Electron Trapping at Point Defects on Hydroxylated Silica Surfaces

Giordano, Livia; Sushko, Peter V.; Pacchioni, Gianfranco; Shluger, Alexander L.

doi:10.1103/physrevlett.99.136801

Cited by 47 publications

(40 citation statements)

References 29 publications

(25 reference statements)

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“…S1). High affinities for both surface and bulk electron polaron trapping have been reported for substrates such as HfO 2 (13) and SiO 2 (14). This has been used to explain a number of surface properties, such as SiO 2 discharge phenomena, although not the surface chemistry.…”

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.

267

271

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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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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.

267

271

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“…345 For the siloxyl results show that the singly occupied orbital is mainly centered on one of the 2p orbitals of the non-bridging oxygen 345,394 and that theory is able to reproduce the essential EPR properties of these centers, 345,395 particularly when using hybrid functionals. Moreover, the overall absorption spectra calculated with time dependent DFT are in qualitative agreement with the experimental data.…”

Section: Surface Sitesmentioning

confidence: 99%

“…345,[393][394][395][396] In these cases, however, where radical species are present, the use of hybrid functionals is especially important to get accurate results.…”

Section: Dft Based Methodsmentioning

confidence: 99%

“…345 For silyl sites, the unpaired electron is also localized in a sp 3 orbital of the three coordinated silicon but, the EPR properties, particularly a iso , are more sensitive to the surface structure, mostly to the O-Si • -O angle and the Si • -O distance. 395 Remarkably, calculations have also shown that these two well-known paramagnetic centers of silica surface are deep electron traps able to form stable negatively charged surface centers, 394 which may have important consequences on their physical-chemical properties.…”

Section: Surface Sitesmentioning

confidence: 99%

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Silica Surface Features and Their Role in the Adsorption of Biomolecules: Computational Modeling and Experiments

et al. 2013

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“…19 Recent calculations have also demonstrated that the two dominant neutral paramagnetic defects at surfaces of a-SiO 2 , the non-bridging oxygen center and the silicon dangling bond, are deep electron traps and can form the corresponding negatively charged defects. 20 However, these theoretical predictions have not yet been confirmed experimentally due to challenges in identifying defect centers.…”

Section: Introductionmentioning

confidence: 99%

Nature of intrinsic and extrinsic electron trapping in SiO2

et al. 2014

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Using ab initio calculations we demonstrate that extra electrons in pure amorphous SiO2 can be trapped in deep band gap states. Classical potentials were used to generate amorphous silica models and density functional theory to characterize the geometrical and electronic structures of trapped electrons. Extra electrons can trap spontaneously on pre-existing structural precursors in amorphous SiO2 and produce ≈ 3.2 eV deep states in the band gap. These precursors comprise wide (≥132 • ) O-Si-O angles and elongated Si-O bonds at the tails of corresponding distributions. The electron trapping in amorphous silica structure results in an opening of the O-Si-O angle (up to almost 180 • ). We estimate the concentration of these electron trapping sites to be ≈ 4 × 10 19 cm −3 . The structure of these centers is similar to that of Ge and Li electron centers in α-quartz.

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Electron Trapping at Point Defects on Hydroxylated Silica Surfaces

Cited by 47 publications

References 29 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)

Silica Surface Features and Their Role in the Adsorption of Biomolecules: Computational Modeling and Experiments

Nature of intrinsic and extrinsic electron trapping in SiO2

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Electron Trapping at Point Defects on Hydroxylated Silica Surfaces

Cited by 47 publications

References 29 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)

Silica Surface Features and Their Role in the Adsorption of Biomolecules: Computational Modeling and Experiments

Nature of intrinsic and extrinsic electron trapping in SiO2

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Product

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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)