Transient Mobility of Oxygen Adatoms upon O2 Dissociation on

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J. Phys. Chem. C 2008, 112, 2649-2653

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Transient Mobility of Oxygen Adatoms upon O2 Dissociation on Reduced TiO2(110) Yingge Du,† Zdenek Dohna´ lek,‡ and Igor Lyubinetsky*,† EnVironmental Molecular Science Laboratory, and Fundamental Science Directorate, Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352 ReceiVed: September 24, 2007; In Final Form: NoVember 12, 2007

Tracking the same region of the reduced TiO2(110) surface by scanning tunneling microscopy before and after oxygen exposure at room temperature (RT) confirms that O2 molecules dissociate only at the bridging oxygen vacancies, with one O atom healing a vacancy and other O atom bonding at the neighboring Ti site as an adatom. The majority of O adatoms (∼81%) are found separated from the original vacancy positions by up to two lattice constants along the [001] direction. Since at RT the thermal diffusivity of O adatoms has been found to be rather small, with an experimentally estimated activation energy of ∼1.1 eV, we conclude that the observed lateral distribution of the oxygen adatoms is attained through a nonthermal, transient mobility during the course of O2 dissociation. Unlike for other known cases of the dissociation of diatomic molecules where both “hot” adatoms accommodate at equivalent sites, in the studied system, the oxygen atoms filling the vacancies are locked into the bridging oxygen rows, and only the O adatoms are relatively free to move. The transient motion of the hyperthermal oxygen adatoms on the TiO2(110) surface occurs exclusively along the Ti troughs.

1. Introduction The interaction of molecular oxygen with TiO2-based materials can affect many chemical and photochemical processes. In particular, it plays an important role in a wide range of applications such as photocatalysis, degradation of organic pollutants, and water splitting for hydrogen production.1-3 Consequently, the systematic studies of O2 surface chemistry have been carried out on the prototypical metal oxide surface of rutile TiO2(110).4,5 The electronic structure and reactivity of the TiO2(110) surface are known to be influenced to a great extent by the bridging oxygen vacancies, the most common point defects resulting, for example, from the reduction by the vacuum annealing.4-6 In particular, O2 adsorbs molecularly at T e 150 K or, at higher temperatures, dissociates on the TiO2(110) surface only when the surface oxygen vacancies are present.4,6-8 Oxygen-vacancy-mediated dissociative adsorption of O2 molecules on TiO2(110) is shown to be energetically favorable by a number of theoretical investigations.9-12 A single bridging oxygen vacancy exposes two undercoordinated Ti atoms. These vacancies are removed by O2 exposure at temperatures above 150 K.7,13 However, it turns out that the O2 interaction with the reduced TiO2(110) surface is much more complex than a straightforward vacancy healing process.7,14,15 On the basis of the temperature-programmed desorption (TPD) observations, Henderson and co-workers have suggested that for each O2 molecule dissociated at the vacancy, one O atom fills a vacancy while the other O atom resides on the surface as an adatom, bound to the nearest-neighbor fivefold-coordinated Ti site in the adjacent titanium row.7,15 The O adatom species are not detected by the surface spectroscopy techniques of photoemission or electron energy loss spectroscopies,15,16 although they have been observed by the scanning tunneling microscopy * To whom correspondence should be addressed. E-mail: igor. [email protected]. † Environmental Molecular Science Laboratory. ‡ Fundamental Science Directorate.

(STM) as bright protrusions on the Ti rows.11,14,17 Two recent STM studies at and below room temperature have confirmed that each O2 dissociation event causes the filling of a single oxygen vacancy and the deposition of an O adatom at the nearby Ti atom site.11,17 The oxygen adatoms are found to significantly perturb the surface chemistry of adsorbed water, ammonia, and methanol15,18 and may alter the surface chemistry of other adsorbate species on the TiO2(110) as well. Despite the remarkable progress that has been made in the fundamental understanding of the O2 dissociation process on the TiO2(110) surface, surprisingly little is known about the mobility and/or the lateral distribution of the O adatoms. In the recent STM observation, virtually no oxygen adatom motion was detected at temperatures e 300 K.11 Density functional theory (DFT) calculations have estimated the energy barrier of ∼1.14 eV for the O adatom diffusion along the Ti rows.11 DFT calculations have also revealed that there is a large energy release of ∼3.6 eV associated with the O2 dissociative chemisorption process. A considerable part of this energy can be transferred to the translation degrees of freedom, which may lead to transient mobility of the dissociation products along the surface. Such kinetically hyperthermal (“hot”) adsorbates have been observed in several other systems involving O2 dissociation on Al(111), Pt(111),19,20 or Cl2 dissociation on TiO2(110).21 STM studies have found widely separated adatoms upon dissociation of O2 on Al(111) (>8 nm) and Cl2 on TiO2(110) (>2 nm),19,21 while only a small average distance of two lattice constants has been detected between adatom pairs for O2 on Pt(111).20 To date, it is not known whether this process is relevant to the O2 dissociation on the TiO2(110) surface. A detailed answer to this question is highly important because it may have a profound effect on the kinetics and reaction mechanisms of many chemical and photochemical processes involving oxygen interactions with TiO2. It has been suggested that hyperthermal adsorbate species may operate as “hot”

10.1021/jp077677u CCC: $40.75 © 2008 American Chemical Society Published on Web 01/05/2008

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Du et al.

Figure 1. STM images of the same (11 × 10) nm2 area of the TiO2(110) surface (a) before and (b) after adsorption of 0.03 ML of O2 at 300 K. One of the oxygen vacancies, double hydroxyl group, and oxygen adatom are labeled with I, II, and III, respectively.

precursors in surface chemical reactions and have a large effect on corresponding reaction rates.22 In this paper, we report the results of the STM statistical analysis of the initial stages of the O2 dissociation on the reduced TiO2(110) surface. The observed lateral distribution of the O adatoms, separated from the original vacancies by up to two lattice constants along the [001] direction, has been interpreted in terms of a limited nonthermal, transient mobility resulting from O2 dissociation. 2. Experimental Section The STM experiments were conducted in an ultrahigh vacuum (UHV) system (base pressure 3 × 10-11 Torr) equipped with a variable-temperature STM (Omicron), a semispherical electron energy analyzer (Omicron), a mass spectrometer (Ametek), and electron and ion guns (VG and SPECS, respectively). The single-crystal rutile TiO2(110)-(1 × 1) surface (Princeton Scientific) was prepared by multiple cycles of Ar ion sputtering (2 keV) and UHV annealing (800-900 K), with cleanness monitored by Auger electron spectroscopy. Special care has been taken to reduce the contamination by background water, and at the beginning of each experiment, the sample was flash-annealed to 600 K with a pressure rise not more than 6 × 10-11 Torr. The initial stages of the oxygen adsorption have been studied, with all experiments carried out at RT. We have analyzed the same surface area before and after adsorption, which allowed us to follow the initial defects (oxygen vacancies) and to monitor changes caused by the adsorption of single O2 molecules. Oxygen was introduced using a movable directional doser connected to a 1 mm i.d. tube that terminated 3 mm from the STM tunneling junction. To overcome the STM tip shadow effect, the tip was retracted 1 µm from the surface during O2 exposures. The adsorbate coverage and vacancy concentration were obtained by a direct counting from STM images and expressed in monolayer units (1 ML corresponds to 5.2 × 1014 cm-2 Ti atoms). STM tips were made from electrochemically etched W wire and cleaned in situ by the annealing and ion sputtering. Presented STM (empty state) images were collected in a constant-current (0.1-0.3 nA) mode at positive sample bias voltages of 1.5-1.8 V. 3. Results and Discussion Figure 1 shows STM images of the same area of the TiO2(110) surface before and after adsorption of 0.03 ML of oxygen at 300 K. The STM empty state image contrast of a clean TiO2(110) surface has been known to be dominated by electronic effects rather than topography, with the rows of twofoldcoordinated bridging oxygen atoms (the features protruding most above the terminal surface plane) being imaged dark and the

Figure 2. STM images of the same (4 × 4) nm2 area (a) before and (b) after O2 adsorption. Images (a) and (b) are shown with a superimposed lattice grid in (c) and (d), respectively; (e) and (f) are the corresponding schematic models. Large, blue spheres denote bridging oxygen atoms, small red spheres denote fivefold Ti atoms and, oxygen atoms resulting from O2 dissociation are represented with yellow spheres. Different configurations of the oxygen adatoms, A, B, and C, are shown in (f).

rows of the terminal fivefold-coordinated titanium atoms being imaged bright.5 There are ∼12% bridging oxygen vacancies (seen as bright spots on dark rows, e.g., encircled such as I) as a result of the vacuum annealing treatment. Only a very small number of the hydroxyl species from background water dissociation at oxygen vacancies has been detected. Two separated single OH species, labeled as II and distinguished from O vacancies by relatively larger size/brightness,11,23 appear in Figure 1. The large, bright spot to the left of the image center is a minority feature possibly due to a different titanium oxide surface phase which is often observed on a sputtered and UHVannealed surface.24 After oxygen dosing, the new features that appear are small well-defined bright spots centered on the Ti rows, for example, marked as III in Figure 1b. These features are attributed to the oxygen adatoms residing at the fivefoldcoordinated Ti sites (Oa),11,14,17 and they originate from O2 dissociation.15 A magnified view of a smaller area before and after O2 dosing is displayed in Figure 2a and b for a more detailed analysis. These two STM images are duplicated in Figure 2c and d, respectively, with a lattice grid superimposed to identify the exact positions of the surface atoms. As a result of such an assessment, the one-to-one schematic models of the analyzed area have been constructed, Figure 2e and f, unraveling the positions of the oxygen vacancies and oxygen adatoms. Comparing Figure 2e and f, it is clear that each Oa species can be related to a disappeared (healed) O vacancy, which has been filled by the other oxygen atom (Ov) of the O2 molecule as a result of its dissociation at the vacancy. This observation agrees with previous STM studies11,17 and supports the proposed reaction mechanism.15 Although the corresponding Ov and Oa

Transient Mobility of Oxygen Adatoms on TiO2(110)

Figure 3. (a) Relative abundances of three distinctive configurations of O adatoms. (b) Schematic model showing the proposed transient motion paths.

atoms from the dissociation of a single O2 molecule remain close, it is evident that there are three different configurations for Oa adatoms relative to the position of the corresponding Ov atoms. There is one Oa atom (top left corner) sitting directly next to the annihilated vacancy, marked as configuration A in Figure 2f. Using the lattice constant (∼3.0 Å) as a reference, three Ov and Oa pairs are found to be separated by one lattice constant along the [001] direction, and one pair (bottom right corner) is two lattice constants apart. Here, we define these two configurations as B and C, respectively. It should be noted that no Oa/Ov pairs apart by more than two lattice constants along the [001] direction or Oa atoms on a next-nearest Ti row have been found. Also, there has been no evidence for one O2 molecule healing two separate O vacancies. To obtain a statistical distribution of the observed configurations, we have analyzed STM images from six separate experiments on two different samples. Figure 3a shows the relative abundances of three distinctive configurations. We have found that the configuration A accounts for 19%, B accounts for 74%, and C accounts for 7% (out of 110 total counts). Remarkably, the majority (81%) of the oxygen adatoms are found separated along the [001] direction, albeit by a small distance, from the original vacancy positions. This observation somewhat deviates from the existing model, where Ov and Oa pairs have been believed to occupy solely the nearest-neighbor sites (configurations A).15 In two recent STM studies, the authors have claimed that dissociated oxygen atoms reside next to each other based on the images acquired at 135 K11 and RT.17 Both STM works have shown no more than a few oxygen adatoms though and have not given any statistical information. In fact, re-examination of the published RT STM image has revealed that Oa atoms in a configuration B are also present there.17

J. Phys. Chem. C, Vol. 112, No. 7, 2008 2651 One possible explanation for the formation of the different configurations of the oxygen adatoms is a surface thermal diffusion. To explore this possibility, we have tracked each chosen area for a prolonged period of time following the oxygen exposure. Systematic examination of a series of STM timelapsed images (movies), acquired from several separate experiments, reveals that in about 3 h, only ∼9% changed their position along Ti rows (each hop corresponding to one lattice constant), resulting in an average hoping rate of ∼8 × 10-6 s-1 at 300 K. If the observed Oa motion is a direct result of the thermally induced diffusion with an Arrhenius-type process, the hopping rate, ν, is given as ν ) ν0 exp(-∆E/kBT), where ν0 is the attempt frequency, ∆E the activation energy barrier, kB the Boltzmann constant, and T the temperature. Assuming the typical attempt frequency of ∼1013 s-1, the activation energy for the oxygen adatom diffusion along a titanium row is estimated to be ∼1.1 ( 0.1 eV. This value is comparable with a previously calculated energy barrier of 1.14 eV for this process.11 Such a relatively large energy barrier could explain the rarely found motion of the Oa atoms at RT. Interestingly, this energy barrier is rather close to the activation energy barrier of 1.15 eV for the diffusion of the bridging oxygen vacancies,25 which involves the motion of the bridging oxygen atoms, also along the [001] direction. Additionally, it should be noted that the above-described infrequent Oa atom hops could be caused by another possible mechanism, namely, the oxygen scrambling during reaction between H2O (from a minor background adsorption) and O adatoms.15,26 The net result of the proton transfer and hydroxyls recombination may also lead to an apparent oxygen adatom shift of one lattice constant along the [001] direction. Our future STM studies conducted in the range of elevated temperatures should be able to distinguish between these two different mechanisms. However, keeping in mind a practically immobile nature of the Oa atoms at RT, the observation of both configurations B and C is a direct proof that those Oa atoms reach their final positions via a different mechanism. We conclude that a limited transient mobility of the hyperthermal (“hot”) oxygen adatoms, created in the course of O2 dissociative adsorption, is responsible for Oa atom migration along the Ti rows. Without such a mobility mechanism, the Oa and Ov atoms would always have been detected on the closestneighbor sites, in contrast to the observations. In fact, when an O2 molecule dissociates on the TiO2(110) at a bridging oxygen vacancy, the molecular bond between two oxygen atoms is broken, three O-Ti bonds are formed (two inside of the O vacancy and another with the fivefold-coordinated Ti), and there is a large energy release of ∼3.6 eV.11 A substantial part of the energy gain from the O2 dissociation can be transformed into the kinetic energy of the products. Since the excess kinetic energy typically thermalizes within a few picoseconds,27,28 the acquired STM images can be considered as effective snapshots of the O adatoms final locations. Unlike other known cases of the surface migration of the “hot” atoms, where both atoms of a diatomic molecule upon dissociation accommodate on the equivalent sites, in the system reported here, one atom (Ov) fills the vacancy and is locked into the twofold-coordinated bridging oxygen row, while the other atom (Oa adatom) is relatively free to move, likely with the most kinetic energy. Nevertheless, even if the kinetic energy gain is considered to be distributed evenly between two O atoms (∼1.8 eV per atom), the Oa adatom should still have enough kinetic energy to move across the ∼1.1 eV energy barrier determined above and accommodate at more distant Ti sites.

2652 J. Phys. Chem. C, Vol. 112, No. 7, 2008 The schematic model depicted in Figure 3b describes how we picture the transient motion paths that result in the observed distribution of the oxygen adatoms. According to the calculation results, the most stable configuration for an O2 molecule at the O vacancy is when its axis is parallel to the surface and perpendicular to O/Ti rows,9,10 as shown in the second panel of Figure 3b. This would favor an initial translational motion of the Oa atom parallel to the surface (along the O-O bond direction) upon dissociation as well. Hence, immediately after a dissociation, an Oa atom most likely moves toward a nearestneighbor fivefold-coordinated Ti site (located on either side of the bridging oxygen row), configuration A. Having an excess kinetic energy leads to a nonthermal, transient motion of the adatom and brings it along the [001] direction to the next Ti site (as depicted in configuration B). The theoretical calculations show that the excess kinetic energy of a hyperthermal atom is efficiently randomized and dissipated through scattering in a highly corrugated chemisorption potential.27-29 Thus, after reaching its position in the configuration B, an Oa atom may stay, move in the same direction to the next Ti site, configuration C, or be scattered back to configuration A, as illustrated in Figure 3b. If we assume Oa adatoms to have the equal probability to move in either direction because of scattering, adatoms reaching configurations C should be already immobile as no Oa and Ov pairs with three lattice constant distances are found. Consequently, the configuration B should contribute the same amount of adatoms (∼7%) to the configuration A as well. This should be considered as a top limit estimation since an Ov atom, residing in the vacancy with some nondissipated yet excess energy (e.g., vibrational), may somewhat hinder Oa motion back to configuration A. Anyway, out of the observed 19% population of the starting configuration A, at least 12% of the Oa atoms have stayed permanently while the majority (88%) of the oxygen adatoms move to the configuration B immediately after the dissociation. Ultimately, a balance among the diffusion barrier height, excess hyperthermal kinetic energy, and dissipation efficiency results in the configuration B being the most probable. Note also that possible O2 mobility in the course of oxygen dissociation at RT may cause a deviation of the Oa initial trajectory from the most probable direction toward a nearestneighbor Ti site. For this reason, O2 dissociation directly into the configuration B (in minority cases) cannot be completely excluded, while a direct dissociation into the configuration C is improbable due to an apparent steric constraint. The scattering by the corrugated chemisorption potential apparently also assists the change (by 90°) of the direction during the oxygen adatom motion, which (on the TiO2(110) surface) is confined entirely to the Ti troughs between the bridging oxygen rows. The latter represents an important difference in a comparison with the only prior study of the “hot” adsorbate transient mobility on the TiO2(110), where Cl2 dissociates in an upright position and adatoms follow a “cannonball” trajectories.21 As a result, Cl adatoms may be separated by up to three rows, even though less energy is released in this case than upon O2 dissociation.21 4. Conclusions Using in situ scanning tunneling microscopy, we have studied the initial stages of O2 dissociative adsorption at the atomic level on a partially reduced TiO2(110) surface at RT by tracking the same surface area before and after oxygen exposure. Our results confirm that O2 molecules dissociate only at the bridging oxygen vacancies, resulting in the healing of a vacancy by one oxygen atom, Ov, and the deposition of the other O as an adatom, Oa, on a neighboring fivefold-coordinated Ti site. Remarkably, the

Du et al. majority (∼81%) of O adatoms are found separated from the original vacancy positions by up to two lattice constants along the [001] direction. The greater part of the adatoms (∼74%) is one lattice constant apart along the [001] direction from the original vacancy positions, while the other Oa’s are separated by two lattice constants (∼7%) or bonded at the nearest-neighbor Ti sites (∼19%). Thermal diffusion of O adatoms at RT is very slow, with an experimentally estimated activation energy of ∼1.1 eV. As a result, thermal diffusion cannot induce the observed lateral distribution of the O adatoms, which has been interpreted in terms of a limited nonthermal, transient mobility resulting from O2 dissociation. Unlike for other known cases of the dissociation of diatomic molecules where both “hot” adatoms accommodate at the equivalent sites, in the studied system, Ov atoms filling the vacancy are locked into the twofoldcoordinated bridging oxygen rows, and only hyperthermal Oa adatoms are allowed to move. Following a dissociation event, the transient motion of the hyperthermal O adatoms on the TiO2(110) surface is steered along the Ti troughs between the bridging O rows. The effect of the hyperthermal transient mobility of the O adatoms may lead to an enhanced reactivity, which could be of general relevance for the chemical and photochemical processes involving oxygen interaction with TiO2-based systems. Acknowledgment. We would like to thank M. A. Henderson, G. A. Kimmel, N. Petrik, Z. Zhang, and S. Li for stimulating discussions. This work was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, and performed at the W. R. Wiley Environmental Molecular Science Laboratory, a DOE User Facility sponsored by the Office of Biological and Environmental Research. References and Notes (1) Mills, A.; Davies, R. H.; Worsley, D. Chem. Soc. ReV. 1993, 22, 417. (2) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigashi, M.; Watanabe, T. Nature 1997, 388, 431. (3) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1. (4) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, U.K., 1994. (5) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (6) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. J. Chem. ReV. 1995, 95, 735. (7) Henderson, M. A.; Epling, W. S.; Perkins, C. L.; Peden, C. H. F. J. Phys. Chem. B 1999, 103, 5328. (8) Dohna´lek, Z.; Kim, J.; Bondarchuk, O.; White, J. M.; Kay, B. D. J. Phys. Chem. B 2006, 110, 6229. (9) Wu, X.; Selloni, A.; Lazzeri, M.; Nayak, S. K. Phys. ReV. B 2003, 68, 241402. (10) Rasmussen, M. D.; Molina, L. M.; Hammer, B. J. Chem. Phys. 2004, 120, 988. (11) Wendt, S.; Schaub, R.; Matthiesen, J.; Vestergaard, E. K.; Wahlstrom, E.; Rasmussen, M. D.; Thostrup, P.; Molina, L. M.; Lægsgaard, E.; Stensgaard, I.; Hammer, B.; Besenbacher, F. Surf. Sci. 2005, 598, 226. (12) Menetrey, M.; Markovits, A.; Minot, C. J. Mol. Struct.: THEOCHEM 2007, 808, 71. (13) Kurtz, R. L.; Stockbauer, R.; Madey, T. E.; Roman, E.; Segovia, J. L. d. Surf. Sci. 1989, 218, 178. (14) Diebold, U.; Lehman, J.; Mahmoud, T.; Kuhn, M.; Leonardelli, G.; Hebenstreit, W.; Schmid, M.; Varga, P. Surf. Sci. 1998, 411, 137. (15) Epling, W. S.; Peden, C. H. F.; Henderson, M. A.; Diebold, U. Surf. Sci. 1998, 412/413, 333. (16) Shultz, A. N.; Jang, W.; Hetherington, W. M.; Baer, D. R.; Wang, L.-Q.; Engelhard, M. H. Surf. Sci. 1995, 339, 114. (17) Bikondoa, O.; Pang, C. L.; Ithnin, R.; Muryn, C. A.; Onishi, H.; Thornton, G. Nat. Mater. 2006, 5, 189. (18) Henderson, M. A.; Otero-Tapia, S.; Castro, M. E. Faraday Discuss. 1999, 313.

Transient Mobility of Oxygen Adatoms on TiO2(110) (19) Brune, H.; Wintterlin, J.; Behm, R. J.; Ertl, G. Phys. ReV. Lett. 1992, 68, 624. (20) Wintterlin, J.; Schuster, R.; Ertl, G. Phys. ReV. Lett. 1996, 77, 123. (21) Diebold, U.; Hebenstreit, W.; Leonardelli, G.; Schmid, M.; Varga, P. Phys. ReV. Lett. 1998, 81, 405. (22) Au, C.-T. In Adsorption on Ordered Syrfaces of Ionic Solids and Thin Films; Umbach, E., Freund, H.-J., Eds.; Springer: Berlin, Germany, 1993; Vol. 33. (23) Brookes, I. M.; Muryn, C. A.; Thornton, G. Phys. ReV. Lett. 2001, 87, 266103.

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