J. Phys. Chem. C 2009, 113, 7355–7363
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NO2 Adsorption on Ag(100) Supported MgO(100) Thin Films: Controlling the Adsorption State with Film Thickness David E. Starr,⊥,† Christoph Weis,3,‡ Susumu Yamamoto,# Anders Nilsson,| and Hendrik Bluhm*,† Chemical Sciences DiVision, Lawrence Berkeley National Laboratory, Berkeley, California 94720, Materials Sciences DiVision, Lawrence Berkeley National Laboratory, Berkeley, California 94720, Stanford Synchrotron Radiation Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, Fysikum, AlbanoVa UniVersity Center, Stockholm UniVersity, SE-106 91 Stockholm, Sweden ReceiVed: January 14, 2009
Using photoemission and X-ray absorption spectroscopy, we compare the adsorption properties of NO2 at 300 K on MgO(100)/Ag(100) films with thicknesses varying from 2 to 8 ML and NO2 exposures ranging from 0 L to over 25 000 L. We find that NO2 is stable on 2 ML MgO(100) films, where it is the most abundant adsorbate on the surface (∼0.35 ML) for exposures up to at least ∼25 000 L. At high exposures, NO3 also forms on the surface of 2 ML thick films but is a minority species. In contrast, films thicker than ∼5 ML show conversion to NO3 beginning already at low exposures. At high exposure to NO2, NO3 is the only species present on the surface. Shifts to lower binding energy of the O 1s spectra with adsorbed species indicate that the NO2 adsorbed on the thin MgO(100) films is likely negatively charged and forms NO2-. A more gradual binding energy shift is observed on thicker films and is likely associated with the slower formation of NO3-. Measurements on MgO(100) films of various thicknesses indicate that for films thicker than 5 ML, the NO2 adsorption properties are similar and most likely correspond to surfaces of bulk MgO(100). We discuss potential mechanisms for NO2 charging and stabilization on the thin MgO(100) films in the context of recent literature. Introduction Of central importance to improving catalyst performance is the ability to modify and control the adsorption state of a molecule. To this end, over the past decade, model catalyst systems have been developed to investigate catalytic properties on a molecular level.1-3 These model catalysts are often composed of metal particles deposited onto oxide thin films grown on a metal substrate. Such systems mimic the reactive properties of oxide-supported metal catalysts while remaining structurally simple enough for molecular level characterization using traditional surface science techniques. Recent research has established that metal-supported oxide thin films may possess unique properties, different from those of the bulk oxide, if the oxide film is only a few layers thick.4-9 This has provided the possibility of tuning the catalytic properties of such model oxide surfaces by varying the thickness of metal-supported oxide thin films. MgO(100) thin films supported on either Mo(100) or Ag(100) are probably the most studied metal-supported oxide thin film systems to date. When these films are approximately 1 nm thick or less, they possess unique properties that differ from their bulk oxide counterpart.4,6,8,10,11 Both experimental and theoretical investigations have demonstrated that Au deposited onto thin * Corresponding author. Phone (510) 486 5431. E-mail:
[email protected]. † Chemical Sciences Division, Lawrence Berkeley National Laboratory. ‡ Materials Sciences Division, Lawrence Berkeley National Laboratory. | Stockholm University. ⊥ Current address: Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973. 3 Current address: Accelerator and Fusion Research Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720. # Current address: FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ, Amsterdam, The Netherlands.
MgO films grown on either Mo(100) or Ag(100) may become charged.6-8,10,12,13 The charge state of the Au can be modified by changing the thickness of the MgO(100) film.6,10 On MgO films with a thickness of only a few layers, two-dimensional growth of deposited Au is observed as a result of charging, which differs from the typical three-dimensional growth of Au on thicker, bulklike films.6,10 That has been explained by the compression of the metal electron density upon formation of the metal/oxide interface, leading to a reduction in the metal’s work function9 which aids charge transfer into the deposited Au. The charging of metal atoms also depends on the deposited metal’s electron affinity. Metals with high electron affinities, such as Au, are more easily charged than those of lower electron affinities. Scanning tunneling microscopy (STM) experiments showed that Au deposited onto 3 monolayer (ML) thick MgO(100)/Ag(100) form ordered arrays due to repulsive interactions between the charged Au species, whereas Pd atoms, with their much smaller electron affinity, are randomly dispersed.8 Similarly, recent theoretical calculations have predicted unique adsorption properties for NO2 on MgO(100) thin films grown on either Ag(100) or Pt(100), as well as Pt(100)supported BaO(100) thin films.4,5,14 Experimental evidence for these unique properties, however, is still lacking. These calculations predict that when NO2 is adsorbed onto a 2 ML thick MgO(100) film on Ag(100), NO2 abstracts near unit charge from the oxide thin film (instead of the metal support as predicted for Au deposition), thereby leading to the formation of NO2-.4 The adsorption energy of NO2 on the supported thin oxide film increases from 0.28 to 1.81 eV, as compared to an unsupported film (representative of the bulk oxide). The abstraction of an electron is also accompanied by changes at the MgO(100)/
10.1021/jp900410v CCC: $40.75 2009 American Chemical Society Published on Web 04/01/2009
7356 J. Phys. Chem. C, Vol. 113, No. 17, 2009 Ag(100) interface. Upon NO2 adsorption, an increase in the overlap between the O 2p states and the Ag metal electronic states is predicted and leads to a contraction of the bond distance between the oxygen anions and the Ag atoms by 0.19 to a value of 2.39 Å. This increases the adhesion energy between the oxide film and the metal and provides overall stabilization of the adsorption system. Only fractional charge transfer (0.3 e-) is predicted for NO2 adsorption on bulk MgO. These calculations have indicated that the presence of the metal substrate modifies the adsorption properties of NO2, resulting in charging of the adsorbed NO2 and a large gain in energy that stabilizes both the adsorbed NO2 and the oxide-metal interface. Due to the use of alkaline earth oxides in NOx storage catalysts,15,16 NO2 adsorption onto bulk MgO(100), Mo(100)supported bulklike MgO(100) films, and MgO powders has been studied previously.17,18 Photoemission measurements by Rodriguez et al., following multilayer adsorption of NO2 at 100 K and subsequent formation of physisorbed N2O4, found conversion of NO2 to NO3 when the sample was heated from 150 to 300 K.17 At 150 K, there is approximately twice as much NO3 on the surface as NO2. Between 150 and 300 K, the amount of NO3 on the surface increases while the amount of NO2 decreases. At 300 K, NO3 is the main species on the surface, with only small amounts of NO2 remaining. Small amounts of NO2 persist on the surface up to ∼500-600 K, above which only NO3 remains. Similar studies on (100)-oriented MgO powders also showed that NO3 is the dominant adsorbate in the temperature range from 300 to 770 K.18 In the present study, we compare the adsorption properties of NO2 on MgO(100)/Ag(100) films with thicknesses ranging from 2 to 8 ML. All measurements were done at 300 K, in contrast to previous low-temperature adsorption studies on bulklike films. We find that NO2 is stable on MgO(100) thin films when these films are ∼2 ML thick and is the dominant adsorbate on the surface for exposures up to at least ∼25 000 L, thus confirming previous theoretical predictions of the increased stability of NO2 on Ag(100) supported MgO(100) films with ∼2 ML thickness. In contrast, on films thicker than ∼5 ML, conversion to NO3 begins already at lower exposures. Measurements on MgO(100) films of varying thicknesses indicate that at film thicknesses of about 5 ML, the NO2 adsorption properties become similar to bulk MgO(100). Experimental The experiments were performed at beamline 11.0.2 of the Advanced Light Source at Lawrence Berkeley National Laboratory using the Ambient Pressure Photoemission Spectroscopy endstation.19,20 The Ag(100) single crystal was cleaned by cycles of Ar+ sputtering (1.5 keV, PAr ) 1 × 10-5 Torr), followed by annealing to 725 K. MgO(100) films were grown by vapordepositing Mg (99.9% purity, Goodfellow) at a rate of ∼1 ML MgO(100)/min in 1.0 × 10 -6 Torr O2(g). During MgO(100) film growth, the Ag(100) substrate was kept at 575 K. Growth at this temperature has been found to produce films with better epitaxy.21 The film thickness was determined using the O 1s/ Ag 3d signal intensity ratio and literature values for the inelastic mean free path of electrons22 and the photoionization cross sections.23 The O 1s and Ag 3d signal intensity was measured at photon energies of 680, 830, and 980 eV for the O 1s spectra and 520, 670, and 870 eV for the Ag 3d spectra, corresponding to photoelectron kinetic energies of 150, 300, and 450 eV, respectively (i.e., inelastic mean free paths of 6.2, 8.8, and 11.7 Å, respectively, in MgO22). Film thicknesses estimated at the different kinetic energies varied by ∼(0.1 ML of MgO(100)
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Figure 1. N K-edge NEXAFS spectra for increasing NO2 exposure at 300 K and p(NO2) ) 1 × 10-6 Torr. (a) 2.2 ML MgO(100)/Ag(100), (b) 8.4 ML MgO(100)/Ag(100), and (c) Ag(100).
for thin films (5 ML thick). MgO(100) films for all data presented here had sharp LEED patterns, indicating epitaxial growth of the film on the Ag(100) substrate. Due to the finite size of the X-ray spot, all film thickness values given in this paper represent average values measured over the size of the X-ray spot. NO2(g) was leaked into an ultrahigh vacuum chamber (base pressure 2 × 10-10 Torr) to a pressure of 1 × 10-6 Torr at 300 K. Both near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoemission spectroscopy (XPS) spectra were then collected at various times during the exposure to NO2. All exposures are given in units of Langmuirs (1 L ) 1 × 10-6 Torr sec). N 1s, O 1s, and Ag 3d XPS spectra were collected with photon energies of 550, 680, and 520 eV, respectively, ensuring the same probing depth of ∼6 Å, at photoelectron kinetic energies of ∼150 eV. All core-level photoemission spectra were normalized by the synchrotron ring current. The binding energy scale was calibrated by the Fermi edge of the Ag substrate. N K-edge NEXAFS spectra were recorded using the electron energy analyzer (Specs Phoibos 150) as an Auger yield detector, set to a kinetic energy of 382 at 40 eV pass energy. The photon energy presented in Figure 1 is not calibrated, but we estimate an error of, at most, 0.2 eV using the difference in peak positions of the Mg 2p core level taken at photon energies of 350 and 490 eV. Initial measurements on the NO2/MgO(100)/Ag(100) system indicated that it is particularly susceptible to beam damage. To minimize effects such as MgO film and NOx decomposition, great care, such as beam defocusing and frequently changing sample position, was taken. Possible beam damage was also routinely checked by retaking spectra in different sample positions. All spectra presented here show no effects from beam damage.
NO2 Adsorption on Ag-Supported MgO Thin Films
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Results Figure 1 presents N K-edge NEXAFS results for NO2 exposure to MgO(100) films grown on Ag(100) of thicknesses (a) 2.2 ML, (b) 8.42 ML and (c) clean Ag(100). In all cases, two distinct peaks at ∼402 and ∼405.5 eV are evident, as well as broad features from ∼407 eV up to ∼420 eV. Although all three of these features are present in Figure 1a, b, and c, their behavior with exposure differs for the two MgO film thicknesses and the clean Ag(100) sample. The spectra for the 2.2 ML thick MgO(100) film in Figure 1a are dominated by the peak at ∼402 eV up to exposures of at least 17 600 L. The peak at ∼405.5 eV grows slightly in intensity with exposure, whereas the broad feature from ∼407 to 420 eV remains largely unchanged. In Figure 1b, the peak at ∼402 eV decreases in intensity with increasing exposure of NO2. At ∼9 100 L exposure, the peaks at ∼402 and ∼405.5 eV are of nearly equal intensity, whereas at larger exposures, the spectrum is dominated by the peak at 405.5 eV. In addition, the broad envelope from ∼407 to 420 eV shows a slight shift in the position of maximum intensity to higher energy and develops a shoulder in the range from ∼414 to 420 eV. The spectra for NO2 exposure to a clean Ag(100) crystal of Figure 1c show similar but slower changes compared to the spectra in Figure 1b. The spectrum in 1c, which was acquired after 21 300 L exposure of NO2 to clean Ag(100), is similar to the spectrum after 9 100 L exposure of NO2 to the 8.4 ML thick MgO(100) film. Rodriguez et al. have studied NO2 adsorption on bulk oxide surfaces using N K-edge NEXAFS and compared their spectra to those from KNO2 and KNO3 standard samples.18 For KNO3 they observed an intense resonance at ∼407 eV and a broad envelope from ∼410 to 420 eV. The intense resonance at ∼407 eV was attributed to 1s f 2a′′2(π) transitions, and the broad features from 410 to 420 eV were attributed to excitations into 5a′1 and 5e′ empty states. The KNO2 spectrum showed an intense resonance at ∼403 eV and a broad envelope from ∼407 to 417 eV. In this case, the low-energy peak at ∼403 eV was attributed to 1s f 2b1(π) transitions and the broad envelope to excitations into the empty 7a1 and 5b2 states. From these measurements, an energy splitting of ∼4 eV between the intense resonances for NO2 and NO3 species can be assigned. In addition, the high-energy features are located, on average, at slightly higher energies for NO3 as compared to NO2. The general characteristics of these spectra allow the assignment of the peaks in Figure 1. The peak located at 405.5 eV in all spectra can be assigned to NO3-like species, and the peak located at 402 eV can be assigned to NO2-like species. We can rule out the presence of NO species on our surface by comparison with the data by Geisler et al., who have investigated NO2 adsorption on a Ni(100) surface where the NO2 dissociates to form NO and atomic O.24 They observed a difference in energy for the intense resonances from NO2 and NO of 1 eV, with NO being lower in energy. This would correspond to a splitting between NO3 and NO of 4.5 eV or a location of ∼401 eV for an intense resonance if NO were present in our spectra. The absence of such a resonance indicates that our NEXAFS data are consistent with the adsorption of NO2 on all surfaces at low exposures and formation of NO3 on the thick MgO film as well as clean Ag surfaces at higher exposures. Also consistent with this behavior are the shifts in the broad envelope to higher energy for the thick film and the clean Ag surface with increasing exposure and little change for the thin MgO film. In Figure 2, N 1s XPS spectra are presented for NO2 adsorption onto a 2.2 ML thin MgO(100) film supported by
Figure 2. N 1s spectra for increasing NO2 exposure to a 2.2 ML thick MgO(100) film supported on Ag(100) at 300 K and p(NO2) ) 1 × 10-6 Torr.
Ag(100). At low NO2 exposures, a single peak at 402.5 eV is observed. Once an exposure of ∼10 000 L is reached, a second peak appears at 406.2 eV, which grows in intensity with exposure. At 19 000 L exposure, the peak area of the 406.2 eV peak is approximately 18% of the total N 1s peak intensity. The binding energy of the peak at 402.5 eV is slightly lower than that previously observed for NO2 adsorbed on Mo(100)supported thick MgO films at ∼404 eV, but higher than that found for NO adsorption on MgO(100) at ∼401.5 eV.17 The 406.2 eV peak position is also lower than previously observed for NO3 adsorbed on MgO(100) at ∼407 eV.18 The splitting between the two peaks observed here, 3.7 eV, is closer to the previously observed splitting between NO3 and NO2 (∼3.2 eV) than to that expected for NO3 and NO peaks, ∼5.5 eV.17 In previous photoemission studies, molecules adsorbed on metalsupported thin film oxides have shown lower binding energies than that observed on bulk oxides or bulklike oxide films.25 This has generally been attributed to increased screening of the core hole due to the proximity of the metal substrate. Core-level binding energies may also shift to lower values upon formation of negatively charged species on the surface (see below).26-28 Therefore, similar to the NEXAFS spectra of Figure 1, the N 1s spectra for NO2 adsorption as a function of exposure to a 2.2 ML thick MgO(100) film are most consistent with the adsorption of exclusively NO2 at low exposures and the formation of NO3 at larger exposures, amounting to ∼18% of the N-containing species at 19 000 L total exposure. In contrast to the 2.2 ML thick MgO(100) film in Figure 2, a 5.7 ML thick MgO(100) film exhibits very different behavior for N 1s spectra as a function of NO2 exposure, as shown in Figure 3. In this case, a small peak located at 402.8 eV is present
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Figure 3. N 1s spectra for increasing NO2 exposure to a 5.7 ML thick MgO(100) film supported on Ag(100) at 300 K and p(NO2) ) 1 × 10-6 Torr.
at low exposures. At exposures of ∼9 000 L, this peak has grown slightly in intensity, and a peak of similar intensity at 406.8 eV is present. At 16 000 L exposure, the low-energy peak has disappeared, and the higher-energy peak has grown in intensity and continues to grow for exposures of at least 24 000 L. Similar to the spectra for the 2.2 ML MgO(100) film, we assign the low binding energy peak to adsorbed NO2 and the higher binding energy peak to adsorbed NO3. The N 1s spectra are consistent with the interpretation of the NEXAFS spectra presented in Figure 1 for NO2 adsorption on the thick MgO(100) film. This data indicates that in contrast to the ∼2 ML film, only small amounts of NO2 (relative to NO3) are ever present on the surface. For exposures exceeding ∼10 000 L, NO3 becomes the dominant species, and by ∼15 000 L exposure, it is the only species present on the surface. It is interesting to note that the NO3 peak continues to grow in intensity at large exposures but there is little or no NO2 present on the surface (below our detection limit of a few percent of a ML). This seems to indicate that the increase in adsorbed NO3 does not occur via a disproportionation reaction at 300 K, as has been previously argued for NO2 adsorption studies on oxide surfaces.17,18,29 These studies may have been affected by kinetic or coverage-dependent effects, since exposure was carried out at low temperatures followed by heating and formation of NO3 in vacuum. We are currently exploring the details of possible reaction mechanisms for the formation of NO3 at 300 K and will present our results in a future publication.30 Here, we emphasize only that the species present on the surface of the thick MgO(100) film behaves quite differently with NO2 exposure from those observed upon NO2 exposure for the thin film.
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Figure 4. O 1s spectra for increasing NO2 exposure to a 2.2 ML thick MgO(100) film supported on Ag(100) at 300 K and p(NO2) ) 1 × 10-6 Torr.
O 1s spectra for NO2 adsorption at 300 K onto the 2.2 ML thick MgO(100) film for exposures ranging from 0 to 10 000 L are shown in Figure 4. The O 1s spectrum before NO2 exposure (0 L) is characteristic for clean, thin MgO(100) films on Ag(100).11,31,32 The oxide O 1s binding energy (529.5 eV) is lower than that typically observed for thick, bulklike films (binding energy ∼530.6 eV) but consistent with previous observations.32 The reduction in the oxide peak binding energy is associated with reductions in the on-site Coulomb interactions and charge transfer energies in the thin films compared to the thick or bulklike films. The clean MgO(100) film also shows a small shoulder at 1.9 eV higher binding energy than the oxide peak. This shoulder is present in nearly all O 1s spectra of MgO(100) thin films supported on Ag(100) and has been attributed to the formation of a small amount of hydroxyl groups from reaction with residual H2(g) or H2O(g) in the vacuum chamber. This assignment has been based mainly on the peak position, which correlates well with the binding energy difference between bulk oxide O 1s binding energies and the O 1s binding energy of adsorbed hydroxyl groups, as well as the observed increase in intensity of this peak with exposure to the residual gas in the vacuum chamber.31 There may be other interpretations for the origin of this peak (see below), which we are currently exploring. Upon exposure to NO2, the thin film O 1s spectra changes (see upper three spectra in Figure 4). The oxide peak attenuates to ∼0.7 times the original intensity of the clean MgO film oxide peak already at 630 L NO2 exposure, as expected for adsorption of molecules on top of the film. The oxide peak also shifts to a 0.9 eV lower binding energy (528.6 eV). Such a shift has been attributed to the formation of negatively charged adsorbates
NO2 Adsorption on Ag-Supported MgO Thin Films
Figure 5. O 1s spectra for increasing NO2 exposure to an 8.4 ML thick MgO(100) film supported on Ag(100) at 300 K and p(NO2) ) 1 × 10-6 Torr.
and has been observed for low NO2 exposures to Al2O3 thin films grown on NiAl(110).26,28 It is argued that this shift is a result of changes in the electrostatic potential upon formation of negative NO2 ions on the surface, which influence the outermost oxide core levels. We have observed a similar shift in the Mg 2p spectra (not shown), which is consistent with this picture and with previously observed shifts of Al 2p core levels.26,28 In addition, at low exposures (630 L) a broad peak at 531.8 eV (3.2 eV higher than the oxide binding energy) appears. This peak can be associated with the adsorption of NOx species on the surface.29,33 This spectral feature remains largely unchanged upon further exposure up to at least 10 000 L. O 1s spectra for exposure of an 8.4 ML thick MgO(100) film to NO2 are shown in Figure 5. The clean film has an O 1s binding energy of 530.5 eV, which is close to the expected bulk value of ∼530.6 eV.32 In addition, these spectra show distinct differences when compared to the O 1s spectra for NO2 adsorption on the ∼2 ML thick film in Figure 4. The attenuation of the oxide peak is more gradual with exposure instead of the abrupt change in intensity observed on the thin film. At 700 L, the peak intensity is ∼0.9 times its original value, whereas at 15 000 L, it is ∼0.6 times its original value. For comparison, in the case of 2.2 ML thick MgO(100) film, the intensity of oxide peak is ∼0.7 its original intensity already at 630 L NO2 exposure. In addition, the shift in the spectra to lower binding energy is more gradual than observed for the thin film. After 700 L, the oxide O 1s peak has shifted by 0.75 eV to lower binding energy; after 5 800 L, to 0.95 eV lower binding energy; and after 15 000 L, to 1.5 eV lower binding energy. This more gradual shift, compared to the 2.2 ML film, may be associated with a more gradual buildup of negatively charged species on
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Figure 6. Ag 3d spectra for increasing NO2 exposure to a 2.2 ML thick MgO(100) film supported on Ag(100) at 300 K and p(NO2) ) 1 × 10-6 Torr. The top spectrum is for 16 600 L NO2 exposure to an 8.4 ML thick MgO(100) film.
the surface.26,27 Considering the N 1s spectra in Figure 3, this is most likely related to the more gradual formation of NO3rather than NO2-, since at large exposures, we observe exclusively NO3-like species on the surface. Consistent with this is the gradual increase in intensity of a peak at 532.4 eV, which is associated with the formation of NOx species.33 The peak located at ∼1.9 eV above the position of the oxide O 1s peak also grows initially with exposure. At large exposures, where significant amounts of NO3- are being formed in the absence of NO2, this peak begins to decrease in intensity. It is tempting, therefore, to assign this peak to an O-containing species that reacts with NO2 from the gas phase to form NO3-(ads). We are currently investigating this possibility further.30 In summary, the O 1s spectra for increasing exposure of a thick, bulklike MgO(100) film to NO2 are consistent with a gradual build-up of negatively charged adsorbates on the surface. Considering the binding energy of the peaks in the N 1s spectra as well as the N K-edge NEXAFS spectra, the negatively charged species present on the surface at large exposures are most likely NO3-. Ag 3d spectra for increasing NO2 exposure of the 2.2 ML thick MgO(100) film are shown in Figure 6. In addition, at the top of Figure 6, we show the Ag 3d spectrum for high NO2 exposure to a thick (8.4 ML) MgO(100) film. The unexposed film (bottom spectrum, Figure 6) has two peaks corresponding to the Ag 3d5/2 (368.3 eV) and the Ag 3d3/2 (374.3 eV). Upon NO2 exposure, the total Ag 3d intensity decreases due to the additional attenuation from the NO2 adsorbates. In addition, the main peaks in the spectra remain at the same position as for the unexposed film, but a shoulder appears at lower binding energy. This shoulder arises from peaks that appear at 367.8 and 373.8 eV for the Ag 3d5/2 and Ag 3d3/2, respectively. The
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spectra remain largely unchanged for additional exposures up to 19 000 L. The peaks appearing at 0.5 eV lower binding energy match well the expected binding energies of 367.8 and 373.8 eV for Ag2O.34 The Ag 3d5/2 binding energy of AgO is expected to appear at 367.4, or 0.9 eV lower than bulk Ag. The integrated area under these lower binding energy peaks is ∼0.24 times of the total Ag 3d area. Assuming a mean free path of 4.8 Å for 150 eV kinetic energy electrons in Ag22 and using an interlayer distance of 2.1 Å35 for Ag in the [100] direction, we can estimate the amount of Ag in the top layer of Ag that has been modified by the adsorption of NO2. We calculate this to be 0.45 ML of Ag. These additional peaks are absent in Ag 3d spectra taken for large NO2 exposures of the thick MgO(100) films (top spectrum of Figure 6). This indicates that the appearance of the additional peaks is unique to the adsorption of NO2 on the thin MgO(100) films. Discussion Our results indicate that the characteristics of NO2 adsorption as a function of exposure differ for 2 ML thick MgO(100) films supported on Ag(100) as compared to MgO(100) films with thicknesses > 5 ML. The position of the most intense resonance in N K-edge NEXAFS spectra and the position of the N 1s peaks for NO2 adsorption onto a 2.2 ML thick film are consistent with the adsorption of a predominantly NO2-like species, with some NO3-like adsorbates appearing at large exposures. Further, the shift of the O 1s spectra to lower binding energy upon NO2 exposure indicates that the adsorbed species are charged on 2.2 ML thick films. Since this shift occurs when there are exclusively NO2-like species on the surface (i.e., for low exposures), our results indicate that for thin MgO films (∼2 ML thick), NO2 exposure leads to the presence of NO2- on the surface. The absence of significant changes in the spectra with exposure implies that the NO2- is stable on the surface at 300 K for exposures up to at least 19 000 L. In contrast, the N K-edge and N 1s core-level spectra for exposure of thick (∼5 ML or thicker) MgO(100) films supported on Ag(100) are consistent with the more gradual formation of NO3-like species with exposure and only small amounts of NO2-like species at low exposures. The more gradual shift of the O 1s spectra to lower binding energy with exposure indicates that the formation of negatively charged species on the thick films occurs more slowly with exposure than on the thin MgO(100) films. Most likely, the shift in the O 1s spectra is associated with the formation of NO3- species on the surface. The small amount of adsorbed NO2 at low exposures may or may not be charged. Trends in the behavior of NO2 adsorption onto MgO thin films at varying thicknesses are summarized in Figure 7, where we have plotted the area under the NO2 N K-edge NEXAFS 402 eV peak, normalized by the sum of the areas of the NO2 and NO3 peaks. For NO2 adsorption onto the thin MgO films, this ratio has decreased to a value of ∼0.93 after 20 000 L exposure. This is higher than the expected ratio (∼0.82) from the XPS data of Figure 2. This may be a consequence of differences in the overlap of the incident E-field and the 2b1(π) and 2a′′2(π) orbitals of the adsorbed NO2 and NO3, respectively. Due to a limited number of XPS data sets, we have used the NEXAFS data to discuss trends in the thickness dependence of NO2 on MgO films; the data in Figure 7 are likely only a qualitative representation of the fraction of NOx species present as NO2. Measurements on films of varying thicknesses (Figure 7) indicate that the unique adsorption properties of these films toward NO2 disappear when films reach thicknesses of at least 5 ML, consistent with previous experimental and theoretical
Figure 7. Area of NO2 NEXAFS peak (∼402 eV) normalized by the sum of the NO2 and NO3 NEXAFS peak (405.5 eV) areas as a function of exposure to NO2 for various MgO(100) film thicknesses at 300 K and p(NO2) ) 1 × 10-6 Torr. Also included in the figure are similar data for exposure of Ag(100) to NO2. The data are color-coded into three groups: black for films ∼2 ML thick or less, blue for films 3.5 ML or thicker, and red for Ag(100). The lines are guides for the eye.
studies.5,6 Even for MgO(100) films that are ∼3-4 ML thick, our measurements indicate the formation of mostly NO3- at large exposures. The limited amount of data for MgO films in this thickness range, however, does not allow us to determine whether these films adsorb NO2 similar to thicker bulklike films or form NO3 more gradually with exposure compared to thicker bulklike films. Theoretical calculations by Gro¨nbeck4 have indicated that the adsorption energy of NO2 on 2 ML thick MgO(100) films increases from 0.28 to 1.81 eV when these films are supported by Ag(100), as opposed to when they are free-standing. The free-standing films of two-layer thickness were used to model the bulklike MgO(100) surface. For identical pressures and temperatures of exposure (as in our measurements), such a large increase in adsorption energy should lead to an increase in coverage of adsorbates on the surface. We have quantified the coverage of NO2- on the surface of the thin films by using the N 1s spectra to calculate the fraction of NOx species present as NO2- and NO3. These fractions were then used to calculate the area fractions of the NOx peak in the O 1s spectra that are due to each species, taking into account that one NO3 has 3/2 the O 1s intensity of one NO2-. These areas were then normalized by the fraction of the O 1s oxide peak that is due to the top layer of MgO(100) in the MgO film, using the mean free path of 150 eV electrons in MgO (6.2 Å)21 and an interlayer distance in the MgO film equal to the bulk, 2.1 Å.22,35 Approximating coverages in this way eliminates errors that may arise from uncertainties in photon flux or core-level cross sections, since intensities are compared only for specific core levels (either N 1s or O 1s). This normalization provides the coverage of NOx species on the surface in terms of the O-density in the top layer of MgO(100). These coverages are slightly overestimated, since the attenuation of the MgO O 1s peak due to the adsorption of NOx species on top of the film is not included in this analysis. Figure 8 shows the coverage in terms of the O density in MgO(100) of NO2- and NO3- on the surface of the 2.2 ML
NO2 Adsorption on Ag-Supported MgO Thin Films
Figure 8. Coverage of NO2 and NO3 on a 2.2 ML thick MgO(100) film grown on Ag(100) as a function of NO2 exposure at 300 K and p(NO2) ) 1 × 10-6 Torr.
thick MgO(100) film as a function of exposure at 300 K. The coverage of NO2- is approximately 0.35 ML for the entire exposure range, from 0 to 19 000 L. The coverage of NO3, on the other hand, starts at ∼0.02 ML and increases to ∼0.08 ML at 19 000 L exposure. A similar analysis on the thick film shows that the amount of NO2 species on the surface is a maximum of ∼0.05 ML in the exposure range from 0 to 10 000 L and completely disappears above 10 000 L exposure.30 This would be consistent with a large increase in stabilization of adsorbed NO2 species on the thin MgO films compared to the thick MgO films. There are a number of possible mechanisms that can lead to the increased stabilization and formation of NO2- on Ag(100)supported MgO(100) thin films, as well as to the modification of the Ag layer at the Ag/MgO interface (see Figure 6). We will discuss possible mechanisms in the following in detail. (a) Incomplete Coverage of Ag(100) Substrate by MgO(100) and NO2 Induced MgO(100) Thin Film Dewetting. First, we have to rule out the possibility that the special adsorption behavior of the 2.2 ML film is caused by holes in the MgO film that lead to partial NO2 adsorption and stabilization on the exposed Ag surface. Scanning tunneling microscopy studies of MgO(100) film grown on Ag(100) have indicated that for 1.2 ML thick films, >90% of the surface is covered by MgO.36 Our coverage estimate for the amount of NO2- (∼0.35 ML), as well as the amount of Ag in the top Ag layer that has been modified (0.45 ML), therefore seems to exceed the amount of bare Ag that would be available for adsorption (∼0.10 ML). In addition, our measurements for NO2 adsorption onto clean Ag(100) (see NEXAFS results in Figures 1c and 7) indicate that for large exposures, we would expect that a significant fraction of the NOx species adsorbed in the exposed Ag patches are NO3 species. This would further reduce the expected amount of NO2 species if uncovered Ag patches were responsible for the observed stabilization of NO2-. Similar arguments can be made to rule out the stabilization of NO2 at defect sites in the MgO(100) film, since the concentration of defects, even for the 2 ML thick films, is expected to be low.37 In the presence of NO2, it may be possible that thin MgO films dewet the surface of the Ag(100) support. Such restructuring would likely cause a change in the ratio of intensities of
J. Phys. Chem. C, Vol. 113, No. 17, 2009 7361 the MgO O 1s to Ag 3d peaks. In our experiments, upon NO2 adsorption, the O 1s/Ag 3d peak area ratio changes from 0.21 to 0.19 for the 2.2 ML thick MgO film. Such a small change would seem to indicate that significant MgO film dewetting does not occur. However, there may be possible scenarios in which this ratio does not change with film dewetting, so we cannot entirely exclude this possibility. (b) NO2 Dissociation, O Diffusion and Ag Oxidation. Another possible mechanism for the increased stability of NO2 on thin MgO films is NO2 adsorption onto the surface, followed by subsequent dissociation into NO(ads) and O. In this scenario, NO must remain adsorbed on the surface (since we observe NOx species other than NO3) while O diffuses through the film to oxidize Ag. Studies of NO adsorption onto bulk MgO and bulklike MgO films have shown that NO(ads) is less stable than NO2(ads).17,18 Temperature-programmed desorption experiments indicate that NO desorbs at temperatures of about 150 K, whereas NO2 is not fully desorbed until ∼400 K. Note that in these studies, the TPD experiments on NO2 adsorption/desorption were done through low-temperature (∼100 K) adsorption of multilayer, physisorbed N2O4 followed by heating and, thus, potentially suffer from kinetic limitations compared to the results of the present study. On the basis of the TPD studies, we therefore expect that NO is less stable than NO2 on thin MgO films. The strongest evidence for NO2 adsorption (as opposed to NO2 dissociation) comes from the positions of the N K-edge NEXAFS resonances (Figure 1) and the positions of the N 1s peaks. The observed energy splitting of 3.5 eV in the N K-edge NEXAFS data is most consistent with the formation of NO2and NO3-like species on the surface. The energy splitting between NO3 and NO species is expected to be 4.5 eV. The energy difference between the peaks in the N 1s spectra of Figure 2 (∼3.7 eV) is more consistent with the energy difference expected for peaks arising from NO2- and NO3-like species (∼3.2 eV) than for peaks arising from NO3- and NO-like species (∼5.5 eV). (Note that the energy differences listed here are for NO2 adsorption and reaction on bulk MgO and bulklike MgO thin films and may differ from those for NO2 adsorption on thin MgO(100) films.) (c) Electron Tunneling through the MgO Film into NO2. STM measurements by Sterrer et al.8 along with theoretical calculations from Pacchioni et al.7,9 have demonstrated a novel mechanism for controlling the charge state of high-electronaffinity metals, such as Au, deposited onto MgO(100) thin films supported by either Mo(100) or Ag(100). Au deposited onto MgO thin films of thicknesses less than 3 ML becomes charged via direct electron tunneling from the metal substrate through the thin oxide film. This mechanism holds true for high-electronaffinity metals, such as Au (EA ∼ 2.3 eV),7 but not for lowelectron-affinity metals, such as Pd (EA ∼ 0.56 eV).7 The electron affinity of NO2 is 2.4-2.5 eV, slightly larger than that of Au.38,39 Solely on the basis of the electron affinity, we may therefore expect that NO2 will also become charged when adsorbed onto thin MgO(100) film. The peak shifts in the O 1s spectra to lower binding energy upon NO2 adsorption are consistent with the formation of a negatively charged surface species; that is, NO2-. Direct electron transfer from the Ag substrate into adsorbed NO2 via tunneling through the oxide film is therefore a possibility. An important contribution to charging and enhanced stabilization of Au on thin MgO(100) films is the interaction of the charged Au metal with its image charge in the metal support.6 Image charge stabilization is expected to be more effective for thinner MgO films than thicker films. Charge transfer from Ag into NO2 is thus consistent with
7362 J. Phys. Chem. C, Vol. 113, No. 17, 2009 the shoulder in the Ag 3d spectra (Figure 6), which is located at approximately the binding energy of positively charged Ag, such as in Ag2O. The Ag+ species could be due to either or both the localization of holes at the Ag-MgO interface upon charge transfer from the Ag into NO2 or contributions from image charges due to the presence of NO2- on the MgO surface. With increasing MgO film thickness, the ability for electrons to tunnel through the oxide is reduced, and the additional electrostatic stabilizing effect due to the charge interaction with the underlying Ag support decays. The formation of predominantly NO2- species on thin MgO(100) films and their absence on thick films is consistent with this mechanism. (d) Charge Transfer from the Oxide Side of the MgO/Ag Interface into NO2. Another closely related mechanism has been postulated by Gro¨nbeck, specifically for NO2 adsorption onto Ag(100) and Pt(100) supported thin MgO(100) films.4,5 In this mechanism, the charge transferred to NO2 originates from the oxide film side of the metal-oxide interface, as opposed to transfer from the metal support. Gro¨nbeck’s calculations show that charge transfer leads to an increase in the adhesion of the oxide film to the metal substrate. The increased adhesion is the result of increased overlap between O 2p states and metal states at the metal-oxide interface. The increased overlap leads to charge accumulation between the oxide anion and the Ag atoms at the interface and a reduction of the mean distance between the oxide anions and Ag atoms by 0.19 Å.4 Such a mechanism would also be consistent with our data. However, in this case, the observed shoulder in the Ag 3d spectra would be due to the increased overlap of the O 2p states with the metal states, forming effectively an Ag oxide-like bond at the interface instead of positive charge localization at the interface following charge transfer to form NO2-. Our data are consistent with either of the mechanisms outlined in sections c or d. The distinguishing characteristic between either of these two mechanisms is the nature of the oxide-Ag interface. In one case (c), there is positive charge localized at the interface, and in the other (d), there is an increase in the overlap between the Ag metal states and the O 2p states. Previous theoretical calculations indicate that the mechanism outlined in section d is more likely.4 The data presented here alone, however, are not sufficient to distinguish between either of the mechanisms in section c or d. Conclusion In summary, the data presented here provide evidence for unique adsorption properties of NO2 onto 2.2 ML MgO(100) thin films supported by Ag(100), which differs from either NO2 adsorption onto Ag(100) or thick MgO(100) films. Our data are consistent with the formation of a stable NO2- species on the thin MgO(100) films, which dominate the surface up to exposures of at least 19 000 L, the highest exposure in our experiments. On clean Ag(100) and MgO(100) films 5 ML or thicker, we observe predominantly adsorption of an NO3-like species, most likely NO3-, at similarly large exposures. Therefore, the theoretically predicted ability to tune the adsorption properties of not only deposited metals but also molecules by controlling oxide thin film thickness has been demonstrated. The change in adsorption properties is due to the unique charge transfer capabilities of the metal-supported oxide thin film systems. Charge transfer is accompanied by a change in the electronic structure of the metal-oxide interface, as demonstrated by the appearance of a distinct Ag+ peak in the Ag 3d spectra. The charge transferred to NO2 upon adsorption may originate from either the MgO film followed by increased
Starr et al. adhesion and overlap between the O 2p states and metal states or from the Ag metal via electron tunneling and positive charge accumulation at the metal-oxide interface. Acknowledgment. We thank Thomas Risse and Martin Sterrer of the Fritz Haber Institute, Berlin, as well as Miquel Salmeron of Lawrence Berkeley National Laboratory (LBNL) for helpful and insightful discussions. Ed Wong and Tolek Tyliszczak (both LBNL) are acknowledged for their continued support at the beamline. This work was supported by the Office of Science, Biological and Environmental Research, Environmental Remediation Sciences Division (ERSD), U.S. Department of Energy under Contract no. DE-AC02-05CH11231 and by the National Science Foundation under Contract no. CHE0431425 (Stanford Environmental Molecular Science Institute). References and Notes (1) Ba¨umer, M.; Freund, H.-J. Prog. Surf. Sci. 1999, 61, 127. (2) Street, S. C.; Xu, C.; Goodman, D. W. Annu. ReV. Phys. Chem. 1997, 48, 43. (3) Campbell, C. T. Surf. Sci. Rep. 1997, 27, 1. (4) Gro¨nbeck, H. J. Phys. Chem. B 2006, 110, 11977. (5) Frondelius, P.; Hellman, A.; Honkala, K.; Ha¨kkinen, H.; Gro¨nbeck, H. Phys. ReV. B 2008, 78, 085426. (6) Ricci, D.; Bongiorno, A.; Pacchioni, G.; Landman, U. Phys. ReV. Lett. 2006, 97, 036106. (7) Pacchioni, G.; Giordano, L.; Baistrocchi, M. Phys. ReV. Lett. 2005, 94, 226104. (8) Sterrer, M.; Risse, T.; Pozzoni, U. M.; Giordano, L.; Heyde, M.; Rust, H.-P.; Pacchioni, G.; Freund, H.-J. Phys. ReV. Lett. 2007, 98, 096107. (9) Giordano, L.; Cinquini, F.; Pacchioni, G. Phys. ReV. B 2005, 73, 045414. (10) Sterrer, M.; Risse, T.; Heyde, M.; Rust, H.-P.; Freund, H.-J. Phys. ReV. Lett. 2007, 98, 206103. (11) Altieri, S.; Tjeng, L. H.; Swatzky, G. A. Thin Solid Films 2001, 400, 9. (12) Giordano, L.; Pacchioni, G. Phys. Chem. Chem. Phys. 2006, 8, 3335. (13) Simic-Milosevic, M.; Heyde, M.; Lin, X.; Rust, H.-P.; Sterrer, M.; Risse, T.; Nilius, N.; Freund, H.-J.; Giordano, L.; Pacchioni, G. Phys. ReV. B 2008, 78, 235429. (14) Broqvist, P.; Gro¨nbeck, H. Surf. Sci. 2006, 600, L214. (15) Takahashi, N.; Shinjoh, H.; Iijima, T.; Suzuki, T.; Yamazaki, K.; Yokota, K.; Suzuki, H.; Miyoshi, N.; Matsumoto, S.; Tanizawa, T.; Tanaka, T.; Tateishi, S.; Kasahara, K. Catal. Today 1996, 27, 63. (16) Matsumoto, S. Catal. Today 1996, 29, 43. (17) Rodriguez, J. A.; Jirsak, T.; Kim, J.-Y.; Larese, J. Z.; Maiti, A. Chem. Phys. Lett. 2000, 330, 475. (18) Rodriguez, J. A.; Jirsak, T.; Sambasivan, S.; Fischer, D.; Maiti, A. J. Chem. Phys. 2000, 112, 9929. (19) Bluhm, H.; Andersson, K.; Araki, T.; Benzerara, K.; Brown, G. E.; Dynes, J. J.; Ghosal, S.; Gilles, M. K.; Hansen, H.-Ch.; Hemminger, J. C.; Hitchcock, A. P.; Ketteler, G.; Kilcoyne, A. L. D.; Kneedler, E.; Lawrence, J. R.; Leppard, G. G.; Majzlam, J.; Mun, B. S.; Myneni, S. C. B.; Nilsson, A.; Ogasawara, H.; Ogletree, D. F.; Pecher, K.; Salmeron, M.; Shuh, D. K.; Toner, B.; Tyliszczak, T.; Warwick, T.; Yoon, T.-H. J. Electron Spectrosc. Relat. Phenom. 2006, 150, 86. (20) Ogletree, D. F.; Bluhm, H.; Hebenstreit, E. L. D.; Salmeron, M. Nucl. Instrum. Methods 2009, in press. (21) Wollschla¨ger, J.; Viernow, J.; Tegenkamp, C.; Erdo¨s, D.; Schro¨der, K. M.; Pfnu¨r, H. App. Surf. Sci. 1999, 142, 129. (22) NIST X-ray Photoelectron Spectroscopy Database, Version 3.4 (Web version); http://srdata.nist.gov/xps/. (23) Yeh, J. J.; Lindau, I. At. Data Nucl. Data Tables 1985, 32, 1. (24) Geisler, H.; Odo¨rfer, G.; Illing, G.; Jaeger, R.; Freund, H.-J.; Watson, G.; Plummer, E. W.; Neuber, M.; Neumann, M. Surf. Sci. 1990, 234, 237. (25) Starr, D. E.; Mendes, F. M. T.; Middeke, J.; Blum, R.-P.; Niehus, H.; Lahav, D.; Guimond, S.; Uhl, A.; Kluener, T.; Schmal, M.; Kuhlenbeck, H.; Shaikhutdinov, S.; Freund, H.-J. Surf. Sci. 2005, 599, 14. (26) Staudt, T.; Desikusumastuti, A.; Happel, M.; Vesselli, E.; Baraldi, A.; Gardonio, S.; Lizzit, S.; Rohr, F.; Libuda, J. J. Phys. Chem. C 2008, 112, 9835. (27) Libuda, J.; Frank, M.; Sandell, A.; Andersson, S.; Bru¨hwiler, P. A.; Ba¨umer, M.; Mårtensson, N.; Freund, H.-J. Surf. Sci. 1997, 384, 106. (28) Ozensoy, E.; Peden, C. H. F.; Szanyi, J. J. Phys. Chem. B 2005, 109, 15977. (29) Rodriguez, J. A.; Jirsak, T.; Liu, G.; Hrbek, J.; Dvorak, J.; Maiti, A. J. Am. Chem. Soc. 2001, 123, 9597.
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