Defect States at the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>TiO</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mo stretchy="false">(</mml:mo><mml:mn>110</mml:mn><mml:mo stretchy="false">)</mml:mo></mml:math>Surface Probed by Resonant Photoelectron Diffraction

Krüger, Péter; Bourgeois, S.; Domenichini, B.; Magnan, H.; Chandesris, D.; Fèvre, P. Le; Flank, A.‐M.; Jupille, Jacques; Floreano, Luca; Cossaro, Albano; Verdini, Alberto; Morgante, A.

doi:10.1103/physrevlett.100.055501

Cited by 191 publications

(157 citation statements)

References 14 publications

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“…This asymmetry is not observed in data recorded at 78 K (27). Presumably this arises because of significant polaronic hopping of the type recently identified at room temperature (25).…”

Section: Resultsmentioning

confidence: 73%

“…To further validate our model of the electronic structure, we probed the electronic charge distribution associated with O bvac experimentally using scanning tunneling spectroscopy (STS) at ∼5 K. This low temperature was chosen to deliberately freeze out polaronic hopping effects that are thought to average out asymmetries that are predicted to arise at 0 K (17,18,25). During the acquisition of the STM image shown in Fig.…”

Section: Resultsmentioning

confidence: 99%

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Electron traps and their effect on the surface chemistry of TiO ₂ (110)

Papageorgiou

Beglitis

Pang

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

268

281

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Oxygen vacancies on metal oxide surfaces have long been thought to play a key role in the surface chemistry. Such processes have been directly visualized in the case of the model photocatalyst surface TiO 2 ð110Þ in reactions with water and molecular oxygen. These vacancies have been assumed to be neutral in calculations of the surface properties. However, by comparing experimental and simulated scanning tunneling microscopy images and spectra, we show that oxygen vacancies act as trapping centers and are negatively charged. We demonstrate that charging the defect significantly affects the reactivity by following the reaction of molecular oxygen with surface hydroxyl formed by water dissociation at the vacancies. Calculations with electronically charged hydroxyl favor a condensation reaction forming water and surface oxygen adatoms, in line with experimental observations. This contrasts with simulations using neutral hydroxyl where hydrogen peroxide is found to be the most stable product.The rutile TiO 2 ð110Þ surface, which we use as a model photocatalytic system here, is displayed as a ball model in Fig. 1A where the reduction of one oxygen atom of O 2 ðgÞ to one bridging oxide species (O 2− b ) is accomplished by oxidation of the two Ti 3þ sites associated with O b -vac to Ti 4þ (3), on the basis of a purely ionic model. (Formal charges are written in reactions 1 and 2 to highlight the redox processes involved.)The interaction of O 2 with OH b , on the other hand, is still a matter of controversy. Following the reaction of these species at temperatures ≤240 K, water is seen to desorb at ∼310 K in temperature programmed desorption (TPD) spectra (3, 4). Henderson et al. (3) concluded that this water evolution is a consequence of the formation of oxygen adatoms (O ad ) at the surface as follows:where the two Ti 3þ species provide the two electrons necessary to reduce one oxygen atom of O 2 ðgÞ to H 2 OðgÞ (3). In stark contrast to the TPD results, previous calculations find H 2 O 2 to be by far the most stable product (5). Moreover, on the basis of these calculations, water desorption is not expected up to the highest temperature computed, 350 K (5). This discrepancy provided the initial motivation for the present work. Results and DiscussionWe use STM to provide an additional experimental test of the picture that has emerged thus far. Fig. 1B shows a surface containing both O b -vac and OH b , alongside the same surface in Fig. 1C after it was exposed to 90 Langmuirs (L) O 2 at 300 K (1 L ¼ 1.33 × 10 −6 mbar · s, 1 mbar ¼ 100 Pa). A number of small, bright spots can be seen on the Ti 5c sites (bright rows) in the latter image. The histogram of the height distribution of these bright spots, shown in Fig. 1D, indicates that these bright spots are almost entirely due to one final product.It should be noted that at lower O 2 exposures we see a number of different types of species on Ti 5c rows that are likely to arise from terminal hydroxyls (OH t ) and other metastable species such as O 2 H. These latter results ...

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“…This asymmetry is not observed in data recorded at 78 K (27). Presumably this arises because of significant polaronic hopping of the type recently identified at room temperature (25).…”

Section: Resultsmentioning

confidence: 73%

Section: Resultsmentioning

confidence: 99%

Electron traps and their effect on the surface chemistry of TiO ₂ (110)

Papageorgiou

Beglitis

Pang

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

268

281

View full text Add to dashboard Cite

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“…Charge dynamics in particular has been investigated in terms of the alternative mechanisms of ohmic and polaronic electronic conduction 56,57 while the nature of localized electron centres has been tackled trying to understand the space distribution of the trapped electron wave function. This question is still debated and opposite conclusions (electron localized on a single titanium ion or delocalized on several ions in a relatively large volume) have been proposed on the basis of different approaches 58,59 . The recent activity of our group has challenged the problem of localisation vs. delocalisation of excess electrons in both anatase and rutile 20,60 exploring the EPR spectra of the corresponding centres generated either by irradiation or by chemical reactions (electron injection, reduction by annealing and consequent oxygen depletion).…”

Section: Excess Electron Injection In N-type Doped Tiomentioning

confidence: 99%

Fluorine- and Niobium-Doped TiO₂: Chemical and Spectroscopic Properties of Polycrystalline n-Type-Doped Anatase

et al. 2014

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“…They can occupy the nearby localized Ti 3d orbit and therefore convert Ti 4+ ions to Ti 3+ ions. In a reduced rutile TiO 2 (110) surface, such a defect complex, Ti 3+ -O V , has been well studied by first-principles calculations 9,10 and experimentally by resonant photoelectron diffraction 11 . Therefore, experimental work is needed to clarify whether the magnetic moments in defective TiO 2 is due to unpaired 3d electrons localized on Ti 3+ ions.…”

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confidence: 99%

Origin of magnetic moments in defectiveTiO2single crystals

et al. 2009

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In this paper we show that ferromagnetism can be induced in pure TiO2 single crystals by oxygen ion irradiation. By combining x-ray diffraction, Raman-scattering, and electron spin resonance spectroscopy, a defect complex, i.e. Ti 3+ ions on the substitutional sites accompanied by oxygen vacancies, has been identified in irradiated TiO2. This kind of defect complex results in a local (TiO6−x) stretching Raman mode. We elucidate that Ti 3+ ions with one unpaired 3d electron provide the local magnetic moments.Recently, ferromagnetism has been observed in nonmagnetically doped, but defective oxides, including TiO 2 1,2,3,4 . This kind of observation challenges the conventional understanding of ferromagnetism, which is rather due to spin-split states or bands. Thus, one fundamental question must be answered: where are the moments located? Intensive theoretical work has been performed to understand the ferromagnetism in defective oxides 5,6,7 . In these papers, the triplet states of p-like electrons, located at cation or oxygen vacancies, yield the local moments, leading to a kind of ferromagnetism without the involvement of 3d electrons. Experimentally the ferromagnetism in undoped TiO 2 has been found to relate with oxygen vacancies (O V ) 2,3 , however, its mechanism remains unclear. It is worth to note that Ti 3+ ions with one 3d electron are usually generated in slightly reduced TiO 2 . When O is removed, the excess electrons are unpaired 8 . They can occupy the nearby localized Ti 3d orbit and therefore convert Ti 4+ ions to Ti 3+ ions. In a reduced rutile TiO 2 (110) surface, such a defect complex, Ti 3+ -O V , has been well studied by first-principles calculations 9,10 and experimentally by resonant photoelectron diffraction 11 . Therefore, experimental work is needed to clarify whether the magnetic moments in defective TiO 2 is due to unpaired 3d electrons localized on Ti 3+ ions.Ion irradiation is a non-equilibrium and reproducible method of inducing defects. Energetic ions displace atoms from their equilibrium lattice sites, thus creating mainly vacancies and interstitials. The amount of defects can be controlled by the ion fluence and energy. In this paper, we irradiated rutile TiO 2 single crystals with 2-MeV O ions, resulting in a projected range of 1.52 µm and a longitudinal straggling of 0.16 µm as calculated by SRIM code (The Stopping and Range of Ions in Matter) 12 . As a result of this irradiation, the formation of Ti/O vacancies/interstitials is expected 12 . We selected high-energy oxygen ions as projectiles to avoid the introduction of foreign elements. Moreover, from a ballistic point of view, the creation of oxygen vacancies is more efficient, e.g., by a factor of 1.5 larger than the Ti-vacancy creation. From SRIM calculations it is also evident that, at the given energy, the maximum atomic concentration of the implanted oxygen ions is by a factor of 500 smaller than the concentration of oxygen recoils. For the region of maximum defect creation, i.e., at the end of the ion range, those project...

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Defect States at theTiO2(110)Surface Probed by Resonant Photoelectron Diffraction

Cited by 191 publications

References 14 publications

Electron traps and their effect on the surface chemistry of TiO ₂ (110)

Electron traps and their effect on the surface chemistry of TiO ₂ (110)

Fluorine- and Niobium-Doped TiO₂: Chemical and Spectroscopic Properties of Polycrystalline n-Type-Doped Anatase

Origin of magnetic moments in defectiveTiO2single crystals

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Defect States at theTiO2(110)Surface Probed by Resonant Photoelectron Diffraction

Cited by 191 publications

References 14 publications

Electron traps and their effect on the surface chemistry of TiO 2 (110)

Electron traps and their effect on the surface chemistry of TiO 2 (110)

Fluorine- and Niobium-Doped TiO2: Chemical and Spectroscopic Properties of Polycrystalline n-Type-Doped Anatase

Origin of magnetic moments in defectiveTiO2single crystals

Contact Info

Product

Resources

About

Electron traps and their effect on the surface chemistry of TiO ₂ (110)

Electron traps and their effect on the surface chemistry of TiO ₂ (110)

Fluorine- and Niobium-Doped TiO₂: Chemical and Spectroscopic Properties of Polycrystalline n-Type-Doped Anatase