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J. Phys. Chem. B 2001, 105, 2412-2416
Effect of Platinum Nanocluster Size and Titania Surface Structure upon CO Surface Chemistry on Platinum-Supported TiO2 (110) S. Gan, Y. Liang,* D. R. Baer, M. R. Sievers,† G. S. Herman, and C. H. F. Peden EnVironmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352 ReceiVed: August 30, 2000; In Final Form: January 12, 2001
The adsorption chemistry of CO on clean and Pt-supported TiO2 (110) was investigated. It was found that surface structure of TiO2 plays an important role in the chemistry that takes place at the surface. On the reduced (1 x 2)-reconstructed surface, CO desorbed at 140 and 170 K, while only desorption at 140 K was observed on the stoichiometric (1 x 1) surface. Additionally, CO dissociation, possibly due to the reduction by Ti3+, was observed on the Pt-supported (1 x 2) surface. On the Pt-covered surfaces, the chemistry of CO adsorption and desorption strongly depends on the size of Pt nanoclusters. With a decrease in cluster size, CO was found to desorb at higher temperatures. This unusual desorption chemistry is likely related to quantum size effects of Pt nanoclusters. Scanning tunneling spectra revealed that clusters below 20 Å in diameter exhibited nonmetallic behavior, while those above 40 Å were metallic. This transition of the properties of Pt nanoclusters from metallic to nonmetallic as the cluster size decreases correlates with stronger interaction of CO with Pt observed in temperature-programmed desorption spectra.
Titanium dioxide is a key material for photocatalysts and has a wide range of other technological applications.1,2 As a model metal oxide, studies using supported-metal clusters on TiO2 surfaces can shed light on the metal-oxide interface and how it influences the catalytic activity of a metal surface. In particular, when metal clusters are only a few atomic layers thick, the size of clusters and surface structure of the oxide can have significant impact on the chemisorptive properties of the metal. Among a variety of metals (Pt, Pd, Rh, Cu, Al, etc.) that can be supported on TiO2 surfaces,2-6 platinum has the highest work function and is expected to enhance the photocatalytic activity of TiO2.7 So far only a few studies have been carried out to investigate Pt thin films on single-crystal rutile TiO2 surfaces.7-13 The results of these studies suggest that Pt forms three-dimensional islands on the stoichiometric surface of TiO2 (110) and that these islands are subject to encapsulation by titanium suboxides when the material is thermally annealed. However, the electronic and geometric structures of the clusters are not well understood. Furthermore, several critical issues, such as how the catalytic activity of the metal is affected by the cluster size (especially when it is within the nanometer region) and surface structure of TiO2, remain undetermined. In this paper, we report on a study in which we use CO as a probe molecule to investigate the chemistry of the Pt/TiO2 system. We observed an unusual size effect on the CO chemistry, which we attribute to a quantum size effect of the nanoclusters. Additionally, the surface structure of TiO2 was found to affect the chemistry of Pt supported TiO2. On the reduced (1 x 2) surface, carbon was found to increase substantially after thermal annealing, suggesting probable CO dissociation. The experiments were conducted in two separate ultrahigh vacuum (UHV) systems that enable the use of scanning * Corresponding author. E-mail:
[email protected]. † Current address: IBM Microelectronics, East Fishkill, NY 12533-0999.
tunneling microscopy (STM) in one system and temperatureprogrammed desorption (TPD) in the other. Both systems were equipped with a low-energy electron diffractometer (LEED), a commercial UHV platinum evaporator, and a sputtering gun. In addition, the UHV-STM system was equipped with capabilities for X-ray photoelectron spectroscopy (XPS) and an UHV oxygen plasma source as described elsewhere.14 The TPD system15 had a UTI quadruple mass spectrometer (QMS) and a Perkin-Elmer single-pass cylindrical mirror analyzer for Auger electron spectroscopy (AES). TiO2 (110) crystals ((0.5°, Princeton Scientific Corp.) were ultrasonically cleaned in acetone and methanol before loading into the UHV systems. In the STM system, the samples were gently sputtered with argon to remove surface contaminants and then heated in 2 x 10-5 Torr of O2 plasma for 15 min at 773 K. This preparation method produced a stoichiometric (1 x 1) surface.14 Further sputtering with argon and annealing in a vacuum at 1073 K resulted in a (1 x 2) surface. The surfaces were characterized by LEED, XPS, and STM. Once an atomically clean and smooth surface was obtained, submonolayer Pt was deposited onto the sample, which then was analyzed further using the techniques identified above. STM images were acquired in the constant-current mode typically with a sample bias of +2 V and a tunneling current of 1 nA. The absolute Pt coverage on the surface was determined from the integrated area ratios of the Pt 4f and Ti 2p peaks in the XPS spectra, taking into account the monolayer matrix factor QPt.16 The QPt factor was determined to be 4.0 from the mean free path of electrons λ ) 10 Å and the layer spacing of platinum dPt ) 2.5 Å.11,16 For calibration, the XPS results were compared with the Pt signal measured by Rutherford backscattering spectroscopy. The samples used in the TPD system were different from those in the STM system. For the TPD system, the (1 x 1) surfaces were subjected to many cycles of argon sputtering and thermal annealing. The (1 x 2) and Pt-covered (1 x 1) and (1 x 2) surfaces were obtained using the same procedure as in the
10.1021/jp003125z CCC: $20.00 © 2001 American Chemical Society Published on Web 03/03/2001
CO Surface Chemistry on TiO2 (110)
Figure 1. The (1 x 2) surface of TiO2 (110): (a) an STM image, 1000 x 1000 Å2 and (b) the TPD spectra of CO desorption at different CO dosages. The insets in (a) show an STM image (80 x 80 Å2) of the (1 x 1) surface and a ball-and-stick model of the (1 x 2) surface. Big and small balls in the model represent oxygen and titanium atoms, respectively. The inset in (b) shows an STM image (1000 x 1000 Å2) of a (1 x 2) surface that is not well-developed.
STM system, so we expect that the Pt distribution on these samples is similar to those in the STM system. This assumption is reasonable because our STM revealed that different depositions of Pt under the same conditions produced clusters with a mean size variance of less than 15%. The surface cleanliness and structure were characterized by AES and LEED. CO was dosed onto the samples through a leak valve at a pressure of 5 x 10-7 Torr. After each TPD measurement of CO desorption from a Pt-supported surface, several cycles of sputtering and subsequent thermal annealing were carried out to remove Pt and/or carbon on the surface. Therefore, CO was dosed on freshly deposited Pt clusters in all the TPD experiments of CO desorption from the Pt-supported surfaces. Atomically clean and smooth TiO2 (110) surfaces were prepared by the method described earlier. Atomic-resolution STM images of the (1 x 2) and (1 x 1) (the inset) surfaces are shown in Figure 1(a). The inset in the upper right-hand portion of the figure is a ball-and-stick model for the (1 x 2) surface proposed by Pang et al.17 The (1 x 1) structure18 can be obtained
J. Phys. Chem. B, Vol. 105, No. 12, 2001 2413 by taking out the top two layers in the (1 x 2) model. However, while the (1 x 1) surface is understood well, the (1 x 2) structure is still controversial in the literature. The two models currently in favor for describing the (1 x 2) surface are the added-(1 x 1)-row (Ti3O5) model17 and the added-Ti2O3-row model.19 Recent work by Bennett et al. suggests that there might exist two kinds of (1 x 2) structures, represented by the Ti2O3 and Ti3O5 models.20,21 The (1 x 2) surface in our study was prepared by many cycles of sputtering and high temperature annealing in a vacuum and, therefore, is heavily reduced as evidenced by Ti3+ states in the Ti 2p XPS that were absent for the (1 x 1) surface. As is consistent with a (1 x 2) LEED pattern, the STM image revealed bright rows that are 13 Å apart, twice the distance between two neighboring atomic rows on the (1 x 1) surface. With a sharp STM tip, each (1 x 2) row could be resolved, having two thinner rows separated by the 1x distance.22 This evidence indicates that the (1 x 2) structure observed in our study is better represented by the Ti3O5 model. The atomic corrugation of the (1 x 2) rows is 2.5 Å, which is significantly higher than that of the (1 x 1) rows. The consensus opinion is that under positive sample biases the bright rows in the STM images are due to titanium-derived empty states.18 The different corrugations observed for the (1 x 2) and (1 x 1) surfaces in STM images may be due to Ti atoms with different local bonding environments. For the (1 x 2) surface structure, as seen in the model,18 some 4-fold coordinated Ti atoms are exposed due to the removal of bridging oxygen atoms, while on the (1 x 1) surface, all the Ti atoms are 5- and 6-fold coordinated. Figure 1b shows CO TPD spectra from the clean (1 x 2) surface at various CO dosages. Two peaks can be seen in the spectra, one at 140 K (I) and the other at 170 K (II), which suggests the presence of two different CO adsorption sites. The CO TPD peak at 140 K (I) was also observed for CO adsorption on the (1 x 1) surface, consistent with previous work.7 On the other hand, the peak at 170 K (II) was present only during CO desorption from the (1 x 2) surface. Previous work by Yates et al.8 suggested that CO adsorbs more strongly with lattice Ti sites in the vicinity of oxygen vacancies. Careful examination of structural models for the (1 x 1)18 and (1 x 2)17 surfaces reveals that they differ mainly in coordination of Ti atoms, with the (1 x 2) surface having one row of 4-fold Ti atoms exposed in one surface unit cell. This finding suggests that the peak at 170 K is likely due to CO desorption from 4-fold Ti sites, which interact more strongly with CO than 5-fold Ti sites. This assignment also agrees with the desorption peak observed for CO desorption from the (1 x 1) surface with oxygen vacancies.8,23 However, Figure 1b also shows that the sites bonded more strongly to CO (II) were not filled before adsorption of CO on the more weakly bonded sites (II). One possible explanation is that the (1 x 2) surface consists of mixed large (1 x 1) domains and not well-developed (1 x 2) rows as seen in the inset. Since the width of these (1 x 1) domains (500-1000 Å) is likely larger than the diffusion length of CO molecules on the surface, the preferential adsorption of CO on sites (II) was not realized. Additionally, it is worth noting that the clean TiO2 (110)-(1 x 2) surface does not dissociate CO under our experimental conditions. This was judged by the AES measurements of surface carbon coverage before and after the TPD experiments. Presented in Figure 2 are STM images of the (1 x 2) (a) and (1 x 1) [(b) and (c)] surfaces after Pt deposition. The Pt coverages in (a), (b), and (c) are 0.1, 0.3, and 5 ML, respectively. The STM images clearly show that Pt forms three-dimensional clusters on both TiO2 (110) (1 x 1) and (1 x 2) surfaces. The
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Gan et al.
Figure 2. Scanning tunneling micrographs of TiO2 (110) surfaces: (a) 0.1 mL of Pt deposited on the (1 x 2) surface, (b) 0.1 mL of Pt deposited on the (1 x 1) surface, and (c) 5 mL of Pt deposited on the (1 x 1) surface. The inset in (c) shows a smaller scale of the same surface. The image size is 400 x 400 Å2 in (a) and (b), 5000 x 5000 Å2 in (c), and 1000 x 1000 Å2 in the inset of (c).
average cluster size was measured to be 8, 14, and 45 Å in (a), (b), and in (c), respectively. We found that, at fractional monolayer coverages, Pt adsorbed on top of Ti atomic rows on the (1 x 2) surface, while on the (1 x 1) surface, the Pt clusters were randomly distributed.22 When several monolayers of Pt were deposited on either surface, most of the surface area was covered by Pt clusters, although there were a considerable number of pinholes between the clusters. Nonetheless, the step structure of the underlying TiO2 surface remained visible. The step height of the Pt-covered surface is 3.2 Å, the same as that of the clean (1 x 1) surface. TPD spectra obtained after 1-L CO doses to the Pt- supported TiO2 surfaces similar to those seen in Figure 2 are shown in Figure 3a. From the bottom to the top, the size of Pt nanoclusters in the corresponding spectra is estimated to be 8, 14, and 45 Å, respectively. The abbreviations SC and BC stand for “small cluster” and “big cluster”. Two sets of desorption peaks can be seen in this figurespeaks at approximately 150 K and those near 500 K. Based on a comparison of the TPD spectra shown in Figure 3a with spectra obtained from clean surfaces, such as those in Figure 1b, we believe that the peaks at 500 K can be readily assigned to CO desorption from Pt clusters. This is consistent with previously reported work.7 Furthermore, for small Pt clusters, only a single CO desorption peak was observed at approximately 510 and 490 K on the corresponding (1 x 2) and (1 x 1) surfaces, respectively. The peak position on both
surfaces is independent of the CO dosages for small clusters. As an example, the inset shows the TPD spectra for various CO doses to the (1 x 2) surface covered by Pt clusters of 8 Å in diameter on the (1 x 2) surface. It can be seen that the peak position does not shift when the CO dosage increases from 0.1 to 5 L. However, when the cluster size increased significantly to 45 Å, CO was found to desorb at 415 and 470 K, on both the Pt/(1 x 1) and Pt/(1 x 2) surfaces. The results in Figure 3a reveal that CO chemistry is strongly influenced by Pt nanocluster size. With decrease of the cluster size, CO desorbs from Pt clusters at higher temperatures. The CO desorption peaks of Pt clusters larger than 40 Å in the TPD spectrum are similar to that observed for CO thermal desorption from the Pt (112) surface.24 The peak at 470 K most likely is due to the desorption of CO from cluster edges, while the peak at 415 K is due to CO desorption from cluster terraces. However, when clusters became smaller than 20 Å, we observed only one CO desorption peak, which also shifted to a significantly higher temperature. Furthermore, the peak position of CO desorption from the small clusters does not change with the CO coverage. This is in contrast to CO desorption from the Pt (112) surface, where the high-temperature desorption state was observed at both low and high CO coverages, whereas the low-temperature state was observed only at high CO coverage. Both peak positions changed with the CO coverage. These results indicate that CO molecules bind more strongly to small Pt clusters.
CO Surface Chemistry on TiO2 (110)
J. Phys. Chem. B, Vol. 105, No. 12, 2001 2415 gap for smaller Pt clusters which may contribute to the enhanced interaction between Pt and CO molecules, we do not have experimental evidence to exclude the geometric effects. Additionally, we observed a significant increase in carbon coverage after CO desorption from the (1 x 2) surface covered by Pt. On the clean (1 x 2) surface, the intensity of the AES signal of C was 0.03 of Ti, which increased slightly to about 0.05-0.08 of Ti after Pt deposition. After CO desorption from the Pt-supported (1 x 2) surface, the carbon level increased to 0.3 (of Ti). This result can be contrasted with previous studies of CO on the Pt-supported TiO2-(1 x 1) surface, which showed no increase in the carbon coverage after the desorption of CO.28 Importantly, a recent study on Rh-supported TiO2-(1 x 2) surfaces reported the formation of carbon nanoclusters due to CO dissociation.29 This finding suggests that the increased carbon level observed in our experiments is due to the dissociation of CO. Since CO does not dissociate on Pt24 or on clean TiO2 (110)-(1 x 2) alone, we speculate that the dissociation observed in our results might be due to the combination of reduced Ti3+ sites and Pt clusters, Pt
CO + Ti3+ + / 9 8 C + Ti4+ + / T
Figure 3. (a) TPD spectra of CO desorption from Pt-deposited TiO2 (110) surfaces. BC and SC refer to big (diameter > 40 Å) and small (diameter < 20 Å) clusters. The inset shows TPD spectra of the SC (1 x 2) surface at 0.1, 0.3, and 5 L CO dosages. (b) Tunneling spectra of the clean TiO2 (110) (1 x 1) surface (I) and of Pt nanoclusters of different sizes, 8 Å (II), 14 Å (III), and 45 Å (IV).
To examine the mechanism that accounts for the enhanced CO bonding with Pt nanocluster size, we investigated the electronic structure of Pt nanoclusters. Figure 3b shows scanning tunneling spectra (STS) of the clean TiO2 (110)-(1 x 1) surface (I) and of Pt nanoclusters of different sizes, 8 Å (II), 14 Å (III), and 45 Å (IV), as seen in Figure 3a. The STS were taken by stopping scanning of an STM tip during tunneling, placing the tip on top of a surface feature of interest (for instance, Ti atomic rows and Pt clusters), and then collecting the corresponding I-V characteristics. It can be seen that the clean surface shows an apparent band gap of 3 eV. However, the small Pt clusters (II and III) exhibit much smaller apparent band gaps that also decrease with an increase of the cluster size. For 45-Å clusters, the apparent band gap essentially diminishes, which is an I-V characteristic of metals. These results indicate that Pt nanoclusters on the TiO2 surfaces go through a metallic to nonmetallic transition as cluster size decreases. Furthermore, the higher desorption temperature of CO from smaller clusters could be explained to be the result of stronger interaction of CO with nonmetallic clusters than with metallic clusters. The sizedependent interaction between metal clusters and molecules has been previously reported.25 Alternatively, the stronger interaction with smaller clusters could be due to geometric effects. Previous work of CO desorption on R-Al2O3-supported Pt clusters26 and TiO2-supported Pd clusters27 suggests that geometric effects of metal clusters might also play a role in the size-dependent CO chemistry. Although our STS results suggest an apparent band
where / refers to surface sites and T the CO dissociation temperature. It is worth emphasizing that the dissociation only occurred on the (1 x 2) surface covered by Pt at elevated temperatures. However, the detailed dissociation mechanism is currently unclear. In summary, we found that both TiO2 surface structure and Pt nanocluster size have profound effects on CO surface chemistry. Our results suggest that the reduced (1 x 2) surface not only has stronger interaction with CO molecules but also causes CO to dissociate in the presence of Pt. The reactivity of CO with Pt-supported TiO2 surfaces depends on the size of the Pt nanoclusters. With a decrease of the cluster size, CO desorbs from the clusters at higher temperatures. The enhanced CO bonding observed on the small Pt nanoclusters could be due to a quantum size effect of the clusters, which become nonmetallic as confirmed by tunneling spectra. Acknowledgment. This research was supported by PNNL Laboratory Directed R&D funds. CHFP also acknowledges support from the Department of Energy (DOE), Office of Basic Energy Sciences, Division of Material Sciences. The work was performed in the William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological & Environmental Research. We thank Dr. S. Thevuthasan for characterizing the platinum coverage by RBS and Dr. M. A. Henderson for useful discussion on surface chemistry. PNNL is operated for DOE by Battelle. References and Notes (1) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, 1994. (2) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (3) Xu, C.; Lai, X.; Zajac, G. W.; Goodman, D. W. Phys. ReV. B 1997, 20, 56. (4) Berko, A.; Solymosi, F. Surf. Sci. 1998, 281, 400. (5) Pan, J. M.; Maschhoff, B. L.; Diebold, U.; Madey, T. E. Surf. Sci. 1993, 381, 291. (6) Lai, X.; Xu, C.; Goodman, G. W. J. Vac. Sci. Technol. A 1998, 2562, 16. (7) Linsebigler, A.; Rusu, C.; Yates, J. T. Jr. J. Am. Chem. Soc. 1996, 118, 5284.
2416 J. Phys. Chem. B, Vol. 105, No. 12, 2001 (8) Linsebigler, A.; Lu, G.; Yates, J. T. Jr. J. Chem. Phys. 1995, 103, 9438. (9) Dulub, O.; Hebenstreit, W.; Diebold, U. Phys. ReV. Lett. 2000, 84, 3646. (10) Pesty, F.; Steinruck, H.; Madey, T. E. Surf. Sci. 1995, 83, 339. (11) Schierbaum, K. D.; Fischer, S.; Torquemada, M. C.; de Segovia, J. L.; Roman, E.; Martin-Gago, J. A. Surf. Sci. 1996, 261, 345. (12) Fischer, S.; Schiebaum, K. D.; Gopel, W. Vacuum 1997, 601, 48. (13) Sun, Y.-M.; Belton, D. N.; White, J. M. J. Phys. Chem. 1986, 90, 5178. (14) Gan, S.; Liang, Y.; Baer, D. R. Surf. Sci. Lett. 2000, 459, 498. (15) Herman, G. S.; Peden, C. H. F. J. Vac. Sci. Technol. A 1994, 12, 2087. (16) Briggs, D.; Seah, M. P. Practical Surface Analysis, 2nd ed.; Wiley: New York, 1990; Vol. 1. (17) Pang, C. L.; Haycock, S. A.; Raza, H.; Murray, P. W.; Thornton, G.; Gulseren, O.; James, R.; Bullett, D. W. Phys. ReV. B 1998, 58, 1586.
Gan et al. (18) Diebold, U.; Anderson, J. F.; Ng, K.-O.; Vanderbilt, D. Phys. ReV. Lett. 1996, 77, 1322. (19) Onishi, H.; Iwasawa, Y. Phys. ReV. Lett. 1996, 76, 79. (20) Bennett, R. A.; Stone, P.; Price, N. J.; Bowker, M. Phys. ReV. Lett. 1999, 82, 3831. (21) Bennett, R. A.; Stone, P.; Bowker, M. Faraday Discuss. 2000, 114, 267. (22) Gan, S.; Liang, Y.; Baer, D. R.; Grant, A. W. Surf. Sci., in press. (23) Henderson, M. A., unpublished data. (24) Siddiqui, H. R.; Guo, X.; Chorkendorff, I.; Yates, J. T. Jr. Surf. Sci. 1987, 191, L813. (25) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (26) Altman, E. I.; Gorte, R. J. Surf. Sci. 1987, 195, 392. (27) Evans, J.; Hayden, B. E.; Lu, G. Surf. Sci. 1996, 360, 61. (28) Steinruck, H.; Pesty, F.; Zhang, L.; Madey, T. E. Phys. ReV. B 1995, 51, 2427. (29) Berko, A.; Biro, T.; Solymosi, F. J. Phys. Chem. B 2000, 104, 2506.
