J. Phys. Chem. C 2010, 114, 20189–20194
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Atomic Scale Analysis of Ultrathin SiO2 Films Prepared on TiO2(100) Surfaces Akira Sasahara,* Chi Lun Pang,† and Masahiko Tomitori Japan AdVanced Institute of Science and Technology (JAIST), Nomi, Ishikawa, 923-1292 Japan ReceiVed: September 2, 2010; ReVised Manuscript ReceiVed: October 13, 2010
Rutile titanium dioxide (TiO2) (100) surfaces covered by silicon oxide (SiO2) ultrathin films were examined by X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and frequency modulation atomic force microscopy (FM-AFM) techniques. The SiO2 films were fabricated on TiO2 crystals by annealing in a quartz case which was used as a SiO2 source. The amount of Si increased with annealing time, and a (3 × 4) LEED pattern was observed on surfaces with the XPS peak intensity ratio of Si 2p to O 1s of TiO2 larger than 0.023. FM-AFM observation in pure water showed that the (3 × 4) surface consists of atomically flat terraces. Within the terraces, rows which extend in the [001] direction were observed. Every fourth row appeared brighter, consistent with the ×4 periodicity in the [010] direction observed in LEED. Models where rutile SiO2 units are accumulated on the TiO2(100) surface via rutile Ti1-xSixO2 units are consistent with the results. Introduction Titanium-silicon binary oxides have been studied as a route to enhancing the catalytic performance of pure TiO2.1 The high catalytic activity of the Ti-Si binary oxides has been reported for many reactions including oxidation of methanol;2 epoxidation of hexane,3 oct-1-ene, and cyclohexane;4 amination of phenol;5 and isomerization of 1-butane.5 It has been expected that the Ti-O-Si structure in the binary oxides give rise to acidity. The enhanced photocatalytic activity of the Ti-Si binary oxides has been shown in the decomposition of salicylic acid,6 rhodamine 6G,7 and phenol and its chrolo derivatives8 and reduction of CO2 in 2-propanol.9 The photocatalytic activity is attributed to the increased surface concentration of reactants on the SiO2 domains as well as the size effect of the TiO2 particles stabilized by SiO2. Notari et al. pointed out that an accurate estimation of the surface area determined by surface structure is necessary in the study of catalysis of the Ti-Si binary oxides.10 Identification of the origin of the catalytic activity of the Ti-Si binary oxides will help to reveal its mechanisms, optimize its performance, and extend its application. One promising approach to the study of catalysis is to apply surface sensitive analytical methods to model catalysts prepared using single crystal surfaces.11 In the present study, SiO2-vapor deposited rutile TiO2(100) surfaces prepared in air were examined by low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and frequency modulation atomic force microscopy (FM-AFM) in water. The ultrathin SiO2 film showed a (3 × 4) LEED pattern with respect to the (100)-(1 × 1) surface. FM-AFM images showed bright rows arranged with an interval corresponding to four-times the periodicity of the (1 × 1) unit cell in the [010] direction. Models where rutile SiO2 units are accumulated on the TiO2(100) surface via rutile Ti1-xSixO2 units are consistent with the experimental results. The analysis of the atomically ordered SiO2 thin film on TiO2 * To whom correspondence should be addressed. E-mail: sasahara@ jaist.ac.jp. † Current address: Surface Science Research Centre, University of Liverpool, L69 3BX United Kingdom.
promotes an atomic scale analysis of the hetero-oxide surface formed under atmospheric-pressure and high temperature conditions. Ordered silicon oxide monolayers are formed on a Mo(112) surface. Weissenrieder et al. reported a SiO2-c(2 × 2) monolayer which gives a honeycomb structure in scanning tunneling microscopy (STM) images.12 To explain the structure, a monolayer consisting of corner-sharing SiO4 tetrahedra was concluded. Density functional theory calculation showed the model is energetically most favorable among the prospective structures. Infrared reflection absorption spectroscopy (IRAS) data indicating the presence of Si-O-Si linkages, and the Si-O-Mo linkages were consistent with the model. Two components in the O 1s region of XPS supported the presence of O in two kinds of chemical environments. Chen et al. also reported a SiO2-c(2 × 2) monolayer which exhibits an STM image consisting of isolated spots arranged in a c(2 × 2) symmetry with sample bias voltages from -2.5 to +4.0 V.13 Both highresolution electron energy loss spectroscopy (HREELS) and IRAS indicated that the monolayer contains Si-O-Mo linkages but not Si-O-Si linkages. The authors proposed a model which consists of isolated SiO4 units arranged in a c(2 × 2) symmetry. Such atomically ordered SiO2 films have not been found on the TiO2 surface. Barranco et al. performed XPS analysis of a SiO2 layer on TiO2(110) prepared by evaporation from SiO powder.14 An increase of the Si 2p binding energy by 1.3 eV and a decrease of the Si Auger parameter by 0.9 eV were observed with the increase of the amount of SiO2. By comparing with the results on a SiO2 layer on Al2O3(0001), perturbation of electronic state of Si at the SiO2/TiO2 interface was concluded. Abad et al. examined evaporation of Si onto TiO2(110)-(1 × 2) surfaces by using several techniques including Auger electron spectroscopy (AES) and STM.15 The Si LVV Auger peak appeared at 75 eV when the Si was less than 0.5 ML, which indicated the formation of SiO2 at room temperature. The positive shift of the Si peak with the increase of the Si coverage was attributed to the formation of less oxidized species SiOx (1 < x < 2). No ordered structure was observed for the SiOx layer by STM, and the subsequent annealing of the SiOx layer at 723 K induced aggregation of the SiOx.
10.1021/jp108380r 2010 American Chemical Society Published on Web 11/04/2010
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Figure 1. Ball-and-stick models of the TiO2(100)-(1 × 1) surface where the coordination of Ti and O atoms are represented. Small and large spheres represent Ti and O atoms, respectively. The O atoms are shaded according to their depth from the surface.
The (100) surface of rutile TiO2 used as a substrate here has been extensively examined in surface science studies.16 The (100) surface gives a nonreconstructed (1 × 1) structure by annealing at a temperature lower than 1000 K in ultrahigh vacuum or in 10-6 Torr of O2.17,18 Figure 1 shows models in side views of the nonreconstructed TiO2(100)-(1 × 1) surface including three TiO2 units. The surface is corrugated with [001]oriented ridges which consist of bridging O atoms and 5-fold coordinated Ti atoms. The unit cell of the surface is 0.30 nm ×0.46 nm, and the minimum step height expected from the crystal structure is 0.23 nm. When reduced by annealing in vacuum, the (100) surface gives a (1 × 3) structure.17,18 Experimental Methods TiO2(100) crystals (Shinko-sha) were placed in a quartz case and annealed at 1273 K in an electric furnace. The annealed crystals were cooled for 7 h to room temperature in the furnace and moved to the vacuum analysis or microscope systems. X-ray photoelectron spectroscopy analysis was conducted by using a commercial XPS system (Kratos, Axis Ultra DLD) with a monochromatic Al KR X-ray source. The base pressure of the system was 1 × 10-7 Pa. The photoelectron emission angle, θ, with respect to surface normal was set to 0°. The pass energies were 160 and 20 eV for wide and narrow scans, respectively. Surface charging of the insulating TiO2 crystals was reduced by using a neutralizer. The binding energy was referenced to the O 1s peak from TiO2 at 530.3 eV.19 Peak deconvolution of the narrow-scan spectra was performed using a mixed Gaussian-Lorentzian (70:30) function after Shirley-type background subtraction. Optics for LEED (Omicron, SPECTALEED) were installed in a homemade ultrahigh vacuum (UHV) chamber with a base pressure of 2 × 10-7 Pa. FM-AFM imaging was performed in water by using a multipurpose scanning probe microscope (Agilent, SPM 5500). Although the intermittent contact mode of the microscope with magnetic amplitude control was invoked, the cantilever oscillation was controlled by an FM detector (Nanosurf, easyPLL plus).20 A cantilever with a tip mounted at one end was oscillated
Figure 2. Wide-scan XPS spectra of an identical TiO2(100) surface from (i) an as-received surface and (ii) after annealing for 24 h.
with its resonant frequency (f0) and the frequency shift signal (∆f) from the detector was fed into the input of the microscope controller in place of the amplitude signal used in the intermittent contact mode. The resonant frequency and the quality factor of silicon cantilevers in pure water were ∼70 kHz and ∼70, respectively. The oscillation amplitude was estimated to be 4 nm peak-to-peak from the dependence of the tip-sample separation on the voltage applied to the piezoelectric oscillator. The images are presented without filtering, and the cross sections are measured from images smoothed by a nine-point median filter. Results and Discussion Spectrum (i) in Figure 2 shows a wide-scan XPS spectrum taken from the as-received TiO2(100) surface. The intense peaks at about 460 and 530 eV are Ti 2p and O 1s peaks, respectively. In addition to Ti and O, peaks for C, Na, Si, P, K, Ca, Fe, and Zn were detected. Carbon and trace amounts of Ca could not be removed by repetition of etching by HF solution, and such etching was not employed in the preparation of the surfaces reported here. The presence of carbon is attributed to organic contaminants from laboratory air whereas Ca is one of the possible intrinsic impurities in TiO2 crystals.21 Spectrum (ii) in Figure 2 shows a wide-scan of the surface following annealing at 1273 K for 24 h in air. The relative intensity of the Si 2p peak with respect to the O 1s peak increased, whereas the intensity of the C 1s peak was reduced. Such an increase of Si was not observed on a TiO2 sample annealed in an alumina case. Hence, we conclude that Si evaporates from the quartz case during annealing and covered the TiO2. Other impurities were below the detection limit, presumably due to diffusion into the TiO2 bulk during annealing or sublimation as oxides. Figure 3a-c shows narrow scans of Ti 2p, O 1s, and Si 2p regions on an identical TiO2 surface. In the Ti 2p region spectra
Analysis of Ultrathin SiO2 Films
Figure 3. Narrow-scan XPS spectra of (a) Ti 2p, (b) O 1s, and (c) Si 2p regions obtained on an identical TiO2(100) surface. (i) An as-received surface, (ii) after annealing for 24 h, (iii) after annealing for 48 h, and (iv) after annealing for 72 h. (d) Dependence of the Si/OTiO2 peak intensity ratio on the annealing time. (e) Si/O-Si peak intensity ratio on TiO2(100) surfaces with different Si/OTiO2 peak intensity ratio. (f) Dependence of Si/O-Si and Si/OTiO2 peak intensity ratios on photoelectron emission angle, θ, for a sample annealed for 216 h.
(a), two peaks were observed at 458.9 and 464.8 eV. These binding energies are typical of the Ti 2p3/2 and Ti 2p1/2 peaks, respectively.19 No appreciable change in binding energy and peak shape resulted from annealing. The spectrum of the O 1s region taken from the as-received surface, (i) in Figure 3b, shows an intense O 1s peak that arises from bulk TiO2. This peak has been calibrated to 530.3 eV and is designated as OTiO2. There is also a tail on the high binding energy side of this peak. Species contributing to the tail could be surface hydroxyl groups22 and oxides such as Na3PO4, SiO2, KPO4, CaO, Fe2O3, and ZnO.23 After annealing for 24 h, the tail reduced in width, appearing as a shoulder at around 532.2 eV, as shown in spectrum (ii). The shoulder became a distinct peak after annealing for 48 and 72 h as shown in spectra (iii)
J. Phys. Chem. C, Vol. 114, No. 47, 2010 20191 and (iv), respectively. The O 1s peak region was well fitted by the sum of three components: the OTiO2 peak, a peak at 532.2 eV, and a third component at about 531 eV. The third component can be assigned to hydroxyl groups on the surface.22 These three components are superimposed on spectrum (iv). Figure 3c shows narrow scans of the Si 2p region with the vertical scale normalized with respect to the height of the OTiO2 peak in each measurement. A broad peak centered at 102.1 eV was observed in spectrum (i) from the as-received surface. After annealing at 1273 K for 24 h, the binding energy of the peak shifted to 102.6 eV, and the peak height increased as shown in spectrum (ii). Further annealing led to an increase of the intensity and a slight positive shift in binding energy (to 102.8 eV) of the Si 2p peak as shown in spectra (iii) and (iv). The dependence of the Si/OTiO2 XPS peak intensity ratio on the annealing time determined on identical TiO2 surfaces is shown in Figure 3d. The Si/OTiO2 XPS peak intensity ratio increased with annealing time, and remained at 0.042 on surfaces annealed longer than 72 h. This indicates the self-limited growth of a Si-containing film. Binding energies of the O 1s peak at 532.2 eV and the Si 2p peak at 102.8 eV indicate the Si-O coordination. The energy difference between the O 1s and Si 2p peaks, 429.4 eV, are consistent with that of SiO2 in previous reports.24-26 The Ti-Si binary oxides gave O 1s peaks around 532.3-533.0 eV and Si 2p peaks at 101.9-103.6 eV.27 The O giving rise to the peak at 532.2 eV is designated hereafter as O-Si. The binding energy of the O-Si peak was higher than that of the OTiO2 peak by 1.9 eV. The positive shift was smaller than the difference in O 1s binding energy between quartz built from tetrahedral SiO4 units and rutile TiO2, 2.9 eV.24 The disagreement of the O 1s binding energy indicates that the O-Si atoms are in a different chemical state from the O atoms in the amorphous SiO2. The O 1s peak energy from the rutile-type SiO2, in which O atoms are in 3-fold coordination, is lower than that from quartz in which O atoms are in 2-fold coordination by 0.8 eV.25 Hence, the coordination number of the O-Si atoms is probably larger than two. Figure 3e shows the Si/O-Si XPS peak intensity ratio on surfaces with different Si/OTiO2 peak intensity ratios obtained by a repetition of the annealing. The Si/O-Si peak intensity ratio remained equivalent (at 0.15) on surfaces with a Si/OTiO2 peak intensity ratio larger than 0.013. This suggests that above a certain Si coverage an oxide with an identical Si/O atom ratio forms. Using the factory-provided relative sensitivity factors of 0.78 for O 1s and 0.33 for Si 2p, the Si/O-Si atom ratio is estimated to be 0.35. The XPS measurements can be made more surface sensitive by increasing the photoemisssion angle, θ. Figure 3f shows the dependence of the Si/OTiO2 and Si/O-Si peak intensity ratios on θ, obtained from a TiO2 surface annealed for 216 h. While the Si/OTiO2 peak intensity ratio increased with θ, the Si/O-Si peak intensity ratio was independent of θ. This indicates that the Si and O-Si atoms are concentrated at the surface layers and that the Si/O-Si atom ratio is constant within the escape depth of the O 1s photoelectron with θ of 60°. In order to estimate the amount of Si present, a continuous SiO2 film is assumed to grow on the TiO2 substrate. The thickness of the SiO2 film d is obtained by the following equation:
SSi{1 - exp(-d/λSi)} ISi ) IO SO exp(-d/λO)
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Figure 4. LEED patterns of (a) the as-received TiO2(100) surface and (b) the TiO2(100) surface annealed at 1273 K. Incident electron energy was 130 eV. Dotted rectangles indicate the unit cell of the TiO2(100)(1 × 1) surface.
ISi and IO are the peak intensities of Si and OTiO2. The ISi/IO intensity ratio was 0.042 on the (3 × 4) surface as shown in Figure 3d. λSi and λO are the attenuation lengths of electrons of Si 2p and O 1s photoelectrons in SiO2 and are 3.5 and 2.6 nm, respectively.28 SSi and SO are the relative sensitivity factors of Si 2p and O 1s, respectively. d is calculated to be 0.32 nm. Figure 4a shows the LEED pattern of the as-received TiO2(100) surface. A diffuse (1 × 1) LEED pattern was observed with the high background reflecting disordered regions as well as the insulating character of the TiO2. Figure 4b shows the (3 × 4) LEED pattern observed on the surface annealed at 1273 K for 48 h. The sharp (3 × 4) pattern is indicative of long-range ordering of the near-surface lattice. This (3 × 4) pattern appeared when the Si/OTiO2 peak intensity ratio became greater than 0.023. The (3 × 4) pattern was still observable after exposing the surface to laboratory air for one week, suggesting that the atom arrangement of the (3 × 4) surface is insensitive to the adsorption of air components. Previous scanning tunneling microscope (STM) measurements showed that the (1 × 1) structure of a TiO2(110) surface prepared in UHV was maintained following 10 min exposure to laboratory air.29 Figure 5a shows an FM-AFM image of the as-received TiO2(100) surface and the cross sections along the lines in the image. The surface consists of uneven terraces separated by winding steps with a height of about 0.1 nm. The steps extend in the [010] direction on the surface. The direction of the steps and the step-to-step distance varied from sample to sample, possibly reflecting the polishing. Figure 5b shows an FM-AFM image and the cross sections of the (3 × 4) surface. The surface consists of flat terraces separated by straight steps with heights of 0.2 nm. Figure 5c shows a closeup image of a terrace from the (3 × 4) surface. Rows running along the [001] direction were observed. Bright rows coexisted with partly resolved darker rows. The bright rows had an interval of 1.80 nm, about four times the unit cell of the TiO2(100)-(1 × 1) surface in the [010] direction, 1.84 nm. The intervals between the darker rows were approximately 0.45 nm, which is about the periodicity of the unit cell of the TiO2(100)-(1 × 1) surface in the [010] direction, 0.46 nm. Structures giving 3-fold periodicity in the [001] direction could not be identified in the images. The FM-AFM topography shows either the atom arrangement of the annealed TiO2 surface or H2O-derived species adsorbed on the TiO2 surface. When the TiO2(110) surface was observed by FM-AFM in water, spots with diameters of ∼1 nm were observed as well as the O atom rows of the TiO2 top layer.20 The spots were assigned to the H2O clusters. The subnanometer separation of the rows in the [010]
Figure 5. Constant frequency shift topography images of the TiO2(100) surfaces and cross sections along the solid lines in the images. (a) Asreceived surface (1000 × 1000 nm2). Frequency shift (∆f) ) +284 Hz, peak-to-peak amplitude of the cantilever oscillation (Ap-p) ) 4 nm. (b) (3 × 4) surface (1000 × 1000 nm2). ∆f ) +102 Hz, Ap-p ) 4 nm. (c) (3 × 4) surface (5 × 5 nm2). ∆f ) +838 Hz, Ap-p ) 4 nm. Two bright rows arranged with an interval of 1.80 nm were indicated by the arrowheads.
direction in Figure 5c is too small to be assigned to the arrangement of the H2O clusters. It is likely that the rows correspond to the surface atoms or the smaller H2O-derived species such as the OH group. The irregular contrast in the rows and bright spots on the rows suggest contribution of surface OH groups. FM-AFM observation in UHV may reveal the effect of the OH groups on the contrast. We now consider the atomic scale structure of the (3 × 4) surface. The (3 × 4) structure indicates that the oxide film grows commensurately with the bulk structure of the TiO2. Hence, we assume a rutile structure for the oxide film. The binding energy of the O-Si peak supports the rutile structure in which the O atoms are coordinated to three Si atoms. The surface of the silicon oxide film should be terminated in the same way as the TiO2(100)-(1 × 1) surface (Figure 1) thereby meeting the rules of charge-neutral cleavage and creating a nonpolar surfaces.30 The Si-O bond length in rutile SiO2 is 0.18 nm31 and is 10% smaller than the Ti-O bond length in rutile TiO2.16 The periodicity of the Si layers in the [100] direction is 0.21 nm in rutile SiO2, which coincides with the step height in Figure 5b. In XPS film thickness measurements, the error can reach several 10% due to uncertainties in the parameters used in the calculation. Allowing an error margin of (20% for the calculated d obtained by assuming a pure SiO2 layer, we consider the top two cation layers of the oxide film grown on the bulk TiO2.
Analysis of Ultrathin SiO2 Films
J. Phys. Chem. C, Vol. 114, No. 47, 2010 20193 atom ratio would reduce to 0.30 by such an exchange. Thus models (b), (c), or some combination of the two seems most appropriate. The arrangement of Ti and Si atoms in the second Ti1-xSixO2 unit in models (b) and (c) would give rise to the ×3 periodicity in the [001] direction. The Si-O bond is shorter than the Ti-O bond, and therefore the Si atoms in the second Ti1-xSixO2 unit are relaxed downward compared to the Ti atoms. The periodic relaxation in the Ti1-xSixO2 unit induces the periodic relaxation in the SiO2 unit above. The FM-AFM image indicates that the height of the bridging O atom row maximizes every four rows. This may be due to a relaxation of the bridging O atom rows periodically in the [010] direction in order to minimize the lateral stress arising at the Ti1-xSixO2/TiO2 interface. The limited growth of SiO2 film is probably due to the mismatch between the layers. Conclusion
Figure 6. Ball-and-stick models of the oxide layer with a rutile structure. Filled and open small circles represent Ti and Si atoms, respectively. Solid, filled, large circles represent O atoms coordinated to Si atom, and dotted, open, large circles represent those not coordinated to Si atom.
Figure 6 shows ball-and-stick models of the oxide film. Filled and open small circles represent Ti and Si atoms, respectively. Solid, filled, large circles and dotted, open, large circles represent O atoms coordinated to Si (O-Si), and those not coordinated to Si, respectively. Model (a) shows an oxide film consisting of two SiO2 units supported on the TiO2 surface. The repetitive growth of the SiO2 unit on the TiO2 surface is expected to be difficult due to the lattice mismatch of 10%. The Si/O-Si atom ratio is 0.4 which is 14% larger than that estimated from the Si/O-Si XPS peak intensity ratio of 0.35. Hence, model (a) is unlikely. In model (b), the topmost oxide unit is SiO2, and the second oxide unit is Ti0.33Si0.67O2. Considering the ×3 periodicity along the [001] direction, Ti atoms are arranged to fill every third cation site in the Ti0.33Si0.67O2 unit. The Si/O-Si atom ratio is 0.36 and matches the Si/O-Si XPS peak intensity ratio. This Ti0.33Si0.67O2 unit which includes both Ti-O and Si-O bonds is likely to reduce the mismatch between the topmost SiO2 unit and the bulk TiO2. Exchanging the first and the second oxide units in model (b) gives the number ratio of Si atoms to O-Si atoms of 0.33. However, like in model (a), the expectation is that such a film would be unfavorable due to the lattice mismatch between the SiO2 units and TiO2. Model (c) consists of the SiO2 unit on top of a Ti0.67Si0.33O2 unit. In this case, the ×3 periodicity along the [001] direction is accommodated by placing Si in every third cation site. The Si/O-Si atom ratio is 0.33, matching to the Si/O-Si XPS peak intensity ratio. Again, exchange of the SiO2 unit and the Ti0.67Si0.33O2 unit seems unlikely on the basis of the lattice mismatch between SiO2 and TiO2. Furthermore, the Si/O-Si
An atomic scale analysis using XPS, LEED, and FM-AFM was performed on SiO2 vapor deposited TiO2(100) surfaces. Models based on rutile SiO2 units supported on the TiO2(100) surface via Ti1-xSixO2 units are consistent with the obtained results. The study shows the potential for atomic scale understanding of oxide surfaces prepared under atmospheric pressure and high-temperature conditions by ex situ analysis using surface science techniques. The simple method of preparing oxide films using a case as a source of evaporating material will promote surface science studies of binary oxides. Acknowledgment. This work was supported by Grant-inAid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) and the Japan Society for the Promotion of Science (JSPS). C.L.P.’s visit to JAIST was supported by a JSPS BRIDGE Fellowship. References and Notes (1) Gao, X.; Wachs, I. E. Catal. Today 1999, 51, 233–254. (2) Gao, X.; Bare, S. R.; Fierro, J. L. G.; Banares, M. A.; Wachs, I. E. J. Phys. Chem. B 1998, 102, 5653–5666. (3) Liu, Z.; Crumbaugh, G. M.; Davis, R. J. J. Catal. 1996, 159, 83– 89. (4) Imamura, S.; Nakai, T.; Kanai, H.; Ito, T. J. Chem. Soc. Faraday Trans. 1995, 91, 1261–1266. (5) Ito, M.; Hattori, H.; Tanabe, K. J. Catal. 1974, 35, 225–231. (6) Anderson, C.; Bard, A. J. J. Phys. Chem. B 1997, 101, 2611–2616. (7) Anderson, C.; Bard, A. J. J. Phys. Chem. 1995, 99, 9882–9885. (8) Dagan, G.; Sampath, S.; Lev, O. Chem. Mater. 1995, 7, 446–453. (9) Inoue, H.; Matsuyama, T.; Liu, B.-J.; Sakata, T.; Mori, H.; Yoneyama, H. Chem. Lett. 1994, 23, 653–656. (10) Notari, B.; Willey, R. J.; Panizza, M.; Busca, G. Catal. Today 2006, 116, 99–110. (11) Gates, B. C., Kno¨zinger, H., Eds.; Impact of Surface Science on Catalysis; Academic Press: San Diego, CA, 2000. (12) Weissenrieder, J.; Kaya, S.; Lu, J.-L.; Gao, H.-J.; Shaikhutdinov, S.; Freund, H.-J.; Sierka, M.; Todorova, T. K.; Sauer, J. Phys. ReV. Lett. 2005, 95, 076103. 1-4. (13) Chen, M.; Goodman, D. W. Surf. Sci. 2006, 600, L255–L259. (14) Barranco, A.; Yubero, F.; Mejı´as, J. A.; Espino´s, J. P.; Gonza´lezElipe, A. R. Surf. Sci. 2001, 482-485, 680–686. (15) Abad, J.; Rogero, C.; Me´ndez, J.; Lo´pez, M. F.; Martı´n-Gago, J. A.; Roma´n, E. Surf. Sci. 2006, 600, 2696–2704. (16) Diebold, U. Surf. Sci. Rep. 2003, 48, 53–229. (17) Raza, H.; Pang, C. L.; Haycock, S. A.; Thornton, G. Phys. ReV. Lett. 1999, 82, 5265–5268. (18) Henderson, M. Surf. Sci. 1994, 319, 315–328. (19) McCafferty, E.; Wightman, J. P. Appl. Surf. Sci. 1999, 143, 92– 100. (20) Sasahara, A.; Tomitori, M. J. Vac. Sci. Technol. B 2010, 28, C4C5– C4C10. (21) Zhang, L. P.; Li, M.; Diebold, U. Surf. Sci. 1998, 412/413, 242– 251.
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