Article pubs.acs.org/JPCC
Nanoscale Domain Structure and Defects in a 2‑D WO3 Layer on Pd(100) N. Doudin,† D. Kuhness,† M. Blatnik,† G. Barcaro,‡ F. R. Negreiros,§ L. Sementa,‡ A. Fortunelli,*,‡ S. Surnev,*,† and F. P. Netzer† †
Surface and Interface Physics, Institute of Physics, Karl-Franzens University Graz, A-8010 Graz, Austria CNR-ICCOM & IPCF, Consiglio Nazionale delle Ricerche, via Giuseppe Moruzzi 1, 56124 Pisa, Italy § Centro de Ciencias Naturais e Humanas, Universidade Federal do ABC, Santo André, 09210-580 SP, Brazil ‡
ABSTRACT: A stoichiometric two-dimensional (2-D) WO3 layer has been fabricated by vapor-phase deposition of (WO3)3 clusters onto a Pd(100) surface and characterized by a combined experimental/theoretical multitechnique approach. The oxide forms a WO2 + O bilayer with a well-ordered c(2 × 2) structure, displaying at the full monolayer coverage a regular nanoscale pattern of antiphase domain boundaries, as revealed by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) and rationalized by DFT analysis as a consequence of elastic strain relief. The stability of the WO2 + O bilayer is provided by polarity compensation via charge rearrangement at the WO3/Pd interface and allows for surface redox chemistry via reversible release and restoration of oxygen atoms of the tungstyl or WO groups.
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INTRODUCTION Ultrathin films of oxide materials used as components in sophisticated nanotechnological applications have nowadays reached the two-dimensional (2-D) limit; i.e., the respective films consist of only one to a few atomic layers or of a singlepolyhedron thick sheet.1−3 As with other 2-D materials,4,5 these 2-D oxide phases display novel physical and chemical properties, which are different from their bulk counterparts: their study is at the forefront of advanced materials research, and they are on the verge of industrial implementation.1,2 In contrast to typical 2-D materials derived from van der Waals solids,4,5 which as a norm feature weak overlayer−substrate interactionsthe latter is the result of strong intralayer covalent bonding and internal bond saturationtransition metal oxide overlayers typically display stronger coupling with the supporting surface, in particular if the substrate is a metal. The oxide−metal interface thus becomes a decisive element in determining the properties of the oxide overlayer system.1−3 Another difference and a major issue of 2D oxides with respect to exfoliated van der Waals layered solids is thatbeing often produced under conditions of high temperature and extremely low pressurethey are usually strongly reduced (understoichiometric) and may undergo drastic changes when utilized in standard environment. Here we report the fabrication of a stoichiometric 2-D tungsten oxide phase supported on a Pd(100) substrate, which not only shows novel structural, electronic, and elastic behavior as a result of 2-D dimensionality and interfacial coupling but also exhibits a robust framework due to organization into nanoscopic islands and the presence of © 2016 American Chemical Society
topmost tungstyls or WO groups, which can be released or restored in response to environmental stimuli. Tungsten trioxide, WO3, has been utilized in various morphologies in diverse applications, such as in electrochromic devices,6,7 in chemical sensors,8 and as flexible catalysts,9−12 and has been proposed as a photocatalyst for water splitting.13−16 The crystal structure of WO3 is derived from the cubic ReO3 lattice with corner-sharing regular WO6 octahedra, but various distortions as a function of temperature give rise to symmetry reduction and a very complex structure behavior and phase diagram.17 The (001) surface of monoclinic WO3 bulk crystals has been studied by surface science techniques:18−21 a variety of superstructures have been observed by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED), depending on reductive/oxidative environments during surface preparation. These superstructures have been ascribed to reduction and the creation of ordered vacancies in the top oxygen layer of the WO3(001) surface termination. Surface defect structures upon reduction have also been studied by STM on epitaxial (001)-oriented WO3 thin films grown on LaAlO3 substrates.22,23 The formation of O vacancies has been discussed in terms of the compensation of polarity of the WO3(001) surface termination;19 moreover, it also testifies that WO3 can be easily reduced23 and is chemically active. Ordered ultrathin tungsten oxide films on Pt(111) have been grown by Li et al.,24 using a new growth methodology involving the Received: October 18, 2016 Revised: November 28, 2016 Published: November 28, 2016 28682
DOI: 10.1021/acs.jpcc.6b10504 J. Phys. Chem. C 2016, 120, 28682−28693
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lower binding energy than the respective core level peak. A linear background was subtracted from the core level spectra prior to the peak decomposition analysis. The latter has been performed with the help of free-ware least-squares minimization software, FitXPS.28 The core level spectra have been decomposed into several components, whose individual shape consists of a Donjach−Šunjić profile convoluted with a Gaussian distribution.29 Ultraviolet photoemission spectra (UPS) were taken in our home laboratory, in a UHV chamber equipped with a He I (21.2 eV) source (UVS 10/35, SPECS) and a hemispherical electron analyzer (PHOIBOS 100, SPECS). The sample work function has been measured from the low-energy secondary electron cutoff in the ultraviolet photoelectron spectra. HREELS experiments have been conducted in a customdesigned system consisting of two UHV chambers with base pressures of 1 × 10−10 mbar, separated by a gate valve. The upper chamber is used for sample preparation and is equipped with LEED optics (SPECS, Germany), a quadrupole mass spectrometer (Pfeiffer, Prisma), and facilities for sample manipulation, surface cleaning, and thin-film evaporation. The lower chamber houses the high-resolution electron energy loss spectrometer (Delta 0.5, SPECS) capable of a total energy resolution of ∼1 meV, which was operated in specular reflection geometry at 60° incidence from the surface normal and with a primary energy of 4.0 eV. The instrumental resolution used in the present HREELS experiments was typically set to 3−4 meV. The Pd surface has been cleaned by cycles of Ar ion sputtering (1.5 keV) and annealing to 1000 K. In order to remove residual carbon contaminations, the Pd crystal was exposed to oxygen at 570 K, followed by a brief flash to 950 K in UHV. Surface order and cleanliness were monitored by LEED, STM and AES, and XPS. The W oxide overlayer has been prepared by the deposition of (WO3)3 clusters in UHV onto the clean Pd(100) crystal surface held at room temperature, followed by annealing at 823 K for 5 min in 2 × 10−7 mbar O2. The (WO3)3 cluster beam has been generated by thermal sublimation of WO3 powder at 1300 K in a thermal evaporator; the evaporation flux was monitored by a quartz microbalance. The evaporated amount of (WO3)3 is given in monolayers (ML) of WO3 units, whereby 1 ML is defined here by the number density of Pd(100) surface atoms (i.e., 1.32 × 1015 atoms·cm−2). Note that for the c(2 × 2) tungsten oxide structure (see below), 0.5 ML of deposited (WO3)3 generates a full monolayer of the oxide. DFT calculations have been performed using the QuantumEspresso suite of computational codes,30 employing a basis set of plane waves, ultrasoft pseudopotentials,31 and the Perdew− Burke−Ernzerhof (PBE) exchange-correlation (xc-) functional.32 Values of 40 Ry (Ry, 1 Ry = 13.606 eV) and 400 Ry were chosen as the energy cutoff for the selection of the plane waves for the description of the wave function and the electronic density, respectively. When studying the (1 × 1) unit cell of WOx on Pd(100) (which corresponds to the experimentally observed c(2 × 2) unit cell), the description of the first Brillouin zone was achieved by employing a (4,4,1) k grid; when studying larger unit cells (in the case of localized oxygen vacancies and antiphase boundaries domains), the number of k points has been maintained as 1 per Å. The electronic levels were broadened with a Gaussian smearing of about 0.002 Ry, and all the calculations were performed spinunpolarized, having verified that zero spin corresponds to the
deposition of molecular cyclic (WO3)3 clusters from the gas phase. The opening of the cyclic clusters and their condensation lead to the formation of a tungsten oxide layer with a zigzag chain structure with a c(4 × 2) periodicity, in which half of the W atoms are in the 6+ oxidation state but half of them are partially reduced to the 5+ oxidation state due to the bonding to the Pt substrate. In the present work, we have prepared a 2-D tungsten oxide phase on a Pd(100) substrate surface. The Pd(100) lattice constant in the ⟨001⟩ direction (a⟨001⟩ = 3.89 Å) fits well to the cubic WO3 bulk lattice (a⟨001⟩ = 3.78 Å), yielding a formal lattice mismatch of −2.8% (tensile strain of the overlayer). Although this comparison of bulk lattice constants for strain determination is questionable in the case of 2-D overlayers (see ref 25 and further below), it gives some guidance for the expectation of epitaxial growth. Following the growth procedure of Li et al.,24 i.e., deposition of cyclic (WO3)3 clusters onto Pd(100) and their condensation by oxidizing treatment at elevated temperature, 2-D W oxide deposits have been generated and atomically characterized by a combined experimental−theoretical multitechnique approach: STM, LEED, X-ray (UV) photoelectron spectroscopy, and surface vibrational spectroscopy (high-resolution electron energy loss spectroscopy (HREELS)), in conjunction with density functional theory (DFT) calculations. We find that the W oxide grows as a single phase in a c(2 × 2) superstructure with square island morphology at submonolayer coverages, but once a continuous 2-D overlayer is established, the c(2 × 2) structure forms a nanoscale domain lattice with well-ordered antiphase domain boundaries. The atomic structure of the W oxide overlayer as determined by experiment and theory consists of a WO2 + O bilayer, with overall WO3 stoichiometry; i.e., it resembles a polar 2-D sheet cut out of a cubic WO3 lattice perpendicular to the [001] direction. The electronic structure of the 2-D WO3 sheet, polarity stabilization of the c(2 × 2) structure and of domain boundaries, as well as strain release as a driving force for the high-order domain superlattice formation have been investigated. The role and energetics of structure defects and point defects in the 2-D layer are discussed and elucidated. This stoichiometric WO3 layer on Pd(110) is a robust 2-D phase, with however catalytic potential due to removable tungstyl groups.
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EXPERIMENTAL AND COMPUTATIONAL DETAILS The STM experiments have been performed in an ultrahigh vacuum (UHV) system (base pressure ∼1 × 10−10 mbar) containing a VT-STM/AFM (Omicron), a LEED optics, and a cylindrical mirror analyzer (CMA) for Auger electron spectroscopy (AES), as well as the usual facilities for sample manipulation, surface cleaning, and thin-film evaporation.26 STM images were recorded at room temperature in constant current mode with electrochemically etched W tips; the bias values cited are referred with respect to the sample. High-resolution XPS experiments have been measured at beamline I311 in the Swedish synchrotron radiation laboratory MAX-Lab, Lund, Sweden.27 The photoemission spectra were taken at photon energies of 140 and 625 eV for the W 4f and O 1s core levels, respectively, with a total energy resolution of ∼200 meV. All core level spectra were collected at normal emission. The binding energy scale was calibrated with respect to the Fermi energy of the metal substrate, and all spectra were normalized to the secondary electron background at a few eV 28683
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Figure 1. Large-scale STM images (65 nm × 65 nm, + 2.5 V, 5 pA) of different W oxide coverages on Pd(100): (a) 0.5 ML; (b) 0.7 ML; (c) 0.9 ML; and (d) 1.0 ML. The images have been differentially filtered to enhance the topographic contrast. The average island (domain) size estimated from the side lengths in the STM images is (a) 9.5 nm; (b) 8.0 nm; (c) 6.5 nm; and (d) 4.0 nm.
lowest-energy spin configuration of the undefective oxide. STM images were simulated applying the Tersoff−Hamann approach33 at a constant height of about 2 Å above the oxide surface. Along the z axis, a minimum empty space of 15 Å was chosen to avoid interactions between replicated cells. A dipole correction34 was applied to cancel spurious Columbic interactions among replicated images in the case of asymmetric unit cells. Core−electron energies of the W atoms were evaluated as in ref 35 by considering explicitly only the valence electrons and creating a core-hole in the 4f levels and using a DFT+U Hamiltonian by adding a Hubbard U term36 on the W atoms, with a U = 6 eV value taken from the literature,37 to better predict the core−electron energies of the W atoms. In order to estimate the stability of the (1 × 1) oxide structure, we have also performed three runs of AIMD (Ab Initio Molecular Dynamics) of 5 ps each at temperatures of 300, 600, and 900 K by adopting a similar numerical approach but a reduction on the energy cutoffs of the wave function and electronic density to 30/300 Ry, respectively. Vibrational frequencies were calculated using the Phonon code of the QuantumEspresso suite.30
The unit cell consists of a metallic slab covered by an oxide overlayer. For the metal support our DFT-GGA approach predicts an equilibrium value of the metal lattice constant of Pd of about 2.80 Å, in good agreement with the experimental value of 2.75 Åin our calculations we used the equilibrium value predicted by DFT. To model the Pd(100) support a slab made by five layers in FCC stacking was used, with the central one frozen in the crystalline positions of bulk metal. All the other atoms of the metallic slab and of the oxide overlayer were optimized until the forces on the individual atoms were smaller than 10−3 eV Å−1. In each layer of the minimal (1 × 1) unit cell only two metal atoms are present, plus one W atom and three O atoms per oxide deposit (in symmetric cells two deposits are adsorbed on each opposite side of the metallic slab avoiding the formation of the surface dipole moment at the metal/oxide interface).
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RESULTS AND DISCUSSION Morphology and Structure. The STM images of Figure 1 illustrate the growth morphology of the W oxide overlayer on Pd(100) for various oxide coverages, from submonolayer to the full monolayer range. At 0.5 ML (Figure 1a) the W oxide film 28684
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Figure 2. 0.5 ML W oxide on Pd(100): (a) LEED pattern (E = 55 eV). The reciprocal unit cell vectors of the Pd(100) substrate are indicated, and the c(2 × 2) oxide overlayer spot at the (1/2, 1/2) position is encircled. (b) STM image (18 nm × 18 nm, −1.0 V, 40 pA) of separated WOx islands. (c) STM image (14 nm × 14 nm, −0.4 V, 140 pA) of coalesced oxide islands. (d) Line profile taken across a WOx island (black solid line in Figure 2b). (e) Plot of the oxide island size (approximated by the side length) and island density as a function of oxide coverage.
condensation of (WO3)3 clusters at elevated temperatures following their deposition at room temperature. The oxide islands develop during this condensation process, and their density and size are thus also determined by thermodynamic effects. The STM images in Figure 2 also reveal that the oxide islands at submonolayer coverage are almost defect-free. This is in contrast to the monolayer surface, as shown below. Figure 3 gives a detailed picture of the W oxide monolayer on Pd(100). The LEED pattern (Figure 3a) shows that the fractional c(2 × 2) superstructure spots are split into four satellites (the (1/2,1/2) spot is encircled), while no splitting is observed for the integer order spots. This is a clear indication for the presence of antiphase domain boundaries in the overlayer,38 which is confirmed by the high-resolution STM images in Figure 3(b,c). It is noted paranthetically that similar spot splitting has been reported for c(2 × 2) reconstructed monoclinic WO3(001) surfaces, which has been associated with the twinning of monoclinic microdomains.19,21 The image in Figure 3b displays the domain structure of the oxide layer: the domains exhibit a quasi-square shape with an average size of 4.0 ± 0.5 nm and are separated by narrow (∼0.3 nm width) trenches, oriented along the substrate [010] and [001] directions. The atom-resolved image in Figure 3c reveals a square lattice of bright protrusions with a periodicity of 3.9 Å, which is equal to √2 times the lattice parameter of the Pd(100) surface and thus corresponds to a (√2 × √2)R45° or alternatively a c(2 × 2) superstructure (unit cell indicated in white). These protrusions will be associated with the terminal oxygen atoms sitting directly above the W atoms, so-called
grows in the form of square-shaped 2D islands, with their edges oriented along the [010] and [001] substrate directions. The average island size as expressed by their side length is 9.5 ± 0.5 nm, and some adjacent islands are seen to coalesce, forming larger islands extended in either [010] or [001] directions. Increasing the oxide coverage (Figures 1b and c) causes an increase of the island density and a decrease of the mean island size to 8.0 ± 0.5 nm at 0.7 ML and 6.5 ± 0.5 nm at 0.9 ML. At 1 ML the Pd(100) surface is completely covered by a wetting W oxide layer (Figure 1d), which now displays a well-ordered network of small (∼4 nm) square domains, separated by straight and narrow trenches. Figure 2 shows details of the W oxide islands at submonolayer coverages (0.5 ML). The LEED pattern in Figure 2(a) reveals a c(2 × 2) superlattice with respect to Pd(100)1 × 1, and the sharp reflections indicate good structural order. The STM images from this surface (showing three separated oxide islands in Figure 2(b), two oxide islands that have coalesced in (c)) reveal atomic-like lines and protrusions on the island surfaces, consistent with the c(2 × 2) superstructure of the oxide. The STM line profile in Figure 2(c), taken along the dark line AB in Figure 2(b), gives an apparent height of 1.6 ± 0.2 Å for the oxide layer. Panel 2(e) summarizes the evolution of the oxide island morphology (size and number density) as a function of oxide coverage: the island sizes decrease, but their number density increases with coverage. This is surprising since usually the island density is determined by nucleation during initial growth which fixes the island density. However, here the oxide layer is formed by 28685
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Figure 3. W oxide monolayer on Pd(100): (a) LEED pattern (E = 60 eV). The reciprocal unit cell vectors of the Pd(100) substrate are indicated, and the four satellite spots, centered around the (1/2, 1/2) position, are encircled; (b) STM image (25 nm × 25 nm, +1.5 V, 30 pA); (c) highresolution STM image (10 nm × 10 nm, −0.9 V, 60 pA) with the c(2 × 2) unit cell indicated. The dashed white lines are a guide for the eye, showing that neighboring domains are antiphase aligned. (d) Top: high-resolution STM image (4.2 nm × 2.7 nm, +0.3 V, 0.3 nA) with the surface defects clearly resolved. Bottom: line profile, taken along the line across a single defect.
tungstyls or WO groups, as discussed below in conjunction with the structure model. A careful inspection of the STM images in Figure 3b and 3c shows that neighboring domains are laterally shifted to each other by a 1/2 unit cell vector in the [010] and [001] directions (see dotted lines in Figure 3c); i.e., they are antiphase aligned, and the trenches separating such domains correspond to antiphase domain boundaries. Apart from the structural defects of antiphase domain boundaries, which are discussed below in terms of a strain release, another interesting feature of the WOx monolayer is the presence of dark depressions inside the domains. These depressions are located at the regular lattice positions and are most naturally associated with point defects, most likely oxygen vacancies.19 Their density as estimated from the STM images is ∼6%. At high resolution (Figure 3d, top), these defects appear as X-shaped depressions in the STM, which are 10−15 pm deep (see line profile in Figure 3d, bottom). The protrusions surrounding these defects display a brighter contrast than at the regular lattice positions, indicating a modified electronic structure in the vicinity of the defect. The apparent absence
of point defects in the submonolayer WOx islands (see Figure 2b,c) may be due to a kinetic effect. The presence of bare Pd areas in the submonolayer regime, where O2 molecules can easily dissociate, provides oxygen atoms which are available for oxidation during the growth of the WOx islands, healing out eventual oxygen vacancies. In contrast, on the WOx monolayer O2 dissociation is restricted, and O vacancies that are formed during the condensation process of the (WO3)3 clusters cannot be as easily reoxidized. A lattice model describing the antiphase domain structure of the W oxide monolayer on Pd(100) is presented in Figure 4(a). Here, four domains with the (√2 × √2)R45° lattice (blue circles) are depicted, with the overlayer lattice points positioned in 4-fold hollow Pd (gray circles) sites. This adsorption geometry follows from the absence of spot splitting of the integer order spots in the LEED pattern (Figure 3a).38 Each domain is shifted with respect to its neighbor by a 1/2 unit cell vector of the overlayer in either [001] or [010] directions (see inset in Figure 4a), to account for the experimental observation. This results in a large superlattice unit cell, as indicated by the 28686
DOI: 10.1021/acs.jpcc.6b10504 J. Phys. Chem. C 2016, 120, 28682−28693
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Figure 4. (a) Lattice model of the W oxide monolayer on Pd(100), used in the simulation of the LEED pattern shown in (b). Four antiphase domains with the (√2 × √2)R45° lattice (blue) are shown, and the gray circles correspond to the Pd atoms. The domain superlattice unit cell is highlighted. The inset in Figure 3a represents an enlarged view of the antiphase domain boundary region.
black solid line in Figure 4(a). The latter generates a LEED pattern (Figure 4b), where only the satellite spots in the intermediate vicinity of the (±1/2, ±1/2) reflections are displayed for clarity. This is justified by the fact that the higherorder satellite spots, only dimly seen in the experimental LEED pattern (Figure 3a), are generally of lower intensity, which decays with increasing distance from the (±1/2, ±1/2) positions. The simulated LEED pattern reproduces well the spot splitting observed in the experiment. Since the separation of the split spots depends on the size of the domains, the latter was varied until the calculated spot splitting matched the experimental one. The domains with the optimum size of 3.7 nm are shown in Figure 4a, and the corresponding superlattice 11 10 . This domain size is in good matrix is M = −11 10 agreement with the estimated mean size from the STM images (4.0 nm). Spectroscopic Characterization. Figure 5 shows W 4f core level spectra of the c(2 × 2)-WO3 overlayer taken at two different coverages, 0.6 ML (bottom spectrum) and 1.0 ML, and the latter was prepared at two different oxidation temperatures of 673 K (middle spectrum) and 823 K (top spectrum). The W 4f spectra consist of a 4f7/2−4f5/2 doublet (separated by a spin−orbit splitting of 2.15 eV and a peak ratio of 4:3), which is decomposed into different doublet components, as described below. The major component (red line) has a 4f7/2 peak located at a binding energy (BE) of 34.3 eV, which would formally correspond to an oxidation state of W5+ in bulk compounds;39 however, core level binding energies of ultrathin oxide films on metal surfaces are difficult to compare to bulk oxide values, due to the proximity of the underlying metal substrate and corresponding differential initial and final state shifts.25,40 We attribute this component to the regular W atoms in the interior of the oxide domains (islands), which are coordinated to four oxygen atoms at the W oxide− Pd(100) interface and terminated by one O atom on top in a WO groupsee structure model below. At 1.0 ML prepared at the typical oxidation temperature of 823 K (top spectrum in Figure 5) a small shoulder can be resolved at the low binding energy side of the W 4f peak; therefore, a minor component doublet with a 4f7/2 BE of 33.2 eV has been included in the
(
)
Figure 5. W 4f core level spectra of 0.6 ML (bottom spectrum) and 1.0 ML oxidized at 673 K (middle spectrum) and 823 K (top spectrum) W oxide on Pd(100). The W 4f spectra are decomposed into different spin−orbit doublet components, ascribed to W atoms inside the domains (red), with missing terminal O atoms (green) and at the domain boundaries (blue).
peak decomposition analysis (green line). Its integrated intensity amounts to ∼5% of the total W 4f peak intensity, which correlates well with the density of the oxygen vacancies as estimated from the STM images (∼6%). This minor component is interpreted as due to W atoms that have lost their terminal oxygen atoms and thus are in a less oxidized state and therefore characterized by a reduced W 4f binding energy. It is worth noting that this low BE component is absent in the W 4f spectra at 0.6 ML oxide coverage (bottom spectrum in Figure 5); this is in agreement with STM observation of submonolayer oxide surfaces, where no O vacancies are observed (Figure 2). The vacancy component is also completely missing in the W 4f spectrum of the 1 ML films oxidized at a lower temperature of 673 K (middle spectrum), which corresponds to a higher chemical potential of oxygen and thus to more oxidizing 28687
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Figure 6. Left panel: HREELS phonon spectra of p(2 × 2)-O (bottom), 0.5 ML WO3 (middle), and 1.0 ML W oxide (top) on Pd(100). The vertical dashed lines correspond to the calculated phonon frequencies. Right panel: principal vibrational modes of the 2-D WO3 structure model on Pd(100).
WO groups in the W oxide structure on Pd(100). This is confirmed by the DFT calculations, which give a frequency of 124 meV for the stretching vibration of perpendicular WO groups according to the 2-D structure model discussed below (see right panel of Figure 6). According to theory, the vibrational structures at 95 and 52 meV are then assigned to inplane and out-of-plane vibrations of the basal oxygen atoms, respectively (with frequency of 93.1 and 48.0 meV, respectively, from theory). The principal displacement vectors and calculated frequencies are indicated in the right panel of Figure 6. This also explains the lesser intensity of the peak at 95 meV as associated with in-plane vibrations. The HREELS spectrum of the 0.5 ML WOx surface shows an additional loss at 44 meV, which is due to the O−Pd stretching vibration of O adatoms on the free Pd surface areas. The fact that this peak nearly overlaps with the out-of-plane vibrations of basal oxygens is consistent with the fact that they both basically correspond to Pd−O stretching modes. Finally, a low-frequency perpendicular vibration of WO units is also predicted by theory, which is however covered by the tail of the elastic peak in the experiment. Structure Model and Theoretical Considerations. Figure 7 shows a model of the cubic WO3 bulk structure (top), in which a 2-D sheet consisting of a WO2 plane plus an O layer on top, cut perpendicular to the [100] direction, is highlighted. This 2-D WO2 + O bilayer forms the basis of our structure model (bottom of Figure 7), which has been refined and substantiated in the DFT calculations. Figure 8 presents the main structural models resulting from the present DFT study. Figure 8(a) reports the most stable equilibrium registry of the WO2 + O bilayer model shown in Figure 7 on the Pd(100) surface: this corresponds to the basal oxygens of the WO3 phase lying on top of surface palladium atoms. The preference for on-top positions of oxygens is reasonable considering the strong affinity of Pd toward O other local minima with O atoms in hollow or bridge sites with respect to the underlying substrate lie 0.21 and 0.25 eV per unit cell higher in energy, respectively. The stability of this registry has also been checked via short AIMD runs at 300, 600, and 900 K of 2.6, 2.8, and 4.8 ps in time, respectively, during which it was found that the atoms basically oscillate around the global minimum positionsoccasional jumps of the whole phase by
conditions. This suggests that the vacancy concentration of the WO3 monolayer can be precisely controlled by the oxygen chemical potential. Finally, a good fit of the experimental W 4f spectra requires the addition of a third component at the higher binding energy side, with a 4f7/2 BE of 35.2 eV (blue line). This latter component exhibits a significantly broader line shape than the main component (full width at half-maximum, fwhm = 1.4 eV versus fwhm = 0.7 eV, respectively), and its spectral weight increases from 20% at 0.6 ML to 38% at 1.0 ML. So far, we have considered the contribution of the W atoms inside the domains/islands (major component) and of W atoms with one terminal oxygen missing (oxygen vacancies, minor component) in the W 4f spectral shape. The W atoms located at the domain/island boundaries, however, have not yet been included in the physical explanation of the peak decomposition analysis. The percentage of these atoms depends on the W oxide islands size: ∼19% for the size of 8 nm (0.7 ML) and ∼36% for 4 nm (1.0 ML). In fact, these values agree very well with the corresponding spectral weights of the third component in the W 4f spectra, and it is most natural to associate the latter with the W atoms at the oxide boundaries. An intuitive explanation of the higher 4f BE of these W atoms at the island/ domain boundaries is that the oxygen atoms at the boundaries are coordinatively unsaturated, and electron density is withdrawn from the respective W atoms, yielding a higher binding energy. The broader line shape of this component may be caused by a higher degree of structural disorder at the domain boundaries (see Figure 3c). The HREELS phonon spectra of W oxide (0.5 and 1 ML) on Pd(100) are reported in Figure 6 (left panel), together with a spectrum of the p(2 × 2)-O adlayer for comparison. The spectra of the W oxide contain a sharp and prominent peak at 125 meV and two broader phonon loss structures, centered around 52 and 95 meV. The frequency of 125 meV is characteristic of the stretching vibration of a terminal WO group, a mode which is common for WO3 hydrates.41−45 Li et al. have also observed two peaks at 982 cm−1 (122 meV) and 1014 cm−1 (126 meV) in their infrared absorption spectrum of the 2-D c(4 × 2)-WOx phase on Pt(111),24 which they have associated with W(5+)O and W(6+)O stretching modes, respectively. The 125 meV phonon is therefore indicative of 28688
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pattern in very good agreement with the experimental images in Figures 2 and 3. When an oxygen vacancy is created, as in Figure 8(b) in a (4 × 4) unit cell a bright spot is missing in the simulated STM image, again in good agreement with Figure 3. The predicted work function of the bare Pd(100) phase according to our computational approach is 5.17 eV, while that of the (1 × 1) WO3/Pd(100) phase is 6.55 eV, somewhat larger than the experimental value of 6.2 eVthis discrepancy may be attenuated by the presence of oxygen vacancies, e.g., taking into account that the work function of the defective structure in Figure 8(b) is reduced to 6.46 eV. The substantial increase of the work function of the oxide-covered surface with respect to the bare metal surface is consistent with the appreciable polarity of the bilayer phase. To further rationalize the origin of this increase, we performed a Bader charge density analysis of the regular WO3/Pd(100) phase as well as of the bulk WO3 phase. We find that the W atoms in the bilayer have a charge of +2.92 e (vs +3.25 e in the bulk); the basal O atoms have an almost identical charge of −1.08 e as in the bulk; whereas the tungstyl O atoms have a reduced negative charge of −0.79 e. In comparison with bulk WO3, there is therefore a reduced polarity of the WO bond, while the electron charge transfer from the Pd substrate to the WO3/Pd(100) phase is modest: only 0.04 e overall. In more detail, we find that the outermost Pd atoms lose in total about 0.22 e in favor of bulk Pd (0.19 e) and W (0.04 e) atoms. The charge transfer from topmost to inner Pd atoms is consistent with a charge compression effect due to the interfacial O atoms and with the image charge effect mirroring and therefore stabilizing the polar W3+O−0.8 tungstyl bonds oriented perpendicular to the surface. To complete this analysis of the WO3/Pd(100) electronic
Figure 7. Cubic WO3 bulk structure: a 2-D sheet consisting of a bilayer of a WO2 plane plus an O plane is highlighted (top). Bottom: rigid-sphere model of the 2-D WO3 bilayer.
one lattice parameter also occurred at 900 K, in keeping with a predicted barrier of ≈0.2 eV. The stability of the phase structure during the AIMD simulations should exclude the occurrence of octahedral distortions, indicating the possible tendency of a monoclinic stabilization of the surface oxide structure; such distortions have been previously observed in the case of heteroepitaxial growth of WO3 thin films.46 In the simulated STM image of this structure, also included in Figure 8(a), only the topmost tungstyls are visible as bright spots, due to both topographic and electronic reasons, thus producing a
Figure 8. (a) Top-view (left panel), side-view (center panel), and STM image simulated at +1.00 V (right panel) of the (1 × 1) WO3 unit cell (corresponding to the experimentally observed c(2 × 2) unit cell). (b) Top-view (left side), side-view (center panel), and STM image simulated at +1.00 V (right panel) of a (4 × 4) cell containing a single oxygen vacancy (a single missing tungstyl)the (4 × 4) cell is highlighted in yellow. 28689
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Figure 9. PDOS of cubic WO3 (a) and of the WO3/Pd(100) phase (b). Contributions from O 2p states and W 5d states are depicted in red and blue, respectively, together with the total DOS, which is depicted in black in (a) and in gray in (b), respectively, where the different colors for the total PDOS mark that the Pd-orbital contribution is included in (b). In (a) the zero of energy has been positioned in correspondence of the HOMO level, while in (b) it corresponds to the system Fermi energy.
structure, we present in Figure 9 a comparison of the projected densities of states (PDOS) of WO3/Pd(100) and bulk WO3. Interestingly, two opposite phenomena come out from an inspection of Figure 9. On the one hand, we observe a metallization25 of the electronic structure of the bilayer due to proximity of the conducting metallic substrate leading to a weak background of oxygen-centered electronic states in the gap: the band gap predicted by our GGA+U approach is about 1 eV for bulk WO3 in its cubic form, whereas it is zero when the oxide bilayer is grown on the Pd surface, as it can be evidenced by comparing the topmost panels of Figures 9a and 9b. On the other hand, an increase in the distance between the main PDOS peaks associated with the oxide film with respect to bulk WO3 can also be observed in Figure 9 (it should be noted that the contribution of Pd-centered states to the total PDOS is not explicitly shown in Figure 9(b), although obviously dominant). These phenomena could lead to interesting chemical properties due to the simultaneous widened band gap and increased availability of low-energy states on the oxygen atoms, which are deemed essential for catalysis.24 It is interesting to note in this context that in an analogous 2-D CuWO4−Cu(110) phase a reduction of the band gap was instead observed with respect to the bulk,35 thus showing once more the particular behavior of 2-D oxide layers. To understand the formation and apparent stability and regularity of the antiphase domain boundaries at 1 ML coverage, we have first analyzed the (1 × 1) ordered phase and its optimal lattice parameter. To this end, we calculated the intrinsic energy of the bilayer phase as the difference between the total energy of the WO3/Pd(100) system minus the energy of the Pd(100) support as proposed in ref 25 and plotted it as a function of contracting/elongating the unit cell (see Figure 10). For the estimation of the lowest-energy lattice constant of the supported oxide, the entire unit cell has been globally contracted; then, for each value of the unit cell lattice constant, the energy of the contracted metallic Pd slab has been subtracted from the total energy of the metal/oxide system, yielding the total oxide + oxide/metal interaction energy
Figure 10. Plot of the oxide + oxide/metal interaction energy (in eV) for a WO3 unit supported on Pd(100) as a function of the Pd−Pd nearest-neighbor (n.n.) distance (in Å).
reported in Figure 10. The minimum of the intrinsic energy occurs at a lattice parameter which in this case is rather close to the bulk one, thus confirming a small tensile strain of ≈−2.8%. This simulation additionally provides the intrinsic elastic modulus of the bilayer phase25 and thus the energy penalty for stretching the bilayer phase to put it in registry with Pd(100), which turns out to be 0.35 eV per unit cell. There is therefore a significant tendency toward the formation of dislocations or phase boundaries. This tendency is confirmed when explicit models of antiphase boundaries are investigated as illustrated in Figure 11(a) in one example. To model phase boundaries, we have constructed several stripe models of the type shown in Figure 11(a) with varying width: from a (13 × 1) to a (25 × 1) unit cellthe picture in Figure 11(a) corresponds to a (21 × 1) unit cell. It can be noted that the overall stoichiometry of the system slightly deviates from WO3 and is, e.g., in fact W20O62 in Figure 11(a). We have also explored boundary models with different stoichiometry by adding or removing O atoms in different locations, but the resulting configurations were found 28690
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Figure 11. (a) Top-view and (b) STM image simulated at +1.00 V of a (21 × 1) cellan antiphase boundary is apparent in the center of the cell. Color coding: Pd atoms in white, oxygen atoms in red, and W atoms in light blue.
semiquantitative agreement with experiment, thus confirming the validity of the proposed assignment. It can be noted that the experimentally observed spread of values for W atoms at the boundaries may also suggest some disorder that is not taken into account in our modeling and could justify the lessened accuracy of the predicted core level energies for these atoms.
to be at higher energy. It is also noted that the domain boundaries are in antiphase, with adjacent stripes being shifted by half a lattice parameter, which reduces the electrostatic repulsion between negatively charged O anions at the boundary on adjacent stripes. Indeed, the resulting simulated STM image (Figure 11b) is in good agreement with the experimental ones in Figure 3. To assess the free energy difference associated with the formation of such boundaries, we use the chemical potential of gas-phase O 2, evaluated by assuming as before given experimental conditions of a temperature of 823 K and an O2 pressure of 2 × 10−7 mbar and the total energy difference between the stripe phases such as Figure 11(a) and the regular (1 × 1) phase in Figure 8(a). We so obtain
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CONCLUSIONS It is shown that a stoichiometric 2-D WO3 layer can be grown on a Pd(100) surface by vapor-phase deposition of (WO3)3 clusters followed by oxidative treatment at elevated temperature. This WO 3 sheet forms a robust well-ordered commensurate c(2 × 2) overlayer of a WO2 + O bilayer; however, residual epitaxial strain leads to a regular lattice of antiphase boundaries and a nanoscale domain pattern at monolayer coverage. The novel atomic geometry, electronic structure, and elastic properties of this WO2 + O bilayer have been elucidated by a combined experimental−theoretical multitechnique study. The DFT calculations reveal that the stability of this bilayer is based on polarity compensation via charge rearrangement at the interface and that the formation of antiphase boundaries is an energetically favorable strain release mechanism. The electronic structure of the 2-D WO3 exhibits two opposite effects: a somewhat widened principal band gap and metallization of the overlayer with low-energy oxygen states in the gap. This may be a favorable situation for surface redox reactions and thus oxidation catalysis. Indeed, the present experiments already preliminarily show that the WO2 + O bilayer is chemically active and displays reversible redox behavior via removal and restoration of tungstyl WO groups.
20WO3 /Pd(100) − (1 × 1) [Fig. 8a] + O2 → W20O62 − (21 × 1) [Fig. 11a]
ΔG = −0.41 eV
The formation of antiphase boundaries is thus thermodynamically favored. We have calculated the formation free energy also for models with a different width and found a shallow minimum at the (21 × 1) unit cell: ΔG(13 × 1) = −0.13 eV; ΔG(17 × 1) = −0.38 eV; ΔG(21 × 1) = −0.41 eV; ΔG(25 × 1) = −0.41 eV, thus predicting an optimal width of ∼4 nm, in excellent agreement with experimental observations. The W core level analysis as specified in section 2 gave the following estimates: (1) 35.28 eV for W in the (1 × 1) regular phase of Figure 8(a); (2) 34.62 eV for the W atom missing a tungstyl O in the (4 × 4) defective structure of Figure 8(b) (the other W atoms in this structure exhibit the same core level energies as in the regular phase with maximum differences of ±0.03 eV); (3) 35.45 eV for the W atoms at the antiphase boundaries of Figure 11a. The core level energy of the regular (1 × 1) phase is in excellent agreement with the experimental valuenote that a scalar relativistic Hamiltonian is used, so that the predicted value should be compared with a weighted average of the 4f7/2 and 4f5/2 experimental energies, i.e., 35.2 eV for the regular phase. The downshift of core levels for W atoms with missing tungstyl O and especially the upward shift of core levels for W atoms at the antiphase boundaries are instead somewhat underestimated by our DFT approach but are in
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
A. Fortunelli: 0000-0001-5337-4450 S. Surnev: 0000-0002-9756-0674 Notes
The authors declare no competing financial interest. 28691
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ACKNOWLEDGMENTS This work has been supported by the FWF Project P26633− N20 and the EU COST Action CM 1104. Computational research was performed in part using EMSL, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory, and PNNL Institutional Computing at Pacific Northwest National Laboratory.
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