First Principles Study of O2 Adsorption on Reduced Rutile TiO2-(110

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First Principles Study of O Adsorption on Reduced Rutile TiO(110) Surface Under UV Illumination and Its Role on CO Oxidation Yongfei Ji, Bing Wang, and Yi Luo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp310443p • Publication Date (Web): 18 Dec 2012 Downloaded from http://pubs.acs.org on December 21, 2012

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The Journal of Physical Chemistry

First Principles Study of O2 Adsorption on Reduced Rutile TiO2-(110) Surface Under UV Illumination and Its Role on CO Oxidation Yongfei Ji,†,‡ Bing Wang,† and Yi Luo∗,†,‡ Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Anhui 230026, China, and Department of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, SE-106 91 Stockholm, Sweden E-mail: [email protected]

∗ To

whom correspondence should be addressed National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Anhui 230026, China ‡ Department of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, SE106 91 Stockholm, Sweden † Hefei

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Abstract Oxidation of CO by O2 on the reduced rutile TiO2 (110) surface under UV illumination has been explored by first-principles simulations. It is found that at the ground state, the O2 molecule prefers to be adsorbed at the oxygen vacancy horizontally; whereas under the photo excitation, it can capture a hole as it transforms itself into a near perpendicular geometry. Such a photo-excited O2 can be effectively connected to the CO molecule to form a O-O-CO complex, which can then convert to CO2 by overcoming a small barrier. This mechanism can be applied to both low and high O2 coverage and in consistent with the off-normal desorption behavior of the CO2 observed in recent experiments.

Keywords: DFT, surface chemistry, Photocatalysis, CO abatement, potential energy surface

Introduction Carbon monooxide (CO) is a highly toxic gas for human and animals, and its oxidation to CO2 has been the optimal method for CO abatement. 1 TiO2 is one of the most popular photocatalysis that has been extensively applied and studied. 2 CO photo-oxidation by O2 on TiO2 , which is probably the simplest bimolecular photo-reaction, has drawn intense scientific interest in recent decades. 3–8 A good understanding of its underlying mechanism is of fundamental importance, which can help to improve the performance of existing materials and the design of new ones. O2 was suggested to have two chemisorption states on the reduced TiO2 (110) surface: the α -O2 that undergoes slow photodesorption can be photo-activated to oxide CO, resulting in CO2 ; whereas the β -O2 experiences a fast photodesorption. 3,4 Much theoretical effort has been devoted to identify the structure of the two chemisorption states. 9–11 Although many candidates were proposed, little about the mechanism of CO photo-oxidation at the molecular level is known because of the complicated interaction between O2 and TiO2 surface under the illumination. O2 only physisorbs weakly on fully oxidized TiO2 (110) surface 12 but chemisorbs strongly at oxygen vacancy (Ov ) at the bridge oxygen row. In an early temperature programmed desorption (TPD) experiment, 13 Henderson et al. suggested that each Ov can adsorb at most three O2 molecules. 2 ACS Paragon Plus Environment

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Based on first-principles calculations, Pillay et al. suggested that the chemistry of O2 on the surface is coverage dependent. 6 They also proposed a tetraoxygen species 14 which was latter claimed to be confirmed by Kimmel et al. 15 in their TPD and electron-stimulated desorption experiments. In the same work, Kimmel et al. 15 provided strong evidence that each Ov can only chemisorb at most two O2 molecules which convert to tetraoxygen at the temperature window 200 K ∼ 400 K. Below 200 K, one O2 was suggested to be adsorbed at Ov and the other on the Ti5c row both as superoxide 16 (O2 − ). The one adsorbed at Ov was shown to be responsible for the photo-oxidation of CO from the off-normal desorption behavior of CO2 . 7 In recent experiment, Yates et al. concluded that CO photo-oxidation is electron mediated, because it is suppressed by upward band bending via the coadsorption of acceptor molecules. 8 The oxidizing species was also proposed to be O2 − . The off-normal desorption behavior of CO2 was explained with the ground state potential energy surface (PES), 7 but the PES of the excited state is needed in order to fully understand the molecular mechanism of CO photo-oxidation and the role of electron in the reaction. On the other hand, at low coverage 1 O2 /Ov , the O2 was suggested to be peroxide 14,16 O2 2− , it also has the ability to photo-oxidize CO. 7 But the corresponding mechanism and the role of electron and hole are not known neither. In this work, we perform first-principles calculations to model the adsorption of O2 at low and higher coverage with and without UV-illumination. Then, PES at the ground and excited states for the transformation of O2 between different geometries and CO oxidation are calculated. And finally, we propose a possible mechanism at molecular level for CO photo-oxidation which is applicable to both low and high O2 coverage and in consistent with the off-normal desorption behavior of CO2 .

Computational details We performed spin-polarized calculation at generalized gradient approximation (GGA) with PW91 17 exchange-correlation functional implanted in VASP 18,19 with a plane-wave basis cutoff 400 eV. Ion-electron interaction was described by Vanderbilt ultra-soft pseudo-potential. 20 To model



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the adsorption of O2 molecule on the surface with light illumination, the electron-hole pair was created by fixing the net number of spin up electrons of the system to two, 21 i.e. using the triplet state to mimic the excited state. 22,23 Although both singlet and triplet states can be generated under the illumination, the difference between two states should be quite small considering the fact that the large number of electrons exist in the system. Moreover, for the reaction under investigations, the hole plays a vital role in the excited state. In this case, the electron spin in the conduction band has very small effects. In many cases, a fraction of the photo-excited electron and hole are proposed to be trapped soon after the excitation. 24 Some reactions are believed to be initiated by these trapped charge carriers. 25,26 Modeling of these trapped electron and hole may require the use of the hybrid functional 27 or DFT+U method. 28 However, calculations with the hybrid functional is extremely time consuming, and the DFT+U method may induce uncontrollable errors. Fortunately, for the photooxidation of CO, there is no experimental evidence to suggest that the reaction should be mediated by the trapped charged carriers. For the chemisorbed molecules, only the free or shallowly trapped charge carriers that can be easily activated to continuum band can react with them effectively. 2 Moreover, O2 itself is an effective charge trapping center, 16 the charge carriers do not need to be trapped at other place before transferring to O2 . For example, the majority of the excited electron was found to remain in the conduction band and they can transfer to O2 directly; 29 the photodesorption of O2 was proposed to be mediated by the free hole. 30 Thus, we focus on the reaction between the delocalized electron/hole with the molecules and all the calculations are performed at the GGA level. The TiO2 surface was described by a slab model with a vacuum layer of 13 Å. The four TiO2 layer slab model with bottom two layers fixed is probably the most frequently applied one in theoretical calculations 21,31,32 as a compromise of the computational efficiency and the accuracy. 33,34 However, in our previous study, 28 we found that this surface model may induce some artifacts for the calculation of the excited state properties because the photo-excited hole has a tendency to distribute on the unrelaxed side for its lower electrostatic potential. In this study, a five layer


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model with the center layer fixed and other four layers on the two sides relaxed were applied. Calculations on the four-layer (4L) surface were also performed to compare with previous theoretical studies. 21,35 A 3 × 2 supercell was used and all calculations were performed at Γ point only due to the large size of the system. The convergence criterion for self-consistent electronic minimization is 1.0 E-5 eV. The geometries were optimized till all forces on atoms are smaller than 0.02 eV/Å. Transition states were searched with nudged elastic band method with climbing images 36,37 (CNEB). The pre-exponential factors of the reactions are assumed to be the same in the following discussions.

Results and Discussion Adsorption of O2 Low Coverage: 1 O2 /Ov At low coverage, O2 prefers to be adsorbed at the Ov in a horizontal style (with its molecular axis parallel to the surface and perpendicular to the bridge oxygen row). This geometry was predicted by many theoretical calculations 6,21,35,38,39 and was confirmed by recent STM experiments. 40,41 Under thermal or photo activation the half of these molecules will undergo dissociation, leaving a repaired Ov and an oxygen adatom on the Ti5c row. 42–44 The electron spin resonance (ESR) experiment 45 suggested that, an electron can be transferred reversibly between this molecule and adjacent Ti atom under visible light illumination: Ti4+ + O2 2− ↔ Ti3+ + O2 − . This indicates that the molecule can capture a photogenerated hole very easily. We optimized three possible adsorption geometries of O2 at Ov at both the ground (singlet) and excited (triplet) states (Figure 1). Their relative energies ( in table I) calculated with 4L surface agree well with those from other theoretical studies. At the ground state, the horizontal structure A is the most stable one, whereas the vertical structure C is the most unstable one. The structure B bridges the Ov and Ti5c atoms and has an energy higher than horizontal one by 0.27 eV. The bond



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length of the O2 molecule in structure A and B is 1.44 Å and in structure C 1.33 Å, which indicate that O2 adsorbed as peroxide in structures A and B, but as superoxide in structure C. 14 At the excited state, the horizontal structure A does not change much of the geometry, but its energy becomes very close to the bridging structure B, within only 0.03 eV. The O–O bond length of the bridging O2 decreases from 1.44 Å to 1.34 Å, which implies that the molecule has captured a hole and become a superoxide. The vertical structure C tilts a little from the surface normal and is always in the charge state -1 at both ground and excited states. This structure has become the most favored geometry at the excited state. Similar geometries with this near perpendicular (NP) one was also found in two previous DFT calculations 11,46 and proposed to be the α -O2 . The partial density of states (PDOS) on the 4L model was shown in Figure 2A. The fixed O and Ti atoms produce a surface state that lies right at the top of the valence band (TVB) and the bottom of the conduction band. This makes the calculated highest occupied molecular orbital (HOMO) of the adsorbed horizontal O2 molecule to appear -0.7 eV (Figure 2A). At the triplet state, one electron will be taken out from the surface state at the fixed oxygen atoms. Thus, this triplet state can not describe the real excited state very well. When to compare the relative energies of the triplet state between the structures A and B/C with the captured hole, the 4L surface model with the bottom two layers fixed may induce some errors. We have therefore tested three more surface models: (1) four layer slab with the top two and bottom one layer relaxed and the middle layer fixed (4L-II); (2) five layer slab with the center layer fixed and other four relaxed (5L); (3) six layer slab with center two layers fixed and other four relaxed (6L). The relative energies of the three geometries are summarized in table I. We can see that, as the other side of the slab is also relaxed, the energies of structures B/C become much lower than that of the structure A. The structure C is always the most favored one. The relative energies converges very well as the number of surface layer increased. To saving computation time without loss of accuracy, the 5L surface model is chosen for the calculations. The calculated PDOS on the 5L surface (Figure 2B) shows that, the HOMO of adsorbed O2 is quite near the TVB. This implies that it would be very easy to capture a photogenerated hole on the


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TVB. The PES from the horizontal geometry to the near perpendicular (NP) one was calculated with CNEB method. As shown in the Figure 2C. The O2 first transform from the horizontal geometry to a tilted one by overcoming a very small barrier of 0.06 eV. The structure of the transition state is quite similar to the initial one which indicates that the O2 molecule only needs to adjust the geometry a little bit for the charge transfer (the tilted O2 is a superoxide). From the tilted geometry to the NP one, there is a barrier of 0.07 eV, which is much smaller than the released heat (0.47 eV) in the first step. Thus the overall rate-limiting barrier for the transformation is just 0.06 eV. The PDOS of the NP O2 is shown together with the horizontal O2 in Figure 2B. The LUMO of the NP O2 is just below the bottom of the conduction band and slightly hybridized. If we perform singlet optimization at the NP geometry, the electron at the conduction band (spin up) will be added back to the NP O2 and it will transform back to the horizontal geometry again. This agrees well with the ESR experiment except that the electron is not localized at a single Ti atom due to self-interaction error 47 in GGA calculation.

High Coverage: 2 O2 /Ov At high coverage, one O2 molecule was suggested to be adsorbed at Ov and the other one at the Ti5c row that can eventually convert to tetraoxygen (Figure 3A) when heated above 200 K. 15 Photooxidation of CO can happen at temperature lower than 120 K, 7,8,48 so it is important to know their adsorption geometry as two O2 − species. The tetraoxygen was predicted to be the most 14 stable geometry at high coverage, for example, it is stable than two separated O2 on Ti row by 0.23 eV (Figure 3B). The energy of tetraoxygen is taken as the energy zero in following discussions. We have also considered other possible configurations with one O2 at Ov with horizontal geometry and the other O2 on Ti5c with parallel (Figure 3C), diagonal and vertical geometries, respectively. We found that in spin-polarized calculation, all these geometries have lower energies than the tetraoxygen. But as long as the first O2 at Ov is adsorbed horizontally, it stays as O2 2− and the second O2 can no longer be chemisorbed on the Ti5c row. Experiments suggested that the charge state of the O2 should change as the second O2 adsorbed. 16 In a test calculation without considering 7 ACS Paragon Plus Environment


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spin-polarization, we observed a spontaneous electron transfer from the O2 at Ov to the one on Ti row and the O2 at Ov changes to a tilted geometry (Figure 3D). This indicates that the charge transfer between these two O2 molecule is also very easy. When we consider other configurations with the O2 adopting the NP geometry, the most stable one is found to be with the O2 on Ti5c adsorbed diagonally (Figure 3E). It is worth noting that the NP O2 appears in the most stable geometry at the excited state at the low coverage, and at the ground state of the high coverage(when the two O2 molecules adsorbed as two O2 − species). If this species is responsible for CO photo-oxidation, the mechanism at both low and high O2 coverage could indeed be the same.

CO Oxidation and Photo-oxidation Adsorption of O2 on the surface can results in many oxygen species such as O2 2− and O2 − adsorbed at Ov or Ti5c row; besides,UV illumination can induce the adsorption, desorption and dissociation of O2 results in O2 − , O2 in the gas phase and oxygen adatoms on the Ti5c row. 16,29,30,42,43 These oxygen species might all have contributions to the photo-oxidation of CO to different extend. But isotope experiment suggested that only the molecular O2 is involved in the reaction, the oxygen adatom results from the dissociation of O2 is not. 48 Petrik et al. showed that the CO can not be photo-oxidized on the fully hydrogenated surface, 7 and the off-normal desorption behavior of the product CO2 indicates that it is mainly the O2 adsorbed at the Ov that is responsible for the CO photo-oxidation. 7 Thus, we will focus on the reactions involve the O2 adsorbed at the Ov . Before investigating the role of the NP O2 molecule in CO oxidation, we have calculated the PES for CO oxidation by the horizontal O2 at both ground and excited states. The results are shown in Figure 4 (left part). At the ground state, the most stable geometry for O2 at Ov is the horizontal one (state a). It can transform to the bridging structure (state c) by overcoming a barrier of 0.83 eV. This value is much larger than the one calculated by Pillay et al. 6 (0.45 eV). In the absence of CO on the adjacent Ti row, the barrier was found to be 0.81 eV that is also much larger than that by Plillay el al.(0.35 eV) but agrees well with other three theoretical results 21,39,49 8 ACS Paragon Plus Environment

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( 0.69 eV ∼ 1.1 eV). It was suggested that the barrier might be overestimated, but with unknown reason. 39,49 We have found that the bridging O2 can further oxidizes the CO to CO2 (state e) by overcoming a barrier of 0.34 eV, resulting in an overall barrier of 0.83 eV. Thus CO can not be oxidized at the ground state because of the high barrier for the horizontal O2 to transforms to the tilted configuration to reach the CO molecule. At the excited state, the barrier for the transformation from the horizontal (state a’) to bridging geometry (state c’) is significantly reduced to 0.12 eV. In this process, a hole is captured by O2 and makes it bind less strongly to the surface which results in a lower transition barrier. But the barrier from the state c’ towards the product CO2 (state e’) becomes very high 0.81 eV. This is because in the state c’, O2 is in the charge state O2 −1 , while it is O2 −2 in the state c at the ground state. This makes the O–O bond in the state c’ much stronger than that in the state c. It is thus much more difficult to break the O–O bond. This might explain why the CO photo-oxidation is electron mediated. These calculations suggest that the use of the PES at either the ground state or the excited states alone can not properly describe the photo-oxidation of CO. Indeed, by combining the PES at the ground and excited states, an possible pathway for the CO oxidation can be revealed: At the initial state a, O2 adsorb in horizontal style, under UV illumination it is excited to the state a’, the O2 can then capture a hole when transform to the bridging geometry as in the state c’; then it captures an electron (falls back to the state c) and oxidizes the CO molecule to CO2 (state e) by overcoming a barrier about 0.34 eV. It is noted that the energy of the bridging structure (state c’) is higher than the NP one (state i’) by 0.35 eV . The pathway that involves the NP O2 could be more efficient. The overall barrier for transition from the state a’ to the state i’ is calculated to be 0.08 eV. This O2 molecule could then capture an electron to oxidize CO as suggested by the experiment. The question is whether the charge transfer and the CO oxidation happen in a stepwise way or concerted way during the reaction. Theoretically, the concerted way is difficult to treat. Hear we only consider the stepwise mechanism.



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As discussed above, the electron which was originally at the conduction band will fall back to the NP O2 molecule if the system is brought back from the triplet state to the singlet state. After further relaxation, the O2 molecule will then transform to the horizontal geometry again in the absence of CO. But in the presence of CO, we found that the O2 molecule could gradually connect to the CO molecule nearby and a complex O-O-CO can be formed in the state i (circled in Figure 4). The structure of the state i is close to the state c, but the energy of the state i is higher by 0.13 eV. The barrier for the transformation from the state i to c was calculated to be 0.48 eV(transition state not shown), whereas the transition barrier from the state i to the final state k was calculated to be 0.13 eV which is considerably small. The pathway(a → a’→ i’ → i → k) should be more efficient than the one suggested previously (a → a’→ c’ → c → e). Firstly, the state i’ is more stable than the state c’ and the transition barrier to transform to the state i’ is also lower; secondly, the barrier from the state i to k is smaller than from the state c to e possibly due to the formation of the O-O-CO complex in the state i. It is interesting to see that in a recent study for the catalytic oxidation of CO on Au deposited TiO2 surface, 50 the formation of a similar O-O-CO complex was proposed to be essential for the CO oxidation. The mechanism can also be applied to high coverage case, because the NP O2 is the most stable one at high coverage as well. It can directly capture an extra electron and oxidize the CO molecule; or the other O2 molecule at the Ti5c site photodesorbs first and then the situation become similar to the low coverage case. The barriers in the pathway are very small and it is also consistent with the off-normal desorption behavior of the CO2 molecule because the molecule plane of the intermediate complex is perpendicular to bridge oxygen row. So the pathway we propose here should at least be an important one in the photo-oxidation of CO.

Conclusion In summary, we have studied the adsorption of O2 at the oxygen vacancy on the TiO2 (110) surface at both low and high coverage. The surface model with both sides of the slab relaxed is essential


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for correctly modeling the processes involving the charge transfer at the triplet state. It is found that the O2 adsorbed at Ov can capture the photogenerated hole or transfer an electron to the other O2 on the Ti row and then transform itself to the NP one. PESs for CO oxidation at ground and excited states show that CO oxidation at ground state is hindered by a high barrier, whereas at the excited state more energy is required to break the O–O bond which results in a higher reaction barrier. But if the O2 molecule transforms to the NP geometry first, and by further capturing an extra electron, the O–O bond of the NP O2 will be weakened and it then gradually connects to the CO molecule at the adjacent Ti site, forming a complex O-O-CO that can converts easily to CO2 . The mechanism proposed here certainly helps to understand the oxidation of CO by O2 in general.

Acknowledgement This work was supported by NBRP (grants 2010CB923300 and 2011CB921400), NSFC (grant 20925311) of China, and Göran Gustafsson Foundation for Research in Natural Sciences and Medicine. The Swedish National Infrastructure for Computing (SNIC) is acknowledged for computer time.

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Page 15 of 21

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


Table 1: Relative energy (eV) of the three structures in Figure 1 Model 4L-I 4L-I 4L-II 5L 6L

Fixed layers Bottom two Bottom two Third layer Center one Center two

Spin state Singlet Triplet Triplet Triplet Triplet

A 0 0 0 0 0


B 0.27 0.03 -0.23 -0.46 -0.34

C 1.33 -0.07 -0.35 -0.52 -0.43


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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List of Figures 1

Adsorption geometries of single O2 adsorbed at Ov . left for singlet and right triplet

2

Partial density of states of O2 adsorbed at Ov on 4L surface (A) and 5L surface (B),

17

Ev is the energy of the top of valence band. The PES at the triplet state is shown in C 18 3

Adsorption of two O2 adsorbed at Ov , spin states with lower energies are labeled in the brackets, configuration D is from spin restricted calculation . . . . . . . . . 19

4

Potential energy surface for CO oxidation and photo-oxidation, the initial state is a horizontal O2 adsorbed at the Ov with an CO molecule adsorbed at the adjacent Ti site. The energy of the initial state is taken as zero. The O-O-CO complex formed in the circled state i. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20


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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


A

singlet Ob

triplet

Ti5c

B

C

Ti of TiO2

O of TiO2

O from O2

Figure 1: Adsorption geometries of single O2 adsorbed at Ov . left for singlet and right triplet



A

HOMO

B

LUMO

C

0.2

0.0

Energy /eV

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

-0.2

-0.4

-0.6

-0.8

Figure 2: Partial density of states of O2 adsorbed at Ov on 4L surface (A) and 5L surface (B), Ev is the energy of the top of valence band. The PES at the triplet state is shown in C


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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


A

E = 0.0 eV (S)

B

E = 0.23 eV (S)

C

E = -0.12 eV (T)

D

E = 0.82 eV

E

E = 0.13 eV (S)

F

E = 0.23 eV (T)

Figure 3: Adsorption of two O2 adsorbed at Ov , spin states with lower energies are labeled in the brackets, configuration D is from spin restricted calculation



2.5

2.5

d' 2.0

2.0

c'

1.5

b'

a'

f' 1.5

g'

i'

h' 1.0

1.0

b Energy /eV

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

Page 20 of 21

0.5

d

i

c

0.5

j

a

0.0

0.0

Initial state

-2.0

-2.0

e' -2.5

-2.5

PES at the singlet state PES at the triplet state Excitation

-3.5

-3.5

Recombination and capture of an electron by O2 -4.0

k

e

-4.0

Figure 4: Potential energy surface for CO oxidation and photo-oxidation, the initial state is a horizontal O2 adsorbed at the Ov with an CO molecule adsorbed at the adjacent Ti site. The energy of the initial state is taken as zero. The O-O-CO complex formed in the circled state i.


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Graphical TOC Entry

Energy /eV

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


2.0

2.0

1.5

1.5

1.0

1.0

0.5

0.5

0.0

0.0

-3.5

-3.5

-4.0

-4.0


First Principles Study of O2 Adsorption on Reduced Rutile TiO2-(110

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