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Strongly-Bound Oxygen on Silver Surfaces: A Molybdenum Oxide Contamination? R. Reichelt,† S. G€unther,†,§ and J. Wintterlin*,†,‡ † ‡
Department Chemie, Ludwig-Maximilians-Universit€at M€unchen, Butenandtstr. 5-13, D-81377 Munich, Germany Center for NanoScience (CeNS), Schellingstr. 4, D-80799 Munich, Germany
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ABSTRACT: “Strongly-bound oxygen” or “Oγ” plays an important role in the literature on the Ag-catalyzed formaldehyde and ethylene oxide synthesis reactions. However, the high thermal stability of this oxygen species on Ag surfaces is an unsolved problem. We have investigated oxygen adsorption on an Ag(111) surface by X-ray photoemission spectroscopy and scanning tunneling microscopy. NO2 was used as oxygen source. By applying a special dosing procedure an oxygen species with EB(O 1s) = 529.5 eV was obtained that is close to the EB of the “strongly-bound oxygen”. The species is stable up to >800 K. Several structures √ ordered √ surface √ √ were resolved, among others a (3 3 3 3)R30°, a ( 52 52)R13.9°, and a quadratic structure. However, a small coexisting oxidic Mo 3d peak indicates that the species is caused by a Mo oxide contamination. During NO2 dosing, Mo oxide is transported from the sample holder to the sample surface.
1. INTRODUCTION The interaction of oxygen with Ag surfaces is one of the beststudied, but also one of the most complicated adsorption systems. The interest is driven by two silver-catalyzed reactions, the partial oxidation of ethylene to give ethylene oxide and the partial oxidation of methanol to give formaldehyde, both largescale industrial processes.1,2 For both reactions one central question is the chemical state of oxygen on the active Ag surface. The interaction of oxygen with Ag has therefore been investigated with all kinds of samples, supported catalysts, Ag powder, polycrystalline Ag, and Ag single crystals, over a wide range of temperatures, at high pressure, under ultrahigh vacuum (UHV) conditions, and by using the entire arsenal of surface science techniques. From the investigations in which thermal desorption spectroscopy (TDS) has been used one can, for temperatures above 300 K, extract three oxygen species that are discriminated by their thermal stabilities. The first species desorbs at 500600 K, the second in a broad peak centered at 700800 K, and the third very roughly at 9001000 K, or it may even remain on the surface at these temperatures. The first desorption state was seen in almost all of these investigations,319 whereas the second3,4,1113,16,18 and third states 1113,16 were only detected in a subset of investigations as additional peaks; a further group of papers only reported one or both of the latter two states2025. The differences are partially explicable by the experimental conditions, but there also seem to be variations between the different groups of authors. The first state is usually assumed to represent an atomic oxygen species on or close to the surface. This includes the oxygeninduced surface reconstructions known from the low-index single r 2011 American Chemical Society
crystal surfaces of Ag. The second state is mostly attributed to O atoms dissolved in the bulk of Ag that migrate to the surface during the TDS and desorb. The third state suggests an extremely stable binding configuration that has been termed “highly stable oxygen”, “strongly bound oxygen”,13,20,23,2530 or “Oγ” according to its appearance in TDS.13,16,2025,2733 We have investigated an oxygen species possibly related to this high-temperature state by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). The high-temperature species is well documented in the literature, and it has been found on small Ag particles, polycrystalline Ag, and Ag single crystal surfaces, mainly on Ag(111). In XPS it was characterized by an O 1s peak at 528.9 or 529.0 eV,27,29,30 and in surface-enhanced Raman spectroscopy (SERS) it displayed a prominent vibration band at 803810 cm1.23,27,28,31,3436 There is strong evidence that the high-temperature oxygen species is localized at the surface. The angular dependence of the intensity of the 529.0 eV peak was characteristic for a surface species,29 the species was detected in ion scattering spectroscopy (ISS),27,29 and it was removed by sputtering.27,29 Furthermore, on the Ag(111) surface it was resolved√by STM √ as a hexagonal moire pattern superimposed by a ( 3 3)R30° structure.26,29,32 These structures were also detected by scanning electron microscopy and by reflection high-energy electron diffraction.2628 All publications in which the high-temperature species is described report that high oxygen exposures were required for preparation. Received: April 12, 2011 Revised: July 12, 2011 Published: July 20, 2011 17417
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The Journal of Physical Chemistry C There have also been reports that the high-temperature species plays a role in the Ag-catalyzed partial oxidation reactions. At the reaction temperature of the formaldehyde synthesis on Ag, 870990 K 2, it is the only species with a significant lifetime on the surface and it has been shown to react with methanol.13,2527,32,37 For the ethylene epoxide synthesis, although performed at lower temperatures,1 it has been detected by in situ XPS, together with other oxygen states, but here it rather blocked active sites.38 The temperature stability of this species is one of the major unsolved problems in the Ag/O adsorption system. All other surface phases of O on Ag and all 3D Ag oxides decompose at temperatures below 600 K. The activated diffusion of O atoms through the Ag lattice, which accounts for the higher desorption temperature of bulk-dissolved oxygen, should not play a role if the species is actually located at the surface. It has been suggested that the stability may be explained by special binding sites of O atoms within the topmost Ag layer (“surface embedded oxygen”)13,25,30,32 or directly below the topmost layer (“subsurface oxygen”).11,13,23,24,26,29,33,34 Such sites, and combinations of such sites with surface sites, have been analyzed in extensive density functional theory (DFT) calculations on the Ag(111) surface.3942 It was found that the most stable site, at low oxygen coverage, is the simple 3-fold (fcc) hollow site on the surface (binding energy of the O atom 3.6 eV at low coverage; this value and the following values are with respect to atomic O in the gas phase). With increasing coverage, the binding energy decreases and subsurface states also become stable. However, more stable at high coverage is another structure, a (4 4)O phase, for which an oxide-like structure had been assumed (binding energy 3.5 eV). It was later shown that the (4 4)O structure does not represent a surface oxide but a surface reconstruction, but the binding energy of the O atoms is almost the same.4345 The experimental values, 3.4 eV for the (4 4)O structure on Ag(111),10 3.4 to 3.5 eV for other reconstructions on the Ag(110) surface,5,9 and 3.3 eV for oxygen on a supported Ag catalyst,19 are all in the same range as these calculated values. What would be needed to explain a desorption temperature of >900 K is a binding energy of higher than 5.3 eV [using the experimental binding energy of 3.4 eV of the (4 4)O structure and the desorption temperature of 580 K as reference points for the extrapolation10]. We have not found such an extreme value in any of the theoretical investigations of silver (e.g., refs 4652). Even if one takes into account that TDS measures an activation energy rather than a binding energy, so that the desorption might involve additional activated processes, >5.3 eV appears unrealistically high. Moreover, the other Ag/O surface phases did not display such differences between desorption energies and calculated binding energies. We here suggest a partial solution to this problem. We report about an O species on Ag(111) that is thermally very stable and XP-spectroscopically identical to the “strongly bound oxygen” (“Oγ”). However, it is caused by a small Mo oxide or Ag molybdate contamination. Mo oxide is transported from the sample holder to the crystal under the conditions of the oxygen treatment of the Ag sample. The effect could also play a role in other surface science experiments when mobile oxides can form at elevated oxygen pressures.
2. EXPERIMENTAL SECTION The experiments were carried out in a two-chamber UHV system that has been described in detail before.53 One of the
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chambers is a small cell with the STM and can be operated under UHV (base pressure 6 1010 mbar) and at pressures of up to 1 bar. The second chamber (base pressure 2 1010 mbar) is used for sample preparation and characterization, for the present investigation mainly by XPS. A transfer rod was used to move the sample from the preparation chamber to the STM cell. The XPS measurements were performed with a monochromatic Al KR X-ray source (Omicron XM 1000) and a hemispherical analyzer (SPECS Phoibos 100). The energy resolution was 0.3 eV according to the width of the Fermi edge; the measured fwhm of the Ag 3d5/2 peak was 0.6 eV. Binding energies were calibrated using EB(Au 4f7/2) = 83,98 eV and EB(Ag 3d5/2) = 368.26 eV.54 The XPS peaks were fitted with a Doniach-Sunjic function55 [fitting parameters: Lorentzian 0.3 eV, asymmetry parameter 0.05, Gaussian 0.41 eV, 0.47 eV, 0.6 eV, 0.7 eV for Ag 3d, Mo 3d, O 1s (528.3 eV), O 1s (529.5 eV), respectively]. The intensities of the signals were normalized to the Ag 3d5/2 signal and were corrected by the respective cross sections. Differential cross sections accounting for the angle of 66° between X-ray source and analyzer in our setup were used for quantitative analysis. [With tabulated total cross sections56 and asymmetry parameters57 the following values were obtained: dσ/dΩ(O 1s) = 0.0032 Mbarn/ Sterad, dσ/dΩ(Mo 3d5/2) = 0.0072 Mbarn/Sterad, dσ/dΩ(Ag 3d5/2) = 0.0136 Mbarn/Sterad.] The analyzer transmission function was determined with a clean Au(111) sample by the intensities of the signals Au 4f7/2, Au 4d5/2, Au 4p3/2, Au 4p1/2, and Au 4s. A Shirley background was subtracted from the Au signals,58 the signals were numerically integrated, and the intensities were normalized by the respective cross sections and by including the number of detected layers Λ(Au) = [1 exp(d/λ)]1. (d = 2.36 Å is the layer distance of Au(111), λ the effective mean free path of the photoelectrons for which the tabulated inelastic mean free paths of the respective Au photoelectrons were taken.59) For the applied iris lens mode the obtained transmission function did not display the typical T(Ekin) ≈ Ekin1 form but only varied by (20% within the employed kinetic energy range between 700 and 1400 eV. Therefore, for the O 1s, Mo 3d, and Ag 3d electrons, the analyzer transmission function was considered as constant, and the stoichiometry of a homogeneous surface phase could be directly obtained from the ratio of the corrected intensities. The STM was a home-built beetle-type setup.53 It can be operated at elevated temperatures and high pressures, but all data presented here were taken at room temperature and under UHV. The STM experiments were performed with the same Ag(111) crystal and without changing the mounting of the crystal, so that, after the STM had been carefully calibrated with the (4 4)O structure, the orientations and lattice constants of all other surface phases could be precisely determined. TD spectra were measured with a quadrupole mass spectrometer (Balzers QMA 200) mounted behind a pseudodifferentially pumped cover. To record the spectra the sample was placed in front of a small orifice in the cover and heated by electron bombardment (heating rate 5 K/s). The Ag(111) sample was mounted to a sample holder that provided the ramps for the beetle STM and fitted to mountings in the manipulator, the STM, and the transfer rod.53 The sample holder was a round Mo mask to which the hat-shaped crystal was mounted at its “brim”. In this geometry, there was a 0.5 mm wide gap between the Ø 5 mm polished surface and the holder, so that only the “brim” was in mechanical contact with the holder. The temperature was measured with a Ni/NiCr thermocouple 17418
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clamped to the back side of the crystal. The sample was prepared by bombardment with Ar ions (500600 eV) at 300 K, annealing at 800 K for about 30 min, and cooling at a rate of about 0.1 K/s. This procedure was repeated until the sample showed sharp spots in low-energy electron diffraction and was clean in XPS. XPS experiments with a second Ag(111) crystal led to the same “high-temperature” oxygen species. For dosing, we used NO2 instead of O2, an established procedure to avoid the large O2 exposures otherwise required because of the low sticking coefficient of O2 on Ag(111).14 The NO2 molecules dissociate on the surface into O atoms and NO molecules with high efficiency, the NO desorbs. Exposures were further lowered by means of a capillary array doser that created a partially directed NO2 beam, corresponding to a locally enhanced pressure at the sample surface. This led to a considerably lower background pressure during and after dosing. The exposures [in Langmuirs (L), with 1 L = 1.33 106 mbar s] contain the estimated enhancement factor of the doser of approximately 100.
3. XPS RESULTS For the data presented here the following procedure gave reproducible results. A 600-L portion of NO2 was dosed at a sample temperature of 500 K, followed by flash annealing to 570 K. Dosing and flash annealing were repeated several times, and then the sample was annealed at 800 K for 2 min, which concluded one preparation cycle. After such a cycle, an O 1s peak at 529.5 eV remained [Figure 1(a)], although during the 800 K annealing, all chemisorbed oxygen and all oxygen from Ag oxide phases should have desorbed. By repeating the preparation cycle, the 529.5 eV peak increased (cycle numbers indicated in the figure). For cycle 7 the figure shows, as an example, a spectrum recorded after the NO2 dosing step at 500 K, but before the 570 K flash annealing. The large peak at 528.3 eV is caused by the (4 4)O phase.10 Hence a preparation cycle first created chemisorbed oxygen in the (4 4)O or similar reconstruction that, during the 570 K annealing, partially desorbed, but partially seemed to convert into the 529.5 eV species that was stable up to at least 800 K. The small peak at 530.7 eV may be due to traces of a species termed “OR” in the literature,29 the weak intensity at 531.5 eV could be caused by minor readsorption of NO2 from the residual gas.14 The EB of the stable species, 529.5 eV, is close to the EB reported by Bao et al. for the “Oγ” species, 529.0 eV.29 It even turns out that the energies become identical when the different energy scales of the spectrometers are considered. The authors of ref 29 used the same calibration point EB(Au 4f7/2) = 84.0 eV as we, but in their spectra the Ag 3d5/2 peak occurred at 368.0 eV, a slightly lower EB than the literature value 368.26 eV,54 the value used by us. If one recalibrates the data of Bao et al. by linearly extrapolating this deviation to the O 1s energy, then the peak shifts from 529.0 to 529.4 eV, i.e., within a typical error of (0.1 eV, to the value found by us. However, in our case, the 529.5 eV peak was clearly connected with a Mo signal [Figure 1(b)]. Two, slightly shifted Mo 3d doublets evolved during the preparation cycles, one with EB(Mo 3d5/2) = 231.0 eV, the second, less intense, with EB(Mo 3d5/2) = 231.7 eV. Mo 3d5/2 binding energies in this range are characteristic for Mo oxides.60 Both, the O 1s peak at 529.5 eV and the Mo doublet, continuously increased with further preparation cycles (Figure 2). However, the ratio of the 529.5 eV O 1s and Mo 3d5/2 normalized peak intensities remained at an almost constant value of [I(O 1s): I(Mo 3d5/2)] [dσ/dΩ(Mo 3d5/2): dσ/dΩ(O 1s)] = 3.88 ( 0.44
Figure 1. XP spectra of the Ag(111) surface taken after selected preparation cycles (dosing of NO2 at 500 K, flash annealing to 570 K, 2 min annealing at 800 K); cycle numbers are indicated. (a) Energy range of the O 1s peak. Gray shading marks the peak at EB(O 1s) = 529.5 eV. The spectrum from cycle 7 was recorded before the 570 K annealing step and shows an additional peak at 528.3 eV caused by the (4 4)O structure. The small peaks at 530.7 eV (red) and 531.5 eV (green) are probably due to small amounts of “OR” and readsorbed NO2, respectively. (b) Energy range of the Mo 3d doublet. Two doublets were resolved, one at EB(Mo 3d5/2) = 231.0 eV/EB(Mo 3d3/2) = 234.1 eV, the second at EB(Mo 3d5/2) = 231.7 eV/EB(Mo 3d3/2) = 234.8 eV.
during the preparation (Figure 2, top). We have to conclude that in the course of the NO2-dosing/annealing cycles, a Mo oxide compound formed on the Ag surface with a stoichiometry of Mo: O = 1: (3.88 ( 0.44). (This assumes that both species are located on the surface, rather than in positions below the surface, and that a homogeneous surface phase exists.) The maximum Mo 3d5/2 intensity was always lower than 1% of the corresponding Ag 3d5/2 substrate signal. We initially assumed a crosstalk signal from the Mo sample holder to explain this low signal, although the sample position had been carefully adjusted to restrict the electron collection area to the sample. Of course, the fact that the sputter-cleaned sample showed no Mo signal and that the intensity of the Mo signal increased in the course of the NO2 dosing/annealing cycles (Figure 2), contradicted such an explanation. Nevertheless, the possibility of a crosstalk was 17419
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Figure 2. Intensities of the O 1s peak at 529.5 eV and of the Mo 3d5/2 peak, normalized to the respective Ag 3d5/2 peaks and corrected by the respective cross sections, as functions of the preparation cycles. Also shown is the ratio of the normalized, corrected intensities of the two peaks. For a homogeneous mixture of the two elements this ratio is equivalent to the O: Mo stoichiometry.
further investigated by changing the sample position in front of the analyzer. When the sample was moved away from the optimal position in any lateral direction a Mo 3d5/2 signal occurred at 228.0 eV, characteristic for metallic Mo and much larger than the oxidic Mo peaks. Because the holder thus mainly showed a metallic Mo signal the oxidic Mo signal on the sample could not be caused by crosstalk from the holder. To finally remove the likely Mo source, we replaced the Mo sample holder by a holder made from stainless steel. The 529.5 eV oxygen species could then no longer be prepared, and no Mo signal was observed. We will discuss below how the Mo might be transported from the holder to the sample surface. The coverage Θ(O) of the 529.5 eV oxygen species was determined from the normalized O 1s intensity x(O 1s) given by xðO 1sÞ ¼
IðO 1sÞ dσ=dΩðAg 3d5=2 Þ ΘðOÞ ¼ IðAg 3d5=2 Þ 3 dσ=dΩðO 1sÞ ΛðAgÞ
ð1Þ
I(O 1s) and I(Ag 3d5/2) are the measured intensities and dσ/ dΩ(O 1s) and dσ/dΩ(Ag 3d5/2) the respective differential cross sections. The number of detected Ag layers Λ(Ag) was estimated by Λ(Ag) = [1 exp(d/λ)]1 (neglecting the damping of the Ag 3d intensity by the Mo oxide). With the layer distance of Ag(111), d = 2.36 Å, and the effective mean free path of the Ag 3d photoelectrons, λ = 16.3 Å, (kinetic energy ∼1120 eV according to the Al KR excitation),59 one obtains Λ(Ag) = 7.4 (layers). The last data point in Figure 2 (preparation cycle 11) thus gives a coverage of the 529.5 eV species of Θ(O) = x 3 Λ(Ag) = 0.037 3 7.4 = 0.27. In the whole set of experiments, the maximum coverage obtained was Θ(O) = 0.50. This nominal value may not yet represent the maximum possible coverage, and it may be subject to errors, e.g., by photoelectron diffraction in the oxide layer. To find out if the 529.5 eV oxygen species is located on or below the Ag surface the intensities of the XPS peaks were measured as a function of the emission angle of the photoelectrons, by tilting the sample in front of the analyzer. Figure 3 shows that the normalized O 1s and Mo 3d5/2 intensities both increased with increasing
Figure 3. Intensities of the O 1s peak at 529.5 eV and of the Mo 3d5/2 peak, normalized to the respective Ag 3d5/2 peaks and corrected by the respective cross sections, and intensity ratio, as functions of the photoelectron emission angle (with respect to the surface normal). The last pair of data points, measured at 0°, shows the same intensities as the first pair at 0°, proving that the surface had not changed by radiation effects during the experiment.
photoelectron emission angle (measured with respect to the surface normal). For a surface species, one expects an increasing normalized intensity with more grazing electron emission angle because the relative peak intensity caused by electron emission from a bulk species used for normalization (here the Ag 3d intensity) decreases because of the greater path length of the emitted photoelectrons in the solid at grazing emission. This is what was observed here. Moreover, the ratio of the O 1s and Mo 3d5/2 signals stayed at a constant value of approximately four when the angle was changed (Figure 3). Oxygen and molybdenum thus display the same angular dependence, indicating that they actually form a compound and that this compound is located on the surface. To further substantiate this conclusion, we performed experiments in which the (4 4)O phase was prepared on a surface that was partially covered by the 529.5 eV species. The preparation followed our standard (4 4)O preparation procedure (600 L of NO2 at 500 K, keeping the sample at 500 K for up to 1 h until the pressure had dropped to below 5.0 1010 mbar to avoid NO2 readsorption, and then cooling down to record the XP spectra). The shape and intensity of the 529.5 eV peak were only marginally affected by this (4 4)O preparation. The (4 4)O coverage was determined from the normalized intensity of the 528.3 eV peak. Figure 4(a) shows that the higher the initial coverage of the 529.5 eV oxygen was the less of the 528.3 eV oxygen could be prepared. This anticorrelation indicates that the two phases compete for the same sites. As the (4 4)O structure is a surface phase,4345 the 529.5 eV species must also be located on the surface. Figure 4(b) shows the same data, but plotted as function of {1 [x(528.3 eV)/x(528.3 eV)max]}, which is a quantitative measure of the surface area not covered by the (4 4)O phase [x as defined in eq 1]. In this plot, the 529.5 eV intensity increases linearly with the (4 4)O-free surface area, meaning that each surface element not covered by the (4 4)O structure is occupied by the same number of 529.5 eV oxygen atoms. In other words, the Mo oxide compound has a constant local coverage during the entire set of experiments. Because the straight line of the 529.5 eV species has a 3.14 times higher slope than the curve of the 528.3 eV oxygen [Figure 4(b)] the local oxygen coverage of the Mo oxide compound should be 3.14 times the local O coverage Θ(O) = 0.3754345 of the (4 4)O 17420
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Figure 5. Changes of the O 1s peak of the Mo oxide layer with time. Spectrum 1 is from the freshly prepared Mo oxide layer, spectrum 2 from the sample kept in UHV for 8 days (at 300 K) and after flash annealing to 460 K, spectrum 3 from the sample after flash annealing to 870 K. The EB of the shifted peak in spectrum 2 is 530.1 eV. (Small red and green peaks have probably the same origin as in Figure 1.).
Figure 4. (a) O 1s peak intensities recorded during a series of experiments in which the Ag(111) surface partially covered by the 529.5 eV species was dosed with further NO2 at 500 K to prepare the (4 4)O phase. The resulting 528.3 eV intensity, characterizing the amount of (4 4)O phase, drops with the amount of 529.5 eV oxygen. Both intensities are normalized to the respective Ag 3d5/2 intensities and corrected by the respective cross sections. (b) Plot of the normalized 528.3 and 529.5 eV intensities vs {1 [x(528.3 eV)/x(528.3 eV)max]}; x(528.3 eV) is the normalized 528.3 eV intensity.
structure. The result, Θ(O) = 1.2, is based on the assumption that the proportionality factors of the O 1s intensities and O coverages are the same for the two oxygen phases. The actual value is probably somewhat lower because we know from other STM experiments that the standard (4 4)O preparation procedure only led to ∼70% (4 4)O-covered surfaces, the rest was empty silver. Assuming the same 70% filling factor for the (4 4)O areas in the Mo oxide/(4 4)O coadsorption experiments this lower reference value reduces the local O coverage in the Mo oxide to 0.7 1.2 = 0.84. [The 529.5 eV oxygen species was also observed in another project in which low-energy electron microscopy (LEEM) and X-ray photoemission electron microscopy (XPEEM) experiments were performed at the nanospectroscopy beamline at ELETTRA. (Data will be published elsewhere.) The sample was also mounted to a Mo sample holder, and NO2 was used as oxygen source. It was found that the 529.5 eV species was almost uniformly distributed on the surface, except for a moderate accumulation at step bunches, confirming a two-dimensional nature of the Mo oxide. As expected from the above behavior the concentration of the 529.5 eV species could be strongly diminished by a stainless steel spacer ring between the sample and the Mo holder.] TD spectra recorded after preparation of the 529.5 eV species did not show any oxygen desorption up to 800 K, and the 529.5 eV
peak only became marginally smaller after repeated TD spectra up to this temperature. This also rules out that oxygen significantly dissolved in the bulk. The Mo oxide compound on the Ag surface is thus stable up to at least 800 K. When the 529.5 eV oxygen-covered surface was dosed with CO (up to an exposure of 1500 L) or with CO2 (up to an exposure of 5000 L) at temperatures between 300 and 670 K no changes of the O 1s intensity and energy were observed. The Mo oxide compound does not therefore react with these gases under these conditions. However, long-term effects were observed (Figure 5). The first XP spectrum in the figure is from a surface after fresh preparation of the 529.5 eV species. The second spectrum was recorded after keeping this sample for 8 days at 300 K in UHV and after flash annealing to 460 K to remove adsorbed carbonate. The peak shifted to 530.1 eV and the intensity was lower. The third spectrum was recorded after flash annealing to 870 K, after which the energy position and shape of the original peak at 529.5 eV were restored and the intensity went up to almost the initial value. No desorption of H2, H2O, CO, O2, or CO2 was detected during the flash annealing to 870 K. Possibly the Mo layer had reacted with H2 or CO from the residual gas that, during the 460 K annealing step, removed part of the surface oxygen by desorption as H2O or CO2, and the 870 K annealing restored the lost oxygen by O atoms from the bulk. We mainly show these data here because of the confusion they may create in spectra of Ag samples covered by a Mo oxide layer.
4. STM RESULTS For STM experiments the sample was again annealed at 800 K for 1 min and then transferred to the STM chamber. A large-scale overview image of the surface covered by the 529.5 eV species [Θ(O) was approximately 0.3] shows extended, flat terraces, separated by monatomic steps [Figure 6(a)], not very different from the typical morphology of uncovered metal surfaces. For an oxide-covered surface one may have expected a restructured, rough surface, possibly covered by three-dimensional clusters, but this was not the case. The terraces are partially covered by 17421
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Figure 6. STM images of the Mo oxide-covered Ag(111) surface. (a) Large scale overview, showing terraces and monatomic steps; (2940 Å)2, Vt = 1.3 V, It = 0.7 nA. √ (b) Detail √ from the marked area in (a),√showing islands with the (3 3 3 3)R30° phase and the ( 52) √ 52)R13.9° phase in the contrast-enhanced window; (900 Å)2, Vt = 1.3 V, It = 0.7 nA.
ordered phases that become visible in a close-up [Figure 6(b)]. The central terrace in Figure 6(a) is mainly disordered but also shows three large islands with a hexagonal structure. On the same terrace level, at the right edge of Figure 6(b), there are two small islands with a different hexagonal structure that corresponds to a (4 4) lattice. However, this structure, most likely caused by the (4 4)O reconstruction, was a minority structure. Chemisorbed oxygen should be completely absent under these conditions, but some NO2 readsorption may have occurred during cooling down. Great parts of the next lower terrace [medium gray area in Figure 6(a)] are covered by yet another hexagonally ordered phase [contrast-enhanced window in the lower left corner of Figure 6(b)]. The two major ordered phases were investigated in detail. Figure 7(a) shows an island with the same lattice parameters as the bright islands in Figure 6(b). That the structure appears dark here is caused by changed imaging conditions of the tip. The Fourier transform of this image [Figure 7(b)] displays perfectly √ hexagonal symmetry; the spots correspond to a (3 3 √ 3 3)R30° superstructure. As already mentioned, the lattice constants and orientations of all superstructures could be precisely determined because of the calibration with the (4 4)O structure in the same series of experiments. In other experiments similar dark features were resolved, but disordered [Figure 7(c)].
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Closer inspection of these images suggests locally ordered arrangements, which is confirmed by the Fourier transform √ √ [Figure 7(d)]. It shows spots at the same (3 3 3 3)R30° positions as Figure 7(b), but weaker and on a higher background. It is therefore likely that this partially ordered phase is formed by the same structure elements as the ordered islands. We have also recorded longer time sequences of the partially ordered phase and compiled these into movies. The sequence (Figure 7 movie) shows a detail from a similar image as Figure 7(c). On the small scale of the movie one can see that the phase is more complex than it appears on the larger scale images; it also contains various bright features. The dark and bright features are seen to change positions in the course of the movie, indicating that both of them are mobile on the time scale of the experiment (T = 300 K). The second major ordered phase [that covers the area on the left of Figure 6(b)] is shown in detail in Figure 8. The hexagonal structure of round features [Figure 8(a), green lines] corresponds to the hexagonal arrangement of spots in the Fourier transform [Figure 8(b), green circles]. The periodicity is 12.1 Å, 4.2 times the lattice constant of Ag(111), and the lattice is rotated by approximately 15° with respect to the substrate. The (4 4)O reconstruction can therefore be ruled out. Moreover, the Fourier transform shows additional, hexagonally arranged spots [Figure 8(b), red circles] corresponding to a lattice with a periodicity of 20.7 Å [7.2 times the lattice constant of Ag(111)] that is rotated √ by 30° √ with respect to the 12.1 Å lattice. It represents a ( 3 3)R30° superstructure with respect√ to the “green” superstructure. In the real-space image, this ( 3 √ 3)R30° superstructure is formed by short bright lines, located on sites between triples of the round √ features √ [Figure 8(a), red circles], and indeed arranged in a ( 3 3)R30° lattice with respect to the hexagonal pattern. That the features only appear as thin lines suggests dynamic effects, i.e, the features rearrange on the time scale of the individual scanning lines. The appearance of this structure in STM strongly changed when the tip changed; Figure 8(c) shows one example [the image is from the same measurement as Figure 8(a)]. However, the Fourier transform [Figure 8(d)] shows the same spots as Figure 8(c), so that the lattice has not changed. The unit cell√of the √ combined structures [Figure 8(e)] corresponds to a ( 52 52)R13.9° superstructure. Whereas these structures were repeatedly observed, there were several other structures which were only resolved in individual experiments. A striking example is a structure observed after heating the sample to 480 K (Figure 9). Figure 9(a) shows islands with a quadratic lattice that are rotated (by 60°) with respect to each other. The lattice constant is 28.8 Å, 10 times the Ag(111) lattice constant, and one side of the quadratic lattice runs along one of the close-packed directions of the substrate. The detail from such an island [Figure 9(b), from another location] shows that the unit cells are almost exact squares and that the fine structure displays 4-fold symmetry. It is unclear how such a structure can be reconciled with the trigonal symmetry of the substrate. Similar features that form the squares also appear along the step edges, however, without any order. We also took STM data from the surface with the coadsorbed Mo oxide and (4 4)O phase. In these experiments first the 529.5 eV species was prepared, then additional 600 L of NO2 were dosed at 500 K following the standard (4 4)O preparation technique. The XP spectra then showed both species [similar to Figure 1(a), spectrum 7]. The STM data of the (4 4)O-covered areas were indistinguishable from the pure (4 4)O phase, but the second 17422
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√ √ W STM images of the Mo oxide-covered Ag(111) surface. (a) Island of the (3 3 3 3)R30° superstructure; (250 Å)2, Vt = 0.4 V, It = Figure 7. b 2 2.0 nA. (b) Fourier transform of (a). (c) Area with partially ordered features; (500 Å) , Vt = 0.5 V, It = 1.0 nA. (d) Fourier transform of (c). (Figure 7 movie, available in mov format). STM movie (at enhanced rate) of a surface state similar to (c), recorded at 51 s per image. The shifting of the scanning area is caused by thermal drift. (150 Å)2, Vt = 0.5 V, It = 1.0 nA.
phase (Figure 10) displayed a different structure than the pure 529.5 eV species. Figure 10(a) shows an area with two domains with quasi-hexagonal structures that are slightly rotated with respect to each other (white and black lines). The Fourier transforms of the two domains are accordingly different [Figure 10(b),(c)]. One domain corresponds to a (5 6) superstructure, the other domain to a (6 5) superstructure. That the structure is actually not exactly hexagonal became more apparent with different tip conditions [Figure 10(d)]. The marked unit cell is clearly a parallelogram with unequal side lengths. A frequent observation for this coadsorbed state was extended narrow stripes in the neighborhood of islands of the (5 6) structure [Figure 10(e)]. The (5 6) structure, which appeared only after further NO2 dosing to prepare the (4 4)O phase may correspond to a Mo oxide compound that contains additional oxygen which desorbs during the final annealing to 800 K.
5. DISCUSSION The main result of the XPS measurements was an oxidic Mo 3d doublet that evolved simultaneously with the O 1s signal at 529.5 eV. The oxidic Mo signal did not originate from photoelectrons from the metallic Mo holder but from the sample, and the 529.5 eV signal disappeared when the Mo holder was replaced by a stainless steel holder. These observations leave no doubt that the 529.5 eV signal stemmed from a thin Mo oxide layer on the Ag(111) surface, and that the source of the Mo oxide was the sample holder.
How was the Mo transported from the holder to the sample surface? The Mo oxide signal appeared in the course of the NO2 dosing/annealing cycles, indicating that oxygen was involved. The most likely responsible species is MoO3 that was formed on the holder and traveled to the sample. Metallic Mo is much too stable to have played a role under these conditions, whereas MoO3 has relatively low melting and boiling temperatures. For supported MoO3 catalysts, the spreading of MoO3 from the original particles to form a monolayer on the support is actually a well-known process.61 It has recently been shown that in a gas atmosphere, the transport happens via the gas phase. MoO3 sublimates to give (MoO3)3 molecules that diffuse through the gas phase by molecular collisions and recondensate on the support.61 Similar sublimation processes of Mo oxide are effective even for metallic Mo as long as there is an oxygen source to oxidize the metal. Experiments in which Ag was evaporated from a (metallic) Mo boat or basket and condensed on a target showed that the condensed Ag was afterward covered by a molybdate film.62,63 The oxygen most likely originated from the O2 atmosphere, but even a modest vacuum was sufficient. However, for our case we rule out a transport of (MoO3)3 through the gas phase by a random-walk process. During NO2 dosing the pressure was lower than 106 mbar (under the doser), so that the mean free path of the (MoO3)3 molecules was approximately 30 m. If gas transport was involved, the molecules must therefore have survived collisions with the chamber walls or with the capillary array in front of the sample before reaching the sample surface. Direct line-of-sight trajectories from the holder to the sample 17423
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√ √ Figure 8. STM images of the Mo oxide-covered Ag(111) surface, showing the ( 52 52)R13.9° structure. (a) The green lines mark a hexagonal substructure of this phase, the red circles the full structure; the black line indicates the close-packed direction of the Ag(111) substrate; (180 Å)2, Vt = 1.0 V, It = 0.7 nA. (b) Fourier transform of (a), showing the periodicities corresponding to the substructure (green circles) and the full structure (red circles). (c) Roughly same area as (a) after a tip change. The features marked by red lines appear much clearer than in (a); (180 Å)2, Vt = 1.1 V, It = 0.7 nA. (d) Fourier transform √ of (c), showing that the same periodicities are present as in (b). (e) Schematic of the lattice, showing the √ Ag(111) substrate lattice points (black), the ( 52 52)R13.9° unit cell (red), and the hexagonal sublattice (green).
surface are excluded because the polished sample surface protrudes above the level of the holder. Alternatively, the (MoO3)3 molecules may have condensed at the sample edge and then crept by surface diffusion to the surface, or the entire transport was by surface diffusion via the “brim” of the hat-shaped crystal. What is the exact form of the Mo oxide on the Ag surface? The angular dependence of the O 1s peak intensity, the competing adsorption with the (4 4)O phase, and the structures resolved by STM evidence a surface species. Together with the coverage range of 0.5 e Θ(O) e 1.2 this speaks for a single layer of MoOn
polyhedra. The measured stoichiometry, Mo: O = 1: (3.88 ( 0.44) would be compatible with the Ag molybdates Ag2MoO4, Ag2Mo2O7, Ag6Mo10O33, and Ag2Mo4O13,6467 but also MoO3 cannot be excluded. Ag molybdates may have formed by the reaction of MoO3 with Ag and O atoms from the intermediately present (4 4)O structure. Ag2Mo2O7, Ag6Mo10O33, and MoO3 contain MoO6 octahedra that are connected by edges and corners to give chains or 2D sheets.65,66,68 The structures of these chains and sheets do not , however, fit to the surface phases observed by STM. The 2D lattice constants within the sheets are 17424
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Figure 9. STM image of the quadratic superstructure of the Mo oxide layer. (a) Overview; 980 Å 600 Å, Vt = 1.3 V, It = 0.7 nA. (b) Detail from another location; (240 Å)2, Vt = 1.3 V, It = 0.7 nA.
small (7.6 Å 11.4 Å for Ag6Mo10O33 and 4.0 Å 3.7 Å for MoO3) as well as the lattice constant along the chains in Ag2Mo2O7 (6.1 Å), much smaller than the periodicities √ √ of the surface phases [15.0 Å 15.0 Å for the√(3 3 √3 3)R30° structure, 20.8 Å 20.8 Å for the ( 52 52)R13.9° structure, 28.8 Å 28.8 Å for the quadratic structure, and 14.4 Å 17.3 Å for the (5 6) structure]. Moreover, the sheets or chains all have low symmetries that do not fit to the hexagonal or other relatively high symmetries observed by STM. Finally, the sheets and chains are relatively thick; placing them on the Ag(111) surface creates O coverages between 2.4 and 3.0, distinctly higher than the experimental value. The monomolybdate, Ag2MoO4, has a spinel structure containing MoO4 tetrahedra.64 The (111) plane has a symmetric, hexagonal arrangement of MoO4 tetrahedra, and the O coverage of a single (111) layer of tetrahedra, Θ(O) = 1.6, deviates less from the experiment. However, the periodicity of the (equally oriented) MoO4 tetrahedra in the (111) plane of Ag2MoO4 of 6.6 Å is again too small. The mobility of some of the features that was observed by STM favors a structure in which the MoOn polyhedra are not connected by the strong, direct MoOMo bonds. This is only the case for the tetrahedra in Ag2MoO4. However, an alternative explanation for the mobility is that individual Ag atoms, or possibly molecules from the residual gas, diffuse across the Mo oxide layer. In all of these compounds, Mo has an oxidation number of +VI. The splitting of the Mo 3d doublet into two components [Figure 1(b)] suggests that the Mo oxide layer contains Mo atoms in two different oxidation states. The absolute Mo 3d5/2 binding energies of MoO360,61 and of Ag molybdates69 that are reported in the literature vary considerably, making conclusions about absolute oxidation numbers from the EB values difficult. However, it has been shown that a shift of 0.8 eV, approximately
Figure 10. STM images of the Mo oxide-covered surface after adsorption of additional NO2 at 500 K. (a) Image with two domains of the (5 6) superstructure; the lines indicate that the domains are rotated with respect to each other; (480 Å)2, Vt = 0.7 V, It = 2.0 nA. (b) Fourier transform of the upper right domain in (a). (c) Fourier transform of the lower left domain of (a). (d) STM image of the (5 6) structure under different tip conditions and (5 6) unit cell; (200 Å)2, Vt = 1.3 V, It = 2.0 nA. (e) Other area, showing the (5 6) phase and a 1D structure; (290 Å)2, Vt = 1.3 V, It = 2.0 nA. 17425
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The Journal of Physical Chemistry C the separation of the two components in Figure 1(b), corresponds to a change of the relative oxidation number of one.60 Mo actually forms many intermediate oxides between MoO3 and MoO2, which thus may also play a role here. However, as for structure, all of these oxides lead to the same problems with periodicity, symmetry, and coverage as the cases discussed. It appears that the Mo oxide layer on Ag(111) cannot be derived from one of the known bulk phases in the AgMoO system but is a specific surface phase. Can the species termed “highly stable oxygen”, “strongly bound oxygen”, or “Oγ” in the Ag/O literature be caused by a Mo oxide or Ag molybdate layer? Mo parts that may have become hot during oxygen adsorption are not mentioned in any of the “Oγ” publications, but Mo is a usual material in UHV systems. It is also possible that W and Ta, two other standard UHV materials, lead to similar effects. The only case we found in the literature where such a material is mentioned in connection with “Oγ” is ref,29 where Ta foil has been used to clamp the Ag crystal to the manipulator. A small peak by a foreign metal—in our case the Mo peak was smaller than 1% of the Ag signal—may have been overlooked or considered as originating from the sample holder. Of course, from the data given in the literature we cannot really prove that the “Oγ” species can be explained in this way. Nevertheless, several properties of the surface Mo oxide are consistent with the properties of the “strongly bound oxygen”: (1) Both species are stable at high temperatures. Ag2Mo2O7 decomposes into Ag and MoO3 at 670 to 770 K, at yet higher temperatures MoO3 starts to sublimate,70 and Ag tungstenates are even more stable.70 From our data, the surface Mo oxide is stable at least up to 800 K. Its thermal stability is thus considerably higher than of any of the pure Ag/O phases, so that a contamination by Mo oxide or another metal oxide would solve the stability problem of the “strongly bound oxygen”. (2) In XPS both species show an O 1s peak at 529.5 eV. For “Oγ” this energy was obtained after recalibration of published spectra (the published value is 529.0 eV).29 (3) The preparation conditions are comparable. “Strongly bound oxygen” has been prepared at O2 pressures between 0.01 mbar and 1 bar and at temperatures between 770 and 1070 K,12,13,16,2032,3436 the Mo oxide film was prepared by repeated dosing of 600 L (at up to 106 mbar) of NO2 and annealing to 800 K. NO2 is a much more efficient atomic oxygen source than O2 that compensates the lower pressure. High oxygen activity and elevated temperatures are required to induce the formation and transport of Mo oxide. (4) Both species are located on the surface. For the Mo oxide film, the evidence is the angular dependent XPS, the coadsorption experiments with the (4 4)O structure, and the fact that surface phases were resolved in STM. That “Oγ” is a surface species followed from STM,26,29,32 angular dependent XPS,29 ISS,27,29 ion sputtering,27,29 and SERS.27,28,31 (5) The Mo oxide compound did not react with CO, which was also reported for the “strongly bound oxygen”.25 (6) The STM√data of√“Oγ” on Ag(111) showed a moire structure and a ( 3 3)R30° structure,26,29,32 two highly symmetric, hexagonal structures, albeit not identical to the structures found by us. However, our STM data point to considerable structural variability of the Mo surface oxide, and W or Ta might form surface oxides with different structures. We do not claim that the problem of the “strongly-bound oxygen” on silver can be fully resolved by a Mo (or W/Ta) oxide contamination because not all of the previous reports of “Oγ”
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have necessarily been dealing with the same species. The data presented here are only meaningful with respect to an oxygen species located at or close to the surface. Our data cannot rule out that the high-temperature peak in TDS, at 9001000 K, is caused by bulk-dissolved oxygen like the 700800 K peak. That bulk oxygen may desorb in two peaks rather than in one could result from a temperature-dependent change of the diffusion mechanism in the bulk; a change from “interstitial” to “substitutional” diffusion has been suggested.20,21 We can also not rule out a combination of a bulk and a surface species that has been proposed in the literature.21,23,36 The idea appears to be based on the assumption of a transient binding state at the surface that, at 900 - 1000 K, is permanently populated by a flux of oxygen atoms from the bulk and depopulated by desorption. If the steady-state concentration of such a transient species is high enough, then it might have been detected by XPS and SERS at high temperatures. However, it is not obvious why such a transient binding state should be different from the known chemisorbed or oxide-like phases. Spectroscopically it is different.
6. CONCLUSIONS (1) Repeated dosing of an Ag(111) sample with 600 L of NO2 at 500 K, flash annealing to 570 K, and finally annealing at 800 K for 2 min led to the development of an O 1s peak at 529.5 eV in XPS. At the same time, a Mo 3d doublet evolved with an intensity of
