111) surface. Regularly arranged triangular O dislocation loops covered the entire surface. To obtain this high density of triangular O dislocation loops (one dislocation loop per three moiré unit cells), a clean FeO/Pd(111) surface was exposed to O at 500 K [p(O2) = 1 × 10−8 mbar for 300 s]. The STM image in Figure 4b also provides evidence for domain inversion: A zigzag stripe runs through the middle of the STM image, connecting the HCP domains and separating two areas with upward-pointing (left) and downward-pointing (right) small triangular O 15158



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dislocations. The assignment of the areas corresponding to the original FeO film and the inverted FeO film in this STM image was possible because the orientation of the Pd crystal in the STM imstrument was known. The initial formation of O dislocation loops on the stoichiometric FeO film with triangular features pointing downward is shown in Figure 3b. Thus, the imaged area to the right in Figure 4b corresponds to the original orientation, whereas the area on the left is inverted. This example illustrates that large areas of the film can be reconstructed, resulting in a domain reversal between the original and inverted parts of the film, making it possible to incorporate additional O atoms.22 Figure 4c shows a high-resolution STM image of a single small triangular O-adatom dislocation loop that was recorded in socalled “mixed-imaging” mode.22 In this tip-dependent imaging mode, both Fe and O atoms are imaged as protrusions, and the pocket sites of the FeO film correspond to the lowest apparent height. Combining this information with the known orientation of the stoichiometric FeO film allowed the imaged protrusions to be assigned to specific surface atoms, namely, Fe and O (Figure 4c). Outside the O-adatom dislocation loop, guide lines are placed on the positions corresponding to Fe atoms. The inset in the lower right of Figure 4c shows a magnification of an area located outside the dislocation loop with the atomic unit cell superimposed. The bright protrusion in the lower left of the unit cell corresponds to the O atom (circle). Inside the dislocation loop (as shown in the magnification in the lower left of Figure 4c), the protrusion arising from the O atom appears in the upper right half of the unit cell. This finding points to a relocation of the enclosed O atoms relative to the Fe lattice. The proposed atomic model of the O-adatom dislocation loop shown in Figure 4d illustrates how the FeO film enclosed by the O-adatom dislocation loop is inverted with respect to the original FeO film outside the O-adatom dislocation loop. An additional row of O atoms parallel to one side of the triangle has been added. An O row needed to form a typical triangle consists of eight O atoms, and to make this extra O row fit, a triangular domain of 28 O atoms must be shifted with respect to the Fe lattice (to the adjacent hollow sites). The excess O atoms and the shifting of the O lattice result in the formation of four-fold O-coordinated Fe atoms. These four-fold-coordinated Fe atoms appear in the STM images as the bright edges of the triangles. After an O dislocation loop has formed, most of the Fe atoms are still three-fold Ocoordinated in the surface layer, whereas the Fe atoms forming the edges of the triangles are four-fold O-coordinated. In most cases, nine Fe atoms were found along each of the edges of the triangles. Note that, in Figure 4c, the bright protrusions along the edges also lie on the superimposed lines. These bright protrusions are assigned to four-fold O-coordinated Fe atoms along the edges. This assignment is consistent with the previously observed dark two-fold O-coordinated Fe edges of the O-vacancy dislocation loops found upon reduction of the FeO film with atomic hydrogen.22,24,25 That is, low-coordinated Fe species appear dark, and high-coordinated Fe species appear bright in the STM images. Furthermore, this assignment is supported by the DFT calculations presented next. To model the structure of the O-adatom dislocation loop by means of DFT, five O adatoms were introduced in the FeO(111)−(7 × 7)/Pd(111)−(8 × 8) unit cell and the 10 O atoms inside were shifted, forming a triangle that consists of six Fe atoms coordinated with four O atoms at each edge of the triangle. Thus, the modeled triangles were three Fe atoms shorter along the edges than the experimentally observed dislocations so that the calculations could be performed at reasonable cost.

Figure 5. (a) Optimized, DFT-based structure of a small triangularshaped O dislocation loop (top view). (b) Simulated STM image at a bias of +0.5 V. The bright protrusions arise from Fe atoms coordinated to four O atoms, as indicated in panel a by green balls. The white parallelogram indicates the unit cell.

Figure 5a shows the optimized structure of this dislocation loop (54 O atoms, 49 Fe atoms, and 192 Pd atoms). The average Fe− Fe distance at the edges of the triangle was found to be 2.99 Å, which is even smaller than the Fe−Fe distance in the FCC domains (3.01 Å) of stoichiometric FeO/Pd(111). After energy minimization, all of the Fe atoms coordinated with four O atoms were found essentially at either the Pd bridge or atop sites. Thus, in accordance with the experimental results, the corners of the triangular O-adatom dislocations are preferentially located at the HCP domains. The calculated formation energy of the considered triangular dislocation loop, Ef = [E050‑FeO/Pd(111) − E0FeO/Pd(111) − 5/2(E0O2)(gas)]/5, of −0.75 eV/O indicates that the O dislocation loop is stable under O-rich conditions. Figure 5b shows a simulated STM image of the triangular Oadatom dislocation loop, based on DFT calculations using the Tersoff−Hamann approximation. The four-fold O-coordinated Fe atoms at the edges of the triangle appear as bright protrusions, whereas the three O lattice atoms inside the triangle appear with a smaller apparent height. These characteristics in the simulated STM image are in good agreement with the mixed-imaging mode shown in the experimentally acquired STM image in Figure 4c. Thus, the DFT calculations corroborate the proposed model of the experimentally observed O-adatom dislocation loops depicted in Figure 4d. Formation of FeO2−x Trilayers upon Excessive O Exposure. After the highest O exposures performed in this study [300 s at p(O2) = 1 × 10−7 mbar at 300 K], nearly circular bright patches were observed that covered almost the entire surface, forming a regular pattern (see Figures 3d and 6a). Although these bright patches were not perfectly uniform in size 15159



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Figure 7. (a,b) STM images (scale bars are 25 Å) showing O−Fe−O trilayer islands coexisting with O-adatom dislocations. Images were obtained after 300 s of O exposure at p(O2) = 1 × 10−7 mbar at 300 K using It = 229 nA and Vt = 0.290 mV. (b) Enlargement of the area indicated by the dotted square in image a. Guidelines mark the Fe positions of the pristine FeO film that coincide with protrusions on the O−Fe−O islands. Recorded using It = 229 nA and Vt = 0.300 mV. (c) Proposed structural changes upon formation of O−Fe−O trilayer islands.

STM image that the edges of larger O dislocations were not straight but could bulge or even consist of partly overlapping line segments. In addition, STM images of this high resolution enabled a better comparison of O dislocations and the FeO2−x trilayer structures. Enlarging the area indicated by the dotted square in Figure 7a, we fixed guide lines on bright protrusions at the edges of the O dislocation (Figure 7b); that is, the grid positions mark the positions of Fe atoms (see the atomic model of the O-adatom dislocation loops in Figure 4d for confirmation). The brighter protrusions seen on the enclosed trilayer island are assigned to O atoms in the top O layer of the O−Fe−O trilayer islands. These protrusions are not aligned with the Fe grid and tend to appear in the lower left of the atomic unit cells of the Fe grid. Thus, assuming that the lowest O layer does not shift during island formation, the protrusions within the O−Fe−O trilayer islands appear atop O atoms in the lowest O layer rather than over hollow sites. These results are consistent with the O−Fe−O trilayer structure previously suggested for FeO/Pt(111) (see sketch in Figure 7c).26,34,36 Because the topmost O atoms in the FeO2−x trilayer islands are more weakly bound than the O atoms in the FeO monolayer film,26,34 it can be expected that these O atoms are catalytically active. Reactivity of FeO2−x Trilayer Islands. Finally, reactivity studies addressing the CO oxidation reaction were conducted on FeO/Pd(111) covered with O−Fe−O trilayer structures (see Figure 8). The sample was exposed to CO at various pressures (up to 10−6 mbar) and temperatures in the main chamber and at high pressures at room temperature in a separate small, gold-

Figure 6. (a) STM image (500 Å × 500 Å, 0.840 nA, 585 mV, scale bar = 100 Å) showing an FeO film saturated with O−Fe−O trilayer islands obtained after 300 s of O exposure at p(O2) = 1 × 10−7 mbar at 300 K followed by vacuum annealing at 600 K for 120 s. The line profiles in panel b reveal that the moiré periodicity of ∼22 Å is preserved after formation of the trilayer islands. (c) Line profile showing the corrugation of the trilayer islands compared with a step edge on the Pd substrate.

and shape and the degree of order was not as high as observed for the stoichiometric FeO film, the periodicity of the moiré pattern (∼22 Å) was preserved, as illustrated by the STM line profiles shown in Figure 6b. This finding suggests that the formation of the bright O-rich patches is initiated exclusively at one of the three types of high-symmetry domains. The present long-range-ordered O-rich film supported on Pd(111) exhibits similarities to the O−Fe−O trilayer structure observed on FeO/Pt(111) after exposure to 20 mbar O2 at 450 K.26,34,36 Therefore, we assign the bright patches to FeO2−x trilayer structures that locally protrude from the otherwise still persisting FeO monolayer. The line profile depicted in Figure 6c illustrates the corrugation of the trilayer islands compared with a step on the Pd(111) substrate. The maxima on the terraces arise from the FeO2−x trilayer patches, whereas the minima are tentatively assigned to monolayered FeO. An atomically resolved STM image of trilayer islands coexisting with O-adatom dislocations is shown in Figure 7a. This high-resolution STM image was obtained after a 5-min O exposure (1 × 10−7 mbar O2 at 300 K). It is apparent from this 15160



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relevance for forthcoming STM studies addressing FeO monolayer films and FeO islands grown both on Pd(111) and P(111), as the assignment of new structural features in the STM images, such as step edges, can be done only if the appearance of known (reference) structures in STM has been established. For example, the O-adatom dislocation loops allowed the assignment of the topmost O atoms at enclosed O−Fe−O trilayer islands (see Figure 7b), which was not previously possible based on experimental data alone and which would be difficult to achieve otherwise. For an improved understanding of the catalysis triggered by both the O−Fe−O trilayer islands and the monolayer FeO islands on Pd(111) and Pt(111) substrates, the structural information provided here is very useful. Given the atomic-scale nature of catalytic processes, precise structural information is essential. Furthermore, considering the apparent structural similarities of monolayer FeO films grown on Pd(111) and Pt(111), the possibility of comparative studies arises. For example, by comparing the reactivity of O−Fe−O trilayer structures on FeO/Pd(111) and FeO/Pt(111), the influence of the metal substrate can be probed. Although the O-rich structures appear to be essentially identical, the electronic properties of these two systems are different, with the work function of Pd(111) being 5.6 eV, which is 0.5 eV lower than that of Pt(111). Thus, it would be interesting to directly compare the reactivity of O−Fe−O trilayer structures grown on Pd(111) supports with that of O− Fe−O trilayer structures grown on Pt(111). In this way, it might be possible to disentangle the influence of structural and electronic effects. Similarly, the reactivities of FeO islands supported on Pd(111) and Pt(111) can be compared.

Figure 8. CO oxidation over O−Fe−O trilayer islands: (a) STM image (0.17 nA, 896.3 mV) of an FeO film covered with trilayer islands after exposure to 1.33 mbar CO for 5 min at 300 K (no reactivity). (b) STM image (0.24 nA, 354.3 mV) obtained after exposure of a trilayer saturated FeO/Pd(111) sample to 1.3 × 10−6 mbar CO for 2 min at 500 K. All trilayer islands disappeared. Scale bars are 25 Å.

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CONCLUSIONS The structural properties of a stoichiometric monolayer FeO film grown on Pd(111) were studied by high-resolution STM and LEED measurements and DFT calculations. The smaller nearestneighbor distance in Pd(111) (2.75 Å) as compared to Pt(111) (2.77 Å) induces subtle differences in the size of the unit cell of the monolayer FeO film. Based on STM and LEED, it was found that FeO/Pd(111) exhibits a surface lattice constant of 3.1 Å, which is similar to that reported for FeO/Pt(111), and a moiré periodicity of ∼22 Å, which is smaller than the ∼25 Å periodicity reported for FeO/Pt(111). For FeO/Pd(111), the rotation of the FeO atomic unit cell with respect to the substrate is less than 0.6°. Our detailed STM studies revealed that the stoichiometric FeO film reconstructs upon O exposure, leading to different structures depending on the O exposure time and pressure. For low O exposures, the formation of ordered triangular O-adatom dislocation loops was observed. STM measurements and DFT calculations revealed that the sides of these triangular structures were composed of four-fold O-coordinated Fe atoms as opposed to the original three-fold O-coordinated Fe atoms in the stoichiometric FeO film. The incorporation of these excess O atoms led to an inversion of the high-symmetry domains of the FeO film enclosed by the dislocation. Upon further O exposure, the dislocations first spread in a disordered manner, before the formation of FeO2−x trilayer island structures was observed. The O-rich island structures resembled those previously reported for FeO/Pt(111) and were found to be catalytically active toward the CO oxidation. The FeO monolayer film supported on Pd(111) is an interesting model system that is suitable for the investigation of catalytic processes at the atomic scale.

coated reaction chamber, and subsequently, the samples were studied by STM. At 300 K and low CO pressures (up to 10−6 mbar), no changes to the O−Fe−O trilayer structures were found (data not shown), and even after exposure to a CO partial pressure of 1.33 mbar at 300 K for 5 min (see Figure 8a), the O− Fe−O trilayer islands remained essentially intact. In contrast, when an FeO/Pd(111) sample covered with O−Fe−O trilayer islands was exposed to CO [60 s at p(CO) = 1.3 × 10−8 mbar] at 400 K, clear indications of the removal of the O−Fe−O trilayer structures were observed (see the Supporting Information, Figure S1). After exposure to an even more reactive environment [2 min at p(CO) = 1.3 × 10−6 mbar at 500 K], all of the O−Fe− O trilayer island were reacted off, and an FeO monolayer film was restored (see Figure 8b). Control experiments showed that vacuum annealing of an FeO/Pd(111) sample covered with O− Fe−O trilayer islands at temperatures up to 600 K did not lead to the removal of the O−Fe−O trilayer islands (see Figure S2, Supporting Information). Accordingly, regarding the reaction experiment described in Figure 8, it can be concluded that the weakly bound excess oxygen in the O−Fe−O trilayer structures reacted with CO, indicating substantial catalytic reactivity of the O−Fe−O trilayer structures on FeO/Pd(111) through a Mars− van Krevelen-type reaction. This result agrees well with the findings reported for O-rich structures prepared on FeO/ Pt(111).26,34−36 Relevance of the Provided Structural Information and Outlook. The presented STM and DFT data reveal unprecedented atomic-scale insight into stoichiometric and Orich FeO films grown on Pd(111). This insight is of particular 15161



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ASSOCIATED CONTENT

S Supporting Information *

Additional STM data showing the reduction of an O−Fe−O trilayer saturated surface by an intermediate CO exposure (Figure S1) and revealing the thermal stability of O-rich FeO/ Pd(111) in UHV (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.B.), [email protected] (S.W.). Phone: +45 2338 2204 (F.B.), +45 8715 6731 (S.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Danish Research Agency, the Strategic Research Council, the Villum Kahn Rasmussen Foundation, the Lundbeck Foundation, the Carlsberg Foundation, Haldor Topsøe, and the European Research Council through an Advanced ERC grant (F.B.). The work at UWMadison was supported by the U.S. Department of Energy, Basic Energy Sciences (DOE-BES), Division of Chemical Sciences. Supercomputing resources from the following institutions were used: Environmental Molecular Sciences Laboratory (EMSL), a National scientific user facility at Pacific Northwest National Laboratory (PNNL); the Center for Nanoscale Materials (CNM) at Argonne National Laboratory (ANL); the National Center for Computational Sciences (NCCS) at Oak Ridge National Laboratory (ORNL); and the National Energy Research Scientific Computing Center (NERSC). EMSL is sponsored by the Department of Energy’s Office of Biological and Environmental Research located at PNNL. CNM, NCCS, and NERSC are supported by the U.S. Department of Energy, Office of Science, under contracts DE-AC02-06CH11357, DEAC05-00OR22725, and DE-AC02-05CH11231, respectively.

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Structure of Stoichiometric and Oxygen-Rich Ultrathin FeO (111) Films

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