Tetraoxygen on Reduced TiO2�110�: Oxygen Adsorption and Reactionswith Bridging Oxygen Vacancies

Greg A. Kimmel* and Nikolay G. PetrikChemical and Materials Sciences Division, Pacific Northwest National Laboratory Richland, Washington 99352, USA

(Received 11 January 2008; published 16 May 2008)

Oxygen adsorption on reduced TiO2�110� is investigated using temperature programed desorption andelectron-stimulated desorption. At low temperatures, 2 O2 molecules can be chemisorbed in each oxygenvacancy. These molecules do not desorb upon annealing to 700 K. Instead, for 200 K< T < 400 K, the 2O2 convert to another species, which has four oxygen atoms, i.e., tetraoxygen, that decomposes at highertemperatures. In contrast, when only 1 O2 is adsorbed in an oxygen vacancy, the molecule dissociatesupon annealing above �150 K to heal the vacancy in agreement with previous results.

DOI: 10.1103/PhysRevLett.100.196102 PACS numbers: 68.43.Mn, 68.43.Rs, 68.47.Gh

The interaction of oxygen with TiO2 is critical forapplications such as the photooxidation of organics [1,2],purification of water and air [3], and (potentially) photo-catalytic water splitting [1]. In photocatalysis, O2 is oftenused as an electron scavenger [2], but its exact role isunclear. The interactions of ‘‘simple’’ molecules, such asO2 and H2O, with TiO2�110� are also scientific bench-marks for testing our understanding of the physical andchemical processes occurring on oxide surfaces.

Oxygen physisorbs on defect-free TiO2�110� [4] butinteracts strongly with vacancies, Ov, in the bridging oxy-gen rows [5–13]. At �30 K, O2 adsorption is precursormediated and up to 1.5 monolayers (ML) can be adsorbed(1 ML � 5:2� 1014 cm�2) [4]. O2 dissociatively adsorbsin Ov’s at room temperature healing the vacancy andleaving an oxygen adatom, Oad, at nearby Ti4� sites[6,7,14]. Henderson et al. proposed that up to 3 O2 adsorbat each Ov at �120 K [5]. Upon heating, 2 O2 desorb at�410 K and the other O2 dissociates to heal the vacancy.Electron energy loss measurements showed that the elec-tronic defect from Ti3� disappeared upon O2 adsorptionand was replaced by a feature assigned to O�2 [5]. Photon-stimulated desorption of O2 at �100 K [15,16] indicatesthere are two forms of O2: one that can oxidize CO, whilethe other photodesorbs.

Theory provides conflicting models for the structure ofoxygen at oxygen vacancies [8–13]. Periodic unrestrictedHartree-Fock predicts that O�2 adsorbed perpendicular tothe surface in an Ov is the most stable configuration for 1O2, and that 2 more O2 can be adsorbed at adjacent Ti sites[8]. In contrast, density functional theory (DFT) has pre-dicted that O2

2� adsorbed parallel to the surface is the moststable [9–11,13]. The DFT calculations also predict thatthe molecular O2

2� state is more stable than the dissociatedstate. However, a recent DFT calculation has predicted thatO4

2� is the most stable species [12].In this Letter, we investigate oxygen’s reactions with

reduced, rutile TiO2�110� using temperature programmeddesorption (TPD) and electron-stimulated desorption

(ESD). For O2 coverages of 2 O2=Ov or less, almost noO2 desorbs for annealing temperatures, Tanneal, less than700 K. The amount of this chemisorbed oxygen, �chem,increases proportionally with increasing vacancy concen-tration. For �chem � 2 O2=Ov, the results indicate that astate with 2 O2 converts to a new species with 4 oxygenatoms—‘‘tetraoxygen’’—as the film is heated above�200 K, and that this species decomposes for Tanneal >400 K. However, for �chem < 1 O2=Ov, annealing above280 K heals the vacancies and prevents further O2 chemi-sorption. These results, which provide a new model for theinteraction of oxygen with TiO2�110�, are consistent withthe recent prediction that O4

2� is the most stable form ofoxygen in the vacancies [12].

The experiments were performed in an ultrahigh-vacuum system (base pressure <1� 10�10 Torr) that hasbeen described previously [17,18]. The TiO2�110� crystalwas mounted on a resistively heated tantalum plate, cooledwith a closed cycle helium cryostat, and prepared byrepeated cycles of ion sputtering (2 keV Ne�) and anneal-ing (typically 120 s at 950 K). O2 TPD and ESD spectrawere measured with a quadrupole mass spectrometer. Forthe ESD measurements, a 100 eV electron beam wasrastered across the surface, and neutral O2 was detected.For one experiment (Fig. 1, squares), electron irradiation ofbare TiO2�110� [17] with a fluence of �8� 1016 e�=cm2

was used to increase the oxygen vacancy concentration,�Ov. However, for measuring the O2 ESD, the electronfluences used were at least a factor of 10 smaller, resultingin no appreciable damage to the surface. Based on themagnitude of the OH recombination peak in water TPD[19], �Ov � 0:08 ML 0:01 ML for the annealedsurface.

Inset A of Fig. 1 shows a series of O2 TPD spectra thatagree with previous results [4] except that here no O2

desorbs for small O2 exposures. This difference arisesbecause the fully oxidized TiO2�110� used by Dohnaleket al. [4] had no vacancies and thus no O2 chemisorbed onthat surface. Figure 1 (circles) shows the amount of phys-

PRL 100, 196102 (2008) P H Y S I C A L R E V I E W L E T T E R S week ending16 MAY 2008

0031-9007=08=100(19)=196102(4) 196102-1 © 2008 The American Physical Society

isorbed O2 versus O2 exposure, �O2. The remarkable

feature is the large exposure required before desorptionof physisorbed O2 is first observed. As discussed below,this exposure threshold, �t � 1� 1014 cm�2, correspondsto�2 O2=Ov. Above �t, the signal increases linearly untila total coverage of �1:5 ML is obtained (and the conden-sation coefficient decreases to zero). The O2 ESD for O2

dosed and irradiated at 25 K increases linearly from �O2�

0 showing that the O2 coverage increases linearly withexposure (Fig. 1, triangles). For O2 dosed at 25 K andthen annealed to 100 K to remove any physisorbed O2, theO2 ESD also increases linearly for �<�t but is constantthereafter (Fig. 1, crosses). Finally, Fig. 1 (squares) showsthe integrated O2 TPD for a surface with a higher vacancyconcentration—�Ov � 0:145 ML 0:01 ML—producedby electron irradiation of TiO2�110� [17]. For this surface,�t is proportionally larger showing that more O2 is chem-isorbed when �Ov increases.

For �O2<�t, there is also no appreciable O2 desorp-

tion at higher temperatures. For example, for �O2� 9�

1013 O2=cm2, a very small desorption peak at �440 K isapparent, but no desorption is observed for 500 K< T <800 K (inset B of Fig. 1). This peak quickly saturates for�O2

>�t with an integrated intensity of <0:025 ML(Fig. 2, triangles). For O2 adsorbed at 100 K, Hendersonet al. observed a similar O2 TPD peak at�410 K [5]. Theyproposed that �chem � 3 O2=Ov and that upon heating 1 O2

dissociated in the vacancy and 2 O2 desorbed at 410 K.However, in those experiments the amount of desorbing O2

was not calibrated. Instead, the oxygen coverage was esti-mated from the O2 sticking coefficient versus exposure

(which decreases rapidly as �O2increases). At �25 K,

the initial sticking is large ( � 0:76 0:05) and nearlyconstant until �1:5 ML O2 has adsorbed (data not shown)[4], and the amount of O2 desorbing at �440 K can becalibrated relative to the 1.5 ML saturation coverage forphysisorbed O2. The results in Fig. 1 (inset B) and Fig. 2(triangles) show that the amount of O2 desorbing between100 and 700 K is small for all O2 exposures, and practicallyzero for oxygen coverages <2 O2=Ov.

From the measured sticking versus �O2, the amount of

adsorbed O2, �O2, can be calculated. Figure 2 (solid circles)

shows the O2 TPD data from Fig. 1 (circles) versus �O2for

small coverages. The solid line (m � 1) shows a fit to thedata for 0:22 ML � �O2

� 1:1 ML. The fit extrapolates tozero desorption, corresponding to �chem, at �O2

�

0:188 ML. Thus for the annealed surface, the number ofmolecules chemisorbed per vacancy, Nv, is 2:35 0:3,where Nv � �chem=�Ov. Of these chemisorbed molecules,�0:2 O2=Ov desorb at�440 K. For the electron-irradiatedsurface with �Ov � 0:145 ML, �chem � 0:30 ML, Nv �2:1 0:3, and a small amount of O2 also desorbs at�440 K. Therefore, both experiments show that 2 O2 arestrongly bound in each vacancy.

While 2 O2=Ov can adsorb at low temperature, anexperiment shows that if a vacancy with only 1 O2 isannealed, then no more O2 can be chemisorbed in thatvacancy. For this experiment, O2 was dosed at 25 K andannealed to 400 K for 240 s. The sample was then exposedto a fixed O2 dose (0.17 ML adsorbed) at 25 K, and thephysisorbed O2 was measured via a second O2 TPD (Fig. 2,open circles). The purpose of the second O2 dose and TPDis to measure �Ov remaining after annealing: If vacanciesremain, some O2 will chemisorb and thus reduce the in-

0

0.05

0.1

0.15

0.2

0 0.1 0.2 0.3 0.4

O2 T

PD

and

ES

D S

igna

ls (

ML)

Absorbed O2 (ML)

2 O

2/Ov

1 O

2/Ov

440 K peak

Physisorbed O

2

m = 2

m = 1

(x 13)

FIG. 2 (color online). Integrated O2 TPD (circles, triangles)and ESD spectra (squares) vs O2 coverage. The amount of O2

that desorbs at �440 K is small (triangles). The O2 ESD signalafter annealing to 380 K is initially small, increases for1 O2=Ov < �O2

< 2 O2=Ov, and saturates thereafter (squares).After annealing to 400 K and adsorbing more O2, the physi-sorbed O2 saturates for �O2

> 1 O2=Ov (open circles).

0

0.5

1

1.5

0 5x1014 1x1015 1.5x1015

Inte

grat

ed O

2 TP

D a

nd E

SD

(M

L)

O2 Exposure (molecules/cm2)

x 2

200 300 400 500 600Temperature (K)

ΦO2

= 2 x 1014 cm-2

ΦO2

= 9 x 1013 cm-2

B)

20 30 40 50 60 70 80Temperature (K)

A) O2 TPD Signal

(arb. linear units)

0.5 ML1 ML

FIG. 1 (color online). Integrated O2 TPD for T < 95 K vs O2

exposure from annealed TiO2�110� (circles) and TiO2�110� witha higher vacancy concentration (squares). For both surfaces, noO2 desorption is observed at any temperature for small O2

exposures. The O2 ESD for O2 adsorbed and irradiated at25 K (triangles) and annealed to 100 K (crosses) is also shown.Inset A: O2 TPD spectra (ramp rate � 2 K=s). Inset B: Two O2

TPD spectra for higher temperatures.

PRL 100, 196102 (2008) P H Y S I C A L R E V I E W L E T T E R S week ending16 MAY 2008

196102-2

tegral of the physisorbed O2 in the second TPD. Thesecond TPD signal increases linearly (Fig. 2, open circles)with twice the slope of the first TPD (Fig. 2, solid circles)and saturates at half the threshold for desorption for thefirst TPD, i.e., at �1 O2=Ov. For �O2

> 1 O2=Ov, themagnitude of the second TPD signal and measurementsof �Ov using water TPD (data not shown) indicate that novacancies remain after annealing to 400 K.

O2 ESD is a useful probe of the chemisorbed oxygensince ESD is often sensitive to adsorbate properties such asthe electronic structure and bonding geometry [21]. Thelinear increase in the O2 ESD versus �O2

for chemisorbedand physisorbed O2 (Fig. 1, triangles) indicates that O2

molecularly adsorbs at �25 K independent of the cover-age. The O2 ESD signals versus irradiation time also sup-port this conclusion: The ESD kinetics after annealing to100 K for �O2

< 1 O2=Ov and �O2� 2 O2=Ov are very

similar [Fig. 3(a) (inset), (i) and (ii)]. However, forTanneal > 100 K, different O2 coverages lead to differentreactions. Because there is no molecular O2 on cleanTiO2�110�, the O2 ESD signal is small. For �O2

<1 O2=Ov and Tanneal � 400 K, the ESD signal [Fig. 3(a)(inset), (iii)] is indistinguishable from that of cleanTiO2�110�, which, consistent with STM results showingthat 1 O2 dissociatively adsorbs in each vacancy at roomtemperature [6,7,14], suggests that no O2 remains.However, for �O2

� 2 O2=Ov and Tanneal � 400 K, theO2 ESD signal is large compared to clean TiO2�110�[Fig. 3(a) (inset), (iv); note semilogarithmic scale].Furthermore, for 2 O2=Ov, the initial ESD signal afterannealing is small compared to the initial signal frommolecular O2 at 100 K. But the signal for the annealedfilm also decays more slowly, such that after�40 s it is thelargest signal [22]. These results show that the state afterannealing depends on the initial number of O2 in thevacancy and suggest that a new chemical species is formedwhen �O2

� 2 O2=Ov.Figure 3(a) shows the integrated O2 ESD versus Tanneal

for �O2� 0:27 ML (solid circles) and �O2

� 0:03 ML(triangles). The larger O2 coverage results in 2 O2=Ov afterthe physisorbed O2 has desorbed, while there should besingly occupied vacancies for the smaller coverage. The O2

was dosed at 35 K, the sample was annealed for 240 s, andthen the O2 ESD was measured at 35 K. For both O2

coverages, the O2 ESD decreases over a broad temperaturerange as the molecular oxygen reacts. However, for �O2

�

2 O2=Ov [Fig. 3(a), solid circles], the film must be an-nealed to significantly higher temperatures to suppress theO2 ESD, showing again that the final state depends on theinitial oxygen coverage. As mentioned above, the changein the ESD kinetics upon annealing for �O2

� 2 O2=Ov

indicates a new species is formed [Fig. 3(a), inset]. The O2

ESD integrated over the end of the irradiation period (e.g.,40 to 140 s), which has a maximum at �390 K, highlightsthe evolution of this new oxygen chemisorbed state versusTanneal [Fig. 3(a), open circles].

For 2 O2=Ov and Tanneal > 200 K, the chemisorbed oxy-gen changes state. If 1 O2 dissociates in a vacancy, then theother O2 should desorb. However, no O2 desorption is seenfor 100 K< T & 400 K (see Fig. 1 and [5,16]). But thelarge decrease in the O2 ESD signal (Fig. 3, solid circles)and the change in kinetics upon annealing to �400 K[Fig. 3(a), inset] suggest the loss of molecular O2.Therefore, the results indicate that 2 O2 convert to anotherchemisorbed species which contains 4 oxygen atoms, i.e.,some form of tetraoxygen. A recent calculation predictsthat O4

2� is �0:9 eV more stable than 2 O2� with an

�0:35 eV barrier for converting 2 O2� to O4

2� [12].This suggests that at low temperature, each vacancy holds2 O2

�, which converts to O42� in the temperature range

from�200 to 400 K [Fig. 3(b)]. While our results indicatethe formation of tetraoxygen, they do not provide a spec-troscopic identification. Since the oxygen vacancies have 2excess electrons, the formation O4

2� is plausible eventhough it has not been observed previously [23]. How-ever, O4

� is also consistent with our observations [24].Figure 2 (squares) shows the O2 ESD (measured at

100 K) versus �O2for O2 dosed at 25 K and annealed at

380 K for 240 s. Tanneal � 380 K was chosen to promote

1 O2

Oad - Ov

2 O2 O4

200 –400 K

2 O2(v)

O4(v)

low T

O2(v) + O2(phys)

O2(v) + O2(gas)

400 –500 K

TiOx – Ov

Ti3+

Ene

rgy

B)

A)

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500 600 700

Inte

grat

ed O

2 ES

D S

igna

l (ar

b. u

nits

)

Annealing Temperature (K)

x 6

x 7

10-3

10-2

10-1

100

0 40 80 120 160Time (s)

iv) 400 K: O4

i) 100 K: O2

iii) 400 K: TiO2 + O

a

ii) 100 K: O2

FIG. 3 (color online). (a) Integrated O2 ESD vs annealingtemperature for �O2

� 0:27 ML (circles) and 0.03 ML (tri-angles). The total O2 ESD yield (solid circles) decreases as thefilm is heated above �200 K, but the integrated signal at latertimes (open circles) has a maximum at �390 K. Inset: O2 ESDvs time for �O2

< 1 O2=Ov annealed to 100 K (ii) and 400 K(iii); and �O2

� 2 O2=Ov annealed to 100 K (i) and 400 K (iv).For �O2

� 2 O2=Ov, the stimulated desorption kinetics changeas 2 O2 converts to tetraoxygen. (b) Potential energy schematicfor 2 O2 and an oxygen vacancy on TiO2�110�.

PRL 100, 196102 (2008) P H Y S I C A L R E V I E W L E T T E R S week ending16 MAY 2008

196102-3

the formation of tetraoxygen [Fig. 3(a), open circles]. TheO2 ESD is small for �O2

< 0:09 ML again indicating that 1O2 dissociates in each vacancy upon annealing for �O2

<1 O2=Ov. However, for 1 O2=Ov < �O2

< 2 O2=Ov, theO2 ESD signal rapidly increases as the number of vacan-cies initially occupied with 2 O2 increases and more tet-raoxygen is formed upon annealing. For �O2

> 0:18 ML,all the vacancies start with 2 O2 and the amount of tet-raoxygen formed is almost constant.

From 400 to 500 K, the O2 ESD signal from tetraoxygenrapidly decreases [Fig. 3(a), open circles] indicating itdecomposes. This is also the temperature range where,for initial O2 coverages >2 O2=Ov, a small amount ofO2 desorbs, suggesting that it is also associated with thedecomposition of tetraoxygen. Apparently some—but notall—of the vacancies can adsorb 3 O2 in agreement withprevious reports [5]. However, for those sites with 3 O2,only 1 O2 desorbs at �450 K when the tetraoxygen de-composes. Above �500 K, oxygen vacancies are regener-ated [20,25], but no O2 desorption is observed there either[see Fig. 1 (inset B) and [5]]. The lack of O2 desorption for500 K< T < 700 K is consistent with calculations pre-dicting that the energy needed to form O2 from a bridgingoxygen atom and an oxygen adatom is >2:5 eV [10].However, above �470 K, bulk interstitial Ti3� reactswith adsorbed oxygen producing a variety of adlayer struc-tures [26]. This suggests that reactions with Ti3� areresponsible for the decomposition of tetraoxygen withoutappreciable O2 desorption [Fig. 3(b)], as well as the regen-eration of the vacancies above �500 K.

How O2 populates the vacancies at low temperatures isnot yet known. However, since the results in Fig. 2 havereasonably well-defined changes at �O2

� 1 O2=Ov and 2O2=Ov, they indicate that O2 is sufficiently mobile uponannealing to 400 K to sequentially fill the vacancies.Furthermore, when �O2

is �1 O2=Ov, the physisorbedprecursor is most likely to find a vacancy already filledwith another O2. In that case, sequential filling suggeststhat the physisorbed O2 does not readily chemisorb at asingly occupied vacancy (i.e., there is an energetic barrier).Instead it continues diffusing until it finds an empty va-cancy. Since chemisorbing a second O2 at a vacancy shouldrequire both an electronic and structural rearrangement, abarrier is not too surprising. However, since the physi-sorbed molecule does chemisorb once all the vacanciesare singly occupied instead of desorbing, this barriershould be below the vacuum level [Fig. 3(b)].

In summary, 2 O2 can be strongly bound in each oxygenvacancy at low temperature. These O2 do not desorb at orbelow the temperature at which vacancies are thermallyregenerated. Instead the results suggest that 2 O2 are con-verted to tetraoxygen, perhaps O4

2� [12], upon annealingto�400 K. In contrast, when a vacancy initially has only 1O2, it dissociates to heal the vacancy upon annealing above

�280 K. These results provide a new model for the inter-actions of oxygen on TiO2�110� and should help elucidatethe critical role that oxygen plays in many photocatalyticapplications.

This work was supported by the U.S. Department ofEnergy (DOE), Office of Basic Energy Sciences, ChemicalSciences Division. Experiments were performed in theW. R. Wiley Environmental Molecular SciencesLaboratory at Pacific Northwest National Laboratory,which is operated for the DOE by Battelle MemorialInstitute under Contract No. DE-AC06-76RLO 1830.

*Corresponding author.gregory.kimmel@pnl.gov

[1] A. L. Linsebigler, G. Q. Lu, and J. T. Yates, Chem. Rev. 95,735 (1995).

[2] M. A. Fox and M. T. Dulay, Chem. Rev. 93, 341 (1993).[3] D. F. Ollis and H. Al-Ekabi, Photocatalytic Purification

and Treatment of Water and Air (Elsevier, Amsterdam,1993).

[4] Z. Dohnalek et al., J. Phys. Chem. B 110, 6229 (2006).[5] M. A. Henderson et al., J. Phys. Chem. B 103, 5328

(1999).[6] O. Bikondoa et al., Nat. Mater. 5, 189 (2006).[7] Y. Du, Z. Dohnalek, and I. Lyubinetsky, J. Phys. Chem. C

112, 2649 (2008).[8] M. P. de Lara-Castells and J. L. Krause, Chem. Phys. Lett.

354, 483 (2002).[9] A. Tilocca and A. Selloni, Chem. Phys. Chem. 6, 1911

(2005).[10] X. Y. Wu et al., Phys. Rev. B 68, 241402 (2003).[11] M. D. Rasmussen, L. M. Molina, and B. Hammer,

J. Chem. Phys. 120, 988 (2004).[12] D. Pillay, Y. Wang, and G. S. Hwang, J. Am. Chem. Soc.

128, 14 000 (2006).[13] Y. Wang, D. Pillay, and G. S. Hwang, Phys. Rev. B 70,

193410 (2004).[14] U. Diebold et al., Surf. Sci. 411, 137 (1998).[15] G. Q. Lu, A. Linsebigler, and J. T. Yates, J. Chem. Phys.

102, 3005 (1995).[16] G. Q. Lu, A. Linsebigler, and J. T. Yates, J. Chem. Phys.

102, 4657 (1995).[17] C. D. Lane et al., J. Phys. Chem. C 111, 16 319 (2007).[18] N. G. Petrik and G. A. Kimmel, J. Chem. Phys. 121, 3736

(2004).[19] M. A. Henderson, Surf. Sci. 355, 151 (1996).[20] W. S. Epling et al., Surf. Sci. 412/413, 333 (1998).[21] R. D. Ramsier and J. T. Yates, Surf. Sci. Rep. 12, 243

(1991).[22] For �O2

< 1 O2=Ov, the kinetics does not change as thesurface is annealed.

[23] H. J. Zhai et al., J. Am. Chem. Soc. 124, 6742 (2002).[24] G. V. Chertihin and L. Andrews, J. Chem. Phys. 108, 6404

(1998).[25] U. Diebold, Surf. Sci. Rep. 48, 53 (2003).[26] M. Li et al., Surf. Sci. 437, 173 (1999).

PRL 100, 196102 (2008) P H Y S I C A L R E V I E W L E T T E R S week ending16 MAY 2008

196102-4