TiO2(110): Effects on Au Mobility

May 21, 2008 - Initial and Final State Effects in the Ultraviolet and X-ray Photoelectron Spectroscopy (UPS and XPS) of Size-Selected Pdn Clusters Sup...
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J. Phys. Chem. C 2008, 112, 9006–9015

Water on Rutile TiO2(110) and Au/TiO2(110): Effects on Au Mobility and the Isotope Exchange Reaction Tianpin Wu, William E. Kaden, and Scott L. Anderson* Department of Chemistry, UniVersity of Utah, 315 South 1400 East, Rm 2020, Salt Lake City, Utah 84112 ReceiVed: January 18, 2008; ReVised Manuscript ReceiVed: March 8, 2008

The behavior of adsorbed water on rutile TiO2 (110) and 0.05 ML-equivalent Au/TiO2 (110) have been probed over the temperature range from 110 to 800 K using a combination of temperature-programmed desorption (TPD), ion scattering spectroscopy (ISS), and X-ray photoelectron spectroscopy (XPS). Water, either predosed in order to hydroxylate vacancy sites on the surface prior to Au deposition, or dosed after Au deposition onto clean vacuum-annealed TiO2, results in more facile agglomeration of the initially dispersed Au atoms compared to Au deposited on vacuum-annealed TiO2. TPD indicates that the presence of Au binding at oxygen vacancy sites blocks dissociative chemisorption of water. I. Introduction In a previous report, we used deposition of size-selected Aun+ to demonstrate strongly size-dependent activity for CO oxidation over Au/TiO2 (110) model catalysts with significant activity first appearing for Au3 and a sharp increase in activity at Au7.1 An important issue in interpreting such results is the nature of the cluster binding to the surface and the extent to which cluster breakup or agglomeration occurs. To probe morphology, we used ion-scattering spectroscopy (ISS), which gives a measure of the relative degree of gold dispersion on the surface. From the pattern of ISS vs deposited cluster size, we argued that gold atoms were highly dispersed on the surface, Au2, Au3, and Au4 formed one- or two-dimensional clusters lying on the surface and that a three-dimensional (i.e., multilayer) morphology became important for Au5 and larger clusters. Subsequent scanning tunneling microscopy (STM) work by Buratto and coworkers2 supported our interpretation of the Aun/TiO2(110) binding morphology following Aun+ deposition with one important exception. While we observed the highest degree of gold dispersion when depositing Au+, their results showed agglomeration into large Au nanoparticles when Au+ was deposited under nominally similar conditions (room-temperature surface, low-energy Au+). Furthermore, experiments by other groups3,4 wherein Au atoms were evaporated onto TiO2(110) also showed significant agglomeration at room temperature. In an effort to understand this discrepancy, we carried out a study where Au+ was deposited at cryogenic temperatures and then ISS was used to monitor dispersion following annealing to various higher temperatures.5 In that study, we concluded that Au atoms did begin to diffuse at room temperature; however, trapping of Au at oxygen vacancy sites, present in ∼8% (see below) of the surface unit cells, prevented growth of large clusters. Au association with the electron-rich vacancy sites was suggested by the shifts to lower Au XPS binding energy when Au/TiO2 was annealed to room temperature. Significant agglomeration, as shown by a decrease in Au ISS signal and Au XPS binding energies near the bulk limit, occurred only when samples were annealed above room temperature. The idea that Au and small Au clusters tend to bind at oxygen vacancy sites was further supported by the observation that Au+ * To whom correspondence should be addressed.

and Au2+ deposition eliminated CO oxidation that would normally occur at the vacancies.1 At the time, it was unclear why our results differed from those in the STM experiments, and we proposed that the rapid time scale of ISS, compared to STM, might have been important in providing us with a snapshot of the sample morphology before significant agglomeration could occur. Recently, Besenbacher and co-workers reported a reinterpretation of their earlier STM experiments regarding O-atom mobility on TiO2(110).6–8 One result was that oxygen vacancies found on the freshly annealed surface were found to react very efficiently with adventitious water, replacing each initial O vacancy with a pair of surface hydroxyl groups. The hydroxylated sites diffuse by H-atom migration to adjacent bridging O atoms. Similar results have been reported by other STM investigators.9,10 Hydroxylation of vacancy sites was observed to occur on a time scale of a few hours even in quite good UHV conditions and raised the possibility that the discrepancy between our results and the STM results for Au agglomeration might be attributable to a greater degree of hydroxylation occurring during the longer time scale of STM experiments. The purpose of this report is to examine the effects of water exposure, both before and after Au+ deposition, on Au agglomeration. Additionally, the effects of Au on water temperature-programmed desorption provide some insight into Au binding on the vacuum-annealed surface. II. Experimental Methods The experiments were performed in a vacuum system consisting of a mass-selected ion deposition beamline described elsewhere,11 connected to an ultrahigh vacuum (UHV) surface analysis chamber with base pressure of ∼8 × 10-11 Torr. The UHV chamber incorporates facilities for sample preparation, X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and ion-scattering spectroscopy (ISS). It also includes a differentially pumped mass spectrometer for temperature-programmed desorption (TPD) and other reactivity studies. The sample can be inserted through a seal into a second chamber that serves as a high-pressure cell and load lock. The operating and analysis procedures are similar to those previously described.12–14

10.1021/jp800521q CCC: $40.75  2008 American Chemical Society Published on Web 05/21/2008

Water on rutile TiO2(110) and Au/TiO2(110) The rutile TiO2(110) single crystal (Commercial Crystal Laboratories) was mounted on a 0.2 mm thick molybdenum backing plate and could be heated to 1200 K by resistive heating and cooled to 100 K by thermal conduction to a liquid nitrogen reservoir. Sample temperatures were measured by a K-type thermocouple attached to the edge of the TiO2 crystal with UHV-compatible ceramic glue (Aremco, Ceramabond 571). The crystal was cleaned by repeated cycles of Ar+ bombardment and 1100 K annealing, not only removing contaminants but also leaving the crystal with sufficient bulk conductivity to perform electron spectroscopy and ion beam deposition without surface charging problems. The cleanliness of the sample was checked by XPS and ISS before deposition experiments. Before gold deposition, the TiO2 sample was cleaned by bombarding for 15 min with 1 keV Ar+ followed by a 15-min annealing period at 850 K in UHV. The state of the surface was probed by Ti XPS and water TPD (see below), both methods indicating that the TiO2 is nearly stoichiometric with ∼8% of oxygen vacancies in the bridging oxygen rows.15 Mass-selected Au+ was deposited at a deposition energy of ∼1 eV with the TiO2 held at 110 K. The Au+ was deposited in a ∼2 mm diameter spot to a density equivalent to 5% of a closepacked Au monolayer (7.0 × 1013 Au atoms/cm2), as determined by integrating the ion current measurements. In previous experiments,5 the Au+ sticking coefficient was found to be unity within experimental error. For our beam current, deposition times were a few minutes, and the sample is conductive enough to neutralize the Au+ on impact. For ISS, He+ at 500 eV kinetic energy was directed at the sample spot with a 45° angle of incidence, and scattered He+ was collected along the surface normal (scattering angle ) 135°) and energy analyzed using a hemispherical analyzer. For water adsorption and dissociation studies, H218O (98.5%, Isotec) was purified by several freeze-pump-thaw cycles using both liquid nitrogen and ice-water baths and then dosed through a pulse valve into the UHV chamber. During dosing, the sample was positioned such that it was directly exposed to H218O effusing from the inlet tube. The actual water coverage was determined by comparing the total integrated desorption intensity with the saturation intensity associated with binding to the twocoordinate O (2cO) and five-coordinate Ti (5cTi) sites (see below). For TPD, the sample was moved into position ∼1 mm in front of the 3 mm skimmer cone aperture of the differentially pumped mass spectrometer and heated to 600 K at a rate of 3 K/s. We observed some run-to-run variation in TPD sensitivity (variation in integrated intensity for desorption from saturated sites), attributed to effects of water exposure on electron multiplier gain. To facilitate comparison between TPD for TiO2 and Au/TiO2, the spectra were normalized to constant total integrated desorption from 2cO and 5cTi sites. One of the motivations for this study was the report from Besenbacher and co-workers that vacuum-annealed TiO2(110) reacts efficiently with background water at room temperature with significant conversion of the oxygen vacancies to hydroxyl groups on the 30-60 min time scale, even though their base pressure is only 3 × 10-11 Torr.7 For these experiments, our system was baked until the base pressure in the main analysis chamber was in the high 10-11 Torr range. Mass spectra of the background suggest that the water partial pressure is in the high 10-12 Torr range; however, this estimate is uncertain because the mass spectrometer probes the background gas in its own differentially pumped chamber. Nonetheless, although our base pressure is somewhat higher than in the Besenbacher experiments, both deposition and ISS

J. Phys. Chem. C, Vol. 112, No. 24, 2008 9007 analysis are fast (total experiment time ≈ 6 min), resulting in an adventitious water exposure of only ∼1.5 × 1012 molecules/ cm2, which would hydroxylate only ∼4% of the initial oxygen vacancies, even if we assume that all water impinging on the surface diffuses to and hydroxylates vacancy sites. To minimize adventitious adsorption during the time it takes to cool the sample holder following TiO2 cleaning/annealing, the sample was flashed to 600 K just prior to the start of each experimental sequence. III. Results and Discussion A. Temperature-Programmed Desorption of H218O and H216O. Before discussing effects of water on Au agglomeration, it is necessary to examine the nature of water binding to the vacuum-annealed TiO2(110) surface and how the binding is affected by Au deposition. The choice of water dose for the experiments below was based on a dose-dependent study of water TPD from TiO2(110). Because the dose-TPD behavior is essentially identical to that reported in previous studies,16,17 those results are relegated to the Supporting Information. All experiments below were carried out using a water dose sufficient to essentially saturate the first monolayer adsorption sites with an additional 0.5-0.6 H2O/unit cell in second layer sites. This second-layer water provides an internal standard for correcting mass spectral intensities, as described below. Intensities are obtained by fitting the peaks to estimate contributions in ranges where they overlap and then integrating. Figure 1 presents TPD spectra (mass 20 ) H218O+) following ∼3 L H218O exposure to freshly vacuum-annealed TiO2(110) and TiO2(110) with 0.05 ML-equivalent of Au deposited as Au+ at 1 eV, respectively. There are three main features, all assigned and analyzed in detail in previous studies of water TPD from rutile TiO2(110).16,18 The broad peak centered at ∼250 K was assigned to water desorbing from five-coordinate Ti (5cTi) sites. The peak at ∼160 K was assigned as water desorbing from two-coordinate O2- (2cO ) bridging oxygen) sites, and the peak at ∼145 K is attributed to water adsorbed in multilayer (i.e., second layer) sites. The integrated intensities of the 5cTi and 2cO desorption peaks are identical within the uncertainty associated with deconvolving the overlapping peaks, as expected because each TiO2 unit cell has one of each site. In the 0.05 ML-equivalent Au/TiO2 sample, there is one Au atom for every ∼8 unit cells; thus, we might expect to see a peak for water desorption from Au sites with an intensity of ∼13% of that for 5cTi. As shown in Figure 1, there is little obvious difference in TPD pattern for TiO2 and Au/TiO2: certainly no distinct peak or obvious shoulder. The increase in second-layer peak for Au/TiO2 is simple, the result of a few percent run-to-run variability in our water dose. Since it seems unlikely that water does not stick at these temperatures, the results suggest that water desorption from Au/TiO2 sites must be buried under one or more of the features associated with the TiO2 surface. Note that there is a broadening to higher temperatures of all the TPD peaks (particularly 2cO), suggesting that the presence of 5% ML Au slightly increases the water binding energy at nearby TiO2 sites. There is also a slight decrease in the 5cTi-to-2cO intensity ratio, relative to Au-free TiO2, which suggests that Au on the surface may block a fraction of the 5cTi sites or at least modify the water binding energy at nearby 5cTi sites. Water interactions at oxygen vacancies, with and without Au, are discussed below. Figure 2 compares the TPD signals for masses 18 and 20 following a ∼3 L H218O dose on the vacuum-annealed TiO2(110) surface at ∼110 K. The inset (right-hand scale) shows

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Figure 1. Full-temperature-range TPD of H218O after ∼3 L H218O dose on TiO2(110) and 5% ML Au/TiO2(110) deposited at ∼110 K. (Inset) Schematic model of the TiO2(110) surface.

Figure 2. Full-temperature-range TPD of after ∼3 L H218O dose on TiO2(110) at ∼110 K. Main figure: H218O signal, total mass 18 signal, and background-corrected H216O signal (“true H216O”). Expanded view: H218O and background-corrected H216O.

a magnified view of the 300-600 K temperature range. For mass 20, the only significant contributor is H218O+ generated by electron impact of desorbing H218O. For mass 18, however, there are significant background contributions in addition to the H216O+ produced by ionizing H216O desorbing from the surface (the signal of interest). There is ∼1.5% H216O contamination in the H218O sample, and there may be some O18-O16 isotope exchange (generating H216O) on inlet system or mass spectrometer surfaces, although in the case of the inlet system, most exchangeable oxygen was probably preconverted to 18O during a series of initial H218O passivation pulses. The biggest source

of mass 18 background is 18O+ generated by dissociative ionization of H218O in the mass spectrometer. To correct for these background signals, we take advantage of the secondlayer desorption signals at both masses. The idea is that secondlayer H218O, separated from Ti16O2 by H218O in the first monolayer, is unlikely to undergo significant 18O f 16O exchange; thus, the ratio of masses 18 and 20 for the secondlayer TPD component (∼0.15) provides a measure of the total background contribution to the mass 18 signal. Henderson’s water/TiO2 TPD study, which found no isotope exchange for second-layer water, supports this assumption.18 If we assume

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Figure 3. Full-temperature-range TPD after ∼3 L H218O dose on 5% ML Au/TiO2(110) at ∼110 K. Main figure: H218O signal, total mass 18 signal, and background-corrected H216O signal (“true H216O”). Expanded view: comparison of background-corrected H216O from Au/TiO2(110) and TiO2(110).

that the relative background contribution is the same for all desorption features, the “true H216O” intensity can be obtained by subtraction, as shown in Figure 2. As shown in the figure, in the temperature range from 120 to 350 K, the “true H216O” signal is only a few percent of the H218O signal, probably within the uncertainty associated with thebackgroundsubtraction.Theabsenceofsignificantwater-surface oxygen exchange for water bound at 5cTi and 2cO sites is consistent with both TPD data, and HREELS results demonstrating that water in these sites is molecularly adsorbed.17 The inset shows the much weaker desorption feature appearing between 300 and 600 K, plotted against the right-hand scale, showing signals for both of H218O+ and background-corrected H216O+ (“true H216O”). In previous work, this feature has been shown to result from recombinative desorption of water dissociatively chemisorbed at defects, predominantly oxygen vacancy sites.16,18,19 Because of a tail extending into this temperature region from the lower temperature TPD features, there is significant uncertainty in the baseline for the mass 20 signal (smooth solid curves); however, it is clear that at least 90% of the recombinative desorption component has undergone 18O-16O exchange on the surface, desorbing as H 16O. The 2 integrated intensity of this recombinative desorption component (H218O + H216O) is ∼8% of the intensity of the 5cTi or 2cO components. Assuming, following previous authors,16,18,19 that this component corresponds to water dissociative adsorption, mostly at vacancies in the bridging oxygen rows, then the intensity ratio implies that ∼8% of unit cells have a bridging oxygen vacancy. An 8% vacancy density is similar to what has been estimated for vacuum-annealed TiO2 by other researchers based on STM20 and TPD.15 The small experiment-to-experiment differences in vacancy density presumably reflect differences in preparation conditions and possibly sample history. Figure 3 gives analogous results for the sample with 0.05 ML-equivalent Au deposited as Au+. As for Au-free TiO2, there is no significant 18O f 16O exchange associated with molecularly bound water for T < 350 K, whereas there is essentially

complete exchange for the recombinative desorption feature above 350 K. To facilitate comparison of the TiO2 and Au/ TiO2 sample, the inset to this figure shows background-corrected H216O desorption signals for both samples: the H218O signals for both show no obvious desorption feature in this temperature range, as in Figure 2. In addition to recombinative desorption associated with O-vacancy sites on TiO2, the high-temperature desorption feature for Au/TiO2 may include desorption of water bound to gold. Indeed, ISS/annealing presented results below show that ∼21% of the Au ISS signal is blocked by some adsorbate that desorbs between 300 and 450 K. Given the lack of H218O signal in TPD, the adsorbate must correspond to water fragments (OH?) that undergo oxygen exchange with the surface at some point before desorbing. It is unclear what fraction of the recombinative desorption is contributed by Au sites because the ISS attenuation per water fragment depends on how the adsorbate is bound. If we assume that the attenuation results from OH bound directly atop Au, where it would completely block ISS, then roughly one-third of the recombinative desorption would be attributable to Au-bound water. Despite this contribution from Au sites, the total recombinative desorption signal for Au/TiO2 is ∼40% smaller than that for TiO2. The initial density of O-vacancy sites on the two surfaces should be identical because they are prepared identically prior to Au deposition. Therefore, the reduction in recombinative desorption suggests that Au must block water dissociation at a substantial fraction (∼40%) of the initial vacancy sites, presumably by binding there. Furthermore, the blocked vacancies apparently do not become unblocked during heating up to ∼250 K because in that case we would expect nearby molecularly adsorbed water to dissociate at the empty vacancies. In our previous study of Au+ on TiO2 we concluded, based on XPS binding energies, that Au deposited as Au+ at cryogenic surface temperatures tends to bind in a variety of sites, resulting in a broad Au XPS peak.5 Upon annealing to 300 K, the Au XPS peak narrowed and shifted to lower binding energy, indicating enough Au mobility to remove much of the hetero-

9010 J. Phys. Chem. C, Vol. 112, No. 24, 2008 geneity in the initial binding site distribution. Accompanying ISS data showed no evidence of sintering at 300 K, and separately, we found that Au deposited at 300 K blocks CO oxidation attributed to O2 activation at vacancy sites on TiO2.21 We proposed, therefore, that most of the Au mobilized by heating to 300 K was trapped at nearby oxygen vacancy sites, where the binding energy is relatively high compared to other sites.22,23 In the present experiments Au+ is deposited at cryogenic temperatures, and we expect Au mobility to be limited, such that it is unclear if it is reasonable that 40% of the vacancy sites should be blocked by Au. The Au deposition density in these experiments is 0.13 Au atom/unit cell, compared to an oxygen vacancy density of 0.08 vacancy/unit cell. Given these densities, the average Au diffusion length needs to be only ∼1 unit cell length for 40% of the vacancies to capture an Au atom. Filling 40% of the vacancies leaves ∼75% of the Au bound in other sites, consistent with the binding site heterogeneity inferred from the XPS. When water dissociates at an oxygen vacancy, the OH group should fill the vacancy site, with the H atom binding to an adjacent oxygen site, creating a second OH moiety. This is, essentially, what is seen in STM experiments of Besenbacher and co-workers7,8 and Dohna´lek and co-workers.9 Furthermore, in the STM experiments, the H atoms are observed to hop between bridging O atoms, effectively allowing the hydroxyl groups to migrate. Given that our surface is initially Ti16O2 and the dissociative chemisorption of H218O should only add ∼8% of 18O to the surface layer, then ∼92% of the recombining water should be H216O, which is consistent with the observed ratio of H216O+ to H218O+ in the high-temperature TPD peak. Oxygen isotope scrambling was also examined in the study by Henderson et al.18 In that experiment, the surface was 18Oenriched by annealing TiO2 in 18O2 and then dosed with H216O at 130 K prior to TPD. This combination, essentially the opposite of that used here, is relatively background/cracking-free in the isotope-exchanged (mass 20) channel with the background/ cracking contributing instead to the unexchanged (mass 18) signal. As in the results here, no significant 16O/18O exchange was observed in the multilayer and monolayer peaks. In the recombinative desorption feature, they reported signals for both masses 18 and 20, and while they did not attempt to correct for background/cracking contributions to the mass 18 signal, it is clear that the recombinative desorption is dominated by H218O, i.e., by isotope-exchanged water. Comparison of our results with those from Henderson indicates that there are no significant isotope effects on the recombinative desorption processes. Furthermore, the presence of 0.05 ML-equivalent Au appears to have little effect on water adsorption/desorption with the exception of blocking dissociative adsorption at Au-occupied vacancy sites. B. Ion Scattering Spectroscopy (ISS) and He+-Impact Effects. TPD reveals the (minimal) effects of a small coverage of Au on water adsorption/desorption behavior from TiO2. As noted in the Introduction, our main interest here is in effects of water adsorption on Au binding and agglomeration behavior. ISS was used as our morphology characterization tool; thus, it is useful to briefly review the factors that influence ISS signal. ISS involves scattering low-energy ions from the surface and measuring the residual kinetic energy (E/E0) for the small fraction of ions surviving scattering from the surface ( 200 K. The increased post-TPD 18O/16O ISS ratio induced by He+ impact on the water layer indicates that ISS followed by heating leaves behind a small amount of 18O from the water dose. ISS-driven oxygen exchange between TiO2 and water is a reasonable explanation for the increased 18O/16O ratio. Sputtering is known to preferentially remove O from the TiO2 surface, and dissociative adsorption of H218O at the resulting O vacancies, followed by TPD, would result in 16O f 18O exchange. The alternative explanationsthat strongly bound 18O-containing adsorbates are left behind on the surface after TPDsseems unlikely in light

of the fact that the TPD spectrum above 200 K is essentially unchanged by He+ impact on the water layer. If such strongly bound adsorbates were present in numbers sufficient to explain the increase in 18O/16O ISS ratio, we might expect them to modify the TPD behavior of the coadsorbed water. C. Water Effects on Au Annealing Behavior. To probe the effects of water on the Au annealing/sintering behavior, we did three sets of ISS experiments. For each annealing step, the sample was heated to the indicated temperature for 5 min, cooled back to ∼120 K, and reanalyzed by ISS. ISS spectra were background corrected and integrated, as above, to extract intensities for each type of atom in the surface layer. To compensate for day-to-day variation in He+ flux (∼4%), the integrated intensities were all scaled such that the sum of 16O and Ti intensity measured for freshly vacuum-annealed TiO2 at the beginning of each data set was constant. To remove adventitious adsorbates sticking during the time it takes to cool the sample holder following annealing, the sample was flashed to 600 K just prior to the start of each experimental sequence. In Figure 5 results are shown for an experiment where Au was deposited on vacuum-annealed TiO2 and then dosed with H218O and subjected to a sequence of annealing/ISS experiments described as below. The percentage numbers given at the points are the changes in intensities relative to the initial intensity for each type of atom. The first set of ISS intensities (TiO2) is for vacuum-annealed TiO2 and only shows Ti and 16O as expected. The second set (Au/TiO2) is for as-deposited 5% ML Au/TiO2 and shows significant intensity for Au as well as attenuation of the Ti and 16O intensities due to Au coverage. The percent attenuation for Ti is ∼50% larger than for O, suggesting that the Au binding at 120 K tends to block Ti more than O but not dramatically. This result is consistent with our XPS study, which found a very broad Au XPS peak under similar conditions, suggesting a heterogeneous distribution of binding sites.5 Note that the relative ISS intensity of Au (∼23% of the 16O peak for Au/TiO2) is significantly higher than might be expected from the Au dose (∼0.13 Au atom per unit cell). This reflects higher scattering cross section for high Z elements and also the fact that He+ survival probability is relatively high for gold.

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Figure 6. ISS intensities for 5% ML Au deposited on prehydroxylated TiO2(110) and after a series of annealing steps.

After Au deposition, the sample was exposed to the same ∼3 L H218O dose used in TPD, i.e., sufficient to saturate the 5cTi and 2cO water binding sites, with a substantial fraction of a second layer on the surface. To avoid ISS-induced water desorption and 18O/16O exchange with the surface, no ISS run was done at this point. Instead, the third date set labeled water300 K was taken after the water-dosed sample was annealed to 300 K for 5 min, thereby desorbing water bound in multilayer, 2cO sites, and 5cTi sites but leaving strongly adsorbed water mostly dissociatively adsorbed forming hydroxyl groups. There is a further reduction in the 16O signal, reflecting the fact that the H atoms from dissociatively adsorbed water will have diffused during the 300 K anneal,9 such that they mostly are bound atop the more abundant 16O sites. In our geometry (He+ detected along the surface normal), signal from the underlying O in these hydroxyl groups will be attenuated relative to bare oxygen atoms. There is also an additional reduction in Ti signal. This reduction most likely results from filling of vacancy sites, which expose underlying Ti, by dissociatively adsorbed water. As expected, there is 18O signal (∼9% of total O), expected for 18O atoms left by dissociative adsorption of H 18O at vacancies, 2 followed by H diffusion. Some of the 18O signal might also be from an adsorbate associated with the gold. Note that the Au signal is attenuated by ∼21% relative to the freshly deposited Au/TiO2 sample. Such a decrease could result either from an Au-bound adsorbate (stable to 300 K) that attenuates ion scattering from the gold or from agglomeration of gold atoms into clusters. The TPD results above showed no distinct feature for Au-bound water above 300 K; however, the adsorbate coverage required to attenuate 0.05 ML Au by 21% is small. As discussed in section A above, such Au-bound water fragments could desorb within the broad recombinative desorption feature. The next data set, taken following annealing at 450 K, provides further insight into this issue. If the reduction in Au intensity after 300 K annealing were due to agglomeration, then we would expect 450 K annealing to cause additional agglomeration and reduction in Au ISS signal. Instead, the Au ISS signal recovers, nearly to the level for freshly deposited Au/TiO2, suggesting that some adsorbate was lost from the gold.

The fact that the Au signal is still 5% below the initial value suggests that some agglomeration occurs but loss of adsorbates dominates. In addition to the recovery of Au signal, the signals for Ti and total O (16O + 18O) are nearly back to their values for clean TiO2. Finally, annealing to 600 K leads to significant reductions in Au ISS signal, a small reduction in 18O signal, and increases in Ti, 16O, and total O signals. Annealing to 800 K reduces Au signal further but has little additional effect on Ti or total O. The temperature of 800 K is only 50 K below the temperature used to anneal TiO2 after sputtering; thus, bulk diffusion is expected to be significant and presumably accounts for the decrease in 18O/16O ratio as surface oxygen is replaced by 16O from the bulk. We speculate that the small decrease in 18O/16O ratio observed after annealing to 600 K reflects the onset of bulk diffusion. The final data set in Figure 5 is from ISS taken immediately after a TPD experiment using our usual water dose and heating to 600 K at 3 K/s. Note that after TPD, the Au signal is reduced 28% relative to the as-deposited value, which is essentially identical to the effect of annealing 5 min at 600 K, indicating that the extent of Au agglomeration is not significantly affected by the longer time at 600 K for the annealed sample. Note, however, that the Ti and oxygen ISS signals for the post-TPD sample are closer to the signals obtained after annealing at 450 K, while the sample annealed at 600 K has a surface composition much closer to that of freshly prepared TiO2. This difference is taken as evidence that the TPD sample does not spend enough time at 600 K for significant bulk diffusion to occur and therefore retains the surface composition determined by the isotope exchange occurring at lower temperatures. The reductions in Au ISS signal during annealing and TPD are attributed to agglomeration of Au into 3-dimensional clusters, leaving only a fraction of the Au atoms in the ISSprobed top layer. It is not clear why the Ti signal and to a lesser extent the total O signal after high-temperature annealing slightly exceed those for vacuum-annealed Au-free TiO2. Note that there is no reason that the total (Au + Ti + O) ISS intensities should be constant because detection sensitivity varies substantially between elements and is particularly high for Au.

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Figure 7. ISS Au/total O ratios after a series of annealing steps for 5% ML Au/TiO2(110) and 5% ML Au/TiO2(110) with water exposure after and before Au deposition.

Figure 6 shows results of an experiment in which the surface was hydroxylated prior to, rather than after, Au deposition. As in the experiment summarized in Figure 5, the first set of ISS intensities (TiO2) in Figure 6 is for vacuum-annealed TiO2. Next, the sample was dosed with ∼ 2 ML of H218O at ∼110 K and then flashed to 350 K to desorb all molecularly adsorbed water. This should leave the hydroxyl groups that result from dissociative chemisorption at oxygen vacancies as well as perhaps a small amount of water bound at other defect sites. ISS was not done at this point to minimize damage to the surface. Au was then deposited on the hydroxylated surface at ∼120 K, and the second set of ISS data (Au/waterTiO2) shows the intensities just after Au deposition. As expected there is a decrease in Ti and 16O signals from the TiO2 and growth of signals for Au and for 18O from the water dose. Note that the magnitude of the Au signal at this point is significantly lower than for freshly deposited Au on clean TiO2 (second data set in Figure 5), suggesting that Au deposition on the prehydroxylated surface leads to either some agglomeration even at 120 K or accumulation of adsorbates on the deposited Au. It is interesting that the signal for Au deposited on hydroxylated TiO2 is essentially identical to that in the third data set in Figure 5, where the Au/ TiO2 sample was exposed to water and annealed to 300 K (which should also leave the surface hydroxylated). The similarity suggests that, as far as the gold is concerned, water exposure before or after Au deposition has similar effects. Note that the Au signal does not change significantly for annealing to 300 K but increases slightly for annealing at 450 K with signal levels essentially identical to those in Figure 5 after 450 K annealing. Again, we assume that the increase reflects desorption of some Au-bound adsorbate. For higher temperature annealing, the Au signal decreases, attributed to annealing into large 3D structures on the surface. Figure 7 compares the Au ISS results from the above experiments with water exposure before or after Au deposition and from a third experiment where the sample was not given any deliberate water exposure. To put all three experiments on the same scale and allow comparison with our previous study,5 we normalized the Au intensity at each point to the total O intensity. From TPD (above) and XPS results,5 it appears that

only a fraction of the Au deposited at ∼120 K on vacuumannealed TiO2 binds to vacancies and the rest is bound to nondefective sites. As in our previously study, with no water exposure, the Au signal remains high (high Au dispersion) for annealing up to 300 K (the slight increase probably reflects desorption of some adventitious adsorbate at 300 K). XPS under these conditions (no water) suggests5 that upon heating to room temperature nonvacancy-bound Au atoms tend to diffuse and bind at vacancy sites (either empty or with existing Au present) and are stable there at least up to room temperature on a 1 h time scale. The stability of vacancy-bound gold atoms at room temperature was recently verified by the STM work of Matthey et al.25 At 450 K or higher temperatures, the strong decreases in Au ISS intensity were attributed to agglomeration, based on the observation that the Au XPS binding energy shifts to near the bulk value. XPS after the TPD experiments described above also found a bulk-like Au XPS binding energy (84 eV). In the two experiments with water exposure before or after Au deposition, the Au/total O ratio is lower than the water-free result prior to annealing and always remains significantly smaller throughout the annealing/agglomeration process. In particular, the ratio is significantly smaller after annealing to 450 K to remove Au-bound adsorbates. The conclusion is that water exposure/surface hydroxylation does contribute to Au diffusion and agglomeration at room temperature, presumably by filling vacancy sites that would otherwise tend to stabilize Au atoms or small clusters. It is not unreasonable, therefore, that the fast time scale of ISS experiments, with less time for both diffusion and adventitious water adsorption, leads to ISS observing less agglomeration than STM experiments. IV. Conclusions Recombinative desorption of water initially dissociatively adsorbed at oxygen vacancy sites undergoes efficient 18O-16O exchange via H-atom migration on the surface. The reduction in the amount of recombinative desorption from Au/TiO2 suggests that Au blocks the initial oxygen vacancy sites, presumably by binding there. High-energy He+ impact on the adsorbed water bilayer has little effect on desorption features

Water on rutile TiO2(110) and Au/TiO2(110) associated with water bound directly to the surface (5cTi and 2cO sites) but substantially reduces the coverage in secondlayer sites and also drives 18O-16O exchange between water and the surface. In our ISS experiments water exposure before or after Au deposition has a similar effect, i.e., more diffusion and agglomeration is observed in both cases, compared to Au deposited on vacuum-annealed TiO2. This result suggests that the origin of the more extensive Au agglomeration observed in STM experiments is probably the result of the longer STM experimental time scale giving more time for water to displace the deposited Au from vacancy sites which otherwise would tend to stabilize atoms and small clusters on the surface. Acknowledgment. We gratefully acknowledge helpful discussions and unpublished results from Michael Henderson and support by Grant No. DEFG03- 99ER15003 from the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy. Supporting Information Available: TPD spectra from various doses of H218O on the TiO2(110) surface at ∼110 K. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Lee, S.; Fan, C.; Wu, T.; Anderson, S. L. J. Am. Chem. Soc. 2004, 126, 5682. (2) Tong, X.; Benz, L.; Kemper, P.; Metiu, H.; Bowers, M. T.; Buratto, S. K. J. Am. Chem. Soc. 2005, 127, 13516. (3) Wahlstro¨m, E.; Lopez, N.; Schaub, R.; Thostrup, P.; Ronnau, A.; Africh, C.; Laegsgaard, E.; Nørskov, J. K.; Besenbacher, F. Phys. ReV. Lett. 2003, 90, 026101/1.

J. Phys. Chem. C, Vol. 112, No. 24, 2008 9015 (4) Mitchell, C. E. J.; Howard, A.; Carney, M.; Egdell, R. G. Surf. Sci. 2001, 490, 196. (5) Lee, S.; Fan, C.; Wu, T.; Anderson, S. L. Surf. Sci. 2005, 578, 5. (6) Schaub, R.; Wahlstro¨m, E.; Rønnau, A.; Lægsgaard, E.; Stensgaard, I.; Besenbacher, F. Science 2003, 299, 377. (7) Wendt, S.; Schaub, R.; Matthiesen, J.; Vestergaard, E. K.; Wahlstro¨m, E.; Rasmussen, M. D.; Thostrup, P.; Molina, L. M.; Lægsgaard, E.; Stensgaard; Hammer, B.; Besenbacher, F. Surf. Sci. 2005, 598, 226. (8) Wendt, S.; Matthiesen, J.; Schaub, R.; Vestergaard, E. K.; Lægsgaard, E.; Bensenbacher, F.; Hammer, B. Phys. ReV. Lett. 2006, 96, 066107. (9) Zhang, Z.; Bondarchuk, O.; Kay, B. D.; White, J. M.; Dohna´lek, Z. J. Phys. Chem. B 2006, 110, 21840. (10) Bikondoa, O.; Pang, C. L.; Ithnin, R.; Muryn, C. A.; Onishi, H.; Thornton, G. Nat. Mater. 2006, 5, 189. (11) Boyd, K. J.; Lapicki, A.; Aizawa, M.; Anderson, S. L. ReV. Sci. Instrum. 1998, 69, 4106. (12) Aizawa, M.; Lee, S.; Anderson, S. L. J. Chem. Phys. 2002, 117, 5001. (13) Aizawa, M.; Lee, S.; Anderson, S. L. Surf. Sci. 2003, 542, 253. (14) Fan, C.; Wu, T.; Kaden, W. E.; Anderson, S. L. Surf. Sci. 2006, 600, 461. (15) Epling, W. S.; Peden, C. H. F.; Henderson, M. A.; Diebold, U. Surf. Sci. 1998, 412/413, 333. (16) Hugenschmidt, M. B.; Gamble, L.; Campbell, C. T. Surf. Sci. 1994, 302, 329. (17) Henderson, M. A. Surf. Sci. 1996, 355, 151. (18) Henderson, M. A. Langmuir 1996, 12, 5093. (19) Schaub, R.; Thostrup, P.; Lopez, N.; Lægsgaard, E.; Stensgaard, I.; Nørskov, J. K.; Besenbacher, F. Phys. ReV. Lett. 2001, 87, 266104. (20) Diebold, U.; Lehman, J.; Mahmoud, T.; Kuhn, M.; Leonardelli, G.; Hebenstreit, W.; Schmid, M.; Varga, P. Surf. Sci. 1998, 411, 137. (21) Lee, S.; Fan, C.; Wu, T.; Anderson, S. L. J. Chem. Phys. 2005, 123, 1. (22) Yang, Z.; Wu, R.; Goodman, D. W. Phys. ReV. B 2000, 61, 14066. (23) Giordano, L.; Pacchioni, G.; Bredow, T.; Sanz, J. F. Surf. Sci. 2001, 471, 21. (24) Bertrand, P. G.; Rabalais, J. W. Ion scattering and recoiling for elemental analysis and structure determination. In Low Energy Ion-Surface Interactions; Rabalais, J. W. Ed.; Wiley: Chichester, 1994; p 55. (25) Matthey, D.; Wang, J. G.; Wendt, S.; Matthiesen, J.; Schaub, R.; Lagsgaard, E.; Hammer, B.; Besenbacher, F. Science 2007, 315, 1692.

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