Agglomeration, Sputtering, and Carbon Monoxide Adsorption

Department of Chemistry, UniVersity of Utah, 315 South 1400 East Room 2020 ... Agglomeration is more extensive in room-temperature deposition, compare...
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J. Phys. Chem. B 2005, 109, 11340-11347

Agglomeration, Sputtering, and Carbon Monoxide Adsorption Behavior for Au/Al2O3 Prepared by Aun+ Deposition on Al2O3/NiAl(110) Sungsik Lee, Chaoyang Fan, Tianpin Wu, and Scott L. Anderson* Department of Chemistry, UniVersity of Utah, 315 South 1400 East Room 2020, Salt Lake City, Utah 84112-0850 ReceiVed: January 13, 2005; In Final Form: March 9, 2005

Size-selected gold clusters, Aun+ (n ) 1, 3, 4), were deposited on an ordered Al2O3 film grown on NiAl(110), and changes in morphology and electronic properties with deposition/annealing temperature and cluster size were investigated by X-ray photoelectron spectroscopy (XPS) and ion-scattering spectroscopy (ISS). Extensive agglomeration was observed by ISS for annealing temperatures above 300 K, accompanied by large shifts in the Au XPS binding energy. Agglomeration is more extensive in room-temperature deposition, compared to samples prepared by low-temperature deposition, then annealed to room temperature. Agglomeration is also observed to be dependent on deposited cluster size. CO adsorption was studied by ISS and temperatureprogrammed desorption, and we looked for CO oxidation under conditions where substantial activity is seen for Aun/TiO2. No activity was observed for Aun/Al2O3. The differences between the two systems are interpreted in terms of the nature of the metal-support interactions.

I. Introduction Small gold particles on metal oxide supports are attracting intense attention in the surface chemistry and catalyst communities, because they show interesting and strongly size-dependent catalytic activity,1-3 despite bulk gold being quite inert.4,5 Although gold-based catalysts have been examined for many different reactions,6 CO adsorption and the CO oxidation reaction have been used by many groups as relatively simple probes of chemical activity. For example, Shaikhutdinov et al.7 and Lemire et al.8 studied CO adsorption on different size gold clusters grown on reducible FeO and Fe3O4 supports as well as the nonreducible Al2O3 support. They concluded that CO adsorption depends strongly on the size of gold clusters but not on the reducibility of the support. A reflection-absorption IR spectroscopy (RAIRS) and temperature-programmed desorption (TPD) study by Winkler et al.9 also showed that CO adsorption is dependent on the size of gold particles and that bigger particles (>500 atoms) showed adsorption behavior similar to that of a gold single crystal. A study by Gottfried et al.10 indicated that CO adsorption behavior on bulk gold is sensitive to the surface structure and gold coordination number based on comparing results for a sputtered gold surface and an annealed single-crystal surface. It has been shown that the reactivity of gold catalysts is very sensitive to sample preparation and pretreatment conditions.11-15 This sensitivity makes it difficult to separate the effects of gold particle size from effects of support condition, which may change as preparation conditions are varied to shift the cluster size distribution. To avoid this complication, we are studying model catalysts prepared on single crystal or thin-film oxide supports, where the support preparation, deposited cluster size, and deposition conditions can be varied independently. Recently we observed that samples prepared by deposition of Aun+ (n e 7) on TiO2 (110) are active for room-temperature CO oxidation, with activity first appearing for deposition of Au3+.16 One interesting point is that the activity is so sharply dependent on deposited cluster size that there cannot be much agglomeration

of the deposited clusters. This result is somewhat surprising, particularly for Au+ deposition, because STM studies of evaporated Au/TiO2 indicate considerable agglomeration at room temperature.17,18 We recently reported an annealing study of agglomeration of Au+ deposited on TiO2, finding no evidence of agglomeration for Tanneal e 300 K. These results raise the question of whether hyperthermal (1 eV/atom) ion deposition might result in samples that are more sinter-resistant than evaporated samples. For comparison, here we report a study of the morphology, electronic structure, and CO adsorption properties of samples prepared by deposition of Aun+ (n ) 1, 3, 4) on alumina. In this case, agglomeration is observed for all sizes at room temperature, although the final state still depends strongly on size. Morphology is characterized by a combination of lowenergy ion-scattering spectroscopy (ISS) and X-ray photoelectron spectroscopy (XPS) as a function deposition and annealing temperature. CO adsorption is probed by ISS and TPD. Finally, CO oxidation was probed under conditions where Aun/TiO2 samples show substantial activity. II. Experimental Methods The experiments were performed in an ultrahigh vacuum (UHV) chamber with a base pressure of 1300 K. Sample temperatures were measured by a K-type thermocouple, which was first spot welded to a thin (0.01 mm) tantalum foil to prevent nickel and aluminum diffusion between the thermocouple and the NiAl sample. The thin tantalum foil was spot welded to the back of the NiAl(110) crystal. Temperature measurement accuracy was confirmed by CO TPD, obtaining a peak temperature (310 K) for CO on NiAl(110) in good agreement with the literature.27 Before each deposition experiment, the NiAl(110) sample was cleaned by 1 keV Ar+ sputtering to remove the Al2O3 film and previously deposited Au, along with any other contaminants. After sputtering, there was no contamination or remaining Au on the sample surface within the measurement sensitivity for XPS, AES, and ISS. The NiAl crystal was then annealed for 15 min at 1000 K and then for 10 min at 1220 K in UHV. After the annealing, the Al2O3 film was grown by exposure to 1200 L of O2 at a sample temperature of 550 K, followed by annealing at 950 K. To fill any open patches in the Al2O3 layer, the oxidation treatment was repeated. The as-prepared Al2O3 thin film was characterized with XPS and ISS. The Al XPS shows growth of shoulders to high binding energy of both the Al 2s and 2p spectra, corresponding to Al3+. A large O 1s peak also appears, and the Ni 2p signal is attenuated relative to its intensity for unoxidized NiAl. From the attenuation, we can estimate the Al2O3 film thickness to be ∼5 Å, consistent with the two-layer film reported by Jaeger et al.23 The continuity of the oxide film surface was verified by ISS measurements, which showed no exposed Ni after growth of the oxide film. Neither XPS nor ISS show any carbon, gold, or other contamination for the as-prepared film. In addition, CO TPD was performed to check the adsorption behavior of the nascent Al2O3 film. For these experiments, the samples were dosed with CO, then moved just behind the 3-mm-diameter entrance to the differentially pumped mass spectrometer, prior to heating. No CO desorption was observed above 100 K, indicating the absence of any strongly interacting defects in the film. In particular, there is no desorption at ∼310 K, where CO desorbs from the NiAl(110) surface. This behavior can be contrasted to the TiO2(110) surface, where oxygen vacancies result in a tail of CO desorption extending to ∼300 K. For all experiments, the integrated Au dose was 7 × 1013 atoms/cm2, equivalent to ∼0.05 of a close-packed Au monolayer (ML), deposited as various size clusters in a 2-mm spot. The deposition dose was determined directly by computer integrating the ion current during the course of deposition. XPS experiments were carried out using Mg KR (1253.6 eV) radiation from a dual-anode X-ray source, together with a hemispherical energy analyzer, with analysis spot area set slightly smaller than the deposition spot. The XPS calibration is periodically checked using the Au 4f spectrum of gold foil. ISS was done with 1 keV He+ incident at 45°, with ion detection along the surface normal.

J. Phys. Chem. B, Vol. 109, No. 22, 2005 11341

Figure 1. Comparison of Au loss for Au/TiO2 and Au/Al2O3/NiAl by sputtering during ISS scanning for 75 s.

III. Results and Discussion A. Au Sputtering. In the annealing experiments discussed below, ISS was used to monitor Au morphology changes. One finding is that the sputter rate for Au/Al2O3 is high, even for the 1-keV He+ ions used for ISS. Rapid sputtering complicates the ISS analysis; however, comparison of sputter rates provides a semiquantitative way to judge the relative strength of gold adatom binding to different surfaces. Figure 1 shows the Au region of two ISS scans for samples of gold atoms deposited on rutile TiO2(110) and on Al2O3/NiAl at low temperature. The spectra indicated with solid symbols show the Au signal during the first scan on each sample, where the aggregate exposure to the He+ beam was only ∼15 s. The spectra indicated with open symbols show the Au signal measured in the 5th of a set of sequential scans, where the integrated exposure time was ∼75 s. The decrease in integrated Au signal observed for Au/Al2O3 (36%) is about 12 times larger than that for Au/TiO2 (∼3%). Relative sputtering rates primarily reflect differences in goldsubstrate bond energies but are also affected by the energy deposition dynamics for He+ impacting the substrates. To account for differences in dynamical effects for TiO2 vs Al2O3, gold sputter yields were calculated using the Stopping and Range of Ions in Matter program (SRIM 200328). The calculations were run for models consisting of a single ML of Au on top of either 100 Å of TiO2 or on 5 Å of Al2O3 on 100 Å of NiAl. If the Au-substrate bond energy is assumed to be the same for TiO2 and Al2O3, then the sputtering rate is predicted to be higher for Au/Al2O3 but only by ∼30%. The ∼12 times faster sputtering observed experimentally, therefore, indicates that the Au-Al2O3 bond energy is substantially weaker than that for Au-TiO2. In particular, the sputter rate for Au/TiO2 probably reflects the

11342 J. Phys. Chem. B, Vol. 109, No. 22, 2005

Figure 2. Au ISS intensity after annealing at various temperatures for Au deposited on alumina at 95 K (LT) and 300 K (RT). Error bars are shown for the LT-deposited data at 300 and 600 K. Also shown are results for Au deposited on TiO2 at 115 K.

strong bonding of Au complexed to oxygen vacancies on the TiO2 surface. Vijay et al.48 have calculated the Au-vacancy bond energy to be ∼2.4 eV, compared to only 0.6 eV for Au binding to perfect TiO2. If the SRIM simulation is run with the Au/TiO2 and Au/alumina binding energies set to 2.4 and 0.6 eV, respectively, the sputter rate for Au/alumina is still only estimated to be three times that for Au/TiO2. Clearly Aualumina bonding is considerably weaker than Au-vacancy bonding in Au/TiO2. B. Annealing Effects for Low-Temperature Deposited Au1/ Al2O3. Two sets of annealing experiments were carried out to probe agglomeration behavior of samples prepared by Au+ deposition on Al2O3/NiAl(110) at 95-K surface temperature. In both sets, the Al2O3/NiAl(110) films were prepared as above, cooled to 95 K, then briefly flashed to 600 K to remove any adventitious adsorbates that might have accumulated during the slow cooling of our cryostat. Au+ was then deposited at a density of 7.0 × 1013/cm2 (0.05 ML equivalents) and at 1-eV impact energy. The as-deposited sample was characterized by either XPS or ISS, annealed at one of a set of temperatures for 5 min, rapidly recooled, then recharacterized. The XPS and ISS experiments were done in separate runs to avoid complications to the XPS analysis from ISS sputter damage. These experiments with deposition at 95-K surface temperature will be referred to as the “LTdeposited” experiments, for later comparison to analogous depositions done at 300 K surface temperature (“RT-deposited”). To minimize complications from sputtering, several steps were taken for the LT-deposited experiment. To minimize exposure to the He+ beam, only the Au region of the ISS spectrum was scanned. In addition, the LT-deposited annealing experiment was broken into two parts, to avoid exposing a single sample to five sequential ISS scans. One sample was characterized as deposited (95 K) and then after annealing to 200 and 450 K. The other sample was characterized as deposited and after annealing to 300 and 600 K. The magnitude of the sputtering effect is easily estimated by simply measuring the effect of sequential ISS scans identical to those in the annealing experiments but without actually annealing the sample. For the LT-deposited experiments, the sputtering effect on Au ISS is estimated to be less than 3%. The LT-deposited Au ISS intensities plotted in Figure 2 have been corrected for sputtering and are normalized to the intensity for the as-deposited sample. The RT-deposited results are discussed in the next section. As shown in the figure, there is a slight increase in Au ISS intensity after 300 K annealing, followed by a decline at higher

Lee et al. temperatures. To check reproducibility of this pattern, the 300 and 600 K annealing experiment was reproduced on a fresh sample. The 300-K points had Au ISS intensities of 1.06 and 1.08 times the as-deposited values, suggesting an increase from 300 K annealing of ∼7 ( 1%. The 600-K points were 0.68 and 0.77, i.e., the effect of 600-K annealing is a decrease in Au ISS to 73 ( 5% of the as-deposited intensity. The higher variability of the 600-K point is not unexpected, because these data points were taken later in the experimental sequence, with more sputter damage and more chance for cumulative effects of small run-to-run variations in experimental conditions. The Au ISS intensity reflects the amount of Au present in the topmost layer of the sample, exposed to the He+ beam. The maximum Au signal is expected for Au atomically dispersed on top of the substrate. If Au is aggregated into 1-D or 2-D islands, the Au ISS signal is diminished by shadowing,29 where Au atoms in clusters interfere with He+ scattering from neighboring Au atoms, decreasing the Au ISS signal. The shadowing effect is small for our steep angle of incidence. For example, in the Aun/TiO2 system, where STM experiments of Buratto and co-workers30 clearly indicate that small clusters (n e 4) have 1-D or 2-D structures, we observe that the Au ISS signal decreases ∼9% for deposition of Au3 and Au4, relative to samples prepared by Au1 deposition. Much larger decreases in Au signals indicate that some fraction of the Au is no longer in the top sample layer, i.e., Au is forming multilayer surface structures, desorbing, or becoming adsorbate covered. Examples of these effects can be seen in ISS studies of size-selected Nin and Irn clusters on TiO2.19,31 STM data indicate that Au mobility on Al2O3/NiAl films is not high at room temperature,32,33 suggesting that, at 95 K, the Au mobility should be negligible. Therefore, the Au ISS intensity for the as-deposited, unannealed sample is assumed to represent Au mostly in the form of atoms dispersed on the surface. If diffusion and agglomeration were facile at 200 or 300 K, we would expect a significant drop in the Au ISS intensity after the 200- or 300-K annealing experiments. Instead, the ISS intensities increase slightly, particularly after 300-K annealing. This increase is attributed to thermal desorption of background gas adsorbed on the sample during the 95-K deposition (∼30 min). This adventitious adsorption was not observed in LTdeposition on TiO2, and the origin was probed by TPD and XPS. CO and H2 adsorption are negligible under our conditions. The culprit is H2O, apparently generated by wall reactions of the O2 used in growing the Al2O3/NiAl film. TPD of Al2O3/NiAl exposed to chamber background at 95 K gives a water peak at ∼160 K, in good agreement with TPD measured by Tzvetkov et al.,34 following 100-K water exposure. In XPS, we see an O 1s shoulder roughly 2.5 eV to higher binding energy from the oxide peak, also consistent with the high-resolution XPS of Tzvetkov et al., which demonstrated that adsorption is molecular at these low temperatures. Dissociative adsorption does occur for exposure temperatures above 170 K; however, this process should be negligible for our conditions.35,36 From the intensity of the XPS shoulder, we estimate a water coverage of 5-10%, comparable to the 8% enhancement of Au ISS signal when the water is desorbed. The results, therefore, suggest that molecular water is weakly adsorbed to 600 K, based on changes in lowenergy electron diffraction patterns. XPS binding energies result from a combination of initialand final-state effects, and for gold/alumina, final-state effects are expected to dominate.39 In that case, the XPS binding energies are expected to be near the bulk gold limit for large clusters but increase significantly as cluster size decreases. On the basis of the ISS results, we would expect to see Au XPS binding energies substantially higher than the bulk limit for deposition at 95 K, where Au is highly dispersed. The binding energy should gradually shift toward the bulk limit with increasing Tanneal as clusters agglomerate on the surface. This is essentially what is observed. The best-fit binding energy of the Au 4f7/2 fine structure component is 85.2 eV for as-deposited Au at 95 K, compared to the 84.0 eV for bulk gold.40 Annealing to 300 K results in a negligible change in binding energy (85.1 eV), even though the adventitious water is desorbed in the process. The negligible change presumably reflects both the weak H2OAu interaction and also the small fraction of gold atoms with attached water (∼8%). In addition, the negligible XPS changes indicate that 5 min at 300 K does not result in enough agglomeration to affect the final state shift, consistent with the ISS results. After annealing the sample to 450 K, however, there is a significant shift of the Au 4f spectrum toward lower binding energy (84.5 eV) and some narrowing as well. Annealing to 600 K results in a spectrum similar to that after 450 K annealing. The XPS features are considerably wider than our ∼1.2 eV instrumental resolution. The large width (2.7 eV) for the asdeposited Au sample could be attributed to heterogeneity in Au binding sites (terrace, step, deposition-induced defects, with adsorbed water); however, in that case annealing should narrow the XPS features. In the Au+/TiO2 experiments, the Au features do narrow nearly to the instrumental limit for Tanneal g 450 K; however for Au+/alumina, the widths are still ∼2.0 eV for Tanneal ) 600 K, although the peak position approaches the bulk limit. Given the evidence that Au goes subsurface at Tanneal > 600 K, the XPS widths at Tanneal ) 600 K may indicate the beginnings of this process, introducing additional heterogeneity in the Au bonding environment. Further discussion of XPS widths is deferred to section D, below. There are similarities, but also an important difference in the annealing behavior as viewed by XPS for Au/alumina compared to Au/TiO2.37 For both systems, the binding energy after annealing to 450 K or above is near the bulk limit, suggesting formation of large clusters. Both systems also show no changes in binding energy for annealing to 200 K, suggesting that the

11344 J. Phys. Chem. B, Vol. 109, No. 22, 2005 as-deposited Au is stable at this temperature. The most interesting difference is for Tanneal ) 300 K, where Au/TiO2 shows a substantial (0.4 eV) shift toward higher binding energy, while Au/Al2O3 does not shift significantly. As shown in Figure 2, there is no change in Au ISS for Au/TiO2 at Tanneal ) 300 K, suggesting that agglomeration is not the explanation for the XPS shift. Instead, the shift was attributed to diffusion of Au atoms to nearby oxygen vacancies and formation of Au-vacancy complexes. Because the vacancies are electron-rich, they tend to transfer electron density to adsorbates, consistent with a shift to lower binding energy. The strong Au-vacancy binding also accounts for the slow Au sputtering observed on TiO2. C. Annealing Effects for Room-Temperature Deposited Au1/Al2O3. Annealing experiments were also carried out for Au+ deposited at room temperature, where molecular adsorption of adventitious water is not an issue. On the basis of the water dissociative sticking coefficient of Elam et al. (∼0.1 on R-Al2O3),35 we estimate that only ∼0.5-1% of surface sites are occupiedstoo small to be detected by any means at our disposal. The RT-deposited sample was characterized as deposited and after annealing to 450, 600, and 800 K. This experiment was done sequentially, and the full ISS spectrum (Au, Al, O) was measured at each temperature point. As a consequence, the exposure to the He+ beam was substantially larger than in the LT-deposited experiments, particularly for the high temperature points measured late in the sequence. As in the LT experiments, the ISS data were approximately corrected for Au sputtering, based on sputtering experiments carried out at fixed temperature, and both raw and corrected values are shown in Figure 2. This process overcorrects the ISS data, because we assume that sputtering rates are identical for the annealed sample, where clustering clearly occurs, and the control sample, where Au is dispersed. In reality, however, our ISS experiments (below) show that Au sputtering rates decrease with increasing size of deposited Aun+, similar to the effect documented in detail for Irn/TiO2.19 Because we do not know what size clusters agglomerate during annealing, we can only say that the true Au ISS intensity values must lie between the raw and corrected values shown in Figure 2. It would be interesting to compare Au ISS intensities for RTdeposited and LT-deposited Au+, because that would tell us if there is significant agglomeration during the RT deposition. For direct comparison it is necessary to correct for day-to-day variations in the He+ beam intensity, and we do this by normalizing the Au signal to the Al and O signals, which are approximately independent of Au morphology for our 5% Au coverage. In this case, however, normalization is complicated by adsorption of adventitious water in the LT-deposited experiment. The normalization shown in Figure 2 was under the assumption that water attenuates ISS from gold and substrate atoms equally in the LT-deposited sample, i.e., equal probability of water binding to Au and alumina sites. Comparison of XPS and ISS results discussed above shows that this assumption is at least roughly correct. The result of this approximate normalization is that the Au ISS intensities are roughly similar for LT- and RT-deposited samples; however, the RT-deposited intensity is ∼7% lower than that for the LT-deposited sample after desorption of adventitious water at 300 K. This intensity relation indicates that there probably is some agglomeration during RT deposition but not into multilayer clusters, where Au ISS attenuation would much larger. Regardless of the uncertainties in the relative intensity scales for LT- and RTdeposited samples, it is clear that the dependence on annealing temperature is quite similar for the two samples.

Lee et al.

Figure 4. Au 4f X-ray photoelectron spectra of samples prepared by deposition of Aun+, n ) 1, 3, 4.

Our conclusion that agglomeration during RT deposition is limited to 1-D or 2-D clusters should be compared with the STM work of Winkler et al.33 They find that a broad size distribution of small particles, up to ∼3 nm in diameter, form during Au evaporation onto room-temperature Al2O3/NiAl and that the growth mode is three-dimensional with particle height ∼0.24* diameter. They also conclude that “Au atoms apparently stick more or less straight to the surface, and they travel at most a couple of lattice distances”. This conclusion rests on observation that their gold clusters are substantially smaller than copper clusters grown under identical conditions and do not appear concentrated at step edges (unlike Cu). Both observations suggest relatively low Au mobility at room temperature, consistent with our results as well. On the other hand, the presence of multilayer clusters in their experiments appears inconsistent with our conclusion that only single layer clusters form; however, there are several experimental differences that may play a role. The most obvious is the higher (1 eV) deposition energy in our experiments, which might conceivably result in pinning of Au to defects on the surface. D. XPS and ISS Following Deposition of Size-Selected Aun+ on Al2O3 at Room Temperature. In this section, we compare XPS and ISS of samples generated by room-temperature deposition of Aun+ (n )1, 3, 4), with the results discussed above for LT-deposited Au+. There are two points of interest. Comparison of the behavior of RT-deposited and LT-deposited Au+ provides additional insight into the agglomeration of Au at room temperature. Comparison of samples prepared with Au+ and Aun+ addresses the question of whether preformed deposited clusters agglomerate differently than atoms deposited on the surface. The XPS measurements are shown in Figure 4. We first compare the data for RT Au+ deposition with the results for LT-deposited Au+ in Figure 3. The best fit Au 4f7/2 peak position for RT deposition is 84.9 eV, compared to 85.2 eV for as-deposited Au+ at 95 K, which is assumed to represent highly dispersed gold. Another obvious difference is that the XPS spectral features are significantly narrower for RT-deposited Au+ (∼2 eV) compared to the LT-deposited result (∼2.7 eV), indicating that RT deposition leads to a more homogeneous gold environment. Both binding energy and width, therefore, are consistent with the conclusion from ISS that RT deposition results in samples with significant agglomeration. More surprisingly, when the LT-deposited Au+ is annealed to 300 K, the binding energy (85.1 eV) and width (2.7 eV) are almost

Agglomeration, Sputtering, and Carbon Monoxide Behavior unchanged and quite distinct from the RT-deposited values. Only after annealing at 450 or 600 K does the binding energy for LT-deposited Au+ (84.5 eV) drop below that for RT-deposited Au+, and at the same time the width also narrows significantly. Comparison of the XPS and ISS results suggests the following. RT-deposited Au+ is agglomerated into 1-D or 2-D clusters, providing a reasonably homogeneous Au electronic environment (narrow XPS width). LT-deposited Au+ is clearly more dispersed, with considerable binding site inhomogeneity as indicated by the large XPS width. Annealing at 300 K leads to little change in Au ISS or XPS, suggesting that changes to the sample, including agglomeration, are minimal during the 5-min annealing period. During high-temperature annealing, both ISS and XPS are consistent with agglomeration into large, multilayer particles. There are two obvious factors that might account for the different levels of agglomeration in samples prepared by RT deposition vs 300 K annealing of LT-deposited samples. One is the presence of 5-10% of adsorbed water on the lowtemperature deposition substrate, which might affect diffusion and sintering of gold atoms on the substrate. We are not aware of any prior studies of the effects of molecular water on metal diffusion/sintering on alumina, but it is not unreasonable to assume that water might act as a trap site or diffusion barrier, inhibiting agglomeration. (Norskov and co-workers have examined hydroxylation effects on film growth on alumina,41 but our adventitious water is clearly molecular). The question, then, is why 300 K annealing of the LT-deposited sample results in less agglomeration than RT deposition, even though the water is desorbed at this temperature. One possibility may be differences in cluster nucleation for the two experiments. For homogeneous nucleation of metal on oxide surfaces, the density of cluster nuclei during evaporation experiments scales such as the square root of the metal flux.22 In our RT-deposition experiments, the flux is low (∼0.05 ML/30 min), thus the density of nucleating clusters should be low and the average size large. For deposition at 95 K, the actual flux is identical; however, the Au atoms are immobile at this temperature, and cluster nucleation takes place only when the sample is annealed. Because all the atoms begin diffusing simultaneously as the sample temperature is rapidly ramped (>5 K/s) to Tanneal, the nucleation behavior is as if there were a very large effective atomic flux. In this case, the density of cluster nuclei should be large, and the corresponding small average cluster size would account for the negligible shift and narrowing of the XPS following 300 K annealing. To drive sufficient cluster coarsening to shift the XPS toward the bulk limit in five minutes of annealing, Tanneal g 450 K is required. Having concluded, based on both ISS and XPS, that Au+ deposited at room-temperature tends to agglomerate to a limited extent, we next address the differences between Au+ and Aun+ deposition. Agglomeration is driven by the higher stability of supported clusters relative to isolated adatoms. In deposition of preformed clusters, therefore, it is not unreasonable to expect that the presence of pre-existing Au-Au bonds should reduce the tendency to diffuse and agglomerate. Aun+ were deposited on the Al2O3/NiAl(110) surface at room temperature and at an impact energy of 1 eV/atom. Adventitious water adsorption should be negligible under these conditions. The total gold coverage is identical in all samples (7 × 1013 atoms/cm2). Figure 5 compares ISS spectra taken for samples prepared by deposition of Au+, Au3+, and Au4+. Au2 was not studied, as our primary interest is in comparing Au+ with Aun+ found to be active for CO oxidation on TiO2 (i.e, n g 3). The spectra are similar;

J. Phys. Chem. B, Vol. 109, No. 22, 2005 11345

Figure 5. ISS spectra for samples prepared by deposition of Aun+, n ) 1, 3, 4.

however, the Au intensities (relative to O and Al) are ∼12% lower for the samples prepared with Au3+ and Au4+, relative to the sample prepared by Au+ deposition. This decrease indicates that, on average, gold deposited as clusters is more agglomerated than gold in samples prepared by Au+ deposition at room temperature. Given that RT deposition of Au+ already leads to some agglomeration, the ISS results suggest that deposited clusters sinter more readily than atoms, counter to intuition. The XPS data are consistent with this conclusion. The spectrum for the sample prepared by Au4+ deposition is relatively sharp, and the 4f7/2 component peaks at 84.3 eV near the bulk limit. This value is substantially below that for RT Au+ deposition and also slightly lower than that resulting from 600-K annealing of the LT-deposited sample (84.5 eV). The implication is that the size of gold structures formed by RT Au4+ deposition is significantly larger than those formed by RT Au+ deposition or by 600-K annealing of the LT-deposited Au+. We note that Carrey et al. reported a TEM study of sintering of gold on amorphous alumina, where they tentatively conclude, based on indirect evidence, that small clusters may be more mobile than atoms.42 The mechanism proposed was based on the usual idea that defects on the surface tend to act as trap sites and nucleate cluster growth. They proposed, however, that the trap depth decreases with cluster size such that clusters above some size are not trapped, allowing facile cluster agglomeration. The sample prepared by RT Au3+ deposition has an unusual XPS spectrum. Both Au 4f fine-structure components for this sample are significantly broader (∼2.6 eV) than those for Au+ or Au4+ deposition (∼2 eV) and have a somewhat flat-topped appearance suggestive of more than one spectral component. The fit shown in Figure 4 is based on a linear combination of the single-component fits used for Au+ (84.9 eV) and Au4+ (84.3 eV), with roughly equal contribution from each. It is unclear why Au3+ deposition is so different from that of Au4+. One obvious scenario would be that Au3 has an ∼50% probability to fragment to atoms on impact, such that half the Au is initially in intact clusters and half as scattered atoms. Such behavior might be expected if Au3 were substantially less stable with respect to fragmentation than Au4. Calculations of Wang et al.43 suggest that this is the case: the atomization energy is 1.28 eV/atom for Au3 and 1.74 eV/atom for Au4. On the other

11346 J. Phys. Chem. B, Vol. 109, No. 22, 2005 hand, the ionization energy for Au3 (∼7.3 eV44,45) is substantially lower than that for Au4 (∼8.5 eV44), thus when the clusters neutralize during deposition, the resulting Au4 is likely to be internally hotter. Considering these off-setting factors, cluster energetics do not appear to provide a convincing rationalization for the odd behavior of Au3, and we leave this as a question for further investigation. The only previous experiments we know of examining XPS of size-selected deposited gold clusters are those of DiCenzo et al.46 for an amorphous carbon substrate at room temperature. Although they had no independent method for characterizing the cluster dispersion following deposition, their results showed a shift from ∼84.7 eV for Au5+, toward the bulk limit with increasing deposited cluster size. Given that the supports are quite different, their results appear quite consistent with ours. E. CO Adsorption and Oxidation. The initial motivation for this study was to examine CO oxidation activity, for comparison with Aun/TiO2, where strongly size-dependent activity is observed.16 Several types of CO adsorption experiments were carried out, both with size-selected Aun+ deposited at room temperature and with Au+ deposited at 95 K. If CO adsorbs on the sample surface, then ISS signals for atoms where binding occurs are attenuated by the adsorbate. The extent of the attenuation depends on the binding geometry. For example, CO binding on top of Au will tend to cause a large attenuation of the Au ISS signal, with little or no effect on the substrate signal. Conversely, CO binding to the substrate should have little effect on the Au ISS. For samples prepared by Au+ deposition on TiO2, substantial Au ISS attenuation is observed upon room-temperature CO exposure, indicating that CO is adsorbing atop the gold. As cluster size increases, however, the CO-induced Au ISS attenuation decreases, becoming negligible for Au5 and Au6.47 On the other hand, for these larger Au clusters deposited on TiO2, CO exposure results in a significant drop in Ti and O signals, suggesting that CO is binding to the support in association with the gold (no CO-induced attenuations are observed for clean TiO2 at room temperature). For Aun+ (n ) 1, 3, 4) deposited on Al2O3 at room temperature, no CO-induced attenuation of Au ISS signal is observed, indicating that CO is not binding atop the gold at room temperature. Furthermore no attenuation is observed for Al or O ISS, i.e., CO is not adsorbing on these samples at all. As a check, we also looked at the effects of CO exposure on a sample prepared by Au+ deposition at 95 K. At 95 K, there is a ∼25% attenuation of Au signal upon exposure to 5 L of CO, indicating that CO is binding to Au. To minimize damage to the sample, we only scanned the Au ISS region, therefore, possible adsorption on O or Al sites was not recorded. Five L of CO exposure corresponds to ∼1.25 CO molecules impinging on each surface atom. If we assume that CO binds atop gold atoms, where Au ISS would be strongly attenuated,19 then 25% attenuation corresponds to ∼20% sticking probability ()25%/ 1.25). We also examined the Au ISS intensity during repeated ISS scans following the CO dose. As expected for CO bound atop Au, the Au ISS signal initially increased with He+ exposure, as sputtering removed CO, exposing Au. Because Au is, itself, easily sputtered, the Au signal recovered only to about 80% of the pre-CO value and then began to sputter away. We also attempted to look for CO oxidation on the Au/Al2O3 model catalysts, using the same conditions used in our study of CO oxidation on Aun/TiO2.16 After Aun+ deposition, the samples were dosed with 600 L of 18O2 at room temperature, in an effort to create a population of reactive surface oxygen species. The samples were then subjected to a series of ∼150-ms pulses of

Lee et al. each amounting to ∼0.2 L 13C18O exposure, and desorption of 13C18O2 was monitored as a function of time during and after the CO pulse. The temperature was ramped slowly during the sequence of CO pulses, from room temperature (where activity was studied for Au/TiO2) to 600 K. No CO2 was observed. This lack of reactivity is consistent with results of Winkler et al.,9 who reported no CO oxidation on Au/Al2O3/ NiAl(110). Lack of activity is, perhaps, not surprising given that CO does not bind atop or in association with the 1-D or 2-D Au clusters that agglomerate during RT deposition on Al2O3. 13C18O,

IV. Summary Sputter-rate measurements show that Au adatoms are weakly bound on alumina. Nonetheless, ISS and XPS indicate that highly dispersed gold results from LT deposition of Au+. Annealing such samples to 300 K does not result in agglomeration detectable by ISS and XPS. For Tanneal g 450 K, however, substantial agglomeration into three-dimensional clusters, with near-bulk XPS binding energies, is observed. Deposition of Au+ at room temperature results in gold agglomerated into 1-D or 2-D clusters. Both ISS and XPS indicate that LT-deposited Au+ is much less agglomerated than RT-deposited Au+ even after annealing to 300 K. This retention of dispersion is attributed to nucleation effects. For Tanneal g 450 K, LT- and RT-deposited gold agglomerate similarly. Deposition of Aun+ (n ) 3, 4) at room temperature leads to a higher degree of agglomeration than Au+ deposition, counter to the idea that preformed clusters should be more stable against diffusion than isolated adatoms. CO is found not to adsorb on, or in association with, Au clusters on the alumina surface at room temperature. CO adsorption on Au at 95 K is observed, but is quite weak, as shown by rapid sputter removal. The behavior of Au/alumina has been contrasted with analogous results for Au/TiO2, where agglomeration is clearly less efficient and substantial CO oxidation activity is observed for clusters as small as Au3. The reduced sintering behavior on TiO2, and probably the chemical activity, can be related to the presence of electron-rich oxygen vacancies, which bind gold strongly. We have no direct evidence regarding the defects on our alumina films; however, the absence of CO adsorption for T g 100 K, and desorption of water at cryogenic temperatures, suggests that the films do not have significant concentrations of defects capable of interacting strongly with adsorbates. This absence, presumably, also accounts for the tendency of gold to agglomerate. Acknowledgment. We gratefully acknowledge support by Grant DE-FG03-99ER15003 from the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U. S. Department of Energy. References and Notes (1) Haruta, M. Catal. Today 1997, 36, 153-66. (2) Sanchez, A.; Abbet, S.; Heiz, U.; Schneider, W.-D.; Ha¨kkinen, H.; Barnett, R. N.; Landman, U. J. Phys. Chem. A 1999, 103, 9573-78. (3) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 164750. (4) Hammer, B.; Norskov, J. K. Nature 1995, 376, 238-40. (5) Kulkarni, G. U.; Vinod, C. P.; Rao, C. N. R. Nanoscale catalysis by gold. In Surface Chemistry and Catalysis; Carley, A. F., Vavies, P. R., Hutchings, G. J., Spencer, M. S., Eds.; Klewer Academic/Plenum Publishers: 2002; pp 191-206. (6) Hutchings, G. J. Gold Bull. 2004, 37, 3-11. (7) Shaikhutdinov, S. K.; Meyer, R.; Naschitzki, M.; Ba¨umer, M.; Freund, H.-J. Catal. Lett. 2003, 86, 211-19.

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