Electronic States of Chemisorbed Oxygen Species and Their Mutually

Apr 19, 2001 - Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai, Kizu-cho, Soraku-gun, Kyoto 619-0292 Japan, Nara Ins...
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J. Phys. Chem. B 2001, 105, 4239-4244

4239

Electronic States of Chemisorbed Oxygen Species and Their Mutually Related Studies on SnO2 Thin Film Takashi Kawabe,† Kenji Tabata,*,†,‡ Eiji Suzuki,†,‡ Yoichi Yamaguchi,§ and Yousuke Nagasawa| Research Institute of InnoVatiVe Technology for the Earth (RITE), 9-2 Kizugawadai, Kizu-cho, Soraku-gun, Kyoto 619-0292 Japan, Nara Institute of Science and Technology, Graduate School of Materials Science (NAIST), 8916-5 Takayama-cho, Ikoma, Nara 630-0101 Japan, Kansai Research Institute, Computational Science Laboratory, Information Communication Research Center, Kyoto Research Park 17, Chudoji Minami-machi, Shimogyo-ku, Kyoto, 600-8813 Japan, and Osaka Gas Co. Ltd. Utilization Technology DeVelopment, Gas Appliance R&D Team, 1-1-3 Hokkou-Shiratsu, Konohana-ku, Osaka 554-0041 Japan ReceiVed: September 13, 2000; In Final Form: December 28, 2000

Not only the clarification of chemisorbed oxygen species on oxides but also the variation of electronic states of these oxygen species under reaction condition are quite important to both science and applications. We proved the presence of four kinds of chemisorbed oxygen species (Ob, O22-, O-, O2-) on the topmost layer of SnO2 thin film with X-ray photoelectron spectroscopy (XPS). Ob was assigned to be O2- ion at the bridging site on the topmost layer of SnO2 (110) surface without oxygen vacancies around it by comparison between the binding energy values of the peak in XPS spectra and that obtained from the convoluted partial electronic density of states which were calculated using the density functional theory (DFT). The intensity variation of Ob during heat treatment up to 773 K with evacuating was weakly and inversely related to that of O22- and O-. The proportion of Ob and O- decreased after CH4 exposure at 473 K. We considered that highly reactive oxygen species, O- reacted with CH4 at 473 K, then oxygen vacancies at the topmost bridging site were produced. Some of Ob species coupled with these vacancies, and the electronic states of Ob modified to Ofrom O2-. This newly produced O- reacted with CH4 at 473 K.

1. Introduction The electronic states of chemisorbed oxygen species on oxides are quite important as basic science and applications. Many studies of chemisorbed oxygen species on oxides at a specialized condition have been carried out using many kinds of surfacesensitive analyzing apparatus.1 SnO2 has been developed widely as the gas sensing device and the catalyst for the oxidation of hydrocarbons.2-4 The presence of different types of chemisorbed oxygen species over SnO2 has been suggested.5,6 Nagasawa et al. reported that three different types of surface oxygen species (O-, O22-, O2-) were observed after O2 exposure on the surface of reduced SnO2 film by XPS measurements, and these oxygen species were thermally desorbed in the order of O-, O22-, and O2-.7 To investigate the reactivity of chemisorbed oxygen species, we recently studied the interaction of oxygen with methane on the surface of SnO2 thin film.8 In that paper, we reported that O- species was observed in the O (1s) level spectra after O2 exposure at 473 K, which was highly reactive enough to produce methoxide and methanol from methane at even room temperature. Very recently, Yamaguchi et al. theoretically studied the interaction of oxygen with the reduced SnO2 (110) surface using the density functional theory (DFT).9 In that paper, they predicted the presence of two kinds of chemisorbed oxygen species (O22-, O2-) on the surface, and also predicted that * Author to whom all correspondence should be addressed. Tel. +81774-75-2305. Fax: +81-774-75-2318. E-mail: [email protected]. † Research Institute of Innovative Technology for the Earth (RITE). ‡ Nara Institute of Science and Technology, Graduate School of Materials. § Kansai Research Institute, Computational Science Laboratory, Information Communication Research Center. | Osaka Gas Co. Ltd. Utilization Technology Development.

atomically adsorbed oxygen species coupled with the bridging oxygen vacant site could be modified to O-. This result suggested that highly oxidized defect-free SnO2 thin film had only a few O- species because of a smaller number of the oxygen vacant sites on the surface. There have been few attempts to study the relations between the presence of oxygen vacancies at the topmost bridging site and that of a highly reactive O- species. Furthermore, mutual relations of electronic states between chemisorbed oxygen species under various reaction conditions are still unclear. In this paper, we systematically examine the relations between the amount of bridging oxygen vacancies and the electronic states of chemisorbed oxygen species over SnO2 thin film. The amount of vacancies at the bridging oxygen site should be controlled by O2 exposure pressure on the reduced SnO2 thin film and also by heat treatment with evacuating in a XPS chamber. The chemisorbed oxygen species measured with XPS are assigned with the theoretical calculations with the DFT calculation procedures with a three-layered cluster model (Sn31O50). 2. Experimental Section SnO2 thin film was deposited on a sapphire substrate by the reactive RF magnetron sputtering method using an SPF-430 (Nichiden Anelva Co. Ltd.). The sputtering target was sintered SnO2 (99.9% purity). We selected the sputtering condition for preparing a dense texture film. Details of the sputtering condition were written in the previous paper.10 XPS measurements were performed with an angle-resolved ESCA-KM spectrometer (Shimadzu Corporation), which was equipped with a concentric hemispherical analyzer. An Al KR (1486.6 eV) X-ray source

10.1021/jp003234d CCC: $20.00 © 2001 American Chemical Society Published on Web 04/19/2001

4240 J. Phys. Chem. B, Vol. 105, No. 19, 2001 was used for the excitation. O2 exposure was carried out in the second chamber of the XPS instrument. O2 gas (>99.8% purity) for oxidization was obtained from Sumitomo Seika Chemicals Co. Ltd. We used it after purification by passing through a liquid nitrogen trap. Argon ion etching was carried out for 5 min at 2 keV (Ar dose: 3.58 × 1019 cm-2) in order to clean the surface. Heat treatments during O2 exposure was carried out with a halogen lamp, which was located over the sample in the second chamber of the XPS instrument. The estimated measurement error was within (2 K. CH4 exposure was carried out in the third chamber of the XPS instrument. Methane gas (>99.9995% purity) was obtained from Teisan Co. Ltd. The electric resistive heater was built in the sample holder of the third preparation chamber and analyzing one. Heat treatments during CH4 exposure in the preparation chamber and during XPS measurement in the analyzing chamber were performed with each electric resistive heater. The estimated measurement error of temperatures was within (10 K. The binding energy (EB) was calibrated with the Sn4+ (3d5/2) peak as 486.7 eV.11,12 Photoelectron spectroscopy was detected at an angle of 15° to the surface of the sample so as to obtain a spectrum from the shallow region of a sample. Treatment of acquired XPS spectra was performed with the software “Vision” which was produced by Kratos Analytical. 3. DFT Calculation Method The theoretical calculation procedure was carried out with a plane-wave pseudopotential formation within the density functional theory (DFT), using the DMol3 program package provided by the Molecular Simulations Inc. (MSI).13 All electrons were treated in the geometrical optimization for the monolayered reduced SnO2 (Sn10O15) cluster with oxygen within the generalized gradient approximation (GGA) level of theory using BeckeLee-Yang-Parr nonlocal type functional (BLYP) and the relative corrections.14,15 The geometries of all the present SnO2 clusters were fixed on an SnO2 crystal.1 On the basis of the optimized geometry, single-point calculation of the GGA energy for the three-layered reduced SnO2 (Sn31O50) cluster with oxygen was conducted using effective core potential (ECP) and BLYP level of theory. A Fermi surface thermal smearing of 0.54 eV was used. The size of the used basis set was the double numerical plus d-functions (DND). We used three-layered reduced cluster, Sn31O50 with D2h symmetry. This cluster was electrically neutral. The stoichiometric cluster was formed by the addition of bridging oxygen atoms onto each reduced cluster. The Madelung potential was not applied to the present calculations, although we have checked that by the Coulomb interaction from the outside. It was significantly weakened by the screening effect. A Silicon Graphics Origin 2000 R10000 workstation was used for the present calculations. 4. Results and Discussion Oxygen Exposure over an Ar-Ion-Etched SnO2 Thin Film. The surface states of SnO2 thin film after Ar ion bombardment were measured at 473 K by using XPS. No other peaks except for those derived from Sn and O were observed in the spectrum. Figure 1 shows the obtained valence band (VB) spectra for the etched sample and for O2 exposed sample at 473 K from 6.75 × 105 L (Langmuir) up to 6.75 × 109 L. Figure 1a shows the spectrum detected after ion bombardment. A shoulder peak at ca. 2-3 eV was observed clearly in the figure, which was derived from the rehybridization of Sn (5s)-Sn (5p).16-18 This characteristic feature in this region was in good accordance with the shape of VB spectrum reported for SnO,16,17 therefore, we

Kawabe et al.

Figure 1. Valence band region XPS spectra obtained after ion bombardment (a), after O2 exposure at 473 K at 6.75 × 105 L (b), 6.75 × 107 L (c), and 6.75 × 109 L (d), respectively.

assumed that the surface of the sample was reduced during the etching. Figure 1 b,c,d shows the VB spectra after O2 exposure at 6.75 × 105 L (b), 6.75 × 107 L (c), and 6.75 × 109 L (d), respectively. All the O2 exposures were carried out at 473 K. Although the value of charge shift for the etched sample was 0 eV, those in Figures 1b-d were 0.1, 0.2, and 1.7 eV, respectively. The shoulder peak at ca. 2-3 eV gradually disappeared with O2 exposure from 6.75 × 105 L up to 6.75 × 109 L. The shape of VB level spectrum at ca. EB ) 6-8 eV in Figure 1d was lowered by the suppression of Sn (5s)-O (2p) interactions with the O2 exposure.17 We assumed that the surface was reoxidized after O2 exposure at 473 K. The protuberant part at around 0-2 eV in Figure 1d could be derived from the accumulation in the surface oxygen concentration. The intensity of the broad peak at ca. 10-12 eV varied through these oxygen treatments, therefore, we assumed that these variations of the broad peak reflected the variations of chemisorbed oxygen species. It was reported that chemisorbed O- had the peaks at EB ) 10 and 12 eV, and O22- had those at EB ) 7.5 and 11.7 eV in the UPS spectra.19,20 We assumed that O- and O22- ions coexisted on Sn sites after O2 exposure at 473 K at 6.75 × 109 L. Figure 2a-d shows the Sn (3d) core level spectra which were obtained after performing the same treatments in Figure 1a-d. In Figure 1, we suggested the presence of SnO after ion bombardment (Figure 1a), and it was oxidized by O2 exposure at 473 K. We examined the variations of the full width at halfmaximum (fwhm) values of Sn (3d5/2) for these spectra. All the values of fwhm were 1.9 eV in Figure 2. The observed spinorbit splitting values of Sn (3d) were 8.4-8.5 eV. Since the observed values of both fwhm and spin-orbit splitting in the core Sn (3d) level spectra hardly changed in Figure 2a-d, we could not indicate the variation of Sn state through these treatments. Figure 3a-d show the O (1s) core level spectra which were obtained after ion bombardment (a), after O2 exposure at 473 K at 6.75 × 105 L (b), 6.75 × 107 L (c), and 6.75 × 109 L (c), respectively. All the spectra had larger values than 2.0 eV as fwhm. To indicate the presented species of chemisorbed oxygen

Electronic States of Chemisorbed Oxygen Species

J. Phys. Chem. B, Vol. 105, No. 19, 2001 4241

Figure 2. Sn (3d) core level XPS spectra obtained after ion bombardment (a), after O2 exposure at 473 K at 6.75 × 105 L (b), 6.75 × 107 L (c), and 6.75 × 109 L (d), respectively.

TABLE 1: Peak Position and Proportion of Resolved Oxygen Species Obtained after Ion Bombardment (a), after O2 Exposure at 473 K at 6.75 × 105 L (b), at 6.75 × 107 L (c) and 6.75 × 109 L (d), Respectively O2O22O2OOb species (530.5 eV) (532.7 eV) (534.5 eV) (531.5 eV) (528-529 eV) (a) (b) (c) (d)

95% 81% 78% 51%

4% 7% 7% 8%

1% 1% 1% 1%

5% 4% 6%

6% 10% 34%

on the SnO2 thin film, we resolved the spectra into several peaks using the software as to match the original one. Every resolved peak position and its intensity proportion were listed in Table 1. All the fwhm values of resolved peaks were included in the region of 1.5-2.2 eV. Concerning Figure 3a, three peaks were needed to match the original one. The peak at EB ) 530.5 eV was assigned to the lattice oxygen of SnO2 crystal (O2-). We assumed the resolved peak at EB ) 532.7 eV was the O22species because the peak positions of molecularly chemisorbed O22- species on nickel and copper were reported as EB ) 532.7 eV and around 533 eV, respectively.20,21 Another resolved peak at EB ) 534.5 eV could be assigned to the O2- species because Qiu et al. reported the peak position of superoxo, O2-, was at ca. 534.5 eV from their experiments on Li-O2 complexes.22 We also resolved the spectra in Figure 3b-d with the same manner. Five resolved peaks were needed to match with the original spectra for Figure 3b,c and six resolved peaks were needed for Figure 3 d. The position of every resolved peak was EB ) 530.5, 532.7, 534.5, 531.5, and ca. 529 eV. Two newly appeared peaks at EB ) 531.5 and ca. 529 eV were observed after O2 exposure at 6.75 × 105 L, (b) and 6.75 × 107 L, (c). Furthermore, a resolved peak at EB ) ca. 528 eV appeared clearly after O2 exposure at 6.75 × 109 L in Figure 3d. The total percentage of intensity of the peak, at ca. 528-529 eV increased clearly with the rise of exposed oxygen Langmuir (Table 1). Since the peak position of atomically adsorbed Oon nickel was reported to be at EB ) 531.0-531.5 eV,23-25 and that of hydroxyl ion (OH-) was reported at ∼532.0 eV,11,26 we considered two oxygen species, i.e., O- and OH- were included in the resolved peak at 531.5 eV. However, since the resolved peak at 531.5 eV was able to be seen in the spectrum after O2 exposure at 473 K, the contribution from OH- ions to

Figure 3. O (1s) core level XPS spectra obtained after ion bombardment (a), after O2 exposure at 473 K at 6.75 × 105 L (b), 6.75 × 107 L (c), and 6.75 × 109 L (d), respectively.

this peak could be small. Therefore, the new peak at 531.5 eV was mainly assignable to the atomically chemisorbed Ospecies. The presence of resolved peaks at 528-529 eV after O2 exposure on SnO2 thin film with a higher pressure in Figure 3c,d had not been reported. The peak position of the oxygen species (Ob) was lower than that of the lattice oxygen, O2- of SnO2 (530.5 eV). It has been well-known that EB of the O (1s) level of O2- were strongly affected by the electron negativities of metal ions.27 Generally, the peak positions of O (1s) level of ionically bonded oxygen species usually shift to a lower binding energy side.27 The oxygen species (Ob) therefore appeared at 528-529 eV and was assumed to be ionically chemisorbed one on the topmost Sn ions. The peak position of this oxygen species for the O (1s) level shifted gradually to a lower binding energy side with a higher exposure pressure. We suspected that this oxygen species (Ob) at ca. 529.0 eV could be the bridging oxygen at the topmost layer of SnO2 (110) surface in the absence of oxygen vacancies around this chemisorbed oxygen. The proportion of Ob which was a total amount of the peaks at 528529 eV in the O (1s) level spectra increased with a higher pressure; however, those of O22-, O- hardly changed through the variation of O2 pressure. We performed theoretical calculations with the density functional theories (DFT) so as to indicate precisely the states of Ob. Theoretical Calculation of SnO2 (110) Cluster Model. Figure 4 shows the schematic structure of the three-layered cluster model of the stoichiometric SnO2 (110) (TYPE-1). We used this model as a basis of theoretical calculations. We calculated the total electronic density of state (TDOS) of the O (1s) core level and the partial DOS (PDOS) of each oxygen ion from O1 to O6 in Figure 4. O1 and O2 were referred to as bridging oxygen. There was a plane containing both tin and oxygen ions below the bridging oxygen ions. Oxygen ions in this plane (O3) were called in-plane oxygen. The oxygen below this plane, lying vertically beneath the bridging oxygen (O4)

4242 J. Phys. Chem. B, Vol. 105, No. 19, 2001

Kawabe et al. TABLE 2: Calculated Peak Position of O (1s) Level with a sToichiometric Cluster Model (TYPE-1), the First Reduced TYPE-2 Model (O1 ions were removed), and the Second reduced TYPE-3 model (O2 ions were removed) species

O1(eV)

O2(eV)

O3(eV)

O4(eV)

O5(eV)

O6(eV)

TYPE-1 TYPE-2 TYPE-3

509.95

510.01 510.06

511.07 511.35 511.19

510.75 511.35 510.66

511.65 511.57 511.38

510.92 510.95 510.76

510.10

Figure 6. The proportion of oxygen species during O2 exposure. (a) after ion bombardment, (b) after O2 exposure at 473 K at 6.75 × 105 L, (c) 6.75 × 107 L, and (d) 6.75 × 109 L , respectively. Figure 4. A schematic structure of the three-layered cluster model of stoichiometric SnO2 (110) (TYPE-1).

Figure 5. DOS of O (1s) core level of three-layered cluster model.

was referred to sub-bridging oxygen. The oxygen ions notified as O5 and O6 were included in the second layer of the cluster model in Figure 4. The convoluted TDOS and PDOS of the core O (1s) level notified as O1-O6 are shown in Figure 5. The predicted peak positions from the convoluted PDOS of the O (1s) level of oxygen ions at from O1 to O6 are shown in Table 2. The predicted peak positions of bridging oxygen species at O1 and O2 sites of the TYPE-1 cluster model were 509.95 and 510.01 eV, respectively (Figure 5). The predicted peak positions of the convoluted PDOS of the oxygen species from O3 to O6 for the TYPE-1 model are also shown in Figure 5.

Oxygen ions at the second layer (O5, O6) were assumed to be lattice oxygen. We normalized the average value (511.3 eV) of these predicted positions of O5 and O6 with the experimentally observed lattice oxygen, O2- at 530.5 eV. The converted peak position of the convoluted PDOS of O1 and O2 by this procedure was assignable to the bridging oxygen at the topmost layer of SnO2 (110). The calculated values of Mulliken net charge of O1 and O2 were -0.746 and -0.724 for the TYPE-1 cluster model. Although these values were slightly smaller than those of O5 (-0.836) and O6 (-0.892), we considered the formal charge of this species was O2-. In the previous paper, we reported that an adsorbed oxygen atom coupled with the nearest-neighboring bridging oxygen vacant site was O- species that displayed catalytic activity on the surface.9 We prepared partly reduced cluster model by removing two (TYPE-2) and one (TYPE-3) oxygen ions from the O2 site and the O1 site of the TYPE-1 cluster model so as to indicate the predicted peak position of O- species. The predicted peak positions of the convoluted PDOS of both TYPE-2 and TYPE-3 models were listed (Table 2). The predicted peak positions of the convoluted PDOS of O1 and O2 in these reduced models were hardly changed in comparison to those with the original TYPE-1 model. The net charge values of oxygen species on the topmost layer, i.e., O1 and O2 of the reduced model slightly changed from those in the TYPE-1 model. We assumed that these unexpected results of the peak positions of O1 and O2 in the reduced cluster models were derived from the algorithm used in the calculation procedures. The DOS of each atom was optimized as to minimize the total potential energy level of the cluster model, therefore, the DOS of the O (1s) core level of O- which was accompanied by oxygen vacancies around the nearest neighboring site could not be fixed precisely in our calculations. The O (1s) level spectra at ca. 528-529 eV in Figure 3 was assigned to the bridging oxygen ions at the topmost layer of SnO2 (110) surface. We considered that the lowering of binding energy of the resolved peak (Ob) from ca. 529 eV to ca. 528 eV through the change of O2 exposure from 6.75 × 107 L to 6.75 × 109 L was brought from a smaller number of oxygen vacancies on the surface. However, the proportion of O22- and O- slightly increased after the same exposure. Therefore the variation of percentage of Ob and those of both O22- and Odid not show any inversely relations (Figure 6).

Electronic States of Chemisorbed Oxygen Species

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TABLE 3: Peak Position and Proportion of Resolved Carbonaceous Species Obtained after CH4 Exposure at 473 K at 6.75 × 105 L onto the Preoxidized SnO2 Thin Film at 6.75 × 109 L species ratio

CH3-/CH4(284.8 eV) 76%

CH3O-(286.2 eV) 8%

Figure 7. O (1s) core level XPS spectra obtained after O2 exposure at 6.75 × 109 L at 473 K (a), after heat treatment with evacuating for 1h at 573 K (b), for 1h at 773 K (c), and for 20 h at 773 K (d), respectively.

H2COO2-(288.9 eV) 12%

CH3OH(287.7 eV) 4%

Figure 9. O (1s) core level XPS spectra obtained after O2 exposure at 6.75 × 109 L at 473 K (a), and after CH4 exposure at 6.75 × 105 L at 473 K (b) after the same O2 exposure with (a).

TABLE 4: Peak Position and Proportion of Resolved Oxygen Species Obtained after O2 Exposure at 473 K at 6.75 × 109 L (a) and after Further CH4 Exposure at 473 K at 6.75 × 105 L onto the Preoxidized SnO2 Thin Film (b), Respectively O2O22O2OOb species (530.5 eV) (532.7 eV) (534.5 eV) (531.5 eV) (528-529 eV) (a) (b)

Figure 8. The proportion of oxygen species during heat treatment with evacuating after O2 exposure at 6.75 × 109 L at 473 K (a), after heat treatment with evacuating for 1 h at 573 K (b), and heat treatment with evacuating for 1 h at 773 K (c), and for 20 h at 773 K (d), respectively.

Heat Treatment Evacuating for Preoxidized SnO2 Thin Film. In Figure 3, we exposed oxygen on the ion-etched SnO2 thin film. O22-, O-, and Ob species were observed in the O (1s) level spectrum as the main chemisorbed oxygen species. We performed heat treatment with evacuating onto the preoxidized SnO2 at 6.75 × 109 L from 573 to 773 K in order to indicate the relations between Ob and O22- and O- (Figures 7 and 8). We expected that the produced vacant site on the topmost layer through heat treatment could be favorable to produce Oand/or aggregated O22- as expected from our previous report.9 The proportion of Ob decreased with the rise of temperature and with a longer time treatment (Figure 8), therefore the increase of the vacant sites on the topmost layer after heat treatment was expected. Meaningful increase of proportions of O22- and O- was observed after heat treatment with evacuating at 773 K for 20 h in comparison to those after heat treatment at 573 K for 1 h. Some percentages of chemisorbed oxygen species may be desorbed from the surface through heat treatment as reported in our previous paper.7 Even so, the increase of O22-

47% 72%

7% 7%

1% 2%

10%

35% 19%

and O- were observed after heat treatment at 773 K for 20 h. This increase of O22- and O- proportions supported our previous predictions, i.e., Ob at the bridging site on the topmost layer coupled with the nearest neighboring vacancy, and modified to O-. Modifications of Electronic States of Chemisorbed Oxygen Species on SnO2 Thin Film during CH4 Exposure at 473 K. We assured the presence of Ob, O22-, O2-, and O- species on the surface of the SnO2 thin film after O2 exposure (Table 1). In these oxygen species, a quite high reactivity of O- species in the oxidation reactions has been reported.5 We performed CH4 exposure at 473 K onto the preoxidized SnO2 thin film at 6.75 × 109 L so as to discuss the variation of observed oxygen species during the oxidation reaction. CH4 exposure was carried out at 6.75 × 105 L. Table 3 shows the proportion of each oxidized species after CH4 exposure at 473 K. The assignment of each oxidized species in the C (1s) level was carried out with the same manner in our previous paper.28 CH4 was clearly oxidized at 473 K on the preoxidized SnO2 thin film. Figure 9a,b shows the O (1s) level spectra obtained before and after CH4 exposure. All the resolved peak positions in Table 4 were the same with those in Table 1. The difference of proportion between Table 4 (a) and Table 1 (d) could be derived from that of experimental lot number. The oxidized species in Table 3 should affect the spectrum in Figure 9b; however we could not

4244 J. Phys. Chem. B, Vol. 105, No. 19, 2001 indicate. Even so, it should be notified that O- species was used up after CH4 exposure at 473 K. This also meant that the produced OH species (∼532.0 eV) desorbed from the surface at 473 K. Furthermore, the proportion of Ob decreased clearly after CH4 exposure. This decrease could be explained by the modification of Ob to O-, i.e., Ob coupled with newly produced vacancies at the bridging site of SnO2 (110) surface. The produced O- could be subsequently consumed as a highly oxygen species during the exposure of CH4 at 473 K. 5. Conclusion Four kinds of oxygen species (O22-, O2-, O-, Ob) were observed on the surface of SnO2 thin film after O2 exposure at 473 K. Ob was assigned to be O2- ion at the bridging site on the topmost layer of SnO2 (110) surface using the DFT calculation procedures. The variation of proportion of Ob through heat treatment with evacuating was weakly and inversely related to the proportion of O- and O22-. The electronic state of Ob was modified from O2- to O- during CH4 exposure at 473 K. This modification was derived from the coupling between Ob and newly produced vacancy on the bridging site of SnO2 (110) surface. Acknowledgment. This study was financially supported by the NEDO. References and Notes (1) Henrich, V. E.; Cox P. A. The Surface Science of Metal Oxides; Cambridge University Press: New York, 1994. (2) Wagner, C. J. Chem. Phys. 1950, 18, 69. (3) Haffe, K. AdV. Catal. 1955, 7, 366. (4) Bielanski, A.; Deren, J.; Haber, J. Nature 1957, 179, 668. (5) Che, M.; Tench, A. J. AdV. Catal. 1982, 31, 77.

Kawabe et al. (6) Che, M.; Tench, A. J. AdV. Catal. 1983, 32, 1. (7) Nagasawa, Y.; Choso, T.; Karasuda, T.; Shimomura, S.; Ouyang, F.; Tabata, K.; Yamaguchi, Y. Surf. Sci. 1999, 433-435, 226-229. (8) Kawabe, T.; Shimomura, S.; Karasuda, T.; Tabata, K.; Suzuki, E.; Yamaguchi, Y. Surf. Sci. 2000, 448, 101. (9) Yamaguchi, Y.; Nagasawa, Y.; Shimomura, S.; Tabata, K.; Suzuki, E. Chem. Phys. Lett. 2000, 316, 477. (10) Nagasawa, Y.; Tabata, K.; Ohnishi, H. Appl. Surf. Sci. 1997, 121/ 122, 327. (11) Gaggiotti, G.; Galdikas, A.; Kae`iulis, S.; Mattogno, G.; Sˇ etkus, A. J. Appl. Phys. 1994, 76 (8), 4467. (12) Ko¨ve´r, L.; Kova´cs, Zs.; Sanjine´s, R.; Moretti, G.; Cserny, I.; Margaritondo, G.; Pa´linka´s, J.; Adachi, H. Surf. Interface Anal. 1995, 23, 461. (13) Delley, B. J. Chem. Phys. 1990, 92, 503. (14) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (15) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. 1988, B37, 785. (16) Themlin, J. M.; Sporken, R.; Darville, J.; Candano, R.; Gilles, J. M.; Johnson, R. L. Phys. ReV. B 1990, 42, 11914. (17) Themlin, J. M.; Chtaib, M.; Henrard, L.; Lombin, P.; Darville, J.; Gilles, J. M. Phys. ReV. B 1992, 46, 2460. (18) Jime´nez, V. M.; Lassaletta, G.; Ferna´ndez, A.; Espino´s, J. P.; Yubero, F.; Gonza´lez-Elipe, A. R.; Soriano, L.; Sanz, J. M.; Papaconatantopoulos, D. A. Phys. ReV. B 1999, 60, 11171. (19) Hegde, M. S.; Ayyoob, M. Surf. Sci. 1986, 173, L635. (20) Badyal, J. P. S.; Zhang, X.; Lambert, R. M. Surf. Sci. Lett. 1990, 225, L15. (21) Kamath, P. V.; Rao, C. N. R. J. Phys. Chem. 1984, 88, 464. (22) Qiu, S. L.; Lin, C. L.; Chen, J.; Strongin, M. Phys. ReV. B 1989, 39, 6194. (23) Hedge, M. S.; Ayyoob, M. Surf. Sci. 1986, 173, L635. (24) Rao, C. N. R.; Vijayakrishnam, V.; Kulkarni, G. U.; Rajumon, M. K. Appl. Surf. Sci. 1995, 84, 285. (25) Kulkarni, G. U.; Rao, C. N. R.; Roberts, M. W. Langmuir 1995, 11, 2572. (26) Mitchel, H. J.; Leiste, H.; Schierbaum, K. D.; Halbritter, J. Appl. Surf. Sci. 1998, 126, 57. (27) Barr, T. L. Modern ESCA; CRC Press: Boca Raton, 1994; Chapter 8, p 189. (28) Kawabe, T.; Shimomura, S.; Karasuda, T.; Tabata, K.; Suzuki, E. Surf. Sci. 2000, 454-456, 374.