An Atomic-Scale Study of TiO2(110) Surfaces Exposed to Humid

An Atomic-Scale Study of TiO2(110) Surfaces Exposed to Humid Environments. Akira Sasahara† and Masahiko ... Publication Date (Web): September 2, 201...
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An Atomic-Scale Study of TiO2(110) Surfaces Exposed to Humid Environments Akira Sasahara*,† and Masahiko Tomitori Japan Advanced Institute of Science and Technology, Nomi, Ishikawa 923-1292, Japan ABSTRACT: Rutile titanium dioxide (TiO2) (110)-(1 × 1) surfaces prepared in an ultrahigh vacuum (UHV) were transferred to humid environments and thereafter returned to UHV to be examined by the scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) techniques. STM images showed that hydroxyl groups terminating 5-foldcoordinated Ti atoms (OHt groups) and hydroperoxyl (OOH) groups, with densities of 0.9 and 0.2 nm−2, respectively, formed after exposure to water vapor at a pressure of several tens of pascals. The density of the OHt groups was found to exceed that predicted when they were assumed to have originated from another type of OH groups involved in bridging O atoms (OHb groups) intrinsically present on the (1 × 1) surface. On the basis of this observation, we hypothesize that dissociation of H2O molecules by evacuation of the water layer produces OHt groups. The OHb groups, produced with the OHt groups by this dissociation, react with O2 molecules dissolved in the water layer, forming OOH groups. Surfaces exposed to laboratory air or immersed in liquid water were also found to be rich in OHt groups; consequently, evacuation-induced processes occurred on those surfaces. Clarification of the effects of evacuation of the water layer implies the possibility of characterizing (1 × 1) surfaces in humid environments by ex situ analysis under UHV.

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INTRODUCTION The adsorption and subsequent reaction of H2O molecules have been actively studied phenomena in the surface science of titanium dioxide (TiO2) because of their extensive roles in surface processes. In addition to being a major target of photolysis,1 H2O molecules also take part in thermal and photooxidation reactions as reactants.2−4 The formation of active O species and subsequent removal of carbonate at active sites by H2O molecules on TiO2-supported Au catalysts has been proposed.5−7 Understanding how H2O molecules behave on TiO2 surfaces will lead to a thorough comprehension of the industrially important properties of TiO2. The rutile TiO2(110) surface has been predominantly used for the study of the adsorption of H2O molecules.8 Sputter− anneal cleaning of the (110) surface in an ultrahigh vacuum (UHV) results in a (1 × 1) structure characteristic of simple termination of the bulk crystal.9,10 The atomically well-defined (1 × 1) surface has qualities that aid the highly sensitive monitoring of the behaviors of adspecies using UHV analytical techniques. In the (1 × 1) surface structure (Figure 1), O atoms coordinate to two Ti atoms protruding on the surface and form rows along the [001] direction. These bridging O atoms are referred to herein as Ob atoms. Between the Ob atom rows, Ti atoms coordinate to five O atoms (hereafter denoted as Ti5c atoms) aligned in the [001] direction. Some of the Ob atoms are removed to form O vacancies, whereas the four stable in-plane O atoms surrounding the Ti5c atoms are left intact.11 The unit cell has dimensions of 0.30 nm × 0.65 nm, and the densities of the Ob and Ti5c atoms are both 5.1 nm−2. © XXXX American Chemical Society

Figure 1. Ball model of the rutile TiO2(110)-(1 × 1) surface. Light blue, red, and gray spheres represent Ti, O, and H atoms, respectively. The size of the unit cell is 0.3 nm × 0.65 nm.

Under an UHV, H2O molecules adsorb onto the (1 × 1) surface at temperatures below 140 K,12,13 with the O atoms oriented toward the surface.14 The H2O admolecules produce a desorption peak at about 270 K in temperature-programmed desorption spectra.13,14 This manner of desorption agrees with that observed in a variable-temperature scanning tunneling microscopy (STM) study15 in which the number of H2O admolecules on the Ti5c atoms decreased as the temperature was increased from 150 to 195 K and eventually disappeared from the surface at 290 K. Received: June 6, 2016 Revised: September 1, 2016

A

DOI: 10.1021/acs.jpcc.6b05661 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

(1 × 1) surface in humid environments with ex situ analysis using UHV conditions.

O vacancies actively participate in the dissociation of H2O molecules, forming two OH groups involving the O b atoms.9,16,17 Such OH groups, hereafter referred to as OHb groups, bridge two Ti atoms

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EXPERIMENTAL METHODS Preparation of the surfaces and STM imaging were performed using a commercial UHV−STM system (JSPM4500S, JEOL). The system is composed of a microscope chamber, a sample preparation chamber, and a small-volume load-lock chamber that are separated by gate valves. The microscope chamber includes an STM stage and a sample preparation chamber equipped with an Ar+ sputtering gun (EX03, Thermo) and a quadrupole mass analyzer (e-Vision+, MKS Instruments). The base pressure in the microscope chamber and in the sample preparation chamber was 2 × 10−8 Pa. The view ports of the chambers were shaded unless it was necessary to view the inside. The temperature and humidity in the environment surrounding the microscope system were approximately 297 K and ∼30%, respectively. Mirror-polished TiO2 wafers (Shinkosha) were clamped onto a Si wafer that was used as a resistive heater. The (1 × 1) surface was prepared by cycles of Ar+ sputtering and annealing at 1070 K. The temperature of the side faces of the wafers was monitored with an infrared thermometer. The temperature of the TiO2 surface was lower than the monitored temperature probably because of incomplete contact between the two wafers. During exposure of the (1 × 1) surface to water vapor after it had been cooled to room temperature, the ion pump of the sample preparation chamber was stopped. Milli-Q water vapor that had been de-aired by freeze−pump−thaw cycles was then introduced into the sample preparation chamber through a leak valve. The exposure time was measured after the leak valve was fully opened. The water vapor pressure was monitored with a crystal pressure gauge (M-320XG, Canon Anelva) in a separate experiment. The vapor pressure increased monotonically to 100 Pa in 600 s. After the leak valve was closed, the sample preparation chamber was evacuated with a turbomolecular pump through the load-lock chamber. The ion pump was then started, and the TiO2 wafer was quickly transferred into the microscope chamber. The pressure of the microscope chamber temporarily increased to the range of 10−6 Pa during sample transfer but returned to the 10−8 Pa range within a few minutes. A preliminary check of the water vapor at 5 × 10−6 Pa showed that O2 had a pressure ratio of 3 × 10−3. Exposure to laboratory air was performed in the shaded load-lock chamber, which was open to the air, and immersion was done in fresh Milli-Q water (pH 6.8) without de-airing. Constant-current STM images were obtained at room temperature using an electrochemically etched tungsten wire as a probe. Wide-area observation of the surfaces was performed in air by contact-mode atomic force microscopy (AFM) using an SPM 5500 microscope (Agilent Technologies) using Si3N4 cantilevers having a spring constant of 0.6 N/m (OMCL-TR800PSA-1, Olympus) as a probe. Image analysis was performed with WSxM software,25 and images are presented without filtering. XPS analysis using an Axis Ultra DLD system (Kratos) was performed at room temperature. The base pressure of the system was 6 × 10−7 Pa. The TiO2 wafers removed from the microscope system were placed in a shaded container and carried to the XPS system while allowing exposure of the wafers to air. The photoelectron emission angle with respect to the perpendicular line on the surface (θ) was typically set to 0°. Monochromatic Al Kα radiation was used as the excitation

−Ob−Ti−■−Ti−Ob− + H 2O → −Ob−Ti−OHb−Ti−OHb−

(R1)

The solid square in reaction R1 represents the O vacancy. OHb groups originally present on the (1 × 1) surface, which are from residual H2O molecules in the vacuum chamber, are referred to as intrinsic OHb groups in this article. The observed density of the intrinsic OHb groups on the (1 × 1) surfaces prepared by our procedure was found to be about 0.3 nm−2 and to depend on the surface-cleaning conditions. Another type of OH group terminates the Ti5c atom (represented as the OHt group in Figure 1). The OHt groups are formed by the reaction of OHb groups with O adatoms12,18 −Ob−Ti−OHb−Ti−Ob− + Ti−Oa → −Ob−Ti−Ob−Ti−Ob− + Ti−OH t

(R2)

Oa in reaction R2 represents an O adatom. The Oa atoms form by dissociation of O2 molecules at the Ti5c atoms19 along with O vacancies.9 OHt groups are not present on the (1 × 1) surface prepared in an O2-free UHV environment. Studies on the (1 × 1) surface now also consider humid environments because of interest in the behaviors of H2O molecules under conditions in which TiO2 materials are used. Uetsuka et al. removed a (1 × 1) surface from a UHV chamber, immersed it in distilled water, and reintroduced it into the chamber for study by STM.20 From their observation of nanosized pits on the terraces, they concluded that the surface became eroded in the water. Using STM analysis performed in vacuo at 10−4 Pa, our group observed atomic-scale corrugation on a (1 × 1) surface exposed to laboratory air.21 This observation suggests retention of the (1 × 1) structure of the terraces upon its brief exposure to air. We also explored the imaging of the (1 × 1) surface immersed in water by noncontact atomic force microscopy (NC-AFM).22 Ob atom rows appeared, along with nanosized particles, hypothesized to be H2O clusters, aligned along the [001] direction. Ketteler et al. also deduced this nucleation of H2O molecules from X-ray photoelectron spectroscopy (XPS) analysis of H2O molecule adsorption under ambient conditions of up to 200 Pa.23 Recently, Serrano et al. reported the STM imaging of the (1 × 1) surface in water.24 They observed spots with 2-fold periodicity along the [001] direction, hypothesizing that the spots are due to H2O dimers aligned on Ti5c atom rows. This work presents an analysis of (1 × 1) surfaces under an UHV upon their exposure to humid environments. STM imaging revealed that molecule-sized spots covered the surfaces. The spots might be OHt groups and hydroperoxyl (OOH) groups, as indicated in previous reports.18 OHt and OOH groups aligned along the [001] direction indicate retention of the (1 × 1) structure prepared under UHV after exposure to humid environments. Dissociation of H2O molecules induced by evacuation of the water layer might be the mechanism by which the OHt groups form. Identifying the evacuation-induced species from H 2 O molecules helps to verify surface modifications specific to humid environments. These results suggest the possibility of complementing in situ analysis of the B

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The Journal of Physical Chemistry C source. Charging of the samples was minimized through the use of a neutralizer. The pass energy of the analyzer and the energy step were 160 and 1.0 eV, respectively, for wide scans and 20 and 0.1 eV, respectively, for narrow scans. The binding energy of the spectra was calibrated such that the major O 1s peak was 530.3 eV.26 With this calibration, the Ti 2p3/2 peak was corrected to 459.1 eV. The spectra were deconvoluted into mixed Gaussian−Lorentzian curves (70:30) after Shirley-type background subtraction.

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RESULTS AND DISCUSSION Figure 2a,b shows topographical images of the (1 × 1) surface. Monatomic 0.33-nm-high steps separate flat terraces, with bright lines along the [001] direction corresponding to the Ti5c atom rows. Short lines bridging the adjacent Ti5c atom rows are either intrinsic OHb groups or O vacancies. These are typical features of STM images of the (1 × 1) surface.9,10 The density of the short lines is 0.4 nm−2, which is roughly 10 times that of the O vacancies (0.03 nm−2), as determined in our previous NC-AFM study.27 Most of the short lines are therefore intrinsic OHb groups. Bright strings and particles, indicated by arrows in Figure 2a, are reduced TiOx species. After exposure to water vapor for 60 s, the (1 × 1) surface was observed to be covered by molecule-sized circular spots (Figure 2c). The spots were almost uniformly distributed, and their density, as estimated from a 300 nm2 area, was 1.2 nm−2. Figure 2d shows a magnified image of the terrace. Spots with different heights appeared to be aligned along the [001] direction. The minimum distance between the spots along the [001] direction was 0.6 nm, which is twice the Ti5c−Ti5c distance. The spots in the region shown in Figure 2e, where their density was accidentally low, were classified according to their heights. Ti5c atom rows, which occur as lines with low brightness in the spot-free areas, indicate retention of the (1 × 1) structure under high-pressure H2O vapor. Most of the spots were found to be located on the Ti5c atom rows. The height distribution of 202 of these spots, as measured from the Ti5c atom row, is shown in Figure 2f. The shortest (darkest) spots, with heights of 0.06 ± 0.02 nm, are referred to as A spots. The second shortest spots, having heights of 0.12 ± 0.02 nm, are referred to as B spots. Those with heights of 0.17 ± 0.02 nm, which comprise about 70% of the spots, are referred to as C spots. The tallest spots, having heights of 0.22 ± 0.02 nm, are referred to as D spots. The D spots exhibited slightly larger lateral diameters than the A−C spots. A schematic model of the image in Figure 2e is shown in Figure 2g, where solid lines represent the Ti5c atom rows and symbols correspond to spots of different heights. The densities of the spots are listed in Table 1. Strings indicated by the arrows in Figure 2e, which are 0.1 nm higher than the Ti5c atom row, can be assigned as B spots migrating on the Ti5c atom rows. Spots that shifted from the Ti5c atom rows are represented by crosses and are referred to as E spots. On the basis of previous STM studies,18,28 A, B, and C spots can be explained by the presence of Oa atoms, OOH groups, and OHt groups, respectively. The heights of these species roughly agree with those reported by Du et al., namely, 0.03 ± 0.1 nm for Oa atoms, 0.05 ± 0.1 nm for OOH groups, and 0.09 ± 0.2 nm for OHt groups.18 The OOH groups, Oa atoms, and OHt groups exhibited circular shapes. Du et al. observed OOH groups as spots that were slightly elongated in the [001] direction. On the basis of density functional theory (DFT)

Figure 2. (a,b) STM images of the (1 × 1) surface with areas of (a) 40 × 40 and (b) 10 × 10 nm2. (c,d) STM images of the (1 × 1) surface exposed to water vapor for 60 s with areas of (c) 40 × 40 and (d) 10 × 10 nm2. (e) STM image (10 × 10 nm2) of the (1 × 1) surface exposed to water vapor for 60 s. The density of spots was found to be accidentally low in the imaged area. (f) Heights of the 202 spots from the Ti5c atom rows measured in STM images of the low-density area. (g) Model of the image in panel e. Solid lines along the [001] direction represent the Ti5c atom rows. White squares, black circles, gray circles, and white circles represent A, B, C, and D spots, respectively. The crosses mark the spots that were shifted from the Ti5c atom rows. (h) STM image (40 × 40 nm2) of the (1 × 1) surface exposed to water vapor for 600 s; a closeup of the terrace (10 × 10 nm2) is shown in the inset. Sample bias voltage, +1.4 V; tunneling current, 0.2 nA.

energy calculations, they hypothesized that the OOH groups rotate about the Ti5c−O bond, taking the stable “transverse conformation” (Figure 3a) and the less stable “up conformation” (Figure 3b). The OOH plane is perpendicular to the [001] direction when oriented in the transverse configuration, whereas it is parallel to the [001] direction when oriented in C

DOI: 10.1021/acs.jpcc.6b05661 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Table 1. Densities (nm−2) of Surface Species Estimated from STM Images surface exposure conditions

A spots (Oa atoms)

B spots (OOH groups)

C spots (OHt groups)

D spots (OOH groups)

water vapor, 60 s water vapor, 600 s water vapor, 600 s + O2 laboratory air, 60 s laboratory air, 600 s Milli-Q water, 60 s

0.04 0.03 0.1

0.2 0.2 0.2 0.6 0.7 0.5

0.9 1.1 1.1 1.0 0.9 1.5

0.03 0.02 0.06 0.1 0.1 0.2

middle of adjacent Ti5c atom rows are assigned to paired OHb groups. One such E spot is circled in Figure 2e. Two OHb groups having adjacent Ob atoms can be observed in STM images as single spots, which appear larger than an isolated OHb group.9 Another type of E spot that is shifted from the middle of the adjacent Ti5c atom rows might be Oa atoms at hollow sites consisting of Ob atoms and in-plane O atoms. One such E spot is surrounded by a square in Figure 2e. Similarly, adsorption of Cl atoms on such hollow sites was observed in a previous study.30 On the basis of the above explanations for the different spots, the densities of the OHt and OOH groups were estimated to be 0.9 and 0.2 nm−2, respectively. The high density of OHt groups (0.9 nm−2) was probably achieved by the dissociation of H2O molecules. H2O molecules are coordinated to Ob and Ti5c atoms at the interface between the (1 × 1) surface and the water layer. Two H2O molecules coordinated between the Ob and Ti5c atoms are shown in panel i of Figure 3c. Bridging the nearest Ob and Ti5c atoms is impossible for a single H2O molecule because of the large Ob− Ti5c distance.13 Upon evacuation of the surface, H2O molecules coordinated to the Ob and Ti5c atoms dissociate, forming OHt and OHb groups, as shown in panel ii of Figure 3c −Ob−Ti−Ob−Ti−Ob− + Ti5c + nH 2O → −Ob−Ti−OHb−Ti−Ob− + Ti−OH t + (n − 1)H 2O (R3)

The presence of a hydrogen-bonding network near the surface22 suggests that a high density of H2O molecules coordinate to the Ob and Ti5c atoms, thereby forming the underlying structure for the hydration layer. Some of the OHt groups might associatively desorb, thus leaving Oa atoms18 Figure 3. Schematic models of (a) OOH groups with the transverse conformation,18 (b) OOH groups with the up conformation,18 and (c) desorption of the H2O layer, which leads to formation of the OHb and OHt groups. Solid lines and dotted lines represent covalent bonding and hydrogen bonding, respectively.

Ti−OH t + Ti−OH t → Ti−Oa + Ti5c + H 2O

(R4)

This desorption might be promoted by the instability of OHt groups that coordinated to adjacent Ti5c atoms. We observed that the distance between the two nearest OHt groups is twice the Ti5c−Ti5c distance. OHt groups that immediately migrated to the adjacent Ti5c atoms and separated from the neighboring OHt groups remained on the surface. We observed that the reaction of the intrinsic OHb groups with the Oa atoms (reaction R2) makes a minor contribution to the formation of the OHt groups. The density of Oa atoms formed at the O vacancies might be less than that of the O vacancies, which have a density of 0.03 nm−2. The density of Oa atoms formed by the Ti interstitials is probably about 0.1 nm−2, because the density of the Ti interstitials is probably comparable to that of the (1 × 1) surfaces prepared by the conventional cleaning method. Therefore, Oa atoms forming OHt groups having a density of 0.9 nm−2 were insufficient. OOH groups originating from the intrinsic OHb groups were present on the surface earlier than the evacuation of the water

the up configuration. A longer time in the energetically favorable transverse conformation accounts for the elongated shape observed in STM images. The circular image of the OOH groups observed in the present study therefore implies their uniform rotation between the two conformations. In the transverse conformation, the H atom of the OOH group is coordinated to the nearest Ob atoms by hydrogen bonding. This hydrogen bonding might be lost when Ob atoms nearest to these groups are converted into OHb groups. D spots are assigned to the OOH groups in the up conformation, because this conformation is geometrically higher than the transverse conformation. The conformation of the OOH group might be sensitive to its interaction with neighboring adspecies, such as dipole−dipole repulsion among carboxylates.29 E spots in the D

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The Journal of Physical Chemistry C layer. They form through the reaction of OHb groups with O212,18 −Ob−Ti−OHb−Ti−Ob− + Ti5c + O2 → −Ob−Ti−Ob−Ti−Ob− + Ti−OOH

(R5)

The density of OOH groups (0.2 nm−2) was comparable to that of the intrinsic OHb groups. O2 molecules introduced into the chamber with water vapor subsequently dissolved in the water layer on the surface. The OHb groups and Oa atoms formed through H2O dissociation and through subsequent reactions as described above. The densities of OHb groups, as seen in Figure 2g, were low particularly because OHb and OHt groups formed in equal amounts through reaction R3. The faint features in the gap between adjacent Ti5c atom rows are probably due to isolated OHb groups. The open arrowheads in Figure 2e indicate three instances of this feature. The STM topography of the isolated OHb group depends strongly on the structural/chemical conditions of the tip end. Sánchez-Sánchez et al. predicted on the basis of DFT calculations that the OHb group forms a protuberance when the tip is terminated by a single O atom.31 Oa atoms do not form clear spots probably because of the weak contrast in the image, instead comprising bumps in the Ti5c atom rows. The solid arrowheads indicate three such bumps. In Figure 2e, the Ti5c atom rows appear rugged, whereas in Figure 2b, they form lines with a uniform width on the adsorbate-free (1 × 1) surface. Figure 2h shows an STM image of the surface following exposure to water vapor for 600 s. The densities of OHt and OOH groups in a 300 nm2 area were estimated to be 1.1 and 0.2 nm−2, respectively. These densities are comparable to those of the OHt and OOH groups on the surface exposed to water vapor for 60 s, as shown in Table 1. The independence of the densities of both groups from the exposure time can be explained by the formation of OHt groups through H2O dissociation and the formation of OOH groups from intrinsic OHb groups. Subsequent exposure of the surface to O2 at 1 atm pressure in the preparation chamber did not change the densities of the OHt and OOH groups, as shown in Table 1. The formation of OOH groups as described in reaction R5 is hindered by OHt groups and Oa atoms on the Ti5c atoms near the OHb groups. An increase in the density of the Oa atoms reflects not an actual increase but rather the uncertainty in the identity of the low Oa atoms in the high OHt and OOH groups. The formation of OHt and OOH groups on the (1 × 1) surfaces introduced into the vacuum chamber after exposure to humid environments was expected, because the surface was covered by a water layer in such environments. Figure 4a shows a topographical image of the (1 × 1) surface exposed to laboratory air for 60 s. The surface was covered by moleculesized spots, similarly to the surfaces exposed to water vapor. The spot density, estimated from a 300 nm2 area, was 1.7 nm−2. Some of the spots were packed in a (2 × 1) arrangement. Figure 4b shows the height distribution of 513 spots in three images of different areas. The spots were classified roughly into three groups with separations of 0.05 and 0.03 nm. The height differences were comparable to those between OHt and OOH groups found on the surfaces exposed to H2O vapor. The spots are therefore attributed to the presence of OOH groups with the up conformation, OHt groups, and OOH groups with the transverse conformation as the height increases. Bright particles exhibiting irregular shapes, as indicated by arrows, are

Figure 4. (a) STM image (10 × 10 nm2) of the (1 × 1) surface exposed to laboratory air for 60 s. (b) Heights of molecule-sized spots in three images (10 × 10 nm2) of the (1 × 1) surface exposed to laboratory air for 60 s. (c) Model of the image shown in panel a. Solid lines along the [001] direction represent the Ti5c atom rows. (d) STM image (10 × 10 nm2) of the (1 × 1) surface exposed to laboratory air for 600 s. (e) STM image (10 × 10 nm2) and (f) a model of the image of the (1 × 1) surfaces immersed in Milli-Q water for 60 s. In panels c and f, OOH groups with the transverse conformation, OHt groups, and OOH groups with the up conformation are represented by black, gray, and white circles, respectively. Sample bias voltage, +1.4 V; tunneling current, 0.2 nA.

contaminants. A model of the image in Figure 4a based on the aforementioned classification of spots is shown in Figure 4c. The densities of OHt and OOH groups were 1.0 and 0.7 nm−2, respectively, as listed in Table 1. Extending the exposure time to 600 s produced little change in the topography and densities of the surface species (Figure 4d and Table 1). An increase in the density of OOH groups relative to that of OOH groups on surfaces exposed to H2O vapor was also observed on the (1 × 1) surface immersed in Milli-Q water. A topographical image and model of the (1 × 1) surface immersed in Milli-Q water for 60 s are shown in panels e and f, respectively, of Figure 4. The (2 × 1) arrangement of the OHt and OOH groups indicates that the (1 × 1) structure tolerated short exposure to both air and water. The water layer formed in the atmosphere prevents immediate contamination of the surface. In comparison with the density of the OOH groups on surfaces exposed to H2O vapor, that of OOH groups on the surfaces exposed to both air and water more than doubled. Some of the OHb groups formed by the dissociation of H2O immediately reacted E

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having Ti 2p and O 1s core-level peaks at about 460 and 530 eV, respectively. The only observed impurities were Ar, which was injected during sputtering, and C from adventitious contaminants. The majority of the contaminants were probably present in the form of particles. Particles with diameters of 150 nm and heights of 2 nm can be found in a wide-field view of the surface obtained by contact-mode AFM in air, as shown in Figure 5b. Figure 5c shows narrow-scan spectra of the O 1s region. The intense peak at 530.3 eV is due to O atoms in a bulk environment, hereafter referred to as Obulk atoms. A shoulder accompanies the Obulk peak on the high-bindingenergy side. The relative intensity of the shoulder with respect to the Obulk peak increased at θ = 60°, indicating that the shoulder is due to the surface species. Possible surface species that contribute to the shoulder are Ob atoms, OH groups, Oa atoms, and OOH groups. Ob atoms, OH groups, and Oa atoms on (110) surfaces have been reported to give peaks at binding energies higher than those of Obulk atoms by 1.2−1.3,32−34 1.1−2.4,23,34,35 and 2.5 eV,33 respectively. A peak with a binding energy 3.2 eV higher than that of the Obulk peak was observed in the spectra of TiO2(001) surfaces after their immersion in an aqueous H2O2 solution. This peak is attributed to surface peroxides.36 Organic contaminants containing CO or COO groups also contributed to the O 1s shoulder, as evidenced by the 2.2−2.4 eV shifting of peaks with positive binding energy from the Obulk peak.37 To minimize the number of peaks used for curve fitting, the OHb and OHt groups, Oa atoms and organic contaminants were assumed to produce peaks at the same binding energy. The O 1s spectrum was fitted by adding four peaks at 531.2, 531.6, 532.1, and 532.5 eV to the Obulk peak at 530.3 eV, as shown in Figure 5d. The four peaks are assigned to Ob atoms, OH groups, Oa atoms and contaminants, and OOH groups, respectively, in accordance with the order of binding-energy shifts. The deviation of the binding-energy shifts from those in previous reports is probably due to the chemical states of the species, which are sensitive to the atomic structures of the TiO2 surfaces. According to Peng and Barteau,38 the ratio of the intensity of the peak from the surface species to that of the Obulk peak, I/ Ibulk, is related to the coverage of the surface species, x, by the equation

with O2 molecules in the water layer. The likelihood of reaction R5 occurring increased when the concentration of O 2 molecules was high. Consequently, the densities of OOH groups were approximately equivalent to those of OOH groups on the surfaces exposed to both laboratory air and water. Using a separate XPS system, we performed ex situ chemical analysis of the (1 × 1) surface prepared in the microscope chamber. The distribution ratio of the species on the analyzed surface was expected to be similar to that on the surfaces that had been exposed to air. Figure 5a shows a wide-scan spectrum

I ≈ Ibulk ⎡ ⎣1 − exp

(

x −d λ cos θ

)⎤⎦

−1

− 0.25

(1)

In this equation, an x value of 1 corresponds to the density of O atoms in a unit cell area of the TiO2 layer (20.51 nm−2), and d is the distance between adjacent TiO2 layers along the [110] axis (0.33 nm). λ is the inelastic mean free path of the O 1s photoelectron in TiO2 (1.45 nm).39 The coverage of the Ob atoms on the OHb-free (1 × 1) surface (0.25) was subtracted from the sum of the signals from the TiO2 layers. The coverages of the surface species calculated by eq 1 were converted into densities (listed in Table 2). The calculated

Figure 5. (a) Wide-scan XPS spectrum of the (1 × 1) surface. (b) AFM image (3000 × 3000 nm2) of the (1 × 1) surface. (c) Narrowscan O 1s XPS spectra of the (1 × 1) surface obtained at θ values of 0° and 60°. (d) Magnified shoulder of the O 1s spectrum obtained at θ = 0°. The raw spectrum and the fitted curve are represented by a thick black line and a thin light blue line, respectively. Other colored lines represent the component peaks.

Table 2. Densities (nm−2) of Surface Species on the (1 × 1) Surface Exposed to Air analytical method

531.2 eV Ob atoms

531.6 eV OH groups

OHb groupsa

OHt groupsa

532.1 eV Oa atoms, contaminants

532.5 eV OOH groups

XPS STM

0.6

5.3

(4.5)

(0.8) 0.9

3.8

0.6 0.7

a

Parentheses indicate values estimated from the densities of Ob atoms and OH groups by XPS analysis. F

DOI: 10.1021/acs.jpcc.6b05661 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C density of Ob atoms is 0.6 nm−2. By subtracting the calculated density of the Ob atoms from the density of the Ob atoms on the OHb-free (1 × 1) surface, the density of the OHb groups was estimated to be 4.5 nm−2. The density of OHt groups was estimated to be 0.8 nm−2 from the difference between the calculated density of all OH groups (5.3 nm−2) and the estimated density of the OHb groups. The calculated densities of O atoms of OOH groups and of OOH groups are 1.2 and 0.6 nm−2 based on an equivalent contribution of the two O atoms to the peak. The densities of the OHt and OOH groups agree with those determined from the STM images. The densities of the surface species were based on the evacuation of the water layer, as discussed above. We assume that OHt groups with a density of 4.7 nm−2 formed with the OHb groups upon dissociation of H2O molecules. According to reaction R4, OHt groups with a density of 0.8 nm−2 remained on the surface, and groups having a density of 3.9 nm−2 desorbed. Oa atoms with a density of 2.0 nm−2, half of which were from desorbed OHt groups, remained on the Ti5c atoms. The density of Oa atoms was about one-half that of O atoms calculated from the peak at 532.1 eV, with organic contaminants and Oa atoms contributing to the peak. Prior to dissociation of the H2O molecules, intrinsic OH groups with a density of 0.4 nm−2 reacted with O2 in the water layer, forming OOH groups, as described in reaction R5. OHb groups having a density of 0.2 nm−2 formed through the dissociation of H2O molecules converted into OOH groups, and the density of the OOH groups reached 0.6 nm−2. Thus, the density of the OHb groups was reduced to 4.5 nm−2. The density of OHb groups (4.5 nm−2) corresponds to the replacement of 88% of the Ob atoms with OHb groups. This density appears to be remarkably high compared to the results of previous studies. The dissociation of H2O molecules at the Ob vacancies (reaction R1) has been the most commonly used mechanism for regulating the density of the OHb groups.9,16 The density of the Ob vacancies depends on the cleaning conditions of the system, such as the frequency of the sputter− anneal cycles9 and the annealing temperatures,40 and is generally less than 0.5 nm−2. The density of OHb groups at the Ob vacancies (reaction R1) is therefore 1.0 nm−2 at a maximum. Even with the inclusion of the intrinsic OHb groups, the density of the OHb groups did not exceed 1.5 nm−2. By removing Ob atoms through electron beam irradiation, Tatsumi et al. prepared a (1 × 1) surface having Ob vacancies and intrinsic OHb groups with densities of 0.3 and 0.8 nm−2, respectively.41 Under the electron beam irradiation conditions, the density of OHb groups on the surface can increase up to 1.4 nm−2. To produce OHb groups independently of Ob vacancies, atomic H can be introduced, as reported by Suzuki et al.42 This method can increase the density of these groups up to 1.2 nm−2. Wang et al. recently performed a study using the pivalate [(CH3)3CCOOH] molecule as a H source to produce OHb groups.43 Pivalic acid molecules dissociate on the (1 × 1) surface, producing pivalate anions [(CH3)3CCOO−] and H cations.44 Each pivalate anion coordinates to two Ti5c atoms in a bridging manner, forming a (2 × 1) monolayer, and the H cations coordinate to the Ob atoms, forming OHb groups. Photochemical processes then selectively remove the pivalate anions, leaving the OHb groups behind. Repeating the adsorption−desorption of the pivalate anions twice increased the density of the OHb groups to 4.7 nm−2. The results of this method suggest the densification of OHb groups by using dissociation of molecules that furnish H atoms. The high

density of OHb groups in the present study is therefore possible when the H2O molecules that are coordinated to Ob and Ti5c atoms dissociate.

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CONCLUSIONS We observed the retention of the (1 × 1) surface during its short exposure to humid environments. We also found Ocontaining species on (1 × 1) surfaces that had been reintroduced into the UHV chamber after exposure to humid environments. On the basis of previous reports, these species were hypothesized to be OHb groups, OHt groups, OOH groups, and Oa atoms. These surface species probably form by dissociation of H2O molecules induced by evacuation of the water layer on the surface. The results of this study indicate that the (1 × 1) surface, when exposed to humid environments, can be characterized on an atomic/molecular scale by ex situ analysis performed under an UHV. The many previously established UHV techniques complement in situ analysis of the (1 × 1) surface in humid environments, thus broadening the choice of methods for modifying the surface to high-pressure gas processes and wet processes, which aid in constructing realistic models of TiO2based materials. Moreover, the evacuation-induced dissociation of H2O might be a key for other oxides that are prepared under ambient conditions and subjected to industrial vacuum processes such as vapor deposition, etching, and atom doping. For instance, equipping a UHV system with high-pressure reactors, microscopes, and spectroscopy facilities for further analysis of the surface in a controlled system reinforces the ex situ analysis of these surfaces.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +81-78-803-5674. Fax: +81-78-803-5674. Present Address †

Department of Chemistry, Faculty of Science, Kobe University, Nada-ku, Kobe 657-8501, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research from the Japanese Society for the Promotion of Science KAKENHI (Grants 26600024, 26630330, 24246014, and 16K13624).

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DOI: 10.1021/acs.jpcc.6b05661 J. Phys. Chem. C XXXX, XXX, XXX−XXX