Characterization of the Active Surface Species Responsible for UV

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Characterization of the Active Surface Species Responsible for UV-Induced Desorption of O from the Rutile TiO(110) Surface 2

2

Michael A Henderson, Mingmin Shen, Zhi-Tao Wang, and Igor Lyubinetsky J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp312161y • Publication Date (Web): 25 Feb 2013 Downloaded from http://pubs.acs.org on February 26, 2013

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The Journal of Physical Chemistry

Submitted to: J. Phys. Chem. C Date: January 17, 2013

Characterization of the Active Surface Species Responsible for UV-Induced Desorption of O2 from the Rutile TiO2(110) Surface Michael A. Henderson,*a Mingmin Shen,a Zhi-Tao Wangb and Igor Lyubinetskyb a

Fundamental and Computational Sciences Directorate, Physical Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352 b

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352

* PO Box 999, MS K8-87, Pacific Northwest National Laboratory, Richland, WA 99352; [email protected]; (509) 371-6527

Abstract We have examined the chemical and photochemical properties of molecular oxygen on the (110) surface of rutile TiO2 at 100 K using electron energy loss spectroscopy (EELS), photon stimulated desorption (PSD) and scanning tunneling microscopy (STM). Oxygen chemisorbs on the TiO2(110) surface at 100 K through charge transfer from surface Ti3+ sites. The charge transfer process is evident in EELS by a decrease in the intensity of the Ti3+ d-to-d transition in EELS at ~0.9 eV and formation of a new loss ~2.8 eV. Based on comparisons with the available homogeneous and heterogeneous literature for complexed/adsorbed O2, the species responsible for the 2.8 eV peak can be assigned to a surface peroxo (O22-) state of O2. This species was identified as the active form of adsorbed O2 on TiO2(110) for PSD. The adsorption site of this peroxo species was assigned to that of a regular five-cooridinated Ti4+ (Ti5c) site based on comparisons between the UV exposure dependent behavior of O2 in STM, PSD and EELS data. Assignment of the active form of adsorbed O2 to a peroxo species at normal Ti5c sites necessitates reevaluation of the simple mechanism in which a single valence band hole neutralizes a singly charged O2 species (superoxo or O2-) leading to desorption of O2 from a physisorbed potential energy surface.

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Keywords: TiO2(110), photodesorption, photocatalysis, oxygen, scanning tunneling microscopy, electron

energy

loss

2 ACS Paragon Plus Environment

spectroscopy

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Introduction The surface chemistry and photochemistry of oxygen on the rutile TiO2(110) surface continues to attract the attention of researchers studying oxide surface phenomena.

1-18

Oxygen

has obvious importance in catalysis, photocatalysis and fuel cell applications as an oxidant, but is also important as a product, for example in solar photochemical conversion of water. One of the most well-studied aspects of oxygen on TiO2(110) has been that of its photodesorption properties.

6,7,10-16,18-24

The Yates group

4,6-9,13,18-20,24-27

has extensively examined various aspects

of O2 photodesorption including thermal and coadsorption effects, as well as employing O2 photodesorption as a probe of other important surface phenomena (such as carrier trapping). This group has proposed a mechanism for O2 photodesorption involving the reaction of a valence band (VB) hole (h+), generated by photoexciting an electron above the TiO2 bandgap, with a singly charged chemisorbed O2 species according to Reaction 1: O2-(a) + h+ O2(g)

(1)

(In contrast, the only theoretical studies of O2 photodesorption from TiO2(110) have approached the problem from the perspective of direct photoexcitation of the O2--Ti4+ surface complex. 22,23) Neutralization of the O2- species essentially results in an excited ‘physisorbed’ O2 species that possesses sufficient internal energy to result in desorption. Sporleder et al. have shown with time-of-flight methods that the energy distribution of photodesorbing O2 from TiO2(110) contains both thermalized and non-thermalized components. 11 Aside from issues regarding the mechanism of O2 photodesorption, a major question remains regarding the nature of the adsorbed O2 species that is active for photodesorption. Several studies have shown that at low temperature (~100 K) O2 adsorbs molecularly at both vacancy and non-vacancy sites on the TiO2(110) surface,

28-30

and various STM studies have

been successful at imaging one or both of these forms of adsorbed O2. coworkers

15

15,31-33

Wang and

recently showed with STM that the active species in O2 photodesorption is O2

bound at non-vacancy site (i.e., a normal Ti5c site), whereas an O2 molecule bound at an oxygen vacancy (Ov) site either photodecomposes or is photoinactive. The latter point confirms assertions by Petrik and Kimmel

10,16,17

that both photodesorption and photodissociation occur

for O2 on TiO2(110). 3 ACS Paragon Plus Environment


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It is generally recognized that O2 chemisorbs on TiO2 surfaces by means of charge transfer from available Ti3+ sites, resulting in a formation of an O2 anion, however the exact extent of electron transfer is not well-understood. In fact, there are not many spectroscopic approaches that can be successfully used to characterize the electronic state of chemisorbed O2 on TiO2 surface (especially on single crystal surfaces). Vibrational approaches (FTIR, HREELS, Raman) are hampered by interference from strong phonon modes of the oxide. Similarly, photoionization and photoelectron emission techniques have to contend with much more intense signals from lattice oxygen. Superoxo (O2-) species have been detected in EPR studies of TiO2 powders, 34-36 however peroxo oxygen (O22-) is typically diamagnetic and not detectable with this technique. The interference from lattice vibrational modes makes it difficult to identify chemisorbed O2 species on TiO2 by vibrational techniques.

37

Kimmel and Petrik

10,16,17,29

have

extensively examined the properties of O2 photodesorption from TiO2(110), probing O2 coverage and temperature dependent behavior, as well as utilized physisorbed O2 as probe of chemisorbed O2 photodesorption. These authors were the first to propose two channels of O2 photochemistry on TiO2(110), desorption and dissociation, with the former initiated by holes and the latter by excited electrons. In a later study, Wang et al.

15

showed with STM that O2 molecules bound at

Ov sites on TiO2(110) photodissociate, whereas those bound at Ti5c sites experienced photodesorption. In this study, we combine time- and coverage-dependent measurements from ELS, PSD and STM to provide new insights into the electronic properties of the O2 surface species responsible for O2 photodesorption. Previous EELS measurements for O2 on TiO2(110) revealed a loss feature at 2.8 eV associated with a chemisorbed form of O2 on the surface. We show that this surface O2 species is responsible for the O2 PSD signals and that its surface location is at Ti5c sites. We characterized this species as a peroxo (O22-) form of adsorbed oxygen rather than a superoxo (O2-) species based on comparisons with the inorganic literature, and re-examine the prevailing mechanism of O2 PSD that centers on neutralization of adsorbed superoxo oxygen.

Experimental The reduced TiO2(110) surfaces employed in this study were prepared similarly on two separate single crystals through multiple cycles of Ar+ or Ne+ ion sputtering and annealing at 800-900 K. The surface oxygen vacancy coverages of these reduced surfaces were determined 4 ACS Paragon Plus Environment

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either directly via STM or indirectly using water TPD. Two ultrahigh vacuum (UHV) systems were employed in this study. The EELS/PSD system possessed a base pressure of better than 1×10-10 Torr. ~8x10

−12

38

Torr.

39

STM experiments were conducted in a UHV system with a base pressure of In both systems, research grade O2 was purified with a liquid nitrogen trap. For

the EELS/PSD measurements, O2 was dosed by backfilling the chamber through a leak valve, with exposures expressed in Langmuirs (1 L = 1x10-6 Torr s). For STM measurements, O2 was introduced through a pinhole doser coupled to the STM stage. Coverages (expressed in monolayer units where 1 ML = 5.2×1014 atoms/cm2) were determined directly from the resulting images. 40,41 The UV light source employed in both chambers was a 100 W Hg are lamp coupled to vacuum via fused silica fiber optic cables and feedthroughs. The infrared portion of the Hg emission spectrum was removed using water filters resulting in UV fluxes in the STM chamber of ~2x1015 photons/cm2s and ~5x1015 photons/cm2s in the EELS chamber. In either case (STM or PSD/EELS), the UV flux did not cause a noteworthy sample temperature increase (3 eV), but resulted in two notable changes elsewhere in the spectrum. First, the prominent Ti3+ feature at 0.9 eV lost ~70% of its intensity due to the 0.1 L O2 exposure. This change was due to charge transfer from Ti3+ surface sites to O2, which is well-documented in the TiO2 literature.

1-3,5,50

The second change

was the appearance of a new loss at 2.7 eV that shifted to 2.8 eV with increasing O2 exposure. The 2.8 eV loss was not seen when the surface was exposed to oxygen at RT (see Supplemental section), despite the fact that O2 exposure at RT depleted the 0.9 eV feature. At temperatures above ~150 K, the charge transfer from Ti3+ to O2 results in predominately dissociative O2 adsorption.

28,29,32,51

However, O2 adsorbs on the surface at 100 K in two distinct molecular

forms associated with vacancy and non-vacancy sites. 31-33 The 2.8 eV loss can be assigned to a ligand-to-metal charge transfer event (excited by the scattered EELS electrons) based on comparisons with O2 organometallic complexes (see below). Increased O2 exposure resulted in continued diminishment of the Ti3+ loss feature to 1000 cm-1. 28,38

(Interference from phonon modes is a common problem encountered when attempting

vibrational spectroscopy on oxides.) Although there are no available data on the optical properties of η1-superoxo Ti complexes, one may ask at wavelength the CT transition for such a complex would be. The wavelength of the CT transition will depend on the metal’s d orbital character. Therefore, speculations regarding η1-superoxo Ti complexes based on other transition metal complexes may be problematic. Nevertheless, the CT transitions of η1-O2 (superoxo) complexes of first row transition metals are generally located below 350 nm. 86-88 The O2 exposure-dependent behavior of the O2-related 2.8 eV loss in Figure 1 is noteworthy. The O2 adsorption capacity of the vacuum-annealed TiO2(110) surface used to produce the data of Figure 1 (with ~0.05 ML of vacancies) was far from being reached after only a 0.1 L O2 exposure at 100 K. Based on previously published sticking coefficient measurements at 100 K, a 0.1 L exposure of O2 should translate into an O2 coverage of ~1.8x1013 molecules cm-2 (or ~0.035 ML) on the TiO2(110) surface at 100 K. The maximum adsorption capacity of this surface should be at least 3 times this estimate given that each Ov site is ultimately responsible for binding at least 2 O2 molecules. 28-30 However, the 0.9 eV loss feature associated with surface Ti3+ was diminished by a proportionally greater amount (~70%) by this O2 exposure suggesting that the majority of available surface charge was transferred from the surface to this minority of adsorbed O2. This is consistent with a model proposed by Petrik and Kimmel

17

in

which the average magnitude of the charge transferred to O2 depends on the O2 coverage. At low O2 coverage, the average charge transferred from the surface to each O2 would be greater than when the O2 coverage approached saturation due to competition between coadsorbed O2 9 ACS Paragon Plus Environment


molecules for the (limited) surface charge. The adsorption capacity of the surface is reached when charge sufficient to generate a negatively charged O2 species that can adsorb is no longer available. However, the data in Figure 1 suggests that the first ~1/3 of O2 molecules arriving at the surface (at 100 K) acquire ~70% of the available charge. Because the 2.8 eV EELS feature does not diminish with increasing O2 exposure, as one might expect it to be if charge were taken from existing O22- surface species to be used for new arrivals, it would appear that the magnitude of the charge on the associated O2 species, which sets apart the 2.8 eV feature for what it is, is not reapportioned to newly arriving O2 molecules. That is, the ‘x’ in the O2x- species responsible for the 2.8 eV loss does not appear to significantly change as the surface becomes saturated with additional chemisorbed O2 species. Additional O2, however, did result in a small blueshift of this EELS from 2.7 to 2.8 eV as saturation was reached. The 2.8 eV peak position suggests that these O2 molecules more closely resemble O22- (peroxo) in nature than O2-

UV on



(superoxo). Assignment of an η

17 L O2

structure rather than an η1 structure is consistent

with

theoretical

assessments of O2 on TiO2(110) which suggest the side-on structure is more

stable

structure.

than

41,74,75,89-91

the

‘end-on’

However, several

of these theoretical studies designate the preferred form of O2 to superoxo rather than peroxo.

Mass 32 QMS signal (arb. units)

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UV off

3.2 Photochemistry In this section, we examine the photodesorption of O2 in the context of a direct comparison between EELS

0

10

15

20

Time (seconds)

and STM data. Several authors have extensively published on the PSD

5

Figure 4: PSD spectrum of 17 L O2 adsorbed on TiO2(110) at 100 K. 10

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behavior of O2 from TiO2(110), 6-14,16-18 but direct correlation between the PSD behavior and the 15

actual surface species responsible has only recently been made.

Figure 4 presents a

representative PSD spectrum obtained from UV irradiation of 17 L O2 adsorbed at 100 K on TiO2(110). The salient characteristics of this spectrum are the prompt rise in the O2 quadrupole mass spectrometer (QMS) signal on irradiation of the surface to UV followed by the decay in the signal as the O2 coverage is depleted through photodesorption. The decay follows multiexponential kinetics indicating the photodesorption process does not obey a simple firstorder process (at least not after the first few seconds of irradiation). 13 Even after long irradiation times, the surface still possesses some unreacted O2 species as well as photodissociated O2 fragments. UV on

10,15

The data presented in

Figure 3 were collected with the sample

20 L O2

facing the QMS, which was remotely

4.1

situated from the EELS spectrometer. Attempts to register O2 PSD while the sample was situated in the EELS

5

Elastic peak intensity (x10 cps)

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


4

spectrometer were

unsuccessful.

In

order

a

of

to

comparison

provide

between

‘timeline’ the

O2

PSD

response as measured by the QMS and UV-induced changes in the EELS spectrum of O2 on the TiO2(110) (to be

3.9

discussed), the EELS elastic peak intensity response was followed during UV irradiation. Figure 5 shows that the 3.8 -10

EELS elastic count rate was constant, 0

10

20

30

40

Time (seconds)

Figure 5: EELS elastic peak during O2 photolysis at 100 K.

with a signal-to-noise ratio of ~80 to 1 (at a counter dwell time of 100 ms) prior to UV irradiation of 20 L O2 on

TiO2(110) at 100 K. The elastic peak dropped by ~3% within the first 2 seconds of UV irradiation and then gradually recovered as the irradiation time continued. No response in the elastic peak count rate was observed for UV exposures to the TiO2(110) surface without 11 ACS Paragon Plus Environment


molecular oxygen present on the surface (data not shown) indicating the small response shown in Figure 5 can be linked to surface photochemistry. Similar responses in the elastic peak were noted in each EELS photolysis experiment with O2 present on the surface (see below). The reflectivity of a surface for low energy electron (specular) scattering depends on many factors, such as the metallicity of the surface, the degree of surface order, the surface work function and the electron energy. 92 It is therefore difficult to predict the magnitude and sign of a change in the scattering yield for any given chemical change occurring on a surface. For example, it is unclear why the elastic peak should decrease due to O2 photodesorption and why it should recover as the UV irradiation was sustained. However, it is possible to attribute these changes to two distinct surface processes occurring during photolysis: one that initially decreases the scattering yield and one that increases the scattering yield over long time periods. In

x500

clean surface 0.5 L O2 3 sec UV 13 sec UV 313 sec UV

particular, the initial decrease in the elastic peak signal provides a direct comparison between the initial timelines of the PSD experiment (Figure 4) and the EELS data (to be discussed below), and both the fast and slow regions can be correlated with

distinct

UV-induced

processes based on STM results

Intensity (a.u.)

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2.8

(see below). Figure 6 presents EELS data for the clean TiO2(110) surface, for 0.5 L O2 adsorbed at 100 K and the same O2-covered surface after UV irradiation time periods of 3, 13 and 313 seconds. The spectra are scaled to focus on the response of the

0

1

2 3 4 5 Electron energy loss (eV) Figure 6: EELS spectra from UV irradiation of 0.5L O2 adsorbed on TiO2(110) at 100 K. 12


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2.8 eV loss feature (responses in other regions of the spectrum will be discussed below). The 2.8 eV feature is significantly attenuated after only 3 seconds of UV irradiation and essentially absent after 13 seconds of irradiation. The same photon irradiation conditions were used in Figure 6 as in Figure 5, so the rapid response of the 2.8 eV feature can be placed in the context of the initial (rapid) change in the elastic peak, and by extension the initial ‘spike’ in the O2 PSD signal (Figure 4). By these comparisons, we can reasonably assign the 2.8 eV feature to the same O2 surface species responsible for O2 PSD. The 0.9 eV feature assigned to surface Ti3+ sites showed little or no change during the same irradiation period in which the 2.8 eV feature decreased indicating no significant change in the population of surface Ti3+ sites during O2 photochemistry. This behavior is in sharp contrast with what occurs in EELS during holemediated decomposition of trimethyl acetate (TMA) on TiO2(110) where the 0.9 eV feature rapidly grew during UV irradiation of the TMA-covered surface.

49

In other words, hole-

mediated photodecomposition of TMA is accompanied by surface trapping of the photoexcited electrons (i.e., as Ti3+ sites), but hole-mediated photodesorption of O2 is not. This inconsistency is perhaps rationalized by considering the surface situation from the perspective of the photoexcited electrons. In the TMA case, the surface provides a stable trapping site in the form of bridging OH groups, which are formed during adsorption and deprotonation of trimethyl acetic acid. These sites obviously are not present on the O2-covered surface, and can not be introduced without unwanted thermal reactions between O2 and OH.

3,48,50,51,93,94

It is not clear

whether the OH-covered TiO2(110) surface can offer stable electron trapping sites for the photoexcited electrons. 95 STM results in Figure 7 provide assistance in confirming the identify the 2.8 eV O2related species, in characterizing its rapid response to UV irradation, and potentially in explaining the absence of electron trapping at the surface. Figure 7a is typical of clean surface STM images from TiO2(110) that exhibit an alternating pattern of periodic bright and dark rows corresponding to Ti5c and Ob atoms, respectively. The Ov sites appear as weak bright spots on the dark rows 1 (one is marked by a square). Two brighter spots in Figure 7a, one at a Ti5c site and another on an Ob row, were due to minor background adsorption (e.g., CO or H2O). After O2 exposure (Figure 7b), bright spots appeared on the Ti5c rows (marked by a hexagon) and many of the vacancy sites in Figure 7a were no longer apparent. The latter observation is attributed to O2 molecules residing in the vacancy sites.

31-33

The O2 coverage resulting from the O2 exposure 13



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used in Figure 7b was estimated

at

~0.11

ML by summing the

(a)

(b)

coverage of new spots on the Ti5c rows and the

coverage

vacancies

VO

O2/Ti5c

of

removed.

The majority of this coverage (~0.08 ML) was

from

(c)

(d)

O2

molecules

Oa

chemisorbed at the Ov sites

because

only

Oa or O2/Ti5c

∼0.01 ML of unfilled Ov The

sites

remained.

coverage

O2

molecules chemisorb at regular Ti5c sites in Figure 7b was ~0.03 ML.

Figure

7c

Figure 7: STM images of the TiO2(110) surface (15 nm×15 nm) (a) before and (b) after O2 exposure (~0.11ML), and after UV irradiation for (c) 8 sec and (d) 15 min. The squares, hexagon and circle correspond to oxygen vacancies, O2/Ti5c and Oa, respectively. Images (a) and (d) are acquired at 0.8V and 5pA; images (b) and (c) are acquired at 0.3V and 1pA.

illustrates the impact of an 8 second period of UV irradiation. The number of bright spots on the Ti5c rows decreased from ~0.03 ML to ~0.008 ML with no apparent change in the number of Ov sites. The ~0.008 ML remaining bright features appeared brighter and more well defined than those shown in Figure 7b, possibly indicating they were not the same species as in Figure 7b. The depletion of bright spots on the Ti5c rows in Figure 7c is assigned to photodesorption of O2 molecules. After 15 minutes of UV irradiation (Figure 7d), the population of bright spots on the Ti5c rows dramatically increased without a significant change in the number of Ov sites apparent on the surface. The bright spots in Figure 7d (marked with a circle) possessed greater contrast than those seen in Figure 7b, pointing to these being from a different species. In fact, the bright spots in Figure 7c resemble more those in 7d than those in 7b. Based on our previous STM studies for 14 ACS Paragon Plus Environment

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O2 adsorbed on TiO2(110),

32,40,96,97

we can definitively assign the high contrast bright spots in

Figure 7d to O adatoms (Oa) resulting from O2 photodissociation. 15 The population of Oa species reached ~0.065 ML only after 15 minute of UV irradiation. In contrast, the majority of bright spots on the Ti5c rows in Figure 7b, assigned to molecularly adsorbed O2, were removed after only 8 seconds of UV irradiation. These data clearly show that there are different timescales for the two types of O2 photochemistry on TiO2(110), with the faster being O2 photodesorption and the slower being due to photodissociation of O2 molecules in vacancies. These results are consistent with the two timescales observed in the EELS measurement of Figure 6. Petrik and Kimmel

10,17

have previously assigned these two channels of photochemistry to hole- and

electron-mediated processes, respectively.

At low to intermediate O2 coverages, we can

designate photodesorption of O2 specifically to O2 molecules bound at Ti5c sites and photodissociation of O2 specifically to O2 molecules bound at vacancies.

15

The

photodissociation of the remaining amount of O2 at Ov sites appeared to be hindered, possibly by charge trapping and recombination effects.

15,17

Unfortunately, there is no apparent indicator in

the EELS spectra of Figure 6 for the O2 photodissociation event that is so clearly evident in STM after long irradiation times (Figure 7). This perhaps is not surprising because the filled vacancies return to the typical state of lattice oxygen and oxo ligands of Ti4+ complexes (i.e., Ti=O) are transparent in the visible. 28,61 The two photochemical timescales observed for O2 on TiO2(110) in the STM images of Figure 7 can be mapped on to the EELS results of Figure 6. Because the photon fluxes used in the two different UHV chambers (i.e., EELS and STM) came from similar light sources (100 W Hg arc lamps) and traversed similar delivery systems (fused silica fiber optics) with similar distances between the fiber optic and sample (~0.5-1 cm), we can assign the change in the 2.8 eV EELS feature to photodesorption of O2 and not to photodissociation of O2. Having previously provided the groundwork for assigning the 2.8 eV EELS feature to a surface peroxo form of O2 (i.e., O22-) and not a superoxo species (O2-) in Section 3.1, these results suggest that the photodesorption of O2 from TiO2(110) is more complex than single hole neutralization of O2-. The pathway from a doubly charged peroxo species reacting with a VB hole to a neutralized O2 that can readily desorb on a physisorbed O2 potential energy surface is not clear, but might explain the complex, non-first-order photodesorption kinetics detailed by Thompson and Yates. 13

Recent time-of-flight PSD measurements by Sporleder et al. 15 ACS Paragon Plus Environment

11

reveal that both non-thermal


and thermalized channels exist for O2 photodesorption from TiO2(110). The thermalized component results from energy accommodation of the desorbing O2 with the surface prior to desorption, which could come from collisions or other structural transformations that extend the residence time of the ‘excited’ species on the surface. In contrast to findings by the same group for methyl radical photodesorption from TiO2(110), 98,99 in which a thermalized channel was not detected, the presence of a thermalized channel for O2 PSD in the results of Sporleder and coworkers suggests that the potential energy surface for O2 PSD maybe coupled to other potential energy surfaces that distribute energy away from the 'excited' O2 molecule during desorption. For example, building off the organometallic analogs, a plausible pathway leading to thermalized O2 might be rapid transformation of a ground state η2-O22- state with reaction with a hole (h+) to an excited η2-O2- state that relaxes on the surface to another configuration prior to desorption, as illustrated in Reaction 2: η2-O22- + h+ {η2-O2-} {η1-O2-} O2(g) + e-

3+

O2 at 100 K after 5 min UV

Ti

A side-on bound superoxo is not 53

and such a configuration 5L

(generated by reaction with a hole) might relax to an ‘end-on’ η1-superoxo -

(O2 ) form, which is metastable in coordination to Ti4+.

71

A transition

from a side-on peroxo species through

0.5 L

0.2 L

an end-on superoxo species prior to desorption

30 L

O2

considered stable in coordination to Ti4+,

(2)

2-

Intensity (arb. units)

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Page 16 of 28

could

provide

an 0.1 L

explanation for the multiple nonthermal and thermal forms of O2 seen by Sporleder et al. superoxo

electron

11

The fate of the

in

Reaction

2

remains an issue. In fact, one of the challenges facing model studies of

1

2 3 4 Electron energy loss (V)

5

Figure 8: Changes in EELS for various O2 exposures as a result of UV irradiation at 100 K. 16


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photocatalysis on TiO2 surfaces (not just on single crystals) is being able to account for the fates of both charge carriers, especially in situations where charge carrier trapping is difficult to detect. 2

There is also the possibility that along with thermalization of desorbing oxygen that some

molecules may be desorbed in the singlet state. Unfortunately, we have no means of addressing this possibility in these studies. A closer look at the impact of UV irradiation on the entire EELS spectral region for various exposures of O2 on TiO2(110) is shown in Figure 8. Irrespective of the O2 coverage, the 2.8 eV feature promptly diminished to nothing after only 10 seconds of UV irradiation (not shown in Figure 8). The band-to-band region (>3 eV) remained essentially unchanged irrespective of the irradiation time. However, there was a trend in the response of the Ti3+ feature at 0.9 eV as a function of O2 coverage and irradiation times approaching saturation of the photoeffect (~5 minutes). At low O2 initial coverages (

Characterization of the Active Surface Species Responsible for UV

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