Oxygen Photochemistry on TiO2(110): Recyclable, Photoactive

Oct 20, 2011 - The photochemistry of oxygen adsorbed on TiO2(110) at 30 K and ... (1-21) The interest is driven by both practical and scientific consi...
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LETTER pubs.acs.org/JPCL

Oxygen Photochemistry on TiO2(110): Recyclable, Photoactive Oxygen Produced by Annealing Adsorbed O2 Nikolay G. Petrik and Greg A. Kimmel* Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, MSIN K8-88, P.O. Box 999, Richland, Washington 99352, United States ABSTRACT: The photochemistry of oxygen adsorbed on TiO2(110) at 30 K and annealed up to 600 K is investigated. UV irradiation results in exchange of atoms between chemisorbed and physisorbed oxygen. Annealing chemisorbed oxygen to 350 K maximizes these exchange reactions, while such reactions are not observed for oxygen that is dissociatively adsorbed on TiO2(110) at 300 K. For oxygen annealed to 350 K, the exchange products photodesorb in the plane perpendicular to the bridge-bonded oxygen rows at an angle of 45°. In contrast, chemisorbed O2 photodesorbs normal to the surface. Remarkably, the chemisorbed species is stable under multiple cycles of UV irradiation. Atoms in the chemisorbed species can be changed from 18O to 16O and then back to 18O via the exchange reactions. The results show that annealing oxygen on TiO2(110) to ∼350 K produces a stable chemical species with novel photochemical properties. Possible forms for the photoactive species include O2 adsorbed in a bridging oxygen vacancy or tetraoxygen. SECTION: Surfaces, Interfaces, Catalysis

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he thermal chemistry and photochemistry of oxygen adsorbed on TiO2(110) are areas of active research.1 21 The interest is driven by both practical and scientific considerations. For example, titanium dioxide photocatalysts are used to remove organic pollutants from water and as thin film coatings on selfcleaning surfaces and could potentially be used for photocatalytic splitting of water into H2 and O2.3,6 The applications have motivated years of research that continue to offer new insights into the interactions of oxygen with TiO2(110). The amount of O2 that can chemisorb on TiO2(110) depends on the extent of reduction of the crystal.7,9,11,22 We will use “chemisorbed” oxygen to refer to a species, such as molecular or atomic oxygen, that becomes negatively charged upon adsorption. For stoichiometric TiO2(110), no O2 chemisorbs on the surface.22 However even on stoichiometric TiO2(110), neutral O2 can adsorb at temperatures below ∼60 K due to van der Waals interactions (i.e., physisorb).9,11,22 For reduced crystals, the most common defects on the (110) surface are vacancies, VO, in the bridge-bonded oxygen (BBO) rows.1,13,23 25 The saturation coverage, θsat(O2), for chemisorbed O2 is proportional to the coverage of these vacancies, θsat(O2) = αθ(VO), where θ(VO) is the vacancy coverage and α ≈ 2 3 for surfaces with θ(VO) ≈ 0.05 0.1 ML (1 ML  5.2  1014 cm 2).5,7,9,11 The source of the negative charge for oxygen chemisorption—surface defects or subsurface Ti3+ interstitials or some combination of these—is an area of active research.7,11,13,16,26 30 The oxygen coverage, θ(O2), and the temperature affect the chemisorption state on TiO2(110). Various experiments indicated that O2 molecularly adsorbs below ∼150 K.7,9,17,18,31 33 Density functional theory (DFT) predicts that O2 can chemisorb in vacancies or on five-fold-coordinated Ti (Ti5c) sites.26,33 38 r 2011 American Chemical Society

Adsorption in a vacancy with an O O distance characteristic of O22 is the most stable molecular configuration for θ(O2)/ θ(VO) = 1.33,35 39 However, the state of the chemisorbed oxygen should depend on θ(O2) or, alternatively, on the number of “defect” electrons available per adsorbed O2.9,26,36,40 For θ(O2)/ θ(VO) = 2, charge sharing reduces the charge on each molecule to less than O22 .36,37,40 As a result, the amount of O2 that photodesorbs depends on θ(O2).40 Recent scanning tunneling microscopy (STM) experiments, which suggest that two O2 molecules adsorbed in or near a vacancy are more stable against tip-induced dissociation than a single O2, also indicate that the O2 adsorption state depends on θ(O2).33 For adsorption temperatures above ∼150 K, O2 dissociates in vacancies, healing the vacancy and leaving oxygen adatoms, Oad, on nearby Ti5c sites, or dissociates on Ti5c sites, leaving paired oxygen adatoms.7,9,23,25,41 However, the possible states of chemisorbed oxygen are less clear when O2 is molecularly adsorbed at temperatures e 120 K and then annealed above 150 K. Temperature-programmed desorption (TPD) and STM experiments indicate that O2 dissociates upon annealing.7,13,15 However, the electron-stimulated desorption (ESD) of O2 versus annealing temperature suggested the creation of a new species when a saturation coverage of chemisorbed oxygen was annealed to ∼300 400 K.9 This observation is consistent with an earlier prediction that tetraoxygen, O4, is more stable than two O2 for θ(O2)/θ(VO) = 2.36

Received: September 7, 2011 Accepted: October 20, 2011 Published: October 20, 2011 2790

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Figure 1. O2 PSD signals versus time. (a c) 0.19 ML of 18O2 was adsorbed at 30 K and then annealed at 350 K for 240 s. Next, 0.43 ML of 16 O2 was adsorbed, and the sample was irradiated with UV light (both at 30 K) while the (a) 16O2, (b) 18O2, and (c) 16O18O PSD signals were monitored. (d) 16O18O PSD signal from 0.25 ML of 16O2 physisorbed on TiO2(110) after it was exposed to 9  1015 18O2/cm2 at 300 K.

The photochemistry of O2 on TiO2(110) provides valuable insight into the state of the chemisorbed oxygen.6,42,43 O2 photodesorption is believed to occur when a chemisorbed O2 reacts with a hole, O2 + h+ f O2(g).42,43 However, recent experiments have also shown that electron-mediated reactions are important in processes such as photoinduced dissociation of chemisorbed O2 and the oxidation of CO.19,40,44 Relatively few experiments have investigated the changes in the photochemistry when oxygen is annealed to temperatures above 150 K. In one example, two forms of chemisorbed O2, one that photodesorbed and a second that photooxidized CO, were inferred from changes in the photochemistry versus annealing temperature.31,32 The velocity distributions of photodesorbing O2 for 100 e T e 260 K are also consistent with two forms of chemisorbed oxygen but showed no evidence for the creation of a new species upon annealing.12 From this brief overview, it is apparent that the chemical state of oxygen on TiO2(110) and its photochemistry, following molecular adsorption below 120 K and annealing above ∼150 K, are not well understood. Here, we investigate the photochemical reactions between chemisorbed oxygen and neutral, weakly bound O2 (i.e., physisorbed O2) versus the annealing temperature of the chemisorbed oxygen on TiO2(110). Using O2 isotopologues (i.e., 16O2, 16 18 O O, and 18O2), we find that chemisorbed oxygen participates in photochemical reactions with physisorbed O2 in which oxygen atoms between the two species are exchanged. The amount of O2 produced in these exchange reactions is maximized for annealing temperatures of ∼350 K. The O2 produced in the nonthermal reactions preferentially desorbs at an angle of ∼45° with respect to the surface normal in a plane perpendicular to the BBO rows. Surprisingly, the chemisorbed species retains its photochemical activity through repeated cycles of UV irradiation and O2 adsorption, with atoms from the physisorbed O2 being incorporated into the chemisorbed form via the exchange reaction. The results

Figure 2. Integrated (a) 16O2, 18O2, and (b) 16O18O PSD yields versus Tanneal. Saturation coverages of chemisorbed 18O2 on TiO2(110) were annealed; then, 0.43 ML 16O2 was adsorbed at 30 K, and the O2 PSD signals were monitored during UV irradiation.

show that a stable, photoactive oxygen species is produced when chemisorbed O2 is annealed above ∼100 K. Furthermore, the photochemical properties of the photoactive oxygen are distinct from any previously identified form of oxygen adsorbed on TiO2(110). Possible forms for the reactive oxygen are discussed in relation to previously observed or predicted oxygen species. Figure 1a c shows the 16O2, 18O2, and 16O18O photon-stimulated desorption (PSD) signals versus time for an experiment where a surface with θ(18O2) = θsat(O2) ≈ 0.19 ML was annealed at 350 K and then 16O2 was physisorbed and the sample was irradiated with UV light for 300 s (only the first 80 s is shown). θsat(O2) is determined from the onset of O2 physisorption (see Experimental Methods).9,11,44 The QMS was positioned in a nonline-of-sight geometry (see Experimental Methods). In this configuration, the measured signal is proportional to the total desorption rate integrated over all desorption angles. The 16 18 O O PSD signal results from photon-stimulated reactions involving the exchange of atoms between the chemisorbed oxygen and physisorbed O2. Below, we will refer to these reactions as “exchange reactions.” We have previously shown that no thermal exchange reactions occur between chemisorbed and physisorbed oxygen.40 The PSD kinetics for each O2 isotopologue are distinct. For example, the 18O2 PSD (Figure 1b) is initially large and then decays quickly, while the 16O18O PSD (Figure 1c) is initially smaller but decays more slowly. For all three O2 isotopologues, the signal decay is not a simple exponential. Similarly complicated kinetics are observed for chemisorbed oxygen at T e 100 K.40,42 While the origin of these complicated kinetics is uncertain,40 the differences in the kinetics for the 18O2 and 16O18O PSD seen in 2791

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Figure 3. O2 PSD yields versus the polar desorption angle (°, with 0° = normal to surface) in planes perpendicular (a and c) or parallel (b) to the BBO rows. Chemisorbed 18O2 was annealed to 350 K (a and b) or 100 K (c), 16O2 was adsorbed at 30 K, and the samples were irradiated with UV light. (d and e) Schematic representations of the O2 desorption directions after annealing to 350 or 100 K, respectively.

Figure 1 suggest that they arise from two different photochemical reactions. This hypothesis is supported by the integrated oxygen PSD signals versus annealing temperature (Figure 2) and angular distributions of the photodesorbing molecules (Figure 3). Figure 1d shows the 16O18O PSD signal for an experiment where the crystal was exposed to 9  1015 cm 2 18O2 at 300 K; then, 0.25 ML of 16O2 was physisorbed at 30 K and irradiated. In this case, the 16O18O PSD signal is small. Because O2 exposure at 300 K results in dissociative oxygen adsorption in oxygen vacancies and on Ti5c sites,7,23,25,41 this result indicates that oxygen adatoms on oxidized TiO2(110) do not react with physisorbed O2. Figure 2 shows the time-integrated 16O2, 18O2, and 16O18O PSD yields versus annealing temperature, Tanneal, of the chemisorbed oxygen. The QMS was positioned in a nonline-of-sight geometry. The O2 PSD signals were integrated over the 300 s duration of the UV exposure. (Similar results were obtained for shorter integration times.) A saturation coverage of chemisorbed 18 O2 was adsorbed at 30 K and annealed at Tanneal for 240 s. Then, 0.43 ML of 16O2 was adsorbed, and the sample was irradiated (both at 30 K). The 18O2 PSD yield decreases approximately monotonically as Tanneal increases and is essentially 0 above 400 K (Figure 2a, red triangles). The 16O2 PSD yield is large and nearly independent of Tanneal for Tanneal < 450 K (Figure 2a, blue

squares). In contrast, the 16O18O PSD yield initially increases with increasing annealing temperature, goes through a maximum at Tanneal ≈ 375 K, and then rapidly drops to 0 for high temperatures (Figure 2b, black circles). The 16O2 PSD yield also decreases from 0.43 ML for Tanneal = 450 K to 0.35 ML for Tanneal = 600 K. Annealing above ∼400 K brings Ti interstitials to the surface that react with the adsorbed oxygen to form TiOx islands, resulting in the observed decrease in the 16O2 and 16O18O PSD yields.9,13,45 47 The results in Figure 2 suggest that annealing TiO2(110) with adsorbed oxygen leads to the creation of a distinct, stable species on the surface, and several more experiments support this hypothesis. For example, the angular distribution of the O2 PSD depends on the annealing temperature. Figure 3a and b shows the 16O2, 16O18O, and 18O2 PSD yields versus the desorption angle relative to the surface normal for desorption in planes that are perpendicular and parallel to the BBO rows (i.e., along the [110] and [001] azimuths), respectively. For Figure 3a and b, 18O2 was annealed at 350 K, 16O2 was physisorbed, and the PSD signals were measured with the QMS set at the specified angle. The experiments were performed on two crystals mounted such that QMS rotated in a plane containing the surface normal and either the [110]or [001] axis (see Figure 3d). 2792

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Figure 4. 16O18O PSD for repeated O2 adsorptions and UV irradiations. Chemisorbed 18O2 (∼0.18 ML) was annealed to 350 or 100 K. Then, the sample was repeatedly (i) dosed with 0.1 ML of 16O2 or 18O2 at 30 K, (ii) irradiated with UV light at 30 K, and (iii) heated to 100 K to remove any remaining physisorbed oxygen. (a) For Tanneal = 350 K, the 16O18O PSD signal versus time for the 1st, 3rd, 10th, 11th, 13th, and 20th O2 dose and irradiation cycles. (b) Integrated 16O18O PSD signals versus O2 dose and irradiation cycle for chemisorbed 18O2 initially annealed to 350 (black circles) or 100 K (red triangles).

For desorption in the plane parallel to the BBO rows (Figure 3b), the PSD signals for both 16O2 (blue squares) and 16 18 O O (black circles) are maximized normal to the surface and are broad without any readily apparent structure. In contrast, there is a prominent peak for the photodesorption products perpendicular to the BBO rows at an angle of ∼45° with respect to the surface normal for both 16O2 and 16O18O (Figure 3a, blue squares and black circles, respectively). The angular distribution of the desorbing 18O2 is narrow and peaked toward normal along both azimuths (Figure 3a and b, red triangles). Figure 3d shows a schematic representation of the directions observed for the 16O2, 16 18 O O, and 18O2 photodesorption products for Tanneal = 350 K. In the schematic, 16O18O is depicted as desorbing after exchanging with an 18O2 chemisorbed in a vacancy, while 16O2 desorbs without exchange. However, as discussed below, the state of the chemisorbed oxygen is uncertain. As seen in Figure 2, the photon-stimulated reactions between chemisorbed 18O2 and physisorbed 16O2 that produce 16O18O also occur for Tanneal = 100 K. However, in that case, the angular distributions of the desorbing species are different. Figure 3c shows the PSD yields for 16O2, 16O18O, and 18O2 versus the angle for desorption in the plane perpendicular to the BBO rows. In this case, all three photodesorption products are peaked along the surface normal. However, there are also small shoulders in the PSD yields at ∼45° evident for each species. Figure 3e shows a schematic representation of the 16O2 and 18O2 photodesorption normal to the surface for Tanneal = 100 K. Although not shown here, the angular distribution for photodesorbing chemisorbed O2 annealed to 100 K without physisorbed O2 is also strongly peaked along the surface normal. However, in that case, there is no shoulder in the angular distribution at 45° perpendicular to the BBO rows. The lack of the shoulder/peak when physisorbed O2 is absent suggests that this feature is associated with the reactions between chemisorbed and physisorbed O2. The fact that annealing the chemisorbed oxygen to 350 K increases both the desorption peak for 16O18O at 45° perpendicular to the BBO rows (Figure 3a) and the angle-integrated

16 18 O O PSD yield (Figure 2b) suggests they are related to the same reaction. Remarkably, the chemisorbed oxygen species responsible for the photon-stimulated exchange reactions with physisorbed O2 is not destroyed by repeated cycles of UV irradiation. Figure 4 shows the 16O18O PSD signals versus time for an experiment with repeated UV irradiations. A saturation coverage of chemisorbed 18O2 (∼0.18 ML) was annealed at 350 K for 240 s. The sample was subsequently cooled to 30 K, dosed with ∼0.1 ML of (physisorbed) 16O2, and irradiated with UV light. The black trace in the upper left corner of Figure 4a (labeled “1st”) shows the 16 18 O O PSD versus time for the first irradiation. This was followed by nine more cycles where the sample was heated to 100 K (to desorb any remaining physisorbed O2), dosed with 16 O2, and UV irradiated at 30 K. The red and blue traces on the left side of Figure 4a show the 16O18O PSD signals for the 3rd and 10th irradiation cycles, respectively. By the 10th irradiation cycle, the 16O18O PSD is essentially 0. Following the first 10 UV irradiation cycles, another 10 irradiation cycles were performed; however, the oxygen physisorbed on the sample was switched to 18 O2. For the 11th irradiation cycle (i.e., the first where the physisorbed oxygen is switched to 18O2), the 16O18O PSD signal reappears (Figure 4a, upper right, black line). For irradiation cycles 12 20, the 16O18O PSD signal decreases again. Figure 4b (black circles) shows the integrated 16O18O PSD versus irradiation cycle for the experiments described in Figure 4a. For the first 10 cycles, the 16O18O PSD decreases exponentially and is approximately 0 for the 10th cycle. However, after the 10th cycle when the oxygen adsorbed on the surface is switched to 18O2, the 16O18O PSD signal reappears at 75% of its initial strength. As the cycles of adsorption, irradiation, and heating to 100 K continue, the PSD signal again decays exponentially with a decay constant similar to that observed for the first 10 UV irradiation cycles. For this experiment, the total O2 PSD (i.e., 16O2 + 16O18O + 18O2) decreases linearly versus the irradiation cycle such that for the last irradiation is ∼85% of its initial magnitude (data not shown).

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The Journal of Physical Chemistry Letters Even for Tanneal = 100 K, a significant amount of 16O18O is produced by reactions between the chemisorbed and physisorbed oxygen (see Figure 2). An important question is whether the same chemisorbed oxygen species that reacts with physisorbed O2 for surfaces annealed to 350 K is produced by annealing to only 100 K. If so, the results in Figure 2 indicate that annealing to higher temperatures simply produces more of the reactive species. Experiments similar to those shown in Figure 4 where chemisorbed 18O2 is initially annealed to 100 K instead of 350 K give qualitatively similar results (Figure 4b, red triangles). For this experiment, the 16O18O PSD yield is initially smaller, but it has a similar magnitude by the 11th irradiation cycle when the physisorbed oxygen is switched to 18O2. This result suggests that less of the reactive species is initially present, but that repeated cycles of O2 physisorption and UV irradiation produces the same state of chemisorbed oxygen. The angular distributions observed for the 16O18O PSD are also consistent with the hypothesis that the same species is involved. As already mentioned, the peak in the PSD yield at ∼45° for desorption perpendicular to the BBO rows for Tanneal = 350 K (Figure 3a) is also apparent as a smaller shoulder for Tanneal = 100 K (Figure 3c). The results presented here suggest that oxygen adsorbed on reduced TiO2(110) and then annealed to temperatures above 100 K forms a distinct chemical species with interesting photochemical properties. At present, the chemical state of the oxygen that reacts with physisorbed O2 and many of the details of that reaction are not known. However, the angular distributions of the desorbing products provide valuable information about the photodesorption process and the reaction complex from which the products evolve. First, the angular distributions for all species (16O2, 16O18O, and 18O2) show that the photodesorption processes are primarily nonthermal. If the oxygen desorbed thermally, the angular distributions would be broad along both azimuths. (For example, broad distributions along both azimuths are observed for thermal desorption of physisorbed oxygen; data not shown.) Instead, the angular distributions suggest that some of the species desorb directly from an excited-state or chemical intermediate without significant subsequent interaction with the substrate. Therefore, the angular distributions provide insight into the geometry of the excited-state and/or reaction complex. The reaction intermediate between physisorbed O2 and the chemisorbed oxygen involves at least three oxygen atoms—two from the physisorbed O2 and at least one oxygen atom from the reactive oxygen species. Because oxygen adatoms on oxidized TiO2(110) do not appear to be responsible (see Figure 1d), the reaction intermediate should involve at least two atoms from the chemisorbed species and thus four or more oxygen atoms in total. The angular distributions for the 16O18O PSD indicate that the bond that was broken in the reaction complex to form this product was oriented at ∼45° from the surface normal in a plane perpendicular to the BBO rows. DFT calculations have predicted that tetraoxygen, O4, is stable on TiO2(110) and forms an arched structure that is oriented perpendicular to the BBO rows.36,48 Thus, the calculated structure for O4 qualitatively fits the requirements for the reactive complex deduced from the experiments, that is, at least four oxygen atoms oriented perpendicular to the BBO rows. Because the angular distribution for the physisorbed O2 PSD also has a prominent peak in the same direction as the 16O18O (e.g., 16O2 desorption in Figure 3a), it suggests that desorption of the physisorbed O2 involves the same reactive intermediate but that many excitations do not lead to exchange reactions between the chemisorbed oxygen and physisorbed

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O2. Instead, most excitations lead to photodesorption of unreacted, physisorbed O2 from the complex. Some possible candidates for the chemisorbed oxygen responsible for the reactions include (i) one O2 adsorbed in a vacancy,17,18,33 36,38,48 (ii) two O2 adsorbed in or near a vacancy,36,48 (iii) O2 adsorbed asymmetrically in a vacancy,33,35 (iv) O2 chemisorbed on a Ti5c site not near a vacancy,18 and (v) tetraoxygen.36,48 (We are unaware of any experiments or calculations indicating the formation of ozone on TiO2(110).) Note that all of these oxygen species (except perhaps O2 adsorbed on a Ti5c site) are believed to be oriented perpendicular to the BBO rows on TiO2(110) and are thus consistent with the observed O2 PSD angular distributions. Because the O2 PSD (see Figure 2) signal decreases versus annealing temperature over the temperature range where the exchange reaction with physisorbed O2 increases, the molecular O2 that photodesorbs during UV or electron irradiation9 is not the reactive species. On the basis of the hole-mediated photodesorption model for O2,40,43 we expect that the chemisorbed O2 that photodesorbs is O2 . Given the correlation between the loss of the chemisorbed 18O2 PSD signal and the increase in the 16O18O PSD signal in Figure 2, it is plausible that annealing converts chemisorbed O2 into the reactive oxygen species. This conversion could involve changes in the charge state for the chemisorbed O2 (e.g., O2 f O2 2), changes in the bonding geometry, or reactions with other chemisorbed oxygen (e.g., 2O2 f O4). We have previously investigated the adsorption and thermal reactions of oxygen on TiO2(110) by monitoring the ESD of O2 versus the annealing temperature.9 Those results indicated that some form of oxygen involving O O bonds remains on the surface even after annealing to ∼400 K. By analyzing changes in the kinetics of the O2 ESD versus annealing temperature, we proposed that the chemisorbed oxygen transformed into a different species, possibly tetraoxygen, upon annealing to ∼350 K. The results presented here provide convincing evidence for the formation of a new chemical species upon annealing of chemisorbed oxygen but do not identify the species itself. STM images of TiO2(110) after O2 dosing at ∼120 K and annealing to higher temperature have shown only dissociated oxygen.13,15 However, because recent experiments have shown that STM imaging under “normal” tunneling conditions can easily dissociate chemisorbed O2,17,18,33 the species formed upon annealing might be also be susceptible to tip-induced dissociation, thus making it hard to observe with STM. More research is needed to identify the chemisorbed species responsible for the photon-stimulated exchange reactions with physisorbed O2. UV irradiation of coadsorbed oxygen and CO on TiO2(110) results in the production of CO2 (i.e., photooxidation of CO).32,49,50 Interestingly, the angular distribution for the photodesorbing CO2 is similar to the distribution for the physisorbed 16 O2 PSD (figure 3a).50 On the basis of calculations of the transition-state geometry for the photooxidation reaction,48,51 we proposed that the CO2 evolves from an OOCO precursor that is oriented perpendicular to the BBO rows. Despite the similarities in the angular distributions, several experiments demonstrate that the oxygen species involved in the photooxidation of CO is different than the one responsible for the exchange reactions with the physisorbed O2. For example, the evolution of these two photostimulated reactions versus annealing of the chemisorbed oxygen is very different. As shown by Yates and coworkers32,49 and reproduced in our laboratory (data not shown), the photon-stimulated production of CO2 decreases monotonically 2794

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The Journal of Physical Chemistry Letters versus the annealing temperature for chemisorbed oxygen and is approximately 0 at Tanneal = 350 K. In contrast, the maximum yield for reactions between the chemisorbed oxygen species and physisorbed O2 occurs at Tanneal = 350 K. In summary, the experiments shown here demonstrate that oxygen chemisorbed on TiO2(110) at 30 K and annealed to temperatures above ∼100 K forms a chemical state with novel photochemical properties. The chemisorbed oxygen can react with physisorbed O2 during UV irradiation, leading to exchange of oxygen atoms between the two forms of oxygen. Annealing the chemisorbed oxygen to ∼350 K maximizes these exchange reactions. The chemisorbed species remains reactive with physisorbed O2 after repeated cycles of UV irradiation and annealing to 100 K. The results suggest that the reaction complex for the exchange reaction should contain at least four oxygen atoms, and the angular distributions of the desorbing reaction products show that the reactive complex is oriented perpendicular to the BBO rows. More research is needed to determine the detailed state of the reactive, chemisorbed oxygen. However, two likely candidates are O2 adsorbed in a bridging oxygen vacancy and tetraoxygen. The results presented here, along with other recent results,13,16 18,20,21,33,40,44 demonstrate that the thermal chemistry and photochemistry of oxygen on TiO2(110) are complicated and that important aspects of them are not yet understood.

’ EXPERIMENTAL METHODS The experiments were performed in an ultrahigh vacuum (UHV) system that has been described previously.50,52 The system is equipped with a molecular beamline for O2 dosing and a quadrupole mass spectrometer (QMS) for detecting the desorption products. Samples were prepared by repeated cycles of sputtering (2 keV Ne+) and annealing at 950 K (typically ∼2 10 min). The coverage of the vacancies, θ(VO), was ∼0.08 0.09 ML.9,11 Because the O2 sticking on TiO2(110) was ∼0.8 and nearly independent of coverage at ∼30 K, θ(O2) could be accurately determined from the O2 exposure.9,22 The amount of O2 that desorbed below 100 K during TPD (i.e., the amount of physisorbed O2) for a series of different O2 coverages was observed to increase linearly above a threshold coverage.9,11,44 This threshold coverage corresponds to the saturation (or maximum) coverage of chemisorbed O2, θsat(O2). The UV light source was a 100 W Hg lamp (Oriel #6281) coupled into the UHV chamber via a fiber optic cable. The infrared portion of the lamp’s output spectrum was blocked using a water filter, while the entire UV portion was used to irradiate the sample (thus improving the signal-to-noise ratio). All UV irradiations were performed at the base temperature for the samples (which was typically 25 30 K). Photon fluxes were ∼3  1015 photons/cm2 s (for energies > 3 eV). A small increase in the sample temperature (∼1 2 K) during UV irradiation did not influence any of the results. Angle-resolved measurements of the (neutral) desorbing O2 were performed with a QMS mounted on a rotation stage and equipped with an integrating cup53 with a 5 mm aperture, located ∼20 mm above the surface.50 (Some nonline-of-sight measurements were made with a 15 mm aperture and thus had better signal-to-noise ratios.) Measurements with the QMS out of lineof-sight to the front face of the sample gave the angle-integrated desorption intensities and allowed quantitative comparison of the TPD and PSD signals. As described previously,50 line-of-sight measurements have two contributions, a direct, angle-resolved

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component and an approximately constant contribution from molecules that reach the aperture after scattering off of other surfaces in the UHV chamber. O2 gases with two different isotopic compositions were used. One gas, which we refer to as 16O2, had the natural isotopic abundances for atomic oxygen.54 The second gas, 18O2, had ∼96% 18O2, ∼3.6% 16 18 O O, and ∼0.4% 16O2. The data have been corrected for the isotopic abundances of the source gases. These corrections are typically small (less than 10%).

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: 509-371-6134. Fax: 509-371-6139.

’ ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Chemical and Materials Sciences Division. The work was performed at the W. R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE, Office of Biological and Environmental Research, and located at Pacific Northwest National Laboratory, which is operated for the DOE by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830. ’ REFERENCES (1) Dohnalek, Z.; Lyubinetsky, I.; Rousseau, R. Thermally-Driven Processes on Rutile TiO2(110)-(1  1): A Direct View at the Atomic Scale. Prog. Surf. Sci. 2010, 85, 161–205. (2) Epling, W. S.; Peden, C. H. F.; Henderson, M. A.; Diebold, U. Evidence for Oxygen Adatoms on TiO2(110) Resulting from O2 Dissociation at Vacancy Sites. Surf. Sci. 1998, 412/413, 333–343. (3) Fujishima, A.; Zhang, X. T.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515–582. (4) Henderson, M. A. Photooxidation of Acetone on TiO2(110): Conversion to Acetate via Methyl Radical Ejection. J. Phys. Chem. B 2005, 109, 12062–12070. (5) Henderson, M. A. Relationship of O2 Photodesorption in Photooxidation of Acetone on TiO2. J. Phys. Chem. C 2008, 112, 11433–11440. (6) Henderson, M. A. A Surface Science Perspective on TiO2 Photocatalysis. Surf. Sci. Rep. 2011, 66, 185–297. (7) Henderson, M. A.; Epling, W. S.; Perkins, C. L.; Peden, C. H. F.; Diebold, U. Interaction of Molecular Oxygen with the Vacuum-Annealed TiO2(110) Surface: Molecular and Dissociative Channels. J. Phys. Chem. B 1999, 103, 5328–5337. (8) Henderson, M. A.; White, J. M.; Uetsuka, H.; Onishi, H. Selectivity Changes During Organic Photooxidation on TiO2: Role of O2 Pressure and Organic Coverage. J. Catal. 2006, 238, 153–164. (9) Kimmel, G. A.; Petrik, N. G. Tetraoxygen on Reduced TiO2(110): Oxygen Adsorption and Reactions with Bridging Oxygen Vacancies. Phys. Rev. Lett. 2008, 100, 196102. (10) Lee, J.; Zhang, Z.; Yates, J. T. Electron-Stimulated Positive-Ion Desorption Caused by Charge Transfer from Adsorbate to Substrate Oxygen Adsorbed on TiO2(110). Phys. Rev. B 2009, 79, 081408. (11) Petrik, N. G.; Zhang, Z. R.; Du, Y. G.; Dohnalek, Z.; Lyubinetsky, I.; Kimmel, G. A. Chemical Reactivity of Reduced TiO2(110): The Dominant Role of Surface Defects in Oxygen Chemisorption. J. Phys. Chem. C 2009, 113, 12407–12411. (12) Sporleder, D.; Wilson, D. P.; White, M. G. Final State Distributions of O2 Photodesorbed from TiO2(110). J. Phys. Chem. C 2009, 113, 13180–13191. (13) Wendt, S.; et al. The Role of Interstitial Sites in the Ti3d Defect State in the Band Gap of Titania. Science 2008, 320, 1755–1759. 2795

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