Lateral Distribution of Li Atoms at the Initial Stage of Adsorption on

Jun 2, 2012 - Lithium adatoms were observed as protrusions in empty state images. Three different adsorption sites for Li atoms, a 4-fold hollow site ...
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Lateral Distribution of Li Atoms at the Initial Stage of Adsorption on TiO2(110) Surface Hitomi Tatsumi, Akira Sasahara,* and Masahiko Tomitori Japan Advanced Institute of Science and Technology, Nomi, Ishikawa 923-1292, Japan ABSTRACT: Adsorption of Li atoms on a titanium dioxide (TiO2) (110)-(1×1) surface was examined by a scanning tunneling microscope (STM). Lithium adatoms were observed as protrusions in empty state images. Three different adsorption sites for Li atoms, a 4-fold hollow site of in-plane O atoms, a 3-fold hollow site composed of one bridging O atom and two in-plane O atoms, and a bridging O vacancy, were identified. The Li adatoms were placed predominantly on the 4-fold hollow site. Time-lapse STM observation revealed that the Li adatoms at the three sites were mobile at room temperature. The number density of the Li adatoms on the terraces was reduced around the steps, which was attributed to the perturbation of the surface electronic state by the Li adatoms concentrated at the steps.

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INTRODUCTION Adsorption of alkali metals on titanium dioxide (TiO2) surfaces has been widely studied, motivated by wide-ranging industrial applications including solar cells,1 batteries,2 gas sensors,3 and catalysts.4 Alkali metals with low first ionization potentials readily donate their electrons to the TiO2. The electron donation modifies the electronic state of the TiO2 and can improve the material properties of the TiO2. Because of the ionic character of the Ti−O bond, the charge distribution on the TiO2 surface is thought to be sensitive to the surface atomistic structure. The surface charge distribution affects the adsorption of the alkali metals accompanied by electron donation. A microscopic view of alkali metal adsorption will give clues for fine-tuning of the TiO2-based materials. A (110) plane of a rutile TiO2, structurally and chemically well-defined, is beneficial to obtain an atomic scale view of the alkali atom adsorption.5 The TiO2(110) surface exhibits the (1 × 1) structure shown in Figure 1 after cleaning in ultrahigh vacuum (UHV).6 Topmost O atoms are bound to two 6-foldcoordinated Ti atoms in a bridging coordination (bridging O atoms) and form the rows along the [001] direction. Between the bridging O atom rows, Ti atoms coordinated to five O atoms (5-fold-coordinated Ti atoms) are aligned to the [001] direction. The size of the unit cell is 0.30 nm × 0.65 nm. Less than 1% of the bridging O atoms are missing,7 whereas removal of the in-plane O atoms is less probable.8 Hydrogen atoms, possibly formed by dissociation of residual water molecules in a vacuum chamber, reside on some of the bridging O atoms and form surface hydroxyl (OH) groups.9 Sodium has been frequently employed in the study of alkali metal adsorption on the TiO2(110) surface as a prototypical alkali metal.5 Onishi et al. found c(4×2) arrangement of Na adatoms on the (1×1) surface.10,11 The Na adatoms were observed as bright spots on the Ti atom rows in scanning tunneling microscope (STM) images. The authors proposed © 2012 American Chemical Society

Figure 1. A ball model of the rutile TiO2(110)-(1×1) surface with single height steps. Gray small and gray large balls represent Ti and O atoms, respectively. Oxygen atoms are shaded according to their depth. White small balls represent H atoms coordinated to the bridging O atoms to form surface OH groups. The size of the unit cell indicated by the dotted rectangle is 0.30 nm × 0.65 nm.

Received: April 13, 2012 Revised: May 31, 2012 Published: June 2, 2012 13688

dx.doi.org/10.1021/jp303555s | J. Phys. Chem. C 2012, 116, 13688−13692

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heater was monitored through the transparent TiO2 wafer. The obtained (1×1) surfaces were lightly bluish. The surfaces were cooled to room temperature and exposed to Li vapor released from a commercial dispenser (SAES Getters). The Li dispenser was resistively heated with a constant current, and the pressure in the chamber was kept below 4 × 10−7 Pa during the Li evaporation. The amount of Li evaporated was controlled by changing the exposure time. The STM imaging was performed using an electrochemically etched W wire as a probe. Empty state images were obtained in a constant current mode at room temperature. The images are presented without filtering, and the cross sections were measured on the images smoothed by a nine-point median filter.

that Na atoms were adsorbed at the 4-fold hollow site of the inplane O atoms in a c(4×2) periodicity. On the basis of the Xray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) results, they concluded the complete ionization of Na adatoms to Na+ at the initial adsorption stage, the decrease of the ionization degree with the increase of coverage, and the reduction of the 5-foldcoordinated Ti atoms by the electron donation from the Na adatoms. A similar coverage-dependent ionization degree of the alkali metal adatoms was concluded in a low energy D+ scattering study of Na-adsorbed TiO2(110),12 an angle-resolved UPS and XPS study of K-adsorbed TiO2(110),13 and an X-ray photoelectron and X-ray-excited Auger electron spectroscopy study of Cs-adsorbed TiO2(110) surfaces.14 Nerlov et al. examined Na-adsorbed TiO2(110) surfaces by synchrotronradiation photoelectron spectroscopy.15 Comparing the Na 2p peak components on the (1×1) surface with those on the reconstructed (1×2) surface, the authors proposed an “inbetween” site, a 3-fold hollow site consisting of two bridging O atoms and one in-plane O atom, as an adsorption site for Na atoms. Adsorption of Na at the in-between sites was also concluded in the extended X-ray absorption fine structure analysis by Lagarde et al.16 and in the ab initio calculations by Albaret et al.17 and Sanz et al.18 In contrast, Hird et al. reported that the O+ ion scattering spectra fitted the calculated curves assuming Na atom adsorption at the “adjacent” site, a 3-foldhollow site consisting of one bridging O atom and two in-plane O atoms.19 San Miguel et al. showed a coverage-dependent adsorption site of Na on the basis of ab initio molecular dynamics simulation: the in-between sites were predominant at low coverage, and the ratio of the adatoms at the adjacent sites increased with the coverage.20 Only a limited number of studies have been reported on the Li adsorption on the TiO2(110) surfaces. Semiempirical calculation by Stashans et al. predicted that the gap between two bridging O atoms was energetically favorable for the Li atom adsorption with almost complete ionization.21 The intercalation of the Li atom into the bulk TiO2 was unlikely, due to large displacement of the surrounding Ti and O atoms. Later, San Miguel et al. predicted the coverage-dependent transition from the in-between site to the adjacent site for the Li adatoms.20 Krischok et al. examined the Li-adsorption by using UPS and metastable-induced electron spectroscopy (MIES) techniques.22,23 At low coverage, complete ionization of the Li adatoms was concluded. The absence of the Li 2s induced peak in MIES spectra was interpreted as the insertion of the ionized Li atoms into the TiO2 structure at the initial adsorption stage. This Article reports the first STM study of Li-adsorbed TiO2(110)-(1×1) surfaces. Individual Li adatoms were visualized to examine their lateral distribution in an atomic scale. Adsorption sites, surface migration, and local density of the Li adatoms are revealed.

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RESULTS AND DISCUSSION Figure 2a shows an STM image of the TiO2(110)-(1×1) surface. The surface consists of flat terraces separated by steps

Figure 2. STM images of the TiO2(110)-(1×1) surface exposed to Li source for (a) 0, (b) 120, (c) 300, and (d) 600 s (15 × 15 nm2). The inset in (b) (10 × 10 nm2) shows the steps of the identical surface. Sample bias voltage (Vs), +1.0 V; tunneling current (It), 1.0 nA.

with a height of 0.32 nm. Bright rows running along the [001] direction correspond to the position of the 5-fold-coordinated Ti atoms. The Ti 3d derived unoccupied orbital contributes to the electron tunneling in the empty state imaging of the (1×1) surface.6 Between two adjacent Ti atom rows, faint spots were observed with the number density of 0.3 nm−2. Two of the faint spots are marked with open circles. The faint spots are either surface OH groups or bridging O vacancies, either of which is formed in the cleaning process.24−26 The OH groups are observed higher than the O vacancies by 0.02−0.06 nm with a sample bias voltage of +1.2−1.4 V.24,25 We could not classify the faint spots in the O vacancy or OH group from their image heights. However, most of the faint spots are assigned to the OH groups, because the number density of the O vacancies was estimated to be 0.03 nm−2 in frequency modulation atomic force microscope (FM-AFM) images of the (1×1) surface prepared by the same recipe.7 The FM-AFM detects an attractive force to regulate tip−sample distance and unequivocally visualizes the O vacancies as depressions in the bridging

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EXPERIMENTAL PROCEDURES The experiments were carried out using a UHV-STM system (JSPM4500S, JEOL) equipped with an Ar+ sputtering gun (EX03, Thermo). The base pressure of the system was below 2 × 10−8 Pa. A TiO2(110) wafer (7 × 1 × 0.3 mm3, Shinko-sha) was placed on an Si wafer used as a resistive heater. The surface was cleaned by a repetition of Ar+ sputtering and annealing at 1100 K. The temperature measured by an optical pyrometer was probably overestimated, because the temperature of the Si 13689

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O atom rows. Bright particles indicated by the arrowheads are either TiOx species or unidentified contaminants. Figure 2b shows the (1×1) surface exposed to Li vapor for 120 s. Atom-sized spots appeared on the Ti atom rows with the number density of 0.1 nm−2. The spots were taller than the Ti atom rows by 0.03−0.05 nm. Two of the spots are indicated by the arrows. When the exposure time was extended to 300 s, the number density of the spots increased to 0.5 nm−2 as shown in Figure 2c. In addition to the atom-sized spots, particles, which were larger than those observed on the (1×1) surface, appeared. After 600 s exposure, the surface was covered by the spots and particles as shown in Figure 2d. From these results, we assigned the atom-sized spots and the nanometersized particles to Li adatoms and Li clusters, respectively. The aggregation of Li at coverage less than 3 nm−2 has been concluded on MgO(100) surface.27 The increase of the adatoms on the terrace with the exposure time was slow at the initial adsorption stage, while the growth of the particles was not remarkable. The slow increase of the adatoms indicates that the adatoms were concentrated at specific sites. The steps between the terraces are one possibility of the sites. As shown in the inset in Figure 2b, a part of the step was surrounded with spot-like features. The adatoms on the surface probably increased as shown in Figure 2c after the specific sites were filled up. The left panel of Figure 3a shows an STM image of the (1×1) surface with 300 s evaporation of Li. The Li adatoms were observed as bright spots. Several Ti atoms were resolved in the rows as indicated by the arrowheads. A schematic model of the image is shown in the right panel. The adatoms and the Ti atom rows are represented by circles and solid lines along the [001] direction, respectively. The ovals represent incomplete spots. The dotted lines were drawn in such a way as to pass the center of the 5-fold-coordinated Ti atoms. Therefore, the dotted lines pass the center of the bridging O atoms. We identified three types of Li adatoms on the basis of their positions with respect to the Ti atom rows. Most of the adatoms were observed on the Ti atom rows. We refer to such an adatom as A adatom. The A adatoms are represented by white circles in the model. As shown in the cross section (1) in Figure 3b, the A adatoms were higher than the Ti atom rows by 0.03 nm. The second type of adatom was slightly shifted from the Ti atom rows. Such adatoms are represented by gray circles in the model and are labeled B. The B adatoms were protruding by 0.04 nm as shown in the cross section (2). The third type of adatom is labeled C, which was located in the middle of Ti atom rows. The C adatoms are represented by the black circles in the model. The image height of the C adatoms was 0.03 nm in the cross section (3). Thus, the heights of the A, B, and C adatoms were comparable. The incomplete spots are probably the adatoms moving beneath the tip. The tip scans from left to right and moves from top to bottom in the images. The adatoms migrating along the Ti atom rows were observed as the incomplete spots lining along the rows. The adsorption sites of the A, B, and C adatoms are assigned as shown in Figure 3c. Most of the A adatoms are placed with their centers on the dotted lines in the model in Figure 3a. The intersection of the solid and dotted lines corresponds to the center of the 5-fold-coordinated Ti atoms. Hence, the A adatoms are positioned on the 5-fold-coordinated Ti atoms, that is, at the 4-fold hollow sites of the in-plane O atoms. The center of the B adatoms was also on the dotted lines, which

Figure 3. (a) A set of an STM image of the Li-adsorbed TiO2(110)(1×1) surface (10 × 10 nm2) and a model of the image. In the model, solid lines along the [001] direction represent the Ti atom rows. Dotted lines along the [11̅0] direction pass the center of the 5-foldcoordinated Ti atoms. A, B, and C adatoms are presented by white, gray, and black circles, respectively. (b) Cross sections along the broken lines in the model in (a). (1) A adatom, (2) B adatom, (3) C adatom. Solid triangles indicate the positions of the Ti atom rows. (c) A ball model of the TiO2(110)-(1×1) surface with the Li adatoms. (d, (e) STM images obtained after the image (b) (10 × 10 nm2) and models of the images. Vs, +1.0 V; It, 1.0 nA.

indicates they were at the adjacent sites. The C adatoms were also on the dotted lines. The bridging O vacancy and the top of the bridging O atom are prospective sites. The O vacancy is more likely, because adatom on the bridging O is possibly imaged higher than the A and B adatoms due to reduced tip− sample distance. Consistent with the previous theoretical prediction,21 no feature implying intercalation of Li atom into the subsurface, such as local change of the image contrast of the (1×1) structure, was found. The image height of the Ti atom rows on the Nb-doped TiO2(110) surface increased in a nanometersized area, which was ascribed to the electron delocalization 13690

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from Nb atom to the neighboring Ti atoms.28 Subsurface Li atoms partially increased the image height of the honeycomb structure of the SiO2 single crystal surface.29 In contrast to the theoretical predictions,20,21 the predominant adsorption at the in-between sites and the insertion of Li atom into the gap between bridging O atoms were not observed. A possible cause of the inconsistency is the surface OH group, which was not considered in the calculations. The bridging O atoms, as well as Ti atoms, probably contribute to accept electrons from the Li atom. Sanz et al. performed ab initio calculation on the Na-adsorbed TiO2(110) surface and concluded that the negative charge from the Na adatoms was delocalized over the 5-fold-coordinated Ti atoms, bridging O atoms, and in-plane O atoms.18 When electropositive H atom is bound, the negative charge of the bridging O atom increases. The increase of the negative charge will suppress the electron donation from Li atom to the bridging O atoms. Thus, the OH group is likely to influence the adsorption of Li atoms. Successive imaging of the identical area showed the migration of the Li adatoms. Figure 3d,e was acquired successively after the image in Figure 3a with the acquisition time of 90 s. Approximately 27% of the A adatoms, 80% of the B adatoms, and 60% of the C adatoms changed their positions between the images in Figure 3a,d. Between the images in Figure 3d,e, the percentages of migrated A, B, and C adatoms were 45%, 100%, and 67%, respectively. The adatoms were mobile irrespective of the adsorption sites. A comparison with Pt adatoms30 highlights the specific points of the Li adatoms. Platinum atoms were adsorbed in the bridging O vacancy as well as on the Ti atom row. Time-lapse FM-AFM imaging showed that the Pt adatoms embedded in the O vacancies were not mobile, while the Pt adatoms on the Ti atom rows migrated. In contrast, Li adatoms in the O vacancies changed their positions frame to frame. When assuming that the electron donation is necessary for the stabilization of Li atom, it can be seen that the O vacancies are unfavorable for the electron donation from the Li atom. The O vacancies are negatively charged due to the electrons left by the removed O atoms.31 Figure 4a shows an STM image of the Li-adsorbed (1×1) surface including the step and the model of the image. In the STM image, Li adatoms and the Ti atom rows are observed. The Li adatoms and the step are represented by circles and the broken line, respectively, in the model. Fewer adatoms were observed around the step. Figure 4b shows the distribution of the adatoms measured on the surface with Li adatoms of 1.5 nm−2. The number density was calculated by dividing the terraces into parallelograms. One such parallelogram is shown in the model. One pair of the opposite sides of the parallelograms was set parallel to the steps approximated by straight lines. The other pair of the opposite sides was set parallel to the Ti atom rows and had a length of 1 nm. The distance between the parallelograms and the step d was determined along the [001] direction as indicated by the arrow. 382 adatoms around the 68 nm steps were examined. On the upper terraces, the number density of the adatom was below 1 nm−2 when the distance was less than 3 nm, and was constant at 1.5 nm−2 when the distance was larger than 3 nm. On the lower terraces, the number density was suppressed at the area within 1 nm from the step. The suppressed distribution of the Li adatoms around the step is explained by the perturbation of the surface electronic state by the adatoms at the steps. Theoretical work by

Figure 4. (a) An STM image of the Li-adsorbed TiO2(110) surface including a step (16 × 16 nm2) and a model of the image. The circles and the broken lines in the model represent Li adatoms and the step, respectively. Vs, +1.0 V; It, 1.0 nA. (b) Number density of the Li adatoms plotted as a function of the distance from the step edges. The number density was calculated by dividing the terraces into parallelograms. One of the parallelograms separated from the step by distance d is shown in the model in (a).

Feibelman et al. showed that the Li adatom on the Rh(100) surface slows the falloff of the surface valence electron density toward the vacuum and that the slowing effect extends more than 0.8 nm laterally from the adatom.32 The slowing of the falloff of the electron density corresponds to a decrease of work function. The work function of the TiO2(110) surface, as well as the Rh(100) surface, decreases by a small amount of Li adatoms.22 Hence, the spill out of the valence electrons by the Li adatom is expected on the TiO2(110) surface. The spill out of electrons toward the vacuum is unfavorable for the adsorption of Li atom, which donates the electron to the TiO2. The Li-induced perturbation of the electronic state is likely to be enhanced around the steps at which the Li atoms are concentrated at the early stage of the adsorption. Three of the adatoms attached to the step are indicated by arrows in Figure 4a.

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CONCLUSION Scanning tunneling microscope observation revealed the distribution of the Li adatoms on the TiO2(110)-(1×1) surface. On the terraces, the Li atoms were adsorbed predominantly on the Ti atom rows and migrated along the rows. The number of the Li adatoms on the terraces was reduced around the steps, which was explained by the surface electronic state perturbed by the adatoms concentrated at the steps. The density of the steps, favorable adsorption site for Li, will be a key to controlling the effect of the Li additive on the TiO2-based materials.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +81-761-51-1503. Fax: +81-761-51-1149. E-mail: [email protected]. 13691

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Notes

(30) Sasahara, A.; Pang, C. L.; Onishi, H. J. Phys. Chem. B 2006, 110, 13453−13457. (31) Mackrodt, W. C.; Simson, E.-A.; Harrison, N. M. Surf. Sci. 1997, 384, 192−200. (32) Feibelman, P. J.; Hamann, D. R. Surf. Sci. 1985, 149, 48−66.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT).

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REFERENCES

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