Article pubs.acs.org/JPCC
High-Coverage Oxygen-Induced Surface Structures on Ag(111) N. M. Martin,*,† S. Klacar,‡ H. Grönbeck,‡ J. Knudsen,† J. Schnadt,† S. Blomberg,† J. Gustafson,† and E. Lundgren† †
Division of Synchrotron Radiation Research, Lund University, Lund, Sweden Competence Centre for Catalysis and Department of Applied Physics, Chalmers University of Technology, Göteborg, Sweden
‡
ABSTRACT: Surface structures formed by exposing Ag(111) to atomic oxygen have been studied by X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory calculations. From the combination of the experimental and theoretical results, a model is proposed for the 0.5 ML oxygen coverage with a c(4 × 8) periodicity. Moreover, we find that a bulk-like Ag2O phase starts to form at coverages above 0.5 ML.
atoms in the furrows. In contrast, the c(4 × 8) higher coverage structure is found to have a different structural motif and oxygen coverage.17 Structural models supported by DFT were proposed for the p(4 × 5√3)rect and c(3 × 5√3)rect structures, whereas only a tentative model without DFT support for the c(4 × 8) structure has been proposed.17 Higher oxidation state compounds that may form on silver have previously been reported in the literature. Reicho et al.18 found that bulk Ag2O(111) grows on Ag(111) after exposure to 1 bar of oxygen at 773 K together with a 2D surface oxide forming a p(7 × 7) superstructure. Kaspar et al.19 reported highly oxidized Ag films (Ag2O, AgO) that were formed by codeposition of Ag and atomic O, while Bao et al.4 reported another type of oxygen species denoted Oγ that forms at 1 bar oxygen pressure and temperatures above 500 K. The Oγ species produced a peak at 529.1 eV in the O 1s photoelectron spectrum, found to be strongly bound to the metal with a different interaction compared to the bulk Ag2O. Tjeng et al.20 reported that a thick Ag2O film may be grown at room temperature by exposing a silver sample to a free radical oxygen source at a total oxygen pressure of 8 × 10−5 Torr for 1.5 h. In the present report, we describe our study on the oxidation of Ag(111) by atomic oxygen using X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and DFT calculations, focusing on the higher oxygen coverage structures. As the oxygen coverage increases beyond that of the p(4 × 4), p(4 × 5√3)rect, and c(3 × 5√3)rect structures, new components in the Ag 3d5/2 photoemission spectrum can be observed in conjunction with a broadening of the O 1s level. Upon comparison with the calculated binding energy shifts and the expected intensities from the structural model, we confirm
I. INTRODUCTION Oxygen-induced reconstructions on Ag(111) have been studied intensively since the 1970s. The reason for the interest, apart from fundamental knowledge, is that silver oxides are known to have applications in catalysis (e.g., oxidation of methanol to formaldehyde, epoxidation of propene to propene oxide, and NOx reduction via selective catalytic reduction),1−5 data storage devices, electrolysis, transparent conducting oxides, as well as antimicrobial coatings or compact disk (CD) technologies.6 For a better understanding of these applications, knowledge of the atomic-scale structure of oxidized Ag surfaces is desired. The p(4 × 4) oxygen-induced reconstruction of the Ag(111) surface was first observed in the 1970s by G. Rovida et al.,7 and for a considerable amount of time, a large effort was directed to solve the structure.7−13 A review can be found in ref 14. On the basis of scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations, Carlisle et al.13 proposed a model with a Ag1.83O stoichiometry. Only recently, a more stable model was reported by Schmid et al.15 and Schnadt et al.16 Despite the Ag2O stoichiometry, the structural model does not resemble the bulk silver oxide. Instead the geometry is composed of triangular Ag hexamers separated by furrows with O atoms. In addition to the p(4 × 4) structure, a number of other oxygen-induced structures on the Ag(111) have appeared in the literature.14 In particular, Schnadt et al.17 reported that, upon exposing the Ag(111) surface to atomic oxygen, a multitude of oxygen-induced structures can be observed simultaneously. Apart from the p(4 × 4) surface reconstruction, structures with p(4 × 5√3)rect, c(3 × 5√3)rect, and c(4 × 8) periodicity were reported. The p(4 × 4), p(4 × 5√3)rect, and c(3 × 5√3)rect structures are very similar in terms of structural motif, oxygen coverage, and core-level shifts. As for p(4 × 4), c(3 × 5√3)rect consists of Ag hexamers, whereas p(4 × 5√3)rect is composed of Ag decamers again separated by O © 2014 American Chemical Society
Received: May 5, 2014 Revised: June 16, 2014 Published: June 23, 2014 15324
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three atomic layers of p(7 × 7)-Ag(111). These are fairly large models that contain 193 and 247 atoms, respectively. The p(7 × 7) surface cell was chosen to model the Ag2O(111)/Ag(111) interface thanks to a small lattice mismatch. The mismatch between the p(3 × 3)-Ag2O(111) and p(7 × 7)-Ag(111) is less than 2%. Repeated slabs are separated by at least 12 Å of vacuum. Structural optimization was performed with the two bottom layers in the Ag(111) slab constrained to their bulk positions until convergence criteria of 0.00001 eV, 0.0005 Å, and 0.01 eV/Å were met for the total energy, coordinates, and gradients, respectively. The surface core level shifts (SCLSs) were calculated by use of a pseudopotential generated with an electron hole in the Ag 3d and O 1s shells, respectively. As a Ag bulk reference, a Ag 3d core hole in the center of the slab was used, whereas the O 1s shifts were calculated with respect to the average O 1s binding energy of the p(4 × 4)-O/Ag(111) structure. The present procedure was tested for clean Ag(111) and was in good agreement with previous calculations and experiments.28 The calculated core level shifts for the topmost and the first sublayer of the Ag(111) surface, calculated in a p(4 × 4) cell, were found to be −0.19 and 0.0 eV with respect to the bulk value, respectively. Simulations of the STM images were performed within the Tersoff−Hamann approximation29 and simulated with a negative bias of −0.5 eV and a tip distance of 2.5 Å above the highest atom on the surface. Ab initio thermodynamics has been used extensively in the past to estimate the stability of surface structures as a function of temperature and partial pressures of gas-phase components.30 The surface free energy γ(T, p) is calculated as
the c(4 × 8) model originally proposed by Schnadt et al.17 Further oxygen exposure leads to a higher intensity of the new Ag 3d5/2 components and an O 1s component shifted toward higher binding energy. From these observations, in combination with DFT calculations, we suggest that for coverages above 0.5 ML bulk-like Ag2O islands start to form on the Ag(111) surface. The different oxide structures are found to coexist on the surface with ratios depending on the exposure time to oxygen.
II. EXPERIMENTAL METHOD The experiments were performed at beamline I311 at the MAX IV Laboratory in Lund, Sweden.21 The sample was mounted on a tungsten wire through which it could be heated to high temperatures by applying an electric current. The temperature of the sample was measured by a Chromel−Alumel thermocouple spot-welded on the back side of the crystal, and the sample was cleaned by repeated cycles of sputtering with Ar+ ions and annealing up to 800 K. The surface cleanliness was checked by low-energy electron diffraction (LEED) and XPS. For the oxidation process we used atomic oxygen produced by a thermal gas cracker from Dr. Eberl MBE Komponenten GmbH with a flux of oxygen atoms of about 1015 atoms/s and a cracking efficiency of about 15% for the conditions mentioned below. The background pressure during the atomic oxygen exposures was between 5 × 10−7 and 1 × 10−6 mbar using a sample temperature of 500 K. In agreement with previous studies,17 we find that the oxidation of Ag(111) with atomic oxygen produces a variety of coexisting surface structures and not close-to-perfect single-phase surfaces. The XPS measurements were recorded using normal emission and photon energies between 400 and 600 eV for the Ag 3d5/2 and between 600 and 900 eV for the O 1s line, respectively. For the analysis of the recorded core-level spectra a deconvolution procedure involving a Doniach-Šunijć line shape convoluted with a Gaussian was used. The deconvolution procedure has been described previously.15,22 The binding energies were calibrated to the Fermi edge. The experimental scanning tunneling microscopy images were obtained in the STM laboratory in Aarhus, Denmark.17
γ(T , p) = (EO/Ag − EAg − NAgμAg − NOμO(T , p))/A (1)
Here, the first and second terms are the total energies of the considered O/Ag systems and Ag reference, i.e., the clean Ag(111) surface. The difference in the number of Ag atoms per unit surface cell, with respect to the corresponding reference system, is given by NAg. μAg is the chemical potential for silver and represents the energy cost of transferring NAg atoms to/ from the bulk reservoir. For this term, the enthalpy of a Ag atom in silver bulk is used. A is the surface area of the computational cell. The chemical potential for oxygen (μ0) is given by
III. COMPUTATIONAL METHOD DFT23,24 was used in an implementation with plane waves and pseudopotentials. In particular, the CASTEP code was used.25 The exchange correlation (xc) functional was approximated with the form proposed by Perdew, Burke, and Ernzerhof.26 The interaction between valence electrons and the core was described by ultrasoft, scalar relativistic pseudopotentials27 where the considered elements were treated with 6 (O) and 11 (Ag) valence electrons, respectively. A plane-wave kinetic energy cutoff of 380 eV was used to expand the Kohn−Sham orbitals. This was sufficient to obtain convergence better than 0.01 eV in energy differences. Reciprocal space integration over the Brillouin zone is approximated with a finite sampling corresponding to a spacing smaller than 0.04 1/Å for all considered systems. The p(4 × 4), p(4 × 5√3)rect, c(3 × 5√3)rect, and c(4 × 8) were modeled in appropriate surface cells. The slabs were represented by five atomic layers, including the surface Ag−O overlayer. The Ag2O bulk-like structure was modeled by adhesion of one and two layers of p(3 × 3)-Ag2O(111) onto
μO(T , p) =
⎛ pO ⎞⎞ 1⎛ ⎜⎜EO + μO′ + kBT ln⎜ 02 ⎟⎟⎟ 2 2 2⎝ ⎝ p ⎠⎠
(2)
EO2 is the total energy of the O2 molecule. μ′O2 is calculated with data from thermodynamic tables.31 In this way, contributions from translations, vibrations, and rotations are included for O2. To enable direct comparisons between theoretical and experimental stabilities, all energies have been corrected to reproduce experimental binding energies of O2 and Ag bulk as well as the cohesion energy in Ag2O bulk.32
IV. XPS RESULTS A. Oxide Growth. The evolution of the Ag 3d5/2 and O 1s core level peaks as a function of oxygen exposure time is shown in Figure 1. As a reference, the spectrum of the clean Ag(111) crystal is shown at the bottom of Figure 1. The oxidation was performed by increasing the oxygen exposure time from 20 to 50 min in steps of 10 min at a sample temperature of 500 K and 15325
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signature for adsorbed OH is 1.8 eV with respect to oxygen in the p(4 × 4) structure. As discussed in the Introduction, the p(4 × 4) structure is very similar to the p(4 × 5√3)rect and the c(3 × 5√3)rect phases [see Figure 3]. Moreover, they have a similar O coverage (0.375 ML for the p(4 × 4) and p(4 × 5√3)rect phases and 0.4 ML for the c(3 × 5√3)rect phase).16 Thus, only small variations of the O 1s and Ag 3d5/2 intensities are expected for these structures. Therefore, we conclude that the spectrum acquired after 20 min exposure to atomic oxygen most likely corresponds to a coexisting structure of the p(4 × 4), the p(4 × 5√3)rect, and the c(3 × 5√3)rect phases. Assuming that the oxygen coverage after 20 min exposure corresponds to 0.375 monolayers (ML, 1 ML is equal to the number of Ag atoms in the top layer of the Ag(111) surface), we find oxygen coverages of 0.48, 0.58, and 0.81 ML for the 30, 40, and 50 min spectra, respectively. By increasing the oxygen exposure time from 20 to 30 min, we observe that the emission from the O 1s line broadens from a full width half-maximum (fwhm) of 0.69 eV for the p(4 × 4) structure to 0.84 eV for the 30 min exposure. Simultaneously the components assigned to surface Ag atoms (black at 367.7 eV) and impurities (red at 367.3 eV) in the Ag 3d5/2 spectrum increase in intensity (Figure 1). This suggests that by increasing the oxygen exposure time the surface evolves from p(4 × 4)-like structure to surface structures with higher oxygen content. Increasing the oxygen exposure to 40 min results in a new component in the O 1s level shifted by 0.7 eV toward higher binding energy with respect to the binding energy of the O 1s line for the p(4 × 4)-like structure [see Figure 1]. At the same time the components at −0.5 and −0.9 eV relative to the main line in the Ag 3d5/2 spectrum increase in intensity. In addition, the component at −0.9 eV broadens from a fwhm of 0.47 eV for the p(4 × 4) structure to 0.63 eV for the spectra obtained after 40 min oxygen exposure. The spectra recorded after 50 min oxygen exposure display the same trend. The component at −0.9 eV broadens to 0.66 eV, and the coverage for the 40 and 50 min exposures is estimated to be 0.58 and 0.81 ML, respectively. The observation of two components in the O 1s spectrum could be an indication of the formation of a thicker silver oxide film that contains several oxygen and Ag layers. To investigate whether this is the case, we used energy-dependent measurements from such a film as shown in Figure 2. From the measurements, it is clear that the different components in the Ag 3d as well as in the O 1s show the same energy-dependent variation. Thus, there is no clear depth dependence. Therefore, the energy-dependent measurements suggest that the O and Ag atoms in the structures formed at higher oxygen exposures are in the near surface region.
Figure 1. XPS data from the O 1s (left) and Ag 3d5/2 (right) regions upon stepwise increasing the atomic oxygen exposure of the Ag(111) surface. The emission from the clean Ag(111) surface is shown as a reference at the bottom. As the oxygen exposure increases beyond that of the p(4 × 4) phase (20 min exposure), two low binding energy components shifted −0.5 eV (black) and −0.9 eV (red) from the bulk component of the Ag 3d5/2 line are gradually increasing in intensity. The O 1s component broadens after an exposure time of 30 min, and a new component at a binding energy of 529 eV appears after 40 min of exposure. The Ag 3d spectra are normalized to the peak intensity, and the O 1s are normalized to the background to illustrate the oxide growth.
a background oxygen pressure of 5 × 10−7 mbar. For the last preparation cycle the oxygen pressure was increased to 1 × 10−6 mbar. Upon oxidation of Ag(111) new components toward lower binding energy appear in the Ag 3d5/2 spectra, in contrast to the typically positive core level binding energy shifts observed during the oxidation of other metals.34−36 For a detailed explanation of the mechanism of the reversed shift we refer to ref 37. We would like to emphasize that the oxidation recipe with atomic oxygen used in this work yields coexistence of different surface structures and not single-phase surfaces.17 Hence, the corresponding core-level spectra were always measured on a multitude of different phases. As will be discussed below for certain conditions, it was possible to prepare surface structures with a predominant phase as compared to other coexisting phases. The structure formed after exposing the Ag(111) sample to 5 × 10−7 mbar of oxygen for 20 min possesses the XPS signature of the p(4 × 4)−O−Ag structure [see Figure 3(a) and section 5.A for details], and three new components appear in the Ag 3d5/2 line as reported previously in refs 15, 22, and 38. In the O 1s spectrum, a component located at 528.2 eV is observed which also agrees well with the previously reported values for this phase.10−12,22 The weak broad component centered around 530.0 eV is attributed to OH. We calculate that the O 1s
V. STM MEASUREMENTS AND PROPOSED MODELS As earlier reported by Schnadt et al.,16,17 a variety of coexisting surface structures have been observed upon oxidation of the Ag(111) surface using atomic oxygen (as in the present study), including the p(4 × 4), p(4 × 5√3)rect, c(3 × 5√3)rect, and c(4 × 8) structures. Below we will discuss different models that we confirm for these structures based on the STM and XPS measurements for the more oxidized silver oxide structures. A. p(4 × 4) Phase. As presented in the previous section, the structure formed after 20 min exposure of the Ag(111) surface to atomic oxygen has the XPS signature of the p(4 × 4) or the p(4 × 5√3)rect or the c(3 × × 5√3)rect phases. For 15326
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is not present in the calculations for the p(4 × 4) structure. Below we will show that this component can be assigned to Ag atoms in structures with higher oxygen content. B. c(3 × 5√3)rect and p(4 × 5√3)rect Phases. The relaxed geometries for the c(3 × 5√3)rect and p(4 × 5√3)rect structures are reported in Figures 3(g) and 3(h), respectively. These structures have been characterized previously17 and are here only included for completeness. The structures are similar to p(4 × 4) in the sense that they are composed of twodimensional Ag clusters separated by furrows with oxygen. In similarity to p(4 × 4), c(3 × 5√3)rect consists of hexamers, whereas p(4 × 5√3)rect consists of decamers. The calculated cls for these structures are very similar to the calculated cls for p(4 × 4) as illustrated in Figure 4, making them difficult to distinguish by XPS. The three structures have similar thermodynamical stability and could experimentally be present simultaneously [see Section VI]. C. c(4 × 8) Phase. By further oxidation of the Ag(111) surface, structures with higher oxygen content are formed. Starting with the O 1s spectrum after an oxygen exposure of 30 min, we find an oxygen coverage of approximately 0.5 ML from the O 1s integrated intensity. Interestingly, the c(4 × 8) model proposed by Schnadt et al.17 [Figure 3(d)] has an oxygen coverage of 0.5 ML. The proposed model by Schnadt et al. was based on the STM appearance and the similarity to Pd5O4 structure. No DFT simulated STM image of the c(4 × 8) structure was, however, presented in ref 17. The simulated and experimental STM images for c(4 × 8) structure are shown in Figure 3(e) and (f), respectively. The area of the images is 41 × 30 Å2. The experimental STM image is recorded with 0.34 nA and −110 mV. There is an excellent agreement between the simulated and the experimental STM images. Using this model, we calculated the core-level binding energy shifts [Figure 4 and Table 1]. Two components are found for the O 1s emission for the c(4 × 8) structure, shifted −0.16 and 0.13 eV from the O 1s binding energy of the p(4 × 4) phase. The two components are assigned to O atoms denoted by 1 and 2 in Figure 3(d), respectively. Type 1 atoms are adsorbed approximately on top with respect to the substrate, and atoms 2 are occupying bridge positions. These shifts are, however, too small to be resolved experimentally, but instead we do see a broadening of the O 1s peak from 0.69 to 0.84 eV upon the formation of the c(4 × 8) structure. For the Ag 3d5/2 spectrum [Figure 1, 30 min exposure], two features are measured that are shifted by −0.5 and −0.9 eV with respect to the bulk component, respectively. On the basis of the results of the calculations for the c(4 × 8) structure [Figure 4 and Table 1], the −0.9 eV component, with a theoretical value of −1.05 eV, is assigned to Ag atoms occupying bridge positions and coordinated to two O atoms [α in Figure 3(d)], whereas the component at −0.5 eV, found at −0.54 eV in the calculations, originates from the remaining Ag atoms [β in Figure 3(d)] occupying both hcp and fcc hollow sites. The β atoms in the c(4 × 8) structure are positioned in sites similar to the α atoms in the p(4 × 4) structure, which explains their similar BE shifts [Figure 3(a) and (d)]. In addition, a broadening of the component at −0.9 eV is observed, from 0.47 eV for the p(4 × 4) phase to 0.63 eV for the 30 min spectrum. From the model in Figure 3(d), the expected intensity ratio between the photoemission from the α and β Ag atoms is 0.25. The experimental ratio is 0.26, which is in good agreement, in particular as coexisting phases are expected to exist on the surface and because diffraction effects may occur.
Figure 2. Energy-dependent measurements from the (a) O 1s level and (b) Ag 3d5/2 level. The integrated area from (c) O 1s components and (d) the Ag 3d5/2 components. The spectra are normalized to the peak intensity.
simplicity and since all structures share the same atomic-scale building blocks, we only consider the p(4 × 4) phase in the following. The p(4 × 4) structure is well described in the literature and consists of two Ag triangles, each with six Ag atoms situated in fcc and hcp sites over the Ag(111) substrate, respectively.15,16 Two O atoms are adsorbed in the furrow between the triangles, leading to a Ag12O6 stoichiometry [Figure 3(a)]. We compared the experimental and DFT simulated STM images to validate the computational setup, and we obtained an identical DFT simulated image as in ref 16. Simulated and experimental STM images for the p(4 × 4) are shown in Figure 3(b) and (c), respectively. The areas of the images are 43 × 30 Å2. The experimental STM image is recorded with 0.42 nA and −22 mV.16 The agreement between the simulated and experimental STM images is very good. Returning to the core-level spectrum [Figure 1, 20 min exposure] the components have previously been assigned15 as follows: bulk (green, 368.16 eV), Ag atoms in the triangles [black, −0.5 eV with respect to bulk, at 367.7 eV, denoted α in Figure 3(a)], Ag atoms in the furrow below the O atoms [blue, −0.23 eV, 368 eV denoted β in Figure 3(a)], and an additional peak [red, −0.9 eV, 367.3 eV]), previously assigned to impurities. The calculated core-level shifts (cls) for the p(4 × 4) phase are presented in Figure 4 and Table 1. There is a good agreement between the measured and calculated cls for the p(4 × 4) structure and with the cls previously reported.15 The component shifted −0.9 eV relative to the bulk Ag component 15327
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Figure 3. Structural model for the p(4 × 4) structure as obtained by DFT (a). The simulated (b) and experimental (c) STM images from the p(4 × 4) structure (43 × 30) Å2. Structural model for the c(4 × 8) structure as obtained by DFT (d). The simulated (e) and experimental (f) STM images from the c(4 × 8) structure (41 × 30) Å2. (g) and (h) show the relaxed geometries for the c(3 × 5√3)rect and p(4 × 5√3)rect structures, respectively. Color code: O (red), Ag in overlayer (gray), Ag in (111) substrate (yellow). The unit cells are indicated by the black/white lines. The experimental STM images were previously discussed in ref 17.
Figure 4. Calculated SCLSs of the different structures investigated: p(4 × 4), c(3 × 5√3)rect, p(4 × 5√3)rect, c(4 × 8), and p(7 × 7) (one and two layers). Gaussian broadenings have been performed for the computed shifts and are shown by the dotted lines. The different shifts have been grouped to match the experimental peaks, using the same color code as in Figure 1. 15328
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Table 1. Comparison between the Calculated (Calcd) and Experimentally (Exptl) Observed Core-Level Shifts (cls) for the Oxygen-Induced Structures on Ag(111)a struct.
O cov. (ML)
p(4 × 4)
0.375
c(4 × 8)
0.5
Ag2O ≥ 1 layer
≥0.75
atom
exptl cls (eV)
calcd cls (eV)
Ag (β) Ag (α) O Ag (β) Ag (α) O (2) O (1) Ag Ag Ag O
−0.2 −0.5 0 −0.5 −0.9 0.0 0.0 −0.2 −0.5 −0.9 0.7
−0.3 −0.6 0 −0.5 −1.0 −0.16 0.13 −0.1 −0.5 −1.0 0.5
a
The core level shifts are presented with respect to the binding energy of the bulk Ag (3d5/2) and O (1s) identified in the spectra measured on the p(4 × 4) phase. The calculated oxygen coverage is also given for the different structures (site in parentheses).
D. Bulk-Like Ag2O. After a further increase of the oxygen dose by extending the exposure time to 50 min, the oxygen coverage is increased to 0.81 ML. As shown in Figure 1 (40 and 50 min exposure time), the Ag 3d5/2 components are shifted about −0.5 and −0.9 eV (black and red components at 367.7 and 367.3 eV, respectively) to lower binding energy as compared to the bulk Ag component. In the O 1s emission, we observe a new component shifted 0.7 eV toward higher binding energy with respect to the component ascribed to the less oxygen-containing p(4 × 4) and c(4 × 8) structures. Binding energies at 528.9 eV in the O 1s and 367.6 eV in the Ag 3d5/2 line have previously been observed upon oxidation of a pure silver sample and were assigned to Ag2O.20 In Table 2
we summarize the reported XPS binding energy assignments for various silver oxides. A comparison with the observed core level shifts after 50 min exposure [cf. Figure 1] suggests that it is likely that a surface layer with a structure similar to that of bulk Ag2O is formed under these conditions.18 We find peaks at 528.9 eV in the O 1s level and 367.7 eV in the Ag 3d5/2 spectrum, which have previously been reported for Ag2O [Table 2]. Previously, a p(7 × 7) reconstruction has been observed by surface X-ray diffraction (SXRD), and the structure has been related to a single Ag2O(111) plane on Ag(111) and subsequent formation of Ag2O islands.18 To corroborate the interpretation of the core-level binding energies, calculations were performed for a bulk-like Ag2O film with one and two layers. The optimized structures are shown in Figure 5. The calculated and experimental cls are reported in Figure 4 and Table 1. The calculations for the p(7 × 7) one-layer structure [Figure 4] yield an O 1s signature shifted 0.5 eV with respect to that of photoemission from the oxygen atoms in the p(4 × 4) structure. Moreover, Ag 3d shifts at −0.1 and −0.5 eV are found [with respect to photoemission from the Ag bulk]. The calculations are consistent with the experiments as well as with previous core-level spectroscopy reports on Ag2O.20 Similar shifts for the O 1s are calculated for the two-layer p(7 × 7)
Table 2. Summary of O 1s and Ag 3d Binding Energy Assignments for Different Silver Oxides structure Ag2O AgO Ag−Oγ
O 1s 528.9 528.4 529.0 529.5
± 0.2 eV eV eV eV
Ag 3d 367.6 366.8 368.0 367.3
± 0.2 eVa eVb eV eVc
a As reported by Tjeng et al.20 bAs reported by Kaspar et al.19 cAs reported by Bao et al.4
Figure 5. Structural model for the p(7 × 7) structure as obtained by DFT (a), and the corresponding two-layer structure is shown in (b). 15329
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structure [Table 1 and Figure 4]. In addition, the calculations of the Ag 3d binding energy shifts suggest an additional component at −1.0 eV with respect to photoemission from bulk Ag as compared to the one-layer structure. This agrees well with the experimental data presented in Figure 1. A coverage of about 0.75 ML, close to the experimentally determined coverage, is calculated for the two-layer p(7 × 7) structure, and it is, therefore, likely that a structure of a bulk-like silver oxide consisting of more than one layer forms on the surface for high oxygen coverages. Consequently, we assign the peaks at 528.9 eV in the O 1s and 367.7 and 367.3 eV in the Ag 3d5/2 spectrum to photoemission from a bulk-like Ag2O structure. However, the remaining intensity of the peak close to 528 eV in the O 1s level suggests that the above-mentioned structures [the p(4 × 4), p(4 × 5√3)rect, c(3 × 5√3)rect, and c(4 × 8) phases] still coexist on the surface with a higher ratio as compared to the Ag2O phase and still contribute to the photoemission spectra. In addition, the (7 × 7) one-layer structure reproduces well the O 1s shift and is thermodynamically close to the above-mentioned low-coverage structures (see below). This could, consequently, explain the energy-dependent measurement behavior in Figure 2.
Figure 6. Surface free energies for p(4 × 4) (green) and c(4 × 8) (blue) as a function of oxygen chemical potential. Δμ is calculated as μ0 − 1/2EO2 (eq 2). Also, for reference, calculated surface free energies for oxygen chemisorption over Ag(111) at 1/16 ML (black), 1/4 ML (red) coverages as well as structures c(3 × 5√3)rect (violet) and p(4 × 5√3)rect (orange) from ref 17 are added. The temperature scales are given for two different pressures, corresponding to atmospheric (1 atm) and preparation conditions of the oxides (10−8 atm).
VI. DISCUSSION From the above data it is seen that the photoemission components in the Ag 3d5/2 line shift toward lower energy, as the oxygen coverage is increased beyond that of the p(4 × 4), p(4 × 5√3)rect, and c(3 × 5√3)rect structures. The shifts in the Ag 3d line are accompanied by a broadening of the O 1s line. The observed changes can be explained by the formation of a more oxidized silver surface in comparison to the p(4 × 4) phase. We observed both the c(4 × 8) structure and Ag2O-like phases with a (7 × 7) periodicity. On the basis of DFT calculations, the previously proposed model17 is confirmed for the c(4 × 8) structure, with a coverage of 0.5 ML [see Figure 3(d)]. The simulated STM pattern for the proposed structure of the c(4 × 8) phase agrees well with the measured STM image. Furthermore, we have shown that for oxygen coverage above 0.5 ML emission from bulk-like Ag2O areas on the surface is detected. The correlations between the theoretical binding energy shifts for the film model are in good agreement with the observed cls. In addition, our measurements indicate that the p(4 × 4), p(4 × 5√3)rect, c(3 × 5√3)rect, c(4 × 8), and Ag2O structures coexist with ratios depending on the exposure time to atomic oxygen, with higher coverage structures being predominant for higher exposure times. The similarities in stability are supported by the phase diagram shown in Figure 6. The surface structure having the lowest surface free energy in the calculated phase diagram is the thermodynamically most stable. From Figure 6 it can be deduced that the p(4 × 4), p(4 × 5√3)rect, c(3 × 5√3)rect, c(4 × 8), and (7 × 7) one-layer structures have similar stability. It is therefore not surprising that these structures may coexist on the surface. Which structure that actually dominates may be kinetically determined. At higher oxygen pressures, thicker Ag2O phases may form as supported by both the measurements and thermodynamic analysis. It should be noted that the used approximation to the exchange-correlation functional has the electronic self-interaction error common to other local and semilocal approximations and, furthermore, does not include van der Waals interactions. Given the similar stability of many structures over
a wide range of conditions, it cannot be excluded that addition of exact exchange and/or van der Waals interactions may alter the stabilities. In fact, this uncertainty is one reason why totalenergy calculations are combined with STM and XPS in the assignments of surface structures. A closer inspection of Figures 3 and 5 suggests that the c(4 × 8) and p(7 × 7) are quite similar from a structural point of view. They consist of similar building blocks just placed differently on the surface. This suggests that the c(4 × 8) structure is a precursor state for the formation of bulk silver oxide.
VII. SUMMARY We have presented an experimental and theoretical study of the oxide structures formed by oxidation of the Ag(111) surface. By using core-level spectroscopy and STM we have shown that upon stepwise oxidation of the Ag(111) surface structures with higher oxygen content are obtained. DFT calculations confirm the previously suggested structure of the c(4 × 8) phase with a coverage of 0.5 ML. At higher coverage (above 0.5 ML), islands of bulk-like Ag2O films were found to appear on the surface. The calculated core-level shifts for the two structures are in good agreement with the experimental data.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +46 46 22 287 21. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the foundation for strategic research (SSF), the Swedish Research Council (contract no. 2010-5080 and 2008-5314), the Crafoord Foundation, the Knut and Alice Wallenberg Foundation, and the Anna and Edwin Berger Foundation. The MAX IV Laboratory staff is gratefully acknowledged. The calculations were performed at C3SE in Göteborg. The Competence 15330
dx.doi.org/10.1021/jp504387p | J. Phys. Chem. C 2014, 118, 15324−15331
The Journal of Physical Chemistry C
Article
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Centre for Catalysis is hosted by Chalmers and supported by the Swedish Energy Agency and the member companies. The authors are grateful to F. Besenbacher for allowing them to test the experimental STM data taken at the Interdisciplinary Nanoscience Center (iNANO), University of Aarhus, against DFT calculated data in Figure 3.
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