Mixed Dissociative and Molecular Water Adsorption on Anatase TiO2

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Mixed Dissociative and Molecular Water Adsorption on Anatase TiO2(101) L. E. Walle,† A. Borg,† E. M. J. Johansson,‡ S. Plogmaker,‡ H. Rensmo,‡ P. Uvdal,§ and A. Sandell*,‡ †

Department of Physics, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway Department of Physics and Astronomy, Uppsala University, P.O. Box 516, SE-751 20 Uppsala, Sweden § Chemical Physics, Department of Chemistry, P.O. Box 124, and MAX-lab, P.O. Box 118, Lund University, SE-221 00 Lund, Sweden ‡

ABSTRACT: The adsorption properties of water on the stoichiometric (101) surface of anatase TiO2 in the temperature range 160
400 K has been studied by synchrotron radiation core level photoelectron spectroscopy. O 1s spectra give clear evidence for the formation of a first layer of water that comprises both H2O and OH. The composition is 0.77 ( 0.05 ML H2O and 0.47 ( 0.05 ML OH. Decreasing the coverage by heating leads to a decreased H2O/OH ratio. The results are very similar to those recently reported for water on rutile TiO2(110) and show that the previously proposed model of molecular adsorption only on anatase TiO2(101) must be revised.

1. INTRODUCTION The adsorption of water on a metal oxide surface defines a scientific problem of high general importance.1
3 Understanding the fundamentals in this process is the key for an advancement of the science of oxide surface chemistry, experimentally as well as theoretically. The archetypical system in this respect is water on the (110) surface of rutile titanium dioxide (TiO2).4 The adsorption state of water (molecular vs dissociated) on a rutile (110) surface free from defects has been highly controversial over the years.5
13 Very recently, we were able to provide decisive experimental input to this debate. With the use of careful surface preparation and highly surface-sensitive photoelectron spectra, it was demonstrated that the first water layer contains a significant fraction of dissociated species, even in a total absence of surface oxygen vacancies.14 In the present work we address the adsorption of water on anatase TiO2(101), that is, a TiO2 surface with different atomic structure. The anatase TiO2(101) surface exhibits a bulkterminated, unreconstructed (11) surface when prepared in ultrahigh vacuum (UHV).15 The anatase TiO2(101)-(11) surface is schematically depicted in Figure 1. The relatively few previous experimental and theoretical efforts have agreed upon a scenario in which water adsorbs molecularly on anatase (101), with the water oxygen binding to Ti(5) sites and the hydrogens to the 2-fold-coordinated O atoms [O(2)] on the next ridge.15
19 This configuration is the one labeled A in Figure 1. It has furthermore been demonstrated that the oxygen vacancy defects have a high propensity toward subsurface sites on anatase TiO2(101).20,21 The subsurface defects strongly influence the water adsorption. The adsorption energy at these sites is considerably higher than on the stoichiometric surface, and calculations even predict facile dissociation above subsurface defect sites.22 r 2011 American Chemical Society

In this study, we demonstrate that the prevailing view of pure molecular adsorption on anatase TiO2(101) is not correct. Using highly surface-sensitive synchrotron radiation excited O 1s photoemission spectra, we investigate the temperature-dependent adsorption state of water on the stoichiometric anatase TiO2(101) surface. We find clear evidence for a water monolayer that comprises comparable amounts of molecular and dissociated species. The behavior is essentially the same as that found on rutile (110), with a similar H2O/OH ratio of the monolayer and a H2O/OH ratio that decreases with decreasing coverage.

2. EXPERIMENTAL SECTION The measurements were performed at beamline D1011 at the Swedish National Synchrotron Facility MAX II.23 The end station comprises a Scienta 200 mm radius hemispherical electron energy analyzer. The presented O 1s spectra were recorded in 60° off normal emission using 610 eV photons. Binding energy (BE) calibration of the core level photoelectron (PES) spectra against the Fermi level was achieved by referencing to a Pt foil mounted on the sample holder. Radiation damage of the water layers has been carefully checked for water on rutile (110) and anatase (101) and found to be negligible.14 Pure water was introduced through a leak valve, backfilling the chamber. Annealing of the adsorbate layer was performed by increasing the temperature to the desired value at a rate of about 1 K per second and then kept at this temperature for 60 s. The anatase TiO2(101) single crystal (supplied by SurfaceNet GmbH) was cleaned by cycles of Ar-sputtering and subsequent annealing in oxygen to 650 °C. No contaminations, such as Received: November 29, 2010 Revised: April 7, 2011 Published: April 22, 2011 9545

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Figure 1. Schematic illustration of the anatase TiO2(101)-(11) surface in (a) top view and (b) side view. The rectangle defines an area of (10.24  7.57) Å2. Blue and red spheres denote lattice O and Ti, respectively. The light blue spheres are bridging oxygen atoms (Obr). The coordination number of the Ti atoms is given in parentheses. Green spheres indicate oxygen atoms bonded to H atoms (yellow spheres). The configuration labeled A represents molecular water adsorbed on a Ti(5) site. B represents the most likely configuration of pseudodissociated water according to calculations,22 with hydroxyl groups on a bridging site (OHbr) and terminal (Ti) site (OHt).

carbon or potassium, could be observed in surface-sensitive PES spectra. A sharp (11) low energy electron diffraction (LEED) pattern was observed. Prior to water adsorption, the sample was flashed to 450 °C. This procedure should lead to a surface free from any oxygen ad-atoms remaining after the oxygen treatment. The surface prepared in this way can be considered as stoichiometric, having a very low defect density. In evidence of this, we show Ti 2p and valence photoemission spectra in Figure 2. The Ti 2p spectrum displays one state with a Ti 2p3/2 binding energy of 458.9 eV, which is assigned to Ti4þ. The spectrum shows no sign of reduced Ti (Ti3þ), neither in normal nor grazing emission, the latter being the most surface-sensitive spectrum. (The Ti3þ state would otherwise have appeared at 1.5 eV lower binding energy, i.e., at about 457.5 eV.) Consistently, the valence spectrum displays very low intensity of the states in the band gap region (0
2.5 eV binding energy), where defect states are expected to appear. By comparing to studies of a reduced rutile TiO2(110) surface, we estimate that the concentration of oxygen

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Figure 2. (a) Ti 2p spectra for the clean anatase TiO2(101)-(11) surface, recorded in normal emission and grazing emission (60° off normal). The observed state is assigned to Ti4þ. The expected position of reduced Ti in the form of Ti3þ is indicated. (b) Valence photoemission spectrum for the clean clean anatase TiO2(101)-(11) surface, recorded in normal emission.

vacancies amounts to 3 ( 1%. We find that the oxygen treatment is motivated because (1) the LEED pattern improves significantly, (2) the concentration of oxygen vacancies is dramatically reduced, and (3) the reactivity of Ti interstitials (if present) is reduced.24 Moreover, the previously used method to produce stoichiometric surfaces on rutile (110) involves water dissociation at vacancies followed by O2 dosing.14,25 The first step of this procedure is however unlikely to work in the case of subsurface vacancies,26 which is the predominant form of oxygen vacancies for anatase (101).20,21 The adsorbate coverage is given relative to the density of 5-fold-coordinated Ti [Ti(5)] ions on the surface. One monolayer (1 ML) thus corresponds to a density of 5.2  1014 cm
2. This definition is also valid for the rutile TiO2(110) surface.

3. RESULTS AND DISCUSSION Figure 3 shows surface-sensitive O 1s spectra obtained after adsorption and subsequent annealing of a multilayer of water on anatase TiO2(101)-(11). The spectra, normalized to the 9546

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Figure 3. O 1s spectra for water adsorption on the anatase TiO2(101)-(11) surface. The spectra show the results after progressive heating of a multilayer of water and each spectrum has been delineated into individual contributions from the TiO2 substrate, OH, and H2O. The spectra have been normalized to the number of scans and the beam current in the synchrotron.

number of scans and the beam current in the synchrotron ring, have been delineated into individual contributions from the TiO2 substrate, OH, and H2O. After adsorption of water at 120 K followed by heating to 160 K, in order to desorb multilayer water two O 1s features are observed apart from the TiO2 substrate peak at 530.5 eV. The state at 531.5 eV can be assigned to OH and the state at 534 eV to H2O.14,26,27 When increasing the temperature further, the intensity of the O 1s component of molecular water continues to decrease rapidly, falling below the detection limit somewhere between 300 and 400 K. The intensity of the O 1s peak from OH decreases continuously from about 200 K up to 400 K. The H2O-related component decreases more rapidly with temperature than the OH related component, resulting in a varying H2O/OH ratio. Clearly, the first layer of

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water consists of both molecular and dissociated species, and the amount of OH is far too high to be attributed to oxygen vacancies; see Figure 2 and the discussion below. It is also evident that OH recombination and desorption of the dissociated species on the stoichiometric surface occurs at much lower temperature than expected for water dissociated on oxygen vacancies.16,28 This observation suggests the formation of a pseudodissociated form of water, which, according to calculations, is most probably the one labeled B in Figure 1.22 The observation of water dissociation on the anatase (101) surface is in contradiction to earlier results.15
19 We believe that the reason for this discrepancy to a large extent originates from difficulties to identify the chemical state of the adsorbed species with the techniques used previously. When scrutinizing the literature, it is clear that the spectroscopic basis for the previous assumption of purely molecular water adsorption on anatase (101) is (1) the absence of a visible OH-related O 1s peak in XPS and (2) the absence of a recombinative TPD peak at about 500 K.16 These grounds are highly questionable. With respect to the O 1s spectra, our superior surface sensitivity combined with high spectral resolution allow for observing an OH related peak not clearly visible in the previous XPS study, in which the low surface sensitivity (and worse resolution) causes the OH peak to become obscured by the substrate signal. Clearly, the O 1s peak stemming from hydroxyl groups is hard to observe with XPS unless a very high surface sensitivity is attained.14 It is also important to have a spectrum representing a clean surface free from OH for comparison. Such a spectrum can only be measured at elevated temperature. With respect to the TPD results, it is not possible to identify pseudodissociated water with this technique because, as noted above, the temperature regime for desorption of pseudodissociated species overlaps with that of molecular water. Previous work by other groups also includes extensive STM studies of water on anatase (101), often in conjunction with DFT calculations.18 Several high-quality images have been presented. At low coverages (below 0.1 ML), characteristic white-blackwhite features were observed.18 At higher coverages, these merge, forming lines with alternating black-white contrast on neighboring sites.18 The result is a superstructure with (22) periodicity. With the aid of calculations, a model comprising solely molecular water was proposed.18 However, STM does not provide the chemical identity of adsorbates. The interpretation of the images obtained is therefore not always straightforward when different species as well as chemical transformation between them are involved. These limitations of the STM technique are for instance apparent for two other water/surface adsorbate systems. STM topographs of a water monolayer on ZnO (1010) show a phase of clear (21) periodicity, interpreted as a half-dissociated (mixed) phase.29 Interestingly, it was suggested that only the upper hydrogen atoms of the OH species were visible in STM, whereas the H2O species were invisible in the STM contrast. The reason was suggested to be different molecular geometries; while the H-atoms of the OH groups stick out from the surface, the undissociated water molecules lie flat on the surface. A very recent study of a water
hydroxyl layer on Cu(110) provides another example of the complex mechanisms behind the contrast in STM images.30 The mixed layer contains Bjerrum defects and the hydroxyl groups associated with these defects image brighter than the water molecules, although some of the H2O molecules attached to the hydroxyls also gain intensity with respect to other H2O molecules in the overlayer. 9547

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using the same experimental parameters.14,32 The agreement between the two methods was found to be within (0.02 ML. Dissociation of water at a Ti(5) site on the stoichiometric surfaces of anatase (101) and rutile (110) results in a terminal OH (OHt) and a hydrogen atom adsorbed on a neighboring Obr site, giving a bridging OH (OHbr).9,33 For a mixture of molecular and dissociated water, the O 1s intensity representing the density of occupied Ti sites is therefore obtained as: O 1s ðTiÞ ¼ 1  O 1s ðH2 OÞ þ 0:5  O 1s ðOHÞ

Figure 4. Estimates of the OH and H2O coverages in monolayers on (a) the anatase TiO2(101)-(11) surface and (b) the rutile (110) surface as a function of temperature. Filled (red) squares: OH; filled (blue) circles: H2O. One monolayer corresponds to the density of Ti(5) sites.

The examples above illustrate that the STM contrast within a mixed water
hydroxyl layer is not likely to be understood in a straightforward way. In light of the present O 1s photoemission results, we propose that the interpretation of the STM images of water on anatase (101) should be reconsidered, taking water dissociation into account. After the presence of dissociated species coexisting with molecular species is established, the next step is to quantify the water and hydroxyl coverages based on the O 1s intensities. Consequently, a monolayer reference point is needed. However, the TPD spectrum for H2O/anatase TiO2(101) shows a complex desorption behavior; the structure associated with desorption from bridging oxygen sites is partly overlapping with the structure associated with desorption from Ti sites.16 This makes it difficult to identify a temperature at which the saturation coverage is one monolayer. In contrast, the TPD spectrum for H2O/ rutile TiO2(110) shows a well-defined monolayer point in the form of a clear dip (at 210 K) between the desorption structure from bridging oxygen sites and the desorption structure from Ti sites.28 The coverage estimates in the present work were made in two different, independent ways. First, water adsorption was monitored simultaneously on two different anatase surfaces, (101) and (001), by the O 1s intensity. Because we have previously estimated the water coverage on the (001) surface using the angular dependence of the O 1s intensity,31 the O 1s intensities could be directly related to coverage by normalizing to the results for the (001) surface. Second, the coverage on the (101) surface was also estimated by using the O 1s data recorded for water adsorption on the rutile TiO2(110) surface at 210 K

ð1Þ

Thus, by normalizing according to the procedures described above, OH and H2O coverages on the anatase surfaces can be calculated. The results are shown in Figure 4a. For comparison, the corresponding results for rutile are shown in Figure 4b. The temperature dependences of the H2O and OH coverages are qualitatively very similar for the two surfaces with a decrease in the H2O coverage that is more rapid than the decrease in the OH coverage. However, the decrease in the adsorbate coverages is slower for the anatase (101) surface than for the rutile (110) surface. We ascribe this at least partly to a difference in the temperature reading and that the rutile data were acquired at elevated temperature whereas the heating was switched off when measuring on the anatase sample. Adsorption of small amounts of residual water is therefore expected to lead to an apparently slower response to the increased temperature for anatase (101) vs rutile (110). Using 1, we can interpolate that a coverage of 1 ML on anatase (101) is attained at about 220 K. This is close to the monolayer temperature of 210 K found for rutile (110). The composition of the water monolayer on anatase (101) is estimated to be 0.77 ( 0.05 ML H2O and 0.47 ( 0.05 ML OH. This composition is within the error bars identical to that found on rutile (110).14 More than ten monolayer preparations have been made on rutile (110) in different ways and the composition is always found to lie within 0.79 ( 0.04 ML H2O and 0.42 ( 0.07 ML OH. Another important comparison is made in Figure 5a. Here the binding energies of the O 1s component related to molecular water on anatase (101) and rutile (110) are shown. In both cases, a binding energy maximum is found at 1 ML coverage, and the absolute binding energy and the temperature (coverage)-dependent binding energy shifts are nearly identical. A feasible explanation is that the decrease in binding energy seen below 200 K arises from the adsorption of second- and multilayer water, while the decreased binding energy seen above 200 K is a result of decreased adsorbate
adsorbate repulsion.16 We note that the binding energy variation in Figure 5a appears to be quite different from that found in the earlier experiment,16 both in relative and absolute terms. It is very difficult to assess the reason for the discrepancy because the previous study lacks a spectroscopic characterization of the clean surface, the coverage calibration is different, and the spread of the values is quite large. One possibility is that the surface in the earlier study had a higher density of oxygen vacancies, which the high binding energy value of the substrate peak indicates. The adsorption of water on anatase (101) and rutile (110) surfaces thus shows clear similarities, in several ways. This is not too surprising because these surfaces are quite similar. The surface layer comprises Ti(5) cations and O(2) anions of nearly identical densities, and the remaining species are in both cases 6-fold coordinated Ti cations and 3-fold coordinated O anions. On both surfaces, the H2O/OH ratio decreases when the coverage 9548

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surfaces, a mechanism that does not require the presence of substoichiometric parts (oxygen vacancies) in the surface and near-surface region. The two surfaces have different atomic structures, but these differences do not appear to be crucial for the composition of the first water layer. Although the adsorption behavior of water on anatase (101) and rutile (110) is quite similar, some small differences are observed. The full width at half maximum (fwhm) of the O 1s peak of OH on anatase is larger and shows a different behavior with respect to temperature (i.e., coverage) compared to the O 1s peak of OH on rutile (Figure 5b). The results hence suggest that the (pseudo)dissociated species on anatase is different from that formed on rutile. When considering the large fwhm and the uncertainties in the fit, we cannot exclude the possibility of having chemically inequivalent OH species on anatase (101). Another detail, noted in Figure 4a, is that the OH coverage on anatase (101) displays a maximum of 0.6
0.7 ML OH at 180 K. The maximum appears to be correlated with a dip in the water coverage. We tentatively associate these observations with the presence of a second layer of water because the desorption maximum of the second layer is found at about 185 K.16 Influence on the adsorption state of first layer water from the second layer of water has been proposed to occur for ZnO (1010).34 A corresponding behavior of the OH coverage is not observed for rutile (110). However, a larger data set is needed to rule this out completely. We can therefore conclude that water adsorption on anatase (101) and rutile (110) do display subtle differences, which may be related to the different atomic structures of the surfaces. Further investigations are needed to establish the significance and origin of these observations.

Figure 5. (a) Binding energies of the O 1s peaks from H2O as a function of temperature. Filled (blue) circles: Water on anatase (101); open (blue) circles: water on rutile (110). (b) fwhm of the adsorbate related O 1s peaks as a function of temperature for water on anatase and rutile. Filled (red) squares: OH on anatase (101); open (red) squares: OH on rutile (110); filled blue circles: H2O on anatase (101); open (blue) circles: H2O on rutile (110).

is decreased by heating-induced desorption, which can be understood qualitatively from earlier TPD results.16,28 The water TPD spectrum for both anatase (101) and rutile (110) shows a decreased adsorbate desorption temperature for increasing coverage, which indicates a repulsive adsorbate
adsorbate interaction. A weakening of the adsorbate
substrate interaction could reduce the probability for dissociation, consistent with the results obtained for water on both surfaces. From this follows that the composition and structure of the adsorbate layer is governed by the interplay between the adsorbate
substrate and the adsorbate
adsorbate interactions. A small increase of (partial) dissociation of water at high coverages on anatase (101) has recently been proposed based on DFT calculations.22 These results indicate, however, that the effect is small enough to be ignored for most experimental conditions, in contrast to our observations. The spectra shown in Figure 2 shows furthermore that the surface is to be considered as stoichiometric. Still, a large coverage (0.5 ML) of OH is formed along with the uptake of molecular water. The behavior is similar to that observed for the stoichiometric rutile (110) surface. This points toward a similar mechanism for the formation of the mixed water layers on the two

4. CONCLUSIONS We have employed surface-sensitive photoelectron spectroscopy to investigate the adsorption state of water on the stoichiometric anatase TiO2(101) -(11) surface in the temperature range 160
400 K. With this approach, we have been able to demonstrate that the formation of a water monolayer on the anatase TiO2(101) surface involves both associative and dissociative adsorption. The monolayer is defined as the density of 5-fold coordinated Ti sites at the surface. When the uptake of water molecules corresponds to a monolayer, it consists of 0.77 ( 0.05 ML H2O and 0.47 ( 0.05 ML OH. The H2O/OH ratio decreases when the coverage is decreased. The behavior is similar to that found for water on rutile TiO2(110). The unambiguous observation of partial dissociation within the first layer of water on anatase TiO2(101) demonstrates that the previous assumption of molecular water adsorption only has to be reconsidered. Our results hence illustrate the importance of having spectroscopic data for systems with mixed or unknown chemical composition. It is clear that determining the chemical composition of adsorbate layers based on STM and DFT calculations alone is very difficult because it may leave too many alternative models. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We thank the staff at MAX-lab for their assistance. This work has been supported through the Swedish Science Council (VR), 9549

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The Journal of Physical Chemistry C the Knut and Alice Wallenberg Foundation (KAW), the Crafoord Foundation, NordForsk, and the G€oran Gustafsson Foundation. L.E.W. has been supported through the Strategic Area Materials at NTNU.

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