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Letter

Adsorption and Photodesorption of CO from Charged Point-Defects on TiO(110) 2

Rentao Mu, Arjun Dahal, Zhi-Tao Wang, Zdenek Dohnalek, Greg A Kimmel, Nikolay G Petrik, and Igor Lyubinetsky J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02052 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 10, 2017

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The Journal of Physical Chemistry Letters

Adsorption and Photodesorption of CO from Charged Point-Defects on TiO2(110) Rentao Mu,a, b Arjun Dahal,c Zhi-Tao Wang,c Zdenek Dohnálek,a, d Greg A. Kimmel,a Nikolay G. Petrik,*a and Igor Lyubinetsky†a

a

Physical and Computational Sciences Directorate and Institute for Integrated Catalysis,

Pacific Northwest National Laboratory, Richland, WA 99352 b

Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical

Engineering and Technology, Tianjin University; Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China. c

Environmental Molecular Sciences Laboratory and Institute for Integrated Catalysis, Pacific

Northwest National Laboratory, Richland, WA 99352 d

Voiland School of Chemical Engineering and Bioengineering, Washington State University,

Pullman, Washington 99163, USA

*Corresponding author: [email protected] †

Corresponding author: [email protected]; Current address: School of Chemical,

Biological and Environmental Engineering, Oregon State University, Johnson Hall 105 SW 26th St., Corvallis, OR 97331

1 ACS Paragon Plus Environment


Abstract. The adsorption and photochemistry of CO on rutile TiO2(110) are studied with scanning tunneling microscopy (STM), temperature-programmed desorption and angle-resolved photonstimulated desorption (PSD) at low temperatures. Site occupancies, when weighted by the concentration of each kind of adsorption site on the reduced surface, show that the adsorption probability is the highest for the bridging oxygen vacancies (VO). The probability distribution for the different adsorption sites corresponds to very small differences in CO adsorption energies (< 0.02 eV). UV irradiation stimulates diffusion and desorption of CO at low temperature. CO photodesorbs primarily from the vacancies with a bi-modal angular distribution, indicating some scattering from the surface which also leads to photo-stimulated diffusion. Hydroxylation of VO’s does not significantly change the CO PSD yield or the angular distribution, which suggests that photodesorption can be initiated by recombination of photo-generated holes with excess electrons localized near the charged point defect (either VO or bridging hydroxyl).

TOC Graphic PSD-1 PSD-2

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CO Ti3+

hν

PS-diffusion

CO Ti4+

h+ e- TiO2

CO TiO2


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CO adsorption on the rutile TiO2(110) surface has been extensively studied theoretically1-6 and experimentally.2,7-13 CO, which has a small dipole moment, adsorbs on the five-fold coordinated Ti sites (Ti5c) normal to the surface with the more negative carbon atom down. Experimental binding energies of 0.43-0.44 eV at low coverages on the non-reduced7,8 and ~ 0.42 eV on the reduced TiO2(110) (r-TiO2(110))9 are consistent with most theoretical estimates1-5 indicating a weak physisorption. Furthermore, an efficient CO-CO repulsion leads to decreasing binding energy with CO coverage.7-9 The interaction of CO with the bridging oxygen vacancy (VO) on the reduced surface has been a controversial topic for many years. Many DFT calculations indicate that the binding energy for CO in a vacancy is higher by 0.01 – 0.15 eV than on the Ti5c sites.1,3-5 On the other hand, the first scanning tunneling microscopy (STM) study2 at 80 K reported that CO molecules are mobile and spend relatively little time in the vacancies or in the Ti5c sites directly adjacent to them. Instead, most of the CO was found at Ti5c sites that are next-nearest to the vacancy. The study concluded that these sites are the primary locations for the CO adsorption on reduced TiO2(110). The number of CO molecules at Ti5c sites further decreases with the distance from the vacancy.2 This complicated adsorption distribution was correlated with the experimentally observed distribution of the compensating negative charge around the vacancy from the filled state STM data.14 The updated DFT calculations2 suggested that CO adsorbs more strongly on the less positively charged Ti5c sites due to electron back-donation and the binding energy at the VO site being considerably smaller (by 0.16 - 0.22 eV) than for the Ti5c sites. TiO2 is a widely used photocatalyst with important applications ranging from solar energy conversion to removing contaminants from air and water.15-17 Photo-generation, trapping, diffusion, and reactivity of electrons and holes in TiO2 are important processes for driving both reduction and oxidation reactions.16,18-21 Simple molecules such as O2 and CO on single crystal



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surfaces are attractive candidates for better understanding of fundamental photochemical processes.16,18,22 Despite considerable research, the roles of photo-generated electrons and holes in various photocatalytic reactions continues to be debated.17,18,23-30 For example, CO photooxidation on TiO2(110) with pre-adsorbed O2 produces hyper-thermal CO2 and CO,26,27,29,31,32 but there is a discussion whether the photo-oxidation reaction is a single step process mediated by electrons26 or a multi-step reaction involving both electrons and holes.27 Without co-adsorbed O2, a small photon stimulated desorption (PSD) of CO was observed at 30 K from the r-TiO2(110),29 but the mechanism of this phenomenon remained unclear since the physisorbed CO molecules are not thought to react efficiently with the photogenerated charge carriers. In the current paper we use a combination of low-temperature STM, temperature-programmed desorption (TPD), and angle-resolved photon-stimulated desorption (PSD) to study the sitespecific adsorption and photodesorption of CO molecules from reduced and also hydroxylated TiO2(110) (h-TiO2(110)). The two surface preparations have quite similar electronic defect signatures. With STM, we have measured the CO occupancy of each site on the TiO2(110) surface as a measure of the adsorption and desorption probability, weighting the data by the number of available adsorption sites. After normalization, the data show that vacancies are the preferred adsorption sites for CO on r-TiO2(110). The adsorption probability for Ti5c adsorption sites is comparable, but less than for the vacancies, except for the Ti5c next to the VO where CO has a low probability to adsorb. Under UV irradiation, CO photodesorbs primarily from the vacancies and mostly normal to the surface. The CO PSD yields and angular distributions do not change significantly for h-TiO2(110), indicating that the electronic structure of the surface defects is primarily responsible for the site-specific PSD process (as opposed to the atomic configuration of the point-defects). We suggest that photodesorption can be initiated by recombination of photo-


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generated holes with extra electrons localized near the surface defect (i.e. VO or bridging hydroxyl (OHb)) leading to rearrangement of the surface atoms and ejection of the weakly bound CO molecules. Panels (a) and (b) in Figure 1 show STM images of the same area of r-TiO2(110) before and after CO adsorption at 80 K. Clean r-TiO2(110) in Fig. 1a displays alternating bright and dark rows of Ti5c atoms and bridging oxygen (Ob) atoms, respectively. Bridging oxygen vacancies can also be seen as faint bright spots on the dark Ob rows. In the grid-marked area in Fig. 1a, several VO are marked with green circles. Adsorbed CO molecules (coverage, θ(CO) = 0.1 ML) are seen in Fig. 1b as isolated large bright spots on top of the Ti5c sites, frequently in close proximity to VO. Concurrently, several VO defects are occupied by CO molecules, appearing as notably less bright features marked with black circles. At higher CO coverage (θ(CO) = 0.21 ML), more CO molecules are found on the Ti5c and VO sites as shown in the Fig. 1c (CO-Ti5c and CO-VO respectively).



[001] _ [110]

Ti5c Ob Ti5c

d)

0.06 0.04

2 1 0.4 0 1 0.3 2 3

VO

0.2

0.02 0

0.1

Vo

0

1

2

0.5

3 0.5

3 2 1 0 1 2 3

0.08

0.6

3

Adsorption sites

4

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e)

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0.32

f)

0.01

0.24 0.16

0.3 0.2

0.005

0.08 0

0

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-0.005

0

-0.01

δE, kcal/mol

CO coverage , ML

0.1

Adsorbed CO Available sites

δE, eV

0.12

CO molecules per site

1 nm

Site coverage, ML

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-0.08 -0.16

Vo

0

1

2

3

Adsorption sites

4

Vo

0

1

2

3

Adsorption sites

4

Figure 1. STM images of the same area (a) before and (b) after adsorption of 0.1 ML CO on rTiO2(110) at 80 K. The grid indicates positions of the Ti5c sites. Green and black circles denote the positions of empty and CO-filled VO defects in the marked area, respectively. (c) STM image of 0.21 ML CO in a different area. (d) Coverages of the CO molecules adsorbed at different sites (red bars) and coverages of the corresponding sites (green bars, note the different scale to the right) on the r-TiO2(110). For this experiment, the total CO coverage, θ(CO), was 0.19 ML. The inset illustrates the Ti site labeling according to the distance from the VO. (e) Occupancy of various sites with CO molecules (i.e. molecules per site) on r-TiO2(110) from the data in (d). (f) CO adsorption energy difference (δE) at various sites with respect to the Ti5c1 site calculated from the experimental data in (e). VO coverages are 0.17 ML (a-c) and 0.12 ML (d-f).


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Rather similar STM images for CO adsorption at VO and Ti5c sites on r-TiO2(110) at lower CO coverages (< 0.04 ML) were reported previously by Zhao et al.2 While adsorption in the vacancies was noted in the previous study, the amount of CO adsorbed in the vacancies was not reported.2 Our data indicate that the number of vacancy sites occupied with CO is significant. For a different sample with 0.19 ML CO and θ(VO) = 0.12 ML, we have calculated the coverages of the CO molecules adsorbed at various Ti5c sites and VO defects at 80 K.33 The various Ti5c site positions relative to VO are labeled in the inset in Fig. 1d according to the distance along the [001] azimuth assigning index “0” to the Ti5c nearest to the vacancy (Ti5c0). The site-specific CO coverages are shown in Fig. 1d in red bars. Most of the adsorbed CO molecules are found at two sites: VO and Ti5c1 (the next-closest to the vacancy). Very few CO molecules are found at Ti5c0 (< 3%) or at the higher index sites (Ti5c3 and Ti5c4, < 12% total). Except for adsorption in the vacancy, the CO adsorption distribution between the Ti5c sites is consistent with the earlier study, including the peak for the Ti5c1 site.2 In agreement with previous results,2 we have observed the occasional diffusion of adsorbed CO molecules along Ti5c rows and across Ob rows (via VO) (Fig. S1) at 80 K, but the distribution of the CO molecules between the VO and Ti5c sites remained unchanged over an extended period of time (Fig. S2) indicating an equilibrium adsorption configuration, which is result of the precursor-mediated adsorption mechanism8 and thermal diffusivity of CO molecules. It is also supported by our studies of the thermal diffusion of CO on r- and h-TiO2(110) which will be presented in a future publication. At higher CO coverages, CO - CO repulsion leads to lower binding energies and faster diffusion such that the equilibrium adsorption distribution is achieved more quickly. To obtain the relative probability of CO adsorption at a particular site (i.e. the site occupancy) we have counted the number of available sites of each type in the same STM image. These “site



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coverages” (in ML) are shown as the green bars in the Fig. 1d. The most common site is Ti5c1 and the number of Ti5c sites decrease rapidly for the higher index sites because the odds of being a long way from the nearest vacancy is low. The site occupancy for CO, calculated by dividing the CO coverage for each site (Fig. 1d, red bars) by the amount of those sites (Fig. 1d, green bars), is shown in the Fig. 1e. It is the highest for the vacancy site (~ 0.4), very small for the Ti5c0 site next to it (~ 0.02), and the same (within the experimental uncertainty) for the all other Ti5c sites (~ 0.17 for Ti5c1 – Ti5c4).34 Note that for CO coverages ranging from 0.013 to 0.19 ML, more CO is absorbed in the vacancies than at the Ti5c1 sites on a per site basis (see Fig. S3). For a system in thermal equilibrium, the relative adsorption energies can be estimated assuming a Boltzmann distribution and relative adsorption probability as P(x)/P(Ti5c1) = exp(-δE/kT) (where P(x) is the site “x” occupancy, δE is the CO adsorption energy difference between the sites “x” and Ti5c1, k is the Boltzmann’s constant and T is temperature).2 This analysis shows that while the differences in the occupation probabilities are quite apparent (Fig. 1d), they correspond to very small differences in the adsorption energy between the various sites (< 0.02 eV, see Fig. 1f). The CO coverage distribution shown in Fig. 1d (red bars) is similar to an earlier report,2 except that we have also quantified CO adsorption in the vacancies. However, once this coverage distribution is normalized by the number of available sites (Fig. 1d, green bars), the resulting adsorption probability distribution (Fig. 1e) leads to some different conclusions. First, vacancies are the preferred adsorption sites. Second, the adsorption probabilities at Ti5c sites farther from the vacancies (i.e. sites Ti5c1 – Ti5c4) are all about the same. As reported previously,2 adsorption at site Ti5c0 is suppressed. However, perhaps more importantly, the difference in adsorption energies between all these sites is relatively small (Fig. 1f).


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Theoretical calculations for CO adsorption at Ti5c sites on the reduced TiO2(110) surface yield binding energies ranging from 0.22 to 0.56 eV 1-4,35 for 0.08 - 0.25 ML coverages. These values can be compared with the experimentally determined CO binding energy of ~ 0.4 eV for 0.2 – 0.3 ML.9 The theoretically-determined CO binding energies at the VO site were reported to be higher by 0.15 eV,4 similar to,3 or lower by 0.21 eV2 relative to that on the Ti5c sites.2 In contrast to the experimental observations, such big differences in binding energies would result in a complete redistribution of the CO population at 80 K in favor of only one adsorption site. The largest modulation in the binding energy is observed in the close vicinity to the vacancy (sites VO and Ti5c0 in Fig. 1f) and it is likely that extra unpaired electrons localized near the VO may induce such a modulation. The photochemistry of CO on r-TiO2(110) was studied with STM and PSD methods at 20 - 30 K, to eliminate the CO thermal diffusion at higher temperatures. On the other hand, CO was dosed at 80 K so as to reach the equilibrium population of adsorption sites. Figure 2 shows STM images before (a) and after (b) UV irradiation of the same area for 1 min, revealing photodesorption (left, green circle) and photoinduced diffusion (right, green circle with green arrow) of CO out of the VO defect. Figure 2c shows the results of a statistical analysis of the larger area demonstrating that the photodesorption of CO from the VO sites is the dominant process: ~ 43 % of CO in VO photodesorb, while the Ti5c sites are 5 – 20 times less efficient for photodesorption and just ~ 8 % of the CO at Ti5c1 photodesorb. While no thermal diffusion of CO occurs at 20K, photoinduced diffusion of CO along the Ti5c row is seen (Fig. S4) along with infrequent jumps from the VO to a nearby Ti5c site (Fig. 2b).



[001]

CO-Ti5c 1 nm

CO-Vo

Vo

[1-10]

1 nm 0.5

2

0.4

0.3

0.2

0.1

(d)

UV on 1.5

CO PSD (arb. units)

(c)

CO PSD signal (arb. units)

Fraction of photodesorbed CO

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1

0.5

0

0.1 0.2 0.3 0.4 0.5 0.6 θ(CO) (ML)

0

0

Vo

0

1

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100

Adsorption sites

200

Time (s)

300

400

Figure 2. STM images of the same area of CO-dosed r-TiO2(110) surface (a) before and (b) after UV-light irradiation (UV photon flux ~ 1 × 1015 cm-2 s-1) for 1 min at 20 K. Black circles mark the CO-VO species. Green circles mark the empty VO defects after photo-induced CO desorption and/or diffusion. Diffusion from a VO defect to Ti5c site is illustrated by a green arrow. (c) Fraction of CO molecules photodesorbed from different sites after 1 min UV irradiation. (d)

13

CO PSD

signal versus time from r-TiO2(110) at 30 K. The flux of UV photons is 1.8 × 1016 cm−2 s−1 and θ(13CO) = 0.33 ML. Inset: initial 13CO PSD signal integrated during the first 2 s versus initial 13CO coverage.

Figure 2d shows the

13

CO PSD signal versus time on r-TiO2(110) surface. The

13

CO PSD

signal increases abruptly when the light is turned on and then decays within ~50 s in a bi10 ACS Paragon Plus Environment

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exponential fashion. For the given irradiation conditions, the 13CO photodesorption yield reaches ~ 2.5 × 10-5 molecules/photon. During irradiation, only a small fraction of 13CO desorbs as seen in the TPD spectra of irradiated and non-irradiated spectra shown in the Fig. S5 for various initial CO coverages in the 0.1 – 1.0 ML range. Longer irradiation (up to 6 hours) at 30 K does not substantially increase the amount of photodesorbed 13CO. The initial 13CO PSD signal integrated during the first 2 s of irradiation versus the initial 13CO coverage is presented in the inset in the Fig. 2d. The PSD signal increases quickly with coverage until ∼ 0.08 ML and considerably slower above that. The change in behavior occurs at a coverage that is close to the VO coverage for this particular sample. This result is consistent with the STM observations that CO molecules associated with VO photodesorb most efficiently. Such high sensitivity of the CO PSD to the adsorption position on the surface is indicative of indirect photodesorption mechanism involving photoexcitation of the TiO2 substrate and creation of the electron-hole pairs. To verify this, we used cut-off optical filters with the UV source and did not observe any CO photodesorption for photon energies below the TiO2 band gap (~3.1 eV) – the threshold for the electron-hole generation. Direct photoexcitation and desorption of the adsorbed CO molecule requires more than 6 eV photon energy36-38 which cannot be achieved in our experiments with a mercury lamp source. A bridging oxygen vacancy is both an atomic and an electronic defect. In order to gain insight into the mechanism of CO photodesorption from the VO sites, we have also studied the photoactivity of CO on h-TiO2(110) where the atomic defects are titrated by water at 300 - 400 K yielding two bridging hydroxyls (OHb) per each VO.39-41 It has been shown that the extra-electrons localized at the surrounding Ti5c and Ti6c (six-fold coordinated) sites are not significantly perturbed by the hydroxylation.14,42-45 A typical STM image of h-TiO2(110) at 80 K is shown in Fig. 3a.



Mostly isolated, immobile OHb groups are observed as bright spots on the Ob rows. The virtual absence of VO’s reflects their nearly complete hydroxylation and is consistent with the determined OHb concentration of ~ 0.2 ML.

1 nm

CO coverage , ML

0.14

Adsorbed CO Available sites

c)

0.12 0.1 0.08 0.06

Ti5c Ob Ti5c 3 2 1 0 1 2 3

3 2 OHb 1 0 1 2 3

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d)

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Adsorption sites

Figure 3. STM images of hydroxylated TiO2(110) surface (a) before and (b) after dosing of 0.13 ML CO at 80 K. (c) Site-specific coverages of the CO molecules (red bars) and of the corresponding adsorption sites (green bars) on the h-TiO2(110) for θ(CO) = 0.15 ML. The inset illustrates the site labeling relatively to the OHb position. (e) Occupancy of various sites with CO molecules for on h-TiO2(110) from the data in (c). Adsorption of 0.13 ML CO molecules at 80 K occurs exclusively at Ti5c sites in the vicinity of the OHb groups (Fig. 3b), typically at the second nearest (Ti5c1) and, to a lesser extent, at the third 12 ACS Paragon Plus Environment

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nearest (Ti5c2) sites (the Ti5cn sites of h-TiO2(110) are similarly defined relative to the position of the closest OHb group, see inset in the Fig. 3c). Since the concentration of OHb groups is rather high, Ti5cn sites farther than three lattice constants away from any OHb are rare. Figure 3c displays coverage of CO occupying different Ti5c sites for the sample dosed with 0.15 ML CO.

46

Three

quarters of the CO molecules are found at the Ti5c1 sites, ~ 4 times less – at the Ti5c2 sites and only ~ 3% were adsorbed next to the OHb (red bars in Fig. 3c). The coverages of the available adsorption sites for the same STM images are shown in the Fig. 3c in green bars: they are maximal for Ti5c1 and significantly smaller for Ti5c2 correlating with the decrease of the adsorbed CO coverage for this site. As a result the site occupancy for the Ti5c2 is actually larger than for the Ti5c1.47 A common feature in the distributions of CO adsorption at different sites for h-TiO2(110) and r-TiO2(110) surfaces in Figures 3d and 1e is the very small occupancy of the Ti5c0 sites closest to the point defect while the other available sites have similar or comparable occupancies corresponding to small differences in the binding energies (Fig. 1f). Under UV irradiation at 20 K, a fraction of CO molecules photodesorb from the h-TiO2(110) surface as well. Figures 4 a and b present STM images of the same area of the CO/h-TiO2(110) surface before and after UV irradiation respectively. Green circles mark the events of CO photodesorption and photo-induced diffusion along Ti5c row (marked with a blue arrow). Statistical analysis for a larger area (see Fig. 4c) shows a higher photodesorption rate from the Ti5c1 sites, while the fractions of CO removed from the Ti5c0 and Ti5c2 sites are smaller but still quite comparable.



0.25

HOb

HOb

CO-Ti5c

CO-Ti5c

Fraction of photodesorbed CO

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Figure 4. STM images of same area on h-TiO2(110) surface with θ(CO) = 0.13 ML (a) before and (b) after UV-light irradiation for 1 min at 20 K. Black circles mark the initial positions of adsorbed CO molecules on Ti5c sites. Green circles mark the UV-light induced CO desorption and/or diffusion. Diffusion along a Ti5c row is illustrated by a blue arrow. (c) Fraction of CO molecules photodesorbed from different sites after 1 min UV irradiation. Figure 5a shows the 13CO PSD signal at 30 K versus time from both h-TiO2(110) and rTiO2(110) surfaces with 0.22 ML CO. The yields of CO PSD from the both surfaces are very similar. Similar results are also seen for different CO coverages in the 0.03 – 0.22 ML range. We also conducted angle-resolved PSD measurements that could provide more insight into the photodesorption mechanism.29,32,48-50 Figure 5b shows the integrated PSD yields from 0.22 ML 13

CO dosed on the reduced (blue circles) and hydroxylated (red circles) TiO2(110) versus

desorption angle in a plane perpendicular to the rows of Ob and Ti5c. The angular distributions of the 13CO PSD are also very similar and apparently have two components: a broad component that can be fit with a cosine function and a narrow component peaked along the surface normal (the width of the narrow component is comparable to the angular resolution of our detector, which is determined by the aperture size). Decreasing the 13CO coverage from 0.22 to 0.06 ML reduces the


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overall PSD yield, but does not appreciably change the shape of the angular distribution (Fig. S6(b)).


3.5

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100

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2

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-40

ϕ

0 des

40

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Figure 5. 13CO PSD signal versus (a) time and (b) desorption angle, φdes, from the reduced (blue trace) and hydroxylated (red trace) TiO2(110) surfaces with θ(13CO) = 0.22 ML at 60 K. The φdes is measured relative to the surface normal and varied in the [11� 0] direction (the data are obtained

for φdes > 0° but also reproduced for φdes < 0° for sake of better visualization). The traces are displaced vertically for clarity. 15 ACS Paragon Plus Environment


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A narrow peak in the angular distribution usually indicates the prompt desorption of a hyperthermal molecule. It is noteworthy that the narrow peak observed in the 13CO PSD is quite similar to the angular PSD distribution of O2 from TiO2(110) (Fig. S6(a)).29,50 On the other hand, two mechanisms could lead to the broad cosine distribution. First, electron-hole recombination could lead to local heating that drives thermal desorption of the CO. Alternatively, some of the photodesorbing molecules might collide with the surface prior to desorption leading to the broad distribution (e.g. similar to PSD of physisorbed Kr atoms from TiO2(110), see Fig. S6(a)32). The spatial variation in photodesorption probabilities shown in Fig. 2c argues against the local heating mechanism: given that the CO adsorption energies for these sites are all similar, local heating should produce a slowly varying desorption probability – not the non-monotonic distribution that is observed. The observation that the angular distributions are similar at different CO coverages is also consistent with the hypothesis that the broad and narrow components arise from the same photoinduced process where some species directly desorb while others scattered from the surface prior to desorption. A fairly high degree of scattering may indicate relatively low hyperthermal energy for the desorbed CO molecules. Also note that photoinduced diffusion (which is observed with the STM but corresponds to a minor channel) may be related to unsuccessful desorption attempts of some CO species excited via the same mechanism. The observed similarity of the PSD signals, angular distributions and integrated yields of CO photodesorbed from the h-TiO2(110) and r-TiO2(110) surfaces gives an important insight into photodesorption mechanism. Hydroxylation changes dramatically the atomic structure of the surface defect, converting an oxygen vacancy into a pair of bridging hydroxyls. On the other hand, the excess negative charge associated with VO’s is only slightly perturbed upon hydroxylation.42,44,45 Thus the similarity of the CO photodesorption results suggests that excess


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electrons play a decisive role in the photodesorption of CO molecules on both surfaces. Reaction of these excess electrons with the photogenerated holes was proposed in the earlier study to explain an efficient “screening” of the trimethyl acetate molecules from hole-mediated photooxidation at the VO site on TiO2(110).51 We suggest that photodesorption of CO can be initiated by recombination of photo-generated holes with electrons localized near the surface defect (VO or OHb) leading to rearrangement of the surface atoms and ejection of the weakly-bound CO molecule. While both surfaces display a mild enhancement of the photoactivity for CO molecules residing at the Ti5c1’s relatively to other Ti5cn sites, the substantial increase of the desorption rate for CO species adsorbed at the VO defects needs further understanding. In summary, our study provides an atomically resolved, quantitative analysis of sub-monolayer CO adsorption and photodesorption on reduced and hydroxylated rutile TiO2(110) at low temperatures. STM images reveal CO distributed between various adsorption sites – VO and Ti5c. Quantitative analysis of the site-specific coverages of CO molecules and actual coverages of available adsorption sites yields an adsorption probability which decreases in the row: VO > Ti5c1-4 >> Ti5c0. The adsorption probability distribution corresponds to very small differences in the CO binding energies between the sites: < 0.02 eV. Under UV irradiation, some of the CO photodesorbs and some photodiffuses. Oxygen vacancies are primary photodesorption sites on the r-TiO2(110), being more than 4 times more efficient than the Ti5c sites. Removing the vacancies via hydroxylation does not appreciably affect the CO PSD yield, and the angle-resolved PSD data show both thermal- and non-thermal CO components from both r-TiO2(110) and h-TiO2(110). We suggest that CO photodiffusion occurs via the same mechanism as photodesorption except that collisions with the surface lead to complete accommodation of the initially hot CO to the surface temperature. The data suggest that photodesorption of CO from TiO2(110) is an indirect, substrate-



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mediated, electronic-excitation process involving localized unpaired electrons (nominally Ti3+) from the point defects of VO or OHb and photo-generated charge carriers, most likely holes.

Experimental Methods The STM measurements were conducted in an UHV system described previously.52 The system was equipped with Omicron low-temperature STM, X-ray photoelectron spectrometer (XPS), Auger electron spectrometer (AES), low energy electron diffraction (LEED), quadrupole mass spectrometer, and electron and ion guns. The rutile TiO2(110) crystal (Princeton Scientific) was cleaned by using multiple cycles of Ne ion sputtering (1 KeV) and UHV annealing at 800– 900 K. The initial concentration of bridging oxygen vacancies was ∼0.07 – 0.11 ML. Emptystate STM images were taken in a constant current mode at 10 - 40 pA and a bias voltage of 1.1 – 1.4 V. With these imaging conditions, no movement of CO on the TiO2(110) surface was observed at 30 K indicating that tip-induced diffusion of the CO was not important. STM tips were home-made from electrochemically etched W wire and cleaned in situ by annealing and ion sputtering. CO was deposited using a retractable pin-hole tube doser. The ensemble-averaged experiments were performed in an ultrahigh vacuum (UHV) system that has been described previously.28,29,32 The rutile TiO2(110) (CrysTec GmbH) was exposed to 13 16

C O (Cambridge Isotope Laboratories, 99%) at ∼30 K using a molecular beamline, and the

Extrel quadrupole mass spectrometer detected the photodesorption products. Samples were prepared by repeated cycles of sputtering (2 keV Ne+) and annealing at 950 K. The coverage of oxygen vacancies, θ(VO), was ∼0.05 − 0.08 ML as measured using water TPD.53 The UV light source was a 100 W Hg lamp (Oriel #6281) coupled into the UHV chamber via a fiber optic

cable. The infrared light was blocked with a water filter, while the entire UV portion was used to 18 ACS Paragon Plus Environment

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irradiate the sample. During photon irradiation, the increase in the sample temperature was < 1 K. Typical flux of “active” UV photons with energy > 3 eV was ∼ 1 × 1016 photons/cm2s. Acknowledgements We thank Drs. M. Henderson, R. Rousseau and Y-G. Wang for stimulating discussions. This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, & Biosciences and performed in EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for DOE by Battelle.

Supporting information available: Additional data on the CO thermal and photo-stimulated diffusion, time- and coverage-dependent CO adsorption site distribution, CO TPD after irradiation, and coverage-dependent CO PSD angular distributions are available in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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The Journal of Physical Chemistry PageLetters 30 of 32

a)

UV on

3 2.5 Hydroxylated TiO

2

2

1.5 1 0.5

Reduced TiO

2

0 0

20

40

1

60

Time (s)

1.2

CO PSD (arb. units)

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3.5

80

100

_ [110]

b)

0.8 Hydroxylated TiO

2

0.6 0.4 0.2

Reduced TiO

2

0 ACS Plus Environment -80Paragon -40 0 40 80

ϕ

des

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