An EPR study of thermally and photochemically generated oxygen radicals on hydrated and dehydrated titania surfaces

Attwood, Anna Lisa; Murphy, D. M.; Edwards, J. L.; Egerton, Terry A.; Harrison, R. W.

doi:10.1163/156856703322148991

Cited by 106 publications

(127 citation statements)

References 27 publications

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“…S6, A and B We propose that the surface redox chemistry on reduced titania is to a large extent associated with Ti interstitials in the near-surface region, because these defect sites provide the electronic charge, enabling important reactions. This conclusion is consistent with numerous experimental studies addressing the Ti3d defect state, including the pioneering work by Henrich et al (5, 8), electron paramagnetic resonance spectroscopy (35)(36)(37), bulk photonic (38) and transport (39) studies, recent photoelectron diffraction studies (40), and thin-film growth studies (41).…”

supporting

confidence: 88%

The Role of Interstitial Sites in the Ti 3d Defect State in the Band Gap of Titania

Wendt

Sprunger

Lira

et al. 2008

Science

838

111

1,188

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Titanium dioxide (TiO 2 ) has a number of uses in catalysis, photochemistry, and sensing that are linked to the reducibility of the oxide. Usually, bridging oxygen (O br ) vacancies are assumed to cause the Ti 3d defect state in the band gap of rutile TiO 2 (110). From high-resolution scanning tunneling microscopy and photoelectron spectroscopy measurements, we propose that Ti interstitials in the near-surface region may be largely responsible for the defect state in the band gap. We argue that these donor-specific sites play a key role in and may dictate the ensuing surface chemistry, such as providing the electronic charge required for O 2 adsorption and dissociation. Specifically, we identified a second O 2 dissociation channel that occurs within the Ti troughs in addition to the O 2 dissociation channel in O br vacancies. Comprehensive density functional theory calculations support these experimental observations.

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supporting

confidence: 88%

The Role of Interstitial Sites in the Ti 3d Defect State in the Band Gap of Titania

Wendt

Sprunger

Lira

et al. 2008

Science

838

111

1,188

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“…The contact with H 2 O 2 also modified the signals of TiO 2 -A-uf, since the signal corresponding to the OC À species disappeared. As expected, after incubation in H 2 O 2 , an additional signal having the typical component of superoxide species adsorbed at the surface (g 1 = 2.026, g 2 = 2.009, g 3 = 2.003) [32] appeared in the ESR spectra of the powders containing the anatase phase ( Figure 5A, spectra a, b and d). In the case of TiO 2 -AR-uf a small shift of the g 1 component (g 1 % 2.029) was observed.…”

supporting

confidence: 81%

“…[35] A small shift of the g 1 component (g 1 % 2.032) was also observed for TiO 2 -A-uf: in this case it can be ascribed to the presence of HO 2 C-like species. [32] No adsorbed superoxide was observed for TiO 2 -R-uf ( Figure 5A, spectrum c). In the spectra of TiO 2 -A-f a new unidentified signal at g = 2.003 appeared when the powder was incubated in H 2 O 2 ( Figure 5A, spectrum d).…”

mentioning

confidence: 97%

“…The ESR spectra of the four samples are compared in Figure 4. The samples exhibit different degrees of defects: in the case of TiO 2 -A-uf (Figure 4a) a spectrum resulting from overlapping the signal corresponding to ad- www.chemeurj.org sorbed superoxide (g 1 = 2.026, g 2 = 2.009, g 3 = 2.002) [32] and OC À species (g 1 = 2.014, g 2 = 2.002), the latter being associated with electron holes (h + ), has been recorded. TiO 2 -AR-uf ( Figure 4b) exhibited an intense signal at g 1 = 1.973 and g 2 = 1.949, which may be assigned to Ti 3 + in the rutile phase, [33] and two less intense signals, partially buried in the signal at 1.973, at g = 1.991 and g = 1.979, which correspond to two Ti 3 + species in the anatase phase in different chemical surroundings.…”

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confidence: 99%

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Non‐UV‐Induced Radical Reactions at the Surface of TiO₂ Nanoparticles That May Trigger Toxic Responses

Fenoglio

Greco

Livraghi

et al. 2009

Chemistry A European J

177

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Kept in the dark: The non-photocatalytic generation of free radicals from fine and ultrafine TiO(2) particles has been studied by means of a spin-trapping/ESR spectroscopy technique (see figure). The amount and kind of free radicals generated depends on the crystalline structure, but not on the particle dimensions.Titania is generally considered to be an inert and safe material. Several studies, however, have reported that nanosized TiO(2) may elicit toxic effects. In some cases the observed adverse effects have been related to free radicals. Although new studies mainly concern irradiated titania, the role and the mechanisms of the generation of free radicals by TiO(2) in the absence of UV irradiation are not well known. The purpose of the present study is to investigate the free-radical-generation mechanisms by nano- and micronsized anatase or rutile powders under normal laboratory illumination or in the dark by means of a spin-trapping/ESR spectroscopy technique. This technique is used to identify the nature and the amount of free radicals released in solution, and in the solid-state to characterise the paramagnetic centres at the surface of particles that may participate in the reactions. The following radical-generating mechanisms have been considered: 1) the generation of oxygenated free radicals (HO(2) (.), O(2) (.-), HO(.)) following the reaction of TiO(2) with oxygen, water or H(2)O(2) and 2) the generation of carbon-centred radicals following the cleavage of the C--H bond in a model molecule. Although no free radicals were detected in a simply buffered solution, anatase and rutile generated O(2) (.-) and HO(.), respectively, in the presence of H(2)O(2). Both polymorphs were also active in the cleavage of the C--H bond. Although the formation of O(2) (.-) appears to be related to exposure to sunlight, the generation of HO(.) and carbon-centred free radicals also occurs in the dark. When samples of equal surface area were tested, micron- and nanosized anatase was found to react in the same way indicating that a reduction in diameter does not generate new kinds of reactive sites. The data presented herein may have implications in the assessment of the health risk associated with the exposure to TiO(2) nanoparticles and in the ecotoxicological impact following their possible leakage into the environment.

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“…The difference observed for the values of the g z component confirms different local environments at the surface for the R-TiO 2 and L-TiO 2 nanoparticles. 38,[40][41][42] The system dye-R-TiO 2 shows evidence of charge transfer (CT) complex 43 formation, whereas dye-L-TiO 2 does not, suggesting the presence of strong electronic coupling between near-by defects and the dye in the dark (see Supporting Information Figure S6). In order to determine the role of defect states in photoinduced electron transfer processes, a perylene dye bearing a phosphonic acid group as a binding moiety (dye in Scheme 1) was synthesized and attached to the surface of the nanoparticles.…”

Section: Resultsmentioning

confidence: 99%

Controlling Surface Defects and Photophysics in TiO₂ Nanoparticles

et al. 2014

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Titanium dioxide (TiO 2 ) is widely used for photocatalysis and solar cell applications, and the electronic structure of bulk TiO 2 is well understood. However, surface structure of nanoparticulate TiO 2 , which has a key role in properties such as solubility and catalytic activity, still remains controversial. Detailed understanding of surface defects structures may help explain reactivity and overall materials performance in a wide range of applications. In this work we address the solubility problem and surface defects control on TiO 2 nanoparticles. We report the synthesis and characterization of ~ 4 nm TiO 2 anatase spherical nanoparticles that are soluble and stable in a wide range of organic solvents and water. By controlling the temperature during the synthesis, we are able to tailor the density of defect states on the surface of the TiO 2 nanoparticles without affecting parameters such as size, shape, core crystallinity and solubility.The morphology of both kinds of nanoparticles was determined by TEM. EPR experiments were used to characterize the surface defects, and transient absorption measurements demonstrate the influence of the TiO 2 defect states on photoinduced electron transfer dynamics.3

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An EPR study of thermally and photochemically generated oxygen radicals on hydrated and dehydrated titania surfaces

Cited by 106 publications

References 27 publications

The Role of Interstitial Sites in the Ti 3d Defect State in the Band Gap of Titania

The Role of Interstitial Sites in the Ti 3d Defect State in the Band Gap of Titania

Non‐UV‐Induced Radical Reactions at the Surface of TiO₂ Nanoparticles That May Trigger Toxic Responses

Controlling Surface Defects and Photophysics in TiO₂ Nanoparticles

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An EPR study of thermally and photochemically generated oxygen radicals on hydrated and dehydrated titania surfaces

Cited by 106 publications

References 27 publications

The Role of Interstitial Sites in the Ti 3d Defect State in the Band Gap of Titania

The Role of Interstitial Sites in the Ti 3d Defect State in the Band Gap of Titania

Non‐UV‐Induced Radical Reactions at the Surface of TiO2 Nanoparticles That May Trigger Toxic Responses

Controlling Surface Defects and Photophysics in TiO2 Nanoparticles

Contact Info

Product

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Non‐UV‐Induced Radical Reactions at the Surface of TiO₂ Nanoparticles That May Trigger Toxic Responses

Controlling Surface Defects and Photophysics in TiO₂ Nanoparticles