XPS and STM Study of Nb-Doped TiO2(110)-(1 × 1) Surfaces - The

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XPS and STM Study of Nb-Doped TiO2(110)-(1 × 1) Surfaces Akira Sasahara* and Masahiko Tomitori Japan Advanced Institute of Science and Technology, Nomi, Ishikawa 923-1292, Japan ABSTRACT: The Nb-doped rutile titanium dioxide TiO2(110)-(1 × 1) surface, which was obtained after cleaning by cycles of Ar+ sputtering and annealing in ultrahigh vacuum, was examined by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscope (STM) techniques. The angle-resolved XPS measurement revealed that Nb in a pentavalent state was concentrated in the near-surface region. Preferential removal of O by the Ar+ sputtering induced enrichment of Nb and Ti on the surface, and the Nb cations with a low diffusivity in bulk TiO2 remained in the near-surface region in the subsequent annealing. Atom-sized spots assignable to either Nb atoms incorporated into the rutile lattice or Nb adatoms were observed in the STM images. The density of the spots was almost one-third of that of the surface Nb atoms estimated from the XPS results, which indicated that a large part of the near-surface Nb atoms was in the interstitial sites and was invisible in the STM images. The Nb interstitials were segregated to the surface to form oxide particles when the surface was annealed in O2. The height of the Nb adatom in STM images was reversibly changed, dependent on the adatom−adatom distances. The change in the image height of the adatoms was attributed to the change in the oxidation state.

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INTRODUCTION Niobium doping improves the performance of titanium dioxide (TiO2) nanocrystalline films as high-temperature gas sensors1−6 and photocatalysts.7−10 A rise of the anatase-to-rutile phase transition temperature and a suppression of the coalescence of the grains are induced by the doping,11 which makes it possible to maintain gas-sensing abilities of the film at high temperatures. Increase in the conduction band electrons by substituting the pentavalent Nb cation for the tetravalent Ti cation reduces the lattice O vacancies that act as nucleation sites of the rutile structure.12 Improved photocatalytic activity is also ascribed to the conduction band electrons that enhance the upward band bending and thereby promote the electron−hole separation.10 Besides the bulk properties, surface electronic and structural properties of the TiO2 are expected to be modified by the Nb doping following the analogy of the improved catalysis of the niobium pentaoxide (Nb2O5)-TiO2 mixed oxides. Loading Nb2O5 onto TiO2 promoted the reduction of NO with NH313 and the photocatalytic decomposition of 1,4-dichlorobenzene.14 A Nb oxide with a NbO bond and an acidic OH group was concluded as a unique species of the mixed oxide surface in the infrared absorption spectroscopy study.15 Applying the surface-sensitive analytical techniques to welldefined surfaces of single crystals facilitates a detailed understanding of the surface properties. Fabrication of the single-crystal film of the Nb-doped TiO2 was first achieved by Gao and coworkers in 1996.16−19 The authors employed molecular beam epitaxy, where Ti and Nb were coevaporated onto rutile TiO2 single crystals in an oxygen plasma, and obtained transparent or pale-yellow films. By applying a wide range of analytical techniques including X-ray diffraction (XRD), reflection high-energy and low-energy electron © 2013 American Chemical Society

diffraction (RHEED and LEED), X-ray photoelectron diffraction (XPD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM), the authors concluded that tetravalent Nb cations were incorporated into the rutile lattice. Later, the authors fabricated the Nb-doped rutile TiO2 film by plasma-enhanced metalorganic chemical vapor deposition onto Al2O3 single crystals using titanium isopropoxide (Ti(OC3H7)4) and niobium ethoxide (Nb(OC2H5)5) as the precursors.20 There again, Nb cations substituting for Ti cations were concluded to be tetravalent. Morris et al. doped Nb into a pure rutile TiO2(110) wafer by diffusion using Nb-doped TiO2 powder as a source.21 The doped wafer was dark blue, and its surface exhibited a (1 × 1) LEED pattern after cleaning in ultrahigh vacuum (UHV) by cycles of Ar+ sputtering and annealing. Unlike in the epitaxially grown films, Nb was in a pentavalent state. Four bright spots in a group were observed in scanning tunneling microscope (STM) images and were assigned to four surface Ti atoms electronically modified by one Nb atom substituting for the subsurface Ti atom. The present work examined the Nb-doped TiO2(110)-(1 × 1) surface by XPS and STM techniques, focusing on the chemical compositions of the near-surface region and the atomic scale surface structures. Concentration of Nb in the near-surface region and the formation of Nb adatoms were found. The near-surface Nb atoms are segregated onto the surface by annealing in O2. The Nb adatoms transformed reversibly between neutral and cationic states. Such nanostrucReceived: June 10, 2013 Revised: July 24, 2013 Published: July 29, 2013 17680

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neutralizer. The spectra were collected with the pass energy of 20 eV and the energy step of 0.1 eV. The TiO2 wafers were transferred from the microscope system to the XPS system in laboratory air without any surface protection. Binding energy of the spectra was calibrated so that the major O 1s peak became 530.3 eV.25 With this calibration, the Ti 2p3/2 peak was corrected to 459.1 eV. The binding energy of the spectra of the Nb2O5 powder (>99.99%, Mitsuwa Chemicals) was calibrated by referring to the Ti 2p3/2 peak of the rutile TiO2 powder (>99.99%, Kojundo Chemical Laboratory) mixed with the Nb2O5 powder. The intensities of the peaks were determined for the fitted curves by Gaussian−Lorentzian (70:30) function after Shirley-type background subtraction.

tures potentially contribute to the surface properties of the Nbdoped TiO2. A look at the structure of the undoped TiO2(110) surface is helpful to interpret the results of the Nb-doped TiO2(110) surface. Figure 1 shows the model of the TiO2(110)-(1 × 1)

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RESULTS AND DISCUSSION Figure 2a shows the Nb 3d XPS spectra of the Nb-doped TiO2(110) surfaces with a doping level of 0.43 at %. Spectra of

Figure 1. Ball-and-stick model of the rutile TiO2(110)-(1 × 1) surface. Midsize white balls and large gray balls represent Ti atoms and O atoms, respectively. The smallest white balls bound to a part of the bridging O atoms are H atoms forming surface OH groups. The size of the unit cell is 0.3 nm × 0.65 nm.

surface obtained after cleaning in UHV.22 Oxygen atoms coordinated to two Ti atoms in a bridging fashion (bridging O atom) are protruding out of the surface and forming ridges along the [001] direction (O atom rows). A part of the bridging O atoms is missing (bridging O vacancy).23 Titanium atoms coordinated to five O atoms (five-fold coordinated Ti atoms) are exposed between the O atom rows and are aligned along the [001] direction (Ti atom rows). The OH groups including the bridging O atoms are formed by dissociation of the H2O molecule residual in the vacuum chamber.24

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EXPERIMENTAL METHODS A commercial UHV-microscope system (JSPM4500S, JEOL) was used for the STM experiments. The system was composed of the microscope chamber including the STM stage and the sample preparation chamber equipped with an Ar+ sputtering gun (EX03, Thermo), low-energy electron diffraction optics (BDL600, OCI), and a quadrupole mass analyzer (e-Vision+, MKS instruments). The base pressure of the system was 2 × 10−8 Pa. Constant current images were obtained in the dark at room temperature using an electrochemically etched W wire as a probe. The images are presented without filtering, and the cross sections are measured from images smoothed by a ninepoint median filter. Dark-blue Nb-doped TiO2 wafers (Shinkosha) were placed on a Si wafer of the same size used as a resistive heater. The surface was cleaned by cycles of Ar+ sputtering and vacuum annealing. Beam energy, sample current, and irradiation time for the sputtering were, respectively, 2 keV, 0.5 μA, and 1 min. Annealing temperature and time were 1070 K and 1 min, respectively. The temperature at the side of the wafers was monitored by an infrared thermometer. Hence, the temperature of the TiO2 surface was probably lower than the monitored temperature due to incomplete contact between the wafers. Annealing in O2 was performed in 10−4 Pa O2 ambient. X-ray photoelectron spectroscopy analysis was conducted at room temperature by a commercial system (Axis Ultra DLD, Kratos) with the base pressure of 1 × 10−7 Pa. The photoelectron emission angle, θ, with respect to surface normal was usually set to 0°. Monochromatic Al Kα was used as an excitation source. Charging of the samples was reduced by a

Figure 2. Nb 3d XPS spectra of the (a) 0.43 at % and (b) 0.86 at % Nb-doped surfaces. The spectra of the undoped (1 × 1) surface and Nb2O5 powder are also shown in panel a for comparison. (c,d) Intensity ratio of the Nb 3d to Ti 2p peaks INb/ITi on (c) 0.43 at % and (d) 0.86 at % Nb-doped surfaces. Black and white circles represent the ratios on the as-received and the (1 × 1) surfaces, respectively.

the undoped (1 × 1) surface and the Nb2O5 powder are also shown for comparison. The vertical scales of the spectra of the TiO2 surfaces were normalized by the Ti 2p3/2 peak maximum in each spectrum. Whereas no feature was observed in the spectrum of the undoped surface, the Nb-doped surface of the as-received state exhibited peaks at 207.6 and 210.4 eV. The 17681

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units of atomic percent and nanometers, respectively. z, x, and λNb are, respectively, the depth from the surface, the doping level of Nb, and the inelastic mean free path of the Nb 3d photoelectron in the TiO2. ITi was considered by the second term of the eq 1 for simplicity. Change of the atomic percent of Ti in bulk by the Ti atom diffusion from the sputtered region is expected to be small. The INb/ITi is expressed as a function of the θ as follows.

two peaks are assigned to 3d5/2 and 3d3/2 states of Nb, respectively. The 3d5/2 and 3d3/2 peaks shifted to 207.5 and 210.2 eV on the (1 × 1) surface. The binding energies of the (1 × 1) surface were equal to those of the Nb2O5 powder. Figure 2b shows the Nb 3d spectra on the Nb-doped TiO2 surfaces with a doping level of 0.86 at %. The Nb 3d peaks at 207.7 and 210.4 eV in the as-received state shifted to 207.5 and 210.2 eV on the (1 × 1) surface. The XPS results show that Nb of the Nb-doped (1 × 1) surface is in a pentavalent state. Slightly higher binding energies of the as-received surfaces are due to the surface charging caused by insulating impurity oxides on the surface. Survey scan showed the presence of P, Ca, Mn, Fe, and Zn impurities on the as-received surfaces, and narrow scan of the O 1s region showed a peak assignable to impurity oxides at ∼533 eV. After immersing into HF solution, the impurity-related peaks disappeared, and the Nb 3d peaks shifted to 207.5 and 210.2 eV. Tomizuka et al. reported that the binding energies of the Al 2p and O 1s peaks on insulating α-Al2O3 surfaces were shifted depending on the degree of charging.26 Depth profile of Nb in the Nb-doped (1 × 1) surface was examined by changing θ. Figure 2c shows the ratios of the Nb 3d peak intensity INb to the Ti 2p peak intensity ITi on the 0.43 at % Nb-doped surfaces. The peak intensity ratio INb/ITi monotonously increased on the (1 × 1) surface from 0.0079 to 0.0097 as the probing depth became shallow with the increase in θ. Unlike on the (1 × 1) surface, the INb/ITi remained around 0.0065 with the θ from 0 to 60° on the asreceived surface. Similar dependence of the INb/ITi on the θ was observed on the 0.86 at % Nb-doped surface, as shown in Figure 2d. The INb/ITi increased from 0.020 up to 0.023 with the θ on the (1 × 1) surface, while the INb/ITi was 0.016 independently of the θ on the as-received surface. Increase in the INb/ITi with the θ on the (1 × 1) surface indicates that the Nb is concentrated in the near-surface region. The concentration of Nb probably arises from the lower diffusivity of the pentavalent Nb cations rather than the tetravalent Ti cations in the bulk TiO2. Sasaki et al. reported that the diffusion rate of cations in the TiO2 single crystal is sensitive to their charge density.27 The Ar+ sputtering efficiently removes O,28 and the sputtered layers are enriched with Nb and Ti. It was confirmed that the INb/ITi was not affected by the Ar+ sputtering. The excess Nb atoms as well as Ti atoms29 for a lattice formation diffuse into bulk during the annealing. The less diffusive Nb cations remain in the near-surface region. Assuming a Gaussian distribution to the depth profile of the diffused Nb reproduced the dependence of the INb/ITi on θ. The Gaussian depth profile is seen for the surface-to-bulk diffusion of the impurity of a fixed amount.30 Here INb is considered by the following equation. INb =

=

∞

(

⎪

⎛ ⎛ z2 ⎞ z 1 ⎞ S Nba exp⎜ − 2 ⎟ exp⎜− ⎟ dz ⎝ σ ⎠ ⎝ cos θ λNb ⎠ x +S Nb λNb × cos θ 1 00

∫0

1

)

x

)

The ratio of the constants SNb/STi was determined from the asreceived surface, where the INb/ITi is the ratio of the second term in eq 1. λNb and λTi were estimated to be 1.9 and 1.5 nm, respectively, according to the report by Fuentes et al.31 The dotted lines in Figure 2c,d show the INb/ITi−θ curves of eq 2 fitted to the measured values by the least-squares method. Sets of the Gaussian distribution parameters (a, σ) were (0.72 at %, 0.32 nm) and (1.3 at %, 0.32 nm) for the 0.43 and 0.86 at % Nb-doped surfaces, respectively. The obtained parameters indicate that the Nb concentration reaches 1.2 and 2.2 at % at z = 0 nm for the 0.43 and 0.86 at % Nb-doped (1 × 1) surfaces, respectively, and that the diffused Nb atoms were mainly concentrated in the ∼0.7 nm region from the surface. The Ti 2p XPS spectra of the TiO2 surfaces are shown in Figure 3. The vertical scales of the spectra were reduced by the same ratio. Figure 3a shows the spectra on the undoped (1 × 1) surface. The major peaks at 459.1 and 465.1 eV are the 2p3/2 and 2p1/2 states. The binding energies and shape of the peaks were independent of θ. No structure was recognized at the lower binding energy side of the 2p3/2 peak, as magnified in the inset. Figure 3b shows the spectra on the 0.43 at % Nbdoped (1 × 1) surface. While the binding energies and shapes of the major peaks were similar to those of the undoped (1 × 1) surface, a faint shoulder peak was observed at ∼457.0 eV. The spectra of the undoped surface were superimposed in the inset to highlight the shoulder peak. The shoulder peak is assigned to the Ti cations in a lower valence state, Tin+ (n < 4).28 The Tin+ peak was enhanced with the increase in the θ. The intensity ratios of the Tin+ peak to the major 2p3/2 peak were 0.003, 0.008, and 0.012, at θ = 0, 20, and 40°, respectively. Intensity of the tiny Tin+ peak was estimated as the difference between the spectra of the Nb-doped and undoped surfaces that were normalized by the height of the major 2p2/3 peak. The Tin+ peak was more distinct on the 0.86 at % Nb-doped (1 × 1) surface, as shown in Figure 3c. The Tin+/major 2p3/2 peak intensity ratios were 0.01, 0.02, and 0.02 at θ = 0, 20, and 40°, respectively. The Tin+ peak was not observed on the 0.86 at % Nb-doped surface in the as-received state, as shown in Figure 3d. The Tin+ cations are associated to Nb cations concentrated in the near-surface region, on the basis of the dependence of the Tin+ peak intensity on the doping level and θ. The Tin+/major 2p3/2 peak intensity ratios at θ = 0° were comparable to the INb/ITi of 0.005 and 0.01 on the 0.43 at % and the 0.86 at % Nb-doped (1 × 1) surfaces, respectively, calculated by eq 2 for the regions of 2 nm below the surfaces. The tetravalent Ti cations are reduced by neighboring pentavalent Nb cations to maintain the electric neutrality of the crystal. The Tin+ of 0.86 at % on average was under detection limit, as shown in Figure

∞

⎪

z

(2)

⎧ ⎛ z2 ⎞ x ⎫ ⎬ S Nb⎨a exp⎜ − 2 ⎟ + ⎝ σ ⎠ 100 ⎭ ⎩ ⎛ z 1 ⎞ exp⎜ − ⎟ dz ⎝ cos θ λNb ⎠

∫0

z2

( ) (

∫0 SNba exp − σ 2 exp − cos θ λNb dz+SNb 100 λNb cos θ INb = x ITi STi 1 − 100 λ Ti cos θ

∞

(1)

SNb is the constant related to the sensitivity factor for the Nb 3d peaks. a and σ are the Gaussian distribution parameters in the 17682

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the stripes are point defects, either OH groups or O vacancies.24 One of the point defects is marked with the circle. The specific features observed on the Nb-doped (1 × 1) surface were the atom-sized spots with the heights of 0.03−0.06 nm on the Ti atom rows. One of the spots is indicated by the arrow in Figure 4a. The inset shows the magnified image of the spot. The spots were centered on the five-fold coordinated Ti atom. The density of the spots was 0.02 and 0.04 nm−2 on the 0.43 and 0.86 at % Nb-doped (1 × 1) surfaces, respectively. The spots were occasionally found in a concentrated manner, as shown in Figure 4b, where one of the four colonies in the image was surrounded by the dotted circle. The spots were recognized at sample bias voltages from +0.6 to +1.6 V. The atom-sized spots are assigned to Nb atoms from the dependence of their density on the doping level. A part of the Nb atoms should be adatom because 20% of the Nb atoms changed their positions during time-lapse imaging for 330 s. It is possible that the Nb cations substituting for the five-fold coordinated Ti atom were included in the Nb atoms. It should be noted that the densities of the Nb atoms in the STM images were inconsistent with those estimated from the XPS analysis. Assuming that the Nb atoms substitute for the 2.2% of the fivefold coordinated Ti atoms in accordance with the analysis of the INb/ITi−θ curve in Figure 2d, the density of the surface Nb atoms is estimated to be 0.11 nm−2 The density of the Nb atoms in the STM images is almost one-third of the estimated density. It turns out that a large part of the Nb atoms were not visualized by STM. An interstitial Nb atom is a candidate for the Nb atoms invisible in STM images. To gain insight into the interstitial species, the 0.86 at % Nb-doped (1 × 1) surface was annealed in O2. Figure 5a shows the STM image of the surface annealed in O2 once. Atom-sized spots and nanometer-sized particles appeared. Several rectangular patches with a width of 1 to 2 nm and a height of ∼0.2 nm were also observed as a minor product. The arrows and the arrowheads indicate the spot and the patch, respectively. The particles grew and aggregated after the annealing was repeated three times, as shown in Figure 5b. Figure 5c,d shows the INb/ITi and the Ti 2p XPS spectra of the 0.86 at % Nb-doped (1 × 1) surface annealed in O2 three times. The INb/ITi became larger than that of the as-prepared (1 × 1) surface by 13% at θ = 0°, and the deviation of the INb/ITi reached 64% at θ = 60°. The Tin+ peak at 457.0 eV did not appear in Ti 2p spectra. Figure 5e,f shows the STM images of the undoped (1 × 1) surfaces annealed in O2, which were obtained for comparison purposes. In contrast with the Nbdoped surface, the patch was the major product after annealing in O2 once, as shown in Figure 5e. The surface annealed in O2 for the third time was covered with a rough layer, which was possibly aggregates of the patches, as shown in Figure 5f. The O2-induced atom-sized spots and the particles in Figure 5a,b are specific to the Nb-doped (1 × 1) surface and are assigned to either Nb or oxides of Nb. The increase in the INb/ ITi supports the assignment. The binding energies of Nb 3d5/2 and 3d3/2 peaks were 207.5 and 210.2 eV, respectively. Hence, the Nb on the surface is in a pentavalent state. The patch is the oxide of Ti formed by the oxidation of the interstitial Ti atoms segregated to the surface.32,33 The lower density of the patches on the Nb-doped surface than that on the undoped surface indicates that the Nb-doped surface possessed less interstitial Ti. It is expected that the interstitial sites of the Nb-doped surface were occupied by Nb instead of Ti and that the interstitial Nb atoms segregated to the surface by annealing in

Figure 3. Ti 2p XPS spectra of (a) undoped (1 × 1), (b) 0.43 at % Nb-doped (1 × 1), (c) 0.86 at % Nb-doped (1 × 1), and (d) asreceived 0.86 at % Nb-doped surfaces. The magnified spectra in the region between 458 and 454 eV are inset in each spectrum. The spectra on the undoped (1 × 1) surface in the inset are colored gray and are superimposed on the spectra in the inset in panels b−d.

3d. The bridging O vacancies are not responsible to the Tin+ peak because the vacancies should have been healed by dissociative adsorption of either O2 or H2O molecules24 in the laboratory air during transfer to the XPS system. The empty-state STM image of the Nb-doped (1 × 1) surfaces showed nearly identical features to that of the undoped (1 × 1) surface. Figure 4a shows the image of the 0.86 at % Nbdoped (1 × 1) surface. The surface exhibited [001]-oriented bright rows and faint stripes bridging the adjacent rows. Following the established interpretation of the image of the undoped (1 × 1) surface, the rows are the Ti atom rows,22 and

Figure 4. STM images of the 0.86 at % Nb-doped (1 × 1) surface (25 × 25 nm2). The inset in panel a (3 × 3 nm2) shows the magnified image of the bright spots on the Ti atom row. (a) Sample bias voltage (Vs): +0.6 V, tunneling current (It): 0.2 nA; (b) Vs: +1.0 V, It: 0.2 nA. 17683

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Figure 5. (a,b) STM images of the 0.86 at % Nb-doped (1 × 1) surfaces annealed in O2 (a) once and (b) three times (25 × 25 nm2). Vs: +1.2 V, It: 0.2 nA. (c) INb/ITi on the 0.86 at % Nb-doped (1 × 1) surface, which annealed three times in O2, is shown by gray circles. The INb/ITi on the as-prepared (1 × 1) surface is also shown for comparison by white circles. (d) Ti 2p XPS spectra of the 0.86 at % Nb-doped (1 × 1) annealed in O2. The spectra in the region between 458 and 454 eV are magnified and inset in each spectrum. The spectra on the undoped (1 × 1) surface are colored gray and are superimposed on the spectra in the inset. (e,f) STM images of the undoped (1 × 1) surfaces annealed in O2 (a) once and (b) three times (25 × 25 nm2). Vs: +1.2 V, It: 0.2 nA.

Figure 6. Successive STM images of the 0.86 at % Nb-doped (1 × 1) surface (8 × 5 nm2) with an acquisition time of 55 s. The models of the images are shown in the right panel. Niobium atoms, point defects, and Ti atom rows are presented by white circles, black circles, and solid lines along the [001] direction, respectively. Dotted lines along the [110̅ ] direction pass the center of the five-fold coordinated Ti atoms. Vs: +0.9 V, It: 0.2 nA.

O2. The O2-induced surface segregation of Nb has been reported for polycrystalline Nb-doped TiO2.34 Figure 6a shows an STM image of the colony of the Nb atoms, a schematic model of the image, and cross sections along the broken lines in the model. In the model, the Nb atoms, point defects, and Ti atom rows are represented by white circles, black circles, and solid lines along the [001] direction, respectively. The dotted lines passing the center of the five-fold coordinated Ti atoms were drawn on the basis of the positions of the point defects and the five-fold coordinated Ti atoms. The gray rectangle in the model represents the (1 × 1) unit cell. The Nb atoms were roughly classified into two types. One is the isolated atoms with heights of 0.04 nm or

above. The other is the paired atoms with heights of 0.03 nm or less, which are indicated by the arrows. Figure 6b shows the image obtained after the image in Figure 6a. Three differences were found in the arrangement of the Nb atoms. First, the isolated tall atom labeled A appeared to be transferred from the adjacent Ti atom row observed at the upper side of the image. Second, two isolated tall atoms labeled B were formed from the paired short atoms in Figure 6a. Third, the oval spot labeled C appeared. The height of the oval spots reached 0.07 nm. Figure 6c,d show images consecutively obtained after the image in Figure 6b. The paired short atoms labeled D were regenerated from the B atoms in Figure 6c. The A adatom and another isolated tall atom labeled E moved along the Ti atom rows in Figure 6d. 17684

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interstitial site and was invisible by STM. The Nb adatoms reversibly varied their oxidation states. A part of the adatoms formed the colonies, which suggested nonuniform distribution of the surface charges. The results show that the doping of Nb induces specific nanostructures on the TiO2 surface and that the structures are sensitive to the redox treatment. Controlling the surface structures is one promising approach to improve gas sensitivity and catalysis of the Nb-doped TiO2.

The A atom is a Nb adatom that moved to the adjacent Ti atom row. The A adatom might have migrated to the adjacent Ti atom rows without climbing over the protruding O atom row if the point defect next to A adatom is the bridging O vacancy. Unfortunately, it was impossible to identify from the image height whether the defect was the O vacancy or the OH group. The other manner for the move of A adatom is the tipassisted diffusion including pickup and release by the tip and dragging through the electric field. The E atom is also Nb adatoms migrated along the Ti atom row. The migration on the Ti atom row will occur without the assistance of the tip. The paired short atoms, which are reversibly transformed into tall isolated atoms, are also adatoms. The B and D atoms meet such adatoms. The change of their heights and adatom− adatom distances probably reflects their oxidation states: B adatoms are cationic and the D adatoms are neutral. The cationic B adatom forms a dipole moment directed to the vacuum side, which reduces the barrier height.35 The reduction of the barrier height results in an increase in the tunneling current. The tip is separated from the B adatom to keep the tunneling current constant. Therefore, the B adatom is imaged taller than the D adatom. A repulsive force works between the cationic B atoms, and they are separated from each other by two unit cells or more along the [001] direction. The neutral D adatoms showing a relatively larger barrier height, are imaged as a short spot and are paired by an attractive interatomic force. The cross section across the C spot includes a sharpened apex, which suggests that one atom was located between the two Nb atoms in Figure 6a. A Nb adatom might have migrated from the outside to the scanned area. The colony of the cationic Nb adatoms is likely to be formed at the negatively charged area. A possible cause for the negative charge is a deficiency of Ti atoms. Several depressions in the Ti atom rows were observed in the colonies, as indicated by the arrowheads in Figure 6a. Such depression is one candidate of the Ti atom vacancies. Concentration of Tin+ cations in the near-surface region is likely to destabilize the (1 × 1) structure. However, the assignment of the depression is still questionable. The depression in the Ti atom rows is observed on the undoped (1 × 1) surface as a minor defect and has been assigned to the vacancy of the O atom beneath the five-fold coordinated Ti atom.36 The positively charged O atom vacancy does not contribute to the formation of the colonies. Kelvin probe force microscope,37 which provides a nanometer-scale distribution of the contact potential difference, will help to examine the charge distribution and to identify the origin of the charges. The fact that the group of four bright spots observed on the diffusion-doped TiO2 surface21 was not found indicates the low density of the substituting Nb on the (1 × 1) surfaces examined in this study. The doping level of the diffusion-doped TiO2 was 1.6 at % and was twice as high as that of our samples. Furthermore, the diffusion-doped TiO2 surface was sputtered in the condition where six hundred times as many Ar+ ions as our condition impinge on the surface. Hence, the density of Nb in the sputtered layers is expected to have been much higher than that of our sample. The incorporation of Nb cations into the rutile lattice will be enhanced at a higher density of Nb.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-761-51-1503. Fax: +81761-51-1149. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPM KAKENHI Grant Number 21686004, 23656030, and 24246014. REFERENCES

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CONCLUSIONS Concentration of Nb in the near-surface region and formation of Nb adatoms were found on the Nb-doped TiO2(110)-(1 × 1) surface. A large part of the near-surface Nb atoms was in the 17685

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