Hydrogen Distribution and Electronic Structure of TiO2(110

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Hydrogen Distribution and Electronic Structure of TiO(110) Hydrogenated with Low-Energy Hydrogen Ion 2

Yuki Ohashi, Naoki Nagatsuka, Shohei Ogura, and Katsuyuki Fukutani J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09434 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 2, 2019

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The Journal of Physical Chemistry

Hydrogen Distribution and Electronic Structure of TiO2 (110) Hydrogenated with Low-energy Hydrogen Ion Yuki Ohashi1 , Naoki Nagatsuka1 , Shohei Ogura1 , Katsuyuki Fukutani1,2 1

Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba Meguro-ku, Tokyo 153-8505, Japan and 2

Advanced Science Research Center,

Japan Atomic Energy Agency (JAEA), Tokai, Ibaraki 319-1195, Japan∗ ( Dated: March 28, 2019) The rutile TiO2 surface irradiated by hydrogen ions at 0.5 keV was investigated by ultraviolet photoemission spectroscopy, X-ray photoemission spectroscopy, and nuclear reaction analysis. With hydrogen irradiation, an in-gap state (IGS) was formed at ∼0.8 eV below the Fermi level and the bands were downward bent by 0.5 eV. The H depth profile showed a maximum at about 1 nm extending to ∼30 nm with an average concentration of 5.6 at.%. Upon annealing at 673 K, the IGS intensity was reduced and H with a coverage of 1.4 monolayer remained in the near-surface region suggesting stable H occupation of subsurface sites. After O2 dose to the H-irradiated surface, while the IGS intensity was substantially reduced, the hydrogen distribution was found to be unchanged.

∗ [email protected];

[email protected]

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I.

INTRODUCTION

Titanium dioxide (TiO2 ) is a wide-gap semiconductor with a band gap of ∼3 eV, which finds many applications such as hydrogen sensors and photocatalytic H2 generation due to water splitting [1–6]. While hydrogen adsorption induces an electrical conductivity change in a hydrogen sensor, charge transfer and associated proton dynamics result in H2 formation in a photocatalytic reaction. In these regards, understanding the interaction between hydrogen and TiO2 is of considerable importance. Among the three crystal types of TiO2 , the rutile phase is most stable and its

single-crystal surface has been extensively studied to date. On the rutile TiO2 (110) surface, hydrogen atoms are adsorbed on bridging O sites forming OH species [7–12], which act as an electron donor [12, 13]. Electron doping to TiO2 has been extensively studied both experimentally and theoretically as reviewed in recent articles [14–17]. It has been shown that TiO2 is easily reduced by vacuum annealing, electron irradiation or ultraviolet light irradiation, and that the reduction is associated with presence of oxygen vacancies and Ti-interstitials. The electrons doped to rutile TiO2 surface due to these defects as well as adsorbed H are localized at the Ti site reducing the charged state of Ti [14–17]. According to theoretical studies, the doped electrons tend to be located at the neighboring Ti5c sites or at the Ti site in the subsurface layer[18–22]. The electron doping causes a downward band bending and in-gap state(IGS) formation at ∼ 0.8 eV below the Fermi level (E F ), which corresponds to a polaron state [23–25]. As well as the electric conductivity, this localized state has a significant effect on molecular adsorption such as oxygen and water [15, 26]. In addition to oxygen vacancies and OH species, it is shown that a complex of an oxygen vacancy and a hydrogen atom can act as an electron donor in oxides such as BaTiO3 and SrTiO3 [12, 27–29], where hydrogen is negatively charged at the oxygen vacancy site. In view of photocatalytic applications, the large band gap of TiO2 allows photoabsorption of only ultraviolet light. Band-gap narrowing is therefore required to utilize the visible light in the solar spectrum thereby improving the efficiency of photocatalytic reactions. Hydrogenated TiO2 has acquired much attention in recent years as a visiblelight absorbing material, which is often called black titania[30–32]. Black TiO2 has been produced by various experimental methods such as high-pressure H2 treatment, hydrogen-plasma treatment, and high-power ultrasonic treatment[31, 32]. The visible light absorption has been attributed to a broadening of the valence band, and its origin has been discussed to be a disorder of the TiO2 structure[31, 33, 34]. In a recent study, on the other hand, hydrogenation was performed by high-energy hydrogen ions, and hydrogen in TiO2 is shown to be thermally unstable at 473 K[35]. The detailed role of hydrogen in hydrogenated TiO2 for the change of the electronic structure is yet to be elucidated. In the present article, we investigated hydrogenation of the rutile TiO2 surface by hydrogen ion irradiation with ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), and nuclear reaction anal-


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The Journal of Physical Chemistry 3

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ysis (NRA). Whereas UPS showed appearance of an IGS at ∼ 0.8 eV below EF and downward band bending by 0.5 eV, NRA revealed presence of H within a depth of about 30 nm with an average concentration of 5.6 at.%. Upon annealing at 673 K, it was found that the IGS intensity was reduced and that H with a coverage of 1.4 monolayer (ML) remained in the near-surface region suggesting stable H occupation of subsurface sites. After O2 dose to the H-irradiated surface, the IGS intensity was substantially reduced without changing the hydrogen concentration indicating charge transfer from the substrate to adsorbates.

II.

EXPERIMENTAL

The experiments were conducted in ultrahigh vacuum (UHV) chambers. A rutile TiO2 (110) single crystal (0.05 wt.% Nb- doped) was cleaned by Ar ion sputtering for 10 min with an energy of 1 keV and an ion current of 2 µA followed by annealing at 723 K under an oxygen gas of 1 × 10−4 Pa for 10 min. The cleaning was repeated until the position of the valence band maximum (VBM) became −2.8 eV and no IGS was observed in the UPS spectrum. To prepare a hydrogenated surface, the clean surface was irradiated by hydrogen ions with an energy of 0.5 keV in the normal incidence. The hydrogen ion dose was estimated to be 6.7 × 1014 cm−2 · min−1 from the ion current, and the irradiation time was 5 − 30 min. According to the simulation software, the stopping and range of ions in matter (SRIM-2013), hydrogen molecules and atoms at 0.5 keV have penetration depths of 6.4 and 6.1 nm for rutile TiO2 , respectively. The electronic structure of the surfaces was in situ investigated by UPS and XPS at room temperature in a UHV chamber at a base pressure of 6×10−8 Pa. UPS measurements were conducted with the He I light source (hν = 21.22 eV) at an incidence angle of 60◦ in normal emission. XPS measurements were conducted with the MgKα X-ray (hν = 1253.6 eV) at incidence and emission angles of 60◦ . The spectra were recorded with a hemispherical analyzer with an energy resolution of 10 meV. The depth profile of hydrogen in the sample was in situ measured by NRA in a separate UHV chamber (base pressure: 2×10−8 Pa) at the 2E beam line of Micro Analysis Laboratory (MALT) in the University of Tokyo[36, 37]. A

15 N2+

ion beam at an energy of 6.37 − 6.53 MeV and an ion current of 20 − 50 nA irradiated the sample in

normal incidence, and γ rays of 4.43 MeV due to the 1 H(15 N, αγ)12 C nuclear reaction were detected by Bi4 Ge3 O12 scintillators. The stopping power for rutile TiO2 is calculated to be 2.663 keV/nm [37], which is used for the depth calibration. A kapton film with a known H concentration was used for the reference sample to evaluate the detection efficiency of the system. The H concentration as a function of depth is described as a volume density. On the other hand, it is shown as an areal density when we evaluate the total amount of H irrespective of the depth.



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III.

RESULTS

The UPS spectrum of the clean TiO2 (110) surface is shown in Fig. 1(a). The valence band is observed from -9 to -3 eV and no particular features are observed in the band-gap region below E F as shown in Fig. 1(b). The UPS spectrum of the surface after hydrogen ion irradiation for 30 min shown in Fig. 1(a), on the other hand, shows that the valence band shifts to a lower energy, and that the shape of the O2p valence band changes. The peaks at −8 and −5 eV originate from the O2p σ and π orbitals, respectively[38]. The peak intensity of the O2p σ orbital relatively

increases after hydrogen ion irradiation as compared with the clean surface. The spectrum also shows IGS at −0.85 eV as shown in Fig. 1(b). The valence band shift estimated from VBM of the UPS spectra is plotted as a function of the hydrogen ion irradiation time in Fig. 2. Here, the positive sign of the VBM shift represents a downward shift. Since the probing depth of UPS is as shallow as 1 nm, the VBM shift represents the band bending caused by charge transfer due to H irradiation. The VBM shift saturates at an irradiation time of ∼10 min. Figure 2 also shows the IGS peak area as a function of the hydrogen irradiation time, which continuously increases until 30 min. The changes induced by hydrogen ion irradiation, the downward band bending and IGS formation, are similar to those by hydrogen adsorption[11]. It is worth noting that, however, the VBM shift of 0.5 eV observed for the H-irradiated surface is larger than the value of 0.2 eV measured for the hydrogen-adsorbed surface[11]. Furthermore, the IGS intensity after H irradiation for 30 min is about 10 times larger than that of the hydrogen-adsorbed surface. The XPS spectrum of the clean surface reveals a peak at 530.3 eV due to O1s and a double peak at 464.7 and 459.0 eV due to Ti2p indicating that O and Ti are in the O2− and Ti4+ states, respectively[31, 39](see Fig. S1 in Supporting Information) [40]. The spectrum taken after hydrogen ion irradiation exhibits a tail at a higher-energy side of the O-derived peak[40], which originates from OH species[13, 31, 41]. Furthermore, new peaks are observed at a lowerenergy side of the Ti-derived peaks, which originate from Ti3+ [42]. These results show that hydrogen irradiation reduce Ti4+ to Ti3+ by doping electrons. By evaluating the O1s intensity due to OH species in the XPS spectrum for the surface after the H irradiation for 30 min, the amount of OH species is estimated to be 5.6 ± 0.2 × 1014 cm−2 (1.1 ML) on the assumption of the XPS probing depth as 1 nm. Here, 1 ML is defined as the density of the surface unit cell, 5.17 × 1014 cm−2 . The effects of annealing and an O2 dose on the surface after H irradiation for 30 min (H-irradiated surface) are demonstrated in Fig. 3. After annealing the H-irradiated surface at 673 K under UHV for 10 min (UHV-annealed surface), the UPS spectrum shows an upward shift of VBM and reduction of the IGS intensity as compared to that before annealing. The shape of the valence band is almost recovered to the initial state before H irradiation. The UHV-annealed surface was then exposed to 100 Langmuirs (1 Langmuir=1.33×10−4 Pa s) of O2 (O2 −dosed


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surface), which further shows a VBM shift and reduction of IGS. Figure 4 shows the changes in the VBM shift and the IGS area by UHV-annealing and O2 dose. While the VBM shift is reduced to 0.29 eV by UHV annealing, it is furthermore reduced to 0.036 eV by additional O2 dose. The IGS area is, on the other hand, reduced to 60% of the initial state by UHV annealing, and then reduced to 4.2% by O2 dose. The data on the right hand side of Fig. 4 demonstrate the changes without UHV annealing. In contrast to the changes after UHV annealing, the VBM shift decreases to 0.20 eV and the area of the IGS decreases to 34% with O2 dose. Note that the initial VBM shift for the experiment without UHV annealing was smaller, although the reason is not clear. Figure 5 shows the NRA yield curves measured for the H-irradiated, UHV-annealed (673 K) and O2 −dosed (100 Langmuirs) surfaces. In Fig. 5(a), the curve of the H-irradiated surface shows a maximum at a depth of 1 nm extending to about 30 nm, which reveals a saturating behavior at an H ion irradiation of 20 min. The total amount of hydrogen in the H-irradiated surface for 20 min is obtained to be 4.2 ± 1.4 × 1015 cm−2 by integrating the curve over the measured depth range, which corresponds to 8.2 ± 2.6 ML. The average concentration within 10 nm from the surface is about 5.6 at.%. The observed H depth is more extended than the penetration depth (∼6 nm) of hydrogen at 0.5 keV calculated by SRIM. The H distribution extending to ∼30 nm is considered to be caused by H diffusion after H ion irradiation. By UHV annealing, as shown in Fig. 5(b), the hydrogen concentration near the surface is substantially reduced. The resultant profile is almost symmetric with respect to a depth of 1 nm with a slight intensity in the deeper region with a hydrogen density of 1.5 ±0.6 ×1020 cm−3 . By exposing the surfaces to O2 , on the other hand, the NRA yield curves seem to remain intact with or without UHV annealing. Note that O2 dose significantly affects the UPS spectrum, which will be discussed later.

IV.

DISCUSSION

With H ion irradiation at 0.5 keV, the near-surface region of TiO2 (110) was hydrogenated as confirmed by the NRA results. The average H concentration was found to saturate at about 5.6 at.%. In a previous study, hydrogenation of TiO2 is investigated with H ion irradiation at an energy of 40 keV[35]. It is reported that hydrogen ions penetrate to a depth of a few hundreds of nm and that the maximum H concentration amounts to 2.5 at.% at a depth of 300 nm, which is smaller than the value obtained at 0.5 keV in the present study. The study also shows that the H distribution spreads out by annealing at 373 K and that almost all H is removed from the sample at 523 K. We notice however that there remains a small intensity near the surface[35]. As the depth resolution of the present NRA is better than that adopted in the previous study, the present data unambiguously shows that H is present at the surface even after annealing at 673 K. The amount of hydrogen at the UHV-annealed surface is 7.2 ± 0.4 × 1014 cm−2 ,



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which corresponds to 1.4 ± 0.2 ML. The NRA profile reveals that the hydrogen atoms exist in the close vicinity of the surface. Note that this value is higher than the saturation coverage of 0.47 ± 0.10 ML at 300 K obtained by atomic hydrogen dosage to the TiO2 (110) surface (H-adsorbed surface)[11]. Since the NRA peak is located at 1 nm, furthermore, part of the H atoms observed by NRA after UHV annealing are present in the subsurface region. It is worth noting that these H atoms are strongly bound as compared with those in the deeper region, because these hydrogen atoms remained even after annealing at 673 K. As confirmed by UPS shown in Fig. 1, IGS is formed at the H-irradiated surface suggesting electron doping and associated reduction of Ti4+ to Ti3+ . While the IGS intensity starts to increase at about 7 min, the VBM shift saturates at an H irradiation time of 10 min. This behavior is caused by the difference in the electron doping depth: the VBM shift due to the band bending is caused by electron doping in a deep region, whereas the IGS intensity reflects the electron doping in the near-surface region within the UPS probing depth of ∼1 nm. The experimental results indicate that the H irradiation initially induces an electron doping in a deep region and then causes IGS in the near-surface region. This is probably because H ions at 0.5 keV have a penetration depth of ∼6 nm. The IGS intensity of the H-irradiated and UHV-annealed (673 K) surfaces is estimated to be 10.9 and 6.4 times as large as that observed for the H-adsorbed surface. Although the H depth distribution is not uniquely determined below the NRA resolution (∼5 nm), we tentatively derived the distribution for the H-irradiated surface as shown by the histogram in Fig. S2. From this H distribution, the total amount of H in the UPS probing depth of 1 nm is estimated to be 1.9 ML. Table I summarizes the hydrogen amount in the UPS probing depth (1 nm) and relative IGS intensity. From the analysis of the XPS spectrum of Fig. S1, the amount of OH species in 1 nm is estimated to be 1.1 ML. This value is significantly smaller than the H amount estimated from the NRA result suggesting that H species other than the OH species are present at the H-irradiated surface. As seen in Table I, furthermore, the IGS intensity ratio of 10.9(6.4) between the UHV-annealed (H-irradiated) and H-adsorbed surfaces is obviously larger than the H amount ratio of 1.9(1.4)/0.47. This indicates that in addition to the OH species other species are responsible for electron donation and IGS formation. Possible electron donating species are oxygen vacancies (VO ), hydrides (H− ), and Ti interstitial, where two and one electrons are expected to be doped by VO and H− formation at VO , respectively, which are larger than the value of 0.32 estimated for OH on the surface[11]. According to SRIM simulations, the probability to generate an O or Ti vacancy with an H ion at 0.5 keV is as small as 0.1%. Hence, direct oxygen vacancy formation due to H ion collision is unlikely to occur. Nevertheless, H ions might reactively scatter and remove oxygen atoms, which leads to formation of VO . Previous studies have shown that part of hydrogen atoms thermally desorb as H2 O forming oxygen vacancies by annealing of a hydrogenated surface[35, 43]. If H desorption were accompanied by oxygen vacancy formation in the present case, the IGS intensity would be increased by UHV-annealing. Nevertheless, the IGS intensity was reduced by UHV-annealing of


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the H-irradiated surface, therefore it is considered that hydrogen desorption predominantly occurs as H2 rather than H2 O in the present case.

In previous studies, hydrogenated TiO2 (black titania) was synthesized by various approaches including a highpressure hydrogen treatment and a hydrogen plasma treatment[32]. It is reported that VBM reveals a tailing feature by hydrogenation, which is ascribed to structural disorder caused by high-pressure or plasma treatments[31, 44]. In the present work, on the other hand, no such tailing feature was observed as shown in Fig. 1. This is probably because the crystallographic periodicity of the sample surface is preserved after H irradiation as confirmed by LEED (see Fig. S3). This indicates that hydrogen incorporated in TiO2 without structural disorder does not cause a tailing in VBM. Some other factors including structural disorder seem to be responsible for the VBM tailing and formation of black titania. After the O2 dose to the UHV-annealed surface, both the IGS intensity and VBM shift considerably decreased as shown in Fig. 4. On the other hand, the NRA profile remained unchanged after O2 dose. As discussed earlier, IGS originates from Ti3+ due to charge transfer from OH, oxygen vacancy, or hydride species. Since H is present at the surface after O2 dose, oxygen induces oxidation of Ti3+ without removing H from the surface. The interaction of O2 with the OH species and oxygen vacancies on the rutile TiO2 surface is studied with electron energy loss spectroscopy and temperature-programmed desorption[45–47]. It is shown that the adsorbed O2 molecule induces charge transfer from the substrate Ti3+ species to O2 , which is consistent with the present result. As for the reaction with OH, it is shown that O2 reacts with a pair of OH at the bridging-oxygen site producing two OH species adsorbed on the five-coordinate Ti sites. As discussed above, however, hydrogen atoms are present in the subsurface region as well as on the surface after UHV-annealing of the H-irradiated surface. Considering the total H coverage of 1.4 ML in the UHV-annealed surface, it is difficult for all H atoms to undergo the above reaction, because sufficient five-coordinate Ti sites to accommodate the reaction products are not available. We therefore argue that O2 on the surface accepts electrons and reduces Ti3+ below the surface without causing particular reactions with H. Such charge transfer has recently be demonstrated by STM experiments on an anatase TiO2 surface[48]. Note that part of IGS remains after O2 dose to the H-irradiated surface without UHV annealing. This suggests the charge transfer depth due to O2 adsorption is shorter than the UPS probing depth of ∼1 nm, or sufficient O2 is not accommodated on the surface to reduce all Ti3+ in the H-irradiated surface.

V.

CONCLUSION

Hydrogenation of the rutile TiO2 surface by low-energy hydrogen ions was investigated by UPS, XPS, and NRA. Hydrogen ion irradiation induced an IGS at ∼ 0.8 eV below EF and a downward band bending by 0.5 eV. It was



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found that H atoms are distributed to a depth ∼30 nm with an average concentration of 5.6 at.%. When the hydrogenirradiated surface was annealed at 673 K, the IGS intensity was reduced and H with a coverage of 1.4 ML remained at the surface. This suggests that subsurface sites are stable for hydrogen. By exposure of the H-irradiated surface to O2 , the IGS intensity was substantially reduced without changing the hydrogen distribution.

ACKNOWLEDGMENTS

We thank K. Kato for valuable discussion. This work was supported by Grant-in-Aid for Scientific Research from Japan Society for the Promotion of Sciences (JSPS KAKENHI) Grants JP16K04957, JP17H01057, and JP18H05518.

VI.

SUPPORTING INFORMATION

Figure S1: XPS spectra of (a) O1s and (b) Ti2p levels of the clean TiO2 (110) surface (gray curve) and the surface after hydrogen ion irradiation at 0.5 keV for 30 min (blue curve) using Mg Kα radiation with incidence and emission angles of 60◦ from the surface normal. The dotted and dashed curves are fits to the spectrum for the surface after the H-irradiation. Figure S2: Possible H distributions deduced from the NRA profile. Figure S3: LEED patterns for (a) the clean TiO2 (110) surface and (b) after the H-irradiation for 15 min. The electron energy is 101 eV. This information is available free of charge via the Internet at http://pubs.acs.org

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5

Intensity (x10 )

(a ) 3

Clean H-irrad.

2 1 0 -8

-4 E - EF (eV)

0

(b ) 4

Intensity (x10 )

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

5

Clean H-irrad.

4 3 2 1 0 -2.0

-1.0 0.0 E - EF (eV)

Figure 1. UPS spectra of the clean surface (gray dashed curve) and the H-irradiated surface at 0.5 keV for 30 min (purple solid curve). (a) The spectra near the valence band and (b) near E F .

TABLE 1. Hydrogen amount and relative IGS intensity

H-ads. H-irrad. UHV-annealed H amount (ML) 0.47[11] 1.9 1.4 IGS intensity 1 10.9 6.4



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Energy shift (eV)

0.6 6

0.5 0.4

4

0.3 0.2

2 band bending IGS peak area

0.1 0.0 0

5

10

15

20

25

0

IGS area (arb. units)

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

30

Irradiation time (min.) Figure 2. Changes in the band bending (blue triangle) and peak area of the in-gap state (black square) as a function of the hydrogen ion irradiation time. The blue dashed line and black dot-dash line are guides to eyes.


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4

5

Intensity (x10 )

(a) 3 2 1 0

A: H-irrad. B: A + UHV anneal C: B + O2 dose

-8

-4 E - EF (eV)

0

(b) 4

Intensity (x10 )

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 2 0

A: H-irrad. B: A + UHV anneal C: B + O2 dosed

-2.0 -1.5 -1.0 -0.5 0.0 E - EF (eV)

0.5

Figure 3. UPS spectra of the H-irradiated surface (purple solid line), UHV-annealed at 673 K of the H-irradiated surface (green dotted line) and 100 Langmuirs O2 -dosed to the UHV-annealed surface (red dot-dash line) surfaces. (a) The spectra near the valence band and (b) near E F .



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VBM shift (eV)

0.5

2.0

0.4

1.5

0.3 1.0

0.2 0.1 0.0

0.5 band bending IGS peak area

A

B

0.0 C

D

IGS area (arb. units)

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

E

A+ Figure 4. Changes in the band bending (blue triangle) and the IGS peak area (black square) of H-irradiated surfaces (the surface after the H irradiation for 30 min) by UHV annealing and 100 Langmuir O2 dose. The position of VBM and the IGS peak area of the H-irradiated surface are shown at A and D in this figure. Those after UHV annealing at 673 K for 10 min and post-O2 dose are shown at B and C, respectively. E corresponds to that after O2 dose of D.


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(a)

(b)

Figure 5. (a) NRA yield curves measured for the TiO2 (110) surface after H ion irradiation for 10 min(black filled circle) and 20 min (black open circle). (b) NRA yield curves of 100 Langmuirs O2 dose to the H-irradiated surface (black filled square), after UHV-annealing of the H-irradiated surface (black open circle), and after UHV-annealing and 100 Langmuirs O2 dose (red open triangle).The H irradiation time of each profile is 20 min.



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Hydrogen Distribution and Electronic Structure of TiO2(110

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