J. Phys. Chem. C 2007, 111, 8069-8074
8069
Photoexcitation of Local Surface Structures on Strontium Oxide Grains Slavica Stankic,† Johannes Bernardi,‡ Oliver Diwald,*,† and Erich Kno1 zinger† Institute of Materials Chemistry, Vienna UniVersity of Technology, Veterina¨rplatz 1/GA, A-1210 Vienna, Austria, and UniVersity SerVice Centre for Transmission Electron Microscopy, Vienna UniVersity of Technology, Wiedner Hauptstrasse 8-10/137, A-1040 Vienna, Austria ReceiVed: January 22, 2007; In Final Form: March 19, 2007
The photoluminescence properties of strontium oxide grains result from the photoexcitation of local surface structures and are characterized by intense emission bands in the visible light range. Using diffuse reflectance and photoluminescence spectroscopy, we have investigated SrO nanocrystals obtained by chemical vapor deposition (CVD) and subjected to thermal activation under high vacuum conditions afterward. Transmission electron microscopy reveals that compact and morphologically ill-defined SrO grains with sizes up to 200 nm arise from the coalescence of various misaligned nanocrystals that aggregate and intersect in the course of thermal treatment. However, despite the low specific surface area of less than 1 m2 g-1, intense PL emission bands can be induced by selective excitation of surface anions in (100) planes, edges, and oxygen-terminated corners. Transfer of excitation energy across the SrO surface from sites of higher coordination to those of lower coordination as reported by Coluccia (Stud. Surf. Sci. Catal. 1985, 21, 59) has also been observed for CVD SrO. Comparison of room temperature measurements with those carried out at T ) 77 K reveals a strong temperature dependence of all PL emission effects as well as the presence of a specific radiative deactivation process that is attributed to 4-fold coordinated surface sites and subjected to thermal quenching at room temperature. It was found that the abundance of excitation and emission sites in the surface region of the polycrystalline SrO grains can be controlled by the annealing time. Prolonged thermal treatment at 1170 K depletes low-coordinated surface elements and induces the enhancement of 5-fold coordinated surface anions located in extended (100) planes.
Introduction Utilizing photostimulated processes on oxide surfaces is essential to photocatalysis,1-3 environmentally clean energy production,4-6 functional coatings,7-9 materials’ processing,10-12 and many more fields. All these applications require a better understanding of the mechanisms that convert photon energy into excited surface states that in turn lead to selective bond breaking, to nonradiative exciton deactivation, or to photoluminescence emission from particular surface sites. Beyond that, recent advances in nanocrystal synthesis and functionalization of their surfaces and interfaces have established an unprecedented playground for photophysics and chemistry studies and provide a base for new and unexpected applications.13-18 Consequently, the investigation of photoexcitation effects on highly dispersed solids is very much needed but associated with formidable challenges that arise from the principal difficulty to identify specific surface elements as excitation and emission sites. Therefore, it is a primary objective to establish a direct correlation between local excitation and emission events on one hand and associated surface elements on the other. For nanostructured alkaline earth oxides (AEOs), the combination of experimental studies and theory has allowed for assignments of particular spectroscopic fingerprints to specific surface sites.19-23 In particular, on MgO, various photoluminescence * Corresponding author. E-mail:
[email protected]; fax: 011-43-1-25077 3890. † Institute of Materials Chemistry. ‡ University Service Centre for Transmission Electron Microscopy.
PL studies have been carried out on different types of highly dispersed powders to probe the chemical reactivity of surface defects.24-28 Chemical vapor deposition represents an efficient approach for the production of pure and mixed AEO nanocrystals that dispose a high concentration of specific optically active surface structures.29,30 Since these sites absorb and emit light above λ > 200 nm, their enrichment and manipulation hold the key to produce nanocrystals with controlled optoelectronic and chemical activities. Energy transfer processes occur whenever light emission originates from sites that are different from those of excitation. On oxide surfaces, evidence for energy transfer was obtained for highly dispersed AEOs thatsreasoned by their ionicitys serve as suitable model systems for the investigation of these phenomena.31,32 Coluccia and Tench reported for the first time PL effects on SrO powders and concluded exciton transfer from O2- anions of 5-fold coordination (5C) located in (100) planes or 4-fold coordinated ones (4C) in edges to oxygen-terminated corner sites (3C).33,34 Apart from PL studies,24,25 evidence for energy transfer on MgO stems from combined electron paramagnetic resonance, IR, and UV-vis spectroscopy studies carried out in conjunction with theoretical modeling.35,36 Photoexcited states generated at MgO nanocube edges were found to be mobile, and their localization at corner sites was evidenced via characteristic spectroscopic features.36 In the present study, the optical properties of nanostructured SrO obtained by chemical vapor deposition will be analyzed for three major reasons. (a) Excitation and emission energies related to low-coordinated surface elements on SrO are in the
10.1021/jp070538s CCC: $37.00 © 2007 American Chemical Society Published on Web 05/12/2007
8070 J. Phys. Chem. C, Vol. 111, No. 22, 2007 ranges from 3.5 eV (350 nm) to 5.5 eV (225 nm) and from 2.0 eV (620 nm) to 3.5 eV (350 nm), respectively. As compared to MgO and CaO, these energies are significantly red-shifted. Since PL emission covers a considerable part of the visible light region,37 highly dispersed SrO represents a candidate as an inorganic phosphor with surface-dependent light emission properties.29 (b) Although energy transfer on nanostructured oxide surfaces is relatively unexplored, it offers the potential for tailor-made photosystems for the detection and emission of light as well as for the performance of site selective photochemistry. The first evidence for energy transfer on AEO surfaces was obtained on SrO powders by Coluccia and Tench.33,34 Related information about particle size and morphology that forms the basis for the discussion of the underlying electronic surface properties has not been presented since this time. (c) Clean particle surfaces and consequently adsorbate removal are indispensible for the investigation of surface processes on AEO nanostructures and require vacuum annealing up to 1170 K. As compared to MgO, the thermal stabilities of CaO, SrO, and BaO are significantly decreased.31,37 SrO particles survive thermal treatment to such an extent that the resulting powder can be characterized by photoluminescence spectroscopy and transmission electron microscopy (TEM), whereas unsupported BaO nanoparticles coalesce to one compact grain that renders the investigation of powdered materials impossible.38 Experimental Procedures SrO samples were synthesized by the chemical vapor deposition technique (CVD) based on the combustion of a metal vapor with oxygen within a flow reactor system. Temperatures for Sr evaporation are in the range between 900 and 1010 K, and typical flow rates of 1.22 and 8.2 × 10-3 slm were used for Ar and O2, respectively. Further details about this production technique are given elsewhere.39,40 Quartz glass cells that guarantee vacuum conditions better than p < 10-6 mbar were used for spectroscopic measurements. The powder samples were subjected to thermal treatment up to 1173 K and final pressures below p < 10-5 mbar. In a first step, SrO powders were heated at a rate of 5 K min-1 to 873 K and exposed to oxygen at this temperature to remove organic contaminants. Then, the sample temperature was raised to 1123 K in oxygen and -after pumping to p < 10-5 mbar-was raised to 1173 K and kept at this temperature for 15 min (t1170K ) 15′) or 120 min (t1170K ) 120′). All powders investigated were white, with no absorption in the range of visible light. On this basis, we exclude the presence of color centers that may arise from temperature induced lattice oxygen depletion. The BET surface area SBET of the resulting white powder was less than 1 m2 g-1 as derived from nitrogen sorption isotherms obtained at 77 K in a relative pressure range p/p0 from 0.05 to 0.2 (Micromeritics ASAP 2020). For thermally activated SrO powders, it is impossible to specify the degree of surface hydroxylation by transmission IR spectroscopy. The transparency of the very dense pellets is low, and the S/N ratio in the OH stretching region is insufficient for either identifying or ruling out residual surface OH groups. The presented absorption spectra were acquired using a PerkinElmer Lambda 15 spectrophotometer equipped with an integrating sphere. The measurements were carried out in the presence of 10 mbar O2 to suppress PL artifacts in the absorption curve. The diffuse reflectance curves were then converted into absorption spectra via the Kubelka-Munk transform procedure. A pulsed Xe discharge lamp served as an excitation light source in a PerkinElmer LS 50B system for photoluminescence
Stankic et al.
Figure 1. UV absorption spectrum of vacuum annealed SrO (t1170K ) 15′).
measurements. Low-temperature measurements were carried out using a commercially available low-temperature luminescence accessory where the sample cell was held by a high-purity copper rod that was immersed in liquid nitrogen. After spectroscopy experiments, small amounts of the metal oxide powders were cast on a holey carbon grid for investigation with a TECNAI F20 analytical transmission electron microscope equipped with a field emission electron source and an S-Twin objective lens. Images were recorded with a Gatan 794 MultiScan camera. Results and Discussion A characteristic UV diffuse reflectance spectrum of a SrO powder that was previously activated at 1170 K (t1170K ) 15′) is shown in Figure 1. Total light absorption was measured for energies above 5.5 eV (λ < 225 nm) consistent with band gap (BG) transitions in the SrO bulk.31,41,42 In the lower energy region, two absorption maxima at 4.6 and 3.9 eV were clearly observable and attributed to the excitation of the 5- and 4-coordinated oxygen ions (5C and 4C), respectively.21 The 3.9 eV band shows a shoulder that reaches to 3.5 eV (350 nm), where the electronic excitation of 3-coordinated oxygen ions is expected.21 Emission spectra that were measured at 298 and 77 K are presented in the left and right hand panel of Figure 2, respectively, using (a) 4.6 eV (5C), (b) 3.9 eV (4C), and (c) 3.5 eV (3C) light for photoexcitation.33,34 At room temperature and pressures p < 10-5 mbar, all emission features (curves a-c in the left panel of Figure 2) exhibit a maximum at 2.6 ( 0.1 eV (indicated by an arrow in the bottom up direction) and a full width at half-maximum (fwhm) of 0.25 eV irrespective of the excitation energy used. The presence of O2 at pressures p g 1 mbar completely quenches these emission effects and proves that they originate from the surface.16,29,30 Since photoexcitation at 4.6 eV (5C) and 3.9 eV (4C) gives rise to the same band at 2.6 eV as the 3.5 eV (3C) light does, efficient energy transfer from 5C and 4C sites to oxygenterminated corners (3C) is concluded.33,34 At 77 K (right-hand panel in Figure 2), the measured PL intensity is by nearly an order of magnitude stronger than at room temperature (lefthand panel in Figure 2). An intense band at 3.0 eV (arrow in the top down direction) results from excitation of 5C and 4C oxygen ions (Figure 2a,b, respectively) and is quenched when molecular oxygen is present in the gas phase. In addition, a shoulder at 2.6 eV (arrow in bottom up direction) is present and indicates that cooling to T ) 77 K does not impair the
Local Surface Structures on Strontium Oxide Grains
Figure 2. Photoluminescence emission spectra of SrO polycrystals (t1170K ) 15′). The emission spectra were measured at T ) 298 K (left panel) and at T ) 77 K (right panel) using the following excitation energies: (a) 4.6 eV, (b) 3.9 eV, and (c) 3.5 eV.
Figure 3. Photoluminescence emission spectrum induced by photoexcitation with 4.6 eV light. The measured spectrum (Exp) can be reproduced by two Gaussian curves with maxima at 3.0 and 2.6 eV.
transfer of excitation energy from 5C/4C sites to 3C sites as reported by others.32,33 The low-temperature spectra plotted in the right-hand panel of Figure 2 can be decomposed into two Gaussian curves with maxima at 3.0 and 2.6 eV (Figure 3) as illustrated for the emission spectrum related to 4.6 eV excitation (Figure 2a). The simulation reveals that the band at 2.6 eV is by a factor of 4 more intense at T ) 77 K than at room temperature (Figure 2a, left-hand panel). Since the formation of the 3.0 eV feature does not give rise to the depletion of that at 2.6 eV, we conclude that the underlying process of radiative exciton deactivations which apparently is subject to thermal quenching at room temperaturesis independent from that of the 2.6 eV feature (arrow in the bottom up direction in Figure 2) and involves a completely different electronic transition. Lifetime measurements are needed for further relevant insights. The values of 3.0 and 2.6 eV were selected as emission energies for acquisition of the excitation spectra shown in Figure 4. Coinciding with the absorption spectrum, two excitation bands with maxima at 4.6 and 3.9 eV are associated with the emission feature at 3.0 eV (dark gray curve in Figure 4). The excitation spectrum related to the 2.6 eV emission (light gray curve) reveals an additional feature that is located below 3.9 eV, peaks at 3.7 eV (335 nm), and extends to 3.4 eV (370 nm). Coluccia and Tench observed in the room-temperature spectra of SrO powders, which had been obtained via thermal SrCO3 decomposition, only one emission band with a maximum at 2.7 eV.33,34 That the respective emission process occurs for all surface anion types excited (5C, 4C, or 3C) is consistent with
J. Phys. Chem. C, Vol. 111, No. 22, 2007 8071
Figure 4. UV absorption and photoluminescence excitation spectra of SrO polycrystals (t1170K ) 15′). The excitation spectra were acquired at 77 K by tracking the photoluminescence emission intensity at hνEM ) 3.0 eV (dark gray curve) and at hνEM ) 2.6 eV (light gray curve).
our study (Table 1) and was attributed to energy transfer from the 5C or 4C sites to the 3C sites. At 77 K, the 2.7 eV emission was observed when oxygen-terminated corner sites (3C) were directly excited with 3.5 eV light. Different from our results, they found that the excitation of the 5C or 4C sites produced two bands at 3.1 and 2.8 eV, respectively, and concluded that there was suppression of energy transfer at low temperatures. However, our study reveals that the feature at 3.0 eV (Figure 2a,b, right-hand panel) results from photoexcitation of the 4C or 5C sites and does not relate to that at 2.6 eV, which in turn is also present at 77 K. Since exciton energy only transfers from sites of higher coordination to those of lower, the respective radiative deactivation process is attributed to the 4C sites in edges.31 We also studied the photoluminescence properties of SrO powders that were kept at T ) 1170 K for 2 h (t1170K ) 120′, Figure 5). An intense band at 3.1 eV was measured only when the 5C sites in (100) planes were excited with 4.6 eV light.43 Upon cooling to T ) 77 K, its maximum shifts from 3.1 to 3.2 eV, and the entire band gains intensity by an order of magnitude (Figure 5, right-hand panel). However, PL emission associated with the photoexcitation of the 4C and 3C sites (Figure 5, curves b and c) is very weak. Structural and morphological information related to vacuum annealed SrO particles was obtained by transmission electron microscopy. After thermal activation, SrO exists in the form of morphologically ill-defined grains with sizes between 20 and 200 nm (Figure 6). The observed contrast in these TEM images is attributed to the overlap of different crystallographic domains. The high-resolution TEM image of a typical SrO grain in Figure 6 (bottom part) shows a complex topography and indicates that many interconnected and misoriented nanocrystals make up one grain. The lattice fringes measured are consistent with the (100) and (111) planes of SrO, while Moire´ fringes demonstrate an overlap of different nanocrystals, the diameters of which are estimated to be in the range between 10 and 25 nm. No significant changes in the structural properties were observed for samples that were kept for longer times at T ) 1170 K (t1170K ) 120′). In principle, grain boundaries44 that are located in the interior part of the grain and arise from the intersection of misaligned nanocrystals correspond to extended defects and can contain potential sites that are involved in PL emission. We found that oxygen gas efficiently quenches the PL emission of photoexcited SrO grains, indicating that O2 must have access to the PL emission sites. On the other hand, significant porosity of the material in question can be ruled out because specific surface areas of annealed SrO were found to be below 1 m2
8072 J. Phys. Chem. C, Vol. 111, No. 22, 2007
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Figure 5. Photoluminescence emission spectra of SrO polycrystals (t1170K ) 120′) at p < 10-5 mbar. The emission spectra were measured at room temperature (left panel) and at T ) 77K (right panel) using (a) 4.6 eV, (b) 3.9 eV, and (c) 3.5 eV as excitation energies.
g-1. We therefore conclude that ions that are located in the interfacial region between nanocrystals and have no contact with the gas phase are not involved in PL emission. Obviously, high-temperature treatment induces not only particle agglomeration but also enforces ion diffusion and thus reorganization of the grain surface structure. TEM images showing the surface region of polycrystalline SrO grains (Figures 6 and 7) show flat domains that are very likely terminated by low-energy (100) facets (see, i.e., the inset in Figure 7). We propose thatsas a consequence of extended annealings flat (100) facets grow at the expense of rougher surface elements and cause a significant enhancement of the 5C anion concentration. Such a structural reorganization of nanocrystal surfacess illustrated by the schematic in Figure 7swould explain why PL emission associated with the excitation of 4C and 3C sites is depleted and why that related to excited 5C ions is increased (Figure 5). The origin of the PL emission at hνEM g 3.1 eV remains open. Although the energy is very close to that of the band observed for SrO grains that were subjected to less extended annealing (t1170K ) 15′, Figure 2a,b in the right-hand panel), the following major differences between these features have to be emphasized: (a) in the case of the SrO (t1170K ) 15′) sample, the 3.0 eV emission is only observed at T ) 77 K, while on SrO (t1170K ) 120′), the band at 3.1 eV is already present at room temperature and gains intensity by an order of magnitude after sample cooling to T ) 77 K. (b) On SrO (t1170K ) 15′), the 3.0 eV emission results from the excitation of 5C and 4C sites at variance to the process at hνEM g 3.1 eV on SrO (t1170K ) 120′), which exclusively occurs after 5C excitation. We did not observe morphological differences between SrO grains after 15′ or 120′ of vacuum annealing at 1170 K. Furthermore, the accumulation or depletion of point defects, residual adsorbates, as well as details about the grain boundaries that are in contact with the gas phase cannot be identified on the basis of high-resolution TEM data. However, it is very likely that residual hydroxyls exist on SrO (t1170K ) 15′) and that their
Figure 6. Transmission electron micrographs of SrO (t1170K ) 15′) at different magnifications.
concentration is significantly decreased after extended periods of annealing. One way to explain the feature at hνEM g 3.1 eV that exists on SrO (t1170K ) 120′) but is absent on SrO (t1170K ) 15′) is based on grain boundaries that adjoin misoriented SrO nanocrystals. Different from low-coordinated sites that are present on every SrO nanocrystal surface plane that is exposed to the gas phase (Figure 7), grain boundaries must survive extended annealing. As a consequence of progressive dehydroxylation, ions that are located in such disordered interfacial regions might become active as light emission sites when 3C or 4C oxygen anions are depleted and therefore do not contribute to PL emission anymore. Further chemisorption studies in conjunction with PL emission properties are needed to substantiate or falsify such a hypothesis. Finally, we comment on the PL emission intensity of SrO as compared to MgO and CaO.30 For similar powder densities,
TABLE 1: Photoluminescence Excitation and Emission Energies Observed on Different Polycrystalline SrO Samples at T ) 77 K as Well as Their Assignment to Specific Surface Elementsa this study (SrO, t1170K ) 15′)
Coluccia and Tench33,34 excitation (eV) 3.7 3.9 4.4 a
emission (eV) 2.7 2,8 3.1
emission site O2- (3C) O2- (4C) O2- (5C)
excitation (eV)
emission (eV)
3.5 3.9 4.6
2.6 3.0 3.0
On the basis of width and partial overlap, the accuracy of the band positions is declared with hν ( 0.1 eV.
emission site O2- (3C) O2- (4C) O2- (4C)
Local Surface Structures on Strontium Oxide Grains
J. Phys. Chem. C, Vol. 111, No. 22, 2007 8073 Acknowledgment. This work was financially supported by Fonds zur Fo¨rderung der Wissenschaftlichen Forschung (FWF) P 17770-N11, which is gratefully acknowledged. We thank M. J. Elser for his assistance with the sorption studies and his very useful comments on the manuscript. References and Notes
Figure 7. High-resolution TEM image of the surface region of a SrO grain (t1170K ) 120′) with an inset showing the details of one SrO nanocrystal located in the grain surface. The lattice fringes were measured to be 2.6 ( 0.1 Å, consistent with the (100) spacings of SrO. The schematic diagram illustrates the expected effect of extended vacuum annealing on the surface structure of polycrystalline SrO grains: elements with a high concentration of low-coordinated surface sites are transformed into flat (100) facets of low surface energy.
we found that PL emission light from SrO powders is at least by an order of magnitude more intense, even though the abundance of surface sites must be significantly smaller. Furthermore, excitation and emission energies of SrO are significantly red-shifted, and PL emission covers a considerable part of the visible light region. The pronounced temperature dependence of PL emission (Figure 2) together with annealing induced trends in the abundance of excitation and emission sites (compare Figures 2 and 5) makes AEO powders an interesting class of materials with surface-dependent optical properties. In addition, the availability of various parameters of surface doping29,45 opens up an opportunity region for engineering the electronic surface structure of insulating nanocrystals to potentially employ them as constituents of photoactive sensing devices, inorganic phosphors, as well as electronic parts that are based on energy transfer steps. Conclusion We report a combined structural and spectroscopic investigation of SrO nanocrystals that in the course of vacuum annealing coalesce and form large polycrystalline grains. As a surprising result, we found that although the resulting SrO powder exhibits a low specific surface area, there is significant PL emission that originates from the photoexcitation of surface anions in different coordination states. An analysis of our data in the light of earlier work has revealed that energy transfer processes occur from 5C and 4C sites to oxygen-terminated corner sites (3C) and are not affected when the sample is cooled to T ) 77 K. Moreover, we observed a PL emission process that results from the photoexcitation of 5C and 4C sites and is only active at T ) 77 K. Furthermore, annealing induced trends in the abundance of specific excitation and emissions sites demonstrate that PL spectroscopy can be used as an exceptionally sensitive technique to track surface structure changes of polycrystalline grains.
(1) Thompson, T. L.; Yates, J. T. Chem. ReV. 2006, 106, 4428. (2) Osgood, R. Chem. ReV. 2006, 106, 4379. (3) Bahnemann, D. Solar Energy 2004, 77, 445. (4) Gra¨tzel, M. Inorg. Chem. 2005, 44, 6841. (5) Watson, D. F.; Meyer, G. J. Annu. ReV. Phys. Chem. 2005, 56, 119. (6) Haque, S. A.; Palomares, E.; Cho, B. M.; Green, A. N. M.; Hirata, N.; Klug, D. R.; Durrant, J. R. J. Am. Chem. Soc. 2005, 127, 3456. (7) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (8) Zubkov, T.; Stahl, D.; Thompson, T. L.; Panayotov, D.; Diwald, O.; Yates, J. T. J. Phys. Chem. B 2005, 109, 15454. (9) White, J. M.; Szanyi, J.; Henderson, M. A. J. Phys. Chem. B 2003, 108, 9029. (10) Ba¨uerle, D. Laser Processing and Chemistry; Springer-Verlag: Berlin, 2000. (11) Hess, W. P.; Joly, A. G.; Beck, K. M.; Henyk, M.; Sushko, P. V.; Trevisanutto, P. E.; Shluger, A. L. J. Phys. Chem. B 2005, 109, 19563. (12) Beck, K. M.; Henyk, M.; Wang, C.; Trevisanutto, P. E.; Sushko, P. V.; Hess, W. P.; Shluger, A. L. Phys. ReV. B 2006, 74, 45404. (13) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (14) Jun, Y.-W.; Lee, J.-H.; Choi, J.-S.; Choi, J. J. Phys. Chem. B 2005, 109, 14795. (15) Klimov, V. I. J. Phys. Chem. B 2006, 110, 16827. (16) Stankic, S.; Mu¨ller, M.; Sterrer, M.; Bernardi, J.; Diwald, O.; Kno¨zinger, E. Angew. Chem., Int. Ed. 2005, 44, 4917. (17) Erwin, S. C.; Zu, L.; Haftel, M. I.; Efros, A. L.; Kennedy, T. A.; Norris, D. J. Nature 2005, 436, 91. (18) Riss, A.; Berger, T.; Grothe, H.; Bernardi, J.; Diwald, O.; Kno¨zinger, E. Nano Lett. 2007, 7, 433. (19) Chiesa, M.; Paganini, M. C.; Giamello, E.; Murphy, D. M.; Di Valentin, C.; Pacchioni, G. Acc. Chem. Res. 2006, 39, 861. (20) Gribov, E. N.; Bertarione, S.; Scarano, D.; Lamberti, C.; Spoto, G.; Zecchina, A. J. Phys. Chem. B 2004, 108, 16174. (21) Garrone, E.; Zecchina, A.; Stone, F. S. Philos. Mag. B 1980, 42, 683. (22) Benedetti, S.; Benia, H. M.; Nilius, N.; Valeri, S.; Freund, H.-J. Chem. Phys. Lett. 2006, 430, 330. (23) Carrasco, J.; Sousa, C.; Illas, F.; Sushko, P. V.; Shluger, A. L. J. Chem. Phys. 2006, 125, 74710. (24) Hacquart, R.; Krafft, J.-M.; Costentin, G.; Jupille, J. Surf. Sci. 2005, 595, 172. (25) Chizallet, C.; Costentin, G.; Krafft, J.-M.; Laurent-Pernot, H.; Che, M.; ChemPhysChem 2006, 7, 904. (26) Anpo, M.; Yamada, Y.; Kubokawa, Y.; Coluccia, S.; Zecchina, A.; Che, M. J. Chem. Soc., Faraday Trans. 1 1988, 84, 751. (27) Anpo, M.; Che, M. AdV. Catal. 1999, 44, 119. (28) Bailly, L.-M.; Costentin, G.; Lauron-Pernot, H.; Krafft, J. M.; Che, M. J. Phys. Chem. B 2005, 109, 2404. (29) Stankic, S.; Sterrer, M.; Hofmann, P.; Bernardi, J.; Diwald, O.; Kno¨zinger, E. Nano Lett. 2005, 5, 1889. (30) Stankic, S.; Bernardi, J.; Diwald, O.; Kno¨zinger, E. J. Phys. Chem. B 2006, 110, 13866. (31) Garrone, E.; Zecchina, A.; Stone, M. G. Philos. Mag. B 1980, 42, 683. (32) Coluccia, S.; Deane, A. M.; Tench, A. J. J. Chem. Soc., Faraday Trans. 1 1978, 74, 2913 (33) Coluccia, S. In Adsorption and Catalysis on Oxide Surfaces; Che, M., Bond, G. C., Eds.; Elsevier Science Publishers B.V.: Amsterdam, 1985; p 59. (34) Coluccia, S.; Tench, A. J. J. Chem. Soc., Faraday Trans. 1 1983, 79, 1881. (35) Sterrer, M.; Berger, T.; Diwald, O.; Kno¨zinger, E. J. Am. Chem. Soc. 2003, 125, 195. (36) Sterrer, M.; Berger, T.; Diwald, O.; Kno¨zinger, E.; Sushko, P. V.; Shluger, A. L. J. Chem. Phys. 2005, 123, 64714. (37) Spoto, G.; Gribov, E. N.; Ricchiardi, G.; Damin, A.; Scarano, D.; Bordiga, S.; Lamberti, C.; Zecchina, A. Prog. Surf. Sci. 2004, 76, 71. (38) Stankic, S.; Diwald, O.; Kno¨zinger, E., unpublished data. (39) Hofmann, P.; Kno¨zinger, E.; Diwald, O.; Mustafa, A. Ber. BunsenGes. Phys. Chem. 1997, 101, 1722.
8074 J. Phys. Chem. C, Vol. 111, No. 22, 2007 (40) Becker, A.; Benfer, S.; Hofmann, P.; Jacob, K.-H.; Kno¨zinger, E. Ber. Bunsen-Ges. Phys. Chem. 1995, 99, 1328. (41) Cappellini, G.; Finocchi, F.; Bouette-Russo, S.; Noguera, C. Comp. Mater. Sci. 2001, 20, 401. (42) Cappellini, G.; Bouette-Russo, S.; Amadon, B.; Noguera, C.; Finocchi, F. J. Phys: Condens. Matter 2000, 12, 3671.
Stankic et al. (43) The related excitation spectrum (not shown) shows only one maximum at 4.6 eV. (44) Sayle, D. C.; Watson, G. W. J. Phys. Chem. B 2002, 106, 10973. (45) Berger, T.; Schuh, J.; Sterrer, M.; Diwald, O.; Kno¨zinger, E. J. Catal. 2007, 247, 61.
